GATE | Gate IT 2005 | Question 26

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


Answer: (A)

Explanation: traceroute tracks the route packets taken from an IP network on their way to a given host. It utilizes the IP protocol’s time to live (TTL) field and attempts to elicit an ICMP TIME_EXCEEDED response from each gateway along the path to the host.

Alternative approach –



The options given for this question are partially correct.
Option-A is partially correct in saying that the source progressively queries routers but is wrong in saying that the source queries information about the next router in the path using ICMP packets.
Option-C is also partially correct. Traceroute ensures that a packet is returned from each router by prematurely expiring the TTL which is incremented by 1 each time it gets a response from a router. But it is wrong in saying that it uses an ICMP reply packet since it uses an ICMP error packet.
Out of both the options, Option-C seems the correct choice as it describes the traceroute program in a better way than Option-A.

This explanation is provided by Chirag Manwani.

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