diff --git a/broker/client.go b/broker/client.go
index a06d7f8..405265b 100644
--- a/broker/client.go
+++ b/broker/client.go
@@ -3,6 +3,7 @@ package broker
 import (
 	"context"
 	"errors"
+	"github.com/eapache/queue"
 	"math/rand"
 	"net"
 	"reflect"
@@ -41,29 +42,52 @@ const (
 	Disconnected = 2
 )
 
+const (
+	awaitRelTimeout int64 = 20
+	retryInterval   int64 = 20
+)
+
 var (
 	groupCompile = regexp.MustCompile(_GroupTopicRegexp)
 )
 
 type client struct {
-	typ         int
-	mu          sync.Mutex
-	broker      *Broker
-	conn        net.Conn
-	info        info
-	route       route
-	status      int
-	ctx         context.Context
-	cancelFunc  context.CancelFunc
-	session     *sessions.Session
-	subMap      map[string]*subscription
-	topicsMgr   *topics.Manager
-	subs        []interface{}
-	qoss        []byte
-	rmsgs       []*packets.PublishPacket
-	routeSubMap map[string]uint64
+	typ            int
+	mu             sync.Mutex
+	broker         *Broker
+	conn           net.Conn
+	info           info
+	route          route
+	status         int
+	ctx            context.Context
+	cancelFunc     context.CancelFunc
+	session        *sessions.Session
+	subMap         map[string]*subscription
+	topicsMgr      *topics.Manager
+	subs           []interface{}
+	qoss           []byte
+	rmsgs          []*packets.PublishPacket
+	routeSubMap    map[string]uint64
+	awaitingRel    map[uint16]int64
+	maxAwaitingRel int
+	inflight       map[uint16]*inflightElem
+	mqueue         *queue.Queue
+	retryTimer     *time.Timer
+	retryTimerLock sync.Mutex
 }
 
+type InflightStatus uint8
+
+const (
+	Publish InflightStatus = 0
+	Pubrel  InflightStatus = 1
+)
+
+type inflightElem struct {
+	status    InflightStatus
+	packet    *packets.PublishPacket
+	timestamp int64
+}
 type subscription struct {
 	client    *client
 	topic     string
@@ -108,6 +132,9 @@ func (c *client) init() {
 	c.subMap = make(map[string]*subscription)
 	c.topicsMgr = c.broker.topicsMgr
 	c.routeSubMap = make(map[string]uint64)
+	c.awaitingRel = make(map[uint16]int64)
+	c.inflight = make(map[uint16]*inflightElem)
+	c.mqueue = queue.New()
 }
 
 func (c *client) readLoop() {
@@ -177,9 +204,43 @@ func ProcessMessage(msg *Message) {
 		packet := ca.(*packets.PublishPacket)
 		c.ProcessPublish(packet)
 	case *packets.PubackPacket:
+		packet := ca.(*packets.PubackPacket)
+		if _, found := c.inflight[packet.MessageID]; found {
+			delete(c.inflight, packet.MessageID)
+		} else {
+			log.Error("Duplicated PUBACK PacketId", zap.Uint16("MessageID", packet.MessageID))
+		}
 	case *packets.PubrecPacket:
+		packet := ca.(*packets.PubrecPacket)
+		if _, found := c.inflight[packet.MessageID]; found {
+			if c.inflight[packet.MessageID].status == Publish {
+				c.inflight[packet.MessageID].status = Pubrel
+				c.inflight[packet.MessageID].timestamp = time.Now().Unix()
+			} else if c.inflight[packet.MessageID].status == Pubrel {
+				log.Error("Duplicated PUBREC PacketId", zap.Uint16("MessageID", packet.MessageID))
+			}
+		} else {
+			log.Error("The PUBREC PacketId is not found.", zap.Uint16("MessageID", packet.MessageID))
+		}
+
+		pubrel := packets.NewControlPacket(packets.Pubrel).(*packets.PubrelPacket)
+		pubrel.MessageID = packet.MessageID
+		if err := c.WriterPacket(pubrel); err != nil {
+			log.Error("send pubrel error, ", zap.Error(err), zap.String("ClientID", c.info.clientID))
+			return
+		}
 	case *packets.PubrelPacket:
+		packet := ca.(*packets.PubrelPacket)
+		c.pubRel(packet.MessageID)
+		pubcomp := packets.NewControlPacket(packets.Pubcomp).(*packets.PubcompPacket)
+		pubcomp.MessageID = packet.MessageID
+		if err := c.WriterPacket(pubcomp); err != nil {
+			log.Error("send pubcomp error, ", zap.Error(err), zap.String("ClientID", c.info.clientID))
+			return
+		}
 	case *packets.PubcompPacket:
+		packet := ca.(*packets.PubcompPacket)
+		delete(c.inflight, packet.MessageID)
 	case *packets.SubscribePacket:
 		packet := ca.(*packets.SubscribePacket)
 		c.ProcessSubscribe(packet)
@@ -279,6 +340,17 @@ func (c *client) processClientPublish(packet *packets.PublishPacket) {
 		}
 		c.ProcessPublishMessage(packet)
 	case QosExactlyOnce:
+		if err := c.registerPublishPacketId(packet.MessageID); err != nil {
+			return
+		} else {
+			pubrec := packets.NewControlPacket(packets.Pubrec).(*packets.PubrecPacket)
+			pubrec.MessageID = packet.MessageID
+			if err := c.WriterPacket(pubrec); err != nil {
+				log.Error("send pubrec error, ", zap.Error(err), zap.String("ClientID", c.info.clientID))
+				return
+			}
+			c.ProcessPublishMessage(packet)
+		}
 		return
 	default:
 		log.Error("publish with unknown qos", zap.String("ClientID", c.info.clientID))
@@ -684,3 +756,50 @@ func (c *client) WriterPacket(packet packets.ControlPacket) error {
 	c.mu.Unlock()
 	return err
 }
+
+func (c *client) registerPublishPacketId(packetId uint16) error {
+	if c.isAwaitingFull() {
+		log.Error("Dropped qos2 packet for too many awaiting_rel", zap.Uint16("id", packetId))
+		return errors.New("DROPPED_QOS2_PACKET_FOR_TOO_MANY_AWAITING_REL")
+	}
+
+	if _, found := c.awaitingRel[packetId]; found {
+		return errors.New("RC_PACKET_IDENTIFIER_IN_USE")
+	}
+	c.awaitingRel[packetId] = time.Now().Unix()
+	time.AfterFunc(time.Duration(awaitRelTimeout)*time.Second, c.expireAwaitingRel)
+	return nil
+}
+
+func (c *client) isAwaitingFull() bool {
+	if c.maxAwaitingRel == 0 {
+		return false
+	}
+	if len(c.awaitingRel) < c.maxAwaitingRel {
+		return false
+	}
+	return true
+}
+
+func (c *client) expireAwaitingRel() {
+	if len(c.awaitingRel) == 0 {
+		return
+	}
+	now := time.Now().Unix()
+	for packetId, Timestamp := range c.awaitingRel {
+		if now-Timestamp >= awaitRelTimeout {
+			log.Error("Dropped qos2 packet for await_rel_timeout", zap.Uint16("id", packetId))
+			delete(c.awaitingRel, packetId)
+		}
+	}
+}
+
+func (c *client) pubRel(packetId uint16) error {
+	if _, found := c.awaitingRel[packetId]; found {
+		delete(c.awaitingRel, packetId)
+	} else {
+		log.Error("The PUBREL PacketId is not found", zap.Uint16("id", packetId))
+		return errors.New("RC_PACKET_IDENTIFIER_NOT_FOUND")
+	}
+	return nil
+}
diff --git a/broker/comm.go b/broker/comm.go
index 17182b3..52c05f1 100644
--- a/broker/comm.go
+++ b/broker/comm.go
@@ -152,8 +152,78 @@ func publish(sub *subscription, packet *packets.PublishPacket) {
 	// 	log.Error("process message for psub error,  ", zap.Error(err))
 	// }
 
