Network Working Group A. B. Roach
Internet-Draft Tekelec
Expires: August 23, 2010 February 19, 2010
SIP-Specific Event Notification
draft-ietf-sipcore-rfc3265bis-01
Abstract
This document describes an extension to the Session Initiation
Protocol (SIP). 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.
Status of this Memo
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Copyright (c) 2010 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
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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 . . . . . . 13
4.1.4. Forking of SUBSCRIBE Messages . . . . . . . . . . . . 16
4.2. Notifier Behavior . . . . . . . . . . . . . . . . . . . . 16
4.2.1. Subscription Establishment and Maintenance . . . . . . 16
4.2.2. Sending State Information to Subscribers . . . . . . . 20
4.2.3. PINT Compatibility . . . . . . . . . . . . . . . . . . 22
4.3. Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 22
4.4. Common Behavior . . . . . . . . . . . . . . . . . . . . . 23
4.4.1. Dialog Creation and Termination . . . . . . . . . . . 23
4.4.2. Notifier Migration . . . . . . . . . . . . . . . . . . 23
4.4.3. Polling Resource State . . . . . . . . . . . . . . . . 24
4.4.4. Allow-Events header field usage . . . . . . . . . . . 25
4.5. Targeting Subscriptions at Devices . . . . . . . . . . . . 25
4.5.1. Using GRUUs to Route to Devices . . . . . . . . . . . 26
4.5.2. Sharing Dialogs . . . . . . . . . . . . . . . . . . . 26
4.6. CANCEL Requests for SUBSCRIBE and NOTIFY . . . . . . . . . 28
5. Event Packages . . . . . . . . . . . . . . . . . . . . . . . . 28
5.1. Appropriateness of Usage . . . . . . . . . . . . . . . . . 28
5.2. Event Template-packages . . . . . . . . . . . . . . . . . 29
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5.3. Amount of State to be Conveyed . . . . . . . . . . . . . . 29
5.3.1. Complete State Information . . . . . . . . . . . . . . 30
5.3.2. State Deltas . . . . . . . . . . . . . . . . . . . . . 30
5.4. Event Package Responsibilities . . . . . . . . . . . . . . 30
5.4.1. Event Package Name . . . . . . . . . . . . . . . . . . 31
5.4.2. Event Package Parameters . . . . . . . . . . . . . . . 31
5.4.3. SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . 31
5.4.4. Subscription Duration . . . . . . . . . . . . . . . . 31
5.4.5. NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . 31
5.4.6. Notifier processing of SUBSCRIBE requests . . . . . . 32
5.4.7. Notifier generation of NOTIFY requests . . . . . . . . 32
5.4.8. Subscriber processing of NOTIFY requests . . . . . . . 32
5.4.9. Handling of forked requests . . . . . . . . . . . . . 32
5.4.10. Rate of notifications . . . . . . . . . . . . . . . . 33
5.4.11. State Aggregation . . . . . . . . . . . . . . . . . . 33
5.4.12. Examples . . . . . . . . . . . . . . . . . . . . . . . 33
5.4.13. Use of URIs to Retrieve State . . . . . . . . . . . . 34
6. Security Considerations . . . . . . . . . . . . . . . . . . . 34
6.1. Access Control . . . . . . . . . . . . . . . . . . . . . . 34
6.2. Notifier Privacy Mechanism . . . . . . . . . . . . . . . . 34
6.3. Denial-of-Service attacks . . . . . . . . . . . . . . . . 35
6.4. Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 35
6.5. Man-in-the middle attacks . . . . . . . . . . . . . . . . 35
6.6. Confidentiality . . . . . . . . . . . . . . . . . . . . . 36
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
7.1. Event Packages . . . . . . . . . . . . . . . . . . . . . . 36
7.1.1. Registration Information . . . . . . . . . . . . . . . 37
7.1.2. Registration Template . . . . . . . . . . . . . . . . 38
7.2. Reason Codes . . . . . . . . . . . . . . . . . . . . . . . 38
7.3. Header Field Names . . . . . . . . . . . . . . . . . . . . 39
7.4. Response Codes . . . . . . . . . . . . . . . . . . . . . . 39
8. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
8.1. New Methods . . . . . . . . . . . . . . . . . . . . . . . 40
8.1.1. SUBSCRIBE method . . . . . . . . . . . . . . . . . . . 41
8.1.2. NOTIFY method . . . . . . . . . . . . . . . . . . . . 41
8.2. New Header Fields . . . . . . . . . . . . . . . . . . . . 42
8.2.1. "Event" Header Field . . . . . . . . . . . . . . . . . 42
8.2.2. "Allow-Events" Header Field . . . . . . . . . . . . . 42
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 . . . . . . . . . . . . 43
8.4. Augmented BNF Definitions . . . . . . . . . . . . . . . . 43
