An Extension to Session
Initiation Protocol (SIP) Events for Conditional Event
NotificationNokiaP.O. Box 407NOKIA GROUPFIN00045Finland+358 50 389 1644aki.niemi@nokia.com
Real-Time Applications and Infrastructure
Session Initiation Protocol Working GroupI-DRFCInternet-DraftRequest For CommentsSIP eventssubnot-etagsoptimizationThe 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.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 of 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, message waiting indications and registrations.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 in 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 almost identical to conditional
requests defined in the HyperText Transfer
Protocol (HTTP), and follows the mechanism already defined for
the PUBLISH method for issuing conditional
event publications.This memo is structured as follows. explains the background, motivations and
requirements for the work; gives a
general overview of the mechanism; explains
the underlying model for resources and entities as they apply to
conditional notification; defines the
subscriber behavior; defines the
notifier behavior; includes the protocol
element definitions; includes the IANA
considerations; and includes the
security considerations.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 and indicate requirement levels for
compliant implementations.In addition to the terminology introduced in , and , this specification uses these additional
terms to describe the objects of conditional notification: 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.The representation
of resource state. An entity consists of the event 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.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.In spite of being somewhat distinct operations, 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. Instead, the SUBSCRIBE method is
overloaded to perform all of these functions, and the notifier
behavior is identical in each of them; each SUBSCRIBE request
generates a NOTIFY request containing the latest resource state. In
fact, the only difference between a fetch that does not create a
(lasting) subscription, and a SUBSCRIBE that creates one is in the
Expires header field value of the SUBSCRIBE; a zero-expiry SUBSCRIBE
only generates a single NOTIFY, after which the subscription
immediately terminates.Some subscriber implementations may choose to operate in
semi-stateless mode, in which they immediately upon receiving and
processing the NOTIFY forget the resource state. This operation
necessarily needs every NOTIFY to carry the full resource state.
However, for an implementation that stores the resource state locally,
this mode of operation is inefficient.There are certain conditions that aggravate the problem. Such
conditions usually entail such things as: Large entity bodies in the payloads of notificationsHigh rate of subscription refreshesRelatively low rate of notifications triggered by state
changesIn effect, for an event package that generates few state changes,
and is refreshed relatively often the majority of traffic generated
may be related to subscription maintenance. Especially in networks
where bandwidth consumption and traffic count is at a premium, the
high overhead of subscription maintenance becomes a barrier for
deployment.The same problem affects fetching and polling of resource state as
well. As a benchmark, if we look at the performance of HTTP in similar scenarios, it performs
substantially better using conditional requests. When resources are
tagged with an entity-tag, and each GET is a conditional one using the
"If-None-Match" header field, the entity body need not be sent more
than once; if the resource has not changed between successive polls,
an error response is returned indicating this fact, and the resource
entity is not transmitted again.The SIP PUBLISH method also contains
a similar feature, 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.As a summary, here is the required functionality to solve the
presented issues: 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 the latest event state of the resource.This mechanism must apply to initial subscriptions, in which
the subscriber is attempting to "resume" an earlier
subscription.This mechanism must apply to refreshing a subscription.This mechanism must apply to terminating a subscription (i.e.,
an unsubscribe).This mechanism must apply to fetching or polling of resource
state.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 RFC3261, RFC3265.If the notifier receiving the SUBSCRIBE request supports conditional
subscriptions, it generates a unique entity tag for the event
notification, 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. More on this in .Entity-tags are independent of subscriptions; the notifier remembers
the entity-tags of all versions of entities for a resource regardless of
whether or not there are any active subscription to that resource. 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 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 current entity-tag of the resource state. There is also a
wildcard entity-tag with a special value of "*" that always matches. describes a typical message flow for
conditional notification: The subscriber initiates a subscription by sending a SUBSCRIBE
request for a resource.After proper authentication and authorization, the notifier
accepts the subscription.The notifier then immediately sends the initial event
notification, including a unique entity-tag in a SIP-ETag header
field.The subscriber accepts the notification and stores the entity-tag
value along with the resource state.Later, the subscriber refreshes the subscription, and includes an
entity-tag in a Suppress-If-Match header field.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.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.The subscriber accepts the notification and stores the new
entity-tag along with the resource state.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.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.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. illustrates the model.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.
