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`PROCEEDINGS OF THE IEEE VOL. 71, NO. 12, DECEMBER 1983
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`The OSI Reference Model
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`JOHN D. DAY AND HUBERT ZIMMERMANN
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`Invited Paper
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`I.
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`Abrm—Theeflymsofcomputernetworlsinthemid—lms
`madeitappamntflnttoufilinefltefullpotemiflofeomternetworls,
`intennfionalstandardswouldbereqrfired.lnlW,daeIntemafioml
`StandardsOrgmintionaSO) initiatedworkonOpenSystems Intercon-
`nection(OSI)toIddlessflIssereqm’i-ements.1hiqurerhieflydesuibes
`tlIeOSl ReferenceModelJ‘heCEl Reference Modelisthelu‘ghestlevel
`ofahstracfioninflieOSIscheme.Thepaperfhstdesaibesthebasic
`buildingbbcksusedtoeonstmctthenetworkmodel.1henthepartictflar
`sevemhyermodellsedbyOSIishieflymeoflowedbyadiscusw
`sionofoutstandingissuesandfuhneextensionsforthemodel
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`INTRODUCTION
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`N THE mid-1970’s, the development and use of computer
`Inetworks began to achieve considerable attention. The early
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`successes of the ARPANET [1] and CYCLADES [2],
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`immediate commercial potential of packet switching, satellite and
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`local network technology, and the declining cost of hardware
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`made it apparent that computer networking was quickly becom-
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`ing an important field. It was also apparent that to utilize the full
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`potential of computer networks, international standards to ensure
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`interworking would be required.
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`the International Organization for Standardization
`In 1978,
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`(ISO) Technical Committee 97 on Information Processing, recog-
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`nizing that standards for networks of heterogeneous systems were
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`urgently required, created a new subcommittee (SC16) for “ Open
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`Systems Interconnection.” The term “open” was chosen to em—
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`phasize that by conforming to OSI standards, a system would be
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`open to communication with any other system obeying the same
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`standards anywhere in the world.
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`In 1978, it was clear that the commercial endeavors to exploit
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`the emerging communication technology would wait neither for
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`SC16 to leisurely develop communication standards nor for the
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`research community to answer most of the outstanding questions.
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`If there was to be a consistent set of international standards, OSI
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`would have to lead rather than follow commercial development
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`and make use of the most recent research work when available.
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`The size of the task would require the work to be divided among
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`several working groups each developing standards; however, close
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`overall coordination would also be necessary.
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`The first meeting of SC16 was held in March 1978. Initial
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`discussions revealed [3] that a consensus could be reached rapidly
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`on a basic layered architecture which would satisfy most require-
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`ments of 051 and which could be extended later to meet new
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`requirements. SC16 decided to give the highest priority to the
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`development of a standard Model of Architecture which would
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`constitute the framework for the development of standard proto-
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`cols.
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`Manuscript received March 1, 1983; revised August 20. 1983.
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`J. Day is with Codex Corporation, Mansfield. MA 02046.
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`H. Zimmermann is with the Centre National d'Etudcs des Téléoommunica—
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`tions. 92131 lssy-Lcs-Moulineaux, France.
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`001 8-9219/83/1200-133450100 ©1983 IEEE
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`Petitioner Valve - Ex. 1017, Page 1334
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`Petitioner Riot Games, Inc. - EX. 1017, p. 1334
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`After less than 18 months of discussions, this task was com-
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`pleted, and the Reference Model of Open Systems Interconnec-
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`tion, was transmitted by SC16 to TC97 along with rwommenda—
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`tions to start a number of projects for developing an initial set of
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`standard protocols for OSI. These recommendations were adopted
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`by TC97 at the end of 1979 as the basis for development of
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`standards for Open Systems Interconnection within the ISO. The
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`OSI Reference Model was also recognized by the CCITT Rap-
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`porteur’s Group on Public Data Network Services. At this time,
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`SC16 began development of standard OSI Protocols for the
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`upper four layers. These are discussed in more detail in subse-
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`quent articles in this special issue.
