A header processing engine for a network interface device having a buffer for holding one or more data packets each having one or more headers, the header processing engine comprising: a command memory; a header recognizer configured to parse the headers of a data packet stored at the buffer so as to identify the type and position of each header in the data packet; a constructor unit having read access to the headers of the data packet; and a processor including an execution pipeline; the header recognizer being further configured to, for each header: (a) select in dependence on the type of the header one or more commands stored at the command memory; and (b) form one or more messages for the constructor unit identifying the selected commands and the position of the header in the data packet; the commands selected for the headers of the data packet being collectively such as to, if executed by the constructor unit, cause the constructor unit to generate a data structure which is such as to be operable to cause the processor to effect processing of the headers of the data packet without accessing the data packet at the buffer; and the constructor unit being configured to receive the messages and execute the commands identified therein.