Re: Request for RTGWG Working Group adoption for draft-bashandy-rtgwg-segment-routing-ti-lfa

Stewart Bryant <> Fri, 13 July 2018 12:57 UTC

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Subject: Re: Request for RTGWG Working Group adoption for draft-bashandy-rtgwg-segment-routing-ti-lfa
To:, Alexander Vainshtein <>, DECRAENE Bruno IMT/OLN <>
Cc: Robert Raszuk <>, "" <>, "" <>, "" <>, "" <>, "" <>
References: <> <> <> <> <> <> <> <> <30046_1531388953_5B472419_30046_355_1_53C29892C857584299CBF5D05346208A47AEA48D@OPEXCLILM21.corporate.adroot.infra.ftgroup> <> <9235_1531399667_5B474DF3_9235_106_2_ec03f05d-ff9c-4680-9c6b-8a975430ea9e@OPEXCLILM24.corporate.adroot.infra.ftgroup> <> <29234_1531485472_5B489D20_29234_5_1_7d70ca00-8630-456a-b620-03da4acbf685@OPEXCLILM43.corporate.adroot.infra.ftgroup>
From: Stewart Bryant <>
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Date: Fri, 13 Jul 2018 13:57:22 +0100
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On 13/07/2018 13:37, wrote:
> Hi Stewart,
> TILFA is not intended to provide a loop free path at all phases of the 
> convergence process.
I need to read the document carefully and look at the slides, because I 
am sure that the
original claim was that it prevented loops during convergence.

> As any FRR technique (or most), it may suffer from micro-looping when 
> the PLR has converged and rewrites the path out of the computed FRR path.
> TILFA just encodes the postconvergence path seen from the PLR point of 
> view in a loop free manner and the resulting label stack is used only 
> during the FRR.

So can we have a clear statement in the document of the exact benefits 
and costs of the TI-LFA approach of constraining the path vs the 
alternative approaches?

> Global microloop avoidance is also doable using similar mechanism as 
> TILFA, but that’s another story.
> RFC8333 (local delay) can also be used in conjunction with TILFA to 
> protect against local microloops that are caused by a link down.

Indeed, lets clarify all this in the text.

We also need to clarify in the text that an abandon all hope system is 
required in the event that the failure is worse than anticipated.

As I said earlier, we seem to me agreeing on the limits of the system, 
now we need a new version that puts that in writing.

Best regards


> *From:*Stewart Bryant []
> *Sent:* Thursday, July 12, 2018 15:47
> *To:* LITKOWSKI Stephane OBS/OINIS; Alexander Vainshtein; DECRAENE 
> Bruno IMT/OLN
> *Cc:* Robert Raszuk;;; 
> *Subject:* Re: Request for RTGWG Working Group adoption for 
> draft-bashandy-rtgwg-segment-routing-ti-lfa
> On 12/07/2018 13:47, 
> <> wrote:
>     TILFA helps here as it can use a loopfree IGP metric optimized path
> All IPFRR paths are loop free against the number of failures that they 
> set out to guard against.
> However not all techniques are loop-free at all phases of convergence, 
> and you now recognize that to be the case with this method.
> Some methods are unconditionally stable against any degree of failure 
> (down stream repair), although this is achieved at a cost of reduced 
> coverage. All other methods are potentially looping and need a method 
> of retrieving the situation which I did not see documented in this draft.
> - Stewart
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