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rfc:rfc8048

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): Presence

Abstract

 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.

Saint-Andre Standards Track [Page 1] RFC 8048 SIP-XMPP Interworking: Presence December 2016

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

Saint-Andre Standards Track [Page 2] RFC 8048 SIP-XMPP Interworking: Presence December 2016

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]

Saint-Andre Standards Track [Page 3] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 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]

Saint-Andre Standards Track [Page 4] RFC 8048 SIP-XMPP Interworking: Presence December 2016

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.

Saint-Andre Standards Track [Page 5] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 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

Saint-Andre Standards Track [Page 6] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 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     |
  |            |            |***********>|

Saint-Andre Standards Track [Page 7] RFC 8048 SIP-XMPP Interworking: Presence December 2016

  |            |            | (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.

Saint-Andre Standards Track [Page 8] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 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.

Saint-Andre Standards Track [Page 9] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 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'/>

Saint-Andre Standards Track [Page 10] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 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

Saint-Andre Standards Track [Page 11] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 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     |            |
  |            |<***********|            |
  |            |            |            |

Saint-Andre Standards Track [Page 12] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 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.

Saint-Andre Standards Track [Page 13] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 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.

Saint-Andre Standards Track [Page 14] RFC 8048 SIP-XMPP Interworking: Presence December 2016

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    |            |           |
  |**********>|            |           |
  |           |            |           |

Saint-Andre Standards Track [Page 15] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 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').

Saint-Andre Standards Track [Page 16] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 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'/>

Saint-Andre Standards Track [Page 17] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 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: 0

5.3.2. 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].

Saint-Andre Standards Track [Page 18] RFC 8048 SIP-XMPP Interworking: Presence December 2016

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.

6. Notifications of Presence Information

6.1. Overview

 Both XMPP and presence-aware SIP systems enable entities (often, but
 not necessarily, human users) to send presence notifications to other
 entities.  At its most basic, the term "presence" refers to
 information about an entity's "on/off" availability for communication
 on a network.  Often, this basic concept is supplemented by
 information that further specifies the entity's context or status
 while available for communication; these availability states commonly

Saint-Andre Standards Track [Page 19] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 include "away" and "do not disturb".  Some systems and protocols
 extend the concepts of presence and availability even further and
 refer to any relatively ephemeral information about an entity as a
 kind of presence; categories of such "extended presence" include
 geographical location (e.g., GPS coordinates), user mood (e.g.,
 grumpy), user activity (e.g., walking), and ambient environment
 (e.g., noisy).  This document focuses on the "least common
 denominator" of network availability only.  Future documents might
 address broader notions of presence, including availability states
 and extended presence or so-called "rich presence" as defined in
 specifications such as [RFC4480], [XEP-0107], and [XEP-0108].
 The XMPP instant messaging and presence specification [RFC6121]
 defines how XMPP <presence/> stanzas can indicate availability (via
 the absence of a 'type' attribute) or lack of availability (via a
 'type' attribute with a value of "unavailable").  SIP presence using
 a SIP event package for presence is specified in [RFC3856].
 As described in [RFC6121], XMPP presence information about an entity
 is communicated by means of an XML <presence/> stanza sent over an
 XML stream.  This document assumes that such a <presence/> stanza is
 sent from an XMPP client to an XMPP server over an XML stream
 negotiated between the client and the server, and that the client is
 controlled by a human user.  In general, XMPP presence is sent by the
 user's client to the user's server and then broadcast to all entities
 who are subscribed to the user's presence information.
 As described in [RFC3856], presence information about an entity is
 communicated by means of a SIP NOTIFY event sent from a SIP User
 Agent to an intended recipient who is most generally referenced by a
 Presence URI of the form <pres:user@domain> but who might be
 referenced by a SIP or SIPS URI of the form <sip:user@domain> or
 <sips:user@domain>.

