< draft-ietf-tcpm-rfc793bis-24.txt   draft-ietf-tcpm-rfc793bis-28.txt >
Internet Engineering Task Force W. Eddy, Ed. Internet Engineering Task Force W. Eddy, Ed.
Internet-Draft MTI Systems Internet-Draft MTI Systems
Obsoletes: 793, 879, 2873, 6093, 6429, July 12, 2021 Obsoletes: 793, 879, 2873, 6093, 6429, 6528, 7 March 2022
6528, 6691 (if approved) 6691 (if approved)
Updates: 5961, 1122 (if approved) Updates: 5961, 1011, 1122 (if approved)
Intended status: Standards Track Intended status: Standards Track
Expires: January 13, 2022 Expires: 8 September 2022
Transmission Control Protocol (TCP) Specification Transmission Control Protocol (TCP) Specification
draft-ietf-tcpm-rfc793bis-24 draft-ietf-tcpm-rfc793bis-28
Abstract Abstract
This document specifies the Transmission Control Protocol (TCP). TCP This document specifies the Transmission Control Protocol (TCP). TCP
is an important transport layer protocol in the Internet protocol is an important transport layer protocol in the Internet protocol
stack, and has continuously evolved over decades of use and growth of stack, and has continuously evolved over decades of use and growth of
the Internet. Over this time, a number of changes have been made to the Internet. Over this time, a number of changes have been made to
TCP as it was specified in RFC 793, though these have only been TCP as it was specified in RFC 793, though these have only been
documented in a piecemeal fashion. This document collects and brings documented in a piecemeal fashion. This document collects and brings
those changes together with the protocol specification from RFC 793. those changes together with the protocol specification from RFC 793.
This document obsoletes RFC 793, as well as RFCs 879, 2873, 6093, This document obsoletes RFC 793, as well as RFCs 879, 2873, 6093,
6429, 6528, and 6691 that updated parts of RFC 793. It updates RFC 6429, 6528, and 6691 that updated parts of RFC 793. It updates RFCs
1122, and should be considered as a replacement for the portions of 1011 and 1122, and should be considered as a replacement for the
that document dealing with TCP requirements. It also updates RFC portions of those document dealing with TCP requirements. It also
5961 by adding a small clarification in reset handling while in the updates RFC 5961 by adding a small clarification in reset handling
SYN-RECEIVED state. The TCP header control bits from RFC 793 have while in the SYN-RECEIVED state. The TCP header control bits from
also been updated based on RFC 3168. RFC 793 have also been updated based on RFC 3168.
RFC EDITOR NOTE: If approved for publication as an RFC, this should RFC EDITOR NOTE: If approved for publication as an RFC, this should
be marked additionally as "STD: 7" and replace RFC 793 in that role. be marked additionally as "STD: 7" and replace RFC 793 in that role.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 13, 2022. This Internet-Draft will expire on 8 September 2022.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . 3 1. Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 5 2.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2.2. Key TCP Concepts . . . . . . . . . . . . . . . . . . . . 5 2.2. Key TCP Concepts . . . . . . . . . . . . . . . . . . . . 6
3. Functional Specification . . . . . . . . . . . . . . . . . . 6 3. Functional Specification . . . . . . . . . . . . . . . . . . 6
3.1. Header Format . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Header Format . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Specific Option Definitions . . . . . . . . . . . . . . . 11 3.2. Specific Option Definitions . . . . . . . . . . . . . . . 12
3.2.1. Other Common Options . . . . . . . . . . . . . . . . 13 3.2.1. Other Common Options . . . . . . . . . . . . . . . . 13
3.2.2. Experimental TCP Options . . . . . . . . . . . . . . 13 3.2.2. Experimental TCP Options . . . . . . . . . . . . . . 13
3.3. TCP Terminology Overview . . . . . . . . . . . . . . . . 13 3.3. TCP Terminology Overview . . . . . . . . . . . . . . . . 13
3.3.1. Key Connection State Variables . . . . . . . . . . . 13 3.3.1. Key Connection State Variables . . . . . . . . . . . 13
3.3.2. State Machine Overview . . . . . . . . . . . . . . . 15 3.3.2. State Machine Overview . . . . . . . . . . . . . . . 15
3.4. Sequence Numbers . . . . . . . . . . . . . . . . . . . . 18 3.4. Sequence Numbers . . . . . . . . . . . . . . . . . . . . 18
3.4.1. Initial Sequence Number Selection . . . . . . . . . . 21
3.4.2. Knowing When to Keep Quiet . . . . . . . . . . . . . 23
3.4.3. The TCP Quiet Time Concept . . . . . . . . . . . . . 23
3.5. Establishing a connection . . . . . . . . . . . . . . . . 25 3.5. Establishing a connection . . . . . . . . . . . . . . . . 25
3.5.1. Half-Open Connections and Other Anomalies . . . . . . 28
3.5.2. Reset Generation . . . . . . . . . . . . . . . . . . 31
3.5.3. Reset Processing . . . . . . . . . . . . . . . . . . 32
3.6. Closing a Connection . . . . . . . . . . . . . . . . . . 32 3.6. Closing a Connection . . . . . . . . . . . . . . . . . . 32
3.6.1. Half-Closed Connections . . . . . . . . . . . . . . . 34 3.6.1. Half-Closed Connections . . . . . . . . . . . . . . . 35
3.7. Segmentation . . . . . . . . . . . . . . . . . . . . . . 35 3.7. Segmentation . . . . . . . . . . . . . . . . . . . . . . 35
3.7.1. Maximum Segment Size Option . . . . . . . . . . . . . 36 3.7.1. Maximum Segment Size Option . . . . . . . . . . . . . 37
3.7.2. Path MTU Discovery . . . . . . . . . . . . . . . . . 38 3.7.2. Path MTU Discovery . . . . . . . . . . . . . . . . . 38
3.7.3. Interfaces with Variable MTU Values . . . . . . . . . 38 3.7.3. Interfaces with Variable MTU Values . . . . . . . . . 39
3.7.4. Nagle Algorithm . . . . . . . . . . . . . . . . . . . 39 3.7.4. Nagle Algorithm . . . . . . . . . . . . . . . . . . . 39
3.7.5. IPv6 Jumbograms . . . . . . . . . . . . . . . . . . . 39 3.7.5. IPv6 Jumbograms . . . . . . . . . . . . . . . . . . . 40
3.8. Data Communication . . . . . . . . . . . . . . . . . . . 39 3.8. Data Communication . . . . . . . . . . . . . . . . . . . 40
3.8.1. Retransmission Timeout . . . . . . . . . . . . . . . 40 3.8.1. Retransmission Timeout . . . . . . . . . . . . . . . 41
3.8.2. TCP Congestion Control . . . . . . . . . . . . . . . 41 3.8.2. TCP Congestion Control . . . . . . . . . . . . . . . 41
3.8.3. TCP Connection Failures . . . . . . . . . . . . . . . 41 3.8.3. TCP Connection Failures . . . . . . . . . . . . . . . 42
3.8.4. TCP Keep-Alives . . . . . . . . . . . . . . . . . . . 42 3.8.4. TCP Keep-Alives . . . . . . . . . . . . . . . . . . . 43
3.8.5. The Communication of Urgent Information . . . . . . . 43 3.8.5. The Communication of Urgent Information . . . . . . . 44
3.8.6. Managing the Window . . . . . . . . . . . . . . . . . 44 3.8.6. Managing the Window . . . . . . . . . . . . . . . . . 45
3.9. Interfaces . . . . . . . . . . . . . . . . . . . . . . . 49 3.9. Interfaces . . . . . . . . . . . . . . . . . . . . . . . 50
3.9.1. User/TCP Interface . . . . . . . . . . . . . . . . . 49 3.9.1. User/TCP Interface . . . . . . . . . . . . . . . . . 50
3.9.2. TCP/Lower-Level Interface . . . . . . . . . . . . . . 58 3.9.2. TCP/Lower-Level Interface . . . . . . . . . . . . . . 59
3.10. Event Processing . . . . . . . . . . . . . . . . . . . . 60 3.10. Event Processing . . . . . . . . . . . . . . . . . . . . 61
3.10.1. OPEN Call . . . . . . . . . . . . . . . . . . . . . 62 3.10.1. OPEN Call . . . . . . . . . . . . . . . . . . . . . 63
3.10.2. SEND Call . . . . . . . . . . . . . . . . . . . . . 63 3.10.2. SEND Call . . . . . . . . . . . . . . . . . . . . . 64
3.10.3. RECEIVE Call . . . . . . . . . . . . . . . . . . . . 64 3.10.3. RECEIVE Call . . . . . . . . . . . . . . . . . . . . 65
3.10.4. CLOSE Call . . . . . . . . . . . . . . . . . . . . . 65 3.10.4. CLOSE Call . . . . . . . . . . . . . . . . . . . . . 67
3.10.5. ABORT Call . . . . . . . . . . . . . . . . . . . . . 66 3.10.5. ABORT Call . . . . . . . . . . . . . . . . . . . . . 68
3.10.6. STATUS Call . . . . . . . . . . . . . . . . . . . . 67 3.10.6. STATUS Call . . . . . . . . . . . . . . . . . . . . 69
3.10.7. SEGMENT ARRIVES . . . . . . . . . . . . . . . . . . 68 3.10.7. SEGMENT ARRIVES . . . . . . . . . . . . . . . . . . 70
3.10.8. Timeouts . . . . . . . . . . . . . . . . . . . . . . 81 3.10.8. Timeouts . . . . . . . . . . . . . . . . . . . . . . 84
4. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . 81 4. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5. Changes from RFC 793 . . . . . . . . . . . . . . . . . . . . 86 5. Changes from RFC 793 . . . . . . . . . . . . . . . . . . . . 89
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 92 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 96
7. Security and Privacy Considerations . . . . . . . . . . . . . 93 7. Security and Privacy Considerations . . . . . . . . . . . . . 97
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 95 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 99
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 95 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 100
9.1. Normative References . . . . . . . . . . . . . . . . . . 96 9.1. Normative References . . . . . . . . . . . . . . . . . . 100
9.2. Informative References . . . . . . . . . . . . . . . . . 97 9.2. Informative References . . . . . . . . . . . . . . . . . 102
Appendix A. Other Implementation Notes . . . . . . . . . . . . . 102 Appendix A. Other Implementation Notes . . . . . . . . . . . . . 107
A.1. IP Security Compartment and Precedence . . . . . . . . . 102 A.1. IP Security Compartment and Precedence . . . . . . . . . 108
A.1.1. Precedence . . . . . . . . . . . . . . . . . . . . . 102 A.1.1. Precedence . . . . . . . . . . . . . . . . . . . . . 108
A.1.2. MLS Systems . . . . . . . . . . . . . . . . . . . . . 103 A.1.2. MLS Systems . . . . . . . . . . . . . . . . . . . . . 109
A.2. Sequence Number Validation . . . . . . . . . . . . . . . 103 A.2. Sequence Number Validation . . . . . . . . . . . . . . . 109
A.3. Nagle Modification . . . . . . . . . . . . . . . . . . . 104 A.3. Nagle Modification . . . . . . . . . . . . . . . . . . . 109
A.4. Low Water Mark Settings . . . . . . . . . . . . . . . . . 104 A.4. Low Watermark Settings . . . . . . . . . . . . . . . . . 110
Appendix B. TCP Requirement Summary . . . . . . . . . . . . . . 104 Appendix B. TCP Requirement Summary . . . . . . . . . . . . . . 110
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 108 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 114
1. Purpose and Scope 1. Purpose and Scope
In 1981, RFC 793 [16] was released, documenting the Transmission In 1981, RFC 793 [16] was released, documenting the Transmission
Control Protocol (TCP), and replacing earlier specifications for TCP Control Protocol (TCP), and replacing earlier specifications for TCP
that had been published in the past. that had been published in the past.
Since then, TCP has been widely implemented, and has been used as a Since then, TCP has been widely implemented, and has been used as a
transport protocol for numerous applications on the Internet. transport protocol for numerous applications on the Internet.
For several decades, RFC 793 plus a number of other documents have For several decades, RFC 793 plus a number of other documents have
combined to serve as the core specification for TCP [48]. Over time, combined to serve as the core specification for TCP [50]. Over time,
a number of errata have been filed against RFC 793, as well as a number of errata have been filed against RFC 793. There have also
deficiencies in security, performance, and many other aspects. The been deficiencies found and resolved in security, performance, and
number of enhancements has grown over time across many separate many other aspects. The number of enhancements has grown over time
documents. These were never accumulated together into a across many separate documents. These were never accumulated
comprehensive update to the base specification. together into a comprehensive update to the base specification.
The purpose of this document is to bring together all of the IETF The purpose of this document is to bring together all of the IETF
Standards Track changes that have been made to the base TCP Standards Track changes and other clarifications that have been made
functional specification and unify them into an update of RFC 793. to the base TCP functional specification and unify them into an
updated version of RFC 793.
Some companion documents are referenced for important algorithms that Some companion documents are referenced for important algorithms that
are used by TCP (e.g. for congestion control), but have not been are used by TCP (e.g. for congestion control), but have not been
completely included in this document. This is a conscious choice, as completely included in this document. This is a conscious choice, as
this base specification can be used with multiple additional this base specification can be used with multiple additional
algorithms that are developed and incorporated separately. This algorithms that are developed and incorporated separately. This
document focuses on the common basis all TCP implementations must document focuses on the common basis all TCP implementations must
support in order to interoperate. Since some additional TCP features support in order to interoperate. Since some additional TCP features
have become quite complicated themselves (e.g. advanced loss recovery have become quite complicated themselves (e.g. advanced loss recovery
and congestion control), future companion documents may attempt to and congestion control), future companion documents may attempt to
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explanations and rationale, where appropriate. explanations and rationale, where appropriate.
This document is intended to be useful both in checking existing TCP This document is intended to be useful both in checking existing TCP
implementations for conformance purposes, as well as in writing new implementations for conformance purposes, as well as in writing new
implementations. implementations.
2. Introduction 2. Introduction
RFC 793 contains a discussion of the TCP design goals and provides RFC 793 contains a discussion of the TCP design goals and provides
examples of its operation, including examples of connection examples of its operation, including examples of connection
establishment, connection termination, packet retransmission to establishment, connection termination, and packet retransmission to
repair losses. repair losses.
This document describes the basic functionality expected in modern This document describes the basic functionality expected in modern
TCP implementations, and replaces the protocol specification in RFC TCP implementations, and replaces the protocol specification in RFC
793. It does not replicate or attempt to update the introduction and 793. It does not replicate or attempt to update the introduction and
philosophy content in Sections 1 and 2 of RFC 793. Other documents philosophy content in Sections 1 and 2 of RFC 793. Other documents
are referenced to provide explanation of the theory of operation, are referenced to provide explanation of the theory of operation,
rationale, and detailed discussion of design decisions. This rationale, and detailed discussion of design decisions. This
document only focuses on the normative behavior of the protocol. document only focuses on the normative behavior of the protocol.
The "TCP Roadmap" [48] provides a more extensive guide to the RFCs The "TCP Roadmap" [50] provides a more extensive guide to the RFCs
that define TCP and describe various important algorithms. The TCP that define TCP and describe various important algorithms. The TCP
Roadmap contains sections on strongly encouraged enhancements that Roadmap contains sections on strongly encouraged enhancements that
improve performance and other aspects of TCP beyond the basic improve performance and other aspects of TCP beyond the basic
operation specified in this document. As one example, implementing operation specified in this document. As one example, implementing
congestion control (e.g. [9]) is a TCP requirement, but is a complex congestion control (e.g. [8]) is a TCP requirement, but is a complex
topic on its own, and not described in detail in this document, as topic on its own, and not described in detail in this document, as
there are many options and possibilities that do not impact basic there are many options and possibilities that do not impact basic
interoperability. Similarly, most TCP implementations today include interoperability. Similarly, most TCP implementations today include
the high-performance extensions in [46], but these are not strictly the high-performance extensions in [48], but these are not strictly
required or discussed in this document. Multipath considerations for required or discussed in this document. Multipath considerations for
TCP are also specified separately in [55]. TCP are also specified separately in [59].
A list of changes from RFC 793 is contained in Section 5. A list of changes from RFC 793 is contained in Section 5.
2.1. Requirements Language 2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [3][12] when, and only when, they appear in all capitals, as shown 14 [3][12] when, and only when, they appear in all capitals, as shown
here. here.
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segments, with each TCP segment sent as an Internet Protocol (IP) segments, with each TCP segment sent as an Internet Protocol (IP)
datagram. datagram.
TCP reliability consists of detecting packet losses (via sequence TCP reliability consists of detecting packet losses (via sequence
numbers) and errors (via per-segment checksums), as well as numbers) and errors (via per-segment checksums), as well as
correction via retransmission. correction via retransmission.
TCP supports unicast delivery of data. Anycast applications exist TCP supports unicast delivery of data. Anycast applications exist
that successfully use TCP without modifications, though there is some that successfully use TCP without modifications, though there is some
risk of instability due to changes of lower-layer forwarding behavior risk of instability due to changes of lower-layer forwarding behavior
[45]. [47].
TCP is connection-oriented, though does not inherently include a TCP is connection-oriented, though does not inherently include a
liveness detection capability. liveness detection capability.
Data flow is supported bidirectionally over TCP connections, though Data flow is supported bidirectionally over TCP connections, though
applications are free to send data only unidirectionally, if they so applications are free to send data only unidirectionally, if they so
choose. choose.
TCP uses port numbers to identify application services and to TCP uses port numbers to identify application services and to
multiplex distinct flows between hosts. multiplex distinct flows between hosts.
A more detailed description of TCP features compared to other A more detailed description of TCP features compared to other
transport protocols can be found in Section 3.1 of [51]. Further transport protocols can be found in Section 3.1 of [53]. Further
description of the motivations for developing TCP and its role in the description of the motivations for developing TCP and its role in the
Internet protocol stack can be found in Section 2 of [16] and earlier Internet protocol stack can be found in Section 2 of [16] and earlier
versions of the TCP specification. versions of the TCP specification.
3. Functional Specification 3. Functional Specification
3.1. Header Format 3.1. Header Format
TCP segments are sent as internet datagrams. The Internet Protocol TCP segments are sent as internet datagrams. The Internet Protocol
(IP) header carries several information fields, including the source (IP) header carries several information fields, including the source
and destination host addresses [1] [13]. A TCP header follows the IP and destination host addresses [1] [13]. A TCP header follows the IP
headers, supplying information specific to the TCP protocol. This headers, supplying information specific to the TCP protocol. This
division allows for the existence of host level protocols other than division allows for the existence of host level protocols other than
TCP. In early development of the Internet suite of protocols, the IP TCP. In early development of the Internet suite of protocols, the IP
header fields had been a part of TCP. header fields had been a part of TCP.
This document describes the TCP protocol. The TCP protocol uses TCP This document describes the TCP protocol. The TCP protocol uses TCP
Headers. Headers.
A TCP Header is formatted as follows, using the style from [61]: A TCP Header, followed by any user data in the segment, is formatted
as follows, using the style from [67]:
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port | | Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number | | Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number | | Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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the initial sequence number (ISN) and the first data octet is the initial sequence number (ISN) and the first data octet is
ISN+1. ISN+1.
Acknowledgment Number: 32 bits. Acknowledgment Number: 32 bits.
If the ACK control bit is set, this field contains the value of the If the ACK control bit is set, this field contains the value of the
next sequence number the sender of the segment is expecting to next sequence number the sender of the segment is expecting to
receive. Once a connection is established, this is always sent. receive. Once a connection is established, this is always sent.
Data Offset (DOffset): 4 bits. Data Offset (DOffset): 4 bits.
The number of 32 bit words in the TCP Header. This indicates where The number of 32 bit words in the TCP Header. This indicates where
the data begins. The TCP header (even one including options) is an the data begins. The TCP header (even one including options) is an
integer multiple of 32 bits long. integer multiple of 32 bits long.
Reserved (Rsrvd): 4 bits. Reserved (Rsrvd): 4 bits.
A set of control bits reserved for future use. Must be zero in A set of control bits reserved for future use. Must be zero in
generated segments and must be ignored in received segments, if generated segments and must be ignored in received segments, if
corresponding future features are unimplemented by the sending or corresponding future features are unimplemented by the sending or
receiving host. receiving host.
The control bits are also know as "flags". Assignment is managed The control bits are also known as "flags". Assignment is managed
by IANA from the "TCP Header Flags" registry [57]. The currently by IANA from the "TCP Header Flags" registry [63]. The currently
assigned control bits are CWR, ECE, URG, ACK, PSH, RST, SYN, and assigned control bits are CWR, ECE, URG, ACK, PSH, RST, SYN, and
FIN. FIN.
CWR: 1 bit. CWR: 1 bit.
Congestion Window Reduced (see [6]).
Congestion Window Reduced (see [7]).
ECE: 1 bit. ECE: 1 bit.
ECN-Echo (see [6]).
ECN-Echo (see [7]).
URG: 1 bit. URG: 1 bit.
Urgent Pointer field is significant. Urgent Pointer field is significant.
ACK: 1 bit. ACK: 1 bit.
Acknowledgment field is significant. Acknowledgment field is significant.
PSH: 1 bit. PSH: 1 bit.
Push Function (see the Send Call description in Section 3.9.1). Push Function (see the Send Call description in Section 3.9.1).
RST: 1 bit. RST: 1 bit.
Reset the connection. Reset the connection.
SYN: 1 bit. SYN: 1 bit.
Synchronize sequence numbers. Synchronize sequence numbers.
FIN: 1 bit. FIN: 1 bit.
No more data from sender. No more data from sender.
Window: 16 bits. Window: 16 bits.
The number of data octets beginning with the one indicated in the The number of data octets beginning with the one indicated in the
acknowledgment field that the sender of this segment is willing to acknowledgment field that the sender of this segment is willing to
accept. The value is shifted when the Window Scaling extension is accept. The value is shifted when the Window Scaling extension is
used [46]. used [48].
The window size MUST be treated as an unsigned number, or else The window size MUST be treated as an unsigned number, or else
large window sizes will appear like negative windows and TCP will large window sizes will appear like negative windows and TCP will
not work (MUST-1). It is RECOMMENDED that implementations will not work (MUST-1). It is RECOMMENDED that implementations will
reserve 32-bit fields for the send and receive window sizes in the reserve 32-bit fields for the send and receive window sizes in the
connection record and do all window computations with 32 bits (REC- connection record and do all window computations with 32 bits (REC-
1). 1).
Checksum: 16 bits. Checksum: 16 bits.
The checksum field is the 16 bit ones' complement of the ones'
The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header and text. The complement sum of all 16 bit words in the header and text. The
checksum computation needs to ensure the 16-bit alignment of the checksum computation needs to ensure the 16-bit alignment of the
data being summed. If a segment contains an odd number of header data being summed. If a segment contains an odd number of header
and text octets, alignment can be achieved by padding the last and text octets, alignment can be achieved by padding the last
octet with zeros on its right to form a 16 bit word for checksum octet with zeros on its right to form a 16 bit word for checksum
purposes. The pad is not transmitted as part of the segment. purposes. The pad is not transmitted as part of the segment.
While computing the checksum, the checksum field itself is replaced While computing the checksum, the checksum field itself is replaced
with zeros. with zeros.
The checksum also covers a pseudo header (Figure 2) conceptually The checksum also covers a pseudo header (Figure 2) conceptually
prefixed to the TCP header. The pseudo header is 96 bits for IPv4 prefixed to the TCP header. The pseudo header is 96 bits for IPv4
and 320 bits for IPv6. Including the pseudo header in the checksum and 320 bits for IPv6. Including the pseudo header in the checksum
gives the TCP connection protection against misrouted segments. gives the TCP connection protection against misrouted segments.
This information is carried in IP headers and is transferred across This information is carried in IP headers and is transferred across
the TCP/Network interface in the arguments or results of calls by the TCP/Network interface in the arguments or results of calls by
the TCP implementation on the IP layer. the TCP implementation on the IP layer.
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| Source Address | | Source Address |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| Destination Address | | Destination Address |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
| zero | PTCL | TCP Length | | zero | PTCL | TCP Length |
+--------+--------+--------+--------+ +--------+--------+--------+--------+
Figure 2: IPv4 Pseudo Header Figure 2: IPv4 Pseudo Header
Pseudo header components for IPv4: Pseudo header components for IPv4:
Source Address: the IPv4 source address in network byte order Source Address: the IPv4 source address in network byte order
Destination Address: the IPv4 destination address in network Destination Address: the IPv4 destination address in network
byte order byte order
zero: bits set to zero zero: bits set to zero
PTCL: the protocol number from the IP header PTCL: the protocol number from the IP header
TCP Length: the TCP header length plus the data length in TCP Length: the TCP header length plus the data length in
octets (this is not an explicitly transmitted quantity, but is octets (this is not an explicitly transmitted quantity, but is
computed), and it does not count the 12 octets of the pseudo computed), and it does not count the 12 octets of the pseudo
header. header.
For IPv6, the pseudo header is defined in Section 8.1 of RFC 8200 For IPv6, the pseudo header is defined in Section 8.1 of RFC 8200
skipping to change at page 10, line 31 skipping to change at page 10, line 25
Address, an Upper Layer Packet Length (a 32-bit value otherwise Address, an Upper Layer Packet Length (a 32-bit value otherwise
equivalent to TCP Length in the IPv4 pseudo header), three bytes equivalent to TCP Length in the IPv4 pseudo header), three bytes
of zero-padding, and a Next Header value (differing from the IPv6 of zero-padding, and a Next Header value (differing from the IPv6
header value in the case of extension headers present in between header value in the case of extension headers present in between
IPv6 and TCP). IPv6 and TCP).
The TCP checksum is never optional. The sender MUST generate it The TCP checksum is never optional. The sender MUST generate it
(MUST-2) and the receiver MUST check it (MUST-3). (MUST-2) and the receiver MUST check it (MUST-3).
Urgent Pointer: 16 bits. Urgent Pointer: 16 bits.
This field communicates the current value of the urgent pointer as This field communicates the current value of the urgent pointer as
a positive offset from the sequence number in this segment. The a positive offset from the sequence number in this segment. The
urgent pointer points to the sequence number of the octet following urgent pointer points to the sequence number of the octet following
the urgent data. This field is only be interpreted in segments the urgent data. This field is only to be interpreted in segments
with the URG control bit set. with the URG control bit set.
Options: [TCP Option]; Options#Size == (DOffset-5)*32; present only Options: [TCP Option]; size(Options) == (DOffset-5)*32; present
when DOffset > 5. only when DOffset > 5. Note that this size expression also
includes any padding trailing the actual options present.
Options may occupy space at the end of the TCP header and are a Options may occupy space at the end of the TCP header and are a
multiple of 8 bits in length. All options are included in the multiple of 8 bits in length. All options are included in the
checksum. An option may begin on any octet boundary. There are two checksum. An option may begin on any octet boundary. There are
cases for the format of an option: two cases for the format of an option:
Case 1: A single octet of option-kind.
Case 2: An octet of option-kind (Kind), an octet of option- Case 1: A single octet of option-kind.
length, and the actual option-data octets.
The option-length counts the two octets of option-kind and option- Case 2: An octet of option-kind (Kind), an octet of option-
length as well as the option-data octets. length, and the actual option-data octets.
Note that the list of options may be shorter than the data offset The option-length counts the two octets of option-kind and option-
field might imply. The content of the header beyond the End-of- length as well as the option-data octets.
Option option must be header padding (i.e., zero).
The list of all currently defined options is managed by IANA [56], Note that the list of options may be shorter than the data offset
and each option is defined in other RFCs, as indicated there. That field might imply. The content of the header beyond the End-of-
set includes experimental options that can be extended to support Option option MUST be header padding of zeros (MUST-69).
multiple concurrent usages [44].
A given TCP implementation can support any currently defined The list of all currently defined options is managed by IANA [62],
options, but the following options MUST be supported (MUST-4 - note and each option is defined in other RFCs, as indicated there. That
Maximum Segment Size option support is also part of MUST-19 in set includes experimental options that can be extended to support
Section 3.7.2): multiple concurrent usages [46].
Kind Length Meaning A given TCP implementation can support any currently defined
---- ------ ------- options, but the following options MUST be supported (MUST-4 - note
0 - End of option list. Maximum Segment Size option support is also part of MUST-19 in
1 - No-Operation. Section 3.7.2):
2 4 Maximum Segment Size.
These options are specified in detail in Section 3.2. Kind Length Meaning
---- ------ -------
0 - End of option list.
1 - No-Operation.
2 4 Maximum Segment Size.
A TCP implementation MUST be able to receive a TCP option in any These options are specified in detail in Section 3.2.
segment (MUST-5).
A TCP implementation MUST (MUST-6) ignore without error any TCP A TCP implementation MUST be able to receive a TCP option in any
option it does not implement, assuming that the option has a length segment (MUST-5).
field. All TCP options except End of option list and No-Operation
MUST have length fields, including all future options (MUST-68).
TCP implementations MUST be prepared to handle an illegal option
length (e.g., zero); a suggested procedure is to reset the
connection and log the error cause (MUST-7).
Note: There is ongoing work to extend the space available for TCP A TCP implementation MUST (MUST-6) ignore without error any TCP
options, such as [60]. option it does not implement, assuming that the option has a length
field. All TCP options except End of option list and No-Operation
MUST have length fields, including all future options (MUST-68).
TCP implementations MUST be prepared to handle an illegal option
length (e.g., zero); a suggested procedure is to reset the
connection and log the error cause (MUST-7).
Data: variable length. Note: There is ongoing work to extend the space available for TCP
options, such as [66].
User data carried by the TCP segment. Data: variable length.
User data carried by the TCP segment.
3.2. Specific Option Definitions 3.2. Specific Option Definitions
A TCP Option is one of: an End of Option List Option, a No-Operation A TCP Option, in the mandatory option set, is one of: an End of
Option, or a Maximum Segment Size Option. Option List Option, a No-Operation Option, or a Maximum Segment Size
Option.
An End of Option List Option is formatted as follows: An End of Option List Option is formatted as follows:
0 0
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| 0 | | 0 |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
where: where:
skipping to change at page 13, line 12 skipping to change at page 13, line 18
If this option is present, then it communicates the maximum receive If this option is present, then it communicates the maximum receive
segment size at the TCP endpoint that sends this segment. This segment size at the TCP endpoint that sends this segment. This
value is limited by the IP reassembly limit. This field may be value is limited by the IP reassembly limit. This field may be
sent in the initial connection request (i.e., in segments with the sent in the initial connection request (i.e., in segments with the
SYN control bit set) and MUST NOT be sent in other segments (MUST- SYN control bit set) and MUST NOT be sent in other segments (MUST-
65). If this option is not used, any segment size is allowed. A 65). If this option is not used, any segment size is allowed. A
more complete description of this option is provided in more complete description of this option is provided in
Section 3.7.1. Section 3.7.1.
Length: 1 byte; Length == 4. Length: 1 byte; Length == 4.
Length of the option in bytes. Length of the option in bytes.
Maximum Segment Size (MSS): 2 bytes. Maximum Segment Size (MSS): 2 bytes.
The maximum receive segment size at the TCP endpoint that sends The maximum receive segment size at the TCP endpoint that sends
this segment. this segment.
3.2.1. Other Common Options 3.2.1. Other Common Options
Additional RFCs define some other commonly used options that are Additional RFCs define some other commonly used options that are
recommended to implement for high performance, but not necessary for recommended to implement for high performance, but not necessary for
basic TCP interoperability. These are the TCP Selective basic TCP interoperability. These are the TCP Selective
Acknowledgement (SACK) option [21][24], TCP Timestamp (TS) option Acknowledgement (SACK) option [23][27], TCP Timestamp (TS) option
[46], and TCP Window Scaling (WS) option [46]. [48], and TCP Window Scaling (WS) option [48].
3.2.2. Experimental TCP Options 3.2.2. Experimental TCP Options
Experimental TCP option values are defined in [28], and [44] Experimental TCP option values are defined in [31], and [46]
describes the current recommended usage for these experimental describes the current recommended usage for these experimental
values. values.
3.3. TCP Terminology Overview 3.3. TCP Terminology Overview
This section includes an overview of key terms needed to understand This section includes an overview of key terms needed to understand
the detailed protocol operation in the rest of the document. There the detailed protocol operation in the rest of the document. There
is a traditional glossary of terms in Section 4. is a glossary of terms in Section 4.
3.3.1. Key Connection State Variables 3.3.1. Key Connection State Variables
Before we can discuss very much about the operation of the TCP Before we can discuss very much about the operation of the TCP
implementation we need to introduce some detailed terminology. The implementation we need to introduce some detailed terminology. The
maintenance of a TCP connection requires the remembering of several maintenance of a TCP connection requires maintaining state for
variables. We conceive of these variables being stored in a several variables. We conceive of these variables being stored in a
connection record called a Transmission Control Block or TCB. Among connection record called a Transmission Control Block or TCB. Among
the variables stored in the TCB are the local and remote IP addresses the variables stored in the TCB are the local and remote IP addresses
and port numbers, the IP security level and compartment of the and port numbers, the IP security level and compartment of the
connection (see Appendix A.1), pointers to the user's send and connection (see Appendix A.1), pointers to the user's send and
receive buffers, pointers to the retransmit queue and to the current receive buffers, pointers to the retransmit queue and to the current
segment. In addition several variables relating to the send and segment. In addition, several variables relating to the send and
receive sequence numbers are stored in the TCB. receive sequence numbers are stored in the TCB.
Send Sequence Variables: Send Sequence Variables:
SND.UNA - send unacknowledged SND.UNA - send unacknowledged
SND.NXT - send next SND.NXT - send next
SND.WND - send window SND.WND - send window
SND.UP - send urgent pointer SND.UP - send urgent pointer
SND.WL1 - segment sequence number used for last window update SND.WL1 - segment sequence number used for last window update
SND.WL2 - segment acknowledgment number used for last window SND.WL2 - segment acknowledgment number used for last window
skipping to change at page 15, line 14 skipping to change at page 15, line 14
1 2 3 1 2 3
----------|----------|---------- ----------|----------|----------
RCV.NXT RCV.NXT RCV.NXT RCV.NXT
+RCV.WND +RCV.WND
1 - old sequence numbers that have been acknowledged 1 - old sequence numbers that have been acknowledged
2 - sequence numbers allowed for new reception 2 - sequence numbers allowed for new reception
3 - future sequence numbers that are not yet allowed 3 - future sequence numbers that are not yet allowed
Figure 4: Receive Sequence Space Figure 4: Receive Sequence Space
The receive window is the portion of the sequence space labeled 2 in The receive window is the portion of the sequence space labeled 2 in
Figure 4. Figure 4.
There are also some variables used frequently in the discussion that There are also some variables used frequently in the discussion that
take their values from the fields of the current segment. take their values from the fields of the current segment.
Current Segment Variables: Current Segment Variables:
SEG.SEQ - segment sequence number SEG.SEQ - segment sequence number
skipping to change at page 16, line 36 skipping to change at page 16, line 36
termination request, and to avoid new connections being impacted termination request, and to avoid new connections being impacted
by delayed segments from previous connections. by delayed segments from previous connections.
CLOSED - represents no connection state at all. CLOSED - represents no connection state at all.
A TCP connection progresses from one state to another in response to A TCP connection progresses from one state to another in response to
events. The events are the user calls, OPEN, SEND, RECEIVE, CLOSE, events. The events are the user calls, OPEN, SEND, RECEIVE, CLOSE,
ABORT, and STATUS; the incoming segments, particularly those ABORT, and STATUS; the incoming segments, particularly those
containing the SYN, ACK, RST and FIN flags; and timeouts. containing the SYN, ACK, RST and FIN flags; and timeouts.
