Network Working Group L. Daigle
Request for Comments: 2970 T. Eklof
Category: Informational October 2000
Architecture for Integrated Directory Services - Result from TISDAG
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
A single, unified, global whitepages directory service remains
elusive. Nonetheless, there is increasing call for participation of
widely-dispersed directory servers (i.e., across multiple
organizations) in large-scale directory services. These services
range from national whitepages services, to multi-national indexes of
WWW resources, and beyond. Drawing from experiences with the TISDAG
(Technical Infrastructure for Swedish Directory Access Gateways)
([TISDAG]) project, this document outlines an approach to providing
the necessary infrastructure for integrating such widely-scattered
servers into a single service, rather than attempting to mandate a
single protocol and schema set for all participating servers to use.
The TISDAG project addressed the issue of providing centralized
access to distributed information for whitepages information on a
national scale. The specification of the eventual system is
presented in [TISDAG], and [DAGEXP] outlines some of the practical
experience already gained in implementing a system of this scale and
nature. [DAG-Mesh] considers the issues and possibilities of
networking multiple DAG services. Following on from those, this
document attempts to describe some of the architectural underpinnings
of the system, and propose directions in which the approach can be
generalized, within the bounds of applicability.
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The proposed architecture inserts a coordinated set of modules
between the client access software and participating servers. While
the client software interacts with the service at a single entry
point, the remaining modules are called upon (behind the scenes) to
provide the necessary application support. This may come in the form
of modules that provide query proxying, schema translation, lookups,
referrals, security infrastructure, etc.
Part of this architecture is an "internal protocol" -- called the
"DAG/IP" in the TISDAG project. This document also outlines the
perceived requirements for this protocol in the extended DAG.
Terms used in this document are compliant with those set out in
[ALVE]. For the purposes of this document, important distinctions and
relationships are defined between applications, services, servers and
systems. These are defined as follows:
Application: this is meant in the general sense, as a solution to a
particular (set of) user need(s). That is, the definition is not
tied to a particular piece of software (as in "application
program").
The definition of an application includes the type(s) of
information to be exchanged, expected behavior, etc. Thus, a
whitepages (search) application may expect to receive a name as
input to a query engine, and will return all information associated
with the name. By contrast, a specific security application might
use the same input name to verify access controls.
Service: an operational system providing (controlled) access to
fulfill a particular application's needs.
One service may be changed by configuring location, access
controls, etc. Changing application means changing the service.
Server: a single component offering access through a dedicated
protocol, without regard to a specific service (or services) it may
be supporting in a given configuration. Typically programmed for a
particular application.
System: a set of components with established interconnections.
Thus, a service can be split between several servers. A collection
of services (independently, or interrelated through specified
agreements) act as an implementation of an application. A system
is composed of one or more servers and services.
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A "system architecture" identifies specific software components,
their behavior, communication channels and messages needed to
fulfill a particular service's needs. The TISDAG specification
[TISDAG] includes just such a description, defining a software
system that will meet the needs of a national whitepages directory
service. Here, we outline some of the general principles which
lead to that specific system architecture and discuss ways in which
the principles can be applied in other contexts.
Looking at this bigger picture, we present a "service
architecture", or a framework for assembling components into
systems that meet the needs of a wider variety of services. This
is not a question of developing one or more new protocols for
services, but rather to examine a useful framework of
interoperating components. The goal is to reduce the overall
number of (specialized) protocols that are developed requiring
incorporation of some very general concepts that are common to all
protocols.
The Swedish TISDAG project (described in detail in [TISDAG], with
some experiences reported in [DAGEXP]) was designed to fulfill the
requirements of a particular national directory service. The
experience of developing component-based system for providing a
directory service through a uniform interface (client access point)
provided valuable insight into the possibilities of extending the
system architecture so that services with different base requirements
can benefit from many of the same advantages.
