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https://github.com/moparisthebest/xeps
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349 lines
15 KiB
XML
349 lines
15 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>Stanza Multiplexing</title>
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<abstract>
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This spec provides a mechanism for multiplexing multiple virtual hosts over
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a single XMPP session.
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</abstract>
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&LEGALNOTICE;
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<number>0451</number>
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<status>Experimental</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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<spec>XEP-0220</spec>
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</supersedes>
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<supersededby/>
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<shortname>mux</shortname>
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&sam;
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<revision>
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<version>0.1.0</version>
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<date>2021-01-19</date>
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<initials>XEP Editor (jsc)</initials>
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<remark>Accepted by vote of Council on 2020-12-09.</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>2020-12-03</date>
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<initials>ssw</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>
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The ability to multiplex multiple virtual hosts over a single XMPP session
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(historically known as "piggybacking") was originally defined in &rfc3920;
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and later pulled out into &xep0220; for use with &rfc6120;.
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With the advent of cheap or free TLS certificates the use of dialback
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began falling off on the public XMPP network as more secure authentication
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mechanisms such as SASL EXTERNAL began to become more common.
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However, multiplexing is still a useful technique in constrained
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environments regardless of the authentication mechanism being used.
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</p>
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<p>
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Multiplexing is also useful for reusing connections for additional
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services associated with a domain but hosted at a subdomain.
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For example, both the "montague.example" and the "capulet.example" may be
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hosted by the same XMPP server which may also host &xep0369; services at
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"chat.montague.example" and "rooms.capulet.example" respectively.
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Without multiplexing this would require eight TCP connections for a
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bidirectional exchange of stanzas between two sending domains and two
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target domains.
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However, with multiplexing this can be reduced to two connections, or, at
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the operator's discretion, more than two for operational reasons.
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If multiplexing is not used, the number of server-to-server connections
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needed to exchange stanzas between virtual hosting providers or
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multi-service XMPP servers can increase signficantly.
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This can lead to the number of connections exceeding the maximum number of
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connections allowed from a single address as explained in &xep0205;.
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</p>
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<p>
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This specification defines new mechanisms for advertising and negotiating
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multiple hosts over a single session.
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Furthermore it advances the state of the art over the multiplexing
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solution defined in &xep0220; by working on both client-to-server (c2s)
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and server-to-server (s2s) sessions.
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</p>
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</section1>
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<section1 topic='Advertising Support' anchor='advertising'>
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<p>
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If a server supports receiving multiplexed streams it SHOULD inform the
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connecting entity when returning stream features during the negotiation
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process.
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Two mechanisms exist for authenticating domains that can be multiplexed
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over a connection: domains may be authenticated using the TLS certificate
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(and client certificate if applicable), and domains may be authorized
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using the connection authorization mechanism described later in this
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document.
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</p>
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<p>
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To advertise support for multiplexing all domains present in a TLS
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certificate the server includes a <mux/> element qualified by the
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'urn:xmpp:mux:0' namespace in the stream features list.
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This feature MUST be advertised only after TLS has been negotiated (either
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by opportunistic TLS using the STARTTLS feature or by implicit TLS when
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establishing the TCP socket) an before authentication using SASL EXTERNAL
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has been performed.
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This feature is not mandatory to negotiate.
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</p>
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<example caption='Server advertises support for mux using SASL EXTERNAL'><![CDATA[
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<stream:features>
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<mux xmlns='urn:xmpp:mux:0'/>
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<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
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<mechanism>EXTERNAL</mechanism>
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</mechanisms>
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</stream:features>]]></example>
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<p>
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The mux feature may also be advertised after authentication with SASL
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EXTERNAL.
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If advertised after authentication the feature MUST include a list of
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supported hosts wrapped in <host/> elements.
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</p>
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<example caption='Server advertises support for connection authorization'><![CDATA[
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<stream:features>
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<mux xmlns='urn:xmpp:mux:0'>
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<host>capulet.example</host>
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<host>montague.example</host>
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<host>chat.montague.example</host>
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<host>rooms.capulet.example</host>
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</mux>
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<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
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</stream:features>]]></example>
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</section1>
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<section1 topic='Authentication with SASL EXTERNAL' anchor='authn'>
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<p>
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If the initiating entity wishes to indicate that it intends to use
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multiplexing with SASL EXTERNAL it MUST respond to the empty <mux/>
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element by sending another empty <mux/> element qualified by the
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'urn:xmpp:mux:0' namespace in reply.
