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1.1 RC1 renamed to Session Negotiation (no other changes)

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Ian Paterson 2007-03-12 23:34:19 +00:00
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@ -6,8 +6,8 @@
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
<title>Chat Session Negotiation</title>
<abstract>This document specifies a feature negotiation profile for initiating a one-to-one XMPP chat session.</abstract>
<title>Session Negotiation</title>
<abstract>This document specifies a feature negotiation profile for initiating a one-to-one XMPP communication session.</abstract>
&LEGALNOTICE;
<number>0155</number>
<status>Draft</status>
@ -25,6 +25,12 @@
<shortname>chatneg</shortname>
&ianpaterson;
&stpeter;
<revision>
<version>1.1</version>
<date>2007-03-12</date>
<initials>ip</initials>
<remark><p>Renamed to Session Negotiation</p></remark>
</revision>
<revision>
<version>1.0</version>
<date>2007-01-17</date>
@ -107,7 +113,7 @@
<version>0.2</version>
<date>2005-07-15</date>
<initials>psa</initials>
<remark><p>Further described contexts in which chat session negotiation could be useful; added more examples; added reference to SIP RFC and explained basic mapping to SIP INVITE method; added XMPP Registrar considerations.</p></remark>
<remark><p>Further described contexts in which session negotiation could be useful; added more examples; added reference to SIP RFC and explained basic mapping to SIP INVITE method; added XMPP Registrar considerations.</p></remark>
</revision>
<revision>
<version>0.1</version>
@ -123,22 +129,22 @@
</revision>
</header>
<section1 topic='Introduction' anchor='intro'>
<p>The traditional model for one-to-one chat "sessions" in Jabber/XMPP is for a user to simply send a message to a contact without any formal negotiation of chat session parameters (e.g., see &xmppim;). This informal approach to initiation of a chat session is perfectly acceptable in many contexts, environments, and cultures. However, it may be desirable to formally request the chat and negotiate its parameters before beginning the chat session in some circumstances, such as:</p>
<p>The traditional model for one-to-one chat "sessions" in Jabber/XMPP is for a user to simply send a message to a contact without any formal negotiation of session parameters (e.g., see &xmppim;). This informal approach to initiation of a session is perfectly acceptable in many contexts, environments, and cultures. However, it may be desirable to formally request a chat session (or any other type of session) and negotiate its parameters before beginning the session in some circumstances, such as:</p>
<ul>
<li>Whenever parameters specific to a chat session must be agreed. e.g., security and privacy parameters (see &xep0116; and &xep0136;).</li>
<li>Whenever parameters specific to a session must be agreed. e.g., security and privacy parameters (see &xep0116; and &xep0136;).</li>
<li>The parties are unknown to each other, have not exchanged presence, or have not discovered their respective capabilities via &xep0030; or &xep0115;.</li>
<li>When an XMPP-based system interfaces with a SIP-based system built on top of &rfc3261;. <note>In essence, a chat state negotiation request as specified herein is functionally equivalent to a SIP INVITE request, and acceptance of such a request is functionally equivalent to sending a SIP 200 OK response; see Section 17 of <cite>RFC 3261</cite>.</note></li>
<li>Within an organization or culture in which one would not simply begin chatting with another person (e.g., a superior) without first receiving permission to do so.</li>
<li>When an XMPP-based system interfaces with a SIP-based system built on top of &rfc3261;. <note>In essence, a session negotiation request as specified herein is functionally equivalent to a SIP INVITE request, and acceptance of such a request is functionally equivalent to sending a SIP 200 OK response; see Section 17 of <cite>RFC 3261</cite>.</note></li>
<li>Within an organization or culture in which one would not simply begin interacting with another person (e.g., a superior) without first receiving permission to do so.</li>
</ul>
<p>This proposal defines best practices for such a negotiation, re-using the protocol defined in &xep0020;.</p>
</section1>
<section1 topic='Requirements' anchor='req'>
<p>The specification addresses the following use cases:</p>
<ul>
<li>Negotiating a new chat session</li>
<li>Moving an existing chat session from one resource to another</li>
<li>Renegotiating an existing chat session</li>
<li>Terminating an existing chat session</li>
<li>Negotiating a new session</li>
<li>Moving an existing session from one resource to another</li>