6.2. XMPP to SIP

 When Juliet interacts with her XMPP client to modify her presence
 information (or when her client automatically updates her presence
 information, e.g., via an "auto-away" feature), her client generates
 an XMPP <presence/> stanza.  The syntax of the <presence/> stanza,
 including required and optional elements and attributes, is defined
 in [RFC6121].  The following is an example of such a stanza:
 Example 18: XMPP User Sends Presence Notification
 |  <presence from='juliet@example.com/yn0cl4bnw0yr3vym'/>

Saint-Andre Standards Track [Page 20] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 Upon receiving such a stanza, the XMPP server to which Juliet has
 connected broadcasts it to all subscribers who are authorized to
 receive presence notifications from Juliet and who have indicated a
 current interest in receiving notifications (this is similar to the
 SIP NOTIFY method).  For each subscriber, broadcasting the presence
 notification involves adding the 'to' address of the subscriber and
 then either delivering the notification to a local recipient (if the
 hostname in the subscriber's address matches one of the hostnames
 serviced by the XMPP server) or attempting to route it to the foreign
 domain that services the hostname in the subscriber's address.  If
 the notification is bound for an address at a foreign domain, the
 XMPP server needs to determine the identity of the domainpart in the
 'to' address, which it does by following the procedures discussed in
 [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
 proxy.
 The XMPP-to-SIP gateway is then responsible for translating the XMPP
 <presence/> stanza into a SIP NOTIFY request (including the PIDF
 document) from the XMPP user to the SIP contact.
 Example 19: SIP Transformation of XMPP Presence Notification
 |  NOTIFY sip:juliet@example.com SIP/2.0
 |  Via: SIP/2.0/TCP x2s.example.com;branch=z9hG4bKna998sk
 |  From: <sip:juliet@example.com>;tag=gh19
 |  To: <sip:romeo@example.net>
 |  Contact: <sip:juliet@example.com>;gr=yn0cl4bnw0yr3vym
 |  Call-ID: 2B44E147-3B53-45E4-9D48-C051F3216D14
 |  Event: presence
 |  Subscription-State: active;expires=599
 |  Max-Forwards: 70
 |  CSeq: 2 NOTIFY
 |  Content-Type: application/pidf+xml
 |  Content-Length: 192
 |
 |  <?xml version='1.0' encoding='UTF-8'?>
 |  <presence xmlns='urn:ietf:params:xml:ns:pidf'
 |            entity='pres:juliet@example.com'>
 |    <tuple id='ID-yn0cl4bnw0yr3vym'>
 |      <status>
 |        <basic>open</basic>
 |        <show xmlns='jabber:client'>away</show>
 |      </status>
 |    </tuple>
 |  </presence>

Saint-Andre Standards Track [Page 21] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 The mapping of XMPP syntax elements to SIP syntax elements MUST be as
 shown in the following table.  (Mappings for elements not mentioned
 are undefined and therefore are a matter of implementation.)
    +-----------------------------+---------------------------+
    |  XMPP Element or Attribute  |  SIP Header or PIDF Data  |
    +-----------------------------+---------------------------+
    |  <presence/> stanza         |  "Event: presence" (1)    |
    +-----------------------------+---------------------------+
    |  XMPP resource identifier   |  tuple 'id' attribute (2) |
    +-----------------------------+---------------------------+
    |  from                       |  From                     |
    +-----------------------------+---------------------------+
    |  id                         |  no mapping (3)           |
    +-----------------------------+---------------------------+
    |  to                         |  To                       |
    +-----------------------------+---------------------------+
    |  type                       |  basic status (4) (5)     |
    +-----------------------------+---------------------------+
    |  xml:lang                   |  Content-Language         |
    +-----------------------------+---------------------------+
    |  <priority/>                |  priority for tuple (6)   |
    +-----------------------------+---------------------------+
    |  <show/>                    |  no mapping (7)           |
    +-----------------------------+---------------------------+
    |  <status/>                  |  <note/>                  |
    +-----------------------------+---------------------------+
 Table 1: Presence Syntax Mapping from XMPP to SIP
 Note the following regarding these mappings:
 1.  Only an XMPP <presence/> stanza that lacks a 'type' attribute or
     whose 'type' attribute has a value of "unavailable" is mapped by
     an XMPP-to-SIP gateway to a SIP NOTIFY request, because those are
     the only <presence/> stanzas that represent notifications.
 2.  The PIDF schema defines the tuple 'id' attribute as having a
     datatype of "xs:ID"; because this datatype is more restrictive
     than the "xs:string" datatype for XMPP resourceparts (in
     particular, a number is not allowed as the first character of an
     ID), it is RECOMMENDED to prepend the resourcepart with "ID-" or
     some other alphabetic string when mapping from XMPP to SIP.
 3.  In practice, XMPP <presence/> stanzas often do not include the
     'id' attribute.

