rtmp: Implement check bandwidth notification.

According to the behaviour of librtmp, it is recommended to send this
message to the server after receiving the 'onBWDone' callback in order
to do bandwidth checking and improve compatibility with some servers.

libavformat/rtmpproto.c

@@ -372,6 +372,25 @@ static void gen_server_bw(URLContext *s, RTMPContext *rt)
     ff_rtmp_packet_destroy(&pkt);
 }
 
+/**
+ * Generate check bandwidth message and send it to the server.
+ */
+static void gen_check_bw(URLContext *s, RTMPContext *rt)
+{
+    RTMPPacket pkt;
+    uint8_t *p;
+
+    ff_rtmp_packet_create(&pkt, RTMP_SYSTEM_CHANNEL, RTMP_PT_INVOKE, 0, 21);
+
+    p = pkt.data;
+    ff_amf_write_string(&p, "_checkbw");
+    ff_amf_write_number(&p, ++rt->nb_invokes);
+    ff_amf_write_null(&p);
+
+    ff_rtmp_packet_write(rt->stream, &pkt, rt->chunk_size, rt->prev_pkt[1]);
+    ff_rtmp_packet_destroy(&pkt);
+}
+
 /**
  * Generate report on bytes read so far and send it to the server.
  */
@@ -691,6 +710,8 @@ static int rtmp_parse_result(URLContext *s, RTMPContext *rt, RTMPPacket *pkt)
             if (!t && !strcmp(tmpstr, "NetStream.Play.Stop")) rt->state = STATE_STOPPED;
             if (!t && !strcmp(tmpstr, "NetStream.Play.UnpublishNotify")) rt->state = STATE_STOPPED;
             if (!t && !strcmp(tmpstr, "NetStream.Publish.Start")) rt->state = STATE_PUBLISHING;
+        } else if (!memcmp(pkt->data, "\002\000\010onBWDone", 11)) {
+            gen_check_bw(s, rt);
         }
         break;
     }