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RFC 793 - Transmission Control Protocol


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RFC: 793

                     TRANSMISSION CONTROL PROTOCOL

                         DARPA INTERNET PROGRAM

                         PROTOCOL SPECIFICATION

                             September 1981

                              prepared for

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

                                   by

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

September 1981                                                          
                                           Transmission Control Protocol

                           TABLE OF CONTENTS

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

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

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

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

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

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

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

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

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

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[Page ii]                                                               

September 1981                                                          
                                           Transmission Control Protocol

                                PREFACE

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

                                                           Jon Postel

                                                           Editor

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

                     TRANSMISSION CONTROL PROTOCOL

                         DARPA INTERNET PROGRAM
                         PROTOCOL SPECIFICATION

                            1.  INTRODUCTION

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

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

1.1.  Motivation

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

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

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

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Introduction

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

                           Protocol Layering

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

                                Figure 1

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

1.2.  Scope

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

1.3.  About this Document

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

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                                           Transmission Control Protocol
                                                            Introduction

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

1.4.  Interfaces

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

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

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

1.5.  Operation

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

    Basic Data Transfer
    Reliability
    Flow Control
    Multiplexing
    Connections
    Precedence and Security

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

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  Basic Data Transfer:

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

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

  Reliability:

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

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

  Flow Control:

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

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  Multiplexing:

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

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

  Connections:

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

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

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

  Precedence and Security:

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

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                             2.  PHILOSOPHY

2.1.  Elements of the Internetwork System

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

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

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

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

2.2.  Model of Operation

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

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

  The packet switches may perform further packaging, fragmentation, or

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

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

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

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

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

2.3.  The Host Environment

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

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

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2.4.  Interfaces

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

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

2.5.  Relation to Other Protocols

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

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

                         Protocol Relationships

                               Figure 2.

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

2.6.  Reliable Communication

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

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

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

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

2.7.  Connection Establishment and Clearing

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

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

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

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

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  connection name by which the user refers to the connection in
  subsequent calls.  There are several things that must be remembered
  about a connection.  To store this information we imagine that there
  is a data structure called a Transmission Control Block (TCB).  One
  implementation strategy would have the local connection name be a
  pointer to the TCB for this connection.  The OPEN call also specifies
  whether the connection establishment is to be actively pursued, or to
  be passively waited for.

  A passive OPEN request means that the process wants to accept incoming
  connection requests rather than attempting to initiate a connection.
  Often the process requesting a passive OPEN will accept a connection
  request from any caller.  In this case a foreign socket of all zeros
  is used to denote an unspecified socket.  Unspecified foreign sockets
  are allowed only on passive OPENs.

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

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

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

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

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

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

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

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

2.8.  Data Communication

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

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

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

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

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

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

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

2.9.  Precedence and Security

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

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

2.10.  Robustness Principle

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

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                      3.  FUNCTIONAL SPECIFICATION

3.1.  Header Format

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

  TCP Header Format

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

                            TCP Header Format

          Note that one tick mark represents one bit position.

                               Figure 3.

  Source Port:  16 bits

    The source port number.

  Destination Port:  16 bits

    The destination port number.

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  Sequence Number:  32 bits

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

  Acknowledgment Number:  32 bits

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

  Data Offset:  4 bits

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

  Reserved:  6 bits

    Reserved for future use.  Must be zero.

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

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

  Window:  16 bits

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

  Checksum:  16 bits

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

    The checksum also covers a 96 bit pseudo header conceptually

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

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

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

  Urgent Pointer:  16 bits

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

  Options:  variable

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

      Case 1:  A single octet of option-kind.

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

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

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

    A TCP must implement all options.

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    Currently defined options include (kind indicated in octal):

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

    Specific Option Definitions

      End of Option List

        +--------+
        |00000000|
        +--------+
         Kind=0

        This option code indicates the end of the option list.  This
        might not coincide with the end of the TCP header according to
        the Data Offset field.  This is used at the end of all options,
        not the end of each option, and need only be used if the end of
        the options would not otherwise coincide with the end of the TCP
        header.

      No-Operation

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

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

      Maximum Segment Size

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

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        Maximum Segment Size Option Data:  16 bits

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

  Padding:  variable

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

3.2.  Terminology

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

    Send Sequence Variables

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

    Receive Sequence Variables

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

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

  Send Sequence Space

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

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

                          Send Sequence Space

                               Figure 4.

