Internet Engineering Task Force (IETF) A. Niemi
Request for Comments: 5839 Nokia
Category: Standards Track D. Willis, Ed.
ISSN: 2070-1721 Softarmor Systems
May 2010
An Extension to Session Initiation Protocol (SIP) Events
for Conditional Event Notification
Abstract
The Session Initiation Protocol (SIP) events framework enables
receiving asynchronous notification of various events from other SIP
user agents. This framework defines the procedures for creating,
refreshing, and terminating subscriptions, as well as fetching and
periodic polling of resource state. These procedures provide no
tools to avoid replaying event notifications that have already been
received by a user agent. This memo defines an extension to SIP
events that allows the subscriber to condition the subscription
request to whether the state has changed since the previous
notification was received. When such a condition is true, either the
body of a resulting event notification or the entire notification
message is suppressed.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5839.
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RFC 5839 Entity-Tags for SIP Events May 2010
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
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Without obtaining an adequate license from the person(s) controlling
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not be created outside the IETF Standards Process, except to format
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than English.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Document Conventions . . . . . . . . . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Motivations and Background . . . . . . . . . . . . . . . . . . 5
2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Problem Description . . . . . . . . . . . . . . . . . . . 5
2.3. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6
3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 7
4. Resource Model for Entity-Tags . . . . . . . . . . . . . . . . 10
5. Subscriber Behavior . . . . . . . . . . . . . . . . . . . . . 12
5.1. Detecting Support for Conditional Notification . . . . . . 13
5.2. Generating SUBSCRIBE Requests . . . . . . . . . . . . . . 13
5.3. Receiving NOTIFY Requests . . . . . . . . . . . . . . . . 14
5.4. Polling or Fetching Resource State . . . . . . . . . . . . 15
5.5. Resuming a Subscription . . . . . . . . . . . . . . . . . 17
5.6. Refreshing a Subscription . . . . . . . . . . . . . . . . 18
5.7. Terminating a Subscription . . . . . . . . . . . . . . . . 18
5.8. Handling Transient Errors . . . . . . . . . . . . . . . . 19
6. Notifier Behavior . . . . . . . . . . . . . . . . . . . . . . 20
6.1. Generating Entity-tags . . . . . . . . . . . . . . . . . . 20
6.2. Suppressing NOTIFY Bodies . . . . . . . . . . . . . . . . 20
6.3. Suppressing NOTIFY Requests . . . . . . . . . . . . . . . 21
6.4. State Differentials . . . . . . . . . . . . . . . . . . . 21
6.5. List Subscriptions . . . . . . . . . . . . . . . . . . . . 22
7. Protocol Element Definitions . . . . . . . . . . . . . . . . . 22
7.1. 204 (No Notification) Response Code . . . . . . . . . . . 22
7.2. Suppress-If-Match Header Field . . . . . . . . . . . . . . 22
7.3. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . 22
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
8.1. 204 (No Notification) Response Code . . . . . . . . . . . 23
8.2. Suppress-If-Match Header Field . . . . . . . . . . . . . . 23
9. Security Considerations . . . . . . . . . . . . . . . . . . . 24
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
11.1. Normative References . . . . . . . . . . . . . . . . . . . 24
11.2. Informative References . . . . . . . . . . . . . . . . . . 24
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1. Introduction
The Session Initiation Protocol (SIP) events framework provides an
extensible facility for requesting notification of certain events
from other SIP user agents. This framework includes procedures for
creating, refreshing, and terminating subscriptions, as well as the
possibility to fetch or periodically poll the event resource.
Several instantiations of this framework, called event packages have
been defined, e.g., for presence [RFC3856], message waiting
indications [RFC3842], and registrations [RFC3680].
By default, every SUBSCRIBE request generates a NOTIFY request
containing the latest event state. Typically, a SUBSCRIBE request is
issued by the subscriber whenever it needs a subscription to be
installed, periodically refreshed, or terminated. Once the
subscription has been installed, the majority of the NOTIFYs
generated by the subscription refreshes are superfluous; the
subscriber usually is in possession of the event state already,
except in the unlikely case where a state change exactly coincides
with the periodic subscription refresh. In most cases, the final
event state generated upon terminating the subscription similarly
contains resource state that the subscriber already has.
Fetching or polling of resource state behaves in a similarly
suboptimal way in cases where the state has not changed since the
previous poll occurred. In general, the problem lies with the
inability to persist state across a SUBSCRIBE request.
