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Network Working Group                                       J. Rosenberg
Request for Comments: 3857                                   dynamicsoft
Category: Standards Track                                    August 2004

           A Watcher Information Event Template-Package for
                 the Session Initiation Protocol (SIP)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).


   This document defines the watcher information template-package for
   the Session Initiation Protocol (SIP) event framework.  Watcher
   information refers to the set of users subscribed to a particular
   resource within a particular event package.  Watcher information
   changes dynamically as users subscribe, unsubscribe, are approved, or
   are rejected.  A user can subscribe to this information, and
   therefore learn about changes to it.  This event package is a
   template-package because it can be applied to any event package,
   including itself.

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Table of Contents

   1.   Introduction ........................................    2
   2.   Terminology .........................................    3
   3.   Usage Scenarios .....................................    3
        3.1.  Presence Authorization ........................    4
        3.2.  Blacklist Alerts ..............................    5
   4.   Package Definition ..................................    5
        4.1.  Event Package Name ............................    5
        4.2.  Event Package Parameters ......................    5
        4.3.  SUBSCRIBE Bodies ..............................    6
        4.4.  Subscription Duration .........................    6
        4.5.  NOTIFY Bodies .................................    7
        4.6.  Notifier Processing of SUBSCRIBE Requests......    7
        4.7.  Notifier Generation of NOTIFY Requests ........    8
              4.7.1.  The Subscription State Machine.........    9
              4.7.2.  Applying the State Machine.............   11
        4.8.  Subscriber Processing of NOTIFY Requests ......   12
        4.9.  Handling of Forked Requests ...................   12
        4.10. Rate of Notifications .........................   13
        4.11. State Agents ..................................   13
   5.   Example Usage .......................................   14
   6.   Security Considerations .............................   17
        6.1.  Denial of Service Attacks .....................   17
        6.2.  Divulging Sensitive Information ...............   17
   7.   IANA Considerations .................................   18
   8.   Acknowledgements ....................................   18
   9.   Normative References ................................   18
   10.  Informative References ..............................   19
   11.  Author's Address ....................................   19
   12.  Full Copyright Statement ............................   20

1.  Introduction

   The Session Initiation Protocol (SIP) event framework is described in
   RFC 3265 [1].  It defines a generic framework for subscription to,
   and notification of, events related to SIP systems.  The framework
   defines the methods SUBSCRIBE and NOTIFY, and introduces the notion
   of a package.  A package is a concrete application of the event
   framework to a particular class of events.  Packages have been
   defined for user presence [5], for example.

   This document defines a "template-package" within the SIP event
   framework.  A template-package has all the properties of a regular
   SIP event package.  However, it is always associated with some other
   event package, and can always be applied to any event package,
   including the template-package itself.

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   The template-package defined here is for watcher information, and is
   denoted with the token "winfo".  For any event package, such as
   presence, there exists a set (perhaps an empty set) of subscriptions
   that have been created or requested by users trying to ascertain the
   state of a resource in that package.  This set of subscriptions
   changes over time as new subscriptions are requested by users, old
   subscriptions expire, and subscriptions are approved or rejected by
   the owners of that resource.  The set of users subscribed to a
   particular resource for a specific event package, and the state of
   their subscriptions, is referred to as watcher information.  Since
   this state is itself dynamic, it is reasonable to subscribe to it in
   order to learn about changes to it.  The watcher information event
   template-package is meant to facilitate exactly that - tracking the
   state of subscriptions to a resource in another package.

   To denote this template-package, the name is constructed by appending
   ".winfo" to the name of whatever package is being tracked.  For
   example, the set of people subscribed to presence is defined by the
   "presence.winfo" package.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in BCP14, RFC 2119
   [2] and indicate requirement levels for compliant implementations.

