Re: [hybi] Call for interest: multiplexing dedicated for WebSocket

Simone Bordet <sbordet@intalio.com> Wed, 22 May 2013 16:50 UTC

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From: Simone Bordet <sbordet@intalio.com>
To: "John A. Tamplin" <jat@jaet.org>
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Subject: Re: [hybi] Call for interest: multiplexing dedicated for WebSocket
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Hi,

On Wed, May 22, 2013 at 6:26 PM, John A. Tamplin <jat@jaet.org> wrote:
> I agree -- without flow control, there is very little benefit, and almost
> nothing that an app couldn't easily do themselves by just adding a simple
> framing layer inside WS messages (perhaps just the first byte is a logical
> channel number).

In SPDY, the flow control had an initial window of 64k. This proved way too
small for uploads/downloads, which resulted in a 3x or more slowdown.
So there needs to be in ws a mechanism to configure this window (which SPDY
has).
Turns out that now every client sends a huge initial window, so downloads
are now fast.
This of course impacts memory, since flow control is about memory, but
pretty much solved the flow control problem.

The next problem that arose was that now a stream/channel can take the
whole bandwidth and squat others channels.
So there is the need for a further flow control mechanism to prioritize
channels, that must be dynamic (i.e. changed during the transmissions of
frames).
SPDY is still discussing this, I am guessing it's also being discussed in
HTTP 2.0.

So "simple mux" looks like an oxymoron to me, and makes me wonder what
useful could be a ws mux solution that does not do dynamic channel
prioritization.
A typical use case Joakim presented to me was to tune video download rate
in a websocket server: a big priority initially to download faster than the
client video player plays, to make some buffer, and then reducing the
priority to a steady state with the video player's speed, or reducing
priority to almost zero if the user pauses the play.

--
Simone Bordet
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