Section G.2. TCP Segment Header

This description is taken from pages 15 to 17 of RFC 793, Transmission Control Protocol.

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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