HTTP - Requests

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An HTTP request is a message sent by a client to a server, requesting for a specific resource. It consists of a request line, headers, and optionally a body. An HTTP client sends an HTTP request to a server in the form of a request message.

HTTP request message

HTTP Request Components

An HTTP request message has following four components mentioned below:

  • A Request-line: <Method> <Request-URI> <HTTP-Version>
  • Zero or more header (General|Request|Entity) fields followed by CRLF
  • An empty line (i.e., a line with nothing preceding the CRLF) indicating the end of the header fields
  • Optionally a message-body

The following sections explain each of the entities used in an HTTP request message.

Request-Line

The Request-Line begins with a method token, followed by the Request-URI and the protocol version, and ending with CRLF. The elements are separated by space SP characters.

<Method> <Request-URI> <HTTP-Version>

Let's discuss each of the parts mentioned in the Request-Line.

Request Method

The request method indicates the method to be performed on the resource identified by the given Request-URI. The method is case-sensitive and should always be mentioned in uppercase. The following table lists all the supported methods in HTTP/1.1.

Method Description
GET The GET method is used to retrieve information from the given server using a given URI. Requests using GET should only retrieve data and should have no other effect on the data.
HEAD Same as GET, but it transfers the status line and the header section only.
POST A POST request is used to send data to the server, for example, customer information, file upload, etc. using HTML forms.
PUT Replaces all the current representations of the target resource with the uploaded content.
DELETE Removes all the current representations of the target resource given by URI.
CONNECT Establishes a tunnel to the server identified by a given URI.
OPTIONS Describe the communication options for the target resource.
TRACE Performs a message loop back test along with the path to the target resource.

Request-URI

The Request-URI is a Uniform Resource Identifier and identifies the resource upon which to apply the request. Following are the most commonly used forms to specify an URI:

Request-URI = origin form | absoluteURI | "*" | authority
Method Description
Asterisk(*) form It is used when an HTTP request does not apply to a particular resource, but to the server itself, and is only allowed when the method used does not necessarily apply to a resource. For example: OPTIONS * HTTP/1.1
Absolute form It is used when an HTTP request is being made to a proxy. The proxy is requested to forward the request or service from a valid cache, and return the response. For example: GET https://www.tutorialspoint.com/tailwind_css/index.htm HTTP/1.1
Origin form The most common form of Request-URI is that used to identify a resource on an origin server or gateway. For example, a client wishing to retrieve a resource directly from the origin server would create a TCP connection to port 80 of the host "www.w3.org" and send the following lines:

GET /tailwind_css/index.htm HTTP/1.1

Host: www.tutorialspoint.com

Note that the absolute path cannot be empty; if none is present in the original URI, it MUST be given as "/" (the server root).
Authority form It is only used with CONNECT method.

Request Header Fields

We will study General-header and Entity-header in a separate chapter when we will learn HTTP header fields. For now, let's check what Request header fields are.

The request-header fields allow the client to pass additional information about the request, and about the client itself, to the server. These fields act as request modifiers.Here is a list of some important Request-header fields that can be used based on the requirement:

  • Accept-Charset
  • Accept-Encoding
  • Accept-Language
  • Authorization
  • Expect
  • From
  • Host
  • If-Match
  • If-Modified-Since
  • If-None-Match
  • If-Range
  • If-Unmodified-Since
  • Max-Forwards
  • Proxy-Authorization
  • Range
  • Referer
  • TE
  • User-Agent

You can introduce your custom fields in case you are going to write your own custom Client and Web Server.

Examples of Request Message

Now let's put it all together to form an HTTP request to fetch hello.htm page from the web server running on tutorialspoint.com

GET /hello.htm HTTP/1.1
User-Agent: Mozilla/4.0 (compatible; MSIE5.01; Windows NT)
Host: www.tutorialspoint.com
Accept-Language: en-us
Accept-Encoding: gzip, deflate
Connection: Keep-Alive

Here we are not sending any request data to the server because we are fetching a plain HTML page from the server. Connection is a general-header, and the rest of the headers are request headers. The following example shows how to send form data to the server using request message body:

POST /cgi-bin/process.cgi HTTP/1.1
User-Agent: Mozilla/4.0 (compatible; MSIE5.01; Windows NT)
Host: www.tutorialspoint.com
Content-Type: application/x-www-form-urlencoded
Content-Length: length
Accept-Language: en-us
Accept-Encoding: gzip, deflate
Connection: Keep-Alive

licenseID=string&content=string&/paramsXML=string

Here the given URL /cgi-bin/process.cgi will be used to process the passed data and accordingly, a response will be returned. Here content-type tells the server that the passed data is a simple web form data and length will be the actual length of the data put in the message body. The following example shows how you can pass plain XML to your web server:

POST /cgi-bin/process.cgi HTTP/1.1
User-Agent: Mozilla/4.0 (compatible; MSIE5.01; Windows NT)
Host: www.tutorialspoint.com
Content-Type: text/xml; charset=utf-8
Content-Length: length
Accept-Language: en-us
Accept-Encoding: gzip, deflate
Connection: Keep-Alive

<?xml version="1.0" encoding="utf-8"?>
<string xmlns="http://clearforest.com/">string</string>
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