Internet Engineering Task Force R. Barnes
Internet-Draft M. Lepinski
Intended status: Standards Track BBN Technologies
Expires: July 24, 2010 January 20, 2010
Using Imprecise Location for Emergency Context Resolution
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
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Determining sufficient location precision . . . . . . . . . . 4
3.1. Location filtering . . . . . . . . . . . . . . . . . . . . 6
3.2. Constructing location filters . . . . . . . . . . . . . . 10
3.2.1. Civic address considerations . . . . . . . . . . . . . 11
3.3. Maintaining location filters . . . . . . . . . . . . . . . 12
3.4. Applying location filters . . . . . . . . . . . . . . . . 12
4. Requesting emergency and non-emergency services . . . . . . . 13
4.1. Emergency calling . . . . . . . . . . . . . . . . . . . . 13
4.2. Non-emergency services . . . . . . . . . . . . . . . . . . 14
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1. Normative References . . . . . . . . . . . . . . . . . . . 16
8.2. Informative References . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
Information about the location of an emergency caller is a critical
input to the process of emergency call establshment. Endpoint
location is used to determine which Public Safety Answering Point
(PSAP) should be the destination of the call. (The entire emergency
calling process is described in detail in [6] and [1].) This process
is most likely to work properly when the endpoint is provided with
the most accurate and precise information available about its
location. Using location information with maximal precision and
accuracy minimizes the chance that a call will be mis-routed. In
addition, when that location is provided to the endpoint, the
endpoint is able to verify that the location is correct (to the
extent of the endpoint's knowledge of its own location) prior to an
emergency call, and is able to perform emergency call routing
functions on its own, providing redundancy for network-provided
functions.
However, there may be situations in which it is not feasible for
endpoints to be provided with maximally precise and accurate
location. These cases may arise when computing precise location is
an expensive or time-consuming operation (e.g., in the case of
wireless triangulation), and location is needed quickly, as is often
the case in emergency situations. Or they may arise because the
policy of a location provider does not allow precise location to be
provided to the endpoint. While it is undesirable to use imprecise
location for emergency call routing, the possibility that precise
location may not be available to the calling device must be
accomodated in order to make emergency calling possible in the
largest possible set of circumstances.
This document is concerned with imprecise location only in the
context of routing emergency calls, i.e., for determining the correct
PSAP to receive a given call (e.g., via a LoST query [2]). Depending
on the the structure of the local emergency service network, the
location information provided to the endpoint may also be used to
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
framework to direct the dispatch of emergency responders. This usage
is treated separately from call routing for purposes of this
document, and this document does not place requirements on the
location provided for dispatch, although it should obviously be as
precise as possible. The only provision for dispatch in this
document is a recommendation that the location provider supply
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endpoints with a URI that can be used by a PSAP or other emergency
authority to obtain a different location for use in dispatch,
hopefully more precise than the one used for routing.
This document describes the use of imprecise location information in
the emergency call routing system. Section 3 describes how location
providers can determine the precision necessary to support emergency
call routing, and how they can use this information to optimize
location delivery. Section 4 describes how emergency calls are
placed in such an environment, and how non-emergency services can be
invoked when precise location is not available to the endpoint by
value.
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 [3].
We consider in this document patterns of interaction as described in
[6]. The two main parties of interest are endpoints and location
providers. Endpoints are hosts connected to the Internet that
originate emergency calls in the emergency calling architecture,
while location providers are entities that supply location
information that is used for emergency calling. In addition, we will
discuss how these parties interact with the LoST mapping
infrastructure [7], and with emergency and non-emergency location-
based service providers.
For convenience, we say that location information, either in LoST
queries or in service boundaries, is provided "in geodetic form" if
it is provided in the "geodetic-2d" LoST location profile, and "in
civic form" if it is provided in the "civic" profile.
3. Determining sufficient location precision
A location provider wishing to provide location information usable
for emergency call routing requires a mechanism for determining when
a description of location (e.g., a polygon) is precise enough to be
used for emergency call routing. This mechanism might be used to
decide when to terminate a positioning process that converges over
time, or to choose a polygon larger than the known location of the
endpoint (in order to obscure the known location of the endpoint),
while preserving the utility of the location for emergency call
routing.
