Hi:
Per RFC 4944:
"The UDP header's checksum field is not compressed and is therefore
carried in full."
The UDP checksum is not the only way to protect the IP pseudo header,
the UDP header and the payload.
ISA 100.11a is defining a transport-level security that does all this
and more, since it has a larger signature and provides mutual
authentication at the same time.
Also, the ISA100.11a transport-level security is usually
hardware-assisted, so it requires little power or CPU time on the field
device, whereas UDP checksum will be a costly CPU operation.
So ISA100.11a is an example where the UDP checksum could and actually
should be compressed over the LoWPAN, leaving it up to be reconstructed
by a backbone router should the packet go any further than the LOWPAN
itself.
Since bit 3 in the HC-UDP header is reserved anyway, it makes sense to
standardize it to mean that the UDP checksum is compressed, provided
that the headers and payload are equally or better protected than if the
checksum was used.
Note: that would be bit 7 in my HC3 proposal. As a result, the complete
HC3 proposal could save us up to 4 additional bytes over RFC 4944 for a
UDP packet.
What do you think?
Pascal
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