xeps/xep-0410.xml

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<xep>
<header>
  <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>
  &LEGALNOTICE;
  <number>0410</number>
  <status>Proposed</status>
  <lastcall>2019-01-22</lastcall>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XMPP Core</spec>
    <spec>XEP-0001</spec>
    <spec>Etc.</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>muc-selfping</shortname>
  <author>
    <firstname>Georg</firstname>
    <surname>Lukas</surname>
    <email>georg@op-co.de</email>
    <jid>georg@yax.im</jid>
  </author>
  <revision>
    <version>0.1.0</version>
    <date>2018-08-31</date>
    <initials>XEP Editor (jcb)</initials>
    <remark>Accepted by vote of Council on 2018-08-01.</remark>
  </revision>
  <revision>
    <version>0.0.2</version>
    <date>2018-08-06</date>
    <initials>gl</initials>
    <remark><p>Add handling for another corner case, change title to Council’s liking</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2018-07-31</date>
    <initials>gl</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>
<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 participant.</p>
  <p>Existing approaches for re-synchronization are either inefficient
    (presence updates and "silent" messages are reflected to all participants,
    totalling to O(N²) stanzas per time unit), or mask message /
    presence losses (the implicit join performed via the deprecated GC1.0
    protocol).</p>
  <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.</p>
</section1>
<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>
<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
    participants from the affected MUCs, without the clients being informed.</p>
  <p>To a participant, 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>
    <ol>
      <li><strong>Silent message</strong> (e.g. &xep0085;): this message will be reflected to
	all MUC participants, 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 participants and full room history. The user's client
	will however miss partial history (other participants 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 participant 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>
      <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>
    </ol>
    <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>
  <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'
      to='characters@chat.shakespeare.lit/JuliC'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>
]]></example>
    <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-Participant"><![CDATA[
<iq from='characters@chat.shakespeare.lit/JuliC' id='s2c1' type='error'
      to='juliet@capulet.lit/client' >
  <error type="cancel">
    <not-acceptable xmlns="urn:ietf:params:xml:ns:xmpp-stanzas" />
  </error>
</iq>
]]></example>
    <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'/>
</iq>
]]></example>
    <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>
    <ul>
      <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 participant just changed their name (e.g. initiated by
	a different client).</li>
      <li><strong>Any other error</strong>: the client is probably not
	joined.</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>
    </ul>
  </section2>
  <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 participant's Ping request to the participant's own nickname, as
      opposed to routing it to any of the participant's clients.</p>
  </section2>
</section1>
<section1 topic='Implementation Notes' anchor='impl'>
  <p>In Multi-Session-Nick scenarios, where multiple clients of the same user
    are joined as the same participant, 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>
</section1>
<section1 topic='Security Considerations' anchor='security'>
  <p>A MUC service implementation should not allow a non-participant to obtain
    information about participants. This is however true irregardless of
    implementing this specification.</p>
</section1>
<section1 topic='IANA Considerations' anchor='iana'>
  <p>REQUIRED.</p>
</section1>
<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
  <p>REQUIRED.</p>
</section1>
<section1 topic='XML Schema' anchor='schema'>
  <p>REQUIRED for protocol specifications.</p>
</section1>
</xep>