The OPEN call specifies whether connection establishment is to be
actively pursued, or to be passively waited for.
A passive OPEN request means that the process wants to accept
incoming connection requests, in contrast to an active OPEN
attempting to initiate a connection.
The state diagram in Figure 5 illustrates only state changes, The state diagram in Figure 5 illustrates only state changes,
together with the causing events and resulting actions, but addresses together with the causing events and resulting actions, but addresses
neither error conditions nor actions that are not connected with neither error conditions nor actions that are not connected with
state changes. In a later section, more detail is offered with state changes. In a later section, more detail is offered with
respect to the reaction of the TCP implementation to events. Some respect to the reaction of the TCP implementation to events. Some
state names are abbreviated or hyphenated differently in the diagram state names are abbreviated or hyphenated differently in the diagram
from how they appear elsewhere in the document. from how they appear elsewhere in the document.
NOTA BENE: This diagram is only a summary and must not be taken as NOTA BENE: This diagram is only a summary and must not be taken as
the total specification. Many details are not included. the total specification. Many details are not included.
skipping to change at page 17, line 50 skipping to change at page 17, line 50
+---------+ +---------+ +---------+ +---------+ +---------+ +---------+
|FINWAIT-2| | CLOSING | | LAST-ACK| |FINWAIT-2| | CLOSING | | LAST-ACK|
+---------+ +---------+ +---------+ +---------+ +---------+ +---------+
| rcv ACK of FIN | rcv ACK of FIN | | rcv ACK of FIN | rcv ACK of FIN |
| rcv FIN -------------- | Timeout=2MSL -------------- | | rcv FIN -------------- | Timeout=2MSL -------------- |
| ------- x V ------------ x V | ------- x V ------------ x V
\ snd ACK +---------+delete TCB +---------+ \ snd ACK +---------+delete TCB +---------+
-------------------->|TIME-WAIT|------------------->| CLOSED | -------------------->|TIME-WAIT|------------------->| CLOSED |
+---------+ +---------+ +---------+ +---------+
Figure 5: TCP Connection State Diagram Figure 5: TCP Connection State Diagram
The following notes apply to Figure 5: The following notes apply to Figure 5:
Note 1: The transition from SYN-RECEIVED to LISTEN on receiving a Note 1: The transition from SYN-RECEIVED to LISTEN on receiving a
RST is conditional on having reached SYN-RECEIVED after a passive RST is conditional on having reached SYN-RECEIVED after a passive
open. open.
Note 2: An unshown transition exists from FIN-WAIT-1 to TIME-WAIT Note 2: The figure omits a transition from FIN-WAIT-1 to TIME-WAIT
if a FIN is received and the local FIN is also acknowledged. if a FIN is received and the local FIN is also acknowledged.
Note 3: A RST can be sent from any state with a corresponding Note 3: A RST can be sent from any state with a corresponding
transition to TIME-WAIT (see [64] for rationale). These transition to TIME-WAIT (see [71] for rationale). These
transitions are not not explicitly shown, otherwise the diagram transitions are not explicitly shown, otherwise the diagram would
would become very difficult to read. Similarly, receipt of a RST become very difficult to read. Similarly, receipt of a RST from
from any state results in a transition to LISTEN or CLOSED, though any state results in a transition to LISTEN or CLOSED, though this
this is also omitted from the diagram for legibility. is also omitted from the diagram for legibility.
3.4. Sequence Numbers 3.4. Sequence Numbers
A fundamental notion in the design is that every octet of data sent A fundamental notion in the design is that every octet of data sent
over a TCP connection has a sequence number. Since every octet is over a TCP connection has a sequence number. Since every octet is
sequenced, each of them can be acknowledged. The acknowledgment sequenced, each of them can be acknowledged. The acknowledgment
mechanism employed is cumulative so that an acknowledgment of mechanism employed is cumulative so that an acknowledgment of
sequence number X indicates that all octets up to but not including X sequence number X indicates that all octets up to but not including X
have been received. This mechanism allows for straight-forward have been received. This mechanism allows for straight-forward
duplicate detection in the presence of retransmission. Numbering of duplicate detection in the presence of retransmission. Numbering of
octets within a segment is that the first data octet immediately octets within a segment is that the first data octet immediately
following the header is the lowest numbered, and the following octets following the header is the lowest numbered, and the following octets
are numbered consecutively. are numbered consecutively.
It is essential to remember that the actual sequence number space is It is essential to remember that the actual sequence number space is
finite, though very large. This space ranges from 0 to 2**32 - 1. finite, though large. This space ranges from 0 to 2**32 - 1. Since
Since the space is finite, all arithmetic dealing with sequence the space is finite, all arithmetic dealing with sequence numbers
numbers must be performed modulo 2**32. This unsigned arithmetic must be performed modulo 2**32. This unsigned arithmetic preserves
preserves the relationship of sequence numbers as they cycle from the relationship of sequence numbers as they cycle from 2**32 - 1 to
2**32 - 1 to 0 again. There are some subtleties to computer modulo 0 again. There are some subtleties to computer modulo arithmetic, so
arithmetic, so great care should be taken in programming the great care should be taken in programming the comparison of such
comparison of such values. The symbol "=<" means "less than or values. The symbol "=<" means "less than or equal" (modulo 2**32).
equal" (modulo 2**32).
The typical kinds of sequence number comparisons that the TCP The typical kinds of sequence number comparisons that the TCP
implementation must perform include: implementation must perform include:
(a) Determining that an acknowledgment refers to some sequence (a) Determining that an acknowledgment refers to some sequence
number sent but not yet acknowledged. number sent but not yet acknowledged.
(b) Determining that all sequence numbers occupied by a segment (b) Determining that all sequence numbers occupied by a segment
have been acknowledged (e.g., to remove the segment from a have been acknowledged (e.g., to remove the segment from a
retransmission queue). retransmission queue).
skipping to change at page 19, line 38 skipping to change at page 19, line 38
the inequality below holds: the inequality below holds:
SND.UNA < SEG.ACK =< SND.NXT SND.UNA < SEG.ACK =< SND.NXT
A segment on the retransmission queue is fully acknowledged if the A segment on the retransmission queue is fully acknowledged if the
sum of its sequence number and length is less or equal than the sum of its sequence number and length is less or equal than the
acknowledgment value in the incoming segment. acknowledgment value in the incoming segment.
When data is received the following comparisons are needed: When data is received the following comparisons are needed:
RCV.NXT = next sequence number expected on an incoming segments, RCV.NXT = next sequence number expected on an incoming segment,
and is the left or lower edge of the receive window and is the left or lower edge of the receive window
RCV.NXT+RCV.WND-1 = last sequence number expected on an incoming RCV.NXT+RCV.WND-1 = last sequence number expected on an incoming
segment, and is the right or upper edge of the receive window segment, and is the right or upper edge of the receive window
SEG.SEQ = first sequence number occupied by the incoming segment SEG.SEQ = first sequence number occupied by the incoming segment
SEG.SEQ+SEG.LEN-1 = last sequence number occupied by the incoming SEG.SEQ+SEG.LEN-1 = last sequence number occupied by the incoming
segment segment
skipping to change at page 20, line 51 skipping to change at page 20, line 51
retransmitted and acknowledged without confusion (i.e., one and only retransmitted and acknowledged without confusion (i.e., one and only
one copy of the control will be acted upon). Control information is one copy of the control will be acted upon). Control information is
not physically carried in the segment data space. Consequently, we not physically carried in the segment data space. Consequently, we
must adopt rules for implicitly assigning sequence numbers to must adopt rules for implicitly assigning sequence numbers to
control. The SYN and FIN are the only controls requiring this control. The SYN and FIN are the only controls requiring this
protection, and these controls are used only at connection opening protection, and these controls are used only at connection opening
and closing. For sequence number purposes, the SYN is considered to and closing. For sequence number purposes, the SYN is considered to
occur before the first actual data octet of the segment in which it occur before the first actual data octet of the segment in which it
occurs, while the FIN is considered to occur after the last actual occurs, while the FIN is considered to occur after the last actual
data octet in a segment in which it occurs. The segment length data octet in a segment in which it occurs. The segment length
(SEG.LEN) includes both data and sequence space occupying controls. (SEG.LEN) includes both data and sequence space-occupying controls.
When a SYN is present then SEG.SEQ is the sequence number of the SYN. When a SYN is present then SEG.SEQ is the sequence number of the SYN.
Initial Sequence Number Selection 3.4.1. Initial Sequence Number Selection
A connection is defined by a pair of sockets. Connections can be A connection is defined by a pair of sockets. Connections can be
reused. New instances of a connection will be referred to as reused. New instances of a connection will be referred to as
incarnations of the connection. The problem that arises from this is incarnations of the connection. The problem that arises from this is
-- "how does the TCP implementation identify duplicate segments from -- "how does the TCP implementation identify duplicate segments from
previous incarnations of the connection?" This problem becomes previous incarnations of the connection?" This problem becomes
apparent if the connection is being opened and closed in quick apparent if the connection is being opened and closed in quick
succession, or if the connection breaks with loss of memory and is succession, or if the connection breaks with loss of memory and is
then reestablished. To support this, the TIME-WAIT state limits the then reestablished. To support this, the TIME-WAIT state limits the
rate of connection reuse, while the initial sequence number selection rate of connection reuse, while the initial sequence number selection
described below further protects against ambiguity about what described below further protects against ambiguity about what
incarnation of a connection an incoming packet corresponds to. incarnation of a connection an incoming packet corresponds to.
To avoid confusion we must prevent segments from one incarnation of a To avoid confusion we must prevent segments from one incarnation of a
connection from being used while the same sequence numbers may still connection from being used while the same sequence numbers may still
be present in the network from an earlier incarnation. We want to be present in the network from an earlier incarnation. We want to
assure this, even if a TCP endpoint loses all knowledge of the assure this, even if a TCP endpoint loses all knowledge of the
sequence numbers it has been using. When new connections are sequence numbers it has been using. When new connections are
created, an initial sequence number (ISN) generator is employed that created, an initial sequence number (ISN) generator is employed that
selects a new 32 bit ISN. There are security issues that result if selects a new 32 bit ISN. There are security issues that result if
an off-path attacker is able to predict or guess ISN values. an off-path attacker is able to predict or guess ISN values [43].
TCP Initial Sequence Numbers are generated from a number sequence TCP Initial Sequence Numbers are generated from a number sequence
that monotonically increases until it wraps, known loosely as a that monotonically increases until it wraps, known loosely as a
"clock". This clock is a 32-bit counter that typically increments at "clock". This clock is a 32-bit counter that typically increments at
least once every roughly 4 microseconds, although it is neither least once every roughly 4 microseconds, although it is neither
assumed to be realtime nor precise, and need not persist across assumed to be realtime nor precise, and need not persist across
reboots. The clock component is intended to insure that with a reboots. The clock component is intended to ensure that with a
Maximum Segment Lifetime (MSL), generated ISNs will be unique, since Maximum Segment Lifetime (MSL), generated ISNs will be unique, since
it cycles approximately every 4.55 hours, which is much longer than it cycles approximately every 4.55 hours, which is much longer than
the MSL. the MSL.
A TCP implementation MUST use the above type of "clock" for clock- A TCP implementation MUST use the above type of "clock" for clock-
driven selection of initial sequence numbers (MUST-8), and SHOULD driven selection of initial sequence numbers (MUST-8), and SHOULD
generate its Initial Sequence Numbers with the expression: generate its Initial Sequence Numbers with the expression:
ISN = M + F(localip, localport, remoteip, remoteport, secretkey) ISN = M + F(localip, localport, remoteip, remoteport, secretkey)
where M is the 4 microsecond timer, and F() is a pseudorandom where M is the 4 microsecond timer, and F() is a pseudorandom
function (PRF) of the connection's identifying parameters ("localip, function (PRF) of the connection's identifying parameters ("localip,
localport, remoteip, remoteport") and a secret key ("secretkey") localport, remoteip, remoteport") and a secret key ("secretkey")
(SHLD-1). F() MUST NOT be computable from the outside (MUST-9), or (SHLD-1). F() MUST NOT be computable from the outside (MUST-9), or
an attacker could still guess at sequence numbers from the ISN used an attacker could still guess at sequence numbers from the ISN used
for some other connection. The PRF could be implemented as a for some other connection. The PRF could be implemented as a
cryptographic hash of the concatenation of the TCP connection cryptographic hash of the concatenation of the TCP connection
parameters and some secret data. For discussion of the selection of parameters and some secret data. For discussion of the selection of
a specific hash algorithm and management of the secret key data, a specific hash algorithm and management of the secret key data,
please see Section 3 of [41]. please see Section 3 of [43].
For each connection there is a send sequence number and a receive For each connection there is a send sequence number and a receive
sequence number. The initial send sequence number (ISS) is chosen by sequence number. The initial send sequence number (ISS) is chosen by
the data sending TCP peer, and the initial receive sequence number the data sending TCP peer, and the initial receive sequence number
(IRS) is learned during the connection establishing procedure. (IRS) is learned during the connection establishing procedure.
For a connection to be established or initialized, the two TCP peers For a connection to be established or initialized, the two TCP peers
must synchronize on each other's initial sequence numbers. This is must synchronize on each other's initial sequence numbers. This is
done in an exchange of connection establishing segments carrying a done in an exchange of connection establishing segments carrying a
control bit called "SYN" (for synchronize) and the initial sequence control bit called "SYN" (for synchronize) and the initial sequence
skipping to change at page 22, line 37 skipping to change at page 22, line 45
2) A <-- B ACK your sequence number is X 2) A <-- B ACK your sequence number is X
3) A <-- B SYN my sequence number is Y 3) A <-- B SYN my sequence number is Y
4) A --> B ACK your sequence number is Y 4) A --> B ACK your sequence number is Y
Because steps 2 and 3 can be combined in a single message this is Because steps 2 and 3 can be combined in a single message this is
called the three-way (or three message) handshake (3WHS). called the three-way (or three message) handshake (3WHS).
A 3WHS is necessary because sequence numbers are not tied to a global A 3WHS is necessary because sequence numbers are not tied to a global
clock in the network, and TCP implementations may have different clock in the network, and TCP implementations may have different
mechanisms for picking the ISNs. The receiver of the first SYN has mechanisms for picking the ISNs. The receiver of the first SYN has
no way of knowing whether the segment was an old delayed one or not, no way of knowing whether the segment was an old one or not, unless
unless it remembers the last sequence number used on the connection it remembers the last sequence number used on the connection (which
(which is not always possible), and so it must ask the sender to is not always possible), and so it must ask the sender to verify this
verify this SYN. The three way handshake and the advantages of a SYN. The three-way handshake and the advantages of a clock-driven
clock-driven scheme are discussed in [63]. scheme for ISN selection are discussed in [70].
Knowing When to Keep Quiet 3.4.2. Knowing When to Keep Quiet
A theoretical problem exists where data could be corrupted due to A theoretical problem exists where data could be corrupted due to
confusion between old segments in the network and new ones after a confusion between old segments in the network and new ones after a
host reboots, if the same port numbers and sequence space are reused. host reboots, if the same port numbers and sequence space are reused.
The "Quiet Time" concept discussed below addresses this and the The "Quiet Time" concept discussed below addresses this and the
discussion of it is included for situations where it might be discussion of it is included for situations where it might be
relevant, although it is not felt to be necessary in most current relevant, although it is not felt to be necessary in most current
implementations. The problem was more relevant earlier in the implementations. The problem was more relevant earlier in the
history of TCP. In practical use on the Internet today, the error- history of TCP. In practical use on the Internet today, the error-
prone conditions are sufficiently unlikely that it is felt safe to prone conditions are sufficiently unlikely that it is felt safe to
skipping to change at page 23, line 24 skipping to change at page 23, line 34
remaining in the network, the TCP endpoint must keep quiet for an MSL remaining in the network, the TCP endpoint must keep quiet for an MSL
before assigning any sequence numbers upon starting up or recovering before assigning any sequence numbers upon starting up or recovering
from a situation where memory of sequence numbers in use was lost. from a situation where memory of sequence numbers in use was lost.
For this specification the MSL is taken to be 2 minutes. This is an For this specification the MSL is taken to be 2 minutes. This is an
engineering choice, and may be changed if experience indicates it is engineering choice, and may be changed if experience indicates it is
desirable to do so. Note that if a TCP endpoint is reinitialized in desirable to do so. Note that if a TCP endpoint is reinitialized in
some sense, yet retains its memory of sequence numbers in use, then some sense, yet retains its memory of sequence numbers in use, then
it need not wait at all; it must only be sure to use sequence numbers it need not wait at all; it must only be sure to use sequence numbers
larger than those recently used. larger than those recently used.
The TCP Quiet Time Concept 3.4.3. The TCP Quiet Time Concept
Hosts that for any reason lose knowledge of the last sequence numbers Hosts that for any reason lose knowledge of the last sequence numbers
transmitted on each active (i.e., not closed) connection shall delay transmitted on each active (i.e., not closed) connection shall delay
emitting any TCP segments for at least the agreed MSL in the internet emitting any TCP segments for at least the agreed MSL in the internet
system that the host is a part of. In the paragraphs below, an system that the host is a part of. In the paragraphs below, an
explanation for this specification is given. TCP implementors may explanation for this specification is given. TCP implementors may
violate the "quiet time" restriction, but only at the risk of causing violate the "quiet time" restriction, but only at the risk of causing
some old data to be accepted as new or new data rejected as old some old data to be accepted as new or new data rejected as old
duplicated by some receivers in the internet system. duplicated data by some receivers in the internet system.
TCP endpoints consume sequence number space each time a segment is TCP endpoints consume sequence number space each time a segment is
formed and entered into the network output queue at a source host. formed and entered into the network output queue at a source host.
The duplicate detection and sequencing algorithm in the TCP protocol The duplicate detection and sequencing algorithm in the TCP protocol
relies on the unique binding of segment data to sequence space to the relies on the unique binding of segment data to sequence space to the
extent that sequence numbers will not cycle through all 2**32 values extent that sequence numbers will not cycle through all 2**32 values
before the segment data bound to those sequence numbers has been before the segment data bound to those sequence numbers has been
delivered and acknowledged by the receiver and all duplicate copies delivered and acknowledged by the receiver and all duplicate copies
of the segments have "drained" from the internet. Without such an of the segments have "drained" from the internet. Without such an
assumption, two distinct TCP segments could conceivably be assigned assumption, two distinct TCP segments could conceivably be assigned
the same or overlapping sequence numbers, causing confusion at the the same or overlapping sequence numbers, causing confusion at the
receiver as to which data is new and which is old. Remember that receiver as to which data is new and which is old. Remember that
each segment is bound to as many consecutive sequence numbers as each segment is bound to as many consecutive sequence numbers as
there are octets of data and SYN or FIN flags in the segment. there are octets of data and SYN or FIN flags in the segment.
Under normal conditions, TCP implementations keep track of the next Under normal conditions, TCP implementations keep track of the next
sequence number to emit and the oldest awaiting acknowledgment so as sequence number to emit and the oldest awaiting acknowledgment so as
to avoid mistakenly using a sequence number over before its first use to avoid mistakenly using a sequence number over before its first use
has been acknowledged. This alone does not guarantee that old has been acknowledged. This alone does not guarantee that old
duplicate data is drained from the net, so the sequence space has duplicate data is drained from the net, so the sequence space has
been made very large to reduce the probability that a wandering been made large to reduce the probability that a wandering duplicate
duplicate will cause trouble upon arrival. At 2 megabits/sec. it will cause trouble upon arrival. At 2 megabits/sec. it takes 4.5
takes 4.5 hours to use up 2**32 octets of sequence space. Since the hours to use up 2**32 octets of sequence space. Since the maximum
maximum segment lifetime in the net is not likely to exceed a few segment lifetime in the net is not likely to exceed a few tens of
tens of seconds, this is deemed ample protection for foreseeable seconds, this is deemed ample protection for foreseeable nets, even
nets, even if data rates escalate to 10's of megabits/sec. At 100 if data rates escalate to 10s of megabits/sec. At 100 megabits/sec,
megabits/sec, the cycle time is 5.4 minutes, which may be a little the cycle time is 5.4 minutes, which may be a little short, but still
short, but still within reason. within reason. Much higher data rates are possible today, with
implications described in the final paragraph of this subsection.
The basic duplicate detection and sequencing algorithm in TCP can be The basic duplicate detection and sequencing algorithm in TCP can be
defeated, however, if a source TCP endpoint does not have any memory defeated, however, if a source TCP endpoint does not have any memory
of the sequence numbers it last used on a given connection. For of the sequence numbers it last used on a given connection. For
example, if the TCP implementation were to start all connections with example, if the TCP implementation were to start all connections with
sequence number 0, then upon the host rebooting, a TCP peer might re- sequence number 0, then upon the host rebooting, a TCP peer might re-
form an earlier connection (possibly after half-open connection form an earlier connection (possibly after half-open connection
resolution) and emit packets with sequence numbers identical to or resolution) and emit packets with sequence numbers identical to or
overlapping with packets still in the network, which were emitted on overlapping with packets still in the network, which were emitted on
an earlier incarnation of the same connection. In the absence of an earlier incarnation of the same connection. In the absence of
skipping to change at page 25, line 7 skipping to change at page 25, line 16
bearing sequence numbers in the neighborhood of S1 may arrive and be bearing sequence numbers in the neighborhood of S1 may arrive and be
treated as new packets by the receiver of the new incarnation of the treated as new packets by the receiver of the new incarnation of the
connection. connection.
The problem is that the recovering host may not know for how long it The problem is that the recovering host may not know for how long it
was down between rebooting nor does it know whether there are still was down between rebooting nor does it know whether there are still
old duplicates in the system from earlier connection incarnations. old duplicates in the system from earlier connection incarnations.
One way to deal with this problem is to deliberately delay emitting One way to deal with this problem is to deliberately delay emitting
segments for one MSL after recovery from a reboot - this is the segments for one MSL after recovery from a reboot - this is the
"quiet time" specification. Hosts that prefer to avoid waiting are "quiet time" specification. Hosts that prefer to avoid waiting and
willing to risk possible confusion of old and new packets at a given are willing to risk possible confusion of old and new packets at a
destination may choose not to wait for the "quiet time". given destination may choose not to wait for the "quiet time".
Implementors may provide TCP users with the ability to select on a Implementors may provide TCP users with the ability to select on a
connection by connection basis whether to wait after a reboot, or may connection by connection basis whether to wait after a reboot, or may
informally implement the "quiet time" for all connections. informally implement the "quiet time" for all connections.
Obviously, even where a user selects to "wait," this is not necessary Obviously, even where a user selects to "wait," this is not necessary
after the host has been "up" for at least MSL seconds. after the host has been "up" for at least MSL seconds.
To summarize: every segment emitted occupies one or more sequence To summarize: every segment emitted occupies one or more sequence
numbers in the sequence space, the numbers occupied by a segment are numbers in the sequence space, the numbers occupied by a segment are
"busy" or "in use" until MSL seconds have passed, upon rebooting a "busy" or "in use" until MSL seconds have passed, upon rebooting a
block of space-time is occupied by the octets and SYN or FIN flags of block of space-time is occupied by the octets and SYN or FIN flags of
the last emitted segment, if a new connection is started too soon and any potentially still in-flight segments, and if a new connection is
uses any of the sequence numbers in the space-time footprint of the started too soon and uses any of the sequence numbers in the space-
last segment of the previous connection incarnation, there is a time footprint of those potentially still in-flight segments of the
potential sequence number overlap area that could cause confusion at previous connection incarnation, there is a potential sequence number
the receiver. overlap area that could cause confusion at the receiver.
High performance cases will have shorter cycle times than those in
the megabits per second that the base TCP design described above
considers. At 1 Gbps, the cycle time is 34 seconds, only 3 seconds
at 10 Gbps, and around a third of a second at 100 Gbps. In these
higher performance cases, TCP Timestamp options and Protection
Against Wrapped Sequences (PAWS) [48] provide the needed capability
to detect and discard old duplicates.
3.5. Establishing a connection 3.5. Establishing a connection
The "three-way handshake" is the procedure used to establish a The "three-way handshake" is the procedure used to establish a
connection. This procedure normally is initiated by one TCP peer and connection. This procedure normally is initiated by one TCP peer and
responded to by another TCP peer. The procedure also works if two responded to by another TCP peer. The procedure also works if two
TCP peers simultaneously initiate the procedure. When simultaneous TCP peers simultaneously initiate the procedure. When simultaneous
open occurs, each TCP peer receives a "SYN" segment that carries no open occurs, each TCP peer receives a "SYN" segment that carries no
acknowledgment after it has sent a "SYN". Of course, the arrival of acknowledgment after it has sent a "SYN". Of course, the arrival of
an old duplicate "SYN" segment can potentially make it appear, to the an old duplicate "SYN" segment can potentially make it appear, to the
recipient, that a simultaneous connection initiation is in progress. recipient, that a simultaneous connection initiation is in progress.
Proper use of "reset" segments can disambiguate these cases. Proper use of "reset" segments can disambiguate these cases.
Several examples of connection initiation follow. Although these Several examples of connection initiation follow. Although these
examples do not show connection synchronization using data-carrying examples do not show connection synchronization using data-carrying
segments, this is perfectly legitimate, so long as the receiving TCP segments, this is perfectly legitimate, so long as the receiving TCP
endpoint doesn't deliver the data to the user until it is clear the endpoint doesn't deliver the data to the user until it is clear the
data is valid (e.g., the data is buffered at the receiver until the data is valid (e.g., the data is buffered at the receiver until the
connection reaches the ESTABLISHED state, given that the three-way connection reaches the ESTABLISHED state, given that the three-way
handshake reduces the possibility of false connections). It is the handshake reduces the possibility of false connections). It is a
implementation of a trade-off between memory and messages to provide trade-off between memory and messages to provide information for this
information for this checking. checking.
The simplest 3WHS is shown in Figure 6. The figures should be The simplest 3WHS is shown in Figure 6. The figures should be
interpreted in the following way. Each line is numbered for interpreted in the following way. Each line is numbered for
reference purposes. Right arrows (-->) indicate departure of a TCP reference purposes. Right arrows (-->) indicate departure of a TCP
segment from TCP peer A to TCP peer B, or arrival of a segment at B segment from TCP peer A to TCP peer B, or arrival of a segment at B
from A. Left arrows (<--), indicate the reverse. Ellipsis (...) from A. Left arrows (<--), indicate the reverse. Ellipsis (...)
indicates a segment that is still in the network (delayed). Comments indicates a segment that is still in the network (delayed). Comments
appear in parentheses. TCP connection states represent the state appear in parentheses. TCP connection states represent the state
AFTER the departure or arrival of the segment (whose contents are AFTER the departure or arrival of the segment (whose contents are
shown in the center of each line). Segment contents are shown in shown in the center of each line). Segment contents are shown in
skipping to change at page 26, line 24 skipping to change at page 26, line 42
1. CLOSED LISTEN 1. CLOSED LISTEN
2. SYN-SENT --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED 2. SYN-SENT --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED
3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED 3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED 4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED
5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED 5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED
Figure 6: Basic 3-Way Handshake for Connection Synchronization Figure 6: Basic 3-Way Handshake for Connection Synchronization
In line 2 of Figure 6, TCP Peer A begins by sending a SYN segment In line 2 of Figure 6, TCP Peer A begins by sending a SYN segment
indicating that it will use sequence numbers starting with sequence indicating that it will use sequence numbers starting with sequence
number 100. In line 3, TCP Peer B sends a SYN and acknowledges the number 100. In line 3, TCP Peer B sends a SYN and acknowledges the
SYN it received from TCP Peer A. Note that the acknowledgment field SYN it received from TCP Peer A. Note that the acknowledgment field
indicates TCP Peer B is now expecting to hear sequence 101, indicates TCP Peer B is now expecting to hear sequence 101,
acknowledging the SYN that occupied sequence 100. acknowledging the SYN that occupied sequence 100.
At line 4, TCP Peer A responds with an empty segment containing an At line 4, TCP Peer A responds with an empty segment containing an
ACK for TCP Peer B's SYN; and in line 5, TCP Peer A sends some data. ACK for TCP Peer B's SYN; and in line 5, TCP Peer A sends some data.
skipping to change at page 28, line 36 skipping to change at page 28, line 36
Figure 8. At line 3, an old duplicate SYN arrives at TCP Peer B. Figure 8. At line 3, an old duplicate SYN arrives at TCP Peer B.
TCP Peer B cannot tell that this is an old duplicate, so it responds TCP Peer B cannot tell that this is an old duplicate, so it responds
normally (line 4). TCP Peer A detects that the ACK field is normally (line 4). TCP Peer A detects that the ACK field is
incorrect and returns a RST (reset) with its SEQ field selected to incorrect and returns a RST (reset) with its SEQ field selected to
make the segment believable. TCP Peer B, on receiving the RST, make the segment believable. TCP Peer B, on receiving the RST,
returns to the LISTEN state. When the original SYN finally arrives returns to the LISTEN state. When the original SYN finally arrives
at line 6, the synchronization proceeds normally. If the SYN at line at line 6, the synchronization proceeds normally. If the SYN at line
6 had arrived before the RST, a more complex exchange might have 6 had arrived before the RST, a more complex exchange might have
occurred with RST's sent in both directions. occurred with RST's sent in both directions.
Half-Open Connections and Other Anomalies 3.5.1. Half-Open Connections and Other Anomalies
An established connection is said to be "half-open" if one of the TCP An established connection is said to be "half-open" if one of the TCP
peers has closed or aborted the connection at its end without the peers has closed or aborted the connection at its end without the
knowledge of the other, or if the two ends of the connection have knowledge of the other, or if the two ends of the connection have
become desynchronized owing to a failure or reboot that resulted in become desynchronized owing to a failure or reboot that resulted in
loss of memory. Such connections will automatically become reset if loss of memory. Such connections will automatically become reset if
an attempt is made to send data in either direction. However, half- an attempt is made to send data in either direction. However, half-
open connections are expected to be unusual. open connections are expected to be unusual.
If at site A the connection no longer exists, then an attempt by the If at site A the connection no longer exists, then an attempt by the
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3. SYN-SENT --> <SEQ=400><CTL=SYN> --> (??) 3. SYN-SENT --> <SEQ=400><CTL=SYN> --> (??)
4. (!!) <-- <SEQ=300><ACK=100><CTL=ACK> <-- ESTABLISHED 4. (!!) <-- <SEQ=300><ACK=100><CTL=ACK> <-- ESTABLISHED
5. SYN-SENT --> <SEQ=100><CTL=RST> --> (Abort!!) 5. SYN-SENT --> <SEQ=100><CTL=RST> --> (Abort!!)
6. SYN-SENT CLOSED 6. SYN-SENT CLOSED
7. SYN-SENT --> <SEQ=400><CTL=SYN> --> 7. SYN-SENT --> <SEQ=400><CTL=SYN> -->
Figure 9: Half-Open Connection Discovery Figure 9: Half-Open Connection Discovery
When the SYN arrives at line 3, TCP Peer B, being in a synchronized When the SYN arrives at line 3, TCP Peer B, being in a synchronized
state, and the incoming segment outside the window, responds with an state, and the incoming segment outside the window, responds with an
acknowledgment indicating what sequence it next expects to hear (ACK acknowledgment indicating what sequence it next expects to hear (ACK
100). TCP Peer A sees that this segment does not acknowledge 100). TCP Peer A sees that this segment does not acknowledge
anything it sent and, being unsynchronized, sends a reset (RST) anything it sent and, being unsynchronized, sends a reset (RST)
because it has detected a half-open connection. TCP Peer B aborts at because it has detected a half-open connection. TCP Peer B aborts at
line 5. TCP Peer A will continue to try to establish the connection; line 5. TCP Peer A will continue to try to establish the connection;
the problem is now reduced to the basic 3-way handshake of Figure 6. the problem is now reduced to the basic 3-way handshake of Figure 6.
skipping to change at page 30, line 15 skipping to change at page 30, line 21
connection. connection.
TCP Peer A TCP Peer B TCP Peer A TCP Peer B
1. (REBOOT) (send 300,receive 100) 1. (REBOOT) (send 300,receive 100)
2. (??) <-- <SEQ=300><ACK=100><DATA=10><CTL=ACK> <-- ESTABLISHED 2. (??) <-- <SEQ=300><ACK=100><DATA=10><CTL=ACK> <-- ESTABLISHED
3. --> <SEQ=100><CTL=RST> --> (ABORT!!) 3. --> <SEQ=100><CTL=RST> --> (ABORT!!)
Figure 10: Active Side Causes Half-Open Connection Discovery Figure 10: Active Side Causes Half-Open Connection Discovery
In Figure 11, two TCP Peers A and B with passive connections waiting In Figure 11, two TCP Peers A and B with passive connections waiting
for SYN are depicted. An old duplicate arriving at TCP Peer B (line for SYN are depicted. An old duplicate arriving at TCP Peer B (line
2) stirs B into action. A SYN-ACK is returned (line 3) and causes 2) stirs B into action. A SYN-ACK is returned (line 3) and causes
TCP A to generate a RST (the ACK in line 3 is not acceptable). TCP TCP A to generate a RST (the ACK in line 3 is not acceptable). TCP
Peer B accepts the reset and returns to its passive LISTEN state. Peer B accepts the reset and returns to its passive LISTEN state.
TCP Peer A TCP Peer B TCP Peer A TCP Peer B
1. LISTEN LISTEN 1. LISTEN LISTEN
skipping to change at page 30, line 40 skipping to change at page 31, line 5
4. --> <SEQ=Z+1><CTL=RST> --> (return to LISTEN!) 4. --> <SEQ=Z+1><CTL=RST> --> (return to LISTEN!)
5. LISTEN LISTEN 5. LISTEN LISTEN
Figure 11: Old Duplicate SYN Initiates a Reset on two Passive Sockets Figure 11: Old Duplicate SYN Initiates a Reset on two Passive Sockets
A variety of other cases are possible, all of which are accounted for A variety of other cases are possible, all of which are accounted for
by the following rules for RST generation and processing. by the following rules for RST generation and processing.
Reset Generation 3.5.2. Reset Generation
A TCP user or application can issue a reset on a connection at any A TCP user or application can issue a reset on a connection at any
time, though reset events are also generated by the protocol itself time, though reset events are also generated by the protocol itself
when various error conditions occur, as described below. The side of when various error conditions occur, as described below. The side of
a connection issuing a reset should enter the TIME-WAIT state, as a connection issuing a reset should enter the TIME-WAIT state, as
this generally helps to reduce the load on busy servers for reasons this generally helps to reduce the load on busy servers for reasons
described in [64]. described in [71].
As a general rule, reset (RST) is sent whenever a segment arrives As a general rule, reset (RST) is sent whenever a segment arrives
that apparently is not intended for the current connection. A reset that apparently is not intended for the current connection. A reset
must not be sent if it is not clear that this is the case. must not be sent if it is not clear that this is the case.
There are three groups of states: There are three groups of states:
1. If the connection does not exist (CLOSED) then a reset is sent 1. If the connection does not exist (CLOSED) then a reset is sent
in response to any incoming segment except another reset. A SYN in response to any incoming segment except another reset. A SYN
segment that does not match an existing connection is rejected by segment that does not match an existing connection is rejected by
skipping to change at page 31, line 25 skipping to change at page 31, line 34
If the incoming segment has the ACK bit set, the reset takes its If the incoming segment has the ACK bit set, the reset takes its
sequence number from the ACK field of the segment, otherwise the sequence number from the ACK field of the segment, otherwise the
reset has sequence number zero and the ACK field is set to the sum reset has sequence number zero and the ACK field is set to the sum
of the sequence number and segment length of the incoming segment. of the sequence number and segment length of the incoming segment.
