Internet Engineering Task Force (IETF) A.B. Roach
Request for Comments: 6665 Tekelec
Obsoletes: 3265 July 2012
Updates: 3261, 4660
Category: Standards Track
ISSN: 2070-1721
SIP-Specific Event Notification
Abstract
This document describes an extension to the Session Initiation
Protocol (SIP) defined by RFC 3261. The purpose of this extension is
to provide an extensible framework by which SIP nodes can request
notification from remote nodes indicating that certain events have
occurred.
Note that the event notification mechanisms defined herein are NOT
intended to be a general-purpose infrastructure for all classes of
event subscription and notification.
This document represents a backwards-compatible improvement on the
original mechanism described by RFC 3265, taking into account several
years of implementation experience. Accordingly, this document
obsoletes RFC 3265. This document also updates RFC 4660 slightly to
accommodate some small changes to the mechanism that were discussed
in that document.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6665.
Roach Standards Track [Page 1]RFC 6665 SIP-Specific Event Notification July 2012
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Overview of Operation . . . . . . . . . . . . . . . . . . 5
1.2. Documentation Conventions . . . . . . . . . . . . . . . . 6
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. SIP Methods for Event Notification . . . . . . . . . . . . . . 7
3.1. SUBSCRIBE . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.1. Subscription Duration . . . . . . . . . . . . . . . . 7
3.1.2. Identification of Subscribed Events and Event
Classes . . . . . . . . . . . . . . . . . . . . . . . 8
3.1.3. Additional SUBSCRIBE Header Field Values . . . . . . . 9
3.2. NOTIFY . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2.1. Identification of Reported Events, Event Classes,
and Current State . . . . . . . . . . . . . . . . . . 9
4. Node Behavior . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. Subscriber Behavior . . . . . . . . . . . . . . . . . . . 10
4.1.1. Detecting Support for SIP Events . . . . . . . . . . . 10
4.1.2. Creating and Maintaining Subscriptions . . . . . . . . 10
4.1.3. Receiving and Processing State Information . . . . . . 14
4.1.4. Forking of SUBSCRIBE Requests . . . . . . . . . . . . 16
4.2. Notifier Behavior . . . . . . . . . . . . . . . . . . . . 17
4.2.1. Subscription Establishment and Maintenance . . . . . . 17
4.2.2. Sending State Information to Subscribers . . . . . . . 20
4.2.3. PSTN/Internet Interworking (PINT) Compatibility . . . 23
4.3. Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 23
4.4. Common Behavior . . . . . . . . . . . . . . . . . . . . . 24
4.4.1. Dialog Creation and Termination . . . . . . . . . . . 24
4.4.2. Notifier Migration . . . . . . . . . . . . . . . . . . 24
4.4.3. Polling Resource State . . . . . . . . . . . . . . . . 25
4.4.4. "Allow-Events" Header Field Usage . . . . . . . . . . 26
4.5. Targeting Subscriptions at Devices . . . . . . . . . . . . 26
4.5.1. Using GRUUs to Route to Devices . . . . . . . . . . . 27
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4.5.2. Sharing Dialogs . . . . . . . . . . . . . . . . . . . 27
4.6. CANCEL Requests for SUBSCRIBE and NOTIFY Transactions . . 29
5. Event Packages . . . . . . . . . . . . . . . . . . . . . . . . 29
5.1. Appropriateness of Usage . . . . . . . . . . . . . . . . . 29
5.2. Event Template-Packages . . . . . . . . . . . . . . . . . 30
5.3. Amount of State to Be Conveyed . . . . . . . . . . . . . . 31
5.3.1. Complete State Information . . . . . . . . . . . . . . 31
5.3.2. State Deltas . . . . . . . . . . . . . . . . . . . . . 32
5.4. Event Package Responsibilities . . . . . . . . . . . . . . 32
5.4.1. Event Package Name . . . . . . . . . . . . . . . . . . 33
5.4.2. Event Package Parameters . . . . . . . . . . . . . . . 33
5.4.3. SUBSCRIBE Request Bodies . . . . . . . . . . . . . . . 33
5.4.4. Subscription Duration . . . . . . . . . . . . . . . . 33
5.4.5. NOTIFY Request Bodies . . . . . . . . . . . . . . . . 34
5.4.6. Notifier Processing of SUBSCRIBE Requests . . . . . . 34
5.4.7. Notifier generation of NOTIFY requests . . . . . . . . 34
5.4.8. Subscriber Processing of NOTIFY Requests . . . . . . . 34
5.4.9. Handling of Forked Requests . . . . . . . . . . . . . 34
5.4.10. Rate of Notifications . . . . . . . . . . . . . . . . 35
5.4.11. State Aggregation . . . . . . . . . . . . . . . . . . 35
5.4.12. Examples . . . . . . . . . . . . . . . . . . . . . . . 36
5.4.13. Use of URIs to Retrieve State . . . . . . . . . . . . 36
6. Security Considerations . . . . . . . . . . . . . . . . . . . 36
6.1. Access Control . . . . . . . . . . . . . . . . . . . . . . 36
6.2. Notifier Privacy Mechanism . . . . . . . . . . . . . . . . 36
6.3. Denial-of-Service Attacks . . . . . . . . . . . . . . . . 37
6.4. Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 37
6.5. Man-in-the-Middle Attacks . . . . . . . . . . . . . . . . 37
6.6. Confidentiality . . . . . . . . . . . . . . . . . . . . . 38
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
7.1. Event Packages . . . . . . . . . . . . . . . . . . . . . . 38
7.1.1. Registration Information . . . . . . . . . . . . . . . 39
7.1.2. Registration Template . . . . . . . . . . . . . . . . 40
7.2. Reason Codes . . . . . . . . . . . . . . . . . . . . . . . 40
7.3. Header Field Names . . . . . . . . . . . . . . . . . . . . 41
7.4. Response Codes . . . . . . . . . . . . . . . . . . . . . . 41
8. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
8.1. New Methods . . . . . . . . . . . . . . . . . . . . . . . 42
8.1.1. SUBSCRIBE Method . . . . . . . . . . . . . . . . . . . 42
8.1.2. NOTIFY Method . . . . . . . . . . . . . . . . . . . . 42
8.2. New Header Fields . . . . . . . . . . . . . . . . . . . . 42
8.2.1. "Event" Header Field . . . . . . . . . . . . . . . . . 42
8.2.2. "Allow-Events" Header Field . . . . . . . . . . . . . 43
8.2.3. "Subscription-State" Header Field . . . . . . . . . . 43
8.3. New Response Codes . . . . . . . . . . . . . . . . . . . . 43
8.3.1. 202 (Accepted) Response Code . . . . . . . . . . . . . 43
8.3.2. 489 (Bad Event) Response Code . . . . . . . . . . . . 44
8.4. Augmented BNF Definitions . . . . . . . . . . . . . . . . 44
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9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45
9.1. Normative References . . . . . . . . . . . . . . . . . . . 45
9.2. Informative References . . . . . . . . . . . . . . . . . . 46
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 48
Appendix B. Changes from RFC 3265 . . . . . . . . . . . . . . . . 48
B.1. Bug 666: Clarify use of "expires=xxx" with "terminated" . 48
B.2. Bug 667: Reason code for unsub/poll not clearly
spelled out . . . . . . . . . . . . . . . . . . . . . . . 48
B.3. Bug 669: Clarify: SUBSCRIBE for a duration might be
answered with a NOTIFY/expires=0 . . . . . . . . . . . . . 48
B.4. Bug 670: Dialog State Machine needs clarification . . . . 49
B.5. Bug 671: Clarify timeout-based removal of subscriptions . 49
B.6. Bug 672: Mandate "expires" in NOTIFY . . . . . . . . . . . 49
B.7. Bug 673: INVITE 481 response effect clarification . . . . 49
B.8. Bug 677: SUBSCRIBE response matching text in error . . . . 49
B.9. Bug 695: Document is not explicit about response to
NOTIFY at subscription termination . . . . . . . . . . . . 49
B.10. Bug 696: Subscription state machine needs clarification . 49
B.11. Bug 697: Unsubscription behavior could be clarified . . . 49
B.12. Bug 699: NOTIFY and SUBSCRIBE are target refresh
requests . . . . . . . . . . . . . . . . . . . . . . . . . 50
B.13. Bug 722: Inconsistent 423 reason phrase text . . . . . . . 50
B.14. Bug 741: Guidance needed on when to not include
"Allow-Events" . . . . . . . . . . . . . . . . . . . . . . 50
B.15. Bug 744: 5xx to NOTIFY terminates a subscription, but
should not . . . . . . . . . . . . . . . . . . . . . . . . 50
B.16. Bug 752: Detection of forked requests is incorrect . . . . 50
B.17. Bug 773: Reason code needs IANA registry . . . . . . . . . 50
B.18. Bug 774: Need new reason for terminating subscriptions
to resources that never change . . . . . . . . . . . . . . 50
B.19. Clarify Handling of "Route"/"Record-Route" in NOTIFY . . . 50
B.20. Eliminate Implicit Subscriptions . . . . . . . . . . . . . 51
B.21. Deprecate Dialog Reuse . . . . . . . . . . . . . . . . . . 51
B.22. Rationalize Dialog Creation . . . . . . . . . . . . . . . 51
B.23. Refactor Behavior Sections . . . . . . . . . . . . . . . . 51
B.24. Clarify Sections That Need to Be Present in Event
Packages . . . . . . . . . . . . . . . . . . . . . . . . . 51
B.25. Make CANCEL Handling More Explicit . . . . . . . . . . . . 51
B.26. Remove "State Agent" Terminology . . . . . . . . . . . . . 51
B.27. Miscellaneous Changes . . . . . . . . . . . . . . . . . . 52
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1. Introduction
The ability to request asynchronous notification of events proves
useful in many types of SIP services for which cooperation between
end-nodes is required. Examples of such services include automatic
callback services (based on terminal state events), buddy lists
(based on user presence events), message waiting indications (based
on mailbox state change events), and PSTN and Internet
Internetworking (PINT) [RFC2848] status (based on call state events).
The methods described in this document provide a framework by which
notification of these events can be ordered.
The event notification mechanisms defined herein are NOT intended to
be a general-purpose infrastructure for all classes of event
subscription and notification. Meeting requirements for the general
problem set of subscription and notification is far too complex for a
single protocol. Our goal is to provide a SIP-specific framework for
event notification that is not so complex as to be unusable for
simple features, but that is still flexible enough to provide
powerful services. Note, however, that event packages based on this
framework may define arbitrarily elaborate rules that govern the
subscription and notification for the events or classes of events
they describe.
This document does not describe an extension that may be used
directly; it must be extended by other documents (herein referred to
as "event packages"). In object-oriented design terminology, it may
be thought of as an abstract base class that must be derived into an
instantiable class by further extensions. Guidelines for creating
these extensions are described in Section 5.
1.1. Overview of Operation
The general concept is that entities in the network can subscribe to
resource or call state for various resources or calls in the network,
and those entities (or entities acting on their behalf) can send
notifications when those states change.
A typical flow of messages would be:
Subscriber Notifier
|-----SUBSCRIBE---->| Request state subscription
|<-------200--------| Acknowledge subscription
|<------NOTIFY----- | Return current state information
|--------200------->|
|<------NOTIFY----- | Return current state information
|--------200------->|
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Subscriptions are expired and must be refreshed by subsequent
SUBSCRIBE requests.
