    RFC: 793

                         TRANSMISSION CONTROL PROTOCOL

                             DARPA INTERNET PROGRAM

                             PROTOCOL SPECIFICATION

                                 September 1981

                                  prepared for

                   Defense Advanced Research Projects Agency
                    Information Processing Techniques Office
                             1400 Wilson Boulevard
                           Arlington, Virginia  22209

                                       by

                         Information Sciences Institute
                       University of Southern California
                               4676 Admiralty Way
                       Marina del Rey, California  90291

    September 1981
                                               Transmission Control Protocol

                               TABLE OF CONTENTS

        PREFACE ........................................................ iii

    1.  INTRODUCTION ..................................................... 1

      1.1  Motivation .................................................... 1
      1.2  Scope ......................................................... 2
      1.3  About This Document ........................................... 2
      1.4  Interfaces .................................................... 3
      1.5  Operation ..................................................... 3

    2.  PHILOSOPHY ....................................................... 7

      2.1  Elements of the Internetwork System ........................... 7
      2.2  Model of Operation ............................................ 7
      2.3  The Host Environment .......................................... 8
      2.4  Interfaces .................................................... 9
      2.5  Relation to Other Protocols ................................... 9
      2.6  Reliable Communication ........................................ 9
      2.7  Connection Establishment and Clearing ........................ 10
      2.8  Data Communication ........................................... 12
      2.9  Precedence and Security ...................................... 13
      2.10 Robustness Principle ......................................... 13

    3.  FUNCTIONAL SPECIFICATION ........................................ 15

      3.1  Header Format ................................................ 15
      3.2  Terminology .................................................. 19
      3.3  Sequence Numbers ............................................. 24
      3.4  Establishing a connection .................................... 30
      3.5  Closing a Connection ......................................... 37
      3.6  Precedence and Security ...................................... 40
      3.7  Data Communication ........................................... 40
      3.8  Interfaces ................................................... 44
      3.9  Event Processing ............................................. 52

    GLOSSARY ............................................................ 79

    REFERENCES .......................................................... 85

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    [Page ii]

    September 1981
                                               Transmission Control Protocol

                                    PREFACE

    This document describes the DoD Standard Transmission Control Protocol
    (TCP).  There have been nine earlier editions of the ARPA TCP
    specification on which this standard is based, and the present text
    draws heavily from them.  There have been many contributors to this work
    both in terms of concepts and in terms of text.  This edition clarifies
    several details and removes the end-of-letter buffer-size adjustments,
    and redescribes the letter mechanism as a push function.

                                                               Jon Postel

                                                               Editor

                                                                  [Page iii]

    RFC:  793
    Replaces: RFC 761
    IENs:  129, 124, 112, 81,
    55, 44, 40, 27, 21, 5

                         TRANSMISSION CONTROL PROTOCOL

                             DARPA INTERNET PROGRAM
                             PROTOCOL SPECIFICATION

                                1.  INTRODUCTION

    The Transmission Control Protocol (TCP) is intended for use as a highly
    reliable host-to-host protocol between hosts in packet-switched computer
    communication networks, and in interconnected systems of such networks.

    This document describes the functions to be performed by the
    Transmission Control Protocol, the program that implements it, and its
    interface to programs or users that require its services.

    1.1.  Motivation

      Computer communication systems are playing an increasingly important
      role in military, government, and civilian environments.  This
      document focuses its attention primarily on military computer
      communication requirements, especially robustness in the presence of
      communication unreliability and availability in the presence of
      congestion, but many of these problems are found in the civilian and
      government sector as well.

      As strategic and tactical computer communication networks are
      developed and deployed, it is essential to provide means of
      interconnecting them and to provide standard interprocess
      communication protocols which can support a broad range of
      applications.  In anticipation of the need for such standards, the
      Deputy Undersecretary of Defense for Research and Engineering has
      declared the Transmission Control Protocol (TCP) described herein to
      be a basis for DoD-wide inter-process communication protocol
      standardization.

      TCP is a connection-oriented, end-to-end reliable protocol designed to
      fit into a layered hierarchy of protocols which support multi-network
      applications.  The TCP provides for reliable inter-process
      communication between pairs of processes in host computers attached to
      distinct but interconnected computer communication networks.  Very few
      assumptions are made as to the reliability of the communication
      protocols below the TCP layer.  TCP assumes it can obtain a simple,
      potentially unreliable datagram service from the lower level
      protocols.  In principle, the TCP should be able to operate above a
      wide spectrum of communication systems ranging from hard-wired
      connections to packet-switched or circuit-switched networks.

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    Transmission Control Protocol
    Introduction

      TCP is based on concepts first described by Cerf and Kahn in [1].  The
      TCP fits into a layered protocol architecture just above a basic
      Internet Protocol [2] which provides a way for the TCP to send and
      receive variable-length segments of information enclosed in internet
      datagram "envelopes".  The internet datagram provides a means for
      addressing source and destination TCPs in different networks.  The
      internet protocol also deals with any fragmentation or reassembly of
      the TCP segments required to achieve transport and delivery through
      multiple networks and interconnecting gateways.  The internet protocol
      also carries information on the precedence, security classification
      and compartmentation of the TCP segments, so this information can be
      communicated end-to-end across multiple networks.

                               Protocol Layering

                            +---------------------+
                            |     higher-level    |
                            +---------------------+
                            |        TCP          |
                            +---------------------+
                            |  internet protocol  |
                            +---------------------+
                            |communication network|
                            +---------------------+

                                    Figure 1

      Much of this document is written in the context of TCP implementations
      which are co-resident with higher level protocols in the host
      computer.  Some computer systems will be connected to networks via
      front-end computers which house the TCP and internet protocol layers,
      as well as network specific software.  The TCP specification describes
      an interface to the higher level protocols which appears to be
      implementable even for the front-end case, as long as a suitable
      host-to-front end protocol is implemented.

    1.2.  Scope

      The TCP is intended to provide a reliable process-to-process
      communication service in a multinetwork environment.  The TCP is
      intended to be a host-to-host protocol in common use in multiple
      networks.

    1.3.  About this Document

      This document represents a specification of the behavior required of
      any TCP implementation, both in its interactions with higher level
      protocols and in its interactions with other TCPs.  The rest of this

    [Page 2]

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                                               Transmission Control Protocol
                                                                Introduction

      section offers a very brief view of the protocol interfaces and
      operation.  Section 2 summarizes the philosophical basis for the TCP
      design.  Section 3 offers both a detailed description of the actions
      required of TCP when various events occur (arrival of new segments,
      user calls, errors, etc.) and the details of the formats of TCP
      segments.

    1.4.  Interfaces

      The TCP interfaces on one side to user or application processes and on
      the other side to a lower level protocol such as Internet Protocol.

      The interface between an application process and the TCP is
      illustrated in reasonable detail.  This interface consists of a set of
      calls much like the calls an operating system provides to an
      application process for manipulating files.  For example, there are
      calls to open and close connections and to send and receive data on
      established connections.  It is also expected that the TCP can
      asynchronously communicate with application programs.  Although
      considerable freedom is permitted to TCP implementors to design
      interfaces which are appropriate to a particular operating system
      environment, a minimum functionality is required at the TCP/user
      interface for any valid implementation.

      The interface between TCP and lower level protocol is essentially
      unspecified except that it is assumed there is a mechanism whereby the
      two levels can asynchronously pass information to each other.
      Typically, one expects the lower level protocol to specify this
      interface.  TCP is designed to work in a very general environment of
      interconnected networks.  The lower level protocol which is assumed
      throughout this document is the Internet Protocol [2].

    1.5.  Operation

      As noted above, the primary purpose of the TCP is to provide reliable,
      securable logical circuit or connection service between pairs of
      processes.  To provide this service on top of a less reliable internet
      communication system requires facilities in the following areas:

        Basic Data Transfer
        Reliability
        Flow Control
        Multiplexing
        Connections
        Precedence and Security

      The basic operation of the TCP in each of these areas is described in
      the following paragraphs.

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      Basic Data Transfer:

        The TCP is able to transfer a continuous stream of octets in each
        direction between its users by packaging some number of octets into
        segments for transmission through the internet system.  In general,
        the TCPs decide when to block and forward data at their own
        convenience.

        Sometimes users need to be sure that all the data they have
        submitted to the TCP has been transmitted.  For this purpose a push
        function is defined.  To assure that data submitted to a TCP is
        actually transmitted the sending user indicates that it should be
        pushed through to the receiving user.  A push causes the TCPs to
        promptly forward and deliver data up to that point to the receiver.
        The exact push point might not be visible to the receiving user and
        the push function does not supply a record boundary marker.

      Reliability:

        The TCP must recover from data that is damaged, lost, duplicated, or
        delivered out of order by the internet communication system.  This
        is achieved by assigning a sequence number to each octet
        transmitted, and requiring a positive acknowledgment (ACK) from the
        receiving TCP.  If the ACK is not received within a timeout
        interval, the data is retransmitted.  At the receiver, the sequence
        numbers are used to correctly order segments that may be received
        out of order and to eliminate duplicates.  Damage is handled by
        adding a checksum to each segment transmitted, checking it at the
        receiver, and discarding damaged segments.

        As long as the TCPs continue to function properly and the internet
        system does not become completely partitioned, no transmission
        errors will affect the correct delivery of data.  TCP recovers from
        internet communication system errors.

      Flow Control:

        TCP provides a means for the receiver to govern the amount of data
        sent by the sender.  This is achieved by returning a "window" with
        every ACK indicating a range of acceptable sequence numbers beyond
        the last segment successfully received.  The window indicates an
        allowed number of octets that the sender may transmit before
        receiving further permission.

    [Page 4]

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                                               Transmission Control Protocol
                                                                Introduction

      Multiplexing:

        To allow for many processes within a single Host to use TCP
        communication facilities simultaneously, the TCP provides a set of
        addresses or ports within each host.  Concatenated with the network
        and host addresses from the internet communication layer, this forms
        a socket.  A pair of sockets uniquely identifies each connection.
        That is, a socket may be simultaneously used in multiple
        connections.

        The binding of ports to processes is handled independently by each
        Host.  However, it proves useful to attach frequently used processes
        (e.g., a "logger" or timesharing service) to fixed sockets which are
        made known to the public.  These services can then be accessed
        through the known addresses.  Establishing and learning the port
        addresses of other processes may involve more dynamic mechanisms.

      Connections:

        The reliability and flow control mechanisms described above require
        that TCPs initialize and maintain certain status information for
        each data stream.  The combination of this information, including
        sockets, sequence numbers, and window sizes, is called a connection.
        Each connection is uniquely specified by a pair of sockets
        identifying its two sides.

        When two processes wish to communicate, their TCP's must first
        establish a connection (initialize the status information on each
        side).  When their communication is complete, the connection is
        terminated or closed to free the resources for other uses.

        Since connections must be established between unreliable hosts and
        over the unreliable internet communication system, a handshake
        mechanism with clock-based sequence numbers is used to avoid
        erroneous initialization of connections.

      Precedence and Security:

        The users of TCP may indicate the security and precedence of their
        communication.  Provision is made for default values to be used when
        these features are not needed.

