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Merge pull request #1188 from mar-v-in/xep-0428

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XEP-0428: Fallback Indication v0.2.0
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@ -8,13 +8,14 @@
<xep> <xep>
<header> <header>
<title>Fallback Indication</title> <title>Fallback Indication</title>
<abstract>This specification proposes a mechanism by which message bodies can be marked as being purely for fallback <abstract>This specification proposes a mechanism by which message bodies or parts thereof can be marked as being for fallback
purposes, and therefore to be ignored by intermediaries and anything that understands the remainder of the message.</abstract> purposes, and therefore to be ignored by anything that understands the original intent of the message.</abstract>
&LEGALNOTICE; &LEGALNOTICE;
<number>0428</number> <number>0428</number>
<status>Deferred</status> <status>Experimental</status>
<type>Standards Track</type> <type>Standards Track</type>
<sig>Standards</sig> <sig>Standards</sig>
<approver>Council</approver>
<dependencies> <dependencies>
<spec>XMPP Core</spec> <spec>XMPP Core</spec>
</dependencies> </dependencies>
@ -23,6 +24,19 @@
<shortname>fallback</shortname> <shortname>fallback</shortname>
&dcridland; &dcridland;
&larma; &larma;
<revision>
<version>0.2.0</version>
<date>2022-07-17</date>
<initials>lmw</initials>
<remark>
<ul>
<li>Add 'for' attribute such that entities can discover what the fallback is for.</li>
<li>Allow to specify that only one of &SUBJECT; or &BODY; is meant as a fallback.</li>
<li>Allow to specify the part of respective text that is meant as fallback where applicable.</li>
<li>Don't use encryption example, which should use XEP-0380 instead.</li>
</ul>
</remark>
</revision>
<revision> <revision>
<version>0.1.1</version> <version>0.1.1</version>
<date>2020-03-03</date> <date>2020-03-03</date>
@ -48,17 +62,20 @@
</header> </header>
<section1 topic='Introduction' anchor='intro'> <section1 topic='Introduction' anchor='intro'>
<p>A common and convenient practise for new extensions is to supply a fallback body. This provides immediate backwards <p>A common and convenient practise for new extensions is to supply a fallback in the body. This provides immediate backwards
compatibility for naive clients, since - not understanding the new protocol - they will gracefully degrade to displaying compatibility for naive clients, since - not understanding the new protocol - they will gracefully degrade to displaying
the body as an instant message.</p> 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 <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> 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> <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 as being an indicator that a message is indeed an instant message. They may errornously treat a message this way
purposes, etc, which might not be appropriate.</p> for archival purposes, etc, that only has a <tt>&lt;body/></tt> for fallback purposes, 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 <p>This specification tackles the problem by providing an element to be used as a hint that parts or all of the supplied
<tt>&lt;body/></tt> and <tt>&lt;subject/></tt> elements are only for fallback purposes, and the message SHOULD be <tt>&lt;body/></tt> and <tt>&lt;subject/></tt> elements are for fallback purposes, and the message may be treated as
treated as if they were not present for most purposes.</p> if they were not present if the processing entity understands what the message is a fallback for.</p>
<p>Additionally, the specification allows for transporting information about which parts of a <tt>&lt;body/></tt> are used
for fallback purposes and for which reason, such that supporting clients can hide or dim those parts when displaying them
to the user or otherwise treat those parts special as intended or encouraged by other specifications.</p>
</section1> </section1>
<section1 topic='Overview' anchor="overview"> <section1 topic='Overview' anchor="overview">
@ -67,25 +84,41 @@
of <tt>&ns;</tt>. Note that lack of support will result in the desired fallback behaviour.</p> of <tt>&ns;</tt>. Note that lack of support will result in the desired fallback behaviour.</p>
</section2> </section2>
<section2 topic="Fallback Indicator"> <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 <p>The fallback indicator is an element <tt>&lt;fallback/></tt> qualified by the <tt>&ns;</tt> namespace. It has an attribute
attributes, content, or child elements.</p> <tt>for</tt> that indicates the specification that the fallback is meant to replace. This is typically the primary namespace
of the respective specification, but may be specified otherwise. The <tt>&lt;fallback/></tt> element may have one or multiple
<tt>&lt;body/></tt> or <tt>&lt;subject/></tt> child elements, that indicate the part of the message, that is a fallback. Both
of these child elements may have a <tt>start</tt> and <tt>end</tt> attribute which point to the start and end of a fallback
character sequence as defined in &xep0426; in the respective element in the message. If <tt>start</tt> and <tt>end</tt>
attribute are not supplied, the whole respective message element should be assumed to be there for fallback purposes. If
the <tt>&lt;fallback/></tt> element does not have any childs, it is assumed to apply to every message <tt>&lt;body/></tt> and
<tt>&lt;subject/></tt> present in the message.</p>
<p class="box">A previous version of this specification had an example using an encrypted message. It is suggested to use &xep0380;
instead of this specification for that usecase.</p>
<example><![CDATA[ <example><![CDATA[
<message from="alice@example.org" to="bob@example.net" type="chat"> <message to='anna@example.com' id='message-id2' type='groupchat'>
<fallback xmlns="]]>&ns;<![CDATA["/> <body>
<encrypted xmlns="urn:example:crypto">Abobql jvyy rire qrpelcg bhe fhcre-frperg zrffntr!</encrypted> > Anna wrote:
<body>This message is encrypted.</body> > Hi, how are you?
Great
</body>
<reply to='anna@example.com' id='message-id1' xmlns='urn:xmpp:reply:0' />
<fallback xmlns='urn:xmpp:fallback:0' for='urn:xmpp:reply:0'>
<body start='0' end='33' />
</fallback>
</message> </message>
]]></example> ]]></example>
<p>Receiving the above message, a naive client will naturally display only the <tt>&lt;body/></tt> element text, but <p>Receiving the above message, a naive client will naturally display the full <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 a client which supports this specification and the specification for <tt>urn:xmpp:reply:0</tt> will know that a part of the
(and not display) the content therein.</p> message is merely a fallback placeholder, and to ignore (and not display) that part, if it has other ways to convey the
intended meaning.</p>
</section2> </section2>
<section2 topic="Alternatives"> <section2 topic="Alternatives">
<ul> <ul>
<li>&xep0334; was considered, and would probably be ideal - servers often examine these elements and alter <li>&xep0334; was considered to inform intermediaries, and would probably be ideal - servers often examine these elements
behaviour accordingly, but the specification was rejected by Council.</li> 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 <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> client, but this is likely to be less useful.</li>
</ul> </ul>
</section2> </section2>
</section1> </section1>
@ -94,10 +127,20 @@
<code> <code>
<![CDATA[ <![CDATA[
<?xml version="1.0" encoding="UTF-8"?> <?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:schema attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="]]>&ns;<![CDATA[" xmlns="]]>&ns;<![CDATA[" xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="fallback"> <xs:element name="fallback">
<xs:complexType/> <xs:complexType>
<xs:sequence>
<xs:element name="subject" type="region" minOccurs="0" maxOccurs="unbounded" />
<xs:element name="body" type="region" minOccurs="0" maxOccurs="unbounded" />
</xs:sequence>
<xs:attribute name="for" type="xs:string" />
</xs:complexType>
</xs:element> </xs:element>
<xs:complexType name="region">
<xs:attribute name="start" type="xs:integer" />
<xs:attribute name="end" type="xs:integer" />
</xs:complexType>
</xs:schema> </xs:schema>
]]> ]]>
</code> </code>