Re: [Bloat] sweet tcp

[Lawrence Stewart <lstewart@room52.net>]

On 07/10/13 05:33, Hannes Frederic Sowa wrote:
> On Tue, Jul 09, 2013 at 12:10:48PM -0700, Eric Dumazet wrote:
>> On Tue, 2013-07-09 at 19:38 +0200, Jaume Barcelo wrote:
>>> Hi,
>>>
>>> I was explaining the bufferbloat problem to some undergrad students
>>> showing them the "Bufferbloat: Dark Buffers in the Internet" paper. I
>>> asked them to find a solution for the problem and someone pointed at
>>> Fig. 1 and said "That's easy. All you have to do is to operate in the
>>> sweet point where the throughput is maximum and the delay is minimum".
>>>
>>> It seemed to me that it was a good idea and I tried to think a way to
>>> force TCP to operate close to the optimal point. The goal is to
>>> increase the congestion window until it is larger than the optimal
>>> one. At that point, start decreasing the congestion window until is
>>> lower than the optimal point.
>>>
>>> To be more specific, TCP would be at any time increasing or decreasing
>>> the congestion window. In other words, it will be moving in one
>>> direction (right or left) along the x axis of Fig. 1 of Getty's paper.
>>> Each RTT, the performance is measured in terms of delay and
>>> throughput. If there is a performance improvement, we keep moving in
>>> the same direction. If there is a performance loss, we change the
>>> direction.
>>>
>>> I tried to explain the algorithm here:
>>> https://github.com/jbarcelo/sweet-tcp-paper/blob/master/document.pdf?raw=true
>>>
>>> I am not an expert on TCP, so I decided to share it with this list to
>>> get some expert opinions.
>>
>> Are you familiar with existing delay based algorithms ?
>>
>> A known one is TCP Vegas.
>>
>> Problem is that it would work well only if all flows would use it.
>>
>> Alas, lot of flows (or non flows traffic) will still use Reno/cubic (or
>> no congestion at all) and they will clamp flows that are willing to
>> reduce delays.
>>
>> So that's definitely not 'easy' ...
> 
> FreeBSD recently imported a new CC algorithm. From the commit msg[0]:
> 
>     Import an implementation of the CAIA Delay-Gradient (CDG) congestion control
>     algorithm, which is based on the 2011 v0.1 patch release and described in the
>     paper "Revisiting TCP Congestion Control using Delay Gradients" by David Hayes
>     and Grenville Armitage. It is implemented as a kernel module compatible with the
>     modular congestion control framework.
>     
>     CDG is a hybrid congestion control algorithm which reacts to both packet loss
>     and inferred queuing delay. It attempts to operate as a delay-based algorithm
>     where possible, but utilises heuristics to detect loss-based TCP cross traffic
>     and will compete effectively as required. CDG is therefore incrementally
>     deployable and suitable for use on shared networks.
>     
>     In collaboration with:	David Hayes <david.hayes at ieee.org> and
>     		Grenville Armitage <garmitage at swin edu au>
>     MFC after:	4 days
>     Sponsored by:	Cisco University Research Program and FreeBSD Foundation
> 
> I had no time to play with it myself, yet.
> 
> [0] http://svnweb.freebsd.org/base/head/sys/netinet/cc/cc_cdg.c?revision=252504&view=markup

FYI, I'm the guy who did the import of CDG into FreeBSD - I work at CAIA
with Grenville and formerly with David who is now at the University of Oslo.

CDG will ship as a loadable kernel module in FreeBSD 9.2-RELEASE due out
early September. If anyone is keen to play with it in the meantime, the
post 20130707 9.1-STABLE, 9.2-PRERELEASE and 10-CURRENT snapshots have
the code. You can grab snapshot ISOs or virtual disk images which
contain CDG from:

ftp://ftp.freebsd.org/pub/FreeBSD/snapshots/amd64/amd64/ISO-IMAGES/9.1/
ftp://ftp.freebsd.org/pub/FreeBSD/snapshots/amd64/amd64/ISO-IMAGES/9.2/
ftp://ftp.freebsd.org/pub/FreeBSD/snapshots/amd64/amd64/ISO-IMAGES/10.0/

ftp://ftp.freebsd.org/pub/FreeBSD/snapshots/VM-IMAGES/20130713/10.0-CURRENT/amd64/
ftp://ftp.freebsd.org/pub/FreeBSD/snapshots/VM-IMAGES/20130713/9.2-PRERELEASE/amd64/

I should draw everyone's attention to the caveat in the BUGS section of
the cc_cdg(4) man page:

    The underlying algorithm and parameter values are still a work
    in progress and may not be optimal for some network scenarios.

i.e. CDG is the focus of active research and development and we have
unreleased algorithm/parameter improvements which will be rolled into
the FreeBSD public code as they pass internal muster.

We would certainly welcome any feedback and experiences from anyone who
plays with it.

Cheers,
Lawrence