rfc761.txt

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   |                             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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                                                            January 1980
Transmission Control Protocol
Functional Specification



  Sequence Number:  32 bits

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

  Acknowledgment Number:  32 bits

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

  Data Offset:  4 bits

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

  Reserved:  6 bits

    Reserved for future use.  Must be zero.

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

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

  Window:  16 bits

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

  Checksum:  16 bits

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

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


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January 1980                                                            
                                           Transmission Control Protocol
                                                Functional Specification



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

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

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

  Urgent Pointer:  16 bits

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

  Options:  variable

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

      Case 1:  A single octet of option-kind.

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

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

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

    A TCP must implement all options.




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                                                            January 1980
Transmission Control Protocol
Functional Specification



    Currently defined options include (kind indicated in octal):

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

    Specific Option Definitions

      End of Option List

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

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

      No-Operation

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

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

      Buffer Size

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





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January 1980                                                            
                                           Transmission Control Protocol
                                                Functional Specification



        Buffer Size Option Data:  16 bits

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

  Padding:  variable

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

3.2.  Terminology

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

    Send Sequence Variables

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

    Receive Sequence Variables

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




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                                                            January 1980
Transmission Control Protocol
Functional Specification



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

  Send Sequence Space

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

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

                          Send Sequence Space

                               Figure 4.
    
    

  Receive Sequence Space

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

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

                         Receive Sequence Space

                               Figure 5.
    
    

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










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January 1980                                                            
                                           Transmission Control Protocol
                                                Functional Specification



    Current Segment Variables

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

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

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

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

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

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

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

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

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