Prepaid Extensions to Remote Authentication Dial-In User Service (RADIUS)
draft-lior-radius-prepaid-extensions-22

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Network Working Group                                            A. Lior
Internet-Draft                                               Independent
Intended status: Informational                                 P. Yegani
Expires: August 29, 2013                                         Juniper
                                                            K. Chowdhury
                                               Radio Mobile Access, Inc.
                                                           H. Tschofenig
                                                  Nokia Siemens Networks
                                                           A. Pashalidis
                                                                     KUL
                                                       February 25, 2013

    Prepaid Extensions to Remote Authentication Dial-In User Service
                                (RADIUS)
              draft-lior-radius-prepaid-extensions-22.txt

Abstract

   This document specifies an extension to the Remote Authentication
   Dial-In User Service (RADIUS) protocol that enables service providers
   to charge for prepaid services.  The supported charging models
   supported are volume-based, duration-based, and based on one-time
   events.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 29, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal

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   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  7
       1.2.1.  Architectural Model  . . . . . . . . . . . . . . . . .  7
       1.2.2.  Motivation . . . . . . . . . . . . . . . . . . . . . .  9
     1.3.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . . 11
     1.4.  Example Use Case . . . . . . . . . . . . . . . . . . . . . 11
   2.  Supported Features . . . . . . . . . . . . . . . . . . . . . . 14
     2.1.  Services and Quotas  . . . . . . . . . . . . . . . . . . . 14
     2.2.  Resource Pools . . . . . . . . . . . . . . . . . . . . . . 14
     2.3.  Rating Groups  . . . . . . . . . . . . . . . . . . . . . . 16
     2.4.  Tariff Switching . . . . . . . . . . . . . . . . . . . . . 17
     2.5.  Support for Roaming  . . . . . . . . . . . . . . . . . . . 18
     2.6.  Dynamic Termination  . . . . . . . . . . . . . . . . . . . 19
     2.7.  One Time Event . . . . . . . . . . . . . . . . . . . . . . 19
       2.7.1.  One-Time Charging  . . . . . . . . . . . . . . . . . . 19
       2.7.2.  Resource Consumption Query . . . . . . . . . . . . . . 20
       2.7.3.  Service Price Enquiry  . . . . . . . . . . . . . . . . 20
       2.7.4.  Balance Check  . . . . . . . . . . . . . . . . . . . . 21
       2.7.5.  Refund . . . . . . . . . . . . . . . . . . . . . . . . 21
   3.  Operations . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     3.1.  Capability Discovery . . . . . . . . . . . . . . . . . . . 22
     3.2.  Authentication and Authorization Operation . . . . . . . . 22
     3.3.  Session Start Operation  . . . . . . . . . . . . . . . . . 24
     3.4.  Mid-Session Operation  . . . . . . . . . . . . . . . . . . 24
     3.5.  Dynamic Operations . . . . . . . . . . . . . . . . . . . . 26
       3.5.1.  Unsolicited Session Termination Operation  . . . . . . 26
       3.5.2.  Unsolicited Change of Authorization Operation  . . . . 26
     3.6.  Termination Operation  . . . . . . . . . . . . . . . . . . 27
     3.7.  Mobile IP Operations . . . . . . . . . . . . . . . . . . . 27
     3.8.  Operation Considerations for Multiple Services . . . . . . 28
       3.8.1.  Initial Quota Request  . . . . . . . . . . . . . . . . 28
       3.8.2.  Quota Update . . . . . . . . . . . . . . . . . . . . . 29
       3.8.3.  Termination  . . . . . . . . . . . . . . . . . . . . . 29
       3.8.4.  Dynamic Operations . . . . . . . . . . . . . . . . . . 29
       3.8.5.  Support for Resource Pools . . . . . . . . . . . . . . 30

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       3.8.6.  One-time Charging  . . . . . . . . . . . . . . . . . . 30
       3.8.7.  Error Handling . . . . . . . . . . . . . . . . . . . . 30
       3.8.8.  Accounting Considerations  . . . . . . . . . . . . . . 31
   4.  Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     4.1.  PrePaid Accounting Capability (PPAC) Attribute . . . . . . 32
     4.2.  Session Termination Capability Attribute . . . . . . . . . 34
     4.3.  Prepaid Accounting Operation (PPAQ) Attribute  . . . . . . 36
     4.4.  Fields . . . . . . . . . . . . . . . . . . . . . . . . . . 52
     4.5.  Prepaid Tariff Switching (PTS) Attribute . . . . . . . . . 53
   5.  Diameter RADIUS Interoperability . . . . . . . . . . . . . . . 59
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 60
   7.  Table of Attributes  . . . . . . . . . . . . . . . . . . . . . 61
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 62
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 66
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 67
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 67
     10.2. Informative References . . . . . . . . . . . . . . . . . . 67
   Appendix A.  Example flows . . . . . . . . . . . . . . . . . . . . 69
     A.1.  A simple flow  . . . . . . . . . . . . . . . . . . . . . . 69
     A.2.  A flow with prepaid tariff switching . . . . . . . . . . . 72
     A.3.  Resource pools and Rating Groups . . . . . . . . . . . . . 75
     A.4.  One-time charging  . . . . . . . . . . . . . . . . . . . . 80
     A.5.  Price enquiry  . . . . . . . . . . . . . . . . . . . . . . 81
     A.6.  Balance check  . . . . . . . . . . . . . . . . . . . . . . 82
   Appendix B.  Translation between RADIUS Prepaid and Diameter
                Credit Control  . . . . . . . . . . . . . . . . . . . 84
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 93

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1.  Introduction

   This document specifies an extension to the RADIUS protocol that
   enables service providers to perform accounting and charging in an
   "online" fashion.  In particular, they enable the service provider to

      (a) ensure that subscriber's remaining funds suffice before the
      service is delivered, and

      (b) interrupt service provision when the funds are exhausted.

   These capabilities are typically used in scenarios where the
   subscriber maintains a prepaid account with the service provider;
   hence, this extension is called the "prepaid" extension for RADIUS.
   The functionality described in this document is often referred as
   "online charging" in comparison to "offline charging" support
   provided by RFC 2866 [RFC2866].

   Note that this document does not follow the RADIUS design guidelines
   outlined in RFC 6158 [RFC6158] since it predates the publication of
   RFC 6158.  This document documents existing implementations.

   The extensions were designed with the following goals in mind:

   o  Make use of existing infrastructure as much as possible (including
      enabling the interworking of RADIUS-based and Diameter-based
      infrastructures), and thereby limit the amount of necessary
      capital expenditures,

   o  provide the ability to rate service requests in an "online"
      fashion,

   o  provide the ability to charge the user's account prior to service
      provision,

   o  protect against revenue loss, i.e., to prevent an end user from
      obtaining service when the available funds do not suffice,

   o  protect against fraud, and

   o  be deployable for a number of services independent of the access
      network technology.

   The architecture between the entities that execute the RADIUS
   protocols, with the extensions defined in this document, assumes that
   the rating of chargeable events does not occur in the element that
   provides the service.  Instead, the rating may be performed at a
   dedicated server, termed the "prepaid-enabled AAA server" or simply

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   "prepaid server" (PPS).  Alternatively, the actual rating may occur
   in an entity related to this prepaid server.

   Furthermore, this document assumes that a "quota server" is available
   which, through co-ordination with the rating entity and an account
   balance manager, is able to provide a quota indication for a
   particular user when requested.  This quota server may or may not
   coexist in the prepaid server.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   Prepaid Client (PPC):

      The entity which triggers the RADIUS message exchange, including
      the prepaid extensions defined in this document.  The PPC provides
      the service to the users, and executes the RADIUS client which,
      for the purposes of this document, is termed the "PrePaid Client"
      (PPC).  When the prepaid service is used the PPC collects service
      event information and reports it while the services is provided to
      the user.  This event information is sent to the PPS using the
      extensions defined in this document.

   Prepaid Server (PPS):

      The entity that interacts with the PPC using the RADIUS prepaid
      extensions defined in this document.

   Rating Entity:

      This entity converts the credit that is allocated by the PPS into
      a "quota".  This quota is then returned to the requesting PPC via
      the PPS.  The rating entity may also determine that during service
      provision a tariff switch will occur.  In this case the rating
      entity will include details of when exactly tariff switch will
      occur.

   Quota:

      A quota denotes the amount of granted units to be consumed without
      performing another credit control interaction.

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   Home Network:

      The network which contains the user profile and the user's prepaid
      account.

   Authorize-Only Access Request:

      A RADIUS message of type "Access Request" (code field = 1) that
      contains a "Service-Type" AVP (type = 6) with value "Authorize-
      Only".

   Offline Charging:

      Offline charging is a process where charging information for
      resource usage is collected concurrently with that resource usage.
      The charging information is then passed through a chain of logical
      charging functions.  At the end of this process, Charging Data
      Record (CDR) files are generated, which are then transferred to
      the operator's billing domain for the purpose of subscriber
      billing and/or inter-operator accounting (or additional functions,
      e.g., statistics, at the operator's discretion).  The billing
      domain typically comprises post-processing systems, such as the
      operator's billing system or billing mediation device.  In
      conclusion, offline charging is a mechanism where charging
      information does not affect, in real-time, the service rendered.
      [TS32240]

   Online Charging:

      Online charging is a process where charging information for
      resource usage is collected concurrently with that resource usage
      in the same fashion as in offline charging.  However,
      authorization for the network resource usage must be obtained
      prior to the actual resource usage to occur.  This authorization
      is granted by the PPS upon request from the PPC.  When receiving a
      resource usage request, the PPS assembles the relevant charging
      information and generates a charging event in real-time.  The PPS
      then returns an appropriate resource usage authorization.  The
      resource usage authorization may be limited in its scope (e.g.,
      volume of data or duration), therefore the authorization may have
      to be renewed from time to time as long as the resource usage
      persists.  Note that the charging information utilized in online
      charging is not necessarily identical to the charging information
      employed in offline charging.  In conclusion, online charging is a
      mechanism where charging information can affect, in real-time, the

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      service rendered and therefore a direct interaction of the
      charging mechanism with the control of resource usage is required.
      [TS32240]

1.2.  Overview

   This section provides an overview of the prepaid charging models and
   architectures, which are supported by the extensions described in
   this document.

   A number of models of how to charge customers for services in a
   prepaid manner are supported:

   o  Volume-based charging (e.g., 2 Cents/KiloByte).

   o  Duration-based charging (e.g., 3 Cents/minute).

   o  Resource-based charging (e.g., 3 videos for 10 Euros)

   o  Event-based charging (e.g., 7 Cents/ring tone) .

   This draft assumes that the user maintains a prepaid account with his
   home network.  This account may be used to fund multiple services,
   some of which may use the extensions defined in this document, and
   some may use other mechanisms.  The interworking of these mechanisms
   is outside the scope of this document.  Similarly, the means by which
   the subscriber obtains funds is also outside the scope of this
   document.

1.2.1.  Architectural Model

   This section describes the architectural model of the protocol
   extensions described in this document.  Figure 1 describes the
   involved entities.

   The end user establishes a connection with one of possibly multiple
   PPCs during service access.  The selected PPC communicates with a
   HAAA server (directly or indirectly via a broker network).

   The interface between the HAAA and the PPS is implemented using the
   RADIUS protocol together with the extensions described in this
   document.  However, in cases where the PPS does not implement the
   RADIUS protocol, the implementation would have to map the
   requirements defined in this document to a functionally equivalent
   protocol.

   The requesting PPC meters the consumption of the service according to
   the instructions provided by the PPS.  After service completion, or

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   on reception of a subsequent request for service, the PPS deducts the
   corresponding amount of credit from the user account.  When a user
   terminates an on-going service, the PPC informs the PPS with a
   suitable indication about the unused portion of the allocated quota.
   The PPS then refunds the user account accordingly.  Note that
   multiple PPSs may be deployed for reasons of redundancy and load
   sharing.  The system may also employ multiple rating servers.

                       Service
                    Access Device   Accounting
   +----------+      +---------+    Protocol  +------------+
   |  End     |<---->|+-------+|<------------>| Accounting |
   |  User    |   +->||  PCC  ||              |   Server   |
   |          |   |  ||       ||<----+        |            |
   +----------+   |  |+-------+|     |        +------------+
                  |  +---------+     |
   +----------+   |                  |
   |  End     |<--+                  |
   |  User    |                      |        +----------+
   +----------+                      +------->|          |
                                     Prepapid | PPS      |
                                     Protocol |          |
                                              +----------+

                   Figure 1: Basic Prepaid Architecture

   The PPS and the accounting server in this architecture MAY be
   combined.  The PPC must have the ability to meter the consumption of
   a prepaid data session.  This metering is typically based on time
   (i.e., seconds) or volume (i.e., octets).

   The device running the PPC may also have "Dynamic Session
   Capabilities", such as the ability to terminate a data session or to
   change the filters associated with a specific data session by
   processing "Disconnect" messages and "Change of Authorization"
   messages as per RFC 3576 [RFC3576].

   This document assumes that the PPS is used as the AAA server.  There
   are three types of AAA server, as follows.

   The AAA server in the home network (HAAA) is responsible for
   authentication of the subscriber.  In addition, the HAAA communicates
   with the PPS using the RADIUS protocol in order to authorize
   subscribers.

   This document assumes that the PPS communicates with the HAAA for the
   purposes of authentication and authorisation.  The PPS, in turn,

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   interfaces to entities which

   o  keep the subscriber's account balance (balance manager),

   o  rate access service requests in real-time (rating engine), and

   o  manage quota for a particular prepaid service (quota server).

   The balance manager, the rating engine and the quota server belong to
   the service provider's backend infrastructure and are outside the
   scope of this specification.  In particular, as far as this
   specification is concerned, they are assumed to exist in the PPS.

   Accounting messages are not needed to deliver a prepaid service.
   However, accounting messages can be used to keep the PPS up-to-date
   as to what is happening with the prepaid data session.

1.2.2.  Motivation

   Why not use existing RADIUS attributes to construct a protocol for
   prepaid scenarios?  This could lead to a solution where no code has
   to be modified at existing devices.

   It is indeed possible to construct a solution for prepaid scenarios
   using existing RADIUS attributes.  The RADIUS server would send an
   Access-Accept message containing a Session-Timeout(27) and include a
   Termination-Action(29) in the RADIUS-request.  Upon receiving the
   Access-Accept message, the NAS would meter the duration of the
   session and upon termination of the session the NAS would generate an
   Access-Request message again.  The RADIUS server would then re-
   authenticate the session and reply with an Access-Accept message
   indicating the amount of additional time in a Session-Timeout(27).
   Alternatively, it could respond with an Access-Reject message if
   there were no more resources in the user account.

   Moreover, if the user terminates the session prematurely, the NAS
   could indicate this in the accounting stream so that unused funds can
   be returned into the prepaid user account.

   Unfortunately, the above "solution" has a number of drawbacks,
   including the following.

   o  It only supports time-based charging.  The solution presented in
      this document supports multiple charging metrics.

   o  Using accounting messages to recoup unused time may be problematic
      because RADIUS accounting messages are not delivered in real-time.
      A RADIUS server may store-and-forward accounting messages in

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      batches.  Thus, relying on accounting messages for the purposes of
      prepaid may cause revenue leakage.  The solution presented in this
      document does not rely on Accounting packets at all.  It uses
      Access-Request messages, which are required to flow through any
      network in real-time.

   o  Session-Timeout(27) is not a mandatory attribute.  If a prepaid
      subscriber is served by a NAS that does not adhere to Session-
      Timeout then that subscriber may use the service for an
      undetermined period of time.

   o  Termination-Action(29) presents its own issues.  Firstly, the
      behaviour of Termination-Action(29) is not mandatory.  Secondly,
      according to RFC 2865 [RFC2865], Termination-Action fires when the
      provision of the service has completed.  However, service should
      not be terminated when negotiating additional quota, because this
      should happen in a manner transparent to the subscriber.  Due to
      the fact that Termination-Action occurs when the service is
      completed, it is unclear whether or not user experience would be
      affected if this attribute would be used in a prepaid scenario.
      The RADIUS server might even allocate a new IP address to the
      subscriber device after a Termination-Action.  Also, the RADIUS
      server has no way of telling why a given Access-Request message
      was generated.  The RADIUS server might have to wait for the
      corresponding accounting packet to determine the reason.  Finally,
      re-authenticating the subscriber may take too long.  The solution
      presented in this document allows quota replenishing to occur
      without affecting user experience.  No re-authentication is
      required and quotas can be negotiated before the available credit
      actually runs out.

   o  Due to the fact that the standard RADIUS attributes are not
      mandatory, the correct prepaid operation is really an act of faith
      on the part of the RADIUS server.  If Session-Timeout(27) and/or
      Termination-Action(29) are not supported, the prepaid subscriber
      might be able to obtain the service for free.  The solution
      described in this document requires that a PPC informs the RADIUS
      server, regardless of whether or not the latter supports the
      prepaid extensions.  The RADIUS server can then determine whether
      or not service should be granted.  For example, if a prepaid
      subscriber is connected to a NAS that does not support prepaid,
      the RADIUS server can either instruct the NAS to tunnel the
      traffic to another entity in the home network (e.g., an Home
      Agent) that supports prepaid, or cause it to provide only a
      restricted service.

   The solution presented in this document requires the support of two
   mandatory and one optional attribute.  Furthermore, it does not

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   require a great amount of additional code at a NAS (or similar
   device) that already supports time or volume-based metering.  The
   solution requires that RADIUS entities advertise their prepaid
   capabilities in an Access-Request and that they generate an Access-
   Request packet with Service-Type="Authorize-Only" in order to obtain
   more quota when or before the current quota is used up.  It also
   requires the NAS to send an Access-Request with Service-
   Type="Authorize-Only" when the session terminates in order to refund
   the subscriber account.

