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

Etienne-Victor Depasquale edepa at ieee.org
Sun May 31 11:10:48 UTC 2020

Pascal, thank you, the draft at
is very informative.

You hit the nail on the head with your suggestion of confusion between the
congruence of link and subnet.

However, I followed one of the references (RFC4903) in your draft and
it does not help that it (RFC4903) points to RFC4291's assertion that:
"Currently IPv6 continues the IPv4 model that a subnet prefix is associated
with one link"

RFC4903 further states that:
 "clearly, the notion of a multi-link subnet would be a change to the
existing IP model.".

I confess: your assertion in the draft that:
"In Route-Over Multi-link subnets (MLSN) [RFC4903],
routers federate the links between nodes
that belong to the subnet, the subnet is not on-link and it extends
beyond any of the federated links"

is news to me.

Best regards,


On Sat, May 30, 2020 at 1:39 PM Pascal Thubert (pthubert) <
pthubert at cisco.com> wrote:

> Hello Etienne Victor
> Maybe you’re confusing link and a subnet?
> This is discussed at length here:
> https://datatracker.ietf.org/doc/draft-thubert-6man-ipv6-over-wireless/
> RPL can route inside a subnet using host routes. This is how a multi link
> subnet can be made to work...
> Please let me know if the draft above helped and whether it is clear
> enough. The best way for that discussion would be to cc 6MAN.
> Keep safe,
> Pascal
> Le 30 mai 2020 à 10:03, Etienne-Victor Depasquale <edepa at ieee.org> a
> écrit :
> Thank you Carsten, and thank you Pacal. Your replies are valuable and
> packed with insight.
> I'll wrap up with how I interpret RPL's behaviour in terms of IP hops.
> On one hand, RFC6775 defines a route-over topology as follows:
> "A topology where hosts are connected to the 6LBR through the use of
> intermediate layer-3 (IP) routing.
> Here, hosts are typically multiple IP hops away from a 6LBR.
> The route-over topology typically consists of a 6LBR, a set of 6LRs, and
> hosts."
> If RPL is route-over by definition, then RFC6775 would imply that there
> are typically multiple IP hops between a leaf and the border router.
> On the other hand, there at least two contradictions (which I justify
> after stating them):
> (a) RFC6550 states that "RPL also introduces the capability to bind a
> subnet together with a common prefix and to route within that subnet."
> (b) Reduction of a DODAG to a single subnet prefix, albeit only only one
> parent-child relationship deep, is clearly shown at Contiki-NG's Github
> page (deep dive section).
> The hinge on which my understanding revolves is that an IP hop traverses a
> router and ***results in a change of prefix of the link on which the packet
> travels*** :
> --------<incoming packet; link prefix = p1>------><router>
> --------<outgoing packet; link prefix = p2>------>
> With RPL, the "hop" would look like as shown below:
>   --------<incoming packet; link prefix = p1>------<router>
> --------<outgoing packet; link prefix = p1>------
> There seems to be a change in the meaning associated with "IP hop".
> I guess that I can reconcile both cases through the observation that RPL
> actually does apply to a single, NBMA link and therefore the IP prefix
> ***is*** the same.
> Then again, calling the RPL device involved in the packet forwarding by
> the name "router" feels like an uncomfortable stretch.
> Don't routers sit at the meeting point of different layer 2 links?
> Cheers,
> Etienne
> On Fri, May 29, 2020 at 10:39 PM Pascal Thubert (pthubert) <
> pthubert at cisco.com> wrote:
>> Hello Etienne
>> You may see ND as the host to * interface for any network and RPL as the
>> router to router interface when the network is NBMA.
>> Some of us cared about the interworking.
>> Look at the RPL Unaware leaf I-draft and you’ll see that I’m sure.
>> Keep safe,
>> Pascal
>> > Le 29 mai 2020 à 20:28, Carsten Bormann <cabo at tzi.org> a écrit :
>> >
>> > 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
>> >
> --
> Ing. Etienne-Victor Depasquale
> Assistant Lecturer
> Department of Communications & Computer Engineering
> Faculty of Information & Communication Technology
> University of Malta
> Web. https://www.um.edu.mt/profile/etiennedepasquale

Ing. Etienne-Victor Depasquale
Assistant Lecturer
Department of Communications & Computer Engineering
Faculty of Information & Communication Technology
University of Malta
Web. https://www.um.edu.mt/profile/etiennedepasquale
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