   This document fundamentally deals with recursion - subscriptions to
   subscriptions.  Therefore, the term "subscription" itself can be
   confusing in this document.  To reduce confusion, the term
   "watcherinfo subscription" refers to a subscription to watcher
   information, and the term "watcherinfo subscriber" refers to a user
   that has subscribed to watcher information.  The term "watcherinfo
   notification" refers to a NOTIFY request sent as part of a
   watcherinfo subscription.  When the terms "subscription",
   "subscriber", and "notification" are used unqualified, they refer to
   the "inner" subscriptions, subscribers and notifications - those that
   are being monitored through the watcherinfo subscriptions.  We also
   use the term "watcher" to refer to a subscriber to the "inner"
   resource.  Information on watchers is reported through watcherinfo

3.  Usage Scenarios

   There are many useful applications for the watcher information

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3.1.  Presence Authorization

   The motivating application for this template-package is presence
   authorization.  When user A subscribes to the presence of user B, the
   subscription needs to be authorized.  Frequently, that authorization
   needs to occur through direct user intervention.  For that to happen,
   B's software needs to become aware that a presence subscription has
   been requested.  This is supported through watcher information.  B's
   client software would SUBSCRIBE to the watcher information for the
   presence of B:

   SUBSCRIBE sip:B@example.com SIP/2.0
   Via: SIP/2.0/UDP pc34.example.com;branch=z9hG4bKnashds7
   From: sip:B@example.com;tag=123s8a
   To: sip:B@example.com
   Call-ID: 9987@pc34.example.com
   Max-Forwards: 70
   CSeq: 9887 SUBSCRIBE
   Contact: sip:B@pc34.example.com
   Event: presence.winfo

   The policy of the server is such that it allows B to subscribe to its
   own watcher information.  So, when A subscribes to B's presence, B
   gets a notification of the change in watcher information state:

   NOTIFY sip:B@pc34.example.com SIP/2.0
   Via: SIP/2.0/UDP server.example.com;branch=z9hG4bKna66g
   From: sip:B@example.com;tag=xyz887
   To: sip:B@example.com;tag=123s8a
   Call-ID: 9987@pc34.example.com
   Max-Forwards: 70
   CSeq: 1288 NOTIFY
   Contact: sip:B@server.example.com
   Event: presence.winfo
   Content-Type: application/watcherinfo+xml
   Content-Length: ...

   <?xml version="1.0"?>
   <watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
                version="0" state="full">
      <watcher-list resource="sip:B@example.com" package="presence">
        <watcher id="7768a77s" event="subscribe"

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   This indicates to B that A has subscribed, and that the subscription
   is pending (meaning, it is awaiting authorization).  B's software can
   alert B that this subscription is awaiting authorization.  B can then
   set policy for that subscription.

3.2.  Blacklist Alerts

   Applications can subscribe to watcher information in order to provide
   value-added features.  An example application is "blacklist alerts".
   In this scenario, an application server maintains a list of known
   "bad guys".  A user, Joe, signs up for service with the application
   provider, presumably by going to a web page and entering in his
   presence URI.  The application server subscribes to the watcher
   information for Joe's presence.  When someone attempts to SUBSCRIBE
   to Joe's user presence, the application learns of this subscription
   as a result of its watcher info subscription.  It checks the
   watcher's URI against the database of known bad guys.  If there is a
   match, it sends email to Joe letting him know about this.

   For this application to work, Joe needs to make sure that the
   application is allowed to subscribe to his presence.winfo.

4.  Package Definition

   This section fills in the details needed to specify an event package
   as defined in Section 4.4 of RFC 3265 [1].

4.1.  Event Package Name

   RFC 3265 [1] requires package definitions to specify the name of
   their package or template-package.

   The name of this template-package is "winfo".  It can be applied to
   any other package.  Watcher information for any package foo is
   denoted by the name "foo.winfo".  Recursive template-packaging is
   explicitly allowed (and useful), so that "foo.winfo.winfo" is a valid
   package name.

4.2.  Event Package Parameters

   RFC 3265 [1] requires package and template-package definitions to
   specify any package specific parameters of the Event header field.

   No package specific Event header field parameters are defined for
   this event template-package.

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4.3.  SUBSCRIBE Bodies

   RFC 3265 [1] requires package or template-package definitions to
   define the usage, if any, of bodies in SUBSCRIBE requests.

   A SUBSCRIBE request for watcher information MAY contain a body.  This
   body would serve the purpose of filtering the watcherinfo
   subscription.  The definition of such a body is outside the scope of
   this specification.  For example, in the case of presence, the body
   might indicate that notifications should contain full state every
   time something changes, and that the time the subscription was first
   made should not be included in the watcherinfo notifications.

