Internet Engineering Task Force Adam Roach
Internet Draft dynamicsoft
Category: Standards Track February 2002
Expires August 2002
<draft-ietf-sip-events-02.txt>
SIP-Specific Event Notification
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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This document is an individual submission to the IETF. Comments
should be directed to the authors.
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.
1. Table of Contents
1. Table of Contents...................................... 1
2. Introduction........................................... 3
2.1. Overview of Operation.................................. 4
3. Definitions............................................ 4
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4. Node Behavior.......................................... 5
4.1. Description of SUBSCRIBE Behavior...................... 5
4.1.1. Subscription duration.................................. 5
4.1.2. Identification of Subscribed Events and Event Classes.. 6
4.1.3. Additional SUBSCRIBE Header Values..................... 6
4.1.4. Subscriber SUBSCRIBE Behavior.......................... 7
4.1.5. Proxy SUBSCRIBE Behavior............................... 8
4.1.6. Notifier SUBSCRIBE Behavior............................ 9
4.2. Description of NOTIFY Behavior......................... 11
4.2.1. Identification of reported events, event classes, and c 12
4.2.2. Notifier NOTIFY Behavior............................... 12
4.2.3. Proxy NOTIFY Behavior.................................. 14
4.2.4. Subscriber NOTIFY Behavior............................. 14
4.3. General................................................ 16
4.3.1. Detecting support for SUBSCRIBE and NOTIFY............. 16
4.3.2. CANCEL requests........................................ 16
4.3.3. Forking................................................ 16
4.3.4. Dialog creation and termination........................ 17
4.3.5. State Agents and Notifier Migration.................... 18
4.3.6. Polling Resource State................................. 18
4.3.7. Allow-Events header usage.............................. 19
4.3.8. PINT Compatibility..................................... 19
5. Event Packages......................................... 19
5.1. Appropriateness of Usage............................... 19
5.2. Event Template-packages................................ 20
5.3. Amount of State to be Conveyed......................... 20
5.3.1. Complete State Information............................. 21
5.3.2. State Deltas........................................... 21
5.4. Event Package Responsibilities......................... 22
5.4.1. Event Package Name..................................... 22
5.4.2. Event Package Parameters............................... 22
5.4.3. SUBSCRIBE Bodies....................................... 22
5.4.4. Subscription Duration.................................. 22
5.4.5. NOTIFY Bodies.......................................... 23
5.4.6. Notifier processing of SUBSCRIBE requests.............. 23
5.4.7. Notifier generation of NOTIFY requests................. 23
5.4.8. Subscriber processing of NOTIFY requests............... 23
5.4.9. Handling of forked requests............................ 23
5.4.10. Rate of notifications.................................. 24
5.4.11. State Agents........................................... 24
5.4.12. Examples............................................... 25
5.4.13. URI List handling...................................... 25
6. Security Considerations................................ 25
6.1. Access Control......................................... 25
6.2. Release of Sensitive Policy Information................ 25
6.3. Denial-of-Service attacks.............................. 26
6.4. Replay Attacks......................................... 26
6.5. Man-in-the middle attacks.............................. 26
6.6. Confidentiality........................................ 27
7. IANA Considerations.................................... 27
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7.1. Registration Information............................... 28
7.2. Registration Template.................................. 28
7.3. Syntax................................................. 29
7.4. New Methods............................................ 29
7.4.1. SUBSCRIBE method....................................... 31
7.4.2. NOTIFY method.......................................... 31
7.5. New Headers............................................ 31
7.5.1. "Event" header......................................... 31
7.5.2. "Allow-Events" Header.................................. 32
7.5.3. "Subscription-State" Header............................ 32
7.6. New Response Codes..................................... 33
7.6.1. "202 Accepted" Response Code........................... 33
7.6.2. "489 Bad Event" Response Code.......................... 33
8. Changes................................................ 33
8.1. Changes from draft-ietf-...-01......................... 33
8.2. Changes from draft-ietf-...-00......................... 35
8.3. Changes from draft-roach-...-03........................ 36
8.4. Changes from draft-roach-...-02........................ 38
8.5. Changes from draft-roach-...-01........................ 39
9. References............................................. 40
10. Acknowledgements....................................... 41
11. Author's Address....................................... 41
2. Introduction
The ability to request asynchronous notification of events proves
useful in many types of 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 PINT
status (based on call state events).
