DATA PRO Data Networking 2783 1 Standards

ISO Reference Model for Open Systems Interconnection (OSI)

In this report: Datapro Summary

OSI Standards Progress ..... 6 The goal of Open Systems Interconnection (OS!) was designed to enable dissimilar com­ puters in multivendor environments to share information transparently. The OSI structure OSI Management ...... 8 calls for cooperation among systems of different manufacture and design. There are seven layers of the OSI model that communicate between one end system and another. The layers OSI and the Future •....••....•.. 9 cover nearly all aspects of information flow, from applications-related services provided at the Application Layer to the physical connection of devices to the communications medium Note: This report ex­ at the Physical Layer. All seven layers have long since been defmed and ISO protocols plains the OSI Seven­ ratified for each layer, though extensions have been made occasionally. Although the model Layer Reference Model at has changed the way we look at networking, the dream of complete OSI-compliance has not all layers; compares OSI come to fruition. The causes are varied, but this is essentially because OSI protocols are too to other architectures; expensive and too complex compared with other protocols that have become de facto stan­ rationalizes the need for dards in their own right. Even so, it is important to understand the model because, although standards testing and veri­ the complete stack of protocols is not much used today, the model has formed the way we fication; examines the case for OSI; profiles think of the structure of networks, and the model itself is always referred to in intemetwork­ major testing organiza­ ing matters. tions; and outlines OSI Management standards and status.

standardization effort and confront the issue of Analysis incompatibility head-on. The purpose of TC97! SC 16 was to develop a model and define the protocols and interfaces required to support an The proliferation of computerized data process­ open system. The goal of OS! was, and still is, ing systems in the late 1960s produced a need for to enable dissimilar computers in multivendor compatible data communications networks in environments to share information transparently. the 1970s. Several proprietary network architec­ With this capability, it was thought that global tures were developed for mainframe-to-tenninal digital networks could become a reality. As we communications, including ffiM's SNAin 1974. shall see, however, much of the OSI goal has in Although many of these proprietary architec­ fact come about through widespread use of de tures were based on a layered model, none was facto protocols such as TCPIIP and from multi­ compatible with any other. The CClTI's X.25 vendor initiatives formed to ensure interoper­ host interface to the packet networks standard ability such as the ATM Forum, a standards-set­ was ratified in 1976 but this is not a complete ting body in its own right. network architecture. In 1977 the International Organization for Standardization (ISO) formed The Open System ISO Technical Committee 97 (TC97), Subcom­ mittee 16 (SC16), to embark on a worldwide The ISO defines a system as a set of one or more computers and associated software, peripherals, tenninals, human operators, physical processes, information transfer means, etc., which form an autonomous whole capable of performing infor­ -By Marina Smith mation processing and/or information transfer. Senior Analyst An open system is one that obeys OSI standards email: [email protected] in its communication with other systems.

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An application process is an element within a system that distinct groups. Communications-oriented functions are sepa­ performs information processing for a particular application. The . rated from user-oriented functions; features which move informa­ application process can be manual (a person operating a banking ~on across a network are distinct from features which handle and terminal), computerized (a program executing in a computer cen­ format information. ter and accessing a remote database), or physical (a process con­ There are seven layers of the OSI model that communicate trol program executing in a dedicated cQIllPuter attachedto indus­ between one end system and another end system. The layers trial equipment and linked to a plant control system). cover nearly all aspects of information flow, from applications­ The OSI structure calls for cooperation among systems of dif­ related services provided at the Application Layer to the connec­ ferent manufacture and design. This includes coordinating activi­ tioJ;l of devices to the communications medium at the Physical ties such as the following: Layer. Below the Physical Layer, the media itself corresponding • Interprocess communications-the synchronization between to '1..ayer 0"-such as wire, cable, or through-the-air communi­ OSI application processes and the exchange of information cation-is not addressed by the model. Application, Presentation, Session, Transport, Network, Data Link, and Physical Layers • Data representation-the creation and maintenance of data have been defined (see Table "The Seven Layers of OSI"). The descriptions and transformations for reformatting data ex­ table describes the OS.! model's seven layers and their purposes. changed between systems In the model, information flows down from Layer 7 to Layer 1, • Data storage-storage media, file systems, and database sys­ and then out over a physical transmission medium. At the receiv­ tems for providing access to and management of stored data ing end, the information flows into another end system and up from Layer I to Layer 7, until it is received by a user. • Process and resource management-how application processes The seven layers can be divided into two functional groups: are declared, initiated, controlled, and acquired the Transport Platform (Layers I to 4) and the Application Plat­ • Integrity and security-information processing constraints that form (Layers 5 to 7). The Transport Platform's function is to get must be ensured during open systems operations data from one system to another without errors. The Application • Program support-the definition, compilation, testing, linking, Platform's function is to interpret the data stream and present it to storage, and transfer of and access to programs executed by the the user in a usable form (see Figure "Application and Transport application processes Division"). Each layer contributes functions to the communications task. For example, the Link Layer enables communications across a The OSI model is concerned only with the exchange of informa­ single physical connection, while the Network Layer provides tion between open systems. end-to-end routing and data relay. Services at the upper-layer in­ The Layering Concept terface--providing communications to the next~higher layer­ Layering is a basic structuring technique used in the OSI model. are provided by each layer, usually described by a service speci­ Each layer is composed of an ordered set of subsystems, with fication for the layer. Services at each layer are provided by a logically related functions grouped together. The OSI model layer entity. Each layer entity communicates with its peer at the breaks down internetworking activities between systems into two same layer on another system, providing services specified in the service specification.

The Seven Layers of 051

Layer Name Purpose

7 Application Applications and application interfaces for OSI networks. Provides access to lower-layer functions and services. 6 Presentation Negotiates syntactic representation for the Presentation Layer and performs data transformations. 5 Session Coordinates connection and interaction between applications. Establishes a dialog, manages and synchronizes the direction of data flow. 4 Transport Ensures end-to-end data transfer between applications, data integrity, and service quality. Assembles data packets for routing by Layer 3. 3 Network Routes and relays data units among network nodes. 2 Data Unk Transfers data units from one network node to another over a transmission circuit. Ensures data integrity between nodes. Physical Delimits and encodes the bits onto the physical medium.

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IEEE Corporate Office ITU (International 345 E. 47th Street Telecommunications Union) Standards Organizations New York, NY, 10017, U.S.A. Tel: +1 2127057900 Place des Nations IEEE Operations Center CH - 1211 Geneva 20 445 Hoes Lane Switzerland Piscataway, NJ, 08855-1331, Tel: +412273051 11 Several standards testing and ATMForum U.S.A. Fax (Group 3): +41 22 733 verification bodies have been Tel: +1 908981 0060 7256 organized by vendor consor­ World Headquarters Fax: (Group 4): +41 22 730 tiums, govemment agencies, 2570 West EI Camino Real IEEE European Operations 6500 and independent organiza­ Suite 304 Center (Brussels) http://www.itu.ch tions. They have found that Mountain View, CA 94040- Tel: +32 2 770 2242 developing conformance 1313, U.S.A. Fax: 32 2 770 8505 European Computer specifications, producing test­ Phone: +1 4159496700 E-mail: memservice­ Manufacturers Association [email protected] ing suites, and conducting Fax: +14159496705 (ECMA) comprehensive testing are http://www.atmforum.com complicated, expensive, and ISO (International European Office Rue du Rhone 114 time consuming. Regional dif­ Organization for CH-1204 Geneva, Switzerland ferences can stymie attempts Boulevard Saint-Michel 78 1040 Brussels, BELGIUM Standardization) Tel: +41 22 849 6000 at verification. The trend for Fax: +41 22 849 6001 these organizations, therefore, Phone: +32 2 732 8505 Fax: +32 2 732 8485 ISO Central Secretariat Telex: 413237 ECMA CH is to cooperate with each 1, rue de Varembe http://www.ecma.ch other, sharing resources and Asia-Pacific Office Hamamatsucho Suzuki Case Postale 56 expertise. A primary objective CH-1211 Geneva 20 Electronic Industries is demonstrating interopera­ Bldg.3F 1-2-11 Hamamatsucho, Switzerland Association (EIA) bility among different vendors; Tel: + 4122 749 0111 i.e., proving that standards Minato-ku Tokyo 105, JAPAN Fax: + 4122 733 34 30 Electronic Industries really work and fostering end­ E-mail: user interest. Many agencies Phone: +8133438 3694 Association Fax: +81 3 3438 3698 INTERNET: [email protected] 2500 Wilson Boulevard have tried vainly to involve X.4OO: c=ch; a=4oonet; p=iso; more end users but are Arlington, VA 22201-3834 IEEE (The Institute Of e=isocs; s=central Tel: +1 703 907 7500 backed primarily by the http://www.iso.ch vendors. Electrical And Electronics Fax: +1 703 907 7501 http://www.eia.org Engineers) Please note: Copies of ISO ANSI (American National standards can be ordered Standards Institute) The Institute Of Electrical And from local standards offices. Electronics Engineers, Inc. ANSI 1828 l Street NW, Suite 1202 11 W.42nd Street Washington DC 20036-5104, New York NY 10036, U.S.A. U.S.A. Tel:+12126424900 Tel: +1 908 981 0060 Fax: +12123980023 Fax: +19089810027 http://www.ansi.org http://www.ieee.org

IBM's SNA is also a layered architecture, following rules of with messages from other layers and passed through these other layering similar to OSI and other layered architectures. There are layers on the way to their destination, picking up and then shed­ good reasons for layering: layering simplifies change; compo­ ding these other protocol layers along the way. For example, if nents inside a layer can be changed without affecting any other layer seven at one end system must send a message to layer seven layers in that node. Layers are like structured programming-but at another, it must travel down through six layers at its own end for teleprocessing systems. Because there are rigid interfaces be­ and then up through six layers at the other, until it reaches layer tween levels, fewer people need to react to changes, allowing seven at its opposite (peer) layer. them to be implemented faster. There is no better way of achiev­ Each network node (a network user, computer, terminal) is ing complex functions. Layering allows each network function to equipped with this layer mechanism. However, not all intermedi­ be made "transparent," unaware and independent of other func­ ate nodes need all seven layers. Network nodes, in particular, tions at other layers, thus enabling any layer to be modified with­ must only route and transmit data packets-functions at the bot­ out changing the entire monolithic architecture. tom three layers of the OS! model. Layer 4 through 7 functions Each layer may support one of several different protocols are not required and, therefore, not included in network node soft­ designed for specific network applications; the choice of a spe­ ware. Data packets processed in these nodes reach only Layer 3 cific protocol is optional, allowing users to tailor networks to and are then routed elsewhere (see Figure "Message Movement their own design. Each layer defines functions crucial to the com­ Among OS] l.o.yerS'). A node communicates with its peer in an­ munications process at that layer, independent of the other layers. other node sending or receiving data. Data transfer is routed from However, a layer may perform functions hinging on functions Layer 7 down to Layer 1 at the transmitting node, then along the performed in the layers immediately above or below. A layer can network to Layer 1 at the receiving node, and finally from Layer I only communicate with another device or network node at its peer up to Layer 7. Peer layers communicate by the same method. layer. Messages exchanged between peer layers are "enveloped"

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Figure does not provide services to a layer above it. Some of the services Application and Transport Division provided by this layer, other than information transfer, are the following: • Identifying intended communications partners 7 Application • Determining current availability of the intended partners • Establishing the authority to communicate 6 Application • Agreeing on responsibility for error recovery Presentation Platform • Agreeing on procedures for controlling data integrity 5 The Presentation Layer Session The Presentation Layer (Layer 6) allows an application to inter­ pret the meaning of information exchanged, Information is for­ matted and translated at this layer. Aspects of Layer 6 include data syntax, which is the data to be transferred between layers, and the presentation image syntax, which is the data structure thatappli­ cation entities refer to in their dialog, or the set of actions that may 4 be performed on the data structure. Transport Services provided to the Presentation Layer include the following: 3 • Transforming data syntax, primarily code and character set Network conversion Transport • Transforming and selecting the presentation syntax, the adap­ 2 Platform tation and modification of the presentation data (the OSI view) Data Unk Functions within the Presentation Layer include session estab­ lishment request; data transfer; negotiation and renegotiation of 1 data syntax and presentation image syntax; and session termina­ Physical tion request. The Session Layer The seven layers are divided into two functional groups. The Session Layer (Layer 5) allows cooperating presentation entities to organize and synchronize their dialog and to manage data exchange. It provides the following services: The message initiated at the Application Layer is passed from • Session-connection establishment-creation of an exchange layer to layer, through the various OSI layers, encapsulating con­ between presentation entities trol information in the process. A fully encapsulated message enters the cable at Layer 1. The procedure is reversed at the re­ • Session-connection release ceiving end. Each item of control information is processed at its • Normal data exchange appropriate layer, and the message itself passes up to Layer 7. Data transfer essentially is a packaging process at the transmitting • Expedited data exchange node and an unpackaging process at the receiving node. • Interaction management-allowing presentation entities to take turns exercising control functions The Layers • Session-connection synchronization A number of objectives were considered by the reference model's designers: to limit the number of layers to make the system engi­ • Exception reporting-permitting the presentation entities to be neering task of describing and integrating the layers as simple as notified of exceptional situations possible; to create boundaries between layers at points where the • Activity management description of services can be small and the number of interac­ tions across each boundary is minimized; and to collect similar The Transport Layer functions in the same layer. Table "The Seven Layers of OSf' The Transport Layer (Layer 4) provides transparent data flow summarizes the OSI Reference Model's layers; more detailed between session entities, freeing the Session Layer from respon­ descriptions follow for each layer. sibility for cost-effective and reliable data transfer. Layer 4 pro­ vides information interchange according to a user-specified reli­ The App6cation Layer ability level and end-to-end control. Transport protocols transfer The Application Layer (Layer 7) is the highest layer, providing information from one end of a physical connection to another and the means for the application process to access the OSI environ­ ensure that it is delivered correctly_ Layer 4 protocols are used ment. Its function is to serve as the passageway between applica­ after a route has been established through the network by the tion processes using Open Systems Interconnection to exchange network-layer protocol. information; consequently, all application process parameters are made known to the OSI environment through this layer. The services provided by this layer include the following: All services directly usable by the application process (i.e., systems and applications management functions) are provided by • Transport-connection establishment to complete a connection the Application Layer. It differs from the other layers in that it between session entities

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Figure Message Movement Among OSI Layers

Message moves from Layer 7 through ~ the other layers to reach Layer 7 at the opposite end ,....------, 7 7 Application Application

6 6 Presentation Presentation

5 5 Session Session

4 4 Transport Transport

3 3 Network 3 Network I-...... ;..=.;.;:.;..:.:.----I-~ ------Network ------~~------_4 2 2 Data Link 2 Data Link I------+~------Data Link ------~~------_4 1 1 Physical Physical L.--__r------L~ ------~~--~--~ Transmission Media '--v-----' '--v-----' '--v-----' End System Network Node End System

Intermediate nodes in an OSI network require only bottom-layer functions of the OSI modeL Note how peer layers communicate only with their peers; i.e., Layer I talks to other Layer Is but not to Layer 2s.

• Data transfer, in accordance with the agreed quality of service • Noting errors for reporting unrecoverable errors to the trans- port layer • Transport-connection release • Sequencing network control data units The European Computer Manufacturers Assn. (ECMA) has • Row control defined this layer in its Transport Protocol standard, ECMA-72. In the early 199Os, as the popularity of the OSI protocols be­ • Releasing the network connection gan to wane and TCPIIP began to take over, a number of hybrid stacks were developed whereby data streams could "cross over" The Network Layer is where routers and, nowadays, some LAN from one type of stack to another, in either direction. The cross­ switches operate. They are indifferent to the type of network and over point was at the Transport Layer in all cases. This allowed can therefore be used to pass data from one type of network to applications to reach their intended destinations whichever sys­ another, for example from Ethernet to Token-Ring. Although tem they were using, and encouraged interoperability. Although many routers have OSI protocol support, in fact it is little used, IP this was in keeping with the aims of the ISO, it made possible being by far the preferred protocol for this layer. Given the impor­ migration to the newly-popular TCPIIP stack, and aided the even­ tance of the Internet, which today runs almost entirely on IP, this tual near-demise of the OSI stack as a complete set of protocols. can only increase. However, the remarks made earlier as to how the model has formed the way we think about networks holds The Network Layer particularly true for this layer and Layer 2-nobody can talk co­ The Network Layer (Layer 3) provides the means to establish, herently about networking without mentioning these layers. maintain, and terminate connections between systems. Its basic service is providing transparent data transfer between transport The Data Link Layer entities. Data Link Layer 2 provides the procedural and functional means to establish, maintain, and release data link connections between The services provided by this layer encompass the following: two network nodes or network entities and to transfer data frames (or packets). This layer also detects and may correct errors that • Establishing network connections for transporting data be­ occur in the Physical Layer. tween transport entities through network addresses Services provided by the Data Link Layer to the Network • Identifying connection endpoints Layer include data link connection, sequencing, error notifica­ tion, flow control, and data unit transfer. • Transferring network service data units

