draft-ietf-ecrit-rough-loc-00.txt   draft-ietf-ecrit-rough-loc-01.txt 
Internet Engineering Task Force R. Barnes Internet Engineering Task Force R. Barnes
Internet-Draft M. Lepinski Internet-Draft M. Lepinski
Intended status: Standards Track BBN Technologies Intended status: Standards Track BBN Technologies
Expires: April 10, 2010 October 7, 2009 Expires: July 24, 2010 January 20, 2010
Using Imprecise Location for Emergency Context Resolution Using Imprecise Location for Emergency Context Resolution
draft-ietf-ecrit-rough-loc-00.txt draft-ietf-ecrit-rough-loc-01.txt
Abstract
Emergency calling works best when precise location is available for
emergency call routing. However, there are situations in which a
location provider is unable or unwilling to provide precise location,
yet still wishes to enable subscribers to make emergency calls. This
document describes the level of location accuracy that providers must
provide to enable emergency call routing. In addition, we descibe
how emergency services and non-emergency services can be invoked by
an endpoint that does not have access to its precise location.
Status of this Memo Status of this Memo
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This Internet-Draft will expire on April 10, 2010. This Internet-Draft will expire on July 24, 2010.
Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights publication of this document. Please review these documents
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Abstract include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
Emergency calling works best when precise location is available for described in the BSD License.
emergency call routing. However, there are situations in which a
location provider is unable or unwilling to provide precise location,
yet still wishes to enable subscribers to make emergency calls. This
document describes the level of location accuracy that providers must
provide to enable emergency call routing. In addition, we descibe
how emergency services and non-emergency services can be invoked by
an endpoint that does not have access to its precise location.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Determining sufficient location precision . . . . . . . . . . 4 3. Determining sufficient location precision . . . . . . . . . . 4
3.1. Location filtering . . . . . . . . . . . . . . . . . . . . 5 3.1. Location filtering . . . . . . . . . . . . . . . . . . . . 6
3.2. Constructing location filters . . . . . . . . . . . . . . 7 3.2. Constructing location filters . . . . . . . . . . . . . . 10
3.2.1. Geodetic service boundaries . . . . . . . . . . . . . 8 3.2.1. Civic address considerations . . . . . . . . . . . . . 11
3.2.2. Civic service boundaries . . . . . . . . . . . . . . . 9 3.3. Maintaining location filters . . . . . . . . . . . . . . . 12
3.3. Maintaining location filters . . . . . . . . . . . . . . . 9 3.4. Applying location filters . . . . . . . . . . . . . . . . 12
3.4. Applying location filters . . . . . . . . . . . . . . . . 9 4. Requesting emergency and non-emergency services . . . . . . . 13
4. Requesting emergency and non-emergency services . . . . . . . 10 4.1. Emergency calling . . . . . . . . . . . . . . . . . . . . 13
4.1. Emergency calling . . . . . . . . . . . . . . . . . . . . 10 4.2. Non-emergency services . . . . . . . . . . . . . . . . . . 14
4.2. Non-emergency services . . . . . . . . . . . . . . . . . . 11 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 8.1. Normative References . . . . . . . . . . . . . . . . . . . 16
8.1. Normative References . . . . . . . . . . . . . . . . . . . 13 8.2. Informative References . . . . . . . . . . . . . . . . . . 16
8.2. Informative References . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction 1. Introduction
Information about the location of an emergency caller is a critical Information about the location of an emergency caller is a critical
input to the process of emergency call establshment. Endpoint input to the process of emergency call establshment. Endpoint
location is used to determine which Public Safety Answering Point location is used to determine which Public Safety Answering Point
(PSAP) should be the destination of the call. (The entire emergency (PSAP) should be the destination of the call. (The entire emergency
calling process is described in detail in [1] and [2].) This process calling process is described in detail in [6] and [1].) This process
is most likely to work properly when the endpoint is provided with is most likely to work properly when the endpoint is provided with
the most accurate precise information available about its location. the most accurate and precise information available about its
Using location information with maximal precision and accuracy location. Using location information with maximal precision and
minimizes the chance that a call will be mis-routed. And when that accuracy minimizes the chance that a call will be mis-routed. In
location is provided to the endpoint, the endpoint is able to verify addition, when that location is provided to the endpoint, the
that the location is correct (to the extent of the endpoint's endpoint is able to verify that the location is correct (to the
knowledge of its own location) prior to an emergency call, and is extent of the endpoint's knowledge of its own location) prior to an
able to perform emergency call routing functions on its own, emergency call, and is able to perform emergency call routing
providing redundancy for network-provided functions. functions on its own, providing redundancy for network-provided
functions.
However, there may be situations in which it is not feasible for However, there may be situations in which it is not feasible for
endpoints to be provided with maximally precise and accurate endpoints to be provided with maximally precise and accurate
location. These cases may arise when computing precise location is location. These cases may arise when computing precise location is
an expensive or time-consuming operation (e.g., in the case of an expensive or time-consuming operation (e.g., in the case of
wireless triangulation), and location is needed quickly (as is often wireless triangulation), and location is needed quickly, as is often
the case in emergency situations). Or they may arise because the the case in emergency situations. Or they may arise because the
policy of the location provider does not allow precise location to be policy of a location provider does not allow precise location to be
provided to the endpoint (e.g. due to privacy considerations). While provided to the endpoint. While it is undesirable to use imprecise
it is undesirable to use imprecise location for emergency call location for emergency call routing, the possibility that precise
routing, the possibility that precise location may not be available location may not be available to the calling device must be
to the calling device must be accomodated in order to make emergency accomodated in order to make emergency calling possible in the
calling possible in the largest possible set of circumstances. largest possible set of circumstances.
