RFC 8048
Interworking between the Session Initiation Protocol (SIP) and the Extensible Messaging and Presence Protocol (XMPP): Presence
Internet Engineering Task Force (IETF) P. Saint-Andre Request for Comments: 8048 Filament Obsoletes: 7248 December 2016 Category: Standards Track ISSN: 2070-1721 Interworking between the Session Initiation Protocol (SIP) and the Extensible Messaging and Presence Protocol (XMPP): PresenceAbstract
This document defines a bidirectional protocol mapping for the exchange of presence information between the Session Initiation Protocol (SIP) and the Extensible Messaging and Presence Protocol (XMPP). This document obsoletes RFC 7248. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc8048. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Intended Audience . . . . . . . . . . . . . . . . . . . . . . 4 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Architectural Assumptions . . . . . . . . . . . . . . . . . . 5 5. Presence Authorizations . . . . . . . . . . . . . . . . . . . 6 5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 6 5.2. XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . . 7 5.2.1. Requesting a Presence Authorization . . . . . . . . . 7 5.2.2. Refreshing a Notification Dialog . . . . . . . . . . 11 5.2.3. Cancelling a Presence Authorization . . . . . . . . . 11 5.3. SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . . 15 5.3.1. Requesting a Presence Authorization . . . . . . . . . 15 5.3.2. Refreshing a Notification Dialog . . . . . . . . . . 18 5.3.3. Cancelling a Presence Authorization . . . . . . . . . 19 6. Notifications of Presence Information . . . . . . . . . . . . 19 6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 19 6.2. XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . . 20 6.3. SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . . 25 7. Polling for Presence Information . . . . . . . . . . . . . . 27 7.1. XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . . 27 7.2. SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . . 28 8. Privacy and Security Considerations . . . . . . . . . . . . . 28 8.1. Amplification Attacks . . . . . . . . . . . . . . . . . . 28 8.2. Presence Leaks . . . . . . . . . . . . . . . . . . . . . 29 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 30 9.1. Normative References . . . . . . . . . . . . . . . . . . 30 9.2. Informative References . . . . . . . . . . . . . . . . . 31 Appendix A. Changes from RFC 7248 . . . . . . . . . . . . . . . 33 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 34 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction
Presence is information about the availability of an entity (such as network availability or availability for communication). Presence features in both SIP and XMPP involve several aspects: o A long-lived authorization for a user to receive notifications about a contact's presence across presence and notification sessions; such an authorization is formally requested by the user, approved (or not) by the contact, and often associated with a record in an address list or "buddy list". o An ephemeral presence session, during which the contact is online (i.e., available for interaction) and after which the contact is offline again. o An ephemeral notification session, during which the user requests presence notifications from the contact (these are implicit in XMPP, but explicit in SIP where they are managed by means of notification dialogs). o Notifications that are sent from the contact to the user for the life of either the contact's presence session or the user's notification session. Although specifications for both SIP and XMPP use the term "subscription", they do so in different ways. In SIP, a "subscription" is the specific mechanism whereby a subscriber (or an entity acting on the subscriber's behalf, such as a SIP Presence Server) requests presence notifications from the contact over a relatively short period of time, renewed as necessary to keep receiving presence notifications during a presence session. By contrast, in XMPP a "subscription" is essentially shorthand for a long-lived presence authorization. To prevent confusion, this document uses the term "notification dialog" for a SIP subscription and the term "presence authorization" for an XMPP subscription. In order to help ensure interworking between presence systems that conform to the instant messaging and presence protocol requirements [RFC2779], it is important to clearly define protocol mappings between such systems. Within the IETF, work has proceeded on two presence technologies: o Various extensions to the Session Initiation Protocol ([RFC3261]) for presence, in particular [RFC3856]
o The Extensible Messaging and Presence Protocol (XMPP), which
consists of a formalization of the core XML-streaming protocols
developed originally by the Jabber open-source community; the
relevant specifications are [RFC6120] for the XML-streaming layer
and [RFC6121] for basic presence and instant-messaging extensions
One approach to help ensure interworking between these protocols is
to map each protocol to the abstract semantics described in
[RFC3860]; however, apparently that approach has never been
implemented. The approach taken in this document is to directly map
semantics from one protocol to another (i.e., from SIP/SIMPLE (SIP
for Instant Messaging and Presence Leveraging Extensions) to XMPP and
vice versa), because that is how existing systems solve the
interworking problem.
