ECRIT                                                     H. Schulzrinne
Internet-Draft                                               Columbia U.
Intended status: Informational                              July 8, 2007
Expires: January 9, 2008

           Location-to-URL Mapping Architecture and Framework

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document describes an architecture for a global, scalable,
   resilient and administratively distributed system for mapping
   geographic location information to URLs, using the Location-to-
   Service (LoST) protocol.  The architecture generalizes well-known
   approaches found in hierarchical lookup systems such as DNS.

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Table of Contents

   1.  The Mapping Problem  . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Overview of Architecture . . . . . . . . . . . . . . . . . . .  5
     4.1.  Minimal System Architecture  . . . . . . . . . . . . . . .  6
   5.  Seeker . . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   6.  Resolver . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   7.  Trees: Maintaining Authoritative Knowledge . . . . . . . . . .  7
     7.1.  Basic Operation  . . . . . . . . . . . . . . . . . . . . .  7
     7.2.  Answering Queries  . . . . . . . . . . . . . . . . . . . . 10
     7.3.  Overlapping Coverage Regions . . . . . . . . . . . . . . . 10
     7.4.  Scaling and Reliability  . . . . . . . . . . . . . . . . . 11
   8.  Forest Guides  . . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  Configuring Service Numbers  . . . . . . . . . . . . . . . . . 12
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     13.2. Informative References . . . . . . . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
   Intellectual Property and Copyright Statements . . . . . . . . . . 17

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1.  The Mapping Problem

   It is often desirable to allow users to access a service that
   provides a common function, but is actually offered by a variety of
   local service providers.  In many of these cases, the service
   provider chosen depends on the location of the person wishing to
   access that service.  Among the best-known public services of this
   kind is emergency calling, where emergency calls are routed to the
   most appropriate public safety answering point (PSAP), based on the
   caller's physical location.  Other services, from food delivery to
   directory services and roadside assistance, also follow this general
   pattern.  This is a mapping problem [8], where a geographic location
   and a service identifier (URN) [10] is translated into a set of URIs,
   the service URIs, that allow the Internet system to contact an
   appropriate network entity that provides the service.

   The caller does not need to know where the service is being provided
   from, and the location of the service provider may change over time,
   e.g., to deal with temporary overloads, failures in the primary
   service provider location or long-term changes in system
   architecture.  For emergency services, this problem is described in
   more detail in [6].

   The overall emergency calling architecture [6] separates mapping from
   placing calls or otherwise invoking the service, so the same
   mechanism can be used to verify that a mapping exists ("address
   validation") or to obtain test service URIs.

   Mapping locations to URIs describing services requires a distributed,
   scalable and highly resilient infrastructure.  Authoritative
   knowledge about such mappings is distributed among a large number of
   autonomous entities that may have no direct knowledge of each other.
   In this document, we describe an architecture for such a global
   service.  It allows significant freedom to combine and split
   functionality among actual servers and imposes few requirements as to
   who should operate particular services.

   Besides determining the service URI, end systems also need to
   determine the local service numbers.  As discussed in Section 9, the
   architecture described here can also address that problem.

   The architecture described here uses the Location-to-Service
   Translation (LoST) [2] protocol, although much of the discussion
   would also apply for other mapping protocols satisfying the mapping
   requirements [8].

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2.  Terminology

   In this document, the key words "MUST", "MUSTNOT", "REQUIRED",
   "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and
   indicate requirement levels for compliant implementations.

