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Text of the page (random words):
ttp 2 http 2 extended the usage of persistent connections by multiplexing many concurrent requests responses through a single tcp ip connection http 3 does not use tcp ip connections but quic udp content retrieval optimizations edit in http 0 9 a requested resource was always sent in its entirety http 1 0 added headers to manage resources cached by a client in order to allow conditional get requests a server must return the entire content of the requested resource only if its last modified time is not known by the client or if it changed since the last full response to a get request header content encoding was added to specify whether the returned content is compressed if the size of the content is not known in advance i e because it is dynamically generated then the header content length would not be included the client would assume that transfer was complete when the connection closed but a premature close would leave the client with partial content yet the client would not know it s partial http 1 1 introduced and later versions provide headers to better manage the conditional retrieval of cached resources chunked transfer encoding allows content to be streamed in chunks in order to reliably send it even when the server does not know its length in advance i e because it is dynamically generated etc byte range serving allows a client to request portions ranges of bytes of a resource this is useful to resume an interrupted download when a file is very large when only a part of a content has to be shown or dynamically added to the already visible part by a browser i e only the first or the following n comments of a web page in order to spare time bandwidth and system resources etc application session edit as a stateless protocol http does not require the web server to retain information or status about each user for the duration of multiple requests if a web application needs an application session it implements it via http cookies 21 hidden variables in a web form or another mechanism typically to start a session an interactive login is performed and to end a session a logout is requested by the user these kind of operations use a custom authentication mechanism not http authentication authentication edit 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 18 the authentication mechanisms described above belong to the http protocol and are managed by client and server http software if configured to require authentication before allowing client access to one or more web resources and not by the web applications using an application session the http authentication specification includes realms that provide 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 1 encrypted connection edit the most popular way of establishing an encrypted http connection is https 22 two other methods for establishing an encrypted http connection also exist secure hypertext transfer protocol and using the http 1 1 upgrade header to specify an upgrade to tls browser support for these two is however nearly non existent 23 24 25 message format edit an http 1 1 request made using telnet the parts of the transaction are shown in different colors request in red response header in purple and response body in green this section describes messages for http 1 1 later versions http 2 26 and http 3 use a binary protocol where headers are encoded in a single headers and zero or more continuation frames using hpack 27 http 2 or qpack http 3 which both provide efficient header compression the request or response line from http 1 has also been replaced by several pseudo header fields each beginning with a colon at the highest level a message consists of a header followed by a body header edit a header consists of lines of ascii text each terminated with a carriage return and line feed sequence the layout for both a request and a response header is start line structured data that differs for request vs response header fields zero or more header field lines at least 1 for http 1 1 see below empty line marks the end of the header body edit a body consists of data in any format not limited to ascii the format must match that specified by the content type header field if the message contains one a body is optional or in other words can be blank entity edit before http 2 the term entity was used to mean the body plus header fields that describe the body in particular not all headers were considered part of the entity the term entity header referred to a header that was considered part of the entity and sometimes the body was called the entity body modern documentation uses body and header without using entity header field edit see also list of http header fields a header field represents metadata about the containing message examples include how the body is encoded via content encoding session verification and identification of the client as in browser cookies ip address user agent or their anonymity thereof vpn or proxy masking user agent spoofing how the server should handle data as in do not track or global privacy control and the age the time it has resided in a shared cache of the document being downloaded generally the information of a header field is used by software and not shown to the user a header field line is formatted as a name value pair with a colon separator whitespace is not allowed around the name but leading and trailing whitespace is ignored for the value part unlike a method name that must match exactly case sensitive 28 a header field name is matched ignoring case although often shown with each word capitalized 29 for example the following are header fields for host and accept language host www example com accept language en the standards do not limit the size of a header field or the number of fields in a message however most servers clients and proxy software impose limits for practical and security reasons for example the apache 2 3 server by default limits the size of each field to 8190 bytes and there can be at most 100 header fields in a single request 30 although deprecated by rfc 7230 31 in the past long lines could be split into multiple lines with a continuation line starting with a space or tab character request edit a request is sent by a client to a server the start line includes a method name a request uri and the protocol version with a single space between each field 32 the following request start line specifies method get uri customer 123 and protocol version http 1 1 get customer 123 http 1 1 request header fields allow the client to pass additional information beyond the request line acting as request modifiers similarly to the parameters of a procedure they give information about the client about the target resource or about the expected handling of the request in the http 1 1 protocol all header