[quicwg/base-drafts] unrecoverable loss pattern leads to deadlock (#2863)

[Marten Seemann <[e-mail]>]

There's a loss pattern that QUIC cannot recover from:
Assume client and server perform a handshake up to the point just after the server received the ClientFinished message. No packet loss happens so far. Both peers have now **completed** (but not yet **confirmed**) the handshake.

Right after sending the ClientFinished, the client sends a 1-RTT packet (e.g. containing application data). This packet is lost. The server sends an acknowledgement for the ClientFinished (in a Handshake packet), and this packet is lost as well. Furthermore, the server also sends a 1-RTT packet (e.g. containing a session ticket). This packet is successfully received and acknowledged by the client.

At this point the endpoint are in the following state: Since it received an acknowledgement for a packet sent with 1-RTT keys, the server now **confirmed** the handshake, and drops the Handshake keys. On the other hand, the client is still waiting for an acknowledgement for the ClientFinished, and retransmits it. It will however never receive an acknowledgement, since the server dropped the keys already.

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https://github.com/quicwg/base-drafts/issues/2863