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rfc:rfc8599

Internet Engineering Task Force (IETF) C. Holmberg Request for Comments: 8599 Ericsson Category: Standards Track M. Arnold ISSN: 2070-1721 Metaswitch Networks

                                                              May 2019
    Push Notification with the Session Initiation Protocol (SIP)

Abstract

 This document describes how a Push Notification Service (PNS) can be
 used to wake a suspended Session Initiation Protocol (SIP) User Agent
 (UA) with push notifications, and it also describes how the UA can
 send binding-refresh REGISTER requests and receive incoming SIP
 requests in an environment in which the UA may be suspended.  The
 document defines new SIP URI parameters to exchange PNS information
 between the UA and the SIP entity that will then request that push
 notifications be sent to the UA.  It also defines the parameters to
 trigger such push notification requests.  The document also defines
 new feature-capability indicators that can be used to indicate
 support of this mechanism.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8599.

Holmberg & Arnold Standards Track [Page 1] RFC 8599 SIP PUSH May 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
 (https://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  . . . . . . . . . . . . . . . . . . . . . . . .   4
 2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   8
 3.  Push Resource ID (PRID) . . . . . . . . . . . . . . . . . . .   8
 4.  SIP User Agent (UA) Behavior  . . . . . . . . . . . . . . . .   9
   4.1.  REGISTER  . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.1.  Request Push Notifications  . . . . . . . . . . . . .   9
     4.1.2.  Disable Push Notifications  . . . . . . . . . . . . .  11
     4.1.3.  Receive Push Notifications  . . . . . . . . . . . . .  11
     4.1.4.  Sending Binding-Refresh Requests Using Non-push
             Mechanism . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.5.  Query Network PNS Capabilities  . . . . . . . . . . .  13
 5.  SIP Proxy Behavior  . . . . . . . . . . . . . . . . . . . . .  14
   5.1.  PNS Provider  . . . . . . . . . . . . . . . . . . . . . .  14
   5.2.  SIP Request Push Bucket . . . . . . . . . . . . . . . . .  15
   5.3.  SIP URI Comparison Rules  . . . . . . . . . . . . . . . .  15
   5.4.  Indicate Support of Type of PNS . . . . . . . . . . . . .  15
   5.5.  Trigger Periodic Binding Refresh  . . . . . . . . . . . .  16
   5.6.  SIP Requests  . . . . . . . . . . . . . . . . . . . . . .  17
     5.6.1.  REGISTER  . . . . . . . . . . . . . . . . . . . . . .  17
     5.6.2.  Initial Request for Dialog or Standalone Request  . .  20
 6.  Support of Long-Lived SIP Dialogs . . . . . . . . . . . . . .  23
   6.1.  SIP UA Behavior . . . . . . . . . . . . . . . . . . . . .  25
     6.1.1.  Initial Request for Dialog  . . . . . . . . . . . . .  25
   6.2.  SIP Proxy Behavior  . . . . . . . . . . . . . . . . . . .  25
     6.2.1.  REGISTER  . . . . . . . . . . . . . . . . . . . . . .  25
     6.2.2.  Initial Request for Dialog  . . . . . . . . . . . . .  26
     6.2.3.  Mid-dialog Request  . . . . . . . . . . . . . . . . .  26
 7.  Support of SIP Replaces . . . . . . . . . . . . . . . . . . .  27
 8.  Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . .  28
   8.1.  555 (Push Notification Service Not Supported) Response
         Code  . . . . . . . . . . . . . . . . . . . . . . . . . .  28

