ackhandler: immediately acknowledge ECN-CE marked packets (#4147)

* ackhandler: immediately acknowledge ECN-CE marked packets

* shorter debug statements

internal/ackhandler/received_packet_tracker.go

@@ -58,11 +58,9 @@ func (h *receivedPacketTracker) ReceivedPacket(pn protocol.PacketNumber, ecn pro
 
 	if ackEliciting {
 		h.hasNewAck = true
+		h.maybeQueueACK(pn, rcvTime, ecn, isMissing)
 	}
-	if ackEliciting {
+	//nolint:exhaustive // Only need to count ECT(0), ECT(1) and ECN-CE.
-		h.maybeQueueACK(pn, rcvTime, isMissing)
-	}
-	//nolint:exhaustive // Only need to count ECT(0), ECT(1) and ECNCE.
 	switch ecn {
 	case protocol.ECT0:
 		h.ect0++
@@ -104,7 +102,7 @@ func (h *receivedPacketTracker) hasNewMissingPackets() bool {
 }
 
 // maybeQueueACK queues an ACK, if necessary.
-func (h *receivedPacketTracker) maybeQueueACK(pn protocol.PacketNumber, rcvTime time.Time, wasMissing bool) {
+func (h *receivedPacketTracker) maybeQueueACK(pn protocol.PacketNumber, rcvTime time.Time, ecn protocol.ECN, wasMissing bool) {
 	// always acknowledge the first packet
 	if h.lastAck == nil {
 		if !h.ackQueued {
@@ -143,12 +141,18 @@ func (h *receivedPacketTracker) maybeQueueACK(pn protocol.PacketNumber, rcvTime
 		h.ackAlarm = rcvTime.Add(h.maxAckDelay)
 	}
 
-	// Queue an ACK if there are new missing packets to report.
+	// queue an ACK if there are new missing packets to report
 	if h.hasNewMissingPackets() {
 		h.logger.Debugf("\tQueuing ACK because there's a new missing packet to report.")
 		h.ackQueued = true
 	}
 
+	// queue an ACK if the packet was ECN-CE marked
+	if ecn == protocol.ECNCE {
+		h.logger.Debugf("\tQueuing ACK because the packet was ECN-CE marked.")
+		h.ackQueued = true
+	}
+
 	if h.ackQueued {
 		// cancel the ack alarm
 		h.ackAlarm = time.Time{}

internal/ackhandler/received_packet_tracker_test.go

@@ -50,6 +50,7 @@ var _ = Describe("Received Packet Tracker", func() {
 
 	Context("ACKs", func() {
 		Context("queueing ACKs", func() {
+			// receives and gets ACKs for packet numbers 1 to 10 (including)
 			receiveAndAck10Packets := func() {
 				for i := 1; i <= 10; i++ {
 					Expect(tracker.ReceivedPacket(protocol.PacketNumber(i), protocol.ECNNon, time.Time{}, true)).To(Succeed())
@@ -126,6 +127,16 @@ var _ = Describe("Received Packet Tracker", func() {
 				Expect(tracker.GetAlarmTimeout()).To(Equal(rcvTime.Add(protocol.MaxAckDelay)))
 			})
 
+			It("queues an ACK if the packet was ECN-CE marked", func() {
+				receiveAndAck10Packets()
+				Expect(tracker.ReceivedPacket(11, protocol.ECNCE, time.Now(), true)).To(Succeed())
+				ack := tracker.GetAckFrame(true)
+				Expect(ack).ToNot(BeNil())
+				Expect(ack.AckRanges).To(HaveLen(1))
+				Expect(ack.AckRanges[0].Largest).To(Equal(protocol.PacketNumber(11)))
+				Expect(ack.ECNCE).To(BeEquivalentTo(1))
+			})
+
 			It("queues an ACK if it was reported missing before", func() {
 				receiveAndAck10Packets()
 				Expect(tracker.ReceivedPacket(11, protocol.ECNNon, time.Now(), true)).To(Succeed())