Re: [quicwg/base-drafts] QUIC header format/demultiplexing (#426)

[Colin Perkins <[e-mail]>]

@janaiyengar you can't just use the fingerprint. The algorithm to identify STUN is to check that the first two bits of the packet are 00, then check the magic cookie (bits 33-64 = 0x2112A442), then if necessary find and check the fingerprint attribute. Checking the fingerprint is relatively expensive.

QUIC long header packets don't conflict with STUN, since QUIC long header packets start 0b1x and STUN packets start 0b00. The fingerprint isn't needed.

QUIC short header packets don't conflict if either the C bit is 1 or the C bit is set to 0 and bits 33-64 differ from 0x2112A442. We can avoid collisions without requiring the STUN FINGERPRINT by making the C bit an Omit Connection ID bit (as in #956), and changing the Connection ID to be either (a) a 63-bit value with bit 64 always set to 1; or (b) a 56-bit value in the range 64 to 2^62-1 (i.e., a 64-bit value where the top two and the lower six bits are set to zero) formatted using the variable length integer encoding.

Both these changes seem conceptually clean, and would be QUIC version independent and fit with the invariants. 

That said, I think there's value in demuxing with WebRTC traffic in addition to STUN, at least for QUIC version 1. Renumbering the fields as in #956 is independent of the above discussion of how to demux STUN, doesn't change QUIC semantics, and enables us to do this. 

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