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RFC 8048 - Interworking between the Session Initiation Protocol


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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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Intended Audience . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Architectural Assumptions . . . . . . . . . . . . . . . . . .   5
   5.  Presence Authorizations . . . . . . . . . . . . . . . . . . .   6
     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . .   7
       5.2.1.  Requesting a Presence Authorization . . . . . . . . .   7
       5.2.2.  Refreshing a Notification Dialog  . . . . . . . . . .  11
       5.2.3.  Cancelling a Presence Authorization . . . . . . . . .  11
     5.3.  SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . .  15
       5.3.1.  Requesting a Presence Authorization . . . . . . . . .  15
       5.3.2.  Refreshing a Notification Dialog  . . . . . . . . . .  18
       5.3.3.  Cancelling a Presence Authorization . . . . . . . . .  19
   6.  Notifications of Presence Information . . . . . . . . . . . .  19
     6.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  19
     6.2.  XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . .  20
     6.3.  SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . .  25
   7.  Polling for Presence Information  . . . . . . . . . . . . . .  27
     7.1.  XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . .  27
     7.2.  SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . .  28
   8.  Privacy and Security Considerations . . . . . . . . . . . . .  28
     8.1.  Amplification Attacks . . . . . . . . . . . . . . . . . .  28
     8.2.  Presence Leaks  . . . . . . . . . . . . . . . . . . . . .  29
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  30
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  31
   Appendix A.  Changes from RFC 7248  . . . . . . . . . . . . . . .  33
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  34
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  34

1.  Introduction

   Presence is information about the availability of an entity (such as
   network availability or availability for communication).  Presence
   features in both SIP and XMPP involve several aspects:

   o  A long-lived authorization for a user to receive notifications
      about a contact's presence across presence and notification
      sessions; such an authorization is formally requested by the user,
      approved (or not) by the contact, and often associated with a
      record in an address list or "buddy list".

   o  An ephemeral presence session, during which the contact is online
      (i.e., available for interaction) and after which the contact is
      offline again.

   o  An ephemeral notification session, during which the user requests
      presence notifications from the contact (these are implicit in
      XMPP, but explicit in SIP where they are managed by means of
      notification dialogs).

   o  Notifications that are sent from the contact to the user for the
      life of either the contact's presence session or the user's
      notification session.

   Although specifications for both SIP and XMPP use the term
   "subscription", they do so in different ways.  In SIP, a
   "subscription" is the specific mechanism whereby a subscriber (or an
   entity acting on the subscriber's behalf, such as a SIP Presence
   Server) requests presence notifications from the contact over a
   relatively short period of time, renewed as necessary to keep
   receiving presence notifications during a presence session.  By
   contrast, in XMPP a "subscription" is essentially shorthand for a
   long-lived presence authorization.  To prevent confusion, this
   document uses the term "notification dialog" for a SIP subscription
   and the term "presence authorization" for an XMPP subscription.

   In order to help ensure interworking between presence systems that
   conform to the instant messaging and presence protocol requirements
   [RFC2779], it is important to clearly define protocol mappings
   between such systems.  Within the IETF, work has proceeded on two
   presence technologies:

   o  Various extensions to the Session Initiation Protocol ([RFC3261])
      for presence, in particular [RFC3856]

   o  The Extensible Messaging and Presence Protocol (XMPP), which
      consists of a formalization of the core XML-streaming protocols
      developed originally by the Jabber open-source community; the
      relevant specifications are [RFC6120] for the XML-streaming layer
      and [RFC6121] for basic presence and instant-messaging extensions

   One approach to help ensure interworking between these protocols is
   to map each protocol to the abstract semantics described in
   [RFC3860]; however, apparently that approach has never been
   implemented.  The approach taken in this document is to directly map
   semantics from one protocol to another (i.e., from SIP/SIMPLE (SIP
   for Instant Messaging and Presence Leveraging Extensions) to XMPP and
   vice versa), because that is how existing systems solve the
   interworking problem.

   The architectural assumptions underlying such direct mappings are
   provided in [RFC7247], including mapping of addresses and error
   conditions.  The mappings specified in this document cover basic
   presence functionality.  Mapping of more advanced functionality
   (e.g., so-called "rich presence") is out of scope for this document.

