RFC 5964 PROPOSED STANDARD LOCATION · LOST · GML
LoST holes: Specifying Holes in Location-to-Service Translation (LoST) Service Boundaries
Rules for cutting holes in the map areas that decide which emergency or other service a location maps to.
RFC EDITOR
Winterbottom, J. and M. Thomson, "Specifying Holes in Location-to-Service Translation (LoST) Service Boundaries", RFC 5964, DOI 10.17487/RFC5964, August 2010, <https://www.rfc-editor.org/info/rfc5964>.
BIBTEX
@misc{rfc5964,
series = {Request for Comments},
number = 5964,
howpublished = {RFC 5964},
publisher = {RFC Editor},
doi = {10.17487/RFC5964},
url = {https://www.rfc-editor.org/info/rfc5964},
author = {J. Winterbottom and M. Thomson},
title = {{Specifying Holes in Location-to-Service Translation (LoST) Service Boundaries}},
pagetotal = 11,
year = 2010,
month = aug,
} RFC text
Contents
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.
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.
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.
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.
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.
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).
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
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>
<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.
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.
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
AI ELI5
On this page
PROBLEMA LoST service area can contain exception areas that must map to a different destination, and unconstrained polygon encodings are hard to process.
MECHANISMRFC 5964 profiles GML polygons with three rules for holes, written as interior boundaries, and describes a lookup that checks the exterior boundary and then the holes.
WHY IT MATTERSImplementations can stay simple and still interoperate, and LoST servers can answer location queries quickly.
§1Holes in Service Boundaries
The LoST protocol (RFC 5222) maps a service and a location to destination addresses. A LoST server does this by provisioning boundary maps against service URNs. Each boundary is a polygon of geodetic coordinates, called the exterior polygon. Sometimes it contains exception areas, or holes, that for the same service must yield a different destination.
RFC 5964 describes a profile of Geographic Markup Language (GML) polygons that removes permutations that are difficult to process. This allows simpler implementations, and a fully conformant GML implementation must produce polygons that fit the profile.
The working group noted that such regions could be drawn as polygons without holes, but chose these recommendations to avoid problems with arbitrary division of regions and to align with existing geospatial practice.
A big area has a smaller patch inside where the rules differ. This document says how to describe that patch so every computer reads it the same way.
§2The Three Rules for Holes
Rule 1: two holes MUST NOT have more than one point of intersection. If holes overlap or share a common boundary, they represent a single hole, so common boundaries are removed and one hole is created.
Rule 2: a polygon MUST describe a contiguous region. A hole that overlaps the outer boundary or shares part of a side with it has an inlet, and the region MUST be expressed without the hole. If a hole touches the outer boundary in two places, the region MUST be expressed as two separate polygons. A polygon entirely inside a hole, an island, is likewise a separate polygon.
Rule 3: a hole MUST be a legal linear ring in accordance with the geoshape specification, except that its points go clockwise. The exterior boundary goes counterclockwise, so the upward normals oppose each other. The number of points is not restricted.
Holes that touch merge into one. A hole cannot reach the edge or split the shape. You draw holes in the opposite direction to the outline.
§3Holes in GML
A GML polygon has an enclosed exterior boundary whose first and last points are the same. Vertices can be written with <pos> elements or a <posList> element.
A hole is an interior boundary inside the polygon. In the RFC's example, a hexagon with exterior points A to F gets an interior <gml:interior> ring of four points, w, x, y and z, and the area those points enclose is the hole.
The outline is listed first, then a second list of points draws the cut-out inside it.
Polygon with a hole (abridged)
<gml:Polygon srsName="urn:ogc:def:crs:EPSG::4326">
<gml:exterior>
<gml:LinearRing>
<gml:pos>43.311 -73.422</gml:pos> <!--A-->
... B, C, D, E, 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>§4Boundaries and Selection
A service boundary's enclosed area is where a service URN maps to a single URI. The RFC's example has two boundaries: area-A, the hexagon with a hole, and area-w, the hole itself. Since area-w is also a service boundary, it needs its own separate polygon, with the vertices reversed. In the LoST mappings, area-A is "Outer Area Police" and area-w is "Inner Area Police", both for urn:service:sos.police.
Each polygon is stored in a geospatial database mapped to a service URN and destination URI. Many such databases support interior exclusions natively. Without that, interior boundaries can be stored with the polygon and checked separately. A location falls inside a polygon if it is within the exterior boundary and not within any interior boundary. So a location in the hole maps to Inner Area Police, and one in the hexagon but outside the hole maps to Outer Area Police.
The RFC considers it likely that LoST servers must respond fast enough for real-time queries in an emergency call routing flow, which is why native geospatial queries and easily mapped boundaries are preferred.
To find the right helper, check whether the spot is in the big area and not in any cut-out. If it is in a cut-out, ask that area's helper instead.
§5Security Considerations
The RFC states that constraining the polygon representation does not introduce new security considerations.
Setting tidier drawing rules does not create any new dangers.
§6References
[RFC5222]RFC 5222Hardie, T., Newton, A., Schulzrinne, H., and H. Tschofenig, "LoST: A Location-to-Service Translation Protocol", RFC 5222, August 2008.August 2008[RFC2119]RFC 2119Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.March 1997
[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.
[ISO-19107]ISO, "Geographic information - Spatial Schema", ISO Standard 19107, First Edition, May 2003.