rtc.io transportless signalling for WebRTC


The rtc-signaller module provides a transportless signalling mechanism for WebRTC.

The signaller provides set of client-side tools that assist with the setting up an PeerConnection and helping them communicate. All that is required for the signaller to operate is a suitable messenger. A messenger is simply a function that is able to create a pull-stream Source and/or Sink. From version 5.0.0 the rtc-signaller package will use pull-streams to ensure robust delivery of messages.

By using this approach, we can conduct signalling over any number of mechanisms:

While the signaller is capable of communicating by a number of different messengers (i.e. anything that can send and receive messages over a wire) it comes with support for understanding how to connect to an rtc-switchboard out of the box.

The following code sample demonstrates how:

// create a new signaller, connecting to the target switchboard 
var messenger = require('rtc-switchboard-messenger');
var signaller = require('rtc-signaller')(messenger('//switchboard.rtc.io/'));
// when a new peer is announced, log it 
signaller.on('peer:announce', function(data) {
 console.log('new peer found in room: ', data);
// for our sanity, pop a message once we are connected 
signaller.once('connected', function() {
  console.log('we have successfully connected');
// send through an announce message 
// this will occur once the websocket has been opened and active 
signaller.announce({ room: 'signaller-getting-started' });

There are a number of events that are generating throughout the lifecycle of a signaller.

The following events are generated by the signaller in response to updates n it's own state:

  • connected

    A connection has been established via the underlying messenger to a signalling server (or equivalent).

  • disconnected

    The connection has been lost (possibly temporarily) with the signalling server (or transport). It is possible that the connection will be re-established so this does not necessarily mean the end.

  • local:announce

    This event is trigged when an /announce message is sent via the messenging channel. The event includes a single data argument which contains the object data that has been sent.

The following events relate to information that has been relayed to this signaller about other peers:

  • peer:filter

    The peer:filter event is triggered prior to the peer:announce or peer:update events being fired and provides an application the opportunity to reject a peer. The handler for this event is passed the id of the peer that has connected to the room and a JS data object for the announce data. This data only differs from the peer:announce (or peer:update) data in that an allow attribute is included and controls whether we will acknowledge the presence of this new peer.

    Due to the way event emitters behave in node, the last handler invoked is the authority on whether the peer is accepted or not (so make sure to check the previous state of the allow flag):

    // only accept connections from Bob 
    signaller.on('peer:filter', function(iddata) {
      data.allow = data.allow && (data.name === 'Bob');
  • peer:connected

    If a peer has passed the peer:filter test (either no filtering has been applied, or the allow flag is set to true in the filter events) then a peer:connected event will be emitted:

    signaller.on('peer:connected', function(id) {
      console.log('peer ' + id + ' has connected');

    This event can be useful if you wish to know when a peer has connected to the signalling server, and don't care whether it is a new peer (generating a peer:announce event) or known peer (generating a peer:update event).

  • peer:announce

    While the peer:connected event is triggered each time a peer reconnects and announces to the signalling server, a peer:announce event is only emitted by your local signaller if this is considered a new connection from a peer.

    If you are writing a WebRTC application, then this event is the best place to start creating RTCPeerConnection objects between the local machine and your remote, announced counterpart. You will then be able to couple those connections together using the signaller.

    signaller.on('peer:announce', function(data) {
      console.log('discovered new peer: ' + data.id, data);
      // TODO: create a peer connection with our new friend :) 
  • peer:update

    An existing peer in the system has been "re-announced" possibly with some data changes:

    signaller.on('peer:update', function(data) {
      console.log('data update from peer: ' + data.id, data);

Displayed below are some diagrams how the signalling flow between peers behaves. In each of the diagrams we illustrate three peers (A, B and C) participating discovery and coordinating RTCPeerConnection handshakes.

In each case, only the interaction between the clients is represented not how a signalling server (such as rtc-switchboard) would pass on broadcast messages, etc. This is done for two reasons:

  1. It is out of scope of this documentation.
  2. The rtc-signaller has been designed to work without having to rely on any intelligence in the server side signalling component. In the instance that a signaller broadcasts all messages to all connected peers then rtc-signaller should be smart enough to make sure everything works as expected.

This diagram illustrates the process of how peer A announces itself to peers B and C, and in turn they announce themselves.

Each of the diagrams has been generated using mscgen and the source for these documents can be found in the docs/ folder of this repository.

The rtc-signaller module is designed to be used primarily in a functional way and when called it creates a new signaller that will enable you to communicate with other peers via your messaging network.

// create a signaller from something that knows how to send messages 
var signaller = require('rtc-signaller')(messenger);

As demonstrated in the getting started guide, you can also pass through a string value instead of a messenger instance if you simply want to connect to an existing rtc-switchboard instance.

Manually connect the signaller using the supplied messenger.

NOTE: This should never have to be called if the default setting for autoconnect is used.

The announce function of the signaller will pass an /announce message through the messenger network. When no additional data is supplied to this function then only the id of the signaller is sent to all active members of the messenging network.

To join a room using an announce call you simply provide the name of the room you wish to join as part of the data block that you annouce, for example:

signaller.announce({ room: 'testroom' });

Signalling servers (such as rtc-switchboard) will then place your peer connection into a room with other peers that have also announced in this room.

Once you have joined a room, the server will only deliver messages that you send to other peers within that room.

There may be instances where you wish to send additional data as part of your announce message in your application. For instance, maybe you want to send an alias or nick as part of your announce message rather than just use the signaller's generated id.

If for instance you were writing a simple chat application you could join the webrtc room and tell everyone your name with the following announce call:

  room: 'webrtc',
  nick: 'Damon'

The signaller is written to distinguish between initial peer announcements and peer data updates (see the docs on the announce handler below). As such it is ok to provide any data updates using the announce method also.

For instance, I could send a status update as an announce message to flag that I am going offline:

signaller.announce({ status: 'offline' });

Tell the signalling server we are leaving. Calling this function is usually not required though as the signalling server should issue correct /leave messages when it detects a disconnect event.

Copyright 2013 - 2014 National ICT Australia Limited (NICTA)

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