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Versions: (draft-saintandre-stox-7248bis) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 RFC 8048

Network Working Group                                     P. Saint-Andre
Internet-Draft                                                  Filament
Obsoletes: 7248 (if approved)                             March 20, 2016
Intended status: Standards Track
Expires: September 21, 2016


   Interworking between the Session Initiation Protocol (SIP) and the
      Extensible Messaging and Presence Protocol (XMPP): Presence
                       draft-ietf-stox-7248bis-07

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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 21, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.












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   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 . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Architectural Assumptions . . . . . . . . . . . . . . . . . .   4
   5.  Subscriptions to Presence Information . . . . . . . . . . . .   5
     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.2.  XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . .   6
       5.2.1.  Establishing a Presence Subscription  . . . . . . . .   6
       5.2.2.  Refreshing a Presence Subscription  . . . . . . . . .  10
       5.2.3.  Cancelling a Presence Subscription  . . . . . . . . .  10
     5.3.  SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . .  13
       5.3.1.  Establishing a Presence Subscription  . . . . . . . .  13
       5.3.2.  Refreshing a Presence Subscription  . . . . . . . . .  16
       5.3.3.  Cancelling a Presence Subscription  . . . . . . . . .  18
   6.  Notifications of Presence Information . . . . . . . . . . . .  19
     6.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  19
     6.2.  XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . .  20
     6.3.  SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . .  24
   7.  Polling for Presence Information  . . . . . . . . . . . . . .  26
     7.1.  XMPP to SIP . . . . . . . . . . . . . . . . . . . . . . .  26
     7.2.  SIP to XMPP . . . . . . . . . . . . . . . . . . . . . . .  27
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   9.  Privacy and Security Considerations . . . . . . . . . . . . .  28
     9.1.  Amplification Attacks . . . . . . . . . . . . . . . . . .  28
     9.2.  Presence Leaks  . . . . . . . . . . . . . . . . . . . . .  29
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     10.2.  Informative References . . . . . . . . . . . . . . . . .  30
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  30
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  31









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1.  Introduction

   In order to help ensure interworking between presence systems that
   conform to the instant message / presence 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 helping 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 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]



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   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, subscription,
   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 in 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].

   In SIP, there are two ways that presence services are deployed on the
   server side:

   o  Under the first model, described most fully in [RFC3857], a
      dedicated SIP Presence Server handles subscriptions to the
      presence event package.  Instead of forwarding a SUBSCRIBE message
      to the SIP user, the Presence Server would act on the basis of a
      policy set by the SIP User Agent using the 'presence.winfo' event
      package, to which the SIP user would be subscribed.  The SIP User
      Agent would then authorize the subscriber through some interaction
      with the Presence Server (for instance using 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.



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   o  Under the second 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.  Subscriptions to Presence Information

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 subscriptions are specified in [RFC6121].
   Presence subscriptions using a SIP event package for presence are
   specified in [RFC3856].

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

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

   o  "from" (the user has a subscription from the contact)

   o  "to" (the user has a subscription to the contact's presence
      information)

   o  "both" (both user and contact are subscribed to each other's
      presence information)

   As described in [RFC3856], SIP presence subscriptions are managed
   through the use of SIP SUBSCRIBE events sent from a SIP User Agent or
   Presence Server 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 ((Session Initiation Protocol
   Secure) URI of the form <sip:user@domain> or <sips:user@domain>.  In
   practice, 'pres' URIs are rarely used, which is why the examples in
   this document use 'sip' URIs.

   The subscription models underlying XMPP and SIP differ mainly in the
   fact that XMPP presence subscriptions are long-lived (indeed
   permanent if not explicitly cancelled, so that a subscription need
   never be refreshed during any given presence session [RFC6121]),
   whereas SIP presence subscriptions are short-lived (the default time-



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   to-live of a SIP presence subscription is 3600 seconds, as specified
   in Section 6.4 of [RFC3856], so that a subscription needs to be
   explicitly refreshed if it will have the appearance of being
   permanent or even of lasting as long as the duration of a presence
   session).  This disparity has implications for the handling of
   subscription cancellations in either direction and, from the SIP
   side, subscription refreshes.