3.  Definitions

   In addition to the terms defined in [8], this document uses the
   following terms to describe LoST clients and servers:

   authoritative mapping server (AMS):  An authoritative mapping server
      (AMS) is a LoST server that can provide the authoritative answer
      to a particular set of queries, e.g., covering a set of PIDF-LO
      civic labels or a particular region described by a geometric
      shape.  In some (rare) cases of territorial disputes, two
      resolvers may be authoritative for the same region.  An AMS may
      redirect or forward a query to other AMS within the tree.
   child:  A child is an AMS that is authoritative for a subregion of
      another AMS.  A child can in turn be parent for another AMS.
   (tree node) cluster:  A node cluster is a group of LoST servers that
      all share the same mapping information and return the same results
      for queries.  Clusters provide redundancy and share query load.
      Clusters are fully-meshed, i.e., they all exchange updates with
      each other.
   forest guide (FG):  A forest guide (FG) has knowledge of the coverage
      region of trees for a particular top-level service.
   mapping:  A mapping is a short-hand for 'mapping from a location
      object to one or more URLs describing either another mapping
      server or the desired service URLs'.
   parent:  A mapping server that covers the region of all of its
      children.  A mapping server without a parent is a root AMS.
   resolver:  A resolver is contacted by a seeker, consults a forest
      mapping server and then resolves the query using an appropriate
      tree.  Resolvers may cache query results.
   seeker:  A seeker is a LoST client requesting a mapping.  A seeker
      does not provide mapping services to others, but may cache results
      for its own use.
   region map:  A data object describing a contiguous area covered by an
      AMS, either as a subset of a civic address or a geometric object.
   tree:  A tree consists of a self-contained hierarchy of authoritative
      mapping servers.  Each tree exports its coverage region to the
      forest mapping servers.

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4.  Overview of Architecture

   In short, end users of the LoST-based [2] mapping mechanism, called
   seekers, contact resolvers that cache query results and know one or
   more "forest guides".  Forest guides know the coverage region of
   trees and direct queries to the node at the top of the appropriate
   tree.  Trees consist of authoritative mapping servers and maintain
   the authoritative mapping information.  Figure 1 shows the
   interaction of the components.

          /-\        /-\        +-----+                 +-----+
         | S +******* R *********  FG *-----------------+  FG |
          \-/        \-/        |     |*                |     |
                                +--+--+  *              +--+--+
                                   |      *                |
                                   |       *               |
                                   |        *              |
                                   |        *              |
                     /-\        +--+--+     *           +--+--+
                    | R +------>+  FG +-----*-----------+  FG |
                     \-/        |     |     *           |     |
                                +--+--+    *            +--+--+
                                   |      *                |
                                   |     *                 |
                                   |    *                  |
                                   |***                    ^
                                  / \                     / \
                                 /   \                   /   \
                                /     \                 /     \
                               /       \               /       \
                              -----------             -----------
                                tree                     tree

   Architecture diagram, showing seekers (S), resolvers (R), forest
   guides (FG) and trees.  The star (*) line indicates the flow of the
   query and responses in recursive mode.

                                 Figure 1

   The mapping function for the world is divided among trees.  The
   collection of trees may not cover the whole world and trees are added
   and removed as the organization of mapping data changes.  We call the
   collection of trees a forest.  There is no limit on the number of
   trees within the forest, but the author guesses that the number of
   trees will likely be somewhere between a few hundred and a few
   thousand.  The lower estimate would apply if each country operates
   one tree.  We assume that tree coverage information changes
   relatively slowly, on the order of less than one change per year per

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   tree, although the system imposes no specific threshold.  Tree
   coverage would change, for example, if a country is split or merged
   or if two trees for different regions become part of a larger tree.
   (On the other hand, information within a tree is likely to change
   much more frequently.)

4.1.  Minimal System Architecture

   It is possible to build a functioning system consisting only of
   seekers and resolvers if these resolvers have other means of
   obtaining mapping data.  For example, a company acting as a mapping
   service provider could collect mapping records manually and make them
   available to their customers through the resolver.  While feasible as
   a starting point, such an architecture is unlikely to scale globally.
   Among other problems, it becomes very hard for providers of
   authoritative data to ensure that all such providers have up-to-date
   information.  If new trees are set up, they would somehow make
   themselves known to these providers.  Such a mechanism would be
   similar to the old "hosts.txt" mechanism for distributing host
   information in the early Internet before DNS was developed.

