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TCP Header

 The Transmission Control Protocol (TCP) header is the first 24 bytes of a TCP segment that contains the parameters and state of an end-to-end TCP socket. The TCP header is used to track the state of communication between two TCP endpoints. Since TCP segments are inserted (encapsulated) in the payload of the IP packet the TCP header immediately follows the IP header during transmission. TCP does not need to keep track of which systems are communicating, it only needs to track which end to end sockets are currently open. Internet Protocol handles the logical addressing, routing and host-to-host connectivity.

TCP uses port numbers on each side of the connection to track the connection endpoints, state bits such as SYN, ACK, RST, FIN, sequence numbers and acknowledgement numbers to track the communication at each step in transmission.

An example of a TCP header is shown below.

 

Source Port (0 - 65535)								Destination Port (0 - 65535)
Sequence Number (0 - 4294967295)
Acknowledgement Number (0 - 4294967295)
Data Offset	Reserved	U R G	A C K	P S H	R S T	S Y N	F I N	Window
Checksum (CRC-Check)								Urgent Pointer
Options									Padding
Data

Field	Bits	Usage
Source Port	16	Communication source point
Destination Port	16	Communication end point
Sequence Number	32	Used for segmentation and reassembly of TCP segments.
Data Offset		Indicates number of bytes into segment where data can be found (number of bytes in the TCP header)
Reserved
URG - Urgent Flag	1
ACK - Acknowledgement Flag	1	Used during 3-way handshake and data transfers.
PSH - Push Flag	1	Used for TCP data push
RST - Reset Flag	1	Used to reset a TCP connection
SYN - Synchronize Flag	1	Used during 3-way handshake
FIN - End of data	1	Indicates end of the TCP session
Window	16	Number of octets in the TCP header
Checksum	16
Urgent Pointer	16
Options	Varies
Padding	Varies