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