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No stream restart is necessary.
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</p>
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<p>
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After indicating support for multiplexing by negotiating the mux stream
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feature, authentication can proceed.
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When using SASL EXTERNAL this is done by validating the certificate as
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detailed in &xep0178; except that every domain that is present in the
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certificate is now eligible for multiplexing without further negotiation.
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Further stream features (such as resource binding) still use the JID from
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the original connection (or from the authorization identity).
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</p>
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<p>
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If a bidirectional s2s connection has been negotiated for this session using
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&xep0288;, negotiation of the mux stream feature also implies that the
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receiving entity SHOULD multiplex stanzas sent back to the initiating entity
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for all domains in the verified client certificate.
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If bidi is not negotiated then mux will need to be negotiated again when the
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original receiving entity establishes a connection with the original
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initiating entity.
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</p>
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</section1>
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<section1 topic='Connection Authorization' anchor='authz'>
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<p>
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Often it is not desirable to have one certificate containing every XMPP
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address or host managed by the server, or the use of SASL EXTERNAL may be
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impossible.
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In these cases the initiating entity may request authorization to
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send stanzas over an existing connection.
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</p>
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<p>
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If the initiating entity has an authenticated connection to a server and
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wishes to send stanzas to another server that was listed in the original
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servers post-auth <mux/> stream feature it MAY establish an XMPP
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connection with the new server and verify that new server also lists the
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original server in its post-auth mux stream feature.
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If it does the initiating entity replies with a <mux/> element
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qualified by the 'urn:xmpp:mux:0' namespace with a shared secret as the
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payload and the host being selected included in the 'host' attribute.
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The old server then sends an IQ over its existing connection with the
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initiating entity containing the same mux element and secret, thereby
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confirming its relationship to the new server.
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If the client verifies that the secrets match it sends an empty IQ of type
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"result" in response to indicate success, otherwise the IQ response should
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be a "not-acceptable" stanza error (see &rfc6120; §8.3.3.9).
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</p>
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<p>
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For example, if the server montague.example wishes to establish a
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multiplexed connection with capulet.example and rooms.capulet.example the
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flow would look like this:
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</p>
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<code><![CDATA[
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montague.example capulet.example
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---------------- ---------------
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| |
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| [if necessary, |
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| perform DNS lookup |
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| on Target Domain, |
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| open TCP connection, |
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| and establish stream] |
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| -----------------------> | rooms.capulet.example
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| | ---------------------
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| [if necessary, | |
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| perform DNS lookup, | |
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| on Sender Domain, | |
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| open TCP connection, | |
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| and establish stream] | |
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| -----------------------------------------------> |
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| | |
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| send mux secret | |
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| -----------------------------------------------> |
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| send mux secret IQ | |
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| <----------------------- | |
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| | |
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| send IQ response | |
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| -----------------------> | |
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| | |
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| | close connection |
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| <----------------------------------------------- |]]></code>
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<p>
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The XML for this exchange would look like the following:
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</p>
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<example caption='Initial connection between montague.example and capulet.example'><![CDATA[
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<!--
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Elided: a stream is negotiated between montague.example and capulet.example.
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After authentication is complete capulet.example advertises support for mux:
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-->
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<stream:features>
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<mux xmlns='urn:xmpp:mux:0'>
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<host>rooms.capulet.example</host>
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</mux>
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</stream:features>
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<!--
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Negotiation proceeds and the mux stream feature is not selected. After
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negotiation is complete montague.example tries to establish a connection with
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rooms.capulet example and sends it a secret. The server responds with the same
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secret:
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-->
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<iq to="montague.example" from="capulet.example" type="set" id="1285152">
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<mux xmlns='urn:xmpp:mux:0'>secret</mux>
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</iq>
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<-- The server at montague.example indicates that the secret was verified. -->
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<iq to="capulet.example" from="montague.example" type="result" id="1285152"/>
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]]></example>
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<example caption='Secondary connection between montague.example and rooms.capulet.example'><![CDATA[
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<!--
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Elided: a stream is negotiated between montague.example and
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rooms.capulet.example. After authentication is complete rooms.capulet.example
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advertises support for mux:
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-->
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<stream:features>
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<mux xmlns='urn:xmpp:mux:0'>
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<host>capulet.example</host>
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</mux>
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</stream:features>
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<!--
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The server at montague.example indicates that it wishes to authorize its
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existing connection with capulet.example:
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-->
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<mux xmlns='urn:xmpp:mux:0' host='capulet.example'>
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secret
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</mux>
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<!--
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The server at rooms.capulet.example closes the connection gracefully
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if mux was established and begins using the connection between
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montague.example and capulet.example.
