[quicwg/base-drafts] PTO after handshake completion is not well-defined (#3831)

[Marten Seemann <[e-mail]>]

Assume the following situation: The server sends 0.5-RTT data, but never receives an acknowledgement for that packet. When the handshake completes, the only outstanding packet is that 0.5-RTT packet. The server now needs to arm a PTO timer.

> A sender recomputes and may need to reset its PTO timer every time an ack-eliciting packet is sent or acknowledged, when the handshake is confirmed, or when Initial or Handshake keys are discarded.

This seems to imply that the timer should be set to `handshake_completion_time + PTO`, whereas 

> This ensures the PTO is always set based on the latest RTT information and for the last sent packet in the correct packet number space.

seems to imply that the timer should be set to `outstanding_0.5RTT_packet_sent_time + PTO`.

I think I'd prefer the former interpretation of the latter one. For example, if the entire first flight is lost, the server won't retransmit the Initial for 1s. When the handshake then completes, the RTT is most likely << 1s, which would mean that the timer is set to a time in the past, if we base it on the time the 0.5-RTT packet was sent.

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