A long time issue we've had is how to test protocols under "real" circumstances. For DNS testing, I decided to do just that - see the code in twisted/test/test_names.py. Basically, I start a server listening using UDP, and have a client connect to it, all within the testing code and automatically. It works pretty well, after a bit of debugging. Data is pushed by calling main.iterate(). Now, in a lot of cases twisted.protocols.loopback may be good enough, but this approach is really necessary for thorough testing in certain cases. FTP leaps to mind, but also if we want automated testing of TOC or OSCAR clients against real servers. One drawback at least is that these tests will fail miserably on Jython (main.iterate on Jython doesn't push data), so I'm probably going to add code that makes sure they don't run at all on Jython. Another drawback is the related issue that I'm also testing DNS queries against real servers - this assumes a working net connection.