mirror of
https://github.com/moparisthebest/xeps
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198 lines
9.5 KiB
XML
198 lines
9.5 KiB
XML
<?xml version='1.0' encoding='UTF-8'?>
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<!DOCTYPE xep SYSTEM 'xep.dtd' [
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<!ENTITY % ents SYSTEM 'xep.ent'>
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%ents;
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]>
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<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
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<xep>
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<header>
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<title>MUC Self-Ping (Schrödinger's Chat)</title>
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<abstract>This protocol extension for XEP-0045 Multi User Chat allows clients to check whether they are still joined to a chatroom.</abstract>
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&LEGALNOTICE;
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<number>xxxx</number>
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<status>ProtoXEP</status>
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<type>Standards Track</type>
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<sig>Standards</sig>
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<approver>Council</approver>
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<dependencies>
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<spec>XMPP Core</spec>
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<spec>XEP-0001</spec>
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<spec>Etc.</spec>
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</dependencies>
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<supersedes/>
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<supersededby/>
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<shortname>muc-selfping</shortname>
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<author>
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<firstname>Georg</firstname>
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<surname>Lukas</surname>
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<email>georg@op-co.de</email>
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<jid>georg@yax.im</jid>
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</author>
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<revision>
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<version>0.0.2</version>
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<date>2018-08-06</date>
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<initials>gl</initials>
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<remark><p>Add handling for another corner case, change title to Council’s liking</p></remark>
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</revision>
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<revision>
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<version>0.0.1</version>
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<date>2018-07-31</date>
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<initials>gl</initials>
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<remark><p>First draft.</p></remark>
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</revision>
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</header>
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<section1 topic='Introduction' anchor='intro'>
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<p>The &xep0045; protocol was
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not designed to handle s2s interruptions or message loss well. Rather
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often, the restart of a server or a component causes a client to believe
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that it is still joined to a given chatroom, while the chatroom service
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does not know of this participant.</p>
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<p>Existing approaches for re-synchronization are either inefficient
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(presence updates and "silent" messages are reflected to all participants,
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totalling to O(N²) stanzas per time unit), or mask message /
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presence losses (the implicit join performed via the deprecated GC1.0
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protocol).</p>
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<p>This specification aims to provide the most efficient, albeit not the
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most elegant, way for clients to periodically check whether they are still
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joined to a chatroom.</p>
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</section1>
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<section1 topic='Requirements' anchor='reqs'>
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<p>This specification only makes sense in the context of &xep0045;
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chatrooms. It makes use of &xep0199; to perform periodic self-pings.</p>
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<p>Server support for this extension is optional, but will significantly
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improve the reliability with Multi-Session Nicks and mobile clients.</p>
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</section1>
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<section1 topic='Client Self-Presence Check' anchor='selfpresencecheck'>
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<p>A typical connection between a client and a Multi-User-Chatroom (MUC)
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goes through the client-to-server link, possibly a server-to-server link
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and a typically local server-to-component link. If one of the involved
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servers or the MUC component is restarted, or one of the links is
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disturbed for some time, this can lead to the removal of some or all
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participants from the affected MUCs, without the clients being informed.</p>
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<p>To a participant, this looks like the MUC is silent (there is no chat
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activity and no presence changes), making it hard to realize that the
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connection was interrupted.</p>
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<p>To prevent the bad usability effect (message loss, lack of reaction from
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people in a chatroom), a client needs to actively check whether it is
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still joined to a MUC.</p>
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<section2 topic='Possible Protocol Approaches' anchor='possibleprotocols'>
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<p>There are multiple alternative approaches for a client to test whether
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it is still joined to a MUC:</p>
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<ol>
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<li><strong>Silent message</strong> (e.g. &xep0085;): this message will be reflected to
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all MUC participants, causing unwanted traffic and potentially waking
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up mobile devices without reason. If implemented by all clients, this
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will result in O(N²) messages to the MUC.</li>
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<li><strong>Presence update</strong>: if the MUC service implements the legacy GC1.0 protocol,
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this will be treated as a join attempt, and the MUC will return the
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full list of participants and full room history. The user's client
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will however miss partial history (other participants leaving,
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potentially also messages), and this has the same drawbacks as the
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first solution.</li>
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<li><strong>Private message to self</strong>: the client can send a MUC
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private message to itself. However, not all MUCs support / allow
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private messages, and there is no way to differentiate that from the
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error responses.</li>
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<li><strong>Private IQ to self</strong>: the client can send an IQ to
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its own participant JID. MUCs typically do not forbid those, and
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reflect the IQ request to the client (or another client of the same
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user). Once that client responds to the reflected IQ, the response is
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delivered to the initiating client as a sign of still being joined.
