GEOPRIV Working Group                                            J. Polk
INTERNET-DRAFT                                             Cisco Systems
Obsoletes: 3825 (if approved)                              J. Schnizlein
Category: Standards Track                         Individual Contributor
Expires: May 25, July 4, 2011                                         M. Linsner
27 November 2010
14 January 2011                                            Cisco Systems
                                                              M. Thomson
                                                                  Andrew
                                                           B. Aboba (ed)
                                                   Microsoft Corporation

            Dynamic Host Configuration Protocol Options for
          Coordinate-based Location Configuration Information

                  draft-ietf-geopriv-rfc3825bis-14.txt

                  draft-ietf-geopriv-rfc3825bis-15.txt

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

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   This Internet-Draft will expire on May 25, July 4, 2011.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
       1.1.  Conventions  . . . . . . . . . . . . . . . . . . . . . .  5
       1.2   Resolution and Uncertainty . . . . . . . . . . . . . . .  5
   2.  DHCP Option Format . . . . . . . . . . . . . . . . . . . . . .  6
       2.1   DHCPv6 Option  . . . . . . . . . . . . . . . . . . . . .  6
       2.2   DHCPv4 Option Options . . . . . . . . . . . . . . . . . . . . .  8
       2.3   Latitude and Longitude Fields  . . . . . . . . . . . . . 10 11
       2.4   Altitude . . . . . . . . . . . . . . . . . . . . . . . . 13 14
       2.5   Datum  . . . . . . . . . . . . . . . . . . . . . . . . . 14 16
   3.  Security Considerations. . . . . . . . . . . . . . . . . . . . 15 16
   4.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 16 17
   5.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17 18
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 18
       6.1.  Normative References . . . . . . . . . . . . . . . . . . 17 18
       6.2.  Informational References . . . . . . . . . . . . . . . . 18 19
   Appendix A.  GML Mapping . . . . . . . . . . . . . . . . . . . . . 20 21
       A.1.  GML Templates  . . . . . . . . . . . . . . . . . . . . . 20 21
   Appendix B. Calculations of Resolution . . . . . . . . . . . . . . 23 24
       B.1.  LCI of "White House" (Example 1) . . . . . . . . . . . . 23 25
       B.2.  LCI of "Sears Tower" (Example 2) . . . . . . . . . . . . 26 27
   Appendix C. Calculations of Uncertainty  . . . . . . . . . . . . . 27 28
       C.1   LCI of "Sydney Opera House" (Example 3)  . . . . . . . . 27 28
   Appendix D. Changes from RFC 3825  . . . . . . . . . . . . . . . . 31 32
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32 33

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.

   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] is
   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 DHC server uses RAIO for location
   determination, a wireless host can use this mechanism to discover
   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.

   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.

   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 format formats defined in this document supports both support resolution
   and uncertainty parameters.  Version 0 of the  DHCPv4
   option format 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.  Since version 0 of the DHCPv6
   option format is not defined, the DHCPv6 option does not support a
   resolution parameter.  Version 1 of the
   DHCPv4 and Option TBD as well as the DHCPv6 option format utilizes utilize an
   uncertainty parameter.

   Appendix A describes the mapping of DHCP option values to GML. 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.

   Since the PIDF-LO format Presence Information Data Format Location Object (PIDF-LO)
   [RFC4119][RFC5491] is used to conveying 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:

     version 0

     DHCPv4 Option 123 to PIDF-LO
     version 1
     DHCPv4 Option TBD and DHCPv6 option to PIDF-LO
     PIDF-LO to version 1 DHCP Option TBD and DHCPv6 option

   Conversion to PIDF-LO does not increase uncertainty; conversion from
   PIDF-LO to version 1 DHCPv4 Option TBD and the DHCPv6 option increases
   uncertainty by less than a factor of 2 in each dimension.  Since it
   is not possible to translate an arbitrary PIDF-LO to version 0 DHCP Option 123
   with a bounded increase in uncertainty, the conversion to version 0 is not
   specified.

2.  DHCP Option Format

   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 Option

   The DHCPv6 [RFC3315] option format 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 (TBD)       |            OptLen (16)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  LatUnc   |                  Latitude                         +
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Lat (cont'd)  |  LongUnc  |               Longitude           +
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Longitude (cont'd)         | AType |   AltUnc  |  Altitude +
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Altitude (cont'd)               |Ver| Res |Datum|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Code:      GEOCONF_GEODETIC      DHCP Option Code TBD (16 bits).

   OptLen:    Option Length (16).  This option has a fixed length, so
              that Length.  For version 1, the value of this octet will always be option length is 16.

