GATE2025_CS1_Q22

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?

Note: This is a Multiple Select Question (MSQ). One or more options may be correct.

✅ Final Answer:

The correct options are (B) and (D).

🔹 Short Answer:

The TCP 3-way handshake involves three packets with specific flags:
P1 (Client → Server): SYN=1, ACK=0. The client requests to synchronize.
P2 (Server → Client): SYN=1, ACK=1. The server acknowledges the client's request and sends its own synchronization request.
P3 (Client → Server): SYN=0, ACK=1. The client acknowledges the server's request.
Based on this, statement (B) `P2: SYN=1, ACK=1` and (D) `P1: SYN=1` are true.

🔸 Long Answer:

To solve this question, we must understand the purpose and flag settings of each packet in the TCP 3-way handshake, which is used to establish a reliable connection.

Packet-by-Packet Analysis:

1. Packet 1 (P1): Client to Server
   The client initiates the connection. It sends a segment with the SYN (Synchronize) flag set to 1. This informs the server that the client wants to establish a connection and indicates the client's initial sequence number (ISN), say `Seq=x`. The ACK flag is 0 because there is nothing yet to acknowledge.
   Flags: SYN=1, ACK=0.
2. Packet 2 (P2): Server to Client
   The server, upon receiving P1, must do two things:
   • Acknowledge the client's request. It does this by setting the ACK (Acknowledgment) flag to 1 and setting the acknowledgment number to `x+1`.
   • Initiate its own side of the connection. It also sends a SYN segment to the client with its own ISN, say `Seq=y`.
   These two actions are combined into a single packet.
   Flags: SYN=1, ACK=1.
3. Packet 3 (P3): Client to Server
   Finally, the client acknowledges the server's synchronization request (P2). It sends a segment with the ACK flag set to 1 and the acknowledgment number set to `y+1`. The SYN flag is now 0 because the client is not initiating a new synchronization, merely acknowledging one. At this point, the connection is established.
   Flags: SYN=0, ACK=1.

Evaluating the Options:

• A. P3: SYN = 1, ACK = 1 → FALSE. In P3, SYN=0.
• B. P2: SYN = 1, ACK = 1 → TRUE. This correctly describes the server's SYN-ACK response.
• C. P2: SYN = 0, ACK = 1 → FALSE. In P2, SYN must be 1.
• D. P1: SYN = 1 → TRUE. This correctly describes the initial client request.

Bibliography:
- Kurose, J. F., & Ross, K. W. (2016). Computer Networking: A Top-Down Approach. Pearson. (Chapter 3: Transport Layer).
- Tanenbaum, A. S., & Wetherall, D. J. (2011). Computer Networks. Pearson. (Chapter 6: The Transport Layer).

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