Request for Comments: 3265 dynamicsoft
Updates: 2543 June 2002
Category: Standards Track
Session Initiation Protocol (SIP)-Specific Event Notification
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
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.
Concrete uses of the mechanism described in this document may be
standardized in the future.
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.
Table of Contents
1. Introduction........................................... 3
1.1. Overview of Operation.................................. 4
1.2. Documentation Conventions.............................. 4
2. Definitions............................................ 5
3. Node Behavior.......................................... 6
3.1. Description of SUBSCRIBE Behavior...................... 6
3.1.1. Subscription Duration.................................. 6
3.1.2. Identification of Subscribed Events and Event Classes.. 6
3.1.3. Additional SUBSCRIBE Header Values..................... 7
3.1.4. Subscriber SUBSCRIBE Behavior.......................... 7
3.1.5. Proxy SUBSCRIBE Behavior............................... 9
3.1.6. Notifier SUBSCRIBE Behavior............................ 10
3.2. Description of NOTIFY Behavior......................... 13
3.2.1. Identification of Reported Events, Event Classes, and
Current State.......................................... 13
3.2.2. Notifier NOTIFY Behavior............................... 14
3.2.3. Proxy NOTIFY Behavior.................................. 15
3.2.4. Subscriber NOTIFY Behavior............................. 16
3.3. General................................................ 18
3.3.1. Detecting support for SUBSCRIBE and NOTIFY............. 18
3.3.2. CANCEL requests........................................ 18
3.3.3. Forking................................................ 18
3.3.4. Dialog creation and termination........................ 18
3.3.5. State Agents and Notifier Migration.................... 19
3.3.6. Polling Resource State................................. 20
3.3.7. Allow-Events header usage.............................. 21
3.3.8. PINT Compatibility..................................... 21
4. Event Packages......................................... 21
4.1. Appropriateness of Usage............................... 21
4.2. Event Template-packages................................ 22
4.3. Amount of State to be Conveyed......................... 22
4.3.1. Complete State Information............................. 23
4.3.2. State Deltas........................................... 23
4.4. Event Package Responsibilities......................... 24
4.4.1. Event Package Name..................................... 24
4.4.2. Event Package Parameters............................... 24
4.4.3. SUBSCRIBE Bodies....................................... 24
4.4.4. Subscription Duration.................................. 25
4.4.5. NOTIFY Bodies.......................................... 25
4.4.6. Notifier processing of SUBSCRIBE requests.............. 25
4.4.7. Notifier generation of NOTIFY requests................. 25
4.4.8. Subscriber processing of NOTIFY requests............... 26
4.4.9. Handling of forked requests............................ 26
4.4.10. Rate of notifications.................................. 26
4.4.11. State Agents........................................... 27
4.4.12. Examples............................................... 27
4.4.13. Use of URIs to Retrieve State.......................... 27
5. Security Considerations................................ 28
5.1. Access Control......................................... 28
5.2. Notifier Privacy Mechanism............................. 28
5.3. Denial-of-Service attacks.............................. 28
5.4. Replay Attacks......................................... 29
5.5. Man-in-the middle attacks.............................. 29
5.6. Confidentiality........................................ 29
6. IANA Considerations.................................... 30
6.1. Registration Information............................... 30
6.2. Registration Template.................................. 31
6.3. Header Field Names..................................... 31
6.4. Response Codes......................................... 32
7. Syntax................................................. 32
7.1. New Methods............................................ 32
7.1.1. SUBSCRIBE method....................................... 34
7.1.2. NOTIFY method.......................................... 34
7.2. New Headers............................................ 34
7.2.1. "Event" header......................................... 34
7.2.2. "Allow-Events" Header.................................. 35
7.2.3. "Subscription-State" Header............................ 35
7.3. New Response Codes..................................... 35
7.3.1. "202 Accepted" Response Code........................... 35
7.3.2. "489 Bad Event" Response Code.......................... 35
7.4. Augmented BNF Definitions.............................. 35
8. Normative References................................... 36
9. Informative References................................. 37
10. Acknowledgements....................................... 37
11. Notice Regarding Intellectual Property Rights.......... 37
12. Author's Address....................................... 37
13. Full Copyright Statement............................... 38
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) [2] 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 4.
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------->|
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.
Numbers in square brackets (e.g., [1]) denote a reference to one of
the entries in the reference sections; see sections 8 and 9.
The all-capital terms "MUST", "SHOULD", "MAY", "SHOULD NOT", "MUST
NOT", and "RECOMMENDED" are used as defined in RFC2119 [5].
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.
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.
State Agent: A state agent is a notifier which publishes state
information on behalf of a resource; in order to do so, it may
need to gather such state information from multiple sources.
