draft-ietf-geopriv-rfc3825bis-03.txt   draft-ietf-geopriv-rfc3825bis-04.txt 
GEOPRIV Working Group J. Polk GEOPRIV Working Group J. Polk
INTERNET-DRAFT Cisco Systems INTERNET-DRAFT Cisco Systems
Obsoletes: 3825 (if approved) J. Schnizlein Obsoletes: 3825 (if approved) J. Schnizlein
Category: Standards Track ISOC Category: Standards Track ISOC
Expires: May 11, 2010 M. Linsner Expires: June 21, 2010 M. Linsner
11 November 2009 Cisco Systems 17 December 2009 Cisco Systems
M. Thomson M. Thomson
Andrew Andrew
B. Aboba (ed) B. Aboba (ed)
Microsoft Corporation Microsoft Corporation
Dynamic Host Configuration Protocol Option for Dynamic Host Configuration Protocol Options for
Coordinate-based Location Configuration Information Coordinate-based Location Configuration Information
draft-ietf-geopriv-rfc3825bis-03.txt draft-ietf-geopriv-rfc3825bis-04.txt
Status of This Memo Status of This Memo
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Copyright Notice Copyright Notice
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Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Conventions . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Resolution and Uncertainty . . . . . . . . . . . . . . . 4 1.2 Resolution and Uncertainty . . . . . . . . . . . . . . . 4
2. DHCP Option Format . . . . . . . . . . . . . . . . . . . . . . 4 2. DHCP Option Format . . . . . . . . . . . . . . . . . . . . . . 4
2.1 DHCPv6 Option . . . . . . . . . . . . . . . . . . . . . 5 2.1 DHCPv6 Option . . . . . . . . . . . . . . . . . . . . . 5
2.2 DHCPv4 Option . . . . . . . . . . . . . . . . . . . . . 6 2.2 DHCPv4 Option . . . . . . . . . . . . . . . . . . . . . 6
2.3 Latitude and Longitude Fields . . . . . . . . . . . . . 8 2.3 Latitude and Longitude Fields . . . . . . . . . . . . . 8
2.4 Altitude . . . . . . . . . . . . . . . . . . . . . . . . 10 2.4 Altitude . . . . . . . . . . . . . . . . . . . . . . . . 11
2.5 Datum . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.5 Datum . . . . . . . . . . . . . . . . . . . . . . . . . 12
3. Security Considerations. . . . . . . . . . . . . . . . . . . . 12 3. Security Considerations. . . . . . . . . . . . . . . . . . . . 13
4. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 13 4. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 14
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13 5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Normative References . . . . . . . . . . . . . . . . . . 14 6.1. Normative References . . . . . . . . . . . . . . . . . . 15
6.2. Informational References . . . . . . . . . . . . . . . . 14 6.2. Informational References . . . . . . . . . . . . . . . . 15
Appendix A. Calculations of Imprecision possible with the DHCP LCI 15 Appendix A. Calculations of Resolution . . . . . . . . . . . . . . 16
A.1. LCI of "White House" (Example 1) . . . . . . . . . . . . 15 A.1. LCI of "White House" (Example 1) . . . . . . . . . . . . 16
A.2. LCI of "Sears Tower" (Example 2) . . . . . . . . . . . . 17 A.2. LCI of "Sears Tower" (Example 2) . . . . . . . . . . . . 19
Appendix B. Changes from RFC 3825 . . . . . . . . . . . . . . . . 19 Appendix B. Calculations of Uncertainty . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20 B.1 LCI of "Sydney Opera House" (Example 3) . . . . . . . . 20
Appendix C. Changes from RFC 3825 . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction 1. Introduction
The physical location of a network device has a range of The physical location of a network device has a range of
applications. In particular, emergency telephony applications rely applications. In particular, emergency telephony applications rely
on knowing the location of a caller in order to determine the correct on knowing the location of a caller in order to determine the correct
emergency center. emergency center.
The location of a device can be represented either in terms of The location of a device can be represented either in terms of
geospatial (or geodetic) coordinates, or as a civic address. geospatial (or geodetic) coordinates, or as a civic address.
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might be difficult in practice. might be difficult in practice.
1.1. Conventions used in this document 1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
1.2. Resolution and Uncertainty 1.2. Resolution and Uncertainty
The DHCPv4 option format defined in this document utilizes both The DHCP options defined in this document include fields quantifying
resolution and uncertainty parameters. The DHCPv6 option format only the resolution or uncertainty associated with a target location. No
utilizes an uncertainty parameter. inferences relating to privacy policies can be drawn from either
uncertainty or resolution values.
Version 0 of the DHCPv4 option format defined in this document
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 3
location elements. No inferences relating to privacy policies can be
drawn from either uncertainty or resolution values.
Appendix A of this document provides some arithmetic examples of the As utilized in this document, resolution refers to the accuracy of a
implication of different resolution values on the La/Lo/Alt. reported location, as expressed by the number of valid bits in each
of the Latitude, Longitude and Altitude fields.
The DHCPv6 option format as well as version 1 of the DHCPv4 option In the context of location technology, uncertainty is a
format utilizes an uncertainty parameter. In the context of location quantification of errors. Any method for determining location is
technology, uncertainty is a quantification of errors. Any method subject to some sources of error; uncertainty describes the amount of
for determining location is subject to some sources of error; error that is present. Uncertainty might be the coverage area of a
uncertainty describes the amount of error that is present. wireless transmitter, the extent of a building or a single room.
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 Uncertainty is usually represented as an area within which the target
is located. In this document, each of the three axes can be assigned is located. In this document, each of the three axes can be assigned
an uncertainty value. In effect, this describes a rectangular prism. an uncertainty value. In effect, this describes a rectangular prism.