<li>Renegotiating an existing session</li>
<li>Terminating an existing session</li>
</ul>
</section1>
<section1 topic='State Chart' anchor='statechart'>
@ -171,22 +177,22 @@ PENDING o---------------+
|
o ENDED
</code>
<p>[1] A chat session negotiation is initiated when the user sends a message containing a data form of type "form" with an "accept" field.</p>
<p>[2] A chat session negotiation is accepted when the contact sends a message containing a data form of type "submit" with an "accept" field whose value is "1" or "true".</p>
<p>[3] A chat session negotiation is rejected when the contact sends a message containing a data form of type "submit" with an "accept" field whose value is "0" or "false".</p>
<p>[4] A chat session negotiation is confirmed when the user sends a message containing a data form of type "result" with an "accept" field whose value is "1" or "true".</p>
<p>[5] A chat session negotiation is confirmed when the user sends a message containing a data form of type "result" with an "accept" field whose value is "0" or "false".</p>
<p>[6] An existing chat session is re-negotiated when either party sends a message containing a data form of type "form" with a "renegotiate" field whose value is "1" or "true".</p>
<p>[7] A chat session re-negotiation is accepted when the other party sends a message containing a data form of type "submit" with a "renegotiate" field whose value is "1" or "true".</p>
<p>[8] A chat session re-negotiation is rejected when the other party sends a message containing a data form of type "submit" with a "renegotiate" field whose value is "0" or "false"; however, the session remains in the active state with the previously-negotiated parameters in force.</p>
<p>[9] A chat session is terminated when either party sends a message containing a data form of type "submit" with a "terminate" field whose value is "1" or "true".</p>
<p>[1] A session negotiation is initiated when the user sends a message containing a data form of type "form" with an "accept" field.</p>
<p>[2] A session negotiation is accepted when the contact sends a message containing a data form of type "submit" with an "accept" field whose value is "1" or "true".</p>
<p>[3] A session negotiation is rejected when the contact sends a message containing a data form of type "submit" with an "accept" field whose value is "0" or "false".</p>
<p>[4] A session negotiation is confirmed when the user sends a message containing a data form of type "result" with an "accept" field whose value is "1" or "true".</p>
<p>[5] A session negotiation is confirmed when the user sends a message containing a data form of type "result" with an "accept" field whose value is "0" or "false".</p>
<p>[6] An existing session is re-negotiated when either party sends a message containing a data form of type "form" with a "renegotiate" field whose value is "1" or "true".</p>
<p>[7] A session re-negotiation is accepted when the other party sends a message containing a data form of type "submit" with a "renegotiate" field whose value is "1" or "true".</p>
<p>[8] A session re-negotiation is rejected when the other party sends a message containing a data form of type "submit" with a "renegotiate" field whose value is "0" or "false"; however, the session remains in the active state with the previously-negotiated parameters in force.</p>
<p>[9] A session is terminated when either party sends a message containing a data form of type "submit" with a "terminate" field whose value is "1" or "true".</p>
</section1>
<section1 topic='Negotiating a New Chat Session' anchor='new'>
<section2 topic='Initiating a Chat Session' anchor='new-initiate'>
<p>In order to initiate a negotiated chat session, the initiating party ("user") sends a &MESSAGE; <note>The &MESSAGE; stanza is used because the user does not necessarily know which of the contact's resources is most available (or indeed if the contact is online).</note> stanza to the receiving party ("contact") containing a &lt;feature/&gt; child qualified by the 'http://jabber.org/protocol/feature-neg' namespace. The &MESSAGE; stanza MUST NOT contain a &BODY; child element (as specified in &rfc3921;). The &MESSAGE; stanza type SHOULD be "normal" (either explicitly or by non-inclusion of the 'type' attribute). The stanza MUST contain a &THREAD; element for tracking purposes (where the newly-generated ThreadID is unique to the proposed session). The data form MUST contain a hidden FORM_TYPE field whose value is "urn:xmpp:chatneg" and MUST contain a boolean field named "accept". &BOOLEANNOTE; The inclusion of "logging", "disclosure" and "security" fields is also RECOMMENDED. Note: The options within any 'list-single' fields SHOULD appear in order of preference.</p>