1.3.  Assumptions

   This document makes the following assumptions.

   o  The values carried in the Service Identifiers are pre-configured
      between the PPC and the PPS.

   o  The decision about the service rating happens at the PPS.

   o  The decision whether credit control requests for two services are
      placed in a resource pool are made by the PPS.

   o  The decision which services belong to the same rating group are
      pre-configured at the PPC.  Once a rating group is authorized it
      is not necessary to re-authorize an additional service that
      belongs to the same rating group at the PPS again.

   o  A price enquiry is done purely for the purpose of providing AoC
      for the end user, not for processing at the PPC nor to trigger any
      specific actions.

1.4.  Example Use Case

   This section describes the sequence of events in an example RADIUS
   prepaid transaction.

   1.  When an end host attaches to a network (for example, using IEEE
       802.1X), as usual, the PPC that is servicing the subscriber uses
       the AAA infrastructure in order to authenticate and authorize the
       subscriber with respect to the requested service.  In order to do
       this, it sends a RADIUS Access-Request to the AAA server.  This
       Access-Request contains the subscriber's credentials and may
       contain the prepaid capabilities of the PPC.

   2.  The authentication procedure proceeds.  This may involve several
       message exchanges, as it is the case with the Extensible
       Authentication Protocol (EAP) [RFC2284].  Once the subscriber has
       been successfully authenticated, the home AAA server determines

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       that the subscriber is a prepaid subscriber and requests
       authorisation from the PPS.  This request MUST include the
       prepaid capabilities of the serving PPC.

   3.  The PPS, possibly with the help of the backend infrastructure,
       validates that the subscriber has a prepaid account and that the
       account is active.  It further validates that the PPC has the
       appropriate prepaid capabilities.  If all is in order, the PPS
       authorises the subscriber to use the network.  Otherwise it
       rejects the request.  The decision is sent to the AAA system in
       the form of a response message.  In the case of success, this
       message contains attributes that indicate the allocation of a
       portion of the subscriber credit.  This portion is called the
       "initial quota" and is expressed in units of time or volume.  The
       response may also include a threshold value.  Note that only a
       portion of the user's funds is allocated because the user may be
       engaged in other services that may draw on the same account.  For
       example, the user may be engaged in a data session and a voice
       session.  Although these two services would draw from the same
       account, they form separate parts of the overall system.  If the
       entire quota was allocated to the data session then the user
       would have no more funds for a voice session.

   4.  The AAA system incorporates the attributes received from the PPS
       into an Access-Accept message that it sends to the PPC.  Note
       that the AAA system is responsible for authorizing the service
       whereas the prepaid system is responsible for prepaid
       authorization.

   5.  Upon receiving the Access-Response, the PPC starts the prepaid
       data session and meters the session based on time or volume, as
       indicated in the message.

   6.  Once the consumption approaches the allocated limit (as expressed
       by the threshold), the PPC will request additional quota.  Re-
       authorization for additional quota flows through the AAA system
       to the PPS.  The PPS revalidates the subscriber account and
       subtracts the previously allocated quota from the current
       balance.  If there is remaining balance, it reauthorizes the
       request with an additional quota allotment.  Otherwise, the PPS
       rejects the request.  Note that the replenishment of the quota is
       a re-authorization procedure and does not require the subscriber
       to authenticate himself again.

   7.  Upon receiving a re-allotment of the quota, the PPC continues to
       provide the requested service until the new threshold is reached.
       If the request for additional quota cannot be fulfilled then the
       PPC lets the subscriber use the remaining quota and terminates

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       the session.  Alternatively, instead of terminating the session,
       the PPC may restrict service access in such a way that the
       subscriber can only reach a particular web server.  This web
       server maybe used to allow the subscriber to replenish his
       account.  This restriction can also be used to allow new
       subscribers to set up prepaid accounts in the first place.

   8.  Should the subscriber terminate the session before the quota is
       exhausted, the remaining balance allotted to the session is
       refunded into his account.

   Note that the subscriber may have disconnected while the PPC is
   waiting for the initial quota.  The entire allocated quota will have
   to be credited back to the subscribers account in this case.  Also
   note that the PPS maintains session state for the subscriber.  This
   state includes how much account balance was allocated during the last
   quota enquiry and how much is left in the account.  Therefore, it is
   required that all messages about the session reach the same (and
   correct) PPS.

   For a simple message flow, along the lines of this use case, please
   see Appendix A.

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2.  Supported Features

   This section describes the features that are supported by the
   extensions specified in this document.

2.1.  Services and Quotas

   Examples of services that the user may be using are browsing the web,
   participating in a VoIP conversation, watching streaming video and
   downloading a ring tone.  Some operators may want to distinguish
   between these services and to charge them at different rates and
   meters them differently.  Therefore, the prepaid solution needs to be
   able to distinguish services, and allocate quota to the services
   using different unit types (time, volume) and allow for those quotas
   to be consumed at different rates.

   +---------+            +---------+            +-------+
   |         | 1        N |         | 1        1 |       |
   | Session |<---------->| Service |<---------->| Quota |
   |         |            |         |            |       |
   +---------+            +---------+            +-------+

            Figure 2: Multiple services within a single session

   As shown in Figure 2, a session may be associated with multiple
   services.  Each service is identified by a service identifier
   (Service-ID).  The format of the Service-ID is not in the scope of
   this document.  It may, for example, be expressed as a 5-tuple {i.e.,
   source IP address, destination IP address, source port, destination
   port, and protocol type}.  Each service is associated with a quota
   whereby a quota might be applicable to multiple services.  An example
   message flow that involves multiple services within a single session
   is given in the Appendix A.

2.2.  Resource Pools

   When working with multiple services a new problem arises because one
   service may consume its quota faster than another service.  When the
   user balance is close to exhaustion, a situation could arise where
   one service is unable to obtain quota while another service has
   plenty of quota remaining.  Unless the quotas can be rebalanced, the
   SAD would then have to terminate the former service.  Moreover, it is
   likely that each service generates a certain amount of RADIUS prepaid
   traffic.  In an environment with many users and charged services,
   this amount of traffic may become a considerable overhead that could
   lead to inefficiency.

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   One method to circumvent the above situation is to use a so-called
   "resource pool".  Resource pools enable the allocation of resources
   to multiple services of a session by allocating resources to a pool
   and have services draw their quota from the pool at a rate
   appropriate to that service.  When the quota that has been allocated
   to the pool is close to exhaustion, the entire pool (rather than
   individual services) is replenished.

              +-----------+
              | Service-A |-----+         +--------+
              +-----------+     |    Ma   |        |
                                +-------->|        |
                                          |  Pool  |
                                +-------->|   (1)  |
              +-----------+     |    Mb   |        |
              | Service-B |-----+         +--------+
              +-----------+

                      Figure 3: Resource pool example

   As shown in Figure 3, Service-A and Service-B are bound to Pool(1).
   Ma and Mb are the pool multipliers (that are associated with
   Service-A and Service-B respectively) that determine the rate at
   which Service-A and Service-B draw from the pool.

   The pool is initialized by taking the quota allocated to service n
   and multiplying it by Mn.  Therefore, the amount of resources
   allocated to a pool is given by Poolr = Ma*Qa + Mb*Qb + . . ., where
   Qn denotes the amount of quota that is allocated to service n.
   Further, the pool is considered to be empty if

           Poolr <= Ca*Ma + Cb*Mb + . . .,

                                 Figure 4

   where Ca and Cb are resources consumed by Service-A and Service-B
   respectively.

   Note that the resources assigned to the pool are not associated with
   a metric.  That is, Service-A can be rated at $1 per MB and Service-B
   can rated at $0.10 per minute.  In this case, if $5 worth of
   resources are allocated for service-A to the pool and if Ma = 10,
   then 50 units would be placed into the pool.  If a further $5 are
   allocated for service-B to the pool, then M=1 and 50 units are
   deposited into the pool.  The pool would then have a total sum of 100
   units to be shared between the two services.  The PPC would then

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   mater the services such that each Mbyte used by Service-A will draw
   10 units from the pool and each minute used by Service-B will draw 1
   unit from the pool.

2.3.  Rating Groups

   A Rating Group gathers a set of services, identified by a service
   identifier, and subject to the same cost and rating type (e.g., $0.1/
   minute).

   The rating of a service can be quite complex.  While some operators
   follow linear pricing models, others may wish to apply more complex
   functions.  For example, a service provider may wish to rate a
   service such that the first N MBytes are free, then the next M Mbytes
   are rated at $1 per MB and volume above (N+M) MB be rated at $0.50
   per MB.  Such a function could be implemented by repeated message
   exchanges in the prepaid system.

   To avert the need to exchange many messages while still supporting
   such complex rating functions, the concept of the Rating Group was
   introduced.

   As shown in Figure 5, a Rating Group is associated with one or more
   services and defines the rate that the services associated with the
   Rating Group consume an allocated amount of quota.

                           +--------------+       +--------------+
   +-----------+ N     1   |              | M  1  | Resource Pool|
   | Service-A +---------->| Rating Group |------>|      or      |
   +-----------+           |              |       |    Quota     |
                           +--------------+       +--------------+

                          Figure 5: Rating Group

   During the usage of a service that is associated with a Rating Group,
   the PPC sends the ID of the Rating Group to the PPS.  The PPS
   authorises the Rating Group by allocating a quota to it and assign it
   to a Resource Pool.  When an additional service that belongs to an
   already authorised Rating Group is instantiated, the PPC does not
   need to re-authorize this service.  This effectively means that the
   PPC meters the service such that it draws from the already allocated
   quota.  Therefore, no RADIUS messages need to be exchanged in this
   case.  This limits the amount of traffic between the PPC and the PPS.
   An example of a flow that uses Rating Groups is given in Appendix A.3

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2.4.  Tariff Switching

   Tariff is the set of parameters defining the utilization charges for
   the use of a particular service.

   This mechanism is useful if, for example, as shown in Figure 6,
   traffic before 18:00 is rated at rate r1 and traffic after 18:00 is
   rated at rate r2.  The mechanism requires the PPC to report usage
   before and after the switch occured.

                           18:00
            ------------------+-----------------
                   r1         |      r2
            ------------------+-----------------
                 ^                        ^
                 |<----TSI--->            |
                 |                        |
           Access-Accept            Access-Request
           (quota allocated)        (quota consumed)

                   Figure 6: Example of Tariff Switching

   The PPC indicates support for tariff switching by setting the
   appropriate bit in the PPAC.  If the PPS needs to signal a tariff
   switch time it will send a PTS attribute that indicates the point in
   time when the switch will occur.  This indication represents the
   number of seconds from current time (TariffSwitchInterval TSI).

   At some point after the tariff switch the PPC sends another Access-
   Request, as a result of either the user having logged off or the
   volume threshold being reached.  The PPC reports how much volume was
   used in total (in a PPAQ attribute) and how much volume was used
   after the tariff switch (in a PTS VUATS subtype attribute).

   In situations with multiple tariff switches, the PPS has to specify
   the length of the tariff switch period using the
   TimeIntervalAfterTariffSwitchUpdate (TITSU) field in the PTS
   attribute, as shown in Figure 7.

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                            18:00                 23:30
            ------------------+---------------------+--------------
                   r1         |      r2             |     r3
            ------------------+---------------------+--------------
                 ^                        ^         ^
                 |<----TSI---><-----------|-------->|TITSU
                 |                        |
           Access-Accept            Access-Request

                    Figure 7: Multiple Tariff Switches

   When a TITSU is specified in the PTS, the PPC MUST generate an
   Access-Request within the time after TSI and before TITSU expires.
   Note that, typically, the PPC will be triggered by the Volume
   Threshold.  However, it is possible that, during period r2, resources
   are not entirely consumed and, thus, the threshold is not reached.
   The TITSU attribute ensures that, even in this case, the PPC will
   generate the new Access-Request in good time.

   For time based services, the quota is continuously consumed at the
   regular rate of 60 seconds per minute.  At the time when credit
   resources are allocated, the server already knows how many units will
   be consumed before the tariff time change and how many units will be
   consumed afterward.  Similarly, the server can determine the units
   consumed at the before rate and the units consumed at the rate
   afterward in the event that the end user closes the session before
   the consumption of the allotted quota.  There is no need for
   additional traffic between the PPC and the PPS in the case of tariff
   time changes for continuous time based service.  Therefore, the
   tariff change mechanism is not used for such services.  For time-
   based services in which the quota is not continuously consumed at a
   regular rate, the tariff change mechanism described for volume and
   event units may be used.

2.5.  Support for Roaming

   In certain networks it is essential for prepaid data services to be
   available to roaming subscribers.  Support for both static and
   dynamic roaming models is needed.  In a static roaming scenario the
   subscriber connects to a foreign network which has a roaming
   agreement either directly with the home network, or through a broker
   network.  When the subscriber logs into another foreign network, a
   new login procedure has to be executed.

   In a dynamic roaming scenario the subscriber may move between
   networks while maintaining his connection.  In such a scenario the
   data session is seamlessly handed off between the networks.

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   In both roaming scenarios, the subscriber always authenticates
   himself to the home network.  Authorization for the prepaid session
   and quota replenishing occurs at the home network and more
   specifically at the PPS where state is being maintained.

   Roaming is challenging because a subscriber who established a prepaid
   data session may move to another PPC that does not support the
   prepaid extensions.

2.6.  Dynamic Termination

   When fraud or an error is detected, either only the affected session,
   or all sessions of the affected subscriber should be immediately
   terminated.  Under certain conditions, the system may wish to
   terminate the session in order to make sure that the user is not
   charged for services it does not use.

   Certain handoff procedures used in dynamic roaming scenarios require
   that the system terminates the subscribers prepaid data session at a
   PPC.  This is the case, for example, when time-based prepaid is used
   and the mobile subscriber performs a dormant handoff.

2.7.  One Time Event

2.7.1.  One-Time Charging

   One-time charging is a mode of operation where the RADIUS prepaid
   extensions are used for charging of a service that is provided
   instansteneously.  An example of such an event is the purchase of a
   ring tone.  Subscription based services can also be modeled as a one-
   time event.  In this case the one-time service event is the purchase
   of a subscription.

   For a given user, one-time charging may occur in parallel with other
   charging models.  For example, the subscriber may be connected to the
   Internet, which is metered (based on time or volume), while he also
   purchases a ring tone (a one-time-based event).

   Note that it is up to the service providers to decide whether or not
   the user will be charged for the download of, for example, the video
   and also be charged for the data volume required to download the
   video.  The facilities provided by this document gives the service
   provider the capability to achieve their service charging business
   goals.

   The PPC signals one-time charging to the PPS with an indication that
   identifies the service and the units that should be debited from the
   user account.

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   A PPC may decide to perform one-time charging and the PPC may need to
   authenticate the user before sending the relevant message to the
   user's home AAA server (and to the PPS).

   Note that one-time charging can also be used to credit the prepaid
   account.  For example, the PPC can return resources to the subscriber
   by issuing a one-time charging request that includes the amount of
   resources to be credited into the account.

2.7.2.  Resource Consumption Query

   It should be possible for the PPS to query the PPC for the current
   resource consumption and to adjust the users account balance.  For
   example, a request to the PPS is made (e.g., a one-time charging
   event), the account is depleted and resources have been allocated to
   the PPC.  The PPS should have the ability to query the PPC and, if it
   has the spare resources, to reassign the quotas to the PPC and to the
   pending request.  Note that the PPS does not know resource usage
   until the PPC request for more resources.  This can be a long time.

   In the absence of this capability the PPS can minimize the effect of
   this phenomenon by allocating small quotas, a practice that results
   in more message exchanges.

2.7.3.  Service Price Enquiry

   The PPC may need to know the price of the service event.  Services
   offered by application service providers whose prices are not known
   in the PPC might exist.  The end user might also want to get an
   estimation of the price of a service event before requesting it.

   A PPC issues a PPAQ to the PPS including the Requested-Action SubType
   with the value set to "Price Enquiry" (2).  The request includes
   enough information to identify the service, namely a Service-
   Identifier or a Rating-Group-Identifer.

   The PPS calculates the cost of the requested service event, but it
   does not perform any account balance check or credit reservation from
   the account.

   The estimated cost of the requested service event is returned to the
   PPS with a PPAQ in the Cost-Information SubType.  The PPC may
   transfer the information to the end user as an advice of charge.

   More information regarding the price enquiry functionality is
   provided in Section 4.3.15 and in Section 4.3.17.

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2.7.4.  Balance Check

   The PPC may only have to verify that the end user's account balance
   covers the cost of a certain service without reserving any units from
   the account at the time of the inquiry.  This method does not
   guarantee that credit would be left when the PPC requests the
   debiting of the account with a separate request.

   A PPC issues a PPAQ to the PPS including the Requested-Action SubType
   with the value set to "Balance Check" (1).  The request includes
   enough information to identify the service, namely a Service-
   Identifier or a Rating-Group-Identifer.

   The PPS makes the balance check, but it does not make any credit-
   reservation from the account.

   The result of balance check, namely "Success" (1) or "Failure" (2),
   is returned to the PPC in the Check-Balance-Result SubType conveyed
   in the PPAQ attribute from the PPS to the PPC.

   More information regarding the balance check functionality is
   provided in Section 4.3.15 and in Section 4.3.16.

2.7.5.  Refund

   Some services may refund service units to the end user's account; for
   example, gaming services.

   To initiate refunding the PPC includes the PPAQ attribute in an
   Access-Request packet and the amount (as a negative value) to be
   refunded is specified using the Resource Quota and Resource Quota
   overflow subtypes.  This functionality is similar to one-time
   charging with the difference that refunding uses negative values

   Information about the service need to be provided by the PPC to allow
   service identification, namely the Service-ID field of the PPAQ
   identifies the prepaid service.

   Note that a monetary amount itself to be refunded is not provided but
   rather abstract units.  Based on prior out-of-band agreements between
   the PPC and the PPS these abstract units are translated into a
   monetary amount.

