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Network Working Group                                            A. Lior
INTERNET-DRAFT                                       Bridgewater Systems
Category: Informational                                        P. Yegani
draft-lior-radius-prepaid-extensions-03.txt                        Cisco
Expires: 16 July, 2004                                      K. Chowdhury
                                                                  Nortel
                                                               L. Madour
                                                         Ericsson Canada
                                                                   Y. Li
                                                     Bridgewater Systems
                                                       February 16, 2003


     PrePaid Extensions to Remote Authentication Dial-In User Service
                                 (RADIUS)


Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time.  It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
        http://www.ietf.org/ietf/1id-abstracts.txt
   The list of Internet-Draft Shadow Directories can be accessed at
        http://www.ietf.org/shadow.html.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   The draft presents an extension to the Remote Authentication Dial-In
   User Service (RADIUS) protocol to support PrePaid data services for
   a wide range of deployments such as Dial, Wireless, WLAN.
   Consideration for roaming using mobile-ip is also given.


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                    RADIUS Extensions for PrePaid       February 2004


Table of Contents

   1. Introduction...................................................4
      1.1 Terminology................................................6
      1.2 Requirements language......................................6
   2. Architectural Model............................................6
      2.1 Why not existing RADIUS attributes?.......................14
   3. Use-cases.....................................................16
      3.1 Simple pre-paid access use-case...........................17
      3.2 Simple Service Device use-case............................20
      3.3 Support for concurrent PrePaid sessions...................20
      3.4 Support for Roaming.......................................21
      3.5 PrePaid termination.......................................22
   4. Operations....................................................22
      4.1 General Requirements......................................22
         4.1.1 Broker AAA Requirements..............................22
      4.2 Authentication and Authorization for Prepaid Enabled Access
      Devices.......................................................23
         4.2.1 Single Service Pre-paid..............................24
         4.2.2 Multiple-Session Pre-paid............................25
      4.3 Session Start Operation...................................27
      4.4 Mid-Session Operation.....................................28
      4.5 Dynamic Operations........................................30
         4.5.1 Unsolicited Session Termination Operation............30
         4.5.2 Unsolicited Change of Authorization Operation........31
      4.6 Termination Operation.....................................32
      4.7 Mobile IP Operations......................................32
      4.8 Accounting Considerations.................................33
      4.9 Service Device Operation..................................33
      4.10 Interoperability with Diameter Credit Control Application34
   5. Attributes....................................................34
      5.1 PPAC Attribute............................................34
      5.2 Session Termination Capability............................36
      5.3 PPAQ Attribute............................................36
      5.4 Table of Attributes.......................................40
   6. Security Considerations.......................................41
      6.1 Authentication and Authorization..........................41
      6.2 Replenishing Procedure....................................41
   7. IANA Considerations...........................................41
   8. Normative References..........................................41
   Acknowledgments..................................................42
   Author's Addresses...............................................42
   Intellectual Property Statement..................................43


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   Full Copyright Statement.........................................43
   Expiration Date..................................................44












































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1. Introduction

   This draft describes RADIUS protocol extensions supporting PrePaid
   Data Services.

   PrePaid data services are cropping up in many wireless and wireline
   based networks.  A PrePaid Data Service subscriber is one that
   purchases a contract to deliver a data service for either a period
   of time, or a quantity of data.  Before providing a prepaid data
   service, the service provider checks that the prepaid subscriber has
   sufficient funds to cover the particular service request. Only after
   confirmation that funds are available is the service provided to the
   user.

   The subscriber purchases the Data Service using various means such
   as buying a PrePaid Card, or online.  How the subscriber purchases
   their PrePaid Data Service depends on the deployment and is not in
   scope for this document.

   In some deployments, the PrePaid data service will be combined with
   other Prepaid services such as PrePaid voice service.  This is not
   an issue for this document other than the fact that the PrePaid Data
   Services described in this paper should work with other PrePaid data
   and or voice services.

   The fundamental business driver for a carrier to provide PrePaid
   data services is to increase participation (subscriber base) and
   thus to increase revenues.  Therefore, it makes sense that PrePaid
   services meet the following goals:

   - Leverage existing infrastructure, hence reducing capital
      expenditures typically required when rolling a new service;
   - Ability to rate service requests in real-time;
   - Ability to check that the end userÆs account for coverage for the
      requested service charge prior to execution of that service;
   - Protect against revenue loss, i.e., prevent an end user from
      generating chargeable events when the credit of that account is
      exhausted or expired;
   - Protect against fraud;
   - Be as widely deployable over Dialup, Wireless and WLAN networks.





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   The protocol described in this document maximizes existing
   infrastructure as much as possible ¡ hence the use of the RADIUS
   protocol.  The protocol is used in ways to protect against revenue
   loss or revenue leakage.  This is achieved by defining procedures
   for the real-time delivery of service information to a pre-paid
   enabled AAA server, to minimize the financial risk, for the pre-paid
   enabled AAA server to be able to allocate small quotas to each data
   session and having the ability to update the quotas from a central
   quota server dynamically during the lifetime of the PrePaid data
   session.  As well, mechanisms have been designed to be able to
   recover from errors that occur from time to time.

   Protection against fraud is provided by recording of accounting
   records, by providing mechanisms to thwart replay attacks.  As well,
   mechanisms have been provided to terminate data sessions when fraud
   is detected.

   PrePaid System will become more prevalent and sophisticated as the
   various networks such as Dialup, Wireless and WLAN converge.  This
   protocol extension is designed to meet the challenges of converged
   networks.  The draft mainly addresses how to use the RADIUS protocol
   to achieve a PrePaid Data Service.  The prepaid architecture assumes
   that rating of chargeable events does not occur in the element
   providing the service. This rating could be performed in the prepaid
   enabled AAA server or may exist in an entity behind this AAA server.
   Business logic and service rules may define that tariffing of events
   vary in time, e.g., the particular price per megabyte download may
   be defined to switch at 8pm from a high tariff to a low tariff. The
   RADIUS extensions for prepaid support scenarios enable scalable
   implementation of tariff switched prepaid systems.

   Furthermore, the prepaid architecture assumes that a quota server is
   available which, through co-ordination with the rating entity and
   centralized balance manager is able to provide a quota response in
   response for prepaid data service.  This quota server functionality
   could be performed in the prepaid enabled AAA server or may exist in
   an entity behind this AAA server. Finally, the details of the
   PrePaid System, such as its persistent store, how it maintains its
   accounts are not covered at all.  However, in order to define the
   RADIUS protocol extensions it is necessary to discuss the functional
   behavior of the PrePaid System.





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1.1 Terminology

   Access Device
   PrePaid Client
   PrePaid Server
   Home agent (HA)
   Home network
   Home AAA (HAAA)
   Broker AAA (BAAA)
   Visited AAA (VAAA)
   Foreign Agent (FA)
   WLAN
   Service Device

1.2 Requirements language

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.  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 [RFC2119].

2. Architectural Model

   The architectural model supports prepaid clients on either an access
   device or a service device. An access device (e.g. a NAS) typically
   provides a single service to end-users, corresponding to network
   access. The service device enables finer grain services to be
   defined. For example, a service may be defined for access to a
   particular destination network, which enables further segmentation
   of services within the network. An access device and service device
   may be combined into a single physical entity.

   When pre-paid service is used the access or service device collects
   service event information and reports it while and/or after services
   are provided to the prepaid user. This event information is sent to
   a prepaid server by using the prepaid RADIUS extensions.

   If real-time credit control is required, the access or service
   device (prepaid client) contacts the prepaid server with service
   event information included before the service is provided. The
   prepaid server, depending on the service event information, performs
   credit check and allocates a portion of available credit to the


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   service event. The rating entity converts this credit value into a
   time and/or volume amount, which is then returned to the requesting
   device. The rating entity may determine that during the allocated
   quota, a tariff switch will occur in which case the rating entity
   will include details of the quota allocated prior to the tariff
   switch, details of the quota allocated after the tariff switch
   together with details of when the tariff switch will occur.

   The requesting device (either access or service device) then
   monitors service execution according to the instructions returned by
   the prepaid server. After service completion or on a subsequent
   request for service, the prepaid server deducts the reserved
   allocation of credit from the prepaid userÆs account.

   Similarly, when a user terminates an on-going prepaid service, the
   prepaid client signals the prepaid server with the a value
   corresponding to the unused portion of the allocated quota.  The
   prepaid server is then able to refund unused allocated funds into a
   userÆs prepaid account.

