ntp: reverse poll tracking in interleaved symmetric mode

Unlike in the basic mode, the peer with a higher stratum needs to wait
for a response before sending the next request in order to minimize the
delay of the measurement and error in the measured delay.

Slightly increase the delay adjustment to make it work with older chrony
versions.

diff --git a/ntp_core.c b/ntp_core.c
index 115864247734bec04c03876170bbcf6a721a968f..8df8da3260fb851db0bf72be4c0c0c019146d7ec 100644 (file)
--- a/ntp_core.c
+++ b/ntp_core.c
@@ -213,7 +213,7 @@ static ARR_Instance broadcasts;
 #define SAMPLING_RANDOMNESS 0.02
 
 /* Adjustment of the peer polling interval */
-#define PEER_SAMPLING_ADJ 1.1
+#define PEER_SAMPLING_ADJ 1.15
 
 /* Spacing between samples in burst mode for one server/peer */
 #define BURST_INTERVAL 2.0
@@ -789,13 +789,16 @@ get_transmit_delay(NCR_Instance inst, int on_tx, double last_tx)
 
           delay_time = UTI_Log2ToDouble(poll_to_use);
 
-          /* If the remote stratum is higher than ours, try to lock on the
-             peer's polling to minimize our response time by slightly extending
-             our delay or waiting for the peer to catch up with us as the
-             random part in the actual interval is reduced. If the remote
-             stratum is equal to ours, try to interleave evenly with the peer. */
+          /* If in the basic mode the remote stratum is higher than ours,
+             or in the interleaved mode it is lower, wait a bit for the next
+             packet before responding in order to minimize the delay of the
+             measurement and its error for the peer which has higher stratum.
+             If the remote stratum is equal to ours, try to interleave packets
+             evenly with the peer. */
           stratum_diff = inst->remote_stratum - REF_GetOurStratum();
-          if ((stratum_diff > 0 && last_tx * PEER_SAMPLING_ADJ < delay_time) ||
+          if ((((stratum_diff > 0 && !inst->interleaved) ||
+                (stratum_diff < 0 && inst->interleaved)) &&
+               last_tx * PEER_SAMPLING_ADJ < delay_time) ||
               (!on_tx && !stratum_diff &&
                last_tx / delay_time > PEER_SAMPLING_ADJ - 0.5))
             delay_time *= PEER_SAMPLING_ADJ;