When representing locations from sources that can quantify When representing locations from sources that can quantify
uncertainty, the goal is to find the smallest possible rectangular uncertainty, the goal is to find the smallest possible rectangular
prism that this format can describe. This is achieved by taking the prism that this format can describe. This is achieved by taking the
minimum and maximum values on each axis and ensuring that the final minimum and maximum values on each axis and ensuring that the final
encoding covers these points. This increases the region of encoding covers these points. This increases the region of
uncertainty, but ensures that the region that is described uncertainty, but ensures that the region that is described
encompasses the target location. encompasses the target location.
The DHCPv4 option format defined in this document supports both
resolution and uncertainty parameters. Version 0 of the DHCPv4
option format defined in this document 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 3 location elements.
The DHCPv6 option format as well as version 1 of the DHCPv4 option
format utilizes an uncertainty parameter. Appendix A of this
document provides examples showing the calculation of resolution
values. Appendix B provides an example demonstrating calculation of
uncertainty values.
2. DHCP Option Format 2. DHCP Option Format
This section defines the format for the DHCPv4 and DHCPv6 options. This section defines the format for the DHCPv4 and DHCPv6 options.
These options use the same basic format, differing only in the option These options utilize a similar format, differing primarily in the
code. option code.
2.1. DHCPv6 Option 2.1. DHCPv6 Option
The DHCPv6 [RFC3315] option format is as follows: The DHCPv6 [RFC3315] option format is as follows:
0 1 2 3 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 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) | | Option Code (TBD) | OptLen (16) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Latitude (cont'd) | LongUnc | + | Latitude (cont'd) | LongUnc | +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Longitude | | Longitude |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AType | AltUnc | Altitude + | AType | AltUnc | Altitude +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Alt.(cont'd) |Ver| Res |Datum| | Alt.(cont'd) |Ver| Res |Datum|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code: 8 bits. The code for the DHCPv4 option (123). Code: 8 bits. The code for the DHCPv4 option (123).
Length: 8 bits. The length of the DHCPv4 option, in octets. Length: 8 bits. The length of the DHCPv4 option, in octets.
For versions 0 and 1, the option length is 16. For versions 0 and 1, the option length is 16.
LatUnc: 6 bits. When the Ver field = 0, this field represents LatUnc: 6 bits. When the Ver field = 0, this field represents
Latitude resolution. When the Ver field = 1, Latitude resolution. When the Ver field = 1,
this field represents Latitude uncertainty. this field represents Latitude uncertainty.
Latitude: a 34 bit fixed point value consisting of 9 bits of integer Latitude: a 34 bit fixed point value consisting of 9 bits of
and 25 bits of fraction. Latitude SHOULD be normalized to integer and 25 bits of fraction. Latitude SHOULD be
within +/- 90 degrees. Positive numbers are north of the normalized to within +/- 90 degrees. Positive numbers
equator and negative numbers are south of the equator. are north of the equator and negative numbers are south
of the equator.
LongUnc: 6 bits. When the Ver field = 0, this field represents LongUnc: 6 bits. When the Ver field = 0, this field represents
Longitude resolution. When the Ver field = 1, Longitude resolution. When the Ver field = 1,
this field represents Longitude uncertainty. this field represents Longitude uncertainty.
Longitude: a 34 bit fixed point value consisting of 9 bits of integer Longitude: a 34 bit fixed point value consisting of 9 bits of
and 25 bits of fraction. Longitude SHOULD be normalized integer and 25 bits of fraction. Longitude SHOULD be
to within +/- 180 degrees. Positive values are East of normalized to within +/- 180 degrees. Positive values
the prime meridian and negative (2s complement) numbers are East of the prime meridian and negative
are West of the prime meridian. (2s complement) numbers are West of the prime meridian.
AType: Altitude Type (4 bits). AType: Altitude Type (4 bits).
AltUnc: 6 bits. When the Ver field = 0, this field represents AltUnc: 6 bits. When the Ver field = 0, this field represents
Altitude resolution. When the Ver field = 1, Altitude resolution. When the Ver field = 1,
this field represents Altitude Uncertainty. this field represents Altitude uncertainty.
Altitude: A 30 bit value defined by the AType field. Altitude: A 30 bit value defined by the AType field.
Ver: The Ver field is two bits, providing for four potential Ver: The Ver field is two bits, providing for four potential
versions. This specification defines the behavior of versions. This specification defines the behavior of
version 0 (originally specified in [RFC3825]) as well as version 0 (originally specified in [RFC3825]) as well as
version 1. The Ver field is always located at the same version 1. The Ver field is always located at the same
offset from the beginning of the option, regardless of the offset from the beginning of the option, regardless of
version in use. the version in use.
Res: The Res field which is 3 bits, is reserved. These bits Res: The Res field which is 3 bits, is reserved. These bits
have been used by [IEEE-802.11y], but are not defined have been used by [IEEE-802.11y], but are not defined
within this specification. within this specification.
Datum: 3 bits. The Map Datum used for the coordinates given in Datum: 3 bits. The Map Datum used for the coordinates given in
this Option. this Option.
2.2.1. Version Support 2.2.1. Version Support
2.2.1.1. Client Version Support 2.2.1.1. Client Version Support
Clients implementing this specification MUST support receiving DHCPv4 clients implementing this specification MUST support receiving
responses of versions 0 and 1. Since this specification utilizes the responses of versions 0 and 1. Since this specification utilizes the
same DHCP option code as [RFC3825], the option format does not same DHCPv4 option code as [RFC3825], the option format does not
provide a means for the client to indicate the highest version that provide a means for the client to indicate the highest version that
it supports to the server. it supports to the server.