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Layers are sometimes divided into sublayers, for several rea­ OSI Standards Progress sons. Layer functions are often divided into separate modules to handle the service interface of the layer beneath it. This avoids There are four stages in the developmerit cycle: working paper; "rewriting" the entire layer. The Data Link Layer of the IEEE 802 committee draft (CD), previously known as a draft proposal; draft Local Area Network (LAN) standards is divided into a Logical international standard (DIS); and international standard (IS). A Link Control (LLC) sublayer and a Media Access Control (MAC) working paper is developed in the first stage. When it matures and sublayer. The MAC sublayer depends on characteristics of the contains well-developed technical concepts, it is registered as a underlying" Physical Layer. Any layer may originate a message to CD. Passage advances the CD to the DIS level, and the document fulfill its responsibilities. The message may not bypass any layer is considered sufficiently stable to serve as the basis of initial en route to its destination. If a message leaves the node, it will end implementations. At the DIS level, the document is distributed for up in another node at the same layer that originated the message. a 180-day ballot. The DIS may require multiple ballots. A suc­ It is at Layer 2 that bridges and most LAN switches reside. All cessful ballot elevates the DIS to the level of IS and completes such devices have a MAC address table for all the end stations ISO's process. (which all have MAC addresses). The entire process usually takes between four and eight years. Here we can see one of the reasons for the lack of popularity for The Physical Layer the protocols involved. The ITU-T was formerly the CCITT, The lowest of the OSI layers is Physical Layer I. It provides the under whose aegis these standards were set. The CCITT has now electrical, mechanical, functional, and procedural characteristics been devolved into the International Telecommunications Union for activation, maintenance, and deactivation of a physical con­ (ITU). Telecommunications standards can be set over a long nection. Physical Layer standards specify physical interfaces period of time as the voice and WAN world needs to ensure high (connectors) connected by a physical medium. quality and complete interoperability between systems. Data net­ working, on the other hand, in many cases exists in isolated Services provided by this layer include the following: LANs, and though these are almost always today connected to the outside world in some manner, a gateway of some sort, often IP, • Activating and deactivating physical connections gives enough connectivity. For Wide Area Networks (WANs) the • Data circuit identification same needs may apply as for voice but there is a lot of crossover between the types of device used and the WAN manufacturers • Sequencing have adopted the same standards as are used in LANs in many • Transmitting physical service data units either synchronously cases. With the new standard, Asynchronous Transfer Mode or asynchronously (ATM), the standard was set for WANs, but became developed by • Fault condition notification the LAN vendors far more quickly. Given the fast-moving nature of LAN developments, with low-cost high-volume products Abstract Syntax Notation One (ASN.1) appearing in ever greater numbers and new high-specification ASN.I is a specification language adopted for the OSI Reference models ensuring replacements in ever-shorter times, it is easy to Model, giving standards developers a common method for defin­ see that a standards process that takes up to eight years to com­ ing syntax. ASN.I is somewhat analogous to grammatical rules plete is not workable. LAN standards today are proposed one defining the English language, with the exception that it is not year and a first version out the next. Again, the ISO process that procedural. Just as English grammar specifies notation (punctua­ demands a complete standard before the first release is ignored. tion symbols) and word classifications (such as nouns and verbs), as soon as a halfway working standard can be given out, it is, with ASN.I specifies the rules that help standards developers define a further release the year after. A case in point is the new LAN complex data types in terms of simple building blocks. standard for virtual LANs, being developed by the IEEE 802.lq ASN.I was first formally described and published in 1984, in working group-the first release, planned for mid- to late 1997, the ITU-T X.409 standard entitled "Message Handling Systems: has only the facilities for port-switching LANs, whereas the idea Presentation Syntax and Notation." It is now described (in less of completely separating the logical from the physical LAN readable fashion) in two later documents: ITU-T X.208 (ISO (which is what a virtual LAN can do) needs far more than that to 8824), entitled "Specification of Abstract Syntax Notation One be efficient. (ASN.l)," and X.2OO (ISO 8825), "Basic Encoding Rules for A list of standards organizations, together with contact details, Abstract Syntax Notation One (ASN.l)." associated with the OSI Reference Model and other standards According to ASN.I, each fragment of information must pos­ mentioned here is given at the end of this report. sess a type and a value. For example: OSI Applications Standards .• Device-Status could be a type (in this case, it is a Boolean tyj,e) In the early 1990s, applications at Layer 7 were considered the • Zero or One are the possible values driving force of OSI acceptance. In particular, the ISO electronic mail standard for message handling systems (MHSs)-or This is specified in ASN.I notation as such: X.400-was becoming popular in commercial implementations. Two versions, one in 1984, and another in 1988, were unratified Device-Status ::= Boolean draft international standards. The standard was given a big boost Boolean ::= I (or 0) in 1989, when the Aerospace Industry Assn. (AlA) adopted X.400 to interconnect its diverse electronic mail networks. Gateways to This is a very simple example; ASN.I is a powerful grammar, proprietary E-Mail systems were also developed that year, and capable of specifying very complex data types. Hence, it will dozens of vendors rolled out X.400-based products. In addition, continue to be the grammar of choice for specifying open systems the ISO adapted XAOO as the message medium for electronic data standards and protocols. interchange (EDI). In this context, X.400 was to be used as the communications method to· store and forward trade documents and business forms conforming to ANSI X 12, the European EDI­ FACT, and de facto EDI standards. Alas, MHSIX.400 proved

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costly and complicated to implement, and was overtaken by Management Protocol (SNMP) in the LAN, CMIP is still popular other, simpler, electronic mail standards, particularly, since the for WAN management, showing once again how different the two mid-199Os, Internet mail based on IP. worlds can be. SNMP has taken up the structure for management One component of successful E-Mail internetworking is direc­ that OSI set up with CMIP and now does the job of managing tory services (OS), with the ISO version known as X.500. Again, networks perfectly well. (For more information, see the OSI this seems to be unworkable and, while no international standard Management section featured later in this report.) has taken over, Novell's NOS and Banyan's StreetTalk provide Other Layer 7 protocols include distributed Transaction Pro­ most of the directory functionality in use today. Microsoft is also cessing (TP), designed to interconnect different transaction com­ in the process of developing a similar function. In today's open puting systems across OSI networks; Remote Database Access networking environment, reliance on proprietary software is un­ (RDA), a protocol for integrating database management systems; usual (unless as pre-standard releases) and it can only be con­ and Manufacturing Message Specification (MMS), ISO 9506, a cluded that X.500 proved more or less unworkable, although manufacturing protocol that requires extensions for specific Banyan declared its StreetTalk to be based very much upon X.500 manufacturing device types. and obviously drew upon it for a lot of the ideas. X.500 specifies an on-line directory for message communica­ Connection Methods tions, ultimately allowing network providers to map a common, Every layer of the OSI Reference Model, except the Physical interconnected directory of worldwide users. X.500 dictates nam­ Layer, supports both connection and connectionless mode (this is ing conventions, how users access directory information, and one of the reasons the protocols are so complex and expensive to what services are available. implement-{)ther technologies are one or the other). Connec­ The OSI protocol pair for office automation, Office Document tion-oriented service requires a connection establishment phase, a Architecture (aDA) and Office Document Interchange Format data transfer phase, and a connection termination phase; a logical (ODIF), has been an international standard since 1988 (ISO connection is set up between end systems prior to data exchange. 8613). It was also specified in the various governments' GOSIP These phases define the necessary sequence of events for suc­ procurement standards until the demise of GOSIP worldwide in cessful data transmission. Connection-oriented service capa­ the early 199Os. aDA and ODIF facilitate the exchange of office bilities include data sequencing, flow control, and transparent documents-such as letters, memoranda, and business reports­ error handling. among dissimilar systems. Moreover, the standard specifies the In a connectionless service, such as Switched Multi-megabit formatting and exchange of compound documents-those con­ Data Service (SMDS), each Protocol Data Unit (PDU) isjnde­ taining combinations of text, images, and graphics. Several ISO pendently routed to the destination; no connection establishment working groups are attempting to strengthen and extend the stan­ activities are required, since each data unit is independent of the dard in such areas as the inclusion of audio, spreadsheet data, previous or subsequent one. Connectionless-mode service trans­ color graphics, document security, and various layout and presen­ fers data units without regard to establishing or maintaining con­ tation styles. nections. In connectionless mode, transmission delivery is uncer­ The OSI standard for sending and sharing data files-File tain due to the possibility of errors. This appears contrary to the Transfer, Access, and Management (FTAM)-is also a finalized goal of network design--users want to ensure that messages international standard. It is a Layer 7 component of the Manufac­ reach their destination. In reality, connectionless-mode communi­ turing Automation Protocol (MAP), which was developed from cation simply shifts responsibility for message integrity to a networking efforts in the manufacturing industry. It spread higher layer, which checks integrity only once, rather than requir­ quickly in Europe and made significant progress in domestic ing checks at each lower layer. Alternatively, each data unit might business applications, but the file protocol used with Transmis­ contain the error recovery mechanism. sion Control Protocol/lnternet Protocol (TCPIIP)-File Transfer In connection-oriented networking, such as ATM, a connec­ Protocol (FfP)-was also well established in the U.S., however, tion is established for the whole data stream just once, and all and generally more popular than FTAM. FTPhas since taken over packets or cells follow that path. There is now also what might be from FTAM for most file transfer and is the established way of thought of as a hybrid never dreamed of in OSI-IP switching, downloading large files from Internet sites. It was thought that where the first packet is examined but the rest follow the path of FfAM would become the standard for EDI transfers but, again, it the first through the switch, despite the connectionless nature of became apparent that it was too cumbersome, and that was when the protocols involved. This depends on having a switched net­ X.400 became popular for that. The difference in use between work. Multiprotocol Over ATM (MPOA) will perform the same X.400 (or any messaging system) and FTAM (or any other file function when standardized late in 1997. transfer protocol such as FfP), is that file transfer is real time while X.400 is store and forward. With any store-and-forward Lower-Layer Protocols system, there can be long delays while the information gets Lower-layer OSI protocols for Layers 1 through 3 are well­ through the network-with FfAM and FTP any delays during defined veterans and in many cases borrowed from existing EIA, transmission are usually brief, but are variable depending upon IEEE, or ITU-T standards_ Connectionless communications at the bottlenecks on a large system such as the Internet. lower layers of the OSI model is well established and is found, for The Layer 7 protocol for network management, Common example, in LANs and metropolitan area networks (MANs). Management Information Protocol (CMIP), is now an Interna­ While the original OSI model-described in ISO 7498-was tional Standard. CMIP is a communications protocol between the connection oriented, the ISO foresaw the need for connection­ agent process, and management agents at each managed OSI less service and issued an addendum to that protocol (ISO 74981 node. The real work of managing network processes is located ADI). The ISO standard for Network Layer service, ISO 8348, within each node's managed objects at individual OSI layers. In contains connectionless service (in ADl) in addition to the con­ other words, each layer must have its own network management nection mode. system, which OSI does not specify. CMIP allows a centralized management process to either modify the value of an attribute, or OSI Security request its value (read its status) at each of the layers. While By definition, an open system is one that encourages communi­ CMIP has been more or less superseded by the Simple Network cations between different applications or users. Unfortunately, an

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open system can also encourage illegal eavesdropping and infor­ cannot communicate across different networks if they implement mation theft or destruction. Notorious examples of white-<:ollar different options at these layers. crime, corporate espionage, and network intrusions by computer Despite OSI's early promise, a majority ·of networks no:w use worms and viruses have alarmed information processing profes­ TCPIIP for LAN interconnectivity. The Internet, the. tool and sionals, and raised a general awareness of computer security is­ plaything "Of the 1990s that is ARPANET's grandchild, runs sues. The concepts of information security and open systems are almost exclusively on IP, though ATM is hidden now in the infra­ antithetical; nevertheless, the ISO has taken steps to provide a structure, and we can expect this to increase as ever more band­ secure environment within the OSI Reference Model. width is needed. TCPIIP is also used for Ethernet LANs, and International Standard 7498, Part 2 addresses a security Novell has recently withdrawn its proprietary IPX from new Net­ architecture within the general OSI model. It describes security Ware installations, preferring instead to migrate customers to measures that can be provided by specific layers in the model. pure IP. Specific security standards are not yet defined, however, but are under study by working group JTC1, Subcommittee 27 OSI Management for Information Technology Security Standards, plus other sub­ committees. Since the first draft of the seven-layer ISO model was produced in SC2l, concerned with maintaining and defining the upper 1978, extensions to the basic model have been developed to more three layers of the OSI Reference Model, stabilized several net­ adequately represent all of the functions required by large-scale, work management and security standards in 1991. The Security multivendor networking environments. OSI Management is an model is composed of six frameworks that work together across extension to the original reference model that specifies transfer of all seven layers of the OSI Reference Model: authentication, network management information in the Application Layer and access control, security audit, nonrepudiation, confidentiality, support for network management functions at Layers 4, 5, and 6. and integrity. Advantages and Disadvantages to OSI·Based OSI and MAP/TOP Network Management In addition to solving the problem of managing heterogeneous The Manufacturing Automation Protocol and Technical Office environments, OSI-based network management played a part in Protocol (TOP) were originally developed by General Motors and bringing about a new phenomenon-the unbundling of network Boeing Computer Services, respectively; to automate manufac­ management from network products. In a proprietary enVlron- . turing functions on the factory floor and in the "back office." Both ment, a given vendor's products are primarily manageable only are based on the OSI Reference Model, using formal standards by products developed by that vendor. The promise ofOS I helped for each layer where possible. MAP, in particular, is probably the to split that one-to-one relationship, making it possible for any best-known example of a formal multilevel OSI implementation OSI-based network management system (NMS) to manage any and is achieving substantial industry acceptance. OSI Management-conformant device. Despite being widely sup­ Version 3.0 added a Presentation Layer to the profile and ported, CMIP has in the end lost out to SNMP in the LAN envi­ implemented a version of the Manufacturing Message Specifica­ ronment, but the initiative encouraged the widespread publication tion (MMS), the protocol for transferring factory and robotics of private Management Information Bases (MIBs) so that hetero­ information, ISO 9506. Other Layer 7 protocols specified are geneous networks could be managed under SNMP. In the WAN, FfAM, Network Management, and Directory Service. Middle CMIP is both widespread and popular. Here, there is not such an layers implement ISO connection-oriented protocols, although open environment due to lack of customer demand for it, and such these must be bypassed for time-critical applications. At the lower interoperability as there is must be reliable. WAN management is transport layers, MAP specifies the IEEE 802.4 token bus system vital--LAN management is still often, wrongly, treated as a employing a Type F coaxial connection to a 75-ohm cable. Al­ lUXUry. though MAP was taken up widely in some countries, notably The market (both vendors and users)widely criticized ISO for Japan, it has not been further pursued generally. LANs have moving too slowly in its efforts to ratify OSI Management stan­ moved on and few new LANs use token bus or coaxial cable now dards. Vendors are wisely unwilling to develop products based on (except that in the new field of home LANs, coaxial cable is often standards that are not yet final. In an effort to open the door to employed for its ease of use, but standard Ethernet is universally new OSI~based network management system products, SC21 installed here). WG4 defined groups of network management functions that were to be covered within OSI-based network management: fault, con­ OSI and TCP/IP figuration, performance, accounting, and security management specifications. These principles, if not the actual protocols, have Transmission Control ProtocollInternet Protocol was developed been established as the expected field to be covered by any net­ by the U.S. government's Defense Advanced Research Projects work management system (NMS), and this is one of the areas that Agency (DARPA) for its research network, ARPANET. By 1986, OSI has changed networking profoundly. TCPIIP had gained a following of commercial users seeking a Another disadvantage of standards-based network manage­ protocol that could be used as a common denominator for multi­ ment is that OSI standards merely provide a menu of options. vendor computer networks. TCP and IP are actually two separate There are numerouS gaps and ambiguities in OSI Management protocols, occupying middle layers number Four (Transport) and standards that could be interpreted differently, leading to incom­ number Three (Network), respectively, of the OSI Reference patible implementations. Model. TCPIIP has been implemented on almost every type of com­ Standards Documents puter and is especially successful in commercial Ethernet LAN OSI Management standards can be broadly categorized into four environments. The reason TCPIIP is so popular is because it is areas: free and its development was paid for by the U.S. government. It 1. Functions--what network management is, according to OSI avoids the connection-orientedlconnectionless dilemma by essen­ tially avoiding it. OSI provides a richer set of network options, 2. Services--how network management functions are accom­ but these may not be compatible in different networks. Users plished

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3. Information Structure-terms and categories describing Information Structure what is managed (e.g., "management information") The most important standards in this category are Structure of 4. Protocols-describe means of transporting network manage­ Management Information (SMI), Parts I, 2, and 4 (CD 1Ol6?-I, ment information -2, and -4). (Part 3 is not missing; rather, ISO merged Part 3 mto Part 4.) Included in these standards is an explanation of the Taken together, these four areas describe a generic package for object-oriented paradigm, used to model a network in terms of network management systems, and how these products relate to object classes and attributes. In object-oriented environments, a the network devices they manage (called managed objects in OSI variable (for example, a variable called Bridge) is defined both in terminology). terms of the operations that can be performed on it and the values A blueprint document, Management Framework OMNIPoint, of attributes it can possess. For example, Bridge can have an places the OSI Management environment in perspective by attribute such as Status, which may have a value of Busy; a net­ describing terms and the scope of OSI network management. work management system may obtain this value via a Get opera­ tion, or alter it via a Set operation. OSI Management Functions Objects (including their attributes and operations) are stored OSI Management functions are described in the Systems Man­ in a MIB, sometimes called an Object Library. The SMI docu­ agement standards (IS 10040, IS 10164-1 through 10164-7, and ments just listed provide syntax and semantics for information in N 10164-8 through 10164-12). Management using three models: the MIB; however, no single ISO standard defines exactly what the OSI MIB will contain, nor how vendors and users can register 1. The Organizational Model---describes ways OSI Manage­ objects in the standard MIB. ment can be distributed administratively In the TCPIIP world, an Internet Standard MIB exists for 2. The Information Model-provides guidelines for defining objects managed using SNMP. This MIB functions in an analo­ managed objects and their interrelationships, classes, and gous role to the proposed OSI MIB, although the administration names and rules governing the two are sure to differ. MIB includes all information needed to make management 3. The Functional Model-describes network management decisions. MIB is a conceptual repository of all OSI management functions data in an OSI environment. The MIB concept does not imply any form of physical or logical storage for management infonnation, The Functional Model outlines how ISO has partitioned network however, and its implementation is outside the scope of OSI stan­ management into five functional areas: fault management, con­ dards. Rather, the SMI defines the abstract syntax and the seman­ figuration and name management, performance management, tics of information, so that it can be represented in OSI protocol accounting management, and security management. ISO origi­ exchanges. nally described each of these areas in its own standard. Further studies revealed that functions overlapped; therefore, ISO reorga­ Protocols nized the documents in December 1988 into their present Sys­ Common Management Information Protocol, IS 9596, is the pri­ tems Management form. mary OSI Management protocol. CMIP specifies procedures for Fault management provides the detection, isolation, and cor­ the exchange of basic management information between open rection of abnormalities in network operation. Configuration systems interconnected by OSI protocols. and name management facilities permit network managers to control the configuration of the system, network, or layer entities. X.500.-The Directory Changed configurations may isolate faults, alleviate congestion, The Directory is a related standard designed to manage name­ or meet changing user needs. Performance management enables related infonnation concerning protocol layers and network the network manager to monitor and evaluate the performance of nodes. These services connect the actual names used in the net­ the system, network, and layer entities. Data from performance work with names and addresses understood by human users. The management may be used to initiate configuration changes and Directory is defined in CD 9594. diagnostic testing to allow a satisfactory level of performance. Accounting management facilities help determine and allocate costs for the use of a network manager's communications re­ OSI and the Future sources. Security management facilities permit the management The world needs a network of computers, similar to standards for of those services providing access protection of communications international telephony, to link users across oceans and conti­ resources. nents. A few years ago, most industry analysts perceived OSI as Services the answer. Although endorsed by such prominent vendors as IBM, DIGITAL, and Hewlett-Packard, OSI's once-bright future Services are described, in part, in the Common Management In­ as the premier means of interconnecting multivendor computer formation Protocol (CMIP) standard, IS 9596. Services use networks is now dimmed. Where OSI was too complex, slow, and primitives, or command types, to accomplish network manage­ expensive, TCPIIP stepped in to fill the gaps. The OSI Reference ment functions. Examples of CMIP commands include GET, Model also had some technical glitches and holes that prevented SET, GET REPORT, CREATE, and DELETE. While service it from being widely implemented. These will probably never primitives are somewhat abstract, they are important building now be repaired. As an internetworking protocol, TCPIIP has blocks for composite commands used by network management proved its worth and is a popular and commercially succ~ssful applications to obtain vital data on the status and activity of net­ method of linking users across diverse networks. OSI IDlddle­ work devices. layer protocols 3 through 5, the altematives to TCPIIP, are not as Common Management Infonnation Services (CMIS) include simple to implement in the real world. SNMP, the network man­ a detailed abstract model of open systems management services. agement protocol for TCPIIP networks, is also a proven, comm r­ ( 7 If" These fall into three categories--event notification, information cially successful solution. As long as vendors and users requIre transfer, and control. Event notification allows one system to no­ practical networking products, they will continue using TCPIIP­ tify another that some event of importance has occurred. based protocols-standard or not.

