Re: Naive question on multiple TCP/IP channels

Donald Eastlake <d3e3e3@gmail.com> Wed, 04 February 2015 19:51 UTC

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From: Donald Eastlake <d3e3e3@gmail.com>
Date: Wed, 04 Feb 2015 14:50:43 -0500
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Subject: Re: Naive question on multiple TCP/IP channels
To: Phillip Hallam-Baker <phill@hallambaker.com>
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SCTP

Thanks,
Donald
=============================
 Donald E. Eastlake 3rd   +1-508-333-2270 (cell)
 155 Beaver Street, Milford, MA 01757 USA
 d3e3e3@gmail.com


On Wed, Feb 4, 2015 at 2:22 PM, Phillip Hallam-Baker
<phill@hallambaker.com> wrote:
> Today most Web browsers attempt to optimize download of images etc. by
> opening multiple TCP/IP streams at the same time. This is actually done for
> two reasons, first to reduce load times and second to allow the browser to
> optimize page layout by getting image sizes etc up front.
>
> This approach first appeared round about 1994. I am not sure whether anyone
> actually did a study to see if multiple TCP/IP streams are faster than one
> but the approach has certainly stuck.
>
> But looking at the problem from the perspective of the network it is really
> hard to see why setting up five TCP/IP streams between the same endpoints
> should provide any more bandwidth than one. If the narrow waist is observed,
> then the only parts of the Internet that are taking note of the TCP part of
> the packet are the end points. So having five streams should not provide any
> more bandwidth than one unless the bandwidth bottleneck was at one or other
> endpoint.
>
> Now there are some parts of the deployed Internet that do actually perform
> statefull inspection. But I would expect increasing the number of channels
> to degrade performance at a firewall or any other middle boxen.
>
> So we have a set of behavior that seems at odd with the theory. Has anyone
> done any experiments recently that would show which is right?
>
>
> The reason it makes a difference is that it is becoming clear that modern
> applications are not best served by an application API that is limited to
> one bi-directional stream. There are two possible ways to fix this
> situation. The first is to build something on top of TCP/IP the second is to
> replace single stream TCP with multi-stream.
>
> My preference and gut instinct is that the first is the proper architectural
> way to go regardless of the performance benefits. When Thompson and co were
> arguing that all files are flat sequences of bits, they were saying that was
> the right O/S abstraction because you could build anything you like on top.
>
> But then I started to ask what the performance benefits to a multi-stream
> TCP might be and I am pretty sure there should not be any. But the actual
> Internet does not always behave like it appears it should.
>
> I suspect that the preference for multiple streams probably comes from the
> threading strategies it permits. But that is an argument about where the
> boundary between the kernel and application is placed in the network stack
> rather than where multiplex should live in the stack. Microsoft already
> provides a network stack for .NET where the boundary is in the HTTP layer
> after all.
>
>
> So anyone got hard data they could share?