Understanding the TCP and UDP message format
The TCP message is called a segment. A TCP segment consists of a header and a data section. The TCP header is often 20 bytes long (without TCP options). It can be described using the following figure:
Here is a brief description of each field:
The Source Port and the Destination Port have a length of 16 bits each. The source port is the port on the sending machine that transmits the packet, while the destination port is the port on the target machine that receives the packet.
The Sequence Number (32 bits), in a normal transmission, is the sequence number of the first byte of data of this segment.
The Acknowledgment Number (32 bits) contains the sequence number from the sender, increased by one.
H.Len. (4 bits) is the size of the TCP header in 32-bit words.
Rsvd. is reserved for future use. It is a 4-bit field and must be zero.
The Control Bits (control flags) contains eight 1-bit flags. In the original specification (RFC 793; the RFC can be downloaded...