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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: Is the export policy selective under valley-free?
On 3 sep 2008, at 11:40, Randy Bush wrote: I think that yes, the valley-free property is a necessary but not sufficient criteria for generating the set of in-reality-valid paths on the Internet. i assure you that the actual topology is not valley free. e.g. there Note that valley-freeness is possible in the presence of backup configurations, which are usually called "sibling" relationships in this context. The basis for the valley-freeness rules is the paper "Stable Internet Routing Without Global Coordination" by Lixin Gao and Jennifer Rexford, although the term isn't used in this paper. They start out with Guideline A which says that customers must have a higher local preference than peers. If everyone uses policies like this then BGP will provably converge to a single stable state. But there's more. Assumption P is about clusters of ASes that peer with each other (possibly indirectly). ASes that don't peer are a cluster of their own. Assumption P is then that the graph of these is a directed acyclic graph: = when you follow the provider->customer relationships there are no cycles in the topology and there are no "self cycles". I guess this means you don't sell transit to yourself... or your peers. (Note that it's possible to have one type of relationship for one prefix and another for another prefix, so a European and American network can sell each other transit for their home region and peer for Asian traffic and still conform to the assumption.) With Assumption P in place Guideline A can be relaxed (as Guideline B) such that peers and customers may now have the same local preference. The next step is Guidelince C which allows for mutual backup relationships. Any path that has a hop with a backup link in it is considered a backup path and must have a single local preference value that is lower than that of any non-backup paths. "Note that, unlike Guideline A or B, enforcing Guideline C requires cooperation between ASs. An AS can not tell which routes involve backup links between other AS pairs. Hence, the BGP advertisements must identify these routes. This is typically achieved using the community attribute". So... I guess if we all set and recognize that "IHAZBACKUP" community we should be safe. Oh wait: http://www.iana.org/assignments/bgp-well-known-communities/
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