Re: [Roll] Adoption of draft-goyal-roll-rpl-compression as a new ROLL WG document

["Pascal Thubert (pthubert)" <[e-mail]>]

Hello Angelo:

> Some questions about the design of the current message format:
>1) What are the benefits of having a protocol targeting LLNs with very
big flexible base message format?
>2) Wouldn't a more parsimonious packet format, more defaults, more
in-line fields be a better tradeoff?

We have been quite  a bit more parsimonious than classical IPv6.
Examples: the length is expressed in bytes, the RH has a prefix
factorization...
Still, we have refrained to optimize for one specific type of LLN; as
you know, LP 802.11 and recent variations of 802.15.4 support IPv6 min
MTU.
And we have kept reserved fields. They are set to 0 and easy to
compress, and at the same time the protocol is open to future
extensions.

> However, in my understanding of ROLL, this WG is born targeting LLNs,
where short-frames and low-bandwidth is the norm rather than the
exception.

Many sorts of them actually. In one extreme there can be a really
optimized adaption layer for each type of LLN and for each given
protocol in which case it probably makes sense to leave the work to the
IPv6-over-foo WG. GHC is the other end of that spectrum, where we can
get a reasonable benefit independent of the MAC and for all sorts of
packets.

> In my understanding, GHC is a general purpose compression method, that
should be applied to address the fact that we can't expect that every
protocol will be redesigned or compressed to go on power-constrained
networks.
> Native power-constrained protocols probably can natively get better
results by considering the fact that are being designed for such
networks.

Yes, that's the discussion I had hoped we'd have with this thread, and
the reason why I support this work. You'll note that GHC reuses the
6LoWPAN context and can probably compress efficiently the costly target
and transit options, as well as the routing header.

My own take is that it is a good start for us, and that we should study
its applicability and then figure if/where more specific macros
can/need-to be applied. 

Cheers,

Pascal