A data processing apparatus is provided having a hierarchy of layers comprising at least two data processing layers, each data processing layer configured to receive data and to generate processed data for passing to a next lower layer in said hierarchy, according to a protocol specific to that data processing layer. Each data processing layer is configured intermittently to add synchronization information to its processed data, the synchronization information providing semantic information required to interpret the processed data. Each data processing layer is further configured to output its synchronization information in response to a synchronization request signal received from a lower layer in said hierarchy, and at least one data processing layer is configured, when outputting its synchronization information, to issue its synchronization request signal to a higher layer in the hierarchy.