04/19/23 CS135601 Introduction to Information Engineering 1

Networking and the Internet

04/19/23

Che-Rung Lee

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Network

• An intricately connected system of things or people – Webster dictionary

• Computer network– A linked computer

system

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Outline

• Layered approach for network

• Link layer

• Network layer/Transport layer

• Application layer

Link

Network

Transport

Application

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Layered Approach

What and why?

Link

Network

Transport

Application

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A story

某日，某資工系某家族決定要去竹北某餐廳家聚。他們決定先在某資電館前集合，再出發到餐廳。家族共有 12 人，某學長有一輛車，可以坐 4 人，另外還有 2 個人有摩托車，共可以載 4 人，另外 4 個人決定坐公車。某學長車上有 GPS ，可以找出路要怎麼走；有摩托車的同學上網查出地圖路線；要坐公車的同學查出要轉一次車，再走一小段路就可以到餐廳了…

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Layered approach

• 去竹北某餐廳家聚– Decide where to go and what to do

• 4 人坐車， 4 人騎車， 4 人搭公車 – Decide the transportation methods

• GPS, 地圖路線 , 公車班次– Decide the routes from the source to the

destination

• 汽車 , 摩托車 , 公車 , 司機 , 馬路 , 紅綠燈 , 路標…– Make the real transportation happen

Application layer

Transport layer

Network layer

Link layer

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Network layers

• Application: Constructs message with dest address

• Transport: Provides reliable data transfer services

• Network: Handles routing through the network

• Link: Handles actual transmission of packets

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Layered approach: why?

• Each task in different layer can be handled more easily without considering the details of the tasks in other layers.– For example, they can decide a “best”

restaurant without considering how to go there

• Methods in different layers can be changed easily.– For example, different applications can use the

same transport protocol.

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Protocols

• A set of rules used to communicate with each other across a network. – Example 1: 先在資電館前集合，再出發到餐廳– Example 2: bus route and schedule– Example 3: traffic regulations

• Protocol is a special algorithm that enables and controls the data communications.

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Figure 4.14 Following a message through the Internet

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Message encapsulation

• To transmit the data, each layer appends some information to the original message.

• The corresponding layer in the receiver side needs to “decapsulate” the message.

address 1name 1address 2

name 2address 3name 3address 4name 4

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Link Layer

Link

Network

Transport

Application

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LAN, MAN, WAN

• Network can be classified by the scope– Local area network (LAN)– Metropolitan area (MAN)– Wide area network (WAN)

• Network for different scopesuses different media and has different protocols.

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Network topology

• Popular network topologies used in LAN– Bus (Ethernet)

– Star (Wireless network with central Access Point, AP)

• Different topology uses different protocols.

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Ethernet and CSMA/CD

• CSMA/CD: protocol for Ethernet– Carrier Sense, Multiple Access with Collision

Detection

– Broadcast messages to all machines on the bus

– Resend message if detecting more than one messages sent at the same time (collision)

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WiFi and CSMA/CA

• CSMA/CA: protocol for WiFi– Carrier Sense, Multiple Access

with Collision Avoidance

• Similar to CSMA/CD– Sensing carriers before sending

• WiFi cannot detect collisions perfectly– Self signal drowns out others– Hidden terminal problem

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Collision avoidance

• Before sending data, a node listens to the channel for a period.– If the channel is silent, it sends data.

– If the channel is busy, it waits another random period. After waiting, if the channel is silent, it sends data immediately.

• Cannot avoid collisions totally.– Handshaking: Before sending data, a node sends

a request to AP. AP sends an acknowledgement to all nodes after receiving the request.

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Network equipments for LAN

• Hub: relays signals it received

• Repeater: connects two buses

• Bridge: similar to repeater,but only forwards necessarymessages

• Switch: a bridge that connects multiple buses– All those do not change the protocol

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Network layer/Transport layer

IP, TCP, UDP, and the Internet

Link

Network

Transport

Application

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Terminology

• An internet: – a network of networks– It is different from “the Internet”.

• The Internet: a global internet that uses TCP/IP protocol suite.– TCP/IP protocol suite: IP+TCP+UDP– TCP and UDP: Transport layer protocols – IP : Network layer protocol

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Internet Protocol (IP)

• A protocol used for communicating data across an internet.

• It is a connectionless protocol.– Telephone network is connection-oriented. – For the same source

and destination, different message may be routed differently.

