Discover the new easier way to develop Kurento video applications

JavaScript - Loopback stats

This web application extends the Hello World tutorial showing how statistics are collected.

Running this example

First of all, install Kurento Media Server: Installation Guide. Start the media server and leave it running in the background.


If you will run this tutorial from a remote machine (i.e. not from localhost), then you need to configure Secure WebSocket (wss://) in Kurento Media Server. For instructions, check Securing Kurento Media Server.

This is not an issue if you will run both KMS and the tutorial demo locally, because browsers (at least Chrome at the time of this writing) allow connecting to insecure WebSockets from HTTPS pages, as long as everything happens in localhost.

Install Node.js, Bower, and a web server in your system:

curl -sL | sudo -E bash -
sudo apt-get install -y nodejs
sudo npm install -g bower
sudo npm install -g http-server

Here, we suggest using the simple Node.js http-server, but you could use any other web server.


You need to configure the web server with HTTPS. For more information, check Configure JavaScript applications to use HTTPS.

You also need the source code of this demo; clone it from GitHub, then start the web server:

git clone
cd kurento-tutorial-js/kurento-loopback-stats
git checkout 6.12.0
bower install
http-server -p 8443 --ssl --cert keys/server.crt --key keys/server.key

Finally, access the web application by using a WebRTC-capable browser (Firefox, Chrome) to open the appropriate URL:

  • If KMS is running in your local machine:

  • If KMS is running in a remote server:


Understanding this example

The logic of the application is quite simple: the local stream is sent to the Kurento Media Server, which returns it back to the client without modifications. To implement this behavior we need to create a Media Pipeline composed by the Media Element WebRtcEndpoint, which holds the capability of exchanging full-duplex (bidirectional) WebRTC media flows. This media element is connected to itself so any received media (from browser) is send back (to browser). Using method getStats the application shows all stats of element WebRtcEndpoint.

The complete source code of this demo can be found in GitHub.

JavaScript Logic

This demo follows a Single Page Application architecture (SPA). The interface is the following HTML page: index.html. This web page links two Kurento JavaScript libraries:

  • kurento-client.js : Implementation of the Kurento JavaScript Client.
  • kurento-utils.js : Kurento utility library aimed to simplify the WebRTC management in the browser.

In addition, these two JavaScript libraries are also required:

  • Bootstrap : Web framework for developing responsive web sites.
  • jquery.js : Cross-platform JavaScript library designed to simplify the client-side scripting of HTML.
  • adapter.js : WebRTC JavaScript utility library maintained by Google that abstracts away browser differences.
  • ekko-lightbox : Module for Bootstrap to open modal images, videos, and galleries.
  • demo-console : Custom JavaScript console.

The specific logic of this demo is coded in the following JavaScript page: index.js. In this file, there is a function which is called when the green button labeled as Start in the GUI is clicked.

var startButton = document.getElementById("start");

startButton.addEventListener("click", function() {
   var options = {
     localVideo: videoInput,
     remoteVideo: videoOutput

   webRtcPeer = kurentoUtils.WebRtcPeer.WebRtcPeerSendrecv(options, function(error) {
      if(error) return onError(error)


The function WebRtcPeer.WebRtcPeerSendrecv hides internal details (i.e. PeerConnection and getUserStream) and makes possible to start a full-duplex WebRTC communication, using the HTML video tag with id videoInput to show the video camera (local stream) and the video tag videoOutput to show the remote stream provided by the Kurento Media Server.

Inside this function, a call to generateOffer is performed. This function accepts a callback in which the SDP offer is received. In this callback we create an instance of the KurentoClient class that will manage communications with the Kurento Media Server. So, we need to provide the URI of its WebSocket endpoint. In this example, we assume it’s listening in port 8433 at the same host than the HTTP serving the application.


var args = getopts(,
    ws_uri: 'wss://' + location.hostname + ':8433/kurento',
    ice_servers: undefined


kurentoClient(args.ws_uri, function(error, client){

Once we have an instance of kurentoClient, the following step is to create a Media Pipeline, as follows:

client.create("MediaPipeline", function(error, _pipeline){

If everything works correctly, we have an instance of a media pipeline (variable pipeline in this example). With this instance, we are able to create Media Elements. In this example we just need a WebRtcEndpoint. Then, this media elements is connected itself:

pipeline.create("WebRtcEndpoint", function(error, webRtc) {
   if (error) return onError(error);

   webRtcEndpoint = webRtc;

   setIceCandidateCallbacks(webRtcPeer, webRtc, onError)

   webRtc.processOffer(sdpOffer, function(error, sdpAnswer) {
     if (error) return onError(error);


     webRtcPeer.processAnswer(sdpAnswer, onError);

   webRtc.connect(webRtc, function(error) {
     if (error) return onError(error);

     console.log("Loopback established");

     webRtcEndpoint.on('MediaStateChanged', function(event) {
       if (event.newState == "CONNECTED") {
         console.log("MediaState is CONNECTED ... printing stats...")

In the following snippet, we can see getStats method. This method returns several statistic values of WebRtcEndpoint.

function getBrowserOutgoingVideoStats(webRtcPeer, callback) {
  var peerConnection = webRtcPeer.peerConnection;

  peerConnection.getStats(function(stats) {
    var results = stats.result();

    for (var i = 0; i < results.length; i++) {
      var res = results[i];
      if (res.type != 'ssrc') continue;

      //Publish it to be compliant with W3C stats draft
      var retVal = {
        timeStamp: res.timestamp,
        //StreamStats below
        codecId: "--",
        firCount: res.stat('googFirsReceived'),
        isRemote: false,
        mediaTrackId: res.stat('googTrackId'),
        nackCount: res.stat('googNacksReceived'),
        pliCount: res.stat('googPlisReceived'),
        sliCount: 0,
        ssrc: res.stat('ssrc'),
        transportId: res.stat('transportId'),
        //Specific outbound below
        bytesSent: res.stat('bytesSent'),
        packetsSent: res.stat('packetsSent'),
        roundTripTime: res.stat('googRtt'),
        packetsLost: res.stat('packetsLost'),
        targetBitrate: "??",
        remb: "??"
      return callback(null, retVal);
    return callback("Error: could not find ssrc type on track stats", null);
  }, localVideoTrack);


The TURN and STUN servers to be used can be configured simple adding the parameter ice_servers to the application URL, as follows:



Demo dependencies are located in file bower.json. Bower is used to collect them.

"dependencies": {
   "kurento-client": "6.12.0",
   "kurento-utils": "6.12.0"


We are in active development. You can find the latest version of Kurento JavaScript Client at Bower.