Internet Engineering Task Force (IETF) R. Barnes
Request for Comments: 7199 M. Thomson
Category: Standards Track Mozilla
ISSN: 2070-1721 J. Winterbottom
Location Configuration Extensions for Policy Management
Current location configuration protocols are capable of provisioning
an Internet host with a location URI that refers to the host's
location. These protocols lack a mechanism for the target host to
inspect or set the privacy rules that are applied to the URIs they
distribute. This document extends the current location configuration
protocols to provide hosts with a reference to the rules that are
applied to a URI so that the host can view or set these rules.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Policy URIs . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Policy URI Usage . . . . . . . . . . . . . . . . . . . . 5
3.2. Policy URI Allocation . . . . . . . . . . . . . . . . . . 6
3.3. Policy Defaults . . . . . . . . . . . . . . . . . . . . . 7
4. Location Configuration Extensions . . . . . . . . . . . . . . 8
4.1. HELD . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Client Processing . . . . . . . . . . . . . . . . . . . . 9
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Basic Access Control Policy . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6.1. URN Sub-Namespace Registration for
urn:ietf:params:xml:ns:geopriv:held:policy . . . . . . . 12
6.2. XML Schema Registration . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7.1. Integrity and Confidentiality for Authorization Policy
Data . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.2. Access Control for Authorization Policy . . . . . . . . . 13
7.3. Location URI Allocation . . . . . . . . . . . . . . . . . 15
7.4. Policy URI Handling . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. Example Policy URI Generation Algorithm . . . . . . 18
A critical step in enabling Internet hosts to access location-based
services is to provision those hosts with information about their own
location. This is accomplished via a Location Configuration Protocol
(LCP) [RFC5687], which allows a location provider (e.g., a local
access network) to inform a host about its location.
There are two basic patterns for location configuration, namely
configuration "by value" and "by reference" [RFC5808]. Configuration
by value provisions a host directly with its location, by providing
it location information that is directly usable (e.g., coordinates or
a civic address). Configuration by reference provides a host with a
URI that references the host's location, i.e., one that can be
dereferenced to obtain the location (by value) of the host.
In some cases, location by reference offers a few benefits over
location by value. From a privacy perspective, the required
dereference transaction provides a policy enforcement point so that
if suitable privacy policies have been provisioned, the opaque
location URI can be safely conveyed over untrusted media. (If the
location URI is not subject to privacy rules, then conveying the
location URI may pose even greater risk than sending location by
value [RFC5606].) If the target host is mobile, an application
provider can use a single reference to obtain the location of the
host multiple times, saving bandwidth to the host. For some
configuration protocols, the location object referenced by a location
URI provides a much more expressive syntax for location values than
the configuration protocol itself (e.g., DHCP geodetic location
[RFC6225] versus Geography Markup Language (GML) in a Presence
Information Data Format Location Object (PIDF-LO) [RFC4119]).
From a privacy perspective, however, current LCPs are limited in
their flexibility, in that they do not provide hosts (the clients in
an LCP) with a way to inform the Location Server with policy for how
his location information should be handled. This document addresses
this gap by defining a simple mechanism for referring to and
manipulating policy and by extending current LCPs to carry policy
references. Using the mechanisms defined in this document, an LCP
server (acting for the Location Server (LS) or Location Information
Server (LIS)) can inform a host as to which policy document controls
a given location resource, and the host (in its Rule Maker role) can
inspect this document and modify it as necessary.
In the following figure, adapted from RFC 5808, this document extends
the Location Configuration Protocols (1) and defines a simple
protocol for policy exchange (4).
+---------+---------+ Location +-----------+
| | | Dereference | Location |
| LIS/LS +---------------+ Recipient |
| | | Protocol | |
+----+----+----+----+ (3) +-----+-----+
| | |
| | |
Policy| |Location |Location
Exchange| |Configuration |Conveyance
(4)| |Protocol |Protocol
| |(1) |(2)
| | |
| Rule | Target/ | |
| Maker | Host +---------------------+
| | |
The remainder of this document is structured as follows:
After introducing a few relevant terms, we define policy URIs as a
channel for referencing, inspecting, and updating policy documents.
