diff --git a/frames/ack_frame.go b/frames/ack_frame.go
index 61aac9ff3..775628c89 100644
--- a/frames/ack_frame.go
+++ b/frames/ack_frame.go
@@ -18,8 +18,8 @@ type NackRange struct {
 type AckFrame struct {
 	Entropy         byte
 	LargestObserved protocol.PacketNumber
-	DelayTime       uint16 // Todo: properly interpret this value as described in the specification
-	NackRanges      []NackRange
+	DelayTime       uint16      // Todo: properly interpret this value as described in the specification
+	NackRanges      []NackRange // has to be ordered. The NACK range with the highest FirstPacketNumber goes first, the NACK range with the lowest FirstPacketNumber goes last
 }
 
 // Write writes an ACK frame.
@@ -73,6 +73,14 @@ func (f *AckFrame) HasNACK() bool {
 	return false
 }
 
+// GetHighestInOrderPacket gets the highest in order packet number that is confirmed by this ACK
+func (f *AckFrame) GetHighestInOrderPacket() protocol.PacketNumber {
+	if f.HasNACK() {
+		return (f.NackRanges[len(f.NackRanges)-1].FirstPacketNumber - 1)
+	}
+	return f.LargestObserved
+}
+
 // ParseAckFrame reads an ACK frame
 func ParseAckFrame(r *bytes.Reader) (*AckFrame, error) {
 	frame := &AckFrame{}
diff --git a/frames/ack_frame_test.go b/frames/ack_frame_test.go
index 400013ef5..79cbee0be 100644
--- a/frames/ack_frame_test.go
+++ b/frames/ack_frame_test.go
@@ -73,6 +73,33 @@ var _ = Describe("AckFrame", func() {
 			Expect(frame.NackRanges[2].Length).To(Equal(uint8(1)))
 			Expect(b.Len()).To(Equal(0))
 		})
+
+		It("gets the highest in order packet number for an ACK without NACK ranges", func() {
+			frame := AckFrame{LargestObserved: 5}
+			Expect(frame.GetHighestInOrderPacket()).To(Equal(protocol.PacketNumber(5)))
+		})
+
+		It("gets the highest in order packet number for an ACK with one NACK ranges", func() {
+			nackRange := NackRange{FirstPacketNumber: 3, Length: 2}
+			frame := AckFrame{
+				LargestObserved: 6,
+				NackRanges:      []NackRange{nackRange},
+			}
+			Expect(frame.GetHighestInOrderPacket()).To(Equal(protocol.PacketNumber(2)))
+		})
+
+		It("gets the highest in order packet number for an ACK with one NACK ranges", func() {
+			nackRanges := []NackRange{
+				NackRange{FirstPacketNumber: 9, Length: 3},
+				NackRange{FirstPacketNumber: 7, Length: 1},
+				NackRange{FirstPacketNumber: 4, Length: 2},
+			}
+			frame := &AckFrame{
+				LargestObserved: 15,
+				NackRanges:      nackRanges,
+			}
+			Expect(frame.GetHighestInOrderPacket()).To(Equal(protocol.PacketNumber(3)))
+		})
 	})
 
 	Context("when writing", func() {