Network Working Group H. Schulzrinne
Request for Comments: 5582 Columbia U.
Category: Informational September 2009 Location-to-URL Mapping Architecture and Framework
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
Translation (LoST) protocol. The architecture generalizes well-known
approaches found in hierarchical lookup systems such as DNS.
Status of This Memo
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not specify an Internet standard of any kind. Distribution of this
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The overall emergency calling architecture [ECRIT-FRAME] 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 that describe 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) [RFC5222] protocol, although much of the
discussion would also apply for other mapping protocols satisfying
the mapping requirements [RFC5012].
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] and
indicate requirement levels for compliant implementations.
In addition to the terms defined in [RFC5012], 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 Presence
Information Data Information Location Object (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 another 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
coverage region: The coverage region of an AMS is the geographic
region within which the AMS is able to authoritatively answer
mapping queries. Coverage regions are generally, but not
necessarily, contiguous and may be represented as either a subset
of a civic address or a geometric object.
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 either another mapping server or the desired service
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.
tree: A tree consists of a self-contained hierarchy of authoritative
mapping servers for a particular service. Each tree exports its
coverage region to the forest mapping servers.
4. Overview of Architecture
4.1. The Principal Components
The mapping architecture distinguishes four logical roles: seekers,
resolvers, authoritative mapping servers (AMS), and forest guides
(FGs). End users of the LoST-based [RFC5222] mapping mechanism,
called seekers, contact resolvers that cache query results and know
one or more forest guides. Forest guides form the top level of a
conceptual hierarchy, with one or more trees providing a hierarchical
resolution service for different geographic regions. Forest guides
know the geographic coverage region of all or almost all 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.
Seekers, resolvers, authoritative mapping servers, and forest guides
all communicate using LoST; indeed, it is likely that, in many cases,
the same software can operate as a resolver, authoritative mapping
server, and forest guide. In addition to the basic LoST query
protocol [RFC5222], a synchronization protocol [LOST-SYNC] may be
used to exchange information between forest guides or to push
coverage information from a tree node to its parent.
Seekers may be part of Voice over IP (VoIP) or other end systems, or
of SIP proxies or similar call routing functions.
Figure 1 shows the interaction of the components. The lines
indicating the connection between the forest guides are logical
connections, indicating that they are synchronizing their information
via the synchronization protocol [LOST-SYNC].
/-\ /-\ +-----+ +-----+
| S +******* R ********* FG *-----------------+ FG |
\-/ \-/ | |* | |
+--+--+ * +--+--+
| * |
| * |
| * |
| * |
/-\ +--+--+ * +--+--+
| R +------>+ FG +-----*-----------+ FG |
\-/ | | * | |
+--+--+ * +--+--+
| * |
| * |
| * |
/ \ / \
/ \ / \
/ \ / \
/ \ / \
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, while the lines indicate
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, the higher if different governmental or private
organizations within a country operate independent trees. We assume
that tree coverage information changes relatively slowly, on the
order of less than one change per year per 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.)
4.2. 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.
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 such as SIP user agents, 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 [RFC5223].
In an ad hoc or zero-configuration environment, appropriate service
directories may advertise resolvers.
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.
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 that 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 Voice Service Providers (VSPs) may include the address of a
suitable resolver in their configuration information, e.g., in SIP
configuration for a VSP or DHCP [RFC5223] 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) may potentially need to find out
mapping information for any spot on earth. (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
Each tree can map a location described by either civic or geographic
coordinates, but not both, for one type of service (such as
'sos.police', 'sos.fire' or 'counseling') and one location profile,
although nothing prevents re-using the same servers for multiple,
different services or both types of coordinates. The collection of
all trees for one service and location profile 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 any of the shapes in the LoST
specification expressed in geospatial coordinates, such as circles or
polygons, 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, 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 the Lightweight Directory Access Protocol
(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,
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
country A1 A2 A3 resource or LoST server
US NJ Bergen Leonia sip:firstname.lastname@example.org
US NJ Bergen Fort Lee sip:email@example.com
US NJ Bergen Teaneck sip:firstname.lastname@example.org
US NJ Bergen Englewood englewoodnj.gov
Coverage regions are described by sets of LoST-compatible shapes
enclosing contiguous geographic areas or by descriptors enumerating
groups of civic locations. For the former, the LoST server performs
the same matching operation as described in Section 12.2 of the LoST
specification [RFC5222] to find the tree or AMS.
As a civic location example, a state-level tree node for New Jersey
in the United States may contain the coverage region entries shown
below, indicating that any query matching a location in Bergen
County, for example, would be redirected or forwarded to the node
located at bergen.nj.example.org.
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 queries for all other towns within Essex
county would be directed to the county node for further resolution.
