[spring] A belated comment on end-to-end path protection in draft-ietf-spring-resiliency-use-cases

[Alexander Vainshtein <[e-mail]>]

Hi all,
I have a belated (but hopefully late is still better than never) comment on path protection as defined in Section 2 of the draft<https://datatracker.ietf.org/doc/draft-ietf-spring-resiliency-use-cases/?include_text=1>.

This second para in this section says:
   A first protection strategy consists in excluding any local repair
   but instead use end-to-end path protection where each SPRING path is
   protected by a second disjoint SPRING path.  In this case local
   protection MUST NOT be used.
First of all, I do not think that RFC 2119 language should be used in Informational documents, especially in the documents that describe use cases.

In addition, I specifically disagree with the quoted statement above, because, from my POV:

*         Local repair and end-to-end path protection can be combined for the same path

*         Such a combination may be beneficial for the operators.

One possible way to combine the two is described below:


1.       A pair of SR paths is set up between the given two nodes - later referred to as source and destination -  in the network. These paths are "SR-disjoint" in the sense that their "explicit routes"  do not have any common elements, be they nodes or adjacencies, with exclusion of the final destination

2.       Local repair for these paths is enabled in the network. It is triggered by locally observed events (link failures etc.), applied by the nodes adjacent to the failure and guarantees that, in the case of a link or node failure that is not specified in the explicit route, traffic along the affected path would be restored within <X> milliseconds

3.       End-to-end liveness monitoring is enabled for the two SR paths, and detects end-to-end failures of these paths within <Y> milliseconds where Y >> X. In other words, end-to-end liveness monitoring for these paths will ignore any failures that local repair can fix, but will detect failures that cannot be locally repaired (e.g., failures of nodes or links that have been specified in the explicit route of one of the paths

4.       End-to-end liveness monitoring triggers end-to-end path protection to be applied by the source node in the following way:

a.       If it recognizes both paths as alive, one of them will carry the customer traffic, while the other one will be idle. The rules for selecting the active path in this scenario may vary

b.      If end-to-end failure of one of these paths is detected while the other one remains alive, traffic will be carried across the live path

c.       If end-to-end failure of both paths is detected (e.g., if the final destination node fails, or if the network is partitioned), this is recognized as an unrecoverable failure.

>From my POV the combination of local repair and end-to-end protection for SR paths is one of a few possibilities to protect such paths against failures of nodes and/or links that have been specified in their explicit routes. (Another option has been described in Node Protection for SR-TE Paths<https://tools.ietf.org/html/draft-hegde-spring-node-protection-for-sr-te-paths-00>, but this draft has expired).

Do I miss something substantial?

Regards,
Sasha

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