This document is concerned imprecise location only in the context of This document is concerned with imprecise location only in the
routing emergency calls, i.e., for determining the correct PSAP to context of routing emergency calls, i.e., for determining the correct
receive a given call (e.g., via a LoST query [3]). (More generally, PSAP to receive a given call (e.g., via a LoST query [2]). Depending
the provided location information will be needed to route the call to on the the structure of the local emergency service network, the
an entity that is authorized to request precise location, e.g., an location information provided to the endpoint may also be used to
Emergency Services Routing Proxy.) route the call to an entity that is authorized to request precise
location, e.g., an Emergency Services Routing Proxy. The
requirements and processes described in this document are the same
for both cases.
Location information may also be used in the emergency calling Location information may also be used in the emergency calling
framework to direct the dispatch of emergency responders. This usage framework to direct the dispatch of emergency responders. This usage
is treated separately from call routing for purposes of this is treated separately from call routing for purposes of this
document, and this document does not place requirements on the document, and this document does not place requirements on the
location provided for dispatch (although it should obviously be as location provided for dispatch, although it should obviously be as
precise as possible). The only provision for dispatch in this precise as possible. The only provision for dispatch in this
document is a recommendation that the location provider supply document is a recommendation that the location provider supply
endpoints with a URI that can be used by a PSAP or other emergency endpoints with a URI that can be used by a PSAP or other emergency
authority to obtain a different location for use in dispatch, authority to obtain a different location for use in dispatch,
hopefully more precise than the one used for routing. hopefully more precise than the one used for routing.
This document describes the use of imprecise location information in This document describes the use of imprecise location information in
the emergency call routing system. Section 3 describes how location the emergency call routing system. Section 3 describes how location
providers can determine the precision necessary to support emergency providers can determine the precision necessary to support emergency
call routing, and how they can use this information to optimize call routing, and how they can use this information to optimize
location delivery. Section 4 describes how emergency calls are location delivery. Section 4 describes how emergency calls are
placed in such an environment, and how non-emergency services can be placed in such an environment, and how non-emergency services can be
invoked when precise location is not available to the endpoint by invoked when precise location is not available to the endpoint by
value. value.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [4]. document are to be interpreted as described in [3].
We consider in this document patterns of interaction as described in We consider in this document patterns of interaction as described in
[1]. The two main parties of interest are endpoints and location [6]. The two main parties of interest are endpoints and location
providers. Endpoints are hosts connected to the Internet that providers. Endpoints are hosts connected to the Internet that
originate emergency calls in the emergency calling architecture, originate emergency calls in the emergency calling architecture,
while location providers are entities that supply location while location providers are entities that supply location
information that is used for emergency calling. In addition, we will information that is used for emergency calling. In addition, we will
discuss how these parties interact with the LoST mapping discuss how these parties interact with the LoST mapping
infrastructure [7], and with emergency and non-emergency location- infrastructure [7], and with emergency and non-emergency location-
based service providers. based service providers.
For convenience, we say that location information (either in LoST For convenience, we say that location information, either in LoST
queries or in service boundaries) is provided "in geodetic form" if queries or in service boundaries, is provided "in geodetic form" if
it is provided in the "geodetic-2d" location profile, and "in civic it is provided in the "geodetic-2d" LoST location profile, and "in
form" if it is provided in the "civic" profile. civic form" if it is provided in the "civic" profile.
3. Determining sufficient location precision 3. Determining sufficient location precision
A location provider wishing to provide location information usable A location provider wishing to provide location information usable
for emergency call routing requires a mechanism for determining when for emergency call routing requires a mechanism for determining when
a description of location (e.g., a polygon) is precise enough to be a description of location (e.g., a polygon) is precise enough to be
used for emergency call routing. This mechanism might be used to used for emergency call routing. This mechanism might be used to
decide when to terminate a positioning mechanism that converges over decide when to terminate a positioning process that converges over
time, or to choose a polygon larger than the known location of the time, or to choose a polygon larger than the known location of the
endpoint (in order to obscure the known location of the endpoint), endpoint (in order to obscure the known location of the endpoint),
while preserving the utility of the location for emergency call while preserving the utility of the location for emergency call
routing. routing.
There are two base requirements for a location to be usable for There are three basic requirements for a location to be usable for
emergency call routing: emergency call routing:
1. The location SHOULD be sufficiently precise that a LoST request 1. The location SHOULD be sufficiently precise that a LoST request
with the location and any service URN will return a unique URI with the location and any service URN will return a unique URI
mapping value. This may not be possible in all cases, e.g., mapping value. This may not be possible in all cases, e.g.,
because of overlapping service boundaries (leading to areas that because of overlapping service boundaries creating areas with
do not have a unique mapping) or positioning limitations (leading non-unique mappings, or because of positioning limitations that
to insufficient precision). prevent sufficiently precise positioning.
2. When the location of the endpoint is known by the provider to 2. When the location of the endpoint is known by the provider to
greater precision than is being provided, the provided location greater precision than is being provided, the provided location
MUST return the same mappings from LoST (for all service URNs) as MUST return the same mappings from LoST, for all service URNs, as
the known location. the known location.
