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Versions: 00

Internet Engineering Task Force                              James M. Polk
Internet Draft                                             John Schnizlein
Expiration: July 17th, 2003                                   Marc Linsner
File: draft-ietf-geopriv-dhcp-lo-option-00.txt               Cisco Systems








                   DHC Location Object within GEOPRIV

                          January 17th, 2003




Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

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.






Abstract

This document specifies a Dynamic Host Configuration Protocol Option for
the geographic location of the client. The location object includes
latitude, longitude, and altitude, with resolution indicators for each.







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Table of Contents

Abstract  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1
Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . .  2
1.0  Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .  2
1.1  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
1.2  Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
1.3  Rationale  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
2.0  DHC Location Object Elements . . . . . . . . . . . . . . . . . . .  5
 2.1 Elements of the Location Object  . . . . . . . . . . . . . . . . .  5
3.0  Purpose of Resolution Value per La/Lo/Alt Element  . . . . . . . .  7
4.0  Security Considerations  . . . . . . . . . . . . . . . . . . . . .  7
5.0  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . . .  7
6.0  References . . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
7.0  Author Information . . . . . . . . . . . . . . . . . . . . . . . .  8
Appendix Calculations of Imprecision possible with the DHC LO . . . . .  8
 A.1 Location Object of "White House" (Example 1) . . . . . . . . . . .  8
 A.2 Location Object of "Sears Tower" (Example 2) . . . . . . . . . . . 11


1.0 Introduction

This document specifies a Dynamic Host Configuration Protocol [1] Option
for the 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.


The Location Object (LO) format presented here could be considered a
subset of a larger GEOPRIV LO. Achieving a core set of LO elements is
desired across multiple Protocols which can convey location information.

An important feature of this document is it places the LO completely under
control of the end device rather than storing the object in an outside
service for retrieval by the end device. Storage outside the end device
during times of emergency can cause unnecessary delay, or failure during
communication.

Another important feature of the LO is its inclusion of a resolution
parameter for each of the dimensions of location. The GEOPRIV working
group has a stated requirement [3] to enable decreasing the precision of a
location element. Because this resolution parameter need not apply to all
dimensions equally, a resolution value is included for each of the 3
location elements. GEOPRIV actually calls their requirement: Accuracy. We


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use the term and meaning "Resolution" as being more precise for the intent
here.

When comparing the resolution method described here with other methods to
express geo-location accuracy, the result of this resolution method
describes a region (normally trapezoidal) rather than the possible (error)
distance from the indicated location.  There is good reason not to
describe geo-location accuracy as a potential error in distance from the
latitude, longitude, and altitude values.

The error distance method describes a circular area with a center point.
If this method were used for a LO and the end device wanted to 'hide' from
the center point, it would need to recalculate a location from the stated
center point that remains in the stated circle. This recalculation could
be problematic for the end device depending on that deviceÆs geo-location
computing framework. By using the resolution method described here, the
end device could simply express a lesser resolution value to provide a
less precise location. Or, the end device may or may not need to be
involved with the resolution policy at all, as this could be dictated by
another entity, perhaps that device's domain, simply by providing the end
device with a location with the resolution of the desired value.

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 affects of binary mathematics, using the power
of 2.

The resulting LO using this resolution method is a small fixed length
object that can be easily stored in memory and be easily appended within
other protocols with little regards to packet size as the LO is only 15
bytes long.

Finally, in the appendix this document provides some arithmetic examples
of just how the imprecision can be introduced in any or all of the
La/Lo/Alt values without the IP device needing to be preprogrammed with
bogus LOs, and just how imprecise the La/Lo/Alt values can be.

This document does not cover any policy regarding the potential use of
this other than a few as potential suggestions to convey the meaning
intended by the document.


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 [4].






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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 provide its
location 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 will need
some more exotic mechanisms, maybe GPS, or maybe a future DHCP option,
which includes a list of geo-loc objects like that defined here, which has
the locations of the radio access points that are close to the client.


1.3 Rationale

Within the LO described here, Latitude and Longitude are represented in
fixed-point 2s-complement binary degrees, for the economy of a smaller
option size compared to the string encoding of digits in [5].  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 represented in measurement units (MU) indicated by the MU
field, which is 4 bits long. Two measurement units are defined here,
meters (code=1) and floors (code=2), both of which are 2s-complement
fixed-point with 8 bits of fraction. Additional measurement units MAY be
assigned by IANA. The floor of a building is often the relevant location
information, and not necessarily computable from meters of altitude.

Each of these 3 variables is preceded by an accuracy sub-field of 6 bits,
indicating the number of bits of resolution. This resolution sub-field
accommodates the GEOPRIV requirement [3] 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.






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2.0 DHC Location Object Elements

DHCP is a binary Protocol; GEOPRIV is text-based. Most coordinate systems
translate fairly easily between binary-based and text-based location
output (i.e. even emergency services within the US). The authors believe
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.


The proposed LO format is:

      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    |      15       |   LaRes   |     Latitude      +
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                Latitude (cont'd)              |    LoRes  |   +
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Longitude                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   MU  |   AltRes  |                Altitude                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Alt (cont'd) |
      +-+-+-+-+-+-+-+-+


2.1 Elements of the Location Object

   Code TBD: The code for this DHCP option is TBD by IANA.

   15:       The length of this option is 15 bytes.

   LaRes:    Latitude resolution. 6 bits indicating the valid number of
             valid bits in the fixed-point value of Latitude.

This value is a length of the (left to right) number of 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 device (meaning the GEOPRIV target towards the requestor).

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 (without deception).

