wdiff draft-lior-radius-prepaid-extensions-07.txt draft-lior-radius-prepaid-extensions-08.txt

Network Working Group A. Lior
INTERNET-DRAFT Bridgewater Systems
Category: Informational P. Yegani
draft-lior-radius-prepaid-extensions-07.txt
draft-lior-radius-prepaid-extensions-08.txt Cisco
Expires: 20 July, 2005 18 January, 2006 K. Chowdhury
Starent Networks
Y. Li
Bridgewater Systems
H. Tschofenig
C. Guenther
Siemens
February 20,
July 17, 2005

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

Status of this Memo

By submitting this Internet-Draft, I certify each author represents that any
applicable patent or other IPR claims of which I am he or she is aware
have been or will be disclosed, and any of which I become he or she becomes
aware will be disclosed, in accordance with RFC 3668. Section 6 of BCP 79.

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This Internet-Draft will expire on July 20, 2005 December 29, 2005.

Abstract
This draft presents an extension to the Remote Authentication Dial-
In User Service (RADIUS) protocol to support charging for prepaid
services. The charging models supported are namely: volume-based
charging, duration-based charging and one-time-based charging.

Table of Contents

1. Introduction...................................................4
1.1 Terminology................................................6 Terminology................................................5
1.2 Requirements language......................................6 language......................................5
2. Overview.......................................................6
2.1 PrePaid Prepaid Charging Model.....................................7 Models....................................6
2.2 Architectural Model........................................7 Model........................................6
2.3 Why not existing RADIUS attributes?.......................13 Motivation................................................11
3. Use-cases.....................................................15 Operations....................................................13
3.1 Simple pre-paid access use-case...........................15
3.2 Support for Multi-Services................................17
3.3 Resource Pools............................................18
3.4 Support for Complex Rating Functions......................20
3.5 One-Time-based Prepaid Charging...........................21
3.6 Support for Tariff Switching..............................22
3.7 Support for Roaming.......................................24
3.8 PrePaid termination.......................................24
3.9 Querying and Rebalancing Prepaid Resources................25
4. Operations....................................................25
4.1 General Requirements......................................25
4.1.1 Requirements......................................13
3.1.1 Broker AAA Requirements..............................25
4.2 Requirements..............................13
3.2 Authentication and Authorization for Prepaid Enabled SADs.26
4.3 SADs.14
3.3 Session Start Operation...................................28
4.4 Operation...................................16
3.4 Mid-Session Operation.....................................29
4.5 Operation.....................................16
3.5 Dynamic Operations........................................31
4.5.1 Operations........................................18
3.5.1 Unsolicited Session Termination Operation............31
4.5.2 Operation............19
3.5.2 Unsolicited Change of Authorization Operation........32
4.6 Operation........19
3.6 Termination Operation.....................................32
4.7 Operation.....................................20
3.7 Mobile IP Operations......................................33
4.8 Operations......................................20
3.8 Operation consideration considerations for Multi-Services................34
4.8.1 Multiple prepaid services....21
3.8.1 Initial Quota Request................................34
4.8.2 Request................................22
3.8.2 Quota Update.........................................35
4.8.3 Termination..........................................35
4.8.4 Update.........................................22
3.8.3 Termination..........................................23
3.8.4 Dynamic Operations...................................36
4.8.5 Operations...................................23
3.8.5 Support for Resource Pools...........................36
4.8.6 One-Time-Charging....................................36
4.8.7 Pools...........................23
3.8.6 One-Time-Charging....................................24
3.8.7 Error Handling.......................................37
4.9 Handling.......................................24
3.9 Accounting Considerations.................................38
4.10 Considerations.................................25
3.10 SAD Operation............................................38
4.11 Operation............................................25
3.11 Interoperability with Diameter Credit Control Application38
5. Attributes....................................................38
5.1 Application25
4. Attributes....................................................25
4.1 PPAC Attribute............................................39
5.2 Attribute............................................26
4.2 Session Termination Capability............................40
5.3 Capability............................27
4.3 PPAQ Attribute............................................40
5.4 Attribute............................................27
4.4 Prepaid Tariff Switching (PTS)............................46
5.5 (PTS)............................34
4.5 Table of Attributes.......................................49
6. Attributes.......................................36
5. Security Considerations.......................................49
6.1 Considerations.......................................37
5.1 Authentication and Authorization..........................49
6.2 Authorization..........................37
5.2 Replenishing Procedure....................................50
7. Procedure....................................37
6. IANA Considerations...........................................50
8. Considerations...........................................37
7. Normative References..........................................51
9. References..........................................38
8. Informative References........................................51
10. References........................................39
9. Call Flows...................................................52
10.1 Flows....................................................39
9.1 Simple Concurrent Services...............................53
10.2 Services................................40
9.2 One-time Charging........................................55
Contributor......................................................56
Acknowledgments..................................................56 Charging.........................................43
Contributor......................................................43
Acknowledgments..................................................43
Author's Addresses...............................................56 Addresses...............................................43
Intellectual Property Statement..................................57 Statement..................................44
Disclaimer of Validity...........................................57 Validity...........................................44
Copyright Statement..............................................57 Statement..............................................45
Expiration Date..................................................58 Date..................................................45
10. Appendix A û use cases.......................................45
10.1 Simple prepaid use case..................................45
10.2 Support for Multi-Services...............................47
10.3 Resource Pools...........................................48
10.4 Support for Complex Rating Functions.....................49
10.5 One-Time-based Charging..................................50
10.6 Support for Tariff Switching.............................51
10.7 Support for Roaming......................................53
10.8 Termination of a prepaid session.........................53
10.9 Querying and Rebalancing Prepaid Resources...............54

1.
Introduction

This draft describes RADIUS protocol extensions supporting charging for PrePaid Data Services.

PrePaid data services the RADIUS protocol. These
extensions are cropping up in many wireless meant to enable service providers to charge and wireline
based networks. bill
their customers using prepaid accounts.

A PrePaid Data Service prepaid service subscriber is one that
purchases a user who has purchased a contract
according to which he will receive a particular data service for
either a period of time, time or a quantity of data. Before providing a In the typical
prepaid data
service, scenario, the service provider checks verifies that the prepaid subscriber
has sufficient funds to cover in his account before delivering the particular service request. service.
Only after
confirmation that if sufficient funds are available is the service provided to
the user.

The subscriber purchases

Note that the Data Service using various means such
as buying a PrePaid Card, or online. How by which the subscriber purchases
their PrePaid Data Service depends on the deployment and obtains funds is not in outside
the scope for of this document.

In Also note that, in some deployments, scenarios, the PrePaid data service will
subscriber's account may be combined with
other Prepaid services such as PrePaid circuit voice service. This
is not an issue for used to fund multiple services, some of
which may use the extensions defined in this document documents, and some
may use other than mechanisms. While the fact that interworking of the
PrePaid Data Services mechanisms
described in this paper should work document with other
PrePaid data mechanisms should be possible
and or circuit voice services. straightforward, how this could be done depends on the external
mechanisms and is, as such, outside the scope of this
document.

The fundamental business driver for a carrier to provide PrePaid
data services behind the protocol extensions defined in this
document is to increase participation (subscriber (i.e. a service provider's
subscriber base) and thus to increase revenues. Therefore, it makes sense that PrePaid
services meet In particular, the
extensions were designed with the following goals: goals in mind.

- Leverage Make use of existing infrastructure, hence reducing infrastructure as much as possible, and
thereby limit the amount of necessary capital
expenditures typically required when rolling out a new service; expenditures,
- Ability provide the ability to rate service requests in real-time; real-time,</t>
- Ability provide the ability to check that charge the end userÆs user's account for coverage for the
requested service - charge prior to execution of that service;
service provision,
- Protect protect against revenue loss, i.e., i.e. prevent an end user from
generating chargeable events
obtaining service when the credit of that account is
exhausted or expired; available funds are not sufficient,
- Protect protect against fraud; fraud, and
- Be be as widely deployable over Dialup, Wireless dialup, wireless and WLAN networks.

The protocol described in this document maximizes existing
infrastructure as much as possible û 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 architecture between the PrePaid data
session. As well, mechanisms have been designed to be able to
recover from errors entities that occur from time to time.

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

PrePaid Systems 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 execute the RADIUS protocol
to achieve a PrePaid Data Service. The prepaid architecture
protocols with the extensions defined in this document assumes that
rating of chargeable events does not occur in the element
providing
that provides the service. This Instead, the rating could may be performed in at a
dedicated server, termed the prepaid ôprepaid enabled AAA server serverö or simply
ôprepaid serverö. Alternatively, the actual rating may exist occur in an
entity behind this AAA prepaid server.
Business Furthermore, business logic and service rules may define
dictate a time-dependent tariff model, for example that tariffing of events
vary in time, e.g., the particular price per megabyte download
for a service may
be defined to switch at 8pm from a high tariff to a low tariff. The
RADIUS
extensions for prepaid defined in this document support scenarios enable scalable
implementation of tariff switched prepaid systems. such scenarios.

Furthermore, the prepaid architecture this documents assumes that an architecture where a quota server `quota
server' is available which, through co-ordination with the rating
entity and a centralized account balance manager manager, is able to
provide a quota response in
response indication for prepaid data service. a particular user when requested.
This quota server functionality
could be performed in the prepaid enabled AAA server may 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, coexist in order to define the
RADIUS protocol extensions it is necessary to discuss the functional
behavior of the PrePaid System. prepaid server.

