RFC3334: Policy-Based Accounting

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Network Working Group                                           T. Zseby
Request for Comments: 3334                                     S. Zander
Category: Experimental                                          G. Carle
                                                        Fraunhofer FOKUS
                                                            October 2002

                        Policy-Based Accounting

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   This document describes policy-based accounting which is an approach
   to provide flexibility to 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 Authentication,
   Authorization and Accounting (AAA) entities in order to share
   configuration information.

   This document describes building blocks and message sequences for
   policy-based accounting in the generic AAA architecture (RFC 2903).
   Examples are given for the usage of accounting policies in different
   scenarios.  It is also shown how accounting components can be
   integrated into the AAA authorization framework (RFC 2904).  This
   document does not propose a language for the description of
   accounting policies.  Rather, it is assumed that a suitable policy
   language can be chosen from existing or upcoming standards.

Table of Contents

   1.    Introduction...............................................2
   1.1   Motivation.................................................2
   1.2   Document Scope.............................................3
   2.    Terminology................................................4
   3.    Impact of Provider Network Characteristics on Accounting...7
   4.    Business roles and relations...............................8
   5.    Reference Model and Building Blocks.......................11

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   6.    Accounting Policies.......................................14
   6.1   Accounting Policy Condition...............................15
   6.2   Accounting Policy Action..................................16
   6.3   Example for Meter Configuration...........................17
   7.    Accounting Services.......................................19
   7.1   Integrated Accounting.....................................19
   7.2   Discrete Accounting.......................................21
   7.3   Intra-Domain Accounting...................................22
   7.4   Inter-Domain Accounting...................................23
   8.    Accounting with different Authorization Models............25
   8.1   Agent Sequence............................................25
   8.2   Pull Sequence.............................................26
   8.3   Push Sequence.............................................27
   8.4   Roaming...................................................28
   9.    Examples..................................................29
   9.1   Printing Service Example..................................29
   9.1.1 Intra-Domain Accounting...................................29
   9.1.2 Inter-Domain Accounting...................................30
   9.1.3 User Accounting Indication................................31
   9.2   Mobile/Roaming Example....................................31
   9.3   Diffserv Example..........................................33
   9.4   User Accounting Indication Example........................37
   10.   Security Considerations...................................39
   11.   References................................................41
   12.   Acknowledgments...........................................42
   Author's Addresses..............................................43
   Full Copyright Statement........................................44

1. Introduction

1.1 Motivation

   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 is an
   important concept 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.

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   Service providers have various options for service differentiation,
   charging schemes and the provisioning of accounting services.  The
   applied charging schemes for the provided services are one
   significant feature used by providers to distinguish themselves from
   competitors.  Therefore, providers use different charging schemes and
   may change the schemes in accordance with their business plan.
   Providers can also offer different accounting services (e.g.
   standard, comprehensive, etc.) in order to allow customers/users to
   choose one scheme that meets the customers/users needs.  Furthermore,
   it may be advantageous for a provider to outsource accounting
   functionality to a third party.  Users introduce various traffic
   profiles and may have individual preferences regarding accounting
   services (like itemized invoices, accounting indications, spending
   limits etc.).

   One further challenge for the configuration of accounting services
   are heterogeneous metering and accounting infrastructures within
   provider domains.  Also, the usage of different accounting and
   metering solutions used in different provider networks complicates
   the sharing of configuration parameters (e.g. in roaming scenarios).

   The configuration and dynamic adaptation of the accounting process to
   the business model and specific user demands requires a flexible
   configurable accounting infrastructure.  The utilization of
   standardized policies for the expression of conditions and related
   configuration actions also allows the configuration of heterogeneous
   infrastructures.  For this purpose we propose to use accounting
   policies to configure the accounting infrastructure and use the
   Authentication, Authorization and Accounting (AAA) architecture to
   exchange and to deploy these policies.

1.2 Document Scope

   This document describes the structure and usage of accounting
   policies.  It shows how the characteristics of the provider network
   influence 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
   introduced.  This document describes an architecture and mechanisms
   to configure the accounting service.  It proposes to use the AAA
   protocol for the exchange of accounting configuration information
   expressed in policies.  It does not propose a specific protocol for
   the accounting configuration itself.  The configuration itself can be
   done by existing protocols (e.g. Common Open Policy Service Protocol
   for Support of Policy Provisioning - COPS-PR, Simple Network
   Management Protocol - SNMP, etc.).  Furthermore, it is shown how
   different accounting services can be provided in intra- and inter-
   domain scenarios.  Examples are given for the usage of accounting

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   policies in different scenarios.  They show how accounting components
   can be integrated into the authorization framework proposed in

   Accounting management architectures and objectives as well as the
   transport of accounting records are discussed in [RFC2975] and are
   not further explained here.  This document focuses on the
   configuration of the accounting architecture and measurement devices.

   The policy-based accounting architecture represented in this document
   describes policy-based accounting from the perspective of a Generic
   AAA Server [RFC2903].  Such a server combines into a single entity
   the functions of managing accounting policy, together with the
   functions of managing user-specific authentication, authorization and
   service provisioning.  Some service providers may choose to implement
   an approach that does not combine these functions into a single
   entity or protocol, in which case that particular aspect of this
   architecture does not apply.

