4. Node Behavior 4.1. Subscriber Behavior 4.1.1. Detecting Support for SIP Events The extension described in this document does not make use of the "Require" or "Proxy-Require" header fields; similarly, there is no token defined for "Supported" header fields. Potential subscribers may probe for the support of SIP events using the OPTIONS request defined in [RFC3261]. The presence of "SUBSCRIBE" in the "Allow" header field of any request or response indicates support for SIP events; further, in the absence of an "Allow" header field, the simple presence of an "Allow- Events" header field is sufficient to indicate that the node that sent the message is capable of acting as a notifier (see Section 4.4.4). The "methods" parameter for Contact may also be used to specifically announce support for SUBSCRIBE and NOTIFY requests when registering. (See [RFC3840] for details on the "methods" parameter.) 4.1.2. Creating and Maintaining Subscriptions From the subscriber's perspective, a subscription proceeds according to the following state diagram. Events that result in a transition back to the same state are not represented in this diagram.
+-------------+ | init |<-----------------------+ +-------------+ | | Retry-after Send SUBSCRIBE expires | | V Timer N Fires; | +-------------+ SUBSCRIBE failure | +------------| notify_wait |-- response; --------+ | | +-------------+ or NOTIFY, | | | | state=terminated | | | | | | ++========|===================|============================|==|====++ || | | V | || || Receive NOTIFY, Receive NOTIFY, +-------------+ || || state=active state=pending | terminated | || || | | +-------------+ || || | | Re-subscription A A || || | V times out; | | || || | +-------------+ Receive NOTIFY, | | || || | | pending |-- state=terminated; --+ | || || | +-------------+ or 481 response | || || | | to SUBSCRIBE | || || | Receive NOTIFY, refresh | || || | state=active | || || | | Re-subscription | || || | V times out; | || || | +-------------+ Receive NOTIFY, | || || +----------->| active |-- state=terminated; -----+ || || +-------------+ or 481 response || || to SUBSCRIBE || || Subscription refresh || ++=================================================================++ In the state diagram, "Re-subscription times out" means that an attempt to refresh or update the subscription using a new SUBSCRIBE request does not result in a NOTIFY request before the corresponding Timer N expires. Any transition from "notify_wait" into a "pending" or "active" state results in a new subscription. Note that multiple subscriptions can be generated as the result of a single SUBSCRIBE request (see Section 4.4.1). Each of these new subscriptions exists in its own independent state machine and runs its own set of timers.
18.104.22.168. Requesting a Subscription SUBSCRIBE is a dialog-creating method, as described in [RFC3261]. When a subscriber wishes to subscribe to a particular state for a resource, it forms a SUBSCRIBE request. If the initial SUBSCRIBE request represents a request outside of a dialog (as it typically will), its construction follows the procedures outlined in [RFC3261] for User Agent Client (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 request will be sent immediately. The "Expires" header field in a 200-class response to SUBSCRIBE request indicates the actual duration for which the subscription will remain active (unless refreshed). The received value might be smaller than the value indicated in the SUBSCRIBE request but cannot be larger; see Section 4.2.1 for details. Non-200-class final responses indicate that no subscription or new dialog usage has been created, and no subsequent NOTIFY request will be sent. All non-200-class responses (with the exception of 489 (Bad Event), described herein) have the same meanings and handling as described in [RFC3261]. For the sake of clarity: if a SUBSCRIBE request contains an "Accept" header field, but that field does not indicate a media type that the notifier is capable of generating in its NOTIFY requests, then the proper error response is 406 (Not Acceptable). 22.214.171.124. 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. The handling for such a request is the same as for the initial creation of a subscription except as described below. If a SUBSCRIBE request to refresh a subscription receives a 404, 405, 410, 416, 480-485, 489, 501, or 604 response, the subscriber MUST consider the subscription terminated. (See [RFC5057] for further details and notes about the effect of error codes on dialogs and usages within dialog, such as subscriptions). 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 126.96.36.199).
