Internet Engineering Task Force (IETF) J. Polk Request for Comments: 6225 M. Linsner Obsoletes: 3825 Cisco Systems Category: Standards Track M. Thomson ISSN: 2070-1721 Andrew Corporation B. Aboba, Ed. Microsoft Corporation July 2011 Dynamic Host Configuration Protocol Options for Coordinate-Based Location Configuration Information Abstract This document specifies Dynamic Host Configuration Protocol options (both DHCPv4 and DHCPv6) for the coordinate-based geographic location of the client. The Location Configuration Information (LCI) includes Latitude, Longitude, and Altitude, with resolution or uncertainty indicators for each. Separate parameters indicate the reference datum for each of these values. This document obsoletes RFC 3825. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6225. Polk, et al. Standards Track [Page 1]
RFC 6225 DHCP Options for Coordinate LCI July 2011 Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Polk, et al. Standards Track [Page 2]
RFC 6225 DHCP Options for Coordinate LCI July 2011 Table of Contents 1. Introduction ....................................................3 1.1. Conventions Used in This Document ..........................4 1.2. Resolution and Uncertainty .................................4 2. DHCP Option Formats .............................................6 2.1. DHCPv6 GeoLoc Option .......................................6 2.2. DHCPv4 Options .............................................8 2.3. Latitude and Longitude Fields .............................11 2.4. Altitude ..................................................14 2.5. Datum .....................................................16 3. Security Considerations ........................................17 4. IANA Considerations ............................................17 4.1. DHCP Options ..............................................17 4.2. Altitude Type Registry ....................................18 4.3. Datum Registry ............................................18 4.4. GeoLoc Option Version Registry ............................19 5. Acknowledgments ................................................20 6. References .....................................................20 6.1. Normative References ......................................20 6.2. Informative References ....................................21 Appendix A. GML Mapping ...........................................23 A.1. GML Templates ............................................23 Appendix B. Calculations of Resolution ............................27 B.1. Location Configuration Information of "White House" (Example 1) ..............................................27 B.2. Location Configuration Information of "Sears Tower" (Example 2) ..............................................29 Appendix C. Calculations of Uncertainty ...........................30 C.1. Location Configuration Information of "Sydney Opera House" (Example 3) .......................................30 Appendix D. Changes from RFC 3825 .................................34 1. Introduction The physical location of a network device has a range of applications. In particular, emergency telephony applications rely on knowing the location of a caller in order to determine the correct emergency center. The location of a device can be represented either in terms of geospatial (or geodetic) coordinates, or as a civic address. Different applications may be more suited to one form of location information; therefore, both the geodetic and civic forms may be used simultaneously. Polk, et al. Standards Track [Page 3]
RFC 6225 DHCP Options for Coordinate LCI July 2011 This document specifies Dynamic Host Configuration Protocol v4 (DHCPv4) [RFC2131] and DHCPv6 [RFC3315] options for the coordinate- based geographic location of the client, to be provided by the server. "Dynamic Host Configuration Protocol (DHCPv4 and DHCPv6) Option for Civic Addresses Configuration Information" [RFC4776] specifies DHCP options for civic addresses. The geodetic coordinate options defined in this document and the civic address options defined in RFC 4776 [RFC4776] enable a DHCP client to obtain its location. For example, a wired Ethernet host might use these options for location determination. In this case, the location information could be derived from a wiremap by the DHCP server, using the Circuit ID Relay Agent Information Option (RAIO) defined (as Sub-Option 1) in RFC 3046 [RFC3046]. The DHCP server could correlate the Circuit ID with the geographic location where the identified circuit terminates (such as the location of the wall jack). The mechanism defined here may also be utilized to provide location to wireless hosts. DHCP relay agent sub-options (RAIO) [RFC3046] provide one method a DHCP server might use to perform host location determination. Currently, the relay agent sub-options do not include data sets required for device-level location determination of wireless hosts. In cases where the DHCP server uses RAIO for location determination, a wireless host can use this mechanism to discover the location of the radio access point, or the area of coverage for the radio access point. An important feature of this specification is that after the relevant DHCP exchanges have taken place, the location information is stored on the end device rather than somewhere else, where retrieving it might be difficult in practice. 