The connection remains in the CLOSED state. The connection remains in the CLOSED state.
2. If the connection is in any non-synchronized state (LISTEN, 2. If the connection is in any non-synchronized state (LISTEN,
SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges
something not yet sent (the segment carries an unacceptable ACK), something not yet sent (the segment carries an unacceptable ACK),
or if an incoming segment has a security level or compartment that or if an incoming segment has a security level or compartment
does not exactly match the level and compartment requested for the Appendix A.1 that does not exactly match the level and compartment
connection, a reset is sent. requested for the connection, a reset is sent.
If the incoming segment has an ACK field, the reset takes its If the incoming segment has an ACK field, the reset takes its
sequence number from the ACK field of the segment, otherwise the sequence number from the ACK field of the segment, otherwise the
reset has sequence number zero and the ACK field is set to the sum reset has sequence number zero and the ACK field is set to the sum
of the sequence number and segment length of the incoming segment. of the sequence number and segment length of the incoming segment.
The connection remains in the same state. The connection remains in the same state.
3. If the connection is in a synchronized state (ESTABLISHED, 3. If the connection is in a synchronized state (ESTABLISHED,
FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT),
any unacceptable segment (out of window sequence number or any unacceptable segment (out of window sequence number or
unacceptable acknowledgment number) must be responded to with an unacceptable acknowledgment number) must be responded to with an
empty acknowledgment segment (without any user data) containing empty acknowledgment segment (without any user data) containing
the current send-sequence number and an acknowledgment indicating the current send-sequence number and an acknowledgment indicating
the next sequence number expected to be received, and the the next sequence number expected to be received, and the
connection remains in the same state. connection remains in the same state.
If an incoming segment has a security level, or compartment that If an incoming segment has a security level or compartment that
does not exactly match the level and compartment requested for the does not exactly match the level and compartment requested for the
connection, a reset is sent and the connection goes to the CLOSED connection, a reset is sent and the connection goes to the CLOSED
state. The reset takes its sequence number from the ACK field of state. The reset takes its sequence number from the ACK field of
the incoming segment. the incoming segment.
Reset Processing 3.5.3. Reset Processing
In all states except SYN-SENT, all reset (RST) segments are validated In all states except SYN-SENT, all reset (RST) segments are validated
by checking their SEQ-fields. A reset is valid if its sequence by checking their SEQ-fields. A reset is valid if its sequence
number is in the window. In the SYN-SENT state (a RST received in number is in the window. In the SYN-SENT state (a RST received in
response to an initial SYN), the RST is acceptable if the ACK field response to an initial SYN), the RST is acceptable if the ACK field
acknowledges the SYN. acknowledges the SYN.
The receiver of a RST first validates it, then changes state. If the The receiver of a RST first validates it, then changes state. If the
receiver was in the LISTEN state, it ignores it. If the receiver was receiver was in the LISTEN state, it ignores it. If the receiver was
in SYN-RECEIVED state and had previously been in the LISTEN state, in SYN-RECEIVED state and had previously been in the LISTEN state,
then the receiver returns to the LISTEN state, otherwise the receiver then the receiver returns to the LISTEN state, otherwise the receiver
aborts the connection and goes to the CLOSED state. If the receiver aborts the connection and goes to the CLOSED state. If the receiver
was in any other state, it aborts the connection and advises the user was in any other state, it aborts the connection and advises the user
and goes to the CLOSED state. and goes to the CLOSED state.
TCP implementations SHOULD allow a received RST segment to include TCP implementations SHOULD allow a received RST segment to include
data (SHLD-2). data (SHLD-2). It has been suggested that a RST segment could
contain diagnostic data that explains the cause of the RST. No
standard has yet been established for such data.
3.6. Closing a Connection 3.6. Closing a Connection
CLOSE is an operation meaning "I have no more data to send." The CLOSE is an operation meaning "I have no more data to send." The
notion of closing a full-duplex connection is subject to ambiguous notion of closing a full-duplex connection is subject to ambiguous
interpretation, of course, since it may not be obvious how to treat interpretation, of course, since it may not be obvious how to treat
the receiving side of the connection. We have chosen to treat CLOSE the receiving side of the connection. We have chosen to treat CLOSE
in a simplex fashion. The user who CLOSEs may continue to RECEIVE in a simplex fashion. The user who CLOSEs may continue to RECEIVE
until the TCP receiver is told that the remote peer has CLOSED also. until the TCP receiver is told that the remote peer has CLOSED also.
Thus, a program could initiate several SENDs followed by a CLOSE, and Thus, a program could initiate several SENDs followed by a CLOSE, and
then continue to RECEIVE until signaled that a RECEIVE failed because then continue to RECEIVE until signaled that a RECEIVE failed because
the remote peer has CLOSED. The TCP implementation will signal a the remote peer has CLOSED. The TCP implementation will signal a
user, even if no RECEIVEs are outstanding, that the remote peer has user, even if no RECEIVEs are outstanding, that the remote peer has
closed, so the user can terminate his side gracefully. A TCP closed, so the user can terminate their side gracefully. A TCP
implementation will reliably deliver all buffers SENT before the implementation will reliably deliver all buffers SENT before the
connection was CLOSED so a user who expects no data in return need connection was CLOSED so a user who expects no data in return need
only wait to hear the connection was CLOSED successfully to know that only wait to hear the connection was CLOSED successfully to know that
all their data was received at the destination TCP endpoint. Users all their data was received at the destination TCP endpoint. Users
must keep reading connections they close for sending until the TCP must keep reading connections they close for sending until the TCP
implementation indicates there is no more data. implementation indicates there is no more data.
There are essentially three cases: There are essentially three cases:
1) The user initiates by telling the TCP implementation to CLOSE 1) The user initiates by telling the TCP implementation to CLOSE
the connection (TCP Peer A in Figure 12). the connection (TCP Peer A in Figure 12).
2) The remote TCP endpoint initiates by sending a FIN control 2) The remote TCP endpoint initiates by sending a FIN control
signal (TCP Peer B in Figure 12). signal (TCP Peer B in Figure 12).
3) Both users CLOSE simultaneously (Figure 13). 3) Both users CLOSE simultaneously (Figure 13).
Case 1: Local user initiates the close Case 1: Local user initiates the close
In this case, a FIN segment can be constructed and placed on the In this case, a FIN segment can be constructed and placed on the
outgoing segment queue. No further SENDs from the user will be outgoing segment queue. No further SENDs from the user will be
accepted by the TCP implementation, and it enters the FIN-WAIT-1 accepted by the TCP implementation, and it enters the FIN-WAIT-1
state. RECEIVEs are allowed in this state. All segments state. RECEIVEs are allowed in this state. All segments
preceding and including FIN will be retransmitted until preceding and including FIN will be retransmitted until
acknowledged. When the other TCP peer has both acknowledged the acknowledged. When the other TCP peer has both acknowledged the
FIN and sent a FIN of its own, the first TCP peer can ACK this FIN and sent a FIN of its own, the first TCP peer can ACK this
FIN. Note that a TCP endpoint receiving a FIN will ACK but not FIN. Note that a TCP endpoint receiving a FIN will ACK but not
send its own FIN until its user has CLOSED the connection also. send its own FIN until its user has CLOSED the connection also.
Case 2: TCP endpoint receives a FIN from the network Case 2: TCP endpoint receives a FIN from the network
If an unsolicited FIN arrives from the network, the receiving TCP If an unsolicited FIN arrives from the network, the receiving TCP
endpoint can ACK it and tell the user that the connection is endpoint can ACK it and tell the user that the connection is
closing. The user will respond with a CLOSE, upon which the TCP closing. The user will respond with a CLOSE, upon which the TCP
endpoint can send a FIN to the other TCP peer after sending any endpoint can send a FIN to the other TCP peer after sending any
remaining data. The TCP endpoint then waits until its own FIN is remaining data. The TCP endpoint then waits until its own FIN is
acknowledged whereupon it deletes the connection. If an ACK is acknowledged whereupon it deletes the connection. If an ACK is
not forthcoming, after the user timeout the connection is aborted not forthcoming, after the user timeout the connection is aborted
and the user is told. and the user is told.
Case 3: Both users close simultaneously Case 3: Both users close simultaneously
A simultaneous CLOSE by users at both ends of a connection causes A simultaneous CLOSE by users at both ends of a connection causes
FIN segments to be exchanged (Figure 13). When all segments FIN segments to be exchanged (Figure 13). When all segments
preceding the FINs have been processed and acknowledged, each TCP preceding the FINs have been processed and acknowledged, each TCP
peer can ACK the FIN it has received. Both will, upon receiving peer can ACK the FIN it has received. Both will, upon receiving
these ACKs, delete the connection. these ACKs, delete the connection.
TCP Peer A TCP Peer B TCP Peer A TCP Peer B
1. ESTABLISHED ESTABLISHED 1. ESTABLISHED ESTABLISHED
skipping to change at page 33, line 50 skipping to change at page 34, line 22
3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT 3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT
4. (Close) 4. (Close)
TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK
5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --> CLOSED 5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --> CLOSED
6. (2 MSL) 6. (2 MSL)
CLOSED CLOSED
Figure 12: Normal Close Sequence Figure 12: Normal Close Sequence
TCP Peer A TCP Peer B TCP Peer A TCP Peer B
1. ESTABLISHED ESTABLISHED 1. ESTABLISHED ESTABLISHED
2. (Close) (Close) 2. (Close) (Close)
FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> ... FIN-WAIT-1 FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> ... FIN-WAIT-1
<-- <SEQ=300><ACK=100><CTL=FIN,ACK> <-- <-- <SEQ=300><ACK=100><CTL=FIN,ACK> <--
... <SEQ=100><ACK=300><CTL=FIN,ACK> --> ... <SEQ=100><ACK=300><CTL=FIN,ACK> -->
3. CLOSING --> <SEQ=101><ACK=301><CTL=ACK> ... CLOSING 3. CLOSING --> <SEQ=101><ACK=301><CTL=ACK> ... CLOSING
<-- <SEQ=301><ACK=101><CTL=ACK> <-- <-- <SEQ=301><ACK=101><CTL=ACK> <--
... <SEQ=101><ACK=301><CTL=ACK> --> ... <SEQ=101><ACK=301><CTL=ACK> -->
4. TIME-WAIT TIME-WAIT 4. TIME-WAIT TIME-WAIT
(2 MSL) (2 MSL) (2 MSL) (2 MSL)
CLOSED CLOSED CLOSED CLOSED
Figure 13: Simultaneous Close Sequence Figure 13: Simultaneous Close Sequence
A TCP connection may terminate in two ways: (1) the normal TCP close A TCP connection may terminate in two ways: (1) the normal TCP close
sequence using a FIN handshake (Figure 12), and (2) an "abort" in sequence using a FIN handshake (Figure 12), and (2) an "abort" in
which one or more RST segments are sent and the connection state is which one or more RST segments are sent and the connection state is
immediately discarded. If the local TCP connection is closed by the immediately discarded. If the local TCP connection is closed by the
remote side due to a FIN or RST received from the remote side, then remote side due to a FIN or RST received from the remote side, then
the local application MUST be informed whether it closed normally or the local application MUST be informed whether it closed normally or
was aborted (MUST-12). was aborted (MUST-12).
3.6.1. Half-Closed Connections 3.6.1. Half-Closed Connections
skipping to change at page 34, line 46 skipping to change at page 35, line 19
independently, it is possible for a connection to be "half closed," independently, it is possible for a connection to be "half closed,"
i.e., closed in only one direction, and a host is permitted to i.e., closed in only one direction, and a host is permitted to
continue sending data in the open direction on a half-closed continue sending data in the open direction on a half-closed
connection. connection.
A host MAY implement a "half-duplex" TCP close sequence, so that an A host MAY implement a "half-duplex" TCP close sequence, so that an
application that has called CLOSE cannot continue to read data from application that has called CLOSE cannot continue to read data from
the connection (MAY-1). If such a host issues a CLOSE call while the connection (MAY-1). If such a host issues a CLOSE call while
received data is still pending in the TCP connection, or if new data received data is still pending in the TCP connection, or if new data
is received after CLOSE is called, its TCP implementation SHOULD send is received after CLOSE is called, its TCP implementation SHOULD send
a RST to show that data was lost (SHLD-3). See [22] section 2.17 for a RST to show that data was lost (SHLD-3). See [24] section 2.17 for
discussion. discussion.
When a connection is closed actively, it MUST linger in the TIME-WAIT When a connection is closed actively, it MUST linger in the TIME-WAIT
state for a time 2xMSL (Maximum Segment Lifetime) (MUST-13). state for a time 2xMSL (Maximum Segment Lifetime) (MUST-13).
However, it MAY accept a new SYN from the remote TCP endpoint to However, it MAY accept a new SYN from the remote TCP endpoint to
reopen the connection directly from TIME-WAIT state (MAY-2), if it: reopen the connection directly from TIME-WAIT state (MAY-2), if it:
(1) assigns its initial sequence number for the new connection to (1) assigns its initial sequence number for the new connection to
be larger than the largest sequence number it used on the previous be larger than the largest sequence number it used on the previous
connection incarnation, and connection incarnation, and
(2) returns to TIME-WAIT state if the SYN turns out to be an old (2) returns to TIME-WAIT state if the SYN turns out to be an old
duplicate. duplicate.
When the TCP Timestamp options are available, an improved algorithm When the TCP Timestamp options are available, an improved algorithm
is described in [39] in order to support higher connection is described in [41] in order to support higher connection
establishment rates. This algorithm for reducing TIME-WAIT is a Best establishment rates. This algorithm for reducing TIME-WAIT is a Best
Current Practice that SHOULD be implemented, since timestamp options Current Practice that SHOULD be implemented, since timestamp options
are commonly used, and using them to reduce TIME-WAIT provides are commonly used, and using them to reduce TIME-WAIT provides
benefits for busy Internet servers (SHLD-4). benefits for busy Internet servers (SHLD-4).
3.7. Segmentation 3.7. Segmentation
The term "segmentation" refers to the activity TCP performs when The term "segmentation" refers to the activity TCP performs when
ingesting a stream of bytes from a sending application and ingesting a stream of bytes from a sending application and
packetizing that stream of bytes into TCP segments. Individual TCP packetizing that stream of bytes into TCP segments. Individual TCP
segments often do not correspond one-for-one to individual send (or segments often do not correspond one-for-one to individual send (or
socket write) calls from the application. Applications may perform socket write) calls from the application. Applications may perform
writes at the granularity of messages in the upper layer protocol, writes at the granularity of messages in the upper layer protocol,
but TCP guarantees no boundary coherence between the TCP segments but TCP guarantees no boundary coherence between the TCP segments
sent and received versus user application data read or write buffer sent and received versus user application data read or write buffer
boundaries. In some specific protocols, such as Remote Direct Memory boundaries. In some specific protocols, such as Remote Direct Memory
Access (RDMA) using Direct Data Placement (DDP) and Marker PDU Access (RDMA) using Direct Data Placement (DDP) and Marker PDU
Aligned Framing (MPA) [32], there are performance optimizations Aligned Framing (MPA) [35], there are performance optimizations
possible when the relation between TCP segments and application data possible when the relation between TCP segments and application data
units can be controlled, and MPA includes a specific mechanism for units can be controlled, and MPA includes a specific mechanism for
detecting and verifying this relationship between TCP segments and detecting and verifying this relationship between TCP segments and
application message data structures, but this is specific to application message data structures, but this is specific to
applications like RDMA. In general, multiple goals influence the applications like RDMA. In general, multiple goals influence the
sizing of TCP segments created by a TCP implementation. sizing of TCP segments created by a TCP implementation.
Goals driving the sending of larger segments include: Goals driving the sending of larger segments include:
o Reducing the number of packets in flight within the network. * Reducing the number of packets in flight within the network.
o Increasing processing efficiency and potential performance by * Increasing processing efficiency and potential performance by
enabling a smaller number of interrupts and inter-layer enabling a smaller number of interrupts and inter-layer
interactions. interactions.
o Limiting the overhead of TCP headers. * Limiting the overhead of TCP headers.
Note that the performance benefits of sending larger segments may Note that the performance benefits of sending larger segments may
decrease as the size increases, and there may be boundaries where decrease as the size increases, and there may be boundaries where
advantages are reversed. For instance, on some implementation advantages are reversed. For instance, on some implementation
architectures, 1025 bytes within a segment could lead to worse architectures, 1025 bytes within a segment could lead to worse
performance than 1024 bytes, due purely to data alignment on copy performance than 1024 bytes, due purely to data alignment on copy
operations. operations.
Goals driving the sending of smaller segments include: Goals driving the sending of smaller segments include:
o Avoiding sending a TCP segment that would result in an IP datagram * Avoiding sending a TCP segment that would result in an IP datagram
larger than the smallest MTU along an IP network path, because larger than the smallest MTU along an IP network path, because
this results in either packet loss or packet fragmentation. this results in either packet loss or packet fragmentation.
Making matters worse, some firewalls or middleboxes may drop Making matters worse, some firewalls or middleboxes may drop
fragmented packets or ICMP messages related to fragmentation. fragmented packets or ICMP messages related to fragmentation.
o Preventing delays to the application data stream, especially when * Preventing delays to the application data stream, especially when
TCP is waiting on the application to generate more data, or when TCP is waiting on the application to generate more data, or when
the application is waiting on an event or input from its peer in the application is waiting on an event or input from its peer in
order to generate more data. order to generate more data.
o Enabling "fate sharing" between TCP segments and lower-layer data * Enabling "fate sharing" between TCP segments and lower-layer data
units (e.g. below IP, for links with cell or frame sizes smaller units (e.g. below IP, for links with cell or frame sizes smaller
than the IP MTU). than the IP MTU).
Towards meeting these competing sets of goals, TCP includes several Towards meeting these competing sets of goals, TCP includes several
mechanisms, including the Maximum Segment Size option, Path MTU mechanisms, including the Maximum Segment Size option, Path MTU
Discovery, the Nagle algorithm, and support for IPv6 Jumbograms, as Discovery, the Nagle algorithm, and support for IPv6 Jumbograms, as
discussed in the following subsections. discussed in the following subsections.
3.7.1. Maximum Segment Size Option 3.7.1. Maximum Segment Size Option
TCP endpoints MUST implement both sending and receiving the MSS TCP endpoints MUST implement both sending and receiving the MSS
option (MUST-14). option (MUST-14).
TCP implementations SHOULD send an MSS option in every SYN segment TCP implementations SHOULD send an MSS option in every SYN segment
when its receive MSS differs from the default 536 for IPv4 or 1220 when its receive MSS differs from the default 536 for IPv4 or 1220
for IPv6 (SHLD-5), and MAY send it always (MAY-3). for IPv6 (SHLD-5), and MAY send it always (MAY-3).
If an MSS option is not received at connection setup, TCP If an MSS option is not received at connection setup, TCP
implementations MUST assume a default send MSS of 536 (576-40) for implementations MUST assume a default send MSS of 536 (576 - 40) for
IPv4 or 1220 (1280 - 60) for IPv6 (MUST-15). IPv4 or 1220 (1280 - 60) for IPv6 (MUST-15).
The maximum size of a segment that TCP endpoint really sends, the The maximum size of a segment that TCP endpoint really sends, the
"effective send MSS," MUST be the smaller (MUST-16) of the send MSS "effective send MSS," MUST be the smaller (MUST-16) of the send MSS
(that reflects the available reassembly buffer size at the remote (that reflects the available reassembly buffer size at the remote
host, the EMTU_R [18]) and the largest transmission size permitted by host, the EMTU_R [20]) and the largest transmission size permitted by
the IP layer (EMTU_S [18]): the IP layer (EMTU_S [20]):
Eff.snd.MSS = Eff.snd.MSS =
min(SendMSS+20, MMS_S) - TCPhdrsize - IPoptionsize min(SendMSS+20, MMS_S) - TCPhdrsize - IPoptionsize
where: where:
o SendMSS is the MSS value received from the remote host, or the * SendMSS is the MSS value received from the remote host, or the
default 536 for IPv4 or 1220 for IPv6, if no MSS option is default 536 for IPv4 or 1220 for IPv6, if no MSS option is
received. received.
o MMS_S is the maximum size for a transport-layer message that TCP * MMS_S is the maximum size for a transport-layer message that TCP
may send. may send.
o TCPhdrsize is the size of the fixed TCP header and any options. * TCPhdrsize is the size of the fixed TCP header and any options.
This is 20 in the (rare) case that no options are present, but may This is 20 in the (rare) case that no options are present, but may
be larger if TCP options are to be sent. Note that some options be larger if TCP options are to be sent. Note that some options
might not be included on all segments, but that for each segment might not be included on all segments, but that for each segment
sent, the sender should adjust the data length accordingly, within sent, the sender should adjust the data length accordingly, within
the Eff.snd.MSS. the Eff.snd.MSS.
o IPoptionsize is the size of any IPv4 options or IPv6 extension * IPoptionsize is the size of any IPv4 options or IPv6 extension
headers associated with a TCP connection. Note that some options headers associated with a TCP connection. Note that some options
or extension headers might not be included on all packets, but or extension headers might not be included on all packets, but
that for each segment sent, the sender should adjust the data that for each segment sent, the sender should adjust the data
length accordingly, within the Eff.snd.MSS. length accordingly, within the Eff.snd.MSS.
The MSS value to be sent in an MSS option should be equal to the The MSS value to be sent in an MSS option should be equal to the
effective MTU minus the fixed IP and TCP headers. By ignoring both effective MTU minus the fixed IP and TCP headers. By ignoring both
IP and TCP options when calculating the value for the MSS option, if IP and TCP options when calculating the value for the MSS option, if
there are any IP or TCP options to be sent in a packet, then the there are any IP or TCP options to be sent in a packet, then the
sender must decrease the size of the TCP data accordingly. RFC 6691 sender must decrease the size of the TCP data accordingly. RFC 6691
[42] discusses this in greater detail. [44] discusses this in greater detail.
The MSS value to be sent in an MSS option must be less than or equal The MSS value to be sent in an MSS option must be less than or equal
to: to:
MMS_R - 20 MMS_R - 20
where MMS_R is the maximum size for a transport-layer message that where MMS_R is the maximum size for a transport-layer message that
can be received (and reassembled at the IP layer) (MUST-67). TCP can be received (and reassembled at the IP layer) (MUST-67). TCP
obtains MMS_R and MMS_S from the IP layer; see the generic call obtains MMS_R and MMS_S from the IP layer; see the generic call
GET_MAXSIZES in Section 3.4 of RFC 1122. These are defined in terms GET_MAXSIZES in Section 3.4 of RFC 1122. These are defined in terms
of their IP MTU equivalents, EMTU_R and EMTU_S [18]. of their IP MTU equivalents, EMTU_R and EMTU_S [20].
When TCP is used in a situation where either the IP or TCP headers When TCP is used in a situation where either the IP or TCP headers
are not fixed, the sender must reduce the amount of TCP data in any are not fixed, the sender must reduce the amount of TCP data in any
given packet by the number of octets used by the IP and TCP options. given packet by the number of octets used by the IP and TCP options.
This has been a point of confusion historically, as explained in RFC This has been a point of confusion historically, as explained in RFC
6691, Section 3.1. 6691, Section 3.1.
3.7.2. Path MTU Discovery 3.7.2. Path MTU Discovery
A TCP implementation may be aware of the MTU on directly connected A TCP implementation may be aware of the MTU on directly connected
skipping to change at page 38, line 26 skipping to change at page 38, line 46
PMTUD and PLPMTUD help TCP choose segment sizes that avoid both on- PMTUD and PLPMTUD help TCP choose segment sizes that avoid both on-
path (for IPv4) and source fragmentation (IPv4 and IPv6). path (for IPv4) and source fragmentation (IPv4 and IPv6).
PMTUD for IPv4 [2] or IPv6 [14] is implemented in conjunction between PMTUD for IPv4 [2] or IPv6 [14] is implemented in conjunction between
TCP, IP, and ICMP protocols. It relies both on avoiding source TCP, IP, and ICMP protocols. It relies both on avoiding source
fragmentation and setting the IPv4 DF (don't fragment) flag, the fragmentation and setting the IPv4 DF (don't fragment) flag, the
latter to inhibit on-path fragmentation. It relies on ICMP errors latter to inhibit on-path fragmentation. It relies on ICMP errors
from routers along the path, whenever a segment is too large to from routers along the path, whenever a segment is too large to
traverse a link. Several adjustments to a TCP implementation with traverse a link. Several adjustments to a TCP implementation with
PMTUD are described in RFC 2923 in order to deal with problems PMTUD are described in RFC 2923 in order to deal with problems
experienced in practice [25]. PLPMTUD [29] is a Standards Track experienced in practice [28]. PLPMTUD [32] is a Standards Track
improvement to PMTUD that relaxes the requirement for ICMP support improvement to PMTUD that relaxes the requirement for ICMP support
across a path, and improves performance in cases where ICMP is not across a path, and improves performance in cases where ICMP is not
consistently conveyed, but still tries to avoid source fragmentation. consistently conveyed, but still tries to avoid source fragmentation.
The mechanisms in all four of these RFCs are recommended to be The mechanisms in all four of these RFCs are recommended to be
included in TCP implementations. included in TCP implementations.
The TCP MSS option specifies an upper bound for the size of packets The TCP MSS option specifies an upper bound for the size of packets
that can be received (see [42]). Hence, setting the value in the MSS that can be received (see [44]). Hence, setting the value in the MSS
option too small can impact the ability for PMTUD or PLPMTUD to find option too small can impact the ability for PMTUD or PLPMTUD to find
a larger path MTU. RFC 1191 discusses this implication of many older a larger path MTU. RFC 1191 discusses this implication of many older
TCP implementations setting the TCP MSS to 536 (corresponding to the TCP implementations setting the TCP MSS to 536 (corresponding to the
IPv4 576 byte default MTU) for non-local destinations, rather than IPv4 576 byte default MTU) for non-local destinations, rather than
deriving it from the MTUs of connected interfaces as recommended. deriving it from the MTUs of connected interfaces as recommended.
3.7.3. Interfaces with Variable MTU Values 3.7.3. Interfaces with Variable MTU Values
The effective MTU can sometimes vary, as when used with variable The effective MTU can sometimes vary, as when used with variable
compression, e.g., RObust Header Compression (ROHC) [35]. It is compression, e.g., RObust Header Compression (ROHC) [38]. It is
tempting for a TCP implementation to advertise the largest possible tempting for a TCP implementation to advertise the largest possible
MSS, to support the most efficient use of compressed payloads. MSS, to support the most efficient use of compressed payloads.
Unfortunately, some compression schemes occasionally need to transmit Unfortunately, some compression schemes occasionally need to transmit
full headers (and thus smaller payloads) to resynchronize state at full headers (and thus smaller payloads) to resynchronize state at
their endpoint compressors/decompressors. If the largest MTU is used their endpoint compressors/decompressors. If the largest MTU is used
to calculate the value to advertise in the MSS option, TCP to calculate the value to advertise in the MSS option, TCP
retransmission may interfere with compressor resynchronization. retransmission may interfere with compressor resynchronization.
As a result, when the effective MTU of an interface varies packet-to- As a result, when the effective MTU of an interface varies packet-to-
packet, TCP implementations SHOULD use the smallest effective MTU of packet, TCP implementations SHOULD use the smallest effective MTU of
the interface to calculate the value to advertise in the MSS option the interface to calculate the value to advertise in the MSS option
(SHLD-6). (SHLD-6).
3.7.4. Nagle Algorithm 3.7.4. Nagle Algorithm
The "Nagle algorithm" was described in RFC 896 [17] and was The "Nagle algorithm" was described in RFC 896 [18] and was
recommended in RFC 1122 [18] for mitigation of an early problem of recommended in RFC 1122 [20] for mitigation of an early problem of
too many small packets being generated. It has been implemented in too many small packets being generated. It has been implemented in
most current TCP code bases, sometimes with minor variations (see most current TCP code bases, sometimes with minor variations (see
Appendix A.3). Appendix A.3).
If there is unacknowledged data (i.e., SND.NXT > SND.UNA), then the If there is unacknowledged data (i.e., SND.NXT > SND.UNA), then the
sending TCP endpoint buffers all user data (regardless of the PSH sending TCP endpoint buffers all user data (regardless of the PSH
bit), until the outstanding data has been acknowledged or until the bit), until the outstanding data has been acknowledged or until the
TCP endpoint can send a full-sized segment (Eff.snd.MSS bytes). TCP endpoint can send a full-sized segment (Eff.snd.MSS bytes).
A TCP implementation SHOULD implement the Nagle Algorithm to coalesce A TCP implementation SHOULD implement the Nagle Algorithm to coalesce
short segments (SHLD-7). However, there MUST be a way for an short segments (SHLD-7). However, there MUST be a way for an
application to disable the Nagle algorithm on an individual application to disable the Nagle algorithm on an individual
connection (MUST-17). In all cases, sending data is also subject to connection (MUST-17). In all cases, sending data is also subject to
the limitation imposed by the Slow Start algorithm [9]. the limitation imposed by the Slow Start algorithm [8].
Since there can be problematic interactions between the Nagle Since there can be problematic interactions between the Nagle
Algorithm and delayed acknowledgements, some implementations use Algorithm and delayed acknowledgements, some implementations use
minor variations of the Nagle algorithm, such as the one described in minor variations of the Nagle algorithm, such as the one described in
Appendix A.3. Appendix A.3.
3.7.5. IPv6 Jumbograms 3.7.5. IPv6 Jumbograms
In order to support TCP over IPv6 Jumbograms, implementations need to In order to support TCP over IPv6 Jumbograms, implementations need to
be able to send TCP segments larger than the 64KB limit that the MSS be able to send TCP segments larger than the 64KB limit that the MSS
option can convey. RFC 2675 [5] defines that an MSS value of 65,535 option can convey. RFC 2675 [25] defines that an MSS value of 65,535
bytes is to be treated as infinity, and Path MTU Discovery [14] is bytes is to be treated as infinity, and Path MTU Discovery [14] is
used to determine the actual MSS. used to determine the actual MSS.
The Jumbo Payload option need not be implemented or understood by The Jumbo Payload option need not be implemented or understood by
IPv6 nodes that do not support attachment to links with a MTU greater IPv6 nodes that do not support attachment to links with a MTU greater
than 65,575 [5], and the present IPv6 Node Requirements does not than 65,575 [25], and the present IPv6 Node Requirements does not
include support for Jumbograms [53]. include support for Jumbograms [55].
3.8. Data Communication 3.8. Data Communication
Once the connection is established data is communicated by the Once the connection is established data is communicated by the
exchange of segments. Because segments may be lost due to errors exchange of segments. Because segments may be lost due to errors
(checksum test failure), or network congestion, TCP uses (checksum test failure), or network congestion, TCP uses
retransmission to ensure delivery of every segment. Duplicate retransmission to ensure delivery of every segment. Duplicate
segments may arrive due to network or TCP retransmission. As segments may arrive due to network or TCP retransmission. As
discussed in the section on sequence numbers the TCP implementation discussed in the section on sequence numbers, the TCP implementation
performs certain tests on the sequence and acknowledgment numbers in performs certain tests on the sequence and acknowledgment numbers in
the segments to verify their acceptability. the segments to verify their acceptability.
The sender of data keeps track of the next sequence number to use in The sender of data keeps track of the next sequence number to use in
the variable SND.NXT. The receiver of data keeps track of the next the variable SND.NXT. The receiver of data keeps track of the next
sequence number to expect in the variable RCV.NXT. The sender of sequence number to expect in the variable RCV.NXT. The sender of
data keeps track of the oldest unacknowledged sequence number in the data keeps track of the oldest unacknowledged sequence number in the
variable SND.UNA. If the data flow is momentarily idle and all data variable SND.UNA. If the data flow is momentarily idle and all data
sent has been acknowledged then the three variables will be equal. sent has been acknowledged then the three variables will be equal.
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3.8.1. Retransmission Timeout 3.8.1. Retransmission Timeout
Because of the variability of the networks that compose an Because of the variability of the networks that compose an
internetwork system and the wide range of uses of TCP connections the internetwork system and the wide range of uses of TCP connections the
retransmission timeout (RTO) must be dynamically determined. retransmission timeout (RTO) must be dynamically determined.
The RTO MUST be computed according to the algorithm in [10], The RTO MUST be computed according to the algorithm in [10],
including Karn's algorithm for taking RTT samples (MUST-18). including Karn's algorithm for taking RTT samples (MUST-18).
RFC 793 contains an early example procedure for computing the RTO. RFC 793 contains an early example procedure for computing the RTO,
This was then replaced by the algorithm described in RFC 1122, and based on work mentioned in IEN 177 [72]. This was then replaced by
subsequently updated in RFC 2988, and then again in RFC 6298. the algorithm described in RFC 1122, and subsequently updated in RFC
2988, and then again in RFC 6298.
RFC 1122 allows that if a retransmitted packet is identical to the RFC 1122 allows that if a retransmitted packet is identical to the
original packet (which implies not only that the data boundaries have original packet (which implies not only that the data boundaries have
not changed, but also that none of the headers have changed), then not changed, but also that none of the headers have changed), then
the same IPv4 Identification field MAY be used (see Section 3.2.1.5 the same IPv4 Identification field MAY be used (see Section 3.2.1.5
of RFC 1122) (MAY-4). The same IP identification field may be reused of RFC 1122) (MAY-4). The same IP identification field may be reused
anyways, since it is only meaningful when a datagram is fragmented anyways, since it is only meaningful when a datagram is fragmented
[43]. TCP implementations should not rely on or typically interact [45]. TCP implementations should not rely on or typically interact
with this IPv4 header field in any way. It is not a reasonable way with this IPv4 header field in any way. It is not a reasonable way
to either indicate duplicate sent segments, nor to identify duplicate to either indicate duplicate sent segments, nor to identify duplicate
received segments. received segments.
3.8.2. TCP Congestion Control 3.8.2. TCP Congestion Control
RFC 2914 [6] explains the importance of congestion control for the RFC 2914 [5] explains the importance of congestion control for the
Internet. Internet.
RFC 1122 required implementation of Van Jacobson's congestion control RFC 1122 required implementation of Van Jacobson's congestion control
algorithms slow start and congestion avoidance together with algorithms slow start and congestion avoidance together with
exponential back-off for successive RTO values for the same segment. exponential back-off for successive RTO values for the same segment.
RFC 2581 provided IETF Standards Track description of slow start and RFC 2581 provided IETF Standards Track description of slow start and
congestion avoidance, along with fast retransmit and fast recovery. congestion avoidance, along with fast retransmit and fast recovery.
RFC 5681 is the current description of these algorithms and is the RFC 5681 is the current description of these algorithms and is the
current Standards Track specification providing guidelines for TCP current Standards Track specification providing guidelines for TCP
congestion control. RFC 6298 describes exponential back-off of RTO congestion control. RFC 6298 describes exponential back-off of RTO
skipping to change at page 41, line 32 skipping to change at page 42, line 8
A TCP endpoint MUST implement the basic congestion control algorithms A TCP endpoint MUST implement the basic congestion control algorithms
slow start, congestion avoidance, and exponential back-off of RTO to slow start, congestion avoidance, and exponential back-off of RTO to
avoid creating congestion collapse conditions (MUST-19). RFC 5681 avoid creating congestion collapse conditions (MUST-19). RFC 5681
and RFC 6298 describe the basic algorithms on the IETF Standards and RFC 6298 describe the basic algorithms on the IETF Standards
Track that are broadly applicable. Multiple other suitable Track that are broadly applicable. Multiple other suitable
algorithms exist and have been widely used. Many TCP implementations algorithms exist and have been widely used. Many TCP implementations
support a set of alternative algorithms that can be configured for support a set of alternative algorithms that can be configured for
use on the endpoint. An endpoint MAY implement such alternative use on the endpoint. An endpoint MAY implement such alternative
algorithms provided that the algorithms are conformant with the TCP algorithms provided that the algorithms are conformant with the TCP
specifications from the IETF Standards Track as described in RFC specifications from the IETF Standards Track as described in RFC
2914, RFC 5033 [8], and RFC 8961 [15] (MAY-18). 2914, RFC 5033 [7], and RFC 8961 [15] (MAY-18).