Conditional notification for list subscriptions is addressed in .A resource is identified by zero or more URIs, which can be SIP URIs,
pres URIs 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 what is called an entity. Each
resource may be associated with zero or more entities; however, an
entity is only valid for a single resource. Note that, as can be seen from the illustration, the association
between a resource and an entity follows the typical composition
relationship, i.e., an entity may belong to only one resource, and
it is expected to only exist with that resource.An entity consists of the data carried in the body of a NOTIFY
message, and related meta-data in the message header. This meta-data
includes, but is not limited to the following SIP header fields:Note that the Subscription-State is explicitly not part of the
entity. Event packages may in the future 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 being equal does not indicate identical
entities. In other words, if an entity-tag is received that matches a
previously seen entity-tag, the subscriber cannot assume the event state
to be identical to that received earlier.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.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.This section augments the subscriber behavior defined in RFC3265. It first discusses general issues
related to indicating support for the
mechanism and creating conditions in
SUBSCRIBE requests; it then describes the workflows for the main
three use cases for making the subscription conditional.The mechanism defined in this memo is backwards compatible with
SIP events 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.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 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 regards 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 to either omit
the body of the resulting NOTIFY message (if the SUBSCRIBE is not sent
within an existing dialog), or suppress (i.e., block) the NOTIFY
request that would otherwise be triggered 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 to either
suppress any subsequent NOTIFY request, or if there are reportable
changes in the NOTIFY header, e.g., the Subscription-State has
changed, suppress the body of any subsequent NOTIFY request.If the condition is false, the notifier follows its default
behaviour.If the subscriber receives a 204 (No Notification) to an in-dialog
SUBSCRIBE, the subscriber can clear handle that it may have had
pending on a NOTIFY in response the SUBSCRIBE message.The value of the "Suppress-If-Match" header field is an entity-tag,
which is an opaque token that the subscriber simply copies from a
previously received NOTIFY request.Example: Suppress-If-Match: b4cf7The 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.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. Note that there are valid cases for which identical entity-tag
values indeed imply identical event state. For example, it is
possible to generate entity-tag values using a one-way hash
function.Polling with conditional notification allows a user agent to
efficiently poll resource state. This is accomplished using the
Suppress-If-Match condition:The subscriber polls for resource state by sending a SUBSCRIBE
with zero expiry (expires immediately).The notifier accepts the SUBSCRIBE with a 202 (Accepted)
response.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.The subscriber accepts the notification with a 200 (OK)
response.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.The notifier accepts the SUBSCRIBE with a 202 (Accepted)
response.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.The subscriber accepts the notification with a 200 (OK)
response.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:The subscriber attempts to resume an earlier subscription by
including a Suppress-If-Match header field with the entity-tag it
last received.The notifier accepts the subscription after proper
authentication and authorization, by sending a 202 (Accepted)
response.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.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.To refresh a subscription using conditional notification, the
subscriber creates a subscription refresh before the subscription is
about to expire, and uses the Suppress-If-Match header field:Before the subscription is about to expire, the subscriber
sends a SUBSCRIBE request that includes the Suppress-If-Match
header field with the latest entity-tag it has seen.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) indicates the
new expiry time.To terminate a subscription using conditional notification, the
subscriber creates a SUBSCRIBE request with a Suppress-If-Match
condition: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.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.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 fall back behavior.If subscribers encounter transient errors in conditional
notification, they should disable the feature and fall back to normal
subscription behavior.This section augments the notifier behavior as specified in RFC3265.A notifier MUST generate entity-tags for event notifications of all
resources it is responsible for. The entity-tag MUST be unique across
all versions of all entities for a resource and event package.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.An entity-tag is considered valid for as long as the entity is
valid. An entity becomes stale when its version is no longer the
current one. The notifier MUST remember 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.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; there may be several sources to the
event composition process.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 match)
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.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.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.Some event packages may support a scheme where notifications
contain state differentials, or state
deltas instead of complete resource state.A notifier can optionally keep track of the state changes of a
resource, e.g., storing the changes in a journal. If a condition
fails, the notifier MAY 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.When state differentials are sent, the SIP-ETag header field MUST
contain an entity-tag that corresponds to the full resource state.The Event Notification Extension for
Resource Lists 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.This section describes the protocol extensions required for
conditional notification.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 message grammar.The Suppress-If-Match header field is added to the definition of
the "message-header" rule in the SIP
grammar. Its use is described in ,
and .This header field is allowed to appear in any request, but its
behavior is only defined for the SUBSCRIBE request.This section defines the formal syntax for extensions described in
this memo in Augmented BNF (ABNF). The
rules defined here augment and reference the syntax defined in RFC3261 and RFC3903.This document registers a new response code and a new header field
name.Note to IANA and the RFC editor: please replace all occurrences
of RFCXYZ in this section with the RFC number of this specification
upon publication.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 under
http://www.iana.org/assignments/sip-parameters. This information is to be added under "Successful 2xx"
category.Response CodeReference204 No Notification[RFCXYZ]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
under http://www.iana.org/assignments/sip-parameters. Header NameCompactReferenceSuppress-If-Match[RFCXYZ]The security considerations for SIP event notification are
extensively discussed in RFC 3265. This
specification introduces an optimization to SIP event notification,
which in itself does not alter the security properties of the
protocol.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.