   A SUBSCRIBE request for a watcher information package MAY be sent
   without a body.  This implies the default watcherinfo subscription
   filtering policy has been requested.  The default policy is:

   o  Watcherinfo notifications are generated every time there is any
      change in the state of the watcher information.

   o  Watcherinfo notifications triggered from a SUBSCRIBE contain full
      state (the list of all watchers that the watcherinfo subscriber is
      permitted to know about).  Watcherinfo notifications triggered
      from a change in watcher state only contain information on the
      watcher whose state has changed.

   Of course, the server can apply any policy it likes to the

4.4.  Subscription Duration

   RFC 3265 [1] requires package definitions to define a default value
   for subscription durations, and to discuss reasonable choices for
   durations when they are explicitly specified.

   Watcher information changes as users subscribe to a particular
   resource for some package, or their subscriptions time out.  As a
   result, the state of watcher information can change very dynamically,
   depending on the number of subscribers for a particular resource in a
   given package.  The rate at which subscriptions time out depends on
   how long a user maintains its subscription.  Typically, watcherinfo
   subscriptions will be timed to span the lifetime of the subscriptions
   being watched, and therefore range from minutes to days.

   As a result of these factors, it is difficult to define a broadly
   useful default value for the lifetime of a watcherinfo subscription.
   We arbitrarily choose one hour.  However, clients SHOULD use an
   Expires header field to specify their preferred duration.

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4.5.  NOTIFY Bodies

   RFC 3265 [1] requires package definitions to describe the allowed set
   of body types in NOTIFY requests, and to specify the default value to
   be used when there is no Accept header field in the SUBSCRIBE

   The body of the watcherinfo notification contains a watcher
   information document.  This document describes some or all of the
   watchers for a resource within a given package, and the state of
   their subscriptions.  All watcherinfo subscribers and notifiers MUST
   support the application/watcherinfo+xml format described in [3], and
   MUST list its MIME type, application/watcherinfo+xml, in any Accept
   header field present in the SUBSCRIBE request.

   Other watcher information formats might be defined in the future.  In
   that case, the watcherinfo subscriptions MAY indicate support for
   other formats.  However, they MUST always support and list
   application/watcherinfo+xml as an allowed format.

   Of course, the watcherinfo notifications generated by the server MUST
   be in one of the formats specified in the Accept header field in the
   SUBSCRIBE request.  If no Accept header field was present, the
   notifications MUST use the application/watcherinfo+xml format
   described in [3].

4.6.  Notifier Processing of SUBSCRIBE Requests

   RFC 3265 [1] specifies that packages should define any package-
   specific processing of SUBSCRIBE requests at a notifier, specifically
   with regards to authentication and authorization.

   The watcher information for a particular package contains sensitive
   information.  Therefore, all watcherinfo subscriptions SHOULD be
   authenticated and then authorized before approval.  Authentication
   MAY be performed using any of the techniques available through SIP,
   including digest, S/MIME, TLS or other transport specific mechanisms
   [4].  Authorization policy is at the discretion of the administrator,
   as always.  However, a few recommendations can be made.

   It is RECOMMENDED that user A be allowed to subscribe to their own
   watcher information for any package.  This is true recursively, so
   that it is RECOMMENDED that a user be able to subscribe to the
   watcher information for their watcher information for any package.

   It is RECOMMENDED that watcherinfo subscriptions for some package foo
   for user A be allowed from some other user B, if B is an authorized
   subscriber to A within the package foo.  However, it is RECOMMENDED

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   that the watcherinfo notifications sent to B only contain the state
   of B's own subscription.  In other words, it is RECOMMENDED that a
   user be allowed to monitor the state of their own subscription.

   To avoid infinite recursion of authorization policy, it is
   RECOMMENDED that only user A be allowed to subscribe to
   foo.winfo.winfo for user A, for any foo.  It is also RECOMMENDED that
   by default, a server does not authorize any subscriptions to
   foo.winfo.winfo.winfo or any other deeper recursions.

