<abstract>This document defines a method for one XMPP stanza to provide references to another entity, such as mentioning users, HTTP resources, or other XMPP resources.</abstract>
It's often desirable to encode a reference to another entity within a chat message and to mark up the reference. Examples of this include HTTP URLs, 'mentions' (referring to another user), references to previous messages and references to &xep0346; (FDP) forms. This document provides a mechanism for marking up a section of a message body with information about the target of the reference.
</p>
</section1>
<!--<section1 topic='Requirements' anchor='reqs'>
<ul>
<li></li>
</ul>
</section1>-->
<section1topic='Discovery'anchor='discovery'>
<p>If a client implements references, it MUST specify the 'urn:xmpp:reference:0' feature in its service discovery information features as specified in &xep0030; and the Entity Capabilities profile specified in &xep0115;.</p>
<p>References are provided in a 'reference' element of a message, with a namespace of 'urn:xmpp:reference:0'. The element MUST contain a 'type' attribute denoting the type of the reference and a 'uri' attribute of the thing that is referenced. It MAY contain 'begin', 'end' and 'anchor' elements.</p>
<p>The 'begin' and 'end' attributes are indexes denoting the beginning and end of the referenced substring in the message body. The Dijkstra convention of ranges<note>Dijkstra convention of ranges <<linkurl='https://www.cs.utexas.edu/users/EWD/transcriptions/EWD08xx/EWD831.html'>https://www.cs.utexas.edu/users/EWD/transcriptions/EWD08xx/EWD831.html</link>></note> is used, which means that 'begin' is inclusive and 'end' is exclusive. In other words, the 'begin' attribute is the index of the first unicode code point in the referenced substring, with 0 being the index of the first code point in the body, and the 'end' attribute is one higher than the index of the last code point in the substring.
This convention has three main advantages. It matches subsequence indexing in various programming languages, 'end' minus 'begin' equals the length of the substring, and when two substrings are adjacent, the 'end' attribute of the first one matches the 'begin' attribute of the second one.
Where the reference is not a substring of the message body in the referring stanza, 'begin' and 'end' are not used.</p>
<p>An 'anchor' attribute is used when the referring message is not the one containing the reference element, and points to the previous message containing the reference (the referring message).</p>
<p>Note that the URIs of the reference and anchor do not need to refer to the same mechanism as that in which the reference was received. E.g., a service could listen for mentions in a MIX channels of users outside that channel, and send them messages containing a reference to let them know that they've been mentioned.</p>
<p>Data references are a generic reference without additional information. The URI points to an 'item' that is able to be fetched. This is useful for, for example, fetching an item from pubsub, as in the example below. TODO: check URI syntax for refering to a pubsub item.</p>
<examplecaption='A MIX Channel sends a message that a new FDP form has been submitted elsewhere'><