HTTP, HTML , and the Web

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HTTP, HTML , and the Web

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Basic Ideas of the Web

• Certain machines on the internet have direct
  access to (usually on their disks) a large number
  of documents of varying formats but with
  content others are interested in. Call these web
  server machines. Document formats are identified
  by file name suffixes like file.doc or
  file.html
• Goal is to allow users on remote computers across
  an internet to see these documents displayed
  according to their format.
• To accomplish User program on users remote host
  becomes a client of a web server must contact
  the web server
• User program has to somehow identify which web
  server machine it wants to get to and which
  document on that machine it wants.
• Then it has to set up a connection to that
  machine, ask for the document and wait till the
  server machine sends it back a copy
• Then the users program must display the document
  according to its appropriate format.
• Users application program that does all this is
  called a web browser

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Web Browser-Server Model
Web Server Disk Storage Document resides here
3. Web Server retrieves Web Page from disk storage
Web Server Machine
Web Server Application
Internet (WWW)
2. Client Requests Web Page from Server (GET
request)
HTTP Spoken Here
4. Server downloads web page to client as body
part of reply to GET
Client Machine
Client Browser
Web Page
5. Browser visually renders document User sees
it!
1. User on client machine inputs request to see
document (web page) with a GIVEN URL
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Client/Server

• Uniform Resource Locator uniquely identifies
  ltserver-machine,documentgt with layout
• protocol//computerport/document
• http//clem.mscd.edu/evansell/MAINPOINTS.html
• http//clem.mscd.edu80/evansell/MAINPOINTS.html
• ftp//clem.mscd.edu21/evansell/MAINPOINTS.html
• Given a URL, client contacts server listed as
  computer sets up a TCP connection
• Requests document by sending a message on that
  connection
• Server replies sending back the document
  requested then tears down connection
• Transient connection
• Usually works well as users tend to get one page
  and then browse elsewhere, or slowly

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Client/Server Continued

• If document has embedded images (or other stuff),
  new connection for each embedded entity
• Not the best
• Therefore some persistence has been added
• Can create persistent connections
• Negotiated at connect

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HTTP Protocol used from Browser to Web Server

• Sent of messages and rules for messaging between
  web browsers (clients) and web servers - protocol
• HyperText Transmission Protocol (HTTP) (RFC 2616)
  Text-based protocol easy to read in the clear
• Just a few simple messages
• GET item version CRLF request to read a general
  object
• HEAD (status) - A request to return the response
  header only, without the content. This can
  contain much useful information about the
  requested entity, without the need to actually
  load it -- eg, how big it is.
• POST - Originally defined as a request to "append
  to a named resource" (eg, a Web page), this
  method is extensively used in CGI-based systems
• PUT - Request to store an object (eg, Web page,
  image, etc). Has only ever been used
  experimentally.

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HTTP Get

• GET /index.html HTTP/1.0ltnewlinegtltnewlinegt
• The response from the server consists of a status
  line, then a number of plain text headers,
  followed by a blank line and then the requested
  data object.
• Request GET /index.html HTTP/1.0
• Response
• HTTP/1.0 200 OK Server Netscape-Enterprise/3.5.1C
  
• Date Sun, 16 Mar 2003 114839 GMT
• Content-type text/html
• Last-modified Fri, 14 Mar 2003 022252 GMT
• Content-length 11378
• lt!doctype html public "-//w3c//dtd html 4.0
  transitional//en"gt
• lthtmlgt
• ltheadgt
• ........(etc the actual text of the document
  you asked to get)

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Server Response
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Hypertext Markup Language (HTML)

• What is hypermedia and hypertext? what is a
  hyperlink?
• Need a standard hypertext document format as a
  standard
• HTML is that standard it is a markup language
  that uses tags to specify document content and
  layout
• Child of SGML, which is a meta-language that HTML
  is defined in
• lttaggt lt/taggt

