IPsec Tunneling

I am wanting to learn a little more about IPsec Tunneling. I have been googling and reading as much as I can, but I was just curious if any of you had a good reference for the topic.

I am wanting to find both a technical howto and a theoretical reference.

Like I said, I am already reading some howto's, but I was just curious if any of you knew of a good reference.

Thanks

Basically you are most of the way there IPsec is very gungy under the hood. If you really want to go there the RFC is a good place to start. I read the Building Linux VPNs from New Riders.