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    [Page 6]

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                                               Transmission Control Protocol

                                 2.  PHILOSOPHY

    2.1.  Elements of the Internetwork System

      The internetwork environment consists of hosts connected to networks
      which are in turn interconnected via gateways.  It is assumed here
      that the networks may be either local networks (e.g., the ETHERNET) or
      large networks (e.g., the ARPANET), but in any case are based on
      packet switching technology.  The active agents that produce and
      consume messages are processes.  Various levels of protocols in the
      networks, the gateways, and the hosts support an interprocess
      communication system that provides two-way data flow on logical
      connections between process ports.

      The term packet is used generically here to mean the data of one
      transaction between a host and its network.  The format of data blocks
      exchanged within the a network will generally not be of concern to us.

      Hosts are computers attached to a network, and from the communication
      network's point of view, are the sources and destinations of packets.
      Processes are viewed as the active elements in host computers (in
      accordance with the fairly common definition of a process as a program
      in execution).  Even terminals and files or other I/O devices are
      viewed as communicating with each other through the use of processes.
      Thus, all communication is viewed as inter-process communication.

      Since a process may need to distinguish among several communication
      streams between itself and another process (or processes), we imagine
      that each process may have a number of ports through which it
      communicates with the ports of other processes.

    2.2.  Model of Operation

      Processes transmit data by calling on the TCP and passing buffers of
      data as arguments.  The TCP packages the data from these buffers into
      segments and calls on the internet module to transmit each segment to
      the destination TCP.  The receiving TCP places the data from a segment
      into the receiving user's buffer and notifies the receiving user.  The
      TCPs include control information in the segments which they use to
      ensure reliable ordered data transmission.

      The model of internet communication is that there is an internet
      protocol module associated with each TCP which provides an interface
      to the local network.  This internet module packages TCP segments
      inside internet datagrams and routes these datagrams to a destination
      internet module or intermediate gateway.  To transmit the datagram
      through the local network, it is embedded in a local network packet.

      The packet switches may perform further packaging, fragmentation, or

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    Philosophy

      other operations to achieve the delivery of the local packet to the
      destination internet module.

      At a gateway between networks, the internet datagram is "unwrapped"
      from its local packet and examined to determine through which network
      the internet datagram should travel next.  The internet datagram is
      then "wrapped" in a local packet suitable to the next network and
      routed to the next gateway, or to the final destination.

      A gateway is permitted to break up an internet datagram into smaller
      internet datagram fragments if this is necessary for transmission
      through the next network.  To do this, the gateway produces a set of
      internet datagrams; each carrying a fragment.  Fragments may be
      further broken into smaller fragments at subsequent gateways.  The
      internet datagram fragment format is designed so that the destination
      internet module can reassemble fragments into internet datagrams.

      A destination internet module unwraps the segment from the datagram
      (after reassembling the datagram, if necessary) and passes it to the
      destination TCP.

      This simple model of the operation glosses over many details.  One
      important feature is the type of service.  This provides information
      to the gateway (or internet module) to guide it in selecting the
      service parameters to be used in traversing the next network.
      Included in the type of service information is the precedence of the
      datagram.  Datagrams may also carry security information to permit
      host and gateways that operate in multilevel secure environments to
      properly segregate datagrams for security considerations.

    2.3.  The Host Environment

      The TCP is assumed to be a module in an operating system.  The users
      access the TCP much like they would access the file system.  The TCP
      may call on other operating system functions, for example, to manage
      data structures.  The actual interface to the network is assumed to be
      controlled by a device driver module.  The TCP does not call on the
      network device driver directly, but rather calls on the internet
      datagram protocol module which may in turn call on the device driver.

      The mechanisms of TCP do not preclude implementation of the TCP in a
      front-end processor.  However, in such an implementation, a
      host-to-front-end protocol must provide the functionality to support
      the type of TCP-user interface described in this document.

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                                                                  Philosophy

    2.4.  Interfaces

      The TCP/user interface provides for calls made by the user on the TCP
      to OPEN or CLOSE a connection, to SEND or RECEIVE data, or to obtain
      STATUS about a connection.  These calls are like other calls from user
      programs on the operating system, for example, the calls to open, read
      from, and close a file.

      The TCP/internet interface provides calls to send and receive
      datagrams addressed to TCP modules in hosts anywhere in the internet
      system.  These calls have parameters for passing the address, type of
      service, precedence, security, and other control information.

    2.5.  Relation to Other Protocols

      The following diagram illustrates the place of the TCP in the protocol
      hierarchy:

           +------+ +-----+ +-----+       +-----+
           |Telnet| | FTP | |Voice|  ...  |     |  Application Level
           +------+ +-----+ +-----+       +-----+
                 |   |         |             |
                +-----+     +-----+       +-----+
                | TCP |     | RTP |  ...  |     |  Host Level
                +-----+     +-----+       +-----+
                   |           |             |
                +-------------------------------+
                |    Internet Protocol & ICMP   |  Gateway Level
                +-------------------------------+
                               |
                  +---------------------------+
                  |   Local Network Protocol  |    Network Level
                  +---------------------------+

                             Protocol Relationships

                                   Figure 2.

      It is expected that the TCP will be able to support higher level
      protocols efficiently.  It should be easy to interface higher level
      protocols like the ARPANET Telnet or AUTODIN II THP to the TCP.

    2.6.  Reliable Communication

      A stream of data sent on a TCP connection is delivered reliably and in
      order at the destination.

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    Philosophy

      Transmission is made reliable via the use of sequence numbers and
      acknowledgments.  Conceptually, each octet of data is assigned a
      sequence number.  The sequence number of the first octet of data in a
      segment is transmitted with that segment and is called the segment
      sequence number.  Segments also carry an acknowledgment number which
      is the sequence number of the next expected data octet of
      transmissions in the reverse direction.  When the TCP transmits a
      segment containing data, it puts a copy on a retransmission queue and
      starts a timer; when the acknowledgment for that data is received, the
      segment is deleted from the queue.  If the acknowledgment is not
      received before the timer runs out, the segment is retransmitted.

      An acknowledgment by TCP does not guarantee that the data has been
      delivered to the end user, but only that the receiving TCP has taken
      the responsibility to do so.

      To govern the flow of data between TCPs, a flow control mechanism is
      employed.  The receiving TCP reports a "window" to the sending TCP.
      This window specifies the number of octets, starting with the
      acknowledgment number, that the receiving TCP is currently prepared to
      receive.

    2.7.  Connection Establishment and Clearing

      To identify the separate data streams that a TCP may handle, the TCP
      provides a port identifier.  Since port identifiers are selected
      independently by each TCP they might not be unique.  To provide for
      unique addresses within each TCP, we concatenate an internet address
      identifying the TCP with a port identifier to create a socket which
      will be unique throughout all networks connected together.

      A connection is fully specified by the pair of sockets at the ends.  A
      local socket may participate in many connections to different foreign
      sockets.  A connection can be used to carry data in both directions,
      that is, it is "full duplex".

      TCPs are free to associate ports with processes however they choose.
      However, several basic concepts are necessary in any implementation.
      There must be well-known sockets which the TCP associates only with
      the "appropriate" processes by some means.  We envision that processes
      may "own" ports, and that processes can initiate connections only on
      the ports they own.  (Means for implementing ownership is a local
      issue, but we envision a Request Port user command, or a method of
      uniquely allocating a group of ports to a given process, e.g., by
      associating the high order bits of a port name with a given process.)

      A connection is specified in the OPEN call by the local port and
      foreign socket arguments.  In return, the TCP supplies a (short) local

    [Page 10]

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                                                                  Philosophy

      connection name by which the user refers to the connection in
      subsequent calls.  There are several things that must be remembered
      about a connection.  To store this information we imagine that there
      is a data structure called a Transmission Control Block (TCB).  One
      implementation strategy would have the local connection name be a
      pointer to the TCB for this connection.  The OPEN call also specifies
      whether the 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 rather than attempting to initiate a connection.
      Often the process requesting a passive OPEN will accept a connection
      request from any caller.  In this case a foreign socket of all zeros
      is used to denote an unspecified socket.  Unspecified foreign sockets
      are allowed only on passive OPENs.

      A service process that wished to provide services for unknown other
      processes would issue a passive OPEN request with an unspecified
      foreign socket.  Then a connection could be made with any process that
      requested a connection to this local socket.  It would help if this
      local socket were known to be associated with this service.

      Well-known sockets are a convenient mechanism for a priori associating
      a socket address with a standard service.  For instance, the
      "Telnet-Server" process is permanently assigned to a particular
      socket, and other sockets are reserved for File Transfer, Remote Job
      Entry, Text Generator, Echoer, and Sink processes (the last three
      being for test purposes).  A socket address might be reserved for
      access to a "Look-Up" service which would return the specific socket
      at which a newly created service would be provided.  The concept of a
      well-known socket is part of the TCP specification, but the assignment
      of sockets to services is outside this specification.  (See [4].)

      Processes can issue passive OPENs and wait for matching active OPENs
      from other processes and be informed by the TCP when connections have
      been established.  Two processes which issue active OPENs to each
      other at the same time will be correctly connected.  This flexibility
      is critical for the support of distributed computing in which
      components act asynchronously with respect to each other.

      There are two principal cases for matching the sockets in the local
      passive OPENs and an foreign active OPENs.  In the first case, the
      local passive OPENs has fully specified the foreign socket.  In this
      case, the match must be exact.  In the second case, the local passive
      OPENs has left the foreign socket unspecified.  In this case, any
      foreign socket is acceptable as long as the local sockets match.
      Other possibilities include partially restricted matches.

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      If there are several pending passive OPENs (recorded in TCBs) with the
      same local socket, an foreign active OPEN will be matched to a TCB
      with the specific foreign socket in the foreign active OPEN, if such a
      TCB exists, before selecting a TCB with an unspecified foreign socket.

      The procedures to establish connections utilize the synchronize (SYN)
      control flag and involves an exchange of three messages.  This
      exchange has been termed a three-way hand shake [3].

      A connection is initiated by the rendezvous of an arriving segment
      containing a SYN and a waiting TCB entry each created by a user OPEN
      command.  The matching of local and foreign sockets determines when a
      connection has been initiated.  The connection becomes "established"
      when sequence numbers have been synchronized in both directions.

      The clearing of a connection also involves the exchange of segments,
      in this case carrying the FIN control flag.

    2.8.  Data Communication

      The data that flows on a connection may be thought of as a stream of
      octets.  The sending user indicates in each SEND call whether the data
      in that call (and any preceeding calls) should be immediately pushed
      through to the receiving user by the setting of the PUSH flag.

      A sending TCP is allowed to collect data from the sending user and to
      send that data in segments at its own convenience, until the push
      function is signaled, then it must send all unsent data.  When a
      receiving TCP sees the PUSH flag, it must not wait for more data from
      the sending TCP before passing the data to the receiving process.

      There is no necessary relationship between push functions and segment
      boundaries.  The data in any particular segment may be the result of a
      single SEND call, in whole or part, or of multiple SEND calls.

      The purpose of push function and the PUSH flag is to push data through
      from the sending user to the receiving user.  It does not provide a
      record service.

      There is a coupling between the push function and the use of buffers
      of data that cross the TCP/user interface.  Each time a PUSH flag is
      associated with data placed into the receiving user's buffer, the
      buffer is returned to the user for processing even if the buffer is
      not filled.  If data arrives that fills the user's buffer before a
      PUSH is seen, the data is passed to the user in buffer size units.

