1. Basic Internet and Networking Concepts Representation and Management of Data on the Internet

2. The Internet and the World-Wide Web TCP/IP and Web Browsers

3. The Internet and the Web • Internet means Inter-Network • A world-wide network of many LANs (local-area networks) • The LANs are of various types • Web means World-Wide Web • A large collection of information arranged as hypertext and stored in many computers that are part of the Internet • The two are related but not the same

4. A Bit of History • The Internet grew very rapidly throughout the 1980s and 90s • Less than 600 computers were connected to the Internet in 1983 • Now there are tens (if not hundreds) of millions of computers • The Web started in 1989 and grew very rapidly during the 1990s • The current Web has billions of pages

5. Internet Applications • Email • Telnet • FTP • Newsgroups • World-Wide Web • Chat • ...

6. The Web

7. Web Browsers • Web browsers provide a very convenient interface for viewing the information stored on the Web • Mosaic – the first browser – was introduced in 1993 and sharply increased the popularity of the Web

8. TCP/IP • TCP/IP is the common language of the Internet • IP – Internet Protocol • TCP – Transmission Control Protocol • The IP protocol transmits packets of data from one host (i.e., computer) to another • The TCP protocol uses many packets to transmit a long stream of data

9. TCP vs. IP • IP routes each packet from the source host to the destination host • IP is oblivious to the fact that usually each packet is part of a data stream • TCP handles correctly a long data stream • Divides a long data stream into many packets, at the source • Reassembles the packets, in the right order, at the destination • Handles errors and lost data

10. Sockets • Sockets are a common interface that make TCP streams look like file streams • Modern programming languages support sockets • A read or a write operation to/from a socket may block • Until data arrives, or • Until data can be sent • Use multiple threads so that blocking will not cause the whole GUI to freeze

11. IP Addresses, Host Names and URLs

12. IP Addresses • A computer connected to the Internet is called a host • Every host has a unique IP address • An IP address consists of 32 bits that are written as four decimal numbers, separated by dots • Example: 135.17.98.240 • The numbers denote the four bytes composing this address

13. Internet Addresses • In addition to an IP address, a host may also have a human-readable Internet address (or hostname) • Some examples of hostnames: • www.cs.huji.ac.il • www.cocacola.com • shum.cc.huji.ac.il • The first part is the name of a particular host (i.e., computer) • The rest is the domain name

14. The Hierarchical Structureof Hostnames • Example: www.cs.huji.ac.il • www is a name of a computer • That computer is in the CS Department • That dept. is at The Hebrew University of Jerusalem (huji) • That university is an Academic Campus (ac) in Israel (il) • The rightmost name, il, is the main domain • As we move left, the sub-domains are more specific

15. The First 7 Generic Domains • com - commercial organizations (www.cocacola.com) • edu - educational institutions (www.berkeley.edu) • gov - U.S. governmental organizations (www.cia.gov) • int - international organizations • mil - U.S. military • net - networks (InterNIC) • org - other organizations (www.w3.org) • More domains have been added in recent years

16. Country 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 have sub-domains that correspond to the generic domains, for example: • co.il is the domain of all commercial organizations in Israel • ac.il is the domain of all academic institutions in Israel

17. URLs • Each information piece on the Web has a unique identifying address, 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 file protocol hostname

18. URL Fields • The protocol field (“http” in the previous example) specifies the way in which the information should be accessed • The hostname 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 • More complex forms of URLs are possible

19. Using IP Addresses in URLs • 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 hostname field of the URL, for example: http://135.17.98.240/index.html • However, it is inconvenient for people to remember such addresses

20. From Hostnames to IP Addresses • 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 that hostname to the corresponding IP address of the given host • There is no algorithm for computing the IP address from the hostname • A lookup table provides the IP address of each hostname

21. Where is the Translation Done? • 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 that maps all hostnames to their IP-addresses • Moreover, new hosts are added to the Internet every day and hosts change their names

22. DNS (Domain Name System) • The browser (and other Internet applications) use a DNS Server to map hostnames to IP addresses • DNS is a hierarchical scheme for naming hosts • The command nslookup gets an IP address and returns a hostname or vice-versa • It runs on clients and contacts a DNS server

23. LANS, IP Addresses and Routing How IP Packets are transmitted Across the Internet

24. IP Addresses • An IP address consists of 4 bytes • Each byte is a number in the range 0 – 255 • For example, 132.64.1.10 • The first 1 to 3 bytes identify the network and the remaining 1 to 3 bytes identify hosts on the network • There are several classes of network addresses • Subnet masks effectively increase the number of networks • Network Information Center (NIC) assigns IP addresses to organizations and companies

25. Classes of IP Addresses • Class A: The first byte is 1 – 127 • 1 byte for network and 3 for host • Class B: The first byte is 128 – 191 • 2 bytes for network and 2 for host • Class C: The first byte is 192 – 223 • 3 bytes for network and 1 for host • Classes D and E: 224 – 255 • These classes have special functions, e.g., a multicast packet uses a class D address

26. Subnet Masking • The network part of an IP address identifies a LAN (Local-Area Network) • Hosts in a given LAN can be up to 100 meters from the LAN switch • HU has one class B network address, namely, 132.64 (CS is 132.65) • But HU needs many LANs ! • Subnet masking solves this problem

