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ntp: send two presend packets in interleaved mode

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In a burst of three requests (two presend + one normal) the server can
detect the client is using the interleaved mode and save the transmit
timestamp of the second response for the third response. This shortens
the interval in which the server has to keep the state.
This commit is contained in:
Miroslav Lichvar 2016-11-23 16:34:53 +01:00
parent d23c647e34
commit 2b3d64c31d
2 changed files with 10 additions and 9 deletions
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7
doc/chrony.conf.adoc
View file

@ -171,7 +171,9 @@ interleaved mode requires the servers to keep some state for each client and
the state can be dropped when there are too many clients (e.g.
<<clientloglimit,*clientloglimit*>> is too small), or it can be overwritten by
other clients that have the same IP address (e.g. computers behind NAT or
someone sending requests with a spoofed source address).
someone sending requests with a spoofed source address). The *presend* option
can be used to shorten the interval in which the server has to keep the state
for this computer and be able to respond in the interleaved mode.
*polltarget* _target_:::
Target number of measurements to use for the regression algorithm which
*chronyd* will try to maintain by adjusting the polling interval between
@ -203,7 +205,8 @@ when the polling interval is 512 seconds or more, an extra NTP client packet
will be sent to the server a short time (2 seconds) before making the actual
measurement.
+
The *presend* option cannot be used in the *peer* directive.
The *presend* option cannot be used in the *peer* directive. If it is used
with the *xleave* option, *chronyd* will send two extra packets instead of one.
*minstratum* _stratum_:::
When the synchronisation source is selected from available sources, sources
with lower stratum are normally slightly preferred. This option can be used to

12
ntp_core.c
View file

@ -1050,16 +1050,14 @@ transmit_timeout(void *arg)
/* Check whether we need to 'warm up' the link to the other end by
sending an NTP exchange to ensure both ends' ARP caches are
primed. On loaded systems this might also help ensure that bits
of the program are paged in properly before we start. */
primed or whether we need to send two packets first to ensure a
server in the interleaved mode has a fresh timestamp for us. */
if ((inst->presend_minpoll > 0) &&
(inst->presend_minpoll <= inst->local_poll) &&
!inst->presend_done) {
inst->presend_done = 1;
} else {
/* Reset for next time */
inst->presend_done = 0;
inst->presend_done = inst->interleaved ? 2 : 1;
} else if (inst->presend_done > 0) {
inst->presend_done--;
}
sent = transmit_packet(inst->mode, inst->interleaved, inst->local_poll,