Re: [Pce] Whither Stateless PCE?

"Aissaoui, Mustapha (Nokia - CA)" <mustapha.aissaoui@nokia.com> Thu, 08 September 2016 22:00 UTC

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From: "Aissaoui, Mustapha (Nokia - CA)" <mustapha.aissaoui@nokia.com>
To: Olivier Dugeon <olivier.dugeon@orange.com>, LITKOWSKI Stephane OBS/OINIS <stephane.litkowski@orange.com>, Ina Minei <inaminei@google.com>, "MEURIC Julien IMT/OLN" <julien.meuric@orange.com>
Thread-Topic: [Pce] Whither Stateless PCE?
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Cc: "pce@ietf.org" <pce@ietf.org>
Subject: Re: [Pce] Whither Stateless PCE?
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Hi Olivier,
While I understand the need for consistency across the PCEP messages, I think an empty ERO may just work fine in this case. If I am not mistaken, a valid empty path (computed by PCE or router) will always have one loose hop sub-object corresponding to the destination of the LSP while the empty ERO has no hop sub-objects. So, we can always distinguish the case when there is no current valid path for the LSP from the case where there is a valid empty path.

If we used the NO-PATH object in the PCRpt message, I believe we would need to also define new NI field values and potentially new Flags field values when the PCC is not able to compute a path. For example the SR-TE LSP requested bandwidth but this cannot be performed by the local CSPF since there is no SR-TE LSP state in the LSR to update the available bandwidth consumed by SR-TE LSPs on a link.

The NO-PATH object could indeed be used as is in RFC 5440 in the PCUpd message with the NI field set to 0 (“No path satisfying the set of constraints could be found”). This may be useful for the user to debug the situation from the router without logging in into the PCE. I am OK if this is added an optional object in addition to the empty ERO and can be used to send more information about the lack of a path in the PCUpd message.

Regards,
Mustapha.

From: Pce [mailto:pce-bounces@ietf.org] On Behalf Of Olivier Dugeon
Sent: Friday, September 02, 2016 8:26 AM
To: LITKOWSKI Stephane OBS/OINIS; Ina Minei; MEURIC Julien IMT/OLN
Cc: pce@ietf.org
Subject: Re: [Pce] Whither Stateless PCE?

Hello Stéphane,
Le 02/09/2016 11:43, LITKOWSKI Stephane OBS/OINIS a écrit :
Hi Olivier,

Do you also suggest to use NO-PATH in PCUpdate when PCE has no more path available for a particular LSP ?
Yes indeed. NO-PATH object would be used each time message report/update an LSP without a valid path. In RFC5440, when a PCE received a PCReq message from a PCC, it must answer a PCRep with a NO-PATH object if it would not be able to find a suitable path that meet the requested constraints. So, for the consistency of standard, the PCUpd message must carry a NO-PATH object if no suitable path is available.

Even if I think this could be a right way to go, we need to take care about backward compatibility, there are running codes now that are not using NO-PATH at all for PCRpt/PCUpd and we must not break those implementations.
Agree. But, as we face some interoperability issue of those implementations regarding the empty ERO, it is necessary to update the code. I prefer to clarify the actual standard and obtain interoperable implementations instead of letting the standard unclear or subject to variable interpretation and run into others interoperability issue when new implementations come into the market.

Regards

Olivier


Best Regards,


From: Pce [mailto:pce-bounces@ietf.org] On Behalf Of Olivier Dugeon
Sent: Friday, September 02, 2016 11:29
To: Ina Minei; MEURIC Julien IMT/OLN
Cc: pce@ietf.org<mailto:pce@ietf.org>
Subject: Re: [Pce] Whither Stateless PCE?

Hello Ina, all,

Like for the end of sync, I would suggest to used the NO_PATH object defined in RFC5440 instead of empty ERO. The NO-PATH object has been defined exactly for this meaning: there is no path. An empty ERO has variable interpretation which is not good for a standard i.e. it could mean that there is really no path, no computed ERO, or the path is loose.

So, I would suggest the following text (section 6.1, page 25 line 25)

   "The intended path, represented by the ERO object, is REQUIRED.  If

   the ERO object is missing, the receiving PCE MUST send a PCErr

   message with Error-type=6 (Mandatory Object missing) and Error-value

   to be assigned by IANA (ERO object missing). If the PCC does not have

   a path for a delegated LSP, it MUST replace the ERO object by the

   NO_PATH object defined in RFC5440."
Regards

Olivier

BTW, In the original text, there is a misspelling in line 26 s/ojbect/object/ and I think you would mean PCC instead of PCE in line 29.
Le 02/09/2016 01:22, Ina Minei a écrit :
Thank you all. I will remove the text that states that the ERO must include at least one subobject, and state instead that the ERO may indeed be empty.

