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Scenix Lib IO OSI3 Tcpip Documentation Rfcs RFC793.TXT

 






       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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                                       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




































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       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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         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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         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


       [Page 16]







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                                                       Functional Specification



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

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

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

         Urgent Pointer:  16 bits

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

         Options:  variable

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

             Case 1:  A single octet of option-kind.

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

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

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

           A TCP must implement all options.


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           Currently defined options include (kind indicated in octal):

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


           Specific Option Definitions

             End of Option List

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

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

             No-Operation

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

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

             Maximum Segment Size

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






       [Page 18]







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                                                  Transmission Control Protocol
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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.




       [Page 20]







       September 1981
                                                  Transmission Control Protocol
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           Current Segment Variables

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

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

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

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

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

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

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

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

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

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

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



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                                                                 September 1981
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           TIME-WAIT - represents waiting for enough time to pass to be sure
           the remote TCP received the acknowledgment of its connection
           termination request.

           CLOSED - represents no connection state at all.

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

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

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































       [Page 22]







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

                             TCP Connection State Diagram
                                      Figure 6.


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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).














       [Page 24]







       September 1981
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         In response to sending data the TCP will receive acknowledgments.  The
         following comparisons are needed to process the acknowledgments.

           SND.UNA = oldest unacknowledged sequence number

           SND.NXT = next sequence number to be sent

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

           SEG.SEQ = first sequence number of a segment

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

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

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

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

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

         When data is received the following comparisons are needed:

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

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

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

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

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

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

         or

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



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

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

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

              0       0     SEG.SEQ = RCV.NXT

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

             >0       0     not acceptable

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

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

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







       [Page 26]







       September 1981
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         Initial Sequence Number Selection

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

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

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

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

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

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



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

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

         Knowing When to Keep Quiet

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

         The TCP Quiet Time Concept

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

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


       [Page 28]







       September 1981
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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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                                                                 September 1981
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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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                                                  Transmission Control Protocol
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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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                                                  Transmission Control Protocol
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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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                                                  Transmission Control Protocol
                                                       Functional Specification



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

             Close

               Format:  CLOSE (local connection name)

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

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

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

               Close also implies push function.

             Status

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

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

               This command returns a data block containing the following
               information:

                 local socket,


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                                                                 September 1981
       Transmission Control Protocol
       Functional Specification



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

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

             Abort

               Format:  ABORT (local connection name)

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

           TCP-to-User Messages

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

             The following information is provided:

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


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       September 1981
                                                  Transmission Control Protocol
                                                       Functional Specification



         TCP/Lower-Level Interface

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

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

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

             Time to Live    = one minute, or 00111100.

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

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

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


















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       Functional Specification



       3.9.  Event Processing

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

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

           Events that occur:

             User Calls

               OPEN
               SEND
               RECEIVE
               CLOSE
               ABORT
               STATUS

             Arriving Segments

               SEGMENT ARRIVES

             Timeouts

               USER TIMEOUT
               RETRANSMISSION TIMEOUT
               TIME-WAIT TIMEOUT

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

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

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



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       September 1981
                                                  Transmission Control Protocol
                                                       Functional Specification



         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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                                                                 September 1981
       Transmission Control Protocol
       Functional Specification
                                                                      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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       September 1981
                                                  Transmission Control Protocol
                                                       Functional Specification
       OPEN Call



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

             Return "error:  connection already exists".






































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                                                                 September 1981
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       Functional Specification
                                                                      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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       September 1981
                                                  Transmission Control Protocol
                                                       Functional Specification
       SEND Call



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

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










































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                                                                 September 1981
       Transmission Control Protocol
       Functional Specification
                                                                   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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       September 1981
                                                  Transmission Control Protocol
                                                       Functional Specification
       RECEIVE Call



           CLOSE-WAIT STATE

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

           CLOSING STATE
           LAST-ACK STATE
           TIME-WAIT STATE

             Return "error:  connection closing".




































