Traceroute: Use the file icmp-ethereal-trace-2.pcap to answer the questions

Examine the last three ICMP packets received by the source host, and determine how are these packets different from the ICMP error packets.

The last three ICMP packets are Type [Fill in the blank],
(echo (ping) reply) rather than Type [Fill in the blank],
(TTL expired), because the datagrams have made it all the way to the destination host before the TTL expired.

Question

Traceroute:  Use the file icmp-ethereal-trace-2.pcap to answer the questions

Examine the last three ICMP packets received by the source host, and determine how are these packets different from the ICMP error packets.

The last three ICMP packets are Type [Fill in the blank],
  (echo (ping) reply) rather than Type  [Fill in the blank],
  (TTL expired), because the datagrams have made it all the way to the destination host before the TTL expired.

BlackTom AI · Accepted Answer

To approach this question, we first note that it asks about the last three ICMP packets and how they differ from ICMP error packets, using the given fill-in values.
Option 1 fill: The correct ICMP type for an ICMP echo reply (the response to a ping) is 0. This is widely established in ICMP where Type 0 corresponds to Echo Reply, indicating the destination host successfully received the Echo Request and is replying. The prompt already mentions (echo (ping) reply), so Type 0 is the appropriate fill for that position.
Option 2 fill: The second blank compares to (TTL expired). The ICMP type that indicates a TTL expired condition is 11, which is the ICMP Time Exceeded message. This type is generated by a router or host when a packet’s TTL reaches zero before reaching its final destination, signaling that the packet could not be delivered due to TTL expiration. The sentence notes that the datagrams have made it all the way to the destination host before TTL expired, so it would not be a TTL-expired condition in the last three packets; instead, the last three are Echo Replies, while TTL-exceed messages would be Type 11.
In contrast, if the last three packets were ICMP error packets caused by TTL expiration, you would expect Type 11 entries rather than Type 0 entries, because TTL expiry generates the error, not a normal echo reply. The distinction hinges on whether the destination host actually returns a successful Echo Reply (Type 0) or intermediate hops report TTL expiration (Type 11). The observed behavior described in the prompt—packets reaching the destination before TTL expiration—aligns with echo replies (Type 0) and not with TTL-exceeded errors (Type 11).
Therefore, the rationale for the two fills is: fill the first blank with 0 to denote Echo Reply, and fill the second blank with 11 to denote Time Exceeded, reflecting that the scenario involves successful delivery to the destination rather than TTL expiry at a router. The contrast with ICMP error packets is that error types like Time Exceeded (11) arise when TTLs drop before delivery, whereas Echo Reply (0) indicates successful delivery and response.

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