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OSI model

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OSI model
by layer
7.  Application layer
6.  Presentation layer
5.  Session layer
4.  Transport layer
3.  Network layer
2.  Data link layer
1.  Physical layer

The Open Systems Interconnection (OSI) model is a reference model developed by the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems interconnection."[2]

In the OSI reference model, the components of a communication system are distinguished in seven abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.[3]

The model describes communications from the physical implementation of transmitting bits across a transmission medium to the highest-level representation of data of a distributed application. Each layer has well-defined functions and semantics and serves a class of functionality to the layer above it and is served by the layer below it. Established, well-known communication protocols are decomposed in software development into the model's hierarchy of function calls.

The Internet protocol suite as defined in RFC 1122 and RFC 1123 is a model of networking developed contemporarily to the OSI model, and was funded primarily by the U.S. Department of Defense. It was the foundation for the development of the Internet. It assumed the presence of generic physical links and focused primarily on the software layers of communication, with a similar but much less rigorous structure than the OSI model.

In comparison, several networking models have sought to create an intellectual framework for clarifying networking concepts and activities,[citation needed] but none have been as successful as the OSI reference model in becoming the standard model for discussing and teaching networking in the field of information technology. The model allows transparent communication through equivalent exchange of protocol data units (PDUs) between two parties, through what is known as peer-to-peer networking (also known as peer-to-peer communication). As a result, the OSI reference model has not only become an important piece among professionals and non-professionals alike, but also in all networking between one or many parties, due in large part to its commonly accepted user-friendly framework.[4]

Communication in the OSI model (example with layers 3 to 5)

History

The development of the OSI model started in the late 1970s to support the emergence of the diverse computer networking methods that were competing for application in the large national networking efforts in the world (see OSI protocols and Protocol Wars). In the 1980s, the model became a working product of the Open Systems Interconnection group at the International Organization for Standardization (ISO). While attempting to provide a comprehensive description of networking, the model failed to garner reliance during the design of the Internet, which is reflected in the less prescriptive Internet Protocol Suite, principally sponsored under the auspices of the Internet Engineering Task Force (IETF).

In the early- and mid-1970s, networking was largely either government-sponsored (NPL network in the UK, ARPANET in the US, CYCLADES in France) or vendor-developed with proprietary standards, such as IBM's Systems Network Architecture and Digital Equipment Corporation's DECnet. Public data networks were only just beginning to emerge, and these began to use the X.25 standard in the late 1970s.[5][6]

The Experimental Packet Switched System in the UK c. 1973–1975 identified the need for defining higher-level protocols.[5] The UK National Computing Centre publication, Why Distributed Computing, which came from considerable research into future configurations for computer systems,[7] resulted in the UK presenting the case for an international standards committee to cover this area at the ISO meeting in Sydney in March 1977.[8][9]

Beginning in 1977, the ISO initiated a program to develop general standards and methods of networking. A similar process evolved at the International Telegraph and Telephone Consultative Committee (CCITT, from French: Comité Consultatif International Téléphonique et Télégraphique). Both bodies developed documents that defined similar networking models. The British Department of Trade and Industry acted as the secretariat, and universities in the United Kingdom developed prototypes of the standards.[10]

The OSI model was first defined in raw form in Washington, D.C., in February 1978 by French software engineer Hubert Zimmermann, and the refined but still draft standard was published by the ISO in 1980.[9]

The drafters of the reference model had to contend with many competing priorities and interests. The rate of technological change made it necessary to define standards that new systems could converge to rather than standardizing procedures after the fact; the reverse of the traditional approach to developing standards.[11] Although not a standard itself, it was a framework in which future standards could be defined.[12]

In May 1983,[13] the CCITT and ISO documents were merged to form The Basic Reference Model for Open Systems Interconnection, usually referred to as the Open Systems Interconnection Reference Model, OSI Reference Model, or simply OSI model. It was published in 1984 by both the ISO, as standard ISO 7498, and the renamed CCITT (now called the Telecommunications Standardization Sector of the International Telecommunication Union or ITU-T) as standard X.200.

