Keep dd extension id in the session

Safari could generate different rtp extensions
for video tracks(camera and screenshare), need
to use same extension id for dependency descriptor
in the offer.

src/room/PCTransport.ts

@@ -48,6 +48,8 @@ export default class PCTransport extends EventEmitter {
 
   private loggerOptions: LoggerOptions;
 
+  private ddExtID = 0;
+
   pendingCandidates: RTCIceCandidateInit[] = [];
 
   restartingIce: boolean = false;
@@ -288,7 +290,7 @@ export default class PCTransport extends EventEmitter {
           }
 
           if (isSVCCodec(trackbr.codec)) {
-            ensureVideoDDExtensionForSVC(media);
+            this.ensureVideoDDExtensionForSVC(media);
           }
 
           // TODO: av1 slow starting issue already fixed in chrome 124, clean this after some versions
@@ -503,6 +505,36 @@ export default class PCTransport extends EventEmitter {
       throw new NegotiationError(msg);
     }
   }
+
+  private ensureVideoDDExtensionForSVC(
+    media: {
+      type: string;
+      port: number;
+      protocol: string;
+      payloads?: string | undefined;
+    } & MediaDescription,
+  ) {
+    let maxID = 0;
+    const ddFound = media.ext?.some((ext): boolean => {
+      if (ext.uri === ddExtensionURI) {
+        return true;
+      }
+      if (ext.value > maxID) {
+        maxID = ext.value;
+      }
+      return false;
+    });
+
+    if (!ddFound) {
+      if (this.ddExtID === 0) {
+        this.ddExtID = maxID + 1;
+      }
+      media.ext?.push({
+        value: this.ddExtID,
+        uri: ddExtensionURI,
+      });
+    }
+  }
 }
 
 function ensureAudioNackAndStereo(
@@ -555,32 +587,6 @@ function ensureAudioNackAndStereo(
   }
 }
 
-function ensureVideoDDExtensionForSVC(
-  media: {
-    type: string;
-    port: number;
-    protocol: string;
-    payloads?: string | undefined;
-  } & MediaDescription,
-) {
-  let maxID = 0;
-  const ddFound = media.ext?.some((ext): boolean => {
-    if (ext.uri === ddExtensionURI) {
-      return true;
-    }
-    if (ext.value > maxID) {
-      maxID = ext.value;
-    }
-    return false;
-  });
-
-  if (!ddFound) {
-    media.ext?.push({
-      value: maxID + 1,
-      uri: ddExtensionURI,
-    });
-  }
-}
 
 function extractStereoAndNackAudioFromOffer(offer: RTCSessionDescriptionInit): {
   stereoMids: string[];