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Versions: (draft-ietf-sip-subnot-etags) 00 01 02 03 04 RFC 5839

Session Initiation Protocol                                     A. Niemi
Working Group                                                      Nokia
Internet-Draft                                            April 22, 2009
Intended status: Standards Track
Expires: October 24, 2009


An Extension to Session Initiation Protocol (SIP) Events for Conditional
                           Event Notification
                   draft-ietf-sipcore-subnot-etags-02

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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   The Session Initiation Protocol (SIP) events framework enables
   receiving asynchronous notification of various events from other SIP
   user agents.  This framework defines the procedures for creating,
   refreshing and terminating subscriptions, as well as fetching and
   periodic polling of resource state.  These procedures provide no
   tools to avoid replaying event notifications that have already been
   received by a user agent.  This memo defines an extension to SIP
   events that allows the subscriber to condition the subscription
   request to whether the state has changed since the previous
   notification was received.  When such a condition is true, either the
   body of a resulting event notification or the entire notification
   message is suppressed.































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Document Conventions . . . . . . . . . . . . . . . . . . .  5
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Motivations and Background . . . . . . . . . . . . . . . . . .  5
     2.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  Problem Description  . . . . . . . . . . . . . . . . . . .  5
     2.3.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  7
   4.  Resource Model for Entity-Tags . . . . . . . . . . . . . . . . 11
   5.  Subscriber Behavior  . . . . . . . . . . . . . . . . . . . . . 13
     5.1.  Detecting Support for Conditional Notification . . . . . . 13
     5.2.  Generating SUBSCRIBE Requests  . . . . . . . . . . . . . . 14
     5.3.  Receiving NOTIFY Requests  . . . . . . . . . . . . . . . . 15
     5.4.  Polling or Fetching Resource State . . . . . . . . . . . . 15
     5.5.  Resuming a Subscription  . . . . . . . . . . . . . . . . . 17
     5.6.  Refreshing a Subscription  . . . . . . . . . . . . . . . . 18
     5.7.  Terminating a Subscription . . . . . . . . . . . . . . . . 18
     5.8.  Handling Transient Errors  . . . . . . . . . . . . . . . . 19
   6.  Notifier Behavior  . . . . . . . . . . . . . . . . . . . . . . 19
     6.1.  Generating Entity-tags . . . . . . . . . . . . . . . . . . 20
     6.2.  Suppressing NOTIFY Bodies  . . . . . . . . . . . . . . . . 20
     6.3.  Suppressing NOTIFY Requests  . . . . . . . . . . . . . . . 21
     6.4.  State Differentials  . . . . . . . . . . . . . . . . . . . 21
     6.5.  List Subscriptions . . . . . . . . . . . . . . . . . . . . 21
   7.  Protocol Element Definitions . . . . . . . . . . . . . . . . . 22
     7.1.  204 (No Notification) Response Code  . . . . . . . . . . . 22
     7.2.  Suppress-If-Match Header Field . . . . . . . . . . . . . . 22
     7.3.  Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . 22
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
     8.1.  204 (No Notification) Response Code  . . . . . . . . . . . 23
     8.2.  Suppress-If-Match Header Field . . . . . . . . . . . . . . 23
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 24
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 24
     11.2. Informative References . . . . . . . . . . . . . . . . . . 25
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25












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1.  Introduction

   The Session Initiation Protocol (SIP) events framework provides an
   extensible facility for requesting notification of certain events
   from other SIP user agents.  This framework includes procedures for
   creating, refreshing and terminating of subscriptions, as well as the
   possibility to fetch or periodically poll the event resource.

   Several instantiations of this framework, called event packages have
   been defined, e.g., for presence [RFC3856], message waiting
   indications [RFC3842] and registrations [RFC3680].

   By default, every SUBSCRIBE request generates a NOTIFY request
   containing the latest event state.  Typically, a SUBSCRIBE request is
   issued by the subscriber whenever it needs a subscription to be
   installed, periodically refreshed or terminated.  Once the
   subscription has been installed, the majority of the NOTIFYs
   generated by the subscription refreshes are superfluous; the
   subscriber usually is in possession of the event state already,
   except in the unlikely case where a state change exactly coincides
   with the periodic subscription refresh.  In most cases, the final
   event state generated upon terminating the subscription similarly
   contains resource state that the subscriber already has.

   Fetching or polling of resource state behaves in a similarly
   suboptimal way in cases where the state has not changed since the
   previous poll occurred.  In general, the problem lies in with the
   inability to persist state across a SUBSCRIBE request.

