Basic Internet and Basic Internet and

Networking ConceptsNetworking Concepts

Representation and Management of Data on the Internet

The Internet and the The Internet and the

World-Wide WebWorld-Wide Web

The InternetThe Internet

A worldwide network connecting millions of hosts

Interconnecting many Local Area Networks (LANs) (inter-network or just Internet)

The LANs connected to the Internet can be of various types

A host is a computer that is connected to the Internet

History of the InternetHistory of the Internet It started as a United States government project,

sponsored by the Advanced Research Projects Agency (ARPA), and was originally called the ARPANET

The Internet grew quickly throughout the 1980s and 90s

Less than 600 computers were connected to the Internet in 1983; now there are over 10 million

Internet ApplicationsInternet Applications

Email World-Wide Web FTP Telnet Newsgroups Chat ...

The WebThe Web

The WebThe Web

The term World-Wide Web (Web or WWW) refers to pieces of information found on the Internet

These pieces of information can be reached by hosts connected to the Internet

The Web allows many different types of information to be accessed using a common interface (Web browser)

A Web document usually contains links to other Web documents, creating a hypermedia environment

The term Web comes from the fact that information is not organized in a linear fashion

The WebThe Web

The term World-Wide Web (or simply Web) describes a collection of many pieces of information that are found on the Internet

Internet hosts can access this information The Web allows many different types of

information to be accessed using a common interface (Web browser)

A Web document usually contains links to other Web documents, creating a hypermedia environment

The term Web comes from the fact that information is not organized in a linear fashion

Web ServersWeb Servers

These pieces of information are stored as files on particular hosts of the Internet

These hosts are called Web servers

Information Types on the WebInformation Types on the Web

The information pieces of the Web can be of textual nature, images, video, audio, programs or any other type of information

Every type of information can have different formats for storing it as a file

For example, some formats for storing images are jpeg, bmp, gif, ps, pdf

HTMLHTML

Much of the information that is found on the Web is stored as HTML files

HTML is a markup language for formatting text. In addition, HTML facilitates inclusion of other types of information (such as images) in our text documents

Here is an example of an HTML document This is how it looks like when displayed

inside a browser

BrowsersBrowsers

We use a browser to display HTML documents

The browser is responsible for fetching the documents and displaying their contents according to the HTML rules

BrowsingBrowsing

HTML documents can also contain links to other HTML documents (or files of other types, such as images, etc.). The user can follow these links (by clicking them) to view other related documents and files

Browsing/surfing refers to the activity of viewing documents in the Internet and following their links

URLsURLs

Each information piece on the Web has a unique identifying address which is called a URL (Uniform Resource Locator)

A URL takes the following form: http://www.huji.ac.il/index.html

It has 3 parts: a protocol field, a hostname field and a file field

protocol filehostname

URL FieldsURL Fields The protocol field (“http” in the previous

example) specifies the way in which the information should be accessed

The host field specifies the host on which the information is found

The file field specifies the particular location in the host's file system where the file is found

There could be more complex forms of URLs, but we do not discuss them

Search EnginesSearch Engines

What are search engines? How do they work? Shortcomings of search engines Some popular search engines: Infoseek,

HotBot, Altavista, Excite, Lycos, Yahoo!, Jeeves,...

HTTP DaemonsHTTP Daemons

The information pieces of the Web are stored as files on Web servers

In order to make these information pieces available to other hosts, each server runs an HTTP-daemon

HTTP Daemons (continued)HTTP Daemons (continued)

An HTTP-daemon is an application that is constantly running on the server and waits for requests from remote hosts

A host can request the daemon for a document (a file) that is located on the server

Technically, any host connected to the Internet can act as a Web server by running an HTTP-daemon application

Browser - HTTPD InteractionBrowser - HTTPD Interaction

host www.cs.huji.ac.il

HTTPD

applicationDisk

Browser

user requests

http:// www.cs.huji.ac.il /index.html

GET /index.html

sends the

content of

index.html

Browser - HTTPD InteractionBrowser - HTTPD Interaction The user requests http://www.cs.huji.ac.il/index.html The browser contacts the HTTP-daemon running on

the host www.cs.huji.ac.il and requests the document /index.html

The HTTP-daemon translates the requested name to a specific file in its local file system