Explicit Congestion Notification (ECN) was defined in RFC 3168 and is Explicit Congestion Notification (ECN) was defined in RFC 3168 and is
an IETF Standards Track enhancement that has many benefits [50]. an IETF Standards Track enhancement that has many benefits [52].
A TCP endpoint SHOULD implement ECN as described in RFC 3168 (SHLD- A TCP endpoint SHOULD implement ECN as described in RFC 3168 (SHLD-
8). 8).
3.8.3. TCP Connection Failures 3.8.3. TCP Connection Failures
Excessive retransmission of the same segment by a TCP endpoint Excessive retransmission of the same segment by a TCP endpoint
indicates some failure of the remote host or the Internet path. This indicates some failure of the remote host or the Internet path. This
failure may be of short or long duration. The following procedure failure may be of short or long duration. The following procedure
MUST be used to handle excessive retransmissions of data segments MUST be used to handle excessive retransmissions of data segments
(MUST-20): (MUST-20):
(a) There are two thresholds R1 and R2 measuring the amount of (a) There are two thresholds R1 and R2 measuring the amount of
retransmission that has occurred for the same segment. R1 and R2 retransmission that has occurred for the same segment. R1 and R2
might be measured in time units or as a count of retransmissions might be measured in time units or as a count of retransmissions
(with the current RTO and corresponding backoffs as a conversion (with the current RTO and corresponding backoffs as a conversion
factor, if needed). factor, if needed).
(b) When the number of transmissions of the same segment reaches (b) When the number of transmissions of the same segment reaches
or exceeds threshold R1, pass negative advice (see Section 3.3.1.4 or exceeds threshold R1, pass negative advice (see Section 3.3.1.4
of [18]) to the IP layer, to trigger dead-gateway diagnosis. of [20]) to the IP layer, to trigger dead-gateway diagnosis.
(c) When the number of transmissions of the same segment reaches a (c) When the number of transmissions of the same segment reaches a
threshold R2 greater than R1, close the connection. threshold R2 greater than R1, close the connection.
(d) An application MUST (MUST-21) be able to set the value for R2 (d) An application MUST (MUST-21) be able to set the value for R2
for a particular connection. For example, an interactive for a particular connection. For example, an interactive
application might set R2 to "infinity," giving the user control application might set R2 to "infinity," giving the user control
over when to disconnect. over when to disconnect.
(e) TCP implementations SHOULD inform the application of the (e) TCP implementations SHOULD inform the application of the
delivery problem (unless such information has been disabled by the delivery problem (unless such information has been disabled by the
application; see Asynchronous Reports section), when R1 is reached application; see Asynchronous Reports section), when R1 is reached
and before R2 (SHLD-9). This will allow a remote login (User and before R2 (SHLD-9). This will allow a remote login
Telnet) application program to inform the user, for example. application program to inform the user, for example.
The value of R1 SHOULD correspond to at least 3 retransmissions, at The value of R1 SHOULD correspond to at least 3 retransmissions, at
the current RTO (SHLD-10). The value of R2 SHOULD correspond to at the current RTO (SHLD-10). The value of R2 SHOULD correspond to at
least 100 seconds (SHLD-11). least 100 seconds (SHLD-11).
An attempt to open a TCP connection could fail with excessive An attempt to open a TCP connection could fail with excessive
retransmissions of the SYN segment or by receipt of a RST segment or retransmissions of the SYN segment or by receipt of a RST segment or
an ICMP Port Unreachable. SYN retransmissions MUST be handled in the an ICMP Port Unreachable. SYN retransmissions MUST be handled in the
general way just described for data retransmissions, including general way just described for data retransmissions, including
notification of the application layer. notification of the application layer.
skipping to change at page 43, line 28 skipping to change at page 44, line 10
An implementation SHOULD send a keep-alive segment with no data An implementation SHOULD send a keep-alive segment with no data
(SHLD-12); however, it MAY be configurable to send a keep-alive (SHLD-12); however, it MAY be configurable to send a keep-alive
segment containing one garbage octet (MAY-6), for compatibility with segment containing one garbage octet (MAY-6), for compatibility with
erroneous TCP implementations. erroneous TCP implementations.
3.8.5. The Communication of Urgent Information 3.8.5. The Communication of Urgent Information
As a result of implementation differences and middlebox interactions, As a result of implementation differences and middlebox interactions,
new applications SHOULD NOT employ the TCP urgent mechanism (SHLD- new applications SHOULD NOT employ the TCP urgent mechanism (SHLD-
13). However, TCP implementations MUST still include support for the 13). However, TCP implementations MUST still include support for the
urgent mechanism (MUST-30). Details can be found in RFC 6093 [38]. urgent mechanism (MUST-30). Information on how some TCP
implementations interpret the urgent pointer can be found in RFC 6093
[40].
The objective of the TCP urgent mechanism is to allow the sending The objective of the TCP urgent mechanism is to allow the sending
user to stimulate the receiving user to accept some urgent data and user to stimulate the receiving user to accept some urgent data and
to permit the receiving TCP endpoint to indicate to the receiving to permit the receiving TCP endpoint to indicate to the receiving
user when all the currently known urgent data has been received by user when all the currently known urgent data has been received by
the user. the user.
This mechanism permits a point in the data stream to be designated as This mechanism permits a point in the data stream to be designated as
the end of urgent information. Whenever this point is in advance of the end of urgent information. Whenever this point is in advance of
the receive sequence number (RCV.NXT) at the receiving TCP endpoint, the receive sequence number (RCV.NXT) at the receiving TCP endpoint,
skipping to change at page 44, line 14 skipping to change at page 44, line 44
To send an urgent indication the user must also send at least one To send an urgent indication the user must also send at least one
data octet. If the sending user also indicates a push, timely data octet. If the sending user also indicates a push, timely
delivery of the urgent information to the destination process is delivery of the urgent information to the destination process is
enhanced. Note that because changes in the urgent pointer correspond enhanced. Note that because changes in the urgent pointer correspond
to data being written by a sending application, the urgent pointer to data being written by a sending application, the urgent pointer
can not "recede" in the sequence space, but a TCP receiver should be can not "recede" in the sequence space, but a TCP receiver should be
robust to invalid urgent pointer values. robust to invalid urgent pointer values.
A TCP implementation MUST support a sequence of urgent data of any A TCP implementation MUST support a sequence of urgent data of any
length (MUST-31). [18] length (MUST-31). [20]
The urgent pointer MUST point to the sequence number of the octet The urgent pointer MUST point to the sequence number of the octet
following the urgent data (MUST-62). following the urgent data (MUST-62).
A TCP implementation MUST (MUST-32) inform the application layer A TCP implementation MUST (MUST-32) inform the application layer
asynchronously whenever it receives an Urgent pointer and there was asynchronously whenever it receives an Urgent pointer and there was
previously no pending urgent data, or whenever the Urgent pointer previously no pending urgent data, or whenever the Urgent pointer
advances in the data stream. The TCP implementation MUST (MUST-33) advances in the data stream. The TCP implementation MUST (MUST-33)
provide a way for the application to learn how much urgent data provide a way for the application to learn how much urgent data
remains to be read from the connection, or at least to determine remains to be read from the connection, or at least to determine
whether or not more urgent data remains to be read [18]. whether more urgent data remains to be read [20].
3.8.6. Managing the Window 3.8.6. Managing the Window
The window sent in each segment indicates the range of sequence The window sent in each segment indicates the range of sequence
numbers the sender of the window (the data receiver) is currently numbers the sender of the window (the data receiver) is currently
prepared to accept. There is an assumption that this is related to prepared to accept. There is an assumption that this is related to
the currently available data buffer space available for this the currently available data buffer space available for this
connection. connection.
The sending TCP endpoint packages the data to be transmitted into The sending TCP endpoint packages the data to be transmitted into
segments that fit the current window, and may repackage segments on segments that fit the current window, and may repackage segments on
the retransmission queue. Such repackaging is not required, but may the retransmission queue. Such repackaging is not required, but may
be helpful. be helpful.
In a connection with a one-way data flow, the window information will In a connection with a one-way data flow, the window information will
be carried in acknowledgment segments that all have the same sequence be carried in acknowledgment segments that all have the same sequence
number so there will be no way to reorder them if they arrive out of number, so there will be no way to reorder them if they arrive out of
order. This is not a serious problem, but it will allow the window order. This is not a serious problem, but it will allow the window
information to be on occasion temporarily based on old reports from information to be on occasion temporarily based on old reports from
the data receiver. A refinement to avoid this problem is to act on the data receiver. A refinement to avoid this problem is to act on
the window information from segments that carry the highest the window information from segments that carry the highest
acknowledgment number (that is segments with acknowledgment number acknowledgment number (that is segments with acknowledgment number
equal or greater than the highest previously received). equal or greater than the highest previously received).
Indicating a large window encourages transmissions. If more data Indicating a large window encourages transmissions. If more data
arrives than can be accepted, it will be discarded. This will result arrives than can be accepted, it will be discarded. This will result
in excessive retransmissions, adding unnecessarily to the load on the in excessive retransmissions, adding unnecessarily to the load on the
network and the TCP endpoints. Indicating a small window may network and the TCP endpoints. Indicating a small window may
restrict the transmission of data to the point of introducing a round restrict the transmission of data to the point of introducing a round
trip delay between each new segment transmitted. trip delay between each new segment transmitted.
The mechanisms provided allow a TCP endpoint to advertise a large The mechanisms provided allow a TCP endpoint to advertise a large
window and to subsequently advertise a much smaller window without window and to subsequently advertise a much smaller window without
having accepted that much data. This, so called "shrinking the having accepted that much data. This, so-called "shrinking the
window," is strongly discouraged. The robustness principle [18] window," is strongly discouraged. The robustness principle [20]
dictates that TCP peers will not shrink the window themselves, but dictates that TCP peers will not shrink the window themselves, but
will be prepared for such behavior on the part of other TCP peers. will be prepared for such behavior on the part of other TCP peers.
A TCP receiver SHOULD NOT shrink the window, i.e., move the right A TCP receiver SHOULD NOT shrink the window, i.e., move the right
window edge to the left (SHLD-14). However, a sending TCP peer MUST window edge to the left (SHLD-14). However, a sending TCP peer MUST
be robust against window shrinking, which may cause the "usable be robust against window shrinking, which may cause the "usable
window" (see Section 3.8.6.2.1) to become negative (MUST-34). window" (see Section 3.8.6.2.1) to become negative (MUST-34).
If this happens, the sender SHOULD NOT send new data (SHLD-15), but If this happens, the sender SHOULD NOT send new data (SHLD-15), but
SHOULD retransmit normally the old unacknowledged data between SHOULD retransmit normally the old unacknowledged data between
skipping to change at page 45, line 46 skipping to change at page 46, line 31
reported to the other. This is referred to as Zero-Window Probing reported to the other. This is referred to as Zero-Window Probing
(ZWP) in other documents. (ZWP) in other documents.
Probing of zero (offered) windows MUST be supported (MUST-36). Probing of zero (offered) windows MUST be supported (MUST-36).
A TCP implementation MAY keep its offered receive window closed A TCP implementation MAY keep its offered receive window closed
indefinitely (MAY-8). As long as the receiving TCP peer continues to indefinitely (MAY-8). As long as the receiving TCP peer continues to
send acknowledgments in response to the probe segments, the sending send acknowledgments in response to the probe segments, the sending
TCP peer MUST allow the connection to stay open (MUST-37). This TCP peer MUST allow the connection to stay open (MUST-37). This
enables TCP to function in scenarios such as the "printer ran out of enables TCP to function in scenarios such as the "printer ran out of
paper" situation described in Section 4.2.2.17 of RFC1122. The paper" situation described in Section 4.2.2.17 of [20]. The behavior
behavior is subject to the implementation's resource management is subject to the implementation's resource management concerns, as
concerns, as noted in [40]. noted in [42].
When the receiving TCP peer has a zero window and a segment arrives When the receiving TCP peer has a zero window and a segment arrives
it must still send an acknowledgment showing its next expected it must still send an acknowledgment showing its next expected
sequence number and current window (zero). sequence number and current window (zero).
The transmitting host SHOULD send the first zero-window probe when a The transmitting host SHOULD send the first zero-window probe when a
zero window has existed for the retransmission timeout period (SHLD- zero window has existed for the retransmission timeout period (SHLD-
29) (Section 3.8.1), and SHOULD increase exponentially the interval 29) (Section 3.8.1), and SHOULD increase exponentially the interval
between successive probes (SHLD-30). between successive probes (SHLD-30).
skipping to change at page 46, line 37 skipping to change at page 47, line 18
3.8.6.2.1. Sender's Algorithm - When to Send Data 3.8.6.2.1. Sender's Algorithm - When to Send Data
A TCP implementation MUST include a SWS avoidance algorithm in the A TCP implementation MUST include a SWS avoidance algorithm in the
sender (MUST-38). sender (MUST-38).
The Nagle algorithm from Section 3.7.4 additionally describes how to The Nagle algorithm from Section 3.7.4 additionally describes how to
coalesce short segments. coalesce short segments.
The sender's SWS avoidance algorithm is more difficult than the The sender's SWS avoidance algorithm is more difficult than the
receivers's, because the sender does not know (directly) the receiver's, because the sender does not know (directly) the
receiver's total buffer space RCV.BUFF. An approach that has been receiver's total buffer space RCV.BUFF. An approach that has been
found to work well is for the sender to calculate Max(SND.WND), the found to work well is for the sender to calculate Max(SND.WND), the
maximum send window it has seen so far on the connection, and to use maximum send window it has seen so far on the connection, and to use
this value as an estimate of RCV.BUFF. Unfortunately, this can only this value as an estimate of RCV.BUFF. Unfortunately, this can only
be an estimate; the receiver may at any time reduce the size of be an estimate; the receiver may at any time reduce the size of
RCV.BUFF. To avoid a resulting deadlock, it is necessary to have a RCV.BUFF. To avoid a resulting deadlock, it is necessary to have a
timeout to force transmission of data, overriding the SWS avoidance timeout to force transmission of data, overriding the SWS avoidance
algorithm. In practice, this timeout should seldom occur. algorithm. In practice, this timeout should seldom occur.
The "usable window" is: The "usable window" is:
U = SND.UNA + SND.WND - SND.NXT U = SND.UNA + SND.WND - SND.NXT
i.e., the offered window less the amount of data sent but not i.e., the offered window less the amount of data sent but not
acknowledged. If D is the amount of data queued in the sending TCP acknowledged. If D is the amount of data queued in the sending TCP
endpoint but not yet sent, then the following set of rules is endpoint but not yet sent, then the following set of rules is
recommended. recommended.
Send data: Send data:
(1) if a maximum-sized segment can be sent, i.e, if: (1) if a maximum-sized segment can be sent, i.e., if:
min(D,U) >= Eff.snd.MSS; min(D,U) >= Eff.snd.MSS;
(2) or if the data is pushed and all queued data can be sent now, (2) or if the data is pushed and all queued data can be sent now,
i.e., if: i.e., if:
[SND.NXT = SND.UNA and] PUSHED and D <= U [SND.NXT = SND.UNA and] PUSHED and D <= U
(the bracketed condition is imposed by the Nagle algorithm); (the bracketed condition is imposed by the Nagle algorithm);
skipping to change at page 48, line 39 skipping to change at page 49, line 20
min( Fr * RCV.BUFF, Eff.snd.MSS ) min( Fr * RCV.BUFF, Eff.snd.MSS )
where Fr is a fraction whose recommended value is 1/2, and where Fr is a fraction whose recommended value is 1/2, and
Eff.snd.MSS is the effective send MSS for the connection (see Eff.snd.MSS is the effective send MSS for the connection (see
Section 3.7.1). When the inequality is satisfied, RCV.WND is set to Section 3.7.1). When the inequality is satisfied, RCV.WND is set to
RCV.BUFF-RCV.USER. RCV.BUFF-RCV.USER.
Note that the general effect of this algorithm is to advance RCV.WND Note that the general effect of this algorithm is to advance RCV.WND
in increments of Eff.snd.MSS (for realistic receive buffers: in increments of Eff.snd.MSS (for realistic receive buffers:
Eff.snd.MSS < RCV.BUFF/2). Note also that the receiver must use its Eff.snd.MSS < RCV.BUFF/2). Note also that the receiver must use its
own Eff.snd.MSS, assuming it is the same as the sender's. own Eff.snd.MSS, making the assumption that it is the same as the
sender's.
3.8.6.3. Delayed Acknowledgements - When to Send an ACK Segment 3.8.6.3. Delayed Acknowledgements - When to Send an ACK Segment
A host that is receiving a stream of TCP data segments can increase A host that is receiving a stream of TCP data segments can increase
efficiency in both the Internet and the hosts by sending fewer than efficiency in both the Internet and the hosts by sending fewer than
one ACK (acknowledgment) segment per data segment received; this is one ACK (acknowledgment) segment per data segment received; this is
known as a "delayed ACK". known as a "delayed ACK".
A TCP endpoint SHOULD implement a delayed ACK (SHLD-18), but an ACK A TCP endpoint SHOULD implement a delayed ACK (SHLD-18), but an ACK
should not be excessively delayed; in particular, the delay MUST be should not be excessively delayed; in particular, the delay MUST be
less than 0.5 seconds (MUST-40). An ACK SHOULD be generated for at less than 0.5 seconds (MUST-40). An ACK SHOULD be generated for at
least every second full-sized segment or 2*RMSS bytes of new data least every second full-sized segment or 2*RMSS bytes of new data
(where RMSS is the MSS specified by the TCP endpoint receiving the (where RMSS is the MSS specified by the TCP endpoint receiving the
segments to be acknowledged, or the default value if not specified) segments to be acknowledged, or the default value if not specified)
(SHLD-19). Excessive delays on ACKs can disturb the round-trip (SHLD-19). Excessive delays on ACKs can disturb the round-trip
timing and packet "clocking" algorithms. More complete discussion of timing and packet "clocking" algorithms. More complete discussion of
delayed ACK behavior is in Section 4.2 of RFC 5681 [9], including delayed ACK behavior is in Section 4.2 of RFC 5681 [8], including
recomendations to immediately acknowledge out-of-order segments, recommendations to immediately acknowledge out-of-order segments,
segments above a gap in sequence space, or segments that fill all or segments above a gap in sequence space, or segments that fill all or
part of a gap, in order to accelerate loss recovery. part of a gap, in order to accelerate loss recovery.
Note that there are several current practices that further lead to a Note that there are several current practices that further lead to a
reduced number of ACKs, including generic receive offload (GRO), ACK reduced number of ACKs, including generic receive offload (GRO) [73],
compression, and ACK decimation [26]. ACK compression, and ACK decimation [29].
3.9. Interfaces 3.9. Interfaces
There are of course two interfaces of concern: the user/TCP interface There are of course two interfaces of concern: the user/TCP interface
and the TCP/lower-level interface. We have a fairly elaborate model and the TCP/lower level interface. We have a fairly elaborate model
of the user/TCP interface, but the interface to the lower level of the user/TCP interface, but the interface to the lower level
protocol module is left unspecified here, since it will be specified protocol module is left unspecified here, since it will be specified
in detail by the specification of the lower level protocol. For the in detail by the specification of the lower level protocol. For the
case that the lower level is IP we note some of the parameter values case that the lower level is IP we note some of the parameter values
that TCP implementations might use. that TCP implementations might use.
3.9.1. User/TCP Interface 3.9.1. User/TCP Interface
The following functional description of user commands to the TCP The following functional description of user commands to the TCP
implementation is, at best, fictional, since every operating system implementation is, at best, fictional, since every operating system
will have different facilities. Consequently, we must warn readers will have different facilities. Consequently, we must warn readers
that different TCP implementations may have different user that different TCP implementations may have different user
interfaces. However, all TCP implementations must provide a certain interfaces. However, all TCP implementations must provide a certain
minimum set of services to guarantee that all TCP implementations can minimum set of services to guarantee that all TCP implementations can
support the same protocol hierarchy. This section specifies the support the same protocol hierarchy. This section specifies the
functional interfaces required of all TCP implementations. functional interfaces required of all TCP implementations.
Section 3.1 of [52] also identifies primitives provided by TCP, and Section 3.1 of [54] also identifies primitives provided by TCP, and
could be used as an additional reference for implementers. could be used as an additional reference for implementers.
TCP User Commands The following sections functionally characterize a USER/TCP
interface. The notation used is similar to most procedure or
function calls in high level languages, but this usage is not meant
to rule out trap type service calls.
The following sections functionally characterize a USER/TCP The user commands described below specify the basic functions the TCP
interface. The notation used is similar to most procedure or implementation must perform to support interprocess communication.
function calls in high level languages, but this usage is not Individual implementations must define their own exact format, and
meant to rule out trap type service calls. may provide combinations or subsets of the basic functions in single
calls. In particular, some implementations may wish to automatically
OPEN a connection on the first SEND or RECEIVE issued by the user for
a given connection.
The user commands described below specify the basic functions the In providing interprocess communication facilities, the TCP
TCP implementation must perform to support interprocess implementation must not only accept commands, but must also return
communication. Individual implementations must define their own information to the processes it serves. The latter consists of:
exact format, and may provide combinations or subsets of the basic
functions in single calls. In particular, some implementations
may wish to automatically OPEN a connection on the first SEND or
RECEIVE issued by the user for a given connection.
In providing interprocess communication facilities, the TCP (a) general information about a connection (e.g., interrupts,
implementation must not only accept commands, but must also return remote close, binding of unspecified remote socket).
information to the processes it serves. The latter consists of:
(a) general information about a connection (e.g., interrupts, (b) replies to specific user commands indicating success or
remote close, binding of unspecified remote socket). various types of failure.
(b) replies to specific user commands indicating success or 3.9.1.1. Open
various types of failure.
Open Format: OPEN (local port, remote socket, active/passive [,
timeout] [, DiffServ field] [, security/compartment] [local IP
address,] [, options]) -> local connection name
Format: OPEN (local port, remote socket, active/passive [, If the active/passive flag is set to passive, then this is a call
timeout] [, DiffServ field] [, security/compartment] [local IP to LISTEN for an incoming connection. A passive open may have
address,] [, options]) -> local connection name either a fully specified remote socket to wait for a particular
connection or an unspecified remote socket to wait for any call.
A fully specified passive call can be made active by the
subsequent execution of a SEND.
If the active/passive flag is set to passive, then this is a A transmission control block (TCB) is created and partially filled
call to LISTEN for an incoming connection. A passive open may in with data from the OPEN command parameters.
have either a fully specified remote socket to wait for a
particular connection or an unspecified remote socket to wait
for any call. A fully specified passive call can be made
active by the subsequent execution of a SEND.
A transmission control block (TCB) is created and partially Every passive OPEN call either creates a new connection record in
filled in with data from the OPEN command parameters. LISTEN state, or it returns an error; it MUST NOT affect any
previously created connection record (MUST-41).
Every passive OPEN call either creates a new connection record A TCP implementation that supports multiple concurrent connections
in LISTEN state, or it returns an error; it MUST NOT affect any MUST provide an OPEN call that will functionally allow an
previously created connection record (MUST-41). application to LISTEN on a port while a connection block with the
same local port is in SYN-SENT or SYN-RECEIVED state (MUST-42).
A TCP implementation that supports multiple concurrent On an active OPEN command, the TCP endpoint will begin the
connections MUST provide an OPEN call that will functionally procedure to synchronize (i.e., establish) the connection at once.
allow an application to LISTEN on a port while a connection
block with the same local port is in SYN-SENT or SYN-RECEIVED
state (MUST-42).
On an active OPEN command, the TCP endpoint will begin the The timeout, if present, permits the caller to set up a timeout
procedure to synchronize (i.e., establish) the connection at for all data submitted to TCP. If data is not successfully
once. delivered to the destination within the timeout period, the TCP
endpoint will abort the connection. The present global default is
five minutes.
The timeout, if present, permits the caller to set up a timeout The TCP implementation or some component of the operating system
for all data submitted to TCP. If data is not successfully will verify the user's authority to open a connection with the
delivered to the destination within the timeout period, the TCP specified DiffServ field value or security/compartment. The
endpoint will abort the connection. The present global default absence of a DiffServ field value or security/compartment
is five minutes. specification in the OPEN call indicates the default values must
be used.
The TCP implementation or some component of the operating TCP will accept incoming requests as matching only if the
system will verify the users authority to open a connection security/compartment information is exactly the same as that
with the specified DiffServ field value or security/ requested in the OPEN call.
compartment. The absence of a DiffServ field value or
security/compartment specification in the OPEN call indicates
the default values must be used.
TCP will accept incoming requests as matching only if the The DiffServ field value indicated by the user only impacts
security/compartment information is exactly the same as that outgoing packets, may be altered en route through the network, and
requested in the OPEN call. has no direct bearing or relation to received packets.
The DiffServ field value indicated by the user only impacts A local connection name will be returned to the user by the TCP
outgoing packets, may be altered en route through the network, implementation. The local connection name can then be used as a
and has no direct bearing or relation to received packets. short-hand term for the connection defined by the <local socket,
remote socket> pair.
A local connection name will be returned to the user by the TCP The optional "local IP address" parameter MUST be supported to
implementation. The local connection name can then be used as allow the specification of the local IP address (MUST-43). This
a short hand term for the connection defined by the <local enables applications that need to select the local IP address used
socket, remote socket> pair. when multihoming is present.
The optional "local IP address" parameter MUST be supported to A passive OPEN call with a specified "local IP address" parameter
allow the specification of the local IP address (MUST-43). will await an incoming connection request to that address. If the
This enables applications that need to select the local IP parameter is unspecified, a passive OPEN will await an incoming
address used when multihoming is present. connection request to any local IP address, and then bind the
local IP address of the connection to the particular address that
is used.
A passive OPEN call with a specified "local IP address" For an active OPEN call, a specified "local IP address" parameter
parameter will await an incoming connection request to that will be used for opening the connection. If the parameter is
address. If the parameter is unspecified, a passive OPEN will unspecified, the host will choose an appropriate local IP address
await an incoming connection request to any local IP address, (see RFC 1122 section 3.3.4.2).
and then bind the local IP address of the connection to the
particular address that is used.
For an active OPEN call, a specified "local IP address" If an application on a multihomed host does not specify the local
parameter will be used for opening the connection. If the IP address when actively opening a TCP connection, then the TCP
parameter is unspecified, the host will choose an appropriate implementation MUST ask the IP layer to select a local IP address
local IP address (see RFC 1122 section 3.3.4.2). before sending the (first) SYN (MUST-44). See the function
GET_SRCADDR() in Section 3.4 of RFC 1122.
If an application on a multihomed host does not specify the At all other times, a previous segment has either been sent or
local IP address when actively opening a TCP connection, then received on this connection, and TCP implementations MUST use the
the TCP implementation MUST ask the IP layer to select a local same local address that was used in those previous segments (MUST-
IP address before sending the (first) SYN (MUST-44). See the 45).
function GET_SRCADDR() in Section 3.4 of RFC 1122.
At all other times, a previous segment has either been sent or A TCP implementation MUST reject as an error a local OPEN call for
received on this connection, and TCP implementations MUST use an invalid remote IP address (e.g., a broadcast or multicast
the same local address is used that was used in those previous address) (MUST-46).
segments (MUST-45).
A TCP implementation MUST reject as an error a local OPEN call 3.9.1.2. Send
for an invalid remote IP address (e.g., a broadcast or
multicast address) (MUST-46).
Send Format: SEND (local connection name, buffer address, byte count,
PUSH flag (optional), URGENT flag [,timeout])
This call causes the data contained in the indicated user buffer
to be sent on the indicated connection. If the connection has not
been opened, the SEND is considered an error. Some
implementations may allow users to SEND first; in which case, an
automatic OPEN would be done. For example, this might be one way
for application data to be included in SYN segments. If the
calling process is not authorized to use this connection, an error
is returned.
Format: SEND (local connection name, buffer address, byte A TCP endpoint MAY implement PUSH flags on SEND calls (MAY-15).
count, PUSH flag (optional), URGENT flag [,timeout]) If PUSH flags are not implemented, then the sending TCP peer: (1)
MUST NOT buffer data indefinitely (MUST-60), and (2) MUST set the
PSH bit in the last buffered segment (i.e., when there is no more
queued data to be sent) (MUST-61). The remaining description
below assumes the PUSH flag is supported on SEND calls.
This call causes the data contained in the indicated user If the PUSH flag is set, the application intends the data to be
buffer to be sent on the indicated connection. If the transmitted promptly to the receiver, and the PUSH bit will be set
connection has not been opened, the SEND is considered an in the last TCP segment created from the buffer.
error. Some implementations may allow users to SEND first; in
which case, an automatic OPEN would be done. For example, this
might be one way for application data to be included in SYN
segments. If the calling process is not authorized to use this
connection, an error is returned.
A TCP endpoint MAY implement PUSH flags on SEND calls (MAY-15). The PSH bit is not a record marker and is independent of segment
If PUSH flags are not implemented, then the sending TCP peer: boundaries. The transmitter SHOULD collapse successive bits when
(1) MUST NOT buffer data indefinitely (MUST-60), and (2) MUST it packetizes data, to send the largest possible segment (SHLD-
set the PSH bit in the last buffered segment (i.e., when there 27).
is no more queued data to be sent) (MUST-61). The remaining
description below assumes the PUSH flag is supported on SEND
calls.
If the PUSH flag is set, the application intends the data to be If the PUSH flag is not set, the data may be combined with data
transmitted promptly to the receiver, and the PUSH bit will be from subsequent SENDs for transmission efficiency. When an
set in the last TCP segment created from the buffer. When an application issues a series of SEND calls without setting the PUSH
application issues a series of SEND calls without setting the flag, the TCP implementation MAY aggregate the data internally
PUSH flag, the TCP implementation MAY aggregate the data without sending it (MAY-16). Note that when the Nagle algorithm
internally without sending it (MAY-16). is in use, TCP implementations may buffer the data before sending,
without regard to the PUSH flag (see Section 3.7.4).
The PSH bit is not a record marker and is independent of An application program is logically required to set the PUSH flag
segment boundaries. The transmitter SHOULD collapse successive in a SEND call whenever it needs to force delivery of the data to
bits when it packetizes data, to send the largest possible avoid a communication deadlock. However, a TCP implementation
segment (SHLD-27). SHOULD send a maximum-sized segment whenever possible (SHLD-28),
to improve performance (see Section 3.8.6.2.1).
If the PUSH flag is not set, the data may be combined with data New applications SHOULD NOT set the URGENT flag [40] due to
from subsequent SENDs for transmission efficiency. Note that implementation differences and middlebox issues (SHLD-13).
when the Nagle algorithm is in use, TCP implementations may
buffer the data before sending, without regard to the PUSH flag
(see Section 3.7.4).
An application program is logically required to set the PUSH If the URGENT flag is set, segments sent to the destination TCP
flag in a SEND call whenever it needs to force delivery of the peer will have the urgent pointer set. The receiving TCP peer
data to avoid a communication deadlock. However, a TCP will signal the urgent condition to the receiving process if the
implementation SHOULD send a maximum-sized segment whenever urgent pointer indicates that data preceding the urgent pointer
possible (SHLD-28), to improve performance (see has not been consumed by the receiving process. The purpose of
Section 3.8.6.2.1). urgent is to stimulate the receiver to process the urgent data and
to indicate to the receiver when all the currently known urgent
data has been received. The number of times the sending user's
TCP implementation signals urgent will not necessarily be equal to
the number of times the receiving user will be notified of the
presence of urgent data.
New applications SHOULD NOT set the URGENT flag [38] due to If no remote socket was specified in the OPEN, but the connection
implementation differences and middlebox issues (SHLD-13). is established (e.g., because a LISTENing connection has become
specific due to a remote segment arriving for the local socket),
then the designated buffer is sent to the implied remote socket.
Users who make use of OPEN with an unspecified remote socket can
make use of SEND without ever explicitly knowing the remote socket
address.
If the URGENT flag is set, segments sent to the destination TCP However, if a SEND is attempted before the remote socket becomes
peer will have the urgent pointer set. The receiving TCP peer specified, an error will be returned. Users can use the STATUS
will signal the urgent condition to the receiving process if call to determine the status of the connection. Some TCP
the urgent pointer indicates that data preceding the urgent implementations may notify the user when an unspecified socket is
pointer has not been consumed by the receiving process. The bound.
purpose of urgent is to stimulate the receiver to process the
urgent data and to indicate to the receiver when all the
currently known urgent data has been received. The number of
times the sending user's TCP implementation signals urgent will
not necessarily be equal to the number of times the receiving
user will be notified of the presence of urgent data.
If no remote socket was specified in the OPEN, but the If a timeout is specified, the current user timeout for this
connection is established (e.g., because a LISTENing connection connection is changed to the new one.
has become specific due to a remote segment arriving for the
local socket), then the designated buffer is sent to the
implied remote socket. Users who make use of OPEN with an
unspecified remote socket can make use of SEND without ever
explicitly knowing the remote socket address.
However, if a SEND is attempted before the remote socket In the simplest implementation, SEND would not return control to
becomes specified, an error will be returned. Users can use the sending process until either the transmission was complete or
the STATUS call to determine the status of the connection. the timeout had been exceeded. However, this simple method is
Some TCP implementations may notify the user when an both subject to deadlocks (for example, both sides of the
unspecified socket is bound. connection might try to do SENDs before doing any RECEIVEs) and
offers poor performance, so it is not recommended. A more
sophisticated implementation would return immediately to allow the
process to run concurrently with network I/O, and, furthermore, to
allow multiple SENDs to be in progress. Multiple SENDs are served
in first come, first served order, so the TCP endpoint will queue
those it cannot service immediately.
If a timeout is specified, the current user timeout for this We have implicitly assumed an asynchronous user interface in which
connection is changed to the new one. a SEND later elicits some kind of SIGNAL or pseudo-interrupt from
the serving TCP endpoint. An alternative is to return a response
immediately. For instance, SENDs might return immediate local
acknowledgment, even if the segment sent had not been acknowledged
by the distant TCP endpoint. We could optimistically assume
eventual success. If we are wrong, the connection will close
anyway due to the timeout. In implementations of this kind
(synchronous), there will still be some asynchronous signals, but
these will deal with the connection itself, and not with specific
segments or buffers.
In the simplest implementation, SEND would not return control In order for the process to distinguish among error or success
to the sending process until either the transmission was indications for different SENDs, it might be appropriate for the
complete or the timeout had been exceeded. However, this buffer address to be returned along with the coded response to the
simple method is both subject to deadlocks (for example, both SEND request. TCP-to-user signals are discussed below, indicating
sides of the connection might try to do SENDs before doing any the information that should be returned to the calling process.
RECEIVEs) and offers poor performance, so it is not
recommended. A more sophisticated implementation would return
immediately to allow the process to run concurrently with
network I/O, and, furthermore, to allow multiple SENDs to be in
progress. Multiple SENDs are served in first come, first
served order, so the TCP endpoint will queue those it cannot
service immediately.
We have implicitly assumed an asynchronous user interface in 3.9.1.3. Receive
which a SEND later elicits some kind of SIGNAL or pseudo-
interrupt from the serving TCP endpoint. An alternative is to
return a response immediately. For instance, SENDs might
return immediate local acknowledgment, even if the segment sent
had not been acknowledged by the distant TCP endpoint. We
could optimistically assume eventual success. If we are wrong,
the connection will close anyway due to the timeout. In
implementations of this kind (synchronous), there will still be
some asynchronous signals, but these will deal with the
connection itself, and not with specific segments or buffers.
