TCP Header

2009 Mar 09

16 bit - Source Port Number										16 bit - Destination Port Number
32 bit - Sequence Number
32 bit - Acknowledgement Number
4 bit - Header Length	4 bit - Reserved	C W R	E C E	U R G	A C K	P S H	R S T	S Y N	F I N	16 bit - Window Size
16 bit - TCP Checksum										16 bit - Urgent Pointer
Options (if any)
Data

Three-way handshake

Client creates packet with SYN=1 with value for initial sequence number set.
If server has open port for above, it sends back a packet with ACK=1 and SYN=1 with filled in Sequence Number and Acknowledgement Number fields.
Client responds with packet with ACL=1 with same Ackonwledgement Number field value.

TCP Header and Flag Bits

Reserved : reserved for future use. between systems. It gives each side a way to control the flow of packets from the other side to be sure that all packets are recived and acknowledged properly before new packets are sent.
CWR : Congestion Window Reduced
ECE : Explicit Congestion Notification echo
URG : =1 means the Urgent Pointer field will indicate where in the data stream the really urgent data is. The applications must know what to do about it.
ACK : Acknowledement
PSH : Push (used to flush data through the TCP layer), =1 it means the TCP layer should flush the packet through the stack quickly, not queing it up for later delivery
RST : connection should be Reset due to error or other interruption, used to stop connections and free up the sequence numbers in use. If a machine receives an unexpected packet (e.g. one with ACL=1 when no session is established) it will respond with a packet with RST=1, meaning "Don't know what you are talking about, cease any session".
SYN : system should Synchonrize sequence numbers (used during session establishment)
FIN : Finished (no more data from sender), is used to end a session, each side sends a packet with FIN=1.
Window Size : used to control the number of outstanding packets sent
Checksum : used to verify that the TCP packet (header and data) has not been corrupted in transmission across the network.
Data Offset : specifies where in the TCP packet the header ends and the data starts. It is equal to the length of the TCP header in 32 bit words.
Options : a set variable of variable length fields that can indicate additional information about the TCP processing capabilities if either side of the connection (e.g. maximum size limit). Padding at the end is added to make the resulting header be a multiple of 32 bits.

External Links

TCP, RFCs, Ports

2005-2009