Internet Engineering Task Force Adam Roach Internet Draft dynamicsoft Category: Standards Track February 2002 Expires August 2002 SIP-Specific Event Notification Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering 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 cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This document is an individual submission to the IETF. Comments should be directed to the authors. Abstract This document describes an extension to the Session Initiation Protocol (SIP). The purpose of this extension is to provide an extensible framework by which SIP nodes can request notification from remote nodes indicating that certain events have occurred. Concrete uses of the mechanism described in this document may be standardized in the future. Note that the event notification mechanisms defined herein are NOT intended to be a general-purpose infrastructure for all classes of event subscription and notification. 1. Table of Contents 1. Table of Contents...................................... 1 2. Introduction........................................... 3 2.1. Overview of Operation.................................. 4 3. Definitions............................................ 4 Roach [Page 1] Internet Draft SIP-Specific Event Notification February 2002 4. Node Behavior.......................................... 5 4.1. Description of SUBSCRIBE Behavior...................... 5 4.1.1. Subscription duration.................................. 5 4.1.2. Identification of Subscribed Events and Event Classes.. 6 4.1.3. Additional SUBSCRIBE Header Values..................... 6 4.1.4. Subscriber SUBSCRIBE Behavior.......................... 7 4.1.5. Proxy SUBSCRIBE Behavior............................... 8 4.1.6. Notifier SUBSCRIBE Behavior............................ 9 4.2. Description of NOTIFY Behavior......................... 11 4.2.1. Identification of reported events, event classes, and c 12 4.2.2. Notifier NOTIFY Behavior............................... 12 4.2.3. Proxy NOTIFY Behavior.................................. 14 4.2.4. Subscriber NOTIFY Behavior............................. 14 4.3. General................................................ 16 4.3.1. Detecting support for SUBSCRIBE and NOTIFY............. 16 4.3.2. CANCEL requests........................................ 16 4.3.3. Forking................................................ 16 4.3.4. Dialog creation and termination........................ 17 4.3.5. State Agents and Notifier Migration.................... 18 4.3.6. Polling Resource State................................. 18 4.3.7. Allow-Events header usage.............................. 19 4.3.8. PINT Compatibility..................................... 19 5. Event Packages......................................... 19 5.1. Appropriateness of Usage............................... 19 5.2. Event Template-packages................................ 20 5.3. Amount of State to be Conveyed......................... 20 5.3.1. Complete State Information............................. 21 5.3.2. State Deltas........................................... 21 5.4. Event Package Responsibilities......................... 22 5.4.1. Event Package Name..................................... 22 5.4.2. Event Package Parameters............................... 22 5.4.3. SUBSCRIBE Bodies....................................... 22 5.4.4. Subscription Duration.................................. 22 5.4.5. NOTIFY Bodies.......................................... 23 5.4.6. Notifier processing of SUBSCRIBE requests.............. 23 5.4.7. Notifier generation of NOTIFY requests................. 23 5.4.8. Subscriber processing of NOTIFY requests............... 23 5.4.9. Handling of forked requests............................ 23 5.4.10. Rate of notifications.................................. 24 5.4.11. State Agents........................................... 24 5.4.12. Examples............................................... 25 5.4.13. URI List handling...................................... 25 6. Security Considerations................................ 25 6.1. Access Control......................................... 25 6.2. Release of Sensitive Policy Information................ 25 6.3. Denial-of-Service attacks.............................. 26 6.4. Replay Attacks......................................... 26 6.5. Man-in-the middle attacks.............................. 26 6.6. Confidentiality........................................ 27 7. IANA Considerations.................................... 27 Roach [Page 2] Internet Draft SIP-Specific Event Notification February 2002 7.1. Registration Information............................... 28 7.2. Registration Template.................................. 28 7.3. Syntax................................................. 29 7.4. New Methods............................................ 29 7.4.1. SUBSCRIBE method....................................... 31 7.4.2. NOTIFY method.......................................... 31 7.5. New Headers............................................ 31 7.5.1. "Event" header......................................... 31 7.5.2. "Allow-Events" Header.................................. 32 7.5.3. "Subscription-State" Header............................ 32 7.6. New Response Codes..................................... 33 7.6.1. "202 Accepted" Response Code........................... 33 7.6.2. "489 Bad Event" Response Code.......................... 33 8. Changes................................................ 33 8.1. Changes from draft-ietf-...-01......................... 33 8.2. Changes from draft-ietf-...-00......................... 35 8.3. Changes from draft-roach-...-03........................ 36 8.4. Changes from draft-roach-...-02........................ 38 8.5. Changes from draft-roach-...-01........................ 39 9. References............................................. 40 10. Acknowledgements....................................... 41 11. Author's Address....................................... 41 2. Introduction The ability to request asynchronous notification of events proves useful in many types of services for which cooperation between end-nodes is required. Examples of such services include automatic callback services (based on terminal state events), buddy lists (based on user presence events), message waiting indications (based on mailbox state change events), and PINT status (based on call state events). The methods described in this document allow a framework by which notification of these events can be ordered. The event notification mechanisms defined herein are NOT intended to be a general-purpose infrastructure for all classes of event subscription and notification. Meeting requirements for the general problem set of subscription and notification is far too complex for a single protocol. Our goal is to provide a SIP-specific framework for event notification which is not so complex as to be unusable for simple features, but which is still flexible enough to provide powerful services. Note, however, that event packages based on this framework may define arbitrarily complex rules which govern the subscription and notification for the events or classes of events they describe. This draft does not describe an extension which may be used Roach [Page 3] Internet Draft SIP-Specific Event Notification February 2002 directly; it must be extended by other drafts (herein referred to as "event packages.") In object-oriented design terminology, it may be thought of as an abstract base class which must be derived into an instantiatable class by further extensions. Guidelines for creating these extensions are described in section 5. 2.1. Overview of Operation The general concept is that entities in the network can subscribe to resource or call state for various resources or calls in the network, and those entities (or entities acting on their behalf) can send notifications when those states change. A typical flow of messages would be: Subscriber Notifier |-----SUBSCRIBE---->| Request state subscription |<-------200--------| Acknowledge subscription |<------NOTIFY----- | Return current state information |--------200------->| |<------NOTIFY----- | Return current state information |--------200------->| Subscriptions are expired and must be refreshed by subsequent SUBSCRIBE messages. 3. Definitions Event Package: An event package is an additional specification which defines a set of state information to be reported by a notifier to a subscriber. Event packages also define further syntax and semantics based on the framework defined by this document required to convey such state information. Event Template-Package: An event template-package is a special kind of event package which defines a set of state which may be applied to all possible event packages, including itself. Notification: Notification is the act of a notifier sending a NOTIFY message to a subscriber to inform the subscriber of the state of a resource. Notifier: A notifier is a user agent which generates NOTIFY requests for the purpose of notifying subscribers of the state of a resource. Notifiers typically also accept SUBSCRIBE requests to create subscriptions. State Agent: A state agent is a notifier which publishes state information on behalf of a resource; in order to do so, it Roach [Page 4] Internet Draft SIP-Specific Event Notification February 2002 may need gather such state information from multiple sources. State Agents always have complete state information for the resource for which it is creating notifications. Subscriber: A subscriber is a user agent which receives NOTIFY requests from notifiers; these NOTIFY requests contain information about the state of a resource in which the subscriber is interested. Subscribers typically also generate SUBSCRIBE requests and send them to notifiers to create subscriptions. Subscription: A subscription is a set of application state associated with a dialog. This application state includes a pointer to the associated dialog, the event package name, and possibly an identification token. Event packages will define additional subscription state information. By definition, subscriptions exist in both a subscriber and a notifier. Subscription Migration: Subscription migration is the act of moving a subscription from one notifier to another notifier. 