Re: [manet] Some comments on draft-funkfeuer-manet-olsrv2-etx-01

[Georg Wittenburg <[e-mail]>]

Hi Henning, Teco, David, all,

On Saturday 14 May 2011 20:33:15 Henning Rogge wrote:
> Answer to Georg Wittenburg, Teco Boot and David Young...
> 
> Am Freitag 13 Mai 2011, 16:23:19 schrieb Georg Wittenburg:
> > > The funkfeuer metric drafts suggests to use all RFC 5444 packets for
> > > link loss estimation, not only a certain message type. Your suggestion
> > > would make it necessary to pad all RFC5444 packets to the maximum MTU,
> > > which might be a considerable overhead.
> > 
> > I agree. However, it still remains true that link loss estimation that
> > relies excessively on small packets will lead to suboptimal results. I'm
> > aware that the original ETX paper doesn't cover this, but it doesn't mean
> > that it shouldn't be addressed in this draft.
> > 
> > How about, for example, only incrementing the respective counters for
> > packets whose size is above a certain threshold, say MTU / 2 (or even MTU
> > / 4)?
> 
> What if no OLSR packets are that large ? Do we add padding to get at least
> one or multiple large packets each second ?

Sounds like an acceptable trade-off. Another option would be to include data 
packets in the link loss estimation. Was there any particular reason why you 
decided against including them?


> > > We do this in the Freifunk/Funkfeuer meshs, but in NHDP/OLSRv2 this
> > > would be the job of the link hysteresis (which easily could be
> > > implemented based on the link cost value).
> > 
> > But since the link cost value depends on the metric used, it would be up
> > to this draft to suggest values for this, wouldn't it?
> 
> We had some discussion about this, but it seems that most people here don't
> like to use the link metric to do the hysteresis.
> 
> In Freifunk/Funkfeuer, we ignore each link with an link quality below 0.1
> in one direction.
> 
> Am Freitag 13 Mai 2011, 17:53:53 schrieb Teco Boot:
> > > How about, for example, only incrementing the respective counters for
> > > packets whose size is above a certain threshold, say MTU / 2 (or even
> > > MTU / 4)?
> > 
> > How can packet sizes of lost packets be measured?
> 
> Maybe with a second packet sequence number that is only increased for large
> packets ?

I don't see the problem here. Both sender and receiver can increment their 
respective packet counters based on the packets (above a certain size, or 
matching other conditions) which they see and ETX should just work as 
expected. Am I overlooking something?


> Am Freitag 13 Mai 2011, 18:46:30 schrieb David Young:
> > You can probably derive a much better metric than ETX by tapping into
> > information that the rate adaptation collects.  More than one 802.11
> > rate adaptation in the literature has tried to optimize the so-called
> > goodput of a link by choosing a suitable bitrate for each packet based
> > on packet properties such as size.  Advantages of collecting information
> > from the rate adaptation are that you avoid duplicating any code or
> > computational effort, and you collect the information from a more
> > suitable (IMO) place in the network stack.  All the world's MANETs are
> > not built on 802.11, it is true, but the same general principles apply:
> > don't needlessly compute the same metric twice, and collect the link
> > metric at the most suitable place.
> 
> Doing this on one operation system with different wireless cards is hard...
> doing it for multiple operation systems and different cards will get you
> insane pretty quickly...
> 
> I personally like the idea to get the rate control data of the mac80211
> stack in linux, unfortunately that only solves the problem for a very
> special case, and only for unicast traffic.
> 
> If you rely on the rate control, you need to send unicast probing packets
> to all of your neighbors regularly if you have no traffic in this
> direction.

Best,

	Georg

-- 
Dr. Georg Wittenburg
Postdoctoral Researcher
INRIA / École Polytechnique, HIPERCOM Team
Laboratoire d'Informatique de l'École Polytechnique (LIX)
Route de Saclay, 91128 Palaiseau (CEDEX)
Phone: [phone], Fax: [phone]
http://www.lix.polytechnique.fr/~wittenburg/