path: root/doc/rfc/rfc761.txt

RFC: 761
IEN: 129
                                    
                                    
                                    
                                    
                                    
                                    
                                    
                              DOD STANDARD
                                    
                     TRANSMISSION CONTROL PROTOCOL
                                    
                                    
                                    
                              January 1980















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







                                   by

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



                           TABLE OF CONTENTS

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

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

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

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

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

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

  3.1  Header Format ................................................ 15
  3.2  Terminology .................................................. 19
  3.3  Sequence Numbers ............................................. 24
  3.4  Establishing a connection .................................... 29
  3.5  Closing a Connection ......................................... 35
  3.6  Precedence and Security ...................................... 38
  3.7  Data Communication ........................................... 38
  3.8  Interfaces ................................................... 42
  3.9  Event Processing ............................................. 52

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

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











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                                PREFACE



This document describes the DoD Standard Transmission Control Protocol
(TCP).  There have been eight earlier editions of the ARPA TCP
specification on which this standard is based, and the present text
draws heavily from them.  There have been many contributors to this work
both in terms of concepts and in terms of text.  This edition
incorporates the addition of security, compartmentation, and precedence
concepts into the TCP specification.

                                                           Jon Postel

                                                           Editor




































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

                              DOD STANDARD

                     TRANSMISSION CONTROL PROTOCOL



                            1.  INTRODUCTION

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

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

1.1.  Motivation

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

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

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


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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.  As a practical matter, many computer systems will be
  connected to networks via front-end computers which house the TCP and
  internet protocol layers, as well as network specific software.  The
  TCP specification describes an interface to the higher level protocols
  which appears to be implementable even for the front-end case, as long
  as a suitable host-to-front end protocol is implemented.

1.2.  Scope

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

1.3.  About this Document

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


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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 letters on
  established connections.  It is also expected that the TCP can
  asynchronously communicate with application programs.  Although
  considerable freedom is permitted to TCP implementors to design
  interfaces which are appropriate to a particular operating system
  environment, a minimum functionality is required at the TCP/user
  interface for any valid implementation.

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

1.5.  Operation

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

    Basic Data Transfer
    Reliability
    Flow Control
    Multiplexing
    Connections
    Precedence and Security

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


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

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

  Reliability:

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

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

  Flow Control:

    TCP provides a means for the receiver to govern the amount of data
    sent by the sender.  This is achieved by returning a "window" with
    every ACK indicating a range of acceptable sequence numbers beyond
    the last segment successfully received.  For stream mode, the window
    indicates an allowed number of octets that the sender may transmit
    before receiving further permission.  For record mode, the window
    indicates an allowed amount of buffer space the sender may consume,
    this may be more than the number of data octets transmitted if there
    is a mismatch between letter size and buffer size.







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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.

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

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

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

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

2.2.  Model of Operation

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


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

  The packet switches may perform further packaging, fragmentation, or
  other operations to achieve the delivery of the local packet to the
  destination internet module.

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

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

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

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

2.3.  The Host Environment

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


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  Though it is assumed here that processes are supported by the host
  operating system, the mechanisms of TCP do not preclude implementation
  of the TCP in a front-end processor.  However, in such an
  implementation, a host-to-front-end protocol must provide the
  functionality to support the type of TCP-user interface described
  above.

2.4.  Interfaces

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

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

2.5.  Relation to Other Protocols

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

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



                         Protocol Relationships

                               Figure 2.


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  It is expected that the TCP will be able to support higher level
  protocols efficiently.  It should be easy to interface higher level
  protocols like the ARPANET Telnet [3] or AUTODIN II THP to the TCP.

2.6.  Reliable Communication

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

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

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

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

2.7.  Connection Establishment and Clearing

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

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

  TCPs are free to associate ports with processes however they choose.
  However, several basic concepts seem necessary in any implementation.


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

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

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

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

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

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



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

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

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

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

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

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

2.8.  Data Communication

  The data that flows on a connection may be thought of as a stream of
  octets, or as a sequence of records.  In TCP the records are called
  letters and are of variable length.  The sending user indicates in
  each SEND call whether the data in that call completes a letter by the
  setting of the end-of-letter parameter.

