1. An apparatus, comprising: an optical transceiver including a rate-adaptive forward error correction (FEC) encoder and a rate-adaptive FEC decoder, the rate-adaptive FEC encoder configured to adjust a number of a plurality of known symbols associated with a codeword to achieve rate adaption, a length of the codeword being fixed, the rate-adaptive FEC encoder configured to generate the codeword based on (1) a plurality of information symbols including the plurality of known symbols and a plurality of data symbols, and (2) a fixed number of a plurality of parity symbols, the rate-adaptive FEC encoder configured to remove the plurality of known symbols from the codeword to produce a channel word; the rate-adaptive FEC encoder configured to send the channel word and the number of the plurality of known symbols in response to (1) each data symbol from the plurality of data symbols being sent or (2) an FEC overhead of the rate-adaptive FEC encoder being changed.