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rfc:rfc6225

Internet Engineering Task Force (IETF) J. Polk Request for Comments: 6225 M. Linsner Obsoletes: 3825 Cisco Systems Category: Standards Track M. Thomson ISSN: 2070-1721 Andrew Corporation

                                                         B. Aboba, Ed.
                                                 Microsoft Corporation
                                                             July 2011
          Dynamic Host Configuration Protocol Options for
        Coordinate-Based Location Configuration Information

Abstract

 This document specifies Dynamic Host Configuration Protocol options
 (both DHCPv4 and DHCPv6) for the coordinate-based geographic location
 of the client.  The Location Configuration Information (LCI) includes
 Latitude, Longitude, and Altitude, with resolution or uncertainty
 indicators for each.  Separate parameters indicate the reference
 datum for each of these values.  This document obsoletes RFC 3825.

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
 http://www.rfc-editor.org/info/rfc6225.

Polk, et al. Standards Track [Page 1] RFC 6225 DHCP Options for Coordinate LCI July 2011

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Polk, et al. Standards Track [Page 2] RFC 6225 DHCP Options for Coordinate LCI July 2011

Table of Contents

 1. Introduction ....................................................3
    1.1. Conventions Used in This Document ..........................4
    1.2. Resolution and Uncertainty .................................4
 2. DHCP Option Formats .............................................6
    2.1. DHCPv6 GeoLoc Option .......................................6
    2.2. DHCPv4 Options .............................................8
    2.3. Latitude and Longitude Fields .............................11
    2.4. Altitude ..................................................14
    2.5. Datum .....................................................16
 3. Security Considerations ........................................17
 4. IANA Considerations ............................................17
    4.1. DHCP Options ..............................................17
    4.2. Altitude Type Registry ....................................18
    4.3. Datum Registry ............................................18
    4.4. GeoLoc Option Version Registry ............................19
 5. Acknowledgments ................................................20
 6. References .....................................................20
    6.1. Normative References ......................................20
    6.2. Informative References ....................................21
 Appendix A. GML Mapping ...........................................23
     A.1. GML Templates ............................................23
 Appendix B. Calculations of Resolution ............................27
     B.1. Location Configuration Information of "White House"
          (Example 1) ..............................................27
     B.2. Location Configuration Information of "Sears Tower"
          (Example 2) ..............................................29
 Appendix C. Calculations of Uncertainty ...........................30
     C.1. Location Configuration Information of "Sydney Opera
          House" (Example 3) .......................................30
 Appendix D. Changes from RFC 3825 .................................34

1. Introduction

 The physical location of a network device has a range of
 applications.  In particular, emergency telephony applications rely
 on knowing the location of a caller in order to determine the correct
 emergency center.
 The location of a device can be represented either in terms of
 geospatial (or geodetic) coordinates, or as a civic address.
 Different applications may be more suited to one form of location
 information; therefore, both the geodetic and civic forms may be used
 simultaneously.

Polk, et al. Standards Track [Page 3] RFC 6225 DHCP Options for Coordinate LCI July 2011

 This document specifies Dynamic Host Configuration Protocol v4
 (DHCPv4) [RFC2131] and DHCPv6 [RFC3315] options for the coordinate-
 based geographic location of the client, to be provided by the
 server.  "Dynamic Host Configuration Protocol (DHCPv4 and DHCPv6)
 Option for Civic Addresses Configuration Information" [RFC4776]
 specifies DHCP options for civic addresses.
 The geodetic coordinate options defined in this document and the
 civic address options defined in RFC 4776 [RFC4776] enable a DHCP
 client to obtain its location.  For example, a wired Ethernet host
 might use these options for location determination.  In this case,
 the location information could be derived from a wiremap by the DHCP
 server, using the Circuit ID Relay Agent Information Option (RAIO)
 defined (as Sub-Option 1) in RFC 3046 [RFC3046].  The DHCP server
 could correlate the Circuit ID with the geographic location where the
 identified circuit terminates (such as the location of the wall
 jack).
 The mechanism defined here may also be utilized to provide location
 to wireless hosts.  DHCP relay agent sub-options (RAIO) [RFC3046]
 provide one method a DHCP server might use to perform host location
 determination.  Currently, the relay agent sub-options do not include
 data sets required for device-level location determination of
 wireless hosts.  In cases where the DHCP server uses RAIO for
 location determination, a wireless host can use this mechanism to
 discover the location of the radio access point, or the area of
 coverage for the radio access point.
 An important feature of this specification is that after the relevant
 DHCP exchanges have taken place, the location information is stored
 on the end device rather than somewhere else, where retrieving it
 might be difficult in practice.

1.1. Conventions Used 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 [RFC2119].

1.2. Resolution and Uncertainty

 The DHCP options defined in this document include fields quantifying
 the resolution or uncertainty associated with a target location.  No
 inferences relating to privacy policies can be drawn from either
 uncertainty or resolution values.

Polk, et al. Standards Track [Page 4] RFC 6225 DHCP Options for Coordinate LCI July 2011

 As utilized in this document, resolution refers to the accuracy of a
 reported location, as expressed by the number of valid bits in each
 of the Latitude, Longitude, and Altitude fields.
 The Latitude (LaRes), Longitude (LoRes), and Altitude (AltRes)
 Resolution fields are encoded as 6-bit, unsigned integer values.  In
 the DHCPv4 GeoConf Option 123, the LaRes, LoRes, and AltRes fields
 are used to encode the number of bits of resolution.  The resolution
 sub-fields accommodate the desire to easily adjust the precision of a
 reported location.  Contents beyond the claimed resolution MAY be
 randomized to obscure greater precision that might be available.
 In the context of location technology, uncertainty is a
 quantification of errors.  Any method for determining location is
 subject to some sources of error; uncertainty describes the amount of
 error that is present.  Uncertainty might be the coverage area of a
 wireless transmitter, the extent of a building, or a single room.
 Uncertainty is usually represented as an area within which the target
 is located.  In this document, each of the three axes can be assigned
 an uncertainty value.  In effect, this describes a rectangular prism,
 which may be used as a coarse representation of a more complex shape
 that fits within it.  See Section 2.3.2 for more detail on the
 correspondence between shapes and uncertainty.
 When representing locations from sources that can quantify
 uncertainty, the goal is to find the smallest possible rectangular
 prism that this format can describe.  This is achieved by taking the
 minimum and maximum values on each axis and ensuring that the final
 encoding covers these points.  This increases the region of
 uncertainty, but ensures that the region that is described
 encompasses the target location.
 The DHCPv4 option formats defined in this document support resolution
 and uncertainty parameters.  The DHCPv4 GeoConf Option 123 includes a
 resolution parameter for each of the dimensions of location.  Since
 this resolution parameter need not apply to all dimensions equally, a
 resolution value is included for each of the three location elements.
 The DHCPv4 GeoLoc Option 144 as well as the DHCPv6 GeoLoc Option 63
 format utilize an uncertainty parameter.
 Appendix A describes the mapping of DHCP option values to the
 Geography Markup Language (GML).  Appendix B of this document
 provides examples showing the calculation of resolution values.
 Appendix C provides an example demonstrating calculation of
 uncertainty values.

Polk, et al. Standards Track [Page 5] RFC 6225 DHCP Options for Coordinate LCI July 2011

 Since the Presence Information Data Format Location Object (PIDF-LO)
 [RFC4119] [RFC5491] is used to convey location and the associated
 uncertainty within an emergency call [Convey], a mechanism is needed
 to convert the information contained within the DHCPv4 and DHCPv6
 options to PIDF-LO.  This document describes the following
 conversions:
 o  DHCPv4 GeoConf Option 123 to PIDF-LO
 o  DHCPv4 GeoLoc Option 144 and DHCPv6 GeoLoc Option 63 to PIDF-LO
 o  PIDF-LO to DHCP GeoLoc Option 144 and DHCPv6 GeoLoc Option 63
 Conversion to PIDF-LO does not increase uncertainty; conversion from
 PIDF-LO to the DHCPv4 GeoLoc Option 144 and the DHCPv6 GeoLoc Option
 63 increases uncertainty by less than a factor of 2 in each
 dimension.  Since it is not possible to translate an arbitrary
 PIDF-LO to the DHCP GeoConf Option 123 with a bounded increase in
 uncertainty, the conversion is not specified.

