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<!ENTITY RFC0793 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.0793.xml">
<!ENTITY RFC0896 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.0896.xml">
<!ENTITY RFC1122 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.1122.xml">
<!ENTITY RFC1191 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.1191.xml">
<!ENTITY RFC1981 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.1981.xml">
<!ENTITY RFC2119 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml">
<!ENTITY RFC2675 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2675.xml">
<!ENTITY RFC2923 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2923.xml">
<!ENTITY RFC4821 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4821.xml">
<!ENTITY RFC5044 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5044.xml">
<!ENTITY RFC5681 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5681.xml">
<!ENTITY RFC5795 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5795.xml">
<!ENTITY RFC6093 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.6093.xml">
<!ENTITY RFC6298 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.6298.xml">
<!ENTITY RFC6429 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.6429.xml">
<!ENTITY RFC6528 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.6528.xml">
<!ENTITY RFC6691 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.6691.xml">
<!ENTITY RFC7323 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.7323.xml">
<!ENTITY RFC7414 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.7414.xml">
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<rfc
    category="std"
    obsoletes="793, 879, 6093, 6429, 6528, 6691"
    updates="1122"
    ipr="pre5378Trust200902"
    docName="draft-ietf-tcpm-rfc793bis-04" >
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    <!-- end of list of popular I-D processing instructions -->

    <!-- ***** FRONT MATTER ***** -->
<front>
    <!-- The abbreviated title is used in the page header - it is only necessary if the 
         full title is longer than 42 characters -->
    <title abbrev="TCP Specification">Transmission Control Protocol Specification</title>

    <!-- add 'role="editor"' below for the editors if appropriate -->
    <author
        fullname="Wesley M. Eddy" 
        initials="W." 
        surname="Eddy"
        role="editor">

        <!-- abbrev not needed but can be used for the header
             if the full organization name is too long -->
        <organization abbrev="MTI Systems">MTI Systems</organization>
        <address>
            <postal>
                <!-- I've omitted my street address here -->
                <street/>
                <city/>
                <!--
                    The IETF seems to meet once a year in Minneapolis,
                    so that's practically my US address. If so, I would
                    add the following elements:
                <region>MN</region>
                <code>55403</code>
                However, if I lived in France, the <code> comes before the city.  xml2rfc
                preserves the order of <city>, <region>, <code> and <country> elements in 
                output so that they can reflect any possible the national scheme
                -->
                <!-- The country element is supposed to contain an ISO3166 two letter country
                     code. -->
                <country>US</country>
            </postal>
        <email>wes@mti-systems.com</email>
        <!--
            If I had a phone, fax machine, and a URI, I could add the following:
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                <facsimile>+1-555-911-9111</facsimile>
                <uri>http://www.example.com/</uri>
            -->
        </address>
    </author>
<!--
    <author
        fullname="Andre Oppermann"
        initials="A."
        surname="Oppermann">
        <organization>FreeBSD</organization>
        <address>
            <email>andre@freebsd.org</email>
        </address>
    </author>
-->

    <date year="2016"/> <!-- month="March" is no longer necessary
                                           note also, day="30" is optional -->
    <!-- WARNING: If the month and year are the current ones, xml2rfc will fill in the day for 
         you. If only the year is specified, xml2rfc will fill in the current day and month 
         irrespective of the day.  This silliness should be fixed in v1.31. -->
         
    <!-- Meta-data Declarations -->
    
    <!-- Notice the use of &amp; as an escape for & which would otherwise
         start an entity declaration, whereas we want a literal &. -->
    <area>Transport</area>

    <!-- WG name at the upperleft corner of the doc,
         IETF fine for individual submissions.  You can also
         omit this element in which case in defaults to "Network Working Group" -
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    <workgroup>Internet Engineering Task Force</workgroup>
    
    <!-- The DTD allows multiple area and workgroup elements but only the first one has any
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    <!-- You can add <keyword/> elements here.  They will be incorporated into HTML output
         files in a meta tag but they have no effect on text or nroff output. -->
    
    
    <abstract>
        <t>This document specifies the Internet's Transmission Control Protocol (TCP).  TCP is an important transport layer protocol in the Internet stack, and has continuously evolved over decades of use and growth of the Internet.  Over this time, a number of changes have been made to TCP as it was specified in RFC 793, though these have only been documented in a piecemeal fashion.  This document collects and brings those changes together with the protocol specification from RFC 793.  This document obsoletes RFC 793 and several other RFCs (TODO: list all actual RFCs when finished).</t>

        <t>RFC EDITOR NOTE: If approved for publication as an RFC, this should be marked additionally as "STD: 7" and replace RFC 793 in that role.</t>
    </abstract>

    <note title="Requirements Language">
        <t>The key words &quot;MUST&quot;, &quot;MUST NOT&quot;,
        &quot;REQUIRED&quot;, &quot;SHALL&quot;, &quot;SHALL NOT&quot;,
        &quot;SHOULD&quot;, &quot;SHOULD NOT&quot;, &quot;RECOMMENDED&quot;,
        &quot;MAY&quot;, and &quot;OPTIONAL&quot; in this document are to be
        interpreted as described in <xref target="RFC2119">RFC 2119</xref>.
        </t>
    </note>

</front>

<middle>
    <section title="Purpose and Scope">
        <t>
        In 1981, <xref target="RFC0793">RFC 793</xref> was released, documenting the Transmission Control Protocol (TCP), and replacing earlier specifications for TCP that had been published in the past.
        </t>
        <t>
        Since then, TCP has been implemented many times, and has been used as a transport protocol for numerous applications on the Internet.
        </t>
        <t>
        For several decades, RFC 793 plus a number of other documents have combined to serve as the specification for TCP <xref target="RFC7414"></xref>.  Over time, a number of errata have been identified on RFC 793, as well as deficiencies in security, performance, and other aspects.  A number of enhancements has grown and been documented separately.  These were never accumulated together into an update to the base specification.
        </t>
        <t>
        The purpose of this document is to bring together all of the IETF Standards Track changes that have been made to the basic TCP functional specification and unify them into an update of the RFC 793 protocol specification.  Some companion documents are referenced for important algorithms that TCP uses (e.g. for congestion control), but have not been attempted to include in this document.  This is a conscious choice, as this base specification can be used with multiple additional algorithms that are developed and incorporated separately, but all TCP implementations need to implement this specification as a common basis in order to interoperate.  As some additional TCP features have become quite complicated themselves (e.g. advanced loss recovery and congestion control), future companion documents may attempt to similarly bring these together.
        </t>
        <t>
        In addition to the protocol specification that descibes the TCP segment format, generation, and processing rules that are to be implemented in code, RFC 793 and other updates also contain informative and descriptive text for human readers to understand aspects of the protocol design and operation.  This document does not attempt to alter or update this informative text, and is focused only on updating the normative protocol specification.  We preserve references to the documentation containing the important explanations and rationale, where appropriate.
        </t>
        <t>
        This document is intended to be useful both in checking existing TCP implementations for conformance, as well as in writing new implementations.
        </t>
    </section>
    <section title="Introduction">
        <t>RFC 793 contains a discussion of the TCP design goals and provides examples of its operation, including examples of connection establishment, closing connections, and retransmitting packets to repair losses.
        </t>
        <t>
        This document describes the basic functionality expected in modern implementations of TCP, and replaces the protocol specification in RFC 793.  It does not replicate or attempt to update the examples and other discussion in RFC 793.  Other documents are referenced to provide explanation of the theory of operation, rationale, and detailed discussion of design decisions.  This document only focuses on the normative behavior of the protocol.
        </t>
	<t>
	The &quot;TCP Roadmap&quot; <xref target="RFC7414"/> provides a more extensive guide to the RFCs that define TCP and describe various important algorithms. The TCP Roadmap contains sections on strongly encouraged enhancements that improve performance and other aspects of TCP beyond the basic operation specified in this document.  As one example, implementing congestion control (e.g. <xref target="RFC5681"/>) is a TCP requirement, but is a complex topic on its own, and not described in detail in this document, as there are many options and possibilities that do not impact basic interoperability.  Similarly, most common TCP implementations today include the high-performance extensions in <xref target="RFC7323"/>, but these are not strictly required or discussed in this document.
	</t>
        <t>
        TEMPORARY EDITOR'S NOTE: This is an early revision in the process of updating RFC 793.  Many planned changes are not yet incorporated.<vspace blankLines="1"/>  ***Please do not use this revision as a basis for any work or reference.***
        </t>
        <t>
        A list of changes from RFC 793 is contained in <xref target="changes"/>.
        </t>
        <t>
        TEMPORARY EDITOR'S NOTE: the current revision of this document does not yet collect all of the changes that will be in the final version. The set of content changes planned for future revisions is kept in <xref target="changes"/>.
        </t>
    </section>

    <section title="Functional Specification">

<section title="Header Format">

<t>
  TCP segments are sent as internet datagrams.  The Internet Protocol
  header carries several information fields, including the source and
  destination host addresses <xref target="RFC0791"/>.  A TCP header follows the internet
  header, supplying information specific to the TCP protocol.  This
  division allows for the existence of host level protocols other than
  TCP. (Editorial TODO - this last sentence makes sense in 793 context, but may be a candidate to remove here? ... additionally, Section 2 of 793 is not includeed here, but some parts may be useful, to quickly define basic concepts of ports, bytestream service, etc. at high-level before delving into protocol details?)
</t>
<t>
  TCP Header Format
</t>
<figure anchor="header_format">
 <artwork>                              
    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 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |       Destination Port        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Acknowledgment Number                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Data |           |U|A|P|R|S|F|                               |
   | Offset| Reserved  |R|C|S|S|Y|I|            Window             |
   |       |           |G|K|H|T|N|N|                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Checksum            |         Urgent Pointer        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Options                    |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            TCP Header Format

          Note that one tick mark represents one bit position.
</artwork>
</figure>
<t><list style="hanging" hangIndent="2">
  <t hangText="Source Port:  16 bits">
    <vspace />
    <vspace />
    The source port number.
  </t>
  <t hangText="Destination Port:  16 bits">
    <vspace />
    <vspace />
    The destination port number.
  </t>
  <t hangText="Sequence Number:  32 bits">
    <vspace />
    <vspace />
    The sequence number of the first data octet in this segment (except
    when SYN is present). If SYN is present the sequence number is the
    initial sequence number (ISN) and the first data octet is ISN+1.
  </t>
  <t hangText="Acknowledgment Number:  32 bits">
    <vspace />
    <vspace />
    If the ACK control bit is set this field contains the value of the
    next sequence number the sender of the segment is expecting to
    receive.  Once a connection is established this is always sent.
  </t>
  <t hangText="Data Offset:  4 bits">
    <vspace />
    <vspace />
    The number of 32 bit words in the TCP Header.  This indicates where
    the data begins.  The TCP header (even one including options) is an
    integral number of 32 bits long.
  </t>
  <t hangText="Reserved:  4 bits">
    <vspace />
    <vspace />
    Reserved for future use.  Must be zero.
  </t>
  <t hangText="Control Bits:  8 bits (from left to right):">
   <list>
    <t>
    CWR:  Congestion Window Reduced<vspace />
    ECE:  ECN-Echo<vspace />
    URG:  Urgent Pointer field significant<vspace />
    ACK:  Acknowledgment field significant<vspace />
    PSH:  Push Function<vspace />
    RST:  Reset the connection<vspace />
    SYN:  Synchronize sequence numbers<vspace />
    FIN:  No more data from sender</t>
   </list> 
  </t>
  <t hangText="Window:  16 bits">
    <vspace />
    <vspace />
    The number of data octets beginning with the one indicated in the
    acknowledgment field which the sender of this segment is willing to
    accept.
    <vspace />
    <vspace />
    The window size MUST be treated as an unsigned number, or else
    large window sizes will appear like negative windows and TCP will
    now work.  It is RECOMMENDED that implementations will reserve
    32-bit fields for the send and receive window sizes in the connection
    record and do all window computations with 32 bits.
  </t>
  <t hangText="Checksum:  16 bits">
    <vspace />
    <vspace />
    The checksum field is the 16 bit one's complement of the one's
    complement sum of all 16 bit words in the header and text.  If a
    segment contains an odd number of header and text octets to be
    checksummed, the last octet is padded on the right with zeros to
    form a 16 bit word for checksum purposes.  The pad is not
    transmitted as part of the segment.  While computing the checksum,
    the checksum field itself is replaced with zeros.
  </t>
  <t>
    The checksum also covers a 96 bit pseudo header conceptually
    prefixed to the TCP header.  This pseudo header contains the Source
    Address, the Destination Address, the Protocol, and TCP length.
    This gives the TCP protection against misrouted segments.  This
    information is carried in the Internet Protocol and is transferred
    across the TCP/Network interface in the arguments or results of
    calls by the TCP on the IP.  (TODO: this is IPv4-specific, need to mention IPv6 psuedoheader as well)
  </t>
  <t>
   <figure><artwork>
                +--------+--------+--------+--------+
                |           Source Address          |
                +--------+--------+--------+--------+
                |         Destination Address       |
                +--------+--------+--------+--------+
                |  zero  |  PTCL  |    TCP Length   |
                +--------+--------+--------+--------+
   </artwork></figure>
  </t>
  <t>
      The TCP Length is the TCP header length plus the data length in
      octets (this is not an explicitly transmitted quantity, but is
      computed), and it does not count the 12 octets of the pseudo
      header.
  </t>
  <t>
      The TCP checksum is never optional.  The sender MUST generate it
      and the receiver MUST check it.
  </t>
  <t hangText="Urgent Pointer:  16 bits">
    <vspace />
    <vspace />
    This field communicates the current value of the urgent pointer as a
    positive offset from the sequence number in this segment.  The
    urgent pointer points to the sequence number of the octet following the urgent data.  This field is only be interpreted in segments with
    the URG control bit set.  

  </t>
  <t hangText="Options:  variable">
    <vspace />
    <vspace />
    Options may occupy space at the end of the TCP header and are a
    multiple of 8 bits in length.  All options are included in the
    checksum.  An option may begin on any octet boundary.  There are two
    cases for the format of an option:
    <list>
      <t>Case 1:  A single octet of option-kind.</t>

      <t>Case 2:  An octet of option-kind, an octet of option-length, and
               the actual option-data octets.</t>
    </list>
  </t>
  <t>
    The option-length counts the two octets of option-kind and
    option-length as well as the option-data octets.
  </t>
  <t>
    Note that the list of options may be shorter than the data offset
    field might imply.  The content of the header beyond the
    End-of-Option option must be header padding (i.e., zero).
  </t>
  <t>
    Currently defined options include (kind indicated in octal):
    <figure><artwork>
      Kind     Length    Meaning
      ----     ------    -------
       0         -       End of option list.
       1         -       No-Operation.
       2         4       Maximum Segment Size.
    </artwork></figure></t>
  <t>
TODO - I think we may need to include designated experimental options and RFC 6994 reference here
  </t>
  <t>
    A TCP MUST be able to receive a TCP option in any segment.<vspace />
    A TCP MUST ignore without error any TCP option it does not
    implement, assuming that the option has a length field (all
    TCP options except End of option list and No-Operation have length fields).
    TCP MUST be prepared to handle an illegal option length
    (e.g., zero) without crashing; a suggested procedure is to
    reset the connection and log the reason.
  </t>
  <t hangText="Specific Option Definitions">
   <list>
      <t>End of Option List
        <figure><artwork>
        +--------+
        |00000000|
        +--------+
         Kind=0
        </artwork></figure></t>
        <t>
        This option code indicates the end of the option list.  This
        might not coincide with the end of the TCP header according to
        the Data Offset field.  This is used at the end of all options,
        not the end of each option, and need only be used if the end of
        the options would not otherwise coincide with the end of the TCP
        header.
        </t>
      <t>No-Operation
        <figure><artwork>
        +--------+
        |00000001|
        +--------+
         Kind=1
        </artwork></figure>    
        This option code may be used between options, for example, to
        align the beginning of a subsequent option on a word boundary.
        There is no guarantee that senders will use this option, so
        receivers must be prepared to process options even if they do
        not begin on a word boundary.
      </t>
      <t>Maximum Segment Size (MSS)
       <figure><artwork>
        +--------+--------+---------+--------+
        |00000010|00000100|   max seg size   |
        +--------+--------+---------+--------+
         Kind=2   Length=4
       </artwork></figure></t>
      <t>Maximum Segment Size Option Data:  16 bits
          <vspace />
          <vspace />
          If this option is present, then it communicates the maximum
          receive segment size at the TCP which sends this segment.
          This field may be sent in the initial connection request
          (i.e., in segments with the SYN control bit set) and must not
          be sent in other segments.  If this
          option is not used, any segment size is allowed.
          A more complete description of this option is in <xref target="mss"/>.
      </t>
    </list>
   </t>
   <t hangText="Padding:  variable">
    <vspace />
    <vspace />
    The TCP header padding is used to ensure that the TCP header ends
    and data begins on a 32 bit boundary.  The padding is composed of
    zeros.
   </t>
</list></t>
</section>
<section title="Terminology">
<t>
  Before we can discuss very much about the operation of the TCP we need
  to introduce some detailed terminology.  The maintenance of a TCP
  connection requires the remembering of several variables.  We conceive
  of these variables being stored in a connection record called a
  Transmission Control Block or TCB.  Among the variables stored in the
  TCB are the local and remote socket numbers, the security and
  precedence of the connection, pointers to the user's send and receive
  buffers, pointers to the retransmit queue and to the current segment.
  In addition several variables relating to the send and receive
  sequence numbers are stored in the TCB.
</t>
<t><figure><artwork>
    Send Sequence Variables

      SND.UNA - send unacknowledged
      SND.NXT - send next
      SND.WND - send window
      SND.UP  - send urgent pointer
      SND.WL1 - segment sequence number used for last window update
      SND.WL2 - segment acknowledgment number used for last window
                update
      ISS     - initial send sequence number

    Receive Sequence Variables

      RCV.NXT - receive next
      RCV.WND - receive window
      RCV.UP  - receive urgent pointer
      IRS     - initial receive sequence number
</artwork></figure></t>
<t>
  The following diagrams may help to relate some of these variables to
  the sequence space.
</t>
<figure anchor="send_seq_space">
<artwork>
  Send Sequence Space

                   1         2          3          4      
              ----------|----------|----------|---------- 
                     SND.UNA    SND.NXT    SND.UNA        
                                          +SND.WND        

        1 - old sequence numbers which have been acknowledged  
        2 - sequence numbers of unacknowledged data            
        3 - sequence numbers allowed for new data transmission 
        4 - future sequence numbers which are not yet allowed  

                          Send Sequence Space
 </artwork>   
</figure>
<t>
  The send window is the portion of the sequence space labeled 3 in
  <xref target="send_seq_space" />.
</t>
<figure anchor="recv_seq_space">
<artwork>
  Receive Sequence Space

                       1          2          3      
                   ----------|----------|---------- 
                          RCV.NXT    RCV.NXT        
                                    +RCV.WND        

        1 - old sequence numbers which have been acknowledged  
        2 - sequence numbers allowed for new reception         
        3 - future sequence numbers which are not yet allowed  

                         Receive Sequence Space
</artwork>
</figure> 
<t>
  The receive window is the portion of the sequence space labeled 2 in
  <xref target="recv_seq_space" />.
</t>
<t>
  There are also some variables used frequently in the discussion that
  take their values from the fields of the current segment.
</t>
<t>Current Segment Variables
    <figure><artwork>
    SEG.SEQ - segment sequence number
    SEG.ACK - segment acknowledgment number
    SEG.LEN - segment length
    SEG.WND - segment window
    SEG.UP  - segment urgent pointer
    SEG.PRC - segment precedence value
    </artwork></figure>
</t>
<t>
  A connection progresses through a series of states during its
  lifetime.  The states are:  LISTEN, SYN-SENT, SYN-RECEIVED,
  ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK,
  TIME-WAIT, and the fictional state CLOSED.  CLOSED is fictional
  because it represents the state when there is no TCB, and therefore,
  no connection.  Briefly the meanings of the states are:
</t>
<t><list>
    <t>LISTEN - represents waiting for a connection request from any remote
    TCP and port.</t>

    <t>SYN-SENT - represents waiting for a matching connection request
    after having sent a connection request.</t>

    <t>SYN-RECEIVED - represents waiting for a confirming connection
    request acknowledgment after having both received and sent a
    connection request.</t>

    <t>ESTABLISHED - represents an open connection, data received can be
    delivered to the user.  The normal state for the data transfer phase
    of the connection.</t>

    <t>FIN-WAIT-1 - represents waiting for a connection termination request
    from the remote TCP, or an acknowledgment of the connection
    termination request previously sent.</t>

    <t>FIN-WAIT-2 - represents waiting for a connection termination request
    from the remote TCP.</t>

    <t>CLOSE-WAIT - represents waiting for a connection termination request
    from the local user.</t>

    <t>CLOSING - represents waiting for a connection termination request
    acknowledgment from the remote TCP.</t>

    <t>LAST-ACK - represents waiting for an acknowledgment of the
    connection termination request previously sent to the remote TCP
    (this termination request sent to the remote TCP already included an acknowledgment of the termination request sent from the remote TCP).</t>

    <t>TIME-WAIT - represents waiting for enough time to pass to be sure
    the remote TCP received the acknowledgment of its connection
    termination request.</t>

    <t>CLOSED - represents no connection state at all.</t>
</list></t>
<t>
  A TCP connection progresses from one state to another in response to
  events.  The events are the user calls, OPEN, SEND, RECEIVE, CLOSE,
  ABORT, and STATUS; the incoming segments, particularly those
  containing the SYN, ACK, RST and FIN flags; and timeouts.
</t>
<t>
  The state diagram in <xref target="conn_states" /> illustrates only state changes, together
  with the causing events and resulting actions, but addresses neither
  error conditions nor actions which are not connected with state
  changes.  In a later section, more detail is offered with respect to
  the reaction of the TCP to events.
</t>
<t>
  NOTA BENE:  this diagram is only a summary and must not be taken as
  the total specification.
</t>
<figure anchor="conn_states">
<artwork>
                            +---------+ ---------\      active OPEN  
                            |  CLOSED |            \    -----------  
                            +---------+&lt;---------\   \   create TCB  
                              |     ^              \   \  snd SYN    
                 passive OPEN |     |   CLOSE        \   \           
                 ------------ |     | ----------       \   \         
                  create TCB  |     | delete TCB         \   \       
                              V     |                      \   \     
          rcv RST (note 1)  +---------+            CLOSE    |    \   
       -------------------->|  LISTEN |          ---------- |     |  
      /                     +---------+          delete TCB |     |  
     /           rcv SYN      |     |     SEND              |     |  
    /           -----------   |     |    -------            |     V  
+--------+      snd SYN,ACK  /       \   snd SYN          +--------+
|        |&lt;-----------------           ------------------>|        |
|  SYN   |                    rcv SYN                     |  SYN   |
|  RCVD  |&lt;-----------------------------------------------|  SENT  |
|        |                  snd SYN,ACK                   |        |
|        |------------------           -------------------|        |
+--------+   rcv ACK of SYN  \       /  rcv SYN,ACK       +--------+
   |           --------------   |     |   -----------                  
   |                  x         |     |     snd ACK                    
   |                            V     V                                
   |  CLOSE                   +---------+                              
   | -------                  |  ESTAB  |                              
   | snd FIN                  +---------+                              
   |                   CLOSE    |     |    rcv FIN                     
   V                  -------   |     |    -------                     
+---------+          snd FIN  /       \   snd ACK          +---------+
|  FIN    |&lt;-----------------           ------------------>|  CLOSE  |
| WAIT-1  |------------------                              |   WAIT  |
+---------+          rcv FIN  \                            +---------+
  | rcv ACK of FIN   -------   |                            CLOSE  |  
  | --------------   snd ACK   |                           ------- |  
  V        x                   V                           snd FIN V  
+---------+                  +---------+                   +---------+
|FINWAIT-2|                  | CLOSING |                   | LAST-ACK|
+---------+                  +---------+                   +---------+
  |                rcv ACK of FIN |                 rcv ACK of FIN |  
  |  rcv FIN       -------------- |    Timeout=2MSL -------------- |  
  |  -------              x       V    ------------        x       V  
   \ snd ACK                 +---------+delete TCB         +---------+
    ------------------------>|TIME WAIT|------------------>| CLOSED  |
                             +---------+                   +---------+

note 1: The transition from SYN-RCVD to LISTEN on receiving a RST is
conditional on having reached SYN-RCVD after a passive open.

note 2: An unshown transition exists from FIN-WAIT-1 to TIME-WAIT if
a FIN is received and the local FIN is also acknowledged.

