GEOPRIV M. Thomson
Internet-Draft J. Winterbottom
Intended status: Standards Track Andrew
Expires: June 23, 2008 December 21, 2007
Specifying Location Quality Constraints in Location Protocols
draft-thomson-geopriv-location-quality-00.txt
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Abstract
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . 4
2. Location Quality Operation . . . . . . . . . . . . . . . . . . 5
3. Location Quality Parameters . . . . . . . . . . . . . . . . . 6
3.1. Location Quality Request . . . . . . . . . . . . . . . . . 6
3.1.1. Maximum Uncertainty . . . . . . . . . . . . . . . . . 6
3.1.2. Required Civic Elements . . . . . . . . . . . . . . . 7
3.1.3. Maximum Age . . . . . . . . . . . . . . . . . . . . . 8
3.2. Location Quality Indication . . . . . . . . . . . . . . . 8
4. Location Quality Schema . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
6.1. URN Sub-Namespace Registration for
urn:ietf:params:xml:ns:geopriv:lq . . . . . . . . . . . . 14
6.2. XML Schema Registration for Measurement Schema . . . . . . 14
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. Normative References . . . . . . . . . . . . . . . . . . . 15
7.2. Informative References . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18
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1. Introduction
Location determination methods produce results of varying quality.
In general, the quality of location information increases as the
effort expended in generating the information increases. This
document provides XML extension objects that can be added to any
protocol that provides location information. These elements provide
the ability to communicate location quality constraints to the
location server.
This document provides semantics, examples and security
considerations for the HELD protocol
[I-D.ietf-geopriv-http-location-delivery]. Application of the
parameters described in this document are out of scope.
Means for expressing the quality of location information is outlined
in [I-D.thomson-geopriv-uncertainty] and [GeoShape]. An entity
requesting location information of an LS is unable to specify the
quality of the location that it ultimately receives. This is
inefficient because an LS either provides location information that
is inadequate for the intended task; or the LS could waste resources
generating location information that is of eccessively high quality.
This document describes an optional HELD parameter that communicates
location quality constraints to an LS. These constraints specify a
desired uncertainty at a certain confidence, plus the maximum
acceptable age where location information is stored. Guidelines for
deterministically evaluating location information against these rules
are provided.
[I-D.busin-geopriv-location-qos-req] describes some of the benefits
of providing an LS with location quality constraints. Location
quality constraints provide information that an LS can use in
deciding how to generate location information. For example, an LS
that is able to provide a location estimate with a sufficiently small
uncertainty might be able to provide a response before the time
indicated in the "responseTime".
This document also provides a means by which the LS is able to
indicate if the location quality meets the constraints. These
parameters can be used by a Location Recipient to ensure that the
location is of adequate quality without requiring specific checking
(although the PIDF-LO should include sufficient information to
perform this check). Response parameters are optional, and are also
used to indicate that the LS has understood the location quality
constraints.
This document provides solutions that address a subset of the
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requirements in [I-D.busin-geopriv-location-qos-req].
1.1. Conventions used in this document
Terms and procedures relating to uncertainty and confidence are taken
from [I-D.thomson-geopriv-uncertainty]. Familiarity with terminology
outlined in [I-D.ietf-geopriv-l7-lcp-ps] and [RFC3693] is also
assumed.
The term Location Server (LS) is used as a generic label. From the
perspective of this document, the LS could be a Location Information
Server (LIS). Similarly, the term Location Recipient (LR) is used to
refer to the requester of location information, which could be a
Device or Target for HELD.
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 [RFC2119].
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2. Location Quality Operation
Location quality parameters are provided by a Device or any other
client of an LS in a location request message. Figure 1 shows an
example message.
geodetic
150
1000
PT30S
Figure 1
A LS that supports the location quality element uses the information
contained in the request to choose how it serves the query. The
response to this message contains a quality indicator element that
includes a list of the quality constraints that were met. Figure 2
shows a location response that includes a quality indicator.
maxUncertainty/vertical maxAge
Figure 2
A LS doesn't indicate the quality of the location estimate in the
quality indicator; quality information is included in the PIDF-LO.
The quality indicator provides notice to its recipient that the
requested quality was provided.
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3. Location Quality Parameters
This section defines the format and semantics of the location quality
parameters for requests and the indication that is included with
responses.