In order for the process to distinguish among error or success Format: RECEIVE (local connection name, buffer address, byte
indications for different SENDs, it might be appropriate for count) -> byte count, urgent flag, push flag (optional)
the buffer address to be returned along with the coded response
to the SEND request. TCP-to-user signals are discussed below,
indicating the information that should be returned to the
calling process.
Receive This command allocates a receiving buffer associated with the
specified connection. If no OPEN precedes this command or the
calling process is not authorized to use this connection, an error
is returned.
Format: RECEIVE (local connection name, buffer address, byte In the simplest implementation, control would not return to the
count) -> byte count, urgent flag, push flag (optional) calling program until either the buffer was filled, or some error
occurred, but this scheme is highly subject to deadlocks. A more
sophisticated implementation would permit several RECEIVEs to be
outstanding at once. These would be filled as segments arrive.
This strategy permits increased throughput at the cost of a more
elaborate scheme (possibly asynchronous) to notify the calling
program that a PUSH has been seen or a buffer filled.
This command allocates a receiving buffer associated with the A TCP receiver MAY pass a received PSH flag to the application
specified connection. If no OPEN precedes this command or the layer via the PUSH flag in the interface (MAY-17), but it is not
calling process is not authorized to use this connection, an required (this was clarified in RFC 1122 section 4.2.2.2). The
error is returned. remainder of text describing the RECEIVE call below assumes that
passing the PUSH indication is supported.
In the simplest implementation, control would not return to the If enough data arrive to fill the buffer before a PUSH is seen,
calling program until either the buffer was filled, or some the PUSH flag will not be set in the response to the RECEIVE. The
error occurred, but this scheme is highly subject to deadlocks. buffer will be filled with as much data as it can hold. If a PUSH
A more sophisticated implementation would permit several is seen before the buffer is filled the buffer will be returned
RECEIVEs to be outstanding at once. These would be filled as partially filled and PUSH indicated.
segments arrive. This strategy permits increased throughput at
the cost of a more elaborate scheme (possibly asynchronous) to
notify the calling program that a PUSH has been seen or a
buffer filled.
A TCP receiver MAY pass a received PSH flag to the application If there is urgent data the user will have been informed as soon
layer via the PUSH flag in the interface (MAY-17), but it is as it arrived via a TCP-to-user signal. The receiving user should
not required (this was clarified in RFC 1122 section 4.2.2.2). thus be in "urgent mode". If the URGENT flag is on, additional
The remainder of text describing the RECEIVE call below assumes urgent data remains. If the URGENT flag is off, this call to
that passing the PUSH indication is supported. RECEIVE has returned all the urgent data, and the user may now
leave "urgent mode". Note that data following the urgent pointer
(non-urgent data) cannot be delivered to the user in the same
buffer with preceding urgent data unless the boundary is clearly
marked for the user.
If enough data arrive to fill the buffer before a PUSH is seen, To distinguish among several outstanding RECEIVEs and to take care
the PUSH flag will not be set in the response to the RECEIVE. of the case that a buffer is not completely filled, the return
The buffer will be filled with as much data as it can hold. If code is accompanied by both a buffer pointer and a byte count
a PUSH is seen before the buffer is filled the buffer will be indicating the actual length of the data received.
returned partially filled and PUSH indicated.
If there is urgent data the user will have been informed as Alternative implementations of RECEIVE might have the TCP endpoint
soon as it arrived via a TCP-to-user signal. The receiving allocate buffer storage, or the TCP endpoint might share a ring
user should thus be in "urgent mode". If the URGENT flag is buffer with the user.
on, additional urgent data remains. If the URGENT flag is off,
this call to RECEIVE has returned all the urgent data, and the
user may now leave "urgent mode". Note that data following the
urgent pointer (non-urgent data) cannot be delivered to the
user in the same buffer with preceding urgent data unless the
boundary is clearly marked for the user.
To distinguish among several outstanding RECEIVEs and to take 3.9.1.4. Close
care of the case that a buffer is not completely filled, the
return code is accompanied by both a buffer pointer and a byte
count indicating the actual length of the data received.
Alternative implementations of RECEIVE might have the TCP Format: CLOSE (local connection name)
endpoint allocate buffer storage, or the TCP endpoint might
share a ring buffer with the user.
Close This command causes the connection specified to be closed. If the
connection is not open or the calling process is not authorized to
use this connection, an error is returned. Closing connections is
intended to be a graceful operation in the sense that outstanding
SENDs will be transmitted (and retransmitted), as flow control
permits, until all have been serviced. Thus, it should be
acceptable to make several SEND calls, followed by a CLOSE, and
expect all the data to be sent to the destination. It should also
be clear that users should continue to RECEIVE on CLOSING
connections, since the remote peer may be trying to transmit the
last of its data. Thus, CLOSE means "I have no more to send" but
does not mean "I will not receive any more." It may happen (if
the user level protocol is not well-thought-out) that the closing
side is unable to get rid of all its data before timing out. In
this event, CLOSE turns into ABORT, and the closing TCP peer gives
up.
Format: CLOSE (local connection name) The user may CLOSE the connection at any time on their own
initiative, or in response to various prompts from the TCP
implementation (e.g., remote close executed, transmission timeout
exceeded, destination inaccessible).
This command causes the connection specified to be closed. If Because closing a connection requires communication with the
the connection is not open or the calling process is not remote TCP peer, connections may remain in the closing state for a
authorized to use this connection, an error is returned. short time. Attempts to reopen the connection before the TCP peer
Closing connections is intended to be a graceful operation in replies to the CLOSE command will result in error responses.
the sense that outstanding SENDs will be transmitted (and
retransmitted), as flow control permits, until all have been
serviced. Thus, it should be acceptable to make several SEND
calls, followed by a CLOSE, and expect all the data to be sent
to the destination. It should also be clear that users should
continue to RECEIVE on CLOSING connections, since the remote
peer may be trying to transmit the last of its data. Thus,
CLOSE means "I have no more to send" but does not mean "I will
not receive any more." It may happen (if the user level
protocol is not well thought out) that the closing side is
unable to get rid of all its data before timing out. In this
event, CLOSE turns into ABORT, and the closing TCP peer gives
up.
The user may CLOSE the connection at any time on their own Close also implies push function.
initiative, or in response to various prompts from the TCP
implementation (e.g., remote close executed, transmission
timeout exceeded, destination inaccessible).
Because closing a connection requires communication with the 3.9.1.5. Status
remote TCP peer, connections may remain in the closing state
for a short time. Attempts to reopen the connection before the
TCP peer replies to the CLOSE command will result in error
responses.
Close also implies push function. Format: STATUS (local connection name) -> status data
This is an implementation dependent user command and could be
excluded without adverse effect. Information returned would
typically come from the TCB associated with the connection.
Status This command returns a data block containing the following
information:
Format: STATUS (local connection name) -> status data - local socket,
This is an implementation dependent user command and could be remote socket,
excluded without adverse effect. Information returned would
typically come from the TCB associated with the connection.
This command returns a data block containing the following local connection name,
information:
local socket, receive window,
remote socket,
local connection name,
receive window,
send window,
connection state,
number of buffers awaiting acknowledgment,
number of buffers pending receipt,
urgent state,
DiffServ field value,
security/compartment,
and transmission timeout.
Depending on the state of the connection, or on the send window,
implementation itself, some of this information may not be
available or meaningful. If the calling process is not
authorized to use this connection, an error is returned. This
prevents unauthorized processes from gaining information about
a connection.
Abort connection state,
Format: ABORT (local connection name) number of buffers awaiting acknowledgment,
This command causes all pending SENDs and RECEIVES to be
aborted, the TCB to be removed, and a special RESET message to
be sent to the remote TCP peer of the connection. Depending on
the implementation, users may receive abort indications for
each outstanding SEND or RECEIVE, or may simply receive an
ABORT-acknowledgment.
Flush number of buffers pending receipt,
Some TCP implementations have included a FLUSH call, which will urgent state,
empty the TCP send queue of any data that the user has issued
SEND calls but is still to the right of the current send
window. That is, it flushes as much queued send data as
possible without losing sequence number synchronization. The
FLUSH call MAY be implemented (MAY-14).
Asynchronous Reports DiffServ field value,
There MUST be a mechanism for reporting soft TCP error security/compartment,
conditions to the application (MUST-47). Generically, we
assume this takes the form of an application-supplied
ERROR_REPORT routine that may be upcalled asynchronously from
the transport layer:
ERROR_REPORT(local connection name, reason, subreason) and transmission timeout.
The precise encoding of the reason and subreason parameters is Depending on the state of the connection, or on the implementation
not specified here. However, the conditions that are reported itself, some of this information may not be available or
asynchronously to the application MUST include: meaningful. If the calling process is not authorized to use this
connection, an error is returned. This prevents unauthorized
processes from gaining information about a connection.
* ICMP error message arrived (see Section 3.9.2.2 for 3.9.1.6. Abort
description of handling each ICMP message type, since some
message types need to be suppressed from generating reports
to the application)
* Excessive retransmissions (see Section 3.8.3) Format: ABORT (local connection name)
* Urgent pointer advance (see Section 3.8.5) This command causes all pending SENDs and RECEIVES to be aborted,
the TCB to be removed, and a special RESET message to be sent to
the remote TCP peer of the connection. Depending on the
implementation, users may receive abort indications for each
outstanding SEND or RECEIVE, or may simply receive an ABORT-
acknowledgment.
However, an application program that does not want to receive 3.9.1.7. Flush
such ERROR_REPORT calls SHOULD be able to effectively disable
these calls (SHLD-20).
Set Differentiated Services Field (IPv4 TOS or IPv6 Traffic Class) Some TCP implementations have included a FLUSH call, which will
empty the TCP send queue of any data that the user has issued SEND
calls for but is still to the right of the current send window.
That is, it flushes as much queued send data as possible without
losing sequence number synchronization. The FLUSH call MAY be
implemented (MAY-14).
The application layer MUST be able to specify the 3.9.1.8. Asynchronous Reports
Differentiated Services field for segments that are sent on a
connection (MUST-48). The Differentiated Services field
includes the 6-bit Differentiated Services Code Point (DSCP)
value. It is not required, but the application SHOULD be able
to change the Differentiated Services field during the
connection lifetime (SHLD-21). TCP implementations SHOULD pass
the current Differentiated Services field value without change
to the IP layer, when it sends segments on the connection
(SHLD-22).
The Differentiated Services field will be specified There MUST be a mechanism for reporting soft TCP error conditions
independently in each direction on the connection, so that the to the application (MUST-47). Generically, we assume this takes
receiver application will specify the Differentiated Services the form of an application-supplied ERROR_REPORT routine that may
field used for ACK segments. be upcalled asynchronously from the transport layer:
TCP implementations MAY pass the most recently received - ERROR_REPORT(local connection name, reason, subreason)
Differentiated Services field up to the application (MAY-9).
The precise encoding of the reason and subreason parameters is not
specified here. However, the conditions that are reported
asynchronously to the application MUST include:
- * ICMP error message arrived (see Section 3.9.2.2 for
description of handling each ICMP message type, since some
message types need to be suppressed from generating reports to
the application)
- * Excessive retransmissions (see Section 3.8.3)
- * Urgent pointer advance (see Section 3.8.5)
However, an application program that does not want to receive such
ERROR_REPORT calls SHOULD be able to effectively disable these
calls (SHLD-20).
3.9.1.9. Set Differentiated Services Field (IPv4 TOS or IPv6 Traffic
Class)
The application layer MUST be able to specify the Differentiated
Services field for segments that are sent on a connection (MUST-
48). The Differentiated Services field includes the 6-bit
Differentiated Services Code Point (DSCP) value. It is not
required, but the application SHOULD be able to change the
Differentiated Services field during the connection lifetime
(SHLD-21). TCP implementations SHOULD pass the current
Differentiated Services field value without change to the IP
layer, when it sends segments on the connection (SHLD-22).
The Differentiated Services field will be specified independently
in each direction on the connection, so that the receiver
application will specify the Differentiated Services field used
for ACK segments.
TCP implementations MAY pass the most recently received
Differentiated Services field up to the application (MAY-9).
3.9.2. TCP/Lower-Level Interface 3.9.2. TCP/Lower-Level Interface
The TCP endpoint calls on a lower level protocol module to actually The TCP endpoint calls on a lower level protocol module to actually
send and receive information over a network. The two current send and receive information over a network. The two current
standard Internet Protocol (IP) versions layered below TCP are IPv4 standard Internet Protocol (IP) versions layered below TCP are IPv4
[1] and IPv6 [13]. [1] and IPv6 [13].
If the lower level protocol is IPv4 it provides arguments for a type If the lower level protocol is IPv4 it provides arguments for a type
of service (used within the Differentiated Services field) and for a of service (used within the Differentiated Services field) and for a
time to live. TCP uses the following settings for these parameters: time to live. TCP uses the following settings for these parameters:
DiffServ field: The IP header value for the DiffServ field is DiffServ field: The IP header value for the DiffServ field is
given by the user. This includes the bits of the DiffServ Code given by the user. This includes the bits of the DiffServ Code
Point (DSCP). Point (DSCP).
Time to Live (TTL): The TTL value used to send TCP segments MUST Time to Live (TTL): The TTL value used to send TCP segments MUST
be configurable (MUST-49). be configurable (MUST-49).
Note that RFC 793 specified one minute (60 seconds) as a - Note that RFC 793 specified one minute (60 seconds) as a
constant for the TTL, because the assumed maximum segment constant for the TTL, because the assumed maximum segment
lifetime was two minutes. This was intended to explicitly ask lifetime was two minutes. This was intended to explicitly ask
that a segment be destroyed if it cannot be delivered by the that a segment be destroyed if it cannot be delivered by the
internet system within one minute. RFC 1122 changed this internet system within one minute. RFC 1122 changed this
specification to require that the TTL be configurable. specification to require that the TTL be configurable.
Note that the DiffServ field is permitted to change during a - Note that the DiffServ field is permitted to change during a
connection (Section 4.2.4.2 of RFC 1122). However, the connection (Section 4.2.4.2 of RFC 1122). However, the
application interface might not support this ability, and the application interface might not support this ability, and the
application does not have knowledge about individual TCP application does not have knowledge about individual TCP
segments, so this can only be done on a coarse granularity, at segments, so this can only be done on a coarse granularity, at
best. This limitation is further discussed in RFC 7657 (sec best. This limitation is further discussed in RFC 7657 (sec
5.1, 5.3, and 6) [49]. Generally, an application SHOULD NOT 5.1, 5.3, and 6) [51]. Generally, an application SHOULD NOT
change the DiffServ field value during the course of a change the DiffServ field value during the course of a
connection (SHLD-23). connection (SHLD-23).
Any lower level protocol will have to provide the source address, Any lower level protocol will have to provide the source address,
destination address, and protocol fields, and some way to determine destination address, and protocol fields, and some way to determine
the "TCP length", both to provide the functional equivalent service the "TCP length", both to provide the functional equivalent service
of IP and to be used in the TCP checksum. of IP and to be used in the TCP checksum.
When received options are passed up to TCP from the IP layer, TCP When received options are passed up to TCP from the IP layer, a TCP
implementations MUST ignore options that it does not understand implementation MUST ignore options that it does not understand (MUST-
(MUST-50). 50).
A TCP implementation MAY support the Time Stamp (MAY-10) and Record A TCP implementation MAY support the Time Stamp (MAY-10) and Record
Route (MAY-11) options. Route (MAY-11) options.
3.9.2.1. Source Routing 3.9.2.1. Source Routing
If the lower level is IP (or other protocol that provides this If the lower level is IP (or other protocol that provides this
feature) and source routing is used, the interface must allow the feature) and source routing is used, the interface must allow the
route information to be communicated. This is especially important route information to be communicated. This is especially important
so that the source and destination addresses used in the TCP checksum so that the source and destination addresses used in the TCP checksum
skipping to change at page 59, line 48 skipping to change at page 60, line 41
3.9.2.2. ICMP Messages 3.9.2.2. ICMP Messages
TCP implementations MUST act on an ICMP error message passed up from TCP implementations MUST act on an ICMP error message passed up from
the IP layer, directing it to the connection that created the error the IP layer, directing it to the connection that created the error
(MUST-54). The necessary demultiplexing information can be found in (MUST-54). The necessary demultiplexing information can be found in
the IP header contained within the ICMP message. the IP header contained within the ICMP message.
This applies to ICMPv6 in addition to IPv4 ICMP. This applies to ICMPv6 in addition to IPv4 ICMP.
[33] contains discussion of specific ICMP and ICMPv6 messages [36] contains discussion of specific ICMP and ICMPv6 messages
classified as either "soft" or "hard" errors that may bear different classified as either "soft" or "hard" errors that may bear different
responses. Treatment for classes of ICMP messages is described responses. Treatment for classes of ICMP messages is described
below: below:
Source Quench Source Quench
TCP implementations MUST silently discard any received ICMP Source TCP implementations MUST silently discard any received ICMP Source
Quench messages (MUST-55). See [11] for discussion. Quench messages (MUST-55). See [11] for discussion.
Soft Errors Soft Errors
For ICMP these include: Destination Unreachable -- codes 0, 1, 5, For IPv4 ICMP these include: Destination Unreachable -- codes 0, 1,
Time Exceeded -- codes 0, 1, and Parameter Problem. 5; Time Exceeded -- codes 0, 1; and Parameter Problem.
For ICMPv6 these include: Destination Unreachable -- codes 0 and 3,
Time Exceeded -- codes 0, 1, and Parameter Problem -- codes 0, 1, For ICMPv6 these include: Destination Unreachable -- codes 0, 3;
Time Exceeded -- codes 0, 1; and Parameter Problem -- codes 0, 1,
2. 2.
Since these Unreachable messages indicate soft error conditions, Since these Unreachable messages indicate soft error conditions,
TCP implementations MUST NOT abort the connection (MUST-56), and it TCP implementations MUST NOT abort the connection (MUST-56), and it
SHOULD make the information available to the application (SHLD-25). SHOULD make the information available to the application (SHLD-25).
Hard Errors Hard Errors
For ICMP these include Destination Unreachable -- codes 2-4. For ICMP these include Destination Unreachable -- codes 2-4.
These are hard error conditions, so TCP implementations SHOULD These are hard error conditions, so TCP implementations SHOULD
abort the connection (SHLD-26). [33] notes that some abort the connection (SHLD-26). [36] notes that some
implementations do not abort connections when an ICMP hard error is implementations do not abort connections when an ICMP hard error is
received for a connection that is in any of the synchronized received for a connection that is in any of the synchronized
states. states.
Note that [33] section 4 describes widespread implementation behavior Note that [36] section 4 describes widespread implementation behavior
that treats soft errors as hard errors during connection that treats soft errors as hard errors during connection
establishment. establishment.
3.9.2.3. Source Address Validation 3.9.2.3. Source Address Validation
RFC 1122 requires addresses to be validated in incoming SYN packets: RFC 1122 requires addresses to be validated in incoming SYN packets:
An incoming SYN with an invalid source address MUST be ignored An incoming SYN with an invalid source address MUST be ignored
either by TCP or by the IP layer (MUST-63) (Section 3.2.1.3 of either by TCP or by the IP layer (MUST-63) (Section 3.2.1.3 of
[18]). [20]).
A TCP implementation MUST silently discard an incoming SYN segment A TCP implementation MUST silently discard an incoming SYN segment
that is addressed to a broadcast or multicast address (MUST-57). that is addressed to a broadcast or multicast address (MUST-57).
This prevents connection state and replies from being erroneously This prevents connection state and replies from being erroneously
generated, and implementers should note that this guidance is generated, and implementers should note that this guidance is
applicable to all incoming segments, not just SYNs, as specifically applicable to all incoming segments, not just SYNs, as specifically
indicated in RFC 1122. indicated in RFC 1122.
3.10. Event Processing 3.10. Event Processing
skipping to change at page 61, line 16 skipping to change at page 62, line 16
to events. The events that occur can be cast into three categories: to events. The events that occur can be cast into three categories:
user calls, arriving segments, and timeouts. This section describes user calls, arriving segments, and timeouts. This section describes
the processing the TCP endpoint does in response to each of the the processing the TCP endpoint does in response to each of the
events. In many cases the processing required depends on the state events. In many cases the processing required depends on the state
of the connection. of the connection.
Events that occur: Events that occur:
User Calls User Calls
OPEN - OPEN
SEND SEND
RECEIVE RECEIVE
CLOSE CLOSE
ABORT ABORT
STATUS STATUS
Arriving Segments Arriving Segments
SEGMENT ARRIVES - SEGMENT ARRIVES
Timeouts Timeouts
USER TIMEOUT - USER TIMEOUT
RETRANSMISSION TIMEOUT RETRANSMISSION TIMEOUT
TIME-WAIT TIMEOUT TIME-WAIT TIMEOUT
The model of the TCP/user interface is that user commands receive an The model of the TCP/user interface is that user commands receive an
immediate return and possibly a delayed response via an event or immediate return and possibly a delayed response via an event or
pseudo interrupt. In the following descriptions, the term "signal" pseudo interrupt. In the following descriptions, the term "signal"
means cause a delayed response. means cause a delayed response.
Error responses in this document are identified by character strings. Error responses in this document are identified by character strings.
For example, user commands referencing connections that do not exist For example, user commands referencing connections that do not exist
receive "error: connection not open". receive "error: connection not open".
Please note in the following that all arithmetic on sequence numbers, Please note in the following that all arithmetic on sequence numbers,
acknowledgment numbers, windows, et cetera, is modulo 2**32 the size acknowledgment numbers, windows, et cetera, is modulo 2**32 (the size
of the sequence number space. Also note that "=<" means less than or of the sequence number space). Also note that "=<" means less than
equal to (modulo 2**32). or equal to (modulo 2**32).
A natural way to think about processing incoming segments is to A natural way to think about processing incoming segments is to
imagine that they are first tested for proper sequence number (i.e., imagine that they are first tested for proper sequence number (i.e.,
that their contents lie in the range of the expected "receive window" that their contents lie in the range of the expected "receive window"
in the sequence number space) and then that they are generally queued in the sequence number space) and then that they are generally queued
and processed in sequence number order. and processed in sequence number order.
When a segment overlaps other already received segments we When a segment overlaps other already received segments we
reconstruct the segment to contain just the new data, and adjust the reconstruct the segment to contain just the new data, and adjust the
header fields to be consistent. header fields to be consistent.
Note that if no state change is mentioned the TCP connection stays in Note that if no state change is mentioned the TCP connection stays in
the same state. the same state.
3.10.1. OPEN Call 3.10.1. OPEN Call
CLOSED STATE (i.e., TCB does not exist) CLOSED STATE (i.e., TCB does not exist)
Create a new transmission control block (TCB) to hold - Create a new transmission control block (TCB) to hold
connection state information. Fill in local socket identifier, connection state information. Fill in local socket identifier,
remote socket, DiffServ field, security/compartment, and user remote socket, DiffServ field, security/compartment, and user
timeout information. Note that some parts of the remote socket timeout information. Note that some parts of the remote socket
may be unspecified in a passive OPEN and are to be filled in by may be unspecified in a passive OPEN and are to be filled in by
the parameters of the incoming SYN segment. Verify the the parameters of the incoming SYN segment. Verify the
security and DiffServ value requested are allowed for this security and DiffServ value requested are allowed for this
user, if not return "error: precedence not allowed" or "error: user, if not return "error: DiffServ value not allowed" or
security/compartment not allowed." If passive enter the LISTEN "error: security/compartment not allowed." If passive enter
state and return. If active and the remote socket is the LISTEN state and return. If active and the remote socket
unspecified, return "error: remote socket unspecified"; if is unspecified, return "error: remote socket unspecified"; if
active and the remote socket is specified, issue a SYN segment. active and the remote socket is specified, issue a SYN segment.
An initial send sequence number (ISS) is selected. A SYN An initial send sequence number (ISS) is selected. A SYN
segment of the form <SEQ=ISS><CTL=SYN> is sent. Set SND.UNA to segment of the form <SEQ=ISS><CTL=SYN> is sent. Set SND.UNA to
ISS, SND.NXT to ISS+1, enter SYN-SENT state, and return. ISS, SND.NXT to ISS+1, enter SYN-SENT state, and return.
If the caller does not have access to the local socket - If the caller does not have access to the local socket
specified, return "error: connection illegal for this process". specified, return "error: connection illegal for this process".
If there is no room to create a new connection, return "error: If there is no room to create a new connection, return "error:
insufficient resources". insufficient resources".
LISTEN STATE LISTEN STATE
- If the OPEN call is active and the remote socket is specified,
If the OPEN call is active and the remote socket is specified,
then change the connection from passive to active, select an then change the connection from passive to active, select an
ISS. Send a SYN segment, set SND.UNA to ISS, SND.NXT to ISS+1. ISS. Send a SYN segment, set SND.UNA to ISS, SND.NXT to ISS+1.
Enter SYN-SENT state. Data associated with SEND may be sent Enter SYN-SENT state. Data associated with SEND may be sent
with SYN segment or queued for transmission after entering with SYN segment or queued for transmission after entering
ESTABLISHED state. The urgent bit if requested in the command ESTABLISHED state. The urgent bit if requested in the command
must be sent with the data segments sent as a result of this must be sent with the data segments sent as a result of this
command. If there is no room to queue the request, respond command. If there is no room to queue the request, respond
with "error: insufficient resources". If Foreign socket was with "error: insufficient resources". If the remote socket was
not specified, then return "error: remote socket unspecified". not specified, then return "error: remote socket unspecified".
SYN-SENT STATE SYN-SENT STATE
SYN-RECEIVED STATE SYN-RECEIVED STATE
ESTABLISHED STATE ESTABLISHED STATE
FIN-WAIT-1 STATE FIN-WAIT-1 STATE
FIN-WAIT-2 STATE FIN-WAIT-2 STATE
CLOSE-WAIT STATE CLOSE-WAIT STATE
CLOSING STATE CLOSING STATE
LAST-ACK STATE LAST-ACK STATE
TIME-WAIT STATE TIME-WAIT STATE
Return "error: connection already exists". - Return "error: connection already exists".
3.10.2. SEND Call 3.10.2. SEND Call
CLOSED STATE (i.e., TCB does not exist) CLOSED STATE (i.e., TCB does not exist)
If the user does not have access to such a connection, then - If the user does not have access to such a connection, then
return "error: connection illegal for this process". return "error: connection illegal for this process".
Otherwise, return "error: connection does not exist". - Otherwise, return "error: connection does not exist".
LISTEN STATE LISTEN STATE
If the remote socket is specified, then change the connection - If the remote socket is specified, then change the connection
from passive to active, select an ISS. Send a SYN segment, set from passive to active, select an ISS. Send a SYN segment, set
SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT state. Data SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT state. Data
associated with SEND may be sent with SYN segment or queued for associated with SEND may be sent with SYN segment or queued for
transmission after entering ESTABLISHED state. The urgent bit transmission after entering ESTABLISHED state. The urgent bit
if requested in the command must be sent with the data segments if requested in the command must be sent with the data segments
sent as a result of this command. If there is no room to queue sent as a result of this command. If there is no room to queue
the request, respond with "error: insufficient resources". If the request, respond with "error: insufficient resources". If
Foreign socket was not specified, then return "error: remote the remote socket was not specified, then return "error: remote
socket unspecified". socket unspecified".
SYN-SENT STATE SYN-SENT STATE
SYN-RECEIVED STATE SYN-RECEIVED STATE
Queue the data for transmission after entering ESTABLISHED - Queue the data for transmission after entering ESTABLISHED
state. If no space to queue, respond with "error: insufficient state. If no space to queue, respond with "error: insufficient
resources". resources".
ESTABLISHED STATE ESTABLISHED STATE
CLOSE-WAIT STATE CLOSE-WAIT STATE
Segmentize the buffer and send it with a piggybacked - Segmentize the buffer and send it with a piggybacked
acknowledgment (acknowledgment value = RCV.NXT). If there is acknowledgment (acknowledgment value = RCV.NXT). If there is
insufficient space to remember this buffer, simply return insufficient space to remember this buffer, simply return
"error: insufficient resources". "error: insufficient resources".
If the urgent flag is set, then SND.UP <- SND.NXT and set the - If the urgent flag is set, then SND.UP <- SND.NXT and set the
urgent pointer in the outgoing segments. urgent pointer in the outgoing segments.
FIN-WAIT-1 STATE FIN-WAIT-1 STATE
FIN-WAIT-2 STATE FIN-WAIT-2 STATE
CLOSING STATE CLOSING STATE
LAST-ACK STATE LAST-ACK STATE
TIME-WAIT STATE TIME-WAIT STATE
Return "error: connection closing" and do not service request. - Return "error: connection closing" and do not service request.
3.10.3. RECEIVE Call 3.10.3. RECEIVE Call
CLOSED STATE (i.e., TCB does not exist) CLOSED STATE (i.e., TCB does not exist)
If the user does not have access to such a connection, return - If the user does not have access to such a connection, return
"error: connection illegal for this process". "error: connection illegal for this process".
Otherwise return "error: connection does not exist". - Otherwise return "error: connection does not exist".
LISTEN STATE LISTEN STATE
SYN-SENT STATE SYN-SENT STATE
SYN-RECEIVED STATE SYN-RECEIVED STATE
Queue for processing after entering ESTABLISHED state. If - Queue for processing after entering ESTABLISHED state. If
there is no room to queue this request, respond with "error: there is no room to queue this request, respond with "error:
insufficient resources". insufficient resources".
ESTABLISHED STATE ESTABLISHED STATE
FIN-WAIT-1 STATE FIN-WAIT-1 STATE
FIN-WAIT-2 STATE FIN-WAIT-2 STATE
If insufficient incoming segments are queued to satisfy the - If insufficient incoming segments are queued to satisfy the
request, queue the request. If there is no queue space to request, queue the request. If there is no queue space to
remember the RECEIVE, respond with "error: insufficient remember the RECEIVE, respond with "error: insufficient
resources". resources".
Reassemble queued incoming segments into receive buffer and - Reassemble queued incoming segments into receive buffer and
return to user. Mark "push seen" (PUSH) if this is the case. return to user. Mark "push seen" (PUSH) if this is the case.
If RCV.UP is in advance of the data currently being passed to - If RCV.UP is in advance of the data currently being passed to
the user notify the user of the presence of urgent data. the user notify the user of the presence of urgent data.
When the TCP endpoint takes responsibility for delivering data - When the TCP endpoint takes responsibility for delivering data
to the user that fact must be communicated to the sender via an to the user that fact must be communicated to the sender via an
acknowledgment. The formation of such an acknowledgment is acknowledgment. The formation of such an acknowledgment is
described below in the discussion of processing an incoming described below in the discussion of processing an incoming
segment. segment.
CLOSE-WAIT STATE CLOSE-WAIT STATE
Since the remote side has already sent FIN, RECEIVEs must be - Since the remote side has already sent FIN, RECEIVEs must be
satisfied by data already on hand, but not yet delivered to the satisfied by data already on hand, but not yet delivered to the
user. If no text is awaiting delivery, the RECEIVE will get a user. If no text is awaiting delivery, the RECEIVE will get an
"error: connection closing" response. Otherwise, any remaining "error: connection closing" response. Otherwise, any remaining
text can be used to satisfy the RECEIVE. data can be used to satisfy the RECEIVE.
CLOSING STATE CLOSING STATE
LAST-ACK STATE LAST-ACK STATE
TIME-WAIT STATE TIME-WAIT STATE
Return "error: connection closing". - Return "error: connection closing".
3.10.4. CLOSE Call 3.10.4. CLOSE Call
CLOSED STATE (i.e., TCB does not exist) CLOSED STATE (i.e., TCB does not exist)
If the user does not have access to such a connection, return - If the user does not have access to such a connection, return
"error: connection illegal for this process". "error: connection illegal for this process".
Otherwise, return "error: connection does not exist". - Otherwise, return "error: connection does not exist".
LISTEN STATE LISTEN STATE
Any outstanding RECEIVEs are returned with "error: closing" - Any outstanding RECEIVEs are returned with "error: closing"
responses. Delete TCB, enter CLOSED state, and return. responses. Delete TCB, enter CLOSED state, and return.
SYN-SENT STATE SYN-SENT STATE
Delete the TCB and return "error: closing" responses to any - Delete the TCB and return "error: closing" responses to any
queued SENDs, or RECEIVEs. queued SENDs, or RECEIVEs.
SYN-RECEIVED STATE SYN-RECEIVED STATE
If no SENDs have been issued and there is no pending data to - If no SENDs have been issued and there is no pending data to
send, then form a FIN segment and send it, and enter FIN-WAIT-1 send, then form a FIN segment and send it, and enter FIN-WAIT-1
state; otherwise queue for processing after entering state; otherwise queue for processing after entering
ESTABLISHED state. ESTABLISHED state.
ESTABLISHED STATE ESTABLISHED STATE
Queue this until all preceding SENDs have been segmentized, - Queue this until all preceding SENDs have been segmentized,
then form a FIN segment and send it. In any case, enter FIN- then form a FIN segment and send it. In any case, enter FIN-
WAIT-1 state. WAIT-1 state.
FIN-WAIT-1 STATE FIN-WAIT-1 STATE
FIN-WAIT-2 STATE FIN-WAIT-2 STATE
Strictly speaking, this is an error and should receive a
- Strictly speaking, this is an error and should receive an
"error: connection closing" response. An "ok" response would "error: connection closing" response. An "ok" response would
be acceptable, too, as long as a second FIN is not emitted (the be acceptable, too, as long as a second FIN is not emitted (the
first FIN may be retransmitted though). first FIN may be retransmitted though).
CLOSE-WAIT STATE CLOSE-WAIT STATE
Queue this request until all preceding SENDs have been - Queue this request until all preceding SENDs have been
segmentized; then send a FIN segment, enter LAST-ACK state. segmentized; then send a FIN segment, enter LAST-ACK state.
CLOSING STATE CLOSING STATE
LAST-ACK STATE LAST-ACK STATE
TIME-WAIT STATE TIME-WAIT STATE
Respond with "error: connection closing". - Respond with "error: connection closing".
3.10.5. ABORT Call 3.10.5. ABORT Call
CLOSED STATE (i.e., TCB does not exist) CLOSED STATE (i.e., TCB does not exist)
If the user should not have access to such a connection, return - If the user should not have access to such a connection, return
"error: connection illegal for this process". "error: connection illegal for this process".
Otherwise return "error: connection does not exist". - Otherwise return "error: connection does not exist".
LISTEN STATE LISTEN STATE
Any outstanding RECEIVEs should be returned with "error: - Any outstanding RECEIVEs should be returned with "error:
connection reset" responses. Delete TCB, enter CLOSED state, connection reset" responses. Delete TCB, enter CLOSED state,
and return. and return.
SYN-SENT STATE SYN-SENT STATE
All queued SENDs and RECEIVEs should be given "connection - All queued SENDs and RECEIVEs should be given "connection
reset" notification, delete the TCB, enter CLOSED state, and reset" notification, delete the TCB, enter CLOSED state, and
return. return.
SYN-RECEIVED STATE SYN-RECEIVED STATE
ESTABLISHED STATE ESTABLISHED STATE
FIN-WAIT-1 STATE FIN-WAIT-1 STATE
FIN-WAIT-2 STATE FIN-WAIT-2 STATE
CLOSE-WAIT STATE CLOSE-WAIT STATE
Send a reset segment: - Send a reset segment:
<SEQ=SND.NXT><CTL=RST> o <SEQ=SND.NXT><CTL=RST>
All queued SENDs and RECEIVEs should be given "connection - All queued SENDs and RECEIVEs should be given "connection
reset" notification; all segments queued for transmission reset" notification; all segments queued for transmission
(except for the RST formed above) or retransmission should be (except for the RST formed above) or retransmission should be
flushed, delete the TCB, enter CLOSED state, and return. flushed, delete the TCB, enter CLOSED state, and return.
