GeoPriv R. Marshall, Ed.
Internet-Draft TCS
Intended status: Informational September 5, 2007
Expires: March 8, 2008
Requirements for a Location-by-Reference Mechanism
draft-ietf-geopriv-lbyr-requirements-00
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Abstract
This document defines terminology and provides requirements relating
to a Location-by-Reference approach to handling location information
within SIP signaling and other Internet messaging. The key for a
Location-by-Reference mechanism is the Location URI, which is a
reference to a location, and is used by either an end-device or a
middlebox to represent a location, and is used as a key by a
dereferencing protocol to get a usable form of location. An example
application for which the Location-by-Reference mechanism is used is
emergency call routing with voice-over-IP (VoIP) and general Internet
multimedia systems, where Internet protocols are used end-to-end.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Terminology . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Definition of Terms . . . . . . . . . . . . . . . . . . . 6
4. Basic Actors . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. High-Level Requirements for a Location Configuration
Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6. High-Level Requirements for a Location Dereference
Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Normative References . . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . . . 19
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1. Introduction
Location-based services rely on ready access to location information,
which can be through a direct, or indirect mechanism. While there is
already a direct mechanism which exists to provide location as part
of the SIP signaling protocol, an alternative mechanism has been
developed for handling location indirectly, via a location reference,
a reference which points to the actual location information. This
reference is called the location URI, and is used by the Location-by-
Reference mechanism.
Since possessing the location URI alone is insufficient to perform
location-based routing, the location URI must be dereferenced. Once
the actual location information is returned to a location recipient,
it can then be used as input to some location-based service, such as
in the case of routing a VoIP-based emergency call.
This document lists a set of requirements for a Location-by-Reference
(LbyR) mechanism, using a location URI within the SIP protocol for
the purpose of executing a location-based service routing request.
There are a variety of actions in which a location URI can be used.
Included in this list is the action of 'location configuration', or
the acquisition of the location into an end device or middlebox,
'location conveyance', which is the shuttling of location between SIP
signaling nodes, and, 'location dereferencing', which we define as
the action of exchanging a location URI for the actual location
information it points to at a dereference server, which we call a
Location Information Server, or LIS.
Each of these actions are represented by specific individual
protocols. A Location Configuration Protocol (LCP), is used by a
device or middlebox to acquire a location which already exists
(examples of this protocol include DHCP, LLDP-MED, and HELD). By
conveyance protocol, we mean a protocol which transports a location
URI along from node to node according to specific rules (e.g., SIP).
A Location Dereferencing Protocol (LDP), is used by a client to
resolve a location URI in exchange for location information at a LIS.
Though conveyance of a location URI may be discussed in general
terms, any requirements for conveyance using LbyR are not included,
and are considered out of scope.
In our SIP example, the LbyR is setup, instead of having a content
identifier (cid:) pointing to a location object within a SIP body, to
have a location URI carried in the SIP Geolocation header.
In constrast to LbyR, a direct access to location is equivalent to
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having a location object included along with the signaling, (e.g., a
PIDF-LO), is referred to as the Location-by-Value (LbyV) mechanism,
and is treated as out of scope for this document. A separate draft
document exists which describes, for both LbyR and LbyV scenarios, a
way to convey location within SIP [I-D.ietf-sip-location-conveyance].
The structure of this document first defines terminology in
Section 3. Then a short discussion on the basic elements which show
LbyR. This section on actors, Section 4 includes a basic LbyR model,
and describes the steps which the LbyR mechanism takes.
Requirements are outlined separately for the configuration step
(LCP), (Section 5), followed by an additional list of requirements
targeted toward the dereferencing step (LDP) (Section 6).
Location determination, which may include the processes of manual
provisioning, automated measurements, or location transformations,
(e.g., geo-coding), are beyond the scope of this document.
A detailed discussion of Identity information related to the caller,
subscriber, or device, as associated to location or location URI, is
also out of scope.
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2. Requirements Terminology
In this document, 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].
This document outlines only requirements for an LbyR mechanism which
is used by two different protocols, a Location Configuration
Protocol, and a Location Derferencing Protocol. Each of these
protocols has its own unique client and server interactions, and the
requirements here are not intended to state what either an LCP or LDP
host client or server is expected to do, but rather which
requirements must be met by both the LCP and LDP interface protocols.
