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UDT Initial ProtoXEP

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Dave Cridland 2019-12-30 12:20:46 +00:00
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<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE xep SYSTEM 'xep.dtd' [
<!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
<!ENTITY ns "urn:xmpp:udt:0">
<!ENTITY nsx "urn:example:">
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
<title>User-defined Data Transfer</title>
<abstract>This specification proposes a simple mechanism by which applications can transfer data safely, without
needing additional protocol design work. It is intended to provide a protocol that is trivial to implement and can
be driven with a simple API.</abstract>
&LEGALNOTICE;
<number>XXXX</number>
<status>ProtoXEP</status>
<type>Standards Track</type>
<sig>Standards</sig>
<dependencies>
<spec>XMPP Core</spec>
</dependencies>
<supersedes/>
<supersededby/>
<shortname>udt</shortname>
&dcridland;
<revision>
<version>0.0.1</version>
<date>2019-12-30</date>
<initials>dwd</initials>
<remark>
<ul>
<li>Initial Revision</li>
</ul>
</remark>
</revision>
</header>
<section1 topic='Introduction' anchor='intro'>
<p>Applications written on top of XMPP often need to exchange data that has no existing standard. Such applications are
often written by developers unfamiliar with best practise in designing new extensions for XMPP, making it hard to achieve
this simple design goal without causing longer term problems.</p>
<p>This leads to "solutions" such as stuffing JSON directly in the &lt;body/> element, for example, and recognising
this at the receiver either by heuristics or by a special &lt;subject/>. While this works, it's difficult to then
migrate to something else, and enforces that custom clients are always used.</p>
<p>Therefore this document proposes a very simple (and simplistic) framework for sending such data which - while
very light on features - nevertheless conforms to best practice. Unusually, this specification SHOULD NOT be used
as a base upon which to build other standards, and suggests an API for library developers to implement.</p>
<section2 topic="Terminology">
<p>Data transferred using this specification is encoded using JSON. The type of the data is given by a URI under
the same rules as an XML namespace, and this specification refers to this as the datatype.</p>
<p>Because this document defines mechanisms for sending essentially arbitrary data, no real-world examples are
given.</p>
<p>Instead, example namespaces are used within an XML namespace prefixed by <tt>&nsx;</tt></p>
</section2>
</section1>
<section1 topic='Overview' anchor="overview">
<section2 topic="Discovering Support" anchor="feature">
<p>Support for this protocol is advertised by the Service Discovery protocol defined in &xep0030; using a feature
of <tt>&ns;</tt>.</p>
<p>Support for a particular datatype is given by concatenating the <tt>&ns;</tt> feature with a hash character
('<tt>#</tt>') and the datatype, for example <tt>&ns;#&nsx;foo</tt>.</p>
</section2>
<section2 topic="Data Transfers">
<p>This specification provides for two types of user-defined data transfers. Each uses a similar payload syntax,
the UDT data payload.</p>
<p>Requests are carried within &lt;iq/> stanzas, either of type "get" or "set", and result in a "result" optionally
containing a second UDT data payload.</p>
<p>UDT payloads may also be placed within a &lt;message/> stanza. &lt;message/> stanzas MAY contain multiple UDT
payloads, but typical usage is expected to be that there will be only one. The UDT payload may be ancillary data
to another message, or a standalone message in its own right.</p>
<section3 topic="Protocol Syntax">
<p>A UDT payload consists of a single element, <tt>&lt;payload/></tt>, qualified by the XML namespace
<tt>&ns;</tt>. It has a single, mandatory attribute of <tt>datatype</tt>, which MUST contain a string conformant
to the requirements for XML namespaces (typically a URI under the control of the application developer).</p>
<p>As with XML namespaces, this URI is never expected to be resolved, and is used solely as an indentifier.
