HTTP Protocol Design and Description

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HTTP Protocol Design and
Description
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Topics
HTTP PROTOCOL DESIGN AND DESCRIPTION
Protocol Definition Protocol Properties HTTP Headers HTTP Response Classes
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Protocol Definition
HTTP PROTOCOL DESIGN AND DESCRIPTION
A protocol is a language with a Grammar Syntactic structure Semantic rules HTTP is A request-response protocol An application-level protocol The HTTP protocol evolved along with the Web and URI and HTML. (Table 6.1)
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Table 6.1. Historical timeline of HTTP-related
documents
HTTP PROTOCOL DESIGN AND DESCRIPTION
Document Date
CERN labs document proposing Web HTTP/0.9 specification W3 and WAIS/X.500 Proposal to add MIME to HTTP UDI (Universal Document Identifier) for the Network HTTP/1.0 first draft HTML (1.0 Specification) URL specification HTTP/1.0 second draft URI in WWW HTTP/1.0 Informational, RFC 1945 HTTP/1.1 Proposed Standard, RFC 2068 HTTP/1.1 Draft Standard, RFC 2616 HTTP/1.1 Formal Standard Mar 1990 Jan 1992 Feb 1992 Dec 1992 Feb 1993 Mar 1993 Jun 1993 Oct 1993 Nov 1993 Mar 1994 May 1996 Jan 1997 Jun 1999 2001
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Protocol Properties
HTTP PROTOCOL DESIGN AND DESCRIPTION
Global URI HTTP Request/Response Format Statelessness Resource Metadata HTTP Request Methods
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Global URI
HTTP PROTOCOL DESIGN AND DESCRIPTION
URI (Uniform Resource Identifier) permits resources to reside anywhere on the Internet URL (Uniform Resource Locator) shows the location of a copy of a resource URN (Uniform Resource Name) is a unique name for a resource URI is a superset of both URL and URN
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Global URI
HTTP PROTOCOL DESIGN AND DESCRIPTION

URI
URN
URL
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HTTP Request
HTTP PROTOCOL DESIGN AND DESCRIPTION
Consider the following HTTP request GET /foo.html HTTP/1.o each request message consists of a Request-line Method GET Resource /foo.html HTTP version number 1.0 General/Request/Entity Header (s) CRLF Entity body (optional)
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HTTP Request Format
HTTP PROTOCOL DESIGN AND DESCRIPTION

GET /motd HTTP/1.0
Request line
Date Wed,22 Mar 2000 080901 GMT Pragma
No-cache
General headers
From gorby_at_moskvax.com User-Agent Mozilla/4.03
Request headers
ltno entity bodygt
Figure 6.1. An HTTP request message
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HTTP Request Format
HTTP PROTOCOL DESIGN AND DESCRIPTION

PUT /motd HTTP/1.0
Request line
General header
Date Wed,22 Mar 2000 080901 GMT
From gorby_at_moskvax.com User-Agent Mozilla/4.03
Request headers
Content-Length23 Allow GET, HEAD, PUT
Entity headers
Welcome to Comers Vax
Entity body
Figure 6.2. Another HTTP request message
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HTTP Response
HTTP PROTOCOL DESIGN AND DESCRIPTION
Consider the following HTTP response HTTP/1.o 200 OK Date Wed,22 Mar 2000 080101 GMT Last-Modified Wed,22 Mar 2000 021633 GMT Content-Length 3913 lt3,913 bytes of the current contents of /foo.htmlgt
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HTTP Response
HTTP PROTOCOL DESIGN AND DESCRIPTION
Each response message consists of a Status-line HTTP version number Status code (indicating success or failure) Status phrase General/Response/Entity Header (s) Date Last-Modified Content-Length CRLF Entity body (optional)
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HTTP Response Format
HTTP PROTOCOL DESIGN AND DESCRIPTION

