ARMWARE RFC Archive <- STD Index (1..100)


(also RFC 9293)

Obsoletes RFC 793, RFC 879, RFC 2873, RFC 6093, RFC 6429, RFC 6528, RFC 6691
Updates RFC 1011, RFC 1122, RFC 5961

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

                  Transmission Control Protocol (TCP)


   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

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
   ( in effect on the date of
   publication of this document.  Please review these documents
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   in the Revised BSD License.

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   Contributions published or made publicly available before November
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   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
   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

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",
   "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

   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

   The application byte-stream is conveyed over the network via TCP
   segments, with each TCP segment sent as an Internet Protocol (IP)

   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

   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


   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

   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-

   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-

     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 1 2 3 4 5 6 7
      |       0       |


   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 1 2 3 4 5 6 7
      |       1       |


   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)  |


   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

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

           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

           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,
   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

   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

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

      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:


   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

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




   The first part of this test checks to see if the beginning of the
   segment falls in the window, the second part of the test checks to
   see if the end of the segment falls in the window; if the segment
   passes either part of the test, it contains data in the window.

   Actually, it is a little more complicated than this.  Due to zero
   windows and zero-length segments, we have four cases for the
   acceptability of an incoming segment:

       | Segment | Receive | Test                                 |
       | Length  | Window  |                                      |
       | 0       | 0       | SEG.SEQ = RCV.NXT                    |
       | 0       | >0      | RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND |
       | >0      | 0       | not acceptable                       |
       | >0      | >0      | RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND |
       |         |         |                                      |
       |         |         | or                                   |
       |         |         |                                      |
       |         |         | RCV.NXT =< SEG.SEQ+SEG.LEN-1 <       |
       |         |         | RCV.NXT+RCV.WND                      |

                   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

   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

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

   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

   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

   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

   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


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

       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> ...


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

             Figure 7: Simultaneous Connection Synchronization

   A TCP implementation MUST support simultaneous open attempts (MUST-

   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,
   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


   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

         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-
       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)

                      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

   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

   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

   *  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

   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


   *  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

   *  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

      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

   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

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
   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

   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

   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-

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
        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, 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

   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

   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 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).  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 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).  Silly Window Syndrome Avoidance

   The "Silly Window Syndrome" (SWS) is a stable pattern of small
   incremental window movements resulting in extremely poor TCP
   performance.  Algorithms to avoid SWS are described below for both
   the sending side and the receiving side.  RFC 1122 contains more
   detailed discussion of the SWS problem.  Note that the Nagle
   algorithm and the sender SWS avoidance algorithm play complementary
   roles in improving performance.  The Nagle algorithm discourages
   sending tiny segments when the data to be sent increases in small
   increments, while the SWS avoidance algorithm discourages small
   segments resulting from the right window edge advancing in small
   increments.  Sender's Algorithm -- When to Send Data

   A TCP 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:


   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

   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  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 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               ^

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

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

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

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

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

   Note that the general effect of this algorithm is to advance RCV.WND
   in increments of Eff.snd.MSS (for realistic receive buffers:
   Eff.snd.MSS < RCV.BUFF/2).  Note also that the receiver must use its
   own Eff.snd.MSS, making the assumption that it is the same as the
   sender's.  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.  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

   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

   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-

   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).  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

   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.  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

   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  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.  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.  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

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

   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

   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

   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 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-

   A TCP implementation MAY support the Timestamp (MAY-10) and Record
   Route (MAY-11) Options.  Source Routing

   If the lower level is IP (or other protocol that provides this
   feature) and source routing is used, the interface must allow the
   route information to be communicated.  This is especially important
   so that the source and destination addresses used in the TCP checksum
   be the originating source and ultimate destination.  It is also
   important to preserve the return route to answer connection requests.

   An application MUST be able to specify a source route when it
   actively opens a TCP connection (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).  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

   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,

     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

   Note that [35], Section 4 describes widespread implementation
   behavior that treats soft errors as hard errors during connection
   establishment.  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
   |  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







      Arriving Segments






   The model of the TCP/user interface is that user commands receive an
   immediate return and possibly a delayed response via an event or
   pseudo-interrupt.  In the following descriptions, the term "signal"
   means cause a delayed response.

