Subject: Re: [Isis-wg] too many segments

From: Radia Perlman - Boston Center for Networking <[email-masked]>
Date: Sun, 11 Jun 2000 21:35:54 -0400 (EDT)

The 1 byte fragment number is a field that in retrospect should have
been larger.  As Toni Li pointed out, it should be possible to get around
it with the hack of pretending to be two (or more) nodes with zero cost
between them, and have part of the information reported by each one.
Do people actually do this? Do all implementations calculate correctly
when they see this? Does the Dijkstra calculation do anything weird
with 0-cost adjacencies? (It should work, but I wouldn't be surprised
if some implementations got very confused.)

Certainly it has nothing to do with overflow state if a router has
more information to report than fits into 255 fragments.

If this really is a big problem, it would probably be possible to change
the size of the field, but it's tricky to do it in a way without flag
days, or in a way that would work if someone accidentally plugs in
a router that doesn't know about it.

Radia

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From: Tony Li <[email-masked]>
Date: Sun, 11 Jun 2000 22:11:14 -0700 (PDT)

| The 1 byte fragment number is a field that in retrospect should have
 | been larger.  As Toni Li pointed out, it should be possible to get around
 | it with the hack of pretending to be two (or more) nodes with zero cost
 | between them, and have part of the information reported by each one.
 | Do people actually do this? Do all implementations calculate correctly
 | when they see this? Does the Dijkstra calculation do anything weird
 | with 0-cost adjacencies? (It should work, but I wouldn't be surprised
 | if some implementations got very confused.)

As a pseudo-node adjacency is already at zero cost, there's a fairly high
chance that this will work in all implementations.  And given that the
predominant implementations have a fairly restricted set of authors, who
are reasonably general, there's an excellent chance that it Just Works.

Tony

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From: Mike Shand <[email-masked]>
Date: Mon, 12 Jun 2000 11:10:32 +0100

>are reasonably general, there's an excellent chance that it Just Works.

Yes, the thing about the pseudonode is that it is carefully crafted such 
that the zero cost is only in one direction. Dijkstra's algorithm can get 
upset if the set of LSPs contains a loop who's cost sum is zero. This is 
avoided in the pseudonode case, but here we would want to have zero cost in 
BOTH directions (not least, to satisfy the two way check). Its not clear 
that all implementations would necessarily take kindly to that.

The other thing about the zero cost links in pseudonodes, is that you are 
supposed to give priority to selecting pseudonode LSPs from TENT where 
there are non-pseudonode LSPs present at equal cost. If we didn't do this 
and there were equal cost paths one of which was a direct link and the 
other through the pseudonode, we could fail to detect the path through the 
pseudonode, because the destination node would already have been added to 
PATHs and we can't (don't) increment the adjacency set of node already on 
PATHS.

At least I think that was the rationale. So if we have zero cost links 
which aren't in pseudonodes we might upset an implementation which was 
doing this (as it is supposed to) on the basis of the LSP being a 
pseudonode rather than the more general property that it carried zero cost 
links.

         Mike

         Mike