   There MAY be multiple prepaid servers in the system for reasons of
   redundancy and load balancing. The system MAY also contain separate
   rating server(s) and accounts MAY locate in a centralized database.
   System internal interfaces can exist to relay messages between
   servers and an account manager.  However the detailed architecture
   of prepaid system and its interfaces are implementation specific and
   are out of scope of this specification.

                                           accounting
       +------------+       +-----------+ protocol     +--------------+
       |  Access    |<----->|  Service  |<------------>| Accounting   |
       |  Device    |   +-->|  Device   |<-----+       |   Server     |
       +------------+   |   +-----------+      |       +--------------+
                        |                      |
       +------------+   |                      |
       |  Access    |<--+                      |       +--------------+
       |  Device    |                          +------>|   PrePaid    |
       +------------+                     prepaid      |   Server     |
                                          protocol     +--------------+

   Figure 1 Basic Prepaid Architecture





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   The prepaid server and accounting server in this architecture model
   are logical entities. The real configuration MAY combine them into a
   single host.

   There MAY exist protocol transparent RADIUS Proxies between prepaid
   client and prepaid server. These proxies transparently support the
   prepaid RADIUS extensions.

   In order to generalize the solution, in this paper we generalize the
   Access Devices, which in reality may be a NAS from in Dialup
   deployments, PDSN in CDMA2000 deployments,an 802.11 WLAN Access
   Points or GGSN in GSM deployments. To actively participate in
   Prepaid procedures outlined here, the Access Device MUST have
   Prepaid Client capabilities.  Prepaid Client Capabilities include
   the ability to meter the usage for a prepaid data session; this
   usage includes time or volume usage.

   In circumstances when the Access Device does not support the Prepaid
   client capabilities, prepaid client functionality may be provided
   using either a stand alone service device or, in the case of roaming
   scenarios using mobile IP, the prepaid client functionality may be
   delegated to the Home Agent.  It may also be possible to deliver
   limited prepaid services using RADIUS capabilities specified in
   RFC2865 and RFC2866.

   Furthermore, the device including the prepaid client functionality
   may also have Dynamic Session Capabilities that include the ability
   to terminate a data session and/or change the filters associated
   with a specific data session by processing Disconnect Messages and
   Change of Filter messages as per [RFC3576].

   In this document RADIUS is used as the AAA server.  There are three
   kinds or categories of AAA servers.  The AAA server in the home
   network, the HAAA, is responsible for authentication of the
   subscriber and also authorization of the service.  In addition, the
   HAAA communicates with the Prepaid servers using the RADIUS protocol
   to authorize prepaid subscribers.  In AAA based roaming deployments
   the AAA server in the visited network, the VAAA, is responsible for
   forwarding the RADIUS messages to the HAAA.  The VAAA may also
   modify the messages.  In roaming deployments, the visited network
   may be separated from the home network by one or more broker
   networks.  The AAA servers in the broker networks, BAAA are




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   responsible to route the RADIUS packets and hence donÆt play an
   active roll in the Prepaid Data Service delivery.

   In this document the Prepaid Server is described in functional terms
   related to their interface with the HAAA.  The Prepaid Server
   interfaces to entities which:

   i) Keep the accounting state of the prepaid subscribers (balance
      manager);
   ii) Allow service requests to be rated in real-time (Rating Engine);
      and
   iii) Allow quota to be managed for a particular pre-paid service
      (Quota Server).

   The various deployments for Prepaid are presented in the remainder
   of this section.  The first deployment is the basic Prepaid data
   service and is depicted in figure 2.  Here the Access Device which
   supports the prepaid client functionality, the HAAA and the Prepaid
   Server are collocated in the same provider network.

   The Subscriber Device establishes a connection with one of several
   Access Devices in the network.  The Access Device communicates with
   one or more HAAA servers in the network.  To provide redundancy more
   then one HAAA is available to use by an Access Device.

   The network will have one or more Prepaid Servers.  Multiple Prepaid
   Servers will be used to provide redundancy and load sharing.  The
   interface between the HAAA and the PPS is the RADIUS protocol in
   this specification.  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 whatever protocol is
   used between the HAAA and the PPS.













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                                       +------+     +-----+
                                       |      |     |     |
           +--------+   +--------+  +--| HAAA |--+--| PPS |
           |        |   |        |  |  |      |  |  |     |
           | Sub    |   | Access |  |  +------+  |  +-----+
           |        |---|        |--+            |
           | Device |   | Device |  |  +------+  |  +-----+
           |        |   |        |  |  |      |  |  |     |
           +--------+   +--------+  +--| HAAA |--+--| PPS |
                                       |      |     |     |
                                       +------+     +-----+

      Figure 2 Basic Prepaid Access Architecture

   In the second deployment scenario, the Access Device does not
   support the prepaid client functionality. Instead an independent
   Service Device provides prepaid client functionality as depicted in
   figure 3.  Here the Access Device which dose not supports the
   prepaid client functionality is configured as AAA client to the AAA
   proxy functionality in the Service Device. The Service device, which
   supports the prepaid client functionality then appends prepaid
   extensions in the AAA requests proxied to the HAAA.

   The Subscriber Device establishes a connection with one of several
   Access Devices in the network.  The Authentication and Authorization
   requests from the Access Device are proxied through the Service
   Device which then appends prepaid extensions on to the requests. The
   Service Device communicates with one or more HAAA servers in the
   network.  The Service Device is responsible for removing prepaid
   extensions from messages received from the HAAA before proxying them
   on to the Access Device.  To provide redundancy more then one
   Service Devices are available to use by an Access Device and more
   than one HAAA is available for use by the Service Device.  The
   Service Device is configured to be default gateway to the Access
   Device, enabling all traffic to be correctly metered.










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                         (AAA)     +---------+     +------+     +-----+
                      +------------| Service |     |      |     |     |
    +--------+        |            |         |--+--| HAAA |--+--| PPS |
    |        |   +--------+     +--| Device  |  |  |      |  |  |     |
    | Sub    |   | Access | (IP)|  +---------+  |  +------+  |  +-----+
    |        |---|        |-----+               |            |
    | Device |   | Device |     |  +---------+  |  +------+  |  +-----+
    |        |   +--------+     +--| Service |  |  |      |  |  |     |
    +--------+        |            |         |--+--| HAAA |--+--| PPS |
                      +------------| Device  |     |      |     |     |
                          AAA)     +---------+     +------+     +-----+

      Figure 3 Prepaid Service Architecture

   The following figure 4 shows a static roaming prepaid architecture
   that is typical of a wholesale scenario for Dial-Up users or a
   broker scenario used in Dial-Up or WLAN roaming scenarios.

                           +----+   +----+   +----+   +-----+
                           |    |   |    |   |    |   |     |
      +------+  +------+ +-|VAAA|-+-|BAAA|-+-|HAAA|-+-| PPS |
      |      |  |      | | |    | | |    | | |    | | |     |
      |Sub   |  |Access| | +----+ | +----+ | +----+ | +-----+
      |      |--|      |-+        |        |        |
      |Device|  |Device| | +----+ | +----+ | +----+ | +-----+
      |      |  |      | | |    | | |    | | |    | | |     |
      +------+  +------+ +-|VAAA|-+-|BAAA|-+-|HAAA|-+-| PPS |
                           |    |   |    |   |    |   |     |
                           +----+   +----+   +----+   +-----+

      |     Visited             |  |Broker | |    Home      |
      |     Network             |  |Network| |    Network   |

      Figure 4 Static Roaming Prepaid Architecture

   As in the basic prepaid architecture the subscriberÆs device
   establishes a connection with the Access Device (NAS, WLAN Access
   Point).  The Access Device communicates with the Visiting AAA server
   (VAAA) using the RADIUS protocol.  Again for redundancy there maybe
   more then one VAAA.  The VAAA communicate using the RADIUS protocol
   with AAA servers in the broker network (BAAA).  There maybe more
   then one Broker Network between the Visited Network and the Home




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   Network.  The Home Network is the same as in the simple
   architecture.

   To support dynamic roaming the network will utilize mobile-ip.
   Figure 5 illustrates a typical mobile-ip deployment.  Note that
   typically the mobile device would be moving between networks that
   use the same technology such as Wireless or WLAN.  Increasingly,
   device will be able to roam between networks that use different
   technology such as between WLAN and Wireless and Broadband.
   Fortunately, mobile-ip can address this type of roaming and
   therefore we need not be concerned with the underlying network
   technology.


