Specifying Location Quality Constraints in Location Protocols
draft-thomson-geopriv-location-quality-02

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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 the desired constraints on the quality of the location information provided. 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 constraints were met is defined to be provided by the Location Server alongside location information.

Table of Contents

1.  Introduction
    1.1.  Conventions used in this document
2.  Location Quality Operation
3.  Location Quality Objects
    3.1.  Location Quality Request
        3.1.1.  Maximum Uncertainty
        3.1.2.  Required Civic Elements
        3.1.3.  Maximum Age
    3.2.  Location Quality Indication
4.  Location Quality Schema
5.  Security Considerations
6.  IANA Considerations
    6.1.  URN Sub-Namespace Registration for urn:ietf:params:xml:ns:geopriv:lq
    6.2.  XML Schema Registration for Location Quality Schema
7.  References
    7.1.  Normative References
    7.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

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] (Thomson, M. and J. Winterbottom, “Representation of Uncertainty and Confidence in PIDF-LO,” November 2009.) and [GeoShape] (Thomson, M. and C. Reed, “GML 3.1.1 PIDF-LO Shape Application Schema for use by the Internet Engineering Task Force (IETF),” April 2007.). 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 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.

This document provides semantics, examples and security considerations for the HELD protocol (Barnes, M., Winterbottom, J., Thomson, M., and B. Stark, “HTTP Enabled Location Delivery (HELD),” August 2009.) [I‑D.ietf‑geopriv‑http‑location‑delivery]. Application of the parameters described in this document to other protocols is out of scope.

Location quality constraints 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 constraints 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 constraints might be 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 constraints. 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 constraints.

1.1.  Conventions used in this document

Terms and procedures relating to uncertainty and confidence are taken from [I‑D.thomson‑geopriv‑uncertainty] (Thomson, M. and J. Winterbottom, “Representation of Uncertainty and Confidence in PIDF-LO,” November 2009.). Familiarity with terminology outlined in [I‑D.ietf‑geopriv‑l7‑lcp‑ps] (Tschofenig, H. and H. Schulzrinne, “GEOPRIV Layer 7 Location Configuration Protocol; Problem Statement and Requirements,” July 2009.) and [RFC3693] (Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and J. Polk, “Geopriv Requirements,” February 2004.) 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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  Location Quality Operation

Location quality parameters are provided by a Location Recipient in a location request message. Figure 1 (Example HELD Location Request) shows an example HELD message where a Device requests location information of a specified quality.

  <locationRequest xmlns="urn:ietf:params:xml:ns:geopriv:held">
    <locationType exact="true">geodetic</locationType>
    <quality xmlns="urn:ietf:params:xml:ns:geopriv:lq">
      <maxUncertainty confidence="95">
        <horizontal>150</horizontal>
        <vertical>1000</vertical>
      </maxUncertainty>
      <maxAge>2008-05-27T05:47:55Z</maxAge>
    </quality>
  </locationRequest>

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 constraints that were met. Figure 2 (Example HELD Location Response) shows a location response that includes a quality indicator.

  <locationResponse xmlns="urn:ietf:params:xml:ns:geopriv:held">
    <presence xmlns="urn:ietf:params:xml:ns:pidf"
              entity="pres:user@example.com">
      <!-- Actual location information omitted -->
    </presence>
    <qualityInd xmlns="urn:ietf:params:xml:ns:geopriv:lq">
      maxUncertainty/vertical maxAge
    </qualityInd>
  </locationResponse>

An LS provides an indication of the constraints that have been met. The actual quality of the location estimate SHOULD be included in the actual PIDF-LO document, expressed in the location uncertainty, timestamp, or through the presence of absence of civic address fields.

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 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.

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 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.

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 does not want to share this information. 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. 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] (Thomson, M. and J. Winterbottom, “Representation of Uncertainty and Confidence in PIDF-LO,” November 2009.)—it is RECOMMENDED that a normal PDF be assumed for all location information 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. 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 (Chamberlin, D., Berglund, A., Boag, S., Kay, M., Robie, J., Siméon, J., and M. Fernández, “XML Path Language (XPath) 2.0,” January 2007.) [W3C.REC‑xpath20‑20070123] expressions that are evaluated in the context of the "civicAddress" element (Thomson, M. and J. Winterbottom, “Revised Civic Location Format for Presence Information Data Format Location Object (PIDF-LO),” February 2008.) [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 (Example Specifying Required Civic Address Fields) 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.

  <quality xmlns="urn:ietf:params:xml:ns:geopriv:lq">
    <requiredCivic
        xmlns:ca="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr">
      ca:country ca:A1 ca:PC
    </requiredCivic>
  </quality>

Note that this does not force the LS to restrict civic address information to the indicated fields. Additional fields can 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 either be determined anew, or checked so that the timestamp can be updated. A value of now can be used to indicate that stored location information 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 constraints from a request were met. The qualityInd element contains a list of the quality constraints that the accompanying location information meets.

The list of constraints is represented as a whitespace-separated list of element names. These can be interpreted as XPath (Chamberlin, D., Berglund, A., Boag, S., Kay, M., Robie, J., Siméon, J., and M. Fernández, “XML Path Language (XPath) 2.0,” January 2007.) [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 (Required Civic Elements).

