RED stands for REDundant coding and it is a RTP payload format defined in RFC 2198 for encoding redundant audio or video data.

The primary motivation of sending redundant data is to be able to recover packets lost under lossy network conditions. If a packet is lost then the missing information may be reconstructed at the receiver from the redundant data that arrives in the following packet(s).

REDundancy encoding

The use of RED is negotiated in the SDP as an additional payload type and when used the audio/video RTP packets are packaged using RED format with a 6 bytes header, before the primary and secondary payloads conveying the actual audio/video packet and the redundant information.

An example of a redundant information schema is ULPFEC.

See also FEC.

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