                      TCP Connection State Diagram
</artwork>
</figure>
</section>
<section title="Sequence Numbers">
<t>
  A fundamental notion in the design is that every octet of data sent
  over a TCP connection has a sequence number.  Since every octet is
  sequenced, each of them can be acknowledged.  The acknowledgment
  mechanism employed is cumulative so that an acknowledgment of sequence
  number X indicates that all octets up to but not including X have been
  received.  This mechanism allows for straight-forward duplicate
  detection in the presence of retransmission.  Numbering of octets
  within a segment is that the first data octet immediately following
  the header is the lowest numbered, and the following octets are
  numbered consecutively.
</t>
<t>
  It is essential to remember that the actual sequence number space is
  finite, though very large.  This space ranges from 0 to 2**32 - 1.
  Since the space is finite, all arithmetic dealing with sequence
  numbers must be performed modulo 2**32.  This unsigned arithmetic
  preserves the relationship of sequence numbers as they cycle from
  2**32 - 1 to 0 again.  There are some subtleties to computer modulo
  arithmetic, so great care should be taken in programming the
  comparison of such values.  The symbol "=&lt;" means "less than or equal"
  (modulo 2**32).
</t>
<t>
  The typical kinds of sequence number comparisons which the TCP must
  perform include:
</t>
<t><list>
    <t>(a)  Determining that an acknowledgment refers to some sequence
         number sent but not yet acknowledged.</t>

    <t>(b)  Determining that all sequence numbers occupied by a segment
         have been acknowledged (e.g., to remove the segment from a
         retransmission queue).</t>

    <t>(c)  Determining that an incoming segment contains sequence numbers
         which are expected (i.e., that the segment "overlaps" the
         receive window).</t>
</list></t>
<t>
  In response to sending data the TCP will receive acknowledgments.  The
  following comparisons are needed to process the acknowledgments.
</t>
<t><list>
    <t>SND.UNA = oldest unacknowledged sequence number</t>

    <t>SND.NXT = next sequence number to be sent</t>

    <t>SEG.ACK = acknowledgment from the receiving TCP (next sequence
              number expected by the receiving TCP)</t>

    <t>SEG.SEQ = first sequence number of a segment</t>

    <t>SEG.LEN = the number of octets occupied by the data in the segment
              (counting SYN and FIN)</t>

    <t>SEG.SEQ+SEG.LEN-1 = last sequence number of a segment</t>
</list></t>
<t>
  A new acknowledgment (called an "acceptable ack"), is one for which
  the inequality below holds:
</t>
<t><list>
    <t>SND.UNA &lt; SEG.ACK =&lt; SND.NXT</t>
</list></t>
<t>
  A segment on the retransmission queue is fully acknowledged if the sum
  of its sequence number and length is less or equal than the
  acknowledgment value in the incoming segment.
</t>
<t>
  When data is received the following comparisons are needed:
</t>
<t><list>
    <t>RCV.NXT = next sequence number expected on an incoming segments, and
        is the left or lower edge of the receive window</t>

    <t>RCV.NXT+RCV.WND-1 = last sequence number expected on an incoming
        segment, and is the right or upper edge of the receive window</t>

    <t>SEG.SEQ = first sequence number occupied by the incoming segment</t>

    <t>SEG.SEQ+SEG.LEN-1 = last sequence number occupied by the incoming
        segment</t>
</list></t>
<t>
  A segment is judged to occupy a portion of valid receive sequence
  space if
</t>
<t><list>
    <t>RCV.NXT =&lt; SEG.SEQ &lt; RCV.NXT+RCV.WND</t>
</list></t>
<t>
  or
</t>
<t><list>
    <t>RCV.NXT =&lt; SEG.SEQ+SEG.LEN-1 &lt; RCV.NXT+RCV.WND</t>
</list></t>
<t>
  The first part of this test checks to see if the beginning of the
  segment falls in the window, the second part of the test checks to see
  if the end of the segment falls in the window; if the segment passes
  either part of the test it contains data in the window.
</t>
<t>
  Actually, it is a little more complicated than this.  Due to zero
  windows and zero length segments, we have four cases for the
  acceptability of an incoming segment:
</t>
<t><figure><artwork>
    Segment Receive  Test
    Length  Window
    ------- -------  -------------------------------------------

       0       0     SEG.SEQ = RCV.NXT

       0      >0     RCV.NXT =&lt; SEG.SEQ &lt; RCV.NXT+RCV.WND

      >0       0     not acceptable

      >0      >0     RCV.NXT =&lt; SEG.SEQ &lt; RCV.NXT+RCV.WND
                  or RCV.NXT =&lt; SEG.SEQ+SEG.LEN-1 &lt; RCV.NXT+RCV.WND
</artwork></figure></t>
<t>
  Note that when the receive window is zero no segments should be
  acceptable except ACK segments.  Thus, it is be possible for a TCP to
  maintain a zero receive window while transmitting data and receiving
  ACKs.  However, even when the receive window is zero, a TCP must
  process the RST and URG fields of all incoming segments.
</t>
<t>
  We have taken advantage of the numbering scheme to protect certain
  control information as well.  This is achieved by implicitly including
  some control flags in the sequence space so they can be retransmitted
  and acknowledged without confusion (i.e., one and only one copy of the
  control will be acted upon).  Control information is not physically
  carried in the segment data space.  Consequently, we must adopt rules
  for implicitly assigning sequence numbers to control.  The SYN and FIN
  are the only controls requiring this protection, and these controls
  are used only at connection opening and closing.  For sequence number
  purposes, the SYN is considered to occur before the first actual data
  octet of the segment in which it occurs, while the FIN is considered
  to occur after the last actual data octet in a segment in which it
  occurs.  The segment length (SEG.LEN) includes both data and sequence
  space occupying controls.  When a SYN is present then SEG.SEQ is the
  sequence number of the SYN.
</t>
<t>
  Initial Sequence Number Selection
</t>
<t>
  The protocol places no restriction on a particular connection being
  used over and over again.  A connection is defined by a pair of
  sockets.  New instances of a connection will be referred to as
  incarnations of the connection.  The problem that arises from this is
  -- "how does the TCP identify duplicate segments from previous
  incarnations of the connection?"  This problem becomes apparent if the
  connection is being opened and closed in quick succession, or if the
  connection breaks with loss of memory and is then reestablished.
</t>
<t>
  To avoid confusion we must prevent segments from one incarnation of a
  connection from being used while the same sequence numbers may still
  be present in the network from an earlier incarnation.  We want to
  assure this, even if a TCP crashes and loses all knowledge of the
  sequence numbers it has been using.  When new connections are created,
  an initial sequence number (ISN) generator is employed which selects a
  new 32 bit ISN.  There are security issues that result if an off-path
  attacker is able to predict or guess ISN values.
</t>
<t>
  The recommended ISN generator is based on the combination of a (possibly fictitious) 32 bit clock whose low order bit is incremented roughly every 4 microseconds, and a pseudorandom hash function (PRF).  The clock component is intended to insure that with a Maximum Segment Lifetime (MSL), generated ISNs will be unique, since it cycles approximately every 4.55 hours, which is much longer than the MSL.  This recommended algorithm is further described in RFC 1948 and builds on the basic clock-driven algorithm from RFC 793.
</t>
<t>
  A TCP MUST use a clock-driven selection of initial sequence numbers, and
  SHOULD generate its Initial Sequence Numbers with the expression:
</t>
<t>
      ISN = M + F(localip, localport, remoteip, remoteport, secretkey)
</t>
<t>
   where M is the 4 microsecond timer, and F() is a pseudorandom
   function (PRF) of the connection's identifying parameters ("localip, localport, remoteip, remoteport") and a secret key ("secretkey").  F() MUST NOT be computable from the outside, or an attacker could still guess at sequence numbers from the ISN used for some other connection.  The PRF could be implemented as a cryptographic has of the concatenation of the TCP connection parameters and some secret data.  For discussion of the selection of a specific hash algorithm and management of the secret key data, please see Section 3 of <xref target="RFC6528"/>.
</t>

<t>
  For each connection there is a send sequence number and a receive
  sequence number.  The initial send sequence number (ISS) is chosen by
  the data sending TCP, and the initial receive sequence number (IRS) is
  learned during the connection establishing procedure.
</t>
<t>
  For a connection to be established or initialized, the two TCPs must
  synchronize on each other's initial sequence numbers.  This is done in
  an exchange of connection establishing segments carrying a control bit
  called "SYN" (for synchronize) and the initial sequence numbers.  As a
  shorthand, segments carrying the SYN bit are also called "SYNs".
  Hence, the solution requires a suitable mechanism for picking an
  initial sequence number and a slightly involved handshake to exchange
  the ISN's.
</t>
<t>
  The synchronization requires each side to send it's own initial
  sequence number and to receive a confirmation of it in acknowledgment
  from the other side.  Each side must also receive the other side's
  initial sequence number and send a confirming acknowledgment.
</t>
<t><figure><artwork>
    1) A --> B  SYN my sequence number is X
    2) A &lt;-- B  ACK your sequence number is X
    3) A &lt;-- B  SYN my sequence number is Y
    4) A --> B  ACK your sequence number is Y
</artwork></figure></t>
<t>
  Because steps 2 and 3 can be combined in a single message this is
  called the three way (or three message) handshake.
</t>
<t>
  A three way handshake is necessary because sequence numbers are not
  tied to a global clock in the network, and TCPs may have different
  mechanisms for picking the ISN's.  The receiver of the first SYN has
  no way of knowing whether the segment was an old delayed one or not,
  unless it remembers the last sequence number used on the connection
  (which is not always possible), and so it must ask the sender to
  verify this SYN.  The three way handshake and the advantages of a
  clock-driven scheme are discussed in [3].
</t>
<t>
  Knowing When to Keep Quiet
</t>
<t>
  To be sure that a TCP does not create a segment that carries a
  sequence number which may be duplicated by an old segment remaining in
  the network, the TCP must keep quiet for an MSL before assigning any sequence numbers upon starting up or
  recovering from a crash in which memory of sequence numbers in use was
  lost.  For this specification the MSL is taken to be 2 minutes.  This
  is an engineering choice, and may be changed if experience indicates
  it is desirable to do so.  Note that if a TCP is reinitialized in some
  sense, yet retains its memory of sequence numbers in use, then it need
  not wait at all; it must only be sure to use sequence numbers larger
  than those recently used.
</t>
<t>
  The TCP Quiet Time Concept
</t>
<t>
    This specification provides that hosts which "crash" without
    retaining any knowledge of the last sequence numbers transmitted on
    each active (i.e., not closed) connection shall delay emitting any
    TCP segments for at least the agreed MSL
    in the internet system of which the host is a part.  In the
    paragraphs below, an explanation for this specification is given.
    TCP implementors may violate the "quiet time" restriction, but only
    at the risk of causing some old data to be accepted as new or new
    data rejected as old duplicated by some receivers in the internet
    system.
</t>
<t>
    TCPs consume sequence number space each time a segment is formed and
    entered into the network output queue at a source host. The
    duplicate detection and sequencing algorithm in the TCP protocol
    relies on the unique binding of segment data to sequence space to
    the extent that sequence numbers will not cycle through all 2**32
    values before the segment data bound to those sequence numbers has
    been delivered and acknowledged by the receiver and all duplicate
    copies of the segments have "drained" from the internet.  Without
    such an assumption, two distinct TCP segments could conceivably be
    assigned the same or overlapping sequence numbers, causing confusion
    at the receiver as to which data is new and which is old.  Remember
    that each segment is bound to as many consecutive sequence numbers
    as there are octets of data and SYN or FIN flags in the segment.
</t>
<t>
    Under normal conditions, TCPs keep track of the next sequence number
    to emit and the oldest awaiting acknowledgment so as to avoid
    mistakenly using a sequence number over before its first use has
    been acknowledged.  This alone does not guarantee that old duplicate
    data is drained from the net, so the sequence space has been made
    very large to reduce the probability that a wandering duplicate will
    cause trouble upon arrival.  At 2 megabits/sec. it takes 4.5 hours
    to use up 2**32 octets of sequence space.  Since the maximum segment
    lifetime in the net is not likely to exceed a few tens of seconds,
    this is deemed ample protection for foreseeable nets, even if data
    rates escalate to l0's of megabits/sec.  At 100 megabits/sec, the
    cycle time is 5.4 minutes which may be a little short, but still
    within reason.
</t>
<t>
    The basic duplicate detection and sequencing algorithm in TCP can be
    defeated, however, if a source TCP does not have any memory of the
    sequence numbers it last used on a given connection. For example, if
    the TCP were to start all connections with sequence number 0, then
    upon crashing and restarting, a TCP might re-form an earlier
    connection (possibly after half-open connection resolution) and emit
    packets with sequence numbers identical to or overlapping with
    packets still in the network which were emitted on an earlier
    incarnation of the same connection.  In the absence of knowledge
    about the sequence numbers used on a particular connection, the TCP
    specification recommends that the source delay for MSL seconds
    before emitting segments on the connection, to allow time for
    segments from the earlier connection incarnation to drain from the
    system.
</t>
<t>
    Even hosts which can remember the time of day and used it to select
    initial sequence number values are not immune from this problem
    (i.e., even if time of day is used to select an initial sequence
    number for each new connection incarnation).
</t>
<t>
    Suppose, for example, that a connection is opened starting with
    sequence number S.  Suppose that this connection is not used much
    and that eventually the initial sequence number function (ISN(t))
    takes on a value equal to the sequence number, say S1, of the last
    segment sent by this TCP on a particular connection.  Now suppose,
    at this instant, the host crashes, recovers, and establishes a new
    incarnation of the connection. The initial sequence number chosen is
    S1 = ISN(t) -- last used sequence number on old incarnation of
    connection!  If the recovery occurs quickly enough, any old
    duplicates in the net bearing sequence numbers in the neighborhood
    of S1 may arrive and be treated as new packets by the receiver of
    the new incarnation of the connection.
</t>
<t>
    The problem is that the recovering host may not know for how long it
    crashed nor does it know whether there are still old duplicates in
    the system from earlier connection incarnations.
</t>
<t>
    One way to deal with this problem is to deliberately delay emitting
    segments for one MSL after recovery from a crash- this is the &quot;quiet
    time&quot; specification.  Hosts which prefer to avoid waiting are
    willing to risk possible confusion of old and new packets at a given
    destination may choose not to wait for the &quot;quite time&quot;.
    Implementors may provide TCP users with the ability to select on a
    connection by connection basis whether to wait after a crash, or may
    informally implement the "quite time" for all connections.
    Obviously, even where a user selects to &quot;wait,&quot; this is not
    necessary after the host has been &quot;up&quot; for at least MSL seconds.
</t>
<t>
    To summarize: every segment emitted occupies one or more sequence
    numbers in the sequence space, the numbers occupied by a segment are
    &quot;busy&quot; or &quot;in use&quot; until MSL seconds have passed, upon crashing a
    block of space-time is occupied by the octets and SYN or FIN flags of the last emitted
    segment, if a new connection is started too soon and uses any of the
    sequence numbers in the space-time footprint of the last segment of
    the previous connection incarnation, there is a potential sequence
    number overlap area which could cause confusion at the receiver.
</t>
</section>
<section title="Establishing a connection">
<t>
  The &quot;three-way handshake&quot; is the procedure used to establish a
  connection.  This procedure normally is initiated by one TCP and
  responded to by another TCP.  The procedure also works if two TCP
  simultaneously initiate the procedure.  When simultaneous attempt
  occurs, each TCP receives a "SYN" segment which carries no
  acknowledgment after it has sent a &quot;SYN&quot;.  Of course, the arrival of
  an old duplicate &quot;SYN&quot; segment can potentially make it appear, to the
  recipient, that a simultaneous connection initiation is in progress.
  Proper use of &quot;reset&quot; segments can disambiguate these cases.
</t>
<t>
  Several examples of connection initiation follow.  Although these
  examples do not show connection synchronization using data-carrying
  segments, this is perfectly legitimate, so long as the receiving TCP
  doesn't deliver the data to the user until it is clear the data is
  valid (i.e., the data must be buffered at the receiver until the
  connection reaches the ESTABLISHED state).  The three-way handshake
  reduces the possibility of false connections.  It is the
  implementation of a trade-off between memory and messages to provide
  information for this checking.
</t>
<t>
  The simplest three-way handshake is shown in <xref target="handshake" /> below.  The
  figures should be interpreted in the following way.  Each line is
  numbered for reference purposes.  Right arrows (-->) indicate
  departure of a TCP segment from TCP A to TCP B, or arrival of a
  segment at B from A.  Left arrows (&lt;--), indicate the reverse.
  Ellipsis (...) indicates a segment which is still in the network
  (delayed).  An &quot;XXX&quot; indicates a segment which is lost or rejected.
  Comments appear in parentheses.  TCP states represent the state AFTER
  the departure or arrival of the segment (whose contents are shown in
  the center of each line).  Segment contents are shown in abbreviated
  form, with sequence number, control flags, and ACK field.  Other
  fields such as window, addresses, lengths, and text have been left out
  in the interest of clarity.
</t>
<figure anchor="handshake">
<artwork>
    TCP A                                                TCP B

1.  CLOSED                                               LISTEN

2.  SYN-SENT    --> &lt;SEQ=100>&lt;CTL=SYN>               --> SYN-RECEIVED

3.  ESTABLISHED &lt;-- &lt;SEQ=300>&lt;ACK=101>&lt;CTL=SYN,ACK>  &lt;-- SYN-RECEIVED

4.  ESTABLISHED --> &lt;SEQ=101>&lt;ACK=301>&lt;CTL=ACK>       --> ESTABLISHED

5.  ESTABLISHED --> &lt;SEQ=101>&lt;ACK=301>&lt;CTL=ACK>&lt;DATA> --> ESTABLISHED

        Basic 3-Way Handshake for Connection Synchronization
</artwork>
</figure>
<t>
  In line 2 of <xref target="handshake" />, TCP A begins by sending a SYN segment
  indicating that it will use sequence numbers starting with sequence
  number 100.  In line 3, TCP B sends a SYN and acknowledges the SYN it
  received from TCP A.  Note that the acknowledgment field indicates TCP
  B is now expecting to hear sequence 101, acknowledging the SYN which
  occupied sequence 100.
</t>
<t>
  At line 4, TCP A responds with an empty segment containing an ACK for
  TCP B's SYN; and in line 5, TCP A sends some data.  Note that the
  sequence number of the segment in line 5 is the same as in line 4
  because the ACK does not occupy sequence number space (if it did, we
  would wind up ACKing ACK's!).
</t>
<t>
  Simultaneous initiation is only slightly more complex, as is shown in
  <xref target="simul_connect" />.  Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to ESTABLISHED.
</t>
<figure anchor="simul_connect">
<artwork>
    TCP A                                            TCP B

1.  CLOSED                                           CLOSED

2.  SYN-SENT     --> &lt;SEQ=100>&lt;CTL=SYN>              ...

3.  SYN-RECEIVED &lt;-- &lt;SEQ=300>&lt;CTL=SYN>              &lt;-- SYN-SENT

4.               ... &lt;SEQ=100>&lt;CTL=SYN>              --> SYN-RECEIVED

5.  SYN-RECEIVED --> &lt;SEQ=100>&lt;ACK=301>&lt;CTL=SYN,ACK> ...

6.  ESTABLISHED  &lt;-- &lt;SEQ=300>&lt;ACK=101>&lt;CTL=SYN,ACK> &lt;-- SYN-RECEIVED

7.               ... &lt;SEQ=100>&lt;ACK=301>&lt;CTL=SYN,ACK>  --> ESTABLISHED

              Simultaneous Connection Synchronization
</artwork>
</figure>
<t>
A TCP MUST support simultaneous open attempts.
</t>
<t>
Note that a TCP implementation MUST keep track of whether a
connection has reached SYN_RCVD state as the result of a
passive OPEN or an active OPEN.
</t>
<t>
  The principle reason for the three-way handshake is to prevent old
  duplicate connection initiations from causing confusion.  To deal with
  this, a special control message, reset, has been devised.  If the
  receiving TCP is in a  non-synchronized state (i.e., SYN-SENT,
  SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset.
  If the TCP is in one of the synchronized states (ESTABLISHED,
  FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it
  aborts the connection and informs its user.  We discuss this latter
  case under "half-open" connections below.
</t>
<figure anchor="dup_syn">
<artwork>

