RE: WGLC for draft-ietf-bfd-large-packets

"Les Ginsberg (ginsberg)" <ginsberg@cisco.com> Wed, 18 September 2019 05:24 UTC

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From: "Les Ginsberg (ginsberg)" <ginsberg@cisco.com>
To: Jeffrey Haas <jhaas@pfrc.org>, "Ketan Talaulikar (ketant)" <ketant@cisco.com>
CC: "Reshad Rahman (rrahman)" <rrahman@cisco.com>, "rtg-bfd@ietf.org" <rtg-bfd@ietf.org>
Subject: RE: WGLC for draft-ietf-bfd-large-packets
Thread-Topic: WGLC for draft-ietf-bfd-large-packets
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A few apologies:

1)Sorry to be late in responding - but just back from vacation.
2)As a number of my comments overlap with Ketan's, I decided to respond to this email - apologies if that adds confusion.
3)I will top post my reply in the hopes it will more universally readable - apologies if the lack of context makes things harder to understand.

With that out of the way...

I support this draft - but I do not think it is ready for last call.

There are very legitimate concerns about the impact supporting padded BFD PDUs may have on existing hardware implementations - both functionally and in terms of scalability.
As a standards track document I think more needs to be said about operational considerations - and I think there may be legitimate reasons to consider negotiation of the capability.
In particular the statement in Section 3:

    "Additional changes to the base BFD   protocol may be required to permit negotiation of this functionality
         and the padding value."

If these protocol changes are to be made, shouldn't they be specified in this document?? Otherwise the document would seem just informational.

Section 3 also suggests 

      "support[ing] a lower transmission  interval (bfd.DesiredMinTxInterval)"

as a means of minimizing scalability impacts. But in early discussions of this draft it had been suggested that rather than use BFD for PathMTU validation we could use another protocol (e.g., IS-IS hellos) to do this. But that was rejected because it was stated that the deployment requirement required much faster detection. If that argument holds, then the suggestion in the draft to use longer timers seems inappropriate - or at least without sufficient context.

(BTW, by "lower transmission interval" I assume you mean a longer interval (i.e., a larger value for bfd.DesiredMinTxInterval)??)

I think these points need to be addressed in the draft before it is can be considered complete.