The architectural assumptions underlying such direct mappings are
provided in [RFC7247], including mapping of addresses and error
conditions. The mappings specified in this document cover basic
presence functionality. Mapping of more advanced functionality
(e.g., so-called "rich presence") is out of scope for this document.
This document obsoletes RFC 7248.
2. Intended Audience
The documents in this series (which include [RFC7247], [RFC7572],
[RFC7573], and [RFC7702]) are intended for use by software developers
who have an existing system based on one of these technologies (e.g.,
SIP) and would like to enable communication from that existing system
to systems based on the other technology (e.g., XMPP). We assume
that readers are familiar with the core specifications for both SIP
[RFC3261] and XMPP [RFC6120], with the base document for this series
[RFC7247], and with the following presence-related specifications:
o "A Presence Event Package for the Session Initiation Protocol"
[RFC3856]
o "Presence Information Data Format (PIDF)" [RFC3863]
o "Extensible Messaging and Presence Protocol (XMPP): Instant
Messaging and Presence" [RFC6121]
o "SIP-Specific Event Notification" [RFC6665]
3. Terminology
A number of terms used here ("user", "contact", "notification", etc.) are explained in [RFC3261], [RFC3856], [RFC3857], [RFC6120], and [RFC6121]. This document uses some, but not all, of the presence- related terms defined in the Model for Presence and Instant Messaging [RFC2778]. In particular, the term "presence session" is used as described in [RFC6121] to mean a delimited time period during which an endpoint is online and available for communications. In flow diagrams, SIP traffic is shown using arrows such as "***>", whereas XMPP traffic is shown using arrows such as "...>". As in [RFC7247], the terms "SIP to XMPP Gateway" and "XMPP to SIP Gateway" are abbreviated as "S2X GW" and "X2S GW", respectively. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].4. Architectural Assumptions
The fundamental architectural assumptions underlying SIP-XMPP interworking are described in [RFC7247]. Note that, in SIP, there are two ways that presence services can be deployed on the server side: 1. Under this model, described most fully in [RFC3857], a dedicated SIP Presence Server handles events related to the presence event package. Instead of forwarding a SUBSCRIBE message to the SIP user, the Presence Server would inform the user of subscription activity using the 'presence.winfo' event package. The SIP User Agent would then authorize the subscribing contact through some interaction with the Presence Server (for instance, using XML Configuration Access Protocol (XCAP) [RFC4825]). Therefore, presence updates from the SIP User Agent would not be sent as NOTIFY messages to the XMPP user but as PUBLISH messages to the Presence Server, which would then generate NOTIFY messages to all active subscribers. 2. Under this model, a SIP Presence Server acts in proxy mode and merely passes through the SUBSCRIBE and NOTIFY messages to the SIP User Agent.
