[Olsr-dev] Routing loops [was: Freifunk Testing]

Henning Rogge (spam-protected)
Thu Jun 19 08:44:52 CEST 2008

Am Mittwoch 18 Juni 2008 21:08:04 schrieb Juliusz Chroboczek:
> This points at the fact that OLSR is not designed for sparse networks.
I'm not sure I can agree with this.

LSR (link state routing) are not designed for any special kind of network 
topology, as long as the neighbors can hear each other. OLSR just use the MPR 
mechanism as an optimiziation (maybe too strong, so we had to add some 
redundancy) for dense networks.

> In the dense networks for which OLSR is designed, it is reasonable to
> assume that the LS databases are in synch most of the time; not so in
> sparse networks, which rely on a number of marginal links.
So you are talking about a number of nodes where neighbors cannot hear each 
other well ?

> The original OLSR ensures that SPF is only run in well-connected parts
> of the network using the so-called ``hysteresis'' setting.  Not so
> OLSR-ETX, obviously.
The hysteresis part did never work in practice, especially because of the 
hopcount metric and the small data sample (three hellos).

OLSR-ETX just give bad links a high ETX value, so it's unlikely they are 
choosen for routing.

> I'm not sure why that works.  My guess is that you're assuming that
> given a doubly-connected part of the network (one that may give rise
> to a routing loop), it is at least simply connected with zero-loss
> links to ensure that the LS database is in synch.  Could you clarify this?
Each OLSR node repeats it's topology information regulary and flood it into 
the network. This way even nodes not connected with "zero-loss links" (which 
do not exist in WLAN networks) will get a picture of the topology.


Diplom Informatiker Henning Rogge
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