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>> In case of TCP re-transmission I am seeing that length is getting
>> changed, [...].  I my opinion is that the packet shouldn't get
>> changed on re-transmission in any respect, right?
> TCP doesn't retransmit packets; it retransmits segments which are
> newly assembled into packets.  The new transmission being a different
> length is a clue that the sender is doing PMTU-D.

Maybe.  It depends on what the lengths in question are.  (I don't
recall seeing the original, so this may have been given there.)

Another possible explanation is that the first transmission was at the
end of the data available to send and more octets have been queued to
be sent in the meantime, in which case I think the stack may end up
sending both retransmitted data and new data in a single packet.

Another possibility is that it's PMTU black hole detection in action.

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