OSI had two major components: an abstract model of networking, called the Basic Reference Model or seven-layer model, and a set of specific protocols. The OSI reference model was a major advance in the standardisation of network concepts. It promoted the idea of a consistent model of protocol layers, defining interoperability between network devices and software.

The concept of a seven-layer model was provided by the work of Charles Bachman at Honeywell Information Systems.[14] Various aspects of OSI design evolved from experiences with the NPL network, ARPANET, CYCLADES, EIN, and the International Network Working Group (IFIP WG6.1). In this model, a networking system was divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacted directly only with the layer immediately beneath it and provided facilities for use by the layer above it.

The OSI standards documents are available from the ITU-T as the X.200 series of recommendations.[15] Some of the protocol specifications were also available as part of the ITU-T X series. The equivalent ISO/IEC standards for the OSI model were available from ISO. Not all are free of charge.[16]

OSI was an industry effort, attempting to get industry participants to agree on common network standards to provide multi-vendor interoperability.[17] It was common for large networks to support multiple network protocol suites, with many devices unable to interoperate with other devices because of a lack of common protocols. For a period in the late 1980s and early 1990s, engineers, organizations and nations became polarized over the issue of which standard, the OSI model or the Internet protocol suite, would result in the best and most robust computer networks.[9][18][19] However, while OSI developed its networking standards in the late 1980s,[20][page needed][21][page needed] TCP/IP came into widespread use on multi-vendor networks for internetworking.

The OSI model is still used as a reference for teaching and documentation;[22] however, the OSI protocols originally conceived for the model did not gain popularity. Some engineers argue the OSI reference model is still relevant to cloud computing.[23] Others say the original OSI model does not fit today's networking protocols and have suggested instead a simplified approach.[24][25]

Definitions

Communication protocols enable an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions, like the OSI model, abstractly describe the functionality provided to a layer N by a layer N−1, where N is one of the seven layers of protocols operating in the local host.

At each level N, two entities at the communicating devices (layer N peers) exchange protocol data units (PDUs) by means of a layer N protocol. Each PDU contains a payload, called the service data unit (SDU), along with protocol-related headers or footers.

Data processing by two communicating OSI-compatible devices proceeds as follows:

  1. The data to be transmitted is composed at the topmost layer of the transmitting device (layer N) into a protocol data unit (PDU).
  2. The PDU is passed to layer N−1, where it is known as the service data unit (SDU).
  3. At layer N−1 the SDU is concatenated with a header, a footer, or both, producing a layer N−1 PDU. It is then passed to layer N−2.
  4. The process continues until reaching the lowermost level, from which the data is transmitted to the receiving device.
  5. At the receiving device the data is passed from the lowest to the highest layer as a series of SDUs while being successively stripped from each layer's header or footer until reaching the topmost layer, where the last of the data is consumed.

Standards documents

The OSI model was defined in ISO/IEC 7498 which consists of the following parts:

  • ISO/IEC 7498-1 The Basic Model
  • ISO/IEC 7498-2 Security Architecture
  • ISO/IEC 7498-3 Naming and addressing
  • ISO/IEC 7498-4 Management framework

ISO/IEC 7498-1 is also published as ITU-T Recommendation X.200.

Cross-layer functions

Further information: Cross-layer optimization

Cross-layer functions are services that are not tied to a given layer, but may affect more than one layer.[26] Some orthogonal aspects, such as management and security, involve all of the layers (See ITU-T X.800 Recommendation[27]). These services are aimed at improving the CIA triad—confidentiality, integrity, and availability—of the transmitted data. Cross-layer functions are the norm, in practice, because the availability of a communication service is determined by the interaction between network design and network management protocols.

