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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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
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.
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
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.
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.
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.
(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.
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
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
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).
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.
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.
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:
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.
(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 22.214.171.124 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.
(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:
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.
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.
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.
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].
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] | +-------------------+---------+-----------+
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.
[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: firstname.lastname@example.org Dean Willis (editor) Softarmor Systems 3100 Independence Pkwy #311-164 Plano, TX 75075 USA Phone: +1 214 504 1987 EMail: email@example.com