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

From: <bizzbyster@gmail.com>
Date: Mon, 30 Jun 2014 14:18:53 -0400
Cc: Mike Belshe <mike@belshe.com>, HTTP Working Group <ietf-http-wg@w3.org>
Message-Id: <19909481-EDCC-481C-8136-34DE432597EF@gmail.com>
To: Patrick McManus <mcmanus@ducksong.com>
I see the same problem occurring for Firefox in tests run from this Sydney location. I have trouble seeing how that can be due to WPT since it is just driving Firefox. Will's idea that the network is stripping the window scale option seems the most likely here. And in that case I do think it is an infrastructure thing that we should cope with.

Peter

On Jun 30, 2014, at 12:58 PM, Patrick McManus <mcmanus@ducksong.com> wrote:

> 
> 
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> 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.
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> 
> 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.
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> but maybe my info is dated.
> 
> -P
> 
> 


Received on Monday, 30 June 2014 18:19:20 UTC

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