In retrospect, we can describe the TISDAG system architecture in
terms of 3 key requirements and 4 basic design principles:
R1. The service had to function with (several) existing client and
server software for the white pages application.
R2. It had to be possible to extend the service to accommodate new
client and server protocols if and when they became relevant.
R3. The service had to be easily reconfigurable -- to accommodate
more machines (load-sharing), etc.
D1. As a design principle, it was important to consider the
possibility that queries and information templates (schema)
other than the originally-defined set might eventually be
supported.
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D2. As the architecture was already modular and geared towards
extensibility, it seemed important to keep in mind that the
same (or a similar) system could be applied to other (non-
white pages) applications.
D3. There is an "inside" and an "outside" to the service --
distinguishing between components that are accessible to the
world at large and those that are open only to other
components of the system.
D4. Internally, there is a single protocol framework for all
communications -- this facilitates service support functions
(e.g., security of transmission), ensures distributability,
and provides the base mechanism for allowing/ascertaining
interoperability of components.
The resulting system architecture featured modular component (types)
to fulfill a small number of functional roles, interconnected by a
generic query-response language. The functional roles were defined
as:
CAPs -- "client access points" -- responsible for accepting and
responding to incoming requests through programmed and configured
behavior -- to translate the incoming query into some set of DAG-
internal actions (queries) and dealing with the responses,
filtering and recombining them in such a way as to fulfill the
client request within the scope of the service. In the TISDAG
system, all CAPs are responsible for handling whitepages queries,
but the CAPs are distinguished by the application protocol in
which they will receive queries (e.g., LDAPv2, LDAPv3, HTTP, etc).
To the client software, the TISDAG system appears as a server of
that particular protocol. In the more general case, CAPs may be
configured to handle different aspects of a service (e.g.,
authenticated vs. non-authenticated access). While the TISDAG
CAPs all had a simple control structure, the more general case
would also see CAPs drawing on different subsets of DAG (internal)
servers in order to handle different query types. (See the
"Operator Service" example, in section 5.2 below).
SAPs -- "service access points" -- responsible for proxying DAG-
internal queries to specified services. These are resources drawn
upon by other components within the system. Through programmed
and configured behavior, they translate queries in the internal
protocol into actions against (typically external) servers, taking
care of any necessary overhead or differences in interaction
style, and converting the responses back into the internal
protocol. In the TISDAG system, all SAPs are responsible for
handling whitepages queries, but they are distinguished by the
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application protocol in which they will access remote services.
Further distinctions could be made based on the (remote service's)
schema mappings they handle, and other service differentiators.
Internal Servers respond to queries in the internal protocol and
provide specific types of information. In the TISDAG system,
there is one internal server which provides referral information
in response to queries.
Note that all these components are defined by the functional roles
they play in the system, not the particular protocols they handle, or
even the aspect of the service they are meant to support. That is, a
client access point is responsible for handling client traffic,
whether its for searching, establishing security credentials, or some
other task.
The Requirements and Design principles outlined above are not
particular to a national whitepages service. They are equally
applicable in any application based on a query-response model, in
services where multiple protocols need to be supported, and/or when
the service requires specialized behavior "behind the scenes". In
the TISDAG project, this last was inherent in the way the service
first looks for referrals, then makes queries as appropriate. For
protocols that don't handle the referral concept natively, the TISDAG
system proxies the queries.
Because of its particular application to query-response situations,
the term "Directory Access Gateway", or "DAG" still fits as a label
for this type of system architecture.
Internet applications are evolving, and require more sophisticated
features (e.g., security mechanisms, accounting mechanisms,
integration of historical session data). Continuing to develop a
dedicated protocol per application type results in encumbered and
unwieldy protocols, as each must implement coverage of all of these
common aspects. But creating a single multi-application protocol
seems unlikely at best. The implicit proposal here is that, rather
than overloading protocols to support multiple aspects of a service,
those aspects can be managed by breaking the service into multiple
supporting components to carry out the specialized tasks of
authentication, etc.
