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rfc:rfc8506

Internet Engineering Task Force (IETF) L. Bertz, Ed. Request for Comments: 8506 Sprint Obsoletes: 4006 D. Dolson, Ed. Category: Standards Track Y. Lifshitz, Ed. ISSN: 2070-1721 Sandvine

                                                            March 2019
                Diameter Credit-Control Application

Abstract

 This document specifies a Diameter application that can be used to
 implement real-time credit-control for a variety of end-user services
 such as network access, Session Initiation Protocol (SIP) services,
 messaging services, and download services.  The Diameter Credit-
 Control application as defined in this document obsoletes RFC 4006,
 and it must be supported by all new Diameter Credit-Control
 application implementations.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8506.

Bertz, et al. Standards Track [Page 1] RFC 8506 Diameter Credit-Control Application March 2019

Copyright Notice

 Copyright (c) 2019 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Bertz, et al. Standards Track [Page 2] RFC 8506 Diameter Credit-Control Application March 2019

Table of Contents

 1. Introduction ....................................................6
    1.1. Requirements Language ......................................7
    1.2. Terminology ................................................7
    1.3. Advertising Application Support ............................9
 2. Architecture Models .............................................9
 3. Credit-Control Messages ........................................11
    3.1. Credit-Control-Request (CCR) Command ......................11
    3.2. Credit-Control-Answer (CCA) Command .......................12
 4. Credit-Control Application Overview ............................13
    4.1. Service-Specific Rating Input and Interoperability ........14
         4.1.1. Specifying Rating Input AVPs .......................15
         4.1.2. Service-Specific Documentation .....................16
         4.1.3. Handling of Unsupported/Incorrect Rating Input .....16
         4.1.4. RADIUS Vendor-Specific Rating Attributes ...........17
 5. Session-Based Credit-Control ...................................17
    5.1. General Principles ........................................17
         5.1.1. Basic Support for Tariff Time Change ...............18
         5.1.2. Credit-Control for Multiple Services within
                a (Sub-)Session ....................................19
    5.2. First Interrogation .......................................23
         5.2.1. First Interrogation after Authorization and
                Authentication .....................................25
         5.2.2. First Interrogation Included with
                Authorization Messages .............................27
    5.3. Intermediate Interrogation ................................29
    5.4. Final Interrogation .......................................31
    5.5. Server-Initiated Credit Re-authorization ..................32
    5.6. Graceful Service Termination ..............................34
         5.6.1. Terminate Action ...................................37
         5.6.2. Redirect Action ....................................38
         5.6.3. Restrict Access Action .............................40
         5.6.4. Usage of the Server-Initiated Credit
                Re-authorization ...................................41
    5.7. Failure Procedures ........................................41
 6. One-Time Event .................................................44
    6.1. Service Price Inquiry .....................................45
    6.2. Balance Checks ............................................46
    6.3. Direct Debiting ...........................................46
    6.4. Refunds ...................................................47
    6.5. Failure Procedure .........................................48
 7. Credit-Control Application State Machines ......................50
 8. Credit-Control AVPs ............................................59
    8.1. CC-Correlation-Id AVP .....................................61
    8.2. CC-Request-Number AVP .....................................62
    8.3. CC-Request-Type AVP .......................................62
    8.4. CC-Session-Failover AVP ...................................63

Bertz, et al. Standards Track [Page 3] RFC 8506 Diameter Credit-Control Application March 2019

    8.5. CC-Sub-Session-Id AVP .....................................64
    8.6. Check-Balance-Result AVP ..................................64
    8.7. Cost-Information AVP ......................................64
    8.8. Unit-Value AVP ............................................65
    8.9. Exponent AVP ..............................................65
    8.10. Value-Digits AVP .........................................66
    8.11. Currency-Code AVP ........................................66
    8.12. Cost-Unit AVP ............................................66
    8.13. Credit-Control AVP .......................................66
    8.14. Credit-Control-Failure-Handling AVP (CCFH) ...............67
    8.15. Direct-Debiting-Failure-Handling AVP (DDFH) ..............68
    8.16. Multiple-Services-Credit-Control AVP .....................68
    8.17. Granted-Service-Unit AVP .................................70
    8.18. Requested-Service-Unit AVP ...............................71
    8.19. Used-Service-Unit AVP ....................................71
    8.20. Tariff-Time-Change AVP ...................................72
    8.21. CC-Time AVP ..............................................72
    8.22. CC-Money AVP .............................................72
    8.23. CC-Total-Octets AVP ......................................72
    8.24. CC-Input-Octets AVP ......................................72
    8.25. CC-Output-Octets AVP .....................................73
    8.26. CC-Service-Specific-Units AVP ............................73
    8.27. Tariff-Change-Usage AVP ..................................73
    8.28. Service-Identifier AVP ...................................74
    8.29. Rating-Group AVP .........................................74
    8.30. G-S-U-Pool-Reference AVP .................................74
    8.31. G-S-U-Pool-Identifier AVP ................................75
    8.32. CC-Unit-Type AVP .........................................75
    8.33. Validity-Time AVP ........................................75
    8.34. Final-Unit-Indication AVP ................................76
    8.35. Final-Unit-Action AVP ....................................77
    8.36. Restriction-Filter-Rule AVP ..............................78
    8.37. Redirect-Server AVP ......................................78
    8.38. Redirect-Address-Type AVP ................................79
    8.39. Redirect-Server-Address AVP ..............................79
    8.40. Multiple-Services-Indicator AVP ..........................80
    8.41. Requested-Action AVP .....................................80
    8.42. Service-Context-Id AVP ...................................81
    8.43. Service-Parameter-Info AVP ...............................82
    8.44. Service-Parameter-Type AVP ...............................82
    8.45. Service-Parameter-Value AVP ..............................83
    8.46. Subscription-Id AVP ......................................83
    8.47. Subscription-Id-Type AVP .................................83
    8.48. Subscription-Id-Data AVP .................................84
    8.49. User-Equipment-Info AVP ..................................84
    8.50. User-Equipment-Info-Type AVP .............................84
    8.51. User-Equipment-Info-Value AVP ............................85
    8.52. User-Equipment-Info-Extension AVP ........................85

Bertz, et al. Standards Track [Page 4] RFC 8506 Diameter Credit-Control Application March 2019

    8.53. User-Equipment-Info-IMEISV AVP ...........................86
    8.54. User-Equipment-Info-MAC AVP ..............................86
    8.55. User-Equipment-Info-EUI64 AVP ............................86
    8.56. User-Equipment-Info-ModifiedEUI64 AVP ....................86
    8.57. User-Equipment-Info-IMEI AVP .............................86
    8.58. Subscription-Id-Extension AVP ............................87
    8.59. Subscription-Id-E164 AVP .................................87
    8.60. Subscription-Id-IMSI AVP .................................87
    8.61. Subscription-Id-SIP-URI AVP ..............................88
    8.62. Subscription-Id-NAI AVP ..................................88
    8.63. Subscription-Id-Private AVP ..............................88
    8.64. Redirect-Server-Extension AVP ............................88
    8.65. Redirect-Address-IPAddress AVP ...........................89
    8.66. Redirect-Address-URL AVP .................................89
    8.67. Redirect-Address-SIP-URI AVP .............................89
    8.68. QoS-Final-Unit-Indication AVP ............................89
 9. Result-Code AVP Values .........................................91
    9.1. Transient Failures ........................................91
    9.2. Permanent Failures ........................................92
 10. AVP Occurrence Table ..........................................92
    10.1. Credit-Control AVP Table .................................93
    10.2. Re-Auth-Request/Re-Auth-Answer AVP Table .................94
 11. RADIUS/Diameter Credit-Control Interworking Model .............94
 12. IANA Considerations ...........................................97
    12.1. Application Identifier ...................................97
    12.2. Command Codes ............................................97
    12.3. AVP Codes ................................................97
    12.4. Result-Code AVP Values ...................................98
    12.5. CC-Request-Type AVP ......................................98
    12.6. CC-Session-Failover AVP ..................................98
    12.7. CC-Unit-Type AVP .........................................99
    12.8. Check-Balance-Result AVP .................................99
    12.9. Credit-Control AVP .......................................99
    12.10. Credit-Control-Failure-Handling AVP .....................99
    12.11. Direct-Debiting-Failure-Handling AVP ....................99
    12.12. Final-Unit-Action AVP ...................................99
    12.13. Multiple-Services-Indicator AVP ........................100
    12.14. Redirect-Address-Type AVP ..............................100
    12.15. Requested-Action AVP ...................................100
    12.16. Subscription-Id-Type AVP ...............................100
    12.17. Tariff-Change-Usage AVP ................................100
    12.18. User-Equipment-Info-Type AVP ...........................100
 13. Parameters Related to the Credit-Control Application .........101
 14. Security Considerations ......................................101
    14.1. Direct Connection with Redirects ........................102
    14.2. Application-Level Redirects .............................103

Bertz, et al. Standards Track [Page 5] RFC 8506 Diameter Credit-Control Application March 2019

 15. Privacy Considerations .......................................104
    15.1. Privacy-Sensitive AVPs ..................................104
    15.2. Data Minimization .......................................106
    15.3. Diameter Agents .........................................107
 16. References ...................................................107
    16.1. Normative References ....................................107
    16.2. Informative References ..................................110
 Appendix A. Credit-Control Sequences .............................111
   A.1. Flow I ....................................................111
   A.2. Flow II ...................................................113
   A.3. Flow III ..................................................116
   A.4. Flow IV ...................................................117
   A.5. Flow V ....................................................119
   A.6. Flow VI ...................................................120
   A.7. Flow VII ..................................................121
   A.8. Flow VIII .................................................123
   A.9. Flow IX ...................................................124
 Acknowledgements .................................................130
 Authors' Addresses ...............................................130

1. Introduction

 This document specifies a Diameter application that can be used to
 implement real-time credit-control for a variety of end-user services
 such as network access, Session Initiation Protocol (SIP) services,
 messaging services, and download services.  ("Credit-control" is
 sometimes abbreviated as "CC" in figures and tables throughout this
 document.)  The Diameter Credit-Control application as defined in
 this document obsoletes [RFC4006], and it must be supported by all
 new Diameter Credit-Control application implementations.  This
 document provides a general solution to real-time cost and
 credit-control.
 The prepaid model has been shown to be very successful -- for
 instance, in GSM networks, where network operators offering prepaid
 services have experienced a substantial growth of their customer base
 and revenues.  Prepaid services are now cropping up in many other
 wireless and wire-line-based networks.
 In mobile networks, additional functionality is required beyond that
 specified in the Diameter base protocol [RFC6733].  For example, the
 3GPP charging and billing requirements document [TGPPCHARG] states
 that an application must be able to rate service information in
 real time.  In addition, it is necessary to check that the end user's
 account provides coverage for the requested service prior to
 initiation of that service.  When an account is exhausted or expired,
 the user must be denied the ability to compile additional chargeable
 events.

Bertz, et al. Standards Track [Page 6] RFC 8506 Diameter Credit-Control Application March 2019

 A mechanism has to be provided to allow the user to be informed of
 the charges to be levied for a requested service.  In addition, there
 are services such as gaming and advertising that may credit as well
 as debit a user account.
 The other Diameter applications provide service-specific
 authorization, and they do not provide credit authorization for
 prepaid users.  The credit authorization shall be generic and
 applicable to all the service environments required to support
 prepaid services.
 To fulfill these requirements, it is necessary to facilitate
 credit-control communication between the network element providing
 the service (e.g., Network Access Server (NAS), SIP Proxy,
 Application Server) and a credit-control server.
 The scope of this specification is credit authorization.  Service-
 specific authorization and authentication are out of scope.

1.1. Requirements Language

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

1.2. Terminology

 AAA:  Authentication, Authorization, and Accounting.
 AA-Answer:  "AA-Answer" generically refers to a service-specific
    authorization and authentication answer.  AA-Answer commands are
    defined in service-specific authorization applications, e.g.,
    [RFC7155] [RFC4004].
 AA-Request:  "AA-Request" generically refers to a service-specific
    authorization and authentication request.  AA-Request commands are
    defined in service-specific authorization applications, e.g.,
    [RFC7155] [RFC4004].
 Credit-control:  "Credit-control" is a mechanism that directly
    interacts in real time with an account and controls or monitors
    the charges related to service usage.  Credit-control is a
    process of (1) checking whether or not credit is available,
    (2) credit reservation, (3) deduction of credit from the end-user
    account when service is completed, and (4) refunding of reserved
    credit that is not used.

Bertz, et al. Standards Track [Page 7] RFC 8506 Diameter Credit-Control Application March 2019

 Diameter Credit-Control server:  A Diameter Credit-Control server
    acts as a prepaid server, performing real-time rating and
    credit-control.  It is located in the home domain and is accessed
    by Service Elements or Diameter AAA servers in real time, for the
    purpose of price determination and credit-control before the
    service event is delivered to the end user.  It may also interact
    with Business Support Systems.
 Diameter Credit-Control client:  A Diameter Credit-Control client is
    an entity that interacts with a credit-control server.  It
    monitors the usage of the granted quota according to instructions
    returned by the credit-control server.
 Interrogation:  The Diameter Credit-Control client uses interrogation
    to initiate a session-based credit-control process.  During the
    credit-control process, it is used to report the used quota and
    request a new one.  An interrogation maps to a request/answer
    transaction.
 One-time event:  A charging transaction session comprising a single
    request and single response.
 Rating:  The act of determining the cost of the service event.
 Service:  A type of task performed by a Service Element for an
    end user.
 Service Element:  A network element that provides a service to the
    end users.  The Service Element may include the Diameter
    Credit-Control client or another entity (e.g., a RADIUS AAA
    server) that can act as a credit-control client on behalf of the
    Service Element.  In the latter case, the interface between the
    Service Element and the Diameter Credit-Control client is outside
    the scope of this specification.  Examples of Service Elements
    include NASs, SIP Proxies, and Application Servers such as
    messaging servers, content servers, and gaming servers.
 Service event:  An event relating to a service provided to the
    end user.
 Session-based credit-control:  A credit-control process that makes
    use of several interrogations: the first, a possible intermediate,
    and the final.  The first interrogation is used to reserve money
    from the user's account and to initiate the process.  Intermediate
    interrogations (if any) may be needed to request a new quota while
    the service is being rendered.  The final interrogation is used to
    exit the process.  The credit-control server is required to
    maintain session state for session-based credit-control.