Specific examples of cross-layer functions include the following:

  • Security service (telecommunication)[27] as defined by ITU-T X.800 recommendation.
  • Management functions, i.e. functions that permit to configure, instantiate, monitor, terminate the communications of two or more entities: there is a specific application-layer protocol, Common Management Information Protocol (CMIP) and its corresponding service, Common Management Information Service (CMIS), they need to interact with every layer in order to deal with their instances.
  • Multiprotocol Label Switching (MPLS), ATM, and X.25 are 3a protocols. OSI subdivides the Network Layer into three sublayers: 3a) Subnetwork Access, 3b) Subnetwork Dependent Convergence and 3c) Subnetwork Independent Convergence.[28] It was designed to provide a unified data-carrying service for both circuit-based clients and packet-switching clients which provide a datagram-based service model. It can be used to carry many different kinds of traffic, including IP packets, as well as native ATM, SONET, and Ethernet frames. Sometimes one sees reference to a Layer 2.5.
  • Cross MAC and PHY Scheduling is essential in wireless networks because of the time-varying nature of wireless channels. By scheduling packet transmission only in favourable channel conditions, which requires the MAC layer to obtain channel state information from the PHY layer, network throughput can be significantly improved and energy waste can be avoided.[29][page needed]

Programming interfaces

Neither the OSI Reference Model, nor any OSI protocol specifications, outline any programming interfaces, other than deliberately abstract service descriptions. Protocol specifications define a methodology for communication between peers, but the software interfaces are implementation-specific.

For example, the Network Driver Interface Specification (NDIS) and Open Data-Link Interface (ODI) are interfaces between the media (layer 2) and the network protocol (layer 3).

Comparison to other networking suites

The table below presents a list of OSI layers, the original OSI protocols, and some approximate modern matches. This correspondence is rough: the OSI model contains idiosyncrasies not found in later systems such as the IP stack in modern Internet.[25]

Layer OSI protocols TCP/IP protocols Signaling
System 7[30] 		AppleTalk IPX SNA UMTS HTTP-based protocols Miscellaneous examples
No. Name
7 Application
  • Transaction Services
  • Presentation Services
6 Presentation
  • ISO/IEC 8823
  • X.226

  • ISO/IEC 9576-1
  • X.236
Presentation Services
5 Session
  • ISO/IEC 8327
  • X.225

  • ISO/IEC 9548-1
  • X.235
 Sockets (session establishment in TCP / RTP / PPTP)
  • Presentation Services
  • Data Flow Control
4 Transport
  • ISO/IEC 8073
  • TP0
  • TP1
  • TP2
  • TP3
  • TP4 (X.224)
  • ISO/IEC 8602
  • X.234
  • Data Flow Control
  • Transmission Control
Port number can be specified.
3 Network
ATP (TokenTalk / EtherTalk)
  • Transmission Control
  • Path Control
Out of scope. IP addresses can be used instead of domain names in URLs.
2 Data link
IEEE 802.3 framing
Ethernet II framing
Out of scope.
1 Physical
TCP/IP stack does not care about the physical medium, as long as it provides a way to communicate octets
  • Physical
 UMTS air interfaces Out of scope.

Comparison with TCP/IP model

See also: Internet protocol suite § Comparison of TCP/IP and OSI layering

The design of protocols in the TCP/IP model of the Internet does not concern itself with strict hierarchical encapsulation and layering. RFC 3439 contains a section entitled "Layering considered harmful".[39] TCP/IP does recognize four broad layers of functionality which are derived from the operating scope of their contained protocols: the scope of the software application; the host-to-host transport path; the internetworking range; and the scope of the direct links to other nodes on the local network.[40]

Despite using a different concept for layering than the OSI model, these layers are often compared with the OSI layering scheme in the following manner:

  • The Internet application layer maps to the OSI application layer, presentation layer, and most of the session layer.
  • The TCP/IP transport layer maps to the graceful close function of the OSI session layer as well as the OSI transport layer.
  • The internet layer performs functions as those in a subset of the OSI network layer.
  • The link layer corresponds to the OSI data link layer and may include similar functions as the physical layer, as well as some protocols of the OSI's network layer.

These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in the internal organization of the network layer.