4.7.  Notifier Generation of NOTIFY Requests

   The SIP Event framework requests that packages specify the conditions
   under which notifications are sent for that package, and how such
   notifications are constructed.

   Each watcherinfo subscription is associated with a set of "inner"
   subscriptions being watched.  This set is defined by the URI in the
   Request URI of the watcherinfo SUBSCRIBE request, along with the
   parent event package of the watcherinfo subscription.  The parent
   event package is obtained by removing the trailing ".winfo" from the
   value of the Event header field from the watcherinfo SUBSCRIBE
   request.  If the Event header field in the watcherinfo subscription
   has a value of "presence.winfo", the parent event package is
   "presence".  If the Event header field has a value of
   "presence.winfo.winfo", the parent event package is "presence.winfo".
   Normally, the URI in the Request URI of the watcherinfo SUBSCRIBE
   identifies an address-of-record within the domain.  In that case, the
   set of subscriptions to be watched are all of the subscriptions for
   the parent event package that have been made to the resource in the
   Request URI of the watcherinfo SUBSCRIBE.  However, the Request URI
   can contain a URI that identifies any set of subscriptions, including
   the subscriptions to a larger collection of resources.  For example,
   sip:all-resources@example.com might be defined within example.com to
   refer to all resources.  In that case, a watcherinfo subscription for
   "presence.winfo" to sip:all-resources@example.com is requesting
   notifications any time the state of any presence subscription for any
   resource within example.com changes.  A watcherinfo notifier MAY
   generate a notification any time the state of any of the watched
   subscriptions changes.

   Because a watcherinfo subscription is made to a collection of
   subscriptions, the watcher information package needs a model of
   subscription state.  This is accomplished by specifying a
   subscription Fine State Machine (FSM), described below, which governs
   the subscription state of a user in any package.  Watcherinfo
   notifications MAY be generated on transitions in this state machine.
   It's important to note that this FSM is just a model of the

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   subscription state machinery maintained by a server.  An
   implementation would map its own state machines to this one in an
   implementation-specific manner.

4.7.1.  The Subscription State Machine

   The underlying state machine for a subscription is shown in Figure 1.
   It derives almost entirely from the descriptions in RFC 3265 [1], but
   adds the notion of a waiting state.

   When a SUBSCRIBE request arrives, the subscription FSM is created in
   the init state. This state is transient.  The next state depends on
   whether policy exists for the subscription.  If there is an existing
   policy that determines that the subscription is forbidden, it moves
   into the terminated state immediately, where the FSM can be
   destroyed.  If there is existing policy that determines that the
   subscription is authorized, the FSM moves into the active state.
   This state indicates that the subscriber will receive notifications.

   If, when a subscription arrives, there is no authorization policy in
   existence, the subscription moves into the pending state.  In this
   state, the server is awaiting an authorization decision.  No
   notifications are generated on changes in presence state (an initial
   NOTIFY will have been delivered as per RFC 3265 [1]), but the
   subscription FSM is maintained.  If the authorization decision comes
   back positive, the subscription is approved, and moves into the
   active state.  If the authorization is negative, the subscription is
   rejected, and the FSM goes into the terminated state.  It is possible
   that the authorization decision can take a very long time.  In fact,
   no authorization decision may arrive until after the subscription
   itself expires.  If a pending subscription suffers a timeout, it
   moves into the waiting state.  At any time, the server can decide to
   end a pending or waiting subscription because it is concerned about
   allocating memory and CPU resources to unauthorized subscription
   state.  If this happens, a "giveup" event is generated by the server,
   moving the subscription to terminated.