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 44
9.1. Normative References . . . . . . . . . . . . . . . . . . . 44
9.2. Informative References . . . . . . . . . . . . . . . . . . 45
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 46
Appendix B. Open Issues . . . . . . . . . . . . . . . . . . . . . 46
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B.1. Bug 711: Allow-Events can't express template support . . . 47
B.2. Remove 202 Response Code? . . . . . . . . . . . . . . . . 47
B.3. Timer N and Resubscribes . . . . . . . . . . . . . . . . . 47
Appendix C. Changes . . . . . . . . . . . . . . . . . . . . . . . 48
C.1. Changes from draft-ietf-sipcore-rfc3265bis-00 to
draft-ietf-sipcore-rfc3265bis-01 . . . . . . . . . . . . . 48
C.2. Changes from draft-roach-sipcore-rfc3265bis-00 to
draft-ietf-sipcore-rfc3265bis-00 . . . . . . . . . . . . . 48
C.3. Changes since RFC 3265 . . . . . . . . . . . . . . . . . . 48
C.3.1. Bug 666: Clarify use of expires=xxx with terminated . 48
C.3.2. Bug 667: Reason code for unsub/poll not clearly
spelled out . . . . . . . . . . . . . . . . . . . . . 48
C.3.3. Bug 669: Clarify: SUBSCRIBE for a duration might
be answered with a NOTIFY/expires=0 . . . . . . . . . 48
C.3.4. Bug 670: Dialog State Machine needs clarification . . 48
C.3.5. Bug 671: Clarify timeout-based removal of
subscriptions . . . . . . . . . . . . . . . . . . . . 49
C.3.6. Bug 672: Mandate expires= in NOTIFY . . . . . . . . . 49
C.3.7. Bug 673: INVITE 481 response effect clarification . . 49
C.3.8. Bug 677: SUBSCRIBE response matching text in error . . 49
C.3.9. Bug 695: Document is not explicit about response
to NOTIFY at subscription termination . . . . . . . . 49
C.3.10. Bug 696: Subscription state machine needs
clarification . . . . . . . . . . . . . . . . . . . . 49
C.3.11. Bug 697: Unsubscription behavior could be clarified . 49
C.3.12. Bug 699: NOTIFY and SUBSCRIBE are target refresh
requests . . . . . . . . . . . . . . . . . . . . . . . 49
C.3.13. Bug 722: Inconsistent 423 reason phrase text . . . . . 50
C.3.14. Bug 741: guidance needed on when to not include
Allow-Events . . . . . . . . . . . . . . . . . . . . . 50
C.3.15. Bug 744: 5xx to NOTIFY terminates a subscription,
but should not . . . . . . . . . . . . . . . . . . . . 50
C.3.16. Bug 752: Detection of forked requests is incorrect . . 50
C.3.17. Bug 773: Reason code needs IANA registry . . . . . . . 50
C.3.18. Bug 774: Need new reason for terminating
subscriptions to resources that never change . . . . . 50
C.3.19. Clarify handling of Route/Record-Route in NOTIFY . . . 50
C.3.20. Eliminate implicit subscriptions . . . . . . . . . . . 50
C.3.21. Deprecate dialog re-use . . . . . . . . . . . . . . . 51
C.3.22. Rationalize dialog creation . . . . . . . . . . . . . 51
C.3.23. Refactor behavior sections . . . . . . . . . . . . . . 51
C.3.24. Clarify sections that need to be present in event
packages . . . . . . . . . . . . . . . . . . . . . . . 51
C.3.25. Make CANCEL handling more explicit . . . . . . . . . . 51
C.3.26. Remove State Agent Terminology . . . . . . . . . . . . 51
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 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 which is not so complex as to be unusable for
simple features, but which is still flexible enough to provide
powerful services. Note, however, that event packages based on this
framework may define arbitrarily elaborate rules which govern the
subscription and notification for the events or classes of events
they describe.
This document does not describe an extension which 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 which must be derived into an
instantiatable 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 messages.
1.2. Documentation Conventions
There are several paragraphs throughout this document which 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 all-capital terms "MUST", "SHOULD", "MAY", "SHOULD NOT", "MUST
NOT", and "RECOMMENDED" are used as defined 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 inappropriate to fail to comply with
"SHOULD"-strength requirements whimsically or for ease of
implementation.
The use of quotation marks next to periods and commas follows the
convention used by the American Mathematical Society; although
contrary to traditional American English convention, this usage lends
clarity to certain passages.
