[SOLVED] Why are the messages sent over WebRTC received in a different order sometimes?

Why are the messages sent over WebRTC received in a different order sometimes?

I use ordered set to true, however when many (1000 or more) messages are sent in a short period of time (< 1 second) the messages received are not all received in the same order.

rtcPeerConnection.createDataChannel("app", {
   ordered: true,
   maxPacketLifeTime: 3000
});

I could provide a minimal example to reproduce this strange behavior if necessary.

I also use bufferedAmountLowThreshold and the associated event to delay when the send buffered amount is too big. I chose 2000 but I don't know what the optimal number is. The reason I have so many messages in a short period of time is because I don't want to overflow the maximum amount of data sent at once. So I split the data into 800 Bytes packs and send those. Again I don't know what the maximum size 1 message can be.

const SEND_BUFFERED_AMOUNT_LOW_THRESHOLD = 2000; //Bytes
rtcSendDataChannel.bufferedAmountLowThreshold = SEND_BUFFERED_AMOUNT_LOW_THRESHOLD;
const MAX_MESSAGE_SIZE = 800;

Everything works fine for small data that is not split into too many messages. The error occurs randomly for big files only.

Solution

In 2016/11/01 , there is a bug that lets the dataChannel.bufferedAmount value change during the event loop task execution. Relying on this value can thus cause unexpected results. It is possible to manually cache dataChannel.bufferedAmount, and to use that to prevent this issue.

See https://bugs.chromium.org/p/webrtc/issues/detail?id=6628