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 their 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. It also describes additional measures which may be taken to further reduce the amount of presence traffic.
A client that supports presence versioning needs to keep track and store the presence statuses 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 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 presence versioning is supported, the client MAY include a 'ver' attribute set to the last known presence version in the <{http://jabber.org/protocol/muc}x> tag of the <presence/> stanza, which it sends to join the MUC.
If 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' ver='ver14'/>
</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 'ver' attribute with the latest version number on the <{http://jabber.org/protocol/muc}x> tag. The client must save the version number and use it 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' ver='ver16'>
<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:presence-versioning:0'/>
...
</query>
</iq>If a MUC receives a presence version number that's so old, so that it no longer has the corresponding state available, it needs to send all presence statuses back to the client.
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="").
Even if the client did not include a 'ver' attribute in its "join" <presence/> stanza, the server SHOULD still set a 'ver' attribute on the relevant <presence/> stanzas.
There are a number of Multi-User Chat (XEP-0045) [1] features that a client and server may decide to configure and/or implement in order to further reduce the number of presence stanzas being sent around.
A MUC MAY be configured to only broadcast presence from occupants above a certain affiliation, (see the presence broadcast section of XEP-0045), for example in a MUC where non-affiliated users are allowed to view the discussion but aren't allowed to send messages.
This step can be taken in addition to letting users register themselves as members in the MUC. XEP-45 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.
In some cases, it makes sense to reduce the number of presence statuses to only a subset, in order to reduce to total amount of states the server needs to keep track off. In the simplest case, this would mean keeping track only of two statuses, 'available' and 'unavailable'.
XEP-0045 documents the status code 102, which is used to indicate that a room shows unavailable members. By also sending to newly joined users the presence stanzas of unavailable members, it's possible to provide any necessary presence metadata of all relevant users in a groupchat and not just the currently "present" users.
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.
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>.
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/
First draft
@report{brand2020omnipresent-muc-affiliates,
title = {MUC presence versioning},
author = {Brand, JC and Wild, Matthew},
type = {XEP},
number = {xxxx},
version = {0.0.1},
institution = {XMPP Standards Foundation},
url = {https://xmpp.org/extensions/xep-xxxx.html},
date = {2020-03-30/2020-03-30},
}END