[rtcweb] Data Channel question: correlating streams with channels

[Paul Kyzivat <[e-mail]>]

Section 6.4 of draft-ietf-rtcweb-data-channel-06 says:

    The realization of a bidirectional Data Channel is a pair of one
    incoming stream and one outgoing SCTP stream.

    Note that there's no requirement for the SCTP streams used to create
    a bidirectional channel have the same number in each direction.  How
    stream values are selected is protocol and implementation dependent.

The note seems to be something of a cop-out. But I'll accept for the 
moment that this can be true when external negotiation is used to define 
the channels.

But I've looked through draft-ietf-rtcweb-data-protocol-01, and I don't 
find any specification there of how the pairing is done.

Section 4 of the protocol draft says:

    The data channel protocol uses a two way handshake to open a data
    channel.  The side wanting to open a data channel selects an unused
    Stream and sends a DATA_CHANNEL_OPEN message.  The peer responds with
    a DATA_CHANNEL_ACK message.  Then the data channel is open.

The side doing the OPEN chooses what will be its outgoing stream id. 
That then becomes the incoming stream id of the peer. I had always 
assumed that the same stream id would be used for the ACK. But I find no 
evidence to support that assumption. For it to be anything else I would 
think the ACK would need to carry more information to correlate back to 
the OPEN message.

	Thanks,
	Paul