OSI model
The
Open Systems Interconnection model (
OSI model) was a product of the
Open Systems Interconnection effort at the
International Organization for Standardization. It is a way of sub-dividing a
communications system into smaller parts called
layers. Similar communication functions are grouped into logical layers. A layer provides services to its upper layer while receiving services from the layer below. On each layer, an
instance provides service to the instances at the layer above and requests service from the layer below.
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 that path. Two instances at one layer are connected by a horizontal connection on that layer.
Communication in the OSI-Model (Example with layers 3 to 5)
History
Work on a layered model of network architecture was started and the
International Organization for Standardization (ISO) began to develop its OSI framework architecture. 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.
Note: The standard documents that described the OSI model could be freely downloaded from the ITU-T as the
X.200-series of recommendations.
[1] A number of the protocol specifications were also available as part of the ITU-T X series. The equivalent ISO and ISO/IEC standards for the OSI model were available from ISO, but only some of them at no charge.
[2]
The concept of a 7 layer model was provided by the work of
Charles Bachman, Honeywell Information Services. Various aspects of OSI design evolved from experiences with the ARPANET, the fledgling Internet, NPLNET, EIN,
CYCLADES network and the work in IFIP WG6.1. The new design was documented in ISO 7498 and its various addenda. 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.
Protocols enabled an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions abstractly described the functionality provided to an (N)-layer by an (N-1) layer, where N was one of the seven layers of protocols operating in the local host.
Description of OSI layers
Depending on to recommendation X.200, there are seven layers, each generically known as an N layer. An N+1 entity requests services from the N entity.
At each level, two entities (N-entity peers) interact by means of the N protocol by transmitting
protocol data units (PDU).
A
Service Data Unit (SDU) is a specific unit of data that has been passed down from an OSI layer to a lower layer, and which the lower layer has not yet encapsulated into a protocol data unit (PDU). An SDU is a set of data that is sent by a user of the services of a given layer, and is transmitted semantically unchanged to a peer service user.
The PDU at any given layer, layer N, is the SDU of the layer below, layer N-1. In effect the SDU is the 'payload' of a given PDU. That is, the process of changing a SDU to a PDU, consists of an encapsulation process, performed by the lower layer. All the data contained in the SDU becomes encapsulated within the PDU. The layer N-1 adds headers or footers, or both, to the SDU, transforming it into a PDU of layer N-1. The added headers or footers are part of the process used to make it possible to get data from a source to a destination.
| OSI Model |
|---|
| Data unit | Layer | Function |
|---|
Host
layers | Data | 7. Application | Network process to application |
|---|
| 6. Presentation | Data representation, encryption and decryption, convert machine dependent data to machine independent data |
| 5. Session | Interhost communication |
| Segments | 4. Transport | End-to-end connections and reliability, flow control |
Media
layers | Packet/Datagram | 3. Network | Path determination andlogical addressing |
|---|
| Frame | 2. Data Link | Physical addressing |
| Bit | 1. Physical | Media, signal and binary transmission |
Some orthogonal aspects, such as management and security, involve every layer.
Security services are not related to a specific layer: they can be related by a number of layers, as defined by
ITU-T X.800 Recommendation.
[3]
These services are aimed to improve the
CIA triad (i.e.
confidentiality,
integrity,
availability) of transmitted data. Actually the availability of communication service is determined by
network design and/or
network management protocols. Appropriate choices for these are needed to protect against
denial of service.
Layer 1: Physical Layer
The
Physical Layer defines
electrical and physical specifications for devices. In particular, it defines the relationship between a device and a
transmission medium, such as a copper or optical cable. This includes the layout of
pins,
voltages,
cable specifications,
hubs,
repeaters,
network adapters,
host bus adapters (HBA used in
storage area networks) and more.
To understand the function of the Physical Layer, contrast it with 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, whereas the Data Link Layer is concerned more with the interactions of multiple devices (i.e., at least two) with a shared medium. 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,
ITU-T G.hn and
IEEE 802.11, as well as personal area networks such as
Bluetooth and
IEEE 802.15.4.
Layer 2: 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 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 the 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.
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 which provides both error correction and flow control by means of a
selective repeat Sliding Window Protocol.
Both WAN and LAN service 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 Layer.
WAN Protocol architecture
Connection-oriented WAN data link protocols, in addition to framing, detect and may correct errors. They are also 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 (MAC) 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 3: Network Layer
The
Network Layer provides the functional and procedural means of transferring variable length
data sequences from a source host on one network to a destination host on a different network, while maintaining the
quality of service requested by the Transport Layer (in contrast to the data link layer which connects hosts within the same network). 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 not hierarchical.
Careful analysis of the Network Layer indicated that the Network Layer could have at least three sublayers:
- Subnetwork Access – that considers protocols that deal with the interface to networks, such as X.25;
- Subnetwork Dependent Convergence – when it is necessary to bring the level of a transit network up to the level of networks on either side;
- Subnetwork Independent Convergence – which handles transfer across multiple networks.
The best example of this latter case is CLNP, or IPv7 ISO 8473. 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 erroneous packets so they may be discarded. In this scheme, IPv4 and IPv6 would have to be classed with X.25 as subnet access protocols because they carry interface addresses rather than node addresses.
