KLIPS is an Acronym for Kernel Layer IP Security.
It is the kernel portion of the FreeSwan project, and is available for kernels from 2.0 -> 2.6.
It has never been part of the mainline kernel, mostly because the FreeS/WAN developer was worried about US export restrictions on Cryptography and as such never allowed it to be worked on by US developers. David Miller developed 26sec to replace it and it is in the mainline 2.6 kernels.
The main difference you will see betweeh the two implementations is that KLIPS provides an ipsecN interface where 26sec does not. KLIPS also appears to be much more mature when used with the S/WAN userspace tools.
3 pages link to KLIPS:
