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Versions: (draft-schnizlein-geopriv-binary-lci) 00 01

GEOPRIV                                                    J. Schnizlein
Internet-Draft                                                M. Linsner
Intended status: Informational                             Cisco Systems
Expires: June 10, 2008                                   December 10, 2007


   Binary to Decimal Conversion for Location Configuration Information
                       draft-ietf-geopriv-binary-lci-01


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   Copyright (C) The IETF Trust (2007)












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Abstract

   This document describes the nature of the data expressed in the
   geographic LCI defined in RFC 3825, and includes examples of
   conversion from its binary format to decimal character strings.


Table of Contents

   1.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.   Definitions  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.   Overview   . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.   Programming Hints  . . . . . . . . . . . . . . . . . . . . .   4
   6.   Calculation of LCI values  . . . . . . . . . . . . . . . . .   5
   7.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .   6
   8.   Security . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   9.   References . . . . . . . . . . . . . . . . . . . . . . . . .   7
      9.1   Normative References . . . . . . . . . . . . . . . . . .   7
      9.2   Informative References . . . . . . . . . . . . . . . . .   7
   10.  Author's Address . . . . . . . . . . . . . . . . . . . . . .   7


1.  Terminology

    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 [0].

2.  Definitions

   This document uses the following terms to describe geo LCI binary to
decimal conversion:

   Location Configuration Information: (LCI) An object that carries
location information.  LCI has no ability to express privacy rules as
outlined in [3] and [4], therefore is considered part of the 'sighting'
function.  For purposes of this discussion, all references to LCI refer
to its use in [1].

   GNU Compiler Collection: (GCC) The GNU Compiler Collection is a set
of programming language compilers produced by the GNU Project.








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3.  Introduction

  The LCI encodes a point's latitude, longitude and altitude, along with
the resolution of that point.  LCI does not encode boundaries of an
arbitrary region.  The resolution is nothing more than the
representation of significant digits for the fixed-length, binary values
in the LCI.  This document corrects misinterpretations of the non-
normative examples in [1].

  Format conversion is required between the binary LCI that a host can
receive through DHCP [1] or LLDP-MED [5] and the decimal representation
used by applications, e.g. PIDF-LO [2].  This conversion could be used
by a host that provides its location to another party with the privacy
rules of the [2], including to a server authorized to redistribute the
information.  It is unclear why anyone would need to convert from the
geographic-coordinate location format of [2] to the LCI.


4. Overview

  This section provides an overview of the programming hints in the next
section for the translation from the efficient binary representation of
the LCI [1][5] to the decimal string representation of geographic
location used in PIDF-LO [2], for example.  GCC syntax is used because
it is well known.  The binary values are converted to decimal, with the
invalid bits removed and with the number of significant digits
determined by the resolution of the binary values.

  After unpacking the network-order bytes of the LCI into C variables
sufficiently large to accommodate the fields, the sign bit of the two’s-
complement integers are extended to the size of the variable.  The sign
bit at 34 bits to the left is tested with an octal constant containing
33 bits in 11 octal-digits of zero.  If negative, the sign is extended:
the upper bits are set to ‘1’ by ORing the value with a value of
minus-one with the lower 34 bits inverted to zero with XOR.  This
operation is safe to perform more than once.

  Because [1] says "Contents beyond the claimed resolution MAY be
randomized ...", these contents are erased, i.e. set to zero.  The
number of bits to erase is the field length minus the resolution of the
value in that field.  A mask is constructed by left-shifting a one into
the right of the mask for as many bits as to be erased.  ANDing the
inverse of the mask with the value erases the invalid bits.

  The fixed-point fraction values are scaled into a floating-point
(double for enough precision) by dividing by the constant reflecting the
number of fractional bits.  Note that latitude and longitude have 25
bits of fraction, while altitude has only 22 bits.  The number of
significant digits to the right of the decimal point is the resolution
minus its integer portion, scaled by 3 decimal digits for 10 binary
digits because 10 to the 3rd = 1000 approximates 2 to the 10th = 1024.


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5. Programming hints

    The LCI format is as follows:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Code 123    |      16       |   LaRes   |     Latitude      +
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                Latitude (cont'd)              |    LoRes  |   +
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                             Longitude                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   AT  |   AltRes  |                Altitude                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Alt (cont'd) |     Datum     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Assume the following element values have been unpacked from the 16 bytes
of the wire protocol above.

struct LCIoption {
int8_t code;        /* DHCP LCI option code = 123  */
int8_t length;      /* length 16 bytes - not incl code + length */
int8_t LaRes;       /* Latitude Resolution 6 bits */
int64_t Latitude;   /* Latitude 34 bits, 25 fractional */
int8_t LoRes;       /* Longitude Resolution 6 bits */
int64_t Longitude;  /* Latitude 34 bits, 25 fractional */
int8_t AltType;     /* Altitude Type 4 bits */
int8_t AltRes;      /* Altitude Resolution 6 bits */
int64_t Altitude;   /* Altitude 30 bits 22 bits Fraction */
int8_t Datum;       /* Datum code 8 bits */
};

Because the latitude, longitude, and altitude values are twos complement
of non-standard length, they require sign-extension that is not built
into typical variable types. For the Latitude example:

struct LCIoption OptIn;


