< draft-ietf-l2tpext-l2tp-base-14.txt   draft-ietf-l2tpext-l2tp-base-15.txt >
Network Working Group J. Lau Network Working Group J. Lau, Ed.
Internet-Draft M. Townsley Internet Draft M. Townsley, Ed.
Category: Standards Track cisco Systems Category: Standards Track Cisco Systems
<draft-ietf-l2tpext-l2tp-base-14.txt> I. Goyret I. Goyret, Ed.
Lucent Technologies Lucent Technologies
Editors December 2004
June 2004
Layer Two Tunneling Protocol (Version 3) Layer Two Tunneling Protocol - Version 3 (L2TPv3)
draft-ietf-l2tpext-l2tp-base-15.txt
Status of this Memo Status of this Memo
By submitting this Internet-Draft, I certify that any applicable By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed, patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with and any of which I become aware will be disclosed, in accordance with
RFC 3668. RFC 3668.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
skipping to change at page 1, line 38 skipping to change at page 1, line 38
material or to cite them other than as "work in progress". material or to cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt . http://www.ietf.org/ietf/1id-abstracts.txt .
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html . http://www.ietf.org/shadow.html .
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved. Copyright (C) The Internet Society (2004).
Abstract Abstract
This document describes "version 3" of the Layer Two Tunneling This document describes "version 3" of the Layer Two Tunneling
Protocol (L2TPv3). L2TPv3 defines the base control protocol and Protocol (L2TPv3). L2TPv3 defines the base control protocol and
encapsulation for tunneling multiple layer 2 connections between two encapsulation for tunneling multiple Layer 2 connections between two
IP connected nodes. Additional documents detail the specifics for IP nodes. Additional documents detail the specifics for each data
each link-type being emulated. link type being emulated.
Contents
Status of this Memo.......................................... 1
1. Introduction............................................. 4
1.1 Changes from RFC 2661................................ 5
1.2 Specification of Requirements........................ 5
1.3 Terminology.......................................... 5
2. Topology................................................. 9
3. Protocol Overview........................................ 10
3.1 Control Message Types................................ 11
3.2 L2TP Header Formats.................................. 12
3.2.1 L2TP Control Message Header..................... 12
3.2.2 L2TP Data Message............................... 13
3.3 Control Connection Management........................ 14
3.3.1 Control Connection Establishment................ 14
3.3.2 Control Connection Teardown..................... 15
3.4 Session Management................................... 15
3.4.1 Session Establishment for an Incoming Call...... 15
3.4.2 Session Establishment for an Outgoing Call...... 16
3.4.3 Session Teardown................................ 16
4. Protocol Operation....................................... 17
4.1 L2TP Over Specific Packet-Switched Networks (PSNs)... 17
4.1.1 L2TPv3 over IP.................................. 18
4.1.2 L2TP over UDP................................... 19
4.1.3 L2TP and IPsec................................... 21
4.1.4 IP Fragmentation Issues......................... 22
4.2 Reliable Delivery of Control Messages................ 23
4.3 Control Connection and Control Message Authentication 26
4.4 Keepalive (Hello).................................... 27
4.5 Forwarding Session Data Frames....................... 27
4.6 Default L2-Specific Sublayer......................... 28
4.6.1 Sequencing Data Packets......................... 29
4.7 L2TPv2/v3 Interoperability and Migration............. 29
4.7.1 L2TPv3 over IP.................................. 29
4.7.2 L2TPv3 over UDP................................. 30
4.7.3 Automatic L2TPv2 Fallback....................... 30
5. Control Message Attribute Value Pairs.................... 31
5.1 AVP Format........................................... 31
5.2 Mandatory AVPs and Setting the M Bit................. 33
5.3 Hiding of AVP Attribute Values....................... 34
5.4 AVP Summary.......................................... 36
5.4.1 General Control Message AVPs.................... 36
5.4.2 Result and Error Codes.......................... 41
5.4.3 Control Connection Management AVPs.............. 43
5.4.4 Session Management AVPs......................... 48
5.4.5 Circuit Status AVPs............................. 56
6. Control Connection Protocol Specification................ 59
6.1 Start-Control-Connection-Request (SCCRQ)............. 59
6.2 Start-Control-Connection-Reply (SCCRP)............... 59
6.3 Start-Control-Connection-Connected (SCCCN)........... 60
6.4 Stop-Control-Connection-Notification (StopCCN)....... 60
6.5 Hello (HELLO)........................................ 61
6.6 Incoming-Call-Request (ICRQ)......................... 61
6.7 Incoming-Call-Reply (ICRP)........................... 62
6.8 Incoming-Call-Connected (ICCN)....................... 62
6.9 Outgoing-Call-Request (OCRQ)......................... 63
6.10 Outgoing-Call-Reply (OCRP).......................... 64
6.11 Outgoing-Call-Connected (OCCN)...................... 64
6.12 Call-Disconnect-Notify (CDN)........................ 65
6.13 WAN-Error-Notify (WEN).............................. 65
6.14 Set-Link-Info (SLI)................................. 66
6.15 Explicit-Acknowledgement (ACK)...................... 66
7. Control Connection State Machines........................ 67
7.1 Malformed AVPs and Control Messages.................. 67
7.2 Control Connection States............................ 68
7.3 Incoming Calls....................................... 70
7.3.1 ICRQ Sender States.............................. 71
7.3.2 ICRQ Recipient States........................... 72
7.4 Outgoing Calls....................................... 73
7.4.1 OCRQ Sender States.............................. 74
7.4.2 OCRQ Recipient (LAC) States..................... 75
7.5 Termination of a Control Connection.................. 76
8. Security Considerations.................................. 77
8.1 Control Connection Endpoint and Message Security..... 77
8.2 Data Packet Spoofing................................. 77
9. Internationalization Considerations...................... 78
10. IANA Considerations..................................... 79
10.1 Control Message Attribute Value Pairs (AVPs)........ 79
10.2 Message Type AVP Values............................. 80
10.3 Result Code AVP Values.............................. 80
10.4 AVP Header Bits..................................... 81
10.5 L2TP Control Message Header Bits.................... 81
10.6 Pseudowire Types..................................... 81
10.7 Circuit Status Bits.................................. 82
10.8 Default L2-Specific Sublayer bits.................... 82
10.9 L2-Specific Sublayer Type............................ 82
10.10 Data Sequencing Level............................... 83
11. References.............................................. 83
11.1 Normative References................................ 83
11.2 Informative References.............................. 84
12. Editors' Addresses...................................... 85
13. Acknowledgments......................................... 86
Appendix A: Control Slow Start and Congestion Avoidance...... 87
Appendix B: Control Message Examples......................... 88 Table of Contents
Appendix C: Processing Sequence Numbers...................... 89 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Changes from RFC 2661. . . . . . . . . . . . . . . . . . 4
1.2. Specification of Requirements. . . . . . . . . . . . . . 4
1.3. Terminology. . . . . . . . . . . . . . . . . . . . . . . 5
2. Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Protocol Overview. . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Control Message Types. . . . . . . . . . . . . . . . . . 10
3.2. L2TP Header Formats. . . . . . . . . . . . . . . . . . . 11
3.2.1. L2TP Control Message Header. . . . . . . . . . . 11
3.2.2. L2TP Data Message. . . . . . . . . . . . . . . . 12
3.3. Control Connection Management. . . . . . . . . . . . . . 13
3.3.1. Control Connection Establishment . . . . . . . . 14
3.3.2. Control Connection Teardown. . . . . . . . . . . 14
3.4. Session Management . . . . . . . . . . . . . . . . . . . 15
3.4.1. Session Establishment for an Incoming Call . . . 15
3.4.2. Session Establishment for an Outgoing Call . . . 15
3.4.3. Session Teardown . . . . . . . . . . . . . . . . 16
4. Protocol Operation . . . . . . . . . . . . . . . . . . . . . . 16
4.1. L2TP Over Specific Packet-Switched Networks (PSNs) . . . 16
4.1.1. L2TPv3 over IP . . . . . . . . . . . . . . . . . 17
4.1.2. L2TP over UDP. . . . . . . . . . . . . . . . . . 18
4.1.3. L2TP and IPsec . . . . . . . . . . . . . . . . . 20
4.1.4. IP Fragmentation Issues. . . . . . . . . . . . . 21
4.2. Reliable Delivery of Control Messages. . . . . . . . . . 23
4.3. Control Message Authentication . . . . . . . . . . . . . 25
4.4. Keepalive (Hello). . . . . . . . . . . . . . . . . . . . 26
4.5. Forwarding Session Data Frames . . . . . . . . . . . . . 26
4.6. Default L2-Specific Sublayer . . . . . . . . . . . . . . 27
4.6.1. Sequencing Data Packets. . . . . . . . . . . . . 28
4.7. L2TPv2/v3 Interoperability and Migration . . . . . . . . 28
4.7.1. L2TPv3 over IP . . . . . . . . . . . . . . . . . 29
4.7.2. L2TPv3 over UDP. . . . . . . . . . . . . . . . . 29
4.7.3. Automatic L2TPv2 Fallback. . . . . . . . . . . . 29
5. Control Message Attribute Value Pairs. . . . . . . . . . . . . 30
5.1. AVP Format . . . . . . . . . . . . . . . . . . . . . . . 30
5.2. Mandatory AVPs and Setting the M Bit . . . . . . . . . . 32
5.3. Hiding of AVP Attribute Values . . . . . . . . . . . . . 33
5.4. AVP Summary. . . . . . . . . . . . . . . . . . . . . . . 36
5.4.1. General Control Message AVPs . . . . . . . . . . 36
5.4.2. Result and Error Codes . . . . . . . . . . . . . 40
5.4.3. Control Connection Management AVPs . . . . . . . 43
5.4.4. Session Management AVPs. . . . . . . . . . . . . 48
5.4.5. Circuit Status AVPs. . . . . . . . . . . . . . . 56
6. Control Connection Protocol Specification. . . . . . . . . . . 59
6.1. Start-Control-Connection-Request (SCCRQ) . . . . . . . . 59
6.2. Start-Control-Connection-Reply (SCCRP) . . . . . . . . . 60
6.3. Start-Control-Connection-Connected (SCCCN) . . . . . . . 60
6.4. Stop-Control-Connection-Notification (StopCCN) . . . . . 60
6.5. Hello (HELLO). . . . . . . . . . . . . . . . . . . . . . 61
6.6. Incoming-Call-Request (ICRQ) . . . . . . . . . . . . . . 61
6.7. Incoming-Call-Reply (ICRP) . . . . . . . . . . . . . . . 62
6.8. Incoming-Call-Connected (ICCN) . . . . . . . . . . . . . 63
6.9. Outgoing-Call-Request (OCRQ) . . . . . . . . . . . . . . 63
6.10. Outgoing-Call-Reply (OCRP) . . . . . . . . . . . . . . . 64
6.11. Outgoing-Call-Connected (OCCN) . . . . . . . . . . . . . 65
6.12. Call-Disconnect-Notify (CDN) . . . . . . . . . . . . . . 65
6.13. WAN-Error-Notify (WEN) . . . . . . . . . . . . . . . . . 66
6.14. Set-Link-Info (SLI). . . . . . . . . . . . . . . . . . . 66
6.15. Explicit-Acknowledgement (ACK) . . . . . . . . . . . . . 67
7. Control Connection State Machines. . . . . . . . . . . . . . . 67
7.1. Malformed AVPs and Control Messages. . . . . . . . . . . 67
7.2. Control Connection States. . . . . . . . . . . . . . . . 69
7.3. Incoming Calls . . . . . . . . . . . . . . . . . . . . . 71
7.3.1. ICRQ Sender States . . . . . . . . . . . . . . . 71
7.3.2. ICRQ Recipient States. . . . . . . . . . . . . . 73
7.4. Outgoing Calls . . . . . . . . . . . . . . . . . . . . . 74
7.4.1. OCRQ Sender States . . . . . . . . . . . . . . . 74
7.4.2. OCRQ Recipient (LAC) States. . . . . . . . . . . 76
7.5. Termination of a Control Connection. . . . . . . . . . . 77
8. Security Considerations. . . . . . . . . . . . . . . . . . . . 77
8.1. Control Connection Endpoint and Message Security . . . . 78
8.2. Data Packet Spoofing . . . . . . . . . . . . . . . . . . 78
9. Internationalization Considerations. . . . . . . . . . . . . . 79
10. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 80
10.1. Control Message Attribute Value Pairs (AVPs) . . . . . . 80
10.2. Message Type AVP Values. . . . . . . . . . . . . . . . . 81
10.3. Result Code AVP Values . . . . . . . . . . . . . . . . . 81
10.4. AVP Header Bits. . . . . . . . . . . . . . . . . . . . . 81
10.5. L2TP Control Message Header Bits . . . . . . . . . . . . 82
10.6. Pseudowire Types . . . . . . . . . . . . . . . . . . . . 82
10.7. Circuit Status Bits. . . . . . . . . . . . . . . . . . . 83
10.8. Default L2-Specific Sublayer bits. . . . . . . . . . . . 83
10.9. L2-Specific Sublayer Type. . . . . . . . . . . . . . . . 83
10.10 Data Sequencing Level. . . . . . . . . . . . . . . . . . 84
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 84
11.1. Normative References . . . . . . . . . . . . . . . . . . 84
11.2. Informative References . . . . . . . . . . . . . . . . . 85
12. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 86
Appendix A: Control Slow Start and Congestion Avoidance. . . . . . 88
Appendix B: Control Message Examples . . . . . . . . . . . . . . . 89
Appendix C: Processing Sequence Numbers. . . . . . . . . . . . . . 90
Editors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 92
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 93
1. Introduction 1. Introduction
The Layer Two Tunneling Protocol (L2TP) provides a dynamic mechanism The Layer Two Tunneling Protocol (L2TP) provides a dynamic mechanism
for tunneling Layer 2 (L2) "circuits" across a packet-oriented data for tunneling Layer 2 (L2) "circuits" across a packet-oriented data
network (e.g., over IP). L2TP, as originally defined in RFC 2661, is network (e.g., over IP). L2TP, as originally defined in RFC 2661, is
a standard method for tunneling Point to Point Protocol (PPP) a standard method for tunneling Point-to-Point Protocol (PPP)
[RFC1661] sessions. L2TP has since been adopted for tunneling a [RFC1661] sessions. L2TP has since been adopted for tunneling a
number of other L2 protocols. In order to provide greater number of other L2 protocols. In order to provide greater
modularity, this document describes the base L2TP protocol, modularity, this document describes the base L2TP protocol,
independent of the L2 payload that is being tunneled. independent of the L2 payload that is being tunneled.
The base L2TP protocol defined in this document consists of (1) the The base L2TP protocol defined in this document consists of (1) the
control protocol for dynamic creation, maintenance, and teardown of control protocol for dynamic creation, maintenance, and teardown of
L2TP sessions, and (2) the L2TP data encapsulation to multiplex and L2TP sessions, and (2) the L2TP data encapsulation to multiplex and
demultiplex L2 data streams between two L2TP nodes across an IP demultiplex L2 data streams between two L2TP nodes across an IP
network. Additional documents are expected to be published for each network. Additional documents are expected to be published for each
layer 2 data link emulation type (a.k.a. pseudowire-type) supported L2 data link emulation type (a.k.a. pseudowire-type) supported by
by L2TP (i.e., PPP, Ethernet, Frame Relay, etc.). These documents L2TP (i.e., PPP, Ethernet, Frame Relay, etc.). These documents will
will contain any individual details that are outside the scope of contain any pseudowire-type specific details that are outside the
this base specification. scope of this base specification.
When the designation between L2TPv2 and L2TPv3 is necessary, L2TP as When the designation between L2TPv2 and L2TPv3 is necessary, L2TP as
defined in RFC 2661 will be referred to as "L2TPv2", corresponding to defined in RFC 2661 will be referred to as "L2TPv2", corresponding to
the value in the Version field of an L2TP header. (Layer 2 the value in the Version field of an L2TP header. (Layer 2
Forwarding, L2F, [RFC2341] was defined as "version 1".) At times, Forwarding, L2F, [RFC2341] was defined as "version 1".) At times,
L2TP as defined in this document will be referred to as "L2TPv3". L2TP as defined in this document will be referred to as "L2TPv3".
Otherwise, the acronym "L2TP" will refer to L2TPv3 or L2TP in Otherwise, the acronym "L2TP" will refer to L2TPv3 or L2TP in
general. general.
1.1 Changes from RFC 2661 1.1. Changes from RFC 2661
Many of the protocol constructs described in this document are Many of the protocol constructs described in this document are
carried over from RFC 2661. Changes include clarifications based on carried over from RFC 2661. Changes include clarifications based on
years of interoperability and deployment experience as well as years of interoperability and deployment experience as well as
modifications to either improve protocol operation or provide a modifications to either improve protocol operation or provide a
clearer separation from PPP. The intent of these modifications is to clearer separation from PPP. The intent of these modifications is to
achieve a healthy balance between code reuse, interoperability achieve a healthy balance between code reuse, interoperability
experience, and a directed evolution of L2TP as it is applied to new experience, and a directed evolution of L2TP as it is applied to new
tasks. tasks.
Notable differences between L2TPv2 and L2TPv3 include: Notable differences between L2TPv2 and L2TPv3 include the following:
Separation of all PPP-related AVPs, references, etc., including a Separation of all PPP-related AVPs, references, etc., including a
portion of the L2TP data header that was specific to the needs of portion of the L2TP data header that was specific to the needs of
PPP. The PPP-specific constructs are described in a companion PPP. The PPP-specific constructs are described in a companion
document. document.
Transition from a 16-bit Session ID and Tunnel ID to a 32-bit Transition from a 16-bit Session ID and Tunnel ID to a 32-bit
Session ID and Control Connection ID, respectively. Session ID and Control Connection ID, respectively.
Extension of the Tunnel Authentication mechanism to cover the Extension of the Tunnel Authentication mechanism to cover the
entire control message rather than just a portion of certain entire control message rather than just a portion of certain
messages. messages.
Details of these changes and a recommendation for transitioning to Details of these changes and a recommendation for transitioning to
L2TPv3 are discussed in Section 4.7. L2TPv3 are discussed in Section 4.7.
1.2 Specification of Requirements 1.2. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
1.3 Terminology 1.3. Terminology
Attribute Value Pair (AVP) Attribute Value Pair (AVP)
The variable-length concatenation of a unique Attribute The variable-length concatenation of a unique Attribute
(represented by an integer), a length field, and a Value (represented by an integer), a length field, and a Value
containing the actual value identified by the attribute. Zero or containing the actual value identified by the attribute. Zero or
more AVPs make up the body of control messages, which are used in more AVPs make up the body of control messages, which are used in
the establishment, maintenance, and teardown of control the establishment, maintenance, and teardown of control
connections. This basic construct is sometimes referred to as a connections. This basic construct is sometimes referred to as a
Type-Length-Value (TLV) in some specifications. (See also: Type-Length-Value (TLV) in some specifications. (See also:
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Control Connection.) Control Connection.)
Data Message Data Message
Message used by the data channel. (a.k.a. Data Packet, See also: Message used by the data channel. (a.k.a. Data Packet, See also:
Data Channel.) Data Channel.)
Data Channel Data Channel
The channel for L2TP-encapsulated data traffic that passes between The channel for L2TP-encapsulated data traffic that passes between
two LCCEs over a Packet Switched Network (i.e. IP). (See also: two LCCEs over a Packet-Switched Network (i.e., IP). (See also:
Control Connection, Data Message.) Control Connection, Data Message.)
Incoming Call Incoming Call
The action of receiving a call (circuit up event) on an LAC. The The action of receiving a call (circuit up event) on an LAC. The
call may have been placed by a remote system (e.g., a phone call call may have been placed by a remote system (e.g., a phone call
over a PSTN), or it may have been triggered by a local event over a PSTN), or it may have been triggered by a local event
(e.g., interesting traffic routed to a virtual interface). An (e.g., interesting traffic routed to a virtual interface). An
incoming call that needs to be tunneled (as determined by the LAC) incoming call that needs to be tunneled (as determined by the LAC)
results in the generation of an L2TP ICRQ message. (See also: results in the generation of an L2TP ICRQ message. (See also:
Call, Outgoing Call, Outgoing Call Request.) Call, Outgoing Call, Outgoing Call Request.)
L2TP Access Concentrator (LAC) L2TP Access Concentrator (LAC)
If an L2TP Control Connection Endpoint (LCCE) is being used to If an L2TP Control Connection Endpoint (LCCE) is being used to
cross-connect an L2TP session directly to a data link, we refer to cross-connect an L2TP session directly to a data link, we refer to
it as an L2TP Access Concentrator (LAC). An LCCE may act as both it as an L2TP Access Concentrator (LAC). An LCCE may act as both
an L2TP Network Server (LNS) for some sessions and an LAC for an L2TP Network Server (LNS) for some sessions and an LAC for
others, so these terms must only be used within the context of a others, so these terms must only be used within the context of a
given set of sessions unless the LCCE is in fact single purpose given set of sessions unless the LCCE is in fact single purpose
for a given topology. (See also: LCCE, LNS.) for a given topology. (See also: LCCE, LNS.)
L2TP Control Connection Endpoint (LCCE) L2TP Control Connection Endpoint (LCCE)
An L2TP node which exists at either end of an L2TP control An L2TP node that exists at either end of an L2TP control
connection. May also be referred to as an LAC or LNS, depending on connection. May also be referred to as an LAC or LNS, depending
whether tunneled frames are processed at the data link (LAC) or on whether tunneled frames are processed at the data link (LAC) or
network layer (LNS). (See also: LAC, LNS.) network layer (LNS). (See also: LAC, LNS.)
L2TP Network Server (LNS) L2TP Network Server (LNS)
If a given L2TP session is terminated at the L2TP node and the If a given L2TP session is terminated at the L2TP node and the
encapsulated network layer (L3) packet processed on a virtual encapsulated network layer (L3) packet processed on a virtual
interface, we refer to this L2TP node as an L2TP Network Server interface, we refer to this L2TP node as an L2TP Network Server
(LNS). A given LCCE may act as both an LNS for some sessions and (LNS). A given LCCE may act as both an LNS for some sessions and
an LAC for others, so these terms must only be used within the an LAC for others, so these terms must only be used within the
context of a given set of sessions unless the LCCE is in fact context of a given set of sessions unless the LCCE is in fact
single purpose for a given topology. (See also: LCCE, LAC.) single purpose for a given topology. (See also: LCCE, LAC.)
Outgoing Call Outgoing Call
The action of placing a call by an LAC, typically in response to The action of placing a call by an LAC, typically in response to
policy directed by the peer in an Outgoing Call Request message. policy directed by the peer in an Outgoing Call Request. (See
(See also: Call, Incoming Call, Outgoing Call Request.) also: Call, Incoming Call, Outgoing Call Request.)
Outgoing Call Request Outgoing Call Request
A request sent to an LAC to place an outgoing call. The request A request sent to an LAC to place an outgoing call. The request
contains specific information not known a priori by the LAC (i.e., contains specific information not known a priori by the LAC (e.g.,
a number to dial). (See also: Call, Incoming Call, Outgoing a number to dial). (See also: Call, Incoming Call, Outgoing
Call.) Call.)
Packet-Switched Network (PSN) Packet-Switched Network (PSN)
A network that uses packet-switching technology for data delivery. A network that uses packet switching technology for data delivery.
For L2TPv3, this layer is principally IP. Other examples include For L2TPv3, this layer is principally IP. Other examples include
MPLS, Frame Relay, and ATM. MPLS, Frame Relay, and ATM.
Peer Peer
When used in context with L2TP, Peer refers to the far end of an When used in context with L2TP, Peer refers to the far end of an
L2TP control connection (i.e., the remote LCCE). An LAC's peer L2TP control connection (i.e., the remote LCCE). An LAC's peer
may be either an LNS or another LAC. Similarly, an LNS's peer may may be either an LNS or another LAC. Similarly, an LNS's peer may
be either an LAC or another LNS. (See also: LAC, LCCE, LNS.) be either an LAC or another LNS. (See also: LAC, LCCE, LNS.)
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Pseudowire per L2TP Session. (See also: Packet-Switched Network, Pseudowire per L2TP Session. (See also: Packet-Switched Network,
Session.) Session.)
Pseudowire Type Pseudowire Type
The payload type being carried within an L2TP session. Examples The payload type being carried within an L2TP session. Examples
include PPP, Ethernet, and Frame Relay. (See also: Session.) include PPP, Ethernet, and Frame Relay. (See also: Session.)
Remote System Remote System
An end-system or router connected by a circuit to an LAC. An end system or router connected by a circuit to an LAC.
Session Session
An L2TP session is the entity which is created between two LCCEs An L2TP session is the entity that is created between two LCCEs in
in order to exchange parameters for and maintain an emulated L2 order to exchange parameters for and maintain an emulated L2
connection. Multiple sessions may be associated with a single connection. Multiple sessions may be associated with a single
Control Connection. Control Connection.
Zero-Length Body (ZLB) Message Zero-Length Body (ZLB) Message
A control message with only an L2TP header. ZLB messages are used A control message with only an L2TP header. ZLB messages are used
only to acknowledge messages on the L2TP reliable control channel. only to acknowledge messages on the L2TP reliable control
(See also: Control Message.) connection. (See also: Control Message.)
2. Topology 2. Topology
L2TP operates between two L2TP Control Connection Endpoints (LCCEs), L2TP operates between two L2TP Control Connection Endpoints (LCCEs),
tunneling traffic across a packet network. There are three tunneling traffic across a packet network. There are three
predominant tunneling models in which L2TP operates: LAC-LNS (or vice predominant tunneling models in which L2TP operates: LAC-LNS (or vice
versa), LAC-LAC, and LNS-LNS. These models are diagrammed below. versa), LAC-LAC, and LNS-LNS. These models are diagrammed below.
(Dotted lines designate network connections. Solid lines designate (Dotted lines designate network connections. Solid lines designate
circuit connections.) circuit connections.)
skipping to change at page 9, line 37 skipping to change at page 8, line 50
system system
|<-- emulated service -->| |<-- emulated service -->|
|<----------- L2 service ------------>| |<----------- L2 service ------------>|
(b) LAC-LAC Reference Model: In this model, both LCCEs are LACs. (b) LAC-LAC Reference Model: In this model, both LCCEs are LACs.
Each LAC forwards circuit traffic from the remote system to the peer Each LAC forwards circuit traffic from the remote system to the peer
LAC using L2TP, and vice versa. In its simplest form, an LAC acts as LAC using L2TP, and vice versa. In its simplest form, an LAC acts as
a simple cross-connect between a circuit to a remote system and an a simple cross-connect between a circuit to a remote system and an
L2TP session. This model typically involves symmetric establishment; L2TP session. This model typically involves symmetric establishment;
that is, either side of the connection may initiate a session at any that is, either side of the connection may initiate a session at any
time (or simultaneously, in which a tie-breaking mechanism is time (or simultaneously, in which a tie breaking mechanism is
utilized). utilized).
+-----+ L2 +-----+ +-----+ L2 +-----+ +-----+ L2 +-----+ +-----+ L2 +-----+
| |------| LAC |........[ IP ]........| LAC |------| | | |------| LAC |........[ IP ]........| LAC |------| |
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+ +-----+
remote remote remote remote
system system system system
|<- emulated service ->| |<- emulated service ->|
|<----------------- L2 service ----------------->| |<----------------- L2 service ----------------->|
(c) LNS-LNS Reference Model: This model has two LNSs as the LCCEs. A (c) LNS-LNS Reference Model: This model has two LNSs as the LCCEs. A
user-level, traffic-generated, or signaled event typically drives user-level, traffic-generated, or signaled event typically drives
session establishment from one side of the tunnel. For example, a session establishment from one side of the tunnel. For example, a
tunnel generated from a PC by a user, or automatically by customer tunnel generated from a PC by a user, or automatically by customer
premises equipment. premises equipment.
+-----+ +-----+ +-----+ +-----+
[home network]...| LNS |........[ IP ]........| LNS |...[home network] [home network]...| LNS |........[ IP ]........| LNS |...[home network]
+-----+ +-----+ +-----+ +-----+
|<- emulated service ->| |<- emulated service ->|
|<---- L2 service ---->| |<---- L2 service ---->|
Note: In L2TPv2, user-driven tunneling of this type is often referred Note: In L2TPv2, user-driven tunneling of this type is often referred
to as "voluntary tunneling" [RFC2809]. Further, an LNS acting as part to as "voluntary tunneling" [RFC2809]. Further, an LNS acting as
of a software package on a host is sometimes referred to as an "LAC part of a software package on a host is sometimes referred to as an
Client" [RFC2661]. "LAC Client" [RFC2661].
3. Protocol Overview 3. Protocol Overview
L2TP is comprised of two types of messages, control messages and data L2TP is comprised of two types of messages, control messages and data
messages (sometimes referred to as "control packets" and "data messages (sometimes referred to as "control packets" and "data
packets", respectively). Control messages are used in the packets", respectively). Control messages are used in the
establishment, maintenance, and clearing of control connections and establishment, maintenance, and clearing of control connections and
sessions. These messages utilize a reliable control channel within sessions. These messages utilize a reliable control channel within
L2TP to guarantee delivery (see Section 4.2 for details). Data L2TP to guarantee delivery (see Section 4.2 for details). Data
messages are used to encapsulate the L2 traffic being carried over messages are used to encapsulate the L2 traffic being carried over
the L2TP session. Unlike control messages, data messages are not the L2TP session. Unlike control messages, data messages are not
retransmitted when packet loss occurs. retransmitted when packet loss occurs.
The L2TPv3 control message format defined in this document borrows The L2TPv3 control message format defined in this document borrows
largely from L2TPv2. These control messages are used in conjunction largely from L2TPv2. These control messages are used in conjunction
with the associated protocol state machines that govern the dynamic with the associated protocol state machines that govern the dynamic
setup, maintenance, and teardown for L2TP sessions. The data message setup, maintenance, and teardown for L2TP sessions. The data message
format for tunneling data packets may be utilized with or without the format for tunneling data packets may be utilized with or without the
L2TP control channel, either via manual configuration or other L2TP control channel, either via manual configuration or via other
signaling methods to pre-configure or distribute L2TP session signaling methods to pre-configure or distribute L2TP session
information. Utilization of the L2TP data message format with other information. Utilization of the L2TP data message format with other
signaling methods is outside the scope of this document. signaling methods is outside the scope of this document.
Figure 3.0: L2TPv3 Structure Figure 3.0: L2TPv3 Structure
+-------------------+ +-----------------------+ +-------------------+ +-----------------------+
| Tunneled Frame | | L2TP Control Message | | Tunneled Frame | | L2TP Control Message |
+-------------------+ +-----------------------+ +-------------------+ +-----------------------+
| L2TP Data Header | | L2TP Control Header | | L2TP Data Header | | L2TP Control Header |
+-------------------+ +-----------------------+ +-------------------+ +-----------------------+
| L2TP Data Channel | | L2TP Control Channel | | L2TP Data Channel | | L2TP Control Channel |
skipping to change at page 11, line 18 skipping to change at page 10, line 33
a reliable L2TP control channel, which operates over the same PSN. a reliable L2TP control channel, which operates over the same PSN.
The necessary setup for tunneling a session with L2TP consists of two The necessary setup for tunneling a session with L2TP consists of two
steps: (1) Establishing the control connection, and (2) establishing steps: (1) Establishing the control connection, and (2) establishing
a session as triggered by an incoming call or outgoing call. An L2TP a session as triggered by an incoming call or outgoing call. An L2TP
session MUST be established before L2TP can begin to forward session session MUST be established before L2TP can begin to forward session
frames. Multiple sessions may be bound to a single control frames. Multiple sessions may be bound to a single control
connection, and multiple control connections may exist between the connection, and multiple control connections may exist between the
same two LCCEs. same two LCCEs.
3.1 Control Message Types 3.1. Control Message Types
The Message Type AVP (see Section 5.4.1) defines the specific type of The Message Type AVP (see Section 5.4.1) defines the specific type of
control message being sent. control message being sent.
