ntp: use previous root delay/disp in interleaved mode

When calculating the root delay and dispersion of a sample measured in
the interleaved mode, use the root delay and dispersion values from
the previous response (to which the TX timestamp corresponds). If the TX
timestamp is combined with the RX timestamp of the latest response (e.g.
in the symmetric mode), use the maximum of the previous and latest root
delay/dispersion.

diff --git a/ntp_core.c b/ntp_core.c
index f7c955d2ddc7418ca8903aea0cc833024cc2352a..595118ed4aab33bfaabd534840f5b6def2d8e6e4 100644 (file)
--- a/ntp_core.c
+++ b/ntp_core.c
@@ -88,6 +88,8 @@ struct NCR_Instance_Record {
                                    from received packets) */
   int remote_stratum;           /* Stratum of the server/peer (recovered from
                                    received packets) */
+  double remote_root_delay;     /* Root delay from last valid packet */
+  double remote_root_dispersion;/* Root dispersion from last valid packet */
 
   int presend_minpoll;           /* If the current polling interval is
                                     at least this, an extra client packet
@@ -664,6 +666,8 @@ NCR_ResetInstance(NCR_Instance instance)
 
   instance->remote_poll = 0;
   instance->remote_stratum = 0;
+  instance->remote_root_delay = 0.0;
+  instance->remote_root_dispersion = 0.0;
 
   instance->valid_rx = 0;
   instance->valid_timestamps = 0;
@@ -1566,7 +1570,7 @@ process_response(NCR_Instance inst, NTP_Local_Address *local_addr,
     /* These are the timespec equivalents of the remote and local epochs */
     struct timespec remote_receive, remote_transmit, remote_request_receive;
     struct timespec local_average, remote_average, prev_remote_transmit;
-    double prev_remote_poll_interval;
+    double prev_remote_poll_interval, root_delay, root_dispersion;
 
     /* Select remote and local timestamps for the new sample */
     if (interleaved_packet) {
@@ -1580,10 +1584,14 @@ process_response(NCR_Instance inst, NTP_Local_Address *local_addr,
         UTI_Ntp64ToTimespec(&inst->remote_ntp_rx, &remote_receive);
         remote_request_receive = remote_receive;
         local_transmit = inst->prev_local_tx;
+        root_delay = inst->remote_root_delay;
+        root_dispersion = inst->remote_root_dispersion;
       } else {
         UTI_Ntp64ToTimespec(&message->receive_ts, &remote_receive);
         UTI_Ntp64ToTimespec(&inst->remote_ntp_rx, &remote_request_receive);
         local_transmit = inst->local_tx;
+        root_delay = MAX(pkt_root_delay, inst->remote_root_delay);
+        root_dispersion = MAX(pkt_root_dispersion, inst->remote_root_dispersion);
       }
       UTI_Ntp64ToTimespec(&message->transmit_ts, &remote_transmit);
       UTI_Ntp64ToTimespec(&inst->remote_ntp_tx, &prev_remote_transmit);
@@ -1595,6 +1603,8 @@ process_response(NCR_Instance inst, NTP_Local_Address *local_addr,
       remote_request_receive = remote_receive;
       local_receive = *rx_ts;
       local_transmit = inst->local_tx;
+      root_delay = pkt_root_delay;
+      root_dispersion = pkt_root_dispersion;
     }
 
     /* Calculate intervals between remote and local timestamps */
@@ -1631,9 +1641,11 @@ process_response(NCR_Instance inst, NTP_Local_Address *local_addr,
     /* Calculate skew */
     skew = (source_freq_hi - source_freq_lo) / 2.0;
     
-    /* and then calculate peer dispersion */
+    /* and then calculate peer dispersion and the rest of the sample */
     sample.peer_dispersion = MAX(precision, MAX(local_transmit.err, local_receive.err)) +
                              skew * fabs(local_interval);
+    sample.root_delay = root_delay + sample.peer_delay;
+    sample.root_dispersion = root_dispersion + sample.peer_dispersion;
     
     /* If the source is an active peer, this is the minimum assumed interval
        between previous two transmissions (if not constrained by minpoll) */
@@ -1675,6 +1687,7 @@ process_response(NCR_Instance inst, NTP_Local_Address *local_addr,
   } else {
     remote_interval = local_interval = response_time = 0.0;
     sample.offset = sample.peer_delay = sample.peer_dispersion = 0.0;
+    sample.root_delay = sample.root_dispersion = 0.0;
     sample.time = rx_ts->ts;
     local_receive = *rx_ts;
     local_transmit = inst->local_tx;
@@ -1685,9 +1698,6 @@ process_response(NCR_Instance inst, NTP_Local_Address *local_addr,
      the additional tests passed */
   good_packet = testA && testB && testC && testD;
 
-  sample.root_delay = pkt_root_delay + sample.peer_delay;
-  sample.root_dispersion = pkt_root_dispersion + sample.peer_dispersion;
-
   /* Update the NTP timestamps.  If it's a valid packet from a synchronised
      source, the timestamps may be used later when processing a packet in the
      interleaved mode.  Protect the timestamps against replay attacks in client
@@ -1771,6 +1781,8 @@ process_response(NCR_Instance inst, NTP_Local_Address *local_addr,
     inst->remote_poll = message->poll;
     inst->remote_stratum = message->stratum != NTP_INVALID_STRATUM ?
                            MIN(message->stratum, NTP_MAX_STRATUM) : NTP_MAX_STRATUM;
+    inst->remote_root_delay = pkt_root_delay;
+    inst->remote_root_dispersion = pkt_root_dispersion;
 
     inst->prev_local_poll = inst->local_poll;
     inst->prev_tx_count = inst->tx_count;