   This document obsoletes RFC 7248.

2.  Intended Audience

   The documents in this series (which include [RFC7247], [RFC7572],
   [RFC7573], and [RFC7702]) are intended for use by software developers
   who have an existing system based on one of these technologies (e.g.,
   SIP) and would like to enable communication from that existing system
   to systems based on the other technology (e.g., XMPP).  We assume
   that readers are familiar with the core specifications for both SIP
   [RFC3261] and XMPP [RFC6120], with the base document for this series
   [RFC7247], and with the following presence-related specifications:

   o  "A Presence Event Package for the Session Initiation Protocol"
      [RFC3856]

   o  "Presence Information Data Format (PIDF)" [RFC3863]

   o  "Extensible Messaging and Presence Protocol (XMPP): Instant
      Messaging and Presence" [RFC6121]

   o  "SIP-Specific Event Notification" [RFC6665]

3.  Terminology

   A number of terms used here ("user", "contact", "notification", etc.)
   are explained in [RFC3261], [RFC3856], [RFC3857], [RFC6120], and
   [RFC6121].  This document uses some, but not all, of the presence-
   related terms defined in the Model for Presence and Instant Messaging
   [RFC2778].  In particular, the term "presence session" is used as
   described in [RFC6121] to mean a delimited time period during which
   an endpoint is online and available for communications.

   In flow diagrams, SIP traffic is shown using arrows such as "***>",
   whereas XMPP traffic is shown using arrows such as "...>".  As in
   [RFC7247], the terms "SIP to XMPP Gateway" and "XMPP to SIP Gateway"
   are abbreviated as "S2X GW" and "X2S GW", respectively.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

4.  Architectural Assumptions

   The fundamental architectural assumptions underlying SIP-XMPP
   interworking are described in [RFC7247].

   Note that, in SIP, there are two ways that presence services can be
   deployed on the server side:

   1.  Under this model, described most fully in [RFC3857], a dedicated
       SIP Presence Server handles events related to the presence event
       package.  Instead of forwarding a SUBSCRIBE message to the SIP
       user, the Presence Server would inform the user of subscription
       activity using the 'presence.winfo' event package.  The SIP User
       Agent would then authorize the subscribing contact through some
       interaction with the Presence Server (for instance, using XML
       Configuration Access Protocol (XCAP) [RFC4825]).  Therefore,
       presence updates from the SIP User Agent would not be sent as
       NOTIFY messages to the XMPP user but as PUBLISH messages to the
       Presence Server, which would then generate NOTIFY messages to all
       active subscribers.

   2.  Under this model, a SIP Presence Server acts in proxy mode and
       merely passes through the SUBSCRIBE and NOTIFY messages to the
       SIP User Agent.

   Because the behavior of the XMPP-to-SIP gateway is not changed by the
   SIP architectural model that is used, the diagrams and protocol flows
   in this document cover both options by labeling the end entity a "SIP
   User Agent or Presence Server".

5.  Presence Authorizations

5.1.  Overview

   Both XMPP and presence-aware SIP systems enable entities (often, but
   not necessarily, human users) to subscribe to the presence of other
   entities.  XMPP presence is specified in [RFC6121].  Presence using a
   SIP event package is specified in [RFC3856].

   As described in [RFC6121], XMPP presence authorizations are managed
   using XMPP <presence/> stanzas of type "subscribe", "subscribed",
   "unsubscribe", and "unsubscribed".  The main states are:

   o  "none" (neither the user nor the contact is subscribed to the
      other's presence information)

   o  "from" (the contact will receive presence notifications from the
      user)

   o  "to" (the contact will send presence notifications to the user)

   o  "both" (both user and contact will receive each other's presence
      notifications)

   As described in [RFC3856], in SIP the subscriber does not explicitly
   request the creation or removal of presence authorizations.  Rather,
   the authorizations are triggered by subscription activity.  When a
   SIP user receives an initial SIP SUBSCRIBE event from a contact, the
   recipient's SIP User Agent or SIP Presence Server asks the user to
   make an authorization policy decision.  This decision is recorded in
   the SIP User Agent or SIP Presence Server, so that in the future any
   notification dialogs from the contact are automatically approved.
   (Note that addresses for SIP users and contacts are most generally
   referenced by a Presence URI of the form <pres:user@domain> but might
   be referenced by a SIP or SIPS (Session Initiation Protocol Secure)
   URI of the form <sip:user@domain> or <sips:user@domain>; because, in
   practice, 'pres' URIs are rarely used, the examples in this document
   use 'sip' URIs.)