2. Definitions
Event Package: An event package is an additional specification which
defines a set of state information to be reported by a notifier to
a subscriber. Event packages also define further syntax and
semantics based on the framework defined by this document required
to convey such state information.
Event Template-Package: An event template-package is a special kind
of event package which defines a set of states which may be
applied to all possible event packages, including itself.
Notification: Notification is the act of a notifier sending a NOTIFY
message to a subscriber to inform the subscriber of the state of a
resource.
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Notifier: A notifier is a user agent which generates NOTIFY requests
for the purpose of notifying subscribers of the state of a
resource. Notifiers typically also accept SUBSCRIBE requests to
create subscriptions.
Subscriber: A subscriber is a user agent which 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 is a target refresh request,
as that term is defined in SIP [RFC3261].
3.1.1. Subscription Duration
SUBSCRIBE requests SHOULD contain an "Expires" header field (defined
in SIP [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
message on the same dialog as defined in SIP [RFC3261].
If no "Expires" header field is present in a SUBSCRIBE request, the
implied default is 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 which defines the duration
of the subscription.
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An "expires" parameter on the "Contact" header field has no semantics
for SUBSCRIBE and is explicitly not equivalent to an "Expires" header
field in a SUBSCRIBE request or response.
A natural consequence of this scheme is that a SUBSCRIBE with an
"Expires" of 0 constitutes a request to unsubscribe from an event.
In addition to being a request to unsubscribe, a SUBSCRIBE message
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 SIP [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 which
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 which 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 as defined in SIP
[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 SIP [RFC3261].
3.2. NOTIFY
NOTIFY messages are sent to inform subscribers of changes in state to
which the subscriber has a subscription. Subscriptions are typically
put in place using the SUBSCRIBE method; however, it is possible that
other means have been used.
NOTIFY is a target refresh request, as that term is defined in SIP
[RFC3261].
A NOTIFY 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 "Event" header fields will 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 message.
Event packages may define semantics associated with the body of their
NOTIFY requests; if they do so, those semantics apply. NOTIFY bodies
are expected to provide additional details about the nature of the
event which has occurred and the resultant resource state.
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. 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).
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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 use
of "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 SIP [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 messages
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:
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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 | ||
|| | | +-------------+ ||
|| | | A A ||
|| | V Receive NOTIFY, | | ||
|| | +-------------+ state=terminated; | | ||
|| | | pending |-- or 481 response ----+ | ||
|| | +-------------+ to SUBSCRIBE | ||
|| | | refresh | ||
|| | Receive NOTIFY, | ||
|| | state=active | ||
|| | | | ||
|| | V Receive NOTIFY, | ||
|| | +-------------+ state=terminated; | ||
|| +----------->| active |-- or 481 response -------+ ||
|| +-------------+ to SUBSCRIBE ||
|| refresh ||
|| Subscription ||
++=================================================================++
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.
4.1.2.1. Requesting a Subscription
SUBSCRIBE is a dialog-creating method, as described in SIP [RFC3261].
When a subscriber wishes to subscribe to a particular state for a
resource, it forms a SUBSCRIBE message. If the initial SUBSCRIBE
represents a request outside of a dialog (as it typically will), its
construction follows the procedures outlined in SIP [RFC3261] for UAC
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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 will be sent immediately. A 200 response indicates
that the subscription has been accepted and that the user is
authorized to subscribe to the requested resource. A 202 response
merely indicates that the subscription has been understood, and that
authorization may or may not have been granted.
The "Expires" header field in a 200-class response to SUBSCRIBE
indicates the actual duration for which the subscription will remain
active (unless refreshed).
Non-200 class final responses indicate that no subscription or dialog
has been created, and no subsequent NOTIFY message will be sent. All
non-200 class responses (with the exception of "489", described
herein) have the same meanings and handling as described in SIP
[RFC3261]. For the sake of clarity: if a SUBSCRIBE request contains
an "Accept" header field, but that field does not indicate a MIME
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 should
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.)
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 SUBSCRIBE and its response, or as
communicated by NOTIFY in the "Subscription-State" header field
"expires" parameter.
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Note that many such errors indicate that there may be a problem
with the network or the notifier such that no further NOTIFY
messages will be received.
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 message.
The final NOTIFY may or may not contain information about the state
of the resource; subscribers need to be prepared to receive final
NOTIFY messages both with and without state.
4.1.2.4. Confirmation of Subscription Creation
The subscriber can expect to receive a NOTIFY message 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. If this Timer N expires prior to the receipt of a NOTIFY
message, the subscriber considers the subscription failed, and cleans
up any state associated with the subscription attempt.
