The Open Systems Interconnection protocols are a family of information exchange standards developed jointly by the

ISO ISO is the most common abbreviation for the International Organization for Standardization. ISO or Iso may also refer to: Business and finance * Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007 * Iso ...

and 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 Comm ...

. The standardization process began in 1977. While the seven-layer

OSI model 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 ...

is often used as a reference for teaching and documentation, the protocols originally conceived for the model did not gain popularity, and only X.400, X.500, and

IS-IS Intermediate System to Intermediate System (IS-IS, also written ISIS) is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices. It accomplishes this b ...

have achieved lasting impact. The goal of an open-standard protocol suite instead has been met by the

Internet protocol suite 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 sui ...

, maintained by 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).

Overview

The OSI

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 technolog ...

stack is structured into seven conceptual layers. The layers form a hierarchy of functionality starting with the physical hardware components to the user interfaces at the software application level. Each layer receives information from the layer above, processes it and passes it down to the next layer. Each layer adds encapsulation information ( header) to the incoming information before it is passed to the lower layer. Headers generally include address of source and destination, error control information, protocol identification and protocol parameters such as flow control options and sequence numbers.

Layer 1: physical layer

This layer deals with the physical plugs and sockets and electrical specification of signals only. This is the medium over which the digital signals are transmitted. It can be

twisted pair Twisted pair cabling is a type of wiring used for communications in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted ba ...

, coaxial cable,

optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass ( silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a mea ...

wireless Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more points without the use of an electrical conductor, optical fiber or other continuous guided medium for the transfer. The most ...

, or other transmission media.

Layer 2: data link layer

The

data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer ...

packages raw bits from the physical layer into frames (logical, structured packets for data). It is specified i
ITU-T_Rec._X.212_ ITU-T_Rec._X.212_[ISO/IEC_8886
/nowiki>">SO/IEC_8886">ITU-T_Rec._X.212_[ISO/IEC_8886
/nowiki>br>ITU-T_Rec._X.222
and_others._This_layer_is_responsible_for_transferring_frames_from_one_host_to_another.__It_might_perform_error_checking._This_layer_further_consists_of_two_sublayers:_ ITU-T_Rec._X.212_[ISO/IEC_8886
/nowiki>">SO/IEC_8886">ITU-T_Rec._X.212_[ISO/IEC_8886
/nowiki>br>ITU-T_Rec._X.222
and_others._This_layer_is_responsible_for_transferring_frames_from_one_host_to_another.__It_might_perform_error_checking._This_layer_further_consists_of_two_sublayers:_Media_access_control">MAC_and_Logical_link_control.html" ;"title="Media_access_control.html" ;"title="SO/IEC_8886
/nowiki>.html" ;"title="SO/IEC 8886">ITU-T Rec. X.212 [ISO/IEC 8886
/nowiki>">SO/IEC 8886">ITU-T Rec. X.212 [ISO/IEC 8886
/nowiki>br>ITU-T Rec. X.222
and others. This layer is responsible for transferring frames from one host to another. It might perform error checking. This layer further consists of two sublayers: Media access control">MAC and Logical link control">LLC.

Layer 3: network layer

* Connectionless Network Service (CLNS) �
ITU-T Rec. X.213 [ISO/IEC 8348
/nowiki>]. Signalling Connection Control Part, SCCP is based on X.213. * Connectionless Network Protocol (CLNP) �
ITU-T_Rec._X.233_[ISO/IEC_8473-1
/nowiki>.html" ;"title="/nowiki>ISO/IEC 8473-1">ITU-T Rec. X.233 /nowiki>ISO/IEC_8473-1">ITU-T_Rec._X.233_[ISO/IEC_8473-1
/nowiki> *_Connection-Oriented_Network_Service.html" ;"title="/nowiki>ISO/IEC 8473-1
/nowiki>">/nowiki>ISO/IEC 8473-1">ITU-T Rec. X.233 [ISO/IEC 8473-1
/nowiki> * Connection-Oriented Network Service">/nowiki>ISO/IEC 8473-1
/nowiki>">/nowiki>ISO/IEC 8473-1">ITU-T Rec. X.233 [ISO/IEC 8473-1
/nowiki> * Connection-Oriented Network Service (CONS) �
ITU-T Rec. X.213 [ISO/IEC 8348
/nowiki>]. * Connection-Oriented Network Protocol (

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 ...

