ECRIT K. Wolf
Internet-Draft nic.at
Expires: April 4, 2010 October 1, 2009
Location-to-Service Translation Protocol (LoST) Extension:
ServiceListBoundary
draft-wolf-ecrit-lost-servicelistboundary-02
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Abstract
LoST maps service identifiers and location information to service
contact URIs. If a LoST client wants to discover available services
for a particular location, it will perform a <listServicesByLocation>
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query to the LoST server. However, the response from the LoST server
does not provide information about the geographical region for which
the returned service list is valid. Therefore, this document
proposes a ServiceListBoundary to assist the client to not miss a
change in available services when moving.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. LoST Extensions . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Extensions to <ListServiceByLocation> . . . . . . . . . . . 4
3.2. Retrieving the serviceList Boundary via
getServiceListBoundary . . . . . . . . . . . . . . . . . . 6
3.3. Service List Boundary . . . . . . . . . . . . . . . . . . . 7
3.4. Implementation Considerations . . . . . . . . . . . . . . . 8
3.4.1. Server Side . . . . . . . . . . . . . . . . . . . . . . 8
3.4.2. Client Side . . . . . . . . . . . . . . . . . . . . . . 8
4. Security & Privacy Considerations . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . 9
7. Normative References . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
Location based service providers as well as Public Safety Answering
Points (PSAPs) only serve a specific geographic region. Therefore
the LoST protocol defines the ServiceBoundary, which indicates the
service region for a specific service URL. However, not all services
are available everywhere. Clients can discover available services
for a particular location by the <listServicesByLocation> query. The
LoST server returns a list of services that are available at this
particular location. But the server does not inform the client for
which geographical region the returned service list is valid. This
may lead to the situation where a client initially discovered all
available services by the <listServicesByLocation> query, and then
moves to a different location while refreshing the service mappings,
but does not notice the availability of another service. The
following imaginary example illustrates the problem for emergency
calling:
The client is powered-up, does location determination (resulting in
location A) and performs an initial <listServicesByLocation> query
with location A requesting urn:services:sos.
The LoST server returns the following services list:
urn:service:sos.police
urn:service:sos.ambulance
urn:service:sos.fire
The client does the initial LoST mapping and discovers the
dialstrings for these services. Then the client moves, refreshing
the service mappings when necessary as told by the ServiceBoundary.
However, when arriving in location B (close to a mountain), service
sos.mountainrescue is available, which was not available in location
A. Nevertheless, the client does not detect this, because only the
mapping of the initially discovered services (police, ambulance,
fire) are refreshed. Consequently, the dialstring for the mountain
rescue is not known by the client, and the emergency call to the
mountain rescue service will certainly fail.
Note that the ServiceBoundary (service region for an individual
service) cannot be considered as an indicator for the region a
specific service list is valid for. The service list may even change
within the ServiceBoundary of another service. For example, the
ambulance mapping is valid for a whole state, but for a part of the
state there is an additional mountain rescue service.
Consequently, there are two ways to tackle this issue:
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o clients continuously ask for the service list, although it may not
have changed
o a boundary information (telling the client that the service list
does not change inside this area)
Since the LoST protocol has the ServiceBoundary concept in order to
avoid that clients continuously try to refresh the mapping of a
specific service, a ServiceListBoundary would provide a similar
mechanism for service lists.
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 RFC 2119 [RFC2119].
3. LoST Extensions
This chapter describes the necessary modifications to the LoST
protocol in order to support the proposed ServiceListBoundary in a
similar way as the ServiceBoundary.
3.1. Extensions to <ListServiceByLocation>
The query <listServicesByLocation> may contain an additional
serviceListBoundary element to request the boundary for the service
list, either by value or by reference. In the example below the
value of the serviceListBoundary element ist set to "value":
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<?xml version="1.0" encoding="UTF-8"?>
<listServicesByLocation
xmlns="urn:ietf:params:xml:ns:lost1"
xmlns:gml="http://www.opengis.net/gml"
xmlns:slb="TBD"
recursive="true">
<location id="mylocation" profile="civic">
<civicAddress
xmlns="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr">
<country>AT</country>
<A1>Lower Austria</A1>
<A3>Wolfsthal</A3>
<RD>Hauptplatz</RD>
<HNO>1</HNO>
<PC>2412</PC>
</civicAddress>
</location>
<service>urn:service:sos</service>
<slb:serviceListBoundary>value</slb:serviceListBoundary>
</listServicesByLocation>
A possible response is shown below:
<?xml version="1.0" encoding="UTF-8"?>
<listServicesByLocationResponse
xmlns="urn:ietf:params:xml:ns:lost1">
xmlns:slb="TBD"
<serviceList expires="2010-01-01T00:00:00Z">
urn:service:sos.ambulance
urn:service:sos.fire
urn:service:sos.gas
urn:service:sos.mountain
urn:service:sos.poison
urn:service:sos.police
</serviceList>
<path>
<via source="resolver.example"/>
<via source="authoritative.example"/>
</path>
<locationUsed id="mylocation"/>
<slb:serviceListBoundary profile="civic">
<civicAddress
xmlns="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr">
<country>AT</country>
<A1>Lower Austria</A1>
</civicAddress>
</slb:serviceListBoundary>
</listServicesByLocationResponse>
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This response above indicates that the service list is valid for
Lower Austria. The <listServicesByLocation> request has to be
repeated by the client only when moving out of Lower Austria.
However, the mappings of the services itself may have other service
boundaries. Additionally, the expires attribute indicates the
absolute time when this service list becomes invalid.
