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

<bruno.decraene@orange.com> Thu, 12 July 2018 15:42 UTC

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From: <bruno.decraene@orange.com>
To: Stewart Bryant <stewart.bryant@gmail.com>
CC: "rtgwg-chairs@ietf.org" <rtgwg-chairs@ietf.org>, "pfrpfr@gmail.com" <pfrpfr@gmail.com>, "draft-bashandy-rtgwg-segment-routing-ti-lfa@ietf.org" <draft-bashandy-rtgwg-segment-routing-ti-lfa@ietf.org>, "daniel.voyer@bell.ca" <daniel.voyer@bell.ca>, "rtgwg@ietf.org" <rtgwg@ietf.org>, Ahmed Bashandy <abashandy.ietf@gmail.com>, "Alexander Vainshtein" <Alexander.Vainshtein@ecitele.com>, Robert Raszuk <robert@raszuk.net>, Chris Bowers <cbowers@juniper.net>
Subject: RE: Request for RTGWG Working Group adoption for draft-bashandy-rtgwg-segment-routing-ti-lfa
Thread-Topic: Request for RTGWG Working Group adoption for draft-bashandy-rtgwg-segment-routing-ti-lfa
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Date: Thu, 12 Jul 2018 15:42:44 +0000
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Please see inline [Bruno2]

From: Stewart Bryant [mailto:stewart.bryant@gmail.com]
Sent: Thursday, July 12, 2018 3:29 PM

On 12/07/2018 10:49, bruno.decraene@orange.com<mailto:bruno.decraene@orange.com> wrote:
Stewart,

Please see 1 comment inline [Bruno]
Trimming the text to ease the focus on this point

From: Stewart Bryant [mailto:stewart.bryant@gmail.com]
Sent: Tuesday, July 10, 2018 2:40 PM



On 09/07/2018 20:53, Ahmed Bashandy wrote:
[…]



b.       Selecting the post-convergence path (inheritance from draft-francois) does not provide for any benefits for traffic that will not pass via the PLR after convergence.

                                                               i.      The authors claim to have addressed this issue by stating that “Protection applies to traffic which traverses the Point of Local Repair (PLR). Traffic which does NOT traverse the PLR remains unaffected.”

SB> It is not as simple as that, and I think that the draft needs to provide greater clarity.

I think there will be better examples, but consider

              12
      +--------------+
      |              |
A-----B-----C---//---D----E
        10  |        |
            F--------G

Traffic injected at C will initially go C-D-E at cost 2, will be repaired C-F-G-D-E at cost 4 and will remain on that path post convergence. This congruence of path is what TI-LFA claims.

However, a long standing concern about traffic starting further back in the network needs to be more clearly addressed in the draft to clearly demonstrate the scope of applicability.

For traffic starting at A, before failure the path is A-B-C-D-E cost 13

TI-LFA will repair to make the path A-B-C-F-G-D-E cost 15 because TI-LFA optimises based on local repairs computed at C.

After repair the path will be A-B-D-E cost 14.

[Bruno] The draft is about IP Fast ReRoute (FRR).
FRR is a local reaction to failure, so by hypothesis, all nodes but the PLR are not aware about the failure. This includes all upstream nodes which do keep forwarding traffic through the same path, i.e. via the PLR.
Correct

The argument that the path would have been shorter if upstream node were aware of the failure to reroute before (or that the PLR should send the packet back in time) is not relevant.
That is not the point I am making.

The only question which matter is: from the PLR to the destination, which is the best path to use?
I, and the draft, argue that the best path in IP routing, is the IGP shortest path. Whichever type of metric you choose (e.g. bandwidth, latency, cost…). Do you disagree on this?
Correct, but you miss the point I am making.

The draft goes to considerable effort to constrain the FRR path to the path that the traffic arriving at the PLR will take post failure. However, the point illustrated by the network fragment is that not all that traffic benefits from that effort. In the draft you assert post convergence advantage to you approach, but do not seem to make it clear that this is a partial benefit and not a universal benefit.

[Bruno2] May be the term “post convergence” is misleading as it may refers to IGP convergence, while the draft is limited to a fast re-route solution. i.e. reaction of the PLR. FRR coverage is limited to 1) traffic reaching the PLR 2) before the IGP convergence. Within this FRR limit, the benefit applies to all this traffic.
But you are right that this is not all the traffic in the network, and the text needs to be updated if this is not clear enough



Depending on the specific advantage of constraining the path, this might be worth the complexity, or it might be better to use RLFA, or MRT or any one of the other technologies.

Also you really need to make it much clearer that the uloop avoidance properties (a big selling point of this technology) only apply to the traffic that will continue to arrive at C and that if any traffic will take another path you MUST implement an avoidance strategy.

[Bruno2] Again this draft is limited to IP FRR. As you stated, there is no loops during FRR, except when the failure is more expected than assumed/computed for.
Micro-loops happen during the sub-sequent IGP convergence which is out of scope of this FRR document. There is another draft about avoiding the micro-loop during the IGP convergence (draft-bashandy-rtgwg-segment-routing-uloop). Same issue with RLFA https://tools.ietf.org/html/rfc7490#section-10

I personally tend to agree with you that micro-loop avoidance is a must, but that is not the formulation used in the RLFA RFC: “If it is determined that micro-loops are a significant issue in the deployment, then a suitable loop-free convergence method, such as one of those described in [RFC5715<https://tools.ietf.org/html/rfc5715>]5>], [RFC6976<https://tools.ietf.org/html/rfc6976>]6>], or [ULOOP-DELAY<https://tools.ietf.org/html/rfc7490#ref-ULOOP-DELAY>]Y>], should be implemented.” For consistency, I’d rather favor reusing the same sentence (although on the solution space, I’d rather point to draft-bashandy-rtgwg-segment-routing-uloop which IMO is more attractive in segment routing networks)

--Bruno

- Stewart





Now, eventually we can narrow down the discussion to the choice of terms. We can discuss about the term “post-convergence paths from the point of local repair », which you don’t think to like. Although, the term seems technically true to me, I would also be fine with changing from  “post-convergence path” to “optimal IGP shortest path”



So the draft needs to make it clear to the reader that TI-LFA only provides benefit to traffic which traverses the PLR before and after failure.

[Bruno] No, that is not true. cf above.
--Bruno



Traffic which does not pass through the PLR after the failure will need to be traffic engineered separately from traffic that passes though the PLR in both cases.






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