Re: [Idr] Early allocation for draft-ietf-idr-bgp-gr-notification

"John G. Scudder" <jgs@juniper.net> Mon, 20 March 2017 21:50 UTC

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From: "John G. Scudder" <jgs@juniper.net>
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Date: Mon, 20 Mar 2017 17:49:50 -0400
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To: Jeffrey Haas <jhaas@pfrc.org>, Job Snijders <job@ntt.net>
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Subject: Re: [Idr] Early allocation for draft-ietf-idr-bgp-gr-notification
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On Mar 20, 2017, at 4:41 PM, Jeffrey Haas <jhaas@pfrc.org> wrote:
> On Mon, Mar 20, 2017 at 09:14:55PM +0100, Job Snijders wrote:
> [session culling]
>>> I haven't yet read this draft, so take my further comments with that under
>>> consideration.
>> 
>> ack. Please read the draft. We may have a fundamental problem on our
>> hands here.
> 
> I read it. It's a weird hack, but it works. :-)

With respect to what "culling" calls "Voluntary BGP Session Teardown", there's no problem. A pair of implementations that implement gr-notification would already be expected to do the right thing when the session was administratively shut down, by sending a hard reset.

With respect to what "culling" calls "Involuntary BGP Session Teardown", there is potentially a problem, but I think it's not insurmountable. The problem, to the extent it exists, is that "culling" suggests the IX switch fabric should be configured to block BGP traffic, causing the hold time to expire. gr-notification interferes by putting the session into GR mode:

   When a BGP speaker resets its session due to a HOLDTIME expiry, it
   should generate the relevant BGP NOTIFICATION message as mentioned in
   [RFC4271], but subsequently it MUST follow the rules for the
   Receiving Speaker mentioned in Section 4.1.

At first this might seem alarming, but my observation is that the weird hack already relies on waiting several minutes for the hold time to expire. So, it doesn't seem terrible to add a few more minutes to wait for the stale route timeout as well. Although "culling" doesn't discuss how to determine the maintenance can proceed, it would seem both prudent and straightforward to do so by monitoring the traffic level between the affected peers, since presumably the IX operator doesn't have a priori knowledge of the hold time in use. Once routing has converged away from the path, the traffic level will drop, and the maintenance can proceed, regardless of what perversions are in use in the control plane. 

--John

P.S.: I take no position, in this note, as to whether the suggestion in "culling" to filter BGP control traffic as an IX management practice is a good one.