1.2. Documentation Conventions
There are several paragraphs throughout this document that provide
motivational or clarifying text. Such passages are non-normative and
are provided only to assist with reader comprehension. These
passages are set off from the remainder of the text by being indented
thus:
This is an example of non-normative explanatory text. It does not
form part of the specification and is used only for clarification.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
In particular, implementors need to take careful note of the meaning
of "SHOULD" defined in RFC 2119. To rephrase: violation of "SHOULD"-
strength requirements requires careful analysis and clearly
enumerable reasons. It is a protocol violation to fail to comply
with "SHOULD"-strength requirements whimsically or for ease of
implementation.
2. Definitions
Event Package: An event package is an additional specification that
defines a set of state information to be reported by a notifier to
a subscriber. Event packages also define further syntax and
semantics that are based on the framework defined by this document
and are required to convey such state information.
Event Template-Package: An event template-package is a special kind
of event package that defines a set of states that may be applied
to all possible event packages, including itself.
Notification: Notification is the act of a notifier sending a NOTIFY
request to a subscriber to inform the subscriber of the state of a
resource.
Notifier: A notifier is a user agent that generates NOTIFY requests
for the purpose of notifying subscribers of the state of a
resource. Notifiers typically also accept SUBSCRIBE requests to
create subscriptions.
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Subscriber: A subscriber is a user agent that receives NOTIFY
requests from notifiers; these NOTIFY requests contain information
about the state of a resource in which the subscriber is
interested. Subscribers typically also generate SUBSCRIBE
requests and send them to notifiers to create subscriptions.
Subscription: A subscription is a set of application state
associated with a dialog. This application state includes a
pointer to the associated dialog, the event package name, and
possibly an identification token. Event packages will define
additional subscription state information. By definition,
subscriptions exist in both a subscriber and a notifier.
Subscription Migration: Subscription migration is the act of moving
a subscription from one notifier to another notifier.
3. SIP Methods for Event Notification
3.1. SUBSCRIBE
The SUBSCRIBE method is used to request current state and state
updates from a remote node. SUBSCRIBE requests are target refresh
requests, as that term is defined in [RFC3261].
3.1.1. Subscription Duration
SUBSCRIBE requests SHOULD contain an "Expires" header field (defined
in [RFC3261]). This expires value indicates the duration of the
subscription. In order to keep subscriptions effective beyond the
duration communicated in the "Expires" header field, subscribers need
to refresh subscriptions on a periodic basis using a new SUBSCRIBE
request on the same dialog as defined in [RFC3261].
If no "Expires" header field is present in a SUBSCRIBE request, the
implied default MUST be defined by the event package being used.
200-class responses to SUBSCRIBE requests also MUST contain an
"Expires" header field. The period of time in the response MAY be
shorter but MUST NOT be longer than specified in the request. The
notifier is explicitly allowed to shorten the duration to zero. The
period of time in the response is the one that defines the duration
of the subscription.
An "expires" parameter on the "Contact" header field has no semantics
for the SUBSCRIBE method and is explicitly not equivalent to an
"Expires" header field in a SUBSCRIBE request or response.
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A natural consequence of this scheme is that a SUBSCRIBE request with
an "Expires" of 0 constitutes a request to unsubscribe from the
matching subscription.
In addition to being a request to unsubscribe, a SUBSCRIBE request
with "Expires" of 0 also causes a fetch of state; see
Section 4.4.3.
Notifiers may also wish to cancel subscriptions to events; this is
useful, for example, when the resource to which a subscription refers
is no longer available. Further details on this mechanism are
discussed in Section 4.2.2.
3.1.2. Identification of Subscribed Events and Event Classes
Identification of events is provided by three pieces of information:
Request URI, Event Type, and (optionally) message body.
The Request URI of a SUBSCRIBE request, most importantly, contains
enough information to route the request to the appropriate entity per
the request routing procedures outlined in [RFC3261]. It also
contains enough information to identify the resource for which event
notification is desired, but not necessarily enough information to
uniquely identify the nature of the event (e.g.,
"sip:adam@example.com" would be an appropriate URI to subscribe to
for my presence state; it would also be an appropriate URI to
subscribe to the state of my voice mailbox).
Subscribers MUST include exactly one "Event" header field in
SUBSCRIBE requests, indicating to which event or class of events they
are subscribing. The "Event" header field will contain a token that
indicates the type of state for which a subscription is being
requested. This token will be registered with the IANA and will
correspond to an event package that further describes the semantics
of the event or event class.
If the event package to which the event token corresponds defines
behavior associated with the body of its SUBSCRIBE requests, those
semantics apply.
Event packages may also define parameters for the "Event" header
field; if they do so, they must define the semantics for such
parameters.
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3.1.3. Additional SUBSCRIBE Header Field Values
Because SUBSCRIBE requests create a dialog usage as defined in
[RFC3261], they MAY contain an "Accept" header field. This header
field, if present, indicates the body formats allowed in subsequent
NOTIFY requests. Event packages MUST define the behavior for
SUBSCRIBE requests without "Accept" header fields; usually, this will
connote a single, default body type.
Header values not described in this document are to be interpreted as
described in [RFC3261].
3.2. NOTIFY
NOTIFY requests are sent to inform subscribers of changes in state to
which the subscriber has a subscription. Subscriptions are created
using the SUBSCRIBE method. In legacy implementations, it is
possible that other means of subscription creation have been used.
However, this specification does not allow the creation of
subscriptions except through SUBSCRIBE requests and (for backwards-
compatibility) REFER requests [RFC3515].
NOTIFY is a target refresh request, as that term is defined in
[RFC3261].
A NOTIFY request does not terminate its corresponding subscription;
in other words, a single SUBSCRIBE request may trigger several NOTIFY
requests.
3.2.1. Identification of Reported Events, Event Classes, and Current
State
Identification of events being reported in a notification is very
similar to that described for subscription to events (see
Section 3.1.2).
As in SUBSCRIBE requests, NOTIFY request "Event" header fields MUST
contain a single event package name for which a notification is being
generated. The package name in the "Event" header field MUST match
the "Event" header field in the corresponding SUBSCRIBE request.
Event packages may define semantics associated with the body of their
NOTIFY requests; if they do so, those semantics apply. NOTIFY
request bodies are expected to provide additional details about the
nature of the event that has occurred and the resultant resource
state.
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When present, the body of the NOTIFY request MUST be formatted into
one of the body formats specified in the "Accept" header field of the
corresponding SUBSCRIBE request (or the default type according to the
event package description, if no "Accept" header field was
specified). This body will contain either the state of the
subscribed resource or a pointer to such state in the form of a URI
(see Section 5.4.13).
4. Node Behavior
4.1. Subscriber Behavior
4.1.1. Detecting Support for SIP Events
The extension described in this document does not make use of the
"Require" or "Proxy-Require" header fields; similarly, there is no
token defined for "Supported" header fields. Potential subscribers
may probe for the support of SIP events using the OPTIONS request
defined in [RFC3261].
The presence of "SUBSCRIBE" in the "Allow" header field of any
request or response indicates support for SIP events; further, in the
absence of an "Allow" header field, the simple presence of an "Allow-
Events" header field is sufficient to indicate that the node that
sent the message is capable of acting as a notifier (see
Section 4.4.4).
The "methods" parameter for Contact may also be used to
specifically announce support for SUBSCRIBE and NOTIFY requests
when registering. (See [RFC3840] for details on the "methods"
parameter.)
4.1.2. Creating and Maintaining Subscriptions
From the subscriber's perspective, a subscription proceeds according
to the following state diagram. Events that result in a transition
back to the same state are not represented in this diagram.
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+-------------+
| init |<-----------------------+
+-------------+ |
| Retry-after
Send SUBSCRIBE expires
| |
V Timer N Fires; |
+-------------+ SUBSCRIBE failure |
+------------| notify_wait |-- response; --------+ |
| +-------------+ or NOTIFY, | |
| | state=terminated | |
| | | |
++========|===================|============================|==|====++
|| | | V | ||
|| Receive NOTIFY, Receive NOTIFY, +-------------+ ||
|| state=active state=pending | terminated | ||
|| | | +-------------+ ||
|| | | Re-subscription A A ||
|| | V times out; | | ||
|| | +-------------+ Receive NOTIFY, | | ||
|| | | pending |-- state=terminated; --+ | ||
|| | +-------------+ or 481 response | ||
|| | | to SUBSCRIBE | ||
|| | Receive NOTIFY, refresh | ||
|| | state=active | ||
|| | | Re-subscription | ||
|| | V times out; | ||
|| | +-------------+ Receive NOTIFY, | ||
|| +----------->| active |-- state=terminated; -----+ ||
|| +-------------+ or 481 response ||
|| to SUBSCRIBE ||
|| Subscription refresh ||
++=================================================================++
In the state diagram, "Re-subscription times out" means that an
attempt to refresh or update the subscription using a new SUBSCRIBE
request does not result in a NOTIFY request before the corresponding
Timer N expires.
Any transition from "notify_wait" into a "pending" or "active" state
results in a new subscription. Note that multiple subscriptions can
be generated as the result of a single SUBSCRIBE request (see
Section 4.4.1). Each of these new subscriptions exists in its own
independent state machine and runs its own set of timers.
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4.1.2.1. Requesting a Subscription
SUBSCRIBE is a dialog-creating method, as described in [RFC3261].
When a subscriber wishes to subscribe to a particular state for a
resource, it forms a SUBSCRIBE request. If the initial SUBSCRIBE
request represents a request outside of a dialog (as it typically
will), its construction follows the procedures outlined in [RFC3261]
for User Agent Client (UAC) request generation outside of a dialog.
This SUBSCRIBE request will be confirmed with a final response.
200-class responses indicate that the subscription has been accepted
and that a NOTIFY request will be sent immediately.
The "Expires" header field in a 200-class response to SUBSCRIBE
request indicates the actual duration for which the subscription will
remain active (unless refreshed). The received value might be
smaller than the value indicated in the SUBSCRIBE request but cannot
be larger; see Section 4.2.1 for details.
Non-200-class final responses indicate that no subscription or new
dialog usage has been created, and no subsequent NOTIFY request will
be sent. All non-200-class responses (with the exception of 489 (Bad
Event), described herein) have the same meanings and handling as
described in [RFC3261]. For the sake of clarity: if a SUBSCRIBE
request contains an "Accept" header field, but that field does not
indicate a media type that the notifier is capable of generating in
its NOTIFY requests, then the proper error response is 406 (Not
Acceptable).
4.1.2.2. Refreshing of Subscriptions
At any time before a subscription expires, the subscriber may refresh
the timer on such a subscription by sending another SUBSCRIBE request
on the same dialog as the existing subscription. The handling for
such a request is the same as for the initial creation of a
subscription except as described below.
If a SUBSCRIBE request to refresh a subscription receives a 404, 405,
410, 416, 480-485, 489, 501, or 604 response, the subscriber MUST
consider the subscription terminated. (See [RFC5057] for further
details and notes about the effect of error codes on dialogs and
usages within dialog, such as subscriptions). If the subscriber
wishes to re-subscribe to the state, he does so by composing an
unrelated initial SUBSCRIBE request with a freshly generated Call-ID
and a new, unique "From" tag (see Section 4.1.2.1).
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RFC 6665 SIP-Specific Event Notification July 2012
If a SUBSCRIBE request to refresh a subscription fails with any error
code other than those listed above, the original subscription is
still considered valid for the duration of the most recently known
"Expires" value as negotiated by the most recent successful SUBSCRIBE
transaction, or as communicated by a NOTIFY request in its
"Subscription-State" header field "expires" parameter.