    Les


> -----Original Message-----
> From: Rtg-bfd <rtg-bfd-bounces@ietf.org> On Behalf Of Jeffrey Haas
> Sent: Friday, September 13, 2019 5:34 AM
> To: Ketan Talaulikar (ketant) <ketant@cisco.com>
> Cc: Reshad Rahman (rrahman) <rrahman@cisco.com>om>; rtg-bfd@ietf.org
> Subject: Re: WGLC for draft-ietf-bfd-large-packets
> 
> Ketan,
> 
> Thanks for your comments.
> 
> 
> > On Sep 12, 2019, at 11:55 PM, Ketan Talaulikar (ketant)
> <ketant@cisco.com> wrote:
> >
> > Hi All,
> >
> > I would like to ask some questions and seek clarifications on this draft.
> >
> > 	• I am aware that this draft originates from practical pain points at a
> specific operator. During the adoption calls, the scenarios were debated in
> detail. It was basically a L2 WAN circuit service over a provider network and
> the challenge was that the PMTU of the underlying path in the SP network
> changes dynamically. However, from the Enterprise POV, the L2 circuit is
> seen as a single link and the BFD runs in directly connected mode. The draft
> however, discusses BFD multi-hop which is an entirely different use-case.
> 
> This is perhaps some poor wording choices in the Introduction section.  The
> intent is really that this is usable for all cases where BFD may be used.  The
> Introduction text was intended to note that for single-hop scenarios, BFD
> Echo procedures may be sufficient to solve the problem.  (See comment to
> Carlos elsewhere in the thread.)  However, for any form of multi-hop, you
> can't solve this via Echo.
> 
> General note about the "pain points at a specific author": While Albert
> certainly was the first customer I'd had willing to help stamp their name on
> such a draft, I'd been approached in my chair capacity both in IETF and at my
> employer over the years about similar headaches many times.  This is sadly
> not a rare problem.
> 
> > When doing multi-hop, the BFD packet could go over entirely different
> paths in the network with different PMTUs (especially different from the
> application sending large packets) – this makes things flaky? So shouldn’t this
> mechanism be actually focussed on the single hop/directly connected mode
> instead?
> 
> Even for single-hop this may be slightly flakey.  Consider "directly connected"
> links that are composed from LAGs.
> 
> The draft doesn't try to completely address all such forms of multipath
> problems, even for directly connected solutions.  When we were doing BFD
> for LAGs, we realized that trying to get overly specific in terms of the
> implementation not only got ourselves perilously into the specifics of a given
> vendor, but potentially into conflict with IEEE in terms of discussions about
> distributing traffic.  To that end, we've been silent on the matter in this draft.
> 
> It's reasonable in any BFD implementation dealing with multipath issues to
> potentially interact with the load balancer and decide to exercise specific
> links.  I believe it's the case for some scenarios that your own employer may
> do this in pre BFD-on-LAG implementations for single hop BFD based on
> some old mailing list discussion.  However, it's proven unreasonable over the
> life of the BFD protocol to try to get too pedantic over how to do such a thing;
> some hardware may simply not support it.
> 
> 
> >
> > 	• There are implementations with BFD offload to H/W out there.
> What happens when a particular implementation cannot handle such large
> packet sizes (and I am not specifically aware of one such)? Like other aspects
> of monitoring like the intervals, would there be a value in indicating a support
> for large packets during the signalling? The draft does raise the need for it
> but doesn’t seem to do anything about it – why? Either we need it and this
> draft should define it. Or we don’t need it and it would placing the onus on
> the operator to enable this when they know both ends support it. Then it is
> something for operational consideration section perhaps?
> 
> This is effectively a criticism of BFD in general rather than specifically this
> feature.  Part of the motivation for RFC 7419, Common Interval Support in
> Bidirectional Forwarding Detection, was to specify a number of common
> implementation timings.  But even then, that document doesn't talk much
> about scaling considerations for a given set of intervals.  Common
> considerations for implementors and operators tend to be "my
> implementation can support X sessions at 3.3ms, Y and 10ms, and Z much
> slower if we leave the fully distributed to the line card mode".  This just
> simply means the product sheet gets another axis in its listings.
> 
> What you're asking for in the protocol otherwise is a mechanism to negotiate
> or renegotiate a set of timers over a given set of interfaces based on existing
> or future re-provisioning of session quantity and parameters.  I think you'll
> find operators aren't particularly supportive of that.
> 
> FWIW, we expect exactly these same questions during IESG review.  BFD had
> exactly this type of question when doing even the base RFC 5880 work.  (BFD
> tends to make the current Transport AD unhappy.)
> 
> 
> >
> > 	• There was a discussion on the list about whether this needs to be
> done for every packet or not. I don’t find that discussion or the result
> captured in the draft. The draft just says that perhaps longer intervals should
> be applied to BFD when doing large packets – but this will defeat the purpose
> of fast-detection. What would be good is we have both fast-detection and
> slow PMTU validation? Perhaps we need some analysis of whether large
> packets should be used always or intermittently and the pros/cons or
> guidelines for the same?
> 
> See my comment to Carlos in-thread about why this makes the timers tricky.
> See also how BFD Echo can solve this problem in some circumstances without
> negotiation.
> 
> It's worth noting that BFD is quite capable of dynamically re-negotiating its
> timers.  This means an implementation could have a behavior such as:
> 
> 1. Start by bringing up its session with timers for regular sized packets, and
> 3.3ms
> 2. Decide to shift to large packets, but renegotiate to 10ms timers first.
> 
> The "signaling" that a system is willing to accept the large packets is shifting to
> a longer timer.
> 
> 
> >
> > 	• The draft is missing an operational considerations and
> manageability considerations sections. Some of this info is placed in sec 3, but
> would help if things were elaborated in their individual sections. It would
> provide more insight into how exactly this mechanism in BFD is envisaged to
> be actually deployed and used. More importantly, perhaps how it should
> NOT be used?
> 
> Is there specific information you think should go into such a section that isn't
> otherwise present in the draft?  The "how it should NOT" be used is a
> challenging section to write - somewhat similar to proving negatives - without
> specific content.
> 
> >
> > Can the authors and WG discuss the above? I think it seems too rushed to
> go for WGLC just as yet?
> 
> The purpose of WGLC is to provide the working group an opportunity to drive
> out commentary like that above when things are otherwise quiet. :-)  Thanks
> for providing it.
> 
> -- Jeff