   In both SIP and XMPP, presence authorizations are long-lived (indeed
   permanent if not explicitly cancelled).  In SIP, by default a
   notification session is typically short-lived unless explicitly
   extended (the default time-to-live of a SIP notification dialog is

   3600 seconds, as specified in Section 6.4 of [RFC3856], so that a
   notification dialog needs to be explicitly refreshed in order for a
   user's notification session to last as long as the contact's presence
   session).  In XMPP, a user's notification session with a contact is
   almost always automatically handled by the user's server based on the
   user's presence state (see [RFC6121] for details).

5.2.  XMPP to SIP

5.2.1.  Requesting a Presence Authorization

   The following diagram illustrates the protocol flow necessary to
   establish an authorization for an XMPP user to a receive presence
   notifications from a SIP contact, as further explained in the text
   and examples after the diagram.

   XMPP         XMPP         SIP     SIP UA or
   Client      Server       Proxy    Presence Server
    |        + X2S GW         |            |
    |            |            |            |
    | (F1) XMPP  |            |            |
    | subscribe  |            |            |
    |...........>|            |            |
    |            | (F2) SIP   |            |
    |            | SUBSCRIBE  |            |
    |            |***********>|            |
    |            |            | (F3) SIP   |
    |            |            | SUBSCRIBE  |
    |            |            |***********>|
    |            |            | (F4) SIP   |
    |            |            | 200 OK     |
    |            |            |<***********|
    |            | (F5) SIP   |            |
    |            | 200 OK     |            |
    |            |<***********|            |
    |            |            | (F6) SIP   |
    |            |            | NOTIFY     |
    |            |            | (pending)  |
    |            |            |<***********|
    |            | (F7) SIP   |            |
    |            | NOTIFY     |            |
    |            |<***********|            |
    |            | (F8) SIP   |            |
    |            | 200 OK     |            |
    |            |***********>|            |
    |            |            | (F9) SIP   |
    |            |            | 200 OK     |
    |            |            |***********>|

    |            |            | (F10) SIP  |
    |            |            | NOTIFY     |
    |            |            | (active)   |
    |            |            |<***********|
    |            | (F11) SIP  |            |
    |            | NOTIFY     |            |
    |            |<***********|            |
    |            | (F12) SIP  |            |
    |            | 200 OK     |            |
    |            |***********>|            |
    |            |            | (F13) SIP  |
    |            |            | 200 OK     |
    |            |            |***********>|
    | (F14) XMPP |            |            |
    | subscribed |            |            |
    |<...........|            |            |
    | (F15) XMPP |            |            |
    | presence   |            |            |
    |<...........|            |            |
    |            |            |            |

   An XMPP user (e.g., juliet@example.com) asks for a presence
   authorization by sending a request to a SIP contact (e.g.,
   romeo@example.net), and the contact either accepts or declines the
   request.  If the SIP contact accepts the request, the XMPP user will
   have a long-lived authorization to receive the SIP contact's presence
   information until (1) the XMPP user unsubscribes or (2) the SIP
   contact cancels the authorization.  The request is encapsulated in a
   <presence/> stanza of type "subscribe":

   Example 1: XMPP User Subscribes to SIP Contact (F1)

   |  <presence from='juliet@example.com'
   |            to='romeo@example.net'
   |            type='subscribe'/>

   Upon receiving such a <presence/> stanza, the XMPP server to which
   Juliet has connected needs to determine the identity of the
   domainpart in the 'to' address, which it does by following the
   procedures explained in Section 5 of [RFC7247].  If the domain is a
   SIP domain, the XMPP server will hand off the <presence/> stanza to
   an associated XMPP-to-SIP gateway or connection manager that natively
   communicates with presence-aware SIP proxies.

