GEOPRIV Working Group J. Polk
INTERNET-DRAFT Cisco Systems
Obsoletes: 3825 (if approved) J. Schnizlein
Category: Standards Track Individual Contributor
Expires: September 15, 2010 M. Linsner
7 March 2010 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-09.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.
Status of This Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on September 15, 2010.
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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described in the BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
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material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Conventions . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Resolution and Uncertainty . . . . . . . . . . . . . . . 5
2. DHCP Option Format . . . . . . . . . . . . . . . . . . . . . . 5
2.1 DHCPv6 Option . . . . . . . . . . . . . . . . . . . . . 6
2.2 DHCPv4 Option . . . . . . . . . . . . . . . . . . . . . 7
2.3 Latitude and Longitude Fields . . . . . . . . . . . . . 9
2.4 Altitude . . . . . . . . . . . . . . . . . . . . . . . . 12
2.5 Datum . . . . . . . . . . . . . . . . . . . . . . . . . 14
3. Security Considerations. . . . . . . . . . . . . . . . . . . . 15
4. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 15
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1. Normative References . . . . . . . . . . . . . . . . . . 16
6.2. Informational References . . . . . . . . . . . . . . . . 17
Appendix A. GML Mapping . . . . . . . . . . . . . . . . . . . . . 18
A.1. GML Templates . . . . . . . . . . . . . . . . . . . . . 18
Appendix B. Calculations of Resolution . . . . . . . . . . . . . . 22
B.1. LCI of "White House" (Example 1) . . . . . . . . . . . . 22
B.2. LCI of "Sears Tower" (Example 2) . . . . . . . . . . . . 25
Appendix C. Calculations of Uncertainty . . . . . . . . . . . . . 26
C.1 LCI of "Sydney Opera House" (Example 3) . . . . . . . . 26
Appendix D. Changes from RFC 3825 . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
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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 (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 [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 options defined in this document have limited applicability for
mobile hosts. Typically DHCP clients refresh their configuration in
response to changes in interface state or pending lease expirations.
As a result, when a mobile host changes location without subsequently
completing another DHCP exchange, location configuration information
initially obtained via DHCP could become outdated.
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].
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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 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 (which supports only version 1) as well as
version 1 of the DHCPv4 option format utilizes an uncertainty
parameter. Appendix A describes the mapping of DHCP option values to
GML. Appendix B of this document provides examples showing the
calculation of resolution values. Appendix C provides an example
demonstrating calculation of uncertainty values.
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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) | AT | AltUnc | Altitude +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Altitude (cont'd) |Ver| Res |Datum|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code: GEOCONF_GEODETIC (16 bits).
OptLen: Option Length (16). This option is fixed size, the
value of this octet will always be 16.
LatUnc: Latitude Uncertainty (6 bits).
Latitude: Latitude (34 bits).
LongUnc: Longitude Uncertainty (6 bits).
Longitude: Longitude (34 bits).
AType: Altitude Type (4 bits).
AltUnc: Altitude Uncertainty (6 bits).
Altitude: Altitude (30 bits).
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.
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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 Option
The DHCPv4 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code 123 | Length | LatUnc | Latitude +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Latitude (cont'd) | LongUnc | +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Longitude |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AType | AltUnc | Altitude +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Alt.(cont'd) |Ver| Res |Datum|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code: 8 bits. The code for the DHCPv4 option (123).
Length: 8 bits. The length of the DHCPv4 option, in octets.
For versions 0 and 1, the option length is 16.
LatUnc: 6 bits. When the Ver field = 0, 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
integer and 25 bits of fraction. Latitude SHOULD be
normalized to within +/- 90 degrees. Positive numbers
are north of the equator and negative numbers are south
of the equator.
LongUnc: 6 bits. When the Ver field = 0, 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
integer and 25 bits of fraction. Longitude SHOULD be
normalized to within +/- 180 degrees. Positive values
are East of the prime meridian and negative
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(2s complement) numbers are West of the prime meridian.
AType: Altitude Type (4 bits).
AltUnc: 6 bits. When the Ver field = 0, 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 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 coordinates given in
this Option.
