Internet DRAFT - draft-ietf-geopriv-dhcp-lci-option
draft-ietf-geopriv-dhcp-lci-option
Internet Engineering Task Force J. Polk
Internet Draft J. Schnizlein
Expiration: June 8th, 2004 M. Linsner
File: draft-ietf-geopriv-dhcp-lci-option-03.txt Cisco Systems
Dynamic Host Configuration Protocol Option for
Coordinate-based Location Configuration Information
December 8th, 2003
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document specifies a Dynamic Host Configuration Protocol Option
for the coordinate-based geographic location of the client. The
Location Configuration Information (LCI) includes latitude,
longitude, and altitude, with resolution indicators for each. The
reference datum for these values is also included.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Conventions . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Rationale . . . . . . . . . . . . . . . . . . . . . . . 4
2. Location Configuration Information (LCI) Elements . . . . . . 4
2.1 Elements of the Location Configuration Information . . . 5
3. Security Considerations . . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
Appendix Calculations of Imprecision possible with the DHC LCI . 9
A.1 LCI of "White House" (Example 1) . . . . . . . . . . . . 9
A.2 LCI of "Sears Tower" (Example 2) . . . . . . . . . . . . 12
6. Normative References . . . . . . . . . . . . . . . . . . . . 12
7. Informational References . . . . . . . . . . . . . . . . . . 13
8. Author Information . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
This document specifies a Dynamic Host Configuration Protocol [1]
Option for coordinate-based geographic location of the client, to be
provided by the server.
The DHCP server is assumed to have determined the location from the
Circuit-ID Relay Agent Information Option (RAIO) defined (as SubOpt
1) in [2]. In order to translate the circuit (switch port
identifier) into a location, the DHCP server is assumed to have
access to a service that maps from circuit-ID to the location at
which the circuit connected to that port terminates in the building;
for example, the location of the wall jack.
An important feature of this specification is that after the
relevant DHC 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.
Another important feature of this LCI is its inclusion of a
resolution parameter for each of the dimensions of location.
Because this resolution parameter need not apply to all dimensions
equally, a resolution value is included for each of the 3 location
elements.
This resolution method provides a natural ability for the device to
hide from the center point of the bounding area as this resolution
method is determined via the inherent effects of binary
representation; or, this resolution mechanism could be used to
define a geographic area. This would be useful when a group of
clients would want to be known as the same geo-location, possibly
all users in a room of a building would use the same LCI value.
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Then the using application could use that value as a key for lookup
in another data source. This is similar to one of the mechanisms
utilized in the North American E911 systems today.
Resolution does not define how Geographic Privacy policy should
relate to precision.
The resulting location information using this resolution method is a
small fixed length Configuration Information that can be easily
carried in protocols, such as DHCP, which have limited packet size
because this LCI is only 18 bytes long.
Finally, the appendix this document provides some arithmetic
examples of the implication of different resolution values on the
La/Lo/Alt.
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 [3].
1.2 Motivation
As applications such as IP Telephony are replacing conventional
telephony, users are expecting the same (or greater) level of
services with the new technology. One service offered by
conventional telephony that is missing, in any standardized fashion,
within IP Telephony is for a user to be automatically located by
emergency responders, in a timely fashion, when the user summons
help (by dialing 911 in North America, for example). Unless strict
administrative rules are followed, the mobility of a wired Ethernet
device within a campus negates any opportunity for an emergency
responder to locate the device with any degree of expediency. Users
do not want to give up the mobility IP Telephony offers. Informing
the host device of its geo-location at host configuration time will
allow the device to utilize this geo-location information to inform
others of it's current geo-location, if the user and/or application
so desires.
The goal of this option is to enable a wired Ethernet host to
obtain its location, which it could provide to an emergency
responder, as one example.
Wireless hosts can utilize this option to gain knowledge of the
location of the radio access point used during host configuration,
but would need some more exotic mechanisms, maybe GPS, or maybe a
future DHCP option, which includes a list of geo-locations like that
defined here, containing the locations of the radio access points
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that are close to the client.
1.3 Rationale
Within the LCI described here, Latitude and Longitude are
represented in fixed-point 2s-complement binary degrees, for the
economy of a smaller option size compared to a string encoding of
digits in [7]. The integer parts of these fields are 9 bits long to
accommodate +/- 180 degrees. The fractional part is 25 bits long,
better than the precision of 7 decimal digits. Each parameter is 40
bits total, in length.
