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Versions: 00 01 02 03 04 05 RFC 3857

Internet Engineering Task Force                                SIMPLE WG
Internet Draft                                              J. Rosenberg
                                                             dynamicsoft
draft-ietf-simple-winfo-package-02.txt
May 20, 2002
Expires: November 2002


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

STATUS OF THIS MEMO

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   To view the list Internet-Draft Shadow Directories, see
   http://www.ietf.org/shadow.html.


Abstract

   This document defines the watcher information template-package for
   the 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 ........................................    3
   2          Terminology .........................................    3
   3          Usage Scenarios .....................................    4
   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 ............................    6
   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 ......................    8
   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 .......................................   13
   6          Security Considerations .............................   16
   6.1        Denial of Service Attacks ...........................   16
   6.2        Divulging Sensitive Information .....................   17
   7          IANA Considerations .................................   17
   8          Acknowledgements ....................................   17
   9          Authors Addresses ...................................   18
   10         Normative References ................................   18
   11         Informative References ..............................   18
















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1 Introduction

   The SIP event framework is described in [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 draft defines a "template-package" within the SIP events
   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 sub-package itself.

   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",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in 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



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   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" subscribers, subscriptions, 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
   subscriptions.

3 Usage Scenarios

   There are many useful applications for the watcher information
   template-package.

3.1 Presence Authorization

   The motivating application for this 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 pc34.example.com;branch=z9hG4bKna66g
   From: sip:B@example.com;tag=xyz887
   To: sip:B@example.com;tag=123s8a
   Call-ID: 9987@pc34.example.com



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   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@foo.com" package="presence">
        <watcher id="7768a77s" event="subscribe"
                 status="pending">sip:A@foo.com</watcher>
      </watcher-list>
   </watcherinfo>



   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
   go 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 5.4 of [1].

4.1 Event Package Name

   The SIP Events specification requires package definitions to specify



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   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

   The SIP Events specification requires package and template-package
   definitions to specify any package specific parameters of the Event
   header that are used by it.

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

4.3 SUBSCRIBE Bodies

   The SIP Events specification requires package or template-package
   definitions to define the usage, if any, of bodies in SUBSCRIBE
   requests.

   A SUBSCRIBE 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 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
   subscription.

4.4 Subscription Duration



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   The SIP Events specification 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.

   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 include in
   Expires header specifying their preferred duration.

4.5 NOTIFY Bodies

   The SIP Events specification 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 in the
   SUBSCRIBE request.

   The body of the watcherinfo notification contains a watcher
   information document. This document describes some or all of the
   watchers for a given package, and the state of their subscriptions.
   All watcherinfo subscribers MUST support the
   application/watcherinfo+xml format described in [3], and MUST list
   its MIME type, application/watcherinfo+xml, in any Accept header
   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 in the
   SUBSCRIBE request. If no Accept header was present, the notifications
   MUST use the application/watcherinfo+xml format described in [3].

4.6 Notifier Processing of SUBSCRIBE Requests

   The SIP Events framework 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



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   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 watcherinfo subscriptions for some package foo
   for user A be allowed from user A themselves. That is, it is
   RECOMMENDED that a user be allowed to subscribe to their own watcher
   information. 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
   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.

   Watcherinfo notifications MAY be generated for watcher information on
   package foo, when the subscription state for a user on package foo
   changes. The watcher information package therefore needs a model of
   subscription state. This is accomplished by specifying a subscription
   state machine, described below, which governs the subscription state
   of a user in any package. Watcherinfo notifications MAY be generated
   on transitions in this state machine. Its important to note that this
   FSM is just a model of the 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 the SIP Events



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   framework [1], but adds the notion of a waiting state.

   Initially, there is no state allocated for a subscription (the init
   state). When a SUBSCRIBE request arrives, the subscription FSM is
   created. 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 [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 is destroyed. The purpose of the waiting state is so that a user
   can 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. Lets 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.




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

   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. If, while in the
   waiting state, the subscription is refreshed through another
   SUBSCRIBE, it moves back into the pending state.



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   The giveup event is generated in either the waiting or pending states
   to destroy resources associated with unauthorized subscriptions.
   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 which retain state for unauthorized subscriptions add
   policies which prohibit a particular subscriber from having more than
   some number of pending or waiting subscriptions.

   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 4.2.4 of [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 does 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. 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 state changes which would not be
   reported (because of authorization policy) in the Subscription-State
   header in notifications on foo.

