Network Working Group C. de Laat
Request for Comments: 2903 Utrecht University
Category: EXPerimental G. Gross
LUCent Technologies
L. Gommans
Enterasys Networks EMEA
J. Vollbrecht
D. Spence
Interlink Networks, Inc.
August 2000
Generic AAA Architecture
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This memo proposes an Authentication, Authorization, Accounting (AAA)
architecture that would incorporate a generic AAA server along with
an application interface to a set of Application Specific Modules
that could perform application specific AAA functions. A separation
of AAA functions required in a multi-domain environment is then
proposed using a layered protocol abstraction. The long term goal is
to create a generic framework which allows complex authorizations to
be realized through a network of interconnected AAA servers.
Table of Contents
1. Introduction ................................................ 2
2. Generic AAA Architecture .................................... 4
2.1. Architectural Components of a Generic AAA Server ....... 4
2.1.1. Authorization Rule Evaluation ................... 4
2.1.2. Application Specific Module (ASM) ............... 5
2.1.3. Authorization Event Log ......................... 6
2.1.4. Policy Repository ............................... 6
2.1.5. Request Forwarding .............................. 6
2.2. Generic AAA Server Model ............................... 6
2.2.1. Generic AAA Server Interactions ................. 7
2.2.2. Compatibility with Legacy Protocols ............. 7
2.2.3. Interaction between the ASM and the Service ..... 9
2.2.4. Multi-domain Architecture ....................... 10
2.3. Model Observations ..................................... 10
2.4. Suggestions for Future Work ............................ 11
3. Layered AAA Protocol Model .................................. 12
3.1. Elements of a Layered Architecture ..................... 14
3.1.1. Service Layer Abstract Interface Primitives ..... 14
3.1.2. Service Layer Peer End Point Name Space ......... 14
3.1.3. Peer Registration, Discovery, and Location
Resolution ............................................. 14
3.1.4. Trust Relationships Between Peer End Points ..... 14
3.1.5. Service Layer Finite State Machine .............. 15
3.1.6. Protocol Data Unit Types ........................ 15
3.2. AAA Application Specific Service Layer ................. 15
3.3. Presentation Service Layer ............................. 16
3.4. AAA Transaction/Session Management Service Layer ....... 17
3.5. AAA-TSM Service Layer Program Interface Primitives ..... 20
3.6. AAA-TSM Layer End Point Name Space ..................... 21
3.7. Protocol Stack Examples ................................ 22
4. Security Considerations ..................................... 22
Glossary ....................................................... 23
References ..................................................... 24
Authors' Addresses ............................................. 24
Full Copyright Statement ....................................... 26
1. Introduction
The work for this memo was done by a group that originally was the
Authorization subgroup of the AAA Working Group of the IETF. When
the charter of the AAA working group was changed to focus on MobileIP
and NAS requirements, the AAAarch Research Group was chartered within
the IRTF to continue and expand the architectural work started by the
Authorization subgroup. This memo is one of four which were created
by the subgroup. This memo is a starting point for further work
within the AAAarch Research Group. It is still a work in progress
and is published so that the work will be available for the AAAarch
subgroup and others working in this area, not as a definitive
description of architecture or requirements.
The authorization subgroup of the AAA Working Group proposed an "AAA
Authorization Framework" [2] illustrated with numerous application
examples [3] which in turn motivates a proposed list of authorization
requirements [4]. This memo builds on the framework presented in [2]
by proposing an AAA infrastructure consisting of a network of
cooperating generic AAA servers communicating via a standard
protocol. The protocol should be quite general and should support
the needs of a wide variety of applications requiring AAA
functionality. To realize this goal, the protocol will need to
operate in a multi-domain environment with multiple service providers
as well as entities taking on other AAA roles such as User Home
Organizations and brokers. It should be possible to combine requests
for multiple authorizations of different types in the same
authorization transaction. The AAA infrastructure will be required
to forward the components of such requests to the appropriate AAA
servers for authorization and to collect the authorization decisions
from the various AAA servers consulted. All of this activity is
perfectly general in nature and can be realized in the common
infrastructure.
But the applications requiring AAA services will each have their own
unique needs. After a service is authorized, it must be configured
and initialized. This will require application specific knowledge
and may require application specific protocols to communicate with
application specific service components. To handle these application
specific functions, we propose an application interface between a
generic AAA server and a set of one or more Application Specific
Modules (ASMs) which can carry out the unique functionality required
by each application.
Since the data required by each application for authentication,
authorization, or accounting may have unique structure, the standard
AAA protocol should allow the encapsulation of opaque units of
Application Specific Information (ASI). These units would begin with
a standard header to allow them to be forwarded by the generic
infrastructure. When delivered to the final destination, an ASI unit
would be passed by a generic AAA server across its program interface
to an appropriate ASM for application specific processing.