Note that many such errors indicate that there may be a problem
with the network or the notifier such that no further NOTIFY
requests will be received.
When refreshing a subscription, a subscriber starts Timer N, set to
64*T1, when it sends the SUBSCRIBE request. If this Timer N expires
prior to the receipt of a NOTIFY request, the subscriber considers
the subscription terminated. If the subscriber receives a success
response to the SUBSCRIBE request that indicates that no NOTIFY
request will be generated -- such as the 204 response defined for use
with the optional extension described in [RFC5839] -- then it MUST
cancel Timer N.
4.1.2.3. Unsubscribing
Unsubscribing is handled in the same way as refreshing of a
subscription, with the "Expires" header field set to "0". Note that
a successful unsubscription will also trigger a final NOTIFY request.
The final NOTIFY request may or may not contain information about the
state of the resource; subscribers need to be prepared to receive
final NOTIFY requests both with and without state.
4.1.2.4. Confirmation of Subscription Creation
The subscriber can expect to receive a NOTIFY request from each node
which has processed a successful subscription or subscription
refresh. To ensure that subscribers do not wait indefinitely for a
subscription to be established, a subscriber starts a Timer N, set to
64*T1, when it sends a SUBSCRIBE request. If this Timer N expires
prior to the receipt of a NOTIFY request, the subscriber considers
the subscription failed, and cleans up any state associated with the
subscription attempt.
Until Timer N expires, several NOTIFY requests may arrive from
different destinations (see Section 4.4.1). Each of these requests
establishes a new dialog usage and a new subscription. After the
expiration of Timer N, the subscriber SHOULD reject any such NOTIFY
requests that would otherwise establish a new dialog usage with a 481
(Subscription does not exist) response code.
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Until the first NOTIFY request arrives, the subscriber should
consider the state of the subscribed resource to be in a neutral
state. Event package specifications MUST define this "neutral state"
in such a way that makes sense for their application (see
Section 5.4.7).
Due to the potential for out-of-order messages, packet loss, and
forking, the subscriber MUST be prepared to receive NOTIFY requests
before the SUBSCRIBE transaction has completed.
Except as noted above, processing of this NOTIFY request is the same
as in Section 4.1.3.
4.1.3. Receiving and Processing State Information
Subscribers receive information about the state of a resource to
which they have subscribed in the form of NOTIFY requests.
Upon receiving a NOTIFY request, the subscriber should check that it
matches at least one of its outstanding subscriptions; if not, it
MUST return a 481 (Subscription does not exist) response unless
another 400- or 500-class response is more appropriate. The rules
for matching NOTIFY requests with subscriptions that create a new
dialog usage are described in Section 4.4.1. Notifications for
subscriptions that were created inside an existing dialog match if
they are in the same dialog and the "Event" header fields match (as
described in Section 8.2.1).
If, for some reason, the event package designated in the "Event"
header field of the NOTIFY request is not supported, the subscriber
will respond with a 489 (Bad Event) response.
To prevent spoofing of events, NOTIFY requests SHOULD be
authenticated using any defined SIP authentication mechanism, such as
those described in Sections 22.2 and 23 of [RFC3261].
NOTIFY requests MUST contain "Subscription-State" header fields that
indicate the status of the subscription.
If the "Subscription-State" header field value is "active", it means
that the subscription has been accepted and (in general) has been
authorized. If the header field also contains an "expires"
parameter, the subscriber SHOULD take it as the authoritative
subscription duration and adjust accordingly. The "retry-after" and
"reason" parameters have no semantics for "active".
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RFC 6665 SIP-Specific Event Notification July 2012
If the "Subscription-State" value is "pending", the subscription has
been received by the notifier, but there is insufficient policy
information to grant or deny the subscription yet. If the header
field also contains an "expires" parameter, the subscriber SHOULD
take it as the authoritative subscription duration and adjust
accordingly. No further action is necessary on the part of the
subscriber. The "retry-after" and "reason" parameters have no
semantics for "pending".
If the "Subscription-State" value is "terminated", the subscriber
MUST consider the subscription terminated. The "expires" parameter
has no semantics for "terminated" -- notifiers SHOULD NOT include an
"expires" parameter on a "Subscription-State" header field with a
value of "terminated", and subscribers MUST ignore any such
parameter, if present. If a reason code is present, the client
should behave as described below. If no reason code or an unknown
reason code is present, the client MAY attempt to re-subscribe at any
time (unless a "retry-after" parameter is present, in which case the
client SHOULD NOT attempt re-subscription until after the number of
seconds specified by the "retry-after" parameter). The reason codes
defined by this document are:
deactivated: The subscription has been terminated, but the
subscriber SHOULD retry immediately with a new subscription. One
primary use of such a status code is to allow migration of
subscriptions between nodes. The "retry-after" parameter has no
semantics for "deactivated".
probation: The subscription has been terminated, but the client
SHOULD retry at some later time (as long as the resource's state
is still relevant to the client at that time). If a "retry-after"
parameter is also present, the client SHOULD wait at least the
number of seconds specified by that parameter before attempting to
re-subscribe.
rejected: The subscription has been terminated due to change in
authorization policy. Clients SHOULD NOT attempt to re-subscribe.
The "retry-after" parameter has no semantics for "rejected".
timeout: The subscription has been terminated because it was not
refreshed before it expired. Clients MAY re-subscribe
immediately. The "retry-after" parameter has no semantics for
"timeout". This reason code is also associated with polling of
resource state, as detailed in Section 4.4.3.
giveup: The subscription has been terminated because the notifier
could not obtain authorization in a timely fashion. If a "retry-
after" parameter is also present, the client SHOULD wait at least
Roach Standards Track [Page 15]RFC 6665 SIP-Specific Event Notification July 2012
the number of seconds specified by that parameter before
attempting to re-subscribe; otherwise, the client MAY retry
immediately, but will likely get put back into pending state.
noresource: The subscription has been terminated because the
resource state that was being monitored no longer exists. Clients
SHOULD NOT attempt to re-subscribe. The "retry-after" parameter
has no semantics for "noresource".
invariant: The subscription has been terminated because the resource
state is guaranteed not to change for the foreseeable future.
This may be the case, for example, when subscribing to the
location information of a fixed-location land-line telephone.
When using this reason code, notifiers are advised to include a
"retry-after" parameter with a large value (for example, 31536000
-- or one year) to prevent older clients that are RFC 3265
compliant from periodically re-subscribing. Clients SHOULD NOT
attempt to re-subscribe after receiving a reason code of
"invariant", regardless of the presence of or value of a "retry-
after" parameter.
Other specifications may define new reason codes for use with the
"Subscription-State" header field.
Once the notification is deemed acceptable to the subscriber, the
subscriber SHOULD return a 200 response. In general, it is not
expected that NOTIFY responses will contain bodies; however, they
MAY, if the NOTIFY request contained an "Accept" header field.
Other responses defined in [RFC3261] may also be returned, as
appropriate. In no case should a NOTIFY transaction extend for any
longer than the time necessary for automated processing. In
particular, subscribers MUST NOT wait for a user response before
returning a final response to a NOTIFY request.
4.1.4. Forking of SUBSCRIBE Requests
In accordance with the rules for proxying non-INVITE requests as
defined in [RFC3261], successful SUBSCRIBE requests will receive only
one 200-class response; however, due to forking, the subscription may
have been accepted by multiple nodes. The subscriber MUST therefore
be prepared to receive NOTIFY requests with "From:" tags that differ
from the "To:" tag received in the SUBSCRIBE 200-class response.
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If multiple NOTIFY requests are received in different dialogs in
response to a single SUBSCRIBE request, each dialog represents a
different destination to which the SUBSCRIBE request was forked.
Subscriber handling in such situations varies by event package; see
Section 5.4.9 for details.
4.2. Notifier Behavior
4.2.1. Subscription Establishment and Maintenance
Notifiers learn about subscription requests by receiving SUBSCRIBE
requests from interested parties. Notifiers MUST NOT create
subscriptions except upon receipt of a SUBSCRIBE request. However,
for historical reasons, the implicit creation of subscriptions as
defined in [RFC3515] is still permitted.
[RFC3265] allowed the creation of subscriptions using means other
than the SUBSCRIBE method. The only standardized use of this
mechanism is the REFER method [RFC3515]. Implementation
experience with REFER has shown that the implicit creation of a
subscription has a number of undesirable effects, such as the
inability to signal the success of a REFER request while signaling
a problem with the subscription, and difficulty performing one
action without the other. Additionally, the proper exchange of
dialog identifiers is difficult without dialog reuse (which has
its own set of problems; see Section 4.5).
4.2.1.1. Initial SUBSCRIBE Transaction Processing
In no case should a SUBSCRIBE transaction extend for any longer than
the time necessary for automated processing. In particular,
notifiers MUST NOT wait for a user response before returning a final
response to a SUBSCRIBE request.
This requirement is imposed primarily to prevent the non-INVITE
transaction timeout timer F (see [RFC3261]) from firing during the
SUBSCRIBE transaction, since interaction with a user would often
exceed 64*T1 seconds.
The notifier SHOULD check that the event package specified in the
"Event" header field is understood. If not, the notifier SHOULD
return a 489 (Bad Event) response to indicate that the specified
event/event class is not understood.
The notifier SHOULD also perform any necessary authentication and
authorization per its local policy. See Section 4.2.1.3.
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The notifier MAY also check that the duration in the "Expires" header
field is not too small. If and only if the expiration interval is
greater than zero AND smaller than one hour AND less than a notifier-
configured minimum, the notifier MAY return a 423 (Interval Too
Brief) error that contains a "Min-Expires" header field. The
"Min-Expires" header field is described in [RFC3261].
Once the notifier determines that it has enough information to create
the subscription (i.e., it understands the event package, the
subscription pertains to a known resource, and there are no other
barriers to creating the subscription), it creates the subscription
and a dialog usage, and returns a 200 (OK) response.
When a subscription is created in the notifier, it stores the event
package name as part of the subscription information.
The "Expires" values present in SUBSCRIBE 200-class responses behave
in the same way as they do in REGISTER responses: the server MAY
shorten the interval but MUST NOT lengthen it.
If the duration specified in a SUBSCRIBE request is unacceptably
short, the notifier may be able to send a 423 response, as
described earlier in this section.
200-class responses to SUBSCRIBE requests will not generally contain
any useful information beyond subscription duration; their primary
purpose is to serve as a reliability mechanism. State information
will be communicated via a subsequent NOTIFY request from the
notifier.
The other response codes defined in [RFC3261] may be used in response
to SUBSCRIBE requests, as appropriate.
4.2.1.2. Confirmation of Subscription Creation/Refreshing
Upon successfully accepting or refreshing a subscription, notifiers
MUST send a NOTIFY request immediately to communicate the current
resource state to the subscriber. This NOTIFY request is sent on the
same dialog as created by the SUBSCRIBE response. If the resource
has no meaningful state at the time that the SUBSCRIBE request is
processed, this NOTIFY request MAY contain an empty or neutral body.
See Section 4.2.2 for further details on NOTIFY request generation.
Note that a NOTIFY request is always sent immediately after any
200-class response to a SUBSCRIBE request, regardless of whether the
subscription has already been authorized.
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4.2.1.3. Authentication/Authorization of SUBSCRIBE Requests
Privacy concerns may require that notifiers apply policy to determine
whether a particular subscriber is authorized to subscribe to a
certain set of events. Such policy may be defined by mechanisms such
as access control lists or real-time interaction with a user. In
general, authorization of subscribers prior to authentication is not
particularly useful.
