Tos in IP header

[Marc REYNES <marc.reynes@tuxfamily.org>]

hello,

	i'm looking for a method to evaluate the gain in performance
involved by the sets of TOS bits.

I'm using ferm to build my firewall the rules are : 

chain output policy ACCEPT {
    proto tcp if $WAN_IF goto tosqueue
}

chain tosqueue
{
    # reverse rules to match any connection
    protocol tcp reverse ACCEPT
    {
	# set tos bits according to traffic type
        dport (ssh,ftp) settos min-delay;
        dport (smtp,pop3,domain,8080) settos max-reliability;
        dport (ftp-data) settos max-throughput;
    }
}

I've some extra question : 
 - this exemple is from the official ferm's homepage, the author prevent from
using ToS for UDP, can someone explain me why ? 
 - why it is recommand to set only one bit ? (from ipchains man)


thank you, sorry for my poor english i'm french student