   More information regarding the refund functionality is provided in
   Section 3.8.6.

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3.  Operations

   This section contains the normative text for the prepaid extension.

3.1.  Capability Discovery

   The PPC initiates the authentication and authorization procedure by
   sending a RADIUS Access-Request to the HAAA.  Since the PPC MUST
   include a PPAC attribute in the RADIUS Access-Request.  The PPAC
   attribute indicates to the PPS which prepaid capabilities are
   possessed by the PPC.  These are required in order to complete the
   prepaid authorization procedure.Moreover, if the PPC supports the
   Disconnect-Message or the Change-of-Authorization capabilities, then
   it SHOULD include the Session Termination attribute.

   In certain deployments, there may be other ways to terminate a data
   session, or change authorization of an active session.  For example,
   some PPCs provide a session termination service via Telnet or SNMP.
   In these cases, the AAA server MAY add the Dynamic-Capabilities
   message to the Access-Request.  Upon receiving the Change-of-
   Authorization message, the AAA server would then be responsible for
   terminating the session using the means that are supported by the
   device.

   If the authentication procedure involves multiple message exchanges
   (as it is the case with EAP), the PPC MUST include the PPAC attribute
   in at least the last Access-Request of the authentication procedure.

3.2.  Authentication and Authorization Operation

   Once the Access-Request arrives at the HAAA, the HAAA authenticates
   the subscriber.  If this fails, the HAAA sends an Access-Reject
   message to the client.  If authentication succeeds, the HAAA
   determines whether or not the subscriber is a prepaid subscriber.  If
   the subscriber is not a prepaid subscriber, then the HAAA responds as
   usual with an Access-Accept or an Access-Reject message.  If the
   subscriber is a prepaid subscriber then the HAAA MAY forward the
   Access-Request to the PPS for further authorization.

   The Access-Request contains the PPAC attribute and the Dynamic-
   Capabilities attribute if one was included.  The User-Name(1)
   attribute MAY be set to a value that identifies the subscriber.  This
   attribute is used by the PPS to locate his account.  For added
   security, the HAAA MAY also set the User-Password(2) attribute to the
   password used between the HAAA and the PPS.

   The PPS locates the subscriber account and authorizes him.  During
   this procedure, the PPS takes into consideration the PPCs

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   capabilities.  Upon successful authorization, the PPS generates an
   Access-Accept containing an PPAC attribute and an PPAQ attribute.
   The PPAC attribute returned to the client indicates the type of
   prepaid service to be provided for the session.  The PPAQ attribute
   includes the following information.

   o  The QID, which is set by the PPS to a unique value, is used to
      correlate quota requests.

   o  Volume and/or Time quota, which is set to a value representing a
      portion of the subscriber's credit.

   o  Time or Volume Threshold that indicates when the PPC should
      request additional quota.  This information is optional.

   o  The IP address of the serving PPS and one or more alternative
      PPSs.  This is used by the HAAA to route subsequent quota
      replenishing messages to the appropriate PPS(s).

   o  A State attribute, as defined in RFC 2865 [RFC2865].  This is
      necessary in order to satisfy the requirements of Section 5.44 of
      RFC 2865 [RFC2865], which mandates that an Access-Request with
      Service-Type="Authorize-Only" must contain a State attribute.
      Since the PPC sends subsequent quota replenishment requests in the
      form of such "Authorize-Only" requests, a State attribute MUST be
      present in all Access-Accept messages that also carry a PPAQ
      attribute.

   Note: The Idle-Timeout(28) attribute can be used to trigger the
   premature termination of a prepaid service, for example as a result
   of inactivity.

   Depending on site policies, after failed authorization, the PPS may
   generate an Access-Reject in order to terminate the session
   immediately.  Alternatively, the PPS may generate an Access-Accept
   blocking some or all of the traffic and/or redirect some or all of
   the traffic to a location to a fixed server.  (This feature could be
   used, for example, to prompt the user to replenish their account.)
   Blocking of traffic is achieved by either Filter-ID(11) or NAS-
   Filter-Rule (see [RFC4849]).  A description of the redirect
   functionality is outside the scope of this document.  The time period
   before the session is blocked/redirected is specified by the Session-
   Timeout(27) attribute.

   Upon receiving an Access-Accept from the PPS, the HAAA appends the
   usual service attributes and forward the packet to the PPC.  The HAAA
   SHOULD NOT overwrite any attributes already set by the PPS.  If the
   HAAA receives an Access-Reject message, it will simply forward the

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   packet to its client.  Depending on site policies, if the HAAA does
   not receive an Access-Accept or an Access-Reject message from the PPS
   it MAY do nothing or send an Access-Reject or an Access- Accept
   message back to the PPC.

3.3.  Session Start Operation

   The start of the session is indicated by the arrival of an
   Accounting-Request(Start) packet.  The Accounting-Request (Start) MAY
   be routed to the PPS such that it can confirm the initial quota
   allocation.

   Note that the role of the PPS is not to record accounting messages
   and therefore it SHOULD NOT respond with an Accounting Response
   packet.  If the PPS does not receive the Accounting-Request(start)
   message it will only know that the session has started upon the first
   reception of a quota replenishment operation.

   If the PPS does not receive indication directly (via Accounting-
   Request(start)) or indirectly, it SHOULD, after some configurable
   time, deduce that the session has not started.  If the PPC supports
   termination capabilities, the PPS SHOULD send a Disconnect Message to
   the PPC as a measure to ensure that the session is indeed dead.

3.4.  Mid-Session Operation

   During the lifetime of a prepaid data session the PPC may request the
   replenishment of the quotas using an Authorize-Only Access-Request
   message.  Once either the allocated quota has been exhausted or the
   threshold has been reached, the PPC MUST send an Access-Request with
   Service-Type(6) set to a value of "Authorize-Only" and the PPAQ
   attribute.

   The PPC MUST also include NAS identifiers, and Session Identifier
   attributes in the Authorize-Only Access-Request.  The Session
   Identifier should be the same as the one used during the initial
   Access-Request.  For example, if the User-Name(1) attribute was used
   in the Access-Request it has to be included in the Authorize-Only
   Access-Request as well, especially if the User-Name(1) attribute is
   used to route the Access-Request to the Home AAA server.

   The Authorize-Only Access-Request MUST NOT include a User Password
   and MUST NOT include a CHAP Password.  In order to enable the
   receiver to authenticate the message, the PPC MUST include a Message-
   Authenticator(80).  In order to satisfy the requirements of Section
   5.44 of RFC 2865 [RFC2865], the PPC MUST also include the State
   attribute.  It is anticipated that the inclusion of the State
   attribute will enable the PPS to map the Authorize-Only Access

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   Request to the authentication context that was established when the
   PPC authenticated itself at the beginning of the session.  The PPC
   computes the value for the Message-Authenticator and the State
   attributes according to RFC 2869 [RFC2869] and RFC 2865 [RFC2865]
   respectively.

   When the HAAA receives an Authorize-Only Access-Request that contains
   a PPAQ, it validates the message using the Message-Authenticator(80),
   according to RFC 2869.  If the HAAA receives an Authorize-Only
   Access-Request that contains a PPAQ and either no or an invalid
   Message-Authenticator(80) it SHALL silently discard the message.  An
   Authorize Only Access-Request message that does not contain a PPAQ is
   either erroneous or belongs to another application (for example, a
   Change of Authorization message [RFC3576]).  In this case the
   Authorize-Only Access-Request is either silently discarded or handled
   by another application.

   Once the Authorize-Only Access-Request message is validated, the HAAA
   SHALL forward the Authorize-Only Access-Request to the appropriate
   PPS.  The HAAA MUST forward the Authorize-Only Access-Request to the
   PPS specified in the PPAQ.  The HAAA MUST add a Message-
   Authenticator(80) to the message, according to RFC 2869.  As with the
   Access-Request message, the HAAA MAY modify the User-Name(1)
   attribute such that it identifies the user to the PPS.

   When the PPS receives the Authorize-Only Access-Request containing a
   PPAQ attribute, it MUST validate the Message-Authenticator(80) as
   described in RFC 2869.  If validation fails, the PPS MUST silently
   discard the message.  If it receives an Authorize-Only Access-Request
   message that does not contain a PPAQ, it MUST silently discard the
   message.

   The PPS locates the prepaid session state and uses the QID contained
   within the PPAQ to detect replays.  The PPS takes the most recently
   allocated quota and subtracts it from the user balance.  If
   sufficient balance remains, the PPS authorizes the PPS and allocates
   additional quota.  The PPS may also calculate a new threshold value.
   Upon successful re-authorization, the PPS generates an Access-Accept
   containing the PPAQ attribute.

   Depending on site policies, upon unsuccessful authorization, the PPS
   generates an Access-Reject or an Access-Accept with Filter-Id(11) or
   Ascend-Data-Filter attribute (if supported) and the Session-
   Timeout(27) attribute such that the subscriber can get access to a
   restricted set of locations for a short period of time.  This feature
   could be used to enable users to replenish their accounts, create new
   accounts, or to access free content.

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   Upon receiving an Access-Accept from the PPS, the HAAA forwards the
   message to its client.  If the HAAA receives an Access-Reject
   message, it forwards the message.  Depending on site policies, if the
   HAAA does not receive an Access-Accept or an Access-Reject message
   from the PPS it MAY do nothing or it MAY send an Access-Reject
   message back to its client.

   Upon receiving an Access-Accept, the PPC updates its quotas and
   threshold parameters with the values contained in the PPAQ attribute.
   Note that the PPS MAY update the PrePaidServer attribute(s) and these
   may have to be saved as well.  If the Access-Accept message contains
   a Filter-Id(11), an Ascend-Data-Filter attribute, or Session
   Timeout(27), the PPC SHALL restrict the subscriber session
   accordingly.

3.5.  Dynamic Operations

   The PPS may take advantage of the dynamic capabilities that are
   supported by the PPC as advertised in the Session Termination and the
   PPAC attribute during the initial Access-Request.  There are two
   types of actions that the PPS may perform.  Firstly, it may request
   the session to be terminated.  Secondly, it may request the
   attributes associated with the session to be modified.  More
   specifically, it may modify a previously sent PPAQ.

   Both of these actions require that the session be uniquely identified
   at the PPC as described in [RFC3576].

3.5.1.  Unsolicited Session Termination Operation

   At anytime during a session the PPS may send a Disconnect Message in
   order to terminate a session, see in [RFC3576].  Upon successful
   termination of a session the PPC MUST return any unused quota to the
   PPS by issuing an Authorize-Only Access-Request containing the PPAQ
   which contains any unused quota and the Update-Reason set to "Remote
   Forced Disconnection".

3.5.2.  Unsolicited Change of Authorization Operation

   At any time during the session the PPC may receive a Change of
   Authorization (CoA) message.  A PPS may send a new quota to either
   add or to remove quota that is allocated to the service.  If the
   Change of Authorization contains a PPAQ then that PPAQ overrides a
   previously received PPAQ.  The PPS MUST NOT change the units used in
   the PPAQ.

   If the newly received PPAQ reduces the amount of allocated quota
   beyond what is already used then the PPC accepts the new PPAQ and act

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   as it normally would when the quota is used up.  For example, if the
   threshold is reached then is request a quota update.

3.6.  Termination Operation

   The termination phase is initiated when (i) the subscriber logs off,
   (ii) the subscriber balance is exhausted, or (iii) when the PPC
   receives a Disconnect Message.

   In case the user logged off, or the PPC receives a Disconnect
   Message, the PPC sends an Authorize-Only Access-Request message with
   a PPAQ and Update-Reason attribute set to either "Client Service
   Termination" or "Remote Forced Disconnect".  This message indicates
   the amount of consumed quota.

   In case the currently allocated quota is exhausted, if the PPAQ
   contained the Termination-Action subytype, the PPC follows the
   specified action.

3.7.  Mobile IP Operations

   In roaming scenarios with Mobile-IP, the prepaid data session should
   be maintained transparently if the HA is acting as the access device
   hosting the PPC.  As the subscriber device associates with a new
   access service device (AP or PDSN that supports PPC capability), this
   service access device sends a RADIUS Access-Request and the
   subscriber is re-authenticated and reauthorized.  The service access
   device SHALL include the PPAC attribute in the RADIUS Access-Request.
   In this manner, the procedure follows the Authentication and
   Authorization procedure described earlier.

   If the HA was acting as the service access device before handoff,
   then the prepaid session does not undergo any change after the
   handoff because the Mobile IP session is anchored at the HA and the
   user's Home IP address does not change.

   In the case of a wireless access point or PDSN acting as the service
   access device, it is likely that the user's (care-of) IP address will
   change.  The prepaid session will be affected by this.  In this
   scenario the service access device shall send an Access-Request
   message which is routed to the home network and SHALL reach the PPS
   that is serving this session.  The PPS correlates the new
   authorization request with the existing active session and assigns a
   quota to the new request.  Any outstanding quota at the old service
   access device SHALL be returned to the PPS if the Mobile-IP nodes (HA
   and FA) support registration revocation (Mobile IPv4 only).
   Specifically, the quota SHOULD be returned when the service access
   device sends the Authorize-Only Access-Request with PPAQ Update-

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   Reason set to either "Remote Forced Disconnect" or "Client Service
   Termination".  In order to trigger the sending of this last
   Authorize-Only Access- Request, the PPS may issue a Disconnect
   Message [3576] to the service access device.

   Even if the subscriber moves to a service access device that does not
   have prepaid capabilities can the prepaid data service continue.
   This can be done by requesting the Home Agent (assuming it has such
   capabilities) to take over the responsibilities of the service access
   device (i.e. metering).  This scenario will be discussed in detail in
   a later version of this document.

3.8.  Operation Considerations for Multiple Services

   This section describes the support for multiple prepaid services on a
   single PPC.  Message flows illustrating the various interactions are
   presented in Appendix A.

   A PPC that supports prepaid operations for multi-services SHOULD set
   the "Multi-Services Supported" bit in the PPAC.  When working with
   multi-services, we need to differentiate between the services.  A
   Service-Id attribute is used in the PPAQ in order to uniquely
   differentiate between the services.  The exact definition of the
   Service-Id attribute is outside the scope of this document.

   A PPAQ that contains a Service-Id is associated with that service.  A
   PPAQ that contains a Rating-Group-Id is associated with that Rating-
   Group.  A PPAQ MUST NOT contain both a Rating-Group-Id and a
   Service-Id.  A PPAQ that contains neither a Rating-Group-Id nor a
   Service-Id then the default service is used, i.e., the "Access
   Service".

3.8.1.  Initial Quota Request

   When operations with multiple services is desired then the PPC
   requests the initial quota by sending a PPAQ containing the
   Service-Id in an Authorize-Only Access-Request packet for that
   service.  Similarly, if the PPC supports rating groups then it may
   request a quota for the rating group by sending a PPAQ containing the
   Rating-Group-Id.  In both cases the Update-Reason is set to "Initial-
   Request".  The Authorize-Only Access-Request message MAY contain more
   than one PPAQ.

   Upon receiving an Authorize-Only Access-Accept message containing one
   or more PPAQs, the PPS allocates resources to each PPAQ.  Each PPAQ
   is assigned a unique QID that MUST appear in subsequent PPAQ updates
   for that service or rating group.  Additionally, the PPAQ MUST
   contain the Service-ID or Rating-Group-Id, unless the PPAQ is the

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   generic "Access Service".

3.8.2.  Quota Update

   Once the services start to utilize their allotted quota they will
   eventually need to replenish their quotas (either the threshold is
   reached or no more quota remains).  In order to replenish the quota,
   the PPC sends an Authorize-Only Access-Request message containing one
   or more PPAQs.  Each PPAQ MUST contain the appropriate QID,
   Service-ID or Rating-Group-Id (or neither the Service-ID or Rating-
   Group-Id if the quota replenishment is for the "Access Service").
   The Update-Reason filed indicates either "Threshold reached"(3), or
   "Quota reached"(4).

   Upon receiving an Authorize-Only Access-Request packet with one or
   more PPAQs the PPS responds with a new PPAQ for that service.  The
   PPAQ contains a new QID, the Service-Id or the Rating-Group-Id, and a
   new QID.  If the PPS does not grant additional quota for the service
   it MUST include the Termination-Action subfield in the PPAQ that will
   instruct the PPC to take appropriate actions.

3.8.3.  Termination

   When the allotted quota for a service is exhausted, the PPC shall act
   in accordance with the flags set in the Termination-Action subtype.
   If the Termination-Action subtype is absent then the service MUST be
   terminated.  If the service is to be terminated, then the PPC shall
   send a PPAQ with the appropriate QID, the Service-Id, the used quota,
   and the Update-Reason set to "Client Service Termination".

   If the "Access Service" has terminated, then all other services must
   be terminated as well.  In this case the PPC MUST report on all
   issued quotas for the various services.  The Update-Reason field
   should be set to "Access Service Terminated".

3.8.4.  Dynamic Operations

   Dynamic operations for multi-services are similar to dynamic
   operations described for single service operations.  The PPS MAY send
   a COA message containing a PPAQ for an existing service instance.
   The PPC matches the PPAQ with the service using the Service-ID or the
   Rating-Group-Id attribute.  The new quota could differ from the
   previously allocated value.

   A disconnect message terminates the "Access Service".  As such the
   PPC MUST report all unused quotas by sending an Authorize-Only Access
   Request message containing a PPAQ for each active service.  The
   Update-Reason MAY indicate that the reason for the update.

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3.8.5.  Support for Resource Pools

   If the PPC supports pools as indicated by setting the "Pools
   supported" bit in the PPAC then the PPS may associate a quota with a
   Pool by including the Pool-Id and the Pool-Multiplier in the PPAQ.
   When Resource Pools are used, the PPAQ MUST NOT use the threshold
   field.