Nevertheless, it remains a goal of the design for information units
to be encoded in standard ways as much as possible so as to enable
processing by a generic rule based engine.
The interactions of the generic AAA server with the Application
Specific Modules and with each other to realize complex AAA functions
is explored in section 2. Then, in section 3, we attempt to further
organize the AAA functions into logical groups using a protocol
layering abstraction. This abstraction is not intended to be a
reference model ready to be used for protocol design. At this point
in the work, there are numerous questions that need to be addressed
and numerous problems that remain to be solved. It may be that an
abstraction other than layering will prove to be more useful or, more
likely, that the application layer will require some substructure of
its own.
Finally, in section 4, we show how the security requirements
identified in [4] can be met in the generic server and the
Application Specific Modules by applying security techniques such as
public key encryption or digital signatures to the Application
Specific Information units individually, so that different
stakeholders in the AAA server network can protect selected
information units from being deciphered or altered by other
stakeholders in an authentication, authorization, or accounting
chain.
2. Generic AAA Architecture
For the long term we envision a generic AAA server which is capable
of authenticating users, handling authorization requests, and
collecting accounting data. For a service provider, such a generic
AAA server would be interfaced to an application specific module
which manages the resource for which authorization is required.
Generic AAA components would also be deployed in other administrative
domains performing authorization functions.
2.1. Architectural Components of a Generic AAA Server
2.1.1. Authorization Rule Evaluation
The first step in the authorization process is for the user or an
entity operating on the user's behalf to submit a well-formatted
request to an AAA server. A generic AAA server has rules (logic
and/or algebraic formulas) to inspect the request and come to an
authorization decision. The first problem which arises is that
Application Specific Information (ASI) has to be separated from the
underlying logic for the authorization. Ideally the AAA server would
have a rule based engine at this point which would know the logic
rules and understand some generic information in the request, but it
would not know anything about application specific information except
where this information can be evaluated to give a boolean or
numerical value. It should be possible to create rules that refer to
data elements that were not considered when the application was
created. For example, one could request to do a remote virtual
control room experiment from home using a dialin provider. The
request would only be successful if the dialin Access server allows
it and if there is bandwidth available (bandwidth broker) and if the
experimenter has the money to pay for it (E-Commerce). Possibly the
people who specified the bandwidth broker protocol did not think of
combining quality of service with a network service authorization in
a single AAA request, but this generic model would allow it.
+------+ +-------+ +-------+ +-------+ +-------+
auth auth auth auth
<----> AAA <----> AAA <----> AAA <----> AAA
+-------+ +-------+ +-------+ +-------+
User
+-------+ +-------+ +-------+
BB BB Budget
+-------+ +-------+ +-------+
+-------+
dial in +-------+ +-------+
<====>service<====>network<====>network<===> Experiment
+------+ +-------+ +-------+ +-------+
user <-> dialin <-> backbone with BB <-> <remote experiment>
Fig. 1 -- Example of a Multi Domain Multi Type of Server Request
2.1.2. Application Specific Module (ASM)
Ultimately an AAA server needs to interact with an application
specific module (ASM). In a service provider, the ASM would manage
resources and configure the service equipment to provide the
authorized service. It might also involve itself in the
authorization decision because it has the application specific
knowledge required. A user home organization (UHO) may require ASMs
as well, to perform application specific user authorization
functions. For example, a UHO ASM might be required to access
certain application specific databases or interpret application
specific service level specifications.
Whatever the role of an administration relative to an authorization
decision, the capabilities of the generic AAA server and the
interface between it and the ASMs remains the same. This interface
may be an Application Program Interface (API) or could even be a
protocol based interface. In this model, however, the application
specific module is regarded as as separate architectural component
from the generic AAA server. As such, it must be addressable and
must therefore be part of a global naming space.
2.1.3. Authorization Event Log
For auditing purposes, the generic server must have some form of
database to store time-stamped events which occur in the AAA server.
This database can be used to account for authorizations which were
given, but it can also be used in rules. One can imagine rules in
which an authorization is only given if some other event was logged
in the past. With the aid of certificates, this database could
support non-repudiation.
2.1.4. Policy Repository
A database containing the available services and resources about
which authorization decisions can be made and the policy rules to
make them is also needed. Here too, the naming space for the
services and resources is important since they must be addressable
from other AAA servers to be able to build complex authorization
requests.
2.1.5. Request Forwarding
Due to the multiple administrative domain (multi-kingdom) nature of
the AAA problem, a mechanism to forward messages between AAA servers
is needed. The protocol by which two AAA servers communicate should
be a peer-to-peer protocol.
