Универсальный Пакетный Протокол PARC (PUP)

Early internetworking protocol
The PARC Universal Packet (PUP or PuP, although the original documents usually use Pup) was one of the two earliest internetworking protocol suites; 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 and packet delivery, as well as higher level functions such as a reliable byte stream, along with numerous applications.

The PUP protocol was created in roughly the same time frame as the earliest parts of the development of TCP/IP 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 (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.

The main internetwork layer protocol is PUP, which roughly corresponds to the Internet Protocol (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. 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) is used as both the routing protocol, and for hosts to discover routers. PUP also includes a simple echo protocol at the internetwork layer, similar to IP's ping, but operating at a lower level.

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.

PUP supported a large number of applications. Some of them, such as Telnet and File Transfer Protocol, were basically the same protocols as used on the ARPANET (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 (IMPs) which made it up).