SIPCORE Working Group                                        C. Holmberg
Internet-Draft                                                  Ericsson
Intended status: Standards Track                               M. Arnold
Expires: July 26, August 9, 2019                              Metaswitch Networks
                                                        January 22,
                                                        February 5, 2019

      Push Notification with the Session Initiation Protocol (SIP)
                     draft-ietf-sipcore-sip-push-24
                     draft-ietf-sipcore-sip-push-25

Abstract

   This document describes how a Push Notification Service (PNS) can be
   used to wake suspended Session Initiation Protocol (SIP) User Agents
   (UAs), using push notifications, for the UA to be able to send
   binding-refresh REGISTER requests and to receive receive incoming SIP
   requests.  The document defines new SIP URI parameters and new
   feature-capability indicators that can be used in SIP messages to
   indicate support of the mechanism defined in this document, to
   exchange PNS information between the SIP User Agent (UA) and the SIP
   entity that will request that push notifications are sent to the UA,
   and to trigger such push notification requests.

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 July 26, August 9, 2019.

Copyright Notice

   Copyright (c) 2019 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.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Push Resource ID (PRID) . . . . . . . . . . . . . . . . . . .   7
   4.  SIP User Agent (UA) Behavior  . . . . . . . . . . . . . . . .   7
     4.1.  REGISTER  . . . . . . . . . . . . . . . . . . . . . . . .   7
       4.1.1.  Request Push Notifications  . . . . . . . . . . . . .   8
       4.1.2.  Disable Push Notifications  . . . . . . . . . . . . .   9
       4.1.3.  Receive Push Notifications  . . . . . . . . . . . . .  10
       4.1.4.  Sending Binding-Refresh Requests Using Non-Push
               Mechanism . . . . . . . . . . . . . . . . . . . . . .  10
       4.1.5.  Query Network PNS Capabilities  . . . . . . . . . . .  11
   5.  SIP Proxy Behavior  . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  PNS Provider  . . . . . . . . . . . . . . . . . . . . . .  12
     5.2.  SIP Request Push Queue Bucket . . . . . . . . . . . . . . . . .  12
     5.3.  SIP URI Comparison Rules  . . . . . . . . . . . . . . . .  12  13
     5.4.  Indicate Support of Type of PNS . . . . . . . . . . . . .  13
     5.5.  Trigger Periodic Binding Refresh  . . . . . . . . . . . .  13
     5.6.  SIP Requests  . . . . . . . . . . . . . . . . . . . . . .  14
       5.6.1.  REGISTER  . . . . . . . . . . . . . . . . . . . . . .  14
       5.6.2.  Initial Request for Dialog or Stand-Alone Request . .  17
   6.  Support Of Longlived SIP Dialogs  . . . . . . . . . . . . . .  20
     6.1.  SIP UA Behavior . . . . . . . . . . . . . . . . . . . . .  22
       6.1.1.  Initial Request for Dialog  . . . . . . . . . . . . .  22
     6.2.  SIP Proxy Behavior  . . . . . . . . . . . . . . . . . . .  23
       6.2.1.  REGISTER  . . . . . . . . . . . . . . . . . . . . . .  23
       6.2.2.  Initial Request for Dialog  . . . . . . . . . . . . .  23
       6.2.3.  Mid-Dialog Request  . . . . . . . . . . . . . . . . .  24
   7.  Support Of SIP Replaces . . . . . . . . . . . . . . . . . . .  24
   8.  Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . .  25
     8.1.  555 (Push Notification Service Not Supported) Response
           Code  . . . . . . . . . . . . . . . . . . . . . . . . . .  25
     8.2.  sip.pns Feature-Capability Indicator  . . . . . . . . . .  25
     8.3.  sip.vapid Feature-Capability Indicator  . . . . . . . . .  26
     8.4.  sip.pnsreg Feature-Capability Indicator . . . . . . . . .  26
     8.5.  sip.pnsreg Media Feature Tag  . . . . . . . . . . . . . .  27
     8.6.  sip.pnspurr Feature-Capability Indicator  . . . . . . . .  27
     8.7.  SIP URI Parameters  . . . . . . . . . . . . . . . . . . .  27
   9.  PNS Registration Requirements . . . . . . . . . . . . . . . .  28
   10. pn-provider, pn-param and pn-prid URI Parameters for Apple
       Push Notification service . . . . . . . . . . . . . . . . . .  28
   11. pn-provider, pn-param and pn-prid URI Parameters for Google
       Firebase Cloud Messaging (FCM) push notification service  . .  29
   12. pn-provider, pn-param and pn-prid URI Parameters for RFC 8030
       (Generic Event Delivery Using HTTP Push)  . . . . . . . . . .  29
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  30
   14. IANA considerations . . . . . . . . . . . . . . . . . . . . .  31
     14.1.  SIP URI Parameters . . . . . . . . . . . . . . . . . . .  31
       14.1.1.  pn-provider  . . . . . . . . . . . . . . . . . . . .  31
       14.1.2.  pn-param . . . . . . . . . . . . . . . . . . . . . .  31
       14.1.3.  pn-prid  . . . . . . . . . . . . . . . . . . . . . .  32
       14.1.4.  pn-purr  . . . . . . . . . . . . . . . . . . . . . .  32
     14.2.  SIP Response Codes . . . . . . . . . . . . . . . . . . .  32
       14.2.1.  555 (Push Notification Service Not Supported)  . . .  32
     14.3.  SIP Global Feature-Capability Indicator  . . . . . . . .  32
       14.3.1.  sip.pns  . . . . . . . . . . . . . . . . . . . . . .  32
       14.3.2.  sip.vapid  . . . . . . . . . . . . . . . . . . . . .  33
       14.3.3.  sip.pnsreg . . . . . . . . . . . . . . . . . . . . .  33
       14.3.4.  sip.pnspurr  . . . . . . . . . . . . . . . . . . . .  34
     14.4.  SIP Media Feature Tag  . . . . . . . . . . . . . . . . .  35
       14.4.1.  sip.pnsreg . . . . . . . . . . . . . . . . . . . . .  35
     14.5.  PNS Sub-registry Establishment . . . . . . . . . . . . .  35
   15. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  36
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  36
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  36
     16.2.  Informative References . . . . . . . . . . . . . . . . .  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39

1.  Introduction

   In order to save resources (e.g., battery life) some devices
   (especially mobile devices) and operating systems will suspend
   applications when not used.  In some cases, internal timers cannot be
   used to wake such applications, nor will incoming network traffic
   wake the application.  Instead, one way to wake the application is by
   using a Push Notification Service (PNS).  A PNS is a service from
   where a user application can receive messages, referred to as push
   notifications, requested by other applications.  Push notifications
   might contain payload data, depending on the application.  An
   application can request that a push notification is sent to a single
   user application, or to multiple user applications.

   Typically each operating system uses a dedicated PNS.  For example,
   Apple iOS devices use the Apple Push Notification service (APNs)
   while Android devices use the Firebase Cloud Messaging (FCM) service.

   Because of the restrictions above, Session Initiation Protocol (SIP)
   User Agents (UAs) [RFC3261] can not be awoken, in order to send
   binding-refresh SIP REGISTER requests and to receive incoming SIP
   requests, without using a PNS to wake the UA in order to perform
   those functions.

   Also, without being able to use internal timers in order to wake
   applications, a UA will not be able to maintain connections e.g.,
   using the SIP Outbound Mechanism [RFC5626], as it requires the UA to
   send periodic keep-alive messages.

   This document describes how PNSs can be used to wake suspended UAs,
   using push notifications, to be able to send binding-refresh REGISTER
   requests and to receive incoming SIP requests.  The document defines
   new SIP URI parameters and new feature-capability indicators
   [RFC6809] that can be used in SIP messages to indicate support of the
   mechanism defined in this document, to exchange PNS information
   between the UA and the SIP entity (realized as a SIP proxy in this
   document) that will request that push notifications are sent to the
   UA, and to request such push notification requests.

   NOTE: Even if a UA is able to be awakened by other means than
   receiving push notifications (e.g., by using internal timers) in
   order to send periodic binding-refresh REGISTER requests, it might
   still be useful to suspend the application between the sending of
   binding-refresh requests (as it will save battery life) and use push
   notifications to wake the UA when an incoming SIP request UA arrives.

   When a UA registers to a PNS, it will receive a unique Push Resource
   ID (PRID) associated with the push notification registration.  The UA
   will use a REGISTER request to provide the PRID to the SIP proxy that
   will request push that notifications are sent to the UA.

   When the proxy receives a SIP request for a new dialog or a stand-
   alone SIP request addressed towards a UA, or when the proxy
   determines that the UA needs to send a binding-refresh REGISTER
   request, the proxy will request that a push notification is sent to
   the UA, using the PNS of the UA.  Once the UA receives the push
   notification, it will be able to send a binding-refresh REGISTER
   request and receive the incoming SIP request.  The proxy will receive
   the REGISTER request.  If the push notification request was triggered
   by a SIP request addressed towards the UA (see above), once the
   REGISTER request has been accepted by the SIP registrar [RFC3261],
   and the associated SIP 2xx response has been forwarded by the proxy
   towards the UA, the proxy can forward the SIP request towards the UA
   using normal SIP routing procedures.  In some cases the proxy can
   forward the SIP request without waiting for the SIP 2xx response to
   the REGISTER request.  Note that this mechanism necessarily adds
   delay to responding to requests requiring push notification.  The
   consequences of that delay are discussed in Section 5.6.2.

