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HTTP Header (continued)

Headers
HTTP headers are very important part of both request message and response message. They mainly specify the characteristics of the resource requested and the data that are provided. For example-a client may want to accept image files only in some specified format. Similarly, the server may provide additional information about the resource being sent such as the length of the message or the last modification date of the resource.
Headers are separated by an empty line from the request and the message body. it consists of a single line or multiple lines. Each line is a single header of the following form:
Header-name: Header-value
The headers may be of two types:

·        HTTP Request header format- the request header consist of three parts: General header, request header and entity header.
 General Header
 Request Header
   Entity Header
                                   
·        HTTP Response header format- the request header consist of three parts: General header, Response header and entity header.
               
General Header
 Response Header
   Entity Header

                       

Ø General Header- Http General header provide information about the message themselves instead of what content they carry. They are mainly used to specify how the message is handled and processed.
   
Header Name
Description
Example
Cache-control
It shows whether the cache is used or not
Cache-control :max age=10
Connection
It specifies whether the server should close the connection or not
Connection: close
Date
It shows the date and time when the message is originated
Date: Fri., 31jan 2014  08:12:31 GMT
Mime-version
MIME version used
MIME version: 1.0
Upgrade
Preferred communication protocol
Upgrade: HTTP/2.0
Transfer encoding
Type of transformation that has been used to transfer the message
Transfer-encoding: chunked
Warning
Specifies the status of message transfer
Warning: 199 Galaxy warning
Via
Intermediate host between the messages
Via: 1.0 fred 1.1 source .com
Ø Request Header- It is only part of request message and contain the information about the client sending the request as well as data format that client expect. 
     Header Name
     Description    
       Example
  Accept-range
   Range type   of server support
     Accept-range: bytes
      Age
  Age of the resource in the proxy
     Age: 18
     Public   
      
List of methods
Public: head
   Retry-after
    Date after which the requested resource will be available
    Retry-after: 60
    Server
  Name and version number of the server
     Server: Apache/2.040





Ø Entity Header- It is present in both request and response messages. It contains the information about the message body.

Header Name
Description
Example
Allow
List of valid methods
Allow: GET,HEAD,POST
Content-encoding
Type of encoding
Content-encoding: x-gzip
Content-language
Language of the content
Content-language: en
Content-length
Length of the response body in bytes
Content-length: 2453
Content-range
Location of the partial message
Content-range: bytes 10-15/25-35
Content-type
MIME type of content
Content-type: text/html
Etag
An identifier for the specific version
Etag: “678060c98se84d83dd34e02d9”
Expires
Date and time the content will modified
Expires: Fri., 28 Feb. 2014 12:19:30 GMT
Last-modified
Last modification date of request resource
Last-modified:  wed, 26 Feb. 2014 15:14:30 GMT

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