2.2. Generic AAA Server Model
With the implementation of the above mentioned components, the AAA
server would be able to handle AAA requests. It would inspect the
contents of the request, determine what authorization is requested,
retrieve the policy rules from the repository, perform various local
functions, and then choose one of the following options to further
process each of the components of the request:
a) Let the component be evaluated by an attached ASM.
b) Query the authorization event log or the policy repository for the
answer.
c) Forward the component(s) to another AAA server for evaluation.
In the following sections we present the generic model.
2.2.1. Generic AAA Server Interactions
Figure 2 illustrates a generic AAA Server with connections to the
various architectural components described above. In this model, the
user or another AAA server contacts the AAA server to get
authorization, and the AAA server interacts with the service. The
request is sent to the AAA server using the future AAA protocol. The
server interacts with the service via a second protocol which we have
labeled as type "2" in the figure. We say no more of the type 2
protocol than that it must support some global naming space for the
application specific items. The same holds for the type 3
communication used to access the repository.
+------------------+
request <-----1----->Generic AAA Server<---1---> AAA server
Rule based engine
+------------------+ 3 +------------+
^ \ Policy and
event
2 repository
+------------+
v
+------------------+
Application
Specific
Module
+------------------+
The numbers in the links denote types of communication.
Fig. 2 -- Generic AAA Server Interactions
2.2.2. Compatibility with Legacy Protocols
Because of the widespread deployment of equipment that implements
legacy AAA protocols and the desire to realize the functionality of
the new AAA protocol while protecting the investment in existing
infrastructure, it may be useful to implement a AAA gateway function
that can encapsulate legacy protocol data units within the messages
of the new protocol. Use of this technique, for example, would allow
Radius attribute value pairs to be encapsulated in Application
Specific Information (ASI) units of the new protocol in such a way
that the ASI units can be digitally signed and encrypted for end-to-
end protection between a service provider's AAA server and a home AAA
server communicating via a marginally trusted proxy AAA server. The
service provider's NAS would communicate via Radius to the service
provider's AAA server, but the AAA servers would communicate among
themselves via the new AAA protocol. In this case, the AAA gateway
would be a software module residing in the service provider's AAA
server. Alternatively the AAA gateway could be implemented as a
standalone process.
Figure 3 illustrates an AAA gateway. Communication type 4 is the
legacy protocol. Communication type 1 is the future standard AAA
protocol. And communication type 2 is for application specific
communication to Application Specific Modules (ASMs) or Service
Equipment.
+-------+
AAA <---1---> to AAA server as in fig. 2
request <---4--->GateWay
<---2---> optionally to ASM/service
+-------+
The numbers in the links denote types of communication.
Fig. 3 -- AAA Gateway for Legacy AAA Protocols
2.2.3. Interaction between the ASM and the Service
In a service provider, the Application Specific Module (ASM) and the
software providing the service itself may be tightly bound into a
single "Service Application". In this case, the interface between
them is just a software interface. But the service itself may be
provided by equipment external to the ASM, for example, a router in
the bandwidth broker application. In this case, the ASM communicates
with the service via some protocol. These two possibilities are
illustrated in figure 4. In both cases, we have labeled the
communication between the ASM and the service as communication type
5, which of course, is service specific.
+------------------+
Service 2 2
Application
+-------------+ +-------------+
Application Application
Specific Specific
Module Module
+-------------+ +-------------+
5 5
+-------------+ +-------------+
Service Service
Equipment
+-------------+ +-------------+
+-------------------+
Fig. 4 -- ASM to Service Interaction (two views)
2.2.4. Multi-domain Architecture
The generic AAA server modules can use communication type 1 to
contact each other to evaluate parts of requests. Figure 5
illustrates a network of generic AAA servers in different
administrative domains communicating via communication type 1.
+-----+
o-------- AAA ---->...
/
/ +-----+ / \+----+
/ +-----+ RP
/ ASM +----+
+--------+ +-----+ /
Client ------ AAA -------o +-----+
+--------+ +-----+ +----+ \ +-----+
+-----+ RP o----- AAA ---->...
ASM +----+
+-----+ +-----+ \+----+
+-----+ RP
ASM +----+
+-----+
The AAA servers use only communication type 1 to communicate.
ASM = Application Specific Module
RP = Repository
Fig. 5 -- Multi-domain Multi-type of Service Architecture
2.3. Model Observations
Some key points of the generic architecture are:
1) The same generic AAA server can function in all three
authorization models: agent, pull, and push [2].
2) The rule based engine knows how to evaluate logical formulas and
how to parse AAA requests.
3) The Generic AAA server has no knowledge whatsoever about the
application specific services so the application specific
information it forwards is opaque to it.
4) Communication types 1, 2, and 3 each present their own naming
space problems. Solving these problems is fundamental to
forwarding AAA messages, locating application specific entities,
and locating applicable rules in the rule repositories.