Holmberg & Arnold Standards Track [Page 2] RFC 8599 SIP PUSH May 2019

   8.2.  'sip.pns' Feature-Capability Indicator  . . . . . . . . .  28
   8.3.  'sip.vapid' Feature-Capability Indicator  . . . . . . . .  28
   8.4.  'sip.pnsreg' Feature-Capability Indicator . . . . . . . .  28
   8.5.  'sip.pnsreg' Media Feature Tag  . . . . . . . . . . . . .  29
   8.6.  'sip.pnspurr' Feature-Capability Indicator  . . . . . . .  29
   8.7.  SIP URI Parameters  . . . . . . . . . . . . . . . . . . .  29
 9.  PNS Registration Requirements . . . . . . . . . . . . . . . .  30
 10. 'pn-provider', 'pn-param', and 'pn-prid' URI Parameters for
     Apple Push Notification service . . . . . . . . . . . . . . .  30
 11. 'pn-provider', 'pn-param', and 'pn-prid' URI Parameters for
     Google Firebase Cloud Messaging (FCM) Push Notification
     Service . . . . . . . . . . . . . . . . . . . . . . . . . . .  31
 12. 'pn-provider', 'pn-param', and 'pn-prid' URI Parameters for
     RFC 8030 (Generic Event Delivery Using HTTP Push) . . . . . .  31
 13. Security Considerations . . . . . . . . . . . . . . . . . . .  32
 14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  33
   14.1.  SIP URI Parameters . . . . . . . . . . . . . . . . . . .  33
     14.1.1.  pn-provider  . . . . . . . . . . . . . . . . . . . .  33
     14.1.2.  pn-param . . . . . . . . . . . . . . . . . . . . . .  33
     14.1.3.  pn-prid  . . . . . . . . . . . . . . . . . . . . . .  33
     14.1.4.  pn-purr  . . . . . . . . . . . . . . . . . . . . . .  33
   14.2.  SIP Response Codes . . . . . . . . . . . . . . . . . . .  34
     14.2.1.  555 (Push Notification Service Not Supported)  . . .  34
   14.3.  SIP Global Feature-Capability Indicator  . . . . . . . .  34
     14.3.1.  sip.pns  . . . . . . . . . . . . . . . . . . . . . .  34
     14.3.2.  sip.vapid  . . . . . . . . . . . . . . . . . . . . .  34
     14.3.3.  sip.pnsreg . . . . . . . . . . . . . . . . . . . . .  35
     14.3.4.  sip.pnspurr  . . . . . . . . . . . . . . . . . . . .  35
   14.4.  SIP Media Feature Tag  . . . . . . . . . . . . . . . . .  36
     14.4.1.  sip.pnsreg . . . . . . . . . . . . . . . . . . . . .  36
   14.5.  PNS Subregistry Establishment  . . . . . . . . . . . . .  36
 15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  37
   15.1.  Normative References . . . . . . . . . . . . . . . . . .  37
   15.2.  Informative References . . . . . . . . . . . . . . . . .  39
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  40
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  40

Holmberg & Arnold Standards Track [Page 3] RFC 8599 SIP PUSH May 2019

1. Introduction

 In order to save resources such as battery life, some devices
 (especially mobile devices) and operating systems will suspend an
 application that is not in use.  A suspended application might not be
 able to wake itself with internal timers and might not be awakened by
 incoming network traffic.  In such an environment, a Push
 Notification Service (PNS) is used to wake the application.  A PNS is
 a service that sends messages requested by other applications to a
 user application in order to wake the user application.  These
 messages are called push notifications.  Push notifications might
 contain payload data, depending on the application.  An application
 can request that a push notification be sent to a single user
 application or to multiple user applications.
 Typically, each operating system uses a dedicated PNS.  Different
 PNSs exist today.  Some are based on the standardized mechanism
 defined in [RFC8030], while others are proprietary.  For example,
 Apple iOS devices use the Apple Push Notification service (APNs)
 while Android devices use the Firebase Cloud Messaging (FCM) service.
 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.
 When a Session Initiation Protocol (SIP) User Agent (UA)[RFC3261] is
 suspended in such an environment, it is unable to send binding-
 refresh SIP REGISTER requests, unable to receive incoming SIP
 requests, and might not be able to use internal timers to wake
 itself.  A suspended UA will not be able to maintain connections,
 e.g., using the SIP Outbound Mechanism [RFC5626], because it cannot
 send periodic keep-alive messages.  A PNS is needed to wake the SIP
 UA so that the UA can perform these functions.
 This document describes how a PNS can be used to wake a suspended UA
 using push notifications, so that the UA can send binding-refresh
 REGISTER requests and 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; be used 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 be used to request such push notification
 requests.