   This document 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/Confirmation
           Accounting indication messages are pushed from the
           originating AAA server (the server where the accounting
           information was generated) to the recipient which can be an
           AAA server or a customer/user application.  Accounting
           indications contain accounting records which describe the
           resource consumption for a service.  Accounting indication
           messages can also contain aggregated information for multiple
           services.  There can be interim and end-of-session accounting
           indication messages.  Interim indications are delivered in
           specified intervals to the recipient during the service
           session while end-of-session indications are given to the
           recipient at the end of the session only.  Accounting
           indications may be acknowledged by accounting confirmations
           to provide application layer reliability.

   Accounting Policy Indication/Confirmation
           Accounting policy indication messages contain accounting
           policies and are sent from a customer/user or a AAA server to
           another AAA server.  Accounting policy indications may be
           acknowledged by accounting policy confirmations to provide
           application layer reliability.

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   Accounting Request/Answer
           Accounting requests are sent by an AAA server to another AAA
           server to request the current accounting information for a
           particular session set (polling).  The request is answered
           with an accounting answer which contains the accounting

   Accounting Policy Request/Answer
           Accounting policy requests are sent by an AAA server to
           another AAA server or a customer/user to request accounting
           policies for a service.  The request is answered by an
           accounting policy answer that contains the accounting policy.

   Accounting Policies
           Accounting policies describe rules for generation, transport
           and storage of accounting data.  These rules are used for the
           configuration of the accounting process.

   Application Specific Module (ASM)
           An ASM provides the functionalities required for the user
           configuration of a service to an authenticated and authorized
           user.  It gets application specific information (ASI) (e.g.
           for user 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 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 [RFC2903].

   Charging Schemes
           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.

           This document uses the definition of classifier as given in
           [RFC2475].  Since this document assumes that meters already
           include classification functions, the term classifier is only
           used for entities that perform additional classification
           (e.g. as part of data post processing).

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           This document uses the definition of meter as given in
           [RFC2722].  This meter definition already includes the
           classification of packets.  It differs from the DiffServ
           model [RFC2475] where classifier and meter are considered as
           separate entities.

   Meter Reader/Collector
           This document uses the definition of meter reader and
           collector as given in [RFC2722].

   Meter Manager
           This document uses the definition of meter manager as given
           in [RFC2722].

   Policy, policy condition, policy action
           The terms policy, policy condition and policy action are used
           as defined in [RFC3198].

   QoS Auditing
           Quality of Service (QoS) Auditing is the process of
           evaluating whether a given quality of service guarantee (e.g.
           thresholds for QoS parameters given in a Service Level
           Agreement -  SLA) has been met during the service

   Service Class
           A service class specifies the handling of a service (as
           defined in [RFC3198]) belonging to that class by the service
           provider.  A service class has some kind of identifier (e.g.
           name) and the handling of the service is defined by a Service
           Level Specification (SLS) as described in [RFC3198].

   User Configuration
           We refer to User Configuration as the process of configuring
           a service for a user which has been authenticated and
           authorized by the AAA architecture.  Although an AAA
           architecture is not directly responsible for this user-
           dependent configuration, it may be responsible for triggering
           the process.

   Further definitions of service related terms (Service, Service
   Subscriber, Service User, Network Provider, Service Provider, Broker)
   can be found in section 4 (business roles and their relations).

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3. Impact of Provider Network Characteristics on 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 operators often
   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 economic 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 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 need only be done once.  If so, it may be useful if
   providers can not only exchange accounting data, but also information
   on the configuration of accounting modules (e.g. meters).  It is

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   important for providers to agree beforehand how accounting data will
   be collected and monitored, and how disputes concerning accounting
   data will be resolved.  In order to minimize disputes between
   providers, it is important for them to agree that either both will
   collect accounting data - and will compare it with the other's data
   at regular intervals, e.g. monthly - or both will use a single source
   of accounting data provided by one of them (or by a trusted third

   - 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. Business roles and relations

   In investigating service provisions 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 examples.

   - 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 recognized.

        - 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 (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. video conferencing or IP

        - 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
        QoS enhancement (e.g. Automatic Repeat reQuest -  ARQ, Forward

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        Error Correction -  FEC).  A transport service might also
        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 that provides
   service relevant information.

   - Service User
   The service user is the entity that uses the service.  The service
   user can be identical to 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 she is
   responsible for.

   - 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 the 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 re-sells the service to the service
   subscriber.  The service provider has contractual relationships with

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   other service providers, subscribers, brokers and network providers.
   A service provider provides information services on top of transport
   services provided by network providers.

   - 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

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   The following examples show 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 [RFC2566].  The
   subscriber is a company and 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 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.  In order to get the
   appropriate transport, the user contacts an agency (broker) that
   returns a reference to a network provider which provides the required
   transport service.  As an alternative, the content (video) can be
   delivered in different qualities via different transport mechanisms
   by the service provider.  The service provider chooses an appropriate
   network provider which provides a transport service compliant with
   the conditions the service provider offers to the subscribers.  In
   this case the service provider can use the facilities of a broker to
   get a reference to appropriate network providers.

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
   data from the bottom level metering via accounting, up to the final
   billing process.  Data aggregation is not only done at the collection
   layer, it can also be done at the other layers.  The building blocks
   on the different layers are configured through the policies shown on
   the left side.  Higher layer policies can be translated into lower
   layer policies.  The configuration parameters are extracted from the
   policy and passed 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

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   distinguished: Static meters and configurable meters.  In the 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 large amount of captured data
   makes filtering and/or aggregation after the metering necessary.  In
   case of a configurable meter, the meter collects meter data only for
   flows specified by metering policies.

