In IP header we have header checksum field which is calculated at every hop because some of the fields in IP header like TTL changes in every hop.

In TCP we calculate checksum on pseudo IP header + TCP header + TCP data . Here pseudo IP header doesn't contain the IP header fields that change frequently.

So in IP header checksum why don't we compute checksum on pseudo header or a header excluding fields which change frequently like TTL ? It will also help to reduce the computational overhead .

1 Answer 1


It's "not easy" to change something like IPv4. Better replace it altogether - IPv6 doesn't have a header checksum any more...

  • In Ipv4, when checking the checksum, the receiving router first checks the checksum then decrements TTL and then recomputes the checksum right ?
    – Zephyr
    Aug 10, 2017 at 2:25
  • Yep, pretty much sums it up.
    – Zac67
    Aug 10, 2017 at 5:54

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