This is an awesome resource for understanding the technical architecture of the IPSec environment from a very talented Microsoft MVP. 

Unixwiz.net Tech Tip: An Illustrated Guide to IPSec

QUOTE: IPSec is a suite of protocols for securing network connections, but the details and many variations quickly become overwhelming. This is particularly the case when trying to interoperate between disparate systems, causing more than one engineer to just mindlessly turn the knobs when attempting to bring up a new connection.