Taken from here:
http://linux.die.net/man/8/iptables
The TCP section, –mss, or the tcpmss, can control the size of the packet in each connection. So if a host is streaming some thing, that would be one constant connection from a remote host, and can be contained to a certain spacket size, i.e
Src: “Streaming Server” dst: Internal Host: –mss, state no more than (FW rule) x-size of packet for this conection, if too large, then DROP, or in the QoS section, if large than this, it gets this bandwidth, etc.