Xerox network services
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Xerox network services (XNS) is a protocol suite which provided routing and packet delivery developed by Xerox at Xerox PARC in the later 1970s and early 1980s. During the 1980s the XNS system was used by 3Com and (with modifications) a number of other commercial systems which became more common than XNS itself, including Ungermann-Bass Net/One, Novell NetWare, and Banyan VINES.
Designed from the outset to complement the Ethernet Local Area Network (also developed by Xerox), a full XNS address consists of a 32-bit network address, a 48-bit host address, and a 16-bit socket number. The host address is usually the host's MAC address.
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:
- Sequenced Packet Protocol (SPP) is an acknowledged transport protocol analogous to TCP (and the direct antecedent to Novell's IPX/SPX) except that packets cannot be fragmented and that the sequence numbers count the packets (and not the octets as in TCP).
- Packet Exchange Protocol (PEP) is a connectionless non-reliable protocol similar in nature to UDP (and the antecedent to Novell's PXP).
- 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, socket fields are part of the full network address in the IDP header, so that 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.