3.8.6.  One-time Charging

   To initiate one-time charging the PPC includes the PPAQ attribute in
   an Access-Request packet.  The Service-ID field of the PPAQ
   identifies the prepaid service.  The amount to be charged is
   specified using the Resource Quota and Resource Quota overflow
   subtypes.  If the value specified is negative then the resources are
   credited to the user account.  This functionality corresponds to
   refunding.

   The QID subtype MUST be set to a unique value and is used by the PPS
   to detect duplicates.  The Update Reason field MUST be set to One-
   Time Charging.  Upon receiving a One-Time charge PPAQ, the RADIUS
   server authenticates the user and, if successful, passes the PPAQ to
   the PPS.  The PPS locates the account and debits or credits it
   accordingly.  The PPS MUST respond to the PPS with an Access-Accept
   message if successful, or an Access-Reject message otherwise.

   In case of a successful operation the HAAA forwards the message to
   the PPC with an Access Accept message.  Since this is a one-time
   charge the PPC MUST NOT allow the session to continue.  Therefore,
   the RADIUS server SHOULD include in the Access-Accept a Session-
   Timeout set to 0.  Upon receiving an Access-Accept response the PPC
   SHOULD generate an Accounting Stop message.

   A PPAQ used for One-Time charging MAY appear in an Authorize-Only
   Access Request.  This is the case when the session already exists.
   The PPS responds with an Access-Accept to indicate that the user
   account has been debited or an Access-Reject otherwise.

3.8.7.  Error Handling

   If the PPS receives a PPAQ with an invalid QID it MUST ignore that
   PPAQ.

   If the PPS receives a PPAQ containing a Service-Id, or a Rating-
   Group-Id that it does not recognize, then it MUST ignore that PPAQ.

   If the PPC receives a PPAQ containing a Service-Id, or a Rating-
   Group-Id that it does not recognize, then it MUST ignore that PPAQ.

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   If the PPC receives a PPAQ that contains a Pool-Id without a Pool-
   Multiplier or a Pool-Multiplier without a Pool-Id it MUST ignore that
   PPAQ.

3.8.8.  Accounting Considerations

   Although typically generated, accounting messages are not required to
   deliver a prepaid data service.  When generated, accounting messages
   are used for auditing purposes and for billing.  Accounting messages
   associated with prepaid data sessions should include the PPAQ
   attribute.

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4.  Attributes

   This section specifies the attributes that implement the RADIUS
   extensions for prepaid.

4.1.  PrePaid Accounting Capability (PPAC) Attribute

   The PrepaidAccountingCapability (PPAC) attribute is sent in the
   Access-Request message by a PPC to describe its prepaid capabilities.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |  Length       |            Vendor-Id
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         Vendor-Id (cont)           | Vendor type   | Vendor length |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Continuation | VALUE ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

      The fields have the following meaning and encoding:

      Type:

        26 for Vendor-Specific

      Length:

        6 + 3 + length of SubTypes

      Vendor-Id:

        The Vendor-Id value for WiMAX is 24757.

      Vendor type:

        35 for PPAC

      Vendor length:

        3 + length of VALUE

      Continuation:

        The Continuation Field is defined as follows:

        0

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        0 1 2 3 4 5 6 7
        +-+-+-+-+-+-+-+-+
        |C|r|r|r|r|r|r|r|
        +-+-+-+-+-+-+-+-+

        The C-bit of the continuation field indicates
        if a attribute is being fragmented. When the
        C-bit is set to one '1' this indicates that
        the attribute is being fragmented that is
        the next VSA of the same type is to be appended
        to this attribute. When the C-bit is set to zero
        '0' this indicates that the next attribute is
        not a fragment of this attribute.
        An attribute that is not being fragmented will
        have the C-bit set to '0'. An attribute that is
        being fragmented will have its C-bit set to '1'
        for all fragments until the last fragment, which
        will have its C-bit set to '0' indicating it's
        the last fragment of the attribute. The r-bits
        are reserved for future use. They SHALL be set
        to zero by the sender and SHALL be ignored by
        the receiver.

        The value of the C-bit MUST be 0.

       VALUE :

        The content of the AvailableInClient (AiC) SubType fields
        are encoded using the data type String.

                         Figure 8: PPAC Attribute

   The AvailableInClient (AiC) SubType is encoding as follows:

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       | LENGTH        | AvailableInClient (AiC)      ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  AvailableInClient (AiC)      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value (1)

      LENGTH  : 2 + 4

      AvailableInClient (AiC): The bitmap is encoded as:

      Value          | Description
      ------------  -+-------------------------------------
       0x00000001    | Volume metering supported
       0x00000010    | Duration metering supported
       0x00000100    | Resource metering supported
       0x00001000    | Pools supported
       0x00010000    | Rating groups supported
       0x00100000    | Multi-Services supported
       0x01000000    | Tariff Switch supported
       0x10000000    | Reserved

                 Figure 9: AvailableInClient (AiC) SubType

4.2.  Session Termination Capability Attribute

   The Session Termination Capability attribute is included in the
   RADIUS Access-Request message towards the RADIUS server to indicate
   whether or not the NAS supports Dynamic Authorization.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |  Length       |            Vendor-Id
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         Vendor-Id (cont)           | Vendor type   | Vendor length |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Continuation | String ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

      The fields have the following meaning and encoding:

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      Type:

        26 for Vendor-Specific

      Length:

        6 + 3 + 4

      Vendor-Id:

        The Vendor-Id value for WiMAX is 24757.

      Vendor type:

        36 for Session Termination Capability

      Vendor length:

        3 + 4

      Continuation:

        The Continuation Field is defined as follows:

        0
        0 1 2 3 4 5 6 7
        +-+-+-+-+-+-+-+-+
        |C|r|r|r|r|r|r|r|
        +-+-+-+-+-+-+-+-+

        The C-bit of the continuation field indicates
        if a attribute is being fragmented. When the
        C-bit is set to one '1' this indicates that
        the attribute is being fragmented that is
        the next VSA of the same type is to be appended
        to this attribute. When the C-bit is set to zero
        '0' this indicates that the next attribute is
        not a fragment of this attribute.
        An attribute that is not being fragmented will
        have the C-bit set to '0'. An attribute that is
        being fragmented will have its C-bit set to '1'
        for all fragments until the last fragment, which
        will have its C-bit set to '0' indicating it's
        the last fragment of the attribute. The r-bits
        are reserved for future use. They SHALL be set
        to zero by the sender and SHALL be ignored by
        the receiver.

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        The value of the C-bit MUST be 0.

      String:

        This field is encoded as a bitmap.

      Value          | Description
      ---------------+-------------------------------------
       0x00000000    | Reserved
       0x00000001    | Dynamic Authorization Extensions
                     | (RFC 3576) is supported.
       ...           | All further values are reserved.

            Figure 10: Session Termination Capability Attribute

4.3.  Prepaid Accounting Operation (PPAQ) Attribute

   One or more PPAQ attributes are sent in an Access Request, Authorize-
   Only Access-Request and Access-Accept message.  In an Access Request
   message, the PPAQ attribute is used to facilitate one-time charging
   transactions.  In Authorize-Only Access-Request messages it is used
   for one-time charging, report usage and to request further quota.  It
   is also used in order to request prepaid quota for a new service
   instance.  In an Access-Accept message it is used in order to
   allocate the (initial and subsequent) quotas.

   When multiple services are supported, a PPAQ is associated with a
   specific service as indicated by the presence of a Service-Id, a
   Rating-Group-Id, or the "Access Service" (as indicated by the absence
   of both, the Service-Id and the Rating-Group-Id).

   Note: Either Volume-Quota, Time-Quota, or Resource-Quota SubTypes
   MUST appear in the PPAQ attribute, except for the price enquiry
   message exchange where these subtypes MUST be absent.  A single PPAQ
   attribute MUST NOT contain more than one Service-Id, MUST NOT contain
   more than one Rating-Group-Id, and MUST NOT contain both a Service-Id
   and a Rating-Group-Id.  A PPAQ that does not contain a Service-ID or
   a Rating-Group-Id refers to the "Access Service".  A PPAQ MUST NOT
   contain more than one Pool-Id.  A PPAQ that contains a Pool-Id MUST
   also contain a Pool-Multiplier SubType.

   The PPAQ attribute, as shown in Figure 11, has a variable length
   (greater than 8, encoded into one octet), and consists of a variable
   number of subtypes.  Unused subtypes are omitted from the message.

   The following table summarizes the presence of various SubTypes in
   the PPAQ attribute carried in the Access-Request and Access-Accept

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   messages.

   Request             Accept        #  SubType Name

   0-1(g)              0-1(m,n)      1  Quota Identifier
   0-1(a,g)            0-1(a,k,n)    2  VolumeQuota
    0                  0-1(a,m,n)    3  VolumeThreshold
   0-1(b,g)            0-1(b,k,n)    4  DurationQuota
    0                  0-1(b,m,n)    5  DurationThreshold
   0-1(c,g)            0-1(c,k,n)    6  ResourceQuota
    0                  0-1(c,m,n)    7  ResourceThreshold
   0-1(d,g)              0           8  Update-Reason
   0-n(e,g)            0-n(e,m,n)    9  PrepaidServer
   0-1(g,h,j)          0-1(m,n)     10 Service-ID
   0-1(g,h,j)          0-1(m,n)     11 Rating-Group-ID
    0                  0-1(m,n)     12 Termination-Action
    0                  0-1(m,n)     13 Pool-ID
    0                  0-1(f,m,n)   14 Pool-Multiplier
   0-1(g)                0          15 Requested-Action
    0                  0-1(k,m,n)   16 Check-Balance-Result
    0                  0-1(n)       17 Cost-Information

   None of the above-listed SubTypes appears in the Access-Reject nor in
   the Access-Challenge.

   Notes:

      (a) SHALL be present if volume based charging is used.  SHALL NOT
      be present otherwise.  Volume- Threshold is optional.

      (b) SHALL be present if duration-based charging is used.  SHALL
      NOT be present otherwise.  Duration- Threshold is optional.

      (c) SHALL be present if resource-based charging is used.  SHALL
      NOT be present otherwise.  Resource- Threshold is optional.

      (d) SHALL be present in an Authorize-Only Access-Request.

      (e) MAY be present in an Access-Accept.  If present in Access
      Accept it SHALL be present in Access- Request (except for the
      first Access-Request).

      (f) Pool-Multiplier SHALL be present when Pool-ID is present
      otherwise Pool-Multiplier SHALL NOT be present in the PPAQ.

      (g) If Requested-Action is present then Service-ID SHALL also be
      present and all other attributes SHALL NOT be present.

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      (h) PPAQ SHALL NOT contain both a Service-ID and a
      Rating-Group-ID.

      (j) A PPAQ that does not contain a Service-ID or a Rating-Group-Id
      refers to the "Access Service"(ISF).

      (k) If Balance-Check-Result is present and set to 0 then either
      Volume-Quota, Duration-Quota or Resource- Quota SHALL be present.

      (m) If Balance-Check-Result is present then Service-ID SHALL also
      be present and other attributes (tagged with m) SHALL NOT be
      present.

      (n) The PPAQ in which a Cost-Information occurs SHALL NOT include
      a Quota-Identifier, because no quota is actually reserved by the
      PPS.  The Service-ID SHALL be present with the Cost-Information
      for that Service-ID may not be present if the Cost-Information
      cannot be provided.  All other attribute SHALL not appear.

   In the following subsections the various subtypes of the PPAQ
   attribute are specified.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |  Length       |            Vendor-Id
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        Vendor-Id (cont)           | Vendor type   | Vendor length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Continuation | VALUE ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

     The fields have the following meaning and encoding:

     Type:

       26 for Vendor-Specific

     Length:

       6 + 3 + length of SubTypes

     Vendor-Id:

       The Vendor-Id value for WiMAX is 24757.

     Vendor type:

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       37 for PPAQ

     Vendor length:

       3 + length of SubTypes

     Continuation:

       The Continuation Field is defined as follows:

       0
       0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+
       |C|r|r|r|r|r|r|r|
       +-+-+-+-+-+-+-+-+

       The C-bit of the continuation field indicates
       if a attribute is being fragmented. When the
       C-bit is set to one '1' this indicates that
       the attribute is being fragmented that is
       the next VSA of the same type is to be appended
       to this attribute. When the C-bit is set to zero
       '0' this indicates that the next attribute is
       not a fragment of this attribute.
       An attribute that is not being fragmented will
       have the C-bit set to '0'. An attribute that is
       being fragmented will have its C-bit set to '1'
       for all fragments until the last fragment, which
       will have its C-bit set to '0' indicating it's
       the last fragment of the attribute. The r-bits
       are reserved for future use. They SHALL be set
       to zero by the sender and SHALL be ignored by
       the receiver.

       The value of the C-bit MAY be 0 or 1.

     VALUE:

       Data type String
       Each SubType is then encoded in the following style:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | SubType       | LENGTH        | VALUE                        ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     The fields have the following meaning and encoding:

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     SubType :

        Contains an 8 bit unsigned integer.

     LENGTH  :

        Contains an 8 bit unsigned integer.
        The value of the LENGTH field is calculated as the length of the
        VALUE field plus two octets (one octet for the length of the
        SubType field and another octet for the length of the LENGTH
        field).

                 Figure 11: PPAQ Attribute Encoding Style

4.3.1.  Quota Identifier (QID) SubType

   The Quota Identifier (QID) is generated by the PPS and subsequently
   returned in a PPAQ->QID subtype from the PPC to the PPS.  This field
   has the semantic of a transaction identifier and therefore changes
   with every transaction initiated by the PPS to the PPC.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | VALUE                        ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  VALUE                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(1)

      LENGTH  : 2 + length of VALUE field.

      VALUE : Data type String

                      Quota Identifier (QID) SubType

4.3.2.  VolumeQuota SubType

   TVolumeQuota SubType is only present when volume-based charging is
   used.  In a RADIUS Access-Accept message (PPS to PPC direction), it
   indicates the volume (in octets) allocated for the session by the
   PPS.  In a RADIUS Authorize-Only Access-Request message (PPC to PPS
   direction), it indicates the totally used volume (in octets) for both
   inbound and outbound traffic.  The Exponent Field, if present, MUST

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   NOT encode a negative number or zero.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | VALUE                        ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(2)

      LENGTH  : 2 + (8 or 12)

      VALUE : Data type String

      The content of the VALUE field either contains the
      Value-Digits Field (8 octets long) or the Value-Digits Field
      plus the Exponent Field (12 octets long).

                            VolumeQuota SubType

4.3.3.  VolumeThreshold SubType

   The value of the type field of the VolumeThreshold SubType is TBD and
   its length is 10 or 14 octets.  This SubType is optionally present if
   the VolumeQuota is present in a RADIUS Access-Accept message (PPS to
   PPC direction).  It is generated by the PPS and indicates the volume
   (in octets) that has to be consumed before a new quota is requested.
   This threshold MUST NOT be larger than the VolumeQuota.  The Exponent
   Field, if present, MUST NOT encode a negative number or zero.

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | VALUE                        ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(3)

      LENGTH  : 2 + (8 or 12)

      VALUE : Data type String

      The content of the VALUE field either contains the
      Value-Digits Field (8 octets long) or the Value-Digits
      SubType plus the Exponent Field 12 octets long).

                          VolumeThreshold SubType

4.3.4.  DurationQuota SubType

   The optional DurationQuota SubType is only present if duration-based
   charging is used.  In a RADIUS Access-Accept message (PPS to PPC
   direction), it indicates the duration (in seconds) allocated for the
   session by the PPS.  In a RADIUS Access-Request message (PPC to PPS
   direction), it indicates the total duration (in seconds) since the
   start of the accounting session related to the QID subtype of the
   PPAQ attribute in which it occurs.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | VALUE                        ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  VALUE                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(4)

      LENGTH  : 2 + 4

      VALUE : Data type string.

      The content of this field contains a 32-bit unsigned integer

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      (with the values 0 to +4,294,967,295).

                           DurationQuota SubType

4.3.5.  DurationThreshold SubType

   The DurationThreshold SubType is optionally present if the
   DurationQuota is present in a RADIUS Access-Accept message (PPS to
   PPC direction).  It represents the duration (in seconds) after which
   new quota should be requested.  This threshold MUST NOT be larger
   than the DurationQuota SubType.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | VALUE                        ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  VALUE                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(5)

      LENGTH  : 2 + 4

      VALUE : Data type string.

      The content of this field contains a 32-bit unsigned integer
      (with the values 0 to +4,294,967,295).

                         DurationThreshold SubType

4.3.6.  ResourceQuota SubType

   The optional ResourceQuota SubType is only present if resource-based
   or one-time charging is used.  In the RADIUS Access-Accept message
   (PPS to PPC direction) it indicates the resources allocated for the
   session by the PPS.  In RADIUS Authorize-Only Access-Request message
   (PPC to PPS direction), it indicates the resources used in total,
   including both incoming and outgoing chargeable traffic.  In one-time
   charging scenarios, the subtype represents the number of units to
   charge the user.  The attribute consists of a Value-Digits Field and
   optionally an Exponent Field (as indicated by the length field).

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | VALUE                        ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(6)

      LENGTH  : 2 + (8 or 12)

      VALUE : Data type String

      The content of the VALUE field either contains the
      Value-Digits Field (8 octets long) or the Value-Digits Field
      plus the Exponent Field (12 octets long).

                           ResourceQuota SubType

4.3.7.  ResourceThreshold SubType

   The semantic of the ResourceThreshold SubType follow those of the
   VolumeThreshold and DurationThreshold SubType.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | VALUE                        ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(7)

      LENGTH  : 2 + (8 or 12)

      VALUE : Data type String

      The content of the VALUE field either contains the
      Value-Digits Field (8 octets long) or the Value-Digits Field
      plus the Exponent Field (12 octets long).

