Re: [6lowpan] Fundamental concerns about 6lowpan ND

[Carsten Bormann <[e-mail]>]

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