Hypertext is structured text click the following article uses logical links hyperlinks between nodes containing text.
HTTP is the protocol to exchange or transfer hypertext. HTTP functions as a request—response protocol in the client—server computing model. A web browserfor example, may be the client and an application running on a computer hosting a website may be the server.
The client submits an HTTP request message to the server. The server, which provides resources such as HTML files and other content, or performs other functions on behalf of the client, returns a response message to the client. Here response contains completion status information about the request and may also contain requested content in its message body.
A web browser is an example of a user agent UA. Other types of user agent include the indexing software used by search providers web crawlersvoice browsersmobile app s, and other software that accesses, consumes, or displays web content. HTTP is designed to permit intermediate network elements to improve or enable communications between clients and servers. High-traffic websites often benefit from web cache servers that deliver content on behalf Vannevar Bush Write About In 1945 Essay upstream servers to improve response time.
Web browsers cache previously accessed web resources and reuse them when possible to reduce network traffic. HTTP proxy servers at private network boundaries can facilitate communication for clients without a globally routable address, by relaying messages with external servers.
HTTP is an application layer protocol designed within the framework of the Internet protocol suite. Its definition presumes an underlying and reliable transport layer protocol,  and Transmission Control Protocol TCP is commonly used.
The term hypertext was coined by Ted Nelson in in the Xanadu Projectwhich was in turn inspired by Vannevar Bush 's s vision of the microfilm-based information retrieval and management " memex " system described in his essay " As We May Think ". The first version of the protocol had only one method, namely GET, which would request a page from a server.
Dave Raggett led the HTTP Working Group HTTP WG in and wanted to expand the protocol with extended operations, extended negotiation, richer meta-information, tied with a security protocol which became more efficient by adding additional methods and header fields. An HTTP session is a sequence of network request-response transactions.
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An HTTP server listening on that port click here for a client's request message. The body of this message is typically the requested resource, although an error message or other information may also be returned. HTTP provides multiple authentication schemes such as basic access authentication and digest access authentication which operate via a challenge-response mechanism whereby the server identifies and issues a challenge before serving the requested content.
HTTP provides a general framework for access control and authentication, via an extensible set of challenge-response authentication schemes, which can be used by a server to challenge a client request and by a client to provide authentication information.
The HTTP Authentication specification also provides an arbitrary, implementation specific construct for further dividing resources common to a given root URI.
The realm value string, if present, is combined with the canonical root URI to form the protection space component of the challenge. This in effect allows the server to define separate authentication scopes under one root URI. HTTP defines methods sometimes referred to as verbs to indicate the desired action to be performed on the identified resource.
What this resource represents, whether pre-existing data or data that is generated dynamically, depends on the go here of the server. Often, the resource corresponds to a file or the output of an executable residing on the server.
By being specified in these documents their semantics are well known and can be depended on. Any client can use any method and the server can be configured to support any combination of methods. If a method is unknown to an intermediate it will be treated as an unsafe and non-idempotent method. There is no limit to the number of methods that can be defined and this allows for future methods to be specified without breaking existing infrastructure.
PWLTO#16 – Andrew Louis on The Memex and designing the internet before digital computers
In other words, they should not have side effectsbeyond relatively harmless effects such as loggingcachingthe serving of banner advertisements or incrementing a web counter. Making arbitrary GET requests without regard to the context of the application's state should therefore be considered safe. However, this is not mandated by the standard, and it is explicitly acknowledged that it cannot be guaranteed. Such methods are therefore not usually used by conforming web robots or web crawlers ; some that do not conform tend to make requests click at this page regard to context or consequences.
Despite the prescribed safety of GET requests, in practice their handling by the server is not technically limited in any way. Therefore, careless or deliberate programming can cause non-trivial changes on the server.
This is discouraged, because it can cause problems for web cachingsearch engines and other automated agents, which can make unintended changes on the server. For example, a website might allow deletion of a resource through a URL such as http: One example of this occurring in practice was during the short-lived Google Web Accelerator beta, which prefetched arbitrary URLs on the page a user was viewing, causing records to be automatically altered or deleted en masse. The beta was suspended only weeks after its first release, following widespread criticism.
