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

[Gregory Mirsky <[e-mail]>]

Hi Carlos,
thank you for your comments. I hope I understand your concerns better and am able to address them. Please find my follow up notes in-line tagged GIM2>>.

                Regards,
                                Greg

From: Carlos Pignataro (cpignata) [mailto:[e-mail]]
Sent: Tuesday, August 09, 2016 9:14 PM
To: Gregory Mirsky <[e-mail]>
Cc: Martin Vigoureux <[e-mail]>; mpls <[e-mail]>; [e-mail]; [e-mail]
Subject: Re: [mpls] 2nd WG LC on draft-ietf-mpls-bfd-directed

Hi Greg,

Thanks for the response — please find some follow-ups inline.

On Jul 25, 2016, at 5:06 PM, Gregory Mirsky <[e-mail]<mailto:[e-mail]>> wrote:

Hi Carlos,
thank you for your comments. Please see my responses in-line and tagged GIM>>.

                Regards,
                                Greg

From: mpls [mailto:[e-mail]] On Behalf Of Carlos Pignataro (cpignata)
Sent: Saturday, July 16, 2016 8:35 AM
To: Martin Vigoureux <[e-mail]<mailto:[e-mail]>>
Cc: mpls <[e-mail]<mailto:[e-mail]>>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: Re: [mpls] 2nd WG LC on draft-ietf-mpls-bfd-directed

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.
GIM>> I think that you refer to the following text in RFC 5880:

Not specifically, not only.

My point is that the very first sentence contradicts standards tracks BFD RFCs. BFD also is expected to monitor unidirectional paths.

GIM2>> Would the following change address your concern:
OLD
Bidirectional Forwarding Detection (BFD) is expected to monitor bi- directional paths. 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.
NEW
Bidirectional Forwarding Detection (BFD) is expected to monitor any kind of paths between systems. When a BFD session monitors an explicitly routed uni-directional path there may be a need to direct egress BFD peer to use specific path for the reverse direction of the BFD session.

   BFD can provide failure detection on any kind of path between
   systems, including direct physical links, virtual circuits, tunnels,
   MPLS Label Switched Paths (LSPs), multihop routed paths, and
   unidirectional links (so long as there is some return path, of
   course).
And this is exactly what motivated the work we’re discussing. Consider the situation when the return path, though temporarily, is not available. Consider scenario when node A sends BFD control packets over an LSP to node B and the node B sends its BFD packets over out of band return path, e.g. IP network.  If the loss of continuity between B and A lasts long enough to will detect failure. Should such failure be interpreted as indication of the failure on the monitored LSP or not?

But this is irrespective of wether the return path is explicit or not, or even if the return path is via some out of band channel. A different way, this is also the case for BFD multihop over plain IP (on a tunnel one way, hop-by-hop routed on the return).

GIM2>> True, but we’re not solving these other scenarios, only those where monitored path is explicitly routed. We can add explicit statement listing out of the scope scenarios.


The second paragraph in the Introduction section explains the scenario when an explicitly routed LSP being monitored while the return path is over IP network that is based the shortest path paradigm.

The fact that the return path goes over the same links as the forward path does not mean that the return path is misprogrammed but the forward path is correctly programmed.
GIM2>> The purpose of making BFD session use co-routed path is not to verify how it is instantiated in LFIB on a LSR. That is the task for defect localization, not defect detection OAM. Would you agree?

I still believe that the motivation and use case is not well defined or explained.

GIM2>> I’ll keep trying.

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?
GIM>> Please see my comment above.


I still believe this is a bit of a non sequitur. Why does BFD monitoring an explicit routed path imply a need to direct the egress on a path for the reverse direction? That’s not generally a “need” in all situations.
GIM2>> Please see modified Abstract above. It uses “may be a 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.
GIM>> If the paths used by forward and return paths are not co-routed that may create ambiguous situation when interpreting failure detection on the node that sends BFD control packets onto the monitored path, e.g. LSP.