Holmberg & Arnold Standards Track [Page 4] RFC 8599 SIP PUSH May 2019

 NOTE: Even if a UA is able to be awakened by means other 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 UA 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 with a PNS (Figure 1), 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, which will then request that push notifications are
 sent to the UA.
 When the SIP proxy receives a SIP request for a new dialog or a
 standalone SIP request addressed towards a UA, or when the SIP proxy
 determines that the UA needs to send a binding-refresh REGISTER
 request, the SIP proxy will send a push request containing the PRID
 of the UA to the PNS, which will then send a push notification to the
 UA.  Once the UA receives the push notification, it will be able to
 send a binding-refresh REGISTER request.  The proxy receives the
 REGISTER request from the UA and forwards it to the SIP registrar
 [RFC3261].  After accepting the REGISTER request, the SIP registrar
 sends a 2xx response to the proxy, which forwards the response to the
 UA.  If the push notification request was triggered by a SIP request
 addressed towards the UA, the proxy can then forward the SIP request
 to 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 from the SIP registrar.  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 for the SIP request that triggered the
 push notification to reach the UA.
 Figure 1 shows the generic push notification architecture supported
 by the mechanism in this document.
 The SIP proxy MUST be in the signaling path of REGISTER requests sent
 by the UA towards the registrar, and of SIP requests (for a new
 dialog or a standalone) forwarded by the proxy responsible for the
 UA's domain (sometimes referred to as home proxy, Serving Call

Holmberg & Arnold Standards Track [Page 5] RFC 8599 SIP PUSH May 2019

 Session Control Function (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 standalone) can be matched against contacts in
 REGISTER requests.

Holmberg & Arnold Standards Track [Page 6] RFC 8599 SIP PUSH May 2019

   +--------+      +---------+        +-----------+    +-------------+
   |        |      |         |        |           |    | 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      |                  |                   |
       |<===================================|                   |
       |                 |                  |                   |
  1. —— Push Notification API

======= SIP

                  Figure 1: SIP Push Information Flow

Holmberg & Arnold Standards Track [Page 7] RFC 8599 SIP PUSH May 2019

   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 preconfigured in the operating system of
 the device.

Holmberg & Arnold Standards Track [Page 8] RFC 8599 SIP PUSH May 2019

4. SIP User Agent (UA) Behavior

4.1. REGISTER

 This section describes how a SIP UA sends SIP REGISTER requests
 (either an 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 (Figure 2).
 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: Since a push notification will trigger the UA to refresh all
 bindings, if a SIP UA has created multiple bindings, it is preferable
 if one can ensure that all bindings expire at the same time to help
 prevent some bindings from being 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 requests in order 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
 mechanism defined by the SIP event package for registrations
 [RFC3680].  See Section 13 for more information.

4.1.1. Request Push Notifications

 This section describes the procedures that a SIP UA follows to
 request push notifications from the SIP network.  The procedures
 assume that the UA has retrieved a PRID from a PNS.  The procedures
 for retrieving the PRID from the PNS are PNS-specific and outside the
 scope of this specification.  See PNS-specific documentation for more
 details.

Holmberg & Arnold Standards Track [Page 9] RFC 8599 SIP PUSH May 2019

 This specification does not define a mechanism to explicitly request
 push notifications from the SIP network for usages other than
 triggering binding-refresh REGISTER requests (e.g., for sending
 periodic subscription-refresh SUBSCRIBE requests [RFC6665]), nor does
 it describe how to distinguish push notifications associated with
 such usages from the push notifications used to trigger binding-
 refresh REGISTER requests.  If a SIP UA wants to use push
 notifications for other usages, the UA can perform actions associated
 with such usages (in addition to sending a binding-refresh REGISTER
 request) whenever it receives a push notification by using the same
 refresh interval that is used for the binding refreshes.
 To request push notifications from the SIP network, 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 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 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 another SIP
 Proxy in the SIP network will request that push notifications are
 sent to the UA.  In addition, if the 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 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).

Holmberg & Arnold Standards Track [Page 10] RFC 8599 SIP PUSH May 2019

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), 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 NOT insert the 'pn-prid' SIP URI parameter in
 the Contact header field URI of the REGISTER request.  The lack of
 the parameter informs 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 standalone SIP request (e.g., a SIP MESSAGE) if such a SIP
 request triggered the proxy to request that the push notification was
 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 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.

