This document specifies a feature negotiation profile for initiating a one-to-one XMPP chat session.
NOTICE: This document is currently within Last Call or under consideration by the XMPP Council for advancement to the next stage in the JSF standards process.
Status:
Proposed
Type:
Standards Track
Number: 0155
Version: 0.11
Last Updated: 2006-11-03
Publishing Organization: Jabber Software Foundation
Approving Body: XMPP Council
Dependencies: XMPP Core, XMPP IM, XEP-0020, XEP-0068
Supersedes: None
Superseded By: None
Short Name: chatneg
Wiki Page: <http://wiki.jabber.org/index.php/Chat Session Negotiation (XEP-0155)>
Email:
ian.paterson@clientside.co.uk
JID:
ian@zoofy.com
Email:
stpeter@jabber.org
JID:
stpeter@jabber.org
This XMPP Extension Protocol is copyright 1999 - 2006 by the Jabber Software Foundation (JSF) and is in full conformance with the JSF's Intellectual Property Rights Policy <http://www.xmpp.org/extensions/ipr-policy.shtml>. This material may be distributed only subject to the terms and conditions set forth in the Creative Commons Attribution License (<http://creativecommons.org/licenses/by/2.5/>).
The preferred venue for discussion of this document is the Standards-JIG discussion list: <http://mail.jabber.org/mailman/listinfo/standards-jig>.
The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 3920) and XMPP IM (RFC 3921) specifications contributed by the Jabber Software Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The following keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
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 XMPP IM [1]). 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:
This proposal defines best practices for such a negotiation, re-using the protocol defined in Feature Negotiation [8].
In order to initiate a negotiated chat session, the initiating party ("user") sends a <message/> [9] stanza to the receiving party ("contact") containing a <feature/> child qualified by the 'http://jabber.org/protocol/feature-neg' namespace. The <message/> stanza MUST NOT contain a <body/> child element (as specified in RFC 3921 [10]). 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 "http://jabber.org/protocol/chatneg" and MUST contain a boolean field named "accept". [11] The inclusion of "otr" and "security" fields is also RECOMMENDED. Note: The options within any 'list-single' fields SHOULD appear in order of preference.
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 immediate chat session then the user SHOULD instruct the contact's server not to store the message for later delivery using the Advanced Message Processing [12] protocol.
In the following example of a negotiation request, Romeo requests a chat with Juliet and also queries her regarding whether she wants to enable all message logging (see Message Archiving [13]) [14] and support the XHTML-IM [15] and Chat State Notifications [16] extensions during this chat session. He also requires that they are both connected securely to their servers, and asks which language she prefers amoungst 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 XMPP Registrar Considerations section of this document.)
Example 1. User requests chat session
<message type='normal'
from='romeo@montague.net/orchard'
to='juliet@capulet.com'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<x xmlns='jabber:x:data' type='form'>
<title>Open chat with Romeo?</title>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/chatneg</value>
</field>
<field label='Accept this chat?' type='boolean' var='accept'>
<value>true</value>
<required/>
</field>
<field label='Off-The-Record?' type='list-single' var='otr'>
<value>0</value>
<option label='Allow message logging'>
<value>0</value>
</option>
<option label='Disable all message logging'>
<value>1</value>
</option>
<required/>
</field>
<field label='XHTML Formatting?'
type='list-single'
var='http://jabber.org/protocol/xhtml-im'>
<value>1</value>
<option label='Disable'><value>0</value></option>
<option label='Enable'><value>1</value></option>
</field>
<field label='Chat State Notifications?'
type='list-single'
var='http://jabber.org/protocol/chatstates'>
<value>1</value>
<option label='Disable'><value>0</value></option>
<option label='Enable'><value>1</value></option>
</field>
<field label='Minimum security level'
type='list-single'
var='security'>
<value>c2s</value>
<option label='Both parties must be securely connected to their servers'>
<value>c2s</value>
</option>
<required/>
</field>
<field label='Primary written language of the chat'
type='list-single'
var='language'>
<value>en</value>
<option label='English'><value>en</value></option>
<option label='Italiano'><value>it</value></option>
</field>
</x>
</feature>
<amp xmlns='http://jabber.org/protocol/amp'>
<rule action='drop' condition='deliver' value='stored'/>
</amp>
</message>
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 Advanced Message Processing rule included in the request specifies otherwise) then the server MAY store the request for later delivery.