   This memo defines an extension to optimize the SIP events framework.
   This extension allows a notifier to tag notifications (called entity-
   tags hereafter), and the subscriber to condition its subsequent
   SUBSCRIBE requests for actual changes since a notification carrying
   that entity-tag was issued.  The solution is almost identical to
   conditional requests defined in the HyperText Transfer Protocol
   (HTTP) [RFC2616], and follows the mechanism already defined for the
   PUBLISH [RFC3903] method for issuing conditional event publications.

   This memo is structured as follows.  Section 2 explains the
   background, motivations and requirements for the work; Section 3
   gives a general overview of the mechanism; Section 4 explains the
   underlying model for resources and entities as they apply to
   conditional notification; Section 5 defines the subscriber behavior;
   Section 6 defines the notifier behavior; Section 7 includes the
   protocol element definitions; Section 8 includes the IANA
   considerations; and Section 9 includes the security considerations.





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1.1.  Document Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119
   [RFC2119] and indicate requirement levels for compliant
   implementations.

1.2.  Terminology

   In addition to the terminology introduced in [RFC3261], [RFC3265] and
   [RFC3903], this specification uses these additional terms to describe
   the objects of conditional notification:

   resource
      An object identified by a URI, whose resource state can be
      accessed using the SIP Event Notification framework.  There is a
      single authoritative notifier responsible for communicating the
      resource state.

   entity
      The representation of resource state.  An entity consists of the
      event data carried in the body of a NOTIFY message, as well as
      related meta-data in the message header.  There may be many
      versions of an entity, one current and the others stale.  Each
      version of an entity is identified by an entity-tag, which is
      guaranteed to be unique across all versions of all entities for a
      resource and event package.


2.  Motivations and Background

2.1.  Overview

   A SUBSCRIBE request creates a subscription with a finite lifetime.
   This lifetime is negotiated using the Expires header field, and
   unless the subscription is refreshed by the subscriber before the
   expiration is met, the subscription is terminated.  The frequency of
   these subscription refreshes depends on the event package, and
   typically ranges from minutes to hours.

2.2.  Problem Description

   In spite of being somewhat distinct operations, the SIP events
   framework does not include different protocol methods for initiating
   and terminating of subscriptions, subscription refreshes and fetches
   inside and outside of the SIP dialog.  Instead, the SUBSCRIBE method
   is overloaded to perform all of these functions, and the notifier



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   behavior is identical in each of them; each SUBSCRIBE request
   generates a NOTIFY request containing the latest resource state.  In
   fact, the only difference between a fetch that does not create a
   (lasting) subscription, and a SUBSCRIBE that creates one is in the
   Expires header field value of the SUBSCRIBE; a zero-expiry SUBSCRIBE
   only generates a single NOTIFY, after which the subscription
   immediately terminates.

   Some subscriber implementations may choose to operate in semi-
   stateless mode, in which they immediately upon receiving and
   processing the NOTIFY forget the resource state.  This operation
   necessarily needs every NOTIFY to carry the full resource state.
   However, for an implementation that stores the resource state
   locally, this mode of operation is inefficient.

   There are certain conditions that aggravate the problem.  Such
   conditions usually entail such things as:

   o  Large entity bodies in the payloads of notifications

   o  High rate of subscription refreshes

   o  Relatively low rate of notifications triggered by state changes

   In effect, for an event package that generates few state changes, and
   is refreshed relatively often the majority of traffic generated may
   be related to subscription maintenance.  Especially in networks where
   bandwidth consumption and traffic count is at a premium, the high
   overhead of subscription maintenance becomes a barrier for
   deployment.

   The same problem affects fetching and polling of resource state as
   well.  As a benchmark, if we look at the performance of HTTP
   [RFC2616] in similar scenarios, it performs substantially better
   using conditional requests.  When resources are tagged with an
   entity-tag, and each GET is a conditional one using the "If-None-
   Match" header field, the entity body need not be sent more than once;
   if the resource has not changed between successive polls, an error
   response is returned indicating this fact, and the resource entity is
   not transmitted again.

   The SIP PUBLISH [RFC3903] method also contains a similar feature,
   where a refresh of a publication is done by reference to its assigned
   entity-tag, instead of retransmitting the event state each time the
   publication expiration is extended.






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2.3.  Requirements

   As a summary, here is the required functionality to solve the
   presented issues:

   REQ1:   It must be possible to suppress the NOTIFY request (or at a
           minimum the event body therein) if the subscriber is already
           in possession of the latest event state of the resource.

   REQ2:   This mechanism must apply to initial subscriptions, in which
           the subscriber is attempting to "resume" an earlier
           subscription.