The boundary can also be requested by reference when setting the
attribute serviceListBoundary to "reference". Then the response
contains a serviceListBoundaryReference element, as shown below.
<?xml version="1.0" encoding="UTF-8"?>
<listServicesByLocationResponse
xmlns="urn:ietf:params:xml:ns:lost1">
xmlns:slb="TBD"
<serviceList expires="2010-01-01T00:00:00Z">
urn:service:sos.ambulance
urn:service:sos.fire
urn:service:sos.gas
urn:service:sos.mountain
urn:service:sos.poison
urn:service:sos.police
</serviceList>
<path>
<via source="resolver.example"/>
<via source="authoritative.example"/>
</path>
<locationUsed id="mylocation"/>
<serviceListBoundaryReference
source="authoritative.example"
serviceListKey="123567890123567890123567890" />
</listServicesByLocationResponse>
3.2. Retrieving the serviceList Boundary via getServiceListBoundary
In order to retrieve the boundary corresponding a specific
serviceListKey, the client issues a <getServiceListBoundary> request,
similar to the <getServiceBoundary> request.
An example is shown below:
<?xml version="1.0" encoding="UTF-8"?>
<getServiceListBoundary xmlns="urn:ietf:params:xml:ns:lost1"
serviceListKey="123567890123567890123567890"/>
The LoST server response is shown below:
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<?xml version="1.0" encoding="UTF-8"?>
<getServiceListBoundaryResponse xmlns="TBD">
<serviceListBoundary profile="civic" expires="2010-01-01T00:00:00Z">
<civicAddress
xmlns="urn:ietf:params:xml:ns:pidf:geopriv10:civicAddr">
<country>AT</country>
<A1>Lower Austria</A1>
</civicAddress>
</serviceListBoundary>
<path>
<via source="resolver.example"/>
<via source="authoritative.example"/>
</path>
</getServiceListBoundaryResponse>
The serviceListKey uniquely identifies a serviceListBoundary as the
key does for the service boundary (see Section 5.6 in RFC 5222).
Therefore the serviceListKey is a random token with at least 128 bits
of entropy and can be assumed globally unique. Whenever the boundary
changes, a new serviceListKey MUST be assigned.
3.3. Service List Boundary
The service list boundary indicates a region within which all
<listServicesByLocation> queries with the same service identifiers
result in the same serviceList. A service list boundary may consist
of geometric shapes (both in civic and geodetic location format), and
may be non-contiguous, like the service boundary.
The mapping of the specific services within the service list boundary
may be different at different locations.
The server may return the boundary information in multiple profiles,
but has to use at least one profile that the client used in the
request in order to ensure that the client is able to process the
boundary information.
TBD: For <getServiceListBoundary> an attribute in the request could
be used to indicate which profile the client understands (e.g.
<getServiceListBoundary profile="civic"... )
There is no need to include boundary information to a
<listServicesResponse>. <ListServices> requests are purely for
diagnostic purposes and do not contain location information at all,
so no boundary information is reasonable.
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3.4. Implementation Considerations
The subsections below discuss implementations issues for the LoST
server and client for the serviceListBoundary support.
3.4.1. Server Side
The mapping architecture and framework [RFC5582] describes that each
tree announces its coverage region (for one type of service, e.g.
sos.police) to one or more forest guides. Forest guides peer with
each other and synchronize their data. Hence, a forest guide has
sufficient knowledge (it knows all the services and their coverage
regions) to answer a listServicesByLocation query and additionally
add the serviceListBoundary as well.
The calculation of the largest possible area for which the service
list stays the same might be a complex task. An alternative would be
to return smaller areas that are easier to compute. In such a case
some unneeded queries to the LoST server are the consequence, but
still the main purpose of the serviceListBoundary is achieved: Never
miss a change of available services. So a reasonable trade-off
between the effort to generate the boundary information and the saved
queries to the LoST server has to be considered.
Probably for some countries the county (or disrict, canton, state,
...) borders would be suitable as serviceListBoundary. Some
neighbouring counties may have implemented different services while a
listServicesByLocation query in other neighbouring counties still
results in the same serviceList. So when moving across a county
border, it is at least ensured, that every device fetches a new
service list from the LoST server.
Other countries might have different structures and the generation of
the serviceListBoundary might follow other rules as long as it is
ensured that a client is able to notice any change in the service
list when moving.
3.4.2. Client Side
A mobile client that already implements LoST and evaluates the
serviceBoundary has almost everything that is needed to make use of
the serviceListBoundary. Since the integration into LoST follows the
concept of the serviceBoundary (and also makes use of the same
location profiles), just the additional serviceListBoundary has to be
evaluated. Whenever moving outside a serviceListBoundary, the client
must perform a new listServicesByLocation query with the new location
information in order to determine a change in available services.
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4. Security & Privacy Considerations
Security considerations are discussed in [RFC5222].
5. IANA Considerations
TODO.
6. Acknowledgement
The author would like to thank Henning Schulzrinne for the discussion
on the draft.
7. Normative References
[RFC5222] Hardie, T., Newton, A., Schulzrinne, H., and H.
Tschofenig, "LoST: A Location-to-Service Translation
Protocol", RFC 5222, August 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5582] Schulzrinne, H., "Location-to-URL Mapping Architecture and
Framework", RFC 5582, September 2009.
Author's Address
Karl Heinz Wolf
nic.at GmbH
Karlsplatz 1/2/9
Wien A-1010
Austria
Phone: +43 1 5056416 37
Email: karlheinz.wolf@nic.at
URI: http://www.nic.at/
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