The HTTP-daemon reads the file index.html from the disk and sends the contents of the file to the browser

The browser receives the document, parses it according to the HTML rules and displays it

IP (Internet-Protocol) AddressesIP (Internet-Protocol) Addresses

Hostnames are used by people. The network mechanism uses IP-addresses instead

Every host connected to the Internet has a unique IP address that identifies it

IP addresses are 32-bit numbers that are usually written as four decimal numbers separated by dots, e.g. 135.17.98.240, where the numbers refer to the four bytes composing this address

IP PacketsIP Packets Information that is sent over a network is often

broken down in parts, called packets, which are sent to the receiving machine and then reassembled

In the Internet, data is transferred from one host to another is divided into IP-packets

Routing IP PacketsRouting IP Packets The essential role of the Internet is to enable every

host to send IP-packets to any other host Each IP-packet contains source and target IP-

addresses There is a routing protocol that handles the

transfer of packets to their target hosts, according to the target IP addresses

The sending host only needs to know the IP address of the target host it wishes to communicate with

Using IP AddressesUsing IP Addresses

How does the browser know the IP address of the Web server?

One possibility is that the user explicitly specifies the IP address of the server in the host field of the URL, for example:

http://135.17.98.240/index.html However, it is inconvenient for people to

remember such addresses

Internet AddressesInternet Addresses Many hosts have, in addition to IP address,

human-readable Internet Address (or hostnames) Here are some examples of Internet Addresses:

www.cs.huji.ac.ilwww.cocacola.comwww.yellowpages.co.ilwww.isdn.net.il

The first part is the name of a particular host (i.e., computer)

The rest is the domain name

Internet Addresses (continued)Internet Addresses (continued) Hostnames have a hierarchical structure

www.cs.huji.ac.ilwww is a computer in the Dept. of

Computer Science (cs) at the Hebrew University of Jerusalem, Israel (huji), which is an Academic Campus (ac) of Israel (il)

The rightmost name describes the main domain of the host (il - Israel). Left to it, there is a sub-domain, and then further to the left, there are more specific sub-domains

Generic DomainsGeneric Domains There are 7 special domains that are called generic

domains • com - commercial organizations

(www.cocacola.com) • edu - educational institutions (www.berkeley.com) • gov - U.S. governmental organizations

(www.cia.gov) • int - international organizations • mil - U.S. military • net - networks (InterNIC) • org - other organizations (www.w3.org)

Country DomainsCountry Domains Generic domains usually refer to hosts inside the

U.S. Other countries use two-letter country domains:• il - Israel • uk - United Kingdom • jp - Japan • se - Sweden

These domains usually have sub-domains that correspond to the generic domains. For example, co.il is the domain of all the commercial organizations in Israel, and ac.il is the domain of all the academic institutions inside Israel

Back to the BrowserBack to the Browser When we address a host in the Internet, we usually

use its hostname (e.g., using a hostname in a URL) The browser needs to map this hostname into the

corresponding IP address of the given host There is no one-to-one correspondence between

the sections of an IP address and the sections of a hostname

Translating IP Addresses to Translating IP Addresses to HostnamesHostnames

The translation of IP addresses to hostnames requires a lookup table

Since there are millions of hosts on the Internet, it is not feasible for the browser to hold a table which maps all hostnames to their IP-addresses

Moreover, new hosts are added to the Internet every day and hosts change their names

DNSDNS

The browser (and other Internet applications) use a DNS-Server to map hostnames to IP addresses

DNS (Domain Name System) is an hierarchical scheme for naming hosts

Proxy ServersProxy Servers A proxy server acts as a delegate of browsers for

accessing the Web The browser transfers the requests for a document

to the Proxy The Proxy contacts the suitable Web-server and

fetches the document on behalf of the browser

Proxy ServerProxy Server

Proxy server

Proxy

application

Browser

user requests a document

browser request the document

from the proxy

sends the

content of

index.html

proxy asks the

document from

the HTTPD

Cache

Advantages of Proxy ServersAdvantages of Proxy Servers

Proxy servers have several advantages over direct access: • They can be combined with a firewall to

enable restricted access to the Internet

• They enable caching of popular documents

• They can enlarge the functionality of the browser by translating from one protocol to another (for example, from FTP to HTTP and vice-versa)