1.1
Terminology

Service

Network Access Device
PrePaid Server As in RADIUS.
(NAS)
Prepaid Client(PPC) The Prepaid Client (PPC) is the entity which triggers the RADIUS message
exchange including prepaid extensions
defined in this document. Typically the Prepaid Client
Resides The PPC typically
resides in the NAS.
PrePaid
Prepaid Server(PPS) The Prepaid Server is the entity that interacts with the Prepaid
Client using the RADIUS prepaid extensions
defined in this document.
Home network The network entity which contains maintains the user userÆs
profile and the userÆs prepaid account.
WLAN Wireless Local Area Network
Service Event
Access Service The service that is provided to the user
when the user is authenticated and
authorized. In this document the term is
used to differentiate between authorization
of services that are explicitly identified
by a Service Identifier. Example of Access
Service would be the Main Service instance
of 3GPP2.

Furthermore, we use the following terms are used in this document. Mobile
IP and AAA terminology: Home agent (HA), Home network, Home AAA
(HAAA), Broker AAA (BAAA), Visited AAA (VAAA) and Foreign Agent (FA)

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.
Overview

This section gives a concise provides an overview of the Prepaid Charging models prepaid charging models,
and their associated architectures, that is are supported by this document, and the Architectural model
relevant to
extensions proposed in this draft. document.

2.1
PrePaid Charging Model

There are several PrePaid
Prepaid Charging Models

A number of models of how to charge customers for availing data services: services in a
prepaid manner are supported, as follows.

. Volume-based charging (VBC): (e.g., (e.g. 2 Cents/KiloByte); Cents/KiloByte)
. Duration-based charging (DBC): (e.g., (e.g. 3 Cents/minute); Cents/minute)
. Subscription-based charging (SBC): (e.g.,
Dollars/month+service);), (e.g. Dollars/month)
. Event-based charging (EBC): (e.g., (e.g. 7 Cents/URL or email).

Charging models can be further divided into those with debiting of
prepaid email)

Whether the user accounts and those with debiting of non-prepaid
accounts account is a dedicated prepaid account or a general
account (such as a current accounts at banks). From bank account) is outside the perspective scope of
this document all userÆs as treated as userÆs having a prepaid
accounts. document.

2.2
Architectural Model

The architectural model supports prepaid clients on a service access
device. A SAD (e.g. a NAS) typically assumed in this document encompasses the
following entities.

(1) Service Access Device (SAD): This entity provides a access to data
service to end-users. A the users, and typically coincides with the NAS. The
SAD is a network entity on executes the data path
that includes a RADIUS client and a PrePaid Client. which, for the purposes of this
document, is termed the PPC. When prepaid service is used the
SAD collects service event information and reports it while and/or or
after services are provided to the prepaid user. This event information
is sent to a prepaid
server by the PPS using the prepaid RADIUS extensions. extensions defined in this
document.
(2) The PPS: The RADUIS server. If real-time credit control is
required, the SAD (prepaid client) PPC (SAD) contacts the prepaid server PPS with service event
information included before the service is provided. The prepaid server, depending on the
service event information, PPS
performs a credit check and allocates a portion of available
credit to the service event.
(3) The rating entity: This entity converts this the credit value that is
allocated by the PPS into a time and/or or volume amount, which called the
ôquotaö. This quota is then returned to the requesting SAD. PPC
(SAD) (via the PPS). The rating entity may also determine that
during the allocated quota, service provision a tariff switch will
occur in which occur. In this
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 exactly
tariff switch will occur.

The requesting SAD then (PPC) monitors the provision of the service execution
according to the instructions returned by the prepaid server. PPS. After service
completion or on a subsequent request for service, the prepaid
server PPS deducts
the reserved allocation corresponding amount of credit from the prepaid
userÆs user account.

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

There the user account appropriately.

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

Figure 1 Basic Prepaid Architecture
The prepaid server PPS and the 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
SADs, which in reality may be a NAS in Dialup deployments, PDSN
(Packet Data Serving Node) or HA (Home Agent) in CDMA2000
deployments, an 802.11 WLAN Access Points or GGSN (Gateway GPRS
Serving Node) in GPRS/UMTS deployments. To actively participate in
Prepaid procedures outlined here, the

The SAD MUST have the Prepaid
Client capabilities. Prepaid Client Capabilities include the ability to meter the usage for a prepaid data session; this
session. This usage includes time or volume (e.g. number of bytes) usage. bytes).

In the case of roaming scenarios using mobile IP (in a wireless or
wireline network), the prepaid client functionality PPC may be delegated
to run on 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 running the prepaid client functionality PPC may also have Dynamic
ôDynamic Session Capabilities that include Capabilitiesö such as the ability to terminate a
data session and/or or change the filters associated with a specific data
session by processing Disconnect Messages ôDisconnectö messages and
Change ôChange of Authorization
Authorizationö messages as per [RFC3576].

In this

This document RADIUS assumes that the PPS is used as the AAA server. There
are three
kinds or categories types of AAA servers. server, as follows. (i) The AAA server in the
home
network, the HAAA, network (HAAA), which is responsible for authentication of the
subscriber and also authorization of the service.
subscriber. In addition, the HAAA communicates with the Prepaid servers PPS using
the RADIUS protocol in order to authorize prepaid subscribers. In AAA based roaming deployments
the (ii) The AAA
server in the visited network, the VAAA, network (VAAA) which exists only in roaming
scenarios and is responsible for forwarding the RADIUS messages to
the HAAA. The VAAA may also modify the messages. In Note that, in
certain roaming deployments, the visited network may be separated from connected to
the home network by via one or more broker networks. (iii) The AAA servers
server in one of the aforementioned broker networks, BAAA are networks (BAAA), which is
responsible to route the RADIUS packets transparently for forwarding messages and hence
donÆt does not play an active roll role
in the Prepaid Data Service prepaid data service delivery.

In this A BAAA obviously exists only
in those roaming deployments where the VAAA and the HAAA are
connected via the BAAA of a broker network.

This document assumes that the Prepaid Server is described in functional terms
related to their interface PPS communicates with the HAAA. The Prepaid Server HAAA for
the purposes of authorisation. Additionally, the PPS interfaces to
entities which:

i) which

- Keep the accounting state of the prepaid subscribers subscriberÆs account balance (balance
manager);

ii) Allow manager),
- Rate access service requests to be rated in real-time (Rating
Engine); Engine), and
iii) Allow
- Manage quota to be managed for a particular pre-paid prepaid service (Quota Server).

The various deployments for Prepaid

Three deployment scenarios are presented in the remainder of this
section. The first deployment is the basic Prepaid data
service and scenario is depicted in figure Figure 2. The In this
scenario, the SAD, which supports runs the
prepaid client functionality, PPC, the HAAA HAAA, and the Prepaid Server PPS are
collocated
located in the same provider network.

The Subscriber Device establishes a connection with one of several
Access Devices possibly
multiple SADs in the network. The selected SAD communicates with one or
more a
HAAA servers server. However, in the network. To order to provide redundancy more than
one redundancy, multiple HAAA
may be available to use by a SAD. available.

The network will have has one or more Prepaid Servers. Multiple Prepaid
Servers may be used to provide redundancy and load sharing. PPSs. The interface between the HAAA and
the PPS is implemented using the RADIUS protocol together with the
extensions described in this specification. 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 whatever
protocol is used between the HAAA and the PPS. a
functionally equivalent protocol.

+------+ +-----+
| | | |
+--------+ +--------+ +--| HAAA |--+--| PPS |
| | | | | | | | | |
| Sub | Subscr.| | Service| | +------+ | +-----+
| |---| Access |--+ |
| Device | | Device | | +------+ | +-----+
| | | | | | | | | |
+--------+ +--------+ +--| HAAA |--+--| PPS |
| | | |
+------+ +-----+

Figure 2 Basic Prepaid Access Architecture

The second scenario, depicted in Figure 3 shows 3, is based on 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 | |Service| | +----+ | +----+ | +----+ | +-----+
| |--|Access |-+ | | |
|Device| |Device | | +----+ | +----+ | +----+ | +-----+
| | | | | | | | | | | | | | | |
+------+ +-------+ +-|VAAA|-+-|BAAA|-+-|HAAA|-+-| PPS |
| | | | | | | |
+----+ +----+ +----+ +-----+

Figure 3 Static Roaming Prepaid Architecture

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

To support dynamic
architecture depicted in Figure 2.

The third scenario is a roaming scenario where the network will utilize Mobile-Ip as
illustrated in utilises
Mobile-IP. It is depicted Figure 4. Note that typically In this scenario 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 moves between networks that use different technology technologies such
as between WLAN and Wireless and Broadband. Fortunately, Mobile-Ip can address Mobile-IP addresses this type of roaming
mobility and therefore we need not be concerned with the underlying
network technology.

+------+ +-------+ +----+ +----+ +----+ +-----+
| | |Service| | | | | | | | |
|Sub |
|Subscr| |Access +-----|VAAA|--|BAAA|--|HAAA|--| PPS |
| |--|Device | | | | | | | | |
|Device| | (FA) +--+ +----+ +-+--+ +----+ +-----+
| | | | | |
+------+ +------ + | |
| | | +----+
| | | | |
|ROAMS +------------------+ HA |
| | | |
V +----+ | +----+
+------+ +-------+ | | | |
| | |Service| +-|VAAA+------+ |
|Sub |
|Subscr| |Access | | | | |
| |--|Device +-+ +----+ |
|Device| | (FA) | |
| | | +------------------------+
+------+ +-------+
Figure 4 Roaming using Mobile-IP and pre-paid enabled SADs

In figure Figure 4, the Subscriber device Device establishes a prepaid session
between with the
SAD in the foreign network, which has prepaid
capabilities. The subscriberÆs home address will be anchored at the
Home Agent (HA) in the home network. The setup for this access service is
identical to the cases covered above. Notice Note that the SAD may be
collocated with the Foreign Agent (FA) in case of Mobile-
IPv4. if Mobile-IPv4 is being used.
As the subscriber device moves moves, it establishes a connection with
another SAD in the same foreign network or possibly in another foreign network. The prepaid data
service should continue to be available. When a device associates to
another SAD it MUST re-authenticate at the new SAD and de-associate
or logoff log off from the old SAD. Furthermore, any unused quota at the
old SAD MUST be promptly credited back to into the subscribers account. The reason we say
promptly, is subscriberÆs account immediately.
This has to happen immediately because otherwise, if the subscriber is very low on resources to
start with, the subscriber
subscriberÆs funds are low, he may not have enough resources to log on
to be denied service at the new SAD. The speed at which resources can be returned depend
on the type of handoff procedure that is used. Some of the example
of handoffs in wireless networks are dormant handoff, active handoff
and fast handoff.

