The RTP timestamp is a media-clock value in the header of every RTP packet, marking the sampling instant of the data it carries. It is not wall-clock time and it runs on a per-stream clock — typically 48 kHz for Opus audio — so audio and video in a call advance on independent RTP timelines that can't be compared directly. The bridge is the RTCP sender report, which periodically pairs a stream's RTP timestamp with an absolute NTP wall-clock time; the receiver uses those mappings to place both streams on one true timeline and achieve lip-sync. RTP timestamps also let the jitter buffer order packets and measure jitter, independent of how they arrived.