   REQ3:   This mechanism must apply to refreshing a subscription.

   REQ4:   This mechanism must apply to terminating a subscription
           (i.e., an unsubscribe).

   REQ5:   This mechanism must apply to fetching or polling of resource
           state.


3.  Overview of Operation

   Whenever a subscriber initiates a subscription, it issues a SUBSCRIBE
   request.  The SUBSCRIBE request is sent, routed and processed by the
   notifier normally, i.e., according to RFC3261 [RFC3261], RFC3265
   [RFC3265].

   If the notifier receiving the SUBSCRIBE request supports conditional
   subscriptions, it generates a unique entity tag for the event
   notification, and includes it in a SIP-ETag header field of the
   NOTIFY request.  The entity tag is unique across all versions of all
   entities for a resource and event package.  More on this in
   Section 4.

   Entity-tags are independent of subscriptions; the notifier remembers
   the entity-tags of all versions of entities for a resource regardless
   of whether or not there are any active subscription to that resource.
   This allows notifications generated to a fetch or a poll to have
   valid entity-tags even across subsequent fetches or polls.

   The subscriber will store the entity-tag received in the notification
   along with the resource state.  It can then later use this entity-tag
   to make a SUBSCRIBE contain a condition in the form of a "Suppress-
   If-Match" header field.  Unlike the "If-Match" condition in a PUBLISH
   [RFC3903] request, which applies to whether the PUBLISH succeeds or
   returns an error, this condition applies to the stream of



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   notifications that are sent after the SUBSCRIBE request has been
   processed.

   The "Suppress-If-Match" header field contains the last entity-tag
   seen by the subscriber.  This condition, if true, instructs the
   notifier to suppress either the body of a subsequent notification, or
   the entire notification.

   The condition is evaluated by matching the value of the header field
   against the current entity-tag of the resource state.  There is also
   a wildcard entity-tag with a special value of "*" that always
   matches.







































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      Subscriber                               Notifier
      ----------                               --------

      (1) SUBSCRIBE       -------->
          Expires: 3600
                          <--------       (2) 200 (or 202)

                          <--------       (3) NOTIFY
                                              Subscription-State: active
                                              SIP-ETag: ffee2
      (4) 200             -------->

           ... time passes ...

      (5) SUBSCRIBE       -------->                \ if "ffee2"
          Suppress-If-Match: ffee2                 |   matches
          Expires: 3600                            |   local
                                                   |   entity-tag
                                                   |
                          <--------       (6) 204  / then

         ... time passes ...


                          <--------       (7) NOTIFY
                                              Subscription-State: active
                                              SIP-ETag: ca89a
      (8) 200             -------->


         ... time passes ...


      (9) SUBSCRIBE       -------->                \ if "ca89"
          Suppress-If-Match: ca89a                 |   matches
          Expires: 0                               |   local
                                                   |   entity-tag
                                                   |
                          <--------      (10) 204  / then


                      Figure 1: Example Message Flow

   Figure 1 describes a typical message flow for conditional
   notification:

   1.   The subscriber initiates a subscription by sending a SUBSCRIBE
        request for a resource.



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   2.   After proper authentication and authorization, the notifier
        accepts the subscription.

   3.   The notifier then immediately sends the initial event
        notification, including a unique entity-tag in a SIP-ETag header
        field.

   4.   The subscriber accepts the notification and stores the entity-
        tag value along with the resource state.

   5.   Later, the subscriber refreshes the subscription, and includes
        an entity-tag in a Suppress-If-Match header field.

   6.   The notifier evaluates the condition by matching its local
        entity-tag value for the resource against the value of the
        Suppress-If-Match header field.  If the condition evaluates to
        true, the notifier informs the subscriber that the notification
        will not be sent.

   7.   At some point, the state of the resource changes, e.g., the
        presence status of a user changes from online to busy.  This
        triggers an event notification with a new value in the SIP-ETag
        header field.

   8.   The subscriber accepts the notification and stores the new
        entity-tag along with the resource state.

   9.   After a while, the subscriber decides to terminate the
        subscription.  It adds a condition for Suppress-If-Match, and
        includes the entity-tag it received in the previous NOTIFY.

   10.  The notifier evaluates the condition by matching its entity-tag
        for the resource against the value of the Suppress-If-Match
        header field.  If the condition evaluates to true, the notifier
        informs the subscriber that no notification will be sent.  This
        concludes the subscription.

   The benefit of using conditional notification in this example is in
   the reduction of the number of NOTIFY requests the subscriber can
   expect to receive.  Each event notification that the subscriber has
   already seen is suppressed by the notifier.  This example illustrates
   only one use case for the mechanism; the same principles can be used
   to optimize the flow of messages related to other event notification
   use cases.







