URLs

1
URLs Uniform Resource Locators

• Since web pages may contain pointers to other
  pages, we will see how those pointers are
  implemented
• When the web was first created, it was apparent
  that having one page point to another required
  mechanisms for naming and locating pages. In
  particular there were 3 questions that had to be
  answered before a selected page could be
  displayed
• What is the page called?
• Where is the page located?
• How can the page be accessed?

2
URLs

• The solution chosen identifies pages in a way
  that solves all 3 problems at once.
• Each page is assigned a URL (Uniform Resource
  Locator) that effectively serves as the pages
  worldwide name.

3
URLs

• URLs have 3 parts
• The protocol (also called a scheme)
• The DNS name of the machine on which the page is
  located, and
• A local name uniquely indicating the specific
  page (usually just a file name on the machine
  where it resides)
• For example, the URL for the authors department
  is http//www.cs.vu.nl/welcome.html This URL
  consists of 3 parts the protocol (http), the DNS
  name of the host (www.cs.vu.nl) and the file name
  (welcome.html) with certain punctuation
  separating the pieces

4
URLs

• Many sites have certain shortcuts for file names
  built in. For example, user/ might be mapped
  onto users WWW directory, with the convention
  that a reference to the directory itself implies
  a certain file, say, index.html
• Thus the authors home page can be reached at
  http//www.cs.vu.nl/ast/ even though the actual
  file name is different.
• At many sites a null file name defaults to the
  organizations home page.

5
URLs mechanism

• To make a piece of text clickable the page writer
  must provide 2 items of information
• The clickable text to be displayed, and
• The URL of the page to go to if the text is
  selected
• When the text is selected, the browser looks up
  the host name using DNS. Now armed with the
  hosts IP address, the browser then establishes a
  TCP connection to the host. Over that connection
  it sends the file name using the specified
  protocol. Next, back comes the page.

6
URLs - protocols

• The URL scheme is open ended, in the sense that
  it is straight forward to have protocols other
  than HTTP. In fact, URLs for various other
  protocols have been defined, and many browsers
  understand them
• The next table illustrates slightly simplified
  forms of the more common ones

7
ULRs - Protocols
8
HTTP HyperText Transfer Protocol

• The standard Web transfer protocol is HTTP
  (HyperText Transfer Protocol)
• The HTTP protocol consists of two fairly distinct
  items
• the set of requests from browsers to servers, and
  
• the set of responses going back the other way

9
HTTP

• HTTP is an ASCII protocol (each interaction
  consists of an ASCII request, followed by one
  MIME-like response)
• MIME (Multipurpose Internet Mail Extensions) in
  the early days of the ARPNET email messages
  consisted exclusively of text messages written in
  English and expressed in ASCII. Nowadays on the
  Internet this approach is no longer adequate, as
  the following need to be addressed
• Messages in languages with accents (French,
  German)
• Messages in nonLatin alphabets (e.g. Hebrew,
  Russian)
• Messages in languages withough alphabets (e.g.
  Chinese, Japanese)
• Messages not containing text at all (e.g. audio,
  video)

10
MIME

• The basic idea of MIME is to define encoding
  rules for non-ASCII messages. MIME defines 5
  message headers

Header Meaning
MIME-Version Identifies the MIME version
Content-Description Human readable string telling what is the message
Content-ID Unique identifier
Content-Transfer-Encoding How the body is wrapped for the transmission
Content-Type Nature of the message
11
MIME Content Type
Header Subtype Meaning
Text Plain Richtext Unformatted text Text including simple formatting
Image Gif Jpeg Still picture in GIF format Still picture in JPEG format
Audio Basic Audible sound
Video Mpeg Movie in MPEG format
Application Octet-stream Postscript An uninterpreted byte sequence A printable document in PostScript
Message Rfc822 Partial External-body A MIME RFC 822 message Message has been split for transmission Message must be fetched over the net
Multipart Mixed Alternative Parallel Digest Independent parts Same message in different formats Parts must be viewed simultaneously Each part is a complete RFC 822 message
12
HTTP - request

• Although HTTP was designed for use in the Web, it
  has been intentionally made more general than
  necessary with an eye to future object oriented
  applications. For this reason the first word of a
  request line is simply the name of the method
  (command) to be executed on the Web page (or
  general object)
• The built in methods are as follows

Method Description
GET Request to read a Web page
HEAD Request to read a Web pages header
PUT Request to store a Web page
POST Append to a named resource (web page)
DELETE Remove the Web page
LINK Connects two existing resources
UNLINK Breaks an existing connection between resources
13
HTTP request / response