The methods described in this document allow 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
complex rules which govern the subscription and notification for
the events or classes of events they describe.
This draft does not describe an extension which may be used
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directly; it must be extended by other drafts (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.
2.1. 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.
3. 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 state 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
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may need gather such state information from multiple sources.
State Agents always have complete state information for the
resource for which it is 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.
4. Node Behavior
4.1. Description of SUBSCRIBE Behavior
The SUBSCRIBE method is used to request current state and state
updates from a remote node.
4.1.1. 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 or 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.
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A natural consequence of this scheme is that a SUBSCRIBE with an
"Expires" of 0 constitutes a request to unsubscribe from an
event.
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.
4.1.2. 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. 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.
4.1.3. 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.
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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] .
4.1.4. Subscriber SUBSCRIBE Behavior
4.1.4.1. 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.
4.1.4.2. 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
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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 4.1.4.1. )
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.
4.1.4.3. 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".
4.1.4.4. 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. Event packages 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 4.2.4.
4.1.5. 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
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SUBSCRIBE and NOTIFY requests for a given dialog, it MUST
record-route all SUBSCRIBE and NOTIFY requests.
4.1.6. Notifier SUBSCRIBE Behavior
4.1.6.1. 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 timer F 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 4.1.6.3.
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 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 SHOULD respond with a "423 Subscription Too Brief"
message.
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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.
4.1.6.2. Confirmation of Subscription Creation/Refreshing
Upon successfully accepting or refreshing of 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.
4.1.6.3. 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.
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
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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."
4.1.6.4. Refreshing of Subscriptions
When a notifier receives a subscription refresh, assuming that
the subscriber is still authorized, the notifier updates the
expiration time for 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.
4.2. 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
4.2.4. In other words, knowledge of a subscription must exist in
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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.
4.2.1. 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
4.1.2. ).
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.
If the event package to which the event package name corresponds
defines behavior associated with the body of its NOTIFY requests,
those semantics apply. This information is 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.
4.2.2. 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
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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 11
times (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 6.2. ). The "pending"
value indicates that the subscription has been received, but that
policy information is insufficient to accept or deny the
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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 4.2.4.
4.2.3. 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 all SUBSCRIBE and NOTIFY requests.
4.2.4. 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 is described in section 4.3.4. 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 7.5.1. )
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
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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 client
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."
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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.
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.
4.3. General
4.3.1. 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 [7] for details on the "methods"
parameter).
4.3.2. CANCEL requests
No semantics are associated with cancelling SUBSCRIBE or NOTIFY.
4.3.3. Forking
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 response to a single
SUBSCRIBE message, they represent different destinations to which
the SUBSCRIBE request was forked. For information on subscriber
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handling in such situations, see section 5.4.9.
4.3.4. Dialog creation and termination
If an initial SUBSCRIBE request is not sent on an 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 field, the
same "To" header field, excluding the "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 "From" header field which matches
the "To" header field of the SUBSCRIBE, excluding the "tag," a
"To" header field which matches the "From" header field of the
SUBSCRIBE, and the same "Event" header field. Rules for
comparisons of the "Event" headers are described in section
7.5.1. If a matching NOTIFY request contains a
"Subscription-State" of "active" or "pending," it creates a new
subscription and a new dialog (unless the 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
"Expiration" 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
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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.
4.3.5. State Agents and Notifier Migration
When state agents (see section 5.4.11. ) 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" with an "Subscription-State" header 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 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.
4.3.6. 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
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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 "Expire" headers of 0 will contain a "Subscription-State"
value of "terminated," and a "reason" parameter of "timeout."
4.3.7. 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.
4.3.8. PINT Compatibility
The "Event" header is considered mandatory for the purposes of
this document. However, to maintain compatibility with PINT (see
[3] ), 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.
5. Event Packages
This section covers several issues which should be taken into
consideration when event packages based on SUBSCRIBE and NOTIFY
are proposed.