Holmberg & Arnold Standards Track [Page 11] RFC 8599 SIP PUSH May 2019

 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 binding-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 needs to 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 that require an
 immediate REGISTER request to be sent (e.g., if the UA is assigned
 new contact parameters due to a network configuration change).
 Even if the UA is able 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 binding-
 refresh REGISTER requests, such REGISTER requests will update the
 binding expiration timer, and the proxy does not need to request that
 a push notification be sent to the UA in order to wake the UA.  The
 proxy will still request that a push notification be 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 be suspended (in order to save
 battery resources, etc.) between sending the REGISTER requests and
 using push notifications to wake the UA to process incoming calls.

Holmberg & Arnold Standards Track [Page 12] RFC 8599 SIP PUSH May 2019

   Example of a SIP REGISTER request including a 'sip.pnsreg'
   media feature tag:
   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>;
     +sip.pnsreg
   Expires: 7200
   Content-Length: 0
   Example of a SIP REGISTER response including a 'sip.pnsreg'
   media feature tag and a 'sip.pnsreq' feature-capability indicator:
   SIP/2.0 200 OK
   Via: SIP/2.0/TCP alicemobile.example.com:5060;branch=z9hG4bKnashds7
   To: Alice <sip:alice@example.com>;tag=123987
   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>;
     +sip.pnsreg
   Feature-Caps: *;+sip.pns="acme";+sip.pnsreg="121"
   Expires: 7200
   Content-Length: 0
     Figure 3: SIP REGISTER When Using Non-push Mechanism Example

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.

Holmberg & Arnold Standards Track [Page 13] RFC 8599 SIP PUSH May 2019

 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:
 o  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.
 o  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, and if the UA inserted a
 'pn-provider' SIP URI parameter in the REGISTER request, the response
 indicates that the network does not support the type of PNS that the
 UA indicated support of.  If the UA did not insert a 'pn-provider'
 parameter in the REGISTER request, the response indicates that the
 network does not support any type of PNS while still supporting the
 555 (Push Notification Service Not Supported) response.
 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.

Holmberg & Arnold Standards Track [Page 14] RFC 8599 SIP PUSH May 2019

 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 Bucket

 When a SIP proxy receives a SIP request addressed towards a UA, that
 will trigger the proxy to request that a push notification be sent to
 the UA.  The proxy will place the request in storage (referred to as
 the SIP Request Push Bucket) and the proxy will start a timer
 (referred to as the Bucket Timer) associated with the transaction.  A
 SIP request is removed from the bucket when one of the following has
 occurred: the proxy forwards the request towards the UA, the proxy
 sends an error response to the request, or the Bucket Timer times
 out.  The detailed procedures are described in the sections below.
 Exactly how the SIP Request Push 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 bucket.  Note that mid-dialog requests
 (Section 6) do not carry the PRID in the SIP request itself.

5.3. SIP URI Comparison Rules

 By default, a SIP proxy uses the URI comparison rules defined in
 [RFC3261].  However, when a SIP 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 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 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 Bucket (Section 5.2) if the UA had
 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 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 with the type of PNS.  A proxy MUST use a separate
 Feature-Cap header field for each supported type of PNS.  A feature-

Holmberg & Arnold Standards Track [Page 15] RFC 8599 SIP PUSH May 2019

 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 be 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., by 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 be 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 itself 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 be 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).

Holmberg & Arnold Standards Track [Page 16] RFC 8599 SIP PUSH May 2019

 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.  In other cases, the proxy MUST skip the 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 that contains a Feature-Caps
 header field with a 'sip.pns' feature-capability indicator, it
 indicates that another 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 that 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

Holmberg & Arnold Standards Track [Page 17] RFC 8599 SIP PUSH May 2019

    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.
 o  If the proxy supports the type 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
 binding would expire before the proxy can request that a push
 notification be sent to the UA 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.
 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.

Holmberg & Arnold Standards Track [Page 18] RFC 8599 SIP PUSH May 2019

  • 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 that contains a
 'pn-provider' SIP URI parameter indicating the type of PNS supported
 by the UA, the proxy MUST perform the following actions:
 o  If the proxy supports the type 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 any other proxies between the proxy
    and the registrar that the proxy supports the type of PNS
    supported by the UA.
 o  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.
 o  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.
 When a proxy receives a REGISTER request, and 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.