1.1. Conventions Used in This Document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 1.2. Resolution and Uncertainty The DHCP options defined in this document include fields quantifying the resolution or uncertainty associated with a target location. No inferences relating to privacy policies can be drawn from either uncertainty or resolution values. Polk, et al. Standards Track [Page 4]
RFC 6225 DHCP Options for Coordinate LCI July 2011 As utilized in this document, resolution refers to the accuracy of a reported location, as expressed by the number of valid bits in each of the Latitude, Longitude, and Altitude fields. The Latitude (LaRes), Longitude (LoRes), and Altitude (AltRes) Resolution fields are encoded as 6-bit, unsigned integer values. In the DHCPv4 GeoConf Option 123, the LaRes, LoRes, and AltRes fields are used to encode the number of bits of resolution. The resolution sub-fields accommodate the desire to easily adjust the precision of a reported location. Contents beyond the claimed resolution MAY be randomized to obscure greater precision that might be available. In the context of location technology, uncertainty is a quantification of errors. Any method for determining location is subject to some sources of error; uncertainty describes the amount of error that is present. Uncertainty might be the coverage area of a wireless transmitter, the extent of a building, or a single room. Uncertainty is usually represented as an area within which the target is located. In this document, each of the three axes can be assigned an uncertainty value. In effect, this describes a rectangular prism, which may be used as a coarse representation of a more complex shape that fits within it. See Section 2.3.2 for more detail on the correspondence between shapes and uncertainty. When representing locations from sources that can quantify uncertainty, the goal is to find the smallest possible rectangular prism that this format can describe. This is achieved by taking the minimum and maximum values on each axis and ensuring that the final encoding covers these points. This increases the region of uncertainty, but ensures that the region that is described encompasses the target location. The DHCPv4 option formats defined in this document support resolution and uncertainty parameters. The DHCPv4 GeoConf Option 123 includes a resolution parameter for each of the dimensions of location. Since this resolution parameter need not apply to all dimensions equally, a resolution value is included for each of the three location elements. The DHCPv4 GeoLoc Option 144 as well as the DHCPv6 GeoLoc Option 63 format utilize an uncertainty parameter. Appendix A describes the mapping of DHCP option values to the Geography Markup Language (GML). Appendix B of this document provides examples showing the calculation of resolution values. Appendix C provides an example demonstrating calculation of uncertainty values. Polk, et al. Standards Track [Page 5]
RFC 6225 DHCP Options for Coordinate LCI July 2011 Since the Presence Information Data Format Location Object (PIDF-LO) [RFC4119] [RFC5491] is used to convey location and the associated uncertainty within an emergency call [Convey], a mechanism is needed to convert the information contained within the DHCPv4 and DHCPv6 options to PIDF-LO. This document describes the following conversions: o DHCPv4 GeoConf Option 123 to PIDF-LO o DHCPv4 GeoLoc Option 144 and DHCPv6 GeoLoc Option 63 to PIDF-LO o PIDF-LO to DHCP GeoLoc Option 144 and DHCPv6 GeoLoc Option 63 Conversion to PIDF-LO does not increase uncertainty; conversion from PIDF-LO to the DHCPv4 GeoLoc Option 144 and the DHCPv6 GeoLoc Option 63 increases uncertainty by less than a factor of 2 in each dimension. Since it is not possible to translate an arbitrary PIDF-LO to the DHCP GeoConf Option 123 with a bounded increase in uncertainty, the conversion is not specified. 