3. When the location of the endpoint is known by the provider to 3. When the location of the endpoint is known by the provider to
greater precision than is being provided, the provided location greater precision than is being provided, the provided location
MUST contain the precise location (as a geographic subset). MUST contain the precise location (as a geographic subset).
In this section, we describe how to use a "location filter" to These requirements lead naturally to the idea of a "location filter".
determine whether a given location is usable for emergency call A location filter is a collection of geographical regions satisfying
routing, and how to construct and maintain such a filter. the following criteria:
3.1. Location filtering 1. For any location value that is a subset of a filter region, a
LoST request for any service will return a unique mapping result.
With each service-to-URI mapping, a LoST query provides a service 2. Any two locations within the same filter region receive the same
boundary that represents the set of locations in which that mapping LoST results for all services
is valid. A consequence of this is that given a set of service
boundaries for difference services (say, one mapping
"urn:service:sos.fire" to "sip:fire@example.com" and one mapping
"urn:service:sos.police" to "sip:police@example.com"), the
intersection of those service boundaries is the region in which two
mappings are valid ("urn:service:sos.fire" maps to
"sip:fire@example.com" and "urn:service:sos.police" maps to
"sip:police@example.com"). Outside that area, one or more of the
mappings is invalid. Said differently, any region contained in an
intersection uniquely determines mappings for the services used in
the intersection, and any two locations within the same intersection
are equivalent for the purpose of LoST mapping (i.e., emergency call
routing).
A location filter is thus a set of regions (optionally, each region Given a location filter, it is easy to determine when a given
may be assigned a list of LoST mappings), as illustrated in Figure 1. location value is sufficiently precise, or to create a less precise
Each region is the intersection of the service boundaries for all version of location that is still precise enough. Namely, a location
services available within the region, and the lists represent the value is precise enough when if fits within a given filter region,
mappings that are valid within that region. A filter is used to and any superset of a location value (e.g., a polygon containing a
determine whether a location is useable for emergency call routing in point) can be used as a less precise version of the location value as
the following way: long as it still fits within the same filter region.
1. The location SHOULD be contained in exactly one of the regions in For example, a simple fuzzing algorithm that maintains sufficient
the filter. This guarantees that LoST mappings are unique. precision for emergency services is to replace a given location value
with the filter region that contains it. This way, the server can
compute the filter off-line (as described below), then provision the
location of each possible target by storing a pointer to the filter
region that contains the target's location.
2. When the precise location of the endpoint is known, the provided The remainder of this section discusses the concept of location
location MUST be contained in the same region(s) of the filter as filtering in more detail, and describes how a location server can
the known location. This guarantees that LoST queries with the construct and maintain a location filter based on information from
provided location return the same results as those done with the the LoST mapping infrastructure.
known location.
3. When the precise location of the endpoint is known, the provided 3.1. Location filtering
location MUST contain the precise location (as a geographic
subset).
When the regions are bound to lists of URN-URI mappings, the With each service-to-URI mapping, a LoST query provides a service
resulting filter can also be used as a cache for LoST mappings; the boundary that represents the set of locations in which that mapping
LoST mappings for a location are the mappings bound to the region(s) is valid. A consequence of this is that given a set of service
containing it. boundaries for different services, the intersection of those service
boundaries is the region in which two mappings are valid. If one
service boundary corresponds to the area where "urn:service:sos.fire"
is served by "sip:fire@example.com" and another maps
"urn:service:sos.police" to "sip:police@example.com", then the
intersection is the are where both of these mappings are valid
("urn:service:sos.fire" maps to "sip:fire@example.com" and
"urn:service:sos.police" maps to "sip:police@example.com"). Outside
that area, one or more of the mappings is invalid. So as was
suggested above, the intersection of two service boundaries defines a
set of mappings, and any two locations within that intersection are
equivalent for the purpose of LoST mapping (i.e., emergency call
routing).
Service boundaries for individual services Service boundaries for individual services
urn:service:sos.police urn:service:sos.fire urn:service:sos.police urn:service:sos.fire
+-------+ +-------+ +-------+ +-------+
| A | | C | | A | | C |
| +---+ | +---+---+ | +---+ | +---+---+
| | | | | | | | | | X | |
+---+---+ | +---+ | +---+---+ | +---+ |
| B | | D | | B | | D |
+-------+ +-------+ +-------+ +-------+
| | | |
| | | |
+-----------+------------+ +-----------+------------+
| |
V V
skipping to change at page 6, line 52 skipping to change at page 7, line 37
+---+ |A,D| +---+ +---+ |A,D| +---+
+---+ | | +---+ | |
+---+ | +---+ |
| B,D | | B,D |
+-------+ +-------+
Resulting Location Filter Regions Resulting Location Filter Regions
Figure 1: Generating a filter from service boundaries Figure 1: Generating a filter from service boundaries
The regions in a location filter can thus be constructed by taking
intersections of service boundaries. Figure 1 shows a simple
location filter: Starting with a set of four service boundaries for
two different services. The filter that results from taking
intersections of these boundaries has three regions:
1. A region where police calls are directed to A and fire calls are
directed to C.
2. A region where police calls are directed to A and fire calls are
directed to D.
3. A region where police calls are directed to B and fire calls are
directed to D.
These regions satisfy the criteria for a location filter because each
one has a unique set of mappings and those mappings are valid across
the entire region. The service regions for B and C do not overlap --
there is no place where police calls go to B and fire calls to C --
so there is no (B,C) region.