FirewallsFirewalls A firewall poses restrictions on the traffic in

or out of a local-area network Examples:

Hides sensitive data from the outside world Prevents access of local users to specific sites

outside the local-area network

How a Firewall WorksHow a Firewall Works All the traffic (of IP-packets) in or out of

the local-area network is forced to go through a single host

A firewall application is installed on this host

The firewall examines all the in and out traffic of IP-packets and discards illegal packets

Dynamically Generated Dynamically Generated DocumentsDocuments

host www.excite.com

HTTPD

applicationBrowser

user requests

http://www.excite.com/search?what=something

GET /search?what=something

sends the

contents of

index.html

execution of

search program

Basic Networking ConceptsBasic Networking Concepts

39

Local-Area NetworksLocal-Area Networks

LANLAN

A A Local-Area NetworkLocal-Area Network(LAN) covers a small(LAN) covers a smalldistance and a smalldistance and a smallnumber of computersnumber of computers

A LAN often connects the machinesA LAN often connects the machinesin a single room or buildingin a single room or building

LANs (Local-Area Networks)LANs (Local-Area Networks) Limited size Privately owned

• Centrally managed

• Usually hosts physically connected via a cable

• Homogeneous devices & protocols

• Known features (latency, bandwidth,..)

WANs (Wide Area Networks)WANs (Wide Area Networks)

42

Wide-Area NetworksWide-Area Networks

LANLAN

A A Wide-Area NetworkWide-Area Network (WAN) (WAN)connects two or more LANs,connects two or more LANs,often over long distancesoften over long distances

A LAN is usually ownedA LAN is usually ownedby one organization, butby one organization, buta WAN often connectsa WAN often connectsdifferent groups indifferent groups indifferent countriesdifferent countries

LANLAN

What is a protocol?What is a protocol?

06 7647834

Welcome to Mount Hermon ski site. For ski conditions press 1, for reservation of ski packages press 5, ...

5

Please select the type of your credit card. For Visa press 1, ...

LayeringLayering

models protocol

sketches protocol

modem protocol

CAD protocol

45

TCP/IPTCP/IP A protocol is a set of rules that determine how things

communicate with each other

The software which manages Internet communication follows a suite of protocols called TCP/IP

The Internet Protocol (IP) determines the format of the information as it is transferred

The Transmission Control Protocol (TCP) dictates how messages are reassembled and handles lost information

TCP/IP protocol suiteTCP/IP protocol suite

Application HTTP, FTP, TELNET,...

Transport TCP, UDP

Internet IP

Link Ethernet, Token-Ring,...

TCP/IP protocol suiteTCP/IP protocol suite

Taken from "TCP/IP Illustrated Vol. 1" / Richard Stevens

Packets headersPackets headers

Taken from "TCP/IP Illustrated Vol. 1" / Richard Stevens

IP LayerIP Layer

Transmission of packets between two hosts IP addresses Routing protocol

IP AddressesIP Addresses

Class

A

B

C

D

E

From

0.0.0.0

128.0.0.0

192.0.0.0

224.0.0.0

240.0.0.0

Till

127.255.255.255

191.255.255.255

233.255.255.255

239.255.255.255

247.255.255.255

Net ID

7 bit

14 bit

21 bit

28 bit

27 bit

Host ID

24 bit

16 bit

8 bit

-

-

Class Network ID Host ID

32 bit

InterNIC

RoutingRouting

Transport LayerTransport Layer

TCP • Connection oriented

• Reliable, keeps order UDP

• Connectionless

• Unreliable

• Fast

Client-Server ModelClient-Server Model

Server application Client applicationPort 5746

Server machine

144.12.34.99

Client machine

190.30.42.155

Well-Known PortsWell-Known Ports

FTP 21 Telnet 23 HTTPD 80 ...

