ntp: improve maxdelayratio test

Similarly to the maxdelaydevratio test, include in the maximum delay
dispersion which accumulated in the interval since the last sample.
Also, enable the test for symmetric associations.

doc/chrony.conf.adoc

@@ -115,8 +115,7 @@ round-trip delay of 0.3 seconds or more should be ignored. The default value is
 This option is similar to the maxdelay option above. *chronyd* keeps a record
 of the minimum round-trip delay amongst the previous measurements that it has
 buffered. If a measurement has a round trip delay that is greater than the
-maxdelayratio times the minimum delay, it will be rejected. This option does
+maxdelayratio times the minimum delay, it will be rejected.
-not work in the *peer* directive.
 *maxdelaydevratio* _ratio_:::
 If a measurement has a ratio of the increase in the round-trip delay from the
 minimum delay amongst the previous measurements to the standard deviation of

ntp_core.c

@@ -1325,6 +1325,30 @@ check_packet_auth(NTP_Packet *pkt, int length,
 
 /* ================================================== */
 
+static int
+check_delay_ratio(NCR_Instance inst, SST_Stats stats,
+                struct timespec *sample_time, double delay)
+{
+  double last_sample_ago, predicted_offset, min_delay, skew, std_dev;
+  double max_delay;
+
+  if (inst->max_delay_ratio < 1.0 ||
+      !SST_GetDelayTestData(stats, sample_time, &last_sample_ago,
+                            &predicted_offset, &min_delay, &skew, &std_dev))
+    return 1;
+
+  max_delay = min_delay * inst->max_delay_ratio +
+              last_sample_ago * (skew + LCL_GetMaxClockError());
+
+  if (delay <= max_delay)
+    return 1;
+
+  DEBUG_LOG("maxdelayratio: delay=%e max_delay=%e", delay, max_delay);
+  return 0;
+}
+
+/* ================================================== */
+
 static int
 check_delay_dev_ratio(NCR_Instance inst, SST_Stats stats,
                       struct timespec *sample_time, double offset, double delay)
@@ -1582,9 +1606,7 @@ receive_packet(NCR_Instance inst, NTP_Local_Address *local_addr,
     /* Test B requires in client mode that the ratio of the round trip delay
        to the minimum one currently in the stats data register is less than an
        administrator-defined value */
-    testB = inst->max_delay_ratio <= 1.0 ||
+    testB = check_delay_ratio(inst, stats, &sample_time, delay);
-            !(inst->mode == MODE_CLIENT &&
-              (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

test/simulation/118-maxdelay

@@ -18,7 +18,7 @@ check_source_selection || test_fail
 check_packet_interval || test_fail
 check_sync || test_fail
 
-for client_server_options in "maxpoll 6 maxdelay 2e-5" "maxpoll 6 maxdelayratio 1.001"; do
+for client_server_options in "maxpoll 6 maxdelay 2e-5"; do
 	run_test || test_fail
 	check_chronyd_exit || test_fail
 	check_packet_interval || test_fail