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2 GEOPRIV Working Group J. Polk
3 INTERNET-DRAFT M. Linsner
4 Obsoletes: 3825 (if approved) Cisco Systems
5 Category: Standards Track M. Thomson
6 Expires: August 26, 2011 Andrew Corporation
7 26 February 2011 B. Aboba (ed)
8 Microsoft Corporation
10 Dynamic Host Configuration Protocol Options for
11 Coordinate-based Location Configuration Information
13 draft-ietf-geopriv-rfc3825bis-17.txt
15 Abstract
17 This document specifies Dynamic Host Configuration Protocol Options
18 (both DHCPv4 and DHCPv6) for the coordinate-based geographic location
19 of the client. The Location Configuration Information (LCI) includes
20 Latitude, Longitude, and Altitude, with resolution or uncertainty
21 indicators for each. Separate parameters indicate the reference
22 datum for each of these values. This document obsoletes RFC 3825.
24 Status of This Memo
26 This Internet-Draft is submitted in full conformance with the
27 provisions of BCP 78 and BCP 79.
29 Internet-Drafts are working documents of the Internet Engineering
30 Task Force (IETF), its areas, and its working groups. Note that
31 other groups may also distribute working documents as Internet-
32 Drafts.
34 Internet-Drafts are draft documents valid for a maximum of six months
35 and may be updated, replaced, or obsoleted by other documents at any
36 time. It is inappropriate to use Internet-Drafts as reference
37 material or to cite them other than as "work in progress."
39 The list of current Internet-Drafts can be accessed at
40 http://www.ietf.org/ietf/1id-abstracts.txt.
42 The list of Internet-Draft Shadow Directories can be accessed at
43 http://www.ietf.org/shadow.html.
45 This Internet-Draft will expire on August 26, 2011.
47 Copyright Notice
49 Copyright (c) 2011 IETF Trust and the persons identified as the
50 document authors. All rights reserved.
52 This document is subject to BCP 78 and the IETF Trust's Legal
53 Provisions Relating to IETF Documents
54 (http://trustee.ietf.org/license-info) in effect on the date of
55 publication of this document. Please review these documents
56 carefully, as they describe your rights and restrictions with respect
57 to this document. Code Components extracted from this document must
58 include Simplified BSD License text as described in Section 4.e of
59 the Trust Legal Provisions and are provided without warranty as
60 described in the BSD License.
62 This document may contain material from IETF Documents or IETF
63 Contributions published or made publicly available before November
64 10, 2008. The person(s) controlling the copyright in some of this
65 material may not have granted the IETF Trust the right to allow
66 modifications of such material outside the IETF Standards Process.
67 Without obtaining an adequate license from the person(s) controlling
68 the copyright in such materials, this document may not be modified
69 outside the IETF Standards Process, and derivative works of it may
70 not be created outside the IETF Standards Process, except to format
71 it for publication as an RFC or to translate it into languages other
72 than English.
74 Table of Contents
76 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
77 1.1. Conventions . . . . . . . . . . . . . . . . . . . . . . 5
78 1.2 Resolution and Uncertainty . . . . . . . . . . . . . . . 5
79 2. DHCP Option Formats . . . . . . . . . . . . . . . . . . . . . 6
80 2.1 DHCPv6 GeoLoc Option . . . . . . . . . . . . . . . . . . 6
81 2.2 DHCPv4 Options . . . . . . . . . . . . . . . . . . . . . 8
82 2.3 Latitude and Longitude Fields . . . . . . . . . . . . . 11
83 2.4 Altitude . . . . . . . . . . . . . . . . . . . . . . . . 14
84 2.5 Datum . . . . . . . . . . . . . . . . . . . . . . . . . 16
85 3. Security Considerations. . . . . . . . . . . . . . . . . . . . 17
86 4. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 17
87 4.1 DHCP Options . . . . . . . . . . . . . . . . . . . . . . 17
88 4.2 Altitude Type Registry . . . . . . . . . . . . . . . . . 18
89 4.3 Datum Registry . . . . . . . . . . . . . . . . . . . . . 18
90 4.4 GeoLoc Option Version Registry . . . . . . . . . . . . . 19
91 5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
92 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
93 6.1. Normative References . . . . . . . . . . . . . . . . . . 20
94 6.2. Informational References . . . . . . . . . . . . . . . . 21
95 Appendix A. GML Mapping . . . . . . . . . . . . . . . . . . . . . 23
96 A.1. GML Templates . . . . . . . . . . . . . . . . . . . . . 23
97 Appendix B. Calculations of Resolution . . . . . . . . . . . . . . 26
98 B.1. LCI of "White House" (Example 1) . . . . . . . . . . . . 27
99 B.2. LCI of "Sears Tower" (Example 2) . . . . . . . . . . . . 29
100 Appendix C. Calculations of Uncertainty . . . . . . . . . . . . . 30
101 C.1 LCI of "Sydney Opera House" (Example 3) . . . . . . . . 30
102 Appendix D. Changes from RFC 3825 . . . . . . . . . . . . . . . . 34
103 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35
105 1. Introduction
107 The physical location of a network device has a range of
108 applications. In particular, emergency telephony applications rely
109 on knowing the location of a caller in order to determine the correct
110 emergency center.
112 The location of a device can be represented either in terms of
113 geospatial (or geodetic) coordinates, or as a civic address.
114 Different applications may be more suited to one form of location
115 information; therefore, both the geodetic and civic forms may be used
116 simultaneously.
118 This document specifies Dynamic Host Configuration Protocol v4
119 (DHCPv4) [RFC2131] and DHCPv6 [RFC3315] options for the coordinate-
120 based geographic location of the client, to be provided by the
121 server. "Dynamic Host Configuration Protocol (DHCPv4 and DHCPv6)
122 Option for Civic Addresses Configuration Information" [RFC4776]
123 specifies DHCP options for civic addresses.
125 The geodetic coordinate options defined in this document and the
126 civic address options defined in RFC 4776 [RFC4776] enable a DHCP
127 client to obtain its location. For example, a wired Ethernet host
128 might use these options for location determination. In this case,
129 the location information could be derived from a wiremap by the DHCP
130 server, using the Circuit-ID Relay Agent Information Option (RAIO)
131 defined (as Sub-Option 1) in RFC 3046 [RFC3046]. The DHCP server
132 could correlate the Circuit-ID with the geographic location where the
133 identified circuit terminates (such as the location of the wall
134 jack).
136 The mechanism defined here may also be utilized to provide location
137 to wireless hosts. DHCP relay agent sub-options (RAIO) [RFC3046] is
138 one method a DHCP server might use to perform host location
139 determination. Currently, the relay agent sub-options do not include
140 data sets required for device level location determination of
141 wireless hosts. In cases where the DHC server uses RAIO for location
142 determination, a wireless host can use this mechanism to discover
143 location of the radio access point, or the area of coverage for the
144 radio access point.
146 An important feature of this specification is that after the relevant
147 DHCP exchanges have taken place, the location information is stored
148 on the end device rather than somewhere else, where retrieving it
149 might be difficult in practice.
151 1.1. Conventions used in this document
153 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
154 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
155 document are to be interpreted as described in [RFC2119].
157 1.2. Resolution and Uncertainty
159 The DHCP options defined in this document include fields quantifying
160 the resolution or uncertainty associated with a target location. No
161 inferences relating to privacy policies can be drawn from either
162 uncertainty or resolution values.
164 As utilized in this document, resolution refers to the accuracy of a
165 reported location, as expressed by the number of valid bits in each
166 of the Latitude, Longitude and Altitude fields.
168 In the context of location technology, uncertainty is a
169 quantification of errors. Any method for determining location is
170 subject to some sources of error; uncertainty describes the amount of
171 error that is present. Uncertainty might be the coverage area of a
172 wireless transmitter, the extent of a building or a single room.
