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Re: QUIC use cases

From: Harald Alvestrand <harald@alvestrand.no>
Date: Mon, 15 Jan 2018 23:37:45 +0100
To: Randell Jesup <randell-ietf@jesup.org>, public-webrtc@w3.org
Message-ID: <cd6aa6fc-49a6-42f8-ed39-681d205173ae@alvestrand.no>
Den 15. jan. 2018 20:52, skrev Randell Jesup:
> On 1/15/2018 9:13 AM, Lennart Grahl wrote:
>> On 15.01.2018 06:03, Bernard Aboba wrote:
>>> 1. Competition between SCTP data channels and audio/video, due to building of queues.  Currently, 
>>> data channel implementations use the default SCTP congestion control  (loss-based, defined in RFC 4960),
>>> and don't expose a way of selecting an alternative algorithm.  One developer I talked to expressed an
>>> interest in being able to use an algorithm like LEDBAT for a background file transfer, so I'm not clear
>>> that a new cc algorithm needs to be standardized in IETF.
>> I'm passing this on to Michael: Would it be feasible to do this with the
>> existing API of usrsctp?
> 
> This was expected - we had a number of discussions about how we could
> split up bandwidth between media and data; limiting data rates to
> something proportional to the media rates (to avoid data overwhelming
> media).  There are SCTP congestion algorithms (not necessarily in
> usrsctp today) that are delay-based, per my memory of some of the early
> proposals I made.
> One issue would be what the relative priority/split "should" be, but the
> browser can certainly make a default decision.  The assumption was that
> we'd do "something" here; either a forced split or a compatible CC
> algorithm, or some variation on the shared-CC proposals.
> 
> If you don't want to give (say) 1/2 the bandwidth to data during a
> transfer, we'd need an API point anyways (QUIC or no QUIC), though maybe
> you could add more semantics onto priorities I suppose.

We do have a very coarse API point in the "priority" setting, which we
defined to say "if it's 1 step higher in priority, it should get double
the bandwidth when there's congestion".


> 
>>> 2. Complexity of doing large file transfers on top of RTCDataChannel message implementations. 
>>> You've mentioned a number of the issues with this, and most of them seem solvable by fixing issues
>>> in the existing specification, and perhaps adding some tests to make sure that implementations conform
>>> to the new guidance.
>> Indeed, I believe one of the most pressing issues is the impact of
>> userland fragmentation/reassembly. 
> 
> Yeah... the original plan was for WebSocket-sized transfers for files,
> with fragmentation/reassembly in the DC impl (PPID-fragmentation). 
> Application-level chunking forces a memory spike (and spike in
> processing time) when combining data for delivery or writing to disk. 
> Direct large-Blob transfer allows/allowed the impl to append the data to
> a temporary disk file as it was received, if it wanted to, or move it
> there after some threshold.

Yes, that's what Streams (the W3C API) is good for.

> 
> Note this isn't a protocol issue really; it's an API issue.  The
> mechanism that the bytes use to cross the wire don't matter for this.
> 
> -- 
> Randell Jesup -- rjesup a t mozilla d o t com
> Please please please don't email randell-ietf@jesup.org!  Way too much spam
> 
Received on Monday, 15 January 2018 22:38:12 UTC

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