class output_packet : public tbb::filter {
    ostream& out_file;
public:
    output_packet (ofstream& file) : out_file (file), filter (true) {}
    void* operator() (void* item) {
        packet_trace* packet = static_cast<packet_trace*> (item);
        out_file << *packet;
        delete packet;
        return NULL;
    }
};