Bertz, et al. Standards Track [Page 8] RFC 8506 Diameter Credit-Control Application March 2019

1.3. Advertising Application Support

 Diameter nodes conforming to this specification MUST advertise
 support by including the value of 4 in the Auth-Application-Id of the
 Capabilities-Exchange-Request and Capabilities-Exchange-Answer
 commands [RFC6733].

2. Architecture Models

 The current accounting models specified in the RADIUS accounting and
 Diameter base specifications [RFC2866] [RFC6733] are not sufficient
 for real-time credit-control, where creditworthiness is to be
 determined prior to service initiation.  Also, the existing Diameter
 authorization applications [RFC7155] [RFC4004] only provide service
 authorization; they do not provide credit authorization for prepaid
 users.  In order to support real-time credit-control, a new type of
 server is needed in the AAA infrastructure: the Diameter
 Credit-Control server.  The Diameter Credit-Control server is the
 entity responsible for credit authorization for prepaid subscribers.
 A Service Element may authenticate and authorize the end user with
 the AAA server by using AAA protocols, e.g., RADIUS or the Diameter
 base protocol (possibly extended via a Diameter application).
 Accounting protocols such as RADIUS accounting and the Diameter base
 accounting protocol can be used to provide accounting data to the
 accounting server after service is initiated and to provide possible
 interim reports until service completion.  However, for real-time
 credit-control, these authorization and accounting models are not
 sufficient.
 When real-time credit-control is required, the credit-control client
 contacts the credit-control server with information about a possible
 service event.  The credit-control process is performed to determine
 potential charges and to verify whether the end user's account
 balance is sufficient to cover the cost of the service being
 rendered.
 Figure 1 illustrates the typical credit-control architecture, which
 consists of a Service Element with an embedded Diameter
 Credit-Control client, a Diameter Credit-Control server, and a AAA
 server.  A Business Support System is usually deployed; at a minimum,
 it includes billing functionality.  The credit-control server and AAA
 server in this architecture model are logical entities.  The real
 configuration can combine them into a single host.  The
 credit-control protocol is the Diameter base protocol [RFC6733] with
 the Diameter Credit-Control application.

Bertz, et al. Standards Track [Page 9] RFC 8506 Diameter Credit-Control Application March 2019

 When an end user requests services such as SIP or messaging, the
 request is typically forwarded to a Service Element (e.g., a SIP
 Proxy) in the user's home realm as defined in [RFC6733].  In some
 cases, it might be possible that the Service Element in the local
 realm [RFC6733] can offer services to the end user; however, a
 commercial agreement must exist between the local realm and the home
 realm.  Network access is an example of a service offered in the
 local realm where the NAS, through a AAA infrastructure,
 authenticates and authorizes the user with the user's home network.
                Service Element   AAA and CC
 +----------+      +---------+     Protocols+-----------+  +--------+
 |  End     |<---->|+-------+|<------------>|    AAA    |  |Business|
 |  User    |   +->|| CC    ||              |   Server  |->|Support |
 |          |   |  || Client||<-----+       |           |  |System  |
 +----------+   |  |+-------+|      |       +-----------+  |        |
                |  +---------+      |             ^        +--------+
 +----------+   |                   | CC Protocol |             ^
 |  End     |<--+                   |       +-----v----+        |
 |  User    |                       +------>|Credit-   |        |
 +----------+                Credit-Control |Control   |--------+
                             Protocol       |Server    |
                                            +----------+
             Figure 1: Typical Credit-Control Architecture
 There can be multiple credit-control servers in the system for
 redundancy and load balancing.  The system can also contain separate
 rating server(s), and accounts can be located in a centralized
 database.  To ensure that the end user's account is not debited or
 credited multiple times for the same service event, only one entity
 in the credit-control system should perform duplicate detection.
 System-internal interfaces can exist to relay messages between
 servers and an account manager.  However, the detailed architecture
 of the credit-control system and its interfaces is implementation
 specific and is out of scope for this specification.
 Protocol-transparent Diameter relays can exist between the
 credit-control client and credit-control server.  Also, Diameter
 redirect agents that refer credit-control clients to credit-control
 servers and allow them to communicate directly can exist.  These
 agents transparently support the Diameter Credit-Control application.
 The different roles of Diameter agents are defined in Diameter base
 [RFC6733], Section 2.8.
 If Diameter Credit-Control proxies exist between the credit-control
 client and the credit-control server, they MUST advertise support for
 the Diameter Credit-Control application.

Bertz, et al. Standards Track [Page 10] RFC 8506 Diameter Credit-Control Application March 2019

3. Credit-Control Messages

 This section defines new Diameter message Command Code values that
 MUST be supported by all Diameter implementations that conform to
 this specification.  The Command Codes are as follows:
       +------------------------+---------+------+-------------+
       | Command Name           | Abbrev. | Code | Reference   |
       +------------------------+---------+------+-------------+
       | Credit-Control-Request | CCR     | 272  | Section 3.1 |
       | Credit-Control-Answer  | CCA     | 272  | Section 3.2 |
       +------------------------+---------+------+-------------+
                   Table 1: Credit-Control Commands
 Section 3.2 of [RFC6733] (Diameter base) defines the Command Code
 Format specification.  These formats are observed in credit-control
 messages.

3.1. Credit-Control-Request (CCR) Command

 The Credit-Control-Request message (CCR) is indicated by the Command
 Code field being set to 272 and the 'R' bit being set in the Command
 Flags field.  It is used between the Diameter Credit-Control client
 and the credit-control server to request credit authorization for a
 given service.
 The Auth-Application-Id MUST be set to the value 4, indicating the
 Diameter Credit-Control application.
 The CCR is extensible via the inclusion of one or more
 Attribute-Value Pairs (AVPs).
 Message Format:
 <Credit-Control-Request> ::= < Diameter Header: 272, REQ, PXY >
                              < Session-Id >
                              { Origin-Host }
                              { Origin-Realm }
                              { Destination-Realm }
                              { Auth-Application-Id }
                              { Service-Context-Id }
                              { CC-Request-Type }
                              { CC-Request-Number }
                              [ Destination-Host ]
                              [ User-Name ]
                              [ CC-Sub-Session-Id ]
                              [ Acct-Multi-Session-Id ]

Bertz, et al. Standards Track [Page 11] RFC 8506 Diameter Credit-Control Application March 2019

                              [ Origin-State-Id ]
                              [ Event-Timestamp ]
                             *[ Subscription-Id ]
                             *[ Subscription-Id-Extension ]
                              [ Service-Identifier ]
                              [ Termination-Cause ]
                              [ Requested-Service-Unit ]
                              [ Requested-Action ]
                             *[ Used-Service-Unit ]
                              [ Multiple-Services-Indicator ]
                             *[ Multiple-Services-Credit-Control ]
                             *[ Service-Parameter-Info ]
                              [ CC-Correlation-Id ]
                              [ User-Equipment-Info ]
                              [ User-Equipment-Info-Extension ]
                             *[ Proxy-Info ]
                             *[ Route-Record ]
                             *[ AVP ]

3.2. Credit-Control-Answer (CCA) Command

 The Credit-Control-Answer message (CCA) is indicated by the Command
 Code field being set to 272 and the 'R' bit being cleared in the
 Command Flags field.  It is used between the credit-control server
 and the Diameter Credit-Control client to acknowledge a
 Credit-Control-Request command.
 Message Format:
      <Credit-Control-Answer> ::= < Diameter Header: 272, PXY >
                                  < Session-Id >
                                  { Result-Code }
                                  { Origin-Host }
                                  { Origin-Realm }
                                  { Auth-Application-Id }
                                  { CC-Request-Type }
                                  { CC-Request-Number }
                                  [ User-Name ]
                                  [ CC-Session-Failover ]
                                  [ CC-Sub-Session-Id ]
                                  [ Acct-Multi-Session-Id ]
                                  [ Origin-State-Id ]
                                  [ Event-Timestamp ]
                                  [ Granted-Service-Unit ]
                                 *[ Multiple-Services-Credit-Control ]
                                  [ Cost-Information ]
                                  [ Final-Unit-Indication ]
                                  [ QoS-Final-Unit-Indication ]

Bertz, et al. Standards Track [Page 12] RFC 8506 Diameter Credit-Control Application March 2019

                                  [ Check-Balance-Result ]
                                  [ Credit-Control-Failure-Handling ]
                                  [ Direct-Debiting-Failure-Handling ]
                                  [ Validity-Time ]
                                 *[ Redirect-Host ]
                                  [ Redirect-Host-Usage ]
                                  [ Redirect-Max-Cache-Time ]
                                 *[ Proxy-Info ]
                                 *[ Route-Record ]
                                 *[ Failed-AVP ]
                                 *[ AVP ]

4. Credit-Control Application Overview

 The credit authorization process takes place before and during
 service delivery to the end user and generally requires the user's
 authentication and authorization before any requests are sent to the
 credit-control server.  The credit-control application defined in
 this specification supports two different credit authorization
 models: credit authorization with money reservation and credit
 authorization with direct debiting.  In both models, the
 credit-control client requests credit authorization from the
 credit-control server prior to allowing any services to be delivered
 to the end user.
 In the first model, the credit-control server rates the request,
 reserves a suitable amount of money from the user's account, and
 returns the amount of credit reserved.  Note that credit resources
 may not imply actual monetary credit; credit resources may be granted
 to the credit-control client in the form of units (e.g., data volume
 or time) to be metered.
 Upon receipt of a successful credit authorization answer with a
 certain amount of credit resources, the credit-control client allows
 service delivery to the end user and starts monitoring the usage of
 the granted resources.  When the credit resources granted to the user
 have been consumed or the service has been successfully delivered or
 terminated, the credit-control client reports back to the server the
 used amount.  The credit-control server deducts the used amount from
 the end user's account; it may perform rating and make a new credit
 reservation if the service delivery is continuing.  This process is
 accomplished with session-based credit-control that includes the
 first interrogation, possible intermediate interrogations, and the
 final interrogation.  For session-based credit-control, both the
 credit-control client and the credit-control server are required to
 maintain credit-control session state.  Session-based credit-control
 is described in more detail, with more variations, in Section 5.

Bertz, et al. Standards Track [Page 13] RFC 8506 Diameter Credit-Control Application March 2019

 In contrast, credit authorization with direct debiting is a
 single-transaction process wherein the credit-control server directly
 deducts a suitable amount of money from the user's account as soon as
 the credit authorization request is received.  Upon receipt of a
 successful credit authorization answer, the credit-control client
 allows service delivery to the end user.  This process is
 accomplished with the one-time event.  Session state is not
 maintained.
 In a multi-service environment, an end user can issue an additional
 service request (e.g., data service) during an ongoing service (e.g.,
 voice call) toward the same account.  Alternatively, during an active
 multimedia session, an additional media type is added to the session,
 causing a new simultaneous request toward the same account.
 Consequently, this needs to be considered when credit resources are
 granted to the services.
 The credit-control application also supports operations such as
 service price inquiries, user's balance checks, and refunds of credit
 on the user's account.  These operations are accomplished with the
 one-time event.  Session state is not maintained.
 Flexible failure handling, specific to the credit-control
 application, is defined in the application.  This allows the service
 provider to control the credit-control client's behavior according to
 its own risk management policy.
 The Credit-Control-Failure-Handling AVP (also referred to as the
 CCFH) and the Direct-Debiting-Failure-Handling AVP (also referred to
 as the DDFH) are defined to determine what is done if the sending of
 credit-control messages to the credit-control server has been
 temporarily prevented.  The usage of the CCFH and the DDFH allows
 flexibility, as failure handling for the credit-control session and
 one-time event direct debiting may be different.

4.1. Service-Specific Rating Input and Interoperability

 The Diameter Credit-Control application defines the framework for
 credit-control; it provides generic credit-control mechanisms
 supporting multiple service applications.  The credit-control
 application therefore does not define AVPs that could be used as
 input in the rating process.  Listing the possible services that
 could use this Diameter application is out of scope for this generic
 mechanism.

Bertz, et al. Standards Track [Page 14] RFC 8506 Diameter Credit-Control Application March 2019

 It is reasonable to expect that a service level agreement will exist
 between providers of the credit-control client and the credit-control
 server covering the charging, services offered, roaming agreements,
 agreed-upon rating input (i.e., AVPs), and so on.
 Therefore, it is assumed that a Diameter Credit-Control server will
 provide service only for Diameter Credit-Control clients that have
 agreed beforehand as to the content of credit-control messages.
 Naturally, it is possible that any arbitrary Diameter Credit-Control
 client can interchange credit-control messages with any Diameter
 Credit-Control server, but with a higher likelihood that unsupported
 services/AVPs could be present in the credit-control message, causing
 the server to reject the request with an appropriate Result-Code.

4.1.1. Specifying Rating Input AVPs

 There are two ways to provide rating input to the credit-control
 server: by either using AVPs or including the rating input in the
 Service-Parameter-Info AVP.  The general principles for sending
 rating parameters are as follows:
 1.  Using AVPs:
     A.  The service SHOULD reuse existing AVPs if it can use AVPs
         defined in existing Diameter applications (e.g., [RFC7155]
         for network access services).  [RFC6733] strongly recommends
         the reuse of existing AVPs.
         For AVPs of type Enumerated, the service may require a new
         value to be defined.  Allocation of new AVP values is done as
         specified in [RFC6733], Section 1.3.
     B.  New AVPs can be defined if the existing AVPs do not provide
         sufficient rating information.  In this case, the procedures
         defined in [RFC6733] for creating new AVPs MUST be followed.
     C.  For services specific only to one vendor's implementation, a
         vendor-specific AVP code for private use can be used.  Where
         a vendor-specific AVP is implemented by more than one vendor,
         allocation of global AVPs is encouraged instead; refer to
         [RFC6733].
 2.  The Service-Parameter-Info AVP MAY be used as a container to pass
     legacy rating information in its original encoded form (e.g.,
     ASN.1 BER).  This method can be used to avoid unnecessary
     conversions from an existing data format to an AVP format.  In
     this case, the rating input is embedded in the Service-Parameter-
     Info AVP as defined in Section 8.43.