3.1. Location Quality Request
The "quality" element is included in a HELD request to indicate the
constraints set by the Location Recipient (LR) on the quality of
returned location information. This document defines three elements
that are included.
3.1.1. Maximum Uncertainty
The "maxUncertainty" element describes an upper limit on uncertainty
at a given confidence. Uncertainty is divided in to horizontal and
vertical components. Horizontal uncertainty is the maximum distance
from the centroid of the area to the point in the shape furthest from
the centroid on the horizontal plane. Vertical uncertainty is the
difference in altitude from the centroid to the point in the shape
with the greatest altitude.
Note: A LS MAY provide location information using the Point shape
and indicate that the requested uncertainty is met providing that
the LS has access to uncertainty information. However, this is
NOT RECOMMENDED since the recipient has no way of verifying the
uncertainty.
The "horizontal" and "vertical" elements are numerical values that
contain a decimal value in meters. Maximum uncertainty values MUST
be greater than zero.
A location estimate that does not contain uncertainty (i.e. a Point
shape), never meets location quality constraints. Where uncertainty
is unknown, it MUST be assumed to be infinite at any non-zero
confidence. In particular, this applies to vertical uncertainty
where the location estimate is two-dimensional only; location
estimates without a vertical component of uncertainty never meet
vertical uncertainty constraints.
The "confidence" attribute of this element includes the confidence
level (expressed as a percentage) that the uncertainty is evaluated
at. Confidence is set to a default of 95%.
To evaluate uncertainty, the location estimate is first scaled so
that the confidence of the estimate matches (or exceeds) the
requested confidence. The LS MAY convert the shape of the
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uncertainty to a circle or a sphere prior to scaling to simply the
scaling process. For consistency, and contrary to the rules in
[I-D.thomson-geopriv-uncertainty], it is RECOMMENDED that a normal
PDF be used for all except where confidence is reduced for a
rectangular PDF.
Horizontal uncertainty is evalulated by removing the altitude and
altitude uncertainty components from the location estimate. While
removing altitude components from a location estimate might normally
increase confidence, confidence MUST NOT be increased at this step;
the confidence value has already been considered. The shape is then
converted to a circle, if it is not already in that shape. The
radius of the resulting circle is compared to the maximum horizontal
uncertainty.
Vertical uncertainty is evaluated for shapes that contain altitude
uncertainty. The value used for evaluating vertical uncertainty
depends on the shape type: the vertical axis value for the Ellipsoid
shape; the radius of the Sphere shape; half the height of the Prism
shape.
The LS MAY use location quality parameters to alter the way that it
generates location information and to provide location information
that more closely matches what is requested. If maximum value is
provided for vertical uncertainty, is is RECOMMENDED that the LS
provide a location estimate that includes altitude and altitude
uncertainty. It is RECOMMENDED that the LS provide location
information at the requested confidence, if possible.
3.1.2. Required Civic Elements
The "requiredCivic" element represents the requirements of an LR for
uncertainty in civic address information. An LR can specify the
address elements that need to be present in the civic address in
order for the location information to meet their quality
requirements.
The "requiredCivic" element contains a whitespace-separated list of
element names. These can be interpreted as XPath
[W3C.REC-xpath-19991116] expressions that are evaluated in the
context of the "civicAddress" element
[I-D.ietf-geopriv-revised-civic-lo]. These XPath statements are
restricted to use of qualified names only (using the response
document namespace context) and the "/" separator. All child nodes
of elements (including attributes and textual content) are treated as
belonging to an element.
Figure 3 shows an example request where an LR requires country, state
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(or equivalent) and post code civic address elements in the location
information provided by the LS.
ca:country ca:A1 ca:PC
Figure 3
3.1.3. Maximum Age
Where location information is stored or cached, an LR can specify how
long since the information. This is particularly important if
location information is generated in advance. The "age" of location
information is the taken as the time interval between the location
information timestamp (the "timestamp" element in the PIDF tuple) and
the time that the request is received by the LS.
Location information that has greater age than requested SHOULD
either be determined anew, or checked so that the timestamp can be
updated. If this value is set to zero (i.e. "PT0S"), stored
location information cannot be used.
3.2. Location Quality Indication
The "qualityInd" element contains a list of the quality constraints
that the resulting location information meets.
The list of constraints is represented as a whitespace-separated list
of element names. These can be interpreted as XPath
[W3C.REC-xpath-19991116] expressions that are evaluated in the
context of the original location quality request. These statements
follow the same constraints as the list of elements in Section 3.1.2.