5.1. Appropriateness of Usage
When designing an event package using the methods described in
this draft 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
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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 read "Guidelines for Authors of SIP
Extensions" [2] for further guidance regarding appropriate uses
of SIP.
Further, it is expected that this mechanism is not to be used in
applications where the frequency of reportable events is
excessively rapid (e.g. more than about once per second). A SIP
network is generally going to be provisioned for a reasonable
signalling volume; sending a notification every time a user's GPS
position changes by one hundreth 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
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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.
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 the circumstances that state may not be sufficient for a
particular class of events, the event packages should include
complete state information along with the event that occurred.
For example, "no customer service representatives available" may
not be as useful "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 use
a payload which contains a version number that increases by
exactly one for each NOTIFY message. Note that the state version
number appears in the body of the message, not in a SIP header.
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.
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5.4. Event Package Responsibilities
Event packages are not required to re-iterate any of the behavior
described in this document, although they may choose to do so for
clarity or emphasis. In general, though, such packages are
expected to describe only the behavior that extends or modifies
the behavior described in this document.
Note that any behavior designated with "SHOULD" or "MUST" in this
document is not allowed to be changed 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 by "Instructions to
RFC Authors" [5] and "Guidelines for Authors of SIP Extensions"
[2] , authors of event packages MUST address each of the issues
detailed in the following subsections, whenever applicable.
5.4.1. Event Package Name
This mandatory section of an event package 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 to modify the
behavior of the event package, the syntax and semantics of such
headers 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
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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 a what type or types of event
bodies are expected in NOTIFY requests. Such packages MUST
specify or cite detailed specifications for the syntax and
semantics associated with such event body.
Event packages also MUST define which MIME type is to be assumed
if none are specified in the "Accept" header 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. )
It may optionally describe the behavior used to processes the
subsequent response. Such a section is required.
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
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Each event package SHOULD 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 tag, Call-ID, CSeq, Event, and Event id) but
which do not match the dialog would be rejected with a 481
response.
If installing of multiple subscriptions by way of a single forked
INVITE 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.
If the event package does not specify which mode of operation to
use, the subscriber may employ either mode of operation.
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.
Such packages MAY further define a throttle mechanism which
allows subscribers to further limit the rate of notification.
5.4.11. State Agents
Designers of event packages should consider whether their package
can benefit from network aggregation points ("State Agents")
and/or nodes which act on behalf of other nodes. (For example,
nodes which provide state information about a resource when such
a resource is unable or unwilling to provide such state
information itself). An example of such an application is a node
which tracks the presence and availability of a user in the
network.
If state agents are to be used by the package, the package MUST
specify how such state agents aggregate information and how they
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provide authentication and authorization.
Event packages MAY also outline specific scenarios under which
notifier migrations take place.
5.4.12. Examples
Event packages SHOULD include several demonstrative message flow
diagrams paired with several typical, syntactically correct and
complete messages.
It is RECOMMENDED that documents describing event packages
clearly indicate that such examples are informative and not
normative, with instructions that implementors refer to the main
text of the draft for exact protocol details.
5.4.13. URI List handling
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
use a MIME body type of "application/uri-list" in NOTIFY
messages. The URI or URIs contained in the NOTIFY body will then
be used to retrieve the actual state information.
If an event package elects to use this mechanism, it MUST define
at least one baseline scheme (e.g. http) which is mandatory to
support, as well as one mandatory baseline data format for the
data so retrieved. Event packages using URIs to retrieve state
information also MUST address the duration of the validity of the
URIs passed to a subscriber in this fashion.
6. Security Considerations
6.1. Access Control
The ability to accept subscriptions should be under the direct
control of the 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 draft.
6.2. Release of Sensitive Policy Information
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
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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.
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 [1] .
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).
To prevent such attacks, implementations SHOULD require
authentication. Authentication issues are discussed in SIP [1] .
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" headers 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.
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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
provide integrity protection across the "Call-ID," "CSeq," and
"Subscription-State" headers and the bodies of NOTIFY messages.
Integrity protection of message headers and bodies is discussed
in SIP [1] .
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
[1] .
7. IANA Considerations
(This section is not applicable until this document is published
as an RFC.)
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" [6] , 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.
