I have found the wiki entry for QoS to mark the packets with “EF” such that my ISP prioritizes the voice traffic accordingly: https://wiki.freeswitch.org/wiki/QoS

However, we are using Shorewall (Shoreline Firewall) to manage IPTables on our CentOS 5.x server, and frankly this entry on Packet Marking in Shorewall is confusing: http://shorewall.net/PacketMarking.html

Has anybody managed to implement this (from the FS QOS page) in Shorewall? :

# Mark RTP packets with EF:

iptables -t mangle -A OUTPUT -p udp -m udp --sport 16384:32768 -j DSCP --set-dscp-class ef

Thanks,
Jason