Session Initiation Protocol A. Niemi Working Group Nokia Internet-Draft July 14, 2008 Intended status: Standards Track Expires: January 15, 2009 An Extension to Session Initiation Protocol (SIP) Events for Conditional Event Notification draft-ietf-sip-subnot-etags-03 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on January 15, 2009. 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 have a serious deficiency in that they 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 Niemi Expires January 15, 2009 [Page 1] Internet-Draft Entity-tags for SIP Events July 2008 is true, either the body of a resulting event notification or the entire notification message is suppressed. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Document Conventions . . . . . . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 2. Motivations and Background . . . . . . . . . . . . . . . . . . 4 2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. Problem Description . . . . . . . . . . . . . . . . . . . 4 2.3. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6 3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 6 4. Resource Model for Entity-Tags . . . . . . . . . . . . . . . . 10 5. Subscriber Behavior . . . . . . . . . . . . . . . . . . . . . 12 5.1. Detecting Support for Conditional Notification . . . . . . 12 5.2. Generating SUBSCRIBE Requests . . . . . . . . . . . . . . 13 5.3. Receiving NOTIFY Requests . . . . . . . . . . . . . . . . 14 5.4. Polling or Fetching Resource State . . . . . . . . . . . . 14 5.5. Resuming a Subscription . . . . . . . . . . . . . . . . . 16 5.6. Refreshing a Subscription . . . . . . . . . . . . . . . . 16 5.7. Terminating a Subscription . . . . . . . . . . . . . . . . 17 5.8. Handling Transient Errors . . . . . . . . . . . . . . . . 18 6. Notifier Behavior . . . . . . . . . . . . . . . . . . . . . . 18 6.1. Generating Entity-tags . . . . . . . . . . . . . . . . . . 18 6.2. Suppressing NOTIFY Bodies . . . . . . . . . . . . . . . . 19 6.3. Suppressing NOTIFY Requests . . . . . . . . . . . . . . . 19 6.4. State Differentials . . . . . . . . . . . . . . . . . . . 19 6.5. List Subscriptions . . . . . . . . . . . . . . . . . . . . 20 7. Protocol Element Definitions . . . . . . . . . . . . . . . . . 20 7.1. 204 (No Notification) Response Code . . . . . . . . . . . 20 7.2. Suppress-If-Match Header Field . . . . . . . . . . . . . . 20 7.3. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . 21 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 8.1. 204 (No Notification) Response Code . . . . . . . . . . . 21 8.2. Suppress-If-Match Header Field . . . . . . . . . . . . . . 22 9. Security Considerations . . . . . . . . . . . . . . . . . . . 22 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22 11.1. Normative References . . . . . . . . . . . . . . . . . . . 22 11.2. Informative References . . . . . . . . . . . . . . . . . . 23 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 23 Intellectual Property and Copyright Statements . . . . . . . . . . 24 Niemi Expires January 15, 2009 [Page 2] Internet-Draft Entity-tags for SIP Events July 2008 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 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 [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 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) [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 backround, 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. Niemi Expires January 15, 2009 [Page 3] Internet-Draft Entity-tags for SIP Events July 2008 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 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 accross 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 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 Niemi Expires January 15, 2009 [Page 4] Internet-Draft Entity-tags for SIP Events July 2008 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: o Large entity bodies in the payloads of notifications o High rate of subscription refreshes o Relatively low rate of notifications triggered by state changes In 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 [RFC2616] 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 [RFC3903] 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. Niemi Expires January 15, 2009 [Page 5] Internet-Draft Entity-tags for SIP Events July 2008 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 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. 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 RFC3261 [RFC3261], RFC3265 [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 accross all versions of all entities for a resource and event package. More on this in Section 4. 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 [RFC3903] request, which applies to whether the PUBLISH succeeds or returns an error, this condition applies to the stream of Niemi Expires January 15, 2009 [Page 6] Internet-Draft Entity-tags for SIP Events July 2008 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. Niemi Expires January 15, 2009 [Page 7] Internet-Draft Entity-tags for SIP Events July 2008 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 ... <-------- (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: Niemi Expires January 15, 2009 [Page 8] Internet-Draft Entity-tags for SIP Events July 2008 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. Niemi Expires January 15, 2009 [Page 9] Internet-Draft Entity-tags for SIP Events July 2008 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 | | | +---------+ Niemi Expires January 15, 2009 [Page 10] Internet-Draft Entity-tags for SIP Events July 2008 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 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: entity-header = Content-Disposition ; defined in RFC 3261 / Content-Encoding ; defined in RFC 3261 / Content-Language ; defined in RFC 3261 / Content-Length ; defined in RFC 3261 / Content-Type ; defined in RFC 3261 / Event ; defined in RFC 3265 / extension-header ; defined in RFC 3261 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 Niemi Expires January 15, 2009 [Page 11] Internet-Draft Entity-tags for SIP Events July 2008 identifies it accross 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 [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 RFC3265 [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); it then describes the workflows for the main three use cases for making the subscription conditional. 