174 Uncertainty is usually represented as an area within which the target
175 is located. In this document, each of the three axes can be assigned
176 an uncertainty value. In effect, this describes a rectangular prism,
177 which may be used as a coarse representation of a more complex shape
178 that fits within it. See Section 2.3.2 for more detail on the
179 correspondence between shapes and uncertainty.
181 When representing locations from sources that can quantify
182 uncertainty, the goal is to find the smallest possible rectangular
183 prism that this format can describe. This is achieved by taking the
184 minimum and maximum values on each axis and ensuring that the final
185 encoding covers these points. This increases the region of
186 uncertainty, but ensures that the region that is described
187 encompasses the target location.
189 The DHCPv4 option formats defined in this document support resolution
190 and uncertainty parameters. The DHCPv4 GeoConf Option 123 includes a
191 resolution parameter for each of the dimensions of location. Since
192 this resolution parameter need not apply to all dimensions equally, a
193 resolution value is included for each of the three location elements.
194 The DHCPv4 GeoLoc Option TBD1 as well as the DHCPv6 GeoLoc Option
195 TBD2 format utilize an uncertainty parameter.
197 Appendix A describes the mapping of DHCP option values to the
198 Geography Markup Language (GML). Appendix B of this document
199 provides examples showing the calculation of resolution values.
200 Appendix C provides an example demonstrating calculation of
201 uncertainty values.
203 Since the Presence Information Data Format Location Object (PIDF-LO)
204 [RFC4119][RFC5491] is used to conveying location and the associated
205 uncertainty within an emergency call [Convey], a mechanism is needed
206 to convert the information contained within the DHCPv4 and DHCPv6
207 options to PIDF-LO. This document describes the following
208 conversions:
210 DHCPv4 GeoConf Option 123 to PIDF-LO
211 DHCPv4 GeoLoc Option TBD1 and DHCPv6 GeoLoc Option TBD2 to PIDF-LO
212 PIDF-LO to DHCP GeoLoc Option TBD1 and DHCPv6 GeoLoc Option TBD2
214 Conversion to PIDF-LO does not increase uncertainty; conversion from
215 PIDF-LO to the DHCPv4 GeoLoc Option TBD1 and the DHCPv6 GeoLoc Option
216 TBD2 increases uncertainty by less than a factor of 2 in each
217 dimension. Since it is not possible to translate an arbitrary PIDF-
218 LO to the DHCP GeoConf Option 123 with a bounded increase in
219 uncertainty, the conversion is not specified.
221 2. DHCP Option Formats
223 This section defines the format for the DHCPv4 and DHCPv6 options.
224 These options utilize a similar format, differing primarily in the
225 option code.
227 2.1. DHCPv6 GeoLoc Option
229 The format of the DHCPv6 [RFC3315] GeoLoc Option is as follows:
231 0 1 2 3
232 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
233 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
234 | Option Code (TBD2) | OptLen |
235 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
236 | LatUnc | Latitude +
237 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
238 | Lat (cont'd) | LongUnc | Longitude +
239 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
240 | Longitude (cont'd) | AType | AltUnc | Altitude +
241 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
242 | Altitude (cont'd) |Ver| Res |Datum|
243 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
245 Code: DHCP Option Code TBD2 (16 bits).
247 OptLen: Option Length. For version 1, the option length is 16.
249 LatUnc: 6 bits. When the Ver field = 1, this field represents
250 latitude uncertainty. The contents of this field is
251 undefined for other values of the Ver field.
253 Latitude: a 34 bit fixed point value consisting of 9 bits of
254 integer and 25 bits of fraction, interpreted as
255 described in Section 2.3.
257 LongUnc: 6 bits. When the Ver field = 1, this field represents
258 longitude uncertainty. The contents of this field is
259 undefined for other values of the Ver field.
261 Longitude: a 34 bit fixed point value consisting of 9 bits of
262 integer and 25 bits of fraction, interpreted as
263 described in Section 2.3.
265 AType: Altitude Type (4 bits), defined in Section 2.4.
267 AltUnc: 6 bits. When the Ver field = 1, this field represents
268 altitude uncertainty. The contents of this field is
269 undefined for other values of the Ver field.
271 Altitude: A 30 bit value defined by the AType field, described in
272 Section 2.4.
274 Ver: The Ver field is two bits, providing for four potential
275 versions. This specification defines the behavior of
276 version 1. The Ver field is always located at the same
277 offset from the beginning of the option, regardless of
278 the version in use. DHCPv6 clients implementing this
279 specification MUST support receiving version 1
280 responses. DHCPv6 servers implementing this
281 specification MUST send version 1 responses.
283 Res: The Res field which is 3 bits, is reserved. These bits
284 have been used by [IEEE-802.11y], but are not defined
285 within this specification.
287 Datum: 3 bits. The Map Datum used for the coordinates given in
288 this Option.
290 2.2. DHCPv4 Options
292 2.2.1. DHCPv4 GeoConf Option
294 The format of the DHCPv4 GeoConf Option is as follows:
296 0 1 2 3
297 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
298 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
299 | Code 123 | Length | LaRes | Latitude +
300 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
301 | Latitude (cont'd) | LoRes | +
302 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
303 | Longitude |
304 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
305 | AType | AltRes | Altitude +
306 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
307 | Alt.(cont'd) | Res |Datum|
308 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
310 Code: 8 bits. The code for the DHCPv4 GeoConf Option (123).
312 Length: 8 bits. The length of the option, in octets.
313 The option length is 16.
315 LaRes: 6 bits. This field represents latitude resolution.
317 Latitude: a 34 bit fixed point value consisting of 9 bits of
318 signed integer and 25 bits of fraction, interpreted
319 as described in Section 2.3.
321 LoRes: 6 bits. This field represents longitude resolution.
323 Longitude: a 34 bit fixed point value consisting of 9 bits of
324 signed integer and 25 bits of fraction, interpreted
325 as described in Section 2.3.
327 AType: Altitude Type (4 bits), defined in Section 2.4.
329 AltRes: 6 bits. This field represents altitude resolution.
331 Altitude: A 30 bit value defined by the AType field, described in
332 Section 2.4.
334 Res: The Res field which is 5 bits, is reserved. These bits
335 have been used by IEEE 802.11y [IEEE-802.11y], but are
336 not defined within this specification.
338 Datum: 3 bits. The Map Datum used for the coordinates given in
339 this Option.
341 2.2.2. DHCPv4 GeoLoc Option
343 The format of DHCPv4 GeoLoc Option is as follows:
345 0 1 2 3
346 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
347 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
348 | Code TBD1 | Length | LatUnc | Latitude +
349 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
350 | Latitude (cont'd) | LongUnc | +
351 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
352 | Longitude |
353 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
354 | AType | AltUnc | Altitude +
355 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
356 | Alt.(cont'd) |Ver| Res |Datum|
357 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
359 Code: 8 bits. The code for the DHCPv4 GeoLoc Option (TBD1).
361 Length: 8 bits. The length of the option, in octets.
362 For version 1, the option length is 16.
364 LatUnc: 6 bits. When the Ver field = 1, this field represents
365 latitude uncertainty. The contents of this field is
366 undefined for other values of the Ver field.
368 Latitude: a 34 bit fixed point value consisting of 9 bits of
369 integer and 25 bits of fraction, interpreted as
370 described in Section 2.3.
372 LongUnc: 6 bits. When the Ver field = 1, this field represents
373 longitude uncertainty. The contents of this field is
374 undefined for other values of the Ver field.
376 Longitude: a 34 bit fixed point value consisting of 9 bits of
377 integer and 25 bits of fraction, interpreted as
378 described in Section 2.3.
380 AType: Altitude Type (4 bits), defined in Section 2.4.
382 AltUnc: 6 bits. When the Ver field = 1, this field represents
383 altitude uncertainty. The contents of this field is
384 undefined for other values of the Ver field.
