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.

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;.

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. Earlier versions of this document defined a structured data format to handle the feature negotiation workflow; versions later than 0.4 use Data Forms, i.e., the 'jabber:x:data' namespace.

In order to begin a negotation, the initiator sends an &IQ; stanza of type "get" (or a &MESSAGE; stanza type "normal" - see Stanza Session Negotiation 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".

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").

The following examples show some likely scenarios for feature negotiation between entities. Further examples can be found in "using protocols", such as File Transfer.

If the responding entity responds to the initiating entity or returns an error (other than a &unavailable; response to an &IQ; request), the initiating entity will effectively discover the presence of the responding entity's resource. Due care must therefore be exercised in determining how to respond (or whether to respond at all to a &MESSAGE; request). For examples, the responding entity SHOULD NOT automatically (i.e. without first asking its human user) either respond to the initiating entity's request or return a specific error unless the initiating entity is subscribing to the responding entity's presence (and the responding entity's presence is not currently "invisible" to the initiating entity). Note: There should be no need for the responding entity's client to consult its block list, since if the initiating entity is on the list then the responding entity would not receive any requests from the initiating entity anyway.

This document requires no interaction with &IANA;.

In order for Jabber entities to adequately leverage Data Forms (e.g., by using machine-readable fields), it is RECOMMENDED to register standard x-data fields with the XMPP Registrar via the mechanisms defined in &xep0068;. Whether to do so for any given features and options shall be determined by the "using protocol".

Peter Millard, the primary author of this specification from version 0.1 through version 1.4, died on April 26, 2006. The remaining authors are thankful for Peter's work on this specification.