Internet DRAFT - draft-schulzrinne-ecrit-mapping-arch
draft-schulzrinne-ecrit-mapping-arch
ECRIT H. Schulzrinne
Internet-Draft Columbia U.
Expires: April 19, 2006 October 16, 2005
Location-to-URL Mapping Architecture and Framework
draft-schulzrinne-ecrit-mapping-arch-00
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes an architecture for a global, scalable,
resilient and administratively distributed system for mapping
geographic location information to URLs. The architecture
generalizes well-known approaches found in hierarchical lookup
systems such as DNS. The architecture does not depend on using a
specific protocol, but does require that protocols can summarize the
coverage region of a node.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1 The Mapping Problem . . . . . . . . . . . . . . . . . . . 4
3.2 Overview of Operation . . . . . . . . . . . . . . . . . . 5
3.3 Seekers: The Users of the Mapping System . . . . . . . . . 5
3.4 Trees: Authoritative Knowledge . . . . . . . . . . . . . . 6
3.5 Forest Guides: Finding the Right Tree . . . . . . . . . . 7
3.6 Resolvers: Finding Forest Guides and Caching Data . . . . 7
3.7 Minimal System Architecture . . . . . . . . . . . . . . . 7
4. Seeker . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Resolver . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1 Basic Operation . . . . . . . . . . . . . . . . . . . . . 8
6.2 Answering Queries . . . . . . . . . . . . . . . . . . . . 10
6.3 Overlapping Coverage Regions . . . . . . . . . . . . . . . 10
6.4 Scaling and Reliability . . . . . . . . . . . . . . . . . 11
7. Forest Guides . . . . . . . . . . . . . . . . . . . . . . . 11
8. Configuring Emergency Dial Strings . . . . . . . . . . . . . 11
9. Security . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1 Normative References . . . . . . . . . . . . . . . . . . 14
10.2 Informative References . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . 15
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . 16
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1. Terminology
In this document, the key words "MUST", "MUSTNOT", "REQUIRED",
"SHALL", "SHALLNOT", "SHOULD", "SHOULDNOT", "RECOMMENDED", "MAY", and
"OPTIONAL" are to be interpreted as described in RFC 2119 [1] and
indicate requirement levels for compliant implementations.
2. Definitions
[Note: The terminology below is still evolving and needs
refinement.]
In addition to the terms defined in [11], this document uses the
following terms to describe LUMP:
authoritative mapping server (AMS): Resolver 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.
caching resolver: A caching resolver is contacted by a seeker,
consults a forest mapping server and then resolves the query using
an appropriate tree.
child: A child is a resolver that is authoritative for a subregion of
a particular server. A child can in turn be parent.
cluster: A cluster is a group of resolver (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.
complete: A civic mapping region is considered complete if it covers
a set of hierarchical labels in its entirety, i.e., there is no
other resolver that covers parts of the same region. (A complete
mapping may have children that cover strict subsets of this
region.) For example, a region spanning the whole country is
complete, but a region spanning only some of the streets in a city
is not.
forest guide: A forest guide has knowledge of the coverage region of
all trees.
hint: A hint provides a mapping from a region to a server name, used
to short-cut mapping operations.
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 PSAP URLs.'
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parent: A mapping server that covers the region of all of its
children. A mapping server without a parent is a root resolver.
peer: A resolver maintains associations other resolvers, called
peers. Peers synchronize their region maps.
seeker: The resolver, ESRP or end system requesting a mapping.
region map: A data object describing a contiguous area covered by a
resolver, either as a subset of a civic address or a geometric
object.
root region map: A data object describing a contiguous area covered
by a resolver, with no parent map.
resolver: The server providing (part of) the mapping service.
Resolvers cooperate to offer the mapping service to seekers.
tree: A tree consists of a hierarchy of authoritative mapping
servers. Each tree exports its coverage region to the forest
mapping servers.
3. Introduction
3.1 The Mapping Problem
One of the central problems of providing emergency services to
Internet systems is to map geographic location to a set of emergency
services, represented by PSAPs, that can provide assistance for that
particular location. This is a mapping problem, where a geographic
location is translated into a set of URIs that allow the Internet
system to contact an appropriate network entity. Other services may
also find such location-to-URI mappings of use.
The architecture 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 PSAP URI, end systems also need to determine
the local emergency dial strings. As discussed in Section 8, the
architecture described here can also address that problem.
The architecture described below does not depend on a particular
mapping protocol, but naturally assumes that such protocols provide
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certain features, such as the ability to discover the coverage region
of tree nodes. In this introduction, we describe the four
participants in the system at a high level. Each role will later be
introduced in more detail.
