diff --git a/ack_timer.go b/ack_timer.go
index 879e86df..b6008d18 100644
--- a/ack_timer.go
+++ b/ack_timer.go
@@ -18,7 +18,7 @@ type ackTimerObserver interface {
 	onAckTimeout()
 }
 
-type ackTimerState int
+type ackTimerState uint8
 
 const (
 	ackTimerStopped ackTimerState = iota
@@ -28,10 +28,11 @@ const (
 
 // ackTimer provides the retnransmission timer conforms with RFC 4960 Sec 6.3.1
 type ackTimer struct {
+	timer    *time.Timer
 	observer ackTimerObserver
-	mutex    sync.RWMutex
+	mutex    sync.Mutex
 	state    ackTimerState
-	timer    *time.Timer
+	pending  uint8
 }
 
 // newAckTimer creates a new acknowledgement timer used to enable delayed ack.
@@ -44,7 +45,7 @@ func newAckTimer(observer ackTimerObserver) *ackTimer {
 
 func (t *ackTimer) timeout() {
 	t.mutex.Lock()
-	if t.state == ackTimerStarted {
+	if t.pending--; t.pending == 0 && t.state == ackTimerStarted {
 		t.state = ackTimerStopped
 		defer t.observer.onAckTimeout()
 	}
@@ -62,6 +63,7 @@ func (t *ackTimer) start() bool {
 	}
 
 	t.state = ackTimerStarted
+	t.pending++
 	t.timer.Reset(ackInterval)
 	return true
 }
@@ -73,7 +75,9 @@ func (t *ackTimer) stop() {
 	defer t.mutex.Unlock()
 
 	if t.state == ackTimerStarted {
-		t.timer.Stop()
+		if t.timer.Stop() {
+			t.pending--
+		}
 		t.state = ackTimerStopped
 	}
 }
@@ -84,8 +88,8 @@ func (t *ackTimer) close() {
 	t.mutex.Lock()
 	defer t.mutex.Unlock()
 
-	if t.state == ackTimerStarted {
-		t.timer.Stop()
+	if t.state == ackTimerStarted && t.timer.Stop() {
+		t.pending--
 	}
 	t.state = ackTimerClosed
 }
@@ -93,8 +97,8 @@ func (t *ackTimer) close() {
 // isRunning tests if the timer is running.
 // Debug purpose only
 func (t *ackTimer) isRunning() bool {
-	t.mutex.RLock()
-	defer t.mutex.RUnlock()
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
 
 	return t.state == ackTimerStarted
 }
diff --git a/rtx_timer.go b/rtx_timer.go
index 354825b5..1fea3931 100644
--- a/rtx_timer.go
+++ b/rtx_timer.go
@@ -118,19 +118,28 @@ type rtxTimerObserver interface {
 	onRetransmissionFailure(timerID int)
 }
 
+type rtxTimerState uint8
+
+const (
+	rtxTimerStopped rtxTimerState = iota
+	rtxTimerStarted
+	rtxTimerClosed
+)
+
 // rtxTimer provides the retnransmission timer conforms with RFC 4960 Sec 6.3.1
 type rtxTimer struct {
-	id         int
+	timer      *time.Timer
 	observer   rtxTimerObserver
+	id         int
 	maxRetrans uint
-	stopFunc   stopTimerLoop
-	closed     bool
-	mutex      sync.RWMutex
 	rtoMax     float64
+	mutex      sync.Mutex
+	rto        float64
+	nRtos      uint
+	state      rtxTimerState
+	pending    uint8
 }
 
-type stopTimerLoop func()
-
 // newRTXTimer creates a new retransmission timer.
 // if maxRetrans is set to 0, it will keep retransmitting until stop() is called.
 // (it will never make onRetransmissionFailure() callback.
@@ -146,21 +155,38 @@ func newRTXTimer(id int, observer rtxTimerObserver, maxRetrans uint,
 	if timer.rtoMax == 0 {
 		timer.rtoMax = defaultRTOMax
 	}
+	timer.timer = time.AfterFunc(math.MaxInt64, timer.timeout)
+	timer.timer.Stop()
 	return &timer
 }
 
