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<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
<!ENTITY % ents SYSTEM 'xep.ent'>
<!ENTITY tls13 "<span class='ref'><link url=''>draft-ietf-tls-tls13-21</link></span> <note>The Transport Layer Security (TLS) Protocol Version 1.3 &lt;<link url=''></link>&gt;.</note>" >
<!ENTITY sasl-ht "<span class='ref'><link url=''>draft-schmaus-sasl-ht-03</link></span><note>draft-schmaus-sasl-ht-03: The Hashed Token SASL Mechanism &lt;<link url=''></link>&gt;.</note>" >
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<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>
<type>Standards Track</type>
<spec>XMPP Core</spec>
<remark><p>Minor fixes and clarifications</p></remark>
<initials>XEP Editor (jwi)</initials>
<remark><p>Accepted by council vote from 2017-12-13.</p></remark>
<remark><p>Minor changes</p></remark>
<li>Bump SASL2 namespace to urn:xmpp:sasl:1, and as result:</li>
<li>Rename 'key' to 'token'</li>
<remark><p>Based ISR on SASL2.</p></remark>
<remark><p>Second draft.</p></remark>
<remark><p>First draft.</p></remark>
<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
§ 5</link>, the approach defined herein reduces the round trips
required to resume a stream to exactly <em>one</em>. This is
achieved by using just a secure short-lived token to resume the
<section1 topic='Glossary' anchor='glossary'>
<dd>Instant Stream Resumption.</dd>
<dt>Instant Stream Resumption Token (ISR Token)</dt>
<dd>A shared secret that is exclusively ephemeral and represented as string.</dd>
<dd>Transport Layer Security (&rfc5246;).</dd>
<section1 topic='Use Cases' anchor='usecases'>
<section1 topic='Stream Feature'>
<p>XMPP entities providing Instant Stream Resumption MUST announce
that functionality as stream feature, but only if an instant stream
resumption is possible at this stage. The ISR stream future consists
of an &lt;isr/&gt; element qualified by the 'htpps://'
namespace. And since ISR requires TLS, this means that the
&lt;isr/&gt; stream feature only appears on TLS secured
<p>The ISR stream feature element MUST contain a &lt;mechanisms/&gt;
element as defined in &rfc6120;. This element contains the SASL
mechanism which are available to be used for instant stream
<example caption='Server announces the Instant Stream Resumption Stream Feature'><![CDATA[
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
<sm xmlns='urn:xmpp:sm:3'/>
<isr xmlns=''>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<p>Every ISR enabled entity SHOULD support the HT-SHA-256-ENDP
mechanism, support for HT-SHA-256-UNIQ is RECOMMENDED. The family
of <cite>HT SASL</cite> mechanisms is specified in &sasl-ht;.</p>
<section1 topic='Obtaining a Instant Stream Resumption Token' anchor='obtain'>
<p>In order to obtain an ISR token, the requesting entity must add an
'isr-enable' element qualified by the 'htpps://' namespace to the
&lt;enable/&gt; element as defined in &xep0198; when attempting to
enable Stream Management. This &lt;isr-enable/&gt; element MUST contain a
'mechanism' attribute containing the name of the SASL mechanism the
requesting entity will use when performing ISR with the returned
token. The entities involved in ISR MUST only use or allow this
mechanism when performing ISR with the according token. This
effectively pins the SASL mechanism <note>Pinning the SASL mechanism
is believed to increase the security</note>.</p>
<example caption='An &lt;enable/&gt; Nonza with the ISR &apos;mechanism&apos; element'><![CDATA[
<enable xmlns='urn:xmpp:sm:3'>
<isr-enable xmlns='' mechanism='HT-SHA-256-ENDP'/>
<p>Next, the &lt;enabled/&gt; Nonza (see &xep0360;) which is sent as
positive reply upon a request to enable Stream Management, MUST
contain an 'isr-enabled' element qualified by the ''
namespace containing a ISR token as value of its 'token' attribute. The
token MUST be newly generated by a cryptographically secure random
number generator and MUST contain at least 128 bit of entropy. The
&lt;isr-enabled/&gt; element can optionally also contain a
'location' attribute
which specifies the preferred IP address or hostname, and a TCP port
number of the host which should be used for instant stream
<example caption='An &lt;enabled/&gt; Nonza with a ISR token'><![CDATA[
<enabled xmlns='urn:xmpp:sm:3'>
<isr-enabled xmlns='' token='a0b9162d-0981-4c7d-9174-1f55aedd1f52'/>
<example caption='An &lt;enabled/&gt; Nonza with a ISR token and location'><![CDATA[
<enabled xmlns='urn:xmpp:sm:3'>
<isr-enabled xmlns=''
<p>The &lt;enabled/&gt; Nonza containing an ISR token MUST only be
sent over TLS secured connections.</p>
<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 token. After it has obtained the ISR token, using the process
described in the previous section, it first determines the host for
resumption, and after that, tries to perform the instant stream
<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>
<li>The host provided in the optional 'location' attribute
qualified by the '' namespace found in the
&lt;enabled/&gt; element of <cite>XEP-0198</cite> (the
<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>
<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 '' 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 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>
<li>establishing a TLS session right away, or</li>
<li>performing STARTTLS (&rfc6120; § 5).</li>
<p>Now the initiating entity sends an XMPP &lt;stream&gt; open
element followed by a &lt;authenticate/&gt; Nonza as specified in
the &xep0388;. The initiating entity must also provide a
&lt;inst-resume/&gt; element qualified by the ''
namespace, which must contain a &lt;resume/&gt; element as defined
in &xep0198;.</p>
<p>The only defined attributed of the &lt;inst-resume/&gt; element is the 'with-isr-token'
