The Internet Protocol Suite Also Known as TCP

8/6/2019 The Internet Protocol Suite Also Known as TCP

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The Internet Protocol Suite also known as TCP/IP is the set of communications

protocols used for the Internet and other similar networks. It is named from two of

the most important protocols in it: the Transmission Control Protocol (TCP) and

the Internet Protocol (IP), which were the first two networking protocols defined

in this standard. IP networking represents a synthesis of several developments that

began to evolve in the 1960s and 1970s, namely the Internet and LANs (LocalArea Networks), which emerged in the mid- to late-1980s, together with the

advent of the World Wide Web in early 1990s.

The Internet Protocol Suite, like many protocol suites, may be viewed as a set of

layers. Each layer solves a set of problems involving the transmission of data, and

provides a well-defined service to the upper layer protocols based on using

services from some lower layers. Upper layers are logically closer to the user and

deal with more abstract data, relying on lower layer protocols to translate data into

forms that can eventually be physically transmitted.The main differences between the two models are as follows:

1. OSI is a reference model and TCP/IP is an implementation of OSI model.2.TCP/IP Protocols are considered to be standards around which the internet

has developed. The OSI model however is a "generic, protocol-independent standard."

3. TCP/IP combines the presentation and session layer issues into itsapplication layer.4. TCP/IP combines the OSI data link and physical layers into the network

access layer.

5. TCP/IP appears to be a simpler model and this is mainly due to the fact thatit has fewer layers.

6. TCP/IP is considered to be a more credible model- This is mainly due to the

fact because TCP/IP protocols are the standards around which the internet

was developed therefore it mainly gains creditability due to this

reason. Where as in contrast networks are not usually built around the OSI

model as it is merely used as a guidance tool.7. The OSI model consists of 7 architectural layers whereas the TCP/IP onlyhas 4 layers.

8. In the TCP/IP model of the Internet, protocols are deliberately not as rigidly

designed into strict layers as the OSI model.[6] 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.9. 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.


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10. The TCP/IP design generally favors decisions based on simplicity,

efficiency and ease of implementation.

OSI Model Reference TCP/IP Model

Reference

Service, interfaceand protocol

Service, interface and

protocol are not clearly

defined. For example, the

only real services offered

by the Internet layer are

- Send IP Packet

- Receive IP Packet

Protocols in the OSI

model are better hidden

and can be replaced

relatively easily as the

technology changes,

which is one of the mainobjective of layered

protocols.

Functionalities

Because models were

invented beforeprotocols, functionalities

put in each layer are not

very optimized.

In this case, the protocols

have been invented beforemodels, so the

functionalities are

perfectly described.

Numbers oflayers

Seven layers, Network

(Internet), Transport andApplication layers being

similar to TCP/IP

Only four layers.

Connectionless/ Connection-

orientedcommunication

Both connectionless andconnection-oriented

communication are

supported in the network

layer, but only

connection-oriented

communication in the

transport layer.

Only one mode in the

network layer

(connectionless) but both

modes in the transport

layer are supported, giving

the users a choice.

Internet Protocol Suite:

The Internet Protocol Suite also known as TCP/IP is the set of communications protocols

used for the Internet and other similar networks. It is named from two of the mostimportant protocols in it: the Transmission Control Protocol (TCP) and the Internet

Protocol (IP), which were the first two networking protocols defined in this standard. IPnetworking represents a synthesis of several developments that began to evolve in the

1960s and 1970s, namely the Internet and LANs (Local Area Networks), which emerged

in the mid- to late-1980s, together with the advent of the World Wide Web in early


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1990s.

The Internet Protocol Suite, like many protocol suites, may be viewed as a set of layers.Each layer solves a set of problems involving the transmission of data, and provides a

well-defined service to the upper layer protocols based on using services from somelower layers. Upper layers are logically closer to the user and deal with more abstract

data, relying on lower layer protocols to translate data into forms th at can eventually be

physically transmitted.

The TCP/IP model consists of four layers (RFC 1122). From lowest to highest, these arethe Link Layer, the Internet Layer, the Transport Layer, and the Application Layer.

OSI model:

The Open Systems Interconnection Reference Model also known to be 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. 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.