This document defines the following control message types (see This document defines the following control message types (see
Sections 6.1 through 6.13 for details on the construction and use of Sections 6.1 through 6.15 for details on the construction and use of
each message): each message):
Control Connection Management Control Connection Management
0 (reserved) 0 (reserved)
1 (SCCRQ) Start-Control-Connection-Request 1 (SCCRQ) Start-Control-Connection-Request
2 (SCCRP) Start-Control-Connection-Reply 2 (SCCRP) Start-Control-Connection-Reply
3 (SCCCN) Start-Control-Connection-Connected 3 (SCCCN) Start-Control-Connection-Connected
4 (StopCCN) Stop-Control-Connection-Notification 4 (StopCCN) Stop-Control-Connection-Notification
5 (reserved) 5 (reserved)
6 (HELLO) Hello 6 (HELLO) Hello
TBA-M1 (ACK) Explicit Acknowledgement 20 (ACK) Explicit Acknowledgement
Call Management Call Management
7 (OCRQ) Outgoing-Call-Request 7 (OCRQ) Outgoing-Call-Request
8 (OCRP) Outgoing-Call-Reply 8 (OCRP) Outgoing-Call-Reply
9 (OCCN) Outgoing-Call-Connected 9 (OCCN) Outgoing-Call-Connected
10 (ICRQ) Incoming-Call-Request 10 (ICRQ) Incoming-Call-Request
11 (ICRP) Incoming-Call-Reply 11 (ICRP) Incoming-Call-Reply
12 (ICCN) Incoming-Call-Connected 12 (ICCN) Incoming-Call-Connected
13 (reserved) 13 (reserved)
14 (CDN) Call-Disconnect-Notify 14 (CDN) Call-Disconnect-Notify
Error Reporting Error Reporting
15 (WEN) WAN-Error-Notify 15 (WEN) WAN-Error-Notify
Link Status Change Reporting Link Status Change Reporting
16 (SLI) Set-Link-Info 16 (SLI) Set-Link-Info
3.2 L2TP Header Formats 3.2. L2TP Header Formats
This section defines header formats for L2TP control messages and This section defines header formats for L2TP control messages and
L2TP data messages. All values are placed into their respective L2TP data messages. All values are placed into their respective
fields and sent in network order (high-order octets first). fields and sent in network order (high-order octets first).
3.2.1 L2TP Control Message Header 3.2.1. L2TP Control Message Header
The L2TP control message header provides information for the reliable The L2TP control message header provides information for the reliable
transport of messages that govern the establishment, maintenance, and transport of messages that govern the establishment, maintenance, and
teardown of L2TP sessions. By default, control messages are sent teardown of L2TP sessions. By default, control messages are sent
over the underlying media in-band with L2TP data messages. over the underlying media in-band with L2TP data messages.
The L2TP control message header is formatted as follows: The L2TP control message header is formatted as follows:
Figure 3.2.1: L2TP Control Message Header Figure 3.2.1: L2TP Control Message Header
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The L and S bits MUST be set to 1, indicating that the Length field The L and S bits MUST be set to 1, indicating that the Length field
and sequence numbers are present. and sequence numbers are present.
The x bits are reserved for future extensions. All reserved bits The x bits are reserved for future extensions. All reserved bits
MUST be set to 0 on outgoing messages and ignored on incoming MUST be set to 0 on outgoing messages and ignored on incoming
messages. messages.
The Ver field indicates the version of the L2TP control message The Ver field indicates the version of the L2TP control message
header described in this document. On sending, this field MUST be header described in this document. On sending, this field MUST be
set to 3 for all messages (unless operating in an environment which set to 3 for all messages (unless operating in an environment that
includes L2TPv2 [RFC2661] and/or L2F [RFC2341] as well, see Section includes L2TPv2 [RFC2661] and/or L2F [RFC2341] as well, see Section
4.1 for details). 4.1 for details).
The Length field indicates the total length of the message in octets, The Length field indicates the total length of the message in octets,
always calculated from the start of the control message header itself always calculated from the start of the control message header itself
(beginning with the T bit). (beginning with the T bit).
The Control Connection ID field contains the identifier for the The Control Connection ID field contains the identifier for the
control connection. L2TP control connections are named by control connection. L2TP control connections are named by
identifiers that have local significance only. That is, the same identifiers that have local significance only. That is, the same
skipping to change at page 13, line 26 skipping to change at page 12, line 41
Ns indicates the sequence number for this control message, beginning Ns indicates the sequence number for this control message, beginning
at zero and incrementing by one (modulo 2**16) for each message sent. at zero and incrementing by one (modulo 2**16) for each message sent.
See Section 4.2 for more information on using this field. See Section 4.2 for more information on using this field.
Nr indicates the sequence number expected in the next control message Nr indicates the sequence number expected in the next control message
to be received. Thus, Nr is set to the Ns of the last in-order to be received. Thus, Nr is set to the Ns of the last in-order
message received plus one (modulo 2**16). See Section 4.2 for more message received plus one (modulo 2**16). See Section 4.2 for more
information on using this field. information on using this field.
3.2.2 L2TP Data Message 3.2.2. L2TP Data Message
In general, an L2TP data message consists of a (1) Session Header, In general, an L2TP data message consists of a (1) Session Header,
(2) an optional L2-Specific Sublayer, and (3) the Tunnel Payload, as (2) an optional L2-Specific Sublayer, and (3) the Tunnel Payload, as
depicted below. depicted below.
Figure 3.2.2: L2TP Data Message Header Figure 3.2.2: L2TP Data Message Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Session Header | | L2TP Session Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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which the L2TP traffic is delivered. The Session Header MUST provide which the L2TP traffic is delivered. The Session Header MUST provide
(1) a method of distinguishing traffic among multiple L2TP data (1) a method of distinguishing traffic among multiple L2TP data
sessions and (2) a method of distinguishing data messages from sessions and (2) a method of distinguishing data messages from
control messages. control messages.
Each type of encapsulating PSN MUST define its own session header, Each type of encapsulating PSN MUST define its own session header,
clearly identifying the format of the header and parameters necessary clearly identifying the format of the header and parameters necessary
to setup the session. Section 4.1 defines two session headers, one to setup the session. Section 4.1 defines two session headers, one
for transport over UDP and one for transport over IP. for transport over UDP and one for transport over IP.
The L2 Specific Sublayer is an intermediary layer between the L2TP The L2-Specific Sublayer is an intermediary layer between the L2TP
session header and the start of the tunneled frame. It contains session header and the start of the tunneled frame. It contains
control fields that are used to facilitate the tunneling of each control fields that are used to facilitate the tunneling of each
frame (e.g. sequence numbers or flags). The default L2-Specific frame (e.g., sequence numbers or flags). The Default L2-Specific
Sublayer for L2TPv3 is defined in Section 4.6. Sublayer for L2TPv3 is defined in Section 4.6.
The Data Message Header is followed by the Tunnel Payload, including The Data Message Header is followed by the Tunnel Payload, including
any necessary L2 framing as defined in the payload-specific companion any necessary L2 framing as defined in the payload-specific companion
documents. documents.
3.3 Control Connection Management 3.3. Control Connection Management
The L2TP Control Connection handles dynamic establishment, teardown, The L2TP control connection handles dynamic establishment, teardown,
and maintenance of the L2TP sessions and of the control connection and maintenance of the L2TP sessions and of the control connection
itself. The reliable delivery of control messages is described in itself. The reliable delivery of control messages is described in
Section 4.2. Section 4.2.
This section describes typical control connection establishment and This section describes typical control connection establishment and
teardown exchanges. It is important to note that, in the diagrams teardown exchanges. It is important to note that, in the diagrams
that follow, the reliable control message delivery mechanism exists that follow, the reliable control message delivery mechanism exists
independently of the L2TP state machine. For instance, Explicit independently of the L2TP state machine. For instance, Explicit
Acknowledgement (ACK) messages may be sent after any of the control Acknowledgement (ACK) messages may be sent after any of the control
messages indicated in the exchanges below if an acknowledgment is not messages indicated in the exchanges below if an acknowledgment is not
piggybacked on a later control message. piggybacked on a later control message.
LCCEs are identified during control connection establishment either LCCEs are identified during control connection establishment either
by the Host Name AVP, the Router ID AVP, or a combination of the two by the Host Name AVP, the Router ID AVP, or a combination of the two
(see Section 5.4.3). The identity of a peer LCCE is central to (see Section 5.4.3). The identity of a peer LCCE is central to
selecting proper configuration parameters (i.e. Hello interval, selecting proper configuration parameters (i.e., Hello interval,
window size, etc.) for a control connection, as well as for window size, etc.) for a control connection, as well as for
determination of how to setup associated sessions within the control determining how to set up associated sessions within the control
connection, password lookup for control connection authentication, connection, password lookup for control connection authentication,
control connection level tie-breaking, etc. control connection level tie breaking, etc.
3.3.1 Control Connection Establishment 3.3.1. Control Connection Establishment
Establishment of the control connection involves an exchange of AVPs Establishment of the control connection involves an exchange of AVPs
that identifies the peer and its capabilities. that identifies the peer and its capabilities.
A three-message exchange is used to establish the control connection. A three-message exchange is used to establish the control connection.
The following is a typical message exchange: The following is a typical message exchange:
LCCE A LCCE B LCCE A LCCE B
------ ------ ------ ------
SCCRQ -> SCCRQ ->
<- SCCRP <- SCCRP
SCCCN -> SCCCN ->
3.3.2 Control Connection Teardown 3.3.2. Control Connection Teardown
Control connection teardown may be initiated by either LCCE and is Control connection teardown may be initiated by either LCCE and is
accomplished by sending a single StopCCN control message. As part of accomplished by sending a single StopCCN control message. As part of
the reliable control message delivery mechanism, the recipient of a the reliable control message delivery mechanism, the recipient of a
StopCCN MUST send an ACK message to acknowledge receipt of the StopCCN MUST send an ACK message to acknowledge receipt of the
message and maintain enough control connection state to properly message and maintain enough control connection state to properly
accept StopCCN retransmissions over at least a full retransmission accept StopCCN retransmissions over at least a full retransmission
cycle (in case the ACK message is lost). The recommended time for a cycle (in case the ACK message is lost). The recommended time for a
full retransmission cycle is at least 31 seconds (see Section 4.2). full retransmission cycle is at least 31 seconds (see Section 4.2).
The following is an example of a typical control message exchange: The following is an example of a typical control message exchange:
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(Clean up) (Clean up)
(Wait) (Wait)
(Clean up) (Clean up)
An implementation may shut down an entire control connection and all An implementation may shut down an entire control connection and all
sessions associated with the control connection by sending the sessions associated with the control connection by sending the
StopCCN. Thus, it is not necessary to clear each session StopCCN. Thus, it is not necessary to clear each session
individually when tearing down the whole control connection. individually when tearing down the whole control connection.
3.4 Session Management 3.4. Session Management
After successful control connection establishment, individual After successful control connection establishment, individual
sessions may be created. Each session corresponds to a single data sessions may be created. Each session corresponds to a single data
stream between the two LCCEs. This section describes the typical stream between the two LCCEs. This section describes the typical
call establishment and teardown exchanges. call establishment and teardown exchanges.
3.4.1 Session Establishment for an Incoming Call 3.4.1. Session Establishment for an Incoming Call
A three-message exchange is used to establish the session. The A three-message exchange is used to establish the session. The
following is a typical sequence of events: following is a typical sequence of events:
LCCE A LCCE B LCCE A LCCE B
------ ------ ------ ------
(Call (Call
Detected) Detected)
ICRQ -> ICRQ ->
<- ICRP <- ICRP
(Call (Call
Accepted) Accepted)
ICCN -> ICCN ->
3.4.2 Session Establishment for an Outgoing Call 3.4.2. Session Establishment for an Outgoing Call
A three-message exchange is used to set up the session. The A three-message exchange is used to set up the session. The
following is a typical sequence of events: following is a typical sequence of events:
LCCE A LCCE B LCCE A LCCE B
------ ------ ------ ------
<- OCRQ <- OCRQ
OCRP -> OCRP ->
(Perform (Perform
Call Call
Operation) Operation)
OCCN -> OCCN ->
(Call Operation (Call Operation
Completed Completed
Successfully) Successfully)
3.4.3 Session Teardown 3.4.3. Session Teardown
Session teardown may be initiated by either the LAC or LNS and is Session teardown may be initiated by either the LAC or LNS and is
accomplished by sending a CDN control message. After the last accomplished by sending a CDN control message. After the last
session is cleared, the control connection MAY be torn down as well session is cleared, the control connection MAY be torn down as well
(and typically is). The following is an example of a typical control (and typically is). The following is an example of a typical control
message exchange: message exchange:
LCCE A LCCE B LCCE A LCCE B
------ ------ ------ ------
CDN -> CDN ->
(Clean up) (Clean up)
(Clean up) (Clean up)
4. Protocol Operation 4. Protocol Operation
4.1 L2TP Over Specific Packet-Switched Networks (PSNs) 4.1. L2TP Over Specific Packet-Switched Networks (PSNs)
L2TP may operate over a variety of Packet Switched Networks (PSNs). L2TP may operate over a variety of PSNs. There are two modes
There are two modes described for operation over IP, L2TP directly described for operation over IP, L2TP directly over IP (see Section
over IP (Section 4.1.1) and L2TP over UDP (Section 4.1.2). L2TPv3 4.1.1) and L2TP over UDP (see Section 4.1.2). L2TPv3 implementations
implementations MUST support L2TP over IP and SHOULD support L2TP MUST support L2TP over IP and SHOULD support L2TP over UDP for better
over UDP for better NAT and firewall traversal, and easier migration NAT and firewall traversal, and for easier migration from L2TPv2.
from L2TPv2.
L2TP over other PSNs may be defined, but the specifics are outside L2TP over other PSNs may be defined, but the specifics are outside
the scope of this document. Examples of L2TPv2 over other PSNs the scope of this document. Examples of L2TPv2 over other PSNs
include [RFC3070] and [RFC3355]. include [RFC3070] and [RFC3355].
The following field definitions are defined for use in all L2TP The following field definitions are defined for use in all L2TP
Session Header encapsulations. Session Header encapsulations.
Session ID Session ID
A 32-bit field containing a non-zero identifier for a session. A 32-bit field containing a non-zero identifier for a session.
L2TP sessions are named by identifiers that have local L2TP sessions are named by identifiers that have local
significance only. That is, the same logical session will be significance only. That is, the same logical session will be
given different Session IDs by each end of the control connection given different Session IDs by each end of the control connection
for the life of the session. When the L2TP control connection is for the life of the session. When the L2TP control connection is
used for session establishment, Session IDs are selected and used for session establishment, Session IDs are selected and
exchanged as Local Session ID AVPs during the creation of a exchanged as Local Session ID AVPs during the creation of a
session. The Session ID alone provides the necessary context for session. The Session ID alone provides the necessary context for
all further packet processing, including the presence, size and all further packet processing, including the presence, size, and
value of the Cookie, L2-Specific Sublayer, and the type of payload value of the Cookie, the type of L2-Specific Sublayer, and the
being tunneled. type of payload being tunneled.
Cookie Cookie
The optional Cookie field contains a variable length (maximum 64 The optional Cookie field contains a variable-length value
bits) value used to check the association of a received data (maximum 64 bits) used to check the association of a received data
message with the session identified by the Session ID. The Cookie message with the session identified by the Session ID. The Cookie
MUST be set to the configured or signaled random value for this MUST be set to the configured or signaled random value for this
session. The Cookie provides an additional level of guarantee session. The Cookie provides an additional level of guarantee
that a data message has been directed to the proper session by the that a data message has been directed to the proper session by the
Session ID. A well-chosen Cookie may prevent inadvertent Session ID. A well-chosen Cookie may prevent inadvertent
misdirection of stray packets with recently reused Session IDs, misdirection of stray packets with recently reused Session IDs,
Session IDs subject to packet corruption, etc. The Cookie may Session IDs subject to packet corruption, etc. The Cookie may
also provide protection against some specific malicious packet also provide protection against some specific malicious packet
insertion attacks, as described in section 8.2. insertion attacks, as described in Section 8.2.
When the L2TP control connection is used for session When the L2TP control connection is used for session
establishment, random Cookie values are selected and exchanged as establishment, random Cookie values are selected and exchanged as
Assigned Cookie AVPs during session creation. Assigned Cookie AVPs during session creation.
4.1.1 L2TPv3 over IP 4.1.1. L2TPv3 over IP
L2TPv3 over IP (both versions) utilizes the IANA assigned IP protocol L2TPv3 over IP (both versions) utilizes the IANA-assigned IP protocol
ID 115. ID 115.
4.1.1.1 L2TPv3 Session Header Over IP 4.1.1.1. L2TPv3 Session Header Over IP
Unlike L2TP over UDP, the L2TPv3 session header over IP is free of Unlike L2TP over UDP, the L2TPv3 session header over IP is free of
any restrictions imposed by coexistence with L2TPv2 and L2F. As any restrictions imposed by coexistence with L2TPv2 and L2F. As
such, the header format has been designed to optimize packet such, the header format has been designed to optimize packet
processing. The following session header format is utilized when processing. The following session header format is utilized when
operating L2TPv3 over IP: operating L2TPv3 over IP:
Figure 4.1.1.1: L2TPv3 Session Header Over IP Figure 4.1.1.1: L2TPv3 Session Header Over IP
0 1 2 3 0 1 2 3
skipping to change at page 18, line 35 skipping to change at page 18, line 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cookie (optional, maximum 64 bits)... | Cookie (optional, maximum 64 bits)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Session ID and Cookie fields are as defined in Section 4.1. The The Session ID and Cookie fields are as defined in Section 4.1. The
Session ID of zero is reserved for use by L2TP control messages (see Session ID of zero is reserved for use by L2TP control messages (see
Section 4.1.1.2). Section 4.1.1.2).
4.1.1.2 L2TP Control and Data Traffic over IP 4.1.1.2. L2TP Control and Data Traffic over IP
Unlike L2TP over UDP which uses the T bit to distinguish between L2TP Unlike L2TP over UDP, which uses the T bit to distinguish between
control and data packets, L2TP over IP uses the reserved Session ID L2TP control and data packets, L2TP over IP uses the reserved Session
of zero (0) when sending control messages. It is presumed that ID of zero (0) when sending control messages. It is presumed that
checking for the zero Session ID is more efficient -- both in header checking for the zero Session ID is more efficient -- both in header
size for data packets and in processing speed for distinguishing size for data packets and in processing speed for distinguishing
between control and data messages -- than checking a single bit. between control and data messages -- than checking a single bit.
The entire control message header over IP, including the zero session The entire control message header over IP, including the zero session
ID, appears as follows: ID, appears as follows:
Figure 4.1.1.2: L2TPv3 Control Message Header Over IP Figure 4.1.1.2: L2TPv3 Control Message Header Over IP
0 1 2 3 0 1 2 3
skipping to change at page 19, line 26 skipping to change at page 18, line 38
| Ns | Nr | | Ns | Nr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Named fields are as defined in Section 3.2.1. Note that the Length Named fields are as defined in Section 3.2.1. Note that the Length
field is still calculated from the beginning of the control message field is still calculated from the beginning of the control message
header, beginning with the T bit. It does NOT include the "(32 bits header, beginning with the T bit. It does NOT include the "(32 bits
of zeros)" depicted above. of zeros)" depicted above.
When operating directly over IP, L2TP packets lose the ability to When operating directly over IP, L2TP packets lose the ability to
take advantage of the UDP checksum as a simple packet integrity take advantage of the UDP checksum as a simple packet integrity
check. This is of particular concern for L2TP control messages. check, which is of particular concern for L2TP control messages.
Control Message Authentication (Section 4.3), even with an empty Control Message Authentication (see Section 4.3), even with an empty
password field, provides for a sufficient packet integrity check and password field, provides for a sufficient packet integrity check and
SHOULD always be enabled. SHOULD always be enabled.
4.1.2 L2TP over UDP 4.1.2. L2TP over UDP
L2TPv3 over UDP must consider other L2 tunneling protocols that may L2TPv3 over UDP must consider other L2 tunneling protocols that may
be operating in the same environment, including L2TPv2 [RFC2661] and be operating in the same environment, including L2TPv2 [RFC2661] and
L2F [RFC2341]. L2F [RFC2341].
While there are efficiencies gained by running L2TP directly over IP, While there are efficiencies gained by running L2TP directly over IP,
there are possible side effects as well. For instance, L2TP over IP there are possible side effects as well. For instance, L2TP over IP
is not as NAT-friendly as L2TP over UDP. is not as NAT-friendly as L2TP over UDP.
4.1.2.1 L2TP Session Header Over UDP 4.1.2.1. L2TP Session Header Over UDP
The following session header format is utilized when operating L2TPv3 The following session header format is utilized when operating L2TPv3
over UDP: over UDP:
Figure 4.1.2.1: L2TPv3 Session Header over UDP Figure 4.1.2.1: L2TPv3 Session Header over UDP
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T|x|x|x|x|x|x|x|x|x|x|x| Ver | Reserved | |T|x|x|x|x|x|x|x|x|x|x|x| Ver | Reserved |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The T bit MUST be set to 0, indicating that this is a data message. The T bit MUST be set to 0, indicating that this is a data message.
The x bits and Reserved field are reserved for future extensions. The x bits and Reserved field are reserved for future extensions.
All reserved values MUST be set to 0 on outgoing messages and ignored All reserved values MUST be set to 0 on outgoing messages and ignored
on incoming messages. on incoming messages.
The Ver field MUST be set to 3, indicating an L2TPv3 message. The Ver field MUST be set to 3, indicating an L2TPv3 message.
Note that the initial bits 1, 4, 6 and 7 have meaning in L2TPv2 Note that the initial bits 1, 4, 6, and 7 have meaning in L2TPv2
[RFC2661], and are deprecated and marked as reserved in L2TPv3. Thus, [RFC2661], and are deprecated and marked as reserved in L2TPv3.
for UDP mode on a system that supports both versions of L2TP, it is Thus, for UDP mode on a system that supports both versions of L2TP,
important that the Ver field be inspected first to determine the it is important that the Ver field be inspected first to determine
Version of the header before acting upon any of these bits. the Version of the header before acting upon any of these bits.
The Session ID and Cookie fields are as defined in Section 4.1. The Session ID and Cookie fields are as defined in Section 4.1.
4.1.2.2 UDP Port Selection 4.1.2.2. UDP Port Selection
The method for UDP Port Selection defined in this section is The method for UDP Port Selection defined in this section is
identical to than defined for L2TPv2 [RFC2661]. identical to that defined for L2TPv2 [RFC2661].
When negotiating a control connection over UDP, control messages MUST When negotiating a control connection over UDP, control messages MUST
be sent as UDP datagrams using the registered UDP port 1701 be sent as UDP datagrams using the registered UDP port 1701
[RFC1700]. The initiator of an L2TP control connection picks an [RFC1700]. The initiator of an L2TP control connection picks an
available source UDP port (which may or may not be 1701), and sends available source UDP port (which may or may not be 1701) and sends to
to the desired destination address at port 1701. The recipient picks the desired destination address at port 1701. The recipient picks a
a free port on its own system (which may or may not be 1701) and free port on its own system (which may or may not be 1701) and sends
sends its reply to the initiator's UDP port and address, setting its its reply to the initiator's UDP port and address, setting its own
own source port to the free port it found. source port to the free port it found.
Any subsequent traffic associated with this control connection Any subsequent traffic associated with this control connection
(either control traffic or data traffic from a session established (either control traffic or data traffic from a session established
through this control connection) must use these same UDP ports. through this control connection) must use these same UDP ports.
It has been suggested that having the recipient choose an arbitrary It has been suggested that having the recipient choose an arbitrary
source port (as opposed to using the destination port in the packet source port (as opposed to using the destination port in the packet
initiating the control connection, i.e., 1701) may make it more initiating the control connection, i.e., 1701) may make it more
difficult for L2TP to traverse some NAT devices. Implementations difficult for L2TP to traverse some NAT devices. Implementations
should consider the potential implication of this capability before should consider the potential implication of this capability before
choosing an arbitrary source port. A NAT device that can pass TFTP choosing an arbitrary source port. A NAT device that can pass TFTP
traffic with variant UDP ports should be able to pass L2TP UDP traffic with variant UDP ports should be able to pass L2TP UDP
traffic since both protocols employ similar policies with regard to traffic since both protocols employ similar policies with regard to
UDP port selection. UDP port selection.
4.1.2.3 UDP Checksum 4.1.2.3. UDP Checksum
The tunneled frames which L2TP carry often have their own checksum or The tunneled frames that L2TP carry often have their own checksums or
integrity checks, rendering the UDP checksum redundant for much of integrity checks, rendering the UDP checksum redundant for much of
the L2TP data message contents. Thus, UDP checksums MAY be disabled the L2TP data message contents. Thus, UDP checksums MAY be disabled
in order to reduce the associated packet processing burden at the in order to reduce the associated packet processing burden at the
L2TP endpoints. L2TP endpoints.
The L2TP header itself does not have its own checksum or integrity The L2TP header itself does not have its own checksum or integrity
check. However, use of the L2TP Session ID and Cookie pair guards check. However, use of the L2TP Session ID and Cookie pair guards
against accepting an L2TP data message if corruption of the Session against accepting an L2TP data message if corruption of the Session
ID or associated Cookie has occurred. When the L2-Specific-Sublayer ID or associated Cookie has occurred. When the L2-Specific Sublayer
is present in the L2TP header, there is no built-in integrity check is present in the L2TP header, there is no built-in integrity check
for the information contained therein if UDP checksums or some other for the information contained therein if UDP checksums or some other
integrity check is not employed. IPsec (Section 4.1.3) may be used integrity check is not employed. IPsec (see Section 4.1.3) may be
for strong integrity protection of the entire contents of L2TP data used for strong integrity protection of the entire contents of L2TP
messages. data messages.
UDP checksums MUST be enabled for L2TP control messages. UDP checksums MUST be enabled for L2TP control messages.
4.1.3 L2TP and IPsec 4.1.3. L2TP and IPsec
The L2TP data channel does not provide cryptographic security of any The L2TP data channel does not provide cryptographic security of any
kind. If the L2TP data channel operates over a public or untrusted IP kind. If the L2TP data channel operates over a public or untrusted
network where privacy of the L2TP data is of concern or sophisticated IP network where privacy of the L2TP data is of concern or
attacks against L2TP are expected to occur, IPsec [RFC2401] MUST be sophisticated attacks against L2TP are expected to occur, IPsec
made available to secure the L2TP traffic. [RFC2401] MUST be made available to secure the L2TP traffic.
Either L2TP over UDP or L2TP over IP may be secured with IPsec. Either L2TP over UDP or L2TP over IP may be secured with IPsec.
[RFC3193] defines the recommended method for securing L2TPv2. L2TPv3 [RFC3193] defines the recommended method for securing L2TPv2. L2TPv3
possesses identical characteristics to IPsec as L2TPv2 when running possesses identical characteristics to IPsec as L2TPv2 when running
over UDP and implementations MUST follow the same recommendation. over UDP and implementations MUST follow the same recommendation.
When operating over IP directly, [RFC3193] still applies, though When operating over IP directly, [RFC3193] still applies, though
references to UDP source and destination ports (in particular those references to UDP source and destination ports (in particular, those
in Section 4 "IPsec Filtering details when protecting L2TP") may be in Section 4, "IPsec Filtering details when protecting L2TP") may be
ignored. Instead, the selectors used to identify L2TPv3 traffic are ignored. Instead, the selectors used to identify L2TPv3 traffic are
simply the source and destination IP address for the tunnel endpoints simply the source and destination IP addresses for the tunnel
together with the L2TPv3 IP protocol type, 115. endpoints together with the L2TPv3 IP protocol type, 115.
In addition to IP transport security, IPsec defines a mode of In addition to IP transport security, IPsec defines a mode of
operation that allows tunneling of IP packets. The packet-level operation that allows tunneling of IP packets. The packet-level
encryption and authentication provided by IPsec tunnel mode and that encryption and authentication provided by IPsec tunnel mode and that
provided by L2TP secured with IPsec provide an equivalent level of provided by L2TP secured with IPsec provide an equivalent level of
security for these requirements. security for these requirements.
IPsec also defines access control features that are required of a IPsec also defines access control features that are required of a
compliant IPsec implementation. These features allow filtering of compliant IPsec implementation. These features allow filtering of
packets based upon network and transport layer characteristics such packets based upon network and transport layer characteristics such
as IP address, ports, etc. In the L2TP tunneling model, analogous as IP address, ports, etc. In the L2TP tunneling model, analogous
filtering may be performed at the network layer above L2TP. These filtering may be performed at the network layer above L2TP. These
network layer access control features may be handled at an LCCE via network layer access control features may be handled at an LCCE via
vendor-specific authorization features, or at the network layer vendor-specific authorization features, or at the network layer
itself by using IPsec transport mode end-to-end between the itself by using IPsec transport mode end-to-end between the
communicating hosts. The requirements for access control mechanisms communicating hosts. The requirements for access control mechanisms
are not a part of the L2TP specification and as such are outside the are not a part of the L2TP specification, and as such, are outside
scope of this document. the scope of this document.
Protecting the L2TP packet stream with IPsec does, in turn, also Protecting the L2TP packet stream with IPsec does, in turn, also
protect the data within the tunneled session packets while protect the data within the tunneled session packets while
transported from one LCCE to the other. Such protection must not be transported from one LCCE to the other. Such protection must not be
considered a substitution for end-to-end security between considered a substitution for end-to-end security between
communicating hosts or applications. communicating hosts or applications.
4.1.4 IP Fragmentation Issues 4.1.4. IP Fragmentation Issues
Fragmentation and reassembly in network equipment generally requires Fragmentation and reassembly in network equipment generally require
significantly greater resources than sending or receiving a packet as significantly greater resources than sending or receiving a packet as
a single unit. As such, it should be avoided whenever possible. Ideal a single unit. As such, fragmentation and reassembly should be
solutions for avoiding fragmentation include proper configuration and avoided whenever possible. Ideal solutions for avoiding
management of MTU sizes between the Remote System, LCCE and across fragmentation include proper configuration and management of MTU
the IP network, as well as adaptive measures which operate with the sizes among the Remote System, the LCCE, and the IP network, as well
originating host (e.g. [RFC1191], [RFC1981]) to reduce the packet as adaptive measures that operate with the originating host (e.g.,
sizes at the source. [RFC1191], [RFC1981]) to reduce the packet sizes at the source.
An LCCE MAY fragment a packet before encapsulating it in L2TP. For An LCCE MAY fragment a packet before encapsulating it in L2TP. For
example, if an IPv4 packet arrives at an LCCE from a Remote System example, if an IPv4 packet arrives at an LCCE from a Remote System
which, after encapsulation with its associated framing, L2TP and IP, that, after encapsulation with its associated framing, L2TP, and IP,
does not fit in the available path MTU towards its LCCE peer, the does not fit in the available path MTU towards its LCCE peer, the
local LCCE may perform IPv4 fragmentation on the packet before tunnel local LCCE may perform IPv4 fragmentation on the packet before tunnel
encapsulation. This creates two (or more) L2TP packets, each carrying encapsulation. This creates two (or more) L2TP packets, each
an IPv4 fragment with its associated framing. This ultimately has the carrying an IPv4 fragment with its associated framing. This
affect of placing the burden of fragmentation on the LCCE, but ultimately has the effect of placing the burden of fragmentation on
reassembly on the IPv4 destination host. the LCCE, while reassembly occurs on the IPv4 destination host.
If an IPv6 packet arrives at an LCCE from a Remote System which, If an IPv6 packet arrives at an LCCE from a Remote System that, after
after encapsulation with associated framing, L2TP and IP, does not encapsulation with associated framing, L2TP and IP, does not fit in
fit in the available path MTU towards its L2TP peer, the Generic the available path MTU towards its L2TP peer, the Generic Packet
Packet Tunneling specification section 7.1 [RFC2473] SHOULD be Tunneling specification [RFC2473], Section 7.1 SHOULD be followed. In
followed, leading to either sending an ICMP Packet Too Big message to this case, the LCCE should either send an ICMP Packet Too Big message
the data source, or fragmenting the resultant L2TP/IP packet to be to the data source, or fragment the resultant L2TP/IP packet (for
reassembled by the L2TP peer. reassembly by the L2TP peer).
