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SIPCORE Working Group                                        C. Holmberg
Internet-Draft                                                  Ericsson
Intended status: Standards Track                               M. Arnold
Expires: June 10, 2019                               Metaswitch Networks
                                                        December 7, 2018


      Push Notification with the Session Initiation Protocol (SIP)
                     draft-ietf-sipcore-sip-push-21

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 push notifications towards 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 June 10, 2019.

Copyright Notice

   Copyright (c) 2018 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



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   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 . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Push Resource ID (PRID) . . . . . . . . . . . . . . . . . . .   6
   4.  SIP User Agent (UA) Behavior  . . . . . . . . . . . . . . . .   7
     4.1.  Request Push Notifications from Network . . . . . . . . .   7
     4.2.  Query Network Push Notification Capabilities  . . . . . .  10
   5.  SIP Proxy Behavior  . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  PNS Provider  . . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Trigger Periodic Binding Refresh  . . . . . . . . . . . .  10
     5.3.  SIP Requests  . . . . . . . . . . . . . . . . . . . . . .  11
       5.3.1.  REGISTER  . . . . . . . . . . . . . . . . . . . . . .  11
       5.3.2.  Initial Request for Dialog or Stand-Alone Request . .  13
   6.  Support Of Longlived SIP Dialogs  . . . . . . . . . . . . . .  16
     6.1.  SIP UA Behavior . . . . . . . . . . . . . . . . . . . . .  16
       6.1.1.  Initial Request for Dialog  . . . . . . . . . . . . .  16
     6.2.  SIP Proxy Behavior  . . . . . . . . . . . . . . . . . . .  17
       6.2.1.  REGISTER  . . . . . . . . . . . . . . . . . . . . . .  17
       6.2.2.  Initial Request for Dialog  . . . . . . . . . . . . .  17
       6.2.3.  Mid-Dialog Request  . . . . . . . . . . . . . . . . .  18
   7.  Support Of SIP Replaces . . . . . . . . . . . . . . . . . . .  18
   8.  Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     8.1.  555 (Push Notification Service Not Supported) Response
           Code  . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     8.2.  sip.pns Feature-Capability Indicator  . . . . . . . . . .  19
     8.3.  sip.vapid Feature-Capability Indicator  . . . . . . . . .  19
     8.4.  sip.pnsreg Feature-Capability Indicator . . . . . . . . .  20
     8.5.  sip.pnsreg Media Feature Tag  . . . . . . . . . . . . . .  20
     8.6.  sip.pnspurr Feature-Capability Indicator  . . . . . . . .  21
     8.7.  SIP URI Parameters  . . . . . . . . . . . . . . . . . . .  21
   9.  PNS Registration Requirements . . . . . . . . . . . . . . . .  21
   10. pn-provider, pn-param and pn-prid URI Parameters for Apple
       Push Notification service . . . . . . . . . . . . . . . . . .  22
   11. pn-provider, pn-param and pn-prid URI Parameters for Google
       Firebase Cloud Messaging (FCM) push notification service  . .  22
   12. pn-provider, pn-param and pn-prid URI Parameters for RFC 8030
       (Generic Event Delivery Using HTTP Push)  . . . . . . . . . .  23
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  23
   14. IANA considerations . . . . . . . . . . . . . . . . . . . . .  24
     14.1.  SIP URI Parameters . . . . . . . . . . . . . . . . . . .  24



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       14.1.1.  pn-provider  . . . . . . . . . . . . . . . . . . . .  24
       14.1.2.  pn-param . . . . . . . . . . . . . . . . . . . . . .  24
       14.1.3.  pn-prid  . . . . . . . . . . . . . . . . . . . . . .  24
       14.1.4.  pn-purr  . . . . . . . . . . . . . . . . . . . . . .  25
     14.2.  SIP Response Codes . . . . . . . . . . . . . . . . . . .  25
       14.2.1.  555 (Push Notification Service Not Supported)  . . .  25
     14.3.  SIP Global Feature-Capability Indicator  . . . . . . . .  25
       14.3.1.  sip.pns  . . . . . . . . . . . . . . . . . . . . . .  25
       14.3.2.  sip.vapid  . . . . . . . . . . . . . . . . . . . . .  26
       14.3.3.  sip.pnsreg . . . . . . . . . . . . . . . . . . . . .  26
       14.3.4.  sip.pnspurr  . . . . . . . . . . . . . . . . . . . .  27
     14.4.  SIP Media Feature Tag  . . . . . . . . . . . . . . . . .  28
       14.4.1.  sip.pnsreg . . . . . . . . . . . . . . . . . . . . .  28
     14.5.  PNS Sub-registry Establishment . . . . . . . . . . . . .  28
   15. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  29
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     16.2.  Informative References . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

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).  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



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   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 push notifications towards 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 notifications towards 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 a push notification towards 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 5.3.2.