As well, notice that

Note that, if the SADs could communicate directly with each other then
there could be a way to accelerate a faster the handoff procedure. In
particular, it could accelerate the return of the unused portion
of the quotas from the old Access Device. subscriber could be refunded more quickly.
Unfortunately, standards with regards to handoff procedures are evolving with
each network technology creating their own scheme specific to make the
handoff procedures more efficient. underlying
network technologies and vary significantly in terms of delay.

2.3
Why not existing RADIUS attributes?
Motivation

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

It is indeed possible to build construct a prepaid solution for prepaid billing
scenarios using existing RADIUS attributes. The RADIUS server can simply would
send an Access-Accept message containing a Session-Timeout(27) and set Termination-
Action(29) to
include a Termination-Action(29) in the RADIUS-request. Upon
receiving the Access-Accept message, the NAS will 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 Access-
Accept message with indicating the amount of additional time in
Session-Timeout(27) or a
Session-Timeout(27). Alternatively, it would responds with an
Access-Reject message if there were no more resources in the userÆs
account.

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

There are several problems with such a solution:

-It

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

-Using

- 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
batches. The solution presented in this paper document 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)

- 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) may use the service for an
undetermined period of time.

- Termination-Action(29) presents its own issues. First Firstly the
behaviour of Termination-Action(29) is not mandatory. Second, Secondly,
according to RFC2865 RFC2865, Termination-Action fires when the Service is
complete. But we provision of
the service has completed. However, service should not be terminating the service while terminated
when negotiating additional quota. The refreshing of the time quota quota, because this should be happen in a
manner transparent to the user. subscriber. Because Termination-Action
occurs when the Service is complete completed it is unclear whether or not the
user experience would be transparent. For example, will the affected. The RADIUS server might even
allocate the subscriber a new IP address? address to the subscriberÆs device. Furthermore,
the RADIUS server has no way of telling why the Access-Request
message was generated. The RADIUS server will might have to wait for the
corresponding accounting packet to determine the reason for this
Access-Request message. Lastly Finally, 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. user
perspective. No re-authentication is required and quotas can be
negotiated prior to the quotas available credit running out.

-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.
might be able to obtain the service for free. The solution described
in this document, document requires that a prepaid capable prepaid-aware SAD inform informs the
RADIUS server server, regardless of whether or not it the latter supports the
prepaid
capabilities. extensions. The RADIUS server can now then determine whether or
not service should be granted or not. 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 that does support prepaid client function (e.g. the Home Agent) that supports
prepaid, or it may allow the subscriber to get access but
restrict the traffic. provide only a restricted service.]

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

Lastly refund the subscriberÆs
account appropriately.

The solution provided in this document is extensible. This
document defines For example,
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 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 SAD and not from the UserÆs Device. The
connection between the UserÆs Device, which typically involves
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 SAD 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 SAD. PrePaid capabilities
MUST be included if the SAD supports PrePaid functionality.

The AAA System proceeds with the authentication procedure. This may
involve several transactions such as in EAP [RFC2284]. Once the
subscriber has been authenticated, 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 SAD.

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 SAD 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 to indicate the allocation of a portion of the
subscriberÆs account called the initial quota (in units of time or
volume) and optionally a threshold value.

The reason we allocate a portion of the userÆs account is that the
user may be engaged in other Services that may draw on the same
Prepaid 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 the involved separate parts of the
system. If the entire quota was allocated to the data session then
the user would have no more funds for a voice session.

The AAA system incorporates the PrePaid attributes received from the
PrePaid System into an Access-Accept message that it sends back to
the SAD. 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 SAD 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 SAD 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 account 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
account balance 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 SAD will, continue
the data service session until the new threshold is reached. If the
request for additional quota cannot be fulfilled then the SAD will
let the subscriber use up the remaining quota and terminate the
session.

Alternatively, instead of terminating the session, the SAD 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.

Should the subscriber terminate the session before 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
Support for Multi-Services

Up to now we were looking at session that consisted of a single
service, ôAccess Serviceö. An ôAccess Serviceö is the basic service
that is provided to the user by the SAD after successful
authentication and authorization. When we donÆt differentiate
between different types of services the ôAccess Serviceö aggregates
all the services that the user my be engaged in on a particular SAD.

For example, the user may be browsing the web, and participating in
a VoIP conversation, watching streaming video and downloading a
file.

Some operators may want to distinguish between these services. Some
services are billed at different rates and services maybe metered
differently. Therefore, the prepaid solution needs to be able to
distinguish services, and allocate quotas to the services using
different units (e.g. time, volume) and allow for those quotas to be
utilized at different rates.

+---------+
| Session |
+---------+
|
V N
+--------------+ +-------+
| Service |------>| Quota |
| (service-Id) | +-------+
+--------------+

As shown in the above diagram, a Session can have N Services. Each
service is identified by a Service-Id. The format of the Service-Id
is not extensions described in the scope of this document but the Service-Id could be
expressed as an IP flow using the stand 5-tuple (Source-IP and Port,
the Destination-IP and Port, and the protocol type). Each service
is allocated an appropriate quota.

3.3
Resource Pools

When working with multiple services that results in multiple quota
allocation another problem arises. Even though quotas are portioned
out in fractional parts of the userÆs prepaid account, there could
be a situation where one Service utilizes its quota faster then
another Service. When the userÆs account is used up, there could be
a situation where one Service is unable to obtain additional quota
while another Service has plenty of quota remaining. Unless the
quotas can be rebalanced, the SAD would then have to terminate that
Service. As well, even before that happens, the existence of
several Services could generate an excessive amount of traffic as
the services update their quotas.

One method to solve these problems is to utilize resource pools.
Resource pools allow us to allocate resources to several 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 allocated to the pool runs out, we replenish the
pool.

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

As the figure above shows, 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 determines the rate
at which Service-A and Service-B draw from the pool.

The pool is initialized by taking the quota allocated to each
service and multiplying it by Mn. Therefore, the amount of
resources allocated to a pool is given by:

Poolr = Ma*Qa + Mb*Qb + . . .

A Pool is empty if:

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

where:
Ca,Cb are the consumed resources of Service-A and Service-B
respectively.

Note that the resources assigned to the pool are unit less. That
is, Service-A can be rated at $1 per Mbyte and Service-B can rated
at $0.10 per Minute. In this case if we allocate $5 worth of
resources on behalf of service-A to the pool we would set Ma = 10
and place 50 units into the pool. If we allocate $5 were designed based on behalf of
Service-B to the Pool, then M=1 and place 50 units into the Pool.
The pool would have a total sum of 100 units to be shared between
the two services. 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.

3.4
Support for Complex Rating Functions

The rate of use
number of a resource by a service can be very complex.
Some services use resources (e.g. time, volume) linearly. For
example, a service maybe consuming resources at a rate of $1 per
Mbyte.

In some cases an operator may wish to apply a much more complex
rating function. 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 Mbyte and volume above M bytes be rated
at $0.50 per Mbyte. This rating function could be achieved by
repeated message exchanges with the Prepaid System.

To avert the need to exchange many messages and to support even more
complex rating functions we support Rating Groups. A Rating Group
is provisioned at the SAD. As illustrated in the figure below, a
Rating Group is associated with one or more Services and defines the
rate that the services associated with the Rating Group consume the
quota.

+-----------+
| Service-A |------+
+-----------+ | +--------------+ +-------+
+---->| | | Quota |
| Rating Group |------>| or |
+-----------+ +---->| | | Pool |
| Service-B |------+ +--------------+ +-------+
+-----------+

During authorization of the of a service, if the service is
associated with a Rating Group, the Prepaid Client sends the Rating
Group to the Prepaid Server. The prepaid service authorizes the
Rating Group by assigning it a Quota and optionally assigning it to
a Resource Pool.

When service that belongs to an authorized Rating Group is
instantiated, then the Prepaid Client does not need to authorize
that service. This could greatly reduce the amount of traffic
between the Prepaid Client and the Prepaid Server.

3.5
One-Time-based Prepaid Charging

One-Time-based Prepaid Charging is used for charging of Service
Events where there is no session. That is, the Service Event does
not have a start or an end. scenarios. An example of a one-time service event
is the purchase of a ring-tone. The one-time event in this case is
the userÆs purchasing the right to use a ring-tone. The actual
downloading of the tone is a separate service event totally distinct
from the right to use the ring tone. For example, the user may have
already downloaded the tone and then after being totally satisfied
with the quality, decides to purchase the right to use the 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 to use a service for a month. While the user uses the
service his usage maybe metered especially if there are limits
associated with the service.

For a given user, One-time-based charging may occur in conjunction
with the other charging models. For example, the prepaid user maybe
accessing a website which is being metered based time or volume
while they purchase the right to use a ring tone (a one-time-based
event). Note: it is up to the service providers to decide whether
or not the user will be charged for the download of the tone and
also be charged for the time and volume required to download the
ring-tone. The facilities provided by this document gives the
service provider the capability to achieve their service charging
business goals. For example, should the service provider choose not
to charge for the download volume or time, then they can treat the
download IP flow as a separate service that is exempt from charging.