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In reality, OSI and other layered architectures .do not serve . eye toward future standards developments. As it stands now, OSI every application and are not a panacea. Proprietary architectures will not be the interconnection standard of the future .. Significant , will continue to thrive alongside both de j(lCto and de jure stan­ portions of it, however, have most certainly contributed to the dards-based networks, especially for closed user groups (where evolution of international standards. The OSI 7-Layer model it­ internetworking is not a requirement) or in time-sensitive appli­ self, however, has proved itself over and over. This view of data cations intolerant of layered protocols' high overhead, communication has become universally accepted, with even All others who desire internetworking must realize that the SNA-which is layered differently-often explained in the OSI associated protocols are still evolving-nothing is truly cast in model's terms. While the standards and protocols may not iron. In market-based economies, products that do .not satisfy develop, the structure has added greatly to a common understand­ market needs will not gain widespread favor. Therefore, prospec­ ing of networking and has now also stood the test of time. - tive users must evaluate OSI protocols and their adoption with an

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( ISO Reference Model for Open Systems Interconnection (OSI)

In this report: Datapro Summary

051 Standards Progress ..... 6 The goal of Open Systems Interconnection (OSI) is to enable dissimilar computers in mul­ tivendor environments to share information transparently. The OSI structure calls for coop­ 051 Management ..•..•...... • 9 eration among systems of different manufacture and design. With this capability, global digital networks can become a reality. There are seven layers of the OSI model that commu­ 051 and the Future .....••..... 11 nicate between one end system and another. The layers cover nearly all aspects of informa­ tion flow, from applications-related services provided at the Application Layer to the phys­ Note: This report updates ical connection of devices to the communications medium at the Physical Layer. Although the OSI's status at all all seven layers have long since been defined and ISO protocols ratified for each layer, the seven layers; compares ISO committees must keep refining and extending specific sections of the model by rewrit­ OSI to other architectures; ing existing definitions and adding new protocols. rationalizes the need for standards testing and veri­ fication; profiles major testing organizations; and outlines OSI Management standards and status. Analysis The Open System The ISO defines a system as a set of one or more computers and associated software, peripherals, terminals, human operators, physical processes, The proliferation of computerized data process­ infonnation transfer means, etc., which fonn an ing systems in the late 1960s produced a need for autonomous whole capable of perfonning infor­ compatible data communications networks in mation processing and/or infonnation transfer. the 1970s. Several proprietary network architec­ An open system is one that obeys OSI standards tures were developed for mainframe-to-tenninal in its communication with other systems. communications, including IBM's SNA in 1974. An application process is an element within a Although many of these proprietary architec­ system that performs infonnation processing for tures were based on a layered model, none were a particular application. The application process compatible with any other. The CClTf's X.25 can be manual (a person operating a banking ter­ host interface to the packet networks standard minal), computerized (a program executing in a was ratified in 1976 but is not a complete net­ computer center and accessing a remote data­ work architecture. In 1977 the International Or­ base), or physical (a process control program ex­ ganization for Standardization (ISO) formed ecuting in a dedicated computer attached to in­ ISO Technical Committee 97 (TC97), Subcem­ dustrial equipment and linked to a plant control mittee 16 (SC16), to embark on a worldwide system). standardization effort and confront the issue of The OSI structure calls for cooperation incompatibility head-on. The pUIpOse of TC97/ among systems of different manufacture and de­ SC 16 was to develop a model and define the pro­ sign. This includes coordinating activities such tocols and interfaces required to support an open as the following: system. The goal of OSI was, and still is, to en­ able dissimilar computers in multivendor envi­ • Intetprocess communications-the synchro­ ronments to share infonnation transparently. nization between OSI application processes With this capability, global digital networks can and the exchange of infonnation become a reality. • Data representation-the creation and mainte­ nance of data descriptions and transfonna­ tions for refonnatting data exchanged be­ tween systems

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• Data storage---storage media, file systems, and database sys­ The seven layers can be divided into two functional groups: tems for providing access to and management of stored data the Transport Platform (Layers 1 to 4) and the Application Plat­ • Process and resource management-how application processes form (Layers 5 to 7). The Transport Platform's function is to get are declared, initiated, controlled, and acquired data from one system to another without errors. The Application Platform's function is to interpret the data stream and present it to • Integrity and security-information processing constraints that the user in a usable form (see Figure 1). must be ensured during open systems operations Each layer contributes functions to the communications task. • Program support-the definition, compilation, testing, linking, For example, the Link Layer enables communications across a storage, and transfer of and access to programs executed by the single physical connection, while the Network Layer provides application processes end-to-end routing and data relay. Services at the upper-layer in­ terface-providing communications to the next-higher layer­ The OSI model is concerned only with the exchange of informa­ are provided by each layer, usually described by a service speci­ tion between open systems. fication for the layer. Services at each layer are provided by a layer entity. Each layer entity communicates with its peer at the The Layering Concept same layer on another system, providing services specified in the Layering is a basic structuring technique used in the OSI model. service specification. Each layer is composed of an ordered set of subsystems, with Layers are sometimes divided into sublayers, for several rea­ logically related functions grouped together. The OSI model sons. Layer functions are often divided into separate modules to breaks down internetworking activities between systems into two handle the service interface of the layer beneath it. This avoids distinct groups. Communications-oriented functions are sepa­ "rewriting" the entire layer. For example, the Link Layer of the rated from user-oriented functions; features which move informa­ IEEE 802 Local Area Network (LAN) standards is divided into a tion across a network are distinct from features which handle and Logical Link Control (LLC) sublayer and a Media Access Con­ format information. trol (MAC) sublayer. The MAC sublayer depends on characteris­ There are seven layers of the OSI model that communicate tics of the underlying Physical Layer. Any layer may originate a between one end system and another end system. The layers message to fulfill its responsibilities. The message may not by­ cover nearly all aspects of information flow, from applications­ pass any layer en route to its destination. If a message leaves the related services provided at the Application Layer to the connec­ node, it will end up in another node at the same layer that origi­ tion of devices to the communications medium at the Physical nated the message. Layer. Below the Physical Layer, the media itself corresponding IBM's SNA is also a layered architecture, following rules of to "Layer 0"-such as wire, cable, or through-the-air communi­ layering similar to OSI and other layered architectures. There are cation-is currently not addressed by the model. Application, good reasons for layering: layering simplifies change; compo­ Presentation, Session, Transport, Network, Data Link, and Phys­ nents inside a layer can be changed without affecting any other ical Layers have been dermed (see Table 1). The model described layers in that node. Layers are like structured programming-but in Table 1 is OSI's seven layers with their purposes. In Table I, for teleprocessing systems. Because there are rigid interfaces be­ information flows down from Layer 7 to Layer I, and then out tween levels, fewer people need to react to changes, allowing over a physical transmission medium. At the receiving end, the them to be implemented faster. There is no better way of achiev­ information flows into another end system and up from Layer 1 to ing complex functions. Layering allows each network function to Layer 7, until it is received by a user.

Table 1. The Seven Layers of OSI

Layer Name .Purpose

7 Application Applications and application interfaces for OSI networks. Provides access to lower-layer functions and services. 6 Presentation Negotiates syntactic representation for the Presentation Layer and performs data transformations. 5 Session Coordinates connection and interaction between applications. Establishes a dialog, manages and synchronizes the direction of dataflow. 4 Transport Ensures end-ta-end data transfer between applications, data integrity, and service quality. Assembles data packets for routing by Layer 3. 3 Network Routes and relays data units among network nodes. 2 Data Link Transfers data units from one network node to another over a transmission circuit. Ensures data integrity between nodes. "'-'-- Physical Delimits and encodes the bits onto the physical medium.

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( Suite SOO, 311 First Street, Place des Nations NW CH-1211 Geneva 20, Switzer- Standards Organizations and Washington, DC 20001-2178 land (202) 737-8888 (+41) 22 99 51 11 Fax: (+41) 22 33 72 56 Testing Agencies Be" Communications Re- Telex: 421 000 UIT CH search (Be"core) 60 New England Avenue Institute of Electrical and Piscataway, NJ 08854-4196 Electronics Engineers (908) 699-2000 (IEEE) Although standards are indis- expertise. A primary objective Customer Service Hot Line: Headquarters pensable for computer inter- is demonstrating interoperabil- (800) 521-CORE 345 E. 47th Street networking, they are useless ity among different vendors; New York, NY 10017 Corporation for Open Sys- unless vendor claims of com- i.e., proving that standards (212) 70~7900 tems(COS) patibility can be tested and really work and fostering end- Fax (publications): (212) 70~ Suite 400, 1750 Old Meadow users can be ensured of 00- user interest. Many agencies 7682 quiring products that comply have tried vainly to involve Road fully with the standards. In more end users but are McLean, VA 221 02-4306 IEEE Service Center general, vendor claims of backed primarily by the ven- (703) 883-2700 (published standards): standards compatibility are dors. Telex: WU16503157578 MCI 445 Hoes Lane, P.O. Box suspect unless verified by an UW 1331 In the U.S., primary testing Piscataway, NJ 0885~1331 impartial testing agency. Addi- European Computer Manu- tionally, compatibility claims and certification agencies in- (908) 981-1393 clude the Corporation for facturers Assn. (ECMA) Fax: (908) 981-9667 can mean different things to Rue du Rhone 114 different people. For users, it Open Systems (COS), the Telex: 833-233 National Institute of Science CH-1204 Geneva, Switzerland is a good idea to probe vendor (+41) 22 735 36 34 OSINET claims of standards compati- and Technology (NIST), and Bell Communications Re- Telex: 413237 ECMA CH Mr. Jerry Mulvenna, Chairman bility to determine what is OSINET Steering Committee compatible with what, and at search (Bellcore). Several Electronic Industries Assn. smaller organizations and cer- NIST Building 225, Room what levels. (EIA) B217 tain vendors, however, also 1722 Eye Street NW, Suite Several standards testing and offer testing services. Europe Clopper Road 300 Gaithersburg, MD 20899 verification bodies have been is represented by the Stan- Washington, DC 20006 organized both here and dards Promotion & Application (202) 457-4900 Standards Promotion & Ap- abroad by vendor consor- Group (SPAG); Japan by the plication Group SA (SPAG) tiums, government agencies, Promoting Conference for OSI International Organization Avenue Louise 149, Box 7 and independent organiza- (POSI). Standards organiza- for Standardization (ISO) 10SO Brussels, Belgium tions. They have found that tions are listed below: 1, Rue de Varembe (+32) 2 535 08 11 developing conformance Case Postale 56 Telex: 20307 SPAG B specifications, producing test- American National Stan- C8-1211 Geneva 20, Switzer- ing suites, and conducting dards Institute (ANSI) land National Institute of Stan- comprehensive testing are 1430 Broadway (+41) 22 33 34 30 dards and Technology complicated, expensive, and New York, NY 10018 (NIST) time consuming. Regional dif- (212) 642-4900 International Telegraph and U.S. Department of Com- Telephone Consultative ferences can stymie attempts ANSI X3 Secretariat merce at verification. The trend for Committee (CCITT) Gaithersburg, MD 20899 Computer and Business General Secretariat these organizations, therefore, (301) 97~2000 Equipment Manufacturers International Telecommunica- is to cooperate with each Assn. (CBEMA) other, sharing resources and tions Union (ITU)

be made "transparent," unaware and independent of other func­ and then up through six layers at the other, until it reaches layer tions at other layers, thus enabling any layer to be modified with­ seven at its opposite (peer) layer. out changing the entire monolithic architecture. Each network node (a network user, computer, terminal) is Each layer may support one of several different protocols de­ equipped with this layer mechanism. However, not all intermedi­ signed for specific network applications; the choice of a specific ate nodes need all seven layers. Network nodes, in particular, protocol is optional, allowing users to tailor networks to their own must only route and transmit data packets-functions at the bot­ design. Each layer defines functions crucial to the communica­ tom three layers of the OSI model. Layer 4 through 7 functions tions process at that layer, independent of the other layers. How­ are not required and, therefore, not included in network node soft­ ever, a layer may perform functions hinging on functions per­ ware. Data packets processed in these nodes reach only Layer 3 formed in the layers immediately above or below. A layer can and are then routed elsewhere (see Figure 2). A node communi­ only communicate with another device or network node at its peer cates with its peer in another node sending or receiving data. Data layer. Messages exchanged between peer layers are "enveloped" transfer is routed from Layer 7 down to Layer 1 at the transmit­ with messages from other layers and passed through these other ting node, then along the network to Layer 1 at the receiving layers on the way to their destination, picking up and then shed­ node, and finally from Layer 1 up to Layer 7. Peer layers commu- ding these other protocol layers along the way. For example, if nicate by the same method. . layer seven at one end system must send a message to layer seven at another, it must travel down through six layers at its own end

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Figure 1. does not provide· services to a layer above it. Some of the services. ApplictJlion tuUl Transport Divisions provided by this layer, other than information transfer,are the'-<_ following: • .Identifying intended communications partners 7 • Determining current availability of the intended partners AppHcation • Establishing the authority to communicate. 6 Application • Agreeing on responsibility for error recovery Presentation Platform • Agreeing on procedures for controlling datil integrity The Presentation Layer 5 The Presentation Layer (Layer 6) allows an application·to inter­ pret the meaning of information exchanged. Information is for­ Session matted and translated at this layer. Aspects of Layer 6 include data syntax, which is the data to be transferred between layers, and the presentation image syntax, which is the data structure that appli­ cation entities refer to in their dialog, or the set of actions that may be perfomied on the data structure. Services provided to the Presentation Layer include the fol­ 4 lowing: Transport • Transforming data syntax, primarily code and character set conversion 3 • Transforming and selecting the presentation syntax, the adap­ Network Transport tation and modification of the presentation data (the OSI view) Functions within the Presentation Layer include session estab­ 2 Platform lishmentrequest; data transfer; negotiation and renegotiation of Data Link data syntax and presentation image syntax; and sessiontermina­ tion request.

1 The ~OD Layer Physical The Session Layer (Layer 5) allows cooperating presentation en­ tities to organize and synchronize their dialog and to manage data exchange. It provides the following services: The seven layers are divided into two junctional groups. • Session-connection establishment-creation of an exchange The message initiated at the Application Layer is passed from between presentation entities layer to layer, through the various OSI layers, encapsulating con­ trol information in the process. A fully encapsulated message en­ • Session-connection release ters the cable at Layer 1. The procedure is reversed at the receiv­ • Normal data exchange ing end. Each item of control information is processed at its • Expedited data exchange appropriate layer,and the message itself passes upto Layer 7. Data transfer essentially is a packaging process at the transmitting • Interaction management-allowing presentation entities to node and an unpackaging process at the receiving node. take turns exercising control functions • Session-connection synchronization The Lare,. A number of objectives were considered by the reference model's • Exception reporting-permitting the presentation entities to be designers: to limit the number of layers to make the system engi­ notified of exceptional situations neering task of describing and integrating the layers as simple as • Activity management possible; to create boundaries between layers at points where the description of services can be small and the number of interac­ The Transport Layer tions across each boundary is minimized; and to colleCt similar The Transport Layer (Layer 4) provides transparent data flow functions in the same layer. Table 1 summarizes the OSI Refer­ between session entities, freeing the Session Layer from respon­ ence Model's layers; more detailed descriptions follow for each sibility for cost-effective and reliable data transfer. Layer 4 pro­ layeL . vides information interchange according to Ii user-specified reli­ ability level and end-to-end control. Transport protocols transfer The Application Layer information from one end of a physical connection to another and The Application Layer (Layer 7) is the highest·layer, providing ensure that it is delivered correctly. Layer 4 protocols are used the means for the application process to access the OSI environ­ after a route has been established through the network by· the ment. Its function is to serve as the passageway between applica­ network-layer protocol. tion processes using Open Systems Interconnection to exchange information; consequently, all application process parameters are The services provided by this layer include the following: made known to the OSI environment through this layer. ."---- • Transport-connection establishment to complete a connection All services directly usable by the application process (Le., systems and applications management functions) are provided by between session entities the Application Layer. It differs from the other layers in that it

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Figure 2. Message Movement Among OSI Layers

Message moves from Layer 7 through ------.. the other layers to reach Layer 7 ~ at the opposite end

7 7 Application ~------.. Application 6 6 Presentation ~------.. Presentation 5 5 Session Session ~------.. 4 4 Transport Transport ~------.. 3 3 Network 3 Network ------.. Network ~------~ 2 2 Data Link 2 Data Link - Data Link ------~------1 .. 1 Physical 1 Physical Physical ------.. ~------Transmiss ion~ I I I Media \...... __ ) \. ) \._--,.-~) y--~ V V End System Network Node End System

Intermediate nodes in an OSI network require only bottom-layer functions o/the OSI model. Note how peer layers communicate only with their peers; i.e .• Layer J talks to other lAyer Is but not to lAyer 2s.

