Transcript Document

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
1
Terminal-Host Platform

Processing done at a central host computer
– Mainframe (very large)
– Minicomputer (mid-size)
Programs
Execute on
the Host
Host
2
PC Network

Client PC sits on desks of ordinary users
– Receives services from servers
– Stand-alone PC with hardware, software added
Client
PC
Client
PC
3
4
PC Network

Servers
– Provide services to client PCs
– Usually PCs but not always
Server
Service
Service
Service
Server
Server
5
PC Network

Servers are Specialized
– Optimized for their services for cost, performance, and
reliability
File Service
File
Server
Database
Server
Internet Access
Gateway Server
File Server Program Access

Programs executed on the client PC
– Program and data files downloaded over the network
Program Executed
On Client PC
Downloaded
Over Network
Client
PC
Network
Server
6
Client Workstation
7
Workstations are more powerful than PCs
 Come in client and server versions

21” display
Powerful RISC
microprocessor
Large, fast RAM,
hard disk
Usually UNIX OS, can be
Windows NT
Client
Workstation
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
8
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
9
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
10
Network 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
The
Internet
11
Low-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
12
Networks Mix Traffic of Multiple
Platforms
13
Network
(LAN, WAN, internet)
PC network traffic
Terminal-host traffic
F2-7: Program Interactions on a StandAlone PC
Application Program
Request
Response
Operating System
Commands related to files
Hardware
Files or error messages
14
Client/Server Processing
Client Program
15
Server Program
Platform independent
TWO programs on
TWO machines
Client Machine
(PC)
Server Machine
(UNIX Minicomputer)
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
16
17
Internet Access
Webserver
Internet
Service
Provider
(ISP)
To connect to
the Internet,
you must have
an ISP
The Internet
Backbone
PC with
browser
Internet
Service
Provider
(ISP)
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)
18
19
Internet Access with PPP
Modem,
TCP/IP software
PPP software
PPP link over telephone line
ISP
20
Internet Access with PPP: routers
Router
Router
Router
The Internet:
1. a network of
routers connected
by transmission lines
2. Subnets connected
ISP
by routers
ISP
Router
21
Layered Interactions in Internet Access
User PC
Router
Webserver
Browser
Webserver
Software
TCP
Software
TCP
Software
IP
Software
CSLIP/PPP
Software
Subnet
Layer
(PPP)
IP
Software
IP
Software
PPP
Software
Subnet
Point-to-Point Connection
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
22
23
Layered Interactions in Internet Access
User PC
Browser
TCP
Software
Routers
Webserver
Internet Layer Governs
Routing Across Multiple Routers
On the Internet
IP
Webserver
Software
TCP
Software
IP
IP
Software
IP
Software
IP
Software
PPP
Software
Subnet
Software
Subnet
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
24
25
Layered Interactions in Internet Access
User PC
Browser
Routers
Transport Layer
(TCP)
TCP
Software
Webserver
Webserver
Software
TCP
Software
IP
Software
IP
Software
IP
Software
PPP
Software
Subnet
Software
Subnet
Transport 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
26
27
Layered Interactions in Internet Access
User PC
Browser
Routers
Application Layer
(HTTP, HTML)
TCP
Software
Webserver
Webserver
Software
TCP
Software
IP
Software
IP
Software
IP
Software
PPP
Software
Subnet
Subnet
Protocol Data Units (PDUs)
 At
each layer, messages are strings of 1s
and 0s
 These
 Called
 Each
messages are organized
PDU
protocol data units (PDUs)
layer (except Physical) has a PDU
– E.g., Application PDU or APDU
– E.g., Computer (Transport) PDU or CPDU
28
Organization of PDU

Header (delivery information)

Trailer (sometimes present for error handling)

Data Field
– Often PDU of next higher layer
PDU at Layer N+1
PDU at Layer N
Trailer
Data Field
Header
29
30
PDU Transmissions from the PC
User PC
Application Layer
Browser
APDU Application PDU
Computer (Transport) Layer
TCP
Software
APDU CH (CPDU) TCP PDU
Internet Layer
IP
Software
CSLIP/PPP
Or Other
Subnet
Software
CPDU
SnT
IPDU
IH
Internet PDU (IPDU)
Subnet Layer
SnH Subnet PDU (SnPDU)
CSLIP or PPP
At Each Router
2. Subnet process
removes IP PDU
from SnPDU,
delivers it to the IP
process.
IP PDU
3. IP process
considers
Router
best route
IP
Software
Subnet
Process
Subnet A
31
SnPDU for
Subnet A
1. Router subnet SnA SnB
process accepts
the SnPDU
4. IP process
delivers the
IP PDU IP PDU to
the subnet
process
SnPDU on
Subnet B
Subnet B
At the Webserver
32
Webserver
4. TCP Software delivers APDU
to Webserver Program
3. IP Software delivers CPDU
to Transport Process
2. delivers IPDU
to IP Software Process
1. accepts SnPDU
Subnet
APDU
CPDU
IPDU
SnPDU
Webserver
Software
TCP
Software
IP
Software
Subnet
Software
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
33
Intranets

Must keep out hackers

Yet users need access to Internet services outside
the Internet

Solution: create a firewall to limit access from the
outside
Firewall
OK
Intranet
Hacker
No
34
35
Extranets
 Give
Buyers or Suppliers Access
– Limited to certain resources, such as inventory
and pricing databases
– Uses the Internet for transmission
Customer
OK
Extranet
Resource
Internal
No
Any Other Resource
Extranets
 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
36