In order to initiate a negotiated session, the initiating party ("user") sends a &MESSAGE; 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). 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 &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:ssn" 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.

Note: Sessions may be conducted between entities who are never online at the same time. However, if the user is interested only in an immediate session then the user SHOULD instruct the contact's server not to store the message for later delivery (see &xep0160;) using the &xep0079; protocol.

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;) 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 Message Archiving)., whether she wants to temporarily share presence for this session (see the Sharing Presence 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 Encrypted Session Negotiation). 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. For definitions of these fields, refer to the Defined Parameters section of this document.

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 session request, a contact finds that the request had been stored for later delivery, and if the contact is interested only in an immediate session, then it SHOULD initiate a new stanza 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. The &MESSAGE; stanza MUST NOT contain a &BODY; child element.

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.

In the example below we assume that Juliet accepts the session and specifies that she prefers to speak Italian with Romeo:

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 session.

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 via the "reason" field (which is of type "text-single"). The &MESSAGE; stanza MUST NOT contain a &BODY; child element.

If the contact's client does not support feature negotiation or does not support the "urn:xmpp:ssn" FORM_TYPE, it SHOULD return a &unavailable; error:

If the contact's client does not support one or more of the required features, it SHOULD return a &feature; 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:

If the contact's client supports none of the options for one or more required fields, it SHOULD return a ¬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:

If the contact accepted the session (see Accepting a Session) then the user MUST either complete or cancel the stanza 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. See Encrypted Session Negotiation for example of other instances where the user might find the values submitted by the contact unacceptable. In any case the user's client SHOULD verify that the selected values are acceptable before completing the stanza 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 an explanation or reason via the "reason" field (which is of type "text-single"). The &MESSAGE; stanza MUST NOT contain a &BODY; child element.

Alternatively, if the user decides to cancel the stanza session negotiation then the client MUST reply with a data form containing the FORM_TYPE field and the "accept" field set to "0" or "false":

A client MAY require a human user to approve each stanza 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).

Stanza 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.

If so negotiated via the 'presence' field, two parties who do not have subscriptions to each other's presence (as specified in XMPP-IM) may share presence by sending directed presence after the session is negotiated.

In accordance with the rules specified in XMPP-IM, sharing presence enables one party's server to send unavailable presence to the other party if the sending party goes offline for any reason.

If a party receives an XMPP presence stanza of type "unavailable" from the full JID &LOCALFULL; 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).

However, the receiving party MAY assume that the other client will not be able to continue the session. 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. In that case it MUST explicitly terminate the session (see Terminating a Session) -- since its assumption could be incorrect. If after terminating the session the receiving party later receives available presence (i.e., a &PRESENCE; stanza with no 'type' attribute) 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 XEP-0085.)

When mapping instant messaging flows to SIP, implementations SHOULD adhere to &rfc7572;.

In addition, the following mappings apply to chat session negotiation:

• Initiation of a negotiated chat session maps to the semantics of the SIP INVITE method.
• Renegotiation of a negotiated chat session also maps to the semantics of the SIP INVITE method.
• Termination of a negotiated chat session maps to the semantics of the SIP BYE method.
• The XMPP &THREAD; value maps to the semantics of the SIP Call-ID attribute.

If a contact does not share its presence information with a user through a presence subscription (see RFC 3921) 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 stanza 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.

If a client is configured to show a request <form/> to a human user instead of responding automatically, it SHOULD replace the content of the <title/> element and of all label attributes of the known and registered <field/> and <option/> 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.

Note: If a client fails to localize the form, a malicious contact might, for example, either switch the labels on the 'security' and 'logging' fields, or use the <title/> to mislead the user regarding the identity of the contact.

The ®ISTRAR; includes 'urn:xmpp:ssn' in its registry of protocol namespaces (see &NAMESPACES;).

The XMPP Registrar includes 'urn:xmpp:ssn' in its registry of Service Discovery features (see &DISCOFEATURES;).

&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 Stanza Session Negotiation (see &FORMTYPES;):