Conceptually, decompression with the C bit set means that the elided
field is reconstituted, i.e. a UDP checksum is computed.
No difference between node and router.
*As an optimization*, a node that is going to discard it right away
(a) when re-compressing on the other side of the forwarder or (b) when
checking the checksum, might not do the computation (I would expect
that optimization to be implemented everywhere, but it is not a
feature of the protocol).
I believe the places where I used MUST should be MUST. They are
essential features to make the compression mechanism work.
When you say SHOULD, what special conditions are you envisioning here
where that would not be heeded but the protocol still works?
Field bus protocols don't encompass the pseudo-header in their
checksum, so technically they are outside the condition that allows
the application (yes, I should have said upper layer) to indicate
authorization to elide. If they have their own addressing, that is
rarely out of a global address space like the IPv6 address protected
by the pseudoheader.
If we turn some of the MUSTs into SHOULDs, we need to add text that
explains the purpose of the UDP checksum and the pseudoheader so
readers not as familiar with IPv6 get enough information to make an
informed decision whether they really should be violating the SHOULD.
Gruesse, Carsten
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