2.2.1. Version Support
2.2.1.1. Client Version Support
DHCPv4 clients implementing this specification MUST support receiving
responses of versions 0 and 1. Since this specification utilizes the
same DHCPv4 option code as [RFC3825], the option format does not
provide a means for the client to indicate the highest version that
it supports to the server.
2.2.1.2. Server Version Selection
A DHCPv4 server that provides location information cannot provide
options with both version 0 and version 1 formats in the same
response. This is not useful since receiving two copies of the same
Option (either in the same response or a separate response) causes a
DHCPv4 client to replace the information in the old Option with the
information in the new Option.
A server uses configuration to determine which version to send in a
response. For example, where a mixture of version 0 and version 1
clients are expected, the server could be configured to send version
0 or version 1 depending on configuration (possibly making the choice
based on information such as the client MAC address). Where few
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version 0 clients are expected, the server could be configured to
send only version 1 responses. Version 0 options will provide
resolution, while version 1 options will provide an area of
uncertainty.
An RFC 3825 DHCPv4 client that receives a version 1 option, as
defined in this document, will either reject the Option or will not
understand the additions to the Datum field and will misinterpret the
LongUnc, LatUnc, and AltUnc values. If the RFC 3825 DHCPv4 client
does not reject the option and utilizes the location data it will
most likely assume a datum and interpret the LongUnc/LatUnc/AltUnc
values as significant digits and apply them to the Latitude,
Longitude, and Altitude values. The resultant location value will be
in error up to a full degree of latitude and longitude, and a full
increment of altitude. This results in a version 0-only client
either not obtaining location information (with no ability to
indicate to the server that version 1 was unsupported), or
misinterpreting the option.
Therefore, in situations where some DHCPv4 clients are known to
support only version 0, by default the DHCPv4 server SHOULD send a
version 0 response. It is also RECOMMENDED that DHCPv4 client
implementations support version 1, so the versioning capability added
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
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 is
better than none, this ensures that some (perhaps less accurate)
location is available to the client.
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.
New datums MUST define the way that the 34 bit values and the
respective 6 bit uncertainties are interpreted. This document uses
the same definition for all datums it specifies.
Latitude values MUST be constrained to the range from -90 to +90
degrees. Positive latitudes are north of the equator; negative
latitudes are south of the equator.
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Longitude values SHOULD be normalized to the range from -180 to +180
degrees. Values outside this range are normalized by adding or
subtracting 360 until they fall within this range. Positive
longitudes are east of the Prime Meridian (Greenwich); negative
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), Longitude (LongUnc) and Altitude (AltUnc)
Uncertainty fields are encoded as 6 bit, unsigned integer values. In
the version 0 DHCPv4 Option, the LatUnc, LongUnc and AltUnc 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, 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 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 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 field indicates a precision
of about 3.11 mm in latitude at the equator.
In 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 B). 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
In the DHCPv6 option and the version 1 DHCPv4 option, the Latitude
and Longitude Uncertainty fields (LatUnc and LongUnc) quantify the
amount of uncertainty in each of the Latitude and Longitude values
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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.
The following figure shows a two-dimensional figure that is projected
to 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
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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 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 type of altitude and the selected
datum.
New Altitude Types and datums MUST define the way that the 30 bit
value and the associated 6 bit uncertainty are interpreted.
Three Altitude Types are defined in this document: unknown (0),
meters (1) and floors (2). Additional Altitude Types MUST be defined
in a Standards Track RFC.
2.4.1. No Known Altitude (AT = 0)
In some cases, the altitude of the location might not be provided. An
Altitude Type of 0 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.
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2.4.2. Altitude in Meters (AT = 1)
If the Altitude Type has a value of 1, the Altitude is measured in
meters. The altitude is measured in relation to the zero set by the
vertical datum.
2.4.3. Altitude in Floors (AT = 2)
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
building; the value is relative to the ground level of the building.
In this definition, numbering starts at ground level, which is floor
0 regardless of local convention.
Non-integer values can be used to represent intermediate or sub-
floors, such as mezzanine levels. For instance, a mezzanine between
floors 4 and 5 could be represented as 4.1.
