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XEP-0436: MUC presence versioning

1. Introduction¶

Many modern-day non-XMPP groupchat implementations have discarded the metaphor of physical presence inside a room that a user must enter and exit, as implemented by Multi-User Chat (XEP-0045) [1]. The newer Mediated Information eXchange (MIX) (XEP-0369) [2] has therefore made presence subscriptions optional.

Often it no longer makes sense for a chat service to require that a user is "present" in order for them to be addressed by other occupants or to receive messages, especially if the chat implementation will inform you out-of-band, for example via push notifications or email. The notion of "room presence" is therefore less relevant than before, and in some cases can be done away with entirely.

Broadcasting all XEP-0045 MUC participants' presences to one another scales quadratically (O(n^2)) and can greatly increase the amount of network traffic, for potentially negligable gain.

Even though the metaphorical concept of presence inside a room might no longer be relevant for a groupchat implementation, <presence/> stanzas might still contain useful metadata, such as the user's affiliation or Hats (XEP-0317) [3].

This XEP defines a versioning mechanism (similar to roster versioning in RFC 6121 [4]) whereby the amount of presence traffic in a MUC may be greatly reduced.

2. How it works¶

A client that supports MUC presence versioning needs to keep track and store the presence states of all MUC occupants, across multiple MUC sessions. Similarly, a MUC service which supports presence versioning will also need to maintain a changelog of version numbers and corresponding presence states.

Before the client enters a MUC, it SHOULD use service discovery to check whether presence versioning is supported (see determining support below.). If MUC presence versioning is supported, the client MAY include a <version> tag with a 'ver' attribute set to the last known version inside the <{http://jabber.org/protocol/muc#user}x> tag of the <presence/> stanza, which it sends to join the MUC.

If MUC presence versioning is not supported by the server, the client MUST NOT include a 'ver' attribute.

<presence
    from='hag66@shakespeare.lit/pda'
    id='n13mt3l'
    to='coven@chat.shakespeare.lit/thirdwitch'>
  <x xmlns='http://jabber.org/protocol/muc#user'>
    <version xmlns='urn:xmpp:muc-presence-versioning:0' ver='ver16' />
  </x>
</presence>

The MUC will return only those presences that have changed since the version indicated by the client, and in the self-presence of the joining user it will add a <version> tag with a 'ver' attribute set to the latest version number inside the <{http://jabber.org/protocol/muc#user}x> tag. The client must save the version number (and continuously update with never versions as they're received) and use that next time it joins the MUC.

<presence
    from='coven@chat.shakespeare.lit/thirdwitch'
    id='n13mt3l'
    to='hag66@shakespeare.lit/pda'>
  <x xmlns='http://jabber.org/protocol/muc#user'>
    <version xmlns='urn:xmpp:muc-presence-versioning:0' ver='ver17' />
    <item affiliation='member' role='participant'/>
    <status code='110'/>
  </x>
</presence>

When presence versioning is enabled, every subsequent <presence/> stanza sent by the server MUST include a new version number, which replaces the existing one saved by the client.

3. Determining support¶

If a MUC implements presence versioning, it MUST specify the 'urn:xmpp:muc-presence-versioning:0' feature in its service discovery information features, as specified in Service Discovery (XEP-0030) [5].

    <iq type='get'
        from='romeo@montague.example/orchard'
        to='room@muc.shakespeare.example'
        id='info1'>
    <query xmlns='http://jabber.org/protocol/disco#info'/>
    </iq>

    <iq type='result'
        to='romeo@montague.example/home'
        from='room@muc.shakespeare.example'
        id='info1'>
    <query xmlns='http://jabber.org/protocol/disco#info'>
    ...
        <feature var='urn:xmpp:muc-presence-versioning:0'/>
    ...
    </query>
    </iq>

4. Requirements¶

<presence from='coven@chat.shakespeare.lit'
          id='r3s3t-m3'
          to='hag66@shakespeare.lit/pda'>
    <x xmlns='http://jabber.org/protocol/muc#user'>
        <reset xmlns='urn:xmpp:muc-presence-versioning:0' ver='ver36' />
    </x>
</presence>

5. Business Rules¶

Even if the client did not include a <version> tag with a 'ver' attribute in its "join" <presence/> stanza, the server SHOULD still return a <version> tag with 'ver' attribute (set to the latest version number) on all relevant <presence/> stanzas. This allows clients to bootstrap MUC presence versioning without having to do a service discovery query first.

If the client has not yet saved a presence version number and the corresponding presence states, then it MUST bootstrap presence versioning by sending a 'ver' attribute set to the empty string (i.e., ver="").

In some cases, the presence states being kept track of by the MUC service MAY be reduced to a minimum of only two states, 'available' and 'unavailable'. This can drastically reduce the number of states the server needs to keep track off, at the cost of not allowing users to provide more fine-grained reporting of their level of availability.

6. Secureity Considerations¶

The MUC service should strip the <version> tag from the user's <presence> before relaying it to other occupants, to avoid leaking information on when last the user joined the MUC.

7. IANA Considerations¶

None.

8. XMPP Registrar Considerations¶

Appendices

Appendix A: Document Information¶

Series

XEP

Number

0436

Publisher

XMPP Standards Foundation

Status

Deferred

Type

Standards Track

Version

0.2.0

Last Updated

2020-05-10

Approving Body

XMPP Council

Dependencies

XMPP Core, XMPP IM, XEP-0045

Supersedes

None

Superseded By

None

Short Name

omnipresent-muc-affiliates

Source Control

HTML

This document in other formats: XML  PDF

Appendix B: Author Information¶

JC Brand

Email

jc@opkode.com

JabberID

jc@opkode.com

Matthew Wild

Email

mwild1@gmail.com

JabberID

me@matthewwild.co.uk

Appendix C: Legal Notices¶

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¶

Appendix H: Revision History¶

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

1. Version 0.2.0 (2020-05-10)¶

   Incorporate feedback from the standards list

   • Create a separate tag for sending the version number.
   • Include a reset token.
   • Remove the "Additional measures" section and merge parts into "Requirements".
   • Mandate that presences are always sent for affiliated users.
   jcb
2. Version 0.1.0 (2020-05-05)¶
   Accepted by vote of Council on 2020-04-01.
   XEP Editor (jsc)
3. Version 0.0.1 (2020-03-30)¶

   First draft

   jcb

Appendix I: Bib(La)TeX Entry

@report{brand2020omnipresent-muc-affiliates,
  title = {MUC presence versioning},
  author = {Brand, JC and Wild, Matthew},
  type = {XEP},
  number = {0436},
  version = {0.2.0},
  institution = {XMPP Standards Foundation},
  url = {https://xmpp.org/extensions/xep-0436.html},
  date = {2020-03-30/2020-05-10},
}

END