Re: [quicwg/base-drafts] Looping with multiple Retry packets (#1451)

[Mike Bishop <[e-mail]>]

@nibanks, if the DoS device sends two Retry packets, the client would ignore the second:

- Client sends Initial, stored token, SN = 0
- DoS device responds Retry, token A, SN = 1
- DoS device spuriously sends Retry, token B, SN = 1
- Client processes either Retry, sends new Initial with SN = 1 and token A or B
- Client receives other Retry, ignores because SN != 2.
- Server responds to the Initial with SN = 1 with a Retry, SN = 2
- Client processes Retry, sends another Initial with SN = 2, etc.

Ossification is definitely a concern; someone could come to expect that this field is always zero on the client's first packet.  Encryption would be preferable, but the point of this is to avoid doing decryption work on the server until it's reasonably confident that the client is genuine, so that's problematic.

However, GREASE is an option.  Nothing requires this to start at 0/1 -- client could randomize the value on the first Initial, simply checking that the server incremented.  If we permit wrapping when the server increments, all initial values are acceptable.

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