limit number of tracked skipped packet numbers in SentPacketHandler

ackhandler/sent_packet_handler.go

@@ -127,6 +127,10 @@ func (h *sentPacketHandler) SentPacket(packet *ackhandlerlegacy.Packet) error {
 
 	for p := h.lastSentPacketNumber + 1; p < packet.PacketNumber; p++ {
 		h.skippedPackets = append(h.skippedPackets, p)
+
+		if len(h.skippedPackets) > protocol.MaxTrackedSkippedPackets {
+			h.skippedPackets = h.skippedPackets[1:]
+		}
 	}
 
 	now := time.Now()

ackhandler/sent_packet_handler_test.go

@@ -175,6 +175,17 @@ var _ = Describe("SentPacketHandler", func() {
 				Expect(handler.skippedPackets).To(HaveLen(2))
 				Expect(handler.skippedPackets).To(Equal([]protocol.PacketNumber{2, 3}))
 			})
+
+			It("limits the lengths of the skipped packet slice", func() {
+				for i := 0; i < protocol.MaxTrackedSkippedPackets+5; i++ {
+					packet := ackhandlerlegacy.Packet{PacketNumber: protocol.PacketNumber(2*i + 1), Frames: []frames.Frame{&streamFrame}, Length: 1}
+					err := handler.SentPacket(&packet)
+					Expect(err).ToNot(HaveOccurred())
+				}
+				Expect(handler.skippedPackets).To(HaveLen(protocol.MaxUndecryptablePackets))
+				Expect(handler.skippedPackets[0]).To(Equal(protocol.PacketNumber(10)))
+				Expect(handler.skippedPackets[protocol.MaxTrackedSkippedPackets-1]).To(Equal(protocol.PacketNumber(10 + 2*(protocol.MaxTrackedSkippedPackets-1))))
+			})
 		})
 	})
 

protocol/server_parameters.go

@@ -47,6 +47,9 @@ const RetransmissionThreshold uint8 = 3
 // SkipPacketAveragePeriodLength is the average period length in which one packet number is skipped to prevent an Optimistic ACK attack
 const SkipPacketAveragePeriodLength PacketNumber = 500
 
+// MaxTrackedSkippedPackets is the maximum number of skipped packet numbers the SentPacketHandler keep track of for Optimistic ACK attack mitigation
+const MaxTrackedSkippedPackets int = 10
+
 // STKExpiryTimeSec is the valid time of a source address token in seconds
 const STKExpiryTimeSec = 24 * 60 * 60