   Below, we describe the operation of each component in more detail.

5.  Seeker

   Clients desiring location-to-service mappings are known as seekers.
   Seekers are consumers of mapping data and originate LoST queries as
   LoST protocol clients.  Seekers do not answer LoST queries.  They
   contact either forest guides or resolvers to find the appropriate
   tree that can authoritatively answer their questions.  Seekers can be
   end systems or call routing entities such as SIP proxy servers.

   Seekers may need to obtain mapping information in several steps,
   i.e., they may obtain pointers to intermediate servers that lead them
   closer to the final mapping.  Seekers MAY cache query results for
   later use, but otherwise have no obligations to other entities in the

   Seekers need to be able to identify appropriate resolvers.  The
   mechanism for providing seekers with that information is likely to
   differ depending on who operates the resolvers.  For example, if the
   voice service provider operates the resolver, it might include the
   location of the resolver in the SIP configuration information it
   distributes to its user agents.  An Internet access provider or
   enterprise can provide a pointer to a resolver via DHCP [5].  In an
   ad-hoc or zero-configuration environment, appropriate service
   directories may advertise resolvers.

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   Like other entities in the system, seekers can cache responses.  This
   is particularly useful if the response describes the result for a
   civic or geospatial region, rather than just a point.  For example,
   for mobile nodes, seekers would only have to update their resolution
   results when they leave the coverage area of a service provider, such
   as a PSAP for emergency services, and can avoid repeatedly polling
   for this information whenever the location information changes
   slightly.  (Mobile nodes would also need a location update mechanism
   that is either local or triggered when they leave the current service
   area.)  This will likely be of particular benefit for seekers
   representing a large user population, such as the outbound proxy in a
   corporate network.  For example, rather than having to query
   separately for each cubicle, information provided by the
   authoritative node may indicate that the whole campus is covered by
   the same service provider.

   Given this caching mechanism and cache lifetimes of several days,
   most mobile users traveling to and from work would only need to
   obtain service area information along their commute route once during
   each cache lifetime.

6.  Resolver

   A seeker can contact a forest guide (see below) directly, but may not
   be able to easily locate such a guide.  In addition, seekers in the
   same geographic area may already have asked the same question.  Thus,
   it makes sense to introduce another entity, known as a resolver in
   the architecture, that knows how to contact one or more forest guides
   and caches earlier queries to accelerate the response to mapping
   queries and to improve the resiliency of the system.  Each resolver
   can decide autonomously which FGs to use, with possibly different
   choices for each top-level service.

   ISPs or VSPs would include the address of a suitable resolver in
   their configuration information, e.g., in SIP configuration for a VSP
   or DHCP [5] for an ISP.  Resolvers are manually configured with the
   name of one or more forest guides.

7.  Trees: Maintaining Authoritative Knowledge

7.1.  Basic Operation

   The architecture assumes that authoritative knowledge about the
   mapping data is distributed among many independent administrative
   entities, but clients (seekers) needing the information may
   potentially need to find out mapping about any spot on earth.

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   (Extensions to extra-terrestrial applications are left for future
   exploration.)  Information is organized hierarchically, in a tree,
   with tree nodes representing larger geographic areas pointing to
   several child nodes each representing a smaller area.  Each tree node
   can be a cluster of LoST servers that all contain the same
   information and back up each other.

   Each tree can map a location described by civic and geographic
   coordinates for one type of service (such as 'sos.police', ''
   or 'counseling'), although nothing prevents re-using the same tree
   for multiple different services.  The collection of all trees for one
   service is known as a forest.

   Each tree root announces its coverage region to one or more forest

   Each tree node cluster knows the coverage region of its children and
   sends queries to the appropriate server "down" the tree.  Each such
   tree node knows authoritatively about the service mappings for a
   particular region, typically, but not necessarily, contiguous.  The
   region can be described by a polygon in geospatial coordinates or a
   set of civic address descriptors (e.g., "country = DE, A1 =
   Bavaria").  These coverage regions may be aligned with political
   boundaries, but that is not required.  In most cases, to avoid
   confusion, only one cluster is responsible for a particular
   geographic or civic location, but the system can also deal with cases
   where coverage regions overlap.