    TCP A                                                TCP B

1.  CLOSED                                               LISTEN

2.  SYN-SENT    --> &lt;SEQ=100>&lt;CTL=SYN>               ...

3.  (duplicate) ... &lt;SEQ=90>&lt;CTL=SYN>               --> SYN-RECEIVED

4.  SYN-SENT    &lt;-- &lt;SEQ=300>&lt;ACK=91>&lt;CTL=SYN,ACK>  &lt;-- SYN-RECEIVED

5.  SYN-SENT    --> &lt;SEQ=91>&lt;CTL=RST>               --> LISTEN
  
6.              ... &lt;SEQ=100>&lt;CTL=SYN>               --> SYN-RECEIVED

7.  SYN-SENT    &lt;-- &lt;SEQ=400>&lt;ACK=101>&lt;CTL=SYN,ACK>  &lt;-- SYN-RECEIVED

8.  ESTABLISHED --> &lt;SEQ=101>&lt;ACK=401>&lt;CTL=ACK>      --> ESTABLISHED

                  Recovery from Old Duplicate SYN
</artwork>
</figure>
<t>
  As a simple example of recovery from old duplicates, consider
  <xref target="dup_syn" />.  At line 3, an old duplicate SYN arrives at TCP B.  TCP B
  cannot tell that this is an old duplicate, so it responds normally
  (line 4).  TCP A detects that the ACK field is incorrect and returns a
  RST (reset) with its SEQ field selected to make the segment
  believable.  TCP B, on receiving the RST, returns to the LISTEN state.
  When the original SYN (pun intended) finally arrives at line 6, the
  synchronization proceeds normally.  If the SYN at line 6 had arrived
  before the RST, a more complex exchange might have occurred with RST's
  sent in both directions.
</t>
<t>
  Half-Open Connections and Other Anomalies
</t>
<t>
  An established connection is said to be  "half-open" if one of the
  TCPs has closed or aborted the connection at its end without the
  knowledge of the other, or if the two ends of the connection have
  become desynchronized owing to a crash that resulted in loss of
  memory.  Such connections will automatically become reset if an
  attempt is made to send data in either direction.  However, half-open
  connections are expected to be unusual, and the recovery procedure is
  mildly involved.
</t>
<t>
  If at site A the connection no longer exists, then an attempt by the
  user at site B to send any data on it will result in the site B TCP
  receiving a reset control message.  Such a message indicates to the
  site B TCP that something is wrong, and it is expected to abort the
  connection.
</t>
<t>
  Assume that two user processes A and B are communicating with one
  another when a crash occurs causing loss of memory to A's TCP.
  Depending on the operating system supporting A's TCP, it is likely
  that some error recovery mechanism exists.  When the TCP is up again,
  A is likely to start again from the beginning or from a recovery
  point.  As a result, A will probably try to OPEN the connection again
  or try to SEND on the connection it believes open.  In the latter
  case, it receives the error message "connection not open" from the
  local (A's) TCP.  In an attempt to establish the connection, A's TCP
  will send a segment containing SYN.  This scenario leads to the
  example shown in <xref target="half_open" />.  After TCP A crashes, the user attempts to
  re-open the connection.  TCP B, in the meantime, thinks the connection
  is open.
</t>
<figure anchor="half_open">
<artwork>
      TCP A                                           TCP B

  1.  (CRASH)                               (send 300,receive 100)

  2.  CLOSED                                           ESTABLISHED

  3.  SYN-SENT --> &lt;SEQ=400>&lt;CTL=SYN>              --> (??)

  4.  (!!)     &lt;-- &lt;SEQ=300>&lt;ACK=100>&lt;CTL=ACK>     &lt;-- ESTABLISHED

  5.  SYN-SENT --> &lt;SEQ=100>&lt;CTL=RST>              --> (Abort!!)

  6.  SYN-SENT                                         CLOSED

  7.  SYN-SENT --> &lt;SEQ=400>&lt;CTL=SYN>              -->

                     Half-Open Connection Discovery
</artwork>
</figure>
<t>
  When the SYN arrives at line 3, TCP B, being in a synchronized state,
  and the incoming segment outside the window, responds with an
  acknowledgment indicating what sequence it next expects to hear (ACK
  100).  TCP A sees that this segment does not acknowledge anything it
  sent and, being unsynchronized, sends a reset (RST) because it has
  detected a half-open connection.  TCP B aborts at line 5.  TCP A will
  continue to try to establish the connection; the problem is now
  reduced to the basic 3-way handshake of <xref target="handshake" />.
</t>
<t>
  An interesting alternative case occurs when TCP A crashes and TCP B
  tries to send data on what it thinks is a synchronized connection.
  This is illustrated in <xref target="crash" />.  In this case, the data arriving at
  TCP A from TCP B (line 2) is unacceptable because no such connection
  exists, so TCP A sends a RST.  The RST is acceptable so TCP B
  processes it and aborts the connection.
</t>
  <figure anchor="crash">
  <artwork>

      TCP A                                              TCP B

1.  (CRASH)                                   (send 300,receive 100)

2.  (??)    &lt;-- &lt;SEQ=300>&lt;ACK=100>&lt;DATA=10>&lt;CTL=ACK> &lt;-- ESTABLISHED

3.          --> &lt;SEQ=100>&lt;CTL=RST>                   --> (ABORT!!)

         Active Side Causes Half-Open Connection Discovery
</artwork>
</figure>
<t>
  In <xref target="passive_reset" />, we find the two TCPs A and B with passive connections
  waiting for SYN.  An old duplicate arriving at TCP B (line 2) stirs B
  into action.  A SYN-ACK is returned (line 3) and causes TCP A to
  generate a RST (the ACK in line 3 is not acceptable).  TCP B accepts
  the reset and returns to its passive LISTEN state.
</t>
<figure anchor="passive_reset">
<artwork>

    TCP A                                         TCP B

1.  LISTEN                                        LISTEN

2.       ... &lt;SEQ=Z>&lt;CTL=SYN>                -->  SYN-RECEIVED

3.  (??) &lt;-- &lt;SEQ=X>&lt;ACK=Z+1>&lt;CTL=SYN,ACK>   &lt;--  SYN-RECEIVED

4.       --> &lt;SEQ=Z+1>&lt;CTL=RST>              -->  (return to LISTEN!)

5.  LISTEN                                        LISTEN

     Old Duplicate SYN Initiates a Reset on two Passive Sockets
</artwork>
</figure>
<t>
  A variety of other cases are possible, all of which are accounted for
  by the following rules for RST generation and processing.
</t>
<t>
  Reset Generation
</t>
<t>
  As a general rule, reset (RST) must be sent whenever a segment arrives
  which apparently is not intended for the current connection.  A reset
  must not be sent if it is not clear that this is the case.
</t>
<t>
  There are three groups of states:
</t>
<t><list>
<t>
    1.  If the connection does not exist (CLOSED) then a reset is sent
    in response to any incoming segment except another reset.  In
    particular, SYNs addressed to a non-existent connection are rejected
    by this means.
</t>
<t>
    If the incoming segment has the ACK bit set, the reset takes its
    sequence number from the ACK field of the segment, otherwise the
    reset has sequence number zero and the ACK field is set to the sum
    of the sequence number and segment length of the incoming segment.
    The connection remains in the CLOSED state.
</t>
<t>
    2.  If the connection is in any non-synchronized state (LISTEN,
    SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges
    something not yet sent (the segment carries an unacceptable ACK), or
    if an incoming segment has a security level or compartment which
    does not exactly match the level and compartment requested for the
    connection, a reset is sent.
</t>
<t>
    If our SYN has not been acknowledged and the precedence level of the
    incoming segment is higher than the precedence level requested then
    either raise the local precedence level (if allowed by the user and
    the system) or send a reset; or if the precedence level of the
    incoming segment is lower than the precedence level requested then
    continue as if the precedence matched exactly (if the remote TCP
    cannot raise the precedence level to match ours this will be
    detected in the next segment it sends, and the connection will be
    terminated then).  If our SYN has been acknowledged (perhaps in this
    incoming segment) the precedence level of the incoming segment must
    match the local precedence level exactly, if it does not a reset
    must be sent.
</t>
<t>
    If the incoming segment has an ACK field, the reset takes its
    sequence number from the ACK field of the segment, otherwise the
    reset has sequence number zero and the ACK field is set to the sum
    of the sequence number and segment length of the incoming segment.
    The connection remains in the same state.
</t>
<t>
    3.  If the connection is in a synchronized state (ESTABLISHED,
    FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT),
    any unacceptable segment (out of window sequence number or
    unacceptable acknowledgment number) must elicit only an empty
    acknowledgment segment containing the current send-sequence number
    and an acknowledgment indicating the next sequence number expected
    to be received, and the connection remains in the same state.
</t>
<t>
    If an incoming segment has a security level, or compartment, or
    precedence which does not exactly match the level, and compartment,
    and precedence requested for the connection,a reset is sent and
    the connection goes to the CLOSED state.  The reset takes its sequence
    number from the ACK field of the incoming segment.
</t>
</list></t>
<t>
  Reset Processing
</t>
<t>
  In all states except SYN-SENT, all reset (RST) segments are validated
  by checking their SEQ-fields.  A reset is valid if its sequence number
  is in the window.  In the SYN-SENT state (a RST received in response
  to an initial SYN), the RST is acceptable if the ACK field
  acknowledges the SYN.
</t>
<t>
  The receiver of a RST first validates it, then changes state.  If the
  receiver was in the LISTEN state, it ignores it.  If the receiver was
  in SYN-RECEIVED state and had previously been in the LISTEN state,
  then the receiver returns to the LISTEN state, otherwise the receiver
  aborts the connection and goes to the CLOSED state.  If the receiver
  was in any other state, it aborts the connection and advises the user
  and goes to the CLOSED state.
</t>
<t>
  TCP SHOULD allow a received RST segment to include data.
</t>
<section title="Remote Address Validation">
<t>TODO - figure out if this section would fit better elsewhere, for instance in the more detailed description of the OPEN call later on</t>
<t>
            A TCP implementation MUST reject as an error a local OPEN
            call for an invalid remote IP address (e.g., a broadcast or
            multicast address).
</t>
<t>
            An incoming SYN with an invalid source address must be
            ignored either by TCP or by the IP layer (see Section
            3.2.1.3 of <xref target="RFC1122"/>).
</t>
<t>
            A TCP implementation MUST silently discard an incoming SYN
            segment that is addressed to a broadcast or multicast
            address.
</t>
</section>
</section>
<section title="Closing a Connection">
<t>
  CLOSE is an operation meaning &quot;I have no more data to send.&quot;  The
  notion of closing a full-duplex connection is subject to ambiguous
  interpretation, of course, since it may not be obvious how to treat
  the receiving side of the connection.  We have chosen to treat CLOSE
  in a simplex fashion.  The user who CLOSEs may continue to RECEIVE
  until he is told that the other side has CLOSED also.  Thus, a program
  could initiate several SENDs followed by a CLOSE, and then continue to
  RECEIVE until signaled that a RECEIVE failed because the other side
  has CLOSED.  We assume that the TCP will signal a user, even if no
  RECEIVEs are outstanding, that the other side has closed, so the user
  can terminate his side gracefully.  A TCP will reliably deliver all
  buffers SENT before the connection was CLOSED so a user who expects no
  data in return need only wait to hear the connection was CLOSED
  successfully to know that all his data was received at the destination
  TCP.  Users must keep reading connections they close for sending until
  the TCP says no more data.
</t>
<t>
  There are essentially three cases:
</t>
<t><list>
<t>
    1) The user initiates by telling the TCP to CLOSE the connection
</t>
<t>
    2) The remote TCP initiates by sending a FIN control signal
</t>
<t>
    3) Both users CLOSE simultaneously
</t>
</list></t>
<t><list style="hanging">
<t hangText="Case 1:  Local user initiates the close"><vspace />
    <vspace />
    In this case, a FIN segment can be constructed and placed on the
    outgoing segment queue.  No further SENDs from the user will be
    accepted by the TCP, and it enters the FIN-WAIT-1 state.  RECEIVEs
    are allowed in this state.  All segments preceding and including FIN
    will be retransmitted until acknowledged.  When the other TCP has
    both acknowledged the FIN and sent a FIN of its own, the first TCP
    can ACK this FIN.  Note that a TCP receiving a FIN will ACK but not
    send its own FIN until its user has CLOSED the connection also.
</t>
<t hangText="Case 2:  TCP receives a FIN from the network"><vspace />
    <vspace />
    If an unsolicited FIN arrives from the network, the receiving TCP
    can ACK it and tell the user that the connection is closing.  The
    user will respond with a CLOSE, upon which the TCP can send a FIN to
    the other TCP after sending any remaining data.  The TCP then waits
    until its own FIN is acknowledged whereupon it deletes the
    connection.  If an ACK is not forthcoming, after the user timeout
    the connection is aborted and the user is told.
</t>
<t hangText="Case 3:  both users close simultaneously"><vspace />
    <vspace />
    A simultaneous CLOSE by users at both ends of a connection causes
    FIN segments to be exchanged.  When all segments preceding the FINs
    have been processed and acknowledged, each TCP can ACK the FIN it
    has received.  Both will, upon receiving these ACKs, delete the
    connection.
</t>
</list></t>
<figure anchor="normal_close">
<artwork>
    TCP A                                                TCP B

1.  ESTABLISHED                                          ESTABLISHED

2.  (Close)
    FIN-WAIT-1  --> &lt;SEQ=100>&lt;ACK=300>&lt;CTL=FIN,ACK>  --> CLOSE-WAIT

3.  FIN-WAIT-2  &lt;-- &lt;SEQ=300>&lt;ACK=101>&lt;CTL=ACK>      &lt;-- CLOSE-WAIT

4.                                                       (Close)
    TIME-WAIT   &lt;-- &lt;SEQ=300>&lt;ACK=101>&lt;CTL=FIN,ACK>  &lt;-- LAST-ACK

5.  TIME-WAIT   --> &lt;SEQ=101>&lt;ACK=301>&lt;CTL=ACK>      --> CLOSED

6.  (2 MSL)
    CLOSED                                                      

                       Normal Close Sequence
</artwork>
</figure>
<figure anchor="simul_close">
<artwork>

    TCP A                                                TCP B

1.  ESTABLISHED                                          ESTABLISHED

2.  (Close)                                              (Close)
    FIN-WAIT-1  --> &lt;SEQ=100>&lt;ACK=300>&lt;CTL=FIN,ACK>  ... FIN-WAIT-1
                &lt;-- &lt;SEQ=300>&lt;ACK=100>&lt;CTL=FIN,ACK>  &lt;--
                ... &lt;SEQ=100>&lt;ACK=300>&lt;CTL=FIN,ACK>  -->

3.  CLOSING     --> &lt;SEQ=101>&lt;ACK=301>&lt;CTL=ACK>      ... CLOSING
                &lt;-- &lt;SEQ=301>&lt;ACK=101>&lt;CTL=ACK>      &lt;--
                ... &lt;SEQ=101>&lt;ACK=301>&lt;CTL=ACK>      -->

4.  TIME-WAIT                                            TIME-WAIT
    (2 MSL)                                              (2 MSL)
    CLOSED                                               CLOSED

                    Simultaneous Close Sequence
</artwork>
</figure>
<t>
            A TCP connection may terminate in two ways: (1) the normal
            TCP close sequence using a FIN handshake, and (2) an "abort"
            in which one or more RST segments are sent and the
            connection state is immediately discarded.  If a TCP
            connection is closed by the remote site, the local
            application MUST be informed whether it closed normally or
            was aborted.
</t>
<section title="Half-Closed Connections">
<t>
            The normal TCP close sequence delivers buffered data
            reliably in both directions.  Since the two directions of a
            TCP connection are closed independently, it is possible for
            a connection to be "half closed," i.e., closed in only one
            direction, and a host is permitted to continue sending data
            in the open direction on a half-closed connection.
</t>
<t>
            A host MAY implement a "half-duplex" TCP close sequence, so
            that an application that has called CLOSE cannot continue to
            read data from the connection.  If such a host issues a
            CLOSE call while received data is still pending in TCP, or
            if new data is received after CLOSE is called, its TCP
            SHOULD send a RST to show that data was lost.
</t>
<t>
            When a connection is closed actively, it MUST linger in
            TIME-WAIT state for a time 2xMSL (Maximum Segment Lifetime).
            However, it MAY accept a new SYN from the remote TCP to
            reopen the connection directly from TIME-WAIT state, if it:
<list>
<t>
            (1)  assigns its initial sequence number for the new
                 connection to be larger than the largest sequence
                 number it used on the previous connection incarnation,
                 and
</t>
<t>
            (2)  returns to TIME-WAIT state if the SYN turns out to be
                 an old duplicate.
</t>
</list>
</t>
</section>
</section>
<section title="Precedence and Security">

<t>
  The intent is that connection be allowed only between ports operating
  with exactly the same security and compartment values and at the
  higher of the precedence level requested by the two ports.
</t>
<t>
  The precedence and security parameters used in TCP are exactly those
  defined in the Internet Protocol (IP) <xref target="RFC0791"/>.  Throughout this TCP
  specification the term "security/compartment" is intended to indicate
  the security parameters used in IP including security, compartment,
  user group, and handling restriction.
</t>
<t>
  A connection attempt with mismatched security/compartment values or a
  lower precedence value must be rejected by sending a reset.  Rejecting
  a connection due to too low a precedence only occurs after an
  acknowledgment of the SYN has been received.
</t>
<t>
  Note that TCP modules which operate only at the default value of
  precedence will still have to check the precedence of incoming
  segments and possibly raise the precedence level they use on the
  connection.
</t>
<t>
  The security parameters may be used even in a non-secure environment
  (the values would indicate unclassified data), thus hosts in
  non-secure environments must be prepared to receive the security
  parameters, though they need not send them.
</t>
</section>
<section title="Segmentation">
    <t> The term &quot;segmentation&quot; refers to the activity TCP performs when ingesting a stream of bytes from a sending application and packetizing that stream of bytes into TCP segments.  Individual TCP segments often do not correspond one-for-one to individual send (or socket write) calls from the application.  Applications may perform writes at the granularity of messages in the upper layer protocol, but TCP guarantees no boundary coherence between the TCP segments sent and received versus user application data read or write buffer boundaries.  In some specific protocols, such as RDMA using DDP and MPA <xref target="RFC5044"/>, there are performance optimizations possible when the relation between TCP segments and application data units can be controlled, and MPA includes a specific mechanism for detecting and verifying this relationship between TCP segments and application message data strcutures, but this is specific to applications like RDMA.  In general, multiple goals influence the sizing of TCP segments created by a TCP implementation.</t>
    
    <t>Goals driving the sending of larger segments include:
    <list style="symbols">
      <t>Reducing the number of packets in flight within the network.</t>
      <t>Increasing processing efficiency and potential performance by enabling a smaller number of interrupts and inter-layer interactions.</t>
      <t>Limiting the overhead of TCP headers.</t>
    </list>
    </t>
    <t>Note that the performance benefits of sending larger segments may decrease as the size increases, and there may be boundaries where advantages are reversed.  For instance, on some machines 1025 bytes within a segment could lead to worse performance than 1024 bytes, due purely to data alignment on copy operations.</t>
    <t>Goals driving the sending of smaller segments include:
    <list style="symbols">
    <t>Avoiding sending segments larger than the smallest MTU within an IP network path, because this  results in either packet loss or fragmentation.  Making matters worse, some firewalls or middleboxes may drop fragmented packets or ICMP messages related related to fragmentation.</t>
    <t>Preventing delays to the application data stream, especially when TCP is waiting on the application to generate more data, or when the application is waiting on an event or input from its peer in order to generate more data.</t>
    <t>Enabling &quot;fate sharing&quot; between TCP segments and lower-layer data units (e.g. below IP, for links with cell or frame sizes smaller than the IP MTU).</t>
    </list>
    </t>

    <t>Towards meeting these competing sets of goals, TCP includes several mechanisms, including the Maximum Segment Size option, Path MTU Discovery, the Nagle algorithm, and support for IPv6 Jumbograms, as discussed in the following subsections.</t>

    <section title="Maximum Segment Size Option" anchor="mss">
    <t>
    TCP MUST implement both sending and receiving the MSS option.
    </t>
    <t>
    TCP SHOULD send an MSS option in
    every SYN segment when its receive MSS differs from the
    default 536 for IPv4 or 1220 for IPv6, and MAY send it always.
    </t>
    <t>
    If an MSS option is not received at connection setup, TCP
    MUST assume a default send MSS of 536 (576-40) for IPv4 or 1220 (1280 - 60) for IPv6.
    </t>
    <t>
    The maximum size of a segment that TCP really sends, the
    &quot;effective send MSS,&quot; MUST be the smaller of the send MSS
    (which reflects the available reassembly buffer size at the
    remote host) and the largest size permitted by the IP layer:
    <list style="hanging" hangIndent="4">
        <t>Eff.snd.MSS =
            <list style="hanging" hangIndent="4">
            <t>min(SendMSS+20, MMS_S) - TCPhdrsize - IPoptionsize</t>
            </list>
        </t>
    </list>
    where:
    <list style="symbols">
        <t>
        SendMSS is the MSS value received from the remote host,
        or the default 536 for IPv4 or 1220 for IPv6, if no MSS option is received.
        </t>
        <t>
        MMS_S is the maximum size for a transport-layer message
        that TCP may send.
        </t>
        <t>
        TCPhdrsize is the size of the fixed TCP header and any options.  This is 20 in the (rare) case that no options are present, but may be larger if TCP options are to be sent.  Note that some options may not be included on all segments, but that for each segment sent, the sender should adjust the data length accordingly, within the Eff.snd.MSS.
        </t>
        <t>
        IPoptionsize is the size of any IP options associated with a TCP connection.  Note that some options may not be included on all packets, but that for each segment sent, the sender should adjust the data length accordingly, within the Eff.snd.MSS.
        </t>
    </list>
    </t>
    <t>
   The MSS value to be sent in an MSS option should be equal to the
   effective MTU minus the fixed IP and TCP headers.  By ignoring both
   IP and TCP options when calculating the value for the MSS option, if
   there are any IP or TCP options to be sent in a packet, then the
   sender must decrease the size of the TCP data accordingly.  RFC 6691 <xref target="RFC6691"/>
   discusses this in greater detail.
   </t>
   <t>
    The MSS value to be sent in an MSS option must be less than
    or equal to:
    <list>
        <t>MMS_R - 20</t>
    </list>
    where MMS_R is the maximum size for a transport-layer
    message that can be received (and reassembled).  TCP obtains
    MMS_R and MMS_S from the IP layer; see the generic call
    GET_MAXSIZES in Section 3.4 of RFC 1122.
   </t>
    <t>
    When TCP is used in a situation where either the IP or TCP headers
    are not fixed, the sender must reduce the amount of TCP data in
    any given packet by the number of octets used by the IP and TCP
    options.  This has been a point of confusion historically, as explained in RFC 6691, Section 3.1.
    </t>
   </section>
   <section title="Path MTU Discovery">