                         ResourceThreshold SubType

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4.3.8.  Update-Reason SubType

   The Update-Reason SubType is present in a RADIUS Access-Request
   message (PPC to PPS direction) and indicates the reason for
   initiating the quota update operation.  Update reasons (6), (7), (8)
   and (9) indicate that the associated resources are released at the
   PPC side, and that therefore the PPS MUST NOT allocate a new quota in
   the RADIUS Access-Accept message.

      0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType         | LENGTH        |      VALUE                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(8)

      LENGTH  : 2 + 1

      VALUE : Data type string

      This field contains a byte
      (with the values 0 to 255).

                           Update-Reason SubType

   The following values for the Update-Reason SubType are defined:

    Value        | Description
    -------------+--------------------------------------
      0          | Reserved
      1          | Pre-initialization
      2          | Initial Request
      3          | Threshold Reached
      4          | Quota Reached
      5          | TITSU Approaching
      6          | Remote Forced Disconnect
      7          | Client Service Termination
      8          | "Access Service" Terminated
      9          | Service not established
     10          | One-Time Charging
     11...255    | **Available for IANA registration**

                Figure 12: Values for Update-Reason SubType

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4.3.9.  PrepaidServer SubType

   The optional PrepaidServer SubType indicates the address of the
   serving PPS.  If present, the Home RADIUS server uses this address to
   route the message to the serving PPS.  Multiple instances of this
   subtype MAY be present in a single PPAQ attribute.

   If present in the PrepaidServer SubType of an incoming RADIUS Access-
   Accept message, the PPC returns this SubType back without modifying
   it in the subsequent RADIUS Access-Request message.  If multiple
   values are present, the PPC MUST NOT change their order.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | Address                      ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(9)

      LENGTH  : 2 + (4 or 16)

      Address : Data type String

      For IPv4, the Address field is 4 octets based on the encoding of
      the NAS-IP-Address defined in RFC 2138. For IPv6, the Address
      field is 16 octets long.

                           PrepaidServer SubType

4.3.10.  Service-ID SubType

   The Service-ID SubType is handled as an opaque string that uniquely
   describes the service instance for which prepaid metering should be
   applied.

   A Service-Id could be an IP 5-tuple (source address, source port,
   destination address, destination port, protocol).  If a Service-ID
   SubType is present in the PPAQ, the entire PPAQ attribute refers to
   that service.  If a PPAQ attribute does not contain a Service-Id or
   Rating-Group-ID, then the PPAQ attribute refers to the "Access
   Service".

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | Service                      ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(10)

      LENGTH  : 2 + length (Service)

      Service : Data type String

                            Service-ID SubType

4.3.11.  Rating-Group-ID SubType

   The Rating-Group-ID SubType indicates that this PPAQ attribute is
   associated with resources allocated to a Rating Group with the
   corresponding ID.  This SubType is encoded as a string.  A single
   PPAQ MUST NOT contain more than one Rating-Group-ID.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | VALUE                        ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  VALUE                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(11)

      LENGTH  : 2 + 4

      VALUE : Data type string with a length of 4 octets

                          Rating-Group-ID SubType

4.3.12.  Termination-Action SubType

   The value of the type field of the Termination-Action SubType is TBD.
   The length of this SubType is 3 octets.  This SubType contains an
   enumeration of the action to take when the PPS does not grant
   additional quota.  Valid actions are as follows.

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       0                   1                   2
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | Action        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(12)

      LENGTH  : 2 + 1

      Action : Data type string.

      The content is a byte (with the values 0 to +255).

                        Termination-Action SubType

   The following values for the Termination-Action SubType are defined:

    Value        | Description
    -------------+------------------------------------
      0          | Reserved
      1          | Terminate
      2          | Request More Quota
      3          | Redirect/Filter
      4..255     | **Available for IANA registration**

           Figure 13: Values for the Termination-Action SubType

4.3.13.  Pool-ID SubType

   The Pool-ID SubType identifies the resource pool.  It is encoded as a
   string.

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | Poolid                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(13)

      LENGTH  : 2 + 4

      Poolid : Data type string with length of 4 octets.

                              Pool-ID SubType

4.3.14.  Pool-Multiplier SubType

   The Pool-Multiplier SubType determines the weight of resources when
   they are inserted into the pool that is identified by the
   accompanying Pool-ID SubType, and the rate at which resources are
   taken out of the pool by the relevant Service or Rating-Group.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | VALUE                        ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(14)

      LENGTH  : 2 + (8 or 12)

      VALUE : Data type String

      The content of the VALUE field either contains the
      Value-Digits Field (8 octets long) or the Value-Digits Field
      plus the Exponent Field (12 octets long).

                          Pool-Multiplier SubType

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4.3.15.  Requested-Action SubType

   The Requested-Action SubType is only be present in messages sent from
   the PPC to the PPS.  It indicates that the PPC desires the PPS to
   perform the indicated action and to return the result.  The PPAQ in
   which a Requested-Action SubType occurs MUST NOT contain a QID, and
   MUST contain a Service-Identifier or a Rating-Group-Identifer that
   allows the PPS to uniquely identify the service for which the
   indicated action is requested.

       0                   1                   2
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | Action        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(15)

      LENGTH  : 2 + 1

      Action : Data type string.

      The content is a byte (with the values 0 to +255).
      The values are listed below.

                         Requested-Action SubType

   The following values for the Action field of the Requested-Action
   SubType are defined:

    Value        | Description
    -------------+-------------------------------------
      0          | Reserved
      1          | Balance Check
      2          | Price Enquiry
      3..255     | **Available for IANA registration**

            Figure 14: Values for the Requested-Action SubType

4.3.16.  Check-Balance-Result SubType

   The Check-Balance-Result SubType can only be present in messages sent
   from the PPS to the PPC.  It indicates the balance check decision of
   the PPS about a previously received Balance Check Request (as

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   indicated in a Requested-Action SubType).  The PPAQ attribute in
   which a Check-Balance-Result occurs MUST NOT include a QID beause no
   quota is reserved by the PPS.

       0                   1                   2
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | Decision      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(16)

      LENGTH  : 2 + 1

      Decision : Data type string.

      The content is a byte (with the values 0 to +255).
      The values are listed below.

                       Check-Balance-Result SubType

   The following values for the Decision field of the Check-Balance-
   Result SubType are defined:

    Value        | Description
    -------------+-------------------------------------------
      0          | Success; Sufficient funds available
                 | in the user's prepaid account
      1          | Failure; Insufficient funds available
      2..255     | **Available for IANA registration**

          Figure 15: Values for the Check-Balance-Result SubType

4.3.17.  Cost-Information SubType

   The Cost-Information SubType is used in order to return the cost
   information of a service, which the PPC can transfer transparently to
   the end user.  This SubType is sent from the PPS to the PPC as a
   response to a "Price Enquiry", as indicated by the Requested-Action
   SubType.

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | VALUE                        ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(17)

      LENGTH  : 2 + 12 + 4 + length of the Cost-Unit Field

      VALUE : Data type String

      The content of the VALUE field contains (in this order) the
      Value-Digits Field, Exponent Field, Currency-Code Field
      and the Cost-Unit Field.

                         Cost-Information SubType

   For example, the cost of 7.75 Malawi kwacha per hour would be encoded
   as follows.  Value-Digits = 775, Exponent = -2, Currency Code = 454,
   and Cost-Unit = "hour".

   The PPAQ that carries a Cost-Information MUST NOT include a QID.

   The Currency-Code Field is of type Unsigned32 and used inside the
   Check-Balance-Result SubType and contains a currency code that
   specifies in which currency the values of AVPs containing monetary
   units were given.  It is specified by using the numeric values
   defined in the ISO 3166-1 standard.

   The Cost-Unit Field is used inside the Check-Balance-Result SubType
   and contains a human readable UTF8 encoded string that can be
   displayed to the end user.  It specifies the applicable unit to the
   Cost-Information when the service cost is a cost per unit (e.g., cost
   of the service is $1 per minute).  The Cost-Unit can, for example, be
   minutes, hours, days, kilobytes, megabytes, etc.

4.4.  Fields

4.4.1.  Value-Digits Field

   The Value-Digits Field is an Unsigned64 value (with a length of 8
   octets) that contains the significant digits of the number.  If
   decimal values are needed to present the number, the scaling MUST be
   indicated with a related Exponent Field, see Section 4.4.2.

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   For example, the decimal number 0.05 is encoded by a Value-Digits
   Field set to 5, and a scaling that is indicated with the Exponent
   Field set to -2.

   The encoding of this SubType is not done in an TLV format but rather
   the encoded value is added to existing subtypes.

4.4.2.  Exponent Field

   The Exponent field is a Integer32 value that contains the exponent
   value that is to be applied to the accompanying Value-Digit Field.

4.5.  Prepaid Tariff Switching (PTS) Attribute

   This specification defines the PTS attribute, which allows to switch
   from one rate to another during service provision.  Support for
   tariff switching is optional to implement and to use for the PPC and
   the PPS.  PPCs use the flag "Tariff Switching supported" in the
   AvailableInClient field of the PPAC attribute in order to indicate
   support for tariff switching.  PPSs employ the PTS attribute in order
   to announce their support for tariff switching.

   If a RADIUS message contains a PTS attribute, it MUST also contain at
   least one PPAQ attribute.  If a RADIUS Access-Request message
   contains a PTS attribute or the "Tariff Switching supported" flag in
   the AvailableInClient field of the PPAC attribute, it MUST also
   contain an Event-Timestamp RADIUS attribute (see [RFC2869]).

   Every PTS attribute MUST include a QID SubType, as specified in
   Section 4.3.1.  In a RADIUS Access-Request message sent from the PPC
   to the PPS, the QID SubType MUST contain the value of the Quota
   Identifier SubType that was previously received from the PPS and MUST
   be the same as the value carried in the QID SubType of one of the
   PPAQ attributes included the same RADIUS message.

   If multiple services are supported and if the PPAQ is associated with
   a service as indicated by the Service-ID SubType, then the PTS refers
   to the tariff switch for that service.  If the PPAQ does not have a
   Service-ID, then the PTS refers to tariff switch for the "Access
   Service".

   A PPAQ attribute that is transported along with a PTS attribute and
   has the same value as the QID SubType contained in the PTS attribute
   in its own QID SubType is referred to as the "accompanying PPAQ
   attribute".  If a PPS receives an Access-Request message from a PPC,
   it associates a unique value for the QID SubType to this request.

   The following table summarizes the presence of various SubTypes in

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   the PTS attribute carried in the Access-Request and Access-Accept
   messages.

 Request           Accept        #  SubType Name
  1                  1           1  Quota Identifier
  1                  0           2  VolumeUsedAfterTariffSwitch
  0                 0-1          3  TarrifSwitchInterval
  0                 0-1(a)       4  TimeIntervalAfterTarriffSwitchUpdate

   None of the above-listed SubTypes appears in the Access-Reject nor in
   the Access-Challenge.

   Notes:

      (a) The PPS SHALL include this AVP if there is another tariff
      switch period after the period that ends as indicated by the TSI
      attribute.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |  Length       |            Vendor-Id
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         Vendor-Id (cont)           | Vendor type   | Vendor length |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Continuation | VALUE ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

      The fields have the following meaning and encoding:

      Type:

        26 for Vendor-Specific

      Length:

        6 + 3 + length of SubTypes

      Vendor-Id:

        The Vendor-Id value for WiMAX is 24757.

      Vendor type:

        38 for PTS

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      Vendor length:

        3 + length of SubTypes

      Continuation:

        The Continuation Field is defined as follows:

        0
        0 1 2 3 4 5 6 7
        +-+-+-+-+-+-+-+-+
        |C|r|r|r|r|r|r|r|
        +-+-+-+-+-+-+-+-+

        The C-bit of the continuation field indicates
        if a attribute is being fragmented. When the
        C-bit is set to one '1' this indicates that
        the attribute is being fragmented that is
        the next VSA of the same type is to be appended
        to this attribute. When the C-bit is set to zero
        '0' this indicates that the next attribute is
        not a fragment of this attribute.
        An attribute that is not being fragmented will
        have the C-bit set to '0'. An attribute that is
        being fragmented will have its C-bit set to '1'
        for all fragments until the last fragment, which
        will have its C-bit set to '0' indicating it's
        the last fragment of the attribute. The r-bits
        are reserved for future use. They SHALL be set
        to zero by the sender and SHALL be ignored by
        the receiver.

        The value of the C-bit MAY be 0 or 1.

      VALUE : Variable length content of data type String containing
              one or multiple SubTypes.

      Each SubType is then encoding in the following style:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       | LENGTH        | VALUE                        ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType:

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         Contains an 8 bit unsigned integer.

      LENGTH:

         Contains an 8 bit unsigned integer.

      VALUE:

         Contains the content of the SubType.

                         Figure 16: PTS Attribute

4.5.1.  VolumeUsedAfterTariffSwitch SubType

   The optional VolumeUsedAfterTariffSwitch (VUATS) SubType is used in
   the RADIUS Access-Request messages (PPC to PPS direction).  It
   indicates the volume (in octets) used during a session after the last
   tariff switch for the service specified via the QID SubType and the
   accompanying PPAQ attribute.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | VALUE                        ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(23)

      LENGTH  : variable (8 or 14)

      VALUE : Data type String

      The content of the VALUE field either contains the
      Value-Digits Field or the Value-Digits Field plus the
      Exponent Field. The length field indicates whether one or
      both subtypes are included.

                    VolumeUsedAfterTariffSwitch SubType

4.5.2.  TariffSwitchInterval SubType

   The TariffSwitchInterval (TSI) SubType MUST be present in each PTS
   attribute that is part of a RADIUS Access-Accept message (PPS to PPC
   direction).  It indicates the interval (in seconds) between the value

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   of Event-Timestamp RADIUS attribute (see [RFC2869]) of the
   corresponding RADIUS Access-Request message and the next tariff
   switch condition.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | VALUE                        ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  VALUE                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(24)

      LENGTH  : 6

      VALUE : Data type string

      This field contains a 16-bit unsigned integer
      (with the values 0 to +65,535).

                       TariffSwitchInterval SubType

4.5.3.  TimeIntervalafterTariffSwitchUpdate SubType

   The PPS MUST include TimeIntervalafterTariffSwitchUpdate (TITSU)
   SubType if there is another tariff switch period after the period
   that ends as indicated by the TSI SubType.  The value of the TITSU
   SubType contains the number of seconds of the tariff period that
   begins immediately after the period that ends as indicated by the TSI
   attribute.  If the TITSU SubType is not present, the PPC assumes that
   the tariff period, which ends as indicated by the TSI SubType, lasts
   until further notice.  If TITSU is specified, the PPC MUST send a
   quota update before the point in time specified by the TITSU SubType
   (see Figure 7).

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | SubType       |    LENGTH     | VALUE                        ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  VALUE                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The fields have the following meaning and encoding:

      SubType : Value(25)

      LENGTH  : 6

      VALUE : Data type string

      This field contains a 16-bit unsigned integer
      (with the values 0 to +65,535).

                TimeIntervalafterTariffSwitchUpdate SubType

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5.  Diameter RADIUS Interoperability

   The RADIUS Prepaid extension described in this document may need to
   interoperate with the Diameter Credit Control Application.  Two
   interoperability scenarios exist, as follows.  Either the AAA server
   is Diameter based and the AAA client is RADIUS based, or the AAA
   client is Diameter based and the AAA server is RADIUS based.

   The Diameter Credit Control Application [RFC4006] describes how to
   implement a prepaid accounting system using a Diameter based
   infrastructure.

   A possible procedure for accomplishing the translation of the
   functionality defined in Diameter Credit Control and in the RADIUS
   Prepaid specification is described in Appendix B.

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6.  Security Considerations

   This specification describes the use of RADIUS for purposes of real-
   time accounting.  Threats and security issues for this application
   are described in [RFC3579] and [RFC3580]; security issues encountered
   in roaming are described in [RFC2607].

   This document specifies new attributes that can be included in
   existing RADIUS packets, which are protected as described in
   [RFC3579] and [RFC3576].  See those documents for a more detailed
   description.

   The security mechanisms supported in RADIUS are focused on preventing
   an attacker from spoofing packets or modifying packets in transit.
   They do not prevent an authorized RADIUS/Diameter server or proxy
   from modifying, inserting, or removing attributes with malicious
   intent.  When the attributes defined in this document are modified or
   removed by a RADIUS proxy they may lead to incorrect accounting
   records being delivered to users or wrong resource consumption being
   collected.

   The described mechanism add the mechanism of capability negotiation,
   so that a RADIUS server can automatically discover whether a NAS
   supports the real-time accounting features described in this document
   (and even more detailed capabilities can be learned by the RADIUS
   server).  These mechanisms are being added to ensure that neither the
   NAS nor the RADIUS server make incorrect assumptions about the
   capabilities of the other party; potentially leading to incorrect
   accounting and improper access to the network or other services.

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7.  Table of Attributes

   The following table provides a guide which attributes may be found in
   which RADIUS messages, and in what quantity.

   Request Accept Reject Challenge Accounting  #  Attribute
                                   Request
   0-1     0      0      0         0          35  PPAC
   0-1     0      0      0         0          36  Session Termination
                                                  Capability
   0+      0+     0      0         0+         37  PPAQ
   0+      0+     0      0         0+         38  PTS

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8.  IANA Considerations

   This document contains a number of instructions to IANA.

8.1.  RADIUS Attributes

   This document does not require IANA to register the following four
   RADIUS attributes as the code registered by the Wimax Forum is re-
   used.  The Wimax Forum SMI Network Management Private Enterprise Code
   is 24757.