   There are no assumptions about the coverage region of a tree as a
   whole.  For example, a tree could cover a single city, or a state/
   province or a whole country.  Nodes within a tree need to loosely
   coordinate their operation, but they do not need to be operated by
   the same administrator.

   The tree architecture is roughly similar to the domain name system
   (DNS), except that delegation is not by label, but rather by region.
   (Naturally, DNS does not have the notion of forest guides.)  One can
   also draw analogies to LDAP, when deployed in a distributed fashion.

   Tree nodes maintain two types of information, namely coverage regions
   and mappings.  Coverage regions describe the region served by a child
   node in the tree and point to a child node for further resolution.
   Mappings contain an actual service URI leading to a service provider
   or another signaling server representing a group of service
   providers, which in turn might further route signaling requests to
   more servers covering smaller regions.

   Leaf nodes, i.e., nodes without children, only maintain mappings,

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   while tree nodes above the leaf nodes only maintain coverage regions.
   An example for emergency services of a leaf node entry is shown
   below, indicating how queries for three towns are directed to
   different PSAPs.  Queries for Englewood are directed to another LoST
   server instead.

   country   A1 A2         A3        resource
   US        NJ Bergen     Leonia
   US        NJ Bergen     Fort Lee
   US        NJ Bergen     Teaneck
   US        NJ Bergen     Englewood

   Coverage regions are described by sets of polygons enclosing
   contiguous geographic areas or by descriptors enumerating groups of
   civic locations.  For the former, the LoST server performs a point-
   in-polygon operation to find the polygon that contains the query
   point.  (More complicated geometric matching algorithms may be added
   in the future.)

   For example, a state-level tree node for New Jersey in the United
   States may contain the following coverage region entries, indicating
   that any query matching a location in Bergen County, for example,
   would be redirected or forwarded to the node located at  There is no requirement that all child nodes
   cover the same level within the civic hierarchy.  As an example, in
   the table below, the city of Newark has decided to be listed directly
   within the state node, rather than through the county.  Longest-match
   rules allow partial coverage, so that for queries for all other towns
   within Essex county would be directed to the county node for further

   C  A1 A2         A3     resource
   US NJ Atlantic   *
   US NJ Bergen     *
   US NJ Monmouth   *
   US NJ Essex      *
   US NJ Essex      Newark

   Thus, there is no substantial difference between coverage region and
   mapping data.  The only difference is that coverage regions return
   LoST URLs, while mapping entries contain service URLs.  Mapping
   entries may be specific down to the house or floor level or may only
   contain street-level information.  For example, in the United States,
   civic mapping data for emergency services is generally limited to
   address ranges ("MSAG data"), so initial mapping databases may only
   contain street-level information.

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   To automate the maintenance of trees, the LoST synchronization
   mechanism [11] allows nodes to query other nodes for mapping data and
   coverage regions.  In the example above, the state-run node would
   query the county nodes and use the records returned to distribute
   incoming LoST queries to the county nodes.  Conversely, nodes could
   also contact their parent nodes to tell them about their coverage
   region.  There is some benefit of child nodes contacting their
   parents, as this allows changes in coverage region to propagate
   quickly up the tree.

7.2.  Answering Queries

   Within a tree, the basic operation is straightforward: A query
   reaches the root of the tree.  That node determines which coverage
   region matches that request and forwards the request to the URL
   indicated in the coverage region record, returning a response to the
   querier when it in turns receives an answer (recursion).
   Alternatively, the node returns the URL of that child node to the
   querier (iteration).  This process applies to each node, i.e., a node
   does not need to know whether the original query came from a parent
   node, a seeker, a forest guide or a resolver.

