Re: Is TI-LFA compatible with the default SR algorithm?

[Stewart Bryant <[e-mail]>]

RFC5714 was written when we were focused on routing in base along normal 
Dijkstra shortest paths.

As policy and flexible algorithms become prevalent we need to specify 
that any RFC5714 action is compliant to the policies and intended 
algorithms along the entirety of its path.

We need to think this through carefully, but that may mean that in a 
network where any of these enhanced routing methods are deployed, ALL 
routers doing FRR need to validate the safety of the packet path. There 
may be more computationally efficient ways to do this, but worse case 
looks like needing to precompute the full path to all destinations 
reachable via the failure including applying the policy and algorithms 
installed on all of those routers. What I am unclear of is how we find 
that policy to do that.

Maybe I am being overly pessimistic here, but Sasha makes an interesting 
observation that we need to think through very carefully.

- Stewart


On 14/06/2018 11:56, [e-mail] wrote:
>
> Hi Sasha,
>
> Could you elaborate on :” I strongly suspect that it is not so, and 
> that these mechanisms are only compatible with the Strict-SPF. 
> (Actually, I can provide an example that confirms this suspicion.)” ?
>
> Thanks,
>
> *From:*spring [mailto:[e-mail]] *On Behalf Of 
> *Alexander Vainshtein
> *Sent:* Wednesday, June 13, 2018 17:00
> *To:* [e-mail]
> *Cc:* Stewart Bryant; Michael Gorokhovsky; 
> [e-mail]; [e-mail]; 
> [e-mail]
> *Subject:* [spring] Is TI-LFA compatible with the default SR algorithm?
>
> Hi all,
>
> I have looked up Section 3.1.1 “*Prefix-SID Algorithm*” of the Segment 
> Routing Architecture 
> <https://tools.ietf.org/html/draft-ietf-spring-segment-routing-15> 
> draft (already In the RFC Editor queue) and found there the following 
> statement (the relevant part is highlighted):
>
> This document defines two algorithms:
>
>    o  "Shortest Path": this algorithm is the default behavior.  The
>
>       packet is forwarded along the well-known ECMP-aware SPF algorithm
>
>       employed by the IGPs.  However it is explicitly allowed for a
>
>       midpoint to implement another forwarding based on local policy.
>
>       The "Shortest Path" algorithm is in fact the default and current
>
>       behavior of most of the networks where local policies may override
>
>       the SPF decision.
>
>    o  "Strict Shortest Path (Strict-SPF)": This algorithm mandates that
>
>       the packet is forwarded according to ECMP-aware SPF algorithm and
>
>       instructs any router in the path to ignore any possible local
>
>       policy overriding the SPF decision.  The SID advertised with
>
>       Strict-SPF algorithm ensures that the path the packet is going to
>
>       take is the expected, and not altered, SPF path. Note that Fast
>
> Reroute (FRR) [RFC5714 <https://tools.ietf.org/html/rfc5714>] 
> mechanisms are still compliant with the
>
> Strict Shortest Path.  In other words, a packet received with a
>
> Strict-SPF SID may be rerouted through a FRR mechanism.
>
> At the same time, the TI-LFA draft 
> <https://tools.ietf..org/html/draft-bashandy-rtgwg-segment-routing-ti-lfa-04> 
> discusses protection of active Prefix-SIDs (e.g., in Section 3 that 
> discusses P-Space and Q-space) but, to the best of my understanding, 
> does not mention algorithms that form the context of these SIDs.
>
> My question to the authors of the TI-LFA draft is:
>
> Are the mechanisms defined in the draft (and examples discussed in 
> Section 4) applicable to Prefix-SIDs associated with the default 
> forwarding algorithm as defined in the Segment Routing Architecture draft?
>
> I strongly suspect that it is not so, and that these mechanisms are 
> only compatible with the Strict-SPF. (Actually, I can provide an 
> example that confirms this suspicion.)
>
> Do I miss something substantial here?
>
> Regards, and lots of thanks in advance,
>
> Sasha
>
> Office: [phone]
>
> Cell:      [phone]
>
> Email:   [e-mail]
>
>
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