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Consider a very large network of 10000 routers.Two host A and B are connected to this network,host A sends data to host B and after some unit of time host A receives ICMP time exceeded message for the same data packet. The maximum number of routers that can be travelled by packets when ICMP message reaches back to HOST A ?
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Why it is not 255+255 , since it has already crossed 255th router
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take a shorter example and visualize it.

A-> B-> C -> B-> A.

3+2 =5
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See, the 255 th router will place that packet in next link and make its ttl value=0 so, the 256th router will discard that packet then ICMP will take packet from link so the packet will again cross 255th router na
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no it will cross 254 routers.

A-> B-> C -> B-> A.

2->1->0->1->2.

here C discarded it and placed it in next link so it crosses 2 routers now A and B.

With 8 bit TTL maximum 255 possible, so answer will be (255+254)= 509.
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How 254 in coming back, please explain ?
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It'll not count itself in coming back as it already covered earlier.
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@Ved Sinha i think it is like this, when the packet reaches 255th router it will decrement the TTL field ....ttl will become 0 and so it will not be forwarded and ICMP packet is generated by the 255th one.

and now the ICMP is crossing 254 routers in between coming back.. i have also the same confusion why they have not included the last one

kudos to all those who have solved this question in the first go :)

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