Editing Xerox Network Systems (section)

===Basic internetwork protocol===
The main [[internetwork]] layer [[Network protocol|protocol]] is the '''Internet Datagram Protocol''' ('''IDP'''). IDP is a close descendant of Pup's [[PARC Universal Packet#Basic internetwork protocol|internetwork protocol]], and roughly corresponds to the [[Internet Protocol]] (IP) layer in the Internet protocol suite.{{sfn|Stephens|1989|p=15}}

IDP uses Ethernet's 48-bit address as the basis for its own [[network address]]ing, generally using the machine's [[MAC address]] as the primary unique identifier. To this is added another 48-bit address section provided by the networking equipment; 32 bits are provided by [[Router (computing)|router]]s to identify the network number in the internetwork, and another 16 bits define a socket number for service selection within a single host. The network number portion of the address also includes a special value which meant "this network", for use by hosts which did not (yet) know their network number.{{sfn|cisco}}

Unlike TCP/IP, socket numbers are part of the full network address in the IDP header, so that upper-layer protocols do not need to implement demultiplexing; IDP also supplies packet types (again, unlike IP). IDP also contains a checksum covering the entire packet, but it is optional, not mandatory. 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. Error correction could be optionally added at higher levels in the protocol stack, for instance, in XNS's own SPP protocol. XNS was widely regarded as faster than IP due to this design note.{{sfn|Stephens|1989|p=15}}

In keeping with the low-latency LAN connections it runs on, XNS uses a short packet size, which improves performance in the case of low error rates and short turnaround times. IDP packets are up to 576 bytes long, including the 30 byte IDP [[Header (computing)|header]].{{sfn|cisco}} In comparison, IP requires all hosts to support at ''least'' 576, but supports packets of up to 65K bytes. Individual XNS host pairs on a particular network might use larger packets, but no XNS router is required to handle them, and no mechanism is defined to discover if the intervening routers support larger packets. Also, packets can not be fragmented, as they can in IP.

The [[Routing Information Protocol]] (RIP), a descendant of Pup's ''Gateway Information Protocol'', is used as the router information-exchange system, and (slightly modified to match the syntax of addresses of other protocol suites), remains in use today in other protocol suites, such as the Internet protocol suite.{{sfn|cisco}}

XNS also implements a simple echo protocol at the internetwork layer, similar to IP's [[Ping (networking utility)|ping]], but operating at a lower level in the networking stack. Instead of adding the ICMP data as payload in an IP packet, as in ping, XNS's echo placed the command directly within the underlying IDP packet.{{sfn|cisco}} The same might be achieved in IP by expanding the ICMP [[IPv4#Protocol|Protocol]] field of the IP header.