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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