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

  Receive Sequence Space

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

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

                         Receive Sequence Space

                               Figure 5.

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

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

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    Current Segment Variables

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

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

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

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

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

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

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

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

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

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

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

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

    CLOSED - represents no connection state at all.

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

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

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

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                              +---------+ ---------\      active OPEN  
                              |  CLOSED |            \    -----------  
                              +---------+<---------\   \   create TCB  
                                |     ^              \   \  snd SYN    
                   passive OPEN |     |   CLOSE        \   \           
                   ------------ |     | ----------       \   \         
                    create TCB  |     | delete TCB         \   \       
                                V     |                      \   \     
                              +---------+            CLOSE    |    \   
                              |  LISTEN |          ---------- |     |  
                              +---------+          delete TCB |     |  
                   rcv SYN      |     |     SEND              |     |  
                  -----------   |     |    -------            |     V  
 +---------+      snd SYN,ACK  /       \   snd SYN          +---------+
 |         |<-----------------           ------------------>|         |
 |   SYN   |                    rcv SYN                     |   SYN   |
 |   RCVD  |<-----------------------------------------------|   SENT  |
 |         |                    snd ACK                     |         |
 |         |------------------           -------------------|         |
 +---------+   rcv ACK of SYN  \       /  rcv SYN,ACK       +---------+
   |           --------------   |     |   -----------                  
   |                  x         |     |     snd ACK                    
   |                            V     V                                
   |  CLOSE                   +---------+                              
   | -------                  |  ESTAB  |                              
   | snd FIN                  +---------+                              
   |                   CLOSE    |     |    rcv FIN                     
   V                  -------   |     |    -------                     
 +---------+          snd FIN  /       \   snd ACK          +---------+
 |  FIN    |<-----------------           ------------------>|  CLOSE  |
 | WAIT-1  |------------------                              |   WAIT  |
 +---------+          rcv FIN  \                            +---------+
   | rcv ACK of FIN   -------   |                            CLOSE  |  
   | --------------   snd ACK   |                           ------- |  
   V        x                   V                           snd FIN V  
 +---------+                  +---------+                   +---------+
 |FINWAIT-2|                  | CLOSING |                   | LAST-ACK|
 +---------+                  +---------+                   +---------+
   |                rcv ACK of FIN |                 rcv ACK of FIN |  
   |  rcv FIN       -------------- |    Timeout=2MSL -------------- |  
   |  -------              x       V    ------------        x       V  
    \ snd ACK                 +---------+delete TCB         +---------+
     ------------------------>|TIME WAIT|------------------>| CLOSED  |
                              +---------+                   +---------+

                      TCP Connection State Diagram
                               Figure 6.

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3.3.  Sequence Numbers

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

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

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

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

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

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

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

    SND.UNA = oldest unacknowledged sequence number

    SND.NXT = next sequence number to be sent

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

    SEG.SEQ = first sequence number of a segment

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

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

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

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

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

  When data is received the following comparisons are needed:

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

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

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

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

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

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

  or

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

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

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

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

       0       0     SEG.SEQ = RCV.NXT

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

      >0       0     not acceptable

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

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

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

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  Initial Sequence Number Selection

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

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

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

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

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

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

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

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

  Knowing When to Keep Quiet

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

  The TCP Quiet Time Concept

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

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

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    assigned the same or overlapping sequence numbers, causing confusion
    at the receiver as to which data is new and which is old.  Remember
    that each segment is bound to as many consecutive sequence numbers
    as there are octets of data in the segment.

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

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

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

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

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

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

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

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

3.4.  Establishing a connection

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

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

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

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

      TCP A                                                TCP B

  1.  CLOSED                                               LISTEN

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

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

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

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

          Basic 3-Way Handshake for Connection Synchronization

                                Figure 7.

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

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

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

      TCP A                                            TCP B

  1.  CLOSED                                           CLOSED

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

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

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

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

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

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

                Simultaneous Connection Synchronization

                               Figure 8.

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

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      TCP A                                                TCP B

  1.  CLOSED                                               LISTEN

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

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

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

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

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

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

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

                    Recovery from Old Duplicate SYN

                               Figure 9.