If a SUBSCRIBE request to refresh a subscription fails with any error code other than those listed above, the original subscription is still considered valid for the duration of the most recently known "Expires" value as negotiated by the most recent successful SUBSCRIBE transaction, or as communicated by a NOTIFY request in its "Subscription-State" header field "expires" parameter. Note that many such errors indicate that there may be a problem with the network or the notifier such that no further NOTIFY requests will be received. When refreshing a subscription, a subscriber starts Timer N, set to 64*T1, when it sends the SUBSCRIBE request. If this Timer N expires prior to the receipt of a NOTIFY request, the subscriber considers the subscription terminated. If the subscriber receives a success response to the SUBSCRIBE request that indicates that no NOTIFY request will be generated -- such as the 204 response defined for use with the optional extension described in [RFC5839] -- then it MUST cancel Timer N. 188.8.131.52. Unsubscribing Unsubscribing is handled in the same way as refreshing of a subscription, with the "Expires" header field set to "0". Note that a successful unsubscription will also trigger a final NOTIFY request. The final NOTIFY request may or may not contain information about the state of the resource; subscribers need to be prepared to receive final NOTIFY requests both with and without state. 184.108.40.206. Confirmation of Subscription Creation The subscriber can expect to receive a NOTIFY request from each node which has processed a successful subscription or subscription refresh. To ensure that subscribers do not wait indefinitely for a subscription to be established, a subscriber starts a Timer N, set to 64*T1, when it sends a SUBSCRIBE request. If this Timer N expires prior to the receipt of a NOTIFY request, the subscriber considers the subscription failed, and cleans up any state associated with the subscription attempt. Until Timer N expires, several NOTIFY requests may arrive from different destinations (see Section 4.4.1). Each of these requests establishes a new dialog usage and a new subscription. After the expiration of Timer N, the subscriber SHOULD reject any such NOTIFY requests that would otherwise establish a new dialog usage with a 481 (Subscription does not exist) response code.
Until the first NOTIFY request arrives, the subscriber should consider the state of the subscribed resource to be in a neutral state. Event package specifications 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 out-of-order messages, packet loss, and forking, the subscriber MUST be prepared to receive NOTIFY requests before the SUBSCRIBE transaction has completed. Except as noted above, processing of this NOTIFY request is the same as in Section 4.1.3. 4.1.3. Receiving and Processing State Information Subscribers receive information about the state of a resource to which they have subscribed in the form of NOTIFY requests. 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 usage are described in Section 4.4.1. Notifications for subscriptions that were created inside an existing dialog match if they are in the same dialog and the "Event" header fields match (as described in Section 8.2.1). If, for some reason, the event package designated in the "Event" header field 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, such as those described in Sections 22.2 and 23 of [RFC3261]. NOTIFY requests MUST contain "Subscription-State" header fields that indicate the status of the subscription. If the "Subscription-State" header field value is "active", it means that the subscription has been accepted and (in general) has been authorized. If the header field 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 field 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 MUST consider the subscription terminated. The "expires" parameter has no semantics for "terminated" -- notifiers SHOULD NOT include an "expires" parameter on a "Subscription-State" header field with a value of "terminated", and subscribers MUST ignore any such parameter, if present. 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 reason codes defined by this document are: deactivated: The subscription has been terminated, but the subscriber 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 (as long as the resource's state is still relevant to the client at that 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". This reason code is also associated with polling of resource state, as detailed in Section 4.4.3. 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. noresource: The subscription has been terminated because the resource state that was being monitored no longer exists. Clients SHOULD NOT attempt to re-subscribe. The "retry-after" parameter has no semantics for "noresource". invariant: The subscription has been terminated because the resource state is guaranteed not to change for the foreseeable future. This may be the case, for example, when subscribing to the location information of a fixed-location land-line telephone. When using this reason code, notifiers are advised to include a "retry-after" parameter with a large value (for example, 31536000 -- or one year) to prevent older clients that are RFC 3265 compliant from periodically re-subscribing. Clients SHOULD NOT attempt to re-subscribe after receiving a reason code of "invariant", regardless of the presence of or value of a "retry- after" parameter. Other specifications may define new reason codes for use with the "Subscription-State" header field. 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 field. Other responses defined in [RFC3261] may also be returned, as appropriate. In no case should a NOTIFY transaction extend for any longer than the time necessary for automated processing. In particular, subscribers MUST NOT wait for a user response before returning a final response to a NOTIFY request. 4.1.4. Forking of SUBSCRIBE Requests In accordance with the rules for proxying non-INVITE requests as defined in [RFC3261], 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 that differ from the "To:" tag received in the SUBSCRIBE 200-class response.