Different strings are considered entirely different datatypes, and common prefixes etc MUST be considered
irrelevant for the purposes of interpreting the data. There are no common or standard datatypes.</p>
<p>The <tt>&lt;payload</tt> element contains exactly one mandatory child element, the <tt>&lt;json/></tt> element
defined in &xep0335;. This in turns contains the JSON data.</p>
<example><![CDATA[
<message from="gamer@game-company.example"
to="match-maker.game-company.example"
id="12345">
<payload xmlns="]]>&ns;<![CDATA[" datatype="]]>&nsx;foo<![CDATA[">
<json xmlns="urn:xmpp:json:0">
{
"annoying-teenager-level": 11
}
</json>
</payload>
</message>
]]></example>
<p>Note that the suggested custom &IQ; query payload of &xep0335; is not used as this would generally require
custom handlers within client libraries:</p>
<example><![CDATA[
<iq from="gamer@game-company.example"
to="match-maker.game-company.example"
id="12345"
type="set">
<payload xmlns="]]>&ns;<![CDATA[" datatype="]]>&nsx;foo<![CDATA[">
<json xmlns="urn:xmpp:json:0">
{
"annoying-teenager-percentage": 101
}
</json>
</payload>
</iq>
]]></example>
</section3>
</section2>
</section1>
<section1 topic="API Requirements">
<p>In order to satisfy the goals of this protocol, it is necessary to define an API that can be consistently
implemented across APIs. This allows application developers to use the protocol easily, and encourages this
over using the ad-hoc techniques described in the introduction.</p>
<p>Therefore, while not imposing hard and fast API definitions, this specification proposes naming conventions
for APIs that will hopefully guide application developers toward consistent usage.</p>
<p>While names are specified in "snake_case", API developers are free to use their own naming. This specification
also defines APIs as taking session arguments which will often be implied by method calls, and omits types.</p>
<p>Library developers SHOULD make these calls as simple to use as possible. If these are significantly harder to use
for inexperienced developers than ad-hoc techniques, then ad-hoc techniques will be used instead.</p>
<section2 topic="Arguments and Types">
<ul>
<li><tt>session</tt> - The session, connection or stream object or handle for the library. In cases where the
library uses a single global connection this is omitted. An OO library is likely to elide this and have the call
be a method call on the object instead.</li>
<li><tt>message</tt> - The message object or handle for the library. An OO library is likely to elide this and have the call
be a method call on the object instead.</li>
<li><tt>datatype</tt> - Typically a string containing an XML-style namespace.</li>
<li><tt>jid</tt> - A string or JID object. Any form of legal jid might be used here.</li>
<li><tt>data</tt> - Either a JSON-encoded string, or an object or data structure which can be converted by the
library.</li>
<li><tt>get_or_set</tt> - A flag for indicating whether this is a "get" or "set" request.</li>
<li><tt>callback</tt> - Some callable object or similar as meets the idiom of the language.</li>
</ul>
</section2>
<section2 topic="Advertising Support">
<p>Support is advertised on a session by calling <tt>udt_advertise(session, datatype)</tt>. Calling this MUST
explicitly advertise both the <tt>&ns;</tt> feature and that of the datatype support.</p>
<p>APIs are free to (and encouraged to) implicitly advertise support when other calls are made.</p>
</section2>
<section2 topic="Requests">
<p>Requests are sent to a particular jid by calling <tt>udt_request(session, jid, get_or_set, datatype, data)</tt>.
This might return a UDT payload, or have an additional <tt>callback</tt> argument to call with the response.</p>
<p>Applications may register to handle such requests by calling <tt>udt_request_callback(session, get_or_set,
datatype, callback)</tt>. The callback should be called as <tt>callback(session, jid, get_or_set, datatype,
data)</tt>, and returning a payload should send a <tt>result</tt> containing it. Applications registering this
way SHOULD implicitly advertise support for the datatype.</p>
</section2>
<section2 topic="Messages">
<p>Messages are sent to a particular jid by calling <tt>udt_message(session, jid, get_or_set, datatype, data)</tt>.</p>
<p>Applications may register to handle such requests by calling <tt>udt_message_callback(session, get_or_set,
datatype, callback)</tt>. The callback should be called as <tt>callback(session, jid, get_or_set, datatype,
data)</tt>. Applications registering this way SHOULD implicitly advertise support for the datatype.</p>
<p>Applications may check to see if any message stanza contains a UDT payload by calling <tt>udt_payload(message,
datatype)</tt>, which returns the payload if it exists.</p>
</section2>
</section1>
<section1 topic="Schema">
<code>
<![CDATA[
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="]]>&ns;<![CDATA[" xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="payload" type="udt:payloadType" xmlns:udt="]]>&ns;<![CDATA["/>
<xs:complexType name="payloadType">
<xs:sequence>
<xs:any minOccurs="1" maxOccurs="1"/>
<!-- Always a XEP-0335 json element, but I can't figure that out. -->
</xs:sequence>
<xs:attribute type="xs:string" name="datatype"/>
</xs:complexType>
</xs:schema>
]]>
</code>
</section1>
<section1 topic='Security Considerations' anchor='security'>
<p>All security implications herein are those of the payload.</p>
</section1>
<section1 topic='IANA Considerations' anchor='iana'>
<p>This XEP requires no interaction with &IANA;. </p>
</section1>
<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
<p>None.</p>
</section1>
<section1 topic='Acknowledgements' anchor='ack'>
<p>The authors wish to share any credit with many members of the community, including Florian Schmaus.</p>
</section1>
</xep>