ISO-OSI.pdf

ISO-OSI 7-Layers Network Reference Model by MAZHAR IQBAL BUTT.1


THE OSI REFERENCE MODELThe International Organization for

Standardization (ISO) developed theOpen System Interconnection (OSI)

Reference Model to promotecommunication between dissimilar

computers as a guide for defining a setof open protocols.

This model has served as one of themost basic and essential, elements of

computer networking since 1984.


The OSI model provides a framework(functional guideline) for defining

standards to connect heterogeneouscomputers.

The OSI model solves the complicatedproblem of communication betweencomputer systems by dividing the

communication into sub-tasks (layers).Each task executes a specific function by

using the services of the task below itand providing services to above task.


OSI

LAYER MODEL

http://en.wikipedia.org/wiki/Image:Osi-model-jb.png


Most communication environmentsseparate the communication functions

and application processing. Thisseparation of networking functions is

called LAYERING.

OSI divides the big task of host-to-hostnetworking called INTERNETWORKING

into a vertical stack. This OSI stackcontains these seven numbered layers,

these indicate distinct functions.


The layered approach to OSI offersseveral advantages to system

implementers. By separating the job ofnetworking into logical smaller pieces,vendors can more easily solve network

"problems" through divide-and-conquer.

The OSI layers afford extensibility. Newprotocols and other network services willgenerally be easier to add to a layered

architecture.



Upper layers7. application6. presentation5. sessionLower layers4. transport3. network2. data link1. physical

Higher-level facilitiesApplicationPresentationSessionNetwork communicationTransportNetworkPhysical mediumData LinkPhysical


ADVANTAGES

•Divide the interrelated aspects ofnetwork operation into less complexelements.

•Define standard interfaces for "Plug andPlug" Compatibility and multi-vendorintegration.

•Divide the complexity ofinternetworking into Discrete, moreeasily learned operations.



APPLICATION LAYERThe application layer provides differentservices to user applications. It interfacesdirectly to these services and provideconversion between associated applicationprocesses, after that it issues requests to thepresentation layer. It contains a variety ofprotocols that are commonly needed.Another application layer function is filetransfer. Different file systems have differentfile naming conventions. Transferring a filebetween two different systems requireshandling all incompatibilities.


Application Layer 7 protocols

FTP File Transfer Protocol

TFTP Trivial File Transfer Protocol, a simple file transfer protocol

HTTP HyperText Transport Protocol, used in the World Wide Web

POP3 Post Office Protocol Version 3

SNMP Simple Network Management Protocol

SMTP Simple Mail Transfer Protocol

Telnet A remote terminal access protocol

Gnutella A peer-to-peer file-swapping protocol

DNS Domain Name Service

BOOTP Bootstrap Protocol

Rlogin A UNIX remote login protocol

MIME Multipurpose Internet Mail Extensions

AFP Apple Filing Protocol

SSH Secure SHell

IMAP Internet Message Access Protocol

NTP Network Time Protocol

Gopher A precursor of web search engines

NCP Netware Core Protocol

Finger Gives user profile information

NNTP News Network Transfer Protocol

LDAP Lightweight Directory Access Protocol

DHCP Dynamic Host Configuration Protocol

IRC Internet Relay Chat

Jabber An instant-messaging protocol

WebDAV Web Distributed Authoring and Versioning

DICT Dictionary protocol

BACnet Building Automation and Control Network protocol

RADIUS An authentication, authorization and accounting protocol

DIAMETER An authentication, authorization and accounting protocol

SIP A signaling protocol


PRESENTATION LAYER

This layer provides data representation and codeformatting. It provides a uniform means for a remotehost to inform the local client how to present the datato the application or client.

It ensures that the data that arrives from the networkcan be used by the application, and it ensures thatinformation send by the application can be transmittedon the network.

In some cases, the presentation layer directlytranslates data from one format to another. Otherfunctions that may correspond to the presentationlayer are data encryption/decryption andcompression/decompression.


The presentation layer is responsible for the deliveryand formatting of information to the application layerfor further processing or display. It relieves thesyntactical differences in data representation withinthe end-user systems.

Note: An example of a presentation service would bethe conversion of an EBCDIC-coded text file to anASCII-coded file.

In many widely used applications and protocols, nodistinction is made between the presentation andapplication layers. For example, HTTP is an applicationlayer protocol, also identify character encoding forproper conversion in the presentation layer.


SESSION LAYERThe session layer establishes, manages andterminates session between application. Itresponds to service requests from thepresentation layer and issues service requeststo the transport layer.It provides the mechanism for managing end-user, host, application process operations andestablishes check pointing, termination, andrestart procedures.This layer is responsible for dialog controlbetween nodes. Dialog is a formal conversationin which two nodes agree to exchange data.


The communications between applicationsacross a network can take place and controlledin one of three dialog modes, are Simplex,Half-duplex, and Full-duplex.Sessions canallow traffic to go in both directions at thesame time, or in only one direction at a time.Testing for out-of-sequence packets is handledalso.

Sessions, enable nodes to communicate in anorganized manner. Each session has threephases as Connection establishment, Datatransfer, Connection release.


Session Layer 5 protocols

NCP Network Control Protocol

SMB Server Message Block

NFS Network File System

ASP AppleTalk Session Protocol

ADSP AppleTalk Data Stream Protocol

DLC Data Link Control

NP Named Pipes

NetBIOS

NWLink

PAP Printer Access Protocol

ZIP Zone Information Protocol



TRANSPORT LAYER

It translates system names into addresses anddivides messages into fragments that fit withinthe size limitations established by the network.This layer is responsible for addressing,determining routes for sending, managingnetwork traffic problems, packet switching,routing, data congestion, and reassemblingdata at the receiving end to recover theoriginal message.