386 Altitude: A 30 bit value defined by the AType field, described in
387 Section 2.4.
389 Ver: The Ver field is two bits, providing for four potential
390 versions. This specification defines the behavior of
391 version 1. The Ver field is always located at the same
392 offset from the beginning of the option, regardless of
393 the version in use.
395 Res: The Res field which is 3 bits, is reserved. These bits
396 have been used by [IEEE-802.11y], but are not defined
397 within this specification.
399 Datum: 3 bits. The Map Datum used for the coordinates given in
400 this Option.
402 2.2.3. Option Support
404 2.2.3.1. Client Support
406 DHCPv4 clients implementing this specification MUST support receiving
407 the DHCPv4 GeoLoc Option TBD1 (version 1), and MAY support receiving
408 the DHCPv4 GeoConf Option 123 (originally defined in RFC 3825
409 [RFC3825]).
411 DHCPv4 clients request the DHCPv4 server to send GeoConf Option 123,
412 GeoLoc Option TBD1 or both via inclusion of the Parameter Request
413 List option. As noted in Section 9.8 of RFC 2132 [RFC2132]:
415 This option is used by a DHCP client to request values for
416 specified configuration parameters. The list of requested
417 parameters is specified as n octets, where each octet is a valid
418 DHCP option code as defined in this document.
420 The client MAY list the options in order of preference. The DHCP
421 server is not required to return the options in the requested
422 order, but MUST try to insert the requested options in the order
423 requested by the client.
425 When DHCPv4 and DHCPv6 clients implementing this specification do not
426 understand a datum value, they MUST assume a World Geodesic System
427 1984 (WGS84) [WGS84] datum (EPSG [EPSG] 4326 or 4979, depending on
428 whether there is an Altitude value present) and proceed accordingly.
429 Assuming that a less accurate location value is better than none,
430 this ensures that some (perhaps less accurate) location is available
431 to the client.
433 2.2.3.2. Server Option Selection
435 A DHCPv4 server implementing this specification MUST support sending
436 GeoLoc Option TBD1 version 1 and SHOULD support sending GeoConf
437 Option 123 in responses.
439 A DHCPv4 server that provides location information SHOULD honor the
440 Parameter Request List included by the DHCPv4 client in order to
441 decide whether to send GeoConf Option 123, GeoLoc Option TBD1 or both
442 in the Response.
444 2.3. Latitude and Longitude Fields
446 The Latitude and Longitude values in this specification are encoded
447 as 34 bit, twos complement, fixed point values with 9 integer bits
448 and 25 fractional bits. The exact meaning of these values is
449 determined by the datum; the description in this section applies to
450 the datums defined in this document. This document uses the same
451 definition for all datums it specifies.
453 When encoding, Latitude and Longitude values are rounded to the
454 nearest 34-bit binary representation. This imprecision is considered
455 acceptable for the purposes to which this form is intended to be
456 applied and is ignored when decoding.
458 Positive latitudes are north of the equator and negative latitudes
459 are south of the equator. Positive longitudes are east of the Prime
460 Meridian (Greenwich) and negative (2s complement) longitudes are west
461 of the Prime Meridian.
463 Within the coordinate reference systems defined in this document
464 (Datum values 1-3), longitude values outside the range of -180 to 180
465 decimal degrees or latitude values outside the range of -90 to 90
466 degrees MUST be considered invalid. Server implementations SHOULD
467 prevent the entry of invalid values within the selected coordinate
468 reference system. Location consumers MUST ignore invalid location
469 coordinates and SHOULD log invalid location errors.
471 2.3.1. Latitude and Longitude Resolution
473 The Latitude (LaRes), Longitude (LoRes) and Altitude (AltRes)
474 Resolution fields are encoded as 6 bit, unsigned integer values. In
475 the DHCPv4 GeoConf Option 123, the LaRes, LoRes and AltRes fields are
476 used to encode the number of bits of resolution. The resolution sub-
477 fields accommodate the desire to easily adjust the precision of a
478 reported location. Contents beyond the claimed resolution MAY be
479 randomized to obscure greater precision that might be available.
481 In the DHCPv4 GeoConf Option 123, the LaRes value encodes the number
482 of high-order latitude bits that should be considered valid. Any
483 bits entered to the right of this limit should not be considered
484 valid and might be purposely false, or zeroed by the sender. The
485 examples in Appendix B illustrate that a smaller value in the
486 resolution field increases the area within which the device is
487 located. A value of 2 in the LaRes field indicates a precision of no
488 greater than 1/6th that of the globe (see the first example of
489 Appendix B). A value of 34 in the LaRes field indicates a precision
490 of about 3.11 mm in latitude at the equator.
492 In the DHCPv4 GeoConf Option 123, the LoRes value encodes the number
493 of high-order longitude bits that should be considered valid. Any
494 bits entered to the right of this limit should not be considered
495 valid and might be purposely false, or zeroed by the sender. A value
496 of 2 in the LoRes field indicates precision of no greater than 1/6th
497 that of the globe (see the first example of Appendix B). A value of
498 34 in the LoRes field indicates a precision of about 2.42 mm in
499 Longitude (at the equator). Because lines of longitude converge at
500 the poles, the distance is smaller (better precision) for locations
501 away from the equator.
503 2.3.2. Latitude and Longitude Uncertainty
505 In the DHCPv6 GeoLoc Option TBD2 and the DHCPv4 GeoLoc Option TBD1,
506 the Latitude and Longitude Uncertainty fields (LatUnc and LongUnc)
507 quantify the amount of uncertainty in each of the Latitude and
508 Longitude values respectively. A value of 0 is reserved to indicate
509 that the uncertainty is unknown; values greater than 34 are reserved.
511 A point within the region of uncertainty is selected to be the
512 encoded point; the centroid of the region is often an appropriate
513 choice. The value for uncertainty is taken as the distance from the
514 selected point to the furthest extreme of the region of uncertainty
515 on that axis. This is demonstrated in the figure below, which shows
516 a two-dimensional polygon that is projected on each axis. In the
517 figure, "X" marks the point that is selected; the ranges marked with
518 "U" is the uncertainty.
520 ___ ___________
521 ^ | / |
522 | | / |
523 | | / |
524 U | / |
525 | | ( |
526 V | | |
527 --X | X |
528 | | `---------.
529 | | |
530 | | |
531 | | |
532 - `-------------------------'
534 |---------X---------------|
535 |<------U------>|
537 Key
538 ---
540 V, ^ = vertical arrows, delimiting the vertical uncertainty range.
541 <> = horizontal arrows, delimiting the horizontal uncertainty
542 range.
544 Uncertainty applies to each axis independently.
546 The amount of uncertainty can be determined from the encoding by
547 taking 2 to the power of 8, less the encoded value. As is shown in
548 the following formula, where "x" is the encoded integer value:
550 uncertainty = 2 ^ ( 8 - x )
552 The result of this formula is expressed in degrees of latitude or
553 longitude. The uncertainty is added to the base latitude or
554 longitude value to determine the maximum value in the uncertainty
555 range; similarly, the uncertainty is subtracted from the base value
556 to determine the minimum value. Note that because lines of longitude
557 converge at the poles, the actual distance represented by this
558 uncertainty changes with the distance from the equator.
560 If the maximum or minimum latitude values derived from applying
561 uncertainty are outside the range of -90 to +90, these values are
562 trimmed to within this range. If the maximum or minimum longitude
563 values derived from applying uncertainty are outside the range of
564 -180 to +180, then these values are normalized to this range by
565 adding or subtracting 360 as necessary.
567 The encoded value is determined by subtracting the next highest whole
568 integer value for the base 2 logarithm of uncertainty from 8. As is
569 shown by the following formula, where uncertainty is the midpoint of
570 the known range less the lower bound of that range:
572 x = 8 - ceil( log2( uncertainty ) )
574 Note that the result of encoding this value increases the range of
575 uncertainty to the next available power of two; subsequent repeated
576 encodings and decodings do not change the value. Only increasing
577 uncertainty means that the associated confidence does not have to
578 decrease.
