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inbox: add new protoxep: Stanza Multiplexing

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<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
<!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
<title>Stanza Multiplexing</title>
<abstract>
This spec provides a mechanism for multiplexing multiple virtual hosts over
a single XMPP session.
</abstract>
&LEGALNOTICE;
<number>xxxx</number>
<status>ProtoXEP</status>
<type>Standards Track</type>
<sig>Standards</sig>
<approver>Council</approver>
<dependencies>
<spec>XMPP Core</spec>
</dependencies>
<supersedes>
<spec>XEP-0220</spec>
</supersedes>
<supersededby/>
<shortname>mux</shortname>
&sam;
<revision>
<version>0.0.1</version>
<date>2020-12-03</date>
<initials>ssw</initials>
<remark><p>First draft.</p></remark>
</revision>
</header>
<section1 topic='Introduction' anchor='intro'>
<p>
The ability to multiplex multiple virtual hosts over a single XMPP session
(historically known as "piggybacking") was originally defined in &rfc3920;
and later pulled out into &xep0220; for use with &rfc6120;.
With the advent of cheap or free TLS certificates the use of dialback
began falling off on the public XMPP network as more secure authentication
mechanisms such as SASL EXTERNAL began to become more common.
However, multiplexing is still a useful technique in constrained
environments regardless of the authentication mechanism being used.
</p>
<p>
Multiplexing is also useful for reusing connections for additional
services associated with a domain but hosted at a subdomain.
For example, both the "montague.example" and the "capulet.example" may be
hosted by the same XMPP server which may also host &xep0369; services at
"chat.montague.example" and "rooms.capulet.example" respectively.
Without multiplexing this would require eight TCP connections for a
bidirectional exchange of stanzas between two sending domains and two
target domains.
However, with multiplexing this can be reduced to two connections, or, at
the operator's discretion, more than two for operational reasons.
If multiplexing is not used, the number of server-to-server connections
needed to exchange stanzas between virtual hosting providers or
multi-service XMPP servers can increase signficantly.
This can lead to the number of connections exceeding the maximum number of
connections allowed from a single address as explained in &xep0205;.
</p>
<p>
This specification defines new mechanisms for advertising and negotiating
multiple hosts over a single session.
Furthermore it advances the state of the art over the multiplexing
solution defined in &xep0220; by working on both client-to-server (c2s)
and server-to-server (s2s) sessions.
</p>
</section1>
<section1 topic='Advertising Support' anchor='advertising'>
<p>
If a server supports receiving multiplexed streams it SHOULD inform the
connecting entity when returning stream features during the negotiation
process.
Two mechanisms exist for authenticating domains that can be multiplexed
over a connection: domains may be authenticated using the TLS certificate
(and client certificate if applicable), and domains may be authorized
using the connection authorization mechanism described later in this
document.
</p>
<p>
To advertise support for multiplexing all domains present in a TLS
certificate the server includes a &lt;mux/&gt; element qualified by the
'urn:xmpp:mux:0' namespace in the stream features list.
This feature MUST be advertised only after TLS has been negotiated (either
by opportunistic TLS using the STARTTLS feature or by implicit TLS when
establishing the TCP socket) an before authentication using SASL EXTERNAL
has been performed.
This feature is not mandatory to negotiate.
</p>
<example caption='Server advertises support for mux using SASL EXTERNAL'><![CDATA[
<stream:features>
<mux xmlns='urn:xmpp:mux:0'/>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>EXTERNAL</mechanism>
</mechanisms>
</stream:features>]]></example>
<p>
The mux feature may also be advertised after authentication with SASL
EXTERNAL.
If advertised after authentication the feature MUST include a list of
supported hosts wrapped in &lt;host/&gt; elements.
