RADEXT A. Lior
Internet-Draft Bridgewater Systems
Expires: December 25, 2006 P. Yegani
Cisco
K. Chowdhury
Starent Networks
H. Tschofenig
A. Pashalidis
Siemens
June 23, 2006
Prepaid extensions to Remote Authentication Dial-In User Service
(RADIUS)
draft-lior-radius-prepaid-extensions-11.txt
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Abstract
This document specifies an extension to the Remote Authentication
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Dial-In User Service (RADIUS) protocol that enables service providers
to charge for prepaid services. The supported charging models
supported are volume-based, duration-based, and based on one-time
events.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.1. Architectural Model . . . . . . . . . . . . . . . . . 7
1.2.2. Motivation . . . . . . . . . . . . . . . . . . . . . . 10
1.3. A simple use case . . . . . . . . . . . . . . . . . . . . 12
2. Supported Features . . . . . . . . . . . . . . . . . . . . . . 15
2.1. Multiple Concurrent Services . . . . . . . . . . . . . . . 15
2.2. Resource Pools . . . . . . . . . . . . . . . . . . . . . . 15
2.3. Complex Rating Functions . . . . . . . . . . . . . . . . . 17
2.4. One-time Charging . . . . . . . . . . . . . . . . . . . . 17
2.5. Tariff Switching . . . . . . . . . . . . . . . . . . . . . 18
2.6. Support for Roaming . . . . . . . . . . . . . . . . . . . 20
2.7. Dynamic Termination . . . . . . . . . . . . . . . . . . . 20
2.8. Querying and Rebalancing . . . . . . . . . . . . . . . . . 20
3. Operations . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.1. Authentication and Authorization Operation . . . . . . . . 22
3.2. Session Start Operation . . . . . . . . . . . . . . . . . 24
3.3. Mid-Session Operation . . . . . . . . . . . . . . . . . . 24
3.4. Dynamic Operations . . . . . . . . . . . . . . . . . . . . 26
3.4.1. Unsolicited Session Termination Operation . . . . . . 26
3.4.2. Unsolicited Change of Authorization Operation . . . . 27
3.5. Termination Operation . . . . . . . . . . . . . . . . . . 27
3.6. Mobile IP Operations . . . . . . . . . . . . . . . . . . . 27
3.7. Operation Considerations for Multiple Services . . . . . . 28
3.7.1. Initial Quota Request . . . . . . . . . . . . . . . . 29
3.7.2. Quota Update . . . . . . . . . . . . . . . . . . . . . 29
3.7.3. Termination . . . . . . . . . . . . . . . . . . . . . 30
3.7.4. Dynamic Operations . . . . . . . . . . . . . . . . . . 30
3.7.5. Support for Resource Pools . . . . . . . . . . . . . . 30
3.7.6. One-time Charging . . . . . . . . . . . . . . . . . . 30
3.7.7. Error Handling . . . . . . . . . . . . . . . . . . . . 31
3.7.8. Accounting Considerations . . . . . . . . . . . . . . 31
3.7.9. Interoperability with Diameter Credit Control
Application . . . . . . . . . . . . . . . . . . . . . 31
4. Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.1. PPAC Attribute . . . . . . . . . . . . . . . . . . . . . . 33
4.2. Session Termination Attribute . . . . . . . . . . . . . . 34
4.3. PPAQ Attribute . . . . . . . . . . . . . . . . . . . . . . 35
4.3.1. Quota Identifier AVP . . . . . . . . . . . . . . . . . 35
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4.3.2. VolumeQuota AVP . . . . . . . . . . . . . . . . . . . 36
4.3.3. VolumeThreshold AVP . . . . . . . . . . . . . . . . . 36
4.3.4. DurationQuota AVP . . . . . . . . . . . . . . . . . . 36
4.3.5. DurationThreshold AVP . . . . . . . . . . . . . . . . 36
4.3.6. ResourceQuota AVP . . . . . . . . . . . . . . . . . . 36
4.3.7. ResourceThreshold AVP . . . . . . . . . . . . . . . . 37
4.3.8. Value-Digits AVP . . . . . . . . . . . . . . . . . . . 37
4.3.9. Exponent AVP . . . . . . . . . . . . . . . . . . . . . 37
4.3.10. Update-Reason AVP . . . . . . . . . . . . . . . . . . 37
4.3.11. PrepaidServer AVP . . . . . . . . . . . . . . . . . . 38
4.3.12. Service-ID AVP . . . . . . . . . . . . . . . . . . . . 38
4.3.13. Rating-Group-ID AVP . . . . . . . . . . . . . . . . . 39
4.3.14. Termination-Action AVP . . . . . . . . . . . . . . . . 39
4.3.15. Pool-ID AVP . . . . . . . . . . . . . . . . . . . . . 39
4.3.16. Pool-Multiplier AVP . . . . . . . . . . . . . . . . . 39
4.3.17. Requested-Action AVP . . . . . . . . . . . . . . . . . 39
4.3.18. Check-Balance-Result AVP . . . . . . . . . . . . . . . 40
4.3.19. Cost-Information AVP . . . . . . . . . . . . . . . . . 40
4.4. Prepaid Tariff Switching Attribute (PTS) . . . . . . . . . 41
4.4.1. VolumeUsedAfterTariffSwitch AVP . . . . . . . . . . . 42
4.4.2. TariffSwitchInterval AVP . . . . . . . . . . . . . . . 42
4.4.3. TimeIntervalafterTariffSwitchUpdate AVP . . . . . . . 42
5. Translation between RADIUS prepaid and Diameter Credit
Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
5.1. Session Identification . . . . . . . . . . . . . . . . . . 45
5.2. Translation between RADIUS prepaid client and Diameter
Credit Control AAA infrastructure . . . . . . . . . . . . 45
5.2.1. PPAC (c<->s) . . . . . . . . . . . . . . . . . . . . . 45
5.2.2. Service Termination Attribute (c->s) . . . . . . . . . 46
5.2.3. Quota Identifier Attribute (c<->s) . . . . . . . . . . 46
5.2.4. Volume Quota Attribute (c<->s) . . . . . . . . . . . . 46
5.2.5. Duration Quota Attribute (c<->s) . . . . . . . . . . . 47
5.2.6. Resource Quota Attribute (c<->s) . . . . . . . . . . . 47
5.2.7. Value Digits Attribute (c<->s) . . . . . . . . . . . . 48
5.2.8. Exponent Attribute (c<->s) . . . . . . . . . . . . . . 48
5.2.9. Volume/Duration/Resource Threshold Attributes
(s->c) . . . . . . . . . . . . . . . . . . . . . . . . 48
5.2.10. Update Reason Attribute (c->s) . . . . . . . . . . . . 48
5.2.11. PrepaidServer Attribute (s<->c) . . . . . . . . . . . 50
5.2.12. Service-ID Attribute (s<->c) . . . . . . . . . . . . . 50
5.2.13. Rating-Group-ID Attribute (s<->c) . . . . . . . . . . 50
5.2.14. Termination-Action Attribute (s->c) . . . . . . . . . 50
5.2.15. Pool-ID Attribute (s<->c) . . . . . . . . . . . . . . 51
5.2.16. Multiplier Attribute (s<->c) . . . . . . . . . . . . . 51
5.2.17. Requested-Action Attribute (c->s) . . . . . . . . . . 51
5.2.18. Check-Balance-Result Attribute (s->c) . . . . . . . . 51
5.2.19. Cost-Information Attribute (s->c) . . . . . . . . . . 52
5.2.20. VolumeUsedAfterTariffSwitch attribute (c->s) . . . . . 52
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6. Security Considerations . . . . . . . . . . . . . . . . . . . 53
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 54
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 55
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 56
9.1. Normative References . . . . . . . . . . . . . . . . . . . 56
9.2. Informative References . . . . . . . . . . . . . . . . . . 56
Appendix A. Example flows . . . . . . . . . . . . . . . . . . . . 57
A.1. A simple flow . . . . . . . . . . . . . . . . . . . . . . 57
A.2. A flow with prepaid tariff switching . . . . . . . . . . . 60
A.3. Resource pools and Rating Groups . . . . . . . . . . . . . 64
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 71
Intellectual Property and Copyright Statements . . . . . . . . . . 72
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1. Introduction
This document specifies an extension to the RADIUS protocol that
enables service providers to perform accounting and charging in an
"online" fashion. In particular, they enable the service provider to
(a) ensure that subscriber's remaining funds suffice before the
service is delivered, and (b) interrupt service provision when the
funds are exhausted. Note that these capabilities are typically used
in scenarios where the subscriber maintains a prepaid account with
the service provider; hence, this extension is called the "prepaid"
extension for RADIUS. Also note that the above capabilities are not
provided by the base RADIUS protocol.
It has been observed that subscribers prefer prepaid accounts to
postpaid ones in many circumstances. Indeed, it is expected that
offering a "prepaid mode of operation" will enabe service providers
to expand their existing customer bases. This is the main business
driver behind the extensions defined in this document. The
extensions were designed with the following goals in mind.
o Make use of existing infrastructure as much as possible (including
enabling the interworking of RADIUS-based and Diameter-based
infrastructures), and thereby limit the amount of necessary
capital expenditures,
o provide the ability to rate service requests in an "online"
fashion,
o provide the ability to charge the user's account prior to service
provision,
o protect against revenue loss, i.e. to prevent an end user from
obtaining service when the available funds do not suffice,
o protect against fraud, and
o be deployable over dialup, wired and wireless networks.
The architecture between the entities that execute the RADIUS
protocols, with the extensions defined in this document, assumes that
the rating of chargeable events does not occur in the element that
provides the service. Instead, the rating may be performed at a
dedicated server, termed the "prepaid-enabled AAA server" or simply
"prepaid server". Alternatively, the actual rating may occur in an
entity behind this prepaid server. Furthermore, business logic may
dictate a time-dependent tariff model, for example that the price for
a service may switch at 8pm from a high to a low tariff. The
extensions defined in this document support such scenarios.
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Furthermore, this document assumes an architecture where a "quota
server" is available which, through co-ordination with the rating
entity and a centralized account balance manager, is able to provide
a quota indication for a particular user when requested. This quota
server may or may not coexist in the prepaid server.
1.1. Terminology
o Network Access Server (NAS): As defined in RADIUS.
o Prepaid client (PPC): The entity which triggers the RADIUS message
exchange, including the prepaid extensions defined in this
document. The PPC typically resides in the NAS.
o Prepaid Server (PPS): The entity that interacts with the PPC using
the RADIUS prepaid extensions defined in this document.
o Home Network: The network which contains the user profile and the
user's prepaid account.
o Authorize-Only Access Request: A RADIUS message of type "Access
Request" (code field=1) that contains a "Service-Type" AVP
(type=6) with value "Authorize-Only".
1.2. Overview
This section provides an overview of the prepaid charging models and
architectures, which are supported by the extensions described in
this document.
A number of models of how to charge customers for data services in a
prepaid manner are supported, as follows.
o Volume-based charging (e.g. 2 Cents/KiloByte).
o Duration-based charging (e.g. 3 Cents/minute).
o Subscription-based charging (e.g. 10 Dollars/month).
o Event-based charging (e.g. 7 Cents/URL or email) .
This draft assumes that the user maintains a prepaid account with his
home network. This account may be used to fund multiple services,
some of which may use the extensions defined in this document, and
some may use other mechanisms. The interworking of these mechanisms
is outside the scope of this document. Similarly, the means by which
the subscriber obtains funds is also outside the scope of this
document.
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1.2.1. Architectural Model
The protocol extensions described in this draft assumes that the
following entities are present in the network architecture.
o Service Access Device (SAD): This entity provides a data service
to the users, and typically coincides with the NAS. The SAD
executes the RADIUS client which, for the purposes of this
document, is termed the "PrePaid Client" (PPC). When the prepaid
service is used the SAD collects service event information and
reports it while services are provided to the user. This event
information is sent to the PPS using the extensions defined in
this document.
o The PPS: The RADUIS server that supports the prepaid extensions
defined in this document. If real-time credit control is
required, the PPC (SAD) contacts the PPS with service event
information included before the service is provided. The PPS
performs a credit check and allocates a portion of the available
credit to the service event.
o The rating entity: This entity converts the credit that is
allocated by the PPS into a time or volume amount, called the
"quota". This quota is then returned to the requesting PPC (SAD)
(via the PPS). The rating entity may also determine that during
service provision a tariff switch will occur. In this case the
rating entity will include details of when exactly tariff switch
will occur.
The requesting PPC (SAD) meters the consumption of the service
according to the instructions provided by the PPS. After service
completion, or on reception of a subsequent request for service, the
PPS deducts the corresponding amount of credit from the user account.
When a user terminates an on-going service, the PPC informs the PPS
with a suitable indication about the unused portion of the allocated
quota. The PPS then refunds the user account accordingly. Note that
multiple PPSs may be deployed for reasons of redundancy and load
sharing. The system MAY also employ multiple rating servers.
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accounting
+------------+ +-----------+ protocol +--------------+
| User |<---------->| Service | | |
| | IEEE 802.1x| Access |<------------>| Accounting |
| Device | PANA | Device |<-----+ | Server |
+------------+ IKEv2 +-----------+ | +--------------+
... etc (PPC) |
|
| +--------------+
+------>| Prepaid |
prepaid | Server |
protocol +--------------+
Figure 1: Basic prepaid architecture
The PPS and the accounting server in this architecture MAY be
combined. The SAD must have the ability to meter the consumption of
a prepaid data session. This metering is typically based on time
(i.e. seconds) or volume (i.e. octets).
The device running the PPC may also have "Dynamic Session
Capabilities" such as the ability to terminate a data session or
change the filters associated with a specific data session by
processing "Disconnect" messages and "Change of Authorization"
messages as per RFC 3576.