580 2.4. Altitude
582 How the Altitude value is interpreted depends on the Altitude Type
583 (AType) value and the selected datum. Three Altitude Type values are
584 defined in this document: unknown (0), meters (1) and floors (2).
586 2.4.1. No Known Altitude (AType = 0)
588 In some cases, the altitude of the location might not be provided.
589 An Altitude Type value of zero indicates that the altitude is not
590 given to the client. In this case, the Altitude and Altitude
591 Uncertainty fields can contain any value and MUST be ignored.
593 2.4.2. Altitude in Meters (AType = 1)
595 If the Altitude Type has a value of one, Altitude is measured in
596 meters, in relation to the zero set by the vertical datum. For AType
597 = 1, the Altitude value is expressed as a 30 bit, fixed point, twos
598 complement integer with 22 integer bits and 8 fractional bits.
600 2.4.3. Altitude in Floors (AType = 2)
602 A value of two for Altitude Type indicates that the Altitude value is
603 measured in floors. Since altitude in meters may not be known within
604 a building, a floor indication may be more useful. For AType = 2,
605 the Altitude value is expressed as a 30 bit, fixed point, twos
606 complement integer with 22 integer bits and 8 fractional bits.
608 This value is relevant only in relation to a building; the value is
609 relative to the ground level of the building. Floors located below
610 ground level are represented by negative values. In some buildings
611 it might not be clear which floor is at ground level or an
612 intermediate floor might be hard to identify as such. Determining
613 what floor is at ground level and what constitutes a sub-floor as
614 opposed to an naturally numbered floor is left to local
615 interpretation.
617 Larger values represent floors that are farther away from floor 0
618 such that:
620 - if positive, the floor value is farther above the ground floor.
621 - if negative, the floor value is farther below the ground floor.
623 Non-integer values can be used to represent intermediate or sub-
624 floors, such as mezzanine levels. Example: a mezzanine between floor
625 1 and floor 2 could be represented as a value of 1.25. Example:
626 mezzanines between floor 4 and floor 5 could be represented as values
627 of 4.5 and 4.75.
629 2.4.4. Altitude Resolution
631 In the DHCPv4 GeoConf Option 123, the Altitude Resolution (AltRes)
632 value encodes the number of high-order altitude bits that should be
633 considered valid. Values above 30 (decimal) are undefined and
634 reserved.
636 If the Altitude Type value is one (AType = 1), an AltRes value 0.0
637 would indicate unknown Altitude. The most precise altitude would
638 have an AltRes value of 30. Many values of AltRes would obscure any
639 variation due to vertical datum differences.
641 The AltRes field SHOULD be set to maximum precision when AType = 2
642 (floors) when a floor value is included in the DHCP Reply, or when
643 AType = 0, to denote that the floor isn't known. An altitude coded
644 as AType = 2, AltRes = 30, and Altitude = 0.0 is meaningful even
645 outside a building, and represents ground level at the given latitude
646 and longitude.
648 2.4.5. Altitude Uncertainty
650 In the DHCPv6 GeoLoc Option TBD2 or the DHCPv4 GeoLoc Option TBD1,
651 the AltUnc value quantifies the amount of uncertainty in the Altitude
652 value. As with LatUnc and LongUnc, a value of 0 for AltUnc is
653 reserved to indicate that Altitude Uncertainty is not known; values
654 above 30 are also reserved. Altitude Uncertainty only applies to
655 Altitude Type 1.
657 The amount of Altitude Uncertainty can be determined by the following
658 formula, where x is the encoded integer value:
660 Uncertainty = 2 ^ ( 21 - x )
662 This value uses the same units as the associated altitude.
664 Similarly, a value for the encoded integer value can be derived by
665 the following formula:
667 x = 21 - ceil( log2( uncertainty ) )
669 2.5. Datum
671 The Datum field determines how coordinates are organized and related
672 to the real world. Three datums are defined in this document, based
673 on the definitions in [OGP.Geodesy]:
675 1: WGS84 (Latitude, Longitude, Altitude):
676 The World Geodesic System 1984 [WGS84] coordinate reference
677 system.
679 This datum is identified by the European Petroleum Survey Group
680 (EPSG)/International Association of Oil & Gas Producers (OGP) with
681 the code 4979, or by the URN "urn:ogc:def:crs:EPSG::4979".
682 Without Altitude, this datum is identified by the EPSG/OGP code
683 4326 and the URN "urn:ogc:def:crs:EPSG::4326".
685 2: NAD83 (Latitude, Longitude) + NAVD88:
686 This datum uses a combination of the North American Datum 1983
687 (NAD83) for horizontal (Latitude and Longitude) values, plus the
688 North American Vertical Datum of 1988 (NAVD88) vertical datum.
690 This datum is used for referencing location on land (not near
691 tidal water) within North America.
693 NAD83 is identified by the EPSG/OGP code of 4269, or the URN
694 "urn:ogc:def:crs:EPSG::4269". NAVD88 is identified by the EPSG/
695 OGP code of 5703, or the URN "urn:ogc:def:crs:EPSG::5703".
697 3: NAD83 (Latitude, Longitude) + MLLW:
698 This datum uses a combination of the North American Datum 1983
699 (NAD83) for horizontal (Latitude and Longitude) values, plus the
700 Mean Lower Low Water (MLLW) vertical datum.
702 This datum is used for referencing location on or near tidal water
703 within North America.
705 NAD83 is identified by the EPSG/OGP code of 4269, or the URN
706 "urn:ogc:def:crs:EPSG::4269". MLLW does not have a specific code
707 or URN.
709 All hosts MUST support the WGS84 datum (Datum 1).
711 3. Security Considerations
713 Geopriv requirements (including security requirements) are discussed
714 in "Geopriv Requirements" [RFC3693]. A threat analysis is provided
715 in "Threat Analysis of the Geopriv Protocol" [RFC3694].
717 Since there is no privacy protection for DHCP messages, an
718 eavesdropper who can monitor the link between the DHCP server and
719 requesting client can discover this LCI.
721 To minimize the unintended exposure of location information, the LCI
722 option SHOULD be returned by DHCP servers only when the DHCP client
723 has included this option in its 'parameter request list' (Section 3.5
724 [RFC2131], Section 9.8 [RFC2132]).
726 Where critical decisions might be based on the value of this option,
727 DHCP authentication as defined in "Authentication for DHCP Messages"
728 [RFC3118] and "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)"
729 [RFC3315] SHOULD be used to protect the integrity of the DHCP
730 options.
732 Link layer confidentiality and integrity protection may also be
733 employed to reduce the risk of location disclosure and tampering.
735 4. IANA Considerations
737 4.1. DHCP Options
739 This document defines the DHCPv6 GeoLoc option (see Section 2.1)
740 which requires assignment of DHCPv6 option code TBD2 [RFC3315]:
742 Value Description Reference
743 ---- ------------------ ----------
744 TBD2 OPTION_GEOLOCATION RFC xxxx
745 [RFC Editor: Please replace xxxx with the RFC number
746 assigned to this document.]
748 This document defines the DHCPv4 GeoConf option (see Section 2.2.1)
749 which has been assigned a DHCPv4 option code of 123 from the DHCP
750 Option space.
752 This document also defines the DHCPv4 GeoLoc option (see Section
753 2.2.2) which requires assignment of DHCPv4 option code TBD1
754 [RFC2132][RFC2939]:
756 Data
757 Tag Name Length Meaning Reference
758 ---- ---- ------ ------- ---------
759 TBD1 GeoLoc 16 Geospatial Location RFC xxxx
760 with Uncertainty
761 [RFC Editor: Please replace xxxx with the RFC number
762 assigned to this document.]
