Introduction to HTTP

Derek Bridge

Department of Computer Science,
University College Cork

Introduction to HTTP

Aims:

• to know how URLs are structured
• to know the difference between absolute URLs and relative URLs
• to know how HTTP works

What makes the web tick

Web servers

Web server computers

• host web resources (text files, graphics files, programs,...)
• run web server software

The market leaders (www.netcraft.com):

• Apache: www.apache.org
• Microsoft Internet Information Server (IIS): www.microsoft.com/windowsserversystem/

Both free!

Uniform Resource Locators (URLs)

• IP addresses/hostnames are used to identify hosts
• URLs are used to locate resources (text files, graphics files, programs, ...)

Schemes

• Typically the name of a protocol for accessing a resource
  • E.g. http (default), https, telnet, ftp,...
• With the mailto scheme, a user can send emails from the browser, e.g.:

  <a href="mailto://d.bridge@cs.ucc.ie">Mail me</a>
  <a href="mailto://d.bridge@cs.ucc.ie?subject=Excuse&
   body=Hi Derek%0AI missed the lab because%0A%">Mail me</a>
  

  But be warned about including your email address like this in web pages
• With the file scheme, a user can access files on the local computer (results in three slashes!)

Schemes: HTTPS

• The HTTPS scheme is not a protocol
• HTTPS describes use of HTTP 'on top of' SSL/TSL 'on top of' TCP
• Secure Sockets Layer (SSL), now supposedly called Transport Layer Security (TLS), allows for:
  • authentication of the identity of the server
  • key exchange
  • encryption/decryption using a key generated for this session
• SSL/TLS can also be used 'between', e.g., SMTP and TCP

Hostnames

• The name of the server on which the resource resides, e.g. www.cs.ucc.ie
• Class exercise: Why do all these names begin with www?
• Instead of a hostname, you can use an IP address in either decimal or hexadecimal, e.g.

  http://227.82.157.177/cs/cs1102/labs.html
  

Pathnames

• Typically, a sequence of directories/folders, ending with the file name
• E.g. /cs1/cs1102/labs.html
• Directories/folder are organised hierarchically
• The root directory/folder on a server is usually called /

Absolute URLs

• In absolute URLs, the pathname starts at the root, e.g.

  <a href="http://www.cs.ucc.ie/cs1/cs1102/labs.html">Labs</a>
  

Relative URLs

• In relative URLs, the pathname is relative to some base directory/folder, usually the directory/folder of the current document

  E.g. if lab1.html is the current document then

       <a href="template.html">template</a>
       

  links to http://www.cs.ucc.ie/cs1/cs1102/lab1/template.html

• In a relative URL, .. means go to parent directory/folder
• Class exercise: When would you use relative URLs and what advantage would you gain?

Class exercise

The current document is highlighted

1. Give the absolute pathname for each of the following
   1. The current document
   2. Its base directory/folder
   3. b.html
   4. ../a.html
   5. ../../dirB/a.html
   6. ../../dirA/dirA/a.html
2. Give the relative pathname for b.gif