+func (t *rtxTimer) calculateNextTimeout() time.Duration {
+	timeout := calculateNextTimeout(t.rto, t.nRtos, t.rtoMax)
+	return time.Duration(timeout) * time.Millisecond
+}
+
+func (t *rtxTimer) timeout() {
+	t.mutex.Lock()
+	if t.pending--; t.pending == 0 && t.state == rtxTimerStarted {
+		if t.nRtos++; t.maxRetrans == 0 || t.nRtos <= t.maxRetrans {
+			t.timer.Reset(t.calculateNextTimeout())
+			t.pending++
+			defer t.observer.onRetransmissionTimeout(t.id, t.nRtos)
+		} else {
+			t.state = rtxTimerStopped
+			defer t.observer.onRetransmissionFailure(t.id)
+		}
+	}
+	t.mutex.Unlock()
+}
+
 // start starts the timer.
 func (t *rtxTimer) start(rto float64) bool {
 	t.mutex.Lock()
 	defer t.mutex.Unlock()
 
-	// this timer is already closed
-	if t.closed {
-		return false
-	}
-
-	// this is a noop if the timer is always running
-	if t.stopFunc != nil {
+	// this timer is already closed or aleady running
+	if t.state != rtxTimerStopped {
 		return false
 	}
 
@@ -168,40 +194,11 @@ func (t *rtxTimer) start(rto float64) bool {
 	// fast timeout for the tests. Non-test code should pass in the
 	// rto generated by rtoManager getRTO() method which caps the
 	// value at RTO.Min or at RTO.Max.
-	var nRtos uint
-
-	cancelCh := make(chan struct{})
-
-	go func() {
-		canceling := false
-
-		timer := time.NewTimer(math.MaxInt64)
-		timer.Stop()
-
-		for !canceling {
-			timeout := calculateNextTimeout(rto, nRtos, t.rtoMax)
-			timer.Reset(time.Duration(timeout) * time.Millisecond)
-
-			select {
-			case <-timer.C:
-				nRtos++
-				if t.maxRetrans == 0 || nRtos <= t.maxRetrans {
-					t.observer.onRetransmissionTimeout(t.id, nRtos)
-				} else {
-					t.stop()
-					t.observer.onRetransmissionFailure(t.id)
-				}
-			case <-cancelCh:
-				canceling = true
-				timer.Stop()
-			}
-		}
-	}()
-
-	t.stopFunc = func() {
-		close(cancelCh)
-	}
-
+	t.rto = rto
+	t.nRtos = 0
+	t.state = rtxTimerStarted
+	t.pending++
+	t.timer.Reset(t.calculateNextTimeout())
 	return true
 }
 
@@ -210,9 +207,11 @@ func (t *rtxTimer) stop() {
 	t.mutex.Lock()
 	defer t.mutex.Unlock()
 
-	if t.stopFunc != nil {
-		t.stopFunc()
-		t.stopFunc = nil
+	if t.state == rtxTimerStarted {
+		if t.timer.Stop() {
+			t.pending--
+		}
+		t.state = rtxTimerStopped
 	}
 }
 
@@ -222,21 +221,19 @@ func (t *rtxTimer) close() {
 	t.mutex.Lock()
 	defer t.mutex.Unlock()
 
-	if t.stopFunc != nil {
-		t.stopFunc()
-		t.stopFunc = nil
+	if t.state == rtxTimerStarted && t.timer.Stop() {
+		t.pending--
 	}
-
-	t.closed = true
+	t.state = rtxTimerClosed
 }
 
 // isRunning tests if the timer is running.
 // Debug purpose only
 func (t *rtxTimer) isRunning() bool {
-	t.mutex.RLock()
-	defer t.mutex.RUnlock()
+	t.mutex.Lock()
+	defer t.mutex.Unlock()
 
-	return (t.stopFunc != nil)
+	return t.state == rtxTimerStarted
 }
 
 func calculateNextTimeout(rto float64, nRtos uint, rtoMax float64) float64 {