Altitude is determined by the Altitude Type (AT) indicated by the
AT field, which is 4 bits long. Two Altitude Types are defined
here, meters (code=1) and floors (code=2), both of which are 2s-
complement fixed-point with 8 bits of fraction. Additional
Altitude Types MAY be assigned by IANA. The "floors" Altitude Type
is provided because altitude in meters may not be known within a
building, and a floor indication may be more useful.
GPS systems today can use WGS84 for horizontal and vertical datums,
[6] defines WGS84 as a three-dimensional datum. For other datum
values that do not include a vertical component, both the horizontal
and vertical datum of reference will be specified in the IANA
record.
Each of these 3 elements is preceded by an accuracy sub-field of 6
bits, indicating the number of bits of resolution. This resolution
sub-field accommodates the desire by some 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.
2. DHC Location Configuration Information Elements
DHCP is a binary Protocol; using protocols of LCI are likely to be
text based. Since most coordinate systems translate fairly easily
between binary-based and text-based location output (even emergency
services within the US), translation/conversion is a non-issue with
DHCP's binary format.
This binary format provides a fortunate benefit in a mechanism for
making a true/correct location coordinate imprecise. It further
provides the capability to have this binary representation be
deterministically imprecise.
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The LCI 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 TBD | 16 | LaRes | Latitude +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Latitude (cont'd) | LoRes | +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Longitude |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AT | AltRes | Altitude |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Alt (cont'd) | Datum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.1 Elements of the Location Configuration Information
Code TBD: The code for this DHCP option.
16: The length of this option is 16 bytes.
LaRes: Latitude resolution. 6 bits indicating the number
of valid bits in the fixed-point value of Latitude.
This value is 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 sending.
The examples below in section 4.0, are to illustrate that a smaller
value in the resolution field increases the area within which the
device is located).
LaRes does not define how Geographic Privacy policy should relate to
precision.
Values of resolution above decimal 34 are Undefined and reserved
because that is the largest number of bits in the Latitude field.
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.
A value of 2 in the LaRes field indicates a precision of no greater
than 1/6th that of the globe (detailed in the first example in
section 4.0). A value of 34 in the LaRes field indicates a
precision of about 3.11 mm in Latitude at the equator.
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LoRes: Longitude resolution. 6 bits indicating the number of
valid bits in the fixed-point value of Longitude.
This value is 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 sending.
LoRes does not define how Geographic Privacy policy should relate to
precision.
Values above decimal 34 are undefined and reserved.
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 (2s complement)
numbers are West of the prime meridian.
A value of 2 in the LoRes field indicates precision of no greater
than 1/6th that of the globe (see first example in section 4.0). A
value of 34 in the LoRes field indicates a precision of about
2.42 mm in longitude (at the equator). Because lines of longitude
converge at the poles, the distance is smaller (better precision)
for locations away from the equator.
Altitude: A 30 bit value defined by the AT field
AltRes: Altitude resolution. 6 bits indicating the number of valid
bits in the altitude. Values above 30 (decimal) are
undefined and reserved.
AltRes does not define how Geographic Privacy policy should relate
to precision.
AT: Altitude Type for altitude. Codes defined are:
1: Meters - in 2s-complement fixed-point 22-bit integer part with
8-bit fraction
If AT = 1, an AltRes value 0.0 would indicate unknown altitude.
The most precise Altitude would have an AltRes value of 30. Many
values of AltRes would obscure any variation due to vertical datum
differences.
2: Floors - in 2s-complement fixed-point 22-bit integer part with
8-bit fraction
AT = 2 for Floors enables representing altitude in a form more
relevant in buildings which have different floor-to-floor
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dimensions. 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. Inside a building, 0.0
represents the floor level associated with ground level at the main
entrance. This document defines a number; one must arrive at the
number by counting floors from the floor defined to be 0.0.
The values represented by this AT will be of local significance.
Since buildings and floors can vary due to lack of common control,
the values chosen to represent the characteristics of an individual
building will be derived and agreed upon by the operator of the
building and the intended users of the data. Attempting to
standardize this type of function is beyond the scope this document.
Sub-floors can be represented by non-integer values. Example: a
mezzanine between floor 1 and floor 2 could be represented as a
value=1.1. Example: (2) mezzanines between floor 4 and floor 5
could be represented as values=4.1 and 4.2 respectively.
Floors located below ground level could be represented by negative
values.
Larger values represent floors that are above (higher in altitude)
floors with lower values.
The AltRes field SHOULD be set to maximum precision when AT = 2
(floors) when a floor value is included in the DHCP Reply, or
the AT = 0 to denote the floor isn't known.
Any additional LCI Altitude Types(s) to be defined for use via
this DHC Option MUST be done through a Standards Track RFC.
Datum: The Map Datum used for the coordinates given in this Option
The datum must include both a horizontal and a vertical reference.