   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



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   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.

4.8 Subscriber Processing of NOTIFY Requests

   The SIP Events framework 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 contruct a coherent
   view of the state of the subscribed resource. Typically, the NOTIFY
   for watcherinfo 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,
   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 SUBCRIBE 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



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   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 SUSCRIBE.

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

4.10 Rate of Notifications

   The SIP Events framework 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

   The SIP Events framework 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 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@bar.com provides presence



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   through the bar.com presence server. Joe subscribes to his own
   watcher information, in order to learn about people who subscribe to
   his presence, in order to approve them. Joe sends the following
   SUBSCRIBE request:


   SUBSCRIBE sip:joe@bar.com SIP/2.0
   Via: SIP/2.0/UDP pc34.bar.com;branch=z9hG4bKnashds7
   From: sip:joe@bar.com;tag=123aa9
   To: sip:joe@bar.com
   Call-ID: 9987@pc34.bar.com
   CSeq: 9887 SUBSCRIBE
   Contact: sip:joe@pc34.bar.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.bar.com;branch=z9hG4bKnashds8
     ;received=1.2.3.4
   From: sip:joe@bar.com;tag=123aa9
   To: sip:joe@bar.com;tag=xyzygg
   Call-ID: 9987@pc34.bar.com
   CSeq: 9988 SUBSCRIBE
   Contact: sip:server19.bar.com
   Expires: 3600
   Event: presence.winfo



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


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



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   Event: presence.winfo
   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@bar.com" package="presence">
       <watcher id="77ajsyy76" event="subscribe"
                status="pending">sip:A@example.com</watcher>
     </watcher-list>
   </watcherinfo>



   Joe then responds with a 200 OK to the NOTIFY:


   SIP/2.0 200 OK
   Via: SIP/2.0/UDP server19.bar.com;branch=z9hG4bKnasaii
     ;received=1.2.3.8
   From: sip:joe@bar.com;tag=xyzygg
   To: sip:joe@bar.com;tag=123aa9
   Call-ID: 9987@pc34.bar.com
   CSeq: 1288 NOTIFY



   The NOTIFY tells Joe that 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 notification:


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

   <?xml version="1.0"?>



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




   B then responds with a 200 OK to the NOTIFY:


   SIP/2.0 200 OK
   Via: SIP/2.0/UDP server19.bar.com;branch=z9hG4bKnasaij
     ;received=1.2.3.8
   From: sip:joe@bar.com;tag=xyzygg
   To: sip:joe@bar.com;tag=123aa9
   Call-ID: 9987@pc34.bar.com
   CSeq: 1289 NOTIFY



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.

   Firstly, 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



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   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 contains nformation about 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 set of 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.

   One way in which this information can be revealed is eavesdropping.
   An attacker can observe watcherinfo notifications, and learn this
   information. To prevent that, the notifications can be encrypted
   using SIPs S/MIME feature. 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.

7 IANA Considerations

   This specification registers an event template package as specified
   in Section 7.2 of [1].

        Package Name: winfo

        Template Package: yes

        Published Specification: RFC XXXX (Note to IANA: Please replace
             XXXX with the RFC number of this specification.)

8 Acknowledgements

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



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9 Authors Addresses


   Jonathan Rosenberg
   dynamicsoft
   72 Eagle Rock Avenue
   First Floor
   East Hanover, NJ 07936
   email: jdrosen@dynamicsoft.com



10 Normative References

   [1] A. Roach, "SIP-specific event notification," Internet Draft,
   Internet Engineering Task Force, Mar. 2002.  Work in progress.

   [2] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.

   [3] J. Rosenberg, "An XML based format for watcher information,"
   Internet Draft, Internet Engineering Task Force, Mar. 2002.  Work in
   progress.

   [4] J. Rosenberg, H. Schulzrinne, et al.  , "SIP: Session initiation
   protocol," Internet Draft, Internet Engineering Task Force, Feb.
   2002.  Work in progress.

11 Informative References

   [5] J. Rosenberg et al.  , "Session initiation protocol (SIP)
   extensions for presence," Internet Draft, Internet Engineering Task
   Force, Apr. 2002.  Work in progress.


   Full Copyright Statement

   Copyright (c) The Internet Society (2002). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of



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   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
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   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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J. Rosenberg                                                 [Page 19]


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