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OSI Reference ModelOSI Reference Model Hierarchical Software LayersHierarchical Software Layers

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The ISO/OSI Reference Model

Developed by the International Standards Organization Developed by the International Standards Organization (ISO) to facilitate the international standardization of (ISO) to facilitate the international standardization of communications protocolscommunications protocols

For U.S.: ANSI (American National Standard Institute) - For U.S.: ANSI (American National Standard Institute) - www.ansi.orgwww.ansi.org

OSI is ISO's Basic Reference Model for Open Systems OSI is ISO's Basic Reference Model for Open Systems Interconnect (hence: ISO/OSI)Interconnect (hence: ISO/OSI)

The Reference Model itself is The Reference Model itself is notnot a Network Architecture a Network Architecture (does not specify any protocols or services)(does not specify any protocols or services)

ISO also developed Network Architecture standardsISO also developed Network Architecture standards

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The ISO/OSI Reference Model The model describes computer communications protocols in The model describes computer communications protocols in

a general sense to facilitate discussiona general sense to facilitate discussion No assumptions are made regarding:No assumptions are made regarding:

Programming language bindingsProgramming language bindings Operating system bindingsOperating system bindings Applications programming interfacesApplications programming interfaces

Development of the model started in the mid-1970’sDevelopment of the model started in the mid-1970’s Biggest ProblemsBiggest Problems

Very long time to complete the model and protocol standardsVery long time to complete the model and protocol standards Very hard to understand the detailed standardsVery hard to understand the detailed standards Difficult (expensive) to get the standards documentsDifficult (expensive) to get the standards documents

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ISO/OSI 7-Layer Reference Model

Presentation

Session

Transport

Network

Data Link

Physical

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

Network

Data Link

Physical

Physical Communications Media

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6

5

4

3

2

1

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ISO/OSI Reference Model - Why 7 Layers? One layer for each level of abstractionOne layer for each level of abstraction Each layer performs (ideally) a limited, well defined Each layer performs (ideally) a limited, well defined

functionfunction Functions for each layer are selected with Functions for each layer are selected with International International

StandardizationStandardization as a goal as a goal Layer boundaries are chosen to minimize information Layer boundaries are chosen to minimize information

crossing the interfacecrossing the interface Want to keep the model manageable (5 would have been Want to keep the model manageable (5 would have been

nice) but not have to jumble together distinct functionsnice) but not have to jumble together distinct functions

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OSI Layer 1 - Physical layer Primary function is transmitting raw bits over a physical Primary function is transmitting raw bits over a physical

communications channelcommunications channel Primary design issues include: mechanical, electrical, Primary design issues include: mechanical, electrical,

coding, physical characteristicscoding, physical characteristics How many pins in the connectorHow many pins in the connector what voltage represents a “1” versus a “0”what voltage represents a “1” versus a “0” etc.etc.

By “raw bits” we mean there is no interpretation of the bits By “raw bits” we mean there is no interpretation of the bits - stream of bits in and bits out- stream of bits in and bits out

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OSI Layer 2 - Data Link layer Primary function is to make Layer 1 into what appears to be a Primary function is to make Layer 1 into what appears to be a

channel channel free of undetected errorsfree of undetected errors Deals with data in chunks (typically 100s-1000s of bytes) Deals with data in chunks (typically 100s-1000s of bytes)

generally called generally called FramesFrames This layer must create/recognize frame boundariesThis layer must create/recognize frame boundaries

remember - Physical layer does not careremember - Physical layer does not care often requires special bit patterns to signal boundariesoften requires special bit patterns to signal boundaries may have to deal with possibility of pattern appearing in may have to deal with possibility of pattern appearing in

datadata

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OSI Layer 2 - Data Link layer

Among the key issues dealt with are:Among the key issues dealt with are: Error handling (e.g. corrupted frame)Error handling (e.g. corrupted frame) Flow controlFlow control Providing various qualities of serviceProviding various qualities of service

For For BroadcastBroadcast networks, a key issue is controlling access networks, a key issue is controlling access to the channel:to the channel: Use a sub-layer called the Use a sub-layer called the Media Access ControlMedia Access Control

(MAC) sub-layer(MAC) sub-layer

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OSI Layer 3 - Network layer Primary function is to control the operation of the layers belowPrimary function is to control the operation of the layers below Among the key issues dealt with are:Among the key issues dealt with are:

Routing packets from source to destination through the Routing packets from source to destination through the network (or multiple networks) using static or dynamic network (or multiple networks) using static or dynamic routing algorithmsrouting algorithms

Controlling congestion in the networkControlling congestion in the network Accounting functions (for billing)Accounting functions (for billing) Translating between protocols across heterogeneous Translating between protocols across heterogeneous

networksnetworks Concerned with AddressingConcerned with Addressing

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OSI Layer 4 - Transport layer

First First end-to-endend-to-end layer layer Uses the network to (most often) provide higher layers Uses the network to (most often) provide higher layers

with a connection oriented, reliable, error free channel that with a connection oriented, reliable, error free channel that delivers messages (or byte stream) in orderdelivers messages (or byte stream) in order

May provide other types of servicesMay provide other types of services Often performs multiplexing of multiple transport Often performs multiplexing of multiple transport

connections over one or more network connectionsconnections over one or more network connections Generally requires Address (or naming)Generally requires Address (or naming) May also perform flow control May also perform flow control

