XFRM messages explained?

Hi all,

Is there a document (other than the kernel source) which documents the 
format of XFRM messages and how/when they are generated?

I need to write a program that looks at XFRM messages from the kernel 
(much the way that "ip xfrm monitor" does) and generates detailed 
logging based on the events.

Is there any document I could use?  Otherwise I'll have to grovel 
through the kernel source and sift through the calling orders, etc.

Thanks,

-Philip