This document assumes that the PPS is used as the AAA server. There
are three types of AAA server, as follows.
o The AAA server in the home network (HAAA), which is responsible
for authentication of the subscriber. In addition, the HAAA
communicates with the PPS using the RADIUS protocol in order to
authorize subscribers.
o The AAA server in the visited 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.
Note that, in certain roaming deployments, the visited network may
be connected to the home network via one or more broker networks.
o The AAA server in one of the aforementioned broker networks
(BAAA), which is responsible for forwarding messages and does not
play an active role in the prepaid data service delivery. 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 PPS communicates with the HAAA for the
purposes of authentication and authorisation. The PPS, in turn,
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interfaces to entities which
o keep the subscriber's account balance (balance manager),
o rate access service requests in real-time (Rating Engine), and
o manage quota for a particular prepaid service (Quota Server).
The above entities belong to the service provider's backend
infrastructure and are outside the scope of this specification. In
particular, as far as this specification is concerned, they are
assumed to exist in the PPS. Three deployment scenarios are
presented in the remainder of this section. The first scenario is
depicted in Figure 2. In this scenario, the SAD, which runs the PPC,
the HAAA, and the PPS are located in the same provider network.
The subscriber's device establishes a connection with one of possibly
multiple SADs in the network. The selected SAD communicates with a
HAAA server (directly or indirectly).
The interface between the HAAA and the PPS is implemented using the
RADIUS protocol together with the extensions described in this
document. However, in cases where the PPS does not implement the
RADIUS protocol, the implementation would have to map the
requirements defined in this document to a functionally equivalent
protocol.
+------+ +-----+
| | | |
+--------+ +--------+ +--| HAAA |--+--| PPS |
| | | | | | | | | |
| Subscr.| | Service| | +------+ | +-----+
| |---| Access |--+ |
| Device | | Device | | +------+ | +-----+
| | | | | | | | | |
+--------+ +--------+ +--| HAAA |--+--| PPS |
| | | |
+------+ +-----+
Figure 2: Basic prepaid access architecture
The second scenario, depicted in Figure 3, is based on a static
roaming architecture that is typical of a wholesale scenario for
Dial-Up users or a broker scenario used in Dial-Up or WLAN roaming
scenarios.
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+----+ +----+ +----+ +-----+
| | | | | | | |
+------+ +-------+ +-|VAAA|-+-|BAAA|-+-|HAAA|-+-| PPS |
| | | | | | | | | | | | | | | |
|Sub | |Service| | +----+ | +----+ | +----+ | +-----+
| |--|Access |-+ | | |
|Device| |Device | | +----+ | +----+ | +----+ | +-----+
| | | | | | | | | | | | | | | |
+------+ +-------+ +-|VAAA|-+-|BAAA|-+-|HAAA|-+-| PPS |
| | | | | | | |
+----+ +----+ +----+ +-----+
Figure 3: Static roaming prepaid architecture
Like in the basic prepaid architecture, the subscriber device
establishes a connection with the SAD. The SAD communicates with the
VAAA using the RADIUS protocol. The VAAA, in turn, communicates
using the RADIUS protocol with BAAA servers in the broker network.
There may be more than one Broker Network between the Visited Network
and the Home Network. The Home Network is the same as in the
architecture depicted in Figure 2.
Broker AAA (BAAA) servers MUST support the Message-Authenticator(80)
attribute as defined in RFC 2869. If they are used, they forward the
RADIUS packets as usual to the appropriate RADIUS servers.
Accounting messages are not needed to deliver a prepaid service.
However, accounting messages can be used to keep the PPS up to date
as to what is happening with the prepaid data session. Therefore, a
BAAA SHOULD deliver RADIUS Accounting messages using the pass through
mode described in RFC 2866.
1.2.2. Motivation
Why not use existing RADIUS attributes to construct a protocol for
prepaid scenarios? This could lead to a solution where no code has
to be modified at existing devices.
It is indeed possible to construct a solution for prepaid scenarios
using existing RADIUS attributes. The RADIUS server would send an
Access-Accept message containing a Session-Timeout(27) and include a
Termination-Action(29) in the RADIUS-request. Upon receiving the
Access-Accept message, the NAS would meter the duration of the
session and upon termination of the session the NAS would generate an
Access-Request message again. The RADIUS server would then re-
authenticate the session and reply with an Access-Accept message
indicating the amount of additional time in a Session-Timeout(27).
Alternatively, it could respond with an Access-Reject message if
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there were no more resources in the user account.
Moreover, if the user terminates the session prematurely, the NAS
could indicate this in the accounting stream so that unused funds can
be returned into the prepaid user account.
Unfortunately, the above "solution" has a number of drawbacks,
including the following.
o It only supports time-based charging. The solution presented in
this document supports multiple charging metrics.
o Using accounting messages to recoup unused time may be problematic
because RADIUS accounting messages are not delivered in real-time.
A RADIUS server may store-and-forward accounting messages in
batches. Thus, relying on accounting messages for the purposes of
prepaid may cause revenue leakage. The solution presented in this
document does not rely on Accounting packets at all. It uses
Access-Request messages, which are required to flow through any
network in real-time.
o Session-Timeout(27) is not a mandatory attribute. If a prepaid
subscriber is served by a NAS that does not adhere to Session-
Timeout then that subscriber may use the service for an
undetermined period of time.
o Termination-Action(29) presents its own issues. Firstly, the
behaviour of Termination-Action(29) is not mandatory. Secondly,
according to RFC 2865, Termination-Action fires when the provision
of the service has completed. However, service should not be
terminated when negotiating additional quota, because this should
happen in a manner transparent to the subscriber. Due to the fact
that Termination-Action occurs when the service is completed, it
is unclear whether or not user experience would be affected if
this attribute would be used in a prepaid scenario. The RADIUS
server might even allocate a new IP address to the subscriber
device after a Termination-Action. Also, the RADIUS server has no
way of telling why a given Access-Request message was generated.
The RADIUS server might have to wait for the corresponding
accounting packet to determine the reason. Finally, re-
authenticating the subscriber may take too long. The solution
presented in this document allows quota replenishing to occur
without affecting user experience. No re-authentication is
required and quotas can be negotiated before the available credit
actually runs out.
o Due to the fact that the standard RADIUS attributes are not
mandatory, the correct prepaid operation is really an act of faith
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on the part of the RADIUS server. If Session-Timeout(27) and/or
Termination-Action(29) are not supported, the prepaid subscriber
might be able to obtain the service for free. The solution
described in this document requires that a prepaid-aware SAD
informs the RADIUS server, regardless of whether or not the latter
supports the prepaid extensions. The RADIUS server can then
determine whether or not service should be granted. For example,
if a prepaid subscriber is connected to a NAS that does not
support prepaid, the RADIUS server can either instruct the NAS to
tunnel the traffic to another entity in the home network (e.g. an
Home Agent) that supports prepaid, or cause it to provide only a
restricted service.
The solution presented in this document requires the support of two
mandatory and one optional attribute. Furthermore, it does not
require a great amount of additional code at a NAS that already
supports time or volume metering. The solution requires that RADIUS
entities advertise their prepaid capabilities in an Access-Request
and that they generate an Access-Request packet with Service-
Type="Authorize-Only" in order to obtain more quota when or before
the current quota is used up. It also requires the NAS to send an
Access-Request with Service-Type="Authorize-Only" when the session
terminates in order to refund the subscriber account.
1.3. A simple use case
This section describes the sequence of events in a simple RADIUS
prepaid transaction.
1. When an end host attaches to a network (for example, using PPP or
PANA), as usual, the NAS (SAD) that is servicing the subscriber
uses the AAA infrastructure in order to authenticate and
authorize the subscriber with respect to the requested service.
In order to do this, it sends a RADIUS Access-Request to the AAA
server. This Access-Request contains the subscriber's
credentials and may contain the prepaid capabilities of the SAD.
Prepaid capabilities MUST be included if the SAD supports them.
2. The authentication procedure proceeds. This may involve several
message exchanges such as in EAP [RFC2284]. Once the subscriber
has been successfully authenticated, the home AAA server
determines that the subscriber is a prepaid subscriber and
requests authorisation from the PPS. This request MUST include
the prepaid capabilities of the serving SAD.
3. The PPS, possibly with the help of the backend infrastructure,
validates that the subscriber has a prepaid account and that the
account is active. It further validates that the SAD has the
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appropriate prepaid capabilities. If all is in order, the PPS
authorises the subscriber to use the network. Otherwise it
rejects the request. The decision is sent to the AAA system in
the form of a response message. In the case of success, this
message contains attributes that indicate the allocation of a
portion of the subscriber credit. This portion is called the
"initial quota" and is expressed in units of time or volume. The
response may also include a threshold value. Note that only a
portion of the user's funds is allocated because the user may be
engaged in other services that may draw on the same account. For
example, the user may be engaged in a data session and a voice
session. Although these two services would draw from the same
account, they form separate parts of the overall system. If the
entire quota was allocated to the data session then the user
would have no more funds for a voice session.
4. The AAA system incorporates the attributes received from the PPS
into an Access-Accept message that it sends to the SAD. Note
that the AAA system is responsible for authorizing the service
whereas the prepaid system is responsible for prepaid
authorization.
5. Upon receiving the Access-Response, the SAD starts the prepaid
data session and meters the session based on time or volume, as
indicated in the message.
6. Once the consumption approaches the allocated limit (as expressed
by the threshold), the SAD will request additional quota. Re-
authorization for additional quota flows through the AAA system
to the PPS. The PPS revalidates the subscriber account and
subtracts the previously allocated quota from the current
balance. If there is remaining balance, it reauthorizes the
request with an additional quota allotment. Otherwise, the PPS
rejects the request. Note that the replenishment of the quota is
a re-authorization procedure and does not require the subscriber
to authenticate himself again.
7. Upon receiving a re-allotment of the quota, the 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. 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 his account. This
restriction can also be used to allow new subscribers to set up
prepaid accounts in the first place.
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8. Should the subscriber terminate the session before the quota is
exhausted, the remaining balance allotted to the session MUST be
refunded into his account.
Note that the subscriber may have disconnected while the Access
Device is waiting for the initial quota. The entire allocated quota
will have to be credited back to the subscribers account in this
case. Also note that the PPS maintains session state for the
subscriber. This state includes how much account balance was
allocated during the last quota enquiry and how much is left in the
account. Therefore, it is required that all messages about the
session reach the same (and correct) PPS.
For a simple message flow, along the lines of this use case, please
see Appendix A.
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2. Supported Features
This section describes the features that are supported by the
extensions specified in this document.
2.1. Multiple Concurrent 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 charged at different rates and
services may be metered differently. Therefore, the prepaid solution
needs to be able to distinguish services, and allocate quota to the
services using different unit types (time, volume) and allow for
those quotas to be consumed at different rates.
+---------+
| Session |
+---------+
| 1
V N
+--------------+ 1 : 1 +-------+
| Service |------>| Quota |
| (service-Id) | +-------+
+--------------+
Figure 4: Multiple services within a single session
As shown in Figure 4, a session may be associated with multiple (N)
services. Each service is identified by a service identifier
(Service-ID). The format of the Service-ID is not in the scope of
this document but it could be expressed as an IP flow using the
5-tuple {Source-IP and Port, Destination-IP and Port, protocol type}.
Each service is associated with a quota metric. An example message
flow that involves multiple such services within a single session is
given in the appendix.
2.2. Resource Pools
When working with multiple services a new problem arises because one
service may consume its quota faster than another service. When the
user balance is close to exhaustion, a situation could arise where
one service is unable to obtain quota while another service has
plenty of quota remaining. Unless the quotas can be rebalanced, the
SAD would then have to terminate the former service. Moreover, if
each service generates a certain amount of RADIUS prepaid traffic.
In an environment with many users and chargarble services, this
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amount of traffic is considerable and could cause undesirable network
congestion.
One method to circumvent the above situation is to use a so-called
"resource pool". Resource pools enable the allocation of resources
to multiple services of a session by allocating resources to a pool
and have services draw their quota from the pool at a rate
appropriate to that service. When the quota that has been allocated
to the pool is close to exhaustion, the entire pool (rather than
individual services) is replenished.
+-----------+
| Service-A |-----+ +--------+
+-----------+ | Ma | |
+-------->| |
| Pool |
+-------->| (1) |
+-----------+ | Mb | |
| Service-B |-----+ +--------+
+-----------+
Figure 5: Resource pool example
As shown in Figure 5, Service-A and Service-B are bound to Pool(1).
Ma and Mb are the pool multipliers (that are associated with
Service-A and Service-B respectively) that determine the rate at
which Service-A and Service-B draw from the pool.
The pool is initialized by taking the quota allocated to service n
and multiplying it by Mn. Therefore, the amount of resources
allocated to a pool is given by Poolr = Ma*Qa + Mb*Qb + . . ., where
Qn denotes the amount of quota that is allocated to service n.
Further, the pool is considered to be empty if
Poolr <= Ca*Ma + Cb*Mb + . . .,
Figure 6
where Ca and Cb are resources consumed by Service-A and Service-B
respectively.
Note that the resources assigned to the pool are not associated with
a metric. That is, Service-A can be rated at $1 per MB and Service-B
can rated at $0.10 per minute. In this case if $5 worth of resources
are allocated for service-A to the pool and if Ma = 10, then 50 units
would be placed into the pool. If a further $5 are allocated for
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service-B to the pool, then M=1 and 50 units are deposited into the
pool. The pool would then have a sum of 100 units to be shared
between the two services. The PPC would then meter the services such
that each Mbyte used by Service-A will draw 10 units from the pool
and each minute used by Service-B will draw 1 unit from the pool.
2.3. Complex Rating Functions
The rating of a service can be quite complex. While some operators
follow linear pricing models, others may wish to apply more complex
functions. For example, a service provider may wish to rate a
service such that the first N MBytes are free, then the next M Mbytes
are rated at $1 per MB and volume above (N+M) MB be rated at $0.50
per MB. Such a function could be implemented by repeated message
exchanges in the prepaid system.
