The document provides an overview of HTTP (Hypertext Transfer Protocol). It discusses the history and development of the internet and world wide web. It describes the client-server model used by HTTP and key concepts like URLs, DNS, HTTP requests and responses. It explains how browsers and web servers communicate via HTTP to transfer web pages and other resources.

1. CHAPTER ONE
HTTP
Hyper Text Transfer Protocol
Compiled By: Seble N.

2.  History
 The World Wide Web (WWW)
 The Client/Server Architecture of the WWW
 URL, URI & URN
 The Domain Name System (DNS)
 HTTP Headers
 HTTP Request/Response, the Stateless nature of HTTP

3.  ARPA net
 Internet
 WWW
 Dr. Tim Berners Lee
 Marc Andreessen
 Mosaic

4.  www (world wide web)
 A collection of websites
 A website
 A collection of resources:
 Web pages (static / dynamic)
 Media files (images, animations, sound, …)
 Style files (CSS)
 Documents (pdf, doc, txt, rtf, …)
 …
 Stored on machines called web server

5.  Software involved:
 At the server:
 Web server software : listens for incoming requests for
resources from clients and serves the requests
 Apache - open source, IIS – Microsoft
 At the client:
 Web browser : sends/receives requests/responses
to/from web servers on behalf of the client and renders
content as necessary
 WWW use client / server architecture

6.  Client (web browser) and Server (web server)
communicate via the HTTP to exchange messages
 HTTP is text-based request-response protocol
Page request
Client running
a Web Browser
Server running Web
Server Software
(IIS, Apache, etc.)
Server response
HTTP
HTTP

7.  What is URL?
 The exact address of a resource on the web
 Format:
 <protocol>://<host>[:<port>][<path>][?<query>]
 E.g. http://www.somedomain.com/search.php?q=dns&lang=en
 Protocol – identifies the type of protocol to be used for communication
 http, ftp ….
 Host – identifies the machine on which the requested resource is stored
 Domain names (eg. www.google.com)
 IP address
 Port – identifies the port number of the web server software on the web server
machine
 Default port for http: 80
 Path – identifies the name and path of the resource on the server
 Query – specifies parameters, if any, that should be sent to the server along with
the request
 has the form: ?var_name1=value1&var_name2=value2&…

9.  DNS is a hierarchical naming system for domain name
services, or any resource connected to with internet
 DNS servers translate domain names to IP addresses
 For this purpose, DNS servers store a table containing name-
IP pairs and do a look-up when requested
 Resolves a human friendly name (eg www.google.com) to a machine
friendly IP address (eg 64.233.187.99)
 “Phone book” of the Internet
 A DNS server may communicate with other server to resolve
a given name
 There are about 13 root DNS servers (http://www.root-
servers.org/)

10.  Hierarchical in nature (eg. www.aau.edu.et)
 Labels are separated by . (dot)
 Labels must start with a letter and can only consist of letters,
digits and hypens
 Labels can be up to 63 characters long and are case insensitive
 A maximum of 255 characters is allowed in total
 The last (highest) label of a DNS name can be:
 Three letter code top level domains (TLDs): indicating the type of
organization
 com, edu, gov, net, org, biz, …
 Two letter country codes (CCTLDs): indicating the country
 et, us, za, uk, tv, …

12.  A protocol that enables communication b/n browser and web
server
 A stateless protocol
 When a resource has been sent, the server keeps no record of the
exchange, so that if a second request is made by the same client, it is as
if this was first contact with that client
 Each request a browser sends to a web server is independent of any
other request
 HTTP conversation involves the exchange of HTTP messages

13.  Is either a request from client to server OR a
response from server to client
 Messages are composed of:
 A start line (request/response line)
 One or more header fields
 An empty line – indicating the end of the header fields
 A message body (optional)

14.  HTTP/1.1 200 ok
 Server: Microsoft-IIS/5.0
 Date: Mon, 12 Aug 2007 08:05:30 GMT
 Content-Type: text/html
 Last-Modified: Tue, 13 Aug 2007 09:34:22 GMT
 Content-Length: 240
 <html><head><title>page title</title></head>
 <body>some content</body></html>

15.  The request line contains three parts:
 Request method, Request URI and HTTP Version
 Request methods
 GET (or retrieve) information from the server
 POST (information) back to the server
 HEAD – like GET but only returns meta-information
 PUT (information) at the server
 DELETE (information) from the server
 Request URI
 The URI of the resource requested
 HTTP Version
 Used by the sender to notify the receiver of its abilities
 Format: HTTP/<major>.<minor>
 E.x. HTTP/1.1

16.  GET /hello.html HTTP/1.1
 Host: www.example.com
 User-Agent: Mozilla/4.0
 Accept-Language: en-us
 Accept-Encoding: gzip

17.  The response line contains:
 HTTP version
 Status code
 Status code description
 Example
 HTTP/1.1 200 ok

18.  HTTP/1.1 200 ok
 Server: Microsoft-IIS/5.0
 Date: Mon, 12 Aug 2007 08:05:30 GMT
 Content-Type: text/html
 Last-Modified: Tue, 13 Aug 2007 09:34:22 GMT
 Content-Length: 240
 <html><head><title>page title</title></head>
 <body>some content</body></html>

20.  Status code - has 5 categories
 1xx – request received, processing continues
 100 Continue – tells client to continue with a request
 2xx – success, action was successfully received,
understood and accepted
 200 Ok – request has succeeded
 202 Accepted – request accepted but not processed
 3xx – further action must be taken to complete
the request
 302 Found – resource found but temporarily moved

21.  4xx – client error or invalid request
 E.x.
 401 Unauthorized – request requires authorization
 403 Forbidden – client may not have access to the resource
 404 Not Found– page not found
 5xx – server error occurred
 E.x.
 500 Internal Server Error – server encountered an
unexpected error (error/bug with a server side script)
 505 HTTP Version Not Supported – server doesn’t support
the HTTP version

22.  Contains header fields
 The header fields can be:
 General headers
 Request headers
 Response headers
 Entity headers
 All header fields follow the same generic format
 Each header field consists of a name followed by a colon (:) and a value:
 Header-name:value
 The order of the header field is insignificant

23.  Have general applicability for both request and response messages
 Apply only to the message being transferred
 Examples:
 Connection: close – client wants to close the connection when the first response
is complete
 Cache-control: no-cache - client can specify additional properties if the
requested resource is cached
 Date

24.  Give meta-information about the entity body (message body)
being transferred
 Apply only if a message body exists
 Examples:
 Content-Language – language of the intended audience
 Content-Length – size of the entity body (message body)
 Last-Modified: Tue,15 Nov 1994
 …

25.  Adds additional information about the request
 May include information about the client/sender, including client
capability
 Examples:
 Accept - acceptable media types for response
 Accept-Charset – acceptable character set
 User-Agent – client browser
 Host: - domain name+port
 …

26.  More information, in addition to the status line
 May contain information about the server or resource
 Examples:
 Server – information about the web server software
 Location – used to redirect to a different location (URI)
 Proxy-Authenticate – proxy authentication challenge
 Age – estimate of time since response was generated
 …

27.  HTTP/1.1 200 ok
 Server: Microsoft-IIS/5.0
 Date: Mon, 12 Aug 2007 08:05:30 GMT
 Content-Type: text/html
 Last-Modified: Tue, 13 Aug 2007 09:34:22 GMT
 Content-Length: 240
 <html><head><title>page title</title></head>
 <body>some content</body></html>