Sessions and streams

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Besides adding friends and sending simple messages, carrier also provides a mechanism of session to help transferring large files, of stream raw data.

Please note that transferring files can be implemented in an easier way than what is described here. Using sessions and streams is the generic way, but for convenience, consider using the FileTransfer class for simple file transfers.

Streams described here are the lower level layer. They can be considered as the physical communication layer. You can also create channels in order to push different kind of information (ex: send 2 files simultaneously) inside the same stream.

Peers handshake procedure

Here is how 2 peers can setup a data flow together:

  • Peer 1 creates a new session locally.
  • Peer 1 adds one or more streams to the session. For instance, for a video streaming, one session may have 3 streams: one for the video, one for the audio, and the last one to send remotre control commands.
  • Peer 1 requests a session to carrier.
  • Peer 2 receives the session request.
  • Peer 2 can then create a new session locally.
  • Peer 2 adds the streams it wants to open. This can be the same as peer 1 (video, audio, etc), or only part of it if it wants to receive only some of them.
  • Peer 2 replies to peer 1’s request.
  • Both parties call startSession().
  • Callbacks are called in handlers and peers can start to send or receive data.

Prepare a manager handler

Session manager uses a handler class to receive new session requests:

ManagerHandler mSessionManagerHandler = new ManagerHandler() {
    public void onSessionRequest(Carrier carrier, String fromUserId, String sdp) {
        // A new session request is received from a friend peer.
        // ... Code to handle this request goes here. See below in this guide.

Initialize the session manager

Session manager is initialized using the session manager handler created earlier.

Manager.initializeInstance(Carrier.getInstance(), mSessionManagerHandler);
Manager manager = Manager.getInstance();

Create a stream handler to receive stream callbacks

All callbacks related to a carrier session need a handler class implementation. It looks like this:

AbstractStreamHandler streamHandler = new AbstractStreamHandler {
    public void onStateChanged(Stream stream, StreamState streamState) {
        // Stream state changes arrive here

    public void onStreamData(Stream stream, byte[] data) {
        // Data received from a peer

    public void onChannelOpened(Stream stream, int channel) {

    public void onChannelClose(Stream stream, int channel, CloseReason reason) {

    public boolean onChannelData(Stream stream, int channel, byte[] data) {
        return false;

Create a session handler to receive session callbacks

SessionRequestCompleteHandler sessionHandler = new SessionRequestCompleteHandler {
    public void onCompletion(Session session, int state, String reason, String sdp) {
        // This is called when peer 1 gets session reply from peer 2

Peer 1: create a session and add streams

A new session is created locally. One or several streams are configured (added) in this session.

// Use a peer address to create a new session
Session session = carrierSessionManager.newSession(peerAddr);

// Add a stream to the session. For example, a video session
Stream stream = session.addStream(StreamType.Application, 0, streamHandler);

// Optionally: create more streams and add them. Some stream can be reliable (similar to tcp - no data lost but uses more bandwidth - good for file transfer). By default, streams are unreliable (similar to udp - good for video streaming) a reliable session (for file transfer or data that can't be lost - similar to TCP) 
Stream mySecondStream = session.addStream(StreamType.Text, Stream.PROPERTY_RELIABLE, streamHandler);

Peer 1: request session creation

After session and streams are created, the whole information has to be sent to carrier to let the friend peer know about this initialization.

But here comes a bit of complication: before sending the actual session request, you need to make sure that all the streams you have added have moved to state Initialized. Choose your preferred way to do so using any code synchzoniration mechanism.

// Request actual session creation to carrier

Peer 2: receive session request

Peer 2 will then be notified that a new session is being created by peer 1 inside its ManagerHandler, onSessionRequest(). At that time, peer 2 can do the same as peer 1 in order to create a local session and add streams into it.

Session session = carrierSessionManager.newSession(peerAddr);
Stream stream = session.addStream(StreamType.Application, 0, streamHandler);

Again, we need to wait until all streams reach the TransportReady state here before sending the following response to peer 1:

session.replyRequest(0, null); // Success
session.start(sdp); // sdp is provided in onSessionRequest()

Peer 1: start session

Peer 1 is currently waiting for peer 2, and will be notified in onCompletion() when peer 2 replied to the session request.

before starting its session, peer 1 needs to wait for 2 conditions: 1. All of its streams must be in state TransportReady (this state is reached after calling session.request()). 1. onCompletion() should be received.

When those conditions are met, the following code can be called:

session.start(sdp); // sdp is provided in onCompletion()

Send data

Now it’s almost ready. Your stream handler will receive onStateChanged() with state Connected. At that time, you can start sending data with the following code:

int sentSize = stream.writeData(data);

Receive data

Peer 2 is now able to receive data from stream handler’s onStreamData().