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


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  There is a coupling between letters as sent and the use of buffers of
  data that cross the TCP/user interface.  Each time an end-of-letter
  (EOL) flag is associated with data placed into the receiving user's
  buffer, the buffer is returned to the user for processing even if the
  buffer is not filled.  If a letter is longer than the user's buffer,
  the letter is passed to the user in buffer size units, the last of
  which may be only partly full.  The receiving TCP's buffer size may be
  communicated to the sending TCP when the connection is being
  established.

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

  TCP also provides a means to communicate to the receiver of data that
  at some point further along in the data stream than the receiver is
  currently reading there is urgent data.  TCP does not attempt to
  define what the user specifically does upon being notified of pending
  urgent data, but the general notion is that the receiving process
  should take action to read through the end urgent data quickly.

2.9.  Precedence and Security

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

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

2.10.  Robustness Principle

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

  


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                      3.  FUNCTIONAL SPECIFICATION

3.1.  Header Format

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

  TCP Header Format

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

                            TCP Header Format

          Note that one tick mark represents one bit position.

                               Figure 3.

  Source Port:  16 bits

    The source port number.

  Destination Port:  16 bits

    The destination port number.




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  Sequence Number:  32 bits

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

  Acknowledgment Number:  32 bits

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

  Data Offset:  4 bits

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

  Reserved:  6 bits

    Reserved for future use.  Must be zero.

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

    URG:  Urgent Pointer field significant
    ACK:  Acknowledgment field significant
    EOL:  End of Letter
    RST:  Reset the connection
    SYN:  Synchronize sequence numbers
    FIN:  No more data from sender

  Window:  16 bits

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

  Checksum:  16 bits

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

    The checksum also covers a 96 bit pseudo header conceptually
    prefixed to the TCP header.  This pseudo header contains the Source


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

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

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

  Urgent Pointer:  16 bits

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

  Options:  variable

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

      Case 1:  A single octet of option-kind.

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

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

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

    A TCP must implement all options.




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    Currently defined options include (kind indicated in octal):

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

    Specific Option Definitions

      End of Option List

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

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

      No-Operation

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

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

      Buffer Size

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





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        Buffer Size Option Data:  16 bits

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

  Padding:  variable

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

3.2.  Terminology

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

    Send Sequence Variables

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

    Receive Sequence Variables

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




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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.
    
    

  Receive Sequence Space

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

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

                         Receive Sequence Space

                               Figure 5.
    
    

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










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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, TIME-WAIT, CLOSE-WAIT, CLOSING,
  and the fictional state CLOSED.  CLOSED is fictional because it
  represents the state when there is no TCB, and therefore, no
  connection.  Briefly the meanings of the states are:

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

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

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

    ESTABLISHED - represents an open connection, ready to transmit and
    receive data segments.

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

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

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

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

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

    CLOSED - represents no connection state at all.



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

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

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





































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

                      TCP Connection State Diagram
                               Figure 6.


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

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

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

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

















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  On send connections the following comparisons are needed:

    older sequence numbers                        newer sequence numbers

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

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

                   Sending Sequence Space Information

                               Figure 7.

    SND.UNA = oldest unacknowledged sequence number

    SND.NXT = next sequence number to be sent

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

    SEG.SEQ = first sequence number of a segment

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

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

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

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

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

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




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  On receive connections the following comparisons are needed:

    older sequence numbers                        newer sequence numbers

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

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

                  Receiving Sequence Space Information

                                Figure 8.

    RCV.NXT = next sequence number expected on incoming segments

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

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

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

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

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

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

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










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

       0       0     SEG.SEQ = RCV.NXT

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

      >0       0     not acceptable

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

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

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

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

  Initial Sequence Number Selection

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


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  is -- "how does the TCP identify duplicate segments from previous
  incarnations of the connection?"  This problem becomes apparent if the
  connection is being opened and closed in quick succession, or if the
  connection breaks with loss of memory and is then reestablished.

