Web pages

1. Web pages Programming Language Design and Implementation (4th Edition) by T. Pratt and M. Zelkowitz Prentice Hall, 2001 Section 12.2.1

2. ARPANET • Initial idea was by Defense Advanced Research Project Agency (DARPA) project in late 1960s for a national defense network • began a project to see whether several computers, widely separated geographically, could be linked together to enable users at a terminal on one system to access the resources on another computer. • Initial military concept: provide access to computers if some communications lines are destroyed by building a network where data communications traffic could dynamically adapt to changing conditions. • data communications -sending messages reliably from one computer to another- was the major obstacle. • The initial ARPANET began in 1970 as a three-node network linking BBN in Cambridge, Massachusetts, with UCLA and SRI in California using 56 kilobit lines. • Sites added until several hundred by mid-1970s

3. ARPANET communications Communication between two computers was handled via messages. A message was broken down into fixed-length strings called packets, and the packets were sent from computer to computer until the original message was reassembled at the receiving node. To ensure that messages destined for another computer arrived reliably, a formal communication model -called a protocol -was developed. For the ARPANET, this developed as the Transmission Control Program/ Internet Protocol (TCP/IP). TCP/IP was a low-level communication mechanism that simply determined that a sequence of bytes destined for a specific computer arrived there uncorrupted. It was generally too complex for users to use directly for accessing a computer.

4. User protocols Telnet is a protocol that makes the sending computer -the computer the user is actually working on -behave like a terminal connected to the distant computer. - user is connected to a client computer, which acts like a terminal, and the terminal program is communicating using the telnet protocol to a distant host computer, which is providing the server program. SMTP is Simple Mail Transport Protocol. This provides the basic e-mail (electronic mail) that has become so ubiquitous today FTP is File Transfer Protocol. One would invoke the FTP client on a local machine, log onto the distant server machine using the FTP protocol, and then retrieve the desired documents from the distant machine or send documents from the user's machine to the distant machine.

5. Weaknesses in FTP one had to know explicitly which machine to access to retrieve the desired data. One also had to have access to the files of that machine to retrieve the information. The anonymous login partially solved that. One had to know exactly where on the file system the desired information was. Despite these weaknesses, FTP was the standard file transmission mechanism for many years until the web changed all that.

6. Birth of the Internet In the mid-1980s, ARPA decided to stop supporting the ARPANET. As a research activity, the concept had been proved, and ARPA was not in the business of providing what was becoming a commercial service. The U.S. National Science Foundation (NSF) took over the backbone network in the United States -the set of high-speed telephone lines that provided the basic TCP/IP communications traffic between host computers as a way to link universities together. The name of the network gradually evolved into the Internet. NSF support stopped. Attached to this backbone, local networks (a state, a university, a large company) were added until the Internet became this amorphous collection of computers all continually chattering to one another. Commercial providers, now called Internet Service Providers (ISP) established links to the Internet so that individuals on their home computers could use a modem to dial into their local ISP to be on the Internet.

7. The World Wide Web By the late 1980s, widespread interest to easily transfer files. FTP was a cumbersome process. Systems like gopher, archie, veronica developed Physicists -principally Tim Berners-Lee at CERN in Geneva -desired a mechanism to access and transfer documents by computer that was simpler than the standard FTP server. They developed the concept of a semantic description language. One server program would display a document, and a client program, called a browser, would read and understand the displayed document. The power of their system was that the displayed document contained pointers to other documents called hypertext. An earlier version of hypertext was Apple Computer's HyperCard product for the Macintosh, but the real power of the CERN development was to allow hypertext links to documents that existed on other computers connected to the Internet.

8. HTTP The protocol developed was the HyperText Transport Protocol (HTTP). Http an addition to TELNET, FTP, and SMTP protocols discussed earlier. Release of the first MOSAIC browser in 1993 led to rapid growth of the web. Each pointer became known as a Uniform Resource Locator (URL). Document location was reduced to: • invoking a Web browser on your local machine, • typing in a URL for the document you wanted to access, • connecting to a Web server on the distant machine that contained the location of the typed in URL, • displaying the document obeying the HTTP protocol. HTML language based upon SGML - to be explained later.

9. Web navigation

10. Prentice Hall example of navigation 1. The user types in the URL for the home page. This URL consists of: a Domain name (www.cs.umd.edu) and a file on that machine(users/mvz/pzbook). 2. The Web browser sends the domain name to one of several special Internet machines called Domain Name Servers} (DNS). The DNS returns the Internet Protocol address of the desired web page. 3. The web browser sends the file name to the Web server at IP address 128.8.128.80. A HTTP Daemon (HTTPD) program on this machine is the main interface between a web server and the Internet. 4. The Web server appends the name index.html because the given file was a directory and not a file. 5. The contents of the file are sent back to the Web browser and displayed to the user. 6. If the user now clicks on the URL for Prentice-Hall that appears on the Web page (www.prenticehall.com), the process is repeated and the Prentice-Hall server at IP address 63.69.110.94 is accessed and the appropriate Web page is displayed.

11. Portals To make navigation easier, certain Web sites are now known as portals -entrance sites to the WWW. These sites have programs known as search engines. A search engine is a query processor in which you enter a question. The result of that query is a list of WWW locations that answer the question. Search engines often operate as Web crawlers. Beginning at one location, the Web crawler follows all links on that Web page to find other Web pages.

12. SGML • Structured General Markup Language is basis of SGML • an unstructured sequence of characters • within the text can be SGML elements. The semantics of elements are unspecified, but their syntax is given. • elements are bracketed by a start-tag and an end-tag notation. <zork> I am a zork </zork> identifies “I am a zork” as the contents of the zork element. • A report in SGML: <report> <title} text </title> <author} text </author> <abstract} text </abstract> <body} text </body> </report> • SGML handles semantic content, not presentation

13. HTML • An instance of SGML with a defined syntax for Web pages <html> <title> title of document </title> <body> text of document </body> </html> • Problem: SGML is semantic content, not layout (presentation). • How to handle things like: <h1>Major heading</h1> - What font and font size to use? - Where on page to place heading? • Elements like <font size=...> move away from pure semantic content

14. Links in HTML HTML contains: • Embedded text • URLs: Links to other web pages <http://web address> • Images: <SRC SRC=...> • MAILTO: protocol (Send email) • Executable pages (CGI scripts. To be discussed soon)