Internet Engineering Task Force                             W. Eddy, Ed.
Internet-Draft                                               MTI Systems
Obsoletes: 793, 879, 6093, 6429, 6528,                  December 8, 2016                    March 28, 2017
           6691 (if approved)
Updates: 1122 (if approved)
Intended status: Standards Track
Expires: June 11, September 29, 2017

              Transmission Control Protocol Specification


   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).

   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.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [4].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."
   This Internet-Draft will expire on June 11, September 29, 2017.

Copyright Notice

   Copyright (c) 2016 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Purpose and Scope . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Functional Specification  . . . . . . . . . . . . . . . . . .   5
     3.1.  Header Format . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Sequence Numbers  . . . . . . . . . . . . . . . . . . . .  15
     3.4.  Establishing a connection . . . . . . . . . . . . . . . .  21
       3.4.1.  Remote Address Validation . . . . . . . . . . . . . .  28
     3.5.  Closing a Connection  . . . . . . . . . . . . . . . . . .  28
       3.5.1.  Half-Closed Connections . . . . . . . . . . . . . . .  31
     3.6.  Precedence and Security . . . . . . . . . . . . . . . . .  31
     3.7.  Segmentation  . . . . . . . . . . . . . . . . . . . . . .  32
       3.7.1.  Maximum Segment Size Option . . . . . . . . . . . . .  33
       3.7.2.  Path MTU Discovery  . . . . . . . . . . . . . . . . .  34
       3.7.3.  Interfaces with Variable MTU Values . . . . . . . . .  35
       3.7.4.  Nagle Algorithm . . . . . . . . . . . . . . . . . . .  35
       3.7.5.  IPv6 Jumbograms . . . . . . . . . . . . . . . . . . .  36

     3.8.  Data Communication  . . . . . . . . . . . . . . . . . . .  36
       3.8.1.  Retransmission Timeout  . . . . . . . . . . . . . . .  37
       3.8.2.  TCP Congestion Control  . . . . . . . . . . . . . . .  37
       3.8.3.  TCP Connection Failures . . . . . . . . . . . . . . .  37
       3.8.4.  TCP Keep-Alives . . . . . . . . . . . . . . . . . . .  38
       3.8.5.  The Communication of Urgent Information . . . . . . .  38
       3.8.5.  39
       3.8.6.  Managing the Window . . . . . . . . . . . . . . . . .  39  40
     3.9.  Interfaces  . . . . . . . . . . . . . . . . . . . . . . .  44
       3.9.1.  User/TCP Interface  . . . . . . . . . . . . . . . . .  44
       3.9.2.  TCP/Lower-Level Interface . . . . . . . . . . . . . .  52
     3.10. Event Processing  . . . . . . . . . . . . . . . . . . . .  54
     3.11. Glossary  . . . . . . . . . . . . . . . . . . . . . . . .  78  79
   4.  Changes from RFC 793  . . . . . . . . . . . . . . . . . . . .  83  84
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  87  88
   6.  Security and Privacy Considerations . . . . . . . . . . . . .  87  89
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  88  89
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  88  89
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  88  89
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  89  90
   Appendix A.  TCP Requirement Summary  . . . . . . . . . . . . . .  90  92
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  94  95

1.  Purpose and Scope

   In 1981, RFC 793 [8] [10] was released, documenting the Transmission
   Control Protocol (TCP), and replacing earlier specifications for TCP
   that had been published in the past.

   Since then, TCP has been implemented many times, and has been used as
   a transport protocol for numerous applications on the Internet.

   For several decades, RFC 793 plus a number of other documents have
   combined to serve as the specification for TCP [20]. [23].  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.

   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.

   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.

   This document is intended to be useful both in checking existing TCP
   implementations for conformance, as well as in writing new

2.  Introduction

   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.

   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.

   The "TCP Roadmap" [20] [23] 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. [13]) [16]) 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 [19], [22], but these are not
   strictly required or discussed in this document.

   TEMPORARY EDITOR'S NOTE: This is an early revision in the process of
   updating RFC 793.  Many planned changes are not yet incorporated.

   ***Please do not use this revision as a basis for any work or

   A list of changes from RFC 793 is contained in Section 4.

   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
   Section 4.

3.  Functional Specification

3.1.  Header Format

   TCP segments are sent as internet datagrams.  The Internet Protocol
   header carries several information fields, including the source and
   destination host addresses [1].  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?)

   TCP Header Format
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |          Source Port          |       Destination Port        |
      |                        Sequence Number                        |
      |                    Acknowledgment Number                      |
      |  Data |           |U|A|P|R|S|F|       |C|E|U|A|P|R|S|F|                               |
      | Offset| Reserved  |R|C|S|S|Y|I| Rsrvd |W|C|R|C|S|S|Y|I|            Window             |
      |       |           |G|K|H|T|N|N|       |R|E|G|K|H|T|N|N|                               |
      |           Checksum            |         Urgent Pointer        |
      |                    Options                    |    Padding    |
      |                             data                              |

                               TCP Header Format

             Note that one tick mark represents one bit position.

                                 Figure 1

   Source Port:  16 bits

     The source port number.

   Destination Port:  16 bits

     The destination port number.

   Sequence Number:  32 bits

     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.

   Acknowledgment Number:  32 bits

     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.

   Data Offset:  4 bits
     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.

   Rsrvd - Reserved:  4 bits

     Reserved for future use.  Must be zero.

   Control Bits:  8 bits (from left to right):

        CWR: Congestion Window Reduced (see [7])
        ECE: ECN-Echo (see [7])
        URG: Urgent Pointer field significant
        ACK: Acknowledgment field significant
        PSH: Push Function
        RST: Reset the connection
        SYN: Synchronize sequence numbers
        FIN: No more data from sender

   Window:  16 bits

     The number of data octets beginning with the one indicated in the
     acknowledgment field which the sender of this segment is willing to

     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.

   Checksum:  16 bits

     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.

     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)

                   |           Source Address          |
                   |         Destination Address       |
                   |  zero  |  PTCL  |    TCP Length   |

     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

     The TCP checksum is never optional.  The sender MUST generate it
     and the receiver MUST check it.

   Urgent Pointer:  16 bits

     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.

   Options:  variable

     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:

        Case 1: A single octet of option-kind.

        Case 2: An octet of option-kind, an octet of option-length, and
        the actual option-data octets.

     The option-length counts the two octets of option-kind and option-
     length as well as the option-data octets.

     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).

