fix(SDP): Reset sources to MSID map for plan-b clients.

When source-name signaling is enabbled on plan-b clients, the sources to MSID map needs to be cleared after every transformation since browser in plan-b mode produces a new set of SSRCs and trackID when a track is removed and new track is added to the peerconnection. This fixes an issue where the track index in the source-name gets incremented after a track replace operation.

modules/RTC/TraceablePeerConnection.js

         if (FeatureFlags.isMultiStreamSupportEnabled()) {
             media = media.filter(mline => mline.direction === MediaDirection.SENDONLY
                 || mline.direction === MediaDirection.SENDRECV);
-            logger.warn(`local m-lines is ${media.length}`);
         } else {
             media = [];
             [ MediaType.AUDIO, MediaType.VIDEO ].forEach(mediaType => {

modules/sdp/LocalSdpMunger.js

             this._injectSourceNames(videoMLine);
         }
 
+        // Plan-b clients generate new SSRCs and trackIds whenever tracks are removed and added back to the
+        // peerconnection, therefore local track based map for msids needs to be reset after every transformation.
+        if (FeatureFlags.isSourceNameSignalingEnabled() && !this.tpc._usesUnifiedPlan) {
+            this.audioSourcesToMsidMap.clear();
+            this.videoSourcesToMsidMap.clear();
+        }
+
         return new RTCSessionDescription({
             type: sessionDesc.type,
             sdp: transformer.toRawSDP()

modules/sdp/LocalSdpMunger.spec.js

         expect(videoMsid).toBe('sRdpsdg-video-0-1');
     });
 });
+
+describe('Track replace operations in plan-b', () => {
+    const tpc = new MockPeerConnection('1', false);
+    const localEndpointId = 'sRdpsdg';
+    let desc, newSdp, sdpStr, transformedDesc, videoMsid, videoMsidLine;
+    const localSdpMunger = new LocalSdpMunger(tpc, localEndpointId);
+
+    it('should not increment track index for new tracks', () => {
+        FeatureFlags.init({ sourceNameSignaling: true });
+
+        sdpStr = transform.write(SampleSdpStrings.simulcastRtxSdp);
+        desc = new RTCSessionDescription({
+            type: 'offer',
+            sdp: sdpStr
+        });
+        transformedDesc = localSdpMunger.transformStreamIdentifiers(desc);
+        newSdp = transform.parse(transformedDesc.sdp);
+
+        videoMsidLine = getSsrcLines(newSdp, 'video').find(ssrc => ssrc.attribute === 'msid')?.value;
+        videoMsid = videoMsidLine.split(' ')[0];
+
+        expect(videoMsid).toBe('sRdpsdg-video-0-1');
+
+        sdpStr = transform.write(SampleSdpStrings.simulcastRtxSdpReplacedTrack);
+        desc = new RTCSessionDescription({
+            type: 'offer',
+            sdp: sdpStr
+        });
+        transformedDesc = localSdpMunger.transformStreamIdentifiers(desc);
+        newSdp = transform.parse(transformedDesc.sdp);
+
+        videoMsidLine = getSsrcLines(newSdp, 'video').find(ssrc => ssrc.attribute === 'msid')?.value;
+        videoMsid = videoMsidLine.split(' ')[0];
+
+        expect(videoMsid).toBe('sRdpsdg-video-0-1');
+    });
+});

modules/sdp/SampleSdpStrings.js

 + 'a=ssrc-group:SIM 1757014965 1479742055 1089111804\r\n'
 + 'a=rtcp-mux\r\n';
 
