New specifications can use the message body to convey intended meaning to users of non-supporting clients. This XEP provides a way to indicate which part of the body serves as fallback and which specification it provides a fallback for.
This specification serves a different purpose than the similarly named Fallback Indication (XEP-0428) [1]. Fallback indication tells servers that the body is only a fallback and that clients implementing all the specifications used by the message will not make use of the message body. This specification tells clients that parts of the body are only included to aid clients not supporting a certain specification.
To mark a specific text section in the body as a fallback, a <fallback> element in the urn:xmpp:compat:0 namespace is placed in the message stanza. The <fallback> element has a 'for' attribute with an identifier of the specification the fallback is for. The <fallback> element contains one <body> element for each continuous character sequence in the body that is part of the fallback text. Each body element contains a 'start' and 'end' attribute which point to the start and end of a fallback character sequence as defined in Character counting in message bodies (XEP-0426) [2], respectively.
For example, Juliet might be part of a group that shares news. Breaking news are indicated by a specific element and supporting clients can highlight them accordingly. To also inform users of non-supporting clients about the importance of a piece of news, the information is prefixed by "BREAKING NEWS: " in the body. A supporting client sees the <fallback> element and removes the respective character sequence before highlighting the message to the user.
<message to='news@muc.example.com/juliet' id='message-id2' type='chat'>
<body>BREAKING NEWS: Romeo is dead.</body>
<breaking xmlns='urn:example:breaking-news:0' />
<fallback xmlns='urn:xmpp:compat:0' for='urn:example:breaking-news:0'>
<body start='0' end='15' />
</fallback>
</message>Another example are message replies, where a <reply> element specifies the referenced message. A simple fallback is to include a Message Styling (XEP-0393) [3] quote of the referenced message in the body text. To provide a better fallback, the sender can also include markup information for the quote.
<message to='anna@example.com' id='message-id2' type='groupchat'>
<body>
> Anna wrote:
> Hi, how are you?
Great
</body>
<reply to='anna@example.com' id='message-id1' xmlns='urn:xmpp:reply:0' />
<markup xmlns="urn:example:markup:0">
<quote start='0' end='33' />
</markup>
<fallback xmlns='urn:xmpp:compat:0' for='urn:example:markup:0'>
<body start='0' end='1' />
<body start='14' end='15' />
</fallback>
<fallback xmlns='urn:xmpp:compat:0' for='urn:xmpp:reply:0'>
<body start='0' end='33' />
</fallback>
</message>The exact behavior for a compatibility fallback should be defined in the respective specification. Not displaying the fallback in supporting clients would be an example for a behavior.
An attacker might include a compatibility fallback with a meaning that is different from what would be displayed by a supporting client. While this could also be achieved using other parts of the XMPP specifications (e.g. xml:lang), some environments might want to prevent it. Specifications could standardize some parts of the compatibility text such that the equivalence can be verified by supporting clients.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [4].
This document requires no interaction with XMPP Registrar [5].
This document in other formats: XML PDF
This XMPP Extension Protocol is copyright © 1999 – 2024 by the XMPP Standards Foundation (XSF).
Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.
## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##
In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.
This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).
The HTML representation (you are looking at) is maintained by the XSF. It is based on the YAML CSS Framework, which is licensed under the terms of the CC-BY-SA 2.0 license.
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.
Discussion on other xmpp.org discussion lists might also be appropriate; see <https://xmpp.org/community/> for a complete list.
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-0428: Fallback Indication <https://xmpp.org/extensions/xep-0428.html>.
2. XEP-0426: Character counting in message bodies <https://xmpp.org/extensions/xep-0426.html>.
3. XEP-0393: Message Styling <https://xmpp.org/extensions/xep-0393.html>.
4. 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/>.
5. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.
Note: Older versions of this specification might be available at https://xmpp.org/extensions/attic/
First draft.
@report{wirth2022compat,
title = {Compatibility Fallbacks},
author = {Wirth, Natalie and Wissfeld, Marvin},
type = {XEP},
number = {xxxx},
version = {0.0.1},
institution = {XMPP Standards Foundation},
url = {https://xmpp.org/extensions/xep-xxxx.html},
date = {2022-01-01/2022-01-01},
}END