Re: [quicwg/base-drafts] Compatible version upgrade (#1901)

[Kazuho Oku <[e-mail]>]

kazuho commented on this pull request.



> +
+Client and server exchange information about versions along with transport
+parameters in order to protect against version downgrade attacks (see
+{{version-validation}}).
+
+The `supported_versions` field is a list of the versions the client supports.
+This field enables upgrade to compatible versions (see {{version-upgrade}}).  A
+client MAY omit versions from this list if they appear in the
+`unsupported_versions` field.
+
+The `unsupported_versions` field is a list of the versions the client supports,
+but which the client believes that the server does not support.  This list MUST
+be empty unless the client has received and processed a Version Negotiation
+packet.  A client MUST NOT omit a version that it supports from
+`unsupported_versions`, unless the Version Negotiation packet it processed
+listed that version.

I wonder if this _unless_ clause contradicts with the following sentence added above: _Though different versions might convey information about versions differently, a client MUST NOT change the set of versions it claims to support._

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