   <t>A TCP implementation may be aware of the MTU on directly connected links, but will rarely have insight about MTUs across an entire network path.  For IPv4, RFC 1122 provides an IP-layer recommendation on the default effective MTU for sending to be less than or equal to 576 for destinations not directly connected.  For IPv6, this would be 1280.  In all cases, however, implementation of Path MTU Discovery (PMTUD) and Packetization Layer Path MTU Discovery (PLPMTUD) is strongly recommended in order for TCP to improve segmentation decisions.</t>

   <t>PMTUD for IPv4 <xref target="RFC1191"/> or IPv6 <xref target="RFC1981"/> is implemented in conjunction between TCP, IP, and ICMP protocols.  Several adjustments to a TCP implementation with PMTUD are described in RFC 2923 in order to deal with problems experienced in practice <xref target="RFC2923"/>.  PLPMTUD <xref target="RFC4821"/> is a Standards Track improvement to PMTUD that relaxes the requirement for ICMP support across a path, and improves performance in cases where ICMP is not consistently conveyed.  The mechanisms in all four of these RFCs are recommended to be included in TCP implementations.</t>

   <t>
   The TCP MSS option specifies an upper bound for the size of packets
   that can be received.  Hence, setting the value in the MSS option too
   small can impact the ability for PMTUD or PLPMTUD to find a larger
   path MTU.  RFC 1191 discusses this implication of many older TCP implementations setting MSS to 536 for non-local destinations, rather than deriving it from the MTUs of connected interfaces as recommended.
   </t>

   </section>
   <section title="Interfaces with Variable MTU Values">
   <t>
   The effective MTU can sometimes vary, as when used with variable
   compression, e.g., RObust Header Compression (ROHC) <xref target="RFC5795"/>.  It is
   tempting for TCP to want to advertise the largest possible MSS, to
   support the most efficient use of compressed payloads.
   Unfortunately, some compression schemes occasionally need to transmit
   full headers (and thus smaller payloads) to resynchronize state at
   their endpoint compressors/decompressors.  If the largest MTU is used
   to calculate the value to advertise in the MSS option, TCP
   retransmission may interfere with compressor resynchronization.
   </t>
   <t>
   As a result, when the effective MTU of an interface varies, TCP
   SHOULD use the smallest effective MTU of the interface to calculate
   the value to advertise in the MSS option.
   </t>
   </section>
   <section title="Nagle Algorithm">
    <t>The &quot;Nagle algorithm&quot; was described in RFC 896 <xref target="RFC0896"/> and was recommended in RFC 1122 <xref target="RFC1122"/> for mitigation of an early problem of too many small packets being generated.  It has been implemented in most current TCP code bases, sometimes with minor variations.</t>
    <t>If there is unacknowledged data (i.e., SND.NXT &gt; SND.UNA), then the sending TCP buffers all user data (regardless of the PSH bit), until the   outstanding data has been acknowledged or until the TCP can send a full-sized segment (Eff.snd.MSS bytes).</t>
    <t>TODO - see if SEND description later should be updated to reflect this</t>
    <t>A TCP SHOULD implement the Nagle Algorithm to coalesce short segments.  However, there MUST be a way for an application to disable the Nagle algorithm on an individual connection.  In all cases, sending data is also subject to the limitation imposed by the Slow Start algorithm <xref target="RFC5681"/>.</t>
   </section>
   <section title="IPv6 Jumbograms">
   <t>
   In order to support TCP over IPv6 jumbograms, implementations need to
   be able to send TCP segments larger than the 64KB limit that the MSS option can convey.  RFC 2675 <xref target="RFC2675"/>
   defines that an MSS value of 65,535 bytes is to be treated as infinity, and Path
   MTU Discovery <xref target="RFC1981"/> is used to determine the actual MSS.
   </t>
   </section>
</section>
<section title="Data Communication">
<t>
  Once the connection is established data is communicated by the
  exchange of segments.  Because segments may be lost due to errors
  (checksum test failure), or network congestion, TCP uses
  retransmission (after a timeout) to ensure delivery of every segment.
  Duplicate segments may arrive due to network or TCP retransmission.
  As discussed in the section on sequence numbers the TCP performs
  certain tests on the sequence and acknowledgment numbers in the
  segments to verify their acceptability.
</t>
<t>
  The sender of data keeps track of the next sequence number to use in
  the variable SND.NXT.  The receiver of data keeps track of the next
  sequence number to expect in the variable RCV.NXT.  The sender of data
  keeps track of the oldest unacknowledged sequence number in the
  variable SND.UNA.  If the data flow is momentarily idle and all data
  sent has been acknowledged then the three variables will be equal.
</t>
<t>
  When the sender creates a segment and transmits it the sender advances
  SND.NXT.  When the receiver accepts a segment it advances RCV.NXT and
  sends an acknowledgment.  When the data sender receives an
  acknowledgment it advances SND.UNA.  The extent to which the values of
  these variables differ is a measure of the delay in the communication.
  The amount by which the variables are advanced is the length of the
  data and SYN or FIN flags in the segment.  Note that once in the ESTABLISHED state all
  segments must carry current acknowledgment information.
</t>
<t>
  The CLOSE user call implies a push function, as does the FIN control
  flag in an incoming segment.
</t>
<section title="Retransmission Timeout">
<t>
  Because of the variability of the networks that compose an
  internetwork system and the wide range of uses of TCP connections the
  retransmission timeout (RTO) must be dynamically determined.
</t>
<t>
      The RTO MUST be computed according to the
      algorithm in <xref target="RFC6298"/>, including Karn's algorithm for taking RTT samples.
</t>
<t>
      RFC 793 contains an early example procedure for computing the RTO.  This was then replaced by the algorithm described in RFC 1122, and subsequently updated in RFC 2988, and then again in RFC 6298.
</t>
<t>
If a retransmitted packet is identical to the original
packet (which implies not only that the data boundaries have
not changed, but also that the window and acknowledgment
fields of the header have not changed), then the same IP
Identification field MAY be used (see Section 3.2.1.5 of RFC 1122).
</t>
</section>
<section title="TCP Connection Failures">
<t>
            Excessive retransmission of the same segment by TCP
            indicates some failure of the remote host or the Internet
            path.  This failure may be of short or long duration.  The
            following procedure MUST be used to handle excessive
            retransmissions of data segments:
</t>
<t>
<list>
<t>
            (a)  There are two thresholds R1 and R2 measuring the amount
                 of retransmission that has occurred for the same
                 segment.  R1 and R2 might be measured in time units or
                 as a count of retransmissions.
</t>
<t>
            (b)  When the number of transmissions of the same segment
                 reaches or exceeds threshold R1, pass negative advice
                 (see <xref target="RFC1122"/> Section 3.3.1.4) to the IP layer, to trigger
                 dead-gateway diagnosis.
</t>
<t>
            (c)  When the number of transmissions of the same segment
                 reaches a threshold R2 greater than R1, close the
                 connection.
</t>
<t>
            (d)  An application MUST be able to set the value for R2 for
                 a particular connection.  For example, an interactive
                 application might set R2 to "infinity," giving the user
                 control over when to disconnect.
</t>
<t>
            (d)  TCP SHOULD inform the application of the delivery
                 problem (unless such information has been disabled by
                 the application; see RFC1122 Section 4.2.4.1 - TODO update to error reporting description in this document), when R1 is
                 reached and before R2.  This will allow a remote login
                 (User Telnet) application program to inform the user,
                 for example.
</t>
</list></t>
<t>
            The value of R1 SHOULD correspond to at least 3
            retransmissions, at the current RTO.  The value of R2 SHOULD
            correspond to at least 100 seconds.
</t>
<t>
            An attempt to open a TCP connection could fail with
            excessive retransmissions of the SYN segment or by receipt
            of a RST segment or an ICMP Port Unreachable.  SYN
            retransmissions MUST be handled in the general way just
            described for data retransmissions, including notification
            of the application layer.
</t>
<t>
            However, the values of R1 and R2 may be different for SYN
            and data segments.  In particular, R2 for a SYN segment MUST
            be set large enough to provide retransmission of the segment
            for at least 3 minutes.  The application can close the
            connection (i.e., give up on the open attempt) sooner, of
            course.
</t>
</section>
<section title="TCP Keep-Alives">
<t>
            Implementors MAY include "keep-alives" in their TCP
            implementations, although this practice is not universally
            accepted.  If keep-alives are included, the application MUST
            be able to turn them on or off for each TCP connection, and
            they MUST default to off.
</t>
<t>
            Keep-alive packets MUST only be sent when no data or
            acknowledgement packets have been received for the
            connection within an interval.  This interval MUST be
            configurable and MUST default to no less than two hours.
</t>
<t>
            It is extremely important to remember that ACK segments that
            contain no data are not reliably transmitted by TCP.
            Consequently, if a keep-alive mechanism is implemented it
            MUST NOT interpret failure to respond to any specific probe
            as a dead connection.
</t>
<t>
            An implementation SHOULD send a keep-alive segment with no
            data; however, it MAY be configurable to send a keep-alive
            segment containing one garbage octet, for compatibility with
            erroneous TCP implementations.
</t>
</section>
<section title="The Communication of Urgent Information">
<t>
  As a result of implementation differences and middlebox interactions, new applications SHOULD NOT employ the TCP urgent mechanism.  However, TCP implementations MUST still include support for the urgent mechanism.  Details can be found in RFC 6093 <xref target="RFC6093"/>.
</t>
<t>
  The objective of the TCP urgent mechanism is to allow the sending user
  to stimulate the receiving user to accept some urgent data and to
  permit the receiving TCP to indicate to the receiving user when all
  the currently known urgent data has been received by the user.
</t>
<t>
  This mechanism permits a point in the data stream to be designated as
  the end of urgent information.  Whenever this point is in advance of
  the receive sequence number (RCV.NXT) at the receiving TCP, that TCP
  must tell the user to go into &quot;urgent mode&quot;; when the receive sequence
  number catches up to the urgent pointer, the TCP must tell user to go
  into &quot;normal mode&quot;.  If the urgent pointer is updated while the user
  is in &quot;urgent mode&quot;, the update will be invisible to the user.
</t>
<t>
  The method employs a urgent field which is carried in all segments
  transmitted.  The URG control flag indicates that the urgent field is
  meaningful and must be added to the segment sequence number to yield
  the urgent pointer.  The absence of this flag indicates that there is
  no urgent data outstanding.
</t>
<t>
  To send an urgent indication the user must also send at least one data
  octet.  If the sending user also indicates a push, timely delivery of
  the urgent information to the destination process is enhanced.
</t>
<t>
  A TCP MUST support a sequence of urgent data of any length. <xref target="RFC1122"/>
</t>
<t>
  A TCP MUST inform the application layer asynchronously whenever it receives an Urgent pointer and there was previously no pending urgent data, or whenvever the Urgent pointer advances in the data stream.  There MUST be a way for the application to learn how much urgent data remains to be read from the connection, or at least to determine whether or not more urgent data remains to be read. <xref target="RFC1122"/>
</t>
</section>
<section title="Managing the Window">
<t>
  The window sent in each segment indicates the range of sequence
  numbers the sender of the window (the data receiver) is currently
  prepared to accept.  There is an assumption that this is related to
  the currently available data buffer space available for this
  connection.
</t>
<t>
  The sending TCP packages the data to be transmitted into segments
  which fit the current window, and may repackage segments on the
  retransmission queue.  Such repackaging is not required, but may be
  helpful.
</t>
<t>
  In a connection with a one-way data flow, the window information will
  be carried in acknowledgment segments that all have the same sequence
  number so there will be no way to reorder them if they arrive out of
  order.  This is not a serious problem, but it will allow the window
  information to be on occasion temporarily based on old reports from
  the data receiver.  A refinement to avoid this problem is to act on
  the window information from segments that carry the highest
  acknowledgment number (that is segments with acknowledgment number
  equal or greater than the highest previously received).
</t>
<t>
  Indicating a large window encourages transmissions.  If more data
  arrives than can be accepted, it will be discarded.  This will result
  in excessive retransmissions, adding unnecessarily to the load on the
  network and the TCPs.  Indicating a small window may restrict the
  transmission of data to the point of introducing a round trip delay
  between each new segment transmitted.
</t>
<t>
  The mechanisms provided allow a TCP to advertise a large window and to
  subsequently advertise a much smaller window without having accepted
  that much data.  This, so called &quot;shrinking the window,&quot; is strongly
  discouraged.  The robustness principle dictates that TCPs will not
  shrink the window themselves, but will be prepared for such behavior
  on the part of other TCPs.
</t>
<t>
  A TCP receiver SHOULD NOT shrink the window, i.e., move the
  right window edge to the left.  However, a sending TCP MUST
  be robust against window shrinking, which may cause the
  &quot;useable window&quot; (see <xref target="SWSsender"/>) to become negative.
</t>
<t>
  If this happens, the sender SHOULD NOT send new data, but
  SHOULD retransmit normally the old unacknowledged data
  between SND.UNA and SND.UNA+SND.WND.  The sender MAY also
  retransmit old data beyond SND.UNA+SND.WND, but SHOULD NOT
  time out the connection if data beyond the right window edge
  is not acknowledged.  If the window shrinks to zero, the TCP
  MUST probe it in the standard way (described below).
</t>
<section title="Zero Window Probing" anchor="zwp">
<t>
  The sending TCP must be prepared to accept from the user and send at
  least one octet of new data even if the send window is zero.  The
  sending TCP must regularly retransmit to the receiving TCP even when
  the window is zero, in order to &quot;probe&quot; the window.  Two minutes is recommended for the retransmission
  interval when the window is zero.  This retransmission is essential to
  guarantee that when either TCP has a zero window the re-opening of the
  window will be reliably reported to the other.  This is referred to as Zero-Window Probing (ZWP) in other documents.
</t>
<t>
  Probing of zero (offered) windows MUST be supported.
</t>
<t>
  A TCP MAY keep its offered receive window closed
  indefinitely.  As long as the receiving TCP continues to
  send acknowledgments in response to the probe segments, the
  sending TCP MUST allow the connection to stay open.  This
  enables TCP to function in scenarios such as the &quot;printer
  ran out of paper&quot; situation described in Section 4.2.2.17
  of RFC1122.  The behavior is subject to the implementation's resource
  management concerns, as noted in <xref target="RFC6429"/>.

</t>
<t>
  When the receiving TCP has a zero window and a segment arrives it must
  still send an acknowledgment showing its next expected sequence number
  and current window (zero).
</t>
</section>

<!-- all of this may be overcome by the SWS Avoidance and other 1122 content in this document - TODO, review this with the TCPM WG
<t>
  The window management procedure has significant influence on the
  communication performance.  The following comments are suggestions to
  implementers.
</t>

<t><list style="hanging">
    <t hangText="Window Management Suggestions">
    <vspace />
    <vspace />
      Allocating a very small window causes data to be transmitted in
      many small segments when better performance is achieved using
      fewer large segments.
    <vspace />
    <vspace />
      One suggestion for avoiding small windows is for the receiver to
      defer updating a window until the additional allocation is at
      least X percent of the maximum allocation possible for the
      connection (where X might be 20 to 40).
    <vspace />
    <vspace />
      Another suggestion is for the sender to avoid sending small
      segments by waiting until the window is large enough before
      sending data.  If the user signals a push function then the
      data must be sent even if it is a small segment.
    <vspace />
    <vspace />
      Note that the acknowledgments should not be delayed or unnecessary
      retransmissions will result.  One strategy would be to send an
      acknowledgment when a small segment arrives (with out updating the
      window information), and then to send another acknowledgment with
      new window information when the window is larger.
    <vspace />
    <vspace />
      The segment sent to probe a zero window may also begin a break up
      of transmitted data into smaller and smaller segments.  If a
      segment containing a single data octet sent to probe a zero window
      is accepted, it consumes one octet of the window now available.
      If the sending TCP simply sends as much as it can whenever the
      window is non zero, the transmitted data will be broken into
      alternating big and small segments.  As time goes on, occasional
      pauses in the receiver making window allocation available will
      result in breaking the big segments into a small and not quite so
      big pair. And after a while the data transmission will be in
      mostly small segments.
    <vspace />
    <vspace />
      The suggestion here is that the TCP implementations need to
      actively attempt to combine small window allocations into larger
      windows, since the mechanisms for managing the window tend to lead
      to many small windows in the simplest minded implementations.
    </t>
</list></t>
-->
<section title="Silly Window Syndrome Avoidance">
<t>The &quot;Silly Window Syndrome&quot; (SWS) is a stable pattern of small incremental window movements resulting in extremely poor TCP performance.  Algorithms to avoid SWS are described below for both the sending side and the receiving side.  RFC 1122 contains more detailed discussion of the SWS problem.  Note that the Nagle algorithm and the sender SWS avoidance algorithm play complementary roles in improving performance.  The Nagle algorithm discourages sending tiny segments when the data to be sent increases in small increments, while the SWS avoidance algorithm discourages small segments resulting from the right window edge advancing in small increments.</t>

<section title="Sender's Algorithm - When to Send Data" anchor="SWSsender">
<t>
            A TCP MUST include a SWS avoidance algorithm in the sender.
</t>
<t>
            A TCP SHOULD implement the Nagle Algorithm to
            coalesce short segments.  However, there MUST be a way for
            an application to disable the Nagle algorithm on an
            individual connection.  In all cases, sending data is also
            subject to the limitation imposed by the Slow Start
            algorithm.
</t>
<t>
            The sender's SWS avoidance algorithm is more difficult
            than the receivers's, because the sender does not know
            (directly) the receiver's total buffer space RCV.BUFF.
            An approach which has been found to work well is for
            the sender to calculate Max(SND.WND), the maximum send
            window it has seen so far on the connection, and to use
            this value as an estimate of RCV.BUFF.  Unfortunately,
            this can only be an estimate; the receiver may at any
            time reduce the size of RCV.BUFF.  To avoid a resulting
            deadlock, it is necessary to have a timeout to force
            transmission of data, overriding the SWS avoidance
            algorithm.  In practice, this timeout should seldom
            occur.
</t>
<t>
            The &quot;useable window&quot; is:<list><t>

                      U = SND.UNA + SND.WND - SND.NXT</t></list>

            i.e., the offered window less the amount of data sent
            but not acknowledged.  If D is the amount of data
            queued in the sending TCP but not yet sent, then the
            following set of rules is recommended.
</t>
<t>
            Send data:<list style="hanging" hangIndent="5">

            <t hangText="(1)">
             if a maximum-sized segment can be sent, i.e, if:<list><t>

                  min(D,U) >= Eff.snd.MSS;</t></list>
            </t>
            <t hangText="(2)">
             or if the data is pushed and all queued data can
             be sent now, i.e., if:<list><t>

                 [SND.NXT = SND.UNA and] PUSHED and D &lt;= U</t></list>

             (the bracketed condition is imposed by the Nagle
             algorithm);
            </t>
            <t hangText="(3)">
             or if at least a fraction Fs of the maximum window
             can be sent, i.e., if:<list><t>

                 [SND.NXT = SND.UNA and]<list><t>

                         min(D.U) >= Fs * Max(SND.WND);</t></list></t></list>

            </t>
            <t hangText="(4)">
             or if data is PUSHed and the override timeout
             occurs.
            </t></list>
</t>
<t>
            Here Fs is a fraction whose recommended value is 1/2.
            The override timeout should be in the range 0.1 - 1.0
            seconds.  It may be convenient to combine this timer
            with the timer used to probe zero windows (Section
            <xref target="zwp"/>).
</t>

</section>
<section title="Receiver's Algorithm - When to Send a Window Update">
<t>
            A TCP MUST include a SWS avoidance algorithm in the receiver.
</t>
<t>
            The receiver's SWS avoidance algorithm determines when
            the right window edge may be advanced; this is
            customarily known as &quot;updating the window&quot;.  This
            algorithm combines with the delayed ACK algorithm (see
            <xref target="delACK"/>) to determine when an ACK segment
            containing the current window will really be sent to
            the receiver. 
</t>
<t>
            The solution to receiver SWS is to avoid advancing the
            right window edge RCV.NXT+RCV.WND in small increments,
            even if data is received from the network in small
            segments.
</t>
<t>
            Suppose the total receive buffer space is RCV.BUFF.  At
            any given moment, RCV.USER octets of this total may be
            tied up with data that has been received and
            acknowledged but which the user process has not yet
            consumed.  When the connection is quiescent, RCV.WND =
            RCV.BUFF and RCV.USER = 0.
</t>
<t>
            Keeping the right window edge fixed as data arrives and
            is acknowledged requires that the receiver offer less
            than its full buffer space, i.e., the receiver must
            specify a RCV.WND that keeps RCV.NXT+RCV.WND constant
            as RCV.NXT increases.  Thus, the total buffer space
            RCV.BUFF is generally divided into three parts:
</t>
<t><figure><artwork>

               |&lt;------- RCV.BUFF ----------------&gt;|
                    1             2            3
           ----|---------|------------------|------|----
                      RCV.NXT               ^
                                         (Fixed)

           1 - RCV.USER =  data received but not yet consumed;
           2 - RCV.WND =   space advertised to sender;
           3 - Reduction = space available but not yet
                           advertised.