We then define an extension to the HELD protocol to allow it to carry
policy URIs. Examples are given that demonstrate how policy URIs are
carried in this protocol and how it can be used by clients.
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 [RFC2119].
3. Policy URIs
A policy URI is an HTTP [RFC2616] or HTTPS [RFC2818] URI that
identifies a policy resource that contains the authorization policy
for a linked location resource. Access to the location resource is
governed by the contents of the authorization policy.
A policy URI identifies an HTTP resource that a Rule Maker can use to
inspect and install policy documents that tell a Location Server how
it should protect the associated location resource. A policy URI
always identifies a resource that can be represented as a common-
policy document [RFC4745] (possibly including some extensions; e.g.,
for geolocation policy [RFC6772]).
Note: RFC 3693 [RFC3693] identified the Rule Holder role as the one
that stores policy information. In this document, the Location
Server is also a Rule Holder.
3.1. Policy URI Usage
A Location Server that is the authority for policy URIs MUST support
GET, PUT, and DELETE requests to these URIs, in order to allow
clients to inspect, replace, and delete policy documents. Clients
support the three request methods as they desire to perform these
Knowledge of the policy URI can be considered adequate evidence of
authorization; a policy URI functions as a shared secret between the
client and the server (see Section 7). A Location Server SHOULD
allow all requests, but it MAY deny certain requests based on local
policy. For instance, a Location Server might allow clients to
inspect policy (GET), but not to update it (PUT). Or, a Location
Server might require clients to authenticate using HTTP or Transport
Layer Security (TLS) client authentication. Clients implementing
this specification SHOULD support HTTP client authentication
[RFC2617] and MAY support TLS client certificates.
A GET request to a policy URI is a request for the referenced policy
information. If the request is authorized, then the Location Server
sends an HTTP 200 response containing the complete policy identified
by the URI.
A PUT request to a policy URI is a request to replace the current
policy. The entity-body of a PUT request includes a complete policy
document. When a Location Server receives a PUT request, it MUST
validate the policy document included in the body of the request. If
the request is valid and authorized, then the Location Server MUST
replace the current policy with the policy provided in the request.
A DELETE request to a policy URI is a request to delete the
referenced policy document. If the request is authorized, then the
Location Server MUST delete the policy referenced by the URI and
disallow access to the location URIs it governs until a new policy
document has been put in place via a PUT request.
A policy URI is only valid while the corresponding location URI set
is valid. A Location Server MUST NOT respond to any requests to a
policy URI once the corresponding location URI set has expired. This
expiry time is specified by the 'expires' attribute in the HELD
A location URI can thus become invalid in three ways: By the
expiration of a validity interval in policy, by the removal of a
policy document with a DELETE request, or by the expiry of the
LCP-specified validity interval. The former two are temporary,
since the policy URI can be used to update the policy. The latter
one is permanent, since the expiry causes the policy URI to be
invalidated as well.
The Location Server MUST support policy documents in the common-
policy format [RFC4745], as identified by the MIME media type of
"application/auth-policy+xml". The common-policy format MUST be
provided as the default format in response to GET requests that do
not include specific "Accept" headers, but content negotiation MAY be
used to allow for other formats.
This usage of HTTP is generally compatible with the use of Extensible
Markup Language (XML) Configuration Access Protocol (XCAP) [RFC4825]
or Web Distributed Authoring and Versioning (WebDAV) [RFC4918] to
manage policy documents, but this document does not define or require
the use of these protocols.
3.2. Policy URI Allocation
A Location Server creates a policy URI for a specific location
resource at the time that the location resource is created; that is,
a policy URI is created at the same time as the location URI that it
controls. The URI of the policy resource MUST be different from the
A policy URI is provided in response to location configuration
requests. A policy URI MUST NOT be provided to an entity that is not
authorized to view or set policy. This document does not describe
how policy might be provided to entities other than for location
configuration, for example, in responses to dereferencing requests
[RFC6753] or requests from third parties [RFC6155].
