Editing OSI model (section)

==Layer architecture==
The recommendation X.200 describes seven layers, labelled 1 to 7. Layer 1 is the lowest layer in this model.
{{OSI model table}}

===Layer 1: Physical layer===
{{main|Physical layer}}
The physical layer 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 (analogue 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 [[Lead (electronics)|pins]], [[voltage]]s, line [[Characteristic impedance|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 communication|simplex]], [[half duplex]], and [[full duplex]]. The components of a physical layer can be described in terms of the [[network topology]]. Physical layer specifications are included in the specifications for the ubiquitous [[Bluetooth]], [[Ethernet physical layer|Ethernet]], and [[USB]] standards. An example of a less well-known physical layer specification would be for the [[CAN bus|CAN]] 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 to a 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===
{{main|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 (data)|flow control]] between them.

[[IEEE 802]] divides the data link layer into two sublayers:<ref>{{cite book|title=IEEE Std 802-2014, ''IEEE Standard for Local and Metropolitan Area Networks: Overview and Architecture''|publisher=ieee|section=5.2 RM description for end stations|doi=10.1109/IEEESTD.2014.6847097|isbn=978-0-7381-9219-2}}</ref>
* [[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]] [[Ethernet]], [[802.11]] [[Wi-Fi]], and [[802.15.4]] [[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 serial communication|synchronous]] and [[asynchronous serial communication|asynchronous]] serial lines.

The [[ITU-T]] [[G.hn]] 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]] and flow control by means of a [[selective repeat|selective-repeat]] [[sliding window protocol|sliding-window protocol]].

Security, specifically (authenticated) encryption, at this layer can be applied with [[MACsec]].

===Layer 3: Network layer===
{{main|Network layer}}
The network layer provides the functional and procedural means of transferring [[network packet|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]] 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 does not need to 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.<ref>{{cite ISO standard|csnumber = 14258|title = ISO/IEC 7498-4:1989 – Information technology – Open Systems Interconnection – Basic Reference Model: Management framework|date = 15 November 1989|access-date = 16 November 2024|website = ISO Standards Maintenance Portal|publisher = ISO Central Secretariat.|author = International Organization for Standardization|author-link = International Organization for Standardization}}</ref>

===Layer 4: Transport layer===
{{main|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 [[packet segmentation|segmentation]]; it is an optional function of the transport layer. Some connection-oriented transport protocols, such as [[Transmission Control Protocol|TCP]] and the OSI connection-oriented transport protocol (COTP), perform segmentation and reassembly of segments on the receiving side; connectionless transport protocols, such as [[User Datagram Protocol|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]], 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 [[Communication protocol|protocol]] classes provide expedited data and preservation of record boundaries. Detailed characteristics of TP0–4 classes are shown in the following table:<ref>{{cite web |title=ITU-T Recommendation X.224 (11/1995) ISO/IEC 8073, ''Open Systems Interconnection – Protocol for providing the connection-mode transport service'' |publisher=ITU|url=http://www.itu.int/rec/T-REC-X.224-199511-I/en/}}</ref>

{| class="wikitable" style="margin:0 auto;"
! Feature name
!   TP0   !!   TP1   !!   TP2   !!   TP3   !!   TP4
|-
| Connection-oriented network
| {{Yes}} || {{Yes}} || {{Yes}} || {{Yes}} || {{Yes}}
|-
| Connectionless network
| {{No}}  || {{No}}  || {{No}}  || {{No}}  || {{Yes}}
|-
| Concatenation and separation
| {{No}}  || {{Yes}} || {{Yes}} || {{Yes}} || {{Yes}}
|-
| Segmentation and reassembly
| {{Yes}} || {{Yes}} || {{Yes}} || {{Yes}} || {{Yes}}
|-
| Error recovery
| {{No}}  || {{Yes}} || {{Yes}} || {{Yes}} || {{Yes}}
|-
| Reinitiate connection{{smallsup|a}}
| {{No}}  || {{Yes}} || {{No}}  || {{Yes}} || {{No}}
|-
|style="padding-right:1.0em;"| Multiplexing{{\}}demultiplexing over single [[virtual circuit]]
| {{No}}  || {{No}}  || {{Yes}} || {{Yes}} || {{Yes}}
|-
| Explicit flow control
| {{No}}  || {{No}}  || {{Yes}} || {{Yes}} || {{Yes}}
|-
| Retransmission on timeout
| {{No}}  || {{No}}  || {{No}}  || {{No}}  || {{Yes}}
|-
| Reliable transport service
| {{No}}  || {{Yes}} || {{No}}  || {{Yes}} || {{Yes}}
|-
|colspan="6" style="font-size:90%;"| {{sup|a}} If an excessive number of [[Protocol data unit|PDUs]] are unacknowledged.
|}

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]]s operate at the transport layer, such as carrying non-IP protocols such as [[IBM]]'s [[Systems Network Architecture|SNA]] or [[Novell]]'s [[IPX]] over an IP network, or end-to-end encryption with [[IPsec]]. While [[Generic Routing Encapsulation]] (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]] carries [[Point-to-Point Protocol|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.<ref name="ccnp">{{cite book
 |last1       = Hooper
 |first1      = Howard
 |title       = CCNP Security VPN 642-648 Official Cert Guide
 |date        = 2012
 |publisher   = Cisco Press
 |isbn        = 9780132966382
 |page        = 22
 |edition     = 2
 |url         = https://books.google.com/books?id=5PJisOKJ0k8C&pg=PA22
 }}</ref><ref name="stackex_layer">{{cite web|url=https://security.stackexchange.com/a/93338|title=What layer is TLS?|website=Information Security Stack Exchange|first1=Andrew|last1=Spott|first2=Tom |last2=Leek|display-authors=etal}}</ref>

===Layer 5: Session layer===
{{main|Session layer}}
The session layer creates the setup, controls the connections, and ends the [[teardown (communications)|teardown]], between two or more computers, which is called a "session". Common functions of the session layer include user logon (establishment) and user logoff (termination) functions. Including this matter, authentication methods 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 applications. The session layer also provides for [[full-duplex]], [[half-duplex]], or [[Simplex communication|simplex]] operation,{{citation needed|date=April 2024}} 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 call]]s.

===Layer 6: Presentation layer===
{{main|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 conversion 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 operating systems, 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.<ref name="Grigonis 2000">{{cite encyclopedia |last=Grigonis |first=Richard |title=Open Systems Interconnection (OSI) Model |encyclopedia=Computer Telephony Encyclopedia |publisher=CMP Books |publication-place=New York |year=2000 |isbn=978-1-929629-51-0 |oclc=48138823 |page=[https://archive.org/details/computertelephon0000grig/page/330/mode/2up 331]}}</ref> 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]] of [[object (computer science)|object]]s and other [[data structure]]s from and to [[XML]].<ref name="guide-to-networking-essentials" />

===Layer 7: Application layer===
{{main|Application layer}}
The application layer 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 a 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 browser]]s and [[email program]]s. 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.<ref name="guide-to-networking-essentials" />