RTX
Also written retransmission stream, apt, rtx/90000
Sending retransmissions as their own stream, with their own SSRC and payload type, instead of repeating the original packet.
The naive way to answer a NACK is to send the original packet again, byte for byte. That breaks anything downstream that assumes sequence numbers only ever move forward, and it makes loss statistics meaningless, because a retransmission is indistinguishable from the packet it replaces.
RTX gives retransmissions their own stream. A separate SSRC, a separate payload type declared as a=rtpmap:97 rtx/90000 and bound to the original with a=fmtp:97 apt=96, and its own monotonic sequence numbering. The original packet’s sequence number is preserved inside the payload, so the receiver can put it back where it belongs.
The bookkeeping stays honest as a result. Loss on the primary stream still reads as loss, and you can measure how much of your bandwidth is going on repair work rather than on new media.
See also