   Error responses 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


   *  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".










   *  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".


   *  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



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



   *  Segmentize the buffer and send it with a piggybacked
      acknowledgment (acknowledgment value = RCV.NXT).  If there is
      insufficient space to remember this buffer, simply return "error:
      insufficient resources".

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






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

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".




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




   *  If insufficient incoming segments are queued to satisfy the
      request, queue the request.  If there is no queue space to
      remember the RECEIVE, respond with "error: insufficient

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

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

   *  When the TCP 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


   *  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.




   *  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".


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


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


   *  If no SENDs have been issued and there is no pending data to send,
      then form a FIN segment and send it, and enter FIN-WAIT-1 state;
      otherwise, queue for processing after entering ESTABLISHED state.


   *  Queue this until all preceding SENDs have been segmentized, then
      form a FIN segment and send it.  In any case, enter FIN-WAIT-1



   *  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).


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


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


   *  All queued SENDs and RECEIVEs should be given "connection reset"
      notification.  Delete the TCB, enter CLOSED state, and return.






   *  Send a reset segment:


   *  All queued SENDs and RECEIVEs should be given "connection reset"
      notification; all segments queued for transmission (except for the
      RST formed above) or retransmission should be flushed.  Delete the
      TCB, enter CLOSED state, and return.




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

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".


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


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


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


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


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


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


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


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


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


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


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

      all data in the incoming segment is discarded.  An incoming
      segment containing a RST is discarded.  An incoming segment not
      containing a RST causes a RST to be sent in response.  The
      acknowledgment and sequence field values are selected to make the
      reset sequence acceptable to the TCP endpoint that sent the
      offending segment.

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


      If the ACK bit is on,


      Return.  LISTEN STATE

   If the state is LISTEN, then

      First, check for a RST:

      -  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:

      -  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:


      -  Return.

      Third, check for a SYN:

      -  If the SYN bit is set, check the security.  If the security/
         compartment on the incoming segment does not exactly match the
         security/compartment in the TCB, then send a reset and return.


      -  Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ, and any other
         control or text should be queued for processing later.  ISS
         should be selected and a SYN segment sent of the form:


      -  SND.NXT is set to ISS+1 and SND.UNA to ISS.  The connection
         state should be changed to SYN-RECEIVED.  Note that any other
         incoming control or data (combined with SYN) will be processed
         in the SYN-RECEIVED state, but processing of SYN and ACK should
         not be repeated.  If the listen was not fully specified (i.e.,
         the remote socket was not fully specified), then the
         unspecified fields should be filled in now.

      Fourth, other data or control:

      -  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.  SYN-SENT STATE

   If the state is SYN-SENT, then

      First, check the ACK bit:

      -  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)


         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:

      -  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

         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:

      -  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,


         o  Otherwise,


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

      Fourth, check the SYN bit:

      -  This step should be reached only if the ACK is ok, or there is
         no ACK, and the segment did not contain a RST.

      -  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.

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


      -  and send it.  Data or controls 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 where the URG bit is checked; otherwise,

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


      -  and send it.  Set the variables:

            SND.WND <- SEG.WND

            SND.WL1 <- SEG.SEQ

            SND.WL2 <- SEG.ACK

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

      -  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.  Other States


      First, check sequence number:



      -  FIN-WAIT-1 STATE

      -  FIN-WAIT-2 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):


         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:

      -  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):


             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


         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.


      -  FIN-WAIT-1 STATE

      -  FIN-WAIT-2 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.




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

      Third, check security:


         o  If the security/compartment in the segment does not exactly
            match the security/compartment in the TCB, then send a reset
            and return.


      -  FIN-WAIT-1 STATE

      -  FIN-WAIT-2 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

      -  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:


         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


      -  FIN-WAIT-1 STATE

      -  FIN-WAIT-2 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:


         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

         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:

      -  if the ACK bit is off, drop the segment and return

      -  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:


            +  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


            +  and send it.


            +  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

         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:


      -  FIN-WAIT-1 STATE

      -  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.