More generally, a filter region is the intersection of the service
boundaries for all services available within the region. A filter
can used to determine whether a location is usable for emergency call
routing in the following way:
1. The location SHOULD be contained in exactly one of the regions in
the filter. This guarantees that LoST mappings are unique.
2. When the precise location of the endpoint is known, the provided
location MUST be contained in the same region(s) of the filter as
the known location. This guarantees that LoST queries with the
provided location return the same results as those done with the
known location.
3. When the precise location of the endpoint is known, the provided
location MUST contain the precise location (as a geographic
subset).
Filter regions can be deduced constructed from LoST mappings for a
sample location by intersecting all the service boundaries for
services available at that point. Figure 2 illustrates how the
filter region containing the point X is the intersection of the
service boundaries for police and fire services that serve X.
If the server also stores the lists of URN-URI mappings for each
region, x then the filter can also be used as a cache for LoST
mappings; the LoST mappings for a location are the mappings bound to
the region(s) containing it.
sos.police sos.fire sos.ambulance
+-------+ +---------------+
| A | | B |
| | | | +-------+
| X | | X | | X |
+-------+ +---------------+ | |
| C |
+-------+
| | |
| | |
+-------------------+-------------------+
|
V
+-------+-------+
| A | B |
| +-------+ |
| | X | | |
+-------+-------+
| C |
+-------+
|
|
V
+---+
| X |
+---+
Resulting filter region
(police=>A, fire=>B, ambulance=>C)
Figure 2: Generating a filter region from a sample point
When the location of the endpoint is known to more precision than the When the location of the endpoint is known to more precision than the
location provided to the endpoint, although any location meeting the location provided to the endpoint, although any location meeting the
two criteria above is equivalent to the known location for purposes two criteria above is equivalent to the known location for purposes
of LoST, the provided location MUST contain the known location in of LoST, the provided location MUST contain the known location in
order to avoid errors if the location is used for other purposes in order to avoid errors if the location is used for other purposes in
the course of an emergency (e.g., if the location is provided to the course of an emergency (e.g., if the location is provided to
first responders for dispatch). This guarantee also allows the first responders for dispatch). This guarantee also allows the
endpoint to do some course verification that the provided location is endpoint to do some course verification that the provided location is
correct (in order to prevent very gross errors in routing). Thus, correct (in order to prevent very gross errors in routing). Thus,
any location that (1) contains the known location and (2) is any location that (1) contains the known location and (2) is
contained in the same filter region as the known location is contained in the same filter region as the known location is
allowable. Locations that also are contained in only one filter allowable. Locations that also are contained in only one filter
region are preferred. Adding randomness to the provided locations region are preferred. Adding randomness to the provided locations
may have privacy benefits in some cases, as discussed in the security may have privacy benefits in some cases, as discussed in the security
considerations below. considerations below.
3.2. Constructing location filters 3.2. Constructing location filters
For simplicity, we assume that the entity performing filtering will For simplicity, we assume that the entity performing filtering will
only be using the filter to test locations contained within a only be using the filter to test locations contained within a
particular geographic "coverage area". (In principle, this coverage particular geographic "coverage area". In principle, this coverage
area could be the entire world, but assuming a more limited coverage area could be the entire world, but assuming a more limited coverage
area allows for a filter to be built more quickly) Given a coverage area allows for a filter to be built more quickly.
area and the ability to act as a LoST client, a location service
provider can autonomously compute a location filter using the
following algorithm:
First, the server must obtain mappings and service boundaries for all
services and for all points within the coverage area. For each
emergency service URN, the server goes through the following process
to build a service map: First, the server queries LoST for the
complete coverage map for the desired service. This can be done with
a LoST <findService> query of the following form:
<?xml version="1.0" encoding="UTF-8"?> Given a coverage area and the ability to act as a LoST client, a
<findService location service provider can autonomously compute a location filter
xmlns="urn:ietf:params:xml:ns:lost1" by constructing regions around sample points until it has a
serviceBoundary="value"> collection of filter regions that collectively cover its service
<location> region. (The process for an individual point is illustrated in
<!-- Coverage Area --> Figure 2.)
</location>
<service>
<!-- Service URN -->
</service>
</findService>
If LoST returns a set of mappings whose service boundaries cover the In order to ensure that all services boundaries are taken into
coverage area (i.e., if LoST is configured to return all possible account, the server starts by issuing a <listServicesByLocation>
matches for the queried location), then the process termintes here. query, and caching the list of services that it returns, along with
The coverage map for this service is the set of returned service the corresponding service list boundary [4]. The server then samples
boundaries. points within that service list boundary, retrieving mappings with
service boundaries for each service in the service list and
intersecting the service boundaries to obtain a new filter region.
In pseudocode, the algorithm is as follows:
If service boundaries in the LoST response to the above query do not Set FILTER = the empty set
cover the location provider's coverage area, then the location server While filter does not cover LS coverage area
must perform further queries. The location included in each query is Choose a random uncovered point X in the LS coverage area
the difference between the coverage area and the current coverage Perform a LoST <listServicesByLocation> query for X
map, that is, the coverage area with all currently-known service Set SL = <serviceList> from LoST response
boundaries removed. The server repeats this process (query, then Set SLB = <serviceListBoundary> from LoST response
remove service boundaries from the query location, then query again) If SLB is not provided, choose new point X and re-query
until either (1) the coverage area is covered by the collected If more than 100 points X have been tried
service boundaries or (2) LoST returns a <notFound> error. Set R = uncovered area
Add R to FILTER
END
While filter does not cover SLB
Choose a random uncovered point Y in SLB
Set R = SLB
For each service S in SL
Perform a LoST <findService> query for Y and S
Set SB = <serviceBoundary> from LoST response
If SB is not provided, return an error
Else set R = intersection( R, SB(S,Y) )
After the location server has performed this procedure for each Add R to FILTER
service, it will have a set of LoST mappings for each service, for
every point in its coverage region where that service is offered.