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

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

  For a connection to be established or initialized, the two TCPs must
  synchronize on each other's initial sequence numbers.  This is done in
  an exchange of connection establishing messages carrying a control bit
  called "SYN" (for synchronize) and the initial sequence numbers.  As a
  shorthand, messages carrying the SYN bit are also called "SYNs".
  Hence, the solution requires a suitable mechanism for picking an
  initial sequence number and a slightly involved handshake to exchange
  the ISN's.  A "three way handshake" is necessary because sequence
  numbers are not tied to a global clock in the network, and TCPs may
  have different mechanisms for picking the ISN's.  The receiver of the
  first SYN has no way of knowing whether the segment was an old delayed
  one or not, unless it remembers the last sequence number used on the
  connection (which is not always possible), and so it must ask the
  sender to verify this SYN.

  The "three way handshake" and the advantages of a "clock-driven"
  scheme are discussed in [4].

  Knowing When to Keep Quiet

  To be sure that a TCP does not create a segment that carries a
  sequence number which may be duplicated by an old segment remaining in
  the network, the TCP must keep quiet for a maximum segment lifetime
  (MSL) before assigning any sequence numbers upon starting up or
  recovering from a crash in which memory of sequence numbers in use was


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  lost.  For this specification the MSL is taken to be 2 minutes.  This
  is an engineering choice, and may be changed if experience indicates
  it is desirable to do so.  Note that if a TCP is reinitialized in some
  sense, yet retains its memory of sequence numbers in use, then it need
  not wait at all; it must only be sure to use sequence numbers larger
  than those recently used.

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

3.4.  Establishing a connection

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

  The simplest three-way handshake is shown in figure 9 below.  The
  figures should be interpreted in the following way.  Each line is
  numbered for reference purposes.  Right arrows (-->) indicate
  departure of a TCP segment from TCP A to TCP B, or arrival of a
  segment at B from A.  Left arrows (<--), indicate the reverse.
  Ellipsis (...) indicates a segment which is still in the network
  (delayed).  An "XXX" indicates a segment which is lost or rejected.
  Comments appear in parentheses.  TCP states represent the state AFTER
  the departure or arrival of the segment (whose contents are shown in
  the center of each line).  Segment contents are shown in abbreviated
  form, with sequence number, control flags, and ACK field.  Other
  fields such as window, addresses, lengths, and text have been left out
  in the interest of clarity.







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      TCP A                                                TCP B

  1.  CLOSED                                               LISTEN

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

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

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

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

          Basic 3-Way Handshake for Connection Synchronization

                                Figure 9.

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

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

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

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






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      TCP A                                        TCP B

  1.  CLOSED                                       CLOSED

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

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

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

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

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

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

                Simultaneous Connection Synchronization

                               Figure 10.

  

      TCP A                                                TCP B

  1.  CLOSED                                               LISTEN

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

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

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

  5.  SYN-SENT    --> <SEQ=1001><CTL=RST>              --> LISTEN
  

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

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

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

                    Recovery from Old Duplicate SYN

                               Figure 11.

  As a simple example of recovery from old duplicates, consider


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

  Half-Open Connections and Other Anomalies

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

  If at site A the connection no longer exists, then an attempt by the
  user at site B to send any data on it will result in the site B TCP
  receiving a reset control message.  Such a message should indicate to
  the site B TCP that something is wrong, and it is expected to abort
  the connection.

  Assume that two user processes A and B are communicating with one
  another when a crash occurs causing loss of memory to A's TCP.
  Depending on the operating system supporting A's TCP, it is likely
  that some error recovery mechanism exists.  When the TCP is up again,
  A is likely to start again from the beginning or from a recovery
  point.  As a result, A will probably try to OPEN the connection again
  or try to SEND on the connection it believes open.  In the latter
  case, it receives the error message "connection not open" from the
  local (A's) TCP.  In an attempt to establish the connection, A's TCP
  will send a segment containing SYN.  This scenario leads to the
  example shown in figure 12.  After TCP A crashes, the user attempts to
  re-open the connection.  TCP B, in the meantime, thinks the connection
  is open.










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      TCP A                                           TCP B

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

  2.  CLOSED                                           ESTABLISHED

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

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

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

  6.                                                   CLOSED

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

                     Half-Open Connection Discovery

                               Figure 12.

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

  An interesting alternative case occurs when TCP A crashes and TCP B
  tries to send data on what it thinks is a synchronized connection.
  This is illustrated in figure 13.  In this case, the data arriving at
  TCP A from TCP B (line 2) is unacceptable because no such connection
  exists, so TCP A sends a RST.  The RST is acceptable so TCP B
  processes it and aborts the connection.