Until Timer N expires, several NOTIFY messages may arrive from
different destinations (see Section 4.4.1). Each of these messages
establish a new dialog and a new subscription. After the expiration
of Timer N, the subscriber SHOULD reject any such NOTIFY messages
that would otherwise establish a new dialog with a "481" response
code.
Until the first NOTIFY message arrives, the subscriber should
consider the state of the subscribed resource to be in a neutral
state. Documents which define new event packages 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 both out-of-order messages and forking, the
subscriber MUST be prepared to receive NOTIFY messages before the
SUBSCRIBE transaction has completed.
Except as noted above, processing of this NOTIFY 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.
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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 are described in Section 4.4.1. Notifications for
subscriptions which 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.
NOTIFY requests MUST contain "Subscription-State" header fields which
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".
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
should 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:
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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. 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
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 which 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, RFC 3265-compliant clients from
periodically resubscribing. Clients SHOULD NOT attempt to
resubscribe 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.
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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 SIP [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 Messages
In accordance with the rules for proxying non-INVITE requests as
defined in SIP [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 which differ from the "To:" tag received in the
SUBSCRIBE 200-class response.
If multiple NOTIFY messages are received in different dialogs in
response to a single SUBSCRIBE message, 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 message. 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 SUBSCRIBE. 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 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 re-use (which has its own set of
problems; see Section 4.5).
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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.
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 which contains a "Min-Expires" header field field. The
"Min-Expires" header field is described in SIP [RFC3261].
If the notifier is able to immediately determine that it understands
the event package, that the authenticated subscriber is authorized to
subscribe, and that there are no other barriers to creating the
subscription, it creates the subscription and a dialog (if
necessary), and returns a "200 OK" response (unless doing so would
reveal authorization policy in an undesirable fashion; see
Section 6.2).
If the notifier cannot immediately create the subscription (e.g., it
needs to wait for user input for authorization, or is acting for
another node which is not currently reachable), or wishes to mask
authorization policy, it will return a "202 Accepted" response. This
response indicates that the request has been received and understood,
but does not necessarily imply that the subscription has been
authorized yet.
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
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shorten the interval, but MUST NOT lengthen it.
If the duration specified in a SUBSCRIBE message 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 SIP [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 message immediately to communicate the current
resource state to the subscriber. This NOTIFY message 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 message is
processed, this NOTIFY message MAY contain an empty or neutral body.
See Section 4.2.2 for further details on NOTIFY message generation.
Note that a NOTIFY message is always sent immediately after any 200-
class response to a SUBSCRIBE request, regardless of whether the
subscription has already been authorized.
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 SIP [RFC3261]. Note
that, even if the notifier node typically acts as a proxy,
authentication for SUBSCRIBE requests will always be performed via a
"401" response, not a "407;" notifiers always act as a user agents
when accepting subscriptions and sending notifications.
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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 UAs, and as such perform UA
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 "202 Accept" response is returned
immediately. Note that a NOTIFY message 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 message 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 field in the response. If the duration specified in a
SUBSCRIBE message is unacceptably short, the notifier SHOULD respond
with a "423 Interval Too Brief" message.
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 message with a
"Subscription-State" value of "terminated" to inform it that the
subscription is being removed. If such a message 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 with an "Expires" header field set to '0'.
Notifiers largely treat this the same way as any other subscription
expiration: they send a NOTIFY message containing a "Subscription-
State" of "terminated", with a reason code of "timeout." For
consistency with state polling (see Section 4.4.3) and subscription
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refreshes, the notifier may choose to include resource state in this
final NOTIFY. 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 message.
The sending of a NOTIFY when a subscription expires allows the
corresponding dialog 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:
+-------------+
| 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
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notifier SHOULD immediately construct and send a NOTIFY request,
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 SIP [RFC3261]
(instead of the typical single transmission for functioning
clients), continuing to service them when no client is available
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 messages to refresh
potentially stale state information; such messages will re-install
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, 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 SUBSCRIBE refreshes will re-
install 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 which indicates the time
remaining on the subscription. The notifier MAY use this mechanism
to shorten a subscription; however, this mechanism MUST NOT be used
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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 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 an "Expires" header field.
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 with a non-zero expires to be answered with a NOTIFY
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. 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 messages without an "Event" header field.
4.3. Proxy Behavior
Proxies need no additional behavior beyond that described in SIP
[RFC3261] to support SUBSCRIBE and NOTIFY. 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 messages; 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.
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Note that subscribers and notifiers may elect to use 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 SIP [RFC3261].