      +------+  +------+     +----+  +----+  +----+  +-----+
      |      |  |      |     |    |  |    |  |    |  |     |
      |Sub   |  |Access+-----|VAAA|--|BAAA|--|HAAA|--| PPS |
      |      |--|Device|     |    |  |    |  |    |  |     |
      |Device|  | (FA) +--+  +----+  +-+--+  +----+  +-----+
      |      |  |      |  |            |
      +------+  +------+  |            |
         |                |            |     +----+
         |                |            |     |    |
         |ROAMS           +------------------+ HA |
         |                             |     |    |
         V                 +----+      |     +----+
      +------+  +------+   |    |      |        |
      |      |  |      | +-|VAAA+------+        |
      |Sub   |  |Access| | |    |               |
      |      |--|Device+-+ +----+               |
      |Device|  | (FA) |                        |
      |      |  |      +------------------------+
      +------+  +------+


      Figure 5 Roaming using mobile-ip and pre-paid enabled Access
               Devices

   In figure 5, the Subscriber device establishes a prepaid session
   between the Access Device in the foreign network, which has prepaid
   capabilities and the Home Agent (HA).  The setup for this service is
   identical to the cases covered above.  Notice that the Access Device
   is known as the Foreign Agent (FA).  As the subscriber device moves


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   to another network it establishes a connection with another Access
   Device in another foreign network.  The prepaid data service should
   continue to be available.  When a device associates to another
   Access Device it MUST re-authenticate at the new Access Device and
   de-associate or logoff the old Access Device.  Furthermore, any
   unused quota at the old Access Device MUST be promptly credited back
   to the subscribers account.  The reason we say promptly, is because
   if the subscriber is very low on resources to start with, the
   subscriber may not have enough resources to log on to the new Access
   Device.  The speed at which resources can be returned depend on the
   type of handoff procedure that is used: dormant handoff vs. active
   handoff vs. fast handoff.

   As well, notice that if the Access Devices could communicate with
   each other then there could be a way to accelerate a faster handoff
   procedure.  In particular, it could accelerate the return of the
   unused portion of the quotas from the old Access Device.

   Unfortunately, standards are evolving with each network technology
   creating their own scheme to make the handoff procedures more
   efficient.

   Finally, pre-paid service may be provided in a roaming scenario
   where the Access Devices do not support the prepaid client
   capabilities. In such a scenario, a Service Device is configured as
   AAA proxy to the Home Agent and also as default gateway for the home
   agent, see Figure 6.


















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    +------+ +------+   +----+ +----+                +----+
    |      | |      |   |    | |    |                |    |
    |Sub   | |Access+---|VAAA|-|BAAA|----------------|    |
    |      |-|Device|   |    | |    |                |    |
    |Device| | (FA) +-+ +----+ +-+--+      (AAA)     |    |
    |      | |      | |          |    +---------+    |HAAA|
    +------+ +------+ |          |    |         |    |    |
       |              |          |  +----+    +---+  |    |  +-----+
       |              |  (IP)    |  |    |(IP)|Ser|  |    |  |     |
       |ROAMS         +-------------+ HA |----|   |--|    |--| PPS |
       |                         |  |    |    |Dev|  |    |  |     |
       V                +----+   |  +----+    +---+  +----+  +-----+
    +------+ +------+   |    |   |    |
    |      | |      | +-|VAAA+---+    |
    |Sub   | |Access| | |    |        |
    |      |-|Device+-+ +----+        |
    |Device| | (FA) |         (IP)    |
    |      | |      +-----------------+
    +------+ +------+

      Figure 6 Roaming using mobile-ip and prepaid enabled Service
               Device behind the Home Agent.

2.1 Why not existing RADIUS attributes?

   It has been asked ôWhy not use existing RADIUS attributes to build a
   prepaid solution? This will allow us to have a solution with
   existing devices without code modification.ö

   It is possible to build a prepaid solution using existing RADIUS
   attributes. The RADIUS server can simply send an Access-Accept
   message containing Session-Timeout(27) and set Termination-
   Action(29) to RADIUS-request.  Upon receiving the Access-Accept
   message, the NAS will time the session and upon termination of the
   session the NAS generate an Access-Request message again.  The
   RADIUS server would re-authenticate the session and reply with an
   Access-Accept message with additional time in Session-Timeout(27) or
   an Access-Reject message if there were no more resources in the
   userÆs account.

   If the user terminates the session before the time expressed in
   Session-Timeout(27).  The NAS will recover any unused time from the
   accounting stream.


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   There are several problems with such a solution:

   -It only allows for time-based prepaid.  The solution presented in
   this document allows for both time and volume based prepaid.  As
   well as extensibility for other features such as tarified based
   solutions.

   -This solution only allows for prepaid based on Access.  The
   solution presented in this document allows the use of this protocol
   to support prepaid solutions for other services not just Access.

   -Using accounting messages to recoup unused time may be problematic
   because RADIUS accounting messages are not real-time.  A RADIUS
   server may store-and-forward accounting messages in batches.  The
   solution presented in this paper does not rely on Accounting Packets
   at all.  It uses Access-Request, messages which do flow through any
   network in real-time.  Delaying accounting messages may cause
   revenue leakage.

   -Session-Timeout(27) is not a mandatory attribute.  If a prepaid
   subscriber is being serviced by a NAS that does not adhere to
   Session-Timeout then that subscriber will obtain unlimited service.

   -Termination-Action(29) presents its own issues.  First the
   behaviour of Termination-Action(29) is not mandatory.  Second,
   according to RFC2865 Termination-Action fires when the Service is
   complete.  But we should not be terminating the service we really
   should only be terminating a session when we are negotiating
   additional quota. The refreshing of the time quota should be
   transparent to the user.  Because Termination-Action occurs when the
   Service is complete it is unclear whether or not the user experience
   would be transparent. For example, will the RADIUS server allocate
   the subscriber a new IP address? Furthermore, the RADIUS server has
   no way of telling why the Access-Request message was generated.  The
   RADIUS server will have to wait for the corresponding accounting
   packet to determine the reason for this Access-Request message.
   Lastly re-authenticating the subscriber may take far too long. The
   solution presented in this document allows quota replenishing to
   occur in an undisruptive manner from the perspective of the user.
   No re-authentication is required and quotas can be negotiated prior
   to the quotas running out.



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   -Prepaid ambiguity.  Implementing prepaid using existing RADIUS
   attributes presents another problem. Due to the fact that the
   standard RADIUS attributes are not mandatory, then 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 will get free access. The
   solution described in this document, requires that a prepaid capable
   NAS inform the RADIUS server whether or not it supports prepaid
   capabilities.  The RADIUS server can now determine whether service
   should be granted or not.  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 a gateway that
   does support prepaid metering, or it may allow the subscriber on but
   restrict their traffic.

   The prepaid solution we present is a robust carrier grade prepaid
   solution.  It only requires the support of 2 mandatory attributes
   and one optional attribute.   Furthermore, it does not really
   require much code support at the NAS. NASes already support
   measurement of time and volume.  This solution requires that they
   advertise their prepaid capabilities in an Access-Request; that they
   generate an Access-Request Authorize-Only packet to obtain more
   quota at or before the quota is used up.  It also requires that the
   NAS send an Access-Request with Authorize-Only when the session
   terminates to return any unused quota to the prepaid system.

   Lastly the solution provided in this document is extensible. This
   document defines the basic exchanges between a prepaid capable NAS
   and a RADIUS server.  The protocol can easily be extended to support
   tariff switching and other prepaid business models.

3. Use-cases

   In this section we present a set of use cases that will help
   establish the requirements needed to deliver PrePaid data services.
   These use cases donÆt address how the PrePaid account is established
   or maintained.  It is assumed that the PrePaid subscriber has
   obtained a valid account from a service provider such as a wireless
   operator or a WLAN operator.

   To make the document as general as possible, the use cases cover the
   experience from the Access Device and not from the UserÆs Device.
   The connection between the UserÆs Device, which typically involves


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   setting up a layer 2 session, e.g., PPP session or GPRS PDP Context,
   is specific to a given network technology and the details are not
   required to deliver a PrePaid service.





3.1 Simple pre-paid access use-case

   A PrePaid subscriber connects to his home network.  As usual, the
   Access Device that is servicing the subscriber will use the AAA
   infrastructure to authenticate and authorize the subscriber.

