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{{OSIModel}}
The ''''''Open Systems Interconnection Basic Reference Model'''''' (OSI Reference Model or '''OSI Model''') is an abstract description for layered communications and computer [[network protocol]] design. It was developed as part of the [[Open Systems Interconnection]] (OSI) initiative<ref>http://www.itu.int/rec/T-REC-X.200-199407-I/en</ref>. In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the '''''OSI Seven Layer Model'''''. (Note: The data security industry sometimes refers to a hypothetical "Layer 8", or Human Interface Layer. This is considered the point at which the user interacts with the application. This is not a part of the official OSI standard, but is frequently referenced to exemplify the weakness of the human component in the overall process.)

A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path.

Even though it has been largely superseded by newer [[IETF]], [[IEEE]], and indeed OSI protocol developments (subsequent to the publication of the original architectural standards), the basic OSI model is considered an excellent place to begin the study of network architecture. Not understanding that the pure seven-layer model is more historic than current, many beginners make the mistake of trying to fit every protocol under study into one of the seven basic layers. Especially the attempts of [[cross-layer optimization]] break the boundaries of the original layer scheme. Describing the actual layer concept with implemented systems is not always easy to do as most of the protocols in use on the [[Internet]] were designed as part of the [[TCP/IP model]], and may not fit cleanly into the OSI Model.

== History ==
In 1977, work on a layered model of network architecture was started, and the [[International Organization for Standardization]] (ISO) began to develop its OSI framework architecture (The ISO is a worldwide federation of national standards bodies from some 130 countries, one from each country).{{Fact|date=June 2008}}
OSI has two major components: 1) an abstract model of networking, called the Basic Reference Model or seven-layer model, and 2) a set of specific protocols.

Note: The standard documents that describe the OSI model can be freely downloaded from the ITU-T as the '''X.200'''-series of recommendations <ref>[http://www.itu.int/rec/T-REC-X/en ITU-T X-Series Recommendations].</ref>. A number of the protocol specifications are also available as part of the ITU-T X series. The equivalent ISO and ISO/IEC standards for the OSI model are available from the ISO, but only some of the ISO/IEC standards are available as cost-free downloads.<ref>[http://standards.iso.org/ittf/PubliclyAvailableStandards/index.html Publicly Available Standards<!-- Bot generated title -->]</ref>

all aspects of OSI design evolved from experiences with the [[CYCLADES]] network, which also influenced Internet design. The new design was documented in ISO 7498 and its various addenda. In this model, a networking system is divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacts directly only with the layer immediately beneath it, and provides facilities for use by the layer above it.

Protocols enable an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions 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.

== Description of OSI layers ==
{|class="wikitable" align="right"
! colspan="4" | OSI Model
|-
!
!Data unit
!Layer
!style="width:9em;"|Function
|-
!rowspan="4"|Host<br />layers
|rowspan="3"|Data
|7. [[Application Layer|Application]]
|<small>Network process to application</small>
|-
|6. [[Presentation Layer|Presentation]]
|<small>Data representation and encryption</small>
|-
|5. [[Session Layer|Session]]
|<small>Interhost communication</small>
|-
|Segment/Datagram
|4. [[Transport Layer|Transport]]
|<small>End-to-end connections and reliability</small>
|-
!rowspan="3"|Media<br />layers
|Packet
|3. [[Network Layer|Network]]
|<small>Path determination and [[logical address]]ing</small>
|-
|Frame
|2. [[Data Link Layer|Data Link]]
|<small>Physical addressing (MAC & LLC)</small>
|-
|Bit
|1. [[Physical Layer|Physical]]
|<small>Media, signal and binary transmission</small>
|}

=== Layer 7: Application Layer ===
{{main | Application Layer}}
The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application layer implementations include [[Telnet]], File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).

=== Layer 6: Presentation Layer ===
{{main | Presentation Layer}}
The [[Presentation Layer]] establishes a context between Application Layer entities, in which the higher-layer entities can use different syntax and semantics, as long as the Presentation Service understands both and the mapping between them. The presentation service data units are then encapsulated into Session Protocol Data Units, and moved down the stack.

The original presentation structure used the Basic Encoding Rules of [[Abstract Syntax Notation One]] (ASN.1), with capabilities such as converting an [[EBCDIC]]-coded text [[Computer file|file]] to an [[ASCII]]-coded file, or [[Serialization|serializing]] [[object (computer science)|objects]] and other [[data structure]]s into and out of [[XML]]. ASN.1 has a set of cryptographic encoding rules that allows end-to-end encryption between application entities.

===Layer 5: Session Layer===
{{main | Session Layer}}
The [[Session Layer]] controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for [[Duplex (telecommunications)|full-duplex]], [[half-duplex]], or [[Simplex communication|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 [[Transmission Control Protocol|TCP]], 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 call|remote procedure calls (RPCs)]].

