Set max average bitrate on PTT calls (#2499)

* Set max average bitrate on PTT calls

Via SDP munging. Also makes the SDP munging a bit more generic and
codec-specific (we were previously adding usedtx to any codec that had an fmtp
line already, which was probably not really the intention).

* Make SDP munging for codecs that don't already have fmtp lines

* Use sensible typescript syntax

Co-authored-by: Šimon Brandner <[email protected]>

Co-authored-by: Šimon Brandner <[email protected]>

src/webrtc/call.ts

@@ -93,6 +93,14 @@ interface AssertedIdentity {
     displayName: string;
 }
 
+// Used internally to specify modifications to codec parameters in SDP
+interface CodecParams {
+    enableDtx?: boolean; // true to enable discontinuous transmission, false to disable, undefined to leave as-is
+    maxAverageBitrate?: number; // sets the max average bitrate, or undefined to leave as-is
+}
+
+type CodecParamMods = Record<string, CodecParams>;
+
 export enum CallState {
     Fledgling = 'fledgling',
     InviteSent = 'invite_sent',
@@ -261,6 +269,18 @@ export function genCallID(): string {
     return Date.now().toString() + randomString(16);
 }
 
+function getCodecParamMods(isPtt: boolean): CodecParamMods {
+    const mods = {
+        'opus': {
+            enableDtx: true,
+        },
+    } as CodecParamMods;
+
+    if (isPtt) mods.opus.maxAverageBitrate = 12000;
+
+    return mods;
+}
+
 export type CallEventHandlerMap = {
     [CallEvent.DataChannel]: (channel: RTCDataChannel) => void;
     [CallEvent.FeedsChanged]: (feeds: CallFeed[]) => void;
@@ -1456,26 +1476,59 @@ 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 {
+    private mungeSdp(description: RTCSessionDescriptionInit, mods: CodecParamMods): 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");
+            const payloadTypeToCodecMap = new Map<number, string>();
+            const codecToPayloadTypeMap = new Map<string, number>();
+            for (const rtp of media.rtp) {
+                payloadTypeToCodecMap.set(rtp.payload, rtp.codec);
+                codecToPayloadTypeMap.set(rtp.codec, rtp.payload);
+            }
+
+            for (const [codec, params] of Object.entries(mods)) {
+                if (!codecToPayloadTypeMap.has(codec)) {
+                    logger.info(`Ignoring SDP modifications for ${codec} as it's not present.`);
+                    continue;
+                }
+
+                const extraconfig: string[] = [];
+                if (params.enableDtx !== undefined) {
+                    extraconfig.push(`usedtx=${params.enableDtx ? '1' : '0'}`);
+                }
+                if (params.maxAverageBitrate !== undefined) {
+                    extraconfig.push(`maxaveragebitrate=${params.maxAverageBitrate}`);
+                }
+
+                let found = false;
+                for (const fmtp of media.fmtp) {
+                    if (payloadTypeToCodecMap.get(fmtp.payload) === codec) {
+                        found = true;
+                        fmtp.config += ";" + extraconfig.join(";");
+                    }
+                }
+                if (!found) {
+                    media.fmtp.push({
+                        payload: codecToPayloadTypeMap.get(codec),
+                        config: extraconfig.join(";"),
+                    });
+                }
             }
         });
         description.sdp = writeSdp(sdp);
     }
 
     private async createOffer(): Promise<RTCSessionDescriptionInit> {
         const offer = await this.peerConn.createOffer();
-        this.enableDtx(offer);
+        this.mungeSdp(offer, getCodecParamMods(this.isPtt));
         return offer;
     }
 
     private async createAnswer(): Promise<RTCSessionDescriptionInit> {
         const answer = await this.peerConn.createAnswer();
-        this.enableDtx(answer);
+        this.mungeSdp(answer, getCodecParamMods(this.isPtt));
         return answer;
     }