3.2 Overview of Operation
In short, end users of the 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
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
3.3 Seekers: The Users of the Mapping System
Clients desiring mappings are known as seekers. Thus, seekers are
the end users of the mapping information. Examples of such clients
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include SIP proxy servers or SIP end systems wishing to place an
emergency call. Seekers provide location information describing a
small geographic area and obtain one or more URIs describing the
service. Seekers may need to obtain this 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 system.
3.4 Trees: Authoritative Knowledge
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.
(Extensions to extra-terrestrial applications are left for future
exploration.) Different types of services may divide responsibility
differently and are independent of each other. Each node
participating in the system has authoritative knowledge about
mappings within its coverage region, typically, but not necessarily,
a contiguous geographic region 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 node is responsible for a particular
geographic or civic location, but the system can also deal with cases
where coverage regions overlap.
The architecture assumes that knowledge about mappings is
hierarchical, represented as a tree. Each tree node knows the
coverage region of its children and sends queries to the appropriate
server "down" the tree. There are no assumptions about the coverage
region of a tree. 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.
Thus, 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 pictures 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 a few changes per year per tree,
although the system imposes no specific threshold. (To be sure,
information within a tree is likely to change much more frequently.)
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3.5 Forest Guides: Finding the Right Tree
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". 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 any forest guide and will then be directed to the
right tree or, rarely, set of trees.
3.6 Resolvers: Finding Forest Guides and Caching Data
A seeker can contact a forest guide 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, a resolver, that knows how
to contact one or more forest guides and caches earlier queries to
accelerate the response to mapping queries.
3.7 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.
4. Seeker
Seekers are consumers of mapping data and originate queries. Seekers
do not answer queries. They contact either forest guides or
resolvers to find the appropriate tree that can authoritatively
answer their questions. As noted in the introduction, seekers can be
end systems or call routing entities such as SIP proxy servers.
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
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location of the resolver in the SIP configuration information it
distributes to its user agents. An Internet access provider might
provide a pointer to a resolver via DHCP. In an ad-hoc or zero-
configuration environment, appropriate service directories may
advertise resolvers.
For emergency calling, seekers could issue queries at boot time,
periodically when cached information expires or only when placing an
emergency call. It is probably unnecessary to continuously update
mapping information for seekers representing a small user population,
e.g., a single phone or residential SIP proxy.
Like other entities in the system, seekers can cache responses. This
is particularly useful if the response describes the result for a
region, not 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 PSAP and can avoid polling for this
information. 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 PSAP.
5. Resolver
Resolvers mediate between seekers and forest guides. Their primary
role is to avoid having seekers find forest guides on their own.
Unlike forest guides, resolvers do not store worldwide coverage maps,
but they may cache regions returned as part of query results.
As noted earlier, seekers can contact forest guides directly. From a
protocol perspective, a resolver acts in the same way as a seeker,
except that it knows one or more forest guide.
6. Trees
6.1 Basic Operation
As noted in the introduction, trees are the authoritative source of
mapping data. Each tree can map location information for one type of
service (such as 'police' or 'fire'), although nothing prevents re-
using the same tree for multiple different services. The collection
of trees for one service is known as a forest.
The tree architecture is similar to the domain name system, except
that delegation is not by label, but rather by region.
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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 PSAP or another
signaling server representing a group of PSAPs.
Leaf nodes, i.e., nodes without children, only maintain mappings,
while tree nodes above the leaf nodes only maintain coverage regions.
An example of a leaf node entry is shown below, indicating how
queries for three towns are directed to different PSAPs.
country A1 A2 A3 resource
US NJ Bergen Leonia sip:psap@leonianj.gov
US NJ Bergen Fort Lee sip:emergency@fortleenj.org
US NJ Bergen Teaneck sip:police@teanecknjgov.org
....
Coverage regions are described by sets of polygons enclosing
contiguous geographic areas or by descriptors enumerating groups of
civic locations.
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
bergen.nj.example.org. There is no requirement that all child nodes
cover the same level within the civic hierarchy. For 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
resolution.
C A1 A2 A3 resource
US NJ Atlantic * lump://atlantic.nj.example.org/sos
US NJ Bergen * lump://bergen.nj.example.org/sos
US NJ Monmouth * lump://monmouth.nj.example.org/sos
US NJ Essex * lump://essex.nj.example.org/sos
US NJ Essex Newark lump://newark.example.com/sos
....
Thus, there is no substantial difference between coverage region and
mapping data. The only difference is that coverage regions return
mapping protocol URLs, while mapping entries contain PSAP 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 is generally limited to address
ranges ("MSAG data"), so initial mapping databases may only contain
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street-level information.
To automate operations, a suitable mapping protocol would thus need
to be able to query nodes for their coverage region. In the example
above, the state-run node would query the county nodes and thus
aggregagate the coverage data. Conversely, nodes could also contact
their parent nodes. There is some benefit of child nodes contacting
their parents, as this allows changes in coverage region to propagate
quickly up the tree.