   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



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   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 UA |           | Push Service |       |    SIP Proxy    |
     +--------+           +--------------+       +-----------------+
         |                      |                         |
         |      Subscribe       |                         |
         |--------------------->|                         |
         |                      |                         |
         |    Push Resource ID  |                         |
         |<---------------------|                         |
         |                      |                         |
         |          SIP REGISTER (Push Resource ID)       |
         |===============================================>|
         |                      |                         | SIP REGISTER
         |                      |                         |============>
         |                      |                         |
         |                      |                         | SIP 200 OK
         |                      |                         |<============
         |          SIP 200 OK  |                         |
         |<===============================================|
         |                      |                         |
         |                      |                         |
         |                      |                         |
         |                      |                         |  SIP INVITE
         |                      |     Push Message        |<============
         |                      |   (Push Resource ID)    |
         |    Push Message      |<------------------------|
         |  (Push Resource ID)  |                         |
         |<---------------------|                         |
         |                      |                         |
         |          SIP REGISTER (Push Resource ID)       |
         |===============================================>|
         |                      |                         | SIP REGISTER
         |                      |                         |============>
         |                      |                         |
         |                      |                         | SIP 200 OK



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         |                      |                         |<============
         |          SIP 200 OK  |                         |
         |<===============================================|
         |                      |                         |
         |          SIP INVITE  |                         |
         |<===============================================|
         |                      |                         |


         ------- Push Notification API

         ======= SIP

     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 1: SIP Push Notification Architecture

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



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   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.  Request Push Notifications from Network

   The procedures in this section apply to bindings associated with a
   given PNS.

   The mechanism defined in this document assumes that when a SIP UA
   registers with a PNS it has received the PRID (using the protocol and
   procedures associated with the PNS).  Then, if the UA wants to
   receive push notifications (requested by the proxy), the UA MUST
   include 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
   identifies the type of PNS, the pn-prid URI parameter contains the
   PRID value and the pn-param URI parameter contains additional PNS-
   specific information.

   NOTE: If the SIP UA creates multiple bindings (e.g., one for IPv4 and
   one for IPv6), since a push notification will trigger to 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.

   When the UA receives a 2xx response to the REGISTER request, if the
   response contains a Feature-Caps header field [RFC6809] with a
   'sip.pns' feature-capability indicator with a parameter value
   identifying the same type of PNS that was identified by the pn-
   provider URI parameter in the REGISTER request, the UA can assume
   that a SIP proxy will request push notifications towards the UA.  In
   other cases, the UA MUST NOT assume that push notifications will be
   requested, and the actions taken by the UA might be dependent on
   implementation or deployment architecture, and are outside the scope
   of this document.

   In addition, if the response contains a Feature-Caps header field
   with a 'sip.vapid' feature-capability indicator, the proxy supports
   use of the Voluntary Application Server Identification (VAPID)
   mechanism [RFC8292] to restrict push notifications to the proxy.

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





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   When the UA receives a push notification, it MUST send a binding-
   refresh REGISTER request, using normal SIP procedures.  If there are
   Network Address Translators (NATs) between the UA and the proxy, the
   REGISTER request will create NAT bindings that will allow incoming
   SIP requests to reach the UA.  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 triggered the push
   notification request.  Note that, depending on which transport
   protocol is used, the SIP request might reach the UA before the
   REGISTER response.

   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.

   If the SIP UA has created multiple bindings (e.g., one for IPv4 and
   one for IPv6), the UA MUST send a binding-refresh REGISTER request
   for each of those bindings when it receives a push notification.

