> From: [email protected]
> Yes, but do hierarchies right.
> ...
> I have my doubts that the routing folks have a proper understanding wrt
> hierarchies.
This is probably true, because we just haven't had that much experience with
them; the _very_ limited amount of hierarchy in use in the current system is
just not enough to fully educate us about them.
(I'd like to make an analogy to congestion control - back in the '77-'79 time
period, not only did we not know much, we didn't know how much we didn't
know... Well, a few people probably had a clue, but the rest of us did not
really... :-)
But I do think this particular topic is something that the RRG _should_ be
thinking about, since it's closer to what I think a 'routing research' group
should be thinking of.
(The location/identity separation stuff is not, to me, really routing - it's
more basic architecture, but since the I* doesn't seem to have a forum for
basic architecture discussion, it seems to have wound up here...)
> What it takes is a "sliding hierarchy" as provided by TARA so that each
> router is fairly in the middle of the hierarchy and never at the very
> rim.
Sorry, I don't know enough about TARA to respond to this. Your Istanbul
analogy didn't really help me much. Can you say more?
> Hierarchy is by no means a reason to enforce a path which is longer
> than the shortest one!
True, but.... it seems to me fairly fundamental that the basic idea behind
hierarchy and routing is that it allows one to discard _some_ information, to
make the amount of data one has to process in the path-selection computation
more reasonable. That being the case, it seems inevitable that _some_ stretch
(over the theoretical minimum) should result.
To return to your real-world map analogy, nobody planning a trip from Madrid
to Gdansk goes out and obtains a map that shows _every_ road in Europe, and
does their path-selection based on that. Any realistic approach would use
'high-level maps' from which many of the minor roads have been omitted.
However, ignoring those minor roads may result in a path which is longer
than the theoretical minimum.
(And yes, I know, a theoretical-minimum length path would probably be slower
than a path which took the larger limited-access highways... most people want
to optimize time, and the road system itself has been designed to optimize
that goal, e.g. with the constuction of those larger limited-access highways,
so in the real-world there's an interesting interaction between the
map-abstraction process and the infrastructure planning process - maybe the
network world needs to do something similar?)
Noel
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