To avert the need to exchange many messages while still supporting
such complex rating functions, the notion of a "Rating Group" is
introduced. A Rating Group are typically configured at the SAD. As
shown in Figure 7, a Rating Group is associated with one or more
services and defines the rate that the services associated with the
Rating Group consume an allocated amount of quota.
+--------------+ +--------------+
+-----------+ N 1 | | M 1 | Resource Pool|
| Service-A +---------->| Rating Group |------>| or |
+-----------+ | | | Quota |
+--------------+ +--------------+
Figure 7: Example of a rating group
During the usage of a service that is associated with a Rating Group,
the PPC sends the ID of the Rating Group to the PPS. The PPS
authorises the Rating Group by allocating a quota to it and
optionally assigning it to a Resource Pool. When an additional
service that belongs to an already authorised Rating Group is
instantiated, the PPC does not need to authorize this service. This
effectively means that the PPC meters the service such that it draws
from the already allocated quota. Therefore, no RADIUS messages need
to be exchanged in this case. This limits the amount of traffic
between the PPC and the PPS. An example of a flow that uses Rating
Groups is given in Appendix A.3
2.4. One-time Charging
One-time charging is a mode of operation of where the RADIUS prepaid
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extensions are used for charging of a service that is provided
instansteneously, i.e. without an ongoing session. An example of
such an event is the purchase of a ring-tone. Subscription based
services can also be modeled as a one-time event. In this case the
one-time service event is the purchase of a subscription.
For a given user, one-time charging may occur in parallel with other
charging models. For example, the subscriber may access a website
which is metered (based on time or volume) while he also purchases
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 not subject to charging.
The SAD signals one-time charging to the PPS with an indication that
identifies the service and the units that should be debited from the
user account.
A SAD may decide to perform one-time charging for an event that was
triggered by an unauthenticated user. In this case case the SAD will
have to authenticate the user before sending the relevant message to
the user's home AAA server.
Note that one-time charging can also be used to credit the prepaid
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.
2.5. Tariff Switching
The PPC and the PPS may support tariff switching mechanism described
in this section. This mechanism is useful if, for example, as shown
in Figure 8, traffic before 18:00 is rated at rate r1 and traffic
after 18:00 is rated at rate r2. The mechanism requires the PPC to
report usage before and after the switch occured.
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18:00
------------------+-----------------
r1 | r2
------------------+-----------------
^ ^
|<----TSI---> |
| |
Access-Accept Access-Request
(quota allocated) (quota consumed)
Figure 8: Example of tariff switching
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 (TariffSwitchInterval TSI).
At some point after the tariff switch the PPC sends another Access-
Request, as a result of either the user having logged off or the
volume threshold being reached. The PPC reports how much volume was
used in total (in a PPAQ attribute) and how much volume was used
after the tariff switch (in a PTS VUATS subtype attribute).
In situations with multiple tariff switches, the PPS must specify the
length of the tariff switch period using the
TimeIntervalAfterTariffSwitchUpdate (TITSU) in the PTS attribute as
shown below.
18:00 23:30
------------------+---------------------+--------------
r1 | r2 | r3
------------------+---------------------+--------------
^ ^ ^
|<----TSI---><-----------|-------->|TITSU
| |
Access-Accept Access-Request
Figure 9: Multiple tariff switches
When a TITSU is specified in the PTS, the PPC MUST generate an
Access-Request within the time after TSI and before TITSU expires.
Note that, typically, the PPC will be triggered by the Volume
Threshold. However, it is possible that, during period r2, resources
are not entirely consumed and, thus, the threshold is not reached.
The TITSU attribute ensures that, even in this case, the PPC will
generate the new Access-Request in good time.
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Note that it makes no sense to use the tariff switching mechanism
described in this section for services that are metered based on time
and the consumption of which is continuous (i.e. without
interruption). Also note that separate services flows may have
individual tariff periods.
2.6. 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 PPS 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 extensions. Even in this case the system should
be able to continue the prepaid session.
2.7. Dynamic Termination
When fraud or an error is detected, either only the affected session,
or all sessions of the affected subscriber should be immediately
terminated. It may further happen that the prepaid system enters a
state where it is unclear whether or not the data session is in
progress. Under certain conditions, the system may wish to terminate
the session in order to make sure that the user is not charged 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.
2.8. Querying and Rebalancing
It should be possible for the PPS to Query the current resource
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consumption at a SAD and adjust the user account balance. For
example, a request to the PPS is made (e.g. a one-time charging
event), the account is depleted and resources have been allocated to
the 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 does not 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, a practice that results
in more message exchanges.
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3. Operations
This section describes the operations that are implemented by a
prepaid-enabled NAS (SAD).
3.1. Authentication and Authorization Operation
The SAD initiates the authentication and authorization procedure by
sending a RADIUS Access-Request to the HAAA. Since the SAD has PPC
capabilities, it MUST include a PPAC attribute in the RADIUS Access-
Request. The PPAC attribute indicates to the PPS which prepaid
capabilities are possessed by the SAD. These are required in order
to complete the prepaid authorization procedure. Moreover, 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 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 message, the AAA server would then be responsible for
terminating the session using the means that are supported by the
device.
If the authentication procedure involves multiple message exchanges
(as 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 is sent, as usual, to the HAAA, possibly through
one or more BAAA.
Once the Access-Request arrives at the HAAA, the HAAA authenticates
the subscriber. If this fails, the HAAA sends an Access-Reject
message to the client. If authentication succeeds, the HAAA
determines whether or not the subscriber is a prepaid subscriber.
(How this is done is beyond the scope of this document.) If the
subscriber is not a prepaid subscriber, then the HAAA responds as
usual with an Access-Accept or an Access-Reject message. If the
subscriber is a prepaid subscriber then the HAAA SHALL forward the
Access-Request to the PPS for further authorization.
The Access-Request contains the PPAC(TBD) attribute and the Dynamic-
Capabilities attribute if one was included. The User-Name(1)
attribute MAY be set to a value that identifies the subscriber. This
attribute is used by the PPS to locate his account. For added
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security, the HAAA MAY also set the User-Password(2) attribute to the
password used between the HAAA and the PPS.
The PPS locates the subscriber account and authorizes him. During
this procedure, the PPS takes into consideration the SAD PPC
Capabilities. Upon successful authorization, the PPS generates an
Access-Accept containing an PPAC attribute and an PPAQ attribute.
The PPAC attribute returned to the client indicates the type of
prepaid service to be provided for the session. The PPAQ attribute
includes the following information.
o The QUOTA-ID, which is set by the PPS to a unique value that is
used to correlate subsequent quota requests.
o Volume and/or Time quota, which is set to a value representing a
portion of the subscriber's credit.
o It MAY contain a Time or Volume Threshold that indicates when the
SAD should request additional quota.
o The IP address of the Serving PPS and one or more alternative
PPSs. This is used by the HAAA to route subsequent quota
replenishing messages to the appropriate PPS(s).
o A State attribute, as defined in RFC 2865. This is necessary in
order to satisfy the requirements of section 5.44 of RFC 2865,
which mandates that an Access-Request with Service-
Type="Authorize-Only" must contain a State attribute. Since the
SAD sends subsequent quota replenishment requests in the form of
such "Authorize-Only" requests, a State attribute MUST be present
in all Access-Accept messages that also carry a PPAQ attribute.
Note: The Idle-Timeout(28) attribute can be used to trigger the
premature termination of a prepaid service, for example as a result
of inactivity.
Depending on site policies, after failed authorization, the PPS may
generate an Access-Reject in order to terminate the session
immediately. Alternatively, the PPS may generate an Access-Accept
blocking some or all of the traffic and/or redirect some or all of
the traffic to a location to a fixed server. (This feature could be
used, for example, to prompt the user to replenish their account.)
Blocking of traffic is achieved by either Filter-ID(11) or NAS-
Filter-Rule(see Redirect I-d). Redirection is achieved by sending
Redirect-Id or Redirect-Rule, HTTP Redirection defined in the
Redirect I-d. The time period before the session is blocked/
redirected is specified by the Session-Timeout(27) attribute.
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Upon receiving an Access-Accept from the PPS, the HAAA appends the
usual service attributes and forward the packet to the SAD. The HAAA
SHOULD NOT overwrite any attributes already set by the PPS. If the
HAAA receives an Access-Reject message, it will simply forward the
packet to its client. Depending on site policies, if the HAAA does
not receive an Access-Accept or an Access-Reject message from the PPS
it MAY do nothing or send an Access-Reject or an Access- Accept
message back to the PPC.
3.2. Session Start Operation
The start of the session is indicated by the arrival of an
Accounting-Request(Start) packet. The Accounting-Request (Start) MAY
be routed to the PPS such that it can confirm the initial quota
allocation.
Note that the role of the PPS is not to record accounting messages
and therefore it SHOULD NOT respond with an Accounting Response
packet. If the PPS does not receive the Accounting-Request(start)
message it will only know that the session has started upon the first
reception of a quota replenishment operation.
If the PPS does not receive indication directly (via Accounting-
Request(start)) or indirectly, it SHOULD, after some configurable
time, deduce that the Session has not started. If the SAD supports
termination capabilities, the PPS SHOULD send a Disconnect Message to
the SAD as a measure to ensure that the session is indeed dead.
3.3. Mid-Session Operation
During the lifetime of a prepaid data session the SAD may request the
replenishment of the quotas using an Authorize-Only Access-Request
message. Once either the allocated quota has been exhausted or the
threshold has been reached, the SAD MUST send an Access-Request with
Service-Type(6) set to a value of "Authorize-Only" and the PPAQ
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 the one used during the initial
Access-Request. For example, if the User-Name(1) attribute was used
in the Access-Request it MUST be included in the Authorize-Only
Access-Request, especially if the User-Name(1) attribute is used to
route the Access-Request to the Home AAA server.
The Authorize-Only Access-Request MUST NOT include a User Password
and MUST NOT include a Chap Password. In order to enable the
receiver to authenticate the message, the SAD MUST include a Message-
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Authenticator(80). In order to satisfy the requirements of section
5.44 of RFC 2865, the SAD MUST also include the State attribute. It
is anticipated that the inclusion of the State attribute will enable
the PPS to map the Authorize-Only Access Request to the
authentication context that was established when the PPC
authenticated itself at the beginning of the session. The SAD
computes the value for the Message-Authenticator and the State
attributes according to RFC 2869 and 2865 respectively.
When the HAAA receives an Authorize-Only Access-Request that contains
a PPAQ(TBD), it SHALL validate the message using the Message-
Authenticator(80), according to RFC 2869. If the HAAA receives an
Authorize-Only Access-Request that contains a PPAQ(TBD) and either no
or an invalid Message-Authenticator(80) it SHALL silently discard the
message. An Authorize Only Access-Request message that does not
contain a PPAQ(TBD) is either erroneous or belongs to another
application (for example, a Change of Authorization message
[RFC3576]). In this case the Authorize-Only Access-Request is either
silently discarded or handled by another application.
Once the Authorize-Only Access-Request message is validated, the HAAA
SHALL forward the Authorize-Only Access-Request to the appropriate
PPS. The HAAA MUST forward the Authorize-Only Access-Request to the
PPS specified in the PPAQ(TBD). The HAAA MUST add a Message-
Authenticator(80) to the message, according to RFC 2869. As with the
Access-Request message, the HAAA MAY modify the User-Name(1)
attribute such that it identifies the user to the PPS. Note that the
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 PPS MUST validate the Message-
Authenticator(80) as described in RFC 2869. If validation fails, the
PPS MUST silently discard the message. If it receives an Authorize-
Only Access-Request message that does not contain a PPAQ(TBD), it
MUST silently discard the message.
The PPS locates the prepaid session state using the Quota Id
contained within the PPAQ(TBD). The PPS takes the most recently
allocated quota and subtracts it from the user balance. If
sufficient balance remains, the PPS authorizes the PPS and allocates
additional quota. The PPS may also calculate a new threshold value.
Upon successful re-authorization, the PPS generates an Access-Accept
containing the PPAQ(TBD) attribute. The Access-Accept message MAY
contain Servicetype(6) set to Authorize-Only and MAY contain the
Message-Authenticator(80).
Depending on site policies, upon unsuccessful authorization, the PPS
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generates an Access-Reject or an Access-Accept with Filter-Id(11) or
Ascend-Data-Filter (if supported) attribute and the Session-
Timeout(27) attribute such that the subscriber can get access to a
restricted set of locations for a short period of time. This feature
could be used to enable users to replenish their accounts, create new
accounts, or to browse free content.
Upon receiving the Access-Accept from the PPS, the HAAA SHALL return
the packet to its client. If the HAAA receives an Access-Reject
message, it forwards the packet. Depending on site policies, if the
HAAA does not receive an Access-Accept or an Access-Reject message
from the PPS it MAY do nothing or it MAY send an Access-Reject
message back to its client.
Upon receiving an Access-Accept, the SAD SHALL update its quotas and
threshold parameters with the values contained in the PPAQ(TBD)
attribute. Note that the PPS MAY update the PrePaidServer
attribute(s) and these may have to be saved as well. If the Access-
Accept message contains a Filter-Id(11), an Ascend-Data-Filter
attribute, or Session Timeout(27), the SAD SHALL restrict the
subscriber session accordingly.
3.4. Dynamic Operations
The PPS may 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
action that the PPS may perform. Firstly, it may request the session
to be terminated. Secondly, it may request the attributes associated
with the session to be modified. More specifically, it may modify a
previously sent PPAQ(TBD).
Both of these actions require that the session be uniquely identified
at the SAD. As a minimum, the PPS MUST provide
1. either the NAS-IP-Address(4) or the NAS-Identifier(32), and
2. at least one session identifier such as User-Name(1), Framed-IP-
Address(), the Accounting-Session-Id(44).
Other attributes could also be used to uniquely identify a prepaid
data session.