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Details on HTML

• HTML is a markup language -- documents are (in
  general) plain ASCII textfiles, with certain
  characters reserved to denote markup. Such
  languages have a long and venerable history in
  computing (eg starting with roff, TeX, (see also
  here), LaTeX, SGML and subsequently XML.
• The structure (or, to a somewhat lesser extent,
  the displayed appearance) of a HTML document (or
  Web page) is described using embedded formatting
  codes (or tags) intermingled with the information
  in the document.  
• In HTML, the markup tags are delimited by the
  special characters "lt" and "gt" -- the "less than"
  and "greater than" characters, often (rather
  clumsily IMHO) called "angle brackets". If either
  of these characters must appear as part of the
  actual data, they are written as lt and gt
  respectively.  
• HTML introduced a uniform, and revolutionary, way
  of specifying hyperlinks in a document, using the
  ltA HREF"...some URL..."gtlink textlt/Agt
  structure. This was revolutionary!  
• Modern HTML standards have evolved to support
  incredibly complex document layouts (using the
  ltTABLEgt markup, style sheets, client-side
  scripting, etc), seemlessly mingling text and
  graphics into what has become an entirely new
  form of media.

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Simple HTML Example See it formatted
ltHTMLgt ltHEADgt ltTITLEgt Elliott's Web Page
lt/TITLEgt lt/HEADgt ltBODY BGCOLOR"bb00bb"
LINK"ffff00" ALINK"ff0000" VLINK"00ff00"gt ltH
1gt Elliott on the Web lt/H1gt ltULgt ltLIgt See my web
page at ltA HREF"http//clem.mscd.edu/evansell"gt
Metro State lt/Agt lt/LIgt ltLIgt My personal home
page link will be added here - later lt/LIgt ltLIgt
Interests Jai-lai, outdated technology, older
movies lt/LIgt lt/ULgt ltHRgt Email me at ltA
HREF"mailtoevansell_at_mscd.edu"gt
evansell_at_mscd.edu lt/Agt lt/BODYgt lt/HTMLgt
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Web Browsers and Server

• Server application fairly simple just receives
  individual requests on temporary connections
  responds and tears down connection
• Usually multi-threaded
• HTTP server on port 80
• Web client browser much more complex
• Must be able to talk various protocols to get
  documents from web servers http, ftp, etc.
• Must be able to format and render documents at
  least HTML type if not others
• Must do caching to save on real time
• Must do other things like be able to handle
  embedded documents or support bookmarks
  (favorites)

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Cache

• Browsers cache documents for faster repeat access
• Save a copy on the local disk
• Faster than going through network
• Files kept for awhile
• Under user control
• Client and server interact for document
  refreshing
• Server can specify how long a document is valid
  for
• From always reload to never reload
• Client can check with server on date of document
  and reload only if newer
• Browsers uses HEAD message to get timeliness info

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Security

• https
• Uses public-key cryptography to assure secure
  connection
• Using 128-bit key

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Emerging Trends

• Browsing from other devices memory and display
  limited devices
• Wide usage of XML
• eXtensible Markup Language
• Specifies structure and meaning of data
• Does not specify formatting is really about
  content of data
• Need style sheets to format for visually
  rendering XSL, XSLT
• Wireless Internet

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Telnet HTTP Session
telnet clem.mscd.edu 80 Trying
147.153.1.3... Connected to clem.mscd.edu. Escape
character is ''. GET /evansell/NETWORKS/simpleh
tml.html HTTP/1.0 HTTP/1.1 200 OK Server
Netscape-Enterprise/6.0 Date Mon, 05 May 2003
221334 GMT Content-type text/html Last-modified
Mon, 28 Apr 2003 220729 GMT Content-length
483 Accept-ranges bytes Connection
close ltHTMLgt ltHEADgt ltTITLEgt Elliott's Web Page
lt/TITLEgt lt/HEADgt ltBODY BGCOLOR"bb00bb"
LINK"ffff00" ALINK"ff0000" VLINK"00ff00"gt ltH
1gt Elliott on the Web lt/H1gt ltULgt ltLIgt See my web
page at ltA HREF"http//clem.mscd.edu/evansell"gt
Metro State lt/Agt lt/LIgt ltLIgt My personal home
page link will be added here - later lt/LIgt ltLIgt
Interests Jai-lai, outdated technology, older
movies lt/LIgt lt/ULgt ltHRgt Email me at ltA
HREF"mailtoevansell_at_mscd.edu"gt
evansell_at_mscd.edu lt/Agt lt/BODYgt lt/HTMLgt Connection
closed by foreign host.
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Exercises

• Chapter 35 3, 7, 13, 14
• Make sure you telnet to a web server and ask for
  a simple HTML document