Basic Internet and Networking Concepts Representation and Management of Data on the Internet 1
The Internet and the World-Wide Web 2
The Internet A worldwide network connecting millions of hosts Ø Interconnecting many Local Area Networks (LANs) (inter-network or just Internet) Ø The LANs connected to the Internet can be of various types Ø A host is a computer that is connected to the Internet Ø 3
History of the Internet Ø It started as a United States government project, sponsored by the Advanced Research Projects Agency (ARPA), and was originally called the ARPANET Ø The Internet grew quickly throughout the 1980 s and 90 s Ø Less than 600 computers were connected to the Internet in 1983; now there are over 10 million 4
Internet Applications Email Ø World-Wide Web Ø FTP Ø Telnet Ø Newsgroups Ø Chat Ø. . . Ø 5
The Web 6
The Web Ø Ø Ø 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 7
The Web Ø Ø Ø The term World-Wide Web (or simply Web) describes a collection of many pieces of information that are found on the 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 8
Web Servers These pieces of information are stored as files on particular hosts of the Internet Ø These hosts are called Web servers Ø 9
Information 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 Ø Every type of information can have different formats for storing it as a file Ø For example, some formats for storing images are jpeg, bmp, gif, ps, pdf Ø 10
HTML Much of the information that is found on the Web is stored as HTML files Ø HTML is a markup language formatting text. In addition, HTML facilitates inclusion of other types of information (such as images) in our text documents Ø Here is an example of an HTML document Ø This is how it looks like when displayed inside a browser Ø 11
Browsers We use a browser to display HTML documents Ø The browser is responsible for fetching the documents and displaying their contents according to the HTML rules Ø 12
Browsing 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 Ø Browsing/surfing refers to the activity of viewing documents in the Internet and following their links Ø 13
URLs Each information piece on the Web has a unique identifying address which is called a URL (Uniform Resource Locator) Ø A URL takes the following form: Ø http: //www. huji. ac. il/index. html Ø protocol Ø hostname file It has 3 parts: a protocol field, a hostname field and a file field 14
URL Fields The protocol field (“http” in the previous example) specifies the way in which the information should be accessed Ø The host field specifies the host on which the information is found Ø The file field specifies the particular location in the host's file system where the file is found Ø There could be more complex forms of URLs, but we do not discuss them Ø 15
Search Engines What are search engines? Ø How do they work? Ø Shortcomings of search engines Ø Some popular search engines: Infoseek, Hot. Bot, Altavista, Excite, Lycos, Yahoo!, Jeeves, . . . Ø 16
HTTP Daemons The information pieces of the Web are stored as files on Web servers Ø In order to make these information pieces available to other hosts, each server runs an HTTP-daemon Ø 17
(HTTP Daemons (continued An HTTP-daemon is an application that is constantly running on the server and waits for requests from remote hosts Ø A host can request the daemon for a document (a file) that is located on the server Ø Technically, any host connected to the Internet can act as a Web server by running an HTTP-daemon application Ø 18
Browser - HTTPD Interaction user requests http: // www. cs. huji. ac. il /index. html GET /index. html host www. cs. huji. ac. il sends the content of index. html HTTPD application Browser Disk 19
Browser - HTTPD Interaction Ø Ø Ø 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 20
IP (Internet-Protocol) Addresses Hostnames are used by people. The network mechanism uses IP-addresses instead Ø Every host connected to the Internet has a unique IP address that identifies it Ø 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 Ø 21
IP Packets Ø Ø 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 22
Routing IP Packets Ø Ø 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 IPaddresses 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 23
Using IP Addresses How does the browser know the IP address of the Web server? Ø 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 Ø However, it is inconvenient for people to remember such addresses Ø 24
Internet Addresses 1. 2. Many hosts have, in addition to IP address, human-readable Internet Address (or hostnames) Here are some examples of Internet Addresses: 1. www. cs. huji. ac. il 2. www. cocacola. com 3. www. yellowpages. co. il 4. www. isdn. net. il 3. 4. The first part is the name of a particular host (i. e. , computer) The rest is the domain name 25
(Internet Addresses (continued Ø Hostnames have a hierarchical structure www. cs. huji. ac. il Øwww 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) Ø 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 26
Generic Domains Ø 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 (Inter. NIC) • org - other organizations (www. w 3. org) 27
Country Domains Ø Generic domains usually refer to hosts inside the U. S. Other countries use two-letter country domains: • • Ø 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 28
Back to the Browser Ø Ø Ø 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 29
Translating IP Addresses to Hostnames Ø Ø Ø 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 30
DNS The browser (and other Internet applications) use a DNS-Server to map hostnames to IP addresses Ø DNS (Domain Name System) is an hierarchical scheme for naming hosts Ø 31
Proxy Servers Ø Ø Ø 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 32
Proxy Server proxy asks the document from the HTTPD user requests a document browser request the document from the proxy sends the content of index. html Proxy server Browser Proxy application Cache 33
Advantages of Proxy Servers Ø 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) 34