SIP authentication mechanisms are discussed in [RFC3261]. Note that,
even if the notifier node typically acts as a proxy, authentication
for SUBSCRIBE requests will always be performed via a 401
(Unauthorized) response, not a 407 (Proxy Authentication Required).
Notifiers always act as user agents when accepting subscriptions and
sending notifications.
Of course, when acting as a proxy, a node will perform normal
proxy authentication (using 407). The foregoing explanation is a
reminder that notifiers are always user agents and, as such,
perform user agent authentication.
If authorization fails based on an access list or some other
automated mechanism (i.e., it can be automatically authoritatively
determined that the subscriber is not authorized to subscribe), the
notifier SHOULD reply to the request with a 403 (Forbidden) or 603
(Decline) response, unless doing so might reveal information that
should stay private; see Section 6.2.
If the notifier owner is interactively queried to determine whether a
subscription is allowed, a 200 (OK) response is returned immediately.
Note that a NOTIFY request is still formed and sent under these
circumstances, as described in the previous section.
If subscription authorization was delayed and the notifier wishes to
convey that such authorization has been declined, it may do so by
sending a NOTIFY request containing a "Subscription-State" header
field with a value of "terminated" and a reason parameter of
"rejected".
4.2.1.4. Refreshing of Subscriptions
When a notifier receives a subscription refresh, assuming that the
subscriber is still authorized, the notifier updates the expiration
time for subscription. As with the initial subscription, the server
MAY shorten the amount of time until expiration but MUST NOT increase
it. The final expiration time is placed in the "Expires" header
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field in the response. If the duration specified in a SUBSCRIBE
request is unacceptably short, the notifier SHOULD respond with a 423
(Interval Too Brief) response.
If no refresh for a notification address is received before its
expiration time, the subscription is removed. When removing a
subscription, the notifier SHOULD send a NOTIFY request with a
"Subscription-State" value of "terminated" to inform it that the
subscription is being removed. If such a request is sent, the
"Subscription-State" header field SHOULD contain a "reason=timeout"
parameter.
Clients can cause a subscription to be terminated immediately by
sending a SUBSCRIBE request with an "Expires" header field set to
'0'. Notifiers largely treat this the same way as any other
subscription expiration: they send a NOTIFY request containing a
"Subscription-State" of "terminated", with a reason code of
"timeout." For consistency with state polling (see Section 4.4.3)
and subscription refreshes, the notifier may choose to include
resource state in this final NOTIFY request. However, in some cases,
including such state makes no sense. Under such circumstances, the
notifier may choose to omit state information from the terminal
NOTIFY request.
The sending of a NOTIFY request when a subscription expires allows
the corresponding dialog usage to be terminated, if appropriate.
4.2.2. Sending State Information to Subscribers
Notifiers use the NOTIFY method to send information about the state
of a resource to subscribers. The notifier's view of a subscription
is shown in the following state diagram. Events that result in a
transition back to the same state are not represented in this
diagram.
Roach Standards Track [Page 20]RFC 6665 SIP-Specific Event Notification July 2012
+-------------+
| init |
+-------------+
|
Receive SUBSCRIBE,
Send NOTIFY
|
V NOTIFY failure,
+-------------+ subscription expires,
+------------| resp_wait |-- or terminated ----+
| +-------------+ per local policy |
| | |
| | |
| | V
Policy grants Policy needed +-------------+
permission | | terminated |
| | +-------------+
| | A A
| V NOTIFY failure, | |
| +-------------+ subscription expires,| |
| | pending |-- or terminated -------+ |
| +-------------+ per local policy |
| | |
| Policy changed to |
| grant permission |
| | |
| V NOTIFY failure, |
| +-------------+ subscription expires, |
+----------->| active |-- or terminated ---------+
+-------------+ per local policy
When a SUBSCRIBE request is answered with a 200-class response, the
notifier MUST immediately construct and send a NOTIFY request to the
subscriber. When a change in the subscribed state occurs, the
notifier SHOULD immediately construct and send a NOTIFY request,
unless the state transition is caused by a NOTIFY transaction
failure. The sending of this NOTIFY message is also subject to
authorization, local policy, and throttling considerations.
If the NOTIFY request fails due to expiration of SIP Timer F
(transaction timeout), the notifier SHOULD remove the subscription.
This behavior prevents unnecessary transmission of state
information for subscribers who have crashed or disappeared from
the network. Because such transmissions will be sent multiple
times, per the retransmission algorithm defined in [RFC3261]
(instead of the typical single transmission for functioning
clients), continuing to service them when no client is available
Roach Standards Track [Page 21]RFC 6665 SIP-Specific Event Notification July 2012
to acknowledge them could place undue strain on a network. Upon
client restart or reestablishment of a network connection, it is
expected that clients will send SUBSCRIBE requests to refresh
potentially stale state information; such requests will reinstall
subscriptions in all relevant nodes.
If the NOTIFY transaction fails due to the receipt of a 404, 405,
410, 416, 480-485, 489, 501, or 604 response to the NOTIFY request,
the notifier MUST remove the corresponding subscription. See
[RFC5057] for further details and notes about the effect of error
codes on dialogs and usages within dialog (such as subscriptions).
A notify error response would generally indicate that something
has gone wrong with the subscriber or with some proxy on the way
to the subscriber. If the subscriber is in error, it makes the
most sense to allow the subscriber to rectify the situation (by
re-subscribing) once the error condition has been handled. If a
proxy is in error, the periodic sending of SUBSCRIBE requests to
refresh the expiration timer will reinstall subscription state
once the network problem has been resolved.
NOTIFY requests MUST contain a "Subscription-State" header field with
a value of "active", "pending", or "terminated". The "active" value
indicates that the subscription has been accepted and has been
authorized (in most cases; see Section 6.2). The "pending" value
indicates that the subscription has been received, but that policy
information is insufficient to accept or deny the subscription at
this time. The "terminated" value indicates that the subscription is
not active.
If the value of the "Subscription-State" header field is "active" or
"pending", the notifier MUST also include in the "Subscription-State"
header field an "expires" parameter that indicates the time remaining
on the subscription. The notifier MAY use this mechanism to shorten
a subscription; however, this mechanism MUST NOT be used to lengthen
a subscription.
Including expiration information for active and pending
subscriptions is necessary in case the SUBSCRIBE request forks,
since the response to a forked SUBSCRIBE request may not be
received by the subscriber. [RFC3265] allowed the notifier some
discretion in the inclusion of this parameter, so subscriber
implementations are warned to handle the lack of an "expires"
parameter gracefully. Note well that this "expires" value is a
parameter on the "Subscription-State" header field NOT the
"Expires" header field.
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The period of time for a subscription can be shortened to zero by
the notifier. In other words, it is perfectly valid for a
SUBSCRIBE request with a non-zero expires to be answered with a
NOTIFY request that contains "Subscription-Status:
terminated;reason=expired". This merely means that the notifier
has shortened the subscription timeout to zero, and the
subscription has expired instantaneously. The body may contain
valid state, or it may contain a neutral state (see
Section 5.4.7).
If the value of the "Subscription-State" header field is
"terminated", the notifier SHOULD also include a "reason" parameter.
The notifier MAY also include a "retry-after" parameter, where
appropriate. For details on the value and semantics of the "reason"
and "retry-after" parameters, see Section 4.1.3.
4.2.3. PSTN/Internet Interworking (PINT) Compatibility
The "Event" header field is considered mandatory for the purposes of
this document. However, to maintain compatibility with PINT (see
[RFC2848]), notifiers MAY interpret a SUBSCRIBE request with no
"Event" header field as requesting a subscription to PINT events. If
a notifier does not support PINT, it SHOULD return 489 (Bad Event) to
any SUBSCRIBE requests without an "Event" header field.
4.3. Proxy Behavior
Proxies need no additional behavior beyond that described in
[RFC3261] to support SUBSCRIBE and NOTIFY transactions. If a proxy
wishes to see all of the SUBSCRIBE and NOTIFY requests for a given
dialog, it MUST add a "Record-Route" header field to the initial
SUBSCRIBE request and all NOTIFY requests. It MAY choose to include
"Record-Route" in subsequent SUBSCRIBE requests; however, these
requests cannot cause the dialog's route set to be modified.
Proxies that did not add a "Record-Route" header field to the initial
SUBSCRIBE request MUST NOT add a "Record-Route" header field to any
of the associated NOTIFY requests.
Note that subscribers and notifiers may elect to use Secure/
Multipurpose Internet Mail Extensions (S/MIME) encryption of
SUBSCRIBE and NOTIFY requests; consequently, proxies cannot rely
on being able to access any information that is not explicitly
required to be proxy-readable by [RFC3261].
Roach Standards Track [Page 23]RFC 6665 SIP-Specific Event Notification July 2012
4.4. Common Behavior
4.4.1. Dialog Creation and Termination
Dialogs usages are created upon completion of a NOTIFY transaction
for a new subscription, unless the NOTIFY request contains a
"Subscription-State" of "terminated."
Because the dialog usage is established by the NOTIFY request, the
route set at the subscriber is taken from the NOTIFY request itself,
as opposed to the route set present in the 200-class response to the
SUBSCRIBE request.
NOTIFY requests are matched to such SUBSCRIBE requests if they
contain the same "Call-ID", a "To" header field "tag" parameter that
matches the "From" header field "tag" parameter of the SUBSCRIBE
request, and the same "Event" header field. Rules for comparisons of
the "Event" header fields are described in Section 8.2.1.
A subscription is destroyed after a notifier sends a NOTIFY request
with a "Subscription-State" of "terminated", or in certain error
situations described elsewhere in this document. The subscriber will
generally answer such final requests with a 200 (OK) response (unless
a condition warranting an alternate response has arisen). Except
when the mechanism described in Section 4.5.2 is used, the
destruction of a subscription results in the termination of its
associated dialog.
A subscriber may send a SUBSCRIBE request with an "Expires" header
field of 0 in order to trigger the sending of such a NOTIFY
request; however, for the purposes of subscription and dialog
lifetime, the subscription is not considered terminated until the
NOTIFY transaction with a "Subscription-State" of "terminated"
completes.
4.4.2. Notifier Migration
It is often useful to allow migration of subscriptions between
notifiers. Such migration may be effected by sending a NOTIFY
request with a "Subscription-State" header field of "terminated" and
a reason parameter of "deactivated". This NOTIFY request is
otherwise normal and is formed as described in Section 4.2.2.
Upon receipt of this NOTIFY request, the subscriber SHOULD attempt to
re-subscribe (as described in the preceding sections). Note that
this subscription is established on a new dialog, and does not reuse
the route set from the previous subscription dialog.
Roach Standards Track [Page 24]RFC 6665 SIP-Specific Event Notification July 2012
The actual migration is effected by making a change to the policy
(such as routing decisions) of one or more servers to which the
SUBSCRIBE request will be sent in such a way that a different node
ends up responding to the SUBSCRIBE request. This may be as simple
as a change in the local policy in the notifier from which the
subscription is migrating so that it serves as a proxy or redirect
server instead of a notifier.
Whether, when, and why to perform notifier migrations may be
described in individual event packages; otherwise, such decisions are
a matter of local notifier policy and are left up to individual
implementations.
4.4.3. Polling Resource State
A natural consequence of the behavior described in the preceding
sections is that an immediate fetch without a persistent subscription
may be effected by sending a SUBSCRIBE with an "Expires" of 0.
Of course, an immediate fetch while a subscription is active may be
effected by sending a SUBSCRIBE request with an "Expires" equal to
the number of seconds remaining in the subscription.
