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Internet Engineering Task Force (IETF) W. Eddy, Ed. STD: 7 MTI Systems Request for Comments: 9293 August 2022 Obsoletes: 793, 879, 2873, 6093, 6429, 6528,

         6691                                                         

Updates: 1011, 1122, 5961 Category: Standards Track ISSN: 2070-1721

                Transmission Control Protocol (TCP)

Abstract

 This document specifies the Transmission Control Protocol (TCP).  TCP
 is an important transport-layer protocol in the Internet protocol
 stack, and it 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, as well as RFCs 879, 2873, 6093,
 6429, 6528, and 6691 that updated parts of RFC 793.  It updates RFCs
 1011 and 1122, and it should be considered as a replacement for the
 portions of those documents dealing with TCP requirements.  It also
 updates RFC 5961 by adding a small clarification in reset handling
 while in the SYN-RECEIVED state.  The TCP header control bits from
 RFC 793 have also been updated based on RFC 3168.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc9293.

Copyright Notice

 Copyright (c) 2022 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
 (https://trustee.ietf.org/license-info) 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 Revised BSD License text as described in Section 4.e of the
 Trust Legal Provisions and are provided without warranty as described
 in the Revised 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
 2.  Introduction
   2.1.  Requirements Language
   2.2.  Key TCP Concepts
 3.  Functional Specification
   3.1.  Header Format
   3.2.  Specific Option Definitions
     3.2.1.  Other Common Options
     3.2.2.  Experimental TCP Options
   3.3.  TCP Terminology Overview
     3.3.1.  Key Connection State Variables
     3.3.2.  State Machine Overview
   3.4.  Sequence Numbers
     3.4.1.  Initial Sequence Number Selection
     3.4.2.  Knowing When to Keep Quiet
     3.4.3.  The TCP Quiet Time Concept
   3.5.  Establishing a Connection
     3.5.1.  Half-Open Connections and Other Anomalies
     3.5.2.  Reset Generation
     3.5.3.  Reset Processing
   3.6.  Closing a Connection
     3.6.1.  Half-Closed Connections
   3.7.  Segmentation
     3.7.1.  Maximum Segment Size Option
     3.7.2.  Path MTU Discovery
     3.7.3.  Interfaces with Variable MTU Values
     3.7.4.  Nagle Algorithm
     3.7.5.  IPv6 Jumbograms
   3.8.  Data Communication
     3.8.1.  Retransmission Timeout
     3.8.2.  TCP Congestion Control
     3.8.3.  TCP Connection Failures
     3.8.4.  TCP Keep-Alives
     3.8.5.  The Communication of Urgent Information
     3.8.6.  Managing the Window
   3.9.  Interfaces
     3.9.1.  User/TCP Interface
     3.9.2.  TCP/Lower-Level Interface
   3.10. Event Processing
     3.10.1.  OPEN Call
     3.10.2.  SEND Call
     3.10.3.  RECEIVE Call
     3.10.4.  CLOSE Call
     3.10.5.  ABORT Call
     3.10.6.  STATUS Call
     3.10.7.  SEGMENT ARRIVES
     3.10.8.  Timeouts
 4.  Glossary
 5.  Changes from RFC 793
 6.  IANA Considerations
 7.  Security and Privacy Considerations
 8.  References
   8.1.  Normative References
   8.2.  Informative References
 Appendix A.  Other Implementation Notes
   A.1.  IP Security Compartment and Precedence
     A.1.1.  Precedence
     A.1.2.  MLS Systems
   A.2.  Sequence Number Validation
   A.3.  Nagle Modification
   A.4.  Low Watermark Settings
 Appendix B.  TCP Requirement Summary
 Acknowledgments
 Author's Address

1. Purpose and Scope

 In 1981, RFC 793 [16] was released, documenting the Transmission
 Control Protocol (TCP) and replacing earlier published specifications
 for TCP.
 Since then, TCP has been widely implemented, and it 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 core specification for TCP [49].  Over time,
 a number of errata have been filed against RFC 793.  There have also
 been deficiencies found and resolved in security, performance, and
 many other aspects.  The number of enhancements has grown over time
 across many separate documents.  These were never accumulated
 together into a comprehensive update to the base specification.
 The purpose of this document is to bring together all of the IETF
 Standards Track changes and other clarifications that have been made
 to the base TCP functional specification (RFC 793) and to unify them
 into an updated version of the specification.
 Some companion documents are referenced for important algorithms that
 are used by TCP (e.g., for congestion control) but have not been
 completely included 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.  This
 document focuses on the common basis that all TCP implementations
 must support in order to interoperate.  Since 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 describes 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 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.  This document preserves
 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 purposes, as well as in writing new
 implementations.

2. Introduction

 RFC 793 contains a discussion of the TCP design goals and provides
 examples of its operation, including examples of connection
 establishment, connection termination, and packet retransmission to
 repair losses.
 This document describes the basic functionality expected in modern
 TCP implementations and replaces the protocol specification in RFC
 793.  It does not replicate or attempt to update the introduction and
 philosophy content in Sections 1 and 2 of RFC 793.  Other documents
 are referenced to provide explanations 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" [49] 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., [8]) is a TCP requirement, but it 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 TCP implementations
 today include the high-performance extensions in [47], but these are
 not strictly required or discussed in this document.  Multipath
 considerations for TCP are also specified separately in [59].
 A list of changes from RFC 793 is contained in Section 5.

2.1. Requirements Language

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [3] [12] when, and only when, they appear in all capitals, as
 shown here.
 Each use of RFC 2119 keywords in the document is individually labeled
 and referenced in Appendix B, which summarizes implementation
 requirements.
 Sentences using "MUST" are labeled as "MUST-X" with X being a numeric
 identifier enabling the requirement to be located easily when
 referenced from Appendix B.
 Similarly, sentences using "SHOULD" are labeled with "SHLD-X", "MAY"
 with "MAY-X", and "RECOMMENDED" with "REC-X".
 For the purposes of this labeling, "SHOULD NOT" and "MUST NOT" are
 labeled the same as "SHOULD" and "MUST" instances.

2.2. Key TCP Concepts

 TCP provides a reliable, in-order, byte-stream service to
 applications.
 The application byte-stream is conveyed over the network via TCP
 segments, with each TCP segment sent as an Internet Protocol (IP)
 datagram.
 TCP reliability consists of detecting packet losses (via sequence
 numbers) and errors (via per-segment checksums), as well as
 correction via retransmission.
 TCP supports unicast delivery of data.  There are anycast
 applications that can successfully use TCP without modifications,
 though there is some risk of instability due to changes of lower-
 layer forwarding behavior [46].
 TCP is connection oriented, though it does not inherently include a
 liveness detection capability.
 Data flow is supported bidirectionally over TCP connections, though
 applications are free to send data only unidirectionally, if they so
 choose.
 TCP uses port numbers to identify application services and to
 multiplex distinct flows between hosts.
 A more detailed description of TCP features compared to other
 transport protocols can be found in Section 3.1 of [52].  Further
 description of the motivations for developing TCP and its role in the
 Internet protocol stack can be found in Section 2 of [16] and earlier
 versions of the TCP specification.

3. Functional Specification

3.1. Header Format

 TCP segments are sent as internet datagrams.  The Internet Protocol
 (IP) header carries several information fields, including the source
 and destination host addresses [1] [13].  A TCP header follows the IP
 headers, supplying information specific to TCP.  This division allows
 for the existence of host-level protocols other than TCP.  In the
 early development of the Internet suite of protocols, the IP header
 fields had been a part of TCP.
 This document describes TCP, which uses TCP headers.
 A TCP header, followed by any user data in the segment, is formatted
 as follows, using the style from [66]:
     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 |       |C|E|U|A|P|R|S|F|                               |
    | Offset| Rsrvd |W|C|R|C|S|S|Y|I|            Window             |
    |       |       |R|E|G|K|H|T|N|N|                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Checksum            |         Urgent Pointer        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           [Options]                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               :
    :                             Data                              :
    :                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Note that one tick mark represents one bit position.
                      Figure 1: TCP Header Format
 where:
 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 the SYN flag is set).  If SYN is set, 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 (DOffset):  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
   integer multiple of 32 bits long.
 Reserved (Rsrvd):  4 bits
   A set of control bits reserved for future use.  Must be zero in
   generated segments and must be ignored in received segments if the
   corresponding future features are not implemented by the sending or
   receiving host.
 Control bits:  The control bits are also known as "flags".
   Assignment is managed by IANA from the "TCP Header Flags" registry
   [62].  The currently assigned control bits are CWR, ECE, URG, ACK,
   PSH, RST, SYN, and FIN.
   CWR:  1 bit
       Congestion Window Reduced (see [6]).
   ECE:  1 bit
       ECN-Echo (see [6]).
   URG:  1 bit
       Urgent pointer field is significant.
   ACK:  1 bit
       Acknowledgment field is significant.
   PSH:  1 bit
       Push function (see the Send Call description in Section 3.9.1).
   RST:  1 bit
       Reset the connection.
   SYN:  1 bit
       Synchronize sequence numbers.
   FIN:  1 bit
       No more data from sender.
 Window:  16 bits
   The number of data octets beginning with the one indicated in the
   acknowledgment field that the sender of this segment is willing to
   accept.  The value is shifted when the window scaling extension is
   used [47].
   The window size MUST be treated as an unsigned number, or else
   large window sizes will appear like negative windows and TCP will
   not work (MUST-1).  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 (REC-
   1).
 Checksum:  16 bits
   The checksum field is the 16-bit ones' complement of the ones'
   complement sum of all 16-bit words in the header and text.  The
   checksum computation needs to ensure the 16-bit alignment of the
   data being summed.  If a segment contains an odd number of header
   and text octets, alignment can be achieved by padding the last
   octet with zeros on its right to form a 16-bit word for checksum
   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 pseudo-header (Figure 2) conceptually
   prefixed to the TCP header.  The pseudo-header is 96 bits for IPv4
   and 320 bits for IPv6.  Including the pseudo-header in the checksum
   gives the TCP connection protection against misrouted segments.
   This information is carried in IP headers and is transferred across
   the TCP/network interface in the arguments or results of calls by
   the TCP implementation on the IP layer.
                   +--------+--------+--------+--------+
                   |           Source Address          |
                   +--------+--------+--------+--------+
                   |         Destination Address       |
                   +--------+--------+--------+--------+
                   |  zero  |  PTCL  |    TCP Length   |
                   +--------+--------+--------+--------+
                       Figure 2: IPv4 Pseudo-header
   Pseudo-header components for IPv4:
     Source Address:  the IPv4 source address in network byte order
     Destination Address:  the IPv4 destination address in network
        byte order
     zero:  bits set to zero
     PTCL:  the protocol number from the IP header
     TCP Length:  the TCP header length plus the data length in octets
        (this is not an explicitly transmitted quantity but is
        computed), and it does not count the 12 octets of the pseudo-
        header.
   For IPv6, the pseudo-header is defined in Section 8.1 of RFC 8200
   [13] and contains the IPv6 Source Address and Destination Address,
   an Upper-Layer Packet Length (a 32-bit value otherwise equivalent
   to TCP Length in the IPv4 pseudo-header), three bytes of zero
   padding, and a Next Header value, which differs from the IPv6
   header value if there are extension headers present between IPv6
   and TCP.
   The TCP checksum is never optional.  The sender MUST generate it
   (MUST-2) and the receiver MUST check it (MUST-3).
 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 to be interpreted in segments
   with the URG control bit set.
 Options:  [TCP Option]; size(Options) == (DOffset-5)*32; present only
   when DOffset > 5.  Note that this size expression also includes any
   padding trailing the actual options present.
   Options may occupy space at the end of the TCP header and are a
   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 (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 List Option MUST be header padding of zeros (MUST-69).
   The list of all currently defined options is managed by IANA [62],
   and each option is defined in other RFCs, as indicated there.  That
   set includes experimental options that can be extended to support
   multiple concurrent usages [45].
   A given TCP implementation can support any currently defined
   options, but the following options MUST be supported (MUST-4 --
   note Maximum Segment Size Option support is also part of MUST-14 in
   Section 3.7.1):
             +======+========+============================+
             | Kind | Length | Meaning                    |
             +======+========+============================+
             | 0    | -      | End of Option List Option. |
             +------+--------+----------------------------+
             | 1    | -      | No-Operation.              |
             +------+--------+----------------------------+
             | 2    | 4      | Maximum Segment Size.      |
             +------+--------+----------------------------+
                     Table 1: Mandatory Option Set
   These options are specified in detail in Section 3.2.
   A TCP implementation MUST be able to receive a TCP Option in any
   segment (MUST-5).
   A TCP implementation MUST (MUST-6) 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 Option (EOL) and
   No-Operation (NOP) MUST have length fields, including all future
   options (MUST-68).  TCP implementations MUST be prepared to handle
   an illegal option length (e.g., zero); a suggested procedure is to
   reset the connection and log the error cause (MUST-7).
   Note: There is ongoing work to extend the space available for TCP
   Options, such as [65].
 Data:  variable length
   User data carried by the TCP segment.

3.2. Specific Option Definitions

 A TCP Option, in the mandatory option set, is one of an End of Option
 List Option, a No-Operation Option, or a Maximum Segment Size Option.
 An End of Option List Option is formatted as follows:
     0
     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |       0       |
    +-+-+-+-+-+-+-+-+
 where:
 Kind:  1 byte; Kind == 0.
   This option code indicates the end of the option list.  This might
   not coincide with the end of the TCP header according to the Data
   Offset field.  This is used at the end of all options, not the end
   of each option, and need only be used if the end of the options
   would not otherwise coincide with the end of the TCP header.
 A No-Operation Option is formatted as follows:
     0
     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |       1       |
    +-+-+-+-+-+-+-+-+
 where:
 Kind:  1 byte; Kind == 1.
   This option code can 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 (MUST-64).
 A Maximum Segment Size Option is formatted as follows:
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       2       |     Length    |   Maximum Segment Size (MSS)  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 where:
 Kind:  1 byte; Kind == 2.
   If this option is present, then it communicates the maximum receive
   segment size at the TCP endpoint that sends this segment.  This
   value is limited by the IP reassembly limit.  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 (MUST-
   65).  If this option is not used, any segment size is allowed.  A
   more complete description of this option is provided in
   Section 3.7.1.
 Length:  1 byte; Length == 4.
   Length of the option in bytes.
 Maximum Segment Size (MSS):  2 bytes.
   The maximum receive segment size at the TCP endpoint that sends
   this segment.

3.2.1. Other Common Options

 Additional RFCs define some other commonly used options that are
 recommended to implement for high performance but are not necessary
 for basic TCP interoperability.  These are the TCP Selective
 Acknowledgment (SACK) Option [22] [26], TCP Timestamp (TS) Option
 [47], and TCP Window Scale (WS) Option [47].

3.2.2. Experimental TCP Options

 Experimental TCP Option values are defined in [30], and [45]
 describes the current recommended usage for these experimental
 values.

3.3. TCP Terminology Overview

 This section includes an overview of key terms needed to understand
 the detailed protocol operation in the rest of the document.  There
 is a glossary of terms in Section 4.

3.3.1. Key Connection State Variables

 Before we can discuss the operation of the TCP implementation in
 detail, we need to introduce some detailed terminology.  The
 maintenance of a TCP connection requires maintaining state for
 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 IP addresses
 and port numbers, the IP security level, and compartment of the
 connection (see Appendix A.1), 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.
  +==========+=====================================================+
  | Variable | Description                                         |
  +==========+=====================================================+
  | SND.UNA  | send unacknowledged                                 |
  +----------+-----------------------------------------------------+
  | SND.NXT  | send next                                           |
  +----------+-----------------------------------------------------+
  | SND.WND  | send window                                         |
  +----------+-----------------------------------------------------+
  | SND.UP   | send urgent pointer                                 |
  +----------+-----------------------------------------------------+
  | SND.WL1  | segment sequence number used for last window update |
  +----------+-----------------------------------------------------+
  | SND.WL2  | segment acknowledgment number used for last window  |
  |          | update                                              |
  +----------+-----------------------------------------------------+
  | ISS      | initial send sequence number                        |
  +----------+-----------------------------------------------------+
                   Table 2: Send Sequence Variables
            +==========+=================================+
            | Variable | Description                     |
            +==========+=================================+
            | RCV.NXT  | receive next                    |
            +----------+---------------------------------+
            | RCV.WND  | receive window                  |
            +----------+---------------------------------+
            | RCV.UP   | receive urgent pointer          |
            +----------+---------------------------------+
            | IRS      | initial receive sequence number |
            +----------+---------------------------------+
                 Table 3: Receive Sequence Variables
 The following diagrams may help to relate some of these variables to
 the sequence space.
                    1         2          3          4
               ----------|----------|----------|----------
                      SND.UNA    SND.NXT    SND.UNA
                                           +SND.WND
         1 - old sequence numbers that have been acknowledged
         2 - sequence numbers of unacknowledged data
         3 - sequence numbers allowed for new data transmission
         4 - future sequence numbers that are not yet allowed
                     Figure 3: Send Sequence Space
 The send window is the portion of the sequence space labeled 3 in
 Figure 3.
                        1          2          3
                    ----------|----------|----------
                           RCV.NXT    RCV.NXT
                                     +RCV.WND
         1 - old sequence numbers that have been acknowledged
         2 - sequence numbers allowed for new reception
         3 - future sequence numbers that are not yet allowed
                    Figure 4: Receive Sequence Space
 The receive window is the portion of the sequence space labeled 2 in
 Figure 4.
 There are also some variables used frequently in the discussion that
 take their values from the fields of the current segment.
             +==========+===============================+
             | Variable | Description                   |
             +==========+===============================+
             | SEG.SEQ  | segment sequence number       |
             +----------+-------------------------------+
             | SEG.ACK  | segment acknowledgment number |
             +----------+-------------------------------+
             | SEG.LEN  | segment length                |
             +----------+-------------------------------+
             | SEG.WND  | segment window                |
             +----------+-------------------------------+
             | SEG.UP   | segment urgent pointer        |
             +----------+-------------------------------+
                  Table 4: Current Segment Variables

3.3.2. State Machine Overview

 A connection progresses through a series of states during its
 lifetime.  The states are: LISTEN, SYN-SENT, SYN-RECEIVED,
 ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK,
 TIME-WAIT, and the fictional state CLOSED.  CLOSED is fictional
 because it represents the state when there is no TCB, and therefore,
 no connection.  Briefly the meanings of the states are:
 LISTEN -  represents waiting for a connection request from any remote
    TCP peer 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 peer, or an acknowledgment of the connection
    termination request previously sent.
 FIN-WAIT-2 -  represents waiting for a connection termination request
    from the remote TCP peer.
 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 peer.
 LAST-ACK -  represents waiting for an acknowledgment of the
    connection termination request previously sent to the remote TCP
    peer (this termination request sent to the remote TCP peer already
    included an acknowledgment of the termination request sent from
    the remote TCP peer).
 TIME-WAIT -  represents waiting for enough time to pass to be sure
    the remote TCP peer received the acknowledgment of its connection
    termination request and to avoid new connections being impacted by
    delayed segments from previous connections.
 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 OPEN call specifies whether connection establishment is to be
 actively pursued, or to be passively waited for.
 A passive OPEN request means that the process wants to accept
 incoming connection requests, in contrast to an active OPEN
 attempting to initiate a connection.
 The state diagram in Figure 5 illustrates only state changes,
 together with the causing events and resulting actions, but addresses
 neither error conditions nor actions that are not connected with
 state changes.  In a later section, more detail is offered with
 respect to the reaction of the TCP implementation to events.  Some
 state names are abbreviated or hyphenated differently in the diagram
 from how they appear elsewhere in the document.
 NOTA BENE:  This diagram is only a summary and must not be taken as
    the total specification.  Many details are not included.
                             +---------+ ---------\      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  |
                           +---------+                    +---------+
                 Figure 5: TCP Connection State Diagram
 The following notes apply to Figure 5:
 Note 1:  The transition from SYN-RECEIVED to LISTEN on receiving a
    RST is conditional on having reached SYN-RECEIVED after a passive
    OPEN.
 Note 2:  The figure omits a transition from FIN-WAIT-1 to TIME-WAIT
    if a FIN is received and the local FIN is also acknowledged.
 Note 3:  A RST can be sent from any state with a corresponding
    transition to TIME-WAIT (see [70] for rationale).  These
    transitions are not explicitly shown; otherwise, the diagram would
    become very difficult to read.  Similarly, receipt of a RST from
    any state results in a transition to LISTEN or CLOSED, though this
    is also omitted from the diagram for legibility.