If the amount of traffic requiring fragmentation and reassembly is If the amount of traffic requiring fragmentation and reassembly is
rather light, or there are sufficiently optimized mechanisms at the rather light, or there are sufficiently optimized mechanisms at the
tunnel endpoints, fragmentation of the L2TP/IP packet may be tunnel endpoints, fragmentation of the L2TP/IP packet may be
sufficient for accommodating mismatched MTUs which cannot be managed sufficient for accommodating mismatched MTUs that cannot be managed
by more efficient means. This method effectively emulates a larger by more efficient means. This method effectively emulates a larger
MTU between tunnel endpoints and should work for any type of layer 2 MTU between tunnel endpoints and should work for any type of L2-
encapsulated packet. Note that IPv6 does not support "in-flight" encapsulated packet. Note that IPv6 does not support "in-flight"
fragmentation of data packets. Thus, unlike IPv4, the MTU of the path fragmentation of data packets. Thus, unlike IPv4, the MTU of the
towards an L2TP peer must be known in advance (or the last resort path towards an L2TP peer must be known in advance (or the last
IPv6 minimum MTU of 1280 bytes utilized) so that IPv6 fragmentation resort IPv6 minimum MTU of 1280 bytes utilized) so that IPv6
may occur at the LCCE. fragmentation may occur at the LCCE.
In summary, attempting to control the source MTU by communicating In summary, attempting to control the source MTU by communicating
with the originating host, forcing that an MTU be sufficiently large with the originating host, forcing that an MTU be sufficiently large
on the path between LCCE peers to tunnel a frame from any other on the path between LCCE peers to tunnel a frame from any other
interface without fragmentation, fragmenting IP packets before interface without fragmentation, fragmenting IP packets before
encapsulation with L2TP/IP, or fragmenting the resultant L2TP/IP encapsulation with L2TP/IP, or fragmenting the resultant L2TP/IP
packet between the tunnel endpoints, are all valid methods for packet between the tunnel endpoints, are all valid methods for
managing MTU mismatches. Some are clearly better than others managing MTU mismatches. Some are clearly better than others
depending on the given deployment. For example, a passive monitoring depending on the given deployment. For example, a passive monitoring
application using L2TP would certainly not wish to have ICMP messages application using L2TP would certainly not wish to have ICMP messages
sent to a traffic source. Further, if the links connecting a set of sent to a traffic source. Further, if the links connecting a set of
LCCEs has a very large MTU (e.g., SDH/SONET) and it is known that the LCCEs have a very large MTU (e.g., SDH/SONET) and it is known that
MTU of all links being tunneled by L2TP have smaller MTUs (e.g. 1500 the MTU of all links being tunneled by L2TP have smaller MTUs (e.g.,
bytes), then any IP fragmentation and reassembly enabled on the 1500 bytes), then any IP fragmentation and reassembly enabled on the
participating LCCEs would never be utilized. An implementation MUST participating LCCEs would never be utilized. An implementation MUST
implement at least one of the methods described in this section for implement at least one of the methods described in this section for
managing mismatched MTUs, based on careful consideration of how the managing mismatched MTUs, based on careful consideration of how the
final product will be deployed. final product will be deployed.
L2TP-specific fragmentation and reassembly methods (which may or may L2TP-specific fragmentation and reassembly methods, which may or may
not depend on the characteristics of the type of link being tunneled, not depend on the characteristics of the type of link being tunneled
i.e., judicious packing of ATM cells) may be defined as well, but are (e.g., judicious packing of ATM cells), may be defined as well, but
outside the scope of this document. these methods are outside the scope of this document.
4.2 Reliable Delivery of Control Messages 4.2. Reliable Delivery of Control Messages
L2TP provides a lower level reliable delivery service for all control L2TP provides a lower level reliable delivery service for all control
messages. The Nr and Ns fields of the control message header (see messages. The Nr and Ns fields of the control message header (see
Section 3.2.1) belong to this delivery mechanism. The upper level Section 3.2.1) belong to this delivery mechanism. The upper level
functions of L2TP are not concerned with retransmission or ordering functions of L2TP are not concerned with retransmission or ordering
of control messages. The reliable control messaging mechanism is a of control messages. The reliable control messaging mechanism is a
sliding window mechanism that provides control message retransmission sliding window mechanism that provides control message retransmission
and congestion control. Each peer maintains separate sequence number and congestion control. Each peer maintains separate sequence number
state for each control connection. state for each control connection.
skipping to change at page 24, line 30 skipping to change at page 23, line 39
acknowledged by the reliable delivery mechanism. This acknowledgment acknowledged by the reliable delivery mechanism. This acknowledgment
may either piggybacked on a message in queue or sent explicitly via may either piggybacked on a message in queue or sent explicitly via
an ACK message. an ACK message.
All control messages take up one slot in the control message sequence All control messages take up one slot in the control message sequence
number space, except the ACK message. Thus, Ns is not incremented number space, except the ACK message. Thus, Ns is not incremented
after an ACK message is sent. after an ACK message is sent.
The last received message number, Nr, is used to acknowledge messages The last received message number, Nr, is used to acknowledge messages
received by an L2TP peer. It contains the sequence number of the received by an L2TP peer. It contains the sequence number of the
message the peer expects to receive next (e.g. the last Ns of a non- message the peer expects to receive next (e.g., the last Ns of a
ACK message received plus 1, modulo 65536). While the Nr in a non-ACK message received plus 1, modulo 65536). While the Nr in a
received ACK message is used to flush messages from the local received ACK message is used to flush messages from the local
retransmit queue (see below), the Nr of the next message sent is not retransmit queue (see below), the Nr of the next message sent is not
updated by the Ns of the ACK message. Nr SHOULD be sanity-checked updated by the Ns of the ACK message. Nr SHOULD be sanity-checked
before flushing the retransmit queue. For instance, if the Nr before flushing the retransmit queue. For instance, if the Nr
received in a control message is greater than the last Ns sent plus 1 received in a control message is greater than the last Ns sent plus 1
modulo 65536, the control message is clearly invalid. modulo 65536, the control message is clearly invalid.
The reliable delivery mechanism at a receiving peer is responsible The reliable delivery mechanism at a receiving peer is responsible
for making sure that control messages are delivered in order and for making sure that control messages are delivered in order and
without duplication to the upper level. Messages arriving out of without duplication to the upper level. Messages arriving out-of-
order may be queued for in-order delivery when the missing messages order may be queued for in-order delivery when the missing messages
are received. Alternatively, they may be discarded, thus requiring a are received. Alternatively, they may be discarded, thus requiring a
retransmission by the peer. When dropping out of order control retransmission by the peer. When dropping out-of-order control
packets, Nr MAY be updated before the packet is discarded. packets, Nr MAY be updated before the packet is discarded.
Each control connection maintains a queue of control messages to be Each control connection maintains a queue of control messages to be
transmitted to its peer. The message at the front of the queue is transmitted to its peer. The message at the front of the queue is
sent with a given Ns value and is held until a control message sent with a given Ns value and is held until a control message
arrives from the peer in which the Nr field indicates receipt of this arrives from the peer in which the Nr field indicates receipt of this
message. After a period of time (a recommended default is 1 second message. After a period of time (a recommended default is 1 second
but SHOULD be configurable) passes without acknowledgment, the but SHOULD be configurable) passes without acknowledgment, the
message is retransmitted. The retransmitted message contains the message is retransmitted. The retransmitted message contains the
same Ns value, but the Nr value MUST be updated with the sequence same Ns value, but the Nr value MUST be updated with the sequence
number of the next expected message. number of the next expected message.
Each subsequent retransmission of a message MUST employ an Each subsequent retransmission of a message MUST employ an
exponential backoff interval. Thus, if the first retransmission exponential backoff interval. Thus, if the first retransmission
occurred after 1 second, the next retransmission should occur after 2 occurred after 1 second, the next retransmission should occur after 2
seconds has elapsed, then 4 seconds, etc. An implementation MAY seconds has elapsed, then 4 seconds, etc. An implementation MAY
place a cap upon the maximum interval between retransmissions. This place a cap upon the maximum interval between retransmissions. This
cap SHOULD be no less than 8 seconds per retransmission. If no peer cap SHOULD be no less than 8 seconds per retransmission. If no peer
response is detected after several retransmissions (a recommended response is detected after several retransmissions (a recommended
default is 10, but MUST be configurable), the control connection and default is 10, but MUST be configurable), the control connection and
all associated sessions MUST be cleared. As it is the first message all associated sessions MUST be cleared. As it is the first message
to establish a control connection, the SCCRQ MAY employ a different to establish a control connection, the SCCRQ MAY employ a different
retransmission maximum than other control messages in order to help retransmission maximum than other control messages in order to help
facilitate failover to alternate LCCEs in a timely fashion. facilitate failover to alternate LCCEs in a timely fashion.
When a control connection is being shut down for reasons other than When a control connection is being shut down for reasons other than
loss of connectivity, the state and reliable delivery mechanisms MUST loss of connectivity, the state and reliable delivery mechanisms MUST
be maintained and operated for the full retransmission interval after be maintained and operated for the full retransmission interval after
the final message StopCCN message has been sent (e.g. 1 + 2 + 4 + 8 + the final message StopCCN message has been sent (e.g., 1 + 2 + 4 + 8
8... seconds), or until the StopCCN message itself has been + 8... seconds), or until the StopCCN message itself has been
acknowledged. acknowledged.
A sliding window mechanism is used for control message transmission A sliding window mechanism is used for control message transmission
and retransmission. Consider two peers, A and B. Suppose A and retransmission. Consider two peers, A and B. Suppose A
specifies a Receive Window Size AVP with a value of N in the SCCRQ or specifies a Receive Window Size AVP with a value of N in the SCCRQ or
SCCRP message. B is now allowed to have a maximum of N outstanding SCCRP message. B is now allowed to have a maximum of N outstanding
(e.g. unacknowledged) control messages. Once N messages have been (i.e., unacknowledged) control messages. Once N messages have been
sent, B must wait for an acknowledgment from A that advances the sent, B must wait for an acknowledgment from A that advances the
window before sending new control messages. An implementation may window before sending new control messages. An implementation may
advertise a non-zero receive window as small or as large as it advertise a non-zero receive window as small or as large as it
wishes, depending on its own ability to process incoming messages wishes, depending on its own ability to process incoming messages
before sending an acknowledgement. Each peer MUST limit the number of before sending an acknowledgement. Each peer MUST limit the number
unacknowledged messages it will send before receiving an of unacknowledged messages it will send before receiving an
acknowledgement by this Receive Window Size. The actual internal acknowledgement by this Receive Window Size. The actual internal
unacknowledged message send-queue depth may be further limited by unacknowledged message send-queue depth may be further limited by
local resource allocation or by dynamic slow-start and congestion- local resource allocation or by dynamic slow-start and congestion-
avoidance mechanisms. avoidance mechanisms.
When retransmitting control messages, a slow start and congestion When retransmitting control messages, a slow start and congestion
avoidance window adjustment procedure SHOULD be utilized. A avoidance window adjustment procedure SHOULD be utilized. A
recommended procedure is described in Appendix A. A peer MAY drop recommended procedure is described in Appendix A. A peer MAY drop
messages, but MUST NOT actively delay acknowledgment of messages as a messages, but MUST NOT actively delay acknowledgment of messages as a
technique for flow control of control messages. Appendix B contains technique for flow control of control messages. Appendix B contains
examples of control message transmission, acknowledgment, and examples of control message transmission, acknowledgment, and
retransmission. retransmission.
4.3 Control Connection and Control Message Authentication 4.3. Control Message Authentication
L2TP incorporates an optional authentication and integrity check for L2TP incorporates an optional authentication and integrity check for
all control messages. This mechanism consists of a computed one-way all control messages. This mechanism consists of a computed one-way
hash over the header and body of the L2TP control message, a pre- hash over the header and body of the L2TP control message, a pre-
configured shared secret, and a local and remote nonce (random value) configured shared secret, and a local and remote nonce (random value)
exchanged via the Nonce AVP. This per-message authentication and exchanged via the Control Message Authentication Nonce AVP. This
integrity check is designed to perform a mutual authentication per-message authentication and integrity check is designed to perform
between L2TP nodes, integrity checking of all control messages, and a mutual authentication between L2TP nodes, perform integrity
guard against control message spoofing and replay attacks that would checking of all control messages, and guard against control message
otherwise be trivial to mount. spoofing and replay attacks that would otherwise be trivial to mount.
At least one shared secret (password) MUST exist between At least one shared secret (password) MUST exist between
communicating L2TP nodes to enable control message authentication. communicating L2TP nodes to enable Control Message Authentication.
See Section 5.4.3 for details on calculation of the Message Digest See Section 5.4.3 for details on calculation of the Message Digest
and construction of the Nonce and Message Digest AVPs. and construction of the Control Message Authentication Nonce and
Message Digest AVPs.
L2TPv3 Control Connection and Control Message Authentication is L2TPv3 Control Message Authentication is similar to L2TPv2 [RFC2661]
similar to L2TPv2 [RFC2661] Tunnel Authentication in its use of a Tunnel Authentication in its use of a shared secret and one-way hash
shared secret and one-way hash calculation. The principal difference calculation. The principal difference is that, instead of computing
is that, instead of computing the hash over selected contents of a the hash over selected contents of a received control message (e.g.,
received control message (e.g. the Challenge AVP and Message Type) as the Challenge AVP and Message Type) as in L2TPv2, the entire message
in L2TPv2, the entire message is used in the hash in L2TPv3. In is used in the hash in L2TPv3. In addition, instead of including the
addition, instead of including the hash digest in just the SCCRP and hash digest in just the SCCRP and SCCCN messages, it is now included
SCCCN messages, it is now included in all L2TP messages. in all L2TP messages.
The Control Message Authentication mechanism is optional, and may be The Control Message Authentication mechanism is optional, and may be
disabled if both peers agree. For example, if IPsec is already being disabled if both peers agree. For example, if IPsec is already being
used for security and integrity checking between the LCCEs, the used for security and integrity checking between the LCCEs, the
function of the L2TP mechanism becomes redundant and may be disabled. function of the L2TP mechanism becomes redundant and may be disabled.
Presence of the Control Message Authentication Nonce AVP in an SCCRQ Presence of the Control Message Authentication Nonce AVP in an SCCRQ
or SCCRP message serves as indication to a peer that Control Message or SCCRP message serves as indication to a peer that Control Message
Authentication is enabled. If an SCCRQ or SCCRP contains a Control Authentication is enabled. If an SCCRQ or SCCRP contains a Control
Message Authentication Nonce AVP, the receiver of the message MUST Message Authentication Nonce AVP, the receiver of the message MUST
respond with a Message Digest AVP in all subsequent messages sent. respond with a Message Digest AVP in all subsequent messages sent.
Control Connection and Control Message Authentication is always Control Message Authentication is always bidirectional; either both
bidirectional, either both sides participate in authentication or sides participate in authentication, or neither does.
neither do.
If the Control Message Authentication is disabled, the Message Digest If Control Message Authentication is disabled, the Message Digest AVP
AVP still MAY be sent as an integrity check of the message. The still MAY be sent as an integrity check of the message. The
integrity check is calculated as in Section 5.4.3, with an empty integrity check is calculated as in Section 5.4.3, with an empty
zero-length shared secret, local nonce, and remote nonce. If an zero-length shared secret, local nonce, and remote nonce. If an
invalid Message Digest is received, it should be assumed that the invalid Message Digest is received, it should be assumed that the
message has been corrupted in transit and the message dropped message has been corrupted in transit and the message dropped
accordingly. accordingly.
Implementations MAY rate-limit control messages, particularly SCCRQ Implementations MAY rate-limit control messages, particularly SCCRQ
messages, upon receipt for performance reasons or to protect against messages, upon receipt for performance reasons or for protection
denial of service attacks. against denial of service attacks.
4.4 Keepalive (Hello) 4.4. Keepalive (Hello)
L2TP employs a keepalive mechanism to detect loss of connectivity L2TP employs a keepalive mechanism to detect loss of connectivity
between a pair of LCCEs. This is accomplished by injecting Hello between a pair of LCCEs. This is accomplished by injecting Hello
control messages (see Section 6.5) after a period of time has elapsed control messages (see Section 6.5) after a period of time has elapsed
since the last data message or control message was received on an since the last data message or control message was received on an
L2TP session or control connection, respectively. As with any other L2TP session or control connection, respectively. As with any other
control message, if the Hello message is not reliably delivered, the control message, if the Hello message is not reliably delivered, the
sending LCCE declares that the control connection is down and resets sending LCCE declares that the control connection is down and resets
its state for the control connection. This behavior ensures that a its state for the control connection. This behavior ensures that a
connectivity failure between the LCCEs is detected independently by connectivity failure between the LCCEs is detected independently by
skipping to change at page 27, line 35 skipping to change at page 26, line 46
the control connection. the control connection.
Periodic keepalive for the control connection MUST be implemented by Periodic keepalive for the control connection MUST be implemented by
sending a Hello if a period of time (a recommended default is 60 sending a Hello if a period of time (a recommended default is 60
seconds, but MUST be configurable) has passed without receiving any seconds, but MUST be configurable) has passed without receiving any
message (data or control) from the peer. An LCCE sending Hello message (data or control) from the peer. An LCCE sending Hello
messages across multiple control connections between the same LCCE messages across multiple control connections between the same LCCE
endpoints MUST employ a jittered timer mechanism to prevent grouping endpoints MUST employ a jittered timer mechanism to prevent grouping
of Hello messages. of Hello messages.
4.5 Forwarding Session Data Frames 4.5. Forwarding Session Data Frames
Once session establishment is complete, circuit frames are received Once session establishment is complete, circuit frames are received
at an LCCE, encapsulated in L2TP (with appropriate attention to at an LCCE, encapsulated in L2TP (with appropriate attention to
framing as described in documents for the particular pseudowire framing, as described in documents for the particular pseudowire
type), and forwarded over the appropriate session. For every type), and forwarded over the appropriate session. For every
outgoing data message, the sender places the identifier specified in outgoing data message, the sender places the identifier specified in
the Local Session ID AVP (received from peer during session the Local Session ID AVP (received from peer during session
establishment) in the Session ID field of the L2TP data header. In establishment) in the Session ID field of the L2TP data header. In
this manner, session frames are multiplexed and demultiplexed between this manner, session frames are multiplexed and demultiplexed between
a given pair of LCCEs. Multiple control connections may exist a given pair of LCCEs. Multiple control connections may exist
between a given pair of LCCEs, and multiple sessions may be between a given pair of LCCEs, and multiple sessions may be
associated with a given control connection. associated with a given control connection.
The peer LCCE receiving the L2TP data packet identifies the session The peer LCCE receiving the L2TP data packet identifies the session
with which the packet is associated by the Session ID in the data with which the packet is associated by the Session ID in the data
packet's header. The LCCE then checks the Cookie field in the data packet's header. The LCCE then checks the Cookie field in the data
packet against the Cookie value received in the Assigned Cookie AVP packet against the Cookie value received in the Assigned Cookie AVP
during session establishment. It is important for implementers to during session establishment. It is important for implementers to
note that the Cookie field check occurs after looking up the session note that the Cookie field check occurs after looking up the session
context by the Session ID, and as such consists merely of a value context by the Session ID, and as such, consists merely of a value
match of the Cookie field and that stored in the retrieved context. match of the Cookie field and that stored in the retrieved context.
There is no need to perform a lookup across the Session ID and Cookie There is no need to perform a lookup across the Session ID and Cookie
as a single value. Any received data packets that contain invalid as a single value. Any received data packets that contain invalid
Session IDs or associated Cookie values MUST be dropped. Finally, Session IDs or associated Cookie values MUST be dropped. Finally,
the LCCE either forwards the network packet within the tunneled frame the LCCE either forwards the network packet within the tunneled frame
(e.g., as an LNS) or switches the frame to a circuit (e.g., as an (e.g., as an LNS) or switches the frame to a circuit (e.g., as an
LAC). LAC).
4.6 Default L2-Specific Sublayer 4.6. Default L2-Specific Sublayer
This document defines a default L2-Specific Sublayer (see Section This document defines a Default L2-Specific Sublayer format (see
3.2.2) format that a pseudowire may use for features such as Section 3.2.2) that a pseudowire may use for features such as
sequencing support, L2 interworking, OAM, or other per-data-packet sequencing support, L2 interworking, OAM, or other per-data-packet
operations. The default L2-Specific Sublayer SHOULD be used by a operations. The Default L2-Specific Sublayer SHOULD be used by a
given PW type to support these features if it is adequate, and its given PW type to support these features if it is adequate, and its
presence is requested by a peer during session negotiation. presence is requested by a peer during session negotiation.
Alternative sublayers MAY be defined (e.g. an encapsulation with a Alternative sublayers MAY be defined (e.g., an encapsulation with a
larger Sequence Number field or timing information) and identified larger Sequence Number field or timing information) and identified
for use via the L2-Specific Sublayer Type AVP. for use via the L2-Specific Sublayer Type AVP.
Figure 4.6: Default L2-Specific Sublayer Format Figure 4.6: Default L2-Specific Sublayer Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x|S|x|x|x|x|x|x| Sequence Number | |x|S|x|x|x|x|x|x| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
skipping to change at page 29, line 5 skipping to change at page 28, line 15
The Sequence Number field contains a free-running counter of 2^24 The Sequence Number field contains a free-running counter of 2^24
sequence numbers. If the number in this field is valid, the S bit sequence numbers. If the number in this field is valid, the S bit
MUST be set to 1. The Sequence Number begins at zero, which is a MUST be set to 1. The Sequence Number begins at zero, which is a
valid sequence number. (In this way, implementations inserting valid sequence number. (In this way, implementations inserting
sequence numbers do not have to "skip" zero when incrementing.) The sequence numbers do not have to "skip" zero when incrementing.) The
sequence number in the header of a received message is considered sequence number in the header of a received message is considered
less than or equal to the last received number if its value lies in less than or equal to the last received number if its value lies in
the range of the last received number and the preceding (2^23-1) the range of the last received number and the preceding (2^23-1)
values, inclusive. values, inclusive.
4.6.1 Sequencing Data Packets 4.6.1. Sequencing Data Packets
The Sequence Number field may be used to detect lost, duplicate, or The Sequence Number field may be used to detect lost, duplicate, or
out-of-order packets within a given session. out-of-order packets within a given session.
When L2 frames are carried over an L2TP-over-IP or L2TP-over-UDP/IP When L2 frames are carried over an L2TP-over-IP or L2TP-over-UDP/IP
data channel, this part of the link has the characteristic of being data channel, this part of the link has the characteristic of being
able to reorder, duplicate, or silently drop packets. Reordering may able to reorder, duplicate, or silently drop packets. Reordering may
break some non-IP protocols or L2 control traffic being carried by break some non-IP protocols or L2 control traffic being carried by
the link. Silent dropping or duplication of packets may break the link. Silent dropping or duplication of packets may break
protocols that assume per-packet indications of error, such as TCP protocols that assume per-packet indications of error, such as TCP
header compression. While a common mechanism for packet sequence header compression. While a common mechanism for packet sequence
detection is provided, the sequence dependency characteristics of detection is provided, the sequence dependency characteristics of
individual protocols are outside the scope of this document. individual protocols are outside the scope of this document.
If any protocol being transported by over L2TP data channels cannot If any protocol being transported by over L2TP data channels cannot
tolerate misordering of data packets, packet duplication, or silent tolerate misordering of data packets, packet duplication, or silent
packet loss, sequencing may be enabled on some or all packets by packet loss, sequencing may be enabled on some or all packets by
using the S bit and Sequence Number field defined in the default L2- using the S bit and Sequence Number field defined in the Default L2-
Specific Sublayer(see Section 4.6). For a given L2TP session, each Specific Sublayer (see Section 4.6). For a given L2TP session, each
LCCE is responsible for communicating to its peer the level of LCCE is responsible for communicating to its peer the level of
sequencing support that it requires of data packets that it receives. sequencing support that it requires of data packets that it receives.
Mechanisms to advertise this information during session negotiation Mechanisms to advertise this information during session negotiation
are provided (see Data Sequencing AVP in Section 5.4.4). are provided (see Data Sequencing AVP in Section 5.4.4).
When determining whether a packet is in or out of sequence, an When determining whether a packet is in or out of sequence, an
implementation SHOULD utilize a method that is resilient to temporary implementation SHOULD utilize a method that is resilient to temporary
dropouts in connectivity coupled with high per-session packet rates. dropouts in connectivity coupled with high per-session packet rates.
The recommended method is outlined in Appendix C. The recommended method is outlined in Appendix C.
4.7 L2TPv2/v3 Interoperability and Migration 4.7. L2TPv2/v3 Interoperability and Migration
L2TPv2 and L2TPv3 environments should be able to coexist while a L2TPv2 and L2TPv3 environments should be able to coexist while a
migration to L2TPv3 is made. Migration issues are discussed for each migration to L2TPv3 is made. Migration issues are discussed for each
media type in this section. Most issues apply only to media type in this section. Most issues apply only to
implementations that require both L2TPv2 and L2TPv3 operation. implementations that require both L2TPv2 and L2TPv3 operation.
However, even L2TPv3-only implementations must at least be mindful of However, even L2TPv3-only implementations must at least be mindful of
these issues in order to interoperate with implementations that these issues in order to interoperate with implementations that
support both versions. support both versions.
4.7.1 L2TPv3 over IP 4.7.1. L2TPv3 over IP
L2TPv3 implementations running strictly over IP with no desire to L2TPv3 implementations running strictly over IP with no desire to
interoperate with L2TPv2 implementations may safely disregard most interoperate with L2TPv2 implementations may safely disregard most
migration issues from L2TPv2. All control messages and data messages migration issues from L2TPv2. All control messages and data messages
are sent as described in this document, without normative reference are sent as described in this document, without normative reference
to RFC2661. to RFC 2661.
If one wishes to tunnel PPP over L2TPv3, and fallback to L2TPv2 only If one wishes to tunnel PPP over L2TPv3, and fallback to L2TPv2 only
if it is not available, then L2TPv3 over UDP with the automatic if it is not available, then L2TPv3 over UDP with automatic fallback
fallback as described in section 4.7.3 MUST be used. There is no (see Section 4.7.3) MUST be used. There is no deterministic method
deterministic method for automatic fallback from L2TPv3 over IP to for automatic fallback from L2TPv3 over IP to either L2TPv2 or L2TPv3
either L2TPv2 or L2TPv3 over UDP. One could infer whether L2TPv3 over over UDP. One could infer whether L2TPv3 over IP is supported by
IP is supported by sending an SCCRQ and waiting for a response, but sending an SCCRQ and waiting for a response, but this could be
this could be problematic during periods of packet loss between L2TP problematic during periods of packet loss between L2TP nodes.
nodes.
4.7.2 L2TPv3 over UDP 4.7.2. L2TPv3 over UDP
The format of the L2TPv3 over UDP header is defined in Section The format of the L2TPv3 over UDP header is defined in Section
4.1.2.1. 4.1.2.1.
When operating over UDP, L2TPv3 uses the same port (1701) as L2TPv2 When operating over UDP, L2TPv3 uses the same port (1701) as L2TPv2
and shares the first two octets of header format with L2TPv2. The Ver and shares the first two octets of header format with L2TPv2. The
field is used to distinguish L2TPv2 packets from L2TPv3 packets. If Ver field is used to distinguish L2TPv2 packets from L2TPv3 packets.
an implementation is capable of operating in L2TPv2 or L2TPv3 modes, If an implementation is capable of operating in L2TPv2 or L2TPv3
it is possible to automatically detect whether a peer can support modes, it is possible to automatically detect whether a peer can
L2TPv2 or L2TPv3 and operate accordingly. The details of this support L2TPv2 or L2TPv3 and operate accordingly. The details of
fallback capability is defined in the following section. this fallback capability is defined in the following section.
4.7.3 Automatic L2TPv2 Fallback 4.7.3. Automatic L2TPv2 Fallback
When running over UDP, an implementation may detect whether a peer is When running over UDP, an implementation may detect whether a peer is
L2TPv3-capable by sending a special SCCRQ that is properly formatted L2TPv3-capable by sending a special SCCRQ that is properly formatted
for both L2TPv2 and L2TPv3. This is accomplished by sending an SCCRQ for both L2TPv2 and L2TPv3. This is accomplished by sending an SCCRQ
with its Ver field set to 2 (for L2TPv2), and ensuring that any with its Ver field set to 2 (for L2TPv2), and ensuring that any
L2TPv3-specific AVPs (i.e. AVPs present within this document and not L2TPv3-specific AVPs (i.e., AVPs present within this document and not
defined within RFC 2661) within the message are sent with each M bit defined within RFC 2661) in the message are sent with each M bit set
set to 0, and all L2TPv2 AVPs present as they would be for L2TPv2. to 0, and that all L2TPv2 AVPs are present as they would be for
This is done so that L2TPv3 AVPs will be ignored by an L2TPv2-only L2TPv2. This is done so that L2TPv3 AVPs will be ignored by an
implementation. Note that, in both L2TPv2 and L2TPv3, the value L2TPv2-only implementation. Note that, in both L2TPv2 and L2TPv3,
contained in the space of the control message header utilized by the the value contained in the space of the control message header
32-bit Control Connection ID in L2TPv3, and the 16-bit Tunnel ID and utilized by the 32-bit Control Connection ID in L2TPv3, and the 16-
16-bit Session ID in L2TPv2, is always 0 for an SCCRQ. This bit Tunnel ID and
16-bit Session ID in L2TPv2, are always 0 for an SCCRQ. This
effectively hides the fact that there are a pair of 16-bit fields in effectively hides the fact that there are a pair of 16-bit fields in
L2TPv2, and a single 32-bit field in L2TPv3. L2TPv2, and a single 32-bit field in L2TPv3.
If the peer implementation is L2TPv3-capable, a control message with If the peer implementation is L2TPv3-capable, a control message with
Ver set to 3 and corresponding header and message format will be sent the Ver field set to 3 and an L2TPv3 header and message format will
in response to the SCCRQ. Operation may then continue as L2TPv3. If be sent in response to the SCCRQ. Operation may then continue as
a message is received with Ver set to 2, it must be assumed that the L2TPv3. If a message is received with the Ver field set to 2, it
peer implementation is L2TPv2-only and fallback to L2TPv2 mode may must be assumed that the peer implementation is L2TPv2-only, thus
safely occur. enabling fallback to L2TPv2 mode to safely occur.
Note Well: The L2TPv2/v3 auto-detection mode requires that all L2TPv3 Note Well: The L2TPv2/v3 auto-detection mode requires that all L2TPv3
implementations over UDP be liberal in acceptance of an SCCRQ control implementations over UDP be liberal in accepting an SCCRQ control
message with the Ver field set to 2 or 3 and the presence of L2TPv2- message with the Ver field set to 2 or 3 and the presence of L2TPv2-
specific AVPs. An L2TPv3-only implementation MUST ignore all L2TPv2 specific AVPs. An L2TPv3-only implementation MUST ignore all L2TPv2
AVPs (e.g. those defined in RFC 2661 and not in this document) within AVPs (e.g., those defined in RFC 2661 and not in this document)
an SCCRQ with the Ver field set to 2 (even if the M bit is set on the within an SCCRQ with the Ver field set to 2 (even if the M bit is set
L2TPv2-specific AVPs). on the L2TPv2-specific AVPs).
5. Control Message Attribute Value Pairs 5. Control Message Attribute Value Pairs
To maximize extensibility while permitting interoperability, a To maximize extensibility while permitting interoperability, a
uniform method for encoding message types is used throughout L2TP. uniform method for encoding message types is used throughout L2TP.
This encoding will be termed AVP (Attribute Value Pair) for the This encoding will be termed AVP (Attribute Value Pair) for the
remainder of this document. remainder of this document.
5.1 AVP Format 5.1. AVP Format
Each AVP is encoded as follows: Each AVP is encoded as follows:
Figure 5.1: AVP Format Figure 5.1: AVP Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|H| rsvd | Length | Vendor ID | |M|H| rsvd | Length | Vendor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
skipping to change at page 32, line 9 skipping to change at page 31, line 23
Section 5.3 describes the procedure for performing AVP hiding. Section 5.3 describes the procedure for performing AVP hiding.
Length: Contains the number of octets (including the Overall Length Length: Contains the number of octets (including the Overall Length
and bit mask fields) contained in this AVP. The Length may be and bit mask fields) contained in this AVP. The Length may be
calculated as 6 + the length of the Attribute Value field in octets. calculated as 6 + the length of the Attribute Value field in octets.