2. DHCP Option Formats This section defines the format for the DHCPv4 and DHCPv6 options. These options utilize a similar format, differing primarily in the option code. 2.1. DHCPv6 GeoLoc Option The format of the DHCPv6 [RFC3315] GeoLoc Option is as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Code (63) | OptLen | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LatUnc | Latitude + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Lat (cont'd) | LongUnc | Longitude + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Longitude (cont'd) | AType | AltUnc | Altitude + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Altitude (cont'd) |Ver| Res |Datum| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code: 16 bits. The code for the DHCP Option Code (63). OptLen: Option Length. For version 1, the option length is 16. Polk, et al. Standards Track [Page 6]
RFC 6225 DHCP Options for Coordinate LCI July 2011 LatUnc: 6 bits. When the Ver field = 1, this field represents latitude uncertainty. The contents of this field are undefined for other values of the Ver field. Latitude: A 34-bit fixed-point value consisting of 9 bits of integer and 25 bits of fraction, interpreted as described in Section 2.3. LongUnc: 6 bits. When the Ver field = 1, this field represents longitude uncertainty. The contents of this field are undefined for other values of the Ver field. Longitude: A 34-bit fixed-point value consisting of 9 bits of integer and 25 bits of fraction, interpreted as described in Section 2.3. AType: 4 bits. Altitude Type, defined in Section 2.4. AltUnc: 6 bits. When the Ver field = 1, this field represents altitude uncertainty. The contents of this field are undefined for other values of the Ver field. Altitude: A 30-bit value defined by the AType field, described in Section 2.4. Ver: The Ver field is 2 bits, providing for four potential versions. This specification defines the behavior of version 1. The Ver field is always located at the same offset from the beginning of the option, regardless of the version in use. DHCPv6 clients implementing this specification MUST support receiving version 1 responses. DHCPv6 servers implementing this specification MUST send version 1 responses. Res: 3 bits. The Res field is reserved. These bits have been used by [IEEE-802.11y], but are not defined within this specification. Datum: 3 bits. The Map Datum used for the coordinates given in this option. Polk, et al. Standards Track [Page 7]
RFC 6225 DHCP Options for Coordinate LCI July 2011 2.2. DHCPv4 Options 2.2.1. DHCPv4 GeoConf Option The format of the DHCPv4 GeoConf Option is as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code 123 | Length | LaRes | Latitude + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Latitude (cont'd) | LoRes | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Longitude | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AType | AltRes | Altitude + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Alt.(cont'd) | Res |Datum| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code: 8 bits. The code for the DHCPv4 GeoConf Option (123). Length: 8 bits. The length of the option, in octets. The option length is 16. LaRes: 6 bits. This field represents latitude resolution. Latitude: A 34-bit fixed-point value consisting of 9 bits of signed integer and 25 bits of fraction, interpreted as described in Section 2.3. LoRes: 6 bits. This field represents longitude resolution. Longitude: A 34-bit fixed-point value consisting of 9 bits of signed integer and 25 bits of fraction, interpreted as described in Section 2.3. AType: 4 bits. Altitude Type, defined in Section 2.4. AltRes: 6 bits. This field represents altitude resolution. Altitude: A 30-bit value defined by the AType field, described in Section 2.4. Polk, et al. Standards Track [Page 8]
RFC 6225 DHCP Options for Coordinate LCI July 2011 Res: 5 bits. The Res field is reserved. These bits have been used by IEEE 802.11y [IEEE-802.11y], but are not defined within this specification. Datum: 3 bits. The Map Datum used for the coordinates given in this option. 2.2.2. DHCPv4 GeoLoc Option The format of the DHCPv4 GeoLoc Option 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 144 | Length | LatUnc | Latitude + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Latitude (cont'd) | LongUnc | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Longitude | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AType | AltUnc | Altitude + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Alt.(cont'd) |Ver| Res |Datum| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Code: 8 bits. The code for the DHCPv4 GeoLoc Option (144). Length: 8 bits. The length of the option, in octets. For version 1, the option length is 16. LatUnc: 6 bits. When the Ver field = 1, this field represents latitude uncertainty. The contents of this field are undefined for other values of the Ver field. Latitude: A 34-bit fixed-point value consisting of 9 bits of integer and 25 bits of fraction, interpreted as described in Section 2.3. LongUnc: 6 bits. When the Ver field = 1, this field represents longitude uncertainty. The contents of this field are undefined for other values of the Ver field. Longitude: A 34-bit fixed-point value consisting of 9 bits of integer and 25 bits of fraction, interpreted as described in Section 2.3. AType: 4 bits. Altitude Type, defined in Section 2.4. Polk, et al. Standards Track [Page 9]
