> From: Curtis Villamizar <curtis@workhorse.fictitious.org>
>> 1) Topology element naming.
>> 2) Topology information distribution.
>> 3) Path computation.
>> 4) Path setup.
>> 5) User packet forwarding.
> Those are subsystems of a specific architecture, one that exists
> mostly on paper and in a few research projects.
You mean PNNI, right? :-)
> Not all architectures use a path setup before path forwarding and
> it is arguably a bad idea in many cases.
First, let me point out that path setup may not be triggered by "reception of
user data packet"; some may happen at system startup, to provide "default"
forwarding for all destinations, so that "best-effort" packets just use those
pre-setup paths. In other words, "best-effort" packets can be handled by the
system *without* any path setup.
Second, all routing architectures do these steps - it's just that they may
intermingle some of them. E.g. even our old pal BGP, there is a "setup" phase
(in which routing table entries are created), it's just that it happens at the
same time as 2 and 3 above (and in a distributed way). Those "classical"
routing table entries form a (hopefully :-) tree-structured set of paths
toward the destination. So the only question is not whether you will do path
setup, but only whether it will be done as a separate, visible (and
potentially user-accessible) step.
> For example, path setup before a single DNS UDP packet to port 53
> is not part of the current architecture and would be inefficient.
And the system which I suspect you're thinking, the one that actually embodied
that 5-part division, did not do that. Perhaps you should actually know
something about it before you make any judgements about it?
> your "list of subsystems" constrain the architecture
I think my "list of subsystems" is *the* fundamental model of how all
routing systems more or less have to work. Of course, some designs have
lumped more than one phase together in a single mechanism (all DV/PV
systems mingle 2, 3 and to some degree 4) - but those elements are all
still there.
Perhaps I'm wrong, and one of the steps can be decomposed further. If so, I'd
dearly love to hear about it (really).
(I should point out that some of these "top-level" subsystems have some
further internal divisions; e.g. the entire subject of topology information
distribution is a world in and of itself. However, I don't consider those
sub-divisions to be on the same level, in architectural terms, as the ones I
listed.)
Noel
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