The BRTP (Barix RTP) was designed by Barix AG (http://www.barix.com) as a simple extension to the RTP protocol to support the following:
Allow a client to register at a server to subscribe to a RTP stream
Detect client dropout quickly and reliably to prevent loading of the network with a stream that is actually not consumed
To provide a primitive plaintext password-based access control
To penetrate routers and firewalls employing Network Address Translation (NAT)
BRTP is RTP with subscription request packets added. The subscription request packet is an UDP datagram which contains only a keyword. If the receiver requires no access control then any UDP packet suffices.
The subscription request packets are sent periodically by the client to the server. If subscription is not renewed within 30 seconds, then the client is removed from the target table for the RTP stream. The RTP stream is sent to the return address of the BRTP packets. The BRTP packet on it's way out penetrates a NAT (for NATs where it is possible) and opens a back channel for the RTP. This way RTP can be received even on clients which don't have public IP addresses.