    Attribute Name                        |  Attribute Type Value
    --------------------------------------+-----------------------------
    Prepaid-Accounting-Capability (PPAC)  |  35
    Session Termination Capability        |  36
    Prepaid-Accounting-Operation (PPAQ)   |  37
    Prepaid Tariff Switching (PTS)        |  38

8.2.  New Registry: PPAC SubTypes

   Section 4.1 defines the SubTypes used within the PPAC attribute.
   IANA is asked to create a registry for these SubTypes.  Each registry
   entry consists of a 8 bit number together with a description of the
   PPAC SubType.  This document creates the following PPAC SubTypes for
   this registry:

    Value    |  SubType Name
    ---------+-----------------------------
      0      |  Reserved
      1      |  AvailableInClient
      2..255 |  **Available for IANA registration**

   The semantic of the above-listed SubType is described in Section 4.1.

   Following the policies outline in [RFC3575] the available SubTypes
   (i.e., value 0 and values 2-255) with a description of their semantic
   will be assigned after the expert review process.  Updates can be
   provided based on expert approval only.  Based on expert approval it
   is possible to mark entries as "deprecated".  A designated expert
   will be appointed by the IESG.

   Each registration must include a number for the SubType and the
   semantic of the SubType.

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8.3.  New Registry: PPAQ SubTypes

   Section 4.3 defines the SubTypes used within the PPAQ attribute.
   IANA is asked to create a registry for these SubTypes.  Each registry
   entry consists of a 8 bit number together with a description of the
   PPAQ SubType.  This document creates the following PPAQ SubTypes for
   this registry:

    Value    |  SubType Name
    ---------+-----------------------------
      0      |  Reserved
      1      |  Quota Identifier
      2      |  VolumeQuota
      3      |  VolumeThreshold
      4      |  DurationQuota
      5      |  DurationThreshold
      6      |  ResourceQuota
      7      |  ResourceThreshold
      8      |  Update-Reason
      9      |  PrepaidServer
     10      |  Service-ID
     11      |  Rating-Group-ID
     12      |  Termination-Action
     13      |  Pool-ID
     14      |  Pool-Multiplier
     15      |  Requested-Action
     16      |  Check-Balance-Result
     17..255 |  **Available for IANA registration**

   The semantic of the above-listed SubTypes is described in
   Section 4.3.

   Following the policies outline in [RFC3575] the available SubTypes
   (i.e., value 0 and values 22-255) with a description of their
   semantic will be assigned after the expert review process.  Updates
   can be provided based on expert approval only.  Based on expert
   approval it is possible to mark entries as "deprecated".  A
   designated expert will be appointed by the IESG.

   Each registration must include a number for the SubType and the
   semantic of the SubType.

8.4.  New Registry: PTS SubTypes

   Section 4.5 defines the SubTypes used within the PTS attribute.  IANA
   is asked to create a registry for these SubTypes.  Each registry
   entry consists of a 8 bit number together with a description of the

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   PTS SubType.  This document creates the following PTS SubTypes for
   this registry:

    Value    |  SubType Name
    ---------+-----------------------------
      0      |  Reserved
      1      |  Quota Identifier
      2      |  VolumeUsedAfterTariffSwitch
      3      |  TariffSwitchInterval
      4      |  TimeIntervalafterTariffSwitchUpdate
      5..255 |  **Available for IANA registration**

   The semantic of the above-listed SubTypes is described in
   Section 4.5.

   Following the policies outline in [RFC3575] the available SubTypes
   (i.e., value 0 and values 5-255) with a description of their semantic
   will be assigned after the expert review process.  Updates can be
   provided based on expert approval only.  Based on expert approval it
   is possible to mark entries as "deprecated".  A designated expert
   will be appointed by the IESG.

   Each registration must include a number for the SubType and the
   semantic of the SubType.

8.5.  New Registry: Update-Reason

   Section 4.3.8 defines the Update-Reason SubType.  IANA is asked to
   create a registry for the values contained in the Update-Reason
   SubType, as shown in Figure 12.  Each registry entry consists of a 16
   bit number together with a description of the update reason.

   Following the policies outline in [RFC3575] the available values
   together with a description of their semantic will be assigned after
   the expert review process.  Updates can be provided based on expert
   approval only.  Based on expert approval it is possible to mark
   entries as "deprecated".  A designated expert will be appointed by
   the IESG.

8.6.  New Registry: Termination-Action

   Section 4.3.12 defines the Termination-Action SubType.  IANA is asked
   to create a registry for the values contained in the Termination-
   Action SubType, as shown in Figure 13.  Each registry entry consists
   of a 8 bit number together with a description of the termination
   action.

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   Following the policies outline in [RFC3575] the available values
   together with a description of their semantic will be assigned after
   the expert review process.  Updates can be provided based on expert
   approval only.  Based on expert approval it is possible to mark
   entries as "deprecated".  A designated expert will be appointed by
   the IESG.

8.7.  New Registry: Requested-Action

   Section 4.3.15 defines the Requested-Action SubType.  IANA is asked
   to create a registry for the values contained in the Requested-Action
   SubType, as shown in Figure 14.  Each registry entry consists of a 8
   bit number together with a description of the requested reason.

   Following the policies outline in [RFC3575] the available values
   together with a description of their semantic will be assigned after
   the expert review process.  Updates can be provided based on expert
   approval only.  Based on expert approval it is possible to mark
   entries as "deprecated".  A designated expert will be appointed by
   the IESG.

8.8.  New Registry: Check-Balance-Result

   Section 4.3.16 defines the Check-Balance-Result SubType.  IANA is
   asked to create a registry for the values contained in the Check-
   Balance-Result SubType, as shown in Figure 15.  Each registry entry
   consists of a 8 bit number together with a description of the
   requested reason.

   Following the policies outline in [RFC3575] the available values
   together with a description of their semantic will be assigned after
   the expert review process.  Updates can be provided based on expert
   approval only.  Based on expert approval it is possible to mark
   entries as "deprecated".  A designated expert will be appointed by
   the IESG.

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9.  Acknowledgements

   The authors would like to thank Bernard Aboba, Christian Guenther,
   Dirk Kroeselberg and John Loughney for their feedback throughout the
   development of this document.  Additionally, the authors would like
   to thank the members of the Wimax Forum and the members of 3GPP2 for
   their help with this specification.

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10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "RFC 2119: Key words for use in RFCs to
              Indicate Requirement Levels", March 1997.

   [RFC2865]  Rigney, C., Rubens, A., Simpson, W., and S. Willens, "RFC
              2865: Remote Authentication Dial In User Server (RADIUS)",
              June 2000.

   [RFC3576]  Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
              Adoba, "RFC 3576: Dynamic Authorization Extensions to
              Remote Authentication Dial-In User Service (RADIUS)",
              February 2003.

10.2.  Informative References

   [RFC2284]  Blunk, L. and J. Vollbrecht, "PPP Extensible
              Authentication Protocol (EAP)", RFC 2284, March 1998.

   [RFC2607]  Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
              Implementation in Roaming", RFC 2607, June 1999.

   [RFC2866]  Rigney, C., "RFC 2866: RADIUS Accounting", June 2000.

   [RFC2869]  Rigney, C., Willats, W., and P. Calhoun, "RFC 2869: RADIUS
              Extensions", June 2000.

   [RFC3575]  Aboba, B., "IANA Considerations for RADIUS (Remote
              Authentication Dial In User Service)", RFC 3575,
              July 2003.

   [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible
              Authentication Protocol (EAP)", RFC 3579, September 2003.

   [RFC3580]  Congdon, P., Aboba, B., Smith, A., Zorn, G., and J. Roese,
              "IEEE 802.1X Remote Authentication Dial In User Service
              (RADIUS) Usage Guidelines", RFC 3580, September 2003.

   [RFC3748]  Adoba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "RFC 3748: Extensible Authentication Protocol",
              June 2004.

   [RFC4006]  Hakala, H., Mattila, L., Koskinen, J-P., Stura, M., and J.
              Loughney, "RFC 4006: Diameter Credit Control Application",
              August 2005.

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   [RFC4849]  Congdon, P., Sanchez, M., and B. Aboba, "RADIUS Filter
              Rule Attribute", RFC 4849, April 2007.

   [RFC6158]  DeKok, A. and G. Weber, "RADIUS Design Guidelines",
              BCP 158, RFC 6158, March 2011.

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Appendix A.  Example flows

   Note: This section is informative.

   This section presents certain example flows that involve the RADIUS
   prepaid extensions.  By no means is the intent of this section to
   specify or recommend business logic, rating strategies, and
   application-level behaviour.  The intent of this section is purely to
   illustrate some fictive scenarios and the RADIUS prepaid message
   flows that could be associated with these scenarios.  The contents of
   this section should be regarded as a collection of informative
   examples that aim to provide guidance to implementors.

A.1.  A simple flow

      End user          PPC                  PPS

      user logs on
      ------(1)--------->
                            Access Request
                            {RADIUS BASE AVPS,
                             PPAC=00...00011}
                            -------(2)-------->

                                               [allocates
                                               5MB quota]

                                Access Accept
                               {RADIUS BASE AVPS,
                               PPAQ={QID=5, VQ = 5MB,
                               VTH = 4.5 MB}}
                               <-------(3)--------

      service provision/metering
      -------(4)--------->

      4.5 MB consumed

                          Access Request
                          {RADIUS BASE AVPS,
                          PPAQ={QID=5, VQ=4.5MB,
                          REASON=THRESHOLD REACHED}}
                          -------(5)--------->

                                       [allocates another 7MB
                                      to the access service]

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                              Access Accept
                              {RADIUS BASE AVPS,
                              PPAQ={QID=8, VQ=12MB,
                              VTH = 11.5 MB}}
                          <----------(6)--------------
        user logs off
      ------(7)-------
                          Access Request
                          {RADIUS BASE AVPS,
                          PPAQ={QID=8, VQ=7 MB,
                          REASON=ACCESS SERV TERMINATED}}
                          -------(8)--------->

                                        [reimburses
                                        user account]

                              AA Accept
                             {RADIUS BASE AVPS}
                            <-------(9)--------

                 Figure 17: A simple example message flow

   The user logs on (1).  The PPC sends a RADIUS Access Request message
   to the PPS (2), and includes the prepaid-specific PPAC AVP.  This AVP
   indicates that both duration-based and volume-based metering is
   supported.  However, it also indicated that multiple services, rating
   groups and resource pools are not supported.  Note that, since this
   is not an "Authorize-Only" message, no PPAQ attribute with Update
   Reason="initial request" is included (see Section 3.7.1).  The PPS
   then authenticates the user and authorizes the access service, as is
   usual in RADIUS.  Note that the PPAC AVP is appended by the PPC in at
   least the last message that is sent to the home AAA server during
   this possibly multiple-round exchange.

   If authentication and authorization is successful (in this example
   this is assumed), then the PPS identifies the user's prepaid account
   from the included base RADIUS AVPs, and determines the capabilities
   of the PPC from the PPAC attribute.  Assuming that sufficient funds
   are available in the user's prepaid account, the PPS reserves some of
   these and rates the service.  In this example, the PPS reserves, say,
   2 Euros and determines that the access service is rated at 0.4 Euro
   per MB.  This results in 5 MB of quota being granted.  The PPS also
   determines that the PPC should ask for this quota to be replenished
   once 4.5 MB have been consumed.  Thus, it creates an Access Accept
   message with a Volume-Threshold indication of 4.5MB.  It further
   associates the QID=5 to this reservation.  This identifier can be
   used to later uniquely identify the prepaid session, user, account,
   etc.  The resulting Access Accept message is sent to the PPC (3).

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   Upon reception of message (4), the PPC provides the access service to
   the user and meters it accordingly.  At some point in time, the
   threshold is reached, i.e., 4.5MB of "access service" have been
   consumed by the user.  At that point, the PPC generates an Authorize-
   Only Access Request that contains the usual RADIUS attributes and a
   PPAQ attributes that reports the amount of consumed quota, and the
   request for replenishment, i.e., the Update-Reason= THRESHOLD REACHED
   (5).  Note that the QID in this message is the same as the one
   previously received from the PPS.

   Upon reception of message (5), the PPS identifies the user and his
   account from the QID.  In also determines that a prepaid session is
   ongoing, and that enough credit remains in the prepaid account in
   order for the access service to continue being provided.  Since 4.5
   MB have been consumed, the PPS subtracts 1.8 Euros from the user's
   prepaid account.  The PPS decides to reserve another 2.8 euros from
   the user's account.  (This results in 3 euros being reserved in total
   at this point in time.)  As the access service is rated at 0.4 euros
   per MB, the PPS determines that another 7 MB of quota should be
   granted.  This results in a total cumulative quota allocation of 12
   MB for the access service.  The PPS further calculates the new
   threshold value of 11.5 MB.  Since this is a new quota reservation,
   the PPS also allocates a new QID to it, in this example QID=8.  The
   resulting RADIUS message is sent to the PPC (6).

   Upon reception of message (6), the PPC updates its records and
   continues provisioning access to the user.  At some point the user
   logs off (7).  The PPC must then report how many resources were
   consumed, so that the PPC can subtract the appropriate monetary
   amount from the user's prepaid account.  To this end the PPC
   constructs an Authorize-Only Access Request message with a PPAQ
   attributes for the access service.  In this example, 7 MB were
   consumed by the access service in total.  The PPC reports 7 MB its
   final message (8).  The PPS correlates the report, using the QID, to
   the previous session state.  It determines, from the previous
   records, that the access service had consumed another 4.5 MB before
   (as indicated in message (6)).  This means that, of the 7 MB, only
   3.5 MB have not yet been subtracted from the user's account.  Thus,
   the PPS subtracts another 1.4 euros from the user's account and,
   since the session is to be terminated (REASON=ACCESS SERVICE
   TERMINATED), releases any reserved monetary amount.

   The PPS responds with an Access Response as required by the RADIUS
   base specification (9).

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A.2.  A flow with prepaid tariff switching

   End user          PPC                  PPS

      user logs on
      ------(1)--------->
                            Access Request
                            {RADIUS BASE AVPS,
                             PPAC=00...00111}
                            -------(2)-------->

                                        [allocates
                                        20MB quota]

                                   Access Accept
                                {RADIUS BASE AVPS,
                           PPAQ={QID=5, VQ = 20MB,
                              VTH = 18 MB}, PTS={
                            QID=5, PTS{TSI=300sec,
                                   TITSU=6000sec}}
                              <-------(3)-------

      service provision/metering
      -------(4)--------->

      5900 seconds
      passed
                          Access Request
                          {RADIUS BASE AVPS,
                          PPAQ={QID=5, VQ=14MB,
                          REASON=TITSU APPROACH.},
                          TSI={QID=5, VUATS=11MB}}
                          -------(5)--------->

                                  [allocates another 10MB
                                  to the access service]

                          Access Accept
                          {RADIUS BASE AVPS,
                          PPAQ={QID=8, VQ=30MB,
                          VTH = 28 MB},PTS={
                          QUD=8, PTS=95 sec}}
                     <----------(6)--------------

        user logs off

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      ------(7)-------

                          Access Request
                          {RADIUS BASE AVPS,
                          PPAQ={QID=8, VQ=17 MB,
                          REASON=ACCESS SERV TERMINATED},
                          PTS={QID=8, VUATS=2.5 MB}
                          -------(8)--------->

                                          [reimburses
                                          user account]

                                AA Accept
                               {RADIUS BASE AVPS}
                              <-------(9)--------

            Figure 18: Example message flow with Tariff Switch

   The user logs on (1).  The PPC sends a RADIUS Access Request message
   to the home AAA server (2), and includes the prepaid-specific PPAC
   AVP.  This AVP indicates that both duration-based and volume-based
   metering is supported, as well as tariff switching.  The home AAA
   server then may authenticate and user and authorize the access
   service, as is usual in RADIUS.  Note that the PPAC AVP is appended
   by the PPC in at least the last message that is sent to the PPS
   during this possibly multiple-round exchange.

   If authentication and authorization is successful (in this example
   this is assumed), the PPS identifies the user's prepaid account from
   the included base RADIUS AVPs, and determines the capabilities of the
   PPC from the PPAC attribute.  In this example, it is assumed that a
   tariff switch is about to occur in 300 seconds from the current time.
   Suppose that the access service is currently rated at 0.5 euros per
   MB and in the next tariff period it is rated at 0.6 euros per MB.
   Suppose further that a third tariff period is about to start in 6000
   seconds from current time and that that access service is rated at
   0.8 euros per MB in that period.  The PPS then decides to reserve 12
   euros from the user's account.  Since it is conceivable that the user
   may consume all allocated quota in the (more expensive) "0.6-euro"
   period, the PPS reserves 20 MB of quota, and determines a threshold
   value of 18 MB.  It constructs a Radius Access Accept message with a
   PPAQ attribute that reflects these choices, and carries a Quota
   Identifier QID=5.  It further adds a PTS AVP in the message which is
   linked to the PPAQ via the common QID value.  The PTS AVP contains a
   TSI attribute indicating that a tariff switch will occur in 300
   seconds.  It also includes a TITSU attribute with the value of 6000
   seconds.  This is included in order to make sure that the PPC will

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   report the consumed quota before the "2-euro" tariff period will
   start.  The message is sent to the PPC (3).

   Upon reception of message (3), the PPC provides the access service to
   the user and meters it accordingly (4).  It also keeps track of time.
   That is, it remembers how many octets are consumed before and how
   many after the tariff switch that will take place in 300 seconds.

   In this example it is assumed that the user consumes the allocated
   quota rather slowly.  In particular, nearly 6000 seconds (the value
   indicated by TITSU SubType) pass without the threshold of 18 MB being
   reached.  The PPC notices this and must therefore report usage and
   request the quota to be replenished despite the fact that the
   threshold has not been reached.  In this example, it decides to do so
   100 seconds before the 6000 seconds are reached.  To this end, it
   constructs an Authorization Access Request message including a PPAQ
   that indicates that 14 MB have been consumed up to now.  It also
   includes a PTS attribute in order to indicate, using the VUATS
   SubType, that 11 MB of these were consumed after the tariff switch.
   The message is sent to the the PPS (5).

