ntp: add burst option

When the burst option is specified in the server/pool directive and the
current poll is longer than the minimum poll, initiate on each poll a
burst with 1 good sample and 2 or 4 total samples according to the
difference between the current and minimum poll.

candm.h

@@ -252,6 +252,7 @@ typedef struct {
 #define REQ_ADDSRC_TRUST 0x20
 #define REQ_ADDSRC_REQUIRE 0x40
 #define REQ_ADDSRC_INTERLEAVED 0x80
+#define REQ_ADDSRC_BURST 0x100
 
 typedef struct {
   IPAddr ip_addr;

client.c

@@ -1109,6 +1109,7 @@ process_cmd_add_server_or_peer(CMD_Request *msg, char *line)
           (data.params.auto_offline ? REQ_ADDSRC_AUTOOFFLINE : 0) |
           (data.params.iburst ? REQ_ADDSRC_IBURST : 0) |
           (data.params.interleaved ? REQ_ADDSRC_INTERLEAVED : 0) |
+          (data.params.burst ? REQ_ADDSRC_BURST : 0) |
           (data.params.sel_options & SRC_SELECT_PREFER ? REQ_ADDSRC_PREFER : 0) |
           (data.params.sel_options & SRC_SELECT_NOSELECT ? REQ_ADDSRC_NOSELECT : 0) |
           (data.params.sel_options & SRC_SELECT_TRUST ? REQ_ADDSRC_TRUST : 0) |

cmdmon.c

@@ -801,6 +801,7 @@ handle_add_source(NTP_Source_Type type, CMD_Request *rx_message, CMD_Reply *tx_m
   params.auto_offline = ntohl(rx_message->data.ntp_source.flags) & REQ_ADDSRC_AUTOOFFLINE ? 1 : 0;
   params.iburst = ntohl(rx_message->data.ntp_source.flags) & REQ_ADDSRC_IBURST ? 1 : 0;
   params.interleaved = ntohl(rx_message->data.ntp_source.flags) & REQ_ADDSRC_INTERLEAVED ? 1 : 0;
+  params.burst = ntohl(rx_message->data.ntp_source.flags) & REQ_ADDSRC_BURST ? 1 : 0;
   params.sel_options =
     (ntohl(rx_message->data.ntp_source.flags) & REQ_ADDSRC_PREFER ? SRC_SELECT_PREFER : 0) |
     (ntohl(rx_message->data.ntp_source.flags) & REQ_ADDSRC_NOSELECT ? SRC_SELECT_NOSELECT : 0) |

cmdparse.c

@@ -51,6 +51,7 @@ CPS_ParseNTPSourceAdd(char *line, CPS_NTP_Source *src)
   src->params.online = 1;
   src->params.auto_offline = 0;
   src->params.presend_minpoll = SRC_DEFAULT_PRESEND_MINPOLL;
+  src->params.burst = 0;
   src->params.iburst = 0;
   src->params.min_stratum = SRC_DEFAULT_MINSTRATUM;
   src->params.poll_target = SRC_DEFAULT_POLLTARGET;
@@ -84,6 +85,8 @@ CPS_ParseNTPSourceAdd(char *line, CPS_NTP_Source *src)
 
     if (!strcasecmp(cmd, "auto_offline")) {
       src->params.auto_offline = 1;
+    } else if (!strcasecmp(cmd, "burst")) {
+      src->params.burst = 1;
     } else if (!strcasecmp(cmd, "iburst")) {
       src->params.iburst = 1;
     } else if (!strcasecmp(cmd, "offline")) {

doc/chrony.conf.adoc

@@ -83,6 +83,13 @@ With this option, the interval between the first four requests sent to the
 server will be 2 seconds instead of the interval specified by the *minpoll*
 option, which allows *chronyd* to make the first update of the clock shortly
 after start.
+*burst*:::
+With this option, *chronyd* will shorten the interval between up to four
+requests to 2 seconds when it cannot get a good measurement from the server.
+The number of requests in the burst is limited by the current polling interval
+to keep the average interval at or above the minimum interval, i.e. the current
+interval needs to be at least two times longer than the minimum interval in
+order to allow a burst with two requests.
 *key* _ID_:::
 The NTP protocol supports the inclusion of checksums in the packets, to prevent
 computers having their system time upset by rogue packets being sent to them.

ntp_core.c

@@ -88,6 +88,7 @@ struct NCR_Instance_Record {
   SCH_TimeoutID tx_timeout_id;  /* Timeout ID for next transmission */
   int tx_suspended;             /* Boolean indicating we can't transmit yet */
 
+  int auto_burst;               /* If 1, initiate a burst on each poll */
   int auto_offline;             /* If 1, automatically go offline if server/peer
                                    isn't responding */
 
@@ -236,6 +237,10 @@ static ARR_Instance broadcasts;
 #define IBURST_GOOD_SAMPLES 4
 #define IBURST_TOTAL_SAMPLES SOURCE_REACH_BITS
 
+/* Number of samples in automatic burst */
+#define BURST_GOOD_SAMPLES 1
+#define MAX_BURST_TOTAL_SAMPLES 4
+
 /* Time to wait after sending packet to 'warm up' link */
 #define WARM_UP_DELAY 2.0
 
@@ -557,6 +562,7 @@ NCR_GetInstance(NTP_Remote_Address *remote_addr, NTP_Source_Type type, SourcePar
   result->max_delay_ratio = CLAMP(0.0, params->max_delay_ratio, MAX_MAXDELAYRATIO);
   result->max_delay_dev_ratio = CLAMP(0.0, params->max_delay_dev_ratio, MAX_MAXDELAYDEVRATIO);
   result->offset_correction = params->offset;
+  result->auto_burst = params->burst;
   result->auto_offline = params->auto_offline;
   result->poll_target = params->poll_target;
 
@@ -1121,6 +1127,14 @@ transmit_timeout(void *arg)
       if (inst->burst_total_samples_to_go <= 0)
         take_offline(inst);
       break;
+    case MD_ONLINE:
+      /* Start a new burst if the burst option is enabled and the average
+         polling interval including the burst will not fall below the
+         minimum polling interval */
+      if (inst->auto_burst && inst->local_poll > inst->minpoll && inst->local_poll > 1)
+        NCR_InitiateSampleBurst(inst, BURST_GOOD_SAMPLES,
+                                MIN(1 << (inst->local_poll - inst->minpoll),
+                                    MAX_BURST_TOTAL_SAMPLES));
     default:
       break;
   }

srcparams.h

@@ -35,6 +35,7 @@ typedef struct {
   int online;
   int auto_offline;
   int presend_minpoll;
+  int burst;
   int iburst;
   int min_stratum;
   int poll_target;