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

      Seventh, process the segment text:


      -  FIN-WAIT-1 STATE

      -  FIN-WAIT-2 STATE

         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:


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





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

      Eighth, check the FIN bit:

      -  Do not process the FIN if the state is CLOSED, LISTEN, or SYN-
         SENT since the SEG.SEQ cannot be validated; drop the segment
         and return.

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



            +  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


   *  For any state if the user timeout expires, flush all queues,
      signal the user "error: connection aborted due to user timeout" in
      general and for any outstanding calls, delete the TCB, enter the
      CLOSED state, and return.


   *  For any state if the retransmission timeout expires on a segment
      in the retransmission queue, send the segment at the front of the
      retransmission queue again, reinitialize the retransmission timer,
      and return.


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

4.  Glossary

           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.

           A logical communication path identified by a pair of sockets.

           A message sent in a packet-switched computer communications

   Destination Address
           The network-layer address of the endpoint intended to receive
           a segment.

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

           To remove all of the contents (data or segments) from a store
           (buffer or queue).

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

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

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

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

   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.

           Internet Protocol.  See [1] and [13].

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

           The Initial Sequence Number.  The first sequence number used
           on a connection (either ISS or IRS).  Selected in a way that
           is unique within a given period of time and is unpredictable
           to attackers.

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

   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.

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

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

           An eight-bit byte.

           An Option field may contain several options, and each option
           may be several octets in length.

           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.

           The portion of a connection identifier used for
           demultiplexing connections at an endpoint.

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

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

           receive next sequence number

           receive urgent pointer

           receive window

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

           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.

           segment acknowledgment

           segment length

           segment sequence

           segment urgent pointer field

           segment window field

           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

   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.)

           send sequence

           left sequence

           send urgent pointer

           segment sequence number at last window update

           segment acknowledgment number at last window update

           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.

           A control bit in the incoming segment, occupying one sequence
           number, used at the initiation of a connection to indicate
           where the sequence numbering will start.

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

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

           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".

           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

   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 <>.

   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

   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

   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/RFC791, September 1981,

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

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

   [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,

   [5]        Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,

   [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,

   [7]        Floyd, S. and M. Allman, "Specifying New Congestion
              Control Algorithms", BCP 133, RFC 5033,
              DOI 10.17487/RFC5033, August 2007,

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

   [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,

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

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

   [12]       Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [13]       Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

   [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,

   [15]       Allman, M., "Requirements for Time-Based Loss Detection",
              BCP 233, RFC 8961, DOI 10.17487/RFC8961, November 2020,

8.2.  Informative References

   [16]       Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC793, September 1981,

   [17]       Nagle, J., "Congestion Control in IP/TCP Internetworks",
              RFC 896, DOI 10.17487/RFC896, January 1984,

   [18]       Reynolds, J. and J. Postel, "Official Internet protocols",
              RFC 1011, DOI 10.17487/RFC1011, May 1987,

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

   [20]       Almquist, P., "Type of Service in the Internet Protocol
              Suite", RFC 1349, DOI 10.17487/RFC1349, July 1992,

   [21]       Braden, R., "T/TCP -- TCP Extensions for Transactions
              Functional Specification", RFC 1644, DOI 10.17487/RFC1644,
              July 1994, <>.

   [22]       Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018,
              DOI 10.17487/RFC2018, October 1996,

   [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,

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

   [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,

   [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,

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

   [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, <>.

   [29]       Allman, M., "TCP Congestion Control with Appropriate Byte
              Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
              2003, <>.

   [30]       Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
              ICMPv6, UDP, and TCP Headers", RFC 4727,
              DOI 10.17487/RFC4727, November 2006,

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

   [32]       Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,

   [33]       Touch, J., "Defending TCP Against Spoofing Attacks",
              RFC 4953, DOI 10.17487/RFC4953, July 2007,

   [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, <>.

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

   [36]       StJohns, M., Atkinson, R., and G. Thomas, "Common
              Architecture Label IPv6 Security Option (CALIPSO)",
              RFC 5570, DOI 10.17487/RFC5570, July 2009,

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

   [38]       Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <>.

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

   [40]       Gont, F., "Reducing the TIME-WAIT State Using TCP
              Timestamps", BCP 159, RFC 6191, DOI 10.17487/RFC6191,
              April 2011, <>.

