reworked version negotiation

draft-ietf-sidrops-8210bis.xml

@@ -1177,6 +1177,49 @@
     </section>
 
     <section anchor="version" title="Protocol Version Negotiation">
+      <t>
+        Once a router has established a transport connection to a cache,
+        it MUST attempt to open a RPKI-Router 'session' by issuing
+        either a Reset Query <xref target="resetquery"/>) or a Serial
+        Query (<xref target="serialquery"/>) with the highest version of
+        this protocol the router implements in the Protocol Version
+        field.  If the cache supports that version, it responds with a
+        Cache Response (<xref target="cacheresponse"/>) of that version
+        and the session is considered open.
+      </t>
+      <t>
+        If a cache which supports version N receives a query with
+        Protocol Version Q &lt; N, and the cache does not support
+        versions &lt;= Q, the cache MUST send an Error Report (<xref
+        target="error"/>) with Protocol Version N and Error Code 4
+        ("Unsupported Protocol Version") and disconnect the transport
+        session.
+      </t>
+      <t>
+        If a cache which supports version N receives a query with
+        Protocol Version Q &lt; N, the cache MUST downgrade to protocol
+        version Q, <xref target="RFC6810"/> or <xref target="RFC8210"/>,
+        and respond with a Cache Response (<xref
+        target="cacheresponse"/>) of that Protocol Version and the
+        session is considered open.
+      </t>
+      <t>
+        If the router query has version Q > N, the cache MUST send an
+        Error Report (<xref target="error"/>) with Protocol Version
+        N and Error Code 4 ("Unsupported Protocol Version"), and the
+        router SHOULD send another query with a Protocol Version Q of
+        the version N in the Error Report, unless it has already
+        failed at that version.  This MAY repeat with the router
+        attempting to negotiate lower and lower versions until they
+        agree.
+      </t>
+      <t>
+        If the router requests Q == 0 and it still fails with the cache
+        sending an Error Report (<xref target="error"/>) with Error Code
+        4 ("Unsupported Protocol Version"), then the router MUST abort
+        the transport connection.
+      </t>
+<!--
       <t>
         A router MUST start each transport connection by issuing either a
         Reset Query or a Serial Query.  This query MUST tell the cache
@@ -1212,6 +1255,7 @@
           </t>
         </list>
       </t>
+-->      
       <t>
         In any of the downgraded combinations above, the new features of
         the higher version will not be available, and all PDUs MUST have