   For efficiency, a node MAY return region information instead of a
   point answer.  Thus, instead of returning that a particular
   geospatial coordinate maps to a service or mapping URL, it MAY return
   a polygon indicating the region for which this answer would be
   returned, along with expiration time (time-to-live) information.  The
   querying node can then cache this information for future use.

   For civic coordinates, trees may not include individual mapping
   records for each floor, house number or street.  To avoid giving the
   wrong indication that a particular location has been found valid,
   LoST can indicate which parts of the location information have
   actually been used to look up a mapping.

7.3.  Overlapping Coverage Regions

   In some cases, coverage regions may overlap, either because there is
   a dispute as to who handles a particular geographic region or, more
   likely, since the resolution of the coverage map may not be
   sufficiently high.  For example, a node may "shave some corners" off
   its polygon, so that its coverage region appears to overlap with its
   geographic neighbor.  For civic coordinates, houses on the same
   street may be served by different PSAPs.  The mapping mechanism needs
   to work even if a coverage map is imprecise or if there are disputes
   about coverage.

   The solution for overlapping coverage regions is relatively simple.

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   If a query matches multiple coverage regions, the node returns all
   URLs, in redirection mode, or queries both children, if in recursive
   mode.  If the overlapping coverage is caused by imprecise coverage
   maps, only one will return a result and the others will return an
   error indication.  If the particular location is disputed territory,
   the response will contain all answers, leaving it to the querier to
   choose the preferred solution or trying to contact all services in

7.4.  Scaling and Reliability

   Since they provide authoritative information, tree nodes need to be
   highly reliable.  Thus, while this document refers to tree nodes as
   logical entities within the tree, an actual implementation would
   likely replicate node information across several servers, forming a
   cluster.  Each such node would have the same information.  Standard
   techniques such as DNS SRV records can be used to select one of the
   servers.  Replication within the cluster can use any suitable
   protocol mechanism, but a standardized incremental update mechanism
   makes it easier to spread those nodes across multiple independently-
   administered locations.  The techniques developed for meshed SLP [3]
   are applicable here.

8.  Forest Guides

   Unfortunately, just having trees covering various regions of the
   world is not sufficient as a client of the mapping protocol would not
   generally be able to keep track of all the trees in the forest.  To
   facilitate orientation among the trees, we introduce a "forest guide"
   (FG).  It is a server that keeps track of the coverage regions of the
   trees.  For scalability and reliability, there will need to be a
   large number of forest guides, all providing the same information.  A
   seeker can contact a suitable forest guide and will then be directed
   to the right tree or, rarely, set of trees.  Forest guides do not
   provide mapping information themselves, but rather redirect to
   mapping servers.  In some configurations, not all forest guides may
   provide the same information, due to policy reasons.

   Introducing forest guides avoids creating a global root, with the
   attendant management and control issues.  Trees can also restrict
   their cooperation to parts of the information.  For example, if
   country C does not recognize country T, C can propagate tree regions
   for all but T.

   For authenticity, the records SHOULD be digitally signed.  They are
   used by resolvers and possibly seekers to find the appropriate tree
   for a particular area.  All forest guides should have consistent

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   information, i.e., a collection of all the coverage regions of all
   the trees.  A tree node at the top of a tree can contact any forest
   guide and inject new coverage region information into the system.
   One would expect that each tree announces its coverage to more than
   one forest guide.  Each forest guide peers with one or more other
   guides and distributes new coverage region announcements to all other

   Forest guides fulfill a similar role to root servers in DNS.
   However, their number is likely to be larger, possibly counted in
   hundreds.  They distribute information, signed for authenticity,
   offered by trees.

   Forest guides can, in principle, be operated by anybody, including
   voice service providers, Internet access providers, dedicated
   services providers and enterprises.