The regions in the location filter are computed separately for If the LoST servers have been provisioned properly then this
service boundaries provided in civic form and in geodetic form. If algorithm will terminate successfully. If LoST mapping do not cover
all service boundaries are provided in one form (e.g., if all part of the service region, then the <serviceListBoundary> will not
boundaries are provided in geodetic form, even if some are also be returned, and the algorithm will give up after 100 queries. This
provided in civic form), the server MAY perform the algorithm for limit on queries introduces some risk that a small covered area will
that form. If both algorithms are being performed, and some mappings be left out of the filter and marked as uncovered; if this is a
provide both civic and geodetic service boundaries, the server MUST concern, then the query limit can be increased.
input those mappings to both the civic and geodetic computations.
3.2.1. Geodetic service boundaries Of course, if the location server operator has information about
service boundaries through some channel other than LoST, then the
LoST queries above can be replaced by queries to a local store of
mapping information. The choice of random points can also be guided
to ensure that all mapped areas are covered even if there are some
uncovered areas. The location server can also cache service
boundaries acquired during the algorithm to avoid unnecessary LoST
queries.
The regions in the location filter are computed from these mappings 3.2.1. Civic address considerations
by iterating over URI tuples: For each service URN, let uris(urn) be
the set of PSAP URIs for that service URN (collected from the
mappings). The set of URI tuples is then the cartesian product of
these sets; if the set of servuce URNs is {urn1,...,urnN}, then the
set of URI tuples is uris(urn1) x ... x uris(urnN). The server
computes the regions in the filter by iterating through the set of
URI tuples, either by constructing the set of URI tuples and directly
iterating, or by using nested iteration through all the sets
uris(urn).
For each URI tuple, the server MUST compute the intersection of the This algorithm actually results in two filters -- one for geodetic
service boundaries for the URIs in the tuple. This becomes an entry service boundaries and one for civic service boundaries -- since
in the location filter: The stored region is the intersection of the civic and geodetic boundaries cannot be directly compared or
service boundaries, and the corresponding mapping table is the list intersected. It is RECOMMENDED that location servers always compute
of (URN, URI) pairs, where the URIs are the URIs from the tuple and a geodetic filter for use with emergency services, since the notion
the URNs are the services used to obtain them from LoST. (Empty of civic service boundaries have some inherent ambiguity.
filter regions, corresponding to URIs in a tuple with disjoint
service boundaries, can of course be discarded.)
3.2.2. Civic service boundaries Indeed, the notion of intersection of civic service boundaries has
some dependence on the jurisdiction within which the service
boundaries are defined. Civic service boundaries are comprised of a
set of <civicAddress> elements, each defining a set of civic
addresses that are within the boundary, namely those that match the
civic elements provided.
As in the case of geodetic location, regions of a civic address When computing the intersection of two civic service boundaries, any
filter are computed based on URI-tuples. For tuples where all <civicAddress> elements that are shared between the two service
mappings have the same service boundary, that service boundary MUST boundaries MUST be included in the resulting intersection. When two
be used as the filter region for that type. For all other cases <civicAddress> elements in the service boundaries being compared are
(i.e., tuples with different civic locations), the regions of the different from each other, then their intersection must be computed
filter must be computed as the intersections of the locations according to local addressing standards.
according to an algorithm that is determined by local addressing
standards.
Note that the resulting filter regions SHOULD still cover the Note that the resulting filter regions SHOULD still cover the
location server's coverage area, i.e., there should be a filter location server's coverage area, i.e., there should be a filter
region that contains every civic address within the coverage area. region that contains every civic address within the coverage area.
In particular, the server SHOULD NOT use a specific address to In particular, the server SHOULD NOT use a specific address to
represent a filter region: Such an address would not include many represent a filter region: Such an address would not include many
points in the service region (i.e., it would not meet the third rules points in the service region (i.e., it would not meet the third rules
from both lists of rules above). If the server chooses to return a from the lists of rules above). If the server creates a PIDF-LO
civic address that does not, then it MUST set the 'method' element of document describing a civic address that does not contain the precise
the PIDF-LO it returns to value 'area-representative' registered in location of the target, then it MUST set the 'method' element of the
PIDF-LO it returns to value 'area-representative' registered in
Section 7. Section 7.
3.3. Maintaining location filters 3.3. Maintaining location filters
As the LoST mappings that underlie the filter change, the filter will As the LoST mappings that underlie the filter change, the filter will
need to be updated. The entity maintaining the filter MUST obtain a need to be updated. The entity maintaining the filter MUST obtain a
new mapping for a region when an existing mapping expires. The new mapping for a region when an existing mapping expires. The
service boundary from the new mapping is compared to the service service boundary from the new mapping is compared to the service
boundary from the old mapping: If they are the same, then the filter boundary from the old mapping: If they are the same, then the filter
need not be updated. If they differ, then regions in the filter that need not be updated. If they differ, then regions in the filter that
intersect either the old service boundary or the new service boundary intersect either the old service boundary or the new service boundary
will need to be recomputed. Note that since this operation only will need to be recomputed. Note that since this operation only
requires the server to determine if two service boundaries are requires the server to determine if two service boundaries are
identical, the server need only store a hash of the old boundary (to identical, the server need only store a hash of the old boundary to
which it can compare a hash of the new boundary). which it can compare a hash of the new boundary.
