ntp: add client support for network correction

If the network correction is known for both the request and response,
and their sum is not larger that the measured peer delay, allowing the
transparent clocks to be running up to 100 ppm faster than the client's
clock, apply the corrections to the NTP offset and peer delay. Don't
correct the root delay to not change the estimated maximum error.

ntp_core.c

@@ -314,6 +314,9 @@ static ARR_Instance broadcasts;
 /* Maximum acceptable change in server mono<->real offset */
 #define MAX_MONO_DOFFSET 16.0
 
+/* Maximum assumed frequency error in network corrections */
+#define MAX_NET_CORRECTION_FREQ 100.0e-6
+
 /* Invalid socket, different from the one in ntp_io.c */
 #define INVALID_SOCK_FD -2
 
@@ -1661,6 +1664,53 @@ parse_packet(NTP_Packet *packet, int length, NTP_PacketInfo *info)
 
 /* ================================================== */
 
+static void
+apply_net_correction(NTP_Sample *sample, NTP_Local_Timestamp *rx, NTP_Local_Timestamp *tx,
+                     double precision)
+{
+  double rx_correction, tx_correction, low_delay_correction;
+
+  /* Require some correction from transparent clocks to be present
+     in both directions (not just the local RX timestamp correction) */
+  if (rx->net_correction <= rx->rx_duration || tx->net_correction <= 0.0)
+    return;
+
+  /* With perfect corrections from PTP transparent clocks and short cables
+     the peer delay would be close to zero, or even negative if the server or
+     transparent clocks were running faster than client, which would invert the
+     sample weighting.  Adjust the correction to get a delay corresponding to
+     a direct connection to the server.  For simplicity, assume the TX and RX
+     link speeds are equal.  If not, the reported delay will be wrong, but it
+     will not cause an error in the offset. */
+  rx_correction = rx->net_correction - rx->rx_duration;
+  tx_correction = tx->net_correction - rx->rx_duration;
+
+  /* Use a slightly smaller value in the correction of delay to not overcorrect
+     if the transparent clocks run up to 100 ppm fast and keep a part of the
+     uncorrected delay for the sample weighting */
+  low_delay_correction = (rx_correction + tx_correction) *
+                         (1.0 - MAX_NET_CORRECTION_FREQ);
+
+  /* Make sure the correction is sane.  The values are not authenticated! */
+  if (low_delay_correction < 0.0 || low_delay_correction > sample->peer_delay) {
+    DEBUG_LOG("Invalid correction %.9f peer_delay=%.9f",
+              low_delay_correction, sample->peer_delay);
+    return;
+  }
+
+  /* Correct the offset and peer delay, but not the root delay to not
+     change the estimated maximum error */
+  sample->offset += (rx_correction - tx_correction) / 2.0;
+  sample->peer_delay -= low_delay_correction;
+  if (sample->peer_delay < precision)
+    sample->peer_delay = precision;
+
+  DEBUG_LOG("Applied correction rx=%.9f tx=%.9f dur=%.9f",
+            rx->net_correction, tx->net_correction, rx->rx_duration);
+}
+
+/* ================================================== */
+
 static int
 check_delay_ratio(NCR_Instance inst, SST_Stats stats,
                 struct timespec *sample_time, double delay)
@@ -1923,10 +1973,11 @@ process_response(NCR_Instance inst, int saved, NTP_Local_Address *local_addr,
   int parsed, ef_length, ef_type, ef_body_length;
   void *ef_body;
   NTP_EFExpMonoRoot *ef_mono_root;
+  NTP_EFExpNetCorrection *ef_net_correction;
 
   NTP_Local_Timestamp local_receive, local_transmit;
   double remote_interval, local_interval, response_time;
-  double delay_time, precision, mono_doffset;
+  double delay_time, precision, mono_doffset, net_correction;
   int updated_timestamps;
 
