Xerox Network Services

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Xerox Network Services (XNS) is a protocol suite developed by Xerox within the Xerox Network Systems Architecture. It provided general purpose network communications, internetwork routing and packet delivery, including higher level functions such as a reliable stream, and remote procedure calls. XNS predated and influenced the development of the Open Systems Interconnect (OSI) networking model.

XNS was developed at Xerox PARC in the early 1980s, based heavily on the earlier (and extremely influential) PARC Universal Packet (PUP) protocol suite done there in the late 1970s; some of the protocols in the XNS suite were lightly modified versions of the ones in the PUP suite. XNS was intended to be a commercial descendant of the research/development oriented PUP. The protocol suite specifications were placed in the public domain.

Being in the public domain, XNS became a canonical local area networking protocol in the 1980s, copied to various degrees by practically all networking systems in use into the 1990s. It had little impact on TCP/IP, however. During the 1980s XNS was used by 3Com and, with modifications, by a number of other commercial systems which became more common than XNS itself, including Ungermann-Bass Net/One, Novell NetWare, and Banyan VINES.

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Basic internetwork protocol

The main internetwork layer protocol was the Internet Datagram Protocol (IDP). IDP is a close descendant of PUP's internetwork protocol, and roughly corresponds to the Internet Protocol (IP) layer in TCP/IP.

Designed from the outset to complement the Ethernet local area network (also developed by Xerox), a full XNS network address consisted of a 32-bit network number, a 48-bit host address, and a 16-bit socket number; the host address was usually the host's MAC address. The network number had a particular special value which meant 'this network', for use by hosts which did not (yet) know their network number.

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 demultiplexing; IDP also supplied packet types (again, unlike IP). IDP also contained a checksum covering the entire packet, but it was optional, not mandatory.

IDP packets were up to 576 bytes long, including the 30 byte IDP header, but smaller than IP which required all hosts to support at least 576, but supports packets of up to 65K bytes. Individual PUP host pairs on a particular network might use larger packets, but no PUP router was required to handle them, and no mechanism was defined to discover if the intervening routers would support larger packets. Also, packets could not be fragmented, as in IP.

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