Firewalls A firewall poses restrictions on the traffic in or out of a local-area network Ø Examples: Ø ØHides sensitive data from the outside world ØPrevents access of local users to specific sites outside the local-area network 35
How 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 Ø A firewall application is installed on this host Ø The firewall examines all the in and out traffic of IP-packets and discards illegal packets Ø 36
Dynamically Generated Documents user requests http: //www. excite. com/search? what=something GET /search? what=something host www. excite. com sends the contents of index. html HTTPD application execution of search program Browser 37
Basic Networking Concepts 38
Local-Area Networks A Local-Area Network (LAN) covers a small distance and a small number of computers LAN A LAN often connects the machines in a single room or building 39
LANs (Local-Area Networks) Ø Ø Limited size Privately owned • • Centrally managed Usually hosts physically connected via a cable Homogeneous devices & protocols Known features (latency, bandwidth, . . ) 40
WANs (Wide Area Networks) 41
Wide-Area Networks A Wide-Area Network (WAN) connects two or more LANs, often over long distances LAN A LAN is usually owned by one organization, but a WAN often connects different groups in different countries 42
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, . . . 43
Layering models protocol sketches protocol CAD protocol modem protocol 44
TCP/IP Ø A protocol is a set of rules that determine how things communicate with each other Ø The software which manages Internet communication follows a suite of protocols called TCP/IP Ø The Internet Protocol (IP) determines the format of the information as it is transferred Ø The Transmission Control Protocol (TCP) dictates how messages are reassembled and handles lost information 45
TCP/IP protocol suite Application HTTP, FTP, TELNET, . . . Transport TCP, UDP Internet IP Link Ethernet, Token-Ring, . . . 46
TCP/IP protocol suite Taken from "TCP/IP Illustrated Vol. 1" / Richard Stevens 47
Packets headers Taken from "TCP/IP Illustrated Vol. 1" / Richard Stevens 48
IP Layer Ø Ø Ø Transmission of packets between two hosts IP addresses Routing protocol 49
IP Addresses Class Network ID Host ID 32 bit Class A B C D E Ø From 0. 0 128. 0. 0. 0 192. 0. 0. 0 224. 0. 0. 0 240. 0 Till 127. 255 191. 255 233. 255 239. 255 247. 255 Net ID 7 bit 14 bit 21 bit 28 bit 27 bit Host ID 24 bit 16 bit 8 bit - Inter. NIC 50
Routing 51
Transport Layer TCP • Connection oriented • Reliable, keeps order Ø UDP • Connectionless • Unreliable • Fast Ø 52
Client-Server Model Server application Server machine 144. 12. 34. 99 Port 5746 Client application Client machine 190. 30. 42. 155 53
Well-Known Ports FTP 21 Ø Telnet 23 Ø HTTPD 80 Ø. . . Ø 54
End of Lecture 1 55
HTTP Protocol Hypertext Transfer Protocol Ø Used between Web-clients (e. g. , browsers) and Web-servers (and proxies) Ø Text based Ø Built on top of TCP Ø Stateless protocol Ø 56
HTTP Transaction -- Client Ø 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 57
HTTP Transaction -- Server Ø 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) Ø sends document data Ø 58
Reacting to Responses of Clients HTML pages are static documents Ø 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 Ø 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 Ø 59
Server Technologies Ø Ø 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) 60
Creating Pages on the Fly in the Server Ø 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) 61
CGI Programming Ø CGI is a scripting language Ø A cgi script works with an application that runs on the server and creates HTML code Ø An early technology 62
Java Servlets Ø Servlets are java applications that some Web servers can run Ø A Servlet creates pages on the fly and these pages are returned to the requesting browser 63
JSP and ASP Ø 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 Ø Microsoft ASP (Active Server Pages) • The code is VB (Visual Basic) scripts • The Web server must be Microsoft IIS server 64
Client Technologies Ø Ø Ø 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 65
Separating Contents from Style In HTML, the contents and the style of pages are inseparable • HTML tags actually refer only to the style Ø XML (e. Xtensible Markup Language) is a new markup language for marking the semantics (meaning) of the data Ø XML tags describe the meaning of each portion of text in an XML document Ø 66
XML Tags XML tags are similar to attributes in a relation Ø However, the attributes are the same for all the records of the relation Ø In XML documents, each portion of text has its own tag • <course> databases </course> • <course> operating systems </course> Ø XML tags can be nested Ø 67
Parsing XML Documents XML facilitates easy parsing of documents according to their semantics 2. For example, the CS Department has many Web pages of courses 3. Can we write a program that reads all these pages and prints a list of the names of courses? 4. If XML tags are used, it is easy to do that 1. 68
Using XML is important in the context of data exchange between applications Ø It is possible to define a common set of tags that are suited for specific applications Ø For example, Math. ML is used for exchanging mathematical information Ø 69
Showing XML Document in Browsers Ø XML documents contain data with semantic tags Ø For a graphical representation, information about the style must be added • For example, HTML tags provide information about the style 70
Style Sheets ØStyle is added to XML documents by means of style sheets ØThere 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 71
Putting it All Together Ø 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) 72
? How Should XML be Used Ø Ø 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 Ø a general style, and Ø information that is semantically well defined without making the process of creating documents too cumbersome? 73
Course topics Ø Server-side programming • JDBC for connecting to the DBMS • Servlets • JSP Ø Client-side programming • Java Scripts • CSS Ø Data storage and processing on the Web • XML • XSL 74
Скачать презентацию Basic Internet and Networking Concepts Representation and Management
75bbd477a6f0a676c1d1c7879548c2c1.ppt