     Currently defined options include (kind indicated in octal):

         Kind     Length    Meaning
         ----     ------    -------
          0         -       End of option list.
          1         -       No-Operation.
          2         4       Maximum Segment Size.

     TODO - I think we may need to include designated experimental
     options and RFC 6994 reference here

     A TCP MUST be able to receive a TCP option in any segment.
     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.

   Specific Option Definitions

        End of Option List


        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



        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.

        Maximum Segment Size (MSS)

           |00000010|00000100|   max seg size   |
            Kind=2   Length=4

        Maximum Segment Size Option Data: 16 bits

        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
        Section 3.7.1.

   Padding:  variable

     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

3.2.  Terminology

   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.

       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
         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

   The following diagrams may help to relate some of these variables to
   the sequence space.

     Send Sequence Space

                      1         2          3          4
                        SND.UNA    SND.NXT    SND.UNA

           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

                                 Figure 2

   The send window is the portion of the sequence space labeled 3 in
   Figure 2.

     Receive Sequence Space

                          1          2          3
                             RCV.NXT    RCV.NXT

           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

                                 Figure 3

   The receive window is the portion of the sequence space labeled 2 in
   Figure 3.

   There are also some variables used frequently in the discussion that
   take their values from the fields of the current segment.

   Current Segment Variables

       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

   A connection progresses through a series of states during its
   lifetime.  The states are: LISTEN, SYN-SENT, SYN-RECEIVED,
   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:

      LISTEN - represents waiting for a connection request from any
      remote TCP and port.

      SYN-SENT - represents waiting for a matching connection request
      after having sent a connection request.

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

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

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

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

      CLOSE-WAIT - represents waiting for a connection termination
      request from the local user.

      CLOSING - represents waiting for a connection termination request
      acknowledgment from the remote TCP.

      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

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

      CLOSED - represents no connection state at all.

   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.

   The state diagram in Figure 4 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.

   NOTA BENE: this diagram is only a summary and must not be taken as
   the total specification.

                              +---------+ ---------\      active OPEN
                              |  CLOSED |            \    -----------
                              +---------+<---------\   \   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          +--------+
  |        |<-----------------           ------------------>|        |
  |  SYN   |                    rcv SYN                     |  SYN   |
  |  RCVD  |<-----------------------------------------------|  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    |<-----------------           ------------------>|  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
                                 Figure 4

3.3.  Sequence Numbers

   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.

   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 "=<" means "less than or
   equal" (modulo 2**32).

   The typical kinds of sequence number comparisons which the TCP must
   perform include:

      (a) Determining that an acknowledgment refers to some sequence
      number sent but not yet acknowledged.

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

      (c) Determining that an incoming segment contains sequence numbers
      which are expected (i.e., that the segment "overlaps" the receive

   In response to sending data the TCP will receive acknowledgments.
   The following comparisons are needed to process the acknowledgments.

      SND.UNA = oldest unacknowledged sequence number

      SND.NXT = next sequence number to be sent

      SEG.ACK = acknowledgment from the receiving TCP (next sequence
      number expected by the receiving TCP)
      SEG.SEQ = first sequence number of a segment

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

      SEG.SEQ+SEG.LEN-1 = last sequence number of a segment

   A new acknowledgment (called an "acceptable ack"), is one for which
   the inequality below holds:


   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.

   When data is received the following comparisons are needed:

      RCV.NXT = next sequence number expected on an incoming segments,
      and is the left or lower edge of the receive window

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

      SEG.SEQ = first sequence number occupied by the incoming segment

      SEG.SEQ+SEG.LEN-1 = last sequence number occupied by the incoming

   A segment is judged to occupy a portion of valid receive sequence
   space if




   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.

   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:

       Segment Receive  Test
       Length  Window
       ------- -------  -------------------------------------------

          0       0     SEG.SEQ = RCV.NXT

          0      >0     RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND

         >0       0     not acceptable

         >0      >0     RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
                     or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND

   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.

   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.

   Initial Sequence Number Selection

   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.

   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.

   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.

   A TCP MUST use a clock-driven selection of initial sequence numbers,
   and SHOULD generate its Initial Sequence Numbers with the expression:

   ISN = M + F(localip, localport, remoteip, remoteport, secretkey)

   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 [17]. [20].

   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.

   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.

   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.

       1) A --> B  SYN my sequence number is X
       2) A <-- B  ACK your sequence number is X
       3) A <-- B  SYN my sequence number is Y
       4) A --> B  ACK your sequence number is Y

   Because steps 2 and 3 can be combined in a single message this is
   called the three way (or three message) handshake.

   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].

   Knowing When to Keep Quiet

   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.

   The TCP Quiet Time Concept

   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.

   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.

   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.

   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.

   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).

   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.

   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.

   One way to deal with this problem is to deliberately delay emitting
   segments for one MSL after recovery from a crash- this is the "quiet
   time" 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 "quite time".
   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 "wait," this is not necessary
   after the host has been "up" for at least MSL seconds.

   To summarize: every segment emitted occupies one or more sequence
   numbers in the sequence space, the numbers occupied by a segment are
   "busy" or "in use" 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.

3.4.  Establishing a connection

   The "three-way handshake" 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 "SYN".  Of course, the arrival of
   an old duplicate "SYN" segment can potentially make it appear, to the
   recipient, that a simultaneous connection initiation is in progress.
   Proper use of "reset" segments can disambiguate these cases.

   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.

   The simplest three-way handshake is shown in Figure 5 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 (<--), indicate the reverse.
   Ellipsis (...) indicates a segment which is still in the network
   (delayed).  An "XXX" 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.

       TCP A                                                TCP B

   1.  CLOSED                                               LISTEN

   2.  SYN-SENT    --> <SEQ=100><CTL=SYN>               --> SYN-RECEIVED


   4.  ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK>       --> ESTABLISHED


           Basic 3-Way Handshake for Connection Synchronization

                                 Figure 5

   In line 2 of Figure 5, 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.

   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!).

   Simultaneous initiation is only slightly more complex, as is shown in
   Figure 6.  Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to

       TCP A                                            TCP B

   1.  CLOSED                                           CLOSED

   2.  SYN-SENT     --> <SEQ=100><CTL=SYN>              ...

   3.  SYN-RECEIVED <-- <SEQ=300><CTL=SYN>              <-- SYN-SENT

   4.               ... <SEQ=100><CTL=SYN>              --> SYN-RECEIVED

   5.  SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...


   7.               ... <SEQ=100><ACK=301><CTL=SYN,ACK>  --> ESTABLISHED

                 Simultaneous Connection Synchronization

                                 Figure 6

   A TCP MUST support simultaneous open attempts.