4.4. Common Behavior
4.4.1. Dialog Creation and Termination
Dialogs are created upon completion of a NOTIFY transaction for a new
subscription, unless the NOTIFY contains a "Subscription-State" of
"terminated."
Because the dialog 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 which
matches the "From" header field "tag" parameter of the SUBSCRIBE, 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." 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
message 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 message, the subscriber SHOULD attempt to
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re-subscribe (as described in the preceding sections). Note that
this subscription is established on a new dialog, and does not re-use
the route set from the previous subscription dialog.
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 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 messages triggered by SUBSCRIBE messages 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.
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Due to the requirement on notifiers to send a NOTIFY 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. If, for example,
the notifier generally needs to retrieve state from another
network server, then that state will be absent from the NOTIFY
that results from polling.
4.4.4. Allow-Events header field usage
The "Allow-Events" header field, if present, includes a list of
tokens which indicates the event packages supported by 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 NOTIFY-established dialog 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.
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 re-use of dialogs has proven to be exceedingly
confusing. [RFC5057] attempted to clarify proper behavior in a
variety of circumstances; however, the ensuing rules remain confusing
and prone to implementation error. At the same time, the mechanism
described in [I-D.ietf-sip-gruu] now provides a far more elegant and
unambiguous means to achieve the same goal.
Consequently, the dialog re-use technique described in RFC 3265 is
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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 re-use, 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 [RFC4528], 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 GRUU extension defined in
[I-D.ietf-sip-gruu], 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.
Because GRUUs are guaranteed to route to a 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 re-use dialogs whose remote target is
a GRUU.
Note that the techniques described in this section are included
for backwards compatibility purposes only. Because subscribers
cannot re-use 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.
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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 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 are expire independently, and
require independent SUBSCRIBE 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 dialog are re-
used, then 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.
o Subscribers MAY include an "id" parameter in SUBSCRIBE request
"Event" header field to allow differentiation between multiple
subscriptions in the same dialog. This "id" parameter, if
present, contains an opaque token which 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 message used to
establish a subscription, that "id" parameter MUST also be present
in all corresponding NOTIFY messages.
o When a subscriber refreshes a the subscription timer, the
SUBSCRIBE MUST contain the same "Event" header field "id"
parameter as was present in the initial subscription. (Otherwise,
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the notifier will interpret the SUBSCRIBE message as a request for
a new subscription in the same dialog).
o When a subscription is created in the notifier, it stores the any
"Event" header field "id" parameter as part of the subscription
information (along with the event package name).
o If an initial SUBSCRIBE 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
Neither SUBSCRIBE nor NOTIFY messages can be canceled. If a 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 which should be taken into
consideration when event packages based on SUBSCRIBE and NOTIFY are
proposed. Event package definitions contain sections addressing each
of these issues, ideally in the same order and with the same titles
as the following sections.
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
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.
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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 which 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 which 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 "Event" and "Allow-Events" would be
"presence.winfo".
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 which 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.
There are two possible solutions to this problem that event packages
may choose to implement.
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5.3.1. Complete State Information
For packages which 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
when 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".
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 messages
contain state deltas instead of complete state.
Such a scheme would work as follows: any NOTIFY sent in immediate
response to a SUBSCRIBE contains full state information. NOTIFY
messages 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 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 message containing the state delta
(except for the version number, which it retains to detect message
loss), and re-sends a SUBSCRIBE to force a NOTIFY 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.
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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" strength 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, whenever applicable.
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
which 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.
5.4.3. SUBSCRIBE Bodies
It is expected that most, but not all, event packages will define
syntax and semantics for SUBSCRIBE method 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 re-use existing MIME types for message bodies
where practical.
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.
5.4.5. NOTIFY Bodies
The NOTIFY body is used to report state on the resource being
monitored. Each package MUST define what type or types of event
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bodies are expected in NOTIFY requests. Such packages MUST specify
or cite detailed specifications for the syntax and semantics
associated with such event body.
Event packages also MUST define which MIME 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. Such
authorization issues may include, for example, whether all SUBSCRIBE
requests for this package are answered with 202 responses (see
Section 6.2).
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 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
deltas 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
messages which correspond to the SUBSCRIBE message (i.e., match "To",
"From", "From" header field "tag" parameter, "Call-ID", "Event", and
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"Event" header field "id" parameter) but which do not match the
dialog would be rejected with a 481 response. Note that the 200-
class response to the SUBSCRIBE can arrive after a matching NOTIFY
has been received; such responses might not correlate to the same
dialog established by the NOTIFY. 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 is allowed, the subscriber establishes a new dialog towards
each notifier by returning a 200-class response to each NOTIFY. Each
dialog is then handled as its own entity, and is refreshed
independent 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
which 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) which 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 which 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.