fields except host are optional a request line containing only the path name is accepted by servers to maintain compatibility with http clients before the http 1 0 specification in rfc 1945 33 resource edit the protocol structures transaction as operating on resources what a resource represents whether pre existing data or data that is generated dynamically depends on the implementation of the server often the resource corresponds to a file or the output of an executable running on the server method edit a request identifies a method sometimes informally called verb to classify the desired action to be performed on a resource the http 1 0 specification 3 8 defined the get head and post methods as well as listing the put delete link and unlink methods under additional methods however the http 1 1 specification 34 9 added five new methods put delete connect options and trace 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 which allows for future methods to be specified without breaking existing infrastructure for example webdav defined seven new methods and rfc 5789 specified the patch method a general purpose web server is required to implement at least get and head and all other methods are considered optional by the specification 18 9 1 method names are case sensitive 4 3 18 9 1 this is in contrast to http header field names which are case insensitive 18 6 3 get the request is for a representation of a resource the server should only retrieve data not modify state 1 for retrieving without making changes get is preferred over post as it can be addressed through a url clarification needed this enables bookmarking and sharing and makes get responses eligible for caching which can save bandwidth the w3c has published guidance principles on this distinction saying web application design should be informed by the above principles but also by the relevant limitations 35 head the request is like a get except that the response should not include the representation data in the body this is useful for retrieving the representation metadata in the response header without having to transfer the entire representation uses include checking whether a page is available via the status code and getting the size of a file via header field content length post the request is to process a resource in some way for example it is used for posting a message to an internet forum subscribing to a mailing list or completing an online shopping transaction 18 9 3 3 put the request is to create or update a resource with the state in the request a distinction from post is that the client specifies the target location on the server 18 9 3 4 delete the request is to delete a resource connect requests that the intermediary establish a tcp ip tunnel to the origin server identified by the request target it is often used to secure connections through one or more http proxies with tls 18 9 3 6 36 see http connect method options request is for a report of the http methods that are supported for a resource this can be used to check the functionality of a web server by requesting instead of a specific resource trace requests the server to respond with the received request in the response body that way a client can see what if any changes or additions have been made by intermediaries useful for debugging patch the request is to modify a resource according to its partial state in the request compared to put this can save bandwidth by sending only part of a resource s representation instead of all of it 37 query requests that the server process the enclosed content in a safe and idempotent manner and then respond with the result this is similar to post requests but query requests can be automatically repeated or restarted without concern for partial state changes 38 properties of request methods method rfc request has payload body response has payload body safe idempotent cacheable get rfc 9110 optional yes yes yes yes head rfc 9110 optional no yes yes yes post rfc 9110 yes yes no no yes put rfc 9110 yes yes no yes no delete rfc 9110 optional yes no yes no connect rfc 9110 optional yes no no no options rfc 9110 optional yes yes yes no trace rfc 9110 no yes yes yes no patch rfc 5789 yes yes no no no query rfc 10008 yes yes yes yes yes safe method edit a request method is safe if a request with that method has no intended effect on the server the methods get head options trace and query are defined as safe in other words safe methods are intended to be read only safe methods can still have side effects not seen by the client such as appending request information to a log file or charging an advertising account in contrast the methods post put delete connect and patch are not safe they may modify the state of the server or have other effects such as sending an email such methods are therefore not usually used by conforming web robots or web crawlers some that do not conform tend to make requests without 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 careless or deliberately irregular programming can allow get requests to cause non trivial changes on the server this is discouraged because of the problems which can occur when web caching search engines and other automated agents make unintended changes on the server for example a website might allow deletion of a resource through a url such as https example com article 1234 delete which if arbitrarily fetched even using get would simply delete the article 39 a properly coded website would require a delete or post method for this action which non malicious bots would not make 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 40 idempotent method edit see also idempotence computer science meaning a request method is idempotent if multiple identical requests with that method have the same effect as a single such request the methods put and delete and safe methods are defined as idempotent safe methods are trivially idempotent since they are intended to have no effect on the server whatsoever the put and delete methods meanwhile are idempotent since successive identical requests will be ignored a website might for instance set up a put endpoint to modify a user s recorded email address if this endpoint is configured correctly any requests which ask to change a user s email address to the same email address which is already recorded e g duplicate requests following a successful request will have no effect similarly a request to delete a certain user will have no effect if that user has already been deleted in contrast the methods post connect and patch are not necessarily idempotent and therefore sending an identical post request multiple times may further modify the state of the server or have further effects such as sending multiple emails in some cases this is the desired effect but in other cases it may occur 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