Subject: [Roll] Question/clarification about applicability of OF0

From: Ralph Droms <[email-masked]>
Date: Tue, 9 Aug 2011 14:39:41 -0400

After reading through the mailing list threads on OF0, I have a couple of questions about the applicability of and use cases for OF0.

OF0, when it generates a rank_increase based on hop count, is characterized as inappropriate for wireless meshes.  I wonder if "wireless" is really the right descriptive term here.  More generally, a hop count metric is inappropriate for subnets composed of links in which a route with a higher hop count might perform better than a router with a lower hop count.  Such subnets can be either wireless or wired; for example, as I understand the technology, it's possible for an IEEE1901.2 network to exhibit connectivity and transmission quality characteristics for which a route through more higher quality hops might be better than a router through fewer lower quality hops.

I'm also confused about some of the use cases for RPL.  Is there a use case in which RPL would be used in a non-LLN environment in place of other routing protocols like RIP, OSPF and ISIS?

- Ralph

---

From: "Pascal Thubert (pthubert)" <[email-masked]>
Date: Wed, 10 Aug 2011 10:03:40 +0200

Hi Ralph

> OF0, when it generates a rank_increase based on hop count, is
characterized
> as inappropriate for wireless meshes.  I wonder if "wireless" is
really the right
> descriptive term here.  More generally, a hop count metric is
inappropriate
> for subnets composed of links in which a route with a higher hop count
might
> perform better than a router with a lower hop count.  Such subnets can
be
> either wireless or wired; for example, as I understand the technology,
it's
> possible for an IEEE1901.2 network to exhibit connectivity and
transmission
> quality characteristics for which a route through more higher quality
hops
> might be better than a router through fewer lower quality hops.

[Pascal] You're correct.
OF0 does not know the type of link it can be applied to so it leaves the
computation of the step_of_rank to the implementation that knows better.

> I'm also confused about some of the use cases for RPL.  Is there a use
case in
> which RPL would be used in a non-LLN environment in place of other
routing
> protocols like RIP, OSPF and ISIS?

[Pascal] RPL is a DV routing protocol. It has the pros and cons of its
family. But yes, it could apply in many places. It is a matter of
network engineering to select the most adapted tool for a given
situation, knowing that:
-As opposed to the classical bunch, it does not compute the optimal end
to end path for all nodes. 
-RPL does not try to fix any broken situation as soon as it occurs. 
-RPL requires looking inside the packets for datapath validation, and
RPL adds a (set of) L3 "VLANs" we call instances

 If you look at it, a spanning tree is not much different, just a lot
simpler (just a tree, no TTL).
So in a situation where a (mesh_under) spanning tree is used today a
(route_over) RPL solution could actually provide a positive replacement.
Benefits would be:
- no risk of meltdown (that's TTL for you) thus faster convergence.
- end to end IP (that's route_over vs. mesh_under)
- multipath ( that's coming from RPL's DAG)
- Goal (the root is picked at a sensible place)

Another example: If you have a number of access points at home, and you
want to mesh them, then the link quality is asserted by the association
process so hop count could work. You'll note that with hop count, a cost
is traditionally associated to link speed and that would apply too since
WIFI reduces the link speed to maintain the link quality. This is a case
where RPL could easily replace a proprietary mesh technology, either
using (hop count, line speed), something a bit fancier, like ETX, or
something more proprietary if that can provide a market advantage. OF0
will allow all those to interoperate and you'll get a DAG that reaches
the goal, if that's possible at all.

Cheers,

Pascal

---

From: Michael Richardson <[email-masked]>
Date: Wed, 10 Aug 2011 09:22:03 -0400

>>>>> "Ralph" == Ralph Droms <[email-masked]> writes:
    Ralph> I'm also confused about some of the use cases for RPL.  Is
    Ralph> there a use case in which RPL would be used in a non-LLN
    Ralph> environment in place of other routing protocols like RIP,
    Ralph> OSPF and ISIS?

RPL fills a different niche than RIP/OSPF/ISIS.  RIP/OSPF primarily is
about connecting broadcast subnets together.  While you can inject
/32s(/128s) into OSPF, and maybe you can do this for RIPv2 (I never
tried), it's not really the problem space that they were designed for.

RPL really provides a replacement in some sense for layer-2 STP, doing
the work at layer-3, and optimizing it for the case of not much
bandwidth, and also for limited broadcast/multicast availability.
If power and bandwidth were free, one could imagine using things like
MSTP, and indeed there are things like 802.1s which do this at layer 2.

The key thing is that the LLN wants to be a single subnet, not a series
of layer-3 subnets which are tied together.

The closest thing to RPL in a non-LLN situation today is, I think,
TRILL.  It however, is a layer-3-technology solution to a layer-2
problem, while RPL is a layer-2-technology solution at a layer-3.

But, to answer your direct question, I don't think any of the use
cases written imagine a purely non-LLN network.  Some of them will, I
think, have RPL running over a network in which parts of it are
distinctly not low power or lossy, but that will be only *part* of the
network.   

I do not believe, btw, the documented use cases are comprehensive.
The space is very much open, and RPL excites me because it can
potentially bridge many layer-2 technologies.  The use cases represent a
set of requirements from industries that have sufficiently matured to
articulate their problems.  

-- 
]       He who is tired of Weird Al is tired of life!           |  firewalls  [
]   Michael Richardson, Sandelman Software Works, Ottawa, ON    |net architect[
] [email-masked] http://www.sandelman.ottawa.on.ca/ |device driver[
   Kyoto Plus: watch the video <http://www.youtube.com/watch?v=kzx1ycLXQSE>
	               then sign the petition.

---

From: "Pascal Thubert (pthubert)" <[email-masked]>
Date: Wed, 10 Aug 2011 18:34:17 +0200

Hello Michael

I like the text a lot : ) and deeply agree for the most part.

Please note that RPL is not limited to intra-subnet though it has
special methods for that case.
RPL can also route between subnets/aggregations, which is actually a
simpler case. 
For instance, this can be used between mobile routers for nested NEMO
route optimization.

The key things are rather:
- the shape of network RPL builds with the concept of root that plays a
major role. RPL builds an edge network, as opposed to incumbent IGPs
that build a core with any-to-any optimization.
- the metrics (RPL is a lot richer than incumbents).
- the instances (RPL allows routing and forwarding along multiple
topologies)

The edge used to be the PC to router like. In a home network that was a
PC with a modem. Now it's getting a lot more than that as the HOMENET
people are figuring out at this very moment. 

Cheers,

Pascal

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]] On Behalf
Of
> Michael Richardson
> Sent: Wednesday, August 10, 2011 3:22 PM
> To: Ralph Droms
> Cc: [email-masked]
> Subject: Re: [Roll] Question/clarification about applicability of OF0
> 
> 
>    Kyoto Plus: watch the video
> <http://www.youtube.com/watch?v=kzx1ycLXQSE>
> 	               then sign the petition.
>