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<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
<!ENTITY % ents SYSTEM 'xep.ent'>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<title>MUC Self-Ping (Schrödinger's Chat)</title>
<abstract>This protocol extension for XEP-0045 Multi User Chat allows clients to check whether they are still joined to a chatroom.</abstract>
<type>Standards Track</type>
<spec>XMPP Core</spec>
<remark>Treat remote-server-not-found and remote-server-timeout like timeout errors (i.e. undecided).</remark>
<remark>Make feature string reference more clear</remark>
<remark>Move to Draft per Council Vote from 2019-02-06.</remark>
<remark>Incorporate council feedback: use @by on error elements</remark>
<remark>Incorporate feedback from Last Call.</remark>
<initials>XEP Editor (jcb)</initials>
<remark>Accepted by vote of Council on 2018-08-01.</remark>
<remark><p>Add handling for another corner case, change title to Council’s liking</p></remark>
<remark><p>First draft.</p></remark>
<section1 topic='Introduction' anchor='intro'>
<p>The &xep0045; protocol was
not designed to handle s2s interruptions or message loss well. Rather
often, the restart of a server or a component causes a client to believe
that it is still joined to a given chatroom, while the chatroom service
does not know of this occupant.</p>
<p>Existing approaches for re-synchronization are either inefficient
(presence updates and "silent" messages are reflected to all occupants,
totalling to O(N²) stanzas per time unit), or mask message /
presence losses (the implicit join performed via the deprecated GC1.0
<p>This specification aims to provide the most efficient, albeit not the
most elegant, way for clients to periodically check whether they are still
joined to a chatroom. However, it can not ensure that a client remains
joined to a room without any interruptions.</p>
<section1 topic='Requirements' anchor='reqs'>
<p>This specification only makes sense in the context of &xep0045;
chatrooms. It makes use of &xep0199; to perform periodic self-pings.</p>
<p>Server support for this extension is optional, but will significantly
improve the reliability with Multi-Session Nicks and mobile clients.</p>
<section1 topic='Client Self-Presence Check' anchor='selfpresencecheck'>
<p>A typical connection between a client and a Multi-User-Chatroom (MUC)
goes through the client-to-server link, possibly a server-to-server link
and a typically local server-to-component link. If one of the involved
servers or the MUC component is restarted, or one of the links is
disturbed for some time, this can lead to the removal of some or all
occupants from the affected MUCs, without the clients being informed.</p>
<p>To an occupant, this looks like the MUC is silent (there is no chat
activity and no presence changes), making it hard to realize that the
connection was interrupted.</p>
<p>To prevent the bad usability effect (message loss, lack of reaction from
people in a chatroom), a client needs to actively check whether it is
still joined to a MUC.</p>
<section2 topic='Possible Protocol Approaches' anchor='possibleprotocols'>
<p>There are multiple alternative approaches for a client to test whether
it is still joined to a MUC:</p>
<li><strong>Silent message</strong> (e.g. &xep0085;): this message will be reflected to
all MUC occupants, causing unwanted traffic and potentially waking
up mobile devices without reason. If implemented by all clients, this
will result in O(N²) messages to the MUC.</li>
<li><strong>Presence update</strong>: if the MUC service implements the legacy GC1.0 protocol,
this will be treated as a join attempt, and the MUC will return the
full list of occupants and full room history. The user's client
will however miss partial history (other occupants leaving,
potentially also messages), and this has the same drawbacks as the
first solution.</li>
<li><strong>Private message to self</strong>: the client can send a MUC
private message to itself. However, not all MUCs support / allow
private messages, and there is no way to differentiate that from the
error responses.</li>
<li><strong>Private IQ to self</strong>: the client can send an IQ to
its own occupant JID. MUCs typically do not forbid those, and
reflect the IQ request to the client (or another client of the same
user). Once that client responds to the reflected IQ, the response is
delivered to the initiating client as a sign of still being joined.
