code: function(hdr) {
                      //The content of this header doesn’t affect us
                      //since all it does is tell us details of how
                      //the sender will ZDLE-encode binary data. Our
                      //ZDLE parser doesn’t need to know in advance.
  
                      sess._next_subpacket_handler = function(spkt) {
                          sess._next_subpacket_handler = null;
                          sess._consume_ZSINIT_data(spkt);
                          sess._send_header('ZACK');
                          sess._next_header_handler = between_files_handler;
                      };
                  }
docs: We use this as a keep-alive. Maybe other implementations do, too?