An apparatus for processing digital service signals includes a receiving module configured to receive contents having watermarks, where the watermarks are inserted in audio or video frames of the contents, the watermarks carry channel information related to the contents, and content information having URL information for additional data of the contents. An extracting module is configured to extract the watermarks from the audio or video frames, and an obtaining module is configured to obtain the additional data over an IP(Internet Protocol) network using the extracted watermarks.