=== Layer 4: Transport Layer ===
{{main | Transport Layer}}
The [[Transport Layer]] provides transparent transfer of [[data]] between end users, providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the Transport Layer can keep track of the segments and retransmit those that fail.

Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer, the best known examples of a Layer 4 protocol are the [[Transmission Control Protocol]] (TCP) and [[User Datagram Protocol]] (UDP).

Of the actual OSI protocols, there are five classes of transport protocols ranging from class 0 (which is also known as '''TP0''' and provides the least error recovery) to class 4 (which is also known as '''TP4''' and is designed for less reliable networks, similar to the Internet). Class 0 is closest to UDP. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the Session Layer. Detailed characteristics of TP0-4 classes are shown in the following table:<ref>{{cite web
| title = ITU-T Recommendation X.224 (11/1993) [ISO/IEC 8073]
| url = http://www.itu.int/rec/T-REC-X.224-199311-I/en/
| accessdate = 2008-06-18}}</ref>

{| class="wikitable"
|-
! Feature Name
! TP0
! TP1
! TP2
! TP3
! TP4
|-
| Connection oriented
| -
| -
| -
| {{Yes}}
| {{Yes}}
|-
| Connectionless
| -
| -
| -
| {{No}}
| {{Yes}}
|-
| Segmentation/Fragmentation
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
|-
| Reassembly
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
|-
| Error Recovery
| {{No}}
| {{Yes}}
| {{No}}
| {{No}}
| {{Yes}}
|-
| Reinitiate connection (if an excessive number of [[Protocol_data_unit|PDUs]] are unacknowledged)
| {{No}}
| {{Yes}}
| {{No}}
| {{Yes}}
| {{Yes}}
|-
| multiplexing and demultiplexing over a single [[virtual circuit]]
| {{No}}
| {{No}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
|-
| Reliable Transport Service
| {{No}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
|}

Perhaps 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. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking, [[Tunneling protocol|tunneling protocols]] operate at the Transport Layer, such as carrying non-IP protocols such as [[IBM]]'s [[IBM Systems Network Architecture|SNA]] or [[Novell]]'s [[Internetwork Packet Exchange|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 endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint. [[Layer 2 Tunneling Protocol|L2TP]] carries [[Point-to-Point Protocol|PPP]] frames inside transport packet.

=== Layer 3: Network Layer ===
{{main | Network Layer}}
The [[Network Layer]] provides the functional and procedural means of transferring variable length [[data]] sequences from a source to a destination via one or more networks, while maintaining the [[quality of service]] requested by the Transport Layer. The Network Layer performs network [[routing]] functions, and might also perform fragmentation and reassembly, and report delivery errors. [[Router]]s operate at this layer—sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is hierarchical.

<!-- STRANGE THAT A PROTOCOL THAT DOESN'T ACUTALLY BELONG TO THE OSI MODEL IS ITS BEST KNOWN EXAMPLE, WHY NOT DISCUSS OSI ON ITS OWN MERITS? -->The best-known example of a Layer 3 protocol is the [[Internet Protocol]] (IP). It manages the [[Connectionless protocol | connectionless]] transfer of data one hop at a time, from end system to ingress router, router to router, and from egress router to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of errored packets so they may be discarded. When the medium of the next hop cannot accept a packet in its current length, IP is responsible for '''fragmenting''' into sufficiently small packets that the medium can accept it.

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 2: Data Link Layer ===
{{main | Data Link Layer}}
The [[Data Link Layer]] provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multiaccess media, was developed independently of the ISO work, in [[IEEE 802 |IEEE Project 802]]. IEEE work assumed sublayering and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in modern data link protocols such as [[Point-to-Point Protocol]] (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on Ethernet, and, on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the Transport Layer by protocols such as [[Transmission Control Protocol|TCP]], but is still used in niches where [[X.25]] offers performance advantages.

Both WAN and LAN services arrange bits, from the Physical Layer, into logical sequences called frames. Not all Physical Layer bits necessarily go into frames, as some of these bits are purely intended for Physical Layer functions. For example, every fifth bit of the [[FDDI]] bit stream is not used by the Data Link Layer.

==== WAN Protocol architecture ====
[[Connection-oriented]] WAN data link protocols, in addition to framing, detect and may correct errors. They also are capable of controlling the rate of transmission. A WAN Data Link Layer might implement a [[sliding window]] flow control and acknowledgment mechanism to provide reliable delivery of frames; that is the case for [[Synchronous Data Link Control|SDLC]] and [[HDLC]], and derivatives of HDLC such as [[LAPB]] and [[Link Access Procedures, D channel|LAPD]].

