[quicwg/base-drafts] Description of the use of Preferred Address is unclear (#3353)

[Kazuho Oku <[e-mail]>]

At the moment, [section 9.6.1](https://quicwg.org/base-drafts/draft-ietf-quic-transport.html#name-communicating-a-preferred-a) states:
> Once the handshake is finished, the client SHOULD select one of the two server's preferred addresses and initiate path validation (see Section 8.2) of that address using the connection ID provided in the preferred_address transport parameter.
>
> If path validation succeeds, the client SHOULD immediately begin sending all future packets to the new server address using the new connection ID and discontinue use of the old server address. If path validation fails, the client MUST continue sending all future packets to the server's original IP address.

I think these paragraphs have two issues:

* It begins with "once handshake is finished," but I am not sure if we define when a handshake finishes. I think we should change this to "once handshake is confirmed," as that is the point where we allow connection migration.
* It states that the connection ID to be used on the new path would be the one using TP.preferred_address. I think that is incorrect, as the server might have requested retirement of the CID provided by that transport parameter by the time the client initiates (or finishes) path validation. It should either be an address provided by that transport parameter, _or an unused CID provided by the server (i.e. through NCID frames)_. Note that this problem is orthogonal to #3348, because these actions in relation to preferred address happens at some point after the handshake is confirmed.

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