3.4. 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 straightforward
 duplicate detection in the presence of retransmission.  The numbering
 scheme of octets within a segment is as follows: 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 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 that the TCP
 implementation 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
      that are expected (i.e., that the segment "overlaps" the receive
      window).
 In response to sending data, the TCP endpoint 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 peer (next
    sequence number expected by the receiving TCP peer)
    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:
    SND.UNA < SEG.ACK =< SND.NXT
 A segment on the retransmission queue is fully acknowledged if the
 sum of its sequence number and length is less than or equal to 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 segment,
    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
    segment
 A segment is judged to occupy a portion of valid receive sequence
 space if
    RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
 or
    RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
 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                      |
     +---------+---------+--------------------------------------+
                 Table 5: Segment Acceptability Tests
 Note that when the receive window is zero no segments should be
 acceptable except ACK segments.  Thus, it is possible for a TCP
 implementation to maintain a zero receive window while transmitting
 data and receiving ACKs.  A TCP receiver MUST process the RST and URG
 fields of all incoming segments, even when the receive window is zero
 (MUST-66).
 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.

3.4.1. Initial Sequence Number Selection

 A connection is defined by a pair of sockets.  Connections can be
 reused.  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 implementation 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 support this, the TIME-WAIT state limits the
 rate of connection reuse, while the initial sequence number selection
 described below further protects against ambiguity about which
 incarnation of a connection an incoming packet corresponds to.
 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 endpoint loses all knowledge of the
 sequence numbers it has been using.  When new connections are
 created, an initial sequence number (ISN) generator is employed that
 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 [42].
 TCP initial sequence numbers are generated from a number sequence
 that monotonically increases until it wraps, known loosely as a
 "clock".  This clock is a 32-bit counter that typically increments at
 least once every roughly 4 microseconds, although it is neither
 assumed to be realtime nor precise, and need not persist across
 reboots.  The clock component is intended to ensure 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.  Please note that for modern networks that support high data
 rates where the connection might start and quickly advance sequence
 numbers to overlap within the MSL, it is recommended to implement the
 Timestamp Option as mentioned later in Section 3.4.3.
 A TCP implementation MUST use the above type of "clock" for clock-
 driven selection of initial sequence numbers (MUST-8), 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")
 (SHLD-1).  F() MUST NOT be computable from the outside (MUST-9), or
 an attacker could still guess at sequence numbers from the ISN used
 for some other connection.  The PRF could be implemented as a
 cryptographic hash 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 [42].
 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 peer, and the initial receive sequence number
 (IRS) is learned during the connection-establishing procedure.
 For a connection to be established or initialized, the two TCP peers
 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 ISNs.
 The synchronization requires each side to send its own initial
 sequence number and to receive a confirmation of it in acknowledgment
 from the remote TCP peer.  Each side must also receive the remote
 peer'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 (3WHS).
 A 3WHS is necessary because sequence numbers are not tied to a global
 clock in the network, and TCP implementations may have different
 mechanisms for picking the ISNs.  The receiver of the first SYN has
 no way of knowing whether the segment was an old 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 for ISN selection are discussed in [69].

3.4.2. Knowing When to Keep Quiet

 A theoretical problem exists where data could be corrupted due to
 confusion between old segments in the network and new ones after a
 host reboots if the same port numbers and sequence space are reused.
 The "quiet time" concept discussed below addresses this, and the
 discussion of it is included for situations where it might be
 relevant, although it is not felt to be necessary in most current
 implementations.  The problem was more relevant earlier in the
 history of TCP.  In practical use on the Internet today, the error-
 prone conditions are sufficiently unlikely that it is safe to ignore.
 Reasons why it is now negligible include: (a) ISS and ephemeral port
 randomization have reduced likelihood of reuse of port numbers and
 sequence numbers after reboots, (b) the effective MSL of the Internet
 has declined as links have become faster, and (c) reboots often
 taking longer than an MSL anyways.
 To be sure that a TCP implementation does not create a segment
 carrying a sequence number that may be duplicated by an old segment
 remaining in the network, the TCP endpoint must keep quiet for an MSL
 before assigning any sequence numbers upon starting up or recovering
 from a situation where memory of sequence numbers in use was lost.
 For this specification the MSL is taken to be 2 minutes.  This is an
 engineering choice, and may be changed if experience indicates it is
 desirable to do so.  Note that if a TCP endpoint is reinitialized in
 some sense, yet retains its memory of sequence numbers in use, then
 it need not wait at all; it must only be sure to use sequence numbers
 larger than those recently used.

3.4.3. The TCP Quiet Time Concept

 Hosts that for any reason lose 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 that the host is a part of.  In the paragraphs below, an
 explanation for this specification is given.  TCP implementers 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 data by some receivers in the internet system.
 TCP endpoints 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 TCP 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, TCP implementations keep track of the next
 sequence number to emit and the oldest awaiting acknowledgment so as
 to avoid mistakenly reusing a sequence number 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 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 10s of megabits/sec.  At 100 megabits/sec.,
 the cycle time is 5.4 minutes, which may be a little short but still
 within reason.  Much higher data rates are possible today, with
 implications described in the final paragraph of this subsection.
 The basic duplicate detection and sequencing algorithm in TCP can be
 defeated, however, if a source TCP endpoint does not have any memory
 of the sequence numbers it last used on a given connection.  For
 example, if the TCP implementation were to start all connections with
 sequence number 0, then upon the host rebooting, a TCP peer 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 that can remember the time of day and use 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 endpoint on a particular connection.  Now suppose,
 at this instant, the host reboots 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
 was down between rebooting 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 reboot -- this is the
 "quiet time" specification.  Hosts that prefer to avoid waiting and
 are willing to risk possible confusion of old and new packets at a
 given destination may choose not to wait for the "quiet time".
 Implementers may provide TCP users with the ability to select on a
 connection-by-connection basis whether to wait after a reboot, or may
 informally implement the "quiet 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, and the numbers occupied by a segment
 are "busy" or "in use" until MSL seconds have passed.  Upon
 rebooting, a block of space-time is occupied by the octets and SYN or
 FIN flags of any potentially still in-flight segments.  If a new
 connection is started too soon and uses any of the sequence numbers
 in the space-time footprint of those potentially still in-flight
 segments of the previous connection incarnation, there is a potential
 sequence number overlap area that could cause confusion at the
 receiver.
 High-performance cases will have shorter cycle times than those in
 the megabits per second that the base TCP design described above
 considers.  At 1 Gbps, the cycle time is 34 seconds, only 3 seconds
 at 10 Gbps, and around a third of a second at 100 Gbps.  In these
 higher-performance cases, TCP Timestamp Options and Protection
 Against Wrapped Sequences (PAWS) [47] provide the needed capability
 to detect and discard old duplicates.

3.5. Establishing a Connection

 The "three-way handshake" is the procedure used to establish a
 connection.  This procedure normally is initiated by one TCP peer and
 responded to by another TCP peer.  The procedure also works if two
 TCP peers simultaneously initiate the procedure.  When simultaneous
 open occurs, each TCP peer receives a SYN segment that 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
 endpoint doesn't deliver the data to the user until it is clear the
 data is valid (e.g., the data is buffered at the receiver until the
 connection reaches the ESTABLISHED state, given that the three-way
 handshake reduces the possibility of false connections).  It is a
 trade-off between memory and messages to provide information for this
 checking.
 The simplest 3WHS is shown in Figure 6.  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 Peer A to TCP Peer B or arrival of a segment at B
 from A.  Left arrows (<--) indicate the reverse.  Ellipses (...)
 indicate a segment that is still in the network (delayed).  Comments
 appear in parentheses.  TCP connection 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 Peer A                                           TCP Peer B
 1.  CLOSED                                               LISTEN
 2.  SYN-SENT    --> <SEQ=100><CTL=SYN>               --> SYN-RECEIVED
 3.  ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK>  <-- SYN-RECEIVED
 4.  ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK>       --> ESTABLISHED
 5.  ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED
   Figure 6: Basic Three-Way Handshake for Connection Synchronization
 In line 2 of Figure 6, TCP Peer A begins by sending a SYN segment
 indicating that it will use sequence numbers starting with sequence
 number 100.  In line 3, TCP Peer B sends a SYN and acknowledges the
 SYN it received from TCP Peer A.  Note that the acknowledgment field
 indicates TCP Peer B is now expecting to hear sequence 101,
 acknowledging the SYN that occupied sequence 100.
 At line 4, TCP Peer A responds with an empty segment containing an
 ACK for TCP Peer B's SYN; and in line 5, TCP Peer A sends some data.
 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 ACKs!).
 Simultaneous initiation is only slightly more complex, as is shown in
 Figure 7.  Each TCP peer's connection state cycles from CLOSED to
 SYN-SENT to SYN-RECEIVED to ESTABLISHED.
     TCP Peer A                                       TCP Peer 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> ...
 6.  ESTABLISHED  <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
 7.               ... <SEQ=100><ACK=301><CTL=SYN,ACK> --> ESTABLISHED
           Figure 7: Simultaneous Connection Synchronization
 A TCP implementation MUST support simultaneous open attempts (MUST-
 10).
 Note that a TCP implementation MUST keep track of whether a
 connection has reached SYN-RECEIVED state as the result of a passive
 OPEN or an active OPEN (MUST-11).
 The principal 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, is specified.  If the
 receiving TCP peer is in a non-synchronized state (i.e., SYN-SENT,
 SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset.
 If the TCP peer is in one of the synchronized states (ESTABLISHED,
 FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it
 aborts the connection and informs its user.  We discuss this latter
 case under "half-open" connections below.
     TCP Peer A                                           TCP Peer 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.  ESTABLISHED <-- <SEQ=400><ACK=101><CTL=SYN,ACK>  <-- SYN-RECEIVED
 8.  ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK>      --> ESTABLISHED
               Figure 8: Recovery from Old Duplicate SYN
 As a simple example of recovery from old duplicates, consider
 Figure 8.  At line 3, an old duplicate SYN arrives at TCP Peer B.
 TCP Peer B cannot tell that this is an old duplicate, so it responds
 normally (line 4).  TCP Peer A detects that the ACK field is
 incorrect and returns a RST (reset) with its SEQ field selected to
 make the segment believable.  TCP Peer B, on receiving the RST,
 returns to the LISTEN state.  When the original SYN 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 RSTs sent in both directions.

3.5.1. Half-Open Connections and Other Anomalies

 An established connection is said to be "half-open" if one of the TCP
 peers has closed or aborted the connection at its end without the
 knowledge of the other, or if the two ends of the connection have
 become desynchronized owing to a failure or reboot 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.
 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
 endpoint receiving a reset control message.  Such a message indicates
 to the site B TCP endpoint that something is wrong, and it is
 expected to abort the connection.
 Assume that two user processes A and B are communicating with one
 another when a failure or reboot occurs causing loss of memory to A's
 TCP implementation.  Depending on the operating system supporting A's
 TCP implementation, it is likely that some error recovery mechanism
 exists.  When the TCP endpoint 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
 implementation.  In an attempt to establish the connection, A's TCP
 implementation will send a segment containing SYN.  This scenario
 leads to the example shown in Figure 9.  After TCP Peer A reboots,
 the user attempts to reopen the connection.  TCP Peer B, in the
 meantime, thinks the connection is open.
       TCP Peer A                                      TCP Peer B
   1.  (REBOOT)                              (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>              -->
                Figure 9: Half-Open Connection Discovery
 When the SYN arrives at line 3, TCP Peer B, being in a synchronized
 state, and the incoming segment outside the window, responds with an
 acknowledgment indicating what sequence it next expects to hear (ACK
 100).  TCP Peer 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 Peer B aborts at
 line 5.  TCP Peer A will continue to try to establish the connection;
 the problem is now reduced to the basic three-way handshake of
 Figure 6.
 An interesting alternative case occurs when TCP Peer A reboots and
 TCP Peer B tries to send data on what it thinks is a synchronized
 connection.  This is illustrated in Figure 10.  In this case, the
 data arriving at TCP Peer A from TCP Peer B (line 2) is unacceptable
 because no such connection exists, so TCP Peer A sends a RST.  The
 RST is acceptable so TCP Peer B processes it and aborts the
 connection.
       TCP Peer A                                         TCP Peer B
 1.  (REBOOT)                                  (send 300,receive 100)
 2.  (??)    <-- <SEQ=300><ACK=100><DATA=10><CTL=ACK> <-- ESTABLISHED
 3.          --> <SEQ=100><CTL=RST>                   --> (ABORT!!)
      Figure 10: Active Side Causes Half-Open Connection Discovery
 In Figure 11, two TCP Peers A and B with passive connections waiting
 for SYN are depicted.  An old duplicate arriving at TCP Peer B (line
 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
 Peer B accepts the reset and returns to its passive LISTEN state.
     TCP Peer A                                    TCP Peer 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
 Figure 11: Old Duplicate SYN Initiates a Reset on Two Passive Sockets
 A variety of other cases are possible, all of which are accounted for
 by the following rules for RST generation and processing.

3.5.2. Reset Generation

 A TCP user or application can issue a reset on a connection at any
 time, though reset events are also generated by the protocol itself
 when various error conditions occur, as described below.  The side of
 a connection issuing a reset should enter the TIME-WAIT state, as
 this generally helps to reduce the load on busy servers for reasons
 described in [70].
 As a general rule, reset (RST) is sent whenever a segment arrives
 that apparently is not intended for the current connection.  A reset
 must not be sent if it is not clear that this is the case.
 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.  A SYN
     segment that does not match an existing connection is 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
     (Appendix A.1) that does not exactly match the level and
     compartment requested for the connection, a reset is 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, FIN-
     WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT),
     any unacceptable segment (out-of-window sequence number or
     unacceptable acknowledgment number) must be responded to with an
     empty acknowledgment segment (without any user data) 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 that
     does not exactly match the level and compartment requested for
     the connection, a reset is sent and the connection goes to the
     CLOSED state.  The reset takes its sequence number from the ACK
     field of the incoming segment.

3.5.3. 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 implementations SHOULD allow a received RST segment to include
 data (SHLD-2).  It has been suggested that a RST segment could
 contain diagnostic data that explains the cause of the RST.  No
 standard has yet been established for such data.

3.6. Closing a Connection

 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 the TCP receiver is told that the remote peer 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 remote peer has CLOSED.  The TCP implementation will signal a
 user, even if no RECEIVEs are outstanding, that the remote peer has
 closed, so the user can terminate their side gracefully.  A TCP
 implementation 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 their data was received at the destination TCP endpoint.  Users
 must keep reading connections they close for sending until the TCP
 implementation indicates there is no more data.
 There are essentially three cases:
 1)  The user initiates by telling the TCP implementation to CLOSE the
     connection (TCP Peer A in Figure 12).
 2)  The remote TCP endpoint initiates by sending a FIN control signal
     (TCP Peer B in Figure 12).
 3)  Both users CLOSE simultaneously (Figure 13).
 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 implementation, 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 peer has both acknowledged the
    FIN and sent a FIN of its own, the first TCP peer can ACK this
    FIN.  Note that a TCP endpoint receiving a FIN will ACK but not
    send its own FIN until its user has CLOSED the connection also.
 Case 2:  TCP endpoint receives a FIN from the network
    If an unsolicited FIN arrives from the network, the receiving TCP
    endpoint can ACK it and tell the user that the connection is
    closing.  The user will respond with a CLOSE, upon which the TCP
    endpoint can send a FIN to the other TCP peer after sending any
    remaining data.  The TCP endpoint 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 (Figure 13).  When all segments
    preceding the FINs have been processed and acknowledged, each TCP
    peer can ACK the FIN it has received.  Both will, upon receiving
    these ACKs, delete the connection.
     TCP Peer A                                           TCP Peer 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)
     CLOSED
                    Figure 12: Normal Close Sequence
     TCP Peer A                                           TCP Peer 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
                 Figure 13: Simultaneous Close Sequence
 A TCP connection may terminate in two ways: (1) the normal TCP close
 sequence using a FIN handshake (Figure 12), and (2) an "abort" in
 which one or more RST segments are sent and the connection state is
 immediately discarded.  If the local TCP connection is closed by the
 remote side due to a FIN or RST received from the remote side, then
 the local application MUST be informed whether it closed normally or
 was aborted (MUST-12).