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

  Half-Open Connections and Other Anomalies

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

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

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

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

      TCP A                                           TCP B

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

  2.  CLOSED                                           ESTABLISHED

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

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

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

  6.  SYN-SENT                                         CLOSED

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

                     Half-Open Connection Discovery

                               Figure 10.

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

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

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

        TCP A                                              TCP B

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

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

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

           Active Side Causes Half-Open Connection Discovery

                               Figure 11.

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

      TCP A                                         TCP B

  1.  LISTEN                                        LISTEN

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

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

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

  5.  LISTEN                                        LISTEN

       Old Duplicate SYN Initiates a Reset on two Passive Sockets

                               Figure 12.

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

  Reset Generation

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

  There are three groups of states:

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

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

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

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

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

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

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

  Reset Processing

  In all states except SYN-SENT, all reset (RST) segments are validated
  by checking their SEQ-fields.  A reset is valid if its sequence number
  is in the window.  In the SYN-SENT state (a RST received in response
  to an initial SYN), the RST is acceptable if the ACK field
  acknowledges the SYN.

  The receiver of a RST first validates it, then changes state.  If the
  receiver was in the LISTEN state, it ignores it.  If the receiver was
  in SYN-RECEIVED state and had previously been in the LISTEN state,
  then the receiver returns to the LISTEN state, otherwise the receiver
  aborts the connection and goes to the CLOSED state.  If the receiver
  was in any other state, it aborts the connection and advises the user
  and goes to the CLOSED state.

3.5.  Closing a Connection

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

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  There are essentially three cases:

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

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

    3) Both users CLOSE simultaneously

  Case 1:  Local user initiates the close

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

  Case 2:  TCP receives a FIN from the network

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

  Case 3:  both users close simultaneously

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

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      TCP A                                                TCP B

  1.  ESTABLISHED                                          ESTABLISHED

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

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

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

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

  6.  (2 MSL)
      CLOSED                                                      

                         Normal Close Sequence

                               Figure 13.

      TCP A                                                TCP B

  1.  ESTABLISHED                                          ESTABLISHED

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

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

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

                      Simultaneous Close Sequence

                               Figure 14.

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3.6.  Precedence and Security

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

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

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

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

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

3.7.  Data Communication

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

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

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

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  acknowledgment it advances SND.UNA.  The extent to which the values of
  these variables differ is a measure of the delay in the communication.
  The amount by which the variables are advanced is the length of the
  data in the segment.  Note that once in the ESTABLISHED state all
  segments must carry current acknowledgment information.

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

  Retransmission Timeout

  Because of the variability of the networks that compose an
  internetwork system and the wide range of uses of TCP connections the
  retransmission timeout must be dynamically determined.  One procedure
  for determining a retransmission time out is given here as an
  illustration.

    An Example Retransmission Timeout Procedure

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

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

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

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

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

  The Communication of Urgent Information

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

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

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

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

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

  Managing the Window

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

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

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

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

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

  The sending TCP packages the data to be transmitted into segments

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  which fit the current window, and may repackage segments on the
  retransmission queue.  Such repackaging is not required, but may be
  helpful.

  In a connection with a one-way data flow, the window information will
  be carried in acknowledgment segments that all have the same sequence
  number so there will be no way to reorder them if they arrive out of
  order.  This is not a serious problem, but it will allow the window
  information to be on occasion temporarily based on old reports from
  the data receiver.  A refinement to avoid this problem is to act on
  the window information from segments that carry the highest
  acknowledgment number (that is segments with acknowledgment number
  equal or greater than the highest previously received).

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

    Window Management Suggestions

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

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

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

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

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

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

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

3.8.  Interfaces

  There are of course two interfaces of concern:  the user/TCP interface
  and the TCP/lower-level interface.  We have a fairly elaborate model
  of the user/TCP interface, but the interface to the lower level
  protocol module is left unspecified here, since it will be specified
  in detail by the specification of the lowel level protocol.  For the
  case that the lower level is IP we note some of the parameter values
  that TCPs might use.

  User/TCP Interface

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

    TCP User Commands

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

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

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

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

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

      Open

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

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

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

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

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

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

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

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

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

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

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

      Send

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

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

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

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

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

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

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

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

        In the simplest implementation, SEND would not return control to
        the sending process until either the transmission was complete
        or the timeout had been exceeded.  However, this simple method
        is both subject to deadlocks (for example, both sides of the
        connection might try to do SENDs before doing any RECEIVEs) and
        offers poor performance, so it is not recommended.  A more
        sophisticated implementation would return immediately to allow
        the process to run concurrently with network I/O, and,
        furthermore, to allow multiple SENDs to be in progress.
        Multiple SENDs are served in first come, first served order, so
        the TCP will queue those it cannot service immediately.

