Basic Internetwork Protocol
The main internetwork layer protocol was PUP, which roughly corresponds to the Internet Protocol (IP) layer in TCP/IP. A full PUP network address consisted of an 8-bit network number, an 8-bit host number, and a 16-bit socket number. 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 were part of the full network address in the PUP header, so that upper-layer protocols did not need to implement their own demultiplexing; PUP also supplied packet types (again, unlike IP). Also, an optional 2-byte checksum covered the entire packet.
PUP packets were up to 554 bytes long (including the 20 byte PUP header), and the checksum. This was a smaller packet size than IP, which requires all hosts to support at least 576 (but supports packets of up to 65K bytes, if the hosts support them); individual PUP host pairs on a particular network might use larger packets, but no PUP router was required to handle them. Larger packets could be fragmented.
A protocol named the Gateway Information Protocol (a remote ancestor of RIP) was used as both the routing protocol, and for hosts to discover routers.
PUP also included a simple echo protocol at the internetwork layer, similar to IP's ping, but operating at a lower level.
Read more about this topic: PARC Universal Packet
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