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

[<[e-mail]>]

RFC7916 was written at a time when TILFA did not exist. LFA and RLFA provide a set of candidate paths where we need to pick one or more to be installed.

With segment routing, we bring the ability to use any alternate path compared to LFA and RLFA which have a limited amount of candidate paths. As a consequence, we are not exactly in the same backup path selection logic as for LFA and RLFA. If we try to mimic RFC7916 logic with SR, we need to consider the list of candidate paths to be all alternate paths available through the network (so many many !) which may provide some scaling concern especially if we try to involve path attribute collection for the candidate paths. That’s why the logic of TILFA is to focus on the postconvergence path.

To answer your question, IMO, TILFA perfectly fits the criteria: lowest IGP metric.

We could think (as a theoretical exercice) of TILFA using other criterias than the lowest IGP metric. Note that if we think of TILFA in the context of flexalgo, the backup path may require to fit the constraint defined by the flexalgo.



From: Alexander Vainshtein [mailto:[e-mail]]
Sent: Thursday, July 12, 2018 15:16
To: LITKOWSKI Stephane OBS/OINIS
Cc: Robert Raszuk; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; DECRAENE Bruno IMT/OLN
Subject: RE: Request for RTGWG Working Group adoption for draft-bashandy-rtgwg-segment-routing-ti-lfa

Stephane,
Lots of thanks for a prompt response.

We seem to agree on at least one of my objections and the need to remove the associated text from the draft.

Regarding the other one:

1.       Lots of thanks for pointing to RFC 7916 that describes problems with LFA/RLFA selection. BTW, this RFC is not referenced by the TI-LFA draft

2.       One of the mandatory criteria in RFC 7916 is lowest IGP metric used to reach the destination:

a.       Is this equivalent to giving precedence to post-convergence paths?

b.       This is just one of mandatory criteria in RFC 7916 with some recommended criteria as well. These criteria should be evaluated based on their preferences.

3.       One modification of the draft that I can think about could be:

a.       Provide a Normative reference to 7916. In particular, clarify that all mandatory criteria listed in this RFC MUST  be supported

b.       RECOMMEND giving highest preference to the criteria of lowest IGP metric to the destination.

Regards,
Sasha

Office: [phone]
Cell:      [phone]
Email:   [e-mail]

From: [e-mail] [mailto:[e-mail]]
Sent: Thursday, July 12, 2018 3:48 PM
To: Alexander Vainshtein <[e-mail]>; DECRAENE Bruno IMT/OLN <[e-mail]>
Cc: Robert Raszuk <[e-mail]>; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]
Subject: RE: Request for RTGWG Working Group adoption for draft-bashandy-rtgwg-segment-routing-ti-lfa

Hi Sasha,

> This flow will experience two path changes (pre-convergence--> FRR and FRR --> post-convergence
+1, I think that the current  statement in the draft is more a “marketing” one rather than a reality and IMO it may be worth removing it.
As Stewart and you pointed, from an end-to-end point of view the path may change (so the statement is wrong), a node upstream from the failure may reroute the traffic out of the FRR path. And in anyway, the label stack used will change (except in one case) even if the hop by hop logical path does not.

> Post-convergence path is taken into account in the operator’s panning (e.g., by allocating sufficient resources for traffic flows on both pre-convergence and post-convergence paths).

This argument is worth to mention. First of all, the draft does not say that TILFA is magic and prevents the requirement of additional tuning. It says : “there is much less need for the operator
   to tune the decision among which protection path to choose.”.
This statement is perfectly true. With LFA and rLFA, you have high chance to pick a P-PE to protect a core link and depending on your topology, a lot of tuning and policies is required (see RFC7916) to ensure you get a good backup (or sometimes we prefer not having a backup).
TILFA helps here as it can use a loopfree IGP metric optimized path which requires less tuning. I do not say that there will never be a requirement for tuning but it is unlikely.

Brgds,

Stephane



From: rtgwg [mailto:[e-mail]] On Behalf Of Alexander Vainshtein
Sent: Thursday, July 12, 2018 13:26
To: DECRAENE Bruno IMT/OLN
Cc: Robert Raszuk; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: RE: Request for RTGWG Working Group adoption for draft-bashandy-rtgwg-segment-routing-ti-lfa

Bruno,
It seems there is some misunderstanding, and I will try to clarify it.

To the best of my understanding, the following text in Section 1 of the draft presents the benefits of using post-convergence path for FRR:

   As the capacity of the post-convergence path is typically planned by
   the operator to support the post-convergence routing of the traffic
   for any expected failure, there is much less need for the operator
   to tune the decision among which protection path to choose.  The
   protection path will automatically follow the natural backup path
   that would be used after local convergence.  This also 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).

I see two different claims of benefits from using post-convergence path in this test fragment

1.       One path change and therefore one service transient instead of two

2.       Post-convergence path is taken into account in the operator’s panning (e.g., by allocating sufficient resources for traffic flows on both pre-convergence and post-convergence paths).


Speaking just for myself, I think that neither of these claims is justified for traffic flows that do not originate at the PLR.

E.g., consider Stewart’s example and the traffic flow from A to E

1.       This flow will experience two path changes (pre-convergence--> FRR and FRR --> post-convergence

2.       The network operator will not take links C-F, F-G and G-D for consideration in its planning of pre-convergence and post-convergence paths for this flow.

Did I miss something substantial?

Regards,
Sasha

Office: [phone]
Cell:      [phone]
Email:   [e-mail]<mailto:[e-mail]>

From: [e-mail]<mailto:[e-mail]> [mailto:[e-mail]]
Sent: Thursday, July 12, 2018 12:49 PM
To: Stewart Bryant <[e-mail]<mailto:[e-mail]>>
Cc: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; Ahmed Bashandy <[e-mail]<mailto:[e-mail]>>; Alexander Vainshtein <[e-mail]<mailto:[e-mail]>>; Robert Raszuk <[e-mail]<mailto:[e-mail]>>; Chris Bowers <[e-mail]<mailto:[e-mail]>>
Subject: RE: Request for RTGWG Working Group adoption for draft-bashandy-rtgwg-segment-routing-ti-lfa

Stewart,

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

From: Stewart Bryant [mailto:[e-mail]]
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.
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.
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?


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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