(usagi-users 03924) A question about XFRM architecture and API

Hello,
 - I have a little question which and I hope somebody can help.

- I am trying to learn and understand what XFRM (which is used in
IPSec, for example ) API is about. Is there some reliable
document/RFC/link which explains in detail more about it ?

As I understand , XFRM is used by the USAGI project;
 in OLS 2004  proceeding , I saw a lecture tilted:
"IPv6 IPsec and Mobile IPv6 implementation of Linux",
by  Kazunori MIYAZAWA and Masahide NAKAMURA from USAGI Project."

It says there:
"XFRM architecture and stackable destination were introduced
into the kernel for IPsec packet processing
[7]. They can be not only for IPsec
packet processing, but also general packet processing
such as MIPv6. USAGI Project decided
to expand the architecture to implement
MIPv6".

I googled for it and did not find a detailed doc about XFRM architecture.
 and stackable destination.
Any ideas ?

Regards,
Ian