North American Network Operators Group|
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Re: multi-homing fixes
| > I'm pretty sure I need further explanation to "get it"./ I probably still don't get it, but let me see if I understand the mechanism. First, assign a prefix to a particular non-topological "locus", such as a metropolitan area, or a continent. Second, networks inside that locus will announce only the prefix, but with these exception bits. [Implied, but not stated: third, all these networks will exchange full information so as to be able to generate these exception bits]. Fourth, receivers of these prefixes, with the exception bits, will expand the longest-match trie (a Patricia tree is a compact representation of a trie, in common use when you have data with many nodes with just one child) so that lookups will only match in the case where there is no exception. If I understand you, what you are trying to do is to reduce the requirement for EVERY network operating within the aggregate to carry traffic to the ENTIRE aggregate at all times. This ordinarily would require announcing more specifics. So you propose a scheme where you use an attribute instead of the more specifics. Unfortunately, your attribute will cause the same behaviour in a receiver as would the list of more specifics, and therefore is merely a compression of the representation on the line that is somewhat better than, say, gzip. IOW, I think you are solving the wrong problem. We really have nearly zero experience with aggregates containing disjoint topology (i.e., non-provieder-based aggregation), largely because there is no obvious way to contain an explosion of more specifics when complete internal connectivity and complete transit break down. Steve Deering does propose a (partial) solution for this, but (in my opinion) it involves a complete reversal of current financial arrangements to work, in that a sender would have to compensate a transit network for carrying its traffic to anything within that aggregate, rather than the transit network collecting from the other (or both) parties. This is only a partial solution, since even where there is an incentive to maintain complete interconnectivity and carry traffic to all the consitituent subnets of the aggregate, failures will still cause black holes to arise even though other valid paths exist. Your scheme does let one warn of black holes in this eventuality, takes a bit less bandwith on the line, probably allows for the "slosh" to happen all at once rather than in dribs and drabs, and so forth, but it represents the same amount of work for the routers processing the attribute. That is, those routers are effectively brought inside the abstraction boundary of the "locus", and as a result the goal of hiding information from those routers is not met. My gut feeling is that for any sizable "locus", almost all of what we consider the core of the global routing system would be contained within the new abstraction boundary, so we're no better off than not aggregating in the first place. That is, we are MUCH better off with PA addressing. Sean.