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Re: Size of window variables

From: Willy Tarreau <w@1wt.eu>
Date: Thu, 3 Jul 2014 21:34:35 +0200
To: Poul-Henning Kamp <phk@phk.freebsd.dk>
Cc: Daniel Sommermann <dcsommer@fb.com>, Martin Thomson <martin.thomson@gmail.com>, Greg Wilkins <gregw@intalio.com>, HTTP Working Group <ietf-http-wg@w3.org>
Message-ID: <20140703193435.GB18168@1wt.eu>
On Thu, Jul 03, 2014 at 06:59:57PM +0000, Poul-Henning Kamp wrote:
> In message <53B5A00F.9030705@fb.com>, Daniel Sommermann writes:
> >6.9.1 has it:
> >
> >A sender MUST NOT allow a flow control window to exceed 2^31 - 1 bytes. 
> >If a sender receives a WINDOW_UPDATE that causes a flow control window 
> >to exceed this maximum it MUST terminate either the stream or the 
> >connection, as appropriate.
> I suggest we raise that limit to 2^63-1 bytes.
> 2^32 bytes only take a few seconds seconds at 100 Gbit, and files
> larger than 2^32 are common as muck these days.

Well, 2^31 should be about 160ms at 100Gbps, but you can only achieve
such rates over TCP on very short links because TCP itself will not
allow you to grow a window that large, so in practice you'll need some
round trips at least for TCP acks. So in practice there will be more
than one TCP connection at these rates when observed out of the
datacenter, thus I don't think there's any gain in mandating a window
larger than 31 or 32 bits.

Received on Thursday, 3 July 2014 19:35:05 UTC

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