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ntp: add server time smoothing

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Time smoothing determines an offset that needs to be applied to the
cooked time to make it smooth for external observers. Observed offset
and frequency change slowly and there are no discontinuities. This can
be used on an NTP server to make it easier for the clients to track the
time and keep their clocks close together even when large offset or
frequency corrections are applied to the server's clock (e.g. after
being offline for longer time).

Accumulated offset and frequency are smoothed out in three stages. In
the first stage, the frequency is changed at a constant rate (wander) up
to a maximum, in the second stage the frequency stays at the maximum for
as long as needed and in the third stage the frequency is brought back
to zero.

Time smoothing is configured by the smoothtime directive. It takes two
arguments, maximum frequency offset and maximum wander. It's disabled by
default.
This commit is contained in:
Miroslav Lichvar
2015-03-31 15:47:11 +02:00
parent 78283dd822
commit 7f45eb7957
11 changed files with 381 additions and 6 deletions
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29
ntp_core.c
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@@ -36,6 +36,7 @@
#include "sched.h"
#include "reference.h"
#include "local.h"
#include "smooth.h"
#include "sources.h"
#include "util.h"
#include "conf.h"
@@ -757,14 +758,14 @@ transmit_packet(NTP_Mode my_mode, /* The mode this machine wants to be */
{
NTP_Packet message;
int leap, auth_len, length, ret;
struct timeval local_transmit;
struct timeval local_receive, local_transmit;
/* Parameters read from reference module */
int are_we_synchronised, our_stratum;
int are_we_synchronised, our_stratum, smooth_time;
NTP_Leap leap_status;
uint32_t our_ref_id, ts_fuzz;
struct timeval our_ref_time;
double our_root_delay, our_root_dispersion;
double our_root_delay, our_root_dispersion, smooth_offset;
/* Don't reply with version higher than ours */
if (version > NTP_VERSION) {
@@ -781,6 +782,15 @@ transmit_packet(NTP_Mode my_mode, /* The mode this machine wants to be */
&our_ref_id, &our_ref_time,
&our_root_delay, &our_root_dispersion);
/* Get current smoothing offset when sending packet to a client */
if (SMT_IsEnabled() && (my_mode == MODE_SERVER || my_mode == MODE_BROADCAST)) {
smooth_time = 1;
smooth_offset = SMT_GetOffset(&local_transmit);
} else {
smooth_time = 0;
smooth_offset = 0.0;
}
if (are_we_synchronised) {
leap = (int) leap_status;
} else {
@@ -807,6 +817,14 @@ transmit_packet(NTP_Mode my_mode, /* The mode this machine wants to be */
message.reference_id = htonl((NTP_int32) our_ref_id);
/* Now fill in timestamps */
if (smooth_time) {
UTI_AddDoubleToTimeval(&our_ref_time, smooth_offset, &our_ref_time);
UTI_AddDoubleToTimeval(local_rx, smooth_offset, &local_receive);
} else {
local_receive = *local_rx;
}
UTI_TimevalToInt64(&our_ref_time, &message.reference_ts, 0);
/* Originate - this comes from the last packet the source sent us */
@@ -816,7 +834,7 @@ transmit_packet(NTP_Mode my_mode, /* The mode this machine wants to be */
This timestamp will have been adjusted so that it will now look to
the source like we have been running on our latest estimate of
frequency all along */
UTI_TimevalToInt64(local_rx, &message.receive_ts, 0);
UTI_TimevalToInt64(&local_receive, &message.receive_ts, 0);
/* Prepare random bits which will be added to the transmit timestamp. */
ts_fuzz = UTI_GetNTPTsFuzz(message.precision);
@@ -826,6 +844,9 @@ transmit_packet(NTP_Mode my_mode, /* The mode this machine wants to be */
from the source we're sending to now. */
LCL_ReadCookedTime(&local_transmit, NULL);
if (smooth_time)
UTI_AddDoubleToTimeval(&local_transmit, smooth_offset, &local_transmit);
length = NTP_NORMAL_PACKET_LENGTH;
/* Authenticate */