27. Defining a Subnet Mask • The subnet mask is a four-byte sequence of 1’s followed by 0’s, e.g., 255.255.255.0 • IP addresses are interpreted as follows: • Any bit that is 1 in the mask identifies the network and any bit that is 0 identifies the host • When the subnet mask 255.255.255.0 is applied to an IP address of Class B, e.g., 132.64.112.52, it means that • The first 2 bytes identify a network (HU) • The third byte identifies a subnet, i.e., a specific LAN • The fourth byte identifies a host

28. Local-Area Networks (LANs) • LANs are typically built by connecting hosts to a 100Mbit Ethernet LAN switch, using Category 5 cables • Maximal distance between switch and host is 100 meters • LAN switches transmit IP packets between hosts on the same LAN • LAN switches translate IP addresses to physical addresses (MAC addresses)

29. Routers • LAN switches are connected using fiber optics to routers • Routers route IP packets across LANs • A router is connected directly to two or more LANs and it can transmit IP packets between this LANs (local routing) • Some routers are connected to each other via WANs (Wide-Area Networks) and do backbone routing

30. Hop-by-Hop Routing • Suppose that an IP packet is sent from a LAN to another far-away LAN • The message gets to the router that is directly connected to the source LAN • The router sends it to the next hop, i.e., • A router on the same LAN that is also connected to some other LANs, or • A router on the same WAN

31. Routing Tables • Each router has routing table with prefixes of IP address • Each prefix has a router address for the router that handles that prefix • Given an IP packet with some IP address, the next-hop router is determined by matching the longest prefix (of an IP address) from the routing table with the given IP address • There is a default entry for the largest routers in the backbone of the Internet

32. Updating the Routing Tables • The routing table includes local information provided by the local network administrator • Router periodically update their routing tables by exchanging information with their neighboring routers • Routing protocols: Distance Vector (Bellman-Ford), Open Shortest Path First (OSPF)

33. A Short Overview of How the Web Works The HTTP Protocol, Web Proxies, Dynamic HTML Pages

34. The HTTP 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 (it doesn’t remember your previous requests)

35. Browsers Are Clients • We use a browser to display HTML pages • The browser is responsible for fetching the HTML pages and displaying their contents according to the HTML rules

36. Web Servers • HTML pages are stored in file systems • Some hosts, called Web servers, can access these HTML pages • Each Web server runs an HTTP-daemon in order to make its HTML pages available to other hosts • The term “Web server” refers to the software that implements the HTTP daemon, but sometimes it also refers to the host that runs that software

37. HTTP Daemons • An HTTP-daemon is an application that is constantly running on a Web server, waiting for requests from remote hosts • Technically, any host connected to the Internet can act as a Web server by running an HTTP-daemon application • A Web client (e.g., browser) connects to a Web server through the HTTP protocol and requests an HTML page

38. Browser-HTTPD Interaction user requests http:// www.cs.huji.ac.il /index.html GET /index.html host www.cs.huji.ac.il Web server sends the content of index.html HTTP daemon Browser Disk

39. Browser-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 HTML page /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 content of the file to the browser • The browser receives the HTML page, parses it according to the HTML rules and displays it

40. HTTP Transaction – Client • Client request: • The request GET /index.html HTTP/1.0 • Optional header information User-Agent: browser name Accept:formats the browser understands ... • A blank line (\n) • The client can also send data (e.g., the data that the user entered into an HTML form)

41. HTTP Transaction – Server • Server response: • Status line HTTP/1.0 200 OK • Header information Content-type: text/html Content-length: 3022 ... • A blank line (\n) • Document data

42. Proxy Servers • A proxy server acts as a delegate of browsers for accessing the Web • The browser transfers the request for a document to the Proxy • The Proxy contacts the Web server and fetches the document on behalf of the browser

43. Proxy Server proxy asks the document from the HTTPD user requests a document browser requests the document from the proxy sends the content of index.html Proxy server Browser Proxy application Cache

44. Advantages 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)

45. Responding to Clients’ Inputs • HTML pages are static documents • Sometimes users supply input, for example, keywords submitted to a search engine • The Web server has to react to this input • The output is an HTML page that is not known in advance • In order to react to the input, the Web server may have to use some applications (e.g., database queries)

46. Server-Side Programming • Writing applications that react to clients’ inputs by creating HTML pages on the fly is known as server-side programming • A client 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 response to the above request is a dynamic HTML page and generating it may involve interaction with other applications (e.g., database queries)

47. Generating Dynamic HTML Pages user requests: http://www.excite.com/search?what=something GET /search?what=something host www.excite.com sends the content of index.html HTTPD application Browser execution of a search program

48. Server-Side and Client-Side Technologies Servlets, JSP and Java Scripts

49. Server-Side Technologies • There are five common tools for server-side programming; each one works with some of the available Web servers • CGI (Common Gateway Interface) programming • Java Servlets • JSP – Java Server Pages, or • Microsoft ASP – ActiveServer Pages (similar to JSP) • PHP

50. CGI Programming • CGI is a scripting language • A cgi script works with an application that runs on the server and creates HTML pages • An early technology