5) A standard AAA protocol for use in communication type 1 should be
a peer-to-peer protocol without imposing client and server roles
on the communicating entities.
6) A standard AAA protocol should allow information units for
multiple different services belonging to multiple different
applications in multiple different administrative domains to be
combined in a single AAA protocol message.
2.4. Suggestions for Future Work
It is hoped that by using this generic model it will be feasible to
design a AAA protocol that is "future proof", in a sense, because
much of what we do not think about now can be encoded as application
specific information and referenced by policy rules stored in a
policy repository. From this model, some generic requirements arise
that will require some further study. For example, suppose a new
user is told that somewhere on a specific AAA server a certain
authorization can be oBTained. The user will need a AAA protocol
that can:
1) send a query to find out which authorizations can be obtained from
a specific server,
2) provide a mechanism for determining what components must be put in
an AAA request for a specific authorization, and
3) formulate and transmit the authorization request.
Some areas where further work is particularly needed are in
identifying and designing the generic components of a AAA protocol
and in determining the basis upon which component forwarding and
policy retrieval decisions are made.
In addition to these areas, there is a need to explore the management
of rules in a multi-domain AAA environment because the development
and future deployment of a generic multi-domain AAA infrastructure is
largely dependent on its manageability. Multi-domain AAA
environments housing many rules distributed over several AAA servers
quickly become unmanageable if there is not some form of automated
rule creation and housekeeping. Organizations that allow their
services to be governed by rules, based on some form of commercial
contract, require the contract to be implemented with the least
possible effort. This can, for example, be achieved in a scalable
fashion if the individual user or user organization requesting a
service is able to establish the service itself. This kind of
interaction requires policy rule establishment between AAA servers
belonging to multiple autonomous administrative domains.
3. Layered AAA Protocol Model
In the previous section, we proposed the idea of a generic AAA server
with an interface to one or more Application Specific Modules (ASMs).
The generic server would handle many common functions including the
forwarding of AAA messages between servers in different
administrative domains. We envision message transport, hop-by-hop
security, and message forwarding as clearly being functions of the
generic server. The application specific modules would handle all
application specific tasks such as communication with service
equipment and access to special purpose databases. Between these two
sets of functions is another set of functions that presumably could
take place in either the generic server or an ASM or possibly by a
collaboration of both. These functions include the evaluation of
authorization rules against data that may reside in various places
including attributes from the authorization request itself. The more
we can push these functions down into the generic server, the more
powerful the generic server can be and the simpler the ASMs can be.
One way of organizing the different functions mentioned above would
be to assign them to a layered hierarchy. In fact, we have found the
layer paradigm to be a useful one in understanding AAA functionality.
This section explores the use of a layered hierarchy consisting of
the following AAA layers as a way of organizing the AAA functions:
Application Specific Service Layer
Presentation Service Layer
Transaction/Session Management Service Layer
Reliable/Secure Transport Service Layer
Nevertheless, the interface between the generic AAA server and the
ASMs proposed in the previous section may be more complex than a
simple layered model would allow. Even the division of functionality
proposed in this section goes beyond a strict understanding of
layering. Therefore this paper can probably best be understood as
the beginnings of a work to understand and organize the common
functionality required for a general purpose AAA infrastructure
rather than as a mature reference model for the creation of AAA
protocols.
In our view of AAA services modeled as a hierarchy of service layers,
there is a set of distributed processes at each service layer that
cooperate and are responsible for implementing that service layer's
functions. These processes communicate with each other using a
protocol specialized to carry out the functions and responsibilities
assigned to their service layer. The protocol at service layer n
communicates to its peers by depending on the services available to
it from service layer n-1. The service layer n also has a protocol
end point address space, through which the peer processes at service
layer n can send messages to each other. Together, these AAA service
layers can be assembled into an AAA protocol stack.
The advantage of this approach is that there is not just one
monolithic "AAA protocol". Instead there is a suite of protocols,
and each one is optimized to solve the problems found at its layer of
the AAA protocol stack hierarchy.
This approach realizes several key benefits:
- The protocol used at any particular layer in the protocol stack
can be substituted for another functionally equivalent protocol
without disrupting the services in adjacent layers.
- Requirements in one layer may be met without impact on protocols
operating in other layers. For example, local security
requirements may dictate the substitution of stronger or weaker
"reliable secure transport" layer security algorithms or
protocols. These can be introduced with no change or awareness of
the substitution by the layers above the Reliable/Secure Transport
layer.
- The protocol used for a given layer is simpler because it is
focused on a specific narrow problem that is assigned to its
service layer. In particular, it should be feasible to leverage
existing protocol designs for some ASPects of this protocol stack
(e.g. CORBA GIOP/CDR for the presentation layer).