3.6.1. 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
 connection.
 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 (MAY-1).  If such a host issues a CLOSE call while
 received data is still pending in the TCP connection, or if new data
 is received after CLOSE is called, its TCP implementation SHOULD send
 a RST to show that data was lost (SHLD-3).  See [23], Section 2.17
 for discussion.
 When a connection is closed actively, it MUST linger in the TIME-WAIT
 state for a time 2xMSL (Maximum Segment Lifetime) (MUST-13).
 However, it MAY accept a new SYN from the remote TCP endpoint to
 reopen the connection directly from TIME-WAIT state (MAY-2), if it:
 (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
      duplicate.
 When the TCP Timestamp Options are available, an improved algorithm
 is described in [40] in order to support higher connection
 establishment rates.  This algorithm for reducing TIME-WAIT is a Best
 Current Practice that SHOULD be implemented since Timestamp Options
 are commonly used, and using them to reduce TIME-WAIT provides
 benefits for busy Internet servers (SHLD-4).

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 correlation between the boundaries of TCP
 segments sent and received and the boundaries of the read or write
 buffers of user application data.  In some specific protocols, such
 as Remote Direct Memory Access (RDMA) using Direct Data Placement
 (DDP) and Marker PDU Aligned Framing (MPA) [34], 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
 structures, 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:
  • Reducing the number of packets in flight within the network.
  • Increasing processing efficiency and potential performance by

enabling a smaller number of interrupts and inter-layer

    interactions.
  • 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 implementation
 architectures, 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:
  • Avoiding sending a TCP segment that would result in an IP datagram

larger than the smallest MTU along an IP network path because this

    results in either packet loss or packet fragmentation.  Making
    matters worse, some firewalls or middleboxes may drop fragmented
    packets or ICMP messages related to fragmentation.
  • 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.
  • 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 endpoints MUST implement both sending and receiving the MSS
 Option (MUST-14).
 TCP implementations SHOULD send an MSS Option in every SYN segment
 when its receive MSS differs from the default 536 for IPv4 or 1220
 for IPv6 (SHLD-5), and MAY send it always (MAY-3).
 If an MSS Option is not received at connection setup, TCP
 implementations MUST assume a default send MSS of 536 (576 - 40) for
 IPv4 or 1220 (1280 - 60) for IPv6 (MUST-15).
 The maximum size of a segment that a TCP endpoint really sends, the
 "effective send MSS", MUST be the smaller (MUST-16) of the send MSS
 (that reflects the available reassembly buffer size at the remote
 host, the EMTU_R [19]) and the largest transmission size permitted by
 the IP layer (EMTU_S [19]):
 Eff.snd.MSS = min(SendMSS+20, MMS_S) - TCPhdrsize - IPoptionsize
 where:
  • SendMSS is the MSS value received from the remote host, or the

default 536 for IPv4 or 1220 for IPv6, if no MSS Option is

    received.
  • MMS_S is the maximum size for a transport-layer message that TCP

may send.

  • 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
    might 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.
  • IPoptionsize is the size of any IPv4 options or IPv6 extension

headers associated with a TCP connection. Note that some options

    or extension headers might 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
 [43] discusses this in greater detail.
 The MSS value to be sent in an MSS Option must be less than or equal
 to:
    MMS_R - 20
 where MMS_R is the maximum size for a transport-layer message that
 can be received (and reassembled at the IP layer) (MUST-67).  TCP
 obtains MMS_R and MMS_S from the IP layer; see the generic call
 GET_MAXSIZES in Section 3.4 of RFC 1122.  These are defined in terms
 of their IP MTU equivalents, EMTU_R and EMTU_S [19].
 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 recommends an IP-layer default
 effective MTU of less than or equal to 576 for destinations not
 directly connected, and for IPv6 this would be 1280.  Using these
 fixed values limits TCP connection performance and efficiency.
 Instead, implementation of Path MTU Discovery (PMTUD) and
 Packetization Layer Path MTU Discovery (PLPMTUD) is strongly
 recommended in order for TCP to improve segmentation decisions.  Both
 PMTUD and PLPMTUD help TCP choose segment sizes that avoid both on-
 path (for IPv4) and source fragmentation (IPv4 and IPv6).
 PMTUD for IPv4 [2] or IPv6 [14] is implemented in conjunction between
 TCP, IP, and ICMP.  It relies both on avoiding source fragmentation
 and setting the IPv4 DF (don't fragment) flag, the latter to inhibit
 on-path fragmentation.  It relies on ICMP errors from routers along
 the path whenever a segment is too large to traverse a link.  Several
 adjustments to a TCP implementation with PMTUD are described in RFC
 2923 in order to deal with problems experienced in practice [27].
 PLPMTUD [31] 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, but
 still tries to avoid source fragmentation.  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 (see [43]).  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 the TCP MSS to 536 (corresponding to the
 IPv4 576 byte default MTU) for non-local destinations, rather than
 deriving it from the MTUs of connected interfaces as recommended.

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) [37].  It is
 tempting for a TCP implementation 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 packet-to-
 packet, TCP implementations SHOULD use the smallest effective MTU of
 the interface to calculate the value to advertise in the MSS Option
 (SHLD-6).

3.7.4. Nagle Algorithm

 The "Nagle algorithm" was described in RFC 896 [17] and was
 recommended in RFC 1122 [19] 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 (see
 Appendix A.3).
 If there is unacknowledged data (i.e., SND.NXT > SND.UNA), then the
 sending TCP endpoint buffers all user data (regardless of the PSH
 bit) until the outstanding data has been acknowledged or until the
 TCP endpoint can send a full-sized segment (Eff.snd.MSS bytes).
 A TCP implementation SHOULD implement the Nagle algorithm to coalesce
 short segments (SHLD-7).  However, there MUST be a way for an
 application to disable the Nagle algorithm on an individual
 connection (MUST-17).  In all cases, sending data is also subject to
 the limitation imposed by the slow start algorithm [8].
 Since there can be problematic interactions between the Nagle
 algorithm and delayed acknowledgments, some implementations use minor
 variations of the Nagle algorithm, such as the one described in
 Appendix A.3.

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 64-KB limit that the MSS
 Option can convey.  RFC 2675 [24] defines that an MSS value of 65,535
 bytes is to be treated as infinity, and Path MTU Discovery [14] is
 used to determine the actual MSS.
 The Jumbo Payload Option need not be implemented or understood by
 IPv6 nodes that do not support attachment to links with an MTU
 greater than 65,575 [24], and the present IPv6 Node Requirements does
 not include support for Jumbograms [55].

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 to ensure delivery of every segment.  Duplicate
 segments may arrive due to network or TCP retransmission.  As
 discussed in the section on sequence numbers (Section 3.4), the TCP
 implementation 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 (see Section 3.9.1), 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 [10],
 including Karn's algorithm for taking RTT samples (MUST-18).
 RFC 793 contains an early example procedure for computing the RTO,
 based on work mentioned in IEN 177 [71].  This was then replaced by
 the algorithm described in RFC 1122, which was subsequently updated
 in RFC 2988 and then again in RFC 6298.
 RFC 1122 allows that if a retransmitted packet is identical to the
 original packet (which implies not only that the data boundaries have
 not changed, but also that none of the headers have changed), then
 the same IPv4 Identification field MAY be used (see Section 3.2.1.5
 of RFC 1122) (MAY-4).  The same IP Identification field may be reused
 anyways since it is only meaningful when a datagram is fragmented
 [44].  TCP implementations should not rely on or typically interact
 with this IPv4 header field in any way.  It is not a reasonable way
 to indicate duplicate sent segments nor to identify duplicate
 received segments.

3.8.2. TCP Congestion Control

 RFC 2914 [5] explains the importance of congestion control for the
 Internet.
 RFC 1122 required implementation of Van Jacobson's congestion control
 algorithms slow start and congestion avoidance together with
 exponential backoff for successive RTO values for the same segment.
 RFC 2581 provided IETF Standards Track description of slow start and
 congestion avoidance, along with fast retransmit and fast recovery.
 RFC 5681 is the current description of these algorithms and is the
 current Standards Track specification providing guidelines for TCP
 congestion control.  RFC 6298 describes exponential backoff of RTO
 values, including keeping the backed-off value until a subsequent
 segment with new data has been sent and acknowledged without
 retransmission.
 A TCP endpoint MUST implement the basic congestion control algorithms
 slow start, congestion avoidance, and exponential backoff of RTO to
 avoid creating congestion collapse conditions (MUST-19).  RFC 5681
 and RFC 6298 describe the basic algorithms on the IETF Standards
 Track that are broadly applicable.  Multiple other suitable
 algorithms exist and have been widely used.  Many TCP implementations
 support a set of alternative algorithms that can be configured for
 use on the endpoint.  An endpoint MAY implement such alternative
 algorithms provided that the algorithms are conformant with the TCP
 specifications from the IETF Standards Track as described in RFC
 2914, RFC 5033 [7], and RFC 8961 [15] (MAY-18).
 Explicit Congestion Notification (ECN) was defined in RFC 3168 and is
 an IETF Standards Track enhancement that has many benefits [51].
 A TCP endpoint SHOULD implement ECN as described in RFC 3168 (SHLD-
 8).

3.8.3. TCP Connection Failures

 Excessive retransmission of the same segment by a TCP endpoint
 indicates some failure of the remote host or the internetwork path.
 This failure may be of short or long duration.  The following
 procedure MUST be used to handle excessive retransmissions of data
 segments (MUST-20):
 (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 (with the current RTO and corresponding backoffs
      as a conversion factor, if needed).
 (b)  When the number of transmissions of the same segment reaches or
      exceeds threshold R1, pass negative advice (see Section 3.3.1.4
      of [19]) to the IP layer, to trigger dead-gateway diagnosis.
 (c)  When the number of transmissions of the same segment reaches a
      threshold R2 greater than R1, close the connection.
 (d)  An application MUST (MUST-21) be able to set the value for R2
      for a particular connection.  For example, an interactive
      application might set R2 to "infinity", giving the user control
      over when to disconnect.
 (e)  TCP implementations SHOULD inform the application of the
      delivery problem (unless such information has been disabled by
      the application; see the "Asynchronous Reports" section
      (Section 3.9.1.8)), when R1 is reached and before R2 (SHLD-9).
      This will allow a remote login application program to inform the
      user, for example.
 The value of R1 SHOULD correspond to at least 3 retransmissions, at
 the current RTO (SHLD-10).  The value of R2 SHOULD correspond to at
 least 100 seconds (SHLD-11).
 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 (MUST-23).  The application can close the connection (i.e.,
 give up on the open attempt) sooner, of course.

3.8.4. TCP Keep-Alives

 A TCP connection is said to be "idle" if for some long amount of time
 there have been no incoming segments received and there is no new or
 unacknowledged data to be sent.
 Implementers MAY include "keep-alives" in their TCP implementations
 (MAY-5), although this practice is not universally accepted.  Some
 TCP implementations, however, have included a keep-alive mechanism.
 To confirm that an idle connection is still active, these
 implementations send a probe segment designed to elicit a response
 from the TCP peer.  Such a segment generally contains SEG.SEQ =
 SND.NXT-1 and may or may not contain one garbage octet of data.  If
 keep-alives are included, the application MUST be able to turn them
 on or off for each TCP connection (MUST-24), and they MUST default to
 off (MUST-25).
 Keep-alive packets MUST only be sent when no sent data is
 outstanding, and no data or acknowledgment packets have been received
 for the connection within an interval (MUST-26).  This interval MUST
 be configurable (MUST-27) and MUST default to no less than two hours
 (MUST-28).
 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 (MUST-29).
 An implementation SHOULD send a keep-alive segment with no data
 (SHLD-12); however, it MAY be configurable to send a keep-alive
 segment containing one garbage octet (MAY-6), for compatibility with
 erroneous TCP implementations.

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 (SHLD-
 13).  However, TCP implementations MUST still include support for the
 urgent mechanism (MUST-30).  Information on how some TCP
 implementations interpret the urgent pointer can be found in RFC 6093
 [39].
 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 endpoint 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 endpoint,
 then the TCP implementation must tell the user to go into "urgent
 mode"; when the receive sequence number catches up to the urgent
 pointer, the TCP implementation 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 an urgent field that 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
 enhanced.  Note that because changes in the urgent pointer correspond
 to data being written by a sending application, the urgent pointer
 cannot "recede" in the sequence space, but a TCP receiver should be
 robust to invalid urgent pointer values.
 A TCP implementation MUST support a sequence of urgent data of any
 length (MUST-31) [19].
 The urgent pointer MUST point to the sequence number of the octet
 following the urgent data (MUST-62).
 A TCP implementation MUST (MUST-32) inform the application layer
 asynchronously whenever it receives an urgent pointer and there was
 previously no pending urgent data, or whenever the urgent pointer
 advances in the data stream.  The TCP implementation MUST (MUST-33)
 provide a way for the application to learn how much urgent data
 remains to be read from the connection, or at least to determine
 whether more urgent data remains to be read [19].

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 data buffer space currently available for this connection.
 The sending TCP endpoint packages the data to be transmitted into
 segments that fit the current window, and may repackage segments on
 the retransmission queue.  Such repackaging is not required but may
 be helpful.
 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 an acknowledgment
 number equal to 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 TCP endpoints.  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 endpoint 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 [19]
 dictates that TCP peers will not shrink the window themselves, but
 will be prepared for such behavior on the part of other TCP peers.
 A TCP receiver SHOULD NOT shrink the window, i.e., move the right
 window edge to the left (SHLD-14).  However, a sending TCP peer MUST
 be robust against window shrinking, which may cause the "usable
 window" (see Section 3.8.6.2.1) to become negative (MUST-34).
 If this happens, the sender SHOULD NOT send new data (SHLD-15), but
 SHOULD retransmit normally the old unacknowledged data between
 SND.UNA and SND.UNA+SND.WND (SHLD-16).  The sender MAY also
 retransmit old data beyond SND.UNA+SND.WND (MAY-7), but SHOULD NOT
 time out the connection if data beyond the right window edge is not
 acknowledged (SHLD-17).  If the window shrinks to zero, the TCP
 implementation MUST probe it in the standard way (described below)
 (MUST-35).

3.8.6.1. Zero-Window Probing

 The sending TCP peer must regularly transmit at least one octet of
 new data (if available), or retransmit to the receiving TCP peer even
 if the send window is zero, in order to "probe" the window.  This
 retransmission is essential to guarantee that when either TCP peer
 has a zero window the reopening 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 (MUST-36).
 A TCP implementation MAY keep its offered receive window closed
 indefinitely (MAY-8).  As long as the receiving TCP peer continues to
 send acknowledgments in response to the probe segments, the sending
 TCP peer MUST allow the connection to stay open (MUST-37).  This
 enables TCP to function in scenarios such as the "printer ran out of
 paper" situation described in Section 4.2.2.17 of [19].  The behavior
 is subject to the implementation's resource management concerns, as
 noted in [41].
 When the receiving TCP peer has a zero window and a segment arrives,
 it must still send an acknowledgment showing its next expected
 sequence number and current window (zero).
 The transmitting host SHOULD send the first zero-window probe when a
 zero window has existed for the retransmission timeout period (SHLD-
 29) (Section 3.8.1), and SHOULD increase exponentially the interval
 between successive probes (SHLD-30).

3.8.6.2. 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.

3.8.6.2.1. Sender's Algorithm – When to Send Data

 A TCP implementation MUST include a SWS avoidance algorithm in the
 sender (MUST-38).
 The Nagle algorithm from Section 3.7.4 additionally describes how to
 coalesce short segments.
 The sender's SWS avoidance algorithm is more difficult than the
 receiver's because the sender does not know (directly) the receiver's
 total buffer space (RCV.BUFF).  An approach that has been found to
 work well is for the sender to calculate Max(SND.WND), which is 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 "usable window" is:
    U = SND.UNA + SND.WND - SND.NXT
 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
 endpoint 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 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 3.8.6.1).

3.8.6.2.2. Receiver's Algorithm – When to Send a Window Update

 A TCP implementation MUST include a SWS avoidance algorithm in the
 receiver (MUST-39).
 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
 (Section 3.8.6.3) 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 that 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               ^
                                          (Fixed)
            1 - RCV.USER =  data received but not yet consumed;
            2 - RCV.WND =   space advertised to sender;
            3 - Reduction = space available but not yet
                            advertised.
 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
 RCV.BUFF-RCV.USER.
 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, making the assumption that it is the same as the
 sender's.

3.8.6.3. Delayed Acknowledgments – When to Send an ACK Segment

 A host that is receiving a stream of TCP data segments can increase
 efficiency in both the network and the hosts by sending fewer than
 one ACK (acknowledgment) segment per data segment received; this is
 known as a "delayed ACK".
 A TCP endpoint SHOULD implement a delayed ACK (SHLD-18), but an ACK
 should not be excessively delayed; in particular, the delay MUST be
 less than 0.5 seconds (MUST-40).  An ACK SHOULD be generated for at
 least every second full-sized segment or 2*RMSS bytes of new data
 (where RMSS is the MSS specified by the TCP endpoint receiving the
 segments to be acknowledged, or the default value if not specified)
 (SHLD-19).  Excessive delays on ACKs can disturb the round-trip
 timing and packet "clocking" algorithms.  More complete discussion of
 delayed ACK behavior is in Section 4.2 of RFC 5681 [8], including
 recommendations to immediately acknowledge out-of-order segments,
 segments above a gap in sequence space, or segments that fill all or
 part of a gap, in order to accelerate loss recovery.
 Note that there are several current practices that further lead to a
 reduced number of ACKs, including generic receive offload (GRO) [72],
 ACK compression, and ACK decimation [28].

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 TCP implementations might use.

3.9.1. User/TCP Interface

 The following functional description of user commands to the TCP
 implementation 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 TCP implementations must provide a certain
 minimum set of services to guarantee that all TCP implementations can
 support the same protocol hierarchy.  This section specifies the
 functional interfaces required of all TCP implementations.
 Section 3.1 of [53] also identifies primitives provided by TCP and
 could be used as an additional reference for implementers.
 The following sections functionally characterize a user/TCP
 interface.  The notation used is similar to most procedure or
 function calls in high-level languages, but this usage is not meant
 to rule out trap-type service calls.
 The user commands described below specify the basic functions the TCP
 implementation 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
 implementation 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 remote socket).
 (b)  replies to specific user commands indicating success or various
      types of failure.

