[olsr-dev] Re: [OLSR-users] Differing metrics for route decision

Thomas Lopatic (spam-protected)
Mon Feb 27 08:50:02 CET 2006


> If I'm correct OLSRD today have three methods to calculate
> metric/routing: OLSR, ETX and Fisheye.

Well, I wouldn't count Fish Eye as a separate metric. It just limits the
number of hops that a TC message is allowed to travel, i.e. its time to
live, so that not every node sees every TC message, so that close nodes
get updated about my status more often than nodes that are a couple of
hops away from me. The information contained in the TC messages is the
same as with "pure" ETX, though.

> Would it be simpler to maintain (less code duplication and so on) if
> olsrd had a common generic framework for the metric/routing calculation
> and plugin-like support for the actual calculation? Just an idea..

Yes, I believe it would. However, one of the things that we'd need to
think through for this is the following. Metrics come in a couple of
variants. They differ in how the metric of a multi-hop path is derived
from the metrics of the individual hops. I think that we have three
different kinds.

  * Additive metrics. Simply add the metrics of the individual hops to
obtain the metric of the full path. An example for this is ETX. If we
have a two-hop path from node A via node B to node C, the ETX for the
path from A to C is the sum of the ETX between A and B and the ETX
between B and C.

  * Multiplicative metrics. Packet loss is multiplicative, I'd say. If
50% of the packets make it from A to B and 40% make it from B to C then
the probability of a successful packet transmission on the path A -> B
-> C is 50% * 40% = 20%.

  * Minimum metrics. If we look at the bandwidth of a path then the hop
that has the lowest bandwidth is the bottleneck and dictates the
bandwidth of the complete path. The bandwidth of A -> B -> C then is
minimum(bandwidth(A -> B), bandwidth (B -> C)).

For additive metrics the Dijkstra algorithm applies. But does it also
apply to the other two types? What about combinations of metrics of
different kinds, e.g. something based on ETX and the bandwidth? Hmmm.



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