Evolution of Xerox's network systems architecture
Authors: Garlick Lawrence 
Published: October 2012

Research and development at Xerox's Palo Alto Research Center (PARC) and Office Systems Division (OSD) have led to many advances in an evolving network architecture. As early experimenters in local area networking, researchers at PARC built a prototype of what is becoming the basis for an international standard. Their early work with networked personal workstations, distributed networked services, and local network interconnection provided valuable insight into requirements for higher level protocols. As designers of products such as the Star professional workstation and the Network Systems services, developers at OSD expanded the original PARC network architecture to include features required for the commercial marketplace.
Contrasting the PARC Universal Packet (PUP) architecture with OSD's Network Systems (NS) architecture is itself an interesting study of protocol evolution and compatibility. No level of the PUP architecture was untouched in its transformation to the NS architecture. One of the major changes was to the addressing conventions at both the Ethernet and the internetwork levels. It is believed that the NS addressing conventions are adequate for world-wide heterogeneous internetworks. The introduction of a remote procedure calling protocol changed the foundation for developing application protocols.
Modelling distributed operations as procedure calls has simplified the development and specification of application protocols. The NS remote procedure calling protocol, <i>Courier</i>, is oriented toward a heterogeneous processor and language environment.
The evolution did not stop with the first version of the NS protocols. Xerox OSD gained experience with the initial NS architecture through use in its development system (the Mesa Development Environment) and early products. Most of the experience was encouraging, reinforcing the strength of the architecture. During this period the benefits and problems of protocol evolution became apparent, and each protocol was examined for its ability to support evolution and backward compatibility.
Minor changes at the physical and data link layers (<i>Ethernet</i>) were made to accommodate standardization. The convergence of the Ethernet Specification and standards from IEEE 802 and ECMA have received adequate attention elsewhere.
At the network layer, Xerox's <i>Internet work Datagram Protocol</i> has changed little since its inception. This protocol is distinguished by its simplicity and power. The <i>Internetwork Datagram Protocol</i> is the foundation of the internetwork architecture. It defines the addressing structure and routing functions that permit higher level protocols to virtually ignore the problems of the location of communicating entities.
The primary protocol in the transport layer, the <i>Sequenced Packet Protocol</i>, has enjoyed stability since its publication; though implementation strategies have been refined, especially in the areas of flow control and retransmission strategies. Early changes included a delimiting feature to facilitate treating a stream of packets as a message.
Most of Xerox's application protocols rely on <i>Courier</i>, the NS remote procedure calling protocol. <i>Courier</i> defines a set of universal data types, the messaging mechanism for performing calls and returns, and a syntax for the specification of higher level protocols. Since its introduction, <i>Courier</i> has been enhanced to include protocol version numbers (both for itself and protocols built upon it), to support more varied data types, and to directly integrate the streaming of bulk data with a procedure call.
Binding of user processes to application services within the NS architecture has changed dramatically with the development of the <i>Clearinghouse</i>. The <i>Clearinghouse</i> serves as an internetwork white pages and yellow pages for naming and locating distributed resources. It permits both simple mappings of names to addresses and the enumeration of instances of a particular type of resource. Distribution lists for electronic mail and access control lists are also supported by the <i>Clearinghouse</i>.
The evolution of the NS architecture, from its beginnings at PARC to its present form in Xerox products has been influenced by a basic philosophy. <i>An architecture or protocol must pass the test of stressful use in its intended environment before it can be considered for standardization.</i>