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4.  Resource Model for Entity-Tags

   The key to understanding how conditional notification works is
   understanding the underlying resource model of event notification.
   In general, this model is similar to the resource model of HTTP with
   some key differences.  This section explains in detail the model as
   it applies to SIP events.  Figure 2 illustrates the model.


                     +-----+
     ............    |     |
     .          .    | URI |
     . Represen .    |     |
     . tation   .    +-----+
     .          .       |*
     ............       |
          .             |
          .             V
          .        +----------+            +---------+
       composition |          |*           |  Event  |
          +------<>| Resource |----------->| Package |<----.
          |        |          |            |         |     |
          |        +----------+            +----.----+     |
          |                                    /_\         |
          |*                                    | classification
      +--------+                                |          |
      |        |        .----------------.------'          |
      | Entity |        |                |                 |
      |        |        |                |                 |*
      +--------+   +----------+    +------------+     +----------+
          ^        |          |    |            |     |          |
          |        | Presence |    | Conference |     | Template |
          |        |          |    |            |     |          |
          |1..*    +----------+    +------------+     +----.-----+
     +---------+                                          /_\
     |         |                                           |
     | Version |                                           |
     |         |                                      +---------+
     +---------+                                      | Watcher |
          |1                                          |  Info   |
          |                                           |         |
          |                                           +---------+
          V
     +---------+
     | Entity- |
     |   Tag   |
     |         |
     +---------+



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                     Figure 2: Resource Model Diagram

   For a given event package, there is a single authoritative agent
   responsible for zero or more resources.  That is, even for a
   distributed agent, the resource state is uniform across all
   instances.  The resource itself can be a list of resources [RFC4662].
   Conditional notification for list subscriptions is addressed in
   Section 6.5.

   A resource is identified by zero or more URIs, which can be SIP URIs,
   pres URIs [RFC3859] or similar.  Subscribers use this URI to
   subscribe to the resource for certain types of events, identified by
   the event package.

   With a successful subscription, a subscriber receives event
   notifications that communicate the resource state and the changes
   thereto.  Each event notification carries a representation of the
   current resource state.  This representation is influenced by many
   factors, e.g., authorization and filtering rules, and the event
   composition rules of the notifier.

   This representation is realized in what is called an entity.  Each
   resource may be associated with zero or more entities; however, an
   entity is only valid for a single resource.

      Note that, as can be seen from the illustration, the association
      between a resource and an entity follows the typical composition
      relationship, i.e., an entity may belong to only one resource, and
      it is expected to only exist with that resource.

   An entity consists of the data carried in the body of a NOTIFY
   message, and related meta-data in the message header.  This meta-data
   includes, but is not limited to the following SIP header fields:

   entity-header  =  Content-Disposition   ; defined in RFC 3261
                  /  Content-Encoding      ; defined in RFC 3261
                  /  Content-Language      ; defined in RFC 3261
                  /  Content-Length        ; defined in RFC 3261
                  /  Content-Type          ; defined in RFC 3261
                  /  Event                 ; defined in RFC 3265
                  /  extension-header      ; defined in RFC 3261

   Note that the Subscription-State is explicitly not part of the
   entity.  Event packages may in the future define additional fields
   that implementations need to consider as part of the entity.

   An entity has one or more versions of which only one is current and
   all others stale.  Each version has an entity-tag, which uniquely



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   identifies it across all versions of all entities pertaining to a
   single resource and event package.

   Note that two entity-tags being equal does not indicate identical
   entities.  In other words, if an entity-tag is received that matches
   a previously seen entity-tag, the subscriber cannot assume the event
   state to be identical to that received earlier.

   With partial event notification, the NOTIFY message only carries the
   delta state, or the set of changes to the previous version of the
   entity.  In that case, implementations MUST consider the full event
   state as the version of the entity to which the entity-tag in the
   NOTIFY message applies.

   The conditional notification mechanism is independent of the way in
   which subscriptions are installed.  In other words, the mechanism
   supports implicit subscriptions, such as those associated with the
   REFER method [RFC3515].

   It is possible that the same resource is in some shape or form
   accessible through another mechanism in addition to SIP Event
   Notification, e.g., HTTP or the SIP PUBLISH method.  In general,
   implementations MUST NOT expect the entity-tags to be shared between
   the mechanisms, unless event packages or specific applications of SIP
   Events explicitly define such dependencies.