This memo defines an extension to optimize the SIP events framework.
This extension allows a notifier to tag notifications (called entity-
tags hereafter) and the subscriber to condition its subsequent
SUBSCRIBE requests for actual changes since a notification carrying
that entity-tag was issued. The solution is similar to conditional
requests defined in the Hypertext Transfer Protocol (HTTP) [RFC2616],
and follows the mechanism already defined for the PUBLISH [RFC3903]
method for issuing conditional event publications.
This memo is structured as follows. Section 2 explains the
background, motivations, and requirements for the work; Section 3
gives a general overview of the mechanism; Section 4 explains the
underlying model for resources and entities as they apply to
conditional notification; Section 5 defines the subscriber behavior;
Section 6 defines the notifier behavior; Section 7 includes the
protocol element definitions; Section 8 includes the IANA
considerations; and Section 9 includes the security considerations.
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RFC 5839 Entity-Tags for SIP Events May 2010
1.1. Document Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC2119] and indicate requirement levels for compliant
implementations.
1.2. Terminology
In addition to the terminology introduced in [RFC3261], [RFC3265],
and [RFC3903], this specification uses these additional terms to
describe the objects of conditional notification:
resource
An object identified by a URI whose resource state can be accessed
using the SIP Event Notification framework. There is a single
authoritative notifier responsible for communicating the resource
state.
entity
The representation of resource state. An entity consists of the
state data carried in the body of a NOTIFY message, as well as
related meta-data in the message header. There may be many
versions of an entity, one current and the others stale. Each
version of an entity is identified by an entity-tag, which is
guaranteed to be unique across all versions of all entities for a
resource and event package.
2. Motivations and Background
2.1. Overview
A SUBSCRIBE request creates a subscription with a finite lifetime.
This lifetime is negotiated using the Expires header field, and
unless the subscription is refreshed by the subscriber before the
expiration is met, the subscription is terminated. The frequency of
these subscription refreshes depends on the event package, and
typically ranges from minutes to hours.
2.2. Problem Description
The SIP events framework does not include different protocol methods
for initiating and terminating of subscriptions, subscription
refreshes, and fetches inside and outside of the SIP dialog. The
SUBSCRIBE method is overloaded to perform all of these functions.
The difference between a fetch that does not create a (lasting)
subscription and a SUBSCRIBE that creates one is in the Expires
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RFC 5839 Entity-Tags for SIP Events May 2010
header field value of the SUBSCRIBE; a zero-expiry SUBSCRIBE only
generates a single NOTIFY, after which the subscription immediately
terminates. Lasting subscriptions typically have relatively short
expiry periods, requiring periodic sending of new SUBSCRIBE requests
in order to refresh the subscription.
Each new SUBSCRIBE request generates a NOTIFY request containing the
latest resource state. Even if the state has not changed, it is sent
again in response to each poll or subscription refresh. This is very
similar to the HTTP [RFC2616] problem of repeated GET operations on a
resource. HTTP solves the problem using conditional requests. The
server versions each entity with an entity-tag that identifies a
specific instance of that entity. Clients making GET requests can
then include the entity-tag for the version of the entity that they
believe to be current in an "If-None-Match" header field. The server
can compare this entity-tag to the entity it believes to be current
and suppress resending the entity in the response if the server
believes the client's version matches. In other words, the server
doesn't resend information that the client has already received.
The SIP PUBLISH [RFC3903] method uses a similar mechanism, where a
refresh of a publication is done by reference to its assigned entity-
tag, instead of retransmitting the event state each time the
publication expiration is extended.
2.3. Requirements
As a summary, here is the required functionality to solve the
presented issues:
REQ1: It must be possible to suppress the NOTIFY request (or at a
minimum, the event body therein) if the subscriber is already
in possession of (or has previously received and discarded)
the latest event state of the resource.
REQ2: This mechanism must apply to initial subscriptions in which
the subscriber is attempting to resume an earlier
subscription that has been paused.
REQ3: This mechanism must apply to refreshing a subscription.
REQ4: This mechanism must apply to terminating a subscription
(i.e., an unsubscribe).
REQ5: This mechanism must apply to fetching or polling of resource
state.
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3. Overview of Operation
Whenever a subscriber initiates a subscription, it issues a SUBSCRIBE
request. The SUBSCRIBE request is sent, routed, and processed by the
notifier normally, i.e., according to the Session Initiation Protocol
[RFC3261] and SIP-Specific Event Notification [RFC3265].