   The XMPP-to-SIP gateway is then responsible for translating the XMPP
   request into a SIP SUBSCRIBE request addressed from the XMPP user to
   the SIP contact:

   Example 2: SIP Transformation of XMPP Presence Authorization Request
   (F2)

   |  SUBSCRIBE sip:romeo@example.net SIP/2.0
   |  Via: SIP/2.0/TCP x2s.example.com;branch=z9hG4bKna998sk
   |  From: <sip:juliet@example.com>;tag=j89d
   |  Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C
   |  Event: presence
   |  Max-Forwards: 70
   |  CSeq: 1 SUBSCRIBE
   |  Contact: <sip:juliet@example.com>;gr=yn0cl4bnw0yr3vym
   |  Accept: application/pidf+xml
   |  Expires: 3600
   |  Content-Length: 0

   Once the SIP proxy has delivered the SIP SUBSCRIBE to the SIP User
   Agent or Presence Server (F3, no example shown), the SIP User Agent
   would then send a response indicating acceptance of the request:

   Example 3: SIP User Accepts Presence Authorization Request (F4)

   |  SIP/2.0 200 OK
   |  Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk
   |  From: <sip:juliet@example.com>;tag=j89d
   |  To: <sip:romeo@example.net>;tag=ffd2
   |  Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C
   |  CSeq: 1 SUBSCRIBE
   |  Contact: <sip:romeo@example.net>;gr=dr4hcr0st3lup4c
   |  Expires: 3600
   |  Content-Length: 0

   In accordance with Section 6.7 of [RFC3856], the XMPP-to-SIP gateway
   needs to consider the state to be "neutral" until it receives a
   NOTIFY message with a Subscription-State header [RFC6665] whose value
   is "active".  Therefore, the SIP User Agent or Presence Server SHOULD
   immediately send such a NOTIFY message (see Section 6 below).  If the
   XMPP-to-SIP gateway initially receives one or more NOTIFY messages
   with a Subscription-State header whose value is "pending" (F6), then
   it MUST respond to them on the SIP side but refrain from sending any
   presence stanzas from the SIP contact to the XMPP user.

   Example 4: SIP User Agent or Presence Server Sends Presence
   Notification (F10)

   |  NOTIFY sip:juliet@example.com SIP/2.0
   |  Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk
   |  From: <sip:juliet@example.com>;tag=j89d
   |  To: <sip:romeo@example.net>;tag=ffd2
   |  Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C
   |  Event: presence
   |  Subscription-State: active;expires=499
   |  Max-Forwards: 70
   |  CSeq: 2 NOTIFY
   |  Content-Type: application/pidf+xml
   |  Content-Length: 193
   |
   |  <?xml version='1.0' encoding='UTF-8'?>
   |  <presence xmlns='urn:ietf:params:xml:ns:pidf'
   |            entity='pres:romeo@example.net'>
   |    <tuple id='ID-dr4hcr0st3lup4c'>
   |      <status>
   |        <basic>open</basic>
   |        <show xmlns='jabber:client'>away</show>
   |      </status>
   |    </tuple>
   |  </presence>

   Upon receiving the first NOTIFY with a state of active, the XMPP-to-
   SIP gateway returns a 200 OK to the SIP User Agent or Presence Server
   (F12, no example shown).

   The XMPP-to-SIP gateway also generates a <presence/> stanza of type
   "subscribed":

   Example 5: XMPP User Receives Acknowledgement from SIP Contact (F14)

   |  <presence from='romeo@example.net'
   |            to='juliet@example.com'
   |            type='subscribed'/>

   As described in Section 6, if this first NOTIFY in the notification
   session contains a body, then the XMPP-to-SIP gateway also generates
   a presence notification addressed to the XMPP user (if the NOTIFY
   does not contain a body, then the gateway would interpret it as
   unknown or "closed"):