   If there are Network Address Translators (NATs) between the UA and
   the proxy, the REGISTER request sent by the UA will create NAT
   bindings that will allow the incoming SIP request that triggered the
   push notification to reach the UA.

   NOTE: The lifetime of any NAT binding created by the REGISTER request
   only needs to be long enough in order for the SIP request that
   triggered the push notification to reach the UA.

   Different PNSs exist today.  Some are based on the standardized
   mechanism defined in [RFC8030], while others are proprietary (e.g.,
   the Apple Push Notification service).  Figure 1 shows the generic
   push notification architecture supported by the mechanism in this
   document.

   Each PNS uses PNS-specific terminology and function names.  The
   terminology in this document is meant to be PNS-independent.  If the
   PNS is based on [RFC8030], the SIP proxy takes the role of the
   application server.

   The proxy MUST be in the signalling path of REGISTER requests sent by
   the UA towards the registrar, and of SIP requests (for a new dialog
   or a stand-alone) forwarded by the proxy responsible for the UA's
   domain (sometimes referred to as home proxy, S-CSCF, etc) towards the
   UA.  The proxy can also be co-located with the proxy responsible for
   the UA's domain.  This will also ensure that the Request-URI of SIP
   requests (for a new dialog or a stand-alone) can be matched against
   contacts in REGISTER requests.

     +--------+      +---------+        +-----------+    +-------------+
     |        |      |         |        |           |    | SIP         |
     | SIP UA |      | Push    |        | SIP Proxy |    | Registrar / |
     |        |      | Service |        |           |    | Home Proxy  |
     +--------+      +---------+        +-----------+    +-------------+
         |                 |                  |                   |
         | Subscribe       |                  |                   |
         |---------------->|                  |                   |
         |                 |                  |                   |
         | PRID            |                  |                   |
         |<----------------|                  |                   |
         |                 |                  |                   |
         | SIP REGISTER (PRID)                |                   |
         |===================================>|                   |
         |                 |                  |SIP REGISTER (PRID)|
         |                 |                  |==================>|
         |                 |                  |                   |
         |                 |                  | SIP 200 OK        |
         |                 |                  |<==================|
         | SIP 200 OK      |                  |                   |
         |<===================================|                   |
         |                 |                  |                   |
         |                 |                  |                   |
         |                 |                  |                   |
         |                 |                  | SIP INVITE (PRID) |
         |                 |                  |<==================|
         |                 |                  |                   |
         |                 |Push Request (PRID)                   |
         |                 |<-----------------|                   |
         |Push Message (PRID)                 |                   |
         |<----------------|                  |                   |
         |                 |                  |                   |
         | SIP REGISTER (PRID)                |                   |
         |===================================>|                   |
         |                 |                  |SIP REGISTER (PRID)|
         |                 |                  |==================>|
         |                 |                  |                   |
         |                 |                  | SIP 200 OK        |
         |                 |                  |<==================|
         | SIP 200 OK      |                  |                   |
         |<===================================|                   |
         |                 |                  |                   |
         | SIP INVITE      |                  |                   |
         |<===================================|                   |
         |                 |                  |                   |

         ------- Push Notification API

         ======= SIP

                    Figure 1: SIP Push Information Flow

     Example of a SIP REGISTER request in the flow above:

     REGISTER sip:alice@example.com SIP/2.0
     Via: SIP/2.0/TCP alicemobile.example.com:5060;branch=z9hG4bKnashds7
     Max-Forwards: 70
     To: Alice <sip:alice@example.com>
     From: Alice <sip:alice@example.com>;tag=456248
     Call-ID: 843817637684230@998sdasdh09
     CSeq: 1826 REGISTER
     Contact: <sip:alice@alicemobile.example.com;
       pn-provider=acme;
       pn-param=acme-param;
       pn-prid=ZTY4ZDJlMzODE1NmUgKi0K>
     Expires: 7200
     Content-Length: 0

                      Figure 2: SIP REGISTER Example

2.  Conventions

   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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Push Resource ID (PRID)

   When a SIP UA registers with a PNS it receives a unique Push Resource
   ID (PRID), which is a value associated with the registration that can
   be used to generate push notifications.

   The format of the PRID varies depending on the PNS.

   The details regarding discovery of the PNS, and the procedures
   regarding the push notification registration and maintenance are
   outside the scope of this document.  The information needed to
   contact the PNS is typically pre-configured in the operating system
   of the device.

4.  SIP User Agent (UA) Behavior

4.1.  REGISTER

   This section describes how a SIP UA sends SIP REGISTER requests
   (initial REGISTER request for a binding, or a binding-refresh
   REGISTER request) in order to request and disable push notifications
   from a SIP network, and to query the types of PNSs supported by the
   SIP network.

   Unless specified otherwise, the normal SIP UA registration procedures
   [RFC3261] apply.  The additional procedures described in this section
   apply when the REGISTER request contains a pn-provider SIP URI
   parameter in the Contact header field URI of the request.

   The procedures in this section apply to individual bindings
   [RFC3261].  If a UA creates multiple bindings (e.g., one for IPv4 and
   one for IPv6) the UA needs to perform the procedures for each
   binding.

   NOTE: If a SIP UA creates multiple bindings, since a push
   notification will trigger the UA to refresh all bindings, it is
   preferable if one can ensure that all bindings expire at the same
   time.  That will help preventing that some bindings are refreshed
   earlier than needed.

   For privacy and security reasons, a UA MUST NOT insert the SIP URI
   parameters (except for the pn-purr parameter) defined in this
   specification in non-REGISTER request, to prevent the PNS information
   associated with the UA from reaching the remote peer.  For example,
   the UA MUST NOT insert the pn-prid SIP URI parameter in the Contact
   header field URI of an INVITE request.  REGISTER requests will not
   reach the remote peer, as they will be terminated by the registrar of
   the UA.  However, the registrar MUST still ensure that the parameters
   are not sent to other users, e.g., using the SIP event package for
   registrations mechanism [RFC3680].  See Section 13 for more
   information.

4.1.1.  Request Push Notifications

   This section describes the procedures when a SIP UA requests push
   notifications from the SIP network.  The procedures assume that the
   UA has retrieved a PRID from a PNS.  The procedures how the UA
   retrieves the PRID from the PNS are PNS-specific, and outside the
   scope of this specification.  See PNS specific documentation for more
   details.

   If a SIP UA wants to use push notifications for other usages than to
   trigger binding-refresh REGISTER requests (e.g., for sending periodic
   subscription-refresh SUBSCRIBE requests [RFC6665]), this
   specification does not define a mechanism to explicitly request push
   notifications from the SIP network for such usages, and how to
   distinguish push notifications associated with such usages from the
   push notifications used to trigger binding-refresh REGISTER requests.
   However, by using the same refresh interval that is used for the
   binding-refreshes, the UA can perform actions associated with such
   usages (in addition to sending a binding-refresh REGISTER request)
   whenever it receives a push notification.

   When a UA requests push notifications, the UA MUST insert the
   following SIP URI parameters in the SIP Contact header field URI of
   the REGISTER request: pn-provider, pn-prid and pn-param (if required
   for the specific PNS).  The pn-provider URI parameter parameter
   indicates the type of PNS to be used for the push notifications.

   If the UA receives a 2xx response to the REGISTER request that
   contains a Feature-Caps header field [RFC6809] with a 'sip.pns'
   feature-capability indicator, with a an indicator value identifying the
   same type of PNS that was identified by the pn-provider URI parameter
   in the REGISTER request, it indicates that a SIP Proxy in the SIP
   network will request that push notifications are sent to the UA.  In
   addition, if the samd same Feature-Caps header field contains a
   'sip.vapid' feature-capability indicator, it indicates that the proxy
   supports use of the Voluntary Application Server Identification
   (VAPID) mechanism [RFC8292] to restrict push notifications to the UA.

   NOTE: The VAPID specific procedures of the SIP UA are outside the
   scope of this document.

   If the UA receives a non-2xx response to the REGISTER, or if the UA
   receives a 2xx response that does not contain a Feature-Caps header
   field [RFC6809] with a 'sip.pns' feature-capability indicator, the UA
   MUST NOT assume a the proxy will request that push notifications are
   sent to the UA.  The actions taken by the UA in such cases are
   outside the scope of this document.

   If the PRID is only valid for a limited time then the UA is
   responsible for retrieving a new PRID from the PNS and sending a
   binding-refresh REGISTER request with the updated pn- parameters.  If
   a PRID is no longer valid, and the UA is not able to retrieve a new
   PRID, the UA MUST disable the push notifications associated with the
   PRID (Section 4.1.2).

