RTP defines a standardized packet format for delivering audio and video over the Internet. It was originally designed as a multicast protocol, but has since been applied in many unicast applications. It's frequently used in streaming media systems (in conjunction with RTSP) as well as videoconferencing systems (in conjunction with H.323 or SIP), making it the technical foundation of the Voice over IP industry.
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RTP is also a common initialism for Research Triangle Park, North Carolina.