path: root/doc/rfc/rfc6225.txt

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.
















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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.

























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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.






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   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.






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   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.





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   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.




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    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.











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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.








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    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.




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    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.









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   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.






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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.











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           ___          ___________
           ^ |         /           |
           | |        /            |
           | |       /             |
           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.





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   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





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   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:

      - if positive, the floor value is farther above the ground floor.

      - 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.







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   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.





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      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.







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   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.



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   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].




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   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.





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   [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>.




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   [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.












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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):




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      <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".





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   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).




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      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











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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:













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      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.



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   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




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   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




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   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:












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      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).






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   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>




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   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.






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   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.





































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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
















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