It provided general purpose network communications, internetwork routing and packet delivery, and higher level functions such as a reliable stream, and remote procedure calls.
Specifically, XNS's Physical layer is really the Ethernet local area network system, also being developed by Xerox at the same time, and a number of its design decisions reflect that fact.
This reflects the fact that LANs generally have low-error rates, so XNS removed error correction from the lower-level protocols in order to improve performance.
[2] XNS also implements a simple echo protocol at the internetwork layer, similar to IP's ping, but operating at a lower level in the networking stack.
Courier contained primitives to implement most of the features of Xerox's Mesa programming language function calls.
[2] Due to its tight integration with Mesa as an underlying technology, many of the traditional higher-level protocols were not part of the XNS system itself.
The designers of this language, John Warnock and Chuck Geschke, later left Xerox PARC to start Adobe Systems.
To realize the value of specifying both a programmable and easily debug-able print job in ASCII, Warnock and Geschke created the Postscript language as one of their first products at Adobe.
Because all 8000+ machines in the Xerox corporate Intranet ran the Wildflower architecture (designed by Butler Lampson), there was a remote-debug protocol for microcode.
[4] This protocol could, via the debugger "nub", freeze a workstation and then peek and poke various parts of memory, change variables, and continue execution.
A seminal event took place when professors from MIT's famed Artificial Intelligence Laboratory approached Xerox in 1974 with the intention of buying Ethernet for use in their lab.
[6] Metcalfe eventually left Xerox November 1975 for Transaction Technology, a division of Citibank tasked with advanced product development.
Metcalfe immediately began re-designing Ethernet to work at 20 Mbit/s and started an effort to re-write Pup in a production quality version.
Looking for help on Pup, Metcalfe approached Yogen Dalal, who was at that time completing his PhD thesis under Vint Cerf at Stanford University.
[7] Dalal built a team including William Crowther and Hal Murray, and started with a complete review of Pup.
Dalal also attempted to remain involved in the TCP efforts underway at DARPA, but eventually gave up and focussed fully on Pup.
Dalal combined his experience with ARPANET with the concepts from Pup and by the end of 1977 they had published the first draft of the Xerox Network System specification.
A wide variety of proprietary networking systems were directly based on XNS or offered minor variations on the theme.
[11] Additional BSD modifications were eventually necessary to support the full range of Open Systems Interconnection (OSI) protocols.
[13] A series of sixteen individual protocol descriptions are listed in the Xerox Systems Institute Literature Catalog.