4.1.2.  Disable Push Notifications

   When a UA wants to disable previously requested push notifications,
   the UA SHOULD remove the binding [RFC3261], unless the UA is no
   longer able to perform SIP procedures (e.g., due to a forced shutdown
   of the UA). UA) in which case the registrar will remove the binding once
   it expires.  When the UA sends the REGISTER request for removing the
   binding, the UA MUST include the pn-pric pn-prid SIP URI parameter in the
   Contact header field URI of the REGISTER request, in order to inform
   the SIP network that the UA no longer wants to receive push
   notifications associated with the PRID.

4.1.3.  Receive Push Notifications

   When a UA receives a push notification, the UA MUST send a binding-
   refresh REGISTER request.  The UA MUST insert the same set of pn- SIP
   URI parameters in the SIP Contact header field URI of the REGISTER
   request that it inserted when it requested push notifications
   (Section 4.1.1).  Note that, in some cases the PNS might update the
   PRID value, in which case the UA will insert the new value in the pn-
   prid SIP URI parameter of the binding-refresh REGISTER request.

   Once the UA has received a 2xx response to the REGISTER request, the
   UA might receive a SIP request for a new dialog (e.g., a SIP INVITE),
   or a stand-alone SIP request (e.g., a SIP MESSAGE), if such SIP
   request has triggered a proxy to request that the push notification
   is sent to the UA.. Note that, depending on which transport protocol
   is used, the SIP request might reach the UA before the REGISTER
   response.

   If the SIP UA has created multiple bindings, the UA MUST send a
   binding-refresh REGISTER request for each of those bindings when it
   receives a push notification.

   This specification does not define any usage of push notification
   payload.  If a SIP UA receives a push notification that contains a
   payload the UA can discard the payload, but the UA will still send a
   binding-refresh REGISTER request.

4.1.4.  Sending Binding-Refresh Requests Using Non-Push Mechanism

   If a UA is able to send binding-refresh REGISTER requests using a
   non-push mechanism (e.g., using an internal timer that periodically
   wakes the UA) the UA MUST insert a 'sip.pnsreg' media feature tag
   [RFC3840] in the Contact header field of each REGISTER request.

   If the UA receives a 2xx response to the REGISTER request that
   contains a Feature-Caps header field with a 'sip.pnsreq' feature-
   capability indicator, the UA MUST send a binging-refresh REGISTER
   request prior to binding expiration.  The indicator value indicates
   the minimum time (given in seconds), prior to the binding expiration
   when the UA MUST send the REGISTER request.

   If the UA receives a 2xx response to the REGISTER request that does
   not contain a Feature-Caps header field with a 'sip.pnsreq' feature-
   capability indicator, the UA SHOULD only send a binding-refresh
   REGISTER request when it receives a push notification (even if the UA
   is able to use a non-push mechanism for sending binding-refresh
   REGISTER requests), or when there are circumstances (e.g., if the UA
   is assigned new contact parameters due to a network configuration
   change) that require an immediate REGISTER request to be sent.

   Even if the UA is able to to send binding-refresh REGISTER requests
   using a non-push mechanism, the UA MUST still send a binding-refresh
   REGISTER request whenever it receives a push notification
   (Section 4.1.3).

   NOTE: If the UA uses a non-push mechanism to wake and send a binding-
   refresh REGISTER request, such REGISTER request will update the
   binding expiration timer, and the proxy does not need to request that
   a push notification is sent to the UA in order to wake the UA.  The
   proxy will still request that a push notification is sent to the UA
   when the proxy receives a SIP request addressed towards the UA
   (Section 5.6.2).  This allows the UA to e.g., use timers for sending
   binding-refresh REGISTER requests, but to be suspended (in order to
   save battery resources etc) between sending the REGISTER requests and
   use push notification to wake the UA to process incoming calls.

4.1.5.  Query Network PNS Capabilities

   This section describes how a SIP UA can query the types of PNSs
   supported by a SIP network, and PNS-related capabilities (e.g.,
   support of the VAPID mechanism).  When a UA performs a query, it does
   not request push notifications from the SIP network.  Therefore, the
   UA can perform the query before it has registered to a PNS and
   received a PRID.

   In order to perform a query, the UA MUST insert a pn-provider SIP URI
   parameter in the Contact header field URI of the REGISTER request:

      If the UA inserts a pn-provider parameter value, indicating
      support of a type of PNS, the SIP network will only inform the UA
      whether that type of PNS is supported.
      If the UA does not insert a pn-provider parameter value (i.e., it
      inserts an "empty pn-provider parameter") the SIP network will
      inform the UA about all types of PNSs supported by the network.
      This is useful e.g., if the UA supports more than one type of PNS.
      Note that it is not possible to insert multiple parameter values
      in the pn-provider parameter.

   The UA MUST NOT insert a pn-prid SIP URI parameter in the Contact
   header field URI of the REGISTER request.

   If the UA receives a 2xx response to the REGISTER request, the
   response will contain one or more Feature-Caps header fields with a
   'sip.pns' feature-capability indicator, indicating the types of PNSs
   supported by the SIP network.  If the UA inserted a pn-provider SIP
   URI parameter value in the REGISTER request, the response will only
   indicate whether the SIP network supports the type of PNS supported
   by the UA.

   If the UA receives a 555 (Push Notification Service Not Supported)
   response to the REGISTER request, the response will contain the
   response will contain one or more Feature-Caps header fields with a
   'sip.pns' feature-capability indicator, indicating the types of PNSs
   supported by the SIP network.

   NOTE: It is optional for a UA to perform a query before it requests
   push notifications from the SIP network.

5.  SIP Proxy Behavior

5.1.  PNS Provider

   The type of PNS is identified by the pn-provider SIP URI parameter.
   In some cases there might only be one PNS provider for a given type
   of PNS, while in other cases there might be multiple providers.  The
   pn-param SIP URI parameter will provide more details associated with
   the actual PNS provider to be used.

   The protocol and format used for the push notification requests are
   PNS-specific, and the details for constructing and sending a push
   notification request are outside the scope of this specification.

5.2.  SIP Request Push Queue Bucket

   When a SIP proxy receives a SIP request, addressed towards a UA, that
   will trigger the proxy to request that a push notification is sent to
   the UA, the proxy will place the request in a queue storage referred to as
   the SIP Request Push Queue. Bucket.  The proxy starts a timer (referred to
   as the Bucket Timer) associated with the transaction.  A SIP request
   is removed from the queue bucket once the proxy either forwards the request
   towards the UA or UA, when the proxy sends an error response to the request.
   request, or when the Bucket Timer times out.  The detailed procedures
   are described in the sections below.

   Exactly how the SIP Request Push Queue Bucket is implemented is outside the
   scope of this document.  One option is to use the PRID as a key to
   search for SIP requests in the queue. bucket.  Note that mid-dialog requests
   (Section 6) do not carry the PRID in the SIP request itself.

5.3.  SIP URI Comparison Rules

   When

   By default, a SIP proxy compares two SIP URIs, the proxy uses the URI comparison rules defined in [RFC3261], meaning that in order for
   [RFC3261].  However, when a
   comparison SIP proxy compares the Contact header
   field URI of a 2xx response to be successful a REGISTER request with a Request-URI
   of a SIP request in the SIP Request Push Bucket (Section 5.2), the
   proxy uses the URI comparison rules with the following additions: the
   pn-prid, pn-provider and pn-param SIP URI parameters must MUST also match.
   If a pn- parameter is present in one of the compared URIs but not in
   the other URI there is no match.

   If only the pn- SIP URI parameters listed above match, but other
   parts of the compared URIs do not match, a proxy MAY still consider
   the comparison successful, based on local policy.  This can occur in
   a race condition when the proxy compares the Contact header field URI
   of a 2xx response to a REGISTER request with a Request-URI of a SIP
   request in the SIP Request Push Queue Bucket (Section 5.2), if the UA that
   sent the associated REGISTER request modified some parts of the
   Contact header field URI in the REGISTER request, but the Request-URI
   of the SIP request in the SIP Request Push Queue Bucket still contains the
   old parts.

5.4.  Indicate Support of Type of PNS

   A SIP proxy uses feature-capability indicators [RFC6809] to indicate
   support of types of PNSs, and additional features (e.g., VAPID)
   associated associated with a type of PNS.  A proxy MUST use a
   separate Feature-Cap header fields for each supported type of PNS.  A
   feature-capability indicator that indicates support of an additional
   feature associated with a given type of PNS MUST be inserted in the
   same Feature-Caps header field that is used to indicate support of
   the type of PNS.

   This specification defines the following feature-capability
   indicators that a proxy can use to indicate support of additional
   features associated with a given type of PNS: 'sip.vapid',
   'sip.pnsreg' and 'sip.pnspurr'.  These feature-capability indicators
   MUST only be inserted in a Feature-Caps header field that also
   contains a 'sip.pns' feature-capability indicator.