   If the 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 each Contact header field URI of each REGISTER request.
   Then, if the response to the REGISTER request contains a 'sip.pnsreg'
   feature-capability indicator with an indicator value, the UA
   refreshes the binding prior to the binding expires.  The indicator
   value indicates a minimum time (given in seconds), prior to the
   binding expires when the UA MUST send the REGISTER request.  Even if
   the UA is able to to send REGISTER requests using a non-push
   mechanism, the UA MUST still send a REGISTER request when it receives
   a push notification, following the procedures in this section.  If
   the REGISTER response does not contain a a 'sip.pnsreg' feature-
   capability indicator, the UA SHOULD only send a re-registration
   REGISTER request when it receives a push notification (even if the UA
   is able to use a non-push mechanism for sending re-registration
   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.

   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 a
   push notification towards the UA in order to wake the UA.  The proxy
   will still request a push notification towards the UA when the proxy
   receives a SIP request addressed towards the UA (Section 5.3.2).
   This allows the UA to e.g., use timers for sending binding-refresh
   REGISTER requests, but to be suspended (in order to save battery



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   resources etc) between sending the REGISTER requests and use push
   notification to wake the UA to process incoming calls.

   NOTE: This specification does not define any usage of a push
   notification payload.  As defined in Section 5.3.2, a proxy must not
   include any payload in the push notification request.  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.

   NOTE: If the SIP UA application wants to use push notifications for
   other purposes than to trigger binding-refresh requests, it needs to
   be able to distinguish between the different purposes when receiving
   push notifications.  Mechanisms for doing that are outside the scope
   of this specification.

   As long as the UA wants to receive push notifications (requested by
   the proxy), the UA MUST include a pn-provider, pn-prid and a pn-param
   (if required for the specific PNS provider) SIP URI parameter in each
   binding-refresh REGISTER request.  Note that, in some cases, the PNS
   might update the PRID value, in which case the UA will include the
   new value in the pn-prid SIP URI parameter in the binding-refresh
   REGISTER request.

   If the UA no longer wants to receive push notifications (requested by
   the proxy), the UA MUST send a binding-refresh REGISTER request
   without including the SIP URI parameters described above, or the UA
   MUST remove the registration.

   If a UA's subscription to a PNS (and the associated PRID) is only
   valid for a limited time then the UA is responsible for retrieving
   new subscription details from the PNS and sending a REGISTER request
   with the updated pn parameters to the proxy prior to the expiry of
   the existing subscription.  If a UA's subscription to a PNS expires
   then the UA MUST send a REGISTER without the pn parameters (to tell
   the proxy that it no longer wants push notifications) or terminate
   the registration.  The exact mechanisms for expiry and renewal of
   PRIDs will be PNS-specific and are outside the scope of this
   document.

   For privacy and security reasons, the UA MUST NOT include the SIP URI
   parameters defined in this document in non-REGISTER request, to
   prevent the PNS information associated with the UA from reaching the
   remote peer.  For example, the UA MUST NOT include the SIP URI
   parameters in the Contact header field of an INVITE request.






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4.2.  Query Network Push Notification Capabilities

   A SIP UA might need to query the SIP network for push notification
   capabilities (e.g., related to VAPID) before it registers with the
   PNS, and before it indicates that it wants to receive push
   notifications.  The UA can do so by only including the pn-provider
   SIP URI parameter in the SIP Contact header field URI of the REGISTER
   request, but without including the pn-prid SIP URI parameter.  Later,
   once the UA has received a response to the REGISTER request with the
   push notification information from the network, if the UA wants to
   receive push notifications, the UA will send a binding-refresh
   REGISTER request, including all pn- SIP URI parameters required by
   the specific PNS, following the procedures in Section 4.1.

5.  SIP Proxy Behavior

5.1.  PNS Provider

   The type of PNS is identified by the pn-provider SIP URI parameter.

   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.  Trigger Periodic Binding Refresh

   In order to request push notifications towards a SIP UA that will
   trigger the UA to send binding-refresh SIP REGISTER requests, the SIP
   proxy needs to have information about when a registration will
   expire.  The proxy needs to be able to retrieve the information from
   the registrar using some mechanism.  Such mechanisms are outside the
   scope of this document, but could be implemented e.g., using the
   SIPregistration event package mechanism [RFC3680].