   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) flow granularity (e.g. micro flows or traffic
   aggregates) (c) metered flow attributes (i.e. which data is to be
   collected for a specific flow), and (d) meter accuracy (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).  Collected
   data can be aggregated before being passed to the accounting layer.
   Metering policies define how collection and aggregation is done.

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     +---------------+                   +-------------------------+
     |               |------------------>|        Billing          |
     |  Billing &    |                   +-------------------------+
     |  Charging     |                             ^ charging
     |               |                             | data
     |               |                   +-------------------------+
     |               |------------------>|        Charging         |
     +---------------+                   +-------------------------+
             |                                     ^ acct
             V                                     | data
     +---------------+                   +-------------------------+
     |  Accounting   |                   |                         |
     |               |------------------>|        Accounting       |
     +---------------+                   +-------------------------+
             |                                     ^ aggr. meter
             V                                     | data
     +---------------+                   +-------------------------+
     |               |------------------>|        Collection       |
     |  Metering     |                   |                         |
     |               |                   +-------------------------+
     |               |                             ^ meter
     |               |                             | data
     |               |                   +-------------------------+
     |               |------------------>|        Metering         |
     +---------------+                   +-------------------------+

   Figure 2: Reference 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.  The accounting
   process is configured through accounting policies.

   - Charging
   Charging derives non-monetary costs for accounting data sets based on
   service and customer specific tariff parameters.  Different cost
   metrics may be applied to the same accounting records even in
   parallel.  Charging policies define the tariffs and parameters which
   are applied.

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   - Billing
   Billing translates costs calculated by the Charging into monetary
   units and generates a final bill for the customer.  Billing policies
   define among others the type (e.g. invoice, credit card), the form of
   the bill (e.g. itemized or not, partial anyomization, etc.) and the
   time for billing (e.g. weekly, monthly, etc.).

   We propose to use policies expressed in a standardized way to
   appropriately 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 can be exchanged between AAA
   instances at the user or provider premises.  They provide a
   standardized representation of configuration information that can be
   converted into the appropriate settings for different elements of the
   accounting infrastructures (e.g. different meters).

   As shown in Figure 2, accounting policies configure the accounting
   process.  Policies for the configuration of the metering and
   collection process can be derived from accounting policies.
   Accounting policies are not used to configure the charging or billing
   process.  Accounting policies reside in the AAA server (local
   policies) or are received from other AAA servers (extra-domain
   policies) or customers/users.  Two different models of obtaining
   accounting policies can be differentiated: push and pull model.

   Push Model
   In the push model, accounting policies are pushed from another AAA
   server or customer/user in order to establish the policies in the
   local accounting infrastructure.  The acceptance and use of pushed
   policies requires special security considerations.  The evaluation of
   the policy should not take place without an appropriate security
   check of the policy in advance.  Also, the evaluation of the
   condition can lead to unwanted actions in the AAA server if the
   condition contains critical data either intentionally (to attack the
   system) or by accident.  Even the evaluation of the condition can
   cause problems (e.g. DoS).  Therefore, not only the action, but also
   the condition, has to be checked for potential security hazards
   before it is evaluated.

   Pull Model
   In the pull model, the AAA server requests the policy from a remote
   AAA server or customer/user by sending an accounting policy request.
   The remote AAA server sends an accounting policy reply as an answer
   that contains the appropriate policy.

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   Accounting policies are enforced by the network elements that are
   configured in accordance with the policies.  They influence the
   following settings in the accounting architecture:

   - meter configuration
   - data collection and aggregation
   - accounting record distribution and storage

6.1 Accounting Policy Condition

   An accounting policy consists of one or more rules, each having a
   condition part and an action part.  The condition part expresses
   under which condition the policy should be enforced.  The following
   attributes are examples for variables in a policy condition

   - customer/user ID
   The customer/user ID identifies the customer or user of the service.
   It can be used in a policy condition in order to select a customer or
   user specific accounting configuration (as policy action).  For
   example, it can be user-dependent whether accounting indications are
   sent to the user or not.

   - IP address
   IP addresses specify the devices or networks from which the service
   usage takes place.  The address of specific hosts or subnets can be
   used to select accounting strategies specific to the customer or a
   user group associated with this address (e.g. all customers of an
   ISP, all public terminals etc.).

   - time of day
   The time of day can be used, for instance, to configure the level of
   detail for the accounting record, the report interval and the

   - service class
   Service classes are defined by the provider.  They describe different
   levels or different kinds of services that are offered by the
   provider and are usually defined based on a business model.
   Customers/users select a service class.  This selected class can be
   used in accounting policies to define appropriate accounting settings
   per class.  With this it is possible, for instance, to provide more
   detailed accounting records for higher prioritized services than for
   standard services.

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   - accounting type
   Accounting types combine multiple accounting settings under one
   keyword.  Like service classes, the offered accounting types are
   defined by the provider in accordance with the business model.  With
   this, providers can offer, for instance, different accounting types
   for one service and allow the customer/user to select one.  The
   combination of settings under one keyword simplifies the selection
   for users.  An example is the combination of high granular accounting
   records with short report intervals under a keyword (e.g.
   "comprehensive accounting"), or less frequent generation of less
   detailed records accessed by another keyword ("standard accounting").
   The definition of accounting types can also help in inter-domain
   scenarios if providers agree on accounting types.