   LatUnc:    6 bits.  When the Ver field = 1, this field represents
              latitude uncertainty.  The contents of this field is
              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 is
              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:     Altitude Type (4 bits). bits), defined in Section 2.4.

   AltUnc:    6 bits.  When the Ver field = 1, this field represents
              altitude uncertainty.  The contents of this field is
              undefined for other values of the Ver field.

   Altitude:  A 30 bit value defined by the AType field. field, described in
              Section 2.4.

   Ver:       The Ver field is two 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:       The Res field which is 3 bits, 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.  DHCPv4 Options

2.2.1.  DHCPv4 Option 123

   The DHCPv4 option format of DHCPv4 Option 123 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     |   LatUnc   LaRes   |     Latitude      +
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Latitude (cont'd)              |  LongUnc   LoRes   |   +
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Longitude                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AType |   AltUnc   AltRes  |                Altitude                   +
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Alt.(cont'd)  |Ver|  |    Res  |Datum|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Code:      8 bits.  The code for the DHCPv4 option (123).

   Length:    8 bits.  The length of the DHCPv4 option, in octets.
              For versions 0 and 1, the
              The option length is 16.

   LatUnc:

   LaRes:     6 bits.  When the Ver field = 0, this  This field represents latitude resolution.  When the Ver field = 1,
              this field represents latitude uncertainty.

   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.

   LongUnc:

   LoRes:     6 bits.  When the Ver field = 0, this  This field represents longitude resolution.  When the Ver field = 1,
              this field represents longitude uncertainty.

   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:     Altitude Type (4 bits).

   AltUnc: bits), defined in Section 2.4.

   AltRes:    6 bits.  When the Ver field = 0, this  This field represents altitude resolution.  When the Ver field = 1,
              this field represents altitude uncertainty.

   Altitude:  A 30 bit value defined by the AType field.

   Ver:       The Ver field is two bits, providing for four potential
              versions.  This specification defines the behavior of
              version 0 (originally specified in [RFC3825]) as well as
              version 1.  The Ver field is always located at the same
              offset from the beginning of the option, regardless of
              the version field, described in use.
              Section 2.4.

   Res:       The Res field which is 3 5 bits, 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.

2.2.1.  Version Support

2.2.1.1.  Client Version Support

   DHCPv6 clients implementing this specification MUST support receiving
   version 1 responses.

2.2.2.  DHCPv4 clients implementing this specification
   MUST support receiving responses Option TBD

   The format of versions 0 and 1.  It is required
   that DHCPv4 client implementations support version option TBD is as follows:

    0                   1 so the
   versioning capability added by this document does not cause errors
   interpreting the Latitude,                   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 TBD    |    Length     |   LatUnc  |     Latitude      +
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Latitude (cont'd)              |  LongUnc  |   +
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Longitude and                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | AType |   AltUnc  |                Altitude values.  Since this
   specification utilizes the same DHCPv4 option                   +
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Alt.(cont'd)  |Ver| Res |Datum|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Code:      8 bits.  The code as [RFC3825], the
   option format does not provide a means for the DHCPv4 client to
   indicate option (TBD).

   Length:    8 bits.  The length of the highest DHCPv4 option, in octets.
              For version that it supports to 1, the DHCPv4 server.

   Moving forward, DHCPv4 and DHCPv6 clients not understanding a datum
   value MUST assume a World Geodesic System 1984 (WGS84) [WGS84] datum
   (EPSG [EPSG] 4326 or 4979, depending on whether there is an Altitude
   value present) and proceed accordingly.  Assuming that a less
   accurate location value option length is better than none, 16.

   LatUnc:    6 bits.  When the Ver field = 1, this ensures that some
   (perhaps less accurate) location field represents
              latitude uncertainty.  The contents of this field is available to
              undefined for other values of the client.

2.2.1.2.  Server Version Selection

   DHCPv6 servers implementing 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 specification MUST send version 1
   responses.  A DHCPv4 server implementing field represents
              longitude uncertainty.  The contents of this specification MUST
   support sending version 1 responses and SHOULD support sending
   version 0 responses.  A DHCPv4 server that provides location
   information cannot provide options with both version 0 field is
              undefined for other values of the Ver field.

   Longitude: a 34 bit fixed point value consisting of 9 bits of
              integer and version 1
   formats 25 bits of fraction, interpreted as
              described in Section 2.3.

   AType:     Altitude Type (4 bits), defined in Section 2.4.

   AltUnc:    6 bits.  When the same response.  This Ver field = 1, this field represents
              altitude uncertainty.  The contents of this field is not useful since receiving two
   copies
              undefined for other values of the same Option (either Ver field.