      TCP also provides a means to communicate to the receiver of data that
      at some point further along in the data stream than the receiver is

    [Page 12]

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                                               Transmission Control Protocol
                                                                  Philosophy

      currently reading there is urgent data.  TCP does not attempt to
      define what the user specifically does upon being notified of pending
      urgent data, but the general notion is that the receiving process will
      take action to process the urgent data quickly.

    2.9.  Precedence and Security

      The TCP makes use of the internet protocol type of service field and
      security option to provide precedence and security on a per connection
      basis to TCP users.  Not all TCP modules will necessarily function in
      a multilevel secure environment; some may be limited to unclassified
      use only, and others may operate at only one security level and
      compartment.  Consequently, some TCP implementations and services to
      users may be limited to a subset of the multilevel secure case.

      TCP modules which operate in a multilevel secure environment must
      properly mark outgoing segments with the security, compartment, and
      precedence.  Such TCP modules must also provide to their users or
      higher level protocols such as Telnet or THP an interface to allow
      them to specify the desired security level, compartment, and
      precedence of connections.

    2.10.  Robustness Principle

      TCP implementations will follow a general principle of robustness:  be
      conservative in what you do, be liberal in what you accept from
      others.

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    [Page 14]

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                          3.  FUNCTIONAL SPECIFICATION

    3.1.  Header Format

      TCP segments are sent as internet datagrams.  The Internet Protocol
      header carries several information fields, including the source and
      destination host addresses [2].  A TCP header follows the internet
      header, supplying information specific to the TCP protocol.  This
      division allows for the existence of host level protocols other than
      TCP.

      TCP Header Format

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

                                TCP Header Format

              Note that one tick mark represents one bit position.

                                   Figure 3.

      Source Port:  16 bits

        The source port number.

      Destination Port:  16 bits

        The destination port number.

                                                                   [Page 15]

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      Sequence Number:  32 bits

        The sequence number of the first data octet in this segment (except
        when SYN is present). If SYN is present the sequence number is the
        initial sequence number (ISN) and the first data octet is ISN+1.

      Acknowledgment Number:  32 bits

        If the ACK control bit is set this field contains the value of the
        next sequence number the sender of the segment is expecting to
        receive.  Once a connection is established this is always sent.

      Data Offset:  4 bits

        The number of 32 bit words in the TCP Header.  This indicates where
        the data begins.  The TCP header (even one including options) is an
        integral number of 32 bits long.

      Reserved:  6 bits

        Reserved for future use.  Must be zero.

      Control Bits:  6 bits (from left to right):

        URG:  Urgent Pointer field significant
        ACK:  Acknowledgment field significant
        PSH:  Push Function
        RST:  Reset the connection
        SYN:  Synchronize sequence numbers
        FIN:  No more data from sender

      Window:  16 bits

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

      Checksum:  16 bits

        The checksum field is the 16 bit one's complement of the one's
        complement sum of all 16 bit words in the header and text.  If a
        segment contains an odd number of header and text octets to be
        checksummed, the last octet is padded on the right with zeros to
        form a 16 bit word for checksum purposes.  The pad is not
        transmitted as part of the segment.  While computing the checksum,
        the checksum field itself is replaced with zeros.

        The checksum also covers a 96 bit pseudo header conceptually

    [Page 16]

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                                               Transmission Control Protocol
                                                    Functional Specification

        prefixed to the TCP header.  This pseudo header contains the Source
        Address, the Destination Address, the Protocol, and TCP length.
        This gives the TCP protection against misrouted segments.  This
        information is carried in the Internet Protocol and is transferred
        across the TCP/Network interface in the arguments or results of
        calls by the TCP on the IP.

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

          The TCP Length is the TCP header length plus the data length in
          octets (this is not an explicitly transmitted quantity, but is
          computed), and it does not count the 12 octets of the pseudo
          header.

      Urgent Pointer:  16 bits

        This field communicates the current value of the urgent pointer as a
        positive offset from the sequence number in this segment.  The
        urgent pointer points to the sequence number of the octet following
        the urgent data.  This field is only be interpreted in segments with
        the URG control bit set.

      Options:  variable

        Options may occupy space at the end of the TCP header and are a
        multiple of 8 bits in length.  All options are included in the
        checksum.  An option may begin on any octet boundary.  There are two
        cases for the format of an option:

          Case 1:  A single octet of option-kind.

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

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

        Note that the list of options may be shorter than the data offset
        field might imply.  The content of the header beyond the
        End-of-Option option must be header padding (i.e., zero).

        A TCP must implement all options.

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        Currently defined options include (kind indicated in octal):

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

        Specific Option Definitions

          End of Option List

            +--------+
            |00000000|
            +--------+
             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.

          No-Operation

            +--------+
            |00000001|
            +--------+
             Kind=1

            This option code may be used between options, for example, to
            align the beginning of a subsequent option on a word boundary.
            There is no guarantee that senders will use this option, so
            receivers must be prepared to process options even if they do
            not begin on a word boundary.

          Maximum Segment Size

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

    [Page 18]

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                                               Transmission Control Protocol
                                                    Functional Specification

            Maximum Segment Size Option Data:  16 bits

              If this option is present, then it communicates the maximum
              receive segment size at the TCP which sends this segment.
              This field must only be sent in the initial connection request
              (i.e., in segments with the SYN control bit set).  If this
              option is not used, any segment size is allowed.

      Padding:  variable

        The TCP header padding is used to ensure that the TCP header ends
        and data begins on a 32 bit boundary.  The padding is composed of
        zeros.

    3.2.  Terminology

      Before we can discuss very much about the operation of the TCP we need
      to introduce some detailed terminology.  The maintenance of a TCP
      connection requires the remembering of several variables.  We conceive
      of these variables being stored in a connection record called a
      Transmission Control Block or TCB.  Among the variables stored in the
      TCB are the local and remote socket numbers, the security and
      precedence of the connection, pointers to the user's send and receive
      buffers, pointers to the retransmit queue and to the current segment.
      In addition several variables relating to the send and receive
      sequence numbers are stored in the TCB.

        Send Sequence Variables

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

        Receive Sequence Variables

          RCV.NXT - receive next
          RCV.WND - receive window
          RCV.UP  - receive urgent pointer
          IRS     - initial receive sequence number

                                                                   [Page 19]

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

      Send Sequence Space

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

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

                              Send Sequence Space

                                   Figure 4.

      The send window is the portion of the sequence space labeled 3 in
      figure 4.

      Receive Sequence Space

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

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

                             Receive Sequence Space

                                   Figure 5.

      The receive window is the portion of the sequence space labeled 2 in
      figure 5.

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

    [Page 20]

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                                               Transmission Control Protocol
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        Current Segment Variables

          SEG.SEQ - segment sequence number
          SEG.ACK - segment acknowledgment number
          SEG.LEN - segment length
          SEG.WND - segment window
          SEG.UP  - segment urgent pointer
          SEG.PRC - segment precedence value

      A connection progresses through a series of states during its
      lifetime.  The states are:  LISTEN, SYN-SENT, SYN-RECEIVED,
      ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK,
      TIME-WAIT, and the fictional state CLOSED.  CLOSED is fictional
      because it represents the state when there is no TCB, and therefore,
      no connection.  Briefly the meanings of the states are:

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

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

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

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

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

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

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

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

        LAST-ACK - represents waiting for an acknowledgment of the
        connection termination request previously sent to the remote TCP
        (which includes an acknowledgment of its connection termination
        request).

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

        CLOSED - represents no connection state at all.

      A TCP connection progresses from one state to another in response to
      events.  The events are the user calls, OPEN, SEND, RECEIVE, CLOSE,
      ABORT, and STATUS; the incoming segments, particularly those
      containing the SYN, ACK, RST and FIN flags; and timeouts.

      The state diagram in figure 6 illustrates only state changes, together
      with the causing events and resulting actions, but addresses neither
      error conditions nor actions which are not connected with state
      changes.  In a later section, more detail is offered with respect to
      the reaction of the TCP to events.

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

    [Page 22]

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                                  +---------+ ---------\      active OPEN
                                  |  CLOSED |            \    -----------
                                  +---------+<---------\   \   create TCB
                                    |     ^              \   \  snd SYN
                       passive OPEN |     |   CLOSE        \   \
                       ------------ |     | ----------       \   \
                        create TCB  |     | delete TCB         \   \
                                    V     |                      \   \
                                  +---------+            CLOSE    |    \
                                  |  LISTEN |          ---------- |     |
                                  +---------+          delete TCB |     |
                       rcv SYN      |     |     SEND              |     |
                      -----------   |     |    -------            |     V
     +---------+      snd SYN,ACK  /       \   snd SYN          +---------+
     |         |<-----------------           ------------------>|         |
     |   SYN   |                    rcv SYN                     |   SYN   |
     |   RCVD  |<-----------------------------------------------|   SENT  |
     |         |                    snd 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  |
                                  +---------+                   +---------+

                          TCP Connection State Diagram
                                   Figure 6.

                                                                   [Page 23]

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    Functional Specification

    3.3.  Sequence Numbers

      A fundamental notion in the design is that every octet of data sent
      over a TCP connection has a sequence number.  Since every octet is
      sequenced, each of them can be acknowledged.  The acknowledgment
      mechanism employed is cumulative so that an acknowledgment of sequence
      number X indicates that all octets up to but not including X have been
      received.  This mechanism allows for straight-forward duplicate
      detection in the presence of retransmission.  Numbering of octets
      within a segment is that the first data octet immediately following
      the header is the lowest numbered, and the following octets are
      numbered consecutively.

      It is essential to remember that the actual sequence number space is
      finite, though very large.  This space ranges from 0 to 2**32 - 1.
      Since the space is finite, all arithmetic dealing with sequence
      numbers must be performed modulo 2**32.  This unsigned arithmetic
      preserves the relationship of sequence numbers as they cycle from
      2**32 - 1 to 0 again.  There are some subtleties to computer modulo
      arithmetic, so great care should be taken in programming the
      comparison of such values.  The symbol "=<" means "less than or equal"
      (modulo 2**32).

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

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

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

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

    [Page 24]

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

        SND.UNA = oldest unacknowledged sequence number

        SND.NXT = next sequence number to be sent

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

        SEG.SEQ = first sequence number of a segment

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

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

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

        SND.UNA < SEG.ACK =< SND.NXT

      A segment on the retransmission queue is fully acknowledged if the sum
      of its sequence number and length is less or equal than the
      acknowledgment value in the incoming segment.

      When data is received the following comparisons are needed:

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

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

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

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

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

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

      or

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

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

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

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

           0       0     SEG.SEQ = RCV.NXT

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

          >0       0     not acceptable

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

      Note that when the receive window is zero no segments should be
      acceptable except ACK segments.  Thus, it is be possible for a TCP to
      maintain a zero receive window while transmitting data and receiving
      ACKs.  However, even when the receive window is zero, a TCP must
      process the RST and URG fields of all incoming segments.

      We have taken advantage of the numbering scheme to protect certain
      control information as well.  This is achieved by implicitly including
      some control flags in the sequence space so they can be retransmitted
      and acknowledged without confusion (i.e., one and only one copy of the
      control will be acted upon).  Control information is not physically
      carried in the segment data space.  Consequently, we must adopt rules
      for implicitly assigning sequence numbers to control.  The SYN and FIN
      are the only controls requiring this protection, and these controls
      are used only at connection opening and closing.  For sequence number
      purposes, the SYN is considered to occur before the first actual data
      octet of the segment in which it occurs, while the FIN is considered
      to occur after the last actual data octet in a segment in which it
      occurs.  The segment length (SEG.LEN) includes both data and sequence
      space occupying controls.  When a SYN is present then SEG.SEQ is the
      sequence number of the SYN.