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 immediate 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.
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.
If the request is accepted then the 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.
In the example below we assume that Juliet accepts the chat and specifies that she prefers to speak Italian with Romeo:
Example 2. Contact accepts chat and specifies parameters
<message type='normal'
from='juliet@capulet.com/balcony'
to='romeo@montague.net/orchard'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<x xmlns='jabber:x:data' type='submit'>
<field var='FORM_TYPE'>
<value>http://jabber.org/protocol/chatneg</value>
</field>
<field var='accept'><value>true</value></field>
<field var='otr'><value>0</value></field>
<field var='http://jabber.org/protocol/xhtml-im'>
<value>0</value>
</field>
<field var='http://jabber.org/protocol/chatstates'>
<value>0</value>
</field>
<field var='security'><value>c2s</value></field>
<field var='language'><value>it</value></field>
</x>
</feature>
</message>
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.
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 Presence Leaks). 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.
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:
Example 3. Contact declines offer and specifies reason
<message type='normal'
from='juliet@capulet.com/balcony'
to='romeo@montague.net/orchard'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<x xmlns='jabber:x:data' type='submit'>
<field var='FORM_TYPE'>
<value>http://jabber.org/protocol/chatneg</value>
</field>
<field var='accept'><value>0</value></field>
</x>
</feature>
<body>Sorry, can't chat now! How about tonight?</body>
</message>
If the contact's client does not support feature negotiation or does not support the "http://jabber.org/protocol/chatneg" FORM_TYPE, it SHOULD return a <service-unavailable/> error:
Example 4. Contact returns service unavailable error
<message type='error'
from='juliet@capulet.com/balcony'
to='romeo@montague.net/orchard'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<x xmlns='jabber:x:data' type='form'>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/chatneg</value>
</field>
...
</x>
</feature>
<error code='503' type='cancel'>
<service-unavailable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</message>
If the contact's client does not support one or more of the required features, it SHOULD return a <feature-not-implemented/> error, specifying the field(s) not implemented using the 'var' attribute of one or more <field/> child elements of a <feature/> child element of the <error/> scoped by the 'http://jabber.org/protocol/feature-neg' namespace:
Example 5. Contact returns feature not implemented error
<message type='error'
from='juliet@capulet.com/balcony'
to='romeo@montague.net/orchard'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<x xmlns='jabber:x:data' type='form'>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/chatneg</value>
</field>
...
</x>
</feature>
<error code='501' type='cancel'>
<feature-not-implemented xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<field var='otr'/>
</feature>
</error>
</message>
If the contact's client supports none of the options for one or more fields, it SHOULD return a <not-acceptable/> error, specifying the field(s) with unsupported options using the 'var' attribute of one or more <field/> child elements of a <feature/> child element of the <error/> scoped by the 'http://jabber.org/protocol/feature-neg' namespace:
Example 6. Contact returns options not acceptable error
<message type='error'
from='juliet@capulet.com/balcony'
to='romeo@montague.net/orchard'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<x xmlns='jabber:x:data' type='form'>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/chatneg</value>
</field>
...