   The Access Device sends a RADIUS Access-Request to the AAA system to
   authenticate the subscriber, and identify and authorize the service.
   The Access-Request includes the subscriberÆs credentials and may
   include the PrePaid capabilities of the Access Device.  PrePaid
   capabilities MUST be included if the Access Device supports PrePaid
   functionality.

   The AAA System proceeds with the authentication procedure.  This may
   involve several transactions such as in EAP.  Once the subscriber
   has been validated, the AAA system determines that the subscriber is
   a PrePaid subscriber and requests that the PrePaid System authorize
   the PrePaid subscriber.  The request MUST include the PrePaid
   Capabilities of the serving Access Device.  These capabilities will
   include whether the Access Device support optional granular prepaid
   service.  Granular prepaid service allows an Access Device to offer
   service differentiation above plain network access, for example
   discriminating between a prepaid service request for access to the
   public Internet from access to a particular application server
   hosted in the private domain of the home provider network.  In the
   simple prepaid access scenario, such capabilities are not required
   to be supported by the Access Device.

   The PrePaid System will validate that the subscriber has a PrePaid
   Account; it will validate that the account is active; and will
   validate that the Access Device has the appropriate PrePaid
   capabilities.  If all is in order, the PrePaid System will authorize
   the subscriber to use the network.  Otherwise it will reject the
   request.  The response is sent back to the AAA System.  The response
   includes attributes such as, definition of what services are


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   authorized.  The exact definition of the service may define vanilla
   network access or more granular service definition.  The exact
   definition of these services is not the focus of this draft.  This
   definition MAY include a ôservice keyö which can be used to
   correlate prepaid requests for access to a service with the service
   definition in the prepaid system.  Such service key information MUST
   be included when the prepaid user has subscribed to more than one
   prepaid service.  If a user has subscribed to only a single service,
   the response MAY also include an allocation of a portion of the
   subscriberÆs account called the initial quota (in units of time or
   volume) and optionally a threshold value.

   [Editor comments: we should leave tariff switch issues to another
   document.  One way to deal with a tariff switch is to set the
   threshold or quota such that a new allocation is requested just
   before or at the tariff switch period.]

   When the rating engine has determined that a tariff switch will
   shortly occur, the initial quota may be segmented into that which
   SHOULD be used before the tariff switch, that which SHOULD be used
   after the tariff switch together with details describing the tariff
   switching instant.

   The Access Device is responsible for requesting quota to be allocate
   for a particular prepaid user.

   In order to support concurrent PrePaid sessions, at any time, the
   PrePaid System allocates a portion of the subscribers account to a
   given PrePaid session.  For example, in a multi-service environment
   it might happen that an end user with an already ongoing service
   (e.g., browsing the Internet) issues a new service request (e.g.,
   for downloading a ring-tone) towards the same account.  Throughout
   the lifetime of a session the Access Device will monitor usage
   according to the quota(s) returned from the prepaid server and will
   request further quota updates from the PrePaid System as previously
   allocated quotas are consumed. Conditions may be included with
   quotas, which indicate when an allocated quota should be returned to
   the prepaid system. These conditions can include an Idle-Timeout(28)
   associated with the provided quota. In this case, the Access device
   monitors the service for activity. When a single inactivity period
   exceeds that provided in the quota conditions, the unused quota is
   returned to the prepaid server.



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   The AAA system incorporates the PrePaid attributes received from the
   PrePaid System with the service attributes into an Access-Accept
   message that it sends back to the Access Device.  Note the AAA
   System is responsible for authorizing the service whereas the
   PrePaid System is responsible for PrePaid authorization.

   Upon receiving the Access-Response, the Access Device allows the
   PrePaid data session to start and it starts to meter the session
   based on time or volume, as indicated in the returned Quota

   Once the usage for the session approaches the allotted quota (as
   expressed by the threshold), the Access Device will request an
   additional quota.  The re-authorization for additional quota flows
   through the AAA system to the PrePaid System.  The PrePaid System
   revalidates the subscriberÆs account; it will subtract the previous
   quota allocation from the userÆs balance and if there is a balance
   remaining it will reauthorize the request with an additional quota
   allotment.  Otherwise, the PrePaid System will reject the request.
   Note the replenishing of the quotas is a re-authorization procedure
   and does not involve re-authentication of the subscriber.

   It is important to note that the PrePaid System is maintaining
   session state for the subscriber.  This state includes how much was
   allocated during the last quota allocation for a particular session
   and how much is left in the account.  Therefore, it is required that
   all subsequent messages about the PrePaid session reach the correct
   PrePaid System.

   Upon receiving a re-allotment of the quota, the Access Device will,
   continue the data service session until the new threshold is
   reached.  If the request for additional quota cannot be fulfilled
   then the Access Device will let the subscriber use up the remaining
   quota and terminate the session.

   Alternatively, instead of terminating the session, the Access Device
   may restrict the data session such that the subscriber can only
   reach a particular web server.  This web server maybe used to allow
   the subscriber to replenish their account.  This restriction can
   also be used to allow new subscribers to purchase their initial
   PrePaid Service.






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   Should the subscriber terminate the session before the session the
   quota is used up, the remaining balance allotted to the session must
   be credited back to the subscriberÆs account.

   As well, while the Access Device is waiting for the initial quota,
   the subscriber may have dropped the session.  The initial quota must
   be credited back to the subscribers account.

3.2 Simple Service Device use-case

   When the Access Device does not support the prepaid extensions, an
   operator may still offer prepaid services to subscribers by using a
   service device configured as default IP gateway to the Access
   device.

   A Prepaid subscriber connects to his home network in the usual way.
   The non-prepaid enabled Access Device that is servicing the
   subscriber will use the AAA infrastructure to authenticate and
   authorize the subscriber.  The Service device will be configured as
   AAA proxy to the Access Device.

   The Access Device sends an Access Request to the Service Device
   acting as AAA proxy to authenticate the subscriber, and identify and
   authorize the service.  The Service Device will proxy the Access
   Request and append its own Prepaid capabilities to the Access
   Request message.  These prepaid capabilities are defined identically
   to the simple access device user-case.

   The prepaid system performs functions as with prepaid support in the
   Access Device, e.g., the AAA system incorporates the prepaid
   attributes received from the Prepaid System with the service
   attributes into an Access-Accept message that it sends back to the
   Service Device.  The Service device removes these attributes before
   forwarding the Access-Accept message to the Access Device.

   Upon receiving the Access-Accept with allocated quota, the Service
   Device allows the prepaid data service session to start and since it
   is configured as default gateway to the access device, it starts to
   meter the session based on time and/or volume.

3.3 Support for concurrent PrePaid sessions





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   Both prepaid support using Access Devices and prepaid support using
   Service Devices can be configured to support a prepaid multi-service
   environment. In such circumstances, the prepaid client capabilities
   will indicate that the Access or Service Device supports a multi-
   service environment [Editor: need to add this to the PPAC]. [Editor:
   This needs to be reworked. DonÆt believe that this step is required.
   The Service Ids should be known a priori ¡ the Access Request should
   include the Service Key being requested.] In such circumstances,
   instead of returning a quota, the prepaid service provides a list of
   authorized services corresponding to a list of service keys to the
   prepaid client. The Access/Service device then uses these service
   keys to request prepaid authorization to the corresponding services.
   The prepaid server responds with an individual quota for the
   requested service key [Editor: add service key to PPAQ]. The
   Access/Service Device may in parallel request prepaid authorization
   to a second service key. In which case a separate authorization
   exchange is used to provide an independent quota for this second
   service.

   Each session is treated independently.

   The method by which a prepaid user activates a service and the
   method for signaling this information to the Access/Service Device
   is out of scope of this draft.

   The method by which a granular service is defined is out of scope of
   this draft. Only service key correlation information is required to
   enable the prepaid server to authorize and rate a particular
   request.

3.4 Support for Roaming

   For some networks it is essential that PrePaid Data Services be
   offered to roaming subscribers.  Support for static and dynamic
   roaming models are needed.  Static roaming is where the subscriber
   logs onto a foreign network.  The foreign network has a roaming
   agreement directly with the home network or through a broker network
   or networks.  The subscriber remains logged into the network until
   the subscriber changes location.  When changing location a new
   connection and a new login procedure is required.

   Dynamic roaming allows to subscriber to move between foreign
   networks while maintaining a connection with the home network


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   seamlessly.  As the subscriber moves between networks, the data
   session is handed off between the networks.

   In both roaming scenarios, the subscriber always authenticates with
   the home network.  PrePaid authorization and quota replenishing for
   the session need to be received at the home network and more
   specifically at the PrePaid System where state is being maintained.

