<abstract>This specification defines an XMPP protocol extension that enables two entities to mutually negotiate feature options, such as parameters related to a file transfer or a communications session.</abstract>
<remark>Added FORM_TYPEs to examples; added service-unavailable error and Security Considerations; various other updates and corrections avoiding material changes.</remark>
<remark>Per Council discussion, changed root element from <query/> to <feature/> for the sake of consistency with using protocols; moved some text from XEP-0030 to this document.</remark>
<p>A discovery protocol such as &xep0030; enables Jabber entities to query other entities regarding the features they support, but does not provide a means for the two entities to negotiate specific options related to the advertised features.</p>
<p>The protocol defined herein enables Jabber entities to negotiate options for specific features. These features could be negotiated between any two endpoints on the Jabber network, such as two clients, a client and a component, two components, a client and a server, or two servers. The protocol is generic enough that it can be used whenever options need to be negotiated between two Jabber entities. For examples, &xep0095;, &xep0096; or &xep0155;.</p>
<p>Features are negotiated through the exchange of &IQ; or &MESSAGE; stanzas containing <feature/> child elements qualified by the 'http://jabber.org/protocol/feature-neg' namespace. However, this <feature/> element is simply a wrapper for structured data encapsulated in the &xep0004; protocol. <note>Earlier versions of this document defined a structured data format to handle the feature negotiation workflow; versions later than 0.4 use <cite>Data Forms</cite>, i.e., the 'jabber:x:data' namespace.</note></p>
<p>In order to begin a negotation, the initiator sends an &IQ; stanza of type "get" (or a &MESSAGE; stanza type "normal" - see <cite>Stanza Session Negotiation</cite> for examples) to the recipient with a single <feature/> element containing a data form of type "form" which defines the available options for one or more features. Each feature is represented as an x-data "field".</p>
<p>The recipient SHOULD examine each feature and the values of the options provided. In order to indicate preferred values, the recipient then SHOULD specify one value for each feature and return a data form of type "submit" to the initiator in an &IQ; stanza of type "result" (or a &MESSAGE; stanza type "normal").</p>
<p>The following examples show some likely scenarios for feature negotiation between entities. Further examples can be found in "using protocols", such as <cite>File Transfer</cite>.</p>
<p>Note: If the responding entity does not want to reveal presence to the initiating entity for whatever reason then the responding entity's client SHOULD return a &unavailable; error (or return no response or error whatsoever if the offer was wrapped in a &MESSAGE; stanza) - see <linkurl='#security'>Security Considerations</link>.</p>
<p>If the responding entity does not support <strong>Feature Negotiation</strong> or does not support the specified FORM_TYPE, it SHOULD also return a &unavailable; error:</p>
<examplecaption="Responding entity does not support feature negotiation"><