I will post shortly, comments welcome.

On Fri, Aug 19, 2016 at 8:56 AM, Julien Meuric <julien.meuric@orange.com<mailto:julien.meuric@orange.com>> wrote:
Hi Mustapha,

You are right: the information is already there, this is an non-issue. As a result, an ERO which is not empty can be considered as loose.

Cheers,

Julien


Aug. 17, 2016 - mustapha.aissaoui@nokia.com<mailto:mustapha.aissaoui@nokia.com>:
Julien, Stephane, and Ina,
The lsp-identifiers tlv has the information about source and destination of the lsp. So no need to use the END-POINTS object for that.

I think it may be just fine to continue using an empty ERO in the PCRpt message to mean there is no current path for a non-zero plsp-id. When the plsp-id is zero we know that it is the end of synchronization marker.

Mustapha.
Sent from my iPhone

On Aug 12, 2016, at 5:31 AM, Julien Meuric <julien.meuric@orange.com<mailto:julien.meuric@orange.com>> wrote:
Hi Ina, hi Stéphane,

I am glad to see this discussion progressing, sorry for interrupting.

RFC 5440 defines the END-POINTS object, which includes an egress ID. Do not you think it could be considered to unambiguously convey the egress destination-attached ID in the PCRpt, without colliding with the loose ERO case?

Cheers,

Julien


Aug. 12, 2016 - <stephane.litkowski@orange.com<mailto:stephane.litkowski@orange.com>:
Hi Ina,

%%% The PCC must at the minimum know what the destination is, and a loose hop is a way to encode this. I cannot think of any situation in which the PCC does not know the destination. I don't think the PCE should worry about whether the PCC will request a path or not, since we are mandating a PCReq to request a path, the PCE does not have to imply it from the ERO

[SLI] PCC knows the destination but when LSP is delegated, PCC does not own path of the LSP anymore, so ERO should reflect what PCE sent to PCC. Again in case of no path available from PCE, PCE will send empty ERO, PCE will not really understand if PCC reports back an ERO with the destination in the ERO as loose hop as this may imply that the PCC is trying to establish a path to the destination using loose hop which is not compliant with what PCE tells. You may fall into PCUpd/PCRpt loop in such scenario because PCE will try to update PCC until PCC reports the same path as PCE sent in ERO.

Best Regards,

Stephane


From: Ina Minei [mailto:inaminei@google.com]
Sent: Thursday, August 11, 2016 19:25

Stephane,

Please see inline %%%


[snip]

Example :

Case#1 : PCC has no path, it reports empty ERO in PCRpt (case Olivier was mentioning), PCE vendor does not compute path and does not send update

### In this case, the ERO must at least have one loose hop, the destination. This will be clarified in the next version of the draft in (the new) section 6.1. See proposed text below:

The intended path, represented by the ERO object, is REQUIRED.  If
   the ERO ojbect is missing, the receiving PCE MUST send a PCErr
   message with Error-type=6 (Mandatory Object missing) and Error-value
   to be assigned by IANA (ERO object missing).  When present, the ERO
   object SHOULD contain at least one subobject, representing the
   destination of the LSP. "

This is a SHOULD and not a MUST because an empty ERO is allowed for
the end of synchronization marker.

[SLI] Having PCC sending an ERO with the destination as loose hop does not make sense to me while PCC has no path. How do you differentiate with the case the PCC reports a loose path to the destination from the case it has no path ?
%%% The PCC must at the minimum know what the destination is, and a loose hop is a way to encode this. I cannot think of any situation in which the PCC does not know the destination. I don't think the PCE should worry about whether the PCC will request a path or not, since we are mandating a PCReq to request a path, the PCE does not have to imply it from the ERO.


=>  Using PCReq could make sense here as Mustapha proposed or clearly mentioning that PCRpt with empty ERO needs to trigger path computation and sending PCUpd.
### This will be clarified in the next version of the draft as well, the mode of operation is to mandate a PCReq before the delegation.

[SLI] Ok thanks, that’s the good way to go.

