    RFC: 761
    IEN: 129

                                  DOD STANDARD

                         TRANSMISSION CONTROL PROTOCOL

                                  January 1980

                                  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

    January 1980
                                               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 ....................................... 10
      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 .................................... 29
      3.5  Closing a Connection ......................................... 35
      3.6  Precedence and Security ...................................... 38
      3.7  Data Communication ........................................... 38
      3.8  Interfaces ................................................... 42
      3.9  Event Processing ............................................. 52

    GLOSSARY ............................................................ 75

    REFERENCES .......................................................... 83

                                                                    [Page i]

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    Transmission Control Protocol

    [Page ii]

    January 1980
                                               Transmission Control Protocol

                                    PREFACE

    This document describes the DoD Standard Transmission Control Protocol
    (TCP).  There have been eight 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
    incorporates the addition of security, compartmentation, and precedence
    concepts into the TCP specification.

                                                               Jon Postel

                                                               Editor

                                                                  [Page iii]

    January 1980
    RFC:761
    IEN:129
    Replaces:  IENs 124, 112,
    81, 55, 44, 40, 27, 21, 5

                                  DOD STANDARD

                         TRANSMISSION CONTROL PROTOCOL

                                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 especially 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 primarily focuses its attention 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.

                                                                    [Page 1]

                                                                January 1980
    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.  As a practical matter, many 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]

    January 1980
                                               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 letters 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.

                                                                    [Page 3]

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    Transmission Control Protocol
    Introduction

      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 this
        stream mode, the TCPs decide when to block and forward data at their
        own convenience.

        For users who desire a record-oriented service, the TCP also permits
        the user to submit records, called letters, for transmission.  When
        the sending user indicates a record boundary (end-of-letter), this
        causes the TCPs to promptly forward and deliver data up to that
        point to the receiver.

      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 users.  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.  For stream mode, the window
        indicates an allowed number of octets that the sender may transmit
        before receiving further permission.  For record mode, the window
        indicates an allowed amount of buffer space the sender may consume,
        this may be more than the number of data octets transmitted if there
        is a mismatch between letter size and buffer size.

    [Page 4]

    January 1980
                                               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.

                                                                    [Page 5]

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    Transmission Control Protocol

    [Page 6]

    January 1980
                                               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.

      We specifically assume that data is transmitted from host to host
      through means of a set of  networks.  When we say network, we have in
      mind a packet switched network (PSN).  This assumption is probably
      unnecessary, since a circuit switched network or a hybrid combination
      of the two could also be used; but for concreteness, we explicitly
      assume that the hosts are connected to one or more packet switches of
      a PSN.

      The term packet is used generically here to mean the data of one
      transaction between a host and a packet switch.  The format of data
      blocks exchanged between the packet switches in 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.

                                                                    [Page 7]

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    Transmission Control Protocol
    Philosophy

      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
      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 broken
      into smaller ones at intermediate 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 a time sharing 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.

    [Page 8]

    January 1980
                                               Transmission Control Protocol
                                                                  Philosophy

      Though it is assumed here that processes are supported by the host
      operating system, 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
      above.

    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        |  Gateway Level
                +-------------------------------+
                               |
                  +---------------------------+
                  |   Local Network Protocol  |    Network Level
                  +---------------------------+
                               |

                             Protocol Relationships

                                   Figure 2.

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    Transmission Control Protocol
    Philosophy

      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 [3] 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.

      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 the sequence number 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, 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 into a TCP, a flow control mechanism is
      employed.  The the data receiving TCP reports a window to the sending
      TCP.  This window specifies the number of octets, starting with the
      acknowledgment number that the data 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 operating system, TCP, or user, they might not
      be unique.  To provide for unique addresses at 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 seem necessary in any implementation.

    [Page 10]

    January 1980
                                               Transmission Control Protocol
                                                                  Philosophy

      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 only initiate connections 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
      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 could 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 might be permanently assigned to a particular
      socket, and other sockets might be 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.

      Processes can issue passive OPENs and wait for matching calls from
      other processes and be informed by the TCP when connections have been
      established.  Two processes which issue calls to each other at the
      same time are correctly connected.  This flexibility is critical for

                                                                   [Page 11]

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    Transmission Control Protocol
    Philosophy

      the support of distributed computing in which components act
      asynchronously with respect to each other.

      There are two cases for matching the sockets in the local request and
      an incoming segment.  In the first case, the local request has fully
      specified the foreign socket.  In this case, the match must be exact.
      In the second case, the local request has left the foreign socket
      unspecified.  In this case, any foreign socket is acceptable as long
      as the local sockets match.

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

      The procedures to establish and clear connections utilize synchronize
      (SYN) and finis (FIN) control flags and involve an exchange of three
      messages.  This exchange has been termed a three-way hand shake [4].

      A connection is initiated by the rendezvous of an arriving segment
      containing a SYN and a waiting TCB entry 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, or as a sequence of records.  In TCP the records are called
      letters and are of variable length.  The sending user indicates in
      each SEND call whether the data in that call completes a letter by the
      setting of the end-of-letter parameter.

      The length of a letter may be such that it must be broken into
      segments before it can be transmitted to its destination.  We assume
      that the segments will normally be reassembled into a letter before
      being passed to the receiving process.  A segment may contain all or a
      part of a letter, but a segment never contains parts of more than one
      letter.  The end of a letter is marked by the appearance of an EOL
      control flag in a segment.  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 end of letter is signaled then it must send all
      unsent data.  When a receiving TCP has a complete letter, it must not
      wait for more data from the sending TCP before passing the letter to
      the receiving process.

    [Page 12]

    January 1980
                                               Transmission Control Protocol
                                                                  Philosophy

      There is a coupling between letters as sent and the use of buffers of
      data that cross the TCP/user interface.  Each time an end-of-letter
      (EOL) 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 a letter is longer than the user's buffer,
      the letter is passed to the user in buffer size units, the last of
      which may be only partly full.  The receiving TCP's buffer size may be
      communicated to the sending TCP when the connection is being
      established.

