1Computer (Transport) Layer

Manages communication between two computers

Platform independence: one computer does not know even what platform the other is (mainframe, PC, etc.)

TCP/IP’s most popular transport standard is TCP

2

Terminal-Host Platform

Processing done at a central host computer– Mainframe (very large)

– Minicomputer (mid-size)

Host

ProgramsExecute onthe Host

3PC Network Client PC sits on desks of ordinary users

– Receives services from servers– Stand-alone PC with hardware, software added

ClientPC

ClientPC

4PC Network

Servers– Provide services to client PCs– Usually PCs but not always

Server

Server

Server

Service

Service

Service

5

PC Network Servers are Specialized

– Optimized for their services for cost, performance, and reliability

FileServer

DatabaseServer

Internet AccessGateway Server

File Service

6File Server Program Access

Programs executed on the client PC– Program and data files downloaded over the network

ClientPC

Server

Program ExecutedOn Client PC

DownloadedOver Network

Network

7Client Workstation Workstations are more powerful than PCs Come in client and server versions

ClientWorkstation

21” display

Powerful RISCmicroprocessor

Large, fast RAM,hard disk

Usually UNIX OS, can beWindows NT

8

Networked PC

The Dominant Computer Platform Today is the Networked PC– High penetration of PCs on desktops

– Most are already networked

“Legacy” Systems Must be Maintained– Mainframe terminal-host systems

– Minicomputer terminal-host systems

– Many are being “downsized” to PC networking

9

Managed PCs

Designed to be Managed Remotely

– Slashes maintenance costs

– Slashes software upgrade costs

– Users have less control over their machines

– Company can search for illegal games

– Management standards are still immature

– Eventually, all networked PCs will be managed

10

NetPC

Standard Pushed by Microsoft, Intel

– Closed box with limited upgradability

– Should ease maintenance by reducing incompatibilities and add-in problems

– Will users be content?

– Remote management built in

11Network Computers (NCs) Designed for Use on the Internet (Java)

– Little or no local disk drive storage

– Programs downloaded over Internet when needed (or from corporate servers using Internet standards)

– User always gets the latest version of the software

– Downloading can take time; small applications would load faster but might lack functionality

– When the network is down, so are you

TheInternet

TheInternet

12Low-Cost PCs for the Home

WebTV (Microsoft)– Uses TV for its display

– User only needs $500 box

– Problem: TV display resolution is far lower than that of a TV screen; webpages look bad

– Problem: Need special Internet service provider who cannot work with your regular PC

– Problem: No printing

– Problem: Limited to WWW and email

Sub $1,000 PCs

13Networks Mix Traffic of Multiple Platforms

Network(LAN, WAN, internet)

Terminal-host trafficPC network traffic

14F2-7: Program Interactions on a Stand-Alone PC

Application ProgramApplication Program

Operating SystemOperating System

HardwareHardware

Commands related to files Files or error messages

ResponseRequest

15

Client/Server Processing

Client Program Server Program

Client Machine(PC)

Server Machine(UNIX Minicomputer)

TWO programs onTWO machines

Platform independent

16

Client/Server Processing

Application Layer Protocols– Control the requests and responses

– Application independence: any browser can work with any webserver program

Transport Layer Protocols– Allow two computers with different

architectures to work together

– Brings platform independence

17Internet Access

Webserver

PC withbrowser

InternetServiceProvider

(ISP)

InternetServiceProvider

(ISP)

InternetServiceProvider

(ISP)

InternetServiceProvider

(ISP)

The InternetBackbone

The InternetBackbone

To connect to the Internet, you must have an ISP

18

Internet Service Provider (ISP)

Basic Service: Links You to the Internet– Provides a connection

– Pays for your use. (The Internet is NOT Free.)

Most Provide Additional Services– E-mail

– World Wide Web Homepage Hosting

– Premium Providers have Specialized Services(America Online, CompuServ, MSN)

19Internet Access with PPP

Modem,TCP/IP software

PPP software

PPP link over telephone line

ISP

20Internet Access with PPP: routers

ISP

The Internet: 1. a network ofrouters connected by transmission lines

Router

Router

Router

ISPRouter

2. Subnets connected by routers

21Layered Interactions in Internet Access

BrowserBrowser

TCPSoftware

TCPSoftware

IPSoftware

IPSoftware

CSLIP/PPPSoftware

CSLIP/PPPSoftware

User PC

IPSoftware

IPSoftware

PPPSoftware

PPPSoftware

Router

WebserverSoftware

WebserverSoftware

TCPSoftware

TCPSoftware

IPSoftware

IPSoftware

SubnetSubnet

Webserver

SubnetLayer(PPP)