CLOSING STATE LAST-ACK STATE TIME-WAIT STATE CLOSING STATE LAST-ACK STATE TIME-WAIT STATE
- Respond with "ok" and delete the TCB, enter CLOSED state, and
Respond with "ok" and delete the TCB, enter CLOSED state, and
return. return.
3.10.6. STATUS Call 3.10.6. STATUS Call
CLOSED STATE (i.e., TCB does not exist) CLOSED STATE (i.e., TCB does not exist)
If the user should not have access to such a connection, return - If the user should not have access to such a connection, return
"error: connection illegal for this process". "error: connection illegal for this process".
Otherwise return "error: connection does not exist". - Otherwise return "error: connection does not exist".
LISTEN STATE LISTEN STATE
Return "state = LISTEN", and the TCB pointer. - Return "state = LISTEN", and the TCB pointer.
SYN-SENT STATE SYN-SENT STATE
Return "state = SYN-SENT", and the TCB pointer. - Return "state = SYN-SENT", and the TCB pointer.
SYN-RECEIVED STATE SYN-RECEIVED STATE
Return "state = SYN-RECEIVED", and the TCB pointer. - Return "state = SYN-RECEIVED", and the TCB pointer.
ESTABLISHED STATE ESTABLISHED STATE
Return "state = ESTABLISHED", and the TCB pointer. - Return "state = ESTABLISHED", and the TCB pointer.
FIN-WAIT-1 STATE FIN-WAIT-1 STATE
Return "state = FIN-WAIT-1", and the TCB pointer. - Return "state = FIN-WAIT-1", and the TCB pointer.
FIN-WAIT-2 STATE FIN-WAIT-2 STATE
Return "state = FIN-WAIT-2", and the TCB pointer. - Return "state = FIN-WAIT-2", and the TCB pointer.
CLOSE-WAIT STATE CLOSE-WAIT STATE
Return "state = CLOSE-WAIT", and the TCB pointer. - Return "state = CLOSE-WAIT", and the TCB pointer.
CLOSING STATE CLOSING STATE
Return "state = CLOSING", and the TCB pointer.
- Return "state = CLOSING", and the TCB pointer.
LAST-ACK STATE LAST-ACK STATE
Return "state = LAST-ACK", and the TCB pointer. - Return "state = LAST-ACK", and the TCB pointer.
TIME-WAIT STATE TIME-WAIT STATE
Return "state = TIME-WAIT", and the TCB pointer. - Return "state = TIME-WAIT", and the TCB pointer.
3.10.7. SEGMENT ARRIVES 3.10.7. SEGMENT ARRIVES
3.10.7.1. CLOSED State 3.10.7.1. CLOSED State
If the state is CLOSED (i.e., TCB does not exist) then If the state is CLOSED (i.e., TCB does not exist) then
all data in the incoming segment is discarded. An incoming all data in the incoming segment is discarded. An incoming
segment containing a RST is discarded. An incoming segment not segment containing a RST is discarded. An incoming segment not
containing a RST causes a RST to be sent in response. The containing a RST causes a RST to be sent in response. The
acknowledgment and sequence field values are selected to make the acknowledgment and sequence field values are selected to make the
reset sequence acceptable to the TCP endpoint that sent the reset sequence acceptable to the TCP endpoint that sent the
offending segment. offending segment.
If the ACK bit is off, sequence number zero is used, If the ACK bit is off, sequence number zero is used,
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK> - <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If the ACK bit is on, If the ACK bit is on,
<SEQ=SEG.ACK><CTL=RST> - <SEQ=SEG.ACK><CTL=RST>
Return. Return.
3.10.7.2. LISTEN State 3.10.7.2. LISTEN State
If the state is LISTEN then If the state is LISTEN then
first check for an RST first check for an RST
An incoming RST segment could not be valid, since it could not - An incoming RST segment could not be valid, since it could not
have been sent in response to anything sent by this incarnation have been sent in response to anything sent by this incarnation
of the connection. An incoming RST should be ignored. Return. of the connection. An incoming RST should be ignored. Return.
second check for an ACK second check for an ACK
Any acknowledgment is bad if it arrives on a connection still - Any acknowledgment is bad if it arrives on a connection still
in the LISTEN state. An acceptable reset segment should be in the LISTEN state. An acceptable reset segment should be
formed for any arriving ACK-bearing segment. The RST should be formed for any arriving ACK-bearing segment. The RST should be
formatted as follows: formatted as follows:
<SEQ=SEG.ACK><CTL=RST> o <SEQ=SEG.ACK><CTL=RST>
Return. - Return.
third check for a SYN third check for a SYN
If the SYN bit is set, check the security. If the security/ - If the SYN bit is set, check the security. If the security/
compartment on the incoming segment does not exactly match the compartment on the incoming segment does not exactly match the
security/compartment in the TCB then send a reset and return. security/compartment in the TCB then send a reset and return.
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK> o <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ and any other - Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ and any other
control or text should be queued for processing later. ISS control or text should be queued for processing later. ISS
should be selected and a SYN segment sent of the form: should be selected and a SYN segment sent of the form:
<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK> o <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
SND.NXT is set to ISS+1 and SND.UNA to ISS. The connection - SND.NXT is set to ISS+1 and SND.UNA to ISS. The connection
state should be changed to SYN-RECEIVED. Note that any other state should be changed to SYN-RECEIVED. Note that any other
incoming control or data (combined with SYN) will be processed incoming control or data (combined with SYN) will be processed
in the SYN-RECEIVED state, but processing of SYN and ACK should in the SYN-RECEIVED state, but processing of SYN and ACK should
not be repeated. If the listen was not fully specified (i.e., not be repeated. If the listen was not fully specified (i.e.,
the remote socket was not fully specified), then the the remote socket was not fully specified), then the
unspecified fields should be filled in now. unspecified fields should be filled in now.
fourth other data or control fourth other data or control
This should not be reached. Drop the segment and return. Any - This should not be reached. Drop the segment and return. Any
other control or data-bearing segment (not containing SYN) must other control or data-bearing segment (not containing SYN) must
have an ACK and thus would have been discarded by the ACK have an ACK and thus would have been discarded by the ACK
processing in the second step, unless it was first discarded by processing in the second step, unless it was first discarded by
RST checking in the first step. RST checking in the first step.
3.10.7.3. SYN-SENT State 3.10.7.3. SYN-SENT State
If the state is SYN-SENT then If the state is SYN-SENT then
first check the ACK bit first check the ACK bit
If the ACK bit is set - If the ACK bit is set
If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset o If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset
(unless the RST bit is set, if so drop the segment and (unless the RST bit is set, if so drop the segment and
return) return)
<SEQ=SEG.ACK><CTL=RST>
and discard the segment. Return. + <SEQ=SEG.ACK><CTL=RST>
If SND.UNA < SEG.ACK =< SND.NXT then the ACK is acceptable. o and discard the segment. Return.
o If SND.UNA < SEG.ACK =< SND.NXT then the ACK is acceptable.
Some deployed TCP code has used the check SEG.ACK == SND.NXT Some deployed TCP code has used the check SEG.ACK == SND.NXT
(using "==" rather than "=<", but this is not appropriate (using "==" rather than "=<", but this is not appropriate
when the stack is capable of sending data on the SYN, when the stack is capable of sending data on the SYN,
because the TCP peer may not accept and acknowledge all of because the TCP peer may not accept and acknowledge all of
the data on the SYN. the data on the SYN.
second check the RST bit second check the RST bit
If the RST bit is set - If the RST bit is set
A potential blind reset attack is described in RFC 5961 o A potential blind reset attack is described in RFC 5961 [9].
[37]. The mitigation described in that document has The mitigation described in that document has specific
specific applicability explained therein, and is not a applicability explained therein, and is not a substitute for
substitute for cryptographic protection (e.g. IPsec or TCP- cryptographic protection (e.g. IPsec or TCP-AO). A TCP
AO). A TCP implementation that supports the RFC 5961 implementation that supports the RFC 5961 mitigation SHOULD
mitigation SHOULD first check that the sequence number first check that the sequence number exactly matches RCV.NXT
exactly matches RCV.NXT prior to executing the action in the prior to executing the action in the next paragraph.
next paragraph.
If the ACK was acceptable then signal the user "error: o If the ACK was acceptable then signal the user "error:
connection reset", drop the segment, enter CLOSED state, connection reset", drop the segment, enter CLOSED state,
delete TCB, and return. Otherwise (no ACK) drop the segment delete TCB, and return. Otherwise (no ACK), drop the
and return. segment and return.
third check the security third check the security
If the security/compartment in the segment does not exactly - If the security/compartment in the segment does not exactly
match the security/compartment in the TCB, send a reset match the security/compartment in the TCB, send a reset
If there is an ACK o If there is an ACK
<SEQ=SEG.ACK><CTL=RST> + <SEQ=SEG.ACK><CTL=RST>
Otherwise o Otherwise
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK> + <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If a reset was sent, discard the segment and return. - If a reset was sent, discard the segment and return.
fourth check the SYN bit fourth check the SYN bit
This step should be reached only if the ACK is ok, or there is - This step should be reached only if the ACK is ok, or there is
no ACK, and the segment did not contain a RST. no ACK, and the segment did not contain a RST.
If the SYN bit is on and the security/compartment is acceptable - If the SYN bit is on and the security/compartment is acceptable
then, RCV.NXT is set to SEG.SEQ+1, IRS is set to SEG.SEQ. then, RCV.NXT is set to SEG.SEQ+1, IRS is set to SEG.SEQ.
SND.UNA should be advanced to equal SEG.ACK (if there is an SND.UNA should be advanced to equal SEG.ACK (if there is an
ACK), and any segments on the retransmission queue that are ACK), and any segments on the retransmission queue that are
thereby acknowledged should be removed. thereby acknowledged should be removed.
If SND.UNA > ISS (our SYN has been ACKed), change the - If SND.UNA > ISS (our SYN has been ACKed), change the
connection state to ESTABLISHED, form an ACK segment connection state to ESTABLISHED, form an ACK segment
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> o <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
and send it. Data or controls that were queued for - and send it. Data or controls that were queued for
transmission MAY be included. Some TCP implementations transmission MAY be included. Some TCP implementations
suppress sending this segment when the received segment suppress sending this segment when the received segment
contains data that will anyways generate an acknowledgement in contains data that will anyways generate an acknowledgement in
the later processing steps, saving this extra acknowledgement the later processing steps, saving this extra acknowledgement
of the SYN from being sent. If there are other controls or of the SYN from being sent. If there are other controls or
text in the segment then continue processing at the sixth step text in the segment then continue processing at the sixth step
under Section 3.10.7.4 where the URG bit is checked, otherwise under Section 3.10.7.4 where the URG bit is checked, otherwise
return. return.
Otherwise enter SYN-RECEIVED, form a SYN,ACK segment - Otherwise enter SYN-RECEIVED, form a SYN,ACK segment
<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK> o <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
and send it. Set the variables: - and send it. Set the variables:
o SND.WND <- SEG.WND
SND.WND <- SEG.WND
SND.WL1 <- SEG.SEQ SND.WL1 <- SEG.SEQ
SND.WL2 <- SEG.ACK SND.WL2 <- SEG.ACK
If there are other controls or text in the segment, queue them If there are other controls or text in the segment, queue them
for processing after the ESTABLISHED state has been reached, for processing after the ESTABLISHED state has been reached,
return. return.
Note that it is legal to send and receive application data on - Note that it is legal to send and receive application data on
SYN segments (this is the "text in the segment" mentioned SYN segments (this is the "text in the segment" mentioned
above. There has been significant misinformation and above. There has been significant misinformation and
misunderstanding of this topic historically. Some firewalls misunderstanding of this topic historically. Some firewalls
and security devices consider this suspicious. However, the and security devices consider this suspicious. However, the
capability was used in T/TCP [20] and is used in TCP Fast Open capability was used in T/TCP [22] and is used in TCP Fast Open
(TFO) [47], so is important for implementations and network (TFO) [49], so is important for implementations and network
devices to permit. devices to permit.
fifth, if neither of the SYN or RST bits is set then drop the fifth, if neither of the SYN or RST bits is set then drop the
segment and return. segment and return.
3.10.7.4. Other States 3.10.7.4. Other States
Otherwise, Otherwise,
first check sequence number first check sequence number
SYN-RECEIVED STATE - SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
Segments are processed in sequence. Initial tests on ESTABLISHED STATE
arrival are used to discard old duplicates, but further
processing is done in SEG.SEQ order. If a segment's
contents straddle the boundary between old and new, only the
new parts should be processed.
In general, the processing of received segments MUST be FIN-WAIT-1 STATE
implemented to aggregate ACK segments whenever possible
(MUST-58). For example, if the TCP endpoint is processing a
series of queued segments, it MUST process them all before
sending any ACK segments (MUST-59).
There are four cases for the acceptability test for an FIN-WAIT-2 STATE
incoming segment:
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
o Segments are processed in sequence. Initial tests on
arrival are used to discard old duplicates, but further
processing is done in SEG.SEQ order. If a segment's
contents straddle the boundary between old and new, only the
new parts are processed.
o In general, the processing of received segments MUST be
implemented to aggregate ACK segments whenever possible
(MUST-58). For example, if the TCP endpoint is processing a
series of queued segments, it MUST process them all before
sending any ACK segments (MUST-59).
o There are four cases for the acceptability test for an
incoming segment:
Segment Receive Test Segment Receive Test
Length Window Length Window
------- ------- ------------------------------------------- ------- ------- -------------------------------------------
0 0 SEG.SEQ = RCV.NXT 0 0 SEG.SEQ = RCV.NXT
0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND 0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
>0 0 not acceptable >0 0 not acceptable
>0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND >0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
In implementing sequence number validation as described o In implementing sequence number validation as described
here, please note Appendix A.2. here, please note Appendix A.2.
If the RCV.WND is zero, no segments will be acceptable, but o If the RCV.WND is zero, no segments will be acceptable, but
special allowance should be made to accept valid ACKs, URGs special allowance should be made to accept valid ACKs, URGs
and RSTs. and RSTs.
If an incoming segment is not acceptable, an acknowledgment o If an incoming segment is not acceptable, an acknowledgment
should be sent in reply (unless the RST bit is set, if so should be sent in reply (unless the RST bit is set, if so
drop the segment and return): drop the segment and return):
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> + <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
After sending the acknowledgment, drop the unacceptable o After sending the acknowledgment, drop the unacceptable
segment and return. segment and return.
Note that for the TIME-WAIT state, there is an improved o Note that for the TIME-WAIT state, there is an improved
algorithm described in [39] for handling incoming SYN algorithm described in [41] for handling incoming SYN
segments, that utilizes timestamps rather than relying on segments, that utilizes timestamps rather than relying on
the sequence number check described here. When the improved the sequence number check described here. When the improved
algorithm is implemented, the logic above is not applicable algorithm is implemented, the logic above is not applicable
for incoming SYN segments with timestamp options, received for incoming SYN segments with timestamp options, received
on a connection in the TIME-WAIT state. on a connection in the TIME-WAIT state.
In the following it is assumed that the segment is the o In the following it is assumed that the segment is the
idealized segment that begins at RCV.NXT and does not exceed idealized segment that begins at RCV.NXT and does not exceed
the window. One could tailor actual segments to fit this the window. One could tailor actual segments to fit this
assumption by trimming off any portions that lie outside the assumption by trimming off any portions that lie outside the
window (including SYN and FIN), and only processing further window (including SYN and FIN), and only processing further
if the segment then begins at RCV.NXT. Segments with higher if the segment then begins at RCV.NXT. Segments with higher
beginning sequence numbers SHOULD be held for later beginning sequence numbers SHOULD be held for later
processing (SHLD-31). processing (SHLD-31).
second check the RST bit, - second check the RST bit,
o RFC 5961 [9] section 3 describes a potential blind reset
attack and optional mitigation approach. This does not
provide a cryptographic protection (e.g. as in IPsec or TCP-
AO), but can be applicable in situations described in RFC
5961. For stacks implementing the RFC 5961 protection, the
three checks below apply, otherwise processing for these
states is indicated further below.
RFC 5961 [37] section 3 describes a potential blind reset + 1) If the RST bit is set and the sequence number is
attack and optional mitigation approach. This does not outside the current receive window, silently drop the
provide a cryptographic protection (e.g. as in IPsec or TCP- segment.
AO), but can be applicable in situations described in RFC
5961. For stacks implementing the RFC 5961 protection, the
three checks below apply, otherwise processing for these
states is indicated further below.
1) If the RST bit is set and the sequence number is + 2) If the RST bit is set and the sequence number exactly
outside the current receive window, silently drop the matches the next expected sequence number (RCV.NXT), then
segment. TCP endpoints MUST reset the connection in the manner
prescribed below according to the connection state.
2) If the RST bit is set and the sequence number exactly + 3) If the RST bit is set and the sequence number does not
matches the next expected sequence number (RCV.NXT), then exactly match the next expected sequence value, yet is
TCP endpoints MUST reset the connection in the manner within the current receive window, TCP endpoints MUST
prescribed below according to the connection state. send an acknowledgement (challenge ACK):
3) If the RST bit is set and the sequence number does not <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
exactly match the next expected sequence value, yet is
within the current receive window, TCP endpoints MUST
send an acknowledgement (challenge ACK):
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> After sending the challenge ACK, TCP endpoints MUST drop
the unacceptable segment and stop processing the incoming
packet further. Note that RFC 5961 and Errata ID 4772
contain additional considerations for ACK throttling in
an implementation.
After sending the challenge ACK, TCP endpoints MUST drop o SYN-RECEIVED STATE
the unacceptable segment and stop processing the incoming
packet further. Note that RFC 5961 and Errata ID 4772
contain additional considerations for ACK throttling in
an implementation.
SYN-RECEIVED STATE + If the RST bit is set
If the RST bit is set * If this connection was initiated with a passive OPEN
(i.e., came from the LISTEN state), then return this
connection to LISTEN state and return. The user need
not be informed. If this connection was initiated
with an active OPEN (i.e., came from SYN-SENT state)
then the connection was refused, signal the user
"connection refused". In either case, the
retransmission queue should be flushed. And in the
active OPEN case, enter the CLOSED state and delete
the TCB, and return.
If this connection was initiated with a passive OPEN o ESTABLISHED
(i.e., came from the LISTEN state), then return this
connection to LISTEN state and return. The user need
not be informed. If this connection was initiated
with an active OPEN (i.e., came from SYN-SENT state)
then the connection was refused, signal the user
"connection refused". In either case, all segments on
the retransmission queue should be removed. And in
the active OPEN case, enter the CLOSED state and
delete the TCB, and return.
ESTABLISHED FIN-WAIT-1
FIN-WAIT-1 FIN-WAIT-2
FIN-WAIT-2
CLOSE-WAIT
If the RST bit is set then, any outstanding RECEIVEs and CLOSE-WAIT
SEND should receive "reset" responses. All segment
queues should be flushed. Users should also receive an
unsolicited general "connection reset" signal. Enter the
CLOSED state, delete the TCB, and return.
CLOSING STATE + If the RST bit is set then, any outstanding RECEIVEs and
LAST-ACK STATE SEND should receive "reset" responses. All segment
TIME-WAIT queues should be flushed. Users should also receive an
unsolicited general "connection reset" signal. Enter the
CLOSED state, delete the TCB, and return.
If the RST bit is set then, enter the CLOSED state, o CLOSING STATE
delete the TCB, and return.
third check security LAST-ACK STATE
SYN-RECEIVED
If the security/compartment in the segment does not TIME-WAIT
exactly match the security/compartment in the TCB then
send a reset, and return.
ESTABLISHED + If the RST bit is set then, enter the CLOSED state,
FIN-WAIT-1 delete the TCB, and return.
FIN-WAIT-2
CLOSE-WAIT
CLOSING
LAST-ACK
TIME-WAIT
If the security/compartment in the segment does not - third check security
exactly match the security/compartment in the TCB then
send a reset, any outstanding RECEIVEs and SEND should
receive "reset" responses. All segment queues should be
flushed. Users should also receive an unsolicited
general "connection reset" signal. Enter the CLOSED
state, delete the TCB, and return.
Note this check is placed following the sequence check to o SYN-RECEIVED
prevent a segment from an old connection between these port
numbers with a different security from causing an abort of
the current connection.
fourth, check the SYN bit, + If the security/compartment in the segment does not
exactly match the security/compartment in the TCB then
send a reset, and return.
SYN-RECEIVED o ESTABLISHED
If the connection was initiated with a passive OPEN, then FIN-WAIT-1
return this connection to the LISTEN state and return.
Otherwise, handle per the directions for synchronized
states below.
ESTABLISHED STATE FIN-WAIT-2
FIN-WAIT STATE-1
FIN-WAIT STATE-2
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
If the SYN bit is set in these synchronized states, it CLOSE-WAIT
may be either a legitimate new connection attempt (e.g.
in the case of TIME-WAIT), an error where the connection
should be reset, or the result of an attack attempt, as
described in RFC 5961 [37]. For the TIME-WAIT state, new
connections can be accepted if the timestamp option is
used and meets expectations (per [39]). For all other
cases, RFC 5961 provides a mitigation with applicability
to some situations, though there are also alternatives
that offer cryptographic protection (see Section 7). RFC
5961 recommends that in these synchronized states, if the
SYN bit is set, irrespective of the sequence number, TCP
endpoints MUST send a "challenge ACK" to the remote peer:
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> CLOSING
After sending the acknowledgement, TCP implementations LAST-ACK
MUST drop the unacceptable segment and stop processing
further. Note that RFC 5961 and Errata ID 4772 contain
additional ACK throttling notes for an implementation.
For implementations that do not follow RFC 5961, the TIME-WAIT
original RFC 793 behavior follows in this paragraph. If
the SYN is in the window it is an error, send a reset,
any outstanding RECEIVEs and SEND should receive "reset"
responses, all segment queues should be flushed, the user
should also receive an unsolicited general "connection
reset" signal, enter the CLOSED state, delete the TCB,
and return.
If the SYN is not in the window this step would not be + If the security/compartment in the segment does not
reached and an ACK would have been sent in the first step exactly match the security/compartment in the TCB then
(sequence number check). send a reset, any outstanding RECEIVEs and SEND should
receive "reset" responses. All segment queues should be
flushed. Users should also receive an unsolicited
general "connection reset" signal. Enter the CLOSED
state, delete the TCB, and return.
fifth check the ACK field, o Note this check is placed following the sequence check to
prevent a segment from an old connection between these port
numbers with a different security from causing an abort of
the current connection.
if the ACK bit is off drop the segment and return - fourth, check the SYN bit,
if the ACK bit is on o SYN-RECEIVED
RFC 5961 [37] section 5 describes a potential blind data + If the connection was initiated with a passive OPEN, then
injection attack, and mitigation that implementations MAY return this connection to the LISTEN state and return.
choose to include (MAY-12). TCP stacks that implement Otherwise, handle per the directions for synchronized
RFC 5961 MUST add an input check that the ACK value is states below.
acceptable only if it is in the range of ((SND.UNA -
MAX.SND.WND) =< SEG.ACK =< SND.NXT). All incoming
segments whose ACK value doesn't satisfy the above
condition MUST be discarded and an ACK sent back. The
new state variable MAX.SND.WND is defined as the largest
window that the local sender has ever received from its
peer (subject to window scaling) or may be hard-coded to
a maximum permissible window value. When the ACK value
is acceptable, the processing per-state below applies:
SYN-RECEIVED STATE ESTABLISHED STATE
If SND.UNA < SEG.ACK =< SND.NXT then enter ESTABLISHED FIN-WAIT STATE-1
state and continue processing with variables below set
to:
SND.WND <- SEG.WND FIN-WAIT STATE-2
SND.WL1 <- SEG.SEQ
SND.WL2 <- SEG.ACK
If the segment acknowledgment is not acceptable, form CLOSE-WAIT STATE
a reset segment,
<SEQ=SEG.ACK><CTL=RST> CLOSING STATE
and send it. LAST-ACK STATE
ESTABLISHED STATE TIME-WAIT STATE
If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <- + If the SYN bit is set in these synchronized states, it
SEG.ACK. Any segments on the retransmission queue may be either a legitimate new connection attempt (e.g.
that are thereby entirely acknowledged are removed. in the case of TIME-WAIT), an error where the connection
Users should receive positive acknowledgments for should be reset, or the result of an attack attempt, as
buffers that have been SENT and fully acknowledged described in RFC 5961 [9]. For the TIME-WAIT state, new
(i.e., SEND buffer should be returned with "ok" connections can be accepted if the timestamp option is
response). If the ACK is a duplicate (SEG.ACK =< used and meets expectations (per [41]). For all other
SND.UNA), it can be ignored. If the ACK acks cases, RFC 5961 provides a mitigation with applicability
something not yet sent (SEG.ACK > SND.NXT) then send to some situations, though there are also alternatives
an ACK, drop the segment, and return. that offer cryptographic protection (see Section 7). RFC
5961 recommends that in these synchronized states, if the
SYN bit is set, irrespective of the sequence number, TCP
endpoints MUST send a "challenge ACK" to the remote peer:
If SND.UNA =< SEG.ACK =< SND.NXT, the send window + <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
should be updated. If (SND.WL1 < SEG.SEQ or (SND.WL1
= SEG.SEQ and SND.WL2 =< SEG.ACK)), set SND.WND <-
SEG.WND, set SND.WL1 <- SEG.SEQ, and set SND.WL2 <-
SEG.ACK.
Note that SND.WND is an offset from SND.UNA, that + After sending the acknowledgement, TCP implementations
SND.WL1 records the sequence number of the last MUST drop the unacceptable segment and stop processing
segment used to update SND.WND, and that SND.WL2 further. Note that RFC 5961 and Errata ID 4772 contain
records the acknowledgment number of the last segment additional ACK throttling notes for an implementation.
used to update SND.WND. The check here prevents using
old segments to update the window.
FIN-WAIT-1 STATE + For implementations that do not follow RFC 5961, the
In addition to the processing for the ESTABLISHED original RFC 793 behavior follows in this paragraph. If
state, if the FIN segment is now acknowledged then the SYN is in the window it is an error, send a reset,
enter FIN-WAIT-2 and continue processing in that any outstanding RECEIVEs and SEND should receive "reset"
state. responses, all segment queues should be flushed, the user
should also receive an unsolicited general "connection
reset" signal, enter the CLOSED state, delete the TCB,
and return.
FIN-WAIT-2 STATE + If the SYN is not in the window this step would not be
reached and an ACK would have been sent in the first step
(sequence number check).
In addition to the processing for the ESTABLISHED - fifth check the ACK field,
state, if the retransmission queue is empty, the
user's CLOSE can be acknowledged ("ok") but do not
delete the TCB.
CLOSE-WAIT STATE o if the ACK bit is off drop the segment and return
Do the same processing as for the ESTABLISHED state. o if the ACK bit is on
CLOSING STATE + RFC 5961 [9] section 5 describes a potential blind data
injection attack, and mitigation that implementations MAY
choose to include (MAY-12). TCP stacks that implement
RFC 5961 MUST add an input check that the ACK value is
acceptable only if it is in the range of ((SND.UNA -
MAX.SND.WND) =< SEG.ACK =< SND.NXT). All incoming
segments whose ACK value doesn't satisfy the above
condition MUST be discarded and an ACK sent back. The
new state variable MAX.SND.WND is defined as the largest
window that the local sender has ever received from its
peer (subject to window scaling) or may be hard-coded to
a maximum permissible window value. When the ACK value
is acceptable, the processing per-state below applies:
In addition to the processing for the ESTABLISHED + SYN-RECEIVED STATE
state, if the ACK acknowledges our FIN then enter the
TIME-WAIT state, otherwise ignore the segment.
LAST-ACK STATE * If SND.UNA < SEG.ACK =< SND.NXT then enter ESTABLISHED
state and continue processing with variables below set
to:
The only thing that can arrive in this state is an - SND.WND <- SEG.WND
acknowledgment of our FIN. If our FIN is now
acknowledged, delete the TCB, enter the CLOSED state,
and return.
TIME-WAIT STATE SND.WL1 <- SEG.SEQ
The only thing that can arrive in this state is a SND.WL2 <- SEG.ACK
retransmission of the remote FIN. Acknowledge it, and
restart the 2 MSL timeout.
sixth, check the URG bit, * If the segment acknowledgment is not acceptable, form
a reset segment,
- <SEQ=SEG.ACK><CTL=RST>
ESTABLISHED STATE * and send it.
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
If the URG bit is set, RCV.UP <- max(RCV.UP,SEG.UP), and + ESTABLISHED STATE
signal the user that the remote side has urgent data if
the urgent pointer (RCV.UP) is in advance of the data
consumed. If the user has already been signaled (or is
still in the "urgent mode") for this continuous sequence
of urgent data, do not signal the user again.
CLOSE-WAIT STATE * If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <-
CLOSING STATE SEG.ACK. Any segments on the retransmission queue
LAST-ACK STATE that are thereby entirely acknowledged are removed.
TIME-WAIT Users should receive positive acknowledgments for
buffers that have been SENT and fully acknowledged
(i.e., SEND buffer should be returned with "ok"
response). If the ACK is a duplicate (SEG.ACK =<
SND.UNA), it can be ignored. If the ACK acks
something not yet sent (SEG.ACK > SND.NXT) then send
an ACK, drop the segment, and return.
This should not occur, since a FIN has been received from * If SND.UNA =< SEG.ACK =< SND.NXT, the send window
the remote side. Ignore the URG. should be updated. If (SND.WL1 < SEG.SEQ or (SND.WL1
= SEG.SEQ and SND.WL2 =< SEG.ACK)), set SND.WND <-
SEG.WND, set SND.WL1 <- SEG.SEQ, and set SND.WL2 <-
SEG.ACK.
seventh, process the segment text, * Note that SND.WND is an offset from SND.UNA, that
SND.WL1 records the sequence number of the last
segment used to update SND.WND, and that SND.WL2
records the acknowledgment number of the last segment
used to update SND.WND. The check here prevents using
old segments to update the window.
ESTABLISHED STATE + FIN-WAIT-1 STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
Once in the ESTABLISHED state, it is possible to deliver * In addition to the processing for the ESTABLISHED
segment text to user RECEIVE buffers. Text from segments state, if the FIN segment is now acknowledged then
can be moved into buffers until either the buffer is full enter FIN-WAIT-2 and continue processing in that
or the segment is empty. If the segment empties and state.
carries a PUSH flag, then the user is informed, when the
buffer is returned, that a PUSH has been received.
When the TCP endpoint takes responsibility for delivering + FIN-WAIT-2 STATE
the data to the user it must also acknowledge the receipt
of the data.
Once the TCP endpoint takes responsibility for the data * In addition to the processing for the ESTABLISHED
it advances RCV.NXT over the data accepted, and adjusts state, if the retransmission queue is empty, the
RCV.WND as appropriate to the current buffer user's CLOSE can be acknowledged ("ok") but do not
availability. The total of RCV.NXT and RCV.WND should delete the TCB.
not be reduced.
A TCP implementation MAY send an ACK segment + CLOSE-WAIT STATE
acknowledging RCV.NXT when a valid segment arrives that
is in the window but not at the left window edge (MAY-
13).
Please note the window management suggestions in * Do the same processing as for the ESTABLISHED state.
Section 3.8.
Send an acknowledgment of the form: + CLOSING STATE
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> * In addition to the processing for the ESTABLISHED
state, if the ACK acknowledges our FIN then enter the
TIME-WAIT state, otherwise ignore the segment.
This acknowledgment should be piggybacked on a segment + LAST-ACK STATE
being transmitted if possible without incurring undue
delay.
CLOSE-WAIT STATE * The only thing that can arrive in this state is an
CLOSING STATE acknowledgment of our FIN. If our FIN is now
LAST-ACK STATE acknowledged, delete the TCB, enter the CLOSED state,
TIME-WAIT STATE and return.
This should not occur, since a FIN has been received from + TIME-WAIT STATE
the remote side. Ignore the segment text.
eighth, check the FIN bit, * The only thing that can arrive in this state is a
retransmission of the remote FIN. Acknowledge it, and
restart the 2 MSL timeout.
Do not process the FIN if the state is CLOSED, LISTEN or - sixth, check the URG bit,
SYN-SENT since the SEG.SEQ cannot be validated; drop the
segment and return.
If the FIN bit is set, signal the user "connection closing" o ESTABLISHED STATE
and return any pending RECEIVEs with same message, advance
RCV.NXT over the FIN, and send an acknowledgment for the
FIN. Note that FIN implies PUSH for any segment text not
yet delivered to the user.
SYN-RECEIVED STATE FIN-WAIT-1 STATE
ESTABLISHED STATE
Enter the CLOSE-WAIT state. FIN-WAIT-2 STATE
FIN-WAIT-1 STATE + If the URG bit is set, RCV.UP <- max(RCV.UP,SEG.UP), and
signal the user that the remote side has urgent data if
the urgent pointer (RCV.UP) is in advance of the data
consumed. If the user has already been signaled (or is
still in the "urgent mode") for this continuous sequence
of urgent data, do not signal the user again.
If our FIN has been ACKed (perhaps in this segment), o CLOSE-WAIT STATE
then enter TIME-WAIT, start the time-wait timer, turn
off the other timers; otherwise enter the CLOSING
state.
FIN-WAIT-2 STATE CLOSING STATE
Enter the TIME-WAIT state. Start the time-wait timer, LAST-ACK STATE
turn off the other timers.
CLOSE-WAIT STATE TIME-WAIT
Remain in the CLOSE-WAIT state. + This should not occur, since a FIN has been received from
the remote side. Ignore the URG.
CLOSING STATE - seventh, process the segment text,
Remain in the CLOSING state. o ESTABLISHED STATE
FIN-WAIT-1 STATE
LAST-ACK STATE FIN-WAIT-2 STATE
Remain in the LAST-ACK state. + Once in the ESTABLISHED state, it is possible to deliver
segment data to user RECEIVE buffers. Data from segments
can be moved into buffers until either the buffer is full
or the segment is empty. If the segment empties and
carries a PUSH flag, then the user is informed, when the
buffer is returned, that a PUSH has been received.
TIME-WAIT STATE + When the TCP endpoint takes responsibility for delivering
the data to the user it must also acknowledge the receipt
of the data.
Remain in the TIME-WAIT state. Restart the 2 MSL + Once the TCP endpoint takes responsibility for the data
time-wait timeout. it advances RCV.NXT over the data accepted, and adjusts
RCV.WND as appropriate to the current buffer
availability. The total of RCV.NXT and RCV.WND should
not be reduced.
and return. + A TCP implementation MAY send an ACK segment
acknowledging RCV.NXT when a valid segment arrives that
is in the window but not at the left window edge (MAY-
13).
+ Please note the window management suggestions in
Section 3.8.
+ Send an acknowledgment of the form:
* <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
+ This acknowledgment should be piggybacked on a segment
being transmitted if possible without incurring undue
delay.
o CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
+ This should not occur, since a FIN has been received from
the remote side. Ignore the segment text.
- eighth, check the FIN bit,
o Do not process the FIN if the state is CLOSED, LISTEN or
SYN-SENT since the SEG.SEQ cannot be validated; drop the
segment and return.
o If the FIN bit is set, signal the user "connection closing"
and return any pending RECEIVEs with same message, advance
RCV.NXT over the FIN, and send an acknowledgment for the
FIN. Note that FIN implies PUSH for any segment text not
yet delivered to the user.