4. Node Behavior 4.1. Description of SUBSCRIBE Behavior The SUBSCRIBE method is used to request current state and state updates from a remote node. 4.1.1. Subscription duration SUBSCRIBE requests SHOULD contain an "Expires" header (defined in SIP [1] ). This expires value indicates the duration of the subscription. In order to keep subscriptions effective beyond the duration communicated in the "Expires" header, subscribers need to refresh subscriptions on a periodic basis using a new SUBSCRIBE message on the same dialog as defined in SIP [1] . If no "Expires" header is present in a SUBSCRIBE request, the implied default is defined by the event package being used. 200-class responses to SUBSCRIBE requests also MUST contain an "Expires" header. The period of time in the response MAY be shorter or longer than specified in the request. The period of time in the response is the one which defines the duration of the subscription. An "expires" parameter on the "Contact" header has no semantics for SUBSCRIBE and is explicitly not equivalent to an "Expires" header in a SUBSCRIBE request or response. Roach [Page 5] Internet Draft SIP-Specific Event Notification February 2002 A natural consequence of this scheme is that a SUBSCRIBE with an "Expires" of 0 constitutes a request to unsubscribe from an event. Notifiers may also wish to cancel subscriptions to events; this is useful, for example, when the resource to which a subscription refers is no longer available. Further details on this mechanism are discussed in section 4.2.2. 4.1.2. Identification of Subscribed Events and Event Classes Identification of events is provided by three pieces of information: Request URI, Event Type, and (optionally) message body. The Request URI of a SUBSCRIBE request, most importantly, contains enough information to route the request to the appropriate entity. It also contains enough information to identify the resource for which event notification is desired, but not necessarily enough information to uniquely identify the nature of the event (e.g. "sip:adam@dynamicsoft.com" would be an appropriate URI to subscribe to for my presence state; it would also be an appropriate URI to subscribe to the state of my voice mailbox). Subscribers MUST include exactly one "Event" header in SUBSCRIBE requests, indicating to which event or class of events they are subscribing. The "Event" header will contain a token which indicates the type of state for which a subscription is being requested. This token will be registered with the IANA and will correspond to an event package which further describes the semantics of the event or event class. The "Event" header MAY also contain an "id" parameter. This "id" parameter, if present, contains an opaque token which identifies the specific subscription within a dialog. An "id" parameter is only valid within the scope of a single dialog. If the event package to which the event token corresponds defines behavior associated with the body of its SUBSCRIBE requests, those semantics apply. Event packages may also define parameters for the Event header; if they do so, they must define the semantics for such parameters. 4.1.3. Additional SUBSCRIBE Header Values Because SUBSCRIBE requests create a dialog as defined in SIP [1] , they MAY contain an "Accept" header. This header, if present, indicates the body formats allowed in subsequent NOTIFY requests. Roach [Page 6] Internet Draft SIP-Specific Event Notification February 2002 Event packages MUST define the behavior for SUBSCRIBE requests without "Accept" headers; usually, this will connote a single, default body type. Header values not described in this document are to be interpreted as described in SIP [1] . 4.1.4. Subscriber SUBSCRIBE Behavior 4.1.4.1. Requesting a Subscription SUBSCRIBE is a dialog-creating method, as described in SIP [1] . When a subscriber wishes to subscribe to a particular state for a resource, it forms a SUBSCRIBE message. If the initial SUBSCRIBE represents a request outside of a dialog (as it typically will), its construction follows the procedures outlined in SIP [1] for UAC request generation outside of a dialog. This SUBSCRIBE request will be confirmed with a final response. 200-class responses indicate that the subscription has been accepted, and that a NOTIFY will be sent immediately. A 200 response indicates that the subscription has been accepted and that the user is authorized to subscribe to the requested resource. A 202 response merely indicates that the subscription has been understood, and that authorization may or may not have been granted. The "Expires" header in a 200-class response to SUBSCRIBE indicates the actual duration for which the subscription will remain active (unless refreshed). Non-200 class final responses indicate that no subscription or dialog has been created, and no subsequent NOTIFY message will be sent. All non-200 class responses (with the exception of "489," described herein) have the same meanings and handling as described in SIP [1] . A SUBSCRIBE request MAY include an "id" parameter in its "Event" header to allow differentiation between multiple subscriptions in the same dialog. 4.1.4.2. Refreshing of Subscriptions At any time before a subscription expires, the subscriber may refresh the timer on such a subscription by sending another SUBSCRIBE request on the same dialog as the existing subscription, and with the same "Event" header "id" parameter (if one was present in the initial subscription). The handling for such a request is the same as for the initial creation of a Roach [Page 7] Internet Draft SIP-Specific Event Notification February 2002 subscription except as described below. If the initial SUBSCRIBE message contained an "id" parameter on the "Event" header, then refreshes of the subscription must also contain an identical "id" parameter; they will otherwise be considered new subscriptions in an existing dialog. If a SUBSCRIBE request to refresh a subscription receives a "481" response, this indicates that the subscription has been terminated and that the subscriber did not receive notification of this fact. In this case, the subscriber should consider the subscription invalid. If the subscriber wishes to re-subscribe to the state, he does so by composing an unrelated initial SUBSCRIBE request with a freshly-generated Call-ID and a new, unique "From" tag (see section 4.1.4.1. ) If a SUBSCRIBE request to refresh a subscription fails with a non-481 response, the original subscription is still considered valid for the duration of the most recently known "Expires" value as negotiated by SUBSCRIBE and its response, or as communicated by NOTIFY in the "Subscription-State" header "expires" parameter. 4.1.4.3. Unsubscribing Unsubscribing is handled in the same way as refreshing of a subscription, with the "Expires" header set to "0." Note that a successful unsubscription will also trigger a final "NOTIFY". 4.1.4.4. Confirmation of Subscription Creation The subscriber can expect to receive a NOTIFY message from each node which has processed a successful subscription or subscription refresh. Until the first NOTIFY message arrives, the subscriber should consider the state of the subscribed resource to be in a neutral state. Event packages which define new event packages MUST define this "neutral state" in such a way that makes sense for their application (see section 5.4.7. ). Due to the potential for both out-of-order messages and forking, the subscriber MUST be prepared to receive NOTIFY messages before the SUBSCRIBE transaction has completed. Except as noted above, processing of this NOTIFY is the same as in section 4.2.4. 4.1.5. Proxy SUBSCRIBE Behavior Proxies need no additional behavior beyond that described in SIP [1] to support SUBSCRIBE. If a proxy wishes to see all of the Roach [Page 8] Internet Draft SIP-Specific Event Notification February 2002 SUBSCRIBE and NOTIFY requests for a given dialog, it MUST record-route all SUBSCRIBE and NOTIFY requests. 