Windows Vista and Windows Server 2008 both support Authenticated Internet Protocol (AuthIP), an enhanced version of the Internet Key Exchange (IKE) protocol. Both AuthIP and IKE are protocols used to determine keying material and negotiate security parameters for communications protected using Internet Protocol security (IPsec).
In the October issue of TechNet Magazine, Joseph Davies describes the protocol details of AuthIP and coexistence behaviors between IPsec peers that support either both AuthIP and IKE or only IKE.
Don't forget to take a look at Joe's past installments of The Cable Guy for more information about networking technologies in Windows.