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New ProtoXEP: MUC presence versioning

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JC Brand 2020-03-30 12:35:16 +02:00
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<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
<!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
<title>MUC presence versioning</title>
<abstract>This specification defines a versioning mechanism which reduces the amount of presence traffic in a XEP-0045 MUC</abstract>
&LEGALNOTICE;
<number>xxxx</number>
<status>ProtoXEP</status>
<type>Standards Track</type>
<sig>Standards</sig>
<approver>Council</approver>
<dependencies>
<spec>XMPP Core</spec>
<spec>XMPP IM</spec>
<spec>XEP-0045</spec>
</dependencies>
<supersedes/>
<supersededby/>
<shortname>omnipresent-muc-affiliates</shortname>
&jcbrand;
&mwild;
<revision>
<version>0.0.1</version>
<date>2020-03-30</date>
<initials>jcb</initials>
<remark><p>First draft</p></remark>
</revision>
</header>
<section1 topic='Introduction' anchor='intro'>
<p>
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 &xep0045;.
The newer &xep0369; has therefore made presence subscriptions optional.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
Even though the metaphorical concept of presence inside a room might no longer be relevant for a groupchat implementation,
&lt;presence/&gt; stanzas might still contain useful metadata, such as the user's affiliation or their &xep0317;.
</p>
<p>
This XEP defines a versioning mechanism (similar to roster versioning in &rfc6121;) 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.
</p>
</section1>
<section1 topic="How it works">
<p>
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.
</p>
<p>
Before the client enters a MUC, it SHOULD use service discovery to check whether presence versioning is supported
(see <link url="#disco">determining support</link> below.).
If presence versioning is supported, the client MAY include a 'ver' attribute set to the last known presence version
in the &lt;{http://jabber.org/protocol/muc}x&gt; tag of the &lt;presence/&gt; stanza, which it sends to join the MUC.
</p>
<p>
If presence versioning is not supported by the server, the client MUST NOT include a 'ver' attribute.
</p>
<p>
</p>
<example caption='User specifies the last known presence version when seeking to enter a MUC'><![CDATA[
<presence
from='hag66@shakespeare.lit/pda'
id='n13mt3l'
to='coven@chat.shakespeare.lit/thirdwitch'>
<x xmlns='http://jabber.org/protocol/muc' ver='ver14'/>
</presence>
]]></example>
<p>
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 &lt;{http://jabber.org/protocol/muc}x&gt; tag. The client must save the version number and use it next time it joins the MUC.
</p>
<example caption="Service Sends New Occupant's Presence to New Occupant"><![CDATA[
<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>
]]></example>
<p>
When presence versioning is enabled, every subsequent &lt;presence/&gt; stanza sent by the server MUST include a new version number, which replaces the existing one saved by the client.
</p>
</section1>
<section1 topic='Determining support' anchor='disco'>
<p>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 &xep0030;.</p>
<example caption='Client queries for information about a specific MUC'><![CDATA[
<iq type='get'
from='romeo@montague.example/orchard'
to='room@muc.shakespeare.example'
id='info1'>
<query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>]]></example>
<example caption='The MUC advertises support for presence versioning'><![CDATA[
<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>]]></example>
</section1>
<section1 topic='Business Rules' anchor='rules'>
<p>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.</p>
<p>
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="").
</p>
<p>
Even if the client did not include a 'ver' attribute in its "join" &lt;presence/&gt; stanza, the server SHOULD still set a 'ver' attribute on the relevant &lt;presence/&gt; stanzas.
</p>
</section1>
<section1 topic="Additional measures">
<p>
There are a number of &xep0045; 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.
</p>
<section2 topic='Only broadcast presence for affiliated users' anchor='affiliate-presence'>
<p>
A MUC MAY be configured to only broadcast presence from occupants above a certain affiliation,
(see the <link url='https://xmpp.org/extensions/xep-0045.html#enter-pres'>presence broadcast</link> section of <cite>XEP-0045</cite>),
for example in a MUC where non-affiliated users are allowed to view the discussion but aren't allowed to send messages.
</p>
<section3 topic="Let users register themselves with a MUC">
<p>
This step can be taken in addition to letting users register themselves as members in the MUC.
XEP-45 describes in <link url='https://xmpp.org/extensions/xep-0045.html#register'>section 7.10 "Registering with a Room</link> how a user may register themselves with a room,
thereby receiving the "member" affiliation and having their preferred nickname reserved in that room.
</p>
</section3>
</section2>
<section2 topic='Restrict presence states to available and unavailable' anchor='restrict-states'>
<p>
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'.
</p>
<section3 topic='Send presence stanzas for unavailable members' anchor='send-unavailable-presence'>
<p>
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.
</p>
</section3>
</section2>
</section1>
<section1 topic='IANA Considerations' anchor='iana'>
<p>None.</p>
</section1>
<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
<section2 topic='Protocol Namespaces' anchor='ns'>
<p>The &REGISTRAR; includes 'urn:xmpp:muc-presence-versioning:0' in its registry of protocol namespaces (see &NAMESPACES;).</p>
<ul>
<li>urn:xmpp:muc-presence-versioning:0</li>
</ul>
</section2>
<section2 topic='Protocol Versioning' anchor='registrar-versioning'>
&NSVER;
</section2>
</section1>
</xep>