Traceroute reports a possible route that is taken by packets moving from some…
2005
Traceroute reports a possible route that is taken by packets moving from some host A to some other host B. Which of the following options represents the technique used by traceroute to identify these hosts
- A.
By progressively querying routers about the next router on the path to B using ICMP packets, starting with the first router
- B.
By requiring each router to append the address to the ICMP packet as it is forwarded to B. The list of all routers en-route to B is returned by B in an ICMP reply packet
- C.
By ensuring that an ICMP reply packet is returned to A by each router en-route to B, in the ascending order of their hop distance from A
- D.
By locally computing the shortest path from A to B
Attempted by 190 students.
Show answer & explanation
Correct answer: A
Key idea: traceroute discovers each hop by sending probes with increasing time-to-live (TTL) values and reading ICMP Time Exceeded replies from intermediate routers.
Send probes with TTL starting at 1 and increment the TTL on each set of probes.
Each router that forwards a probe decrements the TTL. When TTL reaches zero that router returns an ICMP Time Exceeded message containing its IP address.
The sender records the replying router’s address and the round-trip time for that probe.
Repeat with increasing TTL until the destination replies (e.g., with an ICMP Echo Reply or a UDP Port Unreachable), which marks the final hop.
Implementations differ: some send ICMP Echo probes, others send UDP probes to high-numbered ports; both rely on TTL expiry and ICMP responses.
Limitations: routers may block or rate-limit ICMP, routes can be asymmetric or change during probing, and some routers do not reveal addresses, so traceroute may show partial or missing hops.