Re: Naive question on multiple TCP/IP channels

Phillip Hallam-Baker <phill@hallambaker.com> Thu, 05 February 2015 14:30 UTC

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Subject: Re: Naive question on multiple TCP/IP channels
From: Phillip Hallam-Baker <phill@hallambaker.com>
To: Brian E Carpenter <brian.e.carpenter@gmail.com>
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On Wed, Feb 4, 2015 at 6:54 PM, Brian E Carpenter <
brian.e.carpenter@gmail.com> wrote:

> On 05/02/2015 08:49, Eggert, Lars wrote:
> > Hi,
> >
> > CC'ing tsvwg, which would be a better venue for this discussion.
> >
> > On 2015-2-4, at 20:22, 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.
> >
> > There have been many studies; for example,
> http://www.aqualab.cs.northwestern.edu/publications/106-modeling-and-taming-parallel-tcp-on-the-wide-area-network
>
> GridFTP only exists because of lots of experience that several parallel FTP
> streams achieve better throughput than a single stream, especially on paths
> with a high bandwidth-delay product. I'm guessing that since buffer bloat
> creates an artificially high BDP, that could apply pretty much anywhere.
>
> SCTP is not the only one-acronym answer: try MPTCP. The interesting thing
> there
> is that because there is explicit coupling between the streams, the
> throughput
> increases sub-linearly with the number of streams.


Buffer bloat is a great example of the consequences of the co-operative
nature
of the Internet for one of the audiences I am writing for (Folk trying to
make/understand Title II regulations).

The Internet is actually a demonstration that the commons are not such a
tragedy
after all.


 If we look at the problem of buffer bloat there are several possible
solutions
and the problem is picking one:

* Persuade manufacturers to reduce buffer sizes so the old congestion
algorithms work.

* Change the congestion algorithm.

* Break with the pure end to end principle and have the middleboxen with
the
huge buffers do some reporting back when they start to fill.


The first is difficult unless you get control of the benchmarking suites
that are
going to be used to measure performance. Which would probably mean getting
the likes of nVidia or Steam or some of the gaming companies on board.

The second is certainly possible. There is no reason that we all have to
use the
same congestion algorithm. In fact a mixture might be beneficial. Instead
of looking
for packets being dropped, the sender could look at the latency and the
difference
between the rate packets are being sent and the rate at which
acknowledgements
are being received.

Whether folk are willing to consider the third depends on how serious the
buffer bloat problem is considered to be. If the stability of the Internet
really
is at stake then everything should be on the table. But my gut instinct is
that
a middlebox does not actually have any more useful information available
to it than can be derived from the acknowledgements.