Saint-Andre Standards Track [Page 22] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 4.  Because the lack of a 'type' attribute indicates that an XMPP
     entity is available for communication, the XMPP-to-SIP gateway
     MUST map that information to a PIDF basic status of "open".
     Because a 'type' attribute with a value of "unavailable"
     indicates that an XMPP entity is not available for communication,
     the XMPP-to-SIP gateway MUST map that information to a PIDF
     <basic/> status of "closed".
 5.  When the XMPP-to-SIP gateway receives an XMPP presence of type
     "unavailable" from the XMPP contact, it sends a SIP NOTIFY
     request from the XMPP contact to the SIP User Agent containing a
     PIDF document specifying that the XMPP contact now has a basic
     status of "closed".
 6.  The value of the XMPP <priority/> element is an integer between
     -128 and +127, whereas the value of the PIDF <contact/> element's
     'priority' attribute is a decimal number from zero to one
     inclusive, with a maximum of three decimal places.  If the value
     of the XMPP <priority/> element is negative, an XMPP-to-SIP
     gateway MUST NOT map the value.  If an XMPP-to-SIP gateway maps
     positive values, it SHOULD treat XMPP priority 0 as PIDF priority
     0 and XMPP priority 127 as PIDF priority 1, mapping intermediate
     values appropriately so that they are unique (e.g., XMPP priority
     1 to PIDF priority 0.007, XMPP priority 2 to PIDF priority 0.015,
     and so on up through mapping XMPP priority 126 to PIDF priority
     0.992; note that this is an example only and that the exact
     mapping is up to the implementation).
 7.  Some implementations support custom extensions to encapsulate
     detailed information about availability; however, there is no
     need to standardize a PIDF extension for this purpose, because
     PIDF is already extensible, and thus the XMPP <show/> element
     (qualified by the 'jabber:client' namespace) can be included
     directly in the PIDF XML.  The examples in this document
     illustrate this usage, which is RECOMMENDED.  The most useful
     values are likely "away" and "dnd" (both defined in [RFC6121]),
     although note that in XMPP a value of "dnd" (short for "do not
     disturb") merely means "busy" and does not imply that a server or
     client ought to block incoming traffic while the user is in that
     state.  Naturally, an XMPP-to-SIP gateway can choose to translate
     a custom extension into an established value of the XMPP <show/>
     element (as defined in [RFC6121]) or translate a <show/> element
     into a custom extension that the XMPP-to-SIP gateway knows is
     supported by the SIP User Agent of the intended recipient.
     Unfortunately, this behavior does not guarantee that information
     will not be lost; to help prevent information loss, an XMPP-to-
     SIP gateway ought to include both the <show/> element and the
     custom extension if it cannot suitably translate the custom value

Saint-Andre Standards Track [Page 23] RFC 8048 SIP-XMPP Interworking: Presence December 2016

     into a <show/> value.  However, there is no guarantee that the
     SIP receiver will render a standard XMPP <show/> value or custom
     extension.
 In XMPP, a user can connect with multiple clients at the same time
 [RFC6120]; for presence notification purposes [RFC6121], each client
 is associated with a distinct resourcepart [RFC7622] and a contact's
 SIP User Agent will receive a separate presence notification from
 each of the XMPP user's clients.  Although the interpretation of
 multiple presence notifications from a single user is a matter of
 implementation by the contact's SIP User Agent, typically the SIP
 User Agent will show the "most available" status for the contact
 (e.g., if the user is online with three devices, one of which is
 "away", one of which is in "do not disturb" mode, and one of which is
 "available" with no qualifications, then the status shown will simply
 be "available").  In SIP, it is reasonable for a SIP User Agent to
 model multiple presence notifications from an XMPP user in the same
 way that it would handle multiple tuples from a SIP user.

