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/ > > > > _______________________________________________ > > I-D-Announce mailing list > > I-D-Announce@ietf.org <mailto:I-D-Announce@ietf.org> > > https://www.ietf.org/mailman/listinfo/i-d-announce > > Internet-Draft directories:http://www.ietf.org/shadow.html > > orftp://ftp.ietf.org/ietf/1shadow-sites.txt >
- Fwd: I-D Action: draft-bashandy-rtgwg-segment-rou… Ahmed Bashandy (bashandy)
- RE: I-D Action: draft-bashandy-rtgwg-segment-rout… Sikhivahan Gundu
- Re: I-D Action: draft-bashandy-rtgwg-segment-rout… Ahmed Bashandy (bashandy)
- RE: I-D Action: draft-bashandy-rtgwg-segment-rout… Sikhivahan Gundu
- Re: I-D Action: draft-bashandy-rtgwg-segment-rout… Stewart Bryant
- Re: I-D Action: draft-bashandy-rtgwg-segment-rout… Ahmed Bashandy (bashandy)
- Re: I-D Action: draft-bashandy-rtgwg-segment-rout… Ahmed Bashandy (bashandy)
- Re: I-D Action: draft-bashandy-rtgwg-segment-rout… Stewart Bryant
- Re: I-D Action: draft-bashandy-rtgwg-segment-rout… Stewart Bryant
- RE: I-D Action: draft-bashandy-rtgwg-segment-rout… Ahmed Bashandy (bashandy)
- Re: I-D Action: draft-bashandy-rtgwg-segment-rout… Stewart Bryant
- Re: I-D Action: draft-bashandy-rtgwg-segment-rout… Ahmed Bashandy (bashandy)
- Re: I-D Action: draft-bashandy-rtgwg-segment-rout… Stewart Bryant