   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.

   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-
   aborts the connection and informs its user.  We discuss this latter
   case under "half-open" connections below.

       TCP A                                                TCP B

   1.  CLOSED                                               LISTEN

   2.  SYN-SENT    --> <SEQ=100><CTL=SYN>               ...

   3.  (duplicate) ... <SEQ=90><CTL=SYN>               --> SYN-RECEIVED

   4.  SYN-SENT    <-- <SEQ=300><ACK=91><CTL=SYN,ACK>  <-- SYN-RECEIVED

   5.  SYN-SENT    --> <SEQ=91><CTL=RST>               --> LISTEN

   6.              ... <SEQ=100><CTL=SYN>               --> SYN-RECEIVED

   7.  SYN-SENT    <-- <SEQ=400><ACK=101><CTL=SYN,ACK>  <-- SYN-RECEIVED

   8.  ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK>      --> ESTABLISHED

                     Recovery from Old Duplicate SYN

                                 Figure 7

   As a simple example of recovery from old duplicates, consider
   Figure 7.  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.

   Half-Open Connections and Other Anomalies

   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.

   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

   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 Figure 8.  After TCP A crashes, the user attempts to
   re-open the connection.  TCP B, in the meantime, thinks the
   connection is open.

         TCP A                                           TCP B

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

     2.  CLOSED                                           ESTABLISHED

     3.  SYN-SENT --> <SEQ=400><CTL=SYN>              --> (??)

     4.  (!!)     <-- <SEQ=300><ACK=100><CTL=ACK>     <-- ESTABLISHED

     5.  SYN-SENT --> <SEQ=100><CTL=RST>              --> (Abort!!)

     6.  SYN-SENT                                         CLOSED

     7.  SYN-SENT --> <SEQ=400><CTL=SYN>              -->

                        Half-Open Connection Discovery

                                 Figure 8

   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 Figure 5.

   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 Figure 9.  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.

         TCP A                                              TCP B

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

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

   3.          --> <SEQ=100><CTL=RST>                   --> (ABORT!!)

            Active Side Causes Half-Open Connection Discovery

                                 Figure 9

   In Figure 10, 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.

       TCP A                                         TCP B

   1.  LISTEN                                        LISTEN

   2.       ... <SEQ=Z><CTL=SYN>                -->  SYN-RECEIVED

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

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

   5.  LISTEN                                        LISTEN

        Old Duplicate SYN Initiates a Reset on two Passive Sockets

                                 Figure 10

   A variety of other cases are possible, all of which are accounted for
   by the following rules for RST generation and processing.

   Reset Generation
   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.

   There are three groups of states:

      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.

      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.

      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.

      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.

      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.

      3.  If the connection is in a synchronized state (ESTABLISHED,
      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.

      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

   Reset Processing

   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.

   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.

   TCP SHOULD allow a received RST segment to include data.

3.4.1.  Remote Address Validation

   TODO - figure out if this section would fit better elsewhere, for
   instance in the more detailed description of the OPEN call later on

   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).

   An incoming SYN with an invalid source address must be ignored either
   by TCP or by the IP layer (see Section of [10]). [12]).

   A TCP implementation MUST silently discard an incoming SYN segment
   that is addressed to a broadcast or multicast address.

3.5.  Closing a Connection

   CLOSE is an operation meaning "I have no more data to send."  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.

   There are essentially three cases:

      1) The user initiates by telling the TCP to CLOSE the connection

      2) The remote TCP initiates by sending a FIN control signal

      3) Both users CLOSE simultaneously

   Case 1:  Local user initiates the close

      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

   Case 2:  TCP receives a FIN from the network

      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.

   Case 3:  both users close simultaneously

      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.

       TCP A                                                TCP B

   1.  ESTABLISHED                                          ESTABLISHED

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

   3.  FIN-WAIT-2  <-- <SEQ=300><ACK=101><CTL=ACK>      <-- CLOSE-WAIT

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

   5.  TIME-WAIT   --> <SEQ=101><ACK=301><CTL=ACK>      --> CLOSED

   6.  (2 MSL)

                          Normal Close Sequence

                                 Figure 11

       TCP A                                                TCP B

   1.  ESTABLISHED                                          ESTABLISHED

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

   3.  CLOSING     --> <SEQ=101><ACK=301><CTL=ACK>      ... CLOSING
                   <-- <SEQ=301><ACK=101><CTL=ACK>      <--
                   ... <SEQ=101><ACK=301><CTL=ACK>      -->

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

                       Simultaneous Close Sequence

                                 Figure 12

   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

3.5.1.  Half-Closed Connections

   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

   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.

   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:

      (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

      (2) returns to TIME-WAIT state if the SYN turns out to be an old

3.6.  Precedence and Security

   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.

   The precedence and security parameters used in TCP are exactly those
   defined in the Internet Protocol (IP) [1].  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.

   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.

   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

   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.

3.7.  Segmentation

   The term "segmentation" 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 [12], [14], 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.

   Goals driving the sending of larger segments include:

   o  Reducing the number of packets in flight within the network.

   o  Increasing processing efficiency and potential performance by
      enabling a smaller number of interrupts and inter-layer

   o  Limiting the overhead of TCP headers.

   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.

   Goals driving the sending of smaller segments include:

   o  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.

   o  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.

   o  Enabling "fate sharing" between TCP segments and lower-layer data
      units (e.g. below IP, for links with cell or frame sizes smaller
      than the IP MTU).

   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.

3.7.1.  Maximum Segment Size Option

   TCP MUST implement both sending and receiving the MSS option.

   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.

   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

   The maximum size of a segment that TCP really sends, the "effective
   send MSS," 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:

       Eff.snd.MSS =

           min(SendMSS+20, MMS_S) - TCPhdrsize - IPoptionsize


   o  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

   o  MMS_S is the maximum size for a transport-layer message that TCP
      may send.

   o  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.

   o  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.

   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
   [21] discusses this in greater detail.

   The MSS value to be sent in an MSS option must be less than or equal

      MMS_R - 20

   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

   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.

3.7.2.  Path MTU Discovery

   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

   PMTUD for IPv4 [2] or IPv6 [3] 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 [6].  PLPMTUD [11] [13] 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.

   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

3.7.3.  Interfaces with Variable MTU Values

   The effective MTU can sometimes vary, as when used with variable
   compression, e.g., RObust Header Compression (ROHC) [14]. [17].  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.

   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.

