ntp: allow reusing timestamps of unused samples

When switching from basic mode to interleaved mode following a response
which wasn't accumulated due to failed test A, B, C, or D, allow
timestamps of the failed sample to be reused in interleaved mode, i.e.
replacing the server's less accurate transmit timestamp with a more
accurate timestamp server can turn a failed sample into acceptable one.

Move the presend check into test A to simplify the code.

diff --git a/ntp_core.c b/ntp_core.c
index 3b21f3625129cc82497db03e2784d91a3975d76d..023e60b24c2185efec08a99801fae2ecf207b3c9 100644 (file)
--- a/ntp_core.c
+++ b/ntp_core.c
@@ -2208,8 +2208,10 @@ process_response(NCR_Instance inst, int saved, NTP_Local_Address *local_addr,
 
     /* Additional tests required to pass before accumulating the sample */
 
-    /* Test A requires that the minimum estimate of the peer delay is not
-       larger than the configured maximum, in both client modes that the server
+    /* Test A combines multiple tests to avoid changing the measurements log
+       format and ntpdata report.  It requires that the minimum estimate of the
+       peer delay is not larger than the configured maximum, it is not a
+       response in the 'warm up' exchange, in both client modes that the server
        processing time is sane, in interleaved client/server mode that the
        previous response was not in basic mode (which prevents using timestamps
        that minimise delay error), and in interleaved symmetric mode that the
@@ -2217,6 +2219,7 @@ process_response(NCR_Instance inst, int saved, NTP_Local_Address *local_addr,
        a missed response */
     testA = sample.peer_delay - sample.peer_dispersion <= inst->max_delay &&
             precision <= inst->max_delay &&
+            inst->presend_done <= 0 &&
             !(inst->mode == MODE_CLIENT && response_time > MAX_SERVER_INTERVAL) &&
             !(inst->mode == MODE_CLIENT && interleaved_packet &&
               UTI_IsZeroTimespec(&inst->prev_local_tx.ts) &&
@@ -2301,8 +2304,9 @@ process_response(NCR_Instance inst, int saved, NTP_Local_Address *local_addr,
 
     inst->local_tx.net_correction = net_correction;
 
-    /* Don't use the same set of timestamps for the next sample */
-    if (interleaved_packet || inst->presend_done > 0)
+    /* Avoid reusing timestamps of an accumulated sample when switching
+       from basic mode to interleaved mode */
+    if (interleaved_packet || !good_packet)
       inst->prev_local_tx = inst->local_tx;
     else
       zero_local_timestamp(&inst->prev_local_tx);
@@ -2321,15 +2325,11 @@ process_response(NCR_Instance inst, int saved, NTP_Local_Address *local_addr,
   /* Accept at most one response per request.  The NTP specification recommends
      resetting local_ntp_tx to make the following packets fail test2 or test3,
      but that would not allow the code above to make multiple updates of the
-     timestamps in symmetric mode.  Also, ignore presend responses. */
+     timestamps in symmetric mode. */
   if (inst->valid_rx) {
     test2 = test3 = 0;
     valid_packet = synced_packet = good_packet = 0;
   } else if (valid_packet) {
-    if (inst->presend_done) {
-      testA = 0;
-      good_packet = 0;
-    }
     inst->valid_rx = 1;
   }