2020-01-28 12:34:58 -05:00
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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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<!ENTITY ns "urn:xmpp:fallback:0">
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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>Fallback Indication</title>
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<abstract>This specification proposes a mechanism by which message bodies can be marked as being purely for fallback
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purposes, and therefore to be ignored by intermediaries and anything that understands the remainder of the message.</abstract>
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&LEGALNOTICE;
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<number>0428</number>
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<status>Experimental</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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<spec>XMPP Core</spec>
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</dependencies>
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<supersedes/>
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<supersededby/>
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<shortname>fallback</shortname>
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&dcridland;
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2020-03-03 12:40:07 -05:00
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<revision>
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<version>0.1.1</version>
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<date>2020-03-03</date>
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<initials>ps</initials>
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<remark>Minor editorial fixes</remark>
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</revision>
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2020-01-28 12:34:58 -05:00
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<revision>
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<version>0.1.0</version>
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<date>2020-01-28</date>
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<initials>XEP Editor (jsc)</initials>
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<remark>Accepted by vote of Council on 2020-01-02.</remark>
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</revision>
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<revision>
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<version>0.0.1</version>
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<date>2019-12-30</date>
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<initials>dwd</initials>
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<remark>
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<ul>
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<li>Initial Revision</li>
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</ul>
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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>A common and convenient practise for new extensions is to supply a fallback body. This provides immediate backwards
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compatibility for naive clients, since - not understanding the new protocol - they will gracefully degrade to displaying
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the body as an instant message.</p>
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<p>By way of example, a recent Reactions proposal suggested including the emoji as a <tt><body/></tt> element, so
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that existing clients would simply display it as a normal message.</p>
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<p>The downside of this approach is that servers and other intermediaries treat the presence of a <tt><body/></tt>
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as being an indicator that a message is indeed an instant message. They will then treat it this way for archival
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purposes, etc, which might not be appropriate.</p>
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<p>This specification tackles the problem by providing an element to be used as a hint that the supplied
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2020-02-25 13:01:32 -05:00
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<tt><body/></tt> and <tt><subject/></tt> elements are only for fallback purposes, and the message SHOULD be
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2020-01-28 12:34:58 -05:00
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treated as if they were not present for most purposes.</p>
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</section1>
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<section1 topic='Overview' anchor="overview">
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<section2 topic="Discovering Support" anchor="feature">
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<p>Support for this protocol MAY be advertised by the Service Discovery protocol defined in &xep0030; using a feature
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of <tt>&ns;</tt>. Note that lack of support will result in the desired fallback behaviour.</p>
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</section2>
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<section2 topic="Fallback Indicator">
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<p>The fallback indicator is an element <tt><fallback/></tt> qualified by the <tt>&ns;</tt> namespace. It has no
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attributes, content, or child elements.</p>
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<example><![CDATA[
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<message from="alice@example.org" to="bob@example.net" type="chat">
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<fallback xmlns="]]>&ns;<![CDATA["/>
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<encrypted xmlns="urn:example:crypto">Abobql jvyy rire qrpelcg bhe fhcre-frperg zrffntr!</encrypted>
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<body>This message is encrypted.</body>
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</message>
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]]></example>
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<p>Receiving the above message, a naive client will naturally display only the <tt><body/></tt> element text, but
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a client or server which supports this specification will know this is merely a fallback placeholder, and to ignore
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(and not display) the content therein.</p>
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</section2>
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<section2 topic="Alternatives">
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<ul>
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<li>&xep0334; was considered, and would probably be ideal - servers often examine these elements and alter
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behaviour accordingly, but the specification was rejected by Council.</li>
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<li>Placing fallback elements within the <tt><fallback/></tt> element would shift the onus from server to
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client, but this is likely to be less useful.</li>
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</ul>
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</section2>
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</section1>
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<section1 topic="Schema">
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<code>
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<![CDATA[
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<?xml version="1.0" encoding="UTF-8"?>
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<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="]]>&ns;<![CDATA[" xmlns:xs="http://www.w3.org/2001/XMLSchema">
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<xs:element name="fallback">
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<xs:complexType/>
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</xs:element>
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</xs:schema>
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]]>
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</code>
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</section1>
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<section1 topic='Security Considerations' anchor='security'>
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<p>This specification allows messages with a body (and real message content therein) to be treated by a server as
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if that body text does not exist. Servers MAY, particularly in a secure setting, wish to archive copies of the message
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even if they ordinarily would not archive a message with no body.</p>
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</section1>
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<section1 topic='IANA Considerations' anchor='iana'>
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<p>This XEP 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>None.</p>
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</section1>
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<section1 topic='Acknowledgements' anchor='ack'>
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<p>The author wishes to share any credit with many members of the community.</p>
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</section1>
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</xep>
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