fix: prevent active SV pacing interval from ratcheting slower

src/AR.Iec61850.Transports.Npcap/SvTransmitIntervalEstimator.cs

@@ -1,9 +1,10 @@
 namespace AR.Iec61850.Transports.Npcap;
 
 /// <summary>
-/// Learns a stable SV transmit interval without allowing one scheduler-late observation
+/// Learns a stable SV transmit interval without allowing scheduler-late observations
 /// to become the permanent wire rate. The estimator activates only after several
-/// mutually consistent intervals have been observed.
+/// mutually consistent unpaced intervals have been observed, then remains immutable
+/// for the lifetime of that stream session.
 /// </summary>
 internal sealed class SvTransmitIntervalEstimator
 {
@@ -35,23 +36,16 @@ public SvTransmitIntervalEstimator(
 
     public void Observe(long intervalTicks)
     {
-        if (intervalTicks < _minimumIntervalTicks || intervalTicks > _maximumIntervalTicks)
-        {
-            if (NominalIntervalTicks == 0)
-                ResetCandidate();
+        // Once pacing is active, every subsequent interval is influenced by the pacer
+        // itself and by injection/scheduler lateness. Feeding those observations back
+        // into the nominal would create a one-way ratchet toward a slower wire rate.
+        // A deliberate sample-rate change therefore starts a new transport/session.
+        if (NominalIntervalTicks > 0)
             return;
-        }
 
-        if (NominalIntervalTicks > 0)
+        if (intervalTicks < _minimumIntervalTicks || intervalTicks > _maximumIntervalTicks)
         {
-            var minimumAccepted = NominalIntervalTicks * 3 / 4;
-            var maximumAccepted = NominalIntervalTicks * 3 / 2;
-            if (intervalTicks >= minimumAccepted && intervalTicks <= maximumAccepted)
-            {
-                NominalIntervalTicks = (long)Math.Round(
-                    (NominalIntervalTicks * 0.9) + (intervalTicks * 0.1));
-            }
-
+            ResetCandidate();
             return;
         }
 

tests/AR.Iec61850.Tests/Transports/SvTransmitIntervalEstimatorTests.cs

@@ -37,25 +37,45 @@ public void InconsistentIntervals_DoNotActivatePacing()
     [Fact]
     public void ActiveNominalRate_IgnoresLongSchedulerStall()
     {
-        var estimator = new SvTransmitIntervalEstimator(20, 5_000, requiredConsistentIntervals: 4);
-        foreach (var interval in new long[] { 250, 249, 251, 250 })
-            estimator.Observe(interval);
-
+        var estimator = CreateActiveEstimator();
         var nominalBeforeStall = estimator.NominalIntervalTicks;
+
         estimator.Observe(4_000);
 
         Assert.Equal(nominalBeforeStall, estimator.NominalIntervalTicks);
     }
 
     [Fact]
-    public void ActiveNominalRate_TracksOnlyNearbySteadyIntervals()
+    public void ActiveNominalRate_DoesNotRatchetOnModerateLateness()
     {
-        var estimator = new SvTransmitIntervalEstimator(20, 5_000, requiredConsistentIntervals: 4);
-        foreach (var interval in new long[] { 250, 250, 250, 250 })
-            estimator.Observe(interval);
+        var estimator = CreateActiveEstimator();
+        var nominal = estimator.NominalIntervalTicks;
+
+        foreach (var delayedInterval in new long[] { 300, 310, 290, 320, 300, 305, 295, 315 })
+            estimator.Observe(delayedInterval);
 
+        Assert.Equal(nominal, estimator.NominalIntervalTicks);
+    }
+
+    [Fact]
+    public void ActiveNominalRate_DoesNotFeedPacedIntervalsBackIntoEstimator()
+    {
+        var estimator = CreateActiveEstimator();
+        var nominal = estimator.NominalIntervalTicks;
+
+        estimator.Observe(248);
+        estimator.Observe(252);
         estimator.Observe(260);
 
-        Assert.InRange(estimator.NominalIntervalTicks, 250, 252);
+        Assert.Equal(nominal, estimator.NominalIntervalTicks);
+    }
+
+    private static SvTransmitIntervalEstimator CreateActiveEstimator()
+    {
+        var estimator = new SvTransmitIntervalEstimator(20, 5_000, requiredConsistentIntervals: 4);
+        foreach (var interval in new long[] { 250, 249, 251, 250 })
+            estimator.Observe(interval);
+        Assert.InRange(estimator.NominalIntervalTicks, 249, 251);
+        return estimator;
     }
 }