A layer is a collection of conceptually similar functions that provide services to the layerabove 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 applicationsabove it, while it calls the next lower layer to send and receive packets that make up thecontents of the path.

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 eachlayer, 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 functionalityprovided to an ( N)-layer by an (N-1) layer, where N is one of the seven layers of

protocols operating in the local host.

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02-02-2009

Layers in the TCP/IP Suite:

The TCP/IP suite uses encapsulation to provide abstraction of protocols and services.Such encapsulation usually is aligned with the division of the protocol suite into layers ofgeneral functionality. In general, an application (the highest level of the model ) uses a

set of protocols to send its data down the layers, being further encapsulated at each

level.

This may be illustrated by an example network scenario, in which two Internet hostcomputers communicate across local network boundaries constituted by their

internetworking gateways (routers).


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The functional groups of protocols and methods are the Application Layer, the TransportLayer, the Internet Layer, and the Link Layer (RFC 1122). It should be noted that thismodel was not intended to be a rigid reference model into which new protocols have tofit in order to be accepted as a standard.

y Application Layer:

DNS, TFTP, TLS/SSL, FTP, Gopher, HTTP, IMAP, IRC, NNTP, POP3, SIP,SMTP,SMPP, SNMP, SSH, Telnet, Echo, RTP, PNRP, rlogin, ENRPRouting protocols like BGP and RIP which run over TCP/UDP, may also be

considered part of the Internet Layer.y Transport Layer:

TCP, UDP, DCCP, SCTP, IL, RUDP, RSVPy Internet Layer:

IP (IPv4, IPv6) ICMP, IGMP, and ICMPv6

OSPF for IPv4 was initially considered IP layer protocol since it runs per IP-subnet, but has been placed on the Link since RFC 2740.

y Link Layer:

ARP, RARP, OSPF (IPv4/IPv6), IS-IS, NDP

Different authors have interpreted the RFCs differently regarding whether the Link Layerand the four-layer TCP/IP model covers physical layer issues or a hardware layer is

assumed below the link layer. Some authors have tried to use other names for the link

layer, such as Network interface layer, in effort to avoid confusion with the Data linklayer of the seven-layer OSI model. Others have attempted to map the Internet Protocol

model onto the seven-layer OSI Model. The mapping often results in a five-layer TCP/IP

model, wherein the Link Layer is split into a Data Link Layer on top of a Physical Layer.Especially in literature with a bottom-up approach to computer networking, where

physical layer issues are emphasized, an evolution towards a five-layer Internet model

can be observed out of pedagogical reasons.

The Internet Layer is usually directly mapped to the OSI's Network Layer. At the top ofthe hierarchy, the Transport Layer is always mapped directly into OSI Layer 4 of the

same name. OSIs Application Layer, Presentation Layer, and Session Layer are collapsed

into TCP/IP's Application Layer. As a result, these efforts r esult in either a four- or five-layer scheme with a variety of layer names. This has caused considerable confusion in

the application of these models. Other authors dispense with rigid pedagogy[17]focusing instead on functionality and behavior.

The Internet protocol stack has never been altered by the Internet Engineering TaskForce (IETF) from the four layers defined in RFC 1122. The IETF makes no effort to

follow the seven-layer OSI model and does not refer to it in standards-track protocol

specifications and other architectural documents. The IETF has repeatedly stated thatInternet protocol and architecture development is not intended to be OSI -compliant.

Description of OSI layers:

Layer 1: 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


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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 toreceive from it (in most cases it does not tell the device how to connect to the medium).

Standards such as RS-232 do use physical wires to control access to the medium.

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 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.

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 DataLink 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 multi access media.

Layer 3: 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 layersending data

throughout the extended network and making the Internet possible. This is a logical


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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 theymay be discarded. When the medium of the next hop cannot accept a packet in its

current length, IP is responsible for fragmenting the packet into sufficiently small

packets that the medium can accept.

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.

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).[citation

needed]

Layer 5: 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 madethis 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 6: 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.

