HTTP Message

HTTP message is the information transaction between the client and server.

 Two types of HTTP Message:

 1. Requests

a. Client to server

2. Responses

a. Server to client


· Request line or Response line

· General header

· Request header or Response header

· Entity header

· Entity body

Request Message:

 Request Line:

 •  A request line has three parts, separated by spaces o a method name

 o the local path of the requested resource o the version of HTTP being used

 • A typical request line is:

o GET /path/to/file/index.html HTTP/1.1

• Notes:

 o GET is the most common HTTP method; it says “give me this resource”. Other methods include POST and HEAD. Method names are always uppercase

o The path is the part of the URL after the host name, also called the request URI o The HTTP version always takes the form “HTTP/x.x“, uppercase.

Request Header:

Response Message:

Response Line:

 •  A request line has three parts, separated by spaces o the HTTP version,

 o a response status code that gives the result of the request, and o an English reason phrase describing the status code

 • Typical status lines are:

o HTTP/1.0 200 OK or

o HTTP/1.0 404 Not Found

• Notes:

o The HTTP version is in the same format as in the request line, “HTTP/x.x“.

o The status code is meant to be computer-readable; the reason phrase is meant to be human-readable, and may vary.

HTTP Request Header:


 HTTP Method:

•  HTTP method is supplied in the request line and specifies the operation that the client has requested.

Some common methods:

• Options

• Get

• Head

• Post

• Put

• Move

• Delete

Two methods that are mostly used are the GET and POST: GET for queries that can be safely repeated

 o POST for operations that may have side effects (e.g. ordering a book from an on-line store).

The GET Method

 •   It is used to retrieve information from a specified URI and is assumed to be a safe, repeatable operation by browsers, caches and other HTTP aware components

 • Operations have no side effects and GET requests can be re-issued.

 •  For example, displaying the balance of a bank account has no effect on the account and can be safely repeated.

 •  Most browsers will allow a user to refresh a page that resulted from a GET, without displaying any kind of warning

 •  Proxies may automatically retry GET requests if they encounter a temporary network connection problem.

 •  GET requests is that they can only supply data in the form of parameters encoded in the URI (known as a Query String) – [downside]

 Cannot be unused for uploading files or other operations that require large amounts of data to be sent to the server.

 The POST Method

 • Used for operations that have side effects and cannot be safely repeated.

 •  For example, transferring money from one bank account to another has side effects and should not be repeated without explicit approval by the user.

If you try to refresh a page in Internet Explorer that resulted from a POST, it displays the following message to warn you that there may be side effects:

The POST request message has a content body that is normally used to send parameters and data

• The IIS server returns two status codes in its response for a POST request

o The first is 100 Continue to indicate that it has successfully received the POST request

o The second is 200 OK after the request has been processed.

HTTP response status codes

• Informational (1xx)

• Successful (2xx)

• Redirection (3xx)

o 301: moved permanently

• Client error (4xx)

o 403 : forbidden o 404: Not found

• Server error (5xx)

o 503: Service unavailable

o 505: HTTP version not supported

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