6.2 Accounting Policy Action

   The action part defines the action that takes place if the condition
   is true.  The action for an accounting policy is usually the
   configuration of the accounting infrastructure.  This can already
   include settings for meters and collection entities.  The following
   list gives examples for parameters of the accounting infrastructure
   that can be configured by an accounting policy action:

   - 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
   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 [RFC2924].

   - 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 ensure that unauthorized
   entities cannot get access to confidential data.

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

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   - 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 permissions to read the stored
   accounting records.

   - flow granularity
   The flow granularity determines how fine grained (in coverage) the
   flows in the network are measured.  The granularity usually is
   configured by installing specific classification rules in the meter.
   It is also possible to set a specific granularity by configuring
   aggregation schemes that are applied after the metering process.  The
   granularity can range from individual micro flows (e.g. determined by
   the quintuple <src, dest, proto, src-port, dest-port>) up to coarse
   granular traffic aggregates (e.g. all traffic from one network).

   - meter accuracy
   The parameters for the meter accuracy can determine, for instance,
   how often measurements take place at the meter, how accurate
   timestamps should be, etc.  Meter accuracy parameters can also be
   used to configure sampling schemes.

6.3 Example for Meter Configuration

   Note: In the following examples, the use of NeTraMet or NetFlow to
         collect accounting information does not guarantee exact
         accounting data, so it is not recommended for use in situations
         where exact accounting data are needed.

   The following two examples show how accounting policies can be used
   to configure different meters.  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 the meter NeTraMet [RFC2123] 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 the meter and meter
   reader in accordance with the given configuration.

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

   Figure 3: Policy based Accounting with NeTraMet

   If the meter NetFlow [NetFlow] is used, 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 information.

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

   Figure 4: Policy based Accounting with NetFlow

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.

7.1 Integrated Accounting

   In the integrated accounting model, the accounting is seen as part of
   the provisioned service.  That means the accounting is coupled with a
   specific service.  Therefore, the accounting process is tailored to
   the specific service and might collect accounting information by

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   directly exploiting some service specific entities.  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 user configuration of the service equipment.
   Accounting policies are defined as part of the contractual agreement.
   The ASM converts the instructions from the AAA server into the
   appropriate user configuration including settings for the accounting

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

   Figure 5: AAA Server with Integrated Accounting

   Data about the resource consumption is sent back to the AAA server
   via the ASM.  The accounting process within the service converts the
   metered data into accounting records which are sent to the AAA
   server.  For generating accounting records data conversion,
   aggregation and filtering of data might be performed.

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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.  In this
   case the accounting does not have to be coupled with a specific
   service.  Discrete Accounting can be used for outsourcing the
   accounting task.  The accounting service 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.  Accounting is offered just like
   any other service.  This means authentication and authorization might
   be required prior to the accounting service provisioning.
   Furthermore, it is important that the involved parties agree
   beforehand how the accounting service is provided, what parameters
   can be set and how disputes will be resolved.  After the accounting
   service has been configured, the AAA server can do the user
   configuration of the service.

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

   Figure 6: AAA Server with Discrete Accounting

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   A service provider that has outsourced the accounting service has to
   request this service from an accounting service provider.  The
   generated accounting records are sent from the accounting provider to
   the service provider who may make modifications to the records before
   sending them to the final destination.  Having such a general
   accounting service might speed up the creation of new services -
   especially specialized content services - in the Internet.  This
   separation is also beneficial to support special accounting services
   (e.g. sending accounting indications to users) 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

   Another option is to outsource only the metering service.  The meter
   service provider generates meter data and sends them to the service
   provider who has requested them.  The service provider then generates
   accounting records based on the received meter data.  A separate
   accounting or metering service provider can be used to validate the
   accounting data generated by a service provider.  If the customer
   does not trust a service provider, or in the case of a legal action,
   a trusted accounting or metering provider is able to validate the
   correctness of the accounting data generated by the service provider.

7.3 Intra-Domain Accounting

   In Intra-Domain accounting [RFC2975], the data about resource
   consumption is collected in one administrative domain for usage in
   that domain.  Accounting policies are enforced locally.  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   |
                                |     ASM     |
                                        |            ..............
                                +--------------+     : Accounting :
                                |    AAA       |<--->: Policies   :
                                +--------------+     :............:
                                     |      ^
                                     |      |
                                     V      |
                                |     ASM      |
                                     |      ^
                              config |      | Accounting Records
                                     V      |
   +------------+               +-----------|----------+
   |            | Service usage |  +--------+-------+  |
   | End System |-------------->|  | Accounting     |  |
   |            |               |  +----------------+  |
   +------------+               |                      |
                                |  Service             |
        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
   [RFC2002] 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 as shown in the figure below.