• Data transfer, in accordance with the agreed quality of service The Data Link Layer • Transport-connection release Data Link Layer 2 provides the procedural and functional means to establish, maintain, and release data link connections between The European Computer Manufacturers Assn. (ECMA) has de­ two network nodes or network entities and to transfer data fmmes fined this layer in its Transport Protocol standard, ECMA-72. (or packets). This layer also detects and may correct errors that This standard has gained the support of a number of North Amer­ occur in the Physical Layer. ican and European computer manufacturers. Services provided by the Data Link Layer to the Network Layer include data link connection, sequencing, error notifica­ The Network Layer tion, flow control, and data unit transfer. The Network Layer (Layer 3) provides the means to establish, maintain, and tenninate connections between systems. Its basic The Physical Layer service is providing transparent data transfer between transport The lowest ofthe OSI layers is Physical Layer 1. It provides the entities. electrical, mechanical, functional, and proceduml chamcteristics for activation, maintenance, and deactivation of a physical con­ The services provided by this layer encompass the following: nection. Physical Layer standards specify physical interfaces (connectors) connected by a physical medium. • Establishing network connections for transporting data be- tween transport entities through network addresses Services provided by this layer include the following: • Identifying connection endpoints • Activating and deactivating physical connections • Transferring network service data units • Data circuit identification • Noting errors for reporting unrecovemble errors to the trans- • Sequencing port layer • Transmitting physical service data units either synchronously • Sequencing network control data units or asynchronously • Flow control • Fault condition notification • Releasing the network connection

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Abstract Syntax Notation One (ASN.1) with Technical Committee 83 (TC83) of the International Electro­ ASN.l is a specification language adopted for the OSI Reference technical Commission (IEC). The me is a coalition of industrial Model, giving standards developers a common method for defin­ standards bodies that is co-located with the ISO in Geneva, Swit­ ing syntax. ASN.l is somewhat analogous to grammatical rules zerland. The new JTCI held its first meeting in 1987. The stan­ defining the English language, with the exception that it is not dardization activities of SC21 report to JTC I. procedural. Just as English grammar specifies notation (punctua­ SC21 is composed of member bodies (MBs) from 23 different tion symbols) and word classifications (such as nouns and verbs), countries. Each MB has its own national standards organization; ASN.l specifies the rules that help standards developers define for example, ANSI represents the United States in JTCI. The complex data types in terms of simple building blocks. individuals or "national correspondents" comprising the MB ASN.l was first formally described and published in 1984, in delegations come from different groups including user organiza­ the CCITI X.409 standard entitled "Message Handling Systems: tions, manufacturing firms, government agencies, and common Presentation Syntax and Notation." It is now described (in less carriers or PTfs. As such, they bring varying perspectives and readable fashion) in two later documents: CCITI X.208 (ISO concerns to the committee sessions. 8824), entitled "Specification of Abstract Syntax Notation One When an OSI committee or working group produces a docu­ (ASN.l)," and X.2OO (ISO 8825), "Basic Encoding Rules for ment such as a CD, the document is circulated among the MBs for Abstract Syntax Notation One (ASN.l)." a vote and to the liaison organizations (LOs) for review. LOs are According to ASN.1, each fragment of information must pos­ independent organizations which also have a vested interest in sess a type and a value. For example: OSI development. LOs provide comments on the content of OSI documents but do not have voting privileges. • Device-Status could be a type (in this case, it is a Boolean type) Status of OSI Protocols • Zero or One are the possible values Protocol standards for all seven layers of the OSI model have been approved; however, OSI committees are refining and ex­ This is specified in ASN.1 notation as such: tending some standards as required and may add new standards at specific layers (particularly Layer 7). Additionally, other stan­ Device-Status ::= Boolean dards groups-such as the CCITI, ANSI, and IEEE-may adopt Boolean ::- 1 (or 0) OSI protocol standards as their own and vice versa. Conse­ quently, many OSI standards are known by more than one stan­ This is a very simple example; ASN.1 is a powerful grammar, dard designation. Table 2 shows some major ISO protocols ap­ capable of specifying very complex data types. Hence, it will proved for each OSI layer and lists corresponding appellations continue to be the grammar of choice for specifying open systems from ANSI, the CCITI, and the ECMA, where applicable. standards and protocols. OSI Applications Standards ISO committees are working hard at Layer 7, the Application OSI Standards Progress Layer. In fact, OSI application standards are perceived as poten­ There are four stages in the development cycle: working paper; tially powerful and versatile and are the driving force for OSI committee draft (CD), previously known as a draft proposal; market acceptance. We devote considerable space reviewing draft international standard (DIS); and international standard (IS). some of the most important ones here. A working paper is developed in the first stage. When it matures In particular, the ISO electronic mail standard for message and contains well-developed technical concepts, it is registered as handling systems (MHSs)-or CCITI X.400-is becoming pop­ a CD. Passage advances the CD to the DIS level, and the docu­ ular in commercial implementations. Two versions, one in 1984, ment is considered sufficiently stable to serve as the basis of ini­ and another in 1988, are draft international standards that have tial implementations. At the DIS level, the document is distrib­ not been ratified. The standard was given a big boost in 1989, uted for a 180-day ballot. The DIS may require multiple ballots. A when the Aerospace Industry Assn. (AlA) adopted X.400 to inter­ successful ballot elevates the DIS to the level of IS and completes connect its diverse electronic mail networks. Gateways to propri­ ISO's process. etary E-Mail systems were also developed that year, and dozens The entire process usually takes between four and eight years. of vendors have rolled out X.400-based products. Most public A list of standards organizations associated with the OSI Refer­ E-Mail carriers have also adopted the standard and are migrating ence Model is given at the end of this report. to the 1988 version. In addition, the ISO is adapting X.400 as the message medium The Evolution of OSI CommiHees for electronic data interchange (EDI). In this context, X.400 In the spring of 1977, ISO Technical Committee 97 (TC97) would be used as the communications method to store and for­ formed a special subcommittee (SC16) charged with developing ward trade documents and business forms conforming to ANSI an architectural model that would extend from applications-layer X12, the European EDIFACT, and de facto EDI standards. communications clear down to the connection with the physical One component of successful E-Mail intemetworking is direc­ interface. The first draft of the seven-layer OSI Reference Model tory services (OS), commonly known as CCITI X.500. X.500 was completed in 1978. Between 1978 and 1983, the Basic Ref­ specifies an on-line directory for message communications, ulti­ erence Model and many of the standards for the individual layers mately allowing network providers to map a common, intercon­ approached or attained draft international standard status. By the nected directory of worldwide users. X.500 dictates naming con­ end of 1984, SC16 was reorganized to form Subcommittee 21 ventions, how users access directory information, and what (SC21). Working groups within SC16 were also realigned. services are available. The OSI Basic Reference Model became an international stan­ Since the 1988 standard is not flexible, SC21 WG4 is working dard in 1984. During 1985 a number of vendors demonstrated to ease the transition to the new 1992 version. Older 1985 X.500 products that implemented these standards and, by the end of systems will require a software modification to work with the 1986, many of these products were commercially introduced. 1992 version. Realistically, the vision of a worldwide messaging In July 1987 the Joint Technical Committee for Information directory probably will not be realized until the late 199Os. Technology (JTel) was formed when ISOITC97 joined forces

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f Table 2. ISO Protocols and Equivalent Standards

Layer ISO CCITT ANSI (1) ECMA

7 8571 (FTAM) Application 10021 (MHS) X.400 9041 (VT) 10026 (OTP) 9594 (OS) X.500 8613 (OOA) T.410 Series, T.73 ECMA-101 9579 (ROA) 9596 (CMIP)

6 8823 (connection) X.226 Presentation 9596 (connectionless)

5 8327 (connection) X.225 X3.153 ECMA-75 Session 9548 (connectionless)

4 8073 (TPO-TP4) X.224 X3.140 ECMA-72 Transport (connection) 860218072 (connectionless)

3 8208 (Layers 1-3) X.25 Network 8348 (connection) X.213 8473 (connectionless) ECMA-92 9542 (15-15) 8878 (use w/8208) (X.25) 8880 (LAN) 8881 (X.25 on LANs)

2 7n6(LAPB) X.25 OataUnk 3309 (HOLC) X3.66 ECMA-40 8802.2-.7 (LAN) ECMA-82, -81, -90, -89 (IEEE 802.2-.7)

9314 (FOOl) X3.148, X3.139, X3.166 Physical 2110 (EIA-2320) V.24, V.28 4902 (EIA-449) V.24, V.28 2593 V.35 4903 X-Series interfaces, Other V-Series

(1) No ANSI standards exist by policy in most instances, as U.S. follows International standards.

The OSI protocol pair for office automation, Office Document working groups are attempting to strengthen and extend the Stan- Architecture (ODA) and Office Document Interchange Format dard in such areas as the inclusion of audio, spreadsheet data, (ODIF), has been an international standard since 1988 (ISO color graphics, document security, and various layout and presen- 8613). It is also specified in the U.S. government's GOSIP stan- tation styles. dard. ODA and ODIF facilitate the exchange of office docu- The OSI standard for sending and sharing data files-File (C- ments-such as letters, memoranda, and business reports- Transfer, Access, and Management (FfAM)-is also a finalized among dissimilar systems. Moreover, the standard specifies the international standard. It is a Layer 7 component of the Manufac- formatting and exchange of compound documents-those con- turing Automation Protocol (MAP), which was developed from taining combinations of text, images, and graphics. Several ISO networking efforts in .the manufacturing industry. FfAM has spread quickly in Europe and has made significant progress in domestic business applications. The file protocol used with

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Transmission Control Protocol/lntemet Protocol (TCP/IP)-File OSI Security Transfer Protocol (FfP)-is also well established in the U.S., By definition, an open system is one that encourages communi­ however, and generally more popular than FTAM. cations between different applications or users. Unfortunately, an The Layer 7 protocol for network management, Common open system can also encourage illegal eavesdropping and infor­ Management Information Protocol (CMIP), is now an Interna­ mation theft or destruction. Recently, notorious examples of tional Standard. CMIP is a communications protocol between the white-collar crime, corporate espionage, and network intrusions agent process and management agents at each 'managed OSI by computer worms and viruses have alarmed information pro­ node. The real work of managing network processes is located cessing professionals and raised a general awareness of computer within each node's managed objects at individual OSI layers; in security issues. The concepts of information security and open other words, each layer must have its own network management systems are antithetical; nevertheless, the ISO has taken steps to system, which OSI does not specify. CMIP allows a centralized provide a secure environment within the OSI Reference Model. management process to either modify the value of an attribute or International Standard 7498, Part 2 addresses a security archi­ request its value (read its status) at each of the layers. Definitions tecture within the general OSI model. It describes security mea­ and descriptions of management structures and managed infor­ sures that can be provided by specific layers in the model. Spe­ mation are contained in other OSI standards yet to be completed. cific security standards are not yet defined, however, but are (For more information, see the OSI Management section featured under study by working group JTCI, Subcommittee 27 for Infor­ later in this report.) mation Technology Security Standards, plus other subcommit­ Other Layer 7 protocols include distributed Transaction Pro­ tees. cessing (TP), designed to interconnect different transaction com­ SC21, concerned with maintaining and defining the upper puting systems across OSI networks; Remote Database Access three layers of the OSI Reference Model, stabilized several net­ (RDA), a protocol for integrating database management systems; work management and security standards in 1991. The Security and Manufacturing Message Specification (MMS), ISO 9506, a model is composed of six frameworks that work together across manufacturing protocol that requires extensions for specific man­ all seven layers of the OSI Reference Model: authentication, ac­ ufacturing device types. cess control, security audit, nonrepudiation, confidentiality, and integrity. SC21 is working to establish two of the six security Connection Methods standards as Draft International Standards (OISs), and the re­ Every layer of the OSI Reference Model, except the Physical maining four standards, which are working drafts, will progress Layer, supports connection and connectionless mode. Connec­ to CD status. tion-oriented service requires a connection establishment phase, a data transfer phase, and a connection termination phase; a logical connection is set up between end systems prior to data exchange. OSI and MAP/TOP These phases define the necessary sequence of events for suc­ The Manufacturing Automation Protocol and Technical Office cessful data transmission. Connection-oriented service capabili­ Protocol (TOP) were originally developed by General Motors and ties include data sequencing, flow control, and transparent error Boeing Computer Services, respectively, to automate manufac­ handling. turing functions on the factory floor and in the "back office." In a connectionless service, such as new Switched Multi­ Both are based on the OSI Reference Model, using formal stan­ megabit Data Service (SMDS), each Protocol Data Unit is inde­ dards for each layer where possible. MAP, in particular, is proba­ pendently routed to the destination; no connection establishment bly the best-known example of a formal multilevel OSI imple­ activities are required, since each data unit is independent of the mentation and is achieving substantial industry acceptance. Many previous or subsequent one. Connectionless-mode service trans­ vendors now offer MAP 3.0 products, which have nearly elimi­ fers data units without regard to establishing or maintaining con­ nated proprietary "shop floor" automated factory solutions. nections. In connectionless mode, transmission delivery is uncer­ Today, manufacturing networking standards are directed by tain due to the possibility of errors. This appears contrary to the the MAPITOP users group. MAP Version 3.0 was released in June goal of network design-users want to ensure that messages 1988 and will remain free from major changes until 1994. Version reach their destination. In reality, connectionless-mode communi­ 3.0 added a Presentation Layer to the profile and implemented a cation simply shifts responsibility for message integrity to a version of the Manufacturing Message Specification (MMS), the higher layer, which checks integrity only once, rather than requir­ protocol for transferring factory and robotics information, ISO ing checks at each lower layer. Alternatively, each data unit might 9506. The ISO is currently working to extend MMS in support of contain the error recovery mechanism. realtime applications. Other Layer 7 protocols specified are FTAM, Network Management, and Directory Service. Middle Lower-Layer Protocols layers implement ISO connection-oriented protocols, although Lower-layer OSI protocols for Layers I through 3 are well-de­ these must be bypassed for time-critical applications. At the lower fmed veterans and in many cases borrowed from existing EIA, transport layers, MAP specifies the IEEE 802.4 token bus system IEEE, or CCITT standards. Connectionless communications at employing a Type F coaxial connection to a 75-ohm cable. the lower layers of the OSI model is well established and is found, for example, in LANs and metropolitan area networks (MANs). While the original OSI model-described in ISO 7498-was OSI and TCP/IP connection oriented, the ISO foresaw the need for connectionless Transmission Control Protocol/lntemet Protocol was developed service and issued an addendum to that protocol (ISO by the U.S. government's Defense Advanced Research Projects 7498/ADI). The ISO is now working to update the Connection­ Agency (DARPA) for its research network, ARPANET. By 1986, less Addendum, and CCITT SG VII pursues a parallel process. TCP/IP had gained a following of commercial users seeking a The CCITT, however, has been reluctant to insert connectionless­ protocol that could be used as a common denominator for multi­ mode data transmission concepts into CCITT X.200-its version vendor computer networks. TCP and IP are actually two separate of the OSI model. The ISO standard for Network Layer service, protocols, occupying middle layers number Four (Transport) and ISO 8348, contains connectionless service (in AD I) in addition to number Three (Network), respectively, of the OSI Reference the connection mode. Model.

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TCP/IP has been implemented on almost every type of com­ Another disadvantage to standards-based network manage­ puter and is especially successful in commercial Ethernet LAN ment is that OSI standards merely provide a menu of options. environments. The reason TCP/IP is so popular is because it is There are numerous gaps and ambiguities in OSI Management free and its development was paid for by the U.S. government. In standards that could be interpreted differently, leading to incom­ fact, it is actually more complex than TP4. It avoids the connec­ patible implementations. Industry consensus is the only hope for tion-orientedlconnectionless dilemma by essentially avoiding it. interoperable implementations. Currently, this consensus is build­ OSI provides a richer set of network options, but these may not be ing around the Network Management Forum and the Network compatible in different networks. Users cannot communicate Management Special Interest Group (NMSIG) of the OIW, spon­ across different networks if they implement different options at sored by NIST and the IEEE. The NMSIG is developing Imple­ these layers. mentation Agreements (lAs) based on emerging network man­ Already, some proprietary stripped-down versions of OSI agement standards. lAs are being introduced in phases that have been developed to operate over TCP/IP, and some pundits coincide with ISO/IEC standards as they progress from CD to believe that TCP/IP will evolve to resemble OSI in the future. international standards. The OIW NM Phase I IA became stable TCP/IP's future could have been jeopardized, since the U.S. gov­ in December 1990. ernment mandated OSI compliance in government procurements, To further simplify government procurement of network man­ had it not been for Novell's introduction earlier this year of a new agement products, NIST introduced a proposal in 1991, called the version of its NetWare network operating software that supports Government Network Management Profile (GNMP). GNMP will TCP/IP. also be introduced in phases that will cross-reference the latest A majority of users still use TCP/IP networks for LAN inter­ GOSIP versions. GNMP Phase I, II, and III will address the fol­ connectivity. However, the consensus is that TCP/IP is not the lowing categories of management information: ultimate solution-a feat attributed to OSI. The trend is toward a • Phase I-IEEE 802 LAN standards, X.25, ISDN, FDOI, mo­ migration to OS I-based applications running on a TCP/IP infra­ dems, mUltiplexers, bridges, and the physical link of the OSI structure. As a result, more vendors, including Unisys and Am­ model. dahl, are introducing products that support multiple protocols. • Phase II-protocol software operating in Layers 3 to 7, routers, terminal servers, MTAs, PBX, and circuit switches. OSI Management • Phase I1I--applications, services, operating systems, comput­ Since the first draft of the seven-layer ISO model was produced in ers, networks, and database management systems. 1978, extensions to the basic model have been developed to more adequately represent all of the functions required by large-scale, GNMP Phase I specifies CMISIP, management definitions in multivendor networking environments. OSI Management is an GNMP section 4, and five systems management functions: object extension to the original reference model that specifies transfer of management function, state management function, attributes for network management information in the Application Layer and representing relationships, alarm reporting, and event reporting. support for network management functions at Layers 4, 5, and 6. Since SNMP is already widely implemented, it is likely that SNMP will be deployed to manage routers. Future versions of Advantages to OSI·Based Network GNMP will specify a network management architecture incorpo­ Management rating both SNMP and GNMP protocols. OSI-based network management continues to capture attention as the premier solution for multivendor network management. Ven­ Standards Documents dors such as AT&T, Digital Equipment, Hewlett-Packard, and OSI Management standards can be broadly categorized into four NCR are now designing their network management architectures areas: to accommodate OSI Management standards and protocols. In addition to solving the problem of managing heterogeneous 1. Functions--what network management is, according to OSI environments, OSI-based network management will bring about 2. Services--how network management functions are accom­ a new phenomenon-unbundling network management from net­ plished work products. In a proprietary environment, a given vendor's 3. Information Structure-terms and categories describing products are primarily manageable only by products developed what is managed (e.g., "management information") by that vendor. Widespread use of OSI will split that one-to-one relationship, making it possible for any OS I-based network man­ 4. Protocols--describe means o!transporting network manage­ agement system (NMS) to manage any OSI Management-confor­ ment information mant device. Thken together, these four areas describe a generic package for Disadvantages to OSI·Baseci Network network management systems, and how these products relate to Management the network devices they manage (called managed objects in OSI The market (both vendors and users) has widely criticized ISO for terminology). moving too slowly in its efforts to ratify OSI Management stan­ A blueprint document, Management Framework OMNIPoint, dards. Indeed, the greatest disadvantage to OS I-based network places the OSI Management environment in perspective by de­ management is that the demand for it far exceeds the available scribing terms and the scope of OSI network management. products-and vendors are wisely unwilling to develop products based on standards that are not yet final. In an effort to open the 051 Management Functions door to new OSI-based network management system products, OSI Management functions are described in the Systems Man­ SC21 WG4 is currently working to finalize fault, configuration, agement standards (IS 10040, IS 10164-1 through 10164-7, and performance, accounting, and security management specifica­ N 10164-8 through 10164-12). Management using three models: tions. These standards will assist in differentiating OSI-based sys­ 1. The Organizational Model-describes ways OSI Manage­ tems from Simple Network Management Protocol (SNMP)-based ment can be distributed administratively products.

