Add sequence number to DataChannelMessage

[epes]

According to https://en.wikipedia.org/wiki/Stream_Control_Transmission_Protocol

SCTP assigns a sequence number or a message-id to each message sent in a stream.

It would be great if in v4 the DataChannelMessage struct (https://pkg.go.dev/github.com/pion/webrtc/v3#DataChannelMessage) also included this sequence number/message ID. This would be helpful in a redundant data channel scenario where deduplicated messages would be required.

[Sean-Der]

This would be nice! I think this would be worth bringing up in the W3C even.

It would be nice for a web app to be able to know if packet loss is occuring. It is a shame that people have to implement themselves (and waste the space/add complexity)

If you get a chance you should file something against https://github.com/w3c/webrtc-nv-use-cases/issues would love to expose in a way consistent with the browser.

[epes]

Done. w3c/webrtc-nv-use-cases#72

Thanks, didn't know about this repo! I'll make sure to throw my other spec ideas there too :)

[epes]

TIL that the sequence number in unordered SCTP messages is optional so it might not make sense to add it here.