2. DHCP Option Formats

 This section defines the format for the DHCPv4 and DHCPv6 options.
 These options utilize a similar format, differing primarily in the
 option code.

2.1. DHCPv6 GeoLoc Option

 The format of the DHCPv6 [RFC3315] GeoLoc Option is as follows:
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       Option Code (63)        |            OptLen             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  LatUnc   |                  Latitude                         +
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Lat (cont'd)  |  LongUnc  |               Longitude           +
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |    Longitude (cont'd)         | AType |   AltUnc  |  Altitude +
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |               Altitude (cont'd)               |Ver| Res |Datum|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  Code:      16 bits.  The code for the DHCP Option Code (63).
  OptLen:    Option Length.  For version 1, the option length is 16.

Polk, et al. Standards Track [Page 6] RFC 6225 DHCP Options for Coordinate LCI July 2011

  LatUnc:    6 bits.  When the Ver field = 1, this field represents
             latitude uncertainty.  The contents of this field are
             undefined for other values of the Ver field.
  Latitude:  A 34-bit fixed-point value consisting of 9 bits of
             integer and 25 bits of fraction, interpreted as described
             in Section 2.3.
  LongUnc:   6 bits.  When the Ver field = 1, this field represents
             longitude uncertainty.  The contents of this field are
             undefined for other values of the Ver field.
  Longitude: A 34-bit fixed-point value consisting of 9 bits of
             integer and 25 bits of fraction, interpreted as described
             in Section 2.3.
  AType:     4 bits.  Altitude Type, defined in Section 2.4.
  AltUnc:    6 bits.  When the Ver field = 1, this field represents
             altitude uncertainty.  The contents of this field are
             undefined for other values of the Ver field.
  Altitude:  A 30-bit value defined by the AType field, described in
             Section 2.4.
  Ver:       The Ver field is 2 bits, providing for four potential
             versions.  This specification defines the behavior of
             version 1.  The Ver field is always located at the same
             offset from the beginning of the option, regardless of
             the version in use.  DHCPv6 clients implementing this
             specification MUST support receiving version 1 responses.
             DHCPv6 servers implementing this specification MUST send
             version 1 responses.
  Res:       3 bits.  The Res field is reserved.  These bits have been
             used by [IEEE-802.11y], but are not defined within this
             specification.
  Datum:     3 bits.  The Map Datum used for the coordinates given in
             this option.

Polk, et al. Standards Track [Page 7] RFC 6225 DHCP Options for Coordinate LCI July 2011

2.2. DHCPv4 Options

2.2.1. DHCPv4 GeoConf Option

 The format of the DHCPv4 GeoConf Option is as follows:
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   Code 123    |    Length     |   LaRes   |     Latitude      +
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                Latitude (cont'd)              |   LoRes   |   +
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                             Longitude                         |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | AType |   AltRes  |                Altitude                   +
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Alt.(cont'd)  |    Res  |Datum|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  Code:      8 bits.  The code for the DHCPv4 GeoConf Option (123).
  Length:    8 bits.  The length of the option, in octets.
             The option length is 16.
  LaRes:     6 bits.  This field represents latitude resolution.
  Latitude:  A 34-bit fixed-point value consisting of 9 bits of signed
             integer and 25 bits of fraction, interpreted as described
             in Section 2.3.
  LoRes:     6 bits.  This field represents longitude resolution.
  Longitude: A 34-bit fixed-point value consisting of 9 bits of signed
             integer and 25 bits of fraction, interpreted as described
             in Section 2.3.
  AType:     4 bits.  Altitude Type, defined in Section 2.4.
  AltRes:    6 bits.  This field represents altitude resolution.
  Altitude:  A 30-bit value defined by the AType field, described in
             Section 2.4.

Polk, et al. Standards Track [Page 8] RFC 6225 DHCP Options for Coordinate LCI July 2011

  Res:       5 bits.  The Res field is reserved.  These bits have been
             used by IEEE 802.11y [IEEE-802.11y], but are not defined
             within this specification.
  Datum:     3 bits.  The Map Datum used for the coordinates given in
             this option.

2.2.2. DHCPv4 GeoLoc Option

 The format of the DHCPv4 GeoLoc Option is as follows:
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |   Code 144    |    Length     |   LatUnc  |     Latitude      +
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                Latitude (cont'd)              |  LongUnc  |   +
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                             Longitude                         |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | AType |   AltUnc  |                Altitude                   +
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  | Alt.(cont'd)  |Ver| Res |Datum|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  Code:      8 bits.  The code for the DHCPv4 GeoLoc Option (144).
  Length:    8 bits.  The length of the option, in octets.
             For version 1, the option length is 16.
  LatUnc:    6 bits.  When the Ver field = 1, this field represents
             latitude uncertainty.  The contents of this field are
             undefined for other values of the Ver field.
  Latitude:  A 34-bit fixed-point value consisting of 9 bits of
             integer and 25 bits of fraction, interpreted as described
             in Section 2.3.
  LongUnc:   6 bits.  When the Ver field = 1, this field represents
             longitude uncertainty.  The contents of this field are
             undefined for other values of the Ver field.
  Longitude: A 34-bit fixed-point value consisting of 9 bits of
             integer and 25 bits of fraction, interpreted as described
             in Section 2.3.
  AType:     4 bits.  Altitude Type, defined in Section 2.4.

Polk, et al. Standards Track [Page 9] RFC 6225 DHCP Options for Coordinate LCI July 2011

  AltUnc:    6 bits.  When the Ver field = 1, this field represents
             altitude uncertainty.  The contents of this field are
             undefined for other values of the Ver field.
  Altitude:  A 30-bit value defined by the AType field, described in
             Section 2.4.
  Ver:       The Ver field is 2 bits, providing for four potential
             versions.  This specification defines the behavior of
             version 1.  The Ver field is always located at the same
             offset from the beginning of the option, regardless of
             the version in use.
  Res:       3 bits.  The Res field is reserved.  These bits have been
             used by [IEEE-802.11y], but are not defined within this
             specification.
  Datum:     3 bits.  The Map Datum used for the coordinates given in
             this option.

2.2.3. Option Support

2.2.3.1. Client Support

 DHCPv4 clients implementing this specification MUST support receiving
 the DHCPv4 GeoLoc Option 144 (version 1), and MAY support receiving
 the DHCPv4 GeoConf Option 123 (originally defined in RFC 3825
 [RFC3825]).
 DHCPv4 clients request the DHCPv4 server to send GeoConf Option 123,
 GeoLoc Option 144, or both via inclusion of the Parameter Request
 List option.  As noted in Section 9.8 of RFC 2132 [RFC2132]:
    This option is used by a DHCP client to request values for
    specified configuration parameters.  The list of requested
    parameters is specified as n octets, where each octet is a valid
    DHCP option code as defined in this document.
    The client MAY list the options in order of preference.  The DHCP
    server is not required to return the options in the requested
    order, but MUST try to insert the requested options in the order
    requested by the client.

Polk, et al. Standards Track [Page 10] RFC 6225 DHCP Options for Coordinate LCI July 2011

 When DHCPv4 and DHCPv6 clients implementing this specification do not
 understand a datum value, they MUST assume a World Geodetic System
 1984 (WGS84) [WGS84] datum (European Petroleum Survey Group (EPSG)
 [EPSG] 4326 or 4979, depending on whether there is an altitude value
 present) and proceed accordingly.  Assuming that a less accurate
 location value is better than none, this ensures that some (perhaps
 less accurate) location is available to the client.

2.2.3.2. Server Option Selection

 A DHCPv4 server implementing this specification MUST support sending
 GeoLoc Option 144 version 1 and SHOULD support sending GeoConf Option
 123 in responses.
 A DHCPv4 server that provides location information SHOULD honor the
 Parameter Request List included by the DHCPv4 client in order to
 decide whether to send GeoConf Option 123, GeoLoc Option 144, or both
 in the Response.

