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427 lines
22 KiB
XML
427 lines
22 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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<!--
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© XMPP Standards Foundation, 2015
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Author: Peter Waher
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-->
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<xep>
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<header>
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<title>Quality of Service</title>
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<abstract>This specification describes a generic method whereby messages can be sent between clients using a predefined Quality of Service level.</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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<approver>Council</approver>
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<dependencies>
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<spec>XMPP Core</spec>
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<spec>XEP-0030</spec>
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<spec>XEP-0337</spec>
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</dependencies>
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<supersedes/>
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<supersededby/>
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<shortname>qos</shortname>
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&peterwaher;
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<revision>
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<version>0.0.1</version>
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<date>2015-11-12</date>
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<initials>pw</initials>
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<remark>
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<p>First draft.</p>
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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>
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Sometimes it is important for the sender of a message to control the Quality of Service level (or QoS level) of the message.
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In other protocols, such as queueing, such QoS levels are built into the protocol itself. XMPP lacks an intrinsic QoS level management feature,
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which has left application developers to define proprietary solutions when such have been required. This document however, provides a generic
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solution and can be combined with any type of message structure defined either proprietarily or in another XEP.
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The handling of the Quality of Service level is done prior to parsing the contents of the message. It defines three generic
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Quality of Service levels that can be used regardless of what type of message is being sent:
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</p>
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<dl>
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<di>
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<dt>Unacknowledged service - At most once</dt>
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<dd>
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<p>
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The message is sent at most once to its destination. The delivery of the message is not guaranteed. This QoS level
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should only be used of the loss of messages is not critical to the application.
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</p>
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<p>
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Example: A sensor sending a stream of current values. If a value is lost might not matter much since the next message
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provides a new current value, overriding any lost value.
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</p>
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</dd>
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</di>
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<di>
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<dt>Acknowledged service - At least once</dt>
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<dd>
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<p>
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The message is sent at least once to its destination. Sending of the message is repeated until message has been acknowledged.
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It is not guaranteed that the message is delivered only once. Asynchronous conditions may arise where the message is delivered more
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than once. If an acknowledgement is not received, at least the sender may become aware that the message was not delivered
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properly. The At least once QoS level requires twice as many messages to be transported in the network as compared to
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the At most once QoS level.
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</p>
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<p>
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Example: Any idempotent control action such as setting the light to a certain level. It is important that the control action
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is received, but not how many of them are received. Any identical control action that follows makes no difference in output or
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performance.
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</p>
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</dd>
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</di>
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<di>
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<dt>Assured service - Exactly once</dt>
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<dd>
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<p>
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The message is sent and delivered exactly once to its destination. The delivery of the message is divided into two acknowledged
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message transfers. First, the message is transferred to the other side, but not parsed. The second operation, the message is requested
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to be parsed and delivered. Both these steps are idempotent, making sure that the contents of the message is delivered exactly once.
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If not possible to deliver the message, the sender will be made aware of this fact. The Exactly once QoS level requires
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four times as many messages to be transported in the network as compared to the At most once QoS level and twice as many
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messages to be transported as compared to the At least once QoS level.
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</p>
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<p>
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Example: Any control action or counting operation that is not idempotent, such as sending a control action when a button is
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pressed, whenever an event has occurred or a transaction. If counting events, transactions, or accumulating consumption, it is
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vital that messages are not processed twice, since it will change the end result.
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</p>
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</dd>
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</di>
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</dl>
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</section1>
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<section1 topic='Use Cases' anchor='usecases'>
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<section2 topic='Unacknowledged service - At most once' anchor='unack'>
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<p>
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To send a message that is received at most once to its destination, a normal message stanza is used, as defined by the XMPP
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protocol itself. The delivery of the message is not guaranteed. This QoS level should only be used of the loss of messages is not critical
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to the application.
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</p>
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<p>
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Example: A sensor sending a stream of current values. If a value is lost might not matter much since the next message
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provides a new current value, overriding any lost value.