Holmberg & Arnold Standards Track [Page 19] RFC 8599 SIP PUSH May 2019

 When a proxy receives a 2xx response to the REGISTER request, if the
 proxy had 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 those PNSs in the
 response.

5.6.2. Initial Request for Dialog or Standalone Request

 The procedures in this section apply when a SIP proxy has indicated
 that 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 standalone 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 be 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 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:
 o  The proxy processes the REGISTER response as described in
    Section 5.6.1.
 o  The proxy checks whether the SIP Request Push Bucket contains a
    SIP request associated with the REGISTER 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 bucket.
    If there is a match, the proxy MUST remove the SIP request from
    the bucket and forward it towards the UA.

Holmberg & Arnold Standards Track [Page 20] RFC 8599 SIP PUSH May 2019

 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 Bucket.  If there is a match, the proxy SHOULD remove the
 associated request from the 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
 bucket.
 If the push notification request fails (see PNS-specific
 documentation for details), the proxy MUST remove the SIP request
 from the 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.
 After the proxy has requested that a push notification be 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 before the Bucket
 Timer (Section 5.2) associated with the SIP request times out, the
 proxy MUST remove the SIP request from the SIP Request Push Bucket
 (Section 5.2) and send a 480 (Temporarily Unavailable) response.  The
 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, which 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, if the proxy forwards the SIP request towards the SIP
 UA, the SIP UA accepts the request, but the transaction times out at
 the sender before it receives the successful response, this will
 cause state misalignment between the endpoints (the sender considers
 the transaction a failure, while the receiver considers the
 transaction a success).  The SIP proxy needs to take this into
 account when it sets the value of the Bucket Timer associated with
 the transaction, to make sure that the error response (triggered by a

Holmberg & Arnold Standards Track [Page 21] RFC 8599 SIP PUSH May 2019

 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 cannot be 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 a
 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 the 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 either waiting for a 2xx response to a REGISTER request
 or requesting that a push notification be sent to the UA:
 o  If the proxy is able to authenticate the sender of the REGISTER
    request and verify that it is allowed by authorization policy, 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 Bucket.
 o  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 be 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).

Holmberg & Arnold Standards Track [Page 22] RFC 8599 SIP PUSH May 2019

6. Support of Long-Lived 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 the sending of mid-dialog
 requests.  Because of this, a SIP UA may be suspended and may need to
 be awakened in order to be able to receive mid-dialog requests.
 SIP requests for a new dialog and standalone SIP requests addressed
 towards a UA with 'pn-*' SIP URI parameters allow the proxy to
 request that a push notification be sent to the UA (Section 5.6.2).
 However, 'pn-*' SIP URI parameters will not be present in mid-dialog
 requests addressed towards the UA.  Instead, the proxy needs to
 support a mechanism to store the information needed to request that a
 push notification be 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 a 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  |
   +--------+      +---------+        +-----------+    +-------------+
       |                 |                  |                   |
       | PNS Register    |                  |                   |
       |---------------->|                  |                   |
       |                 |                  |                   |
       | PRID            |                  |                   |
       |<----------------|                  |                   |
       |                 |                  |                   |
       | SIP REGISTER (PRID)                |                   |
       |===================================>|                   |
       |                 |                  |SIP REGISTER (PRID)|
       |                 |                  |==================>|
       |                 |                  |                   |
       |                 |      +-----------------------+       |
       |                 |      | Store PRID (key=PURR) |       |
       |                 |      +-----------------------+       |
       |                 |                  |                   |
       |                 |                  | SIP 200 OK        |
       |                 |                  |<==================|
       | SIP 200 OK (PURR)                  |                   |
       |<===================================|                   |
       |                 |                  |                   |
       |                 |                  |                   |

Holmberg & Arnold Standards Track [Page 23] RFC 8599 SIP PUSH May 2019

       | 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 (PURR)                  |                   |
       |<===================================|                   |
       |                 |                  |                   |
       | SIP UPDATE      |                  |                   |
       |<===================================|                   |
       |                 |                  |                   |
  1. —— Push Notification API
       ======= SIP
               Figure 4: SIP Push Long-Lived Dialog Flow

