While not necessary for supporting IP and ARP, all implementations are required to support IEEE 802.2 standard Class I service. This requires supporting Unnumbered Information (UI) Commands, eXchange IDentification (XID) Commands and Responses, and TEST link (TEST) Commands and Responses.
When either an XID or a TEST command is received a response must be returned; with the Destination and Source addresses, and the DSAP and SSAP swapped.
When responding to an XID or a TEST command the sense of the poll/final bit must be preserved. That is, a command received with the poll/final bit reset must have the response returned with the poll/final bit reset and vice versa.
The XID command or response has an LLC control field value of 175 (decimal) if poll is off or 191 (decimal) if poll is on. (See Appendix on Numbers.)
The TEST command or response has an LLC control field value of 227 (decimal) if poll is off or 243 (decimal) if poll is on. (See Appendix on Numbers.)
A command frame is identified with high order bit of the SSAP address reset. Response frames have high order bit of the SSAP address set to one.
XID response frames should include an 802.2 XID Information field of 129.1.0 indicating Class I (connectionless) service. (type 1).
TEST response frames should echo the information field received in the corresponding TEST command frame.