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        TCP A                                              TCP B

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

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

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

           Active Side Causes Half-Open Connection Discovery

                               Figure 13.

  In figure 14, we find the two TCPs A and B with passive connections
  waiting for SYN.  An old duplicate arriving at TCP B (line 2) stirs B
  into action.  A SYN-ACK is returned (line 3) and causes TCP A to
  generate a RST (the ACK in line 3 is not acceptable).  TCP B accepts
  the reset and returns to its passive LISTEN state.

  

      TCP A                                         TCP B

  1.  LISTEN                                        LISTEN

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

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

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

  5.  LISTEN                                        LISTEN

       Old Duplicate SYN Initiates a Reset on two Passive Sockets

                               Figure 14.

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

  Reset Generation

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


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

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

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

  Reset Processing

  All reset (RST) segments are validated by checking their SEQ-fields.
  A reset is valid if its sequence number is in the window.  In the case
  of a RST received in response to an initial SYN any sequence number is
  acceptable if the ACK field acknowledges the SYN.

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

3.5.  Closing a Connection

  CLOSE is an operation meaning "I have no more data to send."  The
  notion of closing a full-duplex connection is subject to ambiguous
  interpretation, of course, since it may not be obvious how to treat
  the receiving side of the connection.  We have chosen to treat CLOSE
  in a simplex fashion.  The user who CLOSEs may continue to RECEIVE
  until he is told that the other side has CLOSED also.  Thus, a program
  could initiate several SENDs followed by a CLOSE, and then continue to
  RECEIVE until signaled that a RECEIVE failed because the other side
  has CLOSED.  We assume that the TCP will signal a user, even if no
  RECEIVEs are outstanding, that the other side has closed, so the user


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  can terminate his side gracefully.  A TCP will reliably deliver all
  buffers SENT before the connection was CLOSED so a user who expects no
  data in return need only wait to hear the connection was CLOSED
  successfully to know that all his data was received at the destination
  TCP.

  There are essentially three cases:

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

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

    3) Both users CLOSE simultaneously

  Case 1:  Local user initiates the close

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

  Case 2:  TCP receives a FIN from the network

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

  Case 3:  both users close simultaneously

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







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      TCP A                                                TCP B

  1.  ESTABLISHED                                          ESTABLISHED

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

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

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

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

  6.  (2 MSL)
      CLOSED

                         Normal Close Sequence

                               Figure 15.

  

      TCP A                                                TCP B

  1.  ESTABLISHED                                          ESTABLISHED

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

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

  4.  CLOSED                                               CLOSED

                      Simultaneous Close Sequence

                               Figure 16.







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3.6.  Precedence and Security

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

  The precedence levels are:

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

  The security levels are:

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

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

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

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

3.7.  Data Communication

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

  The sender of data keeps track of the next sequence number to use in
  the variable SND.NXT.  The receiver of data keeps track of the next


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

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

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

  End of Letter Sequence Number Adjustments

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

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

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

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










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    older sequence numbers                        newer sequence numbers

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

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

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

                        End of Letter Adjustment

                               Figure 17.

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

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

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

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

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

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

  The Communication of Urgent Information

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


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

  The method employs a urgent field which is carried in all segments
  transmitted.  The URG control flag indicates that the urgent field is
  meaningful and should be added to the segment sequence number to yield
  the urgent pointer.  The absence of this flag indicates that the
  urgent pointer has not changed.

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

  Managing the Window

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

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

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

  The sending TCP must be prepared to accept and send at least one octet
  of new data even if the send window is zero.  The sending TCP should
  regularly retransmit to the receiving TCP even when the window is
  zero.  Two minutes is recommended for the retransmission interval when
  the window is zero.  This retransmission is essential to guarantee



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

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

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

  In a connection with a one-way data flow, the window information will
  be carried in acknowledgment segments that all have the same sequence
  number so there will be no way to reorder them if they arrive out of
  order.  This is not a serious problem, but it will allow the window
  information to be on occasion temporarily based on old reports from
  the data receiver.

