Re: [Idr] draft-ietf-idr-rs-bfd-05 - 2 Week WG LC from 4/13 to 4/27

Nick Hilliard <nick@foobar.org> Wed, 09 May 2018 15:23 UTC

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Subject: Re: [Idr] draft-ietf-idr-rs-bfd-05 - 2 Week WG LC from 4/13 to 4/27
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The WGLC is officially over, but not announced as over. I meant to 
comment on this ID sooner, but here are some thoughts anyway.

The IDR WG needs to take on board the Camel presentation from dnsops.

BGP is already a hugely convoluted protocol, and this convolution breeds 
both complexity and fragility.  It's not that any individual chunk of 
the protocol is especially complicated in itself: it's more that in 
order to produce a working BGP stack, it's necessary to transcode 
voluminous tomes of RFCs and then figure out all the implementation 
subtleties required for reasonable interoperability with other stacks.

Occasionally I get to talk to people who attempt to write new BGP stacks 
and their opinion is that it's a daunting prospect.  The sheer volume of 
features in BGP is now such that building new BGP implementations is 
prohibitively expensive, and the possibility of starting again with a 
new protocol (bgp5?) is receding further into the distance due to the 
scale of what it would take to replicate the bits of BGP that people 
need and use.

There's no doubt all these protocol developments are useful, but the 
cost of implementing them is not linear.  Probably as a working group, 
we need to take stock of this and try to decide where we want BGP to be 
in 20 years?  Tacking on more arms and legs?  Or do we ever envisage 
taking out the secateurs or a saw?

Regarding draft-ietf-idr-rs-bfd implementations, or lack thereof, there 
isn't a compelling reason why the normal guidelines for WG LC should be 
waived.  In fact, John's argument suggests the opposite - there have 
been some major protocol design changes in what's been suggested since 
the draft was first mooted, and there's still no code to prove that it 
will work well in real life situations, or that the spec is defined well 
enough that one implementation might interoperate with another.

Interoperability in this case is more important than usual because the 
ID is targetted at ixp route servers, which often have weird, ancient 
and tragically broken devices connected.  On AS43760, which is a small 
RS, we still have ~5% of devices not supporting the AS4 extension. 
Think about what that means in terms of code rot.

Regarding the content of the ID, there are two aspects to it: bfd 
brokerage and NH reachability propagation.

The draft states that both problems need to be fixed, but do they 
really?  Is BFD brokerage not sufficient?  Putting it another way, is 
the complexity required by the NH reachability propagation really 
justified by the gain in benefit?  I am having trouble understanding 
why.  Although I can see what it does and how it works, it looks like 
it's optimising an edge case rather than dealing with the meat of the 
issue which is bfd brokerage.

It would dramatically simplify the proposal if the NH reachability 
propagation part could be stripped out.  If it's left in, then the 
overall mechanism seems to be a very complicated and very new departure 
for BGP, which gets back to the point above about the point at which the 
straw breaks the camel's back.  I really think the authors need to make 
it clear why this part of the specification cannot be done without.

Regarding WGLC, I can understand the desire to get this ID into the bag, 
get it on customer feature spec lists and get it out the door from a 
vendor point of view, but I query the wisdom of pushing a draft like 
this through IETF WGLC without any running code in the general case, and 
a good deal more in this specific case.

Right now, I'm inclined to think that it shouldn't proceed until there 
is consensus that the extenuating circumstances are sufficient that it 
should get special treatment, or, as John says below, whether we should 
open up a wider discussion on what part running code plays in protocol 
development.  We may also wish to discuss protocol over-development and 
the balance between complexity vs gain.

Nick

John G. Scudder wrote:
> (As a co-author, yes, really. Maybe the co-chair would even disagree 
> with some of these points, who knows.[*])
> 
>> On Apr 13, 2018, at 1:31 PM, Job Snijders <job@ntt.net 
>> <mailto:job@ntt.net>> wrote:
>>
>> Dear all,
>>
>> Are there implementations? Is there an implemention report as per IDR 
>> tradition?
>>
>> If not, I don’t support progressing this draft at this moment.
>>
>> The machinery described in this draft is not entirely straight 
>> forward, without running code we should hold off on progressing. 
> 
> For those who missed it, there is a bit more context in Job's earlier 
> note, https://www.ietf.org/mail-archive/web/idr/current/msg19261.html, 
> including
> 
> "ps. yes, i know IDR can have WGLC without running code, but given that
> this is a non-trivial extension to BGP; I'd really like to see multiple
> implementations that are able to interact with each other in the correct
> way and a report detailing the interopability."
> 
> In this case the authors are, exactly and without pretense, requesting 
> WGLC without running code. Which seems a funny thing coming from me -- 
> sometimes all these hats I wear get too tight and give me a headache. 
> Anyway, in this case we think it's worth doing the WGLC because it 
> indicates as clearly as we can to potential implementors that the spec 
> is "done" and will not continue to mutate. As you might recall, this 
> particular spec has changed quite a lot over its history, more than 
> many, and I wouldn't blame an implementor for standing back and waiting 
> for the dust to settle before writing code.
> 
> As we all know (and have a history of doing, as a WG) if post-WGLC 
> implementation experience indicates a need for changes, the draft can 
> and will be updated to take them onboard, and of course if those changes 
> are substantive enough, I would expect a new WGLC would be needed at 
> that point. Other groups do this same review/implementation cycle by 
> progressing things to RFC, then implementing, then bis-ing or otherwise 
> updating the RFCs. I am not (again, as a WG member) a fan of that, and 
> I'm glad we don't do things that way, although our way is also not 
> perfect as witness this discussion.
> 
> Fundamentally a WGLC is a way of asking the WG, "the authors think this 
> is done, do you agree?" We are asking. "We have implementations" is a 
> strong argument to support the "we are done" argument, but it's neither 
> necessary nor sufficient and so far the WG has deemed it desirable but 
> not required for WGLC. We think there would be no benefit, and some 
> harm, from letting the document sit in "active WG draft" status when 
> it's *not* in fact active and the authors have moved on to other things. 
> Moving it to "waiting for implementation" status, if the results of the 
> WGLC support that, is more honest and more clear. I suppose another way 
> of doing this would be to invent some IDR pre-WGLC-WGLC that feeds into 
> the "waiting for implementation" document state, but right now we don't 
> have that.
> 
> Of course you and any other WG member are free to use all the facts at 
> your disposal (including the document itself, and including any reported 
> implementation) to decide if you want to support a WGLC, and that's fine.
> 
> Finally I'd like to ask (and this *is* with the co-chair hat on) that if 
> we continue a wide-ranging philosophical discussion of the IDR 
> implementation tradition as opposed to a discussion of whether the doc 
> is ready for WGLC, that we change the subject line accordingly.
> 
> Thanks!
> 
> --John
> 
> [*] "Do I contradict myself? Very well, then I contradict myself, I am 
> large, I contain multitudes." --Walt Whitman
> 
> 
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