5.4.12. Examples
Event packages SHOULD include several demonstrative message flow
diagrams paired with several typical, syntactically correct, and
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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 which 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 sensitive for the purposes of privacy. 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 is outside the
scope of this document.
6.2. Notifier Privacy Mechanism
The mere act of returning a 200 or certain 4xx and 6xx 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 202 response. While the
subsequent NOTIFY message 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.
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
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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.
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 SIP [RFC3261].
6.4. Replay Attacks
Replaying of either SUBSCRIBE or NOTIFY can have detrimental effects.
In the case of SUBSCRIBE messages, attackers may be able to install
any arbitrary subscription which it witnessed being installed at some
point in the past. Replaying of NOTIFY messages may be used to spoof
old state information (although a good versioning mechanism in the
body of the NOTIFY messages may help mitigate such an attack). Note
that the prohibition on sending NOTIFY messages to nodes which 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 SIP [RFC3261].
6.5. Man-in-the middle attacks
Even with authentication, man-in-the-middle attacks using SUBSCRIBE
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 of
SUBSCRIBE messages, at a minimum. If SUBSCRIBE 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 messages to
spoof arbitrary state information and/or terminate outstanding
subscriptions. To prevent such attacks, implementations SHOULD
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provide integrity protection across the "Call-ID", "CSeq", and
"Subscription-State" header fields and the bodies of NOTIFY messages.
Integrity protection of message header fields and bodies is discussed
in SIP [RFC3261].
6.6. Confidentiality
The state information contained in a NOTIFY message 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 message 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 messages.
The mechanisms for providing confidentiality are detailed in SIP
[RFC3261].
7. IANA Considerations
(This section is not applicable until this document is published as
an RFC.)
7.1. Event Packages
This document defines an event-type namespace which requires a
central coordinating body. The body chosen for this coordination is
the Internet Assigned Numbers Authority (IANA).
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 MUST NOT share a name with a
package.
Following the policies outlined in "Guidelines for Writing an IANA
Considerations Section in RFCs" [RFC2434], normal event package
identification tokens are allocated as First Come First Served, and
event template-package identification tokens are allocated on a IETF
Consensus basis.
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Registrations with the IANA MUST 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 which describes the
event package. Event template-package token registrations MUST
include a pointer to the published RFC which defines the event
template-package.
Registered tokens to designate packages and template-packages MUST
NOT contain the character ".", which is used to separate template-
packages from packages.
7.1.1. Registration Information
As this document specifies no package or template-package names, the
initial IANA registration for event types will be empty. 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
five possible permutations of package type, contact, and reference.
The table below lists the event packages and template-packages
defined in "SIP-Specific Event Notification" [RFC xxxx]. Each name
is designated as a package or a template-package under "Type".
Package Name Type Contact Reference
------------ ---- ------- ---------
example1 package [Roach]
example2 package [Roach] [RFC xxxx]
example3 package [RFC xxxx]
example4 template [Roach] [RFC xxxx]
example5 template [RFC xxxx]
PEOPLE
------
[Roach] Adam Roach <adam.roach@tekelec.com>
REFERENCES
----------
[RFC xxxx] A.B. Roach, "SIP-Specific Event Notification", RFC XXXX,
Monthname 20XX
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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:
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" [RFC2434], new reason codes require a
Standards Action.
Registrations with the IANA include the reason code being registered
and a reference to a published document which describes the event
package. Insertion of such values takes place as part of the RFC
publication process or as the result of inter-SDO liaison activity.
New reason codes must conform to the syntax of the ABNF "token"
element defined in SIP [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 code will be initialized with the
following values:
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Reason Code Reference
----------- ---------
deactivated [RFC xxxx]
probation [RFC xxxx]
rejected [RFC xxxx]
timeout [RFC xxxx]
giveup [RFC xxxx]
noresource [RFC xxxx]
invariant [RFC xxxx]
badfilter [RFC 4660]
REFERENCES
----------
[RFC xxxx] A.B. Roach, "SIP-Specific Event Notification", RFC XXXX,
Monthname 20XX
[RFC 4660] 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 SIP [RFC3261], and contain
references to elements defined therein.
8.1. New Methods
This document describes two new SIP methods: SUBSCRIBE and NOTIFY.
This table expands on tables 2 and 3 in SIP [RFC3261].
