Chapter 2

The OSI Model and TCP/IP Protocol Suite

Outlineo THE OSI MODELo LAYERS IN THE OSI MODELo TCP/IP PROTOCOL SUITEo ADDRESSINGo TCP/IP VERSIONS

THE OSI

MODEL

2.12.1

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Figure 2-1

OSI Model

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Layered Architectureo Within a single machinen Each layer calls upon the services of the layer just

below itn For example, layer 3 uses the services provided

by layer 2 and provides services for layer 4

Layered Architecture (Cont.)o Between machinesn Layer x on one machine communicates with layer

x on another machine

n This communication is governed by a series of rules and conventions called protocols

Layered Architecture (Cont.)o As the message travels from A to B, it may

pass through intermediate nodesn These intermediate nodes usually involve only

the first three layers of the OSI model

n As shown in the next slide

Figure 2-2

OSI Layers

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Peer-to-Peer Processes

o The processes on each machine that communicate at a given layer are called peer-to-peer processes

o Each layer in the sending device added its own information to the message it receives from the layer just above it

o At the receiving machine, the message is unwrapped layer by layer

Headers are added to the data at layers

6, 5, 4, 3, and 2. Trailers are usually

added only at layer 2.

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Figure 2-3

An Exchange Using the OSI Model

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LAYERS IN

THE OSI

MODEL

2.22.2

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Figure 2-4

Physical Layer

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Physical Layero Physical characteristics of interface and median Interface between the devices and transmission

median The type of transmission media

o Representation of bitsn Define the type of encoding (how 0s and 1s are

changed to signals)

Physical Layer (Cont.)

o Data raten The number of bits sent each second

o Synchronization of bitsn The sender and receiver must not only use the

same bit rate but must also be synchronized at the bit level

Physical Layer (Cont.)o Line configuration

n Be concerned with the connection of devices to the media

n Point-to-point configurationo Two devices are connected through a dedicated link

n Multipoint configurationo A link is shared between several devices

o Physical topologyn Define how devices are connected to make a networkn For example, mesh, star, ring, or bus topology

Physical Layer (Cont.)o Transmission mode

n Define the direction of transmission between two devicesn Simple mode

o only one device can send

n Half-duplex modeo two devices can send and receive but not at the same time

n Full-duplex modeo Two devices can send and receive at the same time

Data Link Layero Transform the physical layer to a reliable link

o Make the physical layer appear error free to the upper layer

Figure 2-5

Data Link Layer

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Data Link Layero Framingn Divide the stream of bits into frames

o Physical addressingn only one hop

o Flow controlo Error controln Detect and retransmit damaged or lost framesn Prevent duplication of frames

Data Link Layer (Cont.)o Access controln Determine which device has control over the

link at any given time

Figure 2-6

Node-to-Node (Hop-to-Hop) Delivery

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Network Layero Be responsible for the source-to-destination

delivery of a packet possibly across multiple networks (links)n Data link layer take care of the delivery of the

packet between two systems on the same network (links)

n Network layer ensures the delivery of a packet from its point of origin to it final destination

Figure 2-7

Network Layer

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Network Layero Logical addressingn Physical addressing in the data link layer handles

the addressing problem locallyn If a packet pass the network boundary

o We need another addressing system to distinguish the source and destination systems

o Routing

Figure 2-8

End-to-End Delivery by the Network Layer

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Transport Layero Be responsible for source-to-destination (end-

to-end) delivery of the entire messagen Network layer only oversees the end-to-end

delivery of individual packetso Oversee both error control and flow control at

the source-to-destination level

Figure 2-9

Transport Layer

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Transport Layero Service-point addressing

n Service-point address: port addressn The network layer gets each packet to the correct

computern The transport layer gets the entire message to the correct

process on that computero Segmentation and reassembly

Transport Layer (Cont.)o Connection control

n Connectionlessn Connection-oriented

o Flow controln End to end flow controln The flow control, however, at the data link layer is across

a single link

Transport Layer (Cont.)o Error control

n End to end error control that makes sure that the entire message arrives without error (damage, loss, or duplication)

n Error correction is usually achieved through retransmission

n The error control, however, at the data link layer is across a single link and applies to a single frame