If the notifier receiving the SUBSCRIBE request supports conditional
subscriptions, it generates an entity-tag for the current entity, and
includes it in a SIP-ETag header field of the NOTIFY request. The
entity-tag is unique across all versions of all entities for a
resource and event package. See Section 4 for more on this.
Entity-tags are independent of subscriptions. This allows
notifications generated to a fetch or a poll to have valid entity-
tags even across subsequent fetches or polls.
The subscriber will store the entity-tag received in the notification
along with the resource state. It can then later use this entity-tag
to make a SUBSCRIBE contain a condition in the form of a "Suppress-
If-Match" header field. Unlike the "If-Match" condition in a PUBLISH
[RFC3903] request, which applies to whether the PUBLISH succeeds or
returns an error, this condition applies to the stream of
notifications that are sent after the SUBSCRIBE request has been
processed.
The Suppress-If-Match header field contains the last entity-tag seen
by the subscriber. This condition, if true, instructs the notifier
to suppress either the body of a subsequent notification, or the
entire notification.
The condition is evaluated by matching the value of the header field
against the entity-tag of the entity that would normally be sent in
the associated NOTIFY message. There is also a wildcard entity-tag
with a special value of "*" that always matches.
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RFC 5839 Entity-Tags for SIP Events May 2010
Subscriber Notifier
---------- --------
(1) SUBSCRIBE -------->
Expires: 3600
<-------- (2) 200 (or 202)
<-------- (3) NOTIFY
Subscription-State: active
SIP-ETag: ffee2
(4) 200 -------->
... time passes ...
(5) SUBSCRIBE --------> \ if "ffee2"
Suppress-If-Match: ffee2 | matches
Expires: 3600 | local
| entity-tag
|
<-------- (6) 204 / then
... time passes and resource state (entity) changes...
<-------- (7) NOTIFY
Subscription-State: active
SIP-ETag: ca89a
(8) 200 -------->
... time passes ...
(9) SUBSCRIBE --------> \ if "ca89"
Suppress-If-Match: ca89a | matches
Expires: 0 | local
| entity-tag
|
<-------- (10) 204 / then
Figure 1: Example Message Flow
Figure 1 describes a typical message flow for conditional
notification:
(1) The subscriber initiates a subscription by sending a SUBSCRIBE
request for a resource.
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RFC 5839 Entity-Tags for SIP Events May 2010
(2) After proper authentication and authorization, the notifier
accepts the subscription.
(3) The notifier then immediately sends the initial event
notification, including a unique entity-tag in a SIP-ETag
header field.
(4) The subscriber accepts the notification and stores the entity-
tag value along with the resource state.
(5) Later, the subscriber refreshes the subscription, and includes
an entity-tag in a Suppress-If-Match header field.
(6) The notifier evaluates the condition by matching its local
entity-tag value for the resource against the value of the
Suppress-If-Match header field. If the condition evaluates to
true, the notifier informs the subscriber that the notification
will not be sent.
(7) At some point, the state of the resource changes, e.g., the
presence status of a user changes from online to busy. This
triggers an event notification with a new value in the SIP-ETag
header field.
(8) The subscriber accepts the notification and stores the new
entity-tag along with the resource state.
(9) After a while, the subscriber decides to terminate the
subscription. It adds a condition for Suppress-If-Match, and
includes the entity-tag it received in the previous NOTIFY.
(10) The notifier evaluates the condition by matching its entity-tag
for the resource against the value of the Suppress-If-Match
header field. If the condition evaluates to true, the notifier
informs the subscriber that no notification will be sent. This
concludes the subscription.
The benefit of using conditional notification in this example is in
the reduction of the number of NOTIFY requests the subscriber can
expect to receive. Each event notification that the subscriber has
already seen is suppressed by the notifier. This example illustrates
only one use case for the mechanism; the same principles can be used
to optimize the flow of messages related to other event notification
use cases.
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RFC 5839 Entity-Tags for SIP Events May 2010
4. Resource Model for Entity-Tags
The key to understanding how conditional notification works is
understanding the underlying resource model of event notification.
In general, this model is similar to the resource model of HTTP with
some key differences. This section explains in detail the model as
it applies to SIP events. Figure 2 illustrates the model.