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`In late 1980, SC16 recommended that the Reference Model be
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`forwarded as a Draft Proposal (DP) for an International Stan-
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`dard. After two rounds of comments, the Reference Model was
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`progressed as a Draft International Standard (DIS) in the Spring
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`of 1982. Comments on this vote were processed late in 1982, and
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`the Basic Reference Model became an International Standard
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`(ISO 7498) [4] in the Spring of 1983.
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`In most cases, the job of a standards committee is to take sets
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`of commercial practices and the current research results when
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`applicable and codify these procedures into a single standard that
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`can be utilized by commercial products. SC16 was presented with
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`a somewhat different problem: develop a set of standards which
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`emerging products could converge to before the commercial
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`practices were in place and while many of the more fundamental
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`research problems remained unsolved It would be presumptious
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`to say that SC16 solved this problem. They did, however, find a
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`way to cope with the problem in such a way as to maximize
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`flexibility and to minimize the impact of change brought on by
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`new technologies or new techniques.
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`The approach adopted by SC16 was to use a layered architec-
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`ture to break up the problem into manageable pieces. The OSI
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`Reference Model is a framework for coordinating the develop-
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`ment of OSI standards. In OSI, the problem is approached in a
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`top—down fashion, starting with a description at a high level of
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`abstraction which imposes few constraints, and proweding to
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`more and more refined descriptions with tighter and tighter
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`constraints. In the world of OSI, three levels of abstractions are
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`explicitly recognized: the architecture, the service specifications,
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`and the protocol specifications (see Fig 1).
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`The OSI Architecture is the highest level of abstraction in the
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`OSI scheme. The term “architecture" can be a very tricky term. It
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`has been used to describe everything from a framework for
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`development, to a particular form of organization, to hardware. A
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`good way to think about the term is to consider the difference
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`between an architecture and a building built to that architecture.
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`For example, Victorian architecture is a set of rules and stylistic
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`conventions that characterize a particular form. A Victorian
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`building is a building built to those rules and conventions. You
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`cannot walk into a Victorian architecture; you can walk into a
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`Petitioner Riot Games, Inc. - Ex. 1017, p. 1334
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`Petitioner Valve - Ex. 1017, Page 1334
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`DAY AND ZINIMERMANN: THE OSI REFERENCE MODEL
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`OSl Reference Model
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`OSI
`Protocols
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`3 ol Implementations
`services, protocols,
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`and implementations
`g 0| protocols and
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`Implementations
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`The OSI Reference Model, Services. and Protocols are successively
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`more detailed and therefore more constraining specifications.
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`Fig. 1.
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`1335
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`Fig. 2. Network layering.
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`protocols. The OSI Reference Model cannot be implemented,
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`and it does not represent a preferred implementation approach. It
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`is a model for describing the concepts for coordinating the
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`parallel development of interprocess communication standards.
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`One must remember that in the world of OSI, only OSI protocols
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`can be implemented and products can only conform to OSI
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`protocols. Thus the statement “this product conforms to the OSI
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`Reference Model" does not imply the ability to interwork with
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`other products which may make the same claim.
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`The purpose of OSI is to allow any computer anywhere in the
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`world to communicate with any other, as long as both obey the
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`OSI standards. OSI standards and the degree of compatibility
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`required to meet this goal make formal description methods a
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`necessity. SC16/WGl on Architecture established a group early
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`in its work to develop formal description methods for defining
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`the protocols so that they could be implemented unambiguously
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`by people all over the world without having to consult with a few
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`experts on how to interpret the standard. The article in this issue
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`by Vissers, Tenney, and Bochmann gives more details on the OSI
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`formal description methods as used by ISO.
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`In the remainder of this paper we describe the basic concepts
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`used in the Reference Model, then give a brief description of each
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`of the seven layers and identify a few of the outstanding archi-
`tectural issues.