4.1.6. Notifier SUBSCRIBE Behavior 4.1.6.1. Initial SUBSCRIBE Transaction Processing In no case should a SUBSCRIBE transaction extend for any longer than the time necessary for automated processing. In particular, notifiers MUST NOT wait for a user response before returning a final response to a SUBSCRIBE request. This requirement is imposed primarily to prevent timer F from firing during the SUBSCRIBE transaction, since interaction with a user would often exceed 64*T1 seconds. The notifier SHOULD check that the event package specified in the "Event" header is understood. If not, the notifier SHOULD return a "489 Bad Event" response to indicate that the specified event/event class is not understood. The notifier SHOULD also perform any necessary authentication and authorization per its local policy. See section 4.1.6.3. If the notifier is able to immediately determine that it understands the event package, that the authenticated subscriber is authorized to subscribe, and that there are no other barriers to creating the subscription, it creates the subscription and a dialog (if necessary), and returns a "200 OK" response (unless doing so would reveal authorization policy in an undesirable fashion; see section 6.2. ). If the notifier cannot immediately create the subscription (e.g. it needs to wait for user input for authorization, or is acting for another node which is not currently reachable), or wishes to mask authorization policy, it will return a "202 Accepted" response. This response indicates that the request has been received and understood, but does not necessarily imply that the subscription has been authorized yet. When a subscription is created in the notifier, it stores the event package name and the "Event" header "id" parameter (if present) as part of the subscription information. The "Expires" values present in SUBSCRIBE 200-class responses behave in the same way as they do in REGISTER responses: the server MAY shorten the interval, but MUST NOT lengthen it. If the duration specified in a SUBSCRIBE message is unacceptably short, the notifier SHOULD respond with a "423 Subscription Too Brief" message. Roach [Page 9] Internet Draft SIP-Specific Event Notification February 2002 200-class responses to SUBSCRIBE requests will not generally contain any useful information beyond subscription duration; their primary purpose is to serve as a reliability mechanism. State information will be communicated via a subsequent NOTIFY request from the notifier. The other response codes defined in SIP [1] may be used in response to SUBSCRIBE requests, as appropriate. 4.1.6.2. Confirmation of Subscription Creation/Refreshing Upon successfully accepting or refreshing of a subscription, notifiers MUST send a NOTIFY message immediately to communicate the current resource state to the subscriber. This NOTIFY message is sent on the same dialog as created by the SUBSCRIBE response. If the resource has no meaningful state at the time that the SUBSCRIBE message is processed, this NOTIFY message MAY contain an empty or neutral body. See section 4.2.2. for further details on NOTIFY message generation. Note that a NOTIFY message is always sent immediately after any 200-class response to a SUBSCRIBE request, regardless of whether the subscription has already been authorized. 4.1.6.3. Authentication/Authorization of SUBSCRIBE requests Privacy concerns may require that notifiers apply policy to determine whether a particular subscriber is authorized to subscribe to a certain set of events. Such policy may be defined by mechanisms such as access control lists or real-time interaction with a user. In general, authorization of subscribers prior to authentication is not particularly useful. SIP authentication mechanisms are discussed in SIP [1] . Note that, even if the notifier node typically acts as a proxy, authentication for SUBSCRIBE requests will always be performed via a "401" response, not a "407;" notifiers always act as a user agents when accepting subscriptions and sending notifications. If authorization fails based on an access list or some other automated mechanism (i.e. it can be automatically authoritatively determined that the subscriber is not authorized to subscribe), the notifier SHOULD reply to the request with a "403 Forbidden" or "603 Decline" response, unless doing so might reveal information that should stay private; see section 6.2. If the notifier owner is interactively queried to determine whether a subscription is allowed, a "202 Accept" response is returned immediately. Note that a NOTIFY message is still formed Roach [Page 10] Internet Draft SIP-Specific Event Notification February 2002 and sent under these circumstances, as described in the previous section. If subscription authorization was delayed and the notifier wishes to convey that such authorization has been declined, it may do so by sending a NOTIFY message containing a "Subscription-State" header with a value of "terminated" and a reason parameter of "rejected." 4.1.6.4. Refreshing of Subscriptions When a notifier receives a subscription refresh, assuming that the subscriber is still authorized, the notifier updates the expiration time for subscription. As with the initial subscription, the server MAY shorten the amount of time until expiration, but MUST NOT increase it. The final expiration time is placed in the "Expires" header in the response. If the duration specified in a SUBSCRIBE message is unacceptably short, the notifier SHOULD respond with a "423 Subscription Too Brief" message. If no refresh for a notification address is received before its expiration time, the subscription is removed. When removing a subscription, the notifier SHOULD send a NOTIFY message with a "Subscription-State" value of "terminated" to inform it that the subscription is being removed. If such a message is sent, the "Subscription-State" header SHOULD contain a "reason=timeout" parameter. The sending of a NOTIFY when a subscription expires allows the corresponding dialog to be terminated, if appropriate. 4.2. Description of NOTIFY Behavior NOTIFY messages are sent to inform subscribers of changes in state to which the subscriber has a subscription. Subscriptions are typically put in place using the SUBSCRIBE method; however, it is possible that other means have been used. If any non-SUBSCRIBE mechanisms are defined to create subscriptions, it is the responsibility of the parties defining those mechanisms to ensure that correlation of a NOTIFY message to the corresponding subscription is possible. Designers of such mechanisms are also warned to make a distinction between sending a NOTIFY message to a subscriber who is aware of the subscription, and sending a NOTIFY message to an unsuspecting node. The latter behavior is invalid, and MUST receive a "481 Subscription does not exist" response (unless some other 400- or 500-class error code is more applicable), as described in section 4.2.4. In other words, knowledge of a subscription must exist in Roach [Page 11] Internet Draft SIP-Specific Event Notification February 2002 both the subscriber and the notifier to be valid, even if installed via a non-SUBSCRIBE mechanism. A NOTIFY does not terminate its corresponding subscription; in other words, a single SUBSCRIBE request may trigger several NOTIFY requests. 4.2.1. Identification of reported events, event classes, and current state Identification of events being reported in a notification is very similar to that described for subscription to events (see section 4.1.2. ). As in SUBSCRIBE requests, NOTIFY "Event" headers will contain a single event package name for which a notification is being generated. The package name in the "Event" header MUST match the "Event" header in the corresponding SUBSCRIBE message. If an "id" parameter was present in the SUBSCRIBE message, that "id" parameter MUST also be present in the corresponding NOTIFY messages. If the event package to which the event package name corresponds defines behavior associated with the body of its NOTIFY requests, those semantics apply. This information is expected to provide additional details about the nature of the event which has occurred and the resultant resource state. When present, the body of the NOTIFY request MUST be formatted into one of the body formats specified in the "Accept" header of the corresponding SUBSCRIBE request. This body will contain either the state of the subscribed resource or a pointer to such state in the form of a URI. 4.2.2. Notifier NOTIFY Behavior When a SUBSCRIBE request is answered with a 200-class response, the notifier MUST immediately construct and send a NOTIFY request to the subscriber. When a change in the subscribed state occurs, the notifier SHOULD immediately construct and send a NOTIFY request, subject to authorization, local policy, and throttling considerations. A NOTIFY request is considered failed if the response times out, or a non-200 class response code is received which has no "Retry-After" header and no implied further action which can be taken to retry the request (e.g. "401 Authorization Required.") If the NOTIFY request fails (as defined above) due to a timeout condition, and the subscription was installed using a soft-state Roach [Page 12] Internet Draft SIP-Specific Event Notification February 2002 mechanism (such as SUBSCRIBE), the notifier SHOULD remove the subscription. This behavior prevents unnecessary transmission of state information for subscribers who have crashed or disappeared from the network. Because such transmissions will be sent 11 times (instead of the typical single transmission for functioning clients), continuing to service them when no client is available to acknowledge them could place undue strain on a network. Upon client restart or reestablishment of a network connection, it is expected that clients will send SUBSCRIBE messages to refresh potentially stale state information; such messages will re-install subscriptions in all relevant nodes. If the NOTIFY request fails (as defined above) due to an error response, and the subscription was installed using a soft-state mechanism, the notifier MUST remove the corresponding subscription. A notify error response would generally indicate that something has gone wrong with the subscriber or with some proxy on the way to the subscriber. If the subscriber is in error, it makes the most sense to allow the subscriber to rectify the situation (by re-subscribing) once the error condition has been handled. If a proxy is in error, the periodic SUBSCRIBE refreshes will re-install subscription state once the network problem has been resolved. If a NOTIFY request receives a 481 response, the notifier MUST remove the corresponding subscription even if such subscription was installed by non-SUBSCRIBE means (such as an administrative interface). If the above behavior were not required, subscribers receiving a notify for an unknown subscription would need to send an error status code in response to the NOTIFY and also send a SUBSCRIBE