764 4.2. Altitude Type Registry
766 IANA is asked to create and maintain the Altitude Type registry
767 following the guidelines below.
769 The registry consists of three values: Altitude Type, Description
770 and Reference. These are described below.
772 Altitude Type: an integer, refers to the value used in the DHCPv4
773 GeoConf and the DHCPv4 and DHCPv6 GeoLoc Options described in this
774 document. Values from 0 to 15 are assigned.
776 Description: the description of the altitude described by this code.
778 Reference: the reference to the document that describes the altitude
779 code. This reference MUST define the way that the 30 bit altitude
780 values and the associated 6 bit uncertainty are interpreted.
782 Initial values are given below; new assignments are to be made
783 following the "Standards Action" policies [RFC5226].
785 +------+---------------------+--------------+
786 | # | Description | Reference |
787 +------+---------------------+--------------+
788 | 0 | No known altitude | RFC xxxx |
789 | 1 | Altitude in meters | RFC xxxx |
790 | 2 | Altitude in floors | RFC xxxx |
791 | 3-15 | Unassigned | RFC xxxx |
792 +------+---------------------+--------------+
793 [RFC Editor: Please replace xxxx with the RFC number
794 assigned to this document.]
796 4.3. Datum Registry
798 IANA is asked to create and maintain the Datum registry following the
799 guidelines below.
801 The registry consists of three values: Datum, Description and
802 Reference. These are described below.
804 Datum: an integer, refers to the value used in the DHCPv4 GeoConf and
805 the DHCPv4 and DHCPv6 GeoLoc Options described in this document.
806 Values from 1 to 7 are assigned.
808 Description: the description of the altitude described by this code.
810 Reference: the reference to the document that describes the Datum
811 code. This reference MUST include specification of both the
812 horizontal and vertical datum, and MUST define the way that the 34
813 bit values and the respective 6 bit uncertainties are interpreted.
815 Initial values are given below; new assignments are to be made
816 following the "Standards Action" policies [RFC5226].
818 +------+----------------------------------------+--------------+
819 | # | Description | Reference |
820 +------+----------------------------------------+--------------+
821 | 0 | Reserved | RFC xxxx |
822 +------+----------------------------------------+--------------+
823 | 1 | Vertical datum WGS 84 defined by EPSG | RFC xxxx |
824 | | CRS Code 4327 | |
825 +------+----------------------------------------+--------------+
826 | 2 | Vertical datum NAD83 defined by EPSG | RFC xxxx |
827 | | CRS Code 4269 with North American | |
828 | | Vertical Datum of 1988 (NAVD88) | |
829 +------+----------------------------------------+--------------+
830 | 3 | Vertical datum NAD83, defined by EPSG | RFC xxxx |
831 | | CRS Code 4269 with Mean Lower Low Water| |
832 | | (MLLW) as associated vertical datum | |
833 +------+----------------------------------------+--------------+
834 | 4-7 | Unassigned | RFC xxxx |
835 +------+----------------------------------------+--------------+
836 [RFC Editor: Please replace xxxx with the RFC number
837 assigned to this document.]
839 4.4. GeoLoc Option Version Registry
841 IANA is asked to create and maintain the GeoLoc Option Version
842 registry following the guidelines below.
844 The registry consists of three values: GeoLoc Option Version,
845 Description and Reference. These are described below.
847 GeoLoc Option Version: an integer, refers to the version used in the
848 DHCPv4 and DHCPv6 GeoLoc Options described in this document. Values
849 from 1 to 3 are assigned.
851 Description: the description of the version described by this code.
853 Reference: the reference to the document that describes the Version
854 code.
856 Initial values are given below; new assignments are to be made
857 following the "Standards Action" policies [RFC5226].
859 +------+---------------------------------------+--------------+
860 | # | Description | Reference |
861 +------+---------------------------------------+--------------+
862 | 0 | Reserved | RFC xxxx |
863 +------+---------------------------------------+--------------+
864 | 1 | Implementations utilizing uncertainty | RFC xxxx |
865 | | parameters for both DHCPv4 and DHCPv6 | |
866 | | GeoLoc options | |
867 +------+---------------------------------------+--------------+
868 | 2-3 | Unassigned | RFC xxxx |
869 +------+---------------------------------------+--------------+
870 [RFC Editor: Please replace xxxx with the RFC number
871 assigned to this document.]
873 5. Acknowledgments
875 The authors would like to thank Randall Gellens, Patrik Falstrom,
876 Ralph Droms, Ted Hardie, Jon Peterson, Robert Sparks, Ralph Droms,
877 Nadine Abbott and Mykyta Yevstifeyev for their inputs and
878 constructive comments regarding this document. Additionally, the
879 authors would like to thank Greg Troxel for the education on vertical
880 datums, as well as Carl Reed. Thanks to Richard Barnes for his
881 contribution on GML mapping for resolution.
883 6. References
885 6.1. Normative References
887 [EPSG] European Petroleum Survey Group, http://www.epsg.org/ and
888 http://www.epsg-registry.org/
890 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
891 Requirement Levels", BCP 14, RFC 2119, March 1997.
893 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
894 March 1997.
896 [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
897 Extensions", RFC2132, March 1997.
899 [RFC2939] Droms, R., "Procedures and IANA Guidelines for Definition
900 of New DHCP Options and Message types", BCP 43, RFC 2939,
901 September 2000.
903 [RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
904 3046, January 2001.
906 [RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
907 Messages", RFC 3118, June 2001.
909 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and
910 M. Carney, "Dynamic Host Configuration Protocol for IPv6
911 (DHCPv6)", RFC 3315, July 2003.
913 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
914 IANA Considerations Section in RFCs", RFC 5226, May 2008.
916 [WGS84] US National Imagery and Mapping Agency, "Department of
917 Defense (DoD) World Geodetic System 1984 (WGS 84), Third
918 Edition", NIMA TR8350.2, January 2000,
919 https://www1.nga.mil/PRODUCTSSERVICES/GEODESYGEOPHYSICS/
920 WORLDGEODETICSYSTEM/Pages/default.aspx and
921 http://www.ngs.noaa.gov/faq.shtml#WGS84
923 6.2. Informational References
925 [Convey] Polk, J., Rosen, B. and J. Peterson, "Location Conveyance
926 for the Session Initiation Protocol", Internet draft (work
927 in progress), draft-ietf-sipcore-location-
928 conveyance-06.txt, February 23, 2011.
930 [GeoShape] Thomson, M. and C. Reed, "GML 3.1.1 PIDF-LO Shape
931 Application Schema for use by the Internet Engineering Task
932 Force (IETF)", Candidate OpenGIS Implementation
933 Specification 06-142, Version: 0.0.9, December 2006.
935 [IEEE-802.11y]
936 Information technology - Telecommunications and information
937 exchange between systems - Local and metropolitan area
938 networks - Specific requirements - Part 11: Wireless LAN
939 Medium Access Control (MAC) and Physical Layer (PHY)
940 specifications Amendment 3: 3650-3700 MHz Operation in USA,
941 November 2008.
943 [NENA] National Emergency Number Association (NENA) www.nena.org
944 NENA Technical Information Document on Model Legislation
945 Enhanced 911 for Multi-Line Telephone Systems.
947 [RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
948 3046, January 2001.
950 [RFC3693] Cuellar, J., Morris, J., Mulligan, D., Peterson, J. and J.