Because the behavior of the XMPP-to-SIP gateway is not changed by the SIP architectural model that is used, the diagrams and protocol flows in this document cover both options by labeling the end entity a "SIP User Agent or Presence Server".5. Presence Authorizations
5.1. Overview
Both XMPP and presence-aware SIP systems enable entities (often, but not necessarily, human users) to subscribe to the presence of other entities. XMPP presence is specified in [RFC6121]. Presence using a SIP event package is specified in [RFC3856]. As described in [RFC6121], XMPP presence authorizations are managed using XMPP <presence/> stanzas of type "subscribe", "subscribed", "unsubscribe", and "unsubscribed". The main states are: o "none" (neither the user nor the contact is subscribed to the other's presence information) o "from" (the contact will receive presence notifications from the user) o "to" (the contact will send presence notifications to the user) o "both" (both user and contact will receive each other's presence notifications) As described in [RFC3856], in SIP the subscriber does not explicitly request the creation or removal of presence authorizations. Rather, the authorizations are triggered by subscription activity. When a SIP user receives an initial SIP SUBSCRIBE event from a contact, the recipient's SIP User Agent or SIP Presence Server asks the user to make an authorization policy decision. This decision is recorded in the SIP User Agent or SIP Presence Server, so that in the future any notification dialogs from the contact are automatically approved. (Note that addresses for SIP users and contacts are most generally referenced by a Presence URI of the form <pres:user@domain> but might be referenced by a SIP or SIPS (Session Initiation Protocol Secure) URI of the form <sip:user@domain> or <sips:user@domain>; because, in practice, 'pres' URIs are rarely used, the examples in this document use 'sip' URIs.) In both SIP and XMPP, presence authorizations are long-lived (indeed permanent if not explicitly cancelled). In SIP, by default a notification session is typically short-lived unless explicitly extended (the default time-to-live of a SIP notification dialog is
3600 seconds, as specified in Section 6.4 of [RFC3856], so that a notification dialog needs to be explicitly refreshed in order for a user's notification session to last as long as the contact's presence session). In XMPP, a user's notification session with a contact is almost always automatically handled by the user's server based on the user's presence state (see [RFC6121] for details).5.2. XMPP to SIP
5.2.1. Requesting a Presence Authorization
The following diagram illustrates the protocol flow necessary to establish an authorization for an XMPP user to a receive presence notifications from a SIP contact, as further explained in the text and examples after the diagram. XMPP XMPP SIP SIP UA or Client Server Proxy Presence Server | + X2S GW | | | | | | | (F1) XMPP | | | | subscribe | | | |...........>| | | | | (F2) SIP | | | | SUBSCRIBE | | | |***********>| | | | | (F3) SIP | | | | SUBSCRIBE | | | |***********>| | | | (F4) SIP | | | | 200 OK | | | |<***********| | | (F5) SIP | | | | 200 OK | | | |<***********| | | | | (F6) SIP | | | | NOTIFY | | | | (pending) | | | |<***********| | | (F7) SIP | | | | NOTIFY | | | |<***********| | | | (F8) SIP | | | | 200 OK | | | |***********>| | | | | (F9) SIP | | | | 200 OK | | | |***********>|
| | | (F10) SIP |
| | | NOTIFY |
| | | (active) |
| | |<***********|
| | (F11) SIP | |
| | NOTIFY | |
| |<***********| |
| | (F12) SIP | |
| | 200 OK | |
| |***********>| |
| | | (F13) SIP |
| | | 200 OK |
| | |***********>|
| (F14) XMPP | | |
| subscribed | | |
|<...........| | |
| (F15) XMPP | | |
| presence | | |
|<...........| | |
| | | |
An XMPP user (e.g., juliet@example.com) asks for a presence
authorization by sending a request to a SIP contact (e.g.,
romeo@example.net), and the contact either accepts or declines the
request. If the SIP contact accepts the request, the XMPP user will
have a long-lived authorization to receive the SIP contact's presence
information until (1) the XMPP user unsubscribes or (2) the SIP
contact cancels the authorization. The request is encapsulated in a
<presence/> stanza of type "subscribe":
Example 1: XMPP User Subscribes to SIP Contact (F1)
| <presence from='juliet@example.com'
| to='romeo@example.net'
| type='subscribe'/>
Upon receiving such a <presence/> stanza, the XMPP server to which
Juliet has connected needs to determine the identity of the
domainpart in the 'to' address, which it does by following the
procedures explained in Section 5 of [RFC7247]. If the domain is a
SIP domain, the XMPP server will hand off the <presence/> stanza to
an associated XMPP-to-SIP gateway or connection manager that natively
communicates with presence-aware SIP proxies.