Holmberg & Arnold Standards Track [Page 24] RFC 8599 SIP PUSH May 2019

6.1. SIP UA Behavior

6.1.1. Initial Request for Dialog

 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 (Section 6.2.1) in the Contact
 header field URI of the initial request for a dialog or the 2xx
 response to such requests.  The UA MUST insert a parameter value
 identical to the last 'sip.pnspurr' feature-capability indicator
 (Section 6.2.1) that it received in a REGISTER response.  If the UA
 has not received a 'sip.pnspurr' feature-capability indicator, the UA
 MUST NOT insert a 'pn-purr' SIP URI parameter in a request or
 response.
 The UA makes the decision to receive push notifications triggered by
 incoming mid-dialog requests 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

 If the proxy supports requesting push notifications triggered by mid-
 dialog requests being 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 when a proxy receives an
 initial REGISTER request for a binding from 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 is unforgeable,
 anonymous, and unlinkable to entities other than the proxy.  It must
 not be possible for an attacker to generate a valid PURR, to

Holmberg & Arnold Standards Track [Page 25] RFC 8599 SIP PUSH May 2019

 associate a PURR with a specific user, or to determine when two PURRs
 correspond to the same user.  It can be generated, e.g., by utilizing
 a cryptographically secure random function with an appropriately
 large output size.
 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 that
 contains a 'pn-purr' SIP URI parameter in the Contact header field
 URI with a PURR value that the proxy has generated (Section 6.2.1),
 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, and 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 that contains a 'pn-purr' SIP URI parameter, and the proxy is
 able to retrieve the stored information needed to request that a push
 notification be sent to the UA (Section 6.2.1), the proxy MUST place
 the SIP request in the SIP Request Push Bucket and request that a
 push notification be 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 Bucket contains a mid-
 dialog SIP request associated with the REGISTER transaction.  If the
 bucket contains such a request, the proxy MUST remove the SIP request
 from the 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

Holmberg & Arnold Standards Track [Page 26] RFC 8599 SIP PUSH May 2019

 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 then for 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 (User Agent Client),
 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 the Contact header field URI
 of the peer.
 If a SIP proxy wants to be able to request that a push notification
 be 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 either 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 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 be sent to the UA).  The procedures for
 doing that are operator-specific and are outside the scope of this
 specification.

Holmberg & Arnold Standards Track [Page 27] RFC 8599 SIP PUSH May 2019

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 does 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.  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, denotes that the entity
 associated with the indicator supports the Voluntary Application
 Server Identification (VAPID) [RFC8292] mechanism when the entity
 requests that a push notification be 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, denotes that the entity
 associated with the indicator expects to receive binding-refresh
 REGISTER requests from the SIP UA associated with the binding before

Holmberg & Arnold Standards Track [Page 28] RFC 8599 SIP PUSH May 2019

 the binding expires, even if the entity does not request that a push
 notification be sent to the SIP UA in order to trigger the binding-
 refresh REGISTER requests.  The indicator value conveys 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 awakened 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, denotes 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

 This section defines new SIP URI parameters by extending the grammar
 for "uri-parameter" as defined in [RFC3261].  The ABNF [RFC5234] 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]>

Holmberg & Arnold Standards Track [Page 29] RFC 8599 SIP PUSH May 2019

 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 subregistry, 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, see
 <https://developer.apple.com/library/archive/documentation/
 NetworkingInternet/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 of 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 Voice over IP
 (VoIP) applications, the service value is "voip".
 Example: pn-param=DEF123GHIJ.com.example.yourexampleapp.voip

Holmberg & Arnold Standards Track [Page 30] RFC 8599 SIP PUSH May 2019

 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, see
 <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.

Holmberg & Arnold Standards Track [Page 31] RFC 8599 SIP PUSH May 2019

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) are 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 requires 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
 signaling might be able to request that push notifications are sent
 to a UA.  Such push notifications will impact the battery life of the
 UA and trigger unnecessary SIP traffic.
 [RFC8292] defines a mechanism that allows a proxy to identify itself
 to a PNS by signing a JSON Web Token (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 signaling is properly secured,
 e.g., using encryption, from malicious network entities.  TLS MUST be
 used unless the operators know that the signaling 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 SIP event package for
 registrations mechanism [RFC3680].  [RFC3680] defines generic
 security considerations for the SIP event package for registrations.
 As the PNS-related SIP URI parameters conveyed in the REGISTER

