1. HTTP HyperText Transfer Protocol

2. HTTP Uses TCP as its underlying transport protocol Uses port 80 Stateless protocol (i.e. HTTP Server maintains no information about the clients) Text-based protocol Two types of messages: – Request messages – Response messages

3. Web pages Web pages consist of objects (HTML files, images etc.) Web pages are addressable by a single URL Example A web page that consists of an HTML file and 5 images, has 6 objects. images. The base HTML file references the other objects in the page with the objects' URLs.

4. URLs Each URL has two components: The host name of the server that houses the object The objects path name www.someSchool.edu/someDepartment/page.html Host name Path name

5. HTTP Request Messages GET /somedir/page.html HTTP/1.1 Connection: close User-agent: Mozilla/4.0 Accept: text/html, image/gif, image/jpeg > (other types of request messages include POST and HEAD messages)

6. HTTP Response Messages HTTP/1.1 200 OK Connection: close Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1988 09:23:23 HMT Content-Length: 6821 Content-Type: text/html data data data data data

7. Status Codes 200 OK 301 Moved Permanently 400 Bad Request 404 Not Found 505 HHTP Version Not Supported etc.