Proposed text (new section 5.8.2)
5.8.2. Switching from Passive Stateful to Active Stateful

This section deals with the scenario of an LSP transitioning from a
passive stateful to an active stateful mode of operation. When the
LSP has no working path, prior to delegating the LSP, the PCC MUST
first use the procedure defined in Section 5.8.1 to request the
initial path from the PCE. This is required because the action of
delegating the LSP to a PCE using a PCRpt message is not an explicit
request to the PCE to compute a path for the LSP. The only explicit
way for a PCC to request a path from PCE is to send a PCReq message.
The PCRpt message MUST NOT be used by the PCC to attempt to request a path from the PCE.
When the LSP is delegated after its setup, it may be useful for the
PCC to communicate to the PCE the locally configured intended
configuration parameters, so that the PCE may reuse them in its
computations. Such parameters MAY be acquired through an out of band
channel, or MAY be communicated in the PCRpt message delegating the
LSPs, by including them as part of the intented-attribute-list as
explained in Section 6.1. An implementation MAY allow policies on
the PCC to determine the configuration parameters to be sent to the
PCE.
(the intended-attribute-list is new, see more below).


Case#2 : PCC has a path, it reports ERO, PCE has different constraints configured, we expect PCE to update the path



Case#3 : PCC has a path, it reports ERO, PCE has a different optimization algorithm leading to a different ERO with the same constraint, we expect PCE to update the path





Case#2 and Case#3 requires the PCE to know the existing constraint configured on PCC for the LSP , by using only PCRpt we cannot have such constraint information, we need a PCReq also to describe the configuration (PCRpt only describes operation state => Mustapha’s point again).


### This observation is correct, the PCRpt currently carries the actual state.
As Robert mentioned in his reply to this thread, the issue you are raising is the one of intended versus actual state.
After discussing at the IETF with Julien, Dhruv, Mustapha as well as offline with other implementors, the proposed solution is to include both the configured (intended) and actual state in the PCRpt.

The following text is proposed to section 6.1
  The format of the PCRpt message is as follows:

   <PCRpt Message> ::= <Common Header>
                       <state-report-list>
Where:

   <state-report-list> ::= <state-report>[<state-report-list>]

   <state-report> ::= [<SRP>]
                      <LSP>
                      <path>
 Where:
   <path>::= <intended_path>
             [<actual_attribute_list><actual_path>]
             <intended_attribute_list>

[SLI] Why do you interleave intended and actual parameters ? Why not having :
   <path>::= <intended_path>
                        <intended_attribute_list>
                         [<actual_path><actual_attribute_list>]
%%% This was done in order to keep the encoding backwards compatible with existing implementations



   <actual_attribute-list>::=[<BANDWIDTH>]
                             [<metric-list>]

Where:
   <intended_path> is represented by the ERO object defined in
   section 7.9 of [RFC5440].
   <actual_attribute_list> consists of the actual computed and signaled values
   of the <BANDWIDTH> and <metric-lists> objects defined in [RFC5440].
   <actual_path> is represented by the RRO object defined in
   section 7.10 of [RFC5440].
   <intended_attribute_list> is the attribute-list defined in
   section 6.5 of [RFC5440] and extended by PCEP extensions.

The intended_attribute_list maps to the attribute_list in Section 6.5
   of [RFC5440] and is used to convey the requested parameters of the
   LSP path.  This is needed in order to support the switch from passive
   to active stateful PCE as described in Section 5.8.2.  When included
   as part of the intended_attribute_list, the meaning of the BANDWIDTH
   object is the requested bandwidth as intended by the operator.  In
   this case, the BANDWIDTH Object-Type of 1 SHOULD be used.  Similarly,
   to indicate a limiting constraint, the METRIC object SHOULD be
   included as part of the intended_attribute_list with the B flag set
   and with a specific metric value.  To indicate the optimization
   metric, the METRIC object SHOULD be included as part of the
   intended_attribute_list with the B flag unset and the metric value
   set to zero.  Note that the intended_attribute_list is optional and
   thus may be omitted.  In this case, the PCE MAY use the values in the
   actual_attribute_list as the requested parameters for the path.

  The actual_attribute_list consists of the actual computed and
   signaled values of the BANDWIDTH and METRIC objects defined in
   [RFC5440].  When included as part of the actual_attribute_list,
   Object-Type 2 ([RFC5440]) SHOULD be used for the BANDWIDTH object and
   the C flag SHOULD be set in the METRIC object ([RFC5440]).




Best Regards,



Stephane





From: Fatai Zhang [mailto:zhangfatai@huawei.com<mailto:zhangfatai@huawei.com>]
Sent: Thursday, April 14, 2016 09:39
To: LITKOWSKI Stephane OBS/OINIS; DUGEON Olivier IMT/OLN; Aissaoui, Mustapha (Nokia - CA); adrian@olddog.co.uk<mailto:adrian@olddog.co.uk>; 'Dhruv Dhody'
Cc: pce@ietf.org<mailto:pce@ietf.org>
Subject: 答 复: [Pce] Whither Stateless PCE?



Hi Stephane,



Could you clarify what is interoperability issue in multivendor environment (multivendor in one administrative domain?)?



Do you mean there is interoperability issue when vendor 1 supports stateless PCE and vendor 2 supports stateful PCE?













Thanks



Fatai



发 件人: Pce [mailto:pce-bounces@ietf.org] 代 表 stephane.litkowski@orange.com<mailto:stephane.litkowski@orange.com>
发 送时间: 2016年4月9日 1:04
收 件人: DUGEON Olivier IMT/OLN; Aissaoui, Mustapha (Nokia - CA); adrian@olddog.co.uk<mailto:adrian@olddog.co.uk>; 'Dhruv Dhody'
抄 送: pce@ietf.org<mailto:pce@ietf.org>
主 题: Re: [Pce] Whither Stateless PCE?



Hi,



I fully agree that stateful PCE draft needs to be more clear about how a PCC retrieves a path when the delegation starts and the LSP has just been configured (does it need to compute locally first and then delegate, or do PCReq as Olivier proposed …).

I want to add my voice to what Olivier said about the inconsistent behaviors we see today in implementations leading to lack of interoperability.

The most important point is that we need to find a solution as soon as possible as some people wants to deploy it in multivendor environment and find workaround (btw vendor1 and vendor 2) to fix it interop is not the right way to go …

This is a question that the WG must work on and close asap.



Best Regards,



Stephane





From: Pce [mailto:pce-bounces@ietf.org] On Behalf Of olivier.dugeon@orange.com<mailto:olivier.dugeon@orange.com>
Sent: Friday, April 08, 2016 18:29
To: Aissaoui, Mustapha (Nokia - CA); adrian@olddog.co.uk<mailto:adrian@olddog.co.uk>; 'Dhruv Dhody'
Cc: pce@ietf.org<mailto:pce@ietf.org>
Subject: Re: [Pce] Whither Stateless PCE?



Hello all,

IMHO the discussion must be split into is 2 different subjects:

1/ PCInit message could be seen as an independent message compared to other PCReq/PCRep, PCRpt and PCUp. Indeed, the PCE uses the PCInit message after a request that comes from another interface (e.g. a RestConf API) instead of PCReq that comes from the router itself through PCEP. In fact, when you configure a tunnel on the router, only the path computation part is requested to the PCE. Complements of tunnel configuration still remain in the router configuration. In case of PCInit, all information must be provided to the router. This could be for example the traffic steering. So, IMHO, it is normal that the PCInit message evolves through extensions different from the other PCEP messages, and in particular PCReq, as it is not triggered by the same entity, i.e. an external component instead the PCC router itself.

2/ But, this will not make PCReq message obsolete. Indeed, RFC5440 will continue to be mandatory for stateful both passive and active mode even if it needs clarification in the draft. Let me explain. In passive stateful, a PCReq/PCRep sequence is drawn in Figure 7 of the pce stateful draft prior to the PCRpt message Now, the ambiguity comes from the active stateful mode and figure 8. Why is the PCReq/PCRep sequence not mentioned? Of course the tunnel is delegated in this mode, but, the delegation object has been added as an extension to the PCReq message in the same draft. So, IMHO, at the creation of the tunnel, the draft must precise that a PCReq/PCRep exchange with delegation=1 must be used prior to the PCRpt to be coherent with RFC 5440 and passive stateful mode.

The problem occured during our evaluation of commercial products on which we made interoperability tests. Indeed we observed different behaviours that are due to the draft ambiguity and conduct to some interoperability issues. The different cases are as follow:
 - a/ - PCReq/PCRep exchange to obtain a valid ERO before the PCRpt message
 - b/ - PCReq message to obtain a valid ERO but with no reaction from the PCE which is not conform to RFC5440
 - c/ - PCRpt with empty ERO (looks strange. What is the meaning of an Empty ERO ? a loose path ? no path ? )/PCupd to get a valid path which overlaps with standard RFC5440 PCReq/PCRep.
 - d/ - PCRpt with empty ERO and no PCUpd leaving the tunnel down.