3.9.1.1. Open

 Format: OPEN (local port, remote socket, active/passive [, timeout]
 [, Diffserv field] [, security/compartment] [, local IP address] [,
 options]) -> local connection name
 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 remote socket to wait for a particular connection or
 an unspecified remote socket to wait for any call.  A fully specified
 passive call can be made active by the subsequent execution of a
 SEND.
 A transmission control block (TCB) is created and partially 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 (MUST-41).
 A TCP implementation that supports multiple concurrent connections
 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 (MUST-42).
 On an active OPEN command, the TCP endpoint 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 endpoint will
 abort the connection.  The present global default is five minutes.
 The TCP implementation or some component of the operating system will
 verify the user's authority to open a connection with the specified
 Diffserv field value or security/compartment.  The absence of a
 Diffserv field value or security/compartment specification in the
 OPEN call indicates the default values must be used.
 TCP will accept incoming requests as matching only if the security/
 compartment information is exactly the same as that requested in the
 OPEN call.
 The Diffserv field value indicated by the user only impacts outgoing
 packets, may be altered en route through the network, and has no
 direct bearing or relation to received packets.
 A local connection name will be returned to the user by the TCP
 implementation.  The local connection name can then be used as a
 shorthand term for the connection defined by the <local socket,
 remote socket> pair.
 The optional "local IP address" parameter MUST be supported to allow
 the specification of the local IP address (MUST-43).  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
 will be used for opening the connection.  If the parameter is
 unspecified, the host will choose an appropriate local IP address
 (see RFC 1122, Section 3.3.4.2).
 If an application on a multihomed host does not specify the local IP
 address when actively opening a TCP connection, then the TCP
 implementation MUST ask the IP layer to select a local IP address
 before sending the (first) SYN (MUST-44).  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 implementations MUST use the
 same local address that was used in those previous segments (MUST-
 45).
 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)
 (MUST-46).

3.9.1.2. Send

 Format: SEND (local connection name, buffer address, byte count,
 URGENT flag [, PUSH flag] [, timeout])
 This call causes the data contained in the indicated user buffer to
 be sent on the indicated connection.  If the connection has not been
 opened, the SEND is considered an error.  Some implementations may
 allow users to SEND first; in which case, an automatic OPEN would be
 done.  For example, this might be one way for application data to be
 included in SYN segments.  If the calling process is not authorized
 to use this connection, an error is returned.
 A TCP endpoint MAY implement PUSH flags on SEND calls (MAY-15).  If
 PUSH flags are not implemented, then the sending TCP peer: (1) MUST
 NOT buffer data indefinitely (MUST-60), and (2) MUST set the PSH bit
 in the last buffered segment (i.e., when there is no more queued data
 to be sent) (MUST-61).  The remaining description below assumes the
 PUSH flag is supported on SEND calls.
 If the PUSH flag is set, the application intends the data to be
 transmitted promptly to the receiver, and the PSH bit will be set in
 the last TCP segment created from the buffer.
 The PSH bit is not a record marker and is independent of segment
 boundaries.  The transmitter SHOULD collapse successive bits when it
 packetizes data, to send the largest possible segment (SHLD-27).
 If the PUSH flag is not set, the data may be combined with data from
 subsequent SENDs for transmission efficiency.  When an application
 issues a series of SEND calls without setting the PUSH flag, the TCP
 implementation MAY aggregate the data internally without sending it
 (MAY-16).  Note that when the Nagle algorithm is in use, TCP
 implementations may buffer the data before sending, without regard to
 the PUSH flag (see Section 3.7.4).
 An application program is logically required to set the PUSH flag in
 a SEND call whenever it needs to force delivery of the data to avoid
 a communication deadlock.  However, a TCP implementation SHOULD send
 a maximum-sized segment whenever possible (SHLD-28) to improve
 performance (see Section 3.8.6.2.1).
 New applications SHOULD NOT set the URGENT flag [39] due to
 implementation differences and middlebox issues (SHLD-13).
 If the URGENT flag is set, segments sent to the destination TCP peer
 will have the urgent pointer set.  The receiving TCP peer 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 the URGENT flag is
 to stimulate the receiver to process the urgent data and to indicate
 to the receiver when all the currently known urgent data has been
 received.  The number of times the sending user's TCP implementation
 signals urgent will not necessarily be equal to the number of times
 the receiving user will be notified of the presence of urgent data.
 If no remote socket was specified in the OPEN, but the connection is
 established (e.g., because a LISTENing connection has become specific
 due to a remote segment arriving for the local socket), then the
 designated buffer is sent to the implied remote socket.  Users who
 make use of OPEN with an unspecified remote socket can make use of
 SEND without ever explicitly knowing the remote socket address.
 However, if a SEND is attempted before the remote socket becomes
 specified, an error will be returned.  Users can use the STATUS call
 to determine the status of the connection.  Some TCP implementations
 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 endpoint will queue those it cannot
 service immediately.
 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 endpoint.  An alternative is to return a response
 immediately.  For instance, SENDs might return immediate local
 acknowledgment, even if the segment sent had not been acknowledged by
 the distant TCP endpoint.  We could optimistically assume eventual
 success.  If we are wrong, the connection will close anyway due to
 the timeout.  In implementations of this kind (synchronous), there
 will still be some asynchronous signals, but these will deal with the
 connection itself, and not with specific segments or buffers.
 In order for the process to distinguish among error or success
 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 that should be returned to the calling process.

3.9.1.3. Receive

 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.
 A TCP receiver MAY pass a received PSH bit to the application layer
 via the PUSH flag in the interface (MAY-17), but it is not required
 (this was clarified in RFC 1122, Section 4.2.2.2).  The remainder of
 text describing the RECEIVE call below assumes that passing the PUSH
 indication is supported.
 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 endpoint
 allocate buffer storage, or the TCP endpoint might share a ring
 buffer with the user.

3.9.1.4. Close

 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
 remote peer may be trying to transmit the last of its data.  Thus,
 CLOSE means "I have no more to send" but does not mean "I will not
 receive any more."  It may happen (if the user-level protocol is not
 well thought out) that the closing side is unable to get rid of all
 its data before timing out.  In this event, CLOSE turns into ABORT,
 and the closing TCP peer gives up.
 The user may CLOSE the connection at any time on their own
 initiative, or in response to various prompts from the TCP
 implementation (e.g., remote close executed, transmission timeout
 exceeded, destination inaccessible).
 Because closing a connection requires communication with the remote
 TCP peer, connections may remain in the closing state for a short
 time.  Attempts to reopen the connection before the TCP peer replies
 to the CLOSE command will result in error responses.
 Close also implies push function.

3.9.1.5. Status

 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
 information:
    local socket,
    remote socket,
    local connection name,
    receive window,
    send window,
    connection state,
    number of buffers awaiting acknowledgment,
    number of buffers pending receipt,
    urgent state,
    Diffserv field value,
    security/compartment, and
    transmission timeout.
 Depending on the state of the connection, or on the 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.

3.9.1.6. Abort

 Format: ABORT (local connection name)
 This command causes all pending SENDs and RECEIVES to be aborted, the
 TCB to be removed, and a special RST message to be sent to the remote
 TCP peer of the connection.  Depending on the implementation, users
 may receive abort indications for each outstanding SEND or RECEIVE,
 or may simply receive an ABORT-acknowledgment.

3.9.1.7. Flush

 Some TCP implementations have included a FLUSH call, which will empty
 the TCP send queue of any data that the user has issued SEND calls
 for but is still to the right of the current send window.  That is,
 it flushes as much queued send data as possible without losing
 sequence number synchronization.  The FLUSH call MAY be implemented
 (MAY-14).

3.9.1.8. Asynchronous Reports

 There MUST be a mechanism for reporting soft TCP error conditions to
 the application (MUST-47).  Generically, we assume this takes the
 form of an application-supplied ERROR_REPORT routine that may be
 upcalled asynchronously from the transport layer:
    ERROR_REPORT(local connection name, reason, subreason)
 The precise encoding of the reason and subreason parameters is not
 specified here.  However, the conditions that are reported
 asynchronously to the application MUST include:
  • ICMP error message arrived (see Section 3.9.2.2 for description of

handling each ICMP message type since some message types need to

    be suppressed from generating reports to the application)
  • Excessive retransmissions (see Section 3.8.3)
  • Urgent pointer advance (see Section 3.8.5)
 However, an application program that does not want to receive such
 ERROR_REPORT calls SHOULD be able to effectively disable these calls
 (SHLD-20).

3.9.1.9. Set Differentiated Services Field (IPv4 TOS or IPv6 Traffic

        Class)
 The application layer MUST be able to specify the Differentiated
 Services field for segments that are sent on a connection (MUST-48).
 The Differentiated Services field includes the 6-bit Differentiated
 Services Codepoint (DSCP) value.  It is not required, but the
 application SHOULD be able to change the Differentiated Services
 field during the connection lifetime (SHLD-21).  TCP implementations
 SHOULD pass the current Differentiated Services field value without
 change to the IP layer, when it sends segments on the connection
 (SHLD-22).
 The Differentiated Services field will be specified independently in
 each direction on the connection, so that the receiver application
 will specify the Differentiated Services field used for ACK segments.
 TCP implementations MAY pass the most recently received
 Differentiated Services field up to the application (MAY-9).

3.9.2. TCP/Lower-Level Interface

 The TCP endpoint calls on a lower-level protocol module to actually
 send and receive information over a network.  The two current
 standard Internet Protocol (IP) versions layered below TCP are IPv4
 [1] and IPv6 [13].
 If the lower-level protocol is IPv4, it provides arguments for a type
 of service (used within the Differentiated Services field) and for a
 time to live.  TCP uses the following settings for these parameters:
 Diffserv field:  The IP header value for the Diffserv field is given
    by the user.  This includes the bits of the Diffserv Codepoint
    (DSCP).
 Time to Live (TTL):  The TTL value used to send TCP segments MUST be
    configurable (MUST-49).
  • 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 could not be delivered by the
       internet system within one minute.  RFC 1122 updated RFC 793 to
       require that the TTL be configurable.
  • Note that the Diffserv field is permitted to change during a

connection (Section 4.2.4.2 of RFC 1122). However, the

       application interface might not support this ability, and the
       application does not have knowledge about individual TCP
       segments, so this can only be done on a coarse granularity, at
       best.  This limitation is further discussed in RFC 7657
       (Sections 5.1, 5.3, and 6) [50].  Generally, an application
       SHOULD NOT change the Diffserv field value during the course of
       a connection (SHLD-23).
 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, a TCP
 implementation MUST ignore options that it does not understand (MUST-
 50).
 A TCP implementation MAY support the Timestamp (MAY-10) and Record
 Route (MAY-11) Options.

3.9.2.1. 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 (MUST-51), and this MUST take
 precedence over a source route received in a datagram (MUST-52).
 When a TCP connection is OPENed passively and a packet arrives with a
 completed IP Source Route Option (containing a return route), TCP
 implementations MUST save the return route and use it for all
 segments sent on this connection (MUST-53).  If a different source
 route arrives in a later segment, the later definition SHOULD
 override the earlier one (SHLD-24).

3.9.2.2. ICMP Messages

 TCP implementations MUST act on an ICMP error message passed up from
 the IP layer, directing it to the connection that created the error
 (MUST-54).  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.
 [35] 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
 below:
 Source Quench
   TCP implementations MUST silently discard any received ICMP Source
   Quench messages (MUST-55).  See [11] for discussion.
 Soft Errors
   For IPv4 ICMP, these include: Destination Unreachable -- codes 0,
   1, 5; Time Exceeded -- codes 0, 1; and Parameter Problem.
   For ICMPv6, these include: Destination Unreachable -- codes 0, 3;
   Time Exceeded -- codes 0, 1; and Parameter Problem -- codes 0, 1,
   2.
   Since these Unreachable messages indicate soft error conditions, a
   TCP implementation MUST NOT abort the connection (MUST-56), and it
   SHOULD make the information available to the application (SHLD-25).
 Hard Errors
   For ICMP these include Destination Unreachable -- codes 2-4.
   These are hard error conditions, so TCP implementations SHOULD
   abort the connection (SHLD-26).  [35] notes that some
   implementations do not abort connections when an ICMP hard error is
   received for a connection that is in any of the synchronized
   states.
 Note that [35], Section 4 describes widespread implementation
 behavior that treats soft errors as hard errors during connection
 establishment.

3.9.2.3. Source Address Validation

 RFC 1122 requires addresses to be validated in incoming SYN packets:
 |  An incoming SYN with an invalid source address MUST be ignored
 |  either by TCP or by the IP layer [(MUST-63)] (see
 |  Section 3.2.1.3).
 |  
 |  A TCP implementation MUST silently discard an incoming SYN segment
 |  that is addressed to a broadcast or multicast address [(MUST-57)].
 This prevents connection state and replies from being erroneously
 generated, and implementers should note that this guidance is
 applicable to all incoming segments, not just SYNs, as specifically
 indicated in RFC 1122.

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 endpoint 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 endpoint 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
       OPEN
       SEND
       RECEIVE
       CLOSE
       ABORT
       STATUS
    Arriving Segments
       SEGMENT ARRIVES
    Timeouts
       USER TIMEOUT
       RETRANSMISSION TIMEOUT
       TIME-WAIT TIMEOUT
 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 in this document are identified by 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 connection stays
 in the same state.

3.10.1. 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, remote

    socket, Diffserv field, security/compartment, and user timeout
    information.  Note that some parts of the remote 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
    Diffserv value requested are allowed for this user, if not, return
    "error: Diffserv value not allowed" or "error: security/
    compartment not allowed".  If passive, enter the LISTEN state and
    return.  If active and the remote socket is unspecified, return
    "error: remote socket unspecified"; if active and the remote
    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 return.
  • 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".
 LISTEN STATE
  • If the OPEN call is active and the remote 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 the remote socket was not specified, then return
    "error: remote socket unspecified".
 SYN-SENT STATE
 SYN-RECEIVED STATE
 ESTABLISHED STATE
 FIN-WAIT-1 STATE
 FIN-WAIT-2 STATE
 CLOSE-WAIT STATE
 CLOSING STATE
 LAST-ACK STATE
 TIME-WAIT STATE
  • Return "error: connection already exists".

3.10.2. 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".
 LISTEN STATE
  • If the remote 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 the remote
    socket was not specified, then return "error: remote socket
    unspecified".
 SYN-SENT STATE
 SYN-RECEIVED STATE
  • Queue the data for transmission after entering ESTABLISHED state.

If no space to queue, respond with "error: insufficient

    resources".
 ESTABLISHED STATE
 CLOSE-WAIT STATE
  • 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.

 FIN-WAIT-1 STATE
 FIN-WAIT-2 STATE
 CLOSING STATE
 LAST-ACK STATE
 TIME-WAIT STATE
  • Return "error: connection closing" and do not service request.

3.10.3. RECEIVE 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".
 LISTEN STATE
 SYN-SENT STATE
 SYN-RECEIVED STATE
  • Queue for processing after entering ESTABLISHED state. If there

is no room to queue this request, respond with "error:

    insufficient resources".
 ESTABLISHED STATE
 FIN-WAIT-1 STATE
 FIN-WAIT-2 STATE
  • If insufficient incoming segments are queued to satisfy the

request, queue the request. If there is no queue space to

    remember the RECEIVE, respond with "error: insufficient
    resources".
  • 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 endpoint takes responsibility for delivering data to

the user, that fact must be communicated to the sender via an

    acknowledgment.  The formation of such an acknowledgment is
    described below in the discussion of processing an incoming
    segment.
 CLOSE-WAIT STATE
  • Since the remote side has already sent FIN, RECEIVEs must be

satisfied by data already on hand, but not yet delivered to the

    user.  If no text is awaiting delivery, the RECEIVE will get an
    "error: connection closing" response.  Otherwise, any remaining
    data can be used to satisfy the RECEIVE.
 CLOSING STATE
 LAST-ACK STATE
 TIME-WAIT STATE
  • Return "error: connection closing".

3.10.4. 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".
 LISTEN STATE
  • Any outstanding RECEIVEs are returned with "error: closing"

responses. Delete TCB, enter CLOSED state, and return.

 SYN-SENT STATE
  • Delete the TCB and return "error: closing" responses to any queued

SENDs, or RECEIVEs.

 SYN-RECEIVED STATE
  • 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.
 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.
 FIN-WAIT-1 STATE
 FIN-WAIT-2 STATE
  • Strictly speaking, this is an error and should receive an "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).
 CLOSE-WAIT STATE
  • Queue this request until all preceding SENDs have been

segmentized; then send a FIN segment, enter LAST-ACK state.

 CLOSING STATE
 LAST-ACK STATE
 TIME-WAIT STATE
  • Respond with "error: connection closing".

3.10.5. 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".
 LISTEN STATE
  • Any outstanding RECEIVEs should be returned with "error:

connection reset" responses. Delete TCB, enter CLOSED state, and

    return.
 SYN-SENT STATE
  • All queued SENDs and RECEIVEs should be given "connection reset"

notification. Delete the TCB, enter CLOSED state, and return.

 SYN-RECEIVED STATE
 ESTABLISHED STATE
 FIN-WAIT-1 STATE
 FIN-WAIT-2 STATE
 CLOSE-WAIT STATE
  • Send a reset segment:
    <SEQ=SND.NXT><CTL=RST>
  • 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.
 CLOSING STATE
 LAST-ACK STATE
 TIME-WAIT STATE
  • Respond with "ok" and delete the TCB, enter CLOSED state, and

return.

3.10.6. 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".
 LISTEN STATE
  • Return "state = LISTEN" and the TCB pointer.
 SYN-SENT STATE
  • Return "state = SYN-SENT" and the TCB pointer.
 SYN-RECEIVED STATE
  • Return "state = SYN-RECEIVED" and the TCB pointer.
 ESTABLISHED STATE
  • Return "state = ESTABLISHED" and the TCB pointer.
 FIN-WAIT-1 STATE
  • Return "state = FIN-WAIT-1" and the TCB pointer.
 FIN-WAIT-2 STATE
  • Return "state = FIN-WAIT-2" and the TCB pointer.
 CLOSE-WAIT STATE
  • Return "state = CLOSE-WAIT" and the TCB pointer.
 CLOSING STATE
  • Return "state = CLOSING" and the TCB pointer.
 LAST-ACK STATE
  • Return "state = LAST-ACK" and the TCB pointer.
 TIME-WAIT STATE
  • Return "state = TIME-WAIT" and the TCB pointer.

3.10.7. SEGMENT ARRIVES

3.10.7.1. CLOSED STATE

 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 endpoint that sent the
    offending segment.
    If the ACK bit is off, sequence number zero is used,
       <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
    If the ACK bit is on,
       <SEQ=SEG.ACK><CTL=RST>
    Return.

