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

Re: [Bug 15206] Add API for sending and receiving p2p application data

From: Stefan Hakansson LK <stefan.lk.hakansson@ericsson.com>
Date: Fri, 27 Jan 2012 14:19:52 +0100
Message-ID: <4F22A478.7050007@ericsson.com>
To: public-webrtc@w3.org
On 01/27/2012 12:30 PM, Randell Jesup wrote:
> My apologies up front for the length...  I just worry this is a rabbit
> hole that the entire working group could go into and never return... :-)

I have a similar worry; and that was one reason why I submitted the 
"Alternative/Very simple" Data API design the other day.

Top-posting a couple of comments to the rest:

- I also think that data content is application specific. If you receive 
e.g. a blob, you can not know what to with it based only on standardized 
DataStream set-up, this is application specific (and should definitely 
fall outside the WG scope).

- But if we are to support multi-channel messaging in the way proposed I 
guess we need some kind of signaling for each add/remove of a data 
stream (so that the other end can create/destroy the objects associated 
with that particular stream). (Naturally this is avoided if you have a 
single channel model with application multiplexing.)

- For cases where there is a central node involved, WebSockets could be 
used; so for those scenarios we might consider focusing on the aspects 
that WebSockets can't meet (unreliable, priority, ...).

Stefan (speaking as a contributor).