2.4.4. Altitude Resolution
In 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
In the DHCPv6 option or the version 1 DHCPv4 option, 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 )
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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( x ) )
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).
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New Datum codes can be registered in the IANA registry (Section 4) by
a Standards Track RFC.
3. Security Considerations
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.
Since there is no privacy protection for DHCP messages, an
eavesdropper who can monitor the link between the DHCP server and
requesting client can discover this LCI.
To minimize the unintended exposure of location information, the LCI
option SHOULD be returned by DHCP servers only when the DHCP client
has included this option in its 'parameter request list' (section 3.5
[RFC2131]). Link layer confidentiality may also be employed to reduce
the risk of location disclosure.
4. IANA Considerations
IANA has assigned a DHCPv4 option code of 123 for the GeoConf option
defined in this document. Assignment of a DHCPv6 option code is
requested.
The GeoConf Option defines two fields for which IANA maintains a
registry: The Altitude (AT) field and the Datum field (see Section
2). The datum indicator MUST include specification of both
horizontal and vertical datum. New values for the Altitude (AT)
field are assigned through "Standards Action" [RFC5226]. The initial
values of the Altitude registry are as follows:
AT = 1 meters of altitude defined by the vertical datum specified.
AT = 2 building floors of altitude.
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
Polk, et al. Standards Track [Page 15]
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associated vertical datum for NAD83
Any additional LCI datum(s) to be defined for use via the DHCPv4 or
DHCPv6 Options defined in this document MUST be done through a
Standards Track RFC.
This document defines the Ver field for the DHCPv4 and DHCPv6
Options, with values as follows:
0: DHCPv4 Implementations conforming to [RFC3825]
1: Implementations of this specification (for both DHCPv4 and DHCPv6)
Any additional Ver field values to be defined for use with the DHCPv4
or DHCPv6 Options MUST be done through a Standards Track RFC.
5. Acknowledgments
The authors would like to thank Randall Gellens, Patrik Falstrom,
Ralph Droms, Ted Hardie, Jon Peterson, 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.
[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.
Polk, et al. Standards Track [Page 16]
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[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
[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.
[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.
Polk, et al. Standards Track [Page 17]
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Appendix A. GML Mapping
The GML representation of a decoded DHCP option depends on what
fields are specified. The DHCP format for location logically
describes a geodetic prism, rectangle, or point, depending on whether
Altitude and uncertainty values are provided. In the absence of
uncertainty information, the value decoded from the DHCP form can be
expressed as a single point; this is true regardless of whether the
version 0 or version 1 interpretations of the uncertainty fields are
used. If the point includes Altitude, it uses a three dimensional
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 of the DHCPv4
option, 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 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"
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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,
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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-dimentional 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 fields represent
resolution, 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, the LatUnc, LongUnc
and AltUnc fields indicate uncertainty distances.
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), while the
second section addresses the "uncertainty encoding" (version 1).
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) 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
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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) 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) directly represent the logarithms of uncertainty
distances. So the low and high bounds are computed by first
computing the uncertainty distances, then adding and subtracting
these from the value provided. If "uncertainty" is the unsigned
integer value of the uncertainty field and "value" is the value of
the coordinate field:
distance = 2 ^ (8 - uncertainty)
lowvalue = value - distance
highvalue = value + distance
Altitude uncertainty (AltUnc in version 1) uses the same process with
different constants:
distance = 2 ^ (21 - uncertainty)
lowvalue = value - distance
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Appendix B. Calculations of Resolution
The following examples for two different locations demonstrate how
the Resolution values for Latitude, Longitude, and Altitude (used in
the version 0 DHCPv4 option) can be calculated. In both examples,
the geo-location values were derived from maps using the WGS84 map
datum, therefore in these examples, the Datum field would have a
value = 1 (00000001, or 0x01).
B.1. Location Configuration Information of "White House" (Example 1)
The grounds of the White House in Washington D.C. (1600 Pennsylvania
Ave. NW, Washington, DC 20006) can be found between 38.895375 and
38.898653 degrees North and 77.037911 and 77.035116 degrees West. In
this example, we assume that we are standing on the sidewalk on the
north side of the White House, between driveways. Since we are not
inside a structure, we assume an Altitude value of 15 meters,
interpolated from the US Geological survey map, Washington Washington
West quadrangle.
