GEOPRIV M. Thomson
Internet-Draft J. Winterbottom
Intended status: Standards Track Andrew Corporation
Expires: July 18, 2010 January 14, 2010
Specifying Location Quality Requirements in Location Protocols
draft-thomson-geopriv-location-quality-05
Abstract
Parameters that define the expected quality of location information
are defined for use in location protocols. These parameter can be
used by a requester to indicate to a Location Server quality
requirements for the location information it requests. If
applicable, the Location Server is able to use this information to
control how location information is determined. An optional
indication of whether the quality requirements were met is defined to
be provided by the Location Server alongside location information.
Status of This Memo
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . 4
2. Location Quality Operation . . . . . . . . . . . . . . . . . . 4
3. Location Quality Objects . . . . . . . . . . . . . . . . . . . 5
3.1. Location Quality Request . . . . . . . . . . . . . . . . . 5
3.1.1. Maximum Uncertainty . . . . . . . . . . . . . . . . . 5
3.1.2. Required Civic Elements . . . . . . . . . . . . . . . 7
3.1.3. Maximum Age . . . . . . . . . . . . . . . . . . . . . 8
3.2. Location Quality Indication . . . . . . . . . . . . . . . 8
4. Location Quality Schema . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6.1. URN Sub-Namespace Registration for
urn:ietf:params:xml:ns:geopriv:lq . . . . . . . . . . . . 11
6.2. XML Schema Registration for Location Quality Schema . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . . 13
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1. Introduction
Location determination methods produce results of varying accuracy.
In general, the accuracy of location information increases as the
effort expended in generating the information increases. Accuracy is
the primary aspect of the quality of location information that is
relevant to a Location Recipient (LR). Other aspects of quality can
also be significant, such as the currency of the data.
Means for expressing the quality of location information is outlined
in [I-D.thomson-geopriv-uncertainty] and [GeoShape]. An entity
requesting location information of a Location Server (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 defines XML objects that can be added to any protocol
that provides location information. These elements provide the
ability to communicate location quality requirements to an LS. These
requirements 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.
This document provides semantics, examples and security
considerations for the HELD protocol
[I-D.ietf-geopriv-http-location-delivery]. The parameters and
procedures described in this document are applicable to HELD when
used either as a location configuration protocol (LCP)
[I-D.ietf-geopriv-l7-lcp-ps] or as a location dereference protocol
[I-D.ietf-geopriv-lbyr-requirements]. Application of the parameters
described in this document to other protocols is not described, but
is relatively trivial for protocols that are able to convey XML.
Location quality requirements provide information that an LS can use
in deciding how to generate location information, if the LS has that
capacity, directly or otherwise. This is the case for a Location
Information Server (LIS) and the HELD protocol.
Specifying location quality requirements ensures that a Location
Receipient (LR) receives information that is suited to their needs.
It also provides information that any Location Generator (LG) can use
to better decide how location information is generated. This
provides advantages to both requester and source of the information.
In one example, a LIS that is able to provide a location estimate
with uncertainty that matches the requested requirements might be
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able to provide that response before the time indicated within the
time indicated in the request (the "responseTime").
This document also defines an object that can be used by the LS to
indicate if the location quality meets the requirements. These
parameters can be used by a Location Recipient to ensure that the
location is of adequate quality without requiring specific checking
without having to examine the location object. Response parameters
are an optional optimisation; the presence of a quality indication in
the response also indicates that the LS has understood the location
quality requirements.
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, since these
paramters apply in all cases where location information is served to
a requesting entity. 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].
2. Location Quality Operation
Location quality parameters are provided by a Location Recipient in a
location request message. Figure 1 shows an example HELD message
where a Device requests location information of a specified quality.
geodetic
150
1000
2008-05-27T05:47:55Z
Figure 1: Example HELD Location Request
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An LS that supports the location quality element uses the information
contained in the request to choose how it serves the query. If the
LS sources the information from an LG, this information might be
passed to the LG to determine how it generates the information. The
response to this message contains a quality indicator element that
includes a list of the quality requirements that were understood and
met. Figure 2 shows a location response that includes a quality
indicator.
maxUncertainty/vertical maxAge
Figure 2: Example HELD Location Response
An LS provides an indication of the requirements that have been met.
The actual quality of the location estimate SHOULD be included in the
actual PIDF-LO document, expressed in the uncertainty inherent in the
location information and tuple timestamp.
3. Location Quality Objects
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
requirements set by the Location Recipient (LR) on the quality of
returned location information. This document defines three elements
that are included.
Extensions to this specification MAY specify XML elements that are
included as children of the "quality" element. Elements that are not
understood SHOULD be ignored by the LS.
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
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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.
The "horizontal" and "vertical" elements are numerical values that
contain a decimal value in metres. 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 requirements. 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 requirements.