Upon receipt of this SUBSCRIBE request, the notifier (or notifiers,
if the SUBSCRIBE request was forked) will send a NOTIFY request
containing resource state in the same dialog.
Note that the NOTIFY requests triggered by SUBSCRIBE requests with
"Expires" header fields of 0 will contain a "Subscription-State"
value of "terminated" and a "reason" parameter of "timeout".
Polling of event state can cause significant increases in load on the
network and notifiers; as such, it should be used only sparingly. In
particular, polling SHOULD NOT be used in circumstances in which it
will typically result in more network messages than long-running
subscriptions.
When polling is used, subscribers SHOULD attempt to cache
authentication credentials between polls so as to reduce the number
of messages sent.
Due to the requirement on notifiers to send a NOTIFY request
immediately upon receipt of a SUBSCRIBE request, the state
provided by polling is limited to the information that the
notifier has immediate local access to when it receives the
SUBSCRIBE request. If, for example, the notifier generally needs
to retrieve state from another network server, then that state
will be absent from the NOTIFY request that results from polling.
Roach Standards Track [Page 25]RFC 6665 SIP-Specific Event Notification July 2012
4.4.4. "Allow-Events" Header Field Usage
The "Allow-Events" header field, if present, MUST include a
comprehensive and inclusive list of tokens that indicates the event
packages for which the user agent can act as a notifier. In other
words, a user agent sending an "Allow-Events" header field is
advertising that it can process SUBSCRIBE requests and generate
NOTIFY requests for all of the event packages listed in that header
field.
Any user agent that can act as a notifier for one or more event
packages SHOULD include an appropriate "Allow-Events" header field
indicating all supported events in all methods which initiate dialogs
and their responses (such as INVITE) and OPTIONS responses.
This information is very useful, for example, in allowing user
agents to render particular interface elements appropriately
according to whether the events required to implement the features
they represent are supported by the appropriate nodes.
On the other hand, it doesn't necessarily make much sense to
indicate supported events inside a dialog established by a NOTIFY
request if the only event package supported is the one associated
with that subscription.
Note that "Allow-Events" header fields MUST NOT be inserted by
proxies.
The "Allow-Events" header field does not include a list of the event
template-packages supported by an implementation. If a subscriber
wishes to determine which event template-packages are supported by a
notifier, it can probe for such support by attempting to subscribe to
the event template-packages it wishes to use.
For example: to check for support for the templatized package
"presence.winfo", a client may attempt to subscribe to that event
package for a known resource, using an "Expires" header value of
0. If the response is a 489 error code, then the client can
deduce that "presence.winfo" is unsupported.
4.5. Targeting Subscriptions at Devices
[RFC3265] defined a mechanism by which subscriptions could share
dialogs with invite usages and with other subscriptions. The purpose
of this behavior was to allow subscribers to ensure that a
subscription arrived at the same device as an established dialog.
Unfortunately, the reuse of dialogs has proven to be exceedingly
confusing. [RFC5057] attempted to clarify proper behavior in a
Roach Standards Track [Page 26]RFC 6665 SIP-Specific Event Notification July 2012
variety of circumstances; however, the ensuing rules remain confusing
and prone to implementation error. At the same time, the mechanism
described in [RFC5627] now provides a far more elegant and
unambiguous means to achieve the same goal.
Consequently, the dialog reuse technique described in RFC 3265 is now
deprecated.
This dialog-sharing technique has also historically been used as a
means for targeting an event package at a dialog. This usage can be
seen, for example, in certain applications of the REFER method
[RFC3515]. With the removal of dialog reuse, an alternate (and more
explicit) means of targeting dialogs needs to be used for this type
of correlation. The appropriate means of such targeting is left up
to the actual event packages. Candidates include the "Target-Dialog"
header field [RFC4538], the "Join" header field [RFC3911], and the
"Replaces" header field [RFC3891], depending on the semantics
desired. Alternately, if the semantics of those header fields do not
match the event package's purpose for correlation, event packages can
devise their own means of identifying dialogs. For an example of
this approach, see the Dialog Event Package [RFC4235].
4.5.1. Using GRUUs to Route to Devices
Notifiers MUST implement the Globally Routable User Agent URI (GRUU)
extension defined in [RFC5627], and MUST use a GRUU as their local
target. This allows subscribers to explicitly target desired
devices.
If a subscriber wishes to subscribe to a resource on the same device
as an established dialog, it should check whether the remote contact
in that dialog is a GRUU (i.e., whether it contains a "gr" URI
parameter). If so, the subscriber creates a new dialog, using the
GRUU as the Request URI for the new SUBSCRIBE request.
Because GRUUs are guaranteed to route to a specific device, this
ensures that the subscription will be routed to the same place as
the established dialog.
4.5.2. Sharing Dialogs
For compatibility with older clients, subscriber and notifier
implementations may choose to allow dialog sharing. The behavior of
multiple usages within a dialog are described in [RFC5057].
Subscribers MUST NOT attempt to reuse dialogs whose remote target is
a GRUU.
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Note that the techniques described in this section are included
for backwards-compatibility purposes only. Because subscribers
cannot reuse dialogs with a GRUU for their remote target, and
because notifiers must use GRUUs as their local target, any two
implementations that conform to this specification will
automatically use the mechanism described in Section 4.5.1.
Further note that the prohibition on reusing dialogs does not
exempt implicit subscriptions created by the REFER method. This
means that implementations complying with this specification are
required to use the "Target-Dialog" mechanism described in
[RFC4538] when the remote target is a GRUU.
If a subscriber wishes to subscribe to a resource on the same device
as an established dialog and the remote contact is not a GRUU, it MAY
revert to dialog-sharing behavior. Alternately, it MAY choose to
treat the remote party as incapable of servicing the subscription
(i.e., the same way it would behave if the remote party did not
support SIP events at all).
If a notifier receives a SUBSCRIBE request for a new subscription on
an existing dialog, it MAY choose to implement dialog sharing
behavior. Alternately, it may choose to fail the SUBSCRIBE request
with a 403 (Forbidden) response. The error text of such 403
responses SHOULD indicate that dialog sharing is not supported.
To implement dialog sharing, subscribers and notifiers perform the
following additional processing:
o When subscriptions exist in dialogs associated with INVITE-created
application state and/or other subscriptions, these sets of
application state do not interact beyond the behavior described
for a dialog (e.g., route set handling). In particular, multiple
subscriptions within a dialog expire independently and require
independent subscription refreshes.
o If a subscription's destruction leaves no other application state
associated with the dialog, the dialog terminates. The
destruction of other application state (such as that created by an
INVITE) will not terminate the dialog if a subscription is still
associated with that dialog. This means that, when dialogs are
reused, a dialog created with an INVITE does not necessarily
terminate upon receipt of a BYE. Similarly, in the case that
several subscriptions are associated with a single dialog, the
dialog does not terminate until all the subscriptions in it are
destroyed.
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o Subscribers MAY include an "id" parameter in a SUBSCRIBE request's
"Event" header field to allow differentiation between multiple
subscriptions in the same dialog. This "id" parameter, if
present, contains an opaque token that identifies the specific
subscription within a dialog. An "id" parameter is only valid
within the scope of a single dialog.
o If an "id" parameter is present in the SUBSCRIBE request used to
establish a subscription, that "id" parameter MUST also be present
in all corresponding NOTIFY requests.
o When a subscriber refreshes the subscription timer, the SUBSCRIBE
request MUST contain the same "Event" header field "id" parameter
as was present in the SUBSCRIBE request that created the
subscription. (Otherwise, the notifier will interpret the
SUBSCRIBE request as a request for a new subscription in the same
dialog.)
o When a subscription is created in the notifier, it stores any
"Event" header field "id" parameter as part of the subscription
information (along with the event package name).
o If an initial SUBSCRIBE request is sent on a pre-existing dialog,
a matching NOTIFY request merely creates a new subscription
associated with that dialog.
4.6. CANCEL Requests for SUBSCRIBE and NOTIFY Transactions
Neither SUBSCRIBE nor NOTIFY requests can be canceled. If a User
Agent Server (UAS) receives a CANCEL request that matches a known
SUBSCRIBE or NOTIFY transaction, it MUST respond to the CANCEL
request, but otherwise ignore it. In particular, the CANCEL request
MUST NOT affect processing of the SUBSCRIBE or NOTIFY request in any
way.
UACs SHOULD NOT send CANCEL requests for SUBSCRIBE or NOTIFY
transactions.
5. Event Packages
This section covers several issues that should be taken into
consideration when event packages based on the SUBSCRIBE and NOTIFY
methods are proposed.
5.1. Appropriateness of Usage
When designing an event package using the methods described in this
document for event notification, it is important to consider: is SIP
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an appropriate mechanism for the problem set? Is SIP being selected
because of some unique feature provided by the protocol (e.g., user
mobility) or merely because "it can be done"? If you find yourself
defining event packages for notifications related to, for example,
network management or the temperature inside your car's engine, you
may want to reconsider your selection of protocols.
Those interested in extending the mechanism defined in this
document are urged to follow the development of "Guidelines for
Authors of SIP Extensions" [RFC4485] for further guidance
regarding appropriate uses of SIP.
Further, it is expected that this mechanism is not to be used in
applications where the frequency of reportable events is excessively
rapid (e.g., more than about once per second). A SIP network is
generally going to be provisioned for a reasonable signaling volume;
sending a notification every time a user's GPS position changes by
one hundredth of a second could easily overload such a network.
5.2. Event Template-Packages
Normal event packages define a set of state applied to a specific
type of resource, such as user presence, call state, and messaging
mailbox state.
Event template-packages are a special type of package that define a
set of state applied to other packages, such as statistics, access
policy, and subscriber lists. Event template-packages may even be
applied to other event template-packages.
To extend the object-oriented analogy made earlier, event template-
packages can be thought of as templatized C++ packages that must be
applied to other packages to be useful.
The name of an event template-package as applied to a package is
formed by appending a period followed by the event template-package
name to the end of the package. For example, if a template-package
called "winfo" were being applied to a package called "presence", the
event token used in the "Event" header field would be
"presence.winfo".
This scheme may be arbitrarily extended. For example, application
of the "winfo" package to the "presence.winfo" state of a resource
would be represented by the name "presence.winfo.winfo". It
naturally follows from this syntax that the order in which
templates are specified is significant.
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For example: consider a theoretical event template-package called
"list". The event "presence.winfo.list" would be the application
of the "list" template to "presence.winfo", which would presumably
be a list of winfo state associated with presence. On the other
hand, the event "presence.list.winfo" would represent the
application of winfo to "presence.list", which would be represent
the winfo state of a list of presence information.
Event template-packages must be defined so that they can be applied
to any arbitrary package. In other words, event template-packages
cannot be specifically tied to one or a few "parent" packages in such
a way that they will not work with other packages.
5.3. Amount of State to Be Conveyed
When designing event packages, it is important to consider the type
of information that will be conveyed during a notification.
A natural temptation is to convey merely the event (e.g., "a new
voice message just arrived") without accompanying state (e.g., "7
total voice messages"). This complicates implementation of
subscribing entities (since they have to maintain complete state for
the entity to which they have subscribed), and also is particularly
susceptible to synchronization problems.
This problem has two possible solutions that event packages may
choose to implement.
5.3.1. Complete State Information
In general, event packages need to be able to convey a well-defined
and complete state, rather than just a stream of events. If it is
not possible to describe complete system state for transmission in
NOTIFY requests, then the problem set is not a good candidate for an
event package.