</x>
</feature>
<error code='406' type='modify'>
<not-acceptable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<field var='security'/>
</feature>
</error>
</message>
If the contact accepted the chat (see Accepting a Chat) then the user MUST either complete or cancel the session negotitation. 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. [17] In any case the user's client SHOULD verify that the selected values are acceptable before completing the session negotitation - 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". It MAY contain the other fields received from the contact. The user MAY include other content (e.g., a <body/> element) in the confirmation stanza:
Example 7. User completes negotitation and confirms session is open
<message type='normal'
from='romeo@montague.net/orchard'
to='juliet@capulet.com/balcony'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<x xmlns='jabber:x:data' type='result'>
<field var='FORM_TYPE'>
<value>http://jabber.org/protocol/chatneg</value>
</field>
<field var='accept'><value>true</value></field>
<field var='otr'><value>0</value></field>
<field var='http://jabber.org/protocol/xhtml-im'>
<value>0</value>
</field>
<field var='http://jabber.org/protocol/chatstates'>
<value>0</value>
</field>
<field var='security'><value>c2s</value></field>
<field var='language'><value>it</value></field>
</x>
</feature>
<body>I forgot what I wanted to say!</body>
</message>
Alternatively, if the user decides to cancel the session negotitation then the client MUST reply with a data form containing the FORM_TYPE field and the "accept" field set to "0" or "false":
Example 8. User cancels session negotitation
<message type='normal'
from='romeo@montague.net/orchard'
to='juliet@capulet.com/balcony'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<x xmlns='jabber:x:data' type='result'>
<field var='FORM_TYPE'>
<value>http://jabber.org/protocol/chatneg</value>
</field>
<field var='accept'><value>0</value></field>
</x>
</feature>
</message>
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:
Example 9. One party asks to switch session to another of its resources
<message type='normal'
from='juliet@capulet.com/balcony'
to='romeo@montague.net/orchard'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<x xmlns='jabber:x:data' type='submit'>
<field var='FORM_TYPE'>
<value>http://jabber.org/protocol/chatneg</value>
</field>
<field var='continue'><value>PDA</value></field>
</x>
</feature>
</message>
The requesting party SHOULD NOT send stanzas within the session from either resource until the other party has accepted the switch to the new resource.
The other client SHOULD accept the switch automatically since the requesting party might otherwise be unable to continue the session:
Example 10. Other client accepts switch
<message type='normal'
from='romeo@montague.net/orchard'
to='juliet@capulet.com/balcony'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<x xmlns='jabber:x:data' type='result'>
<field var='FORM_TYPE'>
<value>http://jabber.org/protocol/chatneg</value>
</field>
<field var='continue'><value>PDA</value></field>
</x>
</feature>
</message>
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.
At any time during an existing chat session, either party MAY attempt to renegotiate the parameters of the session using the protocol described in Negotiating a New Chat Session. The requesting party does this by sending a new <message/> stanza containing a feature negotiation form and a <thread/> element with the same value as that of the existing chat session. Note: Although the "accept" field MUST be included in a renegotiation form, the other fields MAY be different from the set of fields included in the initial session negotitation form.
Example 11. One party requests renegotiation
<message type='normal'
from='juliet@capulet.com/balcony'
to='romeo@montague.net/orchard'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<x xmlns='jabber:x:data' type='form'>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/chatneg</value>
</field>
<field label='Accept this change?' type='boolean' var='accept'>
<value>true</value>
<required/>
</field>
<field label='Off-The-Record?' type='list-single' var='otr'>
<value>1</value>
<option label='Disable all message logging'>
<value>1</value>
</option>
<required/>
</field>
</x>
</feature>
</message>
The requesting party MAY continue to send stanzas within the session while it is waiting for the other party to either accept or reject the renegotiation.
Example 12. Other party accepts renegotiation and specifies parameters
<message type='normal'
from='romeo@montague.net/orchard'
to='juliet@capulet.com/balcony'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<x xmlns='jabber:x:data' type='submit'>
<field var='FORM_TYPE'>
<value>http://jabber.org/protocol/chatneg</value>
</field>
<field var='accept'><value>1</value></field>
<field var='otr'><value>1</value></field>
</x>
</feature>
</message>
Note: Both parties MUST consider the renegotiation to be complete as soon as the acceptance message has been sent (or received). The requesting party SHOULD NOT send a renegotiation completion or cancelation message (see Completing or Canceling the Negotitation).
Note: Both parties MUST ensure that they comply with all the chat session negotiation parameters that were not renegotiated but had previously been agreed for this session.
If the other party's client does not support one or more of the required features, it SHOULD return a <feature-not-implemented/> error instead, while if it supports none of the options for one or more fields, it SHOULD return a <not-acceptable/> error instead (see Rejecting a Chat). However, if the other party simply prefers to maintain the existing negotiated parameters then it SHOULD decline the renegotiation as in the example below. 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 Terminating a Chat.
Example 13. Other party declines renegotiation
<message type='normal'
from='romeo@montague.net/orchard'
to='juliet@capulet.com/balcony'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<x xmlns='jabber:x:data' type='submit'>
<field var='FORM_TYPE'>
<value>http://jabber.org/protocol/chatneg</value>
</field>
<field var='accept'><value>0</value></field>
</x>
</feature>
</message>
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".