+-----+
............ | |
. . | URI |
. Represen . | |
. tation . +-----+
. . |*
............ |
. |
. V
. +----------+ +---------+
composition | |* | Event |
+------<>| Resource |----------->| Package |<----.
| | | | | |
| +----------+ +----.----+ |
| /_\ |
|* | classification
+--------+ | |
| | .----------------.------' |
| Entity | | | |
| | | | |*
+--------+ +----------+ +------------+ +----------+
^ | | | | | |
| | Presence | | Conference | | Template |
| | | | | | |
|1..* +----------+ +------------+ +----.-----+
+---------+ /_\
| | |
| Version | |
| | +---------+
+---------+ | Watcher |
|1 | Info |
| | |
V +---------+
+---------+
| Entity- |
| Tag |
| |
+---------+
Figure 2: Resource Model Diagram
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RFC 5839 Entity-Tags for SIP Events May 2010
For a given event package, there is a single authoritative agent
responsible for zero or more resources. That is, even for a
distributed agent, the resource state is uniform across all
instances. The resource itself can be a list of resources [RFC4662].
Conditional notification for list subscriptions is addressed in
Section 6.5.
A resource is identified by zero or more URIs, which can be SIP URIs,
pres URIs [RFC3859], or similar. Subscribers use this URI to
subscribe to the resource for certain types of events, identified by
the event package.
With a successful subscription, a subscriber receives event
notifications that communicate the resource state and the changes
thereto. Each event notification carries a representation of the
current resource state. This representation is influenced by many
factors, e.g., authorization and filtering rules, and the event
composition rules of the notifier.
This representation is realized in an "entity". Each resource may be
associated with zero or more entities. For example, there may be
multiple subscribers to the presence information of a single user (a
resource), and each subscriber may have a different filtered view of
that resource, producing one entity per subscriber. However, each
entity is associated with one and only one resource; there is no
"compositing" of resources at the entity level. Resources may
themselves be made up of information from other resources (be
"composite resources"), but this does not change the one-resource-
per-entity rule.
An entity consists of the data carried in the body of a NOTIFY
message and related meta-data in the message header. Whenever the
data in the body or any of the meta-data changes, the notifier MUST
produce a new entity-tag. This meta-data MUST include, but is not
limited to the following SIP header fields defined in the Session
Initiation Protocol [RFC3261] and SIP Specific Event Notification
[RFC3265]:
1. Content-Disposition
2. Content-Encoding
3. Content-Language
4. Content-Length
5. Content-Type
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RFC 5839 Entity-Tags for SIP Events May 2010
6. Event
Note that the Subscription-State is explicitly not part of the
entity. In the future, event packages may define additional fields
that implementations need to consider as part of the entity.
An entity has one or more versions of which only one is current and
all others stale. Each version has an entity-tag, which uniquely
identifies it across all versions of all entities pertaining to a
single resource and event package.
Note that two entity-tags for different resources being equal does
not indicate identical entities. In other words, if an entity-tag
received for a subscription to a first resource matches an entity-tag
received for a subscription to a second resource, the subscriber
cannot assume that the two entity values are equal.
With partial event notification, the NOTIFY message only carries the
delta state, or the set of changes to the previous version of the
entity. In that case, implementations MUST consider the full event
state as the version of the entity to which the entity-tag in the
NOTIFY message applies.
The conditional notification mechanism is independent of the way in
which subscriptions are installed. In other words, the mechanism
supports implicit subscriptions, such as those associated with the
REFER method [RFC3515].
It is possible that the same resource is in some shape or form
accessible through another mechanism in addition to SIP Event
Notification, e.g., HTTP or the SIP PUBLISH method. In general,
implementations MUST NOT expect the entity-tags to be shared between
the mechanisms, unless event packages or specific applications of SIP
events explicitly define such dependencies.
5. Subscriber Behavior
This section augments the subscriber behavior defined in RFC 3265
[RFC3265]. It first discusses general issues related to indicating
support for the mechanism (Section 5.1) and creating conditions in
SUBSCRIBE requests (Section 5.2). Next, it describes subscriber
behavior for receiving NOTIFY requests (Section 5.3), and specific
client workflows for polling resource state (Section 5.4), resuming a
subscription (Section 5.5), refreshing a subscription (Section 5.6),
and terminating a subscription (Section 5.7). Finally, handling of
transient errors is discussed (Section 5.8).
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RFC 5839 Entity-Tags for SIP Events May 2010
5.1. Detecting Support for Conditional Notification
The mechanism defined in this memo is backwards compatible with SIP
events [RFC3265] in that a notifier supporting this mechanism will
insert a SIP entity-tag in its NOTIFY requests, and a subscriber that
understands this mechanism will know how to use it in creating a
conditional request.