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-->
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</stream:stream>
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]]></example>
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<p>
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The format of the secret is not specified however, see the Security
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Considerations section of this document for some suggestions.
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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>
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Some clients may send stanzas with no "from" attribute specified and rely on
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the server to add the attribute before routing the stanza to its final
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destination.
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If multiplexing is used the lack of a from attribute indicates that the
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client is acting on behalf of the origin JID for the connection, just like
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normal, so clients MUST set the from attribute on any stanzas sent on behalf
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of any multiplexed host.
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</p>
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<p>
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The format of mux secrets is undefined in this document, however, they MUST
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be unpredictable.
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Only the initiating entity should attribute any meaning (if indeed there is
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any) to the format of mux secrets.
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In particular the receiving entity MUST NOT rely on a specific format for
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the secret.
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</p>
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<p>
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One suggestion for generating mux secrets is to generate a key that signs
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information about the stream.
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The format defined in &xep0185; is appropriate for this.
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If the mux secret is a signature it must protect against reuse by at least
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include a random secret generated with a cryptographically secure random
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number source, the origin JID, the JID of the server initially receiving the
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mux secret, and the stream ID for the stream the key will be authenticating
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(this is not the same stream as the receiving entity's JID).
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It is also RECOMMENDED that an expiration time be included in the key after
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which it is no longer valid.
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</p>
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</section1>
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<section1 topic='IANA Considerations' anchor='iana'>
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<p>This document requires no interaction with &IANA;.</p>
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</section1>
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<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
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<section2 topic='Protocol Namespaces' anchor='registrar-ns'>
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<p>This specification defines the following XML namespace:</p>
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<ul>
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<li>urn:xmpp:mux:0</li>
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</ul>
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<p>
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Upon advancement of this specification from a status of Experimental to a
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status of Draft, the ®ISTRAR; shall add the foregoing namespace to the
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registries located at &STREAMFEATURES;, as described in Section 4 of
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&xep0053;.
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</p>
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<code caption='Jabber/XMPP Protocol Stream Features Registry Submission'><![CDATA[
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<feature>
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<ns>urn:xmpp:mux:0</ns>
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<name>mux</name>
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<element>mux</element>
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<desc>Indicate support for connection multiplexing and transmit secret keys to a peer.</desc>
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<doc>Editor to add document reference if accepted</doc>
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<status>provisional</status>
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</feature>]]></code>
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<p>
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The ®ISTRAR; shall also add the foregoing namespace to the Jabber/XMPP
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Protocol Namespaces Registry located at &NAMESPACES;.
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Upon advancement of this specification from a status of Experimental to a
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status of Draft, the ®ISTRAR; shall remove the provisional status from
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this registry entry.
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</p>
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<code caption='Jabber/XMPP Protocol Namespaces Registry Submission'><![CDATA[
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<ns>
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<name>urn:xmpp:mux:0</name>
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<doc>Editor to add document reference if accepted</doc>
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<status>provisional</status>
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</ns>]]></code>
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</section2>
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<section2 topic='Namespace Versioning' anchor='registrar-versioning'>
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&NSVER;
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</section2>
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</section1>
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<section1 topic='Acknowledgements' anchor='ack'>
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<p>
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Thanks to Jeremie Miller, Peter Saint-Andre, and Philipp Hancke for writing
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&xep0220; from which some of the text and techniques used in this document
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were borrowed.
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</p>
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</section1>
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</xep>
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