   As in routing, peering with other forest guides implies a certain
   amount of trust in the peer.  Thus, peering is likely to require some
   negotiation between the administering parties concerned, rather than
   automatic configuration.  The mechanism itself does not imply a
   particular policy as to who gets to advertise a particular coverage

9.  Configuring Service Numbers

   The section below is not directly related to the problem of
   determining service location, but is an instance of the more generic
   problem solved by this architecture, namely mapping location
   information to service-related parameters, such as service numbers.

   For the foreseeable future, some user devices and software will
   emulate the user interface of a telephone, i.e., the only way to
   enter call address information is via a 12-button keypad with digits
   and the asterisk and hash symbol.  These devices use service numbers
   to identify services.  The best-known examples of service numbers are
   emergency numbers, such as 9-1-1 in North America and 1-1-2 in
   Europe.  However, many other public and private service numbers have
   been defined, ranging in the United States from 3-1-1 for non-
   emergency local government services to 4-1-1 for directory assistance
   to various "800" numbers for anything from roadside assistance to
   legal services to home-delivery food.

   Such service numbers are likely to be used until essentially all
   communication devices feature IP connectivity and an alphanumeric
   keyboard.  Unfortunately, for emergency services, more than 60
   emergency numbers are in use throughout the world, with many of those

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   numbers serving non-emergency purposes elsewhere, e.g., identifying
   repair or directory services.  Countries also occasionally change
   their emergency numbers to conform to regional agreements.  An
   example is the introduction of "1-1-2" for countries in Europe.

   Thus, a system that allows devices to be used internationally to
   place calls needs to allow devices to discover service numbers
   automatically.  In the Internet-based system proposed here [6], these
   numbers are strictly used as a human user interface mechanism and are
   generally not visible in call signaling messages, which carry the
   service URN [10] instead.

   For the best user experience, systems should be able to discover two
   sets of service numbers, namely those used in the user's home country
   and in the country the user is currently visiting.  The user is most
   likely to remember the former, but a companion borrowing a device in
   an emergency, say, may only know the local emergency numbers.

   Determining home and local service numbers is a configuration
   problem, but unfortunately, existing configuration mechanisms are
   ill-suited for this purpose.  For example, a DHCP server might be
   able to provide the local service numbers, but not the home numbers.
   When virtual private networks (VPNs) are used, even DHCP may provide
   numbers of uncertain origin, as a user may contact to the home
   network or some local branch office of the corporate network.
   Similarly, SIP configuration [4] would be able to provide the numbers
   valid at the location of the SIP service provider, but even a SIP
   service provider with national footprint may serve customers that are
   visiting any number of other countries.

   Also, while initially there are likely to be only a few service
   numbers, e.g., for emergency services, the LoST architecture can be
   used to support other services, as noted.  Configuring every local
   DHCP or SIP configuration server with that information is likely to
   be error-prone and tedious.

   For these reasons, the LoST-based mapping architecture supports
   providing service numbers to end systems based on caller location.
   The mapping operation is almost exactly the same as for determining
   the service URL.  The mapping can be obtained either along with
   determining the service URL or separately.  The major difference
   between the two requests is that service numbers often have much
   larger regions of validity than the service URL itself.  Also, the
   service number is likely to be valid longer than the service URL.
   Finally, an end system may want to look up the service number for its
   home location, not just the current (visited) location.

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10.  Security Considerations

   Security considerations for emergency services mapping are discussed
   in [9], while [10] discusses issues related to the service URN, one
   of the inputs into the mapping protocol.  LoST-related security
   considerations are naturally discussed in the LoST [2] specification.

