[Olsr-dev] Thoughts about an ETX-FF-Metric plugin

Henning Rogge (spam-protected)
Mon Jun 16 09:16:28 CEST 2008

At the moment we have problems (ETX values are not as stable as they were 
before) between the new olsrd version (0.5.6-rc) and the old ones.

This problem has two reasons:
- we changed the ETX calculation to a pure Hello based system (as described in 
the original MIT paper), the old one is based on packet sequence numbers 
which use Hellos and TCs.
- the old olsrd agents have an instable timer implementation, so the Hellos 
the new implementation depends on are sometimes late (sometimes 80% later).

The problem of the old ETX implementation was that it has an instable timing. 
The number of routing packet transmitted over a link depends on the size of 
the network. Combined with a fixed window size the rate of ETX-change is 
dependant on the net size too, which is a bad idea in my oppinion.

This weekend I had some time to think about a "compromise" that might solve 
our problem. I will try to summarize the algorithm:

- the routing plugin will count the total number of packet and the missing 
packets (calculated by packet sequence numbers, which have NO duplicates !).
- Once every second the plugin calculates the ratio between captured and lost 
packages and use this value to update the LQ of the link.

I see two advantages of this strategy:
1.) we can use all olsr packets on a link to measure it's quality
2.) the rate of ETX-change of a link is independant of the amount of routing 
messages transmitted over a link.

What are you thinking about this idea ?


Diplom-Informatiker Henning Rogge
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Angewandte Naturwissenschaften e. V. (FGAN) 
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