be pedantic about supporting alias versions

draft-ietf-quic-transport.md

@@ -3251,12 +3251,17 @@ using for private experimentation on the GitHub wiki at
 In order to avoid ossification of the version number defined by this draft,
 servers announce a list of version numbers that they interpret as aliases for
 the version number used in this draft. Alias versions MUST NOT be a reserved
-version. Servers SHOULD send at least one version alias, and SHOULD frequently
-change the value that they announce. Each version alias contains a lifetime,
-which indicates how long the server will accept this version alias. It also
-contains an initial salt, which is used instead of the initial salt as defined
-in section 5.2 of {{QUIC-TLS}}. The list of version aliases is sent in the
-server's Transport Parameters (see {{transport-parameter-definitions}}).
+version. A server MUST NOT advertise an alias version number for a version that
+it actually supports. If the server advertises an alias version number that the
+client actually supports, the client MUST assume the server doesn't support
+that version and ignore the alias.
+
+Servers SHOULD send at least one version alias, and SHOULD frequently change the
+value that they announce. Each version alias contains a lifetime, which
+indicates how long the server will accept this version alias. It also contains
+an initial salt, which is used instead of the initial salt as defined in section
+5.2 of {{QUIC-TLS}}. The list of version aliases is sent in the server's
+Transport Parameters (see {{transport-parameter-definitions}}).
 
 Clients SHOULD remember the list of aliases and use it for subsequent
 connections to the same server in the future. This applies to both 0-RTT