Default the generate key frame algorithm to all layers.

index.bs

@@ -1003,26 +1003,39 @@ The <dfn abstract-op>generate key frame algorithm</dfn>, given |promise|, |encod
     in Section 10 of [[!RFC8851]], then reject |promise| with {{TypeError}} and abort
     these steps.
 1. [=In parallel=], run the following steps:
-    1. Gather a list of video encoders, named |videoEncoders| from |encoder|, ordered according negotiated RIDs if any.
-    1. If |rid| is defined, remove from |videoEncoders| any video encoder that does not match |rid|.
-    1. If |rid| is undefined, remove from |videoEncoders| all video encoders except the first one.
+    1. Let |videoEncoders| be a list of video encoders from |encoder|, ordered according to their negotiated RIDs if any.
+    1. Let |allRids| be the list of rids from |videoEncoders|, using `null` in place of any missing rids.
+    1. Assert that |allRids| either [=list/contains=] no `null` values or a single entry that is `null`.
+
+       Note: This algorithm internally uses `null` to identify the single video encoder in unicast.
+
+    1. If |rid| is not undefined, remove from |videoEncoders| any video encoder that does not match |rid|.
+
+       Note: If no |rid| is passed in, keyframes are generated for all layers.
+
     1. If |videoEncoders| is empty, [=queue a task=] to reject |promise| with {{NotFoundError}} and abort these steps.
-        |videoEncoders| is expected to be empty if the corresponding {{RTCRtpSender}} is not active, or the corresponding {{RTCRtpSender}} track is ended.
-    1. Let |videoEncoder| be the first encoder in |videoEncoders|.
-    1. If |rid| is undefined, set |rid| to the RID value corresponding to |videoEncoder|.
-    1. Create a pending key frame task called |task| with |task|.`[[rid]]` set to rid and |task|.`[[promise]]`| set to |promise|.
+
+       Note: |videoEncoders| is expected to be empty if the corresponding {{RTCRtpSender}} encoding is not
+           {{RTCRtpEncodingParameters/active}}, or the corresponding {{RTCRtpSender}} track is ended.
+
+    1. Let |rids| be [|rid|] if |rid| is not undefined, or |allRids| otherwise.
+    1. Create a pending key frame task called |task| with |task|.`[[rids]]` set to |rids| and |task|.`[[promise]]` set to |promise|.
     1. If |encoder|.`[[pendingKeyFrameTasks]]` is undefined, initialize |encoder|.`[[pendingKeyFrameTasks]]` to an empty set.
-    1. Let |shouldTriggerKeyFrame| be <code>false</code> if |encoder|.`[[pendingKeyFrameTasks]]` contains a task whose `[[rid]]`
-        value is equal to |rid|, and <code>true</code> otherwise.
+    1. Remove from |videoEncoders| any video encoder whose identifying |rid| (or `null` value if it has no rid)
+        already [=list/contained|exists=] in any `[[rids]]` of any tasks in |encoder|.`[[pendingKeyFrameTasks]]`.
     1. Add |task| to |encoder|.`[[pendingKeyFrameTasks]]`.
-    1. If |shouldTriggerKeyFrame| is <code>true</code>, instruct |videoEncoder| to generate a key frame for the next provided video frame.
+    1. For each |videoEncoder| in |videoEncoders| (if any), instruct |videoEncoder| to generate a key frame for its
+        next provided video frame.
 
 For any {{RTCRtpScriptTransformer}} named |transformer|, the following steps are run just before any |frame| is enqueued in |transformer|.`[[readable]]`:
 1. Let |encoder| be |transformer|.`[[encoder]]`.
 1. If |encoder| or |encoder|.`[[pendingKeyFrameTasks]]` is undefined, abort these steps.
 1. If |frame| is not a video {{RTCEncodedVideoFrameType/"key"}} frame, abort these steps.
 1. For each |task| in |encoder|.`[[pendingKeyFrameTasks]]`, run the following steps:
-    1. If |frame| was generated by a video encoder identified by |task|.`[[rid]]`, run the following steps:
+    1. If |frame| was generated by a video encoder identified by a value
+        [=list/contained=] in |task|.`[[rids]]`, and this brings to zero the number of
+        video encoders identified in |task|.`[[rids]]` that still haven't finished
+        generating its keyframe, then run the following steps:
         1. Remove |task| from |encoder|.`[[pendingKeyFrameTasks]]`.
         1. Resolve |task|.`[[promise]]` with |frame|'s timestamp.