C A1 A2 A3 LoST server name
US NJ Atlantic * atlantic.nj.example.org/sos
US NJ Bergen * bergen.nj.example.org/sos
US NJ Monmouth * monmouth.nj.example.org/sos
US NJ Essex * essex.nj.example.org/sos
US NJ Essex Newark newark.example.com/sos
Thus, there is no substantial difference between coverage region and
mapping data. The only difference is that coverage regions return
names of LoST servers, 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.
To automate the maintenance of trees, the LoST synchronization
mechanism [LOST-SYNC] allows nodes to query other nodes for mapping
data and coverage regions, both within a cluster and across different
hierarchy levels in a tree. 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 regions 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 server
indicated in the coverage region record, returning a response to the
querier when it in turn receives an answer (recursion).
Alternatively, the node returns the application unique string (server
name) 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
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 URL or server name, 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, because 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
The solution for overlapping coverage regions is relatively simple.
If a query matches multiple coverage regions, the node returns all
URLs or server names, 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 try to contact all
services in turn.
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 the meshed
Service Location Protocol (SLP) [RFC3528] 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), which keeps track of the coverage regions of all the
trees for one service and location profile. 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.
Forest guides fulfill a similar role to root servers in DNS. They
distribute information, signed for authenticity, offered by trees.
However, introducing forest guides avoids creating a global root,
with the attendant management and control issues.
However, unlike DNS root servers, forest guides may offer different
information based on local policy. Forest guides can also restrict
their data synchronization 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 coverage regions SHOULD be digitally signed by
the authorities responsible for the region, as discussed in more
detail in Section 10. They are used by resolvers and possibly
seekers to find the appropriate tree for a particular area. All
forest guides should have consistent 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 other guides. Due to policy and
maybe political reasons, not all forest guides may share the same
coverage region data.
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 symbols. 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
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 in
[ECRIT-FRAME], 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 [RFC5031] 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 those used 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
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 the home network
or some local branch office of the corporate network. Similarly, SIP
configuration [CONFIG-FRAME] would be able to provide the numbers
valid at the location of the SIP service provider, but even a SIP
service provider with a 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 along with the service
URL. 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 its current (visited)
10. Security Considerations
Security considerations for emergency services mapping are discussed
in [RFC5069], while [RFC5031] 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
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 Transport Layer Security (TLS)
query or query result corruption: To avoid the possibility of an
attacker modifying 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 [W3C.REC-xmldsig-core-20020212]. 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.
coverage region corruption: To avoid the possibility of a third
party or an untrustworthy member of a server population claiming a
coverage region that it is not authorized for, any node
introducing a new service boundary MUST sign the object by
protecting the data with an XML digital signature
[W3C.REC-xmldsig-core-20020212]. 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 coverage region 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,
and 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 [PHONE-BCP].
Jari Arkko, Richard Barnes, Cullen Jennings, Jong Yul Kim, Otmar
Lendl, Matt Lepinski, Chris Newman, Andrew Newton, Jon Peterson,
Schida Schubert, Murugaraj Shanmugam, Richard Stastny, Hannes
Tschofenig, and Karl Heinz Wolf provided helpful comments.
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5031] Schulzrinne, H., "A Uniform Resource Name (URN) for
Emergency and Other Well-Known Services", RFC 5031,
[RFC5222] Hardie, T., Newton, A., Schulzrinne, H., and H.
Tschofenig, "LoST: A Location-to-Service Translation
Protocol", RFC 5222, August 2008.
[RFC5223] Schulzrinne, H., Polk, J., and H. Tschofenig, "Discovering
Location-to-Service Translation (LoST) Servers Using the
Dynamic Host Configuration Protocol (DHCP)", RFC 5223,
12.2. Informative References
Channabasappa, S., "A Framework for Session Initiation
Protocol User Agent Profile Delivery", Work in Progress,
Rosen, B., Schulzrinne, H., Polk, J., and A. Newton,
"Framework for Emergency Calling using Internet
Multimedia", Work in Progress, March 2009.
Schulzrinne, H. and H. Tschofenig, "Synchronizing
Location-to-Service Translation (LoST) Protocol based
Service Boundaries and Mapping Elements", Work
in Progress, March 2009.
Rosen, B. and J. Polk, "Best Current Practice for
Communications Services in support of Emergency Calling",
Work in Progress, March 2009.
[RFC3528] Zhao, W., Schulzrinne, H., and E. Guttman, "Mesh-enhanced
Service Location Protocol (mSLP)", RFC 3528, April 2003.
[RFC5012] Schulzrinne, H. and R. Marshall, "Requirements for
Emergency Context Resolution with Internet Technologies",
RFC 5012, January 2008.
[RFC5069] Taylor, T., Tschofenig, H., Schulzrinne, H., and M.
Shanmugam, "Security Threats and Requirements for
Emergency Call Marking and Mapping", RFC 5069,
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
Solo, D., Eastlake, D., and J. Reagle, "XML-Signature
Syntax and Processing", World Wide Web Consortium
FirstEdition REC-xmldsig-core-20020212, February 2002,
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