   Dynamic roaming is particularly challenging.  A subscriber that
   established a PrePaid Data Session may roam to another Access Device
   that doesnÆt not support PrePaid functionality.  The system should
   be capable to continue the PrePaid session.

3.5 PrePaid termination

   When fraud is detected by the PrePaid System, or when an error is
   detected, it may be beneficial for the PrePaid system to terminate a
   specific session for the subscriber or all the sessions of a
   subscriber.

   Some errors can occur such that the PrePaid System is in a state
   where it is not sure whether the session is in progress or not.
   Under conditions such as this, the PrePaid system may wish to
   terminate the PrePaid data session to make sure that resources are
   not being utilized for which it canÆt charge for reliably.

   Some handoff procedure used during dynamic roaming may require that
   the PrePaid system explicitly terminate the subscribers PrePaid data
   session at an Access Device.  For example, if time based PrePaid
   service is being used and the mobile subscriber performs a dormant
   handoff, the PrePaid System needs to explicitly terminate the
   PrePaid session at the old Access Device.


4. Operations

4.1 General Requirements

4.1.1 Broker AAA Requirements

   Broker AAA servers MUST support the Message-Authenticator(80)
   attribute as defined in [RFC2869].  If BAAA servers are used, the




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   BAAA servers function is to forward the RADIUS packets as usual to
   the appropriate RADIUS servers.

   Accounting messages are not needed to deliver a PrePaid service.
   However, accounting messages can be used to keep the PrePaid Server
   current as to what is happening with the PrePaid data session.
   Therefore, BAAA SHOULD deliver RADIUS Accounting messages using the
   pass through mode described in [RFC2866].

4.2 Authentication and Authorization for Prepaid Enabled Access Devices

   The Access Device initiates the authentication and authorization
   procedure by sending a RADIUS Access-Request as usual.

   If the Access Device has PrePaid Client capabilities, it MUST
   include the PPAC(TBD) attribute in the RADIUS Access-Request.  The
   PPAC(TBD) attribute indicates to the PrePaid server the PrePaid
   capabilities possessed by the Access Device.  These are required in
   order to complete the PrePaid authorization procedures.

   [Editor:  add support for the MultiSession-Capabilities attribute]
   If the Access Device supports multiple sessions-keys capabilities,
   then it SHOULD include the MultiSession-Capabilities attribute.  The
   presence of the MultiSession-Capabilities attribute will indicate to
   the PPS that the Access Device support prepaid service
   differentiation above simple prepaid access.

   If the Access Device supports the Disconnect-Message or the Change-
   of-Authorization capabilities, then it SHOULD include the Dynamic-
   Capabilities attribute.

   In certain deployments, there may be other ways in which to
   terminate a data session, or change authorization of an active
   session.  For example, some Access Devices 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 whatever means that are supported by the device.

   If the authentication procedure involves multiple Access-Requests
   (as in EAP), the Access Device MUST include the PPAC(TBD) attribute




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   and the Dynamic-Capabilities attribute (if used) in at least the
   last Access-Request of the authentication procedure.

   The Access-Request will be sent as usual to the HAAA.  The packet
   may be proxied through zero or more BAAA.

   Once the Access-Request arrives at the HAAA, the HAAA will
   authenticate the subscriber.  If the subscriber is cannot be
   authenticated, the HAAA will send an Access-Reject message back to
   the client.  If the subscriber is authenticated, the HAAA will
   determine whether or not the subscriber is a PrePaid subscriber.
   The techniques used to determine whether or not a subscriber is a
   PrePaid subscriber is beyond the scope of this document.  If the
   subscriber is not a PrePaid subscriber, then the HAAA will respond
   as usual with an Access-Accept or Access-Reject message.  If the
   subscriber is a PrePaid Subscriber the HAAA SHALL forward the
   Access-Request to a PrePaid server for further authorization.

   The Access-Request will contain the PPAC(TBD) attribute, the
   Dynamic-Capabilities attribute if one was included; and the
   MultiSession-Capabilities attribute if one was included, the User-
   Name(1) attribute MAY be set to a value that would represent the
   SubscriberÆs PrePaid Identity.  This attribute is used by the
   PrePaid server to locate the PrePaid SubscriberÆs account.  For
   added security, the HAAA MAY also set the User-Password(2) attribute
   to the password used between the HAAA and the PrePaid server.

   The PrePaid server lookups the subscriberÆs PrePaid account and will
   authorize the subscriber taking into consideration the Access Device
   PrePaid Client Capabilities.  The Prepaid Server will decide whether
   single service prepaid access will be provided or a multiple session
   pre-paid access will be provided.

4.2.1 Single Service Pre-paid

   If a single service prepaid access is provided, upon successful
   authorization, the PrePaid server will generate an Access-Accept
   containing the PPAC(TBD) attribute and the PPAQ(TBD) attribute.

   The PPAC attribute returned to the client indicates the type of
   prepaid service to be provided for the session.
    which contains the following sub-attributes:



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   - The QUOTA-Id which is set by the PrePaid server to a unique value
   that is used to correlate subsequent quota requests;

   - Volume and/or Time Quotas, one of which is set to a value
      representing a portion of the subscribers account;

   - MAY contain a Time or Volume Threshold that controls when the
      Access Device requests additional quota;

   - The IP address of the Serving PrePaid Server and one or more
      alternative PrePaid Servers.  This is used by the HAAA to route
      subsequent quota replenishing messages to the appropriate PrePaid
      server(s).

   Note: Idle-Timeout(28) can be used to trigger the premature
   termination of a pre-paid service following subscriber inactivity.

   Depending on site policies, upon unsuccessful authorization, the
   PrePaid server will generate an Access-Reject to terminate the
   session immediately.  Alternatively, the PrePaid server may generate
   an Access-Accept blocking some or all of the traffic and/or redirect
   some or all of the traffic to a location where the subscriber can
   replenish their account for a period of time.  Blocking of traffic
   is achieved by either Filter-Id(11) or NAS-Filter-Rule(see Redirect
   I-d).  Redirection is achieved by sending Redirect-Id or Redirect-
   Rule defined in the Redirect I-d.  The period of time before the
   blocked/redirected session last can be specified by Session-
   Timeout(27) attribute.

   Upon receiving the Access-Accept from the PrePaid Server, the HAAA
   will append the usual service attributes and forward the packet to
   the Access Device.  The HAAA SHALL NOT append or overwrite any
   attributes already set by the PrePaid server.  If the HAAA, receives
   an Access-Reject message, it will simply forward the packet to its
   client.  Depending on site policies, if the HAAA fails to receive an
   Access-Accept or Access-Reject message from the PrePaid server it
   MAY do nothing or send an Access-Reject or an Access-Accept message
   back to its client.


4.2.2 Multiple-Session Pre-paid





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   If the prepaid server decides that multiple-session prepaid service
   is to be provided, upon successful authorization, the Prepaid server
   will generate an Access-Accept containing the PPAQ attribute which
   contains the following sub-attributes:

  - a list of the service keys which the Access Device can subsequently
     use in pre-paid service authorization request.

   [Editor: if this stands (see earlier comments) then instead of
   issuing an access-accept the PPS should issue a Challenge that
   contains the service-keys]

   The first PrepaidServer subtype is set to the IP address of the
   Serving Prepaid Server, the second one is set to an alternate
   Prepaid Server if any.  This way the HAAA will be able to route
   subsequent packets to the serving Prepaid Server or its alternate.

   [Editor: this should only be done on the Access Accept that deliver
   the quota for the specific service.]
   Additionally, the Prepaid server MAY set the Terminate-Action(29) to
   RADIUS-Request(1); and MAY set Acct-Interim-Interval(85) to control
   how often interim Accounting Requests are generated.

   Upon receiving the Access-Accept from the Prepaid Server, the HAAA
   will append the usual service attributes and forward the packet.
   The HAAA SHALL NOT append any attributes already set by the Prepaid
   server.  If the HAAA, receives an Access-Reject message, it will
   simply forward the packet to its client.  Depending on site
   policies, if the HAAA fails to receive an Access-Accept message from
   the Prepaid server it MAY do nothing or send an Access-Reject or an
   Access-Accept message back to its client.

   Upon receiving the Access-Accept with a list of service keys, the
   Access Device can trigger the authorization request for a particular
   service corresponding to a service key. The technique for triggering
   an authorization request for a particular service is out of scope of
   this draft.