• A request is just a GET line, naming the page
  desired and the HTTP protocol version
• GET /hypertext/WWW/TheProject.html HTTP/1.1
• The response is just the raw page, headers, and
  MIME information
• For example, because HTTP is an ASCII protocol,
  it is easy for aperson at a terminal (opposed to
  a browser) to direcly talk to Web servers. All
  that is a needed is a TCP connection to port 80
  on the server. The simplest way to get such
  connection is the Telnet program

14
HTTP - example

• Client Telnet www.w3.org 80
• Trying 18.23.0.23
• Connected to www.w3.org
• Client GET /hypertext/WWW/TheProject.html
  HTTP/1.1
• Server HTTP/1.1 200 Document follows
• Server MIME-Version 1.0
• Server Server CERN/3.0
• Server Content-Type text/html
• Server Content-Length 8247
• Server ltHEADgtltTITLEgtThe World Wide Web
  Consortium (W3C) lt/TITLEgt lt/HEADgt
• Server ltBODYgt

15
HTTP Example

• Or could use a command line browser, (such as
  WFetch) to review the same information

16
(No Transcript)
17
HTML HyperText Markup Language

• HTML is a markup language, a language for
  describing how documents are to be formatted. The
  term markup comes from the old days when
  copyeditors acutally marked up documents to tell
  the printer (in those days a human being) which
  fonts to use, and so on.
• Markup languages thus contain explicit commands
  for formatting. For example, in HTML, ltBgt means
  start boldface mode, and lt/Bgt means leave
  boldface mode.

18
HTML

• The advantage of a markup language over one with
  no explicit markup is that writing a browser for
  it is straightforward the browser simply has to
  understand the markup commands.
• By embedding the markup commands within each HTML
  file and standardizing them, it becomes possible
  for any Web browser to read and reformat any Web
  page.

19
HTML

• HTTP and HTML are constantly evolving. When
  Mosaic was the only browser, the language it
  interpreted, HTML 1.0, was de facto standard.
• When new browsers came along, there was a need
  for a formal Internet standard, so the HTML 2.0
  standard was produced. Next, HTML 3.0 was created
  as a research effort to add many new features to
  HTML 2.0, including tables, toolbars,
  mathematical formulas, advanced style sheets (for
  defining page layout and the meaning of symbols),
  etc.

20
HTML brief introduction

• A proper Web page consists of a head and body
  enclosed by ltHTMLgt and lt/HTMLgt tags (formatting
  commands), although most browsers do not complain
  if these tags are missing.
• The head is bracketed by ltHEADgt lt/HEADgt tags, and
  the body is bracketed by ltBODYgt lt/BODYgt tags
• The commands inside the tags are called
  directives. Most HTML tags have this format, that
  is, ltSOMETHINGgt to mark the beginning of
  something and lt/SOMETHINGgt to mark its end.

21
HTML brief introduction

• Numerous other examples of HTML are easily
  available. Most browsers have a menu item VIEW
  SOURCE or something similar. Selecting this item
  for an HTML page, displays the current HTML
  source, instead of formatted output

22
DNS Domain Name System

• Programs rarely refer to hosts, mailboxes, and
  other resources by their binary network
  addresses. Instead, they use ASCII strings, such
  as tana_at_art.ucsb.edu
• Nevertheless, the network itself only understands
  binary addresses, so some mechanism is required
  to convert the ASCII strings to network
  addresses.

23
DNS

• Way back in the ARPANET, there was simply a file,
  hosts.txt, that listed all the hosts and their IP
  addresses. Every night, all the hosts would fetch
  it from the site and at which it was maintained.
  For a network of a few hundred large timeshareing
  machines, this approach worked reasonably well.
• However, when thousands of workstations were
  connected to the net, everyone realized that this
  approach could not continue to work forever.

24
DNS

• For one thing, the size of the file would become
  too large. However, even more important, host
  name conflicts would occur constantly unless
  names were centrally managed, something
  unthinkable in a huge international network.
• To solve these problems, DNS (the Domain Name
  System) was invented.

25
DNS

• The essence of DNS is the invention of a
  hierarchical, domain-based naming scheme and a
  distributed database system for implementing this
  naming scheme.
• It is primarily used for mapping host names and
  email destinations to IP addresses.

26
DNS how it is used

• To map a name onto an IP address, an application
  program calls a library procedure called the
  resolver, passing it the name as a parameter. The
  resolver sends a UDP packet to a local DNS
  server, which then looks up the name and returns
  the IP address to the resolver, which then
  returns it to the caller.
• Armed with the IP address, the program can then
  establish a TCP connection with the destination,
  or send it UDP packets.