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RFC 0761

DoD standard Transmission Control Protocol

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Obsoleted by:  07937805
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ToP   noToC   RFC0761 - Page 1
RFC: 761
IEN: 129
                              DOD STANDARD

                              January 1980

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


                     Information Sciences Institute
                   University of Southern California
                           4676 Admiralty Way
                   Marina del Rey, California  90291
ToP   noToC   RFC0761 - Page 2
                           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
ToP   noToC   RFC0761 - Page 3

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

ToP   noToC   RFC0761 - Page 4
January 1980 
Replaces:  IENs 124, 112,
81, 55, 44, 40, 27, 21, 5

                              DOD STANDARD


                            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

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

  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.
ToP   noToC   RFC0761 - Page 5
  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

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
ToP   noToC   RFC0761 - Page 6
  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

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
    Flow Control
    Precedence and Security

  The basic operation of the TCP in each of these areas is described in
  the following paragraphs.
ToP   noToC   RFC0761 - Page 7
  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.


    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.
ToP   noToC   RFC0761 - Page 8

    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

    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.


    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.
ToP   noToC   RFC0761 - Page 10
  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
ToP   noToC   RFC0761 - Page 11
  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

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

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

                         Protocol Relationships

                               Figure 2.
ToP   noToC   RFC0761 - Page 12
  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.
ToP   noToC   RFC0761 - Page 13
  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
ToP   noToC   RFC0761 - Page 14
  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.
ToP   noToC   RFC0761 - Page 15
  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

  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
ToP   noToC   RFC0761 - Page 16
                      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 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.
ToP   noToC   RFC0761 - Page 17
  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

  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
ToP   noToC   RFC0761 - Page 18
    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.
ToP   noToC   RFC0761 - Page 19
    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


        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



        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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