OSI reference model
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The Open Systems Interconnection model (OSI model) is a
conceptual model A conceptual model is a representation of a system. It consists of concepts used to help people knowledge, know, understanding, understand, or simulation, simulate a subject the model represents. In contrast, physical models are physical object su ...
that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications between a computing system are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. The model partitions the flow of data in a communication system into seven
abstraction layer In computing, an abstraction layer or abstraction level is a way of hiding the working details of a subsystem. Examples of software models that use layers of abstraction include the OSI model for network protocols, OpenGL, and other graphics libra ...
s to describe networked communication from the physical implementation of transmitting bits across a communications medium to the highest-level representation of data of a distributed application. Each intermediate layer serves a class of functionality to the layer above it and is served by the layer below it. Classes of functionality are realized in all software development through all and any standardized communication protocols. Each layer in the OSI model has its own well-defined functions, and the functions of each layer communicate and interact with the layers immediately above and below it, unless the layer does not have layers below or above. The Internet protocol suite has a separate model, the layers of which are mentioned in and . That model combines the physical and data link layers of the OSI model into a single link layer, and has a single application layer for all protocols above the transport layer, as opposed to the separate application, presentation and session layers of the OSI model. In comparison, several networking models have sought to create an intellectual framework for clarifying networking concepts and activities, but none have been as successful as the OSI reference model in becoming the standard model for discussing, teaching, and learning for the networking procedures in the field of
Information technology Information technology (IT) is the use of computers to create, process, store, retrieve, and exchange all kinds of data . and information. IT forms part of information and communications technology (ICT). An information technology system (I ...
. Additionally, the model allows transparent communication through equivalent exchange of
protocol data unit In telecommunications, a protocol data unit (PDU) is a single unit of information transmitted among peer entities of a computer network. It is composed of protocol-specific control information and user data. In the layered architectures of c ...
s (PDUs) between two parties, through what is known as
peer-to-peer networking Peer-to-peer (P2P) computing or networking is a distributed application architecture that partitions tasks or workloads between peers. Peers are equally privileged, equipotent participants in the network. They are said to form a peer-to-peer n ...
(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.


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
Protocol Wars A long-running debate in computer science known as the Protocol Wars occurred from the 1970s to the 1990s when engineers, organizations and nations became polarized over the issue of which communication protocol would result in the best and most r ...
). In the 1980s, the model became a working product of the Open Systems Interconnection group at the
International Organization for Standardization The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in Ar ...
(ISO). While attempting to provide a comprehensive description of networking, the model failed to garner reliance during the design of the
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...
, which is reflected in the less prescriptive Internet Protocol Suite, principally sponsored under the auspices of the
Internet Engineering Task Force The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements and a ...
(IETF). In the early- and mid-1970s, networking was largely either government-sponsored ( NPL network in the UK,
ARPANET The Advanced Research Projects Agency Network (ARPANET) was the first wide-area packet-switched network with distributed control and one of the first networks to implement the TCP/IP protocol suite. Both technologies became the technical fou ...
in the US, CYCLADES in France) or vendor-developed with proprietary standards, such as IBM's Systems Network Architecture and
Digital Equipment Corporation Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and Harlan Anderson in 1957. Olsen was president unt ...
's
DECnet DECnet is a suite of network protocols created by Digital Equipment Corporation. Originally released in 1975 in order to connect two PDP-11 minicomputers, it evolved into one of the first peer-to-peer network architectures, thus transforming DEC ...
. Public data networks were only just beginning to emerge, and these began to use the
X.25 X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks (WAN). It was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts a ...
standard in the late 1970s. The Experimental Packet Switched System in the UK circa 1973–1975 identified the need for defining higher level protocols. The UK National Computing Centre publication 'Why Distributed Computing' which came from considerable research into future configurations for computer systems, 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. 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 The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
(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 Universities in the United Kingdom have generally been instituted by royal charter, papal bull, Act of Parliament, or an instrument of government under the Further and Higher Education Act 1992 or the Higher Education and Research Act 2017. D ...
developed
prototype A prototype is an early sample, model, or release of a product built to test a concept or process. It is a term used in a variety of contexts, including semantics, design, electronics, and Software prototyping, software programming. A prototyp ...
s of the standards. The OSI model was first defined in raw form in
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, in February 1978 by Hubert Zimmermann of France and the refined but still draft standard was published by the ISO in 1980. 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. Although not a standard itself, it was a framework in which future standards could be defined. In 1983, 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 The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
or
ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
) 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 Honeywell International Inc. is an American publicly traded, multinational conglomerate corporation headquartered in Charlotte, North Carolina. It primarily operates in four areas of business: aerospace, building technologies, performance ma ...
. Various aspects of OSI design evolved from experiences with the NPL network, ARPANET, CYCLADES,
EIN Ein or EIN may refer to: Science and technology * Ein function, in mathematics * Endometrial intraepithelial neoplasia, a lesion of the uterine lining * Equivalent input noise, of a microphone * European Informatics Network, a 1970s computer netw ...
, and the International Networking 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. 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. OSI was an industry effort, attempting to get industry participants to agree on common network standards to provide multi-vendor interoperability. 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. However, while OSI developed its networking standards in the late 1980s, 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; 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. Others say the original OSI model doesn't fit today's networking protocols and have suggested instead a simplified approach.


