Mandate ACK and allow PADDING in Revert

See discussion in #627 for the stronger language for ACK frames.

Closes #1067. Updates #627.

draft-ietf-quic-transport.md

@@ -627,11 +627,11 @@ any version negotiation that occurred (see {{version-negotiation}}).  The client
 MAY also retain any observed RTT or congestion state that it has accumulated for
 the flow, but other transport state MUST be discarded.
 
-The payload of the Retry packet contains a STREAM frame on
-stream 0 with offset 0 containing the server's cryptographic stateless retry
-material. It SHOULD also contain an ACK frame to allow the client a first RTT
-measurement. The next STREAM frame sent by the server will also start at stream
-offset 0.
+The payload of the Retry packet contains at least two frames. It MUST include a 
+STREAM frame on stream 0 with offset 0 containing the server's cryptographic 
+stateless retry material. It MUST also include an ACK frame to allow the client 
+a first RTT measurement. It MAY additionally include one ore more PADDING frames.
+The next STREAM frame sent by the server will also start at stream offset 0.
 
 
 ### Handshake Packet {#packet-handshake}