Re: [quicwg/base-drafts] describe how 0-RTT is accepted and rejected (#2841)

[Kazuho Oku <[e-mail]>]

kazuho commented on this pull request.



>  
-A server rejects 0-RTT by rejecting 0-RTT at the TLS layer.  This also prevents
-QUIC from sending 0-RTT data. A server will always reject 0-RTT if it sends a
-TLS HelloRetryRequest.
+A server rejects 0-RTT by sending a ServerHello without the EarlyDataIndication.
+A server will always reject 0-RTT if it sends a TLS HelloRetryRequest.  When
+rejecting 0-RTT, a server MUST NOT process any 0-RTT packets, even if it is in
+possesion of the keys to do so.  When 0-RTT was rejected, a client MUST treat
+receipt of an acknowledgement for a 0-RTT packet as a connection error of type

@marten-seemann Contrary to what you state, the spec is clear about skipping PN 0 (or any packet number in a packet number space) is a permitted behavior. Quoting from [section 21.3](https://quicwg.org/base-drafts/draft-ietf-quic-transport.html#rfc.section.21.3), _an endpoint MAY skip packet numbers when sending packets to detect this behavior._

Using unified packet number space on the send side is precisely an act of "skipping packet numbers when sending packets," skipping those that have been used in another packet number space.

>> IIRC, the sub-clause "if it is able to detect the condition" was added intentionally to the general rule, because people were concerned about the text being interpreted as to recommend having a way to detect such errors.
> I think we added this text because for 1-RTT packets, you clear out old ACK ranges from time to time. When receiving an ACK for a very old packet you'd then not be able to check if you actually sent this packet, because you already removed the tracking state.

I'm not sure if I agree with the assumption, but granted that it is true, I think it would be an argument _for_ having the same sub-clause for 0-RTT packets as well. ACK ranges are cleared out from time to time. Then, why do we need to handle ranges for 0-RTT packets specially, only when the server rejects 0-RTT?

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