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 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. The Transport layer also provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred.
Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer, typical examples of Layer 4 are the
Transmission Control Protocol (TCP) and
User Datagram Protocol (UDP).
Of the actual OSI protocols, there are five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the least 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 classes provide expedited data and preservation of record boundaries, both of which TCP is incapable. Detailed characteristics of TP0-4 classes are shown in the following table:
[4]
| 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 (if an excessive number of PDUs are unacknowledged) | No | Yes | No | Yes | No |
| Multiplexing and demultiplexing over a 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 |
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 5: Session Layer
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,
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 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
The
Presentation Layer establishes context between Application Layer entities, in which the higher-layer entities may use different syntax and semantics if the presentation service provides a mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units, and passed down the stack.
This layer provides independence from data representation (e.g.,
encryption) by translating between application and network formats. The presentation layer transforms data into the form that the application accepts. This layer formats and encrypts data to be sent across a network. It is sometimes called the syntax layer.
[5]
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
serialization of
objects and other
data structures from and to
XML.
Layer 7: 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 or 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 also include:
- On OSI stack:
- On TCP/IP stack:
Cross-layer functions
There are some functions or services that are not tied to a given layer, but they can affect more than one layer. Examples are
- security service (telecommunication)[3] as defined by ITU-T 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 (CMIP) and its corresponding service common management information service (CMIS), they need to interact with every layer in order to deal with their instances.
- MPLS operates at an OSI Model layer that is generally considered to lie between traditional definitions of Layer 2 (Data Link Layer) and Layer 3 (Network Layer), and thus is often referred to as a "Layer 2.5" protocol. It was designed to provide a unified data-carrying service for both circuit-based clients and packet-switching clients which provide a datagram 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.
- ARP is used to translate IPv4 addresses (OSI Layer 3) into Ethernet MAC addresses (OSI Layer 2)
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
ODIare 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 | OSI protocols | TCP/IP protocols | Signaling System 7[6] | AppleTalk | IPX | SNA | UMTS | Misc. examples |
|---|
| # | Name |
|---|
| 7 | Application | FTAM, X.400, X.500,DAP, ROSE, RTSE,ACSE[7] CMIP[8] | NNTP, SIP, SSI, DNS, FTP,Gopher, HTTP, NFS, NTP,DHCP, SMPP, SMTP,SNMP, Telnet, RIP, BGP | INAP,MAP,TCAP,ISUP,TUP | AFP, ZIP,RTMP,NBP | RIP, SAP | APPC |
| HL7, Modbus |
| 6 | Presentation | ISO/IEC 8823, X.226, ISO/IEC 9576-1, X.236 | MIME, SSL, TLS, XDR |
| AFP |
|
|
| TDI, ASCII, EBCDIC, MIDI, MPEG |
| 5 | Session | ISO/IEC 8327, X.225, ISO/IEC 9548-1, X.235 | Sockets. Session establishment in TCP, RTP |
| ASP,ADSP,PAP | NWLink | DLC? |
| Named pipes, NetBIOS, SAP, half duplex, full duplex, simplex,RPC |
| 4 | Transport | ISO/IEC 8073, TP0, TP1, TP2, TP3, TP4 (X.224), ISO/IEC 8602, X.234 | TCP, UDP, SCTP, DCCP |
|
| DDP, SPX |
|
| NBF |
| 3 | Network | ISO/IEC 8208, X.25(PLP), ISO/IEC 8878,X.223, ISO/IEC 8473-1,CLNP X.233. | IP, IPsec, ICMP, IGMP,OSPF | SCCP,MTP | ATP(TokenTalkorEtherTalk) | IPX |
| RRC (Radio Resource Control)Packet Data Convergence Protocol (PDCP) and BMC(Broadcast/Multicast Control) | NBF, Q.931, IS-IS
Leaky bucket, token bucket |
| 2 | Data Link | ISO/IEC 7666, X.25(LAPB), Token Bus, X.222, ISO/IEC 8802-2LLC Type 1 and 2[9] | PPP, SLIP, PPTP, L2TP | MTP,Q.710 | LocalTalk,AppleTalk Remote Access,PPP | IEEE 802.3framing,Ethernet II framing | SDLC | LLC (Logical Link Control),MAC (Media Access Control) | 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, NDP ARP (maps layer 3 to layer 2 address), ITU-T G.hn DLL
CRC, Bit stuffing, ARQ, Data Over Cable Service Interface Specification (DOCSIS) |
| 1 | Physical | X.25 (X.21bis, EIA/TIA-232, EIA/TIA-449, EIA-530, G.703)[9] |
| MTP,Q.710 | RS-232,RS-422,STP,PhoneNet |
| Twinax | UMTS Physical Layer or L1 | RS-232, Full duplex, RJ45, V.35, V.34, I.430, I.431, T1, E1,10BASE-T, 100BASE-TX, POTS, SONET, SDH, DSL,802.11a/b/g/n PHY, ITU-T G.hn PHY, Controller Area Network,Data Over Cable Service Interface Specification (DOCSIS) |
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.
[10] 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 from the OSI model, 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 (see above), 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 peer layering of the OSI model 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.
See also
References
External links