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Figure 3. ISDN Through OSI Eyes

7 Application-related functions

6 Encryption/decryption Compression/expansion etc • .... Q) >- as U) Session Session Session 5 Session Session connection connection connection transport con- management etc. establishment release synchronization nection mappina .2>1~ c: :I:.2 Layer 4 Layer 4 Layer 4 Error 4 connection connection connection detection/ Flow Segmenting etc. multiplexing establishment release recovery control blocking

Network Network Network Congestion etc. Routing/ connection connection connection Addressing relaying establishment release multiplexing control

Data link Data link connection congestion Flow Error Sequence Framing etc. establishment release control control control synchronization

Physical Physical layer Channel layer connection connection Bit transmission structure etc. activation deactivation multiplex ISDN functions allocated according to Ioyering principles ofRecommendation X.200.

2. The Infonnation Model-provides guidelines for defining Service. managed objects and their interrelationships, classes, and Services are described, in part, in the Common Management In­ names fonnation Protocol (CMIP) standard, IS 9596. Services use prim­ 3. The Functional Model-describes network management itives, or command types, to accomplish network management functions functions. Examples of CMIP commands include GET, SET, GET REPORT, CREATE, and DELETE. While service primi­ The Functional Model outlines how ISO has partitioned network tives are so~ewhat abstract, they are important building blocks management into five functional areas: fault management, con­ for composite commands used by network management applica­ tions to obtain vital data on the status and activity of network figu~tion and name management, perfonnance management, ac­ countmg management, and security management. ISO originally devices. described each of these areas in its own standard. Further studies Common Management Infonnation Services (CMIS) include revealed that functions overlapped; therefore, ISO reorganized a detailed abstract model of open systems management services. the documents in December 1988 into their present Systems These fall into three categories-event notification, infonnation· Management fonn. transfer, and control. Event notification allows one system to no­ Fault management provides the detection, isolation, and cor­ tify another that some event of importance has occurred. rection of abnonnalities in network operation. Configuration and Information Stftlcture name management facilities pennit network managers to control The most important standards in this category are Structure of the configuration of the system, network, or layer entities. Management Infonnation (SMI), Parts 1,2, and 4 (CD 10165-1, Changed configurations may isolate faults, alleviate congestion, -2, and -4). (part 3 is not missing; rather, ISO merged Part 3 into or meet changing user needs. Perfonnance management enables Part 4.) Included in these standards is an explanation of the ob­ the network manager to monitor and evaluate the perfonnance of ject-oriented paradigm, used to model a network in tenns of ob­ the system, network, and layer entities. Data from perfonnance ject classes and attributes. In object-oriented environments a management may be used to initiate configuration changes and variable (for example, a variable called Modem) is defined diagnostic testing to allow a satisfactory level of perfonnance. ~th in tenns of the operations that can be perfonned on it and the Accounting management facilities help detennine and allocate values of attributes it can possess. For example, Modem can have costs for the use of a network manager's communications re­ an attribute such as Status, which may have a value of On-Line or sources. Security management facilities pennit the management Off-Line; a network management system may obtain this value of those services providing access protection of communications via a Get operation or alter it via a Set operation. resources. Objects (including their attributes and operations) are stored in a Management Information Base (Mffi), sometimes called an Object Library. The SMI documents just listed provide syntax

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and semantics for information in the MIB; however, as yet no means of interconnecting multivendor computer networks is now single ISO standard defines exactly what the OSI MIB will con­ uncertain. The tremendous growth of the Internet has given tain, nor how vendors and users will register objects in the stan­ TCP/IP a firm base in the United States and the protocol has also dard MIB. SC21 WG4 is currently working to finalize the SMI, been making significant strides in Europe. OSI applications pro­ providing guidelines that can be used to define management ob­ tocols, such as CCITI X.400, X.500, and EDI, are still very pop­ jects and their attributes. The final SMI will ensure interoperabil­ ular, however, and lend support to OSI. ity among OSI-based network management systems. The OSI Reference Model has some glitches and holes that In the TCP/IP world, an Internet Standard MIB exists for ob­ prevent it from being widely implemented. In the U.S., OSI and jects managed using SNMP. This MIB functions in an analogous TCPIIP proponents are badly divided. As an internetworldng pro­ role to the proposed OSI Mm, although the administration and tocol, TCP/IP has proved its worth and is a popular and commer­ rules governing the two are sure to differ. cially successful method oflinking users across diverse networks­ MIB includes all information needed to make management particularly LANs. OSI middle-layer protocols 3 through 5, the decisions. Mm is a conceptual repository of all OSI management alternatives to TCP/IP, are not as simple to implement in the real data in an OSI environment. The MIB concept does not imply any world. SNMP, the network management protocol for TCP/IP net­ form of physical or logical storage for management information, works, is also a proven, commercially successful solution. As however, and its implementation is outside the scope of OSI stan­ long as vendors and users require practical networking products, dards. Rather, the SMI defines the abstract syntax and the seman­ they will continue using TCP/IP-based protocols-standards or tics of information so that it can be represented in OSI protocol not. exchanges. OSI will most likely evolve to better serve user needs, with an OSI-TCP/IP hybrid a likely compromise. A possible scenario for Protocols wider OSI acceptance is that TCP/IP middle layers will migrate to Common Management Information Protocol, IS 9596, is the pri­ resemble OSI, at least functionally. In many commercial net­ mary OSI Management protocol. CMIP specifies procedures for working applications, however, vendors are blending different the exchange of basic management information between open protocol stacks from different sources to match user needs. For systems interconnected by OSI protocols. CMIP is intended to be instance, one vendor's network protocol might graft together dif­ a general-purpose management protocol suitable for the manage­ ferent layers from OSI, TCP/IP, and IBM's SNA. ment of both OSI resources and the real resources used to provide In reality, OSI and other layered architectures do not serve communications services. every application and are not a panacea. Proprietary architectures will continue to thrive alongside OSI-based networks, especially X.500-The Directory for closed user groups (where internetworking is not a require­ The Directory is a related standard designed to manage name­ ment) or in time-sensitive applications intolerant of layered pro­ related information concerning protocol layers and network tocols' high overhead. nodes. These services connect the actual names used in the net­ All others who desire internetworking must realize that the work with names and addresses understood by human users. The associated protocols are still evolving-nothing is truly cast in Directory is defined in CD 9594 and several other OSI working iron. In market-based economies, products that do not satisfy drafts. CD 9594 attained DIS in March 1988. market needs will not gain widespread favor. Therefore, prospec­ tive users must evaluate OSI protocols and their adoption with an OSI and the Future eye toward future standards developments. As it stands now, OSI will not be the interconnection standard of the future. Significant The world needs a network of computers, similar to standards for portions of it, however, will most certainly contribute to the evo­ international telephony, to link users across oceans and conti­ lution of an international standard. - nents. A few years ago, most industry analysts perceived OSI as the answer. Although endorsed by such prominent vendors as IBM, Digital, and Hewlett-Packard, OSI's future as the premier

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DATAPRO Data Networking 2783 1 Standards

ISO Reference Model for Open Systems Interconnection (OSI)

In this report: Synopsis

The Open Editor's Note Although all seven layers have long System ...... 2 The goal of Open Systems Intercon­ since been defined and ISO protocols nection (OSI) is to enable dissimilar ratified for each layer, the ISO com-

OSI Standards computers in multi vendor environ­ o mittees must keep refining and rede­ Progress ...... 10 ments to share information transpar­ fining specific sections of the model ently. The OSI structure calls for , by rewriting existing definitions and OSI and Other cooperation among systems of differ­ adding new protocols. Network ent manufacture and design. With Architectures ...... 14 this capability, global digital net­ Report Highlights works can become a reality. This report updates the OSI's status Testing and Verification at all seven layers; gives the status of Agencies ...... 16 There are seven layers of the OSI OSI standards progress; compares model that communicate between OSI to other architectures, including OSI one end system and another. The ISDN, SNA, DECnet, MAP/TOP, Management ...... 18 layers cover nearly all aspects of in­ and TCP/IP; rationalizes the need formation flow, from applications­ for standards testing and verifica­ OSI and the related services provided at the tion; profiles major testing organiza­ Future ...... 21 Application Layer to the physical tions; and outlines OSI Management connection of devices to the commu­ standards and status. nications medium at the Physical Layer.

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TheOSI structure calls for cooperation among systems of different manufacture and de­ / sign. This includes coordinating activities such as Analysis the following: • Interprocess communications-the synchroni­ zation between OSI application processes and the exchange of information The proliferation of computerized data processing • Data representation-the creation and mainte­ systems in the late 1960s produced a need for com­ nance of data descriptions and transformations patible data communications networks in the for reformatting data exchanged between sys­ 1970s. Several proprietary network architectures tems were developed for mainframe-to-terminal com­ • Data storage-storage media, file systems, and munications, including IBM's SNA in 1974. Al­ database systems for providing access to and though many of these proprietary architectures management of stored data were based on a layered model, none were compat­ • Process and resource management-how appli­ ible with any other. The CCITT's X.25 host inter­ cation processes are declared, initiated, con­ face to the packet networks standard was ratified trolled, and acquired in 1976 but is not a complete network architecture. • Integrity and security-information processing In 1977, the International Organization for Stan­ constraints that must be ensured during open dardization (ISO) formed ISO Technical Commit­ systems operations tee 97 (TC97), Subcommittee 16 (SC16), to embark on a worldwide standardization effort and • Program support-the definition, compilation, confront the issue of incompatibility head-on. The testing, linking, storage, and transfer of and ac­ purpose of TC97/SC16 was to develop a model and cess to programs executed by the application define the protocols and interfaces required to sup­ processes port an open system. The goal of Open Systems Interconnection (OSI) was, and still is, to enable The OSI model is concerned only with the ex­ dissimilar computers in multivendor environments change of information between open systems. to share information transparently. With this capa­ bility, global digital networks can become a reality. The Layering Concept Layering is a basic structuring technique used in the OSI model. Each layer is composed of an or­ The Open System dered set of subsystems, with logically related func­ The ISO defines a system as a set of one or more tions grouped together. The OSI model breaks computers and associated software, peripherals, down internetworking activities between systems terminals, human operators, physical processes, into two distinct groups. Communications­ information transfer means, etc., which form an oriented functions are separated from user­ autonomous whole capable of performing informa­ oriented functions; features which move tion processing and/or information transfer. An information across a network are distinct from fea­ open system is one that obeys OSI standards in its tures which handle and format information. communication with other systems. There are seven layers of the OSI model that An application process is an element within a communicate between one end system and another system that performs information processing for a end system. The layers cover nearly all aspects of particular application. The application process can information flow, from applications-related ser­ be manual (a person operating a banking terminal), vices provided at the Application Layer to the con­ computerized (a program executing in a computer nection of devices to the communications medium center and accessing a remote database), or physi­ at the Physical Layer. Below the Physical Layer, cal (a process control program executing in a dedi­ the media itself corresponding to "Layer 0"-such cated computer attached to industrial equipment as wire, cable, or through-the-air communication- and linked to a plant control system). is currently not addressed by the model. ~

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Washington, DC 20001- CH-1211 Geneva 20, 2178 Switzerland Standards Organizations and (202) 737-8888 (+41)22995111 Fax: (+41) 22 33 72 56 Testing Agencies Bell Communications Re- Telex: 421 000 UIT CH search (Bellcore) 60 New England Avenue Institute of Electrical and Piscataway, NJ 08854- Electronics Engineers 4196 (IEEE) Although standards are and expertise. A primary (908) 699-2000 Headquarters indispensable for com- objective is demonstrating Customer Service Hot 345 E. 47th Street puter internetworking, interoperability among Line: New York, NY 10017 they are useless unless different vendors; i.e., (800) 521-CORE (212) 705-7900 vendor claims of compati- proving that standards Fax (publications): (212) bility can be tested and really work and fostering Corporation for Open 705-7682 users can be assured of end-user interest. Many Systems (COS) acquiring products that agencies have tried vainly Suite 400, 1750 Old IEEE Service Center comply fully with the stan- to involve more end users Meadow Road (published standards:) dards. In general, vendor but are backed primarily McLean, VA 22102-4306 445 Hoes Lane, P.O. Box claims of standards com- by the vendors. (703) 883-2700 1331 patibility are suspect un- Telex: WUI 6503157578 Piscataway, NJ 08855- In the U.S., primary test- less verified by an MCIUW 1331 ing and certification agen- impartial testing agency. (908) 981-1393 cies include the European Computer Additionally, compatibility Fax: (908) 981-9667 Corporation for Open Manufacturers Associa- claims can mean different Telex: 833-233 Systems (COS), the Na- things to different people. tion(ECMA) tional Institute of Science For users, it is a good Rue du Rhone 114 OSINET and Technology (NIST), r idea to probe vendor CH-1204 Geneva, Swit- Mr. Jerry Mulvenna, 'I r and Bell Communications ',- claims of standards com- zerland Chairman Research (Bellcore). Sev- patibility to determine (+41) 22 735 36 34 OSINET Steering Commit- eral smaller organizations what is compatible with Telex: 413237 ECMA CH tee and certain vendors, how- NIST Building 225, Room what, and at what levels. Electronic Industries As- ever, also offer testing B217 sociation (EIA) Several standards testing services. Europe is repre- Clopper Road 1722 Eye Street NW, and verification bodies sented by the Standards Gaithersburg, MD 20899 have been organized both Promotion & Application Suite 300 here and abroad by ven- Group (SPAG); Japan by Washington, DC 20006 Standards Promotion" dor consortiums, govern- the Promoting Confer- (202) 457-4900 Application Group sa (SPAG) ment agencies, and ence for OSI (POSI). Stan- Intemational Organiza- Avenue Louise 149, Box 7 independent organiza- dards organizations are tion for Standardization 1050 Brussels, Belgium tions. They have found listed below: (ISO) that developing conform- (+32) 2 535 0811 American National Stan- 1, Rue de Varembe ance specifications, pro- Telex: 20307 SPAG B dards Institute (ANSI) Case Postale 56 ducing testing suites, and 1430 Broadway CS-1211 Geneva 20, National Institute of Stan- conducting comprehen- New York, NY 10018 Switzerland dards and Technology sive testing is compli- (212) 642-4900 (+41) 22 33 34 30 (NIST) cated, expensive, and U.S. Department of Com- Intemational Telegraph time consuming. Regional ANSI X3 Secretariat merce and Telephone Consulta- differences can stymie Computer and Business Gaithersburg, MD 20899 tive Committee (CCITT) attempts at verification. Equipment Manufacturers (301) 975-2000 The trend for these orga- Association (CBEMA) General.8ecretariat nizations, therefore, is to Suite 500, 311 First International Telecom- cooperate with each Street, NW munications Union (ITU) other, sharing resources Place des Nations ("

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layer interface-providing connection to the next­ ~ (Analysis continued) higher layer-are provided by each layer, usually described by a service specification for the layer. model. Application, Presentation, Session, Trans­ Services at each layer are provided by a layer en­ port, Network, Data Link, and Physical Layers tity. Each layer entity communicates with its peer have been defined (see Table 1). The model repre­ at the same layer on another system, providing ser­ sented by Table 1 is only one end system, in this vices specified in the service specification. case the transmitting end; most networks have at Layers are sometimes divided into sublayers, least two end systems. In Table 1, information for several reasons. Layers often need sublayers to flows down from Layer 7 to Layer 1, and then out handle the service interface of the layer beneath it. over a physical transmission medium. At the re­ This avoids "rewriting" the entire layer. For exam­ ceiving end, the information flows into another ple, the Link Layer of the IEEE 802 Local Area end system and up from Layer 1 to Layer 7 until it . . , Network (LAN) standards is divided into a Logical IS received by a user. Link Control (LLC) sub layer and a Media Access The seven layers can be divided into two Control (MAC) sub layer. The MAC sublayer de­ functional groups: the Transport Platform (Layers pends on characteristics of the underlying physical 1 to 4) and the Application Platform (Layers 5 to layer. Layer independence and modularity are pro­ 7). The Transport Platform's function is to get data moted by ensuring that layer entities on one system from one system to another without errors. The are not permitted to communicate with nonpeers Application Platform's function is to interpret the on another OSI system. datastream and present it to the user in a usable IBM's SNA is also a layered architecture, fol­ form (see Figure 1). lowing rules of layering common to OSI and other Each layer contributes functions to the com­ layered architectures. Any layer may originate a munications task. For example, the Link Layer en­ message to fulfill its responsibilities. The message ables communications across a single physical may not bypass any layer en route to its destina­ connection, while the Network Layer provides end­ tion. If a message leaves the node it will end up in to-end routing and data relay. Services at the upper another node at the same layer that originated the message. Figure 1. There are good reasons for layering: layering Application and Transport Divisions simplifies change; components inside a layer can be changed without affecting any other layers in 7 that node. Layers are like structured programming­ Application but for teleprocessing systems. Because there Application 6 are rigid interfaces between levels, fewer peo- Presentation Platform ple need to react to changes, allowing them to be implemented faster. There is no better way of 5 Session achieving complex functions. Layering allows each network function to be made "transparent," un­ aware and independent of other functions at other layers, thus enabling any layer to be modified with­ 4 out changing the entire monolithic architecture. Transport Each layer may support one of several differ­ ent protocols designed for specific network applica­ 3 Transport Network tions; the choice of a specific protocol is optional, Platform allowing users to tailor networks to their own de­ 2 sign. Each layer defines functions crucial to the Data Link communications process at that layer, independent 1 ofthe other layers. However, a layer may perform Physical functions hinging on functions performed in the layers immediately above or below. A layer can The seven layers are divided into two func­ tional groups.