Since the WGS 84 reference datum is three-dimensional, it includes
both. For any additional datum parameters, the datum codepoint must
specify both horizontal datum and vertical datum references.
The Datum byte has 256 possibilities, of which 3 are to be
registered with IANA by this document (all derived from
specification in [5]):
1: WGS 84 (Geographical 3D) - World Geodesic System 1984, CRS
Code 4327, Prime Meridian Name: Greenwich
2: NAD83 - North American Datum 1983, CRS Code 4269, Prime
Meridian Name: Greenwich; The associated vertical
datum is the North American Vertical Datum of 1988
(NAVD88)
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This datum pair to be used when referencing
locations on land, not near tidal water (which would
use Datum = 3 below)
3: NAD83 - North American Datum 1983, CRS Code 4269, Prime
Meridian Name: Greenwich; The associated vertical
datum is Mean Lower Low Water (MLLW)
This datum pair to be used when referencing
locations on water/sea/ocean
Any additional LCI datum(s) to be defined for use via this DHC
Option MUST be done through a Standards Track RFC.
3. Security Considerations
Where critical decisions might be based on the value of this
GeoLoc option, DHCP authentication in [4] 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 [1]).
When implementing a DHC server that will serve clients across an
uncontrolled network, one should consider the potential security
risks.
4. IANA Considerations
IANA has assigned a DHCP option code of TBD for the GeoLoc option
defined in this document.
The GeoLoc Option defines two fields for which IANA is to maintain
a registry: The Altitude (AT) field (see Section 2) 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"
[RFC 2434]. The initial values of the Altitude registry are as
follows:
AT = 1 meters of Altitude defined by the vertical datum
specified.
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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 associated vertical datum for NAD83
Any additional LCI datum(s) to be defined for use via this DHC
Option MUST be done through a Standards Track RFC.
5. Acknowledgements
The authors would like to thank 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, and to Carl Reed.
Appendix: Calculations of Imprecision possible with the DHC LCI
The following examples for two different locations demonstrate
how the Resolution values for Latitude, Longitude and Altitude
can be used. 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).
A.1 Location Configuration Information of "White House" (Example 1)
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.
Postal Address:
White House
1600 Pennsylvania Ave. NW
Washington, DC 20006
Standing on the sidewalk, north side of White House, between
driveways.
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Latitude 38.89868 degrees North (or +38.89868 degrees)
Using 2s complement, 34 bit fixed point, 25 bit fraction
Latitude = 0x04dcc1fc8,
Latitude = 0001001101110011000001111111001000
Longitude 77.03723 degrees West (or -77.03723 degrees)
Using 2s complement, 34 bit fixed point, 25 bit fraction
Longitude = 0xf65ecf031,
Longitude = 1101100101111011001111000000110001
Altitude 15
In this example we are not inside a structure, therefore we will
assume an altitude value of 15 meters, interpolated from the US
Geological survey map, Washington West quadrangle.
AltRes = 30, 0x1e, 011110
AT = 1, 0x01, 000001
Altitude = 15, 0x0F00, 00000000000000000000000001111100000000
If: LaRes is expressed as value 2 (0x02 or 000010) and LoRes is
expressed as value 2 (0x02 or 000010), then it would describe a
geo-location region that is north of the equator and extends
from -1 degree (west of the meridian) to -128 degrees. This
would include the area from approximately 600km south of
Saltpond, Ghana, due north to the North Pole and approximately
4400km south-southwest of Los Angeles, CA due north to the North
Pole. This would cover an area of about one-sixth of the globe,
approximately 20 million square nautical miles (nm).
If: LaRes is expressed as value 3 (0x03 or 000011) and LoRes is
expressed as value 3, (0x03 or 000011) then it would describe a
geo-location area that is north from the equator to 63 degrees
north, and -65 degrees to -128 degrees longitude. This area
includes south of a line from Anchorage, AL to eastern Nunavut,
CN. and from the Amazons of northern Brazil to approximately
4400km south-southwest of Los Angeles, CA. This area would
include North America, Central America, and parts of Venezuela
and Columbia, except portions of Alaska and northern and eastern
Canada, approximately 10 million square nm.
If: LaRes is expressed as value 5 (0x05 or 000101) and LoRes is
expressed as value 5 (0x05 or 000101), then it would describe a
geo-location area that is latitude 32 north of the equator to
latitude 48 and extends from -64 degrees to -80 degrees
longitude. This is approximately an east-west boundary of a
time zone, an area of approximately 700,000 square nm.
