RRTR stands for RTCP Receiver Reference Time Report. It is defined in section 4.4 of RFC 3611
and can, together with the “delay since last receiver report block” (DLRR) be used to calculate the round-trip time for receive-only streams similar to the sender report (SR) round-trip time calculation defined in RFC 3550.

RRTR is supported by WebRTC and can be enabled on a media server for the purpose of providing accurate incoming round trip time values.