        We have implicitly assumed an asynchronous user interface in
        which a SEND later elicits some kind of SIGNAL or
        pseudo-interrupt from the serving TCP.  An alternative is to
        return a response immediately.  For instance, SENDs might return
        immediate local acknowledgment, even if the segment sent had not
        been acknowledged by the distant TCP.  We could optimistically
        assume eventual success.  If we are wrong, the connection will
        close anyway due to the timeout.  In implementations of this
        kind (synchronous), there will still be some asynchronous
        signals, but these will deal with the connection itself, and not
        with specific segments or buffers.

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

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

      Receive

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

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

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

        If enough data arrive to fill the buffer before a PUSH is seen,
        the PUSH flag will not be set in the response to the RECEIVE.
        The buffer will be filled with as much data as it can hold.  If
        a PUSH is seen before the buffer is filled the buffer will be
        returned partially filled and PUSH indicated.

        If there is urgent data the user will have been informed as soon
        as it arrived via a TCP-to-user signal.  The receiving user
        should thus be in "urgent mode".  If the URGENT flag is on,
        additional urgent data remains.  If the URGENT flag is off, this
        call to RECEIVE has returned all the urgent data, and the user
        may now leave "urgent mode".  Note that data following the
        urgent pointer (non-urgent data) cannot be delivered to the user
        in the same buffer with preceeding urgent data unless the
        boundary is clearly marked for the user.

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

        Alternative implementations of RECEIVE might have the TCP

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        allocate buffer storage, or the TCP might share a ring buffer
        with the user.

      Close

        Format:  CLOSE (local connection name)

        This command causes the connection specified to be closed.  If
        the connection is not open or the calling process is not
        authorized to use this connection, an error is returned.
        Closing connections is intended to be a graceful operation in
        the sense that outstanding SENDs will be transmitted (and
        retransmitted), as flow control permits, until all have been
        serviced.  Thus, it should be acceptable to make several SEND
        calls, followed by a CLOSE, and expect all the data to be sent
        to the destination.  It should also be clear that users should
        continue to RECEIVE on CLOSING connections, since the other side
        may be trying to transmit the last of its data.  Thus, CLOSE
        means "I have no more to send" but does not mean "I will not
        receive any more."  It may happen (if the user level protocol is
        not well thought out) that the closing side is unable to get rid
        of all its data before timing out.  In this event, CLOSE turns
        into ABORT, and the closing TCP gives up.

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

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

        Close also implies push function.

      Status

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

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

        This command returns a data block containing the following
        information:

          local socket,

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Functional Specification

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

        Depending on the state of the connection, or on the
        implementation itself, some of this information may not be
        available or meaningful.  If the calling process is not
        authorized to use this connection, an error is returned.  This
        prevents unauthorized processes from gaining information about a
        connection.

      Abort

        Format:  ABORT (local connection name)

        This command causes all pending SENDs and RECEIVES to be
        aborted, the TCB to be removed, and a special RESET message to
        be sent to the TCP on the other side of the connection.
        Depending on the implementation, users may receive abort
        indications for each outstanding SEND or RECEIVE, or may simply
        receive an ABORT-acknowledgment.

    TCP-to-User Messages

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

      The following information is provided:

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

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  TCP/Lower-Level Interface

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

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

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

      Time to Live    = one minute, or 00111100.

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

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

    Any lower level protocol will have to provide the source address,
    destination address, and protocol fields, and some way to determine
    the "TCP length", both to provide the functional equivlent service
    of IP and to be used in the TCP checksum.

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3.9.  Event Processing

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

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

    Events that occur:

      User Calls

        OPEN
        SEND
        RECEIVE
        CLOSE
        ABORT
        STATUS

      Arriving Segments

        SEGMENT ARRIVES

      Timeouts

        USER TIMEOUT
        RETRANSMISSION TIMEOUT
        TIME-WAIT TIMEOUT

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

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

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

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

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

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

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                                                               OPEN Call

  OPEN Call

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

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

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

    LISTEN STATE

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

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OPEN Call

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

      Return "error:  connection already exists".