Bertz, et al. Standards Track [Page 15] RFC 8506 Diameter Credit-Control Application March 2019

 New service applications SHOULD favor the use of explicitly defined
 AVPs as described in items 1a and 1b, to simplify interoperability.

4.1.2. Service-Specific Documentation

 The service-specific rating input AVPs, and the contents of the
 Service-Parameter-Info AVP or Service-Context-Id AVP (defined in
 Section 8.42), are not within the scope of this document.  To
 facilitate interoperability, it is RECOMMENDED that the rating input
 and the values of the Service-Context-Id be coordinated via an
 informational RFC or other permanent and readily available reference
 (preferably that of another cooperative standardization body, e.g.,
 3GPP, the Open Mobile Alliance (OMA), or 3GPP2).  However, private
 services may be deployed that are subject to agreements between
 providers of the credit-control server and client.  In this case,
 vendor-specific AVPs can be used.
 This specification, together with the above-mentioned service-
 specific documents, governs the credit-control message.  Service-
 specific documents (i.e., those documents that do not define new
 credit-control applications) define which existing AVPs or new AVPs
 are used as input to the rating process; thus, the AVPs in question
 have to be included in the Credit-Control-Request command by a
 Diameter Credit-Control client supporting a given service as
 "* [AVP]".  Should the Service-Parameter-Info AVP be used, the
 service-specific document MUST specify the exact content of this
 Grouped AVP.
 The Service-Context-Id AVP MUST be included at the command level of a
 Credit-Control-Request to identify the service-specific document that
 applies to the request.  The specific service or rating-group the
 request relates to is uniquely identified by the combination of
 Service-Context-Id and Service-Identifier or rating-group.

4.1.3. Handling of Unsupported/Incorrect Rating Input

 Diameter Credit-Control implementations are required to support
 mandatory rating-related AVPs defined in service-specific documents
 for the services they support, according to the 'M' bit rules in
 [RFC6733].
 If a rating input required for the rating process is incorrect in
 the Credit-Control-Request or if the credit-control server does not
 support the requested service context (identified by the
 Service-Context-Id AVP at the command level), the
 Credit-Control-Answer MUST contain the error code
 DIAMETER_RATING_FAILED.  A CCA message with this error MUST contain
 one or more Failed-AVP AVPs containing the missing and/or unsupported

Bertz, et al. Standards Track [Page 16] RFC 8506 Diameter Credit-Control Application March 2019

 AVPs that caused the failure.  A Diameter Credit-Control client that
 receives the error code DIAMETER_RATING_FAILED in response to a
 request MUST NOT send similar requests in the future.

4.1.4. RADIUS Vendor-Specific Rating Attributes

 When service-specific documents include RADIUS vendor-specific
 attributes that could be used as input in the rating process, the
 rules described in [RFC7155] for formatting the Diameter AVP MUST be
 followed.
 For example, if the AVP code used is the vendor attribute type code,
 the Vendor-Specific flag MUST be set to 1 and the Vendor-Id MUST be
 set to the IANA Vendor identification value.  The Diameter AVP Data
 field contains only the attribute value of the RADIUS attribute.

5. Session-Based Credit-Control

5.1. General Principles

 For session-based credit-control, several interrogations are needed:
 the first, the intermediate (optional), and the final.  This is
 illustrated in Figures 3 and 4 (Sections 5.2.1 and 5.2.2).
 If the credit-control client performs credit reservation before
 granting service to the end user, it MUST use several interrogations
 toward the credit-control server (i.e., session-based
 credit-control).  In this case, the credit-control server MUST
 maintain the credit-control session state.
 Each credit-control session MUST have a globally unique Session-Id as
 defined in [RFC6733]; this Session-Id MUST NOT be changed during the
 lifetime of a credit-control session.
 Certain applications require multiple credit-control sub-sessions.
 These applications would send messages with a constant Session-Id AVP
 but with a different CC-Sub-Session-Id AVP.  If several credit
 sub-sessions will be used, all sub-sessions MUST be closed separately
 before the main session is closed so that units per sub-session may
 be reported.  The absence of the CC-Sub-Session-Id AVP implies that
 no sub-sessions are in use.

Bertz, et al. Standards Track [Page 17] RFC 8506 Diameter Credit-Control Application March 2019

 Note that the Service Element might send a service-specific
 re-authorization message to the AAA server due to expiration of the
 authorization lifetime during an ongoing credit-control session.
 However, the service-specific re-authorization does not influence the
 credit authorization that is ongoing between the credit-control
 client and credit-control server, as credit authorization is
 controlled by the burning rate of the granted quota.
 If service-specific re-authorization fails, the user will be
 disconnected, and the credit-control client MUST send a final
 interrogation to the credit-control server.
 The Diameter Credit-Control server may seek to control the validity
 time of the granted quota and/or the production of intermediate
 interrogations.  Thus, it MAY include the Validity-Time AVP in the
 Answer message to the credit-control client.  Upon expiration of the
 Validity-Time, the credit-control client MUST generate a
 credit-control update request and report the used quota to the
 credit-control server.  It is up to the credit-control server to
 determine the value of the Validity-Time to be used for consumption
 of the granted service unit(s) (G-S-U).  If the Validity-Time is
 used, its value SHOULD be given as input to set the session
 supervision timer Tcc (the session supervision timer MAY be set to
 two times the value of the Validity-Time, as defined in Section 13).
 Since credit-control update requests are also produced at the expiry
 of granted service units and/or for mid-session service events, the
 omission of Validity-Time does not mean that intermediate
 interrogation for the purpose of credit-control is not performed.

5.1.1. Basic Support for Tariff Time Change

 The Diameter Credit-Control server and client MAY optionally support
 a tariff change mechanism.  The Diameter Credit-Control server may
 include a Tariff-Time-Change AVP in the Answer message.  Note that
 the granted units should be allocated based on the worst-case
 scenario, so that the overall reported used units would never exceed
 the credit reservation.  For example, in the case of a forthcoming
 tariff change, in which the new rate is higher, the allocation should
 be given so it does not exceed the credit, assuming that all of it is
 used after the tariff changed.
 When the Diameter Credit-Control client reports the used units and a
 tariff change has occurred during the reporting period, the Diameter
 Credit-Control client MUST separately itemize the units used before
 and after the tariff change.  If the client is unable to distinguish
 whether units straddling the tariff change were used before or after
 the tariff change, the credit-control client MUST itemize those units
 in a third category.

Bertz, et al. Standards Track [Page 18] RFC 8506 Diameter Credit-Control Application March 2019

 If a client does not support the tariff change mechanism and it
 receives a CCA message carrying the Tariff-Time-Change AVP, it MUST
 terminate the credit-control session, giving a reason of
 DIAMETER_BAD_ANSWER in the Termination-Cause AVP.
 For time-based services, the quota is consumed at the rate of the
 passage of real time (ignoring leap seconds).  That is, precisely
 1 second of quota is consumed per second of real time.  At the time
 when credit resources are allocated, the server already knows how
 many units will be consumed before the tariff time change and how
 many units will be consumed afterward.  Similarly, the server can
 determine the units consumed at the "before" rate and the units
 consumed at the "afterward" rate in the event that the end user
 closes the session before the consumption of the allotted quota.
 There is no need for additional traffic between the client and server
 in the case of tariff time changes for continuous time-based service.
 Therefore, the tariff change mechanism is not used for such services.
 For time-based services in which the quota is NOT continuously
 consumed at a regular rate, the tariff change mechanism described for
 volume and event units MAY be used.

5.1.2. Credit-Control for Multiple Services within a (Sub-)Session

 When multiple services are used within the same user session and each
 service or group of services is subject to different cost, it is
 necessary to perform credit-control for each service independently.
 Making use of credit-control sub-sessions to achieve independent
 credit-control will result in increased signaling load and usage of
 resources in both the credit-control client and the credit-control
 server.  For instance, during one network access session, the
 end user may use several HTTP-based services that could be charged
 with different costs.  The network-access-specific attributes, such
 as Quality of Service (QoS), are common to all the services carried
 within the access bearer, but the cost of the bearer may vary,
 depending on its content.
 To support these scenarios optimally, the credit-control application
 enables independent credit-control of multiple services in a single
 credit-control (sub-)session.  This is achieved by including the
 optional Multiple-Services-Credit-Control AVP in Credit-Control-
 Request/Credit-Control-Answer messages.  It is possible to request
 and allocate resources as a credit pool shared between multiple
 services.  The services can be grouped into rating-groups in order to
 achieve even further aggregation of credit allocation.  It is also
 possible to request and allocate quotas on a per-service basis.
 Where quotas are allocated to a pool by means of the Multiple-
 Services-Credit-Control AVP, the quotas remain independent objects

Bertz, et al. Standards Track [Page 19] RFC 8506 Diameter Credit-Control Application March 2019

 that can be re-authorized independently at any time.  Quotas can also
 be given independent result codes, validity times, and Final-Unit-
 Indication AVP values or QoS-Final-Unit-Indication AVP values.
 A rating-group gathers a set of services, identified by a Service-
 Identifier and subject to the same cost and rating type (e.g.,
 $0.1/minute).  It is assumed that the Service Element is provided
 with rating-groups, service-identifiers, and their associated
 parameters that define what has to be metered by means outside the
 scope of this specification.  (Examples of parameters associated to
 service-identifiers are IP 5-tuples and HTTP URLs.)  Service-
 identifiers enable authorization on a per-service-based credit as
 well as itemized reporting of service usage.  It is up to the
 credit-control server whether to authorize credit for one or more
 services or for the whole rating-group.  However, the client SHOULD
 always report used units at the finest supported level of
 granularity.  Where a quota is allocated to a rating-group, all the
 services belonging to that group draw from the allotted quota.
 Figure 2 provides a graphical representation of the relationship
 between service-identifiers, rating-groups, credit pools, and
 credit-control (sub-)sessions.
                Diameter Credit-Control (Sub-)Session
                                |
       +------------+-----------+-------------+--------------- +
       |            |           |             |                |
 Service-Id a Service-Id b Service-Id c Service-Id d.....Service-Id z
       \        /                 \         /                /
        \      /                   \       /                /
         \    /                  Rating-Group 1.......Rating-Group n
          \  /                         |                    |
         Quota       ---------------Quota                 Quota
           |        /                                       |
           |       /                                        |
        Credit Pool                                    Credit Pool
           Figure 2: Multiple-Service (Sub-)Session Example
 If independent credit-control of multiple services is used, the
 Validity-Time AVP, and the Final-Unit-Indication AVP or
 QoS-Final-Unit-Indication AVP, SHOULD be present either in the
 Multiple-Services-Credit-Control AVP(s) or at the command level as
 single AVPs.  However, the Result-Code AVP MAY be present both at the
 command level and within the Multiple-Services-Credit-Control AVP.
 If the Result-Code AVP at the command level indicates a value other
 than SUCCESS, then the Result-Code AVP at the command level takes
 precedence over any other AVPs included in the Multiple-Services-
 Credit-Control AVP.

Bertz, et al. Standards Track [Page 20] RFC 8506 Diameter Credit-Control Application March 2019

 The credit-control client MUST indicate support for independent
 credit-control of multiple services within a (sub-)session by
 including the Multiple-Services-Indicator AVP in the first
 interrogation.  A credit-control server not supporting this feature
 MUST treat the Multiple-Services-Indicator AVP and any received
 Multiple-Services-Credit-Control AVPs as invalid AVPs.
 If the client indicated support for independent credit-control of
 multiple services, a credit-control server that wishes to use the
 feature MUST return the granted units within the Multiple-Services-
 Credit-Control AVP associated to the corresponding service-identifier
 and/or rating-group.
 To avoid a situation where several parallel (and typically also
 small) credit reservations must be made on the same account (i.e.,
 credit fragmentation), and also to avoid unnecessary load on the
 credit-control server, it is possible to provide service units as a
 pool that applies to multiple services or rating-groups.  This is
 achieved by providing the service units in the form of a quota for a
 particular service or rating-group in the Multiple-Services-Credit-
 Control AVP, and also by including a reference to a credit pool for
 that unit type.
 The reference includes a multiplier derived from the rating
 parameter, which translates from service units of a specific type to
 the abstract service units in the pool.  For instance, if the rating
 parameter for service 1 is $1/MB and the rating parameter for
 service 2 is $0.5/MB, the multipliers could be 10 and 5 for
 services 1 and 2, respectively.
 If (1) S is the total service units within the pool, (2) M1, M2, ...,
 Mn are the multipliers provided for services 1, 2, ..., n, and
 (3) C1, C2, ..., Cn are the used resources within the session, then
 the pool's credit is exhausted and re-authorization MUST be sought
 when:
          C1*M1 + C2*M2 + ... + Cn*Mn >= S
 The total credit in the pool, S, is calculated from the quotas, which
 are currently allocated to the pool as follows:
          S = Q1*M1 + Q2*M2 + ... + Qn*Mn
 If services or rating-groups are added to or removed from the pool,
 then the total credit is adjusted appropriately.  Note that when the
 total credit is adjusted because services or rating-groups are
 removed from the pool, the value that needs to be removed is the
 consumed one (i.e., Cx*Mx).