Saint-Andre Standards Track [Page 24] RFC 8048 SIP-XMPP Interworking: Presence December 2016

6.3. SIP to XMPP

 When Romeo changes his presence, his SIP User Agent generates a SIP
 NOTIFY request for any contacts that have presence authorizations and
 notification sessions.  The syntax of the NOTIFY request is defined
 in [RFC3856].  The following is an example of such a request:
 Example 20: SIP User Sends Presence Notification
 |  NOTIFY sip:romeo@example.net SIP/2.0
 |  Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk
 |  From: <sip:romeo@example.net>;tag=yt66
 |  To: <sip:juliet@example.com>;tag=bi54
 |  Contact: <sip:romeo@example.net>;gr=dr4hcr0st3lup4c
 |  Call-ID: C33C6C9D-0F4A-42F9-B95C-7CE86B526B5B
 |  Event: presence
 |  Subscription-State: active;expires=499
 |  Max-Forwards: 70
 |  CSeq: 8 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>closed</basic>
 |      </status>
 |    </tuple>
 |  </presence>
 Upon receiving the NOTIFY, the SIP proxy needs to determine the
 identity of the domain portion of the Request-URI, which it does by
 following the procedures discussed in [RFC7247].  If the domain is an
 XMPP domain, the SIP proxy will hand off the NOTIFY 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
 NOTIFY into an XMPP <presence/> stanza addressed from the SIP user to
 the XMPP contact:
 Example 21: XMPP Transformation of SIP Presence Notification
 |  <presence from='romeo@example.net'
 |            to='juliet@example.com/yn0cl4bnw0yr3vym'
 |            type='unavailable'/>

Saint-Andre Standards Track [Page 25] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 The mapping of SIP syntax elements to XMPP syntax elements MUST be as
 shown in the following table.  (Mappings for elements not mentioned
 are undefined and therefore are a matter of implementation.)
    +---------------------------+-----------------------------+
    |  SIP Header or PIDF Data  |  XMPP Element or Attribute  |
    +---------------------------+-----------------------------+
    |  basic status             |  type (1)                   |
    +---------------------------+-----------------------------+
    |  Content-Language         |  xml:lang                   |
    +---------------------------+-----------------------------+
    |  From                     |  from                       |
    +---------------------------+-----------------------------+
    |  priority for tuple       |  <priority/> (2)            |
    +---------------------------+-----------------------------+
    |  To                       |  to                         |
    +---------------------------+-----------------------------+
    |  <note/>                  |  <status/>                  |
    +---------------------------+-----------------------------+
    |  <show/>                  |  <show/> (3)                |
    +---------------------------+-----------------------------+
 Table 2: Presence Syntax Mapping from SIP to XMPP
 Note the following regarding these mappings:
 1.  A PIDF basic status of "open" MUST be mapped to a <presence/>
     stanza with no 'type' attribute, and a PIDF basic status of
     "closed" MUST be mapped to a <presence/> stanza whose 'type'
     attribute has a value of "unavailable".
 2.  See the notes following Table 1 of this document regarding
     mapping of presence priority.
 3.  If a SIP implementation supports the XMPP <show/> element
     (qualified by the 'jabber:client' namespace) as a PIDF extension
     for availability status as described in the notes following
     Table 1 of this document, the SIP-to-XMPP gateway is responsible
     for including that element in the XMPP presence notification.

Saint-Andre Standards Track [Page 26] RFC 8048 SIP-XMPP Interworking: Presence December 2016

7. Polling for Presence Information

 Both SIP and XMPP provide methods for explicitly requesting one-time
 information about the current presence status of another entity.
 These are "polling" methods as opposed to the publish-subscribe
 methods described in the rest of this document.

7.1. XMPP to SIP

 In XMPP, an explicit request for information about current presence
 status is completed by sending a <presence/> stanza of type "probe":
 Example 22: XMPP Server Sends Presence Probe on Behalf of XMPP User
 |  <presence from='juliet@example.com/chamber'
 |            to='romeo@example.net'
 |            type='probe'/>
 Note: As described in [RFC6121], presence probes are used by XMPP
 servers to request presence on behalf of XMPP users; XMPP clients are
 discouraged from sending presence probes, because retrieving presence
 is a service that XMPP servers provide automatically.
 A SIP-to-XMPP gateway would transform the presence probe into its SIP
 equivalent, which is a SUBSCRIBE request with an Expires header value
 of zero ("0") in a new dialog:
 Example 23: SIP Transformation of XMPP Presence Probe
 |  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: 2398B737-566F-4CBB-A21A-1F8EEF7AF423
 |  Event: presence
 |  Max-Forwards: 70
 |  CSeq: 1 SUBSCRIBE
 |  Contact: <sip:juliet@example.com>;gr=yn0cl4bnw0yr3vym
 |  Accept: application/pidf+xml
 |  Expires: 0
 |  Content-Length: 0
 As described in [RFC3856], this causes a NOTIFY to be sent to the
 subscriber, just as a presence probe does (the transformation rules
 for presence notifications have been previously described in
 Section 6.2 of this document).