   Altitude:  A 30 bit value defined by the AType field, described in
              Section 2.4.

   Ver:       The Ver field is two bits, providing for four potential
              versions.  This specification defines the behavior of
              version 1.  The Ver field is always located at the same response or a separate
   response) causes a DHCPv4 client to replace
              offset from the information in beginning of the
   old Option with option, regardless of
              the information version in use.

   Res:       The Res field which is 3 bits, 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 new coordinates given in
              this Option.

   A

2.2.3.  Option Support

2.2.3.1.  Client Support

   DHCPv4 server uses configuration to determine which version to send
   in a response.  For example, where a mixture of version 0 clients implementing this specification MUST support receiving
   DHCPv4 Option TBD (version 1), and version
   1 MAY support receiving DHCPv4
   Option 123 (originally defined in RFC 3825 [RFC3825]).

   DHCPv4 clients are expected, request the DHCPv4 server could be configured to send
   version 0 Option 123, Option
   TBD or version 1 depending on configuration (possibly making
   the choice based on information such as the client MAC address).
   Where few version 0 clients are expected, both via inclusion of the DHCPv4 server could be
   configured to send only version 1 responses.  Version 0 options will
   provide resolution, while version 1 options will provide an area Parameter Request List option.  As
   noted in Section 9.8 of
   uncertainty.

   An RFC 3825 DHCPv4 2132 [RFC2132]:

      This option is used by a DHCP client that receives to request values for
      specified configuration parameters.  The list of requested
      parameters is specified as n octets, where each octet is a version 1 valid
      DHCP option code as defined in this document will either reject document.

      The client MAY list the Option, or will options in order of preference.  The DHCP
      server is not understand required to return the additions options in the requested
      order, but MUST try to insert the Datum field and will misinterpret requested options in the LongUnc,
   LatUnc, and AltUnc values.  If order
      requested by the RFC 3825 client.

   When DHCPv4 client does not
   reject the option and utilizes the location data it will most likely DHCPv6 clients implementing this specification do not
   understand a datum value, they MUST assume a datum.  Assuming one of the RFC 3825 datums causes correct
   interpretation of Latitude/Longitude/Altitude values.  The values for
   LongUnc/LatUnc/AltUnc are mistakenly interpreted as representing
   significant digits.  The resultant location World Geodesic System
   1984 (WGS84) [WGS84] datum (EPSG [EPSG] 4326 or 4979, depending on
   whether there is an Altitude value will be in error up
   to a full degree of latitude and longitude, present) and proceed accordingly.
   Assuming that a full increment of
   altitude.

   This results in a version 0-only DHCPv4 client either not obtaining less accurate location information (with no ability to indicate to the server that
   version 1 was unsupported), or misinterpreting the option.
   Therefore, if it value is not known whether all DHCPv4 clients support
   version 1, and it better than none,
   this ensures that some (perhaps less accurate) location is not possible for available
   to the client.

2.2.3.2.  Server Option Selection

   A DHCPv4 server to
   distinguish between version 0 and implementing this specification MUST support sending
   Option TBD version 1 and SHOULD support sending Option 123 in
   responses.

   A DHCPv4 clients by some
   means, server that provides location information SHOULD honor the
   Parameter Request List included by default the DHCPv4 server SHOULD client in order to
   decide whether to send a version 0 response. Option 123, Option TBD or both in the
   Response.

2.3.  Latitude and Longitude Fields

   The Latitude and Longitude values in this specification are encoded
   as 34 bit, twos 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.

   Latitude values encoded by the DHCP server MUST be constrained to the
   range from -90 to +90 degrees.  Location consumers MUST be prepared
   to normalize values outside this range.  Values outside the range are
   normalized by clamping (e.g. values less than -90 degrees are set to
   -90; values greater than 90 degrees are set to +90).  Positive
   latitudes are north of the equator and negative latitudes are south
   of the equator.

   Longitude values encoded by the DHCP server MUST be normalized to the
   range from -180 to +180 degrees.  Location consumers MUST be prepared
   to normalize values outside this range.  Values outside the range are
   normalized by wrapping (e.g. adding or subtracting 360 until they
   fall within the range of -180 to 180).  Positive longitudes are east
   of the Prime Meridian (Greenwich) and negative (2s complement)
   longitudes are west of the Prime Meridian.

   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.