5.5.  Trigger Periodic Binding Refresh

   In order to request that a push notification is sent to a SIP UA, a
   SIP proxy needs to have information about when a binding will expire.
   The proxy needs to be able to retrieve the information from the
   registrar using some mechanism, or run its own registration timers.
   Such mechanisms are outside the scope of this document, but could be
   implemented e.g., using the SIP event package for registrations
   mechanism [RFC3680].

   When the proxy receives an indication that the UA needs to send a
   binding-refresh REGISTER request, the proxy will request that a push
   notification is sent to the UA.

   Note that the push notification needs to be requested early enough
   for the associated binding-refresh REGISTER request to reach the
   registrar before the binding expires.  It is RECOMMENDED that the
   proxy requests the push notification at least 120 seconds before the
   binding expires.

   If the UA has indicated, using the 'sip.pnsreg' media feature tag,
   that it is able to wake using a non-push mechanism in order to send
   binding-refresh REGISTER requests, and if the proxy does not receive
   a REGISTER request prior to 120 seconds before the binding expires,
   the proxy MAY request that a push notification is sent to the UA, to
   trigger the UA to send a binding-refresh REGISTER request.

   NOTE: As described in Section 4.1.5, a UA might send a REGISTER
   request without including a pn-prid SIP URI parameter, in order to
   retrieve push notification capabilities from the network before the
   UA expects to receive push notifications from the network.  A proxy
   will not request that push notifications are sent to a UA that has
   not provided a pn-prid SIP URI parameter (Section 5.6.2).

   If the proxy receives information that a binding associated with a
   PRID has expired, or that a binding has been removed, the proxy MUST
   NOT request that further push notifications are sent to the UA using
   that PRID.

5.6.  SIP Requests

5.6.1.  REGISTER

   This section describes how a SIP proxy processes SIP REGISTER
   requests (initial REGISTER request for a binding, or a binding-
   refresh REGISTER request).

   The procedures in this section apply when the REGISTER request
   contains a pn-provider SIP URI parameter in the Contact header field
   URI of the request.  In other cases the proxy MUST skip the procdures
   procedures in this section, and process the REGISTER request using
   normal SIP procedures.

5.6.1.1.  Request Push Notifications

   This section describes the SIP proxy procedures when a SIP UA
   requests push notifications from the SIP network.

   The procedures in this section apply when the SIP REGISTER request
   contains, in addition to the pn-provider SIP URI parameter, a pn-prid
   SIP URI parameter in the Contact header field URI of the request.

   When a proxy receives a REGISTER request, if the REGISTER request
   contains a Feature-Caps header field with a 'sip.pns' feature-
   capability indicator, it indicates that a proxy between this proxy
   and the UA supports the type of PNS supported by the UA, and will
   request that push notifications are sent to the UA.  In such case,
   the proxy MUST skip the rest of the procedures in this section, and
   process the REGISTER request using normal SIP procedures.

   When a proxy receives a REGISTER request, and the request does not
   contain a Feature-Caps header field with a 'sip.pns' feature-
   capability indicator, the proxy processes the request according to
   the procedures below:

   o  If the proxy does not support the type of PNS supported by the UA,
      or if the REGISTER request does not contain all information
      required for the type of PNS, the proxy SHOULD forward the request
      towards the registrar and skip the rest of the procedures in this
      section.  If the proxy knows (by means of local configuration)
      that no other proxies between itself and the registrar support the
      type of PNS supported by the UA, the proxy MAY send a SIP 555
      (Push Notification Service Not Supported) response instead of
      forwarding the request.
   o  If the proxy supports the type of PNS supported by the UA, but
      considers the requested binding expiration interval [RFC3261] to
      be too short (see below), the proxy MUST either send a 423
      (Interval Too Brief) response to the REGISTER request or forward
      the request towards the registrar and skip the rest of the
      procedures in this section.  If the proxy sends a 423 (Interval
      Too Brief) response, the proxy SHOULD insert one or more Feature-
      Caps header fields with a 'sip.pns' feature-capability indicator
      in the response, indicating each type of PNS that the proxy
      supports.
   o  If the proxy supports the type of of PNS supported by the UA, the
      proxy MUST indicate support of that type of PNS (Section 5.4) in
      the REGISTER request before it forwards the request towards the
      registrar.  This will inform proxies between the proxy and the
      registrar that the proxy supports the type of PNS supported by the
      UA, and that the proxy will request that push notifications are
      sent to the UA.

   A binding expiration interval MUST be considered too short if the the
   binding would expire before the proxy would request that a push
   notification is sent to the UA, in order to trigger the UA to send a
   binding-refresh REGISTER request.  The proxy MAY consider the
   interval too short based on its own policy so as to reduce load on
   the system.

   If the proxy sends a SIP 555 (Push Notification Service Not
   Supported) response, the proxy SHOULD indicate each type of PNS that
   the proxy supports in the response.

   When a proxy receives a 2xx response to the REGISTER request, if the
   proxy indicated support of a type of PNS in the REGISTER request (see
   above), the proxy performs the following actions:

   o  If the proxy considers the binding expiration interval indicated
      by the registrar too short (see above), the proxy forwards the
      response towards the UA and MUST skip the rest of the procedures
      in this section.
   o  The proxy MUST indicate support of the same type of PNS in the
      REGISTER response.  In addition:

      *  If the proxy supports the VAPID mechanism [RFC8292], the proxy
         MUST indicate support of the mechanism, using the 'sip.vapid'
         feature-capability indicator, in the REGISTER response.  The
         indicator value contains the public key identifying the proxy .
         The proxy MUST determine whether the PNS provider supports the
         VAPID mechanism before it indicates support of it.
      *  If the proxy received a 'sip.pnsreg' media feature tag in the
         REGISTER request, the proxy SHOULD insert a 'sip.pnsreg'
         feature-capability indicator with an indicator value bigger
         than 120 in the response, unless the proxy always wants to
         request that push notifications are sent to the UA in order to
         trigger the UA to send a binding-refresh REGISTER request.

5.6.1.2.  Query Network PNS Capabilities

   This section describes the SIP proxy procedures when a SIP UA queries
   about the push notification support in the SIP network
   (Section 4.1.5).

   The procedures in this section apply when the REGISTER request
   contains a pn-provider SIP URI parameter, but does not contain a pn-
   prid SIP URI parameter, in the Contact header field URI of the
   REGISTER request.

   When a proxy receives a REGISTER request, if the pn-provider SIP URI
   parameter contains a parameter value that indicates the type of PNS
   supported by the UA, the proxy MUST perform the following actions:

      If the proxy supports the type of of PNS supported by the UA, the
      proxy MUST indicate support of that type of PNS (Section 5.4) in
      the REGISTER request before it forwards the request towards the
      registrar.  This will inform proxies betwen between the proxy and the
      registrar that the proxy supports the type of PNS supported by the
      UA.
      If the proxy does not support the type of PNS supported by the UA,
      and if the REGISTER request contains Feature-Caps header fields
      indicating support of one or more types of PNSs, the proxy
      forwards the request towards the registrar.
      If the proxy does not support the type of PNS supported by the UA,
      and if the REGISTER request does not contain Feature-Caps header
      fields indicating support of one or more types of PNSs, the proxy
      MUST either forward the request towards the registrar, or send a
      SIP 555 (Push Notification Service Not Supported) response towards
      the UA.  The proxy MUST NOT send a SIP 555 (Push Notification
      Service Not Supported) response unless it knows (by means of local
      configuration) that no other proxy supports any of the types of
      PNSs supported by the UA.

   If the proxy sends a SIP 555 (Push Notification Service Not
   Supported) response, the proxy SHOULD indicate each type of PNS that
   the proxy supports in the response.

   When a proxy receives a REGISTER request, if the pn-provider SIP URI
   parameter does not contain a parameter value, the proxy MUST indicate
   support of each type of PNS supported by the proxy before it forwards
   the request towards the registrar.

   When a proxy receives a 2xx response to the REGISTER request, if the
   proxy indicated support of one or more types of PNSs in the REGISTER
   request (see above), the proxy MUST indicate support of the same set
   of types of PNSs in the response.  In addition, if the proxy supports
   the VAPID mechanism for one or more types of PNSs, the proxy MUST
   indicate support of the mechanism for thoses those PNSs in the response.

5.6.2.  Initial Request for Dialog or Stand-Alone Request

   The procedures in this section apply when a SIP proxy has indicated
   that the it will request that push notifications are sent to the SIP
   UA.

   When the proxy receives a SIP request for a new dialog (e.g., a SIP
   INVITE request) or a stand-alone SIP request (e.g., a SIP MESSAGE
   request) addressed towards a SIP UA, if the Request-URI of the
   request contains a pn-provider, a pn-prid and a pn-param (if required
   for the specific PNS provider) SIP URI parameter, the proxy requests
   that a push notification is sent to the UA, using the information in
   the pn- SIP URI parameters.  The proxy then places the SIP request in
   the SIP Request Push Queue (Section 5.2). Bucket.  The push notification will trigger the
   UA to send a binding-refresh REGISTER request, that the proxy will
   process as described in Section 5.6.1.  In addition, the proxy MUST
   store the Contact URI of the REGISTER request during the lifetime of
   the REGISTER transaction.

