diff --git a/connection.go b/connection.go
index 1920bb2a..d4e44be6 100644
--- a/connection.go
+++ b/connection.go
@@ -1832,7 +1832,7 @@ func (s *connection) sendPackets(now time.Time) error {
 		if err != nil {
 			return err
 		}
-		ecn := s.sentPacketHandler.ECNMode()
+		ecn := s.sentPacketHandler.ECNMode(true)
 		s.logShortHeaderPacket(p.DestConnID, p.Ack, p.Frames, p.StreamFrames, p.PacketNumber, p.PacketNumberLen, p.KeyPhase, ecn, buf.Len(), false)
 		s.registerPackedShortHeaderPacket(p, ecn, now)
 		s.sendQueue.Send(buf, 0, ecn)
@@ -1855,7 +1855,7 @@ func (s *connection) sendPackets(now time.Time) error {
 			return err
 		}
 		s.sentFirstPacket = true
-		if err := s.sendPackedCoalescedPacket(packet, s.sentPacketHandler.ECNMode(), now); err != nil {
+		if err := s.sendPackedCoalescedPacket(packet, s.sentPacketHandler.ECNMode(packet.IsOnlyShortHeaderPacket()), now); err != nil {
 			return err
 		}
 		sendMode := s.sentPacketHandler.SendMode(now)
@@ -1876,8 +1876,8 @@ func (s *connection) sendPackets(now time.Time) error {
 func (s *connection) sendPacketsWithoutGSO(now time.Time) error {
 	for {
 		buf := getPacketBuffer()
-		ecn := s.sentPacketHandler.ECNMode()
-		if _, err := s.appendOnePacket(buf, s.mtuDiscoverer.CurrentSize(), ecn, now); err != nil {
+		ecn := s.sentPacketHandler.ECNMode(true)
+		if _, err := s.appendOneShortHeaderPacket(buf, s.mtuDiscoverer.CurrentSize(), ecn, now); err != nil {
 			if err == errNothingToPack {
 				buf.Release()
 				return nil
@@ -1912,8 +1912,8 @@ func (s *connection) sendPacketsWithGSO(now time.Time) error {
 
 	for {
 		var dontSendMore bool
-		ecn := s.sentPacketHandler.ECNMode()
-		size, err := s.appendOnePacket(buf, maxSize, ecn, now)
+		ecn := s.sentPacketHandler.ECNMode(true)
+		size, err := s.appendOneShortHeaderPacket(buf, maxSize, ecn, now)
 		if err != nil {
 			if err != errNothingToPack {
 				return err
@@ -1971,8 +1971,8 @@ func (s *connection) resetPacingDeadline() {
 }
 
 func (s *connection) maybeSendAckOnlyPacket(now time.Time) error {
-	ecn := s.sentPacketHandler.ECNMode()
 	if !s.handshakeConfirmed {
+		ecn := s.sentPacketHandler.ECNMode(false)
 		packet, err := s.packer.PackCoalescedPacket(true, s.mtuDiscoverer.CurrentSize(), s.version)
 		if err != nil {
 			return err
@@ -1983,6 +1983,7 @@ func (s *connection) maybeSendAckOnlyPacket(now time.Time) error {
 		return s.sendPackedCoalescedPacket(packet, ecn, time.Now())
 	}
 
+	ecn := s.sentPacketHandler.ECNMode(true)
 	p, buf, err := s.packer.PackAckOnlyPacket(s.mtuDiscoverer.CurrentSize(), s.version)
 	if err != nil {
 		if err == errNothingToPack {
@@ -2024,12 +2025,12 @@ func (s *connection) sendProbePacket(encLevel protocol.EncryptionLevel, now time
 	if packet == nil || (len(packet.longHdrPackets) == 0 && packet.shortHdrPacket == nil) {
 		return fmt.Errorf("connection BUG: couldn't pack %s probe packet", encLevel)
 	}
-	return s.sendPackedCoalescedPacket(packet, s.sentPacketHandler.ECNMode(), now)
+	return s.sendPackedCoalescedPacket(packet, s.sentPacketHandler.ECNMode(packet.IsOnlyShortHeaderPacket()), now)
 }
 