</artwork></figure></t>
<t>
          The suggested SWS avoidance algorithm for the receiver
          is to keep RCV.NXT+RCV.WND fixed until the reduction
          satisfies:
</t>
<t><figure><artwork>
             RCV.BUFF - RCV.USER - RCV.WND  >=

                    min( Fr * RCV.BUFF, Eff.snd.MSS )
</artwork></figure></t>
<t>
            where Fr is a fraction whose recommended value is 1/2,
            and Eff.snd.MSS is the effective send MSS for the
            connection (see <xref target="mss"/>).  When the inequality
            is satisfied, RCV.WND is set to RCV.BUFF-RCV.USER.
</t>
<t>
            Note that the general effect of this algorithm is to
            advance RCV.WND in increments of Eff.snd.MSS (for
            realistic receive buffers:  Eff.snd.MSS &lt; RCV.BUFF/2).
            Note also that the receiver must use its own
            Eff.snd.MSS, assuming it is the same as the sender's.
</t>

</section>
</section>
<section title="Delayed Acknowledgements - When to Send an ACK Segment" anchor="delACK">

<t>
            A host that is receiving a stream of TCP data segments can
            increase efficiency in both the Internet and the hosts by
            sending fewer than one ACK (acknowledgment) segment per data
            segment received; this is known as a &quot;delayed ACK&quot;.
</t>
<t>
            A TCP SHOULD implement a delayed ACK, but an ACK should not
            be excessively delayed; in particular, the delay MUST be
            less than 0.5 seconds, and in a stream of full-sized
            segments there SHOULD be an ACK for at least every second
            segment. Excessive delays on ACK's can disturb the round-trip
            timing and packet &quot;clocking&quot; algorithms.
</t>
</section>
</section>

</section>
<section title="Interfaces">
<t>
  There are of course two interfaces of concern:  the user/TCP interface
  and the TCP/lower-level interface.  We have a fairly elaborate model
  of the user/TCP interface, but the interface to the lower level
  protocol module is left unspecified here, since it will be specified
  in detail by the specification of the lower level protocol.  For the
  case that the lower level is IP we note some of the parameter values
  that TCPs might use.
</t>
<section title="User/TCP Interface">
<t>
    The following functional description of user commands to the TCP is,
    at best, fictional, since every operating system will have different
    facilities.  Consequently, we must warn readers that different TCP
    implementations may have different user interfaces.  However, all
    TCPs must provide a certain minimum set of services to guarantee
    that all TCP implementations can support the same protocol
    hierarchy.  This section specifies the functional interfaces
    required of all TCP implementations.
</t>
<t>
    TCP User Commands
</t>
<t><list style="hanging">
<t>
      The following sections functionally characterize a USER/TCP
      interface.  The notation used is similar to most procedure or
      function calls in high level languages, but this usage is not
      meant to rule out trap type service calls (e.g., SVCs, UUOs,
      EMTs).
</t>
<t>
      The user commands described below specify the basic functions the
      TCP must perform to support interprocess communication.
      Individual implementations must define their own exact format, and
      may provide combinations or subsets of the basic functions in
      single calls.  In particular, some implementations may wish to
      automatically OPEN a connection on the first SEND or RECEIVE
      issued by the user for a given connection.
</t>
<t>
      In providing interprocess communication facilities, the TCP must
      not only accept commands, but must also return information to the
      processes it serves.  The latter consists of:
      <list>
        <t>
        (a) general information about a connection (e.g., interrupts,
        remote close, binding of unspecified foreign socket).
        </t>
        <t>
        (b) replies to specific user commands indicating success or
        various types of failure.
        </t>
      </list>
</t>

<t>
      Open
    <list>
<t>
        Format:  OPEN (local port, foreign socket, active/passive
        [, timeout] [, precedence] [, security/compartment]
        [local IP address,] [, options])
        -> local connection name
</t>
<t>
        We assume that the local TCP is aware of the identity of the
        processes it serves and will check the authority of the process
        to use the connection specified.  Depending upon the
        implementation of the TCP, the local network and TCP identifiers
        for the source address will either be supplied by the TCP or the
        lower level protocol (e.g., IP).  These considerations are the
        result of concern about security, to the extent that no TCP be
        able to masquerade as another one, and so on.  Similarly, no
        process can masquerade as another without the collusion of the
        TCP.
</t>
<t>
        If the active/passive flag is set to passive, then this is a
        call to LISTEN for an incoming connection.  A passive open may
        have either a fully specified foreign socket to wait for a
        particular connection or an unspecified foreign socket to wait
        for any call.  A fully specified passive call can be made active
        by the subsequent execution of a SEND.
</t>
<t>
        A transmission control block (TCB) is created and partially
        filled in with data from the OPEN command parameters.
</t>
<t>
        Every passive OPEN call either creates a new connection
        record in LISTEN state, or it returns an error; it MUST NOT
        affect any previously created connection record.
</t>
<t>
        A TCP that supports multiple concurrent users MUST provide
        an OPEN call that will functionally allow an application to
        LISTEN on a port while a connection block with the same
        local port is in SYN-SENT or SYN-RECEIVED state.
</t>
<t>
        On an active OPEN command, the TCP will begin the procedure to
        synchronize (i.e., establish) the connection at once.
</t>
<t>
        The timeout, if present, permits the caller to set up a timeout
        for all data submitted to TCP.  If data is not successfully
        delivered to the destination within the timeout period, the TCP
        will abort the connection.  The present global default is five
        minutes.
</t>
<t>
        The TCP or some component of the operating system will verify
        the users authority to open a connection with the specified
        precedence or security/compartment.  The absence of precedence
        or security/compartment specification in the OPEN call indicates
        the default values must be used.
</t>
<t>
        TCP will accept incoming requests as matching only if the
        security/compartment information is exactly the same and only if
        the precedence is equal to or higher than the precedence
        requested in the OPEN call.
</t>
<t>
        The precedence for the connection is the higher of the values
        requested in the OPEN call and received from the incoming
        request, and fixed at that value for the life of the
        connection.Implementers may want to give the user control of
        this precedence negotiation.  For example, the user might be
        allowed to specify that the precedence must be exactly matched,
        or that any attempt to raise the precedence be confirmed by the
        user.
</t>
<t>
        A local connection name will be returned to the user by the TCP.
        The local connection name can then be used as a short hand term
        for the connection defined by the &lt;local socket, foreign socket>
        pair.
</t>
<t>
	The optional &quot;local IP address&quot; parameter MUST be supported
        to allow the specification of the local IP address.  This enables
        applications that need to select the local IP address used when
        multihoming is present.
</t>
<t>
        A passive OPEN call with a specified "local IP address"
        parameter will await an incoming connection request to
        that address.  If the parameter is unspecified, a
        passive OPEN will await an incoming connection request
        to any local IP address, and then bind the local IP
        address of the connection to the particular address
        that is used.
</t>
<t>
For an active OPEN call, a specified &quot;local IP address&quot; parameter
MUST be used for opening the connection.  If the parameter is unspecified, the
TCP will choose an appropriate local IP address (see RFC 1122 section 3.3.4.2).
</t>
<t>TODO - the previous and next paragraphs are mildly in conflict.  Previous paragraph says that the TCP chooses an address, but next paragraph says that it asks IP to choose ... need to make this consistent</t>
<t>
        If an application on a multihomed host does not specify the
        local IP address when actively opening a TCP connection,
        then the TCP MUST ask the IP layer to select a local IP
        address before sending the (first) SYN.  See the function
        GET_SRCADDR() in Section 3.4 of RFC 1122.
</t>
<t>
        At all other times, a previous segment has either been sent
        or received on this connection, and TCP MUST use the same
        local address is used that was used in those previous
        segments.
</t>
</list>
</t>
<t>
      Send
      <list>
<t>
        Format:  SEND (local connection name, buffer address, byte
        count, PUSH flag, URGENT flag [,timeout])
</t>
<t>
        This call causes the data contained in the indicated user buffer
        to be sent on the indicated connection.  If the connection has
        not been opened, the SEND is considered an error.  Some
        implementations may allow users to SEND first; in which case, an
        automatic OPEN would be done.  If the calling process is not
        authorized to use this connection, an error is returned.
</t>
<t>
        If the PUSH flag is set, the data must be transmitted promptly
        to the receiver, and the PUSH bit will be set in the last TCP
        segment created from the buffer.  If the PUSH flag is not set,
        the data may be combined with data from subsequent SENDs for
        transmission efficiency.
</t>
<t>
        New applications SHOULD NOT set the URGENT flag <xref target="RFC6093"/> due to implementation differences and middlebox issues.
</t>
<t>
        If the URGENT flag is set, segments sent to the destination TCP
        will have the urgent pointer set.  The receiving TCP will signal
        the urgent condition to the receiving process if the urgent
        pointer indicates that data preceding the urgent pointer has not
        been consumed by the receiving process.  The purpose of urgent
        is to stimulate the receiver to process the urgent data and to
        indicate to the receiver when all the currently known urgent
        data has been received.  The number of times the sending user's
        TCP signals urgent will not necessarily be equal to the number
        of times the receiving user will be notified of the presence of
        urgent data.
</t>
<t>
        If no foreign socket was specified in the OPEN, but the
        connection is established (e.g., because a LISTENing connection
        has become specific due to a foreign segment arriving for the
        local socket), then the designated buffer is sent to the implied
        foreign socket.  Users who make use of OPEN with an unspecified
        foreign socket can make use of SEND without ever explicitly
        knowing the foreign socket address.
</t>
<t>
        However, if a SEND is attempted before the foreign socket
        becomes specified, an error will be returned.  Users can use the
        STATUS call to determine the status of the connection.  In some
        implementations the TCP may notify the user when an unspecified
        socket is bound.
</t>
<t>
        If a timeout is specified, the current user timeout for this
        connection is changed to the new one.
</t>
<t>
        In the simplest implementation, SEND would not return control to
        the sending process until either the transmission was complete
        or the timeout had been exceeded.  However, this simple method
        is both subject to deadlocks (for example, both sides of the
        connection might try to do SENDs before doing any RECEIVEs) and
        offers poor performance, so it is not recommended.  A more
        sophisticated implementation would return immediately to allow
        the process to run concurrently with network I/O, and,
        furthermore, to allow multiple SENDs to be in progress.
        Multiple SENDs are served in first come, first served order, so
        the TCP will queue those it cannot service immediately.
</t>
<t>
        We have implicitly assumed an asynchronous user interface in
        which a SEND later elicits some kind of SIGNAL or
        pseudo-interrupt from the serving TCP.  An alternative is to
        return a response immediately.  For instance, SENDs might return
        immediate local acknowledgment, even if the segment sent had not
        been acknowledged by the distant TCP.  We could optimistically
        assume eventual success.  If we are wrong, the connection will
        close anyway due to the timeout.  In implementations of this
        kind (synchronous), there will still be some asynchronous
        signals, but these will deal with the connection itself, and not
        with specific segments or buffers.
</t>
<t>
        In order for the process to distinguish among error or success
        indications for different SENDs, it might be appropriate for the
        buffer address to be returned along with the coded response to
        the SEND request.  TCP-to-user signals are discussed below,
        indicating the information which should be returned to the
        calling process.
</t>
</list></t>
<t>
      Receive
      <list>
<t>
        Format:  RECEIVE (local connection name, buffer address, byte
        count) -> byte count, urgent flag, push flag
</t>
<t>
        This command allocates a receiving buffer associated with the
        specified connection.  If no OPEN precedes this command or the
        calling process is not authorized to use this connection, an
        error is returned.
</t>
<t>
        In the simplest implementation, control would not return to the
        calling program until either the buffer was filled, or some
        error occurred, but this scheme is highly subject to deadlocks.
        A more sophisticated implementation would permit several
        RECEIVEs to be outstanding at once.  These would be filled as
        segments arrive.  This strategy permits increased throughput at
        the cost of a more elaborate scheme (possibly asynchronous) to
        notify the calling program that a PUSH has been seen or a buffer
        filled.
</t>
<t>
        If enough data arrive to fill the buffer before a PUSH is seen,
        the PUSH flag will not be set in the response to the RECEIVE.
        The buffer will be filled with as much data as it can hold.  If
        a PUSH is seen before the buffer is filled the buffer will be
        returned partially filled and PUSH indicated.
</t>
<t>
        If there is urgent data the user will have been informed as soon
        as it arrived via a TCP-to-user signal.  The receiving user
        should thus be in &quot;urgent mode&quot;.  If the URGENT flag is on,
        additional urgent data remains.  If the URGENT flag is off, this
        call to RECEIVE has returned all the urgent data, and the user
        may now leave &quot;urgent mode&quot;.  Note that data following the
        urgent pointer (non-urgent data) cannot be delivered to the user
        in the same buffer with preceding urgent data unless the
        boundary is clearly marked for the user.
</t>
<t>
        To distinguish among several outstanding RECEIVEs and to take
        care of the case that a buffer is not completely filled, the
        return code is accompanied by both a buffer pointer and a byte
        count indicating the actual length of the data received.
</t>
<t>
        Alternative implementations of RECEIVE might have the TCP
        allocate buffer storage, or the TCP might share a ring buffer
        with the user.
</t>
</list></t>
<t>
      Close
      <list>
<t>
        Format:  CLOSE (local connection name)
</t>
<t>
        This command causes the connection specified to be closed.  If
        the connection is not open or the calling process is not
        authorized to use this connection, an error is returned.
        Closing connections is intended to be a graceful operation in
        the sense that outstanding SENDs will be transmitted (and
        retransmitted), as flow control permits, until all have been
        serviced.  Thus, it should be acceptable to make several SEND
        calls, followed by a CLOSE, and expect all the data to be sent
        to the destination.  It should also be clear that users should
        continue to RECEIVE on CLOSING connections, since the other side
        may be trying to transmit the last of its data.  Thus, CLOSE
        means "I have no more to send" but does not mean "I will not
        receive any more."  It may happen (if the user level protocol is
        not well thought out) that the closing side is unable to get rid
        of all its data before timing out.  In this event, CLOSE turns
        into ABORT, and the closing TCP gives up.
</t>
<t>
        The user may CLOSE the connection at any time on his own
        initiative, or in response to various prompts from the TCP
        (e.g., remote close executed, transmission timeout exceeded,
        destination inaccessible).
</t>
<t>
        Because closing a connection requires communication with the
        foreign TCP, connections may remain in the closing state for a
        short time.  Attempts to reopen the connection before the TCP
        replies to the CLOSE command will result in error responses.
</t>
<t>
        Close also implies push function.
</t>
</list></t>
<t>
      Status
      <list>
<t>
        Format:  STATUS (local connection name) -> status data
</t>
<t>
        This is an implementation dependent user command and could be
        excluded without adverse effect.  Information returned would
        typically come from the TCB associated with the connection.
</t>
<t>
        This command returns a data block containing the following
        information:
        <list>
          <t>local socket,<vspace />
          foreign socket,<vspace />
          local connection name,<vspace />
          receive window,<vspace />
          send window,<vspace />
          connection state,<vspace />
          number of buffers awaiting acknowledgment,<vspace />
          number of buffers pending receipt,<vspace />
          urgent state,<vspace />
          precedence,<vspace />
          security/compartment,<vspace />
          and transmission timeout.</t>
        </list>
</t>
<t>
        Depending on the state of the connection, or on the
        implementation itself, some of this information may not be
        available or meaningful.  If the calling process is not
        authorized to use this connection, an error is returned.  This
        prevents unauthorized processes from gaining information about a
        connection.
</t>
</list></t>
<t>
      Abort
      <list>
<t>
        Format:  ABORT (local connection name)
</t>
<t>
        This command causes all pending SENDs and RECEIVES to be
        aborted, the TCB to be removed, and a special RESET message to
        be sent to the TCP on the other side of the connection.
        Depending on the implementation, users may receive abort
        indications for each outstanding SEND or RECEIVE, or may simply
        receive an ABORT-acknowledgment.
</t>
</list></t>
<t>
      Flush
    <list>
     <t>
            Some TCP implementations have included a FLUSH call, which
            will empty the TCP send queue of any data for which the user
            has issued SEND calls but which is still to the right of the
            current send window.  That is, it flushes as much queued
            send data as possible without losing sequence number
            synchronization. 
     </t>
    </list>
</t>
<t>
      Set TOS
      <list>
<t>
            The application layer MUST be able to specify the Type-of-
            Service (TOS) for segments that are sent on a connection.
            It not required, but the application SHOULD be able to
            change the TOS during the connection lifetime.  TCP SHOULD
            pass the current TOS value without change to the IP layer,
            when it sends segments on the connection.
</t>
<t>
            The TOS will be specified independently in each direction on
            the connection, so that the receiver application will
            specify the TOS used for ACK segments.
</t>
<t>
            TCP MAY pass the most recently received TOS up to the
            application.
</t>
</list></t>
<t>
    TCP-to-User Messages
    <list>
<t>
      It is assumed that the operating system environment provides a
      means for the TCP to asynchronously signal the user program.  When
      the TCP does signal a user program, certain information is passed
      to the user.  Often in the specification the information will be
      an error message.  In other cases there will be information
      relating to the completion of processing a SEND or RECEIVE or
      other user call.
</t>
<t>
      The following information is provided:
<figure><artwork>
        Local Connection Name                    Always
        Response String                          Always
        Buffer Address                           Send &amp; Receive
        Byte count (counts bytes received)       Receive
        Push flag                                Receive
        Urgent flag                              Receive
</artwork></figure></t>
</list></t>
</list></t>
</section>
<section title="TCP/Lower-Level Interface">
<t>
    The TCP calls on a lower level protocol module to actually send and
    receive information over a network.  One case is that of the ARPA
    internetwork system where the lower level module is the Internet
    Protocol (IP) <xref target="RFC0791"/>.
</t>
<t>
    If the lower level protocol is IP it provides arguments for a type
    of service and for a time to live.  TCP uses the following settings
    for these parameters:
    <list>
<t>
      Type of Service = Precedence: given by user, Delay: normal, Throughput:
      normal, Reliability: normal; or binary XXX00000, where XXX are the three bits determining precedence, e.g. 000 means routine precedence.
</t>
<t>
      Time to Live (TTL): The TTL value used to send TCP segments MUST be configurable.
      <list>
<t>
	Note that RFC 793 specified one minute (60 seconds) as a constant for
the TTL, because the assumed maximum segment lifetime was two minutes.  This was
intended to explicitly ask that a segment be destroyed if it cannot be
delivered by the internet system within one minute.  RFC 1122 changed this specification to require that the TTL be configurable.
</t>
      </list>
</t>
</list></t>
<t>
    Any lower level protocol will have to provide the source address,
    destination address, and protocol fields, and some way to determine
    the "TCP length", both to provide the functional equivalent service
    of IP and to be used in the TCP checksum.
</t>
<t>
    When received options are passed up to TCP from the IP
    layer, TCP MUST ignore options that it does not understand.
</t>
<t>
    A TCP MAY support the Time Stamp and Record Route options.
</t>
<section title="Source Routing">
<t>
    If the lower level is IP (or other protocol that provides this
    feature) and source routing is used, the interface must allow the
    route information to be communicated.  This is especially important
    so that the source and destination addresses used in the TCP
    checksum be the originating source and ultimate destination. It is
    also important to preserve the return route to answer connection
    requests.
</t>
<t>
    An application MUST be able to specify a source route when
    it actively opens a TCP connection, and this MUST take
    precedence over a source route received in a datagram.
</t>
<t>
    When a TCP connection is OPENed passively and a packet
    arrives with a completed IP Source Route option (containing
    a return route), TCP MUST save the return route and use it
    for all segments sent on this connection.  If a different
    source route arrives in a later segment, the later
    definition SHOULD override the earlier one.
</t>
</section>
<section title="ICMP Messages">
<t>TODO - this section is verbatim from 1122, currently.  It should be revised to match the soft-errors RFC, and other updates (e.g. source quench deprecation)</t>
<t>
            TCP MUST act on an ICMP error message passed up from the IP
            layer, directing it to the connection that created the
            error.  The necessary demultiplexing information can be
            found in the IP header contained within the ICMP message.
</t>
<t>
<list style="hanging" hangIndent="2">
<t hangText="Source Quench"><vspace />

                 TCP MUST react to a Source Quench by slowing
                 transmission on the connection.  The RECOMMENDED
                 procedure is for a Source Quench to trigger a "slow
                 start," as if a retransmission timeout had occurred.
</t>
<t hangText="Destination Unreachable -- codes 0, 1, 5"><vspace />

                 Since these Unreachable messages indicate soft error
                 conditions, TCP MUST NOT abort the connection, and it
                 SHOULD make the information available to the
                 application.
</t>
<t hangText="Destination Unreachable -- codes 2-4"><vspace />

                 These are hard error conditions, so TCP SHOULD abort
                 the connection.
</t>
<t hangText="Time Exceeded -- codes 0, 1"><vspace />

                 This should be handled the same way as Destination
                 Unreachable codes 0, 1, 5 (see above).
</t>
<t hangText="Parameter Problem"><vspace />

                 This should be handled the same way as Destination
                 Unreachable codes 0, 1, 5 (see above).
</t>
</list>
</t>
</section>
</section>
</section>
<section title="Event Processing">
<t>
  The processing depicted in this section is an example of one possible
  implementation.  Other implementations may have slightly different
  processing sequences, but they should differ from those in this
  section only in detail, not in substance.
</t>
<t>
  The activity of the TCP can be characterized as responding to events.
  The events that occur can be cast into three categories:  user calls,
  arriving segments, and timeouts.  This section describes the
  processing the TCP does in response to each of the events.  In many
  cases the processing required depends on the state of the connection.
</t>
<t>
    Events that occur:

    <list>
      <t>User Calls
        <list>
        <t>OPEN<vspace />
        SEND<vspace />
        RECEIVE<vspace />
        CLOSE<vspace />
        ABORT<vspace />
        STATUS</t>
        </list>
      </t>
      <t>Arriving Segments
        <list><t>SEGMENT ARRIVES</t></list>
      </t>
      <t>Timeouts
        <list>
        <t>USER TIMEOUT<vspace />
        RETRANSMISSION TIMEOUT<vspace />
        TIME-WAIT TIMEOUT<vspace />
        </t></list>
      </t>
    </list>
</t>
<t>
  The model of the TCP/user interface is that user commands receive an
  immediate return and possibly a delayed response via an event or
  pseudo interrupt.  In the following descriptions, the term "signal"
  means cause a delayed response.
</t>
<t>
  Error responses are given as character strings.  For example, user
  commands referencing connections that do not exist receive "error:
  connection not open".
</t>
<t>
  Please note in the following that all arithmetic on sequence numbers,
  acknowledgment numbers, windows, et cetera, is modulo 2**32 the size
  of the sequence number space.  Also note that "=&lt;" means less than or
  equal to (modulo 2**32).
</t>
<t>
  A natural way to think about processing incoming segments is to
  imagine that they are first tested for proper sequence number (i.e.,
  that their contents lie in the range of the expected "receive window"
  in the sequence number space) and then that they are generally queued
  and processed in sequence number order.
</t>
<t>
  When a segment overlaps other already received segments we reconstruct
  the segment to contain just the new data, and adjust the header fields
  to be consistent.
</t>
<t>
  Note that if no state change is mentioned the TCP stays in the same
  state.
</t>
<t><vspace blankLines="999"/></t>
<t>
  OPEN Call
<list>
<t>CLOSED STATE (i.e., TCB does not exist)
  <list>
    <t>
      Create a new transmission control block (TCB) to hold connection
      state information.  Fill in local socket identifier, foreign
      socket, precedence, security/compartment, and user timeout
      information.  Note that some parts of the foreign socket may be
      unspecified in a passive OPEN and are to be filled in by the
      parameters of the incoming SYN segment.  Verify the security and
      precedence requested are allowed for this user, if not return
      "error:  precedence not allowed" or "error:  security/compartment
      not allowed."  If passive enter the LISTEN state and return.  If
      active and the foreign socket is unspecified, return "error:
      foreign socket unspecified"; if active and the foreign socket is
      specified, issue a SYN segment.  An initial send sequence number
      (ISS) is selected.  A SYN segment of the form &lt;SEQ=ISS>&lt;CTL=SYN>
      is sent.  Set SND.UNA to ISS, SND.NXT to ISS+1, enter SYN-SENT
      state, and return.
    </t>
    <t>
      If the caller does not have access to the local socket specified,
      return "error:  connection illegal for this process".  If there is
      no room to create a new connection, return "error:  insufficient
      resources".
    </t>
  </list>
</t>
<t>LISTEN STATE
  <list>
    <t>
      If active and the foreign socket is specified, then change the
      connection from passive to active, select an ISS.  Send a SYN
      segment, set SND.UNA to ISS, SND.NXT to ISS+1.  Enter SYN-SENT
      state.  Data associated with SEND may be sent with SYN segment or
      queued for transmission after entering ESTABLISHED state.  The
      urgent bit if requested in the command must be sent with the data
      segments sent as a result of this command.  If there is no room to
      queue the request, respond with "error:  insufficient resources".
      If Foreign socket was not specified, then return "error:  foreign
      socket unspecified".
    </t>
  </list>
</t>
<t><vspace blankLines="999"/></t>
<t>SYN-SENT STATE<vspace />
SYN-RECEIVED STATE<vspace />
ESTABLISHED STATE<vspace />
FIN-WAIT-1 STATE<vspace />
FIN-WAIT-2 STATE<vspace />
CLOSE-WAIT STATE<vspace />
CLOSING STATE<vspace />
LAST-ACK STATE<vspace />
TIME-WAIT STATE
  <list>
      <t>Return "error:  connection already exists".</t>
  </list>
</t>
</list>
</t>
<t><vspace blankLines="999"/></t>
<t>
  SEND Call
  <list>
    <t>
    CLOSED STATE (i.e., TCB does not exist)
      <list>
      <t>
      If the user does not have access to such a connection, then return
      "error:  connection illegal for this process".
      </t>
      <t>
      Otherwise, return "error:  connection does not exist".
      </t>
      </list>
    </t>
    <t>
    LISTEN STATE
      <list>
      <t>
      If the foreign socket is specified, then change the connection
      from passive to active, select an ISS.  Send a SYN segment, set
      SND.UNA to ISS, SND.NXT to ISS+1.  Enter SYN-SENT state.  Data
      associated with SEND may be sent with SYN segment or queued for
      transmission after entering ESTABLISHED state.  The urgent bit if
      requested in the command must be sent with the data segments sent
      as a result of this command.  If there is no room to queue the
      request, respond with "error:  insufficient resources".  If
      Foreign socket was not specified, then return "error:  foreign
      socket unspecified".
      </t>
      </list>
    </t>
    <t>
    SYN-SENT STATE<vspace />
    SYN-RECEIVED STATE
      <list><t>
      Queue the data for transmission after entering ESTABLISHED state.
      If no space to queue, respond with "error:  insufficient
      resources".
      </t></list>
    </t>
    <t>
    ESTABLISHED STATE<vspace />
    CLOSE-WAIT STATE
      <list><t>
      Segmentize the buffer and send it with a piggybacked
      acknowledgment (acknowledgment value = RCV.NXT).  If there is
      insufficient space to remember this buffer, simply return "error:
      insufficient resources".
      </t>
      <t>
      If the urgent flag is set, then SND.UP &lt;- SND.NXT and set the
      urgent pointer in the outgoing segments.
      </t></list>
    </t>
    <t>
    FIN-WAIT-1 STATE<vspace />
    FIN-WAIT-2 STATE<vspace />
    CLOSING STATE<vspace />
    LAST-ACK STATE<vspace />
    TIME-WAIT STATE<vspace />
      <list><t>
      Return "error:  connection closing" and do not service request.
      </t></list>
    </t>
  </list>
</t>
<t><vspace blankLines="999"/></t>
<t>
  RECEIVE Call
  <list>
    <t>
    CLOSED STATE (i.e., TCB does not exist)
    <list>
      <t>
      If the user does not have access to such a connection, return
      "error:  connection illegal for this process".
      </t>
      <t>
      Otherwise return "error:  connection does not exist".
      </t>
    </list></t>
    <t>
    LISTEN STATE<vspace />
    SYN-SENT STATE<vspace />
    SYN-RECEIVED STATE
    <list>
      <t>
      Queue for processing after entering ESTABLISHED state.  If there
      is no room to queue this request, respond with "error:
      insufficient resources".
      </t>
    </list></t>
    <t>
    ESTABLISHED STATE<vspace />
    FIN-WAIT-1 STATE<vspace />
    FIN-WAIT-2 STATE
    <list>
      <t>
      If insufficient incoming segments are queued to satisfy the
      request, queue the request.  If there is no queue space to
      remember the RECEIVE, respond with "error:  insufficient
      resources".
      </t>
      <t>
      Reassemble queued incoming segments into receive buffer and return
      to user.  Mark "push seen" (PUSH) if this is the case.
      </t>
      <t>
      If RCV.UP is in advance of the data currently being passed to the
      user notify the user of the presence of urgent data.
      </t>
      <t>
      When the TCP takes responsibility for delivering data to the user
      that fact must be communicated to the sender via an
      acknowledgment.  The formation of such an acknowledgment is
      described below in the discussion of processing an incoming
      segment.
      </t>
    </list></t>
    <t>
    CLOSE-WAIT STATE
    <list>
      <t>
      Since the remote side has already sent FIN, RECEIVEs must be
      satisfied by text already on hand, but not yet delivered to the
      user.  If no text is awaiting delivery, the RECEIVE will get a
      "error:  connection closing" response.  Otherwise, any remaining
      text can be used to satisfy the RECEIVE.
      </t>
    </list></t>
    <t>
    CLOSING STATE<vspace />
    LAST-ACK STATE<vspace />
    TIME-WAIT STATE
    <list>
      <t>
      Return "error:  connection closing".
      </t>
    </list></t>
  </list>
</t>
<t><vspace blankLines="999"/></t>
<t>
  CLOSE Call
  <list>
    <t>
    CLOSED STATE (i.e., TCB does not exist)
      <list>
      <t>
      If the user does not have access to such a connection, return
      "error:  connection illegal for this process".
      </t>
      <t>
      Otherwise, return "error:  connection does not exist".
      </t>
      </list></t>
    <t>
    LISTEN STATE
      <list>
      <t>
      Any outstanding RECEIVEs are returned with "error:  closing"
      responses.  Delete TCB, enter CLOSED state, and return.
      </t></list></t>
    <t>
    SYN-SENT STATE
      <list>
      <t>
      Delete the TCB and return "error:  closing" responses to any
      queued SENDs, or RECEIVEs.
      </t></list></t>
    <t>
    SYN-RECEIVED STATE
      <list>
      <t>
      If no SENDs have been issued and there is no pending data to send,
      then form a FIN segment and send it, and enter FIN-WAIT-1 state;
      otherwise queue for processing after entering ESTABLISHED state.
      </t></list></t>
    <t>
    ESTABLISHED STATE
      <list>
      <t>
      Queue this until all preceding SENDs have been segmentized, then
      form a FIN segment and send it.  In any case, enter FIN-WAIT-1
      state.
      </t></list></t>
    <t>
    FIN-WAIT-1 STATE<vspace />
    FIN-WAIT-2 STATE
      <list>
      <t>
      Strictly speaking, this is an error and should receive a "error:
      connection closing" response.  An "ok" response would be
      acceptable, too, as long as a second FIN is not emitted (the first
      FIN may be retransmitted though).
      </t></list></t>
    <t>
    CLOSE-WAIT STATE
      <list>
      <t>
      Queue this request until all preceding SENDs have been
      segmentized; then send a FIN segment, enter LAST-ACK state.
      </t></list></t>
    <t>
    CLOSING STATE<vspace />
    LAST-ACK STATE<vspace />
    TIME-WAIT STATE
      <list>
      <t>
      Respond with "error:  connection closing".
      </t></list></t>
  </list>
</t>
<t><vspace blankLines="999"/></t>
<t>
  ABORT Call
  <list>
    <t>
    CLOSED STATE (i.e., TCB does not exist)
      <list>
      <t>
      If the user should not have access to such a connection, return
      "error:  connection illegal for this process". 
      </t>
      <t>
      Otherwise return "error:  connection does not exist".
      </t></list></t>
    <t>
    LISTEN STATE
      <list>
      <t>
      Any outstanding RECEIVEs should be returned with "error:
      connection reset" responses.  Delete TCB, enter CLOSED state, and
      return.
      </t></list></t>
    <t>
    SYN-SENT STATE
      <list>
      <t>
      All queued SENDs and RECEIVEs should be given "connection reset"
      notification, delete the TCB, enter CLOSED state, and return.
      </t></list></t>
    <t>
    SYN-RECEIVED STATE<vspace />
    ESTABLISHED STATE<vspace />
    FIN-WAIT-1 STATE<vspace />
    FIN-WAIT-2 STATE<vspace />
    CLOSE-WAIT STATE
      <list>
      <t>
      Send a reset segment:
      <list>
      <t>
        &lt;SEQ=SND.NXT>&lt;CTL=RST>
      </t>
      </list></t>
      <t>
      All queued SENDs and RECEIVEs should be given "connection reset"
      notification; all segments queued for transmission (except for the
      RST formed above) or retransmission should be flushed, delete the
      TCB, enter CLOSED state, and return.
      </t></list></t>
    <t>
    CLOSING STATE
    LAST-ACK STATE
    TIME-WAIT STATE
      <list>
      <t>
      Respond with "ok" and delete the TCB, enter CLOSED state, and
      return.
      </t></list></t>
  </list>
</t>
<t><vspace blankLines="999"/></t>
<t>
  STATUS Call
  <list>
    <t>
    CLOSED STATE (i.e., TCB does not exist)
    <list>
      <t>
      If the user should not have access to such a connection, return
      "error:  connection illegal for this process".
      </t>
      <t>
      Otherwise return "error:  connection does not exist".
      </t></list></t>
    <t>
    LISTEN STATE
    <list>
      <t>
      Return "state = LISTEN", and the TCB pointer.
      </t></list></t>
    <t>
    SYN-SENT STATE
      <list>
      <t>
      Return "state = SYN-SENT", and the TCB pointer.
      </t></list></t>
    <t>
    SYN-RECEIVED STATE
      <list>
      <t>
      Return "state = SYN-RECEIVED", and the TCB pointer.
      </t></list></t>
    <t>
    ESTABLISHED STATE
      <list>
      <t>
      Return "state = ESTABLISHED", and the TCB pointer.
      </t></list></t>
    <t>
    FIN-WAIT-1 STATE
      <list>
      <t>
      Return "state = FIN-WAIT-1", and the TCB pointer.
      </t></list></t>
    <t>
    FIN-WAIT-2 STATE
      <list>
      <t>
      Return "state = FIN-WAIT-2", and the TCB pointer.
      </t></list></t>
    <t>
    CLOSE-WAIT STATE
      <list>
      <t>
      Return "state = CLOSE-WAIT", and the TCB pointer.
      </t></list></t>
    <t>
    CLOSING STATE
      <list>
      <t>
      Return "state = CLOSING", and the TCB pointer.
      </t></list></t>
    <t>
    LAST-ACK STATE
    <list>
    <t>
      Return "state = LAST-ACK", and the TCB pointer.
    </t></list></t>
    <t>
    TIME-WAIT STATE
    <list>
    <t>
      Return "state = TIME-WAIT", and the TCB pointer.
    </t></list></t>
  </list>
</t>
<t><vspace blankLines="999"/></t>
<t>
  SEGMENT ARRIVES
  <list>
    <t>
    If the state is CLOSED (i.e., TCB does not exist) then
    <list>
      <t>
      all data in the incoming segment is discarded.  An incoming
      segment containing a RST is discarded.  An incoming segment not
      containing a RST causes a RST to be sent in response.  The
      acknowledgment and sequence field values are selected to make the
      reset sequence acceptable to the TCP that sent the offending
      segment.
      </t>
      <t>
      If the ACK bit is off, sequence number zero is used,
        <list>
        <t>
        &lt;SEQ=0>&lt;ACK=SEG.SEQ+SEG.LEN>&lt;CTL=RST,ACK>
        </t>
        </list></t>
      <t>
      If the ACK bit is on,
        <list>
        <t>
        &lt;SEQ=SEG.ACK>&lt;CTL=RST>
        </t></list></t>
      <t>
      Return.
      </t>
    </list></t>
    <t>
    If the state is LISTEN then
      <list>
       <t>
      first check for an RST
        <list>
        <t>
        An incoming RST should be ignored.  Return.
        </t></list></t>
      <t>
      second check for an ACK
        <list>
        <t>
        Any acknowledgment is bad if it arrives on a connection still in
        the LISTEN state.  An acceptable reset segment should be formed
        for any arriving ACK-bearing segment.  The RST should be
        formatted as follows:
        <list>
          <t>
          &lt;SEQ=SEG.ACK>&lt;CTL=RST>
          </t></list></t>
        <t>
        Return.
        </t></list></t>
      <t>
      third check for a SYN
      <list>
        <t>
        If the SYN bit is set, check the security.  If the
        security/compartment on the incoming segment does not exactly
        match the security/compartment in the TCB then send a reset and
        return.
          <list>
          <t>
          &lt;SEQ=0>&lt;ACK=SEG.SEQ+SEG.LEN>&lt;CTL=RST,ACK>
          </t></list></t>
        <t>
        If the SEG.PRC is greater than the TCB.PRC then if allowed by
        the user and the system set TCB.PRC&lt;-SEG.PRC, if not allowed
        send a reset and return.
          <list>
          <t>
          &lt;SEQ=0>&lt;ACK=SEG.SEQ+SEG.LEN>&lt;CTL=RST,ACK>
          </t></list></t>
        <t>
        If the SEG.PRC is less than the TCB.PRC then continue.
        </t>
        <t>
        Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ and any other
        control or text should be queued for processing later.  ISS
        should be selected and a SYN segment sent of the form:
        <list>
          <t>
          &lt;SEQ=ISS>&lt;ACK=RCV.NXT>&lt;CTL=SYN,ACK>
          </t></list></t>
        <t>
        SND.NXT is set to ISS+1 and SND.UNA to ISS.  The connection
        state should be changed to SYN-RECEIVED.  Note that any other
        incoming control or data (combined with SYN) will be processed
        in the SYN-RECEIVED state, but processing of SYN and ACK should
        not be repeated.  If the listen was not fully specified (i.e.,
        the foreign socket was not fully specified), then the
        unspecified fields should be filled in now.
        </t></list></t>
      <t>
      fourth other text or control
        <list>
        <t>
        Any other control or text-bearing segment (not containing SYN)
        must have an ACK and thus would be discarded by the ACK
        processing.  An incoming RST segment could not be valid, since
        it could not have been sent in response to anything sent by this
        incarnation of the connection.  So you are unlikely to get here,
        but if you do, drop the segment, and return.
        </t></list></t>
    </list></t>
    <t>
    If the state is SYN-SENT then
      <list>
      <t>
      first check the ACK bit
        <list>
        <t>
        If the ACK bit is set
          <list>
          <t>
          If SEG.ACK =&lt; ISS, or SEG.ACK > SND.NXT, send a reset (unless
          the RST bit is set, if so drop the segment and return)
            <list>
            <t>
            &lt;SEQ=SEG.ACK>&lt;CTL=RST>
            </t></list></t>
          <t>
          and discard the segment.  Return.
          </t>
          <t>
          If SND.UNA &lt; SEG.ACK =&lt; SND.NXT then the ACK is acceptable. (TODO: in processing Errata ID 3300, it was noted that some stacks in the wild that do not send data on the SYN are just checking that SEG.ACK == SND.NXT ... think about whether anything should be said about that here)
          </t></list></t>
        </list></t>
      <t>
      second check the RST bit
        <list>
        <t>
        If the RST bit is set
          <list>
          <t>
          If the ACK was acceptable then signal the user "error:
          connection reset", drop the segment, enter CLOSED state,
          delete TCB, and return.  Otherwise (no ACK) drop the segment
          and return.
          </t></list></t>
        </list></t>
      <t>
      third check the security and precedence
        <list>
        <t>
        If the security/compartment in the segment does not exactly
        match the security/compartment in the TCB, send a reset
          <list>
          <t>
          If there is an ACK
            <list>
            <t>
            &lt;SEQ=SEG.ACK>&lt;CTL=RST>
            </t></list></t>
          <t>
          Otherwise
            <list>
            <t>
            &lt;SEQ=0>&lt;ACK=SEG.SEQ+SEG.LEN>&lt;CTL=RST,ACK>
            </t></list></t>
          </list></t>
        <t>
        If there is an ACK
          <list>
          <t>
          The precedence in the segment must match the precedence in the
          TCB, if not, send a reset
            <list>
            <t>
            &lt;SEQ=SEG.ACK>&lt;CTL=RST>
            </t></list></t>
          </list></t>
        <t>
        If there is no ACK
          <list>
          <t>
          If the precedence in the segment is higher than the precedence
          in the TCB then if allowed by the user and the system raise
          the precedence in the TCB to that in the segment, if not
          allowed to raise the prec then send a reset.
            <list>
            <t>
            &lt;SEQ=0>&lt;ACK=SEG.SEQ+SEG.LEN>&lt;CTL=RST,ACK>
            </t></list></t>
          <t>
          If the precedence in the segment is lower than the precedence
          in the TCB continue.
          </t></list></t>
        <t>
        If a reset was sent, discard the segment and return.
        </t></list></t>
      <t>
      fourth check the SYN bit
        <list>
        <t>
        This step should be reached only if the ACK is ok, or there is
        no ACK, and it the segment did not contain a RST.
        </t>
        <t>
        If the SYN bit is on and the security/compartment and precedence
        are acceptable then, RCV.NXT is set to SEG.SEQ+1, IRS is set to
        SEG.SEQ.  SND.UNA should be advanced to equal SEG.ACK (if there
        is an ACK), and any segments on the retransmission queue which
        are thereby acknowledged should be removed.
        </t>
        <t>
        If SND.UNA > ISS (our SYN has been ACKed), change the connection
        state to ESTABLISHED, form an ACK segment
          <list>
          <t>
          &lt;SEQ=SND.NXT>&lt;ACK=RCV.NXT>&lt;CTL=ACK>
          </t></list></t>
        <t>
        and send it.  Data or controls which were queued for
        transmission may be included.  If there are other controls or
        text in the segment then continue processing at the sixth step
        below where the URG bit is checked, otherwise return.
        </t>
        <t>
        Otherwise enter SYN-RECEIVED, form a SYN,ACK segment
          <list>
          <t>
          &lt;SEQ=ISS>&lt;ACK=RCV.NXT>&lt;CTL=SYN,ACK>
          </t></list></t>
        <t>
        and send it.  Set the variables:
        <list>
        <t>SND.WND &lt;- SEG.WND<vspace />
        SND.WL1 &lt;- SEG.SEQ<vspace />
        SND.WL2 &lt;- SEG.ACK</t>
        </list>

        If there are other controls or text in the
        segment, queue them for processing after the ESTABLISHED state
        has been reached, return.
        </t></list></t>
      <t>
      fifth, if neither of the SYN or RST bits is set then drop the
      segment and return.
      </t></list></t>
    <t>
    Otherwise,
    </t>
    <t>
    first check sequence number
      <list>
      <t>
      SYN-RECEIVED STATE<vspace />
      ESTABLISHED STATE<vspace />
      FIN-WAIT-1 STATE<vspace />
      FIN-WAIT-2 STATE<vspace />
      CLOSE-WAIT STATE<vspace />
      CLOSING STATE<vspace />
      LAST-ACK STATE<vspace />
      TIME-WAIT STATE
        <list>
        <t>
        Segments are processed in sequence.  Initial tests on arrival
        are used to discard old duplicates, but further processing is
        done in SEG.SEQ order.  If a segment's contents straddle the
        boundary between old and new, only the new parts should be
        processed.
        </t>
        <t>
        There are four cases for the acceptability test for an incoming
        segment:
        </t>
<t><figure><artwork>
      Segment Receive  Test
      Length  Window
      ------- -------  -------------------------------------------

