only accept a single Retry packet for a connection

client.go

@@ -24,6 +24,8 @@ type client struct {
 	conn     connection
 	hostname string
 
+	receivedRetry bool
+
 	versionNegotiated                bool // has the server accepted our version
 	receivedVersionNegotiationPacket bool
 	negotiatedVersions               []protocol.VersionNumber // the list of versions from the version negotiation packet
@@ -370,7 +372,14 @@ func (c *client) handleIETFQUICPacket(hdr *wire.Header, packetData []byte, remot
 		return fmt.Errorf("received a packet with an unexpected connection ID (%s, expected %s)", hdr.DestConnectionID, c.srcConnID)
 	}
 	if hdr.IsLongHeader {
-		if hdr.Type != protocol.PacketTypeRetry && hdr.Type != protocol.PacketTypeHandshake {
+		switch hdr.Type {
+		case protocol.PacketTypeRetry:
+			if c.receivedRetry {
+				return nil
+			}
+			c.receivedRetry = true
+		case protocol.PacketTypeHandshake:
+		default:
 			return fmt.Errorf("Received unsupported packet type: %s", hdr.Type)
 		}
 		if protocol.ByteCount(len(packetData)) < hdr.PayloadLen {

client_test.go

@@ -724,6 +724,23 @@ var _ = Describe("Client", func() {
 			Eventually(done).Should(BeClosed())
 		})
 
+		It("only accepts one Retry packet", func() {
+			sess := NewMockPacketHandler(mockCtrl)
+			cl.session = sess
+			h := &wire.Header{
+				IsLongHeader:     true,
+				Type:             protocol.PacketTypeRetry,
+				SrcConnectionID:  connID,
+				DestConnectionID: connID,
+			}
+			// only EXPECT a single call to handlePacket()
+			sess.EXPECT().handlePacket(gomock.Any())
+			err := cl.handleIETFQUICPacket(h, nil, &net.UDPAddr{}, time.Now())
+			Expect(err).ToNot(HaveOccurred())
+			err = cl.handleIETFQUICPacket(h, nil, &net.UDPAddr{}, time.Now())
+			Expect(err).ToNot(HaveOccurred())
+		})
+
 		It("closes the session when encountering an error while reading from the connection", func() {
 			testErr := errors.New("test error")
 			sess := NewMockPacketHandler(mockCtrl)