Document Sample
Networks Powered By Docstoc
Week 2 LBSC 690 Information Technology

Computer Systems
• Hardware
– Types of hardware – Storage hierarchy – Moore’s law

• Software
– Types of software – Types of interfaces

• Computers and devices connected via
– communication devices – transmission media

Why Network?
• • • • • • • Sharing data and information Sharing hardware Sharing software Increase robustness Facilitating communications Make money etc.

Network of Networks
• Local Area Networks (LANs)
– Connect within a department, company – Linked using Ethernet – Robust – can remove/replace machines
• Detects packet collisions (heavy traffic)

• Wide Area Networks (WANs)
– Connect multiple LANs – with gateways, routers

• Internet

– Collection of WANs across backbones

Local Area Networks
• Within a campus or an office complex
– Short-distance lines are fast and cheap – Fast communications makes routing simple

• Ethernet is a common LAN technology
– All computers are connected to the same cable
• Ordinary phone lines can carry 10 Mb/sec

– Every host broadcasts everything to all others
• Collisions limit throughput to about 50% utilization

Common LAN Topologies
• Bus Network • Ring Network • Star Network

Share Network
• • • • • • • • • • • • • • • • Bus: a single central cable Transmit data in both directions Easy attach and detach Robust to failure of computers But status of bus is critical Cable Internet is an example

Ring Network
Computers along the ring Transmit data in one direction Relatively robust to the failure of computers But difficult to install

Switch Network
All connect to hub Transmit data via hub Easy attach and detach Robust to the failure of computers But status of hub is critical DSL is an example

Local Area Networks Wide Area Networks
• Regional, national, or global scale
– Expensive communications must be used well
• Limiting to two hosts to allows 100% utilization

– Routing is complex with point-to-point circuits
• Which path is shortest? Which is least busy? …

• Internet routers exchange routing tables
– Which routes seem fast, which seem slow?

An example of WAN Internet
• • • • World-wide collection of networks Public, cooperative, and independent Each organization is responsible only for maintaining its own network. W3C oversees research and sets standards and issues guidelines

Short history of Internet
• • • • ARPANET, original 4 hosts NSFnet + ARPANET Internet, now 150million hosts Internet2

Data Transfer on the Internet Hosts and Gateway
• Hosts
– The computers that share the network

• Gateway
– A computer on a network serves as an entrance to another network – Routes inside traffics to outside networks – Often as as a proxy server and a firewal

Packets and Routing
• Packet
– Small piece of large chunk data used for communication – Contain data, the recipient, the original and sequence info

• Routing
– Find a path from one host to another – Routers are specialized computers for routing packets

Packet Switching
• Break long messages into short packets
– Keeps one user from hogging a line

• Route each packet separately
– Number them for easy reconstruction

• Request retransmission for lost packets
– Unless the first packet is lost

Router and Routing Tables
• • • • • • A device forwards packets along networks Connected to at least two networks Located at gateways Use headers and Routing Tables to find the best forwarding path Can hold (“buffer”) packets until ready to send Routing table
– A list of <address, associated information>

• A device for communication back to a known state in the event of an error or disagreement. • Insert synchronization point in packets • Reset to previous synchronization point and continue

Phone vs Network
• Phone system (“circuit-switched”)
– Fixed connection between caller-callee: circuit – Assumes <10% of lines in use at once – “Circuits busy” at high load

• Network (“packet-switched”)
– Message split into pieces – “packets” – Sent individually with:
• Where to go – “address” – IP • How to send and reassemble – number - TCP

– Detect and resend lost packets

Layered Architecture
• Hierarchical modularity
– Black box to upper level - encapsulation – System to lower level

– Standardized modules, means cheap and easy to replace

• Services and protocols
– Services: a set of primitives operations provided – Protocols: a set of rules about how to achieve services

• ISO’s OSI architecture –7 layers • TCPIP – 4 layers

• Suite of protocols connect hosts to internet
– Internet Protocol (IP): at network layer – Transmission Control Protocol (TCP): at transport layer

• De facto standard for data transmission over networks

4 Layers of TCP/IP
• Layer 1 - Link
– This layer defines the network hardware and device drivers.

• Layer 2 - Network
– This layer is used for basic communication, addressing and routing. TCP/IP uses IP and ICMP protocols at the network layer.

• Layer 3 - Transport
– Handles communication among programs on a network. TCP and UDP falls within this layer.

• Layer 4 - Application
– End-user applications reside at this layer. Commonly used applications include NFS, DNS, arp, rlogin, talk, ftp, ntp and traceroute.

