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

"Ahmed Bashandy (bashandy)" <bashandy@cisco.com> Mon, 07 August 2017 18:23 UTC

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Date: Mon, 07 Aug 2017 11:23:11 -0700
From: "Ahmed Bashandy (bashandy)" <bashandy@cisco.com>
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To: Sikhivahan Gundu <sikhivahan.gundu@ericsson.com>, "rtgwg@ietf.org" <rtgwg@ietf.org>
CC: "rtgwg-chairs@ietf.org" <rtgwg-chairs@ietf.org>, "pfrpfr@gmail.com" <pfrpfr@gmail.com>, Stewart Bryant <stewart@g3ysx.org.uk>
Subject: Re: I-D Action: draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt
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I am not aware of such thing as "speculative, ambiguous, probabilistic, 
stocastic,..., etc" SRLG.

SRLG is one member fails, all members fail. I presumed that this is 
understood from the many responses and discussions that we had. However 
I will explicitly define the term "SRLG" in the draft

Thanks

Ahmed

On 8/6/2017 10:45 PM, Sikhivahan Gundu wrote:
>
> Hi,
>
> Thanks for your response.
>
> >> - Hence if the primary link fails, only "L1" will fail and L2 will not
>
> L1 _/may/_ fail, with high probability, but it may also not fail. If 
> it does
>
> not fail, there is a second transitioning of the post-primary-failure
>
> link from FRR-backup (L2) to post-convergence link (L1), because L1
>
> has a smaller metric.
>
> 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?
>
> Thanks,
>
> Sikhi
>
> *From:*Ahmed Bashandy (bashandy) [mailto:bashandy@cisco.com]
> *Sent:* 05 August 2017 01:19
> *To:* Sikhivahan Gundu <sikhivahan.gundu@ericsson.com>; rtgwg@ietf.org
> *Cc:* rtgwg-chairs@ietf.org; pfrpfr@gmail.com; Stewart Bryant 
> <stewart@g3ysx.org.uk>
> *Subject:* Re: I-D Action: 
> draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt
>
> HI,
>
> All members of the same SRLG group are assumed to fail if one of them 
> fails.
>
> Going back to you example
> - L1 is in the same SRLG group as the primary link while L2 is belongs 
> a different group
> - Hence if the primary link fails, only "L1" will fail and L2 will not
> - Hence only L2 is candidate to become a backup path while L1 is not
> - Hence there is no ambiguity
>
> Thanks
>
> Ahmed
>
> On 8/1/2017 12:42 AM, Sikhivahan Gundu wrote:
>
>     Hi,
>
>     The draft mandates using "post-convergence path" as the backup path.
>
>     It states one advantage, among others, of doing so as follows:
>
>     "This .. helps to reduce the amount of path changes and hence service
>
>     transients: one transition (pre-convergence to post-convergence)
>     instead
>
>     of two (pre-convergence to FRR and then post-convergence)".
>
>     This suggests to me that the assumption here is that the
>     post-convergence
>
>     path can be uniquely determined in advance.
>
>     However, SRLG introduces ambiguity. To illustrate the point,  let
>     us say a
>
>     loop-free alternative has two options: one  link (L1) is of the
>     same metric
>
>     value as the primary link and is also in the same SRLG as the
>     primary; the
>
>     second option (L2) is in a different SRLG and has higher metric.
>
>     The actual post-convergence path would depend on whether or not L1
>
>     also failed along with the primary, so is not uniquely computed in
>     advance.
>
>     If TI-LFA picks L1, there might not be a guaranteed backup. If it
>     picks L2,
>
>     there'd be two link transitions because L2 would not be in a
>     (strict) SPF-
>
>     computed post-convergence path. A third option, of course, is to
>     give up
>
>     declaring that there is no TI-LFA backup, but it'd be preferable
>     to have
>
>     some backup than have none at all.
>
>     What do the authors suggest for this situation?
>
>     Thanks,
>
>     Sikhi
>
>       
>
>     *From:*rtgwg [mailto:rtgwg-bounces@ietf.org] *On Behalf Of *Ahmed
>     Bashandy (bashandy)
>     *Sent:* 17 July 2017 12:56
>     *To:* rtgwg@ietf.org <mailto:rtgwg@ietf.org>
>     *Cc:* rtgwg-chairs@ietf.org <mailto:rtgwg-chairs@ietf.org>;
>     pfrpfr@gmail.com <mailto:pfrpfr@gmail.com>; Stewart Bryant
>     <stewart@g3ysx.org.uk> <mailto:stewart@g3ysx.org.uk>
>     *Subject:* Fwd: I-D Action:
>     draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt
>
>     Hi,
>
>     A new version of the ti-lfa draft has been posted to address
>     Stewart Bryant's comments
>
>     Thanks
>
>     Ahmed
>
>
>     -------- Original Message --------
>
>     *Subject: *
>
>     	
>
>     I-D Action: draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt
>
>     *Date: *
>
>     	
>
>     Mon, 17 Jul 2017 00:19:37 -0700
>
>     *From: *
>
>     	
>
>     internet-drafts@ietf.org <mailto:internet-drafts@ietf.org>
>
>     *Reply-To: *
>
>     	
>
>     internet-drafts@ietf.org <mailto:internet-drafts@ietf.org>
>
>     *To: *
>
>     	
>
>     <i-d-announce@ietf.org> <mailto:i-d-announce@ietf.org>
>
>     A New Internet-Draft is available from the on-line Internet-Drafts directories.
>
>       
>
>       
>
>              Title           : Topology Independent Fast Reroute using Segment Routing
>
>              Authors         : Ahmed Bashandy
>
>                                Clarence Filsfils
>
>                                Bruno Decraene
>
>                                Stephane Litkowski
>
>                                Pierre Francois
>
>              Filename        : draft-bashandy-rtgwg-segment-routing-ti-lfa-01.txt
>
>              Pages           : 12
>
>              Date            : 2017-07-17
>
>       
>
>     Abstract:
>
>         This document presents Topology Independent Loop-free Alternate Fast
>
>         Re-route (TI-LFA), aimed at providing protection of node and
>
>         adjacency segments within the Segment Routing (SR) framework.  This
>
>         Fast Re-route (FRR) behavior builds on proven IP-FRR concepts being
>
>         LFAs, remote LFAs (RLFA), and remote LFAs with directed forwarding
>
>         (DLFA).  It extends these concepts to provide guaranteed coverage in
>
>         any IGP network.  A key aspect of TI-LFA is the FRR path selection
>
>         approach establishing protection over post-convergence paths from
>
>         the point of local repair, dramatically reducing the operational
>
>         need to control the tie-breaks among various FRR options.
>
>       
>
>       
>
>     The IETF datatracker status page for this draft is:
>
>     https://datatracker.ietf.org/doc/draft-bashandy-rtgwg-segment-routing-ti-lfa/
>
>       
>
>     There are also htmlized versions available at:
>
>     https://tools.ietf.org/html/draft-bashandy-rtgwg-segment-routing-ti-lfa-01
>
>     https://datatracker.ietf.org/doc/html/draft-bashandy-rtgwg-segment-routing-ti-lfa-01
>
>       
>
>     A diff from the previous version is available at:
>
>     https://www.ietf.org/rfcdiff?url2=draft-bashandy-rtgwg-segment-routing-ti-lfa-01
>
>       
>
>       
>
>     Please note that it may take a couple of minutes from the time of submission
>
>     until the htmlized version and diff are available at tools.ietf.org.
>
>       
>
>     Internet-Drafts are also available by anonymous FTP at:
>
>     ftp://ftp.ietf.org/internet-drafts/
>
>       
>
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>
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