The XMPP-to-SIP gateway is then responsible for translating the XMPP request into a SIP SUBSCRIBE request addressed from the XMPP user to the SIP contact: Example 2: SIP Transformation of XMPP Presence Authorization Request (F2) | SUBSCRIBE sip:romeo@example.net SIP/2.0 | Via: SIP/2.0/TCP x2s.example.com;branch=z9hG4bKna998sk | From: <sip:juliet@example.com>;tag=j89d | Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C | Event: presence | Max-Forwards: 70 | CSeq: 1 SUBSCRIBE | Contact: <sip:juliet@example.com>;gr=yn0cl4bnw0yr3vym | Accept: application/pidf+xml | Expires: 3600 | Content-Length: 0 Once the SIP proxy has delivered the SIP SUBSCRIBE to the SIP User Agent or Presence Server (F3, no example shown), the SIP User Agent would then send a response indicating acceptance of the request: Example 3: SIP User Accepts Presence Authorization Request (F4) | SIP/2.0 200 OK | Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk | From: <sip:juliet@example.com>;tag=j89d | To: <sip:romeo@example.net>;tag=ffd2 | Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C | CSeq: 1 SUBSCRIBE | Contact: <sip:romeo@example.net>;gr=dr4hcr0st3lup4c | Expires: 3600 | Content-Length: 0 In accordance with Section 6.7 of [RFC3856], the XMPP-to-SIP gateway needs to consider the state to be "neutral" until it receives a NOTIFY message with a Subscription-State header [RFC6665] whose value is "active". Therefore, the SIP User Agent or Presence Server SHOULD immediately send such a NOTIFY message (see Section 6 below). If the XMPP-to-SIP gateway initially receives one or more NOTIFY messages with a Subscription-State header whose value is "pending" (F6), then it MUST respond to them on the SIP side but refrain from sending any presence stanzas from the SIP contact to the XMPP user.
Example 4: SIP User Agent or Presence Server Sends Presence Notification (F10) | NOTIFY sip:juliet@example.com SIP/2.0 | Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk | From: <sip:juliet@example.com>;tag=j89d | To: <sip:romeo@example.net>;tag=ffd2 | Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C | Event: presence | Subscription-State: active;expires=499 | Max-Forwards: 70 | CSeq: 2 NOTIFY | Content-Type: application/pidf+xml | Content-Length: 193 | | <?xml version='1.0' encoding='UTF-8'?> | <presence xmlns='urn:ietf:params:xml:ns:pidf' | entity='pres:romeo@example.net'> | <tuple id='ID-dr4hcr0st3lup4c'> | <status> | <basic>open</basic> | <show xmlns='jabber:client'>away</show> | </status> | </tuple> | </presence> Upon receiving the first NOTIFY with a state of active, the XMPP-to- SIP gateway returns a 200 OK to the SIP User Agent or Presence Server (F12, no example shown). The XMPP-to-SIP gateway also generates a <presence/> stanza of type "subscribed": Example 5: XMPP User Receives Acknowledgement from SIP Contact (F14) | <presence from='romeo@example.net' | to='juliet@example.com' | type='subscribed'/>
As described in Section 6, if this first NOTIFY in the notification session contains a body, then the XMPP-to-SIP gateway also generates a presence notification addressed to the XMPP user (if the NOTIFY does not contain a body, then the gateway would interpret it as unknown or "closed"): Example 6: XMPP User Receives Presence Notification from SIP Contact (F15) | <presence from='romeo@example.net/dr4hcr0st3lup4c' | to='juliet@example.com'/>5.2.2. Refreshing a Notification Dialog
It is the responsibility of the XMPP-to-SIP gateway to set the value of the Expires header and to periodically renew the notification dialog on the SIMPLE side of the gateway. For example, the XMPP-to- SIP gateway SHOULD send a new SUBSCRIBE request to the SIP contact whenever the XMPP user initiates a presence session with the XMPP server by sending initial presence to its XMPP server (this is functionally equivalent to sending an XMPP presence probe). The XMPP-to-SIP gateway SHOULD also send a new SUBSCRIBE request to the SIP contact sufficiently in advance of when the SIP notification dialog is