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`Victorian building. In computer science we often mistakenly refer
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`to a building as an architecture. More formally this can also be
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`considered as the distinction between the type of an object
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`(architecture) and an instance of that object (building). The OSI
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`Reference Model defines types of objects that are used to de-
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`scribe an open system, the general relations among these types of
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`objects, and the general constraints on these types of objects and
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`relations. Specifications for the lower levels of abstractions may
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`define other relations and fighter constraints for their purposes,
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`but these must be consistent with those defined in the Reference
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`Model.
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`The document which describes the OSI Architecture, ISO 7498,
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`defines these objects, relations, and constraints, and also defines
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`a seven-layer model for interprocess communication constructed
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`from these objects, relations, and constraints. These are used as a
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`framework for coordinating the development of layer standards
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`by OSI committees, as well as the development of standards built
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`on top of OSI.
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`The OSI Service Specifications represent a lower level of ab-
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`straction that define in greater detail the service provided by each
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`layer. This concept is defined in greater detail in the article by
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`Linington in this issue. The service specification will define
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`tighter constraints than the Reference Model on the protocols
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`and implementations that will satisfy the requirements of the
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`layer. A service specification defines the facilities provided to the
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`user of the service independent of the mechanisms used to
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`accomplish the service. It also defines an abstract interface for
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`the layer, in the sense that it defines the primitives that a user of
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`the layer may request with no implication of how or if that
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`interface is implemented.
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`The OSI Protocol Specifications represent the lowest level of
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`A. Systems, Layer, and Entities
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`abstraction in the OSI standards scheme. Each protocol specifica-
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`tion defines precisely what control information is to be sent and
`The OSI Reference Model is an abstract description of inter-
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`what procedures are to be used to interpret this control informa-
`processes communication. OSI is concerned with standards for
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`communication between systems. In the OSI Reference Model,
`tion. The protocol specifications represent the tightest constraints
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`placed on implementations built to conform to OSI standards.
`communication takes place between application processes run-
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`As shown in Fig. 1, the three levels of abstraction used by OSI
`ning in distinct systems. A system is considered to be one or
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`define successively tighter constraints on what will satisfy OSI.
`more autonomous computers and their associated software, pc-
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`There are many services and protocols that satisfy the constraints
`ripherals, and users that are capable of information processing
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`required by the Reference Model. There are fewer protocols that
`and/or transfer. Although OSI techniques could be used within a
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`satisfy both the Reference Model and the OSI Service Specifica-
`system (and it would be desirable for intra- and inter-system
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`tions. Finally, the Protocol Specifications constrain implementa-
`communication to appear as similar as possible to the user), it is
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`tions sufficiently to allow open systems to communicate while
`not the intent of OSI to standardize the internal operation of a
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`system.
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`still allowing differences in implementations.
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`Layering is used as a structuring technique to allow the net-
`Products can satisfy the much weaker constraints imposed by
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`the Reference Model, but may not be able to communicate with
`work of open systems to be logically decomposed in independent,
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`smaller subsystem (see Fig. 2). Each individual system itself is
`open systems unless they also conform the 051 services and
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`11. THE ELEMENTS or THE ARCHITECTURE
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`ISO 7498, the document describing the basic OSI Reference
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`is divided into two major sections. The first of these
`Model,
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`describes the elements of the architecture. These constitute the
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`building blocks that are used to construct the seven-layer model.
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`The second describes the services and functions of the layers.
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`Petitioner Valve - Ex. 1017, Page 1335
`
`Petitioner Riot Games, Inc. - EX. 1017, p. 1335
`
`Petitioner Riot Games, Inc. - Ex. 1017, p. 1335
`
`Petitioner Valve - Ex. 1017, Page 1335
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`

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`1336
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`Fig 4. Entities, service access points (SAP’s). and protocols.
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`PROCEDDIGS OF THE IEE, VOL. 71, NO. 12, DECEMBER 1983
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`important to note that not all functions performed within the
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`(N )-layer are services. Only those capabilities that can be seen
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`from the layer above are services.
