Which ICMP message is elicited when a zero-offset fragment is sent but the full train is not reassembled in time?

• A. ICMP 'Time Exceeded' message
• B. ICMP 'Destination Unreachable' message
• C. ICMP 'IP/fragment reassembly time exceeded' message
• D. ICMP 'Parameter Problem' message

Answer: (C)

Fragment reassembly timeout is the situation here. When an IP datagram is broken into fragments, the receiver waits a limited time for all fragments to arrive so it can reassemble the original packet. If the full set doesn’t arrive in time (for example, the first fragment with offset zero arrives but the remaining fragments never come quickly enough), the receiver discards what it has and sends back an ICMP message indicating the reassembly time was exceeded.

The most precise description of this condition is an ICMP Time Exceeded message with the code for Fragment Reassembly Time Exceeded (often phrased as IP/fragment reassembly time exceeded). This distinguishes it from other ICMP types: Destination Unreachable relates to delivery failures, and Parameter Problem points to bad header details.