FEC stands for Forward Error Correction.
WebRTC uses IP networks to transport its data. IP networks are prone to errors causing packet loss. This means that media data sent by one WebRTC client to another might not be received on the other end due to congestion or network corruption.
FEC is a mechanism in which media packets are duplicated and sent multiple times across the network without any delay. This means that even if some of the packets aren’t received, the media stream can still be parsed and decoded properly. This assumes that the packets that got lost during transit have been sent multiple times.
FEC exist in WebRTC as part of the implementation of the Opus voice codec.
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