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.
By way of example, a recent Reactions proposal suggested including the emoji as a <body/> element, so
that existing clients would simply display it as a normal message.
The downside of this approach is that servers and other intermediaries treat the presence of a <body/>
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.
This specification tackles the problem by providing an element to be used as a hint that the supplied
<body/> and <subject/> elements are only for fallback purposes, and the message SHOULD be
treated as if they were not present for most purposes.
Support for this protocol MAY be advertised by the Service Discovery protocol defined in Service Discovery (XEP-0030) [1] using a feature
of urn:xmpp:fallback:0. Note that lack of support will result in the desired fallback behaviour.
The fallback indicator is an element <fallback/> qualified by the urn:xmpp:fallback:0 namespace. It has no
attributes, content, or child elements.
<message from="alice@example.org" to="bob@example.net" type="chat"> <fallback xmlns="urn:xmpp:fallback:0"/> <encrypted xmlns="urn:example:crypto">Abobql jvyy rire qrpelcg bhe fhcre-frperg zrffntr!</encrypted> <body>This message is encrypted.</body> </message>
Receiving the above message, a naive client will naturally display only the <body/> 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.
<fallback/> element would shift the onus from server to
client, but this is likely to be less useful.
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="urn:xmpp:fallback:0" xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="fallback">
<xs:complexType/>
</xs:element>
</xs:schema>
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.
This XEP requires no interaction with the Internet Assigned Numbers Authority (IANA) [3].
None.
The author wishes to share any credit with many members of the community.
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.
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Errata can be sent to <editor@xmpp.org>.
The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
1. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.
2. XEP-0334: Message Processing Hints <https://xmpp.org/extensions/xep-0334.html>.
3. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
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