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Figure 2. Message Movement among OS] Layers

Message moves from Layer 7 through ~ the other layers to reach Layer 7 ~ at the opposite end

r-----~~~ r------, 7 7 Application Application

6 6 Presentation Presentation

5 5 Session Session

4 4 Transport Transport

3 3 3 Network Network Network

2 2 2 Data Link Data Link Data Link

1 1 1 Physical Physical Physical

Transmission Media '---y---J '---y---J '---y---J End System Network Node End System

Intermediate nodes in an OS] network require only bottom-layer functions ofthe OSI model. Note how peer lay­ ers communicate only with their peers; i.e., Layer I talks to other Layer Is but not to Layer 2s. only communicate with another device or network 2). A node communicates with its peer in the node node at its peer layer. Messages exchanged between sending or receiving data. Interfaces within nodes peer layers are "enveloped" with messages from allow them to accept, process, and route data. Data other layers and passed through these other layers transfer is routed from Layer 7 down to Layer 1 at on the way to their destination, picking up and the transmitting node, then along the network to then shedding these other protocol layers along the Layer 1 at the receiving node, and finally from way. For example, if layer seven at one end system Layer 1 up to Layer 7. Peer layers communicate by must send a message to layer seven at another, it the same route. must travel down through six layers at its own end The message initiated at the transmitting and then up through six layers at the other, until it node (Layer 7) is passed from layer to layer, each reaches layer seven at its opposite (peer) layer. layer adding control information, if required, and Each network node (a network user, com­ acting in accord with control information from its puter, terminal) is equipped with this layer mecha­ peer in the receiving node. A fully prepared mes­ nism. However, not all intermediate nodes need all sage enters the cable at Layer 1. The procedure is seven layers. Network nodes, in particular, must reversed at the receiving end. Each item of control only route and transmit data packets-functions at information stops at its appropriate layer, and the the bottom three layers of the OSI model. Layer message itself passes up to Layer 7. Data transfer four through seven functions are not required and essentially is a cumulating process at the transmit­ therefore not included in network node software. ting node and a diminishing process at the receiv­ Data packets processed in these nodes reach only ing node. Layer 3 and are then routed elsewhere (see Figure

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Table 1. The Seven Layers of OSI

Layer Name Purpose 7 Application Applications and application interfaces for OSI networks. Provides access to lower layer functions and services. 6 Presentation Formats data received from Layer 7; in­ cludes terminal standards, display rules. 5 Session Coordinates connection and interaction between applications. Establishes a dia­ log, manages and synchronizes the direc­ tion of data flow, and terminates the session. 4 Transport Ensures end-to-end data transfer be­ tween applications, data integrity, and service quality. Assembles data packets for routing by Layer 3. 3 Network Routes and relays data units among net­ work nodes. 2 Data Link Transfers data units from one network node to another over a transmission cir­ cuit. Ensures data integrity between nodes. 1 Physical Sends the bit stream to the transmission medium.

The Layers provide services to a layer above it nor is it associ­ A number of objectives were considered by the ref­ ated with a service-access point. Some of the ser­ erence model's designers: to limit the number of vices provided by this layer, other than layers to make the system engineering task of de­ information transfer, are the following: scribing and integrating the layers as simple as pos­ • Identifying intended communications partners sible; to create boundaries between layers at points where the description ofservices can be small and • Determining current availability of the in- the number of interactions across each boundary is tended partners minimized; and to collect similar functions in the • Establishing the authority to communicate same layer. Table I summarizes the OSI Reference • Agreeing on responsibility for error recovery Model's layers; more detailed descriptions follow • Agreeing on procedures for controlling data in­ for each layer. tegrity The Application Layer The Application Layer actually consists of two sub­ The Application Layer (Layer 7) is the highest layers. The uppermost sub layer consists of Specific layer, providing the means for the application pro­ Application Service Elements (SASEs), such as cess to access the OSI environment. Its function is message handling system (X.400), FTAM, Direc­ to serve as the passageway between application tory Services (X.500), DTP, and Virtual Terminal processes using Open Systems Interconnection to (VT). The lower sublayer consists of Common Ap­ exchange information; consequently, all applica­ plication Service Elements (CASEs), which provide tion process parameters are made known to the specific services for the upper applications. Exam­ OSI environment through this layer. ples of CASE protocols include Commitment Con­ All services directdy usable by the application currency and Recovery Service Element (CCR) and process (i.e., systems and applications management Remote Operation Service Element (ROSE). functions) are provided by the Application Layer. It differs from the other layers in that it does not The Presentation Layer The Presentation Layer (Layer 6) allows an appli­ cation to interpret the meaning of information ex­ changed. Information is formatted and translated

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Table 2. ISO Protocols and Equivalent Standards

Layer ISO CCITT ANSI ECMA 7 8571 (FTAM) Application 10021 (MHS) X.400 9041 (VT) 10026 (OTP) 9594 (OS) X.500 8613 (OOA) T.410 Series, T.73 ECMA-101 9579 (ADA) 9596 (CMIP) 6 8823 (connection) X.226 Presentation 9596 (connectionless) 5 8327 (connection) X.225 X3.153 ECMA-75 Session 9548 (connection less) 4 8073 (TPO-TP4) X.224 X3.140 ECMA-72 Transport (connection) 8602/8072 (connection less) 3 8208 (Layers 1-3) X.25 Network 8348 (connection) X.213 8473 (connectionless) ECMA-92 9542 (IS-IS) 8878 (use w/8208) (X.25) 8880 (LAN) 8881 (X.25 on LANs) 2 7776 (LAPB) X.25 Data Link 3309 (HOLC) X3.66 ECMA-40 8802.2-.7 (LAN) ECMA-82, -81, -90, -89 (IEEE 802.2-.7) 1 9314 (FOOl) X3.148, X3.139, X3.166 Physical 2110 (EIA-2320) V.24, V.28 ( 4902 (EIA-449) V.24, V.28 2593 V.35 4903 X-Series interfaces, Other V-Series

at this layer. Aspects of Layer 6 include data syn­ The Session Layer tax, which is the data to be transferred between The Session Layer (Layer 5) allows cooperating layers, and the presentation image syntax, which is presentation entities to organize and synchronize the data structure that application entities refer to their dialog and to manage data exchange. It pro­ in their dialog, or the set of actions that may be vides the following services: performed on the data structure. • Session-connection establishment-creation of Services provided to the Presentation Layer an exchange between presentation entities include the following: • Session-connection release • Transforming data syntax, primarily code and character set conversion • Normal data exchange • Transforming and selecting the presentation • Quarantine service-in which data units sent image syntax, the adaptation and modification by a presentation entity are withheld from the of the presentation image (the OSI view of the receiving presentation entity until released by data structure) the sending presentation entity • Expedited data exchange Functions within the Presentation Layer include • Interaction management-allowing presenta­ session establishment request; data transfer; nego­ tion entities to take turns exercising control tiation and renegotiation of data syntax and pre­ functions sentation image syntax; special data transform­ • Session-connection synchronization ations, such as compression; and session termination request.

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Table 3. OSI Network Management Standards

Title Current Status Finalization Date CMIS/CMIP Common Management Information Service (CMIS) (IS 9595) International Standard Final 11/89 CMIS CancelGet Addendum (AD 9595-1) Addendum Final 11/90 CMIS Add/Remove Addendum (AD 9595-2) Addendum Final 11/90 Support for Allomorphism (Amendment to CMIS) (CD 9595) New Work Items DIS expected in 11/91; IS ex­ pected in 11/92 Access Control (Amendment to CMIS) (CD 9595) New Work Items DIS expected in 11/91; IS ex­ pected in 11/92 Common Management Information Protocol (CMIP) (IS 9596) International Standard Final 11/89 CMIP CancelGet Addendum (AD 9596-1) Addendum Final 11/90 CMIP Add/Remove Addendum (AD 9596-2) Addendum Final 11/90 Support for Allomorphism (Amendment to CMIP) (CD 9596) New Work Items DIS expected in 11/91; IS ex­ pected in 11/92 PICS Proforma (Amendment to CMIP) (CD 9596) New Work Items DIS expected in 11/91; IS ex­ pected in 11/92 OSI Systems Management Functions OSI Systems Management Overview (DIS 10040) Draft International Standard IS 5/91 Object Management Function (DIS 10164-1) Draft International Standard IS 5/91 State Management Function (DIS 10164-2) Draft International Standard IS 5/91 Attributes for Representing Relationships (DIS 10164-3) Draft International Standard IS 5/91 Alarm Reporting Function (DIS 10164-4) Draft International Standard IS 5/91 Event Management Function (DIS 10164-5) Draft International Standard IS 5/91 Log Control Function (DIS 10164-6) Draft International Standard IS 5/91 Security Alarm Reporting Function (DIS 10164-7) Draft International Standard IS 5/91 Security Audit Trail Function (CD 10164-8) Committee Draft· DIS 4/91 Objects and Attributes for Access Control (CD 10164-9) Committee Draft· DIS 4/91 Accounting Meter Function (CD 10164-10) Committee Draft" DIS 4/91 Workload Monitoring Function (CD 10164-11) Committee Draft· DIS 4/91 Confidence and Diagnostic Testing Classes. Committee Draft· DIS expected 8/91; IS expected Test Management Function (N4078. 10164-X) 8/92 Measurement Summarization (N4081. 10164-X) Committee Draft· DIS expected 8/91; IS expected 8/92 Response Time Monitoring (N4079. 10164-X) Committee Draft· DIS expected 8/91; IS expected 8/92 Software Management Function (10164-X) New Work Item DIS expected 7/92; IS expected 7/93 Time Management Function (10164-X) New Work Item DIS expected 8/92; IS expected 8/93

Structure of Management Information (SMI) Management Information Model (DIS 10165-1) Draft International Standard IS 5/91 Definition of Management Information (DIS 10165-2) Draft International Standard IS 5/91 Guidelines for Definition of Managed Objects (DIS 10165-4·') Draft International Standard IS 5/91 Generic Managed Objects (N4075) Working Draft DIS registration dates to be determined ·Draft proposals (DPs) are now referred to as Committee Drafts (CDs). * ·Document DIS 10165-3 was merged with DIS 10165-2 and will not appear in future standards listings. • Exception reporting-permitting the presenta- An example of a working Layer 5 protocol outside tion entities to be notified of exceptional situa- the scope of OSI is the Department of Defense tions Transmission Control Protocol (TCP).

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( Table 3. OSI Network Management Standards (Continued)

Title Current Status Finalization Date Specific Management Functional Areas (SMFAs) Performance Management (N4981) Working Draft Next milestone to be determined Accounting Management (N875R) Working Draft Next milestone to be determined Security Management (N4091) Working Draft Next milestone to be determined Fault Management (N4077) Working Draft Next milestone to be determined Configuration Management (N3311) Working Draft Next milestone to be determined Related Upper Layer Standards OSI Management Framework (IS 7498/4) International Standard Final 4/89 File Transfer, Access and Management (FTAM) (ISO 8571) International Standard Final 10/88 Association Control Service Element (ACSE) (ISO 8649, plus International Standard Final 12/88 addendums) Remote Operations Service Element (ROSE) (IS 9072-1 &2) International Standard Final 11/89

The Transport Layer The Network Layer The Transport Layer (Layer 4) provides transpar­ The Network Layer (Layer 3) provides the means ent data flow between session entities, freeing the to establish, maintain, and terminate connections Session Layer from responsibility for cost-effective between systems. Its basic service is providing and reliable data transfer. Layer 4 provides infor­ transparent data transfer between transport enti­ mation interchange according to a user-specified ties. reliability level and end-to-end control. Transport The services provided by this layer encom­ protocols transfer information from one end of a pass the following: physical connection to another and ensure that it is • Establishing network connections for transport­ delivered correctly. Layer 4 protocols are used after ing data between transport entities through net­ a route has been established through the network work addresses by the network-layer protocol. The services provided by this layer include • Identifying connection endpoints the following: • Transferring network service data units • Transport-connection establishment to com­ • Noting errors for reporting unrecoverable errors plete a connection between session entities to the transport layer • Data transfer, in accordance with the agreed • Sequencing network control data units quality of service • Flow control • Transport-connection release • Releasing the network connection

The Transport Layer is the most defined of the up­ The Data Link Layer per four layers. The European Computer Manufac­ Data Link Layer 2 provides the procedural and turers Association (ECMA) has defined this layer functional means to establish, maintain, and re­ in its Transport Protocol standard, ECMA-72. This lease data link connections between two network standard has gained the support of a number of nodes or network entities and to transfer data North American and European computer manufac­ frames (or packets). This layer also detects and (- turers. An end-ta-end data transport protocol out­ may correct errors that occur in the physical layer. side the scope of OSI is the Department of Defense Internet Protocol (IP).

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Services provided by the Data Link Layer to • Device-Status could be a type (in this case, it is the Network Layer include data link connection, a Boolean type) sequencing, error notification, flow control, and • Zero or One are the possible values data unit transfer. This is specified in ASN.l notation as such: The Physical Layer The lowest of the OSI layers is Physical Layer 1. It Device-Status :: = Boolean provides the electrical, mechanical, functional, and Boolean :: = 011 procedural characteristics for activation, mainte­ nance, and deactivation of a physical connection. This is a very simple example; ASN.l is a powerful Physical Layer standards specify physical inter­ grammar, capable of specifying very complex data faces (connectors) connected by a physical me­ types. Hence, it will continue to be the grammar of dium. choice for specifying open systems standards and Services provided by this layer include the protocols. following: • Activating and deactivating physical connec- tions 051 Standards Progress • Data circuit identification The primary responsibility for developing OSI management standards in the United States rests • Sequencing with American Standards Committee (ASC) • Transmitting physical service data units either X3T5.4. There are four stages in the development synchronously or asynchronously cycle: working paper, committee draft (CD), previ­ • Fault condition notification ously known as a draft proposal), draft interna­ tional standard (DIS), and international standard Abstract Syntax Notation One (ASN.1) (IS). A working paper is developed in the first ASN.l is a specification language adopted for the stage. When it matures and contains well­ OSI Reference Model, giving standards developers developed technical concepts, it is registered as a a common method for defining protocols and re­ CD. Passage advances the CD to the DIS level, and lated standards. ASN.l is somewhat analogous to the document is considered sufficiently stable to grammatical rules defining the English language. serve as the basis of initial implementations. At the Just as English grammar specifies notation (punc­ DIS level, the document is distributed for a 180- tuation symbols) and word classifications (such as day ballot. The DIS may require multiple ballots. nouns and verbs), ASN.l specifies a "grammar" A successful ballot elevates the DIS to the level of and rules that help standards developers define IS and completes ISO's process. complex data types in terms of simple building The entire process usually takes between four blocks. and eight years. A list of standards organizations ASN.l was derived from the Backus-Naur associated with the OSI Reference Model is given Form, used to describe programming languages at the end of this report. such as Pascal and Ada. ASN.l was first formally described and published in 1984, in the CCITT The Evolution of 051 Committees X.409 standard entitled "Message Handling Sys­ In the spring of 1977, ISO Technical Committee 97 tems: Presentation Syntax and Notation." It is now (TC97) formed a special subcommittee (SC 16) described (in less readable fashion) in two later charged with developing an architectural model documents: CCITT X.208 (ISO 8824), entitled that would extend from applications-layer commu­ "Specification of Abstract Syntax Notation One nications clear down to the connection with the (ASN.l)," and X.209 (ISO 8825), "Basic Encoding physical interface. The first draft of the seven-layer Rules for Abstract Syntax Notation One (ASN.l)." OSI Reference Model was completed in 1978. Be­ According to ASN.l, each fragment of infor­ tween 1978 and 1983, the Basic Reference Model mation must possess a type and a value. For exam­ and many of the standards for the individual layers ple:

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approached or attained draft international stan­ European Computer Manufacturers Association dard status. By the end of 1984, SC16 was reorga­ (ECMA), where applicable. nized to form Subcommittee 21 (SC21). Working groups within SC16 were also realigned. OSI Applications Standards The OSI Basic Reference Model became an ISO committees are working hard at Layer 7, the international standard in 1984. During 1985, a Application Layer. In fact, OSI application stan­ number of vendors demonstrated products that dards are perceived as potentially powerful and implemented these standards and, by the end of versatile and are the driving force for OSI market 1986, many of these products were commercially acceptance. We devote considerable space review­ introduced. ing some of the most important ones here. In July 1987, the Joint Technical Committee In particular, the ISO electronic mail stan­ for Information Technology (JTC1) was formed dard for message handling systems (MHSs)-or when ISO/TC97 joined forces with Technical CCITT X.400-is becoming popular in commer­ Committee 83 (TC83) of the International Electro­ cial implementations. Two versions, one in 1984, technical Commission (1EC). The IEC is a coali­ and another in 1988, are draft international stan­ tion of industrial standards bodies that is co­ dards that have not been ratified. The standard located with the ISO in Geneva, Switzerland. The was given a big boost in 1989, when the Aerospace new JTCI held its first meeting in 1987. The stan­ Industry Association (AlA) adopted X.400 to inter­ dardization activities ofSC21 report to JTCI. connect its diverse electronic mail networks. Gate­ SC21 is composed of member bodies (MBs) ways to proprietary E-Mail systems were also from 23 different countries. Each MB has its own developed that year, and dozens of vendors have national standards organization; for example, rolled out X.400-based products. Most public ANSI represents the United States in JTCI. The E-Mail carriers have also adopted the standard and individuals or "national correspondents" compris­ are migrating to the 1988 version. ing the MB delegations come from different groups In addition, the ISO is adapting X.400 as the including user organizations, manufacturing firms, message medium for electronic data interchange government agencies, and common carriers or (EDI). In this context, X.400 would be used as the PITs. As such, they bring varying perspectives and communications method to store and forward concerns to the committee sessions. trade documents and business forms conforming to When an OSI committee or working group ANSI X12, the European EDIFACT, and de facto produces a document such as a CD, the document EDI standards. is circulated among the MBs for a vote and to the One component of successful E-Mail internet­ liaison organizations (Las) for review. Las are in­ working is directory services (OS), commonly dependent organizations which also have a vested known as CCITT X.500. X.500 specifies an online interest in OSI development. Las provide com­ directory for message communications, ultimately ments on the content of OSI documents but do not allowing network providers to map a common, in­ have voting privileges. terconnected directory of worldwide users. X.500 dictates naming conventions, how users access di­ Status of OSI Protocols rectory information, and what services are avail­ Protocol standards for all seven layers of the OSI able. model have been approved; however, OSI commit­ Since the 1988 standard is not flexible, SC21 tees are refining and extending some standards as WG4 is working to ease the transition to the new required and may add new standards at specific 1992 version. Older 1988 X.500 systems will re­ layers (particularly Layer 7). Additionally, other quire a software modification to work with the standards groups-such as the CCITT, ANSI, and 1992 version. Realistically, the vision of a world­ IEEE-may adopt OSI protocol standards as their wide messaging directory probably will not be real­ own and vice versa. Consequently, many OSI stan­ ized until the late 1990s. dards are known by more than one standard desig­ The OSI protocol pair for office automation, nation. Table 2 shows some major ISO protocols Office Document Architecture (aDA) and Office approved for each OSI layer and lists correspond­ Document Interchange Format (ODIF), has been ing appellations from ANSI, the CCITT, and the an international standard since 1988 (ISO 8613). It

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Figure 3. ISDN through OSI Eyes '-.