      The TCP is responsible for regulating the flow of segments on the
      connections, as a way of preventing itself from becoming saturated or
      overloaded with traffic.  This is done using a window flow control
      mechanism.  The data receiving TCP reports to the data sending TCP a
      window which is the range of sequence numbers of data octets that data
      receiving TCP is currently prepared to accept.

      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
      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
      should take action to read through the end 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 should
      properly mark outgoing segments with the security, compartment, and
      precedence.  Such TCP modules should 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 should follow a general principle of robustness:
      be conservative in what you do, be liberal in what you accept from
      others.

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    Transmission Control Protocol

    [Page 14]

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                                               Transmission Control Protocol

                          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|E|R|S|F|                               |
       | Offset| Reserved  |R|C|O|S|Y|I|            Window             |
       |       |           |G|K|L|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]

                                                                January 1980
    Transmission Control Protocol
    Functional Specification

      Sequence Number:  32 bits

        The sequence number of the first data octet in this segment (except
        when SYN is present).

      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 including options is an integral
        number of 32 bits long.

      Reserved:  6 bits

        Reserved for future use.  Must be zero.

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

        URG:  Urgent Pointer field significant
        ACK:  Acknowledgment field significant
        EOL:  End of Letter
        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
        prefixed to the TCP header.  This pseudo header contains the Source

    [Page 16]

    January 1980
                                               Transmission Control Protocol
                                                    Functional Specification

        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 plus the data length in octets
          (this is not an explicitly transmitted quantity, but is computed
          from the total length, and the header length).

      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 should 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 should be header padding (i.e., zero).

        A TCP must implement all options.

                                                                   [Page 17]

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    Transmission Control Protocol
    Functional Specification

        Currently defined options include (kind indicated in octal):

          Kind     Length    Meaning
          ----     ------    -------
           0         -       End of option list.
           1         -       No-Operation.
          100        -       Reserved.
          105        4       Buffer 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.

          Buffer Size

            +--------+--------+---------+--------+
            |01000101|00000100|    buffer size   |
            +--------+--------+---------+--------+
             Kind=105 Length=4

    [Page 18]

    January 1980
                                               Transmission Control Protocol
                                                    Functional Specification

            Buffer Size Option Data:  16 bits

              If this option is present, then it communicates the receive
              buffer size at the TCP which sends this segment.  This field
              should only be sent in the initial connection request (i.e.,
              in segments with the SYN control bit set).  If this option is
              not used, the default buffer size of one octet is assumed.

      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 sequence
          SND.WND - send window
          SND.BS  - send buffer size
          SND.UP  - send urgent pointer
          SND.WL  - send sequence number used for last window update
          SND.LBB - send last buffer beginning
          ISS     - initial send sequence number

        Receive Sequence Variables

          RCV.NXT - receive sequence
          RCV.WND - receive window
          RCV.BS  - receive buffer size
          RCV.UP  - receive urgent pointer
          RCV.LBB - receive last buffer beginning
          IRS     - initial receive sequence number

                                                                   [Page 19]

                                                                January 1980
    Transmission Control Protocol
    Functional Specification

      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.

      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.

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

    [Page 20]

    January 1980
                                               Transmission Control Protocol
                                                    Functional Specification

        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, TIME-WAIT, CLOSE-WAIT, CLOSING,
      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, ready to transmit and
        receive data segments.

        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.

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

        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.

        CLOSED - represents no connection state at all.

                                                                   [Page 21]

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    Transmission Control Protocol
    Functional Specification

      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 and FIN flags; and timeouts.

      The Glossary contains a more complete list of terms and their
      definitions.

      The state diagram in figure 6 only illustrates 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.

    [Page 22]

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                                               Transmission Control Protocol
                                                    Functional Specification

                                  +---------+ ---------\      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  \       /   CLOSE            +---------+
       | rcv ACK of FIN   -------   |     |   -------
       | --------------   snd ACK   |     |   snd FIN
       V        x                   V     V
     +---------+                  +---------+
     |FINWAIT-2|                  | CLOSING |
     +---------+                  +---------+
       | rcv FIN                          | rcv ACK of FIN
       | -------    Timeout=2MSL          | --------------
       V snd ACK    ------------          V   delete TCB
     +---------+     delete TCB   +---------+
     |TIME WAIT|----------------->| CLOSED  |
     +---------+                  +---------+

                          TCP Connection State Diagram
                                   Figure 6.

                                                                   [Page 23]

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    Transmission Control Protocol
    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 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]

    January 1980
                                               Transmission Control Protocol
                                                    Functional Specification

      On send connections the following comparisons are needed:

        older sequence numbers                        newer sequence numbers

            SND.UNA                SEG.ACK                 SND.NXT
               |                      |                       |
           ----|----XXXXXXX------XXXXXXXXXX---------XXXXXX----|----
               |    |            |    |             |         |
                    |            |                  |
                 Segment 1    Segment 2          Segment 3

                          <----- sequence space ----->

                       Sending Sequence Space Information

                                   Figure 7.

        SND.UNA = oldest unacknowledged sequence number

        SND.NXT = next sequence number to be sent

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

        SEG.SEQ = first sequence number of a segment

        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

      All arithmetic is modulo 2**32 and that comparisons are unsigned.
      "=<" means "less than or equal".

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

      SEG.LEN is the number of octets occupied by the data in the segment.
      It is important to note that SEG.LEN must be non-zero; segments which
      do not occupy any sequence space (e.g., empty acknowledgment segments)
      are never placed on the retransmission queue, so would not go through
      this particular test.

