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  <!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
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<xep>
<header>
  <title>Instant Stream Resumption</title>

  <abstract>This specification introduces a mechanism for instant
  stream resumption, based on Stream Management (XEP-0198), allowing
  XMPP entities to instantaneously resume an XMPP stream.</abstract>

  <legal>
    <copyright>This XMPP Extension Protocol is copyright (c) 1999 -
    2016 by the XMPP Standards Foundation (XSF).</copyright>

  <permissions>Permission is hereby granted, free of charge, to any
    person obtaining a copy of this specification (the
    &quot;Specification&quot;), to make use of the Specification
    without restriction, including without limitation the rights to
    implement the Specification in a software program, deploy the
    Specification in a network service, and copy, modify, merge,
    publish, translate, distribute, sublicense, or sell copies of the
    Specification, and to permit persons to whom the Specification is
    furnished to do so, subject to the condition that the foregoing
    copyright notice and this permission notice shall be included in
    all copies or substantial portions of the Specification. Unless
    separate permission is granted, modified works that are
    redistributed shall not contain misleading information regarding
    the authors, title, number, or publisher of the Specification, and
    shall not claim endorsement of the modified works by the authors,
    any organization or project to which the authors belong, or the
    XMPP Standards Foundation.</permissions>

  <warranty>## NOTE WELL: This Specification is provided on an
    &quot;AS IS&quot; BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
    KIND, express or implied, including, without limitation, any
    warranties or conditions of TITLE, NON-INFRINGEMENT,
    MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. In no event
    shall the XMPP Standards Foundation or the authors of this
    Specification be liable for any claim, damages, or other
    liability, whether in an action of contract, tort, or otherwise,
    arising from, out of, or in connection with the Specification or
    the implementation, deployment, or other use of the
    Specification. ##</warranty>

  <liability>In no event and under no legal theory, whether in tort
    (including negligence), contract, or otherwise, unless required by
    applicable law (such as deliberate and grossly negligent acts) or
    agreed to in writing, shall the XMPP Standards Foundation or any
    author of this Specification be liable for damages, including any
    direct, indirect, special, incidental, or consequential damages of
    any character arising out of the use or inability to use the
    Specification (including but not limited to damages for loss of
    goodwill, work stoppage, computer failure or malfunction, or any
    and all other commercial damages or losses), even if the XMPP
    Standards Foundation or such author has been advised of the
    possibility of such damages.</liability>

  <conformance>This XMPP Extension Protocol has been contributed in
    full conformance with the XSF's Intellectual Property Rights
    Policy (a copy of which may be found at &lt;<link
      url='http://xmpp.org/extensions/ipr-policy.shtml'>http://xmpp.org/extensions/ipr-policy.shtml</link>&gt;
    or obtained by writing to XSF, P.O. Box 1641, Denver, CO 80201
    USA).</conformance>
  </legal>

  <number>xxxx</number>
  <status>ProtoXEP</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XMPP Core</spec>
    <spec>XEP-0198</spec>
    <spec>XEP-0300</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>isr</shortname>
  <author>
    <firstname>Florian</firstname>
    <surname>Schmaus</surname>
    <email>flo@geekplace.eu</email>
    <jid>flo@geekplace.eu</jid>
  </author>
  <revision>
    <version>0.0.2</version>
    <date>2016-03-11</date>
    <initials>fs</initials>
    <remark><p>Second draft.</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2016-02-12</date>
    <initials>fs</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>

<section1 topic='Introduction' anchor='intro'>

  <p>This XEP specifies an instant stream resumption mechanism based
  on &xep0198;, allowing XMPP entities to instantaneously resume an
  XMPP stream. This can be seen as the complementary part to &xep0305;
  allowing for fast XMPP session (re-)establishment.</p>

  <p>Compared to the existing stream resumption mechanism of <link
  url='http://xmpp.org/extensions/xep-0198.html#resumption'><cite>XEP-0198</cite>
  § 5</link>, the approach defined herein reduces the round trips
  required to resume a stream to exactly <em>one</em> (besides the
  round trips required by the TLS handshake). This is achieved by
  using just a secure key to resume the stream.</p>

</section1>

<section1 topic='Glossary' anchor='glossary'>

  <dl>
  <di>
    <dt>ISR</dt>
    <dd>Instant Stream Resumption.</dd>
  </di>
  <di>
    <dt>Instant Stream Resumption Key (ISR Key)</dt>
    <dd>A key, represented as string, which contains at least 128
    bits of entropy generated by a cryptographically secure random
    number generator.</dd>
  </di>
  <di>
    <dt>TLS</dt>
    <dd>Transport Layer Security (&rfc5246;).</dd>
  </di>
  </dl>

</section1>

<!--
<section1 topic='Use Cases' anchor='usecases'>
  <p>STRONGLY RECOMMENDED.</p>
</section1>
-->

