1. Internet and Web Programming Basics Web Programming

2. Networking • Early computers were highly centralized. • Single point of failure • User has to access the computer. • Low cost computers made it possible to get past these 2 primary disadvantages with a network. • Network – “ ... communication system for connecting end-systems”

3. Networking • End-systems also known as “hosts” • PCs, workstations • dedicated computers • network components • Advantages of networking • Sharing of resources • Price/Performance • Centralized administration • Computers as communication tools

4. Networking • Mechanisms by which software running on two or more endpoint can exchange messages • Java is a network centric programming language • Java abstracts details of network implementation behind a standard API

5. Networking - Traditional Uses • Communication (email) • Resource Sharing • File exchange, disk sharing • Sharing peripherals (printers, tape drives) • Remote execution • …

6. Networking - New(er) Uses • Information sharing • Peer-to-Peer computing • Entertainment, distributed games • E-Commerce • Collaborative computing • Forums • Chats • WWW

7. LAN - Local Area Network • Connects computers that are physically close together ( < 1 mile). • high speed • Technologies: • Ethernet 10 Mbps, 100Mbps • Token Ring 16 Mbps

8. WAN - Wide Area Network • Connects computers that are physically far apart (long-haul network). • typically slower than a LAN. • typically less reliable than a LAN. • Technologies: • telephone lines • satellite communications

9. Client/Server Architecture • Classical network architecture is a client/server (C/S) architecture • A server is a process (not a machine!) waiting for requests from a client. • A client is a process (not a machine!) sending requests to a server and waiting for a reply. • Both client and server are software entities

10. Client/Server Architecture • Server examples: • finds a document. • prints a file for client. • records a transaction. • Servers are generally more complex • Two basic types of servers: • Iterative - handles one client at a time. • Concurrent - handles many clients in parallel.

11. Iterative Server • Naïve server implementation is sequential. • handles one request at a time. • Consider a server that needs to read data from a disk • Reading a file from disk takes a long time • The server will be idle while it waits for the data to be read • Other clients will be waiting

12. time Start request loading Awaiting disk availability Deliver the data across network Iterative Server

13. Concurrent Server • Threaded servers can process several requests at once. • Each request is handled by a separate thread. • This does not increase the overall amount of work done, but reduces the wastage! • Threaded operation is worthwhile when threads are expected to block, awaiting I/O operations

14. time Start request loading Awaiting disk availability Deliver the data across network Concurrent Server

15. Networking Models • Using a formal model allows us to deal with various aspects of networking abstractly. • We will look at a popular model – OSI reference model. • ISO proposal for the standardization of the various networking protocols (1984) • The OSI reference model is a layered model.

16. Layering • Divide a task into pieces and then solve each piece independently (or nearly so). • Establish a well defined interface between layers . • Major advantages: • Independence • Extensibility

17. Applications Libraries System Calls Kernel Layered System Example – Unix OS

18. OSI 7-Layer Model: High level protocols 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data-Link 1 Physical Low level protocols

19. OSI Communication Model

20. Communication Protocols • Communication between two sides is defined through a protocol • Protocol – An agreed upon convention for communication • Both sides need to understand the protocol. • Examples: TCP/IP, UDP, others • Protocols must be formally defined and unambiguous • Tons of documentation

21. Process Process Interface Protocols Transport Transport Peer-to-peer Protocols Network Network Data Link Data Link Interface vs. Peer-to-Peer Protocols • Interface protocols describe the communication between layers on the same side. • Peer-to-peers protocols describe the communication between the sides at the same layer.

22. Layers • Physical Layer: transmission of raw bits over a communication channel • Data Link Layer: divides data into packets and provides an error-free communication link • Network Layer: selects path between the two sides, fragmentation & reassembly, connection between network types

23. The Transport Layer • Transport Layer: provides virtual end-to-end links between peer processes • TCP – Transmission Control Protocol • Connection oriented • Reliable, keeps order • UDP – User Datagram Protocol • Connectionless • Unreliable • Fast

24. Layers • Session Layer: establishes, manages, and terminates sessions between applications • Presentation Layer: responsible for data compression and encryption • Application Layer: anything above previous layers (specific applications)

25. The Internet • A worldwide network connecting millions of hosts • WAN interconnecting many LANs of various types • Applications • World-Wide Web • Email • FTP • … more and more

26. The Web • The term World-Wide Web (or simply Web) describes a collection of pieces of information that • are stored as files on particular hosts • can be reached by other connected hosts • These hosts are called Web servers

27. Web or Internet? • They are not the same things. • The Internet is a collection of computers or networking devices connected together. • They have communication between each other. • The Web is a collection of documents that are interconnected by hyper-links. • These documents are provided by Web servers and accessed through Web browsers.

28. How does the Web Work? • The Web information is stored in the Web pages • In HTML format. • The Web pages are stored in the hosts called Web servers • In the Web server file system. • The computers reading the pages are called Web clients using specific Web browser • Most commonly Internet Explorer or Netscape. • The Web server waits for the request from the Web clients over the Internet • Internet Information Server (IIS) or Apache.

29. HTML • Much of the information that is found on the Web is stored as HTML files. • HTML is a scripting language for storing formatted text. • allows to combine other types of information (such as images) in the documents. • Allows interconnection (links) between the documents.

