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ProtoXEP editorial help by MattJ
My earliest drafts were on 2021-08-20. Have mosly working implementation that interops with itself.
This commit is contained in:
Kim Alvefur
Jonas Schäfer
parent
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<?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><header>
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<title>Stanza size limit discovery</title>
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<abstract>This documents describes a mechanism for communicating stanza size limits across streams in order to help avoid reaching those limits.</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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<dependencies/>
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<supersedes/>
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<supersededby/>
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<shortname>NOT_YET_ASSIGNED</shortname>
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<author>
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<firstname>Kim</firstname>
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<surname>Alvefur</surname>
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<email>zash@zash.se</email>
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<jid>zash@zash.se</jid>
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</author>
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<revision>
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<version>0.0.1</version>
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<date>2021-08-20</date>
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<initials>ka</initials>
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<remark>
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<p>Early draft</p>
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</remark>
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</revision>
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<revision>
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<version>nil</version>
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<date>2021-08-25</date>
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<initials>ka</initials>
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<remark>
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<p>more work</p>
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</remark>
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</revision>
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<revision>
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<version>nil</version>
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<date>2021-08-28</date>
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<initials>ka</initials>
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<remark>
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<p>more words</p>
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</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>This documents describes a mechanism for communicating the stanza size
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limit that is in effect on a particular stream, in order to allow the
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other party to avoid reaching those limits.</p>
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<section2 topic="Problem statement" anchor="problem-statement">
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<p>When stanza size limits have been deployed, very often this leads to
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problems with large stanzas causing connection outages, most often
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vCards and Avatar stanzas.</p>
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<p>If stanza size limit violations are met with stream errors then this may
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lead to temporary connection outage, which may a few seconds to recover
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from.</p>
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<p>Especially vCard and Avatar stanzas may be very large and sometimes
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exceed the stanza size limit imposed.</p>
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</section2></section1><section1 topic="Requirements" anchor="reqs">
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<ul>
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<li>Enable discovery of the stanza size limit in use on a stream.</li>
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</ul>
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<section2 topic="TODO" anchor="todo">
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<ul>
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<li>bi-directional?</li>
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<li>disco too?</li>
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<li>some sort of path mtu discovery method?</li>
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<li>error condition?</li>
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<li>rewrite large iq-result to error?</li>
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</ul>
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</section2></section1><section1 topic="Glossary" anchor="glossary">
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<p>OPTIONAL.</p>
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</section1><section1 topic="Use Cases" anchor="usecases">
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<ul>
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<li>An XMPP client could attempt to apply more compression if it sees
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that an avatar it is about to upload would be too large.</li>
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<li>A PubSub server could limit the number of items in an <tt><items/></tt>
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query to ensure it can be delivered.</li>
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</ul>
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</section1><section1 topic="Business Rules" anchor="rules">
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<p>If after serialization to XML a stanza is too large, like, don’t send
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it. Synthesize an error condition, most likely a (modify,
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policy-violation), and pretend the remote entity replied with this.</p>
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</section1><section1 topic="Implementation Notes" anchor="impl">
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<p>Something about margins, due to variations in XML serialization, added
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attributes (e.g. the <tt>from</tt> attribute stamped by servers) or elements
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(delay tags)</p>
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<p>Because the stanza size limit is known ahead of time, entities can check
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this against stanzas they are about to send and take appropriate action,
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such as preemptively pretending that the stanza was rejected by the
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receiving entity.</p>
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<p>A client could for example try to apply more compression to an avatar or
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ask the user to select a smaller picture.</p>
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</section1><section1 topic="Accessibility Considerations" anchor="access">
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<p>OPTIONAL.</p>
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<p>Not relevant?</p>
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</section1><section1 topic="Internationalization Considerations" anchor="i18n">
