Re: [bess] draft-mohanty-bess-evpn-bum-opt-00 - clarification on problem description

Eric C Rosen <erosen@juniper.net> Thu, 22 March 2018 13:57 UTC

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To: "Rabadan, Jorge (Nokia - US/Mountain View)" <jorge.rabadan@nokia.com>, Sandy Breeze <sandy.breeze@eu.clara.net>, "bess@ietf.org" <bess@ietf.org>
References: <53C24F41-B86F-4FE1-8041-721C95C7E7F0@nokia.com>
From: Eric C Rosen <erosen@juniper.net>
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Subject: Re: [bess] draft-mohanty-bess-evpn-bum-opt-00 - clarification on problem description
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On 3/21/2018 12:36 PM, Rabadan, Jorge (Nokia - US/Mountain View) wrote:
>
> This scenario definitively helps understand the use-case better. Still 
> a bit specific but I think you should add this scenario to the draft, 
> and again, make it Informational since there is no control plane 
> change for this.
>

If I understand correctly, the draft does make a control plane change, 
since it describes situations in which IMET routes should not be 
originated.  This contradicts RFC 7432, and so would have to be 
considered a update to that, and hence a standards track document.

Since I wasn't at the BESS meeting (but did watch the video), it's 
possible I missed some of the discussion, but from my reading of the 
draft, I have the following concerns.

I'm not sure the draft properly describes the situations in which one 
may omit the IMET route.  It describes the situation in which a PE 
doesn't need to propagate, on any of its ACs, BUM traffic that it 
receives from the backbone.  However, if the PE has IRB interfaces, 
doesn't it need to receive some of the BUM traffic in order to process 
that traffic itself?  For example, if a PE is configured to be  a PIM 
router attached to two Broadcast Domains, BD1 and BD2, won't it need to 
receive PIM Hellos from BD1, even if it doesn't actually propagate those 
out the local AC attaching it to BD1?

At the meeting a DF election scheme was proposed in which, for a given 
<ES,BD> pair, there could be a different DF for each(S,G)  multicast 
flow.  I don't think the draft takes this into account.  I wonder how 
many other scenarios there are which the draft fails to consider.

Many EVPN drafts have been written on the presumption that IMET routes 
will always be originated.  Some of the drafts add flags or communities 
to the IMET routes to advertise capabilities of one sort or another.  
Every one of those drafts would need to be checked to see if it still 
works when some nodes do not originate IMET routes.

As future EVPN drafts are written, the authors (and reviewers) will now 
have to remember that they cannot presume that all the PEs attached to a 
given BD are originating IMET routes for that BD. This creates more 
complexity, more corner cases, and ultimately, more specification bugs.

Still, one might consider adopting this complication if it were a big 
win.  But it only seems to apply to one specific (and not very common) 
scenario, and from the discussion at the microphone it wasn't clear to 
me that the co-authors are even on the same page about just what that 
scenario is (recall the discussion about whether the diagram in the 
draft does or does not depict the intended use case).