RFC 6225 DHCP Options for Coordinate LCI July 2011 AltUnc: 6 bits. When the Ver field = 1, this field represents altitude uncertainty. The contents of this field are undefined for other values of the Ver field. Altitude: A 30-bit value defined by the AType field, described in Section 2.4. Ver: The Ver field is 2 bits, providing for four potential versions. This specification defines the behavior of version 1. The Ver field is always located at the same offset from the beginning of the option, regardless of the version in use. Res: 3 bits. The Res field is reserved. These bits have been used by [IEEE-802.11y], but are not defined within this specification. Datum: 3 bits. The Map Datum used for the coordinates given in this option. 2.2.3. Option Support 2.2.3.1. Client Support DHCPv4 clients implementing this specification MUST support receiving the DHCPv4 GeoLoc Option 144 (version 1), and MAY support receiving the DHCPv4 GeoConf Option 123 (originally defined in RFC 3825 [RFC3825]). DHCPv4 clients request the DHCPv4 server to send GeoConf Option 123, GeoLoc Option 144, or both via inclusion of the Parameter Request List option. As noted in Section 9.8 of RFC 2132 [RFC2132]: This option is used by a DHCP client to request values for specified configuration parameters. The list of requested parameters is specified as n octets, where each octet is a valid DHCP option code as defined in this document. The client MAY list the options in order of preference. The DHCP server is not required to return the options in the requested order, but MUST try to insert the requested options in the order requested by the client. Polk, et al. Standards Track [Page 10]
RFC 6225 DHCP Options for Coordinate LCI July 2011 When DHCPv4 and DHCPv6 clients implementing this specification do not understand a datum value, they MUST assume a World Geodetic System 1984 (WGS84) [WGS84] datum (European Petroleum Survey Group (EPSG) [EPSG] 4326 or 4979, depending on whether there is an altitude value present) and proceed accordingly. Assuming that a less accurate location value is better than none, this ensures that some (perhaps less accurate) location is available to the client. 2.2.3.2. Server Option Selection A DHCPv4 server implementing this specification MUST support sending GeoLoc Option 144 version 1 and SHOULD support sending GeoConf Option 123 in responses. A DHCPv4 server that provides location information SHOULD honor the Parameter Request List included by the DHCPv4 client in order to decide whether to send GeoConf Option 123, GeoLoc Option 144, or both in the Response. 2.3. Latitude and Longitude Fields The latitude and longitude values in this specification are encoded as 34-bit, two's complement, fixed-point values with 9 integer bits and 25 fractional bits. The exact meaning of these values is determined by the datum; the description in this section applies to the datums defined in this document. This document uses the same definition for all datums it specifies. When encoding, latitude and longitude values are rounded to the nearest 34-bit binary representation. This imprecision is considered acceptable for the purposes to which this form is intended to be applied and is ignored when decoding. Positive latitudes are north of the equator, and negative latitudes are south of the equator. Positive longitudes are east of the Prime Meridian, and negative (two's complement) longitudes are west of the Prime Meridian. Within the coordinate reference systems defined in this document (Datum values 1-3), longitude values outside the range of -180 to 180 decimal degrees or latitude values outside the range of -90 to 90 degrees MUST be considered invalid. Server implementations SHOULD prevent the entry of invalid values within the selected coordinate reference system. Location consumers MUST ignore invalid location coordinates and SHOULD log errors related to invalid location. Polk, et al. Standards Track [Page 11]