5.2.  XMPP to SIP

5.2.1.  Establishing a Presence Subscription

   The following diagram illustrates the protocol flow for establishing
   a presence subscription from an XMPP user to 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) initiates a subscription by
   sending a subscription 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 subscription to the SIP contact's presence information until
   (1) the XMPP user unsubscribes or (2) the SIP contact cancels the
   subscription.  The subscription 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
   subscription request into a SIP SUBSCRIBE request addressed from the
   XMPP user to the SIP contact:

   Example 2: SIP Transformation of XMPP Subscription 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
   then would send a response indicating acceptance of the subscription
   request:

   Example 3: SIP User Accepts Subscription 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 subscription 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).  In case the XMPP-to-SIP gateway initially receives one or
   more NOTIFY messages with Subscription-State header whose value is
   "pending" (F6), it MUST respond to them on the SIP side but not
   generate any presence stanzas towards 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 subscription 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, the XMPP-to-SIP gateway also generates a
   presence notification addressed to the XMPP user:

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





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


5.2.2.  Refreshing a Presence Subscription

   It is the responsibility of the XMPP-to-SIP gateway to set the value
   of the Expires header and to periodically renew the subscription on
   the SIMPLE side of the gateway so that the subscription appears to be
   permanent to the XMPP user.  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.  The XMPP-to-SIP gateway
   also SHOULD send a new SUBSCRIBE request to the SIP contact whenever
   the SIP presence subscription 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 presence subscription are provided in
   [RFC6665].  Those rules also apply to XMPP-to-SIP gateways.
   Furthermore, an XMPP-to-SIP gateway MUST consider the XMPP
   subscription 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
   subscription.  [RFC6665] explains more detailed considerations about
   the handling of SIP responses in relation to subscription requests
   and refreshes.

   Finally, see the security considerations section (Section 9) of this
   document for important information and requirements regarding the
   security implications of subscription refreshes.

5.2.3.  Cancelling a Presence Subscription

   The following diagram illustrates the protocol flow for cancelling an
   XMPP user's presence subscription to 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         |            |
    |            |            |            |
    | (F16) XMPP |            |            |
    |unsubscribe |            |            |
    |...........>|            |            |
    |            | (F17) SIP  |            |
    |            | SUBSCRIBE  |            |



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    |            | 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 unsubscribe from the
   contact's presence.  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
   unsubscribe command into a SIP SUBSCRIBE request with the Expires
   header set to a value of zero:





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   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 acknowleding the unsubscribe, 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 with a
   "Subscription-State" of "terminated" in order to formally end the
   subscription and the associated SIP dialog.

   Example 10: XMPP-to-SIP Gateway Sends Presence Notification to
   Terminate Subscription (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-Type: application/pidf+xml
   |  Content-Length: 0





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5.3.  SIP to XMPP

5.3.1.  Establishing a Presence Subscription

   The following diagram illustrates the protocol flow for establishing
   a presence subscription from a SIP user to 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 subscription to a 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)





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   |  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 subscription 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 subscription request to the XMPP user (or, if a subscription
   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 subscription
   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
   subscription request has been approved (F32).



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   Example 13: XMPP User Approves Subscription Request (F31)

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


   Example 14: Subscription 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 subscription 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 message with a Subscription-State header
   [RFC6665] of "terminated" and a reason of "rejected":

   Example 15: XMPP User Rejects Subscription Request

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

















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   Example 16: Subscription 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 Presence Subscription

   For as long as a SIP user is online and interested in receiving
   presence notifications from the XMPP contact, the user's SIP User
   Agent is responsible for periodically refreshing the subscription 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 to the 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].

