Developers periodically wonder why Service Discovery (XEP-0030) [1] does not include more bits of information. For example, why does the <identity/> element not include a 'description' attribute, and can we add one now? The answer is: well, it just doesn't, and at this point it's too late to make further changes (since XEP-0030 is Final). So the best approach is to specify a well-defined extension mechanism.
Let us consider an example. A Multi-User Chat (XEP-0045) [2] room might want to include additional information in its service discovery results, such as the full room description, the current discussion topic (room subject), the number of occupants in the room, and the JID of the room owner.
Adding one new attribute to the service discovery schema (even if that were an option) would not solve the problem, since a MUC service might want to provide certain bits of information, whereas a Publish-Subscribe (XEP-0060) [3] service might want to provide other bits.
A better solution would be to include extended information qualified by a namespace that provides a way to flexibly define structured data formats. Thankfully, we already possess such a protocol: Data Forms (XEP-0004) [4]. In addition, we possess a way to define common fields used in data forms: Field Standardization for Data Forms (XEP-0068) [5]. Using these building blocks, we can define some best practices for extending service discovery results.
If an entity desires to provide extended information about itself in an IQ results stanza within the context of the Service Discovery protocol, it SHOULD do so by including each bit of information as the XML character data of the <value/> child of a distinct <field/> element, with the entire set of fields contained within an <x/> element of type "result" qualified by the 'jabber:x:data' namespace; this <x/> element SHOULD be a child of the <query/> element qualified by the 'http://jabber.org/protocol/disco#info' namespace. Thus the IQ result SHOULD be of the following form:
<iq type='result'>
<query xmlns='http://jabber.org/protocol/disco#info'>
...
<x type='result' xmlns='jabber:x:data'>
<field var='[var-name]' label='[optional]'>
<value>[var-value]</value>
</field>
...
</x>
</query>
</iq>Note: A <field/> element MAY contain more than one <value/> child if appropriate.
If the data fields are to be used in the context of a protocol approved by the XMPP Standards Foundation, they SHOULD be described in the relevant XMPP Extension Protocol specification and registered in accordance with the rules defined in XEP-0068, resulting in the inclusion of a <field/> element whose 'var' attribute has a value of "FORM_TYPE" and whose 'type' attribute has a value of "hidden".
An entity MUST NOT supply extended information about associated children communicated via the 'http://jabber.org/protocol/disco#items' namespace, since a core principle of Service Discovery is that an entity must define its own identity only and must not define the identity of any children associated with the entity.
The following is an example of including a disco extension in the IQ result sent by a standard instant messaging server.
<iq type='get'
from='capulet.com'
to='shakespeare.lit'
id='disco1'>
<query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
<iq type='result'
from='shakespeare.lit'
to='capulet.com'
id='disco1'>
<query xmlns='http://jabber.org/protocol/disco#info'>
<identity
category='server'
type='im'
name='shakespeare.lit jabber server'/>
<feature var='jabber:iq:register'/>
<x xmlns='jabber:x:data' type='result'>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/network/serverinfo</value>
</field>
<field var='c2s_port'>
<value>5222</value>
</field>
<field var='c2s_port_ssl'>
<value>5223</value>
</field>
<field var='http_access'>
<value>http://shakespeare.lit/jabber</value>
</field>
<field var='ip_version'>
<value>ipv4</value>
<value>ipv6</value>
</field>
<field var='info_url'>
<value>http://shakespeare.lit/support.php</value>
</field>
</x>
</query>
</iq>
The following is an example of including a disco extension in the IQ result sent by a Multi-User Chat room.
<iq type='get'
from='hag66@shakespeare.lit/pda'
to='darkcave@macbeth.shakespeare.lit'
id='disco1'>
<query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
<iq type='result'
from='darkcave@macbeth.shakespeare.lit'
to='hag66@shakespeare.lit/pda'
id='disco1'>
<query xmlns='http://jabber.org/protocol/disco#info'>
<identity
category='conference'
type='text'
name='A Dark Cave'/>
<feature var='http://jabber.org/protocol/muc'/>
<feature var='jabber:iq:register'/>
<x xmlns='jabber:x:data' type='result'>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/muc#roominfo</value>
</field>
<field var='muc#roominfo_description' label='Description'>
<value>The place for all good witches!</value>
</field>
<field var='muc#roominfo_subject' label='Subject'>
<value>Spells</value>
</field>
<field var='muc#roominfo_occupants' label='Number of occupants'>
<value>3</value>
</field>
<field var='muc#roominfo_lang' label='Language of discussion'>
<value>en</value>
</field>
</x>
</query>
</iq>
In general, the XMPP Standards Foundation may choose to define at most one FORM_TYPE for each service discovery identity (category+type) registered with the XMPP Registrar. In addition, particular applications may define application-specific FORM_TYPEs as well, and one entity may have multiple service discovery identities (e.g., an XMPP server might also function as a publish-subscribe service). Therefore, it is possible (and allowed) for a single service discovery result to contain multiple service discovery extension elements (potentially up to two elements for each identity). However, in practice it is unlikely that any given service discovery result will contain more than one service discovery extension element.
Applications SHOULD ensure that information disclosed in a disco extension is appropriate for discovery by any entity on the network.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [6].
This document requires no interaction with the XMPP Registrar [7]; however, specifications following the best practices defined herein may register FORM_TYPEs and field values with the XMPP Registrar.
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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 <standards@xmpp.org> 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 <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-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.
2. XEP-0045: Multi-User Chat <https://xmpp.org/extensions/xep-0045.html>.
3. XEP-0060: Publish-Subscribe <https://xmpp.org/extensions/xep-0060.html>.
4. XEP-0004: Data Forms <https://xmpp.org/extensions/xep-0004.html>.
5. XEP-0068: Field Data Standardization for Data Forms <https://xmpp.org/extensions/xep-0068.html>.
6. 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/>.
7. 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 http://xmpp.org/extensions/attic/
END