The field itself is 10 bits, permitting a maximum of 1023 octets of The field itself is 10 bits, permitting a maximum of 1023 octets of
data in a single AVP. The minimum Length of an AVP is 6. If the data in a single AVP. The minimum Length of an AVP is 6. If the
Length is 6, then the Attribute Value field is absent. Length is 6, then the Attribute Value field is absent.
Vendor ID: The IANA assigned "SMI Network Management Private Vendor ID: The IANA-assigned "SMI Network Management Private
Enterprise Codes" [RFC1700] value. The value 0, corresponding to Enterprise Codes" [RFC1700] value. The value 0, corresponding to
IETF adopted attribute values, is used for all AVPs defined within IETF-adopted attribute values, is used for all AVPs defined within
this document. Any vendor wishing to implement its own L2TP this document. Any vendor wishing to implement its own L2TP
extensions can use its own Vendor ID along with private Attribute extensions can use its own Vendor ID along with private Attribute
values, guaranteeing that they will not collide with any other values, guaranteeing that they will not collide with any other
vendor's extensions or future IETF extensions. Note that there are vendor's extensions or future IETF extensions. Note that there are
16 bits allocated for the Vendor ID, thus limiting this feature to 16 bits allocated for the Vendor ID, thus limiting this feature to
the first 65,535 enterprises. the first 65,535 enterprises.
Attribute Type: A 2-octet value with a unique interpretation across Attribute Type: A 2-octet value with a unique interpretation across
all AVPs defined under a given Vendor ID. all AVPs defined under a given Vendor ID.
skipping to change at page 32, line 39 skipping to change at page 32, line 12
specific AVPs with a 32-bit Vendor ID MUST be encapsulated in the specific AVPs with a 32-bit Vendor ID MUST be encapsulated in the
following manner: following manner:
Figure 5.2: Extended Vendor ID AVP Format Figure 5.2: Extended Vendor ID AVP Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|H| rsvd | Length | 0 | |M|H| rsvd | Length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AVP-TBA-0 | 32 bit Vendor ID ... | 58 | 32-bit Vendor ID ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute Type | | Attribute Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute Value ... | Attribute Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(until Length is reached) | (until Length is reached) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This AVP encodes a vendor-specific AVP with a 32-bit Vendor ID space This AVP encodes a vendor-specific AVP with a 32-bit Vendor ID space
within the Attribute Value field. It is necessary only in the event within the Attribute Value field. Multiple AVPs of this type may
that the 16-bit Vendor ID space is exhausted. Multiple AVPs of this exist in any message. The 16-bit Vendor ID MUST be 0, indicating
type may exist in any message. The 16-bit Vendor ID MUST be 0 that this is an IETF-defined AVP, and the Attribute Type MUST be 58,
indicating that this is an IETF defined AVP, and the Attribute Type indicating that what follows is a vendor-specific AVP with a 32-bit
MUST be AVP-TBA-0 indicating that what follows is a vendor-specific Vendor ID code. This AVP MAY be hidden (the H bit MAY be 0 or 1).
AVP with a 32-bit Vendor ID code. This AVP MAY be hidden (the H bit The M bit for this AVP MUST be set to 0. The Length of the AVP is 12
MAY be 0 or 1). The M bit for this AVP MUST be set to 0. The Length plus the length of the Attribute Value.
of the AVP is 12 plus the length of the Attribute Value.
5.2 Mandatory AVPs and Setting the M Bit 5.2. Mandatory AVPs and Setting the M Bit
If the M bit is set on an AVP that is unrecognized by its recipient, If the M bit is set on an AVP that is unrecognized by its recipient,
the session or control connection associated with the control message the session or control connection associated with the control message
containing the AVP MUST be shutdown. If the control message containing the AVP MUST be shut down. If the control message
containing the unrecognized AVP is associated with a session (e.g. an containing the unrecognized AVP is associated with a session (e.g.,
ICRQ, ICRP, ICCN, SLI, etc.) then the session MUST be issued a CDN an ICRQ, ICRP, ICCN, SLI, etc.), then the session MUST be issued a
with a Result Code of 2 and Error Code of 8 as defined in section CDN with a Result Code of 2 and Error Code of 8 (as defined in
5.4.2. and shutdown. If the control message containing the Section 5.4.2) and shut down. If the control message containing the
unrecognized AVP is associated with establishment or maintenance of a unrecognized AVP is associated with establishment or maintenance of a
Control Connection (e.g. SCCRQ, SCCRP, SCCCN, Hello) then the Control Connection (e.g., SCCRQ, SCCRP, SCCCN, Hello), then the
associated Control Connection MUST be issued a StopCCN with Result associated Control Connection MUST be issued a StopCCN with Result
Code of 2 and Error Code of 8 as defined in section 5.4.2. and Code of 2 and Error Code of 8 (as defined in Section 5.4.2) and shut
shutdown. If the M bit is not set on an unrecognized AVP, the AVP down. If the M bit is not set on an unrecognized AVP, the AVP MUST
MUST be ignored when received, processing the control message as if be ignored when received, processing the control message as if the
the AVP was not present. AVP were not present.
Receipt of an unrecognized AVP that has the M bit set is catastrophic Receipt of an unrecognized AVP that has the M bit set is catastrophic
to the session or control connection with which it is associated. to the session or control connection with which it is associated.
Thus, the M bit should only be set for AVPs that are deemed crucial Thus, the M bit should only be set for AVPs that are deemed crucial
to proper operation of the session or control connection by the to proper operation of the session or control connection by the
sender. AVPs that are considered crucial by the sender may vary by sender. AVPs that are considered crucial by the sender may vary by
application and configured options. In no case shall a receiver of application and configured options. In no case shall a receiver of
an AVP "validate" if the M bit is set on a recognized AVP. If the AVP an AVP "validate" if the M bit is set on a recognized AVP. If the
is recognized (as all AVPs defined in this document MUST be for a AVP is recognized (as all AVPs defined in this document MUST be for a
compliant L2TPv3 specification), then by definition the M bit is of compliant L2TPv3 specification), then by definition, the M bit is of
no consequence. no consequence.
The sender of an AVP is free to set its M bit to 1 or 0 based on The sender of an AVP is free to set its M bit to 1 or 0 based on
whether the configured application strictly requires the value whether the configured application strictly requires the value
contained in the AVP to be recognized or not. For example, "Automatic contained in the AVP to be recognized or not. For example,
L2TPv2 Fallback" (Section 4.7.3), requires the setting of the M bit "Automatic L2TPv2 Fallback" in Section 4.7.3 requires the setting of
on all new L2TPv3 AVPs to zero if fallback to L2TPv2 is supported and the M bit on all new L2TPv3 AVPs to zero if fallback to L2TPv2 is
desired, and 1 if not. supported and desired, and 1 if not.
The M bit is useful as extra assurance for support of critical AVP The M bit is useful as extra assurance for support of critical AVP
extensions. However, more explicit methods may be available to extensions. However, more explicit methods may be available to
determine support for a given feature rather than using the M bit determine support for a given feature rather than using the M bit
alone. For example, if a new AVP is defined in a message for which alone. For example, if a new AVP is defined in a message for which
there is always a message reply (i.e. an ICRQ, ICRP, SCCRQ or SCCRP there is always a message reply (i.e., an ICRQ, ICRP, SCCRQ, or SCCRP
message) rather than simply sending an AVP in the message with the M message), rather than simply sending an AVP in the message with the M
bit set, availability of the extension may be identified by sending bit set, availability of the extension may be identified by sending
an AVP in the request message and expecting a corresponding AVP in a an AVP in the request message and expecting a corresponding AVP in a
reply message. This more explicit method, when possible, is reply message. This more explicit method, when possible, is
preferred. preferred.
The M bit also plays a role in determining whether or not a malformed The M bit also plays a role in determining whether or not a malformed
or out-of-range value within an AVP should be ignored or result in or out-of-range value within an AVP should be ignored or should
termination of a session or control channel. See Section 7.1 for more result in termination of a session or control connection (see Section
details on this. 7.1 for more details).
5.3 Hiding of AVP Attribute Values 5.3. Hiding of AVP Attribute Values
The H bit in the header of each AVP provides a mechanism to indicate The H bit in the header of each AVP provides a mechanism to indicate
to the receiving peer whether the contents of the AVP are hidden or to the receiving peer whether the contents of the AVP are hidden or
present in cleartext. This feature can be used to hide sensitive present in cleartext. This feature can be used to hide sensitive
control message data such as user passwords, IDs, or other vital control message data such as user passwords, IDs, or other vital
information. information.
The H bit MUST only be set if (1) a shared secret exists between the The H bit MUST only be set if (1) a shared secret exists between the
LCCEs and (2) Control Connection and Control Message Authentication LCCEs and (2) Control Message Authentication is enabled (see Section
is enabled (see Section 4.3). If the H bit is set in any AVP(s) in a 4.3). If the H bit is set in any AVP(s) in a given control message,
given control message, at least one Random Vector AVP must also be at least one Random Vector AVP must also be present in the message
present in the message and MUST precede the first AVP having an H bit and MUST precede the first AVP having an H bit of 1.
of 1.
The shared secret between LCCEs is used to derive a unique shared key The shared secret between LCCEs is used to derive a unique shared key
for hiding and unhiding calculations. The derived shared key is for hiding and unhiding calculations. The derived shared key is
obtained via an HMAC-MD5 keyed hash [RFC2104], with the key obtained via an HMAC-MD5 keyed hash [RFC2104], with the key
consisting of the shared secret, and with the data being hashed consisting of the shared secret, and with the data being hashed
consisting of a single octet containing the value 1. consisting of a single octet containing the value 1.
shared_key = HMAC_MD5 (shared_secret, 1) shared_key = HMAC_MD5 (shared_secret, 1)
Hiding an AVP value is done in several steps. The first step is to Hiding an AVP value is done in several steps. The first step is to
take the length and value fields of the original (cleartext) AVP and take the length and value fields of the original (cleartext) AVP and
encode them into a Hidden AVP Subformat as follows: encode them into the Hidden AVP Subformat, which appears as follows:
Figure 5.3: Hidden AVP Subformat Figure 5.3: Hidden AVP Subformat
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of Original Value | Original Attribute Value ... | Length of Original Value | Original Attribute Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... | Padding ... ... | Padding ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Length of Original Attribute Value: This is length of the Original Length of Original Attribute Value: This is length of the Original
Attribute Value to be obscured in octets. This is necessary to Attribute Value to be obscured in octets. This is necessary to
determine the original length of the Attribute Value that is lost determine the original length of the Attribute Value that is lost
when the additional Padding is added. when the additional Padding is added.
Original Attribute Value: Attribute Value that is to be obscured. Original Attribute Value: Attribute Value that is to be obscured.
Padding: Random additional octets used to obscure length of the Padding: Random additional octets used to obscure length of the
Attribute Value that is being hidden. Attribute Value that is being hidden.
To mask the size of the data being hidden, the resulting subformat To mask the size of the data being hidden, the resulting subformat
MAY be padded as shown above. Padding does NOT alter the value MAY be padded as shown above. Padding does NOT alter the value
placed in the Length of Original Attribute Value field, but does placed in the Length of Original Attribute Value field, but does
alter the length of the resultant AVP that is being created. For alter the length of the resultant AVP that is being created. For
example, if an Attribute Value to be hidden is 4 octets in length, example, if an Attribute Value to be hidden is 4 octets in length,
the unhidden AVP length would be 10 octets (6 + Attribute Value the unhidden AVP length would be 10 octets (6 + Attribute Value
length). After hiding, the length of the AVP will become 6 + length). After hiding, the length of the AVP would become 6 +
Attribute Value length + size of the Length of Original Attribute Attribute Value length + size of the Length of Original Attribute
Value field + Padding. Thus, if Padding is 12 octets, the AVP length Value field + Padding. Thus, if Padding is 12 octets, the AVP length
will be 6 + 4 + 2 + 12 = 24 octets. would be 6 + 4 + 2 + 12 = 24 octets.
Next, an MD5 [RFC1321] hash is performed (in network byte order) on Next, an MD5 [RFC1321] hash is performed (in network byte order) on
the concatenation of the following: the concatenation of the following:
+ the 2-octet Attribute number of the AVP + the 2-octet Attribute number of the AVP
+ the shared key + the shared key
+ an arbitrary length random vector + an arbitrary length random vector
The value of the random vector used in this hash is passed in the The value of the random vector used in this hash is passed in the
value field of a Random Vector AVP. This Random Vector AVP must be value field of a Random Vector AVP. This Random Vector AVP must be
skipping to change at page 36, line 16 skipping to change at page 35, line 46
If necessary, this operation is repeated, with the shared key used If necessary, this operation is repeated, with the shared key used
along with each XOR result to generate the next hash to XOR the next along with each XOR result to generate the next hash to XOR the next
segment of the value with. segment of the value with.
The hiding method was adapted from [RFC2865], which was taken from The hiding method was adapted from [RFC2865], which was taken from
the "Mixing in the Plaintext" section in the book "Network Security" the "Mixing in the Plaintext" section in the book "Network Security"
by Kaufman, Perlman and Speciner [KPS]. A detailed explanation of by Kaufman, Perlman and Speciner [KPS]. A detailed explanation of
the method follows: the method follows:
Call the shared key S, the Random Vector RV, and the Attribute Type Call the shared key S, the Random Vector RV, and the Attribute Type
A. Break the value field into 16-octet chunks p1, p2, etc., with the A. Break the value field into 16-octet chunks p_1, p_2, etc., with
last one padded at the end with random data to a 16-octet boundary. the last one padded at the end with random data to a 16-octet
Call the ciphertext blocks c(1), c(2), etc. We will also define boundary. Call the ciphertext blocks c_1, c_2, etc. We will also
intermediate values b1, b2, etc. define intermediate values b_1, b_2, etc.
b_1 = MD5 (A + S + RV) c_1 = p_1 xor b_1 b_1 = MD5 (A + S + RV) c_1 = p_1 xor b_1
b_2 = MD5 (S + c_1) c_2 = p_2 xor b_2 b_2 = MD5 (S + c_1) c_2 = p_2 xor b_2
. . . .
. . . .
. . . .
b_i = MD5 (S + c_i-1) c_i = p_i xor b_i b_i = MD5 (S + c_i-1) c_i = p_i xor b_i
The String will contain c_1 + c_2 +...+ c_i, where + denotes The String will contain c_1 + c_2 +...+ c_i, where "+" denotes
concatenation. concatenation.
On receipt, the random vector is taken from the last Random Vector On receipt, the random vector is taken from the last Random Vector
AVP encountered in the message prior to the AVP to be unhidden. The AVP encountered in the message prior to the AVP to be unhidden. The
above process is then reversed to yield the original value. above process is then reversed to yield the original value.
5.4 AVP Summary 5.4. AVP Summary
The following sections contain a list of all L2TP AVPs defined in The following sections contain a list of all L2TP AVPs defined in
this document. this document.
Following the name of the AVP is a list indicating the message types Following the name of the AVP is a list indicating the message types
that utilize each AVP. After each AVP title follows a short that utilize each AVP. After each AVP title follows a short
description of the purpose of the AVP, a detail (including a graphic) description of the purpose of the AVP, a detail (including a graphic)
of the format for the Attribute Value, and any additional information of the format for the Attribute Value, and any additional information
needed for proper use of the AVP. needed for proper use of the AVP.
5.4.1 General Control Message AVPs 5.4.1. General Control Message AVPs
Message Type (All Messages) Message Type (All Messages)
The Message Type AVP, Attribute Type 0, identifies the control The Message Type AVP, Attribute Type 0, identifies the control
message herein and defines the context in which the exact meaning of message herein and defines the context in which the exact meaning
the following AVPs will be determined. of the following AVPs will be determined.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type | | Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type is a 2-octet unsigned integer. The Message Type is a 2-octet unsigned integer.
The Message Type AVP MUST be the first AVP in a message, immediately The Message Type AVP MUST be the first AVP in a message,
following the control message header (defined in Section 3.2.1). See immediately following the control message header (defined in
Section 3.1 for the list of defined control message types and their Section 3.2.1). See Section 3.1 for the list of defined control
identifiers. message types and their identifiers.
The Mandatory (M) bit within the Message Type AVP has special The Mandatory (M) bit within the Message Type AVP has special
meaning. Rather than an indication as to whether the AVP itself meaning. Rather than an indication as to whether the AVP itself
should be ignored if not recognized, it is an indication as to should be ignored if not recognized, it is an indication as to
whether the control message itself should be ignored. If the M bit whether the control message itself should be ignored. If the M
is set within the Message Type AVP and the Message Type is unknown to bit is set within the Message Type AVP and the Message Type is
the implementation, the control connection MUST be cleared. If the M unknown to the implementation, the control connection MUST be
bit is not set, then the implementation may ignore an unknown message cleared. If the M bit is not set, then the implementation may
type. The M bit MUST be set to 1 for all message types defined in ignore an unknown message type. The M bit MUST be set to 1 for
this document. This AVP MAY NOT be hidden (the H bit MUST be 0). all message types defined in this document. This AVP MUST NOT be
The Length of this AVP is 8. hidden (the H bit MUST be 0). The Length of this AVP is 8.
A vendor-specific control message may be defined by setting the A vendor-specific control message may be defined by setting the
Vendor ID of the Message Type AVP to a value other than the IETF Vendor ID of the Message Type AVP to a value other than the IETF
Vendor ID of 0 (see Section 5.1). The Message Type AVP MUST still be Vendor ID of 0 (see Section 5.1). The Message Type AVP MUST still
the first AVP in the control message. be the first AVP in the control message.
Message Digest (All Messages) Message Digest (All Messages)
The Message Digest AVP, Attribute Type AVP-TBA-1, is used as an The Message Digest AVP, Attribute Type 59 is used as an integrity
integrity and authentication check of the L2TP Control Message header and authentication check of the L2TP Control Message header and
and body. body.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Digest Type | Message Digest ... | Digest Type | Message Digest ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... (16 or 20 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where Digest Type is a one octet integer indicating the Digest +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
calculation algorithm: ... (16 or 20 octets) |
0 HMAC-MD5 [RFC2104] +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1 HMAC-SHA-1 [RFC2104]
Digest type 0 (HMAC-MD5) MUST be supported, Digest Type 1 (HMAC-SHA- Digest Type is a one-octet integer indicating the Digest
1) SHOULD be supported. calculation algorithm:
0 HMAC-MD5 [RFC2104]
1 HMAC-SHA-1 [RFC2104]
The Message Digest is of variable length and contains the result of Digest Type 0 (HMAC-MD5) MUST be supported, while Digest Type 1
the control message authenticity and integrity calculation. For (HMAC-SHA-1) SHOULD be supported.
Digest Type 0 (HMAC-MD5) the length of the digest MUST be 16 bytes.
For Digest Type 1 (SHA-1) the digest MUST be 20 bytes.
If Control Connection and Control Message Authentication is enabled, The Message Digest is of variable length and contains the result
at least one Message Digest AVP MUST present in all messages and MUST of the control message authentication and integrity calculation.
be placed immediately after the Message Type AVP. This forces the For Digest Type 0 (HMAC-MD5), the length of the digest MUST be 16
Message Digest AVP to begin at a well-known and fixed offset. A bytes. For Digest Type 1 (HMAC-SHA-1) the length of the digest
second Message Digest AVP MAY be present in a message, and MUST be MUST be 20 bytes.
placed directly after the first Message Digest AVP.
The shared secret between LCCEs is used to derive a unique shared key If Control Message Authentication is enabled, at least one Message
for Control Connection and Control Message Authentication Digest AVP MUST be present in all messages and MUST be placed
calculations. The derived shared key is obtained via an HMAC-MD5 immediately after the Message Type AVP. This forces the Message
keyed hash [RFC2104], with the key consisting of the shared secret, Digest AVP to begin at a well-known and fixed offset. A second
and with the data being hashed consisting of a single octet Message Digest AVP MAY be present in a message and MUST be placed
containing the value 2. directly after the first Message Digest AVP.
The shared secret between LCCEs is used to derive a unique shared
key for Control Message Authentication calculations. The derived
shared key is obtained via an HMAC-MD5 keyed hash [RFC2104], with
the key consisting of the shared secret, and with the data being
hashed consisting of a single octet containing the value 2.
shared_key = HMAC_MD5 (shared_secret, 2) shared_key = HMAC_MD5 (shared_secret, 2)
Calculation of the digest is as follows for all messages other than Calculation of the Message Digest is as follows for all messages
the SCCRQ (where '+' refers to concatenation): other than the SCCRQ (where "+" refers to concatenation):
Digest = HMAC_Hash (shared_key, local_nonce + remote_nonce + Message Digest = HMAC_Hash (shared_key, local_nonce +
control_message) remote_nonce + control_message)
HMAC_Hash: HMAC Hashing algorithm identified by the Digest Type HMAC_Hash: HMAC Hashing algorithm identified by the Digest Type
(MD5 or SHA1) (MD5 or SHA1)
local_nonce: Nonce chosen locally and advertised to the remote local_nonce: Nonce chosen locally and advertised to the remote
LCCE. LCCE.
remote_nonce: Nonce received from the remote LCCE remote_nonce: Nonce received from the remote LCCE
(The local_nonce and remote_nonce are advertised via the Control (The local_nonce and remote_nonce are advertised via the Control
Message Authentication Nonce AVP, also defined in this section.) Message Authentication Nonce AVP, also defined in this section.)
shared_key: Derived shared key for this Control Connection shared_key: Derived shared key for this control connection
control_message: The entire contents of the L2TP Control Message, control_message: The entire contents of the L2TP control message,
including the Control Message header and all AVPs. Note that the including the control message header and all AVPs. Note that the
Control Message header in this case begins after the 0 Session ID control message header in this case begins after the all-zero
(see Section 4.1.1.2) when running over IP, and after the UDP Session ID when running over IP (see Section 4.1.1.2), and after
header when running over UDP (see Section 4.1.2.1). the UDP header when running over UDP (see Section 4.1.2.1).
When calculating the Message Digest, the Message Digest AVP MUST be When calculating the Message Digest, the Message Digest AVP MUST be
present within the control message with the Digest Type set to its present within the control message with the Digest Type set to its
proper value, but the Message Digest itself set to zeros. proper value, but the Message Digest itself set to zeros.
When receiving a control message, the contents of the Message Digest When receiving a control message, the contents of the Message Digest
AVP MUST be compared against the expected digest value based on local AVP MUST be compared against the expected digest value based on local
calculation. This is done by performing the same digest calculation calculation. This is done by performing the same digest calculation
above, with the local_nonce and remote_nonce reversed. This message above, with the local_nonce and remote_nonce reversed. This message
authenticity and integrity checking MUST be performed before authenticity and integrity checking MUST be performed before
utilizing any information contained within the control message. If utilizing any information contained within the control message. If
the calculation fails, the message MUST be dropped. the calculation fails, the message MUST be dropped.
The SCCRQ has special treatment as it is the initial message The SCCRQ has special treatment as it is the initial message
commencing a new Control Connection. As such, there is only one nonce commencing a new control connection. As such, there is only one
available. Since the nonce is present within the message itself as nonce available. Since the nonce is present within the message
part of the Control Message Authentication Nonce AVP, there is no itself as part of the Control Message Authentication Nonce AVP, there
need to use it in the calculation explicitly. Calculation of the is no need to use it in the calculation explicitly. Calculation of
SCCRQ Digest is performed as follows: the SCCRQ Message Digest is performed as follows:
Digest = HMAC_Hash (shared_key, control_message) Message Digest = HMAC_Hash (shared_key, control_message)
To allow for graceful switchover to a new shared secret or hash To allow for graceful switchover to a new shared secret or hash
algorithm, two Message Digest AVPs MAY be present in a control algorithm, two Message Digest AVPs MAY be present in a control
message and two shared secrets MAY be configured for a given LCCE. message, and two shared secrets MAY be configured for a given LCCE.
If two Message Digest AVPs are received in a control message, the If two Message Digest AVPs are received in a control message, the
message MUST be accepted if either Message Digest is valid. If two message MUST be accepted if either Message Digest is valid. If two
shared secrets are configured, each (separately) MUST be used for shared secrets are configured, each (separately) MUST be used for
calculating a digest to be compared to the Message Digest(s) calculating a digest to be compared to the Message Digest(s)
received. When calculating a digest for a control message, the Value received. When calculating a digest for a control message, the Value
for both of the Message Digest AVPs MUST be set to zero. field for both of the Message Digest AVPs MUST be set to zero.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length is 23 for Digest Type 1 (HMAC-MD5), and 27 for Digest Type 2 Length is 23 for Digest Type 1 (HMAC-MD5), and 27 for Digest Type 2
(SHA-1). (HMAC-SHA-1).
Control Message Authentication Nonce (SCCRQ, SCCRP) Control Message Authentication Nonce (SCCRQ, SCCRP)
The Control Message Authentication Nonce AVP, Attribute Type AVP- The Control Message Authentication Nonce AVP, Attribute Type 73,
TBA-15, MUST contain a cryptographically random value [RFC1750]. This MUST contain a cryptographically random value [RFC1750]. This
value is used for Control Message Authentication. value is used for Control Message Authentication.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nonce ... (arbitrary number of octets) | Nonce ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Nonce is of arbitrary length, though at least 16 octets is
recommended. The Nonce contains the random value for use in the
Control Message Authentication hash calculation (see Message
Digest AVP definition in this section).
The Nonce is of arbitrary length, though at least 16 octets is If Control Message Authentication is enabled, this AVP MUST be
recommended. The Nonce contains the random value for use in the present in the SCCRQ and SCCRP messages.
Control Message Authentication hash calculation (see Message Digest
AVP definition in this section).
If Control Connection and Message Authentication is enabled, this AVP This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
MUST be present in the SCCRQ and SCCRP messages. this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length of this AVP is 6 plus the length of the Nonce.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for this Random Vector (All Messages)
AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The Length of
this AVP is 6 plus the length of the Nonce.
Random Vector (All Messages) The Random Vector AVP, Attribute Type 36, MUST contain a
cryptographically random value [RFC1750]. This value is used for
AVP Hiding.
The Random Vector AVP, Attribute Type 36, MUST contain a The Attribute Value field for this AVP has the following format:
cryptographically random value [RFC1750]. This value is used for AVP
Hiding.
The Attribute Value field for this AVP has the following format: 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Random Octet String ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3 The Random Octet String is of arbitrary length, though at least 16
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 octets is recommended. The string contains the random vector for
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ use in computing the MD5 hash to retrieve or hide the Attribute
| Random Octet String ... (arbitrary number of octets) Value of a hidden AVP (see Section 5.3).
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Random Octet String is of arbitrary length, though at least 16 More than one Random Vector AVP may appear in a message, in which
octets is recommended. The string contains the random vector for use case a hidden AVP uses the Random Vector AVP most closely
in computing the MD5 hash to retrieve or hide the Attribute Value of preceding it. As such, at least one Random Vector AVP MUST
a hidden AVP (see Section 5.3). precede the first AVP with the H bit set.
More than one Random Vector AVP may appear in a message, in which This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
case a hidden AVP uses the Random Vector AVP most closely preceding this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
it. As such, at least one Random Vector AVP MUST precede the first Length of this AVP is 6 plus the length of the Random Octet
AVP with the H bit set. String.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for 5.4.2. Result and Error Codes
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length of this AVP is 6 plus the length of the Random Octet String.
5.4.2 Result and Error Codes Result Code (StopCCN, CDN)
Result Code (StopCCN, CDN) The Result Code AVP, Attribute Type 1, indicates the reason for
terminating the control connection or session.
The Result Code AVP, Attribute Type 1, indicates the reason for The Attribute Value field for this AVP has the following format:
terminating the control channel or session.
The Attribute Value field for this AVP has the following format: 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Result Code | Error Code (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Message ... (optional, arbitrary number of octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3 The Result Code is a 2-octet unsigned integer. The optional Error
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 Code is a 2-octet unsigned integer. An optional Error Message can
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ follow the Error Code field. Presence of the Error Code and
| Result Code | Error Code (optional) | Message is indicated by the AVP Length field. The Error Message
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ contains an arbitrary string providing further (human-readable)
| Error Message ... (optional, arbitrary number of octets) | text associated with the condition. Human-readable text in all
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ error messages MUST be provided in the UTF-8 charset [RFC3629]
using the Default Language [RFC2277].
The Result Code is a 2-octet unsigned integer. The optional Error This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
Code is a 2-octet unsigned integer. An optional Error Message can this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
follow the Error Code field. Presence of the Error Code and Message Length is 8 if there is no Error Code or Message, 10 if there is
is indicated by the AVP Length field. The Error Message contains an an Error Code and no Error Message, or 10 plus the length of the
arbitrary string providing further (human-readable) text associated Error Message if there is an Error Code and Message.
with the condition. Human-readable text in all error messages MUST
be provided in the UTF-8 charset [RFC3629] using the Default Language
[RFC2277].
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for Defined Result Code values for the StopCCN message are as follows:
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length is 8 if there is no Error Code or Message, 10 if there is an
Error Code and no Error Message, or 10 plus the length of the Error
Message if there is an Error Code and Message.
Defined Result Code values for the StopCCN message are as follows: 0 - Reserved.
1 - General request to clear control connection.
2 - General error, Error Code indicates the problem.
3 - Control connection already exists.
4 - Requester is not authorized to establish a control connection.
5 - The protocol version of the requester is not supported,
Error Code indicates highest version supported.
6 - Requester is being shut down.
7 - Finite state machine error or timeout
0 - Reserved. General Result Code values for the CDN message are as follows:
1 - General request to clear control connection.
2 - General error, Error Code indicates the problem.
3 - Control channel already exists.
4 - Requester is not authorized to establish a control channel.
5 - The protocol version of the requester is not supported,
Error Code indicates highest version supported.
6 - Requester is being shut down.
7 - Finite state machine error or timeout 0 - Reserved.
1 - Session disconnected due to loss of carrier or
circuit disconnect.
2 - Session disconnected for the reason indicated in Error
Code.
3 - Session disconnected for administrative reasons.
4 - Session establishment failed due to lack of appropriate
facilities being available (temporary condition).
General Result Code values for the CDN message are as follows: 5 - Session establishment failed due to lack of appropriate
facilities being available (permanent condition).
6 - 11 Reserved
13 - Session not established due to losing tie breaker.
14 - Session not established due to unsupported PW type.
15 - Session not established, sequencing required without
valid L2-Specific Sublayer.
16 - Finite state machine error or timeout.
0 - Reserved. Additional service-specific Result Codes are defined outside this
1 - Session disconnected due to loss of carrier or document.
circuit disconnect.
2 - Session disconnected for the reason indicated in Error Code.
3 - Session disconnected for administrative reasons.
4 - Session establishment failed due to lack of appropriate
facilities being available (temporary condition).
5 - Session establishment failed due to lack of appropriate
facilities being available (permanent condition).
6 - 11 Reserved
RC-TBA-1 - Session not established due to losing tie breaker.
RC-TBA-2 - Session not established due to unsupported PW type.
RC-TBA-3 - Session not established, sequencing required without
valid L2-Specific Sublayer.
RC-TBA-4 - Finite state machine error or timeout.
Additional service-specific Result Codes are defined outside this The Error Codes defined below pertain to types of errors that are
document. not specific to any particular L2TP request, but rather to
protocol or message format errors. If an L2TP reply indicates in
its Result Code that a General Error occurred, the General Error
value should be examined to determine what the error was. The
currently defined General Error codes and their meanings are as
follows:
The Error Codes defined below pertain to types of errors that are not 0 - No General Error.
specific to any particular L2TP request, but rather to protocol or 1 - No control connection exists yet for this pair of LCCEs.
message format errors. If an L2TP reply indicates in its Result Code 2 - Length is wrong.
that a general error occurred, the General Error value should be 3 - One of the field values was out of range.
examined to determine what the error was. The currently defined 4 - Insufficient resources to handle this operation now.
General Error codes and their meanings are as follows: 5 - Invalid Session ID.
6 - A generic vendor-specific error occurred.
7 - Try another. If initiator is aware of other possible
responder destinations, it should try one of them. This can
be used to guide an LAC or LNS based on policy.
8 - The session or control connection was shut down due to receipt
of an unknown AVP with the M bit set (see Section 5.2). The
Error Message SHOULD contain the attribute of the offending
AVP in (human-readable) text form.
