Fix a race condition cause by out of band signaling

Signaling flow IDs may happen out of band and cause a race condition
when flow IDs are received before signaling is complete.

draft-ietf-avtcore-rtp-over-quic.md

@@ -517,7 +517,20 @@ the same flow identifier (following the procedures defined in {{?RFC5761}}), or
 they can use different flow identifiers.
 
 The association between flow identifiers and RTP streams MUST be negotiated
-using appropriate signaling. If a receiver cannot associate a flow identifier
+using appropriate signaling. The signaling happens out of band and thus a stream
+or datagram with a given flow identifer may arrive before the signaling
+finished. In that case, an endpoint may not be able to associate the stream or
+datagram with the corresponding RTP stream. The endpoint SHOULD buffer streams
+and datagrams using an unknown flow identifier until they can be associated with
+the corresponding RTP stream. To avoid resource exhaustion, the endpoint SHOULD
+limit the number of streams and datagrams to buffer. If the number of buffered
+streams exceeds the limit, the endpoint SHOULD send a STOP_SENDING with the
+error code ROQ_UNKNOWN_FLOW_ID. It is an implementation's choice on which stream
+to send STOP_SENDING. If the number of buffered datagrams exceeds the limit, the
+endpoint SHOULD drop a datagram. It is an implementation's choice which datagram
+to drop.
+
+If a receiver cannot associate a flow identifier
 with any known RTP stream, it MUST close the connection with the application
 error code ROQ_UNKNOWN_FLOW_ID.