Methods PUT and DELETE are defined to be idempotentmeaning that multiple identical requests should have the same effect as a single request note that idempotence refers to the state of the system after the request has completed, so while the action the server takes e.
In contrast, the POST method is not necessarily idempotent, and therefore sending an identical POST request multiple times may further affect state or cause further side effects such as financial read more. In some cases this may be desirable, but in other cases this could be due to an accident, such as when a user does not realize that their action will result in sending another request, or they Vannevar Bush Write About In 1945 Essay not receive adequate feedback that their first request was successful.
While web browsers may show alert dialog boxes to warn users in some cases where reloading a page may re-submit a POST request, it is generally up to the web application to handle cases where a POST request should not be submitted more than once. Note that whether a method is idempotent is not enforced by the protocol or web server.
It is perfectly possible to write a web application in which for example a database insert or other non-idempotent action is triggered by a GET or other request. Ignoring this recommendation, however, may result in undesirable consequences, if a user agent assumes that repeating the same request is safe when it isn't. The TRACE method can be used as part of a class of attacks known as cross-site tracing ; for that reason, common security advice is for it to be disabled in the server configuration.
The way the user agent handles the response primarily depends on the code and secondarily on the other response header fields.
Custom status codes can be used since, if the user agent encounters a code it does not recognize, it can use the first digit of the code to determine the general class click the response. The standard reason phrases are only recommendations and can be replaced with "local equivalents" at the web developer 's discretion.
If the status code indicated a problem, the user agent might display the reason phrase to the user to provide further information about the nature of the problem. The standard also allows the user agent to attempt to interpret the reason phrasethough this might be unwise since the standard explicitly specifies that status codes are machine-readable and reason phrases are human-readable.
HTTP status code is primarily divided into five groups for better explanation of request and responses between client and server as named: Such persistent connections reduce request latency perceptibly, because the client does not need to re-negotiate the TCP 3-Way-Handshake connection after the first request has been sent.
Another positive side effect is that in general the connection becomes faster with time due to TCP's slow-start -mechanism. HTTP pipelining further reduces lag time, allowing clients to send multiple requests before waiting for each response. Another addition to the protocol was byte servingwhere a server transmits just the portion of a resource explicitly requested by a client. HTTP is a stateless protocol.
A stateless protocol does not require the HTTP server to retain information or status about each user for the duration of multiple requests. However, some web applications implement states or server side sessions using for instance HTTP cookies or Vannevar Bush Write About In 1945 Essay variables within web forms. Browser support for these two is, however, nearly non-existent.
The client sends requests to the server and the server sends responses. A client request consisting in this case of the request line and only one header field is followed by a blank line, so that the request ends with a double newline, each in the form of a carriage return followed by a line feed.
Click the following article "Host" Vannevar Bush Write About In 1945 Essay distinguishes between various DNS names sharing a single IP addressallowing name-based virtual hosting. The ETag entity tag header field is used to determine if a cached version of the requested resource is identical to the current version of the resource on the server.
This is useful, if the client needs to have only certain portions  of a resource sent by the server, which is called byte serving. Most of the header lines are optional. When Content-Length is missing the length is determined in other ways. Chunked transfer encoding uses a chunk size of 0 to mark the end of the content. Identity encoding without Content-Length reads content until the socket is closed. A Content-Encoding like gzip can be used to compress the transmitted data.
From Wikipedia, the free encyclopedia. This is the latest accepted revisionreviewed on 24 January List of HTTP header fields. List of HTTP status codes.
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Mon, 23 May Wed, 08 Jan World Wide Web Consortium. Retrieved 31 August Retrieved 24 July Retrieved 11 June Retrieved 29 September Archived from the original on Technical Architecture Group finding. Retrieved 26 September Retrieved 23 February A common mistake is to use GET for an action that updates a resource. Fundamentals of Networking Security. Retrieved 30 April Semantic analytics Semantic broker Semantic computing Semantic mapper Semantic matching Semantic publishing Semantic reasoner Semantic search Semantic service-oriented architecture Semantic wiki.
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