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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? A. P2 : SYN = 1, ACK = 1 B. P1 : SYN = 1, ACK = 1 C. P3 : SYN = 1, ACK = 1 D. P1 : SYN = 1

GATE 2025 · Computer Networks · TCP · medium

Answer: Options A and D are TRUE: A. P2: SYN=1, ACK=1 and D. P1: SYN=1

  1. Recall the 3-way handshake flag assignments: P1 is the initial SYN from the client — no prior packet to acknowledge, so ACK=0. P2 is SYN-ACK from the server — it acknowledges P1 (ACK=1) and also initiates server side (SYN=1). P3 is the final ACK from client — acknowledges P2, connection established, SYN is no longer needed so SYN=0.
  2. Evaluate each option: A. P2: SYN=1, ACK=1 — TRUE (matches P2 exactly). B. P1: SYN=1, ACK=1 — FALSE (P1 has ACK=0, there is nothing to acknowledge before the first SYN). C. P3: SYN=1, ACK=1 — FALSE (P3 has SYN=0, the connection is being finalized not initiated from server side). D. P1: SYN=1 — TRUE (P1 is the SYN packet).