9 - Try another directed. If an LAC or LNS is aware of other
possible destinations, it should inform the initiator of the
control connection or session. The Error Message MUST contain
a comma-separated list of addresses from which the initiator
may choose. If the L2TP data channel runs over IPv4, then
this would be a comma-separated list of IP addresses in the
canonical dotted-decimal format (e.g., "192.0.2.1, 192.0.2.2,
192.0.2.3") in the UTF-8 charset [RFC3629] using the Default
Language [RFC2277]. If there are no servers for the LAC or
LNS to suggest, then Error Code 7 should be used. For IPv4,
the delimiter between addresses MUST be precisely a single
comma and a single space. For IPv6, each literal address MUST
be enclosed in "[" and "]" characters, following the encoding
described in [RFC2732].
0 - No general error. When a General Error Code of 6 is used, additional information
1 - No control connection exists yet for this pair of LCCEs. about the error SHOULD be included in the Error Message field. A
2 - Length is wrong. vendor-specific AVP MAY be sent to more precisely detail a
3 - One of the field values was out of range. vendor-specific problem.
4 - Insufficient resources to handle this operation now.
5 - Invalid Session ID.
6 - A generic vendor-specific error occurred.
7 - Try another. If initiator is aware of other possible responder
destinations, it should try one of them. This can be
used to guide an LAC or LNS based on policy.
8 - The session or control connection was shut down due to receipt of
an unknown AVP with the M bit set (see Section 5.2). The Error
Message SHOULD contain the attribute of the offending AVP in
(human-readable) text form.
9 - Try another directed. If an LAC or LNS is aware of other
possible destinations, it should inform the initiator of the
control connection or session. The Error Message MUST contain a
comma-separated list of addresses from which the initiator may
choose. If the L2TP data channel runs over IPv4, then this would
be a comma-separated list of IP addresses in the canonical
dotted-decimal format (e.g. "192.0.2.1, 192.0.2.2, 192.0.2.3")
in the UTF-8 charset [RFC3629] using the Default Language
[RFC2277]. If there are no servers for the LAC or LNS to
suggest, then Error Code 7 should be used. For IPv4, the
delimiter between addresses MUST be precisely a single comma
and a single space. For IPv6, each literal address MUST be
enclosed in "[" and "]" characters, following the encoding
described in [RFC2732].
When a General Error Code of 6 is used, additional information about 5.4.3. Control Connection Management AVPs
the error SHOULD be included in the Error Message field. A vendor-
specific AVP MAY be sent to more precisely detail a vendor-specific
problem.
5.4.3 Control Connection Management AVPs Control Connection Tie Breaker (SCCRQ)
Control Connection Tie Breaker (SCCRQ) The Control Connection Tie Breaker AVP, Attribute Type 5,
indicates that the sender desires a single control connection to
exist between a given pair of LCCEs.
The Control Connection Tie Breaker AVP, Attribute Type 5, indicates The Attribute Value field for this AVP has the following format:
that the sender desires a single control connection to exist between
a given pair of LCCEs.
The Attribute Value field for this AVP has the following format: 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Control Connection Tie Breaker Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3 The Control Connection Tie Breaker Value is an 8-octet random
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 value that is used to choose a single control connection when two
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ LCCEs request a control connection concurrently. The recipient of
| Control Connection Tie Breaker Value ... a SCCRQ must check to see if a SCCRQ has been sent to the peer; if
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ so, a tie has been detected. In this case, the LCCE must compare
... (64 bits) | its Control Connection Tie Breaker value with the one received in
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ the SCCRQ. The lower value "wins", and the "loser" MUST discard
its control connection. A StopCCN SHOULD be sent by the winner as
an explicit rejection for the losing SCCRQ. In the case in which
a tie breaker is present on both sides and the value is equal,
both sides MUST discard their control connections and restart
control connection negotiation with a new, random tie breaker
value.
The Control Connection Tie Breaker Value is an 8-octet random value If a tie breaker is received and an outstanding SCCRQ has no tie
that is used to choose a single control connection when two LCCEs breaker value, the initiator that included the Control Connection
request a control connection concurrently. The recipient of a SCCRQ Tie Breaker AVP "wins". If neither side issues a tie breaker,
must check to see if a SCCRQ has been sent to the peer; if so, a tie then two separate control connections are opened.
has been detected. In this case, the LCCE must compare its Control
Connection Tie Breaker value with the one received in the SCCRQ. The
lower value "wins", and the "loser" MUST discard its control
connection, sending a StopCCN if the SCCRQ that it had sent was
acknowledged by the receiving peer. In the case in which a tie
breaker is present on both sides and the value is equal, both sides
MUST discard their control connections and restart control connection
negotiation with a new, random tie breaker value.
If a tie breaker is received and an outstanding SCCRQ has no tie Applications that employ a distinct and well-known initiator have
breaker value, the initiator that included the Control Connection Tie no need for tie breaking, and MAY omit this AVP or disable tie
Breaker AVP "wins". If neither side issues a tie breaker, then two breaking functionality. Applications that require tie breaking
separate control connections are opened. also require that an LCCE be uniquely identifiable upon receipt of
an SCCRQ. For L2TP over IP, this MUST be accomplished via the
Router ID AVP.
Applications which employ a distinct and well-known initiator have no Note that in [RFC2661], this AVP is referred to as the "Tie
need for tie-breaking, and this AVP MAY be omitted and the tie- Breaker AVP" and is applicable only to a control connection. In
breaking functionality disabled. Applications which require tie- L2TPv3, the AVP serves the same purpose of tie breaking, but is
breaking also require that an LCCE be uniquely identifiable upon applicable to a control connection or a session. The Control
receipt of an SCCRQ. For L2TP over IP, this MUST be accomplished via Connection Tie Breaker AVP (present only in Control Connection
the Router ID AVP. messages) and Session Tie Breaker AVP (present only in Session
messages), are described separately in this document, but share
the same Attribute type of 5.
Note that in [RFC2661], this AVP was referred to as the "Tie-Breaker This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
AVP". Here, the AVP serves the same purpose and has the same this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
attribute value and composition. The name was changed simply to length of this AVP is 14.
distinguish between the Session and Control Connection Tie Breaker
AVP.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for Host Name (SCCRQ, SCCRP)
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length of this AVP is 14.
Host Name (SCCRQ, SCCRP) The Host Name AVP, Attribute Type 7, indicates the name of the
issuing LAC or LNS, encoded in the US-ASCII charset.
The Host Name AVP, Attribute Type 7, indicates the name of the The Attribute Value field for this AVP has the following format:
issuing LAC or LNS, encoded in the US-ASCII charset.
The Attribute Value field for this AVP has the following format: 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host Name ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3 The Host Name is of arbitrary length, but MUST be at least 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 octet.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host Name ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Host Name is of arbitrary length, but MUST be at least 1 octet. This name should be as broadly unique as possible; for hosts
participating in DNS [RFC1034], a host name with fully qualified
domain would be appropriate. The Host Name AVP and/or Router ID
AVP MUST be used to identify an LCCE as described in Section 3.3.
This name should be as broadly unique as possible; for hosts This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
participating in DNS [RFC1034], a host name with fully qualified this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
domain would be appropriate. The Host Name AVP and/or Router ID AVP Length of this AVP is 6 plus the length of the Host Name.
MUST be used to identify an LCCE as described in Section 3.3.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for Router ID (SCCRQ, SCCRP)
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length of this AVP is 6 plus the length of the Host Name.
Router ID (SCCRQ, SCCRP) The Router ID AVP, Attribute Type 60, is an identifier used to
identify an LCCE for control connection setup, tie breaking,
and/or tunnel authentication.
The Router ID AVP, Attribute Type AVP-TBA-2, is an identifier used to The Attribute Value field for this AVP has the following format:
identify an LCCE for control connection setup, tie breaking, and/or
tunnel authentication.
The Attribute Value field for this AVP has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 | Router Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Router Identifier is a 4-octet unsigned integer. Its value is The Router Identifier is a 4-octet unsigned integer. Its value is
unique for a given LCCE, per Section 8.1 of [RFC2072]. The Host Name unique for a given LCCE, per Section 8.1 of [RFC2072]. The Host
AVP and/or Router ID AVP MUST be used to identify an LCCE as Name AVP and/or Router ID AVP MUST be used to identify an LCCE as
described in Section 3.3. described in Section 3.3.
Implementations MUST NOT assume that Router Identifier is a valid IP Implementations MUST NOT assume that Router Identifier is a valid
address. The Router Identifier for L2TP over IPv6 can be obtained IP address. The Router Identifier for L2TP over IPv6 can be
from an IPv4 address (if available) or via unspecified obtained from an IPv4 address (if available) or via unspecified
implementation-specific means. implementation-specific means.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length of this AVP is 10. Length of this AVP is 10.
Vendor Name (SCCRQ, SCCRP) Vendor Name (SCCRQ, SCCRP)
The Vendor Name AVP, Attribute Type 8, contains a vendor-specific The Vendor Name AVP, Attribute Type 8, contains a vendor-specific
(possibly human-readable) string describing the type of LAC or LNS (possibly human-readable) string describing the type of LAC or LNS
being used. being used.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor Name ... (arbitrary number of octets) | Vendor Name ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Vendor Name is the indicated number of octets representing the The Vendor Name is the indicated number of octets representing the
vendor string. Human-readable text for this AVP MUST be provided in vendor string. Human-readable text for this AVP MUST be provided
the US-ASCII charset [RFC1958, RFC2277]. in the US-ASCII charset [RFC1958, RFC2277].
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The Length this AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The
(before hiding) of this AVP is 6 plus the length of the Vendor Name. Length (before hiding) of this AVP is 6 plus the length of the
Vendor Name.
Assigned Control Connection ID (SCCRQ, SCCRP, StopCCN) Assigned Control Connection ID (SCCRQ, SCCRP, StopCCN)
The Assigned Control Connection ID AVP, Attribute Type AVP-TBA-3, The Assigned Control Connection ID AVP, Attribute Type 61,
contains the ID being assigned to this control connection by the contains the ID being assigned to this control connection by the
sender. sender.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assigned Control Connection ID | | Assigned Control Connection ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Assigned Control Connection ID is a 4-octet non-zero unsigned The Assigned Control Connection ID is a 4-octet non-zero unsigned
integer. integer.
The Assigned Control Connection ID AVP establishes the identifier The Assigned Control Connection ID AVP establishes the identifier
used to multiplex and demultiplex multiple control connections used to multiplex and demultiplex multiple control connections
between a pair of LCCEs. Once the Assigned Control Connection ID AVP between a pair of LCCEs. Once the Assigned Control Connection ID
has been received by an LCCE, the Control Connection ID specified in AVP has been received by an LCCE, the Control Connection ID
the AVP MUST be included in the Control Connection ID field of all specified in the AVP MUST be included in the Control Connection ID
control packets sent to the peer for the lifetime of the control field of all control packets sent to the peer for the lifetime of
connection. Before the Assigned Control Connection ID AVP is the control connection. Before the Assigned Control Connection ID
received from a peer, all control messages MUST be sent to that peer AVP is received from a peer, all control messages MUST be sent to
with a Control Connection ID value of 0 in the header. Because a that peer with a Control Connection ID value of 0 in the header.
Control Connection ID value of 0 is used in this special manner, the Because a Control Connection ID value of 0 is used in this special
zero value MUST NOT be sent as an Assigned Control Connection ID manner, the zero value MUST NOT be sent as an Assigned Control
value. Connection ID value.
Under certain circumstances, an LCCE may need to send a StopCCN to a Under certain circumstances, an LCCE may need to send a StopCCN to
peer without having yet received an Assigned Control Connection ID a peer without having yet received an Assigned Control Connection
AVP from the peer (i.e. SCCRQ sent, no SCCRP received yet). In this ID AVP from the peer (i.e., SCCRQ sent, no SCCRP received yet).
case, the Assigned Control Connection ID AVP that had been sent to In this case, the Assigned Control Connection ID AVP that had been
the peer earlier (i.e. in the SCCRQ) MUST be sent as the Assigned sent to the peer earlier (i.e., in the SCCRQ) MUST be sent as the
Control Connection ID AVP in the StopCCN. This policy allows the Assigned Control Connection ID AVP in the StopCCN. This policy
peer to try to identify the appropriate control connection via a allows the peer to try to identify the appropriate control
reverse lookup. connection via a reverse lookup.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The Length this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
(before hiding) of this AVP is 10. Length (before hiding) of this AVP is 10.
Receive Window Size (SCCRQ, SCCRP) Receive Window Size (SCCRQ, SCCRP)
The Receive Window Size AVP, Attribute Type 10, specifies the receive The Receive Window Size AVP, Attribute Type 10, specifies the
window size being offered to the remote peer. receive window size being offered to the remote peer.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Window Size | | Window Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Window Size is a 2-octet unsigned integer.
The Window Size is a 2-octet unsigned integer. If absent, the peer must assume a Window Size of 4 for its
transmit window.
If absent, the peer must assume a Window Size of 4 for its transmit The remote peer may send the specified number of control messages
window. before it must wait for an acknowledgment. See Section 4.2 for
more information on reliable control message delivery.
The remote peer may send the specified number of control messages This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
before it must wait for an acknowledgment. See Section 4.2 for more this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
information on reliable control message delivery. Length of this AVP is 8.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for Pseudowire Capabilities List (SCCRQ, SCCRP)
this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
Length of this AVP is 8.
Pseudowire Capabilities List (SCCRQ, SCCRP) The Pseudowire Capabilities List (PW Capabilities List) AVP,
Attribute Type 62, indicates the L2 payload types the sender can
support. The specific payload type of a given session is
identified by the Pseudowire Type AVP.
The Pseudowire Capabilities List (PW Capabilities List) AVP, The Attribute Value field for this AVP has the following format:
Attribute Type AVP-TBA-4, indicates the L2 payload types the sender
can support. The specific payload type of a given session is
identified by the Pseudowire Type AVP.
The Attribute Value field for this AVP has the following format: 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Type 0 | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | PW Type N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3 Defined PW types that may appear in this list are managed by IANA
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 and will appear in associated pseudowire-specific documents for
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ each PW type.
| PW Type 0 | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | PW Type N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Defined PW types that may appear in this list are managed by IANA and If a sender includes a given PW type in the PW Capabilities List
will appear in associated pseudowire-specific documents for each PW AVP, the sender assumes full responsibility for supporting that
type. particular payload, such as any payload-specific AVPs, L2-Specific
Sublayer, or control messages that may be defined in the
appropriate companion document.
If a sender includes a given PW type in the PW Capabilities List AVP, This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
the sender assumes full responsibility for supporting that particular this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
payload, such as any payload-specific AVPs, L2-Specific Sublayer, or Length (before hiding) of this AVP is 8 octets with one PW type
control messages that may be defined in the appropriate companion specified, plus 2 octets for each additional PW type.
document.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this Preferred Language (SCCRQ, SCCRP)
AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The Length
(before hiding) of this AVP is 8 octets with one PW type specified,
plus 2 octets for each additional PW type.
Preferred Language (SCCRQ, SCCRP) The Preferred Language AVP, Attribute Type 72, provides a method
for an LCCE to indicate to the peer the language in which human-
readable messages it sends SHOULD be composed. This AVP contains
a single language tag or language range [RFC3066].
The Preferred Language AVP, Attribute Type AVP-TBD-14, provides a The Attribute Value field for this AVP has the following format:
method for an LCCE to indicate to the peer the language in which
human-readable messages it sends SHOULD be composed. This AVP
contains a single language tag or language range [RFC3066].
The Attribute Value field for this AVP has the following format: 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Preferred Language... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3 The Preferred Language is the indicated number of octets
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 representing the language tag or language range, encoded in the
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ US-ASCII charset.
| Preferred Language... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Preferred Language is the indicated number of octets representing It is not required to send a Preferred Language AVP. If (1) an
the language tag or language range, encoded in the US-ASCII charset. LCCE does not signify a language preference by the inclusion of
this AVP in the SCCRQ or SCCRP, (2) the Preferred Language AVP is
unrecognized, or (3) the requested language is not supported by
the peer LCCE, the default language [RFC2277] MUST be used for all
internationalized strings sent by the peer.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The Length this AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The
(before hiding) of this AVP is 6 plus the length of the Preferred Length (before hiding) of this AVP is 6 plus the length of the
Language. Preferred Language.
5.4.4 Session Management AVPs 5.4.4. Session Management AVPs
Local Session ID (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, CDN, WEN, SLI) Local Session ID (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, CDN, WEN, SLI)
The Local Session ID AVP (analogous to the Assigned Session ID in The Local Session ID AVP (analogous to the Assigned Session ID in
L2TPv2), Attribute Type AVP-TBA-5, contains the identifier being L2TPv2), Attribute Type 63, contains the identifier being assigned
assigned to this session by the sender. to this session by the sender.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local Session ID | | Local Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Local Session ID is a 4-octet non-zero unsigned integer. The Local Session ID is a 4-octet non-zero unsigned integer.
The Local Session ID AVP establishes the two identifiers used to The Local Session ID AVP establishes the two identifiers used to
multiplex and demultiplex sessions between two LCCEs. Each LCCE multiplex and demultiplex sessions between two LCCEs. Each LCCE
chooses any free value it desires, and sends it to the remote LCCE chooses any free value it desires, and sends it to the remote LCCE
using this AVP. The remote LCCE MUST then send all data packets using this AVP. The remote LCCE MUST then send all data packets
associated with this session using this value. Additionally, for all associated with this session using this value. Additionally, for
session-oriented control messages sent after this AVP is received all session-oriented control messages sent after this AVP is
(e.g. ICRP, ICCN, CDN, SLI, etc.), the remote LCCE MUST echo this received (e.g., ICRP, ICCN, CDN, SLI, etc.), the remote LCCE MUST
value in the Remote Session ID AVP. echo this value in the Remote Session ID AVP.
Note that a Session ID value is unidirectional. Because each LCCE Note that a Session ID value is unidirectional. Because each LCCE
chooses its Session ID independent of its peer LCCE, the value does chooses its Session ID independent of its peer LCCE, the value
not have to match in each direction for a given session. does not have to match in each direction for a given session.
See Section 4.1 for additional information about the Session ID. See Section 4.1 for additional information about the Session ID.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
AVP SHOULD be 1 set to 1, but MAY vary (see Section 5.2). The Length this AVP SHOULD be 1 set to 1, but MAY vary (see Section 5.2).
(before hiding) of this AVP is 10. The Length (before hiding) of this AVP is 10.
Remote Session ID (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, CDN, WEN, SLI) Remote Session ID (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, CDN, WEN, SLI)
The Remote Session ID AVP, Attribute Type AVP-TBA-6, contains the The Remote Session ID AVP, Attribute Type 64, contains the
identifier that was assigned to this session by the peer. identifier that was assigned to this session by the peer.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote Session ID | | Remote Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Remote Session ID is a 4-octet non-zero unsigned integer. The Remote Session ID is a 4-octet non-zero unsigned integer.
The Remote Session ID AVP MUST be present in all session-level The Remote Session ID AVP MUST be present in all session-level
control messages. The AVP's value echoes the session identifier control messages. The AVP's value echoes the session identifier
advertised by the peer via the Local Session ID AVP. It is the same advertised by the peer via the Local Session ID AVP. It is the
value that will be used in all transmitted data messages by this side same value that will be used in all transmitted data messages by
of the session. In most cases, this identifier is sufficient for the this side of the session. In most cases, this identifier is
peer to look up session-level context for this control message. sufficient for the peer to look up session-level context for this
control message.
When a session-level control message must be sent to the peer before When a session-level control message must be sent to the peer
the Local Session ID AVP has been received, the value of the Remote before the Local Session ID AVP has been received, the value of
Sesson ID AVP MUST be set to zero. Additionally, the Local Session the Remote Session ID AVP MUST be set to zero. Additionally, the
ID AVP (sent in a previous control message for this session) MUST be Local Session ID AVP (sent in a previous control message for this
included in the control message. The peer must then use the Local session) MUST be included in the control message. The peer must
Session ID AVP to perform a reverse lookup to find its session then use the Local Session ID AVP to perform a reverse lookup to
context. Session-level control messages defined in this document find its session context. Session-level control messages defined
that might be subject to a reverse lookup by a receiving peer include in this document that might be subject to a reverse lookup by a
the CDN, WEN, and SLI. receiving peer include the CDN, WEN, and SLI.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
AVP SHOULD be set to 1, but MAY vary (see Section 5, but MAY vary this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
(see Section 5.2). The Length (before hiding) of this AVP is 10. Length (before hiding) of this AVP is 10.
Assigned Cookie (ICRQ, ICRP, OCRQ, OCRP) Assigned Cookie (ICRQ, ICRP, OCRQ, OCRP)
The Assigned Cookie AVP, Attribute Type AVP-TBA-7, contains the The Assigned Cookie AVP, Attribute Type 65, contains the Cookie
Cookie value being assigned to this session by the sender. value being assigned to this session by the sender.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assigned Cookie (32 or 64 bits) ... | Assigned Cookie (32 or 64 bits) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Assigned Cookie is a 4-octet or 8-octet random value. The Assigned Cookie is a 4-octet or 8-octet random value.
The Assigned Cookie AVP contains the value used to check the The Assigned Cookie AVP contains the value used to check the
association of a received data message with the session identified by association of a received data message with the session identified
the Session ID. All data messages sent to a peer MUST use the by the Session ID. All data messages sent to a peer MUST use the
Assigned Cookie sent by the peer in this AVP. The value's length (0, Assigned Cookie sent by the peer in this AVP. The value's length
32, or 64 bits) is obtained by the Length of the AVP. (0, 32, or 64 bits) is obtained by the length of the AVP.
A missing Assigned Cookie AVP or Assigned Cookie Value of zero length A missing Assigned Cookie AVP or Assigned Cookie Value of zero
indicates that the Cookie field should not be present in any data length indicates that the Cookie field should not be present in
packets sent to the LCCE sending this AVP. any data packets sent to the LCCE sending this AVP.
See Section 4.1 for additional information about the Assigned Cookie. See Section 4.1 for additional information about the Assigned
Cookie.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The Length this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
(before hiding) of this AVP may be 6, 10, or 14 octets. Length (before hiding) of this AVP may be 6, 10, or 14 octets.
Serial Number (ICRQ, OCRQ) Serial Number (ICRQ, OCRQ)
The Serial Number AVP, Attribute Type 15, contains an identifier The Serial Number AVP, Attribute Type 15, contains an identifier
assigned by the LAC or LNS to this session. assigned by the LAC or LNS to this session.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Serial Number | | Serial Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Serial Number is a 32-bit value. The Serial Number is a 32-bit value.
The Serial Number is intended to be an easy reference for The Serial Number is intended to be an easy reference for
administrators on both ends of a control connection to use when administrators on both ends of a control connection to use when
investigating session failure problems. Serial Numbers should be set investigating session failure problems. Serial Numbers should be
to progressively increasing values, which are likely to be unique for set to progressively increasing values, which are likely to be
a significant period of time across all interconnected LNSs and LACs. unique for a significant period of time across all interconnected
LNSs and LACs.
Note that in RFC 2661, this value was referred to as the "Call Serial Note that in RFC 2661, this value was referred to as the "Call
Number AVP". It serves the same purpose and has the same attribute Serial Number AVP". It serves the same purpose and has the same
value and composition. attribute value and composition.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The Length this AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The
(before hiding) of this AVP is 10. Length (before hiding) of this AVP is 10.
Remote End ID (ICRQ, OCRQ) Remote End ID (ICRQ, OCRQ)
The Remote End ID AVP, Attribute Type AVP-TBA-8, contains an The Remote End ID AVP, Attribute Type 66, contains an identifier
identifier used to bind L2TP sessions to a given circuit, interface, used to bind L2TP sessions to a given circuit, interface, or
or bridging instance. It also may be used to detect session-level bridging instance. It also may be used to detect session-level
ties. ties.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote End Identifier ... (arbitrary number of octets) | Remote End Identifier ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Remote End Identifier field is a variable-length field whose The Remote End Identifier field is a variable-length field whose
value is unique for a given LCCE peer, as described in Section 3.3. value is unique for a given LCCE peer, as described in Section
3.3.
A session-level tie is detected if an LCCE receives an ICRQ or OCRQ A session-level tie is detected if an LCCE receives an ICRQ or
with an End ID AVP whose value matches that which was just sent in an OCRQ with an End ID AVP whose value matches that which was just
outgoing ICRQ or OCRQ to the same peer. If the two values match, an sent in an outgoing ICRQ or OCRQ to the same peer. If the two
LCCE recognizes that a tie exists (e.g. both LCCEs are attempting to values match, an LCCE recognizes that a tie exists (i.e., both
establish sessions for the same circuit). The tie is broken by the LCCEs are attempting to establish sessions for the same circuit).
Session Tie Breaker AVP. The tie is broken by the Session Tie Breaker AVP.
By default, the LAC-LAC cross-connect application (see section By default, the LAC-LAC cross-connect application (see Section
2.0(b)) of L2TP over an IP network MUST utilize the Router ID AVP and 2(b)) of L2TP over an IP network MUST utilize the Router ID AVP
Remote End ID AVP to associate a circuit to an L2TP session. Other and Remote End ID AVP to associate a circuit to an L2TP session.
AVPs MAY be used for LCCE or circuit identification as specified in Other AVPs MAY be used for LCCE or circuit identification as
companion documents. specified in companion documents.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The Length this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
(before hiding) of this AVP is 6 plus the length of the Remote End Length (before hiding) of this AVP is 6 plus the length of the
Identifier value. Remote End Identifier value.
Session Tie Breaker (ICRQ, OCRQ) Session Tie Breaker (ICRQ, OCRQ)
The Session Tie Breaker AVP, Attribute Type TBD, is used to break The Session Tie Breaker AVP, Attribute Type 5, is used to break ties
ties when two peers concurrently attempt to establish a session for when two peers concurrently attempt to establish a session for the
the same circuit. same circuit.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Session Tie Breaker Value ... | Session Tie Breaker Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... (64 bits) | ... (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Session Tie Breaker Value is an 8-octet random value that is used The Session Tie Breaker Value is an 8-octet random value that is used
to choose a session when two LCCEs concurrently request a session for to choose a session when two LCCEs concurrently request a session for
the same circuit. A tie is detected by examining the peer's identity the same circuit. A tie is detected by examining the peer's identity
(described in Section 3.3) plus the per-session shared value (described in Section 3.3) plus the per-session shared value
communicated via the End ID AVP. In the case of a tie, the recipient communicated via the End ID AVP. In the case of a tie, the recipient
of an ICRQ or OCRQ must compare the received tie breaker value with of an ICRQ or OCRQ must compare the received tie breaker value with
the one that it sent earlier. The LCCE with the lower value "wins", the one that it sent earlier. The LCCE with the lower value "wins"
and the "loser" MUST send a CDN with result code set to RC-TBA-1 (as and MUST send a CDN with result code set to 13 (as defined in Section
defined in Section 5.4.2) to tear down the session it instigated. In 5.4.2) in response to the losing ICRQ or OCRQ. In the case in which a
the case in which a tie is detected, tie breakers are sent by both tie is detected, tie breakers are sent by both sides, and the tie
sides, and the tie breaker values are equal, both sides MUST discard breaker values are equal, both sides MUST discard their sessions and
their sessions and restart session negotiation with new random tie restart session negotiation with new random tie breaker values.
breaker values.
If a tie is detected but only one side sends a Session Tie Breaker If a tie is detected but only one side sends a Session Tie Breaker
AVP, the session initiator that included the Session Tie Breaker AVP AVP, the session initiator that included the Session Tie Breaker AVP
"wins". If neither side issues a tie breaker, then both sides MUST "wins". If neither side issues a tie breaker, then both sides MUST
tear down the session. tear down the session.
This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for this This AVP MUST NOT be hidden (the H bit MUST be 0). The M bit for
AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The Length of this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
this AVP is 14. Length of this AVP is 14.
Pseudowire Type (ICRQ, OCRQ) Pseudowire Type (ICRQ, OCRQ)
The Pseudowire Type (PW Type) AVP, Attribute Type AVP-TBA-10, The Pseudowire Type (PW Type) AVP, Attribute Type 68, indicates
indicates the L2 payload type of the packets that will be tunneled the L2 payload type of the packets that will be tunneled using
using this L2TP session. this L2TP session.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PW Type | | PW Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A peer MUST NOT request an incoming or outgoing call with a PW Type A peer MUST NOT request an incoming or outgoing call with a PW
AVP specifying a value not advertised in the PW Capabilities List AVP Type AVP specifying a value not advertised in the PW Capabilities
it received during control connection establishment. Attempts to do List AVP it received during control connection establishment.
so MUST result in the call being rejected via a CDN with the Result Attempts to do so MUST result in the call being rejected via a CDN
Code set to RC-TBA-2 (see Section 5.4.2). with the Result Code set to 14 (see Section 5.4.2).
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The Length this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
(before hiding) of this AVP is 8. Length (before hiding) of this AVP is 8.
L2-Specific Sublayer (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN) L2-Specific Sublayer (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN)
The L2-Specific Sublayer AVP, Attribute Type AVP-TBA-11, indicates The L2-Specific Sublayer AVP, Attribute Type 69, indicates the
the presence and format of the L2-Specific Sublayer the sender of presence and format of the L2-Specific Sublayer the sender of this
this AVP requires on all incoming data packets for this L2TP session. AVP requires on all incoming data packets for this L2TP session.
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2-Specific Sublayer Type | | L2-Specific Sublayer Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The L2-Specific Sublayer Type is a 2-octet unsigned integer with the The L2-Specific Sublayer Type is a 2-octet unsigned integer with the
following values defined in this document: following values defined in this document:
0 - There is no L2-Specific Sublayer present. 0 - There is no L2-Specific Sublayer present.
1 - The default L2-Specific Sublayer (defined in Section 4.6) 1 - The Default L2-Specific Sublayer (defined in Section 4.6)
is used. is used.
If this AVP is received and has a value other than zero, the If this AVP is received and has a value other than zero, the
receiving LCCE MUST include the identified L2-Specific Sublayer in receiving LCCE MUST include the identified L2-Specific Sublayer in
its outgoing data messages. If the AVP is not received, it is its outgoing data messages. If the AVP is not received, it is
assumed that there is no sublayer present. assumed that there is no sublayer present.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this
AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The Length AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The Length
(before hiding) of this AVP is 8. (before hiding) of this AVP is 8.
Data Sequencing (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN) Data Sequencing (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN)
The Data Sequencing AVP, Attribute Type AVP-TBA-12, indicates that The Data Sequencing AVP, Attribute Type 70, indicates that the
the sender requires some or all of the data packets that it receives sender requires some or all of the data packets that it receives
to be sequenced. to be sequenced.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Sequencing Level | | Data Sequencing Level |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Data Sequencing Level is a 2-octet unsigned integer indicating The Data Sequencing Level is a 2-octet unsigned integer indicating
the degree of incoming data traffic that the sender of this AVP the degree of incoming data traffic that the sender of this AVP
wishes to be marked with sequence numbers. wishes to be marked with sequence numbers.
The following values are valid data sequencing levels: Defined Data Sequencing Levels are as follows:
0 - No incoming data packets require sequencing. 0 - No incoming data packets require sequencing.
1 - Only non-IP data packets require sequencing. 1 - Only non-IP data packets require sequencing.
2 - All incoming data packets require sequencing. 2 - All incoming data packets require sequencing.
If a data sequencing level of 0 is specified, there is no need to If a Data Sequencing Level of 0 is specified, there is no need to
send packets with sequence numbers. If sequence numbers are sent, send packets with sequence numbers. If sequence numbers are sent,
they will be ignored upon receipt. If no Data Sequencing AVP is they will be ignored upon receipt. If no Data Sequencing AVP is
received, a data sequencing level of 0 is assumed. received, a Data Sequencing Level of 0 is assumed.