Bertz, et al. Standards Track [Page 21] RFC 8506 Diameter Credit-Control Application March 2019

 Re-authorizations for an individual service or rating-group may be
 sought at any time -- for example, if a "non-pooled" quota is used up
 or the Validity-Time expires.
 Where multiple G-S-U-Pool-Reference AVPs (Section 8.30) with the same
 G-S-U-Pool-Identifier are provided within a Multiple-Services-Credit-
 Control AVP (Section 8.16) along with the Granted-Service-Unit AVP,
 these AVPs MUST have different CC-Unit-Type values, and they all draw
 from the credit pool separately.  For instance, if one multiplier for
 time (M1t) and one multiplier for volume (M1v) are given, then the
 used resources from the pool yield the sum of C1t*M1t + C1v*M1v,
 where C1t is the time unit and C1v is the volume unit.
 Where service units are provided within a Multiple-Services-Credit-
 Control AVP without a corresponding G-S-U-Pool-Reference AVP, these
 units are handled independently from any credit pools and from any
 other services or rating-groups within the session.
 The "credit pool" concept is an optimal tool to avoid the
 over-reservation effect of the basic single-quota tariff time change
 mechanism (Section 5.1.1).  Therefore, Diameter Credit-Control
 clients and servers implementing the independent credit-control of
 multiple services SHOULD leverage the credit pool concept when
 supporting the tariff time change.  The Diameter Credit-Control
 server SHOULD include both the Tariff-Time-Change AVP and the
 Tariff-Change-Usage AVP in two quota allocations in the Answer
 message (i.e., two instances of the Multiple-Services-Credit-Control
 AVP).  One of the grants is allocated to be used before the potential
 tariff change, while the second grant is for use after a tariff
 change.  Both granted unit quotas MUST contain the same Service-
 Identifier and/or rating-group.  This dual-quota mechanism ensures
 that the overall reported used units would never exceed the credit
 reservation.  The Diameter Credit-Control client reports the used
 units both before and after the tariff change in a single instance of
 the Multiple-Services-Credit-Control AVP.
 Failure handling for credit-control sessions is defined in
 Section 5.7 and reflected in the basic credit-control state machines
 defined in Section 7.  Credit-control clients and servers
 implementing the functionality of independent credit-control of
 multiple services in a (sub-)session MUST ensure failure handling and
 general behavior fully consistent with Sections 5.7 and 7 while
 maintaining the ability to handle parallel ongoing credit
 re-authorization within a (sub-)session.  Therefore, it is
 RECOMMENDED that Diameter Credit-Control clients maintain a PendingU
 message queue (Section 7) and restart the Tx timer (Section 13) every
 time a CCR message with the value UPDATE_REQUEST is sent while they
 are in PendingU state.  When answers to all pending messages are

Bertz, et al. Standards Track [Page 22] RFC 8506 Diameter Credit-Control Application March 2019

 received, the state machine moves to Open state, and the Tx timer is
 stopped.  Naturally, when a problem is detected and acted upon per
 Section 5.7, all of the ongoing services are affected (e.g., failover
 to a backup server affects all of the CCR messages in the PendingU
 queue).
 Since the client may send CCR messages with the value UPDATE_REQUEST
 while in PendingU state (i.e., without waiting for an answer to
 ongoing credit re-authorization), the time space between these
 requests may be very short, and the server may not have received the
 previous request(s) yet.  Therefore, in this situation the server may
 receive out-of-sequence requests and SHOULD NOT consider this an
 error condition.  A proper answer is to be returned to each of those
 requests.

5.2. First Interrogation

 When session-based credit-control is required (e.g., the
 authentication server indicated a prepaid user), the first
 interrogation MUST be sent before the Diameter Credit-Control client
 allows any service events for the end user.  The CC-Request-Type AVP
 is set to the value INITIAL_REQUEST in the request message.
 If the Diameter Credit-Control client knows the cost of the service
 event (e.g., a content server delivering ringing tones may know their
 cost) the monetary amount to be charged is included in the Requested-
 Service-Unit AVP.  If the Diameter Credit-Control client does not
 know the cost of the service event, the Requested-Service-Unit AVP
 MAY contain the number of requested service events.  Where the
 Multiple-Services-Credit-Control AVP is used, it MUST contain the
 Requested-Service-Unit AVP to indicate that the quota for the
 associated service/rating-group is requested.  In the case of
 multiple services, the Service-Identifier AVP or the Rating-Group AVP
 within the Multiple-Services-Credit-Control AVP always indicates the
 service concerned.  Additional service event information to be rated
 MAY be sent as service-specific AVPs or MAY be sent within the
 Service-Parameter-Info AVP at the command level.  The
 Service-Context-Id AVP indicates the service-specific document
 applicable to the request.
 The Event-Timestamp AVP SHOULD be included in the request and
 contains the time when the service event is requested in the Service
 Element.  The Subscription-Id AVP or the Subscription-Id-Extension
 AVP SHOULD be included to identify the end user in the credit-control
 server.  The credit-control client MAY include the User-Equipment-
 Info AVP or User-Equipment-Info-Extension AVP so that the

Bertz, et al. Standards Track [Page 23] RFC 8506 Diameter Credit-Control Application March 2019

 credit-control server has some indication of the type and
 capabilities of the end-user access device.  How the credit-control
 server uses this information is outside the scope of this document.
 The credit-control server SHOULD rate the service event and make a
 credit reservation from the end user's account that covers the cost
 of the service event.  If the type of the Requested-Service-Unit AVP
 is "money", no rating is needed, but the corresponding monetary
 amount is reserved from the end user's account.
 The credit-control server returns the Granted-Service-Unit AVP in the
 Answer message to the Diameter Credit-Control client.  The Granted-
 Service-Unit AVP contains the amount of service units that the
 Diameter Credit-Control client can provide to the end user until a
 new Credit-Control-Request MUST be sent to the credit-control server.
 If several unit types are sent in the Answer message, the
 credit-control client MUST handle each unit type separately.  The
 type of the Granted-Service-Unit AVP can be time, volume, service-
 specific, or money, depending on the type of service event.  The unit
 type(s) SHOULD NOT be changed within an ongoing credit-control
 session.
 There MUST be a maximum of one instance of the same unit type in one
 Answer message.  However, if multiple quotas are conveyed to the
 credit-control client in the Multiple-Services-Credit-Control AVPs,
 it is possible to carry two instances of the same unit type
 associated to a service-identifier/rating-group.  This is typically
 the case when a tariff time change is expected and the credit-control
 server wants to make a distinction between the granted quota before
 the tariff change and the granted quota after the tariff change.
 If the credit-control server determines that no further control is
 needed for the service, it MAY include the result code indicating
 that the credit-control is not applicable (e.g., if the service is
 free of charge).  This result code, at the command level, implies
 that the credit-control session is to be terminated.
 The Credit-Control-Answer message MAY also include the Final-Unit-
 Indication AVP or the QoS-Final-Unit-Indication AVP to indicate that
 the Answer message contains the final units for the service.  After
 the end user has consumed these units, the Diameter Credit-Control
 client MUST behave as described in Section 5.6.
 This document defines two different approaches for performing the
 first interrogation to be used in different network architectures.
 The first approach uses credit-control messages after the user's
 authorization and authentication take place.  The second approach
 uses (1) service-specific authorization messages to perform the first

Bertz, et al. Standards Track [Page 24] RFC 8506 Diameter Credit-Control Application March 2019

 interrogation during the user's authorization/authentication phase
 and (2) credit-control messages for the intermediate and final
 interrogations.  If an implementation of the credit-control client
 supports both methods, determining which method to use SHOULD be
 configurable.
 In service environments such as NAS environments, it is desired to
 perform the first interrogation as part of the authorization/
 authentication process for the sake of protocol efficiency.  Further
 credit authorizations after the first interrogation are performed
 with credit-control commands defined in this specification.
 Implementations of credit-control clients operating in the
 environments mentioned in this document SHOULD support this method.
 If the credit-control server and AAA server are separate physical
 entities, the Service Element sends the request messages to the AAA
 server, which then issues an appropriate request or proxies the
 received request forward to the credit-control server.
 In other service environments, such as the 3GPP network and some SIP
 scenarios, there is a substantial decoupling between registration/
 access to the network and the actual service request (i.e., the
 authentication/authorization is executed once during registration/
 access to the network and is not executed for every service event
 requested by the subscriber).  In these environments, it is more
 appropriate to perform the first interrogation after the user has
 been authenticated and authorized.  The first, intermediate, and
 final interrogations are executed with credit-control commands
 defined in this specification.
 Other IETF standards or standards developed by other standardization
 bodies may define the most suitable method in their architectures.

5.2.1. First Interrogation after Authorization and Authentication

 The Diameter Credit-Control client in the Service Element may get
 information from the authorization server as to whether
 credit-control is required, based on its knowledge of the end user.
 If credit-control is required, the credit-control server needs to be
 contacted prior to initiating service delivery to the end user.  The
 accounting protocol and the credit-control protocol can be used in
 parallel.  The authorization server may also determine whether the
 parallel accounting stream is required.

Bertz, et al. Standards Track [Page 25] RFC 8506 Diameter Credit-Control Application March 2019

 Figure 3 illustrates the case where both protocols are used in
 parallel and the Service Element sends credit-control messages
 directly to the credit-control server.  More credit-control sequence
 examples are given in Appendix A.
                                        Diameter
 End User      Service Element          AAA Server          CC Server
                  (CC Client)
   | Registration      | AA-Request/Answer(accounting, CC, or both)|
   |<----------------->|<------------------>|                      |
   |        :          |                    |                      |
   |        :          |                    |                      |
   | Service Request   |                    |                      |
   |------------------>|                    |                      |
   |                   | CCR(Initial, Credit-Control AVPs)         |
   |                  +|------------------------------------------>|
   |         CC stream||                    |    CCA(Granted-Units)|
   |                  +|<------------------------------------------|
   | Service Delivery  |                    |                      |
   |<----------------->| ACR(start, Accounting AVPs)               |
   |         :         |------------------->|+                     |
   |         :         |                ACA ||  Accounting stream  |
   |                   |<-------------------|+                     |
   |         :         |                    |                      |
   |         :         |                    |                      |
   |                   | CCR(Update, Used-Units)                   |
   |                   |------------------------------------------>|
   |                   |                    |    CCA(Granted-Units)|
   |                   |<------------------------------------------|
   |         :         |                    |                      |
   |         :         |                    |                      |
   | End of Service    |                    |                      |
   |------------------>| CCR(Termination, Used-Units)              |
   |                   |------------------------------------------>|
   |                   |                    |                 CCA  |
   |                   |<------------------------------------------|
   |                   | ACR(stop)          |                      |
   |                   |------------------->|                      |
   |                   |                ACA |                      |
   |                   |<-------------------|                      |
   ACR: Accounting-Request
   ACA: Accounting-Answer
          Figure 3: Protocol Example with First Interrogation
               after User's Authorization/Authentication

Bertz, et al. Standards Track [Page 26] RFC 8506 Diameter Credit-Control Application March 2019

5.2.2. First Interrogation Included with Authorization Messages

 The Diameter Credit-Control client in the Service Element MUST
 actively co-operate with the authorization/authentication client in
 the construction of the AA-Request by adding appropriate
 Credit-Control AVPs.  The credit-control client MUST add the
 Credit-Control AVP to indicate credit-control capabilities and MAY
 add other relevant credit-control-specific AVPs to the proper
 authorization/authentication command to perform the first
 interrogation toward the home Diameter AAA server.  The
 Auth-Application-Id is set to the appropriate value, as defined in
 service-specific authorization/authentication application document
 (e.g., [RFC7155] [RFC4004]).  The home Diameter AAA server
 authenticates/authorizes the subscriber and determines whether
 credit-control is required.
 If credit-control is not required for the subscriber, the home
 Diameter AAA server will respond as usual, with an appropriate
 AA-Answer message.  If credit-control is required for the subscriber
 and the Credit-Control AVP with the value set to CREDIT_AUTHORIZATION
 was present in the authorization request, the home AAA server MUST
 contact the credit-control server to perform the first interrogation.
 If credit-control is required for the subscriber and the
 Credit-Control AVP was not present in the authorization request, the
 home AAA server MUST send an authorization reject Answer message.
 The Diameter AAA server supporting credit-control is required to send
 the Credit-Control-Request command (CCR) defined in this document to
 the credit-control server.  The Diameter AAA server populates the CCR
 based on service-specific AVPs used for input to the rating process,
 and possibly on Credit-Control AVPs received in the AA-Request.  The
 credit-control server will reserve money from the user's account,
 will rate the request, and will send a Credit-Control-Answer message
 to the home Diameter AAA server.  The Answer message includes the
 Granted-Service-Unit AVP(s) and MAY include other credit-control-
 specific AVPs, as appropriate.  Additionally, the credit-control
 server MAY set the Validity-Time and MAY include the CCFH and the
 DDFH to determine what to do if the sending of credit-control
 messages to the credit-control server has been temporarily prevented.
 Upon receiving the Credit-Control-Answer message from the
 credit-control server, the home Diameter AAA server will populate the
 AA-Answer with the received Credit-Control AVPs and with the
 appropriate service attributes according to the authorization/
 authentication-specific application (e.g., [RFC7155] [RFC4004]).  It
 will then forward the packet to the credit-control client.  If the
 home Diameter AAA server receives a credit-control reject message, it

Bertz, et al. Standards Track [Page 27] RFC 8506 Diameter Credit-Control Application March 2019

 will simply generate an appropriate authorization reject message to
 the credit-control client, including the credit-control-specific
 error code.
 In this model, the credit-control client sends further credit-control
 messages to the credit-control server via the home Diameter AAA
 server.  Upon receiving a successful authorization Answer message
 with the Granted-Service-Unit AVP(s), the credit-control client will
 grant the service to the end user and will generate an intermediate
 Credit-Control-Request, if required, by using credit-control
 commands.  The CC-Request-Number of the first UPDATE_REQUEST MUST be
 set to 1 (for details regarding how to produce a unique value for the
 CC-Request-Number AVP, see Section 8.2).
 If service-specific re-authorization is performed (i.e., the
 authorization lifetime expires), the credit-control client MUST add
 to the service-specific re-authorization request the Credit-Control
 AVP with a value set to RE_AUTHORIZATION to indicate that the
 credit-control server MUST NOT be contacted.  When session-based
 credit-control is used for the subscriber, a constant credit-control
 message stream flows through the home Diameter AAA server.  The home
 Diameter AAA server can make use of this credit-control message flow
 to deduce that the user's activity is ongoing; therefore, it is
 recommended to set the authorization lifetime to a reasonably high
 value when credit-control is used for the subscriber.
 In this scenario, the home Diameter AAA server MUST advertise support
 for the credit-control application to its peers during the capability
 exchange process.