) �
ITU-T Rec. X.233 /nowiki>ISO/IEC 8878
/nowiki> This is the use of the

protocol to provide the CONS. * Network Fast Byte Protocol – ISO/IEC 14700 * End System to Intermediate System Routing Exchange Protocol (ES-IS) - ISO/IEC 9452 (reprinted in RFC 995). * Intermediate System to Intermediate System Intra-domain Routing Protocol (IS-IS) - ISO/IEC 10589 (reprinted in RFC 1142), later adapted for the

TCP/IP 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 ...

model. * End System Routing Information Exchange Protocol for use with ISO/IEC 8878 (SNARE) �
ITU-T_Rec._X.116_[ISO/IEC_10030
/nowiki>.html" ;"title="/nowiki>ISO/IEC 10030">ITU-T Rec. X.116 [ISO/IEC 10030
/nowiki>">/nowiki>ISO/IEC 10030">ITU-T Rec. X.116 [ISO/IEC 10030
/nowiki> This level is in charge of transferring data between systems in a network, using network-layer addresses of machines to keep track of destinations and sources. This layer uses routers and switches to manage its traffic (control flow control, error check, routing etc.) So here it takes all routing decisions, it deals with end to end data transmission.

Layer 4: transport layer

The connection-mode and connectionless-mode transport services are specified b
ITU-T Rec. X.214 [ISO/IEC 8072
/nowiki>]; the protocol that provides the connection-mode service is specified b
ITU-T Rec. X.224 /nowiki>ISO/IEC 8073
/nowiki> and the protocol that provides the connectionless-mode service is specified b
ITU-T_Rec._X.234_[ISO/IEC_8602
/nowiki>.html" ;"title="/nowiki>ISO/IEC 8602">ITU-T Rec. X.234 /nowiki>ISO/IEC_8602">ITU-T_Rec._X.234_[ISO/IEC_8602
/nowiki> *_Transport_Protocol_Class_0_( /nowiki>ISO/IEC_8602">ITU-T_Rec._X.234_[ISO/IEC_8602
/nowiki> *_Transport_Protocol_Class_0_(TP0_(protocol)">TP0) *_Transport_Protocol_Class_1_( /nowiki>ISO/IEC_8602">ITU-T_Rec._X.234_[ISO/IEC_8602
/nowiki> *_Transport_Protocol_Class_0_(TP0_(protocol)">TP0) *_Transport_Protocol_Class_1_(TP1_(protocol)">TP1) *_Transport_Protocol_Class_2_( /nowiki>ISO/IEC_8602">ITU-T_Rec._X.234_[ISO/IEC_8602
/nowiki> *_Transport_Protocol_Class_0_(TP0_(protocol)">TP0) *_Transport_Protocol_Class_1_(TP1_(protocol)">TP1) *_Transport_Protocol_Class_2_(TP2_(protocol)">TP2) *_Transport_Protocol_Class_3_( /nowiki>ISO/IEC_8602">ITU-T_Rec._X.234_[ISO/IEC_8602
/nowiki> *_Transport_Protocol_Class_0_(TP0_(protocol)">TP0) *_Transport_Protocol_Class_1_(TP1_(protocol)">TP1) *_Transport_Protocol_Class_2_(TP2_(protocol)">TP2) *_Transport_Protocol_Class_3_(TP3_(protocol)">TP3) *_Transport_Protocol_Class_4_(TP4_(protocol).html" ;"title="TP3_(protocol).html" ;"title="TP2_(protocol).html" ;"title="TP1_(protocol).html" ;"title="TP0_(protocol).html" ;"title="/nowiki>ISO/IEC 8602
/nowiki>">/nowiki>ISO/IEC 8602">ITU-T Rec. X.234 [ISO/IEC 8602
/nowiki> * Transport Protocol Class 0 (TP0 (protocol)">TP0) * Transport Protocol Class 1 (TP1 (protocol)">TP1) * Transport Protocol Class 2 (TP2 (protocol)">TP2) * Transport Protocol Class 3 (TP3 (protocol)">TP3) * Transport Protocol Class 4 (TP4 (protocol)">TP4) * Transport Fast Byte Protocol – ISO 14699 The transport layer transfers data between source and destination processes. Generally, two connection modes are recognized, connection-oriented or connectionless. Connection-oriented service establishes a dedicated virtual circuit and offers various grades of guaranteed delivery, ensuring that data received is identical to data transmitted. Connectionless mode provides only best-effort service without the built-in ability to correct errors, which includes complete loss of data without notifying the data source of the failure. No logical connection, and no persistent state of the transaction exists between the endpoints, lending the connectionless mode low overhead and potentially better real-time performance for timing-critical applications such as voice and video transmissions.

Layer 5: session layer

* Session service �
ITU-T Rec. X.215 [ISO/IEC 8326
/nowiki>] * Connection-oriented Session protocol �
ITU-T Rec. X.225 SO/IEC 8327-1
/nowiki>* Connectionless Session protocol �
ITU-T_Rec._X.235_ ITU-T_Rec._X.235_[ISO/IEC_9548-1
/nowiki>">SO/IEC_9548-1">ITU-T_Rec._X.235_[ISO/IEC_9548-1
/nowiki>The_session_layer_controls_the_dialogues_(connections)_between_computers._It_establishes,_manages_and_terminates_the_connections_between_the_local_and_remote_application._It_provides_for_full-duplex,_and_half-duplex_or_Simplex_circuit.html" ;"title="SO/IEC_9548-1
/nowiki>.html" ;"title="SO/IEC 9548-1">ITU-T Rec. X.235 [ISO/IEC 9548-1
/nowiki>">SO/IEC 9548-1">ITU-T Rec. X.235 [ISO/IEC 9548-1
/nowiki>The session layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, and half-duplex or Simplex circuit">simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The session layer is commonly implemented explicitly in application environments that use remote procedure calls.