   The Access Device initiates authorization for a particular service
   by sending a RADIUS Access Request including a single service-key
   reference.





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   For the specific service-key reference, the prepaid server will
   check whether funds are available and will, following successful
   allocation of funds, the Prepaid server will generate an Access-
   Accept containing the PPQ-Response attribute which contains the
   following sub-attributes:

   - The QUOTA-Id which is set by the Prepaid server to a unique value
     that is used to correlate subsequent quota updates;

   - The ServiceKey-Id which is set by the Prepaid server to the
     service key requested by the Access Device;

   - Volume and Time Quotas, one of which is set to a value
     representing a portion of the subscribers account;

   - The Time of Volume Threshold that the Prepaid server MAY set to
     control when the Access Device requests additional quota.

   Note: Idle-Timeout(28) can be used to trigger the premature
   termination of a pre-paid service following subscriber inactivity.


4.3 Session Start Operation

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

   Note that the PrePaid Server role is not to record accounting
   messages and therefore it SHOULD not respond with an Accounting
   Response packet.

   If the Prepaid server 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 Prepaid server 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
   Access Device supports termination capabilities, the PPS SHOULD send
   a Disconnect Message to the Access Device to ensure that the session
   is indeed dead.


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4.4 Mid-Session Operation

   During the lifetime of a PrePaid data session the Access Device will
   request to replenish the quotas using Authorize-Only Access-Request
   messages.

   Once the allocated quota has been reached or the threshold has been
   reached, the Access Device MUST send an Access-Request with Service-
   Type(6) set to a value of ôAuthorize Onlyö and the PPAQ(TBD)
   attribute.

   The Access Device MUST also include NAS identifiers, and Session
   identifier attributes in the Authorize Only Access-Request.  The
   Session Identifier should be the same as those used during the
   Access-Request.  For example, if the User-Name(1) attribute was used
   in the Access-Request it MUST be included in the Authorize Only
   Access-Request 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 either User
   Password or Chap Password.  In order to authenticate the message,
   the Access Device MUST include the Message-Authenticator(80)
   attribute.  The Access Device will compute the value for the
   Message-Authenticator based on [RFC2869].

   When the HAAA receives the Authorize-Only Access-Request that
   contains a PPAQ(TBD), it SHALL validate the message using the
   Message-Authenticator(80) as per [RFC2869].  If the HAAA receives an
   Authorize Only Access-Request that contains a PPAQ(TBD) but not a
   Message-Authenticator(80) it SHALL silently discard the message.  An
   Authorize Only Access-Request message that does not contain a
   PPAQ(TBD) is either in error or belongs to another application (for
   example, a Change of Authorization message [RFC3576]).  In this case
   the Authorize Only Access-Request will either be silently discarded
   or handled by another application (not in scope of this document).

   Once the Authorize Only Access-Request message is validated, the
   HAAA SHALL forward the Authorize Only Access-Request to the
   appropriate PrePaid Server.  The HAAA MUST forward the Authorize
   Only Access-Request to the PrePaid server specified in the
   PPAQ(TBD).  The HAAA MUST sign the message using the Message-
   Authenticator(80) and the procedures in [RFC2869].  As with the


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   Access-Request message, the HAAA MAY modify the User-Name(1)
   attribute to a value that represents the userÆs internal PrePaid
   account in the PrePaid server.  Note the PrePaid server could use
   the Quota-ID sub-attribute contained within the PPAQ(TBD) to locate
   the user account.

   Upon receiving the Authorize Only Access-Request containing a
   PPAQ(TBD) attribute, the PrePaid server MUST validate the Message-
   Authenticator(80) as prescribed in [RFC2869].  If the message is
   invalid, the PrePaid server MUST silently discard the message.  If
   it received an Authorize Only Access-Request message that does not
   contain a PPAQ(TBD) it MUST silently discard the message.

   The PrePaid server will lookup the PrePaid session by using the
   PrePaid Quota Id contained within the PPAQ(TBD).  The PrePaid Server
   would, take the last allocated quota and subtract that from the
   UserÆs balance.  If there is remaining balance, the PrePaid server
   re-authorizes the PrePaid session by allocate an additional quota.
   The PrePaid server may want to calculate a different threshold
   values as well.

   Upon successful re-authorization, the PrePaid server will generate
   an Access-Accept containing the PPAQ(TBD) attribute.  The Access-
   Accept message MAY contain Service-Type(6) set to Authorize-Only and
   MAY contain the Message-Authenticator(80).

   Depending on site policies, upon unsuccessful authorization, the
   PrePaid server will generate an Access-Reject or an Access-Accept
   with Filter-Id(11) or Ascend-Data-Filter (if supported) attribute
   and the Session-Timeout(27) attribute such that the PrePaid
   subscriber could get access to a restricted set of locations for a
   short duration to allow them to replenish their account, or create
   an account; or to browse free content.

   Upon receiving the Access-Accept from the PrePaid server, the HAAA
   SHALL return the packet to its client.  If the HAAA, receives an
   Access-Reject message, it will forward the packet.  Depending on
   site policies, if the HAAA fails to receive an Access-Accept or an
   Access-Reject message from the PrePaid server it MAY do nothing or
   it MAY send an Access-Reject message back to its client.

   Upon receiving an Access-Accept, the Access Device SHALL update its
   quotas and threshold parameters with the values contained in the


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   PPAQ(TBD) attribute.  Note that the PrePaid server MAY update the
   PrePaidServer attribute(s) and these may have to be saved as well.

   Upon receiving an Access-Accept message containing either Filter-
   Id(11) or Ascend-Data-Filter attributes, and or Session Timeout(27).
   The Access Device SHALL restrict the subscriber session accordingly.

4.5 Dynamic Operations

   The PrePaid server may want to take advantage of the dynamic
   capabilities that are supported by the Access Device as advertised
   in the Dynamic-Capabilities attribute during the initial Access-
   Request.

   There are two types of actions that the PrePaid server can perform:
   it can request that the session be terminated; or it can request
   that the filters associated with the session be modified.

   Both of these actions require that the session be uniquely
   identified at the Access Device.  As a minimum the PrePaid server:

   -MUST provide either the NAS-IP-Address(4) or NAS-Identifier(32)
   -MUST provide at least one session identifier such as User-Name(1),
   Framed-IP-Address(), the Accounting-Session-Id(44).

   Other attributes could be used to uniquely identify a PrePaid data
   session.

4.5.1 Unsolicited Session Termination Operation

   This capability is described in detail in [RFC3576].  The PrePaid
   server sends a Disconnect Request packet that MUST contain
   identifiers that uniquely identify the subscriberÆs data session and
   the Access Device servicing that session.

   Upon receiving the Disconnect Request packet the HAAA will either
   act on it or will proxy it to another AAA server until it is
   received by the a AAA that is in the same network as the serving
   Access Device.

   Each AAA MUST route the Disconnect Request packet.  How the routing
   decision is made is an implementation detail.



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   Once the Disconnect Request packet reaches AAA that is in the same
   network as the serving Access Device, if the Access Device supports
   Disconnect-Request (as per [RFC3576]), it sends the message directly
   to the Access Device; otherwise it uses other mechanisms such as
   SNMP or Telnet to command the Access Device to terminate the session
   (this is an implementation detail).

   If the Access Device receives a Disconnect-Request packet, it will
   respond with either a Disconnect-ACK packet if it was able to
   terminate the session or else it will respond with a Disconnect-NAK
   packet.

   If the AAA server is performing the disconnect operation, it MUST
   respond with a Disconnect-ACK message if it successfully terminated
   the session or a Disconnect-NAK message if it failed to terminate
   the session with the appropriate Error-Cause(101) set.

   If any AAA server is unable to route the Disconnect-Request it MUST
   respond with a Disconnect-NAK packet with Error-Cause(101) set to
   ôRequest Not Routableö(502).

4.5.2 Unsolicited Change of Authorization Operation

   The PrePaid Server MAY send a Change-of-Authorization message as
   described in [RFC3576] to restrict Internet access when the
   subscriber has no more balance.  The COA packet may contain Filter-
   Id(11) and or attributes defined in Redirect I-d.

   The PrePaid server sends a Change-of-Authorization packet it MUST
   contain identifiers that will uniquely identify the subscriber
   session and the Access Device serving that session.

   Upon receiving the Change-of-Authorization packet the HAAA will
   either act on it or proxy it to another AAA server until it is
   received by a AAA server that is in the same network as the serving
   Access Device.