3.10.7.2. LISTEN STATE

 If the state is LISTEN, then
    First, check for a RST:
  1. 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.  An incoming RST should be ignored.  Return.
    Second, check for an ACK:
  1. 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:
          <SEQ=SEG.ACK><CTL=RST>
  1. Return.
    Third, check for a SYN:
  1. If the SYN bit is set, check the security. If the security/

compartment on the incoming segment does not exactly match the

       security/compartment in the TCB, then send a reset and return.
          <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
  1. 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:
          <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
  1. 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 remote socket was not fully specified), then the
       unspecified fields should be filled in now.
    Fourth, other data or control:
  1. This should not be reached. Drop the segment and return. Any

other control or data-bearing segment (not containing SYN) must

       have an ACK and thus would have been discarded by the ACK
       processing in the second step, unless it was first discarded by
       RST checking in the first step.

3.10.7.3. SYN-SENT STATE

 If the state is SYN-SENT, then
    First, check the ACK bit:
  1. If the ACK bit is set,
       o  If SEG.ACK =< ISS or SEG.ACK > SND.NXT, send a reset (unless
          the RST bit is set, if so drop the segment and return)
             <SEQ=SEG.ACK><CTL=RST>
       o  and discard the segment.  Return.
       o  If SND.UNA < SEG.ACK =< SND.NXT, then the ACK is acceptable.
          Some deployed TCP code has used the check SEG.ACK == SND.NXT
          (using "==" rather than "=<"), but this is not appropriate
          when the stack is capable of sending data on the SYN because
          the TCP peer may not accept and acknowledge all of the data
          on the SYN.
    Second, check the RST bit:
  1. If the RST bit is set,
       o  A potential blind reset attack is described in RFC 5961 [9].
          The mitigation described in that document has specific
          applicability explained therein, and is not a substitute for
          cryptographic protection (e.g., IPsec or TCP-AO).  A TCP
          implementation that supports the mitigation described in RFC
          5961 SHOULD first check that the sequence number exactly
          matches RCV.NXT prior to executing the action in the next
          paragraph.
       o  If the ACK was acceptable, then signal to 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:
  1. If the security/compartment in the segment does not exactly

match the security/compartment in the TCB, send a reset:

       o  If there is an ACK,
             <SEQ=SEG.ACK><CTL=RST>
       o  Otherwise,
             <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
  1. If a reset was sent, discard the segment and return.
    Fourth, check the SYN bit:
  1. This step should be reached only if the ACK is ok, or there is

no ACK, and the segment did not contain a RST.

  1. If the SYN bit is on and the security/compartment is

acceptable, then RCV.NXT is set to SEG.SEQ+1, IRS is set to

       SEG.SEQ.  SND.UNA should be advanced to equal SEG.ACK (if there
       is an ACK), and any segments on the retransmission queue that
       are thereby acknowledged should be removed.
  1. If SND.UNA > ISS (our SYN has been ACKed), change the

connection state to ESTABLISHED, form an ACK segment

          <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
  1. and send it. Data or controls that were queued for

transmission MAY be included. Some TCP implementations

       suppress sending this segment when the received segment
       contains data that will anyways generate an acknowledgment in
       the later processing steps, saving this extra acknowledgment of
       the SYN from being sent.  If there are other controls or text
       in the segment, then continue processing at the sixth step
       under Section 3.10.7.4 where the URG bit is checked; otherwise,
       return.
  1. Otherwise, enter SYN-RECEIVED, form a SYN,ACK segment
          <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
  1. 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.
  1. Note that it is legal to send and receive application data on

SYN segments (this is the "text in the segment" mentioned

       above).  There has been significant misinformation and
       misunderstanding of this topic historically.  Some firewalls
       and security devices consider this suspicious.  However, the
       capability was used in T/TCP [21] and is used in TCP Fast Open
       (TFO) [48], so is important for implementations and network
       devices to permit.
    Fifth, if neither of the SYN or RST bits is set, then drop the
    segment and return.

3.10.7.4. Other States

 Otherwise,
    First, check sequence number:
  1. SYN-RECEIVED STATE
  1. ESTABLISHED STATE
  1. FIN-WAIT-1 STATE
  1. FIN-WAIT-2 STATE
  1. CLOSE-WAIT STATE
  1. CLOSING STATE
  1. LAST-ACK STATE
  1. TIME-WAIT STATE
       o  Segments are processed in sequence.  Initial tests on
          arrival are used to discard old duplicates, but further
          processing is done in SEG.SEQ order.  If a segment's
          contents straddle the boundary between old and new, only the
          new parts are processed.
       o  In general, the processing of received segments MUST be
          implemented to aggregate ACK segments whenever possible
          (MUST-58).  For example, if the TCP endpoint is processing a
          series of queued segments, it MUST process them all before
          sending any ACK segments (MUST-59).
       o  There are four cases for the acceptability test for an
          incoming segment:
          +=========+=========+======================================+
          | Segment | Receive | Test                                 |
          | 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                    |
          +---------+---------+--------------------------------------+
                      Table 6: Segment Acceptability Tests
       o  In implementing sequence number validation as described
          here, please note Appendix A.2.
       o  If the RCV.WND is zero, no segments will be acceptable, but
          special allowance should be made to accept valid ACKs, URGs,
          and RSTs.
       o  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):
          <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
       o  After sending the acknowledgment, drop the unacceptable
          segment and return.
       o  Note that for the TIME-WAIT state, there is an improved
          algorithm described in [40] for handling incoming SYN
          segments that utilizes timestamps rather than relying on the
          sequence number check described here.  When the improved
          algorithm is implemented, the logic above is not applicable
          for incoming SYN segments with Timestamp Options, received
          on a connection in the TIME-WAIT state.
       o  In the following it is assumed that the segment is the
          idealized segment that begins at RCV.NXT and does not exceed
          the window.  One could tailor actual segments to fit this
          assumption by trimming off any portions that lie outside the
          window (including SYN and FIN) and only processing further
          if the segment then begins at RCV.NXT.  Segments with higher
          beginning sequence numbers SHOULD be held for later
          processing (SHLD-31).
    Second, check the RST bit:
  1. RFC 5961 [9], Section 3 describes a potential blind reset

attack and optional mitigation approach. This does not provide

       a cryptographic protection (e.g., as in IPsec or TCP-AO) but
       can be applicable in situations described in RFC 5961.  For
       stacks implementing the protection described in RFC 5961, the
       three checks below apply; otherwise, processing for these
       states is indicated further below.
       1)  If the RST bit is set and the sequence number is outside
           the current receive window, silently drop the segment.
       2)  If the RST bit is set and the sequence number exactly
           matches the next expected sequence number (RCV.NXT), then
           TCP endpoints MUST reset the connection in the manner
           prescribed below according to the connection state.
       3)  If the RST bit is set and the sequence number does not
           exactly match the next expected sequence value, yet is
           within the current receive window, TCP endpoints MUST send
           an acknowledgment (challenge ACK):
           <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
           After sending the challenge ACK, TCP endpoints MUST drop
           the unacceptable segment and stop processing the incoming
           packet further.  Note that RFC 5961 and Errata ID 4772 [99]
           contain additional considerations for ACK throttling in an
           implementation.
  1. SYN-RECEIVED STATE
       o  If the RST bit is set,
          +  If this connection was initiated with a passive OPEN
             (i.e., came from the LISTEN state), then return this
             connection to LISTEN state and return.  The user need not
             be informed.  If this connection was initiated with an
             active OPEN (i.e., came from SYN-SENT state), then the
             connection was refused; signal the user "connection
             refused".  In either case, the retransmission queue
             should be flushed.  And in the active OPEN case, enter
             the CLOSED state and delete the TCB, and return.
  1. ESTABLISHED STATE
  1. FIN-WAIT-1 STATE
  1. FIN-WAIT-2 STATE
  1. CLOSE-WAIT STATE
       o  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.
  1. CLOSING STATE
  1. LAST-ACK STATE
  1. TIME-WAIT STATE
       o  If the RST bit is set, then enter the CLOSED state, delete
          the TCB, and return.
    Third, check security:
  1. SYN-RECEIVED STATE
       o  If the security/compartment in the segment does not exactly
          match the security/compartment in the TCB, then send a reset
          and return.
  1. ESTABLISHED STATE
  1. FIN-WAIT-1 STATE
  1. FIN-WAIT-2 STATE
  1. CLOSE-WAIT STATE
  1. CLOSING STATE
  1. LAST-ACK STATE
  1. TIME-WAIT STATE
       o  If the security/compartment in the segment does not exactly
          match the security/compartment in the TCB, then send a
          reset; any outstanding RECEIVEs and SEND should receive
          "reset" responses.  All segment queues should be flushed.
          Users should also receive an unsolicited general "connection
          reset" signal.  Enter the CLOSED state, delete the TCB, and
          return.
  1. Note this check is placed following the sequence check to

prevent a segment from an old connection between these port

       numbers with a different security from causing an abort of the
       current connection.
    Fourth, check the SYN bit:
  1. SYN-RECEIVED STATE
       o  If the connection was initiated with a passive OPEN, then
          return this connection to the LISTEN state and return.
          Otherwise, handle per the directions for synchronized states
          below.
  1. ESTABLISHED STATE
  1. FIN-WAIT-1 STATE
  1. FIN-WAIT-2 STATE
  1. CLOSE-WAIT STATE
  1. CLOSING STATE
  1. LAST-ACK STATE
  1. TIME-WAIT STATE
       o  If the SYN bit is set in these synchronized states, it may
          be either a legitimate new connection attempt (e.g., in the
          case of TIME-WAIT), an error where the connection should be
          reset, or the result of an attack attempt, as described in
          RFC 5961 [9].  For the TIME-WAIT state, new connections can
          be accepted if the Timestamp Option is used and meets
          expectations (per [40]).  For all other cases, RFC 5961
          provides a mitigation with applicability to some situations,
          though there are also alternatives that offer cryptographic
          protection (see Section 7).  RFC 5961 recommends that in
          these synchronized states, if the SYN bit is set,
          irrespective of the sequence number, TCP endpoints MUST send
          a "challenge ACK" to the remote peer:
          <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
       o  After sending the acknowledgment, TCP implementations MUST
          drop the unacceptable segment and stop processing further.
          Note that RFC 5961 and Errata ID 4772 [99] contain
          additional ACK throttling notes for an implementation.
       o  For implementations that do not follow RFC 5961, the
          original behavior described in RFC 793 follows in this
          paragraph.  If the SYN is in the window it is an error: send
          a reset, any outstanding RECEIVEs and SEND should receive
          "reset" responses, all segment queues should be flushed, the
          user should also receive an unsolicited general "connection
          reset" signal, enter the CLOSED state, delete the TCB, and
          return.
       o  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:
  1. if the ACK bit is off, drop the segment and return
  1. if the ACK bit is on,
       o  RFC 5961 [9], Section 5 describes a potential blind data
          injection attack, and mitigation that implementations MAY
          choose to include (MAY-12).  TCP stacks that implement RFC
          5961 MUST add an input check that the ACK value is
          acceptable only if it is in the range of ((SND.UNA -
          MAX.SND.WND) =< SEG.ACK =< SND.NXT).  All incoming segments
          whose ACK value doesn't satisfy the above condition MUST be
          discarded and an ACK sent back.  The new state variable
          MAX.SND.WND is defined as the largest window that the local
          sender has ever received from its peer (subject to window
          scaling) or may be hard-coded to a maximum permissible
          window value.  When the ACK value is acceptable, the per-
          state processing below applies:
       o  SYN-RECEIVED STATE
          +  If SND.UNA < SEG.ACK =< SND.NXT, then enter ESTABLISHED
             state and continue processing with the variables below
             set to:
                SND.WND <- SEG.WND
                SND.WL1 <- SEG.SEQ
                SND.WL2 <- SEG.ACK
          +  If the segment acknowledgment is not acceptable, form a
             reset segment
                <SEQ=SEG.ACK><CTL=RST>
          +  and send it.
       o  ESTABLISHED STATE
          +  If SND.UNA < SEG.ACK =< SND.NXT, then set SND.UNA <-
             SEG.ACK.  Any segments on the retransmission queue that
             are thereby entirely acknowledged are removed.  Users
             should receive positive acknowledgments for buffers that
             have been SENT and fully acknowledged (i.e., SEND buffer
             should be returned with "ok" response).  If the ACK is a
             duplicate (SEG.ACK =< SND.UNA), it can be ignored.  If
             the ACK acks something not yet sent (SEG.ACK > SND.NXT),
             then send an ACK, drop the segment, and return.
          +  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 <- SEG.ACK.
          +  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.
       o  FIN-WAIT-1 STATE
          +  In addition to the processing for the ESTABLISHED state,
             if the FIN segment is now acknowledged, then enter FIN-
             WAIT-2 and continue processing in that state.
       o  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.
       o  CLOSE-WAIT STATE
          +  Do the same processing as for the ESTABLISHED state.
       o  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.
       o  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.
       o  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:
  1. ESTABLISHED STATE
  1. FIN-WAIT-1 STATE
  1. FIN-WAIT-2 STATE
       o  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.
  1. CLOSE-WAIT STATE
  1. CLOSING STATE
  1. LAST-ACK STATE
  1. TIME-WAIT STATE
       o  This should not occur since a FIN has been received from the
          remote side.  Ignore the URG.
    Seventh, process the segment text:
  1. ESTABLISHED STATE
  1. FIN-WAIT-1 STATE
  1. FIN-WAIT-2 STATE
       o  Once in the ESTABLISHED state, it is possible to deliver
          segment data to user RECEIVE buffers.  Data from segments
          can be moved into buffers until either the buffer is full or
          the segment is empty.  If the segment empties and carries a
          PUSH flag, then the user is informed, when the buffer is
          returned, that a PUSH has been received.
       o  When the TCP endpoint takes responsibility for delivering
          the data to the user, it must also acknowledge the receipt
          of the data.
       o  Once the TCP endpoint 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.
       o  A TCP implementation MAY send an ACK segment acknowledging
          RCV.NXT when a valid segment arrives that is in the window
          but not at the left window edge (MAY-13).
       o  Please note the window management suggestions in
          Section 3.8.
       o  Send an acknowledgment of the form:
          <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
       o  This acknowledgment should be piggybacked on a segment being
          transmitted if possible without incurring undue delay.
  1. CLOSE-WAIT STATE
  1. CLOSING STATE
  1. LAST-ACK STATE
  1. TIME-WAIT STATE
       o  This should not occur since a FIN has been received from the
          remote side.  Ignore the segment text.
    Eighth, check the FIN bit:
  1. 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.
  1. 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.
       o  SYN-RECEIVED STATE
       o  ESTABLISHED STATE
          +  Enter the CLOSE-WAIT state.
       o  FIN-WAIT-1 STATE
          +  If our FIN has been ACKed (perhaps in this segment), then
             enter TIME-WAIT, start the time-wait timer, turn off the
             other timers; otherwise, enter the CLOSING state.
       o  FIN-WAIT-2 STATE
          +  Enter the TIME-WAIT state.  Start the time-wait timer,
             turn off the other timers.
       o  CLOSE-WAIT STATE
          +  Remain in the CLOSE-WAIT state.
       o  CLOSING STATE
          +  Remain in the CLOSING state.
       o  LAST-ACK STATE
          +  Remain in the LAST-ACK state.
       o  TIME-WAIT STATE
          +  Remain in the TIME-WAIT state.  Restart the 2 MSL time-
             wait timeout.
    and return.

3.10.8. Timeouts

 USER TIMEOUT
  • 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.
 RETRANSMISSION TIMEOUT
  • 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.
 TIME-WAIT TIMEOUT
  • If the time-wait timeout expires on a connection, delete the TCB,

enter the CLOSED state, and return.

4. Glossary

 ACK    
         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.
 connection
         A logical communication path identified by a pair of sockets.
 datagram
         A message sent in a packet-switched computer communications
         network.
 Destination Address
         The network-layer address of the endpoint intended to receive
         a segment.
 FIN    
         A control bit (finis) occupying one sequence number, which
         indicates that the sender will send no more data or control
         occupying sequence space.
 flush  
         To remove all of the contents (data or segments) from a store
         (buffer or queue).
 fragment
         A portion of a logical unit of data.  In particular, an
         internet fragment is a portion of an internet datagram.
 header 
         Control information at the beginning of a message, segment,
         fragment, packet, or block of data.
 host   
         A computer.  In particular, a source or destination of
         messages from the point of view of the communication network.
 Identification
         An Internet Protocol field.  This identifying value assigned
         by the sender aids in assembling the fragments of a datagram.
 internet address
         A network-layer address.
 internet datagram
         A unit of data exchanged between internet hosts, together
         with the internet header that allows the datagram to be
         routed from source to destination.
 internet fragment
         A portion of the data of an internet datagram with an
         internet header.
 IP     
         Internet Protocol.  See [1] and [13].
 IRS    
         The Initial Receive Sequence number.  The first sequence
         number used by the sender on a connection.
 ISN    
         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.
 ISS    
         The Initial Send Sequence number.  The first sequence number
         used by the sender on a connection.
 left sequence
         This is the next sequence number to be acknowledged by the
         data-receiving TCP endpoint (or the lowest currently
         unacknowledged sequence number) and is sometimes referred to
         as the left edge of the send window.
 module 
         An implementation, usually in software, of a protocol or
         other procedure.
 MSL    
         Maximum Segment Lifetime, the time a TCP segment can exist in
         the internetwork system.  Arbitrarily defined to be 2
         minutes.
 octet  
         An eight-bit byte.
 Options
         An Option field may contain several options, and each option
         may be several octets in length.
 packet 
         A package of data with a header that may or may not be
         logically complete.  More often a physical packaging than a
         logical packaging of data.
 port   
         The portion of a connection identifier used for
         demultiplexing connections at an endpoint.
 process
         A program in execution.  A source or destination of data from
         the point of view of the TCP endpoint or other host-to-host
         protocol.
 PUSH   
         A control bit occupying no sequence space, indicating that
         this segment contains data that must be pushed through to the
         receiving user.
 RCV.NXT
         receive next sequence number
 RCV.UP 
         receive urgent pointer
 RCV.WND
         receive window
 receive next sequence number
         This is the next sequence number the local TCP endpoint is
         expecting to receive.
 receive window
         This represents the sequence numbers the local (receiving)
         TCP endpoint is willing to receive.  Thus, the local TCP
         endpoint 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 this range are considered duplicates or injection
         attacks and discarded.
 RST    
         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.
 SEG.ACK
         segment acknowledgment
 SEG.LEN
         segment length
 SEG.SEQ
         segment sequence
 SEG.UP 
         segment urgent pointer field
 SEG.WND
         segment window field
 segment
         A logical unit of data.  In particular, a TCP segment is the
         unit of data transferred 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 that 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
         endpoint 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 that the remote
         (receiving) TCP endpoint is willing to receive.  It is the
         value of the window field specified in segments from the
         remote (data-receiving) TCP endpoint.  The range of new
         sequence numbers that may be emitted by a TCP implementation
         lies between SND.NXT and SND.UNA + SND.WND - 1.
         (Retransmissions of sequence numbers between SND.UNA and
         SND.NXT are expected, of course.)
 SND.NXT
         send sequence
 SND.UNA
         left sequence
 SND.UP 
         send urgent pointer
 SND.WL1
         segment sequence number at last window update
 SND.WL2
         segment acknowledgment number at last window update
 SND.WND
         send window
 socket (or socket number, or socket address, or socket identifier)
         An address that specifically includes a port identifier, that
         is, the concatenation of an Internet Address with a TCP port.
 Source Address
         The network-layer address of the sending endpoint.
 SYN    
         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.
 TCB    
         Transmission control block, the data structure that records
         the state of a connection.
 TCP    
         Transmission Control Protocol: a host-to-host protocol for
         reliable communication in internetwork environments.
 TOS    
         Type of Service, an obsoleted IPv4 field.  The same header
         bits currently are used for the Differentiated Services field
         [4] containing the Differentiated Services Codepoint (DSCP)
         value and the 2-bit ECN codepoint [6].
 Type of Service
         See "TOS".
 URG    
         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 by 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 that
         indicates the data octet associated with the sending user's
         urgent call.

