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TCP/IP Network Administration

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Selected TCP/IP Headers
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F.2 TCP Segment Header

This description is taken from pages 15 to 17 of RFC 793, Transmission Control Protocol, by Jon Postel, Information Sciences Institute, University of Southern California.

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

  Source Port:  16 bits

    The source port number.

  Destination Port:  16 bits

    The destination port number.

  Sequence Number:  32 bits

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

  Acknowledgment Number:  32 bits

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

  Data Offset:  4 bits

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

  Reserved:  6 bits

    Reserved for future use.  Must be zero.

  Control Bits:  6  single-bit values (from left to right):

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

  Window:  16 bits

    The number of data octets (bytes) 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.

  Urgent Pointer:  16 bits

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

  Options:  variable

    Options may occupy space at the end of the TCP header and are a
    multiple of 8 bits in length.


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