Subject: [Pce] Comments regarding PCEP extension to support SR Policy

From: "Stone, Andrew (Nokia - CA/Ottawa)" <[email-masked]>
Date: Wed, 18 Jul 2018 04:04:37 +0000

Hello Dhruv & all

Thanks again for meeting today. Capturing some notes here to share with the list.

As discussed, here’s some comments / items to consider for draft-barth-pce-segment-routing-policy-cp:

  1.  Candidate path support for multiple SID Lists + weight of each SID list
     *   draft-barth-pce-segment-routing-policy-cp defines a PLSPSID per candidate path, therefore 1 SID list (expressed as ERO) per candidate path. Therefore, in this draft there's no way to express multiple SID lists (and their weight) per candidate path.
        *   As discussed, one option may be to map one PLSPID for each SID list and to tag these objects with something (such as Tunnel-ID) that binds them to the same candidate path. A new TLV for weight is likely required. It's still not clear to me where/how the attributes of the candidate path (such as originator (see below)) would also be included in the messaging
  2.  PCREPORT should notify which candidate path is active under a given SR Policy
     *   Status UP may be valid on multiple candidate paths, but only one of them is active
     *   SRCPAG-INFO-TLV would therefore contain an active flag which would need to be set true for all reports related to SID list (as per previous bullet) whose parent candidate path is active
  3.  SR Policy null endpoint support + PCREQ?
     *   (theoretically) SR Policy definition could contain null endpoint 0.0.0.0 for steering purposes but wishes to have a dynamic candidate path and use PCE to compute. draft-ietf-spring-segment-routing-policy points to filsfils-spring-sr-policy-considerations for considerations regarding dynamic path optimization objective, but it’s not explicitly stated regarding whether the candidate path definition may contain a destination for local/PCE calculation purposes.
     *   Therefore, in draft-barth-pce-segment-routing-policy it might be worth mentioning/commenting on whether this is intentionally defined or not.
        *   Obviously, For PCE to compute it needs a real destination IP so I assume SRCPAG-INFO-TLV would contain 0.0.0.0 and the END-POINTS object would contain the intended-for-calculation-destination?
        *   For these reasons calling SRCPAG-INFO-TLV attribute “destination end-point” may be confusing (even though it’s within the SRPOL ASSOCIATION object). Perhaps calling it “sr-pol end-point” or something different may be a good idea.
  4.  Protocol-origin attribute is missing and likely needed
     *   If the goal is for PCEP SR-Policy support is to also report state for SR policies from other origins (such as statically configured) then the candidate path protocol-origin should be advertised as well
  5.  Clarification / reference for BSID
     *   draft-sivabalan-pce-binding-label-sid describes a generic path binding TLV for PCEP LSPs. I assume this draft would be referenced inside draft-barth-pce-segment-routing-policy for messaging the candidate path BSID

Cheers
Andrew