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