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                                                               SEND Call

  SEND Call

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

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

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

    LISTEN STATE

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

    SYN-SENT STATE
    SYN-RECEIVED STATE

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

    ESTABLISHED STATE
    CLOSE-WAIT STATE

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

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

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SEND Call

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

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

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                                                            RECEIVE Call

  RECEIVE Call

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

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

      Otherwise return "error:  connection does not exist".

    LISTEN STATE
    SYN-SENT STATE
    SYN-RECEIVED STATE

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

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

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

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

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

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

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RECEIVE Call

    CLOSE-WAIT STATE

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

    CLOSING STATE
    LAST-ACK STATE
    TIME-WAIT STATE

      Return "error:  connection closing".

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                                                              CLOSE Call

  CLOSE Call

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

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

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

    LISTEN STATE

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

    SYN-SENT STATE

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

    SYN-RECEIVED STATE

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

    ESTABLISHED STATE

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

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

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

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CLOSE Call

    CLOSE-WAIT STATE

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

    CLOSING STATE
    LAST-ACK STATE
    TIME-WAIT STATE

      Respond with "error:  connection closing".

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                                                              ABORT Call

  ABORT Call

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

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

      Otherwise return "error:  connection does not exist".

    LISTEN STATE

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

    SYN-SENT STATE

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

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

      Send a reset segment:

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

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

    CLOSING STATE
    LAST-ACK STATE
    TIME-WAIT STATE

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

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STATUS Call

  STATUS Call

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

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

      Otherwise return "error:  connection does not exist".

    LISTEN STATE

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

    SYN-SENT STATE

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

    SYN-RECEIVED STATE

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

    ESTABLISHED STATE

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

    FIN-WAIT-1 STATE

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

    FIN-WAIT-2 STATE

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

    CLOSE-WAIT STATE

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

    CLOSING STATE

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

    LAST-ACK STATE

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

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                                                             STATUS Call

    TIME-WAIT STATE

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

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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 RST

        An incoming RST should be ignored.  Return.

      second check for an ACK

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

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

        Return.

      third check for a SYN

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

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

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                                                         SEGMENT ARRIVES

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

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

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

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

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

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

      fourth other text or control

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

    If the state is SYN-SENT then

      first check the ACK bit

        If the ACK bit is set

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

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

          and discard the segment.  Return.

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

      second check the RST bit

[Page 66]                                                               

September 1981                                                          
                                           Transmission Control Protocol
                                                Functional Specification
SEGMENT ARRIVES

        If the RST bit is set

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

      third check the security and precedence

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

          If there is an ACK

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

          Otherwise

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

        If there is an ACK

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

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

        If there is no ACK

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

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

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

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

      fourth check the SYN bit

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

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

                                                               [Page 67]

                                                          September 1981
Transmission Control Protocol
Functional Specification
                                                         SEGMENT ARRIVES

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

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

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

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

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

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

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

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

[Page 68]                                                               

September 1981                                                          
                                           Transmission Control Protocol
                                                Functional Specification
SEGMENT ARRIVES

    Otherwise,

    first check sequence number

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

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

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

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

           0       0     SEG.SEQ = RCV.NXT

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

          >0       0     not acceptable

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

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

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

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

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

                                                               [Page 69]

                                                          September 1981
Transmission Control Protocol
Functional Specification
                                                         SEGMENT ARRIVES

        In the following it is assumed that the segment is the idealized
        segment that begins at RCV.NXT and does not exceed the window.
        One could tailor actual segments to fit this assumption by
        trimming off any portions that lie outside the window (including
        SYN and FIN), and only processing further if the segment then
        begins at RCV.NXT.  Segments with higher begining sequence
        numbers may be held for later processing.

    second check the RST bit,

      SYN-RECEIVED STATE

        If the RST bit is set

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

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

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

      CLOSING STATE
      LAST-ACK STATE
      TIME-WAIT

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

[Page 70]                                                               

September 1981                                                          
                                           Transmission Control Protocol
                                                Functional Specification
SEGMENT ARRIVES

    third check security and precedence

      SYN-RECEIVED

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

      ESTABLISHED STATE

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

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

    fourth, check the SYN bit,

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

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

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

                                                               [Page 71]

                                                          September 1981
Transmission Control Protocol
Functional Specification
                                                         SEGMENT ARRIVES

    fifth check the ACK field,

      if the ACK bit is off drop the segment and return

      if the ACK bit is on

        SYN-RECEIVED STATE

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

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

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

            and send it.