5.  Subscriber Behavior

   This section augments the subscriber behavior defined in RFC3265
   [RFC3265].  It first discusses general issues related to indicating
   support for the mechanism (Section 5.1) and creating conditions in
   SUBSCRIBE requests (Section 5.2); it then describes the workflows for
   the main three use cases for making the subscription conditional.

5.1.  Detecting Support for Conditional Notification

   The mechanism defined in this memo is backwards compatible with SIP
   events [RFC3265] in that a notifier supporting this mechanism will
   insert a SIP entity-tag in its NOTIFY requests, and a subscriber that
   understands this mechanism will know how to use it in creating a
   conditional request.

   Unaware subscribers will simply ignore the entity-tag, make requests
   without conditions and receive the default treatment from the
   notifier.  Unaware notifiers will simply ignore the conditional
   header fields, and continue normal operation.




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5.2.  Generating SUBSCRIBE Requests

   When creating a conditional SUBSCRIBE request, the subscriber MUST
   include a single conditional header field including an entity-tag in
   the request.  The condition is evaluated by comparing the entity-tag
   of the subscribed resource with the entity-tag carried in the
   conditional header field.  If they match, the condition evaluates to
   true.

   Unlike the condition introduced for the SIP PUBLISH [RFC3903] method,
   these conditions do not apply to the SUBSCRIBE request itself, but to
   the resulting NOTIFY requests.  When true, the condition drives the
   notifier to change its behavior with regards to sending the
   notifications after the SUBSCRIBE.

   This specification defines a new header field called "Suppress-If-
   Match".  This header field introduces a condition to the SUBSCRIBE
   request.  If true, it instructs the notifier to either omit the body
   of the resulting NOTIFY message (if the SUBSCRIBE is not sent within
   an existing dialog), or suppress (i.e., block) the NOTIFY request
   that would otherwise be triggered the SUBSCRIBE (for an established
   dialog).  In the latter case, the SUBSCRIBE message will be answered
   with a 204 (No Notification) response.  As long as the condition
   remains true, it also instructs the notifier to either suppress any
   subsequent NOTIFY request, or if there are reportable changes in the
   NOTIFY header, e.g., the Subscription-State has changed, suppress the
   body of any subsequent NOTIFY request.

   If the condition is false, the notifier follows its default
   behaviour.

   If the subscriber receives a 204 (No Notification) to an in-dialog
   SUBSCRIBE, the subscriber can clear handle that it may have had
   pending on a NOTIFY in response the SUBSCRIBE message.

   The value of the "Suppress-If-Match" header field is an entity-tag,
   which is an opaque token that the subscriber simply copies from a
   previously received NOTIFY request.

   Example:

      Suppress-If-Match: b4cf7

   The header field can also be wildcarded using the special "*" entity-
   tag value.  Such a condition always evaluates to true regardless of
   the value of the current entity-tag for the resource.

   Example:



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      Suppress-If-Match: *

   Such a wildcard condition effectively quenches a subscription; the
   only notifications received are those reporting changes to the
   subscription state and those in response to a SUBSCRIBE message sent
   outside of an existing dialog.  In both cases, the notifications will
   not contain a body.

      A subscription with a wildcard "Suppress-If-Match" condition is
      useful in scenarios where the subscriber wants to temporarily put
      a subscription in dormant mode.  For example, a host may want to
      conserve bandwidth and power when it detects from screen or input
      device inactivity that the user isn't actively monitoring the
      presence statuses of contacts.

5.3.  Receiving NOTIFY Requests

   When a subscriber receives a NOTIFY request that contains a SIP-ETag
   header field, it MUST store the entity-tag if it wishes to make use
   of the conditional notification mechanism.  The subscriber MUST be
   prepared to receive a NOTIFY with any entity-tag value, including a
   value that matches any previous value that the subscriber might have
   seen.

   The subscriber MUST NOT infer any meaning from the value of an
   entity-tag; specifically, the subscriber MUST NOT assume identical
   entities (i.e., event state) for NOTIFYs with identical entity-tag
   values.

      Note that there are valid cases for which identical entity-tag
      values indeed imply identical event state.  For example, it is
      possible to generate entity-tag values using a one-way hash
      function.

5.4.  Polling or Fetching Resource State

   Polling with conditional notification allows a user agent to
   efficiently poll resource state.  This is accomplished using the
   Suppress-If-Match condition:












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   Subscriber                               Notifier
   ----------                               --------

  (1) SUBSCRIBE       -------->
      Expires: 0
                      <--------       (2) 202

                      <--------       (3) NOTIFY
                                          Subscription-State: terminated
                                          SIP-ETag: f2e45
                                          Content-Length: 17539

  (4) 200             -------->


  ... poll interval elapses ...