3.7.4.  Nagle Algorithm

   The "Nagle algorithm" was described in RFC 896 [9] [11] and was
   recommended in RFC 1122 [10] [12] 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.

   If there is unacknowledged data (i.e., SND.NXT > 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).

   TODO - see if SEND description later should be updated to reflect

   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 [13]. [16].

3.7.5.  IPv6 Jumbograms

   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 [5] defines that an MSS value of 65,535
   bytes is to be treated as infinity, and Path MTU Discovery [3] is
   used to determine the actual MSS.

3.8.  Data Communication

   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.

   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.

   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.

   The CLOSE user call implies a push function, as does the FIN control
   flag in an incoming segment.

3.8.1.  Retransmission Timeout

   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.

   The RTO MUST be computed according to the algorithm in [7], [8], including
   Karn's algorithm for taking RTT samples.

   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.

   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 of RFC 1122).

3.8.2.  TCP Congestion Control

   RFC 1122 required implementation of Van Jacobson's congestion control
   algorithm combining slow start with congestion avoidance.  RFC 2581
   provided IETF Standards Track description of this, along with fast
   retransmit and fast recovery.  RFC 5681 is the current description of
   these algorithms and is the current standard for TCP congestion

   A TCP MUST implement RFC 5681.

   Explicit Congestion Notification (ECN) was defined in RFC 3168 and is
   an IETF Standards Track enhancement that has many benefits [24].

   A TCP SHOULD implement ECN as described in RFC 3168.

3.8.3.  TCP Connection Failures

   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:

      (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.

      (b) When the number of transmissions of the same segment reaches
      or exceeds threshold R1, pass negative advice (see [10] [12]
      Section to the IP layer, to trigger dead-gateway

      (c) When the number of transmissions of the same segment reaches a
      threshold R2 greater than R1, close the connection.

      (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

      (d) TCP SHOULD inform the application of the delivery problem
      (unless such information has been disabled by the application; see
      RFC1122 Section - 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.

   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

   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.

   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.


3.8.4.  TCP Keep-Alives

   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.

   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.

   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.

   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


3.8.5.  The Communication of Urgent Information

   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 [15]. [18].

   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.

   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 "urgent mode"; when the receive
   sequence number catches up to the urgent pointer, the TCP must tell
   user to go into "normal mode".  If the urgent pointer is updated
   while the user is in "urgent mode", the update will be invisible to
   the user.

   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.

   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

   A TCP MUST support a sequence of urgent data of any length. [10] [12]

   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. [10]

3.8.5. [12]

3.8.6.  Managing the Window

   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

   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

   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).

   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.

   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 "shrinking the window," 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.

   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 "useable window" (see
   Section to become negative.

   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).  Zero Window Probing

   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 "probe" 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.

   Probing of zero (offered) windows MUST be supported.

   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 "printer ran out of paper" situation described in
   Section of RFC1122.  The behavior is subject to the
   implementation's resource management concerns, as noted in [16]. [19].

   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).  Silly Window Syndrome Avoidance

   The "Silly Window Syndrome" (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.  Sender's Algorithm - When to Send Data

   A TCP MUST include a SWS avoidance algorithm in the sender.

   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.

   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.

   The "useable window" is:


   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.

   Send data:

   (1)  if a maximum-sized segment can be sent, i.e, if:

           min(D,U) >= Eff.snd.MSS;

   (2)  or if the data is pushed and all queued data can be sent now,
        i.e., if:

           [SND.NXT = SND.UNA and] PUSHED and D <= U

        (the bracketed condition is imposed by the Nagle algorithm);

   (3)  or if at least a fraction Fs of the maximum window can be sent,
        i.e., if:

           [SND.NXT = SND.UNA and]

              min(D.U) >= Fs * Max(SND.WND);

   (4)  or if data is PUSHed and the override timeout occurs.

   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 Section  Receiver's Algorithm - When to Send a Window Update

   A TCP MUST include a SWS avoidance algorithm in the receiver.

   The receiver's SWS avoidance algorithm determines when the right
   window edge may be advanced; this is customarily known as "updating
   the window".  This algorithm combines with the delayed ACK algorithm
   (see Section to determine when an ACK segment containing the
   current window will really be sent to the receiver.

   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.

   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.

   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:

                  |<------- RCV.BUFF ---------------->|
                       1             2            3
                         RCV.NXT               ^

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

   The suggested SWS avoidance algorithm for the receiver is to keep
   RCV.NXT+RCV.WND fixed until the reduction satisfies:

                RCV.BUFF - RCV.USER - RCV.WND  >=

                       min( Fr * RCV.BUFF, Eff.snd.MSS )

   where Fr is a fraction whose recommended value is 1/2, and
   Eff.snd.MSS is the effective send MSS for the connection (see
   Section 3.7.1).  When the inequality is satisfied, RCV.WND is set to

   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 < RCV.BUFF/2).  Note also that the receiver must use its
   own Eff.snd.MSS, assuming it is the same as the sender's.  Delayed Acknowledgements - When to Send an ACK Segment

   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 "delayed ACK".

   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 "clocking" algorithms.

3.9.  Interfaces

   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.

3.9.1.  User/TCP Interface

   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

   TCP User Commands

      The following sections functionally characterize a USER/TCP
      interface.  The notation used is similar to most procedure or
      function calls in high level languages, but this usage is not
      meant to rule out trap type service calls (e.g., SVCs, UUOs,

      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.

      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:

         (a) general information about a connection (e.g., interrupts,
         remote close, binding of unspecified foreign socket).

         (b) replies to specific user commands indicating success or
         various types of failure.


         Format: OPEN (local port, foreign socket, active/passive [,
         timeout] [, precedence] [, security/compartment] [local IP
         address,] [, options]) -> local connection name

         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.

         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.

         A transmission control block (TCB) is created and partially
         filled in with data from the OPEN command parameters.

         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.

         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.

         On an active OPEN command, the TCP will begin the procedure to
         synchronize (i.e., establish) the connection at once.

         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

         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.

         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.

         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

         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 <local socket,
         foreign socket> pair.

         The optional "local IP address" 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.

         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.

         For an active OPEN call, a specified "local IP address"
         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

         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

         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.

         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.


         Format: SEND (local connection name, buffer address, byte
         count, PUSH flag, URGENT flag [,timeout])

         This call causes the data contained in the indicated user
         buffer to be sent on the indicated connection.  If the
         connection has not been opened, the SEND is considered an
         error.  Some implementations may allow users to SEND first; in
         which case, an automatic OPEN would be done.  If the calling
         process is not authorized to use this connection, an error is

         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.