Unaware subscribers will simply ignore the entity-tag, make requests
without conditions, and receive the default treatment from the
notifier. Unaware notifiers will simply ignore the conditional
header fields and continue normal operation.
5.2. Generating SUBSCRIBE Requests
When creating a conditional SUBSCRIBE request, the subscriber MUST
include a single conditional header field including an entity-tag in
the request. The condition is evaluated by comparing the entity-tag
of the subscribed resource with the entity-tag carried in the
conditional header field. If they match, the condition evaluates to
true.
Unlike the condition introduced for the SIP PUBLISH [RFC3903] method,
these conditions do not apply to the SUBSCRIBE request itself, but to
the resulting NOTIFY requests. When true, the condition drives the
notifier to change its behavior with regard to sending the
notifications after the SUBSCRIBE.
This specification defines a new header field called Suppress-If-
Match. This header field introduces a condition to the SUBSCRIBE
request. If true, it instructs the notifier either to omit the body
of the resulting NOTIFY message (if the SUBSCRIBE is not sent within
an existing dialog) or to suppress (i.e., block) the NOTIFY request
that would otherwise be triggered by the SUBSCRIBE (for an
established dialog). In the latter case, the SUBSCRIBE message will
be answered with a 204 (No Notification) response. As long as the
condition remains true, it also instructs the notifier either to
suppress any subsequent NOTIFY request or, if there are reportable
changes in the NOTIFY header, e.g., the Subscription-State has
changed, to suppress the body of any subsequent NOTIFY request.
If the condition is false, the notifier follows its default behavior.
If the subscriber receives a 204 (No Notification) response to an in-
dialog SUBSCRIBE, the subscriber MUST consider the event state and
the subscription state unchanged.
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RFC 5839 Entity-Tags for SIP Events May 2010
The value of the Suppress-If-Match header field is an entity-tag,
which is an opaque token that the subscriber simply copies (byte-
wise) from a previously received NOTIFY request. Inclusion of an
entity-tag in a Suppress-If-Match header field of a SUBSCRIBE request
indicates that the client has a copy of, or is capable of recreating
a copy of, the entity associated with that entity-tag.
Example:
Suppress-If-Match: b4cf7
The header field can also be wildcarded using the special "*" entity-
tag value. Such a condition always evaluates to true regardless of
the value of the current entity-tag for the resource.
Example:
Suppress-If-Match: *
Such a wildcard condition effectively quenches a subscription; the
only notifications received are those reporting changes to the
subscription state and those in response to a SUBSCRIBE message sent
outside of an existing dialog. In both cases, the notifications will
not contain a body.
A subscription with a wildcard Suppress-If-Match condition is
useful in scenarios where the subscriber wants to temporarily put
a subscription in dormant mode. For example, a host may want to
conserve bandwidth and power when it detects from screen or input
device inactivity that the user isn't actively monitoring the
presence statuses of contacts.
5.3. Receiving NOTIFY Requests
When a subscriber receives a NOTIFY request that contains a SIP-ETag
header field, it MUST store the entity-tag if it wishes to make use
of the conditional notification mechanism. The subscriber MUST be
prepared to receive a NOTIFY with any entity-tag value, including a
value that matches any previous value that the subscriber might have
seen.
The subscriber MUST NOT infer any meaning from the value of an
entity-tag; specifically, the subscriber MUST NOT assume identical
entities (i.e., event state) for NOTIFYs with identical entity-tag
values when those NOTIFYs result from subscription to different
resources.
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RFC 5839 Entity-Tags for SIP Events May 2010
Note that there are valid cases for which identical entity-tag
values on different resources may occur. For example, it is
possible to generate entity-tag values using a one-way hash
function, resulting in the possibility that two different
resources having the same entity-value will also have the same
entity-tag. Clients however MUST NOT assume that this is the
case, as the algorithm for the generation of entity-tags is
notifier-dependent and not negotiated with the subscriber.
Consequently, the subscriber cannot differentiate between two
entity-tags that have the same value because they are similar
hashes of identical entities, or because two notifiers happen to
have used the same sequential number as an entity-tag. Entity
tags are only required to be unique for a given resource, not
globally unique.
