TCP Segment
A packet in TCP is called a segment. The format of a segment is shown in the following figure.
The segment consists of a 20- to 60-byte header, followed by data from the application program. The header is 20 bytes if there are no options and up to 60 bytes if it contains options. The different sections of the Header are as follows.
Source port address:
This is a 16-bit field that defines the port number of the application program in the host that is sending the segment.
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Destination port address:
This is a 16-bit field that defines the port number of the application program in the host that is receiving the segment.
Sequence number:
This 32-bit field defines the number assigned to the first byte of data contained in this segment. TCP is a stream transport protocol. To ensure connectivity, each byte to be transmitted is numbered. The sequence number tells the destination which byte in this sequence comprises the first byte in the segment.
During connection establishment, each party uses a random number generator to create an initial sequence number (ISN), which is usually different in each direction.
Acknowledgment number:
This 32-bit field defines the byte number that the receiver of the segment is expecting to receive from the other party. If the receiver of the segment has successfully received byte number x from the other party, it defines x + 1 as the acknowledgment number. Acknowledgment and data can be piggybacked together.
Header length:
This 4-bit field indicates the number of 4-byte words in the TCP header. The length of the header can be between 20 and 60 bytes. Therefore, the value of this field can be between 5 (5 x 4 =20) and 15 (15 x 4 =60).
Reserved: This is a 6-bit field reserved for future use.
Control:
This field defines 6 different control bits or flags as shown in the following figure and One or more of these bits can be set at a time. These bits enable flow control, connection establishment and termination, connection abortion, and the mode of data transfer in TCP. A brief description of each bit is as follows:
URG - The value of the urgent pointer field is valid.
ACK - The value of the acknowledgment field is valid.
PSH - Push the data.
RST - Reset the connection.
SYN - Synchronize sequence numbers during connection.
FIN - Terminate the connection.
Window size:
This field defines the size of the window, in bytes, that the other party must maintain. The length of this field is 16 bits, which means that the maximum size of the window is 65,535 bytes. This value is normally referred to as the receiving window (rwnd) and is determined by the receiver. The sender must obey the dictation of the receiver in this case.
Checksum:
This 16-bit field contains the checksum. The calculation of the checksum for TCP follows the same procedure as the one described for UDP. However, the inclusion of the checksum in the UDP datagram is optional, whereas the inclusion of the checksum for TCP is mandatory. The same pseudo header, serving the same purpose, is added to the segment.
For the TCP pseudo header, the value for the protocol field is 6.
Urgent pointer: This l6-bit field, which is valid, only if the urgent flag is set, is used when the segment contains urgent data. It defines the number that must be added to the sequence number to obtain the number of the last urgent byte in the data section of the segment.
Options: There can be up to 40 bytes of optional information in the TCP header.
For Further Reading:
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