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rfc:rfc5964

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.

Winterbottom & Thomson Standards Track [Page 1] RFC 5964 Service Boundary Holes August 2010

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.

Winterbottom & Thomson Standards Track [Page 2] RFC 5964 Service Boundary Holes August 2010

     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.

Winterbottom & Thomson Standards Track [Page 3] RFC 5964 Service Boundary Holes August 2010

     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.

Winterbottom & Thomson Standards Track [Page 4] RFC 5964 Service Boundary Holes August 2010

     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.

Winterbottom & Thomson Standards Track [Page 5] RFC 5964 Service Boundary Holes August 2010

 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).

Winterbottom & Thomson Standards Track [Page 6] RFC 5964 Service Boundary Holes August 2010

     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

Winterbottom & Thomson Standards Track [Page 7] RFC 5964 Service Boundary Holes August 2010

 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>

Winterbottom & Thomson Standards Track [Page 8] RFC 5964 Service Boundary Holes August 2010

             <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.

Winterbottom & Thomson Standards Track [Page 9] RFC 5964 Service Boundary Holes August 2010

 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.

Winterbottom & Thomson Standards Track [Page 10] RFC 5964 Service Boundary Holes August 2010

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

Winterbottom & Thomson Standards Track [Page 11]

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