Subject: [Autoconf] Old description for 6lowpan; thoughts on link vs. subnet

From: Erik Nordmark <[email-masked]>
Date: Tue, 18 Nov 2008 15:21:44 -0800

This document plus discussions with Dave helped me understand why it 
made sense to define link as within radio reachability:

http://tools.ietf.org/html/draft-culler-6lowpan-architecture-00

Some more details are explained in
http://tools.ietf.org/html/draft-shelby-6lowpan-nd-01

I think there are some details about the definition of link that isn't 
explicit anywhere.

One possible definition is a unidirectional one:
N2 is on the same link as N3 iff N2 can hear packets that N3 sends

Another possible definition is to require that they can hear each other.

What is important to me is that there is a way for an application that 
is part of the control plane, be it an implementation of a routing 
protocol, or an implementation of some (address) configuration protocol, 
or something else, has a mechanism by which it can reach its radio-hop 
neighbors. To me it makes sense to define "link" in such a way that 
link-local unicast/multicast can be used to ensure that packets only 
reach the radio-hop neighbors.

The other part that is important to me is that the IP TTL can be used to 
limit the effect of transient routing loops. This implies that the 
routers must decrement ttl.

Finally, from the perspective of local mobility it makes sense to define 
the subnet prefix as a domain of address allocation where a node can get 
an address allocated in that prefix and then use it as it moves around 
inside the domain.

One implication of the above is that it doesn't make sense to announce 
any on-link prefix in Router Advertisements for these types of networks. 
(Depending on how address allocation is done it may or may not make 
sense to advertise prefixes with the "addrconf" bit set in RAs.)

Another implication is that nodes that are in the same subnet prefix 
will see TTL decrement when sending packets to each other. I don't know 
what impact that has on applications, but since the subnet prefix isn't 
advertised as "on-link" the prefix wouldn't be visible to applications 
in the OS platforms I'm familiar with.

    Erik

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