One-time-based charging occurs when the SAD sends an indication to
the PPS identifying the service, and the units that need to be
debited from the account. The units to be debited from the account
and how those units are rated (if they donÆt represent money) is not
in scope of this specification.

One-time-based charging may occur under two conditions: the SAD may
not have a authenticated context (or access to an authenticated
context for the subscriber); the SAD has access to authenticated
context for the subscriber. In the former case the SAD will have to
authenticate the subscriber. For example, the prepaid user maybe
authenticated by the SAD providing access service. However when the
user accesses the subscription server to purchase a subscription,
the subscription server may not have access to the authentication
context of the subscriber and thus will have to authenticate the
subscriber. Authentication of the subscriber and the generation overview of
the one-time charging event will happen at the same time.

Note that one-time-based charging can be used to credit the prepaid
userÆs account. For example, the SAD these can return resources back to
the prepaid subscriber by making a one-time charge request that
includes the amount of resource to be credited back to the user.

3.6
Support for Tariff Switching

The PPC and the PPS may support tariff switching for volume based
prepaid packet service. Tariff switching allows the PPS to signal
the PPC when a change of rating or tariff switch is to occur. For
example as shown in the figure below, traffic before 18:00 hours is
rated at ær1Æ or business rates and traffic after 18:00 hours is
rated at ær2Æ or non-business rates. The PPC will then be able to
report usage before the tariff switch point and usage after the
tariff switch point. Since time is measured in seconds, tariff
switching only makes sense for volume based prepaid service where
the volume is billed at different rates: one rate before the tariff
switch period and another rate after the tariff switch period.

18:00
------------------+-----------------
r1 | r2
------------------+-----------------
^ ^
|<----TSI---> |
| |
Access-Accept Access-Request

When tariff switching is supported by the PPC it indicates support
for tariff switching by setting the appropriate bit in the PPAC. If
the PPS requires to signal a tariff switch time it will send a PTS
attribute which will indicate when the tariff switch period is to
occur as a number of seconds from the current time
(TarrifSwitchInterval TSI).

Sometime after the tariff switch period the PPC will send another
online Access-Request. Either the user has logged off or the volume
threshold has been reached. The PPC will report how much volume was
used using the PPAQ and how much volume was used after the tariff
switch using the PTSÆs VUATS subtype.

If the PPC will send and event before the tariff switch time, say
the Volume threshold has been reached, the PPS will respond with
another PTS with the TSI indicating how many seconds until the
tariff switch time.

In situations where there is going to be another tariff switch
period, as shown below, the PPS MUST specify the length of the
tariff switch period using the TimeIntervalAfterTariffSwitchUpdate
(TITSU) in the PTS attribute.

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

When TITSU is specified in the PTS, the PPC MUST generate an online
Access-Request within the time after TSI and before TITSU expires.
Normally the PPC will be triggered by the Volume Threshold, but
there is no guarantee that the user session will generate any volume
during the period between 18:00 and 23:30 hours. If TITSU was not
specified in this case, then if there was some volume generated
during r2 but not enough to trigger a prepaid update before the next
tariff switch at 23:30. Then the PPC will not be able to report how
much volume was generated during r1, r2, and r3.

When prepaid for multiple flows is supported, then each flow could
have it own tariff switch period. For example, best effort flow may
not have a tariff switch associated with it, yet a voice over IP
call may have a tariff switch period. The Voice over IP call may
bill at one rate for the first 5 minutes and another rate for the
rest of the call.

[EDITORÆS NOTE: Need to consider tariff switching with pools. Is it
worthwhile?]

3.7
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 networks while
maintaining a connection with the home network 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.

3.8
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 found
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 SAD. 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 SAD.

3.9
Querying and Rebalancing Prepaid Resources

It should be possible to allow the Prepaid Server to Query the
current uses state of a prepaid balance at a SAD and adjust the
prepaid resources.

For example, a request to the PPS is made (e.g., a one-time charging
event) but the userÆs account is depleted but resources have been
allocated to the SAD. The PPS should have a the capability to query
the SAD and if it has the spare resources to reassign the quotas to
the SAD and to the pending request. Note that the PPS doesnÆt know
resource usage until the SAD request for more resources. This can
be a long time.

In the absence of this capability the PPS can minimize the
occurrence of this scenario by allocated smaller quotas. But the
result will be many more transactions. The ability to query and to
rebalance resources provides a good trade-off.

4. Appendix A.

3.
Operations

4.1

3.1
General Requirements

4.1.1

3.1.1
Broker AAA Requirements

Broker AAA (BAAA) servers MUST support the Message-Authenticator(80)
attribute as defined in [RFC2869]. If BAAA servers they are used, the
BAAA servers function is to they forward
the RADIUS packets as usual to the appropriate RADIUS servers.

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

4.2

3.2
Authentication and Authorization for Prepaid Enabled SADs

The SAD initiates the authentication and authorization procedure by
sending a RADIUS Access-Request to the HAAA.

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

If the SAD 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 SADs 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 Change-of-
Authorization message, the AAA server would then be responsible for
terminating the session using whatever the means that are supported by the
device.

If the authentication procedure involves multiple Access-Requests message exchanges
(as in EAP), the SAD MUST include the PPAC(TBD) attribute and the
Dynamic-Capabilities attribute (if used) in at least the last
Access-Request of the authentication procedure.

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

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

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

The PrePaid server lookups PPS locates the subscriberÆs PrePaid account and will
authorize authorizes him. During
this procedure, the subscriber taking PPS takes into consideration the SAD PrePaid
Client PPC
Capabilities.

Upon successful authorization, the PrePaid server will generate PPS generates 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. The PPAQ(TBD)
attribute includes: includes the following.

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

- Volume and/or Time quotas, which are set to a value values representing a
portion of the subscribers account; credit;

- It MAY contain a Time or Volume Threshold that controls when the
SAD
requests should request additional quota;

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

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

Depending on site policies, upon unsuccessful after failed authorization, the
PrePaid server will PPS may
generate an Access-Reject to terminate the session immediately.
Alternatively, the PrePaid server 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 where to a fixed server. (This feature could be used, for
example, to prompt the subscriber can user to replenish their account for a period of time. account.) Blocking of
traffic is achieved by either Filter-Id(11) Filter-ID(11) or NAS-Filter-Rule(see
Redirect I-d). Redirection is achieved by sending Redirect-Id or Redirect-
Rule,
Redirect-Rule, HTTP Redirection defined in the Redirect I-d. The period of
time period before the blocked/redirected session last can be is blocked/redirected is specified by
the Session-Timeout(27) attribute.

Upon receiving the an Access-Accept from the PrePaid Server, PPS, the HAAA
will append appends the
usual service attributes and forward the packet to the SAD. The
HAAA SHOULD NOT overwrite any attributes already set by the PrePaid server. PPS. 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
does not receive an Access-Accept or an Access-Reject message from
the PrePaid server PPS it MAY do nothing or send an Access-Reject or an Access-Accept Access-
Accept message back to its
client.

4.3 the PPC.

3.3
Session Start Operation

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

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

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

4.4

3.4
Mid-Session Operation

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

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

The SAD MUST also include NAS identifiers, and Session identifier
attributes in the Authorize Only Access-Request. The Session
Identifier should be the same as those the one 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
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 NOT include either a User Password or
and MUST NOT include a Chap Password. In order to authenticate the
message, the SAD MUST include the a Message-Authenticator(80) attribute.
The SAD will compute computes the value for the Message-Authenticator based on according
to [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 erroneous or belongs to another application (for
example, a Change of Authorization message [RFC3576]). In this case
the Authorize Only Access-Request will is either be silently discarded or
handled by another application (not in scope of this document). application.

Once the Authorize Only Access-Request message is validated, the
HAAA SHALL forward the Authorize Only Access-Request to the
appropriate PrePaid Server. PPS. The HAAA MUST forward the Authorize Only Access-Request Access-
Request to the PrePaid server PPS specified in the PPAQ(TBD). The HAAA MUST sign the message using the Message-
Authenticator(80) and add an
Message-Authenticator(80) to the procedures in message, according to [RFC2869].
As with the Access-Request message, the HAAA MAY modify the User-Name(1) User-
Name(1) attribute to a value such that it represents the userÆs internal PrePaid
prepaid account in the PrePaid server. PPS. Note the PrePaid server could PPS may also 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 PPS MUST validate the Message-
Authenticator(80) as prescribed described in [RFC2869]. If validation fails,
the message is
invalid, the PrePaid server PPS MUST silently discard the message. If it received receives an
Authorize Only Access-Request message that does not contain a
PPAQ(TBD) it MUST silently discard the message.

The PrePaid server will lookup PPS locates the PrePaid prepaid session by state using the
PrePaid Quota Id
contained within the PPAQ(TBD). The PrePaid Server
would, take PPS takes the last most recently
allocated quota and subtract that subtracts it from the
UserÆs userÆs balance. If there is remaining balance,
sufficient balance remains, the PrePaid server
re-authorizes PPS authorizes the PrePaid session by allocate an PPS and allocates
additional quota. The PrePaid server PPS may want to also calculate a different new threshold
values as well. value.

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

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

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

Upon receiving an Access-Accept, the SAD SHALL update its quotas and
threshold parameters with the values contained in the PPAQ(TBD)
attribute. Note that the PrePaid server PPS MAY update the PrePaidServer
attribute(s) and these may have to be saved as well.

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

4.5

3.5
Dynamic Operations
The PrePaid server PPS may want to take advantage of the dynamic capabilities that are
supported by the SAD as advertised in the Dynamic-Capabilities
attribute during the initial Access-Request.

