The source host wants to estimate the time it takes for the datagram to go from one router to another. Therefore, the router uses the timestamp option in IP with Timestamp Flag = 1.
I am confused as to whether the OPTIONS are included in IP checksum calculation at the source?
If yes, is it recalculated at every router it goes through, because the Timestamp data will change?
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Every IPv4 packet checksum is recalculated as it passes through a router because the TTL is decremented. IPv4 header options are, in practice, deprecated because smart businesses configure their routers to ignore or drop packets with IP header options. IPv6 has removed the IP checksum and header options– Ron Maupin ♦Commented Jun 11, 2020 at 13:17
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Thanks @RonMaupin . I was gonna go through IPv6 next.– FabhiCommented Jun 12, 2020 at 9:33
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1 Answer
I am confused as to whether the OPTIONS are included in IP checksum calculation at the source?
The IPv4 options are part of the header, so they're included in checksum calculation.
If yes, is it recalculated at every router it goes through, because the Timestamp data will change?
Yes, see RFC 791.