   The PPS can link the message to previous session state via the QID.
   It now rates the consumed volume as follows.  The 11 MB that were
   consumed after the tariff switch correspond to 11 * 0.6 = 6.6 euros
   and the remaining 14-11=3 MB to 3 * 0.5 = 1.5 euros.  Thus, the PPS
   subtracts the amount of 6.6+1.5=8.1 euros from the user's account,
   which leads to a remainder of 12 - 8.1 = 3.9 euros being reserved.

   The PPS now determines that message (5) was sent in order to
   replenish the quota for this prepaid session.  This can be deduced
   from the UPDATE REASON field, which indicates that the PPC sent this
   message because the time indicated by the TITSU SubType is
   approacing.  The PPS now determines that enough credit remains in the
   user's prepaid account in order for the access service to continue
   being provided and decides to reserve another 8.9 euros from the
   user's account.  Since it is conceivable that the user will consume
   the 6 unused MB of quota from the previous allocation, as well as the
   entire quota that is to be allocated now, entirely in the "0.8-euro"
   period, the quota that should now be granted in addition to the
   previous 20 MB should be 10 MB.  This is because 0.9 of the 8.9 euros
   are being reserved in order to "cover the worst case scenario".  The
   fact that 0.9 euros are reserved for this purpose is due to the fact
   that the unused 6 MB from the previous allocation correspond to 4.8
   euros (with 0.8 euros per MB).  This is 4.8 - 3.9 = 0.9 euros more
   than the amount of funds that are still "reserved" from the previous
   allocation.  (After this reservation, the total amount of reserved
   money is 8.9 + 3.9 = 12.8 euros, which corresponds to 16 (10+6) MB
   being consumed in the "0.8-euro" period.)

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   Since quotas are encoded in a cumulative way in RADIUS, the PPS
   includes a VolumeQuota of 30 MB into the Access Accept message (6).
   The PPS further calculates the new threshold value of 28 MB.  Since
   this is a new quota reservation, the PPS also allocates a new QID to
   it, in this example QID=8.  The resulting RADIUS message is sent to
   the PPC (6).

   Upon reception of message (6), the PPC updates its records and
   continues providing access to the user.  At some point the user logs
   off (7).  The PPC must then report how many resources were consumed,
   so that the PPC can subtract the appropriate monetary amount from the
   user's prepaid account.  To this end the PPC constructs an Authorize-
   Only Access Request message with a PPAQ attributes for the access
   service.  In this example, 17 MB were consumed by the access service
   in total.  The PPC reports 17 MB its final message (8).  The PPS
   correlates the report, using the QID, to the previous session state.
   It determines, from the previous records, that the access service had
   consumed 14 MB before (as indicated in message (5)).  This means
   that, of the 17 MB, only the monetary equivalent for 3 MB have not
   yet been subtracted from the user's account.  The PPS calculates how
   much should be deducted from the user's account as follows.  Since
   the VUATS SubType indicates that 2.5MB were consumed after the tariff
   switch, only 0.5 MB were consumed before that.  Thus, the monetary
   equivalent is 0.5 * 0.6 + 2.5 * 0.8 = 3.6 euros.  That is, the PPS
   subtracts 3.6 euros from the user's prepaid account.  Since the
   session has by now be terminated by the PPC (REASON=ACCESS SERVICE
   TERMINATED), the PPS now releases any reserved monetary amount, in
   this example 12.8 - 3.6 = 9.2 euros.

   The PPS responds with an Access Response as required by the RADIUS
   base specification (9).

   Remark: In this example, two tariff switches take place.  In other
   scenarios, of course, only one tariff switch may occur.  In such
   scenarios the TITSU SubType is not used.

A.3.  Resource pools and Rating Groups

      End user          PPC                  PPS

      user logs on
      ------(1)--------->

                            Access Request
                            {RADIUS BASE AVPS,
                             PPAC=00...00101111}
                            -------(2)-------->

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                                          [allocates
                                          5MB quota]

                            Access Accept
                           {RADIUS BASE AVPS,
                           PPAQ={QID=5, VQ = 5MB,
                           poolID=1,mult=1}}
                          <-------(3)--------

      service provision/metering
      -------(4)--------->

      user requests service A
      -------(5)--------->

                          Access Request
                          {RADIUS BASE AVPS,PPAQ={
                          SID="A", RGROUP=1}}
                           -------(6)-------->

                           [allocates 50 min
                                       quota]

                          Access Accept
                         {RADIUS BASE AVPS,
                         PPAQ={QID=7, DQ=3000sec
                         poolID=1,RGROUP=1, SID="A"
                         mult=1747.63}}
                         <---------(7)---------------

      user requests service B
      -------(8)-------->

      Pool 1 close to exhaustion

                          Access Request
                          {RADIUS BASE AVPS,
                          PPAQ={QID=5, VQ=4MB,
                          REASON=QUOTA REACHED,
                          PoolID=1, mult=1}
                          PPAQ={QID=7, DQ=3300sec
                          REASON=QUOTA REACHED,
                          PoolID=1, mult=1747.63,
                          SID="A",RGROUP=1}}
                          -------(9)--------->

                                       [allocates another
                                       3 MB to access service

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                                       and 30 minutes to
                                       service "A"]

                          Access Accept
                         {RADIUS BASE AVPS,
                         PPAQ={QID=8, VQ=8MB,
                         PoolID=1, mult=1, RGROUP=1},
                         PPAQ={QID=9, DQ=4800sec
                         PoolID=1, mult=1747.63,
                         SID="A"}}
     \                    <----------(10)--------------

        user logs off
      ------(11)-------
                          Access Request
                          {RADIUS BASE AVPS,
                          PPAQ={QID=8, VQ=6.5MB,
                          REASON=ACCESS SERV TERMINATED,
                          PoolID=1, mult=1}
                          PPAQ={QID=9, DQ=5400sec
                          REASON=ACCESS SERV TERMINATED,
                          PoolID=1, mult=1747.63,
                          SID="A",RGROUP=1}}
                          -------(12)--------->

                                            [reimburses
                                            user account]

                          AA Accept
                         {RADIUS BASE AVPS
                         <------(13)--------

   Figure 19: Example message flow with resource pools and rating groups

   The user logs on (1).  The PPC sends a RADIUS Access Request message
   to the PPS (2), and includes the prepaid-specific PPAC AVP,
   indicating that multiple services, rating groups and resource pools
   are supported.  Note that, since this is not an "Authorize- Only"
   message, no PPAQ attribute with Update Reason="initial request" is
   included (see Section 3.7.1).  The PPS then may authenticate the user
   and authorize the access service, as is usual in RADIUS.  Note that
   the PPAC AVP is appended by the PPC in at least the last message that
   is sent to the PPS during this possibly multiple-round exchange.

   If authentication and authorization is successful (in this example
   this is assumed), the PPS identifies the user's prepaid account from
   the included base RADIUS AVPs, and determines the capabilities of the
   PPC from the PPAC attribute.  Assuming that sufficient funds are

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   available in the user's prepaid account, the PPS reserves some of
   these and rates the service.  In this example, the PPS reserves 5
   Euros and determines that the access service is rated at 1 Euro per
   MB.  In anticipation that the user requests more chargeable services
   throughout this prepaid session, and since this is supported by the
   PPC, the PPS further associates a resource pool with this
   reservation, in this example PoolID=1.  The PPC also specifies the
   multiplier = 1 for the access service.  Note that, since 5MB =
   5242880 octets, 1 unit in the resource pool corresponds to 5 /
   5242880 euros, which is about 0.000095367431640625 Eurocents.
   (However, the PPC does not need to know that.)  Moreover, the PPS
   associates the QID=5 to this reservation.  This identifier can be
   used to later uniquely identify the prepaid session, user, account,
   etc.  The resulting Access Accept message is sent to PPC (3).

   Upon reception of message (3), the PPC provides the access service to
   the user and meters it accordingly (4).  That is, for every octet
   consumed, the PPC subtracts 1 unit (since the multiplier is 1) from
   the resouce pool with PoolID=1.

   At some point in time, the user requests another chargeable service,
   namely service A (5).  The PPC generates an Authorize-Only Access
   Request that contains the usual RADIUS attributes and the Service-ID
   identifying service A (6).  The PPC has determined that service A is
   rated in an identical way as at least one more service.  Thus,
   service A has been configured to belong to a rating group, in this
   example the group with Rating-Group-ID=1.  This identifier is
   included is message (6).

   Upon reception of message (6), the PPS identifies the user and his
   account from the base RADIUS attributes, the fact that a prepaid
   session is ongoing, and determines that enough credit remains in the
   prepaid account in order for service A to be provided.  The PPS also
   determines that service A is rated at 0.10 euros per minute.  The PPS
   decides to reserve another 5 euros from the users account; this
   corresponds to 50 minutes or, as encoded in the DurationQuota
   SubType, 3000 seconds.  As service A draws from the same prepaid
   account as the access service, the PPS associates this reservation
   with the same resource pool as the previous reservation (QID=5),
   namely the pool with PoolID=1.  Note that, in order for the abstract
   units in the pool to be consistent, the multiplier has to be 1747.63.
   This is because each second corresponds to about 0.10 / 60 = 0.00167
   euros, which is about 1747.63 times the value of an abstract resource
   pool unit, as this was determined by the first allocation of quota to
   the pool (i.e., 0.000095367431640625 Eurocents).  Since this is a new
   quota reservation, the PPS also allocates a new QID to it, in this
   example QID=7.  The resulting RADIUS message is sent to the PPC (7).

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   Upon reception of message (7), the PPC adjusts the units in resource
   pool 1.  That is, it first determines how much quota had been
   allocated to service A in the past, and subtracts this from the quota
   reservation found in the message.  Since this is the first quota
   reservation for service A, there is nothing to subtract.  Thus, it
   adds 3000 * 1747.63 = 5242890 units to the pool and remembers that
   3000 seconds have been allocated to service A during this prepaid
   session.  The PPC then provides service A to the user, and meters it
   against resource pool 1.  That is, for every second it subtracts
   1747.63 units from the pool.

   At some point in time, the user requests service B (8).  The PPC
   determines that service B is rated exactly in the same way as service
   A, i.e., that they belong to the same rating group, namely the one
   with Rating-Group-ID=1.  Since this rating group has been effectively
   authorised by the allocation of quota with QID=7, the PPC provides
   service B to the user immediately.  It is rated in the same way as
   service A, i.e., for every second provided, 1747.63 units are
   subtracted from credit pool 1.

   At some point in time, resource pool 1 is close to exhaustion.  (For
   example, the PPC may determine that the pool is "close to exhaustion"
   when has less than 10% its initial amount of units.)  At that point,
   the PPC needs to ask for replenishment for the pool.  Suppose that,
   at that point in time, 4MB of "access service", 45 minutes of
   "service A", and 10 minutes of "service B" were provided to the user.
   Note that this corresponds to (4*1048576) + (55*60*1747.63) = 4194304
   + 5767179 = 9961483 abstract service units from the pool.  The PPC
   constructs an Authorize-Only Access Request message that reports the
   usage for the "access service" and "service A".  This message
   contains two PPAQ attributeS, is sent to the PPS (9).  Note that is
   the message it appears that "service A" has consumed more than it was
   allocated (i.e., 55 minutes although only 50 minutes were initially
   allocated to it).  This is not a a problem since the PPS knows that
   "service A" was drawing from the same pool as the "access service"
   and that the "access service" did only consume 4 out of the 5 MB it
   was allocated.

   Upon reception of message (9), the PPS subtracts 4 euros from the
   user's account for the "access service" and another 5.5 euros for
   "service A".  (This includes the charge incurred by "service B" up to
   that point in time, although the PPS is not aware of "service B"
   being provisioned to the user.)  The PPS then determines that
   sufficient funds remain in the prepaid account in order for both
   services to be continued.  The PPS decides to reserve another 3MB for
   the access service and 30 minutes for "service A".  This corresponds
   to 3+3=6 euros.  Since in RADIUS prepaid the quotas are encoded in a
   cumulative manner, the PPAQ attribute that grants the quota for the

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   "access service" contains a Volume-Quota SubType of 8MB (8388608
   octets), which is the 5MB that were initially allocated, plus the 3MB
   allocated now.  The resource pool identifier is, as previously,
   PoolID=1 and the multiplier is 1.  Similarly, the PPAQ that grants
   quota for "service A" contains 4800 seconds (the initial 3000 plus
   1800 that correspond to the 30 additional minutes).  Again, the
   PoolID=1 and multiplier=1747.63.  The resulting Access Response
   message is sent to the PPC (10).

   When the PPC received message (10) it checks how much quota has been
   allocated previously to the "access service".  It finds that the
   answer is 5MB (5242880 octets); thus, out of the 8MB (8388608 octets)
   that are indicated by the PPAQ with QID=8, only 3MB (3145728 octets)
   have not yet been added to resource pool 1.  The PPC thus adds
   3145728 abstract units to resource pool 1 (since the multiplier is
   1).  The PPC then acts similarly on the other PPAQ attribute that
   exists in message (11).  That is, the PPC determines that 3000
   seconds of quota for "service A" had already been added to the pool.
   Thus only 1800 out of the 4800 should be additionally added to the
   pool.  Since the applicable multiplier here is 1747.63, the PPC adds
   further 3145734 abstract units to the pool 1.

   The PPC then continues to provide the access service, "service A" and
   "service B" to the user, and meters them against the pool, as
   previously.

   At some point the user logs off (11).  The PPC must then report how
   many resources were consumed, so that the PPC can subtract the
   appropriate monetary amount from the user's prepaid account.  To this
   end the PPC constructs an Authorize-Only Access Request message with
   two PPAQ attributes; one for the access service and one for "service
   A".  Suppose that, in total, 6.5MB were consumed by the access
   service, 70 minutes were consumed by "service A" and 20 minutes by
   "service B".  The PPC reports 6.5MB (6815744 octets) and 90 minutes
   (5400 seconds) in its final message (12).  The PPS determines, from
   the previous records, that the access service consumed another 2.5MB
   (since 4MB out of the 6.5MB were already reported in message (9), and
   that "service A" consumed further 600 seconds.  This corresponds to
   2.5 + (600/60)*0.1 = 2.5+ 1=3.5 euros.  Thus, the PPS only subtracts
   2.5 out of the 6 previously reserved euros from the user's prepaid
   account and responds with an Access Response as required by the
   RADIUS base specification (13).

A.4.  One-time charging

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     End user          PPC                  PPS

      user requests ring tone
      ------(1)--------->

                          Access Request
                          {RADIUS BASE AVPS,
                          PPAQ={QID=321, SID=X, RQ=650,
                          REASON=10 (ONE-TIME CHARGING}}
                          -------(2)--------->

                                   [rates 650 abstract units
                                   deducts from user's account]

                                Access Accept
                                {RADIUS BASE AVPS}
                      <----------(3)--------------

        ring tone is delivered
      <------(4)-------

          Figure 20: Example message flow with one-time charging

   The user requests a chargeable ring tone (1).  The PPC sends a RADIUS
   Access Request message to the PPS (2), and includes a PPAQ attribute
   with Update Reason="one-time charging" is included (see
   Section 3.8.6).  The Service ID indicates to the PPS that the
   charging event is connected to a ring tone, so that the PPS can rate
   the event accordingly.  The PPAQ also contains a unique Quota
   Identifier.

   The PPS then may authenticate the user as is usual in RADIUS.  If
   authentication is successful (in this example this is assumed), the
   home AAA server forwards the PPC converts the 650 reported abstract
   units into monetary value, according to the service type, and debit
   the user's account accordingly.  This happens, of course, only if
   sufficient funds are available in the user's prepaid account.  The
   PPC then responds with an an Access Accept message (3).  The PPS adds
   a "session timeout = 0 AVP" (see Section 3.8.6).

A.5.  Price enquiry

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      End user          PPC                  PPS

      user requests AoC
      ------(1)--------->

                          Access Request
                          {RADIUS BASE AVPS,
                          PPAQ={SID=X, VQ=10MB,
                          REQ_ACT=2(PRICE ENQUIRY}}
                          -------(2)--------->

                              [rates 10MB for requested
                                                service]

                           Access Accept
                          {RADIUS BASE AVPS,
                          PPAQ={SID=X, VQ=10MB,
                   COST INFORMATION= 0.6 euros
                          per MB}}
                        <----------(3)--------------

        AoC is delivered
      <------(4)-------

   Figure 21: Example message flow with price enquiry (advice of charge)

   Please refer to Section 2.7.3 for an explanation of this message
   flow.

A.6.  Balance check

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   End User               PPC                  PPS

                          Access Request
                          {RADIUS BASE AVPS,
                          PPAQ={SID=X, VQ=10MB,
                          REQ_ACT=BALANCE CHECK}}
                          -------(2)--------->

                                       [rates requested
                                       Service and checks
                                       remaining funds]

                          Access Accept
                         {RADIUS BASE AVPS,
                         PPAQ={SID=X, VQ=10MB,
                   BALANCE_CHECK_RESULT}}
                    <----------(3)--------------

            Figure 22: Example message flow with balance check

   Please refer to Section 2.7.4 for an explanation of this message
   flow.

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Appendix B.  Translation between RADIUS Prepaid and Diameter Credit
             Control

   Note: This section is informative.

   In scenarios where the service metering device uses the "RADIUS
   prepaid" (RPP) protocol for accounting and prepaid charging while the
   AAA infrastructure uses the "Diameter Credit Control" (DCC) protocol,
   a translation agent that enables the interoperation of both systems,
   is desirable.  This also applies vice versa, i.e., in scenarios where
   the AAA infrastructure uses RADIUS and the service metering device
   uses Diameter.

   The idea of such a translation agent would be to convert incoming RPP
   (resp. DCC) messages into outgoing DCC (resp. RPP) messages.  It
   would be, in principle, desirable for the translation agent to be
   stateless.  That is, the agent should not be required to internally
   maintain information about each ongoing RADIUS or Diameter session.
   However, under the current specification of RPP and DCC, this appears
   to be impossible due to a number of reasons.  These include the
   following.

