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

[Stewart Bryant <[e-mail]>]

On 12/07/2018 10:49, [e-mail] wrote:
>
> 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.
>
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.

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.

- 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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