Tacking stuff on to VN packets

[Martin Duke <[e-mail]>]

Section 6 of invariants
<https://quicwg.org/base-drafts/rfc8999.html#name-version-negotiation>
sayeth:

"Version Negotiation packets do not use integrity or confidentiality
protection. Specific QUIC versions might include protocol elements that
allow endpoints to detect modification or corruption in the set of
supported versions."

I also vaguely remember ekr batting around the idea of tacking some data
onto the packet  to solve a problem we were discussing.

*Maybe we just don't want to do this at all and we can safely ignore this
text in invariants.*

If we *do* want to do it, the design is a little problematic. There is no
length field in VN packets to signal that the supported version list is
ending and other stuff is beginning.

The only way I can think of to make this work is to use the VN draft to
reserve a magic version number to mean "this is the end of the supported
version list". That version is not a real version, MUST be the last one
listed, and perhaps can be followed by another magic number to indicate the
content that follows.

Leaving aside legacy stuff negotiating pre-standard versions, which will
hopefully go away someday, someone supporting multiple versions would be
REQUIRED to support the VN draft and therefore incorporate this change.

Thoughts?
Martin