Re: [quicwg/base-drafts] Avoid renewing retired connection IDs (#1799)

[erickinnear <[e-mail]>]

If you're going to use the proposal from NYC: 

> Each CID comes with a sequence number, every CID that you send must have a sequence number higher than any you've previously sent

> This does not requiring anything to happen in lockstep as before, however this solves a big edge case in that you can now forget CIDs immediately instead of waiting for 3xRTO when you retire them, since any NEW_CONNECTION_ID frame crossing paths with the retirement frame will contain a sequence number. Instead, you just remember the highest sequence number NEW_CONNECTION_ID that you've RETIRED.

>For example:
Peer sends NEW_CONNECTION_ID and you use it, then immediately retire it with RETIRE_CONNECTION_ID.
Ack of NEW_CONNECTION_ID gets lost.
RETIRE_CONNECTION_ID frame is still in flight when the peer retransmits NEW_CONNECTION_ID.
If you've got a sequence number you can keep track of the highest sequence that you've gotten and, since this one will not advance the sequence number, you can safely ignore the duplicate NEW_CONNECTION_ID frame.

Note the distinction between *received* and *retired* sequence number, since if you keep track of *received* sequence number for dropping new ones, then reordering between NEW_CONNECTION_ID frames can cause you to ignore some of them when you shouldn't. 

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