Example 14. One party terminates chat
<message type='normal'
from='juliet@capulet.com/balcony'
to='romeo@montague.net/orchard'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<x xmlns='jabber:x:data' type='submit'>
<field var='FORM_TYPE'>
<value>http://jabber.org/protocol/chatneg</value>
</field>
<field var='terminate'><value>1</value></field>
</x>
</feature>
</message>
Both parties MUST then consider the chat session to be ended.
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". The client MUST mirror the <thread/> value it received.
Example 15. Other party acknowledges chat termination
<message type='normal'
from='romeo@montague.net/orchard'
to='juliet@capulet.com/balcony'>
<thread>ffd7076498744578d10edabfe7f4a866</thread>
<feature xmlns='http://jabber.org/protocol/feature-neg'>
<x xmlns='jabber:x:data' type='result'>
<field var='FORM_TYPE'>
<value>http://jabber.org/protocol/chatneg</value>
</field>
<field var='terminate'><value>1</value></field>
</x>
</feature>
</message>
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 Presence Leaks).
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.
If a party receives an XMPP presence stanza of type "unavailable" from the full JID (<node@domain.tld/resource>) 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).
However, the receiving party MAY assume that the other client will not be able to continue the session. [18] In that case it MUST explicitly terminate the session (see Terminating a Chat) - 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 XEP-0085.)
When mapping instant messaging flows to SIP, implementations SHOULD adhere to draft-saintandre-xmpp-simple [19].
In addition, the following mappings apply to chat session negotiation:
If a contact accepts a user's chat session negotiation request or returns an error to the user, the user will effectively discover the presence of the contact's resource. Due care must therefore be exercised in determining whether to accept the request or return an error. For examples, 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 subscribing to the contact's presence (and the contact's presence is not currently "invisible" to the user). Note: There should be no need for the contact's client to consult the contact's block list, since if the user is on the list then the contact would not receive any request messages from the user anyway.
If a client is configured to show a request <form/> to a human user instead of responding automatically, the client SHOULD replace the content of the <title/> element and of all label attributes of the <field/> and <option/> elements with it's own localised versions before showing the form to the client - even if the form already appears to be in the correct language. Note: If a client fails to localize the form then a malicious contact might, for examples, either switch the labels on the 'security' and 'otr' fields, or use the <title/> to mislead the user regarding the identity of the contact.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [20].
The XMPP Registrar [21] shall include 'http://jabber.org/protocol/chatneg' in its registry of Service Discovery features.
Registry Submission
<var>
<name>http://jabber.org/protocol/chatneg</name>
<desc>Support for Chat Session Negotiation and its FORM_TYPE</desc>
<doc>XEP-0155</doc>
</var>
Field Standardization for Data Forms [22] defines a process for standardizing the fields used within Data Forms qualified by a particular namespace. The following fields shall be registered for use in Chat Session Negotiation:
Registry Submission
<form_type>
<name>http://jabber.org/protocol/chatneg</name>
<doc>XEP-0155</doc>
<desc>
Forms enabling negotation of a one-to-one
chat session between two entities.
</desc>
<field
var='accept'
type='boolean'
label='Whether to accept the invitation'/>
<field
var='continue'
type='text-single'
label='Another resource with which to continue the session'/>
<field
var='http://jabber.org/protocol/chatstates'
type='list-single'
label='Whether must send Chat State Notifications per XEP-0085'>
<option label='May Send'>
<value>0</value>
</option>
<option label='Must Send'>
<value>1</value>
</option>
</field>
<field
var='http://jabber.org/protocol/xhtml-im'
type='list-single'
label='Whether allowed to use XHTML-IM formatting per XEP-0071'/>
<option label='Must Not Send'>
<value>0</value>
</option>
<option label='May Send'>
<value>1</value>
</option>
</field>
<field
var='language'
type='list-single'
label='Primary written language of the chat (each
value appears in order of preference and
conforms to RFC 4646 and the IANA registry)'/>
<field
var='otr'
type='list-single'
label='Off-The-Record'>
<option label='May log messages'>
<value>0</value>
</option>
<option label='Must disable absolutely all message
logging including automatic archiving
- see XEP-0136'>
<value>1</value>
</option>
</field>
<field
var='security'
type='list-single'
label='Minimum security level'>
<option label='Secure connections not required'>
<value>none</value>
</option>
<option label='Both parties must be securely connected to their servers'>
<value>c2s</value>
</option>
<option label='Both parties must be securely connected to each other'>
<value>e2e</value>
</option>
</field>
<field
var='terminate'
type='boolean'
label='Whether to terminate the session'/>
</form_type>
This proposal re-uses the format defined in XEP-0020 and therefore does not require a separate schema.
