Understanding IPSec on Windows Server 2003.
Microsoft has released a document discussing their implementation of the Internet Engineering Task Force’s Internet Protocol security (IPSec). IPSec provides network managers with a key line of defense in protecting their networks.
IPSec exists below the Transport layer, so its security services are transparently inherited by applications. IPSec provides the protections of data integrity, data origin authentication, data confidentiality, and replay protection without having to upgrade applications or train users.
If you want to learn more, consider checking it out. [Dana Epp's ramblings at the Sanctuary]