This layer provides independence from differences in data representation (e.g.,

encryption) by translating from application to network format, and vice versa. The

presentation layer works to transform data into the form that the application layer can

accept. This layer formats and encrypts data to be sent across a network, providing

freedom from compatibility problems. It is sometimes called the syntax layer.

Layer 7: Application Layer:


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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 communicationpartners, 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).

General 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.[6] RFC 3439 contains a section entitled "Layeringconsidered harmful." However, TCP/IP does recognize four broad layers of functionality

which are derived from the operating scope of their contained protocols, namely thescope of the software application, the end-to-end transport connection, theinternetworking 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 ApplicationLayer 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 (InternetLayer) 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 arebased 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 routingprotocols (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 oreven an Application Layer protocol in its own right.

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

TCP/IP Model Vs OSI ModelThis article is on TCP/IP model vs OSI model. It is meant to highlight the differences between the two set standards of

the industry. TCP/IP model and the OSI model have been the two protocol suites on which communication industryheavily relies on.


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Both, TCP/IP model and OSI model, work in very similar fashions. But they do have very subtle differences.Knowing these differences is crucial to learning computer networking. This article will try to show the comparisonbetween TCP/IP model vs OSI model.

ABackgroundOSI reference model came into existence way before TCP/IP model was created. Advance research project

agency (ARPA) created OSI reference model so that they can logically group the similarly working components ofthe network into various layers of the protocol. But after the advent of the Internet, there arose the need for astreamlined protocol suite, which would address the need of the ever growing Internet. So the Defense AdvancedResearch Project Agency (DARPA), decided to create TCP/IP protocol suite. This was going to address many, ifnot all the issues that had arisen with OSI reference model.

TCP/IP Model Layers ExplainedSo, what does TCP/IP stand for? It is a suite of protocol which is named after its most significant pair of protocols.That is Transmission Control Protocol and Internet Protocol. TCP/IP are are made up of layers. Each layer isresponsible for a set of computer network related tasks. Every layer provides service to the the layer above it.There are in all four layers in the TCP/IP reference model.

y Application Layer: This is the topmost layer of the TCP/IP suite. This isresponsible for coding of the packet data.

y Transport layer: This layer monitors end to end path selections of the

packets. It also provides service to the application layer.y Internet Layer: This layer is responsible for sending packets through

different networks.

y Link Layer: It is the closest layer to the network hardware. It providesservice to Internet layer.

OSI Model Layers ExplainedIn OSI reference model there seven layers of protocols. Again, in OSI reference model, each layer providesservices to the layer above it. There are in all seven layers of in OSI. They are

y Physical Layer: It is the lower most layer of the OSI reference model. It islayer which is responsible for direct interaction of OSI model with hardware.The hardware provides service to the physical layer and it provides serviceto the datalink layer.

y Datalink Layer: There may be certain errors, which may occur at physicallayer. If possible, these errors are corrected by datalink layer. Datalink layerprovides the way by which various entities can transfer the data to thenetwork.

y Network Layer: It does not allow the quality of the service to be degradedthat was requested by transport layer. It is also responsible for data transfersequence from source to destination.

y Transport Layer: The reliability of the data is ensured by the transport layer.It also retransmits those data that fail to reach the destination.

y Session Layer: The sessions layer is responsible for creating andterminating the connection. Management of such connection is taken careof by sessions layer.


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The OSI (open Systems interconnection) model was developed by the InternationalStandards Organization (ISO) in 1984 in an attempt to provide some standard to theway networking should work. It is a theoretical layered model in which the notion ofnetworking is divided into several layers, each of which define specific functions and

features. However it must be noted that this model only represents a general guidelinefor developing usable network interfaces and protocols. Sometimes it may become very

difficult to distinguish between each layer as some systems do not rigorously adhere to

the model. Despite all this, however the OSI model has earned the honour of being "themodel" upon which all good network protocols are based.

The OSI Model is based upon 7 layers (Application layer, Presentation Layer, Session

Layer , Transport Layer, Network Layer, Data Link Layer and the Physical layer). Eachone of these are used to perform particular tasks, which include "supporting end user

processes, encrypting the data as-well as providing transparent data transfer over a

network" (Croucher P, "Communications and Networks", 1999 British Librarypublication).