- A legacy AAA protocol message (e.g. a RADIUS message) can be
encapsulated within the protocol message(s) of a lower layer
protocol, preserving the investment of a Service Provider or User
Home Organization in their existing AAA infrastructure.
- At each service layer, a suite of alternatives can be designed,
and the service layer above it can choose which alternative makes
sense for a given application. However, it should be a primary
goal of the AAA protocol standardization effort to specify one
mandatory to implement protocol at the AAA Transaction/Session
Management (AAA-TSM) service layer (see section 3.4).
3.1. Elements of a Layered Architecture
At each layer of a layered architecture, a number of elements need to
be defined. These elements are discussed in the following sections.
3.1.1. Service Layer Abstract Interface Primitives
The service layer n is assumed to present a program interface through
which its adjacent service layer n+1 can access its services. The
types of abstract program service primitives and associated
parameters exchanged across the boundary between these service layers
must be specified.
3.1.2. Service Layer Peer End Point Name Space
Each service layer is treated as a set of cooperating processes
distributed across multiple computing systems. The service layer
must manage an end point name space that identifies these peer
processes. The conventions by which a service layer assigns a unique
end point name to each such peer process must be specified.
3.1.3. Peer Registration, Discovery, and Location Resolution
Along with defining an end point name space, a service layer must
also specify how its peers:
- announce their presence and availability,
- discover one another when they first begin operation, and
- detect loss of connectivity or service withdrawal.
It is also necessary to specify what mechanisms, if any, exist to
resolve a set of service layer specific search attributes into one or
more peer end point names that match the search criteria.
3.1.4. Trust Relationships Between Peer End Points
Once an end point has established its initial contact with another
peer, it must decide what authentication policy to adapt. It can
trust whatever authentication was done on its behalf by a lower
service layer or, through a pre-provisioning process, implicitly
trust the peer, or else go through an authentication process with its
peer. The supported mechanisms for establishing a service layer's
end point trust relationships must be specified.
3.1.5. Service Layer Finite State Machine
To the extent that a service layer's internal states are externally
visible, the layer's behavior in terms of a Finite State Machine
(FSM) should be specified. Events that can drive the FSM state
transitions may include:
- service layer n+1 interface primitive requests
- protocol data unit arrivals from peer service layer n end points
received through the layer n-1 access point
- service layer n-1 interface primitives (e.g. call backs or
interrupts)
- timer expirations
3.1.6. Protocol Data Unit Types
Each service layer defines a lexicon of protocol data units (PDUs)
that communicate between the layer's peer processes the information
that controls and/or monitors that service layer's distributed state
and allows the service processes of that layer to perform their
functions. Embedded in the PDUs of each layer are the PDUs of the
higher layers which depend on its services. The PDUs of each service
layer must be specified.
3.2. AAA Application Specific Service Layer
AAA applications have almost unlimited diversity, but imposing some
constraints and commonality is required for them to participate in
this generic AAA architectural framework. To satisfy these
constraints, participating AAA applications would derive their
application specific program logic from a standardized "Authorization
Server" abstract base object class. They would also support an
"Authorized Session" object class. An Authorization Session object
instance represents an approved authorization request that has a
long-lived allocation of services or resources. The generic AAA
architecture could be extended to include other abstract base object
classes in the future (e.g. Authorization Reservation, Authentication
Server, etc.). How to implement the derived Authorization Server
class's public methods for a given problem domain is entirely up to
the application. One technique might be to place a software
"wrapper" around an existing embedded application specific service to
adapt it to the standardized Authorization Server object paradigm.
The major Authorization Server class methods are:
- Publish an advertisement that describes the Authorization Server's
service attributes and its application specific service layer end
point address. Once the Authorization Server has registered, peer
processes can discover its presence or send messages addressed to
it.
- Application Specific Authorization Decision Function (AS-ADF)
method takes a User's application specific authorization request
and returns a decision of approve, deny, or conditionally approve
with referral to another stakeholder. In the latter case, the
application may create a reservation for the requested services or
resources. This method represents the "condition" side of a
policy rule's condition/action pair.
- Commit a service or set of resources to a previously conditionally
approved authorization decision. For those authorization requests
that have a long-term lifecycle (as opposed to being
transactions), this method mobilizes a reservation into an
Authorized Session object instance. This method represents the
"action" side of a policy rule's condition/action pair.
- Cancel a previously conditionally approved Authorization request.
This method releases any associated reservations for services or
resources.
- Withdraw the Authorization Server's service advertisement.
