IF01088

IDP

The main internetwork layer protocol was IDP, the Internet Datagram Protocol. 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 their own 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), 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.

XNS also included a simple echo protocol at the internetwork layer, similar to IP's ping, but operating at a lower level.

RIP, a descendant of PUP's Gateway Information Protocol, was 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.

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