<p>Note: Chat sessions may be conducted between entities who are never online at the same time. However, if the user is interested only in an <em>immediate</em> chat session then the user SHOULD instruct the contact's server not to store the message for later delivery (see &xep0160;) using the &xep0079; protocol.</p>
<p>In the following example of a negotiation request, Romeo requests a chat with Juliet and also queries her regarding whether she is able to disallow all message logging (see &xep0136;) <note>A client MUST NOT set the 'logging' field to 'mustnot' unless it has confirmed that its server will allow it to switch off Automated Archiving (see <cite>Message Archiving</cite>).</note>, whether she wants to temporarily share presence for this session (see the <link url='#impl-presence'>Sharing Presence</link> section of this document), and whether she wants to support the &xep0071; and &xep0085; extensions during this chat session. He asks Juliet's client if it is prepared to make a (legally binding) guarantee that it does not intentionally implement any feature (not even a disabled feature) that might disclose the content of the chat, any associated (decryption) keys, or his identity to any third-party (see <cite>Encrypted Session Negotiation</cite>). He also requires that they are both connected securely to their servers, and asks which language she prefers amongst those he can write. (Note: These fields are examples only; a full set of chat session negotiation parameters will be registered as described in the <link url='#registrar'>XMPP Registrar Considerations</link> section of this document.)</p>
<example caption="User requests chat session"><![CDATA[
<section1 topic='Negotiating a New Session' anchor='new'>
<section2 topic='Initiating a Session' anchor='new-initiate'>
<p>In order to initiate a negotiated session, the initiating party ("user") sends a &MESSAGE; <note>The &MESSAGE; stanza is used because the user does not necessarily know which of the contact's resources is most available (or indeed if the contact is online).</note> stanza to the receiving party ("contact") containing a &lt;feature/&gt; child qualified by the 'http://jabber.org/protocol/feature-neg' namespace. The &MESSAGE; stanza MUST NOT contain a &BODY; child element (as specified in &rfc3921;). The &MESSAGE; stanza type SHOULD be "normal" (either explicitly or by non-inclusion of the 'type' attribute). The stanza MUST contain a &THREAD; element for tracking purposes (where the newly-generated ThreadID is unique to the proposed session). The data form MUST contain a hidden FORM_TYPE field whose value is "urn:xmpp:chatneg" and MUST contain a boolean field named "accept". &BOOLEANNOTE; The inclusion of "logging", "disclosure" and "security" fields is also RECOMMENDED. Note: The options within any 'list-single' fields SHOULD appear in order of preference.</p>
<p>Note: Sessions may be conducted between entities who are never online at the same time. However, if the user is interested only in an <em>immediate</em> session then the user SHOULD instruct the contact's server not to store the message for later delivery (see &xep0160;) using the &xep0079; protocol.</p>
<p>In the following example of a negotiation request, Romeo requests a chat with Juliet and also queries her regarding whether she is able to disallow all message logging (see &xep0136;) <note>A client MUST NOT set the 'logging' field to 'mustnot' unless it has confirmed that its server will allow it to switch off Automated Archiving (see <cite>Message Archiving</cite>).</note>, whether she wants to temporarily share presence for this session (see the <link url='#impl-presence'>Sharing Presence</link> section of this document), and whether she wants to support the &xep0071; and &xep0085; extensions during this session. He asks Juliet's client if it is prepared to make a (legally binding) guarantee that it does not intentionally implement any feature (not even a disabled feature) that might disclose the content of the session, any associated (decryption) keys, or his identity to any third-party (see <cite>Encrypted Session Negotiation</cite>). He also requires that they are both connected securely to their servers, and asks which language she prefers amongst those he can write. (Note: These fields are examples only; a full set of session negotiation parameters will be registered as described in the <link url='#registrar'>XMPP Registrar Considerations</link> section of this document.)</p>
<example caption="User requests session"><![CDATA[
<message type='normal'
from='romeo@montague.net/orchard'
to='juliet@capulet.com'>
@ -197,7 +203,7 @@ PENDING o---------------+
<field var='FORM_TYPE' type='hidden'>
