#### [4.4.3.1方法扩展](http://w3c.github.io/webrtc-pc/#method-extensions)
```
partial interface RTCPeerConnection {
Promise createOffer(RTCSessionDescriptionCallback successCallback,
RTCPeerConnectionErrorCallback failureCallback,
optional RTCOfferOptions options);
Promise setLocalDescription(RTCSessionDescriptionInit description,
VoidFunction successCallback,
RTCPeerConnectionErrorCallback failureCallback);
Promise createAnswer(RTCSessionDescriptionCallback successCallback,
RTCPeerConnectionErrorCallback failureCallback);
Promise setRemoteDescription(RTCSessionDescriptionInit description,
VoidFunction successCallback,
RTCPeerConnectionErrorCallback failureCallback);
Promise addIceCandidate(RTCIceCandidateInit candidate,
VoidFunction successCallback,
RTCPeerConnectionErrorCallback failureCallback);
};
```
**方法**
`createOffer`
调用createOffer方法时，用户代理必须执行以下步骤：
1. 让successCallback成为方法的第一个参数。
2. 让failureCallback成为方法第二个参数指示的回调。
3. 让options成为方法第三个参数指示的回调。
4. 运行RTCPeerConnection的createOffer（）方法指定的步骤
