XEP-xxxx: Disco Feature Attachment

This specification provides a way to indicate that a feature is implemented for a specific namespace
Jérôme Poisson
© 2021 – 2021 XMPP Standards Foundation. SEE LEGAL NOTICES.


WARNING: This document has not yet been accepted for consideration or approved in any official manner by the XMPP Standards Foundation, and this document is not yet an XMPP Extension Protocol (XEP). If this document is accepted as a XEP by the XMPP Council, it will be published at <https://xmpp.org/extensions/> and announced on the <standards@xmpp.org> mailing list.
Standards Track
0.0.1 (2021-07-22)
Document Lifecycle
  1. Experimental
  2. Proposed
  3. Stable
  4. Final

1. Introduction

Service Discovery (XEP-0030) [1] is ubiquitous tools used by nearly every XMPP implementation. It's handy to declare that an entity implements a specific feature, but it can be confusing when a feature may be part of several functionalities. For instance, if a server advertise support Result Set Management (XEP-0059) [2] and supports both Personal Eventing Protocol (XEP-0163) [3] and Message Archive Management (XEP-0313) [4], we don't know if RSM is implemented only for PEP, only for MAM, or for both (it could be also implemented for other functionalities such as disco items).

In Publish-Subscribe (XEP-0060) [5] there is a workaround which proposes to use http://jabber.org/protocol/pubsub#rsm, but this solution doesn't tell if RSM is implemented for MAM or any other protocol, it is Pubsub specific and not extensible.

To fix this situation, this XEP proposes a simple way to advertise that a feature is implemented for a specific functionality.

2. Requirements

It should be easy to implement Disco Feature Attachment both for the advertising entity and the requesting one, thus, the main requirement is to be based on Service Discovery (XEP-0030) [1] without any other extensions.

3. Glossary

4. Use Cases

An XMPP server example.com wants to advertise support for Result Set Management (XEP-0059) [2], it is implemented for:

To do that, the server MUST advertise the root namespace as usual (here http://jabber.org/protocol/rsm), and it MUST advertise the attachment namespace for every supported target feature:

Example 1. Service Discovery information request
<iq from='example.org'
  <query xmlns='http://jabber.org/protocol/disco#info'/>
Example 2. Service Discovery information response
<iq from='example.com'
  <query xmlns='http://jabber.org/protocol/disco#info'>
    <feature var='http://jabber.org/protocol/rsm'/>
    <feature var='http://jabber.org/protocol/rsm@http://jabber.org/protocol/pubsub'/>
    <feature var='http://jabber.org/protocol/rsm@urn:xmpp:mam:2'/>
    <feature var='http://jabber.org/protocol/rsm@http://jabber.org/protocol/disco#items'/>

5. Business Rules

An attachment namespace is build by concatenating those 3 strings

Here the resulting attachment namespace is urn:xmpp:order-by:1@http://jabber.org/protocol/pubsub and it means "the Order-By (XEP-0413) [6] feature is implemented for Publish-Subscribe (XEP-0060) [5]".

If a functionality has several namespaces linked to it (e.g. Publish-Subscribe (XEP-0060) [5] has http://jabber.org/protocol/pubsub, http://jabber.org/protocol/pubsub#owner, http://jabber.org/protocol/pubsub#event, etc.), the one to be chosen it the one used for advertising with Service Discovery (XEP-0030) [1] (here it's http://jabber.org/protocol/pubsub).

If attached feature is only implemented for part of a functionality, only the corresponding namespace must be used in attachment namespace (e.g. if RSM is implemented only for discovering items but not features, the attachment namespace to advertise is only http://jabber.org/protocol/rsm@http://jabber.org/protocol/disco#items).

If a XEP has special requirements regarding disco feature attachments, then they MAY be specified there, in which case they take precedence over the rules defined here.

6. discovering support

If an entity supports the "Disco Feature Attachment" protocol, it MUST advertise it by including the "urn:xmpp:dfa:0" discovery feature (see Protocol Namespaces regarding issuance of one or more permanent namespaces) in response to a Service Discovery (XEP-0030) [1] information request:

Example 3. service discovery information request
<iq from='example.org'
  <query xmlns='http://jabber.org/protocol/disco#info'/>
Example 4. service discovery information response
<iq from='example.com'
  <query xmlns='http://jabber.org/protocol/disco#info'>
    <feature var='urn:xmpp:dfa:0'/>

7. Security Considerations

This document introduces no security considerations or concerns above and beyond those discussed in RFC 6120 and RFC 6125.

8. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [7].

9. XMPP Registrar Considerations


10. XML Schema



Appendix A: Document Information

XMPP Standards Foundation
Standards Track
Last Updated
Approving Body
XMPP Council
XMPP Core, XEP-0030
Superseded By
Short Name

This document in other formats: XML  PDF

Appendix B: Author Information

Jérôme Poisson


This XMPP Extension Protocol is copyright © 1999 – 2024 by the XMPP Standards Foundation (XSF).


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Disclaimer of Warranty

## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##

Limitation of Liability

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IPR Conformance

This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).

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Appendix D: Relation to XMPP

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.

Appendix E: Discussion Venue

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 <https://xmpp.org/community/> for a complete list.

Errata can be sent to <editor@xmpp.org>.

Appendix F: Requirements Conformance

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".

Appendix G: Notes

1. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.

2. XEP-0059: Result Set Management <https://xmpp.org/extensions/xep-0059.html>.

3. XEP-0163: Personal Eventing Protocol <https://xmpp.org/extensions/xep-0163.html>.

4. XEP-0313: Message Archive Management <https://xmpp.org/extensions/xep-0313.html>.

5. XEP-0060: Publish-Subscribe <https://xmpp.org/extensions/xep-0060.html>.

6. XEP-0413: Order-By <https://xmpp.org/extensions/xep-0413.html>.

7. 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/>.

Appendix H: Revision History

Note: Older versions of this specification might be available at https://xmpp.org/extensions/attic/

  1. Version 0.0.1 (2021-07-22)

    First draft.


Appendix I: Bib(La)TeX Entry

  title = {Disco Feature Attachment},
  author = {Poisson, Jérôme},
  type = {XEP},
  number = {xxxx},
  version = {0.0.1},
  institution = {XMPP Standards Foundation},
  url = {https://xmpp.org/extensions/xep-xxxx.html},
  date = {2021-07-22/2021-07-22},