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, [email protected] 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:[email protected]] *On Behalf Of
*Alexander Vainshtein
*Sent:* Wednesday, June 13, 2018 17:00
*To:* [email protected]
*Cc:* Stewart Bryant; Michael Gorokhovsky;
[email protected]; [email protected];
[email protected]
*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: +972-39266302
Cell: +972-549266302
Email: [email protected]
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