custom phy rx_clkout no signal

Hi,I try to trun off the "Enable rate match FIFO",and the 'rx_clkout' signal present,Why can't trun on it? .My application like that,C2C(chip-to-chip) transmission between two chips,the source is TX only and sink is RX only.The chips have different local clock which in the different board.Without the Rate match FIFO,how can I ensure that the FIFO doesnot underflow or overflow due to per million(ppm) difference between the clocks?Waiting for your reply,thanks again.