ignore repeated ACKs in legacy SentPacketHandler

ref #218

ackhandlerlegacy/sent_packet_handler.go

@@ -184,6 +184,11 @@ func (h *sentPacketHandler) ReceivedAck(ackFrame *frames.AckFrameLegacy, withPac
 
 	h.largestReceivedPacketWithAck = withPacketNumber
 
+	// ignore repeated ACK (ACKs that don't have a higher LargestObserved than the last ACK)
+	if ackFrame.LargestObserved <= h.highestInOrderAckedPacketNumber {
+		return nil
+	}
+
 	expectedEntropy, err := h.calculateExpectedEntropy(ackFrame)
 	if err != nil {
 		return err

ackhandlerlegacy/sent_packet_handler_test.go

@@ -375,6 +375,19 @@ var _ = Describe("SentPacketHandler", func() {
 				Expect(handler.BytesInFlight()).To(Equal(protocol.ByteCount(3)))
 			})
 
+			It("ignores repeated ACKs", func() {
+				ack := frames.AckFrameLegacy{
+					LargestObserved: 3,
+				}
+				err := handler.ReceivedAck(&ack, 1337)
+				Expect(err).ToNot(HaveOccurred())
+				Expect(handler.BytesInFlight()).To(Equal(protocol.ByteCount(3)))
+				err = handler.ReceivedAck(&ack, 1337+1)
+				Expect(err).ToNot(HaveOccurred())
+				Expect(handler.LargestObserved).To(Equal(protocol.PacketNumber(3)))
+				Expect(handler.BytesInFlight()).To(Equal(protocol.ByteCount(3)))
+			})
+
 			It("rejects ACKs with a too high LargestObserved packet number", func() {
 				ack := frames.AckFrameLegacy{
 					LargestObserved: packets[len(packets)-1].PacketNumber + 1337,