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

[Alexander Vainshtein <[e-mail]>]

Bruno,
Lots of thanks for your prompt and clarifying response.

I am also trying to differentiate between technical behavior and “motivation” claims. This is not always easy.

Stephane has provided a pointer to RFC 7916 that lists lowest IGP metric of the repair path as one of the mandatory criteria for selecting LFA/RLFA.

Is this roughly equivalent to your statement that “the shortest path from the PLR to the destination is the best path (that we can choose from)”?

If yes, then it would make sense to me state that TI-LFA should use this (and all other) criteria and techniques defined in RFC 7916 (and include a Normative reference to it, of course).


Regards,
Sasha

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From: [e-mail] [mailto:[e-mail]]
Sent: Thursday, July 12, 2018 3:07 PM
To: Alexander Vainshtein <[e-mail]>
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Subject: RE: Request for RTGWG Working Group adoption for draft-bashandy-rtgwg-segment-routing-ti-lfa

Sasha,

Please see inline [Bruno]

From: Alexander Vainshtein [mailto:[e-mail]]
Sent: Thursday, July 12, 2018 1:26 PM
To: DECRAENE Bruno IMT/OLN
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; Robert Raszuk; Chris Bowers; Stewart Bryant
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?
[Bruno] I think we should distinguish 2 aspects:
a) the technical behavior
b) the text/claims in the draft

My answer was about “a”. I see that you are not challenging it.
Regarding “b”, speaking for myself, I’m primarily interesting in the technical specification, less about the text introduced to motivate the solution. That being said:
1) If it were up to me, I would personally be very open to completely rewrite the text you cited. e.g. using my text ;-) (below).
2) I mostly agree with you. Although possibly the text could be rephrased to say that it refers to the local behavior on the PLR rather than the end to end path in the network. Also, capacity planning is very topology dependent and SP dependent. Here, Stewart’s example is custom-built to highlight a case where capacity planning for TI-LFA is different than capacity planning for link failure. I personally don’t find the example very typical of real network, as typically it’s more efficient to try to share the backup capacity for both link and node failure, which is not the case in the example.
That being said, the example is good enough to say that the capacity planning claim is not guaranteed. Again, cf my point 1. i.e. I support changing this text.

Coming back to the technical behavior, if we agree that using the shortest path from the PLR to the destination is the best path (that we can choose from) that’s good enough for me.

Regards,
--Bruno

Regards,
Sasha

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Cell:      [phone]
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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]>>
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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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