         New applications SHOULD NOT set the URGENT flag [15] [18] due to
         implementation differences and middlebox issues.

         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.

         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.

         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.

         If a timeout is specified, the current user timeout for this
         connection is changed to the new one.

         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

         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.

         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.


         Format: RECEIVE (local connection name, buffer address, byte
         count) -> byte count, urgent flag, push flag

         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.

         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.

         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.

         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 "urgent mode".  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 "urgent mode".  Note that data following the
         urgent pointer (non-urgent data) cannot be delivered to the
         user in the same buffer with preceding urgent data unless the
         boundary is clearly marked for the user.

         To distinguish among several outstanding RECEIVEs and to take
         care of the case that a buffer is not completely filled, the
         return code is accompanied by both a buffer pointer and a byte
         count indicating the actual length of the data received.

         Alternative implementations of RECEIVE might have the TCP
         allocate buffer storage, or the TCP might share a ring buffer
         with the user.


         Format: CLOSE (local connection name)

         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.

         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).

         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.

         Close also implies push function.


         Format: STATUS (local connection name) -> status data

         This is an implementation dependent user command and could be
         excluded without adverse effect.  Information returned would
         typically come from the TCB associated with the connection.

         This command returns a data block containing the following

            local socket,
            foreign socket,
            local connection name,
            receive window,
            send window,
            connection state,
            number of buffers awaiting acknowledgment,
            number of buffers pending receipt,
            urgent state,
            and transmission timeout.

         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.


         Format: ABORT (local connection name)

         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.


         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

      Set TOS

         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.

         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.

         TCP MAY pass the most recently received TOS up to the

      TCP-to-User Messages

         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.

         The following information is provided:

           Local Connection Name                    Always
           Response String                          Always
           Buffer Address                           Send & Receive
           Byte count (counts bytes received)       Receive
           Push flag                                Receive
           Urgent flag                              Receive

3.9.2.  TCP/Lower-Level Interface

   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) [1].

   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:

      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.

      Time to Live (TTL): The TTL value used to send TCP segments MUST
      be configurable.

         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.

   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.

   When received options are passed up to TCP from the IP layer, TCP
   MUST ignore options that it does not understand.

   A TCP MAY support the Time Stamp and Record Route options.  Source Routing

   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.

   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.

   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.  ICMP Messages

   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)

   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.

   This applies to ICMPv6 in addition to IPv4 ICMP.

   [15] contains discussion of specific ICMP and ICMPv6 messages
   classified as either "soft" or "hard" errors that may bear different
   responses.  Treatment for classes of ICMP messages is described

   Source Quench
     TCP MUST react to a silently discard any received ICMP Source Quench by slowing transmission on the
     connection.  The RECOMMENDED procedure is messages.
     See [9] for a Source Quench to
     trigger a "slow start," as if a retransmission timeout had
     occurred. discussion.

   Soft Errors
     For ICMP these include: Destination Unreachable -- codes 0, 1, 5 5,
     Time Exceeded -- codes 0, 1, and Parameter Problem.
     For ICMPv6 these include: Destination Unreachable -- codes 0 and 3,
     Time Exceeded -- codes 0, 1, and Parameter Problem -- codes 0, 1, 2
     Since these Unreachable messages indicate soft error conditions,
     TCP MUST NOT abort the connection, and it SHOULD make the
     information available to the application.

   Hard Errors
     For ICMP these include Destination Unreachable -- codes 2-4 2-4">
     These are hard error conditions, so TCP SHOULD abort the

   Time Exceeded -- codes 0, 1
     This should be handled the same way as Destination Unreachable
     codes 0, 1, 5 (see above).

   Parameter Problem
     This should be handled  [15] notes that some implementations do not abort
     connections when an ICMP hard error is received for a connection
     that is in any of the same way synchronized states.

   Note that [15] section 4 describes widespread implementation behavior
   that treats soft errors as Destination Unreachable
     codes 0, 1, 5 (see above). hard errors during connection

3.10.  Event Processing

   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.

   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.

   Events that occur:

      User Calls


      Arriving Segments




   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.

   Error responses are given as character strings.  For example, user
   commands referencing connections that do not exist receive "error:
   connection not open".

   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 "=<" means less than or
   equal to (modulo 2**32).

   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.

   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.

   Note that if no state change is mentioned the TCP stays in the same

   OPEN Call

      CLOSED STATE (i.e., TCB does not exist)

         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 <SEQ=ISS><CTL=SYN> is sent.  Set
         SND.UNA to ISS, SND.NXT to ISS+1, enter SYN-SENT state, and

         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".


         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".


         Return "error: connection already exists".

   SEND Call

      CLOSED STATE (i.e., TCB does not exist)

         If the user does not have access to such a connection, then
         return "error: connection illegal for this process".

         Otherwise, return "error: connection does not exist".


         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".


         Queue the data for transmission after entering ESTABLISHED
         state.  If no space to queue, respond with "error: insufficient


         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".

         If the urgent flag is set, then SND.UP <- SND.NXT and set the
         urgent pointer in the outgoing segments.


         Return "error: connection closing" and do not service request.


      CLOSED STATE (i.e., TCB does not exist)

         If the user does not have access to such a connection, return
         "error: connection illegal for this process".

         Otherwise return "error: connection does not exist".


         Queue for processing after entering ESTABLISHED state.  If
         there is no room to queue this request, respond with "error:
         insufficient resources".


         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

         Reassemble queued incoming segments into receive buffer and
         return to user.  Mark "push seen" (PUSH) if this is the case.

         If RCV.UP is in advance of the data currently being passed to
         the user notify the user of the presence of urgent data.

         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


         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.


         Return "error: connection closing".

   CLOSE Call

      CLOSED STATE (i.e., TCB does not exist)

         If the user does not have access to such a connection, return
         "error: connection illegal for this process".

         Otherwise, return "error: connection does not exist".


         Any outstanding RECEIVEs are returned with "error: closing"
         responses.  Delete TCB, enter CLOSED state, and return.


         Delete the TCB and return "error: closing" responses to any
         queued SENDs, or RECEIVEs.


         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.


         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.


         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).


         Queue this request until all preceding SENDs have been
         segmentized; then send a FIN segment, enter LAST-ACK state.

         Respond with "error: connection closing".

   ABORT Call

      CLOSED STATE (i.e., TCB does not exist)

         If the user should not have access to such a connection, return
         "error: connection illegal for this process".