2.2.1.2. Server Version Selection 2.2.1.2. Server Version Selection
A DHCP server that provides location information cannot provide A DHCPv4 server that provides location information cannot provide
options with both v0 and v1 formats in the same response. This is not options with both version 0 and version 1 formats in the same
useful since receiving two copies of the same Option (either in the response. This is not useful since receiving two copies of the same
same response or a separate response) causes a DHCP client to replace Option (either in the same response or a separate response) causes a
the information in the old Option with the information in the new DHCPv4 client to replace the information in the old Option with the
Option. information in the new Option.
A server uses configuration to determine which version to send in a A server uses configuration to determine which version to send in a
response. For example, where a mixture of v0 and v1 clients are response. For example, where a mixture of version 0 and version 1
expected, the server could be configured to send v0 or v1 depending clients are expected, the server could be configured to send version
on configuration (possibly making the choice based on information 0 or version 1 depending on configuration (possibly making the choice
such as the client MAC address). Where few v0 clients are expected, based on information such as the client MAC address). Where few
the server could be configured to send only v1 responses. Version 0 version 0 clients are expected, the server could be configured to
options will provide resolution, while version 1 options will provide send only version 1 responses. Version 0 options will provide
an area of uncertainty. resolution, while version 1 options will provide an area of
uncertainty.
An RFC 3825 DHCPv4 client that receives a version 1 option, as An RFC 3825 DHCPv4 client that receives a version 1 option, as
defined in this document, will either reject the Option or will not defined in this document, will either reject the Option or will not
understand the additions to the datum field and will misinterpret the understand the additions to the Datum field and will misinterpret the
LoRes, LaRes, and AltRes values. If the RFC 3825 DHCPv4 client does LongUnc, LatUnc, and AltUnc values. If the RFC 3825 DHCPv4 client
not reject the option and utilizes the location data it will most does not reject the option and utilizes the location data it will
likely assume a datum and interpret the LoRes/LaRes/AltRes values as most likely assume a datum and interpret the LongUnc/LatUnc/AltUnc
significant digits and apply them to the Latitude, Longitude, and values as significant digits and apply them to the Latitude,
Altitude values. The resultant location value will be in error up to Longitude, and Altitude values. The resultant location value will be
a full degree of latitude and longitude, and a full increment of in error up to a full degree of latitude and longitude, and a full
altitude. This results in a v0-only client either not obtaining increment of altitude. This results in a version 0-only client
location information (with no ability to indicate to the server that either not obtaining location information (with no ability to
v1 was unsupported), or misinterpreting the option. indicate to the server that version 1 was unsupported), or
misinterpreting the option.
Therefore, in situations where some clients are known to support only Therefore, in situations where some DHCPv4 clients are known to
v0, by default the server SHOULD send a v0 response. It is also support only version 0, by default the DHCPv4 server SHOULD send a
RECOMMENDED that DHCPv4 client implementations support version 1, so version 0 response. It is also RECOMMENDED that DHCPv4 client
the versioning capability added by this document does not cause implementations support version 1, so the versioning capability added
errors interpreting the latitude, longitude and altitude values. by this document does not cause errors interpreting the latitude,
longitude and altitude values.
Moving forward, clients not understanding a datum value MUST assume a Moving forward, clients not understanding a datum value MUST assume a
World Geodesic System 1984 (WGS84) [WGS84] datum (EPSG [EPSG] 4326 or World Geodesic System 1984 (WGS84) [WGS84] datum (EPSG [EPSG] 4326 or
4979, depending on whether there is an altitude value present) and 4979, depending on whether there is an altitude value present) and
proceed accordingly. Assuming that a less accurate location value is proceed accordingly. Assuming that a less accurate location value is
better than none, this ensures that some (perhaps less accurate) better than none, this ensures that some (perhaps less accurate)
location is available to the client. location is available to the client.
2.3. Latitude and Longitude Fields 2.3. Latitude and Longitude Fields
The Latitude and Longitude values in this format are encoded as 34 The Latitude and Longitude values in this specification are encoded
bit, twos complement, fixed point values with 9 integer bits and 25 as 34 bit, twos complement, fixed point values with 9 integer bits
fractional bits. The exact meaning of these values is determined by and 25 fractional bits. The exact meaning of these values is
the datum; the description in this section applies to the datums determined by the datum; the description in this section applies to
defined in this document. the datums defined in this document.
New datums MUST define the way that the 34 bit values and the New datums MUST define the way that the 34 bit values and the
respective 6 bit uncertainties are interpreted. This document uses respective 6 bit uncertainties are interpreted. This document uses
the same definition for all datums it specifies. the same definition for all datums it specifies.
Latitude values MUST be constrained to the range from -90 to +90 Latitude values MUST be constrained to the range from -90 to +90
degrees. Positive latitudes are north of the equator; negative degrees. Positive latitudes are north of the equator; negative
latitude are south of the equator. latitude are south of the equator.
Longitude values SHOULD be normalized to the range from -180 to +180 Longitude values SHOULD be normalized to the range from -180 to +180
degrees. Values outside this range are normalized by adding or degrees. Values outside this range are normalized by adding or
subtracting 360 until they fall within this range. Positive subtracting 360 until they fall within this range. Positive
longitudes are east of the Prime Meridian (Greenwich); negative longitudes are east of the Prime Meridian (Greenwich); negative
longitudes are west of the Prime Meridian. longitudes are west of the Prime Meridian.