Each location URI has either one policy URI or no policy URI. The
initial policy that is referenced by a policy URI MUST be identical
to the policy that would be applied in the absence of a policy URI.
A client that does not support policy URIs can continue to use the
location URI as they would have if no policy URI were provided.
For HELD, the client assumes that the default policy grants any
requester access to location information, as long as the request
possesses the location URI. To ensure that the authorization
policy is less permissive, a client updates the policy prior to
distributing the location URI.
A Location Server chooses whether or not to provide a policy URI
based on local policy. A HELD-specific extension also allows a
requester to specifically ask for a policy URI.
A policy URI is effectively a shared secret between the Location
Server and its clients. Knowledge of a policy URI is all that is
required to perform any operations allowed on the policy. Thus, a
policy URI should be constructed so that it is hard to predict and
confidentiality protected when transmitted (see Section 7). To avoid
reusing these shared secrets, the Location Server MUST generate a new
policy URI whenever it generates a new location URI set.
3.3. Policy Defaults
Client implementors should keep in mind that setting no policy (never
performing an HTTP request to a policy URI) is very different from
setting an empty policy (performing a PUT with the empty policy). By
"the empty policy", we mean a policy containing no rules, which would
be represented by the following policy document:
<?xml version="1.0" encoding="UTF-8"?>
Figure 1: The Empty Policy
If no policy is set, then the client tacitly accepts whatever policy
the server applies to location URIs, including a policy that provides
location to anyone that makes a dereference request. If the empty
policy is set, then the opposite is true; the client directs the
server to never provide access to location. (Since there are no
rules to allow access and the policy language is default-deny.)
Thus, implementors should consider carefully how to handle the case
implementation might treat this case as if the user had no privacy
preferences and, thus, set no policy. On the other hand, another
implementation might decide that if a user provides no positive
authorization, then the empty policy should be installed.
The same reasoning could also be applied to servers, with the caveat
that servers do not know whether a given HELD client supports the use
of policy URIs. A client that does not understand policy URIs will
not be able to set its own policy, so the server must choose a
default that is open enough that clients will find it useful. On the
other hand, once a client indicates that it understands policy URIs
(by including a "requestPolicyUri" element in its HELD request), the
server may change its default policy to something more restrictive --
even the empty, default-deny policy -- since the client can specify
something more permissive if desired.
4. Location Configuration Extensions
Location configuration protocols can provision hosts with location
URIs that refer to the host's location. If the target host is to
control policy on these URIs, it needs a way to access the policy
that the Location Server uses to guide how it serves location URIs.
This section defines extensions to LCPs to carry policy URIs that the
target can use to control access to location resources.
The HELD protocol [RFC5985] defines a "locationUriSet" element, which
contains a set of one or more location URIs that reference the same
resource and share a common access control policy. The schema in
Figure 2 defines two extension elements for HELD: an empty
"requestPolicyUri" element that is added to a location request to
indicate that a Device desires that a policy URI be allocated and a
"policyUri" element that is included in the location response.
<?xml version="1.0" encoding="UTF-8"?>
<xs:element name="policyUri" type="xs:anyURI"/>
Figure 2: XML Schema for the Policy URI Extension
The URI carried in a "policyUri" element refers to the common access
control policy for location URIs in the location response. The URI
MUST be a policy URI as described in Section 3. A policy URI MUST
use the "http:" or "https:" scheme, and the Location Server MUST
support the specified operations on the URI.
A HELD request MAY contain an explicit request for a policy URI. The
presence of the "requestPolicyUri" element in a location request
indicates that a policy URI is desired.
