Henning Rogge <henning.rogge at fkie.fraunhofer.de> a ?it :
Anyhow RFC 3626 was not excluding the use of link quality in neighbor sensing, and was suggesting some. The hop count should not be problem as long as the links that are on the path are good links. In other words I would see the use of path quality as a minor improvement over the use link quality in neighbor sensing. Anyhow this would help traffic engineering.I don't agree. In practice it's very difficult to decide if a link is "good" or "bad" as a binary decission. Maybe the bad link is really important for another node, maybe it is not.
Dear Henning,we are in agreement, protocol solely based on link admissions control have basically two defaults:
1. either the link admission conditions are too loose and the network accepts links that are too instable for sustained data traffics 2. or the conditions are too tight and the network excludes too many links and makes the graph disconnected.
What is difficult is to get experimental situations where you have both cases. I don't say it is impossible, but it is likely marginal, anyhow worthy enough to include path metrics in OLSR. Anyhow an additive path metric won't give the garantee that all links in the path are good enough, one may have one very bad link, hidden by the quality of the other links that are good enough to make the overall path optimal by Dijskstra.
There are no perfect specifications but one cannot say that all buggy situations necessarily come from the absence of good specifications. Some may come from a too strict application of the default values of the tuning parameters parameters proposed in the RFC.
I remember an experiment that concludes that OLSR between high speed cars was great to reduce overhead but led to too faulty links. I remember of another similar experiment that use much faster hellos between high speed cars, that lead to still very good overhead reduction and made link failure detection very good (good enough at least to open succesful TCP/IP connections between cars moving in opposite direction at 100 km/h). Notice that path metric won't help when bad or delayed estimate of link qualities are advertized.
A last point: one must be careful in defining default metrics if it requires some link layer notification that are not easy to implement.
I am looking forward the draft about ETX, it will be useful in improving the practice of use of metrics in OLSR.
Best regards, Philippe ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program.