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In the TISDAG project, the choice was made to use a single "internal
protocol" (DAG/IP). The particular protocol used is not relevant to
the architecture, but the principle is important. By selecting a
single query-response transaction protocol, the needs of the
particular application could be mapped onto it in terms of queries
and data particular to the application. This makes the internal
communications more flexible for configuration to other environments
(services, applications).
It is common today to select an existing, widely deployed protocol
for transferring commands and data between client and server -- e.g.,
HTTP. However, apart from any issues of the appropriateness (or
inappropriateness) of extending HTTP to this use, the work would have
remained to define all the transaction types and data types over that
protocol -- the specification of the interaction semantics and
syntax.
Apart from the potential to divide and conquer service aspects, as
described above, this approach has many perceived benefits:
- For multi-protocol environments, it requires on the order of
N+M inter-protocol mappings, not NxM.
- distribution of development
- distribution of operation
- eventual possibilities of hooking together different
systems (of different backgrounds)
- separation of
- architectural principles
- implementation to a specific application
- configuration for a given service
It is not the goal to say that a standardized system architecture can
be made so that single components can be built for all possible
applications. However, this approach in general permits the
decoupling of access protocols from specific applications, and
facilitates the integration of necessary infrastructure independently
of access protocol (e.g., referrals, security, lookup services,
distribution etc).
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Pictorially, the DAG architecture is as follows:
+-------------------------------------------+
"a" | | +--------+ |
<-----> CAP a | | SAP A | |
| | | | |
|---------+ +-+------+---+ |
| |(Internal)| |
| "DAG/IP" | Server i | |
| +----------+ |
| |
| |
| +--------+ | "B"
| | SAP B <-------------->
| | | |
| +--------+ |
| |
+-------------------------------------------+
Note that the bounding box is conceptual -- all components may or may
not reside on one server, or a set of servers governed by the
provider of the service.
As we saw in the TISDAG project, the provider of this DAG-based
service may be only loosely affiliated with the remote services that
are used (Whitepages Directory Service Providers (WDSPs) in this
case).
Building a service on this architecture requires:
Service implementation:
1. definition of the overall application to be supported by the
system -- whitepages, web resource indexing, medical
information
2. requirements
3. expected behavior
System architecture:
1. nature of deployment -- distributed, security requirements,
etc.
2. identification of necessary CAPs -- in terms of access
protocols to be supported, different service levels to be
provided (e.g., secure and unsecure connections)
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3. identification of necessary services -- e.g., proxying to
remote information search services, lookup services, "AAA[A]"
(Authentication, Authorization, Accounting, [and Access])
servers, etc
4. definition of the transaction process for the service: insofar
as the CAPs represent the service to client software, CAP
modules manage the necessary transactions with other service
modules
Data architecture:
1. selection of schemas to be used (in each protocol)
2. definition of schema and protocol mappings -- into and out of
some DAG/IP representation
Consider the TISDAG project in the light of the above definitions.
Service implementation:
1. A national-scale subset of Whitepages lookups, with specific
query types supported: only certain schema attributes were
permitted in queries, and the expected behavior was limited in
scope.
2. Requirements: the service had to support multiple query
protocols (from clients and for servers), and be capable of
searching the entire space of data without centralizing the
storage of records.
3. Given a query of accepted type, provide referrals to whitepages
servers that might have information to fulfill the query; if
necessary, proxy the referrals (chain) to retrieve the
information for the client.
System architecture:
1. distributable components
2. publicly accessible CAPs in HTTP, SMTP, Whois++, LDAPv2, and
LDAPv3
3. referral proxies to Whois++, LDAPv2 and LDAPv3 WDSPs, as well
as a referral query service
4. the basic transaction process, uniform across all CAPs, is:
- query the RI for relevant referrals
- where necessary, chain referrals through SAPs of
appropriate protocol return, in the native protocol, all
remaining referrals and data
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Data architecture: see the spec.
