GATE CS 2025 Set 1 | Question 22

MSQ · 1 mark

Consider the 3-way handshaking protocol for TCP connection establishment. Let the three packets exchanged during the connection establishment be denoted as P1, P2, and P3, in order. Which of the following option(s) is/are TRUE with respect to TCP header flags that are set in the packets?

  1. Option A: P3: SYN = 1, ACK = 1
  2. Option B: P2: SYN = 1, ACK = 1 (correct)
  3. Option C: P2: SYN = 0, ACK = 1
  4. Option D: P1: SYN = 1 (correct)

Answer

(B), (D)

Explanation

In the 3-way handshake the client opens with a SYN segment (P1: SYN = 1, ACK = 0). The server replies with SYN + ACK (P2: SYN = 1, ACK = 1), and the client finishes with a pure acknowledgement (P3: SYN = 0, ACK = 1).

  • (A) False: P3 has SYN = 0.
  • (B) True: P2 acknowledges the client’s SYN and carries the server’s own SYN.
  • (C) False: P2 must have SYN = 1.
  • (D) True: P1 is the connection request, so SYN = 1.

Answer: B, D