request to remove the subscription. Since this behavior would make subscribers available for use as amplifiers in denial of service attacks, we have instead elected to give the 481 response special meaning: it is used to indicate that a subscription must be cancelled under all circumstances. NOTIFY requests MUST contain a "Subscription-State" header with a value of "active," "pending," or "terminated." The "active" value indicates that the subscription has been accepted and has been authorized (in most cases; see section 6.2. ). The "pending" value indicates that the subscription has been received, but that policy information is insufficient to accept or deny the Roach [Page 13] Internet Draft SIP-Specific Event Notification February 2002 subscription at this time. The "terminated" value indicates that the subscription is not active. If the value of the "Subscription-State" header is "active" or "pending," the notifier SHOULD also include in the "Subscription-State" header an "expires" parameter which indicates the time remaining on the subscription. The notifier MAY use this mechanism to shorten a subscription; however, this mechanism MUST NOT be used to lengthen a subscription. Including expiration information for active and pending subscriptions is useful in case the SUBSCRIBE request forks, since the response to a forked SUBSCRIBE may not be received by the subscriber. Note well that this "expires" value is a parameter on the "Subscription-State" header, NOT an "Expires" header. If the value of the "Subscription-State" header is "terminated," the notifier SHOULD also include a "reason" parameter. The notifier MAY also include a "retry-after" parameter, where appropriate. For details on the value and semantics of the "reason" and "retry-after" parameters, see section 4.2.4. 4.2.3. Proxy NOTIFY Behavior Proxies need no additional behavior beyond that described in SIP [1] to support NOTIFY. If a proxy wishes to see all of the SUBSCRIBE and NOTIFY requests for a given dialog, it MUST record-route all SUBSCRIBE and NOTIFY requests. 4.2.4. Subscriber NOTIFY Behavior Upon receiving a NOTIFY request, the subscriber should check that it matches at least one of its outstanding subscriptions; if not, it MUST return a "481 Subscription does not exist" response unless another 400- or 500-class response is more appropriate. The rules for matching NOTIFY requests with subscriptions that create a new dialog is described in section 4.3.4. Notifications for subscriptions which were created inside an existing dialog match if they are in the same dialog and the "Event" headers match (as described in section 7.5.1. ) If, for some reason, the event package designated in the "Event" header of the NOTIFY request is not supported, the subscriber will respond with a "489 Bad Event" response. To prevent spoofing of events, NOTIFY requests SHOULD be authenticated, using any defined SIP authentication mechanism. NOTIFY requests MUST contain "Subscription-State" headers which Roach [Page 14] Internet Draft SIP-Specific Event Notification February 2002 indicate the status of the subscription. If the "Subscription-State" header value is "active," it means that the subscription has been accepted and (in general) has been authorized. If the header also contains an "expires" parameter, the subscriber SHOULD take it as the authoritative subscription duration and adjust accordingly. The "retry-after" and "reason" parameters have no semantics for "active." If the "Subscription-State" value is "pending," the subscription has been received by the notifier, but there is insufficient policy information to grant or deny the subscription yet. If the header also contains an "expires" parameter, the subscriber SHOULD take it as the authoritative subscription duration and adjust accordingly. No further action is necessary on the part of the subscriber. The "retry-after" and "reason" parameters have no semantics for "pending." If the "Subscription-State" value is "terminated," the subscriber should consider the subscription terminated. The "expires" parameter has no semantics for "terminated." If a reason code is present, the client should behave as described below. If no reason code or an unknown reason code is present, the client MAY attempt to re-subscribe at any time (unless a "retry-after" parameter is present, in which case the client SHOULD NOT attempt re-subscription until after the number of seconds specified by the "retry-after" parameter). The defined reason codes are: deactivated: The subscription has been terminated, but the client SHOULD retry immediately with a new subscription. One primary use of such a status code is to allow migration of subscriptions between nodes. The "retry-after" parameter has no semantics for "deactivated." probation: The subscription has been terminated, but the client SHOULD retry at some later time. If a "retry-after" parameter is also present, the client SHOULD wait at least the number of seconds specified by that parameter before attempting to re-subscribe. rejected: The subscription has been terminated due to change in authorization policy. Clients SHOULD NOT attempt to re-subscribe. The "retry-after" parameter has no semantics for "rejected." timeout: The subscription has been terminated because it was not refreshed before it expired. Clients MAY re-subscribe immediately. The "retry-after" parameter has no semantics for "timeout." Roach [Page 15] Internet Draft SIP-Specific Event Notification February 2002 giveup: The subscription has been terminated because the notifier could not obtain authorization in a timely fashion. If a "retry-after" parameter is also present, the client SHOULD wait at least the number of seconds specified by that parameter before attempting to re-subscribe; otherwise, the client MAY retry immediately, but will likely get put back into pending state. Once the notification is deemed acceptable to the subscriber, the subscriber SHOULD return a 200 response. In general, it is not expected that NOTIFY responses will contain bodies; however, they MAY, if the NOTIFY request contained an "Accept" header. Other responses defined in SIP [1] may also be returned, as appropriate. 4.3. General 4.3.1. Detecting support for SUBSCRIBE and NOTIFY Neither SUBSCRIBE nor NOTIFY necessitate the use of "Require" or "Proxy-Require" headers; similarly, there is no token defined for "Supported" headers. If necessary, clients may probe for the support of SUBSCRIBE and NOTIFY using the OPTIONS request defined in SIP [1] . The presence of the "Allow-Events" header in a message is sufficient to indicate support for SUBSCRIBE and NOTIFY. The "methods" parameter for Contact may also be used to specifically announce support for SUBSCRIBE and NOTIFY messages when registering. (See reference [7] for details on the "methods" parameter). 4.3.2. CANCEL requests No semantics are associated with cancelling SUBSCRIBE or NOTIFY. 4.3.3. Forking Successful SUBSCRIBE requests will receive only one 200-class response; however, due to forking, the subscription may have been accepted by multiple nodes. The subscriber MUST therefore be prepared to receive NOTIFY requests with "From:" tags which differ from the "To:" tag received in the SUBSCRIBE 200-class response. If multiple NOTIFY messages are received in response to a single SUBSCRIBE message, they represent different destinations to which the SUBSCRIBE request was forked. For information on subscriber Roach [Page 16] Internet Draft SIP-Specific Event Notification February 2002 handling in such situations, see section 5.4.9. 4.3.4. Dialog creation and termination If an initial SUBSCRIBE request is not sent on an pre-existing dialog, the subscriber will wait for a response to the SUBSCRIBE request or a matching NOTIFY. Responses are matched to such SUBSCRIBE requests if they contain the same the same "Call-ID," the same "From" header field, the same "To" header field, excluding the "tag," and the same "CSeq." Rules for the comparison of these headers are described in SIP [1] . If a 200-class response matches such a SUBSCRIBE request, it creates a new subscription and a new dialog (unless they have already been created by a matching NOTIFY request; see below). NOTIFY requests are matched to such SUBSCRIBE requests if they contain the same "Call-ID," a "From" header field which matches the "To" header field of the SUBSCRIBE, excluding the "tag," a "To" header field which matches the "From" header field of the SUBSCRIBE, and the same "Event" header field. Rules for comparisons of the "Event" headers are described in section 7.5.1. If a matching NOTIFY request contains a "Subscription-State" of "active" or "pending," it creates a new subscription and a new dialog (unless the have already been created by a matching response, as described above). If an initial SUBSCRIBE is sent on a pre-existing dialog, a matching 200-class response or successful NOTIFY request merely creates a new subscription associated with that dialog. Multiple subscriptions can be associated with a single dialog. Subscriptions may also exist in dialogs associated with INVITE-created application state and other application state created by mechanisms defined in other specifications. These sets of application state do not interact beyond the behavior described for a dialog (e.g. route set handling). A subscription is destroyed when a notifier sends a NOTIFY request with a "Subscription-State" of "terminated". A subscriber may send a SUBSCRIBE request with an "Expiration" header of 0 in order to trigger the sending of such a NOTIFY request; however, for the purposes of subscription and dialog lifetime, the subscription is not considered terminated until the NOTIFY with a "Subscription-State" of "terminated" is sent. If a subscription's