   Note: In SIP there is no necessary connection between a presence
   session and an outbound subscription to a contact's presence; for
   example, a SIP user agent could end its subscription to the contact's
   presence yet still keep sending presence notifications to the
   contact, or it could send a presence notification indicating that it
   is offline (i.e., by sending a PIDF document with a basic status of
   "closed") while still maintaining a subscription to the contact's
   presence.  However, to simplify the description of handling
   notifications and subscriptions this document assumes that if a SIP
   user goes offline it will also terminate its outbound subscriptions.










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   When the SIP user ends its presence session by going offline (e.g.,
   by sending a basic status of "closed") and terminating its outbound
   subscriptions, it is the responsibility of the presence-aware SIP-to-
   XMPP gateway to properly handle the difference between short-lived
   SIP presence subscriptions and long-lived XMPP presence
   subscriptions.  The SIP-to-XMPP gateway has two options when the SIP
   user's subscription expires:

   o  Cancel the subscription (i.e., treat it as temporary) and send an
      XMPP <presence/> stanza of type "unsubscribe" to the XMPP contact;
      this honors the SIP semantic but will seem strange to the XMPP
      contact (because it will appear that the SIP user has cancelled a
      long-lived subscription).

   o  Treat the SIP subscription as if it were long-lived and:

      1.  send 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.  send an XMPP <presence/> stanza of type "unavailable" to the
          XMPP contact; this will seem more natural to the XMPP contact

   Which of these options a presence-aware SIP-to-XMPP gateway chooses
   is up to the implementation.

   If the implementation chooses the first option, the protocol
   generated would be as follows:

   Example 17: XMPP Handling of Temporary Subscription Expiry

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


   If the implementation chooses the second option, the protocol
   generated would be as follows:

   Example 18: SIP Handling of Long-Lived Subscription Expiry










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   |  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=timeout
   |  Max-Forwards: 70
   |  CSeq: 6 NOTIFY
   |  Content-Type: application/pidf+xml
   |  Content-Length: 194
   |
   |  <?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>closed</basic>
   |      </status>
   |    </tuple>
   |  </presence>


   Example 19: XMPP Handling of Long-Lived Subscription Expiry

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


5.3.3.  Cancelling a Presence Subscription

   At any time, the SIP User Agent can cancel the subscription by
   sending a SUBSCRIBE message whose Expires header is set to a value of
   zero ("0"):

   Example 20: SIP User Cancels Subscription














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   |  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


   As above, upon receiving such a request, a presence-aware SIP-to-XMPP
   gateway is responsible for doing one of the following:

   o  Cancel the subscription (i.e., treat it as temporary) and send an
      XMPP <presence/> stanza of type "unsubscribe" to the XMPP contact.

   o  Treat the SIP subscription as it were long-lived and

      1.  send 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.  send an XMPP <presence/> stanza of type "unavailable" to the
          XMPP contact

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, although future documents



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   might address broader notions of presence, including availability
   states and extended presence.

   [RFC6121] defines how XMPP <presence/> stanzas can indicate
   availability (via 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 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>.

   This document addresses basic presence or network availability only,
   not the various extensions to SIP and XMPP for "rich presence" such
   as [RFC4480], [XEP-0107], and [XEP-0108].

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 21: 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 (this is similar to the
   SIP NOTIFY method).  For each subscriber, broadcasting the presence
   notification involves adding the 'to' address of the subscriber and



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   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 (no example shown).

   The XMPP-to-SIP gateway is then responsible for translating the XMPP
   <presence/> stanza into a SIP NOTIFY request and included PIDF
   document from the XMPP user to the SIP contact.

   Example 22: 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.)





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   +-----------------------------+---------------------------+
   |  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" MUST be
       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 communications, the XMPP-to-SIP gateway
       MUST map that information to a PIDF basic status of "open".



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       Because a 'type' attribute with a value of "unavailable"
       indicates that an XMPP entity is not available communications,
       the XMPP-to-SIP gateway MUST map that information to a PIDF
       <basic/> status of "closed".