Saint-Andre Standards Track [Page 27] RFC 8048 SIP-XMPP Interworking: Presence December 2016

7.2. SIP to XMPP

 In SIP, an explicit request for information about current presence
 status is effectively completed by sending a SUBSCRIBE with an
 Expires header value of zero ("0"):
 Example 24: SIP User Sends Presence Request
 |  SUBSCRIBE sip:juliet@example.com SIP/2.0
 |  Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk
 |  From: <sip:romeo@example.net>;tag=yt66
 |  Call-ID: 717B1B84-F080-4F12-9F44-0EC1ADE767B9
 |  Event: presence
 |  Max-Forwards: 70
 |  CSeq: 1 SUBSCRIBE
 |  Contact: <sip:romeo@example.net>;gr=dr4hcr0st3lup4c
 |  Expires: 0
 |  Content-Length: 0
 A presence-aware SIP-to-XMPP gateway translates such a SIP request
 into a <presence/> stanza of type "probe" if it does not already have
 presence information about the contact:
 Example 25: XMPP Transformation of SIP Presence Request
 |  <presence from='romeo@example.net'
 |            to='juliet@example.com'
 |            type='probe'/>

8. Privacy and Security Considerations

 Detailed privacy and security considerations are given for presence
 protocols in [RFC2779], for SIP-based presence in [RFC3856] (see also
 [RFC3261]), and for XMPP-based presence in [RFC6121] (see also
 [RFC6120]).

8.1. Amplification Attacks

 There exists the possibility of an amplification attack launched from
 the XMPP network against a SIP Presence Server, because each long-
 lived XMPP presence authorization would typically result in multiple
 notification dialog refreshes on the SIP side of an XMPP-to-SIP
 gateway.  Therefore, access to an XMPP-to-SIP gateway SHOULD be
 restricted in various ways; for example:

Saint-Andre Standards Track [Page 28] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 o  Only an XMPP service that carefully controls account provisioning
    and provides effective methods for the administrators to control
    the behavior of registered users ought to host an XMPP-to-SIP
    gateway (e.g., not a service that offers open account
    registration).
 o  An XMPP-to-SIP gateway ought to be associated with only a single
    domain or trust realm.  For example, an XMPP-to-SIP gateway hosted
    at simple.example.com ought to allow only users within the
    example.com domain to access the XMPP-to-SIP gateway, not users
    within example.org, example.net, or any other domain (unless they
    are part of the same multi-tenanted environment as example.com).
    This helps to prevent the gateway equivalent of open relays that
    are shared across XMPP domains from different trust realms.
 If a SIP Presence Server receives communications through an XMPP-to-
 SIP gateway from users who are not associated with a domain that is
 so related to the hostname of the XMPP-to-SIP gateway, it SHOULD
 (based on local service provisioning) refuse to service such users or
 refuse to receive traffic from the XMPP-to-SIP gateway.  As a further
 check, whenever an XMPP-to-SIP gateway seeks to refresh an XMPP
 user's long-lived authorization to a SIP user's presence, it first
 sends an XMPP <presence/> stanza of type "probe" from the address of
 the XMPP-to-SIP gateway to the "bare Jabber Identifier (JID)"
 (user@domain.tld) of the XMPP user, to which the user's XMPP server
 responds in accordance with [RFC6121]; this puts an equal burden on
 the XMPP server and the SIP proxy.

8.2. Presence Leaks

 Presence notifications can contain sensitive information (e.g., about
 network availability).  In addition, it is possible in both SIP and
 XMPP for an entity to send different presence notifications to
 different subscribers.  Therefore, a gateway MUST NOT route or
 deliver a presence notification to any entity other than the intended
 recipient (as represented by the 'to' address for XMPP and by the
 Request-URI for SIP), because it does not possess information about
 authorization to receive presence notifications for such entities --
 that information resides at the user's home service, not at the
 receiving gateway.

