Information Technology Reference
In-Depth Information
14
CHAPTER
Troubleshooting XNS
The Xerox Network Systems (XNS) protocols were created by Xerox Corporation in the late 1970s and
early 1980s. They were designed to be used across a variety of communication media, processors, and
office applications. Several XNS protocols resemble the Internet Protocol (IP) and Transmission Control
Protocol (TCP), developed by the Defense Advanced Research Projects Agency (DARPA) for the U.S.
Department of Defense (DoD).
Because of its availability and early entry into the market, XNS was adopted by most of the early LAN
companies, including Novell, Inc., Ungermann-Bass, Inc. (now a part of Tandem Computers), and 3Com
Corporation. Each of these companies has since made various changes to the XNS protocols. Novell
added the Service Advertising Protocol (SAP) to permit resource advertisement and modified the OSI
Layer 3 protocols (which Novell renamed IPX, for Internetwork Packet Exchange) to run on IEEE 802.3
rather than Ethernet networks. Ungermann-Bass modified Routing Information Protocol (RIP) to
support delay as well as hop count and made other small changes. Over time, the XNS implementations
for PC networking have become more popular than XNS as it was designed by Xerox.
Although XNS documentation mentions X.25, Ethernet, and High-Level Data Link Control (HDLC),
XNS does not expressly define what it refers to as a Level 0 protocol. Like many other protocol suites,
XNS leaves media access an open issue, implicitly allowing any such protocol to host the transport of
XNS packets over a physical medium.
The Network Layer
The XNS network-layer protocol is called the Internet Datagram Protocol (IDP). IDP performs standard
Layer 3 functions, including logical addressing and end-to-end datagram delivery across an
internetwork. The format of an IDP packet is shown in Figure 14-1.