Bertz, et al. Standards Track [Page 28] RFC 8506 Diameter Credit-Control Application March 2019

 Figure 4 illustrates the use of authorization/authentication messages
 to perform the first interrogation.  The parallel accounting stream
 is not shown in the figure.
                                          Diameter
                Service Element           AAA Server        CC Server
 End User          (CC Client)
  | Service Request   | AA-Request (CC AVPs) |                     |
  |------------------>|--------------------->|                     |
  |                   |                      | CCR(Initial, CC AVPs)
  |                   |                      |-------------------->|
  |                   |                      |   CCA(Granted-Units)|
  |                   |                      |<--------------------|
  |                   | AA-Answer(Granted-Units)                   |
  | Service Delivery  |<---------------------|                     |
  |<----------------->|                      |                     |
  |         :         |                      |                     |
  |         :         |                      |                     |
  |         :         |                      |                     |
  |                   |                      |                     |
  |                   | CCR(Update, Used-Units)                    |
  |                   |--------------------->| CCR(Update, Used-Units)
  |                   |                      |-------------------->|
  |                   |                      |   CCA(Granted-Units)|
  |                   |    CCA(Granted-Units)|<--------------------|
  |                   |<---------------------|                     |
  |         :         |                      |                     |
  |         :         |                      |                     |
  | End of Service    |                      |                     |
  |------------------>| CCR(Termination, Used-Units)               |
  |                   |--------------------->| CCR(Term., Used-Units)
  |                   |                      |-------------------->|
  |                   |                      |                 CCA |
  |                   |                  CCA |<--------------------|
  |                   |<---------------------|                     |
     Figure 4: Protocol Example with Use of Authorization Messages
                      for the First Interrogation

5.3. Intermediate Interrogation

 When all the granted service units for one unit type are spent by the
 end user or the Validity-Time has expired, the Diameter
 Credit-Control client MUST send a new Credit-Control-Request to the
 credit-control server.  In the event that credit-control for multiple
 services is applied in one credit-control session (i.e., units
 associated to Service-Identifier(s) or the rating-group are granted),
 a new Credit-Control-Request MUST be sent to the credit-control

Bertz, et al. Standards Track [Page 29] RFC 8506 Diameter Credit-Control Application March 2019

 server when the credit reservation has been wholly consumed or upon
 expiration of the Validity-Time.  It is always up to the Diameter
 Credit-Control client to send a new request well in advance of the
 expiration of the previous request in order to avoid interruption in
 the Service Element.  Even if the granted service units reserved by
 the credit-control server have not been spent upon expiration of the
 Validity-Time, the Diameter Credit-Control client MUST send a new
 Credit-Control-Request to the credit-control server.
 There can also be mid-session service events, which might affect the
 rating of the current service events.  In this case, a spontaneous
 update (a new Credit-Control-Request) SHOULD be sent, including
 information related to the service event, even if all the granted
 service units have not been spent or the Validity-Time has not
 expired.
 When the used units are reported to the credit-control server, the
 credit-control client will not have any units in its possession
 before new granted units are received from the credit-control server.
 When the new granted units are received, these units apply from the
 point where the measurement of the reported used units stopped.
 Where independent credit-control of multiple services is supported,
 this process may be executed for one or more services, a single
 rating-group, or a pool within the (sub-)session.
 The CC-Request-Type AVP is set to the value UPDATE_REQUEST in the
 intermediate request message.  The Subscription-Id AVP or
 Subscription-Id-Extension AVP SHOULD be included in the intermediate
 message to identify the end user in the credit-control server.  The
 Service-Context-Id AVP indicates the service-specific document
 applicable to the request.
 The Requested-Service-Unit AVP MAY contain the new amount of
 requested service units.  Where the Multiple-Services-Credit-Control
 AVP is used, it MUST contain the Requested-Service-Unit AVP if a new
 quota is requested for the associated service/rating-group.  The
 Used-Service-Unit AVP contains the amount of used service units
 measured from the point when the service became active or, if interim
 interrogations are used during the session, from the point when the
 previous measurement ended.  The same unit types used in the previous
 message SHOULD be used.  If several unit types were included in the
 previous Answer message, the used service units for each unit type
 MUST be reported.
 The Event-Timestamp AVP SHOULD be included in the request and
 contains the time of the event that triggered the sending of the new
 Credit-Control-Request.

Bertz, et al. Standards Track [Page 30] RFC 8506 Diameter Credit-Control Application March 2019

 The credit-control server MUST deduct the used amount from the
 end user's account.  It MAY rate the new request and make a new
 credit reservation from the end user's account that covers the cost
 of the requested service event.
 A Credit-Control-Answer message with the CC-Request-Type AVP set to
 the value UPDATE_REQUEST MAY include the Cost-Information AVP
 containing the accumulated cost estimation for the session, without
 taking any credit reservations into account.
 The Credit-Control-Answer message MAY also include the Final-Unit-
 Indication AVP or the QoS-Final-Unit-Indication AVP to indicate that
 the Answer message contains the final units for the service.  After
 the end user has consumed these units, the Diameter Credit-Control
 client MUST behave as described in Section 5.6.
 There can be several intermediate interrogations within a session.

5.4. Final Interrogation

 When the end user terminates the service session or when graceful
 service termination (described in Section 5.6) takes place, the
 Diameter Credit-Control client MUST send a final Credit-Control-
 Request message to the credit-control server.  The CC-Request-Type
 AVP is set to the value TERMINATION_REQUEST.  The Service-Context-Id
 AVP indicates the service-specific document applicable to the
 request.
 The Event-Timestamp AVP SHOULD be included in the request and
 contains the time when the session was terminated.
 The Used-Service-Unit AVP contains the amount of used service units
 measured from the point when the service became active or, if interim
 interrogations are used during the session, from the point when the
 previous measurement ended.  If several unit types were included in
 the previous Answer message, the used service units for each unit
 type MUST be reported.
 After final interrogation, the credit-control server MUST refund the
 reserved credit amount not used to the end user's account and deduct
 the used monetary amount from the end user's account.
 A Credit-Control-Answer message with the CC-Request-Type AVP set to
 the value TERMINATION_REQUEST MAY include the Cost-Information AVP
 containing the estimated total cost for the session in question.

Bertz, et al. Standards Track [Page 31] RFC 8506 Diameter Credit-Control Application March 2019

 If the user logs off during an ongoing credit-control session or if
 the user becomes logged off for some other reason (e.g., a final-unit
 indication causes user logoff according to local policy), the Service
 Element, according to application-specific policy, may send a
 Session-Termination-Request (STR) to the home Diameter AAA server as
 usual [RFC6733].  Figure 5 illustrates the case when the final-unit
 indication causes user logoff upon consumption of the final granted
 units and the generation of an STR.
 The Diameter AAA server responds with a Session-Termination-Answer
 (STA).
               Service Element         AAA Server         CC Server
 End User        (CC Client)
  | Service Delivery  |                     |                     |
  |<----------------->|                     |                     |
  |         :         |                     |                     |
  |         :         |                     |                     |
  |         :         |                     |                     |
  |                   |                     |                     |
  |                   | CCR(Update, Used-Units)                   |
  |                   |-------------------->| CCR(Update, Used-Units)
  |                   |                     |-------------------->|
  |                   |                   CCA(Final-Unit, Terminate)
  |               CCA(Final-Unit, Terminate)|<--------------------|
  |                   |<--------------------|                     |
  |         :         |                     |                     |
  |         :         |                     |                     |
  |  Disconnect user  |                     |                     |
  |<------------------| CCR(Termination, Used-Units)              |
  |                   |-------------------->| CCR(Term., Used-Units)
  |                   |                     |-------------------->|
  |                   |                     |                 CCA |
  |                   |                 CCA |<--------------------|
  |                   |<--------------------|                     |
  |                   | STR                 |                     |
  |                   |-------------------->|                     |
  |                   |                STA  |                     |
  |                   |<--------------------|                     |
         Figure 5: User Disconnected Due to Exhausted Account

5.5. Server-Initiated Credit Re-authorization

 The Diameter Credit-Control application supports server-initiated
 re-authorization.  The credit-control server MAY optionally initiate
 the credit re-authorization by issuing a Re-Auth-Request (RAR) as
 defined in the Diameter base protocol [RFC6733].  The

Bertz, et al. Standards Track [Page 32] RFC 8506 Diameter Credit-Control Application March 2019

 Auth-Application-Id in the RAR message is set to 4 to indicate
 "Diameter Credit Control", and the Re-Auth-Request-Type is set to
 AUTHORIZE_ONLY.
 Section 5.1.2 defines the feature to enable credit-control for
 multiple services within a single (sub-)session where the server can
 authorize credit usage at a different level of granularity.  Further,
 the server may provide credit resources to multiple services or
 rating-groups as a pool (see Section 5.1.2 for details and
 definitions).  Therefore, the server, based on its service logic and
 its knowledge of the ongoing session, can decide to request credit
 re-authorization for a whole (sub-)session, a single credit pool, a
 single service, or a single rating-group.  To request credit
 re-authorization for a credit pool, the server includes in the RAR
 message the G-S-U-Pool-Identifier AVP indicating the affected pool.
 To request credit re-authorization for a service or a rating-group,
 the server includes in the RAR message the Service-Identifier AVP or
 the Rating-Group AVP, respectively.  To request credit
 re-authorization for all the ongoing services within the
 (sub-)session, the server includes none of the above-mentioned AVPs
 in the RAR message.
 If a credit re-authorization is not already ongoing (i.e., the
 credit-control session is in Open state), a credit-control client
 that receives an RAR message with Session-Id equal to a currently
 active credit-control session MUST acknowledge the request by sending
 the Re-Auth-Answer (RAA) message and MUST initiate the credit
 re-authorization toward the server by sending a Credit-Control-
 Request message with the CC-Request-Type AVP set to the value
 UPDATE_REQUEST.  The Result-Code 2002 (DIAMETER_LIMITED_SUCCESS)
 SHOULD be used in the RAA message to indicate that an additional
 message (i.e., a CCR message with the value UPDATE_REQUEST) is
 required to complete the procedure.  If a quota was allocated to the
 service, the credit-control client MUST report the used quota in the
 Credit-Control-Request.  Note that the end user does not need to be
 prompted for the credit re-authorization, since the credit
 re-authorization is transparent to the user (i.e., it takes place
 exclusively between the credit-control client and the credit-control
 server).
 Where multiple services in a user's session are supported, the
 procedure in the above paragraph will be executed at the granularity
 requested by the server in the RAR message.
 If credit re-authorization is ongoing at the time when the RAR
 message is received (i.e., an RAR-CCR collision), the credit-control
 client successfully acknowledges the request but does not initiate a
 new credit re-authorization.  The Result-Code 2001 (DIAMETER_SUCCESS)

Bertz, et al. Standards Track [Page 33] RFC 8506 Diameter Credit-Control Application March 2019

 SHOULD be used in the RAA message to indicate that a credit
 re-authorization procedure is already ongoing (i.e., the client was
 in PendingU state when the RAR was received).  The credit-control
 server SHOULD process the Credit-Control-Request as if it was
 received in answer to the server-initiated credit re-authorization
 and should consider the server-initiated credit re-authorization
 process successful upon reception of the RAA message.
 When multiple services are supported in a user's session, the server
 may request credit re-authorization for a credit pool (or for the
 (sub-)session) while a credit re-authorization is already ongoing for
 some of the services or rating-groups.  In this case, the client
 acknowledges the server request with an RAA message and MUST send a
 new Credit-Control-Request message to perform re-authorization for
 the remaining services/rating-groups.  The Result-Code 2002
 (DIAMETER_LIMITED_SUCCESS) SHOULD be used in the RAA message to
 indicate that an additional message (i.e., a CCR message with the
 value UPDATE_REQUEST) is required to complete the procedure.  The
 server processes the received requests and returns an appropriate
 answer to both requests.
 The above-defined procedures are enabled for each of the possibly
 active Diameter Credit-Control sub-sessions.  The server MAY request
 re-authorization for an active sub-session by including the
 CC-Sub-Session-Id AVP in the RAR message in addition to the
 Session-Id AVP.

5.6. Graceful Service Termination

 When the user's account runs out of money, the user may not be
 allowed to compile additional chargeable events.  However, the home
 service provider may offer some services -- for instance, access to a
 service portal where it is possible to refill the account -- from
 which the user is allowed to benefit for a limited time.  The length
 of this time is usually dependent on the home service provider
 policy.
 This section defines the optional graceful service termination
 feature.  This feature MAY be supported by the credit-control server.
 Credit-control client implementations MUST support the Final-Unit-
 Indication AVP or QoS-Final-Unit-Indication AVP with at least the
 teardown of the ongoing service session once the subscriber has
 consumed all the final granted units.

Bertz, et al. Standards Track [Page 34] RFC 8506 Diameter Credit-Control Application March 2019

 Where independent credit-control of multiple services in a single
 credit-control (sub-)session is supported, it is possible to use
 graceful service termination for each of the services/rating-groups
 independently.  Naturally, the graceful service termination process
 defined in the following subsections will apply to the specific
 service/rating-group as requested by the server.
 In some service environments (e.g., NAS), graceful service
 termination may be used to redirect the subscriber to a service
 portal for online balance refill or other services offered by the
 home service provider.  In this case, the graceful service
 termination process installs a set of packet filters to restrict the
 user's access capability only to/from the specified destinations.
 All the IP packets not matching the filters will be dropped or,
 possibly, redirected to the service portal.  The user may also be
 sent an appropriate notification as to why the access has been
 limited.  These actions may be communicated explicitly from the
 server to the client or may be configured "per service" at the
 client.  Explicitly signaled redirection or restriction instructions
 always take precedence over configured ones.
 It is also possible to use graceful service termination to connect
 the prepaid user to a top-up server that plays an announcement and
 prompts the user to replenish the account.  In this case, the
 credit-control server sends only the address of the top-up server
 where the prepaid user shall be connected after the final granted
 units have been consumed.  An example of this case is given in
 Appendix A.7.
 The credit-control server MAY initiate graceful service termination
 by including the Final-Unit-Indication AVP or the
 QoS-Final-Unit-Indication AVP in the Credit-Control-Answer to
 indicate that the message contains the final units for the service.
 When the credit-control client receives the Final-Unit-Indication AVP
 or the QoS-Final-Unit-Indication AVP in the answer from the server,
 its behavior depends on the value indicated in the Final-Unit-Action
 AVP.  The server may request the following actions: TERMINATE,
 REDIRECT, or RESTRICT_ACCESS.
 Figure 6 illustrates the graceful service termination procedure
 described in the following subsections.

