He starts by asking what is a database link, how is it used should it be used etc and goes on to discuss how to set them up in detail including detailed examples of how and also to test all the components are up and running. Howard goes on to test the initialisation parameters db_domain and db_name and explains what they mean and what they should be set to. He concludes by creating example links and testing them with all sorts of SQL including DDL against remote databases. He also talks about auditing and creation of links into your database and also of their use and also discusses the issues of modifying links that already exist.
This is a superb paper explaining links in detail. They should be of great interest to anyone who is interested in the security of their Oracle databases. Any connection that allows remote access to your data should be reviewed and understood. Again Howards paper "Database Links" is here.