I read RFC 4302, IP Authentication Header, Section 3.3.3, Integrity Check Value Calculation. In page 13, it is mentioned that the fields which are mutable during transit, like TTL or hop count, are set to zero.

If they are set to zero then how can we keep track of hop count?

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1 Answer 1


You are misunderstanding what that is telling you. Simply for the purposes of the calculation, you use zero for those fields; you don't actually change those fields to zero for the packet.

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