2.3. Latitude and Longitude Fields

 The latitude and longitude values in this specification are encoded
 as 34-bit, two's complement, fixed-point values with 9 integer bits
 and 25 fractional bits.  The exact meaning of these values is
 determined by the datum; the description in this section applies to
 the datums defined in this document.  This document uses the same
 definition for all datums it specifies.
 When encoding, latitude and longitude values are rounded to the
 nearest 34-bit binary representation.  This imprecision is considered
 acceptable for the purposes to which this form is intended to be
 applied and is ignored when decoding.
 Positive latitudes are north of the equator, and negative latitudes
 are south of the equator.  Positive longitudes are east of the Prime
 Meridian, and negative (two's complement) longitudes are west of the
 Prime Meridian.
 Within the coordinate reference systems defined in this document
 (Datum values 1-3), longitude values outside the range of -180 to 180
 decimal degrees or latitude values outside the range of -90 to 90
 degrees MUST be considered invalid.  Server implementations SHOULD
 prevent the entry of invalid values within the selected coordinate
 reference system.  Location consumers MUST ignore invalid location
 coordinates and SHOULD log errors related to invalid location.

Polk, et al. Standards Track [Page 11] RFC 6225 DHCP Options for Coordinate LCI July 2011

2.3.1. Latitude and Longitude Resolution

 In the DHCPv4 GeoConf Option 123, the LaRes value encodes the number
 of high-order latitude bits that MUST be considered valid.  Any bits
 entered to the right of this limit MUST NOT be considered valid and
 might be purposely false, or zeroed by the sender.  The examples in
 Appendix B illustrate that a smaller value in the resolution field
 increases the area within which the device is located.  A value of 2
 in the LaRes field indicates a precision of no greater than 1/6th
 that of the globe (see the first example of Appendix B).  A value of
 34 in the LaRes field indicates a precision of about 3.11 mm in
 latitude at the equator.
 In the DHCPv4 GeoConf Option 123, the LoRes value encodes the number
 of high-order longitude bits that MUST be considered valid.  Any bits
 entered to the right of this limit MUST NOT be considered valid and
 might be purposely false, or zeroed by the sender.  A value of 2 in
 the LoRes field indicates precision of no greater than 1/6th that of
 the globe (see the first example of Appendix B).  A value of 34 in
 the LoRes field indicates a precision of about 2.42 mm in longitude
 (at the equator).  Because lines of longitude converge at the poles,
 the distance is smaller (better precision) for locations away from
 the equator.

2.3.2. Latitude and Longitude Uncertainty

 In the DHCPv6 GeoLoc Option 63 and the DHCPv4 GeoLoc Option 144, the
 Latitude and Longitude Uncertainty fields (LatUnc and LongUnc)
 quantify the amount of uncertainty in each of the latitude and
 longitude values, respectively.  A value of 0 is reserved to indicate
 that the uncertainty is unknown; values greater than 34 are reserved.
 A point within the region of uncertainty is selected to be the
 encoded point; the centroid of the region is often an appropriate
 choice.  The value for uncertainty is taken as the distance from the
 selected point to the furthest extreme of the region of uncertainty
 on that axis.  This is demonstrated in the figure below, which shows
 a two-dimensional polygon that is projected on each axis.  In the
 figure, "X" marks the point that is selected; the ranges marked with
 "U" indicate the uncertainty.

Polk, et al. Standards Track [Page 12] RFC 6225 DHCP Options for Coordinate LCI July 2011

         ___          ___________
         ^ |         /           |
         | |        /            |
         | |       /             |
         U |      /              |
         | |     (               |
         V |     |               |
         --X     |         X     |
           |     |               `---------.
           |     |                         |
           |     |                         |
           |     |                         |
           -     `-------------------------'
                 |---------X---------------|
                           |<------U------>|
    Key
    ---
    V, ^ = vertical arrows, delimiting the vertical uncertainty range.
    <>   = horizontal arrows, delimiting the horizontal uncertainty
           range.
 Uncertainty applies to each axis independently.
 The amount of uncertainty can be determined from the encoding by
 taking 2 to the power of 8, less the encoded value, as is shown in
 the following formula, where "x" is the encoded integer value:
    uncertainty = 2 ^ ( 8 - x )
 The result of this formula is expressed in degrees of latitude or
 longitude.  The uncertainty is added to the base latitude or
 longitude value to determine the maximum value in the uncertainty
 range; similarly, the uncertainty is subtracted from the base value
 to determine the minimum value.  Note that because lines of longitude
 converge at the poles, the actual distance represented by this
 uncertainty changes with the distance from the equator.
 If the maximum or minimum latitude values derived from applying
 uncertainty are outside the range of -90 to +90, these values are
 trimmed to within this range.  If the maximum or minimum longitude
 values derived from applying uncertainty are outside the range of
 -180 to +180, then these values are normalized to this range by
 adding or subtracting 360 as necessary.

Polk, et al. Standards Track [Page 13] RFC 6225 DHCP Options for Coordinate LCI July 2011

 The encoded value is determined by subtracting the next highest whole
 integer value for the base 2 logarithm of uncertainty from 8, as is
 shown by the following formula, where uncertainty is the midpoint of
 the known range less the lower bound of that range:
    x = 8 - ceil( log2( uncertainty ) )
 Note that the result of encoding this value increases the range of
 uncertainty to the next available power of two; subsequent repeated
 encodings and decodings do not change the value.  Only increasing
 uncertainty means that the associated confidence does not have to
 decrease.

2.4. Altitude

 How the altitude value is interpreted depends on the Altitude Type
 (AType) value and the selected datum.  Three Altitude Type values are
 defined in this document: unknown (0), meters (1), and floors (2).

2.4.1. No Known Altitude (AType = 0)

 In some cases, the altitude of the location might not be provided.
 An Altitude Type value of zero indicates that the altitude is not
 given to the client.  In this case, the Altitude and Altitude
 Uncertainty fields can contain any value and MUST be ignored.

2.4.2. Altitude in Meters (AType = 1)

 If the Altitude Type has a value of one, altitude is measured in
 meters, in relation to the zero set by the vertical datum.  For AType
 = 1, the altitude value is expressed as a 30-bit, fixed-point, two's
 complement integer with 22 integer bits and 8 fractional bits.

2.4.3. Altitude in Floors (AType = 2)

 A value of two for Altitude Type indicates that the altitude value is
 measured in floors.  Since altitude in meters may not be known within
 a building, a floor indication may be more useful.  For AType = 2,
 the altitude value is expressed as a 30-bit, fixed-point, two's
 complement integer with 22 integer bits and 8 fractional bits.
 This value is relevant only in relation to a building; the value is
 relative to the ground level of the building.  Floors located below
 ground level are represented by negative values.  In some buildings,
 it might not be clear which floor is at ground level, or an
 intermediate floor might be hard to identify as such.  Determining

Polk, et al. Standards Track [Page 14] RFC 6225 DHCP Options for Coordinate LCI July 2011

 what floor is at ground level and what constitutes a sub-floor as
 opposed to a naturally numbered floor is left to local
 interpretation.
 Larger values represent floors that are farther away from floor 0
 such that:
  1. if positive, the floor value is farther above the ground floor.
  1. if negative, the floor value is farther below the ground floor.
 Non-integer values can be used to represent intermediate or
 sub-floors, such as mezzanine levels.  Example: a mezzanine between
 floor 1 and floor 2 could be represented as a value of 1.25.
 Example: mezzanines between floor 4 and floor 5 could be represented
 as values of 4.5 and 4.75.

