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Does ICMP packet contain TCP header

asked 2018-03-16 06:26:29 +0000

Vindra gravatar image

I refer to the above question ( number 140 of kindle book (prep guide for WCNA exam)). The answer sheet in the same book says it does not contain TCP header. But I see in my packet capture some ICMP reply packets do contain TCP header. Can someone please explain?

THANKS

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answered 2018-03-16 07:46:01 +0000

updated 2018-03-16 14:19:54 +0000

cmaynard gravatar image

Generally: ICMP acts on top of IP, so ICMP packets don't contain their own(!) TCP header.

BUT the question is tricky in a sense that ICMP packets of some types (those ones reporting different problems, like in your trace - TTL exceeded, Destination unreachable etc.) according to RFC792 must contain some chunk of ORIGINAL datagram causing a problem.

In this case if original datagram used TCP, its TCP header will be present ICMP packet. It is not used for transport purposes though, but only for diagnostics purposes.

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Great explanation. Thanks

Vindra gravatar imageVindra ( 2018-03-16 07:49:02 +0000 )edit

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Asked: 2018-03-16 06:26:29 +0000

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Last updated: Mar 16