        ESTABLISHED STATE

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

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

          Note that SND.WND is an offset from SND.UNA, that SND.WL1
          records the sequence number of the last segment used to update
          SND.WND, and that SND.WL2 records the acknowledgment number of
          the last segment used to update SND.WND.  The check here
          prevents using old segments to update the window.

[Page 72]                                                               

September 1981                                                          
                                           Transmission Control Protocol
                                                Functional Specification
SEGMENT ARRIVES

        FIN-WAIT-1 STATE

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

        FIN-WAIT-2 STATE

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

        CLOSE-WAIT STATE

          Do the same processing as for the ESTABLISHED state.

        CLOSING STATE

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

        LAST-ACK STATE

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

        TIME-WAIT STATE

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

    sixth, check the URG bit,

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

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

                                                               [Page 73]

                                                          September 1981
Transmission Control Protocol
Functional Specification
                                                         SEGMENT ARRIVES

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

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

    seventh, process the segment text,

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

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

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

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

        Please note the window management suggestions in section 3.7.

        Send an acknowledgment of the form:

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

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

[Page 74]                                                               

September 1981                                                          
                                           Transmission Control Protocol
                                                Functional Specification
SEGMENT ARRIVES

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

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

    eighth, check the FIN bit,

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

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

        SYN-RECEIVED STATE
        ESTABLISHED STATE

          Enter the CLOSE-WAIT state.

        FIN-WAIT-1 STATE

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

        FIN-WAIT-2 STATE

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

        CLOSE-WAIT STATE

          Remain in the CLOSE-WAIT state.

        CLOSING STATE

          Remain in the CLOSING state.

        LAST-ACK STATE

          Remain in the LAST-ACK state.

                                                               [Page 75]

                                                          September 1981
Transmission Control Protocol
Functional Specification
                                                         SEGMENT ARRIVES

        TIME-WAIT STATE

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

    and return.

[Page 76]                                                               

September 1981                                                          
                                           Transmission Control Protocol
                                                Functional Specification
USER TIMEOUT

  USER TIMEOUT

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

  RETRANSMISSION TIMEOUT

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

  TIME-WAIT TIMEOUT

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

                                                               [Page 77]

                                                          September 1981
Transmission Control Protocol

[Page 78]                                                               

September 1981                                                          
                                           Transmission Control Protocol

                                GLOSSARY

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

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

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

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

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

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

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

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

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

FTP
          A file transfer protocol.

                                                               [Page 79]

                                                          September 1981
Transmission Control Protocol
Glossary

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

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

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

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

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

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

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

IP
          Internet Protocol.

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

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

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

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

[Page 80]                                                               

September 1981                                                          
                                           Transmission Control Protocol
                                                                Glossary

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

local packet
          The unit of transmission within a local network.

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

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

octet
          An eight bit byte.

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

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

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

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

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

RCV.NXT
          receive next sequence number

                                                               [Page 81]

                                                          September 1981
Transmission Control Protocol
Glossary

RCV.UP
          receive urgent pointer

RCV.WND
          receive window

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

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

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

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

SEG.ACK
          segment acknowledgment

SEG.LEN
          segment length

SEG.PRC
          segment precedence value

SEG.SEQ
          segment sequence

SEG.UP
          segment urgent pointer field

[Page 82]                                                               

September 1981                                                          
                                           Transmission Control Protocol
                                                                Glossary

SEG.WND
          segment window field

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

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

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

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

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

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

SND.NXT
          send sequence

SND.UNA
          left sequence

SND.UP
          send urgent pointer

SND.WL1
          segment sequence number at last window update

SND.WL2
          segment acknowledgment number at last window update

                                                               [Page 83]

                                                          September 1981
Transmission Control Protocol
Glossary

SND.WND
          send window

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

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

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

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

TCB.PRC
          The precedence of the connection.

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

TOS
          Type of Service, an Internet Protocol field.

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

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

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

[Page 84]                                                               

September 1981                                                          
                                           Transmission Control Protocol

                               REFERENCES

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

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

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

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

 

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