ntp: limit number of interleaved responses in symmetric mode

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.

diff --git a/ntp_core.c b/ntp_core.c
index e18e7fd46db7d744573990291fd0585657d382fb..3442d14b977c40eb564d8da422db07d6c5710431 100644 (file)
--- a/ntp_core.c
+++ b/ntp_core.c
@@ -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);

diff --git a/test/simulation/122-xleave b/test/simulation/122-xleave
index e0ce9ecadfb5e9a8e166283ddb8d68cba285f5b0..93f767e97473281a38a52ef5becdf8fe203dfc1e 100755 (executable)
--- a/test/simulation/122-xleave
+++ b/test/simulation/122-xleave
@@ -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