   /* ==================== */
@@ -1934,6 +1985,7 @@ process_response(NCR_Instance inst, int saved, NTP_Local_Address *local_addr,
   stats = SRC_GetSourcestats(inst->source);
 
   ef_mono_root = NULL;
+  ef_net_correction = NULL;
 
   /* Find requested non-authentication extension fields */
   if (inst->ext_field_flags & info->ext_field_flags) {
@@ -1949,6 +2001,12 @@ process_response(NCR_Instance inst, int saved, NTP_Local_Address *local_addr,
                         NTP_EF_EXP_MONO_ROOT_MAGIC))
             ef_mono_root = ef_body;
           break;
+        case NTP_EF_EXP_NET_CORRECTION:
+          if (inst->ext_field_flags & NTP_EF_FLAG_EXP_NET_CORRECTION &&
+              is_exp_ef(ef_body, ef_body_length, sizeof (*ef_net_correction),
+                        NTP_EF_EXP_NET_CORRECTION_MAGIC))
+            ef_net_correction = ef_body;
+          break;
       }
     }
   }
@@ -2055,6 +2113,12 @@ process_response(NCR_Instance inst, int saved, NTP_Local_Address *local_addr,
       mono_doffset = 0.0;
     }
 
+    if (ef_net_correction) {
+      net_correction = UTI_Ntp64ToDouble(&ef_net_correction->correction);
+    } else {
+      net_correction = 0.0;
+    }
+
     /* Select remote and local timestamps for the new sample */
     if (interleaved_packet) {
       /* Prefer previous local TX and remote RX timestamps if it will make
@@ -2074,6 +2138,7 @@ process_response(NCR_Instance inst, int saved, NTP_Local_Address *local_addr,
         UTI_Ntp64ToTimespec(&message->receive_ts, &remote_receive);
         UTI_Ntp64ToTimespec(&inst->remote_ntp_rx, &remote_request_receive);
         local_transmit = inst->local_tx;
+        local_transmit.net_correction = net_correction;
         root_delay = MAX(pkt_root_delay, inst->remote_root_delay);
         root_dispersion = MAX(pkt_root_dispersion, inst->remote_root_dispersion);
       }
@@ -2088,6 +2153,7 @@ process_response(NCR_Instance inst, int saved, NTP_Local_Address *local_addr,
       remote_request_receive = remote_receive;
       local_receive = *rx_ts;
       local_transmit = inst->local_tx;
+      local_transmit.net_correction = net_correction;
       root_delay = pkt_root_delay;
       root_dispersion = pkt_root_dispersion;
     }
@@ -2131,6 +2197,9 @@ process_response(NCR_Instance inst, int saved, NTP_Local_Address *local_addr,
                              skew * fabs(local_interval);
     sample.root_delay = root_delay + sample.peer_delay;
     sample.root_dispersion = root_dispersion + sample.peer_dispersion;
+
+    /* Apply corrections from PTP transparent clocks if available and sane */
+    apply_net_correction(&sample, &local_receive, &local_transmit, precision);
     
     /* If the source is an active peer, this is the minimum assumed interval
        between previous two transmissions (if not constrained by minpoll) */
@@ -2186,6 +2255,7 @@ process_response(NCR_Instance inst, int saved, NTP_Local_Address *local_addr,
     sample.root_delay = sample.root_dispersion = 0.0;
     sample.time = rx_ts->ts;
     mono_doffset = 0.0;
+    net_correction = 0.0;
     local_receive = *rx_ts;
     local_transmit = inst->local_tx;
     testA = testB = testC = testD = 0;
@@ -2229,6 +2299,8 @@ process_response(NCR_Instance inst, int saved, NTP_Local_Address *local_addr,
       inst->mono_doffset = 0.0;
     }
 
+    inst->local_tx.net_correction = net_correction;
+
     /* Don't use the same set of timestamps for the next sample */
     if (interleaved_packet)
       inst->prev_local_tx = inst->local_tx;