5. Changes from RFC 793

 This document obsoletes RFC 793 as well as RFCs 6093 and 6528, which
 updated 793.  In all cases, only the normative protocol specification
 and requirements have been incorporated into this document, and some
 informational text with background and rationale may not have 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 [73], 574 [74], 700 [75], 701 [76], 1283 [77], 1561
 [78], 1562 [79], 1564 [80], 1571 [81], 1572 [82], 2297 [83], 2298
 [84], 2748 [85], 2749 [86], 2934 [87], 3213 [88], 3300 [89], 3301
 [90], 6222 [91]).  Some errata were not applicable due to other
 changes (Errata IDs: 572 [92], 575 [93], 1565 [94], 1569 [95], 2296
 [96], 3305 [97], 3602 [98]).
 Changes to the specification of the urgent pointer described in RFCs
 1011, 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 text on the RTO in RFC 1122 originally replaced the text
 in RFC 793; however, RFC 2988 should have updated RFC 1122 and has
 subsequently been obsoleted by RFC 6298.
 RFC 1011 [18] contains a number of comments about RFC 793, including
 some needed changes to the TCP specification.  These are expanded in
 RFC 1122, which 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.  The present document, which is now the TCP
 specification rather than RFC 793, updates RFC 1011, and the comments
 noted in RFC 1011 have been incorporated.
 RFC 1122 contains more than just TCP requirements, so this document
 can't obsolete RFC 1122 entirely.  It is only marked as "updating"
 RFC 1122; however, it should be understood to effectively obsolete
 all of the material on TCP found in RFC 1122.
 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 management concerns allow connection resources to be
 reclaimed.  RFC 6429 is obsoleted in the sense that the clarification
 it describes has been reflected within this base TCP specification.
 The description of congestion control implementation was added based
 on the set of documents that are IETF BCP or Standards Track on the
 topic and the current state of common implementations.

6. IANA Considerations

 In the "Transmission Control Protocol (TCP) Header Flags" registry,
 IANA has made several changes as described in this section.
 RFC 3168 originally created this registry but only populated it with
 the new bits defined in RFC 3168, neglecting the other bits that had
 previously been described in RFC 793 and other documents.  Bit 7 has
 since also been updated by RFC 8311 [54].
 The "Bit" column has been renamed below as the "Bit Offset" column
 because it references each header flag's offset within the 16-bit
 aligned view of the TCP header in Figure 1.  The bits in offsets 0
 through 3 are the TCP segment Data Offset field, and not header
 flags.
 IANA has added a column for "Assignment Notes".
 IANA has assigned values as indicated below.
    +========+===================+===========+====================+
    | Bit    | Name              | Reference | Assignment Notes   |
    | Offset |                   |           |                    |
    +========+===================+===========+====================+
    | 4      | Reserved for      | RFC 9293  |                    |
    |        | future use        |           |                    |
    +--------+-------------------+-----------+--------------------+
    | 5      | Reserved for      | RFC 9293  |                    |
    |        | future use        |           |                    |
    +--------+-------------------+-----------+--------------------+
    | 6      | Reserved for      | RFC 9293  |                    |
    |        | future use        |           |                    |
    +--------+-------------------+-----------+--------------------+
    | 7      | Reserved for      | RFC 8311  | Previously used by |
    |        | future use        |           | Historic RFC 3540  |
    |        |                   |           | as NS (Nonce Sum). |
    +--------+-------------------+-----------+--------------------+
    | 8      | CWR (Congestion   | RFC 3168  |                    |
    |        | Window Reduced)   |           |                    |
    +--------+-------------------+-----------+--------------------+
    | 9      | ECE (ECN-Echo)    | RFC 3168  |                    |
    +--------+-------------------+-----------+--------------------+
    | 10     | Urgent pointer    | RFC 9293  |                    |
    |        | field is          |           |                    |
    |        | significant (URG) |           |                    |
    +--------+-------------------+-----------+--------------------+
    | 11     | Acknowledgment    | RFC 9293  |                    |
    |        | field is          |           |                    |
    |        | significant (ACK) |           |                    |
    +--------+-------------------+-----------+--------------------+
    | 12     | Push function     | RFC 9293  |                    |
    |        | (PSH)             |           |                    |
    +--------+-------------------+-----------+--------------------+
    | 13     | Reset the         | RFC 9293  |                    |
    |        | connection (RST)  |           |                    |
    +--------+-------------------+-----------+--------------------+
    | 14     | Synchronize       | RFC 9293  |                    |
    |        | sequence numbers  |           |                    |
    |        | (SYN)             |           |                    |
    +--------+-------------------+-----------+--------------------+
    | 15     | No more data from | RFC 9293  |                    |
    |        | sender (FIN)      |           |                    |
    +--------+-------------------+-----------+--------------------+
                       Table 7: TCP Header Flags
 The "TCP Header Flags" registry has also been moved to a subregistry
 under the global "Transmission Control Protocol (TCP) Parameters"
 registry <https://www.iana.org/assignments/tcp-parameters/>.
 The registry's Registration Procedure remains Standards Action, but
 the Reference has been updated to this document, and the Note has
 been removed.

7. Security and Privacy Considerations

 The TCP design includes only rudimentary security features that
 improve the robustness and reliability of connections and application
 data transfer, but there are no built-in cryptographic capabilities
 to support any form of confidentiality, authentication, or other
 typical security functions.  Non-cryptographic enhancements (e.g.,
 [9]) have been developed to improve robustness of TCP connections to
 particular types of attacks, but the applicability and protections of
 non-cryptographic enhancements are limited (e.g., see Section 1.1 of
 [9]).  Applications typically utilize lower-layer (e.g., IPsec) and
 upper-layer (e.g., TLS) protocols to provide security and privacy for
 TCP connections and application data carried in TCP.  Methods based
 on TCP Options have been developed as well, to support some security
 capabilities.
 In order to fully provide confidentiality, integrity protection, and
 authentication for TCP connections (including their control flags),
 IPsec is the only current effective method.  For integrity protection
 and authentication, the TCP Authentication Option (TCP-AO) [38] is
 available, with a proposed extension to also provide confidentiality
 for the segment payload.  Other methods discussed in this section may
 provide confidentiality or integrity protection for the payload, but
 for the TCP header only cover either a subset of the fields (e.g.,
 tcpcrypt [57]) or none at all (e.g., TLS).  Other security features
 that have been added to TCP (e.g., ISN generation, sequence number
 checks, and others) are only capable of partially hindering attacks.
 Applications using long-lived TCP flows have been vulnerable to
 attacks that exploit the processing of control flags described in
 earlier TCP specifications [33].  TCP-MD5 was a commonly implemented
 TCP Option to support authentication for some of these connections,
 but had flaws and is now deprecated.  TCP-AO provides a capability to
 protect long-lived TCP connections from attacks and has superior
 properties to TCP-MD5.  It does not provide any privacy for
 application data or for the TCP headers.
 The "tcpcrypt" [57] experimental extension to TCP provides the
 ability to cryptographically protect connection data.  Metadata
 aspects of the TCP flow are still visible, but the application stream
 is well protected.  Within the TCP header, only the urgent pointer
 and FIN flag are protected through tcpcrypt.
 The TCP Roadmap [49] includes notes about several RFCs related to TCP
 security.  Many of the enhancements provided by these RFCs have been
 integrated into the present document, including ISN generation,
 mitigating blind in-window attacks, and improving handling of soft
 errors and ICMP packets.  These are all discussed in greater detail
 in the referenced RFCs that originally described the changes needed
 to earlier TCP specifications.  Additionally, see RFC 6093 [39] for
 discussion of security considerations related to the urgent pointer
 field, which also discourages new applications from using the urgent
 pointer.
 Since TCP is often used for bulk transfer flows, some attacks are
 possible that abuse the TCP congestion control logic.  An example is
 "ACK-division" attacks.  Updates that have been made to the TCP
 congestion control specifications include mechanisms like Appropriate
 Byte Counting (ABC) [29] that act as mitigations to these attacks.
 Other attacks are focused on exhausting the resources of a TCP
 server.  Examples include SYN flooding [32] or wasting resources on
 non-progressing connections [41].  Operating systems commonly
 implement mitigations for these attacks.  Some common defenses also
 utilize proxies, stateful firewalls, and other technologies outside
 the end-host TCP implementation.
 The concept of a protocol's "wire image" is described in RFC 8546
 [56], which describes how TCP's cleartext headers expose more
 metadata to nodes on the path than is strictly required to route the
 packets to their destination.  On-path adversaries may be able to
 leverage this metadata.  Lessons learned in this respect from TCP
 have been applied in the design of newer transports like QUIC [60].
 Additionally, based partly on experiences with TCP and its
 extensions, there are considerations that might be applicable for
 future TCP extensions and other transports that the IETF has
 documented in RFC 9065 [61], along with IAB recommendations in RFC
 8558 [58] and [67].
 There are also methods of "fingerprinting" that can be used to infer
 the host TCP implementation (operating system) version or platform
 information.  These collect observations of several aspects, such as
 the options present in segments, the ordering of options, the
 specific behaviors in the case of various conditions, packet timing,
 packet sizing, and other aspects of the protocol that are left to be
 determined by an implementer, and can use those observations to
 identify information about the host and implementation.
 Since ICMP message processing also can interact with TCP connections,
 there is potential for ICMP-based attacks against TCP connections.
 These are discussed in RFC 5927 [100], along with mitigations that
 have been implemented.

8. References

8.1. Normative References

 [1]        Postel, J., "Internet Protocol", STD 5, RFC 791,
            DOI 10.17487/RFC0791, September 1981,
            <https://www.rfc-editor.org/info/rfc791>.
 [2]        Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
            DOI 10.17487/RFC1191, November 1990,
            <https://www.rfc-editor.org/info/rfc1191>.
 [3]        Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <https://www.rfc-editor.org/info/rfc2119>.
 [4]        Nichols, K., Blake, S., Baker, F., and D. Black,
            "Definition of the Differentiated Services Field (DS
            Field) in the IPv4 and IPv6 Headers", RFC 2474,
            DOI 10.17487/RFC2474, December 1998,
            <https://www.rfc-editor.org/info/rfc2474>.
 [5]        Floyd, S., "Congestion Control Principles", BCP 41,
            RFC 2914, DOI 10.17487/RFC2914, September 2000,
            <https://www.rfc-editor.org/info/rfc2914>.
 [6]        Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
            of Explicit Congestion Notification (ECN) to IP",
            RFC 3168, DOI 10.17487/RFC3168, September 2001,
            <https://www.rfc-editor.org/info/rfc3168>.
 [7]        Floyd, S. and M. Allman, "Specifying New Congestion
            Control Algorithms", BCP 133, RFC 5033,
            DOI 10.17487/RFC5033, August 2007,
            <https://www.rfc-editor.org/info/rfc5033>.
 [8]        Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
            Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
            <https://www.rfc-editor.org/info/rfc5681>.
 [9]        Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
            Robustness to Blind In-Window Attacks", RFC 5961,
            DOI 10.17487/RFC5961, August 2010,
            <https://www.rfc-editor.org/info/rfc5961>.
 [10]       Paxson, V., Allman, M., Chu, J., and M. Sargent,
            "Computing TCP's Retransmission Timer", RFC 6298,
            DOI 10.17487/RFC6298, June 2011,
            <https://www.rfc-editor.org/info/rfc6298>.
 [11]       Gont, F., "Deprecation of ICMP Source Quench Messages",
            RFC 6633, DOI 10.17487/RFC6633, May 2012,
            <https://www.rfc-editor.org/info/rfc6633>.
 [12]       Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
            2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
            May 2017, <https://www.rfc-editor.org/info/rfc8174>.
 [13]       Deering, S. and R. Hinden, "Internet Protocol, Version 6
            (IPv6) Specification", STD 86, RFC 8200,
            DOI 10.17487/RFC8200, July 2017,
            <https://www.rfc-editor.org/info/rfc8200>.
 [14]       McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
            "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
            DOI 10.17487/RFC8201, July 2017,
            <https://www.rfc-editor.org/info/rfc8201>.
 [15]       Allman, M., "Requirements for Time-Based Loss Detection",
            BCP 233, RFC 8961, DOI 10.17487/RFC8961, November 2020,
            <https://www.rfc-editor.org/info/rfc8961>.

