ECRIT J. Winterbottom
Internet-Draft M. Thomson
Intended status: BCP Andrew Corporation
Expires: April 16, 2009 October 13, 2008
Specifying Holes in LoST Service Boundaries
draft-ietf-ecrit-specifying-holes-01.txt
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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 LoST server, is
described.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Specifying Holes . . . . . . . . . . . . . . . . . . . . . . . 5
4. GML Polygons . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Holes in GML Polygons . . . . . . . . . . . . . . . . . . . . 9
6. Service Boundary Specification and Selection Algorithm . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. Normative References . . . . . . . . . . . . . . . . . . 16
10.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18
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1. Introduction
The LoST protocol [RFC5222] 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].
o--------------o
/ \
/ /\ \
/ + +-----+ \
o | Hole \ o
| | 1 / |
| +-------+ |<--- Primary Polygon
| +-------+ |
| / Hole | |
o \ 2 | o
\ +-----+ + /
\ \/ /
\ /
o--------------o
Figure 1: Holes in a Polygon
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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].
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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
Figure 2: Incorrect Hole Specification with Boundary Sharing
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 3, or the primary polygon SHOULD be expressed as
two polygons as in Figure 4.
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+------- 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: Correct Specification of an Inlet
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
Figure 4: Correct Specification of Hole with Multiple Outer-Boundary
Intersections
Similarly, a polygon containing a hole with an island must be
represented as two polygons mapping to the same service.
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Rule 3: A hole MUST be a legal polygon in accordance with the
geoshape specification [geoshape]. There is no restriction
on the number of points that may be used to express the
perimeter of the hole.
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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 5.
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>
Figure 5: Hexagon and Associated GML
NOTE that polygon vertices in Figure 5 are expressed using <pos>
elements for clarity. The vertices can also be expressed using a
<posList> element.
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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 6 is represented by the 4 points of the interior boundary
specified by (w,z,y,x).
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>
Figure 6: Hexagon with Hole
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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 6 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 6. 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 7.
<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 7: GML for Area-w
If this data were in a LoST server the data mappings may look similar
to the example in Figure 8. This is an example only and does not
represent actual LoST server provisioning or data transfer records.
The example XML will not complie.
<?xml version="1.0" encoding="UTF-8"?>
<entry>
<name> Outer Area Police Department </name>
<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>
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<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>
<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>
</entry>
<entry>
<name> Inner Area Police Department </name>
<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>
</entry>
Figure 8: Service Boundary Specifications
It is considered likely that LoST servers will need to provide
responses sufficiently quickly to allow real-time queries to be
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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.
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7. Security Considerations
This document does not introduce any security issues.
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8. IANA Considerations
There are no specific IANA considerations for this document.
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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.
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10. References
10.1. 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.
10.2. Informative References
[I-D.ietf-ecrit-lost-sync]
Schulzrinne, H., "Synchronizing Location-to-Service
Translation (LoST) Servers", draft-ietf-ecrit-lost-sync-00
(work in progress), July 2008.
[ISO-19107]
ISO, "Geographic information - Spatial Schema", ISO
Standard 19107, First Edition, 5 2003.
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Authors' Addresses
James Winterbottom
Andrew Corporation
PO Box U40
University of Wollongong, NSW 2500
AU
Email: james.winterbottom@andrew.com
Martin Thomson
Andrew Corporation
PO Box U40
University of Wollongong, NSW 2500
AU
Email: martin.thomson@andrew.com
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