Hello,
The resistor Rf is necessary to bias the oscillator into the linear operating region, required during start-up of the oscillator. The value is usually non-critical, somewhere between 1M and 10M.
The resistor Rs is sometimes required to limit the drive applied to the crystal, particularly with low frequency (32kHz) crystals, with a value of typically 47k or 100k. It is not usually required for higher frequency crystals, and should be a short circuit in these cases.
Regards,
Mac