OSI model

The model of interconnection in network of the open Systems of the ISO (International organization of standardization) is a model of communications between Ordinateur S. It describes the functionalities necessary to the communication and the organization of these functions.

The complete standard, of reference ISO 7498 is overall entitled “basic Model of reference for the interconnection of the open Systèmes (OSI)” and is made up of 4 parts:

  1. the basic model
  2. Architecture of safety
  3. Denomination and addressing
  4. general Framework of management

This version of this article as well as the article devoteds to with each layer of the model concentrate on part 1, revision of 1994.

The text of the Norme itself is very abstract because he wants to be applicable to many type of networks. To make it more comprehensible, in addition to presenting the Standard, this made article of the bonds with the concrete achievements such as one finds them in a Ordinateur, i.e. concrete protocolar piles (a " system réel" within the meaning of section 4). Moreover, the Norme does not indicate mechanisms suitable to provide the definite functions whereas this article does it. The examples of service and especially of protocols are taken in world IP (probably most known but also furthest away from the spirit of the standard), world ISDN (including the second generation, more known under name ATM) and sometimes the world OSI (which fact not that models). It will be noted that the combinations offered by the model are much more numerous than those carried out in existing piles of protocol, one cannot thus give real example for all the functions.

Presentation of the standard

The objective of this standard is to specify a general framework for the creation of coherent later standards. The model itself does not define particular service and even less protocol.

Concepts and terminology: services, protocols and interfaces

The model is primarily an architecture in layers definite and delimited with the concepts of service, protocol and Interface.

  • a service is an abstract description of functionalities using primitives (orders or events) such as asks for connection or reception of data.
  • a protocol is a whole of messages and rules of exchanges carrying out a service.
  • an interface (" access point to the service" in the standard) is the concrete means to use the service. In a program, it is typically a whole of functions of library or calls systems. In a material manufacture, it is for example a set of registers at the entry of a circuit.

The details of a service of course vary network architecture to the other. The coarsest classification is made according to whether the service functions in mode connected or not. In spite of this variability, the common functions have conventionally constant names. These names do not come however directly from ISO 7498-1.

; connection.request: is a request for outgoing connection, i.e on the initiative of a local entity. ; connection.indication: to the event “a request for entering connection corresponds was received.”. ; connection.response: is the indication of acceptance or rejection of connection ; connection.confirmation: corresponds to the event “the answer of required was received.”. It is a payment. ; data.request, data.indication and data.confirm: are during for the data.

The abundant data with a primitive of service are called (NR) - SDU (" Service Dated Links ") where NR is the indication of the layer, its number in the standard, sometimes a letter drawn from the name of the layer. The messages of a protocol are called PDU (" Protocol Data Links ").

Structure in layers

The model comprises 7 layers briefly presented below and detailed upwards in their respective articles. These layers are sometimes divided into 2 groups.

The 4 sub-bases are rather directed communication and are typically provided by a Operating system.

The 3 roadbases are rather directed application and rather carried out by libraries or a special program. In world IP, these 3 layers are seldom distinguished. In this case, all the functions of these layers are regarded as integral part of the applicatif protocol.

In addition, the low layers are normally transparent for the Donnée S to transport, whereas the roadbases are not it necessarily, in particular on the level presentation.

In such an architecture, a “entity” of level (N+1) sends Donnée S with the primitive " data.request" entity of level (NR) while providing him as Given S one (N+1) - PDU which will be typically, in its turn encapsulated in one (NR) - PDU. Dimensioned receiving, each entity analyzes the envelope protocol corresponding to its layer and transmits the Donnée S to roadbase in the form of a primitive " data.indication".

Certain functions like the detection of the errors of transmission and their correction, the control of flow can be present in several layers. These functions are described overall further.

Summarized characterization of the layers

The characterization given here is drawn from chapter 7 of ISO 7498-1. Original description gives in more for each layer the functions of handling of orders or significant data among those described low.

  1. the “physical” layer is in charge of the effective transmission of the signals between the interlocutors. Its service is typically limited to the emission and the reception of a bit or a continuous bit train (in particular for the synchronous supports).