Thanks to Thomas Charron and Jean-Louis Seguineau for their feedback.
1. RFC 3921: Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence <http://www.ietf.org/rfc/rfc3921.txt>.
2. XEP-0116: Encrypted Sessions <http://www.xmpp.org/extensions/xep-0116.html>.
3. XEP-0136: Message Archiving <http://www.xmpp.org/extensions/xep-0136.html>.
4. XEP-0030: Service Discovery <http://www.xmpp.org/extensions/xep-0030.html>.
5. XEP-0115: Entity Capabilities <http://www.xmpp.org/extensions/xep-0115.html>.
6. RFC 3261: Session Initiation Protocol (SIP) <http://www.ietf.org/rfc/rfc3261.txt>.
7. 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 RFC 3261.
8. XEP-0020: Feature Negotiation <http://www.xmpp.org/extensions/xep-0020.html>.
9. 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).
10. RFC 3921: Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence <http://www.ietf.org/rfc/rfc3921.txt>.
11. In accordance with Section 3.2.2.1 of XML Schema Part 2: Datatypes, the allowable lexical representations for the xs:boolean datatype are the strings "0" and "false" for the concept 'false' and the strings "1" and "true" for the concept 'true'; implementations MUST support both styles of lexical representation.
12. XEP-0079: Advanced Message Processing <http://www.xmpp.org/extensions/xep-0079.html>.
13. XEP-0136: Message Archiving <http://www.xmpp.org/extensions/xep-0136.html>.
14. A client MUST NOT set the 'otr' field to 'true' unless it has confirmed that its server will allow it to switch off Automated Archiving (see Message Archiving).
15. XEP-0071: XHTML-IM <http://www.xmpp.org/extensions/xep-0071.html>.
16. XEP-0085: Chat State Notifications <http://www.xmpp.org/extensions/xep-0085.html>.
17. See Encrypted Sessions for examples of other instances where the user might find the values submitted by the contact unacceptable.
18. 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.
19. Basic Messaging and Presence Interoperability between the Extensible Messaging and Presence Protocol (XMPP) and Session Initiation Protocol (SIP) for Instant Messaging and Presence Leveraging Extensions (SIMPLE) <http://www.ietf.org/internet-drafts/draft-saintandre-xmpp-simple-05.txt> (work in progress).
20. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
21. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the Jabber Software Foundation. For further information, see <http://www.xmpp.org/registrar/>.
22. XEP-0068: Field Data Standardization for Data Forms <http://www.xmpp.org/extensions/xep-0068.html>.
Removed reason field; added new implementation notes; many clarifications including the handling of required fields
(ip)Defined handling of offline requests; specified localization of the title element and all labels; changed syntax of list of unacceptable fields; removed reason field from some examples; added confirmation message to initial negotiation; clarified the initial participating resources; removed id attributes.
(ip)Added language field; replaced secure field with security field; changed type of otr, XHTML and Chat State fields from boolean to list-single; added not-acceptable error; several clarifications.
(ip)Added continue field and optional terminate acknowledgement; specified renegotiation failure proceedure; added context to Introduction; changed unavailable presence handling; renamed logging field to otr.
(ip)Added secure field from XEP-0116.
(psa)Specified that a client must re-initiate if it receives presence unavailable; changed document type to Standards Track.
(psa)Added renegotiate use case.
(psa)Added terminate use case; further specified mapping to SIP.
(psa)Further specified use of id attribute and thread element.
(psa)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.
(psa)Initial version.
(psa)First draft.
(psa)END