   When the proxy receives an indication that the UA needs to send a
   binding-refresh REGISTER request, the proxy requests a push
   notification towards 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 registration expires.  It is RECOMMENDED that
   the proxy requests the push notification at least 120 seconds before
   the registration expires.

   If the UA has indicated, using the 'sip.pnsreg' media feature tag,
   that it is able to wake using a non-push mechanism for sending
   binding-refresh REGISTER requests, if the proxy does not receive a
   REGISTER request prior to 120 seconds before the registration



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   expires, the proxy MAY request a push notification towards the UA, to
   trigger the UA to send a REGISTER request.

   NOTE: As described in Section 4.2, a SIP 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 push notifications towards a UA that has not
   provided a pn-prid SIP URI parameter.

   If the proxy receives information that a registration has expired,
   the proxy MUST NOT request further push notifications towards the UA.

5.3.  SIP Requests

5.3.1.  REGISTER

   The procedures in this section apply when the SIP proxy receives a
   SIP REGISTER request (initial REGISTER request for a registration, or
   a binding-refresh REGISTER request) that contains a pn-provider SIP
   URI parameter identifying a type of PNS.  If the proxy receives a
   REGISTER request that does not contain a pn-provider SIP URI
   parameter, or that removes the registration, the proxy MUST NOT
   request push notifications towards the UA associated with the
   REGISTER request or perform any other procedures in this section.

   As described in Section 4.2, a SIP UA might send a REGISTER request
   without including a pn-prid SIP URI parameter in the Contact header
   field URI, in order to retrieve push notification capabilities from
   the network before the UA the proxy will request push notifications
   towards the UA.  The procedures in this section apply to both the
   case when the REGISTER request contains a pn-prid SIP URI parameter,
   and when it does not.

   When the 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 requesting push notifications towards the UA.
   The proxy MUST skip the rest of the procedures in this section, and
   process the REGISTER request using normal SIP procedures.

   If the proxy considers the requested registration expiration interval
   [RFC3261] to be too short, the proxy MUST either send a 423 (Interval
   Too Brief) response to the REGISTER request, or skip the rest of the
   procedures in this section and process the REGISTER request using
   normal SIP procedures.  If the proxy sends a 423 (Interval Too Brief)
   response, the proxy SHOULD insert a Feature-Caps header field with a
   'sip.pns' feature-capability indicator in the response, identifying



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   each type of PNS that the proxy supports.  Similarly, when the proxy
   receives a 2xx response to the REGISTER request (see below), if the
   proxy considers the registration expiration interval indicated by the
   registrar too short, the proxy MUST NOT insert a Feature-Caps header
   field with a 'sip.pns' feature-capability indicator in the response,
   and the proxy MUST NOT request push notifications associated with the
   registration.  A registration expiration interval MUST be considered
   too short if the interval is smaller than the time prior to
   expiration that the proxy would request a push notification.  The
   proxy MAY consider the interval too small based on its own policy so
   as to reduce load on the system.

   Otherwise, if the pn-provider SIP URI parameter identifies a type of
   PNS that the proxy does not support, or if the REGISTER request does
   not contain all additional information required for the specific type
   of PNS, the proxy MUST either forward the request (e.g., if the proxy
   knows that another proxy that will receive the REGISTER request
   supports the type of PNS) or send a SIP 555 (Push Notification
   Service Not Supported) response to the REGISTER request.  If the
   proxy sends a SIP 555 (Push Notification Service Not Supported)
   response Section 8.1, the proxy SHOULD insert a Feature-Caps header
   field with a 'sip.pns' feature-capability indicator in the response,
   identifying the type of each PNS that the proxy supports.  The
   decision whether to forward the request, or to send a response, is
   done based on local policy.

   If the proxy supports the type of PNS identified by the pn-provider
   SIP URI parameter, the proxy MUST insert a Feature-Caps header field
   with a 'sip.pns' feature-capability indicator, identifying the type
   of PNS, in the REGISTER request before forwarding the REGISTER
   request towards the registrar.  This will inform proxies that are not
   between the push proxy and the UA that the proxy supports, and will
   request (if the Contact header field URI of the REGISTER request
   contains a pn-prid SIP URI parameter), push notifications towards the
   UA.