   NOTE: If the proxy receives a SIP request that does not contain the
   pn- SIP URI parameters listed above, the proxy processing of the
   request is based on local policy.  If the proxy also serves requests
   for UAs that do not use the SIP push mechanism, the proxy can forward
   the request towards the UA.  Otherwise the proxy can reject the
   request.

   When the proxy receives a 2xx response to the REGISTER request, the
   proxy performs the following actions:

      The proxy processes the REGISTER response as described in
      Section 5.6.1.
      The proxy checks whether the SIP Request Push Queue Bucket contains a
      SIP request associated with the REGISTER transaction.  If the queue
      contains such request the proxy compares transaction, by comparing
      (Section 5.3) the Contact header field URI in the REGISTER
      response with the Request-URIs of the SIP requests in the queue.
      If there is a match, the proxy MUST remove the SIP request from
      the waiting queue bucket and forward it towards the UA.

   The reason the proxy needs to wait for the REGISTER response before
   forwarding a SIP request towards a UA is to make sure that the
   REGISTER request has been accepted by the registrar, and that the UA
   that initiated the REGISTER request is authorized to receive messages
   for the Request-URI.

   If the proxy receives a non-2xx response to the REGISTER request, the
   proxy compares the Contact URI stored from the REGISTER request (see
   above) with the Request-URIs of the SIP requests in the SIP Request
   Push Queue. Bucket.  If there is a match, the proxy SHOULD remove the
   associated request from the queue bucket and send an error response to the
   request.  It is RECOMMENDED that the proxy sends either a 404 (Not
   Found) response or a 480 (Temporarily Unavailable) response to the
   SIP request, but other response codes can be used as well.  However,
   if the REGISTER response is expected to trigger a new REGISTER
   request from the UA (e.g., if the registrar is requesting the UA to
   perform authentication) the proxy MAY keep the SIP request in the
   queue.
   bucket.

   If the push notification request fails (see PNS-specific
   documentation for details), the proxy MUST remove the SIP request
   from the queue bucket and send an error response to the SIP request.  It is
   RECOMMENDED that the proxy sends either a 404 (Not Found) response or
   a 480 (Temporarily Unavailable) response, but other response codes
   can be used as well.

   When the proxy has requested that a push notification is sent to a
   UA, if the proxy does not receive a REGISTER response with a Contact
   URI that matches the Request-URI of the SIP request, the transaction
   timer of Bucket Timer
   (Section 5.2) associated with the SIP request will eventually time
   out.  When that happens the proxy MUST remove the SIP request from
   the queue SIP Request Push Bucket (Section 5.2) and send a 480 (Temporarily
   Unavailable) response.  The timer expiration Bucket Timer time out value is set based
   on local policy, taking the guidelines below into consideration.

   As discussed in [RFC4320] and [RFC4321], non-INVITE transactions must
   complete immediately or risk losing a race that results in stress on
   intermediaries and state misalignment at the endpoints.  The
   mechanism defined in this document inherently delays the final
   response to any non-INVITE request that requires a push notification.
   In particular, while waiting for the push notification request to
   succeed, and the associated REGISTER request to arrive from the SIP
   UA, the proxy needs to take into consideration that the transaction
   associated with the SIP request will eventually time out at the
   sender of the request (UAC), and the sender will consider the
   transaction a failure.  If the proxy forwards the SIP request towards
   the SIP UA, the SIP UA accepts the request and the transaction times
   out at the sender before it receives the successful response, this
   will cause state misalignment between the endpoints (the sender will
   consider the transaction a failure, while the receiver will consider
   the transaction a success).  The SIP proxy needs to take this into
   account when deciding for how long to wait before it considers sets the
   transaction value of the Bucket Timer associated with
   the SIP request a failure, transaction, to make sure that the error response (triggered by a
   Bucket Timer time out) reaches the sender before the transaction
   times out.  If the accumulated delay of this mechanism combined with
   any other mechanisms in the path of processing the non-INVITE
   transaction is not kept short, this mechanism should not be used.
   For networks encountering such conditions, an alternative (left for
   possible future work) would be for the proxy to immediately return an
   new error code meaning "wait at least the number of seconds specified
   in this response, and retry your request" before initiating the push
   notification.

   NOTE: While this work on this document was ongoing, implementation
   test results showed that the time it takes for a proxy to receive the
   REGISTER request, from when the proxy has requested a push
   notification, is typically around 2 seconds.  However, the time might
   vary depending on the characteristics and load of the SIP network and
   the PNS.

   In addition to the procedures described above there are two cases
   where a proxy, as an optimization, can forward a SIP request towards
   a UA without waiting for a 2xx response to a REGISTER request, or
   without even requesting that a push notification is sent to the UA:

      If the proxy is able to authorize authenticate the sender of the REGISTER
      request, the proxy does not need to wait for the 2xx response
      before it forwards the SIP request towards the UA.  In such cases,
      the proxy will use the Contact URI of the REGISTER request when
      comparing it against the Request-URIs of the SIP requests in the
      SIP Request Push Queue. Bucket.
      If the proxy has knowledge that the UA is awake, and that the UA
      is able to receive the SIP request without first sending a
      binding-refresh REGISTER request, the proxy does not need to
      request that a push notification is sent to the UA (the UA will
      not send a binding-refresh REGISTER request) before it forwards
      the SIP request towards the UA.  The mechanisms for getting such
      knowledge might be dependent on implementation or deployment
      architecture, and are outside the scope of this document.

   Some PNS providers allow payload in the push notifications.  This
   specification does not define usage of such payload (in addition to
   any payload that might be required by the PNS itself).

6.  Support Of Longlived SIP Dialogs

   Some SIP dialogs might have a long lifetime, with little activity.
   For example, when the SIP event notification mechanism [RFC6665] is
   used, there might be a long period between mid-dialog requests are
   sent.  Because of this a SIP UA might get suspended, and needs to be
   awaken in order to be able to receive mid-dialog requests.

   When the proxy receives a SIP request for a new dialog, or a stand-
   alone SIP request, addressed towards a UA, the request will contain
   information (pn- SIP URI parameters) that allows proxy to request
   that a push notification is sent to the UA Section 5.6.2.  However,
   this information will not be present in mid-dialog requests addressed
   towards the UA.  Instead, the proxy need to support a mechanism where
   it stores the information needed to request that a push notification
   is sent to the UA, and to be able to retrieve that information when
   it receives a mid-dialog request addressed towards the UA.  This
   section defines such mechanism.  This section describes such
   mechanism.  The SIP UA and SIP proxy procedures in this section are
   applied in addition to the generic procedures defined in this
   specification.

     +--------+      +---------+        +-----------+    +-------------+
     |        |      |         |        |           |    | SIP         |
     | SIP UA |      | Push    |        | SIP Proxy |    | Registrar / |
     |        |      | Service |        |           |    | Home Proxy  |
     +--------+      +---------+        +-----------+    +-------------+
         |                 |                  |                   |
         | Subscribe       |                  |                   |
         |---------------->|                  |                   |
         |                 |                  |                   |
         | PRID            |                  |                   |
         |<----------------|                  |                   |
         |                 |                  |                   |
         | SIP REGISTER (PRID)                |                   |
         |===================================>|                   |
         |                 |                  |SIP REGISTER (PRID)|
         |                 |                  |==================>|
         |                 |                  |                   |
         |                 |      +-----------------------+       |
         |                 |      | Store PRID (key=PURR) |       |
         |                 |      +-----------------------+       |
         |                 |                  |                   |
         |                 |                  | SIP 200 OK        |
         |                 |                  |<==================|
         | SIP 200 OK (PURR)                  |                   |
         |<===================================|                   |
         |                 |                  |                   |
         |                 |                  |                   |
         | SIP INVITE (PURR)                  |                   |
         |===================================>|                   |
         |                 |                  |SIP INVITE (PURR)  |
         |                 |                  |==================>|
         |                 |                  |                   |
         |                 |                  | SIP 200 OK        |
         |                 |                  |<==================|
         | SIP 200 OK      |                  |                   |
         |<===================================|                   |
         |                 |                  |                   |
         |                 |                  |                   |
         |                 |                  |                   |
         |                 |                  |SIP UPDATE (PURR)  |
         |                 |                  |<==================|
         |                 |                  |                   |
         |                 |      +-----------------------+       |
         |                 |      | Fetch PRID (key=PURR) |       |
         |                 |      +-----------------------+       |
         |                 |                  |                   |
         |                 |Push Request (PRID)                   |
         |                 |<-----------------|                   |
         |Push Message (PRID)                 |                   |
         |<----------------|                  |                   |
         |                 |                  |                   |
         | SIP REGISTER (PRID)                |                   |
         |===================================>|                   |
         |                 |                  |SIP REGISTER (PRID)|
         |                 |                  |==================>|
         |                 |                  |                   |
         |                 |                  | SIP 200 OK        |
         |                 |                  |<==================|
         | SIP 200 OK      |                  |                   |
         |<===================================|                   |
         |                 |                  |                   |
         | SIP UPDATE      |                  |                   |
         |<===================================|                   |
         |                 |                  |                   |

         ------- Push Notification API

         ======= SIP

                 Figure 3: SIP Push Longlived Dialog Flow

6.1.  SIP UA Behavior

6.1.1.  Initial Request for Dialog

   When a UA sends an initial request for a dialog, or a 2xx response to
   such requests, if the UA is willing to receive push notifications
   when a proxy receives a mid-dialog request addressed towards the UA,
   the UA MUST insert a 'pn-purr' SIP URI parameter in the Contact
   header field URI of the request or response.  The UA MUST insert a
   parameter value identical to the the last 'sip.pnspurr' feature-
   capability indicator that it received in a REGISTER response
   (Section 6.2.1).  If the UA has not recived a 'sip.pnspurr' feature-
   capability indicator, the UA MUST NOT insert a 'pn-purr' SIP URI
   parameter in a request or response.