There are two types of actions action that the PrePaid server can perform: PPS may perform. Firstly, it can
may request that the session to be terminated; or terminated. Secondly, it can may request
that
the attributes associated with the session to be modified. More
specifically, it can may modify a previously sent PPAQ(TBD)

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

-MUST PPS MUST

- provide either the NAS-IP-Address(4) or the 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 prepaid data
session.

For a discussion on Dynamic Operations as they related Mutli-Service
operations see further on.

4.5.1

3.5.1
Unsolicited Session Termination Operation

At anytime during a session the Prepaid Server PPS may send a Disconnect Message in
order to terminate a session. This capability is described in
detail in [RFC3576]. The PrePaid server PPS sends a Disconnect Message that MUST
contain identifiers that uniquely identify the
subscriberÆs data session and the
SAD servicing that session.

If the SAD receives a Disconnect-Message, it will respond responds with either a
Disconnect-ACK packet if message (if it was is able to terminate the
session session) or else it will respond
with a Disconnect-NAK packet. packet (otherwise).

Upon successful termination of a session the SAD MUST return any
unused quota to the Prepaid Server PPS by issuing an Authorize Only Access-Request
containing the PPAQ which contains any unused Quota and the Update-Reason Update-
Reason set to ôRemote Forced Disconnectö.

4.5.2

3.5.2
Unsolicited Change of Authorization Operation

At anytime any time during the prepaid session the Prepaid Client PPC may receive a Change of
Authorization (CoA) message. A Prepaid Server PPS may send a new Quota to either
add additional quota or to remove quota
already that is allocated for to the service.

If the Change of Authorization contains a PPAQ then that PPAQ will
override
overrides a previously received PPAQ. The PPAQ may contain more
allocated Quota or less allocated quota. 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 currently already used then the SAD will accept accepts the new PPAQ and
act as it normally would when the quota is used up. For example, if
the threshold is reached then is request a quota update;
if the quota received is less then the currently used level then the
SAD would follow the normal procedures followed when a quota is used
up.

4.6 update.

3.6
Termination Operation

The termination phase is initiated when either: (i) the Subscriber subscriber logs
off; off,
(ii) the quotas have been consumed, subscriberÆs balances is exhausted, or (iii) when the SAD
receives a Disconnect Message.

In the case where the user logged off, or the SAD receives a
Disconnect Message, the SAD will send sends an Authorize-Only Access-
Request Access-Request
message with a PPAQ(TBD) and Update-Reason attribute set to either
ôClient Service terminationö or ôRemote Forced disconnectö
and disconnectö. This
message indicates the currently used already consumed quota.

In the case where the currently allocated quota has been reached, is exhausted, if the
PPAQ(TBD) contained Termination-Action field, the SAD will follow follows the
specified action which (which would be to immediately terminate the
service, to request
service), requests more quota, or to Redirect/Filter redirects/filters the service.

4.7

3.7
Mobile IP Operations

In roaming scenarios using with Mobile-IP, as the mobile subscriber roams
between networks, or between different types of networks such as
between WLAN and CDMA2000 networks, the PrePaid prepaid data session should
be maintained transparently if the HA is acting as the SAD.

As the subscriber device associates with the new SAD (AP or PDSN
that supports prepaid client PPC capability), the SAD sends a RADIUS Access-Request
and the subscriber is re-authenticated and reauthorized. The SAD
MUST include the PPAC(TBD) 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 SAD before handoff, the userÆs prepaid
session does not undergo any change after the handoff because the
Mobile IP session is anchored at the HA and the userÆs Home IP
address remains the same.

In the case of AP a wireless access point or PDSN acting as the SAD it
is likely that the userÆs IP address will change (Care of Address). Therefore, the
ongoing
The prepaid session will have some impact. be affected by this. In the case this scenario the
SAD shall send an Access-Request.
The Access-Request message which is routed to the home
network and MUST reach the PrePaid System prepaid system that is serving the PrePaid this
session. The
PrePaid prepaid system will then correlate correlates the new authorization
request with the existing active session and will assign assigns a quota to the
new request. Any outstanding quota at the old SAD MUST be returned
to the PrePaid system. If prepaid system if the Mobile-IP nodes (HA and FA) supports support
registration revocation (Mobile IPv4 only). Specifically, the quota
SHOULD be returned when the SAD sends the Authorize Only Access-
Request with PPAQ(TBD) Update-Reason 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 prepaid
system may issue a Disconnect Message [3576] to the SAD.

Even if the subscriber has roamed moves to an a SAD that does not have any
PrePaid Capabilities, PrePaid prepaid
capabilities can the prepaid data service may still continue. This can be possible done
by requesting the Home Agent (providing it (assuming that has PrePaid Capabilities) such capabilities)
to
assume take over the responsibilities for metering of the service. The procedure for
this SAD (i.e. metering). This
scenario will be given discussed in the next release a later version of this draft.

4.8 document.

3.8
Operation consideration considerations for Multi-Services Multiple prepaid services

This section describes the operation for supporting Prepaid support for
multi-services multiple prepaid services on the same SAD. The operations for multi-services
are very similar to operations for
a single service. SAD. Message flows illustrating the various interactions
are presented at the end of this document.

A SAD that supports prepaid operations for multi-services SHOULD set
the ôMulti-Services Supportedö 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(TBD) to
uniquely differentiate between the services. The exact definition
of the Service-Id attribute is out of outside the scope for 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 or a
Service-Id applies to the ôAccess Serviceö.

4.8.1

3.8.1
Initial Quota Request

When operations with multi-services is desired, the SAD will request requests the
initial quota for the Service by sending a PPAQ containing the
Service-Id for that Service in an Authorize-Only Access-Request
packet. Similarly, if the SAD supports Rating-Groups then it may
request a prepaid quota for the Rating-Group by sending a PPAQ containing
the Rating-Group-Id. In both cases the Update-Reason
will be is set to
ôInitial-Requestö.

The Authorize-Only Access-Request packet message may contain more than one
PPAQ. The Authorize-Only Access-Request MUST include includes one or more
attributes that serve to identify the session so that it can be
linked to the original authentication. Which Session Identifier(s)
is Identifiers
are included is up to specific deployments. The Authorize-Only
message must contain the Message-Authenticator(80) attribute for
integrity protection of the Authorize-Only Access-Request message.

Upon receiving an Authorize-Only Access-Accept message containing
one or more PPAQs PPAQs, the Prepaid System will allocate prepaid system allocates resources to each
PPAQ. The resources, can be in units of time, volume as before. Each PPAQ will be is assigned a unique QID that MUST appear in a
subsequent PPAQ update updates for that service or rating-group. As well,
Additionally, the PPAQ MUST contain the Service-ID; or Group-ID; Service-ID or neither, if Group-ID,
unless the PPAQ applies to the is a generic ôAccess Serviceö.

4.8.2

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). To replenish the quota the
Prepaid Client will send PPC
sends an Authorize-Only Access-Request message containing one or
more PPAQs. Each PPAQ MUST contain the appropriate QID, Service-ID
or Group-ID (or neither the Service-ID or Group-Id if the quota
replenishment is for the ôAccess Serviceö). The Update-Reason filed will indicate
indicates either ôThreshold reachedö(3), or ôQuota reachedö(4). The
Authorize-Only message must contain
identifiers to identify the session. session identifiers.

Upon receiving an Authorize-Only Access-Request packet with one or
more PPAQs the Prepaid Server will respond PPS responds with a new PPAQ for that service. The
PPAQ will contain contains a new QID, the Service-Id or Rating-
Group-Id, Rating-Group-Id, a new
Quota. If the Prepaid Server PPS does not want to grant additional quota to the Service service it
MUST include the Termination-
Action Termination-Action subfield in the PPAQ that will
instruct the SAD what to do with the service.

4.8.3

3.8.3
Termination

When an the allotted quota for the a service is used up exhausted the SAD shall act
in accordance to the Termination-Action field set in the Quota. If
the Termination-Action field is absent then the Service service MUST be
terminated.

If the Service service is to be terminated then the SAD shall send a PPAQ
with the appropriate QID, the Service-Id, the used quota, and
Update-Reason set to ôClient Service Terminationö.

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

Note when sending more then on PPAQ it may be required to send
multiple Authorize Only Access-Requests.

4.8.4

3.8.4
Dynamic Operations

Dynamic operations for multi-services are similar to dynamic
operations described for single service operations. The prepaid
system may send a COA message containing a PPAQ for an existing
service instance. The SAD will match matches the PPAQ to with the service using
the Service-ID attribute. The new quota could be higher then differ from the
last
previously allocated value or it could be lower. value. The SAD must react to the new quota value
accordingly.

A Disconnect message may not be send for a specific service. A disconnect message terminates the ôAccess Serviceö. As such the
SAD
must MUST report back all unused quotas by sending an Authorize Only
Access Request message containing a PPAQ for each active service.
The Update-Reason shall indicate that the reason for the update
reason.

4.8.5 update.

3.8.5
Support for Resource Pools

If the Prepaid Client PPC supports pools as indicated by setting the ôPools
supportedö bit in the PPAC(TBD) then the Prepaid Server PPS may associate a Quota
with a Pool by including the Pool-Id and the Pool-
Multiplier Pool-Multiplier in the
PPAQ(TBD).

When Resource Pools are used, the PPAQ must not use the threshold
field.

4.8.6

3.8.6
One-Time-Charging

To initiate a One-Time charge the PPC include includes the PPAQ attribute in
an Access-Request packet. The Access Request packet MUST include
the a
Message-Authenticator(80) and an Event-Timestamp(55) attributes. attribute.