   Example 6: XMPP User Receives Presence Notification from SIP Contact
   (F15)

   |  <presence from='romeo@example.net/dr4hcr0st3lup4c'
   |            to='juliet@example.com'/>

5.2.2.  Refreshing a Notification Dialog

   It is the responsibility of the XMPP-to-SIP gateway to set the value
   of the Expires header and to periodically renew the notification
   dialog on the SIMPLE side of the gateway.  For example, the XMPP-to-
   SIP gateway SHOULD send a new SUBSCRIBE request to the SIP contact
   whenever the XMPP user initiates a presence session with the XMPP
   server by sending initial presence to its XMPP server (this is
   functionally equivalent to sending an XMPP presence probe).  The
   XMPP-to-SIP gateway SHOULD also send a new SUBSCRIBE request to the
   SIP contact sufficiently in advance of when the SIP notification
   dialog is scheduled to expire during the XMPP user's active presence
   session.

   The rules regarding SIP SUBSCRIBE requests for the purpose of
   establishing and refreshing a notification dialog are provided in
   [RFC6665].  Those rules also apply to XMPP-to-SIP gateways.
   Furthermore, an XMPP-to-SIP gateway MUST consider the XMPP presence
   authorization to be permanently cancelled (and so inform the XMPP
   user) if it receives a SIP response of 403, 489, or 603.  By
   contrast, it is appropriate to consider a SIP response of 423 or 481
   to be a transient error and to honor the long-lived XMPP presence
   authorization.  [RFC6665] explains more detailed considerations about
   the handling of SIP responses in relation to notification dialogs and
   refreshes.

   Finally, see the Privacy and Security Considerations section
   (Section 8) for important information and requirements regarding the
   security implications of notification refreshes.

5.2.3.  Cancelling a Presence Authorization

   The following diagram illustrates the protocol flow by which an XMPP
   user cancels her outbound presence authorization with a SIP contact
   (i.e., indicates that she no longer wishes to be authorized to see
   the SIP contact's presence).  As can be seen, SIMPLE itself does not

   have a construct that enables a user to cancel her outbound presence
   authorization (however, in many SIP/SIMPLE implementations she could
   use a technology such as XCAP [RFC4825] to remove the contact from
   her address list); therefore, this flow instead results in the
   cancellation of the user's notification dialog (with the implication
   on the XMPP side that the user will not request a subsequent
   notification dialog).  Additional details are explained in the text
   and examples after the diagram.

   XMPP         XMPP         SIP        SIP UA or
   Client      Server       Proxy    Presence Server
    |        + X2S GW         |            |
    |            |            |            |
    | (F16) XMPP |            |            |
    |unsubscribe |            |            |
    |...........>|            |            |
    |            | (F17) SIP  |            |
    |            | SUBSCRIBE  |            |
    |            | Expires: 0 |            |
    |            |***********>|            |
    |            |            | (F18) SIP  |
    |            |            | SUBSCRIBE  |
    |            |            | Expires: 0 |
    |            |            |***********>|
    |            |            | (F19) SIP  |
    |            |            | 200 OK     |
    |            |            |<***********|
    |            | (F20) SIP  |            |
    |            | 200 OK     |            |
    |            |<***********|            |
    | (F21) XMPP |            |            |
    |unsubscribed|            |            |
    |<...........|            |            |
    |            | (F22) SIP  |            |
    |            | NOTIFY     |            |
    |            | terminated |            |
    |            |***********>|            |
    |            |            | (F23) SIP  |
    |            |            | NOTIFY     |
    |            |            | terminated |
    |            |            |***********>|
    |            |            | (F24) SIP  |
    |            |            | 200 OK     |
    |            |            |<***********|
    |            | (F25) SIP  |            |
    |            | 200 OK     |            |
    |            |<***********|            |
    |            |            |            |

   At any time after subscribing, the XMPP user can indicate that she no
   longer wishes to be authorized to receive presence notifications from
   the contact.  This is done by sending a <presence/> stanza of type
   "unsubscribe":

   Example 7: XMPP User Unsubscribes from SIP Contact (F16)

   |  <presence from='juliet@example.com'
   |            to='romeo@example.net'
   |            type='unsubscribe'/>