   If the proxy inserted a Feature-Caps header field with a 'sip.pns'
   feature-capability indicator in the REGISTER request (see above), if
   the proxy receives a 2xx response to the REGISTER request, the proxy
   MUST insert a Feature-Caps header field with a 'sip.pns' feature-
   capability indicator in the response, identifying the type of PNS.
   This will inform the UA that the proxy supports, and will request (if
   the Contact header field URI of the REGISTER request contained a pn-
   prid SIP URI parameter), push notifications towards the UA.  The
   proxy MUST indicate support only of the same PNS that was identified
   in the pn-provider SIP URI parameter in the REGISTER request.  If the
   proxy receives a receives a SIP 555 (Push Notification Service Not
   Supported) response, the proxy SHOULD insert a Feature-Caps header



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   field with a 'sip.pns' feature-capability indicator in the response,
   identifying the type of each PNS that the proxy supports.

   In addition, if the proxy receives a 2xx response to the REGISTER
   request:

   o  if the proxy supports, and will use, the VAPID mechanism, the
      proxy MUST insert a Feature-Caps header field with a 'sip.vapid'
      feature-capability indicator in the response.  The header field
      parameter contains the public key identifying the proxy [RFC8292].
      The proxy MUST determine whether the PNS supports the VAPID
      mechanism before it inserts the feature-capability indicator.
   o  if the proxy received a 'sip.pnsreg' media feature tag in the
      REGISTER request, the proxy SHOULD include a 'sip.pnsreg' feature-
      capability indicator with an indicator value bigger than 120 in
      the response, unless the proxy always want to request push
      notifications to trigger the UA to send a REGISTER request.

5.3.2.  Initial Request for Dialog or Stand-Alone Request

   The procedures in this section apply when the SIP proxy has indicated
   that it supports, and will request, push notifications towards 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
   a push notification towards the UA, using the PRID included in the
   pn-prid SIP URI parameter and the PNS identified by the pn-provider
   SIP URI parameter.

   The push notification will trigger the UA to send a binding-refresh
   REGISTER request.  The proxy will process the REGISTER request and
   the associated response as described in Section 5.3.1.  In case of a
   2xx response to the REGISTER request, once the proxy has forwarded
   the REGISTER response towards the UA, if the contact of the SIP
   REGISTER request associated with the REGISTER response matches the
   Request-URI of the SIP request to be forwarded, and the contact was
   also present (and has not expired) in the REGISTER response, the
   proxy can forward the SIP request towards the UA, using normal SIP
   procedures.  If the contact of the REGISTER request does not match
   the Request-URI of the SIP request to be forwarded, or if the contact
   was not present in the REGISTER response, the proxy MUST reject the
   SIP request with a 404 (Not Found) response.  This can happen if the
   UA sends a binding-refresh REGISTER request that does not contain the
   previously registered contact at the same time the registrar forwards



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   a SIP request towards a UA using the previously registered contact in
   the Request-URI.

   NOTE: If the SIP request does not contain the pn- parameters, the
   proxy processing of the request is based on local policy, e.g.,
   depending on whether the proxy servers requests towards UAs that do
   not use the SIP push mechanism, in which case the proxy will forward
   the request using normal SIP procedures.  Otherwise the proxy might
   reject the request.

   When matching the Request-URI of the SIP request to be forwarded with
   a contact of a REGISTER request, the proxy uses the URI comparison
   rules in [RFC8292], with the following addition: the pn-prid SIP URI
   parameter MUST also match.  If the parameter is not present in the
   Request-URI of the SIP request, or in the contact of the REGISTER,
   there is no match.

   The reason the proxy needs to wait for the REGISTER response before
   forwarding the SIP request is to make sure that the REGISTER request
   has been accepted by the registrar, and that the UA which initiated
   the REGISTER request is authorized to receive messages for the
   Request-URI.  However, if the proxy is able to authorize the sender
   of the REGISTER request, it does not need to wait for the associated
   2xx response before it forwards the SIP request towards the UA.  The
   mechanism for authorizing the UA is outside the scope of this
   document.

   As both the SIP request to be forwarded towards the UA and the
   binding-refresh REGISTER request triggered by the push notification
   request will convey pn- SIP URI parameters associated with the SIP
   registration, those can be used to match the SIP request with the
   binding-refresh REGISTER request (even if other parts of the most
   recent contact and the Request-URI of the SIP request do not match).

