GEOPRIV H. Schulzrinne
Internet-Draft Columbia U.
Expires: November 22, 2006 H. Tschofenig
Siemens
J. Morris
CDT
J. Cuellar
Siemens
J. Polk
J. Rosenberg
Cisco
May 21, 2006
Common Policy: An XML Document Format for Expressing Privacy Preferences
draft-ietf-geopriv-common-policy-10.txt
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Abstract
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This document defines a framework for authorization policies
controlling access to application specific data. This framework
combines common location- and presence-specific authorization
aspects. An XML schema specifies the language in which common policy
rules are represented. The common policy framework can be extended
to other application domains.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Modes of Operation . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Passive Request-Response - PS as Server (Responder) . . . 7
3.2. Active Request-Response - PS as Client (Initiator) . . . . 7
3.3. Event Notification . . . . . . . . . . . . . . . . . . . . 7
4. Goals and Assumptions . . . . . . . . . . . . . . . . . . . . 9
5. Non-Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Basic Data Model and Processing . . . . . . . . . . . . . . . 12
6.1. Identification of Rules . . . . . . . . . . . . . . . . . 13
6.2. Extensions . . . . . . . . . . . . . . . . . . . . . . . . 13
7. Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Identity Condition . . . . . . . . . . . . . . . . . . . . 14
7.1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 14
7.1.2. Matching One Entity . . . . . . . . . . . . . . . . . 14
7.1.3. Matching Multiple Entities . . . . . . . . . . . . . . 15
7.2. Single Entity . . . . . . . . . . . . . . . . . . . . . . 19
7.3. Sphere . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.4. Validity . . . . . . . . . . . . . . . . . . . . . . . . . 21
8. Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
9. Transformations . . . . . . . . . . . . . . . . . . . . . . . 24
10. Procedure for Combining Permissions . . . . . . . . . . . . . 25
10.1. Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 25
10.2. Example . . . . . . . . . . . . . . . . . . . . . . . . . 26
11. Meta Policies . . . . . . . . . . . . . . . . . . . . . . . . 29
12. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
13. XML Schema Definition . . . . . . . . . . . . . . . . . . . . 31
14. Security Considerations . . . . . . . . . . . . . . . . . . . 34
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
15.1. Common Policy Namespace Registration . . . . . . . . . . . 35
15.2. Content-type registration for
'application/auth-policy+xml' . . . . . . . . . . . . . . 35
15.3. Common Policy Schema Registration . . . . . . . . . . . . 37
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
16.1. Normative References . . . . . . . . . . . . . . . . . . . 38
16.2. Informative References . . . . . . . . . . . . . . . . . . 38
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 39
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 40
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
Intellectual Property and Copyright Statements . . . . . . . . . . 43
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1. Introduction
This document defines a framework for creating authorization policies
for access to application specific data. This framework is the
result of combining the common aspects of single authorization
systems that more specifically control access to presence and
location information and that previously had been developed
separately. The benefit of combining these two authorization systems
is two-fold. First, it allows to build a system which enhances the
value of presence with location information in a natural way and
reuses the same underlying authorization mechanism. Second, it
encourages a more generic authorization framework with mechanisms for
extensibility. The applicability of the framework specified in this
document is not limited to policies controlling access to presence
and location information data, but can be extended to other
application domains.
The general framework defined in this document is intended to be
accompanied and enhanced by application-specific policies specified
elsewhere. The common policy framework described here is enhanced by
domain-specific policy documents, including presence [7] and location
[8]. This relationship is shown in Figure 1.
+-----------------+
| |
| Common |
| Policy |
| |
+---+---------+---+
/|\ /|\
| |
+-------------------+ | | +-------------------+
| | | enhance | | |
| Location-specific | | | | Presence-specific |
| Policy |----+ +----| Policy |
| | | |
+-------------------+ +-------------------+
Figure 1: Common Policy Enhancements
This document starts with an introduction to the terminology in
Section 2, an illustration of basic modes of operation in Section 3,
a description of goals (see Section 4) and non-goals (see Section 5)
of the policy framework, followed by the data model in Section 6.
The structure of a rule, namely conditions, actions and
transformations, is described in Section 7, in Section 8 and in
Section 9. The procedure for combining permissions is explained in
Section 10 and used when more than one rule fires. A short
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description of meta policies is given in Section 11. An example is
provided in Section 12. The XML schema will be discussed in
Section 13. IANA considerations in Section 15 follow the security
considerations from Section 14.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT","RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
This document introduces the following terms:
PT - Presentity / Target: The PT is the entity about whom information
has been requested. RFC 3693 [9] uses the term Target to identify
the object or person of which location information is requested.
The presence model described in RFC 2778 [10] uses the term
presentity to describe the entity that provides presence
information to a presence service. We introduce a neutral term
here to avoid confusion or loose generality.
RM - Rule Maker: RM is an entity that creates the authorization rules
which restrict access to data items.
PS - (Authorization) Policy Server: This entity has access to both
the authorization policies and to the data items. In location-
specific applications, the entity PS is labeled as location server
(LS).
WR - Watcher / Recipient: This entity requests access to data items
of the PT. An access operation might be either be a read, write
or be any other operation. In case of access to location
information is likely to be a read operation.
The receiver of the requested data items is the Location Recipient
(LR) in the terminology of RFC 3693 [9]. A watcher, i.e., an
entity that requests presence information about a presentity, is a
recipient in presence systems (see [10]).
