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

→

1111

0000→1111

or

0

→

15

0→15

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

1111

would represent

60

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