   The waiting state is similar to pending, in that no notifications are
   generated.  However, if the subscription is approved or denied, the
   FSM enters the terminated state, and is destroyed. Furthermore, if
   another subscription is received to the same resource, from the same
   watcher, for the same event package, event package parameters and
   filter in the body of the SUBSCRIBE request (if one was present
   initially), the FSM enters the terminated state with a "giveup"
   event, and is destroyed.  This transition occurs because, on arrival
   of a new subscription with identical parameters, it will enter the
   pending state, making the waiting state for the prior subscription
   redundant.  The purpose of the waiting state is so that a user can

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   fetch watcherinfo state at any time, and learn of any subscriptions
   that arrived previously (and which may arrive again) which require an
   authorization decision.  Consider an example.  A subscribes to B.  B
   has not defined policy about this subscription, so it moves into the
   pending state.  B is not "online", so that B's software agent cannot
   be contacted to approve the subscription.  The subscription expires.
   Let's say it were destroyed.  B logs in, and fetches its watcherinfo
   state.  There is no record of the subscription from A, so no policy
   decision is made about subscriptions from A.  B logs off.  A
   refreshes its subscription.  Once more, the subscription is pending
   since no policy is defined for it.  This process could continue
   indefinitely.  The waiting state ensures that B can find out about
   this subscription attempt.

         policy=       +----------+
         reject        |          |<------------------------+
         +------------>|terminated|<---------+              |
         |             |          |          |              |
         |             |          |          |noresource    |
         |             +----------+          |rejected      |
         |                  ^noresource      |deactivated   |
         |                  |rejected        |probation     |
         |                  |deactivated     |timeout       |noresource
         |                  |probation       |              |rejected
         |                  |giveup          |              |giveup
         |                  |                |              |approved
      +-------+         +-------+        +-------+          |
      |       |subscribe|       |approved|       |          |
      | init  |-------->|pending|------->|active |          |
      |       |no policy|       |        |       |          |
      |       |         |       |        |       |          |
      +-------+         +-------+        +-------+          |
         |                  |                ^              |
         | subscribe,       |                |              |
         +-----------------------------------+              |
           policy = accept  |            +-------+          |
                            |            |       |          |
                            |            |waiting|----------+
                            +----------->|       |
                             timeout     |       |

                   Figure 1: Subscription State Machine

   The waiting state is also needed to allow for authorization of fetch
   attempts, which are subscriptions that expire immediately.

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   Of course, policy may never be specified for the subscription.  As a
   result, the server can generate a giveup event to move the waiting
   subscription to the terminated state.  The amount of time to wait
   before issuing a giveup event is system dependent.

   The giveup event is generated in either the waiting or pending states
   to destroy resources associated with unauthorized subscriptions.
   This event is generated when a giveup timer fires. This timer is set
   to a timeout value when entering either the pending or waiting
   states.  Servers need to exercise care in selecting this value.  It
   needs to be large in order to provide a useful user experience; a
   user should be able to log in days later and see that someone tried
   to subscribe to them.  However, allocating state to unauthorized
   subscriptions can be used as a source of DoS attacks.  Therefore, it
   is RECOMMENDED that servers that retain state for unauthorized
   subscriptions add policies which prohibit a particular subscriber
   from having more than some number of pending or waiting

   At any time, the server can deactivate a subscription.  Deactivation
   implies that the subscription is discarded without a change in
   authorization policy.  This may be done in order to trigger refreshes
   of subscriptions for a graceful shutdown or subscription migration
   operation.  A related event is probation, where a subscription is
   terminated, and the subscriber is requested to wait some amount of
   time before trying again.  The meaning of these events is described
   in more detail in Section 3.2.4 of RFC 3265 [1].

   A subscription can be terminated at any time because the resource
   associated with that subscription no longer exists.  This corresponds
   to the noresource event.

4.7.2.  Applying the State Machine

   The server MAY generate a notification to watcherinfo subscribers on
   a transition of the state machine.  Whether it does or not is policy
   dependent.  However, several guidelines are defined.

   Consider some event package foo.  A subscribes to B for events within
   that package.  A also subscribes to foo.winfo for B.  In this
   scenario (where the subscriber to foo.winfo is also a subscriber to
   foo for the same resource), it is RECOMMENDED that A receive
   watcherinfo notifications only about the changes in its own
   subscription.  Normally, A will receive notifications about changes
   in its subscription to foo through the Subscription-State header
   field.  This will frequently obviate the need for a separate
   subscription to foo.winfo.  However, if such a subscription is
   performed by A, the foo.winfo notifications SHOULD NOT report any

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   state changes which would not be reported (because of authorization
   policy) in the Subscription-State header field in notifications on

   As a general rule, when a watcherinfo subscriber is authorized to
   receive watcherinfo notifications about more than one watcher, it is
   RECOMMENDED that watcherinfo notifications contain information about
   those watchers which have changed state (and thus triggered a
   notification), instead of delivering the current state of every
   watcher in every watcherinfo notification.  However, watcherinfo
   notifications triggered as a result of a fetch operation (a SUBSCRIBE
   with Expires of 0) SHOULD result in the full state of all watchers
   (of course, only those watchers that have been authorized to be
   divulged to the watcherinfo subscriber) to be present in the NOTIFY.