         0       0     SEG.SEQ = RCV.NXT

         0      >0     RCV.NXT =&lt; SEG.SEQ &lt; RCV.NXT+RCV.WND

        >0       0     not acceptable

        >0      >0     RCV.NXT =&lt; SEG.SEQ &lt; RCV.NXT+RCV.WND
                    or RCV.NXT =&lt; SEG.SEQ+SEG.LEN-1 &lt; RCV.NXT+RCV.WND
</artwork></figure></t>
        <t>
        If the RCV.WND is zero, no segments will be acceptable, but
        special allowance should be made to accept valid ACKs, URGs and
        RSTs.
        </t>
        <t>
        If an incoming segment is not acceptable, an acknowledgment
        should be sent in reply (unless the RST bit is set, if so drop
        the segment and return):
          <list>
          <t>
          &lt;SEQ=SND.NXT>&lt;ACK=RCV.NXT>&lt;CTL=ACK>
          </t></list></t>
        <t>
        After sending the acknowledgment, drop the unacceptable segment
        and return.
        </t>
        <t>
        In the following it is assumed that the segment is the idealized
        segment that begins at RCV.NXT and does not exceed the window.
        One could tailor actual segments to fit this assumption by
        trimming off any portions that lie outside the window (including
        SYN and FIN), and only processing further if the segment then
        begins at RCV.NXT.  Segments with higher beginning sequence
        numbers should be held for later processing.
        </t>
        <t>
            In general, the processing of received segments MUST be
            implemented to aggregate ACK segments whenever possible.
            For example, if the TCP is processing a series of queued
            segments, it MUST process them all before sending any ACK
            segments. (TODO - see if there's a better place for this paragraph - taken from RFC1122)
        </t></list></t>
    <t>
    second check the RST bit,
      <list>
      <t>
      SYN-RECEIVED STATE
        <list>
        <t>
        If the RST bit is set
          <list>
          <t>
          If this connection was initiated with a passive OPEN (i.e.,
          came from the LISTEN state), then return this connection to
          LISTEN state and return.  The user need not be informed.  If
          this connection was initiated with an active OPEN (i.e., came
          from SYN-SENT state) then the connection was refused, signal
          the user "connection refused".  In either case, all segments
          on the retransmission queue should be removed.  And in the
          active OPEN case, enter the CLOSED state and delete the TCB,
          and return.
          </t></list></t>
        </list></t>
      <t>
      ESTABLISHED<vspace />
      FIN-WAIT-1<vspace />
      FIN-WAIT-2<vspace />
      CLOSE-WAIT
        <list>
        <t>
        If the RST bit is set then, any outstanding RECEIVEs and SEND
        should receive "reset" responses.  All segment queues should be
        flushed.  Users should also receive an unsolicited general
        "connection reset" signal.  Enter the CLOSED state, delete the
        TCB, and return.
        </t></list></t>
      <t>
      CLOSING STATE<vspace />
      LAST-ACK STATE<vspace />
      TIME-WAIT<vspace />
        <list>
        <t>
        If the RST bit is set then, enter the CLOSED state, delete the
        TCB, and return.
        </t></list></t>
      </list></t>
    <t>
    third check security and precedence
      <list>
      <t>
      SYN-RECEIVED
        <list>
        <t>
        If the security/compartment and precedence in the segment do not
        exactly match the security/compartment and precedence in the TCB
        then send a reset, and return.
        </t></list></t>
      <t>
      ESTABLISHED<vspace />
      FIN-WAIT-1<vspace />
      FIN-WAIT-2<vspace />
      CLOSE-WAIT<vspace />
      CLOSING<vspace />
      LAST-ACK<vspace />
      TIME-WAIT
        <list>
        <t>
        If the security/compartment and precedence in the segment do not
        exactly match the security/compartment and precedence in the TCB
        then send a reset, any outstanding RECEIVEs and SEND should
        receive "reset" responses.  All segment queues should be
        flushed.  Users should also receive an unsolicited general
        "connection reset" signal.  Enter the CLOSED state, delete the
        TCB, and return.
        </t></list></t>
      <t>
      Note this check is placed following the sequence check to prevent
      a segment from an old connection between these ports with a
      different security or precedence from causing an abort of the
      current connection.
      </t>
    </list></t>
    <t>
    fourth, check the SYN bit,
      <list>
      <t>
      SYN-RECEIVED<vspace />
      ESTABLISHED STATE<vspace />
      FIN-WAIT STATE-1<vspace />
      FIN-WAIT STATE-2<vspace />
      CLOSE-WAIT STATE<vspace />
      CLOSING STATE<vspace />
      LAST-ACK STATE<vspace />
      TIME-WAIT STATE
        <list>
        <t>
            TODO: need to incorporate RFC 1122 4.2.2.20(e) here
        </t>
        <t>
        If the SYN is in the window it is an error, send a reset, any
        outstanding RECEIVEs and SEND should receive "reset" responses,
        all segment queues should be flushed, the user should also
        receive an unsolicited general "connection reset" signal, enter
        the CLOSED state, delete the TCB, and return.
        </t>
        <t>
        If the SYN is not in the window this step would not be reached
        and an ack would have been sent in the first step (sequence
        number check).
        </t>
        </list></t>
      </list></t>
    <t>
    fifth check the ACK field,
      <list>
      <t>
      if the ACK bit is off drop the segment and return
      </t>
      <t>
      if the ACK bit is on
        <list>
        <t>
        SYN-RECEIVED STATE
          <list>
          <t>
          If SND.UNA &lt; SEG.ACK =&lt; SND.NXT then enter ESTABLISHED state
          and continue processing with variables below set to:
          <list>
          <t>SND.WND &lt;- SEG.WND<vspace />
          SND.WL1 &lt;- SEG.SEQ<vspace />
          SND.WL2 &lt;- SEG.ACK</t>
          </list>
            <list>
            <t>
            If the segment acknowledgment is not acceptable, form a
            reset segment,
              <list>
              <t>
              &lt;SEQ=SEG.ACK>&lt;CTL=RST>
              </t></list></t>
            <t>
            and send it.
            </t>
            </list></t></list></t>
        <t>
        ESTABLISHED STATE
          <list>
          <t>
          If SND.UNA &lt; SEG.ACK =&lt; SND.NXT then, set SND.UNA &lt;- SEG.ACK.
          Any segments on the retransmission queue which are thereby
          entirely acknowledged are removed.  Users should receive
          positive acknowledgments for buffers which have been SENT and
          fully acknowledged (i.e., SEND buffer should be returned with
          "ok" response).  If the ACK is a duplicate
          (SEG.ACK =&lt; SND.UNA), it can be ignored.  If the ACK acks
          something not yet sent (SEG.ACK > SND.NXT) then send an ACK,
          drop the segment, and return.
          </t>
          <t>
          If SND.UNA =&lt; SEG.ACK =&lt; SND.NXT, the send window should be
          updated.  If (SND.WL1 &lt; SEG.SEQ or (SND.WL1 = SEG.SEQ and
          SND.WL2 =&lt; SEG.ACK)), set SND.WND &lt;- SEG.WND, set
          SND.WL1 &lt;- SEG.SEQ, and set SND.WL2 &lt;- SEG.ACK.
          </t>
          <t>
          Note that SND.WND is an offset from SND.UNA, that SND.WL1
          records the sequence number of the last segment used to update
          SND.WND, and that SND.WL2 records the acknowledgment number of
          the last segment used to update SND.WND.  The check here
          prevents using old segments to update the window.
          </t>
          </list></t>
        <t>
        FIN-WAIT-1 STATE
          <list>
          <t>
          In addition to the processing for the ESTABLISHED state, if
          our FIN is now acknowledged then enter FIN-WAIT-2 and continue
          processing in that state.
          </t></list></t>
        <t>
        FIN-WAIT-2 STATE
          <list>
          <t>
          In addition to the processing for the ESTABLISHED state, if
          the retransmission queue is empty, the user's CLOSE can be
          acknowledged ("ok") but do not delete the TCB.
          </t></list></t>
        <t>
        CLOSE-WAIT STATE
          <list>
          <t>
          Do the same processing as for the ESTABLISHED state.
          </t></list></t>
        <t>
        CLOSING STATE
          <list>
          <t>
          In addition to the processing for the ESTABLISHED state, if
          the ACK acknowledges our FIN then enter the TIME-WAIT state,
          otherwise ignore the segment.
          </t></list></t>
        <t>
        LAST-ACK STATE
          <list>
          <t>
          The only thing that can arrive in this state is an
          acknowledgment of our FIN.  If our FIN is now acknowledged,
          delete the TCB, enter the CLOSED state, and return.
          </t></list></t>
        <t>
        TIME-WAIT STATE
          <list>
          <t>
          The only thing that can arrive in this state is a
          retransmission of the remote FIN.  Acknowledge it, and restart
          the 2 MSL timeout.
          </t></list></t>
        </list></t>
      </list></t>
    <t>
    sixth, check the URG bit,
      <list>
      <t>
      ESTABLISHED STATE<vspace />
      FIN-WAIT-1 STATE<vspace />
      FIN-WAIT-2 STATE
        <list>
        <t>
        If the URG bit is set, RCV.UP &lt;- max(RCV.UP,SEG.UP), and signal
        the user that the remote side has urgent data if the urgent
        pointer (RCV.UP) is in advance of the data consumed.  If the
        user has already been signaled (or is still in the &quot;urgent
        mode&quot;) for this continuous sequence of urgent data, do not
        signal the user again.
        </t></list></t>
      <t>
      CLOSE-WAIT STATE<vspace />
      CLOSING STATE<vspace />
      LAST-ACK STATE<vspace />
      TIME-WAIT
        <list>
        <t>
        This should not occur, since a FIN has been received from the
        remote side.  Ignore the URG.
        </t></list></t>
    </list></t>
    <t>
    seventh, process the segment text,
      <list>
      <t>
      ESTABLISHED STATE<vspace />
      FIN-WAIT-1 STATE<vspace />
      FIN-WAIT-2 STATE
        <list>
        <t>
        Once in the ESTABLISHED state, it is possible to deliver segment
        text to user RECEIVE buffers.  Text from segments can be moved
        into buffers until either the buffer is full or the segment is
        empty.  If the segment empties and carries an PUSH flag, then
        the user is informed, when the buffer is returned, that a PUSH
        has been received.
        </t>
        <t>
        When the TCP takes responsibility for delivering the data to the
        user it must also acknowledge the receipt of the data.
        </t>
        <t>
        Once the TCP takes responsibility for the data it advances
        RCV.NXT over the data accepted, and adjusts RCV.WND as
        appropriate to the current buffer availability.  The total of
        RCV.NXT and RCV.WND should not be reduced.
        </t>
        <t>
        A TCP MAY send an ACK segment acknowledging RCV.NXT when a
        valid segment arrives that is in the window but not at the
        left window edge.
        </t>
        <t>
        Please note the window management suggestions in section 3.7.
        </t>
        <t>
        Send an acknowledgment of the form:
          <list><t>
          &lt;SEQ=SND.NXT>&lt;ACK=RCV.NXT>&lt;CTL=ACK>
          </t></list></t>
        <t>
        This acknowledgment should be piggybacked on a segment being
        transmitted if possible without incurring undue delay.
        </t>
        </list></t>
      <t>
      CLOSE-WAIT STATE<vspace />
      CLOSING STATE<vspace />
      LAST-ACK STATE<vspace />
      TIME-WAIT STATE
        <list>
        <t>
        This should not occur, since a FIN has been received from the
        remote side.  Ignore the segment text.
        </t>
        </list>
      </t>
      </list></t>
    <t>
    eighth, check the FIN bit,
      <list>
      <t>
      Do not process the FIN if the state is CLOSED, LISTEN or SYN-SENT
      since the SEG.SEQ cannot be validated; drop the segment and
      return.
      </t>
      <t>
      If the FIN bit is set, signal the user "connection closing" and
      return any pending RECEIVEs with same message, advance RCV.NXT
      over the FIN, and send an acknowledgment for the FIN.  Note that
      FIN implies PUSH for any segment text not yet delivered to the
      user.
        <list>
        <t>
        SYN-RECEIVED STATE<vspace />
        ESTABLISHED STATE
          <list>
          <t>
          Enter the CLOSE-WAIT state.
          </t></list></t>
        <t>
        FIN-WAIT-1 STATE
          <list>
          <t>
          If our FIN has been ACKed (perhaps in this segment), then
          enter TIME-WAIT, start the time-wait timer, turn off the other
          timers; otherwise enter the CLOSING state.
          </t></list></t>
        <t>
        FIN-WAIT-2 STATE
          <list>
          <t>
          Enter the TIME-WAIT state.  Start the time-wait timer, turn
          off the other timers.
          </t></list></t>
        <t>
        CLOSE-WAIT STATE
          <list>
          <t>
          Remain in the CLOSE-WAIT state.
          </t></list></t>
        <t>
        CLOSING STATE
          <list>
          <t>
          Remain in the CLOSING state.
          </t></list></t>
        <t>
        LAST-ACK STATE
          <list>
          <t>
          Remain in the LAST-ACK state.
          </t></list></t>
        <t>
        TIME-WAIT STATE
          <list>
          <t>
          Remain in the TIME-WAIT state.  Restart the 2 MSL time-wait
          timeout.
          </t></list></t>
        </list></t>
      </list></t>
    <t>
    and return.
    </t>
    </list>
  </t>
  </list>
</t>
<t><vspace blankLines="999"/></t>
<t>
USER TIMEOUT
<list>
<t>
  USER TIMEOUT
  <list>
  <t>
    For any state if the user timeout expires, flush all queues, signal
    the user "error:  connection aborted due to user timeout" in general
    and for any outstanding calls, delete the TCB, enter the CLOSED
    state and return.
  </t>
  </list>
</t>
<t>
  RETRANSMISSION TIMEOUT
  <list>
  <t>
    For any state if the retransmission timeout expires on a segment in
    the retransmission queue, send the segment at the front of the
    retransmission queue again, reinitialize the retransmission timer,
    and return.
  </t>
  </list>
</t>
<t>
  TIME-WAIT TIMEOUT
  <list>
  <t>
    If the time-wait timeout expires on a connection delete the TCB,
    enter the CLOSED state and return.
  </t>
  </list>
</t>
</list>
</t>
<t><vspace blankLines="999"/></t>
</section>
<section title="Glossary">
<t>
<list style="hanging" hangIndent="8">
<t hangText="1822">
          BBN Report 1822, "The Specification of the Interconnection of
          a Host and an IMP".  The specification of interface between a
          host and the ARPANET.</t>

<t hangText="ACK"><vspace />
          A control bit (acknowledge) occupying no sequence space, which
          indicates that the acknowledgment field of this segment
          specifies the next sequence number the sender of this segment
          is expecting to receive, hence acknowledging receipt of all
          previous sequence numbers.</t>

<t hangText="ARPANET message"><vspace />
          The unit of transmission between a host and an IMP in the
          ARPANET.  The maximum size is about 1012 octets (8096 bits).</t>

<t hangText="ARPANET packet"><vspace />
          A unit of transmission used internally in the ARPANET between
          IMPs.  The maximum size is about 126 octets (1008 bits).</t>

<t hangText="connection"><vspace />
          A logical communication path identified by a pair of sockets.</t>

<t hangText="datagram"><vspace />
          A message sent in a packet switched computer communications
          network.</t>

<t hangText="Destination Address"><vspace />
          The destination address, usually the network and host
          identifiers.</t>

<t hangText="FIN"><vspace />
          A control bit (finis) occupying one sequence number, which
          indicates that the sender will send no more data or control
          occupying sequence space.</t>

<t hangText="fragment"><vspace />
          A portion of a logical unit of data, in particular an internet
          fragment is a portion of an internet datagram.</t>

<t hangText="FTP"><vspace />
          A file transfer protocol.</t>

<t hangText="header"><vspace />
          Control information at the beginning of a message, segment,
          fragment, packet or block of data.</t>

<t hangText="host"><vspace />
          A computer.  In particular a source or destination of messages
          from the point of view of the communication network.</t>

<t hangText="Identification"><vspace />
          An Internet Protocol field.  This identifying value assigned
          by the sender aids in assembling the fragments of a datagram.</t>

<t hangText="IMP"><vspace />
          The Interface Message Processor, the packet switch of the
          ARPANET.</t>

<t hangText="internet address"><vspace />
          A source or destination address specific to the host level.</t>

<t hangText="internet datagram"><vspace />
          The unit of data exchanged between an internet module and the
          higher level protocol together with the internet header.</t>

<t hangText="internet fragment"><vspace />
          A portion of the data of an internet datagram with an internet
          header.</t>

<t hangText="IP"><vspace />
          Internet Protocol.</t>

<t hangText="IRS"><vspace />
          The Initial Receive Sequence number.  The first sequence
          number used by the sender on a connection.</t>

<t hangText="ISN"><vspace />
          The Initial Sequence Number.  The first sequence number used
          on a connection, (either ISS or IRS).  Selected in a way that is unique within a given period of time and is unpredictable to attackers.</t>

<t hangText="ISS"><vspace />
          The Initial Send Sequence number.  The first sequence number
          used by the sender on a connection.</t>

<t hangText="leader"><vspace />
          Control information at the beginning of a message or block of
          data.  In particular, in the ARPANET, the control information
          on an ARPANET message at the host-IMP interface.</t>

<t hangText="left sequence"><vspace />
          This is the next sequence number to be acknowledged by the
          data receiving TCP (or the lowest currently unacknowledged
          sequence number) and is sometimes referred to as the left edge
          of the send window.</t>

<t hangText="local packet"><vspace />
          The unit of transmission within a local network.</t>

<t hangText="module"><vspace />
          An implementation, usually in software, of a protocol or other
          procedure.</t>

<t hangText="MSL"><vspace />
          Maximum Segment Lifetime, the time a TCP segment can exist in
          the internetwork system.  Arbitrarily defined to be 2 minutes.</t>

<t hangText="octet"><vspace />
          An eight bit byte.</t>

<t hangText="Options"><vspace />
          An Option field may contain several options, and each option
          may be several octets in length.  The options are used
          primarily in testing situations; for example, to carry
          timestamps.  Both the Internet Protocol and TCP provide for
          options fields.</t>

<t hangText="packet"><vspace />
          A package of data with a header which may or may not be
          logically complete.  More often a physical packaging than a
          logical packaging of data.</t>

<t hangText="port"><vspace />
          The portion of a socket that specifies which logical input or
          output channel of a process is associated with the data.</t>

<t hangText="process"><vspace />
          A program in execution.  A source or destination of data from
          the point of view of the TCP or other host-to-host protocol.</t>

<t hangText="PUSH"><vspace />
          A control bit occupying no sequence space, indicating that
          this segment contains data that must be pushed through to the
          receiving user.</t>

<t hangText="RCV.NXT"><vspace />
          receive next sequence number</t>

<t hangText="RCV.UP"><vspace />
          receive urgent pointer</t>

<t hangText="RCV.WND"><vspace />
          receive window</t>

<t hangText="receive next sequence number"><vspace />
          This is the next sequence number the local TCP is expecting to
              receive.</t>

<t hangText="receive window"><vspace />
          This represents the sequence numbers the local (receiving) TCP
          is willing to receive.  Thus, the local TCP considers that
          segments overlapping the range RCV.NXT to
          RCV.NXT + RCV.WND - 1 carry acceptable data or control.
          Segments containing sequence numbers entirely outside of this
          range are considered duplicates and discarded.</t>

<t hangText="RST"><vspace />
          A control bit (reset), occupying no sequence space, indicating
          that the receiver should delete the connection without further
          interaction.  The receiver can determine, based on the
          sequence number and acknowledgment fields of the incoming
          segment, whether it should honor the reset command or ignore
          it.  In no case does receipt of a segment containing RST give
          rise to a RST in response.</t>

<t hangText="RTP"><vspace />
          Real Time Protocol:  A host-to-host protocol for communication
          of time critical information.</t>

<t hangText="SEG.ACK"><vspace />
          segment acknowledgment</t>

<t hangText="SEG.LEN"><vspace />
          segment length</t>

<t hangText="SEG.PRC"><vspace />
          segment precedence value</t>

<t hangText="SEG.SEQ"><vspace />
          segment sequence</t>

<t hangText="SEG.UP"><vspace />
          segment urgent pointer field</t>

<t hangText="SEG.WND"><vspace />
          segment window field</t>

<t hangText="segment"><vspace />
          A logical unit of data, in particular a TCP segment is the
          unit of data transfered between a pair of TCP modules.</t>

<t hangText="segment acknowledgment"><vspace />
          The sequence number in the acknowledgment field of the
          arriving segment.</t>

<t hangText="segment length"><vspace />
          The amount of sequence number space occupied by a segment,
          including any controls which occupy sequence space.</t>

<t hangText="segment sequence"><vspace />
          The number in the sequence field of the arriving segment.</t>

<t hangText="send sequence"><vspace />
          This is the next sequence number the local (sending) TCP will
          use on the connection.  It is initially selected from an
          initial sequence number curve (ISN) and is incremented for
          each octet of data or sequenced control transmitted.</t>

<t hangText="send window"><vspace />
          This represents the sequence numbers which the remote
          (receiving) TCP is willing to receive.  It is the value of the
          window field specified in segments from the remote (data
          receiving) TCP.  The range of new sequence numbers which may
          be emitted by a TCP lies between SND.NXT and
          SND.UNA + SND.WND - 1. (Retransmissions of sequence numbers
          between SND.UNA and SND.NXT are expected, of course.)</t>

<t hangText="SND.NXT"><vspace />
          send sequence</t>

<t hangText="SND.UNA"><vspace />
          left sequence</t>

<t hangText="SND.UP"><vspace />
          send urgent pointer</t>

<t hangText="SND.WL1"><vspace />
          segment sequence number at last window update</t>

<t hangText="SND.WL2"><vspace />
          segment acknowledgment number at last window update</t>

<t hangText="SND.WND"><vspace />
          send window</t>

<t hangText="socket"><vspace />
          An address which specifically includes a port identifier, that
          is, the concatenation of an Internet Address with a TCP port.</t>

<t hangText="Source Address"><vspace />
          The source address, usually the network and host identifiers.</t>

<t hangText="SYN"><vspace />
          A control bit in the incoming segment, occupying one sequence
          number, used at the initiation of a connection, to indicate
          where the sequence numbering will start.</t>

<t hangText="TCB"><vspace />
          Transmission control block, the data structure that records
          the state of a connection.</t>

<t hangText="TCB.PRC"><vspace />
          The precedence of the connection.</t>

<t hangText="TCP"><vspace />
          Transmission Control Protocol:  A host-to-host protocol for
          reliable communication in internetwork environments.</t>

<t hangText="TOS"><vspace />
          Type of Service, an Internet Protocol field.</t>

<t hangText="Type of Service"><vspace />
          An Internet Protocol field which indicates the type of service
          for this internet fragment.</t>

<t hangText="URG"><vspace />
          A control bit (urgent), occupying no sequence space, used to
          indicate that the receiving user should be notified to do
          urgent processing as long as there is data to be consumed with
          sequence numbers less than the value indicated in the urgent
          pointer.</t>

<t hangText="urgent pointer"><vspace />
          A control field meaningful only when the URG bit is on.  This
          field communicates the value of the urgent pointer which
          indicates the data octet associated with the sending user's
          urgent call.</t>
</list>
</t>