6.2 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. 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.
6.3 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.
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
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choose the preferred solution or trying to contact all services in
turn.
6.4 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 [7]
are applicable here.
7. Forest Guides
Forest guides distribute records describing the coverage region for
trees. For authenticity, the records are 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
information. A tree node at the top of a tree can contact any forest
guide and inject new coverage region information into the system.
Each forest guide peers with one or more other guides and distributes
new coverage region announcements to all other guides.
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
region.
8. Configuring Emergency Dial Strings
For the foreseeable future, some user devices and software will
emulate the user interface of a telephone, i.e., the only way to
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enter call address information is via a 12-button keypad. Also,
emergency numbers are likely to used until essentially all
communication devices feature IP connectivity and an alphanumeric
keyboard. Unfortunately, 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, for example, by selecting a number already in use in other
countries of a region (such as 112 in Europe).
Thus, a system that allows devices to be used internationally to
place emergency calls needs to allow devices to discover emergency
numbers automatically. In the system proposed, these numbers are
strictly of local significance and are generally not visible in call
signaling messages.
For simplicity of presentation, this section assumes that emergency
numbers are valid throughout a country, rather than, say, be
restricted to a particular city. This appears likely to be true in
countries likely to deploy IP-based emergency calling solutions. In
addition, the solution proposed also works if certain countries do
not use a national emergency number. There is no requirement that a
country uses a single emergency number for all emergency services,
such as fire, police, or rescue.
For the best user experience, systems should be able to discover two
sets of 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 may only know the local emergency numbers.
Determining home and local emergency 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 emergency number, but not the home numbers.
Similarly, SIP configuration 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.
Since dial strings are represented as URLs [5], the problem of
determining local and home emergency numbers is a problem of mapping
locations to a set of URLs, i.e., exactly the problem that the
mapping architecture is solving already.
The mapping operation is almost exactly the same as for determining
the emergency service URL. The only difference is that if a seeker
knows the civic location at least to the country level, it will use a
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query where the PIDF-LO only includes the country code. If it only
knows its geospatial location, it has to include that longitude and
latitude. The querier uses the service identifiers "dialstring.sos",
"dialstring.sos.fire", etc. The resolver returns the appropriate set
of URLs and, if a geospatial location was used in the query, the
current region map for the country.
Within the mapping system, emergency calling regions are global
information, i.e., they are distributed using the forest guide
replication mechanism described earlier. Thus, every forest guide
has access to all region mappings. This makes it possible that a
querier can ask any resolver for this information, reducing the
privacy threat of revealing its location outside of an emergency
call. The privacy threat is further reduced by the long-lived nature
of the information, i.e., in almost all cases, the querier will have
already cached the national boundary information or country
information on its first visit to the country.
9. Security
The architecture addresses the following security issues, usually
through the underlying transport security associations:
Server impersonation: Queriers, cluster members and peers 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.
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.
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10. References
10.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[3] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[4] Peterson, J., "A Presence-based GEOPRIV Location Object Format",
draft-ietf-geopriv-pidf-lo-03 (work in progress),
September 2004.
[5] Rosen, B., "Dialstring parameter for the sip URI",
draft-rosen-iptel-dialstring-02 (work in progress), July 2005.
10.2 Informative References
[6] Guttman, E., Perkins, C., Veizades, J., and M. Day, "Service
Location Protocol, Version 2", RFC 2608, June 1999.
[7] Zhao, W., Schulzrinne, H., and E. Guttman, "Mesh-enhanced
Service Location Protocol (mSLP)", RFC 3528, April 2003.
[8] Newton, A. and M. Sanz, "IRIS: The Internet Registry
Information Service (IRIS) Core Protocol", RFC 3981,
January 2005.
[9] Krochmal, M. and S. Cheshire, "DNS-Based Service Discovery",
draft-cheshire-dnsext-dns-sd-03 (work in progress), July 2005.
[10] Petrie, D., "A Framework for Session Initiation Protocol User
Agent Profile Delivery", draft-ietf-sipping-config-framework-07
(work in progress), July 2005.
[11] Schulzrinne, H. and R. Marshall, "Requirements for Emergency
Context Resolution with Internet Technologies",
draft-schulzrinne-ecrit-requirements-01 (work in progress),
July 2005.
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Author's Address
Henning Schulzrinne
Columbia University
Department of Computer Science
450 Computer Science Building
New York, NY 10027
US
Phone: +1 212 939 7004
Email: hgs+ecrit@cs.columbia.edu
URI: http://www.cs.columbia.edu
Appendix A. Acknowledgments
Richard Stastny, ... provided helpful comments.
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Internet-Draft MapArch October 2005
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