951 Polk, "Geopriv Requirements", RFC 3693, February 2004.
953 [RFC3694] Danley, M., Mulligan, D., Morris, J. and J. Peterson,
954 "Threat Analysis of the Geopriv Protocol", RFC 3694,
955 February 2004.
957 [RFC3825] Polk, J., Schnizlein, J. and M. Linsner, "Dynamic Host
958 Configuration Protocol Option for Coordinate-based Location
959 Configuration Information", RFC 3825, July 2004.
961 [RFC4119] Peterson, J., "A Presence-based GEOPRIV Location Object
962 Format", RFC 4119, December 2005.
964 [RFC4776] Schulzrinne, H., "Dynamic Host Configuration Protocol
965 (DHCPv4 and DHCPv6) Option for Civic Addresses
966 Configuration Information", RFC 4776, November 2006.
968 [RFC5139] Thomson, M. and J. Winterbottom, "Revised Civic Location
969 Format for Presence Information Data Format Location Object
970 (PIDF-LO)", RFC 5139, February 2008.
972 [RFC5491] Winterbottom, J., Thomson, M. and H. Tschofenig, "GEOPRIV
973 PIDF-LO Usage Clarification, Considerations, and
974 Recommendations ", RFC 5491, March 2009
976 Appendix A. GML Mapping
978 The GML representation of a decoded DHCP option depends on what
979 fields are specified. The DHCP format for location logically
980 describes a geodetic prism, rectangle, or point, depending on whether
981 Altitude and uncertainty values are provided. In the absence of
982 uncertainty information, the value decoded from the DHCP form can be
983 expressed as a single point; this is true regardless of whether the
984 version 0 or version 1 interpretations of the uncertainty fields are
985 used. If the point includes Altitude, it uses a three dimensional
986 CRS, otherwise it uses a two dimensional CRS. If all fields are
987 included along with uncertainty, the shape described is a rectangular
988 prism. Note that this is necessary given that uncertainty for each
989 axis is provided independently.
991 If Altitude or Altitude Uncertainty (AltUnc) is not specified, the
992 shape is described as a rectangle using the "gml:Polygon" shape. If
993 Altitude is available, a three dimensional CRS is used, otherwise a
994 two dimensional CRS is used.
996 For Datum values of 2 or 3 (NAD83), there is no available CRS URN
997 that covers three dimensional coordinates. By necessity, locations
998 described in these datums can be represented by two dimensional
999 shapes only; that is, either a two dimensional point or a polygon.
1001 If the Altitude Type is 2 (floors), then this value can be
1002 represented using a civic address object [RFC5139] that is presented
1003 alongside the geodetic object.
1005 This Appendix describes how the location value encoded in DHCP format
1006 for geodetic location can be expressed in GML. The mapping is valid
1007 for the DHCPv6 GeoLoc Option as well as both of the DHCPv4 GeoConf
1008 and GeoLoc options, and for the currently-defined datum values (1, 2,
1009 and 3). Further version or datum definitions should provide similar
1010 mappings.
1012 These shapes can be mapped to GML by first computing the bounds that
1013 are described using the coordinate and uncertainty fields, then
1014 encoding the result in a GML Polygon or Prism shape.
1016 A.1. GML Templates
1018 If Altitude is provided in meters (AType 1) and the datum value is
1019 WGS84 (value 1), then the proper GML shape is a Prism, with the
1020 following form (where $value$ indicates a value computed from the
1021 DHCP option as described below):
1023
1026
1027
1028
1029
1030
1031 $lowLatitude$ $lowLongitude$ $lowAltitude$
1032 $lowLatitude$ $highLongitude$ $lowAltitude$
1033 $highLatitude$ $highLongitude$ $lowAltitude$
1034 $highLatitude$ $lowLongitude$ $lowAltitude$
1035 $lowLatitude$ $lowLongitude$ $lowAltitude$
1036
1037
1038
1039
1040
1041
1042 $highAltitude - lowAltitude$
1043
1044
1046 The Polygon shape is used if Altitude is omitted or specified in
1047 floors, or if either NAD83 datum is used (value 2 or 3). The
1048 corresponding GML Polygon has the following form:
1050 >
1052
1053
1054
1055 $lowLatitude$ $lowLongitude$
1056 $lowLatitude$ $highLongitude$
1057 $highLatitude$ $highLongitude$
1058 $highLatitude$ $lowLongitude$
1059 $lowLatitude$ $lowLongitude$
1060
1061
1062
1063
1065 The value "2D-CRS-URN" is defined by the datum value: If the datum is
1066 WGS84 (value 1), then the 2D-CRS-URN is "urn:ogc:def:crs:EPSG::4326".
1067 If the datum is NAD83 (value 2 or 3), then the 2D-CRS-URN is
1068 "urn:ogc:def:crs:EPSG::4269".
1070 A Polygon shape with the WGS84 three-dimensional CRS is used if the
1071 datum is WGS84 (value 1) and the Altitude is specified in meters
1072 (Altitude type 1), but no Altitude uncertainty is specified (that is,
1073 AltUnc is 0). In this case, the value of the Altitude field is added
1074 after each of the points above, and the srsName attribute is set to
1075 the three-dimensional WGS84 CRS, namely "urn:ogc:def:crs:EPSG::4979".
1077 A simple point shape is used if either Latitude uncertainty (LatUnc)
1078 or Longitude uncertainty (LongUnc) is not specified. With Altitude,
1079 this uses a three-dimensional CRS; otherwise, it uses a two-
1080 dimensional CRS.
1082
1084 $Latitude$ $Longitude$ $[Altitude]$
1085
1087 A.1.1. Finding Low and High Values using Uncertainty Fields
1089 For the DHCPv4 GeoConf Option 123, resolution fields are used (LaRes,
1090 LoRes, AltRes), indicating how many bits of a value contain
1091 information. Any bits beyond those indicated can be either zero or
1092 one.
1094 For the DHCPv6 GeoLoc Option TBD2 and DHCPv4 GeoLoc Option TBD1, the
1095 LatUnc, LongUnc and AltUnc fields indicate uncertainty distances,
1096 denoting the bounds of the location region described by the DHCP
1097 location object.
1099 The two sections below describe how to compute the Latitude,
1100 Longitude, and Altitude bounds (e.g., $lowLatitude$, $highAltitude$)
1101 in the templates above. The first section describes how these bounds
1102 are computed in the "resolution encoding" (DHCPv4 GeoConf Option
1103 123), while the second section addresses the "uncertainty encoding"
1104 (DHCPv6 GeoLoc Option TBD2 and DHCPv4 GeoLoc Option TBD1).
1106 A.1.1.1. Resolution Encoding
1108 Given a number of resolution bits (i.e., the value of a resolution
1109 field), if all bits beyond those bits are set to zero, this gives the
1110 lowest possible value. The highest possible value can be found
1111 setting all bits to one.
1113 If the encoded value of Latitude/Longitude and resolution (LaRes,
1114 LoRes) are treated as 34-bit unsigned integers, the following can be
1115 used (where ">>" is a bitwise right shift, "&" is a bitwise AND, "~"
1116 is a bitwise negation, and "|" is a bitwise OR).
1118 mask = 0x3ffffffff >> resolution
1119 lowvalue = value & ~mask
1120 highvalue = value | mask + 1
1122 Once these values are determined, the corresponding floating point
1123 numbers can be computed by dividing the values by 2^25 (since there
1124 are 25 bits of fraction in the fixed-point representation).
1126 Alternatively, the lowest possible value can be found by using
1127 resolution to determine the size of the range. This method has the
1128 advantage that it operates on the decoded floating point values. It
1129 is equivalent to the first mechanism, to a possible error of 2^-25
1130 (2^-8 for altitude).
1132 scale = 2 ^ ( 9 - resolution )
1133 lowvalue = floor( value / scale ) * scale
1134 highvalue = lowvalue + scale
1136 Altitude resolution (AltRes) uses the same process with different
1137 constants. There are 22 whole bits in the Altitude encoding (instead
1138 of 9) and 30 bits in total (instead of 34).
1140 A.1.1.2. Uncertainty Encoding
1142 In the uncertainty encoding, the uncertainty fields (LongUnc/LatUnc)
1143 directly represent the logarithms of uncertainty distances. So the
1144 low and high bounds are computed by first computing the uncertainty
1145 distances, then adding and subtracting these from the value provided.
