Apple Mac OS X Server (Version 10.6 Snow Leopard) - Manual (Page 23)

Chapter 1 System Overview and Supported Standards 23
 XMPP: Extensible Messaging and Presence Protocol (XMPP) is an open XML-based
messaging protocol used for messaging and presence information. XMPP serves as
the basis for Mac OS X Server’s Push Notification service, as well as iChat Server,
and all publish and subscribe functions for the server.
Mac OS X Server’s UNIX Heritage
Mac OS X Server has a UNIX foundation built around the Mach microkernel and the
latest advances from the Berkeley Software Distribution (BSD) open source community.
This foundation provides Mac OS X Server with a stable, high-performance, 64-bit
computing platform for deploying server-based applications and services.
Mac OS X Server is built on an open source operating system called Darwin, which is
part of the BSD family of UNIX-like systems. BSD is a family of UNIX variants descended
from Berkeley’s version of UNIX. Also, Mac OS X Server incorporates more than
100 open source projects in addition to proprietary enhancements and extended
functionality created by Apple.
The BSD portion of the Mac OS X kernel is derived primarily from FreeBSD, a version
of 4.4BSD that offers advanced networking, performance, security, and compatibility
features.
In general, BSD variants are derived (sometimes indirectly) from 4.4BSD-Lite Release 2
from the Computer Systems Research Group (CSRG) at the University of California at
Berkeley.
Although the BSD portion of Mac OS X is primarily derived from FreeBSD, some
changes have been made. To find out more about the low-level changes made,
see Apple’s Developer documentation for Darwin.