</p>
<example caption='Server advertises support for connection authorization'><![CDATA[
<stream:features>
<mux xmlns='urn:xmpp:mux:0'>
<host>capulet.example</host>
<host>montague.example</host>
<host>chat.montague.example</host>
<host>rooms.capulet.example</host>
</mux>
<bind xmlns='urn:ietf:params:xml:ns:xmpp-bind'/>
</stream:features>]]></example>
</section1>
<section1 topic='Authentication with SASL EXTERNAL' anchor='authn'>
<p>
If the initiating entity wishes to indicate that it intends to use
multiplexing with SASL EXTERNAL it MUST respond to the empty &lt;mux/&gt;
element by sending another empty &lt;mux/&gt; element qualified by the
'urn:xmpp:mux:0' namespace in reply.
No stream restart is necessary.
</p>
<p>
After indicating support for multiplexing by negotiating the mux stream
feature, authentication can proceed.
When using SASL EXTERNAL this is done by validating the certificate as
detailed in &xep0178; except that every domain that is present in the
certificate is now eligible for multiplexing without further negotiation.
Further stream features (such as resource binding) still use the JID from
the original connection (or from the authorization identity).
</p>
<p>
If a bidirectional s2s connection has been negotiated for this session using
&xep0288;, negotiation of the mux stream feature also implies that the
receiving entity SHOULD multiplex stanzas sent back to the initiating entity
for all domains in the verified client certificate.
If bidi is not negotiated then mux will need to be negotiated again when the
original receiving entity establishes a connection with the original
initiating entity.
</p>
</section1>
<section1 topic='Connection Authorization' anchor='authz'>
<p>
Often it is not desirable to have one certificate containing every XMPP
address or host managed by the server, or the use of SASL EXTERNAL may be
impossible.
In these cases the initiating entity may request authorization to
send stanzas over an existing connection.
</p>
<p>
If the initiating entity has an authenticated connection to a server and
wishes to send stanzas to another server that was listed in the original
servers post-auth &lt;mux/&gt; stream feature it MAY establish an XMPP
connection with the new server and verify that new server also lists the
original server in its post-auth mux stream feature.
If it does the initiating entity replies with a &lt;mux/&gt; element
qualified by the 'urn:xmpp:mux:0' namespace with a shared secret as the
payload and the host being selected included in the 'host' attribute.
The old server then sends an IQ over its existing connection with the
initiating entity containing the same mux element and secret, thereby
confirming its relationship to the new server.
If the client verifies that the secrets match it sends an empty IQ of type
"result" in response to indicate success, otherwise the IQ response should
be a "not-acceptable" stanza error (see &rfc6120; §8.3.3.9).
</p>
<p>
For example, if the server montague.example wishes to establish a
multiplexed connection with capulet.example and rooms.capulet.example the
flow would look like this:
</p>
<code><![CDATA[
montague.example capulet.example
---------------- ---------------
| |
| [if necessary, |
| perform DNS lookup |
| on Target Domain, |
| open TCP connection, |
| and establish stream] |
| -----------------------> | rooms.capulet.example
| | ---------------------
| [if necessary, | |
| perform DNS lookup, | |
| on Sender Domain, | |
| open TCP connection, | |
| and establish stream] | |
| -----------------------------------------------> |
| | |
| send mux secret | |
| -----------------------------------------------> |
| | |
| send mux secret IQ | |
| <----------------------- | |
| | |
| send IQ response | |
| -----------------------> | |
| | |
| | close connection |
| <----------------------------------------------- |]]></code>
<p>
The XML for this exchange would look like the following:
</p>
<example caption='Initial connection between montague.example and capulet.example'><![CDATA[
<!--
Elided: a stream is negotiated between montague.example and capulet.example.