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There are three basic requirements for a location to be usable for
emergency call routing:
1. The location SHOULD be sufficiently precise that a LoST request
with the location and any service URN will return a unique URI
mapping value. This may not be possible in all cases, e.g.,
because of overlapping service boundaries creating areas with
non-unique mappings, or because of positioning limitations that
prevent sufficiently precise positioning.
2. When the location of the endpoint is known by the provider to
greater precision than is being provided, the provided location
MUST return the same mappings from LoST, for all service URNs, as
the known location.
3. When the location of the endpoint is known by the provider to
greater precision than is being provided, the provided location
MUST contain the precise location (as a geographic subset).
These requirements lead naturally to the idea of a "location filter".
A location filter is a collection of geographical regions satisfying
the following criteria:
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.
2. Any two locations within the same filter region receive the same
LoST results for all services
Given a location filter, it is easy to determine when a given
location value is sufficiently precise, or to create a less precise
version of location that is still precise enough. Namely, a location
value is precise enough when if fits within a given filter region,
and any superset of a location value (e.g., a polygon containing a
point) can be used as a less precise version of the location value as
long as it still fits within the same filter region.
For example, a simple fuzzing algorithm that maintains sufficient
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.
The remainder of this section discusses the concept of location
filtering in more detail, and describes how a location server can
construct and maintain a location filter based on information from
the LoST mapping infrastructure.
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3.1. Location filtering
With each service-to-URI mapping, a LoST query provides a service
boundary that represents the set of locations in which that mapping
is valid. A consequence of this is that given a set of service
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).
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Service boundaries for individual services
urn:service:sos.police urn:service:sos.fire
+-------+ +-------+
| A | | C |
| +---+ | +---+---+
| | | | X | |
+---+---+ | +---+ |
| B | | D |
+-------+ +-------+
| |
| |
+-----------+------------+
|
V
+-------+
| A,C |
| +---+
| | +---+
+---+ |A,D| +---+
+---+ | |
+---+ |
| B,D |
+-------+
Resulting Location Filter Regions
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
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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.
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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
location provided to the endpoint, although any location meeting the
two criteria above is equivalent to the known location for purposes
of LoST, the provided location MUST contain the known location 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
first responders for dispatch). This guarantee also allows the
endpoint to do some course verification that the provided location is
correct (in order to prevent very gross errors in routing). Thus,
any location that (1) contains the known location and (2) is
contained in the same filter region as the known location is
allowable. Locations that also are contained in only one filter
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region are preferred. Adding randomness to the provided locations
may have privacy benefits in some cases, as discussed in the security
considerations below.
3.2. Constructing location filters
For simplicity, we assume that the entity performing filtering will
only be using the filter to test locations contained within a
particular geographic "coverage area". In principle, this 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 and the ability to act as a LoST client, a
location service provider can autonomously compute a location filter
by constructing regions around sample points until it has a
collection of filter regions that collectively cover its service
region. (The process for an individual point is illustrated in
Figure 2.)
In order to ensure that all services boundaries are taken into
account, the server starts by issuing a <listServicesByLocation>
query, and caching the list of services that it returns, along with
the corresponding service list boundary [4]. The server then samples
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:
Set FILTER = the empty set
While filter does not cover LS coverage area
Choose a random uncovered point X in the LS coverage area
Perform a LoST <listServicesByLocation> query for X
Set SL = <serviceList> from LoST response
Set SLB = <serviceListBoundary> from LoST response
If SLB is not provided, choose new point X and re-query
If more than 100 points X have been tried
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) )
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Add R to FILTER
If the LoST servers have been provisioned properly then this
algorithm will terminate successfully. If LoST mapping do not cover
part of the service region, then the <serviceListBoundary> will not
be returned, and the algorithm will give up after 100 queries. This
limit on queries introduces some risk that a small covered area will
be left out of the filter and marked as uncovered; if this is a
concern, then the query limit can be increased.
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.