5.4. Polling or Fetching Resource State
Polling with conditional notification allows a user agent to
efficiently poll resource state. This is accomplished using the
Suppress-If-Match condition:
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RFC 5839 Entity-Tags for SIP Events May 2010
Subscriber Notifier
---------- --------
(1) SUBSCRIBE -------->
Expires: 0
<-------- (2) 202
<-------- (3) NOTIFY
Subscription-State: terminated
SIP-ETag: f2e45
Content-Length: 17539
(4) 200 -------->
... poll interval elapses ...
(5) SUBSCRIBE -------->
Suppress-If-Match: f2e45
Expires: 0
<-------- (6) 202
<-------- (7) NOTIFY
Subscription-State: terminated
SIP-ETag: f2e45
Content-Length: 0
(8) 200 -------->
Figure 3: Polling Resource State
(1) The subscriber polls for resource state by sending a SUBSCRIBE
with zero expiry (expires immediately).
(2) The notifier accepts the SUBSCRIBE with a 202 (Accepted)
response.
(3) The notifier then immediately sends a first (and last) NOTIFY
request with the current resource state and the current entity-
tag in the SIP-ETag header field.
(4) The subscriber accepts the notification with a 200 (OK)
response.
(5) After some arbitrary poll interval, the subscriber sends another
SUBSCRIBE with a Suppress-If-Match header field that includes
the entity-tag received in the previous NOTIFY.
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RFC 5839 Entity-Tags for SIP Events May 2010
(6) The notifier accepts the SUBSCRIBE with a 202 (Accepted)
response. (202 would be used to indicate that the subscription
request was understood without also indicating that it was
authorized, as per Section 3.1.6.1 of SIP-Specific Event
Notification [RFC3265].)
(7) Since the resource state has not changed since the previous poll
occurred, the notifier sends a NOTIFY message with no body. It
also mirrors the current entity-tag of the resource in the SIP-
ETag header field.
(8) The subscriber accepts the notification with a 200 (OK)
response.
5.5. Resuming a Subscription
Resuming a subscription means the ability to continue an earlier
subscription that either closed abruptly or was explicitly
terminated. When resuming, the subscription is established without
transmitting the resource state. This is accomplished with
conditional notification and the Suppress-If-Match header field:
Subscriber Notifier
---------- --------
(1) SUBSCRIBE -------->
Suppress-If-Match: ega23
Expires: 3600
<-------- (2) 202
<-------- (3) NOTIFY
Subscription-State: active
SIP-ETag: ega23
Content-Length: 0
(4) 200 -------->
Figure 4: Resuming a Subscription
(1) The subscriber attempts to resume an earlier subscription by
including a Suppress-If-Match header field with the entity-tag
it last received.
(2) The notifier accepts the subscription after proper
authentication and authorization, by sending a 202 (Accepted)
response.
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(3) Since the condition is true, the notifier then immediately sends
an initial NOTIFY request that has no body. It also mirrors the
current entity-tag of the resource in the SIP-ETag header field.
(4) The subscriber accepts the NOTIFY and sends a 200 (OK) response.
Had the entity-tag not been valid any longer, the condition would
have evaluated to false, and the NOTIFY would have had a body
containing the latest resource state.
5.6. Refreshing a Subscription
To refresh a subscription using conditional notification, the
subscriber creates a subscription refresh before the subscription
expires, and uses the Suppress-If-Match header field:
Subscriber Notifier
---------- --------
(1) SUBSCRIBE -------->
Suppress-If-Match: aba91
Expires: 3600
<-------- (2) 204
Expires: 3600
Figure 5: Refreshing a Subscription
(1) Before the subscription expires, the subscriber sends a
SUBSCRIBE request that includes the Suppress-If-Match header
field with the latest entity-tag it has seen.
(2) If the condition evaluates to true, the notifier sends a 204 (No
Notification) response and sends no NOTIFY request. The Expires
header field of the 204 (No Notification) response indicates the
new expiry time.
5.7. Terminating a Subscription
To terminate a subscription using conditional notification, the
subscriber creates a SUBSCRIBE request with a Suppress-If-Match
condition:
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Subscriber Notifier
---------- --------
(1) SUBSCRIBE -------->
Suppress-If-Match: ega23
Expires: 0
<-------- (2) 204
Figure 6: Terminating a Subscription
(1) The subscriber decides to terminate the subscription and sends a
SUBSCRIBE request with the Suppress-If-Match condition with the
entity-tag it has last seen.
(2) If the condition evaluates to true, the notifier sends a 204 (No
Notification) response, which concludes the subscription, and
the subscriber can clear all state related to the subscription.
5.8. Handling Transient Errors
This section is non-normative.
