Is the export policy selective under valley-free?

William Waites ww at styx.org
Wed Sep 3 03:36:52 CDT 2008


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Le 08-09-03 à 02:23, Paul Wall a écrit :
>
> That's correct.  A network purchasing transit will advertise its
> internally-originated prefixes, as well as those it's learning from
> downstream customers, to its provider.
>
> I'm not sure what "valley-free" means in this context.  You might want
> to try the Rosetta Stone patches and make sure your copy is up to
> date.


Valley-free is a property of AS mesh models that says that, where edges
are classified as peering (p2p) or transit (c2p) that a valid path
contains zero or one peering link and that the peering link occurs
adjacent to the top of the path. That is that valid paths look like,

	[c2p c2p ... c2p p2p p2c ... p2c p2c]

(slightly abusing the notation for clarity)

The idea is that a small, customer AS should not provide transit between
its upstreams, though this does, apparently happen sometimes.

Barring misconfigurations, I believe that AS paths are normally valley-
free.

See, for example,

"Toward Valley-Free Inter-domain Routing"
http://nsrc.cse.psu.edu/tech_report/NAS-TR-0054-2006.pdf

"AS Relationships: Inference and Validation"
http://www.caida.org/publications/papers/2006/as_relationships_inference/

Cheers,
- -w
- --
William Waites                       <ww at styx.org>
http://www.irl.styx.org/          +49 30 8894 9942
CD70 0498 8AE4 36EA 1CD7  281C 427A 3F36 2130 E9F5

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