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</p>
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<example caption='Sensor sending a momentary value'>
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<![CDATA[
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<message
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from='temperaturesensor@example.org/34892374'
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to='user@example.org/938089023'>
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<fields xmlns='urn:xmpp:iot:sensordata' seqnr='1' done='true'>
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<node nodeId='Temp01'>
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<timestamp value='2015-11-11T22:01:30'>
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<numeric name='Temperature' momentary='true' automaticReadout='true' value='23.4' unit='°C'/>
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</timestamp>
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</node>
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</fields>
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</message>]]>
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</example>
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</section2>
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<section2 topic='Acknowledged service - At least once' anchor='ack'>
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<p>
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To send a message that is received at least once to its destination, an iq stanza of type set is sent containing an
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acknowledged element from the "urn:xmpp:qos" namespace. The acknowledged element in turn contains the message to be delivered.
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The iq stanza must have an id attribute that is used to identify if the stanza has been received or not. When an acknowledged
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message is received, an empty response is returned to acknowledge the receipt. Parsing the message is done after sending the response.
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If no response is received by the sending side within a given time frame, an number of retries should be made, using an interval or drop-off
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algorithm. These time frames and number of attempts are implementation specific.
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</p>
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<p>
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It is not guaranteed that the message is delivered only once. Asynchronous conditions may arise where the message is delivered more
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than once. If an acknowledgement is not received, at least the sender may become aware that the message was not delivered.
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The At least once QoS level requires twice as many messages to be transported in the network as compared to
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the At most once QoS level.
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</p>
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<p>
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Example: Any idempotent control action such as setting the light to a certain level. It is important that the control action
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is received, but not how many of them are received. Any identical control action that follows makes no difference in output or
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performance.
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</p>
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<example caption='Setting light level'>
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<![CDATA[
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<iq id='123' type='set'
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from='controller@example.org/34892374'
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to='lightswitch@example.org/938089023'>
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<qos:acknowledged xmlns:qos='urn:xmpp:qos'>
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<message>
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<set xmlns='urn:xmpp:iot:control'>
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<int name='Light' value='80'/>
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</set>
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</message>
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</qos:acknowledged>
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</iq>
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<iq id='123' type='result'
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from='lightswitch@example.org/938089023'
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to='controller@example.org/34892374'/>
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]]>
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</example>
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<p>
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Note: The to and from attributes in the embedded message stanza are left empty.
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</p>
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</section2>
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<section2 topic='Assured service - Exactly once' anchor='assured'>
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<p>
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To send a message that is received exactly once to its destination, an iq stanza of type set is sent containing an
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assured element from the "urn:xmpp:qos" namespace. The assured element in turn contains the message to be delivered.
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The iq stanza must have an id attribute that is used to identify if the stanza has been received or not, just as in the
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acknowledged case. But to keep track of the message begin sent, a msgId attribute has to be provided as well. When an assured
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message is received, a received response is returned to acknowledge the receipt. This received response echoes the msgId
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attribute as well. But the message is not parsed and delivered at this point, only temporarily stored on the receiving end.
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If multiple messages are received using the same msgId from the same sender, they are simply ignored, as one is already held in
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memory. Still, the received response is returned as normal.
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</p>
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<p>
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When the sender receives the received response, it sends a new iq set stanza, this time with a deliver payload,
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to tell the receiving end to parse and deliver the message, if in memory, and remove it from memory. The receiver acknowledges the message
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by returning an empty iq result stanza. If receiving multiple requests using the same msgId, it will acknowledge each one. But
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only one message will be parsed and delivered, as it will have been removed from the temporary storage on the receiving end. For both the
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assured and deliver messages, a retry mechanism similar to the one used for acknowledged service is to be used.
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</p>
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<p>
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The Exactly once QoS level requires twice as many messages to be transported in the network as compared to the At least once
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QoS level, and four times the number of messages as compared to the At most once QoS level.
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</p>
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<p>
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Example: Any control action or counting operation that is not idempotent, such as sending a control action when a button is
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pressed, or whenever an event has occurred. If counting events, or accumulating consumption, it is vital that messages are not
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processed twice, since it will change the end result.
