/** Starts this <tt>PacketTransformer</tt>. */
  private synchronized void start() {
    if (this.datagramTransport != null) {
      if (this.connectThread == null && dtlsTransport == null) {
        logger.warn(
            getClass().getName()
                + " has been started but has failed to establish"
                + " the DTLS connection!");
      }
      return;
    }

    if (rtcpmux && Component.RTCP == componentID) {
      // In the case of rtcp-mux, the RTCP transformer does not create
      // a DTLS session. The SRTP context (_srtpTransformer) will be
      // initialized on demand using initializeSRTCPTransformerFromRtp().
      return;
    }

    AbstractRTPConnector connector = this.connector;

    if (connector == null) throw new NullPointerException("connector");

    DtlsControl.Setup setup = this.setup;
    SecureRandom secureRandom = DtlsControlImpl.createSecureRandom();
    final DTLSProtocol dtlsProtocolObj;
    final TlsPeer tlsPeer;

    if (DtlsControl.Setup.ACTIVE.equals(setup)) {
      dtlsProtocolObj = new DTLSClientProtocol(secureRandom);
      tlsPeer = new TlsClientImpl(this);
    } else {
      dtlsProtocolObj = new DTLSServerProtocol(secureRandom);
      tlsPeer = new TlsServerImpl(this);
    }
    tlsPeerHasRaisedCloseNotifyWarning = false;

    final DatagramTransportImpl datagramTransport = new DatagramTransportImpl(componentID);

    datagramTransport.setConnector(connector);

    Thread connectThread =
        new Thread() {
          @Override
          public void run() {
            try {
              runInConnectThread(dtlsProtocolObj, tlsPeer, datagramTransport);
            } finally {
              if (Thread.currentThread().equals(DtlsPacketTransformer.this.connectThread)) {
                DtlsPacketTransformer.this.connectThread = null;
              }
            }
          }
        };

    connectThread.setDaemon(true);
    connectThread.setName(DtlsPacketTransformer.class.getName() + ".connectThread");

    this.connectThread = connectThread;
    this.datagramTransport = datagramTransport;

    boolean started = false;

    try {
      connectThread.start();
      started = true;
    } finally {
      if (!started) {
        if (connectThread.equals(this.connectThread)) this.connectThread = null;
        if (datagramTransport.equals(this.datagramTransport)) this.datagramTransport = null;
      }
    }

    notifyAll();
  }
  /** {@inheritDoc} */
  @Override
  public RawPacket reverseTransform(RawPacket pkt) {
    byte[] buf = pkt.getBuffer();
    int off = pkt.getOffset();
    int len = pkt.getLength();

    if (isDtlsRecord(buf, off, len)) {
      if (rtcpmux && Component.RTCP == componentID) {
        // This should never happen.
        logger.warn(
            "Dropping a DTLS record, because it was received on the"
                + " RTCP channel while rtcpmux is in use.");
        return null;
      }

      boolean receive;

      synchronized (this) {
        if (datagramTransport == null) {
          receive = false;
        } else {
          datagramTransport.queueReceive(buf, off, len);
          receive = true;
        }
      }
      if (receive) {
        DTLSTransport dtlsTransport = this.dtlsTransport;

        if (dtlsTransport == null) {
          // The specified pkt looks like a DTLS record and it has
          // been consumed for the purposes of the secure channel
          // represented by this PacketTransformer.
          pkt = null;
        } else {
          try {
            int receiveLimit = dtlsTransport.getReceiveLimit();
            int delta = receiveLimit - len;

            if (delta > 0) {
              pkt.grow(delta);
              buf = pkt.getBuffer();
              off = pkt.getOffset();
              len = pkt.getLength();
            } else if (delta < 0) {
              pkt.shrink(-delta);
              buf = pkt.getBuffer();
              off = pkt.getOffset();
              len = pkt.getLength();
            }

            int received = dtlsTransport.receive(buf, off, len, DTLS_TRANSPORT_RECEIVE_WAITMILLIS);

            if (received <= 0) {
              // No application data was decoded.
              pkt = null;
            } else {
              delta = len - received;
              if (delta > 0) pkt.shrink(delta);
            }
          } catch (IOException ioe) {
            pkt = null;
            // SrtpControl.start(MediaType) starts its associated
            // TransformEngine. We will use that mediaType to signal
            // the normal stop then as well i.e. we will ignore
            // exception after the procedure to stop this
            // PacketTransformer has begun.
            if (mediaType != null && !tlsPeerHasRaisedCloseNotifyWarning) {
              logger.error("Failed to decode a DTLS record!", ioe);
            }
          }
        }
      } else {
        // The specified pkt looks like a DTLS record but it is
        // unexpected in the current state of the secure channel
        // represented by this PacketTransformer. This PacketTransformer
        // has not been started (successfully) or has been closed.
        pkt = null;
      }
    } else if (transformEngine.isSrtpDisabled()) {
      // In pure DTLS mode only DTLS records pass through.
      pkt = null;
    } else {
      // DTLS-SRTP has not been initialized yet or has failed to
      // initialize.
      SinglePacketTransformer srtpTransformer = waitInitializeAndGetSRTPTransformer();

      if (srtpTransformer != null) pkt = srtpTransformer.reverseTransform(pkt);
      else if (DROP_UNENCRYPTED_PKTS) pkt = null;
      // XXX Else, it is our explicit policy to let the received packet
      // pass through and rely on the SrtpListener to notify the user that
      // the session is not secured.
    }
    return pkt;
  }