Unraveling the Web: How Does it All Work?

1. Unraveling the Web:How Does it All Work?

2. Web Enabling Technologies • TCP/IP network (Internet & others) • URLs • HTTP protocol and HTTP Servers • HTML & MIME type system • Other network service protocols and servers • Web browsers

3. Uniform Resource Locators • Naming scheme: unambiguios way of telling where to find “things” • Indicates protocol, host, port, and “path” • Syntax: protocol://hostname:port/path • Examples: http://whitney:8000/lectures/index.html http://www.cs.purdue.edu/

4. Valid Characters in URLs • Valid: Upper & lower case letters, numerals and $, _, @, ., - • Special: = ; / # ? : % & + • Others must be escaped: %xx where xx is two-digit hex code of character • Example: CR “%0D”, space “%20”, percent “%25” • Help from browsers to handle this

5. URL Specificity • Complete URL: all parts of URL • Partial URL: no protocol/host, contains full path. Browsers must interpret relative to current page • Relative URL: only last part of path, similar to relative paths in OSs • Use: relative URLs for related docs, partial URLs for docs on same server and complete URLs for remote docs

6. Types of URLs • (Local) File: file:///path_to_the_file • HTTP: http://hostname[:port]/path • FTP: ftp://[user[:passwd]@]hostname[:port]/path_to_the_file • Gopher: gopher://hostname:port/path • Telnet: telnet://hostname:port/

7. Types of URLs • WAIS: wais://hostname:port/database_name?query • News: news:name.of.group/[article_selection] • MailTo: mailto:email_address

8. URLs • Uniform Resource Locators • Universal Resource Locators • General concept of universal resource identifiers (URIs): URLs and Universal Resource Names (URNs) are implemented

9. HyperText Transfer Protocol • Simple request-reply readable protocol • Five requests: GET, HEAD, POST, PUT, DELETE • Request includes document URL + (possibly) additional info: User info, browser info, capabilities, wishes, ... • Reply includes status information, a reply header and data

10. HTTPD • Implements HTTP • Two popular free implementations: NCSA HTTPD and CERN HTTPD • Commercial: Netscape Server, .. • Special: Netscape Commerce Server, .. • Basic one is simple!

11. HyperText Markup Language • Based on SGML, Standard Generalized Markup Language • Markup is not layout: no fonts, points, ... • Abstract styles indicate parts of docs: level 1 header, level 2 header, paragraph, table, citations • HTML: Specifies structure of document, not format!

12. MIME • Multipurpose Internet Mail Extensions • Mechanism for sending multimedia data over email • Content-Type: field indicates the type • Additional fields for length, encoding, compression etc. • Metamail: MIME reference impl. for range of email systems

13. MIME Types • Indicates the semantic interpretation of some content • Describes a document by referring to standardized list of types organized by type and a subtype • Examples: text/plain, text/html, video/mpeg, image/gif, image/jpeg, application/postscript, */x-*, ...

14. MIME Types and WWW • HTTP request lists preferred doc typs • HTTP reply indicates doc type supplied • Browser uses type-specific handler to “display” document • Some types are natively supported, some not • Helper applications for non-native types • Easy to add new content types!

15. MIME Types and Filename Extensions • Useful mechanism for identifying MIME type of a file object • Server relies on this heavily! • Example: .ps, .eps, .epsi, .epsf => application/postscript • Example: .gif => image/gif

16. Other Network Service Protocols and Servers • FTP client and ftpd • Telnet client and telnetd • NNTP client and nntpd • Goper client and gopherd • ... • Hooked to the Web by smart browsers

17. Web Browsers • Support HTTP and possibly other protocols • Display HTML documents natively • Graphical browsers show in-line images/graphics • Some support “special” HTML extensions • Does all of above to browse the Web!