Add modifications for V9 of the nqptp interface.

nqptp-shm-structures.h

@@ -22,26 +22,36 @@
 
 #define NQPTP_INTERFACE_NAME "/nqptp"
 
-#define NQPTP_SHM_STRUCTURES_VERSION 8
+#define NQPTP_SHM_STRUCTURES_VERSION 9
 #define NQPTP_CONTROL_PORT 9000
 
-// The control port expects a UDP packet with the first space-delimited string
-// being the name of the shared memory interface (SMI) to be used.
-// This allows client applications to have a dedicated named SMI interface with
-// a timing peer list independent of other clients.
-// The name given must be a valid SMI name and must contain no spaces.
-// If the named SMI interface doesn't exist it will be created by NQPTP.
-// The SMI name should be delimited by a space and followed by a command letter.
-// At present, the only command is "T", which must followed by nothing or by
-// a space and a space-delimited list of IPv4 or IPv6 numbers,
-// the whole not to exceed 4096 characters in total.
+// The control port expects a UDP packet with the first character being a command letter
+// and the rest being any arguments, the whole not to exceed 4096 characters.
+// The "T" command,  must followed by nothing or by
+// a space and a space-delimited list of IPv4 or IPv6 numbers.
 // The IPs, if provided, will become the new list of timing peers, replacing any
-// previous list. If the master clock of the new list is the same as that of the old list,
-// the master clock is retained without resynchronisation; this means that non-master
-// devices can be added and removed without disturbing the SMI's existing master clock.
+// previous list. The first IP number is the clock that NQPTP will listen to.
+// The remaining IP address are the addresses of all the timing peers. The timing peers
+// are not used in this version of NQPTP.
 // If no timing list is provided, the existing timing list is deleted.
-// (In future version of NQPTP the SMI interface may also be deleted at this point.)
-// SMI interfaces are not currently deleted or garbage collected.
+// The "B" command is a message that the client -- which generates the clock --
+// is about to start playing.
+// NQPTP uses it to determine that the clock is active and will not sleep.
+// The "E" command signifies that the client has stopped playing and that
+// the clock may shortly sleep.
+// The "P" command signifies that SPS has paused play (buffered audio only).
+// The clock seems to stay running in this state.
+
+// When the clock is active, it is assumed that any decreases in the offset
+// between the local and remote clocks are due to delays in the network.
+// NQPTP smooths the offset by clamping any decreases to a small value.
+// In this way, it can follow clock drift but ignore network delays.
+
+// When the clock is inactive, it can stop running. This causes the offset to decrease.
+// NQPTP clock smoothing would treat this as a network delay, causing true sync to be lost.
+// To avoid this, when the clock goes from inactive to active,
+// NQPTP resets clock smoothing to the new offset. 
+
 
 #include <inttypes.h>
 #include <pthread.h>

player.c

@@ -3606,6 +3606,9 @@ int player_stop(rtsp_conn_info *conn) {
   // debuglev = 3;
   debug(3, "player_stop");
   if (conn->player_thread) {
+#ifdef CONFIG_AIRPLAY_2
+    ptp_send_control_message_string("E"); // signify play is "E"nding
+#endif
     debug(3, "player_thread cancel...");
     pthread_cancel(*conn->player_thread);
     debug(3, "player_thread join...");

rtsp.c

@@ -2022,6 +2022,7 @@ void handle_setrateanchori(rtsp_conn_info *conn, rtsp_message *req, rtsp_message
       pthread_cleanup_push(mutex_unlock, &conn->flush_mutex);
       conn->ap2_rate = rate;
       if ((rate & 1) != 0) {
+        ptp_send_control_message_string("B"); // signify play is "B"eginning or resuming
         debug(2, "Connection %d: Start playing, with anchor clock %" PRIx64 ".",
               conn->connection_number, conn->networkTimeTimelineID);
         activity_monitor_signify_activity(1);
@@ -2031,7 +2032,8 @@ void handle_setrateanchori(rtsp_conn_info *conn, rtsp_message *req, rtsp_message
 #endif
         conn->ap2_play_enabled = 1;
       } else {
-        debug(2, "Connection %d: Stop playing.", conn->connection_number);
+        ptp_send_control_message_string("P"); // signify play is "P"ausing
+        debug(2, "Connection %d: Pause playing.", conn->connection_number);
         conn->ap2_play_enabled = 0;
         activity_monitor_signify_activity(0);
         reset_anchor_info(conn);
@@ -2955,6 +2957,7 @@ void handle_setup_2(rtsp_conn_info *conn, rtsp_message *req, rtsp_message *resp)
         // if it's a full service PTP stream, we get groupUUID, groupContainsGroupLeader and
         // timingPeerList
         if (conn->airplay_stream_category == ptp_stream) {
+          ptp_send_control_message_string("B"); // signify play is "B"eginning
           if (ptp_shm_interface_open() !=
               0) // it should be open already, but just in case it isn't...
             die("Can not access the NQPTP service. Has it stopped running?");