1.2.2 XNS—Xerox Network Systems
Xerox Network Systems (also called Xerox NS, or XNS) is the network architecture developed by Xerox Corporation in the late 1970s for integrating their office products and computer systems. XNS is an open system, in that Xerox has published and made available the protocols used by XNS. Other vendors also provide hardware and software that supports the XNS protocols. Most 4.3BSD systems provide support for the XNS protocol suite. XNS is similar in structure to the TCP/IP protocol suite, so the presentation in this section assumes the reader has read the TCP/IP description in the previous section.
The arrangement of the layers in the XNS protocol suite, and their approximate mapping into the OSI model is shown in Figure 15.
ECHO Echo Protocol. A simple protocol that causes a host to echo the packet that it receives. Most XNS implementations support this protocol.
RIP Routing Information Protocol. A protocol used to maintain a routing data-base for use on a host in the forwarding of IDP packets to another host. Typically a routing process exists on the host, and this process uses RIP to maintain the database.
PEX Packet Exchange Protocol. An unreliable, connectionless, datagram protocol for user processes. Although PEX is not a reliable protocol, it does retransmission but does not perform duplicate detection.
SPP Sequenced Packet Protocol. A connection-oriented, reliable protocol for user processes. It provides a byte stream for the user process with optional message boundaries. SPP is the most commonly used protocol in the XNS suite, similar to TCP in the Internet suite.
ERROR Error Protocol. A protocol that can be used by any process to report that it has discovered an error and therefore discarded a packet.
IDP Internet Datagram Protocol. IDP is the connectionless, unreliable datagram protocol that provides the packet delivery service for all the above protocols.
Figure 15. Layering in the XNS protocol suite
The five protocols that that are shown at OSI layer 4 are termed the "Internet" Transport Protocols by Xerox. Most Xerox applications are built using the Courier remote procedure call protocol. Courier, in turn, is built using SPP. It is shown in Figure 16.
Figure 16. Typical XNS applications that use Courier
A user process has the choice of interacting directly with SPP or using Courier. Note that all XNS protocols use 1DP. Recall from the TCP/IP protocol suite that ARP and RARP did not use IP.