3.8.  Interfaces

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

  User/TCP Interface

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

    TCP User Commands

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

      The user commands described below specify the basic functions the
      TCP must perform to support interprocess communication.
      Individual implementations should define their own exact format,
      and may provide combinations or subsets of the basic functions in


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

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

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

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

      Open

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

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

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

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

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

        The buffer size, if present, indicates that the caller will
        always receive data from the connection in that size of buffers.
        This buffer size is a measure of the buffer between the user and



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

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

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

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

        The precedence for the connection is the higher of the values
        requested in the OPEN call and received from the incoming
        request, and fixed at that value for the life of the connection.

        Depending on the TCP implementation, either a local connection
        name will be returned to the user by the TCP, or the user will
        specify this local connection name (in which case another
        parameter is needed in the call).  The local connection name can
        then be used as a short hand term for the connection defined by
        the <local socket, foreign socket> pair.

      Send

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

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

        If the EOL flag is set, the data is the End Of a Letter, and the
        EOL bit will be set in the last TCP segment created from the



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

        If the URGENT flag is set, segments resulting from this call
        will have the urgent pointer set to indicate that some of the
        data associated with this call is urgent.  This facility, for
        example, can be used to simulate "break" signals from terminals
        or error or completion codes from I/O devices.  The semantics of
        this signal to the receiving process are unspecified.  The
        receiving TCP will signal the urgent condition to the receiving
        process as long as the urgent pointer indicates that data
        preceding the urgent pointer has not been consumed by the
        receiving process.  The purpose of urgent is to stimulate the
        receiver to accept some urgent data and to indicate to the
        receiver when all the currently known urgent data has been
        received.

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

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

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

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

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



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        Multiple SENDs are served in first come, first served order, so
        the TCP will queue those it cannot service immediately.

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

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

      Receive

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

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

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

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



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

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

        The URGENT flag will be set only if the receiving user has
        previously been informed via a TCP-to-user signal, that urgent
        data is waiting.  The receiving user should thus be in
        "read-fast" mode.  If the URGENT flag is on, additional urgent
        data remains.  If the URGENT flag is off, this call to RECEIVE
        has returned all the urgent data, and the user may now leave
        "read-fast" mode.

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

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

      Close

        Format:  CLOSE(local connection name)

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


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        of all its data before timing out.  In this event, CLOSE turns
        into ABORT, and the closing TCP gives up.

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

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

        Close also implies end of letter.

      Status

        Format:  STATUS(local connection name)

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

        This command returns a data block containing the following
        information:

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

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




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      Abort

        Format:  ABORT (local connection name)

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

    TCP-to-User Messages

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

      The following information is provided:

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

  TCP/Network Interface

    The TCP calls on a lower level protocol module to actually send and
    receive information over a network.  One case is that of the ARPA
    internetwork system where the lower level module is the Internet
    Protocol [2].  In most cases the following simple interface would be
    adequate.













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

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

        where:

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

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

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

        where:

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

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

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


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

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

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

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

      We note that the Internet Protocol provides arguments for a type
      of service and for a time to live.  TCP uses the following
      settings for these parameters:

        Type of Service = Precedence:  none, Package:  stream,
        Reliability:  higher, Preference:  speed, Speed:  higher; or
        00011111.

        Time to Live    = one minute, or 00111100.

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




















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3.9.  Event Processing

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

    Events that occur:

      User Calls

        OPEN
        SEND
        RECEIVE
        CLOSE
        ABORT
        STATUS

      Arriving Segments

        SEGMENT ARRIVES

      Timeouts

        USER TIMEOUT
        RETRANSMISSION TIMEOUT

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

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

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

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



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















































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                                                               OPEN Call



  OPEN Call

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

      Create a new transmission control block (TCB) to hold connection
      state information.  Fill in local socket identifier, foreign
      socket, precedence, security/compartment, and user timeout
      information.  Verify the security and precedence requested are
      allowed for this user, if not return "error:  precedence not
      allowed" or "error:  security/compartment not allowed."  If active
      and the foreign socket is unspecified, return "error:  foreign
      socket unspecified"; if active and the foreign socket is
      specified, issue a SYN segment.  An initial send sequence number
      (ISS) is selected and the TCP receive buffer size is selected (if
      applicable).  A SYN segment of the form <SEQ=ISS><CTL=SYN> is sent
      (this may include the buffer size option if applicable).  Set
      SND.UNA to ISS, SND.NXT to ISS+1, SND.LBB to ISS+1, enter SYN-SENT
      state, and return.