8.2. Informative References

 [16]       Postel, J., "Transmission Control Protocol", STD 7,
            RFC 793, DOI 10.17487/RFC0793, September 1981,
            <https://www.rfc-editor.org/info/rfc793>.
 [17]       Nagle, J., "Congestion Control in IP/TCP Internetworks",
            RFC 896, DOI 10.17487/RFC0896, January 1984,
            <https://www.rfc-editor.org/info/rfc896>.
 [18]       Reynolds, J. and J. Postel, "Official Internet protocols",
            RFC 1011, DOI 10.17487/RFC1011, May 1987,
            <https://www.rfc-editor.org/info/rfc1011>.
 [19]       Braden, R., Ed., "Requirements for Internet Hosts -
            Communication Layers", STD 3, RFC 1122,
            DOI 10.17487/RFC1122, October 1989,
            <https://www.rfc-editor.org/info/rfc1122>.
 [20]       Almquist, P., "Type of Service in the Internet Protocol
            Suite", RFC 1349, DOI 10.17487/RFC1349, July 1992,
            <https://www.rfc-editor.org/info/rfc1349>.
 [21]       Braden, R., "T/TCP -- TCP Extensions for Transactions
            Functional Specification", RFC 1644, DOI 10.17487/RFC1644,
            July 1994, <https://www.rfc-editor.org/info/rfc1644>.
 [22]       Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
            Selective Acknowledgment Options", RFC 2018,
            DOI 10.17487/RFC2018, October 1996,
            <https://www.rfc-editor.org/info/rfc2018>.
 [23]       Paxson, V., Allman, M., Dawson, S., Fenner, W., Griner,
            J., Heavens, I., Lahey, K., Semke, J., and B. Volz, "Known
            TCP Implementation Problems", RFC 2525,
            DOI 10.17487/RFC2525, March 1999,
            <https://www.rfc-editor.org/info/rfc2525>.
 [24]       Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
            RFC 2675, DOI 10.17487/RFC2675, August 1999,
            <https://www.rfc-editor.org/info/rfc2675>.
 [25]       Xiao, X., Hannan, A., Paxson, V., and E. Crabbe, "TCP
            Processing of the IPv4 Precedence Field", RFC 2873,
            DOI 10.17487/RFC2873, June 2000,
            <https://www.rfc-editor.org/info/rfc2873>.
 [26]       Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
            Extension to the Selective Acknowledgement (SACK) Option
            for TCP", RFC 2883, DOI 10.17487/RFC2883, July 2000,
            <https://www.rfc-editor.org/info/rfc2883>.
 [27]       Lahey, K., "TCP Problems with Path MTU Discovery",
            RFC 2923, DOI 10.17487/RFC2923, September 2000,
            <https://www.rfc-editor.org/info/rfc2923>.
 [28]       Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
            Sooriyabandara, "TCP Performance Implications of Network
            Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
            December 2002, <https://www.rfc-editor.org/info/rfc3449>.
 [29]       Allman, M., "TCP Congestion Control with Appropriate Byte
            Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
            2003, <https://www.rfc-editor.org/info/rfc3465>.
 [30]       Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
            ICMPv6, UDP, and TCP Headers", RFC 4727,
            DOI 10.17487/RFC4727, November 2006,
            <https://www.rfc-editor.org/info/rfc4727>.
 [31]       Mathis, M. and J. Heffner, "Packetization Layer Path MTU
            Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
            <https://www.rfc-editor.org/info/rfc4821>.
 [32]       Eddy, W., "TCP SYN Flooding Attacks and Common
            Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
            <https://www.rfc-editor.org/info/rfc4987>.
 [33]       Touch, J., "Defending TCP Against Spoofing Attacks",
            RFC 4953, DOI 10.17487/RFC4953, July 2007,
            <https://www.rfc-editor.org/info/rfc4953>.
 [34]       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, <https://www.rfc-editor.org/info/rfc5044>.
 [35]       Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
            DOI 10.17487/RFC5461, February 2009,
            <https://www.rfc-editor.org/info/rfc5461>.
 [36]       StJohns, M., Atkinson, R., and G. Thomas, "Common
            Architecture Label IPv6 Security Option (CALIPSO)",
            RFC 5570, DOI 10.17487/RFC5570, July 2009,
            <https://www.rfc-editor.org/info/rfc5570>.
 [37]       Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
            Header Compression (ROHC) Framework", RFC 5795,
            DOI 10.17487/RFC5795, March 2010,
            <https://www.rfc-editor.org/info/rfc5795>.
 [38]       Touch, J., Mankin, A., and R. Bonica, "The TCP
            Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
            June 2010, <https://www.rfc-editor.org/info/rfc5925>.
 [39]       Gont, F. and A. Yourtchenko, "On the Implementation of the
            TCP Urgent Mechanism", RFC 6093, DOI 10.17487/RFC6093,
            January 2011, <https://www.rfc-editor.org/info/rfc6093>.
 [40]       Gont, F., "Reducing the TIME-WAIT State Using TCP
            Timestamps", BCP 159, RFC 6191, DOI 10.17487/RFC6191,
            April 2011, <https://www.rfc-editor.org/info/rfc6191>.
 [41]       Bashyam, M., Jethanandani, M., and A. Ramaiah, "TCP Sender
            Clarification for Persist Condition", RFC 6429,
            DOI 10.17487/RFC6429, December 2011,
            <https://www.rfc-editor.org/info/rfc6429>.
 [42]       Gont, F. and S. Bellovin, "Defending against Sequence
            Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February
            2012, <https://www.rfc-editor.org/info/rfc6528>.
 [43]       Borman, D., "TCP Options and Maximum Segment Size (MSS)",
            RFC 6691, DOI 10.17487/RFC6691, July 2012,
            <https://www.rfc-editor.org/info/rfc6691>.
 [44]       Touch, J., "Updated Specification of the IPv4 ID Field",
            RFC 6864, DOI 10.17487/RFC6864, February 2013,
            <https://www.rfc-editor.org/info/rfc6864>.
 [45]       Touch, J., "Shared Use of Experimental TCP Options",
            RFC 6994, DOI 10.17487/RFC6994, August 2013,
            <https://www.rfc-editor.org/info/rfc6994>.
 [46]       McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
            "Architectural Considerations of IP Anycast", RFC 7094,
            DOI 10.17487/RFC7094, January 2014,
            <https://www.rfc-editor.org/info/rfc7094>.
 [47]       Borman, D., Braden, B., Jacobson, V., and R.
            Scheffenegger, Ed., "TCP Extensions for High Performance",
            RFC 7323, DOI 10.17487/RFC7323, September 2014,
            <https://www.rfc-editor.org/info/rfc7323>.
 [48]       Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
            Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
            <https://www.rfc-editor.org/info/rfc7413>.
 [49]       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,
            <https://www.rfc-editor.org/info/rfc7414>.
 [50]       Black, D., Ed. and P. Jones, "Differentiated Services
            (Diffserv) and Real-Time Communication", RFC 7657,
            DOI 10.17487/RFC7657, November 2015,
            <https://www.rfc-editor.org/info/rfc7657>.
 [51]       Fairhurst, G. and M. Welzl, "The Benefits of Using
            Explicit Congestion Notification (ECN)", RFC 8087,
            DOI 10.17487/RFC8087, March 2017,
            <https://www.rfc-editor.org/info/rfc8087>.
 [52]       Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
            Ed., "Services Provided by IETF Transport Protocols and
            Congestion Control Mechanisms", RFC 8095,
            DOI 10.17487/RFC8095, March 2017,
            <https://www.rfc-editor.org/info/rfc8095>.
 [53]       Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of
            Transport Features Provided by IETF Transport Protocols",
            RFC 8303, DOI 10.17487/RFC8303, February 2018,
            <https://www.rfc-editor.org/info/rfc8303>.
 [54]       Black, D., "Relaxing Restrictions on Explicit Congestion
            Notification (ECN) Experimentation", RFC 8311,
            DOI 10.17487/RFC8311, January 2018,
            <https://www.rfc-editor.org/info/rfc8311>.
 [55]       Chown, T., Loughney, J., and T. Winters, "IPv6 Node
            Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
            January 2019, <https://www.rfc-editor.org/info/rfc8504>.
 [56]       Trammell, B. and M. Kuehlewind, "The Wire Image of a
            Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
            2019, <https://www.rfc-editor.org/info/rfc8546>.
 [57]       Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack,
            Q., and E. Smith, "Cryptographic Protection of TCP Streams
            (tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019,
            <https://www.rfc-editor.org/info/rfc8548>.
 [58]       Hardie, T., Ed., "Transport Protocol Path Signals",
            RFC 8558, DOI 10.17487/RFC8558, April 2019,
            <https://www.rfc-editor.org/info/rfc8558>.
 [59]       Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
            Paasch, "TCP Extensions for Multipath Operation with
            Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
            2020, <https://www.rfc-editor.org/info/rfc8684>.
 [60]       Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
            Multiplexed and Secure Transport", RFC 9000,
            DOI 10.17487/RFC9000, May 2021,
            <https://www.rfc-editor.org/info/rfc9000>.
 [61]       Fairhurst, G. and C. Perkins, "Considerations around
            Transport Header Confidentiality, Network Operations, and
            the Evolution of Internet Transport Protocols", RFC 9065,
            DOI 10.17487/RFC9065, July 2021,
            <https://www.rfc-editor.org/info/rfc9065>.
 [62]       IANA, "Transmission Control Protocol (TCP) Parameters",
            <https://www.iana.org/assignments/tcp-parameters/>.
 [63]       Gont, F., "Processing of IP Security/Compartment and
            Precedence Information by TCP", Work in Progress,
            Internet-Draft, draft-gont-tcpm-tcp-seccomp-prec-00, 29
            March 2012, <https://datatracker.ietf.org/doc/html/draft-
            gont-tcpm-tcp-seccomp-prec-00>.
 [64]       Gont, F. and D. Borman, "On the Validation of TCP Sequence
            Numbers", Work in Progress, Internet-Draft, draft-gont-
            tcpm-tcp-seq-validation-04, 11 March 2019,
            <https://datatracker.ietf.org/doc/html/draft-gont-tcpm-
            tcp-seq-validation-04>.
 [65]       Touch, J. and W. M. Eddy, "TCP Extended Data Offset
            Option", Work in Progress, Internet-Draft, draft-ietf-
            tcpm-tcp-edo-12, 15 April 2022,
            <https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-
            tcp-edo-12>.
 [66]       McQuistin, S., Band, V., Jacob, D., and C. Perkins,
            "Describing Protocol Data Units with Augmented Packet
            Header Diagrams", Work in Progress, Internet-Draft, draft-
            mcquistin-augmented-ascii-diagrams-10, 7 March 2022,
            <https://datatracker.ietf.org/doc/html/draft-mcquistin-
            augmented-ascii-diagrams-10>.
 [67]       Thomson, M. and T. Pauly, "Long-Term Viability of Protocol
            Extension Mechanisms", RFC 9170, DOI 10.17487/RFC9170,
            December 2021, <https://www.rfc-editor.org/info/rfc9170>.
 [68]       Minshall, G., "A Suggested Modification to Nagle's
            Algorithm", Work in Progress, Internet-Draft, draft-
            minshall-nagle-01, 18 June 1999,
            <https://datatracker.ietf.org/doc/html/draft-minshall-
            nagle-01>.
 [69]       Dalal, Y. and C. Sunshine, "Connection Management in
            Transport Protocols", Computer Networks, Vol. 2, No. 6,
            pp. 454-473, DOI 10.1016/0376-5075(78)90053-3, December
            1978, <https://doi.org/10.1016/0376-5075(78)90053-3>.
 [70]       Faber, T., Touch, J., and W. Yui, "The TIME-WAIT state in
            TCP and Its Effect on Busy Servers", Proceedings of IEEE
            INFOCOM, pp. 1573-1583, DOI 10.1109/INFCOM.1999.752180,
            March 1999, <https://doi.org/10.1109/INFCOM.1999.752180>.
 [71]       Postel, J., "Comments on Action Items from the January
            Meeting", IEN 177, March 1981,
            <https://www.rfc-editor.org/ien/ien177.txt>.
 [72]       "Segmentation Offloads", The Linux Kernel Documentation,
            <https://www.kernel.org/doc/html/latest/networking/
            segmentation-offloads.html>.
 [73]       RFC Errata, Erratum ID 573, RFC 793,
            <https://www.rfc-editor.org/errata/eid573>.
 [74]       RFC Errata, Erratum ID 574, RFC 793,
            <https://www.rfc-editor.org/errata/eid574>.
 [75]       RFC Errata, Erratum ID 700, RFC 793,
            <https://www.rfc-editor.org/errata/eid700>.
 [76]       RFC Errata, Erratum ID 701, RFC 793,
            <https://www.rfc-editor.org/errata/eid701>.
 [77]       RFC Errata, Erratum ID 1283, RFC 793,
            <https://www.rfc-editor.org/errata/eid1283>.
 [78]       RFC Errata, Erratum ID 1561, RFC 793,
            <https://www.rfc-editor.org/errata/eid1561>.
 [79]       RFC Errata, Erratum ID 1562, RFC 793,
            <https://www.rfc-editor.org/errata/eid1562>.
 [80]       RFC Errata, Erratum ID 1564, RFC 793,
            <https://www.rfc-editor.org/errata/eid1564>.
 [81]       RFC Errata, Erratum ID 1571, RFC 793,
            <https://www.rfc-editor.org/errata/eid1571>.
 [82]       RFC Errata, Erratum ID 1572, RFC 793,
            <https://www.rfc-editor.org/errata/eid1572>.
 [83]       RFC Errata, Erratum ID 2297, RFC 793,
            <https://www.rfc-editor.org/errata/eid2297>.
 [84]       RFC Errata, Erratum ID 2298, RFC 793,
            <https://www.rfc-editor.org/errata/eid2298>.
 [85]       RFC Errata, Erratum ID 2748, RFC 793,
            <https://www.rfc-editor.org/errata/eid2748>.
 [86]       RFC Errata, Erratum ID 2749, RFC 793,
            <https://www.rfc-editor.org/errata/eid2749>.
 [87]       RFC Errata, Erratum ID 2934, RFC 793,
            <https://www.rfc-editor.org/errata/eid2934>.
 [88]       RFC Errata, Erratum ID 3213, RFC 793,
            <https://www.rfc-editor.org/errata/eid3213>.
 [89]       RFC Errata, Erratum ID 3300, RFC 793,
            <https://www.rfc-editor.org/errata/eid3300>.
 [90]       RFC Errata, Erratum ID 3301, RFC 793,
            <https://www.rfc-editor.org/errata/eid3301>.
 [91]       RFC Errata, Erratum ID 6222, RFC 793,
            <https://www.rfc-editor.org/errata/eid6222>.
 [92]       RFC Errata, Erratum ID 572, RFC 793,
            <https://www.rfc-editor.org/errata/eid572>.
 [93]       RFC Errata, Erratum ID 575, RFC 793,
            <https://www.rfc-editor.org/errata/eid575>.
 [94]       RFC Errata, Erratum ID 1565, RFC 793,
            <https://www.rfc-editor.org/errata/eid1565>.
 [95]       RFC Errata, Erratum ID 1569, RFC 793,
            <https://www.rfc-editor.org/errata/eid1569>.
 [96]       RFC Errata, Erratum ID 2296, RFC 793,
            <https://www.rfc-editor.org/errata/eid2296>.
 [97]       RFC Errata, Erratum ID 3305, RFC 793,
            <https://www.rfc-editor.org/errata/eid3305>.
 [98]       RFC Errata, Erratum ID 3602, RFC 793,
            <https://www.rfc-editor.org/errata/eid3602>.
 [99]       RFC Errata, Erratum ID 4772, RFC 5961,
            <https://www.rfc-editor.org/errata/eid4772>.
 [100]      Gont, F., "ICMP Attacks against TCP", RFC 5927,
            DOI 10.17487/RFC5927, July 2010,
            <https://www.rfc-editor.org/info/rfc5927>.

Appendix A. Other Implementation Notes

 This section includes additional notes and references on TCP
 implementation decisions that are currently not a part of the RFC
 series or included within the TCP standard.  These items can be
 considered by implementers, but there was not yet a consensus to
 include them in the standard.

A.1. IP Security Compartment and Precedence

 The IPv4 specification [1] includes a precedence value in the (now
 obsoleted) Type of Service (TOS) field.  It was modified in [20] and
 then obsoleted by the definition of Differentiated Services
 (Diffserv) [4].  Setting and conveying TOS between the network layer,
 TCP implementation, and applications is obsolete and is replaced by
 Diffserv in the current TCP specification.
 RFC 793 required checking the IP security compartment and precedence
 on incoming TCP segments for consistency within a connection and with
 application requests.  Each of these aspects of IP have become
 outdated, without specific updates to RFC 793.  The issues with
 precedence were fixed by [25], which is Standards Track, and so this
 present TCP specification includes those changes.  However, the state
 of IP security options that may be used by Multi-Level Secure (MLS)
 systems is not as apparent in the IETF currently.
 Resetting connections when incoming packets do not meet expected
 security compartment or precedence expectations has been recognized
 as a possible attack vector [63], and there has been discussion about
 amending the TCP specification to prevent connections from being
 aborted due to nonmatching IP security compartment and Diffserv
 codepoint values.

A.1.1. Precedence

 In Diffserv, the former precedence values are treated as Class
 Selector codepoints, and methods for compatible treatment are
 described in the Diffserv architecture.  The RFC TCP specification
 defined by RFCs 793 and 1122 included logic intending to have
 connections use the highest precedence requested by either endpoint
 application, and to keep the precedence consistent throughout a
 connection.  This logic from the obsolete TOS is not applicable to
 Diffserv and should not be included in TCP implementations, though
 changes to Diffserv values within a connection are discouraged.  For
 discussion of this, see RFC 7657 (Sections 5.1, 5.3, and 6) [50].
 The obsoleted TOS processing rules in TCP assumed bidirectional (or
 symmetric) precedence values used on a connection, but the Diffserv
 architecture is asymmetric.  Problems with the old TCP logic in this
 regard were described in [25], and the solution described is to
 ignore IP precedence in TCP.  Since RFC 2873 is a Standards Track
 document (although not marked as updating RFC 793), current
 implementations are expected to be robust in these conditions.  Note
 that the Diffserv field value used in each direction is a part of the
 interface between TCP and the network layer, and values in use can be
 indicated both ways between TCP and the application.

A.1.2. MLS Systems

 The IP Security Option (IPSO) and compartment defined in [1] was
 refined in RFC 1038, which was later obsoleted by RFC 1108.  The
 Commercial IP Security Option (CIPSO) is defined in FIPS-188
 (withdrawn by NIST in 2015) and is supported by some vendors and
 operating systems.  RFC 1108 is now Historic, though RFC 791 itself
 has not been updated to remove the IP Security Option.  For IPv6, a
 similar option (Common Architecture Label IPv6 Security Option
 (CALIPSO)) has been defined [36].  RFC 793 includes logic that
 includes the IP security/compartment information in treatment of TCP
 segments.  References to the IP "security/compartment" in this
 document may be relevant for Multi-Level Secure (MLS) system
 implementers but can be ignored for non-MLS implementations,
 consistent with running code on the Internet.  See Appendix A.1 for
 further discussion.  Note that RFC 5570 describes some MLS networking
 scenarios where IPSO, CIPSO, or CALIPSO may be used.  In these
 special cases, TCP implementers should see Section 7.3.1 of RFC 5570
 and follow the guidance in that document.

A.2. Sequence Number Validation

 There are cases where the TCP sequence number validation rules can
 prevent ACK fields from being processed.  This can result in
 connection issues, as described in [64], which includes descriptions
 of potential problems in conditions of simultaneous open, self-
 connects, simultaneous close, and simultaneous window probes.  The
 document also describes potential changes to the TCP specification to
 mitigate the issue by expanding the acceptable sequence numbers.
 In Internet usage of TCP, these conditions rarely occur.  Common
 operating systems include different alternative mitigations, and the
 standard has not been updated yet to codify one of them, but
 implementers should consider the problems described in [64].

A.3. Nagle Modification

 In common operating systems, both the Nagle algorithm and delayed
 acknowledgments are implemented and enabled by default.  TCP is used
 by many applications that have a request-response style of
 communication, where the combination of the Nagle algorithm and
 delayed acknowledgments can result in poor application performance.
 A modification to the Nagle algorithm is described in [68] that
 improves the situation for these applications.
 This modification is implemented in some common operating systems and
 does not impact TCP interoperability.  Additionally, many
 applications simply disable Nagle since this is generally supported
 by a socket option.  The TCP standard has not been updated to include
 this Nagle modification, but implementers may find it beneficial to
 consider.

A.4. Low Watermark Settings

 Some operating system kernel TCP implementations include socket
 options that allow specifying the number of bytes in the buffer until
 the socket layer will pass sent data to TCP (SO_SNDLOWAT) or to the
 application on receiving (SO_RCVLOWAT).
 In addition, another socket option (TCP_NOTSENT_LOWAT) can be used to
 control the amount of unsent bytes in the write queue.  This can help
 a sending TCP application to avoid creating large amounts of buffered
 data (and corresponding latency).  As an example, this may be useful
 for applications that are multiplexing data from multiple upper-level
 streams onto a connection, especially when streams may be a mix of
 interactive/real-time and bulk data transfer.