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</li>
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<li><strong>Dedicated MUC IQ</strong>: a new type of IQ can be deployed
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to let the client explicitly check whether it is still joined to a
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MUC. However, this needs to be supported by the server, and the client
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needs to implement a fallback solution.</li>
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</ol>
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<p>The private IQ is the most robust and traffic-efficient solution, and
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it does not rely on server support. The &xep0199; protocol is
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appropriate to use for this use case.</p>
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</section2>
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<section2 topic='Performing a Self-Ping' anchor='performingselfping'>
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<p>After an adequate amount of silence from a given MUC (e.g. 15 minutes),
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or from all MUCs from a given service domain, a client should initiate a
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self-ping. If Juliet is joined as JuliC in the
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characters@chat.shakespeare.lit MUC, her client will send the following
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ping IQ:</p>
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<example caption="Self-Ping by Juliet's Client"><![CDATA[
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<iq from='juliet@capulet.lit/client' id='s2c1' type='get'
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to='characters@chat.shakespeare.lit/JuliC'>
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<ping xmlns='urn:xmpp:ping'/>
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</iq>
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]]></example>
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<p>If Juliet's client is not joined, the MUC service will respond with a
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<not-acceptable> error. Thus, her client can automatically attempt
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a rejoin.</p>
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<example caption="Server Response to a Non-Participant"><![CDATA[
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<iq from='characters@chat.shakespeare.lit/JuliC' id='s2c1' type='error'
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to='juliet@capulet.lit/client' >
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<error type="cancel">
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<not-acceptable xmlns="urn:ietf:params:xml:ns:xmpp-stanzas" />
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</error>
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</iq>
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]]></example>
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<p>If her client is joined, the IQ request will be forwarded to any one of
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Juliet's joined clients.</p>
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<example caption="Server Reflection of Ping"><![CDATA[
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<iq from='characters@chat.shakespeare.lit/JuliC' id='c0ffee-s2c1' type='get'
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to='juliet@capulet.lit/somerandomclient' >
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<ping xmlns='urn:xmpp:ping'/>
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</iq>
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]]></example>
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<p>Depending on the other client implementation and its connection status,
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the IQ will be responded to eventually, in one of these ways, as
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delivered to the "<tt>client</tt>" resource:</p>
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<ul>
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<li><strong>Successful IQ response</strong>: the client is still joined.</li>
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<li><strong>Error (<service-unavailable> or
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<feature-not-implemented>)</strong>: the client is joined, but
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the pinged client does not implement &xep0199;.</li>
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<li><strong>Error (<item-not-found>)</strong>: the client is
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joined, but the participant just changed their name (e.g. initiated by
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a different client).</li>
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<li><strong>Any other error</strong>: the client is probably not
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joined.</li>
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<li><strong>Timeout (no response)</strong>: the MUC service (or another
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client) is unreachable. The client may indicate the status to the user
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and re-attempt the self-ping after some timeout, until it receives
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either an error or a success response.</li>
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</ul>
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</section2>
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<section2 topic='Server Optimization' anchor='serveroptimization'>
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<p>The normal routing rules of the self-ping impose two round-trips: first
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the initial ping from the client to the MUC, then the reflection of the
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ping and its response (possibly to another client), and finally the
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response to the initial IQ. If the other client is experiencing network
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connectivity issues, which is often the case with mobile devices, the
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ping request might never be responded to.</p>
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<p>Therefore, a MUC service supporting this protocol may directly respond
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to a participant's Ping request to the participant's own nickname, as
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opposed to routing it to any of the participant's clients.</p>
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</section2>
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</section1>
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<section1 topic='Implementation Notes' anchor='impl'>
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<p>In Multi-Session-Nick scenarios, where multiple clients of the same user
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are joined as the same participant, it is possible that another client
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initiates a nickname change while a ping request is pending. In that case,
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the ping might be responded to with <item-not-found>.</p>
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<p>A client should not perform a self-ping after initiating a nickname
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change, and before receiving the response to the nickname change from the
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service, as it is not yet clear whether the new nickname will be accepted.
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</p>
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</section1>
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<section1 topic='Security Considerations' anchor='security'>
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<p>A MUC service implementation should not allow a non-participant to obtain
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information about participants. This is however true irregardless of
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implementing this specification.</p>
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</section1>
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<section1 topic='IANA Considerations' anchor='iana'>
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<p>REQUIRED.</p>
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</section1>
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<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
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<p>REQUIRED.</p>
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</section1>
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<section1 topic='XML Schema' anchor='schema'>
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<p>REQUIRED for protocol specifications.</p>
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</section1>
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</xep>
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