When encoding, latitude and longitude values are rounded to the When encoding, latitude and longitude values are rounded to the
nearest 34-bit binary representation. This imprecision is considered nearest 34-bit binary representation. This imprecision is considered
acceptable for the purposes to which this form is intended to be acceptable for the purposes to which this form is intended to be
applied and is ignored when decoding. applied and is ignored when decoding.
2.3.1. Latitude and Longitude Uncertainty 2.3.1. Latitude and Longitude Resolution
In the version 0 DHCPv4 Option, the Latitude, Longitude and Altitude
fields are each preceded by an accuracy sub-field of 6 bits,
indicating 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.
When encoded within the version 0 DHCPv4 Option, the LatUnc value
encodes the number of 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 A 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 field indicates a
precision of no greater than 1/6th that of the globe (see the first
example of Appendix A). A value of 34 in the LatUnc field indicates
a precision of about 3.11 mm in Latitude at the equator.
When encoded within the version 0 DHCPv4 Option, the LongUnc value
encodes the number of 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 field indicates precision of
no greater than 1/6th that of the globe (see the first example of
Appendix A). A value of 34 in the LongUnc 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
The latitude and longitude uncertainty fields are encoded as 6 bit, The latitude and longitude uncertainty fields are encoded as 6 bit,
unsigned integer values. These values quantify the amount of unsigned integer values. These values quantify the amount of
uncertainty in each of the latitude and longitude values uncertainty in each of the latitude and longitude values
respectively. A value of 0 is reserved to indicate that the respectively. A value of 0 is reserved to indicate that the
uncertainty is unknown; values greater than 34 are reserved. uncertainty is unknown; values greater than 34 are reserved.
A point within the region of uncertainty is selected to be the A point within the region of uncertainty is selected to be the
encoded point; the centroid of the region is often an appropriate encoded point; the centroid of the region is often an appropriate
choice. The value for uncertainty is taken as the distance from the choice. The value for uncertainty is taken as the distance from the
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A value of 2 for altitude type indicates that the altitude value is A value of 2 for altitude type indicates that the altitude value is
measured in floors. This value is relevant only in relation to a measured in floors. This value is relevant only in relation to a
building; the value is relative to the ground level of the building. building; the value is relative to the ground level of the building.
In this definition, numbering starts at ground level, which is floor In this definition, numbering starts at ground level, which is floor
0 regardless of local convention. 0 regardless of local convention.
Non-integer values can be used to represent intermediate or sub- Non-integer values can be used to represent intermediate or sub-
floors, such as mezzanine levels. For instance, a mezzanine between floors, such as mezzanine levels. For instance, a mezzanine between
floors 4 and 5 could be represented as 4.1. floors 4 and 5 could be represented as 4.1.
2.4.4. Altitude Uncertainty 2.4.4. Altitude Resolution
When encoded within the version 0 DHCPv4 Option, the AltUnc value
encodes the number of high-order Altitude bits that should be
considered valid. Values above 30 (decimal) are undefined and
reserved.
If AT = 1, an AltUnc value 0.0 would indicate unknown altitude. The
most precise Altitude would have an AltUnc value of 30. Many values
of AltUnc would obscure any variation due to vertical datum
differences.
The AltUnc field SHOULD be set to maximum precision when AT = 2
(floors) when a floor value is included in the DHCP Reply, or when AT
= 0, to denote that the floor isn't known. An altitude coded as AT =
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
Altitude uncertainty uses the same form of expression as latitude and Altitude uncertainty uses the same form of expression as latitude and
longitude uncertainty. Like latitude and longitude, a value of 0 is longitude uncertainty. Like latitude and longitude, a value of 0 is
reserved to indicate that uncertainty is not known; values above 30 reserved to indicate that uncertainty is not known; values above 30
are also reserved. Altitude uncertainty only applies to altitude are also reserved. Altitude uncertainty only applies to altitude
type 1. type 1.
The amount of altitude uncertainty can be determined by the following The amount of altitude uncertainty can be determined by the following
formula, where x is the encoded integer value: formula, where x is the encoded integer value:
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This datum is used for referencing location on or near tidal water This datum is used for referencing location on or near tidal water
within North America. within North America.
NAD83 is identified by the EPSG/OGP code of 4269, or the URN 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 "urn:ogc:def:crs:EPSG::4269". MLLW does not have a specific code
or URN. or URN.
All hosts MUST support the WGS84 datum (Datum 1). All hosts MUST support the WGS84 datum (Datum 1).
New datum codes can be registered in the IANA registry (Section 5) by New datum codes can be registered in the IANA registry (Section 4) by
a Standards Track RFC. a Standards Track RFC.
3. Security Considerations 3. Security Considerations
Where critical decisions might be based on the value of this GeoConf Where critical decisions might be based on the value of this GeoConf
option, DHCP authentication in [RFC3118] SHOULD be used to protect option, DHCP authentication as defined in "Authentication for DHCP
the integrity of the DHCP options. Messages" [RFC3118] and "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)" [RFC3315] SHOULD be used to protect the integrity of the
DHCP options.
Since there is no privacy protection for DHCP messages, an Since there is no privacy protection for DHCP messages, an
eavesdropper who can monitor the link between the DHCP server and eavesdropper who can monitor the link between the DHCP server and
requesting client can discover this LCI. requesting client can discover this LCI.
To minimize the unintended exposure of location information, the LCI To minimize the unintended exposure of location information, the LCI
option SHOULD be returned by DHCP servers only when the DHCP client option SHOULD be returned by DHCP servers only when the DHCP client
has included this option in its 'parameter request list' (section 3.5 has included this option in its 'parameter request list' (section 3.5
[RFC2131]). [RFC2131]).