<value>urn:xmpp:chatneg</value>
</field>
<field label='Accept this chat?' type='boolean' var='accept'>
<field label='Accept this session?' type='boolean' var='accept'>
<value>true</value>
<required/>
</field>
@ -270,12 +276,12 @@ PENDING o---------------+
]]></example>
<p>The user MAY request a session with a specific resource of the contact. However, if the user specifies no resource (or if the specified resource is not available), then the contact's server delivers the request to the contact's most available resource (which in the examples below happens to be "balcony"). If no resource is available (and no <cite>Advanced Message Processing</cite> rule included in the request specifies otherwise) then the server MAY store the request for later delivery.</p>
</section2>
<section2 topic='Accepting a Chat Session' anchor='new-accept'>
<p>If, upon reception of a user's chat session request, a contact finds that the request had been stored for later delivery, and if the contact is interested only in an <em>immediate</em> chat session, then it SHOULD initiate a new chat session negotiation (including a newly-generated ThreadID) instead of responding to the user's request. Note: Sending any response to the user's original request would leak presence information since it would divulge the fact that the contact had been offline rather than just ignoring the user.</p>
<section2 topic='Accepting a Session' anchor='new-accept'>
<p>If, upon reception of a user's session request, a contact finds that the request had been stored for later delivery, and if the contact is interested only in an <em>immediate</em> session, then it SHOULD initiate a new session negotiation (including a newly-generated ThreadID) instead of responding to the user's request. Note: Sending any response to the user's original request would leak presence information since it would divulge the fact that the contact had been offline rather than just ignoring the user.</p>
<p>In any response to the user's request, the contact's client MUST mirror the &THREAD; value so that the user's client can correctly track the response.</p>
<p>If the request is accepted then the contact's client MUST include in its response values for all the fields that the request indicated are required. If the contact's client does not support one of the default values or if the contact has disabled its support (as for Chat State Notifications and XHTML formatting in the example below), and the client can still accept the request, then it MUST set that field to a value that it can support.</p>
<p>In the example below we assume that Juliet accepts the chat and specifies that she prefers to speak Italian with Romeo:</p>
<example caption="Contact accepts chat and specifies parameters"><![CDATA[
<p>In the example below we assume that Juliet accepts the session and specifies that she prefers to speak Italian with Romeo:</p>
<example caption="Contact accepts session and specifies parameters"><![CDATA[
<message type='normal'
from='juliet@capulet.com/balcony'
to='romeo@montague.net/orchard'>
@ -302,9 +308,9 @@ PENDING o---------------+
]]></example>
<p>Note: Both entities MUST assume the session is being established with the resource of the contact that sends the reply, even if the user sent its request to a different resource of the contact.</p>
</section2>
<section2 topic='Rejecting a Chat Session' anchor='new-reject'>
<p>If the contact does not want to reveal presence to the user for whatever reason then the contact's client SHOULD return no response or error (see <link url='#secure-leak'>Presence Leaks</link>). Also, if the contact is using a legacy client then it MAY not support returning any response or error. In both these cases the user MAY proceed to send stanzas to the contact outside the context of a negotiated chat session.</p>
<p>However, if the contact simply prefers not to chat then the client SHOULD decline the invitation. The data form MUST contain the FORM_TYPE field and the "accept" field set to "0" or "false". It is RECOMMENDED that the form does not contain any other fields even if the request indicated they are required. The client MAY include a reason in the &BODY; child of the &MESSAGE; stanza:</p>
<section2 topic='Rejecting a Session' anchor='new-reject'>
<p>If the contact does not want to reveal presence to the user for whatever reason then the contact's client SHOULD return no response or error (see <link url='#secure-leak'>Presence Leaks</link>). Also, if the contact is using a legacy client then it MAY not support returning any response or error. In both these cases the user MAY proceed to send stanzas to the contact outside the context of a negotiated session.</p>