The architectural framework of each layer is implemented throughout with software and

hardware to establish how data can be transmitted transparently.

Figure 1 below highlights the responsibility of each layer according to the OSI model.

Layer Responsibility

Application

(Layer 7)

The application layer consists of application programs and user interfaces. It is at this layer that user's interac

systems that the model defines. It supports many features that allow exchange of information across users.

provides application services for transfer of e-mails, and other network software services. The applications that us

include FTP and Telnet which exist entirely on this layer. In Addition to this the layer focuses on a

"communication partners, quality of service, constraints on data syntax ,authentication and privacy".

guide.co.uk/tcp-ip)

Presentation

(Layer 6)

The purpose of the presentation layer is to represent data in such a form that it can be exchanged through a net

example of this would be encrypting the data. This layer is also referred to as the syntax layer.

Session

(Layer 5)

The Session layer is used for establishing, managing and terminating connections on a network. Additionally

concerned with setting up, coordinating, terminating and managing conversations between the user and netw

level that the user and machine names are interpreted.

Transport

(Layer 4)

The Transport layer controls the quality and reliability of the data transmission. At this layer each data packet i

acknowledged . In addition to this layer also provides transparent transfer of data between end users and th

responsible for end to end error recovery and flow control.

Network

(Layer 3)

The network layer is used for routing data across configured network nodes.

This layer uses many technologies to enable the data to be transmitted which are known

as switching and routing. These technologies create logical (known as virtual circuits)

paths for transmitting data across a network from a node to node transmission.

Additionally other functions of this layer include addressing, error handling,

congestion control and finally packet sequencing.


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Data Link

(Laye 2)

The Data link laye is primarily c ncerne with packaging and un-packaging data packets s that they can b

dec ded into bits which will enable the data packets to be transmitted through a network. The Data Link la

transmission protocol bysupporting error handling in the physical layer and corruption (through check summin

is also known that the Data Link Layer is divided in to two sub-layers; these are known as The Media Access C

Logical Link Control Layer. The MAC sub-layer controls the flow of how a computer sends data across a netw

network will receive it. Apart from this

it also gives permission to transmit data. Whilst the Logical Link Contrframesynchronization and flow control.

Physical

(Layer 1)

The Physical layer establishes the physical connection between a computer and the network. This layer also contr

transmission of information as it specifies the mechanical and electrical characteristics of the protocol in terms of

pin assignments and voltage levels.

The Transmission Control Protocol/Internet Protocol (TCP/IP) originated in the 1970s. It was initially

developed by the Department of Defence (DOD) in an attempt to connect a number ofdifferent networks

Today however it is morecommonly used as a standard communication protocol used in a number of

networks the most common of which is the Internet.

FIGURE 2

TCP/IP HISTORY


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As it is depicted in Figure 2 above, the TCP/IP model itself consists of four layers and each layer is responsible

for performing prescribed functions which will be discussed in detail below:

Application Layer - This layer is basically concerned with defining the protocols. It consists of application

programs and user interfaces. In practice it works by sending an unbroken "data stream" to the Transport layer

. At this stage the stream is broken into packets , each of which is "framed" with a TCP header (which contains

the sender's and recipient's addresses and error checking information). According to writers such as Buchanan

it supports a number of protocols such as"SMTP, FTP and Telnet".(Buchanan. W, "Distributed Systems and

Networks", McGraw- Hill, 2000)

Transport Layer - This layer is responsible for providing communication session management between host

computers. Generally speaking it defines the level of service and status of the connection used when

transporting the data. The main protocols that are used in this layer include TCP, UDP and RTP.

Internet Layer - This layer is basically concerned with the Packaging of data. Here the data is packaged into IP

datagram's. which contain the source and destination address information. This is what is used to forward the

datagram's between hosts and across networks and the, main protocols used in this layer are IP, ICMP, ARP as

well as RARP.

Network Interface Layer - This layer is responsible for specifying how the data will be sent through the

network physically. This includes assessing "how bits are electrically signalled by the hardware devices that

interface directly with a network medium" (Croucher P, "Communications and Networks", 1999

British Library publication). These devices generally include the types of caballing used such as the

coaxial cable or the twisted pair copper wire. Typical examples of protocols used throughout this layer are the

"Token Ring", "Ethernet" and FDDI.