-	err := sub.client.WriterPacket(packet)
-	if err != nil {
-		log.Error("process message for psub error,  ", zap.Error(err))
+	switch packet.Qos {
+	case QosAtMostOnce:
+		err := sub.client.WriterPacket(packet)
+		if err != nil {
+			log.Error("process message for psub error,  ", zap.Error(err))
+		}
+	case QosAtLeastOnce, QosExactlyOnce:
+		sub.client.inflight[packet.MessageID] = &inflightElem{status: Publish, packet: packet, timestamp: time.Now().Unix()}
+		err := sub.client.WriterPacket(packet)
+		if err != nil {
+			log.Error("process message for psub error,  ", zap.Error(err))
+		}
+		sub.client.ensureRetryTimer()
+	default:
+		log.Error("publish with unknown qos", zap.String("ClientID", sub.client.info.clientID))
+		return
 	}
 }
+
+// timer for retry delivery
+func (c *client) ensureRetryTimer(interval ...int64) {
+	if c.retryTimer != nil {
+		return
+	}
+	if len(interval) > 1 {
+		return
+	}
+	timerInterval := retryInterval
+	if len(interval) == 1 {
+		timerInterval = interval[0]
+	}
+	c.retryTimerLock.Lock()
+	c.retryTimer = time.AfterFunc(time.Duration(timerInterval)*time.Second, c.retryDelivery)
+	c.retryTimerLock.Unlock()
+	return
+}
+
+func (c *client) resetRetryTimer() {
+	if c.retryTimer == nil {
+		return
+	}
+	// reset timer
+	c.retryTimerLock.Lock()
+	c.retryTimer = nil
+	c.retryTimerLock.Unlock()
+
+}
+
+func (c *client) retryDelivery() {
+	c.resetRetryTimer()
+	if c.conn == nil || len(c.inflight) == 0 { //Reset timer when client offline OR inflight is empty
+		return
+	}
+	now := time.Now().Unix()
+	for _, infEle := range c.inflight {
+		age := now - infEle.timestamp
+		if age >= retryInterval {
+			if infEle.status == Publish {
+				c.WriterPacket(infEle.packet)
+				c.inflight[infEle.packet.MessageID].timestamp = now
+			} else if infEle.status == Pubrel {
+				pubrel := packets.NewControlPacket(packets.Pubrel).(*packets.PubrelPacket)
+				pubrel.MessageID = infEle.packet.MessageID
+				c.WriterPacket(pubrel)
+				c.inflight[infEle.packet.MessageID].timestamp = now
+			}
+		} else {
+			if age < 0 {
+				age = 0
+			}
+			c.ensureRetryTimer(retryInterval - age)
+		}
+	}
+	c.ensureRetryTimer()
+}
diff --git a/go.mod b/go.mod
index 9a7f990..644cf5e 100644
--- a/go.mod
+++ b/go.mod
@@ -6,6 +6,7 @@ require (
 	github.com/Shopify/sarama v1.23.0
 	github.com/bitly/go-simplejson v0.5.0
 	github.com/bmizerany/assert v0.0.0-20160611221934-b7ed37b82869 // indirect
+	github.com/eapache/queue v1.1.0
 	github.com/eclipse/paho.mqtt.golang v1.2.0
 	github.com/gin-gonic/gin v1.4.0
 	github.com/golang/protobuf v1.3.2 // indirect