/* if negative 34-bit field, set all one-bits above the 34-bit field */
if (OptIn.Latitude & 0100000000000LL)
          OptIn.Latitude = OptIn.Latitude | (-1 ^ 0177777777777LL)
/* XOR '^' to flip one bits to zero before ORing in the field */




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Translation from the binary resolution of the LCI to the correct number
of significant decimal digits in the character string representation
used for numbers in PIDF-LO is as in the following example for Latitude:

int8_t eraseBits = 34 - OptIn.LaRes;
int64_t mask = 0LL;
if (eraseBits > 0) while (eraseBits--) mask = (mask << 1) | 1;

/* invert mask and AND to zero invalid bits */
OptIn.Latitude &= ~mask;


double latitude = OptIn.Latitude / exp2 (25);
                      /* scale integer for 25 bits of fraction */
int8_t LatFractDigits = (OptIn.LaRes - 9) * 3 / 10;
                      /* deduct integer part, 2 to 10 ~= 10 to 3  */

if (LatFractDigits < 0) LatFractDigits = 0;
                      /* report integer part if resolution is lower */
printf ("%11.*F\n", LatFractDigits, latitude);

6. Calculation of LCI values

Since the Global Positioning System (GPS) or survey methods do not
provide location in the LCI format, this section illustrates how a
network administrator might calculate the values in preparation for
delivering them to hosts connected to her network.

Where geographic location is expressed with the correct number of
significant digits, it is easy to compute resolution because 3 decimal
digits approximate 10 bits.  The number of digits to the right of the
decimal point, times 10, divided by 3 is the number of fractional bits.
Adding 9 for the integer part yields the resolution.

Where a geographic location comes with an explicit error specification,
this error can be translated into the resolution of the LCI.  If the
error measure is in distance (e.g. meters) rather than degrees, the
conversion of longitude to degrees depends on the distance from the
equator.  Dividing the error distance by the distance for one degree
(computed with the method described at [6]) yields the error in
(presumably fractional) degrees.









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double DegreeError;
int64_t FixedPntErrDeg = degreeError * exp2 (25);
/*  convert error to fixed point 25-bit  */

int64_t TopBit = 0100000000000LL;
if (FixedPntErrDeg & TopBit) FixedPntErrDeg = - FixedPntErrDeg;
/* if negative make positive */

/* shift test bit to find first non-zero error */
int8_t resolution = 1;
while ((FixedPntErrDeg & (TopBit >>= 1)) == 0LL) {
     if (TopBit == 0LL) break;
     resolution++;
     }
/* the shift count is the number of valid bits */

If all that is available is the bounding points of a region, the
difference between the extremes and the center in both latitude and
longitude estimates the error in degrees, which can be converted to
resolution as above.  Find the maximum and minimum of both, calculate
the value of the latitude/longitude as the average, and half the
difference as the error.

For the example bounds ranging about 0.5 meters in distances across
about 32 degrees, the binary and decimal values are as follows:

             binary                    decimal
000011111.1111111111111111111001110  31.99999850
000100000.0000000000000000001011100  32.00000274

001000000.0000000000000000000101010  64.00000124  sum
000100000.0000000000000000000010101  32.00000062  average
000000000.0000000000000000010001110  00.00000423  difference



With 26 bits above the difference, which is twice the error, this
example yields 27 bits of resolution (remembering to add 9 bits for left
of the binary point).


7. IANA Considerations

No IANA Considerations


8. Security

This document discusses binary to decimal conversion within an end host,
which raises no particular security considerations.


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9.  References

9.1  Normative References

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

 [1]  RFC 3825 Dynamic Host Configuration Protocol Option for
     Coordinate-based Location Configuration Information. J. Polk, J.
    Schnizlein, M. Linsner. July 2004.

 [2]  RFC 4119 A Presence-based GEOPRIV Location Object Format. J.
     Peterson. December 2005.

 [3]  RFC 3693 Geopriv Requirements. J. Cuellar, J. Morris, D. Mulligan,
     J. Peterson, J. Polk.  February, 2004.

 [4]  RFC 3694 Threat Analysis of the Geopriv Protocol. M. Danley, D.
     Mulligan, J. Morris, J. Peterson. February 2004


9.2  Informative References

  [5]  TIA-1057 (LLDP-MED) The Telecommunications Industry Association
    (TIA) standard, "Telecommunications - IP Telephony Infrastructure -
    Link Layer Discovery Protocol (LLDP) for Media Endpoint Devices.

  [6]  "Problem 2A.: Calculate path length along a meridian given
       starting and ending coordinates". Andy McGovern. April 2004
       http://www.codeguru.com/Cpp/Cpp/algorithms/general/article.php/c5
       115

10.   Author's Address

      John Schnizlein
       Cisco Systems, Inc.
       Fort Washington, MD, USA
       Email: john.schnizlein@cisco.com

      Marc Linsner
       Cisco Systems, Inc.
       Marco Island, Florida, USA
       Email: marc.linsner@cisco.com


Comments are solicited and should be addressed to the working group's
mailing list at geopriv@ietf.org and/or the authors.





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