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. Niemi Expires January 15, 2009 [Page 12] Internet-Draft Entity-tags for SIP Events July 2008 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 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 suppress (i.e., block) the first NOTIFY request following the SUBSCRIBE, and return a 204 (No Notification) response to the SUBSCRIBE request. 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) response to SUBSCRIBE, it MUST consider the subscription handshake as completed. That is, the subscriber can clear any handle that it may have had pending on a NOTIFY to conclude establishing the subsctiption. 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: 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: Niemi Expires January 15, 2009 [Page 13] Internet-Draft Entity-tags for SIP Events July 2008 Suppress-If-Match: * Such a wildcard condition effectively quenches a subscription; the only notifications received are those reporting changes to the subscription state. Such notifications will also 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. 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. 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: Niemi Expires January 15, 2009 [Page 14] Internet-Draft Entity-tags for SIP Events July 2008 Subscriber Notifier ---------- -------- (1) SUBSCRIBE --------> Expires: 0 <-------- (2) 202 <-------- (3) NOTIFY Subscription-State: terminated SIP-ETag: f2e45 (4) 200 --------> ... poll interval elapses ... (5) SUBSCRIBE --------> Suppress-If-Match: f2e45 Expires: 0 <-------- (6) 204 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 subsciber 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. Since the resource state has not changed since the previous poll occurred, the notifier sends a 204 (No Notification) response, which concludes the poll. Niemi Expires January 15, 2009 [Page 15] Internet-Draft Entity-tags for SIP Events July 2008 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 is about to expire, and uses the Suppress-If-Match header field: Niemi Expires January 15, 2009 [Page 16] Internet-Draft Entity-tags for SIP Events July 2008 Subscriber Notifier ---------- -------- (1) SUBSCRIBE --------> Suppress-If-Match: aba91 Expires: 3600 <-------- (2) 204 Expires: 3600 Figure 5: Refreshing a Subscription 1. 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. 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) 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. Niemi Expires January 15, 2009 [Page 17] Internet-Draft Entity-tags for SIP Events July 2008 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 such B2BUAs always expect to see a NOTIFY method to conclude the dialog establishment as specified in SIP Events [RFC3265], and if this NOTIFY request is suppressed, may terminate or block the subscription. Other problems may also arise, e.g., it is possible that some B2BUA devices treat a NOTIFY with suppressed body as an error. 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. 6. Notifier Behavior This section augments the notifier behavior as specified in RFC3265 [RFC3265]. 6.1. Generating Entity-tags 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 (Section 6.4). Niemi Expires January 15, 2009 [Page 18] Internet-Draft Entity-tags for SIP Events July 2008 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. 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 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. 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, 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. 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 may support a scheme where notifications contain state differentials, or state deltas [RFC3265] instead of complete Niemi Expires January 15, 2009 [Page 19] Internet-Draft Entity-tags for SIP Events July 2008 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. 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. 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 Section 5, Section 6.3 and Section 6.2. This header field is allowed to appear in any request, but its Niemi Expires January 15, 2009 [Page 20] Internet-Draft Entity-tags for SIP Events July 2008 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) [RFC4234]. The rules defined here augment and reference the syntax defined in RFC3261 [RFC3261] and RFC3903 [RFC3903]. Success =/ "204" ; No Notification ; Success is defined in RFC3261. message-header =/ Suppress-If-Match ; message-header is defined in RFC3261. Suppress-If-Match = "Suppress-If-Match" ":" entity-tag / "*" ; entity-tag is defined in RFC3903. 8. IANA Considerations 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. 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 under http://www.iana.org/assignments/sip-parameters. This information is to be added under "Successful 2xx" category. +---------------------+-----------+ | Response Code | Reference | +---------------------+-----------+ | 204 No Notification | [RFCXYZ] | +---------------------+-----------+ Niemi Expires January 15, 2009 [Page 21] Internet-Draft Entity-tags for SIP Events July 2008 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 under http://www.iana.org/assignments/sip-parameters. +-------------------+---------+-----------+ | Header Name | Compact | Reference | +-------------------+---------+-----------+ | Suppress-If-Match | | [RFCXYZ] | +-------------------+---------+-----------+ 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 Niemi Expires January 15, 2009 [Page 22] Internet-Draft Entity-tags for SIP Events July 2008 for Event State Publication", RFC 3903, October 2004. [RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. 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. [RFC4660] Khartabil, H., Leppanen, E., Lonnfors, M., and J. Costa- Requena, "Functional Description of Event Notification Filtering", RFC 4660, September 2006. [RFC4662] Roach, A., Campbell, B., and J. Rosenberg, "A Session Initiation Protocol (SIP) Event Notification Extension for Resource Lists", RFC 4662, August 2006. Author's Address Aki Niemi Nokia P.O. Box 407 NOKIA GROUP, FIN 00045 Finland Phone: +358 50 389 1644 Email: aki.niemi@nokia.com Niemi Expires January 15, 2009 [Page 23] Internet-Draft Entity-tags for SIP Events July 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Niemi Expires January 15, 2009 [Page 24]