Definitions

Communication protocols A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchron ...
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 an (N)-layer by an (N-1) layer, 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 unit In telecommunications, a protocol data unit (PDU) is a single unit of information transmitted among peer entities of a computer network. It is composed of protocol-specific control information and user data. In the layered architectures of c ...
s (PDUs) by means of a layer N ''protocol''. Each PDU contains a payload, called the
service data unit In Open Systems Interconnection (OSI) terminology, a service data unit (SDU) is a unit of data that has been passed down from an OSI layer or sublayer to a lower layer. This unit of data (SDU) has not yet been encapsulated into a protocol da ...
(SDU), along with protocol-related headers or footers. Data processing by two communicating OSI-compatible devices proceeds as follows: # The data to be transmitted is composed at the topmost layer of the transmitting device (layer ''N'') into a ''
protocol data unit In telecommunications, a protocol data unit (PDU) is a single unit of information transmitted among peer entities of a computer network. It is composed of protocol-specific control information and user data. In the layered architectures of c ...
'' (''PDU''). # The ''PDU'' is passed to layer ''N-1'', where it is known as the ''
service data unit In Open Systems Interconnection (OSI) terminology, a service data unit (SDU) is a unit of data that has been passed down from an OSI layer or sublayer to a lower layer. This unit of data (SDU) has not yet been encapsulated into a protocol da ...
'' (''SDU''). # 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''. # The process continues until reaching the lowermost level, from which the data is transmitted to the receiving device. # At the receiving device the data is passed from the lowest to the highest layer as a series of ''SDU''s 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.


Layer architecture

The recommendation X.200 describes seven layers, labelled 1 to 7. Layer 1 is the lowest layer in this model.


Layer 1: Physical layer

The
Physical Layer In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer; The layer most closely associated with the physical connection between devices. This layer may be implemented by a PHY chip. The ...
is responsible for the transmission and reception of unstructured raw data between a device, such as a network interface controller, Ethernet hub, or network switch, and a physical transmission medium. It converts the digital bits into electrical, radio, or optical signals. Layer specifications define characteristics such as voltage levels, the timing of voltage changes, physical data rates, maximum transmission distances, modulation scheme, channel access method and physical connectors. This includes the layout of
pins A pin is a device used for fastening objects or material together. Pin or PIN may also refer to: Computers and technology * Personal identification number (PIN), to access a secured system ** PIN pad, a PIN entry device * PIN, a former Dutch de ...
, voltages, line impedance, cable specifications, signal timing and frequency for wireless devices. Bit rate control is done at the physical layer and may define transmission mode as
simplex In geometry, a simplex (plural: simplexes or simplices) is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions. The simplex is so-named because it represents the simplest possible polytope in any given dimension. ...
, half duplex, and full duplex. The components of a physical layer can be described in terms of a network topology. Physical layer specifications are included in the specifications for the ubiquitous Bluetooth, Ethernet, and USB standards. An example of a less well-known physical layer specification would be for the
CAN Can may refer to: Containers * Aluminum can * Drink can * Oil can * Steel and tin cans * Trash can * Petrol can * Metal can (disambiguation) Music * Can (band), West Germany, 1968 ** ''Can'' (album), 1979 * Can (South Korean band) Other * C ...
standard. The Physical Layer also specifies how encoding occurs over a physical signal, such as electrical voltage or a light pulse. For example, a 1 bit might be represented on a copper wire by the transition from a 0-volt to a 5-volt signal, whereas a 0 bit might be represented by the transition from a 5-volt signal to 0-volt signal. As a result, common problems occurring at the Physical Layer are often related to the incorrect media termination, EMI or noise scrambling, and NICs and hubs that are misconfigured or do not work correctly.


