1. Hypertext Transfer Protocol
HTTP
Hypertext Transfer Protocol
5-Apr-19

2. HTTP messages
HTTP is the language that web clients and web servers use to talk to each other
HTTP is largely “under the hood,” but a basic understanding can be helpful
Each message, whether a request or a response, has three parts:
The request or the response line
A header section
The body of the message

3. What the client does, part I
The client sends a message to the server at a particular port (80 is the default)
The first part of the message is the request line, containing:
A method (HTTP command) such as GET or POST
A document address, and
An HTTP version number
Example:
GET /index.html HTTP/1.0

4. Other methods Other methods beside GET and POST are:
HEAD: Like GET, but ask that only a header be returned
PUT: Request to store the entity-body at the URI
DELETE: Request removal of data at the URI
LINK: Request header information be associated with a document on the server
UNLINK: Request to undo a LINK request
OPTIONS: Request information about communications options on the server
TRACE: Request that the entity-body be returned as received (used for debugging)

5. What the client does, part II
The second part of a request is optional header information, such as:
What the client software is
What formats it can accept
All information is in the form Name: Value
Example:
User-Agent: Mozilla/2.02Gold (WinNT; I) Accept: image/gif, image/jpeg, */*
A blank line ends the header

6. Client request headers
Accept: type/subtype, type/subtype, ...
Specifies media types that the client prefers to accept
Accept-Language: en, fr, de
Preferred language (For example: English, French, German)
User-Agent: string
The browser or other client program sending the request
From:
address of user of client program
Cookie: name=value
Information about a cookie for that URL
Multiple cookies can be separated by commas

7. What the client does, part III
The third part of a request (after the blank line) is the entity-body, which contains optional data
The entity-body part is used mostly by POST requests
The entity-body part is always empty for a GET request

8. What the server does, part I
The server response is also in three parts
The first part is the status line, which tells:
The HTTP version
A status code
A short description of what the status code means
Example: HTTP/ Not Found
Status codes are in groups:
Informational
The request was successful
The request was redirected
The request failed
A server error occurred

9. Common status codes 200 OK 301 Moved Permanently 302 Moved temporarily
Everything worked, here’s the data
301 Moved Permanently
URI was moved, but here’s the new address for your records
302 Moved temporarily
URL temporarily out of service, keep the old one but use this one for now
400 Bad Request
There is a syntax error in your request
403 Forbidden
You can’t do this, and we won’t tell you why
404 Not Found
No such document
408 Request Time-out, 504 Gateway Time-out
Request took too long to fulfill for some reason

10. What the server does, part II
The second part of the response is header information, ended by a blank line
Example:
Content-Length: 2532 Connection: Close Server: GWS/2.0 Date: Sun, 01 Dec :24:50 GMT Content-Type: text/html Cache-control: private Set-Cookie: PREF=ID=05302a93093ec661:TM= :LM= :S=yNWNjraftUz299RH; expires=Sun, 17-Jan :14:07 GMT; path=/; domain=.google.com
All on one line

11. Viewing the response
There is a header viewer at (with nasty jittery advertisements)
Example 2.3 (GetResponses) in the Gittleman book does the same thing
Here’s an example (from GetResponses):
% java GetResponses /index.html Status line: HTTP/ OK Response headers: Date: Wed, 10 Sep :26:53 GMT Server: Apache/ (Unix) PHP/4.2.2 mod_perl/ mod_ssl/ OpenSSL/0.9.6e Last-Modified: Tue, 09 Sep :24:50 GMT ETag: "1c1ad f5e2902” Accept-Ranges: bytes Content-Length: Keep-Alive: timeout=15, max= Connection: Keep-Alive Content-Type: text/html

12. The GetResponses program, I
Here’s just the skeleton of the program that provided the output on the last slide:
import java.net.*; import java.io.*; public class GetResponses { public static void main(String [ ] args) { try { interesting code goes here } catch(Exception e) { e.printStackTrace(); } } }

13. The GetResponses program, II
Here’s the interesting part of the code:
URL url = new URL(args[0]); URLConnection c = url.openConnection(); System.out.println("Status line: "); System.out.println('\t' + c.getHeaderField(0)); System.out.println("Response headers:"); String value = ""; int n = 1; while (true){ value = c.getHeaderField(n); if (value == null) break; System.out.println('\t' + c.getHeaderFieldKey(n++) ": " + value); }

14. Server response headers
Server: NCSA/1.3
Name and version of the server
Content-Type: type/subtype
Should be of a type and subtype specified by the client’s Accept header
Set-Cookie: name=value; options
Requests the client to store a cookie with the given name and value

15. What the server does, part III
The third part of a server response is the entity body
This is often an HTML page
But it can also be a jpeg, a gif, plain text, etc.--anything the browser (or other client) is prepared to accept

16. The <meta http-equiv> tag
The <meta http-equiv=string content=string> tag may occur in the <head> of an HTML document
http-equiv and content typically have the same kinds of values as in the HTTP header
This tag asks the client to pretend that the information actually occurred in the header
The information is not really in the header
This tag is available because you have little direct control over what is in the header (unless you write your own server)
As usual, not all browsers handle this information the same way
Example: <meta http-equiv="Set-Cookie" content="value=n;expires=date; path=url">

17. Summary
HTTP is a fairly straightforward protocol with a lot of possible kinds of predefined header information
More kinds can be added, so long as client and server agree
A request from the client consists of three parts:
A header line
A block of header information, ending with a blank line
The (optional) entity body, containing data
A response from the server consists of the same three parts
HTTP headers are “under the hood” information, not normally displayed to the user
As with most of the things covered in CIT597,
We have covered only the fundamentals
Much more detail can be found on the Web

18. The End