A policy is given by a 'rule set' that contains an unordered list of
'rules'. A 'rule' has a 'conditions', an 'actions' and a
'transformations' part.
The term 'permission' refers to the action and transformation
components of a 'rule'.
The term 'using protocol' is defined in [9]. It refers to the
protocol that is used to request access to and to return privacy
sensitive data items.
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3. Modes of Operation
The abstract sequence of operations can roughly be described as
follows. The PS receives a query for data items for a particular PT,
via the using protocol. The using protocol (or more precisely the
authentication protocol) provides the identity of the requestor,
either at the time of the query or at the subscription time. The
authenticated identity of the WR, together with other information
provided by the using protocol or generally available to the server,
is then used for searching through the rule set. All matching rules
are combined according to a permission combining algorithm described
in Section 10. The combined rules are applied to the application
data, resulting in the application of privacy based on the
transformation policies. The resulting application data is returned
to the WR.
Three different modes of operation can be distinguished:
3.1. Passive Request-Response - PS as Server (Responder)
In a passive request-response mode, the WR queries the PS for data
items about the PT. Examples of protocols following this mode of
operation include HTTP, FTP, LDAP, finger or various RPC protocols,
including Sun RPC, DCE, DCOM, Corba and SOAP. The PS uses the
ruleset to determine whether the WR is authorized to access the PTs
information, refusing the request if necessary. Furthermore, the PS
might filter information by removing elements or by reducing the
resolution of elements.
3.2. Active Request-Response - PS as Client (Initiator)
Alternatively, the PS may contact the WR and convey data items.
Examples include HTTP, SIP session setup (INVITE request), H.323
session setup or SMTP.
3.3. Event Notification
Event notification adds a subscription phase to the "Active Request-
Response - PS as Client (Initiator)" mode of operation. A watcher or
subscriber asks to be added to the notification list for a particular
presentity or event. When the presentity changes state or the event
occurs, the PS sends a message to the WR containing the updated
state. (Presence is a special case of event notification; thus, we
often use the term interchangeably.)
In addition, the subscriber may itself add a filter to the
subscription, limiting the rate or content of the notifications. If
an event, after filtering by the rule maker-provided rules and by the
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subscriber-provided rules, only produces the same notification
content that was sent previously, no event notification is sent.
A single PS may authorize access to data items in more than one mode.
Rather than having different rule sets for different modes all three
modes are supported with a one rule set schema.
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4. Goals and Assumptions
Below, we summarize our design goals and constraints.
Table representation:
Each rule must be representable as a row in a relational database.
This design goal should allow efficient policy implementation by
utilizing standard database optimization techniques.
Permit only:
Rules only provide permissions rather than denying them. Removing
a rule can never increase permissions. Allowing both 'permit' and
'deny' actions would require some rule ordering that has
implications on the update operations executed on these rules.
Additionally, it would make distributed rule sets more
complicated. Hence, only 'permit' actions are allowed that result
in more efficient rule processing. This also implies that rule
ordering is not important.
Additive permissions:
A query for access to data items is matched against the rules in
the rule database. If several rules match, then the overall
permissions granted to the WR are the union of those permissions.
A more detailed discussion is provided in Section 10.
Upgradeable:
It should be possible to add additional rules later, without
breaking PSs that have not been upgraded. Any such upgrades must
not degrade privacy constraints, but PSs not yet upgraded may
reveal less information than the rule maker would have chosen.
Capability support:
In addition to the previous goal, a RM should be able to determine
the extensions that are supported by the PS. The mechanism used
to determine the capability of a PS is outside the scope of this
specification.
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Protocol-independence:
The rule set supports constraints on both notifications or queries
as well as subscriptions for event-based systems such as presence
systems.
No false assurance:
It appears more dangerous to give the user the impression that the
system will prevent disclosure automatically, but fail to do so
with a significant probability of operator error or
misunderstanding, than to force the user to explicitly invoke
simpler rules. For example, rules based on weekday and time-of-
day ranges seem particularly subject to misinterpretation and
false assumptions on part of the RM. (For example, a non-
technical RM would probably assume that the rules are based on the
timezone of his current location, which may not be known to other
components of the system.)
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5. Non-Goals
We explicitly decided that a number of possibly worthwhile
capabilities are beyond the scope of this first version. Future
versions may include these capabilities, using the extension
mechanism described in this document. Non-goals include:
No external references:
Attributes within specific rules cannot refer to external rule
sets, databases, directories or other network elements. Any such
external reference would make simple database implementation
difficult and hence they are not supported in this version.
No regular expression:
Conditions are matched on equality or 'greater-than'-style
comparisons, not regular expressions, partial matches such as the
SQL LIKE operator (e.g., LIKE "%foo%") or glob-style matches
("*@example.com"). Most of these are better expressed as explicit
elements.
No repeat times:
Repeat times (e.g., every day from 9am to 4pm) are difficult to
make work correctly, due to the different time zones that PT, WR,
PS and RM may occupy. It appears that suggestions for including
time intervals are often based on supporting work/non-work
distinctions, which unfortunately are difficult to capture by time
alone. Note that this feature must not be confused with the
'Validity' element that provides a mechanism to restrict the
lifetime of a rule.
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6. Basic Data Model and Processing
A rule set (or synonymously, a policy) consists of zero or more
rules. The ordering of these rules is irrelevant. The rule set can
be stored at the PS and conveyed from RM to PS as a single document,
in subsets or as individual rules. A rule consists of three parts -
conditions (see Section 7), actions (see Section 8), and
transformations (see Section 9).
