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ThePARC Universal Packet (PUP orPuP, although the original documents usually usePup) was one of the two earliestinternetworkingprotocol suites; it was created by researchers atXerox PARC in the mid-1970s. (Technically, the namePUP only refers to the internetwork-level protocol, but it is also applied to the whole protocol suite.) The entire suite providedrouting and packet delivery, as well as higher-level functions such as areliable byte stream, along with numerous applications.
The PUP protocol was created in roughly the same time frame as the earliest parts of the development ofTCP/IP for theInternet and also the same time period as the earlyEthernetlocal area network at PARC.[1] The fundamental design of the PUP suite was substantially complete by 1974. PUP was designed to connect theEthernet to theARPANET, which was a forerunner toTCP/IP and theInternet. PUP was designed byDavid Boggs,John Shoch, Edward Taft, andRobert Metcalfe.[2]
In the 1980sXerox used PUP as the base for theXerox Network Systems (XNS) protocol suite; some of the protocols in the XNS suite (such as theInternetwork Datagram Protocol) were lightly modified versions of the ones in the PUP suite, but others are quite different, reflecting the experience gained with PUP and IP.
The main internetwork layerprotocol is PUP, which roughly corresponds to theInternet Protocol (IP) layer in TCP/IP. A full PUPnetwork address consists of an 8-bit network number, an 8-bit host number, and a 16-bit socket number. The network number has a particular special value which means 'this network', for use by hosts which do not (yet) know their network number.
Unlike TCP/IP, socket fields are 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 supplies packet types (again, unlike IP). Also, an optional 2-byte checksum covers the entirepacket.
PUP packets are up to 554 bytes long (including the 20 byte PUPheader), and the checksum. This is a smaller packet size than IP, which requires all hosts to support a minimum of 576 bytes (but allows 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 is required to handle them. Larger packets can be fragmented.
A protocol named theGateway Information Protocol (an ancestor ofRIP) is used as both therouting protocol, and for hosts to discover routers.
PUP also includes a simple echo protocol at the internetwork layer, similar to IP'sping, but operating at a lower level.
To establish a transport connection, two protocols came into play. The first, theRendezvous and Termination Protocol (RTP), which was used to initiate communication between two entities, as well as manage and terminate the connection. The second was the primary transport layer protocol,Byte Stream Protocol (BSP), which was analogous toTCP.
Once RTP had started the connection, BSP took over and managed the data transfer. Like TCP, BSP's semantics and operation were in terms of bytes; this was discarded in favour of packets for the equivalent protocol in XNS,Sequenced Packet Protocol.
PUP supported a large number of applications. Some of them, such asTelnet andFile Transfer Protocol, were basically the same protocols as used on theARPANET (much as occurred with the TCP/IP suite).
Others were novel, including protocols for printer spooling, copying disk packs, page-level remote access to file servers, name lookup, remote management, etc. (although some of these capabilities had been seen before, e.g. the ARPANET already made heavy use of remote management for controlling theInterface Message Processors (IMPs) which made it up).
PUP showed that internetworking ideas were feasible, influenced the design work on TCP/IP, and laid a foundation for the laterXNS protocols. In June and July 1973,Vint Cerf andBob Kahn organizedINWG meetings atStanford, and Xerox researchers Metcalfe and Shoch attended. However, the Xerox attendees were told by a Xerox lawyer that they could not talk about PUP. During design discussions, the Xerox attendees kept pointing out flaws in the ideas that were suggested, until one of the Stanford researchers blurted out, "You guys have already done this, haven’t you?"[3][4] Shoch continued to be involved in the development ofTCP/IP, and Taft was involved in early discussions about email.[5]
The biggest impact of PUP was probably as a key component of theoffice of the future model first demonstrated at Xerox PARC; that demonstration would not have been anywhere near as powerful as it was without all the capabilities that a working internetwork provided.
The Gateway Information Protocol's descendant,RIP (with minor modifications to carry the addresses of any protocol family), remains in use today in other protocol suites, including TCP/IP. One version of RIP served as one of the initial so-calledinterior gateway protocols for the growingInternet, before the arrival of the more modernOSPF andIS-IS. It is still in use as an interior gateway protocol, in small sites with simple requirements.
In terms of flaws, the PUP protocol family was not device independent, in modern terminology the IP and MAC layers were combined into a single layer, which made wide-scale adoption difficult. PUP's 8-bit network and 8-bit host could scale to at most 64k machines, before an inter-network bridge or gateway would be needed. For this reason, a successor, theXerox Networking System (XNS), was developed by theXerox Office Systems Division using many of the ideas of PUP and a globally-unique, 48-bit host identifier (which became theMAC address in DIX v2 and laterIEEE 802.3) to solve these problems:[6]
Despite the misgivings of Xerox Corporation (which intended to make PUP the basis of a proprietary commercial networking product), researchers at Xerox PARC, including ARPANET pioneers Robert Metcalfe and Yogen Dalal, shared the basic contours of their research with colleagues at TCP and Internet working group meetings in 1976 and 1977