Chapter 2Application Layer

Computer Networking: A Top Down Approach, 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.

HTTP Request MessageTwo types of HTTP messages:

Request, ResponseHTTP request message:

Human-readable format

A typical HTTP request message

GET /somedir/page.html HTTP/1.1Host: www.someschool.edu User-agent: Mozilla/4.0Connection: close Accept-language:fr

(Blank Line) Body

HTTP Request Message

• Request Line First Line of HTTP request message The request line has three fields Method field can take on several values

GET,POST, PUT, DELETE etcMajority of HTTP request messages use GET

URL field HTTP version field

• Header Lines– Host: Host on which the object resides– User-agent: Identifies the client program e.g.

Mozilla/4.0– Connection: Shows whether the connection should be

closed or not– Accept-language: Shows the language the client

prefers

HTTP Request Message

• Body is empty with GET method • Used with the POST method

– Web page often includes form input– Input is sent to server in body

PUT method:• To upload objects to

web serverDELETE method• Allows a user or an

application to delete an object on web server

Find out about OPTIONS, TRACE, HEAD?

HTTP Response Message

HTTP/1.1 200 OK Connection: closeDate: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 Content-Length: 6821 Content-Type: text/html Body contains Requested Object

HTTP Response Message

• Status Line– Protocol Version– Status Code and Status Message

The status code and associated phrase indicate the result of the request

• Header Line Connection: Shows whether the connection should be

closed or not Date and time: When the HTTP response was created Server: Analogous to user-agent in request message Last-modified: When the Object was last created or

modified Content-length: Number of bytes in object being sent Content-Type: Indicates the object type (HTML text etc.)

HTTP Response Status Codes

200 OK

– Request succeeded and information is returned in response

301 Moved Permanently

– Requested object moved, new location specified later in this message (Location:)

404 Not Found

– Requested document not found on this server

505 Version not supported

– HTTP version is not supported by the server

The status code and associated phrase indicate the result of the request