<p>However, if the contact simply prefers not to start a session then the client SHOULD decline the invitation. The data form MUST contain the FORM_TYPE field and the "accept" field set to "0" or "false". It is RECOMMENDED that the form does not contain any other fields even if the request indicated they are required. The client MAY include a reason in the &BODY; child of the &MESSAGE; stanza:</p>
<example caption="Contact declines offer and specifies reason"><![CDATA[
<message type='normal'
from='juliet@capulet.com/balcony'
@ -386,7 +392,7 @@ PENDING o---------------+
]]></example>
</section2>
<section2 topic='Completing or Canceling the Negotiation' anchor='new-complete'>
<p>If the contact accepted the chat (see <link url='#new-accept'>Accepting a Chat Session</link>) then the user MUST either complete or cancel the session negotiation. If the contact chose an option other than the default (prefered) value for one or more of the fields, then instead of having the client accept the session automatically the user may prefer to review the values that the contact selected before confirming that the session is open. <note>See <cite>Encrypted Session Negotiation</cite> for example of other instances where the user might find the values submitted by the contact unacceptable.</note> In any case the user's client SHOULD verify that the selected values are acceptable before completing the session negotiation -- and confirming that the chat session is open -- by replying with a form with the form 'type' attribute set to 'result'. The form MUST contain the FORM_TYPE field and the "accept" field set to "1" or "true". The user MAY include other content (e.g., a &BODY; element) in the confirmation stanza:</p>
<p>If the contact accepted the session (see <link url='#new-accept'>Accepting a Session</link>) then the user MUST either complete or cancel the session negotiation. If the contact chose an option other than the default (prefered) value for one or more of the fields, then instead of having the client accept the session automatically the user may prefer to review the values that the contact selected before confirming that the session is open. <note>See <cite>Encrypted Session Negotiation</cite> for example of other instances where the user might find the values submitted by the contact unacceptable.</note> In any case the user's client SHOULD verify that the selected values are acceptable before completing the session negotiation -- and confirming that the session is open -- by replying with a form with the form 'type' attribute set to 'result'. The form MUST contain the FORM_TYPE field and the "accept" field set to "1" or "true". The user MAY include other content (e.g., a &BODY; element) in the confirmation stanza:</p>
<example caption="User completes negotiation and confirms session is open"><![CDATA[
<message type='normal'
from='romeo@montague.net/orchard'
@ -421,7 +427,7 @@ PENDING o---------------+
]]></example>
</section2>
</section1>
<section1 topic='Moving A Chat Session to a Different Resource' anchor='move'>
<section1 topic='Moving A Session to a Different Resource' anchor='move'>
<p>Either party MAY ask to continue the session using another of its resources. The requesting party does this by submitting a form with a "continue" field containing the value of the new resource:</p>
<example caption="One party asks to switch session to another of its resources"><![CDATA[
<message type='normal'
@ -455,10 +461,10 @@ PENDING o---------------+
</feature>
</message>
]]></example>
<p>Once the other party has accepted the switch then all stanzas sent within the chat session MUST be to or from the new resource. Note: Both parties MUST ensure that they comply with all the other chat session negotiation parameters that were previously agreed for this session.</p>
<p>Once the other party has accepted the switch then all stanzas sent within the session MUST be to or from the new resource. Note: Both parties MUST ensure that they comply with all the other session negotiation parameters that were previously agreed for this session.</p>
</section1>
<section1 topic='Renegotiating a Chat Session' anchor='renegotiate'>
<p>At any time during an existing chat session, either party MAY attempt to renegotiate the parameters of the session using the protocol described in <link url='#new'>Negotiating a New Chat Session</link>. The requesting party does this by sending a new &MESSAGE; stanza containing a feature negotiation form and a &THREAD; element with the <em>same</em> value as that of the existing chat session. Note: The "accept" field MUST NOT be included in a renegotiation form. The other fields MAY be different from the set of fields included in the initial session negotiation form.</p>
