how to parse custom data filed of one protocol in lua

Hi all,
when try to parse custom data filed of LLC when ftype is Supervisory Frames, i don't know how to just parse data filed of one protocol. Following picture is format of LLC, data field is what i want to parse.
1) Can i get data filed of LLC directly, then call my custom diseector?
2) If 1 is no, in http://wiki.wireshark.org/Lua/Dissectors, it shows add dissector to table of one UPD port. How could i add my custom dissector? And to which port? 
Thank you.