The conditions part is a set of expressions, each of which evaluates
to either TRUE or FALSE, i.e., each of which is equipped with a value
of either TRUE or FALSE by the PS. When a WR asks for information
about a PT, the PS goes through each rule in the rule set. For each
rule, it evaluates the expressions in the conditions part. If all of
the expressions evaluate to TRUE, then the rule is applicable to this
request. Generally, each expression specifies a condition based on
some variable that is associated with the context of the request.
These variables can include the identity of the WR, the domain of the
WR, the time of day, or even external variables, such as the
temperature or the mood of the PT.
Assuming that the rule is applicable to the request, the actions and
transformations (commonly referred to as permissions) in the rule
specify how the PS is supposed to handle this request. If the
request is to view the location of the PT, or to view its presence,
the typical action is "permit" that allows the request to proceed.
Assuming the action allows the request to proceed, the
transformations part of the rule specifies how the information about
the PT - their location information, their presence, etc. - is
modified before being presented to the WR. These transformations are
in the form of positive permissions. That is, they always specify a
piece of information that is allowed to be seen by the WR. When a PS
processes a request, it takes the transformations specified across
all rules that match, and creates the union of them. For computing
this union the data type, such as Integer, Boolean, Set, or the Undef
data type, plays a role. The details of the algorithm for combining
permissions is described in Section 10. The resulting union
effectively represents a "mask" - it defines what information is
exposed to the WR. This mask is applied to the actual location or
presence data for the PT, and the data which is permitted by the mask
is shown to the WR. If the WR request a subset of information only
(such as city-level civil location data only, instead of the full
civil location information), the information delivered to the WR MUST
be the intersection of the permissions granted to the WR and the data
requested by the WR.
In accordance to this document, rules are encoded in XML. To this
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end, Section 13 contains an XML schema defining the Common Policy
Markup Language. This, however, is purely an exchange format between
RM and PS. The format does not imply that the RM or the PS use this
format internally, e.g., in matching a query with the policy rules.
The rules are designed so that a PS can translate the rules into a
relational database table, with each rule represented by one row in
the database. The database representation is by no means mandatory;
we will use it as a convenient and widely-understood example of an
internal representation. The database model has the advantage that
operations on rows have tightly defined meanings. In addition, it
appears plausible that larger-scale implementations will employ a
backend database to store and query rules, as they can then benefit
from existing optimized indexing, access control, scaling and
integrity constraint mechanisms. Smaller-scale implementations may
well choose different implementations, e.g., a simple traversal of
the set of rules.
6.1. Identification of Rules
Each rule is equipped with a parameter that identifies the rule.
This rule identifier is an opaque token chosen by the RM. A RM MUST
NOT use the same identifier for two rules that are available to the
PS at the same time for a given PT. If more than one RM modifies the
same rule set then it needs to be ensured that a unique identifier is
chosen for each rule. A RM can accomplish this goal by retrieving
the already specified ruleset and to choose a new identifier for a
rule that is different from the values used by the rules in the rule
set.
6.2. Extensions
The policy framework defined in this document is meant to be
extensible towards specific application domains. Such an extension
is accomplished by defining conditions, actions and transformations
that are specific to the desired application domain. Each extension
MUST define its own namespace.
Extensions cannot change the schema defined in this document, and
this schema is not expected to change excepting a revision to this
specification, and that no versioning procedures for this schema or
namespace are therefore provided.
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7. Conditions
The access to data items needs to be matched with the rule set stored
at the PS. Each instance of a request has different attributes
(e.g., the identity of the requestor) that are used for
authorization. A rule in a rule set might have a number of
conditions that need to be met before executing the remaining parts
of a rule (i.e., actions and transformations). Details about rule
matching are described in Section 10. This document specifies only a
few conditions (i.e., identity, sphere, and validity). Further
condition elements can be added via extensions to this document.
7.1. Identity Condition
7.1.1. Overview
The identity condition restricts matching of a rule either to a
single entity or a group of entities. Only authenticated entities
can be matched; acceptable means of authentication are defined in
protocol-specific documents. If the <identity> element is absent, or
it is present but is empty (meaning that there are no child
elements), identities are not considered, and thus, other conditions
in the rule apply to any user, authenticated or not.
The <identity> condition is considered TRUE if any of its child
elements (e.g., the <one/> and the <many/> elements defined in this
document) evaluate to TRUE, i.e., the results of the individual child
element are combined using a logical OR.
If a child element of <identity> is in a namespace that is not known
or not supported, it can be ignored.
7.1.2. Matching One Entity
The <one> element matches the authenticated identity (as contained in
the 'id' attribute) of exactly one entity or user. For
considerations regarding the 'id' attribute refer to Section 7.2.
An example is shown below:
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<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy">
<rule id="f3g44r1">
<conditions>
<identity>
<one id="sip:alice@example.com"/>
<one id="tel:+1-212-555-1234" />
<one id="mailto:bob@example.net" />
</identity>
</conditions>
<actions/>
<transformations/>
</rule>
</ruleset>
This example matches if the authenticated identity of the WR is
either sip:alice@example.com, tel:+1-212-555-1234 or
mailto:bob@example.net.
7.1.3. Matching Multiple Entities
The <many> element is a mechanism to perform authorization decisions
based on the domain part of the authenticated identity. As such, it
allows to match a large and possibly unknown number of users within a
domain.