-// appendOnePacket appends a new packet to the given packetBuffer.
+// appendOneShortHeaderPacket appends a new packet to the given packetBuffer.
 // If there was nothing to pack, the returned size is 0.
-func (s *connection) appendOnePacket(buf *packetBuffer, maxSize protocol.ByteCount, ecn protocol.ECN, now time.Time) (protocol.ByteCount, error) {
+func (s *connection) appendOneShortHeaderPacket(buf *packetBuffer, maxSize protocol.ByteCount, ecn protocol.ECN, now time.Time) (protocol.ByteCount, error) {
 	startLen := buf.Len()
 	p, err := s.packer.AppendPacket(buf, maxSize, s.version)
 	if err != nil {
@@ -2106,7 +2107,7 @@ func (s *connection) sendConnectionClose(e error) ([]byte, error) {
 	if err != nil {
 		return nil, err
 	}
-	ecn := s.sentPacketHandler.ECNMode()
+	ecn := s.sentPacketHandler.ECNMode(packet.IsOnlyShortHeaderPacket())
 	s.logCoalescedPacket(packet, ecn)
 	return packet.buffer.Data, s.conn.Write(packet.buffer.Data, 0, ecn)
 }
diff --git a/connection_test.go b/connection_test.go
index 828f4ced..c4e067af 100644
--- a/connection_test.go
+++ b/connection_test.go
@@ -613,7 +613,7 @@ var _ = Describe("Connection", func() {
 			conn.sendQueue = newSendQueue(sconn)
 			sph := mockackhandler.NewMockSentPacketHandler(mockCtrl)
 			sph.EXPECT().GetLossDetectionTimeout().Return(time.Now().Add(time.Hour)).AnyTimes()
-			sph.EXPECT().ECNMode().Return(protocol.ECT1).AnyTimes()
+			sph.EXPECT().ECNMode(true).Return(protocol.ECT1).AnyTimes()
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).AnyTimes()
 			// only expect a single SentPacket() call
 			sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any())
@@ -1206,7 +1206,7 @@ var _ = Describe("Connection", func() {
 
