RTP: Real-Time Transport Protocol
A protocol for 'unreliable' delivery of Real Time data. Layered on top of UDP. Used a lot for streaming of audio and video often controlled by an RTSP session. Can make use of multicast because it uses UDP. Closely tied to the RTCP protocol.
VLC has built-in support for RTP as a server (streaming output). As a client, VLC uses the LiveMedia library.