When implementing a DHCP server that will serve clients across an When implementing a DHCP server that will serve clients across an
uncontrolled network, one should consider the potential security uncontrolled network, one should consider the potential security
risks. risks.
4. IANA Considerations 4. IANA Considerations
IANA has assigned a DHCP option code of 123 for the GeoConf option IANA has assigned a DHCPv4 option code of 123 for the GeoConf option
defined in this document. defined in this document. Assignment of a DHCPv6 option code is
requested.
The GeoConf Option defines two fields for which IANA maintains a The GeoConf Option defines two fields for which IANA maintains a
registry: The Altitude (AT) field (see Section 2) and the Datum field registry: The Altitude (AT) field and the Datum field (see Section
(see Section 2). The datum indicator MUST include specification of 2). The datum indicator MUST include specification of both
both horizontal and vertical datum. New values for the Altitude (AT) horizontal and vertical datum. New values for the Altitude (AT)
field are assigned through "Standards Action" [RFC5226]. The initial field are assigned through "Standards Action" [RFC5226]. The initial
values of the Altitude registry are as follows: values of the Altitude registry are as follows:
AT = 1 meters of Altitude defined by the vertical datum specified. AT = 1 meters of Altitude defined by the vertical datum specified.
AT = 2 building Floors of Altitude. AT = 2 building Floors of Altitude.
Datum = 1 denotes the vertical datum WGS 84 as defined by the EPSG as 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 their CRS Code 4327; CRS Code 4327 also specifies WGS 84 as
the vertical datum the vertical datum
Datum = 2 denotes the vertical datum NAD83 as defined by the EPSG as Datum = 2 denotes the vertical datum NAD83 as defined by the EPSG as
their CRS Code 4269; North American Vertical Datum of 1988 their CRS Code 4269; North American Vertical Datum of 1988
(NAVD88) is the associated vertical datum for NAD83 (NAVD88) is the associated vertical datum for NAD83
Datum = 3 denotes the vertical datum NAD83 as defined by the EPSG as Datum = 3 denotes the vertical datum NAD83 as defined by the EPSG as
their CRS Code 4269; Mean Lower Low Water (MLLW) is the their CRS Code 4269; Mean Lower Low Water (MLLW) is the
associated vertical datum for NAD83 associated vertical datum for NAD83
Any additional LCI datum(s) to be defined for use via this DHCP Any additional LCI datum(s) to be defined for use via the DHCPv4 or
Option MUST be done through a Standards Track RFC. DHCPv6 Options defined in this document MUST be done through a
Standards Track RFC.
This document defines the Ver field, with values as follows: This document defines the Ver field for the DHCPv4 Option, with
values as follows:
0: Implementations conforming to [RFC3825] 0: Implementations conforming to [RFC3825]
1: Implementations of this specification 1: Implementations of this specification
Any additional Ver field values to be defined for use with this DHCP Any additional Ver field values to be defined for use with the DHCPv4
Option MUST be done through a Standards Track RFC. Option MUST be done through a Standards Track RFC.
5. Acknowledgements 5. Acknowledgments
The authors would like to thank Patrik Falstrom, Ralph Droms, Ted The authors would like to thank Patrik Falstrom, Ralph Droms, Ted
Hardie, Jon Peterson, and Nadine Abbott for their inputs and Hardie, Jon Peterson, and Nadine Abbott for their inputs and
constructive comments regarding this document. Additionally, the constructive comments regarding this document. Additionally, the
authors would like to thank Greg Troxel for the education on vertical authors would like to thank Greg Troxel for the education on vertical
datums, as well as Carl Reed. datums, as well as Carl Reed.
6. References 6. References
6.1. Normative References 6.1. Normative References
skipping to change at page 14, line 37 skipping to change at page 15, line 46
[WGS84] US National Imagery and Mapping Agency, "Department of Defense [WGS84] US National Imagery and Mapping Agency, "Department of Defense
(DoD) World Geodetic System 1984 (WGS 84), Third Edition", (DoD) World Geodetic System 1984 (WGS 84), Third Edition",
NIMA TR8350.2, January 2000, NIMA TR8350.2, January 2000,
https://www1.nga.mil/PRODUCTSSERVICES/GEODESYGEOPHYSICS/ https://www1.nga.mil/PRODUCTSSERVICES/GEODESYGEOPHYSICS/
WORLDGEODETICSYSTEM/Pages/default.aspx and WORLDGEODETICSYSTEM/Pages/default.aspx and
http://www.ngs.noaa.gov/faq.shtml#WGS84 http://www.ngs.noaa.gov/faq.shtml#WGS84
6.2. Informational References 6.2. Informational References
[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-802.11y]
Information technology - Telecommunications and information Information technology - Telecommunications and information
exchange between systems - Local and metropolitan area exchange between systems - Local and metropolitan area
networks - Specific requirements - Part 11: Wireless LAN networks - Specific requirements - Part 11: Wireless LAN
Medium Access Control (MAC) and Physical Layer (PHY) Medium Access Control (MAC) and Physical Layer (PHY)
specifications Amendment 3: 3650-3700 MHz Operation in USA, specifications Amendment 3: 3650-3700 MHz Operation in USA,
November 2008. November 2008.
[NENA] National Emergency Number Association (NENA) www.nena.org NENA [NENA] National Emergency Number Association (NENA) www.nena.org NENA
Technical Information Document on Model Legislation Enhanced Technical Information Document on Model Legislation Enhanced
911 for Multi-Line Telephone Systems. 911 for Multi-Line Telephone Systems.