 		AfterEach(func() {
 			streamManager.EXPECT().CloseWithError(gomock.Any())
-			sph.EXPECT().ECNMode().Return(protocol.ECNCE).MaxTimes(1)
+			sph.EXPECT().ECNMode(gomock.Any()).Return(protocol.ECNCE).MaxTimes(1)
 			packer.EXPECT().PackApplicationClose(gomock.Any(), gomock.Any(), conn.version).Return(&coalescedPacket{buffer: getPacketBuffer()}, nil)
 			expectReplaceWithClosed()
 			cryptoSetup.EXPECT().Close()
@@ -1234,7 +1234,7 @@ var _ = Describe("Connection", func() {
 			sph.EXPECT().TimeUntilSend().AnyTimes()
 			sph.EXPECT().GetLossDetectionTimeout().AnyTimes()
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).AnyTimes()
-			sph.EXPECT().ECNMode().Return(protocol.ECNNon).AnyTimes()
+			sph.EXPECT().ECNMode(true).Return(protocol.ECNNon).AnyTimes()
 			sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any())
 			runConn()
 			p := shortHeaderPacket{
@@ -1262,7 +1262,7 @@ var _ = Describe("Connection", func() {
 			conn.handshakeConfirmed = true
 			sph.EXPECT().GetLossDetectionTimeout().AnyTimes()
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).AnyTimes()
-			sph.EXPECT().ECNMode().AnyTimes()
+			sph.EXPECT().ECNMode(true).AnyTimes()
 			runConn()
 			packer.EXPECT().AppendPacket(gomock.Any(), gomock.Any(), conn.version).Return(shortHeaderPacket{}, errNothingToPack).AnyTimes()
 			conn.receivedPacketHandler.ReceivedPacket(0x035e, protocol.ECNNon, protocol.Encryption1RTT, time.Now(), true)
@@ -1274,7 +1274,7 @@ var _ = Describe("Connection", func() {
 			sph.EXPECT().TimeUntilSend().AnyTimes()
 			sph.EXPECT().GetLossDetectionTimeout().AnyTimes()
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAck)
-			sph.EXPECT().ECNMode().Return(protocol.ECT1).AnyTimes()
+			sph.EXPECT().ECNMode(gomock.Any()).Return(protocol.ECT1).AnyTimes()
 			done := make(chan struct{})
 			packer.EXPECT().PackCoalescedPacket(true, gomock.Any(), conn.version).Do(func(bool, protocol.ByteCount, protocol.VersionNumber) { close(done) })
 			runConn()
@@ -1287,7 +1287,7 @@ var _ = Describe("Connection", func() {
 			sph.EXPECT().TimeUntilSend().AnyTimes()
 			sph.EXPECT().GetLossDetectionTimeout().AnyTimes()
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).AnyTimes()
-			sph.EXPECT().ECNMode().AnyTimes()
+			sph.EXPECT().ECNMode(gomock.Any()).AnyTimes()
 			sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any())
 			fc := mocks.NewMockConnectionFlowController(mockCtrl)
 			fc.EXPECT().IsNewlyBlocked().Return(true, protocol.ByteCount(1337))
@@ -1341,7 +1341,7 @@ var _ = Describe("Connection", func() {
 					sph.EXPECT().SendMode(gomock.Any()).Return(sendMode)
 					sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendNone)
 					sph.EXPECT().QueueProbePacket(encLevel)
-					sph.EXPECT().ECNMode()
+					sph.EXPECT().ECNMode(gomock.Any())
 					p := getCoalescedPacket(123, enc != protocol.Encryption1RTT)
 					packer.EXPECT().MaybePackProbePacket(encLevel, gomock.Any(), conn.version).Return(p, nil)
 					sph.EXPECT().SentPacket(gomock.Any(), protocol.PacketNumber(123), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any())
@@ -1363,7 +1363,7 @@ var _ = Describe("Connection", func() {
 					sph.EXPECT().TimeUntilSend().AnyTimes()
 					sph.EXPECT().SendMode(gomock.Any()).Return(sendMode)
 					sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendNone)
-					sph.EXPECT().ECNMode()
+					sph.EXPECT().ECNMode(gomock.Any())
 					sph.EXPECT().QueueProbePacket(encLevel).Return(false)
 					p := getCoalescedPacket(123, enc != protocol.Encryption1RTT)
 					packer.EXPECT().MaybePackProbePacket(encLevel, gomock.Any(), conn.version).Return(p, nil)
@@ -1407,7 +1407,7 @@ var _ = Describe("Connection", func() {
 
 		AfterEach(func() {
 			// make the go routine return
-			sph.EXPECT().ECNMode().MaxTimes(1)
+			sph.EXPECT().ECNMode(gomock.Any()).MaxTimes(1)
 			packer.EXPECT().PackApplicationClose(gomock.Any(), gomock.Any(), conn.version).Return(&coalescedPacket{buffer: getPacketBuffer()}, nil)
 			expectReplaceWithClosed()
 			cryptoSetup.EXPECT().Close()
@@ -1422,7 +1422,7 @@ var _ = Describe("Connection", func() {
 		It("sends multiple packets one by one immediately", func() {
 			sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any()).Times(2)
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).Times(2)
-			sph.EXPECT().ECNMode().Times(2)
+			sph.EXPECT().ECNMode(gomock.Any()).Times(2)
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendPacingLimited)
 			sph.EXPECT().TimeUntilSend().Return(time.Now().Add(time.Hour))
 			expectAppendPacket(packer, shortHeaderPacket{PacketNumber: 10}, []byte("packet10"))
@@ -1447,7 +1447,7 @@ var _ = Describe("Connection", func() {
 		It("sends multiple packets one by one immediately, with GSO", func() {
 			enableGSO()
 			sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any()).Times(2)
-			sph.EXPECT().ECNMode().Times(3)
+			sph.EXPECT().ECNMode(true).Times(3)
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).Times(3)
 			payload1 := make([]byte, conn.mtuDiscoverer.CurrentSize())
 			rand.Read(payload1)
@@ -1474,7 +1474,7 @@ var _ = Describe("Connection", func() {
 			enableGSO()
 			sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any()).Times(2)
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).Times(2)
-			sph.EXPECT().ECNMode().Times(2)
+			sph.EXPECT().ECNMode(true).Times(2)
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendNone)
 			payload1 := make([]byte, conn.mtuDiscoverer.CurrentSize())
 			rand.Read(payload1)
@@ -1499,7 +1499,7 @@ var _ = Describe("Connection", func() {
 		It("sends multiple packets, when the pacer allows immediate sending", func() {
 			sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any())
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).Times(2)
-			sph.EXPECT().ECNMode().Times(2)
+			sph.EXPECT().ECNMode(gomock.Any()).Times(2)
 			expectAppendPacket(packer, shortHeaderPacket{PacketNumber: 10}, []byte("packet10"))
 			packer.EXPECT().AppendPacket(gomock.Any(), gomock.Any(), conn.version).Return(shortHeaderPacket{}, errNothingToPack)
 			sender.EXPECT().WouldBlock().AnyTimes()
@@ -1518,7 +1518,7 @@ var _ = Describe("Connection", func() {
 			sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any())
 			sph.EXPECT().TimeUntilSend().Return(time.Now().Add(time.Hour))
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendPacingLimited)
-			sph.EXPECT().ECNMode()
+			sph.EXPECT().ECNMode(gomock.Any())
 			packer.EXPECT().PackAckOnlyPacket(gomock.Any(), conn.version).Return(shortHeaderPacket{PacketNumber: 123}, getPacketBuffer(), nil)
 