Values of resolution above decimal 34 are currently Undefined and reserved
because that is the largest number of bits in the Latitude variable.



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   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. Geo-location formats provide for
             positive numbers to be north of the equator and negative
             numbers to be 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 within 3.11 mm in
Latitude.

   LoRes:    Longitude resolution. 6 bits indicating the number of valid
             bits in the fixed-point value of Longitude.

This value is a length value of the (left to right) number of 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 device (meaning the GEOPRIV target towards the
requestor).

             Values above decimal 34 are currently 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. Geo-location formats provide for
             positive numbers to be east of the prime meridian and
             negative numbers to be west of the prime meridian.

Entering a value of 2 in the LoRes field will result in the precision of
no greater than 1/6th that of the globe (see first example in section 4.0
for more here). A value of 34 in the LoRes field indicates a precision
within 2.42 mm in longitude (at the equator). Because lines of longitude
converge at the poles, the distance is smaller (resolution greater) for
locations away from the equator.

   AltRes:   Altitude resolution. 6 bits indicating the number of valid
             bits in the altitude. Values above 30 decimal are undefined
             and reserved.

   MU: Measurement unit for altitude. Codes defined here are:

     1: Meters - in 2s-complement fixed-point 22-bit integer part with
                 8-bit fraction

If MU = 1, then a completely imprecise Altitude would be of AltRes value
0. If the Altitude is known exactly, then the AltRes can be value 30 (or
all ones).

     2: Floors - in 2s-complement fixed-point 22-bit integer part with
                 8-bit fraction



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MU = 2 for Floors was chosen because of the impreciseness of the number of
meters any one floor is above/below the ground floor, or above mean low
tide (the Altitude datum within WGS84). What is fairly understood is which
If MU=2, AltRes = 30, and Altitude = 0, that represents the ground
regardless if there is a building present or not (as well as the ground
floor of a building). This is important because the precise altitude might
not be known, but it is likely known what is the ground level. The non-
obvious case is in a hilly or mountainous area, these values still
represent being on the ground level at the La/Lo coordinates provided.


3.0 Purpose of Resolution Value per La/Lo/Alt Element

GEOPRIV specified [3] the requirement that any location expressed from or
proxied on behalf of a device through the GEOPRIV Protocol can have the
accuracy or precision of that device's location limited. The owner of the
device, or the domain of the device determines the policy for divulging
how precise the location is for any/all given requesters of that device's
location.

One aspect within the GEOPRIV WG is the precision of a device's ability to
present its location coordinates might have a domain policy override the
individual policy in the sense of maximum resolution possible. In other
words, a user of a device might not mind providing a quite precise
location return to a location request, but the local domain might not want
that level of precision by its policy. In this case, the resolution value
provided in the DHCP Reply can set this maximum precision value, perhaps
allowing the user of the Target device to make the values more imprecise
based on who is requesting their location. The document does not go
further down this thought for good reason.


4.0 Security Considerations

Where critical decisions might be based on the value of this GeoLoc
option, DHCP authentication in [7] SHOULD be used to protect the integrity
of the DHCP options.


5.0 IANA Considerations

The DHCP option code for the GeoLoc option is TBD.


6.0 References

 [1] Droms R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997

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



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 [3] Cuellar J., Morris J., Mulligan D., "GEOPRIV Requirements", Internet
     Draft, "Work in Progress", June 2002

 [4] Bradner S., "Key words for use in RFCs to Indicate Requirement
     Levels", RFC 2119, March 1997

 [5] Farrell C., Schulze M., Pleitner S. and Baldoni D., "DNS Encoding of
     Geographical Location", RFC 1712, November 1994.

 [6] NENA û National Emergency Number Association û 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)

 [7] Droms R., "Authentication for DHCP Messages", RFC 3118, June 2001


7.0 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


Appendix: Calculations of Imprecision possible with the DHC LO

The following examples for two different locations demonstrate how the
Resolution values for Latitude, Longitude and Altitude can be used.

A.1 Location Object 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.




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Postal Address:
  White House
  1600 Pennsylvania Ave. NW
  Washington, DC 20006

Standing on the sidewalk, north side of White House, between driveways.

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 û77.03723 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
    MU = 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
    û1 degree (west of the meridian) to û128 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 û128 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 û64 degrees to û80 degrees longitude.
    This is approximately an east-west boundary of a time zone, an area


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Internet Draft       DHCP Option for Geographic Location    Jan 17th, 2002

    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
    integer bits, then it would describe a geo-location area that
    is latitude 38 north of the equator to latitude 39 and extends from
    û77 degrees to û78 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 û77.0390625 degrees to û77.0371094
    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 û77.0372314 degrees to û77.0371094
    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 û77.0372314 degrees to û77.0372296
    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 û77.0372300 degrees to û77.0372296
    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 û77.0372300 degrees to û77.0372296
    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 [6], 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 û77.0371094 to longitude û77.0375977.  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


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provider could enforce that a device send a Location Object with this
minimum amount of resolution for this particular location when calling
emergency services.


A.2 Location Object of "Sears Tower" (Example 2)

Postal Address:
Sears Tower
103th 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 û87.63602 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 measurement
  unit 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
    MU = 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 û87.6367188 degrees to û87.6347657
degrees longitude.  This is an area of approximately 373412 square feet
(713.3 ft. x 523.5 ft.).


"Copyright (C) The Internet Society (February 23rd, 2001).
All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published and


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Internet Draft       DHCP Option for Geographic Location    Jan 17th, 2002

distributed, in whole or in part, without restriction of any kind,
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The Expiration date for this Internet Draft is:

July 17th, 2003























Polk/Schnizlein/Linsner                                            Page 12


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