scheduled to expire during the XMPP user's active presence session. The rules regarding SIP SUBSCRIBE requests for the purpose of establishing and refreshing a notification dialog are provided in [RFC6665]. Those rules also apply to XMPP-to-SIP gateways. Furthermore, an XMPP-to-SIP gateway MUST consider the XMPP presence authorization to be permanently cancelled (and so inform the XMPP user) if it receives a SIP response of 403, 489, or 603. By contrast, it is appropriate to consider a SIP response of 423 or 481 to be a transient error and to honor the long-lived XMPP presence authorization. [RFC6665] explains more detailed considerations about the handling of SIP responses in relation to notification dialogs and refreshes. Finally, see the Privacy and Security Considerations section (Section 8) for important information and requirements regarding the security implications of notification refreshes.5.2.3. Cancelling a Presence Authorization
The following diagram illustrates the protocol flow by which an XMPP user cancels her outbound presence authorization with a SIP contact (i.e., indicates that she no longer wishes to be authorized to see the SIP contact's presence). As can be seen, SIMPLE itself does not
have a construct that enables a user to cancel her outbound presence authorization (however, in many SIP/SIMPLE implementations she could use a technology such as XCAP [RFC4825] to remove the contact from her address list); therefore, this flow instead results in the cancellation of the user's notification dialog (with the implication on the XMPP side that the user will not request a subsequent notification dialog). Additional details are explained in the text and examples after the diagram. XMPP XMPP SIP SIP UA or Client Server Proxy Presence Server | + X2S GW | | | | | | | (F16) XMPP | | | |unsubscribe | | | |...........>| | | | | (F17) SIP | | | | SUBSCRIBE | | | | Expires: 0 | | | |***********>| | | | | (F18) SIP | | | | SUBSCRIBE | | | | Expires: 0 | | | |***********>| | | | (F19) SIP | | | | 200 OK | | | |<***********| | | (F20) SIP | | | | 200 OK | | | |<***********| | | (F21) XMPP | | | |unsubscribed| | | |<...........| | | | | (F22) SIP | | | | NOTIFY | | | | terminated | | | |***********>| | | | | (F23) SIP | | | | NOTIFY | | | | terminated | | | |***********>| | | | (F24) SIP | | | | 200 OK | | | |<***********| | | (F25) SIP | | | | 200 OK | | | |<***********| | | | | |
At any time after subscribing, the XMPP user can indicate that she no
longer wishes to be authorized to receive presence notifications from
the contact. This is done by sending a <presence/> stanza of type
"unsubscribe":
Example 7: XMPP User Unsubscribes from SIP Contact (F16)
| <presence from='juliet@example.com'
| to='romeo@example.net'
| type='unsubscribe'/>
The XMPP-to-SIP gateway is responsible for translating the XMPP
unsubscribe command into a SIP SUBSCRIBE request with the Expires
header set to a value of zero ("0"):
Example 8: SIP Transformation of XMPP Unsubscribe (F17)
| SUBSCRIBE sip:romeo@example.net SIP/2.0
| Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk
| From: <sip:juliet@example.com>;tag=j89d
| To: <sip:romeo@example.com>;tag=ffd2
| Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C
| Event: presence
| Max-Forwards: 70
| CSeq: 42 SUBSCRIBE
| Contact: <sip:juliet@example.com>;gr=yn0cl4bnw0yr3vym
| Accept: application/pidf+xml
| Expires: 0
| Content-Length: 0
Upon receiving the SIP 200 OK acknowledging the cancellation, the
XMPP-to-SIP gateway SHOULD send a <presence/> stanza of type
"unsubscribed" addressed to the XMPP user:
Example 9: XMPP User Receives Unsubscribed Notification (F21)
| <presence from='romeo@example.net'
| to='juliet@example.com'
| type='unsubscribed'/>
In accordance with Section 4.4.1 of [RFC6665], the XMPP-to-SIP
gateway is then responsible for sending a NOTIFY message with a
Subscription-State header of "terminated" in order to formally end
the XMPP user's outbound presence authorization and the associated
SIP dialog.