Saint-Andre Standards Track [Page 29] RFC 8048 SIP-XMPP Interworking: Presence December 2016

9. References

9.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
            A., Peterson, J., Sparks, R., Handley, M., and E.
            Schooler, "SIP: Session Initiation Protocol", RFC 3261,
            DOI 10.17487/RFC3261, June 2002,
            <http://www.rfc-editor.org/info/rfc3261>.
 [RFC3856]  Rosenberg, J., "A Presence Event Package for the Session
            Initiation Protocol (SIP)", RFC 3856,
            DOI 10.17487/RFC3856, August 2004,
            <http://www.rfc-editor.org/info/rfc3856>.
 [RFC3857]  Rosenberg, J., "A Watcher Information Event Template-
            Package for the Session Initiation Protocol (SIP)",
            RFC 3857, DOI 10.17487/RFC3857, August 2004,
            <http://www.rfc-editor.org/info/rfc3857>.
 [RFC3863]  Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr,
            W., and J. Peterson, "Presence Information Data Format
            (PIDF)", RFC 3863, DOI 10.17487/RFC3863, August 2004,
            <http://www.rfc-editor.org/info/rfc3863>.
 [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
            Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
            March 2011, <http://www.rfc-editor.org/info/rfc6120>.
 [RFC6121]  Saint-Andre, P., "Extensible Messaging and Presence
            Protocol (XMPP): Instant Messaging and Presence",
            RFC 6121, DOI 10.17487/RFC6121, March 2011,
            <http://www.rfc-editor.org/info/rfc6121>.
 [RFC6665]  Roach, A.B., "SIP-Specific Event Notification", RFC 6665,
            DOI 10.17487/RFC6665, July 2012,
            <http://www.rfc-editor.org/info/rfc6665>.

Saint-Andre Standards Track [Page 30] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 [RFC7247]  Saint-Andre, P., Houri, A., and J. Hildebrand,
            "Interworking between the Session Initiation Protocol
            (SIP) and the Extensible Messaging and Presence Protocol
            (XMPP): Architecture, Addresses, and Error Handling",
            RFC 7247, DOI 10.17487/RFC7247, May 2014,
            <http://www.rfc-editor.org/info/rfc7247>.
 [RFC7622]  Saint-Andre, P., "Extensible Messaging and Presence
            Protocol (XMPP): Address Format", RFC 7622,
            DOI 10.17487/RFC7622, September 2015,
            <http://www.rfc-editor.org/info/rfc7622>.

9.2. Informative References

 [RFC2778]  Day, M., Rosenberg, J., and H. Sugano, "A Model for
            Presence and Instant Messaging", RFC 2778,
            DOI 10.17487/RFC2778, February 2000,
            <http://www.rfc-editor.org/info/rfc2778>.
 [RFC2779]  Day, M., Aggarwal, S., Mohr, G., and J. Vincent, "Instant
            Messaging / Presence Protocol Requirements", RFC 2779,
            DOI 10.17487/RFC2779, February 2000,
            <http://www.rfc-editor.org/info/rfc2779>.
 [RFC3860]  Peterson, J., "Common Profile for Instant Messaging
            (CPIM)", RFC 3860, DOI 10.17487/RFC3860, August 2004,
            <http://www.rfc-editor.org/info/rfc3860>.
 [RFC4480]  Schulzrinne, H., Gurbani, V., Kyzivat, P., and J.
            Rosenberg, "RPID: Rich Presence Extensions to the Presence
            Information Data Format (PIDF)", RFC 4480,
            DOI 10.17487/RFC4480, July 2006,
            <http://www.rfc-editor.org/info/rfc4480>.
 [RFC4825]  Rosenberg, J., "The Extensible Markup Language (XML)
            Configuration Access Protocol (XCAP)", RFC 4825,
            DOI 10.17487/RFC4825, May 2007,
            <http://www.rfc-editor.org/info/rfc4825>.
 [RFC7572]  Saint-Andre, P., Houri, A., and J. Hildebrand,
            "Interworking between the Session Initiation Protocol
            (SIP) and the Extensible Messaging and Presence Protocol
            (XMPP): Instant Messaging", RFC 7572,
            DOI 10.17487/RFC7572, June 2015,
            <http://www.rfc-editor.org/info/rfc7572>.

