<remark><p>Added two more scenarios; defined business rule about not sending to bare JIDs; specified security consideration regarding presence leaks.</p></remark>
<remark><p>Added text and examples for service discovery; added text and examples for chat session negotiation; added recommendations regarding message processing, retries, etc.</p></remark>
<p>While &xep0079; provides message acknowledgements at the server level, it does not extend that model all the way to the client. However, sometimes client-level acknowledgements are needed, for example to provide "receipts". This document defines a mechanism for XMPP message receipts.</p>
<p>This document addresses the following requirements:</p>
<ol>
<li>Enable a sender to request notification that an XMPP message stanza has been received.</li>
<li>Enable a recipient to provide message receipts if desired.</li>
</ol>
<p>Note: This document explicitly does not define a protocol for "guaranteed delivery", since that term (like "security") means different things to different people. Instead, we define a more focused protocol that addresses the need for message receipts, thus solving one problem that falls under the heading of "guaranteed delivery".</p>
</section1>
<section1topic='Scenarios'anchor='scenarios'>
<p>In general there are seven possible scenarios (where "S" stands for sender and "R" stands for recipient):</p>
<codecaption='Scenario 1: Happy Path'><![CDATA[
S R
| |
| ------------------------------------> |
| send message |
| |
| <------------------------------------|
| send receipt |
| |
]]></code>
<p>In Scenario 1, the use case ends successfully with message delivery, receipt delivery, and no retries.</p>
<codecaption='Scenario 2: Message Not Received'><![CDATA[
S R
| |
| -----------------------------x |
| send message |
| |
| [trigger timeout] |
| |
| ------------------------------------> |
| re-send message |
| |
| <------------------------------------|
| send receipt |
| |
]]></code>
<p>In Scenario 2, the use case ends successfully with message delivery failure, message retry, and receipt delivery.</p>
<codecaption='Scenario 3: Receipt Not Received'><![CDATA[
S R
| |
| ------------------------------------> |
| send message |
| |
| x------------------------------- |
| send receipt |
| |
| [trigger timeout] |
| |
| ------------------------------------> |
| re-send message |
| |
| <------------------------------------|
| send receipt |
| |
]]></code>
<p>In Scenario 3, the use case ends successfully with message delivery, receipt delivery failure, message retry, and receipt delivery.</p>
<p>In Scenario 4, the use case ends unsuccessfully with message delivery and the sender generating presence unavailable (because the sender has gone offline, the recipient has no one to send the receipt to).</p>
<codecaption='Scenario 5: Message Sent, Sender Goes Offline, Presence Not Received'><![CDATA[
S R
| |
| ------------------------------------> |
| send message |
| |
| -------------------------------x |
| send presence unavailable |
| |
| <------------------------------------|
| send receipt |
| |
]]></code>
<p>In Scenario 5, the use case ends unsuccessfully with message delivery and the sender generating presence unavailable; however, the presence unavailable is not delivered, so the recipient sends a receipt, which is not delivered within the sender's timeout period since the sender is now offline.</p>
<p>In Scenario 6, the use case ends unsuccessfully with message delivery and the recipient generating presence unavailable (because the recipient has gone offline before sending a receipt, the sender cannot be sure that the message has been received).</p>
<codecaption='Scenario 7: Message Sent, Recipient Goes Offline, Presence Not Received'><![CDATA[
S R
| |
| ------------------------------------> |
| send message |
| |
| x---------------------------------- |
| send presence unavailable |
| |
| [trigger timeout] |
| |
| ------------------------------------> |
| re-send message |
| |
]]></code>
<p>In Scenario 7, the use case ends unsuccessfully with message delivery and the recipient generating presence unavailable; however, the presence unavailable is not delivered, so the sender retries sending the message and because the recipient is now offline it cannot send a receipt within the sender's timeout period.</p>
<p>In order to make it possible for senders to request, and for recipients to generate, message receipts, we define a new <cite>Advanced Message Processing</cite> rule: "receipt". In accordance with <cite>XEP-0079</cite>, we provide the following information about the receipt rule:</p>
<li>This condition is met if a message processing application (client) controlled by the intended recipient has received and processed the message; the term "processed" is understood to include presentation to a human user if appropriate <note>Therefore this specification does not distinguish between delivery and presentation, as was done in &xep0022;.</note> or any other application-specific client-side processing, including generation of an error response if the application determines that the message contents cannot be handled.</li>
<p>In order to make it possible for senders to request, and for recipients to generate, message receipts, we define a new <cite>Advanced Message Processing</cite> rule: "receipt". In accordance with <cite>XEP-0079</cite>, we provide the following information about the receipt rule:</p>
<li>This condition is met if a message processing application (client) controlled by the intended recipient has received and processed the message; the term "processed" is understood to include presentation to a human user if appropriate <note>Therefore this specification does not distinguish between delivery and presentation, as was done in &xep0022;.</note> or any other application-specific client-side processing, including generation of an error response if the application determines that the message contents cannot be handled.</li>
<p>The general business rules specified for Advanced Message Processing in <cite>XEP-0079</cite> apply to any rule; in addition, the following business rules apply specifically to the receipt rule:</p>
<li><p>A sender SHOULD NOT include a request for message receipts when sending a message to the bare JID (&BAREJID;) of the recipient, only when sending to a full JID (&FULLJID;).</p></li>
<li><p>A sender SHOULD NOT include a request for message receipts unless it knows (via &xep0030; or &xep0115;) that the intended recipient supports the protocol described herein or unless the use of message receipts is negotiated via &xep0155;.</p></li>
<li><p>The sender (i.e., the message generating application controlled by the sender) MUST initiate a timeout upon sending each message, which timeout SHOULD be 30 seconds. If the sender does not receive a message receipt (or failure event) within its timeout period, it MUST re-send the message with an identical value of the XMPP 'id' attribute.</p></li>
<li><p>The sender MUST NOT send a large number of retries. How many retries are appropriate depends on how important the message is perceived to be. In any case, a sender SHOULD NOT send more than five retries.</p></li>
<li><p>The recipient (i.e., the message processing application controlled by the intended recipient that receives a given message) MUST initiate a timeout upon sending each message receipt, which timeout SHOULD be 60 seconds. If the recipient does not receive a re-sent message within its timeout period, it SHOULD stop waiting for a re-sent message and discard memory of that message ID.</p></li>
<li><p>The recipient MUST NOT include a request for message receipts in its acknowledgements. If the sender receives a request for message receipts in an acknowledgement, it MUST NOT acknowledge the acknowledement.</p></li>
<li><p>The recipient SHOULD send the message receipt once it has processed the message, which may include presenting it to a human user (e.g., visually or aurally). The receiving application SHOULD NOT require a human user to positively affirm that he or she has read and understood the message before sending the receipt, since this is unnecessarily intrusive in the context of instant messaging.</p></li>
<p>Naturally, the receipt rule can be combined wiith rules specified in <cite>XEP-0079</cite> (e.g., the deliver rule) for more complete reporting.</p>
<p>This document covers one use case: sending messages with return receipt requested, for which succcess is defined as the sender receiving a message receipt. As described above, there are seven possible scenarios. These are described in more detail in the following sections.</p>
<section2topic='Happy Path'anchor='flow-happy'>
<p>In the "happy path", the sender sends the message and the recipient returns a receipt within the sender's timeout period.</p>
<examplecaption='A message with receipt requested'><