What is the initial sequence number (ISN) in TCP and how is it chosen?

• A. A Fixed value
• B. A random starting sequence value chosen for each connection
• C. Sequential per host
• D. Based on port numbers

Answer: (B)

The initial sequence number is a 32-bit value used to label the first byte of data in a TCP connection, and it is chosen anew for each connection using a random or pseudorandom value. This per-connection randomness helps prevent spoofing and hijacking, since an attacker would have to guess the correct ISN for a new connection. During the three-way handshake, each side presents its own ISN, and the sequence space is advanced as data flows. The ISN is not fixed, not derived from port numbers, and not simply sequential per host, which is why choosing a random starting value for every connection is the best answer.