Re: [mpls] 2nd WG LC on draft-ietf-mpls-bfd-directed

"Carlos Pignataro (cpignata)" <cpignata@cisco.com> Sat, 16 July 2016 15:35 UTC

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From: "Carlos Pignataro (cpignata)" <cpignata@cisco.com>
To: Martin Vigoureux <martin.vigoureux@nokia.com>
Thread-Topic: [mpls] 2nd WG LC on draft-ietf-mpls-bfd-directed
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Date: Sat, 16 Jul 2016 15:35:23 +0000
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Subject: Re: [mpls] 2nd WG LC on draft-ietf-mpls-bfd-directed
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Hi, Martin,

Admittedly, I had not read or followed this document before. However, I just scanned through it, and I Have at best some fundamental questions and likely some major issues and concerns. I wonder also if you need to Cc the BFD WG, copying the chairs on this response. Copying also SPRING chairs for awareness.

I hope these are useful to this WGLC.

Major Concerns:

As I said, I just glanced through the document, and found these issues, questions, or problems.

1. Motivation for the work.

Uni or bi-directional? The document starts with a fallacy, setting the tone for the document, on the very first sentence:

Abstract

   Bidirectional Forwarding Detection (BFD) is expected to monitor bi-
   directional paths.

This is absolutely not the case, as explained in RFC 5880 Section 2, RFC 5883 Section 4.3 (https://tools.ietf.org/html/rfc5883#section-4.3), and many other places.

The sentence that follows says:

   When a BFD session monitors an explicit routed
   path there is a need to be able to direct egress BFD peer to use
   specific path for the reverse direction of the BFD session.

The question is — why is there that need?

The document then goes on to say:

2.  Problem Statement

   BFD is best suited to monitor bi-directional co-routed paths.

But: First, why is this the case? the BFD specifications do not say so.

And: Second, if there is a co-routed bidirectional path, then there is no need to specify the return path! The return path is basically “back on the other way on this co-routed bidirectional path”, so there is no need for what this document specifies.

Next sentence:

   be co-routed, thus
   fulfilling the implicit BFD requirement

But BFD never has this requirement, implicit or explicit.

I am not going to further dissect each sentence, but the point is that if there’s something co-routed, there’s no need to explicitly point to the return path. If there is not, then why?


2. Technical feasibility

The second major problem area is the actual technical feasibility. A main motivation seems to be “3.1.3.  Segment Routing: MPLS Data Plane Case”.

However, looking at an SR path, it can be constructed by Node-SIDs and Adj-SIDs. Please refer to: https://tools.ietf.org/html/draft-ietf-spring-segment-routing-07#section-3.5

First, Adj-SIDs SHOULD (not MUST) be assigned. This means there is the potential of an Adj-SID not assigned to a local Adj. There’s of course also the cases in which Adj-SIDs can be assigned to bundles, to ECMP/UCMP groups, etc.

The implication here is that there is a possibility that there is no way to exactly explicitly construct a co-routed return path. For example, if the forward path is Node A -> Link X -> Node Z, then the return path needs to go to the node Z. Then it is a loose path (i.e., NOT co-routed) to the adjacent node to A through link X, and then that node needs to have an Adj-SID on the same link, which might not.


3. Actual Protocol Mechanics

Section 3.1.1.  BFD Reverse Path TLV, uses “Target FEC sub-TLV” to define the reverse path. This is consistent with the approach in RFC 7110.

In fact, Section 3.1.2 uses the RFC 7110 sub-TLVs for “Static and RSVP-TE sub-TLVs”.

However, Section 3.1.3, which seems to be a key motivation (“3.1.3.  Segment Routing: MPLS Data Plane Case”), uses “Label Entries” to specify the Path!

I believe this is a serious technical issue. Instead of using Label values, it should use Target FEC Stacks (as with the few other cases above). Labels can change. With labels there is no validation possible that what distributed by a given label distribution protocol is what is meant in the data plane.

In fact, Section 5.2 is trying to assign this for the Target FEC Stack:

    | X (TBD2) | Segment Routing MPLS Tunnel sub-TLV | This document |

But that’s not a Target FEC! It’s a label value!

This should be done by also using the Target FEC Stack.

The Target FEC Stack for SR is defined at https://tools.ietf.org/html/draft-ietf-mpls-spring-lsp-ping-00, but surprisingly, draft-ietf-mpls-spring-lsp-ping (and the SR TFC thereby defined) are not even references in this document.

Section 3.3 of this document does include an older version of that draft, before WG adoption, and is somehow trying to Update it. Instead of updating it from here, it should discuss how to updated it on itself.

As an aside, it’s not clear to me why WGLCing this document (twice) before moving forward with I-D.kumarkini-mpls-spring-lsp-ping.

The Section that follows is: “3.2.  Segment Routing: IPv6 Data Plane Case”, but in this case, I am mostly confused and baffled on how a set of IPv6 addresses can be an MPLS Target FEC Stack.


4. IPR

This document has IPR with specific licensing terms. I would like to understand what parts of the document are potentially covered and understand if there’s ways to work around that.

In summary, browsing through the document, I see high-level problem area issues, feasibility issues, technology issues, and others. Happy to be shown incorrect if I missed or confused anything.

Thanks,

— Carlos.

On Jul 10, 2016, at 1:59 PM, Martin Vigoureux <martin.vigoureux@nokia.com<mailto:martin.vigoureux@nokia.com>> wrote:

WG,

as said by Ross [1], I have been appointed Document Shepherd for draft-ietf-mpls-bfd-directed [2]. As such I am also running the second WG LC.

So, this e-mail starts a WG LC which will end on the 31st of July.

I'd like to remind that an IPR disclosure [3] exists against this document.

So it is time to state whether or not you are in favour of progressing the document. Please also take the time to review the document and post comments on its content.

Please respond to this call.

Thank you

Martin


[1]: https://mailarchive.ietf.org/arch/msg/mpls/rxnEv4LnUiwhTiZOhowOC6AcHYU
[2]: https://datatracker.ietf.org/doc/draft-ietf-mpls-bfd-directed/
[2]: https://datatracker.ietf.org/ipr/2769/

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