Layer 2: Data link layer

The data link layer provides node-to-node data transfer—a link between two directly connected nodes. It detects and possibly corrects errors that may occur in the physical layer. It defines the protocol to establish and terminate a connection between two physically connected devices. It also defines the protocol for flow control between them. IEEE 802 divides the data link layer into two sublayers: * Medium access control (MAC) layer – responsible for controlling how devices in a network gain access to a medium and permission to transmit data. * Logical link control (LLC) layer – responsible for identifying and encapsulating network layer protocols, and controls error checking and frame synchronization. The MAC and LLC layers of IEEE 802 networks such as
802.3 IEEE 802.3 is a working group and a collection standards defining the physical layer and data link layer's media access control (MAC) of wired Ethernet. The standards are produced by the working group of Institute of Electrical and Electronics Eng ...
Ethernet, 802.11 Wi-Fi, and
802.15.4 IEEE 802.15.4 is a technical standard which defines the operation of a low-rate wireless personal area network (LR-WPAN). It specifies the physical layer and media access control for LR-WPANs, and is maintained by the IEEE 802.15 working group, ...
ZigBee operate at the data link layer. The Point-to-Point Protocol (PPP) is a data link layer protocol that can operate over several different physical layers, such as synchronous and
asynchronous Asynchrony is the state of not being in synchronization. Asynchrony or asynchronous may refer to: Electronics and computing * Asynchrony (computer programming), the occurrence of events independent of the main program flow, and ways to deal with ...
serial lines. The
ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
G.hn G.hn is a specification for home networking with data rates up to 2 Gbit/s and operation over four types of legacy wires: telephone wiring, coaxial cables, power lines and plastic optical fiber. A single G.hn semiconductor device is able to net ...
standard, which provides high-speed local area networking over existing wires (power lines, phone lines and coaxial cables), includes a complete data link layer that provides both
error correction In information theory and coding theory with applications in computer science and telecommunication, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communica ...
and flow control by means of a selective-repeat sliding-window protocol. Security, specifically (authenticated) encryption, at this layer can be applied with
MACSec IEEE 802.1AE (also known as MACsec) is a network security standard that operates at the medium access control layer and defines connectionless data confidentiality and integrity for media access independent protocols. It is standardized by the I ...
.


Layer 3: Network layer

The network layer provides the functional and procedural means of transferring packets from one node to another connected in "different networks". A network is a medium to which many nodes can be connected, on which every node has an ''address'' and which permits nodes connected to it to transfer messages to other nodes connected to it by merely providing the content of a message and the address of the destination node and letting the network find the way to deliver the message to the destination node, possibly
routing Routing is the process of selecting a path for traffic in a network or between or across multiple networks. Broadly, routing is performed in many types of networks, including circuit-switched networks, such as the public switched telephone netw ...
it through intermediate nodes. If the message is too large to be transmitted from one node to another on the data link layer between those nodes, the network may implement message delivery by splitting the message into several fragments at one node, sending the fragments independently, and reassembling the fragments at another node. It may, but does not need to, report delivery errors. Message delivery at the network layer is not necessarily guaranteed to be reliable; a network layer protocol may provide reliable message delivery, but it need not do so. A number of layer-management protocols, a function defined in the ''management annex'', ISO 7498/4, belong to the network layer. These include routing protocols, multicast group management, network-layer information and error, and network-layer address assignment. It is the function of the payload that makes these belong to the network layer, not the protocol that carries them.


