OSI
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The Open Systems Interconnection
(OSI) model is a product of the Open Systems
Interconnection effort at the International
Organization for Standardization. It is a prescription of
characterizing and standardizing the functions of a communications system in
terms ofabstraction layers.
Similar communication functions are grouped into logical layers. A layer serves
the layer above it and is served by 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 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)
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Contents
[hide]
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[edit]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.
The concept of a seven-layer model was
provided by the work ofCharles Bachman,
Honeywell Information Services. Various aspects of OSI design evolved from
experiences with the ARPANET, the fledgling Internet, NPLNET, EIN, CYCLADESnetwork 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.
The OSI standards documents are available
from the ITU-T as the X.200-series of recommendations.[1] Some 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 without fees.[2]
[edit]Description
of OSI layers
According to recommendation X.200, there are
seven layers, labeled 1 to 7, with layer 1 at the bottom. Each layer is
generically known as an N layer. An "N+1 entity" (at layer N+1)
requests services from an "N entity" (at layer N).
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 a layer N is the SDU of layer N-1.
In effect the SDU is the 'payload' of a given PDU. That is, the process of
changing an 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.
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OSI Model
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Data unit
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Layer
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Function
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Host
layers |
7. Application
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Network process to application
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6. Presentation
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Data representation, encryption and decryption, convert machine
dependent data to machine independent data
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5. Session
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Interhost communication, managing sessions between applications
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4. Transport
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End-to-end connections, reliability and flow control
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Media
layers |
3. Network
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Path determination and logical addressing
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2. Data link
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1. Physical
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Media, signal and binary transmission
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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 (confidentiality, integrity, and 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.
[edit]Layer
1: physical layer
The physical layer defines electrical and physical specifications
for devices. In particular, it defines the relationship between a device and atransmission medium,
such as a copper or fiber 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.
The major functions and services performed by
the physical layer are:
§
Establishment
and termination of a connection to
a communications medium.
§
Participation
in the process whereby the communication resources are effectively shared among
multiple users. For example, contentionresolution
and flow control.
§
Modulation, or conversion between the
representation of digital data in
user equipment and the corresponding signals transmitted over a communications channel.
These are signals operating over the physical cabling (such as copper and optical fiber) or over a 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 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.
[edit]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.
[edit]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 Synchronous
Data Link Control (SDLC) and HDLC,
and derivatives of HDLC such as LAPB andLAPD.
[edit]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.
[edit]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.
The network layer may be divided into three
sublayers:
1. Subnetwork access – that considers protocols
that deal with the interface to networks, such as X.25;
2. Subnetwork-dependent convergence – when it is
necessary to bring the level of a transit network up to the level of networks
on either side
3. Subnetwork-independent convergence – handles
transfer across multiple networks.
An example of this latter case is CLNP, or
IPv6 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.
[edit]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.
OSI defines 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.
Detailed characteristics of TP0-4 classes are shown in the following table:[4]
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Feature Name
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TP0
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TP1
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TP2
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TP3
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TP4
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Connection oriented
network
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Yes
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Yes
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Yes
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Yes
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Yes
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Connectionless network
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No
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No
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No
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No
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Yes
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Concatenation and
separation
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No
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Yes
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Yes
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Yes
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Yes
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Segmentation and
reassembly
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Yes
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Yes
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Yes
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Yes
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Yes
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Error Recovery
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No
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Yes
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Yes
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Yes
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Yes
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Reinitiate connection
(if an excessive number of PDUs are
unacknowledged)
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No
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Yes
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No
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Yes
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No
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Multiplexing and
demultiplexing over a single virtual circuit
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No
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No
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Yes
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Yes
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Yes
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Explicit flow control
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No
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No
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Yes
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Yes
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Yes
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Retransmission on
timeout
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No
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No
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No
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No
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Yes
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Reliable Transport
Service
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No
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Yes
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No
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Yes
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Yes
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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.
Although not developed under the OSI
Reference Model and not strictly conforming to the OSI definition of the
transport layer, theTransmission
Control Protocol (TCP) and the User Datagram
Protocol (UDP) of the Internet Protocol Suite are commonly
categorized as layer-4 protocols within OSI.
[edit]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. On this level, Inter-Process_(computing) communication
happen (SIGHUP, SIGKILL, End Process, etc.).
[edit]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.
[edit]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:
§
FTAM File
Transfer and Access Management Protocol
§
X.400 Mail
§
On
TCP/IP stack:
§
Hypertext
Transfer Protocol (HTTP),
§
File Transfer
Protocol (FTP),
§
Simple Mail
Transfer Protocol (SMTP)
§
Simple
Network Management Protocol (SNMP).
[edit]Cross-layer
functions
 |
This "datagram service model" reference in MPLS may
be confusing or unclear to
readers. Please help clarify the
"datagram service model" reference in MPLS; suggestions
may be found on the talk page. (January
2012)
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There are some functions or services that are
not tied to a given layer, but they can affect more than one layer. Examples
include the following:
§
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.
§
Multiprotocol
Label Switching (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).
[edit]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, known as network sockets 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.
[edit]Examples
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Layer
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OSI protocols
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Misc. examples
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#
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Name
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7
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Application
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6
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Presentation
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ISO/IEC 8823,
X.226, ISO/IEC 9576-1, X.236
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5
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Session
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ISO/IEC 8327,
X.225, ISO/IEC 9548-1, X.235
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DLC?
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4
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Transport
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ISO/IEC 8073,
TP0, TP1, TP2, TP3, TP4 (X.224), ISO/IEC 8602, X.234
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3
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Network
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