A key motivation for structuring an AAA application as an
Authorization Server object instance is to separate the generic
authorization decision logic from the application-specific
authorization decision logic. In many cases, the application can be
divorced from the AAA problem altogether, and its AAA responsibility
can be assigned to an external rules based generic AAA Server. (The
idea is similar to that of a trust management policy server as
defined in [5].) This would facilitate a security administrator
deploying AAA policy in a central repository. The AAA policy is
applied consistently across all users of the applications, resources,
and services controlled by the AAA server. However, it is recognized
that for many problem domains, there are unique rules intrinsic to
the application. In these cases, the generic AAA Server must refer
the User's authorization request to the relevant Application Specific
Module.
3.3. Presentation Service Layer
The presentation service layer solves the data representation
problems that are encountered when communicating peers exchange
complex data structures or objects between their heterogeneous
computing systems. The goal is to transfer semantically equivalent
application layer data structures regardless of the local machine
architecture, operating system, compiler, or other potential inter-
system differences.
One way to better understand the role of the presentation layer is to
evaluate an existing example. The Generic Inter-ORB Protocol (GIOP)
and its Common Data Representation (CDR) is a presentation service
layer protocol developed by the Object Management Group (OMG)
industry consortium. GIOP is one component within the Common Object
Request Broker Architecture (CORBA). Peer Object Request Brokers
(ORB) executing on heterogeneous systems use GIOP to invoke remote
CORBA object interface methods. GIOP encodes an object method's
input and output parameters in the Common Data Representation (CDR).
While there are other presentation service layer protocols in the
industry, GIOP in combination with CDR represents a mature,
comprehensive solution that exhibits many of the presentation service
layer requirements that are applicable within the AAA protocol model.
In the context of Internet access AAA protocols, RADIUS and its
successors use the Attribute Value Pair (AVP) paradigm as the
presentation service layer encoding scheme. While such an approach
is versatile, it is also prone to becoming splintered into many ad
hoc and vendor specific dialects. There is no structure imposed or
method to negotiate the constraints on which AVPs are combined and
interpreted for a given conversation in a consistent way across AAA
protocol implementations or problem domains. At run-time, it can be
hard for the communicating peers to negotiate to a common inter-
operable set of AVPs.
To avoid this pitfall, a primary presentation service layer
responsibility is the ability to let peers negotiate from a base
Authorization Server object class towards a commonly understood
derived Authorization Server object class that both presentation
service layer peers have implemented for their application specific
problem domain. This negotiation implies a requirement for a
globally registered and maintained presentation service layer
hierarchy of Authorization Server object class names.
3.4. AAA Transaction/Session Management Service Layer
The AAA Transaction/Session Management (AAA-TSM) service layer is a
distributed set of AAA Servers, which typically reside in different
administrative domains. Collectively they are responsible for the
following three services:
Authentication -- Execute the procedure(s) needed to confirm the
identity of the other parties with which the AAA TSM entity has a
trust relationship.
Authorization -- Make an authorization decision to grant or deny a
User's request for services or resources. The generic rules based
policy engine described earlier in this document executes the
authorization decision function. When the User's request is
instantaneous and transient, then its authorization approval is
treated as an ephemeral transaction. If the authorization
approval implies a sustained consumption of a service or
resources, then the request is transformed into an Authorized
Session. For the duration of the Authorized Session's lifetime:
- its state may be queried and reported, or
- it may be canceled before service is completed, or
- the service being delivered may be modified to operate under
new parameters and conditions, or
- the service may complete on its own accord.
In each of these cases, the AAA-TSM service layer must synchronize
the Authorized Session's distributed state across all of those AAA
Servers which are implementing that specific Authorized Session.
Accounting -- Generate any relevant accounting information regarding
the authorization decision and the associated Authorized Session
(if any) that represents the ongoing consumption of those services
or resources.
The peer AAA servers and their AAA-TSM end points exchange AAA-TSM
messages to realize these AAA functions. A central AAA-TSM concept
is that there is a set of one or more AAA Server stakeholders who are
solicited to approve/disapprove a User request for application layer
services. The AAA-TSM service layer routes the User's request from
one stakeholder to the next, accumulating the requisite approvals
until they have all been asked to make an authorization decision.
The AAA Servers may also do User authentication (or re-
authentication) as part of this approval process. The overall flow
of the routing from one stakeholder to another may take the form of
the "push", "pull", or "agent" authorization models developed in [2].
However, in principle, it is feasible to have an arbitrary routing
path of an AAA-TSM authorization request among stakeholders. Once the
final approval is received, the AAA-TSM service layer commits the
requested service by notifying all of those stakeholders that require
a confirmation (i.e. turn on a pending reservation and do a
transaction commit). Alternatively, any stakeholder among those on
the consent list can veto the authorization request. In that case,
all stakeholders who previously approved the request and had asked
for a confirmation are told that the request has been denied (i.e.,
cancel reservation and do a transaction rollback).