If multiple NOTIFY requests are received in different dialogs in response to a single SUBSCRIBE request, each dialog represents a different destination to which the SUBSCRIBE request was forked. Subscriber handling in such situations varies by event package; see Section 5.4.9 for details. 4.2. Notifier Behavior 4.2.1. Subscription Establishment and Maintenance Notifiers learn about subscription requests by receiving SUBSCRIBE requests from interested parties. Notifiers MUST NOT create subscriptions except upon receipt of a SUBSCRIBE request. However, for historical reasons, the implicit creation of subscriptions as defined in [RFC3515] is still permitted. [RFC3265] allowed the creation of subscriptions using means other than the SUBSCRIBE method. The only standardized use of this mechanism is the REFER method [RFC3515]. Implementation experience with REFER has shown that the implicit creation of a subscription has a number of undesirable effects, such as the inability to signal the success of a REFER request while signaling a problem with the subscription, and difficulty performing one action without the other. Additionally, the proper exchange of dialog identifiers is difficult without dialog reuse (which has its own set of problems; see Section 4.5). 220.127.116.11. 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 the non-INVITE transaction timeout timer F (see [RFC3261]) 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 field 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 18.104.22.168.
The notifier MAY also check that the duration in the "Expires" header field is not too small. If and only if the expiration interval is greater than zero AND smaller than one hour AND less than a notifier- configured minimum, the notifier MAY return a 423 (Interval Too Brief) error that contains a "Min-Expires" header field. The "Min-Expires" header field is described in [RFC3261]. Once the notifier determines that it has enough information to create the subscription (i.e., it understands the event package, the subscription pertains to a known resource, and there are no other barriers to creating the subscription), it creates the subscription and a dialog usage, and returns a 200 (OK) response. When a subscription is created in the notifier, it stores the event package name 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 request is unacceptably short, the notifier may be able to send a 423 response, as described earlier in this section. 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 [RFC3261] may be used in response to SUBSCRIBE requests, as appropriate. 22.214.171.124. Confirmation of Subscription Creation/Refreshing Upon successfully accepting or refreshing a subscription, notifiers MUST send a NOTIFY request immediately to communicate the current resource state to the subscriber. This NOTIFY request 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 request is processed, this NOTIFY request MAY contain an empty or neutral body. See Section 4.2.2 for further details on NOTIFY request generation. Note that a NOTIFY request is always sent immediately after any 200-class response to a SUBSCRIBE request, regardless of whether the subscription has already been authorized.
126.96.36.199. 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 [RFC3261]. Note that, even if the notifier node typically acts as a proxy, authentication for SUBSCRIBE requests will always be performed via a 401 (Unauthorized) response, not a 407 (Proxy Authentication Required). Notifiers always act as user agents when accepting subscriptions and sending notifications. Of course, when acting as a proxy, a node will perform normal proxy authentication (using 407). The foregoing explanation is a reminder that notifiers are always user agents and, as such, perform user agent authentication. 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 200 (OK) response is returned immediately. Note that a NOTIFY request is still formed 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 request containing a "Subscription-State" header field with a value of "terminated" and a reason parameter of "rejected". 188.8.131.52. 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
field in the response. If the duration specified in a SUBSCRIBE request is unacceptably short, the notifier SHOULD respond with a 423 (Interval Too Brief) response. 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 request with a "Subscription-State" value of "terminated" to inform it that the subscription is being removed. If such a request is sent, the "Subscription-State" header field SHOULD contain a "reason=timeout" parameter. Clients can cause a subscription to be terminated immediately by sending a SUBSCRIBE request with an "Expires" header field set to '0'. Notifiers largely treat this the same way as any other subscription expiration: they send a NOTIFY request containing a "Subscription-State" of "terminated", with a reason code of "timeout." For consistency with state polling (see Section 4.4.3) and subscription refreshes, the notifier may choose to include resource state in this final NOTIFY request. However, in some cases, including such state makes no sense. Under such circumstances, the notifier may choose to omit state information from the terminal NOTIFY request. The sending of a NOTIFY request when a subscription expires allows the corresponding dialog usage to be terminated, if appropriate. 4.2.2. Sending State Information to Subscribers Notifiers use the NOTIFY method to send information about the state of a resource to subscribers. The notifier's view of a subscription is shown in the following state diagram. Events that result in a transition back to the same state are not represented in this diagram.
