Introduction 2 HTTP

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Intro to HyperTextTransfer Protocol (HTTP)Intro to HyperTextTransfer Protocol (HTTP)

Dr. Abzetdin ADAMOV

QAFQAZ UNIVERSITYQAFQAZ UNIVERSITY


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Overview of HTTPOverview of HTTP

HTTP defines a simple request-response

language

A web client establishes a connection witha web server by using HTTP

HTTP defines how to correctly phrase the

request and how the response should look

like

Note: HTTP does not define how the network connection is made or

managed, nor how the information is actually transmitted; it is done by

the lower-level protocols such as TCP/IP


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The Client RequestThe Client Request

1. The method, which must be one of a set of

legal action

2. The Universal Resource Identifier (URI), which

is the name of the information request

3. The protocol version4. Optional supplemental information

An HTTP request consists of the following:


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HTTP methodsHTTP methods

Method Action

GETReturn the object; that is, retrieve the

information

HEADReturn only information about the object, such

as how old it is, but not the object itself

POSTSend information to be stored on the server.

Many servers do not allow information to be

POSTed except as input to scripts

PUTSend a new copy of an existing object to the

server. Many servers do not allow documents to

be PUT.

DELETEPermanently delete the object. Like PUT, this

method is not allowed by most servers


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Example of Client RequestExample of Client Request

For example, to request the document /network/index.htm from

http://www.qafqaz.edu.az/, the web client will send the following

request:

GET /network/index.htm HTTP/1.0

User-Agent: NCSA Mosaic for Windows 95/3.0

Accept: text/plain

Accept: text/html

Accept: application/postscript

Accept: image/gif


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The Server ResponseThe Server Response

1. A status line, which indicates the success or

failure of the request

2. A description of the information in the

response. This is the metadata or

metainformation

3. The actual information requested

An HTTP response consist of the following:


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Selected HTTP status codeSelected HTTP status code

Code Reason Description

200 Document followsThe request succeeded. The information requested

follows.

301 Moved Permanently The document has moved to a new URL

302 Moved Temporarily The document has moved temporarily to a new URL

304 Not ModifiedThe document has not been modified since the date

specified in a GET request with if-modified-

since.

404 Not FoundThe information could not be found or permission

was denied. This error is returned if the

requested URL does not exist or was misspelled

401 Unauthorized The information is restricted; please retry withproper authentication.

402 Payment RequiredThe information requires paying a fee; please retry

with proper payment (not used often)

403 Forbidden Access is forbidden

500Server Error The server experienced an error


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Description of InformationDescription of Information

Field Description

ServerThe type of server software providing the

response

Date The date and time of the responseContent-Length How many bytes of data will be sent to the client

Content-TypeThe MIME type of the information being returned,

such as HTML or an image

Content-Language The language of the information, such as Englishor French

Content-Encoded Additional encoding, such as data compression

Last-ModifiedThe date and time that the information was most

recently modified


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Example of Server ResponseExample of Server Response

For example, the response for the /network/index.htm request

might be the following:

HTTP/1.0 Status 200Document follows

Server: NCSA/2.0Date: Wed, 23 Jun, 1999 18:08:08 GMT

Content-type: text/html

Content-length: 5800

Last-modified: Tue, 22 Jun, 1999 12:00:00 GMT


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HTTP DetailsHTTP Details

Whats the difference between GET and POST?

Form Elements


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Step 2: A Request arrives from a Web Client

Serving a Web Document form a Web Server


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Step 3: The Request is Parsed by the Web Server


The Web Server parses and understand the request is a GET for information.


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Step 4: Parses the Rest of the Header

The web server now understnad the protocol version is 1.0. It is a Netscape

browser for NT, etc. Since this is a normal example, no further action is

needed.



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Step 5: Do the method requested

Serving a Web Document form a Web ServerThe httpd in this stage will fulfills the request or send back error messages. In

this example, web server will search in the file system for /index.htm.

Success: The document is sent


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Step 5: Failure: An error such as the following will be sent


Failure: An error such as the following will be sent:


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Step 6: Finish up: close the file; close the network connection

Close file and connection (socket).

Client will then due with the data received.

Step 7: Back to Step 1 and Loop and Wait


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Introduction 2 HTTP