If: LaRes is expressed as value 9 (0x09 or 001001) and LoRes is
expressed as value 9 (0x09 or 001001), which includes all the
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integer bits, then it would describe a geo-location area that is
latitude 38 north of the equator to latitude 39 and extends from
-77 degrees to -78 degrees longitude. This is an area of
approximately 9600 square km (111.3km x 86.5km).
If: LaRes is expressed as value 18 (0x12 or 010010) and LoRes is
expressed as value 18 (0x12 or 010010), then it would describe a
geo-location area that is latitude 38.8984375 north to latitude
38.9003906 and extends from -77.0390625 degrees to -77.0371094
degrees longitude. This is an area of approximately 36,600
square meters (169m x 217m).
If: LaRes is expressed as value 22 (0x16 or 010110) and LoRes is
expressed as value 22 (0x16 or 010110), then it would describe a
geo-location area that is latitude 38.896816 north to latitude
38.8985596 and extends from -77.0372314 degrees to -77.0371094
degrees longitude. This is an area of approximately 143 square
meters (10.5m x 13.6m).
If: LaRes is expressed as value 28 (0x1c or 011100) and LoRes is
expressed as value 28 (0x1c or 011100), then it would describe a
geo-location area that is latitude 38.8986797 north to latitude
38.8986816 and extends from -77.0372314 degrees to -77.0372296
degrees longitude. This is an area of approximately 339 square
centimeters (20.9cm x 16.23cm).
If: LaRes is expressed as value 30 (0x1e or 011110) and LoRes is
expressed as value 30 (0x1e or 011110), then it would describe a
geo-location area that is latitude 38.8986797 north to latitude
38.8986802 and extends from -77.0372300 degrees to -77.0372296
degrees longitude. This is an area of approximately 19.5 square
centimeters (50mm x 39mm).
If: LaRes is expressed as value 34 (0x22 or 100010) and LoRes is
expressed as value 34 (0x22 or 100010), then it would describe a
geo-location area that is latitude 38.8986800 north to latitude
38.8986802 and extends from -77.0372300 degrees to -77.0372296
degrees longitude. This is an area of approximately 7.5 square
millimeters (3.11mm x 2.42mm).
In the (White House) example, the requirement of emergency
responders in North America via their NENA Model Legislation [8],
could be met by a LaRes value of 21 and a LoRes value of 20.
This would yield a geo-location that is latitude 38.8984375 north
to latitude 38.8988616 north and longitude -77.0371094 to
longitude -77.0375977. This is an area of approximately 89 feet
by 75 feet or 6669 square feet, which is very close to the 7000
square feet asked for 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.
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A.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 AltRes
field would indicate that all bits in the Altitude field are
true, as we want to accurately represent the floor of the
structure where we are located.
AltRes = 30, 0x1e, 011110
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 LaRes as value 18 (0x12 or 010010)
and LoRes as value 18 (0x12 or 010010). This would be describing
a geo-location area that is latitude 41.8769531 to latitude
41.8789062 and extends from -87.6367188 degrees to -87.6347657
degrees longitude. This is an area of approximately 373412
square feet (713.3 ft. x 523.5 ft.).
6. Normative References
[1] Droms R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997
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[2] Patrick M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001
[3] Bradner S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997
[4] Droms R., "Authentication for DHCP Messages", RFC 3118, June
2001
[5] European Petroleum Survey Group, http://www.epsg.org/ and
http://www.ihsenergy.com/epsg/geodetic2.html
[6] World Geodetic System 1984 (WGS 84), MIL-STD-2401,
http://164.214.2.59/publications/specs/printed/WGS84/wgs84.html
and http://www.wgs84.com/
7. Informational References
[7] Farrell C., Schulze M., Pleitner S. and Baldoni D., "DNS
Encoding of Geographical Location", RFC 1712, November 1994.
[8] National Emergency Number Association (NENA) www.nena.org
NENA Technical Information Document on Model Legislation
Enhanced 911 for Multi-Line Telephone Systems
(http://www.nena.org/9%2D1%2D1techstandards/TechInfoDocs/
MLTS_ModLeg_Nov200.PDF)
8. Author Information
James M. Polk
Cisco Systems
2200 East President George Bush Turnpike
Richardson, Texas 75082 USA jmpolk@cisco.com
John Schnizlein
Cisco Systems
9123 Loughran Road
Fort Washington, MD 20744 USA john.schnizlein@cisco.com
Marc Linsner
Cisco Systems
Marco Island, FL 34145 USA marc.linsner@cisco.com
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The limited permissions granted above are perpetual and will not
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The Expiration date for this Internet Draft is:
June 8th, 2004
Polk & Schnizlein & Linsner [Page 14]
