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.