[quicwg/base-drafts] CID ratcheting language is imprecise (#1452)

[Mike Bishop <[e-mail]>]

As currently written, each endpoint MUST switch to a new CID when it sees its peer do so.  If taken literally by each peer, this will cause a change of CID by each peer every round trip; unless a sufficient stream of NCID frames is kept up, they'll exhaust the supply and #1276 would have the connection be closed.  That seems both unnecessarily fragile and not actually the intent of this requirement.

Intuitively, you want to switch when:
- You see the peer do so
- It's not in response to your own switch

Not marking this design, because I'm fairly sure this is the intent of the existing text, albeit not what it actually says.  I'm having trouble coming up with a concise way of wording the second piece of the condition, however.

Ironically, this would have been simple when CIDs had sequence numbers -- you MUST always use a CID with a sequence number >= the sequence number of the CID you're receiving from the peer.  But we weren't actually using them for that.  We could do something similar, though, saying you must count the number of times the peer has changed CIDs post-handshake and you must have changed at least as many times.

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