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

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

      Return "error:  connection already exists".














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SEND Call



  SEND Call

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

      If the user should no have access to such a connection, then
      return "error:  connection illegal for this process".

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

    LISTEN STATE

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

    SYN-SENT STATE

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

    SYN-RECEIVED STATE

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

    ESTABLISHED STATE

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

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




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                                                               SEND Call



        if EOL = 0 then

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

        if EOL = 1 then

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

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

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

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

    CLOSE-WAIT STATE

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

    CLOSING STATE

      Respond with "error:  connection closing"





















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RECEIVE Call



  RECEIVE Call

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

      If the user should no have access to such a connection, return
      "error:  connection illegal for this process".

      Otherwise return "error:  connection does not exist".

    LISTEN STATE
    SYN-SENT STATE
    SYN-RECEIVED STATE

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

    ESTABLISHED STATE

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

      Reassemble queued incoming segments into receive buffer and return
      to user.  Mark "end of letter" (EOL) if this is the case.

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

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

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

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







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                                                            RECEIVE Call



    TIME-WAIT STATE
    CLOSE-WAIT STATE

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

    CLOSING STATE

      Return "error:  connection closing"





































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CLOSE Call



  CLOSE Call

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

      If the user should no have access to such a connection, return
      "error:  connection illegal for this process".

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

    LISTEN STATE

      Any outstanding RECEIVEs should be returned with "error:  closing"
      responses.  Delete TCB, return "ok".

    SYN-SENT STATE

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

    SYN-RECEIVED STATE

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

    ESTABLISHED STATE

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

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

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











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                                                              CLOSE Call



    TIME-WAIT STATE

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

    CLOSE-WAIT STATE

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

    CLOSING STATE

      Respond with "error:  connection closing"


































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ABORT Call



  ABORT Call

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

      If the user should no have access to such a connection, return
      "error:  connection illegal for this process".

      Otherwise return "error:  connection does not exist".

    LISTEN STATE

      Any outstanding RECEIVEs should be returned with "error:
      connection reset" responses.  Delete TCB, return "ok".

    SYN-SENT STATE

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

    SYN-RECEIVED STATE

      Send a RST of the form:

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

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

    ESTABLISHED STATE

      Send a reset segment:

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

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












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                                                              ABORT Call



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

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

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

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

    TIME-WAIT STATE

      Respond with "ok" and delete the TCB.

    CLOSE-WAIT STATE

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

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

      Flush all segment queues and delete the TCB.

    CLOSING STATE

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




















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STATUS Call



  STATUS Call

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

      If the user should no have access to such a connection, return
      "error:  connection illegal for this process".

      Otherwise return "error:  connection does not exist".

    LISTEN STATE

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

    SYN-SENT STATE

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

    SYN-RECEIVED STATE

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

    ESTABLISHED STATE

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

    FIN-WAIT-1 STATE

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

    FIN-WAIT-2 STATE

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

    TIME-WAIT STATE

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

    CLOSE-WAIT STATE

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

    CLOSING STATE

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





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                                                         SEGMENT ARRIVES



  SEGMENT ARRIVES

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

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

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

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

      If the ACK bit is on,

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

      Return.

    If the state is LISTEN then

      first check for an ACK

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

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

        Return.

        An incoming RST should be ignored.  Return.

      if there was no ACK then check for a SYN

        If the SYN bit is set, check the security.  If the
        security/compartment on the incoming segment does not exactly
        match the security/compartment in the TCB then send a reset and
        return.  If the SEG.PRC is less than the TCB.PRC then send a
        reset and return.  If the SEG.PRC is greater than the TCB.PRC
        then set TCB.PRC<-SEG.PRC.  Now RCV.NXT and RCV.LBB are set to
        SEG.SEQ+1, IRS is set to SEG.SEQ and any other control or text
        should be queued for processing later.  ISS should be selected
        and a SYN segment sent of the form:


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SEGMENT ARRIVES



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

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

      if there was no SYN but there was other text or control

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

    If the state is SYN-SENT then

      first check for an ACK

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

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

        and discard the segment.  Return.