The Service Id field of the PPAQ identifies the Service that is be
charged for. prepaid service.
The amount of to be charged is specified using the Resource Quota and
Resource Quota overflow subtypes. If the value specified is
negative then the resources will be are credited to the userÆs account.

The QID field MUST be set to a unique value and will be is used by the PPS
to detect duplicates should the packet be retransmitted. duplicates. The Update Reason field MUST be set to One-Time One-
Time Charging.

Upon receiving a PPAQ configured as a One-Time charge, charge PPAQ, the RADIUS server
authenticates the user and and, if authenticated, pass successful, passes the PPAQ to the
PPS. The PPS shall locate locates the subscriber account and debit debits or
credit the account credits it
accordingly. The PPS MUST repond respond to the PPS with an Access-Accept
message upon success. Or if successful, or an Access-Reject message
if it cant locate the userÆs account or if there is no balance
remaining in the account. otherwise.

The RADIUS server shall respond back to the SAD with an Access Accept
message. Since this is a one-time event charge the SAD must not allow the
session to continue. Therefore, the RADIUS server should include in
the Access-Accept a Session-Timeout set to 0. The Upon receiving an
Access-Accept response the SAD shall generate an Accounting Stop
message.

A PPAQ used for One-Time charging may appear in an Authorize-Only
Access Request. This is the case where a when the session already exists for
the user. exists.
The PPS shall respond back responds with an Access-Accept to indicate that the userÆs
account has been debited or an Access-
Reject indicating that the account could not be debited.

4.8.7 Access-Reject otherwise.

3.8.7
Error Handling

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

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

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

If the Prepaid Client PPC receives a PPAQ that contains a Pool-Id without a Pool-Multiplier; Pool-
Multiplier or a Pool-Multiplier without a Pool-Id it must ignore
that PPAQ.

4.9

3.9
Accounting Considerations

Accounting

Although typically generated, accounting messages are not required
to deliver PrePaid Data
Service. Accounting message will typically be generated for PrePaid
Data Service. This because a prepaid data service. When generated, accounting message
messages are used for auditing purposes as well as and for bill generation. billing.

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

4.10

3.10
SAD Operation

To be completed

4.11

3.11
Interoperability with Diameter Credit Control Application

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

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

4.
Attributes

This draft is using the RADIUS [RFC2865] namespace.

5.1

4.1
PPAC Attribute

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

TYPE : value of PPAC
LENGTH: 8
VALUE : String

The value MUST be encoded as follows:

Sub-Type

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

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

0x00000001 Volume metering supported.
0x00000002 Duration metering supported.
0x00000004 Resource metering supported.
0x00000008 Pools supported
0x00000010 Rating groups supported
0x00000020 Multi-Services supported.
0x00000040 Tariff Switch supported.

Others Reserved

5.2

4.2
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 (rfc3576) is
supported.

5.3

4.3
PPAQ Attribute

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

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

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

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

String: The String value MUST be encoded as follows:

Sub-Type

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

QuotaIDentifier (QID):

The QuotaIDentifier Sub-Type subtype is generated by the PrePaid server
at PPS together with
the allocation of a Volume and/or or Duration Quota. The on-line quota
update RADIUS Access-Request message sent from the SAD to the PPS
shall include a previously received QuotaIDentifier.

Sub-Type

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

VolumeQuota (VQ):

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

Sub-Type traffic.

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

VolumeQuotaOverflow (VQO):

The optional VolumeQuotaOverflow Sub-Type subtype 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

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

VolumeThreshold (VT):

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

Sub-Type

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

VolumeThresholdOverflow (VTO):

The optional VolumeThresholdOverflow Sub-Type subtype 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

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

DurationQuota (DQ):

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

Sub-Type

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

DurationThreshold (DT):

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

Sub-Type

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

Update-Reason attribute (UR):

The Update-Reason Sub-Type subtype shall be present in the on-line RADIUS
Access-Request message (SAD 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.

1. Pre-initialization
2. Initial Request
3. Threshold Reached
4. Quota Reached
5. Remote Forced Disconnect
6. Client Service Termination
7. ôAccess Serviceö Terminated
8. Service not established
9. One-Time Charging

Sub-Type

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

PrePaidServer:

The optional, multi-value PrePaidServer attribute indicates the
address of the serving PrePaid System. prepaid system. If present, the Home
RADIUS server uses this address to route the message to the
serving PrePaid
Server. PPS. 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.

Sub-Type
their order.

Subtype (=10) : Sub-Type Subtype for Service ID
Length : Length of Service ID

Service-Id:

Opaque string that uniquely describes a service instance for which
we want to apply which
prepaid metering to. should be applied. A Service-Id could be an IP
5-tuple (source address, source port, destination address,
destination port, protocol). If Service-ID is present in the PPAQ
the PPAQ applies refers to that Service. service. If a PPAQ does not contain a
Service-Id then the PPAQ applies refers to the Access Service.

Sub-Type

Subtype (=11) : Sub-Type Subtype for Rating-Group-Id
Length : 6

Rating-Group-Id

Identifies that this PPAQ is associated with resources allocated
to a Rating Group with the corresponding ID.

Sub-Type

Subtype (=12) : Sub-Type Subtype for Termination-Action
Length : 6

This field is an enumeration of the action to take when the prepaid
server PPS does
not grant additional quota. Valid actions are as follows:

0 Reserved
1 Terminate
2 Request More Quota
3 Redirect/Filter

Sub-Type

Subtype (=13) : Pool-Id
Length : 6

Identifies the Pool that this quota is to be associated with.

Sub-Type

Subtype (=14) : Pool-Multiplier
Length : 6

The pool-multiplier determines the weight that resources are
inserted into the pool and the rate at which resources are taken out
of the pool by this Service, service or Rating-Group.

Sub-Type (=13)

Subtype (=15) : Sub-Type Subtype for Resource Quota Resource-Quota
Length : 6

The optional ResourceQuota Sub-Type Resource-Quota subtype is only present if Resource
Based charging is used or when One-Time one-time charging is being used. In the RADIUS Access-Accept
message (PPS to SAD direction), direction) it indicates the Resources
allocated for the session by the PrePaid
server. PPS. In RADIUS Authorize Only
Access-Request message (SAD to PPS direction), it indicates the total
resources used resource for in total, including both
forward incoming and reverse traffic applicable to PrePaid accounting. outgoing
chargeable traffic. In one-time charging scenarios, the subtype
represents the number of units to charge the user or to credit the user (negative values).

Sub-Type (=14) user.

Subtype (=16) : Sub-Type Subtype for Resource Quota Overflow
Length : 6
Sub-Type (=15)

Subtype (=18) : Sub-Type Subtype for ResourceThreshold
Length : 6

NOTES:

Either

Volume-Quota, Time-Quota, or Resource-Quota MUST appear in the
attribute. If Volume Quota appears, Volume Threshold may only appear if Volume Quota appears also
appear.

A PPAQ MUST NOT CONTAIN contain both a Service-Id and a Rating-Group-Id.

A PPAQ that does not contain a Service-ID or a Rating-Group-Id
applies to the ôAccess Serviceö.

When the PPAQ contains a Pool-Id it MUST also contain the Pool-
Multiplier.

5.4

4.4
Prepaid Tariff Switching (PTS)

This specification defines the PTS attribute to allow for
changeovers from one service charge rate to another during service
execution. provision.

Support for tariff switching is OPTIONAL for both PPC and PPS. PPCs
use the flag "Tariff Switching supported" of the AvailableInClient
subtype of the PPAC attribute to indicate support for tariff
switching;
switching. PPSs employ the PTS attribute to announce their support
for tariff switching. Details of this issue are will be specified later on,
when after the
format of the PTS attribute has been defined.

If a RADIUS message contains a PTS attribute, it MUST also contain
at least one PPAQ attribute. This requirement is related to the
identification of the service to which tariff switching applies. If a RADIUS Access-Request message
contains a PTS attribute or if it
contains a "Tariff Switching supported" flag, it
MUST also contain an Event-Timestamp RADIUS attribute (see
[RFC2869]). This
requirement is related to the TariffSwitchInterval subtype of the
PTS attribute (see below).

Type : Value of PTS
Length: variable, greater than at least 8
Subtype (=1): QuotaIDentifier (QID)
Length : Length of QuotaIDentifier Subtype (= 6 octets)

The QID subtype MUST be present in each PTS attribute. In an
online RADIUS Access-Request message sent from the PPC to the
PPS, its value MUST be a quota identifier received previously
from the PPS and MUST be the same as a quota identifier of one of
the PPAQ attributes included the same RADIUS message.

A PPAQ attribute that is transported along with a PTS attribute
and has the same quota identifier value as the PTS attribute in
its own QID subfield shall be referred to as "accompanying PPAQ
attribute". If a PPS receives an Access-Request message from a
PPC, it associates a unique quota identifier to this service
access request.
Thus, a quota identifier also identifies a particular service.

Subtype (=2): VolumeUsedAfterTariffSwitch (VUATS)
Length : Length of VolumeUsedAfterTariffSwitch Subtype
(= 6 octets)

The VolumeUsedAfterTariffSwitch subtype SHALL be used in online
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 subfield
and the accompanying PPAQ attribute (see the remarks under
"Subtype 1: QID").

Subtype (=3): VUATSOverflow (VUATSO)
Length : Length of VUATSOverflow Subtype (= 4 octets)

If an online RADIUS Access-Request message contains a VUATS
subfield and if the VolumeUsedAfterTariffSwitch has wrapped
around 2^32 over the course of provisioning the service
identified via the QID subfield, then the VUATSO subfield MUST be
present in the PTS attribute. In this case, it indicates how
many times the VolumeUsedAfterTariffSwitch has wrapped around
2^32. In all other cases, the VUATSO subfield MUST NOT be
present in the PTS attribute.