RFC 6225 DHCP Options for Coordinate LCI July 2011 2.3.1. Latitude and Longitude Resolution In the DHCPv4 GeoConf Option 123, the LaRes value encodes the number of high-order latitude bits that MUST be considered valid. Any bits entered to the right of this limit MUST NOT be considered valid and might be purposely false, or zeroed by the sender. The examples in Appendix B illustrate that a smaller value in the resolution field increases the area within which the device is located. A value of 2 in the LaRes field indicates a precision of no greater than 1/6th that of the globe (see the first example of Appendix B). A value of 34 in the LaRes field indicates a precision of about 3.11 mm in latitude at the equator. In the DHCPv4 GeoConf Option 123, the LoRes value encodes the number of high-order longitude bits that MUST be considered valid. Any bits entered to the right of this limit MUST NOT be considered valid and might be purposely false, or zeroed by the sender. A value of 2 in the LoRes field indicates precision of no greater than 1/6th that of the globe (see the first example of Appendix B). A value of 34 in the 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. 2.3.2. Latitude and Longitude Uncertainty In the DHCPv6 GeoLoc Option 63 and the DHCPv4 GeoLoc Option 144, the Latitude and Longitude Uncertainty fields (LatUnc and LongUnc) quantify the amount of uncertainty in each of the latitude and longitude values, respectively. A value of 0 is reserved to indicate that the uncertainty is unknown; values greater than 34 are reserved. A point within the region of uncertainty is selected to be the encoded point; the centroid of the region is often an appropriate choice. The value for uncertainty is taken as the distance from the selected point to the furthest extreme of the region of uncertainty on that axis. This is demonstrated in the figure below, which shows a two-dimensional polygon that is projected on each axis. In the figure, "X" marks the point that is selected; the ranges marked with "U" indicate the uncertainty. Polk, et al. Standards Track [Page 12]
RFC 6225 DHCP Options for Coordinate LCI July 2011 ___ ___________ ^ | / | | | / | | | / | U | / | | | ( | V | | | --X | X | | | `---------. | | | | | | | | | - `-------------------------' |---------X---------------| |<------U------>| Key --- V, ^ = vertical arrows, delimiting the vertical uncertainty range. <> = horizontal arrows, delimiting the horizontal uncertainty range. Uncertainty applies to each axis independently. The amount of uncertainty can be determined from the encoding by taking 2 to the power of 8, less the encoded value, as is shown in the following formula, where "x" is the encoded integer value: uncertainty = 2 ^ ( 8 - x ) The result of this formula is expressed in degrees of latitude or longitude. The uncertainty is added to the base latitude or longitude value to determine the maximum value in the uncertainty range; similarly, the uncertainty is subtracted from the base value to determine the minimum value. Note that because lines of longitude converge at the poles, the actual distance represented by this uncertainty changes with the distance from the equator. If the maximum or minimum latitude values derived from applying uncertainty are outside the range of -90 to +90, these values are trimmed to within this range. If the maximum or minimum longitude values derived from applying uncertainty are outside the range of -180 to +180, then these values are normalized to this range by adding or subtracting 360 as necessary. Polk, et al. Standards Track [Page 13]
RFC 6225 DHCP Options for Coordinate LCI July 2011 The encoded value is determined by subtracting the next highest whole integer value for the base 2 logarithm of uncertainty from 8, as is shown by the following formula, where uncertainty is the midpoint of the known range less the lower bound of that range: x = 8 - ceil( log2( uncertainty ) ) Note that the result of encoding this value increases the range of uncertainty to the next available power of two; subsequent repeated encodings and decodings do not change the value. Only increasing uncertainty means that the associated confidence does not have to decrease. 2.4. Altitude How the altitude value is interpreted depends on the Altitude Type (AType) value and the selected datum. Three Altitude Type values are defined in this document: unknown (0), meters (1), and floors (2). 2.4.1. No Known Altitude (AType = 0) In some cases, the altitude of the location might not be provided. An Altitude Type value of zero indicates that the altitude is not given to the client. In this case, the Altitude and Altitude Uncertainty fields can contain any value and MUST be ignored. 