Registrations with the IANA MUST include the token being
registered and whether the token is a package or a
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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. 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@dynamicsoft.com>
REFERENCES
----------
[RFC xxxx] A. Roach "SIP-Specific Event Notification", RFC XXXX,
August 2002.
7.2. Registration Template
To: ietf-sip-events@iana.org
Subject: Registration of new SIP event package
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Package Name:
(Package names must conform to the syntax described in
section 7.5.1. )
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:
7.3. 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 defined by [1] , and contain
references to elements defined therein.
7.4. New Methods
This document describes two new SIP methods: "SUBSCRIBE" and
"NOTIFY."
This table expands on tables 2 and 3 in SIP [1] .
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
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Allow r o o
Allow 405 m m
Authentication-Info 2xx o o
Authorization R o o
Call-ID c m m
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
Expires o -
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 o
WWW-Authenticate 401 m m
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Internet Draft SIP-Specific Event Notification February 2002
7.4.1. 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.
7.4.2. NOTIFY method
"NOTIFY" is added to the definition of the element "Method" in
the SIP message grammar.
The NOTIFY method is used to notify a SIP node that an event
which has been requested by an earlier SUBSCRIBE method has
occurred. It may also provide further details about the event.
7.5. New Headers
This table expands on tables 2 and 3 in SIP [1] , as amended by
the changes described in section 7.4.
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
7.5.1. "Event" header
The following header is defined for the purposes of this
specification.
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 | "-" | "!" | "%" | "*"
| "_" | "+" | "`" | "'" | "~" )
event-param = generic-param | ( "id" EQUAL token )
Event is added to the definition of the element "message-header"
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Internet Draft SIP-Specific Event Notification February 2002
in the SIP message grammar.
For the purposes of matching responses and NOTIFY messages with
SUBSCRIBE messages, the event-type portion of the "Event" header
is compared byte-by-byte, and the "id" parameter token (if
present) is compared byte-by-byte. An "Event" header containing
an "id" parameter never matches an "Event" header without an "id"
parameter. No other parameters are considered when performing a
comparison.
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.
Multiple events per message are disallowed.
For the curious, the "o" short form is chosen to represent
"occurrence."
7.5.2. "Allow-Events" Header
The following header is defined for the purposes of this
specification.
Allow-Events = ( "Allow-Events" | "u" ) HCOLON
1*event-type
Allow-Events is added to the definition of the element
"general-header" in the SIP message grammar.
For the curious, the "u" short form is chosen to represent
"understands."
7.5.3. "Subscription-State" Header
The following header is defined for the purposes of this
specification.
Subscription-State = "Subscription-State" HCOLON
substate-value )
*( SEMI subexp-params )
substate-value = "active" | "pending" | "terminated"
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Internet Draft SIP-Specific Event Notification February 2002
subexp-params = ("reason" EQUAL reason-code)
|("expires" EQUAL delta-seconds)
|("retry-after" EQUAL delta-seconds)
| generic-param
reason-code = "deactivated"
| "probation"
| "rejected"
| "timeout"
| "giveup"
| reason-extension
reason-extension = token
Subscription-State is added to the definition of the element
"request-header" in the SIP message grammar.
7.6. New Response Codes
7.6.1. "202 Accepted" Response Code
The 202 response is added to the "Success" header field
definition:
Success = "200" ; OK
| "202" ; Accepted
"202 Accepted" has the same meaning as that defined in HTTP/1.1
[4] .
7.6.2. "489 Bad Event" Response Code
The 489 event response is added to the "Client-Error" header
field definition:
Client-Error = "400" ; Bad Request
...
| "489" ; Bad Event
"489 Bad Event" is used to indicate that the server did not
understand the event package specified in a "Event" header field.
8. Changes
8.1. Changes from draft-ietf-...-01
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Internet Draft SIP-Specific Event Notification February 2002
- Changed dependancy from RFC2543 to new sip bis draft.
This allowed removal of certain sections of text.
- Renamed "sub-packages" to "template-packages" in an
attempt to mitigate exploding rampant misinterpretation.
- Changed "Subscription-Expires" to "Subscription-State,"
and added clearer semantics for "reason" codes.
- Aligned "Subscription-State" "reason" codes with
watcherinfo draft.
