Specifying Holes in LoST Service Boundaries
draft-winterbottom-ecrit-specifying-holes-00.txt

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 April 13, 2008.

Abstract

This document describes how holes can be specified in service boundaries. One means of implementing a solution is described.

Table of Contents

1.  Introduction
2.  Terminology
3.  Specifying Holes
4.  GML Polygons
5.  Holes in GML Polygons
6.  Service Boundary Specification and Selection Algorithm
7.  Security Considerations
8.  IANA Considerations
9.  Acknowledgements
10.  References
    10.1.  Normative References
    10.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

The LoST protocol [I‑D.ietf‑ecrit‑lost] (Hardie, T., Newton, A., Schulzrinne, H., and H. Tschofenig, “LoST: A Location-to-Service Translation Protocol,” May 2008.) describes a protocol that's primary purpose is to map service and locations to destination addresses. LoST 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 how holes SHOULD be specified in service boundaries defined using a GML encoding for the polygons and their internal elements (holes). GML polygons are based on elements defined in [ISO‑19107] (ISO, “Geographic information - Spatial Schema,” 5 2003.).

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

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

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 share a common set of boundaries then to the primary polygon these represent a single hole in the service. 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.

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

       Incorrect                              Correct

Rule 2:

A hole MUST NOT have more than one point of intersection with the outer-boundary of the primary (exterior) polygon. If more than one point of intersection occurs the primary polygon is either doesn't have a hole, it has an inlet as in Figure 1 (Correct Specification of an Inlet), or the primary polygon SHOULD be expressed as two polygons as in Figure 2 (Correct Specification of Hole with Multiple Outer-Boundary Intersections).

           +------- 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

    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

      1 Polgon with a                     2 Polygons that map
       Dividing Hole                      to the same service

Similarly, a polygon containing a hole with an island must be represented as two polygons mapping to the same service.

Rule 3:

A hole MUST be a legal polygon in accordance with the geoshape specification [geoshape] (Thomson, M. and C. Reed, “GML 3.1.1 PIDF-LO Shape Application Schema for use by the Internet Engineering Task Force (IETF),” April 2007.). There is no restriction on the number of points that may be used to express the perimeter of the hole.

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 3 (Hexagon and Associated GML).

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

<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> <!--F-->
      <gml:pos>43.111 -73.222</gml:pos> <!--E-->
      <gml:pos>43.311 -73.122</gml:pos> <!--D-->
      <gml:pos>43.411 -73.222</gml:pos> <!--C-->
      <gml:pos>43.411 -73.322</gml:pos> <!--B-->
      <gml:pos>43.311 -73.422</gml:pos> <!--A-->
    </gml:LinearRing>
  </gml:exterior>
</gml:Polygon>

NOTE that polygon vertices in Figure 3 (Hexagon and Associated GML) 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 4 (Hexagon with Hole) is represented by the 4 points of the interior boundary specified in Figure 5 (GML for Hexagon with Hole).

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

<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> <!--F-->
      <gml:pos>43.111 -73.222</gml:pos> <!--E-->
      <gml:pos>43.311 -73.122</gml:pos> <!--D-->
      <gml:pos>43.511 -73.222</gml:pos> <!--C-->
      <gml:pos>43.511 -73.322</gml:pos> <!--B-->
      <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>43.211 -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:interior>
</gml:Polygon>

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. [I‑D.schulzrinne‑ecrit‑lost‑sync] (Schulzrinne, H., “Synchronizing Location-to-Service Translation (LoST) Servers,” February 2008.) describes how LoST servers may synchronize with one another and provides examples of possible boundary exchanges and data formats. At the time of writing there is no standard format for service provisioning data into a LoST server, the format described in [I‑D.schulzrinne‑ecrit‑lost‑sync] (Schulzrinne, H., “Synchronizing Location-to-Service Translation (LoST) Servers,” February 2008.) is used for the example in this section.

Figure 4 (Hexagon with Hole) shall be 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. Further more area-A is directly represented by the GML encoding provided in Figure 5 (GML for Hexagon with Hole). Area-w is represented as a hole in area-A by the interior boundary. Since area-w is also a service boundary, a separate polygon describing this area is also required and is shown in Figure 6 (GML for Area-w).

<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>

If this data were in a LoST server and was required in a neighbouring LoST server, the data transfer using the format in [I‑D.schulzrinne‑ecrit‑lost‑sync] (Schulzrinne, H., “Synchronizing Location-to-Service Translation (LoST) Servers,” February 2008.) would look similar to Figure 7 (Service Boundary Specifications).

<?xml version="1.0" encoding="UTF-8"?>
  <pushMappingsRequest xmlns="urn:ietf:params:xml:ns:lost1:sync">
   <mappings>
     <mapping sourceId="lost:area-A.nsw.au.example"
       version="1" lastUpdated="2007-11-26T01:00:00Z"
       timeToLive="2007-12-26T01:00:00Z">
       <displayName xml:lang="en">
         Outer Area Police Department
       </displayName>
       <service>urn:service:sos.police</service>
       <serviceBoundary profile="geodetic-2d">
          <gml:Polygon 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>
            <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>
                <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 sourceId="lost:area-w.nsw.au.example"
       version="1" lastUpdated="2007-11-26T01:00:00Z"
       timeToLive="2007-12-26T01:00:00Z">
       <displayName xml:lang="en">
         Inner Area Police Department
       </displayName>
       <service>urn:service:sos.police</service>
       <serviceBoundary profile="geodetic-2d">
          <gml:Polygon 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>
    </mappings>
   </pushMappingsRequest>

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. The format described in this memo makes support for this operation easy, while allowing an arbitrary number of holes in a service boundary to be specified.

Each primary polygon is stored in the geospatial database and mapped to a service URN and destination URI. Holes may be stored as polygons in a separate table and mapped to the primary polygon. When a location is found to map to a polygon, the exceptions table can be checked to see if the primary polygon contains any coverage holes. In general no holes will exist for a service boundary, so this check results in almost no overhead and the service mapping can be returned. Where one or more holes are found to exist, the provided location is checked against each hole. If the location is found to exist in one of the specified holes then the primary polygon can be discarded, and searching of the service boundary database can continue.

7.  Security Considerations

This document does not introduce any security issues

8.  IANA Considerations

There are no specific IANA considerations for this document.

9.  Acknowledgements

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

10.  References

10.1. Normative References

10.2. Informative References

Authors' Addresses

Full Copyright Statement

Copyright © The IETF Trust (2007).

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.

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, THE IETF TRUST AND THE INTERNET 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.

Intellectual Property

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.