2.4.4. Altitude Resolution

 In the DHCPv4 GeoConf Option 123, the altitude resolution (AltRes)
 value encodes the number of high-order altitude bits that should be
 considered valid.  Values above 30 (decimal) are undefined and
 reserved.
 If the Altitude Type value is one (AType = 1), an AltRes value of 0.0
 would indicate an unknown altitude.  The most precise altitude would
 have an AltRes value of 30.  Many values of AltRes would obscure any
 variation due to vertical datum differences.
 The AltRes field SHOULD be set to maximum precision when AType = 2
 (floors) when a floor value is included in the DHCP Reply, or when
 AType = 0, to denote that the floor isn't known.  An altitude coded
 as AType = 2, AltRes = 30, and Altitude = 0.0 is meaningful even
 outside a building, and represents ground level at the given latitude
 and longitude.

2.4.5. Altitude Uncertainty

 In the DHCPv6 GeoLoc Option 63 or the DHCPv4 GeoLoc Option 144, the
 AltUnc value quantifies the amount of uncertainty in the altitude
 value.  As with LatUnc and LongUnc, a value of 0 for AltUnc is
 reserved to indicate that altitude uncertainty is not known; values
 above 30 are also reserved.  Altitude uncertainty only applies to
 Altitude Type 1.

Polk, et al. Standards Track [Page 15] RFC 6225 DHCP Options for Coordinate LCI July 2011

 The amount of altitude uncertainty can be determined by the following
 formula, where x is the encoded integer value:
    Uncertainty = 2 ^ ( 21 - x )
 This value uses the same units as the associated altitude.
 Similarly, a value for the encoded integer value can be derived by
 the following formula:
    x = 21 - ceil( log2( uncertainty ) )

2.5. Datum

 The Datum field determines how coordinates are organized and related
 to the real world.  Three datums are defined in this document, based
 on the definitions in [OGP.Geodesy]:
 1: WGS84 (Latitude, Longitude, Altitude): The World Geodetic System
    1984 [WGS84] coordinate reference system.
    This datum is identified by the European Petroleum Survey Group
    (EPSG)/International Association of Oil & Gas Producers (OGP) with
    the code 4979, or by the URN "urn:ogc:def:crs:EPSG::4979".
    Without altitude, this datum is identified by the EPSG/OGP code
    4326 and the URN "urn:ogc:def:crs:EPSG::4326".
 2: NAD83 (Latitude, Longitude) + NAVD88: This datum uses a
    combination of the North American Datum 1983 (NAD83) for
    horizontal (Latitude and Longitude) values, plus the North
    American Vertical Datum of 1988 (NAVD88) vertical datum.
    This datum is used for referencing location on land (not near
    tidal water) within North America.
    NAD83 is identified by the EPSG/OGP code of 4269, or the URN
    "urn:ogc:def:crs:EPSG::4269".  NAVD88 is identified by the EPSG/
    OGP code of 5703, or the URN "urn:ogc:def:crs:EPSG::5703".
 3: NAD83 (Latitude, Longitude) + MLLW: This datum uses a combination
    of the North American Datum 1983 (NAD83) for horizontal (Latitude
    and Longitude) values, plus the Mean Lower Low Water (MLLW)
    vertical datum.
    This datum is used for referencing location on or near tidal water
    within North America.

Polk, et al. Standards Track [Page 16] RFC 6225 DHCP Options for Coordinate LCI July 2011

    NAD83 is identified by the EPSG/OGP code of 4269, or the URN
    "urn:ogc:def:crs:EPSG::4269".  MLLW does not have a specific code
    or URN.
 All hosts MUST support the WGS84 datum (Datum 1).

3. Security Considerations

 Geopriv requirements (including security requirements) are discussed
 in "Geopriv Requirements" [RFC3693].  A threat analysis is provided
 in "Threat Analysis of the Geopriv Protocol" [RFC3694].
 Since there is no privacy protection for DHCP messages, an
 eavesdropper who can monitor the link between the DHCP server and
 requesting client can discover this LCI.
 To minimize the unintended exposure of location information, the LCI
 option SHOULD be returned by DHCP servers only when the DHCP client
 has included this option in its 'parameter request list' (Section 3.5
 of [RFC2131], Section 9.8 of [RFC2132]).
 Where critical decisions might be based on the value of this option,
 DHCP authentication as defined in "Authentication for DHCP Messages"
 [RFC3118] and "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)"
 [RFC3315] SHOULD be used to protect the integrity of the DHCP
 options.
 Link-layer confidentiality and integrity protection may also be
 employed to reduce the risk of location disclosure and tampering.

4. IANA Considerations

4.1. DHCP Options

 This document defines the DHCPv6 GeoLoc Option (see Section 2.1),
 which has been assigned a DHCPv6 option code of 63 per [RFC3315]:
    Value   Description          Reference
    ----    ------------------   ----------
    63      OPTION_GEOLOCATION   RFC 6225
 This document defines the DHCPv4 GeoConf Option (see Section 2.2.1),
 which has been assigned a DHCPv4 option code of 123 from the DHCP
 Option space.

Polk, et al. Standards Track [Page 17] RFC 6225 DHCP Options for Coordinate LCI July 2011

 This document also defines the DHCPv4 GeoLoc Option (see
 Section 2.2.2), which has been assigned a DHCPv4 option code of 144
 per [RFC2132] [RFC2939]:
                   Data
    Tag    Name    Length   Meaning              Reference
    ----   ----    ------   -------              ---------
    144    GeoLoc   16      Geospatial Location  RFC 6225
                            with Uncertainty

4.2. Altitude Type Registry

 IANA has created and now maintains the Altitude Type registry
 following the guidelines below.
 The registry consists of three values: Altitude Type, Description,
 and Reference.  These are described below.
 Altitude Type: An integer, refers to the value used in the DHCPv4
    GeoConf and the DHCPv4 and DHCPv6 GeoLoc options described in this
    document.  Values 0 - 2 are assigned.  Values 3 - 15 are
    Unassigned [RFC5226].
 Description: The description of the altitude described by this code.
 Reference: The reference to the document that describes the altitude
    code.  This reference MUST define the way that the 30-bit altitude
    values and the associated 6-bit uncertainty are interpreted.
 Initial values are given below; new assignments are to be made
 following the "Standards Action" policies [RFC5226].
    +------+---------------------+--------------+
    |  #   |  Description        |  Reference   |
    +------+---------------------+--------------+
    |  0   | No known altitude   |  RFC 6225    |
    |  1   | Altitude in meters  |  RFC 6225    |
    |  2   | Altitude in floors  |  RFC 6225    |
    | 3-15 | Unassigned          |              |
    +------+---------------------+--------------+

4.3. Datum Registry

 IANA has created and now maintains the Datum registry following the
 guidelines below.
 The registry consists of three values: Datum, Description, and
 Reference.  These are described below.

Polk, et al. Standards Track [Page 18] RFC 6225 DHCP Options for Coordinate LCI July 2011

 Datum: An integer, refers to the value used in the DHCPv4 GeoConf and
    the DHCPv4 and DHCPv6 GeoLoc options described in this document.
    Value 0 is Reserved.  Values 1 - 3 are assigned.  Values 4 - 7 are
    Unassigned [RFC5226].
 Description: The description of the altitude described by this code.
 Reference: The reference to the document that describes the Datum
    code.  This reference MUST include specification of both the
    horizontal and vertical datum, and MUST define the way that the
    34-bit values and the respective 6-bit uncertainties are
    interpreted.
 Initial values are given below; new assignments are to be made
 following the "Standards Action" policies [RFC5226].
    +------+----------------------------------------+--------------+
    |  #   |  Description                           |  Reference   |
    +------+----------------------------------------+--------------+
    |  0   | Reserved                               |  RFC 6225    |
    +------+----------------------------------------+--------------+
    |  1   | Vertical datum WGS 84 defined by EPSG  |  RFC 6225    |
    |      | CRS Code 4327                          |              |
    +------+----------------------------------------+--------------+
    |  2   | Vertical datum NAD83 defined by EPSG   |  RFC 6225    |
    |      | CRS Code 4269 with North American      |              |
    |      | Vertical Datum of 1988 (NAVD88)        |              |
    +------+----------------------------------------+--------------+
    |  3   | Vertical datum NAD83 defined by EPSG   |  RFC 6225    |
    |      | CRS Code 4269 with Mean Lower Low Water|              |
    |      | (MLLW) as associated vertical datum    |              |
    +------+----------------------------------------+--------------+
    | 4-7  | Unassigned                             |              |
    +------+----------------------------------------+--------------+

4.4. GeoLoc Option Version Registry

 IANA has created and now maintains the GeoLoc Option Version registry
 following the guidelines below.
 The registry consists of three values: GeoLoc Option Version,
 Description, and Reference.  These are described below.
 GeoLoc Option Version: An integer; refers to the version used in the
    DHCPv4 and DHCPv6 GeoLoc options described in this document.
    Value 0 is Reserved.  Value 1 has been assigned.  Values 2 - 3 are
    Unassigned [RFC5226].