TCP/IP and the OSI modelThere is brief discussion on mapping the TCP/IP model onto the OSI model. Since the

TCP/IP and OSI protocol suites do not match precisely, there is no one correct answer.

The following diagram attempts to show where various TCP/IP and other protocols

would reside in the original OSI model:

The OSI Model: Application

PresentationSessionTransport

NetworkData Link

PhysicalCommonly, the top three layers of the OSI model (Application, Presentation and Session)

are considered as a single Application Layer in the TCP/IP suite. Because the TCP/IPsuite has no unified session layer on which higher layers are built, these functions are


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typically carried out (or ignored) by individual applications.The most notable difference between TCP/IP and OSI models is the Application layer, as

TCP/IP integrates a few steps of the OSI model into its Application layer. A simplifiedTCP/IP interpretation of the stack is shown below

Application"layer 7"

e.g. HTTP, FTP, DNS4 Transport e.g. TCP, UDP, RTP, SCTP

3 NetworkFor TCP/IP this is the Internet Protocol (IP)

2 Data Linke.g. Ethernet, Token ring, etc.

1 Physical e.g. physical media, and encoding techniques.

Professor Kevin Roark, Computer Science Department (2)

The physical layerThe Physical layer describes the physical characteristics of the communication, such as

conventions about the nature of the medium used for communication (such as wires, fiber

optic links ect.), and all related details such as connectors, channel codes and modulation,and maximum distances. The Internet protocol suite does not cover the physical layer of

any network.

The data link layerThe data link layer specifies how packets are transported over the physical layer,

including the framing(i.e. the special bit patterns which mark the start and end of

packets). Ethernet, for example, includes fields in the packet header which specify which

machine or machines on the network a packet is destined for.

The network layerAs originally defined, the Network layer solved the problem of getting packets across a

single network. With the advent of the concept of internetworking, additional

functionality was added to this layer, such as getting data from the source network to the

destination network.

In the internet protocol suite, IP performs the basic task of getting packets of data from

source to destination.The transport layerThe protocols at the Transport layer can solve problems like reliability ("did the data

reach the destination?") and ensure that data arrives in the correct order. In the TCP/IPprotocol suite, transport protocols also determine which application any given data is

intended for.The dynamic routing protocols which technically fit at this layer in the TCP/IP Protocol

Suite (since they run over IP) are generally considered to be part of the Network layerTCP is a "reliable", connection-oriented, transport mechanism providing a reliable byte

stream, which makes sure data arrives complete, undamaged, and in order. TCP tries to

continuously measure how loaded the network is and throttles its sending rate in order to

avoid overloading the network. Furthermore, TCP will attempt to deliver all data

correctly in the specified sequence.UDP is a connectionless datagram protocol. It is a "best effort" or "unreliable" protocol -

not because it is particularly unreliable, but because it does not verify that packets have

reached their destination, and gives no guarantee that they will arrive in order. If an

Application requires these characteristics, it must provide them itself, or use TCP.

Professor Kevin Roark, Computer Science Department (3)

UDP is typically used for applications such as streaming media (audio and video, etc)

where on-time arrival is more important than reliability, or for simple query/response

applications like DNS lookups, where the overhead of setting up a reliable connection is


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disproportionately large.

T ppli i lThe Application layeris the layerthat most common network-aware programs interface

use in orderto communicate across a network with other programs. Processes that occurin this layer are application specific; data is passed from the network-aware program, in

the format used internally by this application, and is encoded into a standard protocol.

Some specific programs are considered to run in this layer. They provide services thatdirectly support user applications. These programs and their corresponding protocolsinclude: HTTP (The World Wide Web), FTP (File transport), SMTP (Email), SSH

(Secure remote login), DNS (Name IP Address lookups) and many others.

Once the data from an application has been encoded into a standard application layer

protocolit will be passed down to the nextlayer ofthe IP stack.