- Made "Subscription-State" mandatory in NOTIFY
requests, since it is integral to defining the
creation and destruction of subscriptions (and,
consequently, dialogs)
- Heavily revised section on dialog creation and
termination.
- Expanded migration section.
- Added "id" parameter to Event header, to allow
demultiplexing of NOTIFY requests when more than
one subscription is associated with a single dialog.
- Syncronized SUBSCRIBE "Expires" handling with REGISTER
(again)
- Added definitions section.
- Restructuring for clarity.
- Added statement explicitly allowing event
packages to define additional parameters
for the "Event" header.
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Internet Draft SIP-Specific Event Notification February 2002
- Added motivational text in several places.
- Synced up header table modifications with bis draft.
8.2. Changes from draft-ietf-...-00
- Fixed confusing typo in section describing correlation
of SUBSCRIBE requests
- Added explanitory text to clarify tag handling when
generating re-subscriptions
- Expanded general handling section to include specific
discussion of Route/Record-Route handling.
- Included use of "methods" parameter on Contact as
a means for detecting support for SUBSCRIBE and NOTIFY.
- Added definition of term "dialog"; changed "leg" to
"dialog" everwhere.
- Added syntax for "Subscription-Expires" header.
- Changed NOTIFY messages to refer to "Subscription-Expires"
everywhere (instead of "Expires.")
- Added information about generation and handling of
481 responses to SUBSCRIBE requests
- Changed having Expires header in SUBSCRIBE from
MUST to SHOULD; this aligns more closely with
REGISTER behavior
- Removed experimental/private event package names,
per list consensus
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Internet Draft SIP-Specific Event Notification February 2002
- Cleaned up some legacy text left over from very early
drafts that allowed multiple contacts per subscription
- Strengthened language requiring the removal of subscriptions
if a NOTIFY request fails with a 481. Clarified that such
removal is required for all subscriptions, including
administrative ones.
- Removed description of delaying NOTIFY requests until
authorization is granted. Such behavior was inconsistent
with other parts of this document.
- Moved description of event packages to later in document,
to reduce the number of forward references.
- Minor editorial and nits changes
- Added new open issues to open issues section. All
previous open issues have been resolved.
8.3. Changes from draft-roach-...-03
- Added DOS attacks section to open issues.
- Added discussion of forking to open issues
- Changed response to PINT request for notifiers who don't
support PINT from 400 to 489.
- Added sentence to security section to call attention to
potential privacy issues of delayed NOTIFY responses.
- Added clarification: access control list handling is out
of scope.
- (Hopefully) Final resolution on out-of-band subscriptions:
mentioned in section
4.2.
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Internet Draft SIP-Specific Event Notification February 2002
Removed from open issues.
- Made "Contact" header optional for SUBSCRIBE 1xx responses.
- Added description clarifying tag handling (section
4.3.4.
)
- Removed event throttling from open issues.
- Editorial cleanup to remove term "extension draft" and
similar; "event package" is now (hopefully) used consistently
throughout the document.
- Remove discussion of event agents from open issues.
This is covered in the event packages section now.
- Added discussion of forking to open issues.
- Added discussion of sub-packages
- Added clarification that, upon receiving a "NOTIFY"
with an expires of "0", the subscriber can re-subscribe.
This allows trivial migration of subscriptions between
nodes.
- Added preliminary IANA Considerations section
- Changed syntax for experimental event tokens to avoid
possibly ambiguity between experimental tokens and
sub-packages.
- Slight adjustment to "Event" syntax to accommodate sub-packages.
- Added section describing the information which is to be
included in documents describing event packages.
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Internet Draft SIP-Specific Event Notification February 2002
- Made 481 responses mandatory for unexpected notifications
(allowing notifiers to remove subscriptions in error cases)
- Several minor non-semantic editorial changes.
8.4. Changes from draft-roach-...-02
- Clarification under "Notifier SUBSCRIBE behavior" which
indicates that the first NOTIFY message (sent immediately
in response to a SUBSCRIBE) may contain an empty body, if
resource state doesn't make sense at that point in time.
- Text on message flow in overview section corrected
- Removed suggestion that clients attempt to unsubscribe
whenever they receive a NOTIFY for an unknown event.
