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Re: "Selfish Routing"
From: Mike Lloyd <drmike () routescience com>
Date: Sat, 15 Feb 2003 17:15:46 -0800




Iljitsch van Beijnum wrote:
Utilization-based routing between two ASes is an excercise in futility
if it's in a single location (load balancing is simpler and just as
effective) and isn't simpler than across multiple ASes if the
interconnects are in different places of the network topology

Pure utilization-based routing between 2 AS's is, in effect, load balancing, so I'm not sure how to interpret your first instance. But in any case, adding performance objectives to a load optimization problem makes the solution more demanding. (It is possible in practice to observe performance differences between 2 paths from AS A to directly connected AS B - not as many as you see comparing different transit choices to a far point, but they still exist.)

Optimization across topologically distant egress points from AS A to AS B is harder, and if the choice is a nearby link to B and a far link to some other AS C, harder still. But in practical networking, this is not the common problem.

Practically speaking, most AS's fall between your two cases - that is, they have at least one place in their topology where they border several other AS's. This provides multiple pre-computed, loop-free paths to the same end point. Picking the "best" one for near-optimal performance and low congestion in this context is, I would suggest, beneficial, and if done correctly, not prone to the suboptimality discussed in the paper that started this thread.

Besides, the whole point is that you'd be able to go from New York to
London over Tokio if the direct line is congested.

Given good end to end performance measurement as part of the path selection, a planet-circling route is unlikely. Few apps will perform well if packets have to circumnavigate (streaming and email are among the only candidates).

So to an extent, I agree - performing dynamic re-routing over arbitrary paths purely to avoid congestion isn't a great idea. You can indeed end up bouncing of satellites. But if you add performance sensitivity, you have a harder optimization challenge, but a much more worthwhile result.

Mike


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