Subtype (=4): TariffSwitchInterval (TSI)
Length : Length of TSI Subtype (= 6 octets)

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

Subtype (=5): TimeIntervalafterTariffSwitchUpdate (TITSU)
Length : Length of TITSU Subtype
(= 6 octets)

The PPS MUST include the TITSU subtype if there is another tarrif tariff
switch period after this period. The TITSU represents attributes encodes
the length number remaining seconds of
the current tariff switch period in seconds. period. If this
attribute is
omitted zero or omitted, it is zero, assumes that the current
tariff period that commences in TSI
seconds will last indefinitely or lasts until a new PTS is received
with new information. further notice. If TITSU is specified,
the prepaid client PPC must send a quota update before the end of the tariff switch current period as specified by TITSU. ends.

If a RADIUS message contains a PTS attribute, it MUST also contain
at least one PPAQ attribute. The PTS will be is associated to with the PPAQ by
the QID. If multiple services are supported and if the PPAQ is
associated with a service as indicated by the Service-Id sub-
atrribute of the PPAQ, then the PTS will specify refers to the tarrif tariff switch for
that service. If the PPAQ does not have a Service-Id, then the PTS will be the tarrif
refers to tariff switch of for the Access-Service.

If a PPC supports tariff switching then it MUST set the 0x00000040
(Tariff switching supported) flag of the AvailableInClient subtype
of the PPAC attribute that is contained in the Access-Request packet
starting the prepaid session.

5.5

4.5
Table of Attributes

TO BE COMPLETED.

Request Accept Reject Challenge # Attribute

Authorize_Only Request Accept Reject

5.
Security Considerations

The extended RADIUS protocol exchanges described are susceptible in this document is subject
to a number of potential attacks, in a manner similar to the same
vulnerabilities as RADIUS and it
without these extensions. It is recommended that IPsec be employed
to afford better security. protect against certain of the attacks.

If IPsec is not available available, usage of the protocol extensions described in this draft improves
document improve the overall security of RADIUS. The various
security enhancements are explained in the following sections.

6.1

5.1
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

5.2
Replenishing Procedure

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

To be completed.

6.
IANA Considerations

This document requires the assignment of new Radius attributes type
numbers for the following attributes:

1) Prepaid-Accounting-Capability (PPAC)
with subtype:
AvailableInClient

2) Prepaid-Accounting-Operation (PPAQ)
with subtypes:
QuotaID (QID)
VolumeQuota (VQ)
VolumeQuotaOverflow (VQO)
VolumeTreshold (VT)
VolumeTresholdOverflow (VTO)
DurationQuota (DQ)
DurationTreshold (DT)
UpdateReason (UR)
PrePaidServer (PPS)
ServiceID (SID)
RatingGroupId (RGID)
TerminationAction (TA)
PoolID (PID)
PoolMultiplier (PM)
Cost (COST)
TariffChangeTime (TCT)

3) Prepaid-Tariff-Switch (PTS)

4) Session-Termination-Capability (STC)

5) International-Mobile-Subscriber-Identity (IMSI)

7.
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,
"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.

[RFC3748] Aboba, B., et al., "Extensible Authentication
Protocol", RFC 3748, June 2004.

8.
Informative References

[DIAMETERCC] Hakkala, H., et al., "Diamter Credit-Control
Application", Internet Draft, AAA WG, April 2004,
Work in Progress.

[REDIRECT] "RADIUS Redirection", Internet Draft, Work in
progress.

10.

9.
Call Flows

This section includes call describes the flows illustrating associated with various scenarios
enabled by
that are mentioned in this specification. document. The following fields are used
in the call flows:

RADIUS packets:

AR Access Request
ARA Access Accept
AC Accounting Requests
A Authorize-Only Access-Request
AA Access-Accept for Authorize-
Only Access-Request

RADIUS Attributes:

PPAQ PPAQ as defined in this
specification
SID One or more attributes
representing the Session that
the RADIUS packets is correlated
to.
PPAC PPAC as defined in this
specification
ASID Acct-Session-Id as defined by
RADIUS
MSID Acct-Multi-Session-Id as define
by RADIUS

PPAQ fields:

SRVID Service-Id
Reason Update-Reason
QID Quota-Id

10.1

9.1
Simple Concurrent Services

In this scenario the Prepaid Client PPC authenticates and authorizes the user. The Prepaid Server
PSS responds back with Prepaid Quota for the ôAccess Serviceö instance. The NAS
then request quota for Service-
A. Service-A.

Accounting is turned on.

NAS/ RADIUS/
PPC PPS
=== ===
| |
| AR{SID,PPAC} |
A |-------------------------------------------------->|
| |
| ARA{SID,PPAQ(QID=1,Q=100)} |
B |<--------------------------------------------------|
| |
| AC(start){ASID=25,MSID=13} |
C |-------------------------------------------------->|
| |
| A{SID,PPAQ(SRVID=SA, Reason=Initial} |
D |-------------------------------------------------->|
| |
| AA{SID,PPAQ(QID=200,SRVID=SA, Q=50)} |
E |<--------------------------------------------------|
| |
| AC(start){ASID=30,MSID=13, PPAQ } |
F |-------------------------------------------------->|
| |
| A{SID, PPAQ(QID=200 SRVID=SA, Q=50 Reason=Quota)}|
G |-------------------------------------------------->|
| |
| AA{SID,PPAQ(QID=300,SRVID=SA, Q=100)} |
H |<--------------------------------------------------|
| |
| A{SID, |
| PPAQ(QID=1, Q=100 Reason=Quota), |
| PPAQ(QID=300, SRVID=SA Q=100 Reason=Quota)} |
I |-------------------------------------------------->|
| |
| AA{SID,
| PPAQ(QID=3, Q=200), |
| PPAQ(QID=303, SRVID=SA Q=150)} |
J |<--------------------------------------------------|

A This is the initial Access-Request that indicates the Prepaid
Capabilities prepaid
capabilities of the NAS. In this scenario it will indicate example indicates that
Concurrent Session Sessions are supported. Access-Request also
includes SID (Session Id) which is the Session Identifier
assigned by this NAS to session. Session Identifier is out The formal of the session
identifier is outside the scope in of this document. It can be a single attribute such as
3GPP2 Correlation ID or it could be a set of attributes that
define a session.
B RADIUS authenticates the user and determines that the user is
prepaid. he has a
prepaid account. RADIUS responds with a PPAQ for the ôAccess
Serviceö (PPAQ does not contain a Service-ID or Rating-Group-ID). Rating-Group-
ID). The PPAQ has a QID=1 assigned by the Prepaid System and
Quota of Q=100. The quota could be time or volume and may or
may not have a threshold associated with it.
C The NAS starts the Access Service and generates an Accounting-
Request (Start) message as normal. It will include includes the Acct-
Session-Id and may include the Acct-Multi-Session-Id.
D The NAS wants is about to start a new Service, call it Service-A.
It sends an Authorize-Only access request to RADIUS. The SID
links this Authorize-Only access request to the initial
Authentication & Authorization (Step-A and Step-B).The
Authorize-Only message contains a PPAQ requesting quota for
Service-A, Update-Reason = Initial-Request.

E The PPS checks the resources available to the user and assigns
50 units (time/volume etc) to this service. RADIUS sends an
Access Accept message contain a PPAQ assigning quota Q=50 for
Service-A. The PPAQ contains a QID = 200.
F The NAS starts Service-A and sends an Accounting-Request
(Start) message for that service. Acct-Multi-Session-Id can be
used to tie all of the sessions in the accounting streams
together.
G Quota for Service-A requires refreshing, the quota was
completely used). An Authorize-Only message is sent
containing a PPAQ with QID = 200 which corresponds to the
prior QID received for this service. Note QID is sufficient
for the PPS server to link this request to the previous
request and hence to the original authentication steps.
Therefore SID is not really required. The PPAQ will report the
used part of the quota (50 units).
H RADIUS deducts the used quota from the users accounts and
reserves 50 more additional units for a total quota of 100
(Q=100) for Service-A. It sends back a PPAQ with QID=300.
I NAS needs to refresh both the ôAccess Serviceö and Service-A.
It sends an Authorize Only message contain two PPAQs, one for
the Main Service with QID=1 and one for Service-A with
QID=300. Each PPAQ reports the used resources that were consumed
so far and the reason why the update is being sent.
J RADIUS responds back with two PPAQs. The PPAQ without the
Service-Id grants an additional 100 units for a total of 200
units to the ôAccess Serviceö û QID=3; the other PPAQ,
containing SRVID=SA grants an additional 50 units for a total
quota to service-a of 150 units û QID=303.

This step illustrates why SRVID needs to be specified in the
PPAQ. If Without it were not, then the NAS would not be able unable to differentiate
between the PPAQs. QIDs are not sufficient to correlate the
PPAQ to a service since they are may be changed (and
not necessarily sequentially) by the PPS at
every transaction.

In this scenario, notice

Note how each PPAQ attribute represents a sequential conversation
about a service between the Prepaid Client PPC and the Prepaid Server. PPS in this example. The
links between the messages are the QIDs and the Service-Ids.

As well, notice how

Also note that a SID is needed to tie the Authorize-Only messages to
the Authentication steps. This SID is only really needed the first
time a PPAQ is sent û since the PPAQ does not have
a QID.

Accounting sent.

Although accounting messages have an Accounting-Session-ID. But Accounting-Session-ID, that is
not enough to allow enable the back end system to associate that
accounting message with a particular Service. We therefore need the
PPAQ in the accounting message.

10.2

9.2
One-time Charging

In this One-time charging scenario, example, the Prepaid Client (PPC) PPC authenticates and
authorizes the user and requests charging for a service event
requested by the user. The PPC already knows the price to charge
for the service event identified by SRVID=SA.