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IP address

• How to design a global address– 台灣 新竹市 光復路二段 101 號– 886-3-571-5131

• IP address: the global identification for each computer connected to an internet– Pattern of 32 or 128 bits, usually represented

in the dotted decimal notation.– For example: 140.114.87.5 is for 楓橋驛站

nation-city-road-number

nation-area-phone number

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Domain name

• The mnemonic address for human– For example, bbs.cs.nthu.edu.tw is the

domain name for 楓橋驛站– Top level domains (TLD): suffixes in the

domain name: .gov, .edu, .com, .net, .org,…– Country-code TLD: .tw, .ca, .jp, .au, …– Subdomains: prefixes of an domain name,

bbs., www., …

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Domain Name System (DNS)

• Provides translation between IP addresses and domain names.– Name server– DNS lookup

Machines like binary addresses

Humans prefer domain names

DNS

Bookmark 楓橋驛站Domain name bbs.cs.nthu.edu.tw

IP address 140.114.87.5

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Gateway and router

• Gateway: the “point” linked to an internet

• Router: forwards and routes messages– Forwarding: relay messages on an internet– Routing: select a route to reach destination

• Update and maintain the forwarding table

Destination Next hop Cost

140.114.87.5 103.121.1.2 …

140.114.79.3 … …

Examples in windows

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The IP addresses of domain name servers

The IP address of gateway

The mask for gateway

The IP address of your computer

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Transport layer

TCP UDP

Handshaking

Retransmission

Flow control

Congestion control

• Two protocols• TCP (Transmission

Control Protocol)• UDP (User Datagram

Protocol)

Transmission control examples

• Handshaking: – Make reservation before go to a restaurant

• Retransmission: – Ask someone to repeat what he/she said

when you did not hear it clearly

• Flow control: – Ask someone to talk slower

• Congestion control: – 匝道管制 or line up a crowd of people

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Port number

• TCP/UDP need to know which application should a received message be sent to– This is identified by port numbers– IP address is for the address of computer

• ExamplesPort number Application

53/TCP,UDP Domain Name System (DNS)

80/TCP,UDP Hypertext Transfer Protocol (HTTP)

513/TCP Login

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Internet Service Provider (ISP)

• A company/organization that offers its customers access to the Internet.

• The ISP hierarchy

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How ISPs work

• Connection method to ISPs– Direct connection– Dial-up, DSL– Cable modem– Wireless

TANet 台灣學術網路 SeedNet 新世紀資通Hinet 中華電信 Cable modem 東森有線電視

http://rms.twnic.net.tw/twnic/User/Member/Search/main7.jsp?Order=ORG.ID

Exercises

1. Use nslookup to checkout the IPs of google.com

2. Use ipconfig (or ifconfig on Linux) to check out (1) The IP address of the computer (2) The gateway of the LAN (3) DNS server (use /all options) (4) Physical (MAC) address of the computer

3. Use netstat –a to scan port numbers

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Application layer

Client/server, peer to peer, distributed system

The Internet applications and WWW

Link

Network

Transport

Application

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Client/server model

• Server provides service to clients– One server, many clients– Server must execute

continuously– Client initiates

communication

• Examples: – Printer server, – File server, – Web server, – Email server, – BBS server,– …

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Peer to peer (P2P) model

• Two processes communicating as equals

• Examples:– Peer-to-peer file sharing:

• FastTrack, BitTorrent, …

– VoIP: Skype, …– Streaming media: TVant, …– Instant message, online

chat: MSN, google talk, …

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Distributed system

• Consists of software units that execute as processes on different computer.

• Examples:– Multitier architecture– Computer cluster– Cloud computing– Grid computing– BOINC system– …

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The Internet applications

• Telnet, ssh

• Email

• WWW

• File transfer

• Peer2peer

• Internet phone (VoIP), instant message

• Streaming media

• …

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Electronic mail (email)

• Email address

• Email server

• Email client– Outlook– Thunderbird– Pine

• Webmail – Yahoo mail, gmail, hotmail, …

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World Wide Web (WWW)

• A system, invented by Tim Berners-Lee (1989), of interlinked hypertexts via Internet.

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Web browser

• A software application for retrieving, presenting, and traversing information resources on the World Wide Web.– Internet Explore– Mozilla Firefox– Apple Safari– Google Chrome

• Bookmark, RSS, download management

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Hypertext Transfer Protocol (HTTP)& Uniform Resource Locator (URL)

Figure 4.8 A typical URL

• HTTP is an application-level protocol for distributed, collaborative, hypermedia information systems

• URL (web address)

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Hypertext Markup Language (HTML)

• Encoded as text file

• Uses tags to communicate with browsers

• XML: eXtensible Markup Language – A language for constructing markup

languages (meta-language)

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Related courses

• Computer network– 計算機網路概論 , 密碼與網路安全概論 ,

訊號與系統 , 作業系統

• Textbook chap 4 sec1,2,3,4

• The Internet

References