Since this document assumes you already have some experience with packet radio, and that this might not be the case, I've collected a set of references to other information that you might find useful.
You can get general information about Packet Radio from these sites:
AX.25, NET/ROM - Jonathon Naylor has collated a variety of documents that relate to the packet radio protocols themselves. This documentation has been packaged up into ax25-doc-1.0.tar.gz
John Ackermann has a web site with information related to configuring AX.25 on Linux at http://www.febo.com/linux-ax25/index.html.
The Hamsoft Linux Ham Radio Applications and Utilities Database attempts to maintain a complete list of Amateur Radio related applications for Linux. It can be found at http://radio.linux.org.au.