In the TISDAG project, the above diagram could be mapped as follows:
CAP a LDAPv2 CAP
SAP A the Referral Index (RI) interface
Server i the Referral Index (RI)
SAP B LDAPv3 SAP
Note that, in the TISDAG project specification, the designation SAP
referred exclusively to proxy components designed to deal with
external servers. The Referral Index was considered an entity in its
own right. However, generalizing the concepts of the TISDAG
experience lead to the proposal of regarding all DAG/IP-supporting
service components as SAPs, each designed to carry out a particular
type of service functionality, and whether the server is managed
internally to the DAG system or not is immaterial.
Consider the case of "number portability" -- wherein it is necessary
to determine the current service provider of a specific phone number.
The basic assumption is that phone numbers are assigned to be
globally unique, but are not in any way tied to a specific service
provider. Therefore, it is necessary to determine the current
service provider for the given number before being able to retrieve
current information. For the sake of our illustration, let us assume
that the management of numbers is two-tiered -- suppose the system
stores (internally) the mapping between these random digit strings
and the country in which each was originally activated, but relies on
external (country-specific) services to manage the updated
information about which service provider currently manages a given
number. Then, the service data need only be updated when new numbers
are assigned, or national services change their access points.
We can look at a grossly-simplified version of the problem as an
illustration of some of the concepts proposed in this service
architecture. We couple it with the "name search" facet of the
TISDAG example, to underscore that a single service ("operator") may
in fact be supported by several disjunct underlying activities.
Service implementation:
1. Retrieving service information for a particular (unstructured)
phone number digit sequence, or searching for numbers
associated with a particular name (or fragment thereof).
2. Requirements: support IP-telephony through HTTP-based
requests, wireless device requests through WAP [WAP].
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3. Expected behavior: given a name (fragment), return a list of
names and numbers to match the fragment; given a phone number,
return appropriately-structured information re. the current
service mapping for that number.
System architecture:
1. Publicly accessible through CAPs; components widely
distributed.
2. Need one CAP for HTTP, one for WAP.
3. Support services include: an internal service for lookup of
number strings (to identify nation of origin of the number), a
proxy to access national services for registration of numbers
and service providers, and a proxy for remote service provider
for retrieval of detailed information regarding numbers. For
the name searching, we also need a referral index over the
names, and a proxy to whatever remote servers are managing the
whitepages directories.
4. Now, 2 different types of transaction are possible: search for
name, or look-up a number. In the name search case, the CAP
receives a name or name fragment, looks it up in the internal
referral index, and finds associated numbers through external
whitepages services (WDSPs). To look-up a number, the CAP
first uses the internal look-up service to determine the
country of origin of the number, and then uses a SAP to access
that nation's number-service provider directory, and finally
uses a different SAP to access the current service provider to
determine the information required to make the call.
Data architecture:
[Out of scope for the purposes of this illustration]
Note that some elements of the system architecture are
deliberately vague. Per the requirements, no structure is
expected in the number string, and therefore the lookup server
must maintain an index of number-to-country mappings and relies
on an external number-to-service mapping service (in each
country). However, were there any structure to the numbers, the
lookup server could make use of that structure in the indexing,
or in distribution of the index itself. This would have no
effect on the CAPs, which have no inherent reliance on how the
lookup server performs its task.
