Guaranteeing bandwidth through netfilter-based router?

[Rick Mann <rmann@xxxxxxxxxxxxxxx>]

Hi. I have a netfilter-based router set up on Fedora Core 2 that's 
working very well. However, one of the things I run behind it is a VoIP 
box, and sometimes voice quality suffers if other hosts are consuming 
bandwidth.

Is there a way to give priority to UDP traffic to/from a particular 
host, without permanently reserving bandwidth for it?

TIA,

--
Rick