    [Page 26]

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      Initial Sequence Number Selection

      The protocol places no restriction on a particular connection being
      used over and over again.  A connection is defined by a pair of
      sockets.  New instances of a connection will be referred to as
      incarnations of the connection.  The problem that arises from this is
      -- "how does the TCP identify duplicate segments from previous
      incarnations of the connection?"  This problem becomes apparent if the
      connection is being opened and closed in quick succession, or if the
      connection breaks with loss of memory and is then reestablished.

      To avoid confusion we must prevent segments from one incarnation of a
      connection from being used while the same sequence numbers may still
      be present in the network from an earlier incarnation.  We want to
      assure this, even if a TCP crashes and loses all knowledge of the
      sequence numbers it has been using.  When new connections are created,
      an initial sequence number (ISN) generator is employed which selects a
      new 32 bit ISN.  The generator is bound to a (possibly fictitious) 32
      bit clock whose low order bit is incremented roughly every 4
      microseconds.  Thus, the ISN cycles approximately every 4.55 hours.
      Since we assume that segments will stay in the network no more than
      the Maximum Segment Lifetime (MSL) and that the MSL is less than 4.55
      hours we can reasonably assume that ISN's will be unique.

      For each connection there is a send sequence number and a receive
      sequence number.  The initial send sequence number (ISS) is chosen by
      the data sending TCP, and the initial receive sequence number (IRS) is
      learned during the connection establishing procedure.

      For a connection to be established or initialized, the two TCPs must
      synchronize on each other's initial sequence numbers.  This is done in
      an exchange of connection establishing segments carrying a control bit
      called "SYN" (for synchronize) and the initial sequence numbers.  As a
      shorthand, segments carrying the SYN bit are also called "SYNs".
      Hence, the solution requires a suitable mechanism for picking an
      initial sequence number and a slightly involved handshake to exchange
      the ISN's.

      The synchronization requires each side to send it's own initial
      sequence number and to receive a confirmation of it in acknowledgment
      from the other side.  Each side must also receive the other side's
      initial sequence number and send a confirming acknowledgment.

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

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

      A three way handshake is necessary because sequence numbers are not
      tied to a global clock in the network, and TCPs may have different
      mechanisms for picking the ISN's.  The receiver of the first SYN has
      no way of knowing whether the segment was an old delayed one or not,
      unless it remembers the last sequence number used on the connection
      (which is not always possible), and so it must ask the sender to
      verify this SYN.  The three way handshake and the advantages of a
      clock-driven scheme are discussed in [3].

      Knowing When to Keep Quiet

      To be sure that a TCP does not create a segment that carries a
      sequence number which may be duplicated by an old segment remaining in
      the network, the TCP must keep quiet for a maximum segment lifetime
      (MSL) before assigning any sequence numbers upon starting up or
      recovering from a crash in which memory of sequence numbers in use was
      lost.  For this specification the MSL is taken to be 2 minutes.  This
      is an engineering choice, and may be changed if experience indicates
      it is desirable to do so.  Note that if a TCP is reinitialized in some
      sense, yet retains its memory of sequence numbers in use, then it need
      not wait at all; it must only be sure to use sequence numbers larger
      than those recently used.

      The TCP Quiet Time Concept

        This specification provides that hosts which "crash" without
        retaining any knowledge of the last sequence numbers transmitted on
        each active (i.e., not closed) connection shall delay emitting any
        TCP segments for at least the agreed Maximum Segment Lifetime (MSL)
        in the internet system of which the host is a part.  In the
        paragraphs below, an explanation for this specification is given.
        TCP implementors may violate the "quiet time" restriction, but only
        at the risk of causing some old data to be accepted as new or new
        data rejected as old duplicated by some receivers in the internet
        system.

        TCPs consume sequence number space each time a segment is formed and
        entered into the network output queue at a source host. The
        duplicate detection and sequencing algorithm in the TCP protocol
        relies on the unique binding of segment data to sequence space to
        the extent that sequence numbers will not cycle through all 2**32
        values before the segment data bound to those sequence numbers has
        been delivered and acknowledged by the receiver and all duplicate
        copies of the segments have "drained" from the internet.  Without
        such an assumption, two distinct TCP segments could conceivably be

    [Page 28]

    September 1981
                                               Transmission Control Protocol
                                                    Functional Specification

        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 in the segment.

        Under normal conditions, TCPs keep track of the next sequence number
        to emit and the oldest awaiting acknowledgment so as to avoid
        mistakenly using a sequence number over before its first use has
        been acknowledged.  This alone does not guarantee that old duplicate
        data is drained from the net, so the sequence space has been made
        very large to reduce the probability that a wandering duplicate will
        cause trouble upon arrival.  At 2 megabits/sec. it takes 4.5 hours
        to use up 2**32 octets of sequence space.  Since the maximum segment
        lifetime in the net is not likely to exceed a few tens of seconds,
        this is deemed ample protection for foreseeable nets, even if data
        rates escalate to l0's of megabits/sec.  At 100 megabits/sec, the
        cycle time is 5.4 minutes which may be a little short, but still
        within reason.

        The basic duplicate detection and sequencing algorithm in TCP can be
        defeated, however, if a source TCP does not have any memory of the
        sequence numbers it last used on a given connection. For example, if
        the TCP were to start all connections with sequence number 0, then
        upon crashing and restarting, a TCP might re-form an earlier
        connection (possibly after half-open connection resolution) and emit
        packets with sequence numbers identical to or overlapping with
        packets still in the network which were emitted on an earlier
        incarnation of the same connection.  In the absence of knowledge
        about the sequence numbers used on a particular connection, the TCP
        specification recommends that the source delay for MSL seconds
        before emitting segments on the connection, to allow time for
        segments from the earlier connection incarnation to drain from the
        system.

        Even hosts which can remember the time of day and used it to select
        initial sequence number values are not immune from this problem
        (i.e., even if time of day is used to select an initial sequence
        number for each new connection incarnation).

        Suppose, for example, that a connection is opened starting with
        sequence number S.  Suppose that this connection is not used much
        and that eventually the initial sequence number function (ISN(t))
        takes on a value equal to the sequence number, say S1, of the last
        segment sent by this TCP on a particular connection.  Now suppose,
        at this instant, the host crashes, recovers, and establishes a new
        incarnation of the connection. The initial sequence number chosen is
        S1 = ISN(t) -- last used sequence number on old incarnation of
        connection!  If the recovery occurs quickly enough, any old

                                                                   [Page 29]

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    Functional Specification

        duplicates in the net bearing sequence numbers in the neighborhood
        of S1 may arrive and be treated as new packets by the receiver of
        the new incarnation of the connection.

        The problem is that the recovering host may not know for how long it
        crashed nor does it know whether there are still old duplicates in
        the system from earlier connection incarnations.

        One way to deal with this problem is to deliberately delay emitting
        segments for one MSL after recovery from a crash- this is the "quite
        time" specification.  Hosts which prefer to avoid waiting are
        willing to risk possible confusion of old and new packets at a given
        destination may choose not to wait for the "quite time".
        Implementors may provide TCP users with the ability to select on a
        connection by connection basis whether to wait after a crash, or may
        informally implement the "quite time" for all connections.
        Obviously, even where a user selects to "wait," this is not
        necessary after the host has been "up" for at least MSL seconds.

        To summarize: every segment emitted occupies one or more sequence
        numbers in the sequence space, the numbers occupied by a segment are
        "busy" or "in use" until MSL seconds have passed, upon crashing a
        block of space-time is occupied by the octets of the last emitted
        segment, if a new connection is started too soon and uses any of the
        sequence numbers in the space-time footprint of the last segment of
        the previous connection incarnation, there is a potential sequence
        number overlap area which could cause confusion at the receiver.

    3.4.  Establishing a connection

      The "three-way handshake" is the procedure used to establish a
      connection.  This procedure normally is initiated by one TCP and
      responded to by another TCP.  The procedure also works if two TCP
      simultaneously initiate the procedure.  When simultaneous attempt
      occurs, each TCP receives a "SYN" segment which carries no
      acknowledgment after it has sent a "SYN".  Of course, the arrival of
      an old duplicate "SYN" segment can potentially make it appear, to the
      recipient, that a simultaneous connection initiation is in progress.
      Proper use of "reset" segments can disambiguate these cases.

      Several examples of connection initiation follow.  Although these
      examples do not show connection synchronization using data-carrying
      segments, this is perfectly legitimate, so long as the receiving TCP
      doesn't deliver the data to the user until it is clear the data is
      valid (i.e., the data must be buffered at the receiver until the
      connection reaches the ESTABLISHED state).  The three-way handshake
      reduces the possibility of false connections.  It is the

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      implementation of a trade-off between memory and messages to provide
      information for this checking.

      The simplest three-way handshake is shown in figure 7 below.  The
      figures should be interpreted in the following way.  Each line is
      numbered for reference purposes.  Right arrows (-->) indicate
      departure of a TCP segment from TCP A to TCP B, or arrival of a
      segment at B from A.  Left arrows (<--), indicate the reverse.
      Ellipsis (...) indicates a segment which is still in the network
      (delayed).  An "XXX" indicates a segment which is lost or rejected.
      Comments appear in parentheses.  TCP states represent the state AFTER
      the departure or arrival of the segment (whose contents are shown in
      the center of each line).  Segment contents are shown in abbreviated
      form, with sequence number, control flags, and ACK field.  Other
      fields such as window, addresses, lengths, and text have been left out
      in the interest of clarity.

          TCP A                                                TCP B

      1.  CLOSED                                               LISTEN

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

      3.  ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK>  <-- SYN-RECEIVED

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

      5.  ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED

              Basic 3-Way Handshake for Connection Synchronization

                                    Figure 7.

      In line 2 of figure 7, TCP A begins by sending a SYN segment
      indicating that it will use sequence numbers starting with sequence
      number 100.  In line 3, TCP B sends a SYN and acknowledges the SYN it
      received from TCP A.  Note that the acknowledgment field indicates TCP
      B is now expecting to hear sequence 101, acknowledging the SYN which
      occupied sequence 100.

      At line 4, TCP A responds with an empty segment containing an ACK for
      TCP B's SYN; and in line 5, TCP A sends some data.  Note that the
      sequence number of the segment in line 5 is the same as in line 4
      because the ACK does not occupy sequence number space (if it did, we
      would wind up ACKing ACK's!).

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      Simultaneous initiation is only slightly more complex, as is shown in
      figure 8.  Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to
      ESTABLISHED.

          TCP A                                            TCP B

      1.  CLOSED                                           CLOSED

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

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

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

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

      6.  ESTABLISHED  <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED

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

                    Simultaneous Connection Synchronization

                                   Figure 8.

      The principle reason for the three-way handshake is to prevent old
      duplicate connection initiations from causing confusion.  To deal with
      this, a special control message, reset, has been devised.  If the
      receiving TCP is in a  non-synchronized state (i.e., SYN-SENT,
      SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset.
      If the TCP is in one of the synchronized states (ESTABLISHED,
      FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it
      aborts the connection and informs its user.  We discuss this latter
      case under "half-open" connections below.