Ensure that segments delivered will beacknowledge back to sender.


It provides retransmission of segments those

are not acknowledged. Put segments back into

their correct sequence at the destination.

To enable packets reassembly in their original

order, this layer includes a messages sequence

number in its header.

It allows users to segment/reassemble several

upper layer application on to the same

transport data stream. It insures host-to-host

session connection.


In some protocol suites this layer also providesmultiple conversations (multiplex) between thesame two end-points. Delivery may be'guaranteed' (streams, like TCP) or 'best effort'(datagram, like UDP).

The transport layer also is responsible fordelivering messages from a specific process onone computer to the corresponding process onthe destination computer.

In OSI terms the transport layer assigns aservice access point (SAP ID) to each packet.In TCP/IP terms that is called a port.


One more responsibility of the transport layeris detecting errors in transmitting data.

Two general categories of error detection canbe performed by the transport layer:

Reliable delivery: Reliable delivery does notmean that errors cannot occur, only that errorsare detected if they do occur.

Unreliable delivery: Unreliable delivery doesnot mean that errors are likely to occur, butrather indicates that the transport layer doesnot check for errors.


Transport Layer 4 protocols

SPX Sequenced Packet Exchange

TCP Transmission Control Protocol

UDP User Datagram Protocol

SCTP Stream Control Transmission Protocol

RTP Real-time Transport Protocol


NETWORK LAYER

The network layer operates independently ofthe physical medium, which is a concern of thephysical layer.Since routers are network layerdevices, they can be used to forward packetsbetween physically different networks.

When the network layer receives a messagefrom upper layers, the network layer adds aheader to the messages that includes thesource and destination network address. Thiscombination of data plus the network layer iscalled a packet.


It translates logical addresses and names into

physical addresses for Packets delivery.

The addressing which makes this delivery

possible can be thought of as a universal

address as compared to the local addressing of

the MAC.

It determines routes for sending, managing

network traffic problems, packet switching,

routing, data congestion, and reassembling

data.


This layer may also be concerned with end-to-

end flow control and the segmentation and

reassembly (SAR) functions for the

transmission of large packets of data which

exceed the maximum transmission unit (MTU)

of the data link layer.

The Internet Protocol (IP) is the most common

example, and Novell's Internetwork Packet

eXchange (IPX) is another popular

implementation.


Network Layer 3 protocols

IP Internet Protocol version 4

IPv6 Internet Protocol version 6

ARP Address Resolution Protocol

RARP Reverse Address Resolution Protocol

ICMP Internet Control Message Protocol

IGMP Internet Group Management Protocol

MPLS Multi-Protocol Label Switching

RIP Routing Information Protocol

OSPF Open Shortest Path First

EGP Exterior Gateway Protocol

BGP Border Gateway Protocol


DATA LINK LAYERThe data link layer sends data from network layer tophysical layer. Manages physical layer communicationsbetween connecting systems. It is responsible forproviding node-to-node communication on a single,local network.To provide this service, the data link layer mustperform two functions.

1.It must provide an address mechanism thatenables messages to be delivered to the correctnodes, Also, it must translate messages from upperlayer into bits that the physical layer can transmit.

2.Data units at the Data Link layer are mostcommonly called frames, although the term packetis used with some protocols.


The Data Link layer is comprised of a single layer in theOSI model but actually incorporates two sub-layerswhen viewed from the perspective of the IEEE LogicalLink Control (LLC) model. The lower of the two sub-layers is called the media access control (MAC) and theupper sub-layer is the LLC layer.The MAC layer provides local addressing, errorcorrection, and in some cases includes a field todistinguish the frame type from other frame types.This component determines who is allowed to accessthe media at any one time.The LLC provides Service Access points (SAP) forpassing off the frame to higher level. This componentdetermines where one frame of data ends and the nextone starts. Examples of data link standards includeframe relay, ETHERNET, token ring, and FDDI.


Data link Layer 2 protocols

ARP Address Resolution Protocol

CDP Cisco Discovery Protocol

DCAP Data Link Switching Client Access Protocol

Ethernet

FDDI Fiber Distributed Data Interface

HDLC High Level Data Link Control

L2F Layer 2 Forwarding Protocol

L2TP Layer 2 Tunneling Protocol

PPP Point-to-Point Protocol

PPTP Point-to-Point Tunneling Protocol

SLIP Serial Line Internet Protocol

Token ring

X.25

Frame relay A simplified version of X.25

ATM Asynchronous Transfer Mode

MPLS Multi-Protocol label switching


PHYSICAL LAYER

This transmits data over a physical medium or onnetwork hardware, like cables, cards, etc. This layerrelates the electrical, optical, mechanical andfunctional interface to the cable. It is responsible fortransmitting bits (zero & ones) from one computer toanother.

This layer is concerned with the signaling of themessage and the interface between the sender orreceiver and the medium. It is defined by one of thestandards bodies and carries a designation thatindicates the characteristics of the connection. Amongfrequently used physical layers standards are EIA-232-D, ITU V.35, and some of the X series (X.21/X.21bis).


Physical Layer 1 protocols

ISDN Integrated Services Digital Network

PDH Plesiochronous Digital Hierarchy, T-carrier (T1, T3 etc)

RS-232 A serial line interface originally developed to connectmodems and computer terminals

SDH Synchronous Digital Hierarchy

SONET Synchronous Optical NETworking



MNEMONICS

The following mnemonics may help youremember the layers sequence andnames:

1."People Design Networks To SendPackets Accurately"

2."Please Do Not Take Sales People'sAdvice"

3."Always Put Some Thought IntoDesigning Programs"

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ISO-OSI.pdf