Layer 4: Transport layer

The transport layer provides the functional and procedural means of transferring variable-length data sequences from a source host to a destination host from one application to another across a network, while maintaining the quality-of-service functions. Transport protocols may be connection-oriented or connectionless. This may require breaking large protocol data units or long data streams into smaller chunks called "segments", since the network layer imposes a maximum packet size called the maximum transmission unit (MTU), which depends on the maximum packet size imposed by all data link layers on the network path between the two hosts. The amount of data in a data segment must be small enough to allow for a network-layer header and a transport-layer header. For example, for data being transferred across Ethernet, the MTU is 1500 bytes, the minimum size of a TCP header is 20 bytes, and the minimum size of an IPv4 header is 20 bytes, so the maximum segment size is 1500-(20+20) bytes, or 1460 bytes. The process of dividing data into segments is called segmentation; it is an optional function of the transport layer. Some connection-oriented transport protocols, such as
TCP TCP may refer to: Science and technology * Transformer coupled plasma * Tool Center Point, see Robot end effector Computing * Transmission Control Protocol, a fundamental Internet standard * Telephony control protocol, a Bluetooth communication s ...
and the OSI connection-oriented transport protocol (COTP), perform segmentation and reassembly of segments on the receiving side; connectionless transport protocols, such as UDP and the OSI connectionless transport protocol (CLTP), usually do not. The transport layer also controls the reliability of a given link between a source and destination host through flow control, error control, and acknowledgments of sequence and existence. Some protocols are state- and connection-oriented. This means that the transport layer can keep track of the segments and retransmit those that fail delivery through the acknowledgment hand-shake system. The transport layer will also provide the acknowledgement of the successful data transmission and sends the next data if no errors occurred. Reliability, however, is not a strict requirement within the transport layer. Protocols like UDP, for example, are used in applications that are willing to accept some packet loss, reordering, errors or duplication.
Streaming media Streaming media is multimedia that is delivered and consumed in a continuous manner from a source, with little or no intermediate storage in network elements. ''Streaming'' refers to the delivery method of content, rather than the content it ...
, real-time multiplayer games and voice over IP (VoIP) are examples of applications in which loss of packets is not usually a fatal problem. The OSI connection-oriented transport protocol defines five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the fewest features) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the session layer. Also, all OSI TP connection-mode
protocol Protocol may refer to: Sociology and politics * Protocol (politics), a formal agreement between nation states * Protocol (diplomacy), the etiquette of diplomacy and affairs of state * Etiquette, a code of personal behavior Science and technology ...
classes provide expedited data and preservation of record boundaries. Detailed characteristics of TP0-4 classes are shown in the following table: An easy way to visualize the transport layer is to compare it with a post office, which deals with the dispatch and classification of mail and parcels sent. A post office inspects only the outer envelope of mail to determine its delivery. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking,
tunnelling protocol In computer networks, a tunneling protocol is a communication protocol which allows for the movement of data from one network to another. It involves allowing private network communications to be sent across a public network (such as the Internet ...
s operate at the transport layer, such as carrying non-IP protocols such as IBM's
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or
Novell Novell, Inc. was an American software and services company headquartered in Provo, Utah, that existed from 1980 until 2014. Its most significant product was the multi-platform network operating system known as Novell NetWare. Under the lead ...
's
IPX Internetwork Packet Exchange (IPX) is the network layer protocol in the IPX/SPX protocol suite. IPX is derived from Xerox Network Systems' IDP. It also has the ability to act as a transport layer protocol. The IPX/SPX protocol suite was very po ...
over an IP network, or end-to-end encryption with IPsec. While
Generic Routing Encapsulation Generic Routing Encapsulation (GRE) is a tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links or point-to-multipoint links over an Internet Protocol netw ...
(GRE) might seem to be a network-layer protocol, if the encapsulation of the payload takes place only at the endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete Layer 2 frames or Layer 3 packets to deliver to the endpoint.
L2TP In computer networking, Layer 2 Tunneling Protocol (L2TP) is a tunneling protocol used to support virtual private networks (VPNs) or as part of the delivery of services by ISPs. It uses encryption ('hiding') only for its own control messages (using ...
carries PPP frames inside transport segments. Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the transport layer, the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) of the Internet Protocol Suite are commonly categorized as layer-4 protocols within OSI. Transport Layer Security (TLS) does not strictly fit inside the model either. It contains characteristics of the transport and presentation layers.