 			sender.EXPECT().WouldBlock().AnyTimes()
@@ -1539,7 +1539,7 @@ var _ = Describe("Connection", func() {
 			sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any())
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny)
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAck)
-			sph.EXPECT().ECNMode().Times(2)
+			sph.EXPECT().ECNMode(gomock.Any()).Times(2)
 			expectAppendPacket(packer, shortHeaderPacket{PacketNumber: 100}, []byte("packet100"))
 			sender.EXPECT().WouldBlock().AnyTimes()
 			sender.EXPECT().Send(gomock.Any(), gomock.Any(), gomock.Any())
@@ -1557,13 +1557,13 @@ var _ = Describe("Connection", func() {
 			pacingDelay := scaleDuration(100 * time.Millisecond)
 			gomock.InOrder(
 				sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny),
-				sph.EXPECT().ECNMode(),
+				sph.EXPECT().ECNMode(gomock.Any()),
 				expectAppendPacket(packer, shortHeaderPacket{PacketNumber: 100}, []byte("packet100")),
 				sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any()),
 				sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendPacingLimited),
 				sph.EXPECT().TimeUntilSend().Return(time.Now().Add(pacingDelay)),
 				sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny),
-				sph.EXPECT().ECNMode(),
+				sph.EXPECT().ECNMode(gomock.Any()),
 				expectAppendPacket(packer, shortHeaderPacket{PacketNumber: 101}, []byte("packet101")),
 				sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any()),
 				sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendPacingLimited),
@@ -1587,7 +1587,7 @@ var _ = Describe("Connection", func() {
 		It("sends multiple packets at once", func() {
 			sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any()).Times(3)
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).Times(3)
-			sph.EXPECT().ECNMode().Times(3)
+			sph.EXPECT().ECNMode(gomock.Any()).Times(3)
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendPacingLimited)
 			sph.EXPECT().TimeUntilSend().Return(time.Now().Add(time.Hour))
 			for pn := protocol.PacketNumber(1000); pn < 1003; pn++ {
@@ -1628,7 +1628,7 @@ var _ = Describe("Connection", func() {
 				sender.EXPECT().WouldBlock().AnyTimes()
 				sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any())
 				sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).AnyTimes()
-				sph.EXPECT().ECNMode().AnyTimes()
+				sph.EXPECT().ECNMode(gomock.Any()).AnyTimes()
 				expectAppendPacket(packer, shortHeaderPacket{PacketNumber: 1000}, []byte("packet1000"))
 				packer.EXPECT().AppendPacket(gomock.Any(), gomock.Any(), conn.version).Return(shortHeaderPacket{}, errNothingToPack)
 				sender.EXPECT().Send(gomock.Any(), gomock.Any(), gomock.Any()).DoAndReturn(func(*packetBuffer, uint16, protocol.ECN) { close(written) })
@@ -1653,7 +1653,7 @@ var _ = Describe("Connection", func() {
 				conn.handlePacket(receivedPacket{buffer: getPacketBuffer()})
 			})
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).AnyTimes()
-			sph.EXPECT().ECNMode().AnyTimes()
+			sph.EXPECT().ECNMode(gomock.Any()).AnyTimes()
 			expectAppendPacket(packer, shortHeaderPacket{PacketNumber: 10}, []byte("packet10"))
 			packer.EXPECT().AppendPacket(gomock.Any(), gomock.Any(), conn.version).Return(shortHeaderPacket{}, errNothingToPack)
 			sender.EXPECT().Send(gomock.Any(), gomock.Any(), gomock.Any()).DoAndReturn(func(*packetBuffer, uint16, protocol.ECN) { close(written) })
@@ -1667,7 +1667,7 @@ var _ = Describe("Connection", func() {
 		It("stops sending when the send queue is full", func() {
 			sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any())
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny)
-			sph.EXPECT().ECNMode()
+			sph.EXPECT().ECNMode(gomock.Any())
 			expectAppendPacket(packer, shortHeaderPacket{PacketNumber: 1000}, []byte("packet1000"))
 			written := make(chan struct{}, 1)
 			sender.EXPECT().WouldBlock()
@@ -1688,7 +1688,7 @@ var _ = Describe("Connection", func() {
 			// now make room in the send queue
 			sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any())
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).AnyTimes()
-			sph.EXPECT().ECNMode().AnyTimes()
+			sph.EXPECT().ECNMode(gomock.Any()).AnyTimes()
 			sender.EXPECT().WouldBlock().AnyTimes()
 			expectAppendPacket(packer, shortHeaderPacket{PacketNumber: 1001}, []byte("packet1001"))
 			packer.EXPECT().AppendPacket(gomock.Any(), gomock.Any(), conn.version).Return(shortHeaderPacket{}, errNothingToPack)
@@ -1703,7 +1703,7 @@ var _ = Describe("Connection", func() {
 
