Re: [rtcweb] Open data channel issues

Randell Jesup <> Mon, 03 March 2014 02:43 UTC

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Date: Sun, 02 Mar 2014 21:42:21 -0500
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Cc: Michael Tuexen <>
Subject: Re: [rtcweb] Open data channel issues
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On 3/2/2014 4:27 PM, Justin Uberti wrote:
> I'm not sure I understand the actual problem here. We *are* going to 
> handle arbitrary-sized messages once ndata gets done. The max-msg-size 
> stuff is just a solution until ndata is deployed. (To cite your 
> options, this is A followed by C).

So, here's where I think there may be a disconnect (and if I'm wrong, 

ndata solves the monopolization issue between streams, allowing packets 
for different streams to be interleaved on the wire.  It does not (so 
far as I know) relax the usrsctp library's returning EMSGSIZE if the 
sendto() is larger than available buffer space, and the alternative (EOR 
mode) doesn't allow for interleaving of messages when sending at all 
(and I'm not sure it allows interleaving on reception either, but I 
didn't check right now).

Now, this is an implementation/API issue which could be fixed with some 
work, but so far as I know no work has been done on it.  So ndata will 
allow us to expand the max-sending size to min(SCTP_BUFFER_SIZE - N, 
other-sides-max-receive-size).  It does not allow us to expand the size 
to any useful Blob size.

> That said, Send(blob) seems like a bit of a footgun to me anyway; I 
> think apps are going to typically avoid it, a) to avoid memory bloat, 
> and b) to get progress indications. The only way out of that situation 
> is to provide stream semantics, which seems like a lot to take on 
> given that WebSockets hasn't gone that route. I also agree that we 
> shouldn't try to go the (B) route that you mention.

Well, Streams would be a good thing once they're done. Progress 
indication could be added at the W3 level without much problem.  The 
memory issue should be resolvable in a number of ways, per my discussion 
with Sicking.  Note that application chunking has a serious memory issue 
today in that the File Writer API hasn't been agreed to; I think there's 
progress towards eventually adopting a version of the Filesystem API 
(with changes), but that will be a while.  Again, Stream should help - 
and note that the semantics for Blob reception allow it to be written to 
disk as it's received and not held entirely in memory when it's handed 
to the application, which is NOT possible today for application chunking.

> So I still think manual chunking seems like a reasonable thing to 
> support. I don't quite get the congestion control concerns; just 
> because there is a max chunk size doesn't mean the impl can't buffer 
> multiple chunks in bufferedAmount; the app could let that fill up to a 
> degree to avoid having to poll constantly to prevent underrun.

On a slow link this will work if the browser isn't janky.  On a fast 
link GC pauses and other things may cause the buffer to drain out and go 
idle for significant periods.


> On Sun, Mar 2, 2014 at 1:37 AM, Randell Jesup < 
> <>> wrote:
>     On 2/26/2014 2:50 AM, Magnus Westerlund wrote:
>         Thanks Michael,
>         WG, please consider these open issues and try to form your own
>         position
>         on them. They are intended to be discussed at the meeting. If
>         you have
>         proposals on how to resolve them or other input you are very
>         welcome to
>         provide that on the list.
>     One more big issue.  I realize this is very late for pre-meeting
>     discussion; I'd hoped to hash this out earlier but for various
>     reasons (including power outages and my own workload) this didn't
>     happen.
>     We discussed a way to deal with the issues surrounding maximum
>     message sizes at the last IETF.  Right now we have a proposal in
>     the MMUSIC draft for limiting the maximum message size via the SDP.
>     There is a problem with this: it's at odds with the definition of
>     DataChannels in the W3 and with the "duck-typing" of DataChannels
>     to work largely as a superset of WebSockets (outside of channel
>     creation), and the WebAPI folk at Mozilla I talked to don't like
>     the direction we're taking.
>     I've been having talks with the WebAPI people at Mozilla, in
>     particular Jonas Sicking, our WebAPI lead, and they strongly
>     dislike encouraging applications to try to implement their
>     large-data/blob transfer protocols; browsers have considerably
>     more tools available to them to avoid memory hits and to make use
>     of off-main-thread resources than the JS apps do.  "Having
>     Send(blob) fail for any size of blob is crazy and non-sensical"
>     was one comment made when I described the impacts of the current plan.
>     Manual chunking in the application means poorly-implemented
>     congestion control in the app to keep the channel running
>     efficiently (the only feedback available directly is either having
>     the far-end ack at the user level, or trying to estimate sleep
>     times via setTimeout() and bufferedAmount() (which is simply not a
>     great solution), or simply dumping a large amount of smaller
>     transfers into Send() and causing the browser to have to buffer
>     them in memory).  Also GC or other pauses in JS execution may
>     cause hiccups in the transfer and mis-estimation of available
>     bandwidth.  And of course this is being run over a
>     congestion-controlled channel in the first place.
>     Unless and until the W3 side makes DataChannels (and by extension,
>     PeerConnection) APIs available from JS workers (and this is
>     implemented), there will be compromises with packet-level
>     protocols in JS.  One of those will be "it's hard to implement
>     your own congestion control well".  Even with worker support,
>     considerable extension of the APIs would be needed to make it work
>     really well there.  I'll also note that DataChannels-from-worker
>     support is nowhere near implementation in browsers.
>     Another BIG problem as it's currently defined is that there's no
>     lower bound for this limit, so all DataChannel users will need to
>     implement manual chunking even if they use small fixed messages to
>     guarantee spec compliance.  Of course they won't do so...  and
>     even if they did, they wouldn't test it (another big problem).
>      You might say "ok, fine, lets set some small lower bound on this
>     value, say 2 or 4 or 16K".  That doesn't really help much either.
>      Many will send variable-sized messages (because it's easy), and
>     again won't test what happens when the messages trip over the spec
>     limit (or the actual browser implementation limit!)  Those with
>     fixed-size messages larger than the spec lower-bound won't test
>     the against that; they'll test against what Firefox and Chrome
>     implement at the moment.  So the net result is they'll ship
>     applications that can break randomly in the field for no obvious
>     reason (say if IE implements and uses 16K when Chrome used 32K and
>     Firefox used 100MB).
>     Why hand the application a footgun?
>     Jonas Sicking suggested if the IETF insists on not supporting
>     arbitrary Send(blob), we'll need to push in the W3 for a W3
>     protocol run on top of IETF DataChannels that handles
>     fragmentation and reassembly in order to preserve the JS API for
>     Send().  We can do this, but part of the whole partnership between
>     the IETF and W3 on WebRTC was to try to avoid having the W3 define
>     network protocols and keep them in the IETF where they belong.
>     Note: abandoning Send(blob) in W3 doesn't help much, as the
>     comments I made above about arbitrary limits and almost-certain
>     lack of testing of messages violating the negotiated size would
>     still apply.  Send(blob) just makes it easier to trip over the
>     problem (and in fact more likely that the application will test
>     very large sizes).
>     Our options are:
>     A) Just accept this complexity and just hope that people write
>     good code or use good libraries. (See above about testing...)
>     Note: we'd need to set *some* lower bound for the value.
>     B) Make the W3 API implementation add a level of protocol on top
>     of the underlying IETF network protocol. This protocol could then
>     deal with fragmenting messages on the sending side and
>     reassembling them on the receiving side.
>     C) Convince IETF WG to support arbitrarily sized messages at a
>     protocol level, at least in the WebRTC context, similar to
>     WebSockets.
>     -- 
>     Randell Jesup -- rjesup a t mozilla d o t com
>     _______________________________________________
>     rtcweb mailing list
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Randell Jesup -- rjesup a t mozilla d o t com