Atthe Transport Layer, applications will most commonly make use of TCP or UDP, and

are often associated with a well-known port number. The most common ports are listed

below:

File Transfer Protocol (FTP) on port 21

Secure Shell (SSH) on port 22

Telnet on port 23

Simple Mail Transport Protocol (SMTP) for outgoing e-mail on port 25

Domain Name System (DNS) lookups on UDP (or sometimes TCP) port 53

Dynamic HostConfiguration Protocol (DHCP) on ports 67 and 68

Gopher on port 70

Finger on port 79

HTTP on TCP port 80

POP3 read e-mail on port 110

Network News Transfer Protocol (NNTP) on port 119Network Time Protocol (NTP) on port 123

NetBIOS on port 139IMAP read e-mail on port 143

Simple network management protocol (SNMP) on port 161

HTTPS secure HTTP on port 443

IMAPS on port 993Universal Plug and Play (UPnP) on port 5000

IRC on port 6667

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ed and removed. (April 2011)

OSI model


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7. Application Layer

NNTP SIP SSI DNS FTP Gopher HTTP

NFS NTP SMPP SMTP SNMP Telnet

DHCP Netconf RTP (

ore)

6. Presentation Layer

MIME XDR TLS SSL

5. Session Layer

Na

ed Pipes NetBIOS SAP L2TP PPTP SPDY

4. Transport Layer

TCP UDP SCTP DCCP SPX

3. Network Layer

IP (IP 4, IP 6) ICMP IPsec IGMP IPX

AppleTalk

2. Data Link Layer

ATM SDLC HDLC ARP CSLIP SLIP GFP

PLIP IEEE 802.3 Fra

e Relay ITU-

T G.hn DLL PPP X.25 Network Switch

1. Physical Layer

EIA/TIA-232 EIA/TIA-449 ITU-T V-Series

I.430 I.431 POTS PDH SONET/SDH PON

OTN DSL IEEE 802.3 IEEE 802.11

IEEE 802.15 IEEE 802.16 IEEE 1394 ITU-


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T G.hn PHY USB Bluetooth Hubs

This box: viewtal edit

The Op i l (OSI model) was a product ofthe Open

Systems Interconnection effort atthe International Organization for Standardization. Itis a way of sub-dividing a communications systeminto smaller parts called layers. Similar

communication functions are grouped into logicallayers. A layer provides services to itsupperlayer while receiving services from the layer below. On each layer, aninstance

provides service to the instances atthe layer above and requests service from the layer below.

For example, a layerthat provides error-free communications across a network provides thepath needed by applications above it, while it calls the nextlowerlayerto send and receive

packets that make up the contents ofthat path. Two instances at one layer are connected by a

hori ontal connection on thatlayer.

Communication in the OSI-Model (Example with layers3 to 5)


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Contents

[hide]

y 1History

y 2Description of OSI layers

o 2.1Layer 1: Physical Layero 2.2Layer 2: Data Link Layer

2.2.1WAN Protocol architecture

2.2.2IEEE 802 LAN architecture

o 2.3Layer 3: Network Layer

o 2.4Layer 4: Transport Layer

o 2.5Layer 5: Session Layer

o 2.6Layer 6: Presentation Layer

o 2.7Layer 7: Application Layer

y 3Cross-layer functions

y 4Interfaces

y 5Examples

y 6Comparison with TCP/IPy 7See also

y 8References

y 9External links

[edit] History

Work on a layered model of network architecture was started and the International

Organi ation for Standardi ation (ISO) began to develop its OSI framework architecture.

OSI had two major components: an abstract modelof 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 downloadedfrom 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 andISO/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 ofCharles 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 entitiesimplement 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.


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20/29

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/ornetwork management protocols. Appropriate choicesfor 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 a transmission medium, such as a copper oroptical cable. This includes the layout ofpins, voltages, cablespecifications, hubs,

repeaters, network adapters, host bus adapters (HBA used in storage area networks) andmore.

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 asingle 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:

y Establishment and termination of a connection to a co unications ediu .

y Participation in the process whereby the communication resources are effectively shared

among multiple users. For example, contention resolution and flow control.

y Modulation, or conversion between the representation ofdigital data in user equipment

and the corresponding signals transmitted over a communicationschannel. 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 logicalSCSI 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 DataLink Layer. The same applies to other local-area networks, such as token ring, FDDI, ITU-

TG.hn and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE802.15.4.