Note: An LS MAY provide location information using the Point shape
and indicate that the requested uncertainty is met, if the LS has
access to uncertainty information and is prevented from sharing
this information due to policy constraints. However, this is NOT
RECOMMENDED since the LR has no way of independently verifying
that the uncertainty meets their requirements.
The "confidence" attribute of this element includes the confidence
level (expressed as a percentage) that the uncertainty is evaluated
at. Desired confidence has a default value 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 SHOULD convert the shape of the
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 assumed for all location information except where confidence
is reduced for a rectangular PDF. Other scaling methods MAY be
applied where better information about the distribution is known.
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
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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. A constraint on vertical uncertainty cannot be met if
vertical uncertainty is not known.
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, it is RECOMMENDED that the LS
provide a location estimate that includes altitude and altitude
uncertainty if possible. It is RECOMMENDED that the LS provide
location information at the confidence included in the request, if
scaling to a particular confidence is possible. Scaling MAY be
avoided if the location information is significantly degraded by the
scaling process.
3.1.2. Required Civic Elements
The "requiredCivic" element represents the requirements of an LR for
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-xpath20-20070123] expressions that are evaluated in the
context of the "civicAddress" element [RFC5139]. These XPath
statements are restricted to use of qualified names only (using the
response document namespace context) and the "/" separator; that is,
the only permitted axis is the "child::" axis. 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
(or equivalent) and post code civic address elements in the location
information provided by the LS.
ca:country ca:A1 ca:PC
Figure 3: Example Specifying Required Civic Address Fields
This does not force the LS to limit the civic address fields provided
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to just those requested. Additional address fields MAY be provided.
3.1.3. Maximum Age
Where location information is stored or cached, an LR can specify a
limit on the age of this information. This is particularly important
if location information is generated in advance. The "age" of
location information is indicated by the the "timestamp" element in
the PIDF tuple. The age parameter specifies the minimum value for
this field; that is, the oldest location information that is
acceptable.
Location information that has greater age than requested SHOULD be
determined anew. A value of "now" can be used to indicate that
stored location information of any age is not acceptable to the LR.
3.2. Location Quality Indication
The "qualityInd" element is used in responses to indicate which of
the location quality requirements from a request were met. The
presence of this element indicates that a request for a given
location quality was understood and lists the quality requirements
that the accompanying location information meets.
The list of requirements is represented as a whitespace-separated
list of element names. These can be interpreted as XPath
[W3C.REC-xpath20-20070123] 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.
maxUncertainty
maxUncertainty/horizontal maxUncertainty/vertical
Figure 4: Equivalent Quality Indications
A LS that is unable to determine if a constraint is met for any
reason MUST NOT list that constraint in this element. This includes
the case where the constraint is not supported by the LS. This list
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MAY be empty if none of the requested quality requirements could be
met.
Two special values are added to the quality indication element for
convenience. The value "##all" indicates that all quality
requirements were met. A value of "##all" cannot be used if there
are unknown or unsupported elements in the quality request.
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 extension does not add significantly to the security and privacy
concerns of the protocol that the location quality requirements are
attached to.
An entity that is concerned about the privacy implications of making
its preferences known can choose not to specify location quality
requirements.
6. IANA Considerations
This section registers a namespace and schema for the location
quality objects.
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:
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BEGIN
Location Quality
Namespace for 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 Location Quality 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.
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.
[RFC5139] Thomson, M. and J.
Winterbottom, "Revised
Civic Location Format for
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Presence Information Data
Format Location Object
(PIDF-LO)", RFC 5139,
February 2008.
[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-16 (work
in progress), August 2009.
[I-D.thomson-geopriv-uncertainty] Thomson, M. and J.
Winterbottom,
"Representation of
Uncertainty and Confidence
in PIDF-LO", draft-
thomson-geopriv-
uncertainty-04 (work in
progress), November 2009.
7.2. Informative References
[W3C.REC-xpath20-20070123] Boag, S., Berglund, A.,
Kay, M., Robie, J.,
Simeon, J., Fernandez, M.,
and D. Chamberlin, "XML
Path Language (XPath)
2.0", World Wide Web
Consortium Recommendation
REC-xpath20-20070123,
January 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-10 (work
in progress), July 2009.
[I-D.ietf-geopriv-lbyr-requirements] Marshall, R.,
"Requirements for a
Location-by-Reference
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Mechanism", draft-ietf-
geopriv-lbyr-requirements-
09 (work in progress),
November 2009.
[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.
Authors' Addresses
Martin Thomson
Andrew Corporation
Andrew Building (39)
Wollongong University Campus
Northfields Avenue
Wollongong, NSW 2522
AU
EMail: martin.thomson@andrew.com
James Winterbottom
Andrew Corporation
Andrew Building (39)
Wollongong University Campus
Northfields Avenue
Wollongong, NSW 2522
AU
EMail: james.winterbottom@andrew.com
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