Internet Engineering Task Force (IETF)                   J. Winterbottom
Request for Comments: 5964                                    M. Thomson
Category: Standards Track                             Andrew Corporation
ISSN: 2070-1721                                              August 2010


       Specifying Holes in Location-to-Service Translation (LoST)
                           Service Boundaries

Abstract

   This document describes how holes can be specified in geodetic
   service boundaries.  One means of implementing a search solution in a
   service database, such as one might provide with a Location-to-
   Service Translation (LoST) server, is described.

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/rfc5964.

Copyright Notice

   Copyright (c) 2010 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.







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Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. Specifying Holes ................................................3
   4. GML Polygons ....................................................6
   5. Holes in GML Polygons ...........................................6
   6. Service Boundary Specification and Selection Algorithm ..........7
   7. Security Considerations ........................................10
   8. Acknowledgements ...............................................10
   9. References .....................................................10
      9.1. Normative References ......................................10
      9.2. Informative References ....................................10

1.  Introduction

   The LoST protocol [RFC5222] maps service and locations to destination
   addresses.  A LoST server does this by provisioning boundary maps or
   areas against service URNs.  The boundary is a polygon made up of
   sets of geodetic coordinates specifying an enclosed area.  In some
   circumstances, an area enclosed by a polygon, also known as an
   exterior polygon, may contain exception areas, or holes, that for the
   same service must yield a different destination to that described by
   the larger area.

   This document describes a profile of Geographic Markup Language (GML)
   [ISO-19107] polygons that constrains their representation when used
   for describing service boundaries.  The profile removes a number of
   permutations that are difficult to process.  This allows for
   simplified implementations that are not capable of handling all
   potential variations allowed by GML.  A fully conformant GML
   implementation must produce polygons that fit this profile to ensure
   interoperability.


















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       o--------------o
      /                \
     /    /\            \
    /    + +-----+       \
   o     |  Hole  \       o
   |     |    1   /       |
   |     +-------+        |<--- Primary Polygon
   |        +-------+     |
   |       /  Hole  |     |
   o       \   2    |     o
    \       +-----+ +    /
     \             \/   /
      \                /
       o--------------o

   Figure 1: Holes in a Polygon

   This document describes a profile of GML [ISO-19107] polygons that
   constrains their representation when used for describing service
   boundaries.

   The working group considered that the types of regions described in
   this memo could be represented in various ways as polygons without
   holes, but concluded on the recommendations here to avoid potential
   problems with the arbitrary division of regions and to align with
   existing geospatial system practices.

2.  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 [RFC2119].

3.  Specifying Holes

   Holes related to an exterior boundary polygon MUST adhere to the
   following rules:

   Rule 1:   Two holes MUST NOT have more than one point of
             intersection.

   If two or more holes overlap or share a common boundary, then these
   represent a single hole.  The internal elements (holes) should have
   common boundaries removed and a single hole created irrespective of
   whether the excluded area is itself made up of multiple service
   boundaries.





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       o--------------o                      o--------------o
      /                \                    /                \
     /    /\            \                  /    /\            \
    /    + +-----+       \                /    + +-----+       \
   o     |  Hole  \       o              o     |        \       o
   |     |    1    \      |              |     |  One    \      |
   |     +-+-------+      |  =========>  |     +-+  Hole +      |
   |       /  Hole  |     |              |       /        |     |
   o       \   2    |     o              o       \        |     o
    \       +-----+ +    /                \       +-----+ +    /
     \             \/   /                  \             \/   /
      \                /                    \                /
       o--------------o                      o--------------o

          Incorrect                              Correct

            Figure 2: Hole Specification with Boundary Sharing

   Rule 2:   A polygon MUST describe a contiguous region.

   If a hole overlaps with the outer boundary, or it shares part of a
   side with the outer boundary, then it has an inlet and it MUST be
   expressed without the hole.

