RID denotes the RTP Stream ID in RTP.

It is defined and used as a header extension in RFC 8852 section 3.1.

RID, along with MID and RRID, are used to associate between low-level RTP concepts like synchronization source (SSRC) and higher-level WebRTC objects such as RtpSender and RtpReceiver.

These identifiers are important to be able to demultiplex RTP bundling, a common mechanism used in WebRTC sessions.

RID is used specifically to distinguish between different simulcast streams of the same video source.

When RTP packets are received, WebRTC needs to decide to which object/stream to associate the incoming packet. This is done using roughly the following decision diagram:

All header extensions used by MID, RID and RRID have the same basic structure and contain an ASCII string with the MID/RID/RRID value in it.