Contributor

We would like to thank Hannes Tschofenig for his contributions to
this draft.

Acknowledgments

The authors would like to thank Mark Grayson (Cisco), Nagi Jonnala
and Tseno Tsenov 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 Yong Li Hannes Tschofenig
Starent Networks Bridgewater Systems Siemens
30 International Place, 3rd Flr 303 Terry Fox Drive Otto-Hahn-Ring 6
Tewksbury, MA 01876 Suite 100 81739 Munich
kchowdhury@starentnetworks.com Ottawa Ontario
Canada
Yong.li@bridgewatersystems.com Germany
hannes.tschofenig@siemens.com

Christian Guenther
Siemens
Otto-Hahn-Ring 6
81739 Munich
Germany
christian.guenther@siemens.com

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This document and the information contained herein are provided on
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Expiration Date

This memo is filed as draft-lior-radius-extensions-for-prepaid-
07.txt,
06.txt, and will expire 20 July, 2005.

10.
Appendix A û use cases

In this appendix we present a set of use cases and scenarios based
on which the extensions in this document were designed. It is
assumed that the subscriber possesses a valid prepaid account with a
service provider, for example a WLAN operator.

In order to maintain generality, the use cases refer to the
communications between the SAD and the network. The connection
between the UserÆs Device and the SAD, which typically involves
setting up a layer 2 session, e.g. a PPP session or a GPRS PDP
Context, is specific to a given network technology and the details
do not affect the operation of the prepaid service.

10.1
Simple prepaid use case

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

The SAD sends a RADIUS Access-Request to the AAA server in order to
authenticate and authorise the subscriber with respect to the
requested service. The Access-Request contains the subscriber
credentials and may contain the prepaid capabilities of the SAD.
Prepaid capabilities MUST be included if the SAD supports them.

The AAA System proceeds with the authentication procedure. This may
involve several message exchanges such as in EAP [RFC2284]. Once
the subscriber has been authenticated, the AAA system determines
that the subscriber is a prepaid subscriber and requests
authorisation. The request MUST include the prepaid capabilities of
the serving SAD.

The system validates that the subscriber has a prepaid account and
that the account is active. It further validates that the SAD has
the appropriate prepaid capabilities. If all is in order, the
prepaid system authorises the subscriber to use the network.
Otherwise it rejects the request. The decision is sent to the AAA
system. The response includes attributes to indicate the allocation
of a portion of the subscriberÆs credit. This portion is called the
ôinitial quotaö (in units of time or volume) and optionally a
threshold value.

A portion only of the userÆs 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.

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

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

Once the usage for the session approaches the allocated limit (as
expressed by the threshold), the SAD will request additional quota.
Re-authorization for additional quota flows through the AAA system
to the prepaid System. The prepaid System revalidates the
subscriberÆs 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 prepaid System rejects the request. Note the
replenishing of the quotas is a re-authorization procedure and does
not require the subscriber to authenticate himself again.

It is important to note that the prepaid System is maintaining
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) prepaid
system.

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

Should the subscriber terminate the session before the quota is
exhausted, the remaining balance allotted to the session MUST be
refunded into the subscriberÆs account.

While the Access Device is waiting for the initial quota, the
subscriber may have dropped the connection/session. The entire
allocated quota MUST be credited back to the subscribers account in
this case.

10.2
Support for Multi-Services

Examples of services that the user may be using are browsing the
web, participating in a VoIP conversation, watching streaming video
and downloading a file. Some operators may want to distinguish
between these services. Some services are billed at different rates
and services may be metered differently. Therefore, the prepaid
solution needs to be able to distinguish services, and allocate
quotas to the services using different units (e.g. time, volume) and
allow for those quotas to be utilized at different rates.

+---------+
| Session |
+---------+
|
V N
+--------------+ +-------+
| Service |------>| Quota |
| (service-Id) | +-------+
+--------------+

As shown in the above diagram, a Session may be associated with
multiple (N) services. Each service is identified by a Service-ID.
The format of the Service-ID is not in the scope of this document
but the Service-ID could be expressed as an IP flow using the 5-
tuple {Source-IP and Port, Destination-IP and Port, protocol type}.
Each service is allocated an appropriate quota metric.

10.3
Resource Pools

When working with multiple services a new problem arises because one
service may utilize its quota faster than another service. When the
userÆs 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 that service. Indeed, even before
that happens, the services could generate an excessive amount of
traffic as the they update their quotas.

One method to solve these problems is to utilize resource pools.
Resource pools enable the allocation of resources to several
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 allocated to the pool is close to
exhaustion, the entire pool is replenished.

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

As the figure above shows, 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 each
service and multiplying it by Mn. Therefore, the amount of
resources allocated to a pool is given by:

Poolr = Ma*Qa + Mb*Qb + . . .

A Pool is empty if:

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

where:
Ca,Cb are the consumed resources of 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 Mbyte and
Service-B can rated at $0.10 per Minute. In this case if we allocate
$5 worth of resources on behalf of service-A to the pool we would
set Ma = 10 and place 50 units into the pool. If we allocate $5 on
behalf of Service-B to the Pool, then M=1 and place 50 units into
the Pool. The pool would have a total sum of 100 units to be shared
between the two services. 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.

10.4
Support for Complex Rating Functions

The rating of a service can be quite complex. While some operators
follow linear charging 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 Mbyte and volume above M bytes be rated
at $0.50 per Mbyte. Such a function could be implemented by repeated
message exchanges with the prepaid system.

To avert the need to exchange many messages while still supporting
such complex rating functions the notion of a ôRating Groupö is
introduced. A Rating Group is provisioned at the SAD. As
illustrated in the figure below, a Rating Group is associated with
one or more services and defines the rate that the services
associated with the Rating Group consume the quota.

During consumption of a service that is associated with a Rating
Group, the PPC sends the ID of the Rating Group to the PPS. The
prepaid service authorizes the Rating Group by allocating a quota to
it and optionally assigning it to a Resource Pool.

When service that belongs to an authorized Rating Group is
instantiated, the PPC does not need to authorize this service. This
limits the amount of traffic between the PPC and the PPS.

10.5
One-Time-based Charging

One-Time-based Charging is used for charging of service events
without an ongoing session. That is, the service is provisioned
instantaneously, as far as charging is concerned. 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-based charging may occur in parallel with
other charging models. For example, the subscriber may access a
website which is metered (based on time or volume) while he also
purchase the right to use a ring tone (a one-time-based event).
Note: it is up to the service providers to decide whether or not the
user will be charged for the download of the tone and also be
charged for the time and volume required to download the ring-tone.
The facilities provided by this document gives the service provider
the capability to achieve their service charging business goals.
For example, should the service provider choose not to charge for
the download volume or time, then they can treat the download IP
flow as a separate service that is exempt from charging.

The SAD signals one-time-based charging to the PPS with an
indication that identifies the service and the units that need to be
debited from the userÆs account.

One-time-based charging may occur under two conditions: the (a) SAD
may not have a authenticated context (or access to an authenticated
context) for the subscriber), or (b) the SAD has access to
authenticated context for the subscriber. In the former case the
SAD will have to authenticate the subscriber. For example, the user
maybe authenticated by the SAD providing access service. However
when the user accesses the subscription server to purchase a
subscription, the subscription server may not have access to the
authentication context of the subscriber and thus will have to
authenticate the subscriber from scratch. Authentication of the
subscriber and the generation of the one-time charging event will
happen in conjunction.

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

10.6
Support for Tariff Switching

The PPC and the PPS may support tariff switching as described
earlier. For example, as shown in the figure below, traffic before
18:00 may be rated at ær1Æ and traffic after 18:00 hours is rated at
ær2Æ. The PPC reports usage before and after the switch occurs.
Tariff switching only makes sense for volume based metering where
the volume is billed at different rates.

18:00
------------------+-----------------
r1 | r2
------------------+-----------------
^ ^
|<----TSI---> |
| |
Access-Accept Access-Request

The PPC it 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 which indicates the point
in time when the switch will occur. This indication represents the
number of seconds from current time (TarrifSwitchInterval 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 using the PPAQ in total and how much volume was used after the
tariff switch using the PTSÆs VUATS subtype.

If the PPC sends this message before the tariff switch, the PPS will
respond with another PTS where the TSI is appropriately updated.

In situations with multiple tariff switches, as shown below, the PPS
MUST specify the length of the tariff switch period using the
TimeIntervalAfterTariffSwitchUpdate (TITSU) in the PTS attribute.

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,
insufficient traffic is generated and thus the threshold is not
reached. Even in this case PPC MUST generate an Access-Request in
good time. Also note that separate services flows may have
individual tariff periods.

10.7
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.

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 prepaid system where state is being maintained.

Dynamic roaming is challenging because a subscriber who established
a prepaid data session may move to another Access Device that does
not support the prepaid functionality. Even in this case the system
should be able to continue the prepaid session.

10.8
Termination of a prepaid session

When fraud or an error is detected, the either only the affected
session, or all sessions of the affected subscriber should be
terminated.

It may happen that the prepaid system enters a state where it is
unclear whether or not the data session is in progress. Under such a
condition, the system may wish to terminate the session in order to
make sure that the user is not billed for this potential inactivity.

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

10.9
Querying and Rebalancing Prepaid Resources

It should be possible for the PPS to Query the current resource
consumption at a SAD and adjust the userÆs account balance.

For example, a request to the PPS is made (e.g. a one-time charging
event) but the userÆs account is depleted but resources have been
allocated to the SAD. The PPS should have the ability to query the
SAD and if it has the spare resources to reassign the quotas to the
SAD and to the pending request. Note that the PPS doesnÆt know
resource usage until the SAD 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 û a practice that
results in more message exchanges.