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ntp: rework calculation and testing of peer delay

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Instead of a worst-case delay use a mean value and relate it to the
source's time. This makes it more stable in the interleaved and
symmetric modes, which should improve the weighting and asymmetry
correction. Modify the test A and B to work with a minimum estimated
delay (delay - dispersion).
This commit is contained in:
Miroslav Lichvar 2016-12-05 13:26:58 +01:00
parent 979b53866d
commit 23a4e8b38d
1 changed files with 10 additions and 10 deletions
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20
ntp_core.c
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@ -1408,10 +1408,9 @@ receive_packet(NCR_Instance inst, NTP_Local_Address *local_addr,
sample_rx_tss = rx_ts->source; sample_rx_tss = rx_ts->source;
} }
/* In our case, we work out 'delay' as the worst case delay, /* Work out 'delay' relative to the source's time */
assuming worst case frequency error between us and the other delay = (1.0 - (source_freq_lo + source_freq_hi) / 2.0) *
source */ local_interval - remote_interval;
delay = local_interval - remote_interval * (1.0 + source_freq_lo);
/* Clamp delay to avoid misleading results later */ /* Clamp delay to avoid misleading results later */
delay = fabs(delay); delay = fabs(delay);
@ -1440,11 +1439,12 @@ receive_packet(NCR_Instance inst, NTP_Local_Address *local_addr,
/* Additional tests required to pass before accumulating the sample */ /* Additional tests required to pass before accumulating the sample */
/* Test A requires that the peer delay is not larger than the configured /* Test A requires that the minimum estimate of the peer delay is not
maximum, in client mode that the server processing time is sane, and in larger than the configured maximum, in client mode that the server
interleaved symmetric mode that the delay is not longer than half of the processing time is sane, and in the interleaved symmetric mode that
remote polling interval to detect missed packets */ the delay is not longer than half of the remote polling interval to
testA = delay <= inst->max_delay && detect missed packets */
testA = delay - dispersion <= inst->max_delay &&
!(inst->mode == MODE_CLIENT && server_interval > MAX_SERVER_INTERVAL) && !(inst->mode == MODE_CLIENT && server_interval > MAX_SERVER_INTERVAL) &&
!(inst->mode == MODE_ACTIVE && interleaved_packet && !(inst->mode == MODE_ACTIVE && interleaved_packet &&
delay > UTI_Log2ToDouble(message->poll - 1)); delay > UTI_Log2ToDouble(message->poll - 1));
@ -1453,7 +1453,7 @@ receive_packet(NCR_Instance inst, NTP_Local_Address *local_addr,
minimum one currently in the stats data register is less than an minimum one currently in the stats data register is less than an
administrator-defined value */ administrator-defined value */
testB = inst->max_delay_ratio <= 1.0 || testB = inst->max_delay_ratio <= 1.0 ||
delay / SST_MinRoundTripDelay(stats) <= inst->max_delay_ratio; (delay - dispersion) / SST_MinRoundTripDelay(stats) <= inst->max_delay_ratio;
/* Test C requires that the ratio of the increase in delay from the minimum /* Test C requires that the ratio of the increase in delay from the minimum
one in the stats data register to the standard deviation of the offsets one in the stats data register to the standard deviation of the offsets