Polk, et al. Standards Track [Page 19] RFC 6225 DHCP Options for Coordinate LCI July 2011

 Description: The description of the version described by this code.
 Reference: The reference to the document that describes the Version
    code.
 Initial values are given below; new assignments are to be made
 following the "Standards Action" policies [RFC5226].
    +------+---------------------------------------+--------------+
    |  #   |  Description                          |  Reference   |
    +------+---------------------------------------+--------------+
    |  0   | Reserved                              |  RFC 6225    |
    +------+---------------------------------------+--------------+
    |  1   | Implementations utilizing uncertainty |  RFC 6225    |
    |      | parameters for both DHCPv4 and DHCPv6 |              |
    |      | GeoLoc options                        |              |
    +------+---------------------------------------+--------------+
    | 2-3  | Unassigned                            |              |
    +------+---------------------------------------+--------------+

5. Acknowledgments

 The authors would like to thank Randall Gellens, Patrik Falstrom,
 Ralph Droms, Ted Hardie, Jon Peterson, Robert Sparks, Nadine Abbott,
 and Mykyta Yevstifeyev for their inputs and constructive comments
 regarding this document.  Additionally, the authors would like to
 thank Greg Troxel for the education on vertical datums, as well as
 Carl Reed.  Thanks to Richard Barnes for his contribution on GML
 mapping for resolution.

6. References

6.1. Normative References

 [EPSG]         European Petroleum Survey Group,
                <http://www.epsg.org/> and
                <http://www.epsg-registry.org/>.
 [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2131]      Droms, R., "Dynamic Host Configuration Protocol",
                RFC 2131, March 1997.
 [RFC2132]      Alexander, S. and R. Droms, "DHCP Options and BOOTP
                Vendor Extensions", RFC 2132, March 1997.

Polk, et al. Standards Track [Page 20] RFC 6225 DHCP Options for Coordinate LCI July 2011

 [RFC2939]      Droms, R., "Procedures and IANA Guidelines for
                Definition of New DHCP Options and Message Types",
                BCP 43, RFC 2939, September 2000.
 [RFC3118]      Droms, R., Ed., and W. Arbaugh, Ed., "Authentication
                for DHCP Messages", RFC 3118, June 2001.
 [RFC3315]      Droms, R., Ed., Bound, J., Volz, B., Lemon, T.,
                Perkins, C., and M. Carney, "Dynamic Host
                Configuration Protocol for IPv6 (DHCPv6)", RFC 3315,
                July 2003.
 [RFC5226]      Narten, T. and H. Alvestrand, "Guidelines for Writing
                an IANA Considerations Section in RFCs", BCP 26, RFC
                5226, May 2008.
 [WGS84]        US National Imagery and Mapping Agency, "Department of
                Defense (DoD) World Geodetic System 1984 (WGS 84),
                Third Edition", NIMA TR8350.2, January 2000,
                <https://www1.nga.mil/PRODUCTSSERVICES/
                GEODESYGEOPHYSICS/WORLDGEODETICSYSTEM/
                Pages/default.aspx> and
                <http://www.ngs.noaa.gov/faq.shtml#WGS84>.

6.2. Informative References

 [Convey]       Polk, J., Rosen, B., and J. Peterson, "Location
                Conveyance for the Session Initiation Protocol", Work
                in Progress, May 2011.
 [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 06-142, Version: 0.0.9, December 2006.
 [IEEE-802.11y] IEEE Standard for Information technology -
                Telecommunications and information exchange between
                systems - Local and metropolitan area networks -
                Specific requirements - Part 11: Wireless LAN Medium
                Access Control (MAC) and Physical Layer (PHY)
                specifications Amendment 3: 3650-3700 MHz Operation in
                USA, November 2008.
 [NENA]         National Emergency Number Association (NENA), NENA
                Technical Information Document on Model Legislation
                Enhanced 911 for Multi-Line Telephone Systems,
                <www.nena.org>.

Polk, et al. Standards Track [Page 21] RFC 6225 DHCP Options for Coordinate LCI July 2011

 [OGC-GML3.1.1] Portele, C., Cox, S., Daisy, P., Lake, R., and A.
                Whiteside, "Geography Markup Language (GML) 3.1.1",
                OGC 03-105r1, July 2003.
 [OGP.Geodesy]  International Association of Oil & Gas Producers (OGP)
                Geodesy Resources, Geomatics Committee,
                <http://info.ogp.org.uk/geodesy/>.
 [RFC3046]      Patrick, M., "DHCP Relay Agent Information Option",
                RFC 3046, January 2001.
 [RFC3693]      Cuellar, J., Morris, J., Mulligan, D., Peterson, J.,
                and J. Polk, "Geopriv Requirements", RFC 3693,
                February 2004.
 [RFC3694]      Danley, M., Mulligan, D., Morris, J., and J. Peterson,
                "Threat Analysis of the Geopriv Protocol", RFC 3694,
                February 2004.
 [RFC3825]      Polk, J., Schnizlein, J., and M. Linsner, "Dynamic
                Host Configuration Protocol Option for Coordinate-
                based Location Configuration Information", RFC 3825,
                July 2004.
 [RFC4119]      Peterson, J., "A Presence-based GEOPRIV Location
                Object Format", RFC 4119, December 2005.
 [RFC4776]      Schulzrinne, H., "Dynamic Host Configuration Protocol
                (DHCPv4 and DHCPv6) Option for Civic Addresses
                Configuration Information", RFC 4776, November 2006.
 [RFC5139]      Thomson, M. and J. Winterbottom, "Revised Civic
                Location Format for Presence Information Data Format
                Location Object (PIDF-LO)", RFC 5139, February 2008.
 [RFC5491]      Winterbottom, J., Thomson, M., and H. Tschofenig,
                "GEOPRIV Presence Information Data Format Location
                Object (PIDF-LO) Usage Clarification, Considerations,
                and Recommendations", RFC 5491, March 2009.

Polk, et al. Standards Track [Page 22] RFC 6225 DHCP Options for Coordinate LCI July 2011

Appendix A. GML Mapping

 The GML representation of a decoded DHCP option depends on what
 fields are specified.  The DHCP format for location logically
 describes a geodetic prism, rectangle, or point, depending on whether
 altitude and uncertainty values are provided.  In the absence of
 uncertainty information, the value decoded from the DHCP form can be
 expressed as a single point; this is true regardless of whether the
 version 0 or version 1 interpretations of the uncertainty fields are
 used.  If the point includes altitude, it uses a three-dimensional
 Coordinate Reference System (CRS); otherwise, it uses a two-
 dimensional CRS.  If all fields are included along with uncertainty,
 the shape described is a rectangular prism.  Note that this is
 necessary given that uncertainty for each axis is provided
 independently.
 If altitude or altitude uncertainty (AltUnc) is not specified, the
 shape is described as a rectangle using the "gml:Polygon" shape.  If
 altitude is available, a three-dimensional CRS is used; otherwise, a
 two-dimensional CRS is used.
 For Datum values of 2 or 3 (NAD83), there is no available CRS URN
 that covers three-dimensional coordinates.  By necessity, locations
 described in these datums can be represented by two-dimensional
 shapes only; that is, either a two-dimensional point or a polygon.
 If the Altitude Type is 2 (floors), then this value can be
 represented using a civic address object [RFC5139] that is presented
 alongside the geodetic object.
 This Appendix describes how the location value encoded in DHCP format
 for geodetic location can be expressed in GML.  The mapping is valid
 for the DHCPv6 GeoLoc Option as well as both of the DHCPv4 GeoConf
 and GeoLoc options, and for the currently defined datum values (1, 2,
 and 3).  Further version or datum definitions should provide similar
 mappings.
 These shapes can be mapped to GML by first computing the bounds that
 are described using the coordinate and uncertainty fields, then
 encoding the result in a GML Polygon or Prism shape.