   The XMPP-to-SIP gateway is responsible for translating the XMPP
   unsubscribe command into a SIP SUBSCRIBE request with the Expires
   header set to a value of zero ("0"):

   Example 8: SIP Transformation of XMPP Unsubscribe (F17)

   |  SUBSCRIBE sip:romeo@example.net SIP/2.0
   |  Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk
   |  From: <sip:juliet@example.com>;tag=j89d
   |  To: <sip:romeo@example.com>;tag=ffd2
   |  Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C
   |  Event: presence
   |  Max-Forwards: 70
   |  CSeq: 42 SUBSCRIBE
   |  Contact: <sip:juliet@example.com>;gr=yn0cl4bnw0yr3vym
   |  Accept: application/pidf+xml
   |  Expires: 0
   |  Content-Length: 0

   Upon receiving the SIP 200 OK acknowledging the cancellation, the
   XMPP-to-SIP gateway SHOULD send a <presence/> stanza of type
   "unsubscribed" addressed to the XMPP user:

   Example 9: XMPP User Receives Unsubscribed Notification (F21)

   |  <presence from='romeo@example.net'
   |            to='juliet@example.com'
   |            type='unsubscribed'/>

   In accordance with Section 4.4.1 of [RFC6665], the XMPP-to-SIP
   gateway is then responsible for sending a NOTIFY message with a
   Subscription-State header of "terminated" in order to formally end
   the XMPP user's outbound presence authorization and the associated
   SIP dialog.

   Example 10: XMPP-to-SIP Gateway Sends Presence Notification to
   Terminate Authorization (F25)

   |  NOTIFY sip:juliet@example.com SIP/2.0
   |  Via: SIP/2.0/TCP simple.example.net;branch=z9hG4bKna998sk
   |  From: <sip:juliet@example.com>;tag=j89d
   |  To: <sip:romeo@example.net>;tag=ffd2
   |  Call-ID: 5BCF940D-793D-43F8-8972-218F7F4EAA8C
   |  Event: presence
   |  Subscription-State: terminated
   |  Max-Forwards: 70
   |  CSeq: 43 NOTIFY
   |  Content-Length: 0

   Note: When the XMPP user cancels her outbound presence authorization
   to the SIP user, any inbound authorization that she might have
   approved (thus enabling the SIP user to see her presence) remains
   unchanged.

5.3.  SIP to XMPP

5.3.1.  Requesting a Presence Authorization

   The following diagram illustrates the protocol flow for establishing
   an authorization for a SIP user to receive presence notifications
   from an XMPP contact, as further explained in the text and examples
   after the diagram.

   SIP         SIP         XMPP         XMPP
   UA         Proxy       Server        Client
    |       + S2X GW         |           |
    |           |            |           |
    | (F26) SIP |            |           |
    | SUBSCRIBE |            |           |
    |**********>|            |           |
    | (F27) SIP |            |           |
    | 200 OK    |            |           |
    |<**********|            |           |
    |           | (F28) XMPP |           |
    |           | subscribe  |           |
    |           |...........>|           |
    |           |            | (F29) XMPP|
    |           |            | subscribe |
    |           |            |..........>|
    |           |            | (F30) XMPP|
    |           |            | subscribed|
    |           |            |<..........|
    |           | (F31) XMPP |           |
    |           | subscribed |           |
    |           |<...........|           |
    | (F32) SIP |            |           |
    | NOTIFY    |            |           |
    | (active)  |            |           |
    |<**********|            |           |
    | (F33) SIP |            |           |
    | 200 OK    |            |           |
    |**********>|            |           |
    |           |            |           |

   A SIP User Agent initiates a presence authorization to an XMPP
   contact's presence information by sending a SIP SUBSCRIBE request to
   the contact.  The following is an example of such a request:

   Example 11: SIP User Subscribes to XMPP Contact (F26)

   |  SUBSCRIBE sip:juliet@example.com SIP/2.0
   |  Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk
   |  From: <sip:romeo@example.net>;tag=xfg9
   |  To: <sip:juliet@example.net>
   |  Call-ID: AA5A8BE5-CBB7-42B9-8181-6230012B1E11
   |  Event: presence
   |  Max-Forwards: 70
   |  CSeq: 1 SUBSCRIBE
   |  Contact: <sip:romeo@example.net>;gr=dr4hcr0st3lup4c
   |  Accept: application/pidf+xml
   |  Content-Length: 0

   Notice that the Expires header was not included in the SUBSCRIBE
   request; this means that the default value of 3600 (i.e., 3600
   seconds = 1 hour) applies.

