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Re: [quicwg/base-drafts] Does a Retry really need to change the CID? (#2837)

Mike Bishop <notifications@github.com> Tue, 25 June 2019 17:51 UTC

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From: Mike Bishop <notifications@github.com>
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Subject: Re: [quicwg/base-drafts] Does a Retry really need to change the CID? (#2837)
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Consider the case where:
- Client sends Initial packet 0, no token, DCID A
- Client times out the Initial packet and sends Initial packet 1, no token, DCID A
- Client receives a Retry with ODCID = SCID = A.
- Client sends Initial packet 2, with token from Retry, DCID A
- Client receives a Retry with ODCID = SCID = A.

Is this Retry a response to packet 1 from a stateless server with a high RTT, or a prohibited second Retry in response to packet 2?  If packet 2 carries a different DCID, the client can tell by inspecting the ODCID field in the Retry packet.  If packet 2 uses the same DCID as packets 0-1, it can't tell.

Requiring a change allows the client to determine that the server isn't misbehaving in this case.

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