Re: RFC6550 (RPL) and RFC6775 (IPv6 Neighbor Discovery for 6LoWPANs)

[Carsten Bormann <cabo () tzi org>]

Hi Etienne,

I’m also not sure many of the classical network operators assembled in NANOG work with 6LoWPANs today, but I still can 
answer your question.

> While trying to build a holistic view of LoWPANs, I'm consulting the IETF's informational and standards documents.
>
> I'm struck by the impression that, despite the significance of RFC6775's extension of Neighbor Discovery(ND) to 
> low-power and lossy networks (LLNs),
> it is largely ignored by RFC6550 (RPL), with little to no reference to the ontological plane created in RFC6775's 
> terminology section.

Yes, you could say that.

ND (Neighbor discovery) describes interfaces between hosts and between hosts and routers.
6LoWPAN-ND does not use host-to-host interfaces (different from Ethernet, all traffic goes over routers, which RFC 4861 
already forsaw in the L — on-link — bit, which isn’t set in 6LoWPAN-ND).

RFC 6550 was completed at a time when many people who came in from the WSN (wireless sensor network) world thought they 
could get away with a network that is wholly composed of routers.
Even the “leaf” nodes in the RPL world were participating in the routing protocol and therefore didn’t really need a 
host-router interface.  There was no separate host-router interface in that world, because there were no non-router 
hosts.

> (a) router advertisements and router solicitations are substituted by DAG information objects (DIO) and DAG 
> information solicitations (DIS)

Right, DIO and DAO are router-to-router messages.  If there are no hosts (and routers don’t bootstrap themselves as 
hosts), you don’t need ND.

> (b) the terms "mesh-under" and "route-over" (widely cited), defined in RFC6775, are absent from RFC6550

RFC6550 is route over by definition.  Actually, the term was coined by the people working closely with the RPL 
development; RFC 6775 does appropriate it as 6LoWPAN-ND is applicable in either case.

> (c) jarringly: RFC6775 describes the route-over topologies as multi-IP-hop, while RFC6550 gathers DODAG nodes within 
> the confines of the same IPv6 prefix as their border router - no multiple IP hops.

I’m not sure where you get this interpretation: RFC 6550 (RPL) is very much about IP hops.
Maybe you mean the address architecture that was defined explicitly in RFC 6775; RFC 6550 does not really say much 
about addresses.

Note that the RPL people have since proceeded to (at least partially) embrace the host-router concept from the IP 
architecture; RFC 8505 is an update to RFC 6775 that makes 6LoWPAN-ND more palatable to RPL people.

I have CCed Pascal Thubert who, as a co-author to all three RFCs, certainly will have another perspective on this.

Grüße, Carsten