State agents always have complete state information for the
resource for which they are creating notifications.
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. Node Behavior
3.1. Description of SUBSCRIBE Behavior
The SUBSCRIBE method is used to request current state and state
updates from a remote node.
3.1.1. Subscription Duration
SUBSCRIBE requests SHOULD contain an "Expires" header (defined in SIP
[1]). This expires value indicates the duration of the subscription.
In order to keep subscriptions effective beyond the duration
communicated in the "Expires" header, subscribers need to refresh
subscriptions on a periodic basis using a new SUBSCRIBE message on
the same dialog as defined in SIP [1].
If no "Expires" header 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. The period of time in the response MAY be shorter
but MUST NOT be longer than specified in the request. The period of
time in the response is the one which defines the duration of the
subscription.
An "expires" parameter on the "Contact" header has no semantics for
SUBSCRIBE and is explicitly not equivalent to an "Expires" header 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
3.3.6.
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 3.2.2.
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 [1]. 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@dynamicsoft.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 in SUBSCRIBE
requests, indicating to which event or class of events they are
subscribing. The "Event" header 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. The "Event" header MAY also contain an "id" parameter.
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.
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; if
they do so, they must define the semantics for such parameters.
3.1.3. Additional SUBSCRIBE Header Values
Because SUBSCRIBE requests create a dialog as defined in SIP [1],
they MAY contain an "Accept" header. This header, if present,
indicates the body formats allowed in subsequent NOTIFY requests.
Event packages MUST define the behavior for SUBSCRIBE requests
without "Accept" headers; usually, this will connote a single,
default body type.
Header values not described in this document are to be interpreted as
described in SIP [1].
3.1.4. Subscriber SUBSCRIBE Behavior
3.1.4.1. Requesting a Subscription
SUBSCRIBE is a dialog-creating method, as described in SIP [1].
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 [1] for UAC
request generation outside of a dialog.
This SUBSCRIBE request will be confirmed with a final response.
200-class responses indicate that the subscription has been accepted,
and that a NOTIFY 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 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 [1].
A SUBSCRIBE request MAY include an "id" parameter in its "Event"
header to allow differentiation between multiple subscriptions in the
same dialog.
3.1.4.2. 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, and with the same
"Event" header "id" parameter (if one was present in the initial
subscription). The handling for such a request is the same as for
the initial creation of a subscription except as described below.
If the initial SUBSCRIBE message contained an "id" parameter on
the "Event" header, then refreshes of the subscription must also
contain an identical "id" parameter; they will otherwise be
considered new subscriptions in an existing dialog.
If a SUBSCRIBE request to refresh a subscription receives a "481"
response, this indicates that the subscription has been terminated
and that the subscriber did not receive notification of this fact.
In this case, the subscriber should consider the subscription
invalid. 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
3.1.4.1.)
If a SUBSCRIBE request to refresh a subscription fails with a non-481
response, 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 "expires" parameter.
Note that many such errors indicate that there may be a problem
with the network or the notifier such that no further NOTIFY
messages will be received.
3.1.4.3. Unsubscribing
Unsubscribing is handled in the same way as refreshing of a
subscription, with the "Expires" header set to "0". Note that a
successful unsubscription will also trigger a final NOTIFY message.
3.1.4.4. 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. 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 4.4.7.).
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 3.2.4.
3.1.5. Proxy SUBSCRIBE Behavior
Proxies need no additional behavior beyond that described in SIP [1]
to support SUBSCRIBE. If a proxy wishes to see all of the SUBSCRIBE
and NOTIFY requests for a given dialog, it MUST record-route the
initial SUBSCRIBE and any dialog-establishing NOTIFY requests. Such
proxies SHOULD also record-route all other SUBSCRIBE and NOTIFY
requests.
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 [1].
3.1.6. Notifier SUBSCRIBE Behavior
3.1.6.1. 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 [1]) 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 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 3.1.6.3.
The notifier MAY also check that the duration in the "Expires" header
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
small" error which contains a "Min-Expires" header field. The "Min-
Expires" header field is described in SIP [1].
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
5.2.).
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 and the "Event" header "id" parameter (if present) as
part of the subscription information.
The "Expires" values present in SUBSCRIBE 200-class responses behave
in the same way as they do in REGISTER responses: the server MAY
shorten the interval, but MUST NOT lengthen it.
If the duration specified in a SUBSCRIBE 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 [1] may be used in response
to SUBSCRIBE requests, as appropriate.
3.1.6.2. 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 3.2.2. 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.
3.1.6.3. 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 [1]. 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.
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 5.2.
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
with a value of "terminated" and a reason parameter of "rejected".
