looking for pointers to chapter and verse on TEST frames

I'm mucking about with TEST frames - specifically the Linux
implementation of the HP-UX linkloop command (and the linkloop_reply
code of the Linux implementation, since Linux's networking does not
reply to them on its own like HP-UX and perhaps other OSes). I would
like to find a pointer to chapter and verse on the correct processing
thereof as I go to tighten-up what those bits are doing.
thanks,
rick jones