  (5) SUBSCRIBE       -------->
      Suppress-If-Match: f2e45
      Expires: 0
                      <--------       (6) 202

                      <--------       (7) NOTIFY
                                          Subscription-State: terminated
                                          SIP-ETag: f2e45
                                          Content-Length: 0

  (8) 200             -------->




                     Figure 3: Polling Resource State

   1.  The subscriber polls for resource state by sending a SUBSCRIBE
       with zero expiry (expires immediately).

   2.  The notifier accepts the SUBSCRIBE with a 202 (Accepted)
       response.

   3.  The notifier then immediately sends a first (and last) NOTIFY
       request with the current resource state, and the current entity-
       tag in the SIP-ETag header field.

   4.  The subscriber accepts the notification with a 200 (OK) response.





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   5.  After some arbitrary poll interval, the subscriber sends another
       SUBSCRIBE with a Suppress-If-Match header field that includes the
       entity-tag received in the previous NOTIFY.

   6.  The notifier accepts the SUBSCRIBE with a 202 (Accepted)
       response.

   7.  Since the resource state has not changed since the previous poll
       occurred, the notifier sends a NOTIFY message with no body.  It
       also mirrors the current entity-tag of the resource in the SIP-
       ETag header field.

   8.  The subscriber accepts the notification with a 200 (OK) response.

5.5.  Resuming a Subscription

   Resuming a subscription means the ability to continue an earlier
   subscription that either closed abruptly, or was explicitly
   terminated.  When resuming, the subscription is established without
   transmitting the resource state.  This is accomplished with
   conditional notification and the Suppress-If-Match header field:


          Subscriber                               Notifier
          ----------                               --------

      (1) SUBSCRIBE       -------->
          Suppress-If-Match: ega23
          Expires: 3600
                          <--------       (2) 202

                          <--------       (3) NOTIFY
                                              Subscription-State: active
                                              SIP-ETag: ega23
                                              Content-Length: 0
      (4) 200             -------->


                     Figure 4: Resuming a Subscription

   1.  The subscriber attempts to resume an earlier subscription by
       including a Suppress-If-Match header field with the entity-tag it
       last received.

   2.  The notifier accepts the subscription after proper authentication
       and authorization, by sending a 202 (Accepted) response.





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   3.  Since the condition is true, the notifier then immediately sends
       an initial NOTIFY request that has no body.  It also mirrors the
       current entity-tag of the resource in the SIP-ETag header field.

   4.  The subscriber accepts the NOTIFY and sends a 200 (OK) response.

   Had the entity-tag not been valid any longer, the condition would
   have evaluated to false, and the NOTIFY would have had a body
   containing the latest resource state.

5.6.  Refreshing a Subscription

   To refresh a subscription using conditional notification, the
   subscriber creates a subscription refresh before the subscription is
   about to expire, and uses the Suppress-If-Match header field:


          Subscriber                               Notifier
          ----------                               --------

          (1) SUBSCRIBE       -------->
              Suppress-If-Match: aba91
              Expires: 3600

                              <--------       (2) 204
                                                  Expires: 3600


                    Figure 5: Refreshing a Subscription

   1.  Before the subscription is about to expire, the subscriber sends
       a SUBSCRIBE request that includes the Suppress-If-Match header
       field with the latest entity-tag it has seen.

   2.  If the condition evaluates to true, the notifier sends a 204 (No
       Notification) response and sends no NOTIFY request.  The Expires
       header field of the 204 (No Notification) indicates the new
       expiry time.

5.7.  Terminating a Subscription

   To terminate a subscription using conditional notification, the
   subscriber creates a SUBSCRIBE request with a Suppress-If-Match
   condition:







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          Subscriber                               Notifier
          ----------                               --------

          (1) SUBSCRIBE       -------->
              Suppress-If-Match: ega23
              Expires: 0

                              <--------       (2) 204


                   Figure 6: Terminating a Subscription

   1.  The subscriber decides to terminate the subscription and sends a
       SUBSCRIBE request with the Suppress-If-Match condition with the
       entity-tag it has last seen.

   2.  If the condition evaluates to true, the notifier sends a 204 (No
       Notification) response, which concludes the subscription, and the
       subscriber can clear all state related to the subscription.

5.8.  Handling Transient Errors

   This section is non-normative.

   In some deployments, there may be Back-to-Back User Agent (B2BUA)
   devices that track SIP dialogs such as subscription dialogs.  These
   devices may be unaware of the conditional notification mechanism.

