Enable DTX on audio tracks in calls (#2482)

This greatly reduces the amount of bandwidth used when transmitting
silence.

package.json

@@ -54,6 +54,7 @@
   ],
   "dependencies": {
     "@babel/runtime": "^7.12.5",
+    "@types/sdp-transform": "^2.4.5",
     "another-json": "^0.2.0",
     "browser-request": "^0.3.3",
     "bs58": "^4.0.1",
@@ -63,6 +64,7 @@
     "p-retry": "^4.5.0",
     "qs": "^6.9.6",
     "request": "^2.88.2",
+    "sdp-transform": "^2.14.1",
     "unhomoglyph": "^1.0.6"
   },
   "devDependencies": {

src/webrtc/call.ts

@@ -22,6 +22,8 @@ limitations under the License.
  * @module webrtc/call
  */
 
+import { parse as parseSdp, write as writeSdp } from "sdp-transform";
+
 import { logger } from '../logger';
 import * as utils from '../utils';
 import { MatrixEvent } from '../models/event';
@@ -171,6 +173,11 @@ export enum CallErrorCode {
      */
     CreateAnswer = 'create_answer',
 
+    /**
+     * An offer could not be created
+     */
+    CreateOffer = 'create_offer',
+
     /**
      * Error code used when we fail to send the answer
      * for some reason other than there being unknown devices
@@ -1438,6 +1445,31 @@ export class MatrixCall extends TypedEventEmitter<CallEvent, CallEventHandlerMap
         }
     }
 
+    // Enables DTX (discontinuous transmission) on the given session to reduce
+    // bandwidth when transmitting silence
+    private enableDtx(description: RTCSessionDescriptionInit): void {
+        // The only way to enable DTX at this time is through SDP munging
+        const sdp = parseSdp(description.sdp);
+        sdp.media.forEach(media => {
+            if (media.type === "audio") {
+                media.fmtp.forEach(fmtp => fmtp.config += ";usedtx=1");
+            }
+        });
+        description.sdp = writeSdp(sdp);
+    }
+
+    private async createOffer(): Promise<RTCSessionDescriptionInit> {
+        const offer = await this.peerConn.createOffer();
+        this.enableDtx(offer);
+        return offer;
+    }
+
+    private async createAnswer(): Promise<RTCSessionDescriptionInit> {
+        const answer = await this.peerConn.createAnswer();
+        this.enableDtx(answer);
+        return answer;
+    }
+
     private async gotCallFeedsForAnswer(callFeeds: CallFeed[]): Promise<void> {
         if (this.callHasEnded()) return;
 
@@ -1449,18 +1481,18 @@ export class MatrixCall extends TypedEventEmitter<CallEvent, CallEventHandlerMap
 
         this.setState(CallState.CreateAnswer);
 
-        let myAnswer;
+        let answer: RTCSessionDescriptionInit;
         try {
             this.getRidOfRTXCodecs();
-            myAnswer = await this.peerConn.createAnswer();
+            answer = await this.createAnswer();
         } catch (err) {
             logger.debug(`Call ${this.callId} Failed to create answer: `, err);
             this.terminate(CallParty.Local, CallErrorCode.CreateAnswer, true);
             return;
         }
 
         try {
-            await this.peerConn.setLocalDescription(myAnswer);
+            await this.peerConn.setLocalDescription(answer);
             this.setState(CallState.Connecting);
 
             // Allow a short time for initial candidates to be gathered
@@ -1672,8 +1704,17 @@ export class MatrixCall extends TypedEventEmitter<CallEvent, CallEventHandlerMap
             await this.peerConn.setRemoteDescription(description);
 
             if (description.type === 'offer') {
-                this.getRidOfRTXCodecs();
-                await this.peerConn.setLocalDescription();
+                let answer: RTCSessionDescriptionInit;
+                try {
+                    this.getRidOfRTXCodecs();
+                    answer = await this.createAnswer();
+                } catch (err) {
+                    logger.debug(`Call ${this.callId} Failed to create answer: `, err);
+                    this.terminate(CallParty.Local, CallErrorCode.CreateAnswer, true);
+                    return;
+                }
+
+                await this.peerConn.setLocalDescription(answer);
 