==== IEEE 802 LAN architecture ====
Practical, [[connectionless]] LANs began with the pre-IEEE [[Ethernet]] specification, which is the ancestor of [[IEEE 802.3]]. This layer manages the interaction of devices with a shared medium, which is the function of a [[Media Access Control]] sublayer. Above this MAC sublayer is the media-independent [[IEEE 802.2]] [[Logical Link Control]] (LLC) sublayer, which deals with addressing and multiplexing on multiaccess media.

While IEEE 802.3 is the dominant wired LAN protocol and [[IEEE 802.11]] the wireless LAN protocol, obsolescent MAC layers include [[Token Ring]] and [[FDDI]]. The MAC sublayer detects but does not correct errors.

=== Layer 1: Physical Layer ===
{{main | Physical Layer}}
The [[Physical Layer]] defines the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of [[Lead (electronics)|pins]], [[voltage]]s, [[cable]] [[specification]]s, [[Network hub|Hub]]s, [[repeater]]s, [[network card|network adapter]]s, [[Host adapter|Host Bus Adapter]]s (HBAs used in [[Storage area network|Storage Area Network]]s) and more.

To understand the function of the Physical Layer in contrast to the functions of the Data Link Layer, think of the Physical Layer as concerned primarily with the interaction of a single device with a medium, where the Data Link Layer is concerned more with the interactions of multiple devices (i.e., at least two) with a shared medium. The Physical Layer will tell one device how to transmit to the medium, and another device how to receive from it (in most cases it does not tell the device how to connect to the medium). Obsolescent Physical Layer standards such as [[RS-232]] do use physical wires to control access to the medium.

The major functions and services performed by the Physical Layer are:

*Establishment and termination of a [[electrical connector|connection]] to a [[communication]]s [[Transmission medium|medium]].
*Participation in the process whereby the communication resources are effectively shared among multiple users. For example, [[Computer networking|contention]] resolution and [[flow control]].
*[[Modulation]], or conversion between the representation of [[digital data]] in user equipment and the corresponding signals transmitted over a communications [[channel (communications)|channel]]. These are signals operating over the physical cabling (such as copper and [[optical fiber]]) or over a [[electromagnetic wave#Radio waves|radio link]].

[[Parallel SCSI]] buses operate in this layer, although it must be remembered that the logical [[SCSI]] protocol is a Transport Layer protocol that runs over this bus. Various Physical Layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the Data Link Layer. The same applies to other local-area networks, such as [[IBM token ring|Token ring]], [[Fiber distributed data interface|FDDI]], and [[IEEE 802.11]], as well as personal area networks such as [[Bluetooth]] and [[IEEE 802.15#Task group 4 (Low Rate WPAN)|IEEE 802.15.4]].

== Interfaces ==
Neither the OSI Reference Model nor OSI protocols specify any programming interfaces, other than as deliberately abstract service specifications. Protocol specifications precisely define the interfaces between different computers, but the software interfaces inside computers are implementation-specific.

For example, [[Microsoft Windows]]' [[Winsock]], and [[Unix]]'s [[Berkeley sockets]] and [[UNIX System V|System V]] [[Transport Layer Interface]], are interfaces between applications (Layer 5 and above) and the transport (Layer 4). [[Network Driver Interface Specification|NDIS]] and [[Open Data-Link Interface|ODI]] are interfaces between the media (Layer 2) and the network protocol (Layer 3).

Interface standards, except for the Physical Layer to media, are approximate implementations of OSI Service Specifications.