+const simulcastRtxVideoMLineSdpReplacedTrack = ''
++ 'm=video 9 RTP/SAVPF 100 96\r\n'
++ 'c=IN IP4 0.0.0.0\r\n'
++ 'a=rtpmap:100 VP8/90000\r\n'
++ 'a=fmtp:96 apt=100\r\n'
++ 'a=rtcp:9 IN IP4 0.0.0.0\r\n'
++ 'a=rtcp-fb:100 ccm fir\r\n'
++ 'a=rtcp-fb:100 nack\r\n'
++ 'a=rtcp-fb:100 nack pli\r\n'
++ 'a=rtcp-fb:100 goog-remb\r\n'
++ 'a=extmap:3 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time\r\n'
++ 'a=setup:passive\r\n'
++ 'a=mid:video\r\n'
++ 'a=sendonly\r\n'
++ 'a=ice-ufrag:adPg\r\n'
++ 'a=ice-pwd:Xsr05Mq8S7CR44DAnusZE26F\r\n'
++ 'a=fingerprint:sha-256 6A:39:DE:11:24:AD:2E:4E:63:D6:69:D3:85:05:53:C7:3C:38:A4:B7:91:74:C0:91:44:FC:94:63:7F:01:AB:A9\r\n'
++ 'a=ssrc:1234567890 msid:0836cc8e-a7bb-47e9-affb-0599414bc56d abcdesfgh\r\n'
++ 'a=ssrc:1234567890 cname:peDGrDD6WsxUOki/\r\n'
++ 'a=ssrc:2345678901 msid:0836cc8e-a7bb-47e9-affb-0599414bc56d abcdesfgh\r\n'
++ 'a=ssrc:2345678901 cname:peDGrDD6WsxUOki/\r\n'
++ 'a=ssrc:3456789012 msid:0836cc8e-a7bb-47e9-affb-0599414bc56d abcdesfgh\r\n'
++ 'a=ssrc:3456789012 cname:peDGrDD6WsxUOki/\r\n'
++ 'a=ssrc:4567890123 msid:0836cc8e-a7bb-47e9-affb-0599414bc56d abcdesfgh\r\n'
++ 'a=ssrc:4567890123 cname:peDGrDD6WsxUOki/\r\n'
++ 'a=ssrc:5678901234 msid:0836cc8e-a7bb-47e9-affb-0599414bc56d abcdesfgh\r\n'
++ 'a=ssrc:5678901234 cname:peDGrDD6WsxUOki/\r\n'
++ 'a=ssrc:6789012345 msid:0836cc8e-a7bb-47e9-affb-0599414bc56d abcdesfgh\r\n'
++ 'a=ssrc:6789012345 cname:peDGrDD6WsxUOki/\r\n'
++ 'a=ssrc-group:FID 1234567890 2345678901\r\n'
++ 'a=ssrc-group:FID 3456789012 4567890123\r\n'
++ 'a=ssrc-group:FID 5678901234 6789012345\r\n'
++ 'a=ssrc-group:SIM 1757014965 3456789012 5678901234\r\n'
++ 'a=rtcp-mux\r\n';
+
 // An sdp video mline with 1 video ssrc, 1 video rtx ssrc and one flexfec ssrc
 const flexFecVideoMLineSdp = ''
 + 'm=video 9 RTP/SAVPF 100 107 101 96 99 97 109\r\n'
 // A full sdp string representing a client doing simulcast and rtx
 const simulcastRtxSdpStr = baseSessionSdp + baseAudioMLineSdp + simulcastRtxVideoMLineSdp + baseDataMLineSdp;
 
+// A full sdp string representing a client doing simulcast and rtx with a different track and SSRCs.
+const simulcastRtxSdpTrackReplaced = baseSessionSdp + baseAudioMLineSdp + simulcastRtxVideoMLineSdpReplacedTrack;
+
 // A full sdp string representing a client doing a single video stream
 const plainVideoSdpStr = baseSessionSdp + baseAudioMLineSdp + plainVideoMLineSdp + baseDataMLineSdp;
 
         return transform.parse(simulcastRtxSdpStr);
     },
 
+    get simulcastRtxSdpReplacedTrack() {
+        return transform.parse(simulcastRtxSdpTrackReplaced);
+    },
+
     get plainVideoSdp() {
         return transform.parse(plainVideoSdpStr);
     },