Advertise support for opus/red.

modules/browser/BrowserCapabilities.js

                 || window.RTCRtpSender.prototype.createEncodedVideoStreams));
     }
 
+    /**
+     * Whether the browser supports the RED format for audio.
+     */
+    supportsAudioRed() {
+        return Boolean(window.RTCRtpSender
+            && window.RTCRtpSender.getCapabilities
+            && window.RTCRtpSender.getCapabilities('audio').codecs.some(codec => codec.mimeType === 'audio/red')
+            && window.RTCRtpReceiver
+            && window.RTCRtpReceiver.getCapabilities
+            && window.RTCRtpReceiver.getCapabilities('audio').codecs.some(codec => codec.mimeType === 'audio/red'));
+    }
+
     /**
      * Checks if the browser supports the "sdpSemantics" configuration option.
      * https://webrtc.org/web-apis/chrome/unified-plan/

modules/xmpp/moderator.js

 import { getLogger } from 'jitsi-meet-logger';
 import { $iq, Strophe } from 'strophe.js';
 
+import browser from '../browser';
 import Settings from '../settings/Settings';
 
 const AuthenticationEvents
                     value: Boolean(config.enableRemb)
                 }).up();
     }
+    if (config.enableOpusRed === true && browser.supportsAudioRed()) {
+        elem.c(
+                'property', {
+                    name: 'enableOpusRed',
+                    value: true
+                }).up();
+    }
     if (config.minParticipants !== undefined) {
         elem.c(
                 'property', {

modules/xmpp/xmpp.js

         if (!this.options.disableRtx) {
             this.caps.addFeature('urn:ietf:rfc:4588');
         }
+        if (this.options.enableOpusRed === true) {
+            this.caps.addFeature('http://jitsi.org/opus-red');
+        }
 
         // this is dealt with by SDP O/A so we don't need to announce this
         // XEP-0293