3.4. Applying location filters 3.4. Applying location filters
After constructing a location filter, a location server can use it to After constructing a location filter, a location server can use it to
optimize how it delivers location. When the location server is using optimize how it delivers location. When the location server is using
a positioning algorithm that grows more accurate with time, the a positioning algorithm that grows more accurate with time, the
filter tells it how long to run the algorithm. Namely, the algorithm filter tells it how long to run the algorithm. Namely, the algorithm
can be terminated when the estimated location is within one of the can be terminated when the estimated location (that is, an
regions in the filter. uncertainty region containing the target's location) is within one of
the regions in the filter.
When the location provider knows the precise location of the caller, When the location provider knows the precise location of the caller,
a location filter can also be used as a "location cache". That is, a location filter can also be used as a "location cache". That is,
the location provider can simply look up which of the filter regions the location provider can simply look up which of the filter regions
contains the caller's precise location and return that region as the contains the caller's precise location and return that region as the
caller's location (or some subset that contains the precise caller's location, or some subset that contains the precise location.
location).
This allows an additional optimization in some cases: If the location This caching strategy allows an additional optimization in some
server knows that the caller's precise location will be within the cases: If the location server knows that the caller's precise
same region for a period of time, it can instruct the client not to location will be within the same region for a period of time, it can
re-query in that time. For instance, if the server is delivering instruct the client not to re-query in that time. For instance, if
location over HELD, then it can use the HTTP cache-control headers the server is delivering location over HELD, then it can use the HTTP
(e.g., Expires). However, the location server MUST NOT instruct the cache-control headers (e.g., Expires). However, the location server
client to wait for longer than the current filter is valid; the MUST NOT instruct the client to wait for longer than the current
expiry time of the location MUST be before the earliest expiry of a filter is valid; the expiry time of the location MUST be before the
LoST mapping used in the filter. earliest expiry of a LoST mapping used in the filter.
4. Requesting emergency and non-emergency services 4. Requesting emergency and non-emergency services
When a location provider wishes to deliver endpoints location When a location provider wishes to deliver endpoints location
information that is below its maximum available precision while still information that is below its maximum available precision while still
supporting emergency calling, it MUST provide to the endpoint both a supporting emergency calling, it MUST provide to the endpoint both a
location (by value) that is sufficient for emergency call routing location (by value) that is sufficient for emergency call routing (as
(see above) and a location reference (i.e., a URI) that can defined above) and a location reference (i.e., a URI) that can
subsequently be used by authorized parties to obtain more precise subsequently be used by authorized parties to obtain more precise
information about the location of the endpoint. The endpoint then information about the location of the endpoint. The endpoint then
can then use both the location value and the location reference to can then use both the location value and the location reference to
request location-based services (LBS) as described below. request emergency services and other location-based services (LBS).
4.1. Emergency calling 4.1. Emergency calling
The procedure for placing an emergency call is indentical to that The overall procedure for placing an emergency call is identical to
described in [1]. In particular, the endpoint requirements in that described in [6]. In particular, the endpoint requirements in
Sections 8 and 9 of [2] still apply to an endpoint that receives Sections 8 and 9 of [1] still apply to an endpoint that receives
imprecise location. imprecise location.
In addition, an endpoint that receives location both by value and by In addition, an endpoint that receives location both by value and by
reference from its location provider MUST include both the location reference from its location provider MUST include both the location
value and the location reference in the SIP INVITE message that value and the location reference in the SIP INVITE message that
initiates an emergency call, as specified in [5]. When the endpoint initiates an emergency call, as specified in [8]. When the endpoint
supports LoST, it SHOULD use the location value to obtain a PSAP URI supports LoST, it MUST use the location value to obtain a PSAP URI
for LoST queries (as opposed to attempting to dereference the for LoST queries before attempting to dereference the location
location reference). Note that the caller would also have to add the reference. Note that the caller would also have to add the "used-
"used-for-routing" parameter to the geolocation header that points to for-routing" parameter to the geolocation header that points to the
the location value as inserted into the INVITE message. Note that location value as inserted into the INVITE message. Note that this
this process crucially relies on the location value having sufficient process crucially relies on the location value having sufficient
precision for routing emergency calls (see Section 3 for techniques precision for routing emergency calls (see Section 3 for techniques
to ensure the location value is suitable for emergency call routing). to ensure the location value is suitable for emergency call routing).
When a PSAP receives a SIP INVITE that contains both a location value When a PSAP receives a SIP INVITE that contains both a location value
and a location reference, if the value is too imprecise for use in and a location reference, and the value is too imprecise for use in
dispatch then the PSAP SHOULD dereference the LbyR to obtain more dispatch then the PSAP SHOULD dereference the LbyR to obtain more
precise information. In turn, the location provided by the location precise information. In turn, the location provided by the location
provider MUST allow access by all PSAPs whose service boundaries provider MUST allow access by all PSAPs whose service boundaries
overlap with the region served by the location provider. This means overlap with the region served by the location provider. This means
that either the provider must supply a reference that can be that either the provider must supply a reference that can be
dereferenced by any party, or else the provider must establish dereferenced by any party, or else the provider must establish
explicit authentication and authorization relationships with all explicit authentication and authorization relationships with all
PSAPs in its service area. PSAPs in its service area. It is RECOMMENDED that location providers
establish similar relationships with other PSAPs in adjoining
jursidictions -- even if their service regions do not overlap with
the location provider's -- in case such a PSAP needs access to
precise location information, for example, if it is acting as a
backup for one of the location provider's normal PSAPs.