         Otherwise return "error: connection does not exist".


         Any outstanding RECEIVEs should be returned with "error:
         connection reset" responses.  Delete TCB, enter CLOSED state,
         and return.


         All queued SENDs and RECEIVEs should be given "connection
         reset" notification, delete the TCB, enter CLOSED state, and


         Send a reset segment:


         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.


         Respond with "ok" and delete the TCB, enter CLOSED state, and

   STATUS Call

      CLOSED STATE (i.e., TCB does not exist)

         If the user should not have access to such a connection, return
         "error: connection illegal for this process".

         Otherwise return "error: connection does not exist".


         Return "state = LISTEN", and the TCB pointer.


         Return "state = SYN-SENT", and the TCB pointer.


         Return "state = SYN-RECEIVED", and the TCB pointer.


         Return "state = ESTABLISHED", and the TCB pointer.


         Return "state = FIN-WAIT-1", and the TCB pointer.


         Return "state = FIN-WAIT-2", and the TCB pointer.


         Return "state = CLOSE-WAIT", and the TCB pointer.


         Return "state = CLOSING", and the TCB pointer.


         Return "state = LAST-ACK", and the TCB pointer.


         Return "state = TIME-WAIT", and the TCB pointer.


      If the state is CLOSED (i.e., TCB does not exist) then

         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.

         If the ACK bit is off, sequence number zero is used,


         If the ACK bit is on,



      If the state is LISTEN then

         first check for an RST

            An incoming RST should be ignored.  Return.

         second check for an ACK

            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:



         third check for a SYN

            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


            If the SEG.PRC is greater than the TCB.PRC then if allowed
            by the user and the system set TCB.PRC<-SEG.PRC, if not
            allowed send a reset and return.


            If the SEG.PRC is less than the TCB.PRC then continue.

            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:


            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

         fourth other text or control

            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

      If the state is SYN-SENT then

         first check the ACK bit

            If the ACK bit is set

               If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset
               (unless the RST bit is set, if so drop the segment and


               and discard the segment.  Return.

               If SND.UNA < SEG.ACK =< 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

         second check the RST bit

            If the RST bit is set

               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.

         third check the security and precedence

            If the security/compartment in the segment does not exactly
            match the security/compartment in the TCB, send a reset

               If there is an ACK




            If there is an ACK

               The precedence in the segment must match the precedence
               in the TCB, if not, send a reset


            If there is no ACK

               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


               If the precedence in the segment is lower than the
               precedence in the TCB continue.

            If a reset was sent, discard the segment and return.

         fourth check the SYN bit

            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.

            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.

            If SND.UNA > ISS (our SYN has been ACKed), change the
            connection state to ESTABLISHED, form an ACK segment


            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.

            Otherwise enter SYN-RECEIVED, form a SYN,ACK segment


            and send it.  Set the variables:

               SND.WND <- SEG.WND
               SND.WL1 <- SEG.SEQ
               SND.WL2 <- SEG.ACK

            If there are other controls or text in the segment, queue
            them for processing after the ESTABLISHED state has been
            reached, return.

         fifth, if neither of the SYN or RST bits is set then drop the
         segment and return.


      first check sequence number

         FIN-WAIT-1 STATE
         FIN-WAIT-2 STATE

            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.

            There are four cases for the acceptability test for an
            incoming segment:

         Segment Receive  Test
         Length  Window
         ------- -------  -------------------------------------------

            0       0     SEG.SEQ = RCV.NXT

            0      >0     RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND

           >0       0     not acceptable

           >0      >0     RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
                       or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND

            If the RCV.WND is zero, no segments will be acceptable, but
            special allowance should be made to accept valid ACKs, URGs
            and RSTs.

            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):


            After sending the acknowledgment, drop the unacceptable
            segment and return.

            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

            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)

         second check the RST bit,


               If the RST bit is set

                  If this connection was initiated with a passive OPEN
                  (i.e., came from the LISTEN state), then return this
                  connection to LISTEN state and return.  The user need
                  not be informed.  If this connection was initiated
                  with an active OPEN (i.e., came from SYN-SENT state)
                  then the connection was refused, signal the user
                  "connection refused".  In either case, 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.


               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.

            CLOSING STATE
            LAST-ACK STATE

               If the RST bit is set then, enter the CLOSED state,
               delete the TCB, and return.

         third check security and precedence

               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.


               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.

            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.

         fourth, check the SYN bit,

            FIN-WAIT STATE-1
            FIN-WAIT STATE-2
            CLOSE-WAIT STATE
            CLOSING STATE
            LAST-ACK STATE
            TIME-WAIT STATE

               TODO: need to incorporate RFC 1122 here

               If the SYN is in the window it is an error, send a reset,
               any outstanding RECEIVEs and SEND should receive "reset"
               responses, all segment queues should be flushed, the user
               should also receive an unsolicited general "connection
               reset" signal, enter the CLOSED state, delete the TCB,
               and return.

               If the SYN is not in the window this step would not be
               reached and an ack would have been sent in the first step
               (sequence number check).

         fifth check the ACK field,

            if the ACK bit is off drop the segment and return

            if the ACK bit is on

               SYN-RECEIVED STATE

                  If SND.UNA < SEG.ACK =< SND.NXT then enter ESTABLISHED
                  state and continue processing with variables below set

                     SND.WND <- SEG.WND
                     SND.WL1 <- SEG.SEQ
                     SND.WL2 <- SEG.ACK

                     If the segment acknowledgment is not acceptable,
                     form a reset segment,


                     and send it.

               ESTABLISHED STATE

                  If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <-
                  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 =<
                  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.

                  If SND.UNA =< SEG.ACK =< SND.NXT, the send window
                  should be updated.  If (SND.WL1 < SEG.SEQ or (SND.WL1
                  = SEG.SEQ and SND.WL2 =< SEG.ACK)), set SND.WND <-
                  SEG.WND, set SND.WL1 <- SEG.SEQ, and set SND.WL2 <-

                  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.

               FIN-WAIT-1 STATE

                  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.

               FIN-WAIT-2 STATE

                  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.

               CLOSE-WAIT STATE

                  Do the same processing as for the ESTABLISHED state.

               CLOSING STATE

                  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.

               LAST-ACK STATE

                  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.

