Re: [Pce] Adoption of draft-li-pce-sr-bidir-path-06?

[Rakesh Gandhi <[e-mail]>]

Hi Andrew,

Many thanks for your review comments. Please see below with <RG2>..

On Wed, Jan 22, 2020 at 10:57 PM Stone, Andrew (Nokia - CA/Ottawa) <
[e-mail]> wrote:

> Hi Dhruv,
>
> Thanks again for the speedy reply. Comments below under <andrew2>
>
> Cheers
> Andrew
>
> On 2020-01-22, 8:01 AM, "Dhruv Dhody" <[e-mail]> wrote:
>
>     Hi Andrew,
>
>     On Wed, Jan 22, 2020 at 12:06 AM Stone, Andrew (Nokia - CA/Ottawa)
>     <[e-mail]> wrote:
>     >
>     > Hi Dhruv,
>     >
>     > Thanks for the reply and feedback.
>     >
>     > Could an implementation of PCE not simply just return error during
> that situation? In diversity if too many LSPs grouped together and PCE
> computationally can't support it, it returns a PCERR. So I would reason
> that if bidirectionally associated and notify PCCs is necessary, but was
> configured between SR and RSVP, that's also an error due to the (current)
> unsupported feature set. I see this as trying to protect against day-0
> misconfig by changing the wire encoding within the protocol (which I'm not
> necessarily against..). While I have doubt if it's a valid use case or
> requirement in a production deployment, and it may have been acknowledged
> before, but this essentially blocks associating SR LSP with RSVP LSP
> bidirectionally for PCE to compute.
>     >
>     > Since the overall workflow doesn't change by this new type def, and
> if SR<->RSVP associated is not reasonable requirement, and If consensus has
> already been reached on this, and implementation already exist, I'm okay
> with parking this topic.
>     >
>
>     The SR draft does expect all LSPs to be SR for the new association
>     type and thus easier to handle. If you use a common association type,
>     the behavior would be dependent on the PST for the first LSP that is
>     added to the association. We could end up in a situation where Peer 1
>     would add LSP 1 (SR) first and reject LSP 2 (RSVP-TE) and peer 2 would
>     add LSP 2 (RSVP-TE) first and reject LSP 1 (SR). Also there might be a
>     use for this mixed cases in future, which would require different
>     processing to be defined.
>
>
> <Andrew2>
>
> I will ponder on this some more. I'm undecided yet if the reasons to
> protect config/behaviour mismatch, outweigh just having one type encoding
> and defining the individual behaviours separately, which could also be up
> to the local policy defined by the PCE implementation.
>
> It could be possible that a network has nodes currently running RSVP, with
> plans to migrate to SR-TE which could take a very long time due to the size
> of the network. The software running on the nodes may be upgraded, of which
> those PCCs may have an implementation of these bidirectional drafts, but
> have still not yet migrated to SR-TE. An operator may wish to leverage PCE
> feature sets on newer created services sooner than later (for example,
> bidirectional capability) so they begin using bidirectional RSVP to have
> symmetric paths for SLA reasons and aid in avoiding manual traffic
> engineering. Over time, as the network is migrated to SR-TE tunnels you
> have some nodes using SR-TE tunnels and the reverse nodes still operating
> with RSVP. From an operator p.o.v the bidirectional notification behaviour
> here wouldn't matter, they just want the LSPs to take the same resources
> symmetrically. The feature set on PCE, as per the current draft wording and
> types will be broken.
>
> </end andrew2>
>


<RG2> I would think that the migration would happen for the whole
bidirectional LSP on both endpoints at the same time. This is because, all
nodes would have migrated to be SR aware in order for the forward path to
be SR path (I am thinking co-routed). In addition, the complexity to
associate the SR and RSVP paths into a bidirectional path is not small,
given that SR path ingress node would know the reverse path via signaling
whereas RSVP path ingress node would know via PCEP.  Which also means, one
would need to upgrade the RSVP ingress node anyways to learn the reverse SR
path. Traffic steering and other functions for the RSVP path and SR path in
the reverse direction (and vice versa) may bring additional work.

<RG2> Having said that, the same association group type can be used for SR
and RSVP paths (e.g. both are the same PST) in theory, but given the
implementation status in the draft, I will let respective co-authors
comment.



>
>
>     > Still hoping for feedback regarding my comments on section 5. I see
> that as being more significant, since it influences the workflow and at the
> moment I don't see the dependency on draft-li-pce-controlled-id-space as
> necessary to achieve notifying PCCs of reverse paths.
>     >
>
>     And I was hoping that authors would take a bite :)
>     From what I understood PLSP-ID remains a PCC allocated ID. The point
>     that section 5 is making is that the same LSP would be identified by
>     two PLSP-IDs, one allocated by Ingress and another by Egress PCCs.
>     There is no proposal for PCE-controlled PLSP-ID. So PCE-init + R bit
>     is enough  (as you state) and I am in full agreement with you that
>     figures with PLSP-ID could be useful.
>
>
>
>
> <Andrew2>
>
> Thanks for the comments and agreement. draft-li-pce-sr-bidir-path-06
> section 5.1, says "PCE needs to allocate a PLSP-ID". Perhaps it was just a
> typo and should have said PCC.
>
> ===current
>    Since the PLSP-ID space is independent at each PCC, the PLSP-ID
>    allocated by the egress PCC can not be used for the LSP at the
>    ingress PCC (PLSP-ID conflict may occur).  Hence, the PCE needs to
>    allocate a PLSP-ID for LSP2 from the ingress PCC's PLSP-ID space ,
>    say 101.  Similarly for LSP1, it has PLSP-ID 100 at the ingress, and
>    may have say PLSP-ID 201 at the egress node.
> =====end current
>
>
> May I propose the following text  (or something like it) instead:
>
>
> ===new proposal
>
>    Since the PLSP-ID space is independent at each PCC, the PLSP-ID
>    allocated by the egress PCC cannot be used for the LSP at the
>    ingress PCC (PLSP-ID conflict may occur). As per normal PCE-INIT
>    operations, PCC assigns the PLSP-IDs for local LSPs.
>    Hence, when the PCE notifies an ingress PCC of the reverse egress LSP,
> it
>    does so using PCE-INIT operations and sets PLSP-ID to zero and sets the
> R bit in the association
>    object to indicate that this PCE-INIT LSP is a reverse LSP. The PCC
>    upon receiving the PCE-INIT MUST locally assign a PLSP-ID and it MUST
>    issue a PCREPORT to PCE for this LSP containing the new PLSP-ID. This
>    LSP MUST NOT be instantiated on the PCC.
>
>    For example, ingress PCC1 way may report to PCE an LSP with
>    PLSP-ID 100. Egress PCC2 may report to PCE an LSP with PLSP-ID 200.
>    Both of these LSPs are bi-directional associated. When PCE
>    notifies PCC1 of the PCC2 LSP, it does so by sending a PCE-INIT to PCC1
> with
>    PLSP-ID set to zero and R bit set. PCC1 upon reception of this
> generates a PLSP-ID
>    (example PLSP-ID 300) and issues a PCREPORT to PCE.
>
> =====end new proposal
>
>
<RG2> Looks good to me. Thanks for the text.
<RG2> Many thanks Dhruv for the inputs.

Thanks,
Rakesh




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