 		It("doesn't set a pacing timer when there is no data to send", func() {
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).AnyTimes()
-			sph.EXPECT().ECNMode().AnyTimes()
+			sph.EXPECT().ECNMode(gomock.Any()).AnyTimes()
 			sender.EXPECT().WouldBlock().AnyTimes()
 			packer.EXPECT().AppendPacket(gomock.Any(), gomock.Any(), conn.version).Return(shortHeaderPacket{}, errNothingToPack)
 			// don't EXPECT any calls to mconn.Write()
@@ -1723,7 +1723,7 @@ var _ = Describe("Connection", func() {
 			conn.config.DisablePathMTUDiscovery = false
 			sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any())
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny)
-			sph.EXPECT().ECNMode()
+			sph.EXPECT().ECNMode(true)
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendNone)
 			written := make(chan struct{}, 1)
 			sender.EXPECT().WouldBlock().AnyTimes()
@@ -1774,7 +1774,7 @@ var _ = Describe("Connection", func() {
 			sph.EXPECT().GetLossDetectionTimeout().AnyTimes()
 			sph.EXPECT().TimeUntilSend().AnyTimes()
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).AnyTimes()
-			sph.EXPECT().ECNMode().AnyTimes()
+			sph.EXPECT().ECNMode(gomock.Any()).AnyTimes()
 
 			sph.EXPECT().SentPacket(gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any())
 			conn.sentPacketHandler = sph
@@ -1803,7 +1803,7 @@ var _ = Describe("Connection", func() {
 			sph := mockackhandler.NewMockSentPacketHandler(mockCtrl)
 			sph.EXPECT().GetLossDetectionTimeout().AnyTimes()
 			sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).AnyTimes()
-			sph.EXPECT().ECNMode().AnyTimes()
+			sph.EXPECT().ECNMode(gomock.Any()).AnyTimes()
 			sph.EXPECT().SentPacket(gomock.Any(), protocol.PacketNumber(1234), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any(), gomock.Any())
 			conn.sentPacketHandler = sph
 			rph := mockackhandler.NewMockReceivedPacketHandler(mockCtrl)
@@ -1855,7 +1855,7 @@ var _ = Describe("Connection", func() {
 