Point-to-Point Connection

22

Point-to-Point Connection

Physical Layer– Serial Port

– Modem

– Telephone Transmission

Data Link Layer– Packaging of data for delivery over line

– Error detection and correction

– CSLIP or PPP standard

23Layered Interactions in Internet Access

BrowserBrowser

TCPSoftware

TCPSoftware

IPSoftware

IPSoftware

PPPSoftware

PPPSoftware

User PC

WebserverSoftware

WebserverSoftware

TCPSoftware

TCPSoftware

IPSoftware

IPSoftware

SubnetSubnet

Webserver

Internet Layer GovernsRouting Across Multiple Routers

On the Internet

IPIP

Software

IPSoftware

SubnetSoftware

SubnetSoftware

Routers

IP

24

Routing Across the Internet

Routing

– Message (packet) must travel over a network of routers

– The user PC tells the first router the destination address of the webserver (e.g., voyager.cba.hawaii.edu)

– Working together, routers route the packet to its destination host

25Layered Interactions in Internet Access

BrowserBrowser

TCPSoftware

TCPSoftware

IPSoftware

IPSoftware

PPPSoftware

PPPSoftware

User PC

WebserverSoftware

WebserverSoftware

TCPSoftware

TCPSoftware

IPSoftware

IPSoftware

SubnetSubnet

Webserver

Transport Layer(TCP)

IPSoftware

IPSoftware

SubnetSoftware

SubnetSoftware

Routers

26Transport Layer

Connects the Two Computers

– Establishes a temporary connection

– Breaks connection when transmission ends

– Asks for the retransmission of damaged packets

– Asks for the retransmission of lost packets

– Flow control: tell the other computer to pause

– Platform independence: connects computers from different vendors

27Layered Interactions in Internet Access

BrowserBrowser

TCPSoftware

TCPSoftware

IPSoftware

IPSoftware

PPPSoftware

PPPSoftware

User PC

IPSoftware

IPSoftware

SubnetSubnet

Routers

WebserverSoftware

WebserverSoftware

TCPSoftware

TCPSoftware

IPSoftware

IPSoftware

SubnetSubnet

Webserver

Application Layer(HTTP, HTML)

28

Protocol Data Units (PDUs)

At each layer, messages are strings of 1s and 0s

These messages are organized

Called protocol data units (PDUs)

Each layer (except Physical) has a PDU– E.g., Application PDU or APDU

– E.g., Computer (Transport) PDU or CPDU

PDUPDU

29

Organization of PDU

Header (delivery information) TrailerTrailer (sometimes present for error handling) Data Field

– Often PDU of next higher layer

TrailerTrailer Data FieldData Field HeaderHeader

PDU at Layer N

PDU at Layer N+1PDU at Layer N+1

30PDU Transmissions from the PC

BrowserBrowser

IPSoftware

IPSoftware

CSLIP/PPPOr OtherSubnet

Software

CSLIP/PPPOr OtherSubnet

Software

User PC

APDU

APDU CH

Computer (Transport) Layer

(CPDU) TCP PDU

Application Layer

CPDU IH

Internet Layer

TCPSoftware

TCPSoftware

Internet PDU (IPDU)

SnT IPDU SnH Subnet PDU (SnPDU)CSLIP or PPP

Subnet Layer

Application PDU

31At Each Router

IPSoftware

IPSoftware

SubnetProcess

SubnetProcessSnPDU for

Subnet A

SnPDU forSubnet A

SnASnA SnBSnB

IP PDUIP PDU

Router

Subnet A Subnet B

2. Subnet process removes IP PDU from SnPDU, delivers it to the IP process.

1. Router subnet process accepts the SnPDU

3. IP processconsidersbest route

IP PDUIP PDU

SnPDU onSubnet B

SnPDU onSubnet B

4. IP process delivers the IP PDU to the subnet process

32At the Webserver

WebserverSoftware

WebserverSoftware

TCPSoftware

TCPSoftware

IPSoftware

IPSoftware

SubnetSoftware

SubnetSoftware

Webserver

SubnetSnPDUSnPDU1. accepts SnPDU

IPDUIPDU2. delivers IPDUto IP Software Process

CPDUCPDU3. IP Software delivers CPDU

to Transport Process

APDUAPDU

4. TCP Software delivers APDUto Webserver Program

33

Intranets Use of Internet Technology within Firms

– Standards are mature

– Standards for many applications

– Security is good for internal use

– Platform independent: No need to create separate applications for PCs, Macs, Unix

– Browser runs software: no need to install software on PCs

– Major concern of corporations today

34Intranets

Must keep out hackers

Yet users need access to Internet services outside the Internet

Solution: create a firewall to limit access from the outside

Intranet

Hacker

OK

No

Firewall

35Extranets

Give Buyers or Suppliers Access

– Limited to certain resources, such as inventory and pricing databases

– Uses the Internet for transmission

Internal

ExtranetResource

Any Other Resource

OK

No

Customer

36Extranets

Tunneling– Extranet transmission uses the unsecure

Internet

– Add secure transmission through the Internet by means of encryption, other measures

– Creates a secure “tunnel” through the Internet

Tunnel