              +------- Inlet
              |
              v
       o---+-----+----o                     o---o     o----o
      /    |%%%%%|     \                   /    |     |     \
     /    /%%%%%%|      \                 /    /      |      \
    /    +%%%%%%%|       \               /    o       o       \
   o     |%%%%%%%%\       o             o     |        \       o
   |     |%%%%%%%%%\      |             |     |         \      |
   |     +-+%%%%%%%%+     |  ========>  |     o-o        o     |
   |       /%%%%%%%%|     |             |       /        |     |
   o       \%%%%%%%%|     o             o       \        |     o
    \       +-----+ +    /               \       o-----o o    /
     \             \/   /                 \             \/   /
      \                /                   \                /
       o--------------o                     o--------------o

          Incorrect                             Correct

                    Figure 3: Specification of an Inlet

   If a hole touches the outer boundary in two places, the region MUST
   be expressed as two separate polygons.




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       A--q-----------B                     A-q   q----------B
      /  | |           \                   /  |   |           \
     /   | |            \                 /   |   |            \
    /    z r-----s       \               / P  z   r-----s   P   \
   H     |        \       C             H  o  |          \   o   C
   |     |  One    \      |             |  l  |           \   l  |
   |     y-x  Hole  t     |  ========>  |  y  y-x          t  y  |
   |       /        |     |             |  g    /          |  g  |
   G       \        |     D             G  o    \          |  o  D
    \      /    v---u    /               \ n    /      v---u  n /
     \     \   /        /                 \  1  \     /      2 /
      \     \ /        /                   \     \   /        /
       F-----w--------E                     F-----w w--------E

          Incorrect                               Correct

       Figure 4: Specification of Hole with Multiple Outer-Boundary
                               Intersections

   Similarly, a polygon that is enclosed entirely within a hole from
   another polygon (i.e., an "island") is a separate polygon.

          o--------------o
         /                \
        / +--------------+ \
       /  |%%%%%%%%%%%%%%|  \
      o   |%%o--------o%%|   o
      |   |%/  Island  \%|   |
      |   |%\          /%|   |
      |   |%%o--------o%%|   |
      o   |%%%%%%%%%%%%%%|   o
       \  +--------------+  /
        \                  /
         \                /
          o--------------o

   Figure 5: Hole with Enclosed Polygon (Island)

   Rule 3:   A hole MUST be formed from a legal linear ring in
             accordance with [geoshape], except that points are
             specified in a clockwise direction.

   Holes are specified in a clockwise direction so that the upward
   normal is opposed to the upward normal of the exterior boundary of
   the polygon.  Note that [geoshape] stipulates that exterior
   boundaries are specified in counterclockwise order.





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   There is no restriction on the number of points that are used to
   express the perimeter of either exterior or interior boundaries.

4.  GML Polygons

   The GML encoding of a polygon defines a enclosed exterior boundary,
   with the first and last points of boundary being the same.  Consider
   the example in Figure 6.

       F--------------E
      /                \
     /                  \
    /                    \
   A                      D
    \                    /
     \                  /
      \                /
       B--------------C

   <gml:Polygon srsName="urn:ogc:def:crs:EPSG::4326">
     <gml:exterior>
       <gml:LinearRing>
         <gml:pos>43.311 -73.422</gml:pos> <!--A-->
         <gml:pos>43.111 -73.322</gml:pos> <!--B-->
         <gml:pos>43.111 -73.222</gml:pos> <!--C-->
         <gml:pos>43.311 -73.122</gml:pos> <!--D-->
         <gml:pos>43.411 -73.222</gml:pos> <!--E-->
         <gml:pos>43.411 -73.322</gml:pos> <!--F-->
         <gml:pos>43.311 -73.422</gml:pos> <!--A-->
       </gml:LinearRing>
     </gml:exterior>
   </gml:Polygon>

                   Figure 6: Hexagon and Associated GML

   Note that polygon vertices in Figure 6 are expressed using <pos>
   elements for clarity.  The vertices can also be expressed using a
   <posList> element.

5.  Holes in GML Polygons

   A hole is specified in the polygon by defining an interior boundary.
   The points defining the internal boundary define the area represented
   by the hole in the primary (exterior) polygon.  The shaded area in
   Figure 7 is represented by the 4 points of the interior boundary
   specified by (w,z,y,x).