Header Where SUB NOT
------ ----- --- ---
Accept R o o
Accept 2xx - -
Accept 415 o o
Accept-Encoding R o o
Accept-Encoding 2xx - -
Accept-Encoding 415 o o
Accept-Language R o o
Accept-Language 2xx - -
Accept-Language 415 o o
Alert-Info R - -
Alert-Info 180 - -
Allow R o o
Allow 2xx o o
Allow r o o
Allow 405 m m
Authentication-Info 2xx o o
Authorization R o o
Call-ID c m m
Call-Info R o o
Contact R m m
Contact 1xx o o
Contact 2xx m o
Contact 3xx m m
Contact 485 o o
Content-Disposition o o
Content-Encoding o o
Content-Language o o
Content-Length t t
Content-Type * *
CSeq c m m
Date o o
Error-Info 300-699 o o
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Expires o -
Expires 2xx m -
From c m m
In-Reply-To R - -
Max-Forwards R m m
Min-Expires 423 m -
MIME-Version o o
Organization o -
Priority R o -
Proxy-Authenticate 407 m m
Proxy-Authorization R o o
Proxy-Require R o o
RAck R - -
Record-Route R o o
Record-Route 2xx,401,484 o o
Reply-To - -
Require o o
Retry-After 404,413,480,486 o o
Retry-After 500,503 o o
Retry-After 600,603 o o
Route R c c
RSeq 1xx o o
Server r o o
Subject R - -
Supported R o o
Supported 2xx o o
Timestamp o o
To c(1) m m
Unsupported 420 o o
User-Agent o o
Via c m m
Warning R - o
Warning r o o
WWW-Authenticate 401 m m
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.
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The NOTIFY method is used to notify a SIP node that an event which
has been requested by an earlier SUBSCRIBE method has occurred. It
may also provide further details about the event.
8.2. New Header Fields
This table expands on tables 2 and 3 in SIP [RFC3261], as amended by
the changes described in Section 8.1.
Header field where proxy ACK BYE CAN INV OPT REG PRA SUB NOT
-----------------------------------------------------------------
Allow-Events R o o - o o o o o o
Allow-Events 2xx - o - o o o o o o
Allow-Events 489 - - - - - - - m m
Event R - - - - - - - m m
Subscription-State R - - - - - - - - m
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 messages with SUBSCRIBE messages,
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 SIP
[RFC3261].
Note that the forgoing 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.
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
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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.
8.3. New Response Codes
8.3.1. "202 Accepted" Response Code
The 202 response is added to the "Success" header field definition.
"202 Accepted" has the same meaning as that defined in HTTP/1.1
[RFC2616].
8.3.2. "489 Bad Event" Response Code
The 489 event response is added to the "Client-Error" header field
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 definitions for the various new and modified syntax
elements follows. The notation is as used in SIP [RFC3261], and any
elements not defined in this section are as defined in SIP and the
documents to which it refers.
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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.
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.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
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October 1998.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2779] Day, M., Aggarwal, S., Mohr, G., and J. Vincent, "Instant
Messaging / Presence Protocol Requirements", RFC 2779,
February 2000.
[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.
[RFC4483] Burger, E., "A Mechanism for Content Indirection in
Session Initiation Protocol (SIP) Messages", RFC 4483,
May 2006.
[I-D.ietf-sip-gruu]
Rosenberg, J., "Obtaining and Using Globally Routable User
Agent (UA) URIs (GRUU) in the Session Initiation Protocol
(SIP)", draft-ietf-sip-gruu-15 (work in progress),
October 2007.
9.2. Informative References
[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.
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[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.
[RFC4485] Rosenberg, J. and H. Schulzrinne, "Guidelines for Authors
of Extensions to the Session Initiation Protocol (SIP)",
RFC 4485, May 2006.
[RFC4528] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP) Assertion Control", RFC 4528, 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.
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 -00 draft, 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.
Appendix B. Open Issues
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B.1. Bug 711: Allow-Events can't express template support
OPEN ISSUE: There are several things we can do here. I have not
proposed on in particular; I would prefer to solicit initial feedback
from implementors regarding what has been developed and deployed so
far.
The key problem is that support of template event packages can't be
expressed in a complete yet bounded fashion. It would not be
reasonable, for example, to require notifiers that support winfo on
arbitrary packages to include an "Allow-Events" header field with
contents like "presence, presence.winfo, presence.winfo.winfo,
presence.winfo.winfo.winfo, presence.winfo.winfo.winfo.winfo,
presence.winfo.winfo.winfo.winfo.winfo...."
One alternative would be to list event packages and template event
packages, without explicitly indicating which templates can be
applied to which other packages. In such a case, the preceding
example would be collapsed to "Allow-Events: presence, winfo". The
notifier may have local policy that limits how they can be combined
-- but we have plenty of other places where protocol allows
something, but policy forbids it.