A.1. GML Templates

 If altitude is provided in meters (AType 1) and the datum value is
 WGS84 (value 1), then the proper GML shape is a Prism, with the
 following form (where $value$ indicates a value computed from the
 DHCP option as described below):

Polk, et al. Standards Track [Page 23] RFC 6225 DHCP Options for Coordinate LCI July 2011

    <gs:Prism srsName="urn:ogc:def:crs:EPSG::4979"
              xmlns:gs="http://www.opengis.net/pidflo/1.0"
              xmlns:gml="http://www.opengis.net/gml">
      <gs:base>
        <gml:Polygon>
          <gml:exterior>
            <gml:LinearRing>
              <gml:posList>
                $lowLatitude$ $lowLongitude$ $lowAltitude$
                $lowLatitude$ $highLongitude$ $lowAltitude$
                $highLatitude$ $highLongitude$ $lowAltitude$
                $highLatitude$ $lowLongitude$ $lowAltitude$
                $lowLatitude$ $lowLongitude$ $lowAltitude$
              </gml:posList>
            </gml:LinearRing>
          </gml:exterior>
        </gml:Polygon>
      </gs:base>
      <gs:height uom="urn:ogc:def:uom:EPSG::9001">
        $highAltitude - lowAltitude$
      </gs:height>
    </gs:Prism>
 The Polygon shape is used if altitude is omitted or specified in
 floors, or if either NAD83 datum is used (value 2 or 3).  The
 corresponding GML Polygon has the following form:
    <gml:Polygon srsName="$2D-CRS-URN$"
                 xmlns:gml="http://www.opengis.net/gml">>
      <gml:exterior>
        <gml:LinearRing>
          <gml:posList>
            $lowLatitude$ $lowLongitude$
            $lowLatitude$ $highLongitude$
            $highLatitude$ $highLongitude$
            $highLatitude$ $lowLongitude$
            $lowLatitude$ $lowLongitude$
          </gml:posList>
        </gml:LinearRing>
      </gml:exterior>
    </gml:Polygon>
 The value "2D-CRS-URN" is defined by the datum value: If the datum is
 WGS84 (value 1), then the 2D-CRS-URN is "urn:ogc:def:crs:EPSG::4326".
 If the datum is NAD83 (value 2 or 3), then the 2D-CRS-URN is
 "urn:ogc:def:crs:EPSG::4269".

Polk, et al. Standards Track [Page 24] RFC 6225 DHCP Options for Coordinate LCI July 2011

 A Polygon shape with the WGS84 three-dimensional CRS is used if the
 datum is WGS84 (value 1) and the altitude is specified in meters
 (Altitude Type 1), but no altitude uncertainty is specified (that is,
 AltUnc is 0).  In this case, the value of the Altitude field is added
 after each of the points above, and the srsName attribute is set to
 the three-dimensional WGS84 CRS, namely "urn:ogc:def:crs:EPSG::4979".
 A simple point shape is used if either latitude uncertainty (LatUnc)
 or longitude uncertainty (LongUnc) is not specified.  With altitude,
 this uses a three-dimensional CRS; otherwise, it uses a two-
 dimensional CRS.
    <gml:Point srsName="$CRS-URN$"
               xmlns:gml="http://www.opengis.net/gml">
      <gml:pos>$Latitude$ $Longitude$ $[Altitude]$</gml:pos>
    </gml:Point>

A.1.1. Finding Low and High Values Using Uncertainty Fields

 For the DHCPv4 GeoConf Option 123, resolution fields are used (LaRes,
 LoRes, AltRes), indicating how many bits of a value contain
 information.  Any bits beyond those indicated can be either zero or
 one.
 For the DHCPv6 GeoLoc Option 63 and DHCPv4 GeoLoc Option 144, the
 LatUnc, LongUnc, and AltUnc fields indicate uncertainty distances,
 denoting the bounds of the location region described by the DHCP
 location object.
 The two sections below describe how to compute the latitude,
 longitude, and altitude bounds (e.g., $lowLatitude$, $highAltitude$)
 in the templates above.  The first section describes how these bounds
 are computed in the "resolution encoding" (DHCPv4 GeoConf
 Option 123), while the second section addresses the "uncertainty
 encoding" (DHCPv6 GeoLoc Option 63 and DHCPv4 GeoLoc Option 144).

A.1.1.1. Resolution Encoding

 Given a number of resolution bits (i.e., the value of a resolution
 field), if all bits beyond those bits are set to zero, this gives the
 lowest possible value.  The highest possible value can be found
 setting all bits to one.
 If the encoded value of latitude/longitude and resolution (LaRes,
 LoRes) are treated as 34-bit unsigned integers, the following can be
 used (where ">>" is a bitwise right shift, "&" is a bitwise AND, "~"
 is a bitwise negation, and "|" is a bitwise OR).

Polk, et al. Standards Track [Page 25] RFC 6225 DHCP Options for Coordinate LCI July 2011

    mask = 0x3ffffffff >> resolution
    lowvalue = value & ~mask
    highvalue = value | mask + 1
 Once these values are determined, the corresponding floating-point
 numbers can be computed by dividing the values by 2^25 (since there
 are 25 bits of fraction in the fixed-point representation).
 Alternatively, the lowest possible value can be found by using
 resolution to determine the size of the range.  This method has the
 advantage that it operates on the decoded floating-point values.  It
 is equivalent to the first mechanism, to a possible error of 2^-25
 (2^-8 for altitude).
    scale = 2 ^ ( 9 - resolution )
    lowvalue = floor( value / scale ) * scale
    highvalue = lowvalue + scale
 Altitude resolution (AltRes) uses the same process with different
 constants.  There are 22 whole bits in the altitude encoding (instead
 of 9) and 30 bits in total (instead of 34).

A.1.1.2. Uncertainty Encoding

 In the uncertainty encoding, the uncertainty fields (LongUnc/LatUnc)
 directly represent the logarithms of uncertainty distances.  So the
 low and high bounds are computed by first computing the uncertainty
 distances, then adding and subtracting these from the value provided.
 If "uncertainty" is the unsigned integer value of the uncertainty
 field and "value" is the value of the coordinate field:
    distance = 2 ^ (8 - uncertainty)
    lowvalue = value - distance
    highvalue = value + distance
 Altitude uncertainty (AltUnc in version 1) uses the same process with
 different constants:
    distance = 2 ^ (21 - uncertainty)
    lowvalue = value - distance

Polk, et al. Standards Track [Page 26] RFC 6225 DHCP Options for Coordinate LCI July 2011

Appendix B. Calculations of Resolution

 The following examples for two different locations demonstrate how
 the resolution values for latitude, longitude, and altitude (used in
 DHCPv4 GeoConf Option 123) can be calculated.  In both examples, the
 geo-location values were derived from maps using the WGS84 map datum;
 therefore, in these examples, the Datum field would have a value = 1
 (00000001, or 0x01).