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`(N)-entities distributed among the interconnected open sys-
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`tems work collectively to provide the (N )-service to (N + 1)-
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`entities as illustrated in Fig. 4. In other words, the (N )-entities
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`add value to the (N — 1) service they get from the (N - 1)-layer
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`and offer this value-added service, ie., the (N )-service to the
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`(N + 1)-entities.
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`the
`The (N)-services are offered to the (N + 1)—entities at
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`(N )-service access points, or (N )-SAP’s for short, which repre-
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`sent the logical interfaces between the (N )—entities and the (N +
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`1)-entities. An (N + 1)—entity communicates with an (N)—entity
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`in the same system through an (N)-SAP. An (N)—SAP can be
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`served by only one (N)«entity and used by only one '(N + 1)-
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`entity, but one (N)-entity can serve several (N)—SAP’s and one
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`(N + l)—entity can use several (N)-SAP’s. An (N)-SAP is located
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`by its ( N)-address (see Section II-D).
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`C. Functions and Protocols
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`An (N)-function is part of the activity of an (N)—entity. Flow
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`control, sequencing, data transformation are all examples of
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`(N )-functions. Cooperation among (N)—entities is governed by
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`one or more (N )vprotocols. An (N)-pr_otocol is the set of rules
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`and formats which govern the communication between (N)-enti-
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`ties performing the (N)—functions in different open systems. In
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`particular, direct communication between the (N )—entities in the
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`same system, e.g.,
`for sharing resources,
`is not visible from
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`outside the system and thus is not covered by the OSI Architec-
`ture.
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`viewed as being logically composed of a succession of subsys-
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`tems, each subsystem corresponding to the intersection of the
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`system with a layer. In other words, a layer is viewed as being
`D. Naming
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`locally composed of subsystems of the same rank in all inter-
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`Objects Within a layer or at the boundary between adjacent
`connected systems. Each subsystem, in turn, is viewed as being
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`layers need to be uniquely identifiable, e.g., in order to establish a
`made of one or several entities. A layer, therefore; comprises
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`connection between two SAP’s, one must be able to identify them
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`many entities distributed among interconnected open systems.
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`uniquely. The 051 Architecture defines identifiers for entities,
`Entities in the same layer are termed peer entities.
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`SAP’s, and connections as well as relations between these identi-
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`For simplicity, any layer is referred to as the (N )-layer, while
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`fiers, as briefly outlined below.
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`its next
`lower and next higher layers are referred to as the
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`Each (N )-entity is identified with a global title which is unique
`(N — l)-layer and the (N + 1)-layer, respectively. The same no-
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`and identifies the same (N)~entity anywhere in the network of
`tation is used to designate all concepts relating to layers, cg,
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`open systems. Within more limited domains, an (N)—entity can
`entities in the (N)-layer are termed ( N )-entities, and illustrated
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`be identified with a local
`title which uniquely identifies the
`in Figs. 3 and 4.
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`(N )—entity only in that domain. For instance, within the domain
`The basic idea of layering is that each layer adds value to
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`corresponding to the (N)—layer, (N)-entities are identified with
`services provided by the set of lower layers in such a way that the
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`(N )-global titles which are unique within the (N)—layer.
`highest layer is offered the full set of services needed to rim
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`Each (N)—SAP is identified by an (N yaddrsss which tmiquely
`distributed applications. Layering thus divides the total problem
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`locates the (N)-SAP at the boundary between the (N)—layer and
`into smaller pieces.
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`the (N + 1)-layer. The concepts of titles and addresses are il-
`Another basic principle of layering is to ensure independence
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`lustrated in Fig. 6.
`of each layer by defining services provided by a layer to the next
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`they
`Bindings between (N)-entities and the (N — 1)—SAP’s
`higher layer, independent of how these services are performed.
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`use (i.e., SAP’s through which they can access each other and
`This permits changes to be made in the way a layer or a set of
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`communicate) are defined in an (N)-directory which indicates
`layers operate, provided they still offer the same service to the
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`correspondence between global titles of (N)—entities and (N )~
`next higher layer. This technique is similar to the one used in
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`addrmses through which they can be reached.
