files/en-us/web/api/rtcpeerconnection/setremotedescription/index.md
{{APIRef("WebRTC")}}
The setRemoteDescription() method of the {{domxref("RTCPeerConnection")}} interface sets the specified session description as the remote peer's current offer or answer.
The description specifies the properties of the remote end of the connection, including the media format.
The method takes a single parameter—the session description—and it returns a {{jsxref("Promise")}} which is fulfilled once the description has been changed, asynchronously.
This is typically called after receiving an offer or answer from another peer over the signaling server.
Keep in mind that if setRemoteDescription() 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 setRemoteDescription() 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.
setRemoteDescription(sessionDescription)
// deprecated
setRemoteDescription(sessionDescription, successCallback, errorCallback)
sessionDescription
: An object which specifies the remote peer's current offer or answer. It should contain the following properties:
type
sdp {{optional_inline}}
type is "rollback", sdp must be null or an empty string. See {{domxref("RTCSessionDescription.sdp")}}.You can also pass an actual {{domxref("RTCSessionDescription")}} instance, but there's no difference. For this reason, the RTCSessionDescription constructor is deprecated.
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.
You should update any existing code to use the {{jsxref("Promise")}}-based version of setRemoteDescription() instead.
The parameters for the older form of setRemoteDescription() are described below, to aid in updating existing code.
successCallback {{deprecated_inline}}
errorCallback {{deprecated_inline}}
RTCPeerConnectionErrorCallback which gets called if the description can't be set.
It is passed a single {{domxref("DOMException")}} object explaining why the request failed.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.
A {{jsxref("Promise")}} which is fulfilled once the value of the connection's {{domxref("RTCPeerConnection.remoteDescription", "remoteDescription")}} 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 fulfillment handler receives no input parameters.
[!NOTE] 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 the WebRTC Connectivity page for more details on this process.
The following exceptions are reported to the rejection handler for the promise returned by setRemoteDescription():
InvalidAccessError {{domxref("DOMException")}}
InvalidStateError {{domxref("DOMException")}}
type is rollback and the signaling state is one of stable, have-local-pranswer, or have-remote-pranswer because you cannot roll back a connection that's either fully established or is in the final stage of becoming connected.OperationError {{domxref("DOMException")}}
RTCError {{domxref("DOMException")}}
sdp-syntax-error if the {{Glossary("SDP")}} specified by {{domxref("RTCSessionDescription.sdp")}} is not valid.
The error object's {{domxref("RTCError.sdpLineNumber", "sdpLineNumber")}} property indicates the line number within the SDP on which the syntax error was detected.sessionDescription is missing the {{domxref("RTCSessionDescription.type", "type")}} property, or no description parameter was provided at all.When using the deprecated callback-based version of setRemoteDescription(), the following exceptions may occur:
InvalidStateError {{deprecated_inline}}
"closed", indicating that the connection is not currently open, so negotiation cannot take place.InvalidSessionDescriptionError {{deprecated_inline}}
sessionDescription parameter is invalid.When you call setRemoteDescription(), the ICE agent checks to make sure the {{domxref("RTCPeerConnection")}} is in either the stable or have-remote-offer {{domxref("RTCPeerConnection.signalingState", "signalingState")}}.
These states indicate that either an existing connection is being renegotiated or that an offer previously specified by an earlier call to setRemoteDescription() is to be replaced with the new offer.
In either of those two cases, we're at the beginning of the negotiation process, and the offer is set as the remote description.
On the other hand, if we're in the middle of an ongoing negotiation and an offer is passed into setRemoteDescription(), the ICE agent automatically begins an ICE rollback in order to return the connection to a stable signaling state, then, once the rollback is completed, sets the remote description to the specified offer.
This begins a new negotiation session, with the newly-established offer as the starting point.
Upon starting the new negotiation with the newly-established offer, the local peer is now the callee, even if it was previously the caller. This happens instead of throwing an exception, thereby reducing the number of potential errors which might occur, and simplifies the processing you need to do when you receive an offer, by eliminating the need to handle the offer/answer process differently depending on whether the local peer is the caller or callee.
[!NOTE] Earlier implementations of WebRTC would throw an exception if an offer was set outside a
stableorhave-remote-offerstate.
Here we see a function which handles an offer received from the remote peer. This code is derived from the example and tutorial in the article Signaling and video calling; take a look at that for more details and a more in-depth explanation of what's going on.
function handleOffer(msg) {
createMyPeerConnection();
myPeerConnection
.setRemoteDescription(msg.description)
.then(() => navigator.mediaDevices.getUserMedia(mediaConstraints))
.then((stream) => {
document.getElementById("local_video").srcObject = stream;
return myPeerConnection.addStream(stream);
})
.then(() => myPeerConnection.createAnswer())
.then((answer) => myPeerConnection.setLocalDescription(answer))
.then(() => {
// Send the answer to the remote peer using the signaling server
})
.catch(handleGetUserMediaError);
}
After creating our {{domxref("RTCPeerConnection")}} and saving it as myPeerConnection, we pass the description included in the received offer
message, msg, directly into setRemoteDescription() to tell the user agent's WebRTC layer what configuration the caller has proposed using.
When our promise fulfillment handler is called, indicating that this has been done, we create a stream, add it to the connection, then create an SDP answer and call {{domxref("RTCPeerConnection.setLocalDescription", "setLocalDescription()")}} to set that as the configuration at our end of the call before forwarding that answer to the caller.
{{Specifications}}
{{Compat}}