[Cast] Repair receiver playout time calculations and frame skip logic.

media/cast/rtcp/rtcp.h

Index: media/cast/rtcp/rtcp.h
diff --git a/media/cast/rtcp/rtcp.h b/media/cast/rtcp/rtcp.h
index 52c9178acfadf18f4f7f76aefd749bbbfaf98b20..6170c79acb84774d44e18fbd5e069096361da887 100644
--- a/media/cast/rtcp/rtcp.h
+++ b/media/cast/rtcp/rtcp.h
@@ -5,10 +5,8 @@
 #ifndef MEDIA_CAST_RTCP_RTCP_H_
 #define MEDIA_CAST_RTCP_RTCP_H_
 
-#include <list>
 #include <map>
 #include <queue>
-#include <set>
 #include <string>
 
 #include "base/basictypes.h"
@@ -101,22 +99,45 @@ class Rtcp {
            base::TimeDelta* avg_rtt,
            base::TimeDelta* min_rtt,
            base::TimeDelta* max_rtt) const;
-  bool RtpTimestampInSenderTime(int frequency,
-                                uint32 rtp_timestamp,
-                                base::TimeTicks* rtp_timestamp_in_ticks) const;
+
+  // Uses recent reports from the remote to translate remote-provided NTP
+  // timestamps into TimeTicks values relative to the local clock.  Returns a
+  // null TimeTicks value to indicate a usable report has not yet been received.
+  //
+  // Note: The values returned are not guaranteed to be monotonically increasing
+  // for monotonically increasing NTP timestamps.  In fact, it's possible that
+  // calling this method more than once with the same arguments will return
+  // different results.
+  base::TimeTicks ToApproximateLocalTime(uint32 remote_ntp_seconds,

[hubbe, 2014/05/14 23:12:23]

Can this be made private?

[miu, 2014/05/16 22:45:47]

Done.

+                                         uint32 remote_ntp_fraction) const;
+
+  // Maps a frame's |rtp_timestamp| to its approximate capture time, based on
+  // the latest "lip sync" info gleaned from the sender reports.  The returned
+  // TimeTicks value is relative to the local CastEnvironment clock.  Returns a
+  // null TimeTicks value to indicate no lip sync info was available.
+  // |rtp_timebase| is the number of units of |rtp_timestamp| that elapse per
+  // one second (e.g., for audio, this is usually the sampling frequency).
+  //
+  // Note: The values returned are not guaranteed to be monotonically increasing
+  // for monotonically increasing rtp_timestamps.  In fact, it's possible that
+  // calling this method more than once with the same arguments will return
+  // different results.
+  base::TimeTicks ToApproximateCaptureTime(uint32 rtp_timestamp,
+                                           int rtp_timebase) const;
 
   // Set the history size to record Cast receiver events. The event history is
   // used to remove duplicates. The history will store at most |size| events.
   void SetCastReceiverEventHistorySize(size_t size);
 
-  // Update the target delay. Will be added to every sender report.
+  // Update the target delay. Will be added to every report sent back to the
+  // sender.
+  // TODO(miu): Remove this deprecated functionality. The sender ignores this.
   void SetTargetDelay(base::TimeDelta target_delay);
 
   void OnReceivedReceiverLog(const RtcpReceiverLogMessage& receiver_log);
 
  protected:
-  int CheckForWrapAround(uint32 new_timestamp, uint32 old_timestamp) const;
-
+  void OnReceivedNtp(uint32 ntp_seconds, uint32 ntp_fraction);
   void OnReceivedLipSyncInfo(uint32 rtp_timestamp,
                              uint32 ntp_seconds,
                              uint32 ntp_fraction);
@@ -130,8 +151,6 @@ class Rtcp {
                 uint32 media_ssrc,
                 const RtcpCastMessage* cast_message);
 
-  void OnReceivedNtp(uint32 ntp_seconds, uint32 ntp_fraction);
-
   void OnReceivedDelaySinceLastReport(uint32 receivers_ssrc,
                                       uint32 last_report,
                                       uint32 delay_since_last_report);
@@ -147,6 +166,10 @@ class Rtcp {
                            uint32 last_ntp_seconds,
                            uint32 last_ntp_fraction);
 
+  // Returns a truncated 16+16-bit NTP timestamp, computed from the full
+  // 32+32-bit NTP timestamp in |last_report_ntp_time_|.
+  uint32 GetTruncatedLastNtpReportTime() const;

[hubbe, 2014/05/14 23:12:23]

Where is the inverse of this function?

[miu, 2014/05/16 22:45:47]

Not implemented.  It appears to be parsed and delivered to
RtcpReceiver::HandleReportBlock(), but is not consumed for anything.

+
   scoped_refptr<CastEnvironment> cast_environment_;
   transport::CastTransportSender* const transport_sender_;
   const base::TimeDelta rtcp_interval_;
@@ -166,18 +189,27 @@ class Rtcp {
   base::TimeTicks next_time_to_send_rtcp_;
   RtcpSendTimeMap last_reports_sent_map_;
   RtcpSendTimeQueue last_reports_sent_queue_;
+
+  // The NTP timestamp provided in the last report from the remote peer, along
+  // with the local time at which the report was received.  These two values are
+  // used both for: 1) ping-pong'ing NTP timestamps between the peers; and 2)
+  // computing the local clock's offset from the remote clock.
+  uint64 last_report_ntp_time_;

[hubbe, 2014/05/14 23:12:23]

Don't we need some sort of min() calculation for these?

[miu, 2014/05/16 22:45:47]

Nope.  These are just snapshots from the latest sender report.

Since we ignore RTT in the calculation (see TODO comment in
ToApproximateLocalTime()), there's nothing to min() here.  Unless there was
something else we discussed that I'm forgetting?

[hubbe, 2014/05/19 17:52:20]

Hmm, maybe I'm missing something. Isn't this used as a local time base for
converting remote clocks to local clocks? If you just use the last report,
wouldn't the offset change every time you get a new report? Potentially by a
lot?

It seems to me that when we get a new sender report, we should use it's ntp time
to calculate when it was sent in local time. *based on previous sender reports*
If the send time comes out to be in the future, we should set
time_last_report_received_ to Now(), otherwise we set it to 0.995 *
calculated_time + 0.005 * now to account for gradual drift.

[miu, 2014/05/23 03:09:00]

Done.  Created a ClockDriftSmoother, and incorporated your suggested logic into
the clock offset tracking.

   base::TimeTicks time_last_report_received_;
-  uint32 last_report_received_;
 
-  uint32 last_received_rtp_timestamp_;
-  uint32 last_received_ntp_seconds_;
-  uint32 last_received_ntp_fraction_;
+  // Latest "lip sync" info from the sender.  The sender provides the RTP
+  // timestamp of some frame of its choosing and also the frame's capture time.
+  // This allows the receiver to approximate the capture times of other frames
+  // based on their RTP timestamp.  It is expected that the sender will update
+  // this data regularly and in a timely manner (i.e., about once per second).

[hubbe, 2014/05/14 23:12:23]

i.e. -> e.g.

[miu, 2014/05/16 22:45:47]

Done.

+  uint32 lip_sync_rtp_timestamp_;
+  base::TimeTicks lip_sync_capture_time_;
 
   base::TimeDelta rtt_;
   base::TimeDelta min_rtt_;
   base::TimeDelta max_rtt_;
   int number_of_rtt_in_avg_;
-  float avg_rtt_ms_;
+  double avg_rtt_ms_;
   uint16 target_delay_ms_;
   bool is_audio_;