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                                                    |     +-----------+
                                                    |     |  Billing  |
                                                    |     +-----------+
                                                    |           ^
                                                    |           |
                                                    |     +-----------+
                                                    |     |    ASM    |
                                                    |     +-----------+
                                                    |           ^
                                                    |           |
                        +------------------+ 1. AccPolInd +-----------+
                        |                  |<-------------|           |
                        |                  |        |     |           |
                        |     AAAF         | 2.AccPolConf |   AAAH    |
                        |                  |------------->|           |
                        |                  |        |     |           |
                        |                  | 3. AccRec    |           |
                        |                  |------------->|           |
                        +------------------+        |     +-----------+
                    config  |       ^               |           ^
                            |       |               |           |
                            V       |               |           V
                        +--------------+            |     .............
                        |     ASM      |            |     : Acct.     :
                        +--------------+            |     : Policies  :
                            |       ^               |     :...........:
                            |       |               |
                            |       | Acct. Records |
                 Service    V       |               |
   +------------+ usage +-----------|----------+    |
   |            |       |  +--------+-------+  |    |
   | End System |------>|  | Accounting     |  |    |
   |            |       |  +----------------+  |    |
   +------------+       |                      |    |
                        |  Service             |    |
                        +----------------------+    |

        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 indication to the foreign AAA server.  The indication contains

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   the accounting policy.  Instead of sending an indication, the
   accounting policies could also be piggy backed onto an authorization
   reply.  If the foreign AAA server is able to configure devices in a
   way to enforce the desired policy (e.g. the meters are capable of
   metering the requested attributes) the accounting policy indication
   is acknowledged.  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 of measuring 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.

8. Accounting with different Authorization Models

   The AAA authorization framework [RFC2904] 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.

8.1 Agent Sequence

   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 user configuration of the service equipment
   (messages 2 and 3 in Figure 9).  User-specific configuration of 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).

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               +------+      | Service Provider        |
               |      |   1  |  +-------------------+  |
               |      |------+->|    AAA Server     |  |
               |      |<-----+--|                   |  |
               |      |   4  |  +-------------------+  |
               | User |      |       |   ^    ^        |
               |      |      |       |2  |3   |AcctRec |
               |      |      |       V   |    |        |
               |      |      |  +-------------------+  |
               |      |      |  |      Service      |  |
               |      |      |  |     Equipment     |  |
               |      |      |  +-------------------+  |
               +------+      |                         |

   Figure 9: Accounting and Agent Sequence

   In the agent sequence, it is possible to allow the user to send
   accounting policies (e.g. for accounting indications) together with
   the authorization request to the AAA server.  Figure 9 shows the
   agent sequence authorization and accounting messages.

8.2 Pull Sequence

   The configuration of the accounting infrastructure can be done
   similar to the agent sequence during the user configuration 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 Network Access Server (NAS) and the authorization
   sequence simply starts with the dial-in process), it would need
   additional communication to support accounting policy indications
   from users.

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               +------+       | Service Provider        |
               |      |AccPolInd +-------------------+  |
               |      |.........>|    AAA Server     |  |
               |      |<.........|                   |  |
               |      |       |  +-------------------+  |
               | User |       |       ^   |   ^         |
               |      |       |       |2  |3  |AcctRec  |
               |      |       |       |   V   |         |
               |      |   1   |  +-------------------+  |
               |      |-------+->|      Service      |  |
               |      |<------+--|     Equipment     |  |
               |      |   4   |  +-------------------+  |
               +------+       |                         |

   Figure 10: Accounting and Pull Sequence

   This can be, for instance, achieved by a hybrid model of agent and
   pull sequence where the user sends an accounting policy indication to
   the AAA server in addition to the messages exchange for the pull
   sequence.  Figure 10 shows the pull sequence authorization and
   accounting messages.

8.3 Push Sequence

   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 that has been assigned in
   advance for the specific combination of authorized user and service
   is used.

   b) The ticket (message 3 in Figure 11) contains information about the
   accounting policies 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.

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   Figure 11 shows push sequence authorization and accounting messages.

                 +------+      | Service Provider        |
                 |      |   1  |  +-------------------+  |
                 |      |------+->|    AAA Server     |  |
                 |      |<-----+--|                   |  |
                 |      |   2  |  +-------------------+  |
                 | User |      |              ^          |
                 |      |      |              | AcctRec  |
                 |      |      |              |          |
                 |      |   3  |  +-------------------+  |
                 |      |------+->|      Service      |  |
                 |      |<-----+--|     Equipment     |  |
                 |      |   4  |  +-------------------+  |
                 +------+      |                         |

   Figure 11: Accounting and Push Sequence

8.4 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 of collecting 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 the production of 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 possible way of doing this is if 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 can generate accounting records and send 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 9.2.  This
   scenario requires a prior agreement between the involved providers
   about the possible policies and parameters that are allowed to be

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9. Examples

   The following examples illustrate the use of policy-based accounting.
   Please note that the services used in the examples are used only for
   illustration purposes and their use in reality requires different
   messages and parameters.

9.1 Printing Service Example

   The Internet Printing Protocol (IPP) [RFC2566], 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.

   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 [RFC2905].

9.1.1   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 the access to the file and the print server.  This
   means the AAA server enforces the accounting policies and collects
   accounting data for both servers.

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9.1.2   Inter-Domain Accounting

   If two different organizations are involved there are two
   possibilities for trust relationships as shown in [RFC2905]:

   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 which is shown in Figure 12, the accounting
   policies from the user arrive at the print service AAA server.

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

     1: AccPolInd, 2: AccPolConf

   Figure 12: Inter-Domain Accounting and Printing Service

   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 user's 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
   passes it to the print service.  The print service generates the
   final accounting record for the service session based on its 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 user's AAA server.  There it can be
   used for later authorization decisions based on used resources, i.e.
   if the customer is a company and the user is an employee.

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   In case 2, the customer AAA server has an agreement with file and
   print server.  In this case, the user's 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 sent to the user's AAA server for use in
   later authorization decisions.  The user's 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 could be coupled via a session ID) and policies
   that define which activities a certain session is composed of.

9.1.3   User Accounting Indication

   For the printing service, there are a number of possible options for
   sending accounting indications to the user.  Accounting indications
   give the user an indication of how much resources have been used
   until the time of the indication.  A user can receive accounting
   indications or not depending on the accounting 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
   the user has no idea of the costs of the service usage.  If user and
   subscriber are a single entity, accounting indications would help
   users to avoid exceeding their spending limit.  Additionally,
   accounting indications give the user a hint as to which 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 its corresponding charge.

9.2 Mobile/Roaming Example

   In this section, the "Dial-in with Roaming" example from the
   authorization examples [RFC2905], [RFC2002] is used to show how
   accounting functions could interact with authorization functions.
   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 basic configuration of the accounting
   modules is 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.

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      User |         Visited ISP           |        Home  ISP
           |                               |
           |                               |  +-----------+  ..........
    <--------------------12-------------------| Charging, |<-:charging:
           |                               |  | Billing   |  :policies:
           |                               |  +-----------+  :........:
           |                               |        ^
           |                               |        |
           |                               |  +-----------+
           |                               |  |    ASM    |
           |                               |  +-----------+
           |                               |        ^
           |                               |        |11
           |                               |        |
           |          +------------+       |  +-------------+
           |          |            |       |  |             |
           |          |            |---10---->|             |
           |          |            |       |  |             |
       Acct. Records  | AAAF Server|----3---->| AAAH Server |
    <-----------------|            |<---4-----|             |
           |          |            |       |  |             |
           |          |            |       |  |             |
           |          +------------+       |  +-------------+
           |           ^  |      ^         |
           |           |  |      |         |
           |           |  5      9         |
           |           |  |      |         |
           |           |  V      |         |
           |           | +----------------+|
           |           | |     ASM        ||
           |           2 |                ||
           |           | +----------------+|
           |           |  |      ^         |
           |           |  |      |         |
           |           |  6      8         |
           |           |  |      |         |
           | +------------+------+-------+ |
        7  | |  Service   |      |       | |
    <--------| Equipment  |  +----------+| |
        1  | |            |->|Accounting|| |
    -------->|            |  +----------+| |
           | |     config |      |       | |
           | |            |  +---------+ | |
           | |            +->| Meters  | | |
           | |               +---------+ | |
           | +---------------------------+ |
           |                               |
   Figure 13: Roaming Example

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   The exchange of authorization data corresponds to the example in
   [RFC2905].  As an additional component, we introduce an ASM between
   home AAA and service equipment for the user configuration which
   happens after successful authorization.  The extended roaming example
   is shown in Figure 13.  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), user specific service parameters are given to the visited ISP's
   AAA server and are forwarded to the service equipment (5) where the
   user configuration is done.  The user-specific 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 Y has to be generated every 30 seconds".  This accounting policy
   is used by the visited ISP to configure his modules (e.g. metering,
   data collection).

   User-dependent 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 also be offered to
   the user (accounting indication) as a special service.  For this
   special service a separate authorization is required.

9.3 Diffserv Example

   This example explains how integrated accounting is configured via
   policies for a Diffserv service [RFC2475] based on bandwidth brokers
   [I2-BB].  The service is the transport of packets with a higher
   priority and the service includes accounting and QoS auditing.
   Figure 14 shows the service setup.  The user issues a Service Request
   (SR) for a Diffserv service to the AAA server.  The request contains
   a user ID and the parameter for the desired service class.

      User->AAA: user-x@nw-a, service=diffserv, class=gold,
                 amount=2Mbit, dest= nw-b

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   In this example, user-x is located at network A (nw-a) and requests a
   gold class service for all flows from this network to the destination
   network B (nw-b).  After authentication and authorization has been
   completed successfully, the AAA server extracts the ASI from the
   request and passes them to the ASM of the Diffserv service.

      AAA->ASM: service=diffserv, class=gold, amount=2Mbit, src=nw-a

   The ASM takes over the task of translating the application specific
   information into appropriate user configuration information for the
   service equipment.  For the given Diffserv example, the service
   equipment consists of three components: accounting equipment, the QoS
   auditing equipment and the bandwidth broker architecture.  The ASM
   has to address all three components to set up the requested service
   for the user.  The translation of the ASI into configuration
   information for the components can be done by evaluating service
   provisioning policies.  For example, the ASM could have the following
   service provisioning policy:

           if class==gold {
             set bw-request.class = gold
             set accounting.type = comprehensive
             set qos-audit.metric = one-way-delay

   This results in sending a bandwidth request to the BB which asks for
   a gold service with the given parameters.  Furthermore, the ASM
   issues a request to the accounting equipment for comprehensive
   accounting and a request to the QoS auditing equipment for a one-
   way-delay measurement between the given networks.

   ASM->BB: BW-request(gold, src=nw-a, dest=nw-b, amount=2Mbit)

   ASM->Acct: Acct-request(comprehensive, src=nw-a)

   ASM->QoS: QoS-audit-request(one-way-delay, src=nw-a, dest=nw-b)

   The bandwidth broker then sets up the Diffserv infrastructure to
   provide the prioritized forwarding according to the definition of a
   gold class.  This is done in accordance with the actual bandwidth
   broker's architecture and is not further considered here.  For the
   Accounting Configuration and the QoS Audit Control, local
   configuration policies exist for setting up the service.

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                if type==comprehensive {
                  set meter-location = access-point(nw-a)
                  set record type =detailed
                  set report interval = 120 s
                  set report target =
                  ^ indent of last two lines

                  if metric==one-way-delay {
                    set method = passive
                    set timestampsize = 48 bit
                    set ingress-meter-location = access-point(nw-a)
                    set egress-meter-location = access-point(nw-b)

   In this case, the local accounting policy sets the meter location to
   the network access point of network A.  It states that for
   comprehensive accounting, a detailed record type is required with a
   report interval of 120 s.  The resulting records have to be sent to
   the given report target.  The QoS measurement policy sets the
   measurement method to passive measurement.  It sets the size used for
   timestamp representation to 48 bits.  As meter locations, the meters
   at the access points of network A and network B are used.

   After evaluating these policies, the instructions for the meter
   configuration are passed down to the measurement infrastructure.  In
   our example, the accounting configuration instructs the meter at the
   first measurement point (MP1) to add a new rule with the given flow
   attributes and settings for storage and reporting of results.

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      Acct->MI: MP1: add rule dscp=23, src=a.a.a/24, dest=b.b.b.b/24
                     save volume
                     set report interval = 120 s
                     set report target =

              SR          +-------+
   User ----------------->|  AAA  |
                              | ASI
        +-----------------|  ASM  |--------------+--------------+
        |       Policy    +-------+  Policy      |   BW Request |
        |       Parameters           Parameters  |              |
        |                                        |              |
        |       Service Equipment                |              |
        V                                        V              V
   +---------------+    ..............    +-----------+   +-----------+
   | Accounting    |<-->: Local      :<-->| QoS       |   | Bandwidth |
   |               |    : Policies   :    | Auditing  |   | Broker    |
   +---------------+    :............:    +-----------+   +-----------+
        |                                        |
        | Meter Instructions                     | Measurement Setup
        V                                        V
   |  Measurement                                     |
   |  Infrastructure                                  |

   Figure 14: Diffserv Service Provision Setup

   The QoS audit control instructs two meters (at MP1 and MP2) to set up
   a passive one-way-delay measurement.

      QoS->MI: MP1: add rule dscp=23, src=a.a.a.a/24 dest=b.b.b.b/24,
                    save timestamp-48
               MP2: add rule dscp=23, src=a.a.a.a/24, dest=b.b.b.b/24,
                    save timestamp-48

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9.4 User Accounting Indication Example

   This example explains how discrete accounting can be used to provide
   accounting indications for the user.  Accounting indications are sent
   to the user in order to inform the user about current resource
   consumption.  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.  Therefore, the accounting here is seen as a
   separate service.  That means the accounting service is independent
   of the main service and therefore can be applied to different
   services.  It might be used as an addition to an integrated
   accounting that is part of the service.  The authorization process
   for the accounting service is out of the scope of this document and
   therefore is not further explained here.

   Figure 15 illustrates the configuration message sequence for setting
   up the accounting service.  First, the user sends an Accounting
   Service Request (ASR) to the AAA server which includes desired
   parameters for the provisioning of the accounting service (e.g.
   report interval).

      user->AAA: user-x@nw-a, service= accounting indications,
                 report interval= 60 s

   The AAA server passes the ASI to the ASM of the accounting service
   after the user has been authenticated and authorized for the service

      AAA->ASM: user-x@nw-a, service=accounting indications,
                report interval= 60 s

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   The ASM generates an accounting policy based on the ASI and passes
   this policy to the Accounting Configuration.

   ASM->Acct: If src=a.a.a.x {
                acc-indication = on
                report interval = 60s
                report target= a.a.a.x

              ASR       +-------+
   User --------------->|  AAA  |
                            | ASI
                        |  ASM  |
   Service Equipment        | Accounting Policy
                    +-----------------+      ..............
                    |  Accounting     |<---->: Local Acct :
                    |                 |      : Policies   :
                    +-----------------+      :............:
                            | Meter Instructions
                    |  Measurement    |
                    |  Infrastructure |

   Figure 15: Accounting Indication Configuration

   The Accounting Configuration generates meter instructions according
   to the accounting policies from the ASM and local accounting policies
   and passes them to the measurement infrastructure.

      local Acct-Policy: if acc-indication {
                          record type = compact

      Acct->MI: MP1: set report interval = 60 s
                     add report target = a.a.a.x

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10. Security Considerations

   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 introduce potential security hazards.

   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 of
   forgery of information.  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 accounted for, e.g.
   by changing the classification rules of a meter) or produces bogus
   accounting records.  Accounting records contain data about resource
   consumption and provide the basis for billing.  Modifying accounting
   records may lead to erroneous bills.  Furthermore, it is important
   that policies or accounting records are not redirected or removed and
   that forged policies or records are not inserted.

