ntp: add offset option

Add offset option to the server/pool/peer directive. It specifies a
correction which will be applied to offsets measured with the NTP
source. It's particularly useful to compensate for a known asymmetry in
network delay or timestamping errors.

client.c

@@ -1083,6 +1083,8 @@ process_cmd_add_server_or_peer(CMD_Request *msg, char *line)
         opt_name = "maxsources";
       else if (data.params.min_stratum != SRC_DEFAULT_MINSTRATUM)
         opt_name = "minstratum";
+      else if (data.params.offset != 0.0)
+        opt_name = "offset";
       else if (data.params.poll_target != SRC_DEFAULT_POLLTARGET)
         opt_name = "polltarget";
       else if (data.params.version != NTP_VERSION)

cmdparse.c

@@ -63,6 +63,7 @@ CPS_ParseNTPSourceAdd(char *line, CPS_NTP_Source *src)
   src->params.max_delay = SRC_DEFAULT_MAXDELAY;
   src->params.max_delay_ratio = SRC_DEFAULT_MAXDELAYRATIO;
   src->params.max_delay_dev_ratio = SRC_DEFAULT_MAXDELAYDEVRATIO;
+  src->params.offset = 0.0;
 
   hostname = line;
   line = CPS_SplitWord(line);
@@ -123,6 +124,9 @@ CPS_ParseNTPSourceAdd(char *line, CPS_NTP_Source *src)
     } else if (!strcasecmp(cmd, "minstratum")) {
       if (sscanf(line, "%d%n", &src->params.min_stratum, &n) != 1)
         return 0;
+    } else if (!strcasecmp(cmd, "offset")) {
+      if (sscanf(line, "%lf%n", &src->params.offset, &n) != 1)
+        return 0;
     } else if (!strcasecmp(cmd, "port")) {
       if (sscanf(line, "%hu%n", &src->port, &n) != 1)
         return 0;

doc/chrony.conf.adoc

@@ -116,6 +116,13 @@ 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
 the previous measurements that is greater than the specified ratio, it will be
 rejected. The default is 10.0.
+*offset* _offset_:::
+This option specifies a correction (in seconds) which will be applied to
+offsets measured with this source. It's particularly useful to compensate for a
+known asymmetry in network delay or timestamping errors. For example, if
+packets sent to the source were on average delayed by 100 microseconds more
+than packets sent from the source back, the correction would be -0.00005 (-50
+microseconds). The default is 0.0.
 *minsamples* _samples_:::
 Set the minimum number of samples kept for this source. This overrides the
 <<minsamples,*minsamples*>> directive.

ntp_core.c

@@ -119,6 +119,9 @@ struct NCR_Instance_Record {
 
   double max_delay_dev_ratio;   /* Maximum ratio of increase in delay / stddev */
 
+  double offset_correction;     /* Correction applied to measured offset
+                                   (e.g. for asymmetry in network delay) */
+
   int do_auth;                  /* Flag indicating whether we
                                    authenticate packets we send to
                                    this machine (if it's serving us or
@@ -481,6 +484,7 @@ NCR_GetInstance(NTP_Remote_Address *remote_addr, NTP_Source_Type type, SourcePar
   result->max_delay = params->max_delay;
   result->max_delay_ratio = params->max_delay_ratio;
   result->max_delay_dev_ratio = params->max_delay_dev_ratio;
+  result->offset_correction = params->offset;
   result->auto_offline = params->auto_offline;
   result->poll_target = params->poll_target;
 
@@ -1311,6 +1315,9 @@ receive_packet(NTP_Packet *message, struct timeval *now, double now_err, NCR_Ins
        if we are fast of the remote source. */
     UTI_DiffTimevalsToDouble(&offset, &remote_average, &local_average);
 
+    /* Apply configured correction */
+    offset += inst->offset_correction;
+
     /* We treat the time of the sample as being midway through the local
        measurement period.  An analysis assuming constant relative
        frequency and zero network delay shows this is the only possible

srcparams.h

@@ -47,6 +47,7 @@ typedef struct {
   double max_delay;
   double max_delay_ratio;
   double max_delay_dev_ratio;
+  double offset;
 } SourceParameters;
 
 #define SRC_DEFAULT_PORT 123