[RFC3825] Polk, J., Schnizlein, J. and M. Linsner, "Dynamic Host [RFC3825] Polk, J., Schnizlein, J. and M. Linsner, "Dynamic Host
Configuration Protocol Option for Coordinate-based Location Configuration Protocol Option for Coordinate-based Location
Configuration Information", RFC 3825, July 2004. 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 [RFC4776] Schulzrinne, H., "Dynamic Host Configuration Protocol (DHCPv4
and DHCPv6) Option for Civic Addresses Configuration and DHCPv6) Option for Civic Addresses Configuration
Information", RFC 4776, November 2006. 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 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 5226, May 2008. Considerations Section in RFCs", RFC 5226, May 2008.
Appendix A. Calculations of Imprecision Possible with the DHCP LCI Appendix A. Calculations of Resolution
The following examples for two different locations demonstrate how The following examples for two different locations demonstrate how
the Resolution values for Latitude, Longitude, and Altitude can be the Resolution values for Latitude, Longitude, and Altitude (used
used. In both examples the geo-location values were derived from in the version 0 DHCPv4 option) can be used. In both examples,
maps using the WGS84 map datum, therefore in these examples, the the geo-location values were derived from maps using the WGS84
datum field would have a value = 1 (00000001, or 0x01). map datum, therefore in these examples, the Datum field would
have a value = 1 (00000001, or 0x01).
A.1. Location Configuration Information of "White House" (Example 1) A.1. Location Configuration Information of "White House" (Example 1)
The address was NOT picked for any political reason and can easily be 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 found on the Internet or mapping software, but was picked as an
easily identifiable location on our planet. easily identifiable location on our planet.
Postal Address: Postal Address:
White House White House
1600 Pennsylvania Ave. NW 1600 Pennsylvania Ave. NW
skipping to change at page 15, line 50 skipping to change at page 17, line 25
Using 2s complement, 34 bit fixed point, 25 bit fraction Using 2s complement, 34 bit fixed point, 25 bit fraction
Longitude = 0xf65ecf031, Longitude = 0xf65ecf031,
Longitude = 1101100101111011001111000000110001 Longitude = 1101100101111011001111000000110001
Altitude 15 Altitude 15
In this example, we are not inside a structure, therefore we will In this example, we are not inside a structure, therefore we will
assume an altitude value of 15 meters, interpolated from the US assume an altitude value of 15 meters, interpolated from the US
Geological survey map, Washington West quadrangle. Geological survey map, Washington West quadrangle.
AltRes = 30, 0x1e, 011110 AltUnc = 30, 0x1e, 011110
AT = 1, 0x01, 000001 AT = 1, 0x01, 000001
Altitude = 15, 0x0F00, 00000000000000000000000001111100000000 Altitude = 15, 0x0F00, 00000000000000000000000001111100000000
If: LaRes is expressed as value 2 (0x02 or 000010) and LoRes is If: LatUnc is expressed as value 2 (0x02 or 000010) and LongUnc is
expressed as value 2 (0x02 or 000010), then it would describe a expressed as value 2 (0x02 or 000010), then it would describe a
geo-location region that is north of the equator and extends from geo-location region that is north of the equator and extends from
-1 degree (west of the meridian) to -128 degrees. This would -1 degree (west of the meridian) to -128 degrees. This would
include the area from approximately 600km south of Saltpond, include the area from approximately 600km south of Saltpond,
Ghana, due north to the North Pole and approximately 4400km Ghana, due north to the North Pole and approximately 4400km
south-southwest of Los Angeles, CA due north to the North Pole. south-southwest of Los Angeles, CA due north to the North Pole.
This would cover an area of about one-sixth of the globe, This would cover an area of about one-sixth of the globe,
approximately 20 million square nautical miles (nm). approximately 20 million square nautical miles (nm).
If: LaRes is expressed as value 3 (0x03 or 000011) and LoRes is If: LatUnc is expressed as value 3 (0x03 or 000011) and LongUnc is
expressed as value 3 (0x03 or 000011), then it would describe a expressed as value 3 (0x03 or 000011), then it would describe a
geo-location area that is north from the equator to 63 degrees geo-location area that is north from the equator to 63 degrees
north, and -65 degrees to -128 degrees longitude. This area north, and -65 degrees to -128 degrees longitude. This area
includes south of a line from Anchorage, AL to eastern Nunavut, includes south of a line from Anchorage, AL to eastern Nunavut,
CN, and from the Amazons of northern Brazil to approximately CN, and from the Amazons of northern Brazil to approximately
4400km south-southwest of Los Angeles, CA. This area would 4400km south-southwest of Los Angeles, CA. This area would
include North America, Central America, and parts of Venezuela include North America, Central America, and parts of Venezuela
and Columbia, except portions of Alaska and northern and eastern and Columbia, except portions of Alaska and northern and eastern
Canada, approximately 10 million square nm. Canada, approximately 10 million square nm.
If: LaRes is expressed as value 5 (0x05 or 000101) and LoRes is If: LatUnc is expressed as value 5 (0x05 or 000101) and LongUnc is
expressed as value 5 (0x05 or 000101), then it would describe a expressed as value 5 (0x05 or 000101), then it would describe a
geo-location area that is latitude 32 north of the equator to geo-location area that is latitude 32 north of the equator to
latitude 48 and extends from -64 degrees to -80 degrees latitude 48 and extends from -64 degrees to -80 degrees
longitude. This is approximately an east-west boundary of a time longitude. This is approximately an east-west boundary of a time
zone, an area of approximately 700,000 square nm. zone, an area of approximately 700,000 square nm.
