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Re: questions on draft-bryant-ipfrr-tunnels-01.txt

Subject: Re: questions on draft-bryant-ipfrr-tunnels-01.txt
From: Stewart Bryant
Date: Fri, 19 Nov 2004 12:28:53 +0000
OK, we will describe how to handle SRLG in the next version.

However unknown SRLGs will affect all solutions, so if
SRLG is a MUST, then all solutions need text describing
their behavior under conditions of unknown SRLG. In
particular unknown SRLG can result in mutually looping
repairs (which could even be cyclic) and the solution
must describe how to detect and break these loops.

Curtis Villamizar wrote:
In message <[email protected]>
Stewart Bryant writes:

We also have concerns about the relative frequency of known SRLGs
members vs unknown SRLG members. Clearly if the proportion of
unknown SRLG members is significant, then the usefulness of SRLG
aware repairs is questionable. We decided that we really needed
some answer to this question before pursuing this additional
complication to the solution.

If unknown SRLG is a problem and not supporting SRLG makes all SRLG
behave like unknown SRLG, then an approach which does not support SRLG
is more problematic than an approach which does support SRLG.  The
only case where the two are equal (approaches that do and don't
support SRLG) is where SRLG do not exist which we know is not the case
in any real network.

Therefore I think any argument opposing the support of SRLG due to the
possibility (or certainty) that unknown SRLG will exist is quite flawed.

For example, WDM introduces SRLG that have to be considered for a fast
restoration approach to be at all viable.  Logical links that traverse
hundreds of miles on the same fiber are going to have a high
percentage of correlated failure.


ps - we not return you to our regularly scheduled arguments about why
SRLG are a hard problem for some approaches.
If tunnels were to perform a per prefix search for a two hop
solution the results would be identical to that of U-turn.
Therefore tunnels are a true superset of the U-turn approach.
For simplicity we did not describe this per-prefix search,
because we achieved complete coverage without it in all the
networks we examined.

However give that tunnels are a superset of U-turns, then
any SRLG issue applies equally to both.

- Stewart

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