   5.  When the XMPP-to-SIP gateway receives 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 <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", although note that the latter
       value 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 <show/>
       element [RFC6121] or translate a <show/> element into a custom
       extension that the XMPP-to-SIP gateway knows is supported by the
       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.





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   In XMPP, a user can connect with multiple devices at the same time
   [RFC6120]; for presence notification purposes [RFC6121], each device
   is associated with a distinct resourcepart [RFC7622] and a contact's
   user agent will receive a separate presence notification from each of
   the user's devices.  Although the interpretation of multiple presence
   notifications from a single user is a matter of implementation by the
   contact's user agent, typically the 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 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 active subscriptions.  The syntax of the
   NOTIFY request is defined in [RFC3856].  The following is an example
   of such a request:

   Example 23: 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>





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   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 24: 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.)

   +---------------------------+-----------------------------+
   |  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 no 'type'
       attribute, and a PIDF basic status of "closed" MUST be mapped to
       a 'type' attribute whose value is "unavailable".



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   2.  See the notes following Table 1 of this document regarding
       mapping of presence priority.

   3.  If a SIP implementation supports the <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 standing subscription
   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 25: 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 servers provide.

   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:

   Example 26: SIP Transformation of XMPP Presence Probe













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   |  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: 0
   |  Content-Length: 0


   As described in [RFC3856], this cancels any subscription but 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:

   Example 27: 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


   When honoring the long-lived semantics of an XMPP presence
   subscription, 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:









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   Example 28: XMPP Transformation of SIP Presence Request

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


8.  IANA Considerations

   This document makes no requests of IANA.

9.  Privacy and Security Considerations

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

9.1.  Amplification Attacks

   The mismatch between long-lived XMPP presence subscriptions and
   short-lived SIP presence subscriptions introduces the possibility of
   an amplification attack launched from the XMPP network against a SIP
   presence server (because each long-lived XMPP presence subscription
   would typically result in multiple subscription refresh requests 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 only with 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



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   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 subscription 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 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.

9.2.  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 honor data about
   the intended recipient of a presence notification (as represented by
   the 'to' address for XMPP and by the Request-URI for SIP) and it MUST
   NOT route or deliver a presence notification to any other entities,
   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).

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3856]  Rosenberg, J., "A Presence Event Package for the Session
              Initiation Protocol (SIP)", RFC 3856, August 2004.

   [RFC3857]  Rosenberg, J., "A Watcher Information Event Template-
              Package for the Session Initiation Protocol (SIP)", RFC
              3857, August 2004.

   [RFC3863]  Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr,
              W., and J. Peterson, "Presence Information Data Format
              (PIDF)", RFC 3863, August 2004.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.




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   [RFC6121]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and Presence", RFC
              6121, March 2011.

   [RFC6665]  Roach, A., "SIP-Specific Event Notification", RFC 6665,
              July 2012.

   [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, May 2014.

   [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>.

10.2.  Informative References

   [RFC2778]  Day, M., Rosenberg, J., and H. Sugano, "A Model for
              Presence and Instant Messaging", RFC 2778, February 2000.

   [RFC2779]  Day, M., Aggarwal, S., and J. Vincent, "Instant Messaging
              / Presence Protocol Requirements", RFC 2779, February
              2000.

   [RFC3860]  Peterson, J., "Common Profile for Instant Messaging
              (CPIM)", RFC 3860, August 2004.

   [RFC4480]  Schulzrinne, H., Gurbani, V., Kyzivat, P., and J.
              Rosenberg, "RPID: Rich Presence Extensions to the Presence
              Information Data Format (PIDF)", RFC 4480, July 2006.

   [RFC4825]  Rosenberg, J., "The Extensible Markup Language (XML)
              Configuration Access Protocol (XCAP)", RFC 4825, May 2007.

   [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.  Acknowledgements




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   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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