Thus, PCC/PCE that used PCRpt/PCupd messages for active stateful mode are incompatible with PCC/PCE that used standard PCReq/PCrep exchange. We could not mix both behaviours (PCC that use PCReq message with PCE that react to PCRpt with empty ERO and reciprocally). The problem occurs only at the creation of the tunnel. Once created and up the tunnel is reported and updated by means of PCRpt / PCupd messages correctly in all cases.

To summarize: PCInit message could leave independently from other messages. PCReq is the basis of PCE and is mandatory in all use cases included the active stateful mode, but this need to be clarify in the pce stateful draft.

Regards

Olivier

Le 07/04/2016 23:22, Aissaoui, Mustapha (Nokia - CA) a écrit :

Hi Adrian,

I raised in December 2014 the technical issue in draft-ietf-pce-stateful-pce that a PCC must be able to convey the original parameters (constraints) of the LSP path (Bandwidth, Metric, and LSPA objects) using a PCReq message to a PCE and subsequently delegate the LSP to PCE using the PCRpt message. Otherwise, when the LSP is delegated to PCE only the operational values of these parameters can be included in the PCRpt message. The latter means that the PCE will update the path without knowing exactly the original parameters.



For me, PCReq/PCRep are an integral part of operating an LSP in stateful mode.



Here is the link to the archived thread:

https://mailarchive.ietf.org/arch/search/?email_list=pce&so=-date&q=%22+Path+Computation+Request+in+Active+Stateful+PCE%22



Regards,

Mustapha.



From: Pce [mailto:pce-bounces@ietf.org] On Behalf Of EXT Adrian Farrel
Sent: Thursday, April 07, 2016 12:48 AM
To: 'Dhruv Dhody'
Cc: pce@ietf.org<mailto:pce@ietf.org>
Subject: Re: [Pce] Whither Stateless PCE?



I think you are probably right, Dhruv.



But referencing the ways in which customers deploy may be a little limiting.

To say PCE is widely deployed (even after all these years) may be an exaggeration.

Although we do have some clues about what is currently being pushed for deployment.



I think you have mainly grasped my point, however. We need to understand which extensions are definitely only needed in one mode or another, and which should be done in all modes (either because they are needed or because we don't know).



OTOH, I suppose TLVs are just TLVs. Once you specified the TLV it is not rocket science to include it in a message. In fact, it is probably one line of text to include it and only a short paragraph to describe additional processing in other modes once you have described how it is used in one mode.



Where does that leave us?



Adrian



From: dhruvdhody@gmail.com<mailto:dhruvdhody@gmail.com> [mailto:dhruvdhody@gmail.com] On Behalf Of Dhruv Dhody
Sent: 06 April 2016 23:07
To: Farrel Adrian
Cc: pce@ietf.org<mailto:pce@ietf.org>
Subject: Re: [Pce] Whither Stateless PCE?



Hi Adrian,



Even in the brave new world of Stateful PCE, PCReq and PCRep messages do play a role in the passive stateful PCE mode. PCReq/PCRep also play a crucial role in the inter-domain and inter-layer context in the new proposal like stateful H-PCE.



At the same time mandating that every extension (say SFC) must also be specified in a stateless manner when no customer deploy in such a way, might be overkill.



Perhaps we need to look at it case by case!



Dhruv



On Wed, Apr 6, 2016 at 4:00 PM, Adrian Farrel <adrian@olddog.co.uk<mailto:adrian@olddog.co.uk>> wrote:

Once upon a time, in a working group far, far away, PCE was basically stateless.
PCE acted in response to questions asked by PCCs.

These days, everyone is excited by stateful PCEs and there is a lot of
initiation (of LSPs or of control of LSPs).

In the jabber room during today's meeting Ravi noted that not a lot of the new
drafts (maybe none of them) talk about PCReq messages. This raises the question
in our minds as to whether stateless PCE is obsolete.

If (and only if) this mode of PCE usage has gone out of fashion, we *might*
consider cleaning up the protocol and architecture so that we don't need to make
protocol extensions to PCReq and PCRep messages when we make extensions to
PCInit messages.

Thoughts?

Adrian

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