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CS1356 資訊工程導論
Network and the Internet
國立清華大學資訊工程學系
2017/5/25
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
Application
Transport
Network
Link
3
Layered Approach
What and why?
Application
Transport
Network
Link
4
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
Application layer
– 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
Transport
layer
Network layer
– Decide the routes from the source to the
destination
Link layer
• 汽車,摩托車,公車,司機,馬路,紅綠燈,路標…
– Make the real transportation happen
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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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(Fig. 4.14)
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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 1
address32
address
name 1
name
address
name 342
name 4
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Link Layer
Application
Transport
Network
Link
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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 scopes
uses 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
– 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.
– Before sending data, it sends a request to AP.
AP sends an acknowledgement to all
machines after receive the data.
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Network Equipments for LAN
• Hub: relays signals it received
• Repeater: connects two buses
• Bridge: similar to repeater,
but only forwards necessary
messages
• Switch: a bridge that
connects multiple buses
– All those do not change the protocol
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Network layer/Transport
layer
Application
IP, TCP, UDP, and
the Internet
Transport
Network
Link
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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 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
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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 nation-city-roadnumber
– 886-3-571-5131 nation-area-phone number
• 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
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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: update and maintain the forwarding
table
Destination
Next hop
Cost
140.114.87.5
103.121.1.2 …
140.114.79.3
…
…
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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)
Machines like
binary addresses
DNS
Humans prefer
domain names
• Provides translation between IP addresses
and domain names.
– Name server
– DNS lookup
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Transport Layer
• Two protocols
• TCP (Transmission
Control Protocol)
• UDP (User Datagram
Protocol)
TCP
UDP
Handshaking


Retransmission


Flow control


Congestion control


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Transmission Control
• Handshaking:
– Make reservation before go to a restaurant
• Retransmission:
– Ask someone to repeat what he/she said
• Flow control:
– Ask someone to talk slower
• Congestion control:
– Lineup of 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
• Examples
Port number
Application
53/TCP,UDP
Domain Name System (DNS)
80/TCP,UDP
Hypertext Transfer Protocol (HTTP)
513/TCP
Login
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Application
Application Layer
Transport
Network
Link
Client/server, peer to peer,
distributed system
The Internet applications and WWW
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Client/server Model
• Server provides service to clients
– One server, many clients • Examples:
– Server must execute
continuously
– Client initiates
communication
– 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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HTTP and URL
• Hypertext Transfer Protocol (HTTP) is an
application-level protocol for distributed,
collaborative, hypermedia information
• Uniform Resource Locator (URL): web
address
Figure 4.8 A typical URL
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HTML
• 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
References
• Textbook chap 4 sec1,2,3,4
• The Internet
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