WebRTC: Video telephony without a browser plugin
Signaling Server
JavaScript is also used for the signaling server, but server-side on Node.js. On Ubuntu Linux, you can install Node.js (currently version 0.8) and its package manager Npm with the
sudo apt-get install nodejs npm
command. The signaling server also requires the Connect modules (version 2.7.2 or later) as well as version 1.0.8 of WebSocket. Because WebSocket depends on node-gyp
, you first need to install this:
npm install -g node-gyp
Then, you can install websocket
and connect
:
sudo npm install connect websocket
Listing 8 shows the code for the signaling server in Node.js. Lines 2 to 4 integrate the required modules. The channels
variable (line 5) stores the connected clients in a field.
Listing 8
Signaling Server for Node.js
In the next line, the app
variable accepts a connect
application with the static
module added on top. Static tells Node.js to respond to HTTP requests, such as http://localhost:6655/webrtc.html, by returning the webrtc.html
file from the ../
directory.
The WebSocket server binds an httpServer
object in line 9. As an argument in the call to the createServer()
method, the app
variable passes the application logic from the connect
application to the httpServer
object. The listen()
method tells the HTTP server to listen on port 6655.
Connection Events
The callback function in lines 12 to 28 of Listing 8 describes the response of the signaling server when setting up a connection via the WebSocket protocol. To do this, the on()
method in line 12 binds the callback function from the second argument to the instance of the request
event. In line 13, the callback function stores a valid connection in the thisChannel
variable; line 14 adds it to the list of existing connections.
Lines 15 to 22 define a callback function for processing an incoming message. In the loop across all connections (lines 16-21), the sendUTF()
method forwards the message to all other connections. Lines 23 to 27 remove the client from the list on terminating the connection.
As Figure 7 shows, you can watch the signaling server at work: Thanks to console.log(msg.utf8Data)
in line 18, it writes its output to the console.
If you want WebRTC to work over the Internet, you need to send the session descriptions in Listing 5 to the connected browser via the WebSocket protocol and the signaling server, as shown in Figures 1 and 5. Listing 9 shows the HTML document for this example. In contrast to Listing 4, it binds webrtc.js
instead of localwebrtc.js
.
Listing 9
HTML for WebRTC via the Internet
Listing 10 shows the JavaScript code for webrtc.js
: Line 8 opens a connection to the WebSocket server and stores it in the channel
variable. After opening the connection, the code executes the callback function from lines 9 through 15. Within this function, line 10 creates a new peer object and stores it in the global variable pc
. Line 11 uses the addStream()
method to add the media stream, stream
, from the request in line 4 to the peer object.
Listing 10
webrtc.js: WebRTC via the Internet
The next two lines create the callback functions for the onicecandidate
and onaddstream
events in the connection setup. The event handling for onicecandidate
is in line 12, which calls the onice()
function in Listing 11. Like the onadd()
function in Listing 6, it returns a callback function. When called, the send()
function (Listing 13) sends an ICE candidate. Finally, line 14 in Listing 10 uses createOffer()
to generate a session description and passes it to the callback function from the call to desc()
(Listing 12). In doing so, line 3 of Listing 12 stores the session description in the peer object, pc
, and the next line forwards it via the signaling server.
Listing 11
Event Handler
Listing 12
Session Description
Listing 13
Serializing Objects
Receiving and Sending
However, if the browser receives an external message via the WebSocket connection, the callback function in lines 16 to 34 of Listing 10 sees some action. It transforms the incoming JSON message into a JavaScript object that processes the subsequent switch
statement, depending on the message type.
Listing 13 shows the send()
function, which uses a method with the same name in line 2 from the WebSocket object to send a message. Before that happens, stringify()
converts it to a string in JSON format.
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