The OSI protocol suite that was specified as part of the OSI project was considered by many as too complicated and inefficient, and to a large extent unimplementable.[41][page needed] Taking the "forklift upgrade" approach to networking, it specified eliminating all existing networking protocols and replacing them at all layers of the stack. This made implementation difficult and was resisted by many vendors and users with significant investments in other network technologies. In addition, the protocols included so many optional features that many vendors' implementations were not interoperable.[41][page needed]

Although the OSI model is often still referenced, the Internet protocol suite has become the standard for networking. TCP/IP's pragmatic approach to computer networking and to independent implementations of simplified protocols made it a practical methodology.[41][page needed] Some protocols and specifications in the OSI stack remain in use, one example being IS-IS, which was specified for OSI as ISO/IEC 10589:2002 and adapted for Internet use with TCP/IP as RFC 1142.[42]

See also

References

  1. ^ "X.225 : Information technology – Open Systems Interconnection – Connection-oriented Session protocol: Protocol specification". Archived from the original on 1 February 2021. Retrieved 10 March 2023.
  2. ^ ISO/IEC 7498-1:1994 Information technology — Open Systems Interconnection — Basic Reference Model: The Basic Model. June 1999. Introduction. Retrieved 26 August 2022.
  3. ^ "What is the OSI Model?". Forcepoint. 10 August 2018. Retrieved 20 May 2022.
  4. ^ Tomsho, Greg (2016). Guide to Networking Essentials (7th ed.). Cengage. Retrieved 3 April 2022.
  5. ^ a b Davies, Howard; Bressan, Beatrice (26 April 2010). A History of International Research Networking: The People who Made it Happen. John Wiley & Sons. pp. 2–3. ISBN 978-3-527-32710-2.
  6. ^ Roberts, Dr. Lawrence G. (November 1978). "The Evolution of Packet Switching" (PDF). IEEE Invited Paper. Retrieved 26 February 2022.
  7. ^ Down, Peter John; Taylor, Frank Edward (1976). Why distributed computing?: An NCC review of potential and experience in the UK. NCC Publications. ISBN 9780850121704.
  8. ^ Radu, Roxana (2019). "Revisiting the Origins: The Internet and its Early Governance". Negotiating Internet Governance. Oxford University Press. pp. 43–74. doi:10.1093/oso/9780198833079.003.0003. ISBN 9780191871405.
  9. ^ a b c Andrew L. Russell (30 July 2013). "OSI: The Internet That Wasn't". IEEE Spectrum. Vol. 50, no. 8.
  10. ^ Campbell-Kelly, Martin; Garcia-Swartz, Daniel D (2013). "The History of the Internet: The Missing Narratives". Journal of Information Technology. 28 (1): 18–33. doi:10.1057/jit.2013.4. ISSN 0268-3962. S2CID 41013. SSRN 867087.
  11. ^ Sunshine, Carl A. (1989). Computer Network Architectures and Protocols. Springer Science & Business Media. p. 35. ISBN 978-1-4613-0809-6.
  12. ^ Hasman, A. (1995). Education and Training in Health Informatics in Europe: State of the Art, Guidelines, Applications. IOS Press. p. 251. ISBN 978-90-5199-234-2.
  13. ^ "ISO/OSI (Open Systems Interconnection): 1982 - 1983 | History of Computer Communications". historyofcomputercommunications.info. Retrieved 12 July 2024.
  14. ^ J. A. N. Lee. "Computer Pioneers by J. A. N. Lee". IEEE Computer Society.
  15. ^ "ITU-T X-Series Recommendations".
  16. ^ "Publicly Available Standards". Standards.iso.org. 30 July 2010. Retrieved 11 September 2010.
  17. ^ Russell, Andrew L. (28 April 2014). Open Standards and the Digital Age: History, Ideology, and Networks. Cambridge University Press. ISBN 978-1-139-91661-5.