For packages that typically convey state information that is
reasonably small (on the order of 1 KB or so), it is suggested that
event packages are designed so as to send complete state information
whenever an event occurs.
In some circumstances, conveying the current state alone may be
insufficient for a particular class of events. In these cases, the
event packages should include complete state information along with
the event that occurred. For example, conveying "no customer service
representatives available" may not be as useful as conveying "no
customer service representatives available; representative
sip:46@cs.xyz.int just logged off".
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5.3.2. State Deltas
In the case that the state information to be conveyed is large, the
event package may choose to detail a scheme by which NOTIFY requests
contain state deltas instead of complete state.
Such a scheme would work as follows: any NOTIFY request sent in
immediate response to a SUBSCRIBE request contains full state
information. NOTIFY requests sent because of a state change will
contain only the state information that has changed; the subscriber
will then merge this information into its current knowledge about the
state of the resource.
Any event package that supports delta changes to states MUST include
a version number that increases by exactly one for each NOTIFY
transaction in a subscription. Note that the state version number
appears in the body of the message, not in a SIP header field.
If a NOTIFY request arrives that has a version number that is
incremented by more than one, the subscriber knows that a state delta
has been missed; it ignores the NOTIFY request containing the state
delta (except for the version number, which it retains to detect
message loss), and re-sends a SUBSCRIBE request to force a NOTIFY
request containing a complete state snapshot.
5.4. Event Package Responsibilities
Event packages are not required to reiterate any of the behavior
described in this document, although they may choose to do so for
clarity or emphasis. In general, though, such packages are expected
to describe only the behavior that extends or modifies the behavior
described in this document.
Note that any behavior designated with "SHOULD" or "MUST" in this
document is not allowed to be weakened by extension documents;
however, such documents may elect to strengthen "SHOULD" requirements
to "MUST" requirements if required by their application.
In addition to the normal sections expected in Standards Track RFCs
and SIP extension documents, authors of event packages need to
address each of the issues detailed in the following subsections.
For clarity: well-formed event package definitions contain sections
addressing each of these issues, ideally in the same order and with
the same titles as these subsections.
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RFC 6665 SIP-Specific Event Notification July 2012
5.4.1. Event Package Name
This section, which MUST be present, defines the token name to be
used to designate the event package. It MUST include the information
that appears in the IANA registration of the token. For information
on registering such types, see Section 7.
5.4.2. Event Package Parameters
If parameters are to be used on the "Event" header field to modify
the behavior of the event package, the syntax and semantics of such
header fields MUST be clearly defined.
Any "Event" header field parameters defined by an event package MUST
be registered in the "Header Field Parameters and Parameter Values"
registry defined by [RFC3968]. An "Event" header field parameter,
once registered in conjunction with an event package, MUST NOT be
reused with any other event package. Non-event-package
specifications MAY define "Event" header field parameters that apply
across all event packages (with emphasis on "all", as opposed to
"several"), such as the "id" parameter defined in this document. The
restriction of a parameter to use with a single event package only
applies to parameters that are defined in conjunction with an event
package.
5.4.3. SUBSCRIBE Request Bodies
It is expected that most, but not all, event packages will define
syntax and semantics for SUBSCRIBE request bodies; these bodies will
typically modify, expand, filter, throttle, and/or set thresholds for
the class of events being requested. Designers of event packages are
strongly encouraged to reuse existing media types for message bodies
where practical. See [RFC4288] for information on media type
specification and registration.
This mandatory section of an event package defines what type or types
of event bodies are expected in SUBSCRIBE requests (or specify that
no event bodies are expected). It should point to detailed
definitions of syntax and semantics for all referenced body types.
5.4.4. Subscription Duration
It is RECOMMENDED that event packages give a suggested range of times
considered reasonable for the duration of a subscription. Such
packages MUST also define a default "Expires" value to be used if
none is specified.
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RFC 6665 SIP-Specific Event Notification July 2012
5.4.5. NOTIFY Request Bodies
The NOTIFY request body is used to report state on the resource being
monitored. Each package MUST define what type or types of event
bodies are expected in NOTIFY requests. Such packages MUST specify
or cite detailed specifications for the syntax and semantics
associated with such event bodies.
Event packages also MUST define which media type is to be assumed if
none are specified in the "Accept" header field of the SUBSCRIBE
request.
5.4.6. Notifier Processing of SUBSCRIBE Requests
This section describes the processing to be performed by the notifier
upon receipt of a SUBSCRIBE request. Such a section is required.
Information in this section includes details of how to authenticate
subscribers and authorization issues for the package.
5.4.7. Notifier generation of NOTIFY requests
This section of an event package describes the process by which the
notifier generates and sends a NOTIFY request. This includes
detailed information about what events cause a NOTIFY request to be
sent, how to compute the state information in the NOTIFY, how to
generate neutral or fake state information to hide authorization
delays and decisions from users, and whether state information is
complete or what the deltas are for notifications; see Section 5.3.
Such a section is required.
This section may optionally describe the behavior used to process the
subsequent response.
5.4.8. Subscriber Processing of NOTIFY Requests
This section of an event package describes the process followed by
the subscriber upon receipt of a NOTIFY request, including any logic
required to form a coherent resource state (if applicable).
5.4.9. Handling of Forked Requests
Each event package MUST specify whether forked SUBSCRIBE requests are
allowed to install multiple subscriptions.
If such behavior is not allowed, the first potential dialog-
establishing message will create a dialog. All subsequent NOTIFY
requests that correspond to the SUBSCRIBE request (i.e., have
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RFC 6665 SIP-Specific Event Notification July 2012
matching "To", "From", "Call-ID", and "Event" header fields, as well
as "From" header field "tag" parameter and "Event" header field "id"
parameter) but that do not match the dialog would be rejected with a
481 response. Note that the 200-class response to the SUBSCRIBE
request can arrive after a matching NOTIFY request has been received;
such responses might not correlate to the same dialog established by
the NOTIFY request. Except as required to complete the SUBSCRIBE
transaction, such non-matching 200-class responses are ignored.
If installing of multiple subscriptions by way of a single forked
SUBSCRIBE request is allowed, the subscriber establishes a new dialog
towards each notifier by returning a 200-class response to each
NOTIFY request. Each dialog is then handled as its own entity and is
refreshed independently of the other dialogs.
In the case that multiple subscriptions are allowed, the event
package MUST specify whether merging of the notifications to form a
single state is required, and how such merging is to be performed.
Note that it is possible that some event packages may be defined in
such a way that each dialog is tied to a mutually exclusive state
that is unaffected by the other dialogs; this MUST be clearly stated
if it is the case.
5.4.10. Rate of Notifications
Each event package is expected to define a requirement ("SHOULD" or
"MUST" strength) that defines an absolute maximum on the rate at
which notifications are allowed to be generated by a single notifier.
Each package MAY further define a throttle mechanism that allows
subscribers to further limit the rate of notification.
5.4.11. State Aggregation
Many event packages inherently work by collecting information about a
resource from a number of other sources -- either through the use of
PUBLISH [RFC3903], by subscribing to state information, or through
other state-gathering mechanisms.
Event packages that involve retrieval of state information for a
single resource from more than one source need to consider how
notifiers aggregate information into a single, coherent state. Such
packages MUST specify how notifiers aggregate information and how
they provide authentication and authorization.
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5.4.12. Examples
Event packages SHOULD include several demonstrative message flow
diagrams paired with several typical, syntactically correct, and
complete messages.
It is RECOMMENDED that documents describing event packages clearly
indicate that such examples are informative and not normative, with
instructions that implementors refer to the main text of the document
for exact protocol details.
5.4.13. Use of URIs to Retrieve State
Some types of event packages may define state information that is
potentially too large to reasonably send in a SIP message. To
alleviate this problem, event packages may include the ability to
convey a URI instead of state information; this URI will then be used
to retrieve the actual state information.
[RFC4483] defines a mechanism that can be used by event packages to
convey information in such a fashion.
6. Security Considerations
6.1. Access Control
The ability to accept subscriptions should be under the direct
control of the notifier's user, since many types of events may be
considered private. Similarly, the notifier should have the ability
to selectively reject subscriptions based on the subscriber identity
(based on access control lists), using standard SIP authentication
mechanisms. The methods for creation and distribution of such access
control lists are outside the scope of this document.
6.2. Notifier Privacy Mechanism
The mere act of returning certain 400- and 600-class responses to
SUBSCRIBE requests may, under certain circumstances, create privacy
concerns by revealing sensitive policy information. In these cases,
the notifier SHOULD always return a 200 (OK) response. While the
subsequent NOTIFY request may not convey true state, it MUST appear
to contain a potentially correct piece of data from the point of view
of the subscriber, indistinguishable from a valid response.
Information about whether a user is authorized to subscribe to the
requested state is never conveyed back to the original user under
these circumstances.
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RFC 6665 SIP-Specific Event Notification July 2012
Individual packages and their related documents for which such a mode
of operation makes sense can further describe how and why to generate
such potentially correct data. For example, such a mode of operation
is mandated by [RFC2779] for user presence information.
6.3. Denial-of-Service Attacks
The current model (one SUBSCRIBE request triggers a SUBSCRIBE
response and one or more NOTIFY requests) is a classic setup for an
amplifier node to be used in a smurf attack [CERT1998a].
Also, the creation of state upon receipt of a SUBSCRIBE request can
be used by attackers to consume resources on a victim's machine,
rendering it unusable.
To reduce the chances of such an attack, implementations of notifiers
SHOULD require authentication. Authentication issues are discussed
in [RFC3261].
6.4. Replay Attacks
Replaying of either SUBSCRIBE or NOTIFY requests can have detrimental
effects.
In the case of SUBSCRIBE requests, an attacker may be able to install
any arbitrary subscription that it witnessed being installed at some
point in the past. Replaying of NOTIFY requests may be used to spoof
old state information (although a good versioning mechanism in the
body of the NOTIFY requests may help mitigate such an attack). Note
that the prohibition on sending NOTIFY requests to nodes that have
not subscribed to an event also aids in mitigating the effects of
such an attack.
To prevent such attacks, implementations SHOULD require
authentication with anti-replay protection. Authentication issues
are discussed in [RFC3261].
6.5. Man-in-the-Middle Attacks
Even with authentication, man-in-the-middle attacks using SUBSCRIBE
requests may be used to install arbitrary subscriptions, hijack
existing subscriptions, terminate outstanding subscriptions, or
modify the resource to which a subscription is being made. To
prevent such attacks, implementations SHOULD provide integrity
protection across "Contact", "Route", "Expires", "Event", and "To"
header fields (at a minimum) of SUBSCRIBE requests. If SUBSCRIBE
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RFC 6665 SIP-Specific Event Notification July 2012
request bodies are used to define further information about the state
of the call, they SHOULD be included in the integrity protection
scheme.
Man-in-the-middle attacks may also attempt to use NOTIFY requests to
spoof arbitrary state information and/or terminate outstanding
subscriptions. To prevent such attacks, implementations SHOULD
provide integrity protection across the "Call-ID", "CSeq", and
"Subscription-State" header fields and the bodies of NOTIFY requests.
Integrity protection of message header fields and bodies is discussed
in [RFC3261].
6.6. Confidentiality
The state information contained in a NOTIFY request has the potential
to contain sensitive information. Implementations MAY encrypt such
information to ensure confidentiality.
While less likely, it is also possible that the information contained
in a SUBSCRIBE request contains information that users might not want
to have revealed. Implementations MAY encrypt such information to
ensure confidentiality.
To allow the remote party to hide information it considers sensitive,
all implementations SHOULD be able to handle encrypted SUBSCRIBE and
NOTIFY requests.