7 Application-related functions

UJ c: .2 6 ti Encryption/decryption etc. c: Compression/expansion .2 m ~ 5 Session Session Session Session Session .... connection connection CQnnection transport con- etc. Q) management ..c: establishment release synchronization nection mapping Cl :E Layer 4 Layer 4 Layer 4 Error 4 connection connection connection detectionl Flow Segmenting etc. multiplexing establishment release recovery control blocking

Network Network Network 3 Routingl connection connection connection Congestion Addressing etc. c:UJ relaying establishment release multiplexing control .2 ti c: Data link Data link ::l 2 Flow Error Framing .... connection congestion Sequence etc. -Q) establishment release control control control synchronization ~ .... Physical Physical layer Channel ~ layer connection etc. 0 connection Bit transmission structure ...J activation deactivation multiplex

ISDN junctions allocated according to layering principles ofRecommendation X.200. is also specified in the U.S. government's GOSIP The Layer 7 protocol for network manage­ standard. aDA and ODIF facilitate the exchange ment, Common Management Information Proto­ of office documents-such as letters, memoranda, col (CMIP), was approved as a draft standard in and business reports-among dissimilar systems. late 1989. CMIP itself is merely a way of commu­ Moreover, the standard specifies the formatting nicating between the "management process" and and exchange of compound documents-those management agents at each lower layer of the OSI containing combinations of text, images, and model. The real work of managing network pro­ graphics. Several ISO working groups are attempt­ cesses is located within the managed objects at in­ ing to strengthen and extend the standard in such dividual OSI layers; in other words, each layer areas as the inclusion of audio, spreadsheet data, must have its own network management system, color graphics, document security, and various lay­ which OSI does not specify. CMIP allows a central­ out and presentation styles. ized management process to either modify the The OSI standard for sending and sharing value of an attribute or request its value (read its data files-File Transfer, Access, and Management status) at each of the layers. Definitions and de­ (FTAM)-is also a finalized international stan­ scriptions of management structures and managed dard. It was developed from networking efforts in information are contained in other OSI standards the manufacturing industry and is a Layer 7 com­ yet to be completed. (For more information, see ponent of the Manufacturing Automation Protocol the OSI Management section featured later in this (MAP). Although FTAM is spreading quickly in report.) Europe, it is also making some progress in domes­ Other Layer 7 protocols in various develop­ tic business applications. The file protocol used ment stages include distributed Transaction Pro­ with TCP/IP-File Transfer Protocol (FTP)-is cessing (TP), currently a committee draft designed well established in the U.S., however, and generally . to interconnect different transaction computing more popular than FTAM. SC21 is working on an systems across OSI networks; Remote Database enhancement to FTAM, and it plans to establish conformance testing guidelines in the near future.

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Access (RDA), a committee draft describing a pro­ While the original OSI model-described in ISO tocol for integrating database management sys­ 7498-was connection oriented, the ISO foresaw tems; and Manufacturing Message Specification the need for connectionless service and issued an (MMS), ISO 9506, a manufacturing protocol that addendum to that protocol (ISO 7498/AD1). The requires extensions for specific manufacturing de­ ISO is now working to update the Connectionless vice types. Addendum, and CCITT SG VII pursues a parallel process. The CCITT, however, has been reluctant Middle Layer Protocols to insert connectionless-mode data transmission During 1987 and 1988, the ISO finalized protocol concepts into CCITT X.200-its version of the standards for middle layers 4, 5, and 6. These stan­ OSI model. The ISO standard for Network Layer dards were based on a previous agreement that all service, ISO 8348, contains connectionless service connections would conform to a connection­ (in AD1) in addition to the connection mode. oriented method of establishing circuits. Every layer of the OSI Reference Model, ex­ The Government's GOSIP Standard cept the Physical Layer, supports connection and In 1979, the National Bureau of Standards (now connectionless mode. Connection-oriented service the National Institute of Standards and requires a connection establishment phase, a data Technology-NIST) initiated a program to support transfer phase, and a connection termination U.S. government standards for interoperable data phase; a logical connection is set up between end communications. It chose to develop a standard systems prior to data exchange. These phases de­ based on the ISO's OSI Reference Model, named fine the necessary sequence of events for successful the Government OSI Protocol (GOSIP). Since Au­ data transmission. Connection-oriented service gust 1990, NIST has mandated GOSIP as a federal capabilities include data sequencing, flow control, information processing standard (FIPS). All federal and transparent error handling. agencies must conform to GOSIP in procuring net­ In a connectionless service, such as new working products. According to NIST, the stan­ Switched Multi-megabit Data Service (SMDS), dard will be updated every year, and each new each Protocol Data Unit is independently routed version will be compatible with the preceding one. to the destination; no connection establishment GOSIP is also expected to accelerate the de­ activities are required, since each data unit is inde­ velopment of OSI standards and products for the pendent of the previous or subsequent one. private sector. As the largest user of information Connectionless-mode service transfers data units processing systems and services in the world, the without regard to establishing or maintaining con­ federal government greatly influences vendors in nections. In connectionless mode, transmission the computer and communications industries. The delivery is uncertain due to the possibility of er­ need for government GOSIP compliance will spur rors. This appears contrary to the goal of network the development of OSI protocols and software design-users want to ensure that messages reach products. their destination. In reality, connectionless-mode GOSIP Version 1 specifies the following pro­ communication simply shifts responsibility for tocols for each OSI layer: message integrity to a higher layer, which checks • Application Layer 7: File Transfer, Access, and integrity only once, rather than requiring checks at Management (FTAM); X.400 Message Han­ each lower layer. Alternatively, each data unit dling System (MHS); and the Association Con­ might contain the error recovery mechanism. trol Service Element (ACSE)

Lower Layer Protocols • Presentation Layer 6: ISO 8823/CCITT X.226 Lower layer OSI protocols for layers 1 through 3 • Session Layer 5: ISO 8327/CCITT X.225 are well-defined veterans and in many cases bor­ • Transport Layer 4: ISO 8073, Transport Proto­ rowed from existing EIA, IEEE, or CCITT stan­ col Class 4 (TP4) dards. Connectionless communications at the • Network Layer 3: ISO 8473 Connectionless Net­ ( lower layers of the OSI model is well established work Layer Protocol (CLNP) and is found, for example, in local area networks (LANs) and metropolitan area networks (MANs).

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• Data Link Layer 2: ISO 3309 (HDLe); ISO security and open systems are antithetical; never­ 8802.2-5/IEEE 802.2-5 theless, the ISO has taken steps to provide a secure • Physical Layer 1: GOSIP does not mandate spe­ environment within the OSI Reference Model. cific physical interface standards but suggests International Standard 7498, Part 2 addresses standard interfaces such as EIA RS-232-C for a security architecture within the general OSI transmission speeds up to 19.2K bps and model. It describes security measures that can be CCITT V.35 for speeds above 19.2K bps provided by specific layers in the model. Specific security standards are not yet defined, however, GOSIP Version 2.0 became mandatory in August but are under study by working group JTC1, Sub­ 1991. It adds the following protocols to the existing committee 27 for Information Technology Security GOSIP model: Standards, plus other subcommittees. The U.S. participant in this process is ANSI's X3 Commit- • Application Layer 7: Basic Class Virtual Termi­ . tee. nal (VT), ISO 9040, ISO 9041, ISO 9040 AD 1, SC21, concerned with maintaining and defin­ and ISO 9041. Office Document Architecture ing the upper three layers of the OSI Reference (ODA), ISO 8613-8. Model, met in May 1991, in Arles, France, to stabi­ • Transport Layer 4: Connectionless Transport lize several network management and security Service, ISO 8602. standards. The Security model is composed of six • Network Layer 3: Connection-Oriented Net­ frameworks that work together across all seven lay­ work Service for ISDN or X.25 networks, ISO ers of the OSI Reference Model: authentication, 8348 and 8348/ADl. Intermediate System to access control, security audit, nonrepudiation, con­ Intermediate System (IS-IS) intradomain rout­ fidentiality, and integrity. SC21 is working to es­ ing, ISO 9542. Integrated Services Digital Net­ tablish two of the six security standards as Draft work (ISDN), CCITT I.451 and Q.931. International Standards (DISs), and the remaining four standards, which are working drafts, will • Data Link Layer 2: ISDN CCITT 1.441 and progress to CD status. Q.921 (Link Access Protocol D-LAPD).

By supporting ISDN, Version 2.0 will also support OSI and Other Network Architectures CCITT X.25 packet interfaces and IEEE 802.2 to 802.6 LAN networks. GOSIP Version 3.0, which OSland ISDN becomes mandatory in August 1992, adds OSI net­ Functioning as the international voice of the tele­ work management, X.500 directory services, the phone industry, the CCITTworked independently Fiber Distributed Data Interface (FODI) standard, of ISO to develop its Integrated Services Digital and Open Document Architecture (ODA). Com­ Network (ISDN) technology. On the other hand, plete GOSIP details can be obtained from NIST CCITT and ISO efforts are closely related because publications Government Open Systems Intercon­ of expanding digital telephone networks and the nection Profile Users' Guide (SP 500-163) and Gov­ merging of voice and data. ernment Open Systems Interconnection Profile Increasingly, the OSI Reference Model and (FIPS PUB 146). ISDN overlap. The ISO has adopted versions of CCITT X.21 and X.25 standards for the lower lay­ OSI Security ers of the OSI model. ISDN functions are de­ By definition, an open system is one that encour­ scribed in terms of the seven-layer OSI model in ages communications between different applica­ the CCITT's Recommendation X.200 (see Figure tions or users. Unfortunately, an open system can 3). Altogether, about 10 standards adopted by both also encourage illegal eavesdropping and informa­ CCITT and ISO are identical, except for introduc­ tion theft or destruction. Recently, notorious ex­ tory paragraphs and identification numbers. amples of white-collar crime, corporate espionage, ISDN standards are now mapped to the OSI and network intrusions by computer worms and Reference Model and occupy its lower layers. Spe­ viruses have alarmed information processing pro­ cific network applications, such as network man­ fessionals and raised a general awareness of com­ agement and electronic mail, occupy the higher puter security issues. The concepts of information

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OSI layers and can be integrated on top of ISDN study interoperability among network management protocols in telephony networks. ISDN will pro­ systems. In 1990, IBM delivered Release 1.1 of its vide a more versatile communications medium for OSI Communications Subsystem supporting OSI integrating telephony and data processing. The layers 3 to 6 protocols in a multi vendor environ­ U.S. government has already specified ISDN pro­ ment and announced SystemView, an OSI-based tocol options for its GOSIP network architecture, network and systems management architecture. which is modeled on OSI. NIST has successfully IBM now supports three major networking tested ISDN as a transport subnetwork for higher standards: OSI, TCP/IP, and FDDI. However, in layer OSI protocols. ISDN is expected to be imple­ early 1991, the company announced that full de­ mented throughout the public telephone network ployment of its System View will be delayed until by late 1992. OSI and ISDN are complementary 1994 or 1995. standards that will allow effective internetworking. This year, after delaying Phase V product in­ troduction, Digital Equipment introduced a new OSI and Proprietary Architectures networking architecture for incorporating OSI into Until 1976, the only de facto network standards DECnet, called Advantage-Networks. Digital de­ were those developed by IBM for its Systems Net­ signed Advantage-Networks to replace DECnet/ work Architecture (SNA). That year, however, OSI Phase V. The new architecture enables users to CCITT introduced its X.25 standard for host transmit data from among OSI, TCP/IP, and DEC­ interface-to-packet networks, and Digital Equip­ net applications and network management is sup­ ment Corp. brought out its Digital Network Archi­ ported with Digital's Enterprise Management tecture (DNA). When the ISO defined its OSI Architecture (EMA). Basic Reference Model (ISO 7498 and 7498/AD1) In the past, relatively smaller vendors were in 1979, users had a non-IBM alternative for the generally more amenable to adopting OSI stan­ first time. The purpose of the OSI model­ dards. Standard Telephone and Cable/Inter­ interoperability in a multi vendor environment­ national Computers, Ltd. (STC/ICL), for was fundamentally different from the proprietary example, was one of the first European manufac­ nature of SNA. turers to ensure that American and European stan­ Although OSI and SNA are both seven-layer dards efforts are coordinated. STC/ICL is a British network architectures, the layers do not match ex­ information systems firm that perceives adoption actly and are incompatible. At first, IBM paid lip of OSI standards as a way to increase the applica­ service to customer requests for OSI functionality. bility of its own products on an international level. The vendor did not embrace OSI within SNA; in­ NCR is expected to roll out its OSI-based stead, it provided an OSI gateway between SNA router in 1992, as well as its OSI-based Communi­ and other non-SNA networks. Like most gateways, cations Processor software for the 56X5. The com­ this solution proved unwieldy and unsatisfactory. pany was supposed to have made them available IBM also provided partial OSI solutions at various last April, just before the introduction of its new layers, such as X.25 connectivity at Layer 3 and System 3600 parallel processor. However, develop­ below. Another interim OSI product was the Open ment problems have delayed delivery until next Systems Message Exchange (OSME), an E-Mail year. All of NCR's products are based on its Open package conforming to CCITT XAOO. Cooperative Computing Architecture (OCCA), Increasingly, pressure from customers de­ which provides for the coexistence and informa­ manding open, nonproprietary platforms has tion exchange among OSI, SNA, and TCP/IP net­ forced IBM and other major companies, such as works. OCCA will be the platform used by AT&T Digital Equipment Corp. and NCR, into a main­ and NCR to achieve global networking. stream OSI approach. In 1988, IBM announced OSIICommunica­ OSI and MAP/TOP tions Subsystem, mainframe-based (MVS) soft­ The Manufacturing Automation Protocol (MAP) ware supporting layers 3 to 6 OSI protocols in and Technical Office Protocol (TOP) were origi­ an SNA environment. In 1989, both IBM and nally developed by General Motors and Boeing Digital Equipment joined in the OSIINetwork Computer Services, respectively, to automate man­ Management Forum, a group that was formed to ufacturing functions on the factory floor and in the

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"back office." Both are based on the OSI Refer­ Already, some proprietary stripped-down ver­ ence Model, using formal standards for each layer sions of OSI have been developed that resemble where possible. MAP, in particular, is probably the TCP/IP, and some pundits believe that OSI itself best-known exa,mple of a formal multilevel proto­ will evolve to resemble TCP/IP in the future. TCP/ col and has achieved moderate industry accep­ IP's future could have been jeopardized, since the tance. Many vendors now offer MAP 3.0 products, U.S. government mandated OSI compliance in but these compete with proprietary "shop floor" government procurements, had it not been for No­ automated factory solutions. vell's introduction earlier this year of a new ver­ Today, manufacturing networking standards sion of its NetWare network operating software are directed by the MAP/TOP users group. MAP that supports TCP/IP. Version 3.0 was released in June 1988 and will re­ A majority of users still use TCP/IP networks main free from major changes until 1994. Version for LAN interconnectivity. However, the consen­ 3.0 added a Presentation Layer to the protocol and sus is that TCP/IP is not the ultimate solution-a implemented a version of the Manufacturing Mes­ feat attributed to OS1. The trend is toward a migra­ sage Specification (MMS), the protocol for trans­ tion to OSI-based applicaions running on a TCP/IP ferring factory and robotics information, ISO infratsructure. As a result, more vendors, including 9506. The ISO is currently working to extend Unisys and Amdahl, are introducing products that MMS in support of realtime applications. Other support multiple protocols. Layer 7 protocols specified are FfAM, Network Management, and Directory Service. Middle layers implement ISO connection-oriented protocols, al­ Testing and Verification Agencies though these must be bypassed for time-critical applications. At the lower transport layers, MAP The Corporation for Open Systems (COS) specifies the IEEE 802.4 token bus system employ­ During the late 1970s and early 1980s, vendor sup­ ing a Type F coaxial connection to a 75-ohm cable. port for the OSI model ranged from wholehearted to indifferent. By 1985, however, it became appar­ OSI and TCP/IP ent that cooperation among vendors-and users­ Transmission Control Protocol/Internet Protocol would be critical to the success of open standards. (TCP/IP) was developed by the U.S. government's That year, major U.S. vendors officially an­ Defense Advanced Research Projects Agency nounced their support and formed the Corporation (DARPA) for its research network, ARPANET. By for Open Systems (COS) to promote implementa­ 1986, TCP/IP had gained a following of commer­ tion of OSI standards. cial users seeking a protocol that could be used as a COS is a nonprofit research and development common denominator for multi vendor computer consortium located in McLean, VA, with an an­ networks. TCP and IP are actually two separate nual budget of $10 million. Its stated purpose is to protocols, occupying middle layers number four work toward worldwide information systems in­ (transport) and number three (network), respec­ teroperability. Its mission is to open worldwide tively, of the OSI Reference Model. markets for new OSI and ISDN products through TCP/IP has been implemented on almost ev­ certification, by developing conformance test prod­ ery type of computer and is especially successful in ucts, and by cooperating with other international commercial Ethernet LAN environments. The rea­ organizations. COS is not in the business of rein­ son for TCP/IP's popularity is that it is a relatively venting standards. It works with existing standards simple, proven system for intemetworking. Its han­ organizations to accelerate the implementation of dling of the connection-oriented/connectionless present standards by testing and certification. dilemma, which can be problematic in OSI, is COS currently lists about 60 full-fledged straightforward and easy to implement. OSI pro­ member companies composed primarily of infor­ vides a richer set of network options, but these may mation technology vendors such as Apple Com­ not be compatible in different networks. Users can­ puter, AT&T, Digital Equipment Corp., Hewlett­ not communicate across different networks if they Packard, IBM, Intel, Sun Microsystems, Texas implement different options at these layers.

