Re: [Roll] ETX metric

[Richard Kelsey <richard.kelsey at ember.com>]

   From: Henning Rogge <hrogge at googlemail.com>
   Date: Tue, 13 Oct 2009 15:53:43 +0200

   Am Dienstag 13 Oktober 2009 15:35:40 schrieb JP Vasseur:
   > Yes, what we have done so far (in the new revision to be posted) is to
   > define one mandatory metric (hop count) and have all other ones optional.
   > May be "hop count" is one the one that we will keep as mandatory but the
   > spirit is to have a set of metrics people can decide to use based on the OF.

   OLSRv1 did the same way and it shaped the conclusion "OLSR is crap"
   or "OLSR does not work" in huge parts of the industry. Hop count
   metrics can be called "use worst link" metrics for wireless
   networks, because it tries to optimize for the LONGEST links
   possible, which will be (most likely) worst links your network
   knows.

   With OLSR and 802.11 you can summarize it "Hopcount does not work
   in real networks outside the lab". There are some special cases how
   you can improve hopcount, but most times you will still get a poor
   performance and your protocol will be blamed.

   Unless 802.15.4 is totally different (I don't think so) I would
   strongly suggest thinking about something better than hopcount for
   your mandatory metric. At least that's the message I can tell from
   OLSR.

   Those who cannot learn from history are doomed to repeat it.

We have to be careful not to learn the wrong lesson.
Hopcount without careful link assessment definitely doesn't
work.  Hopcount limited to stable, symmetric links is a
different beast entirely.  For example, consider the Cost/PL
column in table 1 of [1], which gives the average
transmissions per link.  For most of the networks it is very
close to 1 and it never goes above 2.  That's puts ETX
pretty close to hopcount for those networks, if routing is
limited to links with good ETX.

So let me propose a reliability metric with two hopcounts,
one for 'good' links and the other for 'okay' ones.  To make
it more concrete:

  'Good' links have a current ETX < 1.5 and are expected to
  stay working throughout a maximum RA-DIO interval (which I
  think is given by 2^(DIOIntervalMin +  DIOIntervalDoublings))
  99% of the time.

  'Okay' links have an ETX < 3.0 and are expected to be
  working throughout a maximum RA-DIO interval 80% of the time.

When comparing, five 'good' hops would be equivalent of one
'okay' one.

I pulled the numbers (1.5, 99%, 3.0, 80% and five) out of
thin air.  Real values would have to be chosen with care.

                               -Richard Kelsey

[1] Omprakash Gnawali, Rodrigo Fonseca, Kyle Jamieson, David
Moss, and Philip Levis "Collection Tree Protocol." Technical
Report SING-09-01