Amruta,
This has to do with how the sequence number is incremented during the TCP session. Let’s say Client A is requesting 900 Bytes of data from Server 1. Once Server 1 starts to send the actual data to Client A, the length of the payload of what is being sent directly influences the next sequence number.
So, let’s say the current Sequence number is 1, and the Server sends Client A, 300 Bytes. This means the sequence number will now be 301 (the original sequence number plus the amount of data in the payload that was just sent). Now, let’s say, after Client A acknowledges the first 300 bytes, Server 1 sends Client A 450 bytes. The new sequence number will now be 751 (301 + 450). Finally, Server 1 sends what is remaining (the last 150 bytes). This means the final sequence number would be 901 (751 + 150).
So, in this respect, the Sequence Number can be used to show the total amount of payload data that was transferred during a TCP session.
--Andrew