Saint-Andre Standards Track [Page 31] RFC 8048 SIP-XMPP Interworking: Presence December 2016

 [RFC7573]  Saint-Andre, P. and S. Loreto, "Interworking between the
            Session Initiation Protocol (SIP) and the Extensible
            Messaging and Presence Protocol (XMPP): One-to-One Text
            Chat Sessions", RFC 7573, DOI 10.17487/RFC7573, June 2015,
            <http://www.rfc-editor.org/info/rfc7573>.
 [RFC7702]  Saint-Andre, P., Ibarra, S., and S. Loreto, "Interworking
            between the Session Initiation Protocol (SIP) and the
            Extensible Messaging and Presence Protocol (XMPP):
            Groupchat", RFC 7702, DOI 10.17487/RFC7702, December 2015,
            <http://www.rfc-editor.org/info/rfc7702>.
 [XEP-0107] Saint-Andre, P. and R. Meijer, "User Mood", XSF XEP 0107,
            October 2008, <http://xmpp.org/extensions/xep-0107.html>.
 [XEP-0108] Meijer, R. and P. Saint-Andre, "User Activity", XSF
            XEP 0108, October 2008,
            <http://xmpp.org/extensions/xep-0108.html>.

Saint-Andre Standards Track [Page 32] RFC 8048 SIP-XMPP Interworking: Presence December 2016

Appendix A. Changes from RFC 7248

 RFC 7248 had already been published when the STOX working group
 discovered that a related document (since published as [RFC7702])
 contained problems that also applied to RFC 7248.  Specifically, the
 diagrams and protocol flows in RFC 7248 contained errors that
 reflected an incorrect architecture with gateways on both sides of
 the protocol exchange; in theory and in practice, presence traffic
 from an XMPP system would be translated by an XMPP-to-SIMPLE gateway
 on the XMPP side and received by a normal SIP/SIMPLE system directly
 (without a receiving gateway on the SIP/SIMPLE side), and traffic
 from a SIP system would be translated by a SIMPLE-to-XMPP gateway on
 the SIP side and received by a normal XMPP system (without a
 receiving gateway on the XMPP side).
 Therefore, this document makes the following substantive changes from
 RFC 7248:
 o  Corrects the architectural assumptions, diagrams, and protocol
    flows to reflect a single-gateway model in each direction.
 o  Adjusts terminology to replace the term "SIP Server" with the term
    "SIP Proxy" or "SIP Presence Server" as appropriate, and to use
    the term "notification dialog" for a SIP subscription and the term
    "presence authorization" for an XMPP subscription instead of the
    generic term "subscription" in both contexts.
 o  Clarifies that SIP notification dialogs are used to handle
    presence authorizations in SIP (e.g., there is no dedicated way to
    signal outbound cancellation of an authorization as there is in
    XMPP).
 o  Clarifies the use of the 'presence.winfo' event package, of the
    SIP Subscription-State headers (specifically with values of
    "pending", "active", "closed", or "terminated"), and of SIP NOTIFY
    messages with no body.
 o  Clarifies the durations of notification dialogs and presence
    authorizations, and how they are extended in SIP and handled in
    XMPP.
 o  Removes the mapping of the XMPP 'id' attribute to the SIP "CSeq"
    header.
 o  Describes the handling of multiple connected resources in XMPP.
 o  Provides information about mitigations for leaks of presence
    information.

Saint-Andre Standards Track [Page 33] RFC 8048 SIP-XMPP Interworking: Presence December 2016

Acknowledgements

 Thanks to the authors, contributors, and other individuals
 acknowledged in RFC 7248.
 Thanks to Saul Ibarra Corretge and Markus Isomaki for their reviews
 during working group consideration.
 Special thanks to Ben Campbell for identifying the underlying
 discrepancy that resulted in the need to obsolete RFC 7248.
 Thanks also to Markus Isomaki and Yana Stamcheva as the working group
 chairs and Alissa Cooper as the sponsoring Area Director.

Author's Address

 Peter Saint-Andre
 Filament
 Email: peter@filament.com
 URI:   https://filament.com/

Saint-Andre Standards Track [Page 34]

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