> Perhaps I'm wrong, of course, or perhaps there's something very tightly
> defined and limited we should do - I'm open to convincing on this.
> On 1/26/2012 5:45 PM, Justin Uberti wrote:
>> On Thu, Jan 26, 2012 at 7:32 PM, Randell Jesup<randell-ietf@jesup.org
>> <mailto:randell-ietf@jesup.org>>  wrote:
>>      I'll note that creating a datastream here requires a full
>>      renegotiation of the entire peerconnection (though one assumes the
>>      other items don't change state and thus would end up being no-ops
>>      after lots of processing occurs.
>> Not true if JSEP is used; with JSEP you can handle this however you see
>> fit, including having a set of pre-allocated channels that you use
>> whenever, or having your own application protocol (like what you mention
>> above) that allocates channels.
> To be honest, you could do the same thing in ROAP I believe.  The point
> was that being application-specific and multiplexed with the other SCTP
> data streams, they may not need any explicit handling by the negotiation
> code.
> As for my assertion about data channels being application-specific:
> The contents of a data channel are not defined in these protocols
> currently.  Even if they're included in the offers and answers, without
> additional context describing them they're inherently application-specific.
> This is, of course, much like how UDP&  TCP ports work - without some
> other way to bind a port to a purpose (portmapper, etc) you have to use
> a fixed "well-known" port or a fixed application port, or you have to
> use an application protocol (like SDP) to define what data in what
> format you're going to put on that port.
> We would need an entire framework for applications to tag and describe
> their data channels (for those cases where the data format is public and
> stable), or a JSON or RPC-like self-descriptive data format).  And the
> description would have to not just be format, but probably also
> semantics - what the object represented and why and what interactivity
> with the data channel was expected.
> We could describe the data channel with a MIME type, which might suffice
> for cases where the data is static (a file type of some sort); that
> doesn't handle all the other cases of dynamic data channels, and in many
> cases wouldn't give you the needed context to tell the receiving party
> what to do with it.  If an app gets a data channel from a different app
> with text/html in it, what does it do with that?
> I don't think all this work would in the end actually add anything
> useful; I think cross-application data-channels are not a problem we
> should try to solve - at best leave that for the applications or another
> spec to layer on top of WebRTC.
> So for me, the one remaining argument is multi-party via a central hub,
> using the same app.  ("Same app" to me also means different apps that
> explicitly know about each other and how to use the data protocols with
> that app.)
> In some cases, the application protocol used would handle data channels
> in a star configuration by having the central node process them and
> distribute data over a single/set of data channels with each user.  A
> typical example would be a game server that processes incoming events
> and distributes state changes and events to each player.
> In other cases, the hub might proxy the data channels from each
> participant through to the others.  An example of this might be a
> conference where a participant shared a document with the other
> participants.  In this case, the hub would need to take an incoming data
> channel and duplicate it to a data channel with each of the other
> participants.  This data channel opened with each of the other
> participants would likely be identical to the one the sender opened to
> it, except that the hub would likely want to also inform each receiver
> as to the origin of the data.  I'll blindly assert that this
> notification should be done through application-specific logic unless,
> again, we want to get deeply into describing the data.
>>      There are alternatives that would not require renegotations of the
>>      peerconnection, as mentioned in a previous message thread titled
>>      "Data channel setup and signalling", starting 11/15/2011, or would
>>      only *require* negotiating one channel and use an
>>      application-specific protocol over that one to open additional ones.
>>      http://www.ietf.org/mail-__archive/web/rtcweb/current/__msg02861.html<http://www.ietf.org/mail-archive/web/rtcweb/current/msg02861.html>
>>      I'll repeat one bit from that which is relevant:
>>            The other option is to have (some) data channels be separate
>>          from media, in
>>            particular app-specific anonymous data channels.  There's no
>>          requirement for
>>            describing the channels if they're private to the app, at
>>          least to the first
>>            approximation.  An app could pre-define data channel 3 as a
>>          private message
>>            structure for game map updates, so long as it knows its
>>          talking to itself.
>>      Unless there's some higher-level structure imposed on the data
>>      somehow, data channels are inherently application-specific, so
>>      generic stream specifications and "negotiation" of them actually
>>      buys you very little (except *maybe* the first one).  This is
>>      different from audio and video streams and their encodings.
>> I thought about this a lot, but I think we probably want to impose
>> higher-level structure, for two reasons:
>> 1) Multiuser. Hasn't been talked about too much in the context of data
>> channels, but without getting into too many details I think this is a
>> critical use case. In multiuser, you need to be informed of what
>> channels are available, changes as new channels become available, what
>> their reliable/unreliable status is, what user they correspond to, etc.
>> I admit that much of this could be done completely by the application,
>> despite being inconsistent with the handling of audio and video, but
>> there is an important exception:
>> 2) 3rd party backend services. WebRTC is already spawning a number of
>> startups creating AWS-style signaling, gatewaying, and conferencing
>> services. For conferencing, the service will need to know the details of
>> how multiple streams are configured within a WebRTC session, as well as
>> the wire protocol, so that it can route the data properly without
>> needing special cases for each application (just like audio and video).
>> This implies we need to specify a way to negotiate the stream configuration.
> This is basically saying (if I understand correctly) that people
> are/will be creating generic conferencing and interconnect
> hubs/gateways, and that these need to understand the data channels in
> order to deal with connecting them in a multi-party conference.  For
> example, the "share a file" example above.
> My suggestion for this case is that the data channels provided by an app
> calling into these would need to somehow be tagged with a
> known-to-the-conference identifier, which would then know what to do
> with the datastream.
> The best usecase for this I can think of is a heterogeneous conference
> server with a mix of clients, where data channels offered by an user
> would be proxied (with added info as to the source) to each of the other
> users, who would only process it if they understood the data
> format/description/purpose.  same-apps in the conference might easily do
> so, while different-apps might well ignore such data channels unless in
> a "well-known" format/tag - which means defining those formats/tags.
> Again, I think instead of pulling all of that into this effort, leave
> that effort to the applications to sort out and standardize separately
> if they wish.
> Maybe find a standard way to define the MIME type of the stream when
> it's opened is the best, mimimal solution, with anything complex or
> application-defined or needing context being opaque binary.  (Even
> standardizing identifying the source is a bit tricky in this context.)
>> I'm certainly not dead-set on this approach, but as we've started
>> building this stuff out, I'm finding that in many respects data has the
>> same needs as audio and video.
> Ok, but we have a well-set language and semantics for those (SDP + RFCs
> defining what a audio/PCMU or audio/Opus, etc type means - data format,
> negotiation semantics, etc, etc) that we're leveraging.  We don't have
> that for "data", and I worry trying to go down that path leads to a
> long, long period defining data interop&  description methods, with
> minimal payback.  I'm not saying it wouldn't be nice to have that,
> especially for the heterogeneous conf case above, but I think that could
> be defined or defacto-standardized in the apps and servers.
Received on Friday, 27 January 2012 13:20:31 UTC

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