This document defines a simple enhancement to support RTP sessions with forward-shifted redundant encodings@ i.e.@ redundant data sent before the corresponding primary data. Forward-shifted redundancy can be used to conceal losses of a large number of consecutive media frames (e.g.@ consecutive loss of seconds or even tens of seconds of media).
RFC 6354-2011 history
2011RFC 6354-2011 Forward-Shifted RTP Redundancy Payload Support