GATE CS 2024 Set 1 Q.29: Transport Layer: UDP, TCP Flow and Congestion Control, Sockets

TCP client P successfully establishes a connection to TCP server Q. Let $N_P$ denote the sequence number in the SYN sent from P to Q. Let $N_Q$ denote the acknowledgement number in the SYN ACK from Q to P. Which of the following statements is/are CORRECT?

The sequence number $N_P$ is chosen randomly by P

The sequence number $N_P$ is always 0 for a new connection

The acknowledgement number $N_Q$ is equal to $N_P$

The acknowledgement number $N_Q$ is equal to $N_P + 1$

Official answer and solution

Official answer: A and D
From: Official GATE 2024 answer key

Solution

Recall TCP 3-way handshake rules for Initial Sequence Number (ISN) selection and SYN acknowledgement numbering.

(A) is CORRECT and (B) is FALSE: In TCP, the client chooses its Initial Sequence Number (ISN) $N_P$ pseudo-randomly (to prevent sequence-number spoofing and stale segment collisions from prior connections).
(D) is CORRECT and (C) is FALSE: Because a TCP SYN flag consumes $1$ sequence number in the sequence space, server Q acknowledges receipt of P's SYN by setting the acknowledgement number (the next expected sequence number from P) to $N_Q = N_P + 1$ .

Final answer

Options (A), (D)

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2024 Computer Science and Information Technology - Set 1 (CS1)
Question: 29 of 65
Section: Computer Science and Information Technology

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