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Re: node disjoint and SRLG (was: questions on draft-bryant-ipfrr-tunnels

Subject: Re: node disjoint and SRLG was: questions on draft-bryant-ipfrr-tunnels-01.txt
From: Alia Atlas
Date: Tue, 23 Nov 2004 15:37:10 -0500
At 01:48 PM 11/23/2004, Curtis Villamizar wrote:
In MPLS/TE you simply remove all links from the topology that share a
common SRLG or for some types of SRLG don't remove the links but
weight those links to encourage avoiding them.  Then run the CSPF.

An interesting problem with IP FRR and SRLG is that an SRLG means that
the links *might* go down concurrently.  It does not indicate that the
links *will* go down concurrently.  If two links share a common
resource they will go down simulaneously if failure of that resource
is the cause of the outage.  If you use the above approach and the
failure is due to some other cause, normal hop by hop forwarding could
cause a loop.
The big difference is that with IPFRR, one can't prune the topology, b/c
the topology is what the other nodes will obey. Instead, I think that one
tracks the SRLGs that are traversed. Then a path can be discarded if a
common SRLG is traversed.
It isn't desirable to go to a node that would also be using an alternate,
b/c then you can get looping alternates or need to consider what alternate
that node would have selected. It's much better to go to a node whose
primary path doesn't traverse the common SRLG.
If no such path is available, then one could give weights to SRLGs and
select an alternate appropriately.
MPLS/TE FRR does not suffer from this problem because tunnels are used
in the backup that terminate at the egress where the path is dictated
by the ingress or the PLR (point of local repair).  Therefore MPLS/TE
FRR can select a path which avoids all links that could potentially
fail without being concerned that a routing loop could form due to an
inconsistent decision.

Alia has addressed the problem of local SRLG and has provided a clear
enough description of how that works.  There is some question whether
the more general SRLG problem can be adequately addressed because of
the above problem.
I think that it can; it requires tracking SRLGs, which is unfortunate.

The algorithm description above doesn't seem complete and therefore
doesn't make sense to me but that might be becuase the description was
sketchy or it might be just because I'm being dense.

For example:

      . . . . X
     .        .
    a --- b   .
    |      \ .
    |       e
    |      /
    c --- d

A is trying to get to e and looking to protect against failure of a-b
but a-b and d-e share a common fiber (a, c, d may be near each other
geographicly and b, e may be near each other).  A needs to use a long
path via X.  I don't see how this "far side of the SRLG" idea fits
into an algorithm that solves this problem.  Note that U-turn doesn't
address this since d-e is not local to A.  The SRLG could also be a-b
and c-d in a slightly different example.
One could address this with loop-free or U-turn, depending on tracking
SRLGs traversed by each neighbor or neighbor's neighbor to the destination.

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