destruction leaves no other application state associated with the dialog, the dialog terminates. The Roach [Page 17] Internet Draft SIP-Specific Event Notification February 2002 destruction of other application state (such as that created by an INVITE) will not terminate the dialog if a subscription is still associated with that dialog. Note that the above behavior means that a dialog created with an INVITE does not necessarily terminate upon receipt of a BYE. 4.3.5. State Agents and Notifier Migration When state agents (see section 5.4.11. ) are used, it is often useful to allow migration of subscriptions between state agents and the nodes for which they are providing state aggregation (or even among various state agents). Such migration may be effected by sending a "NOTIFY" with an "Subscription-State" header of "terminated," and a reason parameter of "deactivated." This NOTIFY request is otherwise normal, and is formed as described in section 4.2.2. Upon receipt of this NOTIFY message, the subscriber SHOULD attempt to re-subscribe (as described in the preceding sections). Note that this subscription is established on a new dialog, and does not re-use the route set from the previous subscription dialog. The actual migration is effected by making a change to the policy (such as routing decisions) of one or more servers to which the SUBSCRIBE request will be sent in such a way that a different node ends up responding to the SUBSCRIBE request. This may be as simple as a change in the local policy in the notifier from which the subscription is migrating so that it serves as a proxy or redirect server instead of a notifier. Whether, when, and why to perform notifier migrations may be described in individual event packages; otherwise, such decisions are a matter of local notifier policy, and are left up to individual implementations. 4.3.6. Polling Resource State A natural consequence of the behavior described in the preceding sections is that an immediate fetch without a persistent subscription may be effected by sending a SUBSCRIBE with an "Expires" of 0. Of course, an immediate fetch while a subscription is active may be effected by sending a SUBSCRIBE with an "Expires" equal to the number of seconds remaining in the subscription. Upon receipt of this SUBSCRIBE request, the notifier (or Roach [Page 18] Internet Draft SIP-Specific Event Notification February 2002 notifiers, if the SUBSCRIBE request was forked) will send a NOTIFY request containing resource state in the same dialog. Note that the NOTIFY messages triggered by SUBSCRIBE messages with "Expire" headers of 0 will contain a "Subscription-State" value of "terminated," and a "reason" parameter of "timeout." 4.3.7. Allow-Events header usage The "Allow-Events" header, if present, includes a list of tokens which indicates the event packages supported by the client (if sent in a request) or server (if sent in a response). In other words, a node sending an "Allow-Events" header is advertising that it can process SUBSCRIBE requests and generate NOTIFY requests for all of the event packages listed in that header. Any node implementing one or more event packages SHOULD include an appropriate "Allow-Events" header indicating all supported events in all methods which initiate dialogs and their responses (such as INVITE) and OPTIONS responses. This information is very useful, for example, in allowing user agents to render particular interface elements appropriately according to whether the events required to implement the features they represent are supported by the appropriate nodes. Note that "Allow-Events" headers MUST NOT be inserted by proxies. 4.3.8. PINT Compatibility The "Event" header is considered mandatory for the purposes of this document. However, to maintain compatibility with PINT (see [3] ), servers MAY interpret a SUBSCRIBE request with no "Event" header as requesting a subscription to PINT events. If a server does not support PINT, it SHOULD return "489 Bad Event" to any SUBSCRIBE messages without an "Event" header. 5. Event Packages This section covers several issues which should be taken into consideration when event packages based on SUBSCRIBE and NOTIFY are proposed. 5.1. Appropriateness of Usage When designing an event package using the methods described in this draft for event notification, it is important to consider: is SIP an appropriate mechanism for the problem set? Is SIP being selected because of some unique feature provided by the protocol (e.g. user mobility), or merely because "it can be done?" If you Roach [Page 19] Internet Draft SIP-Specific Event Notification February 2002 find yourself defining event packages for notifications related to, for example, network management or the temperature inside your car's engine, you may want to reconsider your selection of protocols. Those interested in extending the mechanism defined in this document are urged to read "Guidelines for Authors of SIP Extensions" [2] for further guidance regarding appropriate uses of SIP. Further, it is expected that this mechanism is not to be used in applications where the frequency of reportable events is excessively rapid (e.g. more than about once per second). A SIP network is generally going to be provisioned for a reasonable signalling volume; sending a notification every time a user's GPS position changes by one hundreth of a second could easily overload such a network. 5.2. Event Template-packages Normal event packages define a set of state applied to a specific type of resource, such as user presence, call state, and messaging mailbox state. Event template-packages are a special type of package which define a set of state applied to other packages, such as statistics, access policy, and subscriber lists. Event template-packages may even be applied to other event template-packages. To extend the object-oriented analogy made earlier, event template-packages can be thought of as templatized C++ packages which must be applied to other packages to be useful. The name of an event template-package as applied to a package is formed by appending a period followed by the event template-package name to the end of the package. For example, if a template-package called "winfo" were being applied to a package called "presence," the event token used in "Event" and "Allow-Events" would be "presence.winfo". Event template-packages must be defined so that they can be applied to any arbitrary package. In other words, event template-packages cannot be specifically tied to one or a few "parent" packages in such a way that they will not work with other packages. 5.3. Amount of State to be Conveyed When designing event packages, it is important to consider the Roach [Page 20] Internet Draft SIP-Specific Event Notification February 2002 type of information which will be conveyed during a notification. A natural temptation is to convey merely the event (e.g. "a new voice message just arrived") without accompanying state (e.g. "7 total voice messages"). This complicates implementation of subscribing entities (since they have to maintain complete state for the entity to which they have subscribed), and also is particularly susceptible to synchronization problems. There are two possible solutions to this problem that event packages may choose to implement. 5.3.1. Complete State Information For packages which typically convey state information that is reasonably small (on the order of 1 kb or so), it is suggested that event packages are designed so as to send complete state information when an event occurs. In the circumstances that state may not be sufficient for a particular class of events, the event packages should include complete state information along with the event that occurred. For example, "no customer service representatives available" may not be as useful "no customer service representatives available; representative sip:46@cs.xyz.int just logged off". 5.3.2. State Deltas In the case that the state information to be conveyed is large, the event package may choose to detail a scheme by which NOTIFY messages contain state deltas instead of complete state. Such a scheme would work as follows: any NOTIFY sent in immediate response to a SUBSCRIBE contains full state information. NOTIFY messages sent because of a state change will contain only the state information that has changed; the subscriber will then merge this information into its current knowledge about the state of the resource. Any event package that supports delta changes to states MUST use a payload which contains a version number that increases by exactly one for each NOTIFY message. Note that the state version number appears in the body of the message, not in a SIP header. If a NOTIFY arrives that has a version number that is incremented by more than one, the subscriber knows that a state delta has been missed; it ignores the NOTIFY message containing the state delta (except for the version number, which it retains to detect message loss), and re-sends a SUBSCRIBE to force a NOTIFY containing a complete state snapshot. Roach [Page 21] Internet Draft SIP-Specific Event Notification February 2002 5.4. Event Package Responsibilities Event packages are not required to re-iterate any of the behavior described in this document, although they may choose to do so for clarity or emphasis. In general, though, such packages are expected to describe only the behavior that extends or modifies the behavior described in this document. Note that any behavior designated with "SHOULD" or "MUST" in this document is not allowed to be changed by extension documents; however, such documents may elect to strengthen "SHOULD" requirements to "MUST" strength if required by their application. In addition to the normal sections expected by "Instructions to RFC Authors" [5] and "Guidelines for Authors of SIP Extensions" [2] , authors of event packages MUST address each of the issues detailed in the following subsections, whenever applicable. 