2.3.1.  Latitude and Longitude Resolution

   The Latitude (LatUnc), (LaRes), Longitude (LongUnc) (LoRes) and Altitude (AltUnc)
   Uncertainty (AltRes)
   Resolution fields are encoded as 6 bit, unsigned integer values.  In
   the version 0 DHCPv4 Option, Option 123, the LatUnc, LongUnc LaRes, LoRes and AltUnc 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 version 0 DHCPv4 Option, Option 123, the LatUnc LaRes value encodes the number of high-order high-
   order latitude bits that should be considered valid.  Any bits
   entered to the right of this limit should 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 LatUnc 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 LatUnc LaRes field indicates a precision of about 3.11 mm in
   latitude at the equator.

   In the version 0 DHCPv4 Option, Option 123, the LongUnc LoRes value encodes the number of high-order high-
   order longitude bits that should be considered valid.  Any bits
   entered to the right of this limit should not be considered valid and
   might be purposely false, or zeroed by the sender.  A value of 2 in
   the LongUnc 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 LongUnc 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 option and the version 1 DHCPv4 option, Option TBD, 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" is the uncertainty.

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

   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

   The Altitude value is expressed as a 30 bit, fixed point, twos
   complement integer with 22 integer bits and 8 fractional bits.

   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, twos
   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, twos
   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 this definition, numbering starts some buildings
   it might not be clear which floor is at ground level, which level or an
   intermediate floor might be hard to identify as such.  Determining
   what floor is at ground level and what constitutes a sub-floor as
   opposed to an naturally numbered floor
   0 regardless of is left to local convention.
   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.  For instance,  Example: a mezzanine between
   floors 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 4.1. values
   of 4.5 and 4.75.

2.4.4.  Altitude Resolution

   In the version 0 DHCPv4 Option, Option 123, the Altitude Uncertainty (AltUnc) 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 Altitutde Altitude Type value is one (AType = 1), an AltUnc AltRes value 0.0
   would indicate unknown Altitude.  The most precise altitude would
   have an AltUnc AltRes value of 30.  Many values of AltUnc AltRes would obscure any
   variation due to vertical datum differences.

   The AltUnc 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 option or the version 1 DHCPv4 option, Option TBD, 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.

   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 Geodesic 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.

      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 (Section 3.5
   [RFC2131]).
   [RFC2131], Section 9.8 [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

   IANA has assigned a DHCPv4 option code of 123 for the GeoConf option
   defined in this document. option.
   Assignment of an additional DHCPv4 option code as well as a DHCPv6
   option code is requested.

   The GeoConf Option defines two fields for which

   IANA maintains a
   registry: The registries for the Altitude Type (AType) field and the (AType), Datum field (see
   Section 2).  The datum indicator MUST include specification of both
   horizontal and vertical datum.
   Version fields.  New values for the Altitude Type
   (AType) and Datum fields each of these registries are assigned
   through "Standards Action" Actions" [RFC5226].  New Altitude Types

   Each AType registry entry MUST define the way that the 30 bit
   altitude values and the associated 6 bit uncertainty are interpreted.
   New datums

   Each Datum registry entry MUST include specification of both
   horizontal and vertical datum, and MUST define the way that the 34
   bit values and the respective 6 bit uncertainties are interpreted.

   The initial values
   of the Altitude AType registry are as follows: is:

   AType = 0  No known altitude.

   AType = 1  meters of altitude defined by the vertical datum
           specified.

   AType = 2  building floors of altitude.

   The initial Datum registry is:

   Datum = 1 denotes the vertical datum WGS 84 as defined by the EPSG
           as their CRS Code 4327; CRS Code 4327 also specifies WGS 84
           as the vertical datum.

   Datum = 2 denotes the vertical datum NAD83 as defined by the EPSG as
           their CRS Code 4269; North American Vertical Datum of 1988
           (NAVD88) is the associated vertical datum for NAD83.

   Datum = 3 denotes the vertical datum NAD83 as defined by the EPSG as
           their CRS Code 4269; Mean Lower Low Water (MLLW) is the
           associated vertical datum for NAD83.

   This document defines the Ver field for the DHCPv4 and DHCPv6
   options.  New values for the Ver field are assigned through
   "Standards Action" [RFC5226].  Initial values are as follows:

   0: DHCPv4 Implementations conforming to [RFC3825]

   The initial Version registry is:

   1: Implementations of this specification utilizing uncertainty parameters
      (for both DHCPv4 and DHCPv6) DHCPv6).

5.  Acknowledgments

   The authors would like to thank Randall Gellens, Patrik Falstrom,
   Ralph Droms, Ted Hardie, Jon Peterson, Robert Sparks Sparks, Ralph Droms and
   Nadine Abbott 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", RFC2132, March 1997.

[RFC3046]    Patrick, M., "DHCP Relay Agent Information Option", RFC
             3046, January 2001.