1146 If "uncertainty" is the unsigned integer value of the uncertainty
1147 field and "value" is the value of the coordinate field:
1149 distance = 2 ^ (8 - uncertainty)
1150 lowvalue = value - distance
1151 highvalue = value + distance
1153 Altitude uncertainty (AltUnc in version 1) uses the same process with
1154 different constants:
1156 distance = 2 ^ (21 - uncertainty)
1157 lowvalue = value - distance
1159 Appendix B. Calculations of Resolution
1161 The following examples for two different locations demonstrate how
1162 the Resolution values for Latitude, Longitude, and Altitude (used in
1163 DHCPv4 GeoConf Option 123) can be calculated. In both examples, the
1164 geo-location values were derived from maps using the WGS84 map datum,
1165 therefore in these examples, the Datum field would have a value = 1
1166 (00000001, or 0x01).
1168 B.1. Location Configuration Information of "White House" (Example 1)
1170 The grounds of the White House in Washington D.C. (1600 Pennsylvania
1171 Ave. NW, Washington, DC 20006) can be found between 38.895375 and
1172 38.898653 degrees North and 77.037911 and 77.035116 degrees West. In
1173 this example, we assume that we are standing on the sidewalk on the
1174 north side of the White House, between driveways. Since we are not
1175 inside a structure, we assume an Altitude value of 15 meters,
1176 interpolated from the US Geological survey map, Washington Washington
1177 West quadrangle.
1179 The address was NOT picked for any political reason and can easily be
1180 found on the Internet or mapping software, but was picked as an
1181 easily identifiable location on our planet.
1183 In this example, the requirement of emergency responders in North
1184 America via their NENA Model Legislation [NENA] could be met by a
1185 LaRes value of 21 and a LoRes value of 20. This would yield a geo-
1186 location that is Latitude 38.8984375 north to Latitude 38.8988616
1187 north and Longitude -77.0371094 to Longitude -77.0375977. This is an
1188 area of approximately 89 feet by 75 feet or 6669 square feet, which
1189 is very close to the 7000 square feet requested by NENA. In this
1190 example, a service provider could enforce that a device send a
1191 Location Configuration Information with this minimum amount of
1192 resolution for this particular location when calling emergency
1193 services.
1195 An approximate representation of this location might be provided using
1196 the DHCPv4 GeoConf Option 123 encoding as follows:
1198 0 1 2 3
1199 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
1200 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1201 | Code (123) | OptLen (16) | LaRes | Latitude .
1202 |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.
1203 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1204 . Latitude (cont'd) | LoRes | .
1205 .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.
1206 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1207 . Longitude (cont'd) |
1208 .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|
1209 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1210 | AType | AltRes | Altitude .
1211 |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.
1212 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1213 . Alt (cont'd) | Res |Datum|
1214 .0 0 0 0 0 0 0 0|0 0 0 0 0|0 0 1|
1215 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1217 In hexadecimal, this is 7B10484D CB986347 65ED42C4 1440000F 0001.
1219 Decoding Location Configuration Information with Resolution
1221 Decoding this option gives a latitude of 38.897647 (to 7 decimal
1222 places) with 18 bits of resolution; a longitude of -77.0366000 with
1223 17 bits of resolution; an altitude type of meters with a value of 15
1224 and 17 bits of resolution; version 0 (resolution) and the WGS84
1225 datum.
1227 For the latitude value, 18 bits of resolution allow for values in the
1228 range from 38.8964844 to 38.8984375. For the longitude value, 17
1229 bits of resolution allow for values in the range from -77.0390625 to
1230 -77.0351563. Having 17 bits of resolution in the altitude allows for
1231 values in the range from 0 to 32 meters.
1233 GML Representation of Decoded Location Configuration Information
1235 The following GML shows the value decoded in the previous example as
1236 a point in a three dimensional CRS:
1238
1240 38.897647 -77.0366 15
1241
1243 This representation ignores the values included in the resolution
1244 parameters. If resolution values are provided, a rectangular prism
1245 can be used to represent the location.
1247 The following example uses all of the decoded information from the
1248 previous example:
1250
1253
1254
1255
1256
1257
1258 38.8964844 -77.0390625 0
1259 38.8964844 -77.0351563 0
1260 38.8984375 -77.0351563 0
1261 38.8984375 -77.0390625 0
1262 38.8964844 -77.0390625 0
1263
1264
1266
1267
1268
1269
1270 32
1271
1272
1274 B.2. Location Configuration Information of "Sears Tower" (Example 2)
1276 Postal Address:
1277 Sears Tower
1278 103rd Floor
1279 233 S. Wacker Dr.
1280 Chicago, IL 60606
1282 Viewing the Chicago area from the Observation Deck of the Sears
1283 Tower.
1285 Latitude 41.87884 degrees North (or +41.87884 degrees)
1286 Using 2s complement, 34 bit fixed point, 25 bit fraction
1287 Latitude = 0x053c1f751,
1288 Latitude = 0001010011110000011111011101010001
1289 Longitude 87.63602 degrees West (or -87.63602 degrees)
1290 Using 2s complement, 34 bit fixed point, 25 bit fraction
1291 Longitude = 0xf50ba5b97,
1292 Longitude = 1101010000101110100101101110010111
1294 Altitude 103
1296 In this example, we are inside a structure, therefore we will assume
1297 an Altitude value of 103 to indicate the floor we are on. The
1298 Altitude Type value is 2, indicating floors. The AltRes field would
1299 indicate that all bits in the Altitude field are true, as we want to
1300 accurately represent the floor of the structure where we are located.
1302 AltRes = 30, 0x1e, 011110
1303 AType = 2, 0x02, 000010
1304 Altitude = 103, 0x00006700, 000000000000000110011100000000
1306 For the accuracy of the Latitude and Longitude, the best information
1307 available to us was supplied by a generic mapping service that shows
1308 a single geo-loc for all of the Sears Tower. Therefore we are going
1309 to show LaRes as value 18 (0x12 or 010010) and LoRes as value 18
1310 (0x12 or 010010). This would be describing a geo-location area that
1311 is Latitude 41.8769531 to Latitude 41.8789062 and extends from
1312 -87.6367188 degrees to -87.6347657 degrees Longitude. This is an
1313 area of approximately 373412 square feet (713.3 ft. x 523.5 ft.).
1315 Appendix C. Calculations of Uncertainty
1317 The following example demonstrates how uncertainty values for
1318 Latitude, Longitude, and Altitude (LatUnc, LongUnc and AltUnc
1319 used in the DHCPv6 GeoLoc Option TBD2 as well as DHCPv4 GeoLoc
1320 Option TBD1) can be calculated.
1322 C.1. Location Configuration Information of "Sydney Opera House"
1323 (Example 3)
1325 This section describes an example of encoding and decoding the
1326 geodetic DHCP Option. The textual results are expressed in GML
1327 [OGC.GML-3.1.1] form, suitable for inclusion in PIDF-LO [RFC4119].
1329 These examples all assume a datum of WGS84 (datum = 1) and an
1330 Altitude type of meters (AType = 1).
1332 C.1.1. Encoding a Location into DHCP Geodetic Form
1334 This example draws a rough polygon around the Sydney Opera House.
1335 This polygon consists of the following six points:
1337 33.856625 S, 151.215906 E
1338 33.856299 S, 151.215343 E
1339 33.856326 S, 151.214731 E
1340 33.857533 S, 151.214495 E
1341 33.857720 S, 151.214613 E
1342 33.857369 S, 151.215375 E
1344 The top of the building 67.4 meters above sea level, and a starting
1345 Altitude of 0 meters above the WGS84 geoid is assumed.
1347 The first step is to determine the range of Latitude and Longitude
1348 values. Latitude ranges from -33.857720 to -33.856299; Longitude
1349 ranges from 151.214495 to 151.215906.
