Specify when a data channel's ID is assigned, and what the id attribute returns when no ID is assigned.

[taylor-b]

This is an issue I noticed when working on a solution for issue #746.

The WebRTC data channel establishment spec requires that DTLS clients choose even stream IDs, and DTLS servers choose odd IDs.

However, the DTLS role may not be known at the time of creating a data channel. So, until the role is determined, what does the id attribute return? It's worth noting that both Chrome and Firefox currently return an ID of 65535, which is guaranteed to be out of range (the maximum usable ID is 65534). But it seems more "correct" to make the attribute nullable.

And then, when should a user script expect the ID to be populated? When the stable signaling state is reached with an SCTP media description negotiated? That would seem reasonable to me.

[alvestrand]

@taylor-b There was a discussion at TPAC about this - can you make a PR based on your memory of the conclusion?

[taylor-b]

Are you sure we discussed this at TPAC? My memory's blanking and I can't find anything in the minutes. It's also not in the slide deck. I filed this issue as I was making the slides for issue #746, so it's possible we never touched on it.

[alvestrand]

It seems that nobody is able to figure out a precise issue that needs solving under this title, so closing.

[taylor-b]

This still is an issue as far as I can tell. Does it need a better title?

[alvestrand]

The part we understood seemed to have been covered by #746. Can you try to explain it better?

[taylor-b]

#746 deals with calling createDataChannel with an invalid ID. This issue deals with calling createDataChannel with no ID (letting the browser choose the ID). Maybe an example will clear things up:

pc = new RTCPeerConnection();
dc = pc.createDataChannel("foo");
// What is dc.id? 65535 or null?
// ...
pc.setLocalDescription(offer);
// ...
pc.setRemoteDescription(answer_with_active_role);
// Now we know the DTLS role. An odd value (1) is chosen for dc.id.

I think it would be pretty uncontroversial to say the DTLS role (and thus the data channel ID) is determined when applying an answer. So the main question is what the id attribute returns when no ID is chosen yet.

[alvestrand]

I'd think leaving it as null (which means it has to be nullable) would be the clearest way to do it.
Any "marker" value risks being a possibly-useful value if we change the underlying protocol, but null won't be.

Reopening issue.

[aboba]

@taylor-b Can you submit a PR?