require clients to send RETIRE_CID with sequence number of 0 or 1, wh…

…en the serever uses TP.preferred_address

draft-ietf-quic-transport.md

@@ -2260,6 +2260,12 @@ future packets to the new server and discontinue use of the old server address.
 If path validation fails, the client MUST continue sending all future packets to
 the server's original IP address.
 
+When the client finishes migrating to the preferred address, it retires the
+connection ID that was used for the original path ({{retiring-cids}}).
+Similarly, a client that continues using the original server address SHOULD
+retire the connection ID provided by the preferred_address transport parameter.
+Retirement of either of these connection IDs notifies the server the server
+address the client has chosen.
 
 ### Responding to Connection Migration