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3. Terminology
3.1. Definition of Terms
Several of the terms presented below are based on Geopriv
Requirements [RFC3693], and in some cases, extended to include
additional language to support the LbyR model.
Civic location: A described location based on some understood
location reference system, such a jurisdictions or postal delivery
grid. A street address valid within the USPS system is a common
example.
Coordinate location: A reference to a geographic point which is able
to be located as described by a set of defined coordinates within
a geographic coordinate system, such as latitude and longitude,
within the WGS-84 datum. For example, 2-D geographic location is
defined as an (x,y) coordinate value pair according to the
distance north or south of the equator and east or west of the
prime meridian. A coordinate location may be absolute, or may
have associated uncertainty related to it's exact position,
depending on how it is represented.
Location: Either a geographic coordinate or feature representation
based on a specific coordinate reference system, or by other
identifiable information such as a street number and street name
within a civic, postal, or abstract location reference system.
Location-by-Value: The mechanism of representing location either in
configuration or conveyance protocols, (i.e., the actual location
value is included).
Location-by-Reference: The mechanism of representing location either
in configuration, conveyance, and dereferencing protocols as an
identifier which refers to a fully specified location, (i.e., a
pointer to the actual location value).
Location Configuration Protocol (LCP): A protocol which is used by a
client to acquire location or a location URI from a location
configuration server (e.g., (LIS)), based on information unique to
the client.
Location Dereference Protocol (LDP): A protocol which is used by a
client to query a location dereference server (e.g., (LIS)), based
on location URI input and which returns location information
(e.g., a PIDF-LO).
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Location Information Server (LIS): The entity which receives a
client request for either location or a location reference. In
the latter case, also performs the dereference function for a
Location Refernce Identifier, in the context of the Location-by-
Reference model. May also be referred to as a Location
Information Server (LIS). In the SIP Presence architecture, the
LIS may be referred to as a Presence Server (PS). In this
document the LIS is an instance of an LS.
Location Object (LO): An object conveying location information (and
possibly privacy rules) to which Geopriv security mechanisms and
privacy rules are to be applied.
Location Recipient (LR): The entity that receives location
information. It may have asked for this location explicitly (by
sending a query containing an location URI to a location
configuration server), or it may receive this location
asynchronously.
Location Server (LS): The entity to which a LG publishes location
objects, the recipient of queries from location receivers, and the
entity that applies rules designed by the rule maker.
Location URI: An identifier which serves as a pointer to a location
record on a remote host (e.g., LIS). Used within an Location-by-
Reference (LbyR) mechanism. A location URI is provided by a
location configuration server, based on a client request, and is
the input used by the dereference protocol to retrieve the
associated location from a dereference server. It is assumed that
a LIS can function both as a configuration server and dereference
server.
Rule Maker (RM): The authority that creates rules governing access
to location information for a target (typically, this it the
target themselves).
Target: A person, end device, or other entity whose location is
communicated by a Geopriv Location Object.
Using Protocol: A protocol (e.g., SIP) which carries a Location
Object or an Location Reference Identifier.
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4. Basic Actors
LbyR with Location Subscription
The LbyR mechanism can be used via a normal query/response mode, or
alternatively, by using a subscription model to get updated location.
In mobile wireless networks it is not efficient for the end host to
periodically query the LIS for up-to-date location information. This
is especially the case when power is a constraint or a location
update is not immediately needed. Furthermore, the end host might
want to delegate the task of retrieving and publishing location
information to a third party, such as to a presence server. Finally,
in some deployments, the network operator may not want to make
location information widely available.
These use scenarios motivated the introduction of the LbyR concept.
Depending on the type of reference, such as HTTP/HTTPS or SIP
Presence URI, different operations can be performed. While an HTTP/
HTTPS URI can be resolved to location information, a SIP Presence URI
provides further benefits from the SUBSCRIBE/NOTIFY concept that can
additionally be combined with location filters
[I-D.ietf-geopriv-loc-filters].
+-----------+ Geopriv +-----------+
| | LDP (3) | Location |
| LIS +---------------+ Recipient |
| | | |
+-----+-----+ +----+------+
| --
| --
| Geopriv --
| LCP --
| (1) --
| -- Geopriv
| -- Using Protocol
| -- (e.g., SIP)
+-----+-----+ -- (2)
| Target / |--
| End Host +
| |
+-----------+
Figure 1: Shows the assumed communication model for a layer 7 (L7)
LCP and LDP:
Note that there is no requirement for using the same protocol in (1)
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and (3).