Bertz, et al. Standards Track [Page 35] RFC 8506 Diameter Credit-Control Application March 2019

                                          Diameter
 End User        Service Element          AAA Server         CC Server
                 (CC Client)
   |  Service Delivery |                     |                     |
   |<----------------->|                     |                     |
   |                   |CCR(Update, Used-Units)                    |
   |                   |-------------------->|CCR(Update, Used-Units)
   |         :         |                     |-------------------->|
   |         :         |                     |CCA(Final-Unit, Action)
   |         :         |                     |<--------------------|
   |                   |CCA(Final-Unit, Action)                    |
   |                   |<--------------------|                     |
   |                   |                     |                     |
   |         :         |                     |                     |
   |         :         |                     |                     |
   | ///////////////   |CCR(Update, Used-Units)                    |
   |/Final Units End/->|-------------------->|CCR(Update, Used-Units)
   |/Action and    //  |                     |-------------------->|
   |/Restrictions //   |                     |   CCA(Validity-Time)|
   |/Start       //    |   CCA(Validity-Time)|<--------------------|
   | /////////////     |<--------------------|                     |
   |         :         |                     |                     |
   |         :         |                     |                     |
   |                 Replenish account       |     +-------+       |
   |<--------------------------------------------->|Account|       |
   |                   |                     |     +-------+       |
   |                   |                     |                 RAR |
   |                 + |                 RAR |<====================|
   |                 | |<====================|                     |
   |                 | | RAA                 |                     |
   |  /////////////  | |====================>| RAA                 |
   | /If supported / | | CCR(Update)         |====================>|
   | /by CC Server/  | |====================>| CCR(Update)         |
   | /////////////   | |                     |====================>|
   |                 | |                     |   CCA(Granted-Units)|
   |                 | |   CCA(Granted-Units)|<====================|
   |  Restrictions ->+ |<====================|                     |
   |  removed          |                     |                     |
   |         :         |                     |                     |
   |        OR         | CCR(Update)         |                     |
   |   Validity-Time ->|-------------------->| CCR(Update)         |
   |   expires         |                     |-------------------->|
   |                   |                     |   CCA(Granted-Units)|
   |                   |   CCA(Granted-Units)|<--------------------|
   |    Restrictions ->|<--------------------|                     |
   |    removed        |                     |                     |
       Figure 6: Optional Graceful Service Termination Procedure

Bertz, et al. Standards Track [Page 36] RFC 8506 Diameter Credit-Control Application March 2019

 In addition, the credit-control server MAY reply with the Final-Unit-
 Indication AVP or QoS-Final-Unit-Indication AVP holding a Granted-
 Service-Unit (G-S-U) with a zero grant, indicating that the service
 SHOULD be terminated immediately, and no further reporting is
 required.  Figure 7 illustrates a graceful service termination
 procedure that applies immediately after receiving a zero grant.
                                           Diameter
 End User        Service Element           AAA Server        CC Server
                 (CC Client)
   |  Service Delivery |                      |                     |
   |<----------------->|                      |                     |
   |                   |CCR(Update, Used-Units)                     |
   |                   |--------------------->|CCR(Update, Used-Units)
   |         :         |                      |-------------------->|
   |         :         |                      |CCA(Final-Unit, Action,
   |         :         |                      |            Zero G-S-U)
   |         :         |                      |<--------------------|
   |                   |CCA(Final-Unit, Action,                     |
   |                   |            Zero G-S-U)                     |
   |                   |<---------------------|                     |
   | ///////////////   |                      |                     |
   |/Action and    //  |                      |                     |
   |/Restrictions //   |                      |                     |
   |/Start       //    |                      |                     |
   | /////////////     |                      |                     |
   |         :         |                      |                     |
   |         :         |                      |                     |
  Figure 7: Optional Immediate Graceful Service Termination Procedure

5.6.1. Terminate Action

 The Final-Unit-Indication AVP or the QoS-Final-Unit-Indication AVP
 with Final-Unit-Action set to TERMINATE does not include any other
 information.  When the subscriber has consumed the final granted
 units, the Service Element MUST terminate the service.  This is the
 default handling applicable whenever the credit-control client
 receives an unsupported Final-Unit-Action value and MUST be supported
 by all the Diameter Credit-Control client implementations conforming
 to this specification.  A final Credit-Control-Request message to the
 credit-control server MUST be sent if the Final-Unit-Indication AVP
 or the QoS-Final-Unit-Indication AVP indicating action TERMINATE was
 present at the command level.  The CC-Request-Type AVP in the request
 is set to the value TERMINATION_REQUEST.

Bertz, et al. Standards Track [Page 37] RFC 8506 Diameter Credit-Control Application March 2019

5.6.2. Redirect Action

 The Final-Unit-Indication AVP or the QoS-Final-Unit-Indication AVP
 with Final-Unit-Action set to REDIRECT indicates to the Service
 Element supporting this action that, upon consumption of the final
 granted units, the user MUST be redirected to the address specified
 in the Redirect-Server AVP or Redirect-Server-Extension AVP as
 follows.
 The credit-control server sends the Redirect-Server AVP or Redirect-
 Server-Extension AVP in the Credit-Control-Answer message.  In such a
 case, the Service Element MUST redirect or connect the user to the
 destination specified in the Redirect-Server AVP or Redirect-Server-
 Extension AVP, if possible.  When the end user is redirected (by
 using protocols other than Diameter) to the specified server or
 connected to the top-up server, an additional authorization (and
 possibly authentication) may be needed before the subscriber can
 replenish the account; however, this scenario is out of scope for
 this specification.
 In addition to the Redirect-Server AVP or Redirect-Server-Extension
 AVP, the credit-control server MAY include one or more Restriction-
 Filter-Rule AVPs, one or more Filter-Rule AVPs, or one or more
 Filter-Id AVPs in the Credit-Control-Answer message to enable the
 user to access other services (for example, zero-rated services).  In
 such a case, the access device MUST treat all packets according to
 the Restriction-Filter-Rule AVPs, Filter-Rule AVPs, and the rules
 referred to by the Filter-Id AVP.  After treatment is applied
 according to these rules, all traffic that has not been dropped or
 already forwarded MUST be redirected to the destination specified in
 the Redirect-Server AVP or Redirect-Server-Extension AVP.
 An entity other than the credit-control server may provision the
 access device with appropriate IP packet filters to be used in
 conjunction with the Diameter Credit-Control application.  This case
 is considered in Section 5.6.3.
 When the final granted units have been consumed, the credit-control
 client MUST perform an intermediate interrogation.  The purpose of
 this interrogation is to indicate to the credit-control server that
 the specified action started and to report the used units.  The
 credit-control server MUST deduct the used amount from the end user's
 account but MUST NOT make a new credit reservation.  The
 credit-control client, however, may send intermediate interrogations
 before all the final granted units have been consumed for which
 rating and money reservation may be needed -- for instance, upon
 Validity-Time expiration or upon mid-session service events that
 affect the rating of the current service.  Therefore, the

Bertz, et al. Standards Track [Page 38] RFC 8506 Diameter Credit-Control Application March 2019

 credit-control client MUST NOT include any rating-related AVPs in the
 request sent once all the final granted units have been consumed, as
 an indication to the server that (1) the requested final unit action
 started and (2) rating and money reservation are not required (when
 the Multiple-Services-Credit-Control AVP is used, the Service-
 Identifier AVP or the Rating-Group AVP is included to indicate the
 services concerned).  Naturally, the Credit-Control-Answer message
 does not contain any granted service units and MUST include the
 Validity-Time AVP to indicate to the credit-control client how long
 the subscriber is allowed to use network resources before a new
 intermediate interrogation is sent to the server.
 At the expiry of Validity-Time, the credit-control client sends a
 Credit-Control-Request (UPDATE_REQUEST) as usual.  This message does
 not include the Used-Service-Unit AVP, as there is no allotted quota
 to report.  The credit-control server processes the request and MUST
 perform the credit reservation.  If during this time the subscriber
 did not replenish their account, whether they will be disconnected or
 will be granted access to services not controlled by a credit-control
 server for an unlimited time is dependent on the home service
 provider policy.  (Note: The latter option implies that the Service
 Element should not remove the restriction filters upon termination of
 the credit-control.)  The server will return the appropriate
 Result-Code (see Section 9.1) in the Credit-Control-Answer message in
 order to implement the policy-defined action.  Otherwise, a new quota
 will be returned, and the Service Element MUST remove all the
 possible restrictions activated by the graceful service termination
 process and continue the credit-control session and service session
 as usual.
 The credit-control client may not wait until the expiration of the
 Validity-Time and may send a spontaneous update (a new
 Credit-Control-Request) if the Service Element can determine, for
 instance, that communication between the end user and the top-up
 server took place.  An example of this case is given in Appendix A.8
 (Figure 18).
 Note that the credit-control server may already have initiated the
 above-described process for the first interrogation.  However, the
 user's account might be empty when this first interrogation is
 performed.  In this case, the subscriber can be offered a chance to
 replenish the account and continue the service.  When the
 credit-control client receives (at either the session level or a
 service-specific level) a Final-Unit-Indication AVP or QoS-Final-
 Unit-Indication AVP, together with Validity-Time AVPs, but without a
 Granted-Service-Unit AVP, it immediately starts the graceful service
 termination process without sending any messages to the server.  An
 example of this case is illustrated in Appendix A.8 (Figure 18).

Bertz, et al. Standards Track [Page 39] RFC 8506 Diameter Credit-Control Application March 2019

5.6.3. Restrict Access Action

 A Final-Unit-Indication AVP with Final-Unit-Action set to
 RESTRICT_ACCESS indicates to the device supporting this action that,
 upon consumption of the final granted units, the user's access MUST
 be restricted according to the IP packet filters given in the
 Restriction-Filter-Rule AVP(s) or according to the IP packet filters
 identified by the Filter-Id AVP(s).  The credit-control server SHOULD
 include either the Restriction-Filter-Rule AVP or the Filter-Id AVP
 in the Final-Unit-Indication group AVP of the Credit-Control-Answer
 message.
 A QoS-Final-Unit-Indication AVP with Final-Unit-Action set to
 RESTRICT_ACCESS indicates to the device supporting this action that,
 upon consumption of the final granted units, the actions specified in
 Filter-Rule AVP(s) MUST restrict the traffic according to the
 classifiers in the Filter-Rule AVP(s).  If one or more Filter-Id AVPs
 are provided in the Credit-Control-Answer message, the credit-control
 client MUST restrict the traffic according to the IP packet filters
 identified by the Filter-Id AVP(s).  The credit-control server SHOULD
 include either the Filter-Rule AVP or the Filter-Id AVP in the
 QoS-Final-Unit-Indication group AVP of the Credit-Control-Answer
 message.
 If both the Final-Unit-Indication AVP and the QoS-Final-Unit-
 Indication AVP exist in the Credit-Control-Answer message, a
 credit-control client that supports the QoS-Final-Unit-Indication AVP
 SHOULD follow the directives included in the QoS-Final-Unit-
 Indication AVP and SHOULD ignore the Final-Unit-Indication AVP.
 An entity other than the credit-control server may provision the
 access device with appropriate IP packet filters to be used in
 conjunction with the Diameter Credit-Control application.  Such an
 entity may, for instance, configure the access device with IP flows
 to be passed when the Diameter Credit-Control application indicates
 RESTRICT_ACCESS or REDIRECT.  The access device passes IP packets
 according to the filter rules that may have been received in the
 Credit-Control-Answer message, in addition to those rules that may
 have been configured by the other entity.  However, when the user's
 account cannot cover the cost of the requested service, the action
 taken is the responsibility of the credit-control server that
 controls the prepaid subscriber.
 If another entity working in conjunction with the Diameter
 Credit-Control application already provisions the access device with
 all the required filter rules for the end user, the credit-control
 server presumably need not send any additional filters.  Therefore,
 it is RECOMMENDED that credit-control server implementations

Bertz, et al. Standards Track [Page 40] RFC 8506 Diameter Credit-Control Application March 2019

 supporting graceful service termination be configurable for sending
 the Restriction-Filter-Rule AVP, the Filter-Rule AVP, the Filter-Id
 AVP, or none of the above.
 When the final granted units have been consumed, the credit-control
 client MUST perform an intermediate interrogation.  The
 credit-control client and the credit-control server process this
 intermediate interrogation and execute subsequent procedures, as
 specified in Section 5.6.2.
 The credit-control server may initiate graceful service termination
 when replying with the action RESTRICT_ACCESS for the first
 interrogation.  This is similar to the behavior specified in
 Section 5.6.2.

5.6.4. Usage of the Server-Initiated Credit Re-authorization

 Once the subscriber replenishes the account, they presumably expect
 all the restrictions applied by the graceful service termination
 procedure to be removed immediately and unlimited service access to
 be resumed.  For the best user experience, the credit-control server
 implementation MAY support the server-initiated credit
 re-authorization (see Section 5.5).  In such a case, upon the
 successful account top-up, the credit-control server sends the
 Re-Auth-Request (RAR) message to solicit the credit re-authorization.
 The credit-control client initiates the credit re-authorization by
 sending the Credit-Control-Request message with the CC-Request-Type
 AVP set to the value UPDATE_REQUEST.  The Used-Service-Unit AVP is
 not included in the request, as there is no allotted quota to report.
 The Requested-Service-Unit AVP MAY be included in the request.  After
 the credit-control client successfully receives the Credit-Control-
 Answer with a new Granted-Service-Unit AVP, all the possible
 restrictions activated for the purpose of graceful service
 termination MUST be removed in the Service Element.  The
 credit-control session and the service session continue as usual.

