When a website or app using RTCPeerConnection receives a new ICE candidate from the remote peer over its signaling channel, it delivers the newly-received candidate to the browser's ICE agent by calling RTCPeerConnection.addIceCandidate(). This adds this new remote candidate to the RTCPeerConnection's remote description, which describes the state of the remote end of the connection.

If the candidate parameter is missing or a value of null is given when calling addIceCandidate(), the added ICE candidate is an "end-of-candidates" indicator. The same is the case if the value of the specified object's candidate is either missing or an empty string (""), it signals that all remote candidates have been delivered.

The end-of-candidates notification is transmitted to the remote peer using a candidate with an a-line value of end-of-candidates.

During negotiation, your app will likely receive many candidates which you'll deliver to the ICE agent in this way, allowing it to build up a list of potential connection methods. This is covered in more detail in the articles WebRTC connectivity and Signaling and video calling.

candidate Optional

An RTCIceCandidate object, or an object that has the following properties:

candidate Optional

A string describing the properties of the candidate, taken directly from the SDP attribute "candidate". The candidate string specifies the network connectivity information for the candidate. If the candidate is an empty string (""), the end of the candidate list has been reached; this candidate is known as the "end-of-candidates" marker.

The syntax of the candidate string is described in RFC 5245, section 15.1. For an a-line (attribute line) that looks like this:

a=candidate:4234997325 1 udp 2043278322 192.0.2.172 44323 typ host

the corresponding candidate string's value will be "candidate:4234997325 1 udp 2043278322 192.0.2.172 44323 typ host".

The user agent always prefers candidates with the highest priority, all else being equal. In the example above, the priority is 2043278322. The attributes are all separated by a single space character, and are in a specific order. The complete list of attributes for this example candidate is:

• foundation = 4234997325
• component = "rtp" (the number 1 is encoded to this string; 2 becomes "rtcp")
• protocol = "udp"
• priority = 2043278322
• ip = "192.0.2.172"
• port = 44323
• type = "host"

Additional information can be found in RTCIceCandidate.candidate.

Note: For backward compatibility with older versions of the WebRTC specification, the constructor also accepts this string directly as an argument.

sdpMid Optional

A string containing the identification tag of the media stream with which the candidate is associated, or null if there is no associated media stream. The default is null.

Additional information can be found in RTCIceCandidate.sdpMid.

sdpMLineIndex Optional

A number property containing the zero-based index of the m-line with which the candidate is associated, within the SDP of the media description, or null if no such associated exists. The default is null.

Additional information can be found in RTCIceCandidate.sdpMLineIndex.

usernameFragment Optional

A string containing the username fragment (usually referred to in shorthand as "ufrag" or "ice-ufrag"). This fragment, along with the ICE password ("ice-pwd"), uniquely identifies a single ongoing ICE interaction (including for any communication with the STUN server).

The string is generated by WebRTC at the beginning of the session. It may be up to 256 characters long, and at least 24 bits must contain random data. It has no default value and is not present unless set explicitly.

Additional information can be found in RTCIceCandidate.usernameFragment.

The method will throw a TypeError exception if both sdpMid and sdpMLineIndex are null.

The contents of the object should be constructed from a message received over the signaling channel, describing a newly received ICE candidate that's ready to be delivered to the local ICE agent.

If no candidate object is specified, or its value is null, an end-of-candidates signal is sent to the remote peer using the end-of-candidates a-line, formatted like this:

a=end-of-candidates

In older code and documentation, you may see a callback-based version of this function. This has been deprecated and its use is strongly discouraged. You should update any existing code to use the Promise-based version of addIceCandidate() instead. The parameters for the older form of addIceCandidate() are described below, to aid in updating existing code.

successCallback Deprecated

A function to be called when the ICE candidate has been successfully added. This function receives no input parameters and doesn't return a value.

failureCallback Deprecated

A function to be called if attempting to add the ICE candidate fails. Receives as input a DOMException object describing why failure occurred.

A Promise that is fulfilled when the candidate has been successfully added to the remote peer's description by the ICE agent. The promise does not receive any input parameters.

When an error occurs while attempting to add the ICE candidate, the Promise returned by this method is rejected, returning one of the errors below as the name attribute in the specified DOMException object passed to the rejection handler.

TypeError

Returned if the specified candidate's sdpMid and sdpMLineIndex are both null.

InvalidStateError DOMException

Returned if the RTCPeerConnection currently has no remote peer established (remoteDescription is null).

OperationError DOMException

Returned in one of the following situations:

• The value specified for sdpMid is non-null and doesn't match the media description ID of any media description included within the remoteDescription.
• The specified value of sdpMLineIndex is greater than or equal to the number of media descriptions included in the remote description.
• The specified ufrag doesn't match the ufrag field in any of the remote descriptions being considered.
• One or more of the values in the candidate string are invalid or could not be parsed.
• Attempting to add the candidate fails for any reason.

This code snippet shows how to signal ICE candidates across an arbitrary signaling channel.

// This example assumes that the other peer is using a signaling channel as follows:
//
// pc.onicecandidate = (event) => {
//   if (event.candidate) {
//     signalingChannel.send(JSON.stringify({ice: event.candidate})); // "ice" is arbitrary
//   } else {
//     // All ICE candidates have been sent
//   }
// }

signalingChannel.onmessage = (receivedString) => {
  const message = JSON.parse(receivedString);
  if (message.ice) {
    // A typical value of ice here might look something like this:
    //
    // {candidate: "candidate:0 1 UDP 2122154243 192.0.2.43 53421 typ host", sdpMid: "0", …}
    //
    // Pass the whole thing to addIceCandidate:

    pc.addIceCandidate(message.ice).catch((e) => {
      console.log(`Failure during addIceCandidate(): ${e.name}`);
    });
  } else {
    // handle other things you might be signaling, like sdp
  }
};

The last candidate to be signaled this way by the remote peer will be a special candidate denoting end-of-candidates. Out of interest, end-of-candidates may be manually indicated as follows:

pc.addIceCandidate({ candidate: "" });

However, in most cases you won't need to look for this explicitly, since the events driving the RTCPeerConnection will deal with it for you, sending the appropriate events.

• WebRTC API
• Signaling and video calling
• Introduction to WebRTC protocols
• WebRTC connectivity
• Lifetime of a WebRTC session