NTP (Network Time Protocol) gives a common, absolute wall-clock timestamp that independent media streams can be referenced against. In RTP systems it isn't used so much to set the machine's clock as to provide the shared timeline inside RTCP sender reports: each report maps a stream's media-clock RTP timestamp to a 64-bit NTP-format time, and the receiver uses those mappings to align audio and video that arrived on separate, unsynchronized RTP clocks. The NTP timestamp's job here is purely relative — a common reference both streams point at — so absolute clock accuracy matters less than sender and reports agreeing on the same time base for synchronization.