The following list describes the location subscription approach:
1. The end host discovers the LIS.
2. The target (end host) sends a request to the LIS asking for a
location URI, as shown in (1) of Figure 1.
3. The LIS responds to the request and includes a location object
along with a subscription URI.
4. The Target puts the subscription URI into a SIP message and
forwards it to a Location Recipient via a using protocol, as shown in
(2) of Figure 1. The Location Recipient subscribes to the obtained
subscription URI (see (3) of Figure 1) and potentially uses a
location filter (see [I-D.ietf-geopriv-loc-filters]) to limit the
notification rate.
5. If the Target moves outside a certain area, indicated by a
location filter, the Location Recipient will receive a notification.
Note that the Target may also act in the role of the Location
Recipient whereby it would subscribe to its own location information.
For example, the Target obtains a subscription URI from the Geopriv
L7 LCP protocol. It subscribes to the URI in order to obtain its
current location information. A service boundary indicates the
bounded extent up to which the device can move without the need to
have an updated location, since a re-query with any location within
the boundary would result in the same answer returned from a
location-based service.
For LbyR, the LIS needs to maintain a list of randomized location
URIs for each host, timing out each of these URIs after the reference
expires. Location URIs need to expire to prevent the recipient of
such a URI from being able to (in some cases) permanently track a
host. Furthermore, an expiration mechanism also offers garbage
collection capability for the LIS.
Location URIs must be designed to prevent adversaries from obtaining
a known Target's location. There are at least two approaches: The
location URI contains a random component which helps obscure
sequential updates to location, yet still allows any holder of the
location URI to obtain location information. Alternatively, the
location URI can remain public and the LIS performs access control
via a separate authentication mechanism, such as HTTP digest or TLS
client side authentication, when resolving the reference to a
location object.
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5. High-Level Requirements for a Location Configuration Protocol
Below, we summarize high-level design requirements needed for a
location-by-reference mechanism as used within the LCP.
C1. Location URI support - LCP: The configuration protocol MUST
support a location reference in URI form.
Motivation: It is helpful to have a consistent form of key for the
LbyR mechanism.
C2. Location URI expiration: The LCP MUST support the ability to
specify to the server, the length of time that a location URI will
be valid.
Motivation: Location URIs are not intended to represent a location
forever, and the identifier eventually may need to be recycled, or
may be subject to a specific window of validity, after which the
location reference fails to yield a location, or the location is
determined to be kept confidential. A configurable carried in the
LCP for a location URI ensures that the location reference becomes
invalid based on some internal LIS settings.
C3. Location URI cancellation: The LCP MUST support the ability to
request a cancellation of a specific location URI.
Motivation: If the client determines that in its best interest to
destroy the ability for a location URI to effectively be used to
dereference a location, then there has to be a way to nullify the
location URI. (This may be accomplished by setting the C2
configurable to 'expire=now', for example.)
C4. Random Generated: The location URI MUST be hard to guess, i.e.,
it MUST contain a cryptographically random component.
Motivation: There is some benefit to the client if the location
URI is generated in an obscured manner so that its sequence, for
example in the case of a client's location update, can't be easy
guessed.
C5. Identity Protection - LCP : The location URI MUST NOT contain
any information that identifies the user, device or address of
record within the URI form.
Motivation: It is important to protect caller identity or contact
address from being included in the form of the location URI itself
when it is generated.
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C6. Reuse flag default: The LCP MUST support the default condition
of a requested location URI being repeatedly reused.
Motivation: The requestor of a location URI, shouldn't need to
specify any special flag in order to receive a location URI which
can later be used repeatedly, such as for an updated location.
C7. One-time-use: The LCP MUST support the ability for the client to
request a 'one-time-use' location URI (e.g., via a reuse flag
setting).
Motivation: The client requesting a location URI may request a
location URI which has a 'one-time-use' only characteristic, as
opposed to a location URI having multiple reuse capability.
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6. High-Level Requirements for a Location Dereference Protocol
Below, we summarize high-level design requirements needed for a
location-by-reference mechanism.
D1. Location URI support - LDP: The LDP MUST support a location
reference in URI form.
