Dynamic Host Configuration Protocol Option for Coordinate-based Location Configuration Information
obsoleted by
- rfc-6225 — Dynamic Host Configuration Protocol Options for Coordinate-Based Location Configuration Information
Extracted elements (26)
Latitude and Longitude are encoded as fixed-point 2s-complement binary rather than decimal digit strings (as in RFC 1712) to achieve a smaller option size. The 9-bit integer part accommodates +/- 180 degrees; the 25-bit fraction provides better than 7 decimal digits of precision.
Location information is stored on the end device after DHC exchange rather than retrieved from a remote service, improving availability in emergency scenarios where network access to a location database may be impractical.
Per-dimension resolution subfields (LaRes, LoRes, AltRes) allow independent control of precision for each coordinate axis. This design supports adjusting reported precision without requiring separate option encodings, and allows the LCI to fit in protocols like DHCP that have limited packet size (the full option is only 18 bytes).
The 'floors' Altitude Type (AT=2) was introduced because altitude in meters may not be known or meaningful within a building, making a floor number more operationally useful. Floor values are locally significant and agreed upon between building operators and data users; standardizing floor-to-meter mapping is explicitly out of scope.
The WGS 84 datum (Datum=1) is three-dimensional and therefore inherently includes both horizontal and vertical references. For other datums, both horizontal and vertical references must be explicitly specified in the IANA codepoint record.
Wireless hosts can use this option to learn the location of their radio access point at configuration time, but obtaining their own precise location requires additional mechanisms such as GPS or a future DHCP option carrying a list of nearby access point geo-locations.
Any additional LCI Altitude Types to be defined for use via this DHC Option MUST be done through a Standards Track RFC.
Any additional LCI datum values to be defined for use via this DHC Option MUST be done through a Standards Track RFC. The datum indicator MUST include specification of both horizontal and vertical datum references.
Latitude SHOULD be normalized to within +/- 90 degrees. Positive values are north of the equator and negative values are south.
Longitude SHOULD be normalized to within +/- 180 degrees. Positive values are east of the prime meridian and negative (2s complement) values are west.
The AltRes field SHOULD be set to maximum precision when AT=2 (floors) and a floor value is included in the DHCP Reply; alternatively, AT SHOULD be set to 0 to denote that the floor is not known.
The LCI option SHOULD be returned by DHCP servers only when the DHCP client has included this option in its 'parameter request list', to minimize unintended exposure of location information.
Where critical decisions might be based on the value of this GeoConf option, DHCP authentication (RFC 3118) SHOULD be used to protect the integrity of the DHCP options.
Contents beyond the claimed resolution (LaRes/LoRes/AltRes) MAY be randomized to obscure greater precision that might otherwise be available, providing a built-in mechanism for deterministic location imprecision. However, the resolution fields explicitly do not define Geographic Privacy policy.
AT=2 (Floors) allows altitude to be expressed as a building floor number, using a 2s-complement fixed-point value. Floor 0.0 represents ground level at the main entrance; sub-floors use non-integer values (e.g., 1.1 for a mezzanine); below-grade floors use negative values.
DHCP Option 123 (GeoConf) carries the Location Configuration Information (LCI) for a client's coordinate-based geographic location. The option code is 123 and the data length is 16 bytes, making the full option 18 bytes including code and length octets.
The AltRes field (6 bits) indicates the number of valid bits in the Altitude value. Values above 30 are undefined and reserved. If AT=1 (meters), an AltRes of 0 indicates unknown altitude; maximum precision is AltRes=30.
The LaRes and LoRes resolution fields (6 bits each) indicate the number of high-order bits in Latitude and Longitude that are valid. Bits beyond the resolution limit SHOULD NOT be considered valid and may be deliberately falsified or zeroed. Values above decimal 34 are undefined and reserved.
IANA maintains a registry for the Altitude Type (AT) field in the GeoConf option. New values are assigned through Standards Action (RFC 2434). Initial values: AT=1 (meters), AT=2 (building floors). Additional AT types require a Standards Track RFC.
IANA maintains a registry for the Datum field (8-bit, 256 possible values). Initial values: 1=WGS 84 (CRS 4327), 2=NAD83/NAVD88 (land), 3=NAD83/MLLW (water/tidal). Additional datums require a Standards Track RFC.
DHCP messages have no privacy protection; an eavesdropper on the link between the DHCP server and client can discover the LCI. DHCP authentication (RFC 3118) SHOULD be used when location data is security-critical.
Altitude is a 30-bit field whose interpretation depends on the 4-bit AT (Altitude Type) field. AltRes is a 6-bit resolution subfield preceding Altitude. AT=1 encodes meters; AT=2 encodes floors; both use 2s-complement fixed-point with 22-bit integer and 8-bit fraction.
Latitude is a 34-bit 2s-complement fixed-point value with 9 bits of integer part and 25 bits of fraction, preceded by a 6-bit LaRes resolution field. Positive values are north of the equator; negative values are south.
Longitude is a 34-bit 2s-complement fixed-point value with 9 bits of integer part and 25 bits of fraction, preceded by a 6-bit LoRes resolution field. Positive values are east of the prime meridian; negative (2s complement) values are west.
The Datum field is 8 bits (1 byte) and specifies the map datum for the coordinates. Three values are defined: 1=WGS 84 (3D), 2=NAD83 with NAVD88 vertical datum (land use), 3=NAD83 with MLLW vertical datum (water use).
The LCI option wire format is: Code=123 (8 bits), Length=16 (8 bits), LaRes (6 bits), Latitude (34 bits), LoRes (6 bits), Longitude (34 bits), AT (4 bits), AltRes (6 bits), Altitude (30 bits), Datum (8 bits). Total payload is 128 bits (16 bytes).