  2. the layer “data link” manages the communications between 2 adjacent machines, directly connected between them by a physical support.
  3. the layer “network” manages the communications from beginning to end, generally between machines: routing and addressing of the packages. (cf notes below).
  4. the layer “transport” manages from beginning to end the communications between process (programs in the course of execution).
  5. the layer “session” manages the synchronization of the exchanges and the “transactions”, allows the opening and the closing of session.
  6. the layer “presentation” is in charge of the coding of the applicatives data, precisely of conversion between data handled at the applicatif level and actually transmitted chains of bytes.
  7. the layer “application” is the access point to the services networks, it does not have clean service specific and entering the range of the standard.

Two means Mnemotechnical S to remember the 7 layers:

has close P lusieurs S emaines T out R espire L has P Aix

P artout L E R oi T rouve S has P laces has located

Another means: PHY LI LMBO TRA S P HAS

Some precise details

When the services network and transport function both in connected mode, there is no always clear distinction between these two services. There are however two cases or that is very simple:

  • If the service network authorizes only one connection between 2 machines: in this case, connections of level transport are necessarily multiplexed on a connection of level network and the distinction is clear.
  • the services of the 2 layers relating to the correction of the errors are different: These functions can be present only in only one of the 2 layers.

Common functions

Fiabilisation of the communications

One of the important roles of layers 2 to 4, i.e present in many protocolar piles, is the construction of a connection free from errors of transmission. That means that the transmitted data are received without corruption, loss, regrouping and duplication. That implies that with less one layer, and in practice several, makes detection of error, correction of error or retransmission of data and control of flow.

; Detection of errors: location of the PDU whose at least bit changed value during the transfer. ; Correction of the errors: Compensation of the errors is by correction of the data using code correctors of errors or by destruction of the erroneous PDU and request of retransmission. ; Control flow: Synchronization of the communications intended to prevent that an interlocutor receives more PDU than it cannot treat some.

Controls of flow of layers 2 and 3 can seem redundant, but it is not necessarily the case. Indeed, the control of flow on level 2 guarantees control only on one line. But if a machine is equipped with several interfaces, it is the case in particular of all the routers, and that there is no control of flow on at least one of the interfaces, there is a risk of saturation in the entity of level network. This case arises in particular in the X.25 networks where the control of flow is an option, negotiated with the opening of connection.

Functions of transformation

In addition to the structure in layer, the model defines also a series of standard mechanisms of handling of orders or data, used for the realization of a service. This section defines most current. These transformations are described per pair of operations opposite one of the other.

; Multiplexing and demultiplexing of connection: Use of a connection of level NR to transport the PDU of several connections of N+1 level. Symmetrically, démultiplexer consists in separating to them (N+1) - PDU entering by connection. For example, this mechanism is envisaged in networks ATM by “layer” AAL 3/4. ; Bursting and recombination: similar operations in which (N+1) - PDU of which distributed on several connection of level NR. That is used in particular by the users of access ISDN to increase the flow available. ; Segmentation and re-assembly: When the abundant service by the layer (NR) fixes a limit of size on the data too small compared to the service of the layer (N+1), the layer (N+1) cutting them (N+1) - SDU in several fragments corresponding each one to one (N+1) - PDU before sending. With the reception, the layer (N+1) concatene fragments to find it (N+1) - initial SDU. That is massively used in networks ATM and SSL/TLS. For IP, this function is traditionally called “fragmentation”.

Limitations of the model and uses wide

This section illustrates some cases or an architecture network cannot enter completely within the framework of OSI model.

The model provides that in a concrete pile, there is one and only one protocol by layer. There are however cases or that is quasi-impossible, in particular at the time of the interconnection of heterogeneous networks, i.e. using different plays of protocol. For example, a simple tunnel makes it possible to connect 2 homogeneous networks by treating a network of another type like a point-to-point connection. It is this technique which is used to temporarily connect a machine isolated to Internet (except lines xDSL): A modem manages a telephone connection between 2 distant machines, therefore a connection of level 3 in pile ISDN, and uses it to transmit screens PPP, protocol of level 2 whereas in a canonical pile, that would be PDU of level transport (4).