Appendix B. TCP Requirement Summary

 This section is adapted from RFC 1122.
 Note that there is no requirement related to PLPMTUD in this list,
 but that PLPMTUD is recommended.
  +=================+=========+======+========+=====+========+======+
  |     Feature     |  ReqID  | MUST | SHOULD | MAY | SHOULD | MUST |
  |                 |         |      |        |     |  NOT   | NOT  |
  +=================+=========+======+========+=====+========+======+
  | PUSH flag                                                       |
  +=================+=========+======+========+=====+========+======+
  | Aggregate or    | MAY-16  |      |        |  X  |        |      |
  | queue un-pushed |         |      |        |     |        |      |
  | data            |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Sender collapse | SHLD-27 |      |   X    |     |        |      |
  | successive PSH  |         |      |        |     |        |      |
  | bits            |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | SEND call can   | MAY-15  |      |        |  X  |        |      |
  | specify PUSH    |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  If cannot:   | MUST-60 |      |        |     |        |  X   |
  |    sender       |         |      |        |     |        |      |
  |    buffer       |         |      |        |     |        |      |
  |    indefinitely |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  If cannot:   | MUST-61 |  X   |        |     |        |      |
  |    PSH last     |         |      |        |     |        |      |
  |    segment      |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Notify          | MAY-17  |      |        |  X  |        |      |
  | receiving ALP^1 |         |      |        |     |        |      |
  | of PSH          |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Send max size   | SHLD-28 |      |   X    |     |        |      |
  | segment when    |         |      |        |     |        |      |
  | possible        |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | Window                                                          |
  +=================+=========+======+========+=====+========+======+
  | Treat as        | MUST-1  |  X   |        |     |        |      |
  | unsigned number |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Handle as       | REC-1   |      |   X    |     |        |      |
  | 32-bit number   |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Shrink window   | SHLD-14 |      |        |     |   X    |      |
  | from right      |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Send new     | SHLD-15 |      |        |     |   X    |      |
  |    data when    |         |      |        |     |        |      |
  |    window       |         |      |        |     |        |      |
  |    shrinks      |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Retransmit   | SHLD-16 |      |   X    |     |        |      |
  |    old unacked  |         |      |        |     |        |      |
  |    data within  |         |      |        |     |        |      |
  |    window       |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Time out     | SHLD-17 |      |        |     |   X    |      |
  |    conn for     |         |      |        |     |        |      |
  |    data past    |         |      |        |     |        |      |
  |    right edge   |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Robust against  | MUST-34 |  X   |        |     |        |      |
  | shrinking       |         |      |        |     |        |      |
  | window          |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Receiver's      | MAY-8   |      |        |  X  |        |      |
  | window closed   |         |      |        |     |        |      |
  | indefinitely    |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Use standard    | MUST-35 |  X   |        |     |        |      |
  | probing logic   |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Sender probe    | MUST-36 |  X   |        |     |        |      |
  | zero window     |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  First probe  | SHLD-29 |      |   X    |     |        |      |
  |    after RTO    |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Exponential  | SHLD-30 |      |   X    |     |        |      |
  |    backoff      |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Allow window    | MUST-37 |  X   |        |     |        |      |
  | stay zero       |         |      |        |     |        |      |
  | indefinitely    |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Retransmit old  | MAY-7   |      |        |  X  |        |      |
  | data beyond     |         |      |        |     |        |      |
  | SND.UNA+SND.WND |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Process RST and | MUST-66 |  X   |        |     |        |      |
  | URG even with   |         |      |        |     |        |      |
  | zero window     |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | Urgent Data                                                     |
  +=================+=========+======+========+=====+========+======+
  | Include support | MUST-30 |  X   |        |     |        |      |
  | for urgent      |         |      |        |     |        |      |
  | pointer         |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Pointer         | MUST-62 |  X   |        |     |        |      |
  | indicates first |         |      |        |     |        |      |
  | non-urgent      |         |      |        |     |        |      |
  | octet           |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Arbitrary       | MUST-31 |  X   |        |     |        |      |
  | length urgent   |         |      |        |     |        |      |
  | data sequence   |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Inform ALP^1    | MUST-32 |  X   |        |     |        |      |
  | asynchronously  |         |      |        |     |        |      |
  | of urgent data  |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | ALP^1 can learn | MUST-33 |  X   |        |     |        |      |
  | if/how much     |         |      |        |     |        |      |
  | urgent data Q'd |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | ALP employ the  | SHLD-13 |      |        |     |   X    |      |
  | urgent          |         |      |        |     |        |      |
  | mechanism       |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | TCP Options                                                     |
  +=================+=========+======+========+=====+========+======+
  | Support the     | MUST-4  |  X   |        |     |        |      |
  | mandatory       |         |      |        |     |        |      |
  | option set      |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Receive TCP     | MUST-5  |  X   |        |     |        |      |
  | Option in any   |         |      |        |     |        |      |
  | segment         |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Ignore          | MUST-6  |  X   |        |     |        |      |
  | unsupported     |         |      |        |     |        |      |
  | options         |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Include length  | MUST-68 |  X   |        |     |        |      |
  | for all options |         |      |        |     |        |      |
  | except EOL+NOP  |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Cope with       | MUST-7  |  X   |        |     |        |      |
  | illegal option  |         |      |        |     |        |      |
  | length          |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Process options | MUST-64 |  X   |        |     |        |      |
  | regardless of   |         |      |        |     |        |      |
  | word alignment  |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Implement       | MUST-14 |  X   |        |     |        |      |
  | sending &       |         |      |        |     |        |      |
  | receiving MSS   |         |      |        |     |        |      |
  | Option          |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | IPv4 Send MSS   | SHLD-5  |      |   X    |     |        |      |
  | Option unless   |         |      |        |     |        |      |
  | 536             |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | IPv6 Send MSS   | SHLD-5  |      |   X    |     |        |      |
  | Option unless   |         |      |        |     |        |      |
  | 1220            |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Send MSS Option | MAY-3   |      |        |  X  |        |      |
  | always          |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | IPv4 Send-MSS   | MUST-15 |  X   |        |     |        |      |
  | default is 536  |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | IPv6 Send-MSS   | MUST-15 |  X   |        |     |        |      |
  | default is 1220 |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Calculate       | MUST-16 |  X   |        |     |        |      |
  | effective send  |         |      |        |     |        |      |
  | seg size        |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | MSS accounts    | SHLD-6  |      |   X    |     |        |      |
  | for varying MTU |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | MSS not sent on | MUST-65 |      |        |     |        |  X   |
  | non-SYN         |         |      |        |     |        |      |
  | segments        |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | MSS value based | MUST-67 |  X   |        |     |        |      |
  | on MMS_R        |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Pad with zero   | MUST-69 |  X   |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | TCP Checksums                                                   |
  +=================+=========+======+========+=====+========+======+
  | Sender compute  | MUST-2  |  X   |        |     |        |      |
  | checksum        |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Receiver check  | MUST-3  |  X   |        |     |        |      |
  | checksum        |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | ISN Selection                                                   |
  +=================+=========+======+========+=====+========+======+
  | Include a       | MUST-8  |  X   |        |     |        |      |
  | clock-driven    |         |      |        |     |        |      |
  | ISN generator   |         |      |        |     |        |      |
  | component       |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Secure ISN      | SHLD-1  |      |   X    |     |        |      |
  | generator with  |         |      |        |     |        |      |
  | a PRF component |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | PRF computable  | MUST-9  |      |        |     |        |  X   |
  | from outside    |         |      |        |     |        |      |
  | the host        |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | Opening Connections                                             |
  +=================+=========+======+========+=====+========+======+
  | Support         | MUST-10 |  X   |        |     |        |      |
  | simultaneous    |         |      |        |     |        |      |
  | open attempts   |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | SYN-RECEIVED    | MUST-11 |  X   |        |     |        |      |
  | remembers last  |         |      |        |     |        |      |
  | state           |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Passive OPEN    | MUST-41 |      |        |     |        |  X   |
  | call interfere  |         |      |        |     |        |      |
  | with others     |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Function:       | MUST-42 |  X   |        |     |        |      |
  | simultaneously  |         |      |        |     |        |      |
  | LISTENs for     |         |      |        |     |        |      |
  | same port       |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Ask IP for src  | MUST-44 |  X   |        |     |        |      |
  | address for SYN |         |      |        |     |        |      |
  | if necessary    |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Otherwise,   | MUST-45 |  X   |        |     |        |      |
  |    use local    |         |      |        |     |        |      |
  |    addr of      |         |      |        |     |        |      |
  |    connection   |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | OPEN to         | MUST-46 |      |        |     |        |  X   |
  | broadcast/      |         |      |        |     |        |      |
  | multicast IP    |         |      |        |     |        |      |
  | address         |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Silently        | MUST-57 |  X   |        |     |        |      |
  | discard seg to  |         |      |        |     |        |      |
  | bcast/mcast     |         |      |        |     |        |      |
  | addr            |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | Closing Connections                                             |
  +=================+=========+======+========+=====+========+======+
  | RST can contain | SHLD-2  |      |   X    |     |        |      |
  | data            |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Inform          | MUST-12 |  X   |        |     |        |      |
  | application of  |         |      |        |     |        |      |
  | aborted conn    |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Half-duplex     | MAY-1   |      |        |  X  |        |      |
  | close           |         |      |        |     |        |      |
  | connections     |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Send RST to  | SHLD-3  |      |   X    |     |        |      |
  |    indicate     |         |      |        |     |        |      |
  |    data lost    |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | In TIME-WAIT    | MUST-13 |  X   |        |     |        |      |
  | state for 2MSL  |         |      |        |     |        |      |
  | seconds         |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Accept SYN   | MAY-2   |      |        |  X  |        |      |
  |    from TIME-   |         |      |        |     |        |      |
  |    WAIT state   |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Use          | SHLD-4  |      |   X    |     |        |      |
  |    Timestamps   |         |      |        |     |        |      |
  |    to reduce    |         |      |        |     |        |      |
  |    TIME-WAIT    |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | Retransmissions                                                 |
  +=================+=========+======+========+=====+========+======+
  | Implement       | MUST-19 |  X   |        |     |        |      |
  | exponential     |         |      |        |     |        |      |
  | backoff, slow   |         |      |        |     |        |      |
  | start, and      |         |      |        |     |        |      |
  | congestion      |         |      |        |     |        |      |
  | avoidance       |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Retransmit with | MAY-4   |      |        |  X  |        |      |
  | same IP         |         |      |        |     |        |      |
  | identity        |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Karn's          | MUST-18 |  X   |        |     |        |      |
  | algorithm       |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | Generating ACKs                                                 |
  +=================+=========+======+========+=====+========+======+
  | Aggregate       | MUST-58 |  X   |        |     |        |      |
  | whenever        |         |      |        |     |        |      |
  | possible        |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Queue out-of-   | SHLD-31 |      |   X    |     |        |      |
  | order segments  |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Process all Q'd | MUST-59 |  X   |        |     |        |      |
  | before send ACK |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Send ACK for    | MAY-13  |      |        |  X  |        |      |
  | out-of-order    |         |      |        |     |        |      |
  | segment         |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Delayed ACKs    | SHLD-18 |      |   X    |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Delay < 0.5  | MUST-40 |  X   |        |     |        |      |
  |    seconds      |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Every 2nd    | SHLD-19 |      |   X    |     |        |      |
  |    full-sized   |         |      |        |     |        |      |
  |    segment or   |         |      |        |     |        |      |
  |    2*RMSS ACK'd |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Receiver SWS-   | MUST-39 |  X   |        |     |        |      |
  | Avoidance       |         |      |        |     |        |      |
  | Algorithm       |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | Sending Data                                                    |
  +=================+=========+======+========+=====+========+======+
  | Configurable    | MUST-49 |  X   |        |     |        |      |
  | TTL             |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Sender SWS-     | MUST-38 |  X   |        |     |        |      |
  | Avoidance       |         |      |        |     |        |      |
  | Algorithm       |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Nagle algorithm | SHLD-7  |      |   X    |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Application  | MUST-17 |  X   |        |     |        |      |
  |    can disable  |         |      |        |     |        |      |
  |    Nagle        |         |      |        |     |        |      |
  |    algorithm    |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | Connection Failures                                             |
  +=================+=========+======+========+=====+========+======+
  | Negative advice | MUST-20 |  X   |        |     |        |      |
  | to IP on R1     |         |      |        |     |        |      |
  | retransmissions |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Close           | MUST-20 |  X   |        |     |        |      |
  | connection on   |         |      |        |     |        |      |
  | R2              |         |      |        |     |        |      |
  | retransmissions |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | ALP^1 can set   | MUST-21 |  X   |        |     |        |      |
  | R2              |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Inform ALP of   | SHLD-9  |      |   X    |     |        |      |
  | R1<=retxs<R2    |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Recommended     | SHLD-10 |      |   X    |     |        |      |
  | value for R1    |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Recommended     | SHLD-11 |      |   X    |     |        |      |
  | value for R2    |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Same mechanism  | MUST-22 |  X   |        |     |        |      |
  | for SYNs        |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  R2 at least  | MUST-23 |  X   |        |     |        |      |
  |    3 minutes    |         |      |        |     |        |      |
  |    for SYN      |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | Send Keep-alive Packets                                         |
  +=================+=========+======+========+=====+========+======+
  | Send Keep-alive | MAY-5   |      |   X    |     |        |      |
  | Packets:        |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Application  | MUST-24 |  X   |        |     |        |      |
  |    can request  |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Default is   | MUST-25 |  X   |        |     |        |      |
  |    "off"        |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Only send if | MUST-26 |  X   |        |     |        |      |
  |    idle for     |         |      |        |     |        |      |
  |    interval     |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Interval     | MUST-27 |  X   |        |     |        |      |
  |    configurable |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Default at   | MUST-28 |  X   |        |     |        |      |
  |    least 2 hrs. |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Tolerant of  | MUST-29 |  X   |        |     |        |      |
  |    lost ACKs    |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Send with no | SHLD-12 |      |   X    |     |        |      |
  |    data         |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Configurable | MAY-6   |      |        |  X  |        |      |
  |    to send      |         |      |        |     |        |      |
  |    garbage      |         |      |        |     |        |      |
  |    octet        |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | IP Options                                                      |
  +=================+=========+======+========+=====+========+======+
  | Ignore options  | MUST-50 |  X   |        |     |        |      |
  | TCP doesn't     |         |      |        |     |        |      |
  | understand      |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Timestamp       | MAY-10  |      |   X    |     |        |      |
  | support         |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Record Route    | MAY-11  |      |   X    |     |        |      |
  | support         |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Source Route:   |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  ALP^1 can    | MUST-51 |  X   |        |     |        |      |
  |    specify      |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *     Overrides | MUST-52 |  X   |        |     |        |      |
  |       src route |         |      |        |     |        |      |
  |       in        |         |      |        |     |        |      |
  |       datagram  |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Build return | MUST-53 |  X   |        |     |        |      |
  |    route from   |         |      |        |     |        |      |
  |    src route    |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Later src    | SHLD-24 |      |   X    |     |        |      |
  |    route        |         |      |        |     |        |      |
  |    overrides    |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | Receiving ICMP Messages from IP                                 |
  +=================+=========+======+========+=====+========+======+
  | Receiving ICMP  | MUST-54 |  X   |        |     |        |      |
  | messages from   |         |      |        |     |        |      |
  | IP              |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Dest Unreach | SHLD-25 |  X   |        |     |        |      |
  |    (0,1,5) =>   |         |      |        |     |        |      |
  |    inform ALP   |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Abort on     | MUST-56 |      |        |     |        |  X   |
  |    Dest Unreach |         |      |        |     |        |      |
  |    (0,1,5)      |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Dest Unreach | SHLD-26 |      |   X    |     |        |      |
  |    (2-4) =>     |         |      |        |     |        |      |
  |    abort conn   |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Source       | MUST-55 |  X   |        |     |        |      |
  |    Quench =>    |         |      |        |     |        |      |
  |    silent       |         |      |        |     |        |      |
  |    discard      |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Abort on     | MUST-56 |      |        |     |        |  X   |
  |    Time         |         |      |        |     |        |      |
  |    Exceeded     |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Abort on     | MUST-56 |      |        |     |        |  X   |
  |    Param        |         |      |        |     |        |      |
  |    Problem      |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | Address Validation                                              |
  +=================+=========+======+========+=====+========+======+
  | Reject OPEN     | MUST-46 |  X   |        |     |        |      |
  | call to invalid |         |      |        |     |        |      |
  | IP address      |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Reject SYN from | MUST-63 |  X   |        |     |        |      |
  | invalid IP      |         |      |        |     |        |      |
  | address         |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Silently        | MUST-57 |  X   |        |     |        |      |
  | discard SYN to  |         |      |        |     |        |      |
  | bcast/mcast     |         |      |        |     |        |      |
  | addr            |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | TCP/ALP Interface Services                                      |
  +=================+=========+======+========+=====+========+======+
  | Error Report    | MUST-47 |  X   |        |     |        |      |
  | mechanism       |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | ALP can disable | SHLD-20 |      |   X    |     |        |      |
  | Error Report    |         |      |        |     |        |      |
  | Routine         |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | ALP can specify | MUST-48 |  X   |        |     |        |      |
  | Diffserv field  |         |      |        |     |        |      |
  | for sending     |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | *  Passed       | SHLD-22 |      |   X    |     |        |      |
  |    unchanged to |         |      |        |     |        |      |
  |    IP           |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | ALP can change  | SHLD-21 |      |   X    |     |        |      |
  | Diffserv field  |         |      |        |     |        |      |
  | during          |         |      |        |     |        |      |
  | connection      |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | ALP generally   | SHLD-23 |      |        |     |   X    |      |
  | changing        |         |      |        |     |        |      |
  | Diffserv during |         |      |        |     |        |      |
  | conn.           |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Pass received   | MAY-9   |      |        |  X  |        |      |
  | Diffserv field  |         |      |        |     |        |      |
  | up to ALP       |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | FLUSH call      | MAY-14  |      |        |  X  |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
  | Optional local  | MUST-43 |  X   |        |     |        |      |
  | IP addr param   |         |      |        |     |        |      |
  | in OPEN         |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | RFC 5961 Support                                                |
  +=================+=========+======+========+=====+========+======+
  | Implement data  | MAY-12  |      |        |  X  |        |      |
  | injection       |         |      |        |     |        |      |
  | protection      |         |      |        |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | Explicit Congestion Notification                                |
  +=================+=========+======+========+=====+========+======+
  | Support ECN     | SHLD-8  |      |   X    |     |        |      |
  +=================+=========+======+========+=====+========+======+
  | Alternative Congestion Control                                  |
  +=================+=========+======+========+=====+========+======+
  | Implement       | MAY-18  |      |        |  X  |        |      |
  | alternative     |         |      |        |     |        |      |
  | conformant      |         |      |        |     |        |      |
  | algorithm(s)    |         |      |        |     |        |      |
  +-----------------+---------+------+--------+-----+--------+------+
                   Table 8: TCP Requirements Summary
 FOOTNOTES: (1) "ALP" means Application-Layer Program.

Acknowledgments

 This document is largely a revision of RFC 793, of which Jon Postel
 was the editor.  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
 chairs over the course of work on this document:
 Michael Scharf
 Yoshifumi Nishida
 Pasi Sarolahti
 Michael Tüxen
 During the discussions of this work on the TCPM mailing list, in
 working group meetings, and via area reviews, helpful comments,
 critiques, and reviews were received from (listed alphabetically by
 last name): Praveen Balasubramanian, David Borman, Mohamed Boucadair,
 Bob Briscoe, Neal Cardwell, Yuchung Cheng, Martin Duke, Francis
 Dupont, Ted Faber, Gorry Fairhurst, Fernando Gont, Rodney Grimes, Yi
 Huang, Rahul Jadhav, Markku Kojo, Mike Kosek, Juhamatti Kuusisaari,
 Kevin Lahey, Kevin Mason, Matt Mathis, Stephen McQuistin, Jonathan
 Morton, Matt Olson, Tommy Pauly, Tom Petch, Hagen Paul Pfeifer, Kyle
 Rose, Anthony Sabatini, Michael Scharf, Greg Skinner, Joe Touch,
 Michael Tüxen, Reji Varghese, Bernie Volz, Tim Wicinski, Lloyd Wood,
 and Alex Zimmermann.
 Joe Touch provided additional help in clarifying the description of
 segment size parameters and PMTUD/PLPMTUD recommendations.  Markku
 Kojo helped put together the text in the section on TCP Congestion
 Control.
 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, Charles Deng, Merlin Buge.

Author's Address

 Wesley M. Eddy (editor)
 MTI Systems
 United States of America
 Email: wes@mti-systems.com
/data/webs/external/dokuwiki/data/pages/rfc/std/std7.txt · Last modified: 2022/08/18 13:13 by 127.0.0.1

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