Appendix A - TCP Header

TCP Header [STEVENS94]

• Source Port Number
  16 bits. The source port number.

• Destination Port Number
  16 bits The destination port number.

• Sequence Number
  32 bits. The sequence number of the first data octet 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.

• Header Length
  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 bits. (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 beginning with the one indicated in the
  acknowledgment field which the sender of this segment is willing to
  accept.

• TCP 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
  check summed, 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
  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.
  
  The TCP Length is the TCP header length plus the data length in
  octets (this is not an explicitly transmitted quantity, but is
  computed), and it does not count the 12 octets of the pseudo
  header.

• 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 is only be interpreted in segments with
  the URG control bit set.

• Options