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RTCPeerConnection.setLocalDescription()

This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the specification changes.

The RTCPeerConnection.setLocalDescription() method changes the local description associated with the connection. This description specifies the properties of the local end of the connection, including the media format. The method takes a single parameter—the session description—and it returns a Promise which is fulfilled once the description has been changed, asynchronously.

If setLocalDescription() is called while a connection is already in place, it means renegotiation is underway (possibly to adapt to changing network conditions). Because descriptions will be exchanged until the two peers agree on a configuration, the description submitted by calling setLocalDescription() does not immediately take effect. Instead, the current connection configuration remains in place until negotiation is complete. Only then does the agreed-upon configuration take effect.

Syntax

aPromise = RTCPeerConnection.setLocalDescription(sessionDescription);

pc.setLocalDescription(sessionDescription, successCallback, errorCallback);  

Parameters

sessionDescription
An RTCSessionDescriptionInit or RTCSessionDescription which specifies the configuration to be applied to the local end of the connection.

About the session description parameter

The sessionDescription parameter is technically of type RTCSessionDescriptionInit, but because RTCSessionDescription serializes to be indistinguishable from RTCSessionDescriptionInit, you can also pass in an RTCSessionDescription. This lets you simplify code such as the following:

myPeerConnection.createOffer().then(function(offer) {
  return myPeerConnection.setLocalDescription(new RTCSessionDescription(offer));
});

to simply be:

myPeerConnection.createOffer().then(myPeerConnection.setLocalDescription);

For this reason, the RTCSessionDescription() constructor is deprecated.

Return value

A Promise which is fulfilled once the value of RTCPeerConnection.localDescription is successfully changed or rejected if the change cannot be applied (for example, if the specified description is incompatible with one or both of the peers on the connection). The promise's fulfillment handler receives no input parameters.

The process of changing descriptions actually involves intermediary steps handled by the WebRTC layer to ensure that an active connection can be changed without losing the connection if the change does not succeed. See "Pending and current descriptions" in WebRTC connectivity for more details on this process.

Deprecated parameters

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

successCallback
A JavaScript Function which accepts no input parameters to be be called once the description has been successfully set. At that time, the offer can be sent to a remote peer via the signaling server.
errorCallback
A function matching the signautre RTCPeerConnectionErrorCallback which gets called if the description can't be set. It takes one input parameter:
  • errorInformation: A DOMString describing the reason why the description couldn't be set.

This deprecated form of the method returns instantaneously without waiting for the actual setting to be done: in case of success, the successCallback will be called; in case of failure, the errorCallback will be called.

Deprecated exceptions

When using the deprecated callback-based version of setLocalDescription(), the following exceptions may occur:

InvalidStateError
The connection's signalingState is "closed", indicating that the connection is not currently open, so negotiation cannot take place.
InvalidSessionDescriptionError
The RTCSessionDescription specified by the sessionDescription parameter is invalid.

Example

In the example below, we see the implementation of a handler for the negotiationneeded event.

function handleNegotiationNeededEvent() {
  pc.createOffer().then(function(offer) {
    return pc.setLocalDescription(offer);
  })
  .then(function() {
    // Send the offer to the remote peer using the signaling server
  })
  .catch(reportError);
}

This begins by creating an offer by calling createOffer(); when that succeeds, we call setLocalDescription(). The fulfillment handler for that promise can then send the newly-created offer along to the other peer using the signaling server.

Specifications

Specification Status Comment
WebRTC 1.0: Real-time Communication Between Browsers
The definition of 'RTCPeerConnection.setLocalDescription()' in that specification.
Working Draft Initial specification.
WebRTC 1.0: Real-time Communication Between Browsers
The definition of 'RTCPeerConnection.setLocalDescription()' in that specification.
Working Draft Initial specification.

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes)[1] (Yes) 22 (22) [2] No support (Yes) ?
Promise-based version 50.0 ? 37 (37) ? ? ?
RTCSessionDescriptionInit accepted ? ? 53 (53) ? ? ?
 
Feature Android Webview Chrome for Android Edge Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes)[1] (Yes)[1] (Yes) 22.0 (22) No support ? ?
Promise-based version 50.0 50.0 37.0 (37) ? ? ? ?
RTCSessionDescriptionInit accepted ? ? 53 (53) 53.0 (53) ? ?

[1] Though this method is not prefixed, the interface it belongs to was until Chrome 56.

[2] Though this property is not prefixed, the interface it belongs to is, until Firefox 44.

See also

Document Tags and Contributors

 Contributors to this page: PaulE, Sheppy, jpmedley, abbycar, gaspard, jwhitlock, teoli, Null
 Last updated by: PaulE,