Brian, Bob,
since JP dragged you in at this stage, let me try to summarize four
years of discussion in one message (which probably won't work).
6LoWPANs are non-transitive, but are quite different from 20th century
NBMA links.
We started out in 2006 by trying to modify ("optimize") 4861 to cope.
That work led nowhere.
At the Dublin IETF, 2008, we finally decided to address the issues in
a more radical way, and we now have a spec that is close to completion.
Some people came in later and now wonder why we abandoned the "minimum
change" approach.
Well, we didn't, but the minimum change is a bit different than we
originally thought.
Is an 802.15.4 network that much
more of an onerous environment for the base IPv6 protocols
(modulo some modifications/extensions)?
802.15.4 networks are wireless. Most nodes are extremely resource-
constrained, see RFC 4919 and in particular http://tools.ietf.org/html/draft-ietf-6lowpan-routing-requirements-04
for details.
The main difference from 20c NBMAs (apart from the resource issue) is
that, in a multi-hop wireless network based on low-power nodes, the
definition of "neighbor" is much more tenuous than in ATM or FR. So
we arrived at a somewhat different model of links and address
assignment. This model is also motivated by the extremely small frame
sizes of 802.15.4, which make header compression a core part of the
architecture.
Is it possible that standard IPv6 nodes could operate
on a 6lowpan network as well as a more traditional (e.g.,
ethernet) one?
Some are, we call these "edge routers" if they actually forward
packets. The large majority of nodes (hosts and routers) are 6lowpan-
only (and these are typically the extremely resource constrained 8
MHz, 10 kB $.50 SoCs).
If so, this seems like a lot of complexity
needed in that device to determine the type of link it is
operating on.
I don't follow -- a node would know whether one of its interfaces is
on a LoWPAN or an Ethernet.
More importantly, it is extremely unlikely that a LoWPAN is going to
be set up for the same applications that we run over Ethernet today (.
11 is much better for that purpose).
I agree that non-transitive links are an issue for more
than just
802.15.4 networks. The description given could be applied to
802.11 as well. Yet, IPv6 over 802.11 works relatively well.
For .11, we are talking about 2-3 orders of magnitude higher bit
rates, 3 to 6 orders of magnitude more power (electrical), CPU speed,
and memory per node, much more stable networks held together by
relatively powerful access points mostly directly connected to even
more stable Ethernet backbones etc.
To a laptop, an 802.11 network is pretty much an Ethernet, so it's no
surprise that 4861 works well.
It appears that a cross-WG review would be a good idea
at this point. I would like to see some 6MAN contributors
comment on this work rather than waiting until a Last Call.
I would love to have a Bar BOF/extended hallway meeting in Hiroshima
about this.
The 6lowpan WG meeting is currently scheduled at Monday morning; we
are likely to spend some time on 6lowpan-ND there.
If you think it's worth giving a summary presentation at 6man (Tuesday
morning), that certainly can also be arranged.
Gruesse, Carsten
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