Layer 5: Session layer

The Session Layer creates the setup, controls the connections, and ends the teardown, between two or more computers, which is called a "session". Since DNS and other Name Resolution Protocols operate in this part of the layer, common functions of the Session Layer include user logon (establishment), name lookup (management), and user logoff (termination) functions. Including this matter, authentication protocols are also built into most client software, such as FTP Client and NFS Client for Microsoft Networks. Therefore, the Session layer establishes, manages and terminates the connections between the local and remote application. The Session Layer also provides for full-duplex, half-duplex, or
simplex In geometry, a simplex (plural: simplexes or simplices) is a generalization of the notion of a triangle or tetrahedron to arbitrary dimensions. The simplex is so-named because it represents the simplest possible polytope in any given dimension. ...
operation, and establishes procedures for checkpointing, suspending, restarting, and terminating a session between two related streams of data, such as an audio and a video stream in a web-conferencing application. Therefore, the session layer is commonly implemented explicitly in application environments that use remote procedure calls.


Layer 6: Presentation layer

The Presentation Layer establishes data formatting and data translation into a format specified by the application layer during the encapsulation of outgoing messages while being passed down the protocol stack, and possibly reversed during the deencapsulation of incoming messages when being passed up the protocol stack. For this very reason, outgoing messages during encapsulation are converted into a format specified by the application layer, while the conversation for incoming messages during deencapsulation are reversed. The Presentation Layer handles protocol conversion, data encryption, data decryption, data compression, data decompression, incompatibility of data representation between OSs, and graphic commands. The presentation layer transforms data into the form that the application layer accepts, to be sent across a network. Since the presentation layer converts data and graphics into a display format for the Application Layer, the Presentation Layer is sometimes called the syntax layer. For this reason, the Presentation Layer negotiates the transfer of syntax structure through the Basic Encoding Rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to an ASCII-coded file, or
serialization In computing, serialization (or serialisation) is the process of translating a data structure or object state into a format that can be stored (e.g. files in secondary storage devices, data buffers in primary storage devices) or transmitted (e ...
of
object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ai ...
s and other
data structure In computer science, a data structure is a data organization, management, and storage format that is usually chosen for efficient access to data. More precisely, a data structure is a collection of data values, the relationships among them, a ...
s from and to XML.


Layer 7: Application layer

The
application layer An application layer is an abstraction layer that specifies the shared communications protocols and Interface (computing), interface methods used by Host (network), hosts in a communications network. An ''application layer'' abstraction is speci ...
is the layer of the OSI model that is closest to the end user, which means both the OSI Application Layer and the user interact directly with software application that implements a component of communication between the client and server, such as File Explorer and Microsoft Word. Such application programs fall outside the scope of the OSI model unless they are directly integrated into the Application layer through the functions of communication, as is the case with applications such as Web Browsers and Email Programs. Other examples of software are Microsoft Network Software for File and Printer Sharing and Unix/Linux Network File System Client for access to shared file resources. Application-layer functions typically include file sharing, message handling, and database access, through the most common protocols at the application layer, known as HTTP, FTP, SMB/CIFS, TFTP, and SMTP. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. The most important distinction in the application layer is the distinction between the application-entity and the application. For example, a reservation website might have two application-entities: one using HTTP to communicate with its users, and one for a remote database protocol to record reservations. Neither of these protocols have anything to do with reservations. That logic is in the application itself. The application layer has no means to determine the availability of resources in the network.


