[quicwg/base-drafts] Connection ID Sequence Number and Server's Preferred Address (#1483)

[Mike Bishop <[e-mail]>]

Follow-on issue from #1465.  The existing text (restored but not changed by the PR) defines the CID selected during the handshake to be Sequence -1, and the CIDs issued via NCID frame to begin at zero.

However, the CID issued by the server as part of the Server's Preferred Address doesn't have a suggested sequence number.  If we follow a similar convention and assign it an implicit negative number (e.g. handshake becomes -2, SPA gets -1), that introduces another quirk:  With the existing language, the server will answer with a new connection ID (say, 0) which will immediately force the client into a connection ID rotation anyway.

Possible solutions include:
- Not including a CID with the SPA parameter; client needs to wait for (and server needs to send) a NCID frame before the probe/transition has happened.
- Server includes a sequence number in the SPA parameter and treats it like any other sequence number it has issued (but then why should the client be required to use that particular CID?)
- SPA is implicitly sequence number zero, and server can use the client's sequence number zero in response

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