<section1 topic='Renegotiating a Session' anchor='renegotiate'>
<p>At any time during an existing session, either party MAY attempt to renegotiate the parameters of the session using the protocol described in <link url='#new'>Negotiating a New Session</link>. The requesting party does this by sending a new &MESSAGE; stanza containing a feature negotiation form and a &THREAD; element with the <em>same</em> value as that of the existing session. Note: The "accept" field MUST NOT be included in a renegotiation form. The other fields MAY be different from the set of fields included in the initial session negotiation form.</p>
<example caption="One party requests renegotiation"><![CDATA[
<message type='normal'
from='juliet@capulet.com/balcony'
@ -504,7 +510,7 @@ PENDING o---------------+
]]></example>
<p>Note: Both parties MUST consider the renegotiation to be complete as soon as the parameter acceptance message has been sent (or received).</p>
<p>Note: The requesting party SHOULD NOT send a renegotiation completion or cancelation message (see <link url='#new-complete'>Completing or Canceling the Negotiation</link>).</p>
<p>Note: Both parties MUST ensure that they continue to comply with all the chat session negotiation parameters that were not renegotiated but had previously been agreed for this session.</p>
<p>Note: Both parties MUST ensure that they continue to comply with all the session negotiation parameters that were not renegotiated but had previously been agreed for this session.</p>
<p>In order to reject the renegotiation, the other party shall send a message containing a data form of type "submit" with the 'renegotiate' field set to a value of "0" or "false".</p>
<example caption="Other party rejects renegotiation"><![CDATA[
<message type='normal'
@ -522,11 +528,11 @@ PENDING o---------------+
</feature>
</message>
]]></example>
<p>If the other party's client does not support one or more of the <em>required</em> features, it SHOULD return a &feature; error. If the other party's client supports <em>none</em> of the options for one or more <em>required</em> fields, it SHOULD return a &notacceptable; error (see <link url='#new-reject'>Rejecting a Chat Session</link>). Note: In any of these cases the existing negotiated chat session parameters are maintained. Either party MAY choose to terminate the chat session only as specified in the section <link url='#terminate'>Terminating a Chat Session</link>.</p>
<p>If the other party's client does not support one or more of the <em>required</em> features, it SHOULD return a &feature; error. If the other party's client supports <em>none</em> of the options for one or more <em>required</em> fields, it SHOULD return a &notacceptable; error (see <link url='#new-reject'>Rejecting a Session</link>). Note: In any of these cases the existing negotiated session parameters are maintained. Either party MAY choose to terminate the session only as specified in the section <link url='#terminate'>Terminating a Session</link>.</p>
</section1>
<section1 topic='Terminating a Chat Session' anchor='terminate'>
<p>In order to explicitly terminate a negotiated chat, the party that wishes to end the chat MUST do so by sending a &MESSAGE; containing a data form of type "submit". The &MESSAGE; stanza MUST contain a &THREAD; element with the same XML character data as the original initiation request. The data form containing a boolean field named "terminate" set to a value of "1" or "true".</p>
<example caption="One party terminates chat"><![CDATA[
<section1 topic='Terminating a Session' anchor='terminate'>
<p>In order to explicitly terminate a negotiated session, the party that wishes to end the session MUST do so by sending a &MESSAGE; containing a data form of type "submit". The &MESSAGE; stanza MUST contain a &THREAD; element with the same XML character data as the original initiation request. The data form containing a boolean field named "terminate" set to a value of "1" or "true".</p>
<example caption="One party terminates session"><![CDATA[
<message type='normal'
from='juliet@capulet.com/balcony'
to='romeo@montague.net/orchard'>
@ -541,9 +547,9 @@ PENDING o---------------+
</feature>
</message>
]]></example>
<p>Both parties MUST then consider the chat session to be ended.</p>
<p>The other party's client MAY explicitly acknowledge the termination of the chat by sending a &MESSAGE; containing a data form of type "result", and the value of the "terminate" field set to "1" or "true" (see <cite>Encrypted Session Negotiation</cite> for a practical example). The client MUST mirror the &THREAD; value it received.</p>
<example caption="Other party acknowledges chat termination"><![CDATA[
<p>Both parties MUST then consider the session to be ended.</p>