B.2. Remove 202 Response Code?
In practice, the 202 response code defined in RFC 3265 has proven to
be nearly useless, due to its redundancy with the "pending" state,
and its interaction with the HERFP problem. Given that 202 must be
treated as 200 if an implementation does not understand it: would
removing the 202 response code cause any issues for current
implementations?
B.3. Timer N and Resubscribes
Section 4.1.2.4 defines a new Timer N that is used upon initial
subscription to bound the amount of time that a subscriber needs to
wait for a NOTIFY. Should this also apply to resubscribes? On one
hand, the mechanism is not as necessary, since the subscriber already
has a negotiated expiration time associated with the subscription.
On the other hand, if no NOTIFY arrives in 64*T1, it is highly likely
that the notifier has gone off the rails, which means that the
subscriber can safely clean up state associated with that
subscription. The key question involved in applying Timer N to
resubscriptions is whether doing so makes subscriptions unnecessarily
brittle.
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Appendix C. Changes
This section, and all of its subsections, will be consolidated into a
single "Changes Since RFC 3265" section prior to publication. Bug
numbers refer to the identifiers for the bug reports kept on file at
http://bugs.sipit.net/.
C.1. Changes from draft-ietf-sipcore-rfc3265bis-00 to
draft-ietf-sipcore-rfc3265bis-01
o Renamed Timer L to Timer N, to avoid a naming conflict.
o Added clarification about proper response when the SUBSCRIBE
indicates an unkonwn MIME type in its Accept header field.
o Clarification around 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 "Call-Info" header field to RFC 3261 Table 2 expansion.
C.2. Changes from draft-roach-sipcore-rfc3265bis-00 to
draft-ietf-sipcore-rfc3265bis-00
None
C.3. Changes since RFC 3265
C.3.1. 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.
C.3.2. 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).
C.3.3. 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.
C.3.4. 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
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and moved to Section 4.5.2. This section, in turn, cites [RFC5057],
which addresses all of the issues in Bug 670.
C.3.5. 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).
C.3.6. Bug 672: Mandate expires= in NOTIFY
Changed strength of including of "expires" in a NOTIFY from SHOULD to
MUST in Section 4.2.2.
C.3.7. Bug 673: INVITE 481 response effect clarification
This bug was addressed in [RFC5057].
C.3.8. 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.
C.3.9. 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".
C.3.10. 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.
C.3.11. Bug 697: Unsubscription behavior could be clarified
Added text to Section 4.2.1.4 encouraging (but not requiring) full
state in final NOTIFY message. Also added text to Section 4.1.2.3
warning subscribers that full state may or may not be present in the
final NOTIFY.
C.3.12. Bug 699: NOTIFY and SUBSCRIBE are target refresh requests
Added text to both Section 3.1 and Section 3.2 explicitly indicating
that SUBSCRIBE and NOTIFY are target refresh methods.
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C.3.13. Bug 722: Inconsistent 423 reason phrase text
Changed reason code to "Interval Too Brief" in Section 4.2.1.1 and
Section 4.2.1.4, to match 423 reason code in SIP [RFC3261].
C.3.14. 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.
C.3.15. 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 5057, and cites it for
additional detail and rationale.
C.3.16. Bug 752: Detection of forked requests is incorrect
Removed erroneous "CSeq" from list of matching criteria in
Section 5.4.9.
C.3.17. Bug 773: Reason code needs IANA registry
Added Section 7.2 to create and populate IANA registry.
C.3.18. Bug 774: Need new reason for terminating subscriptions to
resources that never change
Added new "invariant" reason code to Section 4.1.3, ABNF syntax.
C.3.19. Clarify handling of Route/Record-Route in NOTIFY
Changed text in Section 4.3 mandating Record-Route in initial
SUBSCRIBE and all NOTIFY messages, and adding "MAY" level statements
for subsequent SUBSCRIBE messages.
C.3.20. Eliminate implicit subscriptions
Added text to Section 4.2.1 explaining some of the problems
associated with implicit subscriptions, 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.
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C.3.21. Deprecate dialog re-use
Moved handling of dialog re-use and "id" handling to Section 4.5.2.
It is documented only for backwards-compatibility purposes.
C.3.22. 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).
C.3.23. Refactor behavior sections
Reorganized Section 4 to consolidate behavior along role lines
(subscriber/notifier/proxy) instead of method lines.
C.3.24. 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.
C.3.25. 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].
C.3.26. 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
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
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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.
Author's Address
Adam Roach
Tekelec
17210 Campbell Rd.
Suite 250
Dallas, TX 75252
US
Email: adam@nostrum.com
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