Fixed delay problems, if RTP stream jitters too much

The RTP stream is generated or forwarded by OpenBSC to nanoBTS. Due to
switching of streams (hold/retrieve call), packet loss or even stalling
of sender's process, the time stamp must be corrected. If outdated
packets are received, they get dropped.

1 files changed, 30 insertions, 6 deletions

diff --git a/openbsc/src/libtrau/rtp_proxy.c b/openbsc/src/libtrau/rtp_proxy.c
index fdcd5254f..58aadfc3c 100644
--- a/openbsc/src/libtrau/rtp_proxy.c
+++ b/openbsc/src/libtrau/rtp_proxy.c
@@ -242,6 +242,20 @@ static void tv_difference(struct timeval *diff, const struct timeval *from,
 	diff->tv_sec = to->tv_sec - from->tv_sec;
 }
 
+/* add sec,usec to tv */
+static void tv_add(struct timeval *tv, int sec, int usec)
+{
+
+	if (usec < 0)
+		usec += 1000000;
+	tv->tv_sec += sec;
+	tv->tv_usec += usec;
+	if (tv->tv_usec >= 1000000) {
+		tv->tv_sec++;
+		tv->tv_usec -= 1000000;
+	}
+}
+
 /*! \brief encode and send a rtp frame
  *  \param[in] rs RTP socket through which we shall send
  *  \param[in] frame GSM RTP frame to be sent
@@ -265,6 +279,7 @@ int rtp_send_frame(struct rtp_socket *rs, struct gsm_data_frame *frame)
 		rs->transmit.ssrc = rand();
 		rs->transmit.sequence = random();
 		rs->transmit.timestamp = random();
+		gettimeofday(&rs->transmit.last_tv, NULL);
 	}
 
 	switch (frame->msg_type) {
@@ -303,18 +318,27 @@ int rtp_send_frame(struct rtp_socket *rs, struct gsm_data_frame *frame)
 
 		gettimeofday(&tv, NULL);
 		tv_difference(&tv_diff, &rs->transmit.last_tv, &tv);
-		rs->transmit.last_tv = tv;
+		tv_add(&rs->transmit.last_tv, 0, 20000);
 
 		usec_diff = tv_diff.tv_sec * 1000000 + tv_diff.tv_usec;
-		frame_diff = (usec_diff / 20000);
+		frame_diff = ((usec_diff + 10000) / 20000); /* round */
 
-		if (abs(frame_diff) > 1) {
+		if (abs(frame_diff - 1) > 1) {
 			long int frame_diff_excess = frame_diff - 1;
-
+			long int sample_diff_excess = frame_diff_excess * duration;
+
+			/* correct last_tv */
+			tv_add(&rs->transmit.last_tv, sample_diff_excess / 8000,
+				(sample_diff_excess % 8000) * 125);
+			/* drop frame, if time stamp is in the past */
+			if (frame_diff_excess < 0)
+				return 0;
 			LOGP(DLMUX, LOGL_NOTICE,
-				"Correcting frame difference of %ld frames\n", frame_diff_excess);
+				"Correcting timestamp difference of %ld frames "
+				"(to %s)\n", frame_diff_excess,
+				(rs->rx_action == RTP_RECV_L4) ? "app" : "BTS");
 			rs->transmit.sequence += frame_diff_excess;
-			rs->transmit.timestamp += frame_diff_excess * duration;
+			rs->transmit.timestamp += sample_diff_excess;
 		}
 	}