There are also situations where 2 of the same protocols level are used simultaneously because the combination of the abundant service and the awaited service of the sub-base requires it. Thus, in the world IP, protocols SSL and TCP provide both a point-to-point department of communication between process but the only standard protocol carrying out the service awaited by SSL to function is provided only by TCP. One is thus obliged to superimpose SSL on TCP.

In certain architectures network, the service offered to the machines of end is not sufficient to satisfy the internal needs with the network. For example, in a network ATM, the service network is in connected mode. One thus needs a protocolar pile able to transport indication (the messages of management of connections) but the service offered by this pile is not accessible to the machines from end. To model that, one superimposes on “horizontal” cutting in layer, a “vertical” cutting in “plan” in which the protocolar piles are independent. Thus, a model of network ATM consists of 3 plans: the user plan for the ordinary data, the plan of control for the transport of indication and a plan of management for the supervision interns with the network. The phone networks (fixed networks ISDN and networks UMTS) have also a cutting in similar plan.

The world IP and OSI model

If there is well a coarse correspondence between the protocols of pile IP and the layers of the model, one cannot consider that pile IP is really compatible with the model. In particular, the separation of the layers in pile IP is definitely more approximate. Here are 2 illustrations.

To be in conformity with the model, a protocol of a pile should not depend on the protocols of the other layers, but only of the abundant service. As example of nonconformity, let us consider the detection of the errors in a pile IP. 2 protocols TCP and UDP have in their heading a sum of control for the detection of the errors. The calculation of this sum utilizes part of heading IP. Protocols TCP and UDP are thus not independent of IP. That is noticed in particular with the fact that at the time of passage of IP version 4 to IP version 6, it is necessary to redefine the way of calculating these checksum whereas the protocols did not really change to them-even.

When a " datagramme" UDP, protocol of level transport in theory, arrives at an address (even < addresses IP, number of port>) whereas it does not have a process recipient, the error is announced to the transmitter by sending a package ICMP indicating to him “inaccessible port”. However ICMP is in theory a protocol of level network. The machine receiving this package must thus examine the part given of this package to determine the process having to receive the notification of error. Difference in protocol and loss of transparency of the data are 2 cases of bad separation of the layers. Let us note on this occasion that TCP has on the other hand a normal mechanism for this situation: lifting of indicator RST in the error message.

Some protocols

; 7 Layer application: Gopher • HS • NNTP • DNS • SNMP • XMPP • smtp • POP3 • IMAP • IRC • VoIP • WebDAV • SIMPLE • HTTP ; 6 Layer of presentation: Videotex • Unicode • MIME • HTML • TDI • ASN.1 • XDR • UUCP • CPC • AFP • SSP ; 5 Layer of session: RTSP • H.323 • SIP • Appletalk ; 4 Layer of transport: TCP • UDP • ICMP • SCTP • RTP • SPX • TCAP • DCCP ; 3 Layer of network: NetBEUI • IPv4 • IPv6 • ARP • IPX • BGP • ICMP • OSPF • RIP • IGMP • IS-IS • CLNP • WDS • ATM ; 2 Course binder of data: Ethernet • Token ring • LocalTalk • FDDI • X.21 • X.25 • Frame Relay • Bitnet • EDGE • Wi-Fi • PPP • HDLC ; 1 physical Layer: CSMA/CD • CSMA/CA • Coding NRZ • Coding Manchester • Coding Miller • RS-232 • RS-449 • V.21 - V.23 • V.42 - V.90 • Coaxial cable • 10Base2 • 10BASE5 • Twisted pair • 10BaseT • 100BASE-TX • ISDN • PDH • SDH • T-carrier • EIA-422 • EIA-485 • SONNET • ADSL • SDSL • VDSL • DSSS • FHSS • HomeRF • IrDA • USB • IEEE 1394 • Wireless USB, Bluetooth

External bonds

  • ISO: http://standards.iso.org/ittf/PubliclyAvailableStandards/s020269_ISO_IEC_7498-1_1994(E).zip the text of the standard, part 1 only, in pdf compressed with format ZIP

  • OSI: http://www.acm.org/sigs/sigcomm/standards/iso_stds/OSI_MODEL/ Texte rough extracts from parts 1 and 3 of the standard
  • Représentation of Protocol TCP/IP on OSI model

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