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ISRO CS 2025 Q.39: solution and answer

ISRO 2025 · CSQ.39 · 1 mark · MCQ

Suppose the host firewall of a computer is configured to block all incoming ICMP packets. What could be its effect on function of TCP/IP protocol stack of the host?

  1. (A)

    No effect

  2. (B)

    Path MTU discovery fails

  3. (C)

    Slow start fails

  4. (D)

    Congestion control fails

See the question in the paper
ISRO CS 2025 question 39Tap to open full size

Answer and solution

ANSWER

The answer is (b): path MTU discovery fails, because it depends on ICMP 'Fragmentation Needed' messages that the firewall blocks.

Official answer: BSource: ISRO's final answer key of 15 May 2026

  1. Path MTU discovery (PMTUD) sends packets with the Don't Fragment (DF) bit set. A router whose next link has a smaller MTU drops such a packet and returns an ICMP 'Destination Unreachable, Fragmentation Needed' message that carries the smaller MTU.

  2. If the host blocks all incoming ICMP, that message never reaches the TCP/IP stack, so the sender never learns the smaller MTU. Large DF packets keep being dropped and the connection stalls (a 'PMTU black hole'). So path MTU discovery fails.

  3. Slow start is a TCP sender-side algorithm: the congestion window grows by one segment for each ACK received. ACKs are TCP segments, not ICMP messages, so slow start keeps working.

  4. TCP congestion control (slow start, congestion avoidance, fast retransmit, fast recovery) detects loss through timeouts and duplicate ACKs, not through ICMP. ICMP Source Quench is obsolete and TCP does not use it. Congestion control is therefore unaffected.

  5. The only function listed that depends on incoming ICMP messages is path MTU discovery. 'No effect' is wrong because of this, and options (c) and (d) are wrong for the reasons above.

KEY IDEA

Path MTU discovery relies on ICMP 'Destination Unreachable, Fragmentation Needed' messages from routers; TCP's slow start and congestion control rely only on ACKs, timeouts and duplicate ACKs.

WATCH OUT FOR

Assuming TCP congestion control or slow start needs ICMP feedback (confusing it with the obsolete ICMP Source Quench), or assuming ICMP is optional and blocking it has no effect.

From the ISRO CS 2025 paper. Every question of this paper · The full paper (PDF, ISRO) · All 2 ISRO IPv4 questions, with notes