HTTP

Lecture Notes for CS 142
Winter 2014
John Ousterhout

  • Additional reading for this topic: none.

HTTP: HyperText Transfer Protocol

  • Used by browsers to communicate with Web servers.
  • Simple request-response protocol, sent using TCP/IP sockets.
  • Sample request: 
    GET /index.html HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0
Accept: text/xml,application/xml,application/xhtml+xml,text/html*/*
Accept-Language: en-us
Accept-Charset: ISO-8859-1,utf-8
Connection: keep-alive
<blank line>
    • First line contains method, URL, version number
      • GET method: read information from server. Should have no side effects.
      • POST method: uploads data from the browser to the server (typically form data), returns information from the server. Likely to have side effects. Data is in the body of the message, after the blank line.
      • There are several other methods defined besides these two, but we won't use them in CS142.
    • Headers: name-value pairs providing various information that may be useful to the server.
    • A request can also contain data following the headers, but the GET method doesn't have any data (POST does, though).
  • Sample response: 
    HTTP/1.1 200 OK
Date: Thu, 24 Jul 2008 17:36:27 GMT
Server: Apache-Coyote/1.1
Content-Type: text/html;charset=UTF-8
Content-Length: 1846

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC ... >
<html ... >
...
</html>
    • First line contains protocol version number, numerical status code, textual explanation.
    • Headers have same general format as for requests
    • Blank line separates headers from response data.
    • Response body doesn't necessarily have to be HTML.
  • Redirection: causes browser to fetch a new URL in place of the page initially requested.
    • Return a status of 307.
    • Set Location header to some other URL.
    • Useful if information has moved, and for POST requests.

HTTPS

  • Identical to HTTP, except request and response messages are transmitted securely using encryption.
  • Will be discussed in detail later in the course.