   The UA decision whether it is willing to receive push notifications
   triggered by incoming mid-dialog requests is done based on local
   policy.  Such policy might be based on the type of SIP dialog, the
   type of media (if any) negotiated for the dialog [RFC3264], etc.

   NOTE: As the 'pn-purr' SIP URI parameter only applies to a given
   dialog, the UA needs to insert a 'pn-purr' parameter in the Contact
   header field URI of the request or response for each dialog in which
   the UA is willing to receive push notifications triggered by incoming
   mid-dialog requests.

6.2.  SIP Proxy Behavior

6.2.1.  REGISTER

   When a proxy receives an initial REGISTER request for a binding from
   the UA, if the proxy supports requesting that push notifications
   triggered by mid-dialog requests are sent to the registered UA, the
   proxy MUST store the information (the pn- SIP URI parameters) needed
   to request that push notifications are sent to the UA.  In addition,
   the proxy MUST generate a unique (within the context of the proxy)
   value, referred to as the PURR (Proxy Unique Registration Reference),
   that can be used as a key to retrieve the information.

   In order to prevent client fingerprinting, the proxy MUST
   periodically generate a new PURR value (even if pn- parameters did
   not change).  However, as long as there are ongoing dialogs
   associated with the old value, the proxy MUST store it so that it can
   request that push notifications are sent to the UA when it receives a
   mid-dialog request addressed towards the UA.  In addition, the PURR
   value MUST be generated in such a way so that it cannot be used to
   retrieve information about the user or associate it with
   registrations.  It can be generated e.g., by utilizing a
   cryptographically secure random function.

   Whenever the proxy receives a 2xx response to a REGISTER request, the
   proxy MUST insert a 'sip.pnspurr' feature-capability indicator with
   the latest PURR value (see above) in the response.

6.2.2.  Initial Request for Dialog

   When a proxy receives an initial request for a dialog from a UA, if
   the request contains a 'pn-purr' SIP URI parameter in the Contact
   header field URI of the request with a PURR value that the proxy has
   generated (Section 6.2.2), the proxy MUST add a Record-Route header
   to the request, to insert itself in the dialog route [RFC3261] before
   forwarding the request.

   When the proxy receives an initial request for a dialog addressed
   towards the UA, if the proxy has generated a PURR value associated
   with the pn- parameters inserted in the SIP URI of the request
   Section 6.2.2, the proxy MUST add a Record-Route header to the
   request, to insert itself in the dialog route [RFC3261] before
   forwarding the request.

6.2.3.  Mid-Dialog Request

   When the proxy receives a mid-dialog SIP request addressed towards
   the UA, if the request contains a 'pn-purr' SIP URI parameter and if
   the proxy is able to retrieve the stored information needed to
   request that a push notification is sent the UA (Section 6.2.1), the
   proxy MUST place the SIP request in the SIP Request Push Queue Bucket and
   request that a push notification is sent to the UA.

   NOTE: The 'pn-purr' SIP URI parameter will either be carried in the
   Request-URI or in a Route header field [RFC3261] of the SIP request,
   depending on how the route set [RFC3261] of the mid-dialog SIP
   request has been constructed.

   When the proxy receives a 2xx response to a REGISTER request, the
   proxy checks whether the SIP Request Push Queue Bucket contains a mid-dialog mid-
   dialog SIP request associated with the REGISTER transaction.  If the queue
   bucket contains such request the proxy MUST remove the SIP request
   from the
   waiting queue SIP Request Push Bucket and forward it towards the UA.

   Note that the proxy does not perform a URI comparison (Section 5.3)
   when processing mid-dialog requests, as a mid-dialog request will not
   contain the pn-prid, pn-provider and pn-param SIP URI parameters.
   The proxy only checks for a mid-dialog request that contains the PURR
   value associated with the REGISTER 2xx response.

   As described in Section 5.6.2, while waiting for the push
   notification request to succeed, and the associated REGISTER request
   and 2xx response, the proxy needs to take into consideration that the
   transaction associated with the mid-dialog request will eventually
   time out at the sender of the request (UAC), and the sender will
   consider the transaction a failure.

   When a proxy sends an error response to a mid-dialog request (e.g.,
   due to a transaction time out), the proxy SHOULD select a response
   code that only impacts the transaction associated with the request
   ([RFC5079]).

7.  Support Of SIP Replaces

   [RFC3891] defines a mechanism that allows a SIP UA to replace a
   dialog with another dialog.  A UA that wants to replace a dialog with
   another one will send an initial request for the new dialog.  The
   Request-URI of the request will contain Contact header field URI of
   the peer.

   If a SIP proxy wants to be able to request that a push notification
   is sent to a UA when it receives an initial request for a dialog that
   replaces an existing dialog, using the mechanism in [RFC3891], the
   proxy and the UA MUST perform the following actions:

   o  The proxy MUST provide a PURR to the UA during registration
      Section 6.2.1.
   o  The UA MUST insert a 'pn-purr' SIP URI parameter in the Contact
      header field URI of the initial request for a dialog, or a 2xx
      response to such requests Section 6.1.1.  This includes dialogs
      replacing other dialogs, as those dialogs might also get replaced.
   o  The proxy MUST apply the mechanism defined in Section 6.2.3 to
      place and retrieve the request from the waiting queue. SIP Request Push Bucket.

   In addition, the operator needs to make sure that the initial request
   for dialogs, addressed towards the UA using the contact of the
   replaced dialog, will be routed to the SIP proxy (in order to request
   that a push notification is sent to the UA).  The procedures for
   doing that are operator specific, and are outside the scope of this
   specification.

8.  Grammar

8.1.  555 (Push Notification Service Not Supported) Response Code

   The 555 response code is added to the "Server-Error" Status-Code
   definition. 555 (Push Notification Service Not Supported) is used to
   indicate that the server did not support the push notification
   service identified in a 'pn-provider' SIP URI parameter.

   The use of the SIP 555 response code is only defined for SIP REGISTER
   responses.

8.2.  sip.pns Feature-Capability Indicator

   The sip.pns feature-capability indicator, when inserted in a Feature-
   Caps header field of a SIP REGISTER request or a SIP 2xx response to
   a REGISTER request, indicates that the entity associated with the
   indicator supports the SIP push mechanism and the type of push
   notification service indicated by the indicator value.  When inserted
   in a 555 (Push Notification Service Not Supported) response to a
   REGISTER request, the the indicator indicates that the entity associated
   with the indicator supports the SIP push mechanism, and the type of
   push notification service identified by the indicator value.  The
   values defined for the pn-provider SIP URI parameter are used as
   indicator values.

     pns-fc          = "+sip.pns" EQUAL LDQUOT pns RDQUOT
     pns             = tag-value

     tag-value = <tag-value defined in [RFC3840]>

8.3.  sip.vapid Feature-Capability Indicator

   The sip.vapid feature-capability indicator, when inserted in a SIP
   2xx response to a SIP REGISTER request, indicates that the entity
   associated with the indicator supports the Voluntary Application
   Server Identification (VAPID) [RFC8292] mechanism when the entity
   requests that a push notification is sent to a SIP UA.  The indicator
   value is a public key identifying the entity, that can be used by a
   SIP UA to restrict subscriptions to that entity.

     vapid-fc          = "+sip.vapid" EQUAL LDQUOT vapid RDQUOT
     vapid             = tag-value

     tag-value = <tag-value defined in [RFC3840]>

8.4.  sip.pnsreg Feature-Capability Indicator

   The sip.pnsreg feature-capability indicator, when inserted in a SIP
   2xx response to a SIP REGISTER request, indicates that the entity
   associated with the indicator expects to receive binding-refresh
   REGISTER requests for the binding from the SIP UA associated with the
   binding before the binding expires, even if the entity does not
   request that a push notification is sent to the SIP UA in order to
   trigger the binding-refresh REGISTER requests.  The indicator value
   indicates the minimum time (given in seconds), prior to the binding
   expiration when the UA MUST send the REGISTER request.

     pns-fc          = "+sip.pnsreg" EQUAL LDQUOT reg RDQUOT
     reg             = 1*DIGIT

