Network Working Group J. Rosenberg

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).

Abstract

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",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
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
subscriptions.

3. Usage Scenarios

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

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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:[email protected] SIP/2.0
Via: SIP/2.0/UDP pc34.example.com;branch=z9hG4bKnashds7
From: sip:[email protected];tag=123s8a
To: sip:[email protected]
Call-ID: [email protected]
Max-Forwards: 70
CSeq: 9887 SUBSCRIBE
Contact: sip:[email protected]
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:[email protected] SIP/2.0
Via: SIP/2.0/UDP server.example.com;branch=z9hG4bKna66g
From: sip:[email protected];tag=xyz887
To: sip:[email protected];tag=123s8a
Call-ID: [email protected]
Max-Forwards: 70
CSeq: 1288 NOTIFY
Contact: sip:[email protected]
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:[email protected]" package="presence">
<watcher id="7768a77s" event="subscribe"
status="pending">sip:[email protected]</watcher>
</watcher-list>
</watcherinfo>

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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
subscription.

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
request.

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:[email protected] might be defined within example.com to
refer to all resources. In that case, a watcherinfo subscription for
"presence.winfo" to sip:[email protected] 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.

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

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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
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:[email protected] 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:[email protected] SIP/2.0
Via: SIP/2.0/UDP pc34.example.com;branch=z9hG4bKnashds7
From: sip:[email protected];tag=123aa9
To: sip:[email protected]
Call-ID: [email protected]
CSeq: 9887 SUBSCRIBE
Contact: sip:[email protected]
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:[email protected];tag=123aa9
To: sip:[email protected];tag=xyzygg
Call-ID: [email protected]
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 [email protected]:

NOTIFY sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP server19.example.com;branch=z9hG4bKnasaii
From: sip:[email protected];tag=xyzygg
To: sip:[email protected];tag=123aa9
Call-ID: [email protected]
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:[email protected]" package="presence">
<watcher id="77ajsyy76" event="subscribe"
status="pending">sip:[email protected]</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:[email protected];tag=xyzygg
To: sip:[email protected];tag=123aa9
Call-ID: [email protected]
CSeq: 1288 NOTIFY

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RFC 3857 Watcher Information August 2004

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:[email protected] SIP/2.0
Via: SIP/2.0/UDP server19.example.com;branch=z9hG4bKnasaij
From: sip:[email protected];tag=xyzygg
To: sip:[email protected];tag=123aa9
Call-ID: [email protected]
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:[email protected]" package="presence">
<watcher id="77ajsyy76" event="approved"
status="active">sip:[email protected]</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:[email protected];tag=xyzygg
To: sip:[email protected];tag=123aa9
Call-ID: [email protected]
CSeq: 1289 NOTIFY

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RFC 3857 Watcher Information August 2004

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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RFC 3857 Watcher Information August 2004

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, [email protected].

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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RFC 3857 Watcher Information August 2004

[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
dynamicsoft
600 Lanidex Plaza
Parsippany, NJ 07054

EMail: [email protected]

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RFC 3857 Watcher Information August 2004

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
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

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