3.2.1. Civic address considerations
This algorithm actually results in two filters -- one for geodetic
service boundaries and one for civic service boundaries -- since
civic and geodetic boundaries cannot be directly compared or
intersected. It is RECOMMENDED that location servers always compute
a geodetic filter for use with emergency services, since the notion
of civic service boundaries have some inherent ambiguity.
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.
When computing the intersection of two civic service boundaries, any
<civicAddress> elements that are shared between the two service
boundaries MUST be included in the resulting intersection. When two
<civicAddress> elements in the service boundaries being compared are
different from each other, then their intersection must be computed
according to local addressing standards.
Note that the resulting filter regions SHOULD still cover the
location server's coverage area, i.e., there should be a filter
region that contains every civic address within the coverage area.
In particular, the server SHOULD NOT use a specific address to
represent a filter region: Such an address would not include many
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points in the service region (i.e., it would not meet the third rules
from the lists of rules above). If the server creates a PIDF-LO
document describing a civic address that does not contain the precise
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.
3.3. Maintaining location filters
As the LoST mappings that underlie the filter change, the filter will
need to be updated. The entity maintaining the filter MUST obtain a
new mapping for a region when an existing mapping expires. The
service boundary from the new mapping is compared to the service
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
intersect either the old service boundary or the new service boundary
will need to be recomputed. Note that since this operation only
requires the server to determine if two service boundaries are
identical, the server need only store a hash of the old boundary to
which it can compare a hash of the new boundary.
3.4. Applying location filters
After constructing a location filter, a location server can use it to
optimize how it delivers location. When the location server is using
a positioning algorithm that grows more accurate with time, the
filter tells it how long to run the algorithm. Namely, the algorithm
can be terminated when the estimated location (that is, an
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,
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
contains the caller's precise location and return that region as the
caller's location, or some subset that contains the precise location.
This caching strategy allows an additional optimization in some
cases: If the location server knows that the caller's precise
location will be within the same region for a period of time, it can
instruct the client not to re-query in that time. For instance, if
the server is delivering location over HELD, then it can use the HTTP
cache-control headers (e.g., Expires). However, the location server
MUST NOT instruct the client to wait for longer than the current
filter is valid; the expiry time of the location MUST be before the
earliest expiry of a LoST mapping used in the filter.
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4. Requesting emergency and non-emergency services
When a location provider wishes to deliver endpoints location
information that is below its maximum available precision while still
supporting emergency calling, it MUST provide to the endpoint both a
location (by value) that is sufficient for emergency call routing (as
defined above) and a location reference (i.e., a URI) that can
subsequently be used by authorized parties to obtain more precise
information about the location of the endpoint. The endpoint then
can then use both the location value and the location reference to
request emergency services and other location-based services (LBS).
4.1. Emergency calling
The overall procedure for placing an emergency call is identical to
that described in [6]. In particular, the endpoint requirements in
Sections 8 and 9 of [1] still apply to an endpoint that receives
imprecise location.
In addition, an endpoint that receives location both by value and by
reference from its location provider MUST include both the location
value and the location reference in the SIP INVITE message that
initiates an emergency call, as specified in [8]. When the endpoint
supports LoST, it MUST use the location value to obtain a PSAP URI
for LoST queries before attempting to dereference the location
reference. Note that the caller would also have to add the "used-
for-routing" parameter to the geolocation header that points to the
location value as inserted into the INVITE message. Note that this
process crucially relies on the location value having sufficient
precision for routing emergency calls (see Section 3 for techniques
to ensure the location value is suitable for emergency call routing).
When a PSAP receives a SIP INVITE that contains both a location value
and a location reference, and the value is too imprecise for use in
dispatch then the PSAP SHOULD dereference the LbyR to obtain more
precise information. In turn, the location provided by the location
provider MUST allow access by all PSAPs whose service boundaries
overlap with the region served by the location provider. This means
that either the provider must supply a reference that can be
dereferenced by any party, or else the provider must establish
explicit authentication and authorization relationships with all
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.
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4.2. Non-emergency services
Non-emergency LBSs may require more precise information than is
required for emergency call routing. Therefore, when requesting a
non-emergency LBS, the endpoint SHOULD include the location reference
provided by its location provider, and MAY additionally provide the
location value. If the provided location value is not sufficiently
precise to deliver the requested service, then the LBS provider
should then dereference the location value to request location
information of sufficient precision from the location provider. If
the dereference fails, then the request for service may fail as well.