Figure 2-10

Reliable End-to-End Delivery of a Message

end-to-end delivery by the transport layer

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Session Layero Session layer is the network dialog controllern It establishes, maintains, and synchronizes the

interaction between communication systems

Figure 2-11

Session Layer

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Session Layero Dialog controln Allow two systems to enter a dialogn Allow the communication between two processes

ino Half-duplexo Full-duplex

o Synchronizationn Allow a process to add checkpoints

(synchronization points) into a stream of data

Presentation Layero Be concerned with the syntax and semantics

of the information exchanged between two systems

Figure 2-12

Presentation Layer

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Presentation Layero Translationn Different computers use different encoding

systems n The presentation layer is responsible for

interoperability between these different encoding methods

o Encryptiono Compression

Application Layero The application layer enables the user,

whether human or software, to access the network

Figure 2-13

Application Layer

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Application Layero Specific services provided by the application

layern Network virtual terminaln File transfer, access, and management (FTAM)n Mail servicesn Directory services

Figure 2-14

Summary of Layers

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TCP/IP PROTOCOL

SUITE

2.32.3

TCP/IP Protocol Suiteo TCP/IP protocol suite was developed prior to

the OSI model and is made of five layersn Physical, data link, network, transport, and

application layerso The first four layers correspond to the first

four layer of the OSI model

TCP/IP Protocol Suiteo The three topmost layers is OSI model are

represented in TCP by a single layer called the application layer

Figure 2-15

TCP/IP and OSI Model

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Physical and Data Link Layerso TCP/IP does not define any specific protocolo TCP/IP supports all of the standard and

proprietary protocols

Network Layero Internetworking Protocols (IP)o Four supporting protocolsn ARPn RARPn ICMPn IGMP

Internetworking Protocol (IP)o The packets in IP are called datagramso Unreliable and connectionless datagram

protocolo Best-effort delivery servicesn Provide no error checking or tracking

Address Resolution Protocol (ARP)o Associate an IP address with the physical

addressn Data in LAN are transmitted by the physical

address

Reverse Address Resolution Protocol (RARP)o Discover a host’s internet address via its

physical addresso Used whenn A computer is first connected to the networkn When a diskless computer is booted

Internet Control Message Protocol (ICMP)o A mechanism used to send notification of

datagram problems back to the sendero ICMP sends query and error reporting

messages

Internet Group Message Protocol (IGMP)o Facilitate the simultaneous transmission of a

message to a group of recipients

Transport Layero TCP/UDPn A process-to-process protocol

o IPn A host-to-host protocol

User Datagram Protocol (UDP)o Add only n Port numbersn Checksum error controln length

Transmission Control Protocol (TCP)o Reliable connection-oriented protocol

ADDRESSING

2.42.4

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Figure 2-16

Addresses in TCP/IP

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Figure 2-17

Relationship of Layers and Addresses in TCP/IP

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Physical Addresso Also called link address, the address of a node as

defined by its LAN or WANn Ethernet: 6 byten LocalTalk: 1 byte

o Physical address can be eithern Unicast addressn Multicast addressn Broadcast address

Example 1Example 1

Figure 2.18 shows an example of physical addresses.

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Figure 2-18

Physical Addresses

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Example 2Example 2

Most local area networks use a 48-bit (6 bytes) physical address written as 12 hexadecimal digits, with every 2 digitsseparated by a hyphen as shown below:

07-01-02-01-2C-4B A 6-byte (12 hexadecimal digits) physical address

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Internet Addresso Provide universal communication services that

are independent of underlying physical networkn Different networks can have different address formatn A universal addressing system is thus needed

o Internet address can also be either in unicast, multicast and broadcast

o 4 bytes long in IPv4

Example 3Example 3

Next slide shows an example of Internet addresses. o Network address A and physical address 10 =>

send data to => Network address P and physical address 95

o IP address does not change along the tripo However, physical address changes from

network to network

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Figure 2-19

IP Addresses

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Example 4Example 4

As we will see in Chapter 4, an Internet address (in IPv4) is 32 bits in length, normally written as four decimal numbers, with each number representing 1 byte. The numbers are separated by a dot. Below is an example of such an address.

132.24.75.9The McGraw-Hill Companies, Inc., 2000

Port Addresso A label assigns to a process o 16 bits long

Example 5Example 5

Next slide shows an example of transport layer communication.o Process with port address j send data to another

process with port address k

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Figure 2-20

Port Addresses

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Example 6Example 6

As we will see in Chapters 11 and 12, a port address is a 16-bit address represented by one decimal number as shown below.

753 A 16-bit port address

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TCP/IP VERSIONS

2.52.5

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Versiono Version 4n What we current used

o Version 5n Only a proposaln Based on the OSI model

o Version 6n Also called IPng (IP next generation)