Example 10: XMPP-to-SIP Gateway Sends Presence Notification to Terminate Authorization (F25) | NOTIFY sip:juliet@example.com SIP/2.0 | Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk | From: <sip:juliet@example.com>;tag=j89d | To: <sip:romeo@example.net>;tag=ffd2 | Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C | Event: presence | Subscription-State: terminated | Max-Forwards: 70 | CSeq: 43 NOTIFY | Content-Length: 0 Note: When the XMPP user cancels her outbound presence authorization to the SIP user, any inbound authorization that she might have approved (thus enabling the SIP user to see her presence) remains unchanged.
5.3. SIP to XMPP
5.3.1. Requesting a Presence Authorization
The following diagram illustrates the protocol flow for establishing an authorization for a SIP user to receive presence notifications from an XMPP contact, as further explained in the text and examples after the diagram. SIP SIP XMPP XMPP UA Proxy Server Client | + S2X GW | | | | | | | (F26) SIP | | | | SUBSCRIBE | | | |**********>| | | | (F27) SIP | | | | 200 OK | | | |<**********| | | | | (F28) XMPP | | | | subscribe | | | |...........>| | | | | (F29) XMPP| | | | subscribe | | | |..........>| | | | (F30) XMPP| | | | subscribed| | | |<..........| | | (F31) XMPP | | | | subscribed | | | |<...........| | | (F32) SIP | | | | NOTIFY | | | | (active) | | | |<**********| | | | (F33) SIP | | | | 200 OK | | | |**********>| | | | | | |
A SIP User Agent initiates a presence authorization to an XMPP contact's presence information by sending a SIP SUBSCRIBE request to the contact. The following is an example of such a request: Example 11: SIP User Subscribes to XMPP Contact (F26) | SUBSCRIBE sip:juliet@example.com SIP/2.0 | Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk | From: <sip:romeo@example.net>;tag=xfg9 | To: <sip:juliet@example.net> | Call-ID: AA5A8BE5-CBB7-42B9-8181-6230012B1E11 | Event: presence | Max-Forwards: 70 | CSeq: 1 SUBSCRIBE | Contact: <sip:romeo@example.net>;gr=dr4hcr0st3lup4c | Accept: application/pidf+xml | Content-Length: 0 Notice that the Expires header was not included in the SUBSCRIBE request; this means that the default value of 3600 (i.e., 3600 seconds = 1 hour) applies. Upon receiving the SUBSCRIBE, the SIP proxy needs to determine the identity of the domain portion of the Request-URI, which it does by following the procedures explained in Section 5 of [RFC7247]. If the domain is an XMPP domain, the SIP proxy will hand off the SUBSCRIBE to an associated SIP-to-XMPP gateway or connection manager that natively communicates with XMPP servers. The SIP-to-XMPP gateway is then responsible for translating the SUBSCRIBE into an XMPP authorization request addressed from the SIP user to the XMPP contact: Example 12: XMPP Transformation of SIP SUBSCRIBE (F28) | <presence from='romeo@example.net' | to='juliet@example.com' | type='subscribe'/> In accordance with [RFC6121], the XMPP user's server delivers the presence authorization request to the XMPP user (or, if an authorization already exists in the XMPP user's roster, the XMPP server SHOULD auto-reply with a <presence/> stanza of type 'subscribed').
