Merge pull request #110 from youennf/ensure-encodder-order

Ensure receiver-side transform order is the same as sender-side transform order

Author: alvestrand

index.bs

@@ -100,8 +100,9 @@ At construction of each {{RTCRtpSender}} or {{RTCRtpReceiver}}, run the followin
 7. <a dfn for="WritableStream">Set up</a> [=this=].`[[writable]]` with its [=WritableStream/set up/writeAlgorithm=] set to [=writeEncodedData=] given |this| as parameter.
 8. Set [=this=].`[[writable]]`.`[[owner]]` to |this|.
 9. Initialize [=this=].`[[pipeToController]]` to null.
-10. Initialize [=this=].`[[lastReceivedFrameTimestamp]]` to zero.
-11. [=Queue a task=] to run the following steps:
+1. Initialize [=this=].`[[lastReceivedFrameCounter]]` to <code>0</code>.
+1. Initialize [=this=].`[[lastEnqueuedFrameCounter]]` to <code>0</code>.
+1. [=Queue a task=] to run the following steps:
     1. If [=this=].`[[pipeToController]]` is not null, abort these steps.
     2. Set [=this=].`[[pipeToController]]` to a new {{AbortController}}.
     <!-- FIXME: Use pipeTo algorithm when available. -->
@@ -111,19 +112,28 @@ At construction of each {{RTCRtpSender}} or {{RTCRtpReceiver}}, run the followin
 
 The <dfn>readEncodedData</dfn> algorithm is given a |rtcObject| as parameter. It is defined by running the following steps:
 1. Wait for a frame to be produced by |rtcObject|'s encoder if it is a {{RTCRtpSender}} or |rtcObject|'s packetizer if it is a {{RTCRtpReceiver}}.
-2. Let |frame| be the newly produced frame.
-3. Set |frame|.`[[owner]]` to |rtcObject|.
-4. [=ReadableStream/Enqueue=] |frame| in |rtcObject|.`[[readable]]`.
+1. Increment |rtcObject|.`[[lastEnqueuedFrameCounter]]` by <code>1</code>.
+1. Let |frame| be the newly produced frame.
+1. Set |frame|.`[[owner]]` to |rtcObject|.
+1. Set |frame|.`[[counter]]` to |rtcObject|.`[[lastEnqueuedFrameCounter]]`.
+1. [=ReadableStream/Enqueue=] |frame| in |rtcObject|.`[[readable]]`.
 
 The <dfn>writeEncodedData</dfn> algorithm is given a |rtcObject| as parameter and a |frame| as input. It is defined by running the following steps:
 1. If |frame|.`[[owner]]` is not equal to |rtcObject|, abort these steps and return [=a promise resolved with=] undefined. A processor cannot create frames, or move frames between streams.
-2. If the |frame|'s {{RTCEncodedVideoFrame/timestamp}} is equal to or larger than |rtcObject|.`[[lastReceivedFrameTimestamp]]`, abort these steps and return [=a promise resolved with=] undefined. A processor cannot reorder frames, although it may delay them or drop them.
-3. Set |rtcObject|.`[[lastReceivedFrameTimestamp]]` to the |frame|'s {{RTCEncodedVideoFrame/timestamp}}.
+1. If |frame|.`[[counter]]` is equal or smaller than |rtcObject|.`[[lastReceivedFrameCounter]]`, abort these steps and return [=a promise resolved with=] undefined. A processor cannot reorder frames, although it may delay them or drop them.
+1. Set |rtcObject|.`[[lastReceivedFrameCounter]]` to |frame|`[[counter]]`.
 4. Enqueue the frame for processing as if it came directly from the encoded data source, by running one of the following steps:
     * If |rtcObject| is a {{RTCRtpSender}}, enqueue it to |rtcObject|'s packetizer, to be processed [=in parallel=].
     * If |rtcObject| is a {{RTCRtpReceiver}}, enqueue it to |rtcObject|'s decoder, to be processed [=in parallel=].
 5. Return [=a promise resolved with=] undefined.
 
+On sender side, as part of [=readEncodedData=], frames produced by |rtcObject|'s encoder MUST be enqueued in |rtcObject|.`[[readable]]` in the encoder's output order.
+As [=writeEncodedData=] ensures that the transform cannot reorder frames, the encoder's output order is also the order followed by packetizers to generate RTP packets and assign RTP packet sequence numbers.
+
+On receiver side, as part of [=readEncodedData=], frames produced by |rtcObject|'s packetizer MUST be enqueued in |rtcObject|.`[[readable]]` in the same encoder's output order.
+To ensure the order is respected, the depacketizer will typically use RTP packet sequence numbers to reorder RTP packets as needed before enqueuing frames in |rtcObject|.`[[readable]]`.
+As [=writeEncodedData=] ensures that the transform cannot reorder frames, this will be the order expected by |rtcObject|'s decoder.
+
 ## Extension attribute ## {#attribute}
 
 A RTCRtpTransform has two private slots called `[[readable]]` and `[[writable]]`.