Motivation: It is required that there be consistency of use
between location URI formats used in an LCP and those used by a
LDP.
D2. Location URI expiration status: The LDP MUST support a message
indicating that for a location URI which is no longer valid, that
the location URI has expired.
Motivation: Location URIs are expected to expire, based on LCP
parameters, and it is therefore useful to convey the expired
status of the location URI in the LDP.
D3. Authentication: The LDP MUST support either client-side and
server-side authentication between client and server.
Motivation: It is reasonable to expect implementations of
authentication to vary. Some implementations may choose to
implement both client-side and server-side authentication, might
implement one only, or may implement neither.
D4. Dereferenced Location Form: Location URI dereferencing MUST
result in a well-formed PIDF-LO.
Motivation: This is in order to ensure both interoperation
consistancy and that adequate privacy rules can be adhered to,
since the PIDF-LO format comprises the necessary structures to
maintain location privacy.
D5. Repeated use: The LDP MUST support the ability for the same
location URI to be resolved more than once, based on server
settings and LCP parameters.
Motivation: According to LCP parameters, there may or may not be a
limit on the number of dereferencing actions at the dereference
server.
D6. Updated location: The LDP MUST support the ability for the same
location URI to be resolved into a continuum of location values
(e.g., location updates).
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Motivation: A location URI when reused may not always result in
the same location value, but may be a mixture of unchanged and
changed location values.
D7. Location form: The LDP MUST support dereferenced location in
both coordinate and civic forms.
Motivation: It is important that the LDP not limit which type of
location gets dereferenced, since it is assumed that some
dereference servers may provide coordinate form of location only,
others may provide civic only, while some may provide both forms
of location.
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7. Security Considerations
The LbyR mechanism currently addresses security issues as follows.
A location URI, regardless of its randomized construction, if
public, implies no safeguard against anyone being able to
dereference and get the location. The randomization of a location
URI in its naming, does help prevent some potential guessing,
according to some defined pattern. In the instance of one-time-
use location URIs, which function similarly to a pawn ticket, the
argument can be made that with a pawn ticket, possession implies
permission, and location URIs which are public are protected only
by privacy rules enforced at the dereference server.
Additional security issues will be discussed in a separate geopriv
document.
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8. IANA Considerations
This document does not require actions by the IANA.
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9. Acknowledgements
I would like to thank the IETF GEOPRIV working group chairs, Andy
Newton, Allison Mankin and Randall Gellens, for creating the design
team which initiated this requirements work.
I also would like to thank Andrew Newton; Martin Dawson; Henning
Schulzrinne; Marc Linsner; Brian Rosen; Ted Hardie; James M. Polk;
James Winterbottom; Martin Thomson; John Schnizlein; Barbara Stark;
Jon Peterson; Allison Mankin; Randall Gellens; Cullen Jennings;
Richard Barnes; Keith Drage; Rohan Mahy; and Hannes Tschofenig, for
their individual contributions and comments.
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10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
10.2. Informative References
[I-D.ietf-geopriv-http-location-delivery]
Barnes, M., "HTTP Enabled Location Delivery (HELD)",
draft-ietf-geopriv-http-location-delivery-01 (work in
progress), July 2007.
[I-D.ietf-geopriv-l7-lcp-ps]
Tschofenig, H. and H. Schulzrinne, "GEOPRIV Layer 7
Location Configuration Protocol; Problem Statement and
Requirements", draft-ietf-geopriv-l7-lcp-ps-04 (work in
progress), August 2007.
[I-D.ietf-geopriv-loc-filters]
Mahy, R., "A Document Format for Filtering and Reporting
Location Notications in the Presence Information Document
Format Location Object (PIDF-LO)",
draft-ietf-geopriv-loc-filters-01 (work in progress),
March 2007.
[I-D.ietf-sip-location-conveyance]
Polk, J. and B. Rosen, "Location Conveyance for the
Session Initiation Protocol",
draft-ietf-sip-location-conveyance-08 (work in progress),
July 2007.
[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.
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Author's Address
Roger Marshall (editor)
TeleCommunication Systems, Inc.
2401 Elliott Avenue
2nd Floor
Seattle, WA 98121
US
Phone: +1 206 792 2424
Email: rmarshall@telecomsys.com
URI: http://www.telecomsys.com
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