   Upon receiving the SUBSCRIBE, the SIP proxy needs to determine the
   identity of the domain portion of the Request-URI, which it does by
   following the procedures explained in Section 5 of [RFC7247].  If the
   domain is an XMPP domain, the SIP proxy will hand off the SUBSCRIBE
   to an associated SIP-to-XMPP gateway or connection manager that
   natively communicates with XMPP servers.

   The SIP-to-XMPP gateway is then responsible for translating the
   SUBSCRIBE into an XMPP authorization request addressed from the SIP
   user to the XMPP contact:

   Example 12: XMPP Transformation of SIP SUBSCRIBE (F28)

   |  <presence from='romeo@example.net'
   |            to='juliet@example.com'
   |            type='subscribe'/>

   In accordance with [RFC6121], the XMPP user's server delivers the
   presence authorization request to the XMPP user (or, if an
   authorization already exists in the XMPP user's roster, the XMPP
   server SHOULD auto-reply with a <presence/> stanza of type
   'subscribed').

   The "happy path" is for the XMPP user to approve the presence
   authorization request by generating a <presence/> stanza of type
   "subscribed" (F30).  The XMPP server then stamps that presence stanza
   with the 'from' address of the XMPP contact and sends it to the SIP
   user (F31).  Upon receiving the stanza, the SIP-to-XMPP gateway
   generates an empty SIP NOTIFY message with a Subscription-State
   header [RFC6665] of "active", which serves to inform the SIP user
   that the presence authorization request has been approved (F32).

   Example 13: XMPP User Approves Presence Authorization Request (F31)

   |  <presence from='juliet@example.com'
   |            to='romeo@example.net'
   |            type='subscribed'/>

   Example 14: Presence Authorization Request Approved (F32)

   |  NOTIFY sip:romeo@example.net SIP/2.0
   |  Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk
   |  From: <sip:romeo@example.net>;tag=xfg9
   |  To: <sip:juliet@example.com>;tag=ur93
   |  Call-ID: AA5A8BE5-CBB7-42B9-8181-6230012B1E11
   |  Event: presence
   |  Subscription-State: active
   |  Max-Forwards: 70
   |  CSeq: 2 NOTIFY
   |  Content-Length: 0

   As an alternative to the "happy path", the XMPP user could decline
   the presence authorization request by generating a <presence/> stanza
   of type "unsubscribed".  The XMPP server would stamp that presence
   stanza with the 'from' address of the XMPP contact and would send it
   to the SIP user.  The SIP-to-XMPP gateway then transforms that stanza
   into an empty SIP NOTIFY with a Subscription-State header [RFC6665]
   of "terminated" and a reason of "rejected":

   Example 15: XMPP User Rejects Presence Authorization Request

   |  <presence from='juliet@example.com'
   |            to='romeo@example.net'
   |            type='unsubscribed'/>

   Example 16: Presence Authorization Request Rejected

   |  NOTIFY sip:romeo@example.net SIP/2.0
   |  Via: SIP/2.0/TCP s2x.example.net;branch=z9hG4bKna998sk
   |  From: <sip:romeo@example.net>;tag=xfg9
   |  To: <sip:juliet@example.com>;tag=ur93
   |  Call-ID: AA5A8BE5-CBB7-42B9-8181-6230012B1E11
   |  Event: presence
   |  Subscription-State: terminated;reason=rejected
   |  Max-Forwards: 70
   |  CSeq: 2 NOTIFY
   |  Content-Length: 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].

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

   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'/>

   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>

   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.

   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

       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.

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'/>

   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.

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).

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:

   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.

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>.

   [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>.

   [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>.

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.

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/

 

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