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

[Stewart Bryant <[e-mail]>]

So, it sounds like we need a new version of the draft clarifying the 
solution's characteristics and allowing the reader to evaluate its 
applicability to their specific network problem.

Publishing an updated text fully addressing the review comments and 
putting it back to the WG for review of the revised draft would seem to 
be the way forward.

Best Regards

Stewart


On 12/07/2018 13:06, [e-mail] wrote:
>
> 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]; [e-mail]; 
> [e-mail]; 
> [e-mail]; [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
>
> Office: [phone]
>
> Cell:      [phone]
>
> Email:   [e-mail]
>
> *From:*[e-mail] [mailto:[e-mail]]
> *Sent:* Thursday, July 12, 2018 12:49 PM
> *To:* Stewart Bryant <[e-mail]>
> *Cc:* [e-mail]; [e-mail]; 
> [e-mail]; 
> [e-mail]; [e-mail]; Ahmed Bashandy 
> <[e-mail]>; Alexander Vainshtein 
> <[e-mail]>; Robert Raszuk <[e-mail]>; 
> Chris Bowers <[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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