|
| |
What is HTTP?
The HyperText Transfer Protocol is a stateless, TCP/IP-based protocol for
communication over the World-Wide Web. It is typically used between a
client browser and a web server:
- The client browser issues a request to the web server. The client:
- Opens a TCP connection to the web server's HTTP port (usually port 80)
- Sends an HTTP request message to the web server
- The web server:
- Receives the request
- Does the requested work
- Returns the information requested in a response message back to the
client
- The connection is closed
The HTTP Request
The most common form of request from a browser is a GET.
In its simplest form, it looks like:
a single line, where:
- The first item on the line is called the method (here a
GET)
- The second item is the pathname of the requested file
- The third item indicates we are using the HTTP protocol, and specifies the
version.
A GET request can also include a request header. Here's an example:
As you can see, this GET request was generated by Microsoft Internet Explorer
6.0, and there is lots of other information you could extract.
The HTTP Response
In response to the above HTTP request, the web server generates an HTTP
response.
If the request was successfully satisfied by the web server, then the first
line of the response will look like:
HTTP/1.0 200 OK
The 200 is the status code for success. "OK" is a
human-readable description of the meaning of 200.
Following the first line are a number of response header lines, which
communicate a number of useful pieces of information about the server, and a
description of the contents of the file that follows in the response. A blank
line which separates the header from the file contents. For example:
HTTP/1.0 200 OK
Server: Apache/1.3.9 (Win32)
Content-Type: text/html
Content-Length: 94
<HTML>
<HEAD>
<TITLE>HTTP Request Successful</TITLE>
</HEAD>
<BODY>
It Worked!
</BODY>
</HTML> |
|
<----- Note the blank line here |
HTTP Status Codes
If the web server encountered problems -- perhaps it could not find the
requested file -- then the first line of the response contains one of a number of error status
codes. Status codes are organized numerically to indicate the level/type
of error:
| Code
Range |
Category |
Description |
1xx |
Informational |
Mostly experimental application codes.
HTTP/1.0 does not define any Informational status codes; HTTP/1.1
does. |
2xx |
Successful |
Request was received, understood and
accepted. |
3xx |
Redirection |
The server is redirecting the client to
another URL. |
4xx |
Client Error |
The request is improperly formatted, or
cannot be fulfilled. |
5xx |
Server Error |
A valid request was received, but the server
cannot fulfill it. |
Here are some actual status codes (this is not a complete list):
| Status
Code |
Description |
|
200 OK
|
The request succeeded. |
|
201 Created
|
The request was successful, and resulted in the creation
of a new resource. |
|
301 Moved Permanently
|
The resource has been moved permanently. The Location
header field specifies the new location. |
|
302 Moved Temporarily
|
The resource has been moved temporarily. The Location
header field specifies the new location. |
|
400 Bad Request
|
The request used invalid syntax. |
|
401 Unauthorized
|
Authorization, such as username and password, is required
to access this resource. A WWW_Authenticate header field
containing an authorization challenge is returned. The web browser
will normally display a username/password dialog box to the user |
|
403 Forbidden
|
The server refuses to fulfill the response. |
|
404 Not Found
|
No resource matching the requested URL exists on the
server. |
|
500 Internal Server
Error
|
The server encountered an unexpected problem. |
|
501 Not Implemented
|
The server does not implement the requested functionality. |
|