1351 For this example, the point that is encoded is chosen by finding the
1352 middle of each range, that is (-33.8570095, 151.2152005). This is
1353 encoded as (1110111100010010010011011000001101,
1354 0100101110011011100010111011000011) in binary, or (3BC49360D,
1355 12E6E2EC3) in hexadecimal notation (with an extra 2 bits of leading
1356 padding on each). Altitude is set at 33.7 meters, which is
1357 000000000000000010000110110011 (binary) or 000021B3 (hexadecimal).
1359 The Latitude Uncertainty (LatUnc) is given by inserting the
1360 difference between the center value and the outer value into the
1361 formula from Section 2.3.1. This gives:
1363 x = 8 - ceil( log2( -33.8570095 - -33.857720 ) )
1365 The result of this equation is 18, therefore the uncertainty is
1366 encoded as 010010 in binary.
1368 Similarly, Longitude Uncertainty (LongUnc) is given by the formula:
1370 x = 8 - ceil( log2( 151.2152005 - 151.214495 ) )
1372 The result of this equation is also 18, or 010010 in binary.
1374 Altitude Uncertainty (AltUnc) uses the formula from Section 2.4.4:
1376 x = 21 - ceil( log2( 33.7 - 0 ) )
1378 The result of this equation is 15, which is encoded as 001111 in
1379 binary.
1381 Adding an Altitude Type of 1 (meters) and a Datum of 1 (WGS84), this
1382 gives the following DHCPv4 GeoLoc Option TBD1 form:
1384 0 1 2 3
1385 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
1386 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1387 | Code (TBD1) | OptLen (16) | LatUnc | Latitude .
1388 |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.
1389 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1390 . Latitude (cont'd) | LongUnc | .
1391 .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.
1392 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1393 . Longitude (cont'd) |
1394 .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|
1395 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1396 | AType | AltUnc | Altitude .
1397 |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.
1398 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1399 . Alt (cont'd) |Ver| Res |Datum|
1400 .1 0 1 1 0 0 1 1|0 1|0 0 0|0 0 1|
1401 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1403 In hexadecimal, this is 7B104BBC 49360D49 2E6E2EC3 13C00021 B341.
1404 The DHCPv6 form only differs in the code and option length portion.
1406 C.1.2. Decoding a Location from DHCP Geodetic Form
1408 If receiving the binary form created in the previous section, this
1409 section describes how that would be interpreted. The result is then
1410 represented as a GML object, as defined in [GeoShape].
1412 A Latitude value of 1110111100010010010011011000001101 decodes to a
1413 value of -33.8570095003 (to 10 decimal places). The Longitude value
1414 of 0100101110011011100010111011000011 decodes to 151.2152005136.
1416 Decoding Tip: If the raw values of Latitude and Longitude are placed
1417 in integer variables, the actual value can be derived by the
1418 following process:
1420 1. If the highest order bit is set (i.e. the number is a twos
1421 complement negative), then subtract 2 to the power of 34 (the
1422 total number of bits).
1424 2. Divide the result by 2 to the power of 25 (the number of
1425 fractional bits) to determine the final value.
1427 The same principle can be applied when decoding Altitude values,
1428 except with different powers of 2 (30 and 8 respectively).
1430 The Latitude and Longitude Uncertainty are both 18, which gives an
1431 uncertainty value using the formula from Section 2.3.1 of
1432 0.0009765625. Therefore, the decoded Latitudes is -33.8570095003 +/-
1433 0.0009765625 (or the range from -33.8579860628 to -33.8560329378) and
1434 the decoded Longitude is 151.2152005136 +/- 0.0009765625 (or the
1435 range from 151.2142239511 to 151.2161770761).
1437 The encoded Altitude of 000000000000000010000110110011 decodes to
1438 33.69921875. The encoded uncertainty of 15 gives a value of 64,
1439 therefore the final uncertainty is 33.69921875 +/- 64 (or the range
1440 from -30.30078125 to 97.69921875).
1442 C.1.2.1. GML Representation of Decoded Locations
1444 The following GML shows the value decoded in the previous example as
1445 a point in a three dimensional CRS:
1447
1449 -33.8570095003 151.2152005136 33.69921875
1450
1452 The following example uses all of the decoded information from the
1453 previous example:
1455
1458
1459
1460
1461
1462
1463 -33.8579860628 151.2142239511 -30.30078125
1464 -33.8579860628 151.2161770761 -30.30078125
1465 -33.8560329378 151.2161770761 -30.30078125
1466 -33.8560329378 151.2142239511 -30.30078125
1467 -33.8579860628 151.2142239511 -30.30078125
1468
1469
1470
1471
1472
1473
1474 128
1475
1476
1478 Note that this representation is only appropriate if the uncertainty
1479 is sufficiently small. [GeoShape] recommends that distances between
1480 polygon vertices be kept short. A GML representation like this one
1481 is only appropriate where uncertainty is less than 1 degree (an
1482 encoded value of 9 or greater).
1484 Appendix D. Changes from RFC 3825
1486 This section lists the major changes between RFC 3825 and this
1487 document. Minor changes, including style, grammar, spelling and
1488 editorial changes are not mentioned here.
1490 o Section 1 now includes clarifications on wired and wireless uses.
1491 o The former Sections 1.2 and 1.3 have been removed. Section 1.2
1492 now defines the concepts of uncertainty and resolution, as well
1493 as conversion between the DHCP option formats and PIDF-LO.
1494 o A DHCPv6 GeoLoc Option is now defined (Section 2.1) as well
1495 as a new DHCPv4 GeoLoc Option (Section 2.2.2).
1496 o The former Datum field has been split into three fields:
1497 Ver, Res and Datum. These fields are used in both the
1498 DHCPv4 GeoLoc Option and the DHCPv6 GeoLoc Option.
1499 o Section 2.2.3 has been added, describing option support
1500 requirements on DHCP clients and servers.
1501 o Section 2.3 has been added, describing the Latitude and
1502 Longitude fields.
1503 o Section 2.3.1 has been added, covering Latitude and Longitude
1504 resolution.
1505 o Section 2.3.2 has been added, covering Latitude and Longitude
1506 uncertainty.
1507 o Section 2.4 has been added, covering values of the Altitude
1508 field (Sections 2.4.1, 2.4.2 and 2.4.3), Altitude resolution
1509 (Section 2.4.4), and Altitude uncertainty (Section 2.4.5).
1510 o Section 2.5 has been added, covering the Datum field.
1511 o Section 3 (Security Considerations) has added a recommendation
1512 on link layer confidentiality.
1513 o Section 4 (IANA Considerations) has consolidated material
1514 relating to parameter allocation for both the DHCPv4 and
1515 DHCPv6 option parameters, and has been rewritten to
1516 conform to the practices recommended in RFC 5226.
1517 o The material formerly in Appendix A has been updated and
1518 shortened and has been moved to Appendix B.
1519 o An Appendix A on GML mapping has been added.
1520 o Appendix C has been added, providing an example of uncertainty
1521 encoding.
1522 o Appendix D has been added, detailing the changes from RFC 3825.
1524 Authors' Addresses
1526 James M. Polk
1527 Cisco Systems
1528 2200 East President George Bush Turnpike
1529 Richardson, Texas 75082 USA
1530 USA
1532 EMail: jmpolk@cisco.com
1534 Marc Linsner
1535 Cisco Systems
1536 Marco Island, FL 34145 USA
1537 USA
1539 EMail: marc.linsner@cisco.com
1541 Martin Thomson
1542 Andrew Corporation
1543 PO Box U40
1544 Wollongong University Campus, NSW 2500
1545 AU
1547 EMail: martin.thomson@andrew.com
1549 Bernard Aboba
1550 Microsoft Corporation
1551 One Microsoft Way
1552 Redmond, WA 98052 USA
1553 USA
1555 EMail: bernard_aboba@hotmail.com