   Frequently, states in the subscription state machine will be
   transient.  For example, if an authorized watcher performs a fetch
   operation, this will cause the state machine to be created,
   transition from init to active, and then from active to terminated,
   followed by a destruction of the FSM.  In such cases, watcherinfo
   notifications SHOULD NOT be sent for any transient states.  In the
   prior example, the server wouldn't send any notifications, since all
   of the states are transient.

4.8.  Subscriber Processing of NOTIFY Requests

   RFC 3265 [1] expects packages to specify how a subscriber processes
   NOTIFY requests in any package specific ways, and in particular, how
   it uses the NOTIFY requests to construct a coherent view of the state
   of the subscribed resource.  Typically, the watcherinfo NOTIFY will
   only contain information about those watchers whose state has
   changed.  To construct a coherent view of the total state of all
   watchers, a watcherinfo subscriber will need to combine NOTIFYs
   received over time.  This details of this process depend on the
   document format.  See [3] for details on the
   application/watcherinfo+xml format.

4.9.  Handling of Forked Requests

   The SIP Events framework mandates that packages indicate whether or
   not forked SUBSCRIBE requests can install multiple subscriptions.

   When a user wishes to obtain watcher information for some resource
   for package foo, the SUBSCRIBE to the watcher information will need
   to reach a collection of servers that have, unioned together,
   complete information about all watchers on that resource for package
   foo.  If there are a multiplicity of servers handling subscriptions
   for that resource for package foo (for load balancing reasons,

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   typically), it is very likely that no single server will have the
   complete set of watcher information.  There are several solutions in
   this case.  This specification does not mandate a particular one, nor
   does it rule out others.  It merely ensures that a broad range of
   solutions can be built.

   One solution is to use forking.  The system can be designed so that a
   SUBSCRIBE for watcher information arrives at a special proxy which is
   aware of the requirements for watcher information.  This proxy would
   fork the SUBSCRIBE request to all of the servers which could possibly
   maintain subscriptions for that resource for that package.  Each of
   these servers, whether or not they have any current subscribers for
   that resource, would accept the watcherinfo subscription.  Each needs
   to accept because they may all eventually receive a subscription for
   that resource.  The watcherinfo subscriber would receive some number
   of watcherinfo NOTIFY requests, each of which establishes a separate
   dialog.  By aggregating the information across each dialog, the
   watcherinfo subscriber can compute full watcherinfo state.  In many
   cases, a particular dialog might never generate any watcherinfo
   notifications; this would happen if the servers never receive any
   subscriptions for the resource.

   In order for such a system to be built in an interoperable fashion,
   all watcherinfo subscribers MUST be prepared to install multiple
   subscriptions as a result of a multiplicity of NOTIFY messages in
   response to a single SUBSCRIBE.

   Another approach for handling the server multiplicity problem is to
   use state agents.  See Section 4.11 for details.

4.10.  Rate of Notifications

   RFC 3265 [1] mandates that packages define a maximum rate of
   notifications for their package.

   For reasons of congestion control, it is important that the rate of
   notifications not become excessive.  As a result, it is RECOMMENDED
   that the server not generate watcherinfo notifications for a single
   watcherinfo subscriber at a rate faster than once every 5 seconds.

4.11.  State Agents

   RFC 3265 [1] asks packages to consider the role of state agents in
   their design.

