Re: [quicwg/base-drafts] active_connection_id_limit interacts poorly with Retire Prior To (#3193)

Mike Bishop <notifications@github.com> Fri, 08 November 2019 14:58 UTC

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Date: Fri, 08 Nov 2019 06:58:17 -0800
From: Mike Bishop <notifications@github.com>
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Subject: Re: [quicwg/base-drafts] active_connection_id_limit interacts poorly with Retire Prior To (#3193)
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I'm going to contradict me-from-yesterday a little, or at least complicate the point.

The reason we have the 1-RTO window is that some implementations might not be able to actually retire CIDs instantly (packets are queued in hardware offloads, you want to avoid racing in-flight packets against the packet containing the RCID, etc.).  If we require the retirement to be done as part of processing the frame, processing the frame can take up to an RTO.  What happens to other frames in the packet?  Does this packet get ACK'd if you're not done with this process yet?  Plus, you can't actually send the RCID frames until you've got the new CID *unless* you have a race with one more RCID frame in a subsequent packet.  This can get sticky very quickly.

If I had an implementation where I couldn't retire CIDs right away, I would probably tag them for retirement upon receipt of this frame and say those that are tagged don't count toward the limit.  But that's not entirely compliant with the text in your commit, and also assumes I have some headroom I'm not telling the peer about, because I have to remember all of them temporarily.

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