Where elements are nested, such as the "maxUncertainty" element, the
outer element can be included to indicate an entire constraint is
met; or, each individual child element can be identified. Two
equivalent indications are shown in Figure 4.
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maxUncertainty
maxUncertainty/horizontal maxUncertainty/vertical
Figure 4
A LS that is unable to determine if a constraint MUST either omit the
quality indication, or indicate that the constraint was not met.
Two special values are added to the quality indication element for
convenience. The value "##all" indicates that all quality
constraints were met (including any extensions). The value "##none"
indicates that none of the constraints were met.
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4. Location Quality Schema
Note that the pattern rules in the following schema wrap due to
length constraints in RFC documents. None of the patterns contain
whitespace.
HELD Location Quality
This schema defines a framework for location quality requests
and indications of whether they are met.
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5. Security Considerations
This document does not introduce any security considerations.
[[Editor's Note: Please let us know if you can think of some.]]
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6. IANA Considerations
6.1. URN Sub-Namespace Registration for
urn:ietf:params:xml:ns:geopriv:lq
This section registers a new XML namespace,
"urn:ietf:params:xml:ns:geopriv:lq", as per the guidelines in
[RFC3688].
URI: urn:ietf:params:xml:ns:geopriv:lq
Registrant Contact: IETF, GEOPRIV working group,
(geopriv@ietf.org), Martin Thomson (martin.thomson@andrew.com).
XML:
BEGIN
HELD Location Quality
Namespace for HELD Location Quality
urn:ietf:params:xml:ns:geopriv:lq
[[NOTE TO IANA/RFC-EDITOR: Please update RFC URL and replace XXXX
with the RFC number for this specification.]]
See RFCXXXX.
END
6.2. XML Schema Registration for Measurement Schema
This section registers an XML schema as per the guidelines in
[RFC3688].
URI: urn:ietf:params:xml:schema:geopriv:lq
Registrant Contact: IETF, GEOPRIV working group, (geopriv@ietf.org),
Martin Thomson (martin.thomson@andrew.com).
Schema: The XML for this schema can be found in Section 4 of this
document.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[I-D.ietf-geopriv-http-location-delivery]
Barnes, M., Winterbottom, J., Thomson, M., and B. Stark,
"HTTP Enabled Location Delivery (HELD)",
draft-ietf-geopriv-http-location-delivery-03 (work in
progress), November 2007.
[I-D.thomson-geopriv-uncertainty]
Thomson, M. and J. Winterbottom, "Representation of
Uncertainty and Confidence in PIDF-LO",
draft-thomson-geopriv-uncertainty-00 (work in progress),
November 2007.
7.2. Informative References
[W3C.REC-xpath-19991116]
DeRose, S. and J. Clark, "XML Path Language (XPath)
Version 1.0", World Wide Web Consortium
Recommendation REC-xpath-19991116, November 1999,
.
[I-D.ietf-geopriv-revised-civic-lo]
Thomson, M. and J. Winterbottom, "Revised Civic Location
Format for PIDF-LO",
draft-ietf-geopriv-revised-civic-lo-07 (work in progress),
December 2007.
[I-D.busin-geopriv-location-qos-req]
Busin, A., Jin, Y., Mosmondor, M., and S. Loreto,
"Requirements for a Location Quality of Service (QoS)
Information Object",
draft-busin-geopriv-location-qos-req-01 (work in
progress), November 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-06 (work in
progress), November 2007.
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[RFC3693] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and
J. Polk, "Geopriv Requirements", RFC 3693, February 2004.
[GeoShape]
Thomson, M. and C. Reed, "GML 3.1.1 PIDF-LO Shape
Application Schema for use by the Internet Engineering
Task Force (IETF)", Candidate OpenGIS Implementation
Specification 06-142r1, Version: 1.0, April 2007.
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Authors' Addresses
Martin Thomson
Andrew
PO Box U40
Wollongong University Campus, NSW 2500
AU
Phone: +61 2 4221 2915
Email: martin.thomson@andrew.com
URI: http://www.andrew.com/
James Winterbottom
Andrew
PO Box U40
Wollongong University Campus, NSW 2500
AU
Phone: +61 2 4221 2938
Email: james.winterbottom@andrew.com
URI: http://www.andrew.com/
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