[RFC3118]    Droms, R. and W. Arbaugh, "Authentication for DHCP
             Messages", RFC 3118, June 2001.

[RFC3315]    Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and
             M. Carney, "Dynamic Host Configuration Protocol for IPv6
             (DHCPv6)", RFC 3315, July 2003.

[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.  Informational References

[Convey]     Polk, J., Rosen, B. and J. Peterson, "Location Conveyance
             for the Session Initiation Protocol", Internet draft (work
             in progress), draft-ietf-sipcore-location-
             conveyance-04.txt, October 25, 2010.

[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]
             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) www.nena.org
             NENA Technical Information Document on Model Legislation
             Enhanced 911 for Multi-Line Telephone Systems.

[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.

[RFC5226]    Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", RFC 5226, May 2008.

[RFC5491]    Winterbottom, J., Thomson, M. and H. Tschofenig, "GEOPRIV
             PIDF-LO Usage Clarification, Considerations, and
             Recommendations ", RFC 5491, March 2009

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
   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 option as well as versions 0 and 1 both of the DHCPv4
   option, 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 (Altitude Type (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):

       <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".

   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

   The uncertainty fields (LatUnc, LongUnc, AltUnc) indicate the bounds
   of the location region described by a DHCP location object.

   For
   version 0 of the DHCPv4 option, the uncertainty Option 123, resolution fields represent
   resolution, 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 option and version 1 of the DHCPv4 option, Option TBD, the LatUnc, LongUnc and
   AltUnc fields indicate uncertainty distances. 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" (version 0), (DHCPv4 Option 123), while
   the second section addresses the "uncertainty encoding" (version 1). (DHCPv6
   Option and DHCPv4 Option TBD).

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 (LatUnc,
   LongUnc) (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).

        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 (AltUnc for v0) (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
   in version 1) (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

Appendix B. Calculations of Resolution

   The following examples for two different locations demonstrate how
   the Resolution values for Latitude, Longitude, and Altitude (used in
   the version 0
   DHCPv4 option) Option 123) can be calculated.  In both examples, the geo-location 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 NENA Model Legislation [NENA] could be met by a
   LatUnc
   LaRes value of 21 and a LongUnc LoRes value of 20.  This would yield a
   geo-location 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 a
   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 version 0 DHCPv4 Option 123 encoding 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)  |  OptLen (16)  |  LatUnc   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)              |  LongUnc   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 |   AltUnc   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) |Ver| |   Res   |Datum|
    .0 0 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.

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.

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.

   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 2s complement, 34 bit fixed point, 25 bit fraction
   Latitude = 0x053c1f751,
   Latitude = 0001010011110000011111011101010001
   Longitude 87.63602 degrees West (or -87.63602 degrees)
   Using 2s complement, 34 bit fixed point, 25 bit fraction
   Longitude = 0xf50ba5b97,
   Longitude = 1101010000101110100101101110010111

   Altitude 103

   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 AltUnc 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.

   AltUnc

   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 LatUnc LaRes as value 18 (0x12 or 010010) and LongUnc 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 Option option as well as the version 1 DHCPv4 option) Option TBD)
   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.GML-3.1.1] 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

   The top of the building 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 2.3.1.  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 2.4.4:

        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 Option TBD form:

        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) (TBD)  |  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 twos
          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).

   The Latitude and Longitude Uncertainty are both 18, which gives an
   uncertainty value using the formula from Section 2.3.1 of
   0.0009765625.  Therefore, the decoded Latitudes 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>

   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 [RFC3825] 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 format and PIDF-LO.
   o  A DHCPv6 option is now defined (Section 2.1) as well
      as a DHCPv4 option options (Section 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 and DHCPv6 options.
   o  Section 2.2.1 2.2.3 has been added, describing Version Option support.
   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.
   o  Section 4 (IANA Considerations) has consolidated material
      relating to parameter allocation for both the DHCPv4 and
      DHCPv6 option parameters.
   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.

Authors' Addresses

   James M. Polk
   Cisco Systems
   2200 East President George Bush Turnpike
   Richardson, Texas 75082 USA
   USA

   EMail: jmpolk@cisco.com

   John Schnizlein

   EMail: john@schnizlein.org

   Marc Linsner
   Cisco Systems
   Marco Island, FL 34145 USA
   USA

   EMail: marc.linsner@cisco.com

   Martin Thomson
   Andrew
   PO Box U40
   Wollongong University Campus, NSW  2500
   AU

   EMail: martin.thomson@andrew.com

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052 USA
   USA

   EMail: bernarda@microsoft.com bernard_aboba@hotmail.com