<p>The other party's client MAY explicitly acknowledge the termination of the session by sending a &MESSAGE; containing a data form of type "result", and the value of the "terminate" field set to "1" or "true" (see <cite>Encrypted Session Negotiation</cite> for a practical example). The client MUST mirror the &THREAD; value it received.</p>
<example caption="Other party acknowledges session termination"><![CDATA[
<message type='normal'
from='romeo@montague.net/orchard'
to='juliet@capulet.com/balcony'>
@ -561,10 +567,10 @@ PENDING o---------------+
</section1>
<section1 topic='Implementation Notes' anchor='impl'>
<section2 topic='Auto Accept or Reject' anchor='impl-auto'>
<p>A client MAY require a human user to approve each chat session negotiation request, however it is RECOMMENDED that it accepts or rejects automatically as many requests as possible, based on a set of user-configurable policies (see <link url='#secure-leak'>Presence Leaks</link>).</p>
<p>A client MAY require a human user to approve each session negotiation request, however it is RECOMMENDED that it accepts or rejects automatically as many requests as possible, based on a set of user-configurable policies (see <link url='#secure-leak'>Presence Leaks</link>).</p>
</section2>
<section2 topic='Persisting Sessions' anchor='impl-close'>
<p>Chat session negotiation sometimes requires the involvement of either or both human users, and if human input is required but the user is away then session establishment may be delayed indefinitely. So, in order to minimise the number of user interruptions and delays, clients SHOULD reuse existing chat sessions whenever possible. For example, a client SHOULD NOT terminate chat sessions unless the user is going offline, even if its user closes a chat window.</p>
<p>Session negotiation sometimes requires the involvement of either or both human users, and if human input is required but the user is away then session establishment may be delayed indefinitely. So, in order to minimise the number of user interruptions and delays, clients SHOULD reuse existing sessions whenever possible. For example, a client SHOULD NOT terminate sessions unless the user is going offline, even if its user closes a window associated with the session.</p>
</section2>
<section2 topic='Sharing Presence' anchor='impl-presence'>
<p>If so negotiated via the 'presence' field, two parties who do not have subscriptions to each other's presence (as specified in <cite>XMPP-IM</cite>) may share presence by sending directed presence after the session is negotiated.</p>
@ -577,8 +583,8 @@ PENDING o---------------+
<p>In accordance with the rules specified in <cite>XMPP-IM</cite>, sharing presence enables one party's server to send unavailable presence to the other party if the sending party goes offline for any reason.</p>
</section2>
<section2 topic='Unavailable Presence' anchor='impl-unavail'>
<p>If a party receives an XMPP presence stanza of type "unavailable" from the full JID (&FULLJID;) of the other party (i.e., the resource with which it has had an active session) during a chat session, the receiving party SHOULD assume that the other client will still be able to continue the session (perhaps it simply became "invisible", or it is persisting the state of the negotiated chat until it reconnects and receives "offline" messages).</p>
<p>However, the receiving party MAY assume that the other client will <em>not</em> be able to continue the session. <note>In general, if a party is not subscribing to the other party's presence then it will never assume the other party is is unable to continue a session.</note> In that case it MUST explicitly terminate the session (see <link url='#terminate'>Terminating a Chat Session</link>) -- since its assumption could be incorrect. If after terminating the session the receiving party later receives presence of type "available" from that same resource or another resource associated with the other party and the receiving party desires to restart the chat session, then it MUST initiate a new chat session (including a newly-generated ThreadID) with the other party. It MUST NOT renegotiate parameters for the terminated session. (Note: This is consistent with the handling of chat states as specified in <cite>XEP-0085</cite>.)</p>
<p>If a party receives an XMPP presence stanza of type "unavailable" from the full JID (&FULLJID;) of the other party (i.e., the resource with which it has had an active session) during a session, the receiving party SHOULD assume that the other client will still be able to continue the session (perhaps it simply became "invisible", or it is persisting the state of the negotiated session until it reconnects and receives "offline" messages).</p>