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       F-------------E
      /               \
     / w-------------x \
    /  |/////////////|  \
   A   |/////////////|   D
    \  |/////////////|  /
     \ z-------------y /
      \               /
       B-------------C

   <gml:Polygon srsName="urn:ogc:def:crs:EPSG::4326">
     <gml:exterior>
       <gml:LinearRing>
         <gml:pos>43.311 -73.422</gml:pos> <!--A-->
         <gml:pos>43.111 -73.322</gml:pos> <!--B-->
         <gml:pos>43.111 -73.222</gml:pos> <!--C-->
         <gml:pos>43.311 -73.122</gml:pos> <!--D-->
         <gml:pos>43.511 -73.222</gml:pos> <!--E-->
         <gml:pos>43.511 -73.322</gml:pos> <!--F-->
         <gml:pos>43.311 -73.422</gml:pos> <!--A-->
       </gml:LinearRing>
     </gml:exterior>
     <gml:interior>
       <gml:LinearRing>
         <gml:pos>43.411 -73.322</gml:pos> <!--w-->
         <gml:pos>43.411 -73.222</gml:pos> <!--x-->
         <gml:pos>43.211 -73.222</gml:pos> <!--y-->
         <gml:pos>43.211 -73.322</gml:pos> <!--z-->
         <gml:pos>43.411 -73.322</gml:pos> <!--w-->
       </gml:LinearRing>
     </gml:interior>
   </gml:Polygon>

                        Figure 7: Hexagon with Hole

6.  Service Boundary Specification and Selection Algorithm

   A service boundary is represented by a polygon that may have many
   vertices.  The enclosed area of the polygon represents the area in
   which a service, expressed as a service URN, maps to a single URI.

   Figure 7 is used to illustrate two service boundaries.  The first
   service boundary A->F shall be referred to as area-A, and the second
   service boundary w->z shall be referred to as area-w.  Furthermore,
   area-A is directly represented by the GML encoding provided in
   Figure 7.  Area-w is represented as a hole in area-A by the interior





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   boundary.  Since area-w is also a service boundary, a separate
   polygon describing this area is also required and is shown in
   Figure 8 (note the reversal of the vertices).

   <gml:Polygon srsName="urn:ogc:def:crs:EPSG::4326">
     <gml:exterior>
       <gml:LinearRing>
         <gml:pos>43.411 -73.322</gml:pos> <!--w-->
         <gml:pos>43.211 -73.322</gml:pos> <!--z-->
         <gml:pos>43.211 -73.222</gml:pos> <!--y-->
         <gml:pos>43.411 -73.222</gml:pos> <!--x-->
         <gml:pos>43.411 -73.322</gml:pos> <!--w-->
       </gml:LinearRing>
     </gml:exterior>
   </gml:Polygon>

                         Figure 8: GML for Area-w

   Service mappings for these boundaries might be provided by a LoST
   server in the form shown in Figure 9.

     <mapping xmlns="urn:ietf:params:xml:ns:lost1"
              expires="2010-12-25T09:44:33Z"
              lastUpdated="2010-03-08T03:48:22Z"
              source="authoritative.foo.example"
              sourceId="7e3f40b098c711dbb606011111111111">
       <displayName xml:lang="en">Outer Area Police</displayName>
       <service>urn:service:sos.police</service>
       <serviceBoundary profile="geodetic-2d">
         <gml:Polygon xmlns:gml="http://www.opengis.net/gml"
                      srsName="urn:ogc:def:crs:EPSG::4326">
           <gml:exterior>
             <gml:LinearRing>
               <gml:pos>43.311 -73.422</gml:pos>
               <gml:pos>43.111 -73.322</gml:pos>
               <gml:pos>43.111 -73.222</gml:pos>
               <gml:pos>43.311 -73.122</gml:pos>
               <gml:pos>43.511 -73.222</gml:pos>
               <gml:pos>43.511 -73.322</gml:pos>
               <gml:pos>43.311 -73.422</gml:pos>
             </gml:LinearRing>
           </gml:exterior>
           <!-- this is the service boundary hole -->
           <gml:interior>
             <gml:LinearRing>
               <gml:pos>43.411 -73.322</gml:pos>
               <gml:pos>43.211 -73.322</gml:pos>
               <gml:pos>43.211 -73.222</gml:pos>