3.4.1. Unsolicited Session Termination Operation
At anytime during a session the PPS may send a Disconnect Message in
order to terminate a session. This capability is described in detail
in [RFC3576]. The PPS sends a Disconnect Message that MUST contain
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identifiers that uniquely identify the data session and the SAD
servicing that session.
If the SAD receives a Disconnect-Message, it responds with either a
Disconnect-ACK message (if it is able to terminate the session) or
with a Disconnect-NAK packet (otherwise). Upon successful
termination of a session the SAD MUST return any unused quota to the
PPS by issuing an Authorize-Only Access-Request containing the PPAQ
which contains any unused Quota and the Update-Reason set to "Remote
Forced Disconnection".
3.4.2. Unsolicited Change of Authorization Operation
At any time during the session the PPC may receive a Change of
Authorization (CoA) message. A PPS may send a new Quota to either
add or to remove quota that is allocated to the service. If the
Change of Authorization contains a PPAQ then that PPAQ overrides a
previously received PPAQ. The PPS MUST NOT change the units used in
the PPAQ.
If the newly received PPAQ reduces the amount of allocated quota
beyond what is already used then the SAD 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 .
3.5. Termination Operation
The termination phase is initiated when (i) the subscriber logs off,
(ii) the subscriber balance is exhausted, or (iii) when the SAD
receives a Disconnect Message.
In case the user logged off, or the SAD receives a Disconnect
Message, the SAD sends an Authorize-Only Access-Request message with
a PPAQ and Update-Reason attribute set to either "Client Service
Termination" or "Remote Forced Disconnect". This message indicates
the amount of consumed quota.
In case the currently allocated quota is exhausted, if the PPAQ
contained the Termination-Action field, the SAD follows the specified
action (which would be to immediately terminate the service, request
more quota, or redirect/filter the service).
3.6. Mobile IP Operations
In roaming scenarios with Mobile-IP, the prepaid data session should
be maintained transparently if the HA is acting as the SAD. As the
subscriber device associates with a new SAD (AP or PDSN that supports
PPC capability), the SAD sends a RADIUS Access-Request and the
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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 prepaid session
does not undergo any change after the handoff because the Mobile IP
session is anchored at the HA and the user's Home IP address does not
change.
In the case of a wireless access point or PDSN acting as the SAD, it
is likely that the user's (care-of) IP address will change. The
prepaid session will be affected by this. In this scenario the SAD
shall send an Access-Request message which is routed to the home
network and MUST reach the PPS that is serving this session. The PPS
correlates the new authorization request with the existing active
session and assigns a quota to the new request. Any outstanding
quota at the old SAD MUST be returned to the PPS if the Mobile-IP
nodes (HA and FA) support registration revocation (Mobile IPv4 only).
Specifically, the quota SHOULD be returned when the SAD sends the
Authorize-Only Access-Request with PPAQ(TBD) Update-Reason set to
either "Remote Forced Disconnect" or "Client Service Termination".
In order to trigger the sending of this last Authorize-Only Access-
Request, the PPS may issue a Disconnect Message [3576] to the SAD.
Even if the subscriber moves to a SAD that does not have prepaid
capabilities can the prepaid data service continue. This can be done
by requesting the Home Agent (assuming it has such capabilities) to
take over the responsibilities of the SAD (i.e. metering). This
scenario will be discussed in detail in a later version of this
document.
3.7. Operation Considerations for Multiple Services
This section describes the support for multiple prepaid services on a
single SAD. Message flows illustrating the various interactions are
presented in Appendix A.
A SAD that supports prepaid operations for multi-services SHOULD set
the "Multi-Services Supported" bit in the PPAC. When working with
multi-services, we need to differentiate between the services. A
Service-Id attribute is used in the PPAQ(TBD) in order to uniquely
differentiate between the services. The exact definition of the
Service-Id attribute is outside the scope of this document.
A PPAQ that contains a Service-Id is associated with that Service. A
PPAQ that contains a Rating-Group-Id is associated with that Rating-
Group. A PPAQ MUST not contain both a Rating-Group-Id and a
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Service-Id. A PPAQ that contains neither a Rating-Group-Id or a
Service-Id applies to the Access Service.
3.7.1. Initial Quota Request
When operations with multi-services is desired, the SAD 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 quota for the Rating-Group by sending a PPAQ containing the
Rating-Group-Id. In both cases the Update-Reason is set to "Initial-
Request".
The Authorize-Only Access-Request message may contain more than one
PPAQ. The Authorize-Only Access-Request MUST include one or more
attributes that serve to identify the session so that it can be
linked to the original authentication. Which Session 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, the PPS allocates resources to each PPAQ. Each PPAQ
is assigned a unique QID that MUST appear in subsequent PPAQ updates
for that service or rating-group. Additionally, the PPAQ MUST
contain the Service-ID or Group-ID, unless the PPAQ is the generic
"Access Service".
3.7.2. Quota Update
Once the services start to utilize their allotted quota they will
eventually need to replenish their quotas (either the threshold is
reached or no more quota remains). In order to replenish the quota,
the PPC sends an Authorize-Only Access-Request message containing one
or more PPAQs. Each PPAQ MUST contain the appropriate QID,
Service-ID or Group-ID (or neither the Service-ID or Group-Id if the
quota replenishment is for the "Access Service"). The Update-Reason
filed indicates either "Threshold reached"(3), or "Quota reached"(4).
The Authorize-Only message must contain session identifiers.
Upon receiving an Authorize-Only Access-Request packet with one or
more PPAQs the PPS responds with a new PPAQ for that service. The
PPAQ contains a new QID, the Service-Id or Rating-Group-Id, a new
Quota. If the PPS does not grant additional quota to the service it
MUST include the Termination-Action subfield in the PPAQ that will
instruct the SAD what to do with the service.
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3.7.3. Termination
When the allotted quota for a service is exhausted, the SAD shall act
in accordance with the Termination-Action field set in the Quota. If
the Termination-Action field is absent then the service MUST be
terminated. If the service is to be terminated, then the SAD shall
send a PPAQ with the appropriate QID, the Service-Id, the used quota,
and the Update-Reason set to "Client Service Termination".
If the oAccess Serviceoe has terminated, then all other services must
be terminated as well. In this case the SAD MUST report on all
issued quotas for the various services. The Update-Reason field
should be set to "Access Service Terminated".
3.7.4. 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 matches the PPAQ with the service using
the Service-ID attribute. The new quota could differ from the
previously allocated value. The SAD must react to the new value
accordingly.
A disconnect message terminates the "Access Service". As such the
SAD MUST report 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.
3.7.5. Support for Resource Pools
If the PPC supports pools as indicated by setting the "Pools
supported" bit in the PPAC(TBD) then the PPS may associate a Quota
with a Pool by including the Pool-Id and the Pool-Multiplier in the
PPAQ(TBD). When Resource Pools are used, the PPAQ must not use the
threshold field.
3.7.6. One-time Charging
To initiate a One-Time charge the PPC includes the PPAQ attribute in
an Access-Request packet. The Access Request packet MUST include a
Message-Authenticator(80) and an Event-Timestamp(55) attribute. The
Service Id field of the PPAQ identifies the prepaid service. The
amount to be charged is specified using the Resource Quota and
Resource Quota overflow subtypes. If the value specified is negative
then the resources are credited to the user account.
The QID field MUST be set to a unique value and is used by the PPS to
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detect duplicates. The Update Reason field MUST be set to One-Time
Charging. Upon receiving a One-Time charge PPAQ, the RADIUS server
authenticates the user and, if successful, passes the PPAQ to the
PPS. The PPS locates the account and debits or credits it
accordingly. The PPS MUST respond to the PPS with an Access-Accept
message if successful, or an Access-Reject message otherwise.
The RADIUS server shall respond to the SAD with an Access Accept
message. Since this is a one-time 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. 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 when the session already exists.
The PPS responds with an Access-Accept to indicate that the user
account has been debited or an Access-Reject otherwise.
3.7.7. Error Handling
If the PPS receives a PPAQ with an invalid QID it MUST ignore that
PPAQ.
If the PPS receives a PPAQ containing a Service-Id, or a Rating-
Group-Id that it does not recognize, then it MUST ignore that PPAQ.
If the PPC receives a PPAQ containing a Service-Id, or a Rating-
Group-Id that it does not recognize, then it must ignore that PPAQ.
If the PPC receives a PPAQ that contains a Pool-Id without a Pool-
Multiplier or a Pool-Multiplier without a Pool-Id it must ignore that
PPAQ.
3.7.8. Accounting Considerations
Although typically generated, accounting messages are not required to
deliver a prepaid data service. When generated, accounting messages
are used for auditing purposes and for billing. Accounting messages
associated with prepaid data sessions should include the PPAQ(TBD)
attribute.
3.7.9. Interoperability with Diameter Credit Control Application
The RADIUS prepaid extensions need to interoperate with the Diameter
protocol. Two interoperability scenarios exist, as follows. Either
the AAA infrastructure is Diameter based and the SAD are RADIUS
based, or the SAD is Diameter based and the AAA infrastructure is
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RADIUS based.
The Diameter Credit Control Application [DIAMETERCC] describes how to
implement a prepaid accounting system using a Diameter based
infrastructure.
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4. Attributes
This section specifies the attributes that implement the RADIUS
extensions for prepaid. Their general format follow that of the base
RADIUS [RFC2865] and take also account current design guidelines that
are proposed in the RADEXT working group. The type field of these
attributes contains a value that is drawn from the type value space
specified in [RFC2865]. The exact value for the type field of each
attribute is to be allocated by IANA. Note that, unless otherwise
specified, the format of the value field of each of the AVPs defined
in this section adheres to one of the formats specified in section 5
of [RFC2865]. In particular, the labels "string", "integer", and
"address" are used to indicate the format in the remainder of this
document.
4.1. PPAC Attribute
The PrepaidAccountingCapability (PPAC) attribute is sent in the
Access-Request message by a prepaid capable NAS and is used to
describe the prepaid capabilities of the NAS. The PPAC is also
present in an Access-Accept message by the PPS in order to indicate
the type of metering that is to be applied to this session.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE | LENGTH | SUBtype 1 | LENGTH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AvailableInClient (AiC) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TYPE : value of PPAC
LENGTH: 8
VALUE : String
The value MUST be encoded as follows:
Subtype (=1) : Subtype for AvailableInClient attribute
Length : Length of AvailableInClient attribute
(= 6 octets)
AvailableInClient (AiC):
The optional AvailableInClient Subtype, generated by the PPC,
indicates the metering capabilities of the NAS and shall be bitmap
encoded. The possible values are as follows.
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
Figure 10: PPAC Attribute
4.2. Session Termination Attribute
The value is bitmap encoded. This attribute is included in a RADIUS
Access-Request message to the RADIUS server and indicates whether or
not the NAS supports Dynamic Authorization.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TYPE | LENGTH | String |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type : value of Session Termination Capability
Length: = 4
String encoded as follows:
0x00000001 Dynamic Authorization Extensions (rfc3576) is
supported.
Figure 11: Session Termination Attribute
4.3. PPAQ Attribute
One or more PPAQ attributes are sent in an Access Request, Authorize-
Only Access-Request and Access-Accept message. In an Access Request
message, the PPAQ attribute is used to facilitate One-Time charging
transactions. In Authorize-Only Access-Request messages it is used
for One-Time charging, report usage and the request for further
quota. It is also used in order to request prepaid quota for a new
service instance. In an Access-Accept message it is used in order to
allocate the (initial and subsequent) quotas.
When multiple services are supported, a PPAQ is associated with a
specific service as indicated by the presence of a Service-Id, a
Rating-Group-Id, or the "Access Service" (as indicated by the absence
of a Service-Id and a Rating-Group-Id).
The attribute has a variable length (greater than 8, encoded into one
octet), and consists of a variable number of subtypes. Unused
subtypes are omitted from the message. In the following subsections
the various subtypes of the PPAQ attribute are specified.
4.3.1. Quota Identifier AVP
The value of the type field of the QuotaIDentifier AVP is TBD. The
length of this AVP is 6 octets. Its value is encoded as a string.
It is generated by the PPS together with the allocation of new quota.
The online quota update RADIUS Access-Request message that is sent
from the SAD to the PPS includes a previously received
QuotaIdentifier AVP.
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4.3.2. VolumeQuota AVP
The value of the type field of the VolumeQuota AVP is TBD. The
length of this AVP is 12 or 18 octets. The AVP 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 PPS. In an RADIUS Authorize-Only Access-
Request message (SAD to PPS direction), it indicates the total used
volume (in octets) for both inbound and outbound traffic. The
attribute consists of a Value-Digits AVP and optionally an Exponent
AVP (as indicated in the length field). The Exponent AVP, if
present, MUST NOT encode a negative number or zero.
4.3.3. VolumeThreshold AVP
The value of the type field of the VolumeThreshold AVP is TBD and its
length is 12 or 18 octets. This AVP is optionally present if
VolumeQuota is present in a RADIUS Access-Accept message (PPS to SAD
direction). It is generated by the PPS and indicates the volume (in
octets) that shall be consumed before a new quota should be
requested. This threshold should not be larger than the VolumeQuota.
The attribute consists of a Value-Digits AVP and optionally an
Exponent AVP (as indicated by the length field). The Exponent AVP,
if present, MUST NOT encode a negative number or zero.
4.3.4. DurationQuota AVP
The value of the type field of the DurationQuota AVP is TBD and its
length is 6 octets. This optional AVP 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 PPS. It is encoded as an integer. In an on-line
RADIUS Access-Accept message (PPC to PPS direction), it indicates the
total duration (in seconds) since the start of the accounting session
related to the QuotaID of the PPAQ AVP in which it occurs.
4.3.5. DurationThreshold AVP
The value of the type field of the DurationThreshold AVP is TBD and
its length is 6 octets. This AVP shall optionally be present if
DurationQuota is present in a RADIUS Access-Accept message (PPS to
PPC direction). It represents the duration (in seconds) after which
new quota should be requested. This threshold should not be larger
than the DurationQuota. It is encoded as an integer.