== Examples ==
{| class="wikitable"
|-
!colspan="2"| Layer
!rowspan="2"| Misc. examples
!rowspan="2"| [[Internet protocol suite|TCP/IP]] suite
!rowspan="2"| [[Signaling System 7|SS7]]<ref>[http://www.itu.int/rec/T-REC-Q.1400/en/ ITU-T Recommendation Q.1400 (03/1993)], ''Architecture framework for the development of signalling and OA&M protocols using OSI concepts'', pp 4, 7.</ref>
!rowspan="2"| [[AppleTalk]] suite
!rowspan="2"| [[Open Systems Interconnection|OSI]] suite
!rowspan="2"| [[IPX]] suite
!rowspan="2"| [[Systems Network Architecture|SNA]]
!rowspan="2"| [[UMTS]]
|-
! #
! Name
|-
| 7
| Application
| [[HL7]], [[Modbus]]
| [[Network News Transfer Protocol|NNTP]], [[Session Initiation Protocol|SIP]], [[Simple Sensor Interface protocol|SSI]], [[Domain Name Service|DNS]], [[File transfer protocol|FTP]], [[Gopher (protocol)|Gopher]], [[HTTP]], [[Network File System (protocol)|NFS]], [[Network Time Protocol|NTP]], [[Dynamic Host Configuration Protocol|DHCP]], [[Short Message Peer-to-Peer Protocol|SMPP]], [[Simple Mail Transfer Protocol|SMTP]], [[Simple network management protocol|SNMP]], [[Telnet]]
| [[INAP]], [[Mobile Application Part|MAP]], [[Transaction Capabilities Application Part|TCAP]], [[ISDN User Part|ISUP]], [[Telephone User Part|TUP]]
| [[Apple Filing Protocol|AFP]], [[Zone Information Protocol|ZIP]], [[Routing Table Maintenance Protocol|RTMP]], [[AppleTalk|NBP]]
| [[FTAM]], [[X.400]], [[X.500]], [[Directory Access Protocol|DAP]], [[ROSE]], [[RTSE]], [[ACSE]]
| RIP, SAP
| [[APPC]]
|
|-
| 6
| Presentation
| [[Tabbed Document Interface|TDI]], [[ASCII]], [[EBCDIC]], [[Musical Instrument Digital Interface|MIDI]], [[MPEG]]
| [[MIME]], [[External Data Representation|XDR]], [[Secure Sockets Layer|SSL]], [[Transport Layer Security|TLS]] (Not a separate layer)
|
| [[Apple Filing Protocol|AFP]]
| ISO/IEC&nbsp;8823, X.226, ISO/IEC&nbsp;9576-1, X.236
|
|
|
|-
| 5
| Session
| [[Named Pipes]], [[NetBIOS]], [[Session Announcement Protocol|SAP]], [[Half Duplex]], [[Full Duplex]], [[Simplex]], [[Session Description Protocol|SDP]]
| Sockets. Session establishment in [[Transmission Control Protocol|TCP]]. [[Session Initiation Protocol|SIP]]. (Not a separate layer with standardized API.)
|
| [[AppleTalk|ASP]], [[AppleTalk|ADSP]], [[Printer Access Protocol|PAP]]
| ISO/IEC&nbsp;8327, X.225, ISO/IEC&nbsp;9548-1, X.235
| [[NWLink]]
| [[Data Link Control|DLC]]?
|
|-
| 4
| Transport
| [[NetBIOS Frames protocol|NBF]], [[nanoTCP]], [[nanoUDP]]
| [[Transmission Control Protocol|TCP]], [[User Datagram Protocol|UDP]], [[PPTP]], [[L2TP]], [[Stream Control Transmission Protocol|SCTP]], [[Real-time Transport Protocol|RTP]]
|
| [[Datagram Delivery Protocol|DDP]]
| ISO/IEC&nbsp;8073, TP0, TP1, TP2, TP3, TP4 (X.224), ISO/IEC&nbsp;8602, X.234
| [[Sequenced packet exchange|SPX]]
|
|
|-
| 3
| Network
| [[NetBIOS Frames protocol|NBF]], [[Q.931]]
| [[Internet Protocol|IP]], [[IPsec]], [[Address resolution protocol|ARP]], [[Internet Control Message Protocol|ICMP]], [[Routing Information Protocol|RIP]], [[Open Shortest Path First|OSPF]], [[Border Gateway Protocol | BGP]], [[Internet Group Management Protocol | IGMP]], [[IS-IS]]
| [[Signalling Connection Control Part|SCCP]], [[Message Transfer Part|MTP]]
| [[AppleTalk|ATP]] ([[TokenTalk]] or [[EtherTalk]])
| ISO/IEC&nbsp;8208, [[X.25]] ([[Packet-Layer Protocol|PLP]]), ISO/IEC&nbsp;8878, [[CONS|X.223]], ISO/IEC&nbsp;8473-1, [[CLNS|CLNP]] X.233.
| [[IPX]]
|
| [[Radio Resource Control|RRC]] ([[Radio Resource Control]]) [[PDCP|Packet Data Convergence Protocol]] ([[PDCP]]) and [[Broadcast/Multicast Control|BMC]] ([[Broadcast/Multicast Control]])
|-
| 2
| Data Link
| [[Ethernet|802.3 (Ethernet)]], [[IEEE 802.11|802.11a/b/g/n MAC/LLC]], [[IEEE 802.1Q|802.1Q (VLAN)]], [[Asynchronous Transfer Mode|ATM]], [[Hopling Discovery Protocol|HDP]], [[FDDI]], [[Fibre Channel]], [[Frame Relay]], [[HDLC]], [[Cisco Inter-Switch Link|ISL]], [[Point-to-Point Protocol|PPP]], [[Q.921]], [[Token Ring]], [[Cisco Discovery Protocol|CDP]]
| [[Point-to-Point Protocol|PPP]], [[SLIP]]
| [[Message Transfer Part|MTP]], [[Message Transfer Part|Q.710]]
| [[LocalTalk]], [[AppleTalk Remote Access]], [[Point-to-Point Protocol|PPP]]
| ISO/IEC&nbsp;7666, [[X.25]] ([[LAPB]]), [[Token Bus]], X.222, ISO/IEC&nbsp;8802-2 [[IEEE 802.2|LLC]] Type 1 and 2
| [[IEEE 802.3]] framing, [[Ethernet II framing]]
| [[Synchronous Data Link Control|SDLC]]
| [[Logical Link Control|LLC]] ([[Logical Link Control]]), [[Media Access Control|MAC]] ([[Media Access Control]])
|-
| 1
| Physical
| [[RS-232]], [[V.35]], [[V.34]], [[I.430]], [[I.431]], [[T-carrier|T1]], [[E-carrier|E1]], [[10BASE-T]], [[100BASE-TX]], [[Plain old telephone service|POTS]], [[Synchronous optical networking|SONET]], [[Synchronous optical networking|SDH]], [[Digital Subscriber Line|DSL]], [[IEEE 802.11|802.11a/b/g/n PHY]]
|
| [[Message Transfer Part|MTP]], [[Message Transfer Part|Q.710]]
| [[RS-232]], [[RS-422]], [[Twisted pair#Shielded Twisted Pair (STP)|STP]], [[PhoneNet]]
| [[X.25]] ([[X.21bis]], [[EIA/TIA-232]], [[EIA/TIA-449]], [[EIA-530]], [[G.703]])
|
| [[Twinaxial cabling|Twinax]]
| [[UMTS Physical Layer|UMTS L1]] ([[UMTS Physical Layer]])
|}