In some deployments, there may be Back-to-Back User Agent (B2BUA)
devices that track SIP dialogs such as subscription dialogs. These
devices may be unaware of the conditional notification mechanism.
It is possible that some B2BUA devices may treat a NOTIFY with
suppressed body as an error, or may expect all SUBSCRIBE messages to
have an associated NOTIFY message.
In general, there is very little that an endpoint can do to recover
from such transient errors. The most that can be done is to try to
detect such errors, and define a fallback behavior.
If subscribers encounter transient errors in conditional
notification, they should disable the feature and fall back to normal
subscription behavior.
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6. Notifier Behavior
This section augments the notifier behavior as specified in RFC 3265
[RFC3265].
6.1. Generating Entity-tags
An entity-tag is a token carried in the SIP-ETag header field, and it
is opaque to the client. The notifier is free to decide on any means
for generating the entity-tag. It can have any value, except for
"*". For example, one possible method is to implement the entity-tag
as a simple counter, incrementing it by one for each generated
notification per resource.
A notifier MUST generate entity-tags for event notifications of all
resources for which it is responsible. The entity-tag MUST be unique
across all versions of all entities for each state of a resource as
reported by a given event package. Otherwise said, for any
subscription or sequence of subscriptions to a specific resource
using a singular event package, each entity-tag produced MUST map to
one and only one presentation of resource state (entity). Two
identical entities for a specific resource might or might not have
identical entity-tags; this decision is left to the notifier.
An entity-tag is considered valid for as long as the entity exists.
An entity becomes stale when its version is no longer the current
one. The notifier MUST remember (or be able to recalculate) the
entity-tag of an entity as long as the version of the entity is
current. The notifier MAY remember the entity-tag longer than this,
e.g., for implementing journaled state differentials (Section 6.4).
The entity-tag values used in publications are not necessarily shared
with the entity-tag values used in subscriptions. This is because
there may not always be a one-to-one mapping between a publication
and a notification of state change; there may be several sources to
the event composition process, and a publication into a resource may
not affect the resulting entity.
6.2. Suppressing NOTIFY Bodies
When a condition in a SUBSCRIBE request for suppressing notifications
is true (i.e., the local entity-tag for the resource state and the
entity-tag in a Suppress-If-Match header field are byte-wise
identical) but there are reportable changes in the NOTIFY header
(e.g., the Subscription-State has changed), the notifier MUST
suppress the body of the NOTIFY request. That is, the resulting
NOTIFY contains no Content-Type header field, the Content-Length is
set to zero, and no payload is attached to the message.
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Additionally, when a condition in a SUBSCRIBE request for suppressing
notifications is true and the SUBSCRIBE message is not sent within an
established dialog, the notifier MUST send a NOTIFY request with a
suppressed entity body.
Suppressing the entity body of a NOTIFY does not change the current
entity-tag of the resource. Hence, the NOTIFY MUST contain a SIP-
ETag header field that contains the unchanged entity-tag of the
resource state.
A Suppress-If-Match header field that includes an entity-tag with the
value of "*" MUST always evaluate to true.
6.3. Suppressing NOTIFY Requests
When a condition in a SUBSCRIBE request to suppress notifications is
true (i.e., the local entity-tag of the resource and the entity-tag
in a Suppress-If-Match header field match), and the SUBSCRIBE is sent
within an established dialog, then the notifier MUST suppress the
resulting NOTIFY request, and generate a 204 (No Notification)
response. As long as the condition remains true, and there are no
reportable changes in the NOTIFY header, all subsequent NOTIFY
requests MUST also be suppressed.
Notifiers MUST NOT suppress a NOTIFY unless the corresponding
SUBSCRIBE message was sent in an established dialog.
A successful conditional SUBSCRIBE request MUST extend the
subscription expiry time.
Suppressing the entire NOTIFY has no effect on the entity-tag of the
resource. In other words, it remains unchanged.
A Suppress-If-Match header field that includes an entity-tag with the
value of "*" MUST always evaluate to true.
6.4. State Differentials
Some event packages support a scheme where notifications contain
state differentials, or state deltas [RFC3265], instead of complete
resource state.
Further extensions could define means for notifiers to keep track of
the state changes of a resource, e.g., storing the changes in a
journal. If a condition fails, the notifier would then send a state
differential in the NOTIFY rather than the full state of the event
resource. This is only possible if the event package and the
subscriber both support a payload format that has this capability.
