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RFC 8048

Proposed STD
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Interworking between the Session Initiation Protocol (SIP) and the Extensible Messaging and Presence Protocol (XMPP): Presence

Part 1 of 2, p. 1 to 19
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Obsoletes:    7248


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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.

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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

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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]

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   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]

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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.

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   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

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

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    |            |            | (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.

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   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.

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   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'/>

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   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

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

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   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.

Top      ToC       Page 14 
   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.

Top      ToC       Page 15 
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    |            |           |
    |**********>|            |           |
    |           |            |           |

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   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').

Top      ToC       Page 17 
   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'/>

Top      ToC       Page 18 
   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].

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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.



(page 19 continued on part 2)

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