                                                                   [Page 25]

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    Functional Specification

      On receive connections the following comparisons are needed:

        older sequence numbers                        newer sequence numbers

                    RCV.NXT                         RCV.NXT+RCV.WND
                       |                               |
           ---------XXX|XXX------XXXXXXXXXX---------XXX|XX---------
                    |  |         |                  |  |
                    |            |                  |
                 Segment 1    Segment 2          Segment 3

                          <----- sequence space ----->

                      Receiving Sequence Space Information

                                    Figure 8.

        RCV.NXT = next sequence number expected on incoming segments

        RCV.NXT+RCV.WND = last sequence number expected on incoming
            segments, plus one

        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

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

      SEG.SEQ+SEG.LEN-1 is the last sequence number occupied by the segment;
      RCV.NXT is the next sequence number expected on an incoming segment;
      and RCV.NXT+RCV.WND is the right edge of the receive 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:

    [Page 26]

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                                               Transmission Control Protocol
                                                    Functional Specification

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

      Note that the acceptance test for a segment, since it requires the end
      of a segment to lie in the window, is somewhat more restrictive than
      is absolutely necessary.  If at least the first sequence number of the
      segment lies in the receive window, or if some part of the segment
      lies in the receive window, then the segment might be judged
      acceptable.  Thus, in figure 8, at least segments 1 and 2 are
      acceptable by the strict rule, and segment 3 may or may not be,
      depending on the strictness of interpretation of the rule.

      Note that when the receive window is zero no segments should be
      acceptable except ACK segments.  Thus, it should be possible for a TCP
      to maintain a zero receive window while transmitting data and
      receiving ACKs.

      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 includes both data and sequence space
      occupying controls.  When a SYN is present then SEG.SEQ is the
      sequence number of the SYN.

      Initial Sequence Number Selection

      The protocol places no restriction on a particular connection being
      used over and over again.  A connection is defined by a pair of
      sockets.  New instances of a connection will be referred to as
      incarnations of the connection.  The problem that arises owing to this

                                                                   [Page 27]

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    Functional Specification

      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
      tens of seconds or minutes, at worst, 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 messages carrying a control bit
      called "SYN" (for synchronize) and the initial sequence numbers.  As a
      shorthand, messages 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.  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 [4].

      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

    [Page 28]

    January 1980
                                               Transmission Control Protocol
                                                    Functional Specification

      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.

      It should be noted that this strategy does not protect against
      spoofing or other replay type duplicate message problems.

    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, the TCP receives a "SYN" segment which carries no
      acknowledgment after it has sent a "SYN".  Of course, the arrival of
      an old duplicate "SYN" segment can potentially make it appear, to the
      recipient, that a simultaneous connection initiation is in progress.
      Proper use of "reset" segments can disambiguate these cases.  Several
      examples of connection initiation follow.  Although these examples do
      not show connection synchronization using data-carrying segments, this
      is perfectly legitimate, so long as the receiving TCP doesn't deliver
      the data to the user until it is clear the data is valid (i.e., the
      data must be buffered at the receiver until the connection reaches the
      ESTABLISHED state).  The three-way handshake reduces the possibility
      of false connections.  It is the implementation of a trade-off between
      memory and messages to provide information for this checking.

      The simplest three-way handshake is shown in figure 9 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.

                                                                   [Page 29]

                                                                January 1980
    Transmission Control Protocol
    Functional Specification

          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 9.

      In line 2 of figure 9, TCP A begins by sending a SYN segment
      indicating that it will use sequence numbers starting with sequence
      number 100.  In line 3, TCP B sends a SYN and acknowledges the SYN it
      received from TCP A.  Note that the acknowledgment field indicates TCP
      B is now expecting to hear sequence 101, acknowledging the SYN which
      occupied sequence 100.

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

      Simultaneous initiation is only slightly more complex, as is shown in
      figure 10.  Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to
      ESTABLISHED.

      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, TIME-WAIT, CLOSE-WAIT, CLOSING), it aborts the
      connection and informs its user.  We discuss this latter case under
      "half-open" connections below.

    [Page 30]

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                                               Transmission Control Protocol
                                                    Functional Specification

          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=101><ACK=301><CTL=ACK> ...

      6.  ESTABLISHED  <-- <SEQ=301><ACK=101><CTL=ACK> <-- SYN-RECEIVED

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

                    Simultaneous Connection Synchronization

                                   Figure 10.

          TCP A                                                TCP B

      1.  CLOSED                                               LISTEN

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

      3.  (duplicate) ... <SEQ=1000><CTL=SYN>              --> SYN-RECEIVED

      4.  SYN-SENT    <-- <SEQ=300><ACK=1001><CTL=SYN,ACK> <-- SYN-RECEIVED

      5.  SYN-SENT    --> <SEQ=1001><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 11.

      As a simple example of recovery from old duplicates, consider

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    Functional Specification

      figure 11.  At line 3, an old duplicate SYN arrives at TCP B.  TCP B
      cannot tell that this is an old duplicate, so it responds normally
      (line 4).  TCP A detects that the ACK field is incorrect and returns a
      RST (reset) with its SEQ field selected to make the segment
      believable.  TCP B, on receiving the RST, returns to the LISTEN state.
      When the original SYN (pun intended) finally arrives at line 6, the
      synchronization proceeds normally.  If the SYN at line 6 had arrived
      before the RST, a more complex exchange might have occurred with RST's
      sent in both directions.

      Half-Open Connections and Other Anomalies

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

      If at site A the connection no longer exists, then an attempt by the
      user at site B to send any data on it will result in the site B TCP
      receiving a reset control message.  Such a message should indicate 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 12.  After TCP A crashes, the user attempts to
      re-open the connection.  TCP B, in the meantime, thinks the connection
      is open.

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                                                    Functional Specification

          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.                                                   CLOSED

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

                         Half-Open Connection Discovery

                                   Figure 12.