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          TCP A                                                TCP B

      1.  CLOSED                                               LISTEN

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

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

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

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

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

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

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

                        Recovery from Old Duplicate SYN

                                   Figure 9.

      As a simple example of recovery from old duplicates, consider
      figure 9.  At line 3, an old duplicate SYN arrives at TCP B.  TCP B
      cannot tell that this is an old duplicate, so it responds normally
      (line 4).  TCP A detects that the ACK field is incorrect and returns a
      RST (reset) with its SEQ field selected to make the segment
      believable.  TCP B, on receiving the RST, returns to the LISTEN state.
      When the original SYN (pun intended) finally arrives at line 6, the
      synchronization proceeds normally.  If the SYN at line 6 had arrived
      before the RST, a more complex exchange might have occurred with RST's
      sent in both directions.

      Half-Open Connections and Other Anomalies

      An established connection is said to be  "half-open" if one of the
      TCPs has closed or aborted the connection at its end without the
      knowledge of the other, or if the two ends of the connection have
      become desynchronized owing to a crash that resulted in loss of
      memory.  Such connections will automatically become reset if an
      attempt is made to send data in either direction.  However, half-open
      connections are expected to be unusual, and the recovery procedure is
      mildly involved.

      If at site A the connection no longer exists, then an attempt by the

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      user at site B to send any data on it will result in the site B TCP
      receiving a reset control message.  Such a message indicates to the
      site B TCP that something is wrong, and it is expected to abort the
      connection.

      Assume that two user processes A and B are communicating with one
      another when a crash occurs causing loss of memory to A's TCP.
      Depending on the operating system supporting A's TCP, it is likely
      that some error recovery mechanism exists.  When the TCP is up again,
      A is likely to start again from the beginning or from a recovery
      point.  As a result, A will probably try to OPEN the connection again
      or try to SEND on the connection it believes open.  In the latter
      case, it receives the error message "connection not open" from the
      local (A's) TCP.  In an attempt to establish the connection, A's TCP
      will send a segment containing SYN.  This scenario leads to the
      example shown in figure 10.  After TCP A crashes, the user attempts to
      re-open the connection.  TCP B, in the meantime, thinks the connection
      is open.

          TCP A                                           TCP B

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

      2.  CLOSED                                           ESTABLISHED

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

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

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

      6.  SYN-SENT                                         CLOSED

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

                         Half-Open Connection Discovery

                                   Figure 10.

      When the SYN arrives at line 3, TCP B, being in a synchronized state,
      and the incoming segment outside the window, responds with an
      acknowledgment indicating what sequence it next expects to hear (ACK
      100).  TCP A sees that this segment does not acknowledge anything it
      sent and, being unsynchronized, sends a reset (RST) because it has
      detected a half-open connection.  TCP B aborts at line 5.  TCP A will

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      continue to try to establish the connection; the problem is now
      reduced to the basic 3-way handshake of figure 7.

      An interesting alternative case occurs when TCP A crashes and TCP B
      tries to send data on what it thinks is a synchronized connection.
      This is illustrated in figure 11.  In this case, the data arriving at
      TCP A from TCP B (line 2) is unacceptable because no such connection
      exists, so TCP A sends a RST.  The RST is acceptable so TCP B
      processes it and aborts the connection.

            TCP A                                              TCP B

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

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

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

               Active Side Causes Half-Open Connection Discovery

                                   Figure 11.

      In figure 12, we find the two TCPs A and B with passive connections
      waiting for SYN.  An old duplicate arriving at TCP B (line 2) stirs B
      into action.  A SYN-ACK is returned (line 3) and causes TCP A to
      generate a RST (the ACK in line 3 is not acceptable).  TCP B accepts
      the reset and returns to its passive LISTEN state.

          TCP A                                         TCP B

      1.  LISTEN                                        LISTEN

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

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

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

      5.  LISTEN                                        LISTEN

           Old Duplicate SYN Initiates a Reset on two Passive Sockets

                                   Figure 12.

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

      Reset Generation

      As a general rule, reset (RST) must be sent whenever a segment arrives
      which apparently is not intended for the current connection.  A reset
      must not be sent if it is not clear that this is the case.

      There are three groups of states:

        1.  If the connection does not exist (CLOSED) then a reset is sent
        in response to any incoming segment except another reset.  In
        particular, SYNs addressed to a non-existent connection are rejected
        by this means.

        If the incoming segment has 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 CLOSED state.

        2.  If the connection is in any non-synchronized state (LISTEN,
        SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges
        something not yet sent (the segment carries an unacceptable ACK), or
        if an incoming segment has a security level or compartment which
        does not exactly match the level and compartment requested for the
        connection, a reset is sent.

        If our SYN has not been acknowledged and the precedence level of the
        incoming segment is higher than the precedence level requested then
        either raise the local precedence level (if allowed by the user and
        the system) or send a reset; or if the precedence level of the
        incoming segment is lower than the precedence level requested then
        continue as if the precedence matched exactly (if the remote TCP
        cannot raise the precedence level to match ours this will be
        detected in the next segment it sends, and the connection will be
        terminated then).  If our SYN has been acknowledged (perhaps in this
        incoming segment) the precedence level of the incoming segment must
        match the local precedence level exactly, if it does not a reset
        must be sent.

        If the incoming segment has an ACK field, the reset takes its
        sequence number from the ACK field of the segment, otherwise the
        reset has sequence number zero and the ACK field is set to the sum
        of the sequence number and segment length of the incoming segment.
        The connection remains in the same state.

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        3.  If the connection is in a synchronized state (ESTABLISHED,
        FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT),
        any unacceptable segment (out of window sequence number or
        unacceptible acknowledgment number) must elicit only an empty
        acknowledgment segment containing the current send-sequence number
        and an acknowledgment indicating the next sequence number expected
        to be received, and the connection remains in the same state.

        If an incoming segment has a security level, or compartment, or
        precedence which does not exactly match the level, and compartment,
        and precedence requested for the connection,a reset is sent and
        connection goes to the CLOSED state.  The reset takes its sequence
        number from the ACK field of the incoming segment.

      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.

    3.5.  Closing a Connection

      CLOSE is an operation meaning "I have no more data to send."  The
      notion of closing a full-duplex connection is subject to ambiguous
      interpretation, of course, since it may not be obvious how to treat
      the receiving side of the connection.  We have chosen to treat CLOSE
      in a simplex fashion.  The user who CLOSEs may continue to RECEIVE
      until he is told that the other side has CLOSED also.  Thus, a program
      could initiate several SENDs followed by a CLOSE, and then continue to
      RECEIVE until signaled that a RECEIVE failed because the other side
      has CLOSED.  We assume that the TCP will signal a user, even if no
      RECEIVEs are outstanding, that the other side has closed, so the user
      can terminate his side gracefully.  A TCP will reliably deliver all
      buffers SENT before the connection was CLOSED so a user who expects no
      data in return need only wait to hear the connection was CLOSED
      successfully to know that all his data was received at the destination
      TCP.  Users must keep reading connections they close for sending until
      the TCP says no more data.

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      There are essentially three cases:

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

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

        3) Both users CLOSE simultaneously

      Case 1:  Local user initiates the close

        In this case, a FIN segment can be constructed and placed on the
        outgoing segment queue.  No further SENDs from the user will be
        accepted by the TCP, and it enters the FIN-WAIT-1 state.  RECEIVEs
        are allowed in this state.  All segments preceding and including FIN
        will be retransmitted until acknowledged.  When the other TCP has
        both acknowledged the FIN and sent a FIN of its own, the first TCP
        can ACK this FIN.  Note that a TCP receiving a FIN will ACK but not
        send its own FIN until its user has CLOSED the connection also.

      Case 2:  TCP receives a FIN from the network

        If an unsolicited FIN arrives from the network, the receiving TCP
        can ACK it and tell the user that the connection is closing.  The
        user will respond with a CLOSE, upon which the TCP can send a FIN to
        the other TCP after sending any remaining data.  The TCP then waits
        until its own FIN is acknowledged whereupon it deletes the
        connection.  If an ACK is not forthcoming, after the user timeout
        the connection is aborted and the user is told.

      Case 3:  both users close simultaneously

        A simultaneous CLOSE by users at both ends of a connection causes
        FIN segments to be exchanged.  When all segments preceding the FINs
        have been processed and acknowledged, each TCP can ACK the FIN it
        has received.  Both will, upon receiving these ACKs, delete the
        connection.

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          TCP A                                                TCP B

      1.  ESTABLISHED                                          ESTABLISHED

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

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

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

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

      6.  (2 MSL)
          CLOSED

                             Normal Close Sequence

                                   Figure 13.

          TCP A                                                TCP B

      1.  ESTABLISHED                                          ESTABLISHED

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

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

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

                          Simultaneous Close Sequence

                                   Figure 14.

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    3.6.  Precedence and Security

      The intent is that connection be allowed only between ports operating
      with exactly the same security and compartment values and at the
      higher of the precedence level requested by the two ports.

      The precedence and security parameters used in TCP are exactly those
      defined in the Internet Protocol (IP) [2].  Throughout this TCP
      specification the term "security/compartment" is intended to indicate
      the security parameters used in IP including security, compartment,
      user group, and handling restriction.

      A connection attempt with mismatched security/compartment values or a
      lower precedence value must be rejected by sending a reset.  Rejecting
      a connection due to too low a precedence only occurs after an
      acknowledgment of the SYN has been received.

      Note that TCP modules which operate only at the default value of
      precedence will still have to check the precedence of incoming
      segments and possibly raise the precedence level they use on the
      connection.

      The security paramaters may be used even in a non-secure environment
      (the values would indicate unclassified data), thus hosts in
      non-secure environments must be prepared to receive the security
      parameters, though they need not send them.

    3.7.  Data Communication

      Once the connection is established data is communicated by the
      exchange of segments.  Because segments may be lost due to errors
      (checksum test failure), or network congestion, TCP uses
      retransmission (after a timeout) to ensure delivery of every segment.
      Duplicate segments may arrive due to network or TCP retransmission.
      As discussed in the section on sequence numbers the TCP performs
      certain tests on the sequence and acknowledgment numbers in the
      segments to verify their acceptability.

      The sender of data keeps track of the next sequence number to use in
      the variable SND.NXT.  The receiver of data keeps track of the next
      sequence number to expect in the variable RCV.NXT.  The sender of data
      keeps track of the oldest unacknowledged sequence number in the
      variable SND.UNA.  If the data flow is momentarily idle and all data
      sent has been acknowledged then the three variables will be equal.

      When the sender creates a segment and transmits it the sender advances
      SND.NXT.  When the receiver accepts a segment it advances RCV.NXT and
      sends an acknowledgment.  When the data sender receives an

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      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 in the segment.  Note that once in the ESTABLISHED state all
      segments must carry current acknowledgment information.

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

      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 must be dynamically determined.  One procedure
      for determining a retransmission time out is given here as an
      illustration.

        An Example Retransmission Timeout Procedure

          Measure the elapsed time between sending a data octet with a
          particular sequence number and receiving an acknowledgment that
          covers that sequence number (segments sent do not have to match
          segments received).  This measured elapsed time is the Round Trip
          Time (RTT).  Next compute a Smoothed Round Trip Time (SRTT) as:

            SRTT = ( ALPHA * SRTT ) + ((1-ALPHA) * RTT)

          and based on this, compute the retransmission timeout (RTO) as:

            RTO = min[UBOUND,max[LBOUND,(BETA*SRTT)]]

          where UBOUND is an upper bound on the timeout (e.g., 1 minute),
          LBOUND is a lower bound on the timeout (e.g., 1 second), ALPHA is
          a smoothing factor (e.g., .8 to .9), and BETA is a delay variance
          factor (e.g., 1.3 to 2.0).

