diff --git a/icmp/pinger.go b/icmp/pinger.go
index b06fbdc..9e6cb7c 100644
--- a/icmp/pinger.go
+++ b/icmp/pinger.go
@@ -13,6 +13,7 @@ import (
 	"github.com/shallowclouds/omil/metric"
 )
 
+// Monitor sends and receives ICMP packet to the specified host.
 type Monitor struct {
 	from, to string
 	host     string
@@ -22,6 +23,13 @@ type Monitor struct {
 	timeout  time.Duration
 }
 
+// NewMonitor creates a ICMP network monitor.
+// `host` is the target hostname need to test.
+// `from` is the name of this server, used as metric tag `from`.
+// `to` is the name of target host, used as metric tag `to`.
+// `interval` specifies the time interval to send ICMP packets.
+// `timeout` specifies the time to end the loop, 0 for infinite.
+// `client` is the metric client to send data points.
 func NewMonitor(host, from, to string, interval, timeout time.Duration, client metric.Client) (*Monitor, error) {
 	var err error
 	if from == "" {
@@ -40,7 +48,7 @@ func NewMonitor(host, from, to string, interval, timeout time.Duration, client m
 	}
 
 	if client == nil {
-		return nil, fmt.Errorf("nil metric client")
+		return nil, errors.New("nil metric client")
 	}
 
 	return &Monitor{
@@ -53,21 +61,25 @@ func NewMonitor(host, from, to string, interval, timeout time.Duration, client m
 	}, nil
 }
 
-func (m *Monitor) Start(ctx context.Context) error {
+// Start starts the loop to test network and send data points.
+//
+// Return an error if loop failed.
+func (m *Monitor) Start(_ context.Context) error {
 	pinger, err := ping.NewPinger(m.host)
 	if err != nil {
-		return errors.Wrap(err, "failed to create pinger")
+		return errors.WithMessage(err, "failed to create pinger")
 	}
 
 	if m.timeout != 0 {
 		pinger.Timeout = m.timeout
 	}
 	pinger.Interval = m.interval
-	// true for ICMP
+	// True for ICMP
 	pinger.SetPrivileged(true)
 	pinger.Size = 64
 
-	// use startTime to calculate packet sent time: startTime + interval * packet_sequence_number
+	// Use startTime to calculate packet sent time: startTime + interval * packet_sequence_number,
+	// as will cant put the send time in the ICMP packet data at present.
 	// TODO: use a more accurate and graceful way
 	var startTime time.Time
 
@@ -90,9 +102,27 @@ func (m *Monitor) Start(ctx context.Context) error {
 
 	m.pinger = pinger
 
+	sendTicker := time.NewTicker(m.interval)
+	defer sendTicker.Stop()
+
 	startTime = time.Now()
+	go func() {
+		// Metrics for sending packets.
+		for t := range sendTicker.C {
+			if t.IsZero() {
+				break
+			}
+			m.client.Metric("ICMP", t, map[string]string{
+				"from": m.from,
+				"to":   m.to,
+				"host": m.host,
+			}, map[string]interface{}{
+				"sent": 1,
+			})
+		}
+	}()
 	if err := pinger.Run(); err != nil {
-		return errors.Wrap(err, "failed to run pinger")
+		return errors.WithMessage(err, "failed to run pinger")
 	}
 
 	return nil
diff --git a/loop/loop.go b/loop/loop.go
index bea3a50..7a533f0 100644
--- a/loop/loop.go
+++ b/loop/loop.go
@@ -34,35 +34,42 @@ func Loop(ctx context.Context, monitors []*icmp.Monitor) (err error) {
 		cancel()
 	}()
 
+	restart := true
+	var mu sync.RWMutex
+	go func() {
+		<-ctx.Done()
+		mu.Lock()
+		restart = false
+		mu.Unlock()
+		for _, monitor := range monitors {
+			logrus.Infof("stopping monitor %s", monitor.Name())
+			if err := monitor.Stop(); err != nil {
+				logrus.WithError(err).Error("failed to stop monitor")
+			}
+		}
+	}()
 	for _, monitor := range monitors {
 		wg.Add(1)
-		monitor := monitor
+		m := monitor
 		go func() {
-			restart := true
-			var mu sync.RWMutex
-			go func() {
-				<-ctx.Done()
-				logrus.Infof("stopping monitor %s", monitor.Name())
-				if err := monitor.Stop(); err != nil {
-					logrus.WithError(err).Error("failed to stop monitor")
-				}
-				mu.Lock()
-				restart = false
-				mu.Unlock()
-			}()
 			for {
-				if err := monitor.Start(ctx); err != nil {
-					logrus.WithError(err).Error("failed to run monitor")
-				}
 				mu.RLock()
 				if !restart {
-					logrus.Infof("exiting monitor %s", monitor.Name())
+					logrus.Infof("exiting monitor %s", m.Name())
 					wg.Done()
 					break
 				}
 				mu.RUnlock()
+				if err := m.Start(ctx); err != nil {
+					logrus.WithError(err).Error("failed to run monitor")
+				}
 				time.Sleep(restartInterval)
-				logrus.Infof("restarting monitor %s", monitor.Name())
+
+				mu.RLock()
+				if restart {
+					logrus.Infof("restarting monitor %s", m.Name())
+				}
+				mu.RUnlock()
 			}
 		}()
 	}
diff --git a/main.go b/main.go
index b1fba99..0771ef3 100644
--- a/main.go
+++ b/main.go
@@ -33,7 +33,10 @@ func mainAction(ctx *cli.Context) error {
 
 	monitors := make([]*icmp.Monitor, 0, len(conf.Targets))
 	for _, t := range conf.Targets {
-		monitor, err := icmp.NewMonitor(t.Host, conf.Hostname, t.Name, time.Second, time.Hour*12, metricClient)
+		// As pinger stores all packets data in memory,
+		// so too long timeout may cause high memory usage.
+		// Just let it stop and restart.
+		monitor, err := icmp.NewMonitor(t.Host, conf.Hostname, t.Name, time.Second, time.Minute*60, metricClient)
 		if err != nil {
 			logrus.WithError(err).WithFields(logrus.Fields{
 				"target_host": t.Host,