This might help:

16.2. How can I prioritize ACK packets with m0n0wall?

http://doc.m0n0.ch/handbook/faq-ackprio.html

Bart Dubelaar wrote:
> Hello  everyone,
>
> I've set up a wireless hotspot with m0n0wall as a captive portal. I'm 
> using the latest beta version (1.3b2), which is really great as you 
> can run it from a USB stick. The system is up and running for 3 days 
> now and it's working great!
> One thing that I'm not sure about is if I should prevent excessive use 
> of bandwidth by users (p2p) in any way. The connection to the internet 
> is by means of ADSL. One of the nasty things of ADSL is that when the 
> upload channel get's maxed out the download channel stops working 
> also. So to prevent this I could blocks ports used by p2p programs, 
> but I don't like to limit the usability. The other thing is could do 
> is limit bandwidth usage. In the Captive Portal settings page one can 
> define a maximum bandwidth availability per user. The thing is that 
> all the users share one single captive portal account. So now finally 
> coming to my actual question: Is the bandwidth availability that I can 
> set in the captive portal page linked to one user account or is it 
> applied to every physical user that connects regardless of the account 
> that is used?
> For example, if I limit the bandwidth to 100kb and 2 users with 
> account ' guest' connect, will they both get the 100kb at their 
> disposition or will they share it?
>
> Regards,
>
> Bart Dubelaar
>
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