3.1.6.4. Refreshing of Subscriptions
When a notifier receives a subscription refresh, assuming that the
subscriber is still authorized, the notifier updates the expiration
time for the 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 in the response. If the duration specified in a SUBSCRIBE
message is unacceptably short, the notifier SHOULD respond with a
"423 Subscription 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 SHOULD contain a "reason=timeout"
parameter.
The sending of a NOTIFY when a subscription expires allows the
corresponding dialog to be terminated, if appropriate.
3.2. Description of NOTIFY Behavior
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.
If any non-SUBSCRIBE mechanisms are defined to create subscriptions,
it is the responsibility of the parties defining those mechanisms to
ensure that correlation of a NOTIFY message to the corresponding
subscription is possible. Designers of such mechanisms are also
warned to make a distinction between sending a NOTIFY message to a
subscriber who is aware of the subscription, and sending a NOTIFY
message to an unsuspecting node. The latter behavior is invalid, and
MUST receive a "481 Subscription does not exist" response (unless
some other 400- or 500-class error code is more applicable), as
described in section 3.2.4. In other words, knowledge of a
subscription must exist in both the subscriber and the notifier to be
valid, even if installed via a non-SUBSCRIBE mechanism.
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" headers will contain a
single event package name for which a notification is being
generated. The package name in the "Event" header MUST match the
"Event" header in the corresponding SUBSCRIBE message. If an "id"
parameter was present in the SUBSCRIBE message, that "id" parameter
MUST also be present in the corresponding NOTIFY messages.
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 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 4.4.13).
3.2.2. Notifier NOTIFY Behavior
When a SUBSCRIBE request is answered with a 200-class response, the
notifier MUST immediately construct and send a NOTIFY request to the
subscriber. When a change in the subscribed state occurs, the
notifier SHOULD immediately construct and send a NOTIFY request,
subject to authorization, local policy, and throttling
considerations.
A NOTIFY request is considered failed if the response times out, or a
non-200 class response code is received which has no "Retry-After"
header and no implied further action which can be taken to retry the
request (e.g., "401 Authorization Required".)
If the NOTIFY request fails (as defined above) due to a timeout
condition, and the subscription was installed using a soft-state
mechanism (such as SUBSCRIBE), 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 [1]
(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 request fails (as defined above) due to an error
response, and the subscription was installed using a soft-state
mechanism, the notifier MUST remove the corresponding subscription.
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.
If a NOTIFY request receives a 481 response, the notifier MUST remove
the corresponding subscription even if such subscription was
installed by non-SUBSCRIBE means (such as an administrative
interface).
If the above behavior were not required, subscribers receiving a
notify for an unknown subscription would need to send an error
status code in response to the NOTIFY and also send a SUBSCRIBE
request to remove the subscription. Since this behavior would
make subscribers available for use as amplifiers in denial of
service attacks, we have instead elected to give the 481 response
special meaning: it is used to indicate that a subscription must
be cancelled under all circumstances.
NOTIFY requests MUST contain a "Subscription-State" header 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 5.2.). 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 is "active" or
"pending", the notifier SHOULD also include in the "Subscription-
State" header 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
to lengthen a subscription.
Including expiration information for active and pending
subscriptions is useful in case the SUBSCRIBE request forks, since
the response to a forked SUBSCRIBE may not be received by the
subscriber. Note well that this "expires" value is a parameter on
the "Subscription-State" header, NOT an "Expires" header.
If the value of the "Subscription-State" header 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 3.2.4.
3.2.3. Proxy NOTIFY Behavior
Proxies need no additional behavior beyond that described in SIP [1]
to support NOTIFY. If a proxy wishes to see all of the SUBSCRIBE and
NOTIFY requests for a given dialog, it MUST record-route the initial
SUBSCRIBE and any dialog-establishing NOTIFY requests. Such proxies
SHOULD also record-route all other SUBSCRIBE and NOTIFY requests.
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 [1].
3.2.4. Subscriber NOTIFY Behavior
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 3.3.4. Notifications for
subscriptions which were created inside an existing dialog match if
they are in the same dialog and the "Event" headers match (as
described in section 7.2.1.)
If, for some reason, the event package designated in the "Event"
header 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" headers which
indicate the status of the subscription.
If the "Subscription-State" header value is "active", it means that
the subscription has been accepted and (in general) has been
authorized. If the header 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
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". 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 defined reason codes are:
deactivated: The subscription has been terminated, but the subscriber
SHOULD retry immediately with a new subscription. One primary use
of such a status code is to allow migration of subscriptions
between nodes. The "retry-after" parameter has no semantics for
"deactivated".
probation: The subscription has been terminated, but the client
SHOULD retry at some later time. 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".
