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ntp: select timestamps in interleaved mode

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Use previous local TX and remote RX timestamps for the new sample in the
interleaved mode if it will make the local and remote intervals
significantly shorter in order to improve the accuracy of the measured
delay.
This commit is contained in:
Miroslav Lichvar 2017-07-21 10:45:46 +02:00
parent 2e0870ee0c
commit 0baa35eade
1 changed files with 28 additions and 3 deletions
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31
ntp_core.c
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@ -266,6 +266,10 @@ static ARR_Instance broadcasts;
/* Maximum number of missed responses to follow peer's polling interval */
#define MAX_PEER_POLL_TX 8
/* Maximum ratio of local intervals in the timestamp selection of the
interleaved mode to prefer a sample using previous timestamps */
#define MAX_INTERLEAVED_L2L_RATIO 0.1
/* Invalid socket, different from the one in ntp_io.c */
#define INVALID_SOCK_FD -2
@ -1430,11 +1434,23 @@ receive_packet(NCR_Instance inst, NTP_Local_Address *local_addr,
/* Select remote and local timestamps for the new sample */
if (interleaved_packet) {
UTI_Ntp64ToTimespec(&message->receive_ts, &remote_receive);
/* Prefer previous local TX and remote RX timestamps if it will make
the intervals significantly shorter in order to improve the accuracy
of the measured delay */
if (!UTI_IsZeroTimespec(&inst->prev_local_tx.ts) &&
MAX_INTERLEAVED_L2L_RATIO *
UTI_DiffTimespecsToDouble(&inst->local_tx.ts, &inst->local_rx.ts) >
UTI_DiffTimespecsToDouble(&inst->local_rx.ts, &inst->prev_local_tx.ts)) {
UTI_Ntp64ToTimespec(&inst->remote_ntp_rx, &remote_receive);
remote_request_receive = remote_receive;
local_transmit = inst->prev_local_tx;
} else {
UTI_Ntp64ToTimespec(&message->receive_ts, &remote_receive);
UTI_Ntp64ToTimespec(&inst->remote_ntp_rx, &remote_request_receive);
local_transmit = inst->local_tx;
}
UTI_Ntp64ToTimespec(&message->transmit_ts, &remote_transmit);
UTI_Ntp64ToTimespec(&inst->remote_ntp_rx, &remote_request_receive);
local_receive = inst->local_rx;
local_transmit = inst->local_tx;
} else {
UTI_Ntp64ToTimespec(&message->receive_ts, &remote_receive);
UTI_Ntp64ToTimespec(&message->transmit_ts, &remote_transmit);
@ -1546,6 +1562,12 @@ receive_packet(NCR_Instance inst, NTP_Local_Address *local_addr,
inst->local_rx = *rx_ts;
inst->valid_timestamps = synced_packet;
inst->updated_timestamps = 1;
/* Don't use the same set of timestamps for the next sample */
if (interleaved_packet)
inst->prev_local_tx = inst->local_tx;
else
zero_local_timestamp(&inst->prev_local_tx);
}
/* Accept at most one response per request. The NTP specification recommends
@ -2070,6 +2092,9 @@ NCR_SlewTimes(NCR_Instance inst, struct timespec *when, double dfreq, double dof
UTI_AdjustTimespec(&inst->local_rx.ts, when, &inst->local_rx.ts, &delta, dfreq, doffset);
if (!UTI_IsZeroTimespec(&inst->local_tx.ts))
UTI_AdjustTimespec(&inst->local_tx.ts, when, &inst->local_tx.ts, &delta, dfreq, doffset);
if (!UTI_IsZeroTimespec(&inst->prev_local_tx.ts))
UTI_AdjustTimespec(&inst->prev_local_tx.ts, when, &inst->prev_local_tx.ts, &delta, dfreq,
doffset);
}
/* ================================================== */