==Comparison with TCP/IP==
In the [[TCP/IP model]] of the Internet, protocols are deliberately not as rigidly designed into strict layers as the OSI model.<ref>RFC 3439</ref> RFC 3439 contains a section entitled "Layering [[considered harmful]]." However, TCP/IP does recognize four broad layers of functionality which are derived from the operating scope of their contained protocols, namely the scope of the software application, the end-to-end transport connection, the internetworking range, and lastly the scope of the direct links to other nodes on the local network.

Even though the concept is different than in OSI, these layers are nevertheless often compared with the OSI layering scheme in the following way: The Internet [[Application Layer]] includes the OSI Application Layer, Presentation Layer, and most of the Session Layer. Its end-to-end [[Transport Layer]] includes the graceful close function of the OSI Session Layer as well as the OSI Transport Layer. The internetworking layer ([[Internet Layer]]) is a subset of the OSI Network Layer, while the [[Link Layer]] includes the OSI Data Link and Physical Layers, as well as parts of OSI's Network Layer. These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in such things as the internal organization of the Network Layer document.

The presumably strict consumer/producer layering of OSI as it is usually described does not present contradictions in TCP/IP, as it is permissible that protocol usage does not follow the hierarchy implied in a layered model. Such examples exist in some routing protocols (e.g., OSPF), or in the description of [[tunneling protocol]]s, which provide a Link Layer for an application, although the tunnel host protocol may well be a Transport or even an Application Layer protocol in its own right.

The TCP/IP design generally favors decisions based on simplicity, efficiency and ease of implementation.

==Remembering The OSI Layers==
Various [[mnemonic]]s have been created over the years to help remember the order of the OSI layers. Often cited are the following:
<!-- PLEASE DO NOT ADD MORE! They will be removed -->
*'''P'''lease '''D'''o '''N'''ot '''T'''hrow '''S'''ausage '''P'''izza '''A'''way
*'''A'''ll '''P'''eople '''S'''eem '''T'''o '''N'''eed '''D'''ata '''P'''rocessing
*'''P'''lease '''D'''o '''N'''ot '''T'''ell '''S'''ales '''P'''eople '''A'''nything

<!-- PLEASE DO NOT ADD MORE! They will be removed -->

==References==
<references/>

==External links==
* [http://standards.iso.org/ittf/licence.html (license agreement for downloading)] and [http://standards.iso.org/ittf/PubliclyAvailableStandards/s020269_ISO_IEC_7498-1_1994(E).zip ISO/IEC standard 7498-1:1994] ([[ZIP (file format)|ZIP format]])
* [http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-X.200-199407-I!!PDF-E&type=items ITU-T X.200 (the same contents as from ISO)]
* {{PDFlink|1=[http://www.comsoc.org/livepubs/50_journals/pdf/RightsManagement_eid=136833.pdf OSI Reference Model — The ISO Model of Architecture for Open Systems Interconnection]|2=776&nbsp;KB}}, Hubert Zimmermann, IEEE Transactions on Communications, vol. 28, no. 4, April 1980, pp. 425 - 432.
* [http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Intro-to-Internet.html Internetworking Basics]