     DIGIT = <DIGIT defined in [RFC3261]>

8.5.  sip.pnsreg Media Feature Tag

   The sip.pnsreg media feature tag, when inserted in the Contact header
   field of a SIP REGISTER request, indicates that the SIP UA associated
   with the tag is able to send binding-refresh REGISTER requests for
   the associated binding without being awaken by push notifications.
   The media feature tag has no values.

     pnsreg-mt          = "+sip.pnsreg"

8.6.  sip.pnspurr Feature-Capability Indicator

   The sip.pnspurr feature-capability indicator, when inserted in a SIP
   2xx response to a SIP REGISTER request, indicates that the entity
   associated with the indicator will store information that can be used
   to associate a mid-dialog SIP request with the binding information in
   the REGISTER request.

     pnspurr-fc        = "+sip.pnspurr" EQUAL LDQUOT pnspurr RDQUOT
     pnspurr           = tag-value

     tag-value = <tag-value defined in [RFC3840]>

8.7.  SIP URI Parameters

   The section defines new SIP URI parameters, by extending the grammar
   for "uri-parameter" as defined in [RFC3261].  The ABNF is as follows:

     uri-parameter   =/ pn-provider / pn-param / pn-prid / pn-purr
     pn-provider     = "pn-provider" [EQUAL pvalue]
     pn-param        = "pn-param" EQUAL pvalue
     pn-prid         = "pn-prid" EQUAL pvalue
     pn-purr         = "pn-purr" EQUAL pvalue

     pvalue = <pvalue defined in [RFC3261]>
     EQUAL = <EQUAL defined in [RFC3261]>

     The format and semantics of pn-prid and pn-param are
     specific to the pn-provider value.

     Parameter value characters that are not part of pvalue need to be
     escaped, as defined in RFC 3261.

9.  PNS Registration Requirements

   When a new value is registered to the PNS Sub-registry, a reference
   to a specification that describes the usage of the PNS associated
   with the value is provided.  That specification MUST contain the
   following information:

   o  The value of the pn-provider SIP URI parameter.
   o  How the pn-prid SIP URI parameter value is retrieved and set by
      the SIP UA.
   o  How the pn-param SIP URI parameter (if required for the specific
      PNS provider) value is retrieved and set by the SIP UA.

10.  pn-provider, pn-param and pn-prid URI Parameters for Apple Push
     Notification service

   When the Apple Push Notification service (APNs) is used, the PNS-
   related SIP URI parameters are set as described below.

   For detailed information about the parameter values:

   https://developer.apple.com/library/archive/documentation/NetworkingI
   nternet/Conceptual/RemoteNotificationsPG/CommunicatingwithAPNs.html
   [pns-apns])

   The value of the pn-provider URI parameter is "apns".

   Example: pn-provider=apns

   The value of the pn-param URI parameter is a string that is composed
   by two values, separated by a period (.): Team ID and Topic.  The
   Team ID is provided by Apple and is unique to a development team.

   The Topic consists of the Bundle ID, which uniquely identifies an
   application, and a service value that identifies a service associated
   with the application, separated by a period (.).  For VoIP
   applications the service value is "voip".

   Example: pn-param=DEF123GHIJ.com.example.yourexampleapp.voip

   NOTE: The Bundle ID might contain one or more periods (.).  Hence,
   within the pn-param value, the first period will be separating the
   Team ID from the Topic, and within the Topic the last period will be
   separating the Bundle ID from the service.

   The value of the pn-prid URI parameter is the device token, which is
   a unique identifier assigned by Apple to a specific app on a specific
   device.

   Example: pn-prid=00fc13adff78512

11.  pn-provider, pn-param and pn-prid URI Parameters for Google
     Firebase Cloud Messaging (FCM) push notification service

   When Firebase Cloud Messaging (FCM) is used, the PNS related URI
   parameters are set as described below.

   For detailed information about the parameter values:

   https://firebase.google.com/docs/cloud-messaging/concept-options
   ([pns-fcm])

   The value of the pn-provider URI parameter is "fcm".

   The value of the pn-param URI parameter is the Project ID.

   The value of the pn-prid URI parameter is the Registration token,
   which is generated by the FCM SDK for each client app instance.

12.  pn-provider, pn-param and pn-prid URI Parameters for RFC 8030
     (Generic Event Delivery Using HTTP Push)

   When Generic Event Delivery Using HTTP Push is used, the PNS related
   URI parameters are set as described below.

   The value of the pn-provider URI parameter is "webpush".

   The value of the pn-param URI parameter MUST NOT be used.

   The value of the pn-prid URI parameter is the push subscription URI.

   See RFC 8030 [RFC8030] for more details.

   Note that encryption for web push [RFC8291] is not used, therefore
   parameters for message encryption are not defined in this
   specification.  Web push permits the sending of a push message
   without a payload without encryption.

13.  Security Considerations

   The security considerations for the use and operation of any
   particular PNS (e.g., how users and devices are authenticated and
   authorized) is out of scope for this document.  [RFC8030] documents
   the security considerations for the PNS defined in that
   specification.  Security considerations for other PNSs are left to
   their respective specifications.

   Typically, the PNS requires the SIP proxy requesting push
   notifications to be authenticated and authorized by the PNS.  In some
   cases the PNS also require the SIP application (or the SIP
   application developer) to be identified in order for the application
   to request push notifications.  Unless the PNS authenticates and
   authorizes the PNS, a malicious endpoint or network entity that
   managed to get access to the parameters transported in the SIP
   signalling might be able to request that push notifications are sent
   to a UA.  Which such  Such push notifications will not have any security related impacts, they will impact the battery life of the
   UA and trigger unnecessary SIP traffic.

   [RFC8292] defines a mechanism that allows a proxy to identity itself
   to a PNS, by signing a JWT sent to the PNS using a key pair.  The
   public key serves as an identifier of the proxy, and can be used by
   devices to restrict push notifications to the proxy associated with
   the key.

   Operators MUST ensure that the SIP signalling is properly secured,
   e.g., using encryption, from malicious endpoints. network entities.  TLS MUST be
   used, unless the operators know that the signalling is secured using
   some other mechanism that provides strong crypto properties.

   In addition to the information that needs to be exchanged between a
   device and the PNS in order to establish a push notification
   subscription, the mechanism defined in this document does not require
   any additional information to be exchanged between the device and the
   PNS.

   The mechanism defined in this document does not require a proxy to
   insert any payload (in addition to possible payload used for the PNS
   itself) when requesting push notifications.

   Operators MUST ensure that the PNS-related SIP URI parameters
   conveyed by a user in the Contact URI of a REGISTER request are not
   sent to other users, or to non-trusted network entities.  One way to
   convey contact information is by using the the SIP event package for
   registrations mechanism [RFC3680].  [RFC3680] defines generic
   security considerations for the SIP event package for registations. registrations.
   As the PNS-related SIP URI parameters conveyed in the REGISTER
   request contain sensitive information, operators that support the
   event package MUST ensure that event package subscriptions are
   properly authenticated and authorized, and that the SIP URI
   parameters are not inserted in event notifications sent to other
   users, or to non-trusted network entities.

14.  IANA considerations

   [RFC EDITOR NOTE: Please replace all instances of RFCXXXX with the
   RFC number of this document.]

14.1.  SIP URI Parameters

   This section defines new SIP URI Parameters that extend the "SIP/SIPS
   URI Parameters" sub-registry [RFC3969] under the sip-parameters
   registry: http://www.iana.org/assignments/sip-parameters.

14.1.1.  pn-provider

     Parameter Name: pn-provider

     Predefined Values:  No

     Reference:  RFC XXXX

14.1.2.  pn-param

     Parameter Name: pn-param

     Predefined Values:  No

     Reference:  RFC XXXX

14.1.3.  pn-prid

     Parameter Name: pn-prid

     Predefined Values:  No

     Reference:  RFC XXXX

14.1.4.  pn-purr

     Parameter Name: pn-purr

     Predefined Values:  No

     Reference:  RFC XXXX

14.2.  SIP Response Codes

14.2.1.  555 (Push Notification Service Not Supported)

   This section defines a new SIP response code that extends the
   "Response Codes" sub-registry [RFC3261] under the sip-parameters
   registry: http://www.iana.org/assignments/sip-parameters.

      Response Code Number:   555

      Default Reason Phrase:  Push Notification Service Not Supported

14.3.  SIP Global Feature-Capability Indicator

14.3.1.  sip.pns

   This section defines a new feature-capability indicator that extends
   the "SIP Feature-Capability Indicator Registration Tree" sub-registry
   [RFC6809] under the sip-parameters registry:
   http://www.iana.org/assignments/sip-parameters.

    Name: sip.pns

    Description: This feature-capability indicator, when inserted in a
        Feature-Caps header field of a SIP REGISTER request or a SIP 2xx
        response to a REGISTER request, indicates that the entity
        associated with the indicator supports the SIP push mechanism
        and the type of push notification service indicated by the
        indicator value. When inserted in a 555 (Push Notification
        Service Not Supported) response to a REGISTER request, the
        indicator indicates that the entity associated with the
        indicator supports the SIP push mechanism, and the type of push
        notification service indicated by the indicator value.