</section>

    </section>
    <section anchor="changes" title="Changes from RFC 793">
        <?rfc subcompact="yes" ?>
        <t>
            This document obsoletes RFC 793 as well as RFC 6093 and 6528, which updated 793.  In all cases, only the normative protocol specification and requirements have been incorporated into this document, and the informational text with background and rationale has not been carried in.  The informational content of those documents is still valuable in learning about and understanding TCP, and they are valid Informational references, even though their normative content has been incorporated into this document.
        </t>
        <t>
            The main body of this document was adapted from RFC 793's Section 3, titled "FUNCTIONAL SPECIFICATION", with an attempt to keep formatting and layout as close as possible.
        </t>
        <t>
            The collection of applicable RFC Errata that have been reported and either accepted or held for an update to RFC 793 were incorporated (Errata IDs: 573, 574, 700, 701, 1283, 1561, 1562, 1564, 1565, 1571, 1572, 2296, 2297, 2298, 2748, 2749, 2934, 3213, 3300, 3301).  Some errata were not applicable due to other changes (Errata IDs: 572, 575, 1569, 3602).  TODO: 3305
        </t>
        <t>
            Changes to the specification of the Urgent Pointer described in RFC 1122 and 6093 were incorporated.  See RFC 6093 for detailed discussion of why these changes were necessary.
        </t>
        <t>
The discussion of the RTO from RFC 793 was updated to refer to RFC 6298.  The RFC 1122 text on the RTO originally replaced the 793 text, however, RFC 2988 should have updated 1122, and has subsequently been obsoleted by 6298.
        </t>
        <t>
RFC 1122 contains a collection of other changes and clarifications to RFC 793.  The normative items impacting the protocol have been incorporated here, though some historically useful implementation advice and informative discussion from RFC 1122 is not included here.
        </t>
        <t>
RFC 1122 contains more than just TCP requirements, so this document can't obsolete RFC 1122 entirely.  It is only marked as &quot;updating&quot; 1122, however, it should be understood to effectively obsolete all of the RFC 1122 material on TCP.
        </t>
        <t>
            The more secure Initial Sequence Number generation algorithm from RFC 6528 was incorporated.  See RFC 6528 for discussion of the attacks that this mitigates, as well as advice on selecting PRF algorithms and managing secret key data.
        </t>
        <t>
A note based on RFC 6429 was added to explicitly clarify that system resource mangement concerns allow connection resources to be reclaimed.  RFC 6429 is obsoleted in the sense that this clarification has been reflected in this update to the base TCP specification now.
        </t>
        <t>
            RFC EDITOR'S NOTE: the content below is for detailed change tracking and planning, and not to be included with the final revision of the document.
        </t>
        <t>
            This document started as draft-eddy-rfc793bis-00, that was merely a proposal and rough plan for updating RFC 793.
        </t>
        <t>
            The -01 revision of this draft-eddy-rfc793bis incorporates the content of RFC 793 Section 3 titled "FUNCTIONAL SPECIFICATION".  Other content from RFC 793 has not been incorporated.  The -01 revision of this document makes some minor formatting changes to the RFC 793 content in order to convert the content into XML2RFC format and account for left-out parts of RFC 793.  For instance, figure numbering differs and some indentation is not exactly the same.
        </t>
        <t>
            The -02 revision of draft-eddy-rfc793bis incorporates errata that have been verified:
            <list>
                <t>Errata ID 573: Reported by Bob Braden (note: This errata basically is just a reminder that RFC 1122 updates 793.  Some of the associated changes are left pending to a separate revision that incorporates 1122.  Bob's mention of PUSH in 793 section 2.8 was not applicable here because that section was not part of the "functional specification".  Also the 1122 text on the retransmission timeout also has been updated by subsequent RFCs, so the change here deviates from Bob's suggestion to apply the 1122 text.)</t>
                <t>Errata ID 574: Reported by Yin Shuming</t>
                <t>Errata ID 700: Reported by Yin Shuming</t>
                <t>Errata ID 701: Reported by Yin Shuming</t>
                <t>Errata ID 1283: Reported by Pei-chun Cheng</t>
                <t>Errata ID 1561: Reported by Constantin Hagemeier</t>
                <t>Errata ID 1562: Reported by Constantin Hagemeier</t>
                <t>Errata ID 1564: Reported by Constantin Hagemeier</t>
                <t>Errata ID 1565: Reported by Constantin Hagemeier</t>
                <t>Errata ID 1571: Reported by Constantin Hagemeier</t>
                <t>Errata ID 1572: Reported by Constantin Hagemeier</t>
                <t>Errata ID 2296: Reported by Vishwas Manral</t>
                <t>Errata ID 2297: Reported by Vishwas Manral</t>
                <t>Errata ID 2298: Reported by Vishwas Manral</t>
                <t>Errata ID 2748: Reported by Mykyta Yevstifeyev</t>
                <t>Errata ID 2749: Reported by Mykyta Yevstifeyev</t>
                <t>Errata ID 2934: Reported by Constantin Hagemeier</t>
                <t>Errata ID 3213: Reported by EugnJun Yi</t>
                <t>Errata ID 3300: Reported by Botong Huang</t>
                <t>Errata ID 3301: Reported by Botong Huang</t>
                <t>Note: Some verified errata were not used in this update, as they relate to sections of RFC 793 elided from this document.  These include Errata ID 572, 575, and 1569.</t>
                <t>Note: Errata ID 3602 was not applied in this revision as it is duplicative of the 1122 corrections.</t>
                <t>There is an errata 3305 currently reported that need to be verified, held, or rejected by the ADs; it is addressing the same issue as draft-gont-tcpm-tcp-seq-validation and was not attempted to be applied to this document.</t>
            </list>
            Not related to RFC 793 content, this revision also makes small tweaks to the introductory text, fixes indentation of the pseudoheader diagram, and notes that the Security Considerations should also include privacy, when this section is written.
        </t>
        <t>
            The -03 revision of draft-eddy-rfc793bis revises all discussion of the urgent pointer in order to comply with RFC 6093, 1122, and 1011.  Since 1122 held requirements on the urgent pointer, the full list of requirements was brought into an appendix of this document, so that it can be updated as-needed.
        </t>
        <t>
            The -04 revision of draft-eddy-rfc793bis includes the ISN generation changes from RFC 6528.
        </t>
        <t>
            The -05 revision of draft-eddy-rfc793bis incorporates MSS requirements and definitions from RFC 879, 1122, and 6691, as well as option-handling
            requirements from RFC 1122.
        </t>
        <t>
            The -00 revision of draft-ietf-tcpm-rfc793bis incorporates several additional clarifications and updates to the section on segmentation, many of which are based on feedback from Joe Touch improving from the initial text on this in the previous revision.
        </t>
        <t>
            The -01 revision incorporates the change to Reserved bits due to ECN, as well as many other changes that come from RFC 1122.
        </t>
        <t>
            The -02 revision has small formating modifications in order to address xml2rfc warnings about long lines.  It was a quick update to avoid document expiration.  TCPM working group discussion in 2015 also indicated that that we should not try to add sections on implementation advice or similar non-normative information.
        </t>
        <t>
            The -03 revision incorporates more content from RFC 1122: Passive OPEN Calls, Time-To-Live, Multihoming, IP Options, ICMP messages, Data Communications, When to Send Data, When to Send a Window Update, Managing the Window, Probing Zero Windows, When to Send an ACK Segment.  The section on data communications was re-organized into clearer subsections (previously headings were embedded in the 793 text), and windows management advice from 793 was removed (as reviewed by TCPM working group) in favor of the 1122 additions on SWS, ZWP, and related topics.
        </t>
        <t>
           The -04 revision includes reference to RFC 6429 on the ZWP condition, RFC1122 material on TCP Connection Failures, TCP Keep-Alives, Acknowledging Queued Segments, and Remote Address Validation.  RTO computation is referenced from RFC 6298 rather than RFC 1122.

        </t>
        <t>TODO list of other planned changes (these can be added to or made more specific, as the document proceeds):
        <list style="numbers">
            <t>incorporate relevant parts of 3168 (ECN) - beyond just indicating the names of the 2 bits already done</t>
            <t>point to 5461 (soft errors)</t>
            <t>mention 5961 state machine option</t>
            <t>mention 6161 (reducing TIME-WAIT)</t>
            <t>TOS material does not take DSCP changes into account</t>
            <t>there is inconsistency between use of SYN_RCVD and SYNC-RECEIVED in diagrams and text in various places</t>
            <t>make sure that clarifications in RFC 1011 are captured</t>
        </list>
        </t>
        <t>TODO list of other potential changes, if there is TCPM consensus:
        <list style="numbers">
            <t>see draft-gont-tcpm-tcp-seccomp-prec</t>
            <t>incorporate Fernando's new number-checking fixes (if past the IESG in time)</t>
            <t>look at Tony Sabatini suggestion for describing DO field</t>
            <t>clearly specify treatment of reserved bits (see TCPM thread on EDO draft April 25, 2014)</t>
            <t>look at possible mention of draft-minshall-nagle (e.g. as in Linux)</t>
	    <t>per discussion with Joe Touch (TAPS list, 6/20/2015), the description of the API could be revisited</t>
        </list>
        </t> 
        <?rfc subcompact="no" ?>
    </section>

<!-- Possibly a 'Contributors' section ... -->

    <section anchor="IANA" title="IANA Considerations">
        <t>This memo includes no request to IANA.  Existing IANA registries for TCP
            parameters are sufficient.</t>
        <t>TODO: check whether entries pointing to 793 and other documents obsoleted
            by this one should be updated to point to this one instead.</t>
    </section>

    <section anchor="Security" title="Security and Privacy Considerations">
        <t>
        TODO
        </t>
        <t>
            See RFC 6093 <xref target="RFC6093"/> for discussion of security considerations related to the urgent pointer field.
        </t>
        <t>
        Editor's Note: Scott Brim mentioned that this should include a PERPASS/privacy review.
        </t>
    </section>
    <section title="Acknowledgements">
    <t>
    This document is largely a revision of RFC 793, which Jon Postel was the editor of.  Due to his excellent work, it was able to last for three decades before we felt the need to revise it.
    </t>
    <t>
    Andre Oppermann was a contributor and helped to edit the first revision of this document.
    </t>
    <t>
    We are thankful for the assistance of the IETF TCPM working group chairs:
    <list>
        <t>Michael Scharf<vspace />
           Yoshifumi Nishida<vspace />
           Pasi Sarolahti</t>
    </list>
    </t>
    <t>
    During early discussion of this work on the TCPM mailing list, and at the IETF 88 meeting in Vancouver, helpful comments, critiques, and reviews were received from (listed alphebetically): David Borman, Yuchung Cheng, Martin Duke, Kevin Lahey, Kevin Mason, Matt Mathis, Hagen Paul Pfeifer, Anthony Sabatini, Joe Touch, Reji Varghese, Lloyd Wood, and Alex Zimmermann.
    </t>
    <t>
    This document includes content from errata that were reported by (listed chronologically): Yin Shuming, Bob Braden, Morris M. Keesan, Pei-chun Cheng, Constantin Hagemeier, Vishwas Manral, Mykyta Yevstifeyev, EungJun Yi, Botong Huang.
    </t>
    </section>
</middle>

<!--  *****BACK MATTER ***** -->
<back>
    <!-- References split to informative and normative -->
    <references title="Normative References">
         <!-- A *really* full, totally OTT reference - Note, the "target" attribute of the 
	     "reference": if you want a URI printed in the reference, this is where it goes. -->
        <!--
        <reference anchor='RFC2119'
                   target='http://xml.resource.org/public/rfc/html/rfc2119.html'>
            <front>
                <title abbrev='RFC Key Words'>Key words for use in RFCs to Indicate Requirement 
                Levels</title>
                <author initials='S.' surname='Bradner' fullname='Scott Bradner'>
                    <organization>Harvard University</organization>
                    <address>
                        <postal>
                            <street>1350 Mass. Ave.</street>
                            <street>Cambridge</street>
                            <street>MA 02138</street>
                        </postal>
                        <phone>- +1 617 495 3864</phone>
                        <email>sob@harvard.edu</email>
                    </address>
                </author>
                <date year='1997' month='March' />
                <area>General</area>
                <keyword>keyword</keyword>
                <abstract>
                    <t>In many standards track documents several words are used to signify
                    the requirements in the specification.  These words are often
                    capitalized.  This document defines these words as they should be
                    interpreted in IETF documents.  Authors who follow these guidelines
                    should incorporate this phrase near the beginning of their document:

                        <list>
                            <t>
                            The key words &quot;MUST&quot;, &quot;MUST NOT&quot;, 
                            &quot;REQUIRED&quot;, &quot;SHALL&quot;, &quot;SHALL NOT&quot;,
                            &quot;SHOULD&quot;, &quot;SHOULD NOT&quot;, &quot;RECOMMENDED&quot;,
                            &quot;MAY&quot;, and &quot;OPTIONAL&quot; in this document are to be 
                            interpreted as described in RFC 2119.</t>
                        </list>
                    </t>
                    <t> 
                    Note that the force of these words is modified by the requirement level of 
                    the document in which they are used.</t>
                </abstract> 
            </front>

            <seriesInfo name='BCP' value='14' />
            <seriesInfo name='RFC' value='2119' />
            <format type='TXT' octets='4723' target='ftp://ftp.isi.edu/in-notes/rfc2119.txt' />
            <format type='HTML' octets='14486' 
                    target='http://xml.resource.org/public/rfc/html/rfc2119.html' />
            <format type='XML' octets='5661' 
                    target='http://xml.resource.org/public/rfc/xml/rfc2119.xml' />
        </reference>
    -->

        <!-- Right back at the beginning we defined an entity which (we asserted) would contain
             XML needed for a reference... this is where we use it. -->
        &RFC0791;
        &RFC1191;
        &RFC1981;
        &RFC2119;
        &RFC2675;
        &RFC2923;
        &RFC6298;
    </references>

    <references title="Informative References">
        <!-- A reference written by by an organization not a persoN. -->
        <!--
        <reference
            anchor="DOMINATION" >
            <front>
                <title>Ultimate Plan for Taking Over the World</title>
                <author>
                    <organization>Mad Dominators, Inc.</organization>
                </author>
                <date year="1984" />
            </front>
        </reference>
        -->
        &RFC0793;
        &RFC0896;
        &RFC1122;
        <!--&RFC1191;-->
        <!--&RFC1981;-->
        <!--&RFC2675;-->
        <!--&RFC2923;-->
        &RFC4821;
        &RFC5044;
        &RFC5681;
        &RFC5795;
        &RFC6093;

        &RFC6429;
        &RFC6528;
        &RFC6691;
        &RFC7323;
        &RFC7414;
    </references>
    <section title="TCP Requirement Summary">
        <t>This section is adapted from RFC 1122.</t>
        <t>TODO: this needs to be seriously redone, to use 793bis section numbers instead of 1122 ones, the RFC1122 heading should be removed, and all 1122 requirements need to be reflected in 793bis text.</t>
        <t>TODO: NOTE that PMTUD+PLPMTUD is not included in this table of recommendations.</t>
<figure>
    <artwork>

                                                 |        | | | |S| |
                                                 |        | | | |H| |F
                                                 |        | | | |O|M|o
                                                 |        | |S| |U|U|o
                                                 |        | |H| |L|S|t
                                                 |        |M|O| |D|T|n
                                                 |        |U|U|M| | |o
                                                 |        |S|L|A|N|N|t
                                                 |RFC1122 |T|D|Y|O|O|t
FEATURE                                          |SECTION | | | |T|T|e
-------------------------------------------------|--------|-|-|-|-|-|--
                                                 |        | | | | | |
Push flag                                        |        | | | | | |
  Aggregate or queue un-pushed data              |4.2.2.2 | | |x| | |
  Sender collapse successive PSH flags           |4.2.2.2 | |x| | | |
  SEND call can specify PUSH                     |4.2.2.2 | | |x| | |
    If cannot: sender buffer indefinitely        |4.2.2.2 | | | | |x|
    If cannot: PSH last segment                  |4.2.2.2 |x| | | | |
  Notify receiving ALP of PSH                    |4.2.2.2 | | |x| | |1
  Send max size segment when possible            |4.2.2.2 | |x| | | |
                                                 |        | | | | | |
Window                                           |        | | | | | |
  Treat as unsigned number                       |4.2.2.3 |x| | | | |
  Handle as 32-bit number                        |4.2.2.3 | |x| | | |
  Shrink window from right                       |4.2.2.16| | | |x| |
  Robust against shrinking window                |4.2.2.16|x| | | | |
  Receiver's window closed indefinitely          |4.2.2.17| | |x| | |
  Sender probe zero window                       |4.2.2.17|x| | | | |
    First probe after RTO                        |4.2.2.17| |x| | | |
    Exponential backoff                          |4.2.2.17| |x| | | |
  Allow window stay zero indefinitely            |4.2.2.17|x| | | | |
  Sender timeout OK conn with zero wind          |4.2.2.17| | | | |x|
                                                 |        | | | | | |
Urgent Data                                      |        | | | | | |
  Pointer indicates first non-urgent octet       |4.2.2.4 |x| | | | |
  Arbitrary length urgent data sequence          |4.2.2.4 |x| | | | |
  Inform ALP asynchronously of urgent data       |4.2.2.4 |x| | | | |1
  ALP can learn if/how much urgent data Q'd      |4.2.2.4 |x| | | | |1
                                                 |        | | | | | |
TCP Options                                      |        | | | | | |
  Receive TCP option in any segment              |4.2.2.5 |x| | | | |
  Ignore unsupported options                     |4.2.2.5 |x| | | | |
  Cope with illegal option length                |4.2.2.5 |x| | | | |
  Implement sending &amp; receiving MSS option       |4.2.2.6 |x| | | | |
  IPv4 Send MSS option unless 536                |4.2.2.6 | |x| | | |
  IPv6 Send MSS option unless 1220               |   N/A  | |x| | | |
  Send MSS option always                         |4.2.2.6 | | |x| | |
  IPv4 Send-MSS default is 536                   |4.2.2.6 |x| | | | |
  IPv6 Send-MSS default is 1220                  |   N/A  |x| | | | |
  Calculate effective send seg size              |4.2.2.6 |x| | | | |
  MSS accounts for varying MTU                   |   N/A  | |x| | | |
                                                 |        | | | | | |
TCP Checksums                                    |        | | | | | |
  Sender compute checksum                        |4.2.2.7 |x| | | | |
  Receiver check checksum                        |4.2.2.7 |x| | | | |
                                                 |        | | | | | |
ISN Selection                                    |        | | | | | |
  Include a clock-driven ISN generator component |4.2.2.9 |x| | | | |
  Secure ISN generator with a PRF component      |  N/A   | |x| | | |
                                                 |        | | | | | |
Opening Connections                              |        | | | | | |
  Support simultaneous open attempts             |4.2.2.10|x| | | | |
  SYN-RCVD remembers last state                  |4.2.2.11|x| | | | |
  Passive Open call interfere with others        |4.2.2.18| | | | |x|
  Function: simultan. LISTENs for same port      |4.2.2.18|x| | | | |
  Ask IP for src address for SYN if necc.        |4.2.3.7 |x| | | | |
    Otherwise, use local addr of conn.           |4.2.3.7 |x| | | | |
  OPEN to broadcast/multicast IP Address         |4.2.3.14| | | | |x|
  Silently discard seg to bcast/mcast addr       |4.2.3.14|x| | | | |
                                                 |        | | | | | |
Closing Connections                              |        | | | | | |
  RST can contain data                           |4.2.2.12| |x| | | |
  Inform application of aborted conn             |4.2.2.13|x| | | | |
  Half-duplex close connections                  |4.2.2.13| | |x| | |
    Send RST to indicate data lost               |4.2.2.13| |x| | | |
  In TIME-WAIT state for 2MSL seconds            |4.2.2.13|x| | | | |
    Accept SYN from TIME-WAIT state              |4.2.2.13| | |x| | |
                                                 |        | | | | | |
Retransmissions                                  |        | | | | | |
  Jacobson Slow Start algorithm                  |4.2.2.15|x| | | | |
  Jacobson Congestion-Avoidance algorithm        |4.2.2.15|x| | | | |
  Retransmit with same IP ident                  |4.2.2.15| | |x| | |
  Karn's algorithm                               |4.2.3.1 |x| | | | |
  Jacobson's RTO estimation alg.                 |4.2.3.1 |x| | | | |
  Exponential backoff                            |4.2.3.1 |x| | | | |
  SYN RTO calc same as data                      |4.2.3.1 | |x| | | |
  Recommended initial values and bounds          |4.2.3.1 | |x| | | |
                                                 |        | | | | | |
Generating ACK's:                                |        | | | | | |
  Queue out-of-order segments                    |4.2.2.20| |x| | | |
  Process all Q'd before send ACK                |4.2.2.20|x| | | | |
  Send ACK for out-of-order segment              |4.2.2.21| | |x| | |
  Delayed ACK's                                  |4.2.3.2 | |x| | | |
    Delay &lt; 0.5 seconds                          |4.2.3.2 |x| | | | |
    Every 2nd full-sized segment ACK'd           |4.2.3.2 |x| | | | |
  Receiver SWS-Avoidance Algorithm               |4.2.3.3 |x| | | | |
                                                 |        | | | | | |
Sending data                                     |        | | | | | |
  Configurable TTL                               |4.2.2.19|x| | | | |
  Sender SWS-Avoidance Algorithm                 |4.2.3.4 |x| | | | |
  Nagle algorithm                                |4.2.3.4 | |x| | | |
    Application can disable Nagle algorithm      |4.2.3.4 |x| | | | |
                                                 |        | | | | | |
Connection Failures:                             |        | | | | | |
  Negative advice to IP on R1 retxs              |4.2.3.5 |x| | | | |
  Close connection on R2 retxs                   |4.2.3.5 |x| | | | |
  ALP can set R2                                 |4.2.3.5 |x| | | | |1
  Inform ALP of  R1&lt;=retxs&lt;R2                    |4.2.3.5 | |x| | | |1
  Recommended values for R1, R2                  |4.2.3.5 | |x| | | |
  Same mechanism for SYNs                        |4.2.3.5 |x| | | | |
    R2 at least 3 minutes for SYN                |4.2.3.5 |x| | | | |
                                                 |        | | | | | |
Send Keep-alive Packets:                         |4.2.3.6 | | |x| | |
  - Application can request                      |4.2.3.6 |x| | | | |
  - Default is "off"                             |4.2.3.6 |x| | | | |
  - Only send if idle for interval               |4.2.3.6 |x| | | | |
  - Interval configurable                        |4.2.3.6 |x| | | | |
  - Default at least 2 hrs.                      |4.2.3.6 |x| | | | |
  - Tolerant of lost ACK's                       |4.2.3.6 |x| | | | |
                                                 |        | | | | | |
IP Options                                       |        | | | | | |
  Ignore options TCP doesn't understand          |4.2.3.8 |x| | | | |
  Time Stamp support                             |4.2.3.8 | | |x| | |
  Record Route support                           |4.2.3.8 | | |x| | |
  Source Route:                                  |        | | | | | |
    ALP can specify                              |4.2.3.8 |x| | | | |1
      Overrides src rt in datagram               |4.2.3.8 |x| | | | |
    Build return route from src rt               |4.2.3.8 |x| | | | |
    Later src route overrides                    |4.2.3.8 | |x| | | |
                                                 |        | | | | | |
Receiving ICMP Messages from IP                  |4.2.3.9 |x| | | | |
  Dest. Unreach (0,1,5) =&gt; inform ALP            |4.2.3.9 | |x| | | |
  Dest. Unreach (0,1,5) =&gt; abort conn            |4.2.3.9 | | | | |x|
  Dest. Unreach (2-4) =&gt; abort conn              |4.2.3.9 | |x| | | |
  Source Quench => slow start                    |4.2.3.9 | |x| | | |
  Time Exceeded => tell ALP, don't abort         |4.2.3.9 | |x| | | |
  Param Problem => tell ALP, don't abort         |4.2.3.9 | |x| | | |
                                                 |        | | | | | |
Address Validation                               |        | | | | | |
  Reject OPEN call to invalid IP address         |4.2.3.10|x| | | | |
  Reject SYN from invalid IP address             |4.2.3.10|x| | | | |
  Silently discard SYN to bcast/mcast addr       |4.2.3.10|x| | | | |
                                                 |        | | | | | |
TCP/ALP Interface Services                       |        | | | | | |
  Error Report mechanism                         |4.2.4.1 |x| | | | |
  ALP can disable Error Report Routine           |4.2.4.1 | |x| | | |
  ALP can specify TOS for sending                |4.2.4.2 |x| | | | |
    Passed unchanged to IP                       |4.2.4.2 | |x| | | |
  ALP can change TOS during connection           |4.2.4.2 | |x| | | |
  Pass received TOS up to ALP                    |4.2.4.2 | | |x| | |
  FLUSH call                                     |4.2.4.3 | | |x| | |
  Optional local IP addr parm. in OPEN           |4.2.4.4 |x| | | | |
-------------------------------------------------|--------|-|-|-|-|-|--

</artwork></figure>
<t>
FOOTNOTES:

(1)  "ALP" means Application-Layer program.
</t>
    </section>
</back>

</rfc>