   NOTE: The proxy needs to store (or be able to retrieve) the contact
   of the most recent REGISTER 2xx response, to be able to compare it
   with the Request-URI of the request to be forwarded towards the UA.

   In case of non-2xx response to the REGISTER request, the proxy MUST
   reject the SIP request with a 404 (Not Found) response.

   If the push notification request fails (see PNS-specific
   documentation for details), the proxy MUST reject the SIP request
   with a 480 (Temporarily Unavailable) response.

   If the proxy does not receive the REGISTER request from the UA within
   a given time after the proxy has requested the push notification, the




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   proxy MUST reject the request with a 480 (Temporarily Unavailable)
   response.  The time value is set based on local policy.

   As dicussed 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 the
   transaction associated with the SIP request a failure, to make sure
   that the error response 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.

   The proxy MUST NOT include the SIP request as payload in the
   requested push message.

   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 REGISTER
   request, the proxy MAY choose to not request a push notification
   towards the UA (and wait for the associated REGISTER request and 2xx
   response) before it tries to forward 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



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   of this document.  Similarly, if the Request-URI of the SIP request
   only contains any pn-provider SIP URI parameter, but no other pn- SIP
   URI parameters, e.g., because the SIP UA has not included them in a
   REGISTER request (Section 4.2), the proxy is not able to request a
   push notification towards the UA.  If the proxy has knowledge that
   the UA is awake, and that the UA is able to receive the SIP request,
   the proxy MAY forwards the request towards the UA.  Otherwise the
   proxy MUST reject the SIP request with a 480 (Temporarily
   Unavailable) response.

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
   push notifications towards the UA Section 5.3.2.  However, this
   information will not be present in mid-dialog requests towards the
   UA.  Instead, the proxy need to support a mechanism where it stores
   the information needed to request a push notification towards the UA
   and be able to find and retrieve that information when it receives a
   mid-dialog request.  This section defines such mechanism.  The UA and
   proxy procedures in this section are applied in addition to the
   generic procedures defined in this specification.

6.1.  SIP UA Behavior

6.1.1.  Initial Request for Dialog

   When the UA sends in initial request for a dialog, or a 2xx response
   to such requests, if the UA is willing to receive push notifications
   triggered by incoming mid-dialog requests, the UA MUST include a 'pn-
   purr' SIP URI parameter in the Contact header of the request or
   response.  The UA MUST include a parameter value identical to the the
   last 'sip.pnspurr' feature-capability indicator that it received in a
   REGISTER 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.





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   NOTE: As the 'pn-purr' SIP URI parameter only applies to a give
   dialog, the UA needs to include a 'pn-purr' parameter in the Contact
   header 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 the proxy receives an initial REGISTER request for a
   registration from the UA, if the proxy supports requesting push
   notifications, triggered by mid-dialog requests, towards the
   registered UA, the proxy MUST store the information (the pn- SIP URI
   parameters) needed to request push notifications associated with the
   registration towards 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 is unique within the
   context of the proxy and that can be used as a key to retrieve the
   information.  When the proxy receives the associated 2xx REGISTER
   response, it adds a 'sip.pnspurr' feature-capability indicator with
   the PURR value to the associated 2xx REGISTER response.  When the
   proxy receives a binding-refresh REGISTER request, it MUST add a
   'sip.pnspurr' feature-capability indicator with the previously
   generated key as the value to the associated 2xx REGISTER response.

   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.

   In order to prevent client fingerprinting, the proxy MUST
   periodically generate a new PURR value (even if pn- parameters did
   not change) and provide the new value to the UA in a 2xx binding-
   refresh REGISTER response.  However, as long as there are ongoing
   dialogs associated with the old value, the proxy MUST store it so
   that it can request push notifications towards the UA when it
   receives a mid-dialog request addressed towards the UA.

6.2.2.  Initial Request for Dialog

   When the proxy receives an initial request for a dialog from the UA,
   and if the request contains a 'pn-purr' SIP URI parameter in the
   Contact header field 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].





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   When the proxy receives an initial request for a dialog addressed
   towards the UA, and if the proxy has generated a PURR value
   associated with the pn- parameters included 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].