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Instruments, and several software and LAN ven­ • FTAM dors. The list also includes several federal govern­ • MRS ment agencies and a few large end users. Members are committed to accelerate OSI Testing services cost $1,000 per day. Conformance and related standards and to assess how vendors test licenses are available for the protocols men­ can best supply end users with OSI and ISDN solu­ tioned above, plus 10 specific MAP/TOP proto­ tions; they are afforded several privileges not avail­ cols, ranging from $5,500 to $135,000 per protocol able to nonmembers. There are three types of (with discounts for multiple licenses). memberships: COS has been unsuccessful in attracting end­ 1. Regular membership, with an annual fee of user members and is criticized for moving too $25,000 slowly and for not being impartial. COS members 2. Corporate Research membership, for corpora­ represent diverse interests but "theoretically" tions with revenues in excess of $25 million, share a common vision of worldwide interoperabil­ with an annual fee of $25,000 ity as well as a recognition of the potential profit­ ability of open systems products. COS member 3. Senior Research membership, for corporations representatives are working together to translate with revenues in excess of $150 million, with these ideals into several goals. Nevertheless, COS an annual membership fee of $25,000 and an members cannot always reconcile their business annual research fee of$175,000. interests with support of open standards. Accordingly, COS has forged partnerships Besides these three categories, COS provides an with similar testing and conformance groups in the Affiliate Associate Program currently consisting of U.S. and abroad. It has relationships with the over 40 universities, foundations, associations, and MAP/TOP Users Group, NIST, ANSI, SPAG, nonprofit organizations. POSI, and the Interoperability Technology Associ­ COS' major activity is developing and admin­ ation for Information Processing (INTAP) of Ja­ istering the COS Mark Licensing Program, which pan. With ODA becoming mandatory for federal is crucial to the COS mission. Informally known as information system procurements after August the "gold dot," the COS Mark was developed with 1992, the need for OSI conformance testing will the aid of Underwriters Laboratories (UL). It is a increase immensely in the coming years. Recently, "seal of approval," providing impartial verifica­ COS has shifted emphasis from that of a testing tion to users that OSI and ISDN products conform laboratory to producing a model for third-party to standards and ensure multi vendor interoperabil­ testers. ity. COS awards the mark to products that meet In 1991, COS announced an industry-wide COS requirements and pass a set of COS conform­ consensus to develop and deliver ISDN capabili­ ance tests. Since OSI contains many optional ties in the public switched telephone network by classes, subsets, and parameters, conformance tests late 1992. ISDN will be deployed based on stan­ are conducted on a COS Stack Specification-a dard technical specifications and implementation specific profile of optional OSI protocols. agreements. Known collectively as National ISDN COS conformance tests are available through 1, the technical specifications were developed by. three avenues: at an on-site COS testing lab, Bellcore in conjunction with major industry eqUIp- through licenses issued to third-party testing orga­ ment and service providers. nizations, and through licenses issued to vendors. In addition to OSI, the COS Conformance Testing National Institute of Standards and Technology Laboratory offers conformance test services for the (NIST) following protocols: NIST, formerly the National Bureau of Standards • 802.3 CSMAlCD (NBS), is a branch of the U.S. Department of Com­ • 802.4, Layer 1 and Layer 2 merce. It develops federal information processing standards for ISDN and OSI and sponsors the OSI ( • X.25 (OSI 8882) Implementers Workshop (OIW) and several OSI • Internet special interest groups. In 1979, NIST developed • Transport

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formal standards description techniques and proto­ Bellcore is active in public network specifica­ col test methods. It is grappling with the govern­ tions for ISDN, fiber optics, network management, ment's role in standardization, and whether that the intelligent network concept, and related topics. role should be increased to meet the challenges Mter Bellcore drafts network standards, it allows posed by international competition. Unlike other them to be reviewed by the industry at large. After countries, the U.S. does not implement laws for modifications, the standards are then published as standards conformance-a concept vigorously op­ Technical References (TRs), which are listed in posed by u.s. industry. Bellcore's annual Catalog of Technical Informa­ tion. They can be ordered as complete documents NIST and IEEE at modest prices. NIST helps to sponsor an experimental OSI net­ Since its inception, Bellcore has been actively work for OSI product vendors and users, called engaged in ISDN research and testing. It produces OSINET. Originally formed in 1984 to test MAP commercially available testbeds for ISDN protocol protocols, OSINET is a packet switched network compatibility. It also publishes books and video­ with about 40 U.S. nodes. It allows two vendors to tapes on ISDN concepts, planning, and other voluntarily perform brief interoperability tests, ISDN topics. modeled after OSI standards for conformance test­ ing. Abbreviated test results are registered and Standards Promotion & Application Group made available to end users through an online da­ (SPAG) tabase. Complete testing details and results can be SPAG, the European equivalent of COS, was in­ obtained from OSINET. corporated in 1986. Its stated mission is to pave OSINET costs are maintained by the partici­ the way to an open international market for the pating vendors, including Digital Equipment computer and telecommunications industry, based Corp., Hewlett-Packard, IBM, NCR, and Unisys. on harmonized standards and testing and certifica­ NIST is a full member, and the network's chairper­ tion of OSI products. son is a NIST employee. In 1989, OSINET and Like COS, SPAG concentrates its efforts on equivalent networks from Europe, Japan, Austra­ producing conformance testing, accrediting test lia, and Singapore formed a partnership called OS­ laboratories, and certifying OSI products. SPAG lone, which tests global OSI interoperability and has signed international agreements with COS and demonstrates OSI internetworking at trade shows POSI to harmonize tools and testing technology. In and special events around the world. 1988, it signed a joint development agreement with COS to produce the Integrated Tool Set (ITS). The Bell Communications Research (Bellcore) agreement also provided for reciprocal cross­ Bellcore, the R&D arm owned jointly by the Bell licensing and distribution of test tools. Operating Companies (BOCs), is the U.S.'s largest In response to criticism that SPAG and COS research consortium. Splintered from AT&T Bell were competing with tool developers and that test­ Laboratories when AT&T was divested in 1984, its ing was being conducted in a closed environment, goal is to help make it possible for people anywhere SPAG developed the concept of OPEN Integrated in the world to communicate easily and securely in Test Specification (OPEN ITS), an open approach any medium or combination of media. Bellcore's to testing open systems. domain is the public switched telephone network, for which it devises standards and tests vendor products for compliance. Bellcore's role is ex­ 051 Management tremely important: in the absence of a regulated Since the first draft of the seven-layer ISO model telephone monopoly (the Bell system), someone or was produced in 1978, extensions to the basic something must maintain a homogeneous nation­ model have been developed to more adequately wide telephone network. In addition, the BOCs are represent all of the functions required by large­ not allowed to manufacture their own equipment scale, multi vendor networking environments. OSI and must purchase equipment from a variety of manufacturers.

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Management is an extension to the original refer­ the OSI/Network Management Forum and the ence model that specifies transfer of network man­ Network Management Special Interest Group agement information in the Application Layer and (NMSIG) of the OIW, sponsored by NIST and the support for network management functions at Lay­ IEEE. The NMSIG is developing Implementation ers 4, 5, and 6. Agreements (lAs) based on emerging network man­ agement standards. lAs are being introduced in Advantages to OS I-Based Network phases that coincide with ISO/IEC standards as Management they progress from CD to international standards. OSI-based network management continues to cap­ The OIW NM Phase I IA became stable in Decem­ ture attention as the premier solution for multiven­ ber 1990. To further simplify government procure­ dor network management. Vendors such as AT&T, ment of network management products, NIST Digital, Hewlett-Packard, and NCR are now de­ introduced a new proposal in May 1991, called the signing their network management architectures to Government Network Management Profile accommodate OSI Management standards and (GNMP). GNMP will also be introduced in phases protocols. that will cross-reference the latest GOSIP versions. In addition to solving the problem ofmanag­ GNMP Phase I, II, and III will address the follow­ ing heterogeneous environments, OSI-based net­ ing categories of management information: work management will bring about a new • Phase I-IEEE 802 LAN standards, X.25, phenomenon-unbundling network management ISDN, FOOl, modems, multiplexers, bridges, from network products. In a proprietary environ­ and the physical link of the OSI model. ment, a given vendor's products are primarily manageable only by products developed by that • Phase II-protocol software operating in layers vendor. Widespread use of OSI will split that one­ 3 to 7, routers, terminal servers, MTAs, PBX, to-one relationship, making it possible for any OSI­ and circuit switches. based network management system (NMS) to • Phase III-applications, services, operating sys­ manage any OSI management-conformant device. tems, computers, networks, and database man­ agement systems. Disadvantages to OSI-Based Network Management GNMP Phase I specifies CMIS/P, management The market (both vendors and users) has widely definitions in GNMP secion 4, and five systems criticized ISO for moving too slowly in its efforts management functions: object management func­ to ratify OSI Management standards. Indeed, the tion, state management function, attributes for rep­ greatest disadvantage to OSI-based network man­ resenting relationships, alarm reporting, and event agement is that the demand for it far exceeds the reporting. available products-and vendors are wisely unwill­ Since SNMP is already widely implemented, ing to develop products based on standards that it is likely that SNMP will be deployed to manage are not yet final. In an effort to open the door to routers. Future versions of GNMP will specify a new OSI-based network management system prod­ network management architecture incorporating ucts, SC21 WG4 is currently working to finalize both SNMP and GNMP protocols. fault, configuration, performance, accounting, and security management specifications. These stan­ Standards Documents dards will assist in differentiating OSI-based sys­ OSI Management standards can be broadly catego­ tems from SNMP-based products. rized into four areas: Another disadvantage to standards-based net­ 1. Functions-what network management is, ac­ work management is that OSI standards merely cording to OSI provide a menu of options. There are numerous 2. Services-how network management functions gaps and ambiguities in OSI Management stan­ are accomplished dards that could be interpreted differently, leading ( to incompatible implementations. Industry consen­ 3. Information Structure-terms and categories sus is the only hope for interoperable implementa­ describing what is managed (e.g., "manage­ tions. Currently, this consensus is building around ment information")

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4. Protocols-describe means oftransporting net­ initiate configuration changes and diagnostic test­ work management information ing to allow a satisfactory level of performance. Accounting management facilities help determine Taken together, these four areas describe a generic ,and allocate costs for the use of a network manag­ package for network management systems, and er's communications resources. Security Manage­ how these products relate to the network devices ment facilities permit the management of those they manage (called managed objects in OSI termi­ services providing access protection of communi­ nology). cations resources. A blueprint document, OSI Management Framework (IS 7498-4), places the OSI Manage­ Services ment environment in perspective by describing Services are described, in part, in the Common terms and the scope of OSI network management. Management Information Services (CMIS) stan­ dard, IS 9595. Services use primitives, or command OSI Management Functions types, to accomplish network management func­ OSI Management functions are described in the tions. Examples of primitives include INITIAL­ Systems Management standards (CD 10040, CD IZE, EVENT-REPORT, and TERMINATE. While 10164-1 through 10164-7, and N 10164-8 through service primitives are somewhat abstract, they are 10164-12). These documents are listed in Table 3 important building blocks for real commands used and describe the scope of OSI Management using by network management applications to obtain three models: vital data on the status and activity of network de­ 1. The Organizational Model-describes ways vices. OSI Management can be distributed adminis­ CMIS includes a detailed abstract model of tratively open systems management services. These fall into three categories-event notification, information 2. The Information Model-provides guidelines transfer, and control. Event notification allows one for defining managed objects and their interre­ system to notify another that some event of impor­ lationships, classes, and names tance has occurred. Information transfer consists 3. The Functional Model-describes network of a single service element-Get. Control consists management functions of three elements: Set, Action, and Compare.

The Functional Model outlines how ISO has parti­ Information Structure tioned network management into five functional The most important standards in this category are areas: fault management, configuration and name Structure of Management Information (SMI), Parts management, performance management, account­ 1,2, and 4 (CD 10165-1,2, and 4). (Part 3 is not ing management, and security management. ISO missing; rather, ISO merged Part 3 into Part 4.) originally described each of these areas in its own Included in these standards is an explanation of standard. Further studies revealed that functions the object-oriented paradigm, used to model a net­ overlapped; therefore, ISO reorganized the docu­ work in terms of object classes and attributes. In ments in December 1988 into their present Sys­ object-oriented environments, a variable (for ex­ tems Management form. ample, a variable called Modem) is defined both in Fault management provides the detection, terms of the operations that can be performed on it isolation, and correction of abnormalities in net­ and the values of attributes it can possess. For ex­ work operation. Configuration and Name Manage­ ample, Modem can have an attribute such as Sta­ ment facilities permit network managers to control tus, which may have a value of Up or Down; a the configuration of the system, network, or layer network management system may obtain this value entities. Changed configurations may isolate faults, via a Get operation or alter it via a Set operation. alleviate congestion, or meet changing user needs. Objects (including their attributes and opera­ Performance management enables the network tions) are stored in a Management Information manager to monitor and evaluate the performance Base (MIB), sometimes called a Management In­ of the system, network, and layer entities. Data formation Library (MIL). The SMI documents just from performance management may be used to

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listed provide syntax and semantics for informa­ OSI and the Future tion in the MIB; however, as yet no single ISO OSl's future as the premier means of interconnect­ standard defines exactly what the OSI MIB will ing multi vendor computer networks is almost a contain, nor how vendors and users will register certainty. Too many government agencies, ven­ objects in the standard MIB. SC21 WG4 is cur­ dors, trade associations, and users have staked rently working to finalize the Structure of Manage­ their futures on this architecture for it not to suc­ ment Information (SMI), providing guidelines that ceed. OSI applications protocols, such as CCITT can be used to define management objects and X.400, X.500, and EDI, are catching on and spur­ their attributes. The final SMI will ensure interop­ ring OSl's adoption. The world needs a network of erability among OSI-based network management computers, similar to standards for international systems. telephony, to link users across oceans and conti­ In the TCP/IP world, an Internet Standard nents. OSI is perceived as the answer to this need. MIB exists for objects managed using SNMP. This The OSI Reference Model has some glitches MIB functions in an analogous role to the pro­ and holes, however, that prevent it from being posed OSI MIB, although the administration and widely implemented. In the U.S., OSI and TCP/IP rules governing the two are sure to differ. proponents are badly divided. As an internetwork­ MIB includes all information needed to make ing protocol, TCP/IP has proved its worth and is a management decisions. MIB is a conceptual reposi­ popular and commercially successful method of tory of all OSI management data in an OSI envi­ linking users across diverse networks-particularly ronment. The MIB concept does not imply any LANs. OSI middle-layer protocols 3 through 5, the form of physical or logical storage for management alternatives to TCP/IP, are neither as practical nor information, however, and its implementation is as simple to implement in the real world. Simple outside the scope of OSI standards. Rather, the Network Management Protocol (SNMP), the net­ SMI defines the abstract syntax and the semantics work management protocol for TCP/IP networks, of information so that it can be represented in OSI is also a proven, commercially successful solution. protocol exchanges. As long as vendors and users require practical net­ working products, they will continue using TCPI Protocols IP-based protocols-standards or not. Common Management Information Protocol Although nobody can predict the future, OSI (CMIP), IS 9596, is the primary OSI Management will probably evolve to better serve user needs. A protocol. CMIP specifies procedures for the ex­ possible scenario for wider OSI acceptance is that change of basic management information between OSI middle layers will migrate to resemble TCPI open systems interconnected by OSI protocols. IP, at least in functionality. One such implementa­ CMIP is intended to be a general-purpose manage­ tion already exists. A product called Xpress ment protocol suitable for the management of both Transfer Protocol (XTP), a proprietary networking OSI resources and the real resources used to pro­ scheme from Protocol Engines, Inc. (Santa Bar­ vide communications services. bara, CA), is a streamlined version of OSl's middle layers. XTP combines OSI layers 3 and 4 into one X.SOO-The Directory protocol and has been officially proposed to AN­ The Directory is a related standard designed to SI's X3S3 Committee for adoption. manage name-related information concerning pro­ In many commercial networking applications, tocollayers and network nodes. These services con­ however, vendors are blending different protocol nect the actual names used in the network with stacks from different sources to match user needs. names and addresses understood by human users. For instance, one vendor's network protocol might The Directory is defined in CD 9594 and several graft together different layers from OSI, TCP/IP, other OSI working drafts. CD 9594 attained DIS in and IBM's SNA. March 1988. (

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In reality, OSI and other layered architectures All others who desire intemetworking must do not serve every application and are not a pana­ realize that the associated protocols are still cea. Proprietary architectures will continue to evolving-nothing is truly cast in iron. In market­ thrive alongside OSI-based networks, especially for based economies, products that do not satisfy mar­ closed user groups (where intemetworking is not a ket needs will not gain widespread favor. requirement) or in time-sensitive applications in­ Therefore, prospective users must evaluate OSI tolerant oflayered protocols' high overhead. protocols and their adoption with an eye toward future standards developments .•

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