[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 PhysicalLayer. 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 managementfunctions 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-PointProtocol (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


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acknowledgment is used at the Transport Layer by protocols such as TCP, but is still used inniches where X.25 offers performance advantages.

The ITU-TG.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 repeatSliding

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 maycorrect 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 forSDLC and HDLC, andderivatives of HDLC such as LAPB and LAPD.

[edit] IEEE 802 LAN architecture

Practical, connectionless LANs began with the pre-IEEE Ethernet specification, which is

the ancestor ofIEEE 802.3. This layer manages the interaction of devices with a sharedmedium, which is the function of a Media Access Control (MAC) sublayer. Above this

MAC sublayer is the media-independent IEEE 802.2Logical 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 LANprotocol, 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 differentnetwork, 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 NetworkLayer performs networkrouting functions, and might also perform fragmentation and

reassembly, and report delivery errors. Routers operate at this layersending data

throughout the extended network and making the Internet possible. This is a logicaladdressing 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 leastthree sublayers:

1. Subnetwork Access that considers protocols that deal with the interface to networks, such

as X.25


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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 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 reliabledelivery to a next hop, but only for the detection of erroneous packets so they may bediscarded. 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 groupmanagement, 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 theprotocol 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 keeptrack 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 errorsoccurred.

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, whichOSI 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 Na e TP0TP1TP2TP3TP4

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


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Error Recovery No Yes Yes Yes Yes

Reinitiate connection (if an excessive number ofPDUs are unacknowledged) No Yes No Yes No

Multiplexing and demultiplexing over a single

irtual 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 TransportLayer, such as carrying non-IP protocols such as IBM's SNA orNovell'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.

[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

forfull-duplex, half-duplex, orsimplex 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.

[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 translatingbetween 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]


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The original presentation structure used the basic encoding rules ofAbstract SyntaxNotation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to

an ASCII-coded file, orseriali ation ofobjects and otherdata structures from and toXML.

[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 layerinteracts with software applications that implement a communicating component. Such

application programs fall outside the scope of the OSI model. Application layer functionstypically include identifying communication partners, determining resource availability, and

synchronizing communication. When identifying communication partners, the applicationlayer 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:

y On OSI stack:

o FTAM File Transfer and Access Management Protocol

o X.400 Mail

o Co on anage ent infor ation protocol (CMIP)

y On TCP/IP stack:

o Hypertext Transfer Protocol (HTTP),

o File Transfer Protocol (FTP),

o Si j ple Mail Transfer Protocol (SMTP)

o Sik

ple Network Managek

ent Protocol (SNMP).

[edit] Cross-layer functions

There are some functions or services that are not tied to a given layer, but they can affectmore than one layer. Examples are

y security serl

ice (telecom m

unication)[3] as defined by ITU-T X.800 Recommendation.

y 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 Con n

onn

anagen

ent inforn

ation protocol (CMIP) and its corresponding service

con n on n anagen ent infor n ation sero

ice (CMIS), they need to interact with every layer in

order to deal with their instances.

y

MPLS operates at an OSI Model layer that is generally considered to lie between traditionaldefinitions 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.

y ARP is used to translate IPv4 addresses (OSI Layer 3) into Ethernet MAC addresses (OSI Layer

2)


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[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 are implementation-specific.

For example Microsoft Windows' Winsock, and Unix's Berkeley sockets and System VTransport 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 thenetwork protocol (Layer 3).

Interface standards, except for the Physical Layer to media, are approximate implementations

of OSI Service Specifications.

[edit] Examples

Layer

OSIprotocols

TCP/IPprotocols

Signali

ngSystem

7[6]

AppleTalk

IPX SNA UMTS Misc.exa ples

# Na

e

7Applicatio

n

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,

SAPAPPC

HL7,

Modbus

6Presentati

on

ISO/IEC 88

23, X.226,

ISO/IEC 95

76-1, X.236

MIME, SSL,

TLS, XDRAFP

TDI, ASCII,

EBCDIC,

MIDI, MPEG

5Session

ISO/IEC 83

27, X.225,

ISO/IEC 95

48-1, X.235

Sockets.