The "happy path" is for the XMPP user to approve the presence authorization request by generating a <presence/> stanza of type "subscribed" (F30). The XMPP server then stamps that presence stanza with the 'from' address of the XMPP contact and sends it to the SIP user (F31). Upon receiving the stanza, the SIP-to-XMPP gateway generates an empty SIP NOTIFY message with a Subscription-State header [RFC6665] of "active", which serves to inform the SIP user that the presence authorization request has been approved (F32). Example 13: XMPP User Approves Presence Authorization Request (F31) | <presence from='juliet@example.com' | to='romeo@example.net' | type='subscribed'/> Example 14: Presence Authorization Request Approved (F32) | NOTIFY sip:romeo@example.net SIP/2.0 | Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk | From: <sip:romeo@example.net>;tag=xfg9 | To: <sip:juliet@example.com>;tag=ur93 | Call-ID: AA5A8BE5-CBB7-42B9-8181-6230012B1E11 | Event: presence | Subscription-State: active | Max-Forwards: 70 | CSeq: 2 NOTIFY | Content-Length: 0 As an alternative to the "happy path", the XMPP user could decline the presence authorization request by generating a <presence/> stanza of type "unsubscribed". The XMPP server would stamp that presence stanza with the 'from' address of the XMPP contact and would send it to the SIP user. The SIP-to-XMPP gateway then transforms that stanza into an empty SIP NOTIFY with a Subscription-State header [RFC6665] of "terminated" and a reason of "rejected": Example 15: XMPP User Rejects Presence Authorization Request | <presence from='juliet@example.com' | to='romeo@example.net' | type='unsubscribed'/>
Example 16: Presence Authorization Request Rejected | NOTIFY sip:romeo@example.net SIP/2.0 | Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk | From: <sip:romeo@example.net>;tag=xfg9 | To: <sip:juliet@example.com>;tag=ur93 | Call-ID: AA5A8BE5-CBB7-42B9-8181-6230012B1E11 | Event: presence | Subscription-State: terminated;reason=rejected | Max-Forwards: 70 | CSeq: 2 NOTIFY | Content-Length: 05.3.2. Refreshing a Notification Dialog
For as long as a SIP user is online and wishes to maintain a notification session (i.e., receive presence notifications from the XMPP contact), the user's SIP User Agent is responsible for periodically refreshing the notification dialog by sending an updated SUBSCRIBE request with an appropriate value for the Expires header. In response, the presence-aware SIP-to-XMPP gateway sends a SIP NOTIFY message to the SIP User Agent (per [RFC6665]); if the SIP-to- XMPP gateway has meaningful information about the availability state of the XMPP user (e.g., obtained from the core presence session in the XMPP server or learned by sending a presence probe as described under Section 7), then the NOTIFY communicates that information (e.g., by including a PIDF body [RFC3863] with the relevant data), whereas if the SIP-to-XMPP gateway does not have meaningful information about the availability state of the XMPP user, then the NOTIFY MUST be empty as allowed by [RFC6665].
5.3.3. Cancelling a Presence Authorization
SIP does not directly have a construct for cancelling an outbound presence authorization. Instead, the SIP user would terminate his outbound notification dialog by sending a SUBSCRIBE message whose Expires header is set to a value of zero ("0") and then never renew it: Example 17: SIP User Terminates Notification Dialog | SUBSCRIBE sip:juliet@example.com SIP/2.0 | Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk | From: <sip:romeo@example.net>;tag=xfg9 | To: <sip:juliet@example.com>;tag=ur93 | Call-ID: AA5A8BE5-CBB7-42B9-8181-6230012B1E11 | Event: presence | Max-Forwards: 70 | CSeq: 66 SUBSCRIBE | Contact: <sip:romeo@example.net>;gr=dr4hcr0st3lup4c | Expires: 0 | Content-Length: 0 A presence-aware SIP-to-XMPP gateway is then responsible for: 1. Sending a SIP NOTIFY request to the SIP User Agent containing a PIDF document specifying that the XMPP contact now has a basic status of "closed", including a Subscription-State header [RFC6665] of "terminated" with a reason of "timeout". 2. Sending an XMPP <presence/> stanza of type "unavailable" to the XMPP contact. Note: When the SIP user cancels his outbound presence authorization to the XMPP user, any inbound authorization that he might have approved (enabling the XMPP user to see his presence) remains unchanged.
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