55

End of Lecture 1End of Lecture 1

HTTP ProtocolHTTP Protocol

Hypertext Transfer Protocol Used between Web-clients (e.g., browsers)

and Web-servers (and proxies) Text based Built on top of TCP Stateless protocol

HTTP Transaction -- ClientHTTP Transaction -- Client Client request:

• Sends a request

GET /index.html HTTP/1.0• Sends optional header information

User-Agent: browser name

Accept:formats the browser understands

...• Sends a blank line (\n) • Can send post data

HTTP Transaction -- ServerHTTP Transaction -- Server

Server response:• sends status line HTTP/1.0 200 OK

• sends header information Content-type: text/html

Content-length: 3022

...

sends a blank line (\n) sends document data

Reacting to Responses of ClientsReacting to Responses of Clients

HTML pages are static documents To achieve interaction with the user, there

is a need for Internet tools and techniques that get input from the user and react according to this input

Sometimes there is a need to produce output as a result of querying a database. The output in this case is not known in advance

Server TechnologiesServer Technologies Some Web applications use online input to create

pages on the fly (for example, search engines) A request will include, in addition to the URL of

the service provider, a list of parameters For example,

http://www.google.com/search?q=search-word The creation of the pages may also require

interaction with some applications (for example, database queries)

Creating Pages on the Fly Creating Pages on the Fly in the Serverin the Server

There are four common ways to serve page requests that include input parameters:• CGI (Common Gateway Interface) programming

• Java Servlets

• JSP -- Java Server Pages, or

• Microsoft ASP -- Active Server Pages (similar to JSP)

CGI ProgrammingCGI Programming

CGI is a scripting languageA cgi script works with an application that runs on

the server and creates HTML codeAn early technology

Java ServletsJava Servlets

Servlets are java applications that some Web servers can run

A Servlet creates pages on the fly and these pages are returned to the requesting browser

JSP and ASPJSP and ASP JSP (Java Server Pages)

• Create an HTML page that has Java code inside HTML tags

This page is actually a template The code, for example, could issue a database query

and create an HTML table for the result• The Web server executes the code in the template

and produces a pure HTML page that is returned to the client

Microsoft ASP (Active Server Pages)• The code is VB (Visual Basic) scripts• The Web server must be Microsoft IIS server

Client TechnologiesClient Technologies Some technologies interact with the user on the

client level (Web browser) Java Script is a scripting language that can be

added to HTML pages Web browsers can run the script and change the

output accordingly There is a slight interaction of the script with the

file system using cookies Cookies are small files that store some personal

information in the file system of the client

Separating Contents from StyleSeparating Contents from Style

In HTML, the contents and the style of pages are inseparable• HTML tags actually refer only to the style

XML (eXtensible Markup Language) is a new markup language for marking the semantics (meaning) of the data

XML tags describe the meaning of each portion of text in an XML document

XML TagsXML Tags

XML tags are similar to attributes in a relation

However, the attributes are the same for all the records of the relation

In XML documents, each portion of text has its own tag• <course> databases </course>

• <course> operating systems </course> XML tags can be nested

Parsing XML DocumentsParsing XML Documents

XML facilitates easy parsing of documents according to their semantics

For example, the CS Department has many Web pages of courses

Can we write a program that reads all these pages and prints a list of the names of courses?

If XML tags are used, it is easy to do that

Using XMLUsing XML

XML is important in the context of data exchange between applications

It is possible to define a common set of tags that are suited for specific applications

For example, MathML is used for exchanging mathematical information

Showing XML Document in Showing XML Document in BrowsersBrowsers

XML documents contain data with semantic tags

For a graphical representation, information about the style must be added • For example, HTML tags provide information

about the style

Style SheetsStyle Sheets

Style is added to XML documents by means of style sheets

There are two style-sheet languages• CSS -- Cascading Style Sheets

• Describe how to graphically show the data

• XSL -- XML Style-sheet Language• Can also transform the data

Putting it All TogetherPutting it All Together A common architecture for Web applications has

several tiers • DBMS (database management system) for storing

and processing information

• A Web server for producing pages as a result of client requests

• A browser that supports dynamic pages using Java scripts (for creating dynamic pages) and CSS (for creating the desired visual output)

How Should XML be UsedHow Should XML be Used ? ? How can we query easily and effectively XML

documents? How can we store efficiently XML documents? What is the proper way to include other resources

in XML documents (i.e., figures, sounds, etc.)? How can we use

a general style, and information that is semantically well defined

without making the process of creating documents too cumbersome?

Course topicsCourse topics Server-side programming

• JDBC for connecting to the DBMS

• Servlets

• JSP Client-side programming

• Java Scripts

• CSS Data storage and processing on the Web

• XML

• XSL