G. Carle
Internet Draft                                                S. Zander
Document: <draft-irtf-aaaarch-pol-acct-00.txt>                 T. Zseby
Category: Informational                                       GMD FOKUS
                                                              July 2000

                        Policy-based Accounting

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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   This draft describes policy-based accounting which is aimed at the
   provisioning of flexibility for accounting architectures. Accounting
   policies describe the configuration of an accounting architecture in
   a standardized way. They are used to instrument the accounting
   architecture and can be exchanged between AAA entities in order to
   share configuration information.

   This draft describes building blocks and message sequences for
   policy-based accounting in AAA. Examples are given for the usage of
   accounting policies in different scenarios. Furthermore it is shown
   how accounting components can be integrated into the authorization
   framework. This draft does not propose a language for the
   description of accounting policies. It is rather assumed that a
   suitable policy language can be chosen from existing or upcoming

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Table of Contents

   1.    Introduction................................................2
   2.    Terminology.................................................3
   3.    Impact of Provider Network Characteristics to Accounting....4
   4.    Roles and relations between roles...........................5
   5.    Reference Model and Building Blocks.........................8
   6.    Accounting Policies........................................10
   7.    Accounting Services........................................13
   7.1   Integrated Accounting......................................14
   7.2   Discrete Accounting........................................14
   7.3   Intra-Domain Accounting....................................15
   7.4   Inter-Domain Accounting....................................16
   7.5   Accounting Indication......................................18
   7.6   Integration of Accounting Services in Authorization Model..19
   7.6.1 Agent Sequence.............................................19
   7.6.2 Pull Sequence..............................................19
   7.6.3 Push Sequence..............................................19
   7.7   Roaming....................................................20
   8.    Examples...................................................20
   8.1   Printing Service Example...................................20
   8.1.1 Intra-Domain Accounting....................................21
   8.1.2 Inter-Domain Accounting....................................21
   8.1.3 Accounting/Charging Indication.............................22
   8.2   Mobile/Roaming Example.....................................23
   9.    Security Considerations....................................24
   10.   References.................................................26
   11.   Acknowledgments............................................27
   12.   Author's Addresses.........................................27
   13.   Full Copyright Statement...................................28

1. Introduction

   Even if we will have much more bandwidth in the future than now, the
   control of network resource utilization remains essential for the
   support of applications with special demands and for the prevention
   of (malicious or accidental) waste of bandwidth. Charging provides a
   possibility to control utilization and sharing of network resources.
   Charging in multi-service networks can be done based on the reserved
   or the actual used resources. Charging on reserved resources makes
   sense since reservation usually precludes other users from using the
   reserved resources. Nevertheless, if charging is limited to
   reservation parameters only, the applied charge depends on the
   ability of the user to give a good prediction of the expected
   traffic characteristics. This can be extenuated by using a charging
   scheme that is based on both the reserved and the used resources. In
   order to support usage-based charging, the collection of information
   about the resource reservation and utilization is required. The
   collection of data about resource usage is called accounting.
   Accounting management architectures and objectives as well as the
   transport of accounting records are discussed in [2].

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   Service providers have various options for service differentiation,
   charging schemes and the provisioning of accounting services. Users
   introduce various traffic profiles and may have individual
   preferences regarding the accounting services (like itemized
   invoices, accounting indications, spending limits etc.). In order to
   support the different accounting requirements that result from these
   variability an accounting architecture has to be flexible in regard
   to the data collection and distribution process. To support the
   configuration of such an architecture in a standardized way we
   propose to use accounting policies that are exchanged between the
   different entities involved in the accounting process.

   This document describes the structure and usage of accounting
   policies. It shows how the characteristics of the provider network
   influences the requirements for accounting. The relations between
   the different roles that are involved in the accounting process and
   the required building blocks for an accounting architecture are

   It is shown how different accounting services can be provided in
   intra- and inter-domain scenarios. Examples are given for the usage
   of accounting policies in different scenarios. They show how
   accounting components can be integrated into the authorization
   framework proposed in [1]. This draft does not propose a language
   for the description of accounting policies. It is rather assumed
   that a suitable policy language can be chosen from existing or
   upcoming standards.

2. Terminology

   Accounting Indication
        An accounting indication provides information to customers
        and/or to users about currently used resources. Accounting
        indication messages are accounting records that are sent to the
        customers and/or users. The accounting indication information
        can be either a delta in resource consumption (e.g. since the
        last accounting record has been sent) or a cumulated
        information for the whole resource consumption since the
        service usage began. Accounting indications can also contain
        aggregated information for multiple services. There can be
        interim and end-of-session accounting indications. Interim
        indications are delivered in specified intervals to the user
        during the service session while end-of-session indications are
        given to the user at the end of the session.

   Charging Indication
        A charging indication is similar to an accounting indication.
        But instead of giving the user an indication on the resource
        reservation and/or usage the charging indication gives the user
        an indication on the current charge.

   Charging Scheme

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        A charging scheme is an instruction for calculating a charge.
        Usually a charging scheme is represented by a formula that
        consists of charging variables (e.g. volume, time, reserved
        peak rate) and charging coefficients (e.g. price per time
        unit). The charging variables are usually filled by information
        from accounting data.

   Application Specific Module (ASM)
        An ASM provides the functionalities required by a specific
        application for the provisioning of AAA services. It gets
        application specific information (ASI)(e.g. for configuration)
        from the AAA server either in a generic format or in an
        application specific format encapsulated in a standard message
        sent to the ASM. The ASM either extracts the application
        specific information (ASI) from the message or converts
        information given in a generic format into the appropriate
        application specific format. Further information on how the ASM
        is used can be found in [3].

3. Impact of Provider Network Characteristics to Accounting

   There are many options for future service providers for the
   realization of service differentiation and provisioning. Therefore
   provider networks can vary with respect to several characteristics
   that impact accounting and charging:

   - Size and Purpose
   A small ISP that deals with individual customers may charge
   individual users based on single flows. Backbone operator have small
   ISPs and large corporations as customers, and usually charge based
   on traffic aggregates instead of individual flows.

   - QoS provisioning technique
   DiffServ accounting requirements differ from IntServ accounting
   requirements (e.g. meter granularity).

   - Service classes
   The definition of service classes within a network and the degree of
   freedom that customers are given (e.g. gold/silver/bronze service
   vs. a free choice of individual traffic profile parameters) is
   important, e.g. for the flow classification within the network, and
   influences the accounting functions required.

   - Charging scheme
   There exists a wide variety of charging schemes using tariff
   variables based on different technical and/or economical models. The
   chosen charging scheme(s) influence the accounting requirements for
   the provider. While some charging schemes lead to zero or only few
   accounting requirements, other charging schemes may be highly
   demanding. For instance flat rate charging schemes require no
   accounting infrastructure at all. In contrast to this volume-based
   charging schemes require the measurement of the transmitted volume
   and with this increases the complexity for accounting. Tariffs that

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   introduce variable prices may require to provide the users regularly
   with accounting information (e.g. by interim accounting

   - Accounting Services
   Providers may offer different accounting services (e.g. accounting
   indication, itemized invoice, etc.)

   - Accounting agreements with other providers
   Providers may have agreements with other providers in order to share
   accounting tasks and distribute accounting data so that, e.g.,
   metering has to be done only once. For this it may be useful if
   providers can not only exchange accounting data but also information
   on the configuration of accounting modules (e.g. meters).

   - Exploiting Capabilities of Existing Infrastructure (meters, data
   collection points)
   Providers may already have functions within the network that can
   provide accounting functions (e.g. MIB objects, profile meters,
   proprietary accounting solutions). In order to avoid duplicated
   functionality, it should be possible to use these accounting
   resources. Therefore the configuration of different types of
   accounting modules (e.g. meters) should be possible. A common
   language to express accounting module configurations would be useful
   for this purpose.

4. Roles and relations between roles

   In investigating service provision in the current and forthcoming
   Internet we identified different business roles which are part of
   the service usage lifecycle. In this section we first define the
   term service. Afterwards the different roles and their relationships
   are defined. The business roles in this model are used in the later

   - Service
   A service is a set of capabilities offered by a provider to a
   customer. In this definition provider and customer can be one of the
   business roles defined later. Different kinds of services have to be

        - Information services handle the delivery of information to
        the customer on top of transport services. In content-based
        services the service subscriber pays for the content he gets
        (e.g. for a file, an image, a video, etc.). In communication-
        based services the service subscriber pays for the provisioning
        of a certain form of communication (e.g. videoconferencing or
        IP telephony).

        - Transport services describe the provisioning of pure
        transportation of IP packets. At the IP layer this may include
        the differentiation of packets (e.g. number of packets with a
        certain DSCP), Intserv based reservation or other methods for

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        QoS enhancement (e.g. ARQ, FEC). A transport service may as
        well include mechanisms on other layers for improving the
        transport (e.g. MPLS).

        - Management services are responsible for the management of
        resources (e.g. configuration, accounting, security).
        Accounting services describe the provisioning of data about the
        current or previous resource reservation and usage. Accounting
        services are needed by providers to generate a bill or by users
        to monitor their resource usage.

   - Service Subscriber
   The service subscriber is the entity that has subscribed to a
   service and thus has a contractual relationship with a service
   provider and a network provider which provides the underlying
   transport service. A service subscriber can also act as a service
   user. The service subscriber might have a relationship with a broker
   which provides service relevant information.

   - Service User
   The entity that uses the service. The service user can be identical
   with the Service Subscriber. In cases where subscriber and user are
   not identical, the service subscriber should be able to control the
   service usage for all service users he or she is responsible for.
   For this the sending of accounting or charging indications is

   - Network Provider
   A network provider is the entity that provides the underlying
   network infrastructure for the service user, service subscriber,
   service provider and broker. A network provider provides transport
   services. The services are delivered on top of the network
   infrastructure. The service provider has a contractual relationship
   with service subscriber and service provider (and the broker). The
   transport network of a network provider is probably not a global
   network which connects all subscribers, providers and brokers. The
   transport network is segmented into a number of sub-networks or
   domains controlled by different network providers with business
   relations existing between them. Each domain is responsible for
   intra-domain management and accounting. For inter-domain management
   and accounting appropriate communication interfaces between network
   providers must exist.

   - Service Provider
   A service provider entity provides a service. A service provider can
   offer a service directly to the service subscriber/user. A service
   provider can also act like a wholesaler selling a service to another
   service provider (retailer) which resells the service to the service
   subscriber. The service provider has contractual relationships with
   other service providers, subscribers, brokers and network provider.
   A service provider can provide either based services or
   communication based services on top of the transport network of a
   network provider.

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   - Broker
   The broker entity allows the other roles to access the information
   controlled by the broker. The broker can provide different
   information to different business roles. For example a service
   subscriber can get references to appropriate service providers
   and/or network providers (e.g. a broker gives the subscriber a
   reference to a network provider which can provide bandwidth as
   required by the subscriber). A broker can also interact with other
   brokers to complete their information. In this case broker-to-broker
   business relationships exist.

   Figure 1 depicts the different roles and the business relations
   between them.

                                     V    |
                       +---------------+  |
                       |  Broker       |<-+
               +------>|               |<-----------------+
               |       +---------------+                  |
               |               ^                          |
               |               |                          |
               |               V                          V
               |       +------------------+        +---------------+
               |       |  Service         |        |   Service     |
               |       |  Subscriber      |<------>|   Provider    |
               |       |                  |        |               |<-+
               |       | +--------------+ |        +---------------+  |
               |       | | Service User | |               ^      ^    |
               |       | +--------------+ |               |      +----+
               |       +------------------+               |
               |               ^                          |
               |               |                          |
               |               V                          |
               |       +---------------+                  |
               +------>|  Network      |<-----------------+
                       |  Provider     |<-+
                       +---------------+  |
                                     ^    |

   Figure 1: Roles and business relations

   In the rest of this section we give a few examples on how this
   business relationship model can be applied to different services.

   Example 1: This example describes an Internet printing scenario
   according to the "print-by-reference" model [4]. The subscriber is a
   company, the users are the employees of that company. The file
   server and print server belong to two different service providers.
   The company subscribes to the print server service which acts as
   reseller for the file service. The file server service chooses the

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   appropriate transport service (maybe based on user preference), thus
   the file server has a contract with a network provider using the
   offered transport service for downloading the data from the given
   location and sending them to the print server.

   Example 2: A company (service subscriber) has a contract with a
   video archive (service provider). An employee can download clips in
   different qualities from the archive. The employee can use different
   transport mechanisms for the download. For getting the appropriate
   transport the user contacts an agency (broker) that returns a
   reference to a network provider which provides the required
   transport service.

   In the second example the content (video) can be delivered in
   different qualities via different transport mechanisms by the
   service provider. The service provider has some contract with
   appropriate network providers which provide transport according to
   the conditions the service provider offers the customers/
   subscribers. In this case the service provider can use the
   facilities of a broker to get a reference to appropriate network
   providers. But we should not assume in general that the service
   provider resells the transport service to the subscriber. There are
   also lots of opportunities where the subscriber would like to
   individually combine a content-based service with a special
   transport service.

   According to the above discussion there are two approaches for
   offering services in the Internet:

   - Combined service provision
   The service provider sells the service (e.g. some content) and the
   transport of the content to the customer as a combined service.
   Example: The customer orders some goods at a company and can choose
   how fast it is delivered (e.g. normal or 24h delivery). The company
   then uses an appropriate transport mechanism (i.e. service from a
   transport company).

   - Discrete service provision
   The service provider sells the service on top of a transport service
   which must be obtained separately from a network provider. In this
   case the customer has two contractual relationships. Example: The
   customer orders some goods at a company. The customer makes a
   contract with a transport company and instructs the company of the
   desired transport.

5. Reference Model and Building Blocks

   We have developed a reference model for describing the interactions
   between the different metering, accounting and charging processes
   and their configuration via policies. This reference model is shown
   in Figure 2. At the right side five layers show the different
   building blocks. The blocks are layered according to the processing
   of the accounting data from the metering via accounting up to the

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   final billing process. The building blocks on the different layers
   are configured through the policies shown on the left side. The
   configuration parameters are extracted from the policy and passed
   via the control plane to the corresponding building block. The tasks
   of the different building blocks are as follows:

   - Metering
   Meters are needed for capturing data about resource consumption in
   the network (e.g. transmitted volume). They will probably be placed
   at the edges of the network. Two types of meters can be
   distinguished: Static meters, and configurable meters. In case of
   static meters all flows are measured with a fixed granularity, not
   distinguishing if a subsequent charging process needs the specific
   meter data or not. In most cases the huge amount of captured data
   makes a filtering stage after the metering necessary. An example for
   a static meter is Cisco Netflow. In case of a configurable meter,
   the meter collects meter data only for flows specified by meter
   configuration information. An example of a configurable meter is

   For configuration of the meter process the following issues must be
   addressed: (a) metering scope (whether to meter all flows or only
   selected flows), (b) metered flow attributes (i.e. which data is to
   be collected for a specific flow), and (c) meter granularity
   (measurement intervals etc.)

   - Collection
   The data gathered by the meter(s) has to be collected for further
   processing. Collection of meter data can be initiated by the meter
   itself (push model), or by a collector entity (pull model).

   - Accounting
   Accounting describes the collection of data about resource
   consumption. This includes the control of data gathering (via
   metering), transport and storage of accounting data. For subsequent
   charging, the metered data must be associated with a user that is
   the initiator of a flow, and a customer (service subscriber) that is
   responsible for payment. For initiation of an accounting process, a
   user or foreign provider must be authenticated and authorized. These
   three functions can be performed by the AAA server.

   - Charging
   The Charging derives costs for accounting data sets based on service
   specific tariff parameters. Different cost metrics may be applied to
   the same accounting records even in parallel.

   - Billing
   The Billing translates costs calculated by the Charging into
   monetary units and generates a final bill for the customer.

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         Policy          Parameters                 Building Blocks

     +---------------+ User specific    +---------+------------------+
     |               |<---------------->|         | Billing          |
     |               |                  |         |                  |
     |  Pricing      | Service specific |         +------------------+
     |               |<---------------->|         | Charging         |
     |               |                  |         |                  |
     +---------------+ Accounting       |         +------------------+
     |  Accounting   |<---------------->| Control | Accounting       |
     |               |                  |         |                  |
     +---------------+                  | Plane   +------------------+
     |               | Collection       |         | Collection       |
     |  Metering     |<---------------->|         |                  |
     |               |                  |         +------------------+
     |               | Metering         |         | Metering         |
     |               |<---------------->|         |                  |
     +---------------+                  +---------+------------------+

   Figure 2: Reference Model

   We propose to use policies expressed in a standardized way to
   configure the meter, meter data collection and accounting processes

6. Accounting Policies

   Accounting policies describe rules for generation, transport and
   storage of accounting data. They are exchanged between AAA instances
   at the user or provider premises. They give a standardized
   representation of policies that can be converted into the
   appropriate settings for different elements of the accounting
   infrastructures (e.g. different meters).

   An example is given below. The accounting policy is sent from the
   AAA server to the ASM and there converted to the appropriate
   configuration information for the used meter. If NeTraMet is used,
   the policy is converted into a NeTraMet ruleset that contains the
   relevant flows, attributes and reader instructions for the data
   collection. This information is passed to the NeTraMet manager that
   configures meter and meter reader in accordance to the given

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     |     AAA          |
     |                  |
           |         ^
    Policy |         | Accounting Records
           V         |
     |     ASM          |
     |                  |
         |           ^
         |           |
         | config    +-----------------+
         |                             |
    +-------------------------------+  |
    |    |                          |  |
    |    V                          |  |
    | +----------------+            |  |
    | | Meter Manager  |            |  | Meter Records
    | +----------------+            |  |
    |    |      \                   |  |
    |  SNMP      \                  |  |
    |  (conf)+---------------+      |  |
    |    |   | Meter Reader  |---------+
    |    |   +---------------+      |
    |    |              ^           |
    |    V              |           |
    | +-----------+     |           |
    | |   Meter   |-----+           |
    | +-----------+    SNMP(DATA)   |
    |                               |

   Figure 3: Policy based Accounting with NeTraMet

   If NetFlow is used as a meter, the meter policies are translated by
   the ASM into filter instructions for the flow collector. The meter
   itself is static and therefore is not affected by the configuration

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     |    AAA           |
     |                  |
           |         ^
    Policy |         | Accounting Records
           V         |
     |     ASM          |
     |                  |
         |           ^
         |           |
         | config    | Meter Records
         |           |
    |    |    Meter System          |
    |    |                          |
    |    |  +---------------------+ |
    |    |  | Flow Collector      | |
    |    |  |      +------------+ | |
    |    |  |      | Filter     | | |
    |    |  |      | Aggregator | | |
    |    +->|      +------------+ | |
    |       +---------------------+ |
    |                   ^           |
    |                   |           |
    | +-----------+     |           |
    | |   Meter   |-----+           |
    | +-----------+   UDP (DATA)    |
    |                               |

   Figure 4: Policy based Accounting with NetFlow

   The ASM can be realized as a separate component, but it could also
   be integrated in the AAA server or in the meter.

   The following settings can be specified by accounting policies:

   - accounting record type/structure
   The required accounting data depends on the charging scheme.
   Therefore different accounting records should be supported. There
   are two possibilities. Either different record types are defined or
   a flexible record is used that consists of a variable set of
   accounting attributes. Accounting policies can be used to

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   communicate to neighbor providers which kind of accounting record is
   needed to provide appropriate data for the charging scheme. The
   specification of the required accounting attributes can influence
   the settings of different components of the accounting architecture
   (e.g. which attributes have to be measured). An overview of
   accounting attributes and records can be found in [5].

   - accounting record destination
   The accounting record destination describes to which entities
   accounting records are sent. The accounting record destination can
   be a charging entity, a neighbor provider, a user entity or a
   specific database. In these cases authentication and authorization
   mechanisms have to be applied in order to prevent that unauthorized
   entities can get access to confidential data.

   - sending interval
   The sending interval specifies in what time intervals accounting
   records are generated and sent. This influences the configuration of
   meters and collectors in the accounting architecture.

   - storage time
   If the accounting record destination is a database or a log file the
   storage time specifies how long the accounting records have to be

   - access list
   The access list specifies who has the permission to read the stored
   accounting records.

   Accounting policies are enforced by configuring the accounting
   architecture in the appropriate way. They influence the following
   settings in the accounting architecture:

   - Meter configuration (meter accuracy and granularity, relevant
   attributes, meter intervals )

   - Data collection process (from which meters data has to be
   collected, how often, which post-processing has to be applied)

   - accounting record distribution (when are which accounting records
   sent to whom)

   - accounting record storage (location, expiration time, access list)

   - accounting data aggregation (when which records should be
   aggregated and how aggregation is done)

7. Accounting Services

   Accounting can be seen as part of the service provisioning process
   (Integrated Accounting) or as a separate service (Discrete
   Accounting). The different views and their impact on the accounting
   architecture are described below.

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7.1 Integrated Accounting

   In integrated accounting the accounting is seen as part of the
   provisioned service. That means the accounting is coupled to a
   specific service. Therefore the accounting process is tailored to
   the service probably collecting the accounting information directly
   exploiting some service specific entity. For example accounting for
   IP telephony could use call signaling information from a SIP server.
   The configuration of the accounting architecture is done as part of
   the service equipment configuration. Accounting policies are defined
   as part of the service provisioning agreement. The ASM converts the
   instructions from the AAA server into the appropriate service
   equipment configuration including settings for the accounting

   <---1--->|  Generic AAA Server |<---1--->
            |                     |            ............
            |  Rule based engine  |<----|----->:  Policy  :
            |                     |    3|      :..........:
            +---------------------+<----|--+   ............
                        ^                  +-->:  Events  :
                        |                      :..........:
            | Application specific |
            |         Module       |
                 ^             ^
                 |             |
                 5             5
                 |             |
                 V             V
          +-----------+    +----------------+       ..............
          |  Service  |<-->|  Accounting/   |<--3-->: Accounting :
          |           |    |  Metering      |       : Data       :
          +-----------+    +----------------+       :............:

   Figure 5: AAA Server with Integrated Accounting

   Data about the resource consumption is sent back by the meter(s) to
   the ASM. The ASM then converts the metered data into accounting
   records which are sent to the AAA server. For generating accounting
   records the ASM might perform data conversion, aggregation and
   filtering of data.

7.2 Discrete Accounting

   In contrast to the integrated accounting approach, accounting can
   also be seen as a separate or discrete service on its own.

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   Accounting as a separate service can be seen as some form of
   outsourcing the accounting task. It can be provided by a general
   accounting system which is able to account for different services.
   For example a generalized meter can do accounting for web traffic,
   FTP traffic and voice over IP traffic. If accounting is a separate
   service one provider can do the accounting (charging and billing)
   for several other service providers. This might speed up the
   creation of new services in the Internet. This separation is also
   beneficial for instance to support special accounting services (like
   accounting indications or exchange of data between providers) that
   are not directly coupled to a network service. Furthermore this
   separation is useful if the same set of accounting strategies can be
   applied to different services (e.g. different tariffs which can be
   used for a set of services).

   <---1--->|  Generic AAA Server |<---1--->
            |                     |            ............
            |  Rule based engine  |<----|----->:  Policy  :
            |                     |    3|      :..........:
            +---------------------+<----|--+   ............
                ^             ^            +-->:  Events  :
                |             |                :..........:
                2             2
                |             |
                V             V
        +-------------+    +--------------+         .................
        | Application |    | Accounting   |<---3--->: Accounting    :
        | Specific    |    | Module       |         : Data          :
        | Module      |    +--------------+         :...............:
        +-------------+           ^
               ^                  |
               |                  |
               5                  6
               |                  |
               V                  V
        +-------------+    +---------------+
        |  Service    |    |  Accounting/  |
        |             |    |  Metering     |
        +-------------+    +---------------+

   Figure 6: AAA Server with Discrete Accounting

7.3 Intra-Domain Accounting

   In Intra-Domain accounting [2] the data about the resource
   consumption is collected in one administrative domain for the usage
   in that domain. Accounting policies are locally enforced. Since no
   exchange of accounting data with other domains is required in this
   scenario accounting policies do not need to be exchanged with other

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                                  |  Billing  |
                                        ^            ..............
                                        |            : Accounting :
                                        |----------->:  Records   :
                                        |            :............:
                                        |            ..............
                                +--------------+     : Accounting :
                                |    AAA       |<--->: Policies   :
                                +--------------+     :............:
                                     |      ^
                                     |      |
                                     V      |
                                |     ASM      |
                                     |      ^
                              config |      | Meter Records
                                     V      |
   +------------+               +----------------------+
   |            | Service usage |  +----------------+  |
   | End System |-------------->|  | Meter System   |  |
   |            |               |  +----------------+  |
   +------------+               |                      |
                                |  Service Equipment   |

        User                            Provider

   Figure 7: Intra-Domain Accounting

7.4 Inter-Domain Accounting

   For Inter-Domain Accounting at least two administratively separated
   networks are involved in the accounting process. These can be a
   Home- and a Foreign-Provider in a Roaming/Mobile IP Scenario or a
   chain of providers if service provisioning involves data transfer
   and/or services from different domains. In these scenarios the
   exchange of accounting policies between providers is necessary if
   accounting tasks are delegated to one provider or shared among
   multiple providers. The exchange of accounting policies is done by
   the AAA servers. An example is given in the figure below.

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                                                    |     +-----------+
                                                    |     |  Billing  |
                                                    |     +-----------+
                                                    |           ^
                                                    |           |
                        +------------------+ 1. AccPolReq +-----------+
                        |                  |<-------------|           |
                        |                  |        |     |           |
                        |     AAA          | 2. AccPolAck |   AAA     |
                        |                  |------------->|           |
                        |                  |        |     |           |
                        |                  | 3. AccRec    |           |
                        |                  |------------->|           |
                        +------------------+        |     +-----------+
                    config  |       ^               |
                            |       |               |
                            V       |               |
                        +--------------+            |
                        |     ASM      |            |
                        +--------------+            |
                            |       ^               |
                            |       |               |
                            |       | Meter Records |
                 Service    V       |               |
   +------------+ usage +----------------------+    |
   |            |       |  +----------------+  |    |
   | End System |------>|  | Meter System   |  |    |
   |            |       |  +----------------+  |    |
   +------------+       |                      |    |
                        |  Service Equipment   |    |
                        +----------------------+    |

        User                   Foreign-Provider         Home-Provider

   Figure 8: Inter-Domain Accounting (Roaming Example)

   In this example the foreign provider takes over the collection of
   accounting data. The home provider is responsible for applying a
   charging scheme and sending the bill. Therefore the home provider
   needs accounting data from the foreign provider. In order to
   instruct the foreign provider about the desired accounting record
   type and report frequency the home AAA server sends an accounting
   policy request to the foreign AAA server. The request contains the
   accounting policy. If the foreign AAA server is able to enforce the
   desired policy (e.g. the meters are capable of metering the
   requested attributes) the accounting policy request is acknowledged.

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   In case the requested policy cannot be enforced, the accounting
   service is denied. Reasons to deny the enforcement of a specific
   accounting policy could be e.g. because the meter is not capable to
   measure the requested attributes or the frequency of records cannot
   be provided, or the home provider is not authorized to get the
   requested detailed data. In this case procedures would be useful to
   negotiate the smallest common denominator for the involved AAA
   servers regarding the provisioning of accounting data.

7.5 Accounting Indication

   Accounting indications are accounting records that are sent to the
   customer and/or user in order to inform them about the resource
   usage. The accounting indication is a special accounting service
   that can be provided in addition to the standard accounting
   performed by the provider. Like for any other service an
   authorization should take place before the accounting indication
   service provisioning.

          USER                                PROVIDER
     +---------------+                     +----------------+
     |               |                     |                |
     |               | 1. AccPolReq        |                |
     |    End        |-------------------->|   AAA          |
     |   System      |                     |                |
     |               | 2. AccPolAck        |                |
     |               |<--------------------|                |
     |               |                     |                |
     |               | 3. AccRec           |                |
     |               |<--------------------|                |
     |               |                     +----------------+
     |               |                        |        ^
     +---------------+                        |        |
                                              V        |
                                           |   ASM          |
                                           |                |
                                              |        ^
                                       config |        |
                                              V        |
                                       | +----------------+ |
                                       | |  Meter System  | |
                                       | +----------------+ |
                                       |                    |
                                       | Service Equipment  |

   Figure 9: Accounting Indication

   In order to request an accounting indication a user or customer
   sends an accounting policy request to the responsible AAA server.

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   The request contains the preferred accounting attributes and report
   interval. If the user/customer is authorized to use the accounting
   indication service, the policies are enforced by instrumenting the
   metering and the accounting equipment. Accounting records are then
   forwarded to the user/customer.

7.6 Integration of Accounting Services in Authorization Model

   [1] introduces different message sequences for authorization. The
   integration of configurable accounting services for the message
   sequences can be done as described in the following sections.

7.6.1   Agent Sequence

   Since this scheme describes a single domain case, the accounting
   policies from the provider do not need to be transported to AAA
   servers in other domains. The appropriate accounting policy for the
   authorized service is either stored together with the authorization
   policy or in a separate repository.

   The configuration of the accounting infrastructure can be done
   together with the setup of the service equipment. Setting up the
   service equipment and the accounting infrastructure configuration
   might involve the transfer of configuration data to multiple
   entities in the network (e.g. to different routers for setting up
   QoS provisioning or to dedicated accounting meters)

   An ASM might be needed to convert the accounting policies into the
   appropriate configuration information. In the agent sequence it is
   possible to allow the user to send an accounting policy request
   (e.g. for accounting indications) together with the authorization
   request to the AAA server.

7.6.2   Pull Sequence

   The pull sequence also describes a case where service provisioning,
   authorization and accounting are done in one single domain. As in
   the agent sequence the accounting policies do not need to be
   communicated to other domains. The configuration of the accounting
   infrastructure can also be done similar to the agent sequence during
   the setup of the service equipment.

   Since the pull sequence does not involve the sending of a specific
   authorization request (e.g. if the service equipment is a NAS and
   the authorization sequence simply starts with the dial-in process)
   it would need additional communication to support accounting policy
   requests from users. This can be for instance achieved by a hybrid
   model of agent and pull sequence where the user sends an accounting
   policy request to the AAA server in addition to the messages
   exchange for the pull sequence.

7.6.3   Push Sequence

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   In the push sequence there is no direct connection between the AAA
   server and the service equipment. In this sequence there are three
   possibilities for setting up the accounting infrastructure:

   a) A standard fixed accounting procedure is used, that has been
   assigned in advance for the specific combination of authorized user
   and service.

   b) The ticket contains information about the accounting policy used
   (e.g. different tickets for the same service with different
   accounting policies).

   c) The ticket acts as a kind of digital coin and no further
   accounting is needed. This model also supports the anonymous usage
   of a service.

7.7 Roaming

   If the provisioning of the service and the final authentication/
   authorization process is done by different organizations, accounting
   is rather coupled to the service provisioning process than to the
   authentication/authorization process. Since the data doesn't have to
   traverse the home providers network, the home provider has no
   possibility to collect data about the resource consumption.
   Therefore accounting will usually take place in the foreign provider
   domain (i.e. in the domain that does the service provisioning).
   Nevertheless, in order to ensure consistency of the authentication,
   authorization and accounting processes (e.g. allocation of user IDs
   to accounting records) and to produce a bill a connection between
   the accounting process in the service provisioning domain and the
   deciding authentication/authorization process (e.g. at the home
   provider) is needed.

   A possibility to do this is that the foreign provider gets the
   accounting policies from the home provider and sets up the
   accounting architecture in accordance to the given policies. The
   foreign provider generates accounting records and sends them back to
   the home provider. The home provider then can apply charging and can
   produce a bill. An example for this is given in section 8.2.

8. Examples

8.1 Printing Service Example

   The Internet Printing Protocol (IPP) [4], and especially the "print-
   by-reference" model, provides a very interesting example scenario
   for accounting and the interaction between authorization and
   accounting. We will describe possible solutions for the accounting
   of this service and how the accounting is triggered by the
   authorization. We will show how the model presented above can be
   used for this example.

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   IPP "print-by-reference" allows a user to request a print service to
   print a particular file. The file to be printed is not on the client
   system but rather on a public server. That is, the clients print
   request can contain a reference, or pointer, to the document instead
   of the actual document itself. The print service must then read the
   file to a file server (used for spooling) prior to the printing.
   There are two possible setups: The file and print server either
   belong to a single organization (Intra-Domain Accounting) or to two
   different organizations (Inter-Domain Accounting). In the first case
   the user must be authorized by a single service provider for service
   usage. In the second case two different possibilities for
   establishing a trust relationships between the involved entities
   have to be distinguished [6].

8.1.1   Intra-Domain Accounting

   In the case of a single organization the file and print service is
   provided by a single service provider. The service subscriber and
   user role are either one entity (e.g. private home user) or
   different entities (e.g. company as subscriber, employee as user).
   For data transport via the underlying network the transportation
   service of a network provider is used. In this case the AAA Server
   of the provider controls both the file and the print server. This
   means the AAA server enforces the accounting policies and collects
   accounting data for both servers.

8.1.2   Inter-Domain Accounting

   If two different organizations are involved there are two
   possibilities for trust relationships as shown in [6]:

   1. The User has an agreement with the Print Server; the print
      server has an agreement with the file server.
   2. The user has agreements with both print and file server.

   In case 1, the user is first authorized by the print service and the
   request is forwarded to the file server. The file server authorizes
   the print server and determines if the printer is allowed to access
   the file. In this case the accounting policies from the user arrive
   at the print service AAA server. The print service AAA server has to
   decide which policies can be enforced locally and which must be
   passed further to the file service AAA server. The print service can
   add additional accounting policies. In case the file server does not
   support the desired accounting policies the print server must notify
   the users AAA server and some policy conflict resolution must occur.
   After the file server has transferred the file to the print service
   it generates an accounting record according to the accounting policy
   and gives it to the print service. The print service generates the
   final accounting record for the service session based on his own and
   the file service data after finishing printing. This record will be
   used for the later billing process. Additionally the print server
   can send the final record to the users AAA server. There it can be

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   used for later authorization decisions based on used resources i.e.
   if the customer is a company and the user is an employee.


                |                            |
     +------+   |                            |
     |      |   |                            |
     |      |   |                            |
     |      |   |   +--------------------+   |   +-------------------+
     | User |---1-->| Print Service      |---1-->| File Service      |
     | AAA  |<--2---| AAA Server         |<--2---| AAA Server        |
     |Server|   |   +--------------------+   |   +-------------------+
     |      |   |   | Print Server       |   |   | File Server       |
     |      |   |   |  and Printer       |   |   |                   |
     +------+   |   +--------------------+   |   +-------------------+

     1: AccPolReq
     2: AccPolAck

   Figure 10: Inter-Domain Accounting and Printing Service

   In case 2, the customer AAA server has an agreement with file and
   print server. In this case the users AAA server sends accounting
   policies to the file and the print server. After finishing the
   service both servers generate accounting records for the delivered
   services which are used for later billing. As in the former case the
   accounting data can be send to the user AAA server for use in later
   authorization decisions. The user AAA server can tie both accounting
   records together and assign them to the user using audited session
   information (authorization and accounting messages for a particular
   session must have the same session id) and policies which define
   what activities a certain session is composed of.

8.1.3   Accounting/Charging Indication

   For the printing service there are a number of possible options for
   accounting and/or charging indication. Accounting and charging
   indications are usually send to the service user or the service
   subscriber. Charging indications give the user an indication on how
   much money has been spent for the usage so far. Accounting
   indications gives the user an indication on how much resources have
   been used until the time of the indication. A user can receive only
   accounting, only charging, both or no indication depending on the
   indication policy for the user.

   For Internet printing with the "print-by-reference" model such
   indications would be very helpful for the user. Since the file is
   not on the clients site the user might not have information on the
   file size or the number of pages that will be printed. This means

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   the user has no idea of the costs of the service usage. If user and
   subscriber are a single entity charging indications would help the
   user to avoid exceeding his spending limit. Additional accounting
   indications give the user a hint what resource usage has caused the
   charges. This can be compared to an itemized telephony bill where
   not only the monetary sum per month is printed but in addition
   information for every call (start time, duration, distance etc.) and
   the corresponding amount of money.

8.2 Mobile/Roaming Example

      User |         Visited ISP           |        Home  ISP
           |                               |
           |                               |  +------------+ ..........
    <--------------------12----------------|  | Charging   | :charging:
           |                               |  |and Billing | :policies:
           |                               |  +------------+ :........:
           |                               |        ^
           |                               |        |
           |                               |        11
           |                               |        |
           |          +------------+       |  +------------+
           |          |            |       |  |            |
           |          |            |---10---->|            |
           |          |            |       |  |            |
           |   AR's   | AAA Server |----3---->| AAA Server |
    <-----------------|            |<---4-----|            |
           |          |            |       |  |            |
           |          |            |       |  |            |
           |          +------------+       |  +------------+
           |           ^  |      ^         |
           |           |  |      |         |
           |           |  5      9         |
           |           |  |      |         |
           |           |  V      |         |
           |           | +----------------+|
           |           | |     ASM        ||
           |           2 |                ||
           |           | +----------------+|
           |           |  |      ^         |
           |           |  |      |         |
           |           |  |      |         |
           |           |  6      8         |
           |           |  |      |         |
           | +------------+------+-------+ |
        7  | |  Service   |      |       | |
    <--------| Equipment  |  +---------+ | |
        1  | |            |->|Collector| | |
    -------->|            |  +---------+ | |
           | |     config |      |       | |
           | |            |  +---------+ | |
           | |            +->| Meters  | | |
           | |               +---------+ | |

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           | +---------------------------+ |
           |                               |

   Figure 11: Roaming Example

   In this section the "Dial-in with Roaming" example from the
   authorization examples draft [6] is used to show how accounting
   functions could interact with authorization functions. It gives just
   one example from a variety of possibilities. The accounting modules
   (e.g. collectors and meters) are seen here as part of the service
   equipment which is in this example located at the visited ISP
   premises. The basis configuration of the accounting modules would be
   probably done by the visited ISP itself, but the visited ISP can
   allow the home ISP to influence certain parameters (like report
   interval or accounting record format). This is useful if the home
   provider generates the invoice and therefore needs appropriate
   accounting records to calculate the prices.

   The exchange of authorization data corresponds to the example in the
   draft [6]. As an additional component we introduce an ASM between
   home AAA and service equipment for the conversion of the service
   parameters to the appropriate configuration information.
   Steps (1), (2) and (3) describe the forwarding of an
   authentication/authorization request from the user via the AAA sever
   of the visited ISP to the home AAA server. In step (4) service
   parameters are given to the visited ISP's AAA server and are
   forwarded to the service equipment (5). The service parameters could
   additionally include the desired policies for the configuration of
   the accounting infrastructure of the visited ISP. An accounting
   policy could be for instance "for user X one accounting record of
   type x has to be generated all 30 seconds " This accounting policy
   is used by the visited ISP to configure his modules (e.g. metering,
   data collection).

   Service parameters are converted by the ASM into the appropriate
   configuration information (6). Then  the user is informed about the
   completed authentication/authorization process (7). The accounting
   architecture starts metering the resource usage and sends metering
   records to the ASM (8). The ASM uses the metered data to fill the
   required accounting records and sends them to the visited ISP's AAA
   server (9). The visited ISP can either post-process the data or
   directly forward them to the home ISP (10). With this data as input
   an invoice is generated by the charging and billing modules within
   the home providers domain (11) by using charging policies (tariff
   formulas) and then sent to the user/customer (12).

   As an additional option accounting records can be offered also to
   the user (accounting indication) as a special service. For this
   special service a separate authorization is required.

9. Security Considerations

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   Accounting services provide the basis for billing. Therefore the
   incentives (mainly saving money) and potential for fraud is
   extremely high in the field of configuration of the accounting
   architecture and the collection of accounting data.

   In the presented framework two types of data communications are
   required, the exchange of accounting policies and the collection of
   accounting records. Both communications introduces potential
   security hazards. The main hazard is that users try to forge
   accounting data in a way that the final bill shows a lower charge
   than actually occurred. This can be done indirectly by modifying the
   accounting policies or directly by altering accounting records.

   The following potential security hazards can be identified:

   - Forgery of accounting policies and accounting record information
   Both accounting policies and accounting records can be the target
   for information forgery. Accounting policies contain configuration
   information. Modifying this information can lead to a mal-configured
   accounting and metering system which either allows data to traverse
   the accounting system undetected (without being counted) (e.g. by
   changing the classification rules of a meter) or produces bogus
   accounting records. Accounting records contain data about the
   resource consumption and provide the basis for billing. Modifying
   accounting records leads to erroneous bills. Furthermore it should
   be prevented that policies or accounting records are redirected or
   removed or forged policies or records are inserted.

   - Eavesdropping
   It might be desired to keep accounting policies and accounting
   records confidential between the involved parties.

   - Denial of Service (DoS) attacks
   Both the AAA server and the accounting/metering subsystem can be the
   target of denial of service attacks. A denial of service attack
   against the AAA server may lead to malfunction and even breakdown of
   the server. This means the server will not be able to provide proper
   authentication, authorization and accounting functionality. The
   service provided by the AAA server will become unavailable or
   unusable. An attack against the accounting/metering system will
   cause loss of metering data and/or loss of accounting records.

   This leads to the following security requirements:

   - Secrecy of accounting policies and accounting data
   Unauthorized entities should not be able to read or modify
   accounting policies or accounting records. This can be achieved with
   standard encryption methods.

   - Authentication of accounting data and accounting policy sources
   It should be ensured that the data is originated by the original
   source. Source-authentication can be achieved by using digital

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   - Protection of the integrity of accounting policies and accounting
   It should be ensured that the data was not modified on the way from
   sender to receiver. Data-authentication can also be achieved with
   digital signatures.

   - Prevention and protection against Denial of Service attacks
   The AAA protocol and all building blocks should be designed and
   implemented in a way most resistant to denial of service attacks.
   Furthermore a component can be added to the meter system which is
   able to detect an unusual high packet rate. Upon detection further
   actions can be taken according to a defined policy.

   The prevention of these hazards is one requirement for the protocols
   used for accounting policy exchange and the transportation of
   accounting records. Since the needed security requirements
   (authentication, transmission level security, data object
   confidentiality and integrity) are addressed in the criteria for AAA
   protocol evaluation [7] we assume that the future AAA protocol(s)
   will be suited for secure accounting record transfer and probably
   also for secure accounting policy transport. Furthermore we assume
   that existing or upcoming solutions for secure transportation and
   enforcement of policies can be used.


   [1]     John Vollbrecht, et al, "AAA Authorization Framework",
           draft-irtf-aaaarch-authorization-framework-00.txt, January

   [2]     Bernard Aboba, Jari Arkko, David Harrington, "Introduction
           to Accounting Management", <draft-ietf-aaa-acct-03.txt>,
           Work in Progress, 2 May 2000

   [3]     C. de Laat, G. Gross, L. Gommans, J. Vollbrecht, D. Spence,
           "Generic AAA Architecture", draft-irtf-aaaarch-generic-
           01.txt, Work in Progress, March 2000

   [5]     Nevil Brownlee, Alan Blount, "Accounting Attributes and
           Record Formats", Internet-Draft draft-ietf-aaa-accounting-
           attributes-03.txt, Work in Progress, 14 April 2000

   [4]     Roger deBry, "Internet Printing Protocol/1.0: Model and
           Semantics", RFC 2566, April 1999.

   [6]     John Vollbrecht, et al, "AAA Authorization Application
           Examples", draft-irtf-aaaarch-authorization-apps-00.txt,
           Work in Progress, January 2000.

   [7]     Bernard Aboba, et al, "Criteria for Evaluating AAA Protocols
           for Network Access, draft-ietf-aaa-na-reqts-05.txt, Work in
           Progress, 26 April 2000

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   The authors would like to thank the members of the AAAARCH research
   group for the fruitful discussions and comments.

   Author's Addresses

   Georg Carle
   Kaiserin-Augusta-Allee 31
   10589 Berlin
   Phone: +49-30-34 63 7149
   Email: carle@fokus.gmd.de

   Sebastian Zander
   Kaiserin-Augusta-Allee 31
   10589 Berlin
   Phone: +49-30-34 63 7287
   Email: zander@fokus.gmd.de

   Tanja Zseby
   Kaiserin-Augusta-Allee 31
   10589 Berlin
   Phone: +49-30-34 63 7153
   Email: zseby@fokus.gmd.de

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   Full Copyright Statement

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   This document and the information contained herein is provided on an

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