      When the SYN arrives at line 3, TCP B, being in a synchronized state,
      responds with an acknowledgment indicating what sequence it next
      expects to hear (ACK 100).  TCP A sees that this segment does not
      acknowledge anything it sent and, being unsynchronized, sends a reset
      (RST) because it has detected a half-open connection.  TCP B aborts at
      line 5.  TCP A will continue to try to establish the connection; the
      problem is now reduced to the basic 3-way handshake of figure 9.

      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 13.  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.

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            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 13.

      In figure 14, 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 14.

      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) should be sent whenever a segment
      arrives which apparently is not intended for the current or a future
      incarnation of the connection.  A reset should not be sent if it is
      not clear that this is the case.  Thus, if any segment arrives for a
      nonexistent connection, a reset should be sent.  If a segment ACKs

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      something which has never been sent on the current connection, then
      one of the following two cases applies.

      1.  If the connection is in any non-synchronized state (LISTEN,
      SYN-SENT, SYN-RECEIVED) or if the connection does not exist, a reset
      (RST) should be formed and sent for any segment that acknowledges
      something not yet sent.  The RST should take its SEQ field from the
      ACK field of the offending segment (if the ACK control bit was set),
      and its ACK bit should be reset (zero), except to refuse a initial
      SYN.  A reset is also sent if an incoming segment has a security level
      or compartment which does not exactly match the level and compartment
      requested for the connection.  If the precedence of the incoming
      segment is less than the precedence level requested a reset is sent.

      2.  If the connection is in a synchronized state (ESTABLISHED,
      FIN-WAIT-1, FIN-WAIT-2, TIME-WAIT, CLOSE-WAIT, CLOSING), any
      unacceptable segment should elicit only an empty acknowledgment
      segment containing the current send-sequence number and an
      acknowledgment indicating the next sequence number expected to be
      received.

      Reset Processing

      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 case
      of a RST received in response to an initial SYN any sequence number 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

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      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.

      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.  It should be noted 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 should respond with a CLOSE, upon which the TCP can send a FIN
        to the other TCP.  The TCP then waits until its own FIN is
        acknowledged whereupon it deletes the connection.  If an ACK is not
        forthcoming, after a 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><CTL=FIN>               --> CLOSE-WAIT

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

      4.                                                       (Close)
          TIME-WAIT   <-- <SEQ=301><CTL=FIN>               <-- CLOSING

      5.  TIME-WAIT   --> <SEQ=100><ACK=301><CTL=ACK>      --> CLOSED

      6.  (2 MSL)
          CLOSED

                             Normal Close Sequence

                                   Figure 15.

          TCP A                                                TCP B

      1.  ESTABLISHED                                          ESTABLISHED

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

      3.  CLOSING     --> <SEQ=100><ACK=301><CTL=ACK>      ... CLOSING
                      <-- <SEQ=300><ACK=101><CTL=ACK>      <--
                      ... <SEQ=100><ACK=301><CTL=ACK>      -->

      4.  CLOSED                                               CLOSED

                          Simultaneous Close Sequence

                                   Figure 16.

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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 parts.

      The precedence levels are:

        flash override - 111
        flash          - 110
        immediate      - 10X
        priority       - 01X
        routine        - 00X

      The security levels are:

        top secret    - 11
        secret        - 10
        confidential  - 01
        unclassified  - 00

      The compartments are assigned by the Defense Communications Agency.
      The defaults are precedence:  routine, security:  unclassified,
      compartment:  zero.  A host which does not implement precedence or
      security feature should clear these fields to zero for segments it
      sends.

      A connection attempt with mismatched security/compartment values or a
      lower precedence value should be rejected by sending a reset.

      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.

    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

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

      When the sender creates a segment and transmits it the sender advances
      SND.NXT.  When the receiver accepts a segment it advances RCV.NXT and
      sends an acknowledgment.  When the data sender receives an
      acknowledgment it advances SND.UNA.  The extent to which the values of
      these variables differ is a measure of the delay in the communication.

      Normally the amount by which the variables are advanced is the length
      of the data in the segment.  However, when letters are used there are
      special provisions for coordination the sequence numbers, the letter
      boundaries, and the receive buffer boundaries.

      End of Letter Sequence Number Adjustments

      There is provision in TCP for the receiver of data to optionally
      communicate to the sender of data on a connection at the time of the
      connection synchronization the receiver's buffer size.  If this is
      done the receiver must use this fixed size of buffers for the lifetime
      of the connection.  If a buffer size is communicated then there is a
      coordination between receive buffers, letters, and sequence numbers.

      Each time a buffer is completed either due to being filled or due to
      an end of letter, the sequence number is incremented through the end
      of that buffer.

      That is, whenever an EOL is transmitted, the sender advances its send
      sequence number, SND.NXT, by an amount sufficient to consume all the
      unused space in the receiver's buffer.  The amount of space consumed
      in this fashion is subtracted from the send window just as is the
      space consumed by actual data.

      And, whenever an EOL is received, the receiver advances its receive
      sequence number, RCV.NXT, by an amount sufficient to consume all the
      unused space in the receiver's buffer.  The amount of space consumed
      in this fashion is subtracted from the receive window just as is the
      space consumed by actual data.

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        older sequence numbers                        newer sequence numbers

                |           Buffer 1            |   Buffer 2
                |                               |
            ----+-------------------------------+-----------------
                XXXXXXXXXXXXXXXXXXXXX+++++++++++
                |                    |          |
                |<-----SEG.LEN------>|          |
                |                    |          |
                |                    |          |
             SEG.SEQ                 A          B

                        XXX - data octets from segment
                        +++ - phantom data

                          <----- sequence space ----->

                            End of Letter Adjustment

                                   Figure 17.

      In the case illustrated above, if the segment does not carry an EOL
      flag, the next value of SND.NXT or RCV.NXT will be A.  If it does
      carry an EOL flag, the next value will be B.

      The exchange of buffer size and sequencing information is done in
      units of octets.  If no buffer size is stated, then the buffer size is
      assumed to be 1 octet.  The receiver tells the sender the size of the
      buffer in a SYN segment that contains the 16 bit buffer size data in
      an option field in the TCP header.