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<p>OPTIONAL.</p>
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<p>Not relevant?</p>
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</section1><section1 topic="Determining support" anchor="determining-support">
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<p>The responding entity advertises the stanza size limit it enforces
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by including it as an integer in a stream feature element
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<tt>stanza-size-limit</tt> in the namespace <tt>urn:xmpp:tmp:TBD</tt>. An example of
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stream features prior to authentication follows:</p>
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<example><![CDATA[<stream:features>
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<mechanisms xmlns="urn:ietf:params:xml:ns:xmpp-sasl">
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<mechanism>SCRAM-SHA-1</mechanism>
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<mechanism>PLAIN</mechanism>
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</mechanisms>
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<stanza-size-limit xmlns="urn:xmpp:tmp:TBD">10000</stanza-size-limit>
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</stream:features>]]></example>
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<p>Entities may wish to have hire limits after authentication and would
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advertise it the same way after the stream restart:</p>
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<example><![CDATA[<stream:features>
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<bind xmlns="urn:ietf:params:xml:ns:xmpp-bind">
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<required/>
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</bind>
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<stanza-size-limit xmlns="urn:xmpp:tmp:TBD">262144</stanza-size-limit>
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</stream:features>]]></example>
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</section1><section1 topic="Security Considerations" anchor="security">
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<p>REQUIRED.</p>
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<p>Very large stanzas may incur memory and processing costs on the
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receiving entity. Advertising the actual limits could inform an attacker
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of how large a stanza to construct in order to maximize e.g. DoS
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effectiveness. Best combined with network level rate limits on raw
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bytes.</p>
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<p>Also see https://xmpp.org/rfcs/rfc6120.html#rfc.section.13.12</p>
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<p>TBD Recommendations for limits?</p>
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</section1><section1 topic="IANA Considerations" anchor="iana">
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<p>None.</p>
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</section1><section1 topic="XMPP Registrar Considerations" anchor="registrar">
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<p>Need the stream feature registered.</p>
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</section1><section1 topic="Design Considerations" anchor="design">
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<p>RECOMMENDED.</p>
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<p>Design? I just typed words and code into my computer!</p>
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</section1><section1 topic="XML Schema" anchor="schema">
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<p>REQUIRED for protocol specifications.</p>
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<code><![CDATA[type: integer
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xml:
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name: stanza-size-limit
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namespace: urn:xmpp:tmp:TBD]]></code>
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</section1>
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</xep>
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126
inbox/xep-sla.xml
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126
inbox/xep-sla.xml
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<?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>Stream Limits Advertisement</title>
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<abstract>This specification defines a way for an XMPP entity to announce the limits it will enforce for data received on a stream.</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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</dependencies>
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<supersedes/>
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<supersededby/>
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<shortname>sla</shortname>
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<author>
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<firstname>Kim</firstname>
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<surname>Alvefur</surname>
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<email>zash@zash.se</email>
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<jid>zash@zash.se</jid>
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</author>
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&mwild;
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<revision>
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<version>0.0.1</version>
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<date>2022-10-20</date>
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<initials>ka, mw</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>This documents describes a mechanism for communicating limits, such as stanza size limits that is in effect on a particular stream, in order to allow the sending party to avoid reaching those limits.</p>
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<section2 topic='Problem statement' anchor='intro-problem'>
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<p>Where stanza size limits have been deployed, very often this leads to problems with large stanzas causing connection outages, most often &xep0084; and &xep0053; result stanzas, which can be very large due to embedded images.</p>
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<p>If stanza size limit violations are met with stream errors then this may lead to temporary connection outage, which may a few seconds to recover from.</p>
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</section2>
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</section1>
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<section1 topic='Requirements' anchor='reqs'>
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<ul>
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<li>Enable discovery of the stanza size limit in use on a stream.</li>
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<li>Support for bi-directional streams.</li>
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</ul>
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<p>These requirements will enable XMPP clients and servers to adapt data they generate, such that it will fit within the limits required by the recipient, or reject overly large stanzas early, rather than following a trial-and-error approach.</p>
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</section1>
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<section1 topic='Use Cases' anchor='usecases'>
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<section2 topic='Server advertises limits to connecting entity' anchor='advertise'>
