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Xerox network services (XNS) is a protocol stack which provided routing and packet delivery developed by Xerox at Xerox PARC in the later 1970s and early 1980s. During the mid-1980s the XNS system became the basis of a number of commercial systems, including Novell NetWare, Banyan VINES and Ungermann-Bass Net/One, which were much more common than XNS itself.
Key to XNS, and the systems based on it, was the 48-bit addressing scheme. In XNS the 48-bit Ethernet unique ID was used as a key into a hashtable of 48-bit addresses, translating the physical address into a network address. The network address was split into two parts, the lower 32-bits representing a node address, and the upper 16-bits a network address. Thus under XNS networks could have a large number of nodes, over 2 billion of them, but only 65,536 networks could be connected together in an single internet. In retrospect the dividing line almost certainly should have been inverted, allowing for many more, smaller networks.
The main network layer protocol was IDP, the Internet Datagram Protocol. IDP roughly corresponds to the Internet Protocol (IP) layer in TCP/IP. IDP packets are 576 bytes in length, somewhat smaller than IP. XNS also included a simple echo protocol at the connection layer, similar to IP's ping, but operating at a much lower level. RIP was used as the router information-exchange system, and remains in wide use today with other protocols.
There were two primary transport layer protocols, SPP is a streaming protocol similar to TCP (and the direct antecedent to IPX/SPX's SPX), and PEP is a connectionless non-reliable protocol similar in nature to UDP. XNS also used EP, the Error Protocol, as a reporting system for problems such as dropped packets. This provided a unique set of packets which could be filtered to look for problems. Unlike TCP/IP, port fields were provided for in the IDP header, so upper-layer protocols did not need to implement their own demultiplexing.
XNS specifically is no longer in use due to the all pervasiveness of IP. However, it played an important role in the development of networking technology in the 1980s by forcing software and hardware vendors to seriously consider the need for computing platforms to support more than one network protocol stack simultaneously. In particular, it helped to validate the design of the 4.2BSD network subsystem by providing a second protocol suite which was significantly different from the Internet protocols; by implementing both stacks in the same kernel, the Berkeley researchers demonstrated that the design was suitable for more than just IP. (Some modifications were eventually necessary to support the full range of OSI protocols.)
Extensible Name Service also used the acronym XNS. It was an XML-based digital identity architecture developed by the XNS Public Trust Organization (XNSORG (http://www.xns.org)) starting in 2000. The XNS specifications were subsequently contributed by XNSORG to OASIS, where they became part of the [XRI (http://www.oasis-open.org/committees/xri) (Extensible Resource Identifier)] and [XDI (http://www.oasis-open.org/committees/xdi) (XRI Data Interchange)] Technical Committees. XNSORG has since evolved into XDI.ORG (http://www.xdi.org/) and now offers community-based XRI/XDI infrastructure.
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