diff --git a/session.go b/session.go
index a1386fff8..db10e667c 100644
--- a/session.go
+++ b/session.go
@@ -17,6 +17,10 @@ import (
 	"github.com/lucas-clemente/quic-go/utils"
 )
 
+type unpacker interface {
+	Unpack(publicHeaderBinary []byte, hdr *publicHeader, r *bytes.Reader) (*unpackedPacket, error)
+}
+
 type receivedPacket struct {
 	remoteAddr   interface{}
 	publicHeader *publicHeader
@@ -60,7 +64,7 @@ type Session struct {
 	// TODO: remove
 	flowController flowcontrol.FlowController // connection level flow controller
 
-	unpacker *packetUnpacker
+	unpacker unpacker
 	packer   *packetPacker
 
 	cryptoSetup *handshake.CryptoSetup
diff --git a/session_test.go b/session_test.go
index 169f4a9e1..523305ae9 100644
--- a/session_test.go
+++ b/session_test.go
@@ -34,6 +34,15 @@ func (m *mockConnection) write(p []byte) error {
 func (*mockConnection) setCurrentRemoteAddr(addr interface{}) {}
 func (*mockConnection) IP() net.IP                            { return nil }
 
+type mockUnpacker struct{}
+
+func (m *mockUnpacker) Unpack(publicHeaderBinary []byte, hdr *publicHeader, r *bytes.Reader) (*unpackedPacket, error) {
+	return &unpackedPacket{
+		entropyBit: false,
+		frames:     nil,
+	}, nil
+}
+
 var _ = Describe("Session", func() {
 	var (
 		session              *Session
@@ -392,6 +401,33 @@ var _ = Describe("Session", func() {
 		})
 	})
 
+	Context("receiving packets", func() {
+		var hdr *publicHeader
+
+		BeforeEach(func() {
+			session.unpacker = &mockUnpacker{}
+			hdr = &publicHeader{PacketNumberLen: protocol.PacketNumberLen6}
+		})
+
+		It("sets the lastRcvdPacketNumber", func() {
+			hdr.PacketNumber = 5
+			err := session.handlePacketImpl(nil, hdr, nil)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(session.lastRcvdPacketNumber).To(Equal(protocol.PacketNumber(5)))
+		})
+
+		It("sets the lastRcvdPacketNumber, for an out-of-order packet", func() {
+			hdr.PacketNumber = 5
+			err := session.handlePacketImpl(nil, hdr, nil)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(session.lastRcvdPacketNumber).To(Equal(protocol.PacketNumber(5)))
+			hdr.PacketNumber = 3
+			err = session.handlePacketImpl(nil, hdr, nil)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(session.lastRcvdPacketNumber).To(Equal(protocol.PacketNumber(3)))
+		})
+	})
+
 	Context("sending packets", func() {
 		It("sends ack frames", func() {
 			packetNumber := protocol.PacketNumber(0x0135)