Merge branch 'main' into ianswett-subscribe-past

draft-ietf-moq-transport.md

@@ -1433,16 +1433,22 @@ A publisher MUST NOT send objects from outside the requested start and end.
 ## SUBSCRIBE_UPDATE {#message-subscribe-update-req}
 
 A subscriber issues a SUBSCRIBE_UPDATE to a publisher to request a change to
-a prior subscription.  Subscriptions can only become more narrower, not wider,
-because an attempt to widen a subscription could fail.  If Objects before the
-start or after the end of the current subscription are needed, a separate
-subscription can be made. The start Object MUST NOT decrease and when it increases,
-there is no guarantee that a publisher will not have already sent Objects before
-the new start Object.  The end Object MUST NOT increase and when it decreases,
-there is no guarantee that a publisher will not have already sent Objects after
-the new end Object. A publisher SHOULD close the Session as a 'Protocol Violation'
-if the SUBSCRIBE_UPDATE violates either rule or if the subscriber specifies a
-Subscribe ID that does not exist within the Session.
+an existing subscription. Subscriptions can only become more narrow, not wider,
+because an attempt to widen a subscription could fail. If Objects before the
+start or after the end of the current subscription are needed, a fetch might
+be able to retrieve objects before the start. The start Object MUST NOT
+decrease and when it increases, there is no guarantee that a publisher will
+not have already sent Objects before the new start Object.  The end Group
+MUST NOT increase and when it decreases, there is no guarantee that a publisher
+will not have already sent Objects after the new end Object. A publisher SHOULD
+close the Session as a 'Protocol Violation' if the SUBSCRIBE_UPDATE violates
+either rule or if the subscriber specifies a Subscribe ID that has not existed
+within the Session.
+
+There is no control message in response to a SUBSCRIBE_UPDATE, because it is
+expected that it will always succeed and the worst outcome is that it is not
+processed promptly and some extra objects from the existing subscription are
+delivered.
 
 Unlike a new subscription, SUBSCRIBE_UPDATE can not cause an Object to be
 delivered multiple times.  Like SUBSCRIBE, EndGroup MUST specify the