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ntp: limit number of interleaved responses in symmetric mode

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In symmetric mode, don't send a packet in interleaved mode unless it is
the first response to the last valid request received from the peer and
there was just one response to the previous valid request. This prevents
the peer from matching the transmit timestamp with an older response if
it can't detect missed responses.
This commit is contained in:
Miroslav Lichvar 2017-08-01 17:29:47 +02:00
parent 2668a12e4e
commit 64c2fd9888
2 changed files with 17 additions and 3 deletions
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19
ntp_core.c
View file

@ -181,6 +181,7 @@ struct NCR_Instance_Record {
/* Previous values of some variables needed in interleaved mode */
NTP_Local_Timestamp prev_local_tx;
int prev_local_poll;
unsigned int prev_tx_count;
/* The instance record in the main source management module. This
performs the statistical analysis on the samples we generate */
@ -655,8 +656,10 @@ NCR_ResetInstance(NCR_Instance instance)
UTI_ZeroNtp64(&instance->local_ntp_rx);
UTI_ZeroNtp64(&instance->local_ntp_tx);
zero_local_timestamp(&instance->local_rx);
zero_local_timestamp(&instance->prev_local_tx);
instance->prev_local_poll = 0;
instance->prev_tx_count = 0;
}
/* ================================================== */
@ -1072,7 +1075,7 @@ transmit_timeout(void *arg)
{
NCR_Instance inst = (NCR_Instance) arg;
NTP_Local_Address local_addr;
int sent;
int interleaved, sent;
inst->tx_timeout_id = 0;
@ -1112,18 +1115,27 @@ transmit_timeout(void *arg)
local_addr.if_index = INVALID_IF_INDEX;
local_addr.sock_fd = inst->local_addr.sock_fd;
/* In symmetric mode, don't send a packet in interleaved mode unless it
is the first response to the last valid request received from the peer
and there was just one response to the previous valid request. This
prevents the peer from matching the transmit timestamp with an older
response if it can't detect missed responses. */
interleaved = inst->interleaved &&
(inst->mode == MODE_CLIENT ||
(inst->prev_tx_count == 1 && inst->tx_count == 0));
/* 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 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 <= inst->local_poll && !inst->presend_done &&
!inst->burst_total_samples_to_go) {
inst->presend_done = inst->interleaved ? 2 : 1;
inst->presend_done = interleaved ? 2 : 1;
} else if (inst->presend_done > 0) {
inst->presend_done--;
}
sent = transmit_packet(inst->mode, inst->interleaved, inst->local_poll,
sent = transmit_packet(inst->mode, interleaved, inst->local_poll,
inst->version,
inst->auth_mode, inst->auth_key_id,
&inst->remote_ntp_rx, &inst->remote_ntp_tx,
@ -1624,6 +1636,7 @@ receive_packet(NCR_Instance inst, NTP_Local_Address *local_addr,
message->stratum : NTP_MAX_STRATUM;
inst->prev_local_poll = inst->local_poll;
inst->prev_tx_count = inst->tx_count;
inst->tx_count = 0;
SRC_UpdateReachability(inst->source, synced_packet);

1
test/simulation/122-xleave
View file

@ -17,6 +17,7 @@ max_sync_time=500
base_delay="(+ 1e-4 (* -1 (equal 0.1 from 2) (equal 0.1 to 1)))"
client_lpeer_options="xleave minpoll 5 maxpoll 5"
client_rpeer_options="minpoll 5 maxpoll 5"
run_test || test_fail
check_chronyd_exit || test_fail