      Each EOL advances the sequence number (SN) to the next buffer boundary

        While LBB < SEG.SEQ+SEG.LEN
        Do LBB <- LBB + BS End
        SN <- LBB

        where LBB is the Last Buffer Beginning, and BS is the buffer size.

      The CLOSE user call implies an end of letter, as does the FIN control
      flag in an incoming segment.

      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.

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      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
      should tell the user to go into "urgent mode"; when the receive
      sequence number catches up to the urgent pointer, the TCP should tell
      user to go into "normal mode".  If the urgent pointer is updated while
      the user is in "read fast" 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 should be added to the segment sequence number to yield
      the urgent pointer.  The absence of this flag indicates that the
      urgent pointer has not changed.

      To send an urgent indication the user must also send at least one data
      octet.  If the sending user also indicates end of letter, 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 number
      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.  The window
      information is a guideline to be aimed at.

      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 and send at least one octet
      of new data even if the send window is zero.  The sending TCP should
      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

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      that when either TCP has a zero window the re-opening of the window
      will be reliably reported to the other.

      The sending TCP packages the data to be transmitted into segments
      which fit the current window, and may repackage segments on the
      retransmission queue.  Such repackaging is not required, but may be
      helpful.

      Users must keep reading connections they close for sending until the
      TCP says no more data.

      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.

    3.8.  Interfaces

      There are of course two interfaces of concern:  the user/TCP interface
      and the TCP/IP interface.  We have a fairly elaborate model of the
      user/TCP interface, but only a sketch of the interface to the lower
      level protocol module.

      User/TCP Interface

        The 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 should define their own exact format,
          and may provide combinations or subsets of the basic functions in

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          single calls.  In particular, some implementations may wish to
          automatically OPEN a connection on the first SEND or RECEIVE
          issued by the user for a given connection.

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

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

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

          Open

            Format:  OPEN (local port, foreign socket, active/passive
            [, buffer size] [, timeout] [, precedence]
            [, security/compartment]) -> 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 by
            the processes that serve it (e.g., the program which interfaces
            the TCP network).  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 full-duplex 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 buffer size, if present, indicates that the caller will
            always receive data from the connection in that size of buffers.
            This buffer size is a measure of the buffer between the user and

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            the local TCP.  The buffer size between the two TCPs may be
            different.

            The timeout, if present, permits the caller to set up a timeout
            for all buffers transmitted on the connection.  If a buffer is
            not successfully delivered to the destination within the timeout
            period, the TCP will abort the connection.  The present global
            default is 30 seconds.  The buffer retransmission rate may vary;
            most likely, it will be related to the measured time for
            responses from the remote TCP.

            The TCP or some component of the operating system will verify
            the users authority to open a connection with the specified
            precedence or security/compartment.  The absence of precedence
            or security/compartment specification in the OPEN call indicates
            the default values should 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.

            Depending on the TCP implementation, either a local connection
            name will be returned to the user by the TCP, or the user will
            specify this local connection name (in which case another
            parameter is needed in the call).  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,
            EOL 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 EOL flag is set, the data is the End Of a Letter, and the
            EOL bit will be set in the last TCP segment created from the

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            buffer.  If the EOL flag is not set, subsequent SENDs will
            appear to be part of the same letter.

            If the URGENT flag is set, segments resulting from this call
            will have the urgent pointer set to indicate that some of the
            data associated with this call is urgent.  This facility, for
            example, can be used to simulate "break" signals from terminals
            or error or completion codes from I/O devices.  The semantics of
            this signal to the receiving process are unspecified.  The
            receiving TCP will signal the urgent condition to the receiving
            process as long as 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 accept some urgent data and to indicate to the
            receiver when all the currently known urgent data has been
            received.

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

            If no foreign socket was specified in the OPEN, but the
            connection is established (e.g., because a LISTENing connection
            has become specific due to a foreign segment arriving for the
            local socket), then the designated buffer is sent to the implied
            foreign socket.  In general, 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, then the current 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.

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            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 letters.

            NOTA BENE: In order for the process to distinguish among error
            or success indications for different SENDs, it might be
            appropriate for the buffer address to be returned along with the
            coded response to the SEND request.  TCP-to-user signals are
            discussed below, indicating the information which should be
            returned to the calling process.

          Receive

            Format:  RECEIVE (local connection name, buffer address, byte
            count)

            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 letter has been received or a
            buffer filled.

            If insufficient buffer space is given to reassemble a complete
            letter, the EOL flag will not be set in the response to the
            RECEIVE.  The buffer will be filled with as much data as it can
            hold.  The last buffer required to hold the letter is returned
            with EOL signaled.

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                                               Transmission Control Protocol
                                                    Functional Specification

            The remaining parts of a partly delivered letter will be placed
            in buffers as they are made available via successive RECEIVEs.
            If a number of RECEIVEs are outstanding, they may be filled with
            parts of a single long letter or with at most one letter each.
            The return codes associated with each RECEIVE will indicate what
            is contained in the buffer.

            If a buffer size was given in the OPEN call, then all buffers
            presented in RECEIVE calls must be of exactly that size, or an
            error indication will be returned.

            The URGENT flag will be set only if the receiving user has
            previously been informed via a TCP-to-user signal, that urgent
            data is waiting.  The receiving user should thus be in
            "read-fast" 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
            "read-fast" mode.

            To distinguish among several outstanding RECEIVEs and to take
            care of the case that a letter is smaller than the buffer
            supplied, the return code is accompanied by both a buffer
            pointer and a byte count indicating the actual length of the
            letter received.

            Alternative implementations of RECEIVE might have the TCP
            allocate buffer storage, or the TCP might share a ring buffer
            with the user.  Variations of this kind will produce obvious
            variation in user interface to the TCP.

          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

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    Functional Specification

            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 end of letter.