4.3.6. ResourceQuota AVP
The value of the type field of the ResourceQuota AVP is TBD. The
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length of this AVP is 12 or 18 octets. This optional AVP is only
present if resource-based or one-time charging is used. In the
RADIUS Access-Accept message (PPS to SAD direction) it indicates the
resources allocated for the session by the PPS. In RADIUS Authorize-
Only Access-Request message (SAD to PPS direction), it indicates the
resources used in total, including both incoming and outgoing
chargeable traffic. In one-time charging scenarios, the subtype
represents the number of units to charge or credit the user. The
attribute consists of a Value-Digits AVP and optionally an Exponent
AVP (as indicated by the length field).
4.3.7. ResourceThreshold AVP
The value of the type field of the ResourceThreshold AVP is TBD. The
length of this AVP is 12 or 18 octets. The semantics of this AVP
follow those of the VolumeThreshold and DurationThreshold AVPs. It
consists of a Value-Digits AVP and optionally an Exponent AVP.
4.3.8. Value-Digits AVP
The value of the type field of the Value-Digits AVP is TBD and its
length is 10 octets. This AVP encodes the most significant digits of
a number, encoded as an integer. If decimal values are needed to
present the number, the scaling MUST be indicated with a related
Exponent AVP. For example, the decimal number 0.05 is encoded by a
Value-Digits AVP set to 5, and a scaling that is indicated with the
Exponent AVP set to -2.
4.3.9. Exponent AVP
The value of the type field of the Exponent AVP is TBD. The length
of this AVP is 6 octets. This AVP contains the exponent value that
is to be applied to the accompanying Value-Digit AVP. Its value is
encoded as an integer.
4.3.10. Update-Reason AVP
The value of the type field of the Update-Reason AVP is TBD. The
length of this AVP is 4 octets. This AVP shall be present in the on-
line RADIUS Access-Request message (PPC to PPS direction). It
indicates the reason for initiating the on-line quota update
operation. Update reasons 6, 7, 8 and 9 indicate that the associated
resources are released at the client side, and that therefore the PPS
shall not allocate a new quota in the RADIUS Access Accept message.
1. Pre-initialization
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2. Initial Request
3. Threshold Reached
4. Quota Reached
5. TITSU Approaching
6. Remote Forced Disconnect
7. Client Service Termination
8. "Access Service" Terminated
9. Service not established
10. One-Time Charging
4.3.11. PrepaidServer AVP
The value of the type field of the PrepaidServer AVP is TBD. The
length of this AVP is 6 or 18 octets, for IPv4 and IPv6 addresses
respectively. This optional AVP indicates the address of the serving
PPS. If present, the Home RADIUS server uses this address to route
the message to the serving PPS. The attribute may be sent by the
Home RADIUS server. Multiple instances of this subtype MAY be
present in a single PPAQ AVP. The value of this AVP is encoded as an
address.
If present in the incoming RADIUS Access-Accept message, the SAD
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 SAD shall not change their order.
4.3.12. Service-ID AVP
The value of the type and length fields of the Service-ID AVP are
TBD. The value field of this AVP is encoded as a string. This value
is handled as an opaque string that uniquely describes the service
instance to which prepaid metering should be applied. A Service-Id
could be an IP 5-tuple (source address, source port, destination
address, destination port, protocol). If a Service-ID AVP is present
in the PPAQ, the entire PPAQ refers to that service. If a PPAQ does
not contain a Service-Id or Rating-Group-ID, then the PPAQ refers to
the Access Service.
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4.3.13. Rating-Group-ID AVP
The value of the type field of the Rating-Group-ID is TBD. The
length of this AVP is 6 octets. This AVP indicates that this PPAQ is
associated with resources allocated to a Rating Group with the
corresponding ID. This AVP is encoded as a string. A PPAQ MUST NOT
contain more than one Rating-Group-ID.
4.3.14. Termination-Action AVP
The value of the type field of the Termination-Action AVP is TBD.
The length of this AVP is 3 octets. This AVP contains an enumeration
of the action to take when the PPS does not grant additional quota.
Valid actions are as follows. (Note that the value 0 is reserved.)
1. Terminate
2. Request More Quota
3. Redirect/Filter
4.3.15. Pool-ID AVP
The value of the type field of the Pool-ID AVP is TBD. The length of
this AVP is 6 octets. This AVP identifies the resource pool that the
quota included in this PPAQ is associated with. It is encoded as a
string.
4.3.16. Pool-Multiplier AVP
The value of the type field of the Pool-Multiplier AVP is TBD. The
length of this AVP is 12 or 18 octets. The pool-multiplier
determines the weight that resources are inserted into the pool that
is identified by the accompanying Pool-ID AVP, and the rate at which
resources are taken out of the pool by the relevant Service or
Rating-Group. The attribute consists of a Value-Digits AVP and
optionally an Exponent AVP (as indicated by the length field).
4.3.17. Requested-Action AVP
The value of the type field of the Requested-Action AVP is TBD. The
length of this AVP is 3 octets. This AVP can only be present in
messages sent from the PPC to the PPS. It indicates that the user or
the PPC desires the PPS to perform the indicated action and to return
the result. The PPAQ in which a Requested-Action AVP occurs MUST NOT
contain a QID, and MUST contain a Service-Identifier that, possibly
in combination with other AVPS, can be used by the PPS to uniquely
identify the service for which the indicated action is requested.
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The following actions may be requested.
1. Balance Check
2. Price Enquiry
4.3.18. Check-Balance-Result AVP
The value of the type field of the Check-Balance-Result AVP is TBD.
The length of this AVP is 3 octets. This AVP can only be present in
messages sent from the PPS to the PPC. It indicates the balance
check decision of the PPS about a previously received Balance Check
Request (as indicated in a Requested-Action AVP). Possible values
are 0 for "success" and any other value for "failure" and mean that
sufficient funds are available (resp. are not available) in the
user's prepaid account. The PPAQ in which a Check-Balance-Result
occurs MUST NOT include a QID, because no quota is reserved by the
PPS.
4.3.19. Cost-Information AVP
The value of the type field of the Cost-Information AVP is TBD. The
length of this AVP is variable. This AVP is used in order to return
the cost information of a service, which the PPC can transfer
transparently to the end user. This AVP is sent from the PPS to the
PPC as a response to a "Price Enquiry", as indicated by the
Requested-Action AVP. This AVP consists of four further AVPs, as
follows.
1. Value-Digits ASP: this encodes the most significant digits of the
monetery value that represents the cost in question.
2. Exponent AVP: this encodes the exponent that applies to the
Value-Digits AVP.
3. Currency-Code AVP: the value of the type field of this AVP is
TBD. The length of this AVP is 4 octets. It encodes the
currency code, as defined in the ISO 4217 standard.
4. Cost-Unit AVP: the value of the type field of this AVP is TBD.
The length of this AVP is variable. It carries a UTF8String
encoded human readable string that can be displayed to the end
user. It specifies the applicable unit to the Cost-Information
when the service cost is a cost per unit (e.g., cost of the
service is $1 per minute). The Cost-Unit can be minutes, hours,
days, kilobytes, megabytes, etc.
Example: the cost of 7.75 Malawi kwacha per hour would be encoded as
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follows. Value-Digits = 775, Exponent = -2, Currency Code = 103, and
Cost-Unit = "hour".
The PPAQ in which a Cost-Information occurs MUST NOT include a QID,
because no quota is actually reserved by the PPS.
NOTES: Either Volume-Quota, Time-Quota, or Resource-Quota MUST appear
in the PPAQ attribute. A PPAQ MUST NOT contain more than one
Service-Id, MUST NOT contain more than one Rating-Group-Id, and MUST
NOT contain both a Service-Id and a Rating-Group-Id. A PPAQ that
does not contain a Service-ID or a Rating-Group-Id refers to the
"Access Service". A PPAQ MUST NOT contain more than one Pool-Id. A
PPAQ that contains a Pool-Id MUST also contain a Pool-Multiplier AVP.
4.4. Prepaid Tariff Switching Attribute (PTS)
This specification defines the PTS attribute which allows for
changeovers from one rate to another during service provision.
Support for tariff switching is optional for both the PPC and the
PPS. PPCs use the flag "Tariff Switching supported" of the
AvailableInClient subtype of the PPAC attribute in order to indicate
support for tariff switching. PPSs employ the PTS attribute in order
to announce their support for tariff switching. Details of this will
be specified 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. If a RADIUS Access-Request message
contains a PTS attribute or a "Tariff Switching supported" flag, it
MUST also contain an Event-Timestamp RADIUS attribute (see
[RFC2869]).
The value of the type field of the PTS AVP is TBD. The length of
this AVP is variable. It contains one or more subtypes, as follows.
Every PTS AVP MUST include a QuotaIdentifier AVP as specified in
Section 4.3.1. In an online RADIUS Access-Request message sent from
the PPC to the PPS, the QuotaIdentifier AVP must contain a quota
identifier that was previously received 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 is referred to as the "accompanying PPAQ attribute". If
a PPS receives an Access-Request message from a PPC, it associates a
unique quota identifier to this request. Thus, a quota identifier
also identifies a particular service.
The PTS AVP contains a number of other subtype AVPs which are
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specified in the following subsections.
4.4.1. VolumeUsedAfterTariffSwitch AVP
The value of the type field of the VolumeUsedAfterTariffSwitch AVP is
TBD. The length of this AVP is 12 or 18 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. The attribute consists of a Value-Digits AVP and
optionally an Exponent AVP (as indicated in the length field).
4.4.2. TariffSwitchInterval AVP
The type of the TariffSwitchInterval is TBD and its length 6 octets.
This AVP 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.
4.4.3. TimeIntervalafterTariffSwitchUpdate AVP
The value of the type field of the
TimeIntervalafterTariffSwitchUpdate (TITSU) AVP is TBD. The length
of this AVP is 6 octets. The PPS MUST include this AVP if there is
another tariff switch period after the period that ends as indicated
by the TSI attribute. The value of the TITSU AVP in encoded as an
integer, and contains the number of seconds of the tariff period that
begins immediately after the period that ends as indicated by the TSI
attribute. If the TITSU attribute is not present, the PPC assumes
that the tariff period which ends as indicated by the TSI attribute
lasts until further notice. If TITSU is specified, the PPC MUST send
a quota update before the point in time specified by the TITSU
attribute (see Figure 9).
If a RADIUS message contains a PTS attribute, it MUST also contain at
least one PPAQ attribute. The PTS is associated 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 AVP, then
the PTS refers to the tariff switch for that service. If the PPAQ
does not have a Service-ID, then the PTS refers to tariff switch for
the Access-Service.
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
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starting the session.
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5. Translation between RADIUS prepaid and Diameter Credit Control
In scenarios where the service metering device uses the "RADIUS
prepaid" (RPP) protocol for accounting and prepaid charging while the
AAA infrastructure uses the "Diameter Credit Control" (DCC) protocol,
a translation agent that enables the interoperation of both systems,
is desirable. This also applies vice versa, i.e. in scenarios where
the AAA infrastructure uses RADIUS and the service metering device
uses Diameter.
The idea of such a translation agent would be to convert incoming RPP
(resp. DCC) messages into outgoing DCC (resp. RPP) messages. It
would be, in principle, desirable for the translation agent to be
stateless. That is, the agent should not be required to internally
maintain information about each ongoing RADIUS or Diameter session.
However, under the current specification of RPP and DCC, this appears
to be impossible due to a number of reasons. These include the
following.
1. The transport mechanism for DCC is TCP, which requires per-
session state to be maintained at both endpoints of the
communication. Note, however, that, in principle, each DCC
message could be sent over a dedicated TCP connection which is
torn down as soon as the message is sent. This, however, is
likely to be unacceptable in terms of efficiency.
2. While RPP messages encode the cumulative amount of consumed/
requested resources, DCC messages carry the difference from the
previous message. This means that the translation agent has to
maintain the current amount of consumed/requested resources in
order to be able to calculate the correct amount to be put into
an outgoing message.
The translator maps each incoming RPP (resp. DCC) message into an
outgoing DCC (resp. RPP) message, and possibly establishes or updates
local state that is associated with the session. The translated
(i.e. outgoing) message is a function of the incoming message as well
as existing state that is associated with the current session.
Translation occurs on an attribute-by-attribute basis. Certain
attributes are translated without consideration of local per-session
state. Other attributes, namely those that are bound to a particular
session, require such consideration. The translation agent has to
identify the session (and possibly subsession) an incoming message
belongs to in order to consult the appropriate local per-session
state.
Note that certain DCC attributes cannot be translated due to their
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semantics not being present in RPP, and vice versa. This results in
the messages, in which these attributes occur, not being delivered to
their intended destination. In such cases it is desirable to inform
the originator about the failure and terminate the session.
In each scenario (i.e. RPP client / DCC AAA infrastructure and DCC
client / RPP AAA infrastructure), the translator operates in two
directions, namely RPP to DCC and vice versa. In the following
sections, the notation c->s means that the attribute in question may
occur only in the direction from the client to the server. The
notation s->c denotes the converse and the notation c<->s denotes
that the attribute may occur in messages that are directed in either
direction.
5.1. Session Identification
The translation agent has to keep per-session state in order to
perform its task. A session may be identified based on the RPP
identifier or the DCC session identifier. That is, the translation
agent should always maintain a pair of (RPP, DCC) session identifiers
and maintain the per-session state in association with that pair.
This per-session state must be addressable by either of these two
identifiers. Moreover, an RPP session identifier must uniquely
correspond to a DCC identifier. (If this holds, the converse also
holds.) Each subsession identifier within an RPP session must also
uniquely correspond to a subsession identifier within its
corresponding DCC session. (If this holds the converse also holds.)