<section1 topic='Obtaining a Instant Stream Resumption Key' anchor='obtain'>

  <p>If an entity supports ISR, then the &lt;enabled/&gt;
  Nonza<note>XEP-0360: Nonzas (are not Stanzas) &lt;<link
  url='https://xmpp.org/extensions/xep-0360.html'>https://xmpp.org/extensions/xep-0360.html</link>&gt;.</note>,
  which is send as positive reply upon a request to enable Stream
  Management, MUST contain an 'key' attribute qualified by the
  'urn:xmpp:isr:0' namespace containing a ISR Key. The Nonza MAY
  also contain a 'location' attribute qualified by the
  'urn:xmpp:isr:0' namespace which specifies the preferred IP address
  or hostname, and a TCP port number of the host which should be used
  for instant stream resumption.</p>

  <example caption='An &lt;enabled/&gt; Nonza with a ISR key'><![CDATA[
<enabled
  xmlns='urn:xmpp:sm:3'
  xmlns:isr='urn:xmpp:isr:0'
  isr:key='a0b9162d-0981-4c7d-9174-1f55aedd1f52'/>]]></example>

  <example caption='An &lt;enabled/&gt; Nonza with a ISR key and location'><![CDATA[
<enabled
  xmlns='urn:xmpp:sm:3'
  xmlns:isr='urn:xmpp:isr:0'
  isr:key='a0b9162d-0981-4c7d-9174-1f55aedd1f52'
  isr:location='isr.example.org:5222'/>]]></example>

</section1>

<section1 topic='Instant Stream Resumption' anchor='isr'>

  <p>In order to instantaneously resume an XMPP stream the initiating
  entity, which is either an XMPP client or server, must posses a
  valid ISR key. After it has obtained the ISR key, using the process
  described in the previous section, it first determines the host for
  resumption, and after that, tries to perform the instant stream
  resumption.</p>

  <section2 topic='Determing the Host for Resumption' anchor='host'>

  <p>The lookup mechanism order to determine host candidates for ISR
  resumption is as follows:</p>

  <ol>
    <li>The host provided in the optional 'location' attribute
    qualified by the 'urn:xmpp:isr:0' namespace found in the
    &lt;enabled/&gt; element of <cite>XEP-0198</cite> (the
    "isr:location").
    </li>
    <li>The hosts determined by means of &xep0368;.</li>
    <li>The host announced in the 'location' attribute of the
    &lt;enabled/&gt; Nonza defined in <cite>XEP-0198</cite>.</li>
    <li>Standard host lookup mechanisms.</li>
  </ol>

  <p>The host candidates retrieved by those mechanisms SHOULD be
  tried by the initiating entity in this order.</p>

  <p>Note that the hosts announced by the 'location' attribute
  qualified by the 'urn:xmpp:isr:0' namespace MUST be connected to
  using TLS from the beginning, i.e. &lt;starttls/&gt; MUST NOT be
  used, instead the TLS handshake is performed right after
  establishing the connection.</p>

  <p>This order prefers hosts which allow connections where TLS is
  enabled from the beginning. This is desirable to reduce the
  required round trips by skipping the &lt;starttls/&gt; step.</p>

  </section2>

  <section2 topic='Performing Instant Stream Resumption' anchor='resume'>

  <p>After the remote host on which the instant stream resumption
  should be performed was determined, the initiating entity connects
  to the host, and establishes TLS by either</p>

  <ol>
    <li>establishing a TLS session right away, or</li>
    <li>performing STARTTLS (&rfc6120; § 5).</li>
  </ol>

  <p>Next, the initiating entity sends an XMPP &lt;stream&gt; open
  element followed by a &lt;instant-resume/&gt; Nonza qualified by
  the 'urn:xmpp:isr:0' namespace which MUST contain the previous
  stream identifier, the <cite>XEP-0198</cite> "SM-ID", in the
  'previd' attribute, the sequence number of the last by Stream
  Management handled stanza in the 'h' attribute and the
  initiator-hmac as value of at least one &lt;hash/&gt; element as
  specified by &xep0300;, which are put as child elements under the
  &lt;hmac/&gt; element.</p>

  <p>The initiator-hmac is defined as follows:</p>

  <p class='box'>
    initiator-hamc = Base64(HMAC(key, "Initiator" ||
    tls-server-end-point))
  </p>

  <p>The function defined in &rfc2104; is used to compute the HMAC
  using the hash algorithm specified in the 'algo' attribute of the
  &lt;hash/&gt; element as the cryptographic hash function H. The
  ISR Key is used as key of the HMAC. And the bytewise concatnation
  of the ASCII String "Initiator" and the bytes from
  tls-server-end-point, which a TLS Channel Binding defined in
  &rfc5929; § 4, is used a the HMAC text. The resulting bytes of the
  HMAC function are encoded using Base64 as defined in &rfc4648;
  <link url='https://tools.ietf.org/html/rfc4648#section-4'>§
  4</link> and resulting string is used as text value of the
  &lt;hash/&gt; element.</p>

  <p>Note that the initiating entity SHOULD pipeline the instant
  stream resumption request together with then initial
  &lt;stream&gt; open element. The initiating entity is able to do
  so since it already knows that the service supports ISR because it
  announced an ISR key. Servers MUST nevertheless announce that they
  support ISR by including an &lt;isr/&gt; element qualified by the
  'urn:xmpp:isr:0' namespace in their stream features.</p>