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Pictorially, the example can be rendered as follows:
+-------------------------------------------+
"a" | | +--------+ |
<-----> CAP a | | SAP A | |
| | | | |
|---------+ +-+------+---+ |
| |(Internal)| |
| "DAG/IP" | Server i | |
| +----------+ |
| |
| +--------+ | "B"
| | SAP B <-------------->
| | | |
| +--------+ |
| |
| +--------+ | "C"
|---------+ | SAP C <-------------->
"b" | | | | |
<-----> CAP b | +--------+ |
| | |
|---------+ +--------+ |
| | SAP D | |
| | | |
| +-+------+---+ |
| |(Internal)| |
| | Server j | |
| +----------+ |
| |
| +--------+ | "E"
| | SAP E <-------------->
| | | |
| +--------+ |
+-------------------------------------------+
where
CAP a HTTP CAP
CAP b WAP CAP
SAP A the number-nation lookup interface
Server i number-nation lookup server (what country)
SAP B nation-service lookup SAP (what service provider)
SAP C service-number information lookup SAP (current
service details)
SAP D referral index interface
Server j referral index service
SAP E proxy for chaining queries to remote WDSPs
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The service architecture is useful for applications outside the scope
of "telecoms". In another hypothetical illustration, consider the
case of medical information -- records about patients that may be
created and stored at a variety of institutions which they visit. It
is not unusual to need to access all information concerning a
patient, whether or not the person can recollect (or communicate)
conditions that were treated, procedures that were performed, or
medical institutions visited. The data may include everything from
prescriptions, to X-rays and other images, to incident reports and
other elements of medical history, etc. Typically, the information
is stored where it is collected (or by an agency authorized by that
institution) -- not in a central repository. Any service that looks
to provide complete answers to queries must deal with these
realities, and clearly must function with a strong security model.
Service implementation:
1. Retrieving all medical information for a particular person.
2. Requirements: must retrieve, or at least locate, all
available information, regardless of its storage location;
cannot require central repository of information; must
implement authorization and access controls. Must
support a proprietary protocol for secure connections
within hospitals, wireless access for personnel in
emergency vehicles (not considered secure access).
3. Expected behavior: given a patient's national ID, and
authorized access by medical personnel in secure locations,
determine what kinds of records are available, and where;
given a request for a specific type of record, retrieve
the record. Given a patient's national ID, and authorized
access from a wireless device, provide information re.
any known medical flags (e.g., medicine allergies,
conditions, etc).
System architecture:
1. Only 2 CAP types are needed, but instances of these will
be established at major medical institutions.
2. Need one CAP to support the proprietary protocol, one
to support wireless access.
3. Support services include: an internal server to support
security authentication and access control determination;
an internal server to act as referral index for finding
information pertinent to a particular patient, and one
or more proxies for accessing remote data storage servers.
4. The basic transaction requires that the first step be
to authenticate the end-user and determine access privileges.
In the case of wireless access, this last will not involve
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a specific lookup, but rather will be set to allow the
user to see the list of publicized medical conditions.
Depending on the query type, the next step will be to
contact the referral index to determine what records
exist, and then track down information at the remote sources.
Data architecture:
[Out of scope for the purposes of this illustration]
Pictorially, the example can be rendered as follows:
+-------------------------------------------+
"a" | | +--------+ |
<-----> CAP a | | SAP A | |
| | | | |
|---------+ +-+------+---+ |
| |(Internal)| |
| "DAG/IP" | Server i | |
| +----------+ |
| |
| |
| +--------+ | "B"
|---------+ | SAP B <-------------->
"b" | | | | |
<-----> CAP b | +--------+ |
| | |
|---------+ +--------+ |
| | SAP C | |
| | | |
| +-+------+---+ |
| |(Internal)| |
| | Server j | |
| +----------+ |
+-------------------------------------------+
where
CAP a CAP for proprietary protocol, secure clients
CAP b WAP CAP, for roaming access
SAP A authentication and ACL lookup interface
Server i authentication and ACL lookup server
SAP B remote service SAP -- probably LDAPv3
SAP C Referral Index interface
Server j Referral Index
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The role of the DAG/IP is less as a query protocol, and more as a
framework or structure for carrying basic query-response transactions
of different (configurable) types.