Furthermore, it is possible to include one or multiple <except>
elements to exclude either individual users or users belonging to a
specific domain. Excluding individual entities is implemented using
a <except id="..."/> statement. The semantic of the 'id' attribute
of the <except> element has the same meaning as the 'id' attribute of
the <one> element (see Section 7.2). Excluding users belonging to a
specific domain is implemented using the <except domain="..."/>
element that excludes any user from the indicated domain.
If multiple <except> elements are listed as child elements of the
<many> element then the result of each <except> element is combined
using a logical OR.
Common policy MUST either use UTF-8 or UTF-16 to store domain names
in the 'domain' attribute. For non-IDNs, lower-case ASCII SHOULD be
used. For the comparison operation between the value stored in the
'domain' attribute and the domain value provided via the using
protocol (referred as "protocol domain identifier") the following
rules are applicable:
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1. If the values of the 'domain' attribute and the value of the
protocol domain identifier does not begin with xn--, attempt a
string comparison. If the string comparison indicates equality,
the comparison succeeds and the remaining steps are skipped.
2. Translate percent-encoding for either string and repeat (1).
3. Convert both domain strings using the toASCII operation described
in RFC 3490 [2]. (Naturally, if one of the strings already
begins with the ACE prefix xn--, the conversion operation has
already been performed.)
4. Compare the two domain strings for ASCII equality, for each
label. If the string comparison for each label indicates
equality, the comparison succeeds. Otherwise, the domains are
not equal.
If the conversion fails in step (3), the domains are not equal.
7.1.3.1. Matching Any Authenticated Identity
The <many/> element without any child elements or attributes matches
any authenticated user.
The following example shows a rule that matches any authenticated
user:
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy">
<rule id="f3g44r5">
<conditions>
<identity>
<many/>
</identity>
</conditions>
<actions/>
<transformations/>
</rule>
</ruleset>
7.1.3.2. Matching Any User, Authenticated and Unauthenticated
If the <identity> element is used without child elements then it
matches any user, authenticated and unauthenticated. The same is
true for a rule where the <identity> element is omitted.
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The following example shows two rules that match any user,
authenticated and unauthenticated.
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy">
<rule id="f3g44r5">
<conditions>
<identity/>
</conditions>
<actions/>
<transformations/>
</rule>
<rule id="f3g44r57">
<conditions/>
<actions/>
<transformations/>
</rule>
</ruleset>
7.1.3.3. Matching Any Authenticated Identity Excepting Enumerated
Domains/Identities
The <many> element enclosing one or more <except domain="..."/>
elements matches any user from any domain except those enumerated.
The <except id="..."/> element excludes particular users. The
semantic of the 'id' attribute of the <except> element is described
in Section 7.2. The results of the child elements of the <many>
element are combined using a logical OR.
An example is shown below:
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<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy">
<rule id="f3g44r1">
<conditions>
<identity>
<many>
<except domain="example.com"/>
<except domain="example.org"/>
<except id="sip:alice@bad.example.net"/>
<except id="sip:bob@good.example.net"/>
<except id="tel:+1-212-555-1234" />
<except id="sip:alice@example.com"/>
</many>
</identity>
</conditions>
<actions/>
<transformations/>
</rule>
</ruleset>
This example matches all users except any user in example.com, or any
user in example.org or the particular users alice@bad.example.net,
bob@good.example.net and the user with the telephone number
'tel:+1-212-555-1234'. The last 'except' element is redundant since
alice@example.com is already excluded through the following statement
<except domain="example.com"/> in the example.
7.1.3.4. Matching Any Authenticated Identity Within a Domain Excepting
Enumerated Identities
The <many> element with a 'domain' attribute and zero or more <except
id="..."/> elements matches any authenticated user from the indicated
domain except those explicitly enumerated. The semantic of the 'id'
attribute of the <except> element is described in Section 7.2.
It is nonsensical to have domains in the 'id' attribute that do not
match the value of the 'domain' attribute in the enclosing <many>
element.
An example is shown below:
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<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy">
<rule id="f3g44r1">
<conditions>
<identity>
<many domain="example.com">
<except id="sip:alice@example.com"/>
<except id="sip:bob@example.com"/>
</many>
</identity>
</conditions>
<actions/>
<transformations/>
</rule>
</ruleset>
This example matches any user within example.com (such as
carol@example.com) except alice@example and bob@example.com.
7.2. Single Entity
The 'id' attribute used in the <one> and in the <except> element
refers to a single entity. In the subsequent text we use the term
'single-user' entity as a placeholder for the <one> and the <except>
element. The <except> element fulfills the purpose of excluding
elements from the solution set.
A single-user entity matches the authenticated identity (as contained
in the 'id' attribute) of exactly one entity or user. If there is a
match, the single-user entity is considered TRUE. The single-user
entity MUST NOT contain a 'domain' attribute.
The 'id' attribute contains an identity that MUST be expressed as
URI. Applications using this framework must describe how the
identities they are using can be expressed as a URIs.
7.3. Sphere
The <sphere> element belongs to the group of condition elements. It
can be used to indicate a state (e.g., 'work', 'home', 'meeting',
'travel') the PT is currently in. A sphere condition matches only if
the PT is currently in the state indicated. The state may be
conveyed by manual configuration or by some protocol. For example,
RPID [11] provides the ability to inform the PS of its current
sphere. The application domain needs to describe in more detail how
the sphere state is determined. Switching from one sphere to another
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causes a switch between different modes of visibility. As a result
different subsets of rules might be applicable.
