diff --git a/session.go b/session.go
index 8f4ad61..88015e3 100644
--- a/session.go
+++ b/session.go
@@ -17,6 +17,14 @@ const (
 	openCloseTimeout     = 30 * time.Second // stream open/close timeout
 )
 
+// define frame class
+type CLASSID int
+
+const (
+	CLSCTRL CLASSID = iota
+	CLSDATA
+)
+
 var (
 	ErrInvalidProtocol = errors.New("invalid protocol")
 	ErrConsumed        = errors.New("peer consumed more than sent")
@@ -26,7 +34,7 @@ var (
 )
 
 type writeRequest struct {
-	prio   uint32
+	class  CLASSID
 	frame  Frame
 	seq    uint32
 	result chan writeResult
@@ -396,7 +404,7 @@ func (s *Session) keepalive() {
 	for {
 		select {
 		case <-tickerPing.C:
-			s.writeFrameInternal(newFrame(byte(s.config.Version), cmdNOP, 0), tickerPing.C, 0)
+			s.writeFrameInternal(newFrame(byte(s.config.Version), cmdNOP, 0), tickerPing.C, CLSCTRL)
 			s.notifyBucket() // force a signal to the recvLoop
 		case <-tickerTimeout.C:
 			if !atomic.CompareAndSwapInt32(&s.dataReady, 1, 0) {
@@ -515,13 +523,13 @@ func (s *Session) sendLoop() {
 // writeFrame writes the frame to the underlying connection
 // and returns the number of bytes written if successful
 func (s *Session) writeFrame(f Frame) (n int, err error) {
-	return s.writeFrameInternal(f, time.After(openCloseTimeout), 0)
+	return s.writeFrameInternal(f, time.After(openCloseTimeout), CLSCTRL)
 }
 
 // internal writeFrame version to support deadline used in keepalive
-func (s *Session) writeFrameInternal(f Frame, deadline <-chan time.Time, prio uint32) (int, error) {
+func (s *Session) writeFrameInternal(f Frame, deadline <-chan time.Time, class CLASSID) (int, error) {
 	req := writeRequest{
-		prio:   prio,
+		class:  class,
 		frame:  f,
 		seq:    atomic.AddUint32(&s.requestID, 1),
 		result: make(chan writeResult, 1),
diff --git a/session_test.go b/session_test.go
index 3479570..1c49ae7 100644
--- a/session_test.go
+++ b/session_test.go
@@ -867,7 +867,7 @@ func TestWriteFrameInternal(t *testing.T) {
 	session.Close()
 	for i := 0; i < 100; i++ {
 		f := newFrame(1, byte(rand.Uint32()), rand.Uint32())
-		session.writeFrameInternal(f, time.After(session.config.KeepAliveTimeout), 0)
+		session.writeFrameInternal(f, time.After(session.config.KeepAliveTimeout), CLSDATA)
 	}
 
 	// random cmds
@@ -879,14 +879,14 @@ func TestWriteFrameInternal(t *testing.T) {
 	session, _ = Client(cli, nil)
 	for i := 0; i < 100; i++ {
 		f := newFrame(1, allcmds[rand.Int()%len(allcmds)], rand.Uint32())
-		session.writeFrameInternal(f, time.After(session.config.KeepAliveTimeout), 0)
+		session.writeFrameInternal(f, time.After(session.config.KeepAliveTimeout), CLSDATA)
 	}
 	//deadline occur
 	{
 		c := make(chan time.Time)
 		close(c)
 		f := newFrame(1, allcmds[rand.Int()%len(allcmds)], rand.Uint32())
-		_, err := session.writeFrameInternal(f, c, 0)
+		_, err := session.writeFrameInternal(f, c, CLSDATA)
 		if !strings.Contains(err.Error(), "timeout") {
 			t.Fatal("write frame with deadline failed", err)
 		}
@@ -911,7 +911,7 @@ func TestWriteFrameInternal(t *testing.T) {
 			time.Sleep(time.Second)
 			close(c)
 		}()
-		_, err = session.writeFrameInternal(f, c, 0)
+		_, err = session.writeFrameInternal(f, c, CLSDATA)
 		if !strings.Contains(err.Error(), "closed pipe") {
 			t.Fatal("write frame with to closed conn failed", err)
 		}
diff --git a/shaper.go b/shaper.go
index 35773ee..8d52ef7 100644
--- a/shaper.go
+++ b/shaper.go
@@ -6,8 +6,14 @@ func _itimediff(later, earlier uint32) int32 {
 
 type shaperHeap []writeRequest
 
-func (h shaperHeap) Len() int            { return len(h) }
-func (h shaperHeap) Less(i, j int) bool  { return _itimediff(h[j].seq, h[i].seq) > 0 }
+func (h shaperHeap) Len() int { return len(h) }
+func (h shaperHeap) Less(i, j int) bool {
+	if h[i].class != h[j].class {
+		return h[i].class < h[j].class
+	}
+	return _itimediff(h[j].seq, h[i].seq) > 0
+}
+
 func (h shaperHeap) Swap(i, j int)       { h[i], h[j] = h[j], h[i] }
 func (h *shaperHeap) Push(x interface{}) { *h = append(*h, x.(writeRequest)) }
 
