Re: [Teas] Proposed enhancement to draft-ietf-teas-yang-path-computation

"Belotti, Sergio (Nokia - IT/Vimercate)" <> Wed, 21 November 2018 15:31 UTC

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From: "Belotti, Sergio (Nokia - IT/Vimercate)" <>
To: Igor Bryskin <>, Italo Busi <>, "" <>
CC: "" <>, "" <>, "Beller, Dieter (Nokia - DE/Stuttgart)" <>
Thread-Topic: [Teas] Proposed enhancement to draft-ietf-teas-yang-path-computation
Date: Wed, 21 Nov 2018 15:31:01 +0000
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Subject: Re: [Teas] Proposed enhancement to draft-ietf-teas-yang-path-computation
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Hi Igor,
Our assumption , both for our “temporary state” and “compute-only mode” is whar also wrote in tunnel model draft: “a TE tunnel may be provisioned for the only purpose of computing a path and reporting it without the need to instantiate the LSP or commit any resources”.
With this assumption we leave open the remote possibility (how much percentage is not the context) that resources are taken by others and path is no more available. What we need to do in this case?
Our proposal is to re-compute a new path.
Sincerely , it is not fully clear what you mean with “sticky approach” , can you elaborate a bit ?
I suppose auto-discovered topology can be the native topology, right ?


From: Igor Bryskin <>
Sent: Wednesday, November 21, 2018 4:11 PM
To: Italo Busi <>om>;
Cc:; Belotti, Sergio (Nokia - IT/Vimercate) <>om>;
Subject: RE: [Teas] Proposed enhancement to draft-ietf-teas-yang-path-computation


If the server keeps a state for a returned path, it can project said path onto auto-discovered TE topology before computing new paths (this is the essence of “sticky” path computation principle). Therefore, unless in the time interval since the response the resources required for the path are grabbed by some process independent of the server’s PCE in question, it won’t be possible for the returned path to become unavailable. In any case the probability of that would be far less than when returned to a sate -less PCE request.


From: Italo Busi
Sent: Wednesday, November 21, 2018 9:55 AM
To: Igor Bryskin;<>
Subject: RE: [Teas] Proposed enhancement to draft-ietf-teas-yang-path-computation

Hi Igor,

Thanks for your feedbacks

The proposal is quite similar to the stateful solution (in section 3.3.1 of draft-ietf-teas-yang-te) with the only difference that there is no te-tunnel in the config DS

In both cases, since the path reported in the state DS is based on an abstract topology, the server (PNC) also remembers which is the native path (physical path), within its native topology (physical topology), associated with that compute-only te-tunnel. This information is kept internally to the server (PNC) and does not have an impact on the YANG model design

Do you agree?

In both cases, we think that it can still be possible that the path computed at time T1 is no longer available at time T2. We noted that the behavior of the server (PNC) in this case is not described in section 3.3.1 of draft-ietf-teas-yang-te. We would propose that in this case, a new path is computed.

What do you think?

Sergio and Italo

From: Igor Bryskin
Sent: Wednesday, October 31, 2018 10:03 PM
To: Italo Busi <<>>;<>
Subject: Re: [Teas] Proposed enhancement to draft-ietf-teas-yang-path-computation

Not a bad idea, provided that the server is smart when supporting compute-only tunnels. For example it can use the "sticky" approach - project CO tunnel paths onto auto-discovered topology.

From:Italo Busi
Cc:Francesco Lazzeri,Belotti, Sergio (Nokia - IT/Vimercate),Carlo Perocchio,
Date:2018-10-31 12:20:35
Subject:[Teas] Proposed enhancement to draft-ietf-teas-yang-path-computation


Following-up mailing list discussions after IETF 102, we would like to discuss the proposal for a possible enhancement to the path computation RPC defined in draft-ietf-teas-yang-path-computation-03

The stateless path computation solution  requires that the YANG DS server (e.g., a PNC)  computes a path twice before setting up an LSP: at time T1, when its client (e.g., an MDSC) sends a path computation RPC request, and later, at time T2, when the same client (MDSC) creates a te-tunnel requesting the setup of the LSP.

The underlying assumption is that, if network conditions have not changed, the same path that has been computed at time T1 is also computed at time T2, by the server (PNC) and therefore the path that is setup at time T2 is exactly the same path that has been computed at time T1

From the mailing list discussion after IETF 102, it seems that some people think that it is not necessary to guarantee that the path setup at time T2 is exactly the same as the path computed at time T1 but only that it has the same or better metrics, while other people would like to be able to setup at time T2 the path computed at time T1, if still available

One possibility to achieve the latter behavior is to use the stateful path computation solution described in section 3.3.1 of draft-ietf-teas-yang-te-17. However, this approach has some drawbacks as described in section 3.3 of draft-ietf-teas-yang-path-computation-03

We think that it is also possible to define some extensions to the path computation RPC where the server (PNC), after having received a path computation RPC request, maintains some “temporary state” associated to the path computed at time T1 and allows the client (MDSC) to request, at time T2, the setup of that path, if still available.

The solution we are considering is similar to the compute-only mode but, to avoid a stateful approach, is leveraging the path computation RPC and the separation between config and state DS, in line with the NMDA architecture.

At time T1, after having computed a path, as requested by a path computation RPC request, the server (PNC) also creates a compute-only te-tunnel in the state DS reporting the computed path. This would be similar to the stateful solution with the only difference that there is no te-tunnel in the config DS.

At time T2, the client (e.g., MDSC) can request to setup that path by creating a te-tunnel (not in compute-only mode) in the config DS using the same tunnel-name of the existing te-tunnel  in the state datastore: this will trigger the server (PNC) to setup the path that has been computed at time T1, if still available.

Question for the TEAS WG: do you think that a solution that allows requesting the setup a path previously computed, as described above, would be useful?

Thanks, Carlo, Francesco, Sergio and Italo

Italo Busi
Principal Optical Transport Network Research Engineer
Huawei Technologies Italia S.r.l.
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