Cross-layer functions

Cross-layer functions are services that are not tied to a given layer, but may affect more than one layer. Some orthogonal aspects, such as management and
security Security is protection from, or resilience against, potential harm (or other unwanted coercive change) caused by others, by restraining the freedom of others to act. Beneficiaries (technically referents) of security may be of persons and social ...
, involve all of the layers (See
ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
X.800 Recommendation). 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) Security service is a service, provided by a layer of communicating open systems, which ensures adequate security of the systems or of data transfers as defined by ITU-T X.800 Recommendation. X.800 and ISO 7498-2 (Information processing systems †...
as defined by
ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
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 The Common Management Information Protocol (CMIP) is the OSI specified network management protocol. Defined iITU-T Recommendation X.711, ISO/IEC International Standard 9596-1 It provides an implementation for the services defined by the Common ...
(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. 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.


Programming interfaces

Neither the OSI Reference Model, nor any
OSI protocol The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...
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 The Open Data-Link Interface (ODI), developed by Apple and Novell, serves the same function as Microsoft and 3COM's Network Driver Interface Specification (NDIS). Originally, ODI was written for NetWare and Macintosh environments. Like NDIS, ODI ...
(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. It is very important to note that this correspondence is rough: the OSI model contains idiosyncrasies not found in later systems such as the IP stack in modern Internet.


Comparison with TCP/IP model

The design of protocols in the
TCP/IP model The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suit ...
of the Internet does not concern itself with strict hierarchical encapsulation and layering. contains a section entitled "Layering considered harmful". 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. 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 An application layer is an abstraction layer that specifies the shared communications protocols and Interface (computing), interface methods used by Host (network), hosts in a communications network. An ''application layer'' abstraction is speci ...
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 The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...
that was specified as part of the OSI project was considered by many as too complicated and inefficient, and to a large extent unimplementable. Andrew S. Tanenbaum, Computer Networks, § 1.4.4. 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. 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. 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 .


See also

* Common Management Information Service (CMIS) * GOSIP, the (U.S.) Government Open Systems Interconnection Profile *
Hierarchical internetworking model The Hierarchical internetworking model is a three-layer model for network design first proposed by Cisco. It divides enterprise networks into three layers: core, distribution, and access layer. Access layer End-stations and servers connect to t ...
*
Layer 8 Layer 8 is a term used to refer to ''user'' or ''political'' layer on top of the 7-layer OSI model of computer networking. The OSI model is a 7-layer abstract model that describes an architecture of data communications for networked computers. The ...
* List of information technology initialisms *
Management plane In computer networking, the management plane of a networking device is the element of a system that configures, monitors, and provides management, monitoring and configuration services to, all layers of the network stack and other parts of the sys ...
* Recursive Internetwork Architecture *
Service layer In intelligent networks (IN) and cellular networks, service layer is a conceptual layer within a network service provider architecture. It aims at providing middleware that serves third-party value-added services and applications at a higher a ...
* Session multiplexing


Further reading

* John Day, "Patterns in Network Architecture: A Return to Fundamentals" (Prentice Hall 2007, ) *
Marshall Rose Marshall T. Rose (born 1961) is a network protocol and software engineer, author, and speaker who has contributed to the Internet Engineering Task Force (IETF), the Internet, and Internet and network applications. More specifically, he has special ...
, "The Open Book" (Prentice-Hall, Englewood Cliffs, 1990) * David M. Piscitello, A. Lyman Chapin, ''Open Systems Networking'' (Addison-Wesley, Reading, 1993) * Andrew S. Tanenbaum, Computer Networks, 4th Edition, (Prentice-Hall, 2002) * *


References


External links


Microsoft Knowledge Base: The OSI Model's Seven Layers Defined and Functions Explained

ISO/IEC standard 7498-1:1994
( PDF document inside ZIP archive) (requires HTTP cookies in order to accept licence agreement)
ITU-T X.200 (the same contents as from ISO)
* *
Cisco Systems Internetworking Technology Handbook

What is the OSI Model – 7 Layers of OSI Model Explained

Guide to Networking Essentials, 7th Edition - Cengage
{{DEFAULTSORT:Osi Model Reference models Computer-related introductions in 1977 Computer-related introductions in 1979 ISO standards ITU-T recommendations ITU-T X Series Recommendations ISO/IEC 7498