The mechanisms for providing confidentiality are detailed in
[RFC3261].
7. IANA Considerations
With the exception of Section 7.2, the subsections here are for
current reference, carried over from the original specification (RFC
3265). IANA has updated all registry references that pointed to RFC
3265 to instead indicate this document and created the new "reason
code" registry described in Section 7.2.
7.1. Event Packages
This document defines an event-type namespace that requires a central
coordinating body. The body chosen for this coordination is the
Internet Assigned Numbers Authority (IANA).
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RFC 6665 SIP-Specific Event Notification July 2012
There are two different types of event-types: normal event packages
and event template-packages; see Section 5.2. To avoid confusion,
template-package names and package names share the same namespace; in
other words, an event template-package is forbidden from sharing a
name with a package.
Policies for registration of SIP event packages and SIP event package
templates are defined in Section 4.1 of [RFC5727].
Registrations with the IANA are required to include the token being
registered and whether the token is a package or a template-package.
Further, packages must include contact information for the party
responsible for the registration and/or a published document that
describes the event package. Event template-package token
registrations are also required to include a pointer to the published
RFC that defines the event template-package.
Registered tokens to designate packages and template-packages are
disallowed from containing the character ".", which is used to
separate template-packages from packages.
7.1.1. Registration Information
This document specifies no package or template-package names. All
entries in this table are added by other documents. The remainder of
the text in this section gives an example of the type of information
to be maintained by the IANA; it also demonstrates all four possible
permutations of package type, contact, and reference.
The table below lists the event packages and template-packages
defined for use with the "SIP-Specific Event Notification" mechanism
[RFC 6665]. Each name is designated as a package or a template-
package under "Type".
Package Name Type Contact Reference
------------ ---- ------- ---------
example1 package [Doe] [RFCnnnn]
example2 package [RFCnnnn]
example3 template [Doe] [RFCnnnn]
example4 template [RFCnnnn]
PEOPLE
------
[Doe] John Doe <john.doe@example.com>
REFERENCES
----------
[RFCnnnn] Doe, J., "Sample Document", RFC nnnn, Month YYYY.
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RFC 6665 SIP-Specific Event Notification July 2012
7.1.2. Registration Template
To: ietf-sip-events@iana.org
Subject: Registration of new SIP event package
Package name:
(Package names must conform to the syntax described in
Section 8.2.1.)
Is this registration for a Template-Package:
(indicate yes or no)
Published specification(s):
(Template-packages require a published RFC. Other packages may
reference a specification when appropriate.)
Person & email address to contact for further information:
(self-explanatory)
7.2. Reason Codes
This document further defines "reason" codes for use in the
"Subscription-State" header field (see Section 4.1.3).
Following the policies outlined in "Guidelines for Writing an IANA
Considerations Section in RFCs" [RFC5226], new reason codes require a
Standards Action.
Registrations with the IANA include the reason code being registered
and a reference to a published document that describes the event
package. Insertion of such values takes place as part of the RFC
publication process or as the result of liaison activity between
standards development organizations (SDOs), the result of which will
be publication of an associated RFC. New reason codes must conform
to the syntax of the ABNF "token" element defined in [RFC3261].
[RFC4660] defined a new reason code prior to the establishment of an
IANA registry. We include its reason code ("badfilter") in the
initial list of reason codes to ensure a complete registry.
The IANA registry for reason codes has been initialized with the
following values:
Roach Standards Track [Page 40]RFC 6665 SIP-Specific Event Notification July 2012
Reason Code Reference
----------- ---------
deactivated [RFC6665]
probation [RFC6665]
rejected [RFC6665]
timeout [RFC6665]
giveup [RFC6665]
noresource [RFC6665]
invariant [RFC6665]
badfilter [RFC4660]
REFERENCES
----------
[RFC6665] A.B. Roach, "SIP-Specific Event Notification", RFC 6665,
July 2012.
[RFC4660] Khartabil, H., Leppanen, E., Lonnfors, M., and
J. Costa-Requena, "Functional Description of Event
Notification Filtering", September 2006.
7.3. Header Field Names
This document registers three new header field names, described
elsewhere in this document. These header fields are defined by the
following information, which is to be added to the header field sub-
registry under http://www.iana.org/assignments/sip-parameters.
Header Name: Allow-Events
Compact Form: u
Header Name: Subscription-State
Compact Form: (none)
Header Name: Event
Compact Form: o
7.4. Response Codes
This document registers two new response codes. These response codes
are defined by the following information, which is to be added to the
method and response-code sub-registry under
http://www.iana.org/assignments/sip-parameters.
Response Code Number: 202
Default Reason Phrase: Accepted
Response Code Number: 489
Default Reason Phrase: Bad Event
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8. Syntax
This section describes the syntax extensions required for event
notification in SIP. Semantics are described in Section 4. Note
that the formal syntax definitions described in this document are
expressed in the ABNF format used in [RFC3261] and contain references
to elements defined therein.
8.1. New Methods
This document describes two new SIP methods: SUBSCRIBE and NOTIFY.
8.1.1. SUBSCRIBE Method
"SUBSCRIBE" is added to the definition of the element "Method" in the
SIP message grammar.
Like all SIP method names, the SUBSCRIBE method name is case
sensitive. The SUBSCRIBE method is used to request asynchronous
notification of an event or set of events at a later time.
8.1.2. NOTIFY Method
"NOTIFY" is added to the definition of the element "Method" in the
SIP message grammar.
The NOTIFY method is used to notify a SIP node that an event that has
been requested by an earlier SUBSCRIBE method has occurred. It may
also provide further details about the event.
8.2. New Header Fields
8.2.1. "Event" Header Field
Event is added to the definition of the element "message-header
field" in the SIP message grammar.
For the purposes of matching NOTIFY requests with SUBSCRIBE requests,
the event-type portion of the "Event" header field is compared byte
by byte, and the "id" parameter token (if present) is compared byte
by byte. An "Event" header field containing an "id" parameter never
matches an "Event" header field without an "id" parameter. No other
parameters are considered when performing a comparison. SUBSCRIBE
responses are matched per the transaction handling rules in
[RFC3261].
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RFC 6665 SIP-Specific Event Notification July 2012
Note that the foregoing text means that "Event: foo; id=1234"
would match "Event: foo; param=abcd; id=1234", but not "Event:
foo" ("id" does not match) or "Event: Foo; id=1234" ("Event"
portion does not match).
This document does not define values for event-types. These values
will be defined by individual event packages and MUST be registered
with the IANA.
There MUST be exactly one event type listed per "Event" header field.
Multiple events per message are disallowed.
The "Event" header field is defined only for use in SUBSCRIBE and
NOTIFY requests and other requests whose definition explicitly calls
for its use. It MUST NOT appear in any other SIP requests and MUST
NOT appear in responses.
8.2.2. "Allow-Events" Header Field
"Allow-Events" is added to the definition of the element "general-
header field" in the SIP message grammar. Its usage is described in
Section 4.4.4.
User agents MAY include the "Allow-Events" header field in any
request or response, as long as its contents comply with the behavior
described in Section 4.4.4.
8.2.3. "Subscription-State" Header Field
"Subscription-State" is added to the definition of the element
"request-header" field in the SIP message grammar. Its usage is
described in Section 4.1.3. "Subscription-State" header fields are
defined for use in NOTIFY requests only. They MUST NOT appear in
other SIP requests or responses.
8.3. New Response Codes
8.3.1. 202 (Accepted) Response Code
For historical purposes, the 202 (Accepted) response code is added to
the "Success" header field definition.
This document does not specify the use of the 202 response code in
conjunction with the SUBSCRIBE or NOTIFY methods. Previous versions
of the SIP Events Framework assigned specific meaning to the 202
response code.
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Due to response handling in forking cases, any 202 response to a
SUBSCRIBE request may be absorbed by a proxy, and thus it can never
be guaranteed to be received by the UAC. Furthermore, there is no
actual processing difference for a 202 as compared to a 200; a NOTIFY
request is sent after the subscription is processed, and it conveys
the correct state. SIP interoperability tests found that
implementations were handling 202 differently from 200, leading to
incompatibilities. Therefore, the 202 response is being deprecated
to make it clear there is no such difference and 202 should not be
handled differently than 200.
Implementations conformant with the current specification MUST treat
an incoming 202 response as identical to a 200 response and MUST NOT
generate 202 response codes to SUBSCRIBE or NOTIFY requests.
This document also updates [RFC4660], which reiterates the 202-based
behavior in several places. Implementations compliant with the
present document MUST NOT send a 202 response to a SUBSCRIBE request
and will send an alternate success response (such as 200) in its
stead.
8.3.2. 489 (Bad Event) Response Code
The 489 event response is added to the "Client-Error" header field
definition. 489 (Bad Event) is used to indicate that the server did
not understand the event package specified in a "Event" header field.
8.4. Augmented BNF Definitions
The Augmented BNF [RFC5234] definitions for the various new and
modified syntax elements follows. The notation is as used in
[RFC3261], and any elements not defined in this section are as
defined in SIP and the documents to which it refers.
SUBSCRIBEm = %x53.55.42.53.43.52.49.42.45 ; SUBSCRIBE in caps
NOTIFYm = %x4E.4F.54.49.46.59 ; NOTIFY in caps
extension-method = SUBSCRIBEm / NOTIFYm / token
Event = ( "Event" / "o" ) HCOLON event-type
*( SEMI event-param )
event-type = event-package *( "." event-template )
event-package = token-nodot
event-template = token-nodot
token-nodot = 1*( alphanum / "-" / "!" / "%" / "*"
/ "_" / "+" / "`" / "'" / "~" )
; The use of the "id" parameter is deprecated; it is included
; for backwards-compatibility purposes only.
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event-param = generic-param / ( "id" EQUAL token )
Allow-Events = ( "Allow-Events" / "u" ) HCOLON event-type
*(COMMA event-type)
Subscription-State = "Subscription-State" HCOLON substate-value
*( SEMI subexp-params )
substate-value = "active" / "pending" / "terminated"
/ extension-substate
extension-substate = token
subexp-params = ("reason" EQUAL event-reason-value)
/ ("expires" EQUAL delta-seconds)
/ ("retry-after" EQUAL delta-seconds)
/ generic-param
event-reason-value = "deactivated"
/ "probation"
/ "rejected"
/ "timeout"
/ "giveup"
/ "noresource"
/ "invariant"
/ event-reason-extension
event-reason-extension = token
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2848] Petrack, S. and L. Conroy, "The PINT Service Protocol:
Extensions to SIP and SDP for IP Access to Telephone
Call Services", RFC 2848, June 2000.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3968] Camarillo, G., "The Internet Assigned Number Authority
(IANA) Header Field Parameter Registry for the Session
Initiation Protocol (SIP)", BCP 98, RFC 3968,
December 2004.
Roach Standards Track [Page 45]RFC 6665 SIP-Specific Event Notification July 2012
[RFC4483] Burger, E., "A Mechanism for Content Indirection in
Session Initiation Protocol (SIP) Messages", RFC 4483,
May 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5627] Rosenberg, J., "Obtaining and Using Globally Routable
User Agent URIs (GRUUs) in the Session Initiation
Protocol (SIP)", RFC 5627, October 2009.
[RFC5727] Peterson, J., Jennings, C., and R. Sparks, "Change
Process for the Session Initiation Protocol (SIP) and
the Real-time Applications and Infrastructure Area",
BCP 67, RFC 5727, March 2010.
