Editing High-Level Data Link Control (section)

====Peer discovery====
; Exchange identification (XID)
: An XID command includes an information field specifying the primary's capabilities; the secondary responds with an XID response specifying its capabilities.  This is normally done before sending a mode set command.  [[Systems Network Architecture]] defined one format for the information field, in which the most significant bit of the first byte is clear (0), but HDLC implementations normally implement the variant defined in ISO 8885, which has the most significant bit of the first byte set (1).
; TEST
: A TEST command is simply a [[Ping (networking utility)|ping command]] for debugging purposes. The payload of the TEST command is returned in the TEST response.