<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
  <!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
        <!ENTITY ns "urn:xmpp:fallback:0">
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
  <title>Fallback Indication</title>
  <abstract>This specification proposes a mechanism by which message bodies can be marked as being purely for fallback
    purposes, and therefore to be ignored by intermediaries and anything that understands the remainder of the message.</abstract>
  &LEGALNOTICE;
  <number>XXXX</number>
  <status>ProtoXEP</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <dependencies>
    <spec>XMPP Core</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>fallback</shortname>
  &dcridland;
  <revision>
    <version>0.0.1</version>
    <date>2019-12-30</date>
    <initials>dwd</initials>
    <remark>
      <ul>
        <li>Initial Revision</li>
      </ul>
    </remark>
  </revision>
</header>

<section1 topic='Introduction' anchor='intro'>
  <p>A common and convenient practise for new extensions is to supply a fallback body. This provides immediate backwards
    compatibility for naive clients, since - not understanding the new protocol - they will gracefully degrade to displaying
  the body as an instant message.</p>
  <p>By way of example, a recent Reactions proposal suggested including the emoji as a <tt>&lt;body/></tt> element, so
    that existing clients would simply display it as a normal message.</p>
  <p>The downside of this approach is that servers and other intermediaries treat the presence of a <tt>&lt;body/></tt>
    as being an indicator that a message is indeed an instant message. They will then treat it this way for archival
    purposes, etc, which might not be appropriate.</p>
  <p>This specification tackles the problem by providing an element to be used as a hint that the supplied
    <tt>&lt;body/></tt> and <tt>&lt;subject</tt> elements are only for fallback purposes, and the message SHOULD be
    treated as if they were not present for most purposes.</p>
</section1>

<section1 topic='Overview' anchor="overview">
  <section2 topic="Discovering Support" anchor="feature">
    <p>Support for this protocol MAY be advertised by the Service Discovery protocol defined in &xep0030; using a feature
      of <tt>&ns;</tt>. Note that lack of support will result in the desired fallback behaviour.</p>
  </section2>
  <section2 topic="Fallback Indicator">
    <p>The fallback indicator is an element <tt>&lt;fallback/></tt> qualified by the <tt>&ns;</tt> namespace. It has no
      attributes, content, or child elements.</p>
    <example><![CDATA[
<message from="alice@example.org" to="bob@example.net" type="chat">
  <fallback xmlns="]]>&ns;<![CDATA["/>
  <encrypted xmlns="urn:example:crypto">Abobql jvyy rire qrpelcg bhe fhcre-frperg zrffntr!</encrypted>
  <body>This message is encrypted.</body>
</message>
]]></example>
    <p>Receiving the above message, a naive client will naturally display only the <tt>&lt;body/></tt> element text, but
      a client or server which supports this specification will know this is merely a fallback placeholder, and to ignore
      (and not display) the content therein.</p>
  </section2>
  <section2 topic="Alternatives">
    <ul>
      <li>&xep0334; was considered, and would probably be ideal - servers often examine these elements and alter
        behaviour accordingly, but the specification was rejected by Council.</li>
      <li>Placing fallback elements within the <tt>&lt;fallback/></tt> element would shift the onus from server to
      client, but this is likely to be less useful.</li>
    </ul>
  </section2>
</section1>

  <section1 topic="Schema">
    <code>
      <![CDATA[
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="]]>&ns;<![CDATA[" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="fallback">
    <xs:complexType/>
  </xs:element>
</xs:schema>
      ]]>
    </code>
  </section1>

<section1 topic='Security Considerations' anchor='security'>
  <p>This specification allows messages with a body (and real message content therein) to be treated by a server as
  if that body text does not exist. Servers MAY, particularly in a secure setting, wish to archive copies of the message
  even if they ordinarily would not archive a message with no body.</p>
</section1>

<section1 topic='IANA Considerations' anchor='iana'>
  <p>This XEP requires no interaction with &IANA;. </p>
</section1>

<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
  <p>None.</p>
</section1>

<section1 topic='Acknowledgements' anchor='ack'>
  <p>The author wishes to share any credit with many members of the community.</p>
</section1>

</xep>