If a data sequencing level of 1 is specified, only non-IP traffic If a Data Sequencing Level of 1 is specified, only non-IP traffic
carried within the tunneled L2 frame should have sequence numbers carried within the tunneled L2 frame should have sequence numbers
applied. Non-IP traffic here refers to any packets that cannot be applied. Non-IP traffic here refers to any packets that cannot be
classified as an IP packet within their respective L2 framing (i.e., classified as an IP packet within their respective L2 framing (e.g.,
a PPP control packet or NETBIOS frame encapsulated by Frame Relay a PPP control packet or NETBIOS frame encapsulated by Frame Relay
before being tunneled). All traffic that can be classified as IP MUST before being tunneled). All traffic that can be classified as IP
be sent with no sequencing (e.g. the S bit in the L2-Specific MUST be sent with no sequencing (i.e., the S bit in the L2-Specific
Sublayer is set to zero). If a packet is unable to be classified at Sublayer is set to zero). If a packet is unable to be classified at
all (e.g. due to it being compressed or encrypted at layer 2) or if all (e.g., because it has been compressed or encrypted at layer 2) or
an implementation is unable to perform such classification within L2 if an implementation is unable to perform such classification within
frames, all packets MUST be provided with sequence numbers L2 frames, all packets MUST be provided with sequence numbers
(essentially falling back to a data sequencing level of 2). (essentially falling back to a Data Sequencing Level of 2).
If a data sequencing level of 2 is specified, all traffic MUST be If a Data Sequencing Level of 2 is specified, all traffic MUST be
sequenced. sequenced.
Data sequencing may only be requested when there is an L2-Specific Data sequencing may only be requested when there is an L2-Specific
Sublayer present that can provide sequence numbers. If sequencing is Sublayer present that can provide sequence numbers. If sequencing is
requested without requesting a L2-Specific Sublayer AVP, the session requested without requesting a L2-Specific Sublayer AVP, the session
MUST be disconnected with a Result Code of RC-TBA-3. MUST be disconnected with a Result Code of 15 (see Section 5.4.2).
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this
AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The Length AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The Length
(before hiding) of this AVP is 6. (before hiding) of this AVP is 8.
Tx Connect Speed (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN) Tx Connect Speed (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN)
The Tx Connect Speed BPS AVP, Attribute Type AVP-TBA-16, contains the The Tx Connect Speed BPS AVP, Attribute Type 74, contains the
speed of the facility chosen for the connection attempt. speed of the facility chosen for the connection attempt.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Connect Speed in bps... | Connect Speed in bps...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...Connect Speed in bps (64 bits) | ...Connect Speed in bps (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Tx Connect Speed BPS is an 8-octet value indicating the speed in The Tx Connect Speed BPS is an 8-octet value indicating the speed
bits per second. A value of zero indicates that the speed is in bits per second. A value of zero indicates that the speed is
indeterminable or that there is no physical point-to-point link. indeterminable or that there is no physical point-to-point link.
When the optional Rx Connect Speed AVP is present, the value in this When the optional Rx Connect Speed AVP is present, the value in
AVP represents the transmit connect speed from the perspective of the this AVP represents the transmit connect speed from the
LAC (e.g. data flowing from the LAC to the remote system). When the perspective of the LAC (i.e., data flowing from the LAC to the
optional Rx Connect Speed AVP is NOT present, the connection speed remote system). When the optional Rx Connect Speed AVP is NOT
between the remote system and LAC is assumed to be symmetric and is present, the connection speed between the remote system and LAC is
represented by the single value in this AVP. assumed to be symmetric and is represented by the single value in
this AVP.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The Length this AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The
(before hiding) of this AVP is 14. Length (before hiding) of this AVP is 14.
Rx Connect Speed (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN) Rx Connect Speed (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN)
The Rx Connect Speed AVP, Attribute Type AVP-TBA-17, represents the The Rx Connect Speed AVP, Attribute Type 75, represents the speed
speed of the connection from the perspective of the LAC (e.g. data of the connection from the perspective of the LAC (i.e., data
flowing from the remote system to the LAC). flowing from the remote system to the LAC).
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Connect Speed in bps... | Connect Speed in bps...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...Connect Speed in bps (64 bits) | ...Connect Speed in bps (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Connect Speed BPS is an 8-octet value indicating the speed in bits Connect Speed BPS is an 8-octet value indicating the speed in bits
per second. A value of zero indicates that the speed is per second. A value of zero indicates that the speed is
indeterminable or that there is no physical point-to-point link. indeterminable or that there is no physical point-to-point link.
Presence of this AVP implies that the connection speed may be Presence of this AVP implies that the connection speed may be
asymmetric with respect to the transmit connect speed given in the Tx asymmetric with respect to the transmit connect speed given in the
Connect Speed AVP. Tx Connect Speed AVP.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The Length this AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The
(before hiding) of this AVP is 14. Length (before hiding) of this AVP is 14.
Physical Channel ID (ICRQ, ICRP, OCRP) Physical Channel ID (ICRQ, ICRP, OCRP)
The Physical Channel ID AVP, Attribute Type 25, contains the vendor- The Physical Channel ID AVP, Attribute Type 25, contains the
specific physical channel number used for a call. vendor-specific physical channel number used for a call.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Physical Channel ID | | Physical Channel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Physical Channel ID is a 4-octet value intended to be used for Physical Channel ID is a 4-octet value intended to be used for
logging purposes only. logging purposes only.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The Length this AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The
(before hiding) of this AVP is 10. Length (before hiding) of this AVP is 10.
5.4.5 Circuit Status AVPs 5.4.5. Circuit Status AVPs
Circuit Status (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, SLI) Circuit Status (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, SLI)
The Circuit Status AVP, Attribute Type AVP-TBA-13, indicates the The Circuit Status AVP, Attribute Type 71, indicates the initial
initial status of or a status change in the circuit to which the status of or a status change in the circuit to which the session
session is bound. is bound.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |N|A| | Reserved |N|A|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The A (Active) bit indicates whether the circuit is up/active/ready The A (Active) bit indicates whether the circuit is
(1) or down/inactive/not-ready (0). up/active/ready (1) or down/inactive/not-ready (0).
The N (New) bit indicates whether the circuit status indication is The N (New) bit indicates whether the circuit status indication is
for a new circuit (1) or an existing circuit (0). Links which have a for a new circuit (1) or an existing circuit (0). Links that have
similar mechanism available (e.g. Frame Relay) MUST map the setting a similar mechanism available (e.g., Frame Relay) MUST map the
of this bit to the associated signaling for that link. Otherwise, the setting of this bit to the associated signaling for that link.
New bit SHOULD still be set the first time the L2TP session is Otherwise, the New bit SHOULD still be set the first time the L2TP
established after provisioning. session is established after provisioning.
The remaining bits are reserved for future use. Reserved bits MUST The remaining bits are reserved for future use. Reserved bits
be set to 0 when sending and ignored upon receipt. MUST be set to 0 when sending and ignored upon receipt.
The Circuit Status AVP is used to advertise whether a circuit or The Circuit Status AVP is used to advertise whether a circuit or
interface bound to an L2TP session is up and ready to send and/or interface bound to an L2TP session is up and ready to send and/or
receive traffic. Different circuit types have different names for receive traffic. Different circuit types have different names for
status types. For example, HDLC primary and secondary stations refer status types. For example, HDLC primary and secondary stations
to a circuit as being "Receive Ready" or "Receive Not Ready", while refer to a circuit as being "Receive Ready" or "Receive Not
Frame Relay refers to a circuit as "Active" or "Inactive". This AVP Ready", while Frame Relay refers to a circuit as "Active" or
adopts the latter terminology, though the concept remains the same "Inactive". This AVP adopts the latter terminology, though the
regardless of the PW type for the L2TP session. concept remains the same regardless of the PW type for the L2TP
session.
In the simplest case, the circuit referred by this AVP is a single In the simplest case, the circuit to which this AVP refers is a
physical interface, port, or circuit depending on application and how single physical interface, port, or circuit, depending on the
the session was setup. The status indication in this AVP may then be application and the session setup. The status indication in this
used to provide simple ILMI interworking for a variety of circuit AVP may then be used to provide simple ILMI interworking for a
types. For virtual or multipoint interfaces, the Circuit Status AVP variety of circuit types. For virtual or multipoint interfaces,
is still utilized, but effectively refers to the state of an internal the Circuit Status AVP is still utilized, but in this case, it
structure or a logical set of circuits. Each PW-specific companion refers to the state of an internal structure or a logical set of
document MUST then specify precisely how this AVP is translated for circuits. Each PW-specific companion document MUST specify
each circuit type. precisely how this AVP is translated for each circuit type.
If this AVP is received with a Not Active notification for a given If this AVP is received with a Not Active notification for a given
L2TP session, all data traffic for that session MUST cease (or not L2TP session, all data traffic for that session MUST cease (or not
begin) in the direction of the sender of the Circuit Status AVP until begin) in the direction of the sender of the Circuit Status AVP
the circuit is advertised as Active. until the circuit is advertised as Active.
The Circuit Status MUST be advertised by this AVP in ICRQ, ICRP, The Circuit Status MUST be advertised by this AVP in ICRQ, ICRP,
OCRQ, and OCRP messages. Often, the circuit type will be marked OCRQ, and OCRP messages. Often, the circuit type will be marked
Active when initiated, but MAY be advertised as Inactive, indicating Active when initiated, but subsequently MAY be advertised as
that an L2TP session is to be created but that the interface or Inactive. This indicates that an L2TP session is to be created,
circuit is still not ready to pass traffic. The ICCN, OCCN, and SLI but that the interface or circuit is still not ready to pass
control messages all MAY contain this AVP to update the status of the traffic. The ICCN, OCCN, and SLI control messages all MAY contain
circuit after establishment of the L2TP session is requested. this AVP to update the status of the circuit after establishment
of the L2TP session is requested.
If additional circuit status information is needed for a given PW If additional circuit status information is needed for a given PW
type, PW-specific AVPs MUST be defined in a separate document for type, any new PW-specific AVPs MUST be defined in a separate
that information. This AVP is only for general circuit status document. This AVP is only for general circuit status information
information applicable to all circuit/interface types. generally applicable to all circuit/interface types.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The Length this AVP SHOULD be set to 1, but MAY vary (see Section 5.2). The
(before hiding) of this AVP is 8. Length (before hiding) of this AVP is 8.
Circuit Errors (WEN) Circuit Errors (WEN)
The Circuit Errors AVP, Attribute Type 34, conveys circuit error The Circuit Errors AVP, Attribute Type 34, conveys circuit error
information to the peer. information to the peer.
The Attribute Value field for this AVP has the following format: The Attribute Value field for this AVP has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
| Reserved | | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hardware Overruns | | Hardware Overruns |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Buffer Overruns | | Buffer Overruns |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timeout Errors | | Timeout Errors |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Alignment Errors | | Alignment Errors |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following fields are defined: The following fields are defined:
Reserved: 2 octets of Reserved data is present (providing longword Reserved: 2 octets of Reserved data is present (providing longword
alignment within the AVP of the following values). Reserved alignment within the AVP of the following values). Reserved
data MUST be zero on sending and ignored upon receipt. data MUST be zero on sending and ignored upon receipt.
Hardware Overruns: Number of receive buffer overruns since call Hardware Overruns: Number of receive buffer overruns since call
was established. was established.
Buffer Overruns: Number of buffer overruns detected since call was Buffer Overruns: Number of buffer overruns detected since call was
established. established.
Timeout Errors: Number of timeouts since call was established. Timeout Errors: Number of timeouts since call was established.
Alignment Errors: Number of alignment errors since call was Alignment Errors: Number of alignment errors since call was
established. established.
This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for this This AVP MAY be hidden (the H bit MAY be 0 or 1). The M bit for
AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The Length this AVP SHOULD be set to 0, but MAY vary (see Section 5.2). The
(before hiding) of this AVP is 32. Length (before hiding) of this AVP is 32.
6. Control Connection Protocol Specification 6. Control Connection Protocol Specification
The following control messages are used to establish, maintain, and The following control messages are used to establish, maintain, and
tear down L2TP control connections. All data packets are sent in tear down L2TP control connections. All data packets are sent in
network order (high-order octets first). Any "reserved" or "empty" network order (high-order octets first). Any "reserved" or "empty"
fields MUST be sent as 0 values to allow for protocol extensibility. fields MUST be sent as 0 values to allow for protocol extensibility.
The exchanges in which these messages are involved are outlined in The exchanges in which these messages are involved are outlined in
Section 3.3. Section 3.3.
6.1 Start-Control-Connection-Request (SCCRQ) 6.1. Start-Control-Connection-Request (SCCRQ)
Start-Control-Connection-Request (SCCRQ) is a control message used to Start-Control-Connection-Request (SCCRQ) is a control message used to
initiate a control connection between two LCCEs. It is sent by initiate a control connection between two LCCEs. It is sent by
either the LAC or the LNS to begin the control connection either the LAC or the LNS to begin the control connection
establishment process. establishment process.
The following AVPs MUST be present in the SCCRQ: The following AVPs MUST be present in the SCCRQ:
Message Type Message Type
Host Name Host Name
Router ID Router ID
Assigned Control Connection ID Assigned Control Connection ID
Pseudowire Capabilities List Pseudowire Capabilities List
The following AVPs MAY be present in the SCCRQ: The following AVPs MAY be present in the SCCRQ:
Random Vector Random Vector
Nonce Control Message Authentication Nonce
Message Digest Message Digest
Control Connection Tie Breaker Control Connection Tie Breaker
Vendor Name Vendor Name
Receive Window Size Receive Window Size
Preferred Language Preferred Language
6.2 Start-Control-Connection-Reply (SCCRP) 6.2. Start-Control-Connection-Reply (SCCRP)
Start-Control-Connection-Reply (SCCRP) is the control message sent in Start-Control-Connection-Reply (SCCRP) is the control message sent in
reply to a received SCCRQ message. The SCCRP is used to indicate reply to a received SCCRQ message. The SCCRP is used to indicate
that the SCCRQ was accepted and establishment of the control that the SCCRQ was accepted and that establishment of the control
connection should continue. connection should continue.
The following AVPs MUST be present in the SCCRP: The following AVPs MUST be present in the SCCRP:
Message Type Message Type
Host Name Host Name
Router ID Router ID
Assigned Control Connection ID Assigned Control Connection ID
Pseudowire Capabilities List Pseudowire Capabilities List
The following AVPs MAY be present in the SCCRP: The following AVPs MAY be present in the SCCRP:
Random Vector Random Vector
Nonce Control Message Authentication Nonce
Message Digest Message Digest
Vendor Name Vendor Name
Receive Window Size Receive Window Size
Preferred Language Preferred Language
6.3 Start-Control-Connection-Connected (SCCCN) 6.3. Start-Control-Connection-Connected (SCCCN)
Start-Control-Connection-Connected (SCCCN) is the control message Start-Control-Connection-Connected (SCCCN) is the control message
sent in reply to an SCCRP. The SCCCN completes the control sent in reply to an SCCRP. The SCCCN completes the control
connection establishment process. connection establishment process.
The following AVP MUST be present in the SCCCN: The following AVP MUST be present in the SCCCN:
Message Type Message Type
The following AVP MAY be present in the SCCCN: The following AVP MAY be present in the SCCCN:
Random Vector Random Vector
Message Digest Message Digest
6.4 Stop-Control-Connection-Notification (StopCCN) 6.4. Stop-Control-Connection-Notification (StopCCN)
Stop-Control-Connection-Notification (StopCCN) is the control message Stop-Control-Connection-Notification (StopCCN) is the control message
sent by either LCCE to inform its peer that the control connection is sent by either LCCE to inform its peer that the control connection is
being shut down and that the control connection should be closed. In being shut down and that the control connection should be closed. In
addition, all active sessions are implicitly cleared (without sending addition, all active sessions are implicitly cleared (without sending
any explicit session control messages). The reason for issuing this any explicit session control messages). The reason for issuing this
request is indicated in the Result Code AVP. There is no explicit request is indicated in the Result Code AVP. There is no explicit
reply to the message, only the implicit ACK that is received by the reply to the message, only the implicit ACK that is received by the
reliable control message delivery layer. reliable control message delivery layer.
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The following AVPs MAY be present in the StopCCN: The following AVPs MAY be present in the StopCCN:
Random Vector Random Vector
Message Digest Message Digest
Assigned Control Connection ID Assigned Control Connection ID
Note that the Assigned Control Connection ID MUST be present if the Note that the Assigned Control Connection ID MUST be present if the
StopCCN is sent after an SCCRQ or SCCRP message has been sent. StopCCN is sent after an SCCRQ or SCCRP message has been sent.
6.5 Hello (HELLO) 6.5. Hello (HELLO)
The Hello (HELLO) message is an L2TP control message sent by either The Hello (HELLO) message is an L2TP control message sent by either
peer of a control connection. This control message is used as a peer of a control connection. This control message is used as a
"keepalive" for the control connection. See Section 4.2 for a "keepalive" for the control connection. See Section 4.2 for a
description of the keepalive mechanism. description of the keepalive mechanism.
HELLO messages are global to the control connection. The Session ID HELLO messages are global to the control connection. The Session ID
in a HELLO message MUST be 0. in a HELLO message MUST be 0.
The following AVP MUST be present in the HELLO: The following AVP MUST be present in the HELLO:
Message Type Message Type
The following AVP MAY be present in the HELLO: The following AVP MAY be present in the HELLO:
Random Vector Random Vector
Message Digest Message Digest
6.6 Incoming-Call-Request (ICRQ) 6.6. Incoming-Call-Request (ICRQ)
Incoming-Call-Request (ICRQ) is the control message sent by an LCCE Incoming-Call-Request (ICRQ) is the control message sent by an LCCE
to a peer when an incoming call is detected (although the ICRQ may to a peer when an incoming call is detected (although the ICRQ may
also be sent as a result of a local event). It is the first in a also be sent as a result of a local event). It is the first in a
three-message exchange used for establishing a session via an L2TP three-message exchange used for establishing a session via an L2TP
control connection. control connection.
The ICRQ is used to indicate that a session is to be established The ICRQ is used to indicate that a session is to be established
between an LCCE and a peer. The sender of an ICRQ provides the peer between an LCCE and a peer. The sender of an ICRQ provides the peer
with parameter information for the session. However, the sender with parameter information for the session. However, the sender
makes no demands about how the session is terminated at the peer makes no demands about how the session is terminated at the peer
(i.e. whether the L2 traffic is processed locally, forwarded, etc.). (i.e., whether the L2 traffic is processed locally, forwarded, etc.).
The following AVPs MUST be present in the ICRQ: The following AVPs MUST be present in the ICRQ:
Message Type Message Type
Local Session ID Local Session ID
Remote Session ID Remote Session ID
Serial Number Serial Number
Pseudowire Type Pseudowire Type
Remote End ID
Circuit Status Circuit Status
The following AVPs MAY be present in the ICRQ: The following AVPs MAY be present in the ICRQ:
Random Vector Random Vector
Message Digest Message Digest
Assigned Cookie Assigned Cookie
Remote End ID
Session Tie Breaker Session Tie Breaker
L2-Specific Sublayer L2-Specific Sublayer
Data Sequencing Data Sequencing
Tx Connect Speed Tx Connect Speed
Rx Connect Speed Rx Connect Speed
Physical Channel ID Physical Channel ID
6.7 Incoming-Call-Reply (ICRP) 6.7. Incoming-Call-Reply (ICRP)
Incoming-Call-Reply (ICRP) is the control message sent by an LCCE in Incoming-Call-Reply (ICRP) is the control message sent by an LCCE in
response to a received ICRQ. It is the second in the three-message response to a received ICRQ. It is the second in the three-message
exchange used for establishing sessions within an L2TP control exchange used for establishing sessions within an L2TP control
connection. connection.
The ICRP is used to indicate that the ICRQ was successful and that The ICRP is used to indicate that the ICRQ was successful and that
the peer should establish (i.e. answer) the incoming call if it has the peer should establish (i.e., answer) the incoming call if it has
not already done so. It also allows the sender to indicate specific not already done so. It also allows the sender to indicate specific
parameters about the L2TP session. parameters about the L2TP session.
The following AVPs MUST be present in the ICRP: The following AVPs MUST be present in the ICRP:
Message Type Message Type
Local Session ID Local Session ID
Remote Session ID Remote Session ID
Circuit Status Circuit Status
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Random Vector Random Vector
Message Digest Message Digest
Assigned Cookie Assigned Cookie
L2-Specific Sublayer L2-Specific Sublayer
Data Sequencing Data Sequencing
Tx Connect Speed Tx Connect Speed
Rx Connect Speed Rx Connect Speed
Physical Channel ID Physical Channel ID
6.8 Incoming-Call-Connected (ICCN) 6.8. Incoming-Call-Connected (ICCN)
Incoming-Call-Connected (ICCN) is the control message sent by the Incoming-Call-Connected (ICCN) is the control message sent by the
LCCE that originally sent an ICRQ upon receiving an ICRP from its LCCE that originally sent an ICRQ upon receiving an ICRP from its
peer. It is the final message in the three-message exchange used for peer. It is the final message in the three-message exchange used for
establishing L2TP sessions. establishing L2TP sessions.
The ICCN is used to indicate that the ICRP was accepted, that the The ICCN is used to indicate that the ICRP was accepted, that the
call has been established, and that the L2TP session should move to call has been established, and that the L2TP session should move to
the established state. It also allows the sender to indicate the established state. It also allows the sender to indicate
specific parameters about the established call (parameters that may specific parameters about the established call (parameters that may
not have been available at the time the ICRQ is issued). not have been available at the time the ICRQ was issued).
The following AVPs MUST be present in the ICCN: The following AVPs MUST be present in the ICCN:
Message Type Message Type
Local Session ID Local Session ID
Remote Session ID Remote Session ID
The following AVPs MAY be present in the ICCN: The following AVPs MAY be present in the ICCN:
Random Vector Random Vector
Message Digest Message Digest
L2-Specific Sublayer L2-Specific Sublayer
Data Sequencing Data Sequencing
Tx Connect Speed Tx Connect Speed
Rx Connect Speed Rx Connect Speed
Circuit Status Circuit Status
6.9 Outgoing-Call-Request (OCRQ) 6.9. Outgoing-Call-Request (OCRQ)
Outgoing-Call-Request (OCRQ) is the control message sent by an LCCE Outgoing-Call-Request (OCRQ) is the control message sent by an LCCE
to an LAC to indicate that an outbound call at the LAC is to be to an LAC to indicate that an outbound call at the LAC is to be
established based on specific destination information sent in this established based on specific destination information sent in this
message. It is the first in a three-message exchange used for message. It is the first in a three-message exchange used for
establishing a session and placing a call on behalf of the initiating establishing a session and placing a call on behalf of the initiating
LCCE. LCCE.
Note that a call may be any L2 connection requiring well-known Note that a call may be any L2 connection requiring well-known
destination information to be sent from an LCCE to an LAC. This call destination information to be sent from an LCCE to an LAC. This call
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address of another LCCE, or any other destination dictated by the address of another LCCE, or any other destination dictated by the
sender of this message. sender of this message.
The following AVPs MUST be present in the OCRQ: The following AVPs MUST be present in the OCRQ:
Message Type Message Type
Local Session ID Local Session ID
Remote Session ID Remote Session ID
Serial Number Serial Number
Pseudowire Type Pseudowire Type
Remote End ID
Circuit Status Circuit Status
The following AVPs MAY be present in the OCRQ: The following AVPs MAY be present in the OCRQ:
Random Vector Random Vector
Message Digest Message Digest
Assigned Cookie Assigned Cookie
Remote End ID
Tx Connect Speed Tx Connect Speed
Rx Connect Speed Rx Connect Speed
Session Tie Breaker Session Tie Breaker
L2-Specific Sublayer L2-Specific Sublayer
Data Sequencing Data Sequencing
6.10 Outgoing-Call-Reply (OCRP) 6.10. Outgoing-Call-Reply (OCRP)
Outgoing-Call-Reply (OCRP) is the control message sent by an LAC to Outgoing-Call-Reply (OCRP) is the control message sent by an LAC to
an LCCE in response to a received OCRQ. It is the second in a an LCCE in response to a received OCRQ. It is the second in a
three-message exchange used for establishing a session within an L2TP three-message exchange used for establishing a session within an L2TP
control connection. control connection.
OCRP is used to indicate that the LAC has been able to attempt the OCRP is used to indicate that the LAC has been able to attempt the
outbound call. The message returns any relevant parameters regarding outbound call. The message returns any relevant parameters regarding
the call attempt. Data MUST NOT be forwarded until the OCCN is the call attempt. Data MUST NOT be forwarded until the OCCN is
received indicating that the call has been placed. received, which indicates that the call has been placed.
The following AVPs MUST be present in the OCRP: The following AVPs MUST be present in the OCRP:
Message Type Message Type
Local Session ID Local Session ID
Remote Session ID Remote Session ID
Circuit Status Circuit Status
The following AVPs MAY be present in the OCRP: The following AVPs MAY be present in the OCRP:
Random Vector Random Vector
Message Digest Message Digest
Assigned Cookie Assigned Cookie
L2-Specific Sublayer L2-Specific Sublayer
Tx Connect Speed Tx Connect Speed
Rx Connect Speed Rx Connect Speed
Data Sequencing Data Sequencing
Physical Channel ID Physical Channel ID
6.11 Outgoing-Call-Connected (OCCN) 6.11. Outgoing-Call-Connected (OCCN)
Outgoing-Call-Connected (OCCN) is the control message sent by an LAC Outgoing-Call-Connected (OCCN) is the control message sent by an LAC
to another LCCE after the OCRP and after the outgoing call has been to another LCCE after the OCRP and after the outgoing call has been
completed. It is the final message in a three-message exchange used completed. It is the final message in a three-message exchange used
for establishing a session. for establishing a session.
OCCN is used to indicate that the result of a requested outgoing call OCCN is used to indicate that the result of a requested outgoing call
was successful. It also provides information to the LCCE who was successful. It also provides information to the LCCE who
requested the call about the particular parameters obtained after the requested the call about the particular parameters obtained after the
call was established. call was established.
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The following AVPs MAY be present in the OCCN: The following AVPs MAY be present in the OCCN:
Random Vector Random Vector
Message Digest Message Digest
L2-Specific Sublayer L2-Specific Sublayer
Tx Connect Speed Tx Connect Speed
Rx Connect Speed Rx Connect Speed
Data Sequencing Data Sequencing
Circuit Status Circuit Status
6.12 Call-Disconnect-Notify (CDN) 6.12. Call-Disconnect-Notify (CDN)
The Call-Disconnect-Notify (CDN) is a control message sent by an LCCE The Call-Disconnect-Notify (CDN) is a control message sent by an LCCE
to request disconnection of a specific session. Its purpose is to to request disconnection of a specific session. Its purpose is to
inform the peer of the disconnection and the reason for the inform the peer of the disconnection and the reason for the
disconnection. The peer MUST clean up any resources, and does not disconnection. The peer MUST clean up any resources, and does not
send back any indication of success or failure for such cleanup. send back any indication of success or failure for such cleanup.
The following AVPs MUST be present in the CDN: The following AVPs MUST be present in the CDN:
Message Type Message Type
Result Code Result Code
Local Session ID Local Session ID
Remote Session ID Remote Session ID
The following AVP MAY be present in the CDN: The following AVP MAY be present in the CDN:
Random Vector Random Vector
Message Digest Message Digest
6.13 WAN-Error-Notify (WEN) 6.13. WAN-Error-Notify (WEN)
The WAN-Error-Notify (WEN) is a control message sent from an LAC to The WAN-Error-Notify (WEN) is a control message sent from an LAC to
an LNS to indicate WAN error conditions. The counters in this an LNS to indicate WAN error conditions. The counters in this
message are cumulative. This message should only be sent when an message are cumulative. This message should only be sent when an
error occurs, and not more than once every 60 seconds. The counters error occurs, and not more than once every 60 seconds. The counters
are reset when a new call is established. are reset when a new call is established.
The following AVPs MUST be present in the WEN: The following AVPs MUST be present in the WEN:
Message Type Message Type
Local Session ID Local Session ID
Remote Session ID Remote Session ID
Circuit Errors Circuit Errors
The following AVP MAY be present in the WEN: The following AVP MAY be present in the WEN:
Random Vector Random Vector
Message Digest Message Digest
6.14 Set-Link-Info (SLI) 6.14. Set-Link-Info (SLI)
The Set-Link-Info control message is sent by an LCCE to convey link The Set-Link-Info control message is sent by an LCCE to convey link
or circuit status change information regarding the circuit associated or circuit status change information regarding the circuit associated
with this L2TP session. For example, if PPP renegotiates LCP at an with this L2TP session. For example, if PPP renegotiates LCP at an
LNS or between an LAC and a Remote System, or if a forwarded Frame LNS or between an LAC and a Remote System, or if a forwarded Frame
Relay VC transitions to Active or Inactive at an LAC, an SLI message Relay VC transitions to Active or Inactive at an LAC, an SLI message
SHOULD be sent to indicate this event. Precise details of when the SHOULD be sent to indicate this event. Precise details of when the
SLI is sent, what PW type-specific AVPs must be present, and how SLI is sent, what PW type-specific AVPs must be present, and how
those AVPs should be interpreted by the receiving peer are outside those AVPs should be interpreted by the receiving peer are outside
the scope of this document. These details should be described in the the scope of this document. These details should be described in the
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Message Type Message Type
Local Session ID Local Session ID
Remote Session ID Remote Session ID
The following AVPs MAY be present in the SLI: The following AVPs MAY be present in the SLI:
Random Vector Random Vector
Message Digest Message Digest
Circuit Status Circuit Status
6.15 Explicit-Acknowledgement (ACK) 6.15. Explicit-Acknowledgement (ACK)
The Explicit Acknowledgement (ACK) message is used only to The Explicit Acknowledgement (ACK) message is used only to
acknowledge receipt of a message or messages on the Control acknowledge receipt of a message or messages on the control
Connection (e.g. for purposes of updating Ns and Nr values). Receipt connection (e.g., for purposes of updating Ns and Nr values).
of this message does not trigger an event for the L2TP protocol state Receipt of this message does not trigger an event for the L2TP
machine. protocol state machine.
A message received without any AVPs (including the Message Type AVP), A message received without any AVPs (including the Message Type AVP),
is referred to as a Zero Length Body (ZLB) message, and serves the is referred to as a Zero Length Body (ZLB) message, and serves the
same function as the Explicit Acknowledgement. ZLB messages are only same function as the Explicit Acknowledgement. ZLB messages are only
permitted when the Control Message Authentication defined in Section permitted when Control Message Authentication defined in Section 4.3
4.3 is not enabled. is not enabled.
The following AVPs MAY be present in the ACK message: The following AVPs MAY be present in the ACK message:
Message Type Message Type
Message Digest Message Digest
7. Control Connection State Machines 7. Control Connection State Machines
The state tables defined in this section govern the exchange of The state tables defined in this section govern the exchange of
control messages defined in Section 6. Tables are defined for control messages defined in Section 6. Tables are defined for
incoming call placement and outgoing call placement, as well as for incoming call placement and outgoing call placement, as well as for
initiation of the control connection itself. The state tables do not initiation of the control connection itself. The state tables do not
encode timeout and retransmission behavior, as this is handled in the encode timeout and retransmission behavior, as this is handled in the
underlying reliable control message delivery mechanism (see Section underlying reliable control message delivery mechanism (see Section
4.2). 4.2).
7.1 Malformed AVPs and Control Messages 7.1. Malformed AVPs and Control Messages
Receipt of an invalid or unrecoverable malformed control message Receipt of an invalid or unrecoverable malformed control message
SHOULD be logged appropriately and the control connection cleared to SHOULD be logged appropriately and the control connection cleared to
ensure recovery to a known state. The control connection may then be ensure recovery to a known state. The control connection may then be
restarted by the initiator. restarted by the initiator.
An invalid control message is defined as (1) a message that contains An invalid control message is defined as (1) a message that contains
a Message Type marked as mandatory (see Section 5.4.1) but that is a Message Type marked as mandatory (see Section 5.4.1) but that is
unknown to the implementation, or (2) a control message that is unknown to the implementation, or (2) a control message that is
received in the wrong state. received in the wrong state.
Examples of malformed control messages include (1) a message that has Examples of malformed control messages include (1) a message that has
an invalid value in its header, (2) a message that contains an AVP an invalid value in its header, (2) a message that contains an AVP
that is formatted incorrectly or whose value is out of range, and (3) that is formatted incorrectly or whose value is out of range, and (3)
a message that is missing a required AVP. A control message with a a message that is missing a required AVP. A control message with a
malformed header MUST be discarded. malformed header MUST be discarded.