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</p>
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<example caption='Counting cars or individuals in a closed space'>
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<![CDATA[
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<iq id='123' type='set'
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from='entrance@example.org/34892374'
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to='counter@example.org/938089023'>
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<qos:assured xmlns:qos='urn:xmpp:qos' msgId='984232334'>
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<message>
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<log xmlns='urn:xmpp:eventlog' timestamp='2015-11-12T09:07:12Z' id='Entered'>
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<message>Entered.</message>
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</buy>
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</message>
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</qos:assured>
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</iq>
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<iq id='123' type='result'
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from='counter@example.org/938089023'>
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to='entrance@example.org/34892374'>
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<received msgId='984232334'/>
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</iq>
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<iq id='124' type='set'
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from='entrance@example.org/34892374'
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to='counter@example.org/938089023'>
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<qos:deliver xmlns:qos='urn:xmpp:qos' msgId='984232334'/>
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</iq>
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<iq id='124' type='result'
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from='counter@example.org/938089023'>
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to='entrance@example.org/34892374'/>
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</iq>
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...
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<iq id='456' type='set'
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from='exit@example.org/34892374'
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to='counter@example.org/938089023'>
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<qos:assured xmlns:qos='urn:xmpp:qos' msgId='4645623466'>
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<message>
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<log xmlns='urn:xmpp:eventlog' timestamp='2015-11-12T11:23:54Z' id='Left'>
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<message>Left.</message>
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</buy>
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</message>
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</qos:assured>
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</iq>
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<iq id='456' type='result'
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from='counter@example.org/938089023'>
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to='exit@example.org/34892374'>
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<received msgId='4645623466'/>
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</iq>
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<iq id='457' type='set'
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from='exit@example.org/34892374'
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to='counter@example.org/938089023'>
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<qos:deliver xmlns:qos='urn:xmpp:qos' msgId='4645623466'/>
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</iq>
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<iq id='457' type='result'
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from='counter@example.org/938089023'>
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to='exit@example.org/34892374'/>
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</iq>]]>
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</example>
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<p>
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Note: The to and from attributes in the embedded message stanza are left empty.
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</p>
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</section2>
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</section1>
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<section1 topic='Determining Support' anchor='support'>
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<p>
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If an entity supports signing forms as specified herein, it MUST advertise that fact by returning a feature of
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"urn:xmpp:qos" in response to &xep0030; information requests.
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</p>
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<example caption="Service discovery information request">
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<![CDATA[
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<iq type='get'
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from='example.org'
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to='device@example.org'
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id='disco1'>
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<query xmlns='http://jabber.org/protocol/disco#info'/>
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</iq>]]>
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</example>
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<example caption="Service discovery information response">
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<![CDATA[
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<iq type='result'
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from='device@example.org'
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to='example.org'
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id='disco1'>
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<query xmlns='http://jabber.org/protocol/disco#info'>
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...
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<feature var='urn:xmpp:qos'/>
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...
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</query>
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</iq>]]>
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</example>
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<p>
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In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic,
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presence-based profile of service discovery defined in &xep0115;. However, if an application has not received entity capabilities
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information from an entity, it SHOULD use explicit service discovery instead.
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</p>
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</section1>
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<section1 topic='Implementation Notes' anchor='impl'>
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<section2 topic='Offline messages' anchor='offline'>
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<p>
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When sending messages using unacknowledged service, i.e. using the normal message stanza, it can be subject to offline message
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storage if the recipient if offline. This might result in delivery of the message at a later time.
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</p>
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</section2>
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<section2 topic='Resource part' anchor='resource'>
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<p>
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Message routing using unacknowledged service might be handled somewhat differently as compared to acknowledged and assured service,
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since the first uses the message stanza and the latter two the iq stanza, which require a fill JID in order to reach its destination.
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For consistency, full JIDs should be used also for unacknowledged messaging service, if possible.
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</p>
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</section2>
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<section2 topic='Generic implementation' anchor='generic'>
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<p>
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If implementing support for this XEP, make sure implementation is generic and put on a lower level than other XEP-specific processing
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of incoming stanzas. Messages received using this server should be fed into the normal message processing and handling after the QoS level
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has been ascertained. This makes it possible to use QoS levels for any type of message being sent and received.
