diff --git a/transport/internet/kcp/receiving.go b/transport/internet/kcp/receiving.go
index a86f2459..c6a8b6b3 100644
--- a/transport/internet/kcp/receiving.go
+++ b/transport/internet/kcp/receiving.go
@@ -48,3 +48,40 @@ func (this *ReceivingWindow) Advance() {
 		this.start = 0
 	}
 }
+
+type ACKList struct {
+	timestamps []uint32
+	numbers    []uint32
+}
+
+func (this *ACKList) Add(number uint32, timestamp uint32) {
+	this.timestamps = append(this.timestamps, timestamp)
+	this.numbers = append(this.numbers, number)
+}
+
+func (this *ACKList) Clear(una uint32) {
+	count := 0
+	for i := 0; i < len(this.numbers); i++ {
+		if this.numbers[i] >= una {
+			if i != count {
+				this.numbers[count] = this.numbers[i]
+				this.timestamps[count] = this.timestamps[i]
+			}
+			count++
+		}
+	}
+	this.numbers = this.numbers[:count]
+	this.timestamps = this.timestamps[:count]
+}
+
+func (this *ACKList) AsSegment() *ACKSegment {
+	count := len(this.numbers)
+	if count > 16 {
+		count = 16
+	}
+	return &ACKSegment{
+		Count:         byte(count),
+		NumberList:    this.numbers[:count],
+		TimestampList: this.timestamps[:count],
+	}
+}
diff --git a/transport/internet/kcp/receiving_test.go b/transport/internet/kcp/receiving_test.go
index bc060ce5..3945b859 100644
--- a/transport/internet/kcp/receiving_test.go
+++ b/transport/internet/kcp/receiving_test.go
@@ -34,3 +34,20 @@ func TestRecivingWindow(t *testing.T) {
 	assert.Pointer(window.Remove(1)).Equals(seg2)
 	assert.Pointer(window.Remove(2)).Equals(seg3)
 }
+
+func TestACKList(t *testing.T) {
+	assert := assert.On(t)
+
+	ackList := new(ACKList)
+	ackList.Add(1, 2)
+	ackList.Add(3, 4)
+
+	seg := ackList.AsSegment()
+	assert.Byte(seg.Count).Equals(2)
+
+	ackList.Add(1, 3)
+	ackList.Clear(2)
+
+	seg = ackList.AsSegment()
+	assert.Byte(seg.Count).Equals(1)
+}