The PARC Universal Packet (commonly abbreviated to PUP or PuP, although the original documents usually use Pup) was one of the two earliest internetworking

protocol suite The protocol stack or network stack is an implementation of a computer networking protocol suite or protocol family. Some of these terms are used interchangeably but strictly speaking, the ''suite'' is the definition of the communication protoco ...

s; it was created by researchers at Xerox PARC in the mid-1970s. (Technically, the name "PUP" only refers to the internetwork-level protocol, but it is also applied to the whole protocol suite.) The entire suite provided

routing Routing is the process of selecting a path for traffic in a network or between or across multiple networks. Broadly, routing is performed in many types of networks, including circuit-switched networks, such as the public switched telephone netw ...

and packet delivery, as well as higher level functions such as a

reliable byte stream A reliable byte stream is a common service paradigm in computer networking; it refers to a byte stream in which the bytes which emerge from the communication channel at the recipient are exactly the same, and in exactly the same order, as they we ...

, along with numerous applications.

History

The PUP protocol was created in roughly the same time frame as the earliest parts of the development of

TCP/IP The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suit ...

for the Internet and also the same time period as the early Ethernet local area network at PARC. The fundamental design of the PUP suite was substantially complete by 1974. In the 1980s Xerox used PUP as the base for the

Xerox Network Systems Xerox Network Systems (XNS) is a computer networking protocol suite developed by Xerox within the Xerox Network Systems Architecture. It provided general purpose network communications, internetwork routing and packet delivery, and higher level ...

(XNS) protocol suite; some of the protocols in the XNS suite (such as the Internetwork 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.

Basic internetwork protocol

The main internetwork layer

protocol Protocol may refer to: Sociology and politics * Protocol (politics), a formal agreement between nation states * Protocol (diplomacy), the etiquette of diplomacy and affairs of state * Etiquette, a code of personal behavior Science and technology ...

is PUP, which roughly corresponds to the

Internet Protocol The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet. I ...

(IP) layer in TCP/IP. A full PUP network 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 entire

packet Packet may refer to: * A small container or pouch ** Packet (container), a small single use container ** Cigarette packet ** Sugar packet * Network packet, a formatted unit of data carried by a packet-mode computer network * Packet radio, a form ...

. PUP packets are up to 554 bytes long (including the 20 byte PUP header), 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 the ''Gateway Information Protocol'' (an ancestor of

RIP Rest in peace (RIP), a phrase from the Latin (), is sometimes used in traditional Christian services and prayers, such as in the Catholic, Lutheran, Anglican, and Methodist denominations, to wish the soul of a decedent eternal rest and peace. ...

) is used as both the

routing protocol A routing protocol specifies how routers communicate with each other to distribute information that enables them to select routes between nodes on a computer network. Routers perform the traffic directing functions on the Internet; data packets ...

, and for hosts to discover routers. PUP also includes a simple echo protocol at the internetwork layer, similar to IP's

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, but operating at a lower level.

Transport layer protocols

To establish a transport connection, two protocols came into play. The first, the Rendezvous 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 to TCP. 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 Xerox Network Systems (XNS) is a computer networking protocol suite developed by Xerox within the Xerox Network Systems Architecture. It provided general purpose network communications, internetwork routing and packet delivery, and higher level ...

Application protocols

PUP supported a large number of applications. Some of them, such as

Telnet Telnet is an application protocol used on the Internet or local area network to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection. User data is interspersed in-band with Telnet control ...

and

File Transfer Protocol The File Transfer Protocol (FTP) is a standard communication protocol used for the transfer of computer files from a server to a client on a computer network. FTP is built on a client–server model architecture using separate control and data ...

, were basically the same protocols as used on the

ARPANET The Advanced Research Projects Agency Network (ARPANET) was the first wide-area packet-switched network with distributed control and one of the first networks to implement the TCP/IP protocol suite. Both technologies became the technical foun ...

(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 the

Interface Message Processors The Interface Message Processor (IMP) was the packet switching node used to interconnect participant networks to the ARPANET from the late 1960s to 1989. It was the first generation of gateways, which are known today as routers. An IMP was a ...

which made it up).

Impact

PuP showed that internetworking ideas were feasible, influenced the design work on TCP/IP, and laid a foundation for the later XNS protocols. At one point, Vint Cerf and Bob Kahn organized meetings at Stanford, and Xerox Researchers Bob Metcalfe and John 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?" The biggest impact of PuP was probably as a key component of the office of the future model first demonstrated at Xerox PARC; that demonstration would not have been anything like as powerful as it was without all the capabilities that a working internetwork provided. The Gateway Information Protocol's descendant,

(with minor modifications to carry the addresses of any protocol family), remains in use today in other protocol suites, including

. One version of RIP served as one of the initial so-called

interior gateway protocol An interior gateway protocol (IGP) or Interior routing protocol is a type of routing protocol used for exchanging routing table information between gateways (commonly routers) ''within'' an autonomous system (for example, a system of corpora ...

s for the growing Internet, before the arrival of the more modern OSPF and

IS-IS Intermediate System to Intermediate System (IS-IS, also written ISIS) is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices. It accomplishes this b ...

. It is still in use as an interior routing 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, the XNS (Xerox Networking System) was developed by the Xerox Office Systems Division using many of the ideas of PUP, but also including a globally unique 48-bit host identifier (which became MAC address in DIX v2 and later

IEEE 802.3 IEEE 802.3 is a working group and a collection standards defining the physical layer and data link layer's media access control (MAC) of wired Ethernet. The standards are produced by the working group of Institute of Electrical and Electronics Eng ...

) which solved these problems: * Preventing Address collisions / duplicate address allocations. Xerox allocated 24-bit upper MAC address, and manufacturers allocated the lower 24 bits. * Allowing analog repeaters (which were very low cost) to be a more viable network scaling device * Allowing each network interface to generate globally unique id's (UIDs)

References

* Edward A. Taft, Robert M. Metcalfe,
Pup Specifications
' (Xerox Parc, Palo Alto, June, 1978 and October, 1975) * Edward A. Taft,
State Machine for Rendezvous/Termination Protocol
' (Xerox Parc, Palo Alto, July, 1978 and October, 1975) * Edward A. Taft,

' (Xerox Parc, Palo Alto, July, 1978 and October, 1975) * Edward A. Taft,

' (Xerox Parc, Palo Alto, July, 1978 and October, 1975) * Jon A. Hupp, ''Pup Network Constants'' (Xerox Parc, Palo Alto, July, 1979)