Chapter 4: Networking and the...

Chapter 4:

Networking and the Internet

Computer Science: An Overview

Tenth Edition

J. Glenn Brookshear

Presentation files modified by Farn Wang

Chapter 4: Networking and the

Internet

• 4.1 Network Fundamentals

• 4.2 The Internet

• 4.3 The World Wide Web

• 4.4 Internet Protocols

• 4.5 Security

Network Classifications

• Scope

– Local area network (LAN)

– Metropolitan area (MAN)

– Wide area network (WAN)

• Ownership

– Closed versus open

• Topology (configuration)

– Bus (Ethernet)

– Star (Wireless networks with central Access Point)

– Ring (IBM FDDI)

Network topologies

Communication over a bus network

Network topologies (continued)

Protocols

• CSMA/CD (Carrier Sense, Multiple Access with Collision Detection)

– For wired communication.

• CSMA/CA (Carrier Sense, Multiple Access with Collision Avoidance)

– for wireless communication

Protocols

– For wired communication.

– Used in Ethernet

– Silent bus provides right to introduce new

message

– Retry after collection detection.

Ethernet bus arbitration algorithm

IEEE 802.3

• Optimistic – why pessimistic ?

– Use it and withdraw if bad things happen.

• Collision detection bad things

2.5km 51.2s

IEEE 802.3

1. If there is some signals in the bus, then stop and try later.

2. Start sending the message and monitoring the bus.

3. If in 52s the message is corrupted, then stop and try

later.

4. At the 808’th s, complete the message.

Communication over a bus network

through a hub

computer computer

computer

computer computer hub

Protocols

– For wireless communication.

– Used in WiFi (IEEE 802.11)

– Hidden terminal problem

• signal drowning or blocking

• difficulties in collision detection.

– Random stream testing before the real frames

• Significantly reduction in retries.

• may need ack from AP

The hidden terminal problem

collision

detection.

Connecting Networks

• Repeater: Extends a network

• Bridge: Connects two compatible networks

• Switch: Connect several compatible networks

• Router: Connects two incompatible networks

resulting in a network of networks called an

internet

– forwarding table

• Gateway: connecting a network to Internet

Building a large bus network from

smaller ones

Routers connecting two WiFi

networks and an Ethernet network to

form an internet

Inter-process Communication

• Client-server

• Peer-to-peer (P2P)

• Client-server

– One server, many clients

– Ideal for fast network access

– Server must execute continuously

– Client initiates communication

– For large-scale organization

– Win/NT, Linux, NetWare

• Client-server

– Two processes communicating as equals

– Ideal for sharing resources and applications

– For families and small offices

– Peer processes can be short-lived

– text conversation, interactive games, file

sharing

• could be difficult for copyright enforcing

– Win 98, Win 95, …

The client/server model compared to the

peer-to-peer model

Distributed Systems

• Systems with parts that run on different

computers

– Infrastructure can be provided by standardized

toolkits

• Example: Enterprise Java Beans from Sun

Microsystems

• Example: .NET framework from Microsoft

• Example: Cloud computing

The Internet

• The Internet: An internet that spans the

– Original goal was to develop a means of

connecting networks that would not be

disrupted by local disasters.

– Today it has shifted from an academic

research project to a commercial undertaking.

Internet Architecture

• Internet Service Provider (ISP)

– sometimes means the internet.

• Access ISP (intranet)

– Tier-1 • very high-speed, high-capacity, international WAN

– Tier-2 • regional, less potent

• Access ISP (intranet): Provides connectivity to the Internet

– is essentially an independent internet.

– is sometimes called intranet.

– provides connectivity to the Internet

– AOL, Microsoft, local cable companies, telephone companies, ….

– hosts (end systems)

– Traditional telephone (dial up connection)

– Cable connections

– DSL (digital subscriber line) through modems

– Dial-up access (old tech)

– fiber-optic cables

– Wireless through AP • hotspot: the range of an AP

Internet Composition

Internet Addressing

• IP address: pattern often represented in dotted decimal notation

– from 32 to 128 bits

• Mnemonic address:

– Domain names

– Top-Level Domains (TLD) • edu, com, gov, org, info, net, …

– The dots in Mnemonic addresses are not related to the dots in IP address.

Internet Addressing

• Domain name system (DNS)

– How to translate Mnemonic names to IP addresses ?

– Name servers • basically directories

– DNS is an Internet-wide directory system.

– DNS lookup

– Originally domain names are tied to physical regions

– Now individuals and small firms can also have domain presences.

Internet Corporation for Assigned

Names & Numbers (ICANN)

• Allocates IP address blocks to ISPs who

then assign those addresses within their

regions.

• Oversees the registration of domains and

domain names.

– through companies called registrars

– once registered, the domain organization can extend for identifiers.