5.7. Failure Procedures

 The CCFH, as described in this section, determines the behavior of
 the credit-control client in fault situations.  The CCFH may be
 (1) received from the Diameter home AAA server, (2) received from the
 credit-control server, or (3) configured locally.  The CCFH value
 received from the home AAA server overrides the locally configured
 value.  The CCFH value received from the credit-control server in the
 Credit-Control-Answer message always overrides any existing values.

Bertz, et al. Standards Track [Page 41] RFC 8506 Diameter Credit-Control Application March 2019

 The authorization server MAY include the Accounting-Realtime-Required
 AVP to determine what to do if the sending of accounting records to
 the accounting server has been temporarily prevented, as defined in
 [RFC6733].  It is RECOMMENDED that the client complement the
 credit-control failure procedures with a backup accounting flow
 toward an accounting server.  By using different combinations of the
 Accounting-Realtime-Required AVP and the CCFH, different safety
 levels can be built.  For example, by choosing a CCFH equal to
 CONTINUE for the credit-control flow and an Accounting-Realtime-
 Required AVP equal to DELIVER_AND_GRANT for the accounting flow, the
 service can be granted to the end user even if the connection to the
 credit-control server is down, as long as the accounting server is
 able to collect the accounting information and information exchange
 is taking place between the accounting server and credit-control
 server.
 As the credit-control application is based on real-time bidirectional
 communication between the credit-control client and the
 credit-control server, the usage of alternative destinations and the
 buffering of messages may not be sufficient in the event of
 communication failures.  Because the credit-control server has to
 maintain session states, moving the credit-control message stream to
 a backup server requires a complex context transfer solution.
 Whether the credit-control message stream is moved to a backup
 credit-control server during an ongoing credit-control session
 depends on the value of the CC-Session-Failover AVP.  However,
 failover may occur at any point in the path between the
 credit-control client and the credit-control server if a transport
 failure is detected with a peer, as described in [RFC6733].  As a
 consequence, the credit-control server might receive duplicate
 messages.  These duplicate or out-of-sequence messages can be
 detected in the credit-control server based on the credit-control
 server session state machine (Section 7), Session-Id AVP, and
 CC-Request-Number AVP.
 If a failure occurs during an ongoing credit-control session, the
 credit-control client may move the credit-control message stream to
 an alternative server if the credit-control server indicated
 FAILOVER_SUPPORTED in the CC-Session-Failover AVP.  A secondary
 credit-control server name, either received from the home Diameter
 AAA server or configured locally, can be used as an address of the
 backup server.  If the CC-Session-Failover AVP is set to
 FAILOVER_NOT_SUPPORTED, the credit-control message stream MUST NOT be
 moved to a backup server.

Bertz, et al. Standards Track [Page 42] RFC 8506 Diameter Credit-Control Application March 2019

 For new credit-control sessions, failover to an alternative
 credit-control server SHOULD be performed, if possible.  For
 instance, if an implementation of the credit-control client can
 determine primary credit-control server unavailability, it can
 establish the new credit-control sessions with a possibly available
 secondary credit-control server.
 The AAA transport profile [RFC3539] defines an application-layer
 watchdog algorithm that enables failover from a peer that has failed
 and is controlled by a watchdog timer (Tw) (defined in [RFC3539]).
 The recommended default initial value for Tw (Twinit) is 30 seconds.
 Twinit may be set as low as 6 seconds; however, according to
 [RFC3539], setting too low a value for Twinit is likely to result in
 an increased probability of duplicates, as well as an increase in
 spurious failover and failback attempts.  The Diameter base protocol
 [RFC6733] is common to several different types of Diameter AAA
 applications that may be run in the same Service Element.  Therefore,
 tuning the timer for Twinit to a lower value in order to satisfy the
 requirements of real-time applications, such as the Diameter
 Credit-Control application, will certainly cause the above-mentioned
 problems.  For prepaid services, however, the end user expects an
 answer from the network in a reasonable time.  Thus, the Diameter
 Credit-Control client will react more quickly than would the
 underlying base protocol.  Therefore, this specification defines the
 Tx timer (as defined in Section 13), which is used by the
 credit-control client to supervise communication with the
 credit-control server.  When the Tx timer elapses, the credit-control
 client takes action for the end user according to the CCFH.
 When the Tx timer expires, the Diameter Credit-Control client always
 terminates the service if the CCFH is set to the value TERMINATE.
 The credit-control session may be moved to an alternative server only
 if a protocol error DIAMETER_TOO_BUSY or DIAMETER_UNABLE_TO_DELIVER
 is received before the Tx timer expires.  Therefore, the value
 TERMINATE is not appropriate if proper failover behavior is desired.
 If the CCFH is set to the value CONTINUE or RETRY_AND_TERMINATE, the
 service will be granted to the end user when the Tx timer expires.
 An Answer message with granted units may arrive later if the base
 protocol transport failover occurred in the path to the
 credit-control server.  (The Twinit default value is 3 times more
 than the recommended Tx timeout value.)  The credit-control client
 SHOULD grant the service to the end user, start monitoring resource
 usage, and wait for the possible late answer until the timeout of the
 request (e.g., 120 seconds).  If the request fails and the
 CC-Session-Failover AVP is set to FAILOVER_NOT_SUPPORTED, the

Bertz, et al. Standards Track [Page 43] RFC 8506 Diameter Credit-Control Application March 2019

 credit-control client terminates or continues the service --
 depending on the value set in the CCFH -- and MUST free all the
 reserved resources for the credit-control session.  If the protocol
 error DIAMETER_UNABLE_TO_DELIVER or DIAMETER_TOO_BUSY is received or
 the request times out and the CC-Session-Failover AVP is set to
 FAILOVER_SUPPORTED, the credit-control client MAY send the request to
 a backup server, if possible.  If the credit-control client receives
 a successful answer from the backup server, it continues the
 credit-control session with such a server.  If the retransmitted
 request also fails, the credit-control client terminates or continues
 the service -- depending on the value set in the CCFH -- and MUST
 free all the reserved resources for the credit-control session.
 If a communication failure occurs during the graceful service
 termination procedure, the Service Element SHOULD always terminate
 the ongoing service session.
 If the credit-control server detects a failure during an ongoing
 credit-control session, it will terminate the credit-control session
 and return the reserved units back to the end user's account.
 The supervision session timer Tcc (as defined in Section 13) is used
 in the credit-control server to supervise the credit-control session.
 In order to support failover between credit-control servers,
 information transfer about the credit-control session and account
 state SHOULD take place between the primary and secondary
 credit-control servers.  Implementations supporting credit-control
 session failover MUST also ensure proper detection of duplicate or
 out-of-sequence messages.  Communication between the servers is
 regarded as an implementation issue and is outside the scope of this
 specification.

6. One-Time Event

 The one-time event is used when there is no need to maintain any
 state in the Diameter Credit-Control server -- for example, inquiring
 about the price of the service.  The use of a one-time event implies
 that the user has been authenticated and authorized beforehand.

Bertz, et al. Standards Track [Page 44] RFC 8506 Diameter Credit-Control Application March 2019

 The one-time event can be used when the credit-control client wants
 to know the cost of the service event or to check the account balance
 without any credit reservations.  It can also be used for refunding
 service units on the user's account or for direct debiting without
 any credit reservations.  The one-time event is shown in Figure 8.
                                        Diameter
 End User        Service Element        AAA Server        CC Server
                     (CC Client)
   | Service Request   |                    |                    |
   |------------------>|                    |                    |
   |                   | CCR(Event)         |                    |
   |                   |------------------->| CCR(Event)         |
   |                   |                    |------------------->|
   |                   |                    |  CCA(Granted-Units)|
   |                   |  CCA(Granted-Units)|<-------------------|
   |  Service Delivery |<-------------------|                    |
   |<----------------->|                    |                    |
                       Figure 8: One-Time Event
 In environments such as the 3GPP architecture, the one-time event can
 be sent from the Service Element directly to the credit-control
 server.

6.1. Service Price Inquiry

 The credit-control client may need to know the price of the service
 event.  Services offered by application service providers whose
 prices are not known in the credit-control client might exist.  The
 end user might also want to get an estimate of the price of a service
 event before requesting it.
 A Diameter Credit-Control client requesting the cost information MUST
 set the CC-Request-Type AVP equal to EVENT_REQUEST, include the
 Requested-Action AVP set to PRICE_ENQUIRY, and set the requested
 service event information in the Service-Identifier AVP in the
 Credit-Control-Request message.  Additional service event information
 may be sent as service-specific AVPs or within the Service-Parameter-
 Info AVP.  The Service-Context-Id AVP indicates the service-specific
 document applicable to the request.
 The credit-control server calculates the cost of the requested
 service event, but it does not perform any account-balance checks or
 credit reservations from the account.

Bertz, et al. Standards Track [Page 45] RFC 8506 Diameter Credit-Control Application March 2019

 The estimated cost of the requested service event is returned to the
 credit-control client in the Cost-Information AVP in the
 Credit-Control-Answer message.

6.2. Balance Checks

 The Diameter Credit-Control client may only have to verify that the
 end user's account balance covers the cost of a certain service
 without reserving any units from the account at the time of the
 inquiry.  This method does not guarantee that credit would be left
 when the Diameter Credit-Control client requests the debiting of the
 account with a separate request.
 A Diameter Credit-Control client requesting a balance check MUST set
 the CC-Request-Type AVP equal to EVENT_REQUEST, include a Requested-
 Action AVP set to CHECK_BALANCE, and include the Subscription-Id AVP
 or Subscription-Id-Extension AVP in order to identify the end user in
 the credit-control server.  The Service-Context-Id AVP indicates the
 service-specific document applicable to the request.
 The credit-control server makes the balance check, but it does not
 make any credit reservations from the account.
 The result of the balance check (ENOUGH_CREDIT/NO_CREDIT) is returned
 to the credit-control client in the Check-Balance-Result AVP in the
 Credit-Control-Answer message.

6.3. Direct Debiting

 There are certain service events for which service execution is
 always successful in the service environment.  The delay between the
 service invocation and the actual service delivery to the end user
 can be sufficiently long that the use of session-based credit-control
 would lead to unreasonably long credit-control sessions.  In these
 cases, the Diameter Credit-Control client can use the one-time event
 scenario for direct debiting.  The Diameter Credit-Control client
 SHOULD be sure that the requested service event execution would be
 successful when this scenario is used.
 In the Credit-Control-Request message, the CC-Request-Type AVP is set
 to the value EVENT_REQUEST and the Requested-Action AVP is set to
 DIRECT_DEBITING.  The Subscription-Id AVP or Subscription-Id-
 Extension AVP SHOULD be included to identify the end user in the
 credit-control server.  The Event-Timestamp AVP SHOULD be included in
 the request and contain the time when the service event is requested
 in the Service Element.  The Service-Context-Id AVP indicates the
 service-specific document applicable to the request.

Bertz, et al. Standards Track [Page 46] RFC 8506 Diameter Credit-Control Application March 2019

 If it knows the cost of the service event, the Diameter
 Credit-Control client MAY include in the Requested-Service-Unit AVP
 the monetary amount to be charged.  If the Diameter Credit-Control
 client does not know the cost of the service event, the Requested-
 Service-Unit AVP MAY contain the number of requested service events.
 The Service-Identifier AVP always indicates the service concerned.
 Additional service event information to be rated MAY be sent as
 service-specific AVPs or within the Service-Parameter-Info AVP.
 The credit-control server SHOULD rate the service event and deduct
 the corresponding monetary amount from the end user's account.  If
 the type of the Requested-Service-Unit AVP is "money", no rating is
 needed, but the corresponding monetary amount is deducted from the
 end user's account.
 The credit-control server returns the Granted-Service-Unit AVP in the
 Credit-Control-Answer message to the Diameter Credit-Control client.
 The Granted-Service-Unit AVP contains the amount of service units
 that the Diameter Credit-Control client can provide to the end user.
 The type of the Granted-Service-Unit can be time, volume, service-
 specific, or money, depending on the type of service event.
 If the credit-control server determines that no credit-control is
 needed for the service, it can include the result code indicating
 that the credit-control is not applicable (e.g., the service is free
 of charge).
 For informative purposes, the Credit-Control-Answer message MAY also
 include the Cost-Information AVP containing the estimated total cost
 of the requested service.

6.4. Refunds

 Some services may refund service units to the end user's account --
 for example, gaming services.
 The credit-control client MUST set the CC-Request-Type AVP to the
 value EVENT_REQUEST and the Requested-Action AVP to REFUND_ACCOUNT in
 the Credit-Control-Request message.  The Subscription-Id AVP or
 Subscription-Id-Extension AVP SHOULD be included to identify the
 end user in the credit-control server.  The Service-Context-Id AVP
 indicates the service-specific document applicable to the request.
 The Diameter Credit-Control client MAY include the monetary amount to
 be refunded in the Requested-Service-Unit AVP.  The Service-
 Identifier AVP always indicates the service concerned.  If the
 Diameter Credit-Control client does not know the monetary amount to

Bertz, et al. Standards Track [Page 47] RFC 8506 Diameter Credit-Control Application March 2019

 be refunded, in addition to the Service-Identifier AVP it MAY send
 service-specific AVPs or the Service-Parameter-Info AVP containing
 additional service event information to be rated.
 For informative purposes, the Credit-Control-Answer message MAY also
 include the Cost-Information AVP containing the estimated monetary
 amount of refunded units.

6.5. Failure Procedure

 Failover to an alternative credit-control server is allowed for a
 one-time event, as the server is not maintaining session states.  For
 instance, if the credit-control client receives a protocol error
 DIAMETER_UNABLE_TO_DELIVER or DIAMETER_TOO_BUSY, it can resend the
 request to an alternative server, if possible.  There MAY be
 protocol-transparent Diameter relays and redirect agents or Diameter
 Credit-Control proxies between the credit-control client and
 credit-control server.  Failover may occur at any point in the path
 between the credit-control client and the credit-control server if a
 transport failure is detected with a peer, as described in [RFC6733].
 Because there can be duplicate requests for various reasons, the
 credit-control server is responsible for real-time duplicate
 detection.  Implementation issues for duplicate detection are
 discussed in [RFC6733], Appendix C.
 When the credit-control client detects a communication failure with
 the credit-control server, its behavior depends on the requested
 action.  The Tx timer (as defined in Section 13) is used in the
 credit-control client to supervise communication with the
 credit-control server.
 If the requested action is PRICE_ENQUIRY or CHECK_BALANCE and a
 communication failure is detected, the credit-control client SHOULD
 forward the request messages to an alternative credit-control server,
 if possible.  The secondary credit-control server name, if received
 from the home Diameter AAA server, can be used as an address of the
 backup server.
 If the requested action is DIRECT_DEBITING, the DDFH controls the
 credit-control client's behavior.  The DDFH may be received from the
 home Diameter AAA server or may be locally configured.  The
 credit-control server may also send the DDFH in any CCA messages to
 be used for direct-debiting events compiled thereafter.  The DDFH
 value received from the home Diameter AAA server overrides the
 locally configured value, and the DDFH value received from the
 credit-control server in a Credit-Control-Answer message always
 overrides any existing values.