diff --git a/shaper_test.go b/shaper_test.go
index 95d0079..b02e317 100644
--- a/shaper_test.go
+++ b/shaper_test.go
@@ -6,11 +6,11 @@ import (
 )
 
 func TestShaper(t *testing.T) {
-	w1 := writeRequest{prio: 10}
-	w2 := writeRequest{prio: 10}
-	w3 := writeRequest{prio: 20}
-	w4 := writeRequest{prio: 100}
-	w5 := writeRequest{prio: (1 << 32) - 1}
+	w1 := writeRequest{seq: 1}
+	w2 := writeRequest{seq: 2}
+	w3 := writeRequest{seq: 3}
+	w4 := writeRequest{seq: 4}
+	w5 := writeRequest{seq: 5}
 
 	var reqs shaperHeap
 	heap.Push(&reqs, w5)
@@ -19,25 +19,20 @@ func TestShaper(t *testing.T) {
 	heap.Push(&reqs, w2)
 	heap.Push(&reqs, w1)
 
-	var lastPrio = reqs[0].prio
 	for len(reqs) > 0 {
 		w := heap.Pop(&reqs).(writeRequest)
-		if int32(w.prio-lastPrio) < 0 {
-			t.Fatal("incorrect shaper priority")
-		}
-
-		t.Log("prio:", w.prio)
-		lastPrio = w.prio
+		t.Log("sid:", w.frame.sid, "seq:", w.seq)
 	}
 }
 
 func TestShaper2(t *testing.T) {
-	w1 := writeRequest{prio: 10, seq: 1} // stream 0
-	w2 := writeRequest{prio: 10, seq: 2}
-	w3 := writeRequest{prio: 20, seq: 3}
-	w4 := writeRequest{prio: 100, seq: 4}
-	w5 := writeRequest{prio: (1 << 32) - 1, seq: 5}
-	w6 := writeRequest{prio: 10, seq: 1, frame: Frame{sid: 10}} // stream 1
+	w1 := writeRequest{class: CLSDATA, seq: 1} // stream 0
+	w2 := writeRequest{class: CLSDATA, seq: 2}
+	w3 := writeRequest{class: CLSDATA, seq: 3}
+	w4 := writeRequest{class: CLSDATA, seq: 4}
+	w5 := writeRequest{class: CLSDATA, seq: 5}
+	w6 := writeRequest{class: CLSCTRL, seq: 6, frame: Frame{sid: 10}} // ctrl 1
+	w7 := writeRequest{class: CLSCTRL, seq: 7, frame: Frame{sid: 11}} // ctrl 2
 
 	var reqs shaperHeap
 	heap.Push(&reqs, w6)
@@ -46,9 +41,10 @@ func TestShaper2(t *testing.T) {
 	heap.Push(&reqs, w3)
 	heap.Push(&reqs, w2)
 	heap.Push(&reqs, w1)
+	heap.Push(&reqs, w7)
 
 	for len(reqs) > 0 {
 		w := heap.Pop(&reqs).(writeRequest)
-		t.Log("prio:", w.prio, "sid:", w.frame.sid, "seq:", w.seq)
+		t.Log("sid:", w.frame.sid, "seq:", w.seq)
 	}
 }
diff --git a/stream.go b/stream.go
index 94e858e..0d7e045 100644
--- a/stream.go
+++ b/stream.go
@@ -255,7 +255,7 @@ func (s *Stream) sendWindowUpdate(consumed uint32) error {
 	binary.LittleEndian.PutUint32(hdr[:], consumed)
 	binary.LittleEndian.PutUint32(hdr[4:], uint32(s.sess.config.MaxStreamBuffer))
 	frame.data = hdr[:]
-	_, err := s.sess.writeFrameInternal(frame, deadline, 0)
+	_, err := s.sess.writeFrameInternal(frame, deadline, CLSDATA)
 	return err
 }
 
@@ -325,7 +325,7 @@ func (s *Stream) Write(b []byte) (n int, err error) {
 		}
 		frame.data = bts[:sz]
 		bts = bts[sz:]
-		n, err := s.sess.writeFrameInternal(frame, deadline, 0)
+		n, err := s.sess.writeFrameInternal(frame, deadline, CLSDATA)
 		s.numWritten++
 		sent += n
 		if err != nil {
@@ -393,7 +393,7 @@ func (s *Stream) writeV2(b []byte) (n int, err error) {
 				}
 				frame.data = bts[:sz]
 				bts = bts[sz:]
-				n, err := s.sess.writeFrameInternal(frame, deadline, 0)
+				n, err := s.sess.writeFrameInternal(frame, deadline, CLSDATA)
 				atomic.AddUint32(&s.numWritten, uint32(sz))
 				sent += n
 				if err != nil {