+-------------+ | init | +-------------+ | Receive SUBSCRIBE, Send NOTIFY | V NOTIFY failure, +-------------+ subscription expires, +------------| resp_wait |-- or terminated ----+ | +-------------+ per local policy | | | | | | | | | V Policy grants Policy needed +-------------+ permission | | terminated | | | +-------------+ | | A A | V NOTIFY failure, | | | +-------------+ subscription expires,| | | | pending |-- or terminated -------+ | | +-------------+ per local policy | | | | | Policy changed to | | grant permission | | | | | V NOTIFY failure, | | +-------------+ subscription expires, | +----------->| active |-- or terminated ---------+ +-------------+ per local policy 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, unless the state transition is caused by a NOTIFY transaction failure. The sending of this NOTIFY message is also subject to authorization, local policy, and throttling considerations. If the NOTIFY request fails due to expiration of SIP Timer F (transaction timeout), 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 multiple times, per the retransmission algorithm defined in [RFC3261] (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 requests to refresh potentially stale state information; such requests will reinstall subscriptions in all relevant nodes. If the NOTIFY transaction fails due to the receipt of a 404, 405, 410, 416, 480-485, 489, 501, or 604 response to the NOTIFY request, the notifier MUST remove the corresponding subscription. See [RFC5057] for further details and notes about the effect of error codes on dialogs and usages within dialog (such as subscriptions). 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 sending of SUBSCRIBE requests to refresh the expiration timer will reinstall subscription state once the network problem has been resolved. NOTIFY requests MUST contain a "Subscription-State" header field 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 subscription at this time. The "terminated" value indicates that the subscription is not active. If the value of the "Subscription-State" header field is "active" or "pending", the notifier MUST also include in the "Subscription-State" header field an "expires" parameter that 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 necessary in case the SUBSCRIBE request forks, since the response to a forked SUBSCRIBE request may not be received by the subscriber. [RFC3265] allowed the notifier some discretion in the inclusion of this parameter, so subscriber implementations are warned to handle the lack of an "expires" parameter gracefully. Note well that this "expires" value is a parameter on the "Subscription-State" header field NOT the "Expires" header field.
The period of time for a subscription can be shortened to zero by the notifier. In other words, it is perfectly valid for a SUBSCRIBE request with a non-zero expires to be answered with a NOTIFY request that contains "Subscription-Status: terminated;reason=expired". This merely means that the notifier has shortened the subscription timeout to zero, and the subscription has expired instantaneously. The body may contain valid state, or it may contain a neutral state (see Section 5.4.7). If the value of the "Subscription-State" header field 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 4.1.3. 4.2.3. PSTN/Internet Interworking (PINT) Compatibility The "Event" header field is considered mandatory for the purposes of this document. However, to maintain compatibility with PINT (see [RFC2848]), notifiers MAY interpret a SUBSCRIBE request with no "Event" header field as requesting a subscription to PINT events. If a notifier does not support PINT, it SHOULD return 489 (Bad Event) to any SUBSCRIBE requests without an "Event" header field. 4.3. Proxy Behavior Proxies need no additional behavior beyond that described in [RFC3261] to support SUBSCRIBE and NOTIFY transactions. If a proxy wishes to see all of the SUBSCRIBE and NOTIFY requests for a given dialog, it MUST add a "Record-Route" header field to the initial SUBSCRIBE request and all NOTIFY requests. It MAY choose to include "Record-Route" in subsequent SUBSCRIBE requests; however, these requests cannot cause the dialog's route set to be modified. Proxies that did not add a "Record-Route" header field to the initial SUBSCRIBE request MUST NOT add a "Record-Route" header field to any of the associated NOTIFY requests. Note that subscribers and notifiers may elect to use Secure/ Multipurpose Internet Mail Extensions (S/MIME) encryption of SUBSCRIBE and NOTIFY requests; consequently, proxies cannot rely on being able to access any information that is not explicitly required to be proxy-readable by [RFC3261].