The address was NOT picked for any political reason and can easily be
found on the Internet or mapping software, but was picked as an
easily identifiable location on our planet.
In this example, the requirement of emergency responders in North
America via their NENA Model Legislation [NENA] could be met by a
LatUnc value of 21 and a LongUnc value of 20. This would yield a
geo-location that is Latitude 38.8984375 north to Latitude 38.8988616
north and Longitude -77.0371094 to Longitude -77.0375977. This is an
area of approximately 89 feet by 75 feet or 6669 square feet, which
is very close to the 7000 square feet requested by NENA. In this
example, a service provider could enforce that a device send 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 encoding as follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code (123) | OptLen (16) | LatUnc | 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 | .
.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 | 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 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.
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The following example uses all of the decoded information from the
previous example:
<gs:Prism srsName="urn:ogc:def:crs:EPSG::4979"
xmlns:gs="http://www.opengis.net/pidflo/1.0";
xmlns:gml="http://www.opengis.net/gml";>
<gs:base>
<gml:Polygon>
<gml:exterior>
<gml:LinearRing>
<gml:posList>
38.8964844 -77.0390625 0
38.8964844 -77.0351563 0
38.8984375 -77.0351563 0
38.8984375 -77.0390625 0
38.8964844 -77.0390625 0
</gml:posList>
</gml:LinearRing>
</gml:exterior>
</gml:Polygon>
</gs:base>
<gs:height uom="urn:ogc:def:uom:EPSG::9001">
32
</gs:height>
</gs:Prism>
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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 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 = 30, 0x1e, 011110
AT = 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 as value 18 (0x12 or 010010) and LongUnc 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.).
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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 as well as the version 1 DHCPv4 option)
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 (AT = 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:
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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 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) |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].
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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>
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<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).
Polk, et al. Standards Track [Page 29]
INTERNET-DRAFT DHCP Option for Coordinate LCI 7 March 2010
Appendix D. Changes from RFC 3825
Technical changes:
-09: Updated and shortened Appendix B.1.
-08: Added Appendix A on GML mapping.
-06: Added recommendation on link layer confidentiality to the
Security Considerations section.
-05: Added version field to DHCPv6 option.
-04: Added Appendix C 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
a version 1 option and added recommendations for clients and
servers.
-02: Added Section 1.2 introducing uncertainty and resolution
concepts. Added Section 2.1 defining DHCPv6 option format.
-01: Within Section 2.1, split Datum field from RFC 3825 into three
fields: Ver, Res and Datum fields. Explained that the Ver
field is always located at the same offset. Added Section 2.2
relating to Version Support.
-00: None
Editorial changes:
-07: Updated boilerplate, cleaned up security considerations section.
-06: Added corrections to Section 1.2 "Resolution and Uncertainty".
Added the DHCPv6 Option Version field to the IANA
Considerations section.
-05: Corrected length of DHCPv6 option. Added Key to uncertainty
figure.
-04: 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. Reorganized
Section 1.2. Added references to RFC 4119, RFC 5139 and
[GeoShape].
-03: Changed "DHC" to "DHCP" in some usages. Clarified relationship
of resolution and uncertainty to privacy.
-02: Reorganized Sections 1 and 2.
-01: Added references to IEEE 802.11y, RFC 3825.
-00: Changed boilerplate. Added B. Aboba as editor. Re-positioned
Appendix B and Acknowledgments sections. Changed reference
numbers to names, added reference to RFC 5226 (since RFC 3825
was missing a reference to RFC 2434, now obsolete), updated
references (and URLs). Updated author affiliations and email
addresses. Changed references to "the appendix" to Appendix B.
Added this appendix listing changes.
Polk, et al. Standards Track [Page 30]
INTERNET-DRAFT DHCP Option for Coordinate LCI 7 March 2010
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
Polk, et al. Standards Track [Page 31]