[[Category:CISSP]]
[[Category:Network architecture]]
[[Category:ISO standards]]
[[Category:ITU-T recommendations]]
[[Category:OSI protocols]]
[[Category:Reference models]]

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[[cs:Referenční model ISO/OSI]]
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[[el:μοντέλο αναφοράς OSI]]
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[[fr:Modèle OSI]]
[[gl:Modelo OSI]]
[[ko:OSI 모델]]
[[hr:OSI]]
[[id:OSI Reference Model]]
[[it:Open Systems Interconnection]]
[[he:מודל ה-OSI]]
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[[lt:OSI modelis]]
[[ms:Model OSI]]
[[hu:OSI modell]]
[[nl:OSI-model]]
[[ja:OSI参照モデル]]
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[[ta:திறந்த முறைமை வலைப்பின்னல் மாதிரி]]
[[th:OSI Model]]
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[[zh:OSI模型]]

Revision as of 15:42, 10 October 2008

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 Basic Reference Model' (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design. It was developed as part of the Open Systems Interconnection (OSI) initiative[2]. In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model. (Note: The data security industry sometimes refers to a hypothetical "Layer 8", or Human Interface Layer. This is considered the point at which the user interacts with the application. This is not a part of the official OSI standard, but is frequently referenced to exemplify the weakness of the human component in the overall process.)

A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path.

Even though it has been largely superseded by newer IETF, IEEE, and indeed OSI protocol developments (subsequent to the publication of the original architectural standards), the basic OSI model is considered an excellent place to begin the study of network architecture. Not understanding that the pure seven-layer model is more historic than current, many beginners make the mistake of trying to fit every protocol under study into one of the seven basic layers. Especially the attempts of cross-layer optimization break the boundaries of the original layer scheme. Describing the actual layer concept with implemented systems is not always easy to do as most of the protocols in use on the Internet were designed as part of the TCP/IP model, and may not fit cleanly into the OSI Model.

History

In 1977, work on a layered model of network architecture was started, and the International Organization for Standardization (ISO) began to develop its OSI framework architecture (The ISO is a worldwide federation of national standards bodies from some 130 countries, one from each country).[citation needed] OSI has two major components: 1) an abstract model of networking, called the Basic Reference Model or seven-layer model, and 2) a set of specific protocols.

Note: The standard documents that describe the OSI model can be freely downloaded from the ITU-T as the X.200-series of recommendations [3]. A number of the protocol specifications are also available as part of the ITU-T X series. The equivalent ISO and ISO/IEC standards for the OSI model are available from the ISO, but only some of the ISO/IEC standards are available as cost-free downloads.[4]

all aspects of OSI design evolved from experiences with the CYCLADES network, which also influenced Internet design. The new design was documented in ISO 7498 and its various addenda. In this model, a networking system is divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacts directly only with the layer immediately beneath it, and provides facilities for use by the layer above it.

Protocols enable an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions 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.

Description of OSI layers

OSI Model
Data unit Layer Function
Host
layers 		Data 7. Application Network process to application
6. Presentation Data representation and encryption
5. Session Interhost communication
Segment/Datagram 4. Transport End-to-end connections and reliability
Media
layers 		Packet 3. Network Path determination and logical addressing
Frame 2. Data Link Physical addressing (MAC & LLC)
Bit 1. Physical Media, signal and binary transmission

Layer 7: Application Layer

Main article: Application Layer

The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).

Layer 6: Presentation Layer

Main article: Presentation Layer

The Presentation Layer establishes a context between Application Layer entities, in which the higher-layer entities can use different syntax and semantics, as long as the Presentation Service understands both and the mapping between them. The presentation service data units are then encapsulated into Session Protocol Data Units, and moved down the stack.

The original presentation structure used 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 serializing objects and other data structures into and out of XML. ASN.1 has a set of cryptographic encoding rules that allows end-to-end encryption between application entities.

Layer 5: Session Layer

Main article: Session Layer

The Session Layer controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or 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 TCP, 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 (RPCs).

Layer 4: Transport Layer

Main article: Transport Layer

The Transport Layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the Transport Layer can keep track of the segments and retransmit those that fail.

Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer, the best known examples of a Layer 4 protocol are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

Of the actual OSI protocols, there are five classes of transport protocols ranging from class 0 (which is also known as TP0 and provides the least error recovery) to class 4 (which is also known as TP4 and is designed for less reliable networks, similar to the Internet). Class 0 is closest to UDP. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the Session Layer. Detailed characteristics of TP0-4 classes are shown in the following table:[5]

Feature Name TP0 TP1 TP2 TP3 TP4
Connection oriented - - - Yes Yes
Connectionless - - - No Yes
Segmentation/Fragmentation Yes Yes Yes Yes Yes
Reassembly Yes Yes Yes Yes Yes
Error Recovery No Yes No No Yes
Reinitiate connection (if an excessive number of PDUs are unacknowledged) No Yes No Yes Yes
multiplexing and demultiplexing over a single virtual circuit No No Yes Yes Yes
Reliable Transport Service No Yes Yes Yes Yes

Perhaps 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. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the Transport Layer, such as carrying non-IP protocols such as IBM's 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 endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint. L2TP carries PPP frames inside transport packet.

Layer 3: Network Layer

Main article: Network Layer

The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks, while maintaining the quality of service requested by the Transport Layer. The Network Layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer—sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is hierarchical.

The best-known example of a Layer 3 protocol is the Internet Protocol (IP). It manages the connectionless transfer of data one hop at a time, from end system to ingress router, router to router, and from egress router to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of errored packets so they may be discarded. When the medium of the next hop cannot accept a packet in its current length, IP is responsible for fragmenting into sufficiently small packets that the medium can accept it.

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 2: Data Link Layer

Main article: Data Link Layer

The Data Link Layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multiaccess media, was developed independently of the ISO work, in IEEE Project 802. IEEE work assumed sublayering and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in modern data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on Ethernet, and, on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the Transport Layer by protocols such as TCP, but is still used in niches where X.25 offers performance advantages.

Both WAN and LAN services arrange bits, from the Physical Layer, into logical sequences called frames. Not all Physical Layer bits necessarily go into frames, as some of these bits are purely intended for Physical Layer functions. For example, every fifth bit of the FDDI bit stream is not used by the Data Link Layer.

WAN Protocol architecture

Connection-oriented WAN data link protocols, in addition to framing, detect and may correct errors. They also are capable of controlling the rate of transmission. A WAN Data Link Layer might implement a sliding window flow control and acknowledgment mechanism to provide reliable delivery of frames; that is the case for SDLC and HDLC, and derivatives of HDLC such as LAPB and LAPD.

IEEE 802 LAN architecture

Practical, connectionless LANs began with the pre-IEEE Ethernet specification, which is the ancestor of IEEE 802.3. This layer manages the interaction of devices with a shared medium, which is the function of a Media Access Control sublayer. Above this MAC sublayer is the media-independent IEEE 802.2 Logical Link Control (LLC) sublayer, which deals with addressing and multiplexing on multiaccess media.

While IEEE 802.3 is the dominant wired LAN protocol and IEEE 802.11 the wireless LAN protocol, obsolescent MAC layers include Token Ring and FDDI. The MAC sublayer detects but does not correct errors.

Layer 1: Physical Layer

Main article: Physical Layer

The Physical Layer defines the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, cable specifications, Hubs, repeaters, network adapters, Host Bus Adapters (HBAs used in Storage Area Networks) and more.

To understand the function of the Physical Layer in contrast to the functions of the Data Link Layer, think of the Physical Layer as concerned primarily with the interaction of a single device with a medium, where the Data Link Layer is concerned more with the interactions of multiple devices (i.e., at least two) with a shared medium. The Physical Layer will tell one device how to transmit to the medium, and another device how to receive from it (in most cases it does not tell the device how to connect to the medium). Obsolescent Physical Layer standards such as RS-232 do use physical wires to control access to the medium.

The major functions and services performed by the Physical Layer are:

Parallel SCSI buses operate in this layer, although it must be remembered that the logical SCSI protocol is a Transport Layer protocol that runs over this bus. Various Physical Layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the Data Link Layer. The same applies to other local-area networks, such as Token ring, FDDI, and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4.

Interfaces

Neither the OSI Reference Model nor OSI protocols specify any programming interfaces, other than as deliberately abstract service specifications. Protocol specifications precisely define the interfaces between different computers, but the software interfaces inside computers are implementation-specific.

For example, Microsoft Windows' Winsock, and Unix's Berkeley sockets and System V Transport Layer Interface, are interfaces between applications (Layer 5 and above) and the transport (Layer 4). NDIS and ODI are interfaces between the media (Layer 2) and the network protocol (Layer 3).

Interface standards, except for the Physical Layer to media, are approximate implementations of OSI Service Specifications.