4.4. Common Behavior 4.4.1. Dialog Creation and Termination Dialogs usages are created upon completion of a NOTIFY transaction for a new subscription, unless the NOTIFY request contains a "Subscription-State" of "terminated." Because the dialog usage is established by the NOTIFY request, the route set at the subscriber is taken from the NOTIFY request itself, as opposed to the route set present in the 200-class response to the SUBSCRIBE request. NOTIFY requests are matched to such SUBSCRIBE requests if they contain the same "Call-ID", a "To" header field "tag" parameter that matches the "From" header field "tag" parameter of the SUBSCRIBE request, and the same "Event" header field. Rules for comparisons of the "Event" header fields are described in Section 8.2.1. A subscription is destroyed after a notifier sends a NOTIFY request with a "Subscription-State" of "terminated", or in certain error situations described elsewhere in this document. The subscriber will generally answer such final requests with a 200 (OK) response (unless a condition warranting an alternate response has arisen). Except when the mechanism described in Section 4.5.2 is used, the destruction of a subscription results in the termination of its associated dialog. A subscriber may send a SUBSCRIBE request with an "Expires" header field 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 transaction with a "Subscription-State" of "terminated" completes. 4.4.2. Notifier Migration It is often useful to allow migration of subscriptions between notifiers. Such migration may be effected by sending a NOTIFY request with a "Subscription-State" header field 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 request, 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 reuse 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. 4.4.3. 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 request with an "Expires" equal to the number of seconds remaining in the subscription. Upon receipt of this SUBSCRIBE request, the notifier (or notifiers, if the SUBSCRIBE request was forked) will send a NOTIFY request containing resource state in the same dialog. Note that the NOTIFY requests triggered by SUBSCRIBE requests with "Expires" header fields of 0 will contain a "Subscription-State" value of "terminated" and a "reason" parameter of "timeout". Polling of event state can cause significant increases in load on the network and notifiers; as such, it should be used only sparingly. In particular, polling SHOULD NOT be used in circumstances in which it will typically result in more network messages than long-running subscriptions. When polling is used, subscribers SHOULD attempt to cache authentication credentials between polls so as to reduce the number of messages sent. Due to the requirement on notifiers to send a NOTIFY request immediately upon receipt of a SUBSCRIBE request, the state provided by polling is limited to the information that the notifier has immediate local access to when it receives the SUBSCRIBE request. If, for example, the notifier generally needs to retrieve state from another network server, then that state will be absent from the NOTIFY request that results from polling.
4.4.4. "Allow-Events" Header Field Usage The "Allow-Events" header field, if present, MUST include a comprehensive and inclusive list of tokens that indicates the event packages for which the user agent can act as a notifier. In other words, a user agent sending an "Allow-Events" header field is advertising that it can process SUBSCRIBE requests and generate NOTIFY requests for all of the event packages listed in that header field. Any user agent that can act as a notifier for one or more event packages SHOULD include an appropriate "Allow-Events" header field 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. On the other hand, it doesn't necessarily make much sense to indicate supported events inside a dialog established by a NOTIFY request if the only event package supported is the one associated with that subscription. Note that "Allow-Events" header fields MUST NOT be inserted by proxies. The "Allow-Events" header field does not include a list of the event template-packages supported by an implementation. If a subscriber wishes to determine which event template-packages are supported by a notifier, it can probe for such support by attempting to subscribe to the event template-packages it wishes to use. For example: to check for support for the templatized package "presence.winfo", a client may attempt to subscribe to that event package for a known resource, using an "Expires" header value of 0. If the response is a 489 error code, then the client can deduce that "presence.winfo" is unsupported. 4.5. Targeting Subscriptions at Devices [RFC3265] defined a mechanism by which subscriptions could share dialogs with invite usages and with other subscriptions. The purpose of this behavior was to allow subscribers to ensure that a subscription arrived at the same device as an established dialog. Unfortunately, the reuse of dialogs has proven to be exceedingly confusing. [RFC5057] attempted to clarify proper behavior in a
variety of circumstances; however, the ensuing rules remain confusing and prone to implementation error. At the same time, the mechanism described in [RFC5627] now provides a far more elegant and unambiguous means to achieve the same goal. Consequently, the dialog reuse technique described in RFC 3265 is now deprecated. This dialog-sharing technique has also historically been used as a means for targeting an event package at a dialog. This usage can be seen, for example, in certain applications of the REFER method [RFC3515]. With the removal of dialog reuse, an alternate (and more explicit) means of targeting dialogs needs to be used for this type of correlation. The appropriate means of such targeting is left up to the actual event packages. Candidates include the "Target-Dialog" header field [RFC4538], the "Join" header field [RFC3911], and the "Replaces" header field [RFC3891], depending on the semantics desired. Alternately, if the semantics of those header fields do not match the event package's purpose for correlation, event packages can devise their own means of identifying dialogs. For an example of this approach, see the Dialog Event Package [RFC4235]. 4.5.1. Using GRUUs to Route to Devices Notifiers MUST implement the Globally Routable User Agent URI (GRUU) extension defined in [RFC5627], and MUST use a GRUU as their local target. This allows subscribers to explicitly target desired devices. If a subscriber wishes to subscribe to a resource on the same device as an established dialog, it should check whether the remote contact in that dialog is a GRUU (i.e., whether it contains a "gr" URI parameter). If so, the subscriber creates a new dialog, using the GRUU as the Request URI for the new SUBSCRIBE request. Because GRUUs are guaranteed to route to a specific device, this ensures that the subscription will be routed to the same place as the established dialog. 4.5.2. Sharing Dialogs For compatibility with older clients, subscriber and notifier implementations may choose to allow dialog sharing. The behavior of multiple usages within a dialog are described in [RFC5057]. Subscribers MUST NOT attempt to reuse dialogs whose remote target is a GRUU.