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 (Equivalent Quality Indications).

    <qualityInd xmlns="urn:ietf:params:xml:ns:geopriv:lq">
      maxUncertainty
    </qualityInd>

    <qualityInd xmlns="urn:ietf:params:xml:ns:geopriv:lq">
      maxUncertainty/horizontal maxUncertainty/vertical
    </qualityInd>

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). An empty list indicates that none of the constraints were met.

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.

<?xml version="1.0"?>
<xs:schema
    xmlns:lq="urn:ietf:params:xml:ns:geopriv:lq"
    xmlns:conf="urn:ietf:params:xml:ns:geopriv:conf"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    targetNamespace="urn:ietf:params:xml:ns:geopriv:lq"
    elementFormDefault="qualified"
    attributeFormDefault="unqualified">

  <xs:annotation>
    <xs:appinfo
        source="urn:ietf:params:xml:schema:geopriv:lq">
      HELD Location Quality
    </xs:appinfo>
    <xs:documentation source="http://www.ietf.org/rfc/rfcXXXX.txt">
      <!-- [[NOTE TO RFC-EDITOR: Please replace above URL with URL of
           published RFC and remove this note.]] -->
      This schema defines a framework for location quality requests
      and indications of whether they are met.
    </xs:documentation>
  </xs:annotation>

  <xs:import namespace="urn:ietf:params:xml:ns:geopriv:conf"/>

  <xs:element name="quality">
    <xs:complexType>
      <xs:complexContent>
        <xs:restriction base="xs:anyType">
          <xs:sequence>
            <xs:element ref="lq:maxUncertainty" minOccurs="0"/>
            <xs:element ref="lq:requiredCivic" minOccurs="0"/>
            <xs:element ref="lq:maxAge" minOccurs="0"/>
            <xs:any namespace="##other" processContents="lax"
                    minOccurs="0" maxOccurs="unbounded"/>
          </xs:sequence>
          <xs:anyAttribute namespace="##any" processContents="lax"/>
        </xs:restriction>
      </xs:complexContent>
    </xs:complexType>
  </xs:element>

  <xs:element name="maxUncertainty" type="lq:maxUncertaintyType"/>
  <xs:complexType name="maxUncertaintyType">
    <xs:complexContent>
      <xs:restriction base="xs:anyType">
        <xs:sequence>
          <xs:element name="horizontal" type="lq:limitType"/>
          <xs:element name="vertical" type="lq:limitType"/>
        </xs:sequence>
        <xs:attribute name="confidence" type="conf:confidenceBase"
                      default="95"/>
      </xs:restriction>
    </xs:complexContent>
  </xs:complexType>

  <xs:simpleType name="limitType">
    <xs:restriction base="xs:decimal">
      <xs:minExclusive value="0.0"/>
    </xs:restriction>
  </xs:simpleType>

  <xs:element name="maxAge" type="lq:maxAgeType"/>
  <xs:simpleType name="maxAgeType">
    <xs:restriction base="xs:duration">
      <xs:minInclusive value="PT0S"/>
    </xs:restriction>
  </xs:simpleType>

  <xs:element name="requiredCivic" type="lq:elementListType"/>

  <xs:element name="qualityInd" type="lq:qualityIndType"/>
  <xs:simpleType name="qualityIndType">
    <xs:union memberTypes="lq:elementListType">
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:enumeration value="##all"/>
        </xs:restriction>
      </xs:simpleType>
    </xs:union>
  </xs:simpleType>

  <xs:simpleType name="elementListType">
    <xs:list>
      <xs:simpleType>
        <xs:restriction base="xs:token">
          <xs:pattern
              value="(([\i-[:]][\c-[:]]*:)?[\i-[:]][\c-[:]]*/)*
                     ([\i-[:]][\c-[:]]*:)?[\i-[:]][\c-[:]]*"/>
        </xs:restriction>
      </xs:simpleType>
    </xs:list>
  </xs:simpleType>

</xs:schema>

5.  Security Considerations

This document does not introduce any security considerations. [[Editor's Note: Please let us know if you find any.]]

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] (Mealling, M., “The IETF XML Registry,” January 2004.).

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
        <?xml version="1.0"?>
        <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
          "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
        <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
          <head>
            <title>Location Quality</title>
          </head>
          <body>
            <h1>Namespace for Location Quality</h1>
            <h2>urn:ietf:params:xml:ns:geopriv:lq</h2>
[[NOTE TO IANA/RFC-EDITOR: Please update RFC URL and replace XXXX
    with the RFC number for this specification.]]
            <p>See <a href="[[RFC URL]]">RFCXXXX</a>.</p>
          </body>
        </html>
      END

6.2.  XML Schema Registration for Location Quality Schema

This section registers an XML schema as per the guidelines in [RFC3688] (Mealling, M., “The IETF XML Registry,” January 2004.).

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 (Location Quality Schema) of this document.

7.  References

7.1. Normative References

7.2. Informative References

Authors' Addresses

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