4.2. Non-emergency services 4.2. Non-emergency services
Non-emergency LBSs will generally require more precise information Non-emergency LBSs may require more precise information than is
than is required for emergency call routing. Therefore, when required for emergency call routing. Therefore, when requesting a
requesting a non-emergency LBS, the endpoint SHOULD include the non-emergency LBS, the endpoint SHOULD include the location reference
location reference provided by its location provider, and MAY provided by its location provider, and MAY additionally provide the
additionally provide the location value. If the provided location location value. If the provided location value is not sufficiently
value is not sufficiently precise to deliver the requested service, precise to deliver the requested service, then the LBS provider
then the LBS provider should then dereference the location value to should then dereference the location value to request location
request location information of sufficient precision from the information of sufficient precision from the location provider. If
location provider. If the dereference fails, then the request for the dereference fails, then the request for service may fail as well.
service may fail as well.
Note that when the location reference provided by the location Note that when the location reference provided by the location
provider is access-controled, this dereference may require a pre- provider is access-controled, this dereference may require a pre-
existing authentication and authorization agreement between the LBS existing authentication and authorization agreement between the LBS
provider and the location provider. In such a case, the endpoint may provider and the location provider. In such a case, the endpoint may
not know whether a given non-emergency service is authorized to not know whether a given non-emergency service is authorized to
obtain the endpoint's precise location using the location reference. obtain the endpoint's precise location using the location reference.
The endpoint is always capable of requesting services without knowing The endpoint is always capable of requesting services without knowing
whether they are authorized; in this way, the endpoint can discover whether they are authorized; in this way, the endpoint can discover
authorized services by trial and error. In order to simplify this authorized services by trial and error. In order to simplify this
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5. Acknowledgements 5. Acknowledgements
This document generalizes the concept of "rough location" that was This document generalizes the concept of "rough location" that was
originally discussed in the context of the location hiding problem. originally discussed in the context of the location hiding problem.
This concept was put forward by Henning Schulzrinne and Andy Newton, This concept was put forward by Henning Schulzrinne and Andy Newton,
among many others, in a long-running ECRIT discussion. among many others, in a long-running ECRIT discussion.
6. Security Considerations 6. Security Considerations
The use of rough location to support emergency calling enables a The use of imprecise location provides a security trade-off for
location providers. When location providers are required to provide
location in support of emergency services, they have to balance that
requirement against the risk that location information will be
disclosed to an unauthorized party. The use of location
configuration protocols inherently introduces some risk that an
entity other than the target will be able to masquerade as the target
(e.g., another host behind the same NAT or malicious software on the
host) [9]. In some cases, the location provider may not authorize
the target itself to access precise location. At the same time,
because endpoints can roam between networks, it is not generally
possible to have strong client authentication for LCPs.
Using of rough location to support emergency calling enables a
location provider to provide low-precision location with low location provider to provide low-precision location with low
assurance (e.g., of requestor identity) and high-precision location assurance (e.g., without client authentication)and high-precision
with higher assurance. The fact that lower-precision location has location with higher assurance. Because lower-precision location
lower value -- to location providers and LBS providers as a generally has lower value -- to location providers and LBS providers
commercial asset, and to targets as private information -- this as a commercial asset, and to targets as private information -- this
trade-off allows a location provider to avoid the cost of protecting trade-off allows a location provider to avoid the cost of protecting
location with high-assurance access controls when this location has location with high-assurance access controls when this location has
low value. low value.
However, in order to support emergency services, this expense cannot However, in order to support emergency services, location providers
be avoided entirely. Because PSAPs require high-precision location cannot provide only low-precision location; they also have to provide
for emergency response planning, a location provider that normally PSAPs with access to high-precision location information. Because
provides rough location MUST provide a location URI that a PSAP can PSAPs require high-precision location for emergency response, a
use to obtain high-precision location. This constraint means that location provider that normally provides imprecise location to
the provided URI MUST have either no access control at all or a clients MUST also provide them a location URI that a PSAP can use to
policy that allows access by appropriate PSAPs (and other emergency obtain high-precision location. This constraint means that the
response systems, e.g., ESRPs). That is, if such a location URI is provided URI MUST have either no access control at all or a policy
access controlled, then the location provider MUST be able to that allows access by appropriate PSAPs and other emergency response
authenticate requests from PSAPs. systems, e.g., ESRPs. That is, if such a location URI is access
controlled, then the location provider MUST be able to authenticate
requests from PSAPs.
One reason for a location server to provide location information The use of location by reference introduces some risk that the
below its maximum precision is to protect the privacy of the target. reference will be used by an attacker to gain unauthorized access to
Some location provisioning protocols do not enable the location the target's location. These risks are not specific to emergency
provider to obtain strong assurance of the identity of the location service, however; general risks and mitigations for location by
recipient; in particular, the location provider may be unable to reference are discussed in [10]
verify that the recipient is the target of the location being
provided. Therefore, there is a risk that a sufisticated attacker
might be able to spoof the identifier (e.g. IP address) used by the
location provider to identify the target, and obtain the target's
location in this way. One way to mitigate this risk is to provide
only imprecise location information to the end-point (without
authentication), and to provide precise information only to trusted
entities that can authenticate themselves to the location provider.
