In studying R's connections source code, I've put together a series of notes about their workings. Rather than let the notes go where most of my notes go (I have no idea :-)), I decided to do some proofreading and make the notes available to others who might be interested. These notes should make the task of extending or maintaining the connections source code easier for programmers unfamiliar with the code.
R's connections code has been quite stable for 5 years (according to the Subversion log). However, I will do my best to keep these notes up-to-date with changes in to the connections internals, and periodically add notes on other features. The document is in HTML and PDF. I would be happy to provide the texinfo document also, on request.