<DD>
Enforce HTTP 1.1 requests.
+<DT><B>2.0</B>
+
+<DD>
+Enforce HTTP version 2 requests.
+<DT><B>2TLS</B>
+
+<DD>
+Enforce version 2 requests for HTTPS, version 1.1 for HTTP.
+<DT><B>2_PRIOR_KNOWLEDGE</B>
+
+<DD>
+Enforce HTTP 2 requests without performing HTTP/1.1 Upgrade first.
</DL>
</DL>