ECRIT                                                            K. Wolf
Expires: April 4, 2010                                   October 1, 2009

       Location-to-Service Translation Protocol (LoST) Extension:

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   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

   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:


   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",
   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"?>
        <location id="mylocation" profile="civic">
            <A1>Lower Austria</A1>

   A possible response is shown below:

      <?xml version="1.0" encoding="UTF-8"?>
       <serviceList expires="2010-01-01T00:00:00Z">
        <via source="resolver.example"/>
        <via source="authoritative.example"/>
       <locationUsed id="mylocation"/>
       <slb:serviceListBoundary profile="civic">
           <A1>Lower Austria</A1>

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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"?>
       <serviceList expires="2010-01-01T00:00:00Z">
        <via source="resolver.example"/>
        <via source="authoritative.example"/>
       <locationUsed id="mylocation"/>
          serviceListKey="123567890123567890123567890" />

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"

   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">
        <A1>Lower Austria</A1>
       <via source="resolver.example"/>
       <via source="authoritative.example"/>

   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


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 GmbH
   Karlsplatz 1/2/9
   Wien  A-1010

   Phone: +43 1 5056416 37

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