    Reference: [RFCXXXX]

    Contact: IESG (iesg@ietf.org)

14.3.2.  sip.vapid

   This section defines a new feature-capability indicator that extends
   the "SIP Feature-Capability Indicator Registration Tree" sub-registry
   [RFC6809] under the sip-parameters registry:
   http://www.iana.org/assignments/sip-parameters.

     Name: sip.vapid

     Description: This feature-capability indicator, when inserted in a
          SIP 2xx response to a SIP REGISTER request, indicates that the
          entity associated with the indicator supports the Voluntary
          Application Server Identification (VAPID) mechanism when the
          entity requests that a push notifications is sent to a SIP UA.
          The indicator value is a public key identifying the entity,
          that can be used by a SIP UA to restrict subscriptions to that
          entity.

     Reference: [RFCXXXX]

     Contact: IESG (iesg@ietf.org)

14.3.3.  sip.pnsreg

   This section defines a new feature-capability indicator that extends
   the "SIP Feature-Capability Indicator Registration Tree" sub-registry

   [RFC6809] under the sip-parameters registry:
   http://www.iana.org/assignments/sip-parameters.

    Name: sip.pnsreg

    Description: This feature-capability indicator, when inserted in a
         SIP 2xx response to a SIP REGISTER request, indicates that the
         entity associated with the indicator expects to receive
         binding-refresh REGISTER requests for the binding from the SIP
         UA associated with the binding before the binding expires, even
         if the entity does not request that a push notification is sent
         to the SIP UA in order to trigger the binding-refresh REGISTER
         requests. The indicator value indicates the minimum time
         (given in seconds), prior to the binding expiration when the UA
         MUST send the REGISTER request.

    Reference: [RFCXXXX]

    Contact: IESG (iesg@ietf.org)

14.3.4.  sip.pnspurr

   This section defines a new feature-capability indicator that extends
   the "SIP Feature-Capability Indicator Registration Tree" sub-registry
   [RFC6809] under the sip-parameters registry:
   http://www.iana.org/assignments/sip-parameters.

    Name: sip.pnspurr

    Description: This feature-capability indicator, when inserted in a
         SIP 2xx response to a SIP REGISTER request, indicates that
         the entity associated with the indicator will store information
         that can be used to associate a mid-dialog SIP request with the
         binding information in the REGISTER request. The indicator
         value is an identifier that can be used a key to retrieve the
         binding information.

    Reference: [RFCXXXX]

    Contact: IESG (iesg@ietf.org)

14.4.  SIP Media Feature Tag

14.4.1.  sip.pnsreg

   This section defines a new media feature tag that extends the "SIP
   Media Feature Tag Registration Tree" sub-registry [RFC3840] under the
   Media Feature Tag registry: https://www.iana.org/assignments/media-
   feature-tags/media-feature-tags.xhtml.

     Media feature tag name: sip.pnsreg

     Summary of the media feature indicated by this feature tag: This
          media feature tag, when inserted in the Contact header field
          of a SIP REGISTER request, indicates that the SIP UA
          associated with the tag is able to send binding-refresh
          REGISTER requests associated with the registration without
          being awaken by push notifications.

     Values appropriate for use with this feature tag: none

     Related standards or documents: [RFCXXXX]

     Security considerations: This media feature tag does not introduce
        new security considerations, as it simply indicates support for
        a basic SIP feature. If an attacker manages to remove the media
        feature tag, push notifications will not be requested to be sent
        to the client.

     Contact: IESG (iesg@ietf.org)

14.5.  PNS Sub-registry Establishment

   This section creates a new sub-registry, "PNS", under the sip-
   parameters registry: http://www.iana.org/assignments/sip-parameters.

   The purpose of the sub-registry is to register SIP URI pn-provider
   values.

   When a SIP URI pn-provider value is registered in the sub-registry,
   it needs to meet the "Specification Required" policies defined in
   [RFC8126].

     This sub-registry is defined as a table that contains the following
     three columns:

     Value:        The token under registration

     Description:  The name of the Push Notification Service (PNS)

     Document:     A reference to the document defining the registration

     This specification registers the following values:

     Value         Description                             Document
     -------       --------------------------------------  ----------

     apns          Apple Push Notification service         [RFC XXXX]
     fcm           Firebase Cloud Messaging                [RFC XXXX]
     webpush       Generic Event Delivery Using HTTP Push  [RFC XXXX]

15.  Acknowledgements

   Thanks to Mickey Arnold, Paul Kyzivat, Dale Worley, Ranjit Avasarala,
   Martin Thomson, Mikael Klein, Susanna Sjoholm, Kari-Pekka Perttula,
   Liviu Chircu, Roman Shpount and Yehoshua Gev for reading the text,
   and providing useful feedback.

16.  References

16.1.  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, <https://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, <https://www.rfc-
              editor.org/info/rfc3261>.

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840,
              DOI 10.17487/RFC3840, August 2004, <https://www.rfc-
              editor.org/info/rfc3840>.

   [RFC3891]  Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
              Protocol (SIP) "Replaces" Header", RFC 3891,
              DOI 10.17487/RFC3891, September 2004, <https://www.rfc-
              editor.org/info/rfc3891>.

   [RFC3969]  Camarillo, G., "The Internet Assigned Number Authority
              (IANA) Uniform Resource Identifier (URI) Parameter
              Registry for the Session Initiation Protocol (SIP)",
              BCP 99, RFC 3969, DOI 10.17487/RFC3969, December 2004,
              <https://www.rfc-editor.org/info/rfc3969>.

   [RFC5079]  Rosenberg, J., "Rejecting Anonymous Requests in the
              Session Initiation Protocol (SIP)", RFC 5079,
              DOI 10.17487/RFC5079, December 2007, <https://www.rfc-
              editor.org/info/rfc5079>.

   [RFC6809]  Holmberg, C., Sedlacek, I., and H. Kaplan, "Mechanism to
              Indicate Support of Features and Capabilities in the
              Session Initiation Protocol (SIP)", RFC 6809,
              DOI 10.17487/RFC6809, November 2012, <https://www.rfc-
              editor.org/info/rfc6809>.

   [RFC8030]  Thomson, M., Damaggio, E., and B. Raymor, Ed., "Generic
              Event Delivery Using HTTP Push", RFC 8030,
              DOI 10.17487/RFC8030, December 2016, <https://www.rfc-
              editor.org/info/rfc8030>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8292]  Thomson, M. and P. Beverloo, "Voluntary Application Server
              Identification (VAPID) for Web Push", RFC 8292,
              DOI 10.17487/RFC8292, November 2017, <https://www.rfc-
              editor.org/info/rfc8292>.

   [pns-apns]
              Apple Inc, "Apple Push Notification Service", January
              2019, <https://developer.apple.com/library/archive/documen
              tation/NetworkingInternet/Conceptual/
              RemoteNotificationsPG/CommunicatingwithAPNs.html>.

   [pns-fcm]  Google Inc, "Firebase Cloud Messaging", January 2019, <
              https://firebase.google.com/docs/cloud-messaging/concept-
              options>.

16.2.  Informative References

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002, <https://www.rfc-
              editor.org/info/rfc3264>.

   [RFC3680]  Rosenberg, J., "A Session Initiation Protocol (SIP) Event
              Package for Registrations", RFC 3680,
              DOI 10.17487/RFC3680, March 2004, <https://www.rfc-
              editor.org/info/rfc3680>.

   [RFC4320]  Sparks, R., "Actions Addressing Identified Issues with the
              Session Initiation Protocol's (SIP) Non-INVITE
              Transaction", RFC 4320, DOI 10.17487/RFC4320, January
              2006, <https://www.rfc-editor.org/info/rfc4320>.

   [RFC4321]  Sparks, R., "Problems Identified Associated with the
              Session Initiation Protocol's (SIP) Non-INVITE
              Transaction", RFC 4321, DOI 10.17487/RFC4321, January
              2006, <https://www.rfc-editor.org/info/rfc4321>.

   [RFC5626]  Jennings, C., Ed., Mahy, R., Ed., and F. Audet, Ed.,
              "Managing Client-Initiated Connections in the Session
              Initiation Protocol (SIP)", RFC 5626,
              DOI 10.17487/RFC5626, October 2009, <https://www.rfc-
              editor.org/info/rfc5626>.

   [RFC6665]  Roach, A., "SIP-Specific Event Notification", RFC 6665,
              DOI 10.17487/RFC6665, July 2012, <https://www.rfc-
              editor.org/info/rfc6665>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8291]  Thomson, M., "Message Encryption for Web Push", RFC 8291,
              DOI 10.17487/RFC8291, November 2017, <https://www.rfc-
              editor.org/info/rfc8291>.

Authors' Addresses

   Christer Holmberg
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: christer.holmberg@ericsson.com

   Michael Arnold
   Metaswitch Networks
   100 Church Street
   Enfield  EN2 6BQ
   United Kingdom

   Email: Michael.Arnold@metaswitch.com