From 4a11399c2ebb78c9de8b4fca256d00b82416ff32 Mon Sep 17 00:00:00 2001
From: Miroslav Lichvar <mlichvar@redhat.com>
Date: Thu, 23 Mar 2023 17:04:57 +0100
Subject: [PATCH] ntp: rework calculation of transmit timeout

Previously, in the calculation of the next transmission time
corresponding to the current polling interval, the reference point was
the current time in the client mode (i.e. the time when the response is
processed) and the last transmission time in the symmetric mode.

Rework the code to use the last transmission in both modes and make it
independent from the time when the response is processed to avoid extra
delays due to waiting for HW TX timestamps.
---
 ntp_core.c | 45 ++++++++++++++++++++++++---------------------
 1 file changed, 24 insertions(+), 21 deletions(-)

diff --git a/ntp_core.c b/ntp_core.c
index a6e47ac..a3c812e 100644
--- a/ntp_core.c
+++ b/ntp_core.c
@@ -324,7 +324,7 @@ static const char tss_chars[3] = {'D', 'K', 'H'};
 /* Forward prototypes */
 
 static void transmit_timeout(void *arg);
-static double get_transmit_delay(NCR_Instance inst, int on_tx, double last_tx);
+static double get_transmit_delay(NCR_Instance inst, int on_tx);
 static double get_separation(int poll);
 static int parse_packet(NTP_Packet *packet, int length, NTP_PacketInfo *info);
 static void process_sample(NCR_Instance inst, NTP_Sample *sample);
@@ -490,8 +490,7 @@ restart_timeout(NCR_Instance inst, double delay)
 static void
 start_initial_timeout(NCR_Instance inst)
 {
-  double delay, last_tx;
-  struct timespec now;
+  double delay;
 
   if (!inst->tx_timeout_id) {
     /* This will be the first transmission after mode change */
@@ -504,11 +503,7 @@ start_initial_timeout(NCR_Instance inst)
      the interval between packets at least as long as the current polling
      interval */
   if (!UTI_IsZeroTimespec(&inst->local_tx.ts)) {
-    SCH_GetLastEventTime(&now, NULL, NULL);
-    last_tx = UTI_DiffTimespecsToDouble(&now, &inst->local_tx.ts);
-    if (last_tx < 0.0)
-      last_tx = 0.0;
-    delay = get_transmit_delay(inst, 0, 0.0) - last_tx;
+    delay = get_transmit_delay(inst, 0);
   } else {
     delay = 0.0;
   }
@@ -779,7 +774,7 @@ NCR_ResetPoll(NCR_Instance instance)
 
     /* The timer was set with a longer poll interval, restart it */
     if (instance->tx_timeout_id)
-      restart_timeout(instance, get_transmit_delay(instance, 0, 0.0));
+      restart_timeout(instance, get_transmit_delay(instance, 0));
   }
 }
 
@@ -900,10 +895,19 @@ get_transmit_poll(NCR_Instance inst)
 /* ================================================== */
 
 static double
-get_transmit_delay(NCR_Instance inst, int on_tx, double last_tx)
+get_transmit_delay(NCR_Instance inst, int on_tx)
 {
   int poll_to_use, stratum_diff;
-  double delay_time;
+  double delay_time, last_tx;
+  struct timespec now;
+
+  /* Calculate the interval since last transmission if known */
+  if (!on_tx && !UTI_IsZeroTimespec(&inst->local_tx.ts)) {
+    SCH_GetLastEventTime(&now, NULL, NULL);
+    last_tx = UTI_DiffTimespecsToDouble(&now, &inst->local_tx.ts);
+  } else {
+    last_tx = 0;
+  }
 
   /* If we're in burst mode, queue for immediate dispatch.
 
@@ -943,12 +947,6 @@ get_transmit_delay(NCR_Instance inst, int on_tx, double last_tx)
                last_tx / delay_time > PEER_SAMPLING_ADJ - 0.5))
             delay_time *= PEER_SAMPLING_ADJ;
 
-          /* Substract the already spend time */
-          if (last_tx > 0.0)
-            delay_time -= last_tx;
-          if (delay_time < 0.0)
-            delay_time = 0.0;
-
           break;
         default:
           assert(0);
@@ -966,6 +964,12 @@ get_transmit_delay(NCR_Instance inst, int on_tx, double last_tx)
       break;
   }
 
+  /* Subtract elapsed time */
+  if (last_tx > 0.0)
+    delay_time -= last_tx;
+  if (delay_time < 0.0)
+    delay_time = 0.0;
+
   return delay_time;
 }
 
@@ -1310,7 +1314,7 @@ transmit_timeout(void *arg)
   /* Prepare authentication */
   if (!NAU_PrepareRequestAuth(inst->auth)) {
     SRC_UpdateReachability(inst->source, 0);
-    restart_timeout(inst, get_transmit_delay(inst, 1, 0.0));
+    restart_timeout(inst, get_transmit_delay(inst, 1));
     /* Count missing samples for the sample filter */
     process_sample(inst, NULL);
     return;
@@ -1415,7 +1419,7 @@ transmit_timeout(void *arg)
   }
 
   /* Restart timer for this message */
-  restart_timeout(inst, get_transmit_delay(inst, 1, 0.0));
+  restart_timeout(inst, get_transmit_delay(inst, 1));
 
   /* If a client packet was just sent, schedule a timeout to close the socket
      at the time when all server replies would fail the delay test, so the
@@ -2169,8 +2173,7 @@ process_response(NCR_Instance inst, NTP_Local_Address *local_addr,
 
     /* And now, requeue the timer */
     if (inst->opmode != MD_OFFLINE) {
-      delay_time = get_transmit_delay(inst, 0,
-                     UTI_DiffTimespecsToDouble(&inst->local_rx.ts, &inst->local_tx.ts));
+      delay_time = get_transmit_delay(inst, 0);
 
       if (kod_rate) {
         LOG(LOGS_WARN, "Received KoD RATE from %s",