5.2. Translation between RADIUS prepaid client and Diameter Credit
Control AAA infrastructure
This section describes the translator in the "RPP client / DCC AAA
infrastructure" case. In other words, in this section it is assumed
that the client "talks" RPP and the AAA inftrastructure "talks" DCC.
The translator is assumed to sit somewhere in the middle and to
mediate between client and server.
For each RPP AVP (i.e. AVP that is specified in the present
document), the transformation into a semantically equivalent DCC AVP
(if such an AVP exists), along with what per-session state the
translator has to create or consult, is described. For clarity of
exposition, each RPP AVP is addressed in a separate subsection.
Since in this scenario, the PPC is typically the initiator a session,
the focus is on the RPP AVPs.
5.2.1. PPAC (c<->s)
A DCC client is assumed to always support Volume metering, Duration
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metering, Resource metering, Pools, Rating groups, and Tariff
Switching. Thus, if a PPAQ that indicates any of the above is sent
client->server, the translator does the following: It lets message go
through but remembers what exactly the client supports. If the
server later requests (servier -> client direction) an unsupported
metering to be performed, send failure to server and cause the
session to be terminated at the client.
If a PPAC indicates support for multiple services (0x00000020), the
translator maps this onto a DCC Multiple-Services- Indicator AVP.
5.2.2. Service Termination Attribute (c->s)
The Diameter base protocol assumes that the client always supports
dynamic session termination. If this AVP is present, the translator
does not need to do anything, i.e. there exists no DCC AVP that this
AVP can be mapped to. If this AVP is absent, the message in which it
appears should either be discarded and originator should be informed
of a failure, or the message can be passed on (without this AVP being
mapped onto a DCC AVP). However, in the latter case, the translator
has to remember that the client does not support dynamic termination.
Thus, the translatior has to initiate the normal session termination
procedure with the client when (if) dynamic termination is later
initiated by the server.
5.2.3. Quota Identifier Attribute (c<->s)
When quota is allocated for the first time by the DCC server, the
translator has to create a QID AVP, as required by this
specification. The translator later uses a QID AVP that is sent in
the client-to-server direction in order to identify the corresponding
DCC session. The QID has to be saved in the translator's per session
state.
5.2.4. Volume Quota Attribute (c<->s)
If this AVP occurs in a message that is sent in the server-to-client
direction, it is translated into a Granted-Service-Unit AVP with an
embedded CC-Total-Octets AVP. [editor's note: this sentence belongs
to the other translation type, i.e. AAA = RPP and client = DCC.]
If this AVP occurs in a message that is sent in the client-to-server
direction, then it is translated into a Used-Service-Unit AVP with an
embedded CC-Total-Octets AVP. Note that only the difference between
current cumulative quota for the (sub)session and the quota in
incoming messages is indicated in the translated DCC message. Local
state is updated with cumulative consumed resources.
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Conversely, if the server grants quota using the DCC Granted-Service-
Unit AVP with an embedded CC-Total-Octets AVP, then the translation
agent must translate this into a Volume Quota Attribute. Again,
local state must be consulted so that the cumulative amount of octets
is indicated in the Volume Quota attribute.
5.2.5. Duration Quota Attribute (c<->s)
If this AVP occurs in a message that is sent in the server-to-client
direction, it is translated into a Granted-Service-Unit AVP with an
embedded CC-Time AVP. [editor's note: this sentence belongs to the
other translation type, i.e. AAA = RPP and client = DCC.]
If this AVP occurs in a message that is sent in the client-to-server
direction, then it is translated into a Used-Service-Unit AVP with an
embedded CC-Time AVP. Note that only the difference between current
cumulative quota for the (sub)session and the quota in incoming
messages is indicated in the translated DCC message. Local state is
updated with cumulative consumed resources (i.e. time).
Conversely, if the server grants quota using the DCC Granted-Service-
Unit AVP with an embedded CC-Time AVP, then the translation agent
must translate this into a Duration Quota attribute. Again, local
state must be consulted so that the cumulative amount of seconds is
indicated in the Duaration Quota attribute.
5.2.6. Resource Quota Attribute (c<->s)
If this AVP occurs in a message that is sent in the server-to-client
direction, it is translated into a Granted-Service-Unit AVP with an
embedded CC-Service-Specific-Units AVP. [editor's note: this sentence
belongs to the other translation type, i.e. AAA = RPP and client =
DCC.]
If this AVP occurs in a message that is sent in the client-to-server
direction, then it is translated into a Used-Service-Unit AVP with an
embedded CC-Service-Specific-Units AVP. Note that only the
difference between current cumulative quota for the (sub)session and
the quota in incoming messages is indicated in the translated DCC
message. Local state is updated with cumulative consumed resources
(i.e. resources).
Conversely, if the server grants quota using the DCC Granted-Service-
Unit AVP with an embedded CC-Service-Specific-Units AVP, then the
translation agent must translate this into a Resource Quota
attribute. Again, local state must be consulted so that the
cumulative amount of resource units is indicated in the Resource
Quota attribute.
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Note that the "resource" type is application dependent. This means
that a DCC application unit corresponds to n RPP application units,
where n may be any real number. If n is not 1, then the RPP/DCC
translator must be aware of that and translate resource units
accordingly.
5.2.7. Value Digits Attribute (c<->s)
The encoding of this AVP is similar in RPP and DCC, and the value it
holds may have to be evaluated in conjunction with an acommpanying
"Exponent" AVP. It should be kept in mind that, in RPP the
cumulative amount of granted/consumed quota is typically encoded into
an AVP of this type, while in DCC only the difference from a previous
message.
5.2.8. Exponent Attribute (c<->s)
The encoding of this AVP is similar in RPP and DCC, and the value it
holds may have to be evaluated in conjunction with an acommpanying
"Value Digits" AVP. It should be kept in mind that, in RPP the
cumulative amount of granted/consumed quota is typically encoded into
a related "Value Digits" and "Exponent" AVP pair, while in DCC only
the difference from a previous message is encoded into such a pair.
5.2.9. Volume/Duration/Resource Threshold Attributes (s->c)
In DCC the concept of "threshold" does not exist. Instead, the DCC
client is assumed to ask for the replenishment of quota in good time.
In RPP, on the other hand, the server may optionally include a
threshold AVP, as an indication to the PPC about when to ask for
quota replenishment.
Thus, in this scenario, there is no need for the translator to ever
include a threshold attribute into the messages that it sends to the
PPC. If, however, there is a need for a threshold attribute to be
present in order to avoid a possible service provision
5.2.10. Update Reason Attribute (c->s)
The DCC AVP that is semantically closer to the Update Reason AVP than
any other AVP is the CC-Request-Type AVP. This AVP indicates whether
the message is which it appears is intended to indicate an "initial",
an "intermediate" or a "final interrogation". Morever, in case of
the session being terminated at the client, it indicates the reason
for this termination.
The following list lists the possible values of an "Update Reason"
attribute, along with corresponding values for the CC-Request-Type
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AVP.
o Pre-initialization: No action/value defined.
o Initial Request: Typically an "intial interrogation" is triggered
as a result of the reception of the message that contains this
Update Reason AVP. Hence, CC-Request-Type AVP indicates
"INITIAL_REQUEST".
o Threshold Reached: The reception of the message containing this
Update Reason AVP typically triggers an "intermediate
interrogation". Hence, CC-Request-Type AVP indicates
"UPDATE_REQUEST".
o Quota Reached: The reception of the message containing this Update
Reason AVP typically triggers an "intermediate interrogation".
Hence, CC-Request-Type AVP indicates "UPDATE_REQUEST".
o TITSU Approaching: The reception of the message containing this
Update Reason AVP typically triggers an "intermediate
interrogation". Hence, CC-Request-Type AVP indicates
"UPDATE_REQUEST".
o Remote Forced Disconnect: Reception of such an Update Reason
indicates that the client has terminated the session. The
corresponding value for the CC-Request-Type AVP is
"TERMINATION_REQUEST".
o Client Service Termination: Reception of such an Update Reason
indicates that the client has terminated the session. The
corresponding value for the CC-Request-Type AVP is
"TERMINATION_REQUEST".
o "Access Service" Terminated: Reception of such an Update Reason
indicates that the client has terminated the session. The
corresponding value for the CC-Request-Type AVP is
"TERMINATION_REQUEST".
o Service not established: Reception of such an Update Reason
indicates that the client has terminated the session. The
corresponding value for the CC-Request-Type AVP is
"TERMINATION_REQUEST".
o One-Time Charging: Such an Update Reason indicates that a one-time
charging event is initiated by the client. The corresponding
value for the CC-Request-Type AVP is "EVENT_REQUEST". Note that a
"Requested-Action: AVP MUST also be included in the outgoing DCC
message. Typically, this would be of the type "DIRECT_DEBITING",
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or "REFUND_ACCOUNT", depending on other AVPs present in the
message.
5.2.11. PrepaidServer Attribute (s<->c)
The PPC typically never sets the value of a PrepaidServer attribute.
Instead, it repeats those values that it receives from the AAA
infrastructure, in this scenario from the translator. This attribute
is therefore not used in a translation scenario. Nevertheless, the
translator must make sure that messages about the same RPP session
are forwarded to the same DCC server, throughout the whole session.
This may be easy to guarantee since the transport of Diameter is TCP.
5.2.12. Service-ID Attribute (s<->c)
The DCC equivalent of a RPP "Service-ID" AVP is the combination of
Service-Context-Id and Service-Identifier AVPs. The translator must
keep a static equivalence table of the RPP Service-ID and the
corresponding DCC combination in order to correctly translate an RPP
service identifier into DCC and back.
5.2.13. Rating-Group-ID Attribute (s<->c)
The DCC equivalent of a RPP "Rating-Group-ID" AVP is also called a
"Rating-Group-ID". Depending on the configuration, this AVP may
contain the same value on both the RPP and the DCC side of the
communication. If, however, static rating groups are configured
between the RCC client and the translator, and different rating
groups between the DCC server and the translator, then the translator
has to maintain a static translation table for the rating group
identifier. In any case, the translation of a rating group AVP, is
not a function of the translator's local per-session state.
5.2.14. Termination-Action Attribute (s->c)
The DCC equivalent of the "Termination-Action" AVP is called the
"Final-Unit-Action" AVP. In this scenario (RPP client and DCC AAA
infrastructure), a DCC "Final-Unit-Action" AVP is translated into a
"Termination-Action" AVP. The following list contains the possible
"Final-Unit-Action" values along with their "Termination-Action"
equivalent.
o TERMINATE (DCC): This value has a direct equivalent in RPP, also
called "Terminate".
o REDIRECT (DCC): If this value appears in a "Final-Unit-Action"
AVP, then a "Redirect-Server-Address" AVP must also appear in the
same DCC message. The translator translates these two AVPs into a
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"Termination-Action" with value "Redirect/Filter" and an
eqiovalent NAS-Filter-Rule attribute (specified in http://
www.ietf.org/internet-drafts/draft-ietf-radext-ieee802-00.txt).
o RESTRICT_ACCESS (DCC): If this value appears in a "Final-Unit-
Action" AVP, then a "Restriction-Filter-Rule" AVP must also appear
in the same DCC message. The translator translates these two AVPs
into a "Termination-Action" with value "Redirect/Filter" and an
eqiovalent Filter-ID attribute (specified in http://www.ietf.org/
internet-drafts/draft-ietf-radext-ieee802-00.txt).
o In the absence of a "Final-Unit-Action" AVP, the DCC server
assumes that the DCC client will ask for replenishment of quota at
some suitable time. In RPP, this is explicitly conveyed via a
"Termination-Action" AVP with the value "Request More Quota".
Thus, in the absence of a "Final-Unit-Action" AVP, the translator
in this scenario appends such an AVP into the outgoing RPP
message.
5.2.15. Pool-ID Attribute (s<->c)
The DCC equivalent of a RPP "Pool-ID" AVP is also called a "Pool-ID".
Typically, no translation needs to be done to the "Pool-ID"
attribute.
5.2.16. Multiplier Attribute (s<->c)
The multiplier attribute, which is a pair of "Value-Digits" and
"Exponent" AVPs, typically needs no translation, since the value it
carries (inside a "Value-Digits" and an "Exponent" AVP) represents
the rating of the service or rating group to which it refers, with
respect to abstract units. As such, the same multiplier value would
typically applyt be conveyed from a DCC server to an PPC, and vice
versa.
5.2.17. Requested-Action Attribute (c->s)
The "Requested Action" AVP can be directly translated into its DCC
equivalent, which carries the same name.
1. Balance Check (PCC): CHECK_BALANCE (DCC)
2. Price Enquiry (PCC): PRICE_ENQUIRY (DCC)
5.2.18. Check-Balance-Result Attribute (s->c)
This attribute carries only a binary value. Hence, its translation
is straightforward.
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5.2.19. Cost-Information Attribute (s->c)
This attribute consists of a Value-Digits AVP, an Exponent AVP, a
Currency Code AVP, and a Cost-Unit AVP. All these AVPs do likewise
exist in DCC, and carry identical semantics in the context of the
"Cost-Information" AVP. Thus, the translation of this attribute is
straightforward.
5.2.20. VolumeUsedAfterTariffSwitch attribute (c->s)
This attribute carries the amount of octets that were consumed after
a tariff change. It always appears in a message with an accompanying
PPAQ attribute in which the total amount of octets (i.e. those that
were consumed both before and after the tariff switch) is reported.
Thus, the translation agent can compute the amount of octets that
were consumed before the tariff change.
In DCC, the two amounts, i.e. the octets that were consumed before a
tariff change and those that were consumed afterwards, are reported
in separate Used-Service-Unit AVPs. The two Used-Service-Unit AVPs
have an embedded CC-Total-Octets AVP that indicates the appropriate
amount of octets. Furthermore, the Used-Service-Unit AVP that
carries the amount that was consumed before the tariff switch also
carries an embedded Tariff-Change-Usage AVP with the value
UNIT_BEFORE_TARIFF_CHANGE (0). Similarly, the Used-Service-Unit AVP
that carries the amount that was consumed after the tariff switch
also carries an embedded Tariff-Change-Usage AVP with the value
UNIT_AFTER_TARIFF_CHANGE (1).
