Security in Windows Server 2003 exists at different levels—from the protection of entire messages at the application level, to secure channels, to the protection of data packets within IP. The security protocols residing at these different layers provide mechanisms not available at other layers.
S/MIME, for example, provides public-key–based encryption, integrity, and authentication of e-mail messages at the application level. SSL resides beneath the application layer, adding security to application protocols. IPSec, on the other hand, provides application-independent data protection at the IP layer. The following sections examine Windows Server 2003 security protocols.