+ SYN-RECEIVED STATE
ESTABLISHED STATE
* Enter the CLOSE-WAIT state.
+ FIN-WAIT-1 STATE
* If our FIN has been ACKed (perhaps in this segment),
then enter TIME-WAIT, start the time-wait timer, turn
off the other timers; otherwise enter the CLOSING
state.
+ FIN-WAIT-2 STATE
* Enter the TIME-WAIT state. Start the time-wait timer,
turn off the other timers.
+ CLOSE-WAIT STATE
* Remain in the CLOSE-WAIT state.
+ CLOSING STATE
* Remain in the CLOSING state.
+ LAST-ACK STATE
* Remain in the LAST-ACK state.
+ TIME-WAIT STATE
* Remain in the TIME-WAIT state. Restart the 2 MSL
time-wait timeout.
- and return.
3.10.8. Timeouts 3.10.8. Timeouts
USER TIMEOUT USER TIMEOUT
For any state if the user timeout expires, flush all queues, - For any state if the user timeout expires, flush all queues,
signal the user "error: connection aborted due to user timeout" signal the user "error: connection aborted due to user timeout"
in general and for any outstanding calls, delete the TCB, enter in general and for any outstanding calls, delete the TCB, enter
the CLOSED state and return. the CLOSED state and return.
RETRANSMISSION TIMEOUT RETRANSMISSION TIMEOUT
For any state if the retransmission timeout expires on a - For any state if the retransmission timeout expires on a
segment in the retransmission queue, send the segment at the segment in the retransmission queue, send the segment at the
front of the retransmission queue again, reinitialize the front of the retransmission queue again, reinitialize the
retransmission timer, and return. retransmission timer, and return.
TIME-WAIT TIMEOUT TIME-WAIT TIMEOUT
If the time-wait timeout expires on a connection delete the - If the time-wait timeout expires on a connection delete the
TCB, enter the CLOSED state and return. TCB, enter the CLOSED state and return.
4. Glossary 4. Glossary
ACK ACK
A control bit (acknowledge) occupying no sequence space, A control bit (acknowledge) occupying no sequence space,
which indicates that the acknowledgment field of this segment which indicates that the acknowledgment field of this segment
specifies the next sequence number the sender of this segment specifies the next sequence number the sender of this segment
is expecting to receive, hence acknowledging receipt of all is expecting to receive, hence acknowledging receipt of all
previous sequence numbers. previous sequence numbers.
connection connection
A logical communication path identified by a pair of sockets. A logical communication path identified by a pair of sockets.
datagram datagram
A message sent in a packet switched computer communications A message sent in a packet switched computer communications
network. network.
Destination Address Destination Address
The network layer address of the remote endpoint. The network layer address of the endpoint intended to receive
a segment.
FIN FIN
A control bit (finis) occupying one sequence number, which A control bit (finis) occupying one sequence number, which
indicates that the sender will send no more data or control indicates that the sender will send no more data or control
occupying sequence space. occupying sequence space.
flush
To remove all of the contents (data or segments) from a store
(buffer or queue).
fragment fragment
A portion of a logical unit of data, in particular an A portion of a logical unit of data, in particular an
internet fragment is a portion of an internet datagram. internet fragment is a portion of an internet datagram.
header header
Control information at the beginning of a message, segment, Control information at the beginning of a message, segment,
fragment, packet or block of data. fragment, packet or block of data.
host host
A computer. In particular a source or destination of A computer. In particular a source or destination of
messages from the point of view of the communication network. messages from the point of view of the communication network.
Identification Identification
An Internet Protocol field. This identifying value assigned An Internet Protocol field. This identifying value assigned
by the sender aids in assembling the fragments of a datagram. by the sender aids in assembling the fragments of a datagram.
internet address internet address
A network layer address. A network layer address.
internet datagram internet datagram
The unit of data exchanged between an internet module and the A unit of data exchanged between internet hosts, together
higher level protocol together with the internet header. with the internet header that allows the datagram to be
routed from source to destination.
internet fragment internet fragment
A portion of the data of an internet datagram with an A portion of the data of an internet datagram with an
internet header. internet header.
IP IP
Internet Protocol. See [1] and [13]. Internet Protocol. See [1] and [13].
IRS IRS
The Initial Receive Sequence number. The first sequence The Initial Receive Sequence number. The first sequence
skipping to change at page 84, line 16 skipping to change at page 87, line 18
receive next sequence number receive next sequence number
This is the next sequence number the local TCP endpoint is This is the next sequence number the local TCP endpoint is
expecting to receive. expecting to receive.
receive window receive window
This represents the sequence numbers the local (receiving) This represents the sequence numbers the local (receiving)
TCP endpoint is willing to receive. Thus, the local TCP TCP endpoint is willing to receive. Thus, the local TCP
endpoint considers that segments overlapping the range endpoint considers that segments overlapping the range
RCV.NXT to RCV.NXT + RCV.WND - 1 carry acceptable data or RCV.NXT to RCV.NXT + RCV.WND - 1 carry acceptable data or
control. Segments containing sequence numbers entirely control. Segments containing sequence numbers entirely
outside of this range are considered duplicates and outside this range are considered duplicates or injection
discarded. attacks and discarded.
RST RST
A control bit (reset), occupying no sequence space, A control bit (reset), occupying no sequence space,
indicating that the receiver should delete the connection indicating that the receiver should delete the connection
without further interaction. The receiver can determine, without further interaction. The receiver can determine,
based on the sequence number and acknowledgment fields of the based on the sequence number and acknowledgment fields of the
incoming segment, whether it should honor the reset command incoming segment, whether it should honor the reset command
or ignore it. In no case does receipt of a segment or ignore it. In no case does receipt of a segment
containing RST give rise to a RST in response. containing RST give rise to a RST in response.
skipping to change at page 86, line 22 skipping to change at page 89, line 22
the state of a connection. the state of a connection.
TCP TCP
Transmission Control Protocol: A host-to-host protocol for Transmission Control Protocol: A host-to-host protocol for
reliable communication in internetwork environments. reliable communication in internetwork environments.
TOS TOS
Type of Service, an obsoleted IPv4 field. The same header Type of Service, an obsoleted IPv4 field. The same header
bits currently are used for the Differentiated Services field bits currently are used for the Differentiated Services field
[4] containing the Differentiated Services Code Point (DSCP) [4] containing the Differentiated Services Code Point (DSCP)
value and the 2-bit ECN codepoint [7]. value and the 2-bit ECN codepoint [6].
Type of Service Type of Service
See "TOS". See "TOS".
URG URG
A control bit (urgent), occupying no sequence space, used to A control bit (urgent), occupying no sequence space, used to
indicate that the receiving user should be notified to do indicate that the receiving user should be notified to do
urgent processing as long as there is data to be consumed urgent processing as long as there is data to be consumed
with sequence numbers less than the value indicated in the with sequence numbers less than the value indicated by the
urgent pointer. urgent pointer.
urgent pointer urgent pointer
A control field meaningful only when the URG bit is on. This A control field meaningful only when the URG bit is on. This
field communicates the value of the urgent pointer that field communicates the value of the urgent pointer that
indicates the data octet associated with the sending user's indicates the data octet associated with the sending user's
urgent call. urgent call.
5. Changes from RFC 793 5. Changes from RFC 793
skipping to change at page 87, line 11 skipping to change at page 90, line 11
valuable in learning about and understanding TCP, and they are valid valuable in learning about and understanding TCP, and they are valid
Informational references, even though their normative content has Informational references, even though their normative content has
been incorporated into this document. been incorporated into this document.
The main body of this document was adapted from RFC 793's Section 3, The main body of this document was adapted from RFC 793's Section 3,
titled "FUNCTIONAL SPECIFICATION", with an attempt to keep formatting titled "FUNCTIONAL SPECIFICATION", with an attempt to keep formatting
and layout as close as possible. and layout as close as possible.
The collection of applicable RFC Errata that have been reported and The collection of applicable RFC Errata that have been reported and
either accepted or held for an update to RFC 793 were incorporated either accepted or held for an update to RFC 793 were incorporated
(Errata IDs: 573, 574, 700, 701, 1283, 1561, 1562, 1564, 1565, 1571, (Errata IDs: 573, 574, 700, 701, 1283, 1561, 1562, 1564, 1571, 1572,
1572, 2296, 2297, 2298, 2748, 2749, 2934, 3213, 3300, 3301, 6222). 2297, 2298, 2748, 2749, 2934, 3213, 3300, 3301, 6222). Some errata
Some errata were not applicable due to other changes (Errata IDs: were not applicable due to other changes (Errata IDs: 572, 575, 1565,
572, 575, 1569, 3305, 3602). 1569, 2296, 3305, 3602).
Changes to the specification of the Urgent Pointer described in RFC Changes to the specification of the Urgent Pointer described in RFCs
1122 and 6093 were incorporated. See RFC 6093 for detailed 1011, 1122, and 6093 were incorporated. See RFC 6093 for detailed
discussion of why these changes were necessary. discussion of why these changes were necessary.
The discussion of the RTO from RFC 793 was updated to refer to RFC The discussion of the RTO from RFC 793 was updated to refer to RFC
6298. The RFC 1122 text on the RTO originally replaced the 793 text, 6298. The RFC 1122 text on the RTO originally replaced the 793 text,
however, RFC 2988 should have updated 1122, and has subsequently been however, RFC 2988 should have updated 1122, and has subsequently been
obsoleted by 6298. obsoleted by 6298.
RFC 1122 contains a collection of other changes and clarifications to RFC 1011 [19] contains a number of comments about RFC 793, including
RFC 793. The normative items impacting the protocol have been some needed changes to the TCP specification. These are expanded in
incorporated here, though some historically useful implementation RFC 1122, which contains a collection of other changes and
advice and informative discussion from RFC 1122 is not included here. clarifications to RFC 793. The normative items impacting the
protocol have been incorporated here, though some historically useful
implementation advice and informative discussion from RFC 1122 is not
included here. The present document updates RFC 1011, since this is
now the TCP specification rather than RFC 793, and the comments noted
in 1011 have been incorporated.
RFC 1122 contains more than just TCP requirements, so this document RFC 1122 contains more than just TCP requirements, so this document
can't obsolete RFC 1122 entirely. It is only marked as "updating" can't obsolete RFC 1122 entirely. It is only marked as "updating"
1122, however, it should be understood to effectively obsolete all of 1122, however, it should be understood to effectively obsolete all of
the RFC 1122 material on TCP. the RFC 1122 material on TCP.
The more secure Initial Sequence Number generation algorithm from RFC The more secure Initial Sequence Number generation algorithm from RFC
6528 was incorporated. See RFC 6528 for discussion of the attacks 6528 was incorporated. See RFC 6528 for discussion of the attacks
that this mitigates, as well as advice on selecting PRF algorithms that this mitigates, as well as advice on selecting PRF algorithms
and managing secret key data. and managing secret key data.
skipping to change at page 88, line 19 skipping to change at page 91, line 27
content of RFC 793 Section 3 titled "FUNCTIONAL SPECIFICATION". content of RFC 793 Section 3 titled "FUNCTIONAL SPECIFICATION".
Other content from RFC 793 has not been incorporated. The -01 Other content from RFC 793 has not been incorporated. The -01
revision of this document makes some minor formatting changes to the revision of this document makes some minor formatting changes to the
RFC 793 content in order to convert the content into XML2RFC format RFC 793 content in order to convert the content into XML2RFC format
and account for left-out parts of RFC 793. For instance, figure and account for left-out parts of RFC 793. For instance, figure
numbering differs and some indentation is not exactly the same. numbering differs and some indentation is not exactly the same.
The -02 revision of draft-eddy-rfc793bis incorporates errata that The -02 revision of draft-eddy-rfc793bis incorporates errata that
have been verified: have been verified:
Errata ID 573: Reported by Bob Braden (note: This errata basically Errata ID 573: Reported by Bob Braden (note: This errata report
is just a reminder that RFC 1122 updates 793. Some of the basically is just a reminder that RFC 1122 updates 793. Some of
associated changes are left pending to a separate revision that the associated changes are left pending to a separate revision
incorporates 1122. Bob's mention of PUSH in 793 section 2.8 was that incorporates 1122. Bob's mention of PUSH in 793 section 2.8
not applicable here because that section was not part of the was not applicable here because that section was not part of the
"functional specification". Also the 1122 text on the "functional specification". Also, the 1122 text on the
retransmission timeout also has been updated by subsequent RFCs, retransmission timeout also has been updated by subsequent RFCs,
so the change here deviates from Bob's suggestion to apply the so the change here deviates from Bob's suggestion to apply the
1122 text.) 1122 text.)
Errata ID 574: Reported by Yin Shuming Errata ID 574: Reported by Yin Shuming
Errata ID 700: Reported by Yin Shuming Errata ID 700: Reported by Yin Shuming
Errata ID 701: Reported by Yin Shuming Errata ID 701: Reported by Yin Shuming
Errata ID 1283: Reported by Pei-chun Cheng Errata ID 1283: Reported by Pei-chun Cheng
Errata ID 1561: Reported by Constantin Hagemeier Errata ID 1561: Reported by Constantin Hagemeier
Errata ID 1562: Reported by Constantin Hagemeier Errata ID 1562: Reported by Constantin Hagemeier
Errata ID 1564: Reported by Constantin Hagemeier Errata ID 1564: Reported by Constantin Hagemeier
skipping to change at page 89, line 20 skipping to change at page 92, line 28
The -03 revision of draft-eddy-rfc793bis revises all discussion of The -03 revision of draft-eddy-rfc793bis revises all discussion of
the urgent pointer in order to comply with RFC 6093, 1122, and 1011. the urgent pointer in order to comply with RFC 6093, 1122, and 1011.
Since 1122 held requirements on the urgent pointer, the full list of Since 1122 held requirements on the urgent pointer, the full list of
requirements was brought into an appendix of this document, so that requirements was brought into an appendix of this document, so that
it can be updated as-needed. it can be updated as-needed.
The -04 revision of draft-eddy-rfc793bis includes the ISN generation The -04 revision of draft-eddy-rfc793bis includes the ISN generation
changes from RFC 6528. changes from RFC 6528.
The -05 revision of draft-eddy-rfc793bis incorporates MSS The -05 revision of draft-eddy-rfc793bis incorporates MSS
requirements and definitions from RFC 879, 1122, and 6691, as well as requirements and definitions from RFC 879 [17], 1122, and 6691, as
option-handling requirements from RFC 1122. well as option-handling requirements from RFC 1122.
The -00 revision of draft-ietf-tcpm-rfc793bis incorporates several The -00 revision of draft-ietf-tcpm-rfc793bis incorporates several
additional clarifications and updates to the section on segmentation, additional clarifications and updates to the section on segmentation,
many of which are based on feedback from Joe Touch improving from the many of which are based on feedback from Joe Touch improving from the
initial text on this in the previous revision. initial text on this in the previous revision.
The -01 revision incorporates the change to Reserved bits due to ECN, The -01 revision incorporates the change to Reserved bits due to ECN,
as well as many other changes that come from RFC 1122. as well as many other changes that come from RFC 1122.
The -02 revision has small formatting modifications in order to The -02 revision has small formatting modifications in order to
address xml2rfc warnings about long lines. It was a quick update to address xml2rfc warnings about long lines. It was a quick update to
avoid document expiration. TCPM working group discussion in 2015 avoid document expiration. TCPM working group discussion in 2015
also indicated that that we should not try to add sections on also indicated that we should not try to add sections on
implementation advice or similar non-normative information. implementation advice or similar non-normative information.
The -03 revision incorporates more content from RFC 1122: Passive The -03 revision incorporates more content from RFC 1122: Passive
OPEN Calls, Time-To-Live, Multihoming, IP Options, ICMP messages, OPEN Calls, Time-To-Live, Multihoming, IP Options, ICMP messages,
Data Communications, When to Send Data, When to Send a Window Update, Data Communications, When to Send Data, When to Send a Window Update,
Managing the Window, Probing Zero Windows, When to Send an ACK Managing the Window, Probing Zero Windows, When to Send an ACK
Segment. The section on data communications was re-organized into Segment. The section on data communications was re-organized into
clearer subsections (previously headings were embedded in the 793 clearer subsections (previously headings were embedded in the 793
text), and windows management advice from 793 was removed (as text), and windows management advice from 793 was removed (as
reviewed by TCPM working group) in favor of the 1122 additions on reviewed by TCPM working group) in favor of the 1122 additions on
skipping to change at page 92, line 26 skipping to change at page 95, line 41
idnits issues addressed from Michael Scharf. idnits issues addressed from Michael Scharf.
The -24 revision incorporates changes after Martin Duke's AD review, The -24 revision incorporates changes after Martin Duke's AD review,
including further feedback on those comments from Yuchung Cheng and including further feedback on those comments from Yuchung Cheng and
Joe Touch. Important changes for review include (1) removal of the Joe Touch. Important changes for review include (1) removal of the
need to check for the PUSH flag when evaluating the SWS override need to check for the PUSH flag when evaluating the SWS override
timer expiration, (2) clarification about receding urgent pointer, timer expiration, (2) clarification about receding urgent pointer,
and (3) de-duplicating handling of the RST checking between step 4 and (3) de-duplicating handling of the RST checking between step 4
and step 1. and step 1.
The -25 revision incorporates changes based on the GENART review from
Francis Dupont, SECDIR review from Kyle Rose, and OPSDIR review from
Sarah Banks.
The -26 revision incorporates changes stemming from the IESG reviews,
and INTDIR review from Bernie Volz.
The -27 revision fixes a few small editorial incompatibilities that
Stephen McQuistin found related to automated code generation.
The -28 revision addresses some COMMENTs from Ben Kaduk's IESG
review.
Some other suggested changes that will not be incorporated in this Some other suggested changes that will not be incorporated in this
793 update unless TCPM consensus changes with regard to scope are: 793 update unless TCPM consensus changes with regard to scope are:
1. Tony Sabatini's suggestion for describing DO field 1. Tony Sabatini's suggestion for describing DO field
2. Per discussion with Joe Touch (TAPS list, 6/20/2015), the 2. Per discussion with Joe Touch (TAPS list, 6/20/2015), the
description of the API could be revisited description of the API could be revisited
3. Reducing the R2 value for SYNs has been suggested as a possible 3. Reducing the R2 value for SYNs has been suggested as a possible
topic for future consideration. topic for future consideration.
Early in the process of updating RFC 793, Scott Brim mentioned that Early in the process of updating RFC 793, Scott Brim mentioned that
skipping to change at page 93, line 19 skipping to change at page 97, line 13
IANA should assign values indicated below. IANA should assign values indicated below.
TCP Header Flags TCP Header Flags
Bit Name Reference Assignment Notes Bit Name Reference Assignment Notes
Offset Offset
--- ---- --------- ---------------- --- ---- --------- ----------------
4 Reserved for future use (this document) 4 Reserved for future use (this document)
5 Reserved for future use (this document) 5 Reserved for future use (this document)
6 Reserved for future use (this document) 6 Reserved for future use (this document)
7 Reserved for future use [RFC8311] Previously used by Historic [RFC3540] as NS (Nonce Sum) 7 Reserved for future use [RFC8311] [1]
8 CWR (Congestion Window Reduced) [RFC3168] 8 CWR (Congestion Window Reduced) [RFC3168]
9 ECE (ECN-Echo) [RFC3168] 9 ECE (ECN-Echo) [RFC3168]
10 Urgent Pointer field is significant (URG) (this document) 10 Urgent Pointer field is significant (URG) (this document)
11 Acknowledgment field is significant (ACK) (this document) 11 Acknowledgment field is significant (ACK) (this document)
12 Push Function (PSH) (this document) 12 Push Function (PSH) (this document)
13 Reset the connection (RST) (this document) 13 Reset the connection (RST) (this document)
14 Synchronize sequence numbers (SYN) (this document) 14 Synchronize sequence numbers (SYN) (this document)
15 No more data from sender (FIN) (this document) 15 No more data from sender (FIN) (this document)
FOOTNOTES:
[1] Previously used by Historic [RFC3540] as NS (Nonce Sum).
This TCP Header Flags registry should also be moved to a sub-registry This TCP Header Flags registry should also be moved to a sub-registry
under the global "Transmission Control Protocol (TCP) Parameters under the global "Transmission Control Protocol (TCP) Parameters
registry (https://www.iana.org/assignments/tcp-parameters/tcp- registry (https://www.iana.org/assignments/tcp-parameters/tcp-
parameters.xhtml). parameters.xhtml).
The registry's Registration Procedure should remain Standards Action, The registry's Registration Procedure should remain Standards Action,
but the Reference can be updated to this document, and the Note but the Reference can be updated to this document, and the Note
removed. removed.
7. Security and Privacy Considerations 7. Security and Privacy Considerations
The TCP design includes only rudimentary security features that The TCP design includes only rudimentary security features that
improve the robustness and reliability of connections and application improve the robustness and reliability of connections and application
data transfer, but there are no built-in cryptographic capabilities data transfer, but there are no built-in cryptographic capabilities
to support any form of privacy, authentication, or other typical to support any form of confidentiality, authentication, or other
security functions. Non-cryptographic enhancements (e.g. [37]) have typical security functions. Non-cryptographic enhancements (e.g.
been developed to improve robustness of TCP connections to particular [9]) have been developed to improve robustness of TCP connections to
types of attacks, but the applicability and protections of non- particular types of attacks, but the applicability and protections of
cryptographic enhancements are limited (e.g. see section 1.1 of non-cryptographic enhancements are limited (e.g. see section 1.1 of
[37]). Applications typically utilize lower-layer (e.g. IPsec) and [9]). Applications typically utilize lower-layer (e.g. IPsec) and
upper-layer (e.g. TLS) protocols to provide security and privacy for upper-layer (e.g. TLS) protocols to provide security and privacy for
TCP connections and application data carried in TCP. Methods based TCP connections and application data carried in TCP. Methods based
on TCP options have been developed as well, to support some security on TCP options have been developed as well, to support some security
capabilities. capabilities.
In order to fully protect TCP connections (including their control In order to fully provide confidentiality, integrity protection, and
flags) IPsec or the TCP Authentication Option (TCP-AO) [36] are the authentication for TCP connections (including their control flags)
only current effective methods. Other methods discussed in this IPsec is the only current effective method. For integrity protection
section may protect the payload, but either only a subset of the and authentication, the TCP Authentication Option (TCP-AO) [39] is
fields (e.g. tcpcrypt [54]) or none at all (e.g. TLS). Other available, with a proposed extension to also provide confidentiality
security features that have been added to TCP (e.g. ISN generation, for the segment payload. Other methods discussed in this section may
sequence number checks, and others) are only capable of partially provide confidentiality or integrity protection for the payload, but
hindering attacks. for the TCP header only cover either a subset of the fields (e.g.
tcpcrypt [57]) or none at all (e.g. TLS). Other security features
that have been added to TCP (e.g. ISN generation, sequence number
checks, and others) are only capable of partially hindering attacks.
Applications using long-lived TCP flows have been vulnerable to Applications using long-lived TCP flows have been vulnerable to
attacks that exploit the processing of control flags described in attacks that exploit the processing of control flags described in
earlier TCP specifications [31]. TCP-MD5 was a commonly implemented earlier TCP specifications [34]. TCP-MD5 was a commonly implemented
TCP option to support authentication for some of these connections, TCP option to support authentication for some of these connections,
but had flaws and is now deprecated. TCP-AO provides a capability to but had flaws and is now deprecated. TCP-AO provides a capability to
protect long-lived TCP connections from attacks, and has superior protect long-lived TCP connections from attacks, and has superior
properties to TCP-MD5. It does not provide any privacy for properties to TCP-MD5. It does not provide any privacy for
application data, nor for the TCP headers. application data, nor for the TCP headers.
The "tcpcrypt" [54] Experimental extension to TCP provides the The "tcpcrypt" [57] Experimental extension to TCP provides the
ability to cryptographically protect connection data. Metadata ability to cryptographically protect connection data. Metadata
aspects of the TCP flow are still visible, but the application stream aspects of the TCP flow are still visible, but the application stream
is well-protected. Within the TCP header, only the urgent pointer is well-protected. Within the TCP header, only the urgent pointer
and FIN flag are protected through tcpcrypt. and FIN flag are protected through tcpcrypt.
The TCP Roadmap [48] includes notes about several RFCs related to TCP The TCP Roadmap [50] includes notes about several RFCs related to TCP
security. Many of the enhancements provided by these RFCs have been security. Many of the enhancements provided by these RFCs have been
integrated into the present document, including ISN generation, integrated into the present document, including ISN generation,
mitigating blind in-window attacks, and improving handling of soft mitigating blind in-window attacks, and improving handling of soft
errors and ICMP packets. These are all discussed in greater detail errors and ICMP packets. These are all discussed in greater detail
in the referenced RFCs that originally described the changes needed in the referenced RFCs that originally described the changes needed
to earlier TCP specifications. Additionally, see RFC 6093 [38] for to earlier TCP specifications. Additionally, see RFC 6093 [40] for
discussion of security considerations related to the urgent pointer discussion of security considerations related to the urgent pointer
field, that has been deprecated. field, that has been deprecated.
Since TCP is often used for bulk transfer flows, some attacks are Since TCP is often used for bulk transfer flows, some attacks are
possible that abuse the TCP congestion control logic. An example is possible that abuse the TCP congestion control logic. An example is
"ACK-division" attacks. Updates that have been made to the TCP "ACK-division" attacks. Updates that have been made to the TCP
congestion control specifications include mechanisms like Appropriate congestion control specifications include mechanisms like Appropriate
Byte Counting (ABC) [27] that act as mitigations to these attacks. Byte Counting (ABC) [30] that act as mitigations to these attacks.
Other attacks are focused on exhausting the resources of a TCP Other attacks are focused on exhausting the resources of a TCP
server. Examples include SYN flooding [30] or wasting resources on server. Examples include SYN flooding [33] or wasting resources on
non-progressing connections [40]. Operating systems commonly non-progressing connections [42]. Operating systems commonly
implement mitigations for these attacks. Some common defenses also implement mitigations for these attacks. Some common defenses also
utilize proxies, stateful firewalls, and other technologies outside utilize proxies, stateful firewalls, and other technologies outside
of the end-host TCP implementation. the end-host TCP implementation.
The concept of a protocol's "wire image" is described in RFC 8546
[56], which describes how TCP's cleartext headers expose more
metadata to nodes on the path than is strictly required to route the
packets to their destination. On-path adversaries may be able to
leverage this metadata. Lessons learned in this respect from TCP
have been applied in the design of newer transports like QUIC [60].
Additionally, based partly on experiences with TCP and its
extensions, there are considerations that might be applicable for
future TCP extensions and other transports that the IETF has
documented in RFC 9065 [61], along with IAB recommendations in RFC
8558 [58] and [68].
There are also methods of "fingerprinting" that can be used to infer
the host TCP implementation (operating system) version or platform
information. These collect observations of several aspects such as
the options present in segments, the ordering of options, the
specific behaviors in the case of various conditions, packet timing,
packet sizing, and other aspects of the protocol that are left to be
determined by an implementer, and can use those observations to
identify information about the host and implementation.
8. Acknowledgements 8. Acknowledgements
This document is largely a revision of RFC 793, which Jon Postel was This document is largely a revision of RFC 793, which Jon Postel was
the editor of. Due to his excellent work, it was able to last for the editor of. Due to his excellent work, it was able to last for
three decades before we felt the need to revise it. three decades before we felt the need to revise it.
Andre Oppermann was a contributor and helped to edit the first Andre Oppermann was a contributor and helped to edit the first
revision of this document. revision of this document.
skipping to change at page 95, line 20 skipping to change at page 99, line 46
the editor of. Due to his excellent work, it was able to last for the editor of. Due to his excellent work, it was able to last for
three decades before we felt the need to revise it. three decades before we felt the need to revise it.
Andre Oppermann was a contributor and helped to edit the first Andre Oppermann was a contributor and helped to edit the first
revision of this document. revision of this document.
We are thankful for the assistance of the IETF TCPM working group We are thankful for the assistance of the IETF TCPM working group
chairs, over the course of work on this document: chairs, over the course of work on this document:
Michael Scharf Michael Scharf
Yoshifumi Nishida Yoshifumi Nishida
Pasi Sarolahti Pasi Sarolahti
Michael Tuexen Michael Tuexen
During the discussions of this work on the TCPM mailing list and in During the discussions of this work on the TCPM mailing list, in
working group meetings, helpful comments, critiques, and reviews were working group meetings, and via area reviews, helpful comments,
received from (listed alphabetically by last name): Praveen critiques, and reviews were received from (listed alphabetically by
Balasubramanian, David Borman, Mohamed Boucadair, Bob Briscoe, Neal last name): Praveen Balasubramanian, David Borman, Mohamed Boucadair,
Cardwell, Yuchung Cheng, Martin Duke, Ted Faber, Gorry Fairhurst, Bob Briscoe, Neal Cardwell, Yuchung Cheng, Martin Duke, Francis
Fernando Gont, Rodney Grimes, Yi Huang, Rahul Jadhav, Markku Kojo, Dupont, Ted Faber, Gorry Fairhurst, Fernando Gont, Rodney Grimes, Yi
Mike Kosek, Juhamatti Kuusisaari, Kevin Lahey, Kevin Mason, Matt Huang, Rahul Jadhav, Markku Kojo, Mike Kosek, Juhamatti Kuusisaari,
Mathis, Stephen McQuistin, Jonathan Morton, Matt Olson, Tommy Pauly, Kevin Lahey, Kevin Mason, Matt Mathis, Stephen McQuistin, Jonathan
Tom Petch, Hagen Paul Pfeifer, Anthony Sabatini, Michael Scharf, Greg Morton, Matt Olson, Tommy Pauly, Tom Petch, Hagen Paul Pfeifer, Kyle
Skinner, Joe Touch, Michael Tuexen, Reji Varghese, Tim Wicinski, Rose, Anthony Sabatini, Michael Scharf, Greg Skinner, Joe Touch,
Lloyd Wood, and Alex Zimmermann. Michael Tuexen, Reji Varghese, Bernie Volz, Tim Wicinski, Lloyd Wood,
and Alex Zimmermann.
Joe Touch provided additional help in clarifying the description of Joe Touch provided additional help in clarifying the description of
segment size parameters and PMTUD/PLPMTUD recommendations. Markku segment size parameters and PMTUD/PLPMTUD recommendations. Markku
Kojo helped put together the text in the section on TCP Congestion Kojo helped put together the text in the section on TCP Congestion
Control. Control.
This document includes content from errata that were reported by This document includes content from errata that were reported by
(listed chronologically): Yin Shuming, Bob Braden, Morris M. Keesan, (listed chronologically): Yin Shuming, Bob Braden, Morris M. Keesan,
Pei-chun Cheng, Constantin Hagemeier, Vishwas Manral, Mykyta Pei-chun Cheng, Constantin Hagemeier, Vishwas Manral, Mykyta
Yevstifeyev, EungJun Yi, Botong Huang, Charles Deng, Merlin Buge. Yevstifeyev, EungJun Yi, Botong Huang, Charles Deng, Merlin Buge.
skipping to change at page 96, line 25 skipping to change at page 101, line 5
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[4] Nichols, K., Blake, S., Baker, F., and D. Black, [4] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS "Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474, Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998, DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>. <https://www.rfc-editor.org/info/rfc2474>.
[5] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms", [5] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2675, DOI 10.17487/RFC2675, August 1999,
<https://www.rfc-editor.org/info/rfc2675>.
[6] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000, RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/rfc2914>. <https://www.rfc-editor.org/info/rfc2914>.
[7] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition [6] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001, RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>. <https://www.rfc-editor.org/info/rfc3168>.
[8] Floyd, S. and M. Allman, "Specifying New Congestion [7] Floyd, S. and M. Allman, "Specifying New Congestion
Control Algorithms", BCP 133, RFC 5033, Control Algorithms", BCP 133, RFC 5033,
DOI 10.17487/RFC5033, August 2007, DOI 10.17487/RFC5033, August 2007,
<https://www.rfc-editor.org/info/rfc5033>. <https://www.rfc-editor.org/info/rfc5033>.
[9] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion [8] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009, Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>. <https://www.rfc-editor.org/info/rfc5681>.
[9] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
Robustness to Blind In-Window Attacks", RFC 5961,
DOI 10.17487/RFC5961, August 2010,
<https://www.rfc-editor.org/info/rfc5961>.
[10] Paxson, V., Allman, M., Chu, J., and M. Sargent, [10] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298, "Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011, DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/info/rfc6298>. <https://www.rfc-editor.org/info/rfc6298>.
[11] Gont, F., "Deprecation of ICMP Source Quench Messages", [11] Gont, F., "Deprecation of ICMP Source Quench Messages",
RFC 6633, DOI 10.17487/RFC6633, May 2012, RFC 6633, DOI 10.17487/RFC6633, May 2012,
<https://www.rfc-editor.org/info/rfc6633>. <https://www.rfc-editor.org/info/rfc6633>.
[12] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [12] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
skipping to change at page 97, line 33 skipping to change at page 102, line 15
[15] Allman, M., "Requirements for Time-Based Loss Detection", [15] Allman, M., "Requirements for Time-Based Loss Detection",
BCP 233, RFC 8961, DOI 10.17487/RFC8961, November 2020, BCP 233, RFC 8961, DOI 10.17487/RFC8961, November 2020,
<https://www.rfc-editor.org/info/rfc8961>. <https://www.rfc-editor.org/info/rfc8961>.
9.2. Informative References 9.2. Informative References
[16] Postel, J., "Transmission Control Protocol", STD 7, [16] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981, RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>. <https://www.rfc-editor.org/info/rfc793>.
[17] Nagle, J., "Congestion Control in IP/TCP Internetworks", [17] Postel, J., "The TCP Maximum Segment Size and Related
Topics", RFC 879, DOI 10.17487/RFC0879, November 1983,
<https://www.rfc-editor.org/info/rfc879>.
[18] Nagle, J., "Congestion Control in IP/TCP Internetworks",
RFC 896, DOI 10.17487/RFC0896, January 1984, RFC 896, DOI 10.17487/RFC0896, January 1984,
<https://www.rfc-editor.org/info/rfc896>. <https://www.rfc-editor.org/info/rfc896>.
[18] Braden, R., Ed., "Requirements for Internet Hosts - [19] Reynolds, J. and J. Postel, "Official Internet protocols",
RFC 1011, DOI 10.17487/RFC1011, May 1987,
<https://www.rfc-editor.org/info/rfc1011>.
[20] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989, DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>. <https://www.rfc-editor.org/info/rfc1122>.
[19] Almquist, P., "Type of Service in the Internet Protocol [21] Almquist, P., "Type of Service in the Internet Protocol
Suite", RFC 1349, DOI 10.17487/RFC1349, July 1992, Suite", RFC 1349, DOI 10.17487/RFC1349, July 1992,
<https://www.rfc-editor.org/info/rfc1349>. <https://www.rfc-editor.org/info/rfc1349>.
[20] Braden, R., "T/TCP -- TCP Extensions for Transactions [22] Braden, R., "T/TCP -- TCP Extensions for Transactions
Functional Specification", RFC 1644, DOI 10.17487/RFC1644, Functional Specification", RFC 1644, DOI 10.17487/RFC1644,
July 1994, <https://www.rfc-editor.org/info/rfc1644>. July 1994, <https://www.rfc-editor.org/info/rfc1644>.
[21] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP [23] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, Selective Acknowledgment Options", RFC 2018,
DOI 10.17487/RFC2018, October 1996, DOI 10.17487/RFC2018, October 1996,
<https://www.rfc-editor.org/info/rfc2018>. <https://www.rfc-editor.org/info/rfc2018>.
[22] Paxson, V., Allman, M., Dawson, S., Fenner, W., Griner, [24] Paxson, V., Allman, M., Dawson, S., Fenner, W., Griner,
J., Heavens, I., Lahey, K., Semke, J., and B. Volz, "Known J., Heavens, I., Lahey, K., Semke, J., and B. Volz, "Known
TCP Implementation Problems", RFC 2525, TCP Implementation Problems", RFC 2525,
DOI 10.17487/RFC2525, March 1999, DOI 10.17487/RFC2525, March 1999,
<https://www.rfc-editor.org/info/rfc2525>. <https://www.rfc-editor.org/info/rfc2525>.
[23] Xiao, X., Hannan, A., Paxson, V., and E. Crabbe, "TCP [25] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, DOI 10.17487/RFC2675, August 1999,
<https://www.rfc-editor.org/info/rfc2675>.
[26] Xiao, X., Hannan, A., Paxson, V., and E. Crabbe, "TCP
Processing of the IPv4 Precedence Field", RFC 2873, Processing of the IPv4 Precedence Field", RFC 2873,
DOI 10.17487/RFC2873, June 2000, DOI 10.17487/RFC2873, June 2000,
<https://www.rfc-editor.org/info/rfc2873>. <https://www.rfc-editor.org/info/rfc2873>.
[24] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An [27] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
Extension to the Selective Acknowledgement (SACK) Option Extension to the Selective Acknowledgement (SACK) Option
for TCP", RFC 2883, DOI 10.17487/RFC2883, July 2000, for TCP", RFC 2883, DOI 10.17487/RFC2883, July 2000,
<https://www.rfc-editor.org/info/rfc2883>. <https://www.rfc-editor.org/info/rfc2883>.
[25] Lahey, K., "TCP Problems with Path MTU Discovery", [28] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, DOI 10.17487/RFC2923, September 2000, RFC 2923, DOI 10.17487/RFC2923, September 2000,
<https://www.rfc-editor.org/info/rfc2923>. <https://www.rfc-editor.org/info/rfc2923>.
[26] Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M. [29] Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
Sooriyabandara, "TCP Performance Implications of Network Sooriyabandara, "TCP Performance Implications of Network
Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449, Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
December 2002, <https://www.rfc-editor.org/info/rfc3449>. December 2002, <https://www.rfc-editor.org/info/rfc3449>.
[27] Allman, M., "TCP Congestion Control with Appropriate Byte [30] Allman, M., "TCP Congestion Control with Appropriate Byte
Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
2003, <https://www.rfc-editor.org/info/rfc3465>. 2003, <https://www.rfc-editor.org/info/rfc3465>.
[28] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4, [31] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727, ICMPv6, UDP, and TCP Headers", RFC 4727,
DOI 10.17487/RFC4727, November 2006, DOI 10.17487/RFC4727, November 2006,
<https://www.rfc-editor.org/info/rfc4727>. <https://www.rfc-editor.org/info/rfc4727>.
[29] Mathis, M. and J. Heffner, "Packetization Layer Path MTU [32] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>. <https://www.rfc-editor.org/info/rfc4821>.
[30] Eddy, W., "TCP SYN Flooding Attacks and Common [33] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007, Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<https://www.rfc-editor.org/info/rfc4987>. <https://www.rfc-editor.org/info/rfc4987>.
[31] Touch, J., "Defending TCP Against Spoofing Attacks", [34] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, DOI 10.17487/RFC4953, July 2007, RFC 4953, DOI 10.17487/RFC4953, July 2007,
<https://www.rfc-editor.org/info/rfc4953>. <https://www.rfc-editor.org/info/rfc4953>.
[32] Culley, P., Elzur, U., Recio, R., Bailey, S., and J. [35] Culley, P., Elzur, U., Recio, R., Bailey, S., and J.
Carrier, "Marker PDU Aligned Framing for TCP Carrier, "Marker PDU Aligned Framing for TCP
Specification", RFC 5044, DOI 10.17487/RFC5044, October Specification", RFC 5044, DOI 10.17487/RFC5044, October
2007, <https://www.rfc-editor.org/info/rfc5044>. 2007, <https://www.rfc-editor.org/info/rfc5044>.
[33] Gont, F., "TCP's Reaction to Soft Errors", RFC 5461, [36] Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
DOI 10.17487/RFC5461, February 2009, DOI 10.17487/RFC5461, February 2009,
<https://www.rfc-editor.org/info/rfc5461>. <https://www.rfc-editor.org/info/rfc5461>.
[34] StJohns, M., Atkinson, R., and G. Thomas, "Common [37] StJohns, M., Atkinson, R., and G. Thomas, "Common
Architecture Label IPv6 Security Option (CALIPSO)", Architecture Label IPv6 Security Option (CALIPSO)",
RFC 5570, DOI 10.17487/RFC5570, July 2009, RFC 5570, DOI 10.17487/RFC5570, July 2009,
<https://www.rfc-editor.org/info/rfc5570>. <https://www.rfc-editor.org/info/rfc5570>.
[35] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust [38] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795, Header Compression (ROHC) Framework", RFC 5795,
DOI 10.17487/RFC5795, March 2010, DOI 10.17487/RFC5795, March 2010,
<https://www.rfc-editor.org/info/rfc5795>. <https://www.rfc-editor.org/info/rfc5795>.
[36] Touch, J., Mankin, A., and R. Bonica, "The TCP [39] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925, Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>. June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[37] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's [40] Gont, F. and A. Yourtchenko, "On the Implementation of the
Robustness to Blind In-Window Attacks", RFC 5961,
DOI 10.17487/RFC5961, August 2010,
<https://www.rfc-editor.org/info/rfc5961>.
[38] Gont, F. and A. Yourtchenko, "On the Implementation of the
TCP Urgent Mechanism", RFC 6093, DOI 10.17487/RFC6093, TCP Urgent Mechanism", RFC 6093, DOI 10.17487/RFC6093,
January 2011, <https://www.rfc-editor.org/info/rfc6093>. January 2011, <https://www.rfc-editor.org/info/rfc6093>.
[39] Gont, F., "Reducing the TIME-WAIT State Using TCP [41] Gont, F., "Reducing the TIME-WAIT State Using TCP
Timestamps", BCP 159, RFC 6191, DOI 10.17487/RFC6191, Timestamps", BCP 159, RFC 6191, DOI 10.17487/RFC6191,
April 2011, <https://www.rfc-editor.org/info/rfc6191>. April 2011, <https://www.rfc-editor.org/info/rfc6191>.
[40] Bashyam, M., Jethanandani, M., and A. Ramaiah, "TCP Sender [42] Bashyam, M., Jethanandani, M., and A. Ramaiah, "TCP Sender
Clarification for Persist Condition", RFC 6429, Clarification for Persist Condition", RFC 6429,
DOI 10.17487/RFC6429, December 2011, DOI 10.17487/RFC6429, December 2011,
<https://www.rfc-editor.org/info/rfc6429>. <https://www.rfc-editor.org/info/rfc6429>.
[41] Gont, F. and S. Bellovin, "Defending against Sequence [43] Gont, F. and S. Bellovin, "Defending against Sequence
Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February
2012, <https://www.rfc-editor.org/info/rfc6528>. 2012, <https://www.rfc-editor.org/info/rfc6528>.
[42] Borman, D., "TCP Options and Maximum Segment Size (MSS)", [44] Borman, D., "TCP Options and Maximum Segment Size (MSS)",
RFC 6691, DOI 10.17487/RFC6691, July 2012, RFC 6691, DOI 10.17487/RFC6691, July 2012,
<https://www.rfc-editor.org/info/rfc6691>. <https://www.rfc-editor.org/info/rfc6691>.
[43] Touch, J., "Updated Specification of the IPv4 ID Field", [45] Touch, J., "Updated Specification of the IPv4 ID Field",
RFC 6864, DOI 10.17487/RFC6864, February 2013, RFC 6864, DOI 10.17487/RFC6864, February 2013,
<https://www.rfc-editor.org/info/rfc6864>. <https://www.rfc-editor.org/info/rfc6864>.
[44] Touch, J., "Shared Use of Experimental TCP Options", [46] Touch, J., "Shared Use of Experimental TCP Options",
RFC 6994, DOI 10.17487/RFC6994, August 2013, RFC 6994, DOI 10.17487/RFC6994, August 2013,
<https://www.rfc-editor.org/info/rfc6994>. <https://www.rfc-editor.org/info/rfc6994>.
[45] McPherson, D., Oran, D., Thaler, D., and E. Osterweil, [47] McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
"Architectural Considerations of IP Anycast", RFC 7094, "Architectural Considerations of IP Anycast", RFC 7094,
DOI 10.17487/RFC7094, January 2014, DOI 10.17487/RFC7094, January 2014,
<https://www.rfc-editor.org/info/rfc7094>. <https://www.rfc-editor.org/info/rfc7094>.
[46] Borman, D., Braden, B., Jacobson, V., and R. [48] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance", Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014, RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/info/rfc7323>. <https://www.rfc-editor.org/info/rfc7323>.
[47] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP [49] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>. <https://www.rfc-editor.org/info/rfc7413>.
[48] Duke, M., Braden, R., Eddy, W., Blanton, E., and A. [50] Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
Zimmermann, "A Roadmap for Transmission Control Protocol Zimmermann, "A Roadmap for Transmission Control Protocol
(TCP) Specification Documents", RFC 7414, (TCP) Specification Documents", RFC 7414,
DOI 10.17487/RFC7414, February 2015, DOI 10.17487/RFC7414, February 2015,
<https://www.rfc-editor.org/info/rfc7414>. <https://www.rfc-editor.org/info/rfc7414>.
[49] Black, D., Ed. and P. Jones, "Differentiated Services [51] Black, D., Ed. and P. Jones, "Differentiated Services
(Diffserv) and Real-Time Communication", RFC 7657, (Diffserv) and Real-Time Communication", RFC 7657,
DOI 10.17487/RFC7657, November 2015, DOI 10.17487/RFC7657, November 2015,
<https://www.rfc-editor.org/info/rfc7657>. <https://www.rfc-editor.org/info/rfc7657>.
[50] Fairhurst, G. and M. Welzl, "The Benefits of Using [52] Fairhurst, G. and M. Welzl, "The Benefits of Using
Explicit Congestion Notification (ECN)", RFC 8087, Explicit Congestion Notification (ECN)", RFC 8087,
DOI 10.17487/RFC8087, March 2017, DOI 10.17487/RFC8087, March 2017,
<https://www.rfc-editor.org/info/rfc8087>. <https://www.rfc-editor.org/info/rfc8087>.
[51] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind, [53] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
Ed., "Services Provided by IETF Transport Protocols and Ed., "Services Provided by IETF Transport Protocols and
Congestion Control Mechanisms", RFC 8095, Congestion Control Mechanisms", RFC 8095,
DOI 10.17487/RFC8095, March 2017, DOI 10.17487/RFC8095, March 2017,
<https://www.rfc-editor.org/info/rfc8095>. <https://www.rfc-editor.org/info/rfc8095>.
[52] Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of [54] Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of
Transport Features Provided by IETF Transport Protocols", Transport Features Provided by IETF Transport Protocols",
RFC 8303, DOI 10.17487/RFC8303, February 2018, RFC 8303, DOI 10.17487/RFC8303, February 2018,
<https://www.rfc-editor.org/info/rfc8303>. <https://www.rfc-editor.org/info/rfc8303>.
[53] Chown, T., Loughney, J., and T. Winters, "IPv6 Node [55] Chown, T., Loughney, J., and T. Winters, "IPv6 Node
Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504, Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
January 2019, <https://www.rfc-editor.org/info/rfc8504>. January 2019, <https://www.rfc-editor.org/info/rfc8504>.
[54] Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack, [56] Trammell, B. and M. Kuehlewind, "The Wire Image of a
Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
2019, <https://www.rfc-editor.org/info/rfc8546>.
[57] Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack,
Q., and E. Smith, "Cryptographic Protection of TCP Streams Q., and E. Smith, "Cryptographic Protection of TCP Streams
(tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019, (tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019,
<https://www.rfc-editor.org/info/rfc8548>. <https://www.rfc-editor.org/info/rfc8548>.
[55] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C. [58] Hardie, T., Ed., "Transport Protocol Path Signals",
RFC 8558, DOI 10.17487/RFC8558, April 2019,
<https://www.rfc-editor.org/info/rfc8558>.
[59] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
2020, <https://www.rfc-editor.org/info/rfc8684>. 2020, <https://www.rfc-editor.org/info/rfc8684>.
[56] IANA, "Transmission Control Protocol (TCP) Parameters, [60] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
[61] Fairhurst, G. and C. Perkins, "Considerations around
Transport Header Confidentiality, Network Operations, and
the Evolution of Internet Transport Protocols", RFC 9065,
DOI 10.17487/RFC9065, July 2021,
<https://www.rfc-editor.org/info/rfc9065>.
[62] IANA, "Transmission Control Protocol (TCP) Parameters,
https://www.iana.org/assignments/tcp-parameters/tcp- https://www.iana.org/assignments/tcp-parameters/tcp-
parameters.xhtml", 2019. parameters.xhtml", 2019.
[57] IANA, "Transmission Control Protocol (TCP) Header Flags, [63] IANA, "Transmission Control Protocol (TCP) Header Flags,
https://www.iana.org/assignments/tcp-header-flags/tcp- https://www.iana.org/assignments/tcp-header-flags/tcp-
header-flags.xhtml", 2019. header-flags.xhtml", 2019.
[58] Gont, F., "Processing of IP Security/Compartment and [64] Gont, F., "Processing of IP Security/Compartment and
Precedence Information by TCP", draft-gont-tcpm-tcp- Precedence Information by TCP", Work in Progress,
seccomp-prec-00 (work in progress), March 2012. Internet-Draft, draft-gont-tcpm-tcp-seccomp-prec-00, 29
March 2012, <http://www.ietf.org/internet-drafts/draft-
gont-tcpm-tcp-seccomp-prec-00.txt>.
[59] Gont, F. and D. Borman, "On the Validation of TCP Sequence [65] Gont, F. and D. Borman, "On the Validation of TCP Sequence
Numbers", draft-gont-tcpm-tcp-seq-validation-04 (work in Numbers", Work in Progress, Internet-Draft, draft-gont-
progress), March 2019. tcpm-tcp-seq-validation-04, 11 March 2019,
<http://www.ietf.org/internet-drafts/draft-gont-tcpm-tcp-
seq-validation-04.txt>.
[60] Touch, J. and W. Eddy, "TCP Extended Data Offset Option", [66] Touch, J. and W. Eddy, "TCP Extended Data Offset Option",
draft-ietf-tcpm-tcp-edo-10 (work in progress), July 2018. Work in Progress, Internet-Draft, draft-ietf-tcpm-tcp-edo-
10, 19 July 2018, <http://www.ietf.org/internet-drafts/
draft-ietf-tcpm-tcp-edo-10.txt>.
[61] McQuistin, S., Band, V., Jacob, D., and C. Perkins, [67] McQuistin, S., Band, V., Jacob, D., and C. Perkins,
"Describing Protocol Data Units with Augmented Packet "Describing Protocol Data Units with Augmented Packet
Header Diagrams", draft-mcquistin-augmented-ascii- Header Diagrams", Work in Progress, Internet-Draft, draft-
diagrams-08 (work in progress), May 2021. mcquistin-augmented-ascii-diagrams-08, 5 May 2021,
<https://www.ietf.org/archive/id/draft-mcquistin-
augmented-ascii-diagrams-08.txt>.
[62] Minshall, G., "A Proposed Modification to Nagle's [68] Thomson, M. and T. Pauly, "Long-term Viability of Protocol
Algorithm", draft-minshall-nagle-01 (work in progress), Extension Mechanisms", Work in Progress, Internet-Draft,
June 1999. draft-iab-use-it-or-lose-it-02, 23 August 2021,
<https://www.ietf.org/archive/id/draft-iab-use-it-or-lose-
it-02.txt>.
[63] Dalal, Y. and C. Sunshine, "Connection Management in [69] Minshall, G., "A Proposed Modification to Nagle's
Algorithm", Work in Progress, Internet-Draft, draft-
minshall-nagle-01, June 1999,
<https://datatracker.ietf.org/doc/html/draft-minshall-
nagle-01>.
[70] Dalal, Y. and C. Sunshine, "Connection Management in
Transport Protocols", Computer Networks Vol. 2, No. 6, pp. Transport Protocols", Computer Networks Vol. 2, No. 6, pp.
454-473, December 1978. 454-473, December 1978.
[64] Faber, T., Touch, J., and W. Yui, "The TIME-WAIT state in [71] Faber, T., Touch, J., and W. Yui, "The TIME-WAIT state in
TCP and Its Effect on Busy Servers", Proceedings of IEEE TCP and Its Effect on Busy Servers", Proceedings of IEEE
INFOCOM pp. 1573-1583, March 1999. INFOCOM pp. 1573-1583, March 1999.
[72] Postel, J., "Comments on Action Items from the January
Meeting", IEN 177, March 1981,
<https://www.rfc-editor.org/ien/ien177.txt>.
[73] "Segmentation Offloads", Linux Networking Documentation ,
<https://www.kernel.org/doc/html/latest/networking/
segmentation-offloads.html>.
Appendix A. Other Implementation Notes Appendix A. Other Implementation Notes
This section includes additional notes and references on TCP This section includes additional notes and references on TCP
implementation decisions that are currently not a part of the RFC implementation decisions that are currently not a part of the RFC
series or included within the TCP standard. These items can be series or included within the TCP standard. These items can be
considered by implementers, but there was not yet a consensus to considered by implementers, but there was not yet a consensus to
include them in the standard. include them in the standard.
A.1. IP Security Compartment and Precedence A.1. IP Security Compartment and Precedence
The IPv4 specification [1] includes a precedence value in the (now The IPv4 specification [1] includes a precedence value in the (now
obsoleted) Type of Service field (TOS) field. It was modified in obsoleted) Type of Service field (TOS) field. It was modified in
[19], and then obsoleted by the definition of Differentiated Services [21], and then obsoleted by the definition of Differentiated Services
(DiffServ) [4]. Setting and conveying TOS between the network layer, (DiffServ) [4]. Setting and conveying TOS between the network layer,
TCP implementation, and applications is obsolete, and replaced by TCP implementation, and applications is obsolete, and replaced by
DiffServ in the current TCP specification. DiffServ in the current TCP specification.
RFC 793 requires checking the IP security compartment and precedence RFC 793 required checking the IP security compartment and precedence
on incoming TCP segments for consistency within a connection, and on incoming TCP segments for consistency within a connection, and
with application requests. Each of these aspects of IP have become with application requests. Each of these aspects of IP have become
outdated, without specific updates to RFC 793. The issues with outdated, without specific updates to RFC 793. The issues with
precedence were fixed by [23], which is Standards Track, and so this precedence were fixed by [26], which is Standards Track, and so this
present TCP specification includes those changes. However, the state present TCP specification includes those changes. However, the state
of IP security options that may be used by MLS systems is not as of IP security options that may be used by MLS systems is not as
clean. apparent in the IETF currently.
Resetting connections when incoming packets do not meet expected Resetting connections when incoming packets do not meet expected
security compartment or precedence expectations has been recognized security compartment or precedence expectations has been recognized
as a possible attack vector [58], and there has been discussion about as a possible attack vector [64], and there has been discussion about
amending the TCP specification to prevent connections from being amending the TCP specification to prevent connections from being
aborted due to non-matching IP security compartment and DiffServ aborted due to non-matching IP security compartment and DiffServ
codepoint values. codepoint values.
A.1.1. Precedence A.1.1. Precedence
In DiffServ the former precedence values are treated as Class In DiffServ the former precedence values are treated as Class
Selector codepoints, and methods for compatible treatment are Selector codepoints, and methods for compatible treatment are
described in the DiffServ architecture. The RFC 793/1122 TCP described in the DiffServ architecture. The RFC 793/1122 TCP
specification includes logic intending to have connections use the specification includes logic intending to have connections use the
highest precedence requested by either endpoint application, and to highest precedence requested by either endpoint application, and to
keep the precedence consistent throughout a connection. This logic keep the precedence consistent throughout a connection. This logic
from the obsolete TOS is not applicable for DiffServ, and should not from the obsolete TOS is not applicable for DiffServ, and should not
be included in TCP implementations, though changes to DiffServ values be included in TCP implementations, though changes to DiffServ values
within a connection are discouraged. For discussion of this, see RFC within a connection are discouraged. For discussion of this, see RFC
7657 (sec 5.1, 5.3, and 6) [49]. 7657 (sec 5.1, 5.3, and 6) [51].
The obsoleted TOS processing rules in TCP assumed bidirectional (or The obsoleted TOS processing rules in TCP assumed bidirectional (or
symmetric) precedence values used on a connection, but the DiffServ symmetric) precedence values used on a connection, but the DiffServ
architecture is asymmetric. Problems with the old TCP logic in this architecture is asymmetric. Problems with the old TCP logic in this
regard were described in [23] and the solution described is to ignore regard were described in [26] and the solution described is to ignore
IP precedence in TCP. Since RFC 2873 is a Standards Track document IP precedence in TCP. Since RFC 2873 is a Standards Track document
(although not marked as updating RFC 793), current implementations (although not marked as updating RFC 793), current implementations
are expected to be robust to these conditions. Note that the are expected to be robust to these conditions. Note that the
DiffServ field value used in each direction is a part of the DiffServ field value used in each direction is a part of the
interface between TCP and the network layer, and values in use can be interface between TCP and the network layer, and values in use can be
indicated both ways between TCP and the application. indicated both ways between TCP and the application.
A.1.2. MLS Systems A.1.2. MLS Systems
The IP security option (IPSO) and compartment defined in [1] was The IP security option (IPSO) and compartment defined in [1] was
refined in RFC 1038 that was later obsoleted by RFC 1108. The refined in RFC 1038 that was later obsoleted by RFC 1108. The
Commercial IP Security Option (CIPSO) is defined in FIPS-188, and is Commercial IP Security Option (CIPSO) is defined in FIPS-188
supported by some vendors and operating systems. RFC 1108 is now (withdrawn by NIST in 2015), and is supported by some vendors and
Historic, though RFC 791 itself has not been updated to remove the IP operating systems. RFC 1108 is now Historic, though RFC 791 itself
security option. For IPv6, a similar option (CALIPSO) has been has not been updated to remove the IP security option. For IPv6, a
defined [34]. RFC 793 includes logic that includes the IP security/ similar option (CALIPSO) has been defined [37]. RFC 793 includes
compartment information in treatment of TCP segments. References to logic that includes the IP security/compartment information in
the IP "security/compartment" in this document may be relevant for treatment of TCP segments. References to the IP "security/
Multi-Level Secure (MLS) system implementers, but can be ignored for compartment" in this document may be relevant for Multi-Level Secure
non-MLS implementations, consistent with running code on the (MLS) system implementers, but can be ignored for non-MLS
Internet. See Appendix A.1 for further discussion. Note that RFC implementations, consistent with running code on the Internet. See
5570 describes some MLS networking scenarios where IPSO, CIPSO, or Appendix A.1 for further discussion. Note that RFC 5570 describes
CALIPSO may be used. In these special cases, TCP implementers should some MLS networking scenarios where IPSO, CIPSO, or CALIPSO may be
see section 7.3.1 of RFC 5570, and follow the guidance in that used. In these special cases, TCP implementers should see section
document. 7.3.1 of RFC 5570, and follow the guidance in that document.
A.2. Sequence Number Validation A.2. Sequence Number Validation
There are cases where the TCP sequence number validation rules can There are cases where the TCP sequence number validation rules can
prevent ACK fields from being processed. This can result in prevent ACK fields from being processed. This can result in
connection issues, as described in [59], which includes descriptions connection issues, as described in [65], which includes descriptions
of potential problems in conditions of simultaneous open, self- of potential problems in conditions of simultaneous open, self-
connects, simultaneous close, and simultaneous window probes. The connects, simultaneous close, and simultaneous window probes. The
document also describes potential changes to the TCP specification to document also describes potential changes to the TCP specification to
mitigate the issue by expanding the acceptable sequence numbers. mitigate the issue by expanding the acceptable sequence numbers.
In Internet usage of TCP, these conditions are rarely occurring. In Internet usage of TCP, these conditions are rarely occurring.
Common operating systems include different alternative mitigations, Common operating systems include different alternative mitigations,
and the standard has not been updated yet to codify one of them, but and the standard has not been updated yet to codify one of them, but
implementers should consider the problems described in [59]. implementers should consider the problems described in [65].
A.3. Nagle Modification A.3. Nagle Modification
In common operating systems, both the Nagle algorithm and delayed In common operating systems, both the Nagle algorithm and delayed
acknowledgements are implemented and enabled by default. TCP is used acknowledgements are implemented and enabled by default. TCP is used
by many applications that have a request-response style of by many applications that have a request-response style of
communication, where the combination of the Nagle algorithm and communication, where the combination of the Nagle algorithm and
delayed acknowledgements can result in poor application performance. delayed acknowledgements can result in poor application performance.
A modification to the Nagle algorithm is described in [62] that A modification to the Nagle algorithm is described in [69] that
improves the situation for these applications. improves the situation for these applications.
This modification is implemented in some common operating systems, This modification is implemented in some common operating systems,
and does not impact TCP interoperability. Additionally, many and does not impact TCP interoperability. Additionally, many
applications simply disable Nagle, since this is generally supported applications simply disable Nagle, since this is generally supported
by a socket option. The TCP standard has not been updated to include by a socket option. The TCP standard has not been updated to include
this Nagle modification, but implementers may find it beneficial to this Nagle modification, but implementers may find it beneficial to
consider. consider.
A.4. Low Water Mark Settings A.4. Low Watermark Settings
Some operating system kernel TCP implementations include socket Some operating system kernel TCP implementations include socket
options that allow specifying the number of bytes in the buffer until options that allow specifying the number of bytes in the buffer until
the socket layer will pass sent data to TCP (SO_SNDLOWAT) or to the the socket layer will pass sent data to TCP (SO_SNDLOWAT) or to the
application on receiving (SO_RCVLOWAT). application on receiving (SO_RCVLOWAT).
In addition, another socket option (TCP_NOTSENT_LOWAT) can be used to In addition, another socket option (TCP_NOTSENT_LOWAT) can be used to
control the amount of unsent bytes in the write queue. This can help control the amount of unsent bytes in the write queue. This can help
a sending TCP application to avoid creating large amounts of buffered a sending TCP application to avoid creating large amounts of buffered
data (and corresponding latency). As an example, this may be useful data (and corresponding latency). As an example, this may be useful
skipping to change at page 106, line 14 skipping to change at page 111, line 48
Implement sending & receiving MSS option | MUST-14|x| | | | | Implement sending & receiving MSS option | MUST-14|x| | | | |
IPv4 Send MSS option unless 536 | SHLD-5 | |x| | | | IPv4 Send MSS option unless 536 | SHLD-5 | |x| | | |
IPv6 Send MSS option unless 1220 | SHLD-5 | |x| | | | IPv6 Send MSS option unless 1220 | SHLD-5 | |x| | | |
Send MSS option always | MAY-3 | | |x| | | Send MSS option always | MAY-3 | | |x| | |
IPv4 Send-MSS default is 536 | MUST-15|x| | | | | IPv4 Send-MSS default is 536 | MUST-15|x| | | | |
IPv6 Send-MSS default is 1220 | MUST-15|x| | | | | IPv6 Send-MSS default is 1220 | MUST-15|x| | | | |
Calculate effective send seg size | MUST-16|x| | | | | Calculate effective send seg size | MUST-16|x| | | | |
MSS accounts for varying MTU | SHLD-6 | |x| | | | MSS accounts for varying MTU | SHLD-6 | |x| | | |
MSS not sent on non-SYN segments | MUST-65| | | | |x| MSS not sent on non-SYN segments | MUST-65| | | | |x|
MSS value based on MMS_R | MUST-67|x| | | | | MSS value based on MMS_R | MUST-67|x| | | | |
Pad with zero | MUST-69|x| | | | |
| | | | | | | | | | | | | |
TCP Checksums | | | | | | | TCP Checksums | | | | | | |
Sender compute checksum | MUST-2 |x| | | | | Sender compute checksum | MUST-2 |x| | | | |
Receiver check checksum | MUST-3 |x| | | | | Receiver check checksum | MUST-3 |x| | | | |
| | | | | | | | | | | | | |
ISN Selection | | | | | | | ISN Selection | | | | | | |
Include a clock-driven ISN generator component | MUST-8 |x| | | | | Include a clock-driven ISN generator component | MUST-8 |x| | | | |
Secure ISN generator with a PRF component | SHLD-1 | |x| | | | Secure ISN generator with a PRF component | SHLD-1 | |x| | | |
PRF computable from outside the host | MUST-9 | | | | |x| PRF computable from outside the host | MUST-9 | | | | |x|
| | | | | | | | | | | | | |
skipping to change at page 107, line 8 skipping to change at page 112, line 43
Retransmit with same IP ident | MAY-4 | | |x| | | Retransmit with same IP ident | MAY-4 | | |x| | |
Karn's algorithm | MUST-18|x| | | | | Karn's algorithm | MUST-18|x| | | | |
| | | | | | | | | | | | | |
Generating ACKs: | | | | | | | Generating ACKs: | | | | | | |
Aggregate whenever possible | MUST-58|x| | | | | Aggregate whenever possible | MUST-58|x| | | | |
Queue out-of-order segments | SHLD-31| |x| | | | Queue out-of-order segments | SHLD-31| |x| | | |
Process all Q'd before send ACK | MUST-59|x| | | | | Process all Q'd before send ACK | MUST-59|x| | | | |
Send ACK for out-of-order segment | MAY-13 | | |x| | | Send ACK for out-of-order segment | MAY-13 | | |x| | |
Delayed ACKs | SHLD-18| |x| | | | Delayed ACKs | SHLD-18| |x| | | |
Delay < 0.5 seconds | MUST-40|x| | | | | Delay < 0.5 seconds | MUST-40|x| | | | |
Every 2nd full-sized segment or 2*RMSS ACK'd | SHLD-19|x| | | | | Every 2nd full-sized segment or 2*RMSS ACK'd | SHLD-19| |x| | | |
Receiver SWS-Avoidance Algorithm | MUST-39|x| | | | | Receiver SWS-Avoidance Algorithm | MUST-39|x| | | | |
| | | | | | | | | | | | | |
Sending data | | | | | | | Sending data | | | | | | |
Configurable TTL | MUST-49|x| | | | | Configurable TTL | MUST-49|x| | | | |
Sender SWS-Avoidance Algorithm | MUST-38|x| | | | | Sender SWS-Avoidance Algorithm | MUST-38|x| | | | |
Nagle algorithm | SHLD-7 | |x| | | | Nagle algorithm | SHLD-7 | |x| | | |
Application can disable Nagle algorithm | MUST-17|x| | | | | Application can disable Nagle algorithm | MUST-17|x| | | | |
| | | | | | | | | | | | | |
Connection Failures: | | | | | | | Connection Failures: | | | | | | |
Negative advice to IP on R1 retxs | MUST-20|x| | | | | Negative advice to IP on R1 retxs | MUST-20|x| | | | |
Close connection on R2 retxs | MUST-20|x| | | | | Close connection on R2 retxs | MUST-20|x| | | | |
ALP can set R2 | MUST-21|x| | | | |1 ALP can set R2 | MUST-21|x| | | | |1
Inform ALP of R1<=retxs<R2 | SHLD-9 | |x| | | |1 Inform ALP of R1<=retxs<R2 | SHLD-9 | |x| | | |1
Recommended value for R1 | SHLD-10| |x| | | | Recommended value for R1 | SHLD-10| |x| | | |
Recommended value for R2 | SHLD-11| |x| | | | Recommended value for R2 | SHLD-11| |x| | | |
Same mechanism for SYNs | MUST-22|x| | | | | Same mechanism for SYNs | MUST-22|x| | | | |
R2 at least 3 minutes for SYN | MUST-23|x| | | | | R2 at least 3 minutes for SYN | MUST-23|x| | | | |
| | | | | | | | | | | | | |
skipping to change at page 107, line 49 skipping to change at page 113, line 37
Time Stamp support | MAY-10 | | |x| | | Time Stamp support | MAY-10 | | |x| | |
Record Route support | MAY-11 | | |x| | | Record Route support | MAY-11 | | |x| | |
Source Route: | | | | | | | Source Route: | | | | | | |
ALP can specify | MUST-51|x| | | | |1 ALP can specify | MUST-51|x| | | | |1
Overrides src rt in datagram | MUST-52|x| | | | | Overrides src rt in datagram | MUST-52|x| | | | |
Build return route from src rt | MUST-53|x| | | | | Build return route from src rt | MUST-53|x| | | | |
Later src route overrides | SHLD-24| |x| | | | Later src route overrides | SHLD-24| |x| | | |
| | | | | | | | | | | | | |
Receiving ICMP Messages from IP | MUST-54|x| | | | | Receiving ICMP Messages from IP | MUST-54|x| | | | |
Dest. Unreach (0,1,5) => inform ALP | SHLD-25| |x| | | | Dest. Unreach (0,1,5) => inform ALP | SHLD-25| |x| | | |
Dest. Unreach (0,1,5) => abort conn | MUST-56| | | | |x| Abort on Dest. Unreach (0,1,5) =>nn | MUST-56| | | | |x|
Dest. Unreach (2-4) => abort conn | SHLD-26| |x| | | | Dest. Unreach (2-4) => abort conn | SHLD-26| |x| | | |
Source Quench => silent discard | MUST-55|x| | | | | Source Quench => silent discard | MUST-55|x| | | | |
Time Exceeded => tell ALP, don't abort | MUST-56| | | | |x| Abort on Time Exceeded => | MUST-56| | | | |x|
Param Problem => tell ALP, don't abort | MUST-56| | | | |x| Abort on Param Problem => | MUST-56| | | | |x|
| | | | | | | | | | | | | |
Address Validation | | | | | | | Address Validation | | | | | | |
Reject OPEN call to invalid IP address | MUST-46|x| | | | | Reject OPEN call to invalid IP address | MUST-46|x| | | | |
Reject SYN from invalid IP address | MUST-63|x| | | | | Reject SYN from invalid IP address | MUST-63|x| | | | |
Silently discard SYN to bcast/mcast addr | MUST-57|x| | | | | Silently discard SYN to bcast/mcast addr | MUST-57|x| | | | |
| | | | | | | | | | | | | |
TCP/ALP Interface Services | | | | | | | TCP/ALP Interface Services | | | | | | |
Error Report mechanism | MUST-47|x| | | | | Error Report mechanism | MUST-47|x| | | | |
ALP can disable Error Report Routine | SHLD-20| |x| | | | ALP can disable Error Report Routine | SHLD-20| |x| | | |
ALP can specify DiffServ field for sending | MUST-48|x| | | | | ALP can specify DiffServ field for sending | MUST-48|x| | | | |
skipping to change at page 108, line 39 skipping to change at page 114, line 27
Alternative Congestion Control: | | | | | | | Alternative Congestion Control: | | | | | | |
Implement alternative conformant algorithm(s) | MAY-18 | | |x| | | Implement alternative conformant algorithm(s) | MAY-18 | | |x| | |
-------------------------------------------------|--------|-|-|-|-|-|- -------------------------------------------------|--------|-|-|-|-|-|-
FOOTNOTES: (1) "ALP" means Application-Layer Program. FOOTNOTES: (1) "ALP" means Application-Layer Program.
Author's Address Author's Address
Wesley M. Eddy (editor) Wesley M. Eddy (editor)
MTI Systems MTI Systems
US United States of America
Email: wes@mti-systems.com Email: wes@mti-systems.com
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