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RFC 6665

 
 
 

SIP-Specific Event Notification

Part 2 of 4, p. 10 to 29
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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.

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                          +-------------+
                          |    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.

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4.1.2.1.  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).

4.1.2.2.  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.  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 4.1.2.1).

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   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.

4.1.2.3.  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.

4.1.2.4.  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.

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   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".

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   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

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      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.

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   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).

4.2.1.1.  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 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 4.2.1.3.

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   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.

4.2.1.2.  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.

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4.2.1.3.  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 [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".

4.2.1.4.  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

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   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.

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                         +-------------+
                         |    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

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      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.

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      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].

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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.

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   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.

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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

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   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.

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      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.

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   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.



(page 29 continued on part 3)

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