Whatever the syntax or grammar, the basic requirements for the DAG/IP
include that it be:
- lightweight; CAPs, SAPs should be able to be quite small
- flexible enough to carry queries of different paradigms, results
of different types
- able to support authentication, authorization, accounting and
audit mechanisms -- not necessarily native to the protocol
- able to support encryption and end-to-end security within the
DAG system
- sophisticated enough to allow negotiation of capabilities --
querying & identifying application type supported (e.g.,
whitepages vs. service location vs. URN resolution), query types
supported, results types supported
This also means:
Better support for query-passing/other query semantics (need to
balance that against the fact that you don't want DAG-CAPs/SAPs to
have to know a multiplicity of semantic possibilities.
Security infrastructure -- ability to establish security credentials,
maintain a secure transaction, and propagate the security information
forward in the transaction (don't want to reinvent the wheel, just
want to be able to use it!).
Ability to do lookups, instead of searches -- might mean connecting
to different services than the RI and/or presenting things in a
slightly different light -- e.g., lookup <blat> in the <foo> space,
as opposed to search for all things concerning <blat>.
Ability to access other services -- e.g., Norwegian Directory of
Directories [NDD] -- beyond just for specific characteristics of the
service (e.g., security).
In short, the model that seems to stand out from these requirements
one of a protocol framework that looks after establishing secure and
authenticated (authorized, accountable, auditable...) connections,
with transaction negotiation facilities. Within that framework, it
must be possible to identify transaction types, provide suitable
input information (negotiation?) for those transactions, and accept
transaction result objects back.
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In the light of the above proposals, we can revisit the way the
TISDAG CAPs would be defined.
The whitepages-application service known as TISDAG could have SAPs
that supported 2 types of query, and 2 types of result sets:
query types:
. token-based
. phrase-based
result types:
. result data
. referrals
The Whois++ CAP would be configured to contact LDAPv2 and LDAPv3 SAPs
because they are identified as providing that kind of service (i.e.,
if referral protocol == LDAPv2 connect to a particular service). The
query paradigm will be phrase-oriented -- NOT because the Whois++ CAP
understands LDAP, but because that is one of the defined query types.
As it stands, this type of service architecture is limited to query-
response type transactions. This does account for a broad range of
applications and services, although it would be interesting to
consider broadening the concept to make it applicable to tunneling
other protocols (e.g., to connect a call through a SAP, in the number
portability example above).
This document takes a high-level perspective on service architecture,
and as such it neither introduces nor addresses security concerns at
an implementation level.
A distributed service built following this approach must address
issues of authentication of users, authorization for access to
material/components of the system, and encryption of links between
them, as befits the nature of the information and service provided.
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In discussing this perspective on the evolution of DAG/IP, it seemed
to us that the requirements for DAG/IP are falling into line with the
proposed text-based directory access protocol that has variously been
discussed. Whether it survives in a recognizable form or not :-)
some of the above has been drawn from discussions of that protocol
with Michael Mealling and Patrik Faltstrom.
The work described in this document was carried out as part of an on-
going project of Ericsson. For further information regarding that
project, contact:
Bjorn Larsson
bjorn.x.larsson@era.ericsson.se
Request For Comments (RFC) and Internet Draft documents are available
from numerous mirror sites.
[ALVE] Alvestrand, H., "Definitions for Talking about
Directories", Work in Progress.
[TISDAG] Daigle, L. and R. Hedberg "Technical Infrastructure for
Swedish Directory Access Gateways (TISDAG)", RFC 2967,
October 2000.
[DAGEXP] Eklof, T. and L. Daigle, "Wide Area Directory Deployment
Experiences", RFC 2969, September 2000.
[DAG-Mesh] Daigle, L. and T. Eklof, "Networking Multiple DAG servers:
Meshes", RFC 2968, September 2000.
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[NDD] Hedberg, R. and H. Alvestrand, "Technical Specification,
The Norwegian Directory of Directories (NDD)", Work in
Progress.
[WAP] The Wireless Application Protocol, http://www.wapforum.org
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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