Re: [LARTC] TCP Rate Control

[Stef Coene <stef.coene@docum.org>]

> > But I'm still intested in the comparision :)
>
> It's you quest for the truth that endears you to all of us. :)
>
> Googling for "tcp rate control" would lead you to a lot of material,
> including documents "proving" queuing is better than rate control.
>
> Here's the test jig I plan to setup...
>
> Windows client running Gozilla, for crude throughput reporting
>
> |(1)
>
> ->"Gateway" m/c with two NICs [RTL8139 based] running nisnet and tbf,
> iptraf and MRTG--
>
>                  |(2)
>
>                  ->Linux based FTP/HTTP Server
>
> I will try with sustained downloads, perhaps 1GB. The tbf would be set
> to limit at 100kbps.
>
>  From what I expect, I would only get 100kbps on the windows client m/c,
> i.e. at point (1). But between the gateway and server, i.e. point (2),
> the bandwidth consumption would be higher. How much higher I don't know
> - it might be anything from 100kbps to NIC speed. If it is within a 5%
> tolerance, i.e. from 95kbps to 100kbps, I would say that the world is a
> great place and I am happy to be here...i.e. queuing would have achieved
> what I want.
>
> But, I suspect the difference would be much more...
Why?  What will happen with the packets that are arrived too much?  Do you 
really think they all will be lost?  
I'm no tcp specialist, but the traffic at 2 will be exactly 100kbps.  I see it 
like this : client starts download and goes faster and faster.  When it 
reaches 100kbps, all tokens in the bucket are used.  After that, the gateway 
starts delaying packets.  From the tbf manpage :

If no tokens are available, packets are queued, up to a configured limit

Packets are not dropped but delayed.  At that time, the server noticed that 
the packets are not arriving at a speed > 100kbps (they are queued in the 
bucket) at the client and throttle downs.  When the server throttles down 
below 100kbps, the queued packets can be send again because there are tokens 
available in the bucket.  So after some time it will be stable at 100kbps.

Again, I don't know how tbf works.  This is just the way I think it works.

Stef

-- 

stef.coene@docum.org
 "Using Linux as bandwidth manager"
     http://www.docum.org/
     #lartc @ irc.oftc.net

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