RTX stands for Retransmission.
In WebRTC, RTX is the mechanism for retransmitting lost RTP packets. When the receiver detects a missing packet (via sequence number gaps), it sends a NACK request, and the sender responds by retransmitting the packet using the RTX format.
How RTX works
RTX uses a separate SSRC from the original media stream. The association between the original SSRC and the RTX SSRC is declared in the SDP using the FID ssrc-group. The retransmitted packet includes the original sequence number in its payload, allowing the receiver to insert it into the correct position.
RTX vs other packet loss recovery
- RTX (retransmission): Effective when RTT is low enough to retransmit before playback deadline. Adds one RTT of delay
- FEC: Proactive redundancy with zero added delay but uses extra bandwidth
- PLC: Receiver concealment with no overhead but lower quality
RTX is primarily used for video in WebRTC. Audio packet loss is usually handled by Opus's built-in PLC and FEC since audio frames are typically too time-sensitive for retransmission.