The content of the 'value' attribute of the <sphere> element MAY
contain more than one token. The individual tokens MUST be separated
by a blank character. A logical OR is used for the matching the
tokens against the sphere settings of the PT. As an example, if the
content of the 'value' attribute in the sphere attribute contains two
tokens 'work' and 'home' then this part of the rule matches if the
sphere for a particular PT is either 'work' or 'home'. To compare
the content of the 'value' attribute in the <sphere> element with the
stored state information about the PT's sphere setting a case
insensitive string comparison MUST be used for each individual token.
There is no registry for these values nor a language specific
indication of the sphere content. As such, the tokens are treated as
opaque strings.
The rule example below illustrates that the rule with the entity
andrew@example.com matches if the sphere is been set to 'work'. In
the second rule with the entity allison@example.com matches if the
sphere is set to 'home'. The third rule also matches if the sphere
is set to 'home' since the value in the sphere element also contains
the token 'home'.
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<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy">
<rule id="f3g44r2">
<conditions>
<sphere value="work"/>
<identity>
<one id="sip:andrew@example.com"/>
</identity>
</conditions>
<actions/>
<transformations/>
</rule>
<rule id="y6y55r2">
<conditions>
<sphere value="home"/>
<identity>
<one id="sip:allison@example.com"/>
</identity>
</conditions>
<actions/>
<transformations/>
</rule>
<rule id="z6y55r2">
<conditions>
<identity>
<one id="sip:john@doe.example.com"/>
</identity>
<sphere value="home work"/>
</conditions>
<actions/>
<transformations/>
</rule>
</ruleset>
7.4. Validity
The <validity> element is the third condition element specified in
this document. It expresses the rule validity period by two
attributes, a starting and a ending time. The validity condition is
TRUE if the current time is greater than or equal to at least one
<from> child, but less than the <until> child after it. This
represents a logical OR operation across each <from> and <until>
pair. Times are expressed in XML dateTime format.
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A rule maker might not always have access to the PS to invalidate
rules. Hence, this mechanism allows to invalidate granted
permissions automatically without further interaction between the
rule maker and the PS. The PS does not remove the rules instead the
rule maker has to clean them up.
An example of a rule fragment is shown below:
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy">
<rule id="f3g44r3">
<conditions>
<validity>
<from>2003-08-15T10:20:00.000-05:00</from>
<until>2003-09-15T10:20:00.000-05:00</until>
</validity>
</conditions>
<actions/>
<transformations/>
</rule>
</ruleset>
The <validity> element MUST have the <from> and <until> subelements
in pairs. Multiple <from> and <until> elements might appear in pairs
(i.e., without nesting of <from> and <until> elements). Using
multiple <validity> elements as subelements of the <conditions>
element is not useful since all subelements of the <conditions>
element are combined as a logical AND.
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8. Actions
While conditions are the 'if'-part of rules, actions and
transformations build the 'then'-part of them. The actions and
transformations parts of a rule determine which operations the PS
MUST execute after having received from a WR a data access request
that matches all conditions of this rule. Actions and
transformations only permit certain operations; there is no 'deny'
functionality. Transformations exclusively specify PS-side
operations that lead to a modification of the data items requested by
the WR. Regarding location data items, for instance, a
transformation could force the PS to lower the precision of the
location information that is returned to the WR.
Actions, on the other hand, specify all remaining types of operations
the PS is obliged to execute, i.e., all operations that are not of
transformation type. Actions are defined by application specific
usages of this framework. The reader is referred to the
corresponding extensions to see examples of such elements.
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9. Transformations
Two sub-parts follow the conditions part of a rule: transformations
and actions. As defined in Section 8, transformations specify
operations that the PS MUST execute and that modify the result which
is returned to the WR. This functionality is particularly helpful in
reducing the granularity of information provided to the WR, as for
example required for location privacy. Transformations are defined
by application specific usages of this framework.
A simple transformation example is provided in Section 10.
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10. Procedure for Combining Permissions
This section describes the mechanism to evaluate the final result of
a rule evaluation. The result is reflected in the action and
transformation part of a rule. This procedure is sometimes referred
as conflict resolution.
To simplify the description of the algorithm we introduce the term
'item' to refer to child elements and attributes of these child
elements that apear in the condition, action and transformation part
of a rule. An item has a name and a data type. A value may be
assigned to an item or it may be undefined, in case it does not have
a value associated with the item. The values of a particular item
have the same data type. For example, the name of the item <sphere>
discussed in Section 7 is 'sphere', its data type is 'string', and
its value may be set to 'home'. To evaluate a condition means to
associate either TRUE or FALSE to the condition.
When the PS receives a request for access to privacy-sensitive data
then it needs to be matched against a rule set. The conditions part
of each individual rule is evaluated and as a result one or more
rules might match. If only a single rule matches then the result is
determined by executing the actions and the transformations part
following the conditions part of a rule. However, it can also be the
case that two or more matching rules contain a permission of the same
name (e.g., two rules contain a permission named 'precision of
geospatial location information'), but do not specify the same value
for that permission (e.g., the two rule might specify values of '10
km' and '200 km', respectively, for the permission named 'precision
of geospatial location information'). This section describes the
procedure for combining permissions in such cases.
The combining operation will result in the largest value for an
integer type, the OR operation for a boolean type, and union for a
set.
As such, applications should define values such that, for integers,
the lowest value corresponds to the most privacy, for booleans, false
corresponds to the most privacy, and for sets, the empty set
corresponds to the most privacy.