5.4.1. Event Package Name This mandatory section of an event package defines the token name to be used to designate the event package. It MUST include the information which appears in the IANA registration of the token. For information on registering such types, see section 7. 5.4.2. Event Package Parameters If parameters are to be used on the "Event" header to modify the behavior of the event package, the syntax and semantics of such headers MUST be clearly defined. 5.4.3. SUBSCRIBE Bodies It is expected that most, but not all, event packages will define syntax and semantics for SUBSCRIBE method bodies; these bodies will typically modify, expand, filter, throttle, and/or set thresholds for the class of events being requested. Designers of event packages are strongly encouraged to re-use existing MIME types for message bodies where practical. This mandatory section of an event package defines what type or types of event bodies are expected in SUBSCRIBE requests (or specify that no event bodies are expected). It should point to detailed definitions of syntax and semantics for all referenced body types. 5.4.4. Subscription Duration It is RECOMMENDED that event packages give a suggested range of Roach [Page 22] Internet Draft SIP-Specific Event Notification February 2002 times considered reasonable for the duration of a subscription. Such packages MUST also define a default "Expires" value to be used if none is specified. 5.4.5. NOTIFY Bodies The NOTIFY body is used to report state on the resource being monitored. Each package MUST define a what type or types of event bodies are expected in NOTIFY requests. Such packages MUST specify or cite detailed specifications for the syntax and semantics associated with such event body. Event packages also MUST define which MIME type is to be assumed if none are specified in the "Accept" header of the SUBSCRIBE request. 5.4.6. Notifier processing of SUBSCRIBE requests This section describes the processing to be performed by the notifier upon receipt of a SUBSCRIBE request. Such a section is required. Information in this section includes details of how to authenticate subscribers and authorization issues for the package. Such authorization issues may include, for example, whether all SUBSCRIBE requests for this package are answered with 202 responses (see section 6.2. ). 5.4.7. Notifier generation of NOTIFY requests This section of an event package describes the process by which the notifier generates and sends a NOTIFY request. This includes detailed information about what events cause a NOTIFY to be sent, how to compute the state information in the NOTIFY, how to generate "neutral" or "fake" state information to hide authorization delays and decisions from users, and whether state information is complete or deltas for notifications (see section 5.3. ) It may optionally describe the behavior used to processes the subsequent response. Such a section is required. 5.4.8. Subscriber processing of NOTIFY requests This section of an event package describes the process followed by the subscriber upon receipt of a NOTIFY request, including any logic required to form a coherent resource state (if applicable). 5.4.9. Handling of forked requests Roach [Page 23] Internet Draft SIP-Specific Event Notification February 2002 Each event package SHOULD specify whether forked SUBSCRIBE requests are allowed to install multiple subscriptions. If such behavior is not allowed, the first potential dialog-establishing message will create a dialog. All subsequent NOTIFY messages which correspond to the SUBSCRIBE message (i.e. match To, From, From tag, Call-ID, CSeq, Event, and Event id) but which do not match the dialog would be rejected with a 481 response. If installing of multiple subscriptions by way of a single forked INVITE is allowed, the subscriber establishes a new dialog towards each notifier by returning a 200-class response to each NOTIFY. Each dialog is then handled as its own entity, and is refreshed independent of the other dialogs. In the case that multiple subscriptions are allowed, the event package MUST specify whether merging of the notifications to form a single state is required, and how such merging is to be performed. Note that it is possible that some event packages may be defined in such a way that each dialog is tied to a mutually exclusive state which is unaffected by the other dialogs; this MUST be clearly stated if it is the case. If the event package does not specify which mode of operation to use, the subscriber may employ either mode of operation. 5.4.10. Rate of notifications Each event package is expected to define a requirement (SHOULD or MUST strength) which defines an absolute maximum on the rate at which notifications are allowed to be generated by a single notifier. Such packages MAY further define a throttle mechanism which allows subscribers to further limit the rate of notification. 5.4.11. State Agents Designers of event packages should consider whether their package can benefit from network aggregation points ("State Agents") and/or nodes which act on behalf of other nodes. (For example, nodes which provide state information about a resource when such a resource is unable or unwilling to provide such state information itself). An example of such an application is a node which tracks the presence and availability of a user in the network. If state agents are to be used by the package, the package MUST specify how such state agents aggregate information and how they Roach [Page 24] Internet Draft SIP-Specific Event Notification February 2002 provide authentication and authorization. Event packages MAY also outline specific scenarios under which notifier migrations take place. 5.4.12. Examples Event packages SHOULD include several demonstrative message flow diagrams paired with several typical, syntactically correct and complete messages. It is RECOMMENDED that documents describing event packages clearly indicate that such examples are informative and not normative, with instructions that implementors refer to the main text of the draft for exact protocol details. 5.4.13. URI List handling Some types of event packages may define state information which is potentially too large to reasonably send in a SIP message. To alleviate this problem, event packages may include the ability to use a MIME body type of "application/uri-list" in NOTIFY messages. The URI or URIs contained in the NOTIFY body will then be used to retrieve the actual state information. If an event package elects to use this mechanism, it MUST define at least one baseline scheme (e.g. http) which is mandatory to support, as well as one mandatory baseline data format for the data so retrieved. Event packages using URIs to retrieve state information also MUST address the duration of the validity of the URIs passed to a subscriber in this fashion. 6. Security Considerations 6.1. Access Control The ability to accept subscriptions should be under the direct control of the user, since many types of events may be considered sensitive for the purposes of privacy. Similarly, the notifier should have the ability to selectively reject subscriptions based on the subscriber identity (based on access control lists), using standard SIP authentication mechanisms. The methods for creation and distribution of such access control lists is outside the scope of this draft. 6.2. Release of Sensitive Policy Information The mere act of returning a 200 or certain 4xx and 6xx responses to SUBSCRIBE requests may, under certain circumstances, create privacy concerns by revealing sensitive policy information. In Roach [Page 25] Internet Draft SIP-Specific Event Notification February 2002 these cases, the notifier SHOULD always return a 202 response. While the subsequent NOTIFY message may not convey true state, it MUST appear to contain a potentially correct piece of data from the point of view of the subscriber, indistinguishable from a valid response. Information about whether a user is authorized to subscribe to the requested state is never conveyed back to the original user under these circumstances. 6.3. Denial-of-Service attacks The current model (one SUBSCRIBE request triggers a SUBSCRIBE response and one or more NOTIFY requests) is a classic setup for an amplifier node to be used in a smurf attack. Also, the creation of state upon receipt of a SUBSCRIBE request can be used by attackers to consume resources on a victim's machine, rendering it unusable. To reduce the chances of such an attack, implementations of notifiers SHOULD require authentication. Authentication issues are discussed in SIP [1] . 6.4. Replay Attacks Replaying of either SUBSCRIBE or NOTIFY can have detrimental effects. In the case of SUBSCRIBE messages, attackers may be able to install any arbitrary subscription which it witnessed being installed at some point in the past. Replaying of NOTIFY messages may be used to spoof old state information (although a good versioning mechanism in the body of the NOTIFY messages may help mitigate such an attack). To prevent such attacks, implementations SHOULD require authentication. Authentication issues are discussed in SIP [1] . 