Status line
HTTP/1.0 200 OK
General header
Date Wed,22 Mar 2000 080901 GMT
Server Netscape-Enterprise/3.51
Response header
Entity header
Content-Length23
Entity body
Welcome to Comers Vax
Figure 6.2. An HTTP response message
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Statelessness
HTTP PROTOCOL DESIGN AND DESCRIPTION
HTTP is a stateless protocol. NNTP and FTP maintain some amount of state.
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Resource Metadata
HTTP PROTOCOL DESIGN AND DESCRIPTION
Metadata is information that relates to a resource but is not part of a resource itself. Metadata includes The size of a resource The type of the content The last modification time of the resource
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HTTP Request methods
HTTP PROTOCOL DESIGN AND DESCRIPTION
A request method represents what action an HTTP sever should perform on the resource. Some methods are GET,HEAD,POSE,PUT,DELETE,LINK,UNLINK
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HTTP Request methods
HTTP PROTOCOL DESIGN AND DESCRIPTION
Properties of a method are Safety A request method that examines the state of a resource is a safe method. A method that can alter the state of the resource is not safe. Idempotence A method that its side effect is the same as multiple identical requests.
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HTTP Request methods (GET)
HTTP PROTOCOL DESIGN AND DESCRIPTION
Is applied to the resource specified in the URL, and the generated response is the current value of the resource. Is safe and idempotent. Could include arguments on the users input. GET http//www.altavista.com/cgi-bin/query?qfoo Can have modifier If-Modified-Since in header. GET /foo.html HTTP/1.0 If-Modified-Since Sun, 12 Nov 2000 111223 GMT
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HTTP Request methods (HEAD)
HTTP PROTOCOL DESIGN AND DESCRIPTION
Is introduced to obtain just the metadata associated with a resource. Is safe and idempotent. A HEAD request such as HEAD /foo.html HTTP/1.0 might retrun HTTP/1.0 200 OK Content-Length 3219 Last-Modified Sun, 12 Nov 2000 111223 GMT Content-Type text/html
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HTTP Request methods (HEAD)
HTTP PROTOCOL DESIGN AND DESCRIPTION
Uses of HEAD method include Debugging the server Determining recently resource changes Not have request modifier such as If-Modified-Since Has no request body
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HTTP Request methods (POST)
HTTP PROTOCOL DESIGN AND DESCRIPTION
Is used to update an existing resource or provide input to a process handling data. The body of the request includes the data. Is not safe and idempotent. The Content-Length header is required as part of a POST request .
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HTTP Request methods (PUT)
HTTP PROTOCOL DESIGN AND DESCRIPTION
Is similar to POST. Is not safe but idempotent.
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HTTP Request methods (DELETE)
HTTP PROTOCOL DESIGN AND DESCRIPTION
Is used to delete the resource remotely identified in Request-URI. Is not safe but idempotent.
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HTTP Request methods (LINK and UNLINK)
HTTP PROTOCOL DESIGN AND DESCRIPTION
The LINK method permitted creation of links between the Request-URI and other resources. The UNLINK method was used to delete links created via the LINK method.
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HTTP Headers
HTTP PROTOCOL DESIGN AND DESCRIPTION
General Headers Request Headers Response Headers Entity Headers
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HTTP Headers
HTTP PROTOCOL DESIGN AND DESCRIPTION
HTTP header Fieldname Fieldvalue CRLF A header is a free-format ASCII string representing the name with a value. Headers are used to Alter the handling of a request Provide metadata about the resource Parameterize or describe a request or a response. New headers in HTTP have arbitrary length. Headers are limited by CR and LF.
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HTTP Headers
HTTP PROTOCOL DESIGN AND DESCRIPTION
A message header could be A general header A request header A response header An entity header The order of different headers are not significant but it is common to have General header Request/Response header Entity header
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General Headers
HTTP PROTOCOL DESIGN AND DESCRIPTION
General headers appear in both request and response messages. The General headers are significant only to the message itself and not to the entity. A general header has only two fields Date Displayed in three different formats Pragma Are directives for recipient of the message no-cache is the only directive
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Request Headers
HTTP PROTOCOL DESIGN AND DESCRIPTION
A request header can be used by the client to send information with the request or to specify constraints on the sever handling the request. Five request headers are Authorization To include appropriate credentials required to access a resource From To include users email address
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Request Headers
HTTP PROTOCOL DESIGN AND DESCRIPTION
If-Modified-Since Is a conditional header Retrieves resource if it has not changed since the argument specified in the If-Modified-Since header. Referer Lets the client include the URI of the resource from which the request-URI was obtained.
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Request Headers
HTTP PROTOCOL DESIGN AND DESCRIPTION
User-Agent Can be used to include information about Version of the used browser The client machines operating system version Hardware details
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Response Headers
HTTP PROTOCOL DESIGN AND DESCRIPTION
Response headers send additional information about the response and the server that originated the response. If a response header is not recognized, it is assumed to be an entity header. HTTP/1.0 defines three response headers Location Is used to redirect the request to where the resource can be found.
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Response Headers
HTTP PROTOCOL DESIGN AND DESCRIPTION
Server Can be used to include information about Version of the origin server software Configuration details WWW-Authenticate Is used to issue a challenge to the client seeking access to an authenticated resource.
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Entity Headers
HTTP PROTOCOL DESIGN AND DESCRIPTION
An entity header is used to include information about the body of the entity or the resource. Entity headers may be found in requests and in responses. There are six entity headers Allow Is used to indicate the list of valid methods that can be applied to a resource. Content-Type Indicates the media type of the entity body
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Entity Headers
HTTP PROTOCOL DESIGN AND DESCRIPTION
Content-Encoding Indicates how the resource could be decoded into the format indicated in the Content-Type. Content-Length Indicates the length of the entity body in bytes. Expires Indicates that the entity should be considered stale after the time specified in the header. Last-Modified Indicates the time at which the resource was modified last.
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HTTP Response Classes
HTTP PROTOCOL DESIGN AND DESCRIPTION
The various kinds of responses are grouped into a set of response classes Informational class 1xx Success class 2xx are generated after a server received and accepted the HTTP request for processing 200 OK 201 Created 202 Accepted 204 No Content
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HTTP Response Classes
HTTP PROTOCOL DESIGN AND DESCRIPTION
Redirection class 3xx is used to inform the user agent that additional action is needed to complete the request. 300 Multiple Choices 301 Moved Permanently 302 Moved Temporarily 304 Not Modified Client error class 4xx is used for identifying errors that made by clients. 400 Bad Request 401 Unauthorized 403 Forbidden 404 Not Found
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HTTP Response Classes
HTTP PROTOCOL DESIGN AND DESCRIPTION
Server error class 5xx is used for identifying errors that made by the server. 500 Internal Server Error 501 Not Implemented 502 Bad Gateway 503 Service Unavailable
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