ApplicationMessage applicationData = (ApplicationMessage) record.getFragment();
          raw = new RawData(applicationData.getData());
          break;
        case ALERT:
          AlertMessage alert = (AlertMessage) record.getFragment();
          switch (alert.getDescription()) {
          case CLOSE_NOTIFY:
            session.setActive(false);
            
            LOGGER.fine("Received CLOSE_NOTIFY from " + peerAddress.toString());
            
            // server must reply with CLOSE_NOTIFY
            if (!session.isClient()) {
              DTLSMessage closeNotify = new AlertMessage(AlertLevel.WARNING, AlertDescription.CLOSE_NOTIFY);
              flight = new DTLSFlight();
              flight.addMessage(new Record(ContentType.ALERT, session.getWriteEpoch(), session.getSequenceNumber(), closeNotify, session));
              flight.setRetransmissionNeeded(false);
            }
            
            if (dtlsSessions.remove(addressToKey(peerAddress))!=null) {
              LOGGER.info("Closed session with peer: " + peerAddress.toString());
            } else {
              LOGGER.warning("Session to close not found: " + peerAddress.toString());
            }
            
            break;
          
          // remote implementation might use any alert (e.g., against padding oracle attack)
          default:
            LOGGER.warning(alert.getDescription() + " with " + peerAddress.toString());
            // cleaning up
            cancelPreviousFlight(peerAddress);
            dtlsSessions.remove(addressToKey(peerAddress));
            handshakers.remove(addressToKey(peerAddress));
            break;
            
            //TODO somehow tell CoAP endpoint to cancel
          }
          break;
        case CHANGE_CIPHER_SPEC:
        case HANDSHAKE:
          LOGGER.finest(" => handshaker: "+handshaker);
          if (handshaker == null) {
            /*
             * A handshake message received, but no handshaker
             * available: this must mean that we either received
             * a HelloRequest (from server) or a ClientHello
             * (from client) => initialize appropriate
             * handshaker type
             */
            HandshakeMessage handshake = (HandshakeMessage) record.getFragment();
            switch (handshake.getMessageType()) {
            case HELLO_REQUEST:
              /*
               * Client side: server desires a re-handshake
               */
              if (session == null) {
                // create new session
                session = new DTLSSession(peerAddress, true);
                // store session according to peer address
                dtlsSessions.put(addressToKey(peerAddress), session);
                LOGGER.info("Created new session as client with peer: " + peerAddress.toString());
              }
              handshaker = new ClientHandshaker(peerAddress, null, session);
              handshakers.put(addressToKey(peerAddress), handshaker);
              LOGGER.finest("Stored re-handshaker: " + handshaker.toString() + " for " + peerAddress.toString());
              break;
            case CLIENT_HELLO:
              /*
               * Server side: server received a client hello:
               * check first if client wants to resume a
               * session (message must contain session
               * identifier) and then check if particular
               * session still available, otherwise conduct
               * full handshake with fresh session.
               */
              if (!(handshake instanceof FragmentedHandshakeMessage)) {
                // check if session identifier set
                ClientHello clientHello = (ClientHello) handshake;
                session = getSessionByIdentifier(clientHello.getSessionId().getSessionId());
              }
              
              if (session == null) {
                // create new session
                session = new DTLSSession(peerAddress, false);
                // store session according to peer address
                dtlsSessions.put(addressToKey(peerAddress), session);
                LOGGER.info("Created new session as server with peer: " + peerAddress.toString());
                handshaker = new ServerHandshaker(peerAddress, session);
              } else {
                handshaker = new ResumingServerHandshaker(peerAddress, session);
              }
              handshakers.put(addressToKey(peerAddress), handshaker);
              LOGGER.finest("Stored handshaker: " + handshaker.toString() + " for " + peerAddress.toString());
              break;
            default:
              LOGGER.severe("Received unexpected first handshake message (type="+handshake.getMessageType()+") from " + peerAddress.toString() + ":\n" + handshake.toString());
              break;
            }
          }
          flight = handshaker.processMessage(record);
          break;
        default:
          LOGGER.severe("Received unknown DTLS record from " + peerAddress.toString() + ":\n" + ByteArrayUtils.toHexString(data));
          break;
        }
        if (flight != null) {
          cancelPreviousFlight(peerAddress);
          flight.setPeerAddress(peerAddress);
          flight.setSession(session);
          if (flight.isRetransmissionNeeded()) {
            flights.put(addressToKey(peerAddress), flight);
            scheduleRetransmission(flight);
          }
          sendFlight(flight);
        }
        if (raw != null) {
          raw.setAddress(packet.getAddress());
          raw.setPort(packet.getPort());
          return raw;
        }
      }
    } catch (Exception e) {
      /*
       * If it is a known handshake failure, send the specific Alert,
       * otherwise the general Handshake_Failure Alert. 
       */
      DTLSFlight flight = new DTLSFlight();
      flight.setRetransmissionNeeded(false);
      flight.setPeerAddress(peerAddress);
      flight.setSession(session);
      
      AlertMessage alert;
      if (e instanceof HandshakeException) {
        alert = ((HandshakeException) e).getAlert();
        LOGGER.severe("Handshake Exception (" + peerAddress.toString() + "): " + e.getMessage());
      } else {
        alert = new AlertMessage(AlertLevel.FATAL, AlertDescription.HANDSHAKE_FAILURE);
        LOGGER.log(Level.SEVERE, "Unknown Exception (" + peerAddress + ").", e);
      }
      LOGGER.log(Level.SEVERE, "Datagram which lead to exception (" + peerAddress + "): " + ByteArrayUtils.toHexString(data), e);