10.1. Algorithm
The algorithm for combining permissions is simple and depends on the
data types of the values of items: Let P be a rule set. Let M be the
subset of P consisting of rules r in P that match with respect to a
given request. Let n be a name of an item contained in a rule r in
M, and let M(n) be the subset of M consisting of rules r in M that
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have a item of name n. For each rule r in M(n), let v(r,n) and
d(r,n) be the value and the data type, respectively, of the item of r
with name n. Finally, let V(n) be the combined value of all the
values v(r,n), r in M(n). The algorithm that leads to the resulting
value V(n) is the following:
CR 1: If d(r,n)=Boolean for all r in M(n), then V(n) is given as
follows: If there is a r in M(n) with v(r,n)=TRUE, then V(n)=TRUE.
Otherwise, V(n)=FALSE.
CR 2: If d(r,n)=Integer for all r in M(n), then V(n) is given as
follows: If v(r,n)=undefined for all r in M(n), then V(n) is not
specified by this specification. Otherwise, V(n)=max{v(r,n) | r
in M(n)}.
CR 3: If d(r,n)=Set for all r in M(n), then V(n) is given as
follows: V(n)=union of all v(r,n), the union to be computed over all
r in M(n) with v(r,n)!=undefined.
10.2. Example
In the following example we illustrate the process of the combining
permissions algorithm. We will consider three items in the
conditions part in our example, namely identity, sphere, and
validity. For editorial reasons the rule set in this example is
represented in a table. Furthermore, the domain part of the identity
of the WR is omitted. For actions we use two items in the action
part of the rule with names X and Y. The values of X and Y are of
data types boolean and integer, respectively. For transformations we
use the item with the name Z whose value can be set either to '+'(or
1), 'o' (or 2) or '-' (or 3). Item Z allows us to show the
granularity reduction whereby a value of '+' shows the corresponding
information unrestricted and '-' shows nothing. This item might be
related to location information or other presence attributes like
mood. Internally we use the data type integer for computing the
permission of this item.
Conditions Actions/Transformations
+---------------------------------+----------------------+
| Id WR-ID sphere from until | X Y Z |
+---------------------------------+----------------------+
| 1 bob home A1 A2 | TRUE 10 o |
| 2 alice work A1 A2 | FALSE 5 + |
| 3 bob work A1 A2 | TRUE 3 - |
| 4 tom work A1 A2 | TRUE 5 + |
| 5 bob work A1 A3 | undef 12 o |
| 6 bob work B1 B2 | FALSE 10 - |
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+---------------------------------+----------------------+
For editorial reasons we use the items 'from' and 'until' to refer to
validity and we use the following abbreviations for the values:
A1=2003-12-24T17:00:00+01:00
A2=2003-12-24T21:00:00+01:00
A3=2003-12-24T23:30:00+01:00
B1=2003-12-22T17:00:00+01:00
B2=2003-12-23T17:00:00+01:00
Note that B1 < B2 < A1 < A2 < A3.
The entity 'bob' acts as a WR. The policy P consists of the six
rules shown in the table and identified by the values 1 to 6 in the
'Id' column. The PS receives the query at 2003-12-24T17:15:00+01:00
which falls between A1 and A2. The value of the item 'sphere'
indicates that the sphere of PT is currently set to 'work'.
Rule 1 does not match since the sphere condition does not match.
Rule 2 does not match as the identity of the WR (here 'alice') does
not equal 'bob'. Rule 3 matches since all conditions evaluate to
TRUE. Rule 4 does not match as the identity of the WR (here 'tom')
does not equal 'bob'. Rule 5 matches. Rule 6 does not match since
the rule is not valid anymore. Therefore, the set M of matching
rules consists of the rules 3 and 5. These two rules are used to
compute the combined permission V(X), V(Y), and V(Z) for each of the
permissions X, Y, and Z:
Actions/Transformations
+-----+-----------------------+
| Id | X Y Z |
+-----+-----------------------+
| 3 | TRUE 3 - |
| 5 | undef 12 o |
+-----+-----------------------+
The results of the permission combining algorithm is shown below.
The combined value V(X) regarding the permission with name X equals
TRUE according to the first combining rule listed above. The maximum
of 3 and 12 is 12, so that V(Y)=12. For the attribute Z in this
example the maximum between 'o' and '-' (i.e., between 2 and 3) is
'-'.
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Actions/Transformations
+-----+-----------------------+
| Id | X Y Z |
+-----+-----------------------+
| 5 | TRUE 12 - |
+-----+-----------------------+
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11. Meta Policies
Meta policies authorize a rule maker to insert, update or delete a
particular rule or an entire rule set. Some authorization policies
are required to prevent unauthorized modification of rule sets. Meta
policies are outside the scope of this document.
A simple implementation could restrict access to the rule set only to
the PT but more sophisticated mechanisms could be useful. As an
example of such policies one could think of parents configuring the
policies for their children.
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12. Example
This section gives an example of an XML document valid with respect
to the XML schema defined in Section 13. Semantically richer
examples can be found in documents which extend this schema with
application domain specific data (e.g., location or presence
information).
Below a rule is shown with a condition that matches for a given
authenticated identity (bob@example.com) and within a given time
period. Additionally, the rule matches only if the target has set
its sphere to 'work'.