30. Browsers • Are used to display HTML documents. • The browser is responsible for • fetching the documents • displaying them according to the HTML rules. • Browsing refers to the activity of viewing Web documents through following the links.

31. Addresses • Each communication endpoint must have an address. • Consider 2 computers communicating over a network: • the communication protocol must be specified • the name of the host (end-system) must be specified • the specific process of the host must be specified.

32. URLs • Each Web document has a unique identifying address called a URL (Uniform Resource Locator). • A URL takes the following form: http://cs.vu.ac.at/courses/webp/index.htm • URL structure: <scheme>://<user>:<password>@<host>:<port>/<path>;<params>?<query>#<frag> file protocol host

33. URL fields • The protocol field 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 on the host’s disk (path) where the file is found and the name of the file • There could be more complex forms of URLs but we do not discuss them

34. IP Addresses • Hostnames (i.e., URLs) are used by people. • Network mechanisms use IP-addresses instead. • Every host connected to the Web has a unique IP address that identifies it. • IP addresses are • 32-bit (4 byte) numbers • usually written as four decimal numbers separated by dots, e.g. 135.17.98.240, where the numbers refer to the above 4 bytes.

35. Ports • As data traverses the Web, each packet carries not only the address of the host but also the port on that host to which it is aimed. • 65,536 ports are available at each host. • A port does not represent anything physical like a serial or parallel port. • Hosts are responsible for reading the port number from the packets they receive to decide which program should process that data.

36. Ports • On Unix systems, ports between 1 and 1023 are reserved for the OS processes. • Any process can listen for connections on ports of 1025 to 65,535 as long as the port is not already occupied. • In Windows and Mac-OS, any process can listen to any port.

37. Well-Known Ports • Many services run on well-known ports. • Web HTTP servers listen for connections on port 80. • SMTP servers listen on port 25. • Echo servers listen on port 7. • FTP servers listen on port 21. • Telnet servers listen on port 23. • DayTime servers listen on port 13. • whois servers listen on port 43. • finger servers listen on port 79.

38. Client-Server Model Server application Client application Port 5746 Server machine 144.12.34.99 Client machine 190.30.42.155

39. Hostnames • However it is inconvenient for people to remember IP addresses and ports. • Many hosts have in addition to IP address a human readable hostname. • www.vu.ac.at • www.cnn.com

40. Hostnames • Hostnames have hierarchical structure. • Hostname www.cs.vu.ac.at, refers to the host www in the computer science (cs) department of the Vienna University, which is an Academic Campus (ac) in Austria (at). • The rightmost part describes the main domain of the host. Left to it, a sub-domain, and further left more specific sub-domains.

41. Domains • There are generic domains • com commercial organizations • edu educational institutions • gov U.S. governmental organizations • Most countries use country domains: • il Israel • uk United Kingdom • jp Japan

42. DNS Servers • The mapping between the hostnames and the corresponding IP address is done by DNS. • It is not feasible for the Web browser to hold a table mapping all the hostnames to their IP-addresses. • New hosts are added to the Web every day • Hosts change their names and IP addresses.

43. Web Protocols • It is a special set of rules that endpoints (both clients and servers) in the Web use to handle communication. • Transmission Control Protocol (TCP) – To exchange messages with other endpoints at the information packet level. • Internet Protocol (IP) – To send and receive messages at the address level. • Hypertext Transfer Protocol (HTTP) – To deliver HTML, sound, audio files on the World Wide Web.

44. HTTP Protocol • HyperText Transfer Protocol • Used between Web-clients (e.g., browsers) and Web-servers • Text based • Built on top of TCP protocol • Stateless protocol • No data about the communicating sides is stored

45. HTTP Transaction - Request • Client sends a request that looks like • GET /index.html HTTP 1.0 • GET is a keyword • Index.html is the requested document • HTTP 1.0 is the protocol version that the client understands • The request terminates always with \r\n\r\n. • Client may send optional information • For example, <keyword:value> list • User-Agent : browser name • Accept : formats the browser understand

46. HTTP Transaction - Request • Request example: GET /index.html HTTP 1.0 User-Agent: Lynx/2.4 libwww/2.1.4 Accept: text/html Accept: text/plain • In addition to GET, clients can request • HEAD – Retrieve only header for the file • POST – Send data to the server • PUT – Upload a file to the server

47. HTTP Transaction - Response • 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 contents (e.g., html file)

48. HTTP Transaction - Response HTTP/1.1 200 OK Date: Fri, 16 Apr 2004 18:48:13 GMT Server: Apache/1.3.29 (Darwin) Last-Modified: Fri, 16 Apr 2004 10:15:59 GMT ETag: "58db37-89-407fb25f" Accept-Ranges: bytes Content-Length: 137 Connection: close Content-Type: text/html <html> <body> <p>Welcome</p> <img src=“smiley.gif"> </body> </html> HTTP Header Blank line Data

49. HTTP Transaction Example

50. HTTP 1.0 response codes • 2xx Successful • response codes between 200-299 indicate that response understood and accepted • 200 OK – the most popular respond indicate success • 201 created – respond to successful POST request • 202 accepted – respond to POST request, meaning processing is not over yet • 204 no content – the server successfully processed the request, but has no content to send back