2.4.2. Altitude in Meters (AType = 1) If the Altitude Type has a value of one, altitude is measured in meters, in relation to the zero set by the vertical datum. For AType = 1, the altitude value is expressed as a 30-bit, fixed-point, two's complement integer with 22 integer bits and 8 fractional bits. 2.4.3. Altitude in Floors (AType = 2) A value of two for Altitude Type indicates that the altitude value is measured in floors. Since altitude in meters may not be known within a building, a floor indication may be more useful. For AType = 2, the altitude value is expressed as a 30-bit, fixed-point, two's complement integer with 22 integer bits and 8 fractional bits. This value is relevant only in relation to a building; the value is relative to the ground level of the building. Floors located below ground level are represented by negative values. In some buildings, it might not be clear which floor is at ground level, or an intermediate floor might be hard to identify as such. Determining Polk, et al. Standards Track [Page 14]
RFC 6225 DHCP Options for Coordinate LCI July 2011 what floor is at ground level and what constitutes a sub-floor as opposed to a naturally numbered floor is left to local interpretation. Larger values represent floors that are farther away from floor 0 such that: - if positive, the floor value is farther above the ground floor. - if negative, the floor value is farther below the ground floor. Non-integer values can be used to represent intermediate or sub-floors, such as mezzanine levels. Example: a mezzanine between floor 1 and floor 2 could be represented as a value of 1.25. Example: mezzanines between floor 4 and floor 5 could be represented as values of 4.5 and 4.75. 2.4.4. Altitude Resolution In the DHCPv4 GeoConf Option 123, the altitude resolution (AltRes) value encodes the number of high-order altitude bits that should be considered valid. Values above 30 (decimal) are undefined and reserved. If the Altitude Type value is one (AType = 1), an AltRes value of 0.0 would indicate an 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. The AltRes field SHOULD be set to maximum precision when AType = 2 (floors) when a floor value is included in the DHCP Reply, or when AType = 0, to denote that the floor isn't known. An altitude coded as AType = 2, AltRes = 30, and Altitude = 0.0 is meaningful even outside a building, and represents ground level at the given latitude and longitude. 2.4.5. Altitude Uncertainty In the DHCPv6 GeoLoc Option 63 or the DHCPv4 GeoLoc Option 144, the AltUnc value quantifies the amount of uncertainty in the altitude value. As with LatUnc and LongUnc, a value of 0 for AltUnc is reserved to indicate that altitude uncertainty is not known; values above 30 are also reserved. Altitude uncertainty only applies to Altitude Type 1. Polk, et al. Standards Track [Page 15]
RFC 6225 DHCP Options for Coordinate LCI July 2011 The amount of altitude uncertainty can be determined by the following formula, where x is the encoded integer value: Uncertainty = 2 ^ ( 21 - x ) This value uses the same units as the associated altitude. Similarly, a value for the encoded integer value can be derived by the following formula: x = 21 - ceil( log2( uncertainty ) ) 2.5. Datum The Datum field determines how coordinates are organized and related to the real world. Three datums are defined in this document, based on the definitions in [OGP.Geodesy]: 1: WGS84 (Latitude, Longitude, Altitude): The World Geodetic System 1984 [WGS84] coordinate reference system. This datum is identified by the European Petroleum Survey Group (EPSG)/International Association of Oil & Gas Producers (OGP) with the code 4979, or by the URN "urn:ogc:def:crs:EPSG::4979". Without altitude, this datum is identified by the EPSG/OGP code 4326 and the URN "urn:ogc:def:crs:EPSG::4326". 2: NAD83 (Latitude, Longitude) + NAVD88: This datum uses a combination of the North American Datum 1983 (NAD83) for horizontal (Latitude and Longitude) values, plus the North American Vertical Datum of 1988 (NAVD88) vertical datum. This datum is used for referencing location on land (not near tidal water) within North America. NAD83 is identified by the EPSG/OGP code of 4269, or the URN "urn:ogc:def:crs:EPSG::4269". NAVD88 is identified by the EPSG/ OGP code of 5703, or the URN "urn:ogc:def:crs:EPSG::5703". 3: NAD83 (Latitude, Longitude) + MLLW: This datum uses a combination of the North American Datum 1983 (NAD83) for horizontal (Latitude and Longitude) values, plus the Mean Lower Low Water (MLLW) vertical datum. This datum is used for referencing location on or near tidal water within North America. Polk, et al. Standards Track [Page 16]
