HTTP

Derek Bridge

Department of Computer Science,
University College Cork

HTTP

Aims:

• to know a little about simple HTTP requests that request (static) web pages
• to see where DNS fits in

The Web is a client-server application

• Web client computers
  • run web client software (e.g. browsers)
  • the software make requests for web resources (e.g. HTML files, CSS files, image files, ...)
• Web server computers
  • host web resources (text files, graphics files, programs,...)
  • run web server software that responds to requests

Ports

• One server might be offering ('hosting') more than one service
• E.g. a server computer may be running web server software and email server software
• How does a client indicate which server program is to handle its requests?
• Each server program is assigned an identification number, called a port number

Listening on ports

What your browser does when you enter a URL or click on a link...

• If it needs to request a new web page, your browser works out:
  • the scheme (which usually defaults to http)
  • the server name (e.g. www.rte.ie)
  • the port number (which defaults to 80)
  • the pathname of the file you want (e.g. /news/index.html)
• Your browser must now send a request to the web server, asking for this file
• Question: There are times when your browser doesn't need to do this. Give an example
• The request (and the server's response) must be formulated according to a communications protocol (in this case, HTTP)

HTTP requests

• Request line (required): command (method), URL and HTTP version number
• Request header lines (largely optional): info about date, browser,...
• Request message body (optional): empty for most commands (methods)

Example HTTP request from The TCP/IP Guide by Charles M. Kozierok

HTTP responses

• Status line (optional): HTTP version number, status code, short explanation of code
• Response header lines (optional): info about date, server,...
• Response message body (required): the requested resource (web page, image,...)

Example HTTP response from The TCP/IP Guide by Charles M. Kozierok

HTTP response status codes

1XX: Informational (used in 1.1):

e.g. 100 Continue, 101 Switching Protocols

2XX: Success:

e.g. 200 OK, 206 Partial Content

3XX: Redirection:

e.g. 301 Moved Permanently, 304 Not Modified

4XX: Client error:

e.g. 400 Bad Request, 403 Forbidden, 404 Not Found

5XX: Server error:

e.g. 500 Internal Server Error, 503 Service Unavailable

DNS

• Prior to sending the HTTP request, your browser needs to convert from server name to server IP address

Embedded content

• Suppose the web page contains embedded content (e.g. stylesheets, images)
• The server does not send all the content in one go
• The client receives the web page and then sends separate requests for the embedded content