B.1. Location Configuration Information of "White House" (Example 1)

 The grounds of the White House in Washington D.C. (1600 Pennsylvania
 Ave. NW, Washington, DC  20006) can be found between 38.895375 and
 38.898653 degrees North and 77.037911 and 77.035116 degrees West.  In
 this example, we assume that we are standing on the sidewalk on the
 north side of the White House, between driveways.  Since we are not
 inside a structure, we assume an altitude value of 15 meters,
 interpolated from the US Geological survey map, Washington,
 Washington West quadrangle.
 The address was NOT picked for any political reason and can easily be
 found on the Internet or mapping software, but was picked as an
 easily identifiable location on our planet.
 In this example, the requirement of emergency responders in North
 America via their National Emergency Number Association (NENA) Model
 Legislation [NENA] could be met by a LaRes value of 21 and a LoRes
 value of 20.  This would yield a geo-location that is latitude
 38.8984375 north to latitude 38.8988616 north and longitude
 -77.0371094 to longitude -77.0375977.  This is an area of
 approximately 89 feet by 75 feet or 6669 square feet, which is very
 close to the 7000 square feet requested by NENA.  In this example, a
 service provider could enforce that a device send location
 configuration information with this minimum amount of resolution for
 this particular location when calling emergency services.
 An approximate representation of this location might be provided
 using the DHCPv4 GeoConf Option 123 encoding as follows:

Polk, et al. Standards Track [Page 27] RFC 6225 DHCP Options for Coordinate LCI July 2011

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Code (123)  |  OptLen (16)  |   LaRes   |     Latitude      .
   |0 1 1 1 1 0 1 1|0 0 0 1 0 0 0 0|0 1 0 0 1 0|0 0 0 1 0 0 1 1 0 1.
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                Latitude (cont'd)              |   LoRes   |   .
   .1 1 0 0 1 0 1 1 1 0 0 1 1 0 0 0 0 1 1 0 0 0 1 1|0 1 0 0 0 1|1 1.
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                       Longitude (cont'd)                      |
   .0 1 1 0 0 1 0 1 1 1 1 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 0 0 0 1 0 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AType |   AltRes  |                Altitude                   .
   |0 0 0 1|0 1 0 0 0 1|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1.
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .  Alt (cont'd) |   Res   |Datum|
   .0 0 0 0 0 0 0 0|0 0 0 0 0|0 0 1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 In hexadecimal, this is 7B10484D CB986347 65ED42C4 1440000F 0001.

B.1.1. Decoding Location Configuration Information with Resolution

 Decoding this option gives a latitude of 38.897647 (to 7 decimal
 places) with 18 bits of resolution, a longitude of -77.0366000 with
 17 bits of resolution, an Altitude Type of meters with a value of 15
 and 17 bits of resolution, version 0 (resolution), and the WGS84
 datum.
 For the latitude value, 18 bits of resolution allow for values in the
 range from 38.8964844 to 38.8984375.  For the longitude value, 17
 bits of resolution allow for values in the range from -77.0390625 to
 -77.0351563.  Having 17 bits of resolution in the altitude allows for
 values in the range from 0 to 32 meters.

B.1.2. GML Representation of Decoded Location Configuration Information

 The following GML shows the value decoded in the previous example as
 a point in a three-dimensional CRS:
    <gml:Point srsName="urn:ogc:def:crs:EPSG::4979"
               xmlns:gml="http://www.opengis.net/gml">
      <gml:pos>38.897647 -77.0366 15</gml:pos>
    </gml:Point>
 This representation ignores the values included in the resolution
 parameters.  If resolution values are provided, a rectangular prism
 can be used to represent the location.

Polk, et al. Standards Track [Page 28] RFC 6225 DHCP Options for Coordinate LCI July 2011

 The following example uses all of the decoded information from the
 previous example:
    <gs:Prism srsName="urn:ogc:def:crs:EPSG::4979"
        xmlns:gs="http://www.opengis.net/pidflo/1.0"
        xmlns:gml="http://www.opengis.net/gml">
      <gs:base>
        <gml:Polygon>
         <gml:exterior>
            <gml:LinearRing>
             <gml:posList>
                38.8964844 -77.0390625 0
                38.8964844 -77.0351563 0
                38.8984375 -77.0351563 0
                38.8984375 -77.0390625 0
                38.8964844 -77.0390625 0
              </gml:posList>
            </gml:LinearRing>
          </gml:exterior>
        </gml:Polygon>
      </gs:base>
      <gs:height uom="urn:ogc:def:uom:EPSG::9001">
        32
      </gs:height>
    </gs:Prism>

B.2. Location Configuration Information of "Sears Tower" (Example 2)

 Postal Address:
    Sears Tower
    103rd Floor
    233 S. Wacker Dr.
    Chicago, IL  60606
 Viewing the Chicago area from the Observation Deck of the Sears
 Tower:
    Latitude 41.87884 degrees North (or +41.87884 degrees)
    Using two's complement, 34-bit fixed point, 25 bits of fraction
    Latitude = 0x053c1f751,
    Latitude = 0001010011110000011111011101010001
    Longitude 87.63602 degrees West (or -87.63602 degrees)
    Using two's complement, 34-bit fixed point, 25 bits of fraction
    Longitude = 0xf50ba5b97,
    Longitude = 1101010000101110100101101110010111
    Altitude 103

Polk, et al. Standards Track [Page 29] RFC 6225 DHCP Options for Coordinate LCI July 2011

 In this example, we are inside a structure; therefore, we will assume
 an altitude value of 103 to indicate the floor we are on.  The
 Altitude Type value is 2, indicating floors.  The AltRes field would
 indicate that all bits in the Altitude field are true, as we want to
 accurately represent the floor of the structure where we are located.
    AltRes = 30, 0x1e, 011110
    AType = 2, 0x02, 000010
    Altitude = 103, 0x00006700, 000000000000000110011100000000
 For the accuracy of the latitude and longitude, the best information
 available to us was supplied by a generic mapping service that shows
 a single geo-loc for all of the Sears Tower.  Therefore, we are going
 to show LaRes as value 18 (0x12 or 010010) and LoRes as value 18
 (0x12 or 010010).  This would be describing a geo-location area that
 is latitude 41.8769531 to latitude 41.8789062 and extends from
 -87.6367188 degrees to -87.6347657 degrees longitude.  This is an
 area of approximately 373412 square feet (713.3 ft. x 523.5 ft.).

Appendix C. Calculations of Uncertainty

 The following example demonstrates how uncertainty values for
 latitude, longitude, and altitude (LatUnc, LongUnc, and AltUnc used
 in the DHCPv6 GeoLoc Option 63 as well as DHCPv4 GeoLoc Option 144)
 can be calculated.

C.1. Location Configuration Information of "Sydney Opera House"

    (Example 3)
 This section describes an example of encoding and decoding the
 geodetic DHCP Option.  The textual results are expressed in GML
 [OGC-GML3.1.1] form, suitable for inclusion in PIDF-LO [RFC4119].
 These examples all assume a datum of WGS84 (datum = 1) and an
 Altitude Type of meters (AType = 1).

C.1.1. Encoding a Location into DHCP Geodetic Form

 This example draws a rough polygon around the Sydney Opera House.
 This polygon consists of the following six points:
    33.856625 S, 151.215906 E
    33.856299 S, 151.215343 E
    33.856326 S, 151.214731 E
    33.857533 S, 151.214495 E
    33.857720 S, 151.214613 E
    33.857369 S, 151.215375 E

Polk, et al. Standards Track [Page 30] RFC 6225 DHCP Options for Coordinate LCI July 2011

 The top of the building is 67.4 meters above sea level, and a
 starting altitude of 0 meters above the WGS84 geoid is assumed.
 The first step is to determine the range of latitude and longitude
 values.  Latitude ranges from -33.857720 to -33.856299; longitude
 ranges from 151.214495 to 151.215906.
 For this example, the point that is encoded is chosen by finding the
 middle of each range, that is (-33.8570095, 151.2152005).  This is
 encoded as (1110111100010010010011011000001101,
 0100101110011011100010111011000011) in binary, or (3BC49360D,
 12E6E2EC3) in hexadecimal notation (with an extra 2 bits of leading
 padding on each).  Altitude is set at 33.7 meters, which is
 000000000000000010000110110011 (binary) or 000021B3 (hexadecimal).
 The latitude uncertainty (LatUnc) is given by inserting the
 difference between the center value and the outer value into the
 formula from Section 2.3.2.  This gives:
    x = 8 - ceil( log2( -33.8570095 - -33.857720 ) )
 The result of this equation is 18; therefore, the uncertainty is
 encoded as 010010 in binary.
 Similarly, longitude uncertainty (LongUnc) is given by the formula:
    x = 8 - ceil( log2( 151.2152005 - 151.214495 ) )
 The result of this equation is also 18, or 010010 in binary.
 Altitude uncertainty (AltUnc) uses the formula from Section 2.4.5:
    x = 21 - ceil( log2( 33.7 - 0 ) )
 The result of this equation is 15, which is encoded as 001111 in
 binary.
 Adding an Altitude Type of 1 (meters) and a Datum of 1 (WGS84), this
 gives the following DHCPv4 GeoLoc Option 144 form:

