The TCP header consists of the following fields:



Source port A 16-bit field (values from 0 to 65,535) that identifies the source port of the connection.

Destination port A 16-bit field (values from 0 to 65,535) that identifies the destination port of the connection.

Sequence number A 32-bit field (values from 0 to 23 2 ? 1 ) that identifies the current byte count (as of the end of this packet) relative to the initial sequence number (ISN), which is set in the initial SYN packet. Each byte transmitted, as well as the SYN and FIN flags, increment this value.

Acknowledgment number A 32-bit field (values from 0 to 23 2? 1 ) that holds the last sequence number of this communication transmitted by the other party and received correctly by the host. In the first packet sent (the initial SYN), this field is undefined.

Data offset A 4-bit field (values from 0 to 15) that indicates the number of 32-bit words in the header portion of the packet. The maximum size of a TCP header is thus 15 x 4 = 60 bytes. The minimum size of a TCP header (a header without optional arguments) is 5 words, or 20 bytes. Values of 0 through 4 are invalid.

Reserved A 6-bit field reserved for future expansion'it should always be 0.

Flags A 6-bit field that contains the following six 1-bit flags:

URG Urgent

ACK Acknowledgment

PSH Push

RST Reset connection

SYN Synchronize connection

FIN Finish

Window A 16-bit field (values from 0 to 65,535) that indicates the amount of receive buffer space available. This field has important performance implications, as it tells the remote system how much traffic the local host can accept, and thus how many packets the remote host can send before expecting an acknowledgment. The TCP Window Scale option modifies the behavior of this field.

Checksum A 16-bit field (values from 0 to 65,535). This value is computed using ones-complement arithmetic over a pseudo-header, the TCP header, and the TCP data. The receiving system recomputes this value and drops the packet if the checksum is invalid'in this case, the TCP timeout mechanism will cause the packet to be retransmitted by the sender.

Note The pseudo-header used to compute UDP and TCP checksums includes the source and destination IP addresses, as well as the protocol specific header. At the packet's final destination, the checksum is recomputed using the source and destination addresses obtained from the header of the IP packet which transported the protocol-specific packet. If the checksums agree, then we can have a high degree of confidence that the packet reached the intended destination host, as well as the correct protocol- specific port.

Urgent pointer A 16-bit field (values from 0 to 65,535) that points to the last byte of any urgent data that an application wishes to transfer immediately to the receiving host, bypassing normal buffering. This field should have a value of 0 unless the Urgent flag is set.

Options This field can be used for various option capabilities that tune the behavior of the TCP communications.These are padded as necessary to bring the TCP header length to an even multiple of 32 bits.

Data The application data is placed here. The length of this field should match the total packet size identified in the IP header, minus the Internet header length, minus the TCP data offset (all scaled as appropriate to reflectbytes).