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Re: multi-homing fixes

  • From: Sean M. Doran
  • Date: Thu Aug 30 12:27:42 2001

| > 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.