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</p>
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</section2>
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<section2 topic='Persistence' anchor='persistence'>
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<p>
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One decision each implementation of assured QoS has to make, is where to persist incoming messages between reception of message and
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the delivery to underlying message processing logic. Should they be persisted in internal memory, persistent Flash/EEProm, as local files
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or in a database? Such implementation specific concerns are left to the implementor, as this XEP only concerns itself with the communication
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layer between parties. But the choice of solution might affect to what level a message is ascertained to be delivered: Will delivery be
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guaranteed, even beyond a system reset at the receiving end? Or a system reset at the sending side? If delivery of the message has to be
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ascertained even if a system reset at the sending side is allowed, persistance of the request must also be made on the sending side.
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(To some extent, this latter concern even affects acknowledged service.)
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</p>
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</section2>
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<section2 topic='Message IDs' anchor='msdId'>
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<p>
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Outstanding message IDs used in assured delivery, must be unique for the sender (counting the Full JID as the sender). If running
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short on Message IDs, such can be reclaimed after the receipt of a successful delivery of a previous message. Message IDs can be
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sequence numbers, or hash values of the contents of the message.
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</p>
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</section2>
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</section1>
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<section1 topic='Security Considerations' anchor='security'>
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<section2 topic='To and from attributes' anchor='tofrom'>
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<p>
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The to and from attributes of embedded message elements should be ignored and left out by the sender, except in the case of
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unacknowledged service, where the message element is the message stanza itself. In the case of acknowledged and assured quality of
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service levels, the message element MUST have its to and from attributes overwritten by the corresponding attributes of the iq
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stanza being used. This, to avoid the injection of messages with unauthenticated and unauthorized identities.
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</p>
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</section2>
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<section2 topic='Memory attacks' anchor='memory'>
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<p>
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As incoming message using the assured quality of service level is temporarily stored on the receiving end, the receiver might become
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vulnerable to memory attacks if not implementing some basic counter measures:
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</p>
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<ol>
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<li>
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<p>
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Do not accept messages using assured service from untrusted sources, for instance from MUC rooms or senders not in the
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roster. If deemed untrusted, an iq error of type <strong>not-allowed</strong> should be returned instead of the
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<strong>received</strong> element expected in normal flow.
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</p>
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</li>
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<li>
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<p>
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Set a limit on the number and size of assured messages that can be simultaneously received from a single source. If a source
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sends more than this, the <strong>resource-constraint</strong> error should be returned instead of accepting the message.
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</p>
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</li>
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<li>
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<p>
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Set a total limit on the number and size of assured messages that can be simultaneously received from all sources. If anybody
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sends a message that will exceed this amount, the <strong>resource-constraint</strong> error should be returned instead of
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accepting the message.
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</p>
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</li>
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</ol>
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<p>
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Whenever invalid use of the assured messaging service is detected, wether it be from untrusted sources or spam, an event should be
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logged using &xep0337; so that the source of the error can be found and appropriate counter measures taken.
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</p>
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</section2>
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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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<p>
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The <link url="#schema">protocol schema</link> needs to be added to the list of <link url="http://xmpp.org/resources/schemas/">XMPP protocol schemas</link>.
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</p>
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</section1>
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<section1 topic='XML Schema' anchor='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
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xmlns:xs='http://www.w3.org/2001/XMLSchema'
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targetNamespace='urn:xmpp:qos'
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xmlns='urn:xmpp:qos'
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xmlns:jbc='jabber:client'
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elementFormDefault='qualified'>
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<xs:import namespace='jabber:client'/>
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<xs:element name='acknowledged' type='EmbeddedMessage'/>
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<xs:element name='assured' type='Assured'/>
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<xs:element name='received' type='WithMessageId'/>
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<xs:element name='deliver' type='WithMessageId'/>
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<xs:complexType name='EmbeddedMessage'>
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<xs:sequence minOccurs='1' maxOccurs='1'>
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<xs:element ref='jbc:message'/>
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</xs:sequence>
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</xs:complexType>
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<xs:complexType name='Assured'>
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<xs:complexContent>
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<xs:extension base='EmbeddedMessage'>
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<xs:attribute ref='msgId' use='required'/>
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</xs:extension>
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</xs:complexContent>
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</xs:complexType>
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<xs:attribute name='msgId' type='xs:string'/>
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<xs:complexType name='WithMessageId'>
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<xs:attribute ref='msgId' use='required'/>
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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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</xep> |