If: LaRes is expressed as value 9 (0x09 or 001001) and LoRes is If: LatUnc is expressed as value 9 (0x09 or 001001) and LongUnc is
expressed as value 9 (0x09 or 001001), which includes all the expressed as value 9 (0x09 or 001001), which includes all the
integer bits, then it would describe a geo-location area that is integer bits, then it would describe a geo-location area that is
latitude 38 north of the equator to latitude 39 and extends from latitude 38 north of the equator to latitude 39 and extends from
-77 degrees to -78 degrees longitude. This is an area of -77 degrees to -78 degrees longitude. This is an area of
approximately 9600 square km (111.3km x 86.5km). approximately 9600 square km (111.3km x 86.5km).
If: LaRes is expressed as value 18 (0x12 or 010010) and LoRes is If: LatUnc is expressed as value 18 (0x12 or 010010) and LongUnc is
expressed as value 18 (0x12 or 010010), then it would describe a expressed as value 18 (0x12 or 010010), then it would describe a
geo-location area that is latitude 38.8984375 north to latitude geo-location area that is latitude 38.8984375 north to latitude
38.9003906 and extends from -77.0390625 degrees to -77.0371094 38.9003906 and extends from -77.0390625 degrees to -77.0371094
degrees longitude. This is an area of approximately 36,600 degrees longitude. This is an area of approximately 36,600
square meters (169m x 217m). square meters (169m x 217m).
If: LaRes is expressed as value 22 (0x16 or 010110) and LoRes is If: LatUnc is expressed as value 22 (0x16 or 010110) and LongUnc is
expressed as value 22 (0x16 or 010110), then it would describe a expressed as value 22 (0x16 or 010110), then it would describe a
geo-location area that is latitude 38.896816 north to latitude geo-location area that is latitude 38.896816 north to latitude
38.8985596 and extends from -77.0372314 degrees to -77.0371094 38.8985596 and extends from -77.0372314 degrees to -77.0371094
degrees longitude. This is an area of approximately 143 square degrees longitude. This is an area of approximately 143 square
meters (10.5m x 13.6m). meters (10.5m x 13.6m).
If: LaRes is expressed as value 28 (0x1c or 011100) and LoRes is If: LatUnc is expressed as value 28 (0x1c or 011100) and LongUnc is
expressed as value 28 (0x1c or 011100), then it would describe a expressed as value 28 (0x1c or 011100), then it would describe a
geo-location area that is latitude 38.8986797 north to latitude geo-location area that is latitude 38.8986797 north to latitude
38.8986816 and extends from -77.0372314 degrees to -77.0372296 38.8986816 and extends from -77.0372314 degrees to -77.0372296
degrees longitude. This is an area of approximately 339 square degrees longitude. This is an area of approximately 339 square
centimeters (20.9cm x 16.23cm). centimeters (20.9cm x 16.23cm).
If: LaRes is expressed as value 30 (0x1e or 011110) and LoRes is If: LatUnc is expressed as value 30 (0x1e or 011110) and LongUnc is
expressed as value 30 (0x1e or 011110), then it would describe a expressed as value 30 (0x1e or 011110), then it would describe a
geo-location area that is latitude 38.8986797 north to latitude geo-location area that is latitude 38.8986797 north to latitude
38.8986802 and extends from -77.0372300 degrees to -77.0372296 38.8986802 and extends from -77.0372300 degrees to -77.0372296
degrees longitude. This is an area of approximately 19.5 square degrees longitude. This is an area of approximately 19.5 square
centimeters (50mm x 39mm). centimeters (50mm x 39mm).
If: LaRes is expressed as value 34 (0x22 or 100010) and LoRes is If: LatUnc is expressed as value 34 (0x22 or 100010) and LongUnc is
expressed as value 34 (0x22 or 100010), then it would describe a expressed as value 34 (0x22 or 100010), then it would describe a
geo-location area that is latitude 38.8986800 north to latitude geo-location area that is latitude 38.8986800 north to latitude
38.8986802 and extends from -77.0372300 degrees to -77.0372296 38.8986802 and extends from -77.0372300 degrees to -77.0372296
degrees longitude. This is an area of approximately 7.5 square degrees longitude. This is an area of approximately 7.5 square
millimeters (3.11mm x 2.42mm). millimeters (3.11mm x 2.42mm).
In the (White House) example, the requirement of emergency responders In the (White House) example, the requirement of emergency responders
in North America via their NENA Model Legislation [NENA] could be met in North America via their NENA Model Legislation [NENA] could be met
by a LaRes value of 21 and a LoRes value of 20. This would yield a by a LatUnc value of 21 and a LongUnc value of 20. This would yield
geo-location that is latitude 38.8984375 north to latitude 38.8988616 a geo-location that is latitude 38.8984375 north to latitude
north and longitude -77.0371094 to longitude -77.0375977. This is an 38.8988616 north and longitude -77.0371094 to longitude -77.0375977.
area of approximately 89 feet by 75 feet or 6669 square feet, which This is an area of approximately 89 feet by 75 feet or 6669 square
is very close to the 7000 square feet requested by NENA. In this feet, which is very close to the 7000 square feet requested by NENA.
example, a service provider could enforce that a device send a In this example, a service provider could enforce that a device send
Location Configuration Information with this minimum amount of a Location Configuration Information with this minimum amount of
resolution for this particular location when calling emergency resolution for this particular location when calling emergency
services. services.