6.5. Man-in-the middle attacks Even with authentication, man-in-the-middle attacks using SUBSCRIBE may be used to install arbitrary subscriptions, hijack existing subscriptions, terminate outstanding subscriptions, or modify the resource to which a subscription is being made. To prevent such attacks, implementations SHOULD provide integrity protection across "Contact," "Route," "Expires," "Event," and "To" headers of SUBSCRIBE messages, at a minimum. If SUBSCRIBE bodies are used to define further information about the state of the call, they SHOULD be included in the integrity protection scheme. Roach [Page 26] Internet Draft SIP-Specific Event Notification February 2002 Man-in-the-middle attacks may also attempt to use NOTIFY messages to spoof arbitrary state information and/or terminate outstanding subscriptions. To prevent such attacks, implementations SHOULD provide integrity protection across the "Call-ID," "CSeq," and "Subscription-State" headers and the bodies of NOTIFY messages. Integrity protection of message headers and bodies is discussed in SIP [1] . 6.6. Confidentiality The state information contained in a NOTIFY message has the potential to contain sensitive information. Implementations MAY encrypt such information to ensure confidentiality. While less likely, it is also possible that the information contained in a SUBSCRIBE message contains information that users might not want to have revealed. Implementations MAY encrypt such information to ensure confidentiality. To allow the remote party to hide information it considers sensitive, all implementations SHOULD be able to handle encrypted SUBSCRIBE and NOTIFY messages. The mechanisms for providing confidentiality are detailed in SIP [1] . 7. IANA Considerations (This section is not applicable until this document is published as an RFC.) This document defines an event-type namespace which requires a central coordinating body. The body chosen for this coordination is the Internet Assigned Numbers Authority (IANA). There are two different types of event-types: normal event packages, and event template-packages; see section 5.2. To avoid confusion, template-package names and package names share the same namespace; in other words, an event template-package MUST NOT share a name with a package. Following the policies outlined in "Guidelines for Writing an IANA Considerations Section in RFCs" [6] , normal event package identification tokens are allocated as First Come First Served, and event template-package identification tokens are allocated on a IETF Consensus basis. Registrations with the IANA MUST include the token being registered and whether the token is a package or a Roach [Page 27] Internet Draft SIP-Specific Event Notification February 2002 template-package. Further, packages MUST include contact information for the party responsible for the registration and/or a published document which describes the event package. Event template-package token registrations MUST include a pointer to the published RFC which defines the event template-package. Registered tokens to designate packages and template-packages MUST NOT contain the character ".", which is used to separate template-packages from packages. 7.1. Registration Information As this document specifies no package or template-package names, the initial IANA registration for event types will be empty. The remainder of the text in this section gives an example of the type of information to be maintained by the IANA; it also demonstrates all five possible permutations of package type, contact, and reference. The table below lists the event packages and template-packages defined in "SIP-Specific Event Notification" [RFC xxxx]. Each name is designated as a package or a template-package under "Type." Package Name Type Contact Reference ------------ ---- ------- --------- example1 package [Roach] example2 package [Roach] [RFC xxxx] example3 package [RFC xxxx] example4 template [Roach] [RFC xxxx] example5 template [RFC xxxx] PEOPLE ------ [Roach] Adam Roach REFERENCES ---------- [RFC xxxx] A. Roach "SIP-Specific Event Notification", RFC XXXX, August 2002. 7.2. Registration Template To: ietf-sip-events@iana.org Subject: Registration of new SIP event package Roach [Page 28] Internet Draft SIP-Specific Event Notification February 2002 Package Name: (Package names must conform to the syntax described in section 7.5.1. ) Is this registration for a Template Package: (indicate yes or no) Published Specification(s): (Template packages require a published RFC. Other packages may reference a specification when appropriate). Person & email address to contact for further information: 7.3. Syntax This section describes the syntax extensions required for event notification in SIP. Semantics are described in section 4. Note that the formal syntax definitions described in this document are expressed in the ABNF format defined by [1] , and contain references to elements defined therein. 7.4. New Methods This document describes two new SIP methods: "SUBSCRIBE" and "NOTIFY." This table expands on tables 2 and 3 in SIP [1] . Header Where SUB NOT ------ ----- --- --- Accept R o o Accept 2xx - - Accept 415 o o Accept-Encoding R o o Accept-Encoding 2xx - - Accept-Encoding 415 o o Accept-Language R o o Accept-Language 2xx - - Accept-Language 415 o o Alert-Info R - - Alert-Info 180 - - Allow R o o Allow 2xx o o Roach [Page 29] Internet Draft SIP-Specific Event Notification February 2002 Allow r o o Allow 405 m m Authentication-Info 2xx o o Authorization R o o Call-ID c m m Contact R m m Contact 1xx o o Contact 2xx m o Contact 3xx m m Contact 485 o o Content-Disposition o o Content-Encoding o o Content-Language o o Content-Length t t Content-Type * * CSeq c m m Date o o Error-Info 300-699 o o Expires o - From c m m In-Reply-To R - - Max-Forwards R m m Min-Expires 423 m - MIME-Version o o Organization o - Priority R o - Proxy-Authenticate 407 m m Proxy-Authorization R o o Proxy-Require R o o RAck R - - Record-Route R o o Record-Route 2xx,401,484 o o Reply-To - - Require o o Retry-After 404,413,480,486 o o Retry-After 500,503 o o Retry-After 600,603 o o Route R c c RSeq 1xx o o Server r o o Subject R - - Supported R o o Supported 2xx o o Timestamp o o To c(1) m m Unsupported 420 o o User-Agent o o Via c m m Warning r o o WWW-Authenticate 401 m m Roach [Page 30] Internet Draft SIP-Specific Event Notification February 2002 7.4.1. SUBSCRIBE method "SUBSCRIBE" is added to the definition of the element "Method" in the SIP message grammar. Like all SIP method names, the SUBSCRIBE method name is case sensitive. The SUBSCRIBE method is used to request asynchronous notification of an event or set of events at a later time. 7.4.2. NOTIFY method "NOTIFY" is added to the definition of the element "Method" in the SIP message grammar. The NOTIFY method is used to notify a SIP node that an event which has been requested by an earlier SUBSCRIBE method has occurred. It may also provide further details about the event. 7.5. New Headers This table expands on tables 2 and 3 in SIP [1] , as amended by the changes described in section 7.4. Header field where proxy ACK BYE CAN INV OPT REG PRA SUB NOT ----------------------------------------------------------------- Allow-Events R o o - o o o o o o Allow-Events 2xx - o - o o o o o o Allow-Events 489 - - - - - - - m m Event R - - - - - - - m m Subscription-State R - - - - - - - - m 7.5.1. "Event" header The following header is defined for the purposes of this specification. Event = ( "Event" | "o" ) HCOLON event-type *( SEMI event-param ) event-type = event-package *( "." event-template ) event-package = token-nodot event-template = token-nodot token-nodot = 1*( alphanum | "-" | "!" | "%" | "*" | "_" | "+" | "`" | "'" | "~" ) event-param = generic-param | ( "id" EQUAL token ) Event is added to the definition of the element "message-header" Roach [Page 31] Internet Draft SIP-Specific Event Notification February 2002 in the SIP message grammar. For the purposes of matching responses and NOTIFY messages with SUBSCRIBE messages, the event-type portion of the "Event" header is compared byte-by-byte, and the "id" parameter token (if present) is compared byte-by-byte. An "Event" header containing an "id" parameter never matches an "Event" header without an "id" parameter. No other parameters are considered when performing a comparison. This document does not define values for event-types. These values will be defined by individual event packages, and MUST be registered with the IANA. There MUST be exactly one event type listed per event header. Multiple events per message are disallowed. For the curious, the "o" short form is chosen to represent "occurrence." 7.5.2. "Allow-Events" Header The following header is defined for the purposes of this specification. Allow-Events = ( "Allow-Events" | "u" ) HCOLON 1*event-type Allow-Events is added to the definition of the element "general-header" in the SIP message grammar. For the curious, the "u" short form is chosen to represent "understands." 7.5.3. "Subscription-State" Header The following header is defined for the purposes of this specification. Subscription-State = "Subscription-State" HCOLON substate-value ) *( SEMI subexp-params ) substate-value = "active" | "pending" | "terminated" Roach [Page 32] Internet Draft SIP-Specific Event Notification February 2002 subexp-params = ("reason" EQUAL reason-code) |("expires" EQUAL delta-seconds) |("retry-after" EQUAL delta-seconds) | generic-param reason-code = "deactivated" | "probation" | "rejected" | "timeout" | "giveup" | reason-extension reason-extension = token Subscription-State is added to the definition of the element "request-header" in the SIP message grammar. 7.6. New Response Codes 7.6.1. "202 Accepted" Response Code The 202 response is added to the "Success" header field definition: Success = "200" ; OK | "202" ; Accepted "202 Accepted" has the same meaning as that defined in HTTP/1.1 [4] . 7.6.2. "489 Bad Event" Response Code The 489 event response is added to the "Client-Error" header field definition: Client-Error = "400" ; Bad Request ... | "489" ; Bad Event "489 Bad Event" is used to indicate that the server did not understand the event package specified in a "Event" header field. 