[OLSR-users] Bellman-Ford instead of Dijkstra?

Benjamin Henrion (spam-protected)
Tue Jun 13 14:52:19 CEST 2006

Thomas Lopatic <(spam-protected)> [060613]:
> Hey Benjamin,
> I thought through your idea again and - although you already explained
> it to me and I thought that I understood it - I am not quite sure any
> longer whether I really get it. Let me try to put it into my own words
> and see whether I understand what you're trying to accomplish.
> You're trying to establish two links between a node A and a node B. For
> this you have two interfaces per node, say A1 and A2 on node A as well

Two interfaces per node (typically one ethernet and one wireless), here
I make a schematics of 8 nodes where A1 is wireless and A2 is ethernet:

2            2            2            2
|            |            |            |
(-2)        (-2)         (-2)         (-2)
|            |            |            |
2            2            2            2


=== wireless chan1
+++ wireless chan13
| ethernet
() weights on interfaces

> as B1 and B2 on node B. Say, A1 talks to B1, A2 talks to B2.

Nope, see here over. A1 talks to B1 through wireless, but if A2 wants to
talk to B2, it has to go trough A2E2E1F1F2B2

> You would now like to use the A1/B1 link for packets from A to B. You
> would like to use the A2/B2 link for the opposite direction, i.e. for
> packets from B to A. In this way we would have a full-duplex connection
> between A and B, packets could flow in both directions at the same time.

Let's take the example of host A wanting to reach node D.

In my schematics, the fastest road would be in terms of not reusing the
same channel:


Right now, OLSR takes the route ABCD.

> What this boils down to is an asymmetric scenario, doesn't it? From A's
> point of view, we want to leave the A1/B1 link at its original link
> quality and we want to make the A2/B2 link artificially bad, so that
> A1/B1 gets prefered when node A sends packets to B. On B, we'd like to
> make A1/B1 artificially bad and leave A2/B2, so that A2/B2 gets prefered
> when B sends packets to A.

No, I just want the route to be choosen in function of weights on
interfaces. I consider that the radios of chan1 and chan13 have the same
SNR and link quality.

> What this means is that A has another notion of the link quality of link
> A1/B1 and link A2/B2 than B. For A the former is good and the latter is
> bad, for B it's the other way around.
> Based on this, let me ask two questions that came to my mind when
> thinking through the scenario again.
>  * We calculate our routing metric as follows: ETX = 1 / (NLQ x LQ). NLQ
> is the quality of a link as reported by our neighbour (e.g. 0.7 if the
> neighbour sees 70% of our packets), LQ is the quality of a link as
> measured by us. So, I am not sure whether we can use ETX with asymmetric
> routing as described above. The "LinkQualityMult" directive affects the
> link quality that we measure (and then report to our neighbourhood). So,
> if we use "LinkQualityMult" on a given link, then this link becomes
> better or worse for us and for our neighbours. There currently isn't any
> way to make us think that a link is good and at the same time our
> neighbours think that the same link is bad - which is the basis for the
> above asymmetric scenario, unless I am missing something here. Maybe the
> solution is to simply announce the modified link quality to our
> neighbours and ourselves use the original unmodified link quality. Hmm.
>  * Why would the "LinkQualityMult" value have to be negative? Can't we
> stick to Dijkstra, i.e. isn't 0 "bad" enough a multiplier? Hmmm. Could
> you outline the scenario that you have in mind a little further?

LinkQualityMult = weight in olsrd.conf?

I need to think what happens if wireless links are assymetric in the
schema hereover if you also use negative weights on interfaces.

> Hmmm. As I (mis)understand the whole thing now, it rather looks like an
> "asymmetric" kind of challenge and not that much a problem of negative
> weights, or?
> -Thomas
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Benjamin Henrion <(spam-protected)>
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