   The architecture addresses the following security issues, usually
   through the underlying transport security associations:

   Server impersonation:  Seekers, resolvers, fellow tree guides and
      cluster members can assure themselves of the identity of the
      remote party by using the facilities in the underlying channel
      security mechanism, such as TLS.
   Query or query result corruption:  To avoid that an attacker can
      modify the query or its result, the architecture RECOMMENDS the
      use of channel security, such as TLS.  Results SHOULD also be
      digitally signed, e.g., using XML digital signatures.  Note,
      however, that simple origin assertion may not provide the end
      system with enough useful information as it has no good way of
      knowing that a particular signer is authorized to represent a
      particular geographic area.  It might be necessary that certain
      well-known Certificate Authorities (CAs) vet sources of mapping
      information and provide special certificates for that purpose.  In
      many cases, a seeker will have to trust its local resolver to vet
      information for trustworthiness; in turn, the resolver may rely on
      trusted forest guides to steer it to the correct information.
   Region corruption:  To avoid that a third party or an untrustworthy
      member of a server population introduces a region map that it is
      not authorized for, any node introducing a new region map MUST
      sign the object by encapsulating the data into a CMS wrapper.  A
      recipient MUST verify, through a local policy mechanism, that the
      signing entity is indeed authorized to speak for that region.
      Determining who can speak for a particular region is inherently
      difficult unless there is a small set of authorizing entities that
      participants in the mapping architecture can trust.  Receiving
      systems should be particularly suspicious if an existing region
      map is replaced with a new one with a new mapping address.  In
      many cases, trust will be mediated: A seeker will have a trust
      relationship with a resolver.  The resolver, in turn, will contact
      a trusted forest guide.

   Additional threats that need to be addressed by operational measures
   include denial-of-service attacks [7].

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11.  IANA Considerations

   Since this document describes an architecture, not a protocol, it
   does not ask IANA to register any protocol constants.

12.  Acknowledgments

   Richard Barnes, Jong Yul Kim, Otmar Lendl, Andrew Newton, Murugaraj
   Shanmugam, Richard Stastny, and Hannes Tschofenig provided helpful

13.  References

13.1.  Normative References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [2]   Hardie, T., "LoST: A Location-to-Service Translation Protocol",
         draft-ietf-ecrit-lost-05 (work in progress), March 2007.

13.2.  Informative References

   [3]   Zhao, W., Schulzrinne, H., and E. Guttman, "Mesh-enhanced
         Service Location Protocol (mSLP)", RFC 3528, April 2003.

   [4]   Petrie, D. and S. Channabasappa, "A Framework for Session
         Initiation Protocol User Agent Profile Delivery",
         draft-ietf-sipping-config-framework-12 (work in progress),
         June 2007.

   [5]   Schulzrinne, H., "A Dynamic Host Configuration Protocol (DHCP)
         based Location-to-Service  Translation Protocol (LoST)
         Discovery Procedure", draft-ietf-ecrit-dhc-lost-discovery-01
         (work in progress), March 2007.

   [6]   Rosen, B., "Framework for Emergency Calling in Internet
         Multimedia", draft-ietf-ecrit-framework-01 (work in progress),
         March 2007.

   [7]   Rosen, B. and J. Polk, "Best Current Practice for
         Communications Services in support of Emergency  Calling",
         draft-ietf-ecrit-phonebcp-01 (work in progress), March 2007.

   [8]   Schulzrinne, H. and R. Marshall, "Requirements for Emergency
         Context Resolution with Internet Technologies",

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         draft-ietf-ecrit-requirements-13 (work in progress),
         March 2007.

   [9]   Taylor, T., "Security Threats and Requirements for Emergency
         Call Marking and Mapping", draft-ietf-ecrit-security-threats-04
         (work in progress), April 2007.

   [10]  Schulzrinne, H., "A Uniform Resource Name (URN) for Services",
         draft-ietf-ecrit-service-urn-06 (work in progress), March 2007.

   [11]  Schulzrinne, H., "Synchronizing Location-to-Service Translation
         (LoST) Servers", draft-schulzrinne-ecrit-lost-sync-00 (work in
         progress), November 2006.

Author's Address

   Henning Schulzrinne
   Columbia University
   Department of Computer Science
   450 Computer Science Building
   New York, NY  10027

   Phone: +1 212 939 7004

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