[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

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

GEOPRIV                                                    J. Schnizlein
Internet-Draft                                                M. Linsner
Expires: July 1, 2007                                      Cisco Systems
                                                       December 28, 2006


   Binary to Decimal Conversion for Location Configuration Information
                       draft-ietf-geopriv-binary-lci-00

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on July 1, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2006)



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.











Schnizlein, Linsner         Expires July 1, 2007                 [Page 1]


Internet-Draft             Binary to Decimal LCI             December 2006

Table of Contents

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

1.  Terminology

   In this document, the key words "MUST", "MUSTNOT", "REQUIRED",
   "SHALL", "SHALLNOT", "SHOULD", "SHOULDNOT", "RECOMMENDED", "MAY", and
   "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and
   indicate requirement levels for compliant implementations.

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.

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.




Schnizlein, Linsner        Expires July 1, 2007                   [Page 2]


Internet-Draft             Binary to Decimal LCI             December 2006

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.

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     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





Schnizlein, Linsner        Expires July 1, 2007                   [Page 3]


Internet-Draft             Binary to Decimal LCI             December 2006


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 */

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);





Schnizlein, Linsner        Expires July 1, 2007                   [Page 4]


Internet-Draft             Binary to Decimal LCI             December 2006

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.

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


Schnizlein, Linsner        Expires July 1, 2007                   [Page 5]


Internet-Draft             Binary to Decimal LCI             December 2006



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.


9.  References

9.1  Normative References

  [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








Schnizlein, Linsner        Expires July 1, 2007                   [Page 6]


Internet-Draft             Binary to Decimal LCI             December 2006


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.





Appendix A.

Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.


Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET


Schnizlein, Linsner        Expires July 1, 2007                   [Page 7]


Internet-Draft             Binary to Decimal LCI             December 2006

   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.





































Schnizlein, Linsner        Expires July 1, 2007                   [Page 8]


Html markup produced by rfcmarkup 1.129b, available from https://tools.ietf.org/tools/rfcmarkup/