Subject: [Roll] Questions/comments on draft-ietf-roll-of0-03.txt

From: Omprakash Gnawali <[email-masked]>
Date: Sun, 21 Nov 2010 17:58:51 -0800

Thanks for the new version of OF0 draft.

My biggest concern with OF0 is it would compute min-hop routes. There are
many research results and deployment experiences suggesting that
min-hop routing is not a good idea in wireless routing.

The draft notes:

"Using a metric that in essence is similar to
   hop count implies that the quality of the connectivity should be
   asserted so that only neighbors with a good enough connectivity are
   presented to the OF.  How that connectivity is asserted and
   maintained is out of scope."

and:

"   Hop count used in wireless networks will tend to favor paths with
   long distance links and non optimal connectivity properties.  As a
   result, the link selection must be very conservative, and the
   available link set is thus constrained.  In some situations, this
   might end up partitioning the network.  For those reasons, the use of
   hop count only is generally not recommended in wireless networks."

I appreciate mentioning these dangers of OF0 and suggesting that
white-listed links (white-listing is claimed to be out of scope of
this draft and not sure where I can go read how white-listing is
done) might be able to address some of these dangers. It is
unclear how helpful white-listing is.

Right now there is no evidence that OF0-based routing will
actually work. It will be great to look at some arguments
or evidence that this is a reasonable way to do routing
for RPL.

"The metric used in OF0 is the RPL Rank, as defined in
   [I-D.ietf-roll-rpl]."

I am not sure if Rank is defined as a metric. It might be good to be
precise here.

Some comment on how these values are determined would be helpful:

   MinHopRankIncrease:  256
   DEFAULT_RANK_INCREMENT:  4 * MinHopRankIncrease
   MINIMUM_RANK_INCREMENT:  1 * MinHopRankIncrease
   MAXIMUM_RANK_INCREMENT:  16 * MinHopRankIncrease
   MAXIMUM_RANK_STRETCH:  4 * MinHopRankIncrease

- om_p

On Thu, Nov 18, 2010 at 2:41 PM, JP Vasseur <[email-masked]> wrote:
> Dear all,
>
> draft-ietf-roll-of0-03.txt has now been stable for some time.
>
> This starts a 3-week WG Last Call on draft-ietf-roll-of0-03.txt that will end on December 10 at noon ET.
>
> Please send your comment to the authors and copy the list.
>
> We would also appreciate (off-list if you prefer) feed-back on implementations.
>
> Thanks.
>
> JP.
> _______________________________________________
> Roll mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/roll
>

---

From: "Pascal Thubert (pthubert)" <[email-masked]>
Date: Mon, 22 Nov 2010 13:00:30 +0100

Hi Om:

Thanks a lot for your review; please see inline.

> many research results and deployment experiences suggesting that
min-hop
> routing is not a good idea in wireless routing.

[Pascal] Yes, quite explicitly. 
But please remember that RPL is not designed only for a certain radio.
RPL is L2-agnostic, and is supposed to also operate on wires and wire
emulations.

> I appreciate mentioning these dangers of OF0 and suggesting that
white-
> listed links (white-listing is claimed to be out of scope of this
draft and not
> sure where I can go read how white-listing is
> done) might be able to address some of these dangers. It is unclear
how
> helpful white-listing is.

[Pascal] The 802.11 association is an example of white listing. The real
world suggests that it is quite useful. 

> Right now there is no evidence that OF0-based routing will actually
work. It
> will be great to look at some arguments or evidence that this is a
reasonable
> way to do routing for RPL.

[Pascal] Isn't there?

On the contrary, I'd assert that there are (tens of) years of experience
in Distance Vector that indicate that a weighted hop count can be very
useful. The weighted hop count is the dominant metric in the world
today, and OF0 enables to use RPL as an alternate protocol in networks
where other IGPs operate yet are not optimum. A typical example would be
a (recursively) hub and spoke network.

There are also years of practical experience on something very similar
to RPL/OF0 applied to wired links and more controlled radios, such as
WIFI infrastructure mode. You might want to look up work on Layer 3
meshes and MANEMO for instance.

Experience strongly suggest not to use hop count in 802.15.4 or any
other "adhoc" radios. If the text in OF0 is not clear enough on that,
then suggestions are welcome : )

> I am not sure if Rank is defined as a metric. It might be good to be
precise
> here.
[Pascal] Agreed. The Rank is simply analogous to the metric but
conceptually they are different beasts. 

What do you think of :
"
   The metric used in OF0 is an administratively defined scalar cost
   that is trivially added up along a path to compute the RPL Rank, as
   defined in [I-D.ietf-roll-rpl].  As a result, the Rank if a node is
   analogous to a weighted hop count of the path to the root.  " 

?

>    MAXIMUM_RANK_STRETCH:  4 * MinHopRankIncrease

[Pascal] Those numbers come from the classical experience for hop count
used in DV. 
RPL is designed to control count to infinity so we do allow more than
RIP's 16 hops. 
Yet, 16 hops max stays the default behavior, and 256 ops is the max OF0
can allow.
This enables compression of the Rank Floor() in one octet which is a
desirable goal.

- Anything wrong with those values?
- Do you think we should add text like the above in the spec ?

Cheers,

Pascal
> > This starts a 3-week WG Last Call on draft-ietf-roll-of0-03.txt that
will end
> on December 10 at noon ET.
> >

---

From: Philip Levis <[email-masked]>
Date: Mon, 22 Nov 2010 11:14:33 -0800

> RPL is L2-agnostic, and is supposed to also operate on wires and wire

Sure -- but I think the issues hopcount runs into are predominantly  
due to the lossy nature of communication: RPL is for LLNs. Is there  
experience on using hopcount (in an interoperable way) in LLNs?

> world suggests that it is quite useful.

Can you give some data? What "real world" are you referring to? There  
are plenty of times when association != good connectivity.

> a (recursively) hub and spoke network.

Sure -- but this experience is not in LLNs.

> meshes and MANEMO for instance.

Do drafts constitute experience in effective deployments? The major  
work I know about layer 3 meshes is Roofnet and Meraki. Practical  
experience is one thing: practical experience that concluded hopcount  
is a good idea. E.g., there were many years of practical experience  
with OLSR that led its current implementation to discard hopcount.

> then suggestions are welcome : )

What do you mean by "adhoc?" 802.11 can use the same ISM band and has  
similar modulation: its link layer behavior (in the 2.4GHz band) is  
pretty much the same as 802.15.4. Why do you think it is different?

>    analogous to a weighted hop count of the path to the root.  "

My major concern with OF0 is actually quite different: it does not  
say how to compute Rank from metrics. Back in the discussions on  
MRHOF, this emerged as a requirement for an OF, when we separated OFs  
from metrics. Right now, it seems like I could have an OF0 that  
computes Rank from latency, or throughput. Since the computation of  
Rank is very poorly defined, it seems very easy to have two  
incompatible OF0 implementations that work at cross purposes.

For OF0 to be ready to be an RFC, it must actually provide some  
details on how metrics are converted to Rank. Take a look at the  
April discussion entitled "Decoupling Metrics and OF."

Phil