When possible, a malformed AVP should be considered in the same When possible, a malformed AVP should be treated as an unrecognized
manner as an unrecognized AVP as described in Section 5.2. Thus, an AVP (see Section 5.2). Thus, an attempt to inspect the M bit SHOULD
attempt to inspect the M-bit SHOULD be made to determine the be made to determine the importance of the malformed AVP, and thus,
importance of the malformed AVP and thus the severity of the the severity of the malformation to the entire control message. If
malformation to the entire control message. If the M-bit was able to the M bit can be reasonably inspected within the malformed AVP and is
be reasonably inspected within the malformed AVP and found to be 1, determined to be set, then as with an unrecognized AVP, the
then as with an unrecognized AVP the associated Session or Control associated session or control connection MUST be shut down. If the M
Connection MUST be shutdown. If the M-bit was inspected and found to bit is inspected and is found to be 0, the AVP MUST be ignored
be 0, the AVP MUST be ignored, assuming recovery from the AVP (assuming recovery from the AVP malformation is indeed possible).
malformation is indeed possible.
This policy must not be considered a license to send malformed AVPs, This policy must not be considered as a license to send malformed
but rather, a guide towards how to handle an improperly formatted AVPs, but rather, as a guide towards how to handle an improperly
message if one is received. It is impossible to list all potential formatted message if one is received. It is impossible to list all
malformations of a given message and give advice for each. One potential malformations of a given message and give advice for each.
example of a malformed AVP situation which should be recoverable, is One example of a malformed AVP situation that should be recoverable
if the Rx Connect Speed AVP is received with a length of 10 rather is if the Rx Connect Speed AVP is received with a length of 10 rather
than 14, implying that the connect speed bits-per-second is being than 14, implying that the connect speed bits-per-second is being
formatted in 4 octets rather than 8. If the Rx Connect Speed AVP did formatted in 4 octets rather than 8. If the AVP does not have its M
not have its M-bit set (which would typically be the case) this bit set (as would typically be the case), this condition is not
condition would not be considered catastrophic. As such, the control considered catastrophic. As such, the control message should be
message should be accepted as if the AVP had not been present (with accepted as though the AVP were not present (though a local error
the exception of a local error message being logged). message may be logged).
In several cases in the following tables, a protocol message is sent, In several cases in the following tables, a protocol message is sent,
and then a "clean up" occurs. Note that, regardless of the initiator and then a "clean up" occurs. Note that, regardless of the initiator
of the control connection destruction, the reliable delivery of the control connection destruction, the reliable delivery
mechanism must be allowed to run (see Section 4.2) before destroying mechanism must be allowed to run (see Section 4.2) before destroying
the control connection. This permits the control connection the control connection. This permits the control connection
management messages to be reliably delivered to the peer. management messages to be reliably delivered to the peer.
Appendix B.1 contains an example of lock-step control connection Appendix B.1 contains an example of lock-step control connection
establishment. establishment.
7.2 Control Connection States 7.2. Control Connection States
The L2TP control connection protocol is not distinguishable between The L2TP control connection protocol is not distinguishable between
the two LCCEs but is distinguishable between the originator and the two LCCEs but is distinguishable between the originator and
receiver. The originating peer is the one that first initiates receiver. The originating peer is the one that first initiates
establishment of the control connection. (In a tie breaker establishment of the control connection. (In a tie breaker
situation, this is the winner of the tie.) Since either the LAC or situation, this is the winner of the tie.) Since either the LAC or
the LNS can be the originator, a collision can occur. See the the LNS can be the originator, a collision can occur. See the
Control Connection Tie Breaker AVP in Section 5.4.3 for a description Control Connection Tie Breaker AVP in Section 5.4.3 for a description
of this and its resolution. of this and its resolution.
skipping to change at page 69, line 36 skipping to change at page 70, line 12
wait-ctl-reply Receive SCCCN Send StopCCN, idle wait-ctl-reply Receive SCCCN Send StopCCN, idle
clean up clean up
wait-ctl-conn Receive SCCCN, Send control-conn established wait-ctl-conn Receive SCCCN, Send control-conn established
acceptable open event to acceptable open event to
waiting sessions waiting sessions
wait-ctl-conn Receive SCCCN, Send StopCCN, idle wait-ctl-conn Receive SCCCN, Send StopCCN, idle
not acceptable clean up not acceptable clean up
wait-ctl-conn Receive SCCRP, Send StopCCN, idle wait-ctl-conn Receive SCCRQ, Send StopCCN, idle
SCCRQ clean up SCCRP clean up
established Local open Send control-conn established established Local open Send control-conn established
request open event to request open event to
(new call) waiting sessions (new call) waiting sessions
established Administrative Send StopCCN, idle established Administrative Send StopCCN, idle
control-conn clean up control-conn clean up
close event close event
established Receive SCCRQ, Send StopCCN, idle established Receive SCCRQ, Send StopCCN, idle
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Both initiator and recipient start from this state. An initiator Both initiator and recipient start from this state. An initiator
transmits an SCCRQ, while a recipient remains in the idle state transmits an SCCRQ, while a recipient remains in the idle state
until receiving an SCCRQ. until receiving an SCCRQ.
wait-ctl-reply wait-ctl-reply
The originator checks to see if another connection has been The originator checks to see if another connection has been
requested from the same peer, and if so, handles the collision requested from the same peer, and if so, handles the collision
situation described in Section 5.4.3. situation described in Section 5.4.3.
wait-ctl-conn wait-ctl-conn
Awaiting an SCCCN. Upon receipt, the challenge response contained Awaiting an SCCCN. If the SCCCN is valid, the control connection
in the message is checked. The control connection is established is established; otherwise, it is torn down (sending a StopCCN with
if authentication succeeds; otherwise, it is torn down. the proper result and/or error code).
established established
An established connection may be terminated by either a local An established connection may be terminated by either a local
condition or the receipt of a StopCCN. In the event of a local condition or the receipt of a StopCCN. In the event of a local
termination, the originator MUST send a StopCCN and clean up the termination, the originator MUST send a StopCCN and clean up the
control connection. If the originator receives a StopCCN, it MUST control connection. If the originator receives a StopCCN, it MUST
also clean up the control connection. also clean up the control connection.
7.3 Incoming Calls 7.3. Incoming Calls
An ICRQ is generated by an LCCE, typically in response to an incoming An ICRQ is generated by an LCCE, typically in response to an incoming
call or a local event. Once the LCCE sends the ICRQ, it waits for a call or a local event. Once the LCCE sends the ICRQ, it waits for a
response from the peer. However, it may choose to postpone response from the peer. However, it may choose to postpone
establishment of the call (e.g. answering the call, bringing up the establishment of the call (e.g., answering the call, bringing up the
circuit) until the peer has indicated with an ICRP that it will circuit) until the peer has indicated with an ICRP that it will
accept the call. The peer may choose not to accept the call if, for accept the call. The peer may choose not to accept the call if, for
instance, there are insufficient resources to handle an additional instance, there are insufficient resources to handle an additional
session. session.
If the peer chooses to accept the call, it responds with an ICRP. If the peer chooses to accept the call, it responds with an ICRP.
When the local LCCE receives the ICRP, it attempts to establish the When the local LCCE receives the ICRP, it attempts to establish the
call. A final call connected message, the ICCN, is sent from the call. A final call connected message, the ICCN, is sent from the
local LCCE to the peer to indicate that the call states for both local LCCE to the peer to indicate that the call states for both
LCCEs should enter the established state. If the call is terminated LCCEs should enter the established state. If the call is terminated
before the peer can accept it, a CDN is sent by the local LCCE to before the peer can accept it, a CDN is sent by the local LCCE to
indicate this condition. indicate this condition.
When a call transitions to a "disconnected" or "down" state, the call When a call transitions to a "disconnected" or "down" state, the call
is cleared normally, and the local LCCE sends a CDN. Similarly, if is cleared normally, and the local LCCE sends a CDN. Similarly, if
the peer wishes to clear a call, it sends a CDN and cleans up its the peer wishes to clear a call, it sends a CDN and cleans up its
session. session.
7.3.1 ICRQ Sender States 7.3.1. ICRQ Sender States
State Event Action New State State Event Action New State
----- ----- ------ --------- ----- ----- ------ ---------
idle Call signal or Initiate local wait-control-conn idle Call signal or Initiate local wait-control-conn
ready to receive control-conn ready to receive control-conn
incoming conn open incoming conn open
idle Receive ICCN, Clean up idle idle Receive ICCN, Clean up idle
ICRP, CDN ICRP, CDN
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wait-control- control-conn-open Send ICRQ wait-reply wait-control- control-conn-open Send ICRQ wait-reply
conn conn
wait-reply Receive ICRP, Send ICCN established wait-reply Receive ICRP, Send ICCN established
acceptable acceptable
wait-reply Receive ICRP, Send CDN, idle wait-reply Receive ICRP, Send CDN, idle
Not acceptable clean up Not acceptable clean up
wait-reply Receive ICRQ Send CDN, idle
clean up
wait-reply Receive ICRQ, Process as idle wait-reply Receive ICRQ, Process as idle
lose tie breaker ICRQ Recipient lose tie breaker ICRQ Recipient
(Section 7.3.2) (Section 7.3.2)
wait-reply Receive ICRQ, Send CDN wait-reply wait-reply Receive ICRQ, Send CDN wait-reply
win tie breaker for losing win tie breaker for losing
session session
wait-reply Receive CDN, Clean up idle wait-reply Receive CDN, Clean up idle
ICCN ICCN
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win tie breaker for losing win tie breaker for losing
session session
wait-reply Receive CDN, Clean up idle wait-reply Receive CDN, Clean up idle
ICCN ICCN
wait-reply Local close Send CDN, idle wait-reply Local close Send CDN, idle
request clean up request clean up
established Receive CDN Clean up idle established Receive CDN Clean up idle
established Receive ICRQ, Send CDN, idle established Receive ICRQ, Send CDN, idle
ICRP, ICCN clean up ICRP, ICCN clean up
established Local close Send CDN, idle established Local close Send CDN, idle
request clean up request clean up
The states associated with the ICRQ sender are as follows: The states associated with the ICRQ sender are as follows:
idle idle
The LCCE detects an incoming call on one of its interfaces (e.g. The LCCE detects an incoming call on one of its interfaces (e.g.,
an analog PSTN line rings, or an ATM PVC is provisioned), or a an analog PSTN line rings, or an ATM PVC is provisioned), or a
local event occurs. The LCCE initiates its control connection local event occurs. The LCCE initiates its control connection
establishment state machine and moves to a state waiting for establishment state machine and moves to a state waiting for
confirmation of the existence of a control connection. confirmation of the existence of a control connection.
wait-control-conn wait-control-conn
In this state, the session is waiting for either the control In this state, the session is waiting for either the control
connection to be opened or for verification that the control connection to be opened or for verification that the control
connection is already open. Once an indication that the control connection is already open. Once an indication that the control
connection has been opened is received, session control messages connection has been opened is received, session control messages
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call is accepted. In the latter case, the LCCE sends an ICCN and call is accepted. In the latter case, the LCCE sends an ICCN and
enters the established state. enters the established state.
established established
Data is exchanged over the session. The call may be cleared by Data is exchanged over the session. The call may be cleared by
any of the following: any of the following:
+ An event on the connected interface: The LCCE sends a CDN. + An event on the connected interface: The LCCE sends a CDN.
+ Receipt of a CDN: The LCCE cleans up, disconnecting the call. + Receipt of a CDN: The LCCE cleans up, disconnecting the call.
+ A local reason: The LCCE sends a CDN. + A local reason: The LCCE sends a CDN.
7.3.2 ICRQ Recipient States 7.3.2. ICRQ Recipient States
State Event Action New State State Event Action New State
----- ----- ------ --------- ----- ----- ------ ---------
idle Receive ICRQ, Send ICRP wait-connect idle Receive ICRQ, Send ICRP wait-connect
acceptable acceptable
idle Receive ICRQ, Send CDN, idle idle Receive ICRQ, Send CDN, idle
not acceptable clean up not acceptable clean up
idle Receive ICRP Send CDN idle idle Receive ICRP Send CDN idle
clean up clean up
idle Receive ICCN Clean up idle idle Receive ICCN Clean up idle
wait-connect Receive ICCN Prepare for established wait-connect Receive ICCN, Prepare for established
acceptable data acceptable data
wait-connect Receive ICCN Send CDN, idle wait-connect Receive ICCN, Send CDN, idle
not acceptable clean up not acceptable clean up
wait-connect Receive ICRQ, Send CDN, idle wait-connect Receive ICRQ, Send CDN, idle
ICRP clean up ICRP clean up
idle, Receive CDN Clean up idle idle, Receive CDN Clean up idle
wait-connect, wait-connect,
established established
wait-connect Local close Send CDN, idle wait-connect Local close Send CDN, idle
established request clean up established request clean up
established Receive ICRQ, Send CDN, idle established Receive ICRQ, Send CDN, idle
ICRP, ICCN clean up ICRP, ICCN clean up
The states associated with the ICRQ recipient are as follows: The states associated with the ICRQ recipient are as follows:
idle idle
An ICRQ is received. If the request is not acceptable, a CDN is An ICRQ is received. If the request is not acceptable, a CDN is
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wait-connect wait-connect
The local LCCE is waiting for an ICCN from the peer. Upon receipt The local LCCE is waiting for an ICCN from the peer. Upon receipt
of the ICCN, the local LCCE moves to established state. of the ICCN, the local LCCE moves to established state.
established established
The session is terminated either by sending a CDN or by receiving The session is terminated either by sending a CDN or by receiving
a CDN from the peer. Clean up follows on both sides regardless of a CDN from the peer. Clean up follows on both sides regardless of
the initiator. the initiator.
7.4 Outgoing Calls 7.4. Outgoing Calls
Outgoing calls instruct an LAC to place a call. There are three Outgoing calls instruct an LAC to place a call. There are three
messages for outgoing calls: OCRQ, OCRP, and OCCN. An LCCE first messages for outgoing calls: OCRQ, OCRP, and OCCN. An LCCE first
sends an OCRQ to an LAC to request an outgoing call. The LAC MUST sends an OCRQ to an LAC to request an outgoing call. The LAC MUST
respond to the OCRQ with an OCRP once it determines that the proper respond to the OCRQ with an OCRP once it determines that the proper
facilities exist to place the call and that the call is facilities exist to place the call and that the call is
administratively authorized. Once the outbound call is connected, administratively authorized. Once the outbound call is connected,
the LAC sends an OCCN to the peer indicating the final result of the the LAC sends an OCCN to the peer indicating the final result of the
call attempt. call attempt.
7.4.1 OCRQ Sender States 7.4.1. OCRQ Sender States
State Event Action New State State Event Action New State
----- ----- ------ --------- ----- ----- ------ ---------
idle Local open Initiate local wait-control-conn idle Local open Initiate local wait-control-conn
request control-conn-open request control-conn-open
idle Receive OCCN, Clean up idle idle Receive OCCN, Clean up idle
OCRP OCRP
wait-control- control-conn-open Send OCRQ wait-reply wait-control- control-conn-open Send OCRQ wait-reply
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State Event Action New State State Event Action New State
----- ----- ------ --------- ----- ----- ------ ---------
idle Local open Initiate local wait-control-conn idle Local open Initiate local wait-control-conn
request control-conn-open request control-conn-open
idle Receive OCCN, Clean up idle idle Receive OCCN, Clean up idle
OCRP OCRP
wait-control- control-conn-open Send OCRQ wait-reply wait-control- control-conn-open Send OCRQ wait-reply
conn conn
wait-reply Receive OCRP, none wait-connect wait-reply Receive OCRP, none wait-connect
acceptable acceptable
wait-reply Receive OCRP, Send CDN, idle wait-reply Receive OCRP, Send CDN, idle
not acceptable clean up not acceptable clean up
wait-reply Receive OCCN Send CDN, idle wait-reply Receive OCCN Send CDN, idle
clean up clean up
wait-reply Receive ICRQ, Process as idle wait-reply Receive OCRQ, Process as idle
lose tie breaker OCRQ Recipient lose tie breaker OCRQ Recipient
(Section 7.4.2) (Section 7.4.2)
wait-reply Receive ICRQ, Send CDN wait-reply wait-reply Receive OCRQ, Send CDN wait-reply
win tie breaker for losing win tie breaker for losing
session session
wait-connect Receive OCCN none established wait-connect Receive OCCN none established
wait-connect Receive OCRQ, Send CDN, idle wait-connect Receive OCRQ, Send CDN, idle
OCRP clean up OCRP clean up
idle, Receive CDN Clean up idle idle, Receive CDN Clean up idle
wait-reply, wait-reply,
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If a CDN is received, the session is cleaned up and returns to If a CDN is received, the session is cleaned up and returns to
idle state. If an OCCN is received, the call has succeeded, and idle state. If an OCCN is received, the call has succeeded, and
the session may now exchange data. the session may now exchange data.
established established
If a CDN is received, the session is cleaned up and returns to If a CDN is received, the session is cleaned up and returns to
idle state. Alternatively, if the LCCE chooses to terminate the idle state. Alternatively, if the LCCE chooses to terminate the
session, it sends a CDN to the LAC, cleans up the session, and session, it sends a CDN to the LAC, cleans up the session, and
moves the session to idle state. moves the session to idle state.
7.4.2 OCRQ Recipient (LAC) States 7.4.2. OCRQ Recipient (LAC) States
State Event Action New State State Event Action New State
----- ----- ------ --------- ----- ----- ------ ---------
idle Receive OCRQ, Send OCRP, wait-cs-answer idle Receive OCRQ, Send OCRP, wait-cs-answer
acceptable Place call acceptable Place call
idle Receive OCRQ, Send CDN, idle idle Receive OCRQ, Send CDN, idle
not acceptable clean up not acceptable clean up
idle Receive OCRP Send CDN, idle idle Receive OCRP Send CDN, idle
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connection is established, send an OCCN indicating success, and go connection is established, send an OCCN indicating success, and go
to established state. to established state.
established established
If the LAC receives a CDN from the peer, the call MUST be released If the LAC receives a CDN from the peer, the call MUST be released
via appropriate mechanisms, and the session cleaned up. If the via appropriate mechanisms, and the session cleaned up. If the
call is disconnected because the circuit transitions to a call is disconnected because the circuit transitions to a
"disconnected" or "down" state, the LAC MUST send a CDN to the "disconnected" or "down" state, the LAC MUST send a CDN to the
peer and return to idle state. peer and return to idle state.
7.5 Termination of a Control Connection 7.5. Termination of a Control Connection
The termination of a control connection consists of either peer The termination of a control connection consists of either peer
issuing a StopCCN. The sender of this message SHOULD wait a full issuing a StopCCN. The sender of this message SHOULD wait a full
control message retransmission cycle (e.g. 1 + 2 + 4 + 8 ... seconds) control message retransmission cycle (e.g., 1 + 2 + 4 + 8 ...
for the acknowledgment of this message before releasing the control seconds) for the acknowledgment of this message before releasing the
information associated with the control connection. The recipient of control information associated with the control connection. The
this message should send an acknowledgment of the message to the recipient of this message should send an acknowledgment of the
peer, then release the associated control information. message to the peer, then release the associated control information.
When to release a control connection is an implementation issue and When to release a control connection is an implementation issue and
is not specified in this document. A particular implementation may is not specified in this document. A particular implementation may
use whatever policy is appropriate for determining when to release a use whatever policy is appropriate for determining when to release a
control connection. Some implementations may leave a control control connection. Some implementations may leave a control
connection open for a period of time or perhaps indefinitely after connection open for a period of time or perhaps indefinitely after
the last session for that control connection is cleared. Others may the last session for that control connection is cleared. Others may
choose to disconnect the control connection immediately after the choose to disconnect the control connection immediately after the
last call on the control connection disconnects. last call on the control connection disconnects.
8. Security Considerations 8. Security Considerations
This section addresses some of the security issues that L2TP This section addresses some of the security issues that L2TP
encounters in its operation. encounters in its operation.
8.1 Control Connection Endpoint and Message Security 8.1. Control Connection Endpoint and Message Security
If a shared secret (password) exists between two LCCEs, it may be If a shared secret (password) exists between two LCCEs, it may be
used to perform a mutual authentication between the two LCCEs, and used to perform a mutual authentication between the two LCCEs, and
construct an authentication and integrity check of arriving L2TP construct an authentication and integrity check of arriving L2TP
Control Messages. This mechanism is built-in to L2TPv3, and is control messages. The mechanism provided by L2TPv3 is described in
described in section 4.3 and in the definition of the Message Digest Section 4.3 and in the definition of the Message Digest and Control
and Nonce AVPs in section 5.4.1. Message Authentication Nonce AVPs in Section 5.4.1.
This control channel security mechanism provides for (1) mutual This control message security mechanism provides for (1) mutual
endpoint authentication, and (2) individual control message integrity endpoint authentication, and (2) individual control message integrity
and authenticity checking. Mutual endpoint authentication ensures and authenticity checking. Mutual endpoint authentication ensures
that an L2TPv3 Control Connection is only established between two that an L2TPv3 control connection is only established between two
endpoints that are configured with the proper password. The endpoints that are configured with the proper password. The
individual control message and integrity check guards against individual control message and integrity check guards against
accidental or intentional packet corruption (i.e., those caused by a accidental or intentional packet corruption (i.e., those caused by a
control message spoofing or man-in-the-middle attack). control message spoofing or man-in-the-middle attack).
The shared secret that is used for all Control Connection, Control The shared secret that is used for all control connection, control
Message and AVP security features defined in this document never message, and AVP security features defined in this document never
requires the shared secret to be sent in the clear between L2TP needs to be sent in the clear between L2TP tunnel endpoints.
tunnel endpoints.
8.2 Data Packet Spoofing 8.2. Data Packet Spoofing
Packet spoofing for any type of Virtual Private Network (VPN) Packet spoofing for any type of Virtual Private Network (VPN)
protocol is of particular concern as insertion of carefully protocol is of particular concern as insertion of carefully
constructed rogue packets into the VPN transit network could result constructed rogue packets into the VPN transit network could result
in a violation of VPN traffic separation, leaking data into a in a violation of VPN traffic separation, leaking data into a
customer VPN. This is complicated by the fact that it may be customer VPN. This is complicated by the fact that it may be
particularly difficult for the operator of the VPN to even be aware particularly difficult for the operator of the VPN to even be aware
that it has become a point of transit into or between customer VPNs. that it has become a point of transit into or between customer VPNs.
L2TPv3 provides traffic separation for its VPNs via a 32-bit Session L2TPv3 provides traffic separation for its VPNs via a 32-bit Session
ID in the L2TPv3 Header. When present, the L2TPv3 Cookie (described ID in the L2TPv3 data header. When present, the L2TPv3 Cookie
in section 4.1), provides an additional check to ensure that an (described in Section 4.1), provides an additional check to ensure
arriving packet is intended for the identified Session. Thus, use of that an arriving packet is intended for the identified session.
a Cookie with the Session ID provides an extra guarantee that the Thus, use of a Cookie with the Session ID provides an extra guarantee
Session ID lookup was performed properly and that the Session ID that the Session ID lookup was performed properly and that the
itself was not corrupted in transit. Session ID itself was not corrupted in transit.
In the presence of a blind packet spoofing attack, the Cookie may In the presence of a blind packet spoofing attack, the Cookie may
also provide security against inadvertent leaking of frames into a also provide security against inadvertent leaking of frames into a
customer VPN. To illustrate the type of security that it is provided customer VPN. To illustrate the type of security that it is provided
in this case, consider comparing the validation of a 64-bit cookie in in this case, consider comparing the validation of a 64-bit Cookie in
the L2TPv3 header to permitting packets that match a given source and the L2TPv3 header to the admission of packets that match a given
destination IP address. Both the source and destination IP address source and destination IP address pair. Both the source and
validation and Cookie validation consist of a fast check on cleartext destination IP address pair validation and Cookie validation consist
header information on all arriving packets. However, since L2TPv3 of a fast check on cleartext header information on all arriving
uses its own value, it removes the requirement for one to maintain a packets. However, since L2TPv3 uses its own value, it removes the
list of (potentially several) permitted or denied IP addresses, and requirement for one to maintain a list of (potentially several)
to guard knowledge of the permitted IP addresses from hackers who may permitted or denied IP addresses, and moreover, to guard knowledge of
obtain and spoof them. Further, it is far easier to change an L2TPv3 the permitted IP addresses from hackers who may obtain and spoof
Cookie than an IP address if it is compromised, and a them. Further, it is far easier to change a compromised L2TPv3
cryptographically random [RFC1750] value is far less likely to be Cookie than a compromised IP address," and a cryptographically random
discovered by brute-force attacks compared to an IP address. [RFC1750] value is far less likely to be discovered by brute-force
attacks compared to an IP address.
For protection against brute-force, blind, insertion attacks, a 64- For protection against brute-force, blind, insertion attacks, a 64-
bit Cookie MUST be used with all sessions. A 32 bit Cookie is bit Cookie MUST be used with all sessions. A 32-bit Cookie is
vulnerable to brute-force guessing at high packet rates, and as such vulnerable to brute-force guessing at high packet rates, and as such,
should not be considered an effective barrier to blind insertion should not be considered an effective barrier to blind insertion
attacks (it is still useful, however, as an additional verification attacks (though it is still useful as an additional verification of a
of a successful Session ID lookup). The Cookie provides no protection successful Session ID lookup). The Cookie provides no protection
against a sophisticated man-in-the-middle attacker who can sniff and against a sophisticated man-in-the-middle attacker who can sniff and
correlate captured data between nodes for use in a coordinated correlate captured data between nodes for use in a coordinated
attack. attack.
The Assigned Cookie AVP is used to signal the value and size of the The Assigned Cookie AVP is used to signal the value and size of the
Cookie that must be present in all data packets for a given Session. Cookie that must be present in all data packets for a given session.
Each Assigned Cookie MUST be selected in a cryptographically random Each Assigned Cookie MUST be selected in a cryptographically random
manner [RFC1750] such that a series of Assigned Cookies does not manner [RFC1750] such that a series of Assigned Cookies does not
provide any indication of what a future Cookie will be. provide any indication of what a future Cookie will be.
The L2TPv3 Cookie must not be regarded as a substitute for security The L2TPv3 Cookie must not be regarded as a substitute for security
such as that of IPsec when operating over an open or untrusted such as that provided by IPsec when operating over an open or
network where packets may be sniffed, decoded and correlated for use untrusted network where packets may be sniffed, decoded, and
in a coordinated attack. See section 4.1.3 for more information on correlated for use in a coordinated attack. See Section 4.1.3 for
running L2TP over IPsec. more information on running L2TP over IPsec.
9. Internationalization Considerations 9. Internationalization Considerations
The Host Name and Vendor Name AVPs are not internationalized. The The Host Name and Vendor Name AVPs are not internationalized. The
Vendor Name AVP, although intended to be human-readable, would seem Vendor Name AVP, although intended to be human-readable, would seem
to fit in the category of "globally visible names" [RFC2277] and so to fit in the category of "globally visible names" [RFC2277] and so
is represented in US-ASCII. is represented in US-ASCII.
The Preferred Language AVP is not mandatory. If an LCCE does not If (1) an LCCE does not signify a language preference by the
signify a language preference by the inclusion of this AVP in the inclusion of a Preferred Language AVP (see Section 5.4.3) in the
SCCRQ or SCCRP, the Preferred Language AVP is unrecognized, or the SCCRQ or SCCRP, (2) the Preferred Language AVP is unrecognized, or
requested language is not supported by the peer LCCE, the default (3) the requested language is not supported by the peer LCCE, the
language [RFC2277] MUST be used for all internationalized strings default language [RFC2277] MUST be used for all internationalized
sent by the peer. strings sent by the peer.
10. IANA Considerations 10. IANA Considerations
This document defines a number of "magic" numbers to be maintained by This document defines a number of "magic" numbers to be maintained by
the IANA. This section explains the criteria to be used by the IANA the IANA. This section explains the criteria used by the IANA to
to assign additional numbers in each of these lists. The following assign additional numbers in each of these lists. The following
subsections describe the assignment policy for the namespaces defined subsections describe the assignment policy for the namespaces defined
elsewhere in this document. elsewhere in this document.
Sections 10.1 - 10.3 are requests for new values already managed by Sections 10.1 through 10.3 are requests for new values already
IANA according to [RFC3438]. managed by IANA according to [RFC3438].
The remaining sections are for new registries to be added to the The remaining sections are for new registries that have been added to
existing L2TP registry and maintained by IANA accordingly. the existing L2TP registry and are maintained by IANA accordingly.
10.1 Control Message Attribute Value Pairs (AVPs) 10.1. Control Message Attribute Value Pairs (AVPs)
This number space is managed by IANA as per [RFC3438]. This number space is managed by IANA as per [RFC3438].
New AVPs requiring assignment in this document are encoded with
"AVP-TBA-x," where "x" is 1, 2, 3...
A summary of the new AVPs follows: A summary of the new AVPs follows:
Control Message Attribute Value Pairs Control Message Attribute Value Pairs
Attribute Attribute
Type Description Type Description
--------- ------------------ --------- ------------------
AVP-TBA-0 Extended Vendor ID AVP 58 Extended Vendor ID AVP
AVP-TBA-1 Message Digest 59 Message Digest
AVP-TBA-2 Router ID 60 Router ID
AVP-TBA-3 Assigned Control Connection ID 61 Assigned Control Connection ID
AVP-TBA-4 Pseudowire Capabilities List 62 Pseudowire Capabilities List
AVP-TBA-5 Local Session ID 63 Local Session ID
AVP-TBA-6 Remote Session ID 64 Remote Session ID
AVP-TBA-7 Assigned Cookie 65 Assigned Cookie
AVP-TBA-8 Remote End ID 66 Remote End ID
AVP-TBA-9 Application Code 68 Pseudowire Type
AVP-TBA-10 Pseudowire Type 69 L2-Specific Sublayer
AVP-TBA-11 L2-Specific Sublayer 70 Data Sequencing
AVP-TBA-12 Data Sequencing 71 Circuit Status
AVP-TBA-13 Circuit Status 72 Preferred Language
AVP-TBA-14 Preferred Language 73 Control Message Authentication Nonce
AVP-TBA-15 Control Message Authentication Nonce 74 Tx Connect Speed
AVP-TBA-16 Tx Connect Speed 75 Rx Connect Speed
AVP-TBA-17 Rx Connect Speed
10.2 Message Type AVP Values 10.2. Message Type AVP Values
This number space is managed by IANA as per [RFC3438]. There is one This number space is managed by IANA as per [RFC3438]. There is one
new message type, defined in section 3.1, necessary to be allocated new message type, defined in Section 3.1, that was allocated for this
for this specification: specification:
Message Type AVP (Attribute Type 0) Values Message Type AVP (Attribute Type 0) Values
------------------------------------------ ------------------------------------------
Control Connection Management Control Connection Management
TBA-M1 (ACK) Explicit Acknowledgement 20 (ACK) Explicit Acknowledgement
10.3 Result Code AVP Values 10.3. Result Code AVP Values
This number space is managed by IANA as per [RFC3438]. This number space is managed by IANA as per [RFC3438].
New Result Code values for the CDN message are defined in section New Result Code values for the CDN message are defined in Section
5.4. Following is a summary: 5.4. The following is a summary:
Result Code AVP (Attribute Type 1) Values Result Code AVP (Attribute Type 1) Values
----------------------------------------- -----------------------------------------
General Error Codes General Error Codes
RC-TBA-1 - Session not established due to losing 13 - Session not established due to losing
tie breaker (L2TPv3). tie breaker (L2TPv3).
RC-TBA-2 - Session not established due to unsupported 14 - Session not established due to unsupported
PW type (L2TPv3). PW type (L2TPv3).
RC-TBA-3 - Session not established, sequencing required 15 - Session not established, sequencing required
without valid L2-Specific Sublayer (L2TPv3). without valid L2-Specific Sublayer (L2TPv3).
16 - Finite state machine error or timeout.
There are a few cases in Section 5 where these values are referred to
directly within the document text with the RC-TBA-x format. The
assigned values should be inserted within the text for these cases.
10.4 AVP Header Bits 10.4. AVP Header Bits
This is a new registry for IANA to maintain. This is a new registry for IANA to maintain.
Leading Bits of the L2TP AVP Header Leading Bits of the L2TP AVP Header
----------------------------------- -----------------------------------
There six bits at the beginning of the L2TP AVP header. New bits are There six bits at the beginning of the L2TP AVP header. New bits are
assigned via Standards Action [RFC2434]. assigned via Standards Action [RFC2434].