   - Eavesdropping
   It may be required 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 to the server can be worse than an attack to the
   service equipment itself, especially if multiple services use one AAA
   server.  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.

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   - 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

   - Protection of the integrity of accounting policies and records
   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.

   - Verify correctness of generated accounting data
   It must be ensured that the accounting data generated by the service
   provider is correct.  A provider may generate incorrect accounting
   records either deliberately (i.e. forging) or unintentionally (e.g.
   faulty configuration).  These incorrect accounting records probably
   have the consequence of incorrect bills.  Customers can verify the
   correctness of the accounting data through their measurements and/or
   through data collected by a trusted third party.  A trusted third
   party can be an independent accounting service provider as described
   in section 7.2 or a more general entity providing an auditing

   - Prevention and protection against Denial of Service attacks
   The AAA protocol and all building blocks should be designed and
   implemented in a way as resistant as possible to denial of service
   attacks.  An additional strategy to defend against DoS attacks is to
   add a component to the meter system that is able to detect suspicious
   traffic patterns.  Upon detection, further actions can be taken
   according to a pre-defined policy.

   The prevention of these hazards has to be considered for the
   protocols used for accounting policy exchange and the transportation
   of accounting records.  Since the security requirements for
   authentication, transmission level security, data object
   confidentiality and integrity are addressed in the criteria for AAA
   protocol evaluation [RFC2989], 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.  Real
   prevention of DoS attacks is quite difficult.  A selective dropping
   of the attackers packets is impossible if the malicious packets
   cannot be separated from the valid customer traffic.  Dropping of all
   packets of a certain type may prevent authorized customers from using
   the service and therefore help the attacker to achieve her goal.

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11. References

   [I2-BB]     Internet2-QBone Bandwidth Broker,

   [NetFlow]   NetFlow Services and Applications, White Paper, Cisco
               Systems, 1999

   [RFC2002]   Perkins, C., "IP Mobility Support", RFC 3220, October

   [RFC2123]   Brownlee, N., "Traffic Flow Measurement: Experiences with
               NeTraMet", RFC 2123, March 1997.

   [RFC2475]   Blake, S., Black, D., Carlson, M., Davies, E., Wang Z.
               and W. Weiss, "An Architecture for Differentiated
               Services", RFC 2475, December 1998.

   [RFC2566]   DeBry, R., "Internet Printing Protocol/1.0: Model and
               Semantics", RFC 2911, April 1999.

   [RFC2722]   Brownlee, N., Mills, C. and G. Ruth, "Traffic Flow
               Measurement:  Architecture", RFC 2722, October 1999.

   [RFC2903]   de Laat, C., Gross, G., Gommans, L., Vollbrecht, J. and
               D. Spence, "Generic AAA Architecture", RFC 2903, August

   [RFC2904]   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.

   [RFC2905]   Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L.,
               Gross, G., de Bruijn, B., de Laat, C., Holdrege, M. and
               D. Spence, "AAA Authorization Application Examples", RFC
               2905, August 2000.

   [RFC2924]   Brownlee, N. and  A. Blount, "Accounting Attributes and
               Record Formats", RFC 2924, September 2000.

   [RFC2975]   Aboba, B., Arkko, J. and D. Harrington, "Introduction to
               Accounting Management", RFC 2975, October 2000.

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   [RFC2989]   Aboba, B., Calhoun, P., Glass, S., Hiller, T., McCann,
               P., Shiino, H., Walsh, P., Zorn, G., Dommety, G.,
               Perkins, C., Patil, B., Mitton, D.,  Manning, S.,
               Beadles, M., Chen, X., Sivalingham, S., Hameed, A.,
               Munson, M., Jacobs, S., Lim, B., Hirschman, B., Hsu, R.,
               Koo, H., Lipford, M., Campbell, E., Xu, Y., Baba, S. and
               E. Jaques, "Criteria for Evaluating AAA Protocols for
               Network Access", RFC 2989, November 2000.

   [RFC3198]   Westerinen, A., Schnizlein, J., Strassner, J., Scherling,
               M., Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry,
               J. and S. Waldbusser, "Terminology for Policy-Based
               Management", RFC 3198, November 2001.

12. Acknowledgments

   The authors would like to thank the members of the AAAARCH research
   group and in particular, the chairs, John Vollbrecht and Cees de
   Laat, for the fruitful discussions and comments.  Special thanks are
   to Bernard Aboba, Nevil Brownlee and Ed Ellesson for their review and
   valuable input to this document.

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Author's Addresses

   Tanja Zseby
   Fraunhofer Institute for Open Communication Systems
   Kaiserin-Augusta-Allee 31
   10589 Berlin
   Phone: +49-30-34 63 7153
   Fax:   +49-30-34 53 8153
   EMail: zseby@fokus.fhg.de

   Sebastian Zander
   Fraunhofer Institute for Open Communication Systems
   Kaiserin-Augusta-Allee 31
   10589 Berlin
   Phone: +49-30-34 63 7287
   Fax:   +49-30-34 63 8287
   EMail: zander@fokus.fhg.de

   Georg Carle
   Fraunhofer Institute for Open Communication Systems
   Kaiserin-Augusta-Allee 31
   10589 Berlin
   Phone: +49-30-34 63 7149
   Fax:   +49-30-34 63 8149
   EMail: carle@fokus.fhg.de

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Full Copyright Statement

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