   State agents play an important role in this package.  As discussed in
   Section 4.9, there may be a multiplicity of servers sharing the load
   of subscriptions for a particular package.  A watcherinfo

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   subscription might require subscription state spread across all of
   those servers. To handle that, a farm of state agents can be used.
   Each of these state agents would know the entire watcherinfo state
   for some set of resources.  The means by which the state agents would
   determine the full watcherinfo state is outside the scope of this
   specification. When a watcherinfo subscription is received, it would
   be routed to a state agent that has the full watcherinfo state for
   the requested resource.  This server would accept the watcherinfo
   subscription (assuming it was authorized, of course), and generate
   watcherinfo notifications as the watcherinfo state changed.  The
   watcherinfo subscriber would only have a single dialog in this case.

5.  Example Usage

   The following section discusses an example application and call flows
   using the watcherinfo package.

   In this example, a user Joe, sip:joe@example.com provides presence
   through the example.com presence server.  Joe subscribes to his own
   watcher information, in order to learn about people who subscribe to
   his presence, so that he can approve or reject their subscriptions.
   Joe sends the following SUBSCRIBE request:

   SUBSCRIBE sip:joe@example.com SIP/2.0
   Via: SIP/2.0/UDP pc34.example.com;branch=z9hG4bKnashds7
   From: sip:joe@example.com;tag=123aa9
   To: sip:joe@example.com
   Call-ID: 9987@pc34.example.com
   CSeq: 9887 SUBSCRIBE
   Contact: sip:joe@pc34.example.com
   Event: presence.winfo
   Max-Forwards: 70

   The server responds with a 401 to authenticate, and Joe resubmits the
   SUBSCRIBE with credentials (message not shown).  The server then
   authorizes the subscription, since it allows Joe to subscribe to his
   own watcher information for presence.  It responds with a 200 OK:

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP pc34.example.com;branch=z9hG4bKnashds8
   From: sip:joe@example.com;tag=123aa9
   To: sip:joe@example.com;tag=xyzygg
   Call-ID: 9987@pc34.example.com
   CSeq: 9988 SUBSCRIBE
   Contact: sip:server19.example.com
   Expires: 3600
   Event: presence.winfo

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   The server then sends a NOTIFY with the current state of
   presence.winfo for joe@example.com:

   NOTIFY sip:joe@pc34.example.com SIP/2.0
   Via: SIP/2.0/UDP server19.example.com;branch=z9hG4bKnasaii
   From: sip:joe@example.com;tag=xyzygg
   To: sip:joe@example.com;tag=123aa9
   Call-ID: 9987@pc34.example.com
   CSeq: 1288 NOTIFY
   Contact: sip:server19.example.com
   Event: presence.winfo
   Subscription-State: active
   Max-Forwards: 70
   Content-Type: application/watcherinfo+xml
   Content-Length: ...

   <?xml version="1.0"?>
   <watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
                version="0" state="full">
     <watcher-list resource="sip:joe@example.com" package="presence">
       <watcher id="77ajsyy76" event="subscribe"

   Joe then responds with a 200 OK to the NOTIFY:

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP server19.example.com;branch=z9hG4bKnasaii
   From: sip:joe@example.com;tag=xyzygg
   To: sip:joe@example.com;tag=123aa9
   Call-ID: 9987@pc34.example.com
   CSeq: 1288 NOTIFY

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   The NOTIFY tells Joe that user A currently has a pending
   subscription.  Joe then authorizes A's subscription through some
   means.  This causes a change in the status of the subscription (which
   moves from pending to active), and the delivery of another

   NOTIFY sip:joe@pc34.example.com SIP/2.0
   Via: SIP/2.0/UDP server19.example.com;branch=z9hG4bKnasaij
   From: sip:joe@example.com;tag=xyzygg
   To: sip:joe@example.com;tag=123aa9
   Call-ID: 9987@pc34.example.com
   CSeq: 1289 NOTIFY
   Contact: sip:server19.example.com
   Event: presence.winfo
   Subscription-State: active
   Max-Forwards: 70
   Content-Type: application/watcherinfo+xml
   Content-Length: ...