Bit 0 - Mandatory Bit, "M-bit" Bit 0 - Mandatory (M bit)
Bit 1 - Hidden Bit, "H-bit" Bit 1 - Hidden (H bit)
Bit 2 - Reserved Bit 2 - Reserved
Bit 3 - Reserved Bit 3 - Reserved
Bit 4 - Reserved Bit 4 - Reserved
Bit 5 - Reserved Bit 5 - Reserved
10.5 L2TP Control Message Header Bits 10.5. L2TP Control Message Header Bits
This is a new registry for IANA to maintain. This is a new registry for IANA to maintain.
Leading Bits of the L2TP Control Message Header Leading Bits of the L2TP Control Message Header
----------------------------------------------- -----------------------------------------------
There are 12 bits at the beginning of the L2TP Control Message There are 12 bits at the beginning of the L2TP Control Message
Header. Reserved bits should only be defined by Standard Header. Reserved bits should only be defined by Standard
Action [RFC2434]. Action [RFC2434].
Bit 0 - Message Type, "T-bit" Bit 0 - Message Type (T bit)
Bit 1 - Length Field is Present, "L-bit" Bit 1 - Length Field is Present (L bit)
Bit 2 - Reserved Bit 2 - Reserved
Bit 3 - Reserved Bit 3 - Reserved
Bit 4 - Sequence Numbers Present, "S-bit" Bit 4 - Sequence Numbers Present (S bit)
Bit 5 - Reserved Bit 5 - Reserved
Bit 6 - Offset Field is Present Bit 6 - Offset Field is Present [RFC2661]
Bit 7 - Priority Bit, "P-bit" Bit 7 - Priority Bit (P bit) [RFC2661]
Bit 8 - Reserved Bit 8 - Reserved
Bit 9 - Reserved Bit 9 - Reserved
Bit 10 - Reserved Bit 10 - Reserved
Bit 11 - Reserved Bit 11 - Reserved
10.6 Pseudowire Types 10.6. Pseudowire Types
This is a new registry for IANA to maintain, there are no values This is a new registry for IANA to maintain, there are no values
assigned within this document. assigned within this document to maintain.
L2TPv3 Pseudowire Types L2TPv3 Pseudowire Types
----------------------- -----------------------
The Pseudowire Type (PW Type, Section 5.4) is a two-octet value used
in the Pseudowire Type AVP and Pseudowire Capabilities List AVP The Pseudowire Type (PW Type, see Section 5.4) is a 2-octet value
used in the Pseudowire Type AVP and Pseudowire Capabilities List AVP
defined in Section 5.4.3. 0 to 32767 are assignable by Expert Review defined in Section 5.4.3. 0 to 32767 are assignable by Expert Review
[RFC2434], 32768 to 65535 by a First Come First Served policy [RFC2434], while 32768 to 65535 are assigned by a First Come First
[RFC2434]. There are no specific pseudowire types assigned within Served policy [RFC2434]. There are no specific pseudowire types
this document. Each pseudowire-specific document must allocate its assigned within this document. Each pseudowire-specific document
own PW types from IANA as necessary. must allocate its own PW types from IANA as necessary.
10.7 Circuit Status Bits 10.7. Circuit Status Bits
This is a new registry for IANA to maintain. This is a new registry for IANA to maintain.
Circuit Status Bits Circuit Status Bits
------------------- -------------------
The Circuit Status field is a 16 bit mask, with the two high order The Circuit Status field is a 16-bit mask, with the two low order
bits assigned. Additional bits may be assigned by IETF Consensus bits assigned. Additional bits may be assigned by IETF Consensus
[RFC2434]. [RFC2434].
Bit 15 - A (Active) bit Bit 14 - New (N bit)
Bit 16 - N (New) bit Bit 15 - Active (A bit)
10.8 Default L2-Specific Sublayer bits 10.8. Default L2-Specific Sublayer bits
This is a new registry for IANA to maintain. This is a new registry for IANA to maintain.
Default L2-Specific Sublayer bits Default L2-Specific Sublayer Bits
--------------------------------- ---------------------------------
The Default L2 Specific Sublayer contains 8 bits in the low-order The Default L2-Specific Sublayer contains 8 bits in the low-order
portion of the header. Reserved bits may be assigned by IETF portion of the header. Reserved bits may be assigned by IETF
Consensus [RFC2434]. Consensus [RFC2434].
Bit 0 - Reserved Bit 0 - Reserved
Bit 1 - S (Sequence) bit Bit 1 - Sequence (S bit)
Bit 2 - Reserved Bit 2 - Reserved
Bit 3 - Reserved Bit 3 - Reserved
Bit 4 - Reserved Bit 4 - Reserved
Bit 5 - Reserved Bit 5 - Reserved
Bit 6 - Reserved Bit 6 - Reserved
Bit 7 - Reserved Bit 7 - Reserved
10.9 L2-Specific Sublayer Type 10.9. L2-Specific Sublayer Type
This is a new registry for IANA to maintain. This is a new registry for IANA to maintain.
L2-Specific Sublayer Type L2-Specific Sublayer Type
------------------------- -------------------------
The L2-Specific Sublayer Type is a 2 octet unsigned integer.
The L2-Specific Sublayer Type is a 2-octet unsigned integer.
Additional values may be assigned by Expert Review [RFC2434]. Additional values may be assigned by Expert Review [RFC2434].
0 - No L2-Specific Sublayer 0 - No L2-Specific Sublayer
1 - Default L2-Specific Sublayer present 1 - Default L2-Specific Sublayer present
10.10 Data Sequencing Level 10.10. Data Sequencing Level
This is a new registry for IANA to maintain. This is a new registry for IANA to maintain.
Data Sequencing Level Data Sequencing Level
--------------------- ---------------------
The Data Sequencing Level is a 2 octet unsigned integer The Data Sequencing Level is a 2-octet unsigned integer
Additional values may be assigned by Expert Review [RFC2434]. Additional values may be assigned by Expert Review [RFC2434].
0 - No incoming data packets require sequencing. 0 - No incoming data packets require sequencing.
1 - Only non-IP data packets require sequencing. 1 - Only non-IP data packets require sequencing.
2 - All incoming data packets require sequencing. 2 - All incoming data packets require sequencing.
11. References 11. References
11.1 Normative References 11.1. Normative References
[RFC1994] Simpson, W., "PPP Challenge Handshake Authentication
Protocol (CHAP)", RFC 1994, August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998. Languages", BCP 18, RFC 2277, January 1998.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations section in RFCs", BCP 26, RFC 2434, IANA Considerations section in RFCs", BCP 26, RFC 2434,
October 1998. October 1998.
[RFC2661] Townsley, W., et al., "Layer Two Tunneling Layer Two [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6
Tunneling Protocol (L2TP)", RFC 2661, August 1999. Specification", RFC 2473, December 1998.
[RFC2865] Rigney, C., Rubens, A., Simpson, W., and Willens, S., [RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G.,
"Remote Authentication Dial In User Service (RADIUS)", and Palter, B., "Layer Two Tunneling Layer Two Tunneling
RFC 2865, June 2000. Protocol (L2TP)", RFC 2661, August 1999.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)", RFC
2865, June 2000.
[RFC3066] Alvestrand, H., "Tags for the Identification of Languages", [RFC3066] Alvestrand, H., "Tags for the Identification of Languages",
RFC 3066, January 2001. BCP 47, RFC 3066, January 2001.
[RFC3193] Patel, B., Aboba, B., Dixon, W., Zorn, G., and Booth, S., [RFC3193] Patel, B., Aboba, B., Dixon, W., Zorn, G., and Booth, S.,
"Securing L2TP using IPsec", RFC 3193, November 2001. "Securing L2TP using IPsec", RFC 3193, November 2001.
[RFC3438] Townsley, W., "Layer Two Tunneling Protocol (L2TP) Internet
Assigned Numbers Authority (IANA) Considerations Update",
BCP 68, RFC 3438, December 2002.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
RFC 3629, November 2003 STD 63, RFC 3629, November 2003.
11.2 Informative References 11.2. Informative References
[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities", [RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[RFC1191] Mogul, J. C. et al, "Path MTU Discovery", RFC 1191, [RFC1191] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990 November 1990.
[RFC1321] R. Rivest, "The MD5 Message-Digest Algorithm", RFC 1321, [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
04/16/1992 April 1992.
[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, [RFC1661] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)", STD
RFC 1661, July 1994. 51, RFC 1661, July 1994.
[RFC1700] Reynolds, J., and Postel, J., "Assigned Numbers", STD 2, [RFC1700] Reynolds, J. and Postel, J., "Assigned Numbers", STD 2, RFC
RFC 1700, October 1994. See also: 1700, October 1994.
http://www.iana.org/numbers.html.
[RFC1750] D. Eastlake III, S. Crocker, J. Schiller, "Randomness [RFC1750] Eastlake, D., Crocker, S., and Schiller, J., "Randomness
Recommendations for Security", RFC 1750, December 1994 Recommendations for Security", RFC 1750, December 1994.
[RFC1958] Carpenter, B., "Architectural Principles of the Internet", [RFC1958] Carpenter, B., Ed., "Architectural Principles of the
RFC 1958, June 1996. Internet", RFC 1958, June 1996.
[RFC1981] McCann, J. et al, "Path MTU Discovery for IP version [RFC1981] McCann, J., Deering, S., and Mogul, J., "Path MTU Discovery
6", RFC 1981, August 1996 for IP version 6", RFC 1981, August 1996.
[RFC2072] Berkowitz, H., "Router Renumbering Guide", RFC 2072, [RFC2072] Berkowitz, H., "Router Renumbering Guide", RFC 2072,
January 1997. January 1997.
[RFC2104] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing [RFC2104] Krawczyk, H., Bellare, M., and Canetti, R., "HMAC: Keyed-
for Message Authentication", RFC 2104, February 1997 Hashing for Message Authentication", RFC 2104, February
1997.
[RFC2138] Rigney, C., Rubens, A., Simpson, W., and Willens, S.,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2138, April 1997.
[RFC2341] Valencia, A., Littlewood, M., and Kolar, T., [RFC2341] Valencia, A., Littlewood, M., and Kolar, T., "Cisco Layer
"Cisco Layer Two Forwarding (Protocol) L2F", RFC 2341, Two Forwarding (Protocol) L2F", RFC 2341, May 1998.
May 1998.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the [RFC2401] Kent, S. and Atkinson, R., "Security Architecture for the
Internet Protocol", RFC 2401, November 1998. Internet Protocol", RFC 2401, November 1998.
[RFC2581] Allman, M., Paxson, V., Stevens, W., "TCP Congestion [RFC2581] Allman, M., Paxson, V. and Stevens, W., "TCP Congestion
Control", RFC 2581, April 1999 Control", RFC 2581, April 1999.
[RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W., [RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W.,
and Zorn, G., "Point-to-Point Tunneling Protocol (PPTP)", and Zorn, G., "Point-to-Point Tunneling Protocol (PPTP)",
RFC 2637, July 1999. RFC 2637, July 1999.
[RFC2732] Hinden, R., Carpenter, B., Masinter, L., "Format for [RFC2732] Hinden, R., Carpenter, B., and Masinter, L., "Format for
Literal IPv6 Addresses in URL's", RFC 2732, December 1999 Literal IPv6 Addresses in URL's", RFC 2732, December 1999.
[RFC2809] Aboba, B., and Zorn, G., "Implementation of L2TP Compulsory [RFC2809] Aboba, B. and Zorn, G., "Implementation of L2TP Compulsory
Tunneling via RADIUS", RFC 2809, April 2000. Tunneling via RADIUS", RFC 2809, April 2000.
[RFC3070] Rawat, V., Tio, R., Nanji, S., and Verma, R., [RFC3070] Rawat, V., Tio, R., Nanji, S., and Verma, R., "Layer Two
"Layer Two Tunneling Protocol (L2TP) over Frame Relay", Tunneling Protocol (L2TP) over Frame Relay", RFC 3070,
RFC 3070, February 2001. February 2001.
[RFC3355] Singh, A., Turner, R., Tio, R., Nanji, S., "Layer Two [RFC3355] Singh, A., Turner, R., Tio, R., and Nanji, S., "Layer Two
Tunnelling Protocol (L2TP) Over ATM Adaptation Tunnelling Protocol (L2TP) Over ATM Adaptation Layer 5
Layer 5 (AAL5)", RFC 3355, August 2002 (AAL5)", RFC 3355, August 2002.
[KPS] Kaufman, C., Perlman, R., and Speciner, M., "Network [KPS] Kaufman, C., Perlman, R., and Speciner, M., "Network
Security: Private Communications in a Public World", Security: Private Communications in a Public World",
Prentice Hall, March 1995, ISBN 0-13-061466-1. Prentice Hall, March 1995, ISBN 0-13-061466-1.
[STEVENS] Stevens, W. Richard, "TCP/IP Illustrated, Volume I: The [STEVENS] Stevens, W. Richard, "TCP/IP Illustrated, Volume I: The
Protocols", Addison-Wesley Publishing Company, Inc., Protocols", Addison-Wesley Publishing Company, Inc., March
March 1996, ISBN 0-201-63346-9. 1996, ISBN 0-201-63346-9.
12. Editors' Addresses
Jed Lau
cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
jedlau@cisco.com
W. Mark Townsley
cisco Systems
mark@townsley.net
Ignacio Goyret
Lucent Technologies
igoyret@lucent.com
13. Acknowledgments 12. Acknowledgments
Many of the protocol constructs were originally defined in, and the Many of the protocol constructs were originally defined in, and the
text of this document began with, RFC 2661, "L2TPv2". RFC 2661 text of this document began with, RFC 2661, "L2TPv2". RFC 2661
authors are W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn and authors are W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn and
B. Palter. B. Palter.
The basic concept for L2TP and many of its protocol constructs were The basic concept for L2TP and many of its protocol constructs were
adopted from L2F [RFC2341] and PPTP [RFC2637]. Authors of these adopted from L2F [RFC2341] and PPTP [RFC2637]. Authors of these
drafts are A. Valencia, M. Littlewood, T. Kolar, K. Hamzeh, G. Pall, versions are A. Valencia, M. Littlewood, T. Kolar, K. Hamzeh, G.
W. Verthein, J. Taarud, W. Little, and G. Zorn. Pall, W. Verthein, J. Taarud, W. Little, and G. Zorn.
Danny Mcpherson and Suhail Nanji published the first "L2TP Service Danny Mcpherson and Suhail Nanji published the first "L2TP Service
Type" draft which defined the use of L2TP for tunneling of various L2 Type" version, which defined the use of L2TP for tunneling of various
payload types (initially, Ethernet and Frame Relay). L2 payload types (initially, Ethernet and Frame Relay).
The team for splitting RFC 2661 into this base document and the The team for splitting RFC 2661 into this base document and the
companion PPP document consisted of Ignacio Goyret, Jed Lau, Bill companion PPP document consisted of Ignacio Goyret, Jed Lau, Bill
Palter, Mark Townsley, and Madhvi Verma. Skip Booth also provided Palter, Mark Townsley, and Madhvi Verma. Skip Booth also provided
very helpful review and comment. very helpful review and comment.
Some constructs of L2TPv3 were based in part on UTI (Universal Some constructs of L2TPv3 were based in part on UTI (Universal
Transport Interface), which was originally conceived by Peter Transport Interface), which was originally conceived by Peter
Lothberg and Tony Bates. Lothberg and Tony Bates.
Stewart Bryant and Simon Barber provided valuable input for the Stewart Bryant and Simon Barber provided valuable input for the
L2TPv3 over IP header. L2TPv3 over IP header.
Juha Heinanen provided helpful review in the early stages of this Juha Heinanen provided helpful review in the early stages of this
effort. effort.
Jan Vilhuber, Scott Fluhrer, David McGrew, Scott Wainner, Skip Booth Jan Vilhuber, Scott Fluhrer, David McGrew, Scott Wainner, Skip Booth
and Maria Dos Santos contributed to the Control Message and and Maria Dos Santos contributed to the Control Message
Authentication Mechanism as well as general discussions of security. Authentication Mechanism as well as general discussions of security.
James Carlson, Thomas Narten, Maria Dos Santos, Steven Bellovin, Ted James Carlson, Thomas Narten, Maria Dos Santos, Steven Bellovin, Ted
Hardie and Pekka Savola, provided very helpful review of the final Hardie and Pekka Savola provided very helpful review of the final
versions of text. versions of text.
Russ Housley provided valuable review and comment on security, Russ Housley provided valuable review and comment on security,
particularly with respect to the control message authentication particularly with respect to the Control Message Authentication
mechanisms. mechanism.
Pekka Savola contributed to proper alignment with IPv6 and inspired Pekka Savola contributed to proper alignment with IPv6 and inspired
much of section 4.1.4 on fragmentation. much of Section 4.1.4 on fragmentation.
Aside of his original influence and co-authorship of RFC2661, Glen Aside of his original influence and co-authorship of RFC 2661, Glen
Zorn helped get all of the language and character references straight Zorn helped get all of the language and character references straight
in this document. in this document.
A number of people provided valuable input and effort for RFC2661, on A number of people provided valuable input and effort for RFC 2661,
which this document was based: on which this document was based:
John Bray, Greg Burns, Rich Garrett, Don Grosser, Matt Holdrege, John Bray, Greg Burns, Rich Garrett, Don Grosser, Matt Holdrege,
Terry Johnson, Dory Leifer, and Rich Shea provided valuable input and Terry Johnson, Dory Leifer, and Rich Shea provided valuable input and
review at the 43rd IETF in Orlando, FL, which led to improvement of review at the 43rd IETF in Orlando, FL, which led to improvement of
the overall readability and clarity of RFC 2661. the overall readability and clarity of RFC 2661.
Thomas Narten provided a great deal of critical review and Thomas Narten provided a great deal of critical review and
formatting. He wrote the first version of the IANA Considerations formatting. He wrote the first version of the IANA Considerations
section. section.
Dory Leifer made valuable refinements to the protocol definition of Dory Leifer made valuable refinements to the protocol definition of
L2TP and contributed to the editing of early drafts leading to RFC L2TP and contributed to the editing of early versions leading to RFC
2661. 2661.
Steve Cobb and Evan Caves redesigned the state machine tables. Steve Cobb and Evan Caves redesigned the state machine tables.
Barney Wolff provided a great deal of design input on the original Barney Wolff provided a great deal of design input on the original
endpoint authentication mechanism. endpoint authentication mechanism.
Appendix A: Control Slow Start and Congestion Avoidance Appendix A: Control Slow Start and Congestion Avoidance
Although each side has indicated the maximum size of its receive Although each side has indicated the maximum size of its receive
window, it is recommended that a slow start and congestion avoidance window, it is recommended that a slow start and congestion avoidance
method be used to transmit control packets. The methods described method be used to transmit control packets. The methods described
here are based upon the TCP congestion avoidance algorithm as here are based upon the TCP congestion avoidance algorithm as
described in section 21.6 of TCP/IP Illustrated, Volume I, by W. described in Section 21.6 of TCP/IP Illustrated, Volume I, by W.
Richard Stevens [STEVENS] (this algorithm is also described in Richard Stevens [STEVENS] (this algorithm is also described in
[RFC2581]). [RFC2581]).
Slow start and congestion avoidance make use of several variables. Slow start and congestion avoidance make use of several variables.
The congestion window (CWND) defines the number of packets a sender The congestion window (CWND) defines the number of packets a sender
may send before waiting for an acknowledgment. The size of CWND may send before waiting for an acknowledgment. The size of CWND
expands and contracts as described below. Note, however, that CWND expands and contracts as described below. Note, however, that CWND
is never allowed to exceed the size of the advertised window obtained is never allowed to exceed the size of the advertised window obtained
from the Receive Window AVP. (In the text below, it is assumed any from the Receive Window AVP. (In the text below, it is assumed any
increase will be limited by the Receive Window Size.) The variable increase will be limited by the Receive Window Size.) The variable
skipping to change at page 88, line 38 skipping to change at page 89, line 16
B.1: Lock-Step Control Connection Establishment B.1: Lock-Step Control Connection Establishment
In this example, an LCCE establishes a control connection, with the In this example, an LCCE establishes a control connection, with the
exchange involving each side alternating in sending messages. This exchange involving each side alternating in sending messages. This
example shows the final acknowledgment explicitly sent within an ACK example shows the final acknowledgment explicitly sent within an ACK
message. An alternative would be to piggyback the acknowledgment message. An alternative would be to piggyback the acknowledgment
within a message sent as a reply to the ICRQ or OCRQ that will likely within a message sent as a reply to the ICRQ or OCRQ that will likely
follow from the side that initiated the control connection. follow from the side that initiated the control connection.
LCCE A LCCE B LCCE A LCCE B
------ ------ ------ ------
SCCRQ -> SCCRQ ->
Nr: 0, Ns: 0 Nr: 0, Ns: 0
<- SCCRP <- SCCRP
Nr: 1, Ns: 0 Nr: 1, Ns: 0
SCCCN -> SCCCN ->
Nr: 1, Ns: 1 Nr: 1, Ns: 1
<- ACK <- ACK
Nr: 2, Ns: 1 Nr: 2, Ns: 1
B.2: Lost Packet with Retransmission B.2: Lost Packet with Retransmission
An existing control connection has a new session requested by LCCE A. An existing control connection has a new session requested by LCCE A.
The ICRP is lost and must be retransmitted by LCCE B. Note that loss The ICRP is lost and must be retransmitted by LCCE B. Note that loss
of the ICRP has two effects: It not only keeps the upper level state of the ICRP has two effects: It not only keeps the upper level state
machine from progressing, but also keeps LCCE A from seeing a timely machine from progressing, but also keeps LCCE A from seeing a timely
lower level acknowledgment of its ICRQ. lower level acknowledgment of its ICRQ.
LCCE A LCCE B LCCE A LCCE B
------ ------ ------ ------
ICRQ -> ICRQ ->
Nr: 1, Ns: 2 Nr: 1, Ns: 2
(packet lost) <- ICRP (packet lost) <- ICRP
Nr: 3, Ns: 1 Nr: 3, Ns: 1
(pause; LCCE A's timer started first, so fires first) (pause; LCCE A's timer started first, so fires first)
ICRQ -> ICRQ ->
Nr: 1, Ns: 2 Nr: 1, Ns: 2
(Realizing that it has already seen this packet, (Realizing that it has already seen this packet,
LCCE B discards the packet and sends an ACK message) LCCE B discards the packet and sends an ACK message)
<- ACK <- ACK
Nr: 3, Ns: 2 Nr: 3, Ns: 2
(LCCE B's retransmit timer fires) (LCCE B's retransmit timer fires)
<- ICRP <- ICRP
Nr: 3, Ns: 1 Nr: 3, Ns: 1
ICCN -> ICCN ->
Nr: 2, Ns: 3 Nr: 2, Ns: 3
<- ACK <- ACK
Nr: 4, Ns: 2 Nr: 4, Ns: 2
Appendix C: Processing Sequence Numbers Appendix C: Processing Sequence Numbers
The Default L2-Specific Sublayer, defined in Section 4.6, provides a The Default L2-Specific Sublayer, defined in Section 4.6, provides a
24-bit field for sequencing of data packets within an L2TP session. 24-bit field for sequencing of data packets within an L2TP session.
L2TP data packets are never retransmitted, so this sequence is used L2TP data packets are never retransmitted, so this sequence is used
only to detect packet order, duplicate packets, or lost packets. only to detect packet order, duplicate packets, or lost packets.
The 24-bit Sequence Number field of the Default L2-Specific Sublayer The 24-bit Sequence Number field of the Default L2-Specific Sublayer
contains a packet sequence number for the associated session. Each contains a packet sequence number for the associated session. Each
sequenced data packet that is sent must contain the sequence number, sequenced data packet that is sent must contain the sequence number,
incremented by one, of the previous sequenced packet sent on a given incremented by one, of the previous sequenced packet sent on a given
L2TP session. Upon receipt, any packet with a sequence number equal L2TP session. Upon receipt, any packet with a sequence number equal
to or greater than the current expected packet (the last received to or greater than the current expected packet (the last received
in-order packet plus one) should be considered "new" and accepted. in-order packet plus one) should be considered "new" and accepted.
All other packets are considered "old" or "duplicate" and discarded. All other packets are considered "old" or "duplicate" and discarded.
Note that the 24-bit sequence number space includes zero as a valid Note that the 24-bit sequence number space includes zero as a valid
sequence number (as such, it may be implemented with a masked 32-bit sequence number (as such, it may be implemented with a masked 32-bit
counter if desired). All new sessions MUST begin sending sequence counter if desired). All new sessions MUST begin sending sequence
numbers at zero. numbers at zero.
Larger or smaller sequence number fields are possible with L2TP if an Larger or smaller sequence number fields are possible with L2TP if an
alternative format to the Default L2-Specific Sublayer defined in alternative format to the Default L2-Specific Sublayer defined in
this document is used. While 24 bits may be adequate in a number of this document is used. While 24 bits may be adequate in a number of
circumstances, a larger sequence number space will be less circumstances, a larger sequence number space will be less
susceptible to sequence number wrapping problems for very high susceptible to sequence number wrapping problems for very high
session data rates across long dropout periods. The sequence number session data rates across long dropout periods. The sequence number
processing recommendations below should hold for any size sequence processing recommendations below should hold for any size sequence
number field. number field.
When detecting whether a packet sequence number is "greater" or When detecting whether a packet sequence number is "greater" or
"less" than a given sequence number value, wrapping of the sequence "less" than a given sequence number value, wrapping of the sequence
number must be considered. This is typically accomplished by keeping number must be considered. This is typically accomplished by keeping
a window of sequence numbers beyond the current expected sequence a window of sequence numbers beyond the current expected sequence
number for determination of whether a packet is "new" or not. The number for determination of whether a packet is "new" or not. The
window may be sized based on the link speed and sequence number space window may be sized based on the link speed and sequence number space
and SHOULD be configurable with a default equal to one half the size and SHOULD be configurable with a default equal to one half the size
of the available number space (e.g. 2^(n-1), where n is the number of of the available number space (e.g., 2^(n-1), where n is the number
bits available in the sequence number). of bits available in the sequence number).
Upon receipt, packets which exactly match the expected sequence Upon receipt, packets that exactly match the expected sequence number
number are processed immediately and the next expected sequence are processed immediately and the next expected sequence number
number incremented. Packets that fall within the window for new incremented. Packets that fall within the window for new packets may
packets may either be processed immediately and the next expected either be processed immediately and the next expected sequence number
sequence number updated to one plus that received in the new packet, updated to one plus that received in the new packet, or held for a
or held for a very short period of time in hopes of receiving the very short period of time in hopes of receiving the missing
missing packet(s). This 'very short period' should be configurable, packet(s). This "very short period" should be configurable, with a
with a default corresponding to a time lapse which is at least an default corresponding to a time lapse that is at least an order of
order of magnitude less than the retransmission timeout periods of magnitude less than the retransmission timeout periods of higher
higher layer protocols such as TCP. layer protocols such as TCP.
For typical transient packet mis-orderings, dropping out-of-order For typical transient packet mis-orderings, dropping out-of-order
packets alone should suffice and generally requires far less packets alone should suffice and generally requires far less
resources than actively reordering packets within L2TP. An exception resources than actively reordering packets within L2TP. An exception
is a case where a pair of packet fragments are persistently is a case in which a pair of packet fragments are persistently
retransmitted and sent out-of-order. For example, if an IP packet has retransmitted and sent out-of-order. For example, if an IP packet
been fragmented into a very small packet followed by a very large has been fragmented into a very small packet followed by a very large
packet before being tunneled by L2TP, it is possible (though packet before being tunneled by L2TP, it is possible (though
admittedly wrong) that the two resulting L2TP packets may be admittedly wrong) that the two resulting L2TP packets may be
consistently mis-ordered by the PSN in transit between L2TP nodes. If consistently mis-ordered by the PSN in transit between L2TP nodes.
sequence numbers were being enforced at the receiving node without If sequence numbers were being enforced at the receiving node without
any buffering of out-of-order packets, then the fragmented IP packet any buffering of out-of-order packets, then the fragmented IP packet
may never reach its destination. It may be worth noting here that may never reach its destination. It may be worth noting here that
this condition is true for any tunneling mechanism of IP packets this condition is true for any tunneling mechanism of IP packets that
which include sequence number checking on receipt (i.e. GRE includes sequence number checking on receipt (i.e., GRE [RFC2890]).
[RFC2890]).
Utilization of a Data Sequencing Level (see Section 5.4.3) of 1 (only Utilization of a Data Sequencing Level (see Section 5.4.3) of 1 (only
non-IP data packets require sequencing) allows IP data packets being non-IP data packets require sequencing) allows IP data packets being
tunneled by L2TP to not utilize sequence numbers, while utilizing tunneled by L2TP to not utilize sequence numbers, while utilizing
sequence numbers and enforcing packet order for any remaining non-IP sequence numbers and enforcing packet order for any remaining non-IP
data packets. Depending on the requirements of the link-layer being data packets. Depending on the requirements of the link layer being
tunneled, and the network data traversing the data-link, this is tunneled and the network data traversing the data link, this is
sufficient in many cases to enforce packet order on frames which sufficient in many cases to enforce packet order on frames that
require it (such as end-to-end data-link control messages), while not require it (such as end-to-end data link control messages), while not
on IP packets which are known to be resilient to packet reordering. on IP packets that are known to be resilient to packet reordering.
If a large number of packets (e.g. more than one new packet window) If a large number of packets (i.e., more than one new packet window)
are dropped due to an extended outage, or loss of sequence number are dropped due to an extended outage or loss of sequence number
state on one side of the connection (perhaps as part of a forwarding state on one side of the connection (perhaps as part of a forwarding
plane reset or failover to a standby node), it is possible that a plane reset or failover to a standby node), it is possible that a
large number of packets will be sent in-order, but be wrongly large number of packets will be sent in-order, but be wrongly
detected by the peer as out-of-order. This can be generally detected by the peer as out-of-order. This can be generally
characterized for a window size, w, sequence number space, s, and characterized for a window size, w, sequence number space, s, and
number of packets lost in transit between L2TP endpoints, p, as number of packets lost in transit between L2TP endpoints, p, as
follows: follows:
If s > p > w, then an additional (s - p) packets that were otherwise If s > p > w, then an additional (s - p) packets that were otherwise
received in-order, will be incorrectly classified as out-of-order and received in-order, will be incorrectly classified as out-of-order and
dropped. Thus, for a sequence number space, s = 128, window size, w = dropped. Thus, for a sequence number space, s = 128, window size, w
64, and number of lost packets, p = 70; 128 - 70 = 58 additional = 64, and number of lost packets, p = 70; 128 - 70 = 58 additional
packets would be dropped after the outage until the sequence number packets would be dropped after the outage until the sequence number
wrapped back to the current expected next sequence number. wrapped back to the current expected next sequence number.
To mitigate this additional packet loss, one MUST inspect the To mitigate this additional packet loss, one MUST inspect the
sequence numbers of packets dropped due to being classified as "old" sequence numbers of packets dropped due to being classified as "old"
and reset the expected sequence number accordingly. This may be and reset the expected sequence number accordingly. This may be
accomplished by counting the number of "old" packets dropped that accomplished by counting the number of "old" packets dropped that
were in sequence among themselves and upon reaching a threshold, were in sequence among themselves and, upon reaching a threshold,
resetting the next expected sequence number to that seen in the resetting the next expected sequence number to that seen in the
arriving data packets. Packet timestamps may also be used as an arriving data packets. Packet timestamps may also be used as an
indicator to reset the expected sequence number by detecting a period indicator to reset the expected sequence number by detecting a period
of time over which "old" packets have been received in-sequence. The of time over which "old" packets have been received in-sequence. The
ideal thresholds will vary depending on link speed, sequence number ideal thresholds will vary depending on link speed, sequence number
space, and link tolerance to out-of-order packets, and MUST be space, and link tolerance to out-of-order packets, and MUST be
configurable. configurable.
Intellectual Property Statement Editors' Addresses
Jed Lau
cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
EMail: jedlau@cisco.com
W. Mark Townsley
cisco Systems
EMail: mark@townsley.net
Ignacio Goyret
Lucent Technologies
EMail: igoyret@lucent.com
Full Copyright Statement
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment Acknowledgement
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is currently provided by the
Internet Society. Internet Society.
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