`structured programming where only the functions performed by a
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`Correspondence between (N)-addresses served by an (N )—
`module (and not its internal functioning) are known by its users.
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`entity and the (N— lyaddresses used for this purpose is per-
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`formed by an (N)-mapping function. In addition to the simplest
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`case of one-to-one mapping, mapping may,
`in particular, be
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`hierarchical with the (N)—address being made of an (N— 1)-
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`address and an (N )-suffix. Mapping may also be performed “by
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`table lookup.”
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`B. Services and Service Access Points
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`Each layer provides services to the layer above (with the
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`exception of the highest layer). A service is a capability of the
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`(N )-1ayer which is provided to the .(N + 1)—entities. But it is
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`Petitioner Valve - Ex. 1017, Page 1336
`
`Petitioner Riot Games, Inc. - EX. 1017, p. 1336
`
`Petitioner Riot Games, Inc. - Ex. 1017, p. 1336
`
`Petitioner Valve - Ex. 1017, Page 1336
`
`

`

`
`DAY AND ZIMMERMANN: THE 051 REFERENCE MODEL
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`1337
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`Fig. 5. Connections and connection endpoints (CEPs).
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`0““le
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`_____ (m min.
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`Fig. 6. Titlm. address. and CEP identifiers.
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`As work has progressed in 081, two distinctions have come to
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`be recognized as critical
`to the naming problem. First is the
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`recognition that one must distinguish and be able to name
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`separately and uniquely types and instances. Second, there are
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`two types of names: primitive and descriptive. Primitive names
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`are unique and assigned by some domain administrator, e.g.,
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`phone numbers, social security numbers, etc. Descriptive names
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`are composites of primitive names, keywords, etc, that can be
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`resolved to primitive names by interpretation, e.g., my name and
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`address. Relative few types and instances require primitive names.
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`Descriptive names for everything else can be built from these as
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`well as synonyms for objects with primitive names. This greatly
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`simplifies the administration of naming in 081.
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`E. Connections
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`A common service offered by all layers consists of providing
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`associations between peer SAP’s which can be used in particular
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`to transfer data (as well as for other purposes such as to synchro-
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`nize the served entities participating in the association). More
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`precisely (see Fig. 5), the (N)-layer offers (N)—connections be-
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`tween (N)-SAP’s as part of the (N)—servioes. The most usual type
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`of connection is the point-to-point connection, but there are also
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`multi—endpoint connections which correspond to multiple associ-
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`ations between entities (e.g., broadcast or multidrop communica-
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`tions). The end of an (N)-connection at an (N)—SAP is called an
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`(N )-connection-endpoint or (N )—CEP for short. Several connec-
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`tions may coexist between the same pair (or n-tuple) of SAP’s.
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`Each (N)-CEP is uniquely identified within its (N)~SAP by an
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`(N )-CEP identifier which is used by the (N )—entity and the
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`(N + 1)-entity on both sides of the (N )-SAP to identify the
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`(N )-connection, as illustrated in Fig 6. This is necessary since
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`several (N )-connections may end at the same (N )-SAP.
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`The Basic Reference Model currently restricts communications
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`between (N )-entities to “connection mode.” In this mode, the
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`(N — 1)—service requires that an (N — l)-connection be estab-
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`lished between (N — 1)-SAP’s before any communication be-
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`(N — lHfl‘Mwe
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`Fig. 7.
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`Interrelationship between data units.
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`DetaUnlt
`rN- 1Hlyer
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`PCI = Protocoleonthntormatlon
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`PDU = Manual-«um
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`SW = Service-damn“
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`Fig. 8. Logical relationship between data units in adjacent layers.
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`tween (N)-entities can take place. Conversely, when the (N)—enti-
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`ties need no longer communicate, the (N — 1)—connection can be
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`released. This connection mode covers traditional teleprocessing.