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6. Security Considerations
The extended RADIUS protocol described in this document is subject to
a number of potential attacks, in a manner similar to the RADIUS
protocol without these extensions. It is recommended that IPsec be
employed to protect against certain of the attacks.
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7. IANA Considerations
This document requires the assignment of new Radius attributes type
numbers for the following attributes. Prepaid-Accounting-Capability
(PPAC), AvailableInClient, Prepaid-Accounting-Operation (PPAQ),
QuotaIdentifier, (QID), VolumeQuota (VQ), VolumeTreshold (VT),
DurationQuota (DQ), DurationTreshold (DT), UpdateReason (UR),
PrePaidServer (PPS), ServiceID (SID), Rating-Group-ID (RGID),
TerminationAction (TA), PoolID (PID), PoolMultiplier (PM), Cost-
Information (COST), Session-Termination-Capability (STC),
PrepaidTariffSwitch (PTS), TariffSwitchInterval (TSI) and others.
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8. Acknowledgements
The authors would like to thank Christian Guenther for his valuable
insight and feedback and his active and ongoing contributions that he
provided throughout the development of this document.
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9. References
9.1. Normative References
[1] Bradner, S., "RFC 2026: The Internet Standards Process --
Revision 3", October 1996.
[2] Bradner, S., "RFC 2119: Key words for use in RFCs to Indicate
Requirement Levels", March 1997.
[3] Rigney, C., Rubens, A., Simpson, W., and S. Willens, "RFC 2865:
Remote Authentication Dial In User Server (RADIUS)", June 2000.
[4] Rigney, C., "RFC 2866: RADIUS Accounting", June 2000.
[5] Rigney, C., Willats, W., and P. Calhoun, "RFC 2869: RADIUS
Extensions", June 2000.
[6] Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M., and
I. Goyret, "RFC 2868: RADIUS Attributes for Tunnel Protocol
Support", June 2000.
[7] Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B. Adoba,
"RFC 3576: Dynamic Authorization Extensions to Remote
Authentication Dial-In User Service (RADIUS)", February 2003.
[8] Adoba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "RFC 3748: Extensible Authentication Protocol",
June 2004.
9.2. Informative References
[9] Hakala, H., Mattila, L., Koskinen, J-P., Stura, M., and J.
Loughney, "RFC 4006: Diameter Credit Control Application",
August 2005.
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Appendix A. Example flows
This section presents certain example flows that involve the RADIUS
prepaid extensions. By no means is the intent of this section to
specify or recommend business logic, rating strategies, and
application-level behaviour. The intent of this section is purely to
illustrate some fictive scenarios and the RADIUS prepaid message
flows that could be associated with these scenarios. The contents of
this section should be regarded as a collection of informative
examples that aim to provide guidance to implementors.
A.1. A simple flow
End user PPC AAA Server PPS
user logs on
------(1)--------->
Access Request
{RADIUS BASE AVPS,
PPAC=00...00011}
-------(2)-------->
RADIUS authentication
<--------------(3)---------------------->
Access Request
{RADIUS BASE AVPS,
PPAC=00...00011}
------(4)--------->
[allocates
5MB quota]
Access Response
{RADIUS BASE AVPS,
PPAQ={QID=5, VQ = 5MB,
VTH = 4.5 MB}}
<-------(5)--------
forwards message
<-----(6)-----------
service provision/metering
-------(7)--------->
4.5 MB consumed
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Access Request
{RADIUS BASE AVPS,
PPAQ={QID=5, VQ=4.5MB,
REASON=THRESHOLD REACHED}}
-------(8)--------->
forwards message
-------(9)------->
[allocates another 7MB
to the access service]
Access Response
{RADIUS BASE AVPS,
PPAQ={QID=8, VQ=12MB,
VTH = 11.5 MB}}
<----------(10)--------------
forwards message
<------(11)--------
user logs off
------(12)-------
Access Request
{RADIUS BASE AVPS,
PPAQ={QID=8, VQ=7 MB,
REASON=ACCESS SERV TERMINATED}}
-------(13)--------->
forwards message
-------(14)------->
[reimburses
user account]
AA Response
{RADIUS BASE AVPS}
<------(15)--------
AA Response
{RADIUS BASE AVPS}
<------(16)--------
Figure 12: A simple example message flow
The user logs on (1). The PPC sends a RADIUS Access Request message
to the home AAA server (2), and includes the prepaid-specific PPAC
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AVP. This AVP indicates that both duration-based and volume-based
metering is supported. However, it also indicated that multiple
services, rating groups and resource pools are not supported. Note
that, since this is not an "Authorize-Only" message, no PPAQ AVP with
Update Reason="initial request" is included (see Section 3.7.1). The
home AAA server then authenticates the user and authorizes the access
service, as is usual in RADIUS (3). Note that the PPAC AVP is
appended by the PPC in at least the last message that is sent to the
home AAA server during this possibly multiple-round exchange.
If authentication and authorization is successful (in this example
this is assumed), the home AAA server forwards the final Access
Request to the PPS (4). The PPS identifies the user's prepaid
account from the included base RADIUS AVPs, and determines the
capabilities of the PPC from the PPAC attribute. Assuming that
sufficient funds are available in the user's prepaid account, the PPS
reserves some of these and rates the service. In this example, the
PPS reserves, say, 2 Euros and determines that the access service is
rated at 0.4 Euro per MB. This results in 5 MB of quota being
granted. The PPS also determines that the PPC should ask for this
quota to be replenished once 4.5 MB have been consumed. Thus, it
creates an Access Accept message with a Volume-Threshold indication
of 4.5MB. It further associates the QuotaIdentifier QID=5 to this
quota reservation. This identifier can be used to later uniquely
identify the prepaid session, user, account, etc. The resulting
Access Accept message is sent to the home AAA server (5) and
forwarded to the PPC (6).
Upon reception of message (6), the PPC provides the access service to
the user and meters it accordingly.
At some point in time, the threshold is reached, i.e. 4.5MB of
"access service" have been consumed by the user. At that point, the
PPC generates an Authorize-Only Access Request that contains the
usual RADIUS attributes and a PPAQ AVPs that reports the amount of
consumed quota, and the request for replenishment, i.e. the Update-
Reason= THRESHOLD REACHED (8). Note that the QID in this message is
the same as the one previously received from the user's home AAA
server. This message is forwarded to the PPS (9).
Upon reception of message (9), the PPS identifies the user and his
account from the QID. In also determines that a prepaid session is
ongoing, and that enough credit remains in the prepaid account in
order for the access service to continue being provided. Since 4.5
MB have been consumed, the PPS subtracts 1.8 Euros from the user's
prepaid account. The PPS decides to reserve another 2.8 euros from
the user's account. (This results in 3 euros being reserved in total
at this point in time.) As the access service is rated at 0.4 euros
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per MB, the PPS determines that another 7 MB of quota should be
granted. This results in a total cumulative quota allocation of 12
MB for the access service. The PPS further calculates the new
threshold value of 11.5 MB. Since this is a new quota reservation,
the PPS also allocates a new QuotaIdentifier to it, in this example
QID=8. The resulting RADIUS message is sent to the home AAA server
(10) and forwarded to the PPC (11).
Upon reception of message (11), the PPC updates its records and
continues provisioning access to the user. At some point the user
logs off (12). The PPC must then report how many resources were
consumed, so that the PPC can subtract the appropriate monetary
amount from the user's prepaid account. To this end the PPC
constructs an Authorize-Only Access Request message with a PPAQ AVPs
for the access service. In this example, 7 MB were consumed by the
access service in total. The PPC reports 7 MB its final message
(13). This is forwarded to the PPS (14) which correlates the report,
using the QID, to the previous session state. It determines, from
the previous records, that the access service had consumed another
4.5 MB before (as indicated in message (9)). This means that, of the
7 MB, only 3.5 MB have not yet been subtracted from the user's
account. Thus, the PPS subtracts another 1.4 euros from the user's
account and, since the session is to be terminated (REASON=ACCESS
SERVICE TERMINATED), releases any reserved monetary amount.
The PPS responds with an Access Response as required by the RADIUS
base specification (15). So does the home AAA server (16).
A.2. A flow with prepaid tariff switching
End user PPC AAA Server PPS
user logs on
------(1)--------->
Access Request
{RADIUS BASE AVPS,
PPAC=00...00111}
-------(2)-------->
RADIUS authentication
<--------------(3)---------------------->
Access Request
{RADIUS BASE AVPS,
PPAC=00...00011}
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------(4)--------->
[allocates
20MB quota]
Access Response
{RADIUS BASE AVPS,
PPAQ={QID=5, VQ = 20MB,
VTH = 18 MB}, PTS={
QID=5, PTS{TSI=300sec,
TITSU=6000sec}}
<-------(5)--------
forwards message
<-----(6)-----------
service provision/metering
-------(7)--------->
5900 seconds have passed
Access Request
{RADIUS BASE AVPS,
PPAQ={QID=5, VQ=14MB,
REASON=TITSU APPROACH.},
TSI={QID=5, VUATS=11MB}}
-------(8)--------->
forwards message
-------(9)------->
[allocates another 10MB
to the access service]
Access Response
{RADIUS BASE AVPS,
PPAQ={QID=8, VQ=30MB,
VTH = 28 MB},PTS={
QUD=8, PTS=95 sec}}
<----------(10)--------------
forwards message
<------(11)--------
user logs off
------(12)-------
Access Request
{RADIUS BASE AVPS,
PPAQ={QID=8, VQ=17 MB,
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REASON=ACCESS SERV TERMINATED},
PTS={QID=8, VUATS=2.5 MB}
-------(13)--------->
forwards message
-------(14)------->
[reimburses
user account]
AA Response
{RADIUS BASE AVPS}
<------(15)--------
AA Response
{RADIUS BASE AVPS}
<------(16)--------
Figure 13: Example message flow with Tariff Switch
The user logs on (1). The PPC sends a RADIUS Access Request message
to the home AAA server (2), and includes the prepaid-specific PPAC
AVP. This AVP indicates that both duration-based and volume-based
metering is supported, as well as tariff switching. The home AAA
server then authenticates and user and authorizes the access service,
as is usual in RADIUS (3). Note that the PPAC AVP is appended by the
PPC in at least the last message that is sent to the home AAA server
during this possibly multiple-round exchange.
If authentication and authorization is successful (in this example
this is assumed), the home AAA server forwards the final Access
Request to the PPS (4). The PPS identifies the user's prepaid
account from the included base RADIUS AVPs, and determines the
capabilities of the PPC from the PPAC attribute. In this example, it
is assumed that a tariff switch is about to occur in 300 seconds from
the current time. Suppose that the access service is currently rated
at 0.5 euros per MB and in the next tariff period it is rated at 0.6
euros per MB. Suppose further that a third tariff period is about to
start in 6000 seconds from current time and that that access service
is rated at 0.8 euros per MB in that period. The PPS then decides to
reserve 12 euros from the user's account. Since it is conceivable
that the user may consume all allocated quota in the (more expensive)
"0.6-euro" period, the PPS reserves 20 MB of quota, and determines a
threshold value of 18 MB. It constructs a Radius Access Accept
message with a PPAQ attribute that reflects these choices, and
carries a QuotaIdentifier QID=5. It further adds a PTS AVP in the
message which is linked to the PPAQ via the common QID value. The
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PTS AVP contains a TSI attribute indicating that a tariff switch will
occur in 300 seconds. It also includes a TITSU attribute with the
value of 6000 seconds. This is included in order to make sure that
the PPC will report the consumed quota before the "2-euro" tariff
period will start. The message is sent to the AAA server (5) and
forwarded to the PPC (6).
Upon reception of message (6), the PPC provides the access service to
the user and meters it accordingly (7). It also keeps track of time.
That is, it remembers how many octets are consumed before and how
many after the tariff switch that will take place in 300 seconds.
In this example it is assumed that the user consumes the allocated
quota rather slowly. In particular, nearly 6000 seconds (the value
indicated by TITSU) pass without the threshold of 18 MB being
reached. The PPC notices this and must therefore report usage and
request the quota to be replenished despite the fact that the
threshold has not been reached. In this example, it decides to do so
100 seconds before the 6000 seconds are reached. To this end, it
constructs an Authorization Access Request message including a PPAQ
that indicates that 14 MB have been consumed up to now. It also
includes a PTS AVP in order to indicate, using the VUATS AVP, that 11
MB of these were consumed after the tariff switch. The message is
sent to the AAA server (8) and forwarded to the PPS (9).
The PPS can link the message to previous session state via the QID.
It now rates the consumed volume as follows. The 11 MB that were
consumed after the tariff switch correspond to 11 * 0.6 = 6.6 euros
and the remaining 14-11=3 MB to 3 * 0.5 = 1.5 euros. Thus, the PPS
subtracts the amount of 6.6+1.5=8.1 euros from the user's account,
which leads to a remainder of 12 - 8.1 = 3.9 euros being reserved.
The PPS now determines that message (9) was sent in order to
replenish the quota for this prepaid session. This can be deduced
from the UPDATE REASON field, which indicates that the PPC sent this
message because the time indicated by the TITSU AVP is approacing.
The PPS now determines that enough credit remains in the user's
prepaid account in order for the access service to continue being
provided and decides to reserve another 8.9 euros from the user's
account. Since it is conceivable that the user will consume the 6
unused MB of quota from the previous allocation, as well as the
entire quota that is to be allocated now, entirely in the "0.8-euro"
period, the quota that should now be granted in addition to the
previous 20 MB should be 10 MB. This is because 0.9 of the 8.9 euros
are being reserved in order to "cover the worst case scenario". The
fact that 0.9 euros are reserved for this purpose is due to the fact
that the unused 6 MB from the previous allocation correspond to 4.8
euros (with 0.8 euros per MB). This is 4.8 - 3.9 = 0.9 euros more
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than the amount of funds that are still "reserved" from the previous
allocation. (After this reservation, the total amount of reserved
money is 8.9 + 3.9 = 12.8 euros, which corresponds to 16 (10+6) MB
being consumed in the "0.8-euro" period.)
Since quotas are encoded in a cumulative way in RADIUS, the PPS
includes a VolumeQuota of 30 MB into the Access Accept message (10).
The PPS further calculates the new threshold value of 28 MB. Since
this is a new quota reservation, the PPS also allocates a new
QuotaIdentifier to it, in this example QID=8. The resulting RADIUS
message is sent to the home AAA server (10) and forwarded to the PPC
(11).
Upon reception of message (11), the PPC updates its records and
continues providing access to the user. At some point the user logs
off (12). The PPC must then report how many resources were consumed,
so that the PPC can subtract the appropriate monetary amount from the
user's prepaid account. To this end the PPC constructs an Authorize-
Only Access Request message with a PPAQ AVPs for the access service.
In this example, 17 MB were consumed by the access service in total.
The PPC reports 17 MB its final message (13). This is forwarded to
the PPS (14) which correlates the report, using the QID, to the
previous session state. It determines, from the previous records,
that the access service had consumed 14 MB before (as indicated in
message (9)). This means that, of the 17 MB, only the monetary
equivalent for 3 MB have not yet been subtracted from the user's
account. The PPS calculates how much should be deducted from the
user's account as follows. Since the VUATS AVP indicates that 2.5MB
were consumed after the tariff switch, only 0.5 MB were consumed
before that. Thus, the monetary equivalent is 0.5 * 0.6 + 2.5 * 0.8
= 3.6 euros. That is, the PPS subtracts 3.6 euros from the user's
prepaid account. Since the session has by now be terminated by the
PPC (REASON=ACCESS SERVICE TERMINATED), the PPS now releases any
reserved monetary amount, in this example 12.8 - 3.6 = 9.2 euros.
The PPS responds with an Access Response as required by the RADIUS
base specification (15). So does the home AAA server (16).
Remark: In this example, two tariff switches take place. In other
scenarios, of course, only one tariff switch may occur. In such
scenarios the TITSU AVP is not used.
A.3. Resource pools and Rating Groups
End user PPC AAA Server PPS
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user logs on
------(1)--------->
Access Request
{RADIUS BASE AVPS,
PPAC=00...00101111}
-------(2)-------->
RADIUS authentication
<--------------(3)---------------------->
Access Request
{RADIUS BASE AVPS,
PPAC=00...00101111}
------(4)--------->
[allocates
5MB quota]
Access Response
{RADIUS BASE AVPS,
PPAQ={QID=5, VQ = 5MB,
poolID=1,mult=1}}
<-------(5)--------
forwards message
<-----(6)-----------
service provision/metering
-------(7)--------->
user requests service A
-------(8)--------->
Access Request
{RADIUS BASE AVPS,PPAQ={
SID="A", RGROUP=1}}
-------(9)-------->
forwards message
-----(10)--------->
[allocates 50 min
quota]
Access Response
{RADIUS BASE AVPS,
PPAQ={QID=7, DQ=3000sec
poolID=1,RGROUP=1, SID="A"
mult=1747.63}}
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<---------(11)---------------
forwards message
<----(12)--------
user requests service B
-------(13)-------->
Pool 1 close to exhaustion
Access Request
{RADIUS BASE AVPS,
PPAQ={QID=5, VQ=4MB,
REASON=QUOTA REACHED,
PoolID=1, mult=1}
PPAQ={QID=7, DQ=3300sec
REASON=QUOTA REACHED,
PoolID=1, mult=1747.63,
SID="A",RGROUP=1}}
-------(14)--------->
forwards message
-------(15)------->
[allocates another
3 MB to access service
and 30 minutes to
service "A"]
Access Response
{RADIUS BASE AVPS,
PPAQ={QID=8, VQ=8MB,
PoolID=1, mult=1, RGROUP=1},
PPAQ={QID=9, DQ=4800sec
PoolID=1, mult=1747.63,
SID="A"}}
<----------(16)--------------
forwards message
<------(17)--------
user logs off
------(18)-------
Access Request
{RADIUS BASE AVPS,
PPAQ={QID=8, VQ=6.5MB,
REASON=ACCESS SERV TERMINATED,
PoolID=1, mult=1}
PPAQ={QID=9, DQ=5400sec
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REASON=ACCESS SERV TERMINATED,
PoolID=1, mult=1747.63,
SID="A",RGROUP=1}}
-------(19)--------->
forwards message
-------(20)------->
[reimburses
user account]
AA Response
{RADIUS BASE AVPS
<------(21)--------
AA Response
{RADIUS BASE AVPS
<------(22)--------
Figure 14: Example message flow with resource pools and rating groups
The user logs on (1). The PPC sends a RADIUS Access Request message
to the home AAA server (2), and includes the prepaid-specific PPAC
AVP, indicating that multiple services, rating groups and resource
pools are supported. Note that, since this is not an "Authorize-
Only" message, no PPAQ AVP with Update Reason="initial request" is
included (see Section 3.7.1). The home AAA server then authenticates
the user and authorizes the access service, as is usual in RADIUS
(3). Note that the PPAC AVP is appended by the PPC in at least the
last message that is sent to the home AAA server during this possibly
multiple-round exchange.
If authentication and authorization is successful (in this example
this is assumed), the home AAA server forwards the final Access
Request to the PPS (4). The PPS identifies the user's prepaid
account from the included base RADIUS AVPs, and determines the
capabilities of the PPC from the PPAC attribute. Assuming that
sufficient funds are available in the user's prepaid account, the PPS
reserves some of these and rates the service. In this example, the
PPS reserves 5 Euros and determines that the access service is rated
at 1 Euro per MB. In anticipation that the user requests more
chargeable services throughout this prepaid session, and since this
is supported by the PPC, the PPS further associates a resource pool
with this reservation, in this example PoolID=1. The PPC also
specifies the multiplier = 1 for the access service. Note that,
since 5MB = 5242880 octets, 1 unit in the resource pool corresponds
to 5 / 5242880 euros, which is about 0.000095367431640625 Eurocents.
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(However, the PPC does not need to know that.) Moreover, the PPS
associates the QuotaIdentifier QID=5 to this quota reservation. This
identifier can be used to later uniquely identify the prepaid
session, user, account, etc. The resulting Access Accept message is
sent to the home AAA server (5) and forwarded to the PPC (6).
Upon reception of message (6), the PPC provides the access service to
the user and meters it accordingly. That is, for every octet
consumed, the PPC subtracts 1 unit (since the multiplier is 1) from
the resouce pool with PoolID=1.
At some point in time, the user requests another chargeable service,
namely service A (8). The PPC generates an Authorize-Only Access
Request that contains the usual RADIUS attributes and the Service-ID
identifying service A (9). The PPC has determined that service A is
rated in an identical way as at least one more service. Thus,
service A has been configured to belong to a rating group, in this
example the group with Rating-Group-ID=1. This identifier is
included is message (9), which is then forwarded to the PPS (10).
Upon reception of message (10), the PPS identifies the user and his
account from the base RADIUS attributes, the fact that a prepaid
session is ongoing, and determines that enough credit remains in the
prepaid account in order for service A to be provided. The PPS also
determines that service A is rated at 0.10 euros per minute. The PPS
decides to reserve another 5 euros from the users account; this
corresponds to 50 minutes or, as encoded in the DurationQuota AVP,
3000 seconds. As service A draws from the same prepaid account as
the access service, the PPS associates this reservation with the same
resource pool as the previous reservation (QID=5), namely the pool
with PoolID=1. Note that, in order for the abstract units in the
pool to be consistent, the multiplier has to be 1747.63. This is
because each second corresponds to about 0.10 / 60 = 0.00167 euros,
which is about 1747.63 times the value of an abstract resource pool
unit, as this was determined by the first allocation of quota to the
pool (i.e. 0.000095367431640625 Eurocents). Since this is a new
quota reservation, the PPS also allocates a new QuotaIdentifier to
it, in this example QID=7. The resulting RADIUS message is sent to
the home AAA server (11) and forwarded to the PPC (12).
Upon reception of message (12), the PPC adjusts the units in resource
pool 1. That is, it first determines how much quota had been
allocated to service A in the past, and subtracts this from the quota
reservation found in the message. Since this is the first quota
reservation for service A, there is nothing to subtract. Thus, it
adds 3000 * 1747.63 = 5242890 units to the pool and remembers that
3000 seconds have been allocated to service A during this prepaid
session. The PPC then provides service A to the user, and meters it
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against resource pool 1. That is, for every second it subtracts
1747.63 units from the pool.
At some point in time, the user requests service B (13). The PPC
determines that service B is rated exactly in the same way as service
A, i.e. that they belong to the same rating group, namely the one
with Rating-Group-ID=1. Since this rating group has been effectively
authorised by the allocation of quota with QID=7, the PPC provides
service B to the user immediately. It is rated in the same way as
service A, i.e. for every second provided, 1747.63 units are
subtracted from credit pool 1.
At some point in time, resource pool 1 is close to exhaustion. (For
example, the PPC may determine that the pool is "close to exhaustion"
when has less than 10% its initial amount of units.) At that point,
the PPC needs to ask for replenishment for the pool. Suppose that,
at that point in time, 4MB of "access service", 45 minutes of
"service A", and 10 minutes of "service B" were provided to the user.
Note that this corresponds to (4*1048576) + (55*60*1747.63) = 4194304
+ 5767179 = 9961483 abstract service units from the pool. The PPC
constructs an Authorize-Only Access Request message that reports the
usage for the "access service" and "service A". This message
contains two PPAQ AVPS, is sent to the home AAA server (14) and
forwarded to the PPS (15). Note that is the message it appears that
"service A" has consumed more than it was allocated (i.e. 55 minutes
although only 50 minutes were initially allocated to it). This is
not a a problem since the PPS knows that "service A" was drawing from
the same pool as the "access service" and that the "access service"
did only consume 4 out of the 5 MB it was allocated.
Upon reception of message (15), the PPS subtracts 4 euros from the
user's account for the "access service" and another 5.5 euros for
"service A". (This includes the charge incurred by "service B" up to
that point in time, although the PPS is not aware of "service B"
being provisioned to the user.) The PPS then determines that
sufficient funds remain in the prepaid account in order for both
services to be continued. The PPS decides to reserve another 3MB for
the access service and 30 minutes for "service A". This corresponds
to 3+3=6 euros. Since in RADIUS prepaid the quotas are encoded in a
cumulative manner, the PPAQ attribute that grants the quota for the
"access service" contains a Volume-Quota AVP of 8MB (8388608 octets),
which is the 5MB that were initially allocated, plus the 3MB
allocated now. The resource pool identifier is, as previously,
PoolID=1 and the multiplier is 1. Similarly, the PPAQ that grants
quota for "service A" contains 4800 seconds (the initial 3000 plus
1800 that correspond to the 30 additional minutes). Again, the
PoolID=1 and multiplier=1747.63. The resulting Access Response
message is sent to the home AAA server (16) and forwarded to the PPC
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(17).
When the PPC received message (17) it checks how much quota has been
allocated previously to the "access service". It finds that the
answer is 5MB (5242880 octets); thus, out of the 8MB (8388608 octets)
that are indicated by the PPAQ with QID=8, only 3MB (3145728 octets)
have not yet been added to resource pool 1. The PPC thus adds
3145728 abstract units to resource pool 1 (since the multiplier is
1). The PPC then acts similarly on the other PPAQ attribute that
exists in message (17). That is, the PPC determines that 3000
seconds of quota for "service A" had already been added to the pool.
Thus only 1800 out of the 4800 should be additionally added to the
pool. Since the applicable multiplier here is 1747.63, the PPC adds
further 3145734 abstract units to the pool 1.
The PPC then continues to provide the access service, "service A" and
"service B" to the user, and meters them against the pool, as
previously.
At some point the user logs off (18). The PPC must then report how
many resources were consumed, so that the PPC can subtract the
appropriate monetary amount from the user's prepaid account. To this
end the PPC constructs an Authorize-Only Access Request message with
two PPAQ AVPs; one for the access service and one for "service A".
Suppose that, in total, 6.5MB were consumed by the access service, 70
minutes were consumed by "service A" and 20 minutes by "service B".
The PPC reports 6.5MB (6815744 octets) and 90 minutes (5400 seconds)
in its final message (19). This is forwarded to the PPS which
determines, from the previous records, that the access service
consumed another 2.5MB (since 4MB out of the 6.5MB were already
reported in message (15), and that "service A" consumed further 600
seconds. This corresponds to 2.5 + (600/60)*0.1 = 2.5+1=3.5 euros.
Thus, the PPS only subtracts 2.5 out of the 6 previously reserved
euros from the user's prepaid account and responds with an Access
Response as required by the RADIUS base specification.
The home AAA server likewise responds with an Access Response.
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Authors' Addresses
Avi Lior
Bridgewater Systems
303 Terry Fox Drive
Ottawa, Ontario Suite 100
Canada
Email: avi@bridgewatersystems.com
Parviz Yegani
Cisco
Mobile Wireless Group, Cisco Systems
3625 Cisco Way, San Jose, California 95134
USA
Email: pyegani@cisco.com
Kuntal Chowdhury
Starent Networks
30 International Place, 3rd Floor
Tewksbury, MA 01876
USA
Email: kchowdhury@starentnetworks.com
Hannes Tschofenig
Siemens
Otto-Hahn Ring 6
Munich, Bavaria 81739
Germany
Email: hannes.tschofenig@siemens.com
Andreas Pashalidis
Siemens
Otto-Hahn Ring 6
Munich, Bavaria 81739
Germany
Email: andreas.pashalidis@siemens.com
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