<?xml version="1.0" encoding="UTF-8"?>
<ruleset xmlns="urn:ietf:params:xml:ns:common-policy">
<rule id="f3g44r1">
<conditions>
<identity>
<one id="sip:bob@example.com"/>
</identity>
<sphere value="work"/>
<validity>
<from>2003-12-24T17:00:00+01:00</from>
<until>2003-12-24T19:00:00+01:00</until>
</validity>
</conditions>
<actions/>
<transformations/>
</rule>
</ruleset>
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13. XML Schema Definition
This section provides the XML schema definition for the common policy
markup language described in this document.
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema targetNamespace="urn:ietf:params:xml:ns:common-policy"
xmlns:cp="urn:ietf:params:xml:ns:common-policy"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified" attributeFormDefault="unqualified">
<!-- /ruleset -->
<xs:element name="ruleset">
<xs:complexType>
<xs:complexContent>
<xs:restriction base="xs:anyType">
<xs:sequence>
<xs:element name="rule" type="cp:ruleType"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
</xs:element>
<!-- /ruleset/rule -->
<xs:complexType name="ruleType">
<xs:complexContent>
<xs:restriction base="xs:anyType">
<xs:sequence>
<xs:element name="conditions"
type="cp:conditionsType" minOccurs="0"/>
<xs:element name="actions"
type="cp:extensibleType" minOccurs="0"/>
<xs:element name="transformations"
type="cp:extensibleType" minOccurs="0"/>
</xs:sequence>
<xs:attribute name="id" type="xs:ID" use="required"/>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
<!-- //rule/conditions -->
<xs:complexType name="conditionsType">
<xs:complexContent>
<xs:restriction base="xs:anyType">
<xs:choice maxOccurs="unbounded">
<xs:element name="identity"
type="cp:identityType" minOccurs="0"/>
<xs:element name="sphere"
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type="cp:sphereType" minOccurs="0"/>
<xs:element name="validity"
type="cp:validityType" minOccurs="0"/>
<xs:any namespace="##other" processContents="lax"
minOccurs="0" maxOccurs="unbounded"/>
</xs:choice>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
<!-- //conditions/identity -->
<xs:complexType name="identityType">
<xs:complexContent>
<xs:restriction base="xs:anyType">
<xs:choice minOccurs="0" maxOccurs="unbounded">
<xs:element name="one" type="cp:oneType"/>
<xs:element name="many" type="cp:manyType"/>
<xs:any namespace="##other" processContents="lax"/>
</xs:choice>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
<!-- //identity/one -->
<xs:complexType name="oneType">
<xs:complexContent>
<xs:restriction base="xs:anyType">
<xs:sequence>
<xs:any namespace="##other"
minOccurs="0" processContents="lax"/>
</xs:sequence>
<xs:attribute name="id"
type="xs:anyURI" use="required"/>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
<!-- //identity/many -->
<xs:complexType name="manyType">
<xs:complexContent>
<xs:restriction base="xs:anyType">
<xs:choice minOccurs="0" maxOccurs="unbounded">
<xs:element name="except" type="cp:exceptType"/>
<xs:any namespace="##other"
minOccurs="0" processContents="lax"/>
</xs:choice>
<xs:attribute name="domain"
use="optional" type="xs:string"/>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
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<!-- //many/except -->
<xs:complexType name="exceptType">
<xs:attribute name="domain" type="xs:string" use="optional"/>
<xs:attribute name="id" type="xs:anyURI" use="optional"/>
</xs:complexType>
<!-- //conditions/sphere -->
<xs:complexType name="sphereType">
<xs:complexContent>
<xs:restriction base="xs:anyType">
<xs:attribute name="value"
type="xs:string" use="required"/>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
<!-- //conditions/validity -->
<xs:complexType name="validityType">
<xs:complexContent>
<xs:restriction base="xs:anyType">
<xs:sequence minOccurs="0" maxOccurs="unbounded">
<xs:element name="from" type="xs:dateTime"/>
<xs:element name="until" type="xs:dateTime"/>
</xs:sequence>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
<!-- //rule/actions or //rule/transformations -->
<xs:complexType name="extensibleType">
<xs:complexContent>
<xs:restriction base="xs:anyType">
<xs:sequence>
<xs:any namespace="##other"
processContents="lax" minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
</xs:schema>
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14. Security Considerations
This document describes a framework for policies. This framework is
intended to be enhanced elsewhere towards application domain specific
data. Security considerations are to a great extent application data
dependent, and therefore need to be covered by documents that extend
the framework defined in this specification. RFC 3693 [9] and RFC
3694 [3] are good sources to consider for the type of analysis
required by such documents and applications.
Extensions to the action and transformation elements must be defined
in a way so that the usage of the permissions combining rules of
Section 10 does not lower the level of privacy protection. This is
particularly important when defining the semantic of the a more
detailed description of the values for the defined attributes and
elements. See Section 10 for more details on this privacy aspect.
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15. IANA Considerations
This section registers a new XML namespace, a new XML schema and a
new MIME-type. This section registers a new XML namespace per the
procedures in [4].
15.1. Common Policy Namespace Registration
URI: urn:ietf:params:xml:ns:common-policy
Registrant Contact: IETF Geopriv Working Group, Henning Schulzrinne
(hgs+geopriv@cs.columbia.edu).
