ntp: select timestamps in interleaved mode

Use previous local TX and remote RX timestamps for the new sample in the
interleaved mode if it will make the local and remote intervals
significantly shorter in order to improve the accuracy of the measured
delay.

diff --git a/ntp_core.c b/ntp_core.c
index 884a16d4bdeb2ebd16ddf15f7c65c2c3bedcfa47..02bc3b83ed04d56641197f1d069b3e7cb1ed6bfe 100644 (file)
--- a/ntp_core.c
+++ b/ntp_core.c
@@ -266,6 +266,10 @@ static ARR_Instance broadcasts;
 /* Maximum number of missed responses to follow peer's polling interval */
 #define MAX_PEER_POLL_TX 8
 
+/* Maximum ratio of local intervals in the timestamp selection of the
+   interleaved mode to prefer a sample using previous timestamps */
+#define MAX_INTERLEAVED_L2L_RATIO 0.1
+
 /* Invalid socket, different from the one in ntp_io.c */
 #define INVALID_SOCK_FD -2
 
@@ -1430,11 +1434,23 @@ receive_packet(NCR_Instance inst, NTP_Local_Address *local_addr,
 
     /* Select remote and local timestamps for the new sample */
     if (interleaved_packet) {
-      UTI_Ntp64ToTimespec(&message->receive_ts, &remote_receive);
+      /* Prefer previous local TX and remote RX timestamps if it will make
+         the intervals significantly shorter in order to improve the accuracy
+         of the measured delay */
+      if (!UTI_IsZeroTimespec(&inst->prev_local_tx.ts) &&
+          MAX_INTERLEAVED_L2L_RATIO *
+            UTI_DiffTimespecsToDouble(&inst->local_tx.ts, &inst->local_rx.ts) >
+          UTI_DiffTimespecsToDouble(&inst->local_rx.ts, &inst->prev_local_tx.ts)) {
+        UTI_Ntp64ToTimespec(&inst->remote_ntp_rx, &remote_receive);
+        remote_request_receive = remote_receive;
+        local_transmit = inst->prev_local_tx;
+      } else {
+        UTI_Ntp64ToTimespec(&message->receive_ts, &remote_receive);
+        UTI_Ntp64ToTimespec(&inst->remote_ntp_rx, &remote_request_receive);
+        local_transmit = inst->local_tx;
+      }
       UTI_Ntp64ToTimespec(&message->transmit_ts, &remote_transmit);
-      UTI_Ntp64ToTimespec(&inst->remote_ntp_rx, &remote_request_receive);
       local_receive = inst->local_rx;
-      local_transmit = inst->local_tx;
     } else {
       UTI_Ntp64ToTimespec(&message->receive_ts, &remote_receive);
       UTI_Ntp64ToTimespec(&message->transmit_ts, &remote_transmit);
@@ -1546,6 +1562,12 @@ receive_packet(NCR_Instance inst, NTP_Local_Address *local_addr,
     inst->local_rx = *rx_ts;
     inst->valid_timestamps = synced_packet;
     inst->updated_timestamps = 1;
+
+    /* Don't use the same set of timestamps for the next sample */
+    if (interleaved_packet)
+      inst->prev_local_tx = inst->local_tx;
+    else
+      zero_local_timestamp(&inst->prev_local_tx);
   }
 
   /* Accept at most one response per request.  The NTP specification recommends
@@ -2070,6 +2092,9 @@ NCR_SlewTimes(NCR_Instance inst, struct timespec *when, double dfreq, double dof
     UTI_AdjustTimespec(&inst->local_rx.ts, when, &inst->local_rx.ts, &delta, dfreq, doffset);
   if (!UTI_IsZeroTimespec(&inst->local_tx.ts))
     UTI_AdjustTimespec(&inst->local_tx.ts, when, &inst->local_tx.ts, &delta, dfreq, doffset);
+  if (!UTI_IsZeroTimespec(&inst->prev_local_tx.ts))
+    UTI_AdjustTimespec(&inst->prev_local_tx.ts, when, &inst->prev_local_tx.ts, &delta, dfreq,
+                       doffset);
 }
 
 /* ================================================== */