      The Communication of Urgent Information

      The objective of the TCP urgent mechanism is to allow the sending user
      to stimulate the receiving user to accept some urgent data and to
      permit the receiving TCP to indicate to the receiving user when all
      the currently known urgent data has been received by the user.

      This mechanism permits a point in the data stream to be designated as
      the end of urgent information.  Whenever this point is in advance of
      the receive sequence number (RCV.NXT) at the receiving TCP, that TCP
      must tell the user to go into "urgent mode"; when the receive sequence
      number catches up to the urgent pointer, the TCP must tell user to go

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      into "normal mode".  If the urgent pointer is updated while the user
      is in "urgent mode", the update will be invisible to the user.

      The method employs a urgent field which is carried in all segments
      transmitted.  The URG control flag indicates that the urgent field is
      meaningful and must be added to the segment sequence number to yield
      the urgent pointer.  The absence of this flag indicates that there is
      no urgent data outstanding.

      To send an urgent indication the user must also send at least one data
      octet.  If the sending user also indicates a push, timely delivery of
      the urgent information to the destination process is enhanced.

      Managing the Window

      The window sent in each segment indicates the range of sequence
      numbers the sender of the window (the data receiver) is currently
      prepared to accept.  There is an assumption that this is related to
      the currently available data buffer space available for this
      connection.

      Indicating a large window encourages transmissions.  If more data
      arrives than can be accepted, it will be discarded.  This will result
      in excessive retransmissions, adding unnecessarily to the load on the
      network and the TCPs.  Indicating a small window may restrict the
      transmission of data to the point of introducing a round trip delay
      between each new segment transmitted.

      The mechanisms provided allow a TCP to advertise a large window and to
      subsequently advertise a much smaller window without having accepted
      that much data.  This, so called "shrinking the window," is strongly
      discouraged.  The robustness principle dictates that TCPs will not
      shrink the window themselves, but will be prepared for such behavior
      on the part of other TCPs.

      The sending TCP must be prepared to accept from the user and send at
      least one octet of new data even if the send window is zero.  The
      sending TCP must regularly retransmit to the receiving TCP even when
      the window is zero.  Two minutes is recommended for the retransmission
      interval when the window is zero.  This retransmission is essential to
      guarantee that when either TCP has a zero window the re-opening of the
      window will be reliably reported to the other.

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

      The sending TCP packages the data to be transmitted into segments

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      which 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 acknowledgment number
      equal or greater than the highest previously received).

      The window management procedure has significant influence on the
      communication performance.  The following comments are suggestions to
      implementers.

        Window Management Suggestions

          Allocating a very small window causes data to be transmitted in
          many small segments when better performance is achieved using
          fewer large segments.

          One suggestion for avoiding small windows is for the receiver to
          defer updating a window until the additional allocation is at
          least X percent of the maximum allocation possible for the
          connection (where X might be 20 to 40).

          Another suggestion is for the sender to avoid sending small
          segments by waiting until the window is large enough before
          sending data.  If the the user signals a push function then the
          data must be sent even if it is a small segment.

          Note that the acknowledgments should not be delayed or unnecessary
          retransmissions will result.  One strategy would be to send an
          acknowledgment when a small segment arrives (with out updating the
          window information), and then to send another acknowledgment with
          new window information when the window is larger.

          The segment sent to probe a zero window may also begin a break up
          of transmitted data into smaller and smaller segments.  If a
          segment containing a single data octet sent to probe a zero window
          is accepted, it consumes one octet of the window now available.
          If the sending TCP simply sends as much as it can whenever the
          window is non zero, the transmitted data will be broken into
          alternating big and small segments.  As time goes on, occasional
          pauses in the receiver making window allocation available will

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          result in breaking the big segments into a small and not quite so
          big pair. And after a while the data transmission will be in
          mostly small segments.

          The suggestion here is that the TCP implementations need to
          actively attempt to combine small window allocations into larger
          windows, since the mechanisms for managing the window tend to lead
          to many small windows in the simplest minded implementations.

    3.8.  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 lowel level protocol.  For the
      case that the lower level is IP we note some of the parameter values
      that TCPs might use.

      User/TCP Interface

        The following functional description of user commands to the TCP is,
        at best, fictional, since every operating system will have different
        facilities.  Consequently, we must warn readers that different TCP
        implementations may have different user interfaces.  However, all
        TCPs must provide a certain minimum set of services to guarantee
        that all TCP implementations can support the same protocol
        hierarchy.  This section specifies the functional interfaces
        required of all TCP implementations.

        TCP User Commands

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

          The user commands described below specify the basic functions the
          TCP must perform to support interprocess communication.
          Individual implementations must define their own exact format, and
          may provide combinations or subsets of the basic functions in
          single calls.  In particular, some implementations may wish to
          automatically OPEN a connection on the first SEND or RECEIVE
          issued by the user for a given connection.

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                                               Transmission Control Protocol
                                                    Functional Specification

          In providing interprocess communication facilities, the TCP must
          not only accept commands, but must also return information to the
          processes it serves.  The latter consists of:

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

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

          Open

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

            We assume that the local TCP is aware of the identity of the
            processes it serves and will check the authority of the process
            to use the connection specified.  Depending upon the
            implementation of the TCP, the local network and TCP identifiers
            for the source address will either be supplied by the TCP or the
            lower level protocol (e.g., IP).  These considerations are the
            result of concern about security, to the extent that no TCP be
            able to masquerade as another one, and so on.  Similarly, no
            process can masquerade as another without the collusion of the
            TCP.

            If the active/passive flag is set to passive, then this is a
            call to LISTEN for an incoming connection.  A passive open may
            have either a fully specified foreign socket to wait for a
            particular connection or an unspecified foreign socket to wait
            for any call.  A fully specified passive call can be made active
            by the subsequent execution of a SEND.

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

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

            The timeout, if present, permits the caller to set up a timeout
            for all data submitted to TCP.  If data is not successfully
            delivered to the destination within the timeout period, the TCP
            will abort the connection.  The present global default is five
            minutes.

            The TCP or some component of the operating system will verify
            the users authority to open a connection with the specified

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    Transmission Control Protocol
    Functional Specification

            precedence or security/compartment.  The absence of precedence
            or security/compartment specification in the OPEN call indicates
            the default values must be used.

            TCP will accept incoming requests as matching only if the
            security/compartment information is exactly the same and only if
            the precedence is equal to or higher than the precedence
            requested in the OPEN call.

            The precedence for the connection is the higher of the values
            requested in the OPEN call and received from the incoming
            request, and fixed at that value for the life of the
            connection.Implementers may want to give the user control of
            this precedence negotiation.  For example, the user might be
            allowed to specify that the precedence must be exactly matched,
            or that any attempt to raise the precedence be confirmed by the
            user.

            A local connection name will be returned to the user by the TCP.
            The local connection name can then be used as a short hand term
            for the connection defined by the <local socket, foreign socket>
            pair.

          Send

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

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

            If the PUSH flag is set, the data must be transmitted promptly
            to the receiver, and the PUSH bit will be set in the last TCP
            segment created from the buffer.  If the PUSH flag is not set,
            the data may be combined with data from subsequent SENDs for
            transmission efficiency.

            If the URGENT flag is set, segments sent to the destination TCP
            will have the urgent pointer set.  The receiving TCP will signal
            the urgent condition to the receiving process if the urgent
            pointer indicates that data preceding the urgent pointer has not
            been consumed by the receiving process.  The purpose of urgent
            is to stimulate the receiver to process the urgent data and to
            indicate to the receiver when all the currently known urgent

    [Page 46]

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                                               Transmission Control Protocol
                                                    Functional Specification

            data has been received.  The number of times the sending user's
            TCP signals urgent will not necessarily be equal to the number
            of times the receiving user will be notified of the presence of
            urgent data.

            If no foreign socket was specified in the OPEN, but the
            connection is established (e.g., because a LISTENing connection
            has become specific due to a foreign segment arriving for the
            local socket), then the designated buffer is sent to the implied
            foreign socket.  Users who make use of OPEN with an unspecified
            foreign socket can make use of SEND without ever explicitly
            knowing the foreign socket address.

            However, if a SEND is attempted before the foreign socket
            becomes specified, an error will be returned.  Users can use the
            STATUS call to determine the status of the connection.  In some
            implementations the TCP may notify the user when an unspecified
            socket is bound.

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

            In the simplest implementation, SEND would not return control to
            the sending process until either the transmission was complete
            or the timeout had been exceeded.  However, this simple method
            is both subject to deadlocks (for example, both sides of the
            connection might try to do SENDs before doing any RECEIVEs) and
            offers poor performance, so it is not recommended.  A more
            sophisticated implementation would return immediately to allow
            the process to run concurrently with network I/O, and,
            furthermore, to allow multiple SENDs to be in progress.
            Multiple SENDs are served in first come, first served order, so
            the TCP will queue those it cannot service 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.  An alternative is to
            return a response immediately.  For instance, SENDs might return
            immediate local acknowledgment, even if the segment sent had not
            been acknowledged by the distant TCP.  We could optimistically
            assume eventual success.  If we are wrong, the connection will
            close anyway due to the timeout.  In implementations of this
            kind (synchronous), there will still be some asynchronous
            signals, but these will deal with the connection itself, and not
            with specific segments or buffers.

            In order for the process to distinguish among error or success
            indications for different SENDs, it might be appropriate for the

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    Functional Specification

            buffer address to be returned along with the coded response to
            the SEND request.  TCP-to-user signals are discussed below,
            indicating the information which should be returned to the
            calling process.

          Receive

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

            This command allocates a receiving buffer associated with the
            specified connection.  If no OPEN precedes this command or the
            calling process is not authorized to use this connection, an
            error is returned.

            In the simplest implementation, control would not return to the
            calling program until either the buffer was filled, or some
            error occurred, but this scheme is highly subject to deadlocks.
            A more sophisticated implementation would permit several
            RECEIVEs to be outstanding at once.  These would be filled as
            segments arrive.  This strategy permits increased throughput at
            the cost of a more elaborate scheme (possibly asynchronous) to
            notify the calling program that a PUSH has been seen or a buffer
            filled.

            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 preceeding 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

    [Page 48]

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                                               Transmission Control Protocol
                                                    Functional Specification

            allocate buffer storage, or the TCP 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 other side
            may be trying to transmit the last of its data.  Thus, CLOSE
            means "I have no more to send" but does not mean "I will not
            receive any more."  It may happen (if the user level protocol is
            not well thought out) that the closing side is unable to get rid
            of all its data before timing out.  In this event, CLOSE turns
            into ABORT, and the closing TCP gives up.

            The user may CLOSE the connection at any time on his own
            initiative, or in response to various prompts from the TCP
            (e.g., remote close executed, transmission timeout exceeded,
            destination inaccessible).

            Because closing a connection requires communication with the
            foreign TCP, connections may remain in the closing state for a
            short time.  Attempts to reopen the connection before the TCP
            replies to the CLOSE command will result in error responses.