Additionally, in some deployment scenarios, location providers have
concerns about the comprimise of endpoint devices. Providing only
imprecise location to the endpoint, prevents malware on a comprised
device from obtaining the precise location of the target.
As described in Section 3.1 above, the location provider choosing to As described in Section 3.1 above, the location provider choosing to
provide a less precise location than a known location has a provide a less precise location than a known location has a
significant amount of choice in deciding which location to provide: significant amount of choice in deciding which location to provide:
Any location that contains the known location and is in the same Any location that contains the known location and is in the same
filter region will do. When the provider is reducing precision for filter region will do. When the provider is reducing precision for
privacy purposes, there is a signficant benefit to choosing a random privacy purposes, there is a some privacy benefit to choosing a
location meeting these criteria. If a watcher is interested in random location meeting these criteria. If a watcher is interested
whether or not the endpoint is moving, an imprecise location may in whether or not the endpoint is moving, an imprecise location may
still reveal that fact if it is constant when the endpoint is at still reveal that fact if it is constant when the endpoint is at
rest. If the provided location is randomized each time it is rest. If the provided location is randomized each time it is
provided, then the watcher is unable to obtain even this level of provided, then the watcher is unable to obtain even this level of
information. information. An algorithm for securely fuzzing a target's location
can be found in [11]; for emergency services, the additional
constraint must be added that the fuzzed location must remain in the
same filter region as the original.
7. IANA Considerations 7. IANA Considerations
This document requests that IANA register a new PIDF-LO 'method' This document requests that IANA register a new PIDF-LO 'method'
token in the registry defined by RFC 4119 [6] token in the registry defined by RFC 4119 [5]
area-representative: Location chosen as a representative of a region area-representative: Location chosen as a representative of a region
in which the target is located; may not be the target's location in which the target is located; may not be the target's location.
8. References 8. References
8.1. Normative References 8.1. Normative References
[1] Rosen, B., Schulzrinne, H., Polk, J., and A. Newton, "Framework [1] Rosen, B. and J. Polk, "Best Current Practice for
for Emergency Calling using Internet Multimedia", Communications Services in support of Emergency Calling",
draft-ietf-ecrit-framework-10 (work in progress), July 2009. draft-ietf-ecrit-phonebcp-14 (work in progress), January 2010.
[2] Rosen, B. and J. Polk, "Best Current Practice for Communications
Services in support of Emergency Calling",
draft-ietf-ecrit-phonebcp-13 (work in progress), July 2009.
[3] Hardie, T., Newton, A., Schulzrinne, H., and H. Tschofenig, [2] Hardie, T., Newton, A., Schulzrinne, H., and H. Tschofenig,
"LoST: A Location-to-Service Translation Protocol", RFC 5222, "LoST: A Location-to-Service Translation Protocol", RFC 5222,
August 2008. August 2008.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[5] Polk, J. and B. Rosen, "Location Conveyance for the Session [4] Wolf, K., "Location-to-Service Translation Protocol (LoST)
Initiation Protocol", draft-ietf-sip-location-conveyance-13 Extension: ServiceListBoundary",
(work in progress), March 2009. draft-ietf-ecrit-lost-servicelistboundary-01 (work in
progress), November 2009.
[6] Peterson, J., "A Presence-based GEOPRIV Location Object Format", [5] Peterson, J., "A Presence-based GEOPRIV Location Object
RFC 4119, December 2005. Format", RFC 4119, December 2005.
8.2. Informative References 8.2. Informative References
[7] Schulzrinne, H., "Location-to-URL Mapping Architecture and [6] Rosen, B., Schulzrinne, H., Polk, J., and A. Newton, "Framework
Framework", draft-ietf-ecrit-mapping-arch-04 (work in progress), for Emergency Calling using Internet Multimedia",
March 2009. draft-ietf-ecrit-framework-10 (work in progress), July 2009.
[7] Schulzrinne, H., "Location-to-URL Mapping Architecture and
Framework", draft-ietf-ecrit-mapping-arch-04 (work in
progress), March 2009.
[8] Polk, J. and B. Rosen, "Location Conveyance for the Session
Initiation Protocol", draft-ietf-sipcore-location-conveyance-01
(work in progress), July 2009.
[9] Tschofenig, H. and H. Schulzrinne, "GEOPRIV Layer 7 Location
Configuration Protocol; Problem Statement and Requirements",
draft-ietf-geopriv-l7-lcp-ps-10 (work in progress), July 2009.
[10] Marshall, R., "Requirements for a Location-by-Reference
Mechanism", draft-ietf-geopriv-lbyr-requirements-09 (work in
progress), November 2009.
[11] Schulzrinne, H., Tschofenig, H., Morris, J., Cuellar, J., and
J. Polk, "Geolocation Policy: A Document Format for Expressing
Privacy Preferences for Location Information",
draft-ietf-geopriv-policy-21 (work in progress), January 2010.
Authors' Addresses Authors' Addresses
Richard Barnes Richard Barnes
BBN Technologies BBN Technologies
9861 Broken Land Pkwy, Suite 400 9861 Broken Land Pkwy, Suite 400
Columbia, MD 21046 Columbia, MD 21046
USA USA
Phone: +1 410 290 6169 Phone: +1 410 290 6169
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