Sometimes it is desirable for clients to communicate, or to send messages between each other, using a well defined Internet Content Type. Reasons can vary between the purely esthetic to funcional data-exchange. While there are XEPs, such as XHTML-IM (XEP-0071)  that provides means for sending richer content using a specific type, this extensions provides a similar mechanism, but for the general case of any content having a defined Internet Content Type.
Note: While the examples in this extensions uses Markdown as an example, any other text-based content type can be used.
The simplest use case is hinting at the content type of the textual content presented in the message body. This is done by aggregating a content element of namespace urn:xmpp:content to the message, with the attribute type specifying the content type. If the element does not provide a value, it is understood that the body contains the textual body of the content. This method should only be used if there's no risk of misunderstanding the message if the content type is not understood by the receiver, and the textual representation is readable. Example:
If there is a risk of misunderstanding the message if it's content type is not recognized, or the presentation of the message is done in an undesireable fashion, you can provide an alternate encoding of the message in the content element itself. If the content element contains a message, and the content type is recognized, the message should be taken from the content element instead of the body element. The body element in turn, should contain the plain text version of the same message. Example:
By providing multiple content elements in the same message, you can allow the receiver to choose the encoding best suited for its purpose. It also makes it possible to interchange messages that are understood by both humans and machines in the same message. If an empty content element is found, it is interpreted as above, i.e. providing a hint as to the content type of the message in the body element. Example:
If an entity supports content types as specified herein, it MUST advertise that fact by returning a feature of "urn:xmpp:content" in response to Service Discovery (XEP-0030)  information requests.
In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in Entity Capabilities (XEP-0115) . However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.
This document does not specify how content types are to be interpreted, or if content types are valid or well defined. It does not specify which content types are to be understood, or when. It only provides a means to hint or include different encodings in the same message.
It is possible to use custom or vendor-specific content types. These types are marked by prefixing the subtype with x. for custom unregistered types, and with vnd. for registered vendor specific types.
Care has to be taken when sending multiple encodings of the same message, as to not reach the smallest allowed maximum stanza size used by client and server software.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
The protocol schema needs to be added to the list of XMPP protocol schemas.
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This XMPP Extension Protocol is copyright © 1999 – 2020 by the XMPP Standards Foundation (XSF).
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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 <email@example.com> discussion list.
Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.
Errata can be sent to <firstname.lastname@example.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".
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/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/