            Close also implies push function.

          Status

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

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

            This command returns a data block containing the following
            information:

              local socket,

                                                                   [Page 49]

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    Transmission Control Protocol
    Functional Specification

              foreign socket,
              local connection name,
              receive window,
              send window,
              connection state,
              number of buffers awaiting acknowledgment,
              number of buffers pending receipt,
              urgent state,
              precedence,
              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 RESET message to
            be sent to the TCP on the other side of the connection.
            Depending on the implementation, users may receive abort
            indications for each outstanding SEND or RECEIVE, or may simply
            receive an ABORT-acknowledgment.

        TCP-to-User Messages

          It is assumed that the operating system environment provides a
          means for the TCP to asynchronously signal the user program.  When
          the TCP does signal a user program, certain information is passed
          to the user.  Often in the specification the information will be
          an error message.  In other cases there will be information
          relating to the completion of processing a SEND or RECEIVE or
          other user call.

          The following information is provided:

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

    [Page 50]

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                                               Transmission Control Protocol
                                                    Functional Specification

      TCP/Lower-Level Interface

        The TCP calls on a lower level protocol module to actually send and
        receive information over a network.  One case is that of the ARPA
        internetwork system where the lower level module is the Internet
        Protocol (IP) [2].

        If the lower level protocol is IP it provides arguments for a type
        of service and for a time to live.  TCP uses the following settings
        for these parameters:

          Type of Service = Precedence: routine, Delay: normal, Throughput:
          normal, Reliability: normal; or 00000000.

          Time to Live    = one minute, or 00111100.

            Note that the assumed maximum segment lifetime is two minutes.
            Here we explicitly ask that a segment be destroyed if it cannot
            be delivered by the internet system within one minute.

        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.

        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 equivlent service
        of IP and to be used in the TCP checksum.

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    Transmission Control Protocol
    Functional Specification

    3.9.  Event Processing

      The processing depicted in this section is an example of one possible
      implementation.  Other implementations may have slightly different
      processing sequences, but they should differ from those in this
      section only in detail, not in substance.

      The activity of the TCP can be characterized as responding to events.
      The events that occur can be cast into three categories:  user calls,
      arriving segments, and timeouts.  This section describes the
      processing the TCP does in response to each of the events.  In many
      cases the processing required depends on the state of the connection.

        Events that occur:

          User Calls

            OPEN
            SEND
            RECEIVE
            CLOSE
            ABORT
            STATUS

          Arriving Segments

            SEGMENT ARRIVES

          Timeouts

            USER TIMEOUT
            RETRANSMISSION TIMEOUT
            TIME-WAIT TIMEOUT

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

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

      Please note in the following that all arithmetic on sequence numbers,
      acknowledgment numbers, windows, et cetera, is modulo 2**32 the size
      of the sequence number space.  Also note that "=<" means less than or
      equal to (modulo 2**32).

    [Page 52]

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                                               Transmission Control Protocol
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      A natural way to think about processing incoming segments is to
      imagine that they are first tested for proper sequence number (i.e.,
      that their contents lie in the range of the expected "receive window"
      in the sequence number space) and then that they are generally queued
      and processed in sequence number order.

      When a segment overlaps other already received segments we reconstruct
      the segment to contain just the new data, and adjust the header fields
      to be consistent.

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

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    Transmission Control Protocol
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                                                                   OPEN Call

      OPEN Call

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

          Create a new transmission control block (TCB) to hold connection
          state information.  Fill in local socket identifier, foreign
          socket, precedence, security/compartment, and user timeout
          information.  Note that some parts of the foreign socket may be
          unspecified in a passive OPEN and are to be filled in by the
          parameters of the incoming SYN segment.  Verify the security and
          precedence requested are allowed for this user, if not return
          "error:  precedence not allowed" or "error:  security/compartment
          not allowed."  If passive enter the LISTEN state and return.  If
          active and the foreign socket is unspecified, return "error:
          foreign socket unspecified"; if active and the foreign socket is
          specified, issue a SYN segment.  An initial send sequence number
          (ISS) is selected.  A SYN segment of the form <SEQ=ISS><CTL=SYN>
          is sent.  Set SND.UNA to ISS, SND.NXT to ISS+1, enter SYN-SENT
          state, and return.

          If the caller does not have access to the local socket specified,
          return "error:  connection illegal for this process".  If there is
          no room to create a new connection, return "error:  insufficient
          resources".

        LISTEN STATE

          If active and the foreign socket is specified, then change the
          connection from passive to active, select an ISS.  Send a SYN
          segment, set SND.UNA to ISS, SND.NXT to ISS+1.  Enter SYN-SENT
          state.  Data associated with SEND may be sent with SYN segment or
          queued for transmission after entering ESTABLISHED state.  The
          urgent bit if requested in the command must be sent with the data
          segments sent as a result of this command.  If there is no room to
          queue the request, respond with "error:  insufficient resources".
          If Foreign socket was not specified, then return "error:  foreign
          socket unspecified".

    [Page 54]

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                                               Transmission Control Protocol
                                                    Functional Specification
    OPEN Call

        SYN-SENT STATE
        SYN-RECEIVED STATE
        ESTABLISHED STATE
        FIN-WAIT-1 STATE
        FIN-WAIT-2 STATE
        CLOSE-WAIT STATE
        CLOSING STATE
        LAST-ACK STATE
        TIME-WAIT STATE

          Return "error:  connection already exists".

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                                                                   SEND Call

      SEND Call

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

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

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

        LISTEN STATE

          If the foreign socket is specified, then change the connection
          from passive to active, select an ISS.  Send a SYN segment, set
          SND.UNA to ISS, SND.NXT to ISS+1.  Enter SYN-SENT state.  Data
          associated with SEND may be sent with SYN segment or queued for
          transmission after entering ESTABLISHED state.  The urgent bit if
          requested in the command must be sent with the data segments sent
          as a result of this command.  If there is no room to queue the
          request, respond with "error:  insufficient resources".  If
          Foreign socket was not specified, then return "error:  foreign
          socket unspecified".

        SYN-SENT STATE
        SYN-RECEIVED STATE

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

        ESTABLISHED STATE
        CLOSE-WAIT STATE

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

          If the urgent flag is set, then SND.UP <- SND.NXT-1 and set the
          urgent pointer in the outgoing segments.

    [Page 56]

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                                               Transmission Control Protocol
                                                    Functional Specification
    SEND Call

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

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

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                                                                RECEIVE Call

      RECEIVE Call

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

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

          Otherwise return "error:  connection does not exist".

        LISTEN STATE
        SYN-SENT STATE
        SYN-RECEIVED STATE

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

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

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

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

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

          When the TCP takes responsibility for delivering data to the user
          that fact must be communicated to the sender via an
          acknowledgment.  The formation of such an acknowledgment is
          described below in the discussion of processing an incoming
          segment.

    [Page 58]

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                                               Transmission Control Protocol
                                                    Functional Specification
    RECEIVE Call

        CLOSE-WAIT STATE

          Since the remote side has already sent FIN, RECEIVEs must be
          satisfied by text already on hand, but not yet delivered to the
          user.  If no text is awaiting delivery, the RECEIVE will get a
          "error:  connection closing" response.  Otherwise, any remaining
          text can be used to satisfy the RECEIVE.

        CLOSING STATE
        LAST-ACK STATE
        TIME-WAIT STATE

          Return "error:  connection closing".

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                                                                  CLOSE Call

      CLOSE Call

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

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

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

        LISTEN STATE

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

        SYN-SENT STATE

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

        SYN-RECEIVED STATE

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

        ESTABLISHED STATE

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

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

          Strictly speaking, this is an error and should receive a "error:
          connection closing" response.  An "ok" response would be
          acceptable, too, as long as a second FIN is not emitted (the first
          FIN may be retransmitted though).

    [Page 60]

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                                               Transmission Control Protocol
                                                    Functional Specification
    CLOSE Call

        CLOSE-WAIT STATE

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

        CLOSING STATE
        LAST-ACK STATE
        TIME-WAIT STATE

          Respond with "error:  connection closing".

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                                                                  ABORT Call

      ABORT Call

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

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

          Otherwise return "error:  connection does not exist".

        LISTEN STATE

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

        SYN-SENT STATE

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

        SYN-RECEIVED STATE
        ESTABLISHED STATE
        FIN-WAIT-1 STATE
        FIN-WAIT-2 STATE
        CLOSE-WAIT STATE

          Send a reset segment:

            <SEQ=SND.NXT><CTL=RST>

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

        CLOSING STATE
        LAST-ACK STATE
        TIME-WAIT STATE

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

    [Page 62]

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                                               Transmission Control Protocol
                                                    Functional Specification
    STATUS Call

      STATUS Call

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

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

          Otherwise return "error:  connection does not exist".

        LISTEN STATE

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

        SYN-SENT STATE

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

        SYN-RECEIVED STATE

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

        ESTABLISHED STATE

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

        FIN-WAIT-1 STATE

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

        FIN-WAIT-2 STATE

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

        CLOSE-WAIT STATE

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

        CLOSING STATE

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

        LAST-ACK STATE

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

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                                                                 STATUS Call

        TIME-WAIT STATE

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

    [Page 64]

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                                               Transmission Control Protocol
                                                    Functional Specification
    SEGMENT ARRIVES

      SEGMENT ARRIVES

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

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

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

            <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>

          If the ACK bit is on,

            <SEQ=SEG.ACK><CTL=RST>

          Return.

        If the state is LISTEN then

          first check for an RST

            An incoming RST should be ignored.  Return.

          second check for an ACK

            Any acknowledgment is bad if it arrives on a connection still in
            the LISTEN state.  An acceptable reset segment should be formed
            for any arriving ACK-bearing segment.  The RST should be
            formatted as follows:

              <SEQ=SEG.ACK><CTL=RST>

            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.

              <SEQ=SEG.ACK><CTL=RST>

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                                                             SEGMENT ARRIVES

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

              <SEQ=SEG.ACK><CTL=RST>

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

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

              <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>

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

          fourth other text or control

            Any other control or text-bearing segment (not containing SYN)
            must have an ACK and thus would be discarded by the ACK
            processing.  An incoming RST segment could not be valid, since
            it could not have been sent in response to anything sent by this
            incarnation of the connection.  So you are unlikely to get here,
            but if you do, drop the segment, and return.

        If the state is SYN-SENT then

          first check the ACK bit

            If the ACK bit is set

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

                <SEQ=SEG.ACK><CTL=RST>

              and discard the segment.  Return.

              If SND.UNA =< SEG.ACK =< SND.NXT then the ACK is acceptable.

          second check the RST bit

    [Page 66]

    September 1981
                                               Transmission Control Protocol
                                                    Functional Specification
    SEGMENT ARRIVES

            If the RST bit is set

              If the ACK was acceptable then signal the user "error:
              connection reset", drop the segment, enter CLOSED state,
              delete TCB, and return.  Otherwise (no ACK) drop the segment
              and return.

          third check the security and precedence

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

              If there is an ACK

                <SEQ=SEG.ACK><CTL=RST>

              Otherwise

                <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>

            If there is an ACK

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

                <SEQ=SEG.ACK><CTL=RST>

            If there is no ACK

              If the precedence in the segment is higher than the precedence
              in the TCB then if allowed by the user and the system raise
              the precedence in the TCB to that in the segment, if not
              allowed to raise the prec then send a reset.