Polk, et al. Standards Track [Page 31] RFC 6225 DHCP Options for Coordinate LCI July 2011

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Code (144)  |  OptLen (16)  |  LatUnc   |     Latitude      .
   |0 1 1 1 1 0 1 1|0 0 0 1 0 0 0 0|0 1 0 0 1 0|1 1 1 0 1 1 1 1 0 0.
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                Latitude (cont'd)              |  LongUnc  |   .
   .0 1 0 0 1 0 0 1 0 0 1 1 0 1 1 0 0 0 0 0 1 1 0 1|0 1 0 0 1 0|0 1.
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                       Longitude (cont'd)                      |
   .0 0 1 0 1 1 1 0 0 1 1 0 1 1 1 0 0 0 1 0 1 1 1 0 1 1 0 0 0 0 1 1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AType |   AltUnc  |                Altitude                   .
   |0 0 0 1|0 0 1 1 1 1|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1.
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .  Alt (cont'd) |Ver| Res |Datum|
   .1 0 1 1 0 0 1 1|0 1|0 0 0|0 0 1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 In hexadecimal, this is 7B104BBC 49360D49 2E6E2EC3 13C00021 B341.
 The DHCPv6 form only differs in the code and option length portion.

C.1.2. Decoding a Location from DHCP Geodetic Form

 If receiving the binary form created in the previous section, this
 section describes how that would be interpreted.  The result is then
 represented as a GML object, as defined in [GeoShape].
 A latitude value of 1110111100010010010011011000001101 decodes to a
 value of -33.8570095003 (to 10 decimal places).  The longitude value
 of 0100101110011011100010111011000011 decodes to 151.2152005136.
 Decoding Tip: If the raw values of latitude and longitude are placed
 in integer variables, the actual value can be derived by the
 following process:
 1. If the highest order bit is set (i.e., the number is a two's
    complement negative), then subtract 2 to the power of 34 (the
    total number of bits).
 2. Divide the result by 2 to the power of 25 (the number of
    fractional bits) to determine the final value.
    The same principle can be applied when decoding altitude values,
    except with different powers of 2 (30 and 8, respectively).

Polk, et al. Standards Track [Page 32] RFC 6225 DHCP Options for Coordinate LCI July 2011

 The latitude and longitude uncertainty are both 18, which gives an
 uncertainty value of 0.0009765625 using the formula from
 Section 2.3.2.  Therefore, the decoded latitude is -33.8570095003 +/-
 0.0009765625 (or the range from -33.8579860628 to -33.8560329378) and
 the decoded longitude is 151.2152005136 +/- 0.0009765625 (or the
 range from 151.2142239511 to 151.2161770761).
 The encoded altitude of 000000000000000010000110110011 decodes to
 33.69921875.  The encoded uncertainty of 15 gives a value of 64;
 therefore, the final uncertainty is 33.69921875 +/- 64 (or the range
 from -30.30078125 to 97.69921875).

C.1.2.1. GML Representation of Decoded Locations

 The following GML shows the value decoded in the previous example as
 a point in a three-dimensional CRS:
    <gml:Point srsName="urn:ogc:def:crs:EPSG::4979"
               xmlns:gml="http://www.opengis.net/gml">
      <gml:pos>-33.8570095003 151.2152005136 33.69921875</gml:pos>
    </gml:Point>
 The following example uses all of the decoded information from the
 previous example:
    <gs:Prism srsName="urn:ogc:def:crs:EPSG::4979"
        xmlns:gs="http://www.opengis.net/pidflo/1.0"
        xmlns:gml="http://www.opengis.net/gml">
      <gs:base>
        <gml:Polygon>
          <gml:exterior>
            <gml:LinearRing>
              <gml:posList>
                -33.8579860628 151.2142239511 -30.30078125
                -33.8579860628 151.2161770761 -30.30078125
                -33.8560329378 151.2161770761 -30.30078125
                -33.8560329378 151.2142239511 -30.30078125
                -33.8579860628 151.2142239511 -30.30078125
              </gml:posList>
            </gml:LinearRing>
          </gml:exterior>
        </gml:Polygon>
      </gs:base>
      <gs:height uom="urn:ogc:def:uom:EPSG::9001">
        128
      </gs:height>
    </gs:Prism>

Polk, et al. Standards Track [Page 33] RFC 6225 DHCP Options for Coordinate LCI July 2011

 Note that this representation is only appropriate if the uncertainty
 is sufficiently small.  [GeoShape] recommends that distances between
 polygon vertices be kept short.  A GML representation like this one
 is only appropriate where uncertainty is less than 1 degree (an
 encoded value of 9 or greater).

Appendix D. Changes from RFC 3825

 This section lists the major changes between RFC 3825 and this
 document.  Minor changes, including style, grammar, spelling, and
 editorial changes, are not mentioned here.
 o  Section 1 now includes clarifications on wired and wireless uses.
 o  The former Sections 1.2 and 1.3 have been removed.  Section 1.2
    now defines the concepts of uncertainty and resolution, as well as
    conversion between the DHCP option formats and PIDF-LO.
 o  A DHCPv6 GeoLoc Option is now defined (Section 2.1) as well as a
    new DHCPv4 GeoLoc Option (Section 2.2.2).
 o  The former Datum field has been split into three fields: Ver, Res,
    and Datum.  These fields are used in both the DHCPv4 GeoLoc Option
    and the DHCPv6 GeoLoc Option.
 o  Section 2.2.3 has been added, describing option support
    requirements on DHCP clients and servers.
 o  Section 2.3 has been added, describing the Latitude and Longitude
    fields.
 o  Section 2.3.1 has been added, covering latitude and longitude
    resolution.
 o  Section 2.3.2 has been added, covering latitude and longitude
    uncertainty.
 o  Section 2.4 has been added, covering values of the Altitude field
    (Sections 2.4.1, 2.4.2, and 2.4.3), altitude resolution
    (Section 2.4.4), and altitude uncertainty (Section 2.4.5).
 o  Section 2.5 has been added, covering the Datum field.
 o  Section 3 (Security Considerations) has added a recommendation on
    link-layer confidentiality.

Polk, et al. Standards Track [Page 34] RFC 6225 DHCP Options for Coordinate LCI July 2011

 o  Section 4 (IANA Considerations) has consolidated material relating
    to parameter allocation for both the DHCPv4 and DHCPv6 option
    parameters, and has been rewritten to conform to the practices
    recommended in RFC 5226.
 o  The material formerly in Appendix A has been updated and shortened
    and has been moved to Appendix B.
 o  An Appendix A on GML mapping has been added.
 o  Appendix C has been added, providing an example of uncertainty
    encoding.
 o  Appendix D has been added, detailing the changes from RFC 3825.

Polk, et al. Standards Track [Page 35] RFC 6225 DHCP Options for Coordinate LCI July 2011

Authors' Addresses

 James M. Polk
 Cisco Systems
 2200 East President George Bush Turnpike
 Richardson, TX  75082
 USA
 EMail: jmpolk@cisco.com
 Marc Linsner
 Cisco Systems
 Marco Island, FL  34145
 USA
 EMail: marc.linsner@cisco.com
 Martin Thomson
 Andrew Corporation
 PO Box U40
 Wollongong University Campus, NSW  2500
 AU
 EMail: martin.thomson@andrew.com
 Bernard Aboba (editor)
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA  98052
 USA
 EMail: bernard_aboba@hotmail.com

Polk, et al. Standards Track [Page 36]

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