Note that when the location reference provided by the location
provider is access-controled, this dereference may require a pre-
existing authentication and authorization agreement between the LBS
provider and the location provider. In such a case, the endpoint may
not know whether a given non-emergency service is authorized to
obtain the endpoint's precise location using the location reference.
The endpoint is always capable of requesting services without knowing
whether they are authorized; in this way, the endpoint can discover
authorized services by trial and error. In order to simplify this
process, a location provider may supply the endpoint with references
to authorized service providers, although there is currently no
standard protocol for this transaction.
5. Acknowledgements
This document generalizes the concept of "rough location" that was
originally discussed in the context of the location hiding problem.
This concept was put forward by Henning Schulzrinne and Andy Newton,
among many others, in a long-running ECRIT discussion.
6. Security Considerations
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.
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Using of rough location to support emergency calling enables a
location provider to provide low-precision location with low
assurance (e.g., without client authentication)and high-precision
location with higher assurance. Because lower-precision location
generally has lower value -- to location providers and LBS providers
as a commercial asset, and to targets as private information -- this
trade-off allows a location provider to avoid the cost of protecting
location with high-assurance access controls when this location has
low value.
However, in order to support emergency services, location providers
cannot provide only low-precision location; they also have to provide
PSAPs with access to high-precision location information. Because
PSAPs require high-precision location for emergency response, a
location provider that normally provides imprecise location to
clients MUST also provide them a location URI that a PSAP can use to
obtain high-precision location. This constraint means that the
provided URI MUST have either no access control at all or a policy
that allows access by appropriate PSAPs and other emergency response
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.
The use of location by reference introduces some risk that the
reference will be used by an attacker to gain unauthorized access to
the target's location. These risks are not specific to emergency
service, however; general risks and mitigations for location by
reference are discussed in [10]
As described in Section 3.1 above, the location provider choosing to
provide a less precise location than a known location has a
significant amount of choice in deciding which location to provide:
Any location that contains the known location and is in the same
filter region will do. When the provider is reducing precision for
privacy purposes, there is a some privacy benefit to choosing a
random location meeting these criteria. If a watcher is interested
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
rest. If the provided location is randomized each time it is
provided, then the watcher is unable to obtain even this level of
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.
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7. IANA Considerations
This document requests that IANA register a new PIDF-LO 'method'
token in the registry defined by RFC 4119 [5]
area-representative: Location chosen as a representative of a region
in which the target is located; may not be the target's location.
8. References
8.1. Normative References
[1] Rosen, B. and J. Polk, "Best Current Practice for
Communications Services in support of Emergency Calling",
draft-ietf-ecrit-phonebcp-14 (work in progress), January 2010.
[2] Hardie, T., Newton, A., Schulzrinne, H., and H. Tschofenig,
"LoST: A Location-to-Service Translation Protocol", RFC 5222,
August 2008.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Wolf, K., "Location-to-Service Translation Protocol (LoST)
Extension: ServiceListBoundary",
draft-ietf-ecrit-lost-servicelistboundary-01 (work in
progress), November 2009.
[5] Peterson, J., "A Presence-based GEOPRIV Location Object
Format", RFC 4119, December 2005.
8.2. Informative References
[6] Rosen, B., Schulzrinne, H., Polk, J., and A. Newton, "Framework
for Emergency Calling using Internet Multimedia",
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",
Barnes & Lepinski Expires July 24, 2010 [Page 16]
Internet-Draft ECRIT with Rough Location January 2010
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
Richard Barnes
BBN Technologies
9861 Broken Land Pkwy, Suite 400
Columbia, MD 21046
USA
Phone: +1 410 290 6169
Email: rbarnes@bbn.com
Matt Lepinski
BBN Technologies
10 Moulton St
Cambridge, MA 02138
USA
Phone: +1 617 873 5939
Email: mlepinski@bbn.com
Barnes & Lepinski Expires July 24, 2010 [Page 17]
