Re: [quicwg/base-drafts] Desirable behavior when it takes time to derive the traffic keys for the next PN space (#3821)

Kazuho Oku <notifications@github.com> Mon, 06 July 2020 08:53 UTC

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Date: Mon, 06 Jul 2020 01:53:45 -0700
From: Kazuho Oku <notifications@github.com>
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Subject: Re: [quicwg/base-drafts] Desirable behavior when it takes time to derive the traffic keys for the next PN space (#3821)
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> And I have debugged a case where handshakes were failing due to the interaction between PTO timers in different packet number spaces. For that, I don't have a good solution yet; I'm not convinced that deferring PTO is the right answer.

I would argue that a probe packet is expected to send when an endpoint does not receive a response that it expects. That's necessary because it is the only way to guarantee forward progress (and therefore [we use a MUST](https://quicwg.org/base-drafts/draft-ietf-quic-recovery.html#section-6.2.4-1)). But in this particular case that we are discussing, the ball is on the side of the peer. While I am happy to discuss the nuance, I do think that it needs to be something other than "MUST send a probe."

Note also that an endpoint sending PTOs in this case would in turn require the peer to have a larger buffer for storing undecryptable packets in order to avoid buffer overflow (that leads to performance degradation).

> But I don't think that this requires anything in specifications, aside from perhaps a caution, noting that we already have a caution in [Section 5.7 of the TLS draft](https://quicwg.org/base-drafts/draft-ietf-quic-tls.html#pre-hs-protected). That already suggests and permits (though doesn't recommend) buffering.

Aside from relaxing the MUST pointed out above, I think I'd be fine with just adding notes. Though I might argue that it is more important than the existing note that discusses about an "exceptional" case; some endpoints would almost always need time to calculate the next keys.

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