The invention relates to a technique for controlling establishment of a tandem free operation (TFO) for transfer of TFO frames including content frames between a first and a second transcoder unit without transcoding of the content frames. A method aspect of the technique comprises the steps of transmitting, in case a remotely used codec does not meet a codec compatibility condition, a first transparent mode message indicating to in-path equipment a transparent mode for subsequent transparent TFO negotiation, and transmitting, in the transparent mode, a TFO configuration frame indicating a local TFO codec list.