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OSI Layer 5 - Session layer

Sort of an Sort of an unwanted layer,unwanted layer, this layer is usually this layer is usually very very thinthin and little more than a pass through for and little more than a pass through for most protocolsmost protocols

Manages dialog control (e.g. may manage who’s Manages dialog control (e.g. may manage who’s turn it is to talk in a high-level half-duplex turn it is to talk in a high-level half-duplex protocol)protocol)

Manages synchronization of transactions which Manages synchronization of transactions which may need to be able to may need to be able to roll backroll back in case of a crash in case of a crash

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OSI Layer 6 - Presentation layer Rather than being concerned with Rather than being concerned with movingmoving

information, the Presentation layer is concerned information, the Presentation layer is concerned with the with the interpretation of information interpretation of information representationrepresentation

Ensures that the Ensures that the syntaxsyntax and meaning is the same and meaning is the same for each participant in a communicationfor each participant in a communication

Provides for standard representation and may Provides for standard representation and may provide capabilities for conversion of dataprovide capabilities for conversion of data

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OSI Layer 7 - Application layer The layer where end-user applications liveThe layer where end-user applications live This is the highest level of abstraction and the This is the highest level of abstraction and the

level which is of primary importance (for most level which is of primary importance (for most users)users)

All the rest of the layers exist to support these All the rest of the layers exist to support these applicationsapplications

Layering exists so we can move these around to Layering exists so we can move these around to different machines, and so they can communicate different machines, and so they can communicate across any platforms - across any platforms - Open Systems InterconnectOpen Systems Interconnect

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Review - Functions of the OSI LayersLayer 1 (physical): Transmission of bitsLayer 1 (physical): Transmission of bitsLayer 2 (data link): Transmission of Layer 2 (data link): Transmission of

frames on one given linkframes on one given linkLayer 3 (network): Routing of packets Layer 3 (network): Routing of packets

through the networkthrough the networkLayer 4 (transport): End-to-end delivery Layer 4 (transport): End-to-end delivery

of messagesof messages

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Review - Functions of the OSI Layers

Layer 5 (session): Setup and Layer 5 (session): Setup and management of end-to-end management of end-to-end conversation, synchronizationconversation, synchronization

Layer 6 (presentation): Formatting, Layer 6 (presentation): Formatting, encryption, and compression of dataencryption, and compression of data

Layer 7 (application): user applicationsLayer 7 (application): user applications

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TCP/IP Protocol Suite Advanced Research Project Agency Advanced Research Project Agency

(ARPA) of DoD sponsored the (ARPA) of DoD sponsored the development of ARPANET in 1970s. development of ARPANET in 1970s.

TCP/IP has been adopted as the ARPANET TCP/IP has been adopted as the ARPANET protocol suiteprotocol suite

TCP/IP became popular by the inclusion of TCP/IP became popular by the inclusion of this protocol in BSD Unix system (a version this protocol in BSD Unix system (a version of Unix developed by University of of Unix developed by University of California @ Berkley)California @ Berkley)

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TCP/IP (cont.)

Transport Layer-TCP (Transmission Transport Layer-TCP (Transmission Control Protocol)Control Protocol)

• Provides fully reliable, connection-oriented Provides fully reliable, connection-oriented serviceservice

• Byte-stream transmissionByte-stream transmission

Network Layer- IP (Internet Protocol)Network Layer- IP (Internet Protocol)• IP provides datagram service (used in packet IP provides datagram service (used in packet

switching)switching)

• It is connectionless unreliable serviceIt is connectionless unreliable service

• IP handles routingIP handles routing

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TCP/IP suite and OSI 7 Layer Model

Application

Presentation

Session

Transport

Network

Data Link

Physical

ApplicationProcess

TCP

IP

CommunicationNetwork

Most people define TCP/IP as a 5 layer protocol dividing the Communication layer into 2 separate layers (Network & Physical)

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hardwareinterface

TCP

IP

UDP

userprocessor

userprocessor OSI Layer 5-7

OSI Layer 4

OSI Layer 3

OSI Layer 1-2

ARPICMP RARP

UDP: User Datagram Protocol: ICMP: Internet Control Message ProtocolIP: Internet ProtocolARP: Address Resolution ProtocolRARP: Reverse ARP

Network Structure

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Port Numberuserproc.A

userproc.B

userproc.C

userproc.D

userproc.E

userproc.F

userproc.G

IDPIP

SPPPEXTCPUDP

Ethernetinterface

Ethernetinterface

Ethernetinterface

Ethernetinterface

TCP/IPprotocol suite

XNSprotocol suite

Ethernet cable 2

Ethernet cable 1

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Hierarchical Addressing Scheme Connection defines the communication Connection defines the communication

link between two processeslink between two processes

data

dataUDPheader

dataUDPheader

IPheader

UDPheader dataIP

headerEthernettrailer

Ethernetheader

16-bit UDP source port #16-bit UDP dest. port #

protocol = UDPinternet 32-bit source addrinternet 32-bit dest. addr

frame type = IPEthernet 48-bit source addrEthernet 48-bit dest. addr

Ethernet frame

UDP = User Datagram Protocol