Holmberg & Arnold Standards Track [Page 32] RFC 8599 SIP PUSH May 2019

 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

14.1. SIP URI Parameters

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

14.1.1. pn-provider

   Parameter Name: pn-provider
   Predefined Values:  No
   Reference:  RFC 8599

14.1.2. pn-param

   Parameter Name: pn-param
   Predefined Values:  No
   Reference:  RFC 8599

14.1.3. pn-prid

   Parameter Name: pn-prid
   Predefined Values:  No
   Reference:  RFC 8599

14.1.4. pn-purr

   Parameter Name: pn-purr
   Predefined Values:  No
   Reference:  RFC 8599

Holmberg & Arnold Standards Track [Page 33] RFC 8599 SIP PUSH May 2019

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" subregistry [RFC3261] under the SIP Parameters
 registry (https://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" subregistry
 [RFC6809] under the SIP Parameters registry
 (https://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, denotes that the entity
      associated with the indicator supports the SIP push mechanism
      and the type of push notification service conveyed by the
      indicator value.
  Reference: RFC 8599
  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" subregistry
 [RFC6809] under the SIP Parameters registry
 (https://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, denotes that the
        entity associated with the indicator supports the Voluntary
        Application Server Identification (VAPID) mechanism when the
        entity requests that a push notification be sent to a SIP UA.

Holmberg & Arnold Standards Track [Page 34] RFC 8599 SIP PUSH May 2019

        The indicator value is a public key identifying the entity,
        which can be used by a SIP UA to restrict subscriptions to
        that entity.
   Reference: RFC 8599
   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" subregistry
 [RFC6809] under the SIP Parameters registry
 (https://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, denotes 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 be sent
       to the SIP UA in order to trigger the binding-refresh REGISTER
       requests.  The indicator value conveys the minimum time
       (given in seconds) prior to the binding expiration when the UA
       MUST send the REGISTER request.
  Reference: RFC 8599
  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" subregistry
 [RFC6809] under the SIP Parameters registry
 (https://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, conveys 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 as a key to retrieve
       the binding information.

Holmberg & Arnold Standards Track [Page 35] RFC 8599 SIP PUSH May 2019

  Reference: RFC 8599
  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" subregistry [RFC3840] under the
 "Media Feature Tags" registry (https://www.iana.org/assignments/
 media-feature-tags).
   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, conveys that the SIP UA
        associated with the tag is able to send binding-refresh
        REGISTER requests associated with the registration without
        being awakened by push notifications.
   Values appropriate for use with this feature tag: none
   Related standards or documents: RFC 8599
   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 Subregistry Establishment

 This section creates a new subregistry, "PNS", under the SIP
 Parameters registry (https://www.iana.org/assignments/
 sip-parameters).
 The purpose of the subregistry is to register SIP URI 'pn-provider'
 values.
 When a SIP URI 'pn-provider' value is registered in the subregistry,
 it needs to meet the "Specification Required" policies defined in
 [RFC8126].

Holmberg & Arnold Standards Track [Page 36] RFC 8599 SIP PUSH May 2019

 This subregistry 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 8599
   fcm           Firebase Cloud Messaging                RFC 8599
   webpush       Generic Event Delivery Using HTTP Push  RFC 8599

15. References

15.1. 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>.

Holmberg & Arnold Standards Track [Page 37] RFC 8599 SIP PUSH May 2019

 [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>.
 [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234,
            DOI 10.17487/RFC5234, January 2008,
            <https://www.rfc-editor.org/info/rfc5234>.
 [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., "Local and Remote Notification Programming
            Guide: Communicating with APNs", <https://developer.apple.
            com/library/archive/documentation/NetworkingInternet/Conce
            ptual/RemoteNotificationsPG/CommunicatingwithAPNs.html>.
 [pns-fcm]  Google Inc., "Firebase Cloud Messaging",
            <https://firebase.google.com/docs/cloud-messaging/
            concept-options>.

Holmberg & Arnold Standards Track [Page 38] RFC 8599 SIP PUSH May 2019

15.2. 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>.

Holmberg & Arnold Standards Track [Page 39] RFC 8599 SIP PUSH May 2019

Acknowledgements

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

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

Holmberg & Arnold Standards Track [Page 40]

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