Re: [Idr] Queries on draft https://datatracker.ietf.org/doc/html/draft-ietf-idr-te-lsp-distribution-18.txt

[Abhishek Chakraborty <[e-mail]>]

I hope all are doing well.

Hello Ketan,

It is a long due query from my side on this draft but couldn't find time to shoot out an email.

1.
When I see section 6.1 and 6.2 for binding SID, the flag bits for these 2 section overlap each other at 0th bit with 2 different flags:
For SR Binding SID:
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |D|B|U|L|F|                     |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For SRv6 Binding SID:
          0                   1
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |B|U|F|                         |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The D bit is used to identify if the SID is SR-MPLS or SRv6. Now the B flag in SRv6 Binding SID is placed at the 0th Bit in the draft. Is it a typo or intentional which I am missing? Shouldn't the SRv6 flag have the beow:
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |D|B|U|F|                     |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Depending on D flag the rest of the bits can be parsed.

2.
In section 6.6:
      -  P-Flag: Indicates that the CP prefers the use of only protected
         SIDs when set.  This flag is mutually exclusive with the
         U-Flag.

      -  U-Flag: Indicates that the CP prefers the use of only
         unprotected SIDs when set.  This flag is mutually exclusive
         with the P-Flag.
...
..
      -  S-Flag: Indicates that the use of protected (P-Flag) or
         unprotected (U-Flag) SIDs becomes a strict constraint instead
         of a preference when set

P and U flag already says that we use only Protected and Unprotected. How is it different from a strict constraint? Setting P/U means we are using strictly Protected/Unprotected.
Can you please help to clarify.

Thanks and Regards,
Abhishek



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From: Abhishek Chakraborty
Sent: Friday, July 29, 2022 12:03 AM
To: Ketan Talaulikar <[e-mail]>
Cc: [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]; [e-mail]
Subject: RE: Queries on draft https://datatracker.ietf.org/doc/html/draft-ietf-idr-te-lsp-distribution-17

Thanks a lot Ketan for your response.
It has clarified all my doubts.

Thanks and Regards,
Abhishek



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From: Ketan Talaulikar <[e-mail]<mailto:[e-mail]>>
Sent: Thursday, July 28, 2022 6:15 PM
To: Abhishek Chakraborty <[e-mail]<mailto:[e-mail]>>
Cc: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: Re: Queries on draft https://datatracker.ietf.org/doc/html/draft-ietf-idr-te-lsp-distribution-17

[External Email. Be cautious of content]

Hi Abhishek,

Thanks for your email that brings up some very good questions. Please check inline below for responses.


On Thu, Jul 28, 2022 at 3:33 PM Abhishek Chakraborty <[e-mail]<mailto:[e-mail]>> wrote:
Hope everyone is doing well.

My queries are regarding the following flags:

  1.  https://datatracker.ietf.org/doc/html/draft-ietf-idr-te-lsp-distribution-17#section-6.7<https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/html/draft-ietf-idr-te-lsp-distribution-17*section-6.7__;Iw!!NEt6yMaO-gk!AadXpHLhwOmex9PJdephXXL1Ef1EKb2Dama9xDh4w6VooSngX6wI7WQCaQwbcG06enKPN7ok-dWW7tMZXyY$>


E-Flag : Indicates that SID-List is an explicit path when set
        and indicates dynamic path when clear.



         C-Flag : Indicates that SID-List has been computed for a

           dynamic path when set.  It is always reported as set for

           explicit paths.


     Does E flag only get set for Statically configured segment-list (non-compute) on headend router?

KT> Yes, but not "only" that. Please see further.

     Does E flag includes PCE/Controller provisioned segment-lists as well? Because PCE/Controller provisioned segment-lists are dynamically provisioned by controller on the headend.

KT> Yes.

     Does C flag only gets set for locally computed segment-lists? Because if PCE/Controller provisioned segment-lists are also dynamic paths but not locally computed, will this C flag
                    be set for those as well?

KT> The draft is about reporting from the SR Policy headend. If the headend does not see this as a dynamically computed path (e.g., if it is not delegating computation) then the C flag won't be set.


KT> The reference here is explicit and dynamic paths per RFC9256 and not about how they are provisioned. Specifically, please check section 5.1.

                    What is the meaning of "It is always reported as set for explicit paths" in C flag?

KT> The SID list is pre-computed always for explicit path and hence this flag is always set. For a dynamic path, not all paths may be computed (e.g., if it is not the preferred/active then the path may not yet have been computed).

                   In case of delegation how these 2 flags will be set? Because the computation is done in the controller with the local constraint set.

KT> Please check if my prior response clarifies.



  1.  https://datatracker.ietf.org/doc/html/draft-ietf-idr-te-lsp-distribution-17#section-6.8<https://urldefense.com/v3/__https:/datatracker.ietf.org/doc/html/draft-ietf-idr-te-lsp-distribution-17*section-6.8__;Iw!!NEt6yMaO-gk!AadXpHLhwOmex9PJdephXXL1Ef1EKb2Dama9xDh4w6VooSngX6wI7WQCaQwbcG06enKPN7ok-dWWAqrqNz4$>

      *  E-Flag : Indicates the SID value is explicitly provisioned

         value (locally on headend or via controller/PCE) when set and

         is a dynamically resolved value by headend when clear

Does this E flag on each segment means if this segment is explicitly configured in the segment-list locally?
KT> This one is not about a dynamic or explicit path but about whether the headend is doing SID resolution. Even for an explicit path, the operator config could be simply using a node's IP address for a Prefix-SID (Type C). The headend needs to dynamically resolve to a SID value and so this flag is not set. For a dynamic path, if the controller is fully resolving the SID values and providing them to the headend then this flag would be set.


i.e. if a segment-list is a mixture of some statically configured segments (non-compute) and some are computed/translated segments, then segment-list will be set with both segment-list C and E flags and only the non-compute segments will be set with E flag?
KT> I hope my previous comments clarify.


Does this E flag be set on all segments of a PCE/controller provisioned segment-list? If yes then will such PCE/Controller provisioned segment-lists be marked with segment-list E flag? But then such segment-lists are not dynamic anymore right?
KT> I hope my previous comments clarify.

Thanks,
Ketan


Thanks and Regards,
Abhishek



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