After authentication is complete capulet.example advertises support for mux:
-->
<stream:features>
<mux xmlns='urn:xmpp:mux:0'>
<host>rooms.capulet.example</host>
</mux>
</stream:features>
<!--
Negotiation proceeds and the mux stream feature is not selected. After
negotiation is complete montague.example tries to establish a connection with
rooms.capulet example and sends it a secret. The server responds with the same
secret:
-->
<iq to="montague.example" from="capulet.example" type="set" id="1285152">
<mux xmlns='urn:xmpp:mux:0'>secret</mux>
</iq>
<-- The server at montague.example indicates that the secret was verified. -->
<iq to="capulet.example" from="montague.example" type="result" id="1285152"/>
]]></example>
<example caption='Secondary connection between montague.example and rooms.capulet.example'><![CDATA[
<!--
Elided: a stream is negotiated between montague.example and
rooms.capulet.example. After authentication is complete rooms.capulet.example
advertises support for mux:
-->
<stream:features>
<mux xmlns='urn:xmpp:mux:0'>
<host>capulet.example</host>
</mux>
</stream:features>
<!--
The server at montague.example indicates that it wishes to authorize its
existing connection with capulet.example:
-->
<mux xmlns='urn:xmpp:mux:0' host='capulet.example'>
secret
</mux>
<!--
The server at rooms.capulet.example closes the connection gracefully
if mux was established and begins using the connection between
montague.example and capulet.example.
-->
</stream:stream>
]]></example>
<p>
The format of the secret is not specified however, see the Security
Considerations section of this document for some suggestions.
</p>
</section1>
<section1 topic='Security Considerations' anchor='security'>
<p>
Some clients may send stanzas with no "from" attribute specified and rely on
the server to add the attribute before routing the stanza to its final
destination.
If multiplexing is used the lack of a from attribute indicates that the
client is acting on behalf of the origin JID for the connection, just like
normal, so clients MUST set the from attribute on any stanzas sent on behalf
of any multiplexed host.
</p>
<p>
The format of mux secrets is undefined in this document, however, they MUST
be unpredictable.
Only the initiating entity should attribute any meaning (if indeed there is
any) to the format of mux secrets.
In particular the receiving entity MUST NOT rely on a specific format for
the secret.
</p>
<p>
One suggestion for generating mux secrets is to generate a key that signs
information about the stream.
The format defined in &xep0185; is appropriate for this.
If the mux secret is a signature it must protect against reuse by at least
include a random secret generated with a cryptographically secure random
number source, the origin JID, the JID of the server initially receiving the
mux secret, and the stream ID for the stream the key will be authenticating
(this is not the same stream as the receiving entity's JID).
It is also RECOMMENDED that an expiration time be included in the key after
which it is no longer valid.
</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'>
<section2 topic='Protocol Namespaces' anchor='registrar-ns'>
<p>This specification defines the following XML namespace:</p>
<ul>
<li>urn:xmpp:mux:0</li>
</ul>
<p>
Upon advancement of this specification from a status of Experimental to a
status of Draft, the &REGISTRAR; shall add the foregoing namespace to the
registries located at &STREAMFEATURES;, as described in Section 4 of
&xep0053;.
</p>
<code caption='Jabber/XMPP Protocol Stream Features Registry Submission'><![CDATA[
<feature>
<ns>urn:xmpp:mux:0</ns>
<name>mux</name>
<element>mux</element>
<desc>Indicate support for connection multiplexing and transmit secret keys to a peer.</desc>
<doc>Editor to add document reference if accepted</doc>
<status>provisional</status>
</feature>]]></code>
<p>
The &REGISTRAR; shall also add the foregoing namespace to the Jabber/XMPP
Protocol Namespaces Registry located at &NAMESPACES;.
Upon advancement of this specification from a status of Experimental to a
status of Draft, the &REGISTRAR; shall remove the provisional status from
this registry entry.
</p>
<code caption='Jabber/XMPP Protocol Namespaces Registry Submission'><![CDATA[
<ns>
<name>urn:xmpp:mux:0</name>
<doc>Editor to add document reference if accepted</doc>
<status>provisional</status>
</ns>]]></code>
</section2>
<section2 topic='Namespace Versioning' anchor='registrar-versioning'>
&NSVER;
</section2>
</section1>
<section1 topic='Acknowledgements' anchor='ack'>
<p>
Thanks to Jeremie Miller, Peter Saint-Andre, and Philipp Hancke for writing
&xep0220; from which some of the text and techniques used in this document
were borrowed.
</p>
</section1>
</xep>