XML:
BEGIN
<?xml version="1.0"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
"http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="content-type"
content="text/html;charset=iso-8859-1"/>
<title>Common Policy Namespace</title>
</head>
<body>
<h1>Namespace for Common Authorization Policies</h1>
<h2>urn:ietf:params:xml:ns:common-policy</h2>
<p>See <a href="[URL of published RFC]">RFCXXXX
[NOTE TO IANA/RFC-EDITOR:
Please replace XXXX with the RFC number of this
specification.]</a>.</p>
</body>
</html>
END
15.2. Content-type registration for 'application/auth-policy+xml'
This specification requests the registration of a new MIME type
according to the procedures of RFC 4288 [5] and guidelines in RFC
3023 [6].
MIME media type name: application
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MIME subtype name: auth-policy+xml
Mandatory parameters: none
Optional parameters: charset
Indicates the character encoding of enclosed XML.
Encoding considerations:
Uses XML, which can employ 8-bit characters, depending on the
character encoding used. See RFC 3023 [6], Section 3.2.
Security considerations:
This content type is designed to carry authorization policies.
Appropriate precautions should be adopted to limit disclosure of
this information. Please refer to Section 14 of RFCXXXX [NOTE TO
IANA/RFC-EDITOR: Please replace XXXX with the RFC number of this
specification.] and to the security considerations described in
Section 10 of RFC 3023 [6] for more information.
Interoperability considerations: None
Published specification: RFCXXXX [NOTE TO IANA/RFC-EDITOR: Please
replace XXXX with the RFC number of this specification.] this
document
Applications which use this media type:
Presence- and location-based systems
Additional information:
Magic Number: None
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File Extension: .apxml
Macintosh file type code: 'TEXT'
Personal and email address for further information: Hannes
Tschofenig, Hannes.Tschofenig@siemens.com
Intended usage: LIMITED USE
Author/Change controller:
This specification is a work item of the IETF GEOPRIV working
group, with mailing list address <geopriv@ietf.org>.
15.3. Common Policy Schema Registration
URI: urn:ietf:params:xml:schema:common-policy
Registrant Contact: IETF Geopriv Working Group, Henning Schulzrinne
(hgs+geopriv@cs.columbia.edu).
XML: The XML schema to be registered is contained in Section 13. Its
first line is
<?xml version="1.0" encoding="UTF-8"?>
and its last line is
</xs:schema>
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16. References
16.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", March 1997.
[2] Faltstrom, P., Hoffman, P., and A. Costello, "Internationalizing
Domain Names in Applications (IDNA)", RFC 3490, March 2003.
[3] Danley, M., Mulligan, D., Morris, J., and J. Peterson, "Threat
Analysis of the Geopriv Protocol", RFC 3694, February 2004.
[4] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[5] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
[6] Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types",
RFC 3023, January 2001.
16.2. Informative References
[7] Rosenberg, J., "Presence Authorization Rules",
draft-ietf-simple-presence-rules-06 (work in progress),
May 2006.
[8] Schulzrinne, H., "A Document Format for Expressing Privacy
Preferences for Location Information",
draft-ietf-geopriv-policy-08 (work in progress), February 2006.
[9] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and J.
Polk, "Geopriv Requirements", RFC 3693, February 2004.
[10] Day, M., Rosenberg, J., and H. Sugano, "A Model for Presence
and Instant Messaging", RFC 2778, February 2000.
[11] Schulzrinne, H., "RPID: Rich Presence Extensions to the
Presence Information Data Format (PIDF)",
draft-ietf-simple-rpid-10 (work in progress), December 2005.
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Appendix A. Contributors
We would like to thank Christian Guenther for his help with initial
versions of this document.
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Appendix B. Acknowledgments
This document is partially based on the discussions within the IETF
GEOPRIV working group. Discussions at the Geopriv Interim Meeting
2003 in Washington, D.C., helped the working group to make progress
on the authorization policies based on the discussions among the
participants.
We particularly want to thank Allison Mankin <mankin@psg.com>,
Randall Gellens <rg+ietf@qualcomm.com>, Andrew Newton
<anewton@ecotroph.net>, Ted Hardie <hardie@qualcomm.com> and Jon
Peterson <jon.peterson@neustar.biz> for discussing a number of
details with us. They helped us to improve the quality of this
document. Allison, Ted and Andrew also helped us to make good
progress with the internationalization support of the identifier/
domain attributes.
Furthermore, we would like to thank the IETF SIMPLE working group for
their discussions of J. Rosenberg's draft on presence authorization
policies. We would also like to thank Stefan Berg, Murugaraj
Shanmugam, Christian Schmidt, Martin Thomson, Markus Isomaki, Aki
Niemi, Eva Maria Leppanen, Mark Baker, Tim Polk and Brian Carpenter
for their comments. Martin Thomson helped us with the XML schema.
Mark Baker provided a review of the media type. Scott Brim provided
a review on behalf of the General Area Review Team.
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Authors' Addresses
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
USA
Phone: +1 212 939 7042
Email: schulzrinne@cs.columbia.edu
URI: http://www.cs.columbia.edu/~hgs
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Hannes.Tschofenig@siemens.com
URI: http://www.tschofenig.com
John B. Morris, Jr.
Center for Democracy and Technology
1634 I Street NW, Suite 1100
Washington, DC 20006
USA
Email: jmorris@cdt.org
URI: http://www.cdt.org
Jorge R. Cuellar
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Jorge.Cuellar@siemens.com
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James Polk
Cisco
2200 East President George Bush Turnpike
Richardson, Texas 75082
USA
Email: jmpolk@cisco.com
Jonathan Rosenberg
Cisco
600 Lanidex Plaza
Parsippany, New York 07054
USA
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
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Intellectual Property Statement
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