 		sph.EXPECT().GetLossDetectionTimeout().AnyTimes()
 		sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).AnyTimes()
-		sph.EXPECT().ECNMode().Return(protocol.ECT1).AnyTimes()
+		sph.EXPECT().ECNMode(false).Return(protocol.ECT1).AnyTimes()
 		sph.EXPECT().TimeUntilSend().Return(time.Now()).AnyTimes()
 		gomock.InOrder(
 			sph.EXPECT().SentPacket(gomock.Any(), protocol.PacketNumber(13), gomock.Any(), gomock.Any(), gomock.Any(), protocol.EncryptionInitial, protocol.ECT1, protocol.ByteCount(123), gomock.Any()),
@@ -1968,7 +1968,7 @@ var _ = Describe("Connection", func() {
 	It("sends a HANDSHAKE_DONE frame when the handshake completes", func() {
 		sph := mockackhandler.NewMockSentPacketHandler(mockCtrl)
 		sph.EXPECT().SendMode(gomock.Any()).Return(ackhandler.SendAny).AnyTimes()
-		sph.EXPECT().ECNMode().AnyTimes()
+		sph.EXPECT().ECNMode(gomock.Any()).AnyTimes()
 		sph.EXPECT().GetLossDetectionTimeout().AnyTimes()
 		sph.EXPECT().TimeUntilSend().AnyTimes()
 		sph.EXPECT().SetHandshakeConfirmed()
diff --git a/internal/ackhandler/interfaces.go b/internal/ackhandler/interfaces.go
index ba224228..ba8cbbda 100644
--- a/internal/ackhandler/interfaces.go
+++ b/internal/ackhandler/interfaces.go
@@ -29,8 +29,7 @@ type SentPacketHandler interface {
 	// only to be called once the handshake is complete
 	QueueProbePacket(protocol.EncryptionLevel) bool /* was a packet queued */
 
-	ECNMode() protocol.ECN
-
+	ECNMode(isShortHeaderPacket bool) protocol.ECN // isShortHeaderPacket should only be true for non-coalesced 1-RTT packets
 	PeekPacketNumber(protocol.EncryptionLevel) (protocol.PacketNumber, protocol.PacketNumberLen)
 	PopPacketNumber(protocol.EncryptionLevel) protocol.PacketNumber
 
diff --git a/internal/ackhandler/sent_packet_handler.go b/internal/ackhandler/sent_packet_handler.go
index da5d0718..53aba0dd 100644
--- a/internal/ackhandler/sent_packet_handler.go
+++ b/internal/ackhandler/sent_packet_handler.go
@@ -717,10 +717,13 @@ func (h *sentPacketHandler) GetLossDetectionTimeout() time.Time {
 	return h.alarm
 }
 
-func (h *sentPacketHandler) ECNMode() protocol.ECN {
+func (h *sentPacketHandler) ECNMode(isShortHeaderPacket bool) protocol.ECN {
 	if !h.enableECN {
 		return protocol.ECNUnsupported
 	}
+	if !isShortHeaderPacket {
+		return protocol.ECNNon
+	}
 	// TODO: implement ECN logic
 	return protocol.ECNNon
 }
diff --git a/internal/mocks/ackhandler/sent_packet_handler.go b/internal/mocks/ackhandler/sent_packet_handler.go
index 3479d9e6..137ac2ef 100644
--- a/internal/mocks/ackhandler/sent_packet_handler.go
+++ b/internal/mocks/ackhandler/sent_packet_handler.go
@@ -50,17 +50,17 @@ func (mr *MockSentPacketHandlerMockRecorder) DropPackets(arg0 interface{}) *gomo
 }
 
 // ECNMode mocks base method.
-func (m *MockSentPacketHandler) ECNMode() protocol.ECN {
+func (m *MockSentPacketHandler) ECNMode(arg0 bool) protocol.ECN {
 	m.ctrl.T.Helper()
-	ret := m.ctrl.Call(m, "ECNMode")
+	ret := m.ctrl.Call(m, "ECNMode", arg0)
 	ret0, _ := ret[0].(protocol.ECN)
 	return ret0
 }
 