          Status

            Format:  STATUS(local connection name)

            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,
              foreign socket,
              local connection name,
              receive window,
              send window,
              connection state,
              number of buffers awaiting acknowledgment,
              number of buffers pending receipt (including partial ones),
              receive buffer size,
              urgent state,
              precedence,
              security/compartment,
              and default 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.

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                                               Transmission Control Protocol
                                                    Functional Specification

          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
            End-of-Letter flag                       Receive
            End-of-Urgent flag                       Receive

      TCP/Network 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 [2].  In most cases the following simple interface would be
        adequate.

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    Transmission Control Protocol
    Functional Specification

        The following two calls satisfy the requirements for the TCP to
        internet protocol module communication:

          SEND (dest, TOS, TTL, BufPTR, len, Id, DF, options => result)

            where:

              dest = destination address
              TOS = type of service
              TTL = time to live
              BufPTR = buffer pointer
              len = length of buffer
              Id  = Identifier
              DF = Don't Fragment
              options = internet option data
              result = response
                OK = datagram sent ok
                Error = error in arguments or local network error

            Note that the precedence is included in the TOS and the
            security/compartment is passed as an option.

          RECV (BufPTR => result, source, dest, prot, TOS, len)

            where:

              BufPTR = buffer pointer
              result = response
                OK = datagram received ok
                Error = error in arguments
              source = source address
              dest = destination address
              prot = protocol
              TOS = type of service
              options = internet option data
              len = length of buffer

            Note that the precedence is in the TOS, and the
            security/compartment is an option.

          When the TCP sends a segment, it executes the SEND call supplying
          all the arguments.  The internet protocol module, on receiving
          this call, checks the arguments and prepares and sends the
          message.  If the arguments are good and the segment is accepted by
          the local network, the call returns successfully.  If either the
          arguments are bad, or the segment is not accepted by the local
          network, the call returns unsuccessfully.  On unsuccessful
          returns, a reasonable report should be made as to the cause of the

    [Page 50]

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                                               Transmission Control Protocol
                                                    Functional Specification

          problem, but the details of such reports are up to individual
          implementations.

          When a segment arrives at the internet protocol module from the
          local network, either there is a pending RECV call from TCP or
          there is not.  In the first case, the pending call is satisfied by
          passing the information from the segment to the TCP.  In the
          second case, the TCP is notified of a pending segment.

          The notification of a TCP may be via a pseudo interrupt or similar
          mechanism, as appropriate in the particular operating system
          environment of the implementation.

          A TCP's RECV call may then either be immediately satisfied by a
          pending segment, or the call may be pending until a segment
          arrives.

          We note that the Internet Protocol 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:  none, Package:  stream,
            Reliability:  higher, Preference:  speed, Speed:  higher; or
            00011111.

            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.

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    Functional Specification

    3.9.  Event Processing

      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

      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.

      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.

    [Page 52]

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                                               Transmission Control Protocol
                                                    Functional Specification

      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.

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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.  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 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 and the TCP receive buffer size is selected (if
          applicable).  A SYN segment of the form <SEQ=ISS><CTL=SYN> is sent
          (this may include the buffer size option if applicable).  Set
          SND.UNA to ISS, SND.NXT to ISS+1, SND.LBB 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
        SYN-SENT STATE
        SYN-RECEIVED STATE
        ESTABLISHED STATE
        FIN-WAIT-1 STATE
        FIN-WAIT-2 STATE
        TIME-WAIT STATE
        CLOSE-WAIT STATE
        CLOSING STATE

          Return "error:  connection already exists".

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                                               Transmission Control Protocol
                                                    Functional Specification
    SEND Call

      SEND Call

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

          If the user should no 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, and select the receive
          buffer size.  Send a SYN segment, set SND.UNA to ISS, SND.NXT to
          ISS+1 and SND.LBB 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 should be sent with the first data
          segment 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

          Queue for processing after the connection is ESTABLISHED.
          Typically, nothing can be sent yet, anyway, because the send
          window has not yet been set by the other side.  If no space,
          return "error:  insufficient resources".

        SYN-RECEIVED STATE

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

        ESTABLISHED STATE

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

          If remote buffer size is not one octet, and, if this is the end of
          a letter, do the following end-of-letter/buffer-size adjustment
          processing:

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    Functional Specification
                                                                   SEND Call

            if EOL = 0 then

              SND.NXT <- SEG.SEQ + SEG.LEN

            if EOL = 1 then

              While SND.LBB < SEG.SEQ + SEG.LEN
              Do SND.LBB <- SND.LBB + SND.BS End
              SND.NXT <- SND.LBB

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

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

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

        CLOSE-WAIT STATE

          Segmentize any text to be sent and queue for output.  If there is
          insufficient space to remember the SEND, return "error:
          insufficient resources"

        CLOSING STATE

          Respond with "error:  connection closing"

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                                               Transmission Control Protocol
                                                    Functional Specification
    RECEIVE Call

      RECEIVE Call

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

          If the user should no 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

          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 "end of letter" (EOL) 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.

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

          Reassemble and return a letter, or as much as will fit, in the
          user buffer.  Queue the request if it cannot be serviced
          immediately.

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                                                                RECEIVE Call

        TIME-WAIT STATE
        CLOSE-WAIT STATE

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

        CLOSING STATE

          Return "error:  connection closing"

    [Page 58]

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                                               Transmission Control Protocol
                                                    Functional Specification
    CLOSE Call

      CLOSE Call

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

          If the user should no 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:  closing"
          responses.  Delete TCB, return "ok".

        SYN-SENT STATE

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

        SYN-RECEIVED STATE

          Queue for processing after entering ESTABLISHED state or
          segmentize and send FIN segment.  If the latter, enter FIN-WAIT-1
          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).