• ntu.edu.tw

– subdomains for network in the domain • ee.ntu.edu.tw

Traditional Internet Applications

• Electronic Mail (email)

– Domain mail server collects incoming mail and

transmits outing mail

– Mail server delivers collected incoming mail to

clients via POP3 or IMAP

– SMTP for text mails

– MIME for non-ASCII mails to SMTP

• File Transfer Protocol (FTP)

• Telnet and SSH

More Recent Applications

• Voice Over IP (VoIP)

• Internet Radio - webcasting

– streaming audio

– N-unicast

• burden to the sending servers and their immediate

neighbors

– Multicast

• let the routers decide to send copies to destinations

World Wide Web

• Hypertext and HTTP

• Browser gets documents from Web server

• Documents uniquely identified by URLs

– Uniform Resource Locator

2013/04/13 stopped here.

A typical URL

Uniform Resource Locator

• a subset of the Uniform Resource

Identifier (URI)

• the location of an identified resource

• the mechanism for retrieving it.

Hypertext Document Format

• Encoded as text file

• Contains tags to communicate with browser

– Appearance

• <h1> to start a level one heading

• <p> to start a new paragraph

– Links to other documents and content

• <a href = . . . >

– Insert images

• <img src = . . . >

A simple Web page

An enhanced simple Web page

Extensible Markup Language (XML)

• XML: A language for constructing markup

languages similar to HTML

– A descendant of SGML

• (Standard Generalized ML)

– Opens door to a World Wide Semantic Web

– note HTML does not strictly follow XML.

• XHTML does.

Using XML

<staff clef = “treble”> <key>C minor</key>

<measure> < rest> egth </rest> <notes> egth G,

egth G, egth G </notes></measure>

</staff>

Figure 4.11 The first two bars of

Beethoven’s Fifth Symphony

- Client Side Versus Server Side

• Client-side activities

– Examples: java applets, javascript,

Macromedia Flash

• Server-side activities

– Common Gateway Interface (CGI)

– Servlets: a Java class in Java EE

• conforms to the Java Servlet API, a protocol of

Java classes to HTTP requests.

– PHP (personal home page)

• a scripting language for webpages

Package-shipping example

Internet Software Layers

• Application: Constructs message with address

• Transport: Chops message into packets

• Network: Handles routing through the Internet

• Link: Handles actual transmission of packets

The Internet software layers

Following a message through the

Internet

TCP/IP Protocol Suite

• Much more than TCP and IP.

• Transport Layer

– built on top of IP networks

– Users can choose from different versions, e.g.:

• TCP (Transmission Control Protocol)

• UDP (User Datagram Protocol)

• Network Layer

– IP (IPv4 and IPv6)

– deals only with packets

TCP (Transmission Control Protocol)

• connection-oriented.

– deal with streams

• high-quality and reliable.

– enables two hosts to establish a connection

and exchange streams of data.

– guarantees delivery of data

– guarantees that packets will be delivered in

the same order in which they were sent.

UDP (User Datagram Protocol)

• very few error recovery services

• used primarily for broadcasting messages

• minimal overhead

• connectionless

Choosing between TCP and UDP

Security

• Attacks

– Malware • viruses: inserting itself into programs already in the

machine.

• worms: replicating and forwarding themselves

• Trojan horses: disguised as useful programs

• spyware: sniffing to collect private infor.

• phishing software

– Denial of service

– Spam: unwanted junk mails

Security

• Protection

– Firewalls: filtering traffic through a point

– Spam filters: a variation of firewall

– Proxy Servers: • intermediary shielding the clients from suspicious

servers.

• for specific services, elg., FTP, HTTP, telnet, …

– auditing software • for sudden change of system behaviors

– Antivirus software

Encryption

• FTPS, HTTPS, SSL (secure socket layer)

• Private-key Encryption

• Public-key Encryption

– Public key: Used to encrypt messages

– Private key: Used to decrypt messages

• Certificates and Digital Signatures

Private-key encryption

• key: k

• affine transformation modulo operand d

• message m1m2 … mn with mi [0,d),

• Encryption:

((m1+k)% d)((m2+k)% d)…((mn+k)%d)

• Decryption: c1c2 … cn with ci [0,d),

((c1-k)% d)((c2-k)% d)…((cn-k)% d)

remainder operator

Example:

• k = 5

• d = 26

Encryption:

S (int(S)+5)%26 = X

T (int(T)+5)%26 = Y

O (int(O)+5)%26 = T

P (int(P)+5)%26 = U

Drawbacks

• Receivers also must know the private key.

• No authentication

– Once the key is known, messages can be

forged.

• Solution: Need a private key for each

sender-receiver pair.

– Needs a lot of keys.

Public-key encryption

Public key encrytion

RSA encryption method

– Ronald Rivest, Adi Shamir, Leonard

Adleman

• 2 large primes p, q (hundreds of bits

• Ancient Chinese:

n is prime iff 2n-1%n = 1%n

• Fermat’s little theorem: Given a%p0,

ap%p=a%p and ap-1%p = 1%p

incorrect!

Public key encrytion

RSA encryption method

– Ronald Rivest, Adi Shamir, Leonard Adleman

• 2 large primes p, q (hundreds of bits each)

• e: relative prime to (p-1)(q-1)

• Private key: pq

• Public key: d with gcd(d,e%(p-1)(q-1))=1

• Encryption: c = (me) % (pq)

• Decryption: cd=((me)%(pq))d

=m1+k(p-1)(q-1) % (pq) = m%(pq)

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