According to one embodiment, transporting Optical Data Units in an Optical Transport Network comprises receiving a first frame of a first type that includes second frames of a second type. Each second frame is associated with a signal having a client clock rate, and at least two second frames are associated with at least two signals with different client clock rates. The following is repeated for each second frame: determining a client clock rate of the signal associated with the second frame; and inserting clock justification information into the second frame. The clock justification information indicates how to justify the client clock rate of the second frame to synchronize the client clock rate with a system reference clock rate.