mirror of
https://github.com/moparisthebest/xeps
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135 lines
5.4 KiB
XML
135 lines
5.4 KiB
XML
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<?xml version='1.0' encoding='UTF-8'?>
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<!DOCTYPE xep SYSTEM 'xep.dtd' [
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<!ENTITY % ents SYSTEM 'xep.ent'>
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%ents;
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]>
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<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
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<xep>
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<header>
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<title>Compatibility Fallbacks</title>
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<abstract>
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This document defines a way to indicate that a specific part of the body only serves as fallback and which
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specification the fallback is for.
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</abstract>
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&LEGALNOTICE;
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<number>xxxx</number>
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<status>ProtoXEP</status>
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<type>Standards Track</type>
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<sig>Standards</sig>
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<dependencies/>
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<supersedes/>
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<supersededby/>
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<shortname>compat</shortname>
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<author>
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<firstname>Natalie</firstname>
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<surname>Wirth</surname>
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<email>nataliew@laposte.net</email>
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</author>
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<author>
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<firstname>Marvin</firstname>
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<surname>Wissfeld</surname>
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<email>xsf@larma.de</email>
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<jid>jabber@larma.de</jid>
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</author>
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<revision>
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<version>0.0.1</version>
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<date>2022-01-01</date>
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<initials>nw/mw</initials>
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<remark>
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<p>First draft.</p>
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</remark>
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</revision>
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</header>
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<section1 topic='Introduction' anchor='intro'>
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<p>
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New specifications can use the message body to convey intended meaning to users of non-supporting clients. This
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XEP provides a way to indicate which part of the body serves as fallback and which specification it provides a
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fallback for.
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</p>
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<p>
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This specification serves a different purpose than the similarly named &xep0428;. Fallback indication tells
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servers that the body is only a fallback and that clients implementing all the specifications used by the message
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will not make use of the message body. This specification tells clients that parts of the body are only included
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to aid clients not supporting a certain specification.
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</p>
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</section1>
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<section1 topic='Use Cases' anchor='usecases'>
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<p>
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To mark a specific text section in the body as a fallback, a <fallback> element in the urn:xmpp:compat:0
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namespace is placed in the message stanza. The <fallback> element has a 'for' attribute with an identifier
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of the specification the fallback is for. The <fallback> element contains one <body> element for each
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continuous character sequence in the body that is part of the fallback text. Each body element contains a 'start'
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and 'end' attribute which point to the start and end of a fallback character sequence as defined in &xep0426;,
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respectively.
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</p>
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<p>
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For example, Juliet might be part of a group that shares news. Breaking news are indicated by a specific element
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and supporting clients can highlight them accordingly. To also inform users of non-supporting clients about the
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importance of a piece of news, the information is prefixed by "BREAKING NEWS: " in the body. A supporting client
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sees the <fallback> element and removes the respective character sequence before highlighting the message to
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the user.
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</p>
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<example><![CDATA[
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<message to='news@muc.example.com/juliet' id='message-id2' type='chat'>
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<body>BREAKING NEWS: Romeo is dead.</body>
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<breaking xmlns='urn:example:breaking-news:0' />
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<fallback xmlns='urn:xmpp:compat:0' for='urn:example:breaking-news:0'>
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<body start='0' end='15' />
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</fallback>
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</message>]]></example>
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<p>
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Another example are message replies, where a <reply> element specifies the referenced message. A simple
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fallback is to include a &xep0393; quote of the referenced message in the body text. To provide a better fallback,
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the sender can also include markup information for the quote.
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</p>
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<example><![CDATA[
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<message to='anna@example.com' id='message-id2' type='groupchat'>
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<body>
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> Anna wrote:
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> Hi, how are you?
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Great
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</body>
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<reply to='anna@example.com' id='message-id1' xmlns='urn:xmpp:reply:0' />
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<markup xmlns="urn:example:markup:0">
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<quote start='0' end='33' />
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</markup>
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<fallback xmlns='urn:xmpp:compat:0' for='urn:example:markup:0'>
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<body start='0' end='1' />
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<body start='14' end='15' />
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</fallback>
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<fallback xmlns='urn:xmpp:compat:0' for='urn:xmpp:reply:0'>
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<body start='0' end='33' />
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</fallback>
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</message>]]></example>
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</section1>
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<section1 topic='Business Rules' anchor='rules'>
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<p>The exact behavior for a compatibility fallback should be defined in the respective specification. Not displaying
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the fallback in supporting clients would be an example for a behavior.
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</p>
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</section1>
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<section1 topic="Security Considerations" anchor="security">
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<p>
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An attacker might include a compatibility fallback with a meaning that is different from what would be displayed
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by a supporting client. While this could also be achieved using other parts of the XMPP specifications (e.g.
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xml:lang), some environments might want to prevent it. Specifications could standardize some parts of the
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compatibility text such that the equivalence can be verified by supporting clients.
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</p>
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</section1>
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<section1 topic='IANA Considerations' anchor='iana'>
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<p>This document requires no interaction with &IANA;.</p>
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</section1>
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<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
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<p>This document requires no interaction with ®ISTRAR;.</p>
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</section1>
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</xep>
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