First, the same ambiguity exists with bi-directional co-routed paths — because links and nodes are not the only resources that need to be common. FIBs can be programmed well on one direction and wrongly on the reverse.
GIM2>> BFD, as other continuity check protocols, only performs defect detection. Defect localization and characterization usually performed with other tools. But having co-routed path for a test session reduces, not eliminates but reduces, ambiguity of defects of the reverse path.
Second, again, why “best suited”? If the BFD specs say that BFD can monitor unidirectional paths (including MPLS LSPs and unidirectional links), seems BFD is not necessarily “best suited to…"
GIM2>> BFD certainly can and being used to monitor uni-directional paths. And usually we don’t think of how the reverse path may affect defect detection or, in case of PM OAM, measured performance metric. Having test session on bi-directional co-routed path makes the test result interpretation more certain.


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.
GIM>> AFAIK, only MPLS-TP defined p2p bi-directional co-routed LSP. True, co-routed bi-directional tunnel may be constructed by using combination of RSVP’s RRO and ERO as well. But other than that, AFAIK, all objects monitored by multi-hop BFD are not guaranteed to be co-routed.

If the path *is* bi-directional co-routed (by whichever method), you would not need this — the was my point, not that some cases are co-routed and others are not.
GIM>> Yes, and the scope of this proposal is not on already bi-directional co-routed paths, e.g. MPLS-TP p2p bi-directional co-routed LSP. RSVP-TE LSP may be explicitly routed but are unidirectional. And so SR tunnel.


Next sentence:

   be co-routed, thus
   fulfilling the implicit BFD requirement

But BFD never has this requirement, implicit or explicit.
GIM>> If the goal of BFD to ensure reliable detection of failures, then co-routed multi-hop path is implied.

I disagree. Even RFC 5883 says:

   The Bidirectional Forwarding Detection (BFD) protocol [BFD] defines a
   method for liveness detection of arbitrary paths between systems.

and

   BFD can also be useful on arbitrary paths between systems, which may
   span multiple network hops and follow unpredictable paths.

and

   Furthermore, a pair of systems may have multiple paths between them
   that may overlap.  This document describes methods for using BFD in
   such scenarios.

GIM2>> Will remove “… thus fulfilling the implicit BFD requirement” from the document.


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?
GIM>> If there’s no co-routedness between a monitored path and the return path, then this draft provides mechanism that may be used to remove possible ambiguity in interpretation of failure of the return path.


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.
GIM>> Appreciate your detailed analysis of the Segment Routing use case. We didn’t spell it out in such details but will be glad to add applicability clarification based on your comment.


I do not think it is about Applicability. Given the existence of Adj-SIDs, the question is about technical feasibility. What do you do in the example I detailed above? Seems like there are potential cases in which it is not possible to specify the actual return path desired.

GIM2>> There are many scenarios where it is possible and useful to specify strict explicit path for SR tunnel. To use strict or loose paths – that we leave to the operator to decide. The proposal addresses real scenario, though not all possible ones.

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.
GIM>> I don’t see need to use Target FEC Stacks as the purpose of the BFD Reverse Path TLV not to verify mapping between the control and the data planes but to provide the remote BFD peer with pre-computed for the reverse path.

It is not about verification of the control and data planes in the return path.

The reason why RFC 7110 uses FECs and not Labels is that the invariant is the FEC, while the label numerical value can change. Seems like using labels is less robust and more brittle.

GIM2>> If label value changes, i.e. label withdrawn, then BFD return path for the BFD session may be re-signaled via LSP Ping with the same BFD discriminator.

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.
GIM>> We haven’t updated the draft just because I-D.kumarkini-mpls-spring-lsp-ping got adopted by the WG. Will certainly update the reference in the next version.

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.
GIM>> This document enhances LSP Ping for Segment Routing environment and we’ve proposed clarification to use of LSP Ping in Segment Routing when bootstrapping BFD session that, hopefully, will be discussed by MPLS WG in course of this WGLC and used in draft-ietf-mpls-spring-lsp.

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.


draft-ietf-mpls-spring-lsp-ping-00 seems to be a dependency to advance before this.

I still do not understand the technical reason to use MPLS labels and not FECs. Further, using labels can suffer from misdirection if Label assignment changes (for a stable given FEC)
GIM2>> Return path may be re-signaled in such case.

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.
GIM>> Agree, IPv6 case is outside the MPLS WG and I’ll remove it for further study.


OK.

The issue about using labels (along with the other ones) still remain.



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.

Thanks,

— Carlos.

On Jul 10, 2016, at 1:59 PM, Martin Vigoureux <[e-mail]<mailto:[e-mail]>> 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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