The Real-time Transport Protocol, RTP [1], provides quality of service
feedback with reception reports sent alongside the media stream. If the
media is sent via IP multicast it is possible for a third party to snoop 
on these reception reports, displaying reception quality for all members
of a group. The RQM application performs such snooping.

WWW: http://www-mice.cs.ucl.ac.uk/multimedia/software/rqm/