Session

establishm

ent in TCP,

RTP

ASP,

ADSP,

PAP

NWLin

kDLC?

Na

ed

pipes,

NetBIOS,

SAP, half

duplex, full

duplex,

si

plex,


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RPC

4Transport

ISO/IEC 80

73, TP0,

TP1, TP2,

TP3, TP4

(X.224),

ISO/IEC 86

02, X.234

TCP, UDP,

SCTP, DCCP

DDP,

SPXNBF

3Network

ISO/IEC 82

08, X.25

(PLP),

ISO/IEC 88

78, X.223,

ISO/IEC 84

73-1, CLNP

X.233.

IP, IPsec,

ICMP,

IGMP,

OSPF

SCCP,

MTP

ATP

(TokenT

alk or

EtherTal

k)

IPX

RRC (Radio

Resource

Control) Packet

Data

Con

ergence

Protocol (PDCP)

and BMC(Broadcast/Multi

cast Control)

NBF, Q.931,

IS-IS

Leaky

bucket,token

bucket

2Data Link

ISO/IEC 76

66, X.25

(LAPB),

Token Bus,

X.222,

ISO/IEC 88

02-2LLC

Type 1 and

2[9]

PPP, SLIP,

PPTP, L2TP

MTP,

Q.710

LocalTal

k,

AppleTal

k

Re

ote

Access,

PPP

IEEE

802.3

framin

g,

Ethern

et II

fra

in

g

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,

Fra

e

Relay,

HDLC, ISL,

PPP, Q.921,

Token Ring,

CDP, NDP

ARP (maps

layer 3 tolayer 2

address),

ITU-T G.hn

DLL

CRC, Bit

stuffing,

ARQ, Data


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O

er Cable

Ser

ice

Interface

Specificatio

n (DOCSIS)

1Physical

X.25

(X.21bis,

EIA/TIA-

232,

EIA/TIA-

449, EIA-

530,

G.703)[9]

MTP,

Q.710

RS-232,

RS-422,STP,

PhoneN

et

Twin

ax

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 O

er

Cable

Ser

ice

Interface

Specificatio

n (DOCSIS)

[edit] 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


28/29

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 oftunneling 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

Open Systems Interconnection ( OSI ) is a standard reference model for communication

between two end users in a network. The model is used in developing products and

understanding networks. Also see the notes below the figure.

Illustration republished with permission from The manual Page .


29/29

OSI divides telecommunication into seven layers. The layers are in two groups. The upperfour layers are used whenever a message passes from or to a user. The lower three layers are

used when any message passes through the host computer. Messages intended for thiscomputer pass to the upper layers. Messages destined for some other host are not passed up to

the upper layers but are forwarded to another host. The seven layers are:

Layer 7: The application layer ...This is the layer at which communication partners areidentified, quality of service is identified, user authentication and privacy are considered, and

any constraints on data syntax are identified. (This layer is notthe application itself, although

some applications may perform application layer functions.)

Layer 6: The presentation layer ...This is a layer, usually part of an operating system, that

converts incoming and outgoing data from one presentation format to another (for example,

from a text stream into a popup window with the newly arrived text). Sometimes called the

syntax layer.

Layer 5: The session layer ...This layer sets up, coordinates, and terminates conversations,

exchanges, and dialogs between the applications at each end. It deals with session and

connection coordination.

Layer 4: The transport layer ...This layer manages the end-to-end control (for example,

determining whether all packets have arrived) and error-checking. It ensures complete data

transfer.

Layer 3: The network layer ...This layer handles the routing of the data (sending it in the

right direction to the right destination on outgoing transmissions and receiving incoming

transmissions at the packet level). The network layer does routing and forwarding.

Layer 2: The data-link layer ...This layer provides synchronization for the physical leveland does bit-stuffing for strings of 1's in excess of 5. It furnishes transmission protocol

knowledge and management.

Layer 1: The physical layer ...This layer conveys the bit stream through the network at the

electrical and mechanical level. It provides the hardware means of sending and receiving data

on a carrier.

Documents

The Internet Protocol Suite Also Known as TCP