Re: [mpls] Routing directorate QA review of draft-filsfils-spring-segment-routing-ldp-interop

Alexander Vainshtein <> Tue, 28 July 2015 16:37 UTC

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From: Alexander Vainshtein <>
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Thread-Topic: Routing directorate QA review of draft-filsfils-spring-segment-routing-ldp-interop
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Subject: Re: [mpls] Routing directorate QA review of draft-filsfils-spring-segment-routing-ldp-interop
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My name is Sasha Vainshtein, and  I have been asked to perform the QA review of

Due to health problems I have been very late (but, hopefully, not too late) with this review, and here it is.

Does the draft solves a real problem? From my POV, definitely yes.

          1.         The problem stems from the following combination of facts fact that:

a.       LDP distributes labels for FECs represented by IP prefixes (among other things).

b.      LDP-based MPLS network are very widely deployed

c.       SR operating over the MPLS DP allocates labels for SIDs that can represent IP prefixes (again, among other things)

d.      As a consequence, LDP and SR may effectively map the same IP prefix to different labels.

          2.         As SR using the MPLS DP is gaining acceptance in the industry, the following deployment scenarios look as realistic to me:

a.       Coexistence  between LDP-based and SR-based control planes in the same network

b.      Partial deployment of SR in some sub-domains of a network combined with the operators' need to use the benefits of SR where it is already deployed

c.       Gradual transition of services  (especially L2 and L3VPN) tunnelled over LDP-created LSPs to LSPs set up using SR (and, possibly, vice versa)

Is the draft a good enough start for a WG document? From my POV, yes.

1         The draft covers multiple (if not all) realistic use cases for coexistence of LDP-based and SR-based control planes and provides  what looks to me as reasonable solutions for these scenarios.

2         At the same I would think that additional work is required to complete the work.

Specific issues with the current draft:

1         Editorial: SR-related abbreviations (e.g., SRGB, SID etc.) are used without expansion at the first use, and there is no "Abbreviation" section in the draft.

2         Process/Editorial: The draft currently lists 12 authors on its title page exceeding the recommended RFC Editor maximum of 5 authors.

          3.         Technical: The draft does not analyse the use case  of coexistence between SR and LDP with enabled extension for multi-area LSPs as per RFC 5283<>=1>. I think that this use case deserves special consideration in the draft because:

a.       It mentions the use case of seamless MPLS and refers to the (expired) Seamless MPLS Architecture<> draft

b.       The seamless MPLS Architecture draft, in its turn, explicitly refers to RFC 5283 in Section 5.1.1 to facilitate binding of labels received via the DoD LDP session while only default route is configured in the RIB of the access node in the seamless MPLS architecture

c.       It is possible that this use case would not add anything to the draft - but I would prefer to see it in the document with the appropriate explanation

          4.         Technical: Section 8 "Manageability Considerations" is currently left empty. When this section is written, I would expect at least the following:

a.       Some level of detail regarding local  policies defining whether LDP-created or SR-created LSPs should be used etc.

b.      Discussion of using LSP Ping for LSPs that are produced by stitching an LDP segment with an SR one.

c.       Alternatively, it is possible to move this section to a separate dedicated draft

          5.         Technical: I think more information should be provided in Section 5 to explain operational aspects of SR-assisted LDP FR:

a.       In Section 5.1 I would expect the draft to explain that:

                                                               i.      Node D is selected as the RLFA using the same logic  that is defined for selecting RLFA in RFC 7490

                                                             ii.      The "bypass LSP" is set up by the SR CP automatically without any need for management intervention

                                                            iii.      I would like to understand why dynamically set up Targeted LDP sessions are an operational concern. Such sessions appear also when BGP-based auto-discovery for L2VPN is used, and, according to the analysis in RFC 7490, the scalability problems associated with these sessions look as negligible.

b.      In Section 5.2 I would expect the draft to explain that:

                                                               i.      The resulting solution is similar to using a bypass LSP to a manually selected RLFA that is set up by RSVP-TE (the difference is that a Targeted LDP session to such an RLFA would be still needed)

                                                             ii.      Set up of the  "traffic-engineered bypass LSP" by the SR CP is triggered by an appropriate management operation

                                                            iii.      What information should be provided by the Management Plane for computing the RLFA and the traffic-engineered  bypass LSP? Should it be similar to configuration parameters used with RSVP-TE?

                                                           iv.      Which kind of logic should be responsible for computing the RLFA and the path to be taken by the engineered bypass LSP: should it be similar to the CSPF used with RSVP-TE?

                                                             v.      How the traffic-engineered bypass LSP hat is set up by the SR CP would react to additional changes in the network topology (the behaviour of the bypass LSP that is set by RSVP-TE in these situations is well known).

Neither of the issues listed above should be considered as preventing adoption of the draft as a WG document IMO. Actually I think that resolving these issues in the context of a WG document would be more effective.

Hopefully these notes will be useful.



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From: Jonathan Hardwick []
Sent: Wednesday, May 20, 2015 12:46 PM
To: Alexander Vainshtein
Cc:;; Alvaro Retana (aretana); Jon Hudson
Subject: Routing directorate QA review of draft-filsfils-spring-segment-routing-ldp-interop

Hi Sasha

Please would you be the routing directorate QA reviewer for draft-filsfils-spring-segment-routing-ldp-interop?

This initial review has been prompted by the spring WG chairs in parallel with a call for WG adoption.  As such, this document qualifies for "initial QA review".  Please could you provide your initial comments in the next 3 weeks?  As per the QA process, we would also like you to stay with the document and review it again when it goes to WG last call.

The following web page contains a briefing on the QA process, and guidance for the QA reviewer.

Please copy your comments to the spring and rtgdir mailing lists.

Please let me know whether you can do it, or not.

Many thanks