From 5b2df52c6a9a4f4575bbcb234308475a4580b807 Mon Sep 17 00:00:00 2001
From: Jan Schmidt <jan@centricular.com>
Date: Tue, 26 Nov 2013 02:17:36 +1100
Subject: [PATCH] netclock: Implement rolling-average filter on observations.

Keep a rolling average of the round trip time for network clock
observations, favouring shorter round trips as being more accurate.

Don't pass any clock observation to the clock slaving if it has a
round-trip time greater than 2 times the average.

Actual shifts in the network topology will be noticed after some
time, as the rolling average incorporates the new round trip times.
---
 libs/gst/net/gstnetclientclock.c | 36 +++++++++++++++++++++++++++++---
 1 file changed, 33 insertions(+), 3 deletions(-)

diff --git a/libs/gst/net/gstnetclientclock.c b/libs/gst/net/gstnetclientclock.c
index 85fdaa5d95..0d40f332ba 100644
--- a/libs/gst/net/gstnetclientclock.c
+++ b/libs/gst/net/gstnetclientclock.c
@@ -82,6 +82,7 @@ struct _GstNetClientClockPrivate
   GCancellable *cancel;
 
   GstClockTime timeout_expiration;
+  GstClockTime rtt_avg;
 
   gchar *address;
   gint port;
@@ -140,6 +141,7 @@ gst_net_client_clock_init (GstNetClientClock * self)
   priv->thread = NULL;
 
   priv->servaddr = NULL;
+  priv->rtt_avg = GST_CLOCK_TIME_NONE;
 }
 
 static void
@@ -213,21 +215,46 @@ static void
 gst_net_client_clock_observe_times (GstNetClientClock * self,
     GstClockTime local_1, GstClockTime remote, GstClockTime local_2)
 {
+  GstNetClientClockPrivate *priv = self->priv;
   GstClockTime current_timeout;
   GstClockTime local_avg;
   gdouble r_squared;
   GstClock *clock;
+  GstClockTime rtt;
 
   if (local_2 < local_1)
     goto bogus_observation;
 
+  rtt = local_2 - local_1;
+
+  /* Track an average round trip time, for a bit of smoothing */
+  /* Always update before discarding a sample, so genuine changes in
+   * the network get picked up, eventually */
+  if (priv->rtt_avg == GST_CLOCK_TIME_NONE)
+    priv->rtt_avg = rtt;
+  else if (rtt < priv->rtt_avg) // Shorter RTTs carry more weight than longer
+    priv->rtt_avg = (3 * priv->rtt_avg + rtt) / 4;
+  else
+    priv->rtt_avg = (7 * priv->rtt_avg + rtt) / 8;
+
+  if (rtt > 2 * priv->rtt_avg) {
+    GST_LOG_OBJECT (self,
+        "Dropping observation, long RTT %" GST_TIME_FORMAT " > 2 * avg %"
+        GST_TIME_FORMAT, GST_TIME_ARGS (rtt), GST_TIME_ARGS (priv->rtt_avg));
+    goto bogus_observation;
+  }
+
   local_avg = (local_2 + local_1) / 2;
 
+  GST_LOG_OBJECT (self, "local1 %" G_GUINT64_FORMAT " remote %" G_GUINT64_FORMAT
+      " localavg %" G_GUINT64_FORMAT " local2 %" G_GUINT64_FORMAT,
+      local_1, remote, local_avg, local_2);
+
   clock = GST_CLOCK_CAST (self);
 
   if (gst_clock_add_observation (GST_CLOCK (self), local_avg, remote,
           &r_squared)) {
-    /* geto formula */
+    /* ghetto formula - shorter timeout for bad correlations */
     current_timeout = (1e-3 / (1 - MIN (r_squared, 0.99999))) * GST_SECOND;
     current_timeout = MIN (current_timeout, gst_clock_get_timeout (clock));
   } else {
@@ -242,8 +269,11 @@ gst_net_client_clock_observe_times (GstNetClientClock * self,
 bogus_observation:
   {
     GST_WARNING_OBJECT (self, "time packet receive time < send time (%"
-        GST_TIME_FORMAT " < %" GST_TIME_FORMAT ")", GST_TIME_ARGS (local_1),
-        GST_TIME_ARGS (local_2));
+        GST_TIME_FORMAT " < %" GST_TIME_FORMAT ") or too large",
+        GST_TIME_ARGS (local_1), GST_TIME_ARGS (local_2));
+    /* Schedule a new packet again soon */
+    self->priv->timeout_expiration =
+        gst_util_get_timestamp () + (GST_SECOND / 4);
     return;
   }
 }