computer (transport) layer
DESCRIPTION
Computer (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. Terminal-Host Platform. Processing done at a central host computer - PowerPoint PPT PresentationTRANSCRIPT
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