Suppose the host firewall of a computer is configured to block all incoming…
2025
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?
- A.
No effect
- B.
Path MTU discovery fails
- C.
Slow start fails
- D.
Congestion control fails
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Correct answer: B
Blocking incoming ICMP packets disrupts Path MTU Discovery (PMTUD). PMTUD uses ICMP Type 3, Code 4 messages ('Fragmentation Needed') to inform senders when a packet exceeds the MTU of an intermediate link. If these messages are blocked, the sender cannot reduce packet size to fit the path MTU, potentially causing connection failures for large data transfers. TCP congestion control and slow start rely on ACKs and retransmission timeouts, not ICMP.