RE: I-D Action: draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt

Stewart

I already replied to Sikhi explaining the concept of the SRLG used in this draft and the intent to make it even clearer.

IMO the scope of the draft is very clear from the draft itself as well as the numerous responses during the previous IETF and the mailing list.

The issue below is *out of scope* of the draft and hence I have no plans on addressing it.

I hope you don't insist on pushing out-of-scope topics down the throat of this draft :)

Ahmed

From: Stewart Bryant [mailto:stewart.bryant@gmail.com]
Sent: Monday, August 07, 2017 12:48 PM
To: Ahmed Bashandy (bashandy); Stewart Bryant; Sikhivahan Gundu; rtgwg@ietf.org
Cc: rtgwg-chairs@ietf.org; pfrpfr@gmail.com
Subject: Re: I-D Action: draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt

Your answer did not address the issue below, which is one of a class of issues related to SRLG.

- Stewart

On 07/08/2017 19:23, Ahmed Bashandy (bashandy) wrote:
See my reply to Sikhi

Thanks

Ahmed

On 8/7/2017 2:13 AM, Stewart Bryant wrote:

On 07/08/2017 06:45, Sikhivahan Gundu wrote:

By "ambiguity", I meant that backup calculation taking SRLG into
account is  based on speculated topology,  whereas computation of
post-convergence path, ie, SPF, is based on actual topology.  This
seems needs reconciling since in  TI-LFA the backup is by definition
the post-convergence path, with a single path-transition after
link-failure as the intended outcome. Do I understand correctly that
the draft prefers to relax that expectation for SRLG?

Yes, that is a good point, in the event of an incomplete failure
of an SRLG, there may not be congruence between the
FRR path and the post convergence path. This certainly
needs further study.

   *
A--------//---------B
|                   |
|  *                | cost 2
C-------------------D
|                   |
|                   | cost 100
E-------------------F

AB + CD in same SRLG

TiLFA path is ACEFDB

Post convergence path is ACDB

In this case I think that the impact is just more SR hops in the
repair path than might be needed without the SRLG, but we do need to
be sure  that there are no pathological  cases in
topologies that lack the proposed congruence, and as
Sikhivahan notes this effect does need to be clarified in the
text.

- Stewart

_______________________________________________

rtgwg mailing list

rtgwg@ietf.org<mailto:rtgwg@ietf.org>

https://www.ietf.org/mailman/listinfo/rtgwg