RFC 6225 DHCP Options for Coordinate LCI July 2011 NAD83 is identified by the EPSG/OGP code of 4269, or the URN "urn:ogc:def:crs:EPSG::4269". MLLW does not have a specific code or URN. All hosts MUST support the WGS84 datum (Datum 1). 3. Security Considerations Geopriv requirements (including security requirements) are discussed in "Geopriv Requirements" [RFC3693]. A threat analysis is provided in "Threat Analysis of the Geopriv Protocol" [RFC3694]. 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 of [RFC2131], Section 9.8 of [RFC2132]). Where critical decisions might be based on the value of this option, DHCP authentication as defined in "Authentication for DHCP Messages" [RFC3118] and "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)" [RFC3315] SHOULD be used to protect the integrity of the DHCP options. Link-layer confidentiality and integrity protection may also be employed to reduce the risk of location disclosure and tampering. 4. IANA Considerations 4.1. DHCP Options This document defines the DHCPv6 GeoLoc Option (see Section 2.1), which has been assigned a DHCPv6 option code of 63 per [RFC3315]: Value Description Reference ---- ------------------ ---------- 63 OPTION_GEOLOCATION RFC 6225 This document defines the DHCPv4 GeoConf Option (see Section 2.2.1), which has been assigned a DHCPv4 option code of 123 from the DHCP Option space. Polk, et al. Standards Track [Page 17]
RFC 6225 DHCP Options for Coordinate LCI July 2011 This document also defines the DHCPv4 GeoLoc Option (see Section 2.2.2), which has been assigned a DHCPv4 option code of 144 per [RFC2132] [RFC2939]: Data Tag Name Length Meaning Reference ---- ---- ------ ------- --------- 144 GeoLoc 16 Geospatial Location RFC 6225 with Uncertainty 4.2. Altitude Type Registry IANA has created and now maintains the Altitude Type registry following the guidelines below. The registry consists of three values: Altitude Type, Description, and Reference. These are described below. Altitude Type: An integer, refers to the value used in the DHCPv4 GeoConf and the DHCPv4 and DHCPv6 GeoLoc options described in this document. Values 0 - 2 are assigned. Values 3 - 15 are Unassigned [RFC5226]. Description: The description of the altitude described by this code. Reference: The reference to the document that describes the altitude code. This reference MUST define the way that the 30-bit altitude values and the associated 6-bit uncertainty are interpreted. Initial values are given below; new assignments are to be made following the "Standards Action" policies [RFC5226]. +------+---------------------+--------------+ | # | Description | Reference | +------+---------------------+--------------+ | 0 | No known altitude | RFC 6225 | | 1 | Altitude in meters | RFC 6225 | | 2 | Altitude in floors | RFC 6225 | | 3-15 | Unassigned | | +------+---------------------+--------------+ 4.3. Datum Registry IANA has created and now maintains the Datum registry following the guidelines below. The registry consists of three values: Datum, Description, and Reference. These are described below. Polk, et al. Standards Track [Page 18]
RFC 6225 DHCP Options for Coordinate LCI July 2011 Datum: An integer, refers to the value used in the DHCPv4 GeoConf and the DHCPv4 and DHCPv6 GeoLoc options described in this document. Value 0 is Reserved. Values 1 - 3 are assigned. Values 4 - 7 are Unassigned [RFC5226]. Description: The description of the altitude described by this code. Reference: The reference to the document that describes the Datum code. This reference MUST include specification of both the horizontal and vertical datum, and MUST define the way that the 34-bit values and the respective 6-bit uncertainties are interpreted. Initial values are given below; new assignments are to be made following the "Standards Action" policies [RFC5226]. +------+----------------------------------------+--------------+ | # | Description | Reference | +------+----------------------------------------+--------------+ | 0 | Reserved | RFC 6225 | +------+----------------------------------------+--------------+ | 1 | Vertical datum WGS 84 defined by EPSG | RFC 6225 | | | CRS Code 4327 | | +------+----------------------------------------+--------------+ | 2 | Vertical datum NAD83 defined by EPSG | RFC 6225 | | | CRS Code 4269 with North American | | | | Vertical Datum of 1988 (NAVD88) | | +------+----------------------------------------+--------------+ | 3 | Vertical datum NAD83 defined by EPSG | RFC 6225 | | | CRS Code 4269 with Mean Lower Low Water| | | | (MLLW) as associated vertical datum | | +------+----------------------------------------+--------------+ | 4-7 | Unassigned | | +------+----------------------------------------+--------------+