9.2. Informative References
[RFC2779] Day, M., Aggarwal, S., Mohr, G., and J. Vincent,
"Instant Messaging / Presence Protocol Requirements",
RFC 2779, February 2000.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840, August 2004.
[RFC3891] Mahy, R., Biggs, B., and R. Dean, "The Session
Initiation Protocol (SIP) "Replaces" Header", RFC 3891,
September 2004.
[RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension
for Event State Publication", RFC 3903, October 2004.
[RFC3911] Mahy, R. and D. Petrie, "The Session Initiation Protocol
(SIP) "Join" Header", RFC 3911, October 2004.
[RFC4235] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session
Initiation Protocol (SIP)", RFC 4235, November 2005.
Roach Standards Track [Page 46]RFC 6665 SIP-Specific Event Notification July 2012
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288,
December 2005.
[RFC4485] Rosenberg, J. and H. Schulzrinne, "Guidelines for
Authors of Extensions to the Session Initiation Protocol
(SIP)", RFC 4485, May 2006.
[RFC4538] Rosenberg, J., "Request Authorization through Dialog
Identification in the Session Initiation Protocol
(SIP)", RFC 4538, June 2006.
[RFC4660] Khartabil, H., Leppanen, E., Lonnfors, M., and J. Costa-
Requena, "Functional Description of Event Notification
Filtering", RFC 4660, September 2006.
[RFC5057] Sparks, R., "Multiple Dialog Usages in the Session
Initiation Protocol", RFC 5057, November 2007.
[RFC5839] Niemi, A. and D. Willis, "An Extension to Session
Initiation Protocol (SIP) Events for Conditional Event
Notification", RFC 5839, May 2010.
[CERT1998a] CERT, "CERT Advisory CA-1998-01: Smurf IP Denial-of-
Service Attacks", 1998,
<http://www.cert.org/advisories/CA-1998-01.html>.
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Appendix A. Acknowledgements
Thanks to the participants in the Events BOF at the 48th IETF meeting
in Pittsburgh, as well as those who gave ideas and suggestions on the
SIP Events mailing list. In particular, I wish to thank Henning
Schulzrinne of Columbia University for coming up with the final
three-tiered event identification scheme, Sean Olson for
miscellaneous guidance, Jonathan Rosenberg for a thorough scrubbing
of the first draft version, and the authors of the "SIP Extensions
for Presence" document for their input to SUBSCRIBE and NOTIFY
request semantics.
I also owe a debt of gratitude to all the implementors who have
provided feedback on areas of confusion or difficulty in the original
specification. In particular, Robert Sparks' Herculean efforts
organizing, running, and collecting data from the SIPit events have
proven invaluable in shaking out specification bugs. Robert Sparks
is also responsible for untangling the dialog usage mess, in the form
of RFC 5057 [RFC5057].
Appendix B. Changes from RFC 3265
This document represents several changes from the mechanism
originally described in RFC 3265. This section summarizes those
changes. Bug numbers refer to the identifiers for the bug reports
kept on file at http://bugs.sipit.net/.
B.1. Bug 666: Clarify use of "expires=xxx" with "terminated"
Strengthened language in Section 4.1.3 to clarify that "expires"
should not be sent with "terminated", and must be ignored if
received.
B.2. Bug 667: Reason code for unsub/poll not clearly spelled out
Clarified description of "timeout" in Section 4.1.3. (n.b., the text
in Section 4.4.3 is actually pretty clear about this).
B.3. Bug 669: Clarify: SUBSCRIBE for a duration might be answered with
a NOTIFY/expires=0
Added clarifying text to Section 4.2.2 explaining that shortening a
subscription to zero seconds is valid. Also added sentence to
Section 3.1.1 explicitly allowing shortening to zero.
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B.4. Bug 670: Dialog State Machine needs clarification
The issues associated with the bug deal exclusively with the handling
of multiple usages with a dialog. This behavior has been deprecated
and moved to Section 4.5.2. This section, in turn, cites [RFC5057],
which addresses all of the issues in Bug 670.
B.5. Bug 671: Clarify timeout-based removal of subscriptions
Changed Section 4.2.2 to specifically cite Timer F (so as to avoid
ambiguity between transaction timeouts and retransmission timeouts).
B.6. Bug 672: Mandate "expires" in NOTIFY
Changed strength of including of "expires" in a NOTIFY from "SHOULD"
to "MUST" in Section 4.2.2.
B.7. Bug 673: INVITE 481 response effect clarification
This bug was addressed in [RFC5057].
B.8. Bug 677: SUBSCRIBE response matching text in error
Fixed Section 8.2.1 to remove incorrect "...responses and..." --
explicitly pointed to SIP for transaction response handling.
B.9. Bug 695: Document is not explicit about response to NOTIFY at
subscription termination
Added text to Section 4.4.1 indicating that the typical response to a
terminal NOTIFY is a 200 (OK).
B.10. Bug 696: Subscription state machine needs clarification
Added state machine diagram to Section 4.1.2 with explicit handling
of what to do when a SUBSCRIBE never shows up. Added definition of
and handling for new Timer N to Section 4.1.2.4. Added state machine
to Section 4.2.2 to reinforce text.
B.11. Bug 697: Unsubscription behavior could be clarified
Added text to Section 4.2.1.4 encouraging (but not requiring) full
state in final NOTIFY request. Also added text to Section 4.1.2.3
warning subscribers that full state may or may not be present in the
final NOTIFY.
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B.12. Bug 699: NOTIFY and SUBSCRIBE are target refresh requests
Added text to both Sections 3.1 and 3.2 explicitly indicating that
SUBSCRIBE and NOTIFY are target refresh methods.
B.13. Bug 722: Inconsistent 423 reason phrase text
Changed reason phrase to "Interval Too Brief" in Sections 4.2.1.1 and
4.2.1.4, to match 423 reason phrase in SIP [RFC3261].
B.14. Bug 741: Guidance needed on when to not include "Allow-Events"
Added non-normative clarification to Section 4.4.4 regarding
inclusion of "Allow-Events" in a NOTIFY for the one-and-only package
supported by the notifier.
B.15. Bug 744: 5xx to NOTIFY terminates a subscription, but should not
Issue of subscription (usage) termination versus dialog termination
is handled in [RFC5057]. The text in Section 4.2.2 has been updated
to summarize the behavior described by RFC 5057, and cites it for
additional detail and rationale.
B.16. Bug 752: Detection of forked requests is incorrect
Removed erroneous "CSeq" from list of matching criteria in
Section 5.4.9.
B.17. Bug 773: Reason code needs IANA registry
Added Section 7.2 to create and populate IANA registry.
B.18. Bug 774: Need new reason for terminating subscriptions to
resources that never change
Added new "invariant" reason code to Section 4.1.3 and to ABNF syntax
in Section 8.4.
B.19. Clarify Handling of "Route"/"Record-Route" in NOTIFY
Changed text in Section 4.3 in order to mandate "Record-Route" in
initial SUBSCRIBE and all NOTIFY requests, and add "MAY"-level
statements for subsequent SUBSCRIBE requests.
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B.20. Eliminate Implicit Subscriptions
Added text to Section 4.2.1 explaining some of the problems
associated with implicit subscriptions, and added normative language
prohibiting them. Removed language from Section 3.2 describing "non-
SUBSCRIBE" mechanisms for creating subscriptions. Simplified
language in Section 4.2.2, now that the soft-state/non-soft-state
distinction is unnecessary.
B.21. Deprecate Dialog Reuse
Moved handling of dialog reuse and "id" handling to Section 4.5.2.
It is documented only for backwards-compatibility purposes.
B.22. Rationalize Dialog Creation
Section 4.4.1 has been updated to specify that dialogs should be
created when the NOTIFY arrives. Previously, the dialog was
established by the SUBSCRIBE 200 or by the NOTIFY transaction. This
was unnecessarily complicated; the newer rules are easier to
implement (and result in effectively the same behavior on the wire).
B.23. Refactor Behavior Sections
Reorganized Section 4 to consolidate behavior along role lines
(subscriber/notifier/proxy) instead of method lines.
B.24. Clarify Sections That Need to Be Present in Event Packages
Added sentence to Section 5 clarifying that event packages are
expected to include explicit sections covering the issues discussed
in this section.
B.25. Make CANCEL Handling More Explicit
Text in Section 4.6 now clearly calls out behavior upon receipt of a
CANCEL. We also echo the "...SHOULD NOT send..." requirement from
[RFC3261].
B.26. Remove "State Agent" Terminology
As originally planned, we anticipated a fairly large number of event
packages that would move back and forth between end-user devices and
servers in the network. In practice, this has ended up not being the
case. Certain events, like dialog state, are inherently hosted at
end-user devices; others, like presence, are almost always hosted in
the network (due to issues like composition, and the ability to
deliver information when user devices are offline). Further, the
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RFC 6665 SIP-Specific Event Notification July 2012
concept of State Agents is the most misunderstood by event package
authors. In my expert review of event packages, I have yet to find
one that got the concept of State Agents completely correct -- and
most of them start out with the concept being 100% backwards from the
way RFC 3265 described it.
Rather than remove the ability to perform the actions previously
attributed to the widely misunderstood term "State Agent", we have
simply eliminated this term. Instead, we talk about the behaviors
required to create state agents (state aggregation, subscription
notification) without defining a formal term to describe the servers
that exhibit these behaviors. In effect, this is an editorial change
to make life easier for event package authors; the actual protocol
does not change as a result.
The definition of "State Agent" has been removed from Section 2.
Section 4.4.2 has been retooled to discuss migration of subscription
in general, without calling out the specific example of state agents.
Section 5.4.11 has been focused on state aggregation in particular,
instead of state aggregation as an aspect of state agents.
B.27. Miscellaneous Changes
The following changes are relatively minor revisions to the document
that resulted primarily from review of this document in the working
group and IESG, rather than implementation reports.
o Clarified scope of "Event" header field parameters. In RFC 3265,
the scope is ambiguous, which causes problems with the registry in
RFC 3968. The new text ensures that "Event" header field
parameters are unique across all event packages.
o Removed obsoleted language around IANA registration policies for
event packages. Instead, we now cite RFC 5727, which updates RFC
3265, and is authoritative on event package registration policy.
o Several editorial updates after input from working group,
including proper designation of "dialog usage" rather than
"dialog" where needed.
o Clarified two normative statements about subscription termination
by changing from plain English prose to RFC2119 language.
o Removed "Table 2" expansions, per WG consensus on how SIP Table 2
is to be handled.
o Removed 202 response code.
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o Clarified that "Allow-Events" does not list event template-
packages.
o Added clarification about proper response when the SUBSCRIBE
indicates an unknown media type in its "Accept" header field.
o Minor clarifications to "Route" and "Record-Route" behavior.
o Added non-normative warning about the limitations of state
polling.
o Added information about targeting subscriptions at specific
dialogs.
o Added RFC 3261 to list of documents updated by this one (rather
than the "2543" indicated by RFC 3265).
o Clarified text in Section 3.1.1 explaining the meaning of
"Expires: 0".
o Changed text in definition of "probation" reason code to indicate
that subscribers don't need to re-subscribe if the associated
state is no longer of use to them.
o Specified that the termination of a subscription due to a NOTIFY
transaction failure does not require sending another NOTIFY
message.
o Clarified how order of template application affects the meaning of
an "Event" header field value (e.g., "foo.bar.baz" is different
than "foo.baz.bar").
Author's Address
Adam Roach
Tekelec
17210 Campbell Rd.
Suite 250
Dallas, TX 75252
US
EMail: adam@nostrum.com
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