The AAA-TSM authorization request payload must carry its own "Context
State", such that when an AAA server receives it, there is sufficient
information that it is essentially self-contained. Embedding the
Context State within the AAA-TSM message provides two benefits.
First, the message can be immediately processed with respect to the
AAA Server's local policy, and this minimizes or altogether avoids
the need for the AAA Server to exchange additional AAA-TSM messages
with its peers to complete its piece of the overall authorization
decision. The other benefit is that the AAA Server minimizes the
amount of state information resources that it commits to a user's
pending request until it is fully approved. This helps protect
against denial of service attacks.
One can envision many possible message elements that could be part of
the Context State carried within an AAA-TSM request message:
- AAA-TSM session identifier, a unique handle representing this
authorization request. All AAA servers who participate in a
request's approval process and its subsequent monitoring
throughout its Session lifetime refer to this handle.
- permission lists stating which AAA Servers are allowed to modify
which parts of the message.
- User's authorization request, encoded as a presentation layer PDU.
- User authentication information, (e.g. an X.509 public key
certificate).
- User credentials information, or else a pointer to where that
information can be found by an AAA server. An example of such
credentials would be an X.509 attributes certificate.
- the list of AAA Server stakeholders who have yet to be visited to
gain full approval of the User's authorization request. Each
element in that list contains a presentation layer message
encoding how the user authorization request should be evaluated by
its application specific Authorization Decision Function (ADF).
- the current position in the list of AAA Server stakeholders to be
visited.
- a list of those AAA servers which have already conditionally
approved the User's authorization request, but which have
predicated their approval on the request also completing its
approval from those stakeholders who have not yet seen the
request. Each element in the list has a digital signature or
comparable mechanism by which their approval can be subsequently
verified.
- an expiration time stamp, expressed in a universally understood
time reference, which sets a lifetime limit on the AAA-TSM
message's validity. This offers some replay attack protection,
and inhibits messages from circulating indefinitely seeking the
completion of a request's approval.
- a message payload modification audit trail, tracing which parties
introduced changes into the User's authorization request terms and
conditions.
- an AAA-TSM message integrity check, computed across the whole
message rather than its individual elements, and signed by the
most recent AAA-TSM layer end point process to modify the AAA-TSM
message before its transmission to its AAA-TSM peer. This
function may be delegated to the underlying Reliable Secure
Transport layer connection to that destination peer.
3.5. AAA-TSM Service Layer Program Interface Primitives
The AAA-TSM service layer and its adjacent presentation service layer
communicate across their boundary through a set of program interface
primitives. A key design goal is to keep these primitives the same
regardless of the higher level AAA application, analogous to a
callable "plug-in". The two service layers are responsible for
coordinating their state information. This responsibility includes
all of the pending Authorization requests and the Authorization
Sessions that they are both controlling and monitoring. The initial
contact between these two layers is through an abstract object that
is called an AAA-TSM Service Access Point (SAP). A particular
service instance between these two layers is realized in an abstract
object that is called an Authorized Session. The presentation
service layer invokes AAA-TSM interface primitives against an AAA-TSM
SAP.
The AAA-TSM service layer interface primitives can be broadly
characterized as follows:
- Register a presentation end point address identifier and its
associated set of attributes to a service access point.
- Send a presentation layer message to a specified destination
presentation layer peer end point address.
- Receive a presentation layer message from another presentation
layer end point address. A receive operation may select a
specific originating presentation layer end point address from
which the message is expected, or receive a message from any
presentation layer peer.
- The AAA-TSM service layer calls an application specific
authorization decision function, which returns a condition code
expressing an approval, denial, or partially approves with a
referral to another AAA Server.
- AAA-TSM service layer tells the presentation layer to commit an
earlier partially approved authorization request.
- Cancel an earlier partially approved authorization request (i.e.
rollback).
- The presentation service layer notifies the AAA-TSM service layer
that it has terminated an in-progress Authorized Session.
- AAA-TSM service layer notifies the presentation service layer that
another presentation service layer peer has terminated an
Authorized Session.
- Un-register a presentation service layer end point address.
3.6. AAA-TSM Layer End Point Name Space
The AAA-TSM service layer end point name space is the N-tuple formed
by concatenating the following components:
- AAA Server's Reliable/Secure Transport layer end point address
- AAA-TSM authorization request serial number, a unique durable
unsigned integer generated by the AAA Server who first receives
the User's authorization request.
Some AAA applications may require that each assigned AAA-TSM
transaction serial number be stored in persistent storage, and
require that it be recoverable across AAA Server system re-boots.
The serial number generation algorithm must be guaranteed unique even
if the AAA Server does a re-boot.
