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

Subject: Re: questions on draft-bryant-ipfrr-tunnels-01.txt
From: Alia Atlas
Date: Mon, 22 Nov 2004 13:14:36 -0500
At 07:28 AM 11/19/2004, Stewart Bryant wrote:
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.
Unknown SRLGs are uncorrelated failures.
The detection of uncorrelated failures is by the IGP, which can then install the new primaries as quickly as possible. In the meantime, traffic loops until TTL expires.
IPFRR is quite clearly not handling uncorrelated failures; no protection
mechanism that I am aware of does.
Curtis Villamizar wrote:

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.
In terms of alternates provided, but not in terms of forwarding or complexity.

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.
The algorithm for computing U-turn alternates is a bit different from the
algorithm that you are using to look for tunnels.
Therefore, the computational impact of SRLGs may be different as well.


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