Examples

Layer Misc. examples TCP/IP suite SS7[6] AppleTalk suite OSI suite IPX suite SNA UMTS
# Name
7 Application HL7, Modbus NNTP, SIP, SSI, DNS, FTP, Gopher, HTTP, NFS, NTP, DHCP, SMPP, SMTP, SNMP, Telnet INAP, MAP, TCAP, ISUP, TUP AFP, ZIP, RTMP, NBP FTAM, X.400, X.500, DAP, ROSE, RTSE, ACSE RIP, SAP APPC
6 Presentation TDI, ASCII, EBCDIC, MIDI, MPEG MIME, XDR, SSL, TLS (Not a separate layer) AFP ISO/IEC 8823, X.226, ISO/IEC 9576-1, X.236
5 Session Named Pipes, NetBIOS, SAP, Half Duplex, Full Duplex, Simplex, SDP Sockets. Session establishment in TCP. SIP. (Not a separate layer with standardized API.) ASP, ADSP, PAP ISO/IEC 8327, X.225, ISO/IEC 9548-1, X.235 NWLink DLC?
4 Transport NBF, nanoTCP, nanoUDP TCP, UDP, PPTP, L2TP, SCTP, RTP DDP ISO/IEC 8073, TP0, TP1, TP2, TP3, TP4 (X.224), ISO/IEC 8602, X.234 SPX
3 Network NBF, Q.931 IP, IPsec, ARP, ICMP, RIP, OSPF, BGP, IGMP, IS-IS SCCP, MTP ATP (TokenTalk or EtherTalk) ISO/IEC 8208, X.25 (PLP), ISO/IEC 8878, X.223, ISO/IEC 8473-1, CLNP X.233. IPX RRC (Radio Resource Control) Packet Data Convergence Protocol (PDCP) and BMC (Broadcast/Multicast Control)
2 Data Link 802.3 (Ethernet), 802.11a/b/g/n MAC/LLC, 802.1Q (VLAN), ATM, HDP, FDDI, Fibre Channel, Frame Relay, HDLC, ISL, PPP, Q.921, Token Ring, CDP PPP, SLIP MTP, Q.710 LocalTalk, AppleTalk Remote Access, PPP ISO/IEC 7666, X.25 (LAPB), Token Bus, X.222, ISO/IEC 8802-2 LLC Type 1 and 2 IEEE 802.3 framing, Ethernet II framing SDLC LLC (Logical Link Control), MAC (Media Access Control)
1 Physical RS-232, V.35, V.34, I.430, I.431, T1, E1, 10BASE-T, 100BASE-TX, POTS, SONET, SDH, DSL, 802.11a/b/g/n PHY MTP, Q.710 RS-232, RS-422, STP, PhoneNet X.25 (X.21bis, EIA/TIA-232, EIA/TIA-449, EIA-530, G.703) Twinax UMTS L1 (UMTS Physical Layer)

Comparison with TCP/IP

In the TCP/IP model of the Internet, protocols are deliberately not as rigidly designed into strict layers as the OSI model.[7] RFC 3439 contains a section entitled "Layering considered harmful." However, TCP/IP does recognize four broad layers of functionality which are derived from the operating scope of their contained protocols, namely the scope of the software application, the end-to-end transport connection, the internetworking range, and lastly the scope of the direct links to other nodes on the local network.

Even though the concept is different than in OSI, these layers are nevertheless often compared with the OSI layering scheme in the following way: The Internet Application Layer includes the OSI Application Layer, Presentation Layer, and most of the Session Layer. Its end-to-end Transport Layer includes the graceful close function of the OSI Session Layer as well as the OSI Transport Layer. The internetworking layer (Internet Layer) is a subset of the OSI Network Layer, while the Link Layer includes the OSI Data Link and Physical Layers, as well as parts of OSI's Network Layer. These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in such things as the internal organization of the Network Layer document.

The presumably strict consumer/producer layering of OSI as it is usually described does not present contradictions in TCP/IP, as it is permissible that protocol usage does not follow the hierarchy implied in a layered model. Such examples exist in some routing protocols (e.g., OSPF), or in the description of tunneling protocols, which provide a Link Layer for an application, although the tunnel host protocol may well be a Transport or even an Application Layer protocol in its own right.

The TCP/IP design generally favors decisions based on simplicity, efficiency and ease of implementation.

Remembering The OSI Layers

Various mnemonics have been created over the years to help remember the order of the OSI layers. Often cited are the following:

  • Please Do Not Throw Sausage Pizza Away
  • All People Seem To Need Data Processing
  • Please Do Not Tell Sales People Anything


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. ^ http://www.itu.int/rec/T-REC-X.200-199407-I/en
  3. ^ ITU-T X-Series Recommendations.
  4. ^ Publicly Available Standards
  5. ^ "ITU-T Recommendation X.224 (11/1993) [ISO/IEC 8073]". Retrieved 2008-06-18.
  6. ^ ITU-T Recommendation Q.1400 (03/1993), Architecture framework for the development of signalling and OA&M protocols using OSI concepts, pp 4, 7.
  7. ^ RFC 3439

External links

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