• Defines the format of packets • Defines the addressing scheme
– The original address – The destination address

• Current version is IPv4,
– IPng under development

• Enable two hosts
– establish a connection, three-way handshacke – exchange streams of data

• TCP guarantees
– delivery all data, resend data if missing – packets delivered in the same order

• Socket: entire address of a host
– Network ID + host ID + user/process ID

• Port: user/process ID

User Datagram Protocol (UDP)
• Transport packets to another host • Difference to TCP
– – – – – No waiting for connection establishment No connection state Less headers Irregular sending rate No guarantee of sending all the data

• Example: sending clicks on web browser

IP address and Domain Names
• Internet Protocol Address
– 4 groups of numbers ranging from 0-255

• Domain Names
– People prefer to use names for things – Domain Name Server (DNS) coverts names to numbers – Different levels: most general: last, most specific: first

IP address and Domain Names
• Each name server knows one level of names
– – – – WAM’s name server knows rac1, rac2, … UMD’s name server knows wam, glue, ttclass, … .edu name server knows umd, umbc, stanford, … “Top level” name server knows .edu, .com, .mil, …
• Organized by network solutions ( • ICANN (

An Example of IP Address Hands-on: Find Your PC’s IP
• • • • • Click the “start” button on the taskbar Click “Run” Type in “cmd” to display a MS-DOS window Type “ipconfig /all” at the prompt and press enter key See who “owns” your IP address – –

• Trace a route to your PC

Wireless Network
• A network without the need for any wired connections to computers

• Infrastructure mode (BSS and ESS)
– an access point connected to the wired network – a set of wireless end stations.

• Ad hoc mode (IBSS)
– set of wireless computers communicate directly

• IEEE 802.11
– IEEE 802.11b: 10Mbps most wireless network comply – IEEE 802.11g: 54Mbps latest wireless network

Wireless Networks (cont.) Encryption
• Secret-key systems
– Use the same key to encrypt and decrypt – Data Encryption Standard

• Public-key systems
– – – – Public key: open, for encryption Private key: secret, for decryption RSA system: Rivest, Shamir and Adleman Pretty Good Privacy (PGP) is one “standard”

Encryption (cont.)
• Digital Signature Algorithm
– Used for signatures, not encryption

• Further readings

• A set of related programs, located at a network gateway server, that protects the resources of a private network from users from other networks • Against unauthenticated outside access
– block or permit outside access – Permit inside access to outside

• A Firewall is only one part of whole security system • Further readings

• A program designed to copy itself from one computer to another over a network (e.g. by using email) • Difference to virus

– Virus only infect files within one computer – Worms travel over a network

• Two way, computer-person, authenticated • Simulates a dial-up connection
– Read data from another machine

• VT-100 protocol allows only text
– The pine email program is designed for VT-100 – X Windows extension adds graphics
• WAM X-terminals available in CSS 4352

File Transfer Program (FTP)
• Two way, computer-computer, authenticated • Used to move files between machines
– FTP server – Uploading (put) and downloading (get) – Binary transfer

• Two interfaces: command line & GUI
– Command line- DOS “run” or UNIX shell – GUI: Windows WS-FTP

• Two levels of security: anonymous & secure
– Userid “anonymous” provides public access – Web browsers provide one-way anonymous FTP

Hand-on Point: FTP
• Start a cmd window • Type “ftp” • Login in anonymously with
– User: anonymous – Password: your email address

• Go to directory lbsc690
– Type “cd pub/gina/lbsc690/”

• Get file “hwOne.ppt”
– Type “get hwOne.ppt”

• Exit
– Type “quit”

• Secure, Encrypted versions of telnet & ftp • Authenticated telnet/ftp send clear text
– Password visible to “sniffers”

• SSH & SCP
• Secure shell, Secure copy

– Authenticate user with password – Authenticate machines with keys – Encrypt all data - passwords! -between machines

• • • • • HyperText Transfer Protocol Use client server mode Fetch web pages from web server to browser Stateless protocol Define
– how messages are transmitted – What actions Web servers and browsers take

How HTTP works
• • retrieve the file at the URL

pen a socket to, send request
GET /path/file.html HTTP/1.0 From: User-Agent: HTTPTool/1.0 [blank line here]


server response via the same socket
HTTP/1.0 200 OK Date: Fri, 31 Dec 1999 23:59:59 GMT Content-Type: text/html Content-Length: 1354 <html><body> <h1>Happy New Millennium!</h1> (more file contents) . . . </body> </html>


close the socket

Homework 2- Treasure Hunt
• • • • • • Fetch a Web page on a web site end with a .tv address Use URL parsing to see if you can learn what organization hosts the page Look up the country for .tv Determine who has that domain name registered Do a traceroute to see how the packets get there Describe the traceroute geographically

Shared By: