| Rfc | 2905 |
|---|
| Title | AAA Authorization Application Examples |
|---|
| Author | J. Vollbrecht, P. Calhoun,
S. Farrell, L. Gommans, G. Gross, B. de Bruijn, C. de Laat, M.
Holdrege, D. Spence |
|---|
| Date | August 2000 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Network Working Group J. Vollbrecht
Request for Comments: 2905 Interlink Networks, Inc.
Category: Informational P. Calhoun
Sun Microsystems, Inc.
S. Farrell
Baltimore Technologies
L. Gommans
Enterasys Networks EMEA
G. Gross
Lucent Technologies
B. de Bruijn
Interpay Nederland B.V.
C. de Laat
Utrecht University
M. Holdrege
ipVerse
D. Spence
Interlink Networks, Inc.
August 2000
AAA Authorization Application Examples
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
This memo describes several examples of applications requiring
authorization. Each application is described in terms of a
consistent framework, and specific authorization requirements of each
application are given. This material was not contributed by the
working groups responsible for the applications and should not be
considered prescriptive for how the applications will meet their
authorization needs. Rather the intent is to explore the fundamental
needs of a variety of different applications with the view of
compiling a set of requirements that an authorization protocol will
need to meet in order to be generally useful.
Table of Contents
1. Introduction ................................................ 3
2. PPP Dialin with Roaming ..................................... 4
2.1. Descriptive Model ...................................... 4
2.2. Authorization Requirements ............................. 6
3. Mobile-IP ................................................... 6
3.1. Relationship to the Framework .......................... 10
3.2. Minimized Internet Traversal ........................... 10
3.3. Key Distribution ....................................... 10
3.4. Mobile-IP Authorization Requirements ................... 11
4. Bandwidth Broker ............................................ 12
4.1. Model Description ...................................... 13
4.2. Components of the Two-Tier Model ....................... 13
4.3. Identification of Contractual Relationships ............ 13
4.3.1. Single-Domain Case .............................. 14
4.3.2. Multi-Domain Case ............................... 15
4.4. Identification of Trust Relationships .................. 16
4.5. Communication Models and Trust Relationships ........... 18
4.6. Bandwidth Broker Communication Models .................. 19
4.6.1. Concepts ........................................ 19
4.6.1.1. Intra-Domain Authorization ............... 19
4.6.1.2. Inter-Domain Authorization ............... 19
4.6.2. Bandwidth Broker Work Phases .................... 20
4.6.3. Inter-Domain Signaling .......................... 20
4.6.3.1. Phase 0 .................................. 20
4.6.3.2. Phase 1 .................................. 20
4.6.4. Bandwidth Broker Communication Architecture ..... 22
4.6.5. Two-Tier Inter-Domain Model ..................... 23
4.6.5.1. Session Initialization ................... 23
4.6.5.2. Service Setup ............................ 23
4.6.5.3. Service Cancellation ..................... 24
4.6.5.4. Service Renegotiation .................... 24
4.6.5.5. RAR and RAA .............................. 24
4.6.5.6. Session Maintenance ...................... 24
4.6.5.7. Intra-domain Interface Protocol .......... 24
4.7. Requirements ........................................... 24
5. Internet Printing ........................................... 25
5.1. Trust Relationships .................................... 26
5.2. Use of Attribute Certificates .......................... 27
5.3. IPP and the Authorization Descriptive Model ............ 28
6. Electronic Commerce ......................................... 29
6.1. Model Description ...................................... 30
6.1.1. Identification of Components .................... 30
6.1.2. Identification of Contractual Relationships ..... 31
6.1.3. Identification of Trust Relationships ........... 32
6.1.3.1. Static Trust Relationships ............... 33
6.1.3.2. Dynamic Trust Relationships .............. 35
6.1.4. Communication Model ............................. 35
6.2. Multi Domain Model ..................................... 37
6.3. Requirements ........................................... 38
7. Computer Based Education and Distance Learning .............. 40
7.1. Model Description ...................................... 40
7.1.1. Identification of Components .................... 40
7.1.2. Identification of Contractual Relationships ..... 41
7.1.3. Identification of Trust Relationships ........... 43
7.1.4. Sequence of Requests ............................ 44
7.2. Requirements ........................................... 46
8. Security Considerations ..................................... 47
Glossary ....................................................... 47
References ..................................................... 48
Authors' Addresses ............................................. 50
Full Copyright Statement ....................................... 53
1. Introduction
This document is one of a series of three documents under
consideration by the AAAarch RG dealing with the authorization
requirements for AAA protocols. The three documents are:
AAA Authorization Framework [2]
AAA Authorization Requirements [3]
AAA Authorization Application Examples (this document)
In this memo, we examine several important Internet applications that
require authorization. For each application, we present a model
showing how it might do authorization and then map that model back to
the framework presented in [2]. We then present the authorization
requirements of the application as well as we presently understand
them. The requirements presented in this memo have been collected
together, generalized, and presented in [3].
The intent of this memo is to validate and illustrate the framework
presented in [2] and to motivate the requirements presented in [3].
This work is intended to be in alignment with the work of the various
working groups responsible for the authorization applications
illustrated. This memo should not, however, be regarded as
authoritative for any of the applications illustrated. Where
authoritative documents exist or are in development, they are listed
in the references at the end of this document.
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.
This document uses the terms 'MUST', 'SHOULD' and 'MAY', and their
negatives, in the way described in RFC 2119 [4].
2. PPP Dialin with Roaming
In this section, we present an authorization model for dialin network
access in terms of the framework presented in [2]. Included in the
model are the multi-domain considerations required for roaming [5].
Detailed requirements for network access protocols are presented in
[6].
2.1. Descriptive Model
The PPP dialin application uses the pull sequence as discussed in
[2]. The roaming case uses the roaming pull sequence, also discussed
in [2]. This sequence is redrawn using dialin roaming terminology in
figure 1, below.
+------+ +-------------------------+
| | | Home ISP |
| | | (User Home Organization)|
| | | +-------------------+ |
| | | | AAA Server | |
| | | | | |
| | | +-------------------+ |
| | | /|\ | |
| | +--------------+---+------+
| | | |
| | |3 |4
| | | |
| | +--------------+---+------+
| | | Visited ISP | | |
| | | | \|/ |
| User | | +-------------------+ |
| | | | AAA Server | |
| | | | | |
| | | +-------------------+ |
| | | /|\ | |
| | | |2 |5 |
| | | | \|/ |
| | 1 | +-------------------+ |
| |------+->| NAS (Service | |
| |<-----+--| Equipment) | |
| | 6 | +-------------------+ |
| | | (Service Provider) |
+------+ PPP +-------------------------+
Fig. 1 -- Dialin Authorization
Based on Roaming Pull Sequence
In this model, the User dials in to a Network Access Server (NAS)
provided by the visited (or foreign) ISP (the Service Provider in the
general model). The User is authenticated using a protocol such as
PAP, CHAP, or EAP which is encapsulated in PPP frames (1). Because
the User has not yet gained access to the network, he or she cannot
send IP datagrams to a AAA server. At this point, the User can only
communicate with the NAS (Service Equipment). The NAS forwards the
User's authentication/ authorization request including the Network
Access Identifier (NAI) [7] to a AAA server in its own domain via
RADIUS [8] or a successor AAA protocol (2). The visited ISP's AAA
server examines the realm from the NAI and forwards the request to
the User's home domain AAA server (3). The home domain AAA server
authenticates the user and authorizes access according to a roaming
agreement. The home domain AAA server may return service parameters
(e.g. Idle-Timeout) to the visited ISP's AAA server (4) which
forwards them to the NAS, possibly adding additional service
parameters (5). The NAS completes PPP session initialization (6).
In the future, this model may be expanded in several ways [9]. For
instance, Authentication and Authorization may be done in separate
passes using different servers in order to support specialized forms
of authentication. Or to better support roaming, a broker may be
inserted between the visited ISP and the home ISP. Or authorization
may be supported based on other identifiers such as the caller ID and
called ID obtained from the PSTN (e.g., using ANI and DNIS).
2.2. Authorization Requirements
The following requirements are identified in [9] for authorizing PPP
dialin service using roaming.
- Authorization separate from authentication should be allowed when
necessary, but the AAA protocol MUST allow for a single message to
request both authentication and authorization.
- The AAA protocol MUST be "proxyable", meaning that a AAA Server or
PDP MUST be able to forward the request to another AAA Server or
PDP, which may or may not be within the same administrative
domain.
- The AAA protocol MUST allow for intermediate brokers to add their
own local Authorization information to a request or response.
- When a broker is involved, the protocol MUST provide end to end
security.
- The broker MUST be able to return a forwarding address to a
requester, allowing two nodes to communicate together.
- The protocol MUST provide the following features (per user
session):
1. One Authentication, One Authorization
2. One Authentication, Multiple Authorization
3. Multiple Authentication, Multiple Authorization
3. Mobile-IP
The Mobile-IP protocol is used to manage mobility of an IP host
across IP subnets [10]. Recent activity within the Mobile-IP Working
Group has defined the interaction between Mobile-IP and AAA in order
to provide:
- Better scaling of security associations
- Mobility across administrative domain boundaries
- Dynamic assignment of Home Agent
The Mobile IP protocol, as defined in [10], works well when all
mobile nodes belong to the same administrative domain. Some of the
current work within the Mobile IP Working Group is to allow Mobile IP
to scale across administrative domains. This changes the trust model
that is currently defined in [10].
The requirements for Mobile-IP authorization are documented in [11].
In this section, we develop a multi-domain model for Mobile-IP
authorization and present it in the terms of the framework presented
in [2].
Figure 2 depicts the new AAA trust model for Mobile-IP. In this
model each network contains mobile nodes (MN) and a AAA server (AAA).
Each mobility device shares a security association (SA) with the AAA
server within its own home network. This means that none of the
mobility devices initially share a security association. Both
administrative domains' AAA servers can either share a security
association, or can have a security association with an intermediate
broker.
Broker AAA
+--------+
| |
| AAA |
/=====| |=====\
// +--------+ \\
Foreign // SA SA \\ Home
AAA // \\ AAA
+--------+ +--------+
| | SA | |
| AAA |======================| AAA |
| | (in lieu of broker) | |
+--------+ +--------+
|| || ||
SA || SA || || SA
|| || ||
|| || ||
+---------+ +---------+ +---------+
| | | | | |
| FA | | HA | | MN |
| | | | | |
+---------+ +---------+ +---------+
Fig. 2 -- Mobile-IP AAA Trust Model
Figure 3 provides an example of a Mobile-IP network that includes
AAA. In the integrated Mobile-IP/AAA Network, it is assumed that each
mobility agent shares a security association between itself and its
local AAA server. Further, the Home and Foreign AAA servers both
share a security association with the broker's AAA server. Lastly,
it is assumed that each mobile node shares a trust relationship with
its home AAA Server.
Visited Access Broker Home IP
Provider Network Network Network
+--------+ +--------+ +--------+
| | | | | |
| AAA |------| AAA |------| AAA |
| | | | | |
+--------+ +--------+ +--------+
| |
| |
AAA | | AAA
| |
| |
+---------+ +---------+
| | | |
| FA | | HA |
| | | |
+---------+ +---------+
|
| Visited Access Home Network
| Provider Network -Private Network
Mobile | -Home Provider
IP | -Home ISP
|
+--------+
| Mobile |
| Node |
+--------+
Fig. 3 -- General Wireless IP Architecture for Mobile-IP AAA
In this example, a Mobile Node appears within a foreign network and
issues a registration to the Foreign Agent. Since the Foreign Agent
does not share any security association with the Home Agent, it sends
a AAA request to its local AAA server, which includes the
authentication information and the Mobile-IP registration request.
The Mobile Node cannot communicate directly with the home AAA Server
for two reasons:
- It does not have access to the network. The registration
request is sent by the Mobile Node to request access to the
network.
- The Mobile Node may not have an IP address, and may be
requesting that one be assigned to it by its home provider.
The Foreign AAA Server will determine whether the request can be
satisfied locally through the use of the Network Access Identifier
[7] provided by the Mobile Node. The NAI has the format of
user@realm and the AAA Server uses the realm portion of the NAI to
identify the Mobile Node's home AAA Server. If the Foreign AAA Server
does not share any security association with the Mobile Node's home
AAA Server, it may forward the request to its broker. If the broker
has a relationship with the home network, it can forward the request,
otherwise a failed response is sent back to the Foreign AAA Server.
When the home AAA Server receives the AAA Request, it authenticates
the user and begins the authorization phase. The authorization phase
includes the generation of:
- Dynamic Session Keys to be distributed among all Mobility
Agents
- Optional Dynamic assignment of a Home Agent
- Optional Dynamic assignment of a Home Address (note this could
be done by the Home Agent).
- Optional Assignment of QOS parameters for the Mobile Node [12]
Once authorization is complete, the home AAA Server issues an
unsolicited AAA request to the Home Agent, which includes the
information in the original AAA request as well as the authorization
information generated by the home AAA server. The Home Agent
retrieves the Registration Request from the AAA request and processes
it, then generates a Registration Reply that is sent back to the home
AAA server in a AAA response. The message is forwarded through the
broker back to the Foreign AAA server, and finally to the Foreign
Agent.
The AAA servers maintain session state information based on the
authorization information. If a Mobile Node moves to another Foreign
Agent within the foreign domain, a request to the foreign AAA server
can immediately be done in order to immediately return the keys that
were issued to the previous Foreign Agent. This minimizes an
additional round trip through the internet when micro mobility is
involved, and enables smooth hand-off.
3.1. Relationship to the Framework
Mobile-IP uses the roaming pull model described in [2]. The Mobile
Node is the User. The Foreign Network is the Service Provider with
the Foreign Agent as the Service Equipment. The Home Network is the
User Home Organization. Note that the User Home Organization
operates not only a AAA Server, but also the Home Agent. Note, also,
that a broker has been inserted between the Service Provider and the
User Home Organization.
3.2. Minimized Internet Traversal
Although it would have been possible for the AAA interactions to be
performed for basic authentication and authorization, and the
Registration flow to be sent directly to the Home Agent from the
Foreign Agent, one of the key Mobile-IP AAA requirements is to
minimize Internet Traversals. Including the Registration Request and
Replies in the AAA messages allows for a single traversal to
authenticate the user, perform authorization and process the
Registration Request. This streamlined approach is required in order
to minimize the latency involved in getting wireless (cellular)
devices access to the network. New registrations should not increase
the connect time more than what the current cellular networks
provide.
3.3. Key Distribution
In order to allow the scaling of wireless data access across
administrative domains, it is necessary to minimize the security
associations required. This means that each Foreign Agent does not
share a security association with each Home Agent on the Internet.
The Mobility Agents share a security association with their local AAA
server, which in turn shares a security association with other AAA
servers. Again, the use of brokers, as defined by the Roaming
Operations (roamops) Working Group, allows such services to scale by
allowing the number of relationships established by the providers to
be reduced.
After a Mobile Node is authenticated, the authorization phase
includes the generation of Sessions Keys. Specifically, three keys
are generated:
- k1 - Key to be shared between the Mobile Node and the Home
Agent
- k2 - Key to be shared between the Mobile Node and the Foreign
Agent
- k3 - Key to be shared between the Foreign Agent and the Home
Agent
Each Key is propagated to each mobility device through the AAA
protocol (for the Foreign and Home Agent) and via Mobile-IP for the
Mobile Node (since the Mobile Node does not interface directly with
the AAA servers).
Figure 4 depicts the new security associations used for Mobile-IP
message integrity using the keys derived by the AAA server.
+--------+ +--------+
| | k3 | |
| FA |======================| HA |
| | | |
+--------+ +--------+
\\ //
\\ k2 k1 //
\\ +--------+ //
\\ | | //
\=====| MN |=====/
| |
+--------+
Fig. 4 -- Security Association after Key Distribution
Once the session keys have been established and propagated, the
mobility devices can exchange registration information directly
without the need of the AAA infrastructure. However the session keys
have a lifetime, after which the AAA infrastructure must be used in
order to acquire new session keys.
3.4. Mobile-IP Authorization Requirements
To summarize, Mobile-IP has the following authorization requirements:
1. Mobile-IP requires an AAA protocol that makes use of the pull
model.
2. Mobile-IP requires broker support, and data objects must contain
data integrity and confidentiality end-to-end. This means that
neither the broker nor any other intermediate AAA node should be
able to decrypt the data objects, but they must be able to verify
the objects' validity.
3. Authorization includes Resource Management. This allows the AAA
servers to maintain a snapshot of a mobile node's current
location, keying information, etc.
4. Due to the nature of the service being offered, it is imperative
that the AAA transaction add minimal latency to the connect time.
Ideally, the AAA protocol should allow for a single round trip for
authentication and authorization.
5. If the AAA protocol allows for the Mobile-IP registration messages
to be embedded within the authentication/authorization request,
this will further reduce the number of round trips required and
hence reduce the connect time.
6. It must be possible to pass Mobile-IP specific key management data
along with the authorization data. This allows the AAA server to
act as a Key Distribution Center (KDC).
7. It must be possible to pass other application-specific data units
such as home agent selection and home address assignment to be
carried along with the authorization data units.
8. The authorization response should allow for diffserv (QOS)
profiles, which can be used by the mobility agents to provide some
quality of service to the mobile node.
9. The AAA protocol must allow for unsolicited messages to be sent to
a "client", such as the AAA client running on the Home Agent.
4. Bandwidth Broker
This section describes authorization aspects derived from the
Bandwidth Broker architecture as discussed within the Internet2 Qbone
BB Advisory Council. We use authorization model concepts to identify
contract relationships and trust relationships, and we present
possible message exchanges. We will derive a set of authorization
requirements for Bandwidth Brokers from our architectural model. The
Internet 2 Qbone BB Advisory Council researches a single and multi-
domain implementation based on 2-tier authorization concepts. A 3-
tier model is considered as a future work item and therefore not part
of this description. Information concerning the Internet 2 Bandwidth
Broker work and its concepts can be found at:
http://www.merit.edu/working.groups/i2-qbone-bb
The material in this section is based on [13] which is a work in
progress of the Internet2 Qbone BB Advisory Council.
4.1. Model Description
The establishment of a model involves four steps:
1. identification of the components that are involved and what they
are called in this specific environment,
2. identification of the relationships between the involved parties
that are based on some form of agreement,
3. identification of the relationships that are based on trust, and
4. consideration of the sequence of messages exchanged between
components.
4.2. Components of the Two-Tier Model for Bandwidth Brokerage
We will consider the components of a bandwidth broker transaction in
the context of the conceptual entities defined in [2]. The bandwidth
broker two-tier model recognizes a User and the Service Provider
controlling the Service Equipment.
The components are as follows:
- The Service User (User) -- A person or process willing to use
certain level of QoS by requesting the allocation of a
quantifiable amount of resource between a selected destination and
itself. In bandwidth broker terms, the User is called a Service
User, capable of generating a Resource Allocation Request (RAR).
- The Bandwidth Broker (Service Provider) -- a function that
authorizes allocation of a specified amount of bandwidth resource
between an identified source and destination based on a set of
policies. In this context we refer to this function as the
Bandwidth Broker. A Bandwidth Broker is capable of managing the
resource availability within a network domain it controls.
Note: a 3-tier model involving a User Home Organization is recognized
in [13], however its development is left for future study and
therefore it is not discussed in this document.
4.3. Identification of Contractual Relationships
Authorizations to obtain bandwidth are based on contractual
relationships. In both the single and multi-domain cases, the current
Bandwidth Broker model assumes that a User always has a contractual
relationship with the service domain to which it is connected.
4.3.1. Single-Domain Case
In the single-domain case, the User has a contract with a single
Service Provider in a single service domain.
+-------------+
| |
| +---------+ |
| |Bandwidth| |
+-------+ | |Broker | |
| | | | | |
|Service| | +---------+ |
|User |=========| |
| | | +---------+ |
| | | | Network | |
+-------+ | | Routing | |
| | Devices | |
| +---------+ |
| Autonomous |
| Service |
| Domain |
+-------------+
==== contractual
relationship
Fig. 5 -- Two-Tier Single Domain Contractual Relationships
4.3.2. Multi-Domain Case
In the multi-domain case, the User has a contract with a single
Service Provider. This Service Provider has a contract with
neighboring Service Providers. This model is used when independent
autonomous networks establish contracts with each other.
+-------------+ +-------------+
| | | |
| +---------+ | | +---------+ |
| |Bandwidth| | | |Bandwidth| |
+-------+ | |Broker | | | |Broker | |
| | | | | | | | | |
|Service| | +---------+ | | +---------+ |
|User |=========| |========| |
| | | +---------+ | | +---------+ |
| | | | Network | | | | Network | |
+-------+ | | Routing | | | | Routing | |
| | Devices | | | | Devices | |
| +---------+ | | +---------+ |
| Autonomous | | Autonomous |
| Service | | Service |
| Domain A | | Domain B |
+-------------+ +-------------+
==== contractual
relationship
Fig. 6 -- Two-Tier Multi-Domain Contractual Relationships
4.4. Identification of Trust Relationships
Contractual relationships may be independent of how trust, which is
necessary to facilitate authenticated and possibly secure
communication, is implemented. There are several alternatives in the
Bandwidth Broker environment to create trusted relationships.
Figures 7 and 8 show two alternatives that are options in the two-
tier Bandwidth Broker model.
+-------------+ +-------------+
| | | |
| +---------+ | | +---------+ |
| |Bandwidth| | | |Bandwidth| |
+-------+ | |Broker | | | |Broker | |
| O***********O O************O | |
|Service| | +----O----+ | | +----O----+ |
|User |=========| * |========| * |
| | | +----0----+ | | +----O----+ |
| | | |Network | | | |Network | |
+-------+ | |Routing | | | |Routing | |
| |Devices | | | |Devices | |
| +---------+ | | +---------+ |
| Autonomous | | Autonomous |
| Service | | Service |
| Domain A | | Domain B |
+-------------+ +-------------+
==== contractual relationship
O**O trust relationship
Fig. 7 -- Two-Tier Multi-Domain Trust Relationships, alt 1
+-------------+ +-------------+
| | | |
| +---------+ | | +---------+ |
| |Bandwidth| | | |Bandwidth| |
+-------+ | |Broker | | | |Broker | |
| | | | | | | | | |
|Service| | +----O----+ | | +----O----+ |
|User |=========| * |========| * |
| | | +----O----+ | | +----O----+ |
| O***********O Network O************O Network | |
+-------+ | | Routing | | | | Routing | |
| | Devices | | | | Devices | |
| +---------+ | | +---------+ |
| Autonomous | | Autonomous |
| Service | | Service |
| Domain A | | Domain B |
+-------------+ +-------------+
==== contractual relationship
O**O trust relationship
Fig. 8 -- Two-Tier Multi-Domain Trust Relationships, alt 2
Although [13] does not recommend specifics regarding this question,
the document recognizes the need for trust relationships. In the
first model, a trust relationship, based on some form of
authentication method, is created between the User and the Bandwidth
Broker and among Bandwidth Brokers. In the second model, which
enjoys some popularity in enterprise networks, the trust relationship
may be established via the wiring closet and the knowledge of which
physical router port or MAC address is connected to which user. The
router-Bandwidth Broker relationship may be established physically or
by some other authentication method or secure channel.
A Certificate Authority (CA) based trust relationship is shown in
figure 9. In this figure, a CA signs public key certificates, which
then can be used in encrypted message exchanges using public keys
that are trusted by all involved. As a first step, each involved
party must register with the CA so it can join a trust domain. The
Router-Bandwidth Broker relationship may be established as described
in the two previous figures. An interesting observation regarding
this kind of model is that the bandwidth broker in domain B may route
information to the user via the bandwidth broker in domain A without
BB1 being able to read the information (using end-to-end security).
This model creates a meshed trust relationship via a tree like CA
structure.
+-------------------+
| Certificate |
....................| Authority |
: ..| |..
: : +-------------------+ :
: : :
: : :
: ***************:*********************** :
: * +---:---------+ +---*--:------+
: * | : | | * : |
: * | +-:-------+ | | +-O--:----+ |
: * | |{C} | | | | {C} | |
+---:--O+ | |Bandwidth| | | |Bandwidth| |
| {C} O***********O Broker O************O Broker | |
|Service| | +----O----+ | | +----O----+ |
|User |=========| * |========| * |
| | | +----0----+ | | +----O----+ |
| | | |Network | | | |Network | |
+-------+ | |Routing | | | |Routing | |
| |Devices | | | |Devices | |
| +---------+ | | +---------+ |
| Autonomous | | Autonomous |
| Service | | Service |
| Domain A | | Domain B |
+-------------+ +-------------+
==== contractual relationship
O**O trust relationship
{C}. certification process
Fig. 9 -- Two-Tier Multi-Domain Trust Relationships, alt 3
4.5. Communication Models and Trust Relationships
When describing the Bandwidth Broker communication model, it is
important to recognize that trust relationships between components
must ensure secure and authenticated communication between the
involved components. As the Internet 2 Qbone Bandwidth Broker work
does not recommend any particular trust relationship model, we make
the same assumptions as [13]. In theory, the trust model and
communication model can be independent, however communication
efficiency will determine the most logical approach.
4.6. Bandwidth Broker Communication Models
4.6.1. Concepts
The current Internet 2 Qbone Bandwidth Broker discussion describes a
two-tier model, where a Bandwidth Broker accepts Resource Allocation
Requests (RAR's) from users belonging to its domain or RAR's
generated by upstream Bandwidth Brokers from adjacent domains. Each
Bandwidth Broker will manage one service domain and subsequently
provide authorization based on a policy that decides whether a
request can be honored.
4.6.1.1. Intra-Domain Authorization
Admission Authorization or Connection Admission Control (CAC) for
intra-domain communication is performed using whatever method is
appropriate for determining availability of resources within the
domain. Generally a Bandwidth Broker configures its service domain to
certain levels of service. RAR's are subsequently accommodated using
a policy-based decision.
4.6.1.2. Inter-Domain Authorization
Service Level Specifications (SLS's) provide the basis for handling
inter-domain bandwidth authorization requests. A Bandwidth Broker
monitors both the state of its network components and the state of
its connections to neighboring networks. SLS's are translations of
SLA's established between Autonomous Service Domains. Each Bandwidth
Broker will initialize itself so it is aware of existing SLS's.
SLS's are established in a unidirectional sense. Two SLS's must
govern a bi-directional connection. SLS's are established on the
level of aggregate data-flows and the resources (bandwidth)
provisioned for these flows.
A Bandwidth Broker may honor an inter-domain RAR by applying policy
decisions determining that a particular RAR does fit into a pre-
established SLS. If successful, the Bandwidth Broker will authorize
the usage of the bandwidth. If unsuccessful, the Bandwidth Broker
may deny the request or approve the request after it has re-
negotiated the SLS with its downstream Bandwidth Broker.
A separate Policy Manager may be involved in the CAC decision. The
Internet 2 Qbone Bandwidth Broker discussion recognizes an ideal
environment where Bandwidth Brokers and Policy Managers work together
to provide CAC using integrated policy services [13].
4.6.2. Bandwidth Broker Work Phases
The Internet 2 Qbone Bandwidth Broker discussion proposes development
of the Bandwidth Broker model in several phases:
- Phase 0: Local Admission. RAR's are only handled within a local
domain. SLS's are pre-established using manual methods (fax, e-
mail).
- Phase 1: Informed Admission. RAR's spanning multiple domains are
authorized based on information obtained from one or more
Bandwidth Brokers along the path.
- Phase 2: Dynamic SLS admission. Bandwidth Brokers can dynamically
set up new SLS's.
Although the local admission case is addressed, the current Internet
2 Qbone Bandwidth Broker work is currently concerned with solving
multi-domain problems in order to allow individual Bandwidth Brokers
to inter-operate as identified in phase 0 or 1.
4.6.3. Inter-Domain Signaling
4.6.3.1. Phase 0
In phase 0 implementations, no electronic signaling between Bandwidth
Brokers is performed and SLS negotiation will be performed manually
(phone, email etc) by network operators. An RAR is only handled
within the domain and may originate from a User or ingress router.
4.6.3.2. Phase 1
Here a CAC decision is made on information obtained from downstream
Bandwidth Brokers. This information could come from the next hop
Bandwidth Broker or all Bandwidth Brokers downstream to the
destination.
Two fundamental signaling approaches between Bandwidth Brokers have
been identified for the Informed Admission case. These are
illustrated in figure 10.
+-------+ +-------+ +-------+ +-------+
| | | | | | | |
| |RAR | | 1 | | 2 | |
| User |-------->| |-------->| |-------->| |
| | RAA | BB1 | 4 | BB2 | 3 | BB3 |
| |<--------| |<--------| |<--------| |
| | | | | | | |
| | | | | | | |
+-------+ +-------+ +-------+ +-------+
A)End-to-end signaling
+-------+ +-------+ +-------+ +-------+
| | | | | | | |
| |RAR | | 1 | | 3 | |
| User |-------->| |-------->| |-------->| |
| | RAA | BB1 | 2 | BB2 | 4 | BB3 |
| |<--------| |<--------| |<--------| |
| | 7 | | 6 | | 5 | |
| |<--------| |<--------| |<--------| |
+-------+ +-------+ +-------+ +-------+
B) Immediate response signaling.
Fig. 10 -- Fundamental Signalling Approaches
- End to End signaling. An RAR from a User to BB1 is forwarded to
BB2 (1). BB2 will forward the request to BB3 (2). If BB3 is the
destination of the request, BB3 will authorize the request and
reply to BB2 (3). BB2 will then reply to BB1 (4), and BB1 will
send a Resource Allocation Answer (RAA) back to the User to
complete the authorization.
- Immediate response signaling. This is the case where BB1 will
want to authorize an RAR from its domain and forwards the
authorization request to BB2 (1). If BB2 approves, the response
is immediately returned to BB1 (2). BB1 will send an RAA back to
the User. If the authorization was positive BB2 will forward
subsequently a request to the next BB, BB3 (3). BB3 authorizes
the request and responds to BB2 (4). If the response is negative
(5), BB2 will cancel the authorization it previously issued to BB1
(6) and this will result in a cancellation from BB1 to the user
(7). In this case the RAA authorization is valid until revoked by
7.
4.6.4. Bandwidth Broker Communication Architecture
Figure 11 shows components of the discussed Bandwidth Broker
architecture with its interfaces.
- An intra-domain interface allows communication with all the
service components within the network that the Bandwidth Broker
controls.
- An inter-domain interface allows communication between Bandwidth
Brokers of different autonomous networks.
- A user/application interface allows the Bandwidth Broker to be
managed manually. Requests can be sent from the User or a host
application.
- A policy manager interface allows implementation of complex policy
management or admission control.
- A routing table interface allows the Bandwidth Broker to
understand the network topology.
- An NMS interface allows coordination of network provisioning and
monitoring.
adjacent BB <---------------------------> adjacent BB
|
V
+------------------------------+
| | inter-domain | |
| -------------- ------|
application | | PM |
server \ | |iface |
\ |------- ---------+ ------|
->| user/ | | simple | ------|
user/host-->| app | | policy | | NMS |
->| iface | | services| |iface |
/ |------- ---------+ ------|
network / | |
operator | ------- ------- |
| | data | |routing| |
| | store | |info | |
| | | | | |
| ------- ------- |
| |
| ---------------- |
| | intra-domain | |
+------------------------------+
^
|
edge router(s) <---------------------------> edge router(s)
Fig. 11 -- Bandwidth Broker Architecture
4.6.5. Two-Tier Inter-Domain Bandwidth Broker Communication Model
4.6.5.1. Session Initialization
Before Bandwidth Brokers can configure services between two adjacent
domains, they have to establish and initialize a relationship. No
authentication is used; therefore any trust relationship is implicit.
Part of the initialization is an exchange of topology information
(list of adjacent Bandwidth Brokers).
4.6.5.2. Service Setup
The Bandwidth Broker must first be configured in regard to agreed
bi-lateral service levels. All resources allocated to a particular
level of provisioned service must be reserved in each domain.
A Service Setup Request (SSR) is generated (on demand by the
operator or at startup of the system) and forwarded to a downstream
Bandwidth Broker. The downstream Bandwidth Broker will check the
consistency with its own service level specifications and respond
with Setup Answer message (SA) agreements. This message exchange
confirms and identifies pre-established service authorization levels.
4.6.5.3. Service Cancellation
A Service Cancellation (SC) message may cancel a service
authorization. This message may be initiated by the operator or by an
expiration date. A Cancellation Answer (CA) is returned.
4.6.5.4. Service Renegotiation
An (optional) Service-Renegotiation message (SR) may allow a
Bandwidth Broker to re-negotiate an existing service. This message
may be initiated by the operator or automatically when a certain
threshold is reached. Renegotiations happen within the margins of a
pre-established authorization.
4.6.5.5. Resource Allocation Request and Resource Allocation Answer
An RAR allocates a requested level of service on behalf of the User
and when available it will decide on the admittance of a certain User
to the service. A Bandwidth Broker may receive an RAR via either the
intra-domain or inter-domain interface. The RAR must refer to the
Service SetUp Identification (SSU_ID), which binds a request to a
certain authorization. A Resource Allocation Answer (RAA) confirms or
rejects a request or it may indicate an "in progress" state.
4.6.5.6. Session Maintenance
A certain level of session maintenance is required to keep Bandwidth
Brokers aware of each other. This must be implemented using time-
outs and keep-alive messages. This will help Bandwidth Brokers to
notice when other Bandwidth Brokers disappear.
4.6.5.7. Intra-domain Interface Protocol
The Intra-domain interface protocol used between a Bandwidth Broker
and the routers it controls may be COPS, SNMP, or Telnet Command Line
Interface.
4.7. Requirements
From the above descriptions we derive the following requirements.
- The Authorization mechanism may require trust relationships to be
established before any requests can be made from the User to the
Service Provider. Currently trust relationship establishment is
implicit.
- A confirmation of authorization is required in order to initialize
the system.
- A negation of static authorization is required to shut down
certain services.
- A renegotiation of static authorization is required to alter
services (SLS's).
- Dynamic authorization requests (RAR) must fit into pre-established
static authorizations (SLS's).
- Dynamic authorization requests (RAR) may be answered by an "in
progress state" answer.
- Provisions must be made to allow reconstruction of authorization
states after a Bandwidth Broker re-initializes.
5. Internet Printing
The Internet Printing Protocol, IPP [14], has some potentially
complex authorization requirements, in particular with the "print-
by-reference" model. The following attempts to describe some
possible ways in which an authorization solution for this aspect of
IPP might work, and to relate these to the framework described in
[2]. This is not a product of the IPP working group, and is meant
only to illustrate some issues in authorization in order to establish
requirements for a "generic" protocol to support AAA functions across
many applications.
IPP print-by-reference allows a user to request a print service to
print a particular file. The user creates a request to print a
particular file on a printer (or one of a group of printers). The
key aspect is that the request includes only the file name and not
the file content. The print service must then read the file from a
file server prior to printing. Both the file server and the print
server must authorize the request. Once initiated, printing will be
done without intervention of the user; i.e., the file will be sent
directly to the print service rather than through the user to the
printer.
5.1. Trust Relationships
The assumption is that the Printer and File Server may be owned and
operated by different organizations. There appear to be two models
for how "agreements" can be set up.
1. User has agreement with Print Server; Print Server has agreement
with File Server.
2. User has agreements with both File and Print Server directly.
In case 1, the user has a trust relationship with the Print Service
AAA Server. The Printer forwards the request to the File Server. The
File Server authorizes the Printer and determines if the Printer is
allowed access to the file. Note that while there may be some cases
where a Print Server may on its own be allowed access to files
(perhaps some "public files", or that can only be printed on certain
"secure" printers), it is normally the case that files are associated
with users and not with printers. This is not a good "generic" model
as it tends to make the print service an attractive point of attack.
+------+ +----------------------+
| | | File Service |----+
| | | AAA Server |<-+ |
| | +----------------------+ | |
| | | | | |
| | | File Server | | |
| | | | | |
| User | +----------------------+ | |
| | | |
| | | |
| | | |
| | +----------------------+ | |
| |------>| Print Service |--+ |
| |<------| AAA Server |<---+
| | +----------------------+
| | | Print Server |
| | | and Printer |
+------+ +----------------------+
Fig. 12 -- Case 1
User authorizes with Print Service.
Printer authorizes with File Service.
In case 2, the user must have a trust relationship with both the file
and print services so that each can verify the service appropriate to
the User. In this case, the User first contacts the File Service AAA
Server and requests that it enable authorization for the Print
Service to access the file. This might be done in various ways, for
example the File Service AAA Server may return a token to the User
which can (via the Print Service) be presented to the File Server to
enable access.
+------+ +----------------------+
| |------>| File Service |
| |<------| AAA Server |
| | +----------------------+
| |
| | +----------------------+
| | | File Server |
| User | +----------------------+
| | /|\ |
| | | |
| | | \|/
| | +----------------------+
| |------>| Print Service |
| |<------| AAA Server |
| | +----------------------+
| | | Print Server |
| | | and Printer |
+------+ +----------------------+
Fig. 13 -- Case 2
User authorizes File and Print Service.
Must create binding for session between
Print Service and File Service.
5.2. Use of Attribute Certificates in Print-by-Reference
The print-by-reference case provides a good example of the use of
attribute certificates as discussed in [2]. If we describe case 2
above in terms of attribute certificates (ACs) we get the diagram
shown in figure 14.
+------+ +----------------------+
| |------>| File Service |
| |<------| AAA Server |
| |Get AC +----------------------+
| |
| | +----------------------+
| | | File Server |----+
| | | |<-+ |
| User | +----------------------+ | |
| | | |
| | +---authorize passing AC | |<---Create session
| | | | | Using AC
| | V +----------------------+ | |
| |------>| Print Service | | |
| |<------| AAA Server | | |
| | +----------------------+ | |
| | | Print Server |--+ |
| | | and Printer |<---+
+------+ +----------------------+
Fig. 14 -- Using Attribute Certificates in IPP Authorization
In this case, the User gets an AC from the File Service's AAA Server
which is signed by the File Service AAA Server and contains a set of
attributes describing what the holder of the AC is allowed to do. The
User then authorizes with the Print Service AAA Server and passes the
AC in the authorization request. The Printer establishes a session
with the File Server, passing it the AC. The File Server trusts the
AC because it is signed by the File Service AAA Server and allows (or
disallows) the session.
It is interesting to note that an AC could also be created and signed
by the User, and passed from the Print Server to the File Server. The
File Server would need to be able to recognize the User's signature.
Yet another possibility is that the Print Service AAA Server could
simply authenticate the User and then request an AC from the File
Service AAA Server.
5.3. IPP and the Authorization Descriptive Model
The descriptive model presented in [2] includes four basic elements:
User, User Home Organization, Service Provider AAA Server, and
Service Equipment.
Mapping these to IPP, the User is the same, the User Home
Organization (if included) is the same. The Service Provider AAA
Server and the Service Equipment are expected to be closely coupled
on the same processor. In other words, the interface between the
Print Service AAA Server and the Printer as well as that between the
File Service AAA Server and the File Server is an internal one that
will not require a formal protocol (although some standard API might
be useful).
The concept of a Resource Manager (see [2]) has some interesting
twists relative to IPP. Once started, the user is not involved in
the service, but until printing is complete it seems useful that any
of the parties in the authorization process be allowed to query for
status or to cancel the print session. The user needs a way to
"bind" to a particular session, and may have to reauthorize to be
allowed to access Resource Manager information.
6. Electronic Commerce
This section describes the authorization aspects of an e-commerce
architecture typically used in Europe. We will use this model to
identify contractual and trust relationships and message exchanges.
We will then identify a set of authorization requirements for e-
commerce.
Whereas most e-commerce protocols focus on authentication and message
integrity, e-commerce exchanges as described by the Internet Open
Trading Protocol (trade) Working Group in [15] also involve
authorization. This section will examine one e-commerce protocol
called SET (Secure Electronic Transaction) that provides for credit
and debit card payments. We will analyze the authorization aspects
from an architectural viewpoint. We will apply concepts and terms
defined in [2].
We are not here proposing SET as a standard authorization protocol.
Rather, we are examining the SET model as a way of understanding the
e-commerce problem domain so that we can derive requirements that an
authorization protocol would have to meet in order to be used in that
domain.
E-commerce protocols and mechanisms such as those described in [16]
may not only be important to allow customers to shop safely in
Cyberspace, but may also be important for purchases of Internet
services as well. With emerging technologies allowing Internet
transport services to be differentiated, an inherently more complex
pricing model will be required as well as additional payment methods.
Flexible authorization of services will be an important aspect to
allow, for example, globally roaming users ad hoc allocation of
premium bandwidth with an ISP who is authorized to accept certain
credit card brands.
6.1. Model Description
The establishment of a model involves four steps:
1. identification of the components that are involved and what they
are called in this specific environment,
2. identification of the relationships between the involved parties
that are based on some form of agreement,
3. identification of the relationships that are based on trust, and
4. consideration of the sequence of messages exchanged between
components.
6.1.1. Identification of Components
We will consider the components of an electronic commerce transaction
in the context of the conceptual entities defined in [2].
- The Cardholder (User) -- the person or organization that is to
receive and pay for the goods or services after a request to
purchase has been received. In SET terms this is called a
Cardholder.
- The Issuer (User Home Organization) -- the financial organization
that guarantees to pay for authorized transactions to purchase
goods or services on behalf of the User when using a debit or
credit card it issues. The financial organization (typically a
bank or Brand Organization) will transfer money from the user
account to the account the party to which the User instructs it to
send the payment. The issued card authorizes the User to use the
card for payments to merchants who are authorized to accept the
card. In SET terms this organization is called the Issuer. This
organization is considered "home" to the Cardholder.
- The Merchant (Service Provider) -- the organization from whom the
purchase is being made and who is legally responsible for
providing the goods or services and receives the benefit of the
payment made. In SET terms this organization is called a
Merchant. The Cardholder is considered to be "foreign" to the
Merchant.
- The Acquirer (Broker) -- the organization that processes credit or
debit card transactions. Although in reality this function may be
rather complex and may span several organizations, we will simply
assume this organization to be a Brand Organization fulfilling the
role of the Acquirer as defined in SET. The Acquirer establishes
an account with the Merchant. The Acquirer operates a Payment
Gateway that will accept payment authorization requests from
authorized merchants and provide responses from the issuer. The
Acquirer will forward an authorization request to the Issuer. The
Acquirer is considered "home" to the Merchant.
As the SET document [16] notes, a Brand Organization (credit card
organization) may handle both the Issuer function and Acquirer
function that operates a Payment Gateway. For simplicity, we
therefore assume that the authorization role of Broker (Acquirer) and
User Home Organization (Issuer) both belong to the Brand
Organization.
In order to be more descriptive we now use the SET terms. In the
requirements section these terms are mapped back into the
authorization framework terms again.
6.1.2. Identification of Contractual Relationships
Contractual relationships are illustrated in figure 15, below.
- The Cardholder has a contractual relationship with the card
Issuer. The Cardholder holds an account with the Issuer and
obtains an account number.
- The Merchant has a contractual relationship with the Acquirer.
The Merchant obtains a Merchant ID from the Acquirer.
- In the real world there may be no direct contractual relationship
between the Issuer and the Acquirer. The contractual
relationships allowing an Acquirer to relay a payment
authorization request to an Issuer may be very complex and
distributed over multiple organizations. For simplicity, however,
we assume there are contracts in place allowing an Acquirer to
request payment authorization from an Issuer. These contracts are
facilitated by the Brand Organization. Therefore, in our
simplified example, the Acquirer and Issuer belong to the same
Brand Organization. The Acquirer operates a Payment Gateway for
which it needs a Bank Identification Number (BIN).
+----------------+ +------------------------+
| Issuer | | Acquirer |
| (User Home | | (Broker) |
| Organization) | | +------------------+ |
| |=======| | Payment | |
| | | | Gateway | |
| | | +------------------+ |
| | | |
+----------------+ +------------------------+
|| ||
|| ||
|| ||
+----------------+ +--------------------+
| Cardholder | | Merchant |
| (User) | | (Service Provider) |---+
| | | | |
| | | | |
| | +--------------------+ |
| | | |
| | | Fulfillment |
| | | |
+----------------+ +----------------------+
Fig. 15 -- SET Contractual Relationships
6.1.3. Identification of Trust Relationships
It is important to recognize that there are two kinds of trust
relationships: static and dynamic trust relationships. Static trust
relationships in SET are established by means of a registration
process that will request a certificate to be issued to the party
that needs to be trusted and authorized to be part of a SET
transaction. Dynamic trust is created at the time of a payment
transaction and its subsequent authorization request. Note that at
the issue phase of a certificate, based on identification and
registration, the user of the certificate gets an implicit static
authorization and a means of authenticating and securing messages.
For this purpose a Certificate Authority (CA) will issue certificates
that are used to sign and/or encrypt messages exchanged according to
the SET protocol.
6.1.3.1. Static Trust Relationships
In the discussion that follows, refer to figure 16, below.
+-------+
| Root |
| CA |
+-------+ CA = Certificate Authority
| {C} = Certificate
|
+-----------------+
| Brand |
| CA |
+-----------------+
| | |
| | +-------+
| | |Payment|
+----------------+ | | |Gateway| +----------------------+
| Issuer | | | | CA | | Acquirer |
| (User Home | +----------+ | +-------+ | (Broker) |
| Organization) | |Cardholder| | | | +----------------+ |
| | | CA | | +------+--+-{C} Payment | |
| | +----------+ | 3 | | Gateway | |
| | | | | +----------------+ |
| | | +---------+ | |
+----------------+ | | Merchant| +----------------------+
| | CA |
| +---------+
| |
+----------------+ | | +--------------------+
| Cardholder | | | | Merchant |
| (User) | | | | (Service Provider) |--+
| {C}-+-----+ | | | |
| | 1 +-----------+-{C} | |
| | 2 | | |
| | | | |
| | +--------------------+ |
| | | |
| | | Fulfillment |
| | | |
+----------------+ +---------------------+
Fig. 16 -- SET Trust Relationships within a Brand Domain
- The Brand Organization operates a Brand CA and is therefore the
holder of the common trust within the described domain. All
involved parties (Cardholder, Issuer, Merchant and Acquirer) are
members of the same trust domain. We will identify three separate
CA's which issue a certificate on behalf of the Issuer, the
Acquirer and the Brand Organization. The Brand CA, according to a
tree like hierarchy, certifies all underlying CA's. The Brand CA
obtains its trust from a single Root Certificate Authority.
Before any party can obtain a Certificate from a CA, the party
must have some form of contractual relationship.
- After an account has been established with the Issuer, the
Cardholder has to register with a Cardholder CA (CCA) through a
series of registration steps (1) as defined in the SET protocol.
If the CCA approves the registration, the Cardholder will obtain a
Cardholder Certificate. The CCA may be operated by the Brand
Organization on behalf of the Issuer. The Cardholder Certificate
is an electronic representation of the payment card. This process
creates a trust relationship between the Cardholder and the Brand.
After the cardholder has received the Cardholder Certificate, the
Cardholder is authorized to perform payments to an authorized
Merchant.
- After the Merchant has obtained a Merchant ID from the Acquirer,
the Merchant has to register with the Merchant CA (MCA) through a
series of registration steps (2) as defined in the SET protocol.
If the MCA approves the registration, the Merchant will obtain a
Merchant Certificate. This process creates a trust relationship
between the Merchant and the Brand. The MCA may be operated by
the Brand Organization on behalf of the Acquirer. After
registration, the Merchant is authorized to accept payment
requests from Cardholders and to send authorization requests to
the Acquirer's Payment Gateway.
- After the Acquirer has obtained a valid Bank Identification Number
(BIN), the Acquirer must register with the Payment Gateway CA
(PCA) in order to obtain a Payment Gateway Certificate (3). The
Payment Gateway Certificate authorizes the Gateway to accept
payment authorization requests originating from Merchants within
its trust domain.
- The Acquirer and Issuer have a trust relationship via the Brand
Organization. The trust relationship is not ensured by procedures
or a mechanism defined by SET, as this is a problem solved by
agreements between financial organizations facilitating the
payment service. Again, for simplicity, we assume that the
relationship ensures that payment authorization requests received
by the Acquirer's gateway will be forwarded in a secure and
efficient way to the Issuer and its response is handled in the
same way.
6.1.3.2. Dynamic Trust Relationships
Note that there is no prior established static trust relationship
between the Cardholder and the Merchant, as a Cardholder does not
have to register with a Merchant or vice versa. The trust
relationship is dynamically created during the communication process
and is based on the common relationship with the Brand. By means of
digital signatures using public key cryptography, the Cardholder's
software is able to verify that the Merchant is authorized to accept
the Brand Organization's credit card. The merchant is able to verify
that the Cardholder has been authorized to use the Brand
Organization's credit card.
6.1.4. Communication Model
The purchase request from Cardholder to Merchant and subsequent
payment authorization exchange between Merchant and Acquirer is
illustrated in figure 17 and described below.
+----------------+ +------------------------+
| Issuer | | Acquirer |
| (User Home | | (Broker) |
| Organization) | | +------------------+ |
| |<------+--| Payment | |
| | 5 | | Gateway | |
| |-------+->| | |
| | 6 | +------------------+ |
| | | /|\ | |
+----------------+ +---------+---+----------+
|4 |7
| \|/
+----------------+ +--------------------+
| Cardholder | | Merchant |
| (User) | | (Service Provider) |---+
| |------>| | |
| | 1 | | |
| |<------| | |
| | 2 | | |
| |------>| | |
| | 3 | | |
| |<------| | |
| | 8 | | |
| | | | | |
| | +-----------------+--+ |
| | | |9 |
| |<--------| Fulfillment \|/ |
| | 10 | |
+----------------+ +----------------------+
Fig. 17 -- Communication Sequence
1. The Cardholder shops and decides to purchase some goods at
merchant.com. The Cardholder has selected a list of goods and the
Merchant's software has subsequently prepared an order form for
the Cardholder indicating the price, the terms and conditions, and
the accepted payment methods. The SET transaction starts at the
moment the Cardholder indicates that he or she wants to pay for
the goods using a certain payment brand. The Cardholder software
sends a request to the Merchant that initiates the payment
process.
2. The Merchant checks the order and signs it and returns it to the
Cardholder including a certificate from the Acquirer's Gateway
that allows the Cardholder to encrypt payment instructions that
are only relevant to the Gateway and not to the Merchant (e.g.,
the Cardholder's credit card information). The Cardholder also
includes his or her own certificate.
3. The Cardholder now verifies both certificates (the software has
the CA's root certificate). The Cardholder software generates a
message containing the order information and the payment
instructions that is signed by the Cardholder. Using the Gateway
Certificate, it will encrypt the Payment Instruction so that it
will only be readable by the Gateway. The Cardholder will include
his or her certificate.
4. The Merchant verifies the Cardholder certificate and checks the
message integrity. He or she will now process the payment and
issue a payment authorization request to the gateway. The payment
authorization request contains the Cardholder's certificate and
both Merchant certificates.
5. The Gateway verifies the Merchant's signature certificate and that
the Merchant signed the authorization request. Next it will
obtain the account information and payment instructions and will
check the message integrity and the Cardholder's certificate. If
everything is in proper order it will send an authorization
request to the Issuer via a secure bank network.
6. The issuer returns the authorization.
7. The Acquirer's Gateway generates an authorization response which
includes the gateway's certificate.
8. The Merchant checks the authorization response and completes the
process by forwarding a purchase response to the Cardholder.
9. The Merchant software authorizes the delivery of the purchased
goods.
10. The Cardholder receives the purchased goods.
6.2. Multi Domain Model
In the previous "single" domain case we already assume that there are
multiple Cardholders, Merchants, Issuers and Acquirers. However all
these parties belong to a single trust domain as there is only a
single CCA, MCA and PCA. The trust relationship between multiple
cardholders and multiple Issuers go via a single CCA in the same way
as the trust relationship between an Acquirer and a Merchant uses the
same MCA. The multi-domain case arises when there are multiple
domains of CCA's, MCA's and PCA's. In SET these domains reside under
a particular Geopolitical CA (GCA) which is illustrated in figure 18.
+-----------+
| Root CA |
| |
+-----------+
|
|
+----------------------|-------------------------------+
+-----------------------------------------------------+ |
| Brand CA | |
| |-+
+-----------------------------------------------------+
|
|
+----------------------|-------------------------------+
+-----------------------------------------------------+ |
| Geopolitical CA | |
| |-+
+-----------------------------------------------------+
| | |
| | |
+----|--------+ +---|-------+ +-------|----------+
+------------+ | +----------+ | +-----------------+ |
| Cardholder | | | Merchant | | | Payment Gateway | |
| CA |-+ | CA |-+ | CA |-+
+------------+ +----------+ +-----------------+
Fig. 18 -- SET Certificate Management Architecture
A GCA may represent a country or region. The architecture defines a
trust hierarchy needed to manage and verify SET Certificates as these
need to be issued, renewed or revoked. Each geopolitical region may
have different policies for issuing, renewing or revoking
certificates. However once certificates have been issued, Cardholders
and Merchants belonging to different GCA's can still be recognized as
belonging to the same Brand. This will allow a European Cardholder
to purchase goods in the U.S. The U.S. Acquirer's gateway will
recognize that the Cardholder belongs to the same Brand and will
therefore accept a payment authorization request.
6.3. Requirements
Many e-commerce environments do not use SET. Other mechanisms exist
based on SSL, XML, and S/MIME. Also a mechanism that uses SET only
for the payment authorization to the Gateway exists and is known as
half SET. However, using the model described in this document, we
can derive a fairly comprehensive set of protocol requirements for
e-commerce. In these requirements, the SET terms are replaced again
by the descriptive model terms:
Cardholder = User
Merchant = Service Provider
Issuer = User Organization
Acquirer = Broker
1. The Authorization mechanism must allow trust relationships to be
established before any requests can be made from the User to the
Service Provider and from the Service Provider via a Broker to the
User Organization. This process will enable the parties to
communicate securely by creating an authenticated channel and, by
so doing, implicitly authorizing its usage.
2. Upon receipt of any request or response, entities need to be able
to verify whether the transmitting party is still authorized to
send this request or response.
3. The User must be able to authorize the Service Provider to request
an authorization from the User Home Organization.
4. The User must be able to authorize fulfillment of a proposed
service offer from the Service Provider.
Other requirements related to the authorization process:
Integrity
5. For any authorization request or response, the receiving party
needs to verify that the content of the message has not been
altered.
Confidentiality/Privacy
6. The User must be able to pass information relevant to the session
authorization process to the User Home Organization via a Broker
and the Service Provider without allowing the Broker or the
Service Provider to examine its content.
7. The User Home Organization must be able to communicate information
relevant to the session authorization via the Broker and the
Service Provider to the User without allowing the Broker or the
Service Provider to examine its content.
Nonrepudiation
8. There is a need for a recorded, authenticated and authorized
agreement about the request for and delivery of service.
7. Computer Based Education and Distance Learning
This section describes the authorization aspects of computer based
distance learning environments. In this section we will model the
relationships and working practices in a hypothetical university
environment where a student enrolls in courses, attends lectures, and
takes the corresponding exams from remote locations (distance
learning) or via computer equipment (computer based education). When
completed successfully, a student is authorized to enroll in a set of
subsequent courses according to his or her curriculum requirements.
Completion of required courses with passing grades results in
graduation.
Although this section specifically describes an example of a student
taking courses at a faculty (department) of the university, the
resulting requirements should also be valid for other applications in
similar environments, e.g. library loans, electronic abstract and
reprint services, computer and network access, use of copy machines,
budget management, store retrievals, use of coffee machines and
building access.
It is important to recognize that the AAA environment we are
describing also needs to be managed. For example, for an application
such as budget management, it is necessary to delegate budget
authority from a central financial department to budget managers in
education or faculty groups. An AAA environment must allow creation
of policy rules either by certain individuals or by other AAA servers
with authorization to do so.
7.1. Model Description
The establishment of the model involves four steps:
1. identification of the components that are involved and what they
are called in this specific environment,
2. identification of the contractual relationships between the
involved parties,
3. identification of the relationships that are based on trust, and
4. consideration of the sequence of messages exchanged between
components.
7.1.1. Identification of Components
We will consider the components of a distance learning environment in
the context of the conceptual entities defined in [2].
- The Student (User) -- the person enrolling in a course (Service)
and taking the corresponding exam.
- The Educator (Service Equipment) -- the education content server
for which the content is delivered by the Professor.
- The Educator Authorization Module (Service Provider AAA Server).
This module must check at the service access point whether the
student complies with the requirements for enrolling in the
course. The authorization may be based on both local (by the
professor) and remote policies (originating from the faculty).
Rules must allow enough flexibility to prevent students from being
falsely denied access to courses. Strict rules must only be
applied at graduation time.
- The Faculty (Service Provider) -- the organization (department in
U.S. terms) which controls the Service "Equipment" of which the
Educator is one example.
- The Curriculum Commission (Part of User Home Organization) -- body
responsible for creating rules by which a student is allowed to
enroll in a certain course and how this course will count toward
his or her graduation requirements. Students may legally take any
course available at any time, however the Curriculum Commission
will decide whether this course will contribute towards their
graduation. When a Student registers with a certain Educator, the
Educator may check with the Curriculum Commission AAA server
whether the course will count towards graduation and confirm this
with the student.
- The Student Administration (Part of User Home Organization) -- the
administrative organization that authorizes students to enroll in
courses if certain criteria, including financial criteria, are
met. Next to the student, the Student Administration will keep
track of any exam results for the student and will issue a
graduation certificate when all criteria are met.
7.1.2. Identification of Contractual Relationships
Contractual relationships are illustrated in figure 19, below. Based
on contract relationships,specific trust relationships are created as
required.
Although not shown in figure 19, it is assumed that the university
has contractual relationships with the faculties in which every
faculty is allowed and obligated to build, maintain and present one
or more specific studies.
+---------------------------------------------+
| +-----------------------------------------+ |
| | Faculty administration | |
| |+----------------+ +----------------+| |
| |O Student | | Curriculum || |
| *| Administration O*****O Commission || |
|*|| AAA Server | | AAA Server || |
*/|+---O------O-----+ +-----O------O---+| |
*//| * * * * | |
*// +----*---------*-----------*---------*----+ |
*//| * || * * || * |
*// | * || * * || * |
*// | * || * || * |
*// | * || * * || * |
*// | * || * * || * |
*// | +----*---------*--+ +--*---------*----+ |
*// | | * * | | * * |
*// | |+---O------O----+| |+----O------O---+| |
*// | || Educator A || || Educator B || |
*// | || AAA Server || || AAA Server || |
*// | || Service admin.|| || Service admin.|| |
*// | |+---O-----------+| |+-----------O---+| |
*// | | * | | * | |
+-O-------+ | | * | | * | |
| | | |+---O-----------+| |+-----------O---+| |
| Student | | || Educator || || Educator || |
| | | || Course A || || Course B || |
| | | |+---------------+| |+---------------+| |
+---------+ | +-----------------+ +-----------------+ |
| Faculty |
+---------------------------------------------+
// = contractual relationship
** = trust relationship
Fig. 19 -- Contractual relationships - single domain case
As shown in figure 19, the Student has a contractual relationship
with the Faculty. The contract allows the Student to pursue a course
of study consisting of a set of courses. Courses are presented to
the Students by the Educators. A course of study may consist of
courses from different Faculties.
Faculties have contracts among them allowing Students from one
Faculty to enroll in courses from other Faculties.
Faculties instantiate Educators based on a contract between the
Faculty Administration and the professor implementing and managing
the Educator. Authorization is based on policy rules defined by one
or more parties in the contractual relationships. For example, a
professor has a policy to give the course only in the afternoon and
the Faculty has a policy to give the course to their own students and
students from faculty-x but not, when oversubscribed, to faculty-y
students.
7.1.3. Identification of Trust Relationships
Figure 19 illustrates relevant trust relationships which statically
enable AAA entities to communicate certain attributes in our
simplified example. However, in order for the illustrated entities to
work, other trust relationships that are not illustrated must already
be in existence:
- A trust relationship based on a contract between the Faculty and
the university enables a faculty to create and teach specific
courses belonging to a course of study.
- Although not further detailed in this example, it is worth noting
that trust relationships between faculties authorize students from
one faculty to enroll in courses with other faculties.
- A professor responsible for the content of the Educator has a
trust relationship with the administration of the faculty.
Through this relationship, the faculty enables the professor to
teach one or more courses fitting the requirements of the
Curriculum Commission.
Figure 19 illustrates the following trust relationships:
- When a person wants to become a Student of a Faculty, the contract
requires the Student to register with the Student Administration
of the Faculty. If the requirements for registration are met, a
trust relationship with the Faculty enables the Student to
register for courses. For this purpose, the Student
Administration will issue a student card which contains a student
ID and information about the Faculty he or she is admitted to.
The Student Administration will only admit Students who pay the
necessary fees and have met certain prerequisites. The Student
Administration will also keep track of Student grades and will
ultimately issue a certificate at graduation. The Student
Administration AAA server has access to relevant student data and
will only issue grade information and other student-related
information to authorized parties which have a specified means of
authenticating.
- The Curriculum Commission AAA server needs a trust relationship
with the Student Administration AAA server in order to obtain
grade information to check whether a student has met the required
course prerequisites. The Curriculum Commission creates certain
rules within its AAA server which are evaluated when a particular
student attempts to register for a particular course in order to
give an advisory to the student.
- The Educator AAA server needs a trust relationship with the
Student Administrator AAA server in order to verify whether this
particular Student is in good standing with the Faculty. Only
authorized Educator AAA servers may send requests to the Student
Administration AAA server.
- The Educator AAA server needs a trust relationship with the
Curriculum Commission AAA server in order to allow the Educator to
obtain an advisory for the Student whether this course is
consistent with his or her curriculum or whether the student meets
the course prerequisites. Only authorized Educator AAA servers
may send requests to the Curriculum AAA Server.
7.1.4. Sequence of Requests
For the sake of simplicity, we take the example of a student from the
same faculty as the professor.
In this example the following interactions take place for a
hypothetical course (see figure 20).
+----------------------------------------------+
| |
| +----------------+ 6 +----------------+ |
| | Student |----->| Curriculum | |
| | Administration |<-----| Commission | |
| | AAA Server | 5 | AAA Server | |
| +----------------+ _ +----------------+ |
| /|\ | /|/ |
| | | / / |
| 2,8| |3 / /6 |
| | | 4/ / |
| | | / / |
| | | / / |
| | \|/ /|/ |
| +---------------+ -- +---------------+ |
| | Educator A | | Educator B | |
| | AAA Server | | AAA Server | |
| +---------------+ +---------------+ |
| /|\ | |
|2,4,8| |3,6 |
+---------+ | | \|/ |
| | 1,7 | +---------------+ +---------------+ |
| Student |------->| Educator | | Educator | |
| |<-------| Course A | | Course B | |
| | 7,8 | +---------------+ +---------------+ |
+---------+ | Faculty |
+----------------------------------------------+
Fig. 20 -- AAA transactions - single domain case
1. After the Professor has set up the Service Equipment (Educator)
students come to it presenting their ID (college card,
name+faculty) and ask to be admitted to the course.
2. The Educator checks the ID to determine it is indeed dealing with
a student from the faculty. This can include a check with the
Student Administration.
3. The Student Administration replies to the Educator AAA Server, and
the Educator AAA Server replies to the Educator.
4. The Educator checks the request of the Student against its own
policy (courses only in the afternoon) and checks with the
Curriculum Commission whether this student is advised to take the
course. The necessary information is not normally known to or
maintained by the professor.
5. The Curriculum Commission may check against the Student
Administration to see if the Student had the necessary grades for
the previous courses according to the policies set by the
Curriculum Commission.
6. The Student Administration replies to the Curriculum Commission,
the Curriculum Commission replies to the Educator AAA Server, and
the Educator AAA Server replies to the Educator.
7. If now authorized, the Student is presented the material and the
Student returns completed exams.
8. If the Student passes the tests, the Educator informs both the
Student and the Student Administration that the Student has
passed.
7.2. Requirements
We identify the following requirements for an AAA server environment
for this example:
1. It must be possible to delegate authority to contracted partners.
Although this requirement is not explicit in the limited example,
the relationship between University and Faculty may require
delegation of authority regarding the curriculum to the Faculty.
In the case of budget management, this requirement is evident.
2. A system to manage the delegated authority must be established.
It is possible that this is just another AAA server environment.
This comes from the fact that one partner requires the presence of
specific rules to be in the AAA server of another partner. For
example, the Faculty must be sure that certain checks are
performed by the Educator's AAA server.
3. AAA requests must either be evaluated at the AAA server queried or
else parts of the request must be forwarded to another AAA server
which can decide further on the request. As such, it must be
possible to build a network of AAA servers in which each makes the
decisions it is authorized to make by the relationships among the
entities, e.g., a request from the Educator to the Curriculum
Commission may result in a request to the Student Administration.
4. Transaction logs must be maintained to support non-repudiation for
the grades of the students. This recording should be time-stamped
and allow signing by authorized entities. A student should sign
for taking an exam and this should be kept by the Educator's AAA
server. After grading, the professor should be able to sign a
grade and send it to the Student Administrator and the Student
Administrator's AAA server should log and timestamp this event.
5. Three types of AAA messages are required:
- authorization requests and responses for obtaining
authorization,
- notification messages for accounting purposes, and
- information requests and responses for getting information
regarding the correct construction of requests and for querying
the database of notifications.
8. Security Considerations
The authorization applications discussed in this document are modeled
on the framework presented in [2]. Security considerations relative
to the authorization framework are discussed in [2].
Specific security aspects of each authorization application presented
in this document are discussed in the relevant section, above.
Security aspects of the applications, themselves, are discussed in
the references cited below.
Glossary
Attribute Certificate -- structure containing authorization
attributes which is digitally signed using public key
cryptography.
Contract Relationship -- a relation established between two or more
business entities where terms and conditions determine the
exchange of goods or services.
Distributed Service -- a service that is provided by more than one
Service Provider acting in concert.
Dynamic Trust Relationship -- a secure relationship which is
dynamically created between two entities who may never have had
any prior relationship. This relationship can be created if the
involved entities have a mutually trusted third party. Example: A
merchant trusts a cardholder at the time of a payment transaction
because they both are known by a credit card organization.
Policy Decision Point (PDP) -- The point where policy decisions are
made.
Policy Enforcement Point (PEP) -- The point where the policy
decisions are actually enforced.
Resource Manager -- the component of an AAA Server which tracks the
state of sessions associated with the AAA Server or its associated
Service Equipment and provides an anchor point from which a
session can be controlled, monitored, and coordinated.
Roaming -- An authorization transaction in which the Service Provider
and the User Home Organization are two different organizations.
(Note that the dialin application is one for which roaming has
been actively considered, but this definition encompasses other
applications as well.)
Security Association -- a collection of security contexts, between a
pair of nodes, which may be applied to protocol messages exchanged
between them. Each context indicates an authentication algorithm
and mode, a secret (a shared key, or appropriate public/private
key pair), and a style of replay protection in use. [14]
Service Equipment -- the equipment which provides a service.
Service Provider -- an organization which provides a service.
Static Trust Relationship -- a pre-established secure relationship
between two entities created by a trusted party. This
relationship facilitates the exchange of AAA messages with a
certain level of security and traceability. Example: A network
operator (trusted party) who has access to the wiring closet
creates a connection between a user's wall outlet and a particular
network port. The user is thereafter trusted -- to a certain
level -- to be connected to this particular network port.
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, RFC 2026, October 1996.
[2] Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L., Gross,
G., de Bruijn, B., de Laat, C., Holdrege, M. and D. Spence, "AAA
Authorization Framework", RFC 2904, August 2000.
[3] Farrell, S., Vollbrecht, J., Calhoun, P., Gommans, L., Gross,
G., de Bruijn, B., de Laat, C., Holdrege, M. and D. Spence, "AAA
Authorization Requirements", RFC 2906, August 2000.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] Aboba, B. and G. Zorn, "Criteria for Evaluating Roaming
Protocols", RFC 2477, January 1999.
[6] Beadles, Mark Anthony, and David Mitton, "Criteria for
Evaluating Network Access Server Protocols", Work in Progress.
[7] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
2486, January 1999.
[8] Rigney, C., Rubens, A., Simpson, W. and S. Willens, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2138, April
1997.
[9] Calhoun, P. and G. Zorn, "Roamops Authentication/Authorization
Requirements", Work in Progress.
[10] Perkins, C., "IP Mobility Support", RFC 2002, October 1996.
[11] Glass, Steven, et al, "Mobile IP Authentication, Authorization,
and Accounting Requirements", Work in Progress.
[12] Hiller, Tom, et al., "cdma2000 Wireless Data Requirements for
AAA", Work in Progress.
[13] Neilson, Rob, Jeff Wheeler, Francis Reichmeyer, and Susan Hares,
"A Discussion of Bandwidth Broker Requirements for Internet2
Qbone Deployment", ver. 0.7, August 1999,
http://www.merit.edu/working.groups/i2-qbone-bb/doc/BB_Req7.pdf.
[14] deBry, R., "Internet Printing Protocol/1.0: Model and
Semantics", RFC 2566, April 1999.
[15] Burdett, D., "Internet Open Trading Protocol - IOTP", RFC 2801,
April 2000.
[16] "SET Secure Electronic Transaction Specification Book 1:
Business Description", Version 1.0, May 31, 1997,
http://www.setco.org/download/set_bk1.pdf.
Authors' Addresses
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
Pat R. Calhoun
Network and Security Research Center, Sun Labs
Sun Microsystems, Inc.
15 Network Circle
Menlo Park, California, 94025
USA
Phone: +1 650 786 7733
Fax: +1 650 786 6445
EMail: pcalhoun@eng.sun.com
Stephen Farrell
Baltimore Technologies
61 Fitzwilliam Lane
Dublin 2
Ireland
Phone: +353 1 647 7406
Fax: +353 1 647 7499
EMail: stephen.farrell@baltimore.ie
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
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
Betty de Bruijn
Interpay Nederland B.V.
Eendrachtlaan 315
3526 LB Utrecht
The Netherlands
Phone: +31 30 2835104
EMail: betty@euronet.nl
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
Matt Holdrege
ipVerse
223 Ximeno Ave.
Long Beach, CA 90803
EMail: matt@ipverse.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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