                 this.sendVoipEvent(EventType.CallNegotiate, {
                     description: this.peerConn.localDescription,
@@ -1741,7 +1782,6 @@ export class MatrixCall extends TypedEventEmitter<CallEvent, CallEventHandlerMap
     private async wrappedGotLocalOffer(): Promise<void> {
         this.makingOffer = true;
         try {
-            this.getRidOfRTXCodecs();
             await this.gotLocalOffer();
         } catch (e) {
             this.getLocalOfferFailed(e);
@@ -1760,8 +1800,18 @@ export class MatrixCall extends TypedEventEmitter<CallEvent, CallEventHandlerMap
             return;
         }
 
+        let offer: RTCSessionDescriptionInit;
+        try {
+            this.getRidOfRTXCodecs();
+            offer = await this.createOffer();
+        } catch (err) {
+            logger.debug(`Call ${this.callId} Failed to create offer: `, err);
+            this.terminate(CallParty.Local, CallErrorCode.CreateOffer, true);
+            return;
+        }
+
         try {
-            await this.peerConn.setLocalDescription();
+            await this.peerConn.setLocalDescription(offer);
         } catch (err) {
             logger.debug(`Call ${this.callId} Error setting local description!`, err);
             this.terminate(CallParty.Local, CallErrorCode.SetLocalDescription, true);

yarn.lock

@@ -1608,6 +1608,11 @@
   resolved "https://registry.yarnpkg.com/@types/retry/-/retry-0.12.0.tgz#2b35eccfcee7d38cd72ad99232fbd58bffb3c84d"
   integrity sha512-wWKOClTTiizcZhXnPY4wikVAwmdYHp8q6DmC+EJUzAMsycb7HB32Kh9RN4+0gExjmPmZSAQjgURXIGATPegAvA==
 
+"@types/sdp-transform@^2.4.5":
+  version "2.4.5"
+  resolved "https://registry.yarnpkg.com/@types/sdp-transform/-/sdp-transform-2.4.5.tgz#3167961e0a1a5265545e278627aa37c606003f53"
+  integrity sha512-GVO0gnmbyO3Oxm2HdPsYUNcyihZE3GyCY8ysMYHuQGfLhGZq89Nm4lSzULWTzZoyHtg+VO/IdrnxZHPnPSGnAg==
+
 "@types/stack-utils@^2.0.0":
   version "2.0.1"
   resolved "https://registry.yarnpkg.com/@types/stack-utils/-/stack-utils-2.0.1.tgz#20f18294f797f2209b5f65c8e3b5c8e8261d127c"
@@ -5874,6 +5879,11 @@ safer-buffer@^2.0.2, safer-buffer@^2.1.0, safer-buffer@~2.1.0:
   resolved "https://registry.yarnpkg.com/safer-buffer/-/safer-buffer-2.1.2.tgz#44fa161b0187b9549dd84bb91802f9bd8385cd6a"
   integrity sha512-YZo3K82SD7Riyi0E1EQPojLz7kpepnSQI9IyPbHHg1XXXevb5dJI7tpyN2ADxGcQbHG7vcyRHk0cbwqcQriUtg==
 
+sdp-transform@^2.14.1:
+  version "2.14.1"
+  resolved "https://registry.yarnpkg.com/sdp-transform/-/sdp-transform-2.14.1.tgz#2bb443583d478dee217df4caa284c46b870d5827"
+  integrity sha512-RjZyX3nVwJyCuTo5tGPx+PZWkDMCg7oOLpSlhjDdZfwUoNqG1mM8nyj31IGHyaPWXhjbP7cdK3qZ2bmkJ1GzRw==
+
 [email protected]:
   version "7.0.0"
   resolved "https://registry.yarnpkg.com/semver/-/semver-7.0.0.tgz#5f3ca35761e47e05b206c6daff2cf814f0316b8e"