 // ECNMode indicates an expected call of ECNMode.
-func (mr *MockSentPacketHandlerMockRecorder) ECNMode() *gomock.Call {
+func (mr *MockSentPacketHandlerMockRecorder) ECNMode(arg0 interface{}) *gomock.Call {
 	mr.mock.ctrl.T.Helper()
-	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "ECNMode", reflect.TypeOf((*MockSentPacketHandler)(nil).ECNMode))
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "ECNMode", reflect.TypeOf((*MockSentPacketHandler)(nil).ECNMode), arg0)
 }
 
 // GetLossDetectionTimeout mocks base method.
diff --git a/packet_packer.go b/packet_packer.go
index 0483f35b..64081c68 100644
--- a/packet_packer.go
+++ b/packet_packer.go
@@ -71,6 +71,11 @@ type coalescedPacket struct {
 	shortHdrPacket *shortHeaderPacket
 }
 
+// IsOnlyShortHeaderPacket says if this packet only contains a short header packet (and no long header packets).
+func (p *coalescedPacket) IsOnlyShortHeaderPacket() bool {
+	return len(p.longHdrPackets) == 0 && p.shortHdrPacket != nil
+}
+
 func (p *longHeaderPacket) EncryptionLevel() protocol.EncryptionLevel {
 	//nolint:exhaustive // Will never be called for Retry packets (and they don't have encrypted data).
 	switch p.header.Type {
diff --git a/packet_packer_test.go b/packet_packer_test.go
index 6dba31f9..e68906f3 100644
--- a/packet_packer_test.go
+++ b/packet_packer_test.go
@@ -655,6 +655,7 @@ var _ = Describe("Packet packer", func() {
 				Expect(err).ToNot(HaveOccurred())
 				Expect(packet).ToNot(BeNil())
 				Expect(packet.longHdrPackets).To(HaveLen(1))
+				Expect(packet.IsOnlyShortHeaderPacket()).To(BeFalse())
 				// cut off the tag that the mock sealer added
 				// packet.buffer.Data = packet.buffer.Data[:packet.buffer.Len()-protocol.ByteCount(sealer.Overhead())]
 				hdr, _, _, err := wire.ParsePacket(packet.buffer.Data)
@@ -874,6 +875,7 @@ var _ = Describe("Packet packer", func() {
 				p, err := packer.PackCoalescedPacket(false, maxPacketSize, protocol.Version1)
 				Expect(err).ToNot(HaveOccurred())
 				Expect(p).ToNot(BeNil())
+				Expect(p.IsOnlyShortHeaderPacket()).To(BeFalse())
 				parsePacket(p.buffer.Data)
 			})
 
@@ -1047,6 +1049,7 @@ var _ = Describe("Packet packer", func() {
 				packer.retransmissionQueue.addAppData(&wire.PingFrame{})
 				p, err := packer.PackCoalescedPacket(false, maxPacketSize, protocol.Version1)
 				Expect(err).ToNot(HaveOccurred())
+				Expect(p.IsOnlyShortHeaderPacket()).To(BeFalse())
 				Expect(p.buffer.Len()).To(BeEquivalentTo(maxPacketSize))
 				Expect(p.longHdrPackets).To(HaveLen(1))
 				Expect(p.longHdrPackets[0].EncryptionLevel()).To(Equal(protocol.EncryptionInitial))
@@ -1422,6 +1425,7 @@ var _ = Describe("Packet packer", func() {
 				p, err := packer.MaybePackProbePacket(protocol.Encryption1RTT, maxPacketSize, protocol.Version1)
 				Expect(err).ToNot(HaveOccurred())
 				Expect(p).ToNot(BeNil())
+				Expect(p.IsOnlyShortHeaderPacket()).To(BeTrue())
 				Expect(p.longHdrPackets).To(BeEmpty())
 				Expect(p.shortHdrPacket).ToNot(BeNil())
 				packet := p.shortHdrPacket
@@ -1448,6 +1452,7 @@ var _ = Describe("Packet packer", func() {
 				p, err := packer.MaybePackProbePacket(protocol.Encryption1RTT, maxPacketSize, protocol.Version1)
 				Expect(err).ToNot(HaveOccurred())
 				Expect(p).ToNot(BeNil())
+				Expect(p.IsOnlyShortHeaderPacket()).To(BeTrue())
 				Expect(p.longHdrPackets).To(BeEmpty())
 				Expect(p.shortHdrPacket).ToNot(BeNil())
 				packet := p.shortHdrPacket