AV/C command and response data packets are used for asynchronous data block transactions between devices over an IEEE 1394-1995 serial bus network. Extension data fields hold extension data that support new or added features of an IEEE 1394-1995 serial bus network and devices operating therein. The AV/C data packets are encoded for the extension data fields by providing extended fields data within data fields of the Function Control Protocol (FCP) data frames. The extended fields data specifies whether or not the packet includes extension data fields, and if so, how many bytes of extension data may be included within the data field. Extension data supports device and system features such as transaction labels and error/result messages. Transaction data is used to determine the number of other extension data fields to be used during AV/C data transacted between devices and provides for the ability to establish communication protocols based on mutually supported features.