HTTP is the protocol used for communication between web browsers and servers. URIs identify resources, with URLs specifying how to locate resources via protocols, hostnames, directories, and filenames. HTTP uses URLs to retrieve web pages and other resources, with requests containing the method, URL, and headers sent to the server and responses returned with status lines and headers along with the requested content.

2. HTTP
• Hypertext Transport Protocol
• Language of the Web
– protocol used for communication between web
browsers and web servers

3. URI,URN,URL
• Uniform Resource Identifier
– Information about a resource
• Uniform Resource Name
– The name of the resource with in a namespace
• Uniform Resource Locator
– How to find the resource, a URI that says how to
find the resource

4. HTTP - URLs
• URL
– Uniform Resource Locator
• protocol (http)
• host name (name.domain name)
• directory path to the resource
• resource name
– http://abc.myplace.com/www/index.html
– http://abc.myplace.com:80/cgi-bin/t.exe

5. 5
URL and HTTP
• All parts of URL, except parameters, used
with http
• Scheme and host can be omitted when
referenced object is on same machine as
referring document
• Port can be omitted so long as referenced host
is running on port listed in your /etc/services
file.

6. 6
URL and HTTP
• Full path used when referring to another server
– Relative path on same server
• Reference with relative path is a partial URL
• Query passes parameters to CGI
• Fragment jumps to labels within a page

7. Figure HTTP transaction

8. HTTP is an application layer protocol
• The Web client and the Web server are application programs
• Application layer programs do useful work like retrieving Web pages,
sending and receiving email or transferring files
• Lower layers take care of the communication details
• The client and server send messages and data without knowing anything
about the communication network

9. The application layer
Layer Function
Application Do useful work like Web
browsing, email, and file
transfer
Lower layers
Handle communication
between the client and
server

10. Many application layer protocols are used on the
Internet, HTTP is only one
Protocol Application
HTTP: Hypertext Transfer Retrieve and view Web pages
FTP: File Transfer
Copy files from client to server
or from server to client
SMTP: Simple Mail
Transport
Send email
POP: Post Office Read email

11. Figure Request and response messages

12. Example
• This example retrieves a document. We use the GET
method to retrieve an image with the path
/usr/bin/image1. The request line shows the method
(GET), the URL, and the HTTP. The header has two
lines that show that the client can accept images in
the GIF or JPEG format. The request does not have a
body. The response message contains the status line
and four lines of header. The header lines define the
date, server, MIME version, and length of the
document. The body of the document follows the
header