1 Unraveling the Web: How Does it All Work?

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

3 Uniform Resource Locators F Naming scheme: unambiguios way of telling where to find “things” F Indicates protocol, host, port, and “path” F Syntax: protocol://hostname:port/path F Examples:

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

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

6 Types of URLs F (Local) File: file:///path_to_the_file F HTTP: F FTP: ath_to_the_file F Gopher: gopher://hostname:port/path F Telnet: telnet://hostname:port/

7 Types of URLs F WAIS: wais://hostname:port/database_name?q uery F News: news:name.of.group/[article_selection] F MailTo: mailto: _address

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

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

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

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

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

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

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

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

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

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