                <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>

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

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

          fourth check the SYN bit

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

            If the SYN bit is on and the security/compartment and precedence

                                                                   [Page 67]

                                                              September 1981
    Transmission Control Protocol
    Functional Specification
                                                             SEGMENT ARRIVES

            are acceptable then, RCV.NXT is set to SEG.SEQ+1, IRS is set to
            SEG.SEQ.  SND.UNA should be advanced to equal SEG.ACK (if there
            is an ACK), and any segments on the retransmission queue which
            are thereby acknowledged should be removed.

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

              <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>

            and send it.  Data or controls which were queued for
            transmission may be included.  If there are other controls or
            text in the segment then continue processing at the sixth step
            below where the URG bit is checked, otherwise return.

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

              <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>

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

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

    [Page 68]

    September 1981
                                               Transmission Control Protocol
                                                    Functional Specification
    SEGMENT ARRIVES

        Otherwise,

        first check sequence number

          SYN-RECEIVED STATE
          ESTABLISHED STATE
          FIN-WAIT-1 STATE
          FIN-WAIT-2 STATE
          CLOSE-WAIT STATE
          CLOSING STATE
          LAST-ACK STATE
          TIME-WAIT STATE

            Segments are processed in sequence.  Initial tests on arrival
            are used to discard old duplicates, but further processing is
            done in SEG.SEQ order.  If a segment's contents straddle the
            boundary between old and new, only the new parts should be
            processed.

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

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

               0       0     SEG.SEQ = RCV.NXT

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

              >0       0     not acceptable

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

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

            If an incoming segment is not acceptable, an acknowledgment
            should be sent in reply (unless the RST bit is set, if so drop
            the segment and return):

              <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>

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

                                                                   [Page 69]

                                                              September 1981
    Transmission Control Protocol
    Functional Specification
                                                             SEGMENT ARRIVES

            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 begining sequence
            numbers may be held for later processing.

        second check the RST bit,

          SYN-RECEIVED STATE

            If the RST bit is set

              If this connection was initiated with a passive OPEN (i.e.,
              came from the LISTEN state), then return this connection to
              LISTEN state and return.  The user need not be informed.  If
              this connection was initiated with an active OPEN (i.e., came
              from SYN-SENT state) then the connection was refused, signal
              the user "connection refused".  In either case, all segments
              on the retransmission queue should be removed.  And in the
              active OPEN case, enter the CLOSED state and delete the TCB,
              and return.

          ESTABLISHED
          FIN-WAIT-1
          FIN-WAIT-2
          CLOSE-WAIT

            If the RST bit is set then, any outstanding RECEIVEs and SEND
            should receive "reset" responses.  All segment queues should be
            flushed.  Users should also receive an unsolicited general
            "connection reset" signal.  Enter the CLOSED state, delete the
            TCB, and return.

          CLOSING STATE
          LAST-ACK STATE
          TIME-WAIT

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

    [Page 70]

    September 1981
                                               Transmission Control Protocol
                                                    Functional Specification
    SEGMENT ARRIVES

        third check security and precedence

          SYN-RECEIVED

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

          ESTABLISHED STATE

            If the security/compartment and precedence in the segment do not
            exactly match the security/compartment and precedence in the TCB
            then send a reset, any outstanding RECEIVEs and SEND should
            receive "reset" responses.  All segment queues should be
            flushed.  Users should also receive an unsolicited general
            "connection reset" signal.  Enter the CLOSED state, delete the
            TCB, and return.

          Note this check is placed following the sequence check to prevent
          a segment from an old connection between these ports with a
          different security or precedence from causing an abort of the
          current connection.

        fourth, check the SYN bit,

          SYN-RECEIVED
          ESTABLISHED STATE
          FIN-WAIT STATE-1
          FIN-WAIT STATE-2
          CLOSE-WAIT STATE
          CLOSING STATE
          LAST-ACK STATE
          TIME-WAIT STATE

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

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

                                                                   [Page 71]

                                                              September 1981
    Transmission Control Protocol
    Functional Specification
                                                             SEGMENT ARRIVES

        fifth check the ACK field,

          if the ACK bit is off drop the segment and return

          if the ACK bit is on

            SYN-RECEIVED STATE

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

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

                  <SEQ=SEG.ACK><CTL=RST>

                and send it.

            ESTABLISHED STATE

              If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <- SEG.ACK.
              Any segments on the retransmission queue which are thereby
              entirely acknowledged are removed.  Users should receive
              positive acknowledgments for buffers which have been SENT and
              fully acknowledged (i.e., SEND buffer should be returned with
              "ok" response).  If the ACK is a duplicate
              (SEG.ACK < SND.UNA), it can be ignored.  If the ACK acks
              something not yet sent (SEG.ACK > SND.NXT) then send an ACK,
              drop the segment, and return.

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

    [Page 72]

    September 1981
                                               Transmission Control Protocol
                                                    Functional Specification
    SEGMENT ARRIVES

            FIN-WAIT-1 STATE

              In addition to the processing for the ESTABLISHED state, if
              our FIN is now acknowledged then enter FIN-WAIT-2 and continue
              processing in that state.

            FIN-WAIT-2 STATE

              In addition to the processing for the ESTABLISHED state, if
              the retransmission queue is empty, the user's CLOSE can be
              acknowledged ("ok") but do not delete the TCB.

            CLOSE-WAIT STATE

              Do the same processing as for the ESTABLISHED state.

            CLOSING STATE

              In addition to the processing for the ESTABLISHED state, if
              the ACK acknowledges our FIN then enter the TIME-WAIT state,
              otherwise ignore the segment.

            LAST-ACK STATE

              The only thing that can arrive in this state is an
              acknowledgment of our FIN.  If our FIN is now acknowledged,
              delete the TCB, enter the CLOSED state, and return.

            TIME-WAIT STATE

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

        sixth, check the URG bit,

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

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

                                                                   [Page 73]

                                                              September 1981
    Transmission Control Protocol
    Functional Specification
                                                             SEGMENT ARRIVES

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

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

        seventh, process the segment text,

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

            Once in the ESTABLISHED state, it is possible to deliver segment
            text to user RECEIVE buffers.  Text from segments can be moved
            into buffers until either the buffer is full or the segment is
            empty.  If the segment empties and carries an PUSH flag, then
            the user is informed, when the buffer is returned, that a PUSH
            has been received.

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

            Once the TCP takes responsibility for the data it advances
            RCV.NXT over the data accepted, and adjusts RCV.WND as
            apporopriate to the current buffer availability.  The total of
            RCV.NXT and RCV.WND should not be reduced.

            Please note the window management suggestions in section 3.7.

            Send an acknowledgment of the form:

              <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>

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

    [Page 74]

    September 1981
                                               Transmission Control Protocol
                                                    Functional Specification
    SEGMENT ARRIVES

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

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

        eighth, check the FIN bit,

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

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

            SYN-RECEIVED STATE
            ESTABLISHED STATE

              Enter the CLOSE-WAIT state.

            FIN-WAIT-1 STATE

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

            FIN-WAIT-2 STATE

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

            CLOSE-WAIT STATE

              Remain in the CLOSE-WAIT state.

            CLOSING STATE

              Remain in the CLOSING state.

            LAST-ACK STATE

              Remain in the LAST-ACK state.

                                                                   [Page 75]

                                                              September 1981
    Transmission Control Protocol
    Functional Specification
                                                             SEGMENT ARRIVES

            TIME-WAIT STATE

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

        and return.

    [Page 76]

    September 1981
                                               Transmission Control Protocol
                                                    Functional Specification
    USER TIMEOUT

      USER TIMEOUT

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

      RETRANSMISSION TIMEOUT

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

      TIME-WAIT TIMEOUT

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

                                                                   [Page 77]

                                                              September 1981
    Transmission Control Protocol

    [Page 78]

    September 1981
                                               Transmission Control Protocol

                                    GLOSSARY

    1822
              BBN Report 1822, "The Specification of the Interconnection of
              a Host and an IMP".  The specification of interface between a
              host and the ARPANET.

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

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

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

    connection
              A logical communication path identified by a pair of sockets.

    datagram
              A message sent in a packet switched computer communications
              network.

    Destination Address
              The destination address, usually the network and host
              identifiers.

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

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

    FTP
              A file transfer protocol.

                                                                   [Page 79]

                                                              September 1981
    Transmission Control Protocol
    Glossary

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

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

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

    IMP
              The Interface Message Processor, the packet switch of the
              ARPANET.

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

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

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

    IP
              Internet Protocol.

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

    ISN
              The Initial Sequence Number.  The first sequence number used
              on a connection, (either ISS or IRS).  Selected on a clock
              based procedure.

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

    leader
              Control information at the beginning of a message or block of
              data.  In particular, in the ARPANET, the control information
              on an ARPANET message at the host-IMP interface.

    [Page 80]

    September 1981
                                               Transmission Control Protocol
                                                                    Glossary

    left sequence
              This is the next sequence number to be acknowledged by the
              data receiving TCP (or the lowest currently unacknowledged
              sequence number) and is sometimes referred to as the left edge
              of the send window.

    local packet
              The unit of transmission within a local network.

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

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

    octet
              An eight bit byte.

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

    packet
              A package of data with a header which may or may not be
              logically complete.  More often a physical packaging than a
              logical packaging of data.

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

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

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

    RCV.NXT
              receive next sequence number

                                                                   [Page 81]

                                                              September 1981
    Transmission Control Protocol
    Glossary

    RCV.UP
              receive urgent pointer

    RCV.WND
              receive window

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

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

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

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

    SEG.ACK
              segment acknowledgment

    SEG.LEN
              segment length

    SEG.PRC
              segment precedence value

    SEG.SEQ
              segment sequence

    SEG.UP
              segment urgent pointer field

    [Page 82]

    September 1981
                                               Transmission Control Protocol
                                                                    Glossary

    SEG.WND
              segment window field

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

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

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

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

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

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

    SND.NXT
              send sequence

    SND.UNA
              left sequence

    SND.UP
              send urgent pointer

    SND.WL1
              segment sequence number at last window update

    SND.WL2
              segment acknowledgment number at last window update

                                                                   [Page 83]

                                                              September 1981
    Transmission Control Protocol
    Glossary

    SND.WND
              send window

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

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

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

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

    TCB.PRC
              The precedence of the connection.

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

    TOS
              Type of Service, an Internet Protocol field.

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

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

    urgent pointer
              A control field meaningful only when the URG bit is on.  This
              field communicates the value of the urgent pointer which
              indicates the data octet associated with the sending user's
              urgent call.

    [Page 84]

    September 1981
                                               Transmission Control Protocol

                                   REFERENCES

    [1]  Cerf, V., and R. Kahn, "A Protocol for Packet Network
         Intercommunication", IEEE Transactions on Communications,
         Vol. COM-22, No. 5, pp 637-648, May 1974.

    [2]  Postel, J. (ed.), "Internet Protocol - DARPA Internet Program
         Protocol Specification", RFC 791, USC/Information Sciences
         Institute, September 1981.

    [3]  Dalal, Y. and C. Sunshine, "Connection Management in Transport
         Protocols", Computer Networks, Vol. 2, No. 6, pp. 454-473,
         December 1978.

    [4]  Postel, J., "Assigned Numbers", RFC 790, USC/Information Sciences
         Institute, September 1981.

                                                                   [Page 85]