<li><strong>Dedicated MUC IQ</strong>: a new type of IQ can be deployed
to let the client explicitly check whether it is still joined to a
MUC. However, this needs to be supported by the server, and the client
needs to implement a fallback solution.</li>
<p>The private IQ is the most robust and traffic-efficient solution, and
it does not rely on server support. The &xep0199; protocol is
appropriate to use for this use case.</p>
<section2 topic='Performing a Self-Ping' anchor='performingselfping'>
<p>After an adequate amount of silence from a given MUC (e.g. 15 minutes),
or from all MUCs from a given service domain, a client should initiate a
self-ping. If Juliet is joined as JuliC in the
characters@chat.shakespeare.lit MUC, her client will send the following
ping IQ:</p>
<example caption="Self-Ping by Juliet's Client"><![CDATA[
<iq from='juliet@capulet.lit/client' id='s2c1' type='get'
<ping xmlns='urn:xmpp:ping'/>
<p>If Juliet's client is not joined, the MUC service will respond with a
&lt;not-acceptable&gt; error. Thus, her client can automatically attempt
a rejoin.</p>
<example caption="Server Response to a Non-Occupant"><![CDATA[
<iq from='characters@chat.shakespeare.lit/JuliC' id='s2c1' type='error'
to='juliet@capulet.lit/client' >
<error type="cancel" by="characters@chat.shakespeare.lit">
<not-acceptable xmlns="urn:ietf:params:xml:ns:xmpp-stanzas" />
<p>If her client is joined, the IQ request will be forwarded to any one of
Juliet's joined clients.</p>
<example caption="Server Reflection of Ping"><![CDATA[
<iq from='characters@chat.shakespeare.lit/JuliC' id='c0ffee-s2c1' type='get'
to='juliet@capulet.lit/somerandomclient' >
<ping xmlns='urn:xmpp:ping'/>
<p>Depending on the other client implementation and its connection status,
the IQ will be responded to eventually, in one of these ways, as
delivered to the "<tt>client</tt>" resource:</p>
<li><strong>Successful IQ response</strong>: the client is still joined.</li>
<li><strong>Error (&lt;service-unavailable&gt; or
&lt;feature-not-implemented&gt;)</strong>: the client is joined, but
the pinged client does not implement &xep0199;.</li>
<li><strong>Error (&lt;item-not-found&gt;)</strong>: the client is
joined, but the occupant just changed their name (e.g. initiated by
a different client).</li>
<li><strong>Error (&lt;remote-server-not-found&gt; or &lt;remote-server-timeout&gt;)</strong>:
the remote server is unreachable for unspecified reasons; this can
be a temporary network failure or a server outage. No decision can be
made based on this; Treat like a timeout (see below).</li>
<li><strong>Any other error</strong><note>Different service
implementations will send different responses to a client that's not
joined. The recommended error code is &lt;not-acceptable&gt;, however
some servers will respond with &lt;not-allowed&gt; or
&lt;bad-request&gt; as well.</note>: the client is probably not
joined any more. It should perform a re-join.</li>
<li><strong>Timeout (no response)</strong>: the MUC service (or another
client) is unreachable. The client may indicate the status to the user
and re-attempt the self-ping after some timeout, until it receives
either an error or a success response.</li>
<section2 topic='Server Optimization' anchor='serveroptimization'>
<p>The normal routing rules of the self-ping impose two round-trips: first
the initial ping from the client to the MUC, then the reflection of the
ping and its response (possibly to another client), and finally the
response to the initial IQ. If the other client is experiencing network
connectivity issues, which is often the case with mobile devices, the
ping request might never be responded to.</p>
<p>Therefore, a MUC service supporting this protocol may directly respond
to a occupant's Ping request to the occupant's own nickname, as
opposed to routing it to any of the occupant's clients. A service
implementing this optimization needs to advertise the
<tt></tt> feature in the &xep0030; disco#info response on
the individual MUC room JIDs, and it MUST respond to a self-ping request
as follows:</p>
<li><strong>Successful IQ response</strong>: the client is joined to the MUC.</li>
<li><strong>Error (&lt;not-acceptable&gt;)</strong>: the client is not joined to the MUC.</li>
<example caption='MUC Service Advertises Self-Ping Optimization'><![CDATA[
<iq from='darkcave@chat.shakespeare.lit'
<query xmlns=''>
<!-- ... -->
<feature var=''/>
<section1 topic='Implementation Notes' anchor='impl'>
<p>In Multi-Session-Nick scenarios, where multiple clients of the same user
are joined as the same occupant, it is possible that another client
initiates a nickname change while a ping request is pending. In that case,
the ping might be responded to with &lt;item-not-found&gt;.</p>
<p>A client should not perform a self-ping after initiating a nickname
change, and before receiving the response to the nickname change from the
service, as it is not yet clear whether the new nickname will be accepted.
<p>If a client session is in hibernation (&xep0198;), the client should defer
sending of self-ping requests until it is reconnected and re-authenticated.
<section1 topic='Security Considerations' anchor='security'>
<p>A MUC service implementation should not allow a non-occupant to obtain
information about occupants. This is however true irregardless of
implementing this specification.</p>
<section1 topic='IANA Considerations' anchor='iana'>
<p>This document requires no interaction with &IANA;.</p>
<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
<p>Include "<tt></tt>"
as a valid feature in the Registry of Features.</p>
<code caption='Registry Submission'><![CDATA[
<desc>Support for the MUC self-ping optimization</desc>
<section1 topic='XML Schema' anchor='schema'>
<p>This document does not define any new XML structure requiring a schema.</p>