   Each AAA must route the packet to the serving network.  How the
   routing decision is made is an implementation detail.

   Once the Change-of-Authorization packet reaches a AAA that is in the
   same network as the serving Access Device, if the Access Device
   supports Change-of-Authorization message, it will forward the


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   message to the Access Device; otherwise, it will use other
   mechanisms such as SNMP or Telnet to command the Access Device to
   change its filters.

   If the Access Device receives a Change-of-Authorization packet, it
   will respond with either a Change-of-Authorization-ACK packet if it
   was able to change the filter or else it will respond with a Change-
   of-Authorization-NAK packet.

   If the AAA server is performing the change of filter operation, it
   MUST respond with a Change-of-Authorization-ACK message if it
   successfully or a Change-of-Authorization-NAK packet if it failed to
   change the filter.

   If a AAA server was unable to route the Change-of-Authorization it
   MUST respond with a Change-of-Authorization-NAK packet.

4.6 Termination Operation

   The termination phase is initiated when either: the Subscriber logs
   off; the quotas have been consumed, or when the Access Device
   receives a Disconnect Message.  In all of these instances, if the
   session is a PrePaid data session, the Access Device will send an
   Authorize-Only Access-Request message with a PPAQ(TBD) Update-Reason
   attribute set to either ôClient Service terminationö or ôRemote
   Forced disconnectö and the currently used quota.

   The BAAA MUST forward this packet to the next BAAA or the HAAA.

   The HAAA MUST validate the Authorize Only Access-Request using the
   Message-Authenticator(80) as per [RFC2869] and if valid, use the
   PrePaidServer subtype in the PPAQ(TBD) to forward the Authorize Only
   Access-Request packet to the serving PrePaid Server or if needed,
   its alternate.

   The PrePaid Server MUST validate the Authorize Only Access Request
   and use the information contained in the PPAQ(TBD) attribute to
   adjust the subscriberÆs balance and to close the session.  The
   PrePaid Server SHALL respond back with an Access-Accept message.

4.7 Mobile IP Operations





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   In roaming scenarios using mobile-ip, as the mobile subscriber roams
   between networks, or between different types of networks such as
   between WLAN and CDMA2000 networks, the PrePaid data session is
   maintained transparently.

   As the subscriber device associates with the new Access Device, the
   Access Device sends a RADIUS Access-Request and the subscriber is
   re-authenticated and reauthorized.  If the Access Device has PrePaid
   Client capabilities, it MUST include the PPAC(TBD) attribute in the
   RADIUS Access-Request.  In this manner the procedure follows the
   Authentication and Authorization procedure described earlier.

   The Access-Request message is routed to the home network and MUST
   reach the PrePaid System that is serving the PrePaid session.  The
   PrePaid system will then correlate the new authorization request
   with the existing active session and will assign a quota to the new
   request.  Any outstanding quota at the old Access Device will be
   returned to the PrePaid system due to the usual mobile-ip handoff
   procedures.  Specifically, the quota will be returned when the
   Access Device sends the Authorize Only Access-Request with PPAQ(TBD)
   Update-Reason subtype set to either ôRemote Forced disconnectö or
   ôClient Service terminationö.  In order to trigger the sending of
   this last Authorize Only Access-Request, the PrePaid system may
   issue a Disconnect Message [CHIBA] to the Access Device.

   If the subscriber has roamed to an Access Device that does not have
   any PrePaid Capabilities, PrePaid data service may still be possible
   by requesting the Home Agent (providing it has PrePaid Capabilities)
   to assume responsibilities for metering the service.  The procedure
   for this scenario will be given in the next release of this draft.

4.8 Accounting Considerations

   Accounting messages are not required to deliver PrePaid Data
   Service.  Accounting message will typically be generated for PrePaid
   Data Service.  This because accounting message are used for auditing
   purposes as well as for bill generation.

   Accounting messages associated with PrePaid Data Sessions should
   include the PPAQ(TBD) attribute.

4.9 Service Device Operation



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   To be completed

4.10 Interoperability with Diameter Credit Control Application

   RADIUS PrePaid solutions need to interoperate with Diameter
   protocol.  Two possibilities exist: The AAA infrastructure is
   Diameter based and the Access Device are RADIUS based; or the Access
   Device is Diameter based and the AAA infrastructure is RADIUS based.

   The Diameter Credit Control Application [DIAMETERCC] describes how
   to implement a PrePaid using an all Diameter based infrastructure.

   <This section to be completed.>


5. Attributes

   This draft is using the RADIUS [RFC2865] namespace.


5.1 PPAC Attribute

   The PrepaidAccountingCapability (PPAC) attribute is sent in the
   Access-Request message by a Prepaid Capable NAS and is used to
   describe the PrePaid capabilities of the NAS.  The PPAC is available
   to be sent in an Access-Accept message by the Prepaid server to
   indicate the type of prepaid metering that is to be applied to this
   session.

    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        | SUB-TYPE 1    | LENGTH        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    AvailableInClient (AiC)                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SUB-TYPE 2    | LENGTH        |   SelectedForSession (SFS)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   TYPE  : value of PPAC


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   LENGTH: 14
   VALUE : String

   The value MUST be encoded as follows:

   Sub-Type (=1)          : Sub-Type for AvailableInClient attribute
   Length                 : Length of AvailableInClient attribute
                            (= 6 octets)
   AvailableInClient (AiC):

   The optional AvailableInClient Sub-Type, generated by the PrePaid
   client, indicates the PrePaid Accounting capabilities of the NAS and
   shall be bitmap encoded. The possible values are:

      0x00000001  PrePaid Accounting for Volume supported
      0x00000010  PrePaid Accounting for Duration supported
      0x00000011  PrePaid Accounting for Volume and Duration supported
                  (non concurrently)
      Others      Reserved, treat like Not Capable of PrePaid
                  Accounting (=0).

   Sub-Type (=2)          : Sub-Type for SelectedForSession attribute
   Length                 : Length of SelectedForSession attribute
                            (= 6 octets)
   SelectedForSession (SfS):

   The optional SelectedForSession Sub-Type, generated by the PrePaid
   server, indicates the PrePaid Accounting capability to be used for a
   given session. The possible values are:

      0x00000000  PrePaid Accounting not used
      0x00000001  Usage of PrePaid Accounting for Volume.
                  (only possible if the AvailableInClient supports
                  PrePaid Accounting for Volume)
      0x00000010  Usage of PrePaid Accounting for Duration.
                  (only possible if the AvailableInClient supports
                  PrePaid Accounting for Duration)
      0x00000011  Usage of PrePaid Accounting for Volume and Duration
                  (non concurrent) (only possible if the
                  AvailableInClient supports PrePaid Accounting for
                  Volume and duration)
      Others      Reserved, treat like PrePaid Accounting not used(=0).



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5.2 Session Termination Capability

   The value shall be bitmap encoded rather than a raw integer. This
   attribute shall be included RADIUS Access-Request message to the
   RADIUS server and indicates 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        |      String                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type  : value of Session Termination Capability
   Length: = 4
   String encoded as follows:

   0x00000001  Dynamic Authorization Extensions (rrfc3576) is
               supported.


5.3 PPAQ Attribute

   The PPAQ(TBD) attribute is sent in Authorize Only Access-Request and
   Access-Accept messages.  In Authorize Only Access-Request messages
   it is used to report usage and request further quota; in an Access-
   Accept message it is used to allocate the quota (initial quota and
   subsequent quotas).

   The attribute consists of a number of subtypes.  Subtypes not used
   are omitted in the message.













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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | TYPE          | LENGTH        | SUB-TYPE 1    | LENGTH        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        QuotaIdentifier (QID)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SUB-TYPE 2    | LENGTH        |        Volume Quota           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Volume Quota               | SUB-TYPE 3    | LENGTH        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  VolumeQuotaOverflow (VQO)    | SUB-TYPE 4    | LENGTH        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        VolumeThreshold (VT)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SUB-TYPE 5    | LENGTH        | VolumeThresholdOverflow (VTO) |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SUB-TYPE 6    | LENGTH        |      DurationQuota (DQ)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    DurationQuota (DQ)         | SUB-TYPE 7    | LENGTH        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      DurationThreshold (DT)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SUB-TYPE 8    | LENGTH        | Update-Reason attribute (UR)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SUB-TYPE 9    | LENGTH        | PrePaidServer                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            PrePaidServer                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type  : Value of PPAQ
   Length: variable, greater than 8

   String:  The String value MUST be encoded as follows:

   Sub-Type (=1):  Sub-Type for QuotaIDentifier attribute
   Length       :  Length of QuotaIDentifier attribute (= 6 octets)

   QuotaIDentifier (QID):

      The QuotaIDentifier Sub-Type is generated by the PrePaid server
      at allocation of a Volume and/or Duration Quota. The on-line


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      quota update RADIUS Access-Request message sent from the Access
      Device to the PPS shall include a previously received
      QuotaIDentifier.