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               <gml:pos>43.411 -73.222</gml:pos>
               <gml:pos>43.411 -73.322</gml:pos>
             </gml:LinearRing>
           </gml:interior>
         </gml:Polygon>
       </serviceBoundary>
       <uri>sip:area-A-pd@example.com</uri>
       <uri>xmpp:area-A-pd@example.com</uri>
       <serviceNumber>000</serviceNumber>
     </mapping>

     <mapping xmlns="urn:ietf:params:xml:ns:lost1"
              expires="2010-12-25T09:44:33Z"
              lastUpdated="2010-03-08T03:48:22Z"
              source="authoritative.foo.example"
              sourceId="7e3f40b098c711dbb606011111111111">
       <displayName xml:lang="en">Inner Area Police</displayName>
       <service>urn:service:sos.police</service>
       <serviceBoundary profile="geodetic-2d">
         <gml:Polygon xmlns:gml="http://www.opengis.net/gml"
                      srsName="urn:ogc:def:crs:EPSG::4326">
           <gml:exterior>
             <gml:LinearRing>
               <gml:pos>43.411 -73.322</gml:pos>
               <gml:pos>43.211 -73.322</gml:pos>
               <gml:pos>43.211 -73.222</gml:pos>
               <gml:pos>43.411 -73.222</gml:pos>
               <gml:pos>43.411 -73.322</gml:pos>
             </gml:LinearRing>
           </gml:exterior>
         </gml:Polygon>
       </serviceBoundary>
       <uri>sip:area-w-pd@example.com</uri>
       <uri>xmpp:area-w-pd@example.com</uri>
       <serviceNumber>000</serviceNumber>
     </mapping>

                 Figure 9: Service Boundary Specifications

   It is considered likely that LoST servers will need to provide
   responses sufficiently quickly to allow real-time queries to be
   performed as part of an emergency call routing flow.  It is for this
   reason that databases supporting native geospatial query techniques
   are desirable and that service boundary specifications that are
   easily mapped to internal data structures are preferred.  Using
   interior boundaries makes support for this operation easy, while
   allowing an arbitrary number of holes in a service boundary to be
   specified.



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   Each polygon is stored in the geospatial database and mapped to a
   service URN and destination URI.  Many geospatial databases natively
   support polygons with interior exclusions.  Without native support,
   interior boundaries can be stored against the polygon and can checked
   separately.  A location falls within the area described by a polygon
   if it is within the exterior boundary and not within any interior
   boundary.

   In the above example, if a location falls within the interior
   boundary, it maps to the "Inner Area Police" service; likewise, if a
   location falls within the exterior boundary, but not within the
   interior boundary, it maps to the "Outer Area Police" service.

7.  Security Considerations

   Constraining the form of a polygon representation as described in
   this document does not introduce new security considerations.

8.  Acknowledgements

   Thanks to Carl Reed for input provided to the list some months back
   and for reviewing this document.  Thanks to Michael Haberler for
   suggesting that such a specification is required.  Thanks to Avery
   Penniston for review and feedback.

9.  References

9.1.  Normative References

   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5222]    Hardie, T., Newton, A., Schulzrinne, H., and H.
                Tschofenig, "LoST: A Location-to-Service Translation
                Protocol", RFC 5222, August 2008.

   [geoshape]   Thomson, M. and C. Reed, "GML 3.1.1 PIDF-LO Shape
                Application Schema for use by the Internet Engineering
                Task Force (IETF)", Candidate OpenGIS Implementation
                Specification 06-142r1, Version: 1.0, April 2007.

9.2.  Informative References

   [ISO-19107]  ISO, "Geographic information - Spatial Schema", ISO
                Standard 19107, First Edition, May 2003.






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Authors' Addresses

   James Winterbottom
   Andrew Corporation
   Andrew Building (39)
   Wollongong University Campus
   Northfields Avenue
   Wollongong, NSW  2522
   AU

   EMail: james.winterbottom@andrew.com


   Martin Thomson
   Andrew Corporation
   Andrew Building (39)
   Wollongong University Campus
   Northfields Avenue
   Wollongong, NSW  2522
   AU

   EMail: martin.thomson@andrew.com





























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