   It is possible that some B2BUA devices may treat a NOTIFY with
   suppressed body as an error, or may expect all SUBSCRIBE messages to
   have an associated NOTIFY message.

   In general, there is very little that an endpoint can do to recover
   from such transient errors.  The most that can be done is to try to
   detect such errors, and define a fall back behavior.

   If subscribers encounter transient errors in conditional
   notification, they should disable the feature and fall back to normal
   subscription behavior.


6.  Notifier Behavior

   This section augments the notifier behavior as specified in RFC3265
   [RFC3265].






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6.1.  Generating Entity-tags

   A notifier MUST generate entity-tags for event notifications of all
   resources it is responsible for.  The entity-tag MUST be unique
   across all versions of all entities for a resource and event package.

   An entity-tag is a token carried in the SIP-ETag header field, and it
   is opaque to the client.  The notifier is free to decide on any means
   for generating the entity-tag.  It can have any value, except for
   "*".  For example, one possible method is to implement the entity-tag
   as a simple counter, incrementing it by one for each generated
   notification per resource.

   An entity-tag is considered valid for as long as the entity is valid.
   An entity becomes stale when its version is no longer the current
   one.  The notifier MUST remember the entity-tag of an entity as long
   as the version of the entity is current.  The notifier MAY remember
   the entity-tag longer than this, e.g., for implementing journaled
   state differentials (Section 6.4).

   The entity tag values used in publications are not necessarily shared
   with the entity tag values used in subscriptions.  This is because
   there may not always be a one-to-one mapping between a publication
   and a notification; there may be several sources to the event
   composition process.

6.2.  Suppressing NOTIFY Bodies

   When a condition in a SUBSCRIBE request for suppressing notifications
   is true (i.e., the local entity-tag for the resource state and the
   entity-tag in a Suppress-If-Match header field match) but there are
   reportable changes in the NOTIFY header (e.g., the Subscription-State
   has changed), the notifier MUST suppress the body of the NOTIFY
   request.  That is, the resulting NOTIFY contains no Content-Type
   header field, the Content-Length is set to zero, and no payload is
   attached to the message.

   Additionally, when a condition in a SUBSCRIBE request for suppressing
   notifications is true and the SUBSCRIBE message is not sent within an
   established dialog, the notifier MUST send a NOTIFY request with a
   suppressed entity body.

   Suppressing the entity body of a NOTIFY does not change the current
   entity-tag of the resource.  Hence, the NOTIFY MUST contain a SIP-
   Etag header field that contains the unchanged entity-tag of the
   resource state.

   A Suppress-If-Match header field that includes an entity-tag with the



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   value of "*" MUST always evaluate to true.

6.3.  Suppressing NOTIFY Requests

   When a condition in a SUBSCRIBE request to suppress notifications is
   true (i.e., the local entity-tag of the resource and the entity-tag
   in a Suppress-If-Match header field match), and the SUBSCRIBE is sent
   within an established dialog, then the notifier MUST suppress the
   resulting NOTIFY request, and generate a 204 (No Notification)
   response.  As long as the condition remains true, and there are no
   reportable changes in the NOTIFY header, all subsequent NOTIFY
   requests MUST also be suppressed.

   Notifiers MUST NOT suppress a NOTIFY unless the corresponding
   SUBSCRIBE message was sent in an established dialog.

   A successful conditional SUBSCRIBE request MUST extend the
   subscription expiry time.

   Suppressing the entire NOTIFY has no effect on the entity-tag of the
   resource.  In other words, it remains unchanged.

   A Suppress-If-Match header field that includes an entity-tag with the
   value of "*" MUST always evaluate to true.

6.4.  State Differentials

   Some event packages may support a scheme where notifications contain
   state differentials, or state deltas [RFC3265] instead of complete
   resource state.

   A notifier can optionally keep track of the state changes of a
   resource, e.g., storing the changes in a journal.  If a condition
   fails, the notifier MAY send a state differential in the NOTIFY
   rather than the full state of the event resource.  This is only
   possible if the event package and the subscriber both support a
   payload format that has this capability.

   When state differentials are sent, the SIP-ETag header field MUST
   contain an entity-tag that corresponds to the full resource state.

6.5.  List Subscriptions

   The Event Notification Extension for Resource Lists [RFC4662] defines
   a mechanism for subscribing to a homogeneous list of resources using
   the SIP events framework.

   A list subscription delivers event notifications that contain both



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   Resource List Meta-Information (RLMI) documents as well as the
   resource state of the individual resources on the list.

   Implementations MUST consider the full resource state of a resource
   list including RLMI and the entity-header as the entity to which the
   entity-tag applies.