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

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

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

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



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

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

        and send it.  Data or controls which were queued for
        transmission may be included.

        If SEG.PRC is greater than TCB.PRC set TCB.PRC<-SEG.PRC.

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

































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    Otherwise,

    first check sequence number

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

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

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

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

           0       0     SEG.SEQ = RCV.NXT

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

          >0       0     not acceptable

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

        Note that the test above guarantees that the last sequence
        number used by the segment lies in the receive-window.  If the
        RCV.WND is zero, no segments will be acceptable, but special
        allowance should be made to accept valid ACKs, URGs and RSTs.

        If an incoming segment is not acceptable, an acknowledgment
        should be sent in reply:

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

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





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    second check security and precedence

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

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

    third check the ACK field,

      SYN-RECEIVED STATE

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

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

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

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

      ESTABLISHED STATE

        If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <- SEG.ACK.
        Any segments on the retransmission queue which are thereby
        entirely acknowledged are removed.  Users should receive
        positive acknowledgments for buffers which have been SENT and
        fully acknowledged (i.e., SEND buffer should be returned with
        "ok" response).  If the ACK is a duplicate, it can be ignored.

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

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



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

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

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

      TIME-WAIT STATE

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

      CLOSE-WAIT STATE

        Do the same processing as for the ESTABLISHED state.

      CLOSING STATE

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

    fourth check the RST bit,

      SYN-RECEIVED STATE

        If the RST bit is set then, if the segment has passed sequence
        and acknowledgment tests, it is valid.  If this connection was
        initiated with a passive OPEN (i.e., came from the LISTEN
        state), then return this connection to LISTEN state.  The user
        need not be informed.  If this connection was initiated with an
        active OPEN (i.e., came from SYN-SENT state) then the connection
        was refused, signal the user "connection refused".  In either
        case, all segments on the retransmission queue should be
        removed.











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

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

      TIME-WAIT

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

    fifth, check the SYN bit,

      SYN-RECEIVED
      ESTABLISHED STATE

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

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

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

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

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

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




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    sixth, check the URG bit,

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

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

      TIME-WAIT STATE
      CLOSE-WAIT STATE
      CLOSING

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

    seventh, process the segment text,

      ESTABLISHED STATE

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

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

          if EOL = 0 then

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

          if EOL = 1 then

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

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


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

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

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

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

      TIME-WAIT STATE
      CLOSE-WAIT STATE

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

    eighth, check the FIN bit,

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

    and return.


















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USER TIMEOUT



  USER TIMEOUT

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

  RETRANSMISSION TIMEOUT

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

   



































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                                GLOSSARY



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

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

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

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

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

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

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

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

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



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

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

FTP
          A file transfer protocol.

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

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

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

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

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

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

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

IP
          Internet Protocol.

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





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                                                                Glossary



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

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

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

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

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

local packet
          The unit of transmission within a local network.

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

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

octet
          An eight bit byte.

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

packet
          A package of data with a header which may or may not be



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          logically complete.  More often a physical packaging than a
          logical packaging of data.

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

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

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

RCV.BS
          receive buffer size, the remote buffer size

RCV.LBB
          receive last buffer beginning

RCV.NXT
          receive next sequence number

RCV.UP
          receive urgent pointer

RCV.WND
          receive window

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

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

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




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

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

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

SEG.ACK
          segment acknowledgment

SEG.LEN
          segment length

SEG.PRC
          segment precedence value

SEG.SEQ
          segment sequence

SEG.UP
          segment urgent pointer field

SEG.WND
          segment window field

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

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

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



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

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

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

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

SND.BS
           send buffer size, the local buffer size

SND.LBB
          send last buffer beginning

SND.NXT
          send sequence

SND.UNA
          left sequence

SND.UP
          send urgent pointer

SND.WL
          send sequence number at last window update

SND.WND
          send window

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



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

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

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

TCB.PRC
          The precedence of the connection.

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

TOS
          Type of Service, an Internet Protocol field.

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

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

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

          










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                               REFERENCES



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

[2]  Postel, J. (ed.), "DOD Standard Internet Protocol," Defense
     Advanced Research Projects Agency, Information Processing
     Techniques Office, RFC 760, IEN 128, January 1980.

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

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
































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