NACK
Also written Generic NACK, a=rtcp-fb:nack, negative acknowledgement
A receiver telling the sender which RTP sequence numbers never arrived, so it can retransmit them.
The receiver notices a gap in sequence numbers and sends an RTCP feedback message naming the missing packet plus a 16-bit bitmask covering the next 16 after it. One message can therefore ask for up to 17 packets.
Whether it helps depends entirely on the clock. A retransmission has to arrive before the decoder needs that frame, so on a 30ms round trip NACK recovers loss almost invisibly, and on a 300ms satellite link it mostly wastes bandwidth restating what the jitter buffer already gave up on. Implementations cap how long they will keep asking and how long they hold packets available to resend.
Audio usually turns it off and leans on a concealment codec instead. Video wants it, because losing one packet of a keyframe ruins every frame that references it until the next one.
Negotiated per payload type with a=rtcp-fb:96 nack. The retransmitted packets normally come back over RTX rather than as duplicates of the original.
See also