4.2. Client Processing
It is possible that this document will be updated to allow the use of
policy URIs that use protocols other than the HTTP-based protocol
described above. To ensure that they fail safely when presented with
such a URI, clients implementing this specification MUST verify that
a policy URI received from HELD uses either the "http:" or "https:"
scheme. If the URI does not match those schemes, then the client
MUST discard the URI and behave as if no policy URI was provided.
In this section, we provide some brief illustrations of how policy
URIs are delivered to target hosts and used by those hosts to manage
A HELD request that explicitly requests the creation of a policy URI
has the following form:
A HELD response providing a single "locationUriSet", containing two
URIs under a common policy, would have the following form:
5.1. Basic Access Control Policy
Consider a client that gets the policy URI <https://
ls.example.com:9768/policy/357lp6f64prlbvhl5nk3b>, as in the above
LCP example. The first thing this allows the client to do is inspect
the default policy that the LS has assigned to this URI:
GET /policy/357lp6f64prlbvhl5nk3b HTTP/1.1
HTTP/1.1 200 OK
<?xml version="1.0" encoding="UTF-8"?>
This policy allows any requester to obtain location information, as
long as they know the location URI. If the user disagrees with this
policy, and prefers for example, to only provide location to one
friend, at a city level of granularity, then the client can install
this policy on the Location Server:
PUT /policy/357lp6f64prlbvhl5nk3b HTTP/1.1
<?xml version="1.0" encoding="UTF-8"?>
HTTP/1.1 200 OK
Finally, after using the URI for a period, the user wishes to
permanently invalidate the URI.
DELETE /policy/357lp6f64prlbvhl5nk3b HTTP/1.1
HTTP/1.1 200 OK
6. IANA Considerations
This document requires several IANA registrations, detailed below.
6.1. URN Sub-Namespace Registration for
This section registers a new XML namespace,
"urn:ietf:params:xml:ns:geopriv:held:policy", per the guidelines in
Registrant Contact: IETF, GEOPRIV working group,
(email@example.com), Richard Barnes (firstname.lastname@example.org).
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<title>HELD Policy URI Extension</title>
<h1>Namespace for HELD Policy URI Extension</h1>
<p>See <a href="http://www.rfc-editor.org/rfc/rfc7199.txt">
6.2. XML Schema Registration
This section registers an XML schema as per the guidelines in
Registrant Contact: IETF, GEOPRIV working group (email@example.com),
Richard Barnes (firstname.lastname@example.org)
Schema: The XML for this schema can be found in Section 4.1.
7. Security Considerations
There are two main classes of risks associated with access control
policy management: The risk of unauthorized grants or denial of
access to the protected resource via manipulation of the policy
management process, and the risk of disclosure of policy information
Protecting the policy management process from manipulation entails
two primary requirements. First, the policy URI has to be faithfully
and confidentially transmitted to the client; second, the policy
document has to be faithfully and confidentially transmitted to the
Location Server. The mechanism also needs to ensure that only
authorized entities are able to acquire or alter policy.
7.1. Integrity and Confidentiality for Authorization Policy Data
Each LCP ensures integrity and confidentiality through different
means (see [RFC5985]). These measures ensure that a policy URI is
conveyed to the client without modification or interception.
In general, the requirements for TLS on policy transactions are the
same as for the dereference transactions they set policy for
[RFC6753]. To protect the integrity and confidentiality of policy
data during management, the Location Server SHOULD provide policy
URIs with the "https:" scheme and require the use of HTTP over TLS
[RFC2818]. The cipher suites required by TLS [RFC5246] provide both
integrity protection and confidentiality. If other means of
protection are available, an "http:" URI MAY be used, but location
servers SHOULD reject PUT and DELETE requests for policy URIs that
use the "http:" URI scheme.
