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.
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.
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>A MUC that supports presence versioning MUST broadcast presence for all affiliated users, including those who are currently 'unavailable'. Multi-User Chat (XEP-0045) [1] documents the status code '102', which is used to indicate that a MUC shows unavailable members.
The only exception is when a MUC has been explicitly configured to only broadcast presence from occupants above a certain affiliation, (see the presence broadcast section of Multi-User Chat (XEP-0045) [1]).
In order for a user to permanently join a room, and therefore become affiliated so that they are included in presence broadcasts, they MAY be allowed to register themselves as members in the MUC. Multi-User Chat (XEP-0045) [1] describes in section 7.10 "Registering with a Room how a user may register themselves with a room, thereby receiving the "member" affiliation and having their preferred nickname reserved in that room.
If a MUC receives a presence version number that's so old, that it no longer has the corresponding state available, it MUST include a <reset> token in the first <presence> stanza it sends (regardless of whom in the MUC the presence relates to). The first presence stanza MAY be from the MUC itself (as shown in the example below). Afterwards, the MUC MUST then send all presences (including 'unavailable' for affiliated users) as if the client is starting from a blank slate.
<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>
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.
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.
None.
The XMPP Registrar [6] includes 'urn:xmpp:muc-presence-versioning:0' in its registry of protocol namespaces (see <https://xmpp.org/registrar/namespaces.html>).
If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.
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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.
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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-0045: Multi-User Chat <https://xmpp.org/extensions/xep-0045.html>.
2. XEP-0369: Mediated Information eXchange (MIX) <https://xmpp.org/extensions/xep-0369.html>.
3. XEP-0317: Hats <https://xmpp.org/extensions/xep-0317.html>.
4. RFC 6121: Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence <http://tools.ietf.org/html/rfc6121>.
5. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.
6. 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 https://xmpp.org/extensions/attic/
Incorporate feedback from the standards list
First draft
@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