Layer 6: presentation layer

* Presentation service �
ITU-T Rec. X.216 [ISO/IEC 8822
/nowiki>] * Connection-oriented Presentation protocol �
ITU-T Rec. X.226 SO/IEC 8823-1
/nowiki>* Connectionless Presentation protocol �
ITU-T_Rec._X.236_[ISO/IEC_9576-1
/nowiki>.html" ;"title="SO/IEC 9576-1">ITU-T Rec. X.236 [ISO/IEC 9576-1
/nowiki>">SO/IEC 9576-1">ITU-T Rec. X.236 [ISO/IEC 9576-1
/nowiki> This layer defines and encrypts/decrypts data types from the application layer. Protocols such as MIDI, MPEG, and GIF are presentation layer formats shared by different applications.

Layer 7: application layer

Common-Application Service Elements (CASEs)

* Association Control Service Element (ACSE) �
ITU-T Rec. X.217 [ISO/IEC 8649
/nowiki>]
ITU-T_Rec._X.227_[ISO/IEC_8650-1
/nowiki>.html" ;"title="SO/IEC 8650-1">ITU-T Rec. X.227 [ISO/IEC 8650-1
/nowiki>">SO/IEC 8650-1">ITU-T Rec. X.227 [ISO/IEC 8650-1
/nowiki>br>ITU-T Rec. X.237 [ISO/IEC 10035-1
/nowiki>]. * Reliable Transfer Service Element (RTSE) �
ITU-T Rec. X.218 SO/IEC 9066-1
/nowiki>br>ITU-T_Rec._X.228_ ITU-T_Rec._X.228_[ISO/IEC_9066-2
/nowiki>">SO/IEC_9066-2">ITU-T_Rec._X.228_[ISO/IEC_9066-2
/nowiki> *_Remote_Operations_Service_Element_protocol.html" ;"title="SO/IEC_9066-2
/nowiki>.html" ;"title="SO/IEC 9066-2">ITU-T Rec. X.228 [ISO/IEC 9066-2
/nowiki>">SO/IEC 9066-2">ITU-T Rec. X.228 [ISO/IEC 9066-2
/nowiki> * Remote Operations Service Element protocol">Remote Operations Service Element (ROSE) �
ITU-T Rec. X.219 [ISO/IEC 9072-1
/nowiki>]
ITU-T Rec. X.229 [ISO/IEC 9072-2
/nowiki>]. [ TCAP is related to X.219. * Commitment, Concurrency, and Recovery service element (CCRSE) * Security Exchange Service Element (SESE) This keeps track of how each application talks to another application. Destination and source addresses are linked to specific applications.

Application processes

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) – ISO 9596 / X.700 * Directory services (DS) – X.500, later modified for the TCP/IP stack as

LDAP The Lightweight Directory Access Protocol (LDAP ) is an open, vendor-neutral, industry standard application protocol for accessing and maintaining distributed directory information services over an Internet Protocol (IP) network. Directory servi ...

* File transfer, access, and management (FTAM)FTAM support claimed by IBM, Novell, Sun, Unisys etc
,

Joint Interoperability Test Command The Joint Interoperability Test Command (JITC) is a wing of the United States Department of Defense that tests and certifies information technology products for military use. History The JITC had its roots in the TRI-TAC program of the 1970s, wh ...

* Message handling system (MHS) – X.400 * Virtual terminal protocol (VT) - ISO 9040/9041 * Remote Database Access (RDA) * Distributed Transaction Processing ( OSI TP) * Interlibrary Loan Application Protocol (ILAP) * Document Transfer And Manipulation (DTAM) * Document Printing Application (DPA) * Document Filing and Retrieval (DFR)

Routing protocols

Intermediate System to Intermediate System Intermediate System to Intermediate System (IS-IS, also written ISIS) is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices. It accomplishes this b ...

(IS-IS) – ISO 10589 (reprinted in RFC 1142) * End System to Intermediate System (ES-IS) – ISO 9542 (reprinted in RFC 995) *

Interdomain Routing Protocol Inter-domain routing is data flow control and interaction between Primary Domain Controller (PDC) computers. This type of computer uses various computer protocols and services to operate. It is most commonly used to multicast between internet dom ...

(IDRP) – ISO 10747

References

{{DEFAULTSORT:Osi Protocols Network architecture Reference models