A.2. Location Configuration Information of "Sears Tower" (Example 2) A.2. Location Configuration Information of "Sears Tower" (Example 2)
Postal Address: Postal Address:
Sears Tower Sears Tower
103rd Floor 103rd Floor
233 S. Wacker Dr. 233 S. Wacker Dr.
Chicago, IL 60606 Chicago, IL 60606
skipping to change at page 18, line 16 skipping to change at page 19, line 39
Latitude = 0001010011110000011111011101010001 Latitude = 0001010011110000011111011101010001
Longitude 87.63602 degrees West (or -87.63602 degrees) Longitude 87.63602 degrees West (or -87.63602 degrees)
Using 2s complement, 34 bit fixed point, 25 bit fraction Using 2s complement, 34 bit fixed point, 25 bit fraction
Longitude = 0xf50ba5b97, Longitude = 0xf50ba5b97,
Longitude = 1101010000101110100101101110010111 Longitude = 1101010000101110100101101110010111
Altitude 103 Altitude 103
In this example, we are inside a structure, therefore we will assume 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 an altitude value of 103 to indicate the floor we are on. The
Altitude Type value is 2, indicating floors. The AltRes field would Altitude Type value is 2, indicating floors. The AltUnc field would
indicate that all bits in the Altitude field are true, as we want to 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. accurately represent the floor of the structure where we are located.
AltRes = 30, 0x1e, 011110 AltUnc = 30, 0x1e, 011110
AT = 2, 0x02, 000010 AT = 2, 0x02, 000010
Altitude = 103, 0x00006700, 000000000000000110011100000000 Altitude = 103, 0x00006700, 000000000000000110011100000000
For the accuracy of the latitude and longitude, the best information For the accuracy of the latitude and longitude, the best information
available to us was supplied by a generic mapping service that shows 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 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 to show LatUnc as value 18 (0x12 or 010010) and LongUnc as value 18
(0x12 or 010010). This would be describing a geo-location area that (0x12 or 010010). This would be describing a geo-location area that
is latitude 41.8769531 to latitude 41.8789062 and extends from is latitude 41.8769531 to latitude 41.8789062 and extends from
-87.6367188 degrees to -87.6347657 degrees longitude. This is an -87.6367188 degrees to -87.6347657 degrees longitude. This is an
area of approximately 373412 square feet (713.3 ft. x 523.5 ft.). area of approximately 373412 square feet (713.3 ft. x 523.5 ft.).
Appendix B. Changes from RFC 3825 Appendix B. Calculations of Uncertainty
The following example demonstrates how Uncertainty values for
Latitude, Longitude, and Altitude (used in the DHCPv6 Option as
well as the version 1 DHCPv4 option) can be calculated.
B.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 (AT = 1).
B.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 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 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 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 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) | 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) | 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.
B.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).
B.1.2.1. GML Representation of Decoded Locations
The GML representation of a decoded DHCP option depends on what
fields are specified. Uncertainty can be omitted from all of the
respective fields, and altitude can also be absent.
In the absence of uncertainty information, the value decoded from the
DHCP form can be expressed as a single point. If the point includes
altitude, it uses a three dimensional CRS, otherwise it uses a two
dimensional CRS.
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>
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 idependently.
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).
If altitude or altitude uncertainty 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.
Appendix C. Changes from RFC 3825
Technical changes: Technical changes:
-04: Added Appendix B providing an example relating to
uncertainty. Added Section 2.3.1 on Latitude and Longitude
resolution and Section 2.4.4 on Altitude resolution.
Added definition of Resolution to Section 1.2.
-03: Clarified potential behavior of version 0 clients receiving -03: Clarified potential behavior of version 0 clients receiving
a version 1 option and added recommendations for clients and a version 1 option and added recommendations for clients and
servers. servers.
-02: Added Section 1.2 introducing uncertainty and resolution -02: Added Section 1.2 introducing uncertainty and resolution
concepts. Added Section 2.1 defining DHCPv6 option format. concepts. Added Section 2.1 defining DHCPv6 option format.
-01: Within Section 2.1, split Datum field from RFC 3825 into three -01: Within Section 2.1, split Datum field from RFC 3825 into three
fields: Ver, Res and Datum fields. Explained that the Ver fields: Ver, Res and Datum fields. Explained that the Ver
field is always located at the same offset. Added Section 2.2 field is always located at the same offset. Added Section 2.2
relating to Version Support. relating to Version Support.
-00: None -00: None
Editorial changes: Editorial changes:
-03: Changed "DHC" to "DHCP" in some usages. Clarified relatinoship -03: Changed "DHC" to "DHCP" in some usages. Clarified relationship
of resolution and uncertainty to privacy. of resolution and uncertainty to privacy. Changed all uses of
the LoRes/LaRes/AltRes terminology to LongUnc/LatUnc/AltUnc,
and clarified when these parameters were used to encode
resolution vs. uncertainty.
-02: Reorganized Sections 1 and 2. -02: Reorganized Sections 1 and 2.
-01: Added references to IEEE 802.11y, RFC 3825. -01: Added references to IEEE 802.11y, RFC 3825.
-00: Changed boilerplate. Added B. Aboba as editor. Re-positioned -00: Changed boilerplate. Added B. Aboba as editor. Re-positioned
Appendix A and Acknowledgements sections. Changed reference Appendix A and Acknowledgments sections. Changed reference
numbers to names, added reference to RFC 5226 (since RFC 3825 numbers to names, added reference to RFC 5226 (since RFC 3825
was missing a reference to RFC 2434, now obsolete), updated was missing a reference to RFC 2434, now obsolete), updated
references (and URLs). Updated author affiliations and email references (and URLs). Updated author affiliations and email
addresses. Changed references to "the appendix" to Appendix A. addresses. Changed references to "the appendix" to Appendix A.
Added Appendix B listing changes. Added Appendix B listing changes.
Authors' Addresses Authors' Addresses
James M. Polk James M. Polk
Cisco Systems Cisco Systems
 End of changes. 59 change blocks. 
140 lines changed or deleted 417 lines changed or added

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