Subject: [6lowpan] alignment of HC-compressed/inline-carried fields

From: Matus Harvan <[email-masked]>
Date: Tue, 19 Jun 2007 02:06:46 +0200

Hi,

my understanding of the 6lowpan format draft version 13 is that the
Flow Label and UDP Source/Destination Ports would might not end on a
byte-boundary. As a result, all further values carried after such a
field with byte-unaligned end would possibly also get misaligned and bit
shifting would be needed to actually read the correct values. From a
discussion with Carsten Bormann I understood that this may not have been
the intention when writing the format draft and some
byte-alignment/padding could take place earlier than just at the very
end after all the compressed/inline-carried fields. The advantage would
be easier processing of and not having to bit-shift all subsequent fields.

Could someone provide more insights?

IMHO, a possible workaround would be to
1) pad the Flow Label with 4 bits of zeros. It usually is not used
anyway and hence the possible additional wasted byte could be acceptable
in a rather unlikely situation.
2) put the UDP Ports as the last UDP fields rather than the first UDP
ones and hence possibly requiring bit-shifting unless both UDP ports are
compressed. Should more fields follow after the UDP one in a future
version of the draft, padding with zeros could be used after the UDP Ports.

Matus

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