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When state differentials are sent, the SIP-ETag header field MUST
contain an entity-tag that corresponds to the full resource state.
6.5. List Subscriptions
The Event Notification Extension for Resource Lists [RFC4662] defines
a mechanism for subscribing to a homogeneous list of resources using
the SIP events framework.
A list subscription delivers event notifications that contain both
Resource List Meta-Information (RLMI) documents as well as the
resource state of the individual resources on the list.
Implementations MUST consider the full resource state of a resource
list including RLMI and the entity-header as the entity to which the
entity-tag applies.
7. Protocol Element Definitions
This section describes the protocol extensions required for
conditional notification.
7.1. 204 (No Notification) Response Code
The 204 (No Notification) response code indicates that the request
was successful, but the notification associated with the request will
not be sent. It is valid only in response to a SUBSCRIBE message
sent within an established dialog.
The response code is added to the "Success" production rule in the
SIP [RFC3261] message grammar.
7.2. Suppress-If-Match Header Field
The Suppress-If-Match header field is added to the definition of the
"message-header" rule in the SIP [RFC3261] grammar. Its use is
described in Sections 5, 6.3, and 6.2.
This header field is allowed to appear in any request, but its
behavior is only defined for the SUBSCRIBE request.
7.3. Grammar
This section defines the formal syntax for extensions described in
this memo in Augmented BNF (ABNF) [RFC5234]. The rules defined here
augment and reference the syntax defined in RFC 3261 [RFC3261] and
RFC 3903 [RFC3903].
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Success =/ "204" ; No Notification
; Success is defined in RFC 3261.
message-header =/ Suppress-If-Match
; message-header is defined in RFC 3261.
Suppress-If-Match = "Suppress-If-Match" HCOLON ( entity-tag / "*" )
; entity-tag is defined in RFC 3903.
8. IANA Considerations
This document registers a new response code and a new header field
name.
8.1. 204 (No Notification) Response Code
This document registers a new response code. This response code is
defined by the following information, which has been added to the
methods and response-codes sub-registry available from
http://www.iana.org.
This information has been added under "Successful 2xx" category.
+---------------------+-----------+
| Response Code | Reference |
+---------------------+-----------+
| 204 No Notification | [RFC5839] |
+---------------------+-----------+
8.2. Suppress-If-Match Header Field
This document registers a new SIP header field called Suppress-If-
Match. This header field is defined by the following information,
which has been added to the header fields sub-registry available from
http://www.iana.org.
+-------------------+---------+-----------+
| Header Name | Compact | Reference |
+-------------------+---------+-----------+
| Suppress-If-Match | | [RFC5839] |
+-------------------+---------+-----------+
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9. Security Considerations
The security considerations for SIP event notification are
extensively discussed in RFC 3265 [RFC3265]. This specification
introduces an optimization to SIP event notification, which in itself
does not alter the security properties of the protocol.
10. Acknowledgments
The following people have contributed corrections and suggestions to
this document: Adam Roach, Sean Olson, Johnny Vrancken, Pekka Pessi,
Eva Leppanen, Krisztian Kiss, Peili Xu, Avshalom Houri, David
Viamonte, Jonathan Rosenberg, Qian Sun, Dale Worley, Tolga Asveren,
Brian Stucker, Eric Rescorla, Arun Arunachalam, and the SIP and
SIMPLE working groups.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension
for Event State Publication", RFC 3903, October 2004.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
11.2. Informative References
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[RFC3680] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Registrations", RFC 3680, March 2004.
Niemi & Willis Standards Track [Page 24]
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[RFC3842] Mahy, R., "A Message Summary and Message Waiting
Indication Event Package for the Session Initiation
Protocol (SIP)", RFC 3842, August 2004.
[RFC3856] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.
[RFC3859] Peterson, J., "Common Profile for Presence (CPP)",
RFC 3859, August 2004.
[RFC4662] Roach, A., Campbell, B., and J. Rosenberg, "A Session
Initiation Protocol (SIP) Event Notification Extension for
Resource Lists", RFC 4662, August 2006.
Authors' Addresses
Aki Niemi
Nokia
P.O. Box 407
NOKIA GROUP, FIN 00045
Finland
Phone: +358 50 389 1644
EMail: aki.niemi@nokia.com
Dean Willis (editor)
Softarmor Systems
3100 Independence Pkwy #311-164
Plano, TX 75075
USA
Phone: +1 214 504 1987
EMail: dean.willis@softarmor.com
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RFC, FYI, BCP