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Re: [quicwg/base-drafts] Why are we bothering to number CIDs? (#2159)

Mike Bishop <notifications@github.com> Thu, 13 December 2018 22:33 UTC

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Subject: Re: [quicwg/base-drafts] Why are we bothering to number CIDs? (#2159)
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Because you're not realistically going to retain a total history of all CIDs you've ever seen.  You MAY treat it as an error if you notice that one recurs, so that servers are disincented from doing so, but you're not obliged to check that over the whole connection lifetime.  However, without maintaining that history for a sufficiently long time, you then can't differentiate for certain between a retransmission of an old NCID frame and a new issuance of the same CID (an error, but one you might miss).  Therefore, you could wind up in a state where the peers disagree about whether a CID is still live.

The options were a requirement to keep history for a certain length of time (and hope that packets don't get delayed longer than whatever arbitrary timeout we pick) or using sequence numbers and allowing tracking with a similar data structure as that which tracks whether Stream IDs are not-yet-open, open, or closed.

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