Date:	Tue, 13 Feb 2007 19:42:16 +0200
From:	Baruch Even <baruch@...en.org>
To:	SANGTAE HA <sangtae.ha@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	shemminger@...ux-foundation.org, netdev@...r.kernel.org
Subject: Re: [patch 3/3] tcp: remove experimental variants from default list

* SANGTAE HA <sangtae.ha@...il.com> [070213 18:50]:
> Hi Baruch,
> 
> I would like to add some comments on your argument.
> 
> On 2/13/07, Baruch Even <baruch@...en.org> wrote:
> >* David Miller <davem@...emloft.net> [070213 00:53]:
> >> From: Baruch Even <baruch@...en.org>
> >> Date: Tue, 13 Feb 2007 00:12:41 +0200
> >>
> >> > The problem is that you actually put a mostly untested algorithm as the
> >> > default for everyone to use. The BIC example is important, it was the
> >> > default algorithm for a long while and had implementation bugs that no
> >> > one cared for.
> >>
> >> And if our TCP Reno implementation had some bugs, what should
> >> we change the default to?  This is just idiotic logic.
> >>
> >> These kinds of comments are just wanking, and lead to nowhere,
> >> so please kill the noise.
> >>
> >> If we have bugs in a particular algorithm, we should just fix
> >> them.
> >
> >I hope you've finished attempting to insult me. But I hope it won't
> >prevent you from getting back to the topic. The above quote of me was a
> >prelude to show the repeat behaviour where bic was added without
> >testing, modified by Stephen and made default with no serious testing of
> >what was put in the kernel.
> 
> 
> What kind of serious testing you want to? I've been testing all
> highspeed protocols including BIC and CUBIC for two and half years
> now. Even Stephen didn't test CUBIC algorithm by himself, he might see
> the results from our experimental studies. I don't care what algorithm
> is default in kernel, however, it is not appropriate to get back to
> Reno. As Windows decided to go with "Compound TCP", why we want to
> back to 80's algorithm?

I fail to see how Microsoft should be the reason for anything, if
anything Linux started the arms race.

> >It seems this happens again no with cubic. And you failed to respond to
> >this issue.
> >
> >> > The behaviour of cubic wasn't properly verified as the
> >> > algorithm in the linux kernel is not the one that was actually proposed
> >> > and you intend to make it the default without sufficient testing, that
> >> > seems to me to be quite unreasonable.
> >
> >According to claims of Doug Leith the cubic algorithm that is in the
> >kernel is different from what was proposed and tested. That's an
> >important issue which is deflected by personal attacks.
> 
> Did you read that paper?
> http://wil.cs.caltech.edu/pfldnet2007/paper/CUBIC_analysis.pdf
> Then, please read the rebuttal for that paper.
> http://www.csc.ncsu.edu/faculty/rhee/Rebuttal-LSM-new.pdf
> 
> Also, the implementation can be different. The cubic code inside of
> current kernel introduces faster calculation of cubic root. Even
> though we had some bugs on CUBIC implementation, it is fixed now.

We have seen before with bic that the different implementation meant
that things didnt work as expected. I wouldn't like it to happen again.

> 
> >> My main gripe is that there is a run to make an untested algorithm the
> >default for all Linux installations. And saying that I should test it is
> >not an escape route, if it's untested it shouldn't be made the default
> >algorithm.
> 
> What is criteria for "untested"?  Who judges that this algorithm is
> fully tested and is ready to use?

Did you do your tests on 2.6.20? Did you verify that the algorithm
actually behaves as it should? I don't think anyone did any real tests
on the cubic version in the kernel and I fear a repeat of the bic issue.
Code that is untested is likely not to work and as far as I understand
it you didn't test the current kernel version but rather your own code
on an ancient kernel.

I'd be happy to be proven wrong and shown tests of cubic in the latest
kernel. Saying that I shuold do it myself if concerned is the wrong way
to go. I no longer have access to test equipment to do that and we
should not make an algorithm the default without sufficient testing.

> We still do testing with latest kernel version on production
> networks(4ms, 6ms, 9ms, 45ms, and 200ms). I will post the results when
> those are ready.

That would be an important step indeed.

> >My skimming of the PFLDNet 2007 proceedings showed only the works by
> >Injong and Doug on Cubic and Injong tested some version on Linux
> >2.6.13(!) which might noe be the version in the current tree. Doug shows
> >some weaknesses of the Cubic algorithm as implemented in Linux.
> 
> As I mentioned, please read the paper and rebuttal carefully. Also, in

I'll do that later on, but quick reading shows web traffic with a
minimum of 137KB, which doesn't seem to be very realistic. But I need to
read deeper to see what goes there.

> >Do you still think that making Cubic the default is a good idea?
> 
> Then, what do you want to make a default? You want to get back to BIC? or Reno?

I don't claim to have all the right answers, I would prefer to go to
Reno and I don't buy the argument of DaveM that we have to use some
fancy high speed algorithm, even if we do go for one I'd prefer a safer
choice like HSTCP.

Baruch
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