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Re: why not multiple, short-lived HTTP/2 connections?

From: Patrick McManus <mcmanus@ducksong.com>
Date: Mon, 30 Jun 2014 12:58:23 -0400
Message-ID: <CAOdDvNpwAa+ePsKdcxovy7aY5YQfnBJw7wKixsV9aEzBc_qjBg@mail.gmail.com>
To: Peter Lepeska <bizzbyster@gmail.com>
Cc: Mike Belshe <mike@belshe.com>, HTTP Working Group <ietf-http-wg@w3.org>
On Mon, Jun 30, 2014 at 12:04 PM, <bizzbyster@gmail.com> wrote:

> All,
>
> Another huge issue is that for some reason I still see many TCP
> connections that do not advertise support for window scaling in the SYN
> packet. I'm really not sure why this is but for instance WPT test instances
> are running Windows 7 and yet they do not advertise window scaling and so
> TCP connections max out at a send window of 64 KB. I've seen this in tests
> run out of multiple different WPT test locations.
>
> The impact of this is that high latency connections max out at very low
> throughputs. Here's an example (with tcpdump output so you can examine the
> TCP flow on the wire) where I download data from a SPDY-enabled web server
> in Virginia from a WPT test instance running in Sydney:
> http://www.webpagetest.org/result/140629_XG_1JC/1/details/. Average
> throughput is not even 3 Mbps despite the fact that I chose a 20 Mbps FIOS
> connection for my test. Note that when I disable SPDY on this web server, I
> render the page almost twice as fast because I am using multiple
> connections and therefore overcoming the per connection throughput
> limitation: http://www.webpagetest.org/result/140629_YB_1K5/1/details/.
>
> I don't know the root cause (Windows 7 definitely sends windows scaling
> option in SYN in other tests) and have sent a note to the webpagetest.org
> admin but in general there are reasons why even Windows 7 machines
> sometimes appear to not use Windows scaling, causing single connection SPDY
> to perform really badly even beyond the slow start phase.
>
>
I think this is a WPT issue you should take up offlist because , IIRC, the
issue would just be in the application. its not a OS or infrastructure
thing we'll need to cope with.

 IIRC when I last looked at it if you used an explicit SO_RCVBUF on your
socket before opening on win 7 it would set the scaling factor to the
smallest factor that was able to accommodate your desired window. (so if
you set it to 64KB or less, scaling would be disabled). Of course there is
no way to renegotiate scaling, so that sticks with you for the life of the
connection no matter what you might set RCVBUF to along the way. I believe
the correct fix is "don't do that" and any new protocol implementation
should be able to take that into consideration.

but maybe my info is dated.

-P
Received on Monday, 30 June 2014 16:58:49 UTC

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