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                                                                 September 1981
       Transmission Control Protocol
       Functional Specification
                                                                     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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       September 1981
                                                  Transmission Control Protocol
                                                       Functional Specification
       CLOSE Call



           CLOSE-WAIT STATE

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

           CLOSING STATE
           LAST-ACK STATE
           TIME-WAIT STATE

             Respond with "error:  connection closing".







































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                                                                 September 1981
       Transmission Control Protocol
       Functional Specification
                                                                     ABORT Call



         ABORT Call

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

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

             Otherwise return "error:  connection does not exist".

           LISTEN STATE

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

           SYN-SENT STATE

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

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

             Send a reset segment:

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

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

           CLOSING STATE
           LAST-ACK STATE
           TIME-WAIT STATE

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








       [Page 62]







       September 1981
                                                  Transmission Control Protocol
                                                       Functional Specification
       STATUS Call



         STATUS Call

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

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

             Otherwise return "error:  connection does not exist".

           LISTEN STATE

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

           SYN-SENT STATE

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

           SYN-RECEIVED STATE

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

           ESTABLISHED STATE

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

           FIN-WAIT-1 STATE

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

           FIN-WAIT-2 STATE

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

           CLOSE-WAIT STATE

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

           CLOSING STATE

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

           LAST-ACK STATE

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





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       Transmission Control Protocol
       Functional Specification
                                                                    STATUS Call



           TIME-WAIT STATE

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














































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       September 1981
                                                  Transmission Control Protocol
                                                       Functional Specification
       SEGMENT ARRIVES



         SEGMENT ARRIVES

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

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

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

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

             If the ACK bit is on,

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

             Return.

           If the state is LISTEN then

             first check for an RST

               An incoming RST should be ignored.  Return.

             second check for an ACK

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

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

               Return.

             third check for a SYN

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

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



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                                                                 September 1981
       Transmission Control Protocol
       Functional Specification
                                                                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


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               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


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                                                                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.
























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                                                  Transmission Control Protocol
                                                       Functional Specification
       SEGMENT ARRIVES



           Otherwise,

           first check sequence number

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


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       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).






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       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.









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       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.



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                                                                 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.


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                                                                 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.











































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       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.































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       [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.





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                                                                 September 1981
       Transmission Control Protocol
       Glossary



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

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

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

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

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

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

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

       IP
                 Internet Protocol.

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

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

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

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



       [Page 80]







       September 1981
                                                  Transmission Control Protocol
                                                                       Glossary



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

       local packet
                 The unit of transmission within a local network.

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

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

       octet
                 An eight bit byte.

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

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

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

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

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

       RCV.NXT
                 receive next sequence number



                                                                      [Page 81]







                                                                 September 1981
       Transmission Control Protocol
       Glossary



       RCV.UP
                 receive urgent pointer

       RCV.WND
                 receive window

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

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

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

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

       SEG.ACK
                 segment acknowledgment

       SEG.LEN
                 segment length

       SEG.PRC
                 segment precedence value

       SEG.SEQ
                 segment sequence

       SEG.UP
                 segment urgent pointer field





       [Page 82]







       September 1981
                                                  Transmission Control Protocol
                                                                       Glossary



       SEG.WND
                 segment window field

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

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

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

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

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

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

       SND.NXT
                 send sequence

       SND.UNA
                 left sequence

       SND.UP
                 send urgent pointer

       SND.WL1
                 segment sequence number at last window update

       SND.WL2
                 segment acknowledgment number at last window update



                                                                      [Page 83]







                                                                 September 1981
       Transmission Control Protocol
       Glossary



       SND.WND
                 send window

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

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

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

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

       TCB.PRC
                 The precedence of the connection.

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

       TOS
                 Type of Service, an Internet Protocol field.

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

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

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





       [Page 84]







       September 1981
                                                  Transmission Control Protocol



                                      REFERENCES



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

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

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

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

































                                                                      [Page 85]



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* Contributors: Richard Seriani, Sr.
 

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