ntp: rework calculation and testing of peer delay

Instead of a worst-case delay use a mean value and relate it to the
source's time. This makes it more stable in the interleaved and
symmetric modes, which should improve the weighting and asymmetry
correction. Modify the test A and B to work with a minimum estimated
delay (delay - dispersion).

diff --git a/ntp_core.c b/ntp_core.c
index 4fb6677b9d58105815de0af493f912d94c70262f..a1da2c8eaefaa5ce5016975a6b701c2eb7d08aea 100644 (file)
--- a/ntp_core.c
+++ b/ntp_core.c
@@ -1408,10 +1408,9 @@ receive_packet(NCR_Instance inst, NTP_Local_Address *local_addr,
       sample_rx_tss = rx_ts->source;
     }
 
-    /* In our case, we work out 'delay' as the worst case delay,
-       assuming worst case frequency error between us and the other
-       source */
-    delay = local_interval - remote_interval * (1.0 + source_freq_lo);
+    /* Work out 'delay' relative to the source's time */
+    delay = (1.0 - (source_freq_lo + source_freq_hi) / 2.0) *
+              local_interval - remote_interval;
 
     /* Clamp delay to avoid misleading results later */
     delay = fabs(delay);
@@ -1440,11 +1439,12 @@ receive_packet(NCR_Instance inst, NTP_Local_Address *local_addr,
     
     /* Additional tests required to pass before accumulating the sample */
 
-    /* Test A requires that the peer delay is not larger than the configured
-       maximum, in client mode that the server processing time is sane, and in
-       interleaved symmetric mode that the delay is not longer than half of the
-       remote polling interval to detect missed packets */
-    testA = delay <= inst->max_delay &&
+    /* Test A requires that the minimum estimate of the peer delay is not
+       larger than the configured maximum, in client mode that the server
+       processing time is sane, and in the interleaved symmetric mode that
+       the delay is not longer than half of the remote polling interval to
+       detect missed packets */
+    testA = delay - dispersion <= inst->max_delay &&
             !(inst->mode == MODE_CLIENT && server_interval > MAX_SERVER_INTERVAL) &&
             !(inst->mode == MODE_ACTIVE && interleaved_packet &&
               delay > UTI_Log2ToDouble(message->poll - 1));
@@ -1453,7 +1453,7 @@ receive_packet(NCR_Instance inst, NTP_Local_Address *local_addr,
        minimum one currently in the stats data register is less than an
        administrator-defined value */
     testB = inst->max_delay_ratio <= 1.0 ||
-            delay / SST_MinRoundTripDelay(stats) <= inst->max_delay_ratio;
+            (delay - dispersion) / SST_MinRoundTripDelay(stats) <= inst->max_delay_ratio;
 
     /* Test C requires that the ratio of the increase in delay from the minimum
        one in the stats data register to the standard deviation of the offsets