  <example caption='Initiating entity requests instant stream resumption'><![CDATA[
<stream:stream
 from='juliet@im.example.com'
 to='im.example.com'
 version='1.0'
 xml:lang='en'
 xmlns='jabber:client'
 xmlns:stream='http://etherx.jabber.org/streams'>
<inst-resume
  xmlns='urn:xmpp:isr:0'
  previd='some-long-sm-id'
  h='42'>
  <hmac>
    <hash xmlns='urn:xmpp:hashes:1' algo='sha256'>
      initator-hmac
    </hash>
  </hmac>
</inst-resume>]]></example>

  <p>ISR MUST only be performed over TLS secured sessions. What
  follows is that the ISR feature MUST only be announced after
  STARTTLS has been successfully performed or on streams where TLS
  was established from the beginning.</p>

  <section3 topic='Successful Stream Resumption' anchor='isr-success'>

  <p>On success the server replies with &lt;inst-resumed/&gt;
  Nonza which MUST contain a <em>new</em> ISR Key found in the
  'key' attribute, the sequence number of the last by Stream
  Mangement handled stanza in the 'h' attribute and the
  'responder-hmac' as value of the &lt;hash/&gt; element being a
  child of the &lt;hamc/&gt; element.</p>

  <p>The responder-hmac is defined as follows:</p>

  <p class='box'>
    responder-hmac = Base64(HMAC(key, "Responder" ||
    tls-server-end-point))
  </p>

  <p>That is, it is the same as the initiator-hamc, but instead of
    using the ASCII string "Initiator", the ASCII string "Responder"
    is used.</p>

  <p>The initiating entity is required to verify the
    responder-hmac achieve mutual authentication.</p>

  <example caption='Server acknowledges instant stream resumption'><![CDATA[
<stream:stream
  from='im.example.com'
  id='t7AMCin9zjMNwQKDnplntZPIDEI='
  to='juliet@im.example.com'
  version='1.0'
  xml:lang='en'
  xmlns='jabber:client'
  xmlns:stream='http://etherx.jabber.org/streams'>
<stream:features>
 <isr xmlns='urn:xmpp:isr:0'/>
</stream:features>
<inst-resumed
  xmlns='urn:xmpp:isr:0'
  key='006b1a29-c549-41c7-a12c-2a931822f8c0'
  h='21'>
  <hmac>
    <hash xmlns='urn:xmpp:hashes:1' algo='sha-256'>
      responder-hmac
    </hash>
  </hmac>
</inst-resumed>]]></example>

  <p>After the &lt;inst-resumed/&gt; was received and has been
  verified both entities MUST consider the resumed stream to be
  re-established. This includes all previously negotiated stream
  features like &xep0138;. It does however not include the
  specific state of the features: For example in case of Stream
  Compression, the dictionary used by the compression mechanism of
  the resumed stream MUST NOT be considered to be restored after
  instant stream resumption.</p>

  </section3>

  <section3 topic='Failed Stream Resumptino' anchor='isr-failed'>

    <p>If the server is unable to resume the stream instantly it
      MUST reply with a &lt;failed/&gt; Nonza qualified by the
      'urn:xmpp:isr:0' namespace.</p>

    <example caption='Server indicates instant stream resumption failure'><![CDATA[
<failed
  xmlns='urn:xmpp:isr:0'/>]]></example>

    <p>The server MAY also include a 'h' attribute in the
      &lt;failed/&gt; element indicating the number of stanzas it has
      handled so far.</p>

    <example caption='Server indicates failure with handled stanzas count'><![CDATA[
<failed
  xmlns='urn:xmpp:isr:0'
  h='22'/>]]></example>

    <p>Instant stream resumption errors SHOULD be considered
      recoverable, the initiating entity MAY continue with normal
      session establishment; however, misuse of stream management MAY
      result in termination of the stream.</p>

  </section3>

  </section2>

</section1>

<section1 topic='Security Considerations' anchor='security'>

  <p>It is of vital importance that the Instant Stream Resumption Key
  is generated by a cryptographically secure random generator. See
  &rfc4086; for more information about Randomness Requirements for
  Security</p>

</section1>

<section1 topic='IANA Considerations' anchor='iana'>

  <p>This document requires no interaction with &IANA;.</p>

</section1>

<section1 topic='XMPP Registrar Considerations' anchor='registrar'>

  <p>The &REGISTRAR; includes 'urn:xmpp:isr:0' in its registry of protocol namespaces (see &NAMESPACES;).</p>


</section1>

<section1 topic='XML Schema' anchor='schema'>

  <p>TODO: Add after the XEP leaves the 'experimental' state.</p>

</section1>

<section1 topic='Acknowledgements' anchor='acknowledgements'>

  <p>Thanks to Jonas Wielicki and Thijs Alkemade for their feedback.</p>

</section1>

</xep>