7.2. Access Control for Authorization Policy
Access control for the policy resource is based on knowledge of its
URI. The URI of a policy resource operates under the same
constraints as a possession model location URI [RFC5808] and is
subject to the same constraints:
o Knowledge of a policy URI MUST be restricted to authorized Rule
Makers. Confidentiality and integrity protections SHOULD be used
when policy URIs are conveyed in a location configuration protocol
and in the requests that are used to inspect, change, or delete
the policy resource. Note that in some protocols (such as DHCP),
these protections may arise from limiting the use of the protocol
to the local network thus relying on lower-layer security
mechanisms. When neither application-layer nor network-layer
security is provided, location servers MUST reject requests using
the PUT and DELETE methods.
o The Location Server MUST ensure that it is not practical for an
attacker to guess a policy URI value, even if the attacker has
requested many policy URIs from the Location Server over time.
The policy URI MUST NOT be derived solely from information that
might be public, including the Target identity or any location
URI. The addition of 128 bits or more of random entropy is
RECOMMENDED to make it infeasible for a third party to guess a
o Servers SHOULD apply rate limits in order to make brute-force
guessing infeasible. If a server allocates location URIs that
include N bits of entropy with a lifetime of T seconds, then the
server should limit clients to (2^(N/2))/T queries per second.
(The lifetime T of a location URI set is specified by the
"expires" attribute in HELD.)
One possible algorithm for generating appropriately unpredictable
policy URIs for a location URI set is described in Appendix A.
The goal of the above recommendation on rate limiting is to bound the
probability that an attacker can guess a policy URI during its
lifetime. If an attacker is limited to (2^(N/2))/T queries per
second, then he will be able to make at most 2^(N/2) guesses over the
lifetime of the URI. Assuming these guesses are distinct, the
probability of the attacker guessing any given URI is
(2^(N/2))/(2^N), so the probability of compromise over the T-second
lifetime of the URI is at most 2^(-N/2). (Of course, if the attacker
guesses the URI after the policy URI has expired, then there is no
risk.) With N=128, the probability of compromise is 5.4e-20 under
this rate-limiting scheme. Operators should choose values for N so
that the corresponding risk of compromise presents an acceptable
level of risk.
If M distinct URIs are issued within the same namespace, then the
probability of any of the M URIs being compromised is M*2^(N/2). The
example algorithm for generating policy URIs (see Appendix A) places
them in independent namespaces (i.e., below the corresponding
location URIs), so this compounding does not occur.
Note that the chosen entropy level will also affect how quickly
legitimate clients can query a given URI, especially for very long-
lived URIs. If the default lifetime T is greater than 2^(N/2), then
clients will have to wait multiple seconds between queries.
Operators should choose entropy and lifetime values that result in
acceptable high maximum query rates and acceptably low probability of
compromise. For example, with 32 bits of entropy (much less than
recommended above), the one-query-per-second policy URI lifetime is
around 18 hours.
7.3. Location URI Allocation
A policy URI enables the authorization by access control lists model
[RFC5808] for associated location URIs. Under this model, it might
be possible to more widely distribute a location URI, relying on the
authorization policy to constrain access to location information.
To allow for wider distribution, authorization by access control
lists places additional constraints on the construction of location
If multiple Targets share a location URI, an unauthorized location
recipient that acquires location URIs for the Targets can determine
that the Targets are at the same location by comparing location URIs.
With shared policy URIs, Targets are able to see and modify
authorization policy for other Targets.
To allow for the creation of Target-specific authorization policies
that are adequately privacy protected, each location URI and policy
URI that is issued to a different Target MUST be different from other
location URIs and policy URIs. That is, two clients MUST NOT receive
the same location URI or the same policy URI.
In some deployments, it is not always apparent to an LCP server that
two clients are different. In particular, where a middlebox
[RFC3234] exists, two or more clients might appear as a single
client. An example of a deployment scenario of this nature is
described in [RFC5687]. An LCP server MUST create a different
location URI and policy URI for every request, unless the requests
can be reliably identified as being from the same client.
7.4. Policy URI Handling
Although servers may choose to implement access controls on policy
URIs, by default, any holder of a policy URI is authorized to access
and modify the referenced policy document and, thus, to control
access to the associated location resources. Because policy URIs
function as shared secrets, clients SHOULD protect them as they would
passwords. For example, policy URIs SHOULD NOT be transmitted to
other hosts or stored in plaintext.