  18. ^ Russell, Andrew L. (July–September 2006). "Rough Consensus and Running Code' and the Internet-OSI Standards War" (PDF). IEEE Annals of the History of Computing. 28 (3): 48–61. doi:10.1109/MAHC.2006.42.
  19. ^ "Standards Wars" (PDF). 2006.
  20. ^ Network World. IDG Network World Inc. 15 February 1988.
  21. ^ Network World. IDG Network World Inc. 10 October 1988.
  22. ^ Shaw, Keith (22 October 2018). "The OSI model explained: How to understand (and remember) the 7 layer network model". Network World. Archived from the original on 4 October 2020. Retrieved 16 May 2020.
  23. ^ "An OSI Model for Cloud". Cisco Blogs. 24 February 2017. Retrieved 16 May 2020.
  24. ^ Taylor, Steve; Metzler, Jim (23 September 2008). "Why it's time to let the OSI model die". Network World. Retrieved 16 May 2020.
  25. ^ a b Crawford, JB (27 March 2021). "The actual OSI model".
  26. ^ Mao, Stephen (2009). "Chapter 8: Fundamentals of communication networks". In Wyglinski, Alexander; Nekovee, Maziar; Hou, Thomas (eds.). Cognitive Radio Communications and Networks: Principles and Practice. Elsevier. p. 201. ISBN 978-0-08-087932-1. OCLC 635292718, 528550718. Partial preview at Google Books.
  27. ^ a b "ITU-T Recommendation X.800 (03/91), Security architecture for Open Systems Interconnection for CCITT applications". ITU. Retrieved 14 August 2015.
  28. ^ Hegering, Heinz-Gerd; Abeck, Sebastian; Neumair, Bernhard (1999). "Fundamental Structures of Networked Systems". Integrated management of networked systems: concepts, architectures, and their operational application. San Francisco, Calif.: Morgan Kaufmann. p. 54. ISBN 978-1-55860-571-8. OCLC 1341886747 – via Internet Archive.
  29. ^ Miao, Guowang; Song, Guocong (2014). Energy and spectrum efficient wireless network design. New York: Cambridge University Press. ISBN 978-1-139-62677-4. OCLC 898138775 – via Internet Archive.
  30. ^ "ITU-T Recommendation Q.1400 (03/1993)], Architecture framework for the development of signaling and OA&M protocols using OSI concepts". ITU. pp. 4, 7.
  31. ^ "ITU-T X.227 (04/1995)". ITU-T Recommendations. 10 April 1995. Retrieved 12 July 2024.
  32. ^ "ITU-T X.217". Open Systems Interconnection. 10 April 1995. Retrieved 12 July 2024.
  33. ^ "X.700: Management framework for Open Systems Interconnection (OSI) for CCITT applications". ITU. 10 September 1992. Retrieved 12 July 2024.
  34. ^ "X.711". Open Systems Interconnection. 15 May 2014. Retrieved 12 July 2024.
  35. ^ "ISO/IEC 9596-1:1998(en)". ISO. Retrieved 12 July 2024.
  36. ^ "ISO/IEC 9596-2:1993(en)". ISO. Retrieved 12 July 2024.
  37. ^ a b "Internetworking Technology Handbook – Internetworking Basics [Internetworking]". Cisco. 15 January 2014. Retrieved 14 August 2015.
  38. ^ "3GPP specification: 36.300". 3gpp.org. Retrieved 14 August 2015.
  39. ^ "Layering Considered Harmful". Some Internet Architectural Guidelines and Philosophy. December 2002. sec. 3. doi:10.17487/RFC3439. RFC 3439. Retrieved 25 April 2022.
  40. ^ Walter Goralski (2009). The Illustrated Network: How TCP/IP Works in a Modern Network (PDF). Morgan Kaufmann. p. 26. ISBN 978-0123745415.
  41. ^ a b c Tanenbaum, Andrew S. (2003). Computer networks. Upper Saddle River, NJ: Prentice Hall PTR. ISBN 978-0-13-066102-9. OCLC 50166590.
  42. ^ "OSI IS-IS Intra-domain Routing Protocol". IETF Datatracker. doi:10.17487/RFC1142. RFC 1142. Retrieved 12 July 2024.

Further reading

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