3.7. Protocol Stack Examples
The layering paradigm makes it possible to use the most appropriate
syntax for each application for encoding the Application Specific
Information units of that application. This encoding would take
place at the presentation layer. Similarly the application layer can
recognize the semantics specific to each application. Figure 6
illustrates some possible AAA protocol stacks.
+------------++------------++-----------++-----------++----------+
Application E-Commerce Bandwidth Roaming &
AAA specific Internet Broker mobile IP
Applicationobject class Open cross-admin remote
Service interface Trading domain access
Layer specified in Protocol COPS AVP
CORBA IDL (IOTP) extensions lexicons
+------------++------------++-----------++-----------++----------+
CORBA Extensible Common DIAMETER
Presentation Generic Markup Open or
Service Inter-ORB Language Policy RADIUS
Layer Protocol (XML) SpecificatnAttribute
(GIOP) (COPS) Value/Pair
+------------++------------++-----------++-----------++----------+
AAA-TSM Service Layer Application Program Interface (API)
+----------------------------------------------------------------+
AAA Transaction/Session Management (AAA-TSM) Service Layer
+----------------------------------------------------------------+
Reliable Secure Transport Layer
+----------------------------------------------------------------+
Fig. 6 -- Possible AAA Protocol Stacks
4. Security Considerations
Security considerations for the framework on which the work described
in this memo is based are discussed in [2]. Security requirements
for authorization are listed in section 2.2 of [3].
This memo identifies a basic set of AAA functions that are general in
nature and common to many different AAA applications. We propose
that a standard set of security mechanisms should be defined as part
of a base AAA protocol which would include such things as public key
encryption and digital signatures that could be applied to individual
information units within an AAA message. Security with this
granularity is needed to meet the end-to-end security requirement
specified in section 2.2.7 of [3] because a single AAA message may
contain multiple information units each generated by AAA servers from
different administrative domains and destined to AAA servers in
different domains.
In addition, it may be necessary to encrypt or sign an entire AAA
message on a hop-by-hop basis. This could be handled by a standard,
lower layer protocol such as IPSEC. If so, then certain auditing
requirements will have to be met so that it can be established later
that the messages relative to some specific session ID were, in fact,
protected in a particular way. Alternatively, hop-by-hop security
mechanisms may be built into the base AAA protocol itself.
Glossary
Application Specific Information (ASI) -- information in an AAA
protocol message that is specific to a particular application.
Application Specific Module (ASM) -- a software module that
implements a program interface to a generic AAA server which
handles application specific functionality for an AAA protocol
message.
Service Provider -- an organization which provides a service.
User -- the entity seeking authorization to use a resource or a
service.
User Home Organization (UHO) -- An organization with whom the User
has a contractual relationship which can authenticate the User and
may be able to authorize access to resources or services.
References
[1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC2026, October 1996.
[2] Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L., Gross,
G., de Bruijn, B., de Laat, D., Holdrege, M. and D. Spence, "AAA
Authorization Framework", RFC2904, August 2000.
[3] Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L., Gross,
G., de Bruijn, B., de Laat, C., Holdrege, M. and D. Spence, "AAA
Authorization Application Examples", RFC2905, August 2000.
[4] Farrell, S., Vollbrecht, J., Calhoun, P., Gommans, L., Gross,
G., de Bruijn, B., de Laat, C., Holdrege, M. and D. Spence, "AAA
Authorization Requirements", RFC2906, August 2000.
[5] Blaze, M., Feigenbaum, J., Ioannidis, J. and A. Keromytis, "The
KeyNote Trust-Management System Version 2", RFC2704, September
1999.
Authors' Addresses
Cees T.A.M. de Laat
Physics and Astronomy dept.
Utrecht University
Pincetonplein 5,
3584CC Utrecht
Netherlands
Phone: +31 30 2534585
Phone: +31 30 2537555
EMail: delaat@phys.uu.nl
George M. Gross
Lucent Technologies
184 Liberty Corner Road, m.s. LC2N-D13
Warren, NJ 07059
USA
Phone: +1 908 580 4589
Fax: +1 908-580-4991
EMail: gmgross@lucent.com
Leon Gommans
Enterasys Networks EMEA
Kerkplein 24
2841 XM Moordrecht
The Netherlands
Phone: +31 182 379279
email: gommans@cabletron.com
or at University of Utrecht:
l.h.m.gommans@phys.uu.nl
John R. Vollbrecht
Interlink Networks, Inc.
775 Technology Drive, Suite 200
Ann Arbor, MI 48108
USA
Phone: +1 734 821 1205
Fax: +1 734 821 1235
EMail: jrv@interlinknetworks.com
David W. Spence
Interlink Networks, Inc.
775 Technology Drive, Suite 200
Ann Arbor, MI 48108
USA
Phone: +1 734 821 1203
Fax: +1 734 821 1235
EMail: dspence@interlinknetworks.com
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