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<p>For any XMPP stream, there is an "initiating entity" (a client or server) and a "responding entity" that they are connecting to. The responding entity advertises its limits in the <stream:features/> element that it sends at the start of the stream.</p>
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<p>The limits are enclosed in a <limits/> element qualified by the 'urn:xmpp:stream-limits:0' namespace. This element SHOULD contain the following child elements:</p>
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<dl>
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<di>
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<dt><max-bytes/></dt>
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<dd>Contains an integer representing the maximum size of any first-level stream elements (including stanzas), in bytes. Guidance on acceptable limits is provided in &rfc6120; section 13.12.</dd>
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</di>
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<di>
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<dt><idle-seconds/></dt>
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<dd>Contains an integer representing the number of seconds without any traffic from the iniating entity after which the server may consider the stream idle, and either perform liveness checks (using e.g. &xep0198; or &xep0199;) or terminate the stream. Guidance on handling idle connections is provided in &rfc6120; section 4.6.</dd>
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</di>
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</dl>
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<example caption='Advertising limits to connecting entity'><![CDATA[
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<stream:features>
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<mechanisms xmlns="urn:ietf:params:xml:ns:xmpp-sasl">
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<mechanism>SCRAM-SHA-1</mechanism>
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<mechanism>PLAIN</mechanism>
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</mechanisms>
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<limits xmlns="urn:xmpp:stream-limits:0">
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<max-bytes>10000</max-bytes>
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<idle-seconds>1800</idle-seconds>
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</limits>
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</stream:features>
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]]></example>
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</section2>
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<section2 topic='Connecting server announces limits on bidirectional stream' anchor='advertise-bidi'>
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<p>Servers using &xep0288; to establish a bidirectional stream with another server do not get an opportunity to send <stream:features/> to the responding entity. For a server to advertise the limits about what it is willing to accept on such a stream, the <limits/> element can be included in the <bidi/> element.</p>
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<example caption='Advertising limits to responding entity over bidirectional stream'><![CDATA[
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<bidi xmlns='urn:xmpp:bidi'>
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<limits xmlns="urn:xmpp:stream-limits:0">
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<max-bytes>10000</max-bytes>
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<idle-seconds>1800</idle-seconds>
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</limits>
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</bidi>
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]]></example>
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</section2>
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</section1>
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<section1 topic='Business Rules' anchor='rules'>
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<p>If, after serialization, a stanza exceeds the limits that have been advertised on a stream, it SHOULD NOT be sent on that stream. Instead, a server SHOULD return an error to the sender. Such an error SHOULD contain the <policy-violation/> error condition, and SHOULD NOT contain a 'by' attribute (as the policy being violated is not the current entity's). A <text/> may also be included, explaining the limit that would be exceeded.</p>
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<p>It is acceptable for the limits on a stream to change whenever new stream features are announced - such as before and after authentication of the connecting entity.</p>
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</section1>
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<section1 topic='Implementation Notes' anchor='impl'>
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<p>OPTIONAL.</p>
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</section1>
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<section1 topic='Accessibility Considerations' anchor='access'>
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<p>OPTIONAL.</p>
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</section1>
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<section1 topic='Internationalization Considerations' anchor='i18n'>
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<p>OPTIONAL.</p>
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</section1>
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<section1 topic='Security Considerations' anchor='security'>
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<p>REQUIRED.</p>
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<p>Very large stanzas may incur memory and processing costs on the receiving entity. Advertising the actual limits could inform an attacker of how large a stanza to construct in order to maximize e.g. DoS effectiveness. Best combined with network level rate limits on raw bytes.</p>
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</section1>
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<section1 topic='IANA Considerations' anchor='iana'>
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<p>None.</p>
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</section1>
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<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
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<p>This specification defines the following namespace:</p>
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<ul>
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<li>urn:xmpp:stream-limits:0</li>
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</ul>
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<p>Also, the following stream feature:</p>
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<ul>
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<li><code><limits xmlns='urn:xmpp:stream-limits:0'/></code></li>
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</ul>
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</section1>
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<section1 topic='Design Considerations' anchor='design'>
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<p>The ability for a client to announce limits on what it will receive on a client-to-server stream is deliberately not provided by this specification. This vastly simplifies discovery of the maximum limits between any two JIDs, and it avoids situations where the server is unable to deliver incoming stanzas to some or all of an account's connected clients. Clients will already be protected from denial-of-service through excessive stanza sizes due to the server's own limits.</p>
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</section1>
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<section1 topic='XML Schema' anchor='schema'>
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<p>TBD.</p>
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</section1>
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</xep>
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