W3C home > Mailing lists > Public > public-webrtc@w3.org > April 2012

Re: DataChannel API and onopen

From: Tim Panton <thp@westhawk.co.uk>
Date: Mon, 23 Apr 2012 17:22:23 +0100
Cc: "public-webrtc@w3.org" <public-webrtc@w3.org>, Michael Tuexen <tuexen@fh-muenster.de>, Randall Stewart <rrs@lakerest.net>, Salvatore Loreto <salvatore.loreto@ericsson.com>
Message-Id: <28BB1C2C-9CD5-4410-B15E-4268FE022AB6@westhawk.co.uk>
To: Randell Jesup <randell-ietf@jesup.org>

On 23 Apr 2012, at 17:01, Randell Jesup wrote:

> In going over the details for the DataChannel minor protocol and how it works over SCTP, we realized we'll need a 3-way handshake instead of a 2-way.  The reason is that an OPEN_RESPONSE sent ordered followed by a data message sent unordered would allow the data message to come in at the other end before the open response, and since we don't know the input-stream -> output-stream mapping until we get the OPEN_RESPONSE, we might not know what to do with it.  So we plan to add an ACK message to the protocol to provide the 3-way handshake.

> It seems to me that forcing in-order delivery until the handshake is complete is simpler for the user, and only rarely has any impact on the application.  The exception might be if you wanted to dump a bunch of sends that were reliable and out-of-order immediately on creation of the channel - and I don't see it being a major issue there - and you can still wait on onopen if you want.
> Recap: #2 is slightly closer to how WebSockets works (though anything coded for WebSockets' onopen behavior would still work in #1), and #2 avoids potential additional buffering in the stack if the OPEN/OPEN_RESPONSE is lost.  (If it's not lost, there's really no extra buffering occurring, except maybe in the rare case where we need to increase the maximum number of simultaneous streams.)
> Opinions?

Is there any way that the handshake can fail ? If there is, then the apis are significantly different in the way that they report that failure. 
(Since you don't mention a matching NACK I guess this isn't an issue).

A downside of #2 is we need to define what order data gets sent in this case:

   temp = peer.createDataChannel(..)
   temp.data_to_send = "I want to send a magic hello string";
   temp.onopen = function() {
  temp.send("I want to send normal activity");
  temp = null;


Tim Panton - Web/VoIP consultant and implementor
Received on Monday, 23 April 2012 16:22:58 UTC

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