The OSI (Open Systems Interconnection) protocols are a family of information exchange standards developed jointly by the ISO and the ITU-T from 1977 onwards. The OSI model describes seven 'layers' of interconnection: the Physical Layer, the Data Link Layer, the Network Layer, the Transport Layer, the Session Layer, the Presentation Layer, and the Application Layer. These OSI protocols for those layers are described below.

OSI protocols are used to implement various networks. The OSI protocols are split into layers for easier organization and understanding. The layers form a hierarchy starting with the application level, then the presentation level, session level, transport level, network level, and the lowest level is the physical level. Each layer receives information from the layer above, processes it and passes it down to the next layer. Each layer adds its own header to the incoming information before it is passed to the level below. Headers generally include address of destination and source, check sums (for error control), type of protocol used in the current layer, and other options such as flow control options and sequence numbers (used to ensure data is sent in order).

Not all layers are mandatory, it depends on which protocols are implemented. The Internet protocol suite is not intended to be compliant with OSI, and efforts to use it as a model for OSI protocols or the OSI model will eventually fail. They have a better chance of working when additional and lesser-known ISO documents are considered, such as the Internal Organization of the Network Layer.

Nevertheless, there have been real-world examples of pure OSI protocol stacks that do not implement all the layers. The Manufacturing Automation Protocol (MAP) user group, focused on real-time control of manufacturing robots of various types, implements layer 1 (physical), a two-sublayer layer 2 (data link) with LLC Type 3 on top of the medium access layer, and then the Layer 7 Manufacturing Message System on top. Layers 3 through 6 are not present. This high-performance stack is intended just for the robots themselves; the robot controller would load files with a full 7-layer stack with FTAM file transfer on top. TCP/IP is not derived from OSI. Parts of the Signaling System 7 stack are OSI derivatives.

Physical Layer

This layer deals with the physical plugs and sockets and electrical specification of signals.

This is the medium over which the digital signals are transmitted. It can be twisted pair, coaxial cable, optical fiber, wireless, or other transmission media.

Data Link Layer

The Data Link layer packages raw bits from the Physical layer into frames (logical, structured packets for data). It is specified in ITU-T Rec. X.212 [ISO/IEC 8886], ITU-T Rec. X.222 and others. This layer is responsible for transferring frames from one host to another. It might perform error checking.

Network Layer

• Connectionless Network Service (CLNS) – ITU-T Rec. X.213 [ISO/IEC 8348] SCCP is based on X.213.
• Connectionless Network Protocol (CLNP) – ITU-T Rec. X.233 [ISO/IEC 8473-1]
• Connection-Oriented Network Service (CONS) – ITU-T Rec. X.213 [ISO/IEC 8348]
• Connection-Oriented Network Protocol (X.25) – ITU-T Rec. X.233 [ISO/IEC 8878] This is the use of the X.25 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 model.
• End System Routing Information Exchange Protocol for use with ISO/IEC 8878 (SNARE) – ITU-T Rec. X.116 [ISO/IEC 10030]

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).

Transport Layer

The connection-mode and connectionless-mode transport services are specified by ITU-T Rec. X.214 [ISO/IEC 8072]; the protocol that provides the connection-mode service is specified by ITU-T Rec. X.224 [ISO/IEC 8073], and the protocol that provides the connectionless-mode service is specified by ITU-T Rec. X.234 [ISO/IEC 8602]

• Transport Protocol Class 0 (TP0)
• Transport Protocol Class 1 (TP1)
• Transport Protocol Class 2 (TP2)
• Transport Protocol Class 3 (TP3)
• Transport Protocol Class 4 (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.

Session Layer

• Session service – ITU-T Rec. X.215 [ISO/IEC 8326]
• Connection-oriented Session protocol – ITU-T Rec. X.225 [ISO/IEC 8327-1]
• Connectionless Session protocol – ITU-T Rec. X.235 [ISO/IEC 9548-1]

Presentation Layer

• Presentation service – ITU-T Rec. X.216 [ISO/IEC 8822]
• Connection-oriented Presentation protocol – ITU-T Rec. X.226 [ISO/IEC 8823-1]
• Connectionless Presentation protocol – ITU-T Rec. X.236 [ISO/IEC 9576-1]

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.

Application Layer

Common-Application Service Elements (CASEs)

• Association Control Service Element (ACSE) – ITU-T Rec. X.217 [ISO/IEC 8649], ITU-T Rec. X.227 [ISO/IEC 8650-1], ITU-T Rec. X.237 [ISO/IEC 10035-1]
• Reliable Transfer Service Element (RTSE) – ITU-T Rec. X.218 [ISO/IEC 9066-1], ITU-T Rec. X.228 [ISO/IEC 9066-2]
• Remote Operations Service Element (ROSE) – ITU-T Rec. X.219 [ISO/IEC 9072-1], ITU-T Rec. X.229 [ISO/IEC 9072-2] 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 (CMIP) – ISO 9596 / X.700
• Directory services (DS) – X.500, later modified for the TCP/IP stack as LDAP
• File transfer, access, and management (FTAM)
• 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 (IS-IS) – ISO 10589 (reprinted in RFC 1142)
• End System to Intermediate System (ES-IS) – ISO 9542 (reprinted in RFC 996)
• Interdomain Routing Protocol (IDRP) – ISO 10747

See also

• OSI model