               TIME-WAIT STATE

                  The only thing that can arrive in this state is a
                  retransmission of the remote FIN.  Acknowledge it, and
                  restart the 2 MSL timeout.

         sixth, check the URG bit,

            FIN-WAIT-1 STATE
            FIN-WAIT-2 STATE
               If the URG bit is set, RCV.UP <- max(RCV.UP,SEG.UP), and
               signal the user that the remote side has urgent data if
               the urgent pointer (RCV.UP) is in advance of the data
               consumed.  If the user has already been signaled (or is
               still in the "urgent mode") for this continuous sequence
               of urgent data, do not signal the user again.

            CLOSE-WAIT STATE
            CLOSING STATE
            LAST-ACK STATE

               This should not occur, since a FIN has been received from
               the remote side.  Ignore the URG.

         seventh, process the segment text,

            FIN-WAIT-1 STATE
            FIN-WAIT-2 STATE

               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.

               When the TCP takes responsibility for delivering the data
               to the user it must also acknowledge the receipt of the

               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.

               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.

               Please note the window management suggestions in section

               Send an acknowledgment of the form:


               This acknowledgment should be piggybacked on a segment
               being transmitted if possible without incurring undue

            CLOSE-WAIT STATE
            CLOSING STATE
            LAST-ACK STATE
            TIME-WAIT STATE

               This should not occur, since a FIN has been received from
               the remote side.  Ignore the segment text.

         eighth, check the FIN bit,

            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.

            If the FIN bit is set, signal the user "connection closing"
            and return any pending RECEIVEs with same message, advance
            RCV.NXT over the FIN, and send an acknowledgment for the
            FIN.  Note that FIN implies PUSH for any segment text not
            yet delivered to the user.

               SYN-RECEIVED STATE
               ESTABLISHED STATE

                  Enter the CLOSE-WAIT state.

               FIN-WAIT-1 STATE

                  If our FIN has been ACKed (perhaps in this segment),
                  then enter TIME-WAIT, start the time-wait timer, turn
                  off the other timers; otherwise enter the CLOSING

               FIN-WAIT-2 STATE

                  Enter the TIME-WAIT state.  Start the time-wait timer,
                  turn off the other timers.

               CLOSE-WAIT STATE

                  Remain in the CLOSE-WAIT state.

               CLOSING STATE

                  Remain in the CLOSING state.

               LAST-ACK STATE

                  Remain in the LAST-ACK state.

               TIME-WAIT STATE

                  Remain in the TIME-WAIT state.  Restart the 2 MSL
                  time-wait timeout.

         and return.



         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.


         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.


         If the time-wait timeout expires on a connection delete the
         TCB, enter the CLOSED state and return.

3.11.  Glossary

   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.

           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.

   ARPANET message
           The unit of transmission between a host and an IMP in the
           ARPANET.  The maximum size is about 1012 octets (8096 bits).

   ARPANET packet
           A unit of transmission used internally in the ARPANET between
           IMPs.  The maximum size is about 126 octets (1008 bits).

           A logical communication path identified by a pair of sockets.

           A message sent in a packet switched computer communications

   Destination Address
           The destination address, usually the network and host

           A control bit (finis) occupying one sequence number, which
           indicates that the sender will send no more data or control
           occupying sequence space.

           A portion of a logical unit of data, in particular an
           internet fragment is a portion of an internet datagram.

           A file transfer protocol.

           Control information at the beginning of a message, segment,
           fragment, packet or block of data.

           A computer.  In particular a source or destination of
           messages from the point of view of the communication network.

           An Internet Protocol field.  This identifying value assigned
           by the sender aids in assembling the fragments of a datagram.

           The Interface Message Processor, the packet switch of the

   internet address
           A source or destination address specific to the host level.

   internet datagram
           The unit of data exchanged between an internet module and the
           higher level protocol together with the internet header.

   internet fragment
           A portion of the data of an internet datagram with an
           internet header.

           Internet Protocol.

           The Initial Receive Sequence number.  The first sequence
           number used by the sender on a connection.

           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.

           The Initial Send Sequence number.  The first sequence number
           used by the sender on a connection.

           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.

   left sequence
           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.

   local packet
           The unit of transmission within a local network.

           An implementation, usually in software, of a protocol or
           other procedure.

           Maximum Segment Lifetime, the time a TCP segment can exist in
           the internetwork system.  Arbitrarily defined to be 2

           An eight bit byte.

           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.

           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.

           The portion of a socket that specifies which logical input or
           output channel of a process is associated with the data.

           A program in execution.  A source or destination of data from
           the point of view of the TCP or other host-to-host protocol.

           A control bit occupying no sequence space, indicating that
           this segment contains data that must be pushed through to the
           receiving user.

           receive next sequence number

           receive urgent pointer

           receive window

   receive next sequence number
           This is the next sequence number the local TCP is expecting
           to receive.

   receive window
           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.

           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.

           Real Time Protocol: A host-to-host protocol for communication
           of time critical information.

           segment acknowledgment

           segment length

           segment precedence value

           segment sequence

           segment urgent pointer field

           segment window field

           A logical unit of data, in particular a TCP segment is the
           unit of data transfered between a pair of TCP modules.

   segment acknowledgment
           The sequence number in the acknowledgment field of the
           arriving segment.

   segment length
           The amount of sequence number space occupied by a segment,
           including any controls which occupy sequence space.

   segment sequence
           The number in the sequence field of the arriving segment.

   send sequence
           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.

   send window
           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.)

           send sequence

           left sequence

           send urgent pointer

           segment sequence number at last window update

           segment acknowledgment number at last window update

           send window

           An address which specifically includes a port identifier,
           that is, the concatenation of an Internet Address with a TCP

   Source Address
           The source address, usually the network and host identifiers.

           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.

           Transmission control block, the data structure that records
           the state of a connection.

           The precedence of the connection.

           Transmission Control Protocol: A host-to-host protocol for
           reliable communication in internetwork environments.

           Type of Service, an Internet Protocol field.

   Type of Service
           An Internet Protocol field which indicates the type of
           service for this internet fragment.

           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.

   urgent pointer
           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.

4.  Changes from RFC 793

   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.

   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.

   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

   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.

   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.

   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.

   RFC 1122 contains more than just TCP requirements, so this document
   can't obsolete RFC 1122 entirely.  It is only marked as "updating"
   1122, however, it should be understood to effectively obsolete all of
   the RFC 1122 material on TCP.

   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.

   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.

   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

   This document started as draft-eddy-rfc793bis-00, that was merely a
   proposal and rough plan for updating RFC 793.

   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.