   Sub-Type (=2): Sub-Type for VolumeQuota attribute
   Length       : length of VolumeQuota attribute (= 6 octets)

   VolumeQuota (VQ):

      The optional VolumeQuota Sub-Type is only present if Volume Based
      charging is used. In RADIUS Access-Accept message (PPS to Access
      Device direction), it indicates the Volume (in octets) allocated
      for the session by the PrePaid server. In RADIUS Authorize Only
      Access-Request message (Access Device to PPS direction), it
      indicates the total used volume (in octets) for both forward and
      reverse traffic applicable to PrePaid accounting.

   Sub-Type (=3): Sub-Type for VolumeQuotaOverflow
   Length       : length of VolumeQuotaOverflow attribute (= 4 octets)

   VolumeQuotaOverflow (VQO):

      The optional VolumeQuotaOverflow Sub-Type is used to indicate how
      many times the VolumeQuota counter has wrapped around 2^32 over
      the course of the service being provided.

   Sub-Type (=4): Sub-Type for VolumeThreshold attribute
   Length       : length of VolumeThreshold attribute (= 6 octets)

   VolumeThreshold (VT):

      The VolumeThreshold Sub-Type shall always be present if
      VolumeQuota is present in a RADIUS Access-Accept message (PPS to
      Access Device direction). It is generated by the PrePaid server
      and indicates the volume (in octets) that shall be used before
      requesting quota update. This threshold should not be larger than
      the VolumeQuota.

   Sub-Type (=5): Sub-Type for VolumeThresholdOverflow
   Length       : Length of VolumeThresholdOverflow attribute
                   (= 4 octets)

   VolumeThresholdOverflow (VTO):


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      The optional VolumeThresholdOverflow Sub-Type is used to indicate
      how many times the VolumeThreshold counter has wrapped around
      2^32 over the course of the service being provided.

   Sub-Type (=6): Sub-Type for DurationQuota attribute
   Length       : length of DurationQuota attribute (= 6 octets)

   DurationQuota (DQ):

      The optional DurationQuota Sub-Type is only present if Duration
      Based charging is used. In RADIUS Access-Accept message (PPS to
      Access Device direction), it indicates the Duration (in seconds)
      allocated for the session by the PrePaid server. In on-line
      RADIUS Access-Accept message (PPC to PPS direction), it indicates
      the total Duration (in seconds) since the start of the accounting
      session related to the QuotaID.

   Sub-Type (=7): Sub-Type for DurationThreshold attribute
   Length       : length of DurationThreshold attribute (= 6 octets)

   DurationThreshold (DT):

      The DurationThreshold Sub-Type shall always be present if
      DurationQuota is present in a RADIUS Access-Accept message (PPS
      to Access Device direction). It represents the duration (in
      seconds) that shall be used by the session before requesting
      quota update. This threshold should not be larger than the
      DurationQuota and shall always be sent with the DurationQuota.

   Sub-Type (=8): Sub-Type for Update-Reason attribute
   Length       : length of Update-Reason attribute (= 4 octets)

   Update-Reason attribute (UR):

      The Update-Reason Sub-Type shall be present in the on-line RADIUS
      Access-Request message (Access Device to PPS direction). It
      indicates the reason for initiating the on-line quota update
      operation. Update reasons 4, 5, 6, 7 and 8 indicate that the
      associated resources are released at the client side, and
      therefore the PPS shall not allocate a new quota in the RADIUS
      Access_Accept message.



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      1. Pre-initialization
      2. Initial request
      3. Threshold reached
      4. Quota reached
      5. Remote Forced disconnect
      6. Client Service termination
      7. Main SI released
      8. Service Instance not established

   Sub-Type (=9) : Sub-Type for PrePaidServer attribute
   Length        : Length of PrePaidServer
                   (IPv4 = 6 octets, IPv6= 18 octets

   PrePaidServer:

      The optional, multi-value PrePaidServer indicates the address of
      the serving PrePaid System. If present, the Home RADIUS server
      uses this address to route the message to the serving PrePaid
      Server. The attribute may be sent by the Home RADIUS server. If
      present in the incoming RADIUS Access-Accept message, the PDSN
      shall send this attribute back without modifying it in the
      subsequent RADIUS Access-Request message, except for the first
      one. If multiple values are present, the PDSN shall not change
      the order of the attributes.

   NOTES:

   Either Volume-Quota or Time-Quota MUST appear in the attribute.
   Volume Threshold may only appear if Volume Quota appears
   If the Access Device can measure time, and if Time-Threshold appears
   with Volume Quota, then the Access device should trigger a quota
   replenishment when the Current Time >= Time-Threshold.




5.4 Table of Attributes

   TO BE COMPLETED.

   Request   Accept   Reject   Challenge      #    Attribute

   Authorize_Only Request Accept Reject


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6. Security Considerations

   The protocol exchanges described are susceptible to the same
   vulnerabilities as RADIUS and it is recommended that IPsec be
   employed to afford better security.

   If IPsec is not available the protocol in this draft improves the
   security of RADIUS.  The various security enhancements are explained
   in the following sections.

6.1 Authentication and Authorization

   RADIUS is susceptible to replay attacks during the Authentication
   and Authorization procedures.  A successful replay of the initial
   Access-Request could result in an allocation of an initial quota.

   To thwart such an attack...


6.2 Replenishing Procedure

   A successful replay attacks of the Authorize Only Access-Request
   could deplete the subscribers prepaid account.

   To be completed.

7. IANA Considerations

   To be completed.

   This draft does create RADIUS attributes.  However, the authors
   recognize that it may not be possible to obtain such attributes.
   Therefore, in subsequent drafts it will be proposed to use a Vendor
   space as an Application Space.

8. Normative References

   [RFC2026]       Bradner, S., "The Internet Standards Process --
                   Revision 3", RFC 2026, October 1996.
   [RFC2119]       Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", RFC 2119, March 1997.
   [RFC2865]       Rigney, C., Rubens, A., Simpson, W. and S. Willens,


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                    RADIUS Extensions for PrePaid       February 2004


                   "Remote Authentication Dial In User Server
                   (RADIUS)", RFC 2865, June 2000.

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

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

   [RFC2868]       Zorn, G., Leifer, D., Rubens, A., Shriver, J.,
                   Holdrege, M., Goyret, I., "RADIUS Attributes for
                   Tunnel Protocol Support" , RFC 2868, June 2000.
   [RFC3576]       Chiba, M., Dommety, G., Eklund, M., Mitton, D.,
                   Aboba, B., "Dynamic Authorization Extensions to
                   Remote Authentication Dial-In User Service
                   (RADIUS)", RFC 3576, February 2003.

   [DIAMETERCC]    Work in Progress.
   [REDIRECT]      RADIUS Redirection Internet Draft. Work in progress.

Acknowledgments

   The authors would like to thank Mark Grayson (Cisco) and Nagi
   Jonnala for their contribution to this draft.

Author's Addresses

   Avi Lior                           Parviz Yegani, Ph.D.
   Bridgewater Systems                Mobile Wireless Group
   303 Terry Fox Drive                Cisco Systems
   Suite 100                          3625 Cisco Way
   Ottawa Ontario                     San Jose, CA 95134
   Canada                             USA
   avi@bridgewatersystems.com         pyegani@cisco.com

   Kuntal Chowdhury                   Lila Madour
   Nortel Networks                    Ericsson Canada
   2221, Lakeside Blvd,               5400 Decarie, TMR
   Richardson, TX-75082               Quebec, Canada
   chowdury@nortelnetworks.com        Lila.madour@ericsson.ca

   Yong Li
   Bridgewater Systems


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   303 Terry Fox Drive
   Suite 100
   Ottawa Ontario
   Canada
   Yong.li@bridgewatersystems.com



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   developing Internet standards in which case the procedures for


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   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.  The limited permissions granted above are perpetual and
   will not be revoked by the Internet Society or its successors or
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Expiration Date

   This memo is filed as draft-lior-radius-extensions-for-prepaid-
   03.txt, and will expire 16th July, 2004.































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