It should be noted that one of the benefits of the policy URI
construct is that in most cases, there is not a policy URI to leave
the client device to which it is provided. Without policy URIs,
location URIs are subject to a default policy set unilaterally by the
server, and location URIs must be conveyed to another entity in order
to be useful. With policy URIs, location URIs can have more nuanced
access controls, and the shared secret used to authenticate the
client (i.e., the policy URI) can simply be stored on the client and
used to set the access control policy on the location URI. So while
policy URIs do use a default model of authorization by possession,
they reduce the overall risk to location privacy posed by leakage of
shared secret URIs.
Thanks to Mary Barnes and Alissa Cooper for providing critical
commentary and input on the ideas described in this document. Also,
thanks to Ted Hardie and Adam Roach for helping clarify the
relationships between policy URIs, policy documents, and location
resources. Thanks to Stephen Farrell for a helpful discussion on
security and privacy challenges.
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
[RFC4745] Schulzrinne, H., Tschofenig, H., Morris, J., Cuellar, J.,
Polk, J., and J. Rosenberg, "Common Policy: A Document
Format for Expressing Privacy Preferences", RFC 4745,
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5985] Barnes, M., "HTTP-Enabled Location Delivery (HELD)", RFC
5985, September 2010.
9.2. Informative References
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, February 2002.
[RFC3693] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and
J. Polk, "Geopriv Requirements", RFC 3693, February 2004.
[RFC4119] Peterson, J., "A Presence-based GEOPRIV Location Object
Format", RFC 4119, December 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC4825] Rosenberg, J., "The Extensible Markup Language (XML)
Configuration Access Protocol (XCAP)", RFC 4825, May 2007.
[RFC4918] Dusseault, L., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
[RFC5606] Peterson, J., Hardie, T., and J. Morris, "Implications of
'retransmission-allowed' for SIP Location Conveyance", RFC
5606, August 2009.
[RFC5687] Tschofenig, H. and H. Schulzrinne, "GEOPRIV Layer 7
Location Configuration Protocol: Problem Statement and
Requirements", RFC 5687, March 2010.
[RFC5808] Marshall, R., "Requirements for a Location-by-Reference
Mechanism", RFC 5808, May 2010.
[RFC6155] Winterbottom, J., Thomson, M., Tschofenig, H., and R.
Barnes, "Use of Device Identity in HTTP-Enabled Location
Delivery (HELD)", RFC 6155, March 2011.
[RFC6225] Polk, J., Linsner, M., Thomson, M., and B. Aboba, "Dynamic
Host Configuration Protocol Options for Coordinate-Based
Location Configuration Information", RFC 6225, July 2011.
[RFC6753] Winterbottom, J., Tschofenig, H., Schulzrinne, H., and M.
Thomson, "A Location Dereference Protocol Using HTTP-
Enabled Location Delivery (HELD)", RFC 6753, October 2012.
[RFC6772] Schulzrinne, H., Tschofenig, H., Cuellar, J., Polk, J.,
Morris, J., and M. Thomson, "Geolocation Policy: A
Document Format for Expressing Privacy Preferences for
Location Information", RFC 6772, January 2013.
Appendix A. Example Policy URI Generation Algorithm
One possible algorithm for generating appropriately unpredictable
policy URIs for a location URI set is as follows:
1. Choose parameters:
* A cryptographic hash function H, e.g., SHA256
* A number N of bits of entropy to add, such that N is no more
than the length of the output of the hash function
2. On allocation of a location URI, generate a policy URI in the
1. Generate a random value NONCE at least N/8 bytes long
2. Compute hash = H( Location-URI-Set || NONCE ) using some
cryptographic hash function H and some serialization of the
location URI set (e.g., the XML from a HELD response)
3. Form the policy URI by appending the base64url-encoded form
of the hash [RFC4648] to one of the location URIs, e.g., as a
query parameter: "http://example.com/loc/
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