Such behavior opens up DOS attacks, and will lead to
message loops unless additional precautions are taken.
The 481 response to the NOTIFY should serve the same
purpose.
- Changed processing of non-200 responses to NOTIFY from
"SHOULD remove contact" to "MUST remove contact" to support
the above change.
- Re-added discussion of out-of-band subscription mechanisms
(including open issue of resource identification).
- Added text specifying that SUBSCRIBE transactions are not
to be prolonged. This is based on the consensus that non-INVITE
transactions should never be prolonged; such consensus within
the SIP working group was reached at the 49th IETF.
- Added "202 Accepted" response code to support the above
change. The behavior of this 202 response code is a
generalization of that described in the presence draft.
- Updated to specify that the response to an unauthorized
SUBSCRIBE request is 603 or 403.
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Internet Draft SIP-Specific Event Notification February 2002
- Level-4 subheadings added to particularly long sections to
break them up into logical units. This helps make the
behavior description seem somewhat less rambling. This also
caused some re-ordering of these paragraphs (hopefully in a
way that makes them more readable).
- Some final mopping up of old text describing "call related"
and "third party" subscriptions (deprecated concepts).
- Duplicate explanation of subscription duration removed from
subscriber SUBSCRIBE behavior section.
- Other text generally applicable to SUBSCRIBE (instead of just
subscriber handling of SUBSCRIBE) moved to parent section.
- Updated header table to reflect mandatory usage of "Expires"
header in SUBSCRIBE requests and responses
- Removed "Event" header usage in responses
- Added sentence suggesting that notifiers may notify
subscribers when a subscription has timed out.
- Clarified that a failed attempt to refresh a subscription
does not imply that the original subscription has been
cancelled.
- Clarified that 489 is a valid response to "NOTIFY" requests.
- Minor editorial changes to clean up awkward and/or unclear
grammar in several places
8.5. Changes from draft-roach-...-01
- Multiple contacts per SUBSCRIBE message disallowed.
- Contact header now required in NOTIFY messages.
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Internet Draft SIP-Specific Event Notification February 2002
- Distinction between third party/call member events removed.
- Distinction between call-related/resource-related events removed.
- Clarified that subscribers must expect NOTIFY messages before
the SUBSCRIBE transaction completes
- Added immediate NOTIFY message after successful SUBSCRIBE;
this solves a myriad of issues, most having to do with forking.
- Added discussion of "undefined state" (before a NOTIFY arrives).
- Added mechanism for notifiers to shorten/cancel outstanding
subscriptions.
- Removed open issue about appropriateness of new "489" response.
- Removed all discussion of out-of-band subscriptions.
- Added brief discussion of event state polling.
9. References
[1] J. Rosenberg et. al., "SIP: Session Initiation Protocol",
<draft-ietf-sip-rfc2543bis-07>, IETF; February 2002. Work in
progress.
[2] J. Rosenberg, H. Schulzrinne, "Guidelines for Authors of SIP
Extensions", <draft-ietf-sip-guidelines-03.txt>, IETF;
November 2001. Work in progress.
[3] S. Petrack, L. Conroy, "The PINT Service Protocol", RFC 2848,
IETF; June 2000.
[4] R. Fielding et. al., "Hypertext Transfer Protocol --
HTTP/1.1", RFC2068, IETF, January 1997.
[5] J. Postel, J. Reynolds, "Instructions to RFC Authors",
RFC2223, IETF, October 1997.
[6] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
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Internet Draft SIP-Specific Event Notification February 2002
Considerations Section in RFCs", BCP 26, IETF, October 1998.
[7] Schulzrinne/Rosenberg, "SIP Caller Preferences and Callee
Capabilities", <draft-ietf-sip-callerprefs-05.txt>, IETF;
November 2001. Work in progress.
10. 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" draft for their input to SUBSCRIBE and
NOTIFY request semantics.
11. Author's Address
Adam Roach
dynamicsoft
5100 Tennyson Parkway
Suite 1200
Plano, TX 75024
USA
E-Mail: <adam@dynamicsoft.com>
Voice: <sip:adam@dynamicsoft.com>
Roach [Page 41]