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".
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.
Other responses defined in SIP [1] 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.
3.3. General
3.3.1. Detecting support for SUBSCRIBE and NOTIFY
Neither SUBSCRIBE nor NOTIFY necessitate the use of "Require" or
"Proxy-Require" headers; similarly, there is no token defined for
"Supported" headers. If necessary, clients may probe for the support
of SUBSCRIBE and NOTIFY using the OPTIONS request defined in SIP [1].
The presence of the "Allow-Events" header in a message is sufficient
to indicate support for SUBSCRIBE and NOTIFY.
The "methods" parameter for Contact may also be used to
specifically announce support for SUBSCRIBE and NOTIFY messages
when registering. (See reference [8] for details on the "methods"
parameter).
3.3.2. CANCEL requests
No semantics are associated with cancelling SUBSCRIBE or NOTIFY.
3.3.3. Forking
In accordance with the rules for proxying non-INVITE requests as
defined in SIP [1], 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. For
information on subscriber handling in such situations, see section
4.4.9.
3.3.4. Dialog creation and termination
If an initial SUBSCRIBE request is not sent on a pre-existing dialog,
the subscriber will wait for a response to the SUBSCRIBE request or a
matching NOTIFY.
Responses are matched to such SUBSCRIBE requests if they contain the
same the same "Call-ID", the same "From" header "tag", and the same
"CSeq". Rules for the comparison of these headers are described in
SIP [1]. If a 200-class response matches such a SUBSCRIBE request,
it creates a new subscription and a new dialog (unless they have
already been created by a matching NOTIFY request; see below).
NOTIFY requests are matched to such SUBSCRIBE requests if they
contain the same "Call-ID", a "To" header "tag" parameter which
matches the "From" header "tag" parameter of the SUBSCRIBE, and the
same "Event" header field. Rules for comparisons of the "Event"
headers are described in section 7.2.1. If a matching NOTIFY request
contains a "Subscription-State" of "active" or "pending", it creates
a new subscription and a new dialog (unless they have already been
created by a matching response, as described above).
If an initial SUBSCRIBE is sent on a pre-existing dialog, a matching
200-class response or successful NOTIFY request merely creates a new
subscription associated with that dialog.
Multiple subscriptions can be associated with a single dialog.
Subscriptions may also exist in dialogs associated with INVITE-
created application state and other application state created by
mechanisms defined in other specifications. These sets of
application state do not interact beyond the behavior described for a
dialog (e.g., route set handling).
A subscription is destroyed when a notifier sends a NOTIFY request
with a "Subscription-State" of "terminated".
A subscriber may send a SUBSCRIBE request with an "Expires" header
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 with a
"Subscription-State" of "terminated" is sent.
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.
Note that the above behavior means that 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.
3.3.5. State Agents and Notifier Migration
When state agents (see section 4.4.11.) are used, it is often useful
to allow migration of subscriptions between state agents and the
nodes for which they are providing state aggregation (or even among
various state agents). Such migration may be effected by sending a
NOTIFY message with a "Subscription-State" header of "terminated",
and a reason parameter of "deactivated". This NOTIFY request is
otherwise normal, and is formed as described in section 3.2.2.
Upon receipt of this NOTIFY message, the subscriber SHOULD attempt to
re-subscribe (as described in the preceding sections). Note that
this subscription is established on a new dialog, and does not 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.
3.3.6. 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" headers 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.
3.3.7. Allow-Events header usage
The "Allow-Events" header, if present, includes a list of tokens
which indicates the event packages supported by the client (if sent
in a request) or server (if sent in a response). In other words, a
node sending an "Allow-Events" header is advertising that it can
process SUBSCRIBE requests and generate NOTIFY requests for all of
the event packages listed in that header.
Any node implementing one or more event packages SHOULD include an
appropriate "Allow-Events" header 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.
Note that "Allow-Events" headers MUST NOT be inserted by proxies.
3.3.8. PINT Compatibility
The "Event" header is considered mandatory for the purposes of this
document. However, to maintain compatibility with PINT (see [2]),
servers MAY interpret a SUBSCRIBE request with no "Event" header as
requesting a subscription to PINT events. If a server does not
support PINT, it SHOULD return "489 Bad Event" to any SUBSCRIBE
messages without an "Event" header.
4. Event Packages
This section covers several issues which should be taken into
consideration when event packages based on SUBSCRIBE and NOTIFY are
proposed.
4.1. 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" [7] for further guidance regarding
appropriate uses of SIP.
Further, it is expected that this mechanism is not to be used in
applications where the frequency of reportable events is excessively
rapid (e.g., more than about once per second). A SIP network is