Note that the techniques described in this section are included for backwards-compatibility purposes only. Because subscribers cannot reuse dialogs with a GRUU for their remote target, and because notifiers must use GRUUs as their local target, any two implementations that conform to this specification will automatically use the mechanism described in Section 4.5.1. Further note that the prohibition on reusing dialogs does not exempt implicit subscriptions created by the REFER method. This means that implementations complying with this specification are required to use the "Target-Dialog" mechanism described in [RFC4538] when the remote target is a GRUU. If a subscriber wishes to subscribe to a resource on the same device as an established dialog and the remote contact is not a GRUU, it MAY revert to dialog-sharing behavior. Alternately, it MAY choose to treat the remote party as incapable of servicing the subscription (i.e., the same way it would behave if the remote party did not support SIP events at all). If a notifier receives a SUBSCRIBE request for a new subscription on an existing dialog, it MAY choose to implement dialog sharing behavior. Alternately, it may choose to fail the SUBSCRIBE request with a 403 (Forbidden) response. The error text of such 403 responses SHOULD indicate that dialog sharing is not supported. To implement dialog sharing, subscribers and notifiers perform the following additional processing: o When subscriptions exist in dialogs associated with INVITE-created application state and/or other subscriptions, these sets of application state do not interact beyond the behavior described for a dialog (e.g., route set handling). In particular, multiple subscriptions within a dialog expire independently and require independent subscription refreshes. o If a subscription's destruction leaves no other application state associated with the dialog, the dialog terminates. The 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. This means that, when dialogs are reused, a dialog created with an INVITE does not necessarily terminate upon receipt of a BYE. Similarly, in the case that several subscriptions are associated with a single dialog, the dialog does not terminate until all the subscriptions in it are destroyed.
o Subscribers MAY include an "id" parameter in a SUBSCRIBE request's "Event" header field to allow differentiation between multiple subscriptions in the same dialog. This "id" parameter, if present, contains an opaque token that identifies the specific subscription within a dialog. An "id" parameter is only valid within the scope of a single dialog. o If an "id" parameter is present in the SUBSCRIBE request used to establish a subscription, that "id" parameter MUST also be present in all corresponding NOTIFY requests. o When a subscriber refreshes the subscription timer, the SUBSCRIBE request MUST contain the same "Event" header field "id" parameter as was present in the SUBSCRIBE request that created the subscription. (Otherwise, the notifier will interpret the SUBSCRIBE request as a request for a new subscription in the same dialog.) o When a subscription is created in the notifier, it stores any "Event" header field "id" parameter as part of the subscription information (along with the event package name). o If an initial SUBSCRIBE request is sent on a pre-existing dialog, a matching NOTIFY request merely creates a new subscription associated with that dialog. 4.6. CANCEL Requests for SUBSCRIBE and NOTIFY Transactions Neither SUBSCRIBE nor NOTIFY requests can be canceled. If a User Agent Server (UAS) receives a CANCEL request that matches a known SUBSCRIBE or NOTIFY transaction, it MUST respond to the CANCEL request, but otherwise ignore it. In particular, the CANCEL request MUST NOT affect processing of the SUBSCRIBE or NOTIFY request in any way. UACs SHOULD NOT send CANCEL requests for SUBSCRIBE or NOTIFY transactions.