8. Changes 8.1. Changes from draft-ietf-...-01 Roach [Page 33] Internet Draft SIP-Specific Event Notification February 2002 - Changed dependancy from RFC2543 to new sip bis draft. This allowed removal of certain sections of text. - Renamed "sub-packages" to "template-packages" in an attempt to mitigate exploding rampant misinterpretation. - Changed "Subscription-Expires" to "Subscription-State," and added clearer semantics for "reason" codes. - Aligned "Subscription-State" "reason" codes with watcherinfo draft. - Made "Subscription-State" mandatory in NOTIFY requests, since it is integral to defining the creation and destruction of subscriptions (and, consequently, dialogs) - Heavily revised section on dialog creation and termination. - Expanded migration section. - Added "id" parameter to Event header, to allow demultiplexing of NOTIFY requests when more than one subscription is associated with a single dialog. - Syncronized SUBSCRIBE "Expires" handling with REGISTER (again) - Added definitions section. - Restructuring for clarity. - Added statement explicitly allowing event packages to define additional parameters for the "Event" header. Roach [Page 34] Internet Draft SIP-Specific Event Notification February 2002 - Added motivational text in several places. - Synced up header table modifications with bis draft. 8.2. Changes from draft-ietf-...-00 - Fixed confusing typo in section describing correlation of SUBSCRIBE requests - Added explanitory text to clarify tag handling when generating re-subscriptions - Expanded general handling section to include specific discussion of Route/Record-Route handling. - Included use of "methods" parameter on Contact as a means for detecting support for SUBSCRIBE and NOTIFY. - Added definition of term "dialog"; changed "leg" to "dialog" everwhere. - Added syntax for "Subscription-Expires" header. - Changed NOTIFY messages to refer to "Subscription-Expires" everywhere (instead of "Expires.") - Added information about generation and handling of 481 responses to SUBSCRIBE requests - Changed having Expires header in SUBSCRIBE from MUST to SHOULD; this aligns more closely with REGISTER behavior - Removed experimental/private event package names, per list consensus Roach [Page 35] Internet Draft SIP-Specific Event Notification February 2002 - Cleaned up some legacy text left over from very early drafts that allowed multiple contacts per subscription - Strengthened language requiring the removal of subscriptions if a NOTIFY request fails with a 481. Clarified that such removal is required for all subscriptions, including administrative ones. - Removed description of delaying NOTIFY requests until authorization is granted. Such behavior was inconsistent with other parts of this document. - Moved description of event packages to later in document, to reduce the number of forward references. - Minor editorial and nits changes - Added new open issues to open issues section. All previous open issues have been resolved. 8.3. Changes from draft-roach-...-03 - Added DOS attacks section to open issues. - Added discussion of forking to open issues - Changed response to PINT request for notifiers who don't support PINT from 400 to 489. - Added sentence to security section to call attention to potential privacy issues of delayed NOTIFY responses. - Added clarification: access control list handling is out of scope. - (Hopefully) Final resolution on out-of-band subscriptions: mentioned in section 4.2. Roach [Page 36] Internet Draft SIP-Specific Event Notification February 2002 Removed from open issues. - Made "Contact" header optional for SUBSCRIBE 1xx responses. - Added description clarifying tag handling (section 4.3.4. ) - Removed event throttling from open issues. - Editorial cleanup to remove term "extension draft" and similar; "event package" is now (hopefully) used consistently throughout the document. - Remove discussion of event agents from open issues. This is covered in the event packages section now. - Added discussion of forking to open issues. - Added discussion of sub-packages - Added clarification that, upon receiving a "NOTIFY" with an expires of "0", the subscriber can re-subscribe. This allows trivial migration of subscriptions between nodes. - Added preliminary IANA Considerations section - Changed syntax for experimental event tokens to avoid possibly ambiguity between experimental tokens and sub-packages. - Slight adjustment to "Event" syntax to accommodate sub-packages. - Added section describing the information which is to be included in documents describing event packages. Roach [Page 37] Internet Draft SIP-Specific Event Notification February 2002 - Made 481 responses mandatory for unexpected notifications (allowing notifiers to remove subscriptions in error cases) - Several minor non-semantic editorial changes. 8.4. Changes from draft-roach-...-02 - Clarification under "Notifier SUBSCRIBE behavior" which indicates that the first NOTIFY message (sent immediately in response to a SUBSCRIBE) may contain an empty body, if resource state doesn't make sense at that point in time. - Text on message flow in overview section corrected - Removed suggestion that clients attempt to unsubscribe whenever they receive a NOTIFY for an unknown event. Such behavior opens up DOS attacks, and will lead to message loops unless additional precautions are taken. The 481 response to the NOTIFY should serve the same purpose. - Changed processing of non-200 responses to NOTIFY from "SHOULD remove contact" to "MUST remove contact" to support the above change. - Re-added discussion of out-of-band subscription mechanisms (including open issue of resource identification). - Added text specifying that SUBSCRIBE transactions are not to be prolonged. This is based on the consensus that non-INVITE transactions should never be prolonged; such consensus within the SIP working group was reached at the 49th IETF. - Added "202 Accepted" response code to support the above change. The behavior of this 202 response code is a generalization of that described in the presence draft. - Updated to specify that the response to an unauthorized SUBSCRIBE request is 603 or 403. Roach [Page 38] Internet Draft SIP-Specific Event Notification February 2002 - Level-4 subheadings added to particularly long sections to break them up into logical units. This helps make the behavior description seem somewhat less rambling. This also caused some re-ordering of these paragraphs (hopefully in a way that makes them more readable). - Some final mopping up of old text describing "call related" and "third party" subscriptions (deprecated concepts). - Duplicate explanation of subscription duration removed from subscriber SUBSCRIBE behavior section. - Other text generally applicable to SUBSCRIBE (instead of just subscriber handling of SUBSCRIBE) moved to parent section. - Updated header table to reflect mandatory usage of "Expires" header in SUBSCRIBE requests and responses - Removed "Event" header usage in responses - Added sentence suggesting that notifiers may notify subscribers when a subscription has timed out. - Clarified that a failed attempt to refresh a subscription does not imply that the original subscription has been cancelled. - Clarified that 489 is a valid response to "NOTIFY" requests. - Minor editorial changes to clean up awkward and/or unclear grammar in several places 8.5. Changes from draft-roach-...-01 - Multiple contacts per SUBSCRIBE message disallowed. - Contact header now required in NOTIFY messages. Roach [Page 39] Internet Draft SIP-Specific Event Notification February 2002 - Distinction between third party/call member events removed. - Distinction between call-related/resource-related events removed. - Clarified that subscribers must expect NOTIFY messages before the SUBSCRIBE transaction completes - Added immediate NOTIFY message after successful SUBSCRIBE; this solves a myriad of issues, most having to do with forking. - Added discussion of "undefined state" (before a NOTIFY arrives). - Added mechanism for notifiers to shorten/cancel outstanding subscriptions. - Removed open issue about appropriateness of new "489" response. - Removed all discussion of out-of-band subscriptions. - Added brief discussion of event state polling. 9. References [1] J. Rosenberg et. al., "SIP: Session Initiation Protocol", , IETF; February 2002. Work in progress. [2] J. Rosenberg, H. Schulzrinne, "Guidelines for Authors of SIP Extensions", , IETF; November 2001. Work in progress. [3] S. Petrack, L. Conroy, "The PINT Service Protocol", RFC 2848, IETF; June 2000. [4] R. Fielding et. al., "Hypertext Transfer Protocol -- HTTP/1.1", RFC2068, IETF, January 1997. [5] J. Postel, J. Reynolds, "Instructions to RFC Authors", RFC2223, IETF, October 1997. [6] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA Roach [Page 40] Internet Draft SIP-Specific Event Notification February 2002 Considerations Section in RFCs", BCP 26, IETF, October 1998. [7] Schulzrinne/Rosenberg, "SIP Caller Preferences and Callee Capabilities", , IETF; November 2001. Work in progress. 10. Acknowledgements Thanks to the participants in the Events BOF at the 48th IETF meeting in Pittsburgh, as well as those who gave ideas and suggestions on the SIP Events mailing list. In particular, I wish to thank Henning Schulzrinne of Columbia University for coming up with the final three-tiered event identification scheme, Sean Olson for miscellaneous guidance, Jonathan Rosenberg for a thorough scrubbing of the -00 draft, and the authors of the "SIP Extensions for Presence" draft for their input to SUBSCRIBE and NOTIFY request semantics. 11. Author's Address Adam Roach dynamicsoft 5100 Tennyson Parkway Suite 1200 Plano, TX 75024 USA E-Mail: Voice: Roach [Page 41]