   The -02 revision of draft-eddy-rfc793bis incorporates errata that
   have been verified:

      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.)
      Errata ID 574: Reported by Yin Shuming
      Errata ID 700: Reported by Yin Shuming
      Errata ID 701: Reported by Yin Shuming
      Errata ID 1283: Reported by Pei-chun Cheng
      Errata ID 1561: Reported by Constantin Hagemeier
      Errata ID 1562: Reported by Constantin Hagemeier
      Errata ID 1564: Reported by Constantin Hagemeier
      Errata ID 1565: Reported by Constantin Hagemeier
      Errata ID 1571: Reported by Constantin Hagemeier
      Errata ID 1572: Reported by Constantin Hagemeier
      Errata ID 2296: Reported by Vishwas Manral
      Errata ID 2297: Reported by Vishwas Manral
      Errata ID 2298: Reported by Vishwas Manral
      Errata ID 2748: Reported by Mykyta Yevstifeyev
      Errata ID 2749: Reported by Mykyta Yevstifeyev
      Errata ID 2934: Reported by Constantin Hagemeier
      Errata ID 3213: Reported by EugnJun Yi
      Errata ID 3300: Reported by Botong Huang
      Errata ID 3301: Reported by Botong Huang
      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.
      Note: Errata ID 3602 was not applied in this revision as it is
      duplicative of the 1122 corrections.
      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.

   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.

   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.

   The -04 revision of draft-eddy-rfc793bis includes the ISN generation
   changes from RFC 6528.

   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.

   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.

   The -01 revision incorporates the change to Reserved bits due to ECN,
   as well as many other changes that come from RFC 1122.

   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.

   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.

   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.

   The -05 revision includes the requirement to implement TCP congestion
   control with recommendation to implemente ECN, the RFC 6633 update to
   1122, which changed the requirement on responding to source quench
   ICMP messages, and discussion of ICMP (and ICMPv6) soft and hard
   errors per RFC 5461 (ICMPv6 handling for TCP doesn't seem to be
   mentioned elsewhere in standards track).

   TODO list of other planned changes (these can be added to or made
   more specific, as the document proceeds):

   1.  incorporate relevant parts of 3168 (ECN) - beyond just indicating
       the names of the 2 bits already done
   2.  point to 5461 (soft errors)
   3.  mention 5961 state machine option
   2.  mention 6161 (reducing TIME-WAIT)
   3.  TOS material does not take DSCP changes into account
   4.  there is inconsistency between use of SYN_RCVD and SYNC-RECEIVED
       in diagrams and text in various places
   5.  make sure that clarifications in RFC 1011 are captured

   TODO list of other potential changes, if there is TCPM consensus:

   1.  see draft-gont-tcpm-tcp-seccomp-prec
   2.  incorporate Fernando's new number-checking fixes (if past the
       IESG in time)
   3.  look at Tony Sabatini suggestion for describing DO field
   4.  clearly specify treatment of reserved bits (see TCPM thread on
       EDO draft April 25, 2014)
   5.  look at possible mention of draft-minshall-nagle (e.g. as in
   6.  per discussion with Joe Touch (TAPS list, 6/20/2015), the
       description of the API could be revisited

5.  IANA Considerations

   This memo includes no request to IANA.  Existing IANA registries for
   TCP parameters are sufficient.

   TODO: check whether entries pointing to 793 and other documents
   obsoleted by this one should be updated to point to this one instead.

6.  Security and Privacy Considerations


   See RFC 6093 [15] [18] for discussion of security considerations related
   to the urgent pointer field.

   Editor's Note: Scott Brim mentioned that this should include a
   PERPASS/privacy review.

7.  Acknowledgements

   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.

   Andre Oppermann was a contributor and helped to edit the first
   revision of this document.

   We are thankful for the assistance of the IETF TCPM working group

      Michael Scharf
      Yoshifumi Nishida
      Pasi Sarolahti

   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.

   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.

8.  References

8.1.  Normative References

   [1]        Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,

   [2]        Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,

   [3]        McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
              1996, <>.

   [4]        Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [5]        Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
              RFC 2675, DOI 10.17487/RFC2675, August 1999,

   [6]        Lahey, K., "TCP Problems with Path MTU Discovery",
              RFC 2923, DOI 10.17487/RFC2923, September 2000,

   [7]        Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,

   [8]        Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,

   [9]        Gont, F., "Deprecation of ICMP Source Quench Messages",
              RFC 6633, DOI 10.17487/RFC6633, May 2012,

8.2.  Informative References


   [10]       Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,


   [11]       Nagle, J., "Congestion Control in IP/TCP Internetworks",
              RFC 896, DOI 10.17487/RFC0896, January 1984,


   [12]       Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,


   [13]       Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,


   [14]       Culley, P., Elzur, U., Recio, R., Bailey, S., and J.
              Carrier, "Marker PDU Aligned Framing for TCP
              Specification", RFC 5044, DOI 10.17487/RFC5044, October
              2007, <>.


   [15]       Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
              DOI 10.17487/RFC5461, February 2009,

   [16]       Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,


   [17]       Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
              Header Compression (ROHC) Framework", RFC 5795,
              DOI 10.17487/RFC5795, March 2010,


   [18]       Gont, F. and A. Yourtchenko, "On the Implementation of the
              TCP Urgent Mechanism", RFC 6093, DOI 10.17487/RFC6093,
              January 2011, <>.


   [19]       Bashyam, M., Jethanandani, M., and A. Ramaiah, "TCP Sender
              Clarification for Persist Condition", RFC 6429,
              DOI 10.17487/RFC6429, December 2011,


   [20]       Gont, F. and S. Bellovin, "Defending against Sequence
              Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February
              2012, <>.


   [21]       Borman, D., "TCP Options and Maximum Segment Size (MSS)",
              RFC 6691, DOI 10.17487/RFC6691, July 2012,


   [22]       Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, Ed., "TCP Extensions for High Performance",
              RFC 7323, DOI 10.17487/RFC7323, September 2014,


   [23]       Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
              Zimmermann, "A Roadmap for Transmission Control Protocol
              (TCP) Specification Documents", RFC 7414,
              DOI 10.17487/RFC7414, February 2015,

   [24]       Fairhurst, G. and M. Welzl, "The Benefits of Using
              Explicit Congestion Notification (ECN)", RFC 8087,
              DOI 10.17487/RFC8087, March 2017,

Appendix A.  TCP Requirement Summary

   This section is adapted from RFC 1122.

   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.

   TODO: NOTE that PMTUD+PLPMTUD is not included in this table of

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

   FOOTNOTES: (1) "ALP" means Application-Layer program.

Author's Address

   Wesley M. Eddy (editor)
   MTI Systems