A Watcher Information Event Template-Package for the Session Initiation Protocol (SIP)
draft-ietf-simple-winfo-package-05

Internet Engineering Task Force                                SIMPLE WG
Internet Draft                                              J. Rosenberg
                                                             dynamicsoft
draft-ietf-simple-winfo-package-05.txt
January 31, 2003
Expires: July 2003

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

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
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   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
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   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 ....................................   18
   9          Authors Addresses ...................................   18
   10         Normative References ................................   18
   11         Informative References ..............................   18

J. Rosenberg                                                  [Page 2]
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1 Introduction

   The 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 draft 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.

   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

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   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" 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 server.example.com;branch=z9hG4bKna66g
   From: sip:B@example.com;tag=xyz887
   To: sip:B@example.com;tag=123s8a

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   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"
                 status="pending">sip:A@example.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
   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

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   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.

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
   subscription.

4.4 Subscription Duration

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   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 watcher, 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.

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
   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 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

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   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
   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
   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.
   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

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   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.

   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 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 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. 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

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       subscribe,                                                         
       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  | |          +-------+          |               
                          | |subscribe |       |          |               
                          | +----------|waiting|----------+               
                          +----------->|       |                          
                           timeout     |       |                          
                                       +-------+                          
                                                                          

   Figure 1: Subscription State Machine

   about this subscription attempt.

   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

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

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

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   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.

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 contruct 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,
   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

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   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 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

   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 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

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   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
     ;received=192.0.2.8
   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

   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

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   Call-ID: 9987@pc34.example.com
   CSeq: 1288 NOTIFY
   Contact: sip:server19.example.com
   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@example.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.example.com;branch=z9hG4bKnasaii
     ;received=192.0.2.7
   From: sip:joe@example.com;tag=xyzygg
   To: sip:joe@example.com;tag=123aa9
   Call-ID: 9987@pc34.example.com
   CSeq: 1288 NOTIFY

   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
   notification:

   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
   Max-Forwards: 70

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   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"
                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.example.com;branch=z9hG4bKnasaij
     ;received=192.0.2.7
   From: sip:joe@example.com;tag=xyzygg
   To: sip:joe@example.com;tag=123aa9
   Call-ID: 9987@pc34.example.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,

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   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.

   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 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

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   in Section 6.2 of RFC 3265 [1].

        Package Name: winfo

        Template Package: yes

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

        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 Authors Addresses

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

10 Normative References

   [1] A. B. Roach, "Session initiation protocol (sip)-specific event
   notification," RFC 3265, Internet Engineering Task Force, June 2002.

   [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 extensible markup language (XML) based format
   for watcher information," internet draft, Internet Engineering Task
   Force, Dec. 2002.  Work in progress.

   [4] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J.
   Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
   initiation protocol," RFC 3261, Internet Engineering Task Force, June
   2002.

11 Informative References

   [5] J. Rosenberg, "A presence event package for the session

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   initiation protocol (SIP)," internet draft, Internet Engineering Task
   Force, Dec. 2002.  Work in progress.

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J. Rosenberg                                                 [Page 19]
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J. Rosenberg                                                 [Page 20]