title: how can i stop Echo sound in webrtc ios?
body: <p>I am working with Webrtc Video streaming.<br>
  how to audio echo sound disable?
  i write the code <br> </p>
  
  <pre><code>-(RTCMediaConstraints *)defaultOfferConstraints 
   {  
       NSArray *mandatoryConstraints =@[                            
            [[RTCPair alloc] initWithKey:@"OfferToReceiveAudio" value:@"true"],                     
            [[RTCPair alloc] initWithKey:@"OfferToReceiveVideo" value:@"true"],
            [[RTCPair alloc] initWithKey:@"echoCancellation" value:@"false"],
            [[RTCPair alloc] initWithKey:@"googEchoCancellation" value:@"false"]
                                    ];
       RTCMediaConstraints* constraints =[[RTCMediaConstraints alloc]
                     initWithMandatoryConstraints:mandatoryConstraints optionalConstraints:nil];
       return constraints;
  }
  </code></pre>
  
  <p>but,it's not working.
  how can i stop or disable echo sound?</p>