What are TCP ISNs, and how are they used?
TCP sequence numbers are used to provide flow control and data integrity for TCP sessions. Every octet of bits, or byte, in a TCP session has a unique sequence number. Every TCP segment provides, as part of the segment header, the sequence number of the initial byte that it contains. However, the sequence numbers don’t start at zero for each session. Instead, the participants specify initial sequence numbers as part of the handshake process-a different ISN for each direction-and begin numbering the bytes from there. For example, suppose that Host A and Host B have established a session, and the ISN for the Host A-Host B direction was 1111 while that for the Host B-Host A direction was 2222. Now suppose that Host A had already sent 1000 bytes in the session. The next TCP segment Host A sent would carry a sequence number of 2112. Likewise, if Host B had already sent 2000 bytes, the next segment it sent would carry a sequence number of 4223. The important point for our discussion, though,