<p>However, the receiving party MAY assume that the other client will <em>not</em> be able to continue the session. <note>In general, if a party is not subscribing to the other party's presence then it will never assume the other party is is unable to continue a session.</note> In that case it MUST explicitly terminate the session (see <link url='#terminate'>Terminating a Session</link>) -- since its assumption could be incorrect. If after terminating the session the receiving party later receives presence of type "available" from that same resource or another resource associated with the other party and the receiving party desires to restart the session, then it MUST initiate a new session (including a newly-generated ThreadID) with the other party. It MUST NOT renegotiate parameters for the terminated session. (Note: This is consistent with the handling of chat states as specified in <cite>XEP-0085</cite>.)</p>
</section2>
<section2 topic='Mapping to SIP' anchor='impl-sip'>
<p>When mapping instant messaging flows to SIP, implementations SHOULD adhere to &xmppsimple;.</p>
@ -593,7 +599,7 @@ PENDING o---------------+
</section1>
<section1 topic='Security Considerations' anchor='security'>
<section2 topic='Presence Leaks' anchor='secure-leak'>
<p>If a contact does not share its presence information with a user through a presence subscription (see <cite>RFC 3921</cite>) or if it blocks outbound presence notifications to the user (see &xep0016;), then it will effectively expose its presence if it accepts the user's chat session negotiation request or returns an error to the user. Therefore, due care must be exercised in determining whether to accept the request or return an error. The contact's client SHOULD NOT automatically (i.e. without first asking the contact) either accept the user's request or return an error to the user unless the user is subscribed to the contact's presence and the contact is not blocking outbound presence notifications to the user. Note: There should be no need for the contact's client to consult the contact's block list (see &xep0191;), since if the user is on the block list then the contact would not receive the request from the user in the first place.</p>
<p>If a contact does not share its presence information with a user through a presence subscription (see <cite>RFC 3921</cite>) or if it blocks outbound presence notifications to the user (see &xep0016;), then it will effectively expose its presence if it accepts the user's session negotiation request or returns an error to the user. Therefore, due care must be exercised in determining whether to accept the request or return an error. The contact's client SHOULD NOT automatically (i.e. without first asking the contact) either accept the user's request or return an error to the user unless the user is subscribed to the contact's presence and the contact is not blocking outbound presence notifications to the user. Note: There should be no need for the contact's client to consult the contact's block list (see &xep0191;), since if the user is on the block list then the contact would not receive the request from the user in the first place.</p>
</section2>
<section2 topic='Localization' anchor='secure-local'>
<p>If a client is configured to show a request &lt;form/&gt; to a human user instead of responding automatically, it SHOULD replace the content of the &lt;title/&gt; element and of all label attributes of the known and registered &lt;field/&gt; and &lt;option/&gt; elements with its own localised versions before showing the form to the user -- even if the form already appears to be in the correct language.</p>
@ -612,20 +618,20 @@ PENDING o---------------+
<code caption='Registry Submission'><![CDATA[
<var>
<name>urn:xmpp:chatneg</name>
<desc>Support for Chat Session Negotiation and its FORM_TYPE</desc>
<desc>Support for Session Negotiation and its FORM_TYPE</desc>
<doc>XEP-0155</doc>
</var>
]]></code>
</section2>
<section2 topic='Field Standardization' anchor='registrar-formtype'>
<p>&xep0068; defines a process for standardizing the fields used within Data Forms qualified by a particular namespace. The following fields are registered for use in Chat Session Negotiation (see &FORMTYPES;):</p>
<p>&xep0068; defines a process for standardizing the fields used within Data Forms qualified by a particular namespace. The following fields are registered for use in Session Negotiation (see &FORMTYPES;):</p>
<code caption='Registry Submission'><![CDATA[
<form_type>
<name>urn:xmpp:chatneg</name>
<doc>XEP-0155</doc>
<desc>
Forms enabling negotation of a one-to-one
chat session between two entities.
session between two entities.
</desc>
<field
var='accept'