TCP Segment Header

The source and destination port numbers are self-explanatory. They are exactly like the source and destination ports in UDP. Just for a refresher though, the source port is the port of the socket of the application that is sending the segment and the destination port is the port of the socket of the receiving application. The size of each field is two bytes.

Sequence Number

Every byte of the TCP segment’s data is labeled with a number called a sequence number. The sequence number field in the header has the sequence number of the first byte of data in the segment.

Note: The initial sequence number is a randomly generated number between $0$ and $2^{32}-1$.

Acknowledgement Number

The acknowledgment number is a 4-byte field that represents the sequence number of the next expected segment that the sender will send or the receiver will receive.

Example

So if a segment’s sequence number was $42849$ and its data field had $59$ bytes of data, the sequence number of the next expected segment or the acknowledgment number would be $42908$. This helps TCP to identify if a segment was missing or out of order.

Header Length

The length of the TCP header is specified here. This helps the receiving end to identify where the header ends and the data starts from.

Note: The header length is represented by 4 bits, i.e., the numbers $0000\to1111$ or $0\to15$ in decimal which is not enough to represent the potential 60 bytes of the header. Hence, this number is multiplied by 4 upon receiving. So $1111$ would represent $60$. In other words, the way the 4-bit header length field is used to represent a maximum header length of 60, is that this field represents the number of 4-byte words in the header

Reserved Field

The header has a 4-bit field that is reserved and is always set to $0$. This field aligns the total header size to be in multiples of 4 (as we saw was necessary for the header length to be processed).

Quick Quiz!

We’ll continue dissecting the TCP headers in the next lesson!