7.  Protocol Element Definitions

   This section describes the protocol extensions required for
   conditional notification.

7.1.  204 (No Notification) Response Code

   The 204 (No Notification) response code indicates that the request
   was successful, but the notification associated with the request will
   not be sent.  It is valid only in response to a SUBSCRIBE message
   sent within an established dialog.

   The response code is added to the "Success" production rule in the
   SIP [RFC3261] message grammar.

7.2.  Suppress-If-Match Header Field

   The Suppress-If-Match header field is added to the definition of the
   "message-header" rule in the SIP [RFC3261] grammar.  Its use is
   described in Section 5, Section 6.3 and Section 6.2.

   This header field is allowed to appear in any request, but its
   behavior is only defined for the SUBSCRIBE request.

7.3.  Grammar

   This section defines the formal syntax for extensions described in
   this memo in Augmented BNF (ABNF) [RFC5234].  The rules defined here
   augment and reference the syntax defined in RFC3261 [RFC3261] and
   RFC3903 [RFC3903].













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  Success             =/ "204"  ;  No Notification

        ; Success is defined in RFC3261.

  message-header      =/ Suppress-If-Match

        ; message-header is defined in RFC3261.

  Suppress-If-Match   =  "Suppress-If-Match" HCOLON ( entity-tag / "*" )

        ; entity-tag is defined in RFC3903.



8.  IANA Considerations

   This document registers a new response code and a new header field
   name.

      Note to IANA and the RFC editor: please replace all occurrences of
      RFCXYZ in this section with the RFC number of this specification
      upon publication.

8.1.  204 (No Notification) Response Code

   This document registers a new response code.  This response code is
   defined by the following information, which has been added to the
   methods and response-codes sub-registry under
   http://www.iana.org/assignments/sip-parameters.

     This information is to be added under "Successful 2xx" category.

                    +---------------------+-----------+
                    | Response Code       | Reference |
                    +---------------------+-----------+
                    | 204 No Notification | [RFCXYZ]  |
                    +---------------------+-----------+

8.2.  Suppress-If-Match Header Field

   This document registers a new SIP header field called Suppress-If-
   Match.  This header field is defined by the following information,
   which has been added to the header fields sub-registry under
   http://www.iana.org/assignments/sip-parameters.







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                +-------------------+---------+-----------+
                | Header Name       | Compact | Reference |
                +-------------------+---------+-----------+
                | Suppress-If-Match |         | [RFCXYZ]  |
                +-------------------+---------+-----------+


9.  Security Considerations

   The security considerations for SIP event notification are
   extensively discussed in RFC 3265 [RFC3265].  This specification
   introduces an optimization to SIP event notification, which in itself
   does not alter the security properties of the protocol.


10.  Acknowledgments

   The following people have contributed corrections and suggestions to
   this document: Adam Roach, Sean Olson, Johnny Vrancken, Pekka Pessi,
   Eva Leppanen, Krisztian Kiss, Peili Xu, Avshalom Houri, David
   Viamonte, Jonathan Rosenberg, Qian Sun, Dale Worley, Tolga Asveren,
   Brian Stucker, Eric Rescorla, Arun Arunachalam and the SIP and SIMPLE
   working groups.


11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3265]  Roach, A., "Session Initiation Protocol (SIP)-Specific
              Event Notification", RFC 3265, June 2002.

   [RFC3903]  Niemi, A., "Session Initiation Protocol (SIP) Extension
              for Event State Publication", RFC 3903, October 2004.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.






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11.2.  Informative References

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3515]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
              Method", RFC 3515, April 2003.

   [RFC3680]  Rosenberg, J., "A Session Initiation Protocol (SIP) Event
              Package for Registrations", RFC 3680, March 2004.

   [RFC3842]  Mahy, R., "A Message Summary and Message Waiting
              Indication Event Package for the Session Initiation
              Protocol (SIP)", RFC 3842, August 2004.

   [RFC3856]  Rosenberg, J., "A Presence Event Package for the Session
              Initiation Protocol (SIP)", RFC 3856, August 2004.

   [RFC3859]  Peterson, J., "Common Profile for Presence (CPP)",
              RFC 3859, August 2004.

   [RFC4662]  Roach, A., Campbell, B., and J. Rosenberg, "A Session
              Initiation Protocol (SIP) Event Notification Extension for
              Resource Lists", RFC 4662, August 2006.


Author's Address

   Aki Niemi
   Nokia
   P.O. Box 407
   NOKIA GROUP, FIN  00045
   Finland

   Phone: +358 50 389 1644
   Email: aki.niemi@nokia.com














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