   <?xml version="1.0"?>
   <watcherinfo xmlns="urn:ietf:params:xml:ns:watcherinfo"
                version="1" state="partial">
     <watcher-list resource="sip:joe@example.com" package="presence">
       <watcher id="77ajsyy76" event="approved"

   B then responds with a 200 OK to the NOTIFY:

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP server19.example.com;branch=z9hG4bKnasaij
   From: sip:joe@example.com;tag=xyzygg
   To: sip:joe@example.com;tag=123aa9
   Call-ID: 9987@pc34.example.com
   CSeq: 1289 NOTIFY

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6.  Security Considerations

6.1.  Denial of Service Attacks

   Watcher information generates notifications about changes in the
   state of watchers for a particular resource.  It is possible for a
   single resource to have many watchers, resulting in the possibility
   of a large volume of notifications.  This makes watcherinfo
   subscription a potential tool for denial of service attacks.
   Preventing these can be done through a combination of sensible
   authorization policies and good operating principles.

   First, when a resource has a lot of watchers, watcherinfo
   subscriptions to that resource should only be allowed from explicitly
   authorized entities, whose identity has been properly authenticated.
   That prevents a watcherinfo NOTIFY stream from being generated from
   subscriptions made by an attacker.

   Even when watcherinfo subscriptions are properly authenticated, there
   are still potential attacks.  For example, consider a valid user, T,
   who is to be the target of an attack.  T has subscribed to their own
   watcher information.  The attacker generates a large number of
   subscriptions (not watcherinfo subscriptions).  If the server creates
   subscription state for unauthenticated subscriptions, and reports
   those changes in watcherinfo notifications, user T would receive a
   flood of watcherinfo notifications.  In fact, if the server generates
   a watcherinfo notification when the subscription is created, and
   another when it is terminated, there will be an amplification by a
   factor of two.  The amplification would actually be substantial if
   the server generates full state in each watcherinfo notification.
   Indeed, the amount of data sent to T would be the square of the data
   generated by the attacker! Each of the N subscriptions generated by
   the attacker would result in a watcherinfo NOTIFY being sent to T,
   each of which would report on up to N watchers.  To avoid this,
   servers should never generate subscription state for unauthenticated
   SUBSCRIBE requests, and should never generate watcherinfo
   notifications for them either.

6.2.  Divulging Sensitive Information

   Watcher information indicates what users are interested in a
   particular resource.  Depending on the package and the resource, this
   can be very sensitive information.  For example, in the case of
   presence, the watcher information for some user represents the
   friends, family, and business relations of that person.  This
   information can be used for a variety of malicious purposes.

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   One way in which this information can be revealed is eavesdropping.
   An attacker can observe watcherinfo notifications, and learn this
   information.  To prevent that, watchers MAY use the sips URI scheme
   when subscribing to a watcherinfo resource.  Notifiers for
   watcherinfo MUST support TLS and sips as if they were a proxy (see
   Section 26.3.1 of RFC 3261).

   SIP encryption, using S/MIME, MAY be used end-to-end for the
   transmission of both SUBSCRIBE and NOTIFY requests.

   Another way in which this information can be revealed is through
   spoofed subscriptions.  These attacks can be prevented by
   authenticating and authorizing all watcherinfo subscriptions.  In
   order for the notifier to authenticate the subscriber, it MAY use
   HTTP Digest (Section 22 of RFC 3261).  As a result, all watchers MUST
   support HTTP Digest.  This is a redundant requirement, however, since
   all SIP user agents are mandated to support it by RFC 3261.

7.  IANA Considerations

   This specification registers an event template package as specified
   in Section 6.2 of RFC 3265 [1].

   Package Name: winfo

   Template Package: yes

   Published Specification: RFC 3857

   Person to Contact: Jonathan Rosenberg, jdrosen@jdrosen.net.

8.  Acknowledgements

   The authors would like to thank Adam Roach, Allison Mankin and Brian
   Stucker for their detailed comments.

9.  Normative References

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

   [2]  Bradner, S., "Key Words for Use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [3]  Rosenberg, J., "An Extensible Markup Language (XML) Based Format
        for Watcher Information", RFC 3858, August 2004.

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   [4]  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.

10.  Informative References

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

11.  Author's Address

   Jonathan Rosenberg
   600 Lanidex Plaza
   Parsippany, NJ 07054

   EMail: jdrosen@dynamicsoft.com

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12. Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an

Intellectual Property

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   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   The IETF invites any interested party to bring to its attention any
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   Internet Society.

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