System information

CHAPTER
Troubleshooting XNS 14-301
14
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.