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`For newer applications, a “connectionless” mode is currently
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`being developed within ISO as a complement to the connection
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`mode.
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`1) Establishment and Release of Connections: When an (N +
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`1)-entity requests the establishment of an ( N)—connection from
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`one of the (N )—SAP’s it uses to another (N )-SAP, it must provide
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`at the local (N )-SAP the (N )-address of the distant (N )-SAP.
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`When the (N)-connection is established, both the (N + 1)—entity
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`and the (N )—entity will use the (N )-CEP identifier to designate
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`the (N )-connection.
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`(N )-connections may be established and released dynamically
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`on top of (N — 1)—connections. Establishment of an (N )—connec—
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`tion implies the availability of an (N — 1)-connection between
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`the two entities. If not available, the (N — 1)-connection must be
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`established. This
`requires
`the availability of an (N — 2)-
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`connection. The same consideration applies downwards until an
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`available connection is encountered.
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`In some cases, the (N )-connection may be established simulta-
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`neously with its supporting (N — 1)-connection provided the
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`(N — 1)—connection establishment service permits (N )-entil:ies to
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`exchange information necessary to establish the (N)-connection.
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`2) Data Transfer on a Connection:
`Information is transferred
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`in various types of data units between peer entities and between
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`entities attached to a specific service access point. The data units
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`are defined below and the interrelationship among several of
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`them is illustrated in Figs. 7 and 8.
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`(N)-protocol control
`information is information exchanged
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`between two (N )-entities, using an (N — l)-connection, to coor-
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`dinate their joint operation.
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`
`Petitioner Valve - Ex. 1017, Page 1337
`
`Petitioner Riot Games, Inc. - EX. 1017, p. 1337
`
`Petitioner Riot Games, Inc. - Ex. 1017, p. 1337
`
`Petitioner Valve - Ex. 1017, Page 1337
`
`

`

`1338
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`(N )-user-data are the data transferred between two (N)-enti-
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`ties on behalf of the (N + 1)—entities for whom the (N )-entities
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`are providing services.
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`An (N)-protocol-data-unit is a unit of data which contains
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`(N )-protocol-control-information and possibly (N )—user-data.
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`(N )-interface-control-information is
`information exchanged
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`between an (N + 1)-entity and an (N)—entity to coordinate their
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`joint operation.
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`(N )-interface-data-unit is the amount of (N — 1)-interface-data
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`whose identity is preserved from one end of an (N —- 1)-
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`connection to the other. Data may be held within a connection
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`until a complete service data unit is put into the connection.
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`Expedited (N )-service—data-unit
`is a small (N )—service-data-
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`unit whose transfer is expedited. The (N — 1)—layer ensures that
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`an expedited data-unit will not be delivered before any subse-
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`quent service-data-unit or expedited data-unit sent on that con-
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`nection. An expedited (N )-service-data-unit may also be referred
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`to as an (N )-expedited-data-unit.
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`Note: An (N )-protocol-data—unit may be mapped one-
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`to—one onto an (N — l)-service—data—unit (see Fig. 8).
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`3) Elements of Layer Operation: There are a number of func-
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`tions which are recognized as part of layer operation. These
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`include such things as multiplexing,
`flow control, and error
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`control. As the Reference Model matures other elements will be
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`added. In the Reference Model, these elements are described in
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`general without reference to a particular layer.
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`Three particular types of construction of (N )—connections on
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`top of (N — 1)-connections are distinguished.
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`a) One-to-one correspondence, where each (N)-connection is
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`built on one (N — 1)-connection.
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`b) Multiplexing, where several (N )—connections are multi-
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`plexed on one single (N — 1)-connection.
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`c) Splitting, where one single ( N)-connection is built on top of
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`several (N — 1)-connections,
`the traffic on the (N)connection
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`being divided between the various (N — 1)-connections.
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`Two forms of flow control are recognized by the reference
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`model: a peer flow control which regulates the flow of (N )-pro