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

[<[e-mail]>]

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]


___________________________________________________________________________

This e-mail message is intended for the recipient only and contains information which is
CONFIDENTIAL and which may be proprietary to ECI Telecom. If you have received this
transmission in error, please inform us by e-mail, phone or fax, and then delete the original
and all copies thereof.
___________________________________________________________________________

_________________________________________________________________________________________________________________________

Ce message et ses pieces jointes peuvent contenir des informations confidentielles ou privilegiees et ne doivent donc
pas etre diffuses, exploites ou copies sans autorisation. Si vous avez recu ce message par erreur, veuillez le signaler
a l'expediteur et le detruire ainsi que les pieces jointes. Les messages electroniques etant susceptibles d'alteration,
Orange decline toute responsabilite si ce message a ete altere, deforme ou falsifie. Merci.

This message and its attachments may contain confidential or privileged information that may be protected by law;
they should not be distributed, used or copied without authorisation.
If you have received this email in error, please notify the sender and delete this message and its attachments.
As emails may be altered, Orange is not liable for messages that have been modified, changed or falsified.
Thank you.