   1.  The transport mechanism for DCC is TCP, which requires per-
       session state to be maintained at both endpoints of the
       communication.  Note, however, that, in principle, each DCC
       message could be sent over a dedicated TCP connection which is
       torn down as soon as the message is sent.  This, however, is
       likely to be unacceptable in terms of efficiency.

   2.  While RPP messages encode the cumulative amount of consumed/
       requested resources, DCC messages carry the difference from the
       previous message.  This means that the translation agent has to
       maintain the current amount of consumed/requested resources in
       order to be able to calculate the correct amount to be put into
       an outgoing message.

   The translator maps each incoming RPP (resp. DCC) message into an
   outgoing DCC (resp. RPP) message, and possibly establishes or updates
   local state that is associated with the session.  The translated
   (i.e., outgoing) message is a function of the incoming message as
   well as existing state that is associated with the current session.

   Translation occurs on an attribute-by-attribute basis.  Certain
   attributes are translated without consideration of local per-session
   state.  Other attributes, namely those that are bound to a particular
   session, require such consideration.  The translation agent has to
   identify the session (and possibly subsession) an incoming message
   belongs to in order to consult the appropriate local per-session

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   state.

   Note that certain DCC attributes cannot be translated due to their
   semantics not being present in RPP, and vice versa.  This results in
   the messages, in which these attributes occur, not being delivered to
   their intended destination.  In such cases it is desirable to inform
   the originator about the failure and terminate the session.

   In each scenario (i.e., RPP client / DCC AAA infrastructure and DCC
   client / RPP AAA infrastructure), the translator operates in two
   directions, namely RPP to DCC and vice versa.  In the following
   sections, the notation c->s means that the attribute in question may
   occur only in the direction from the client to the server.  The
   notation s->c denotes the converse and the notation c<->s denotes
   that the attribute may occur in messages that are directed in either
   direction.

B.1.  Session Identification

   The translation agent has to keep per-session state in order to
   perform its task.  A session may be identified based on the RPP
   identifier or the DCC session identifier.  That is, the translation
   agent should always maintain a pair of (RPP, DCC) session identifiers
   and maintain the per-session state in association with that pair.
   This per-session state must be addressable by either of these two
   identifiers.  Moreover, an RPP session identifier must uniquely
   correspond to a DCC identifier.  (If this holds, the converse also
   holds.)  Each subsession identifier within an RPP session must also
   uniquely correspond to a subsession identifier within its
   corresponding DCC session.  (If this holds the converse also holds.)

B.2.  Translation between RADIUS Prepaid and Diameter Credit Control

   This section describes the translator in the "RPP client / DCC AAA
   infrastructure" case.  In other words, in this section it is assumed
   that the client "talks" RPP and the AAA inftrastructure "talks" DCC.
   The translator is assumed to sit somewhere in the middle and to
   mediate between client and server.

   For each RPP AVP (i.e., AVPs that are specified in the present
   document), the transformation into a semantically equivalent DCC AVP
   (if such an AVP exists), along with what per-session state the
   translator has to create or consult, is described.  For clarity of
   exposition, each RPP AVP is addressed in a separate subsection.
   Since in this scenario, the PPC is typically the initiator a session,
   the focus is on the RPP AVPs.

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B.2.1.  PPAC (c<->s)

   A DCC client is assumed to always support Volume metering, Duration
   metering, Resource metering, Pools, Rating groups, and Tariff
   Switching.  Thus, if a PPAQ that indicates any of the above is sent
   client->server, the translator does the following: It lets message go
   through but remembers what exactly the client supports.  If the
   server later requests (servier -> client direction) an unsupported
   metering to be performed, send failure to server and cause the
   session to be terminated at the client.

   If a PPAC indicates support for multiple services (0x00000020), the
   translator maps this onto a DCC Multiple-Services- Indicator AVP.

B.2.2.  Service Termination Attribute (c->s)

   The Diameter base protocol assumes that the client always supports
   dynamic session termination.  If this AVP is present, the translator
   does not need to do anything, i.e., there exists no DCC AVP that this
   AVP can be mapped to.  If this AVP is absent, the message in which it
   appears should either be discarded and originator should be informed
   of a failure, or the message can be passed on (without this AVP being
   mapped onto a DCC AVP).  However, in the latter case, the translator
   has to remember that the client does not support dynamic termination.
   Thus, the translatior has to initiate the normal session termination
   procedure with the client when (if) dynamic termination is later
   initiated by the server.

B.2.3.  Quota Identifier Attribute (c<->s)

   When quota is allocated for the first time by the DCC server, the
   translator has to create a QID AVP, as required by this
   specification.  The translator later uses a QID AVP that is sent in
   the client-to-server direction in order to identify the corresponding
   DCC session.  The QID has to be saved in the translator's per session
   state.

B.2.4.  Volume Quota Attribute (c<->s)

   If this AVP occurs in a message that is sent in the server-to-client
   direction, it is translated into a Granted-Service-Unit AVP with an
   embedded CC-Total-Octets AVP.

   If this AVP occurs in a message that is sent in the client-to-server
   direction, then it is translated into a Used-Service-Unit AVP with an
   embedded CC-Total-Octets AVP.  Note that only the difference between
   current cumulative quota for the (sub)session and the quota in
   incoming messages is indicated in the translated DCC message.  Local

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   state is updated with cumulative consumed resources.

   Conversely, if the server grants quota using the DCC Granted-Service-
   Unit AVP with an embedded CC-Total-Octets AVP, then the translation
   agent must translate this into a Volume Quota Attribute.  Again,
   local state must be consulted so that the cumulative amount of octets
   is indicated in the Volume Quota attribute.

B.2.5.  Duration Quota Attribute (c<->s)

   If this AVP occurs in a message that is sent in the server-to-client
   direction, it is translated into a Granted-Service-Unit AVP with an
   embedded CC-Time AVP.

   If this AVP occurs in a message that is sent in the client-to-server
   direction, then it is translated into a Used-Service-Unit AVP with an
   embedded CC-Time AVP.  Note that only the difference between current
   cumulative quota for the (sub)session and the quota in incoming
   messages is indicated in the translated DCC message.  Local state is
   updated with cumulative consumed resources (i.e., time).

   Conversely, if the server grants quota using the DCC Granted-Service-
   Unit AVP with an embedded CC-Time AVP, then the translation agent
   must translate this into a Duration Quota attribute.  Again, local
   state must be consulted so that the cumulative amount of seconds is
   indicated in the Duaration Quota attribute.

B.2.6.  Resource Quota Attribute (c<->s)

   If this AVP occurs in a message that is sent in the server-to-client
   direction, it is translated into a Granted-Service-Unit AVP with an
   embedded CC-Service-Specific-Units AVP.

   If this AVP occurs in a message that is sent in the client-to-server
   direction, then it is translated into a Used-Service-Unit AVP with an
   embedded CC-Service-Specific-Units AVP.  Note that only the
   difference between current cumulative quota for the (sub)session and
   the quota in incoming messages is indicated in the translated DCC
   message.  Local state is updated with cumulative consumed resources
   (i.e., resources).

   Conversely, if the server grants quota using the DCC Granted-Service-
   Unit AVP with an embedded CC-Service-Specific-Units AVP, then the
   translation agent must translate this into a Resource Quota
   attribute.  Again, local state must be consulted so that the
   cumulative amount of resource units is indicated in the Resource
   Quota attribute.

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   Note that the "resource" type is application dependent.  This means
   that a DCC application unit corresponds to n RPP application units,
   where n may be any real number.  If n is not 1, then the RPP/DCC
   translator must be aware of that and translate resource units
   accordingly.

B.2.7.  Value Digits Attribute (c<->s)

   The encoding of this AVP is similar in RPP and DCC, and the value it
   holds may have to be evaluated in conjunction with an acommpanying
   "Exponent" AVP.  It should be kept in mind that, in RPP the
   cumulative amount of granted/consumed quota is typically encoded into
   an AVP of this type, while in DCC only the difference from a previous
   message.

B.2.8.  Exponent Attribute (c<->s)

   The encoding of this AVP is similar in RPP and DCC, and the value it
   holds may have to be evaluated in conjunction with an acommpanying
   "Value Digits" AVP.  It should be kept in mind that, in RPP the
   cumulative amount of granted/consumed quota is typically encoded into
   a related "Value Digits" and "Exponent" AVP pair, while in DCC only
   the difference from a previous message is encoded into such a pair.

B.2.9.  Volume/Duration/Resource Threshold Attributes (s->c)

   In DCC the concept of "threshold" does not exist.  Instead, the DCC
   client is assumed to ask for the replenishment of quota in good time.
   In RPP, on the other hand, the server may optionally include a
   threshold AVP, as an indication to the PPC about when to ask for
   quota replenishment.

   Thus, in this scenario, there is no need for the translator to ever
   include a threshold attribute into the messages that it sends to the
   PPC.  If, however, there is a need for a threshold attribute to be
   present in order to avoid a possible service provision

B.2.10.  Update Reason Attribute (c->s)

   The DCC AVP that is semantically closer to the Update Reason AVP than
   any other AVP is the CC-Request-Type AVP.  This AVP indicates whether
   the message is which it appears is intended to indicate an "initial",
   an "intermediate" or a "final interrogation".  Morever, in case of
   the session being terminated at the client, it indicates the reason
   for this termination.

   The following list lists the possible values of an "Update Reason"
   attribute, along with corresponding values for the CC-Request-Type

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   AVP.

   o  Pre-initialization: No action/value defined.

   o  Initial Request: Typically an "intial interrogation" is triggered
      as a result of the reception of the message that contains this
      Update Reason AVP.  Hence, CC-Request-Type AVP indicates
      "INITIAL_REQUEST".

   o  Threshold Reached: The reception of the message containing this
      Update Reason AVP typically triggers an "intermediate
      interrogation".  Hence, CC-Request-Type AVP indicates
      "UPDATE_REQUEST".

   o  Quota Reached: The reception of the message containing this Update
      Reason AVP typically triggers an "intermediate interrogation".
      Hence, CC-Request-Type AVP indicates "UPDATE_REQUEST".

   o  TITSU Approaching: The reception of the message containing this
      Update Reason AVP typically triggers an "intermediate
      interrogation".  Hence, CC-Request-Type AVP indicates
      "UPDATE_REQUEST".

   o  Remote Forced Disconnect: Reception of such an Update Reason
      indicates that the client has terminated the session.  The
      corresponding value for the CC-Request-Type AVP is
      "TERMINATION_REQUEST".

   o  Client Service Termination: Reception of such an Update Reason
      indicates that the client has terminated the session.  The
      corresponding value for the CC-Request-Type AVP is
      "TERMINATION_REQUEST".

   o  "Access Service" Terminated: Reception of such an Update Reason
      indicates that the client has terminated the session.  The
      corresponding value for the CC-Request-Type AVP is
      "TERMINATION_REQUEST".

   o  Service not established: Reception of such an Update Reason
      indicates that the client has terminated the session.  The
      corresponding value for the CC-Request-Type AVP is
      "TERMINATION_REQUEST".

   o  One-Time Charging: Such an Update Reason indicates that a one-time
      charging event is initiated by the client.  The corresponding
      value for the CC-Request-Type AVP is "EVENT_REQUEST".  Note that a
      "Requested-Action: AVP MUST also be included in the outgoing DCC
      message.  Typically, this would be of the type "DIRECT_DEBITING",

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      or "REFUND_ACCOUNT", depending on other AVPs present in the
      message.

B.2.11.  PrepaidServer Attribute (s<->c)

   The PPC typically never sets the value of a PrepaidServer attribute.
   Instead, it repeats those values that it receives from the AAA
   infrastructure, in this scenario from the translator.  This attribute
   is therefore not used in a translation scenario.  Nevertheless, the
   translator must make sure that messages about the same RPP session
   are forwarded to the same DCC server, throughout the whole session.
   This may be easy to guarantee since the transport of Diameter is TCP.

B.2.12.  Service-ID Attribute (s<->c)

   The DCC equivalent of a RPP "Service-ID" AVP is the combination of
   Service-Context-Id and Service-Identifier AVPs.  The translator must
   keep a static equivalence table of the RPP Service-ID and the
   corresponding DCC combination in order to correctly translate an RPP
   service identifier into DCC and back.

B.2.13.  Rating-Group-ID Attribute (s<->c)

   The DCC equivalent of a RPP "Rating-Group-ID" AVP is also called a
   "Rating-Group-ID".  Depending on the configuration, this AVP may
   contain the same value on both the RPP and the DCC side of the
   communication.  If, however, static rating groups are configured
   between the RCC client and the translator, and different rating
   groups between the DCC server and the translator, then the translator
   has to maintain a static translation table for the rating group
   identifier.  In any case, the translation of a rating group AVP, is
   not a function of the translator's local per-session state.

B.2.14.  Termination-Action Attribute (s->c)

   The DCC equivalent of the "Termination-Action" AVP is called the
   "Final-Unit-Action" AVP.  In this scenario (RPP client and DCC AAA
   infrastructure), a DCC "Final-Unit-Action" AVP is translated into a
   "Termination-Action" AVP.  The following list contains the possible
   "Final-Unit-Action" values along with their "Termination-Action"
   equivalent.

   o  TERMINATE (DCC): This value has a direct equivalent in RPP, also
      called "Terminate".

   o  REDIRECT (DCC): If this value appears in a "Final-Unit-Action"
      AVP, then a "Redirect-Server-Address" AVP must also appear in the
      same DCC message.  The translator translates these two AVPs into a

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      "Termination-Action" with value "Redirect/Filter" and an
      eqiovalent NAS-Filter-Rule attribute (specified in http://
      www.ietf.org/internet-drafts/draft-ietf-radext-ieee802-00.txt).

   o  RESTRICT_ACCESS (DCC): If this value appears in a "Final-Unit-
      Action" AVP, then a "Restriction-Filter-Rule" AVP must also appear
      in the same DCC message.  The translator translates these two AVPs
      into a "Termination-Action" with value "Redirect/Filter" and an
      eqiovalent Filter-ID attribute (specified in http://www.ietf.org/
      internet-drafts/draft-ietf-radext-ieee802-00.txt).

   o  In the absence of a "Final-Unit-Action" AVP, the DCC server
      assumes that the DCC client will ask for replenishment of quota at
      some suitable time.  In RPP, this is explicitly conveyed via a
      "Termination-Action" AVP with the value "Request More Quota".
      Thus, in the absence of a "Final-Unit-Action" AVP, the translator
      in this scenario appends such an AVP into the outgoing RPP
      message.

B.2.15.  Pool-ID Attribute (s<->c)

   The DCC equivalent of a RPP "Pool-ID" AVP is also called a "Pool-ID".
   Typically, no translation needs to be done to the "Pool-ID"
   attribute.

B.2.16.  Multiplier Attribute (s<->c)

   The multiplier attribute, which is a pair of "Value-Digits" and
   "Exponent" AVPs, typically needs no translation, since the value it
   carries (inside a "Value-Digits" and an "Exponent" AVP) represents
   the rating of the service or rating group to which it refers, with
   respect to abstract units.  As such, the same multiplier value would
   typically applyt be conveyed from a DCC server to an PPC, and vice
   versa.

B.2.17.  Requested-Action Attribute (c->s)

   The "Requested Action" AVP can be directly translated into its DCC
   equivalent, which carries the same name.

   1.  Balance Check (PCC): CHECK_BALANCE (DCC)

   2.  Price Enquiry (PCC): PRICE_ENQUIRY (DCC)

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B.2.18.  Check-Balance-Result Attribute (s->c)

   This attribute carries only a binary value.  Hence, its translation
   is straightforward.

B.2.19.  Cost-Information Attribute (s->c)

   This attribute consists of a Value-Digits AVP, an Exponent AVP, a
   Currency Code AVP, and a Cost-Unit AVP.  All these AVPs do likewise
   exist in DCC, and carry identical semantics in the context of the
   "Cost-Information" AVP.  Thus, the translation of this attribute is
   straightforward.

B.2.20.  VolumeUsedAfterTariffSwitch attribute (c->s)

   This attribute carries the amount of octets that were consumed after
   a tariff change.  It always appears in a message with an accompanying
   PPAQ attribute in which the total amount of octets (i.e., those that
   were consumed both before and after the tariff switch) is reported.
   Thus, the translation agent can compute the amount of octets that
   were consumed before the tariff change.

   In DCC, the two amounts, i.e., the octets that were consumed before a
   tariff change and those that were consumed afterwards, are reported
   in separate Used-Service-Unit AVPs.  The two Used-Service-Unit AVPs
   have an embedded CC-Total-Octets AVP that indicates the appropriate
   amount of octets.  Furthermore, the Used-Service-Unit AVP that
   carries the amount that was consumed before the tariff switch also
   carries an embedded Tariff-Change-Usage AVP with the value
   UNIT_BEFORE_TARIFF_CHANGE (0).  Similarly, the Used-Service-Unit AVP
   that carries the amount that was consumed after the tariff switch
   also carries an embedded Tariff-Change-Usage AVP with the value
   UNIT_AFTER_TARIFF_CHANGE (1).

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Authors' Addresses

   Avi Lior
   Independent

   Email: avi.ietf@lior.org

   Parviz Yegani
   Juniper

   Email: pyegani@juniper.net

   Kuntal Chowdhury
   Radio Mobile Access, Inc.

   Email: kc@radiomobiles.com

   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600
   Finland

   Phone: +358 (50) 4871445
   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at

   Andreas Pashalidis
   K.U.Leuven, ESAT/SCD/COSIC
   Kasteelpark Arenberg 10, bus 2446
   Leuven-Heverlee  B-3001
   Belgium

   Email: andreas.pashalidis@esat.kuleuven.be

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