Internet Engineering Task Force (IETF) C. Holmberg

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 | | |
|<===================================| |
| | | |

------- 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:[email protected] SIP/2.0
Via: SIP/2.0/TCP alicemobile.example.com:5060;branch=z9hG4bKnashds7
Max-Forwards: 70
To: Alice <sip:[email protected]>
From: Alice <sip:[email protected]>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:[email protected];
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:[email protected] SIP/2.0
Via: SIP/2.0/TCP alicemobile.example.com:5060;branch=z9hG4bKnashds7
Max-Forwards: 70
To: Alice <sip:[email protected]>
From: Alice <sip:[email protected]>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:[email protected];
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:[email protected]>;tag=123987
From: Alice <sip:[email protected]>;tag=456248
Call-ID: 843817637684230@998sdasdh09
CSeq: 1826 REGISTER
Contact: <sip:[email protected];
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 | | |
|<===================================| |
| | | |

------- 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 ([email protected])

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 ([email protected])

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 ([email protected])

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 ([email protected])

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 ([email protected])

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: [email protected]

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

Email: [email protected]

Holmberg & Arnold Standards Track [Page 40]