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                                                                  CLOSE Call

        TIME-WAIT STATE

          Strictly speaking, this is an error and should receive a "error:
          connection closing" response.  An "ok" response would be
          acceptable, too.  However, since the FIN has been sent and
          acknowledged, nothing should be sent (or retransmitted).

        CLOSE-WAIT STATE

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

        CLOSING STATE

          Respond with "error:  connection closing"

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                                               Transmission Control Protocol
                                                    Functional Specification
    ABORT Call

      ABORT Call

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

          If the user should no 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, return "ok".

        SYN-SENT STATE

          Delete the TCB and return "reset" responses to any queued SENDs,
          or RECEIVEs.

        SYN-RECEIVED STATE

          Send a RST of the form:

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

          and return any unprocessed SENDs, or RECEIVEs with "reset" code,
          delete the TCB.

        ESTABLISHED STATE

          Send a reset segment:

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

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

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                                                                  ABORT Call

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

          A reset segment (RST) should be formed and sent:

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

          Outstanding SENDs, RECEIVEs, CLOSEs, and/or segments queued for
          retransmission, or segmentizing, should be flushed, with
          "connection reset" notification to the user, delete the TCB.

        TIME-WAIT STATE

          Respond with "ok" and delete the TCB.

        CLOSE-WAIT STATE

          Flush any pending SENDs and RECEIVEs, returning "connection reset"
          responses for them.  Form and send a RST segment:

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

          Flush all segment queues and delete the TCB.

        CLOSING STATE

          Respond with "ok" and delete the TCB; flush any remaining segment
          queues.  If a CLOSE command is still pending, respond "error:
          connection reset".

    [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 no 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.

        TIME-WAIT STATE

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

        CLOSE-WAIT STATE

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

        CLOSING STATE

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

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                                                             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 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, except another RST.  The
            RST should be formatted as follows:

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

            Return.

            An incoming RST should be ignored.  Return.

          if there was no ACK then 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.  If the SEG.PRC is less than the TCB.PRC then send a
            reset and return.  If the SEG.PRC is greater than the TCB.PRC
            then set TCB.PRC<-SEG.PRC.  Now RCV.NXT and RCV.LBB are set 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:

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    SEGMENT ARRIVES

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

            SND.NXT and SND.LBB are 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.

          if there was no SYN but there was 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 for an ACK

            If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, or the
            security/compartment in the segment does not exactly match the
            security/compartment in the TCB, or the precedence in the
            segment is less than the precedence in the TCB, send a reset

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

            and discard the segment.  Return.

            If SND.UNA =< SEG.ACK =< SND.NXT and the security/compartment
            and precedence are acceptable then the ACK is acceptable.
            SND.UNA should be advanced to equal SEG.ACK, and any segments on
            the retransmission queue which are thereby acknowledged should
            be removed.

          if the ACK is ok (or there is no ACK), check the RST bit

            If the RST bit is set then signal the user "error:  connection
            reset", enter CLOSED state, drop the segment, delete TCB, and
            return.

          if the ACK is ok (or there is no ACK) and it was not a RST, check
          the SYN bit

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                                                             SEGMENT ARRIVES

            If the SYN bit is on and the security/compartment and precedence
            are acceptable then, RCV.NXT and RCV.LBB are set to SEG.SEQ+1,
            IRS is set to SEG.SEQ.  If SND.UNA > ISS (our SYN has been
            ACKed), change the connection state to ESTABLISHED, otherwise
            enter SYN-RECEIVED.  In any case, 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 SEG.PRC is greater than TCB.PRC set TCB.PRC<-SEG.PRC.

            If there are other controls or text in the segment then continue
            processing at the fifth step below where the URG bit is checked,
            otherwise return.

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                                               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
          TIME-WAIT STATE
          CLOSE-WAIT STATE
          CLOSING 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+SEG.LEN =< RCV.NXT+RCV.WND

            Note that the test above guarantees that the last sequence
            number used by the segment lies in the receive-window.  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:

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

            If the incoming segment is unacceptable, drop it and return.

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    Functional Specification
                                                             SEGMENT ARRIVES

        second check security and precedence

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

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

        third check the ACK field,

          SYN-RECEIVED STATE

            If the RST bit is off and SND.UNA < SEG.ACK =< SND.NXT then set
            SND.UNA <- SEG.ACK, remove any acknowledged segments from the
            retransmission queue, and enter ESTABLISHED state.

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

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

            and send it, unless the incoming segment is an RST (or there is
            no ACK), in which case, it should be discarded, then return.

          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, it can be ignored.

            If the segment passes the sequence number and acknowledgment
            number tests, the send window should be updated.  If
            SND.WL =< SEG.SEQ, set SND.WND <- SEG.WND and set
            SND.WL <- SEG.SEQ.

            If the remote buffer size is not one, then the
            end-of-letter/buffer-size adjustment to sequence numbers may
            have an effect on the next expected sequence number to be
            acknowledged.  It is possible that the remote TCP will
            acknowledge with a SEG.ACK equal to a sequence number of an

    [Page 68]

    January 1980
                                               Transmission Control Protocol
                                                    Functional Specification
    SEGMENT ARRIVES

            octet that was skipped over at the end of a letter.  This a mild
            error on the remote TCPs part, but not cause for alarm.

          FIN-WAIT-1 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.

          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.

          CLOSE-WAIT STATE

            Do the same processing as for the ESTABLISHED state.

          CLOSING STATE

            If the ACK acknowledges our FIN then delete the TCB (enter the
            CLOSED state), otherwise ignore the segment.

        fourth check the RST bit,

          SYN-RECEIVED STATE

            If the RST bit is set then, if the segment has passed sequence
            and acknowledgment tests, it is valid.  If this connection was
            initiated with a passive OPEN (i.e., came from the LISTEN
            state), then return this connection to LISTEN state.  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.

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                                                             SEGMENT ARRIVES

          ESTABLISHED
          FIN-WAIT-1
          FIN-WAIT-2
          CLOSE-WAIT
          CLOSING STATE

            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.