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

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

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

   [44]       Touch, J., "Updated Specification of the IPv4 ID Field",
              RFC 6864, DOI 10.17487/RFC6864, February 2013,

   [45]       Touch, J., "Shared Use of Experimental TCP Options",
              RFC 6994, DOI 10.17487/RFC6994, August 2013,

   [46]       McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
              "Architectural Considerations of IP Anycast", RFC 7094,
              DOI 10.17487/RFC7094, January 2014,

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

   [48]       Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

   [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,

   [50]       Black, D., Ed. and P. Jones, "Differentiated Services
              (Diffserv) and Real-Time Communication", RFC 7657,
              DOI 10.17487/RFC7657, November 2015,

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

   [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,

   [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,

   [54]       Black, D., "Relaxing Restrictions on Explicit Congestion
              Notification (ECN) Experimentation", RFC 8311,
              DOI 10.17487/RFC8311, January 2018,

   [55]       Chown, T., Loughney, J., and T. Winters, "IPv6 Node
              Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
              January 2019, <>.

   [56]       Trammell, B. and M. Kuehlewind, "The Wire Image of a
              Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
              2019, <>.

   [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,

   [58]       Hardie, T., Ed., "Transport Protocol Path Signals",
              RFC 8558, DOI 10.17487/RFC8558, April 2019,

   [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, <>.

   [60]       Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,

   [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,

   [62]       IANA, "Transmission Control Protocol (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, <

   [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,

   [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,

   [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,

   [67]       Thomson, M. and T. Pauly, "Long-Term Viability of Protocol
              Extension Mechanisms", RFC 9170, DOI 10.17487/RFC9170,
              December 2021, <>.

   [68]       Minshall, G., "A Suggested Modification to Nagle's
              Algorithm", Work in Progress, Internet-Draft, draft-
              minshall-nagle-01, 18 June 1999,

   [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, <>.

   [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, <>.

   [71]       Postel, J., "Comments on Action Items from the January
              Meeting", IEN 177, March 1981,

   [72]       "Segmentation Offloads", The Linux Kernel Documentation,

   [73]       RFC Errata, Erratum ID 573, RFC 793,

   [74]       RFC Errata, Erratum ID 574, RFC 793,

   [75]       RFC Errata, Erratum ID 700, RFC 793,

   [76]       RFC Errata, Erratum ID 701, RFC 793,

   [77]       RFC Errata, Erratum ID 1283, RFC 793,

   [78]       RFC Errata, Erratum ID 1561, RFC 793,

   [79]       RFC Errata, Erratum ID 1562, RFC 793,

   [80]       RFC Errata, Erratum ID 1564, RFC 793,

   [81]       RFC Errata, Erratum ID 1571, RFC 793,

   [82]       RFC Errata, Erratum ID 1572, RFC 793,

   [83]       RFC Errata, Erratum ID 2297, RFC 793,

   [84]       RFC Errata, Erratum ID 2298, RFC 793,

   [85]       RFC Errata, Erratum ID 2748, RFC 793,

   [86]       RFC Errata, Erratum ID 2749, RFC 793,

   [87]       RFC Errata, Erratum ID 2934, RFC 793,

   [88]       RFC Errata, Erratum ID 3213, RFC 793,

   [89]       RFC Errata, Erratum ID 3300, RFC 793,

   [90]       RFC Errata, Erratum ID 3301, RFC 793,

   [91]       RFC Errata, Erratum ID 6222, RFC 793,

   [92]       RFC Errata, Erratum ID 572, RFC 793,

   [93]       RFC Errata, Erratum ID 575, RFC 793,

   [94]       RFC Errata, Erratum ID 1565, RFC 793,

   [95]       RFC Errata, Erratum ID 1569, RFC 793,

   [96]       RFC Errata, Erratum ID 2296, RFC 793,

   [97]       RFC Errata, Erratum ID 3305, RFC 793,

   [98]       RFC Errata, Erratum ID 3602, RFC 793,

   [99]       RFC Errata, Erratum ID 4772, RFC 5961,

   [100]      Gont, F., "ICMP Attacks against TCP", RFC 5927,
              DOI 10.17487/RFC5927, July 2010,

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

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.


   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

   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