netdev - Re: Restricting payload size in do_tcp

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Message-ID: <CAJNbMrxjgwuRopSqJkzXnqWZXwBvKNszMrsu-j9fx8NiA3tYLw@mail.gmail.com>
Date:	Thu, 1 Mar 2012 23:00:28 +0100
From:	Ashwin Rao <ashwin.shirvanthe@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: Restricting payload size in do_tcp_sendpages.

On Thu, Mar 1, 2012 at 10:02 PM, David Miller <davem@...emloft.net> wrote:
> From: Ashwin Rao <ashwin.shirvanthe@...il.com>
> Date: Thu, 1 Mar 2012 16:16:40 +0100
>
>> I would like to perform some experiments where I want TCP to send
>> packets of size less than the MSS even more than an MSS worth of data
>> is available in the TCP buffers.
>
> I think if you are trying to make experimental changes to TCP you
> should be at least skilled enough to implement your ideas fully on
> your own rather than having to ask for a crash course on how the stack
> works.

I am not asking for a crash course on how the stack works. I am trying
to figure out what is wrong with my understanding of the code. I admit
I am not as smart as you guys but then the reason for doing
experiments is because I observed a problem based on the measurement
study that I performed on clusters and home networks. I then worked on
developing a small model to capture the behavior I observed. The
problem I observed has its roots in the synchronization of TCP flows.
This can be addressed by using queuing disciplines like RED but then
it is still a problem in networks that will use off the shelf devices
that use drop-tail queues. The numerical resolutions from the model
predict the behavior that I observed. I am now looking a possible
solutions that can address this issue and I am performing experiments
to see whether my proposed solutions are practically useful or not.
This is because the model cannot capture everything.  What I want to
do is to ensure that the packets arriving in the network do not have
the same size or at least the last packet in a window of packets has a
size smaller than the MSS. This according to the model is enough to
break the synchronization that I observed in the clusters and home
networks. To test this I forcefully set the segment size (and size
goal which controls the size in case of offloading) in the function
do_tcp_sendpages. According to my understanding this is the last point
before which the tcp packet is created before being sent to the lower
layer.

> Frankly, if you aren't skilled enough to figure these things out on
> your own I really don't think you should be even suggesting changes
> to a complicated protocol that effects the entire world.

I may not be skilled enough like you guys but then I am not afraid to
admit I do not know the entire picture. But if there is a problem that
I see and a solution in which I believe in then I will not feel shy of
validating my solution.

Ashwin
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