          TIME-WAIT

            Enter the CLOSED state, delete the TCB, and return.

        fifth, check the SYN bit,

          SYN-RECEIVED
          ESTABLISHED STATE

            If the SYN bit is set, check the segment sequence number against
            the receive window.  The segment sequence number must be in the
            receive window; if not, ignore the segment.  If the SYN is on
            and SEG.SEQ = IRS then everything is ok and no action is needed;
            but if they are not equal, there is an error and a reset must be
            sent.

              If a reset must be sent it is formed as follows:

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

              The connection must be aborted as if a RST had been received.

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

            This case should not occur, since a duplicate of the SYN which
            started the current connection incarnation will have been
            filtered in the SEG.SEQ processing.  Other SYN's will have been
            rejected by this test as well (see SYN processing for
            ESTABLISHED state).

    [Page 70]

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                                               Transmission Control Protocol
                                                    Functional Specification
    SEGMENT ARRIVES

        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.

          TIME-WAIT STATE
          CLOSE-WAIT STATE
          CLOSING

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

        seventh, process the segment text,

          ESTABLISHED 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 EOL flag, then the
            user is informed, when the buffer is returned, that an EOL has
            been received.

            If buffer size is not one octet, then do  the following
            end-of-letter/buffer-size adjustment processing:

              if EOL = 0 then

                RCV.NXT <- SEG.SEQ + SEG.LEN

              if EOL = 1 then

                While RCV.LBB < SEG.SEQ+SEG.LEN
                Do RCV.LBB <- RCV.LBB + RCV.BS End
                RCV.NXT <- RCV.LBB

            When the TCP takes responsibility for delivering the data to the
            user it must also acknowledge the receipt of the data.  Send an
            acknowledgment of the form:

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    Transmission Control Protocol
    Functional Specification
                                                             SEGMENT ARRIVES

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

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

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

            If there are outstanding RECEIVEs, they should be satisfied, if
            possible, with the text of this segment; remaining text should
            be queued for further processing.  If a RECEIVE is satisfied,
            the user should be notified, with "end-of-letter" (EOL) signal,
            if appropriate.

          TIME-WAIT STATE
          CLOSE-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,

          Send an acknowledgment for the FIN.  Signal the user "connection
          closing", and return any pending RECEIVEs with same message.  Note
          that FIN implies EOL for any segment text not yet delivered to the
          user.  If the current state is ESTABLISHED, enter the CLOSE-WAIT
          state.  If the current state is FIN-WAIT-1, enter the CLOSING
          state.  If the current state is FIN-WAIT-2, enter the TIME-WAIT
          state.

        and return.

    [Page 72]

    January 1980
                                               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, 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.

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    Transmission Control Protocol

    [Page 74]

    January 1980
                                               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).

    buffer size
              An option (buffer size) used to state the receive data buffer
              size of the sender of this option.  May only be sent in a
              segment that also carries a SYN.

    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.

    EOL
              A control bit (End of Letter) occupying no sequence space,
              indicating that this segment ends a logical letter with the
              last data octet in the segment.  If this end of letter causes
              a less than full buffer to be released to the user and the
              connection buffer size is not one octet then the
              end-of-letter/buffer-size adjustment to the receive sequence
              number must be made.

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    Transmission Control Protocol
    Glossary

    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.

    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.

    [Page 76]

    January 1980
                                               Transmission Control Protocol
                                                                    Glossary

    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.

    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.

    letter
              A logical unit of data, in particular the logical unit of data
              transmitted between processes via TCP.

    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

                                                                   [Page 77]

                                                                January 1980
    Transmission Control Protocol
    Glossary

              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.

    PSN
              A Packet Switched Network.  For example, the ARPANET.

    RCV.BS
              receive buffer size, the remote buffer size

    RCV.LBB
              receive last buffer beginning

    RCV.NXT
              receive next sequence number

    RCV.UP
              receive urgent pointer

    RCV.WND
              receive window

    receive last buffer beginning
              This is the sequence number of the first octet of the most
              recent buffer.  This value is use in calculating the next
              sequence number when a segment contains an end of letter
              indication.

    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.

    [Page 78]

    January 1980
                                               Transmission Control Protocol
                                                                    Glossary

    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.

    Rubber EOL
              An end of letter (EOL) requiring a sequence number adjustment
              to align the beginning of the next letter on a buffer
              boundary.

    SEG.ACK
              segment acknowledgment

    SEG.LEN
              segment length

    SEG.PRC
              segment precedence value

    SEG.SEQ
              segment sequence

    SEG.UP
              segment urgent pointer field

    SEG.WND
              segment window field

    segment
              A logical unit of data, in particular a TCP segment is the
              unit of data 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.

                                                                   [Page 79]

                                                                January 1980
    Transmission Control Protocol
    Glossary

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

    send last buffer beginning
              This is the sequence number of the first octet of the most
              recent buffer.  This value is used in calculating the next
              sequence number when a segment contains an end of letter
              indication.

    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 sequence numbers which may be
              emitted by a TCP lies between SND.NXT and
              SND.UNA + SND.WND - 1.

    SND.BS
               send buffer size, the local buffer size

    SND.LBB
              send last buffer beginning

    SND.NXT
              send sequence

    SND.UNA
              left sequence

    SND.UP
              send urgent pointer

    SND.WL
              send sequence number at last window update

    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.

    [Page 80]

    January 1980
                                               Transmission Control Protocol
                                                                    Glossary

    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.

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    Transmission Control Protocol

    [Page 82]

    January 1980
                                               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.), "DOD Standard Internet Protocol," Defense
         Advanced Research Projects Agency, Information Processing
         Techniques Office, RFC 760, IEN 128, January 1980.

    [3]  Feinler, E. and J. Postel, ARPANET Protocol Handbook, Network
         Information Center, SRI International, Menlo Park, CA,
         January 1978.

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

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