attribute, whose value, if omitted, defaults to 'true'. If is set to 'false', then the
SASL mechanism is performed as when traditionally authenticating
the XMPP session. If the value of the attribute is 'true' then the "password" given to
the SASL mechanism is the ISR token. Note that this implies that only
SASL mechanisms which take a password/token can be used this
<example caption='Initiating entity requests instant stream resumption via the Extensible SASL Profile (XEP-0388)'><![CDATA[
<?xml version='1.0'?>
<authenticate xmlns='urn:xmpp:sasl:1' mechanism='HT-SHA-256-ENDP'>
<initial-response>[base64 encoded SASL data]</initial-response>
<inst-resume xmlns='' with-isr-token='true'/>
<resume xmlns='urn:xmpp:sm:3'
<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 token.</p>
<p>Servers MUST destroy the ISR token of a stream after an instant
stream resumption was attempted for that stream with an invalid ISR
token. Server implementations MUST implement the ISR token comparision in
linear runtime.</p>
<section3 topic='Successful Stream Resumption' anchor='isr-success'>
<example caption='Successful Instant Stream Resumption'><![CDATA[
<success xmlns='urn:xmpp:sasl:1'>z
<inst-resumed xmlns=''
<resumed xmlns='urn:xmpp:sm:3' h='354' previd='123'/>
<p>On success the server replies with a &lt;success/&gt; nonza as
specified in the &xep0388;, which must include a
&lt;inst-resumed/&gt; element qualified by the ''
namespace. This element MUST contain a <em>new</em> ISR Token found in
the 'token' attribute. It also MUST include a &lt;resumed/&gt; as
specified in &xep0198; containing the sequence number of the last by
Stream Management handled stanza in the 'h' attribute and the
'previd' attribute.</p>
<p>In case of an successful Instant Stream Resumption authenticated
by an ISR token, the server MUST immediately destroy the ISR token after
authentication, i.e., it MUST no longer be possible to perform an
ISR using that ISR token and Stream Management ID (SM-ID, see
&xep0198;) tuple.</p>
<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
<p class='box'>Note that this behavior is different from &xep0198;
stream resumption, where "outer stream" features like compression
are not restored. Since such a behavior would be counterproductive
towards the goal of this XEP, it specifies that the negotiation
state of such "outer stream" features is also restored (besides the
features which where already negotiated at ISR-time, i.e. TLS).</p>
<section3 topic='Successful Authentication but failed Stream Resumption' anchor='isr-auth-success-resumption-failed'>
<p>If the server was able to authenticate the initiating entity
but is unable to resume the stream instantly it MUST reply with a
&lt;success/&gt; Nonza as defined in the &xep0388; containing
a &lt;inst-resume-failed/&gt; element qualified by the
'' namespace. This
&lt;inst-resume-failed/&gt; MUST contain a &lt;failed/&gt;
element as defined in &xep0198;.</p>
<example caption='Server indicates instant stream resumption failure'><![CDATA[
<success xmlns='urn:xmpp:sasl:1'>
<inst-resume-failed xmlns=''>
<failed xmlns='urn:xmpp:sm:3'
<item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<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. Since the initiating entity is
authenticated, it could continue with resource binding by using
&rfc6120; § 7. or &xep0386;.</p>
<section3 topic='Multi step authentication ISR' anchor='multi-step-auth-isr'>
<p>As specified in the &xep0388; § 2.6.3, sole SASL authentication
may not be sufficient for authentication. In this case, the remote
entity sends a &lt;continue/&gt; element as defined in &xep0388;
to request the local entity to perform another
<example caption='Server requires Multi SASL Mechanism ISR'><![CDATA[
<continue xmlns='urn:xmpp:sasl:1'>
<section3 topic='Failed ISR Authentication' anchor='isr-auth-failed'>
<p>If the server is unable to authenticate the initiating entity
it replies with a &lt;failure/&gt; Nonza as defined in
&xep0388;. The server MUST delete any state of the stream which
was attempted to resume in case the SM-ID was correct but the
authentication failed.<note>This is to prevent brute force
<example caption='Server indicates instant stream resumption failure'><![CDATA[
<failure xmlns='urn:xmpp:sasl:1'>
<not-authorized xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
<p>After the ISR authentication has failed, the initiating entity
could continue with normal authentication (&xep0388;,
<section1 topic='Security Considerations' anchor='security'>
<p>Any ISR data SHALL NOT be part of <cite>TLS 1.3</cite> 0-RTT
early data. (TODO: Shall we weaken this requirement to allow early
data?. It would be technically possible if the sender does not add
additional data, for example Stanzas, after the ISR/XEP-0388 data at
the end of the early data. And if the receiver does ensure that the
existence of such additional data is causing an ISR failure.)</p>
<p>It is of vital importance that the Instant Stream Resumption
Token is generated by a cryptographically secure random
generator. See &rfc4086; for more information about Randomness
Requirements for Security.</p>
<section1 topic='IANA Considerations' anchor='iana'>
<p>This document requires no interaction with &IANA;.</p>
<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
<p>The &REGISTRAR; includes '' in its registry of protocol namespaces (see &NAMESPACES;).</p>
<section1 topic='XML Schema' anchor='schema'>
<p>TODO: Add after the XEP leaves the 'experimental' state.</p>
<section1 topic='Acknowledgements' anchor='acknowledgements'>
<p>Thanks to Jonas Wielicki, Thijs Alkemade, Dave Cridland, Maxime
Buquet, Alexander Würstlein, Sam Whited and Ivan Vučica for their feedback.</p>
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