Lect-14 (Protocols)

Xerox Network Services (XNS)

Xerox Network Services (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 functions such as a reliable stream, and remote procedure calls. XNS predated and influenced the development of the Open Systems Interconnection (OSI) networking model, and was very influential in local area networking designs.

Xerox used XNS for file transfers, sharing network resources, packet transfers, sharing routing information and remote procedure calls. Its basic working mechanism is almost the same as in the TCP/IP protocol suit, but XNS contains only two network layers. This differs from the seven-layer Open Systems Interconnection (OSI) model, although the functionality is basically the same.

XNS was a public domain technology and therefore became one of the most commonly used networking technologies through 1980s. It was replaced by the Internet Protocol suite.

XNS contains two major layers, a network layer and a transport layer. The network layer provides the packet-carrying service and logical addressing. XNS was developed for many purposes, such as office applications, transmissions, communication media and processors. There is an echo protocol inside the XNS suite, which works as a door knocker, checking the connectivity between the two systems. This is similar to ping in IP systems.

XNS Architectures

XNS Basic Protocol

Internet Datagram Protocol (IDP)

The Internet Datagram Protocol (IDP) delivers a single frame as an independent entity to an Internet address, irrespective of other packets or addressee responses. XNS generally limits the IDP packets to a maximum size of 576 bytes, excluding the data link header.

The following parameters are available for IDP:

Destination network: Four-byte address of the destination network.

Destination socket: Two-byte socket number of the destination port.

Source network: Four-byte address of the source network.

Source socket: Two-byte socket number of the source port.

Hop count: Indicates the number of routers encountered during transport of the packet. Each router handling a packet increments the hop count by one. When the hop count reaches 16, this protocol discards the packet.

Routing Information Protocol (RIP)

XNS uses the Routing Information Protocol (RIP) to maintain a database of network hosts and exchange information about the topology of the network. Each router maintains a list of all networks known to that router along with the routing cost in hops required to reach each network. XNS distributes routing information on the network by routers broadcasting their routing tables every 30 seconds. This protocol sends routing tables as a result of changes in service or topology or in response to a request for routing information.

XNS generally uses the Echo protocol to demonstrate the existence and accessibility of another host on the network, while using the Error protocol to signal routing errors.

Packet Exchange Protocol (PEP)

The Packet Exchange Protocol (PEP) provides a semi-reliable packet delivery service that orients toward single-packet exchanges.

The following parameters are available for PEP:

Packet ID: A unique number used to identify responses as belonging to a particular request. The sending host sets the packet ID field to a fixed value, then looks for PEP responses containing the same packet ID value.

Client type: A registered code used to identify the particular application in use.

Sequenced Packet Protocol (SPP)

The Sequenced Packet Protocol (SPP) provides reliable transport delivery with flow control.

The following parameters are available for SPP:

Source connection ID: Reference number used to identify the source end of a transport connection. This protocol establishes Connection IDs at connect time to distinguish between multiple transport connections.

Destination connection ID: Reference number used to identify the target end of a transport connection.

Sequence number: Sequence number of the packet. Each successive packet transmitted and acknowledged on the transport connection must have a sequence number one higher than the previous sequence number.

Acknowledge number: Sequence number of the last packet that the protocol received properly. Each side of the transport connection uses its own sequence of numbers for transmitted packets, resulting in sequence and acknowledge numbers in the same packet generally being out of phase with each other.

XNS Addresses

An XNS address occupies 12 bytes and is comprised of three parts:

A 32-bit network ID

A 48-bit host ID

A 16-bit port number

The host ID is an absolute number that must be unique to all XNS Internets. The AIX implementation uses the 48-bit Ethernet address as host ID. With unique host IDs, the network ID is redundant but is required for routing purposes.

XNS addresses can be represented by several means, as can be seen in the following examples:

123#9.89.3c.90.45.56

5-124#123-456-900-455-749

0x45:0x9893c9045569:90

0456:9893c9045569H

The first example is in decimal format, and the second example, using - (minus signs), is separated into groups of three digits each. The 0x and H examples are in hex format. Finally, the 0 in front of the last example indicates that the number preceding the colon is in octal format.

System Network Architecture

SNA was developed in the 1970s with an overall structure that parallels the OSI reference model. With SNA, a mainframe running Advanced Communication Facility/Virtual Telecommunication Access Method (ACF/VTAM) serves as the hub of an SNA network. ACF/VTAM is responsible for establishing all sessions and for activating and deactivating resources. In this environment, resources are explicitly predefined, thereby eliminating the requirement for broadcast traffic and minimizing header overhead. The underlying architecture and key components of traditional.

It is a complete protocol stack for interconnecting computers and their resources. SNA describes formats and protocols and is, in itself, not a piece of software. The implementation of SNA takes the form of various communications packages, most notably Virtual Telecommunications Access Method (VTAM), the mainframe software package for SNA communications.

Systems Network Architecture (SNA) is IBM's proprietary networking architecture, created in 1974. It is a complete protocol stack for interconnecting computers and their resources. SNA describes formats and protocols and is, in itself, not a piece of software. The implementation of SNA takes the form of various communications packages, mostly Virtual Telecommunications Access Method (VTAM), the mainframe software package for SNA communications.

IBM SNA Architecture

IBM SNA-model components map closely to the OSI reference model. The descriptions that follow outline the role of each SNA component in providing connectivity among SNA entities.

Data-link control (DLC)---Defines several protocols, including the Synchronous Data Link Control (SDLC) protocol for hierarchical communication, and the Token Ring Network communication protocol for LAN communication between peers

Path control---Performs many OSI network-layer functions, including routing and datagram segmentation and reassembly (SAR)

Transmission control---Provides a reliable end-to-end connection service, as well as encrypting and decrypting services

Data flow control---Manages request and response processing, determines whose turn it is to communicate, groups messages together, and interrupts data flow on request

Presentation services---Specifies data-transformation algorithms that translate data from one format to another, coordinate resource sharing, and synchronize transaction operations

Transaction services---Provides application services in the form of programs that implement distributed processing or management services

Elements of SNA

Network Control Program (NCP):  It is a packet forwarding protocol, acting like modern switch - forwarding data packages to the next node, which might be a mainframe, a terminal or another 3705.  It is a multiplexer that connected multiple terminals into one communication line to the CPU, thus relieved the constraints on the maximum number of communication lines per CPU.

Synchronous Data Link Control (SDLC): It is a protocol which greatly improved the efficiency of data transfer over a single link

SDLC included much more powerful error detection and correction codes. These codes often enabled the communications cards to correct minor transmission errors without requesting re-transmission, and therefore made it possible to pump data down a line much faster.

Virtual Telecommunication Access Method (VTAM): VTAM, a software package to provide log-in, session keeping and routing services within the mainframe. A terminal user would log-in via VTAM to a specific application or application environment (e.g. CICS or TSO). A VTAM device would then route data from that terminal to the appropriate application or application environment until the user logged out and possibly logged into another application. The original versions of IBM hardware could only keep one session per terminal. In the 1980s further software (mainly from third-party vendors) made it possible for a terminal to have simultaneous sessions with different applications or application environments.