GATE CS 2025 Set 1 Q.22: Transport Layer: UDP, TCP Flow and Congestion Control, Sockets

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?

P3: SYN = 1, ACK = 1

P2: SYN = 1, ACK = 1

P2: SYN = 0, ACK = 1

P1: SYN = 1

Official answer and solution

Official answer: B and D
From: Official GATE 2025 answer key

Solution

Recall the SYN and ACK control bits in the three segments (SYN, SYN-ACK, ACK) of TCP connection establishment.

During the TCP 3-way handshake:

P1 (Client $\to$ Server, SYN): SYN = 1, ACK = 0 $\implies$ (D) is TRUE.
P2 (Server $\to$ Client, SYN-ACK): SYN = 1, ACK = 1 $\implies$ (B) is TRUE (and (C) is false).
P3 (Client $\to$ Server, ACK): SYN = 0, ACK = 1 $\implies$ (A) is false.

Final answer

Options (B), (D)

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

Source

Paper: GATE 2025 Computer Science and Information Technology - Session 1 (CS1)
Question: 22 of 65
Section: Computer Science and Information Technology

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