6.2.3.  Mid-Dialog Request

   When the proxy receives a mid-dialog 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 a
   push notification towards the UA (Section 6.2.1), the proxy MUST
   request a push notification towards the UA.  Once the proxy has
   received the triggered REGISTER request, and the associated
   successful response, the proxy can forward the mid-dialog request
   towards the UA.

   As described in Section 5.3.2, 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 NOTIFY request will
   eventually time out at the sender of the request (UAC), and the
   sender will consider the transaction a failure.

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 the SIP Proxy wants to be able to request push notifications
   towards a UA when it receives an initial request for a dialog that
   replaces an existing dialog, using the mechanism in [RFC3891], the
   following apply:

   o  The proxy MUST provide a PURR to the UA during registration
      Section 6.2.1.
   o  The UA MUST include a 'pn-purr' SIP URI parameter in the Contact
      header of 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 for
      the initial request, and use the 'pn-purr' SIP URI parameter to
      retrieve the stored information needed to request a push
      notification towards the UA.




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   In addition, the operator needs to make sure that 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 push
   notifications towards 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 included 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, and will use, the SIP push mechanism and the push
   notification service identified by the indicator value.  When
   included 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 push notification service(s) 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-list RDQUOT
     pns-list        = pns *(COMMA pns)
     pns             = tag-value

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


8.3.  sip.vapid Feature-Capability Indicator

   The sip.vapid feature-capability indicator, when included in a SIP
   2xx response to a SIP REGISTER request, indicates that the entity
   associated with the indicator supports, and will use, the Voluntary
   Application Server Identification (VAPID) [RFC8292] mechanism when
   requesting push notifications towards the SIP UA associated with the



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   SIP registration.  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 included 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 from the SIP UA associated with the registration
   before the registration expires, without the entity having to request
   push notifications towards the SIP UA in order to trigger the
   REGISTER requests.  The indicator value is the minimum value (given
   in seconds) before the registration expiration when the entity
   expects to receive 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 included in the SIP 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.  The media feature tag has no values.



     pnsreg-mt          = "+sip.pnsreg"








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8.6.  sip.pnspurr Feature-Capability Indicator

   The sip.pnspurr feature-capability indicator, when included 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]>
     COLON = <COLON 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 needs 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 which 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.



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   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.

   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 uniquelly identifies an
   appliciation, 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.yourcompany.yourexampleapp.voip

   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

   For more information about the APNs Topic and device token:

   https://developer.apple.com/library/archive/documentation/NetworkingI
   nternet/Conceptual/RemoteNotificationsPG/CommunicatingwithAPNs.html

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.

   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.



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   For more information on the Project ID and Registration token:

   https://firebase.google.com/docs/cloud-messaging/concept-options

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 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

   Different mechanisms exist for authenticating and authorizing devices
   and users registering with a PNS.  The mechanisms for authorizing and
   authenticating the users are PNS-specific, and are outside the scope
   of this document.  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.

   Typically, the PNS also 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.

   Operators MUST ensure that the SIP signalling is properly secured,
   e.g., using encryption, from malicious middlemen.  TLS MUST be used,
   unless the operators know that the signalling is secured using some
   other mechanism.

   [RFC8292] defines a mechanism which 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



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   devices to restrict push notifications to the proxy associated with
   the key.

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

14.  IANA considerations

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





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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.

















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    Name: sip.pns

    Description: This feature-capability indicator, when included 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, and will use, the SIP
        push mechanism and the push notification service identified by
        the indicator value. When included 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 push
        notification service(s) identified 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 included in a
         SIP 2xx response to a SIP REGISTER request, indicates that the
         entity associated with the indicator supports, and will use,
         the Voluntary Application Server Identification (VAPID)
         mechanism when requesting push notifications towards the SIP UA
         associated with the SIP registration. 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




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



    Name: sip.pnsreg

    Description: This feature-capability indicator, when included 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 from the SIP UA associated
         with the registration before the registration expires, without
         the entity having to request push notifications towards the SIP
         UA in order to trigger the REGISTER requests. The indicator
         value is the minimum value (given in seconds) before the
         registration expireation when the entity expects to receive 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 included 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)






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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 included in the SIP 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 towards the client will be
         requested.

      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].







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     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>.







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   [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>.

   [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>.

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>.



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   [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




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