Bertz, et al. Standards Track [Page 48] RFC 8506 Diameter Credit-Control Application March 2019

 If the DDFH is set to TERMINATE_OR_BUFFER, the credit-control client
 SHOULD NOT grant the service if, after a possible retransmission
 attempt to an alternative credit-control server, the credit-control
 client can eventually determine from the result code or error code in
 the Answer message that units have not been debited.  Otherwise, the
 credit-control client SHOULD grant the service to the end user and
 store the request in credit-control application-level non-volatile
 storage.  (Note that resending the request at a later time is not a
 guarantee that the service will be debited, as the user's account may
 be empty when the server successfully processes the request.)  The
 credit-control client MUST mark these request messages as possible
 duplicates by setting the T flag in the command header as described
 in [RFC6733], Section 3.
 If the DDFH is set to CONTINUE, the service SHOULD be granted, even
 if credit-control messages cannot be delivered and messages are not
 buffered.
 If the Tx timer expires, the credit-control client MUST continue the
 service and wait for a possible late answer.  If the request
 times out, the credit-control client retransmits the request (marked
 with the T flag) to a backup credit-control server, if possible.  If
 the retransmitted request also times out or if a temporary error is
 received in answer, the credit-control client buffers the request if
 the value of the DDFH is set to TERMINATE_OR_BUFFER.  If a failed
 answer is received for the retransmitted request, the credit-control
 client frees all the resources reserved for the event message and
 deletes the request regardless of the value of the DDFH.
 The Credit-Control-Request with the requested action REFUND_ACCOUNT
 should always be stored in credit-control application-level
 non-volatile storage in case a temporary failure occurs.  The
 credit-control client MUST mark the retransmitted request message as
 a possible duplicate by setting the T flag in the command header as
 described in [RFC6733], Section 3.
 For stored requests, the implementation may choose to limit the
 number of retransmission attempts and to define a retransmission
 interval.
 Note that only one entity in the credit-control system SHOULD be
 responsible for duplicate detection.  If there is only one
 credit-control server within the given realm, the credit-control
 server may perform duplicate detection.  If there is more than one
 credit-control server in a given realm, only one entity in the
 credit-control system should be responsible, to ensure that the
 end user's account is not debited or credited multiple times for the
 same service event.

Bertz, et al. Standards Track [Page 49] RFC 8506 Diameter Credit-Control Application March 2019

7. Credit-Control Application State Machines

 This section defines five credit-control application state machines.
 The first four state machines are to be observed by credit-control
 clients.
 The first state machine describes session-based credit-control where
 the first interrogation is executed as part of the authorization/
 authentication process.  The second state machine describes
 session-based credit-control where the first interrogation is
 executed after the authorization/authentication process.  The
 requirements regarding what has to be supported for these two state
 machines are discussed in Section 5.2.
 The third state machine describes session-based credit-control for
 the intermediate and final interrogations.  The fourth state machine
 describes event-based credit-control.  These two state machines are
 to be observed by all implementations that conform to this
 specification.
 The fifth state machine describes the credit-control session from a
 credit-control server's perspective.
 Any event not listed in the state machines MUST be considered an
 error condition, and a corresponding answer, if applicable, MUST be
 returned to the originator of the message.
 In Tables 3, 4, and 5, the event "failure to send" means that the
 Diameter Credit-Control client is unable to communicate with the
 desired destination or, if a failover procedure is supported, with a
 possibly defined alternative destination (e.g., the request times out
 and the Answer message is not received).  This could be due to
 (1) the peer being down or (2) a physical link failure in the path to
 or from the credit-control server.
 The event "temporary error" means that the Diameter Credit-Control
 client received a protocol error notification (DIAMETER_TOO_BUSY,
 DIAMETER_UNABLE_TO_DELIVER, or DIAMETER_LOOP_DETECTED) in the
 Result-Code AVP of the Credit-Control-Answer command.  This type of
 notification may ultimately be received in answer to the
 retransmitted request to a defined alternative destination, if
 failover is supported.

Bertz, et al. Standards Track [Page 50] RFC 8506 Diameter Credit-Control Application March 2019

 The event "failed answer" means that the Diameter Credit-Control
 client received a non-transient failure (permanent failure)
 notification in the Credit-Control-Answer command.  This type of
 notification may ultimately be received in answer to the
 retransmitted request to a defined alternative destination, if
 failover is supported.
 The action "store request" means that a request is stored in
 credit-control application-level non-volatile storage.
 The event "not successfully processed" means that the credit-control
 server could not process the message, e.g., due to an unknown
 end user, an account being empty, or errors defined in [RFC6733].
 The event "user service terminated" can be triggered for various
 reasons, e.g., normal user termination, network failure, and ASR
 (Abort-Session-Request).  The Termination-Cause AVP contains
 information about the reason for termination, as specified in
 [RFC6733].
 The Tx timer, which is used to control the waiting time in the
 credit-control client in the Pending state, is stopped upon exit of
 the Pending state.  The stopping of the Tx timer is omitted in the
 state machine when the new state is Idle, as moving to Idle state
 implies the clearing of the session and all the variables associated
 to it.
 The states PendingI, PendingU, PendingT, PendingE, and PendingB stand
 for pending states to wait for an answer to a credit-control request
 related to Initial, Update, Termination, Event, or Buffered request,
 respectively.
 In Table 2, failover to a secondary server upon "temporary error" or
 "failure to send" is not explicitly described.  However, moving an
 ongoing credit-control message stream to an alternative server is
 possible if the CC-Session-Failover AVP is set to FAILOVER_SUPPORTED,
 as described in Section 5.7.
 Resending a credit-control event to an alternative server is
 supported as described in Section 6.5.

Bertz, et al. Standards Track [Page 51] RFC 8506 Diameter Credit-Control Application March 2019

 +----------+-------------------------------+-------------+----------+
 | State    | Event                         | Action      | New      |
 |          |                               |             | State    |
 +----------+-------------------------------+-------------+----------+
 | Idle     | Client or device requests     | Send        | PendingI |
 |          | access/service                | AA-Request  |          |
 |          |                               | with added  |          |
 |          |                               | CC AVPs,    |          |
 |          |                               | start Tx    |          |
 |          |                               | timer       |          |
 |          |                               |             |          |
 | PendingI | Successful answer to          | Grant       | Open     |
 |          | AA-Request received           | service to  |          |
 |          |                               | end user,   |          |
 |          |                               | stop Tx     |          |
 |          |                               | timer       |          |
 |          |                               |             |          |
 | PendingI | Tx timer expired              | Disconnect  | Idle     |
 |          |                               | user/dev    |          |
 |          |                               |             |          |
 | PendingI | Failed AA-Answer received     | Disconnect  | Idle     |
 |          |                               | user/dev    |          |
 |          |                               |             |          |
 | PendingI | AA-Answer received with       | Grant       | Idle     |
 |          | Result-Code equal to          | service to  |          |
 |          | CREDIT_CONTROL_NOT_APPLICABLE | end user    |          |
 |          |                               |             |          |
 | PendingI | User service terminated       | Queue       | PendingI |
 |          |                               | termination |          |
 |          |                               | event       |          |
 |          |                               |             |          |
 | PendingI | Change in rating condition    | Queue       | PendingI |
 |          |                               | changed     |          |
 |          |                               | rating      |          |
 |          |                               | condition   |          |
 |          |                               | event       |          |
 +----------+-------------------------------+-------------+----------+
          Table 2: Session-Based Client State Machine for the
                  First Interrogation with AA-Request

Bertz, et al. Standards Track [Page 52] RFC 8506 Diameter Credit-Control Application March 2019

 +----------+-------------------------------+-------------+----------+
 | State    | Event                         | Action      | New      |
 |          |                               |             | State    |
 +----------+-------------------------------+-------------+----------+
 | Idle     | Client or device requests     | Send CC     | PendingI |
 |          | access/service                | initial     |          |
 |          |                               | req., start |          |
 |          |                               | Tx timer    |          |
 |          |                               |             |          |
 | PendingI | Successful CC initial answer  | Stop Tx     | Open     |
 |          | received                      | timer       |          |
 |          |                               |             |          |
 | PendingI | Failure to send, or temporary | Grant       | Idle     |
 |          | error and CCFH equal to       | service to  |          |
 |          | CONTINUE                      | end user    |          |
 |          |                               |             |          |
 | PendingI | Failure to send, or temporary | Terminate   | Idle     |
 |          | error and CCFH equal to       | end user's  |          |
 |          | TERMINATE or to               | service     |          |
 |          | RETRY_AND_TERMINATE           |             |          |
 |          |                               |             |          |
 | PendingI | Tx timer expired and CCFH     | Terminate   | Idle     |
 |          | equal to TERMINATE            | end user's  |          |
 |          |                               | service     |          |
 |          |                               |             |          |
 | PendingI | Tx timer expired and CCFH     | Grant       | PendingI |
 |          | equal to CONTINUE or to       | service to  |          |
 |          | RETRY_AND_TERMINATE           | end user    |          |
 |          |                               |             |          |
 | PendingI | CC initial answer received    | Terminate   | Idle     |
 |          | with Result-Code equal to     | end user's  |          |
 |          | END_USER_SERVICE_DENIED or to | service     |          |
 |          | USER_UNKNOWN                  |             |          |
 |          |                               |             |          |
 | PendingI | CC initial answer received    | Grant       | Idle     |
 |          | with Result-Code equal to     | service to  |          |
 |          | CREDIT_CONTROL_NOT_APPLICABLE | end user    |          |
 |          |                               |             |          |
 | PendingI | Failed CC initial answer      | Grant       | Idle     |
 |          | received and CCFH equal to    | service to  |          |
 |          | CONTINUE                      | end user    |          |
 |          |                               |             |          |
 | PendingI | Failed CC initial answer      | Terminate   | Idle     |
 |          | received and CCFH equal to    | end user's  |          |
 |          | TERMINATE or to               | service     |          |
 |          | RETRY_AND_TERMINATE           |             |          |
 |          |                               |             |          |

Bertz, et al. Standards Track [Page 53] RFC 8506 Diameter Credit-Control Application March 2019

 | PendingI | User service terminated       | Queue       | PendingI |
 |          |                               | termination |          |
 |          |                               | event       |          |
 |          |                               |             |          |
 | PendingI | Change in rating condition    | Queue       | PendingI |
 |          |                               | changed     |          |
 |          |                               | rating      |          |
 |          |                               | condition   |          |
 |          |                               | event       |          |
 +----------+-------------------------------+-------------+----------+
          Table 3: Session-Based Client State Machine for the
                     First Interrogation with CCR
 +----------+-------------------------------+-------------+----------+
 | State    | Event                         | Action      | New      |
 |          |                               |             | State    |
 +----------+-------------------------------+-------------+----------+
 | Open     | Granted unit elapses and no   | Send CC     | PendingU |
 |          | final-unit indication         | update      |          |
 |          | received                      | req., start |          |
 |          |                               | Tx timer    |          |
 |          |                               |             |          |
 | Open     | Granted unit elapses and      | Terminate   | PendingT |
 |          | final unit action equal to    | end user's  |          |
 |          | TERMINATE received            | service,    |          |
 |          |                               | send CC     |          |
 |          |                               | termination |          |
 |          |                               | req.        |          |
 |          |                               |             |          |
 | Open     | Change in rating condition in | Send CC     | PendingU |
 |          | queue                         | update      |          |
 |          |                               | req., start |          |
 |          |                               | Tx timer    |          |
 |          |                               |             |          |
 | Open     | Service terminated in queue   | Send CC     | PendingT |
 |          |                               | termination |          |
 |          |                               | req.        |          |
 |          |                               |             |          |
 | Open     | Change in rating condition or | Send CC     | PendingU |
 |          | Validity-Time elapses         | update      |          |
 |          |                               | req., start |          |
 |          |                               | Tx timer    |          |
 |          |                               |             |          |
 | Open     | User service terminated       | Send CC     | PendingT |
 |          |                               | termination |          |
 |          |                               | req.        |          |

Bertz, et al. Standards Track [Page 54] RFC 8506 Diameter Credit-Control Application March 2019

 |          |                               |             |          |
 | Open     | RAR received                  | Send RAA    | PendingU |
 |          |                               | followed by |          |
 |          |                               | CC update   |          |
 |          |                               | req., start |          |
 |          |                               | Tx timer    |          |
 |          |                               |             |          |
 | PendingU | Successful CC update answer   | Stop Tx     | Open     |
 |          | received                      | timer       |          |
 |          |                               |             |          |
 | PendingU | Failure to send, or temporary | Grant       | Idle     |
 |          | error and CCFH equal to       | service to  |          |
 |          | CONTINUE                      | end user    |          |
 |          |                               |             |          |
 | PendingU | Failure to send, or temporary | Terminate   | Idle     |
 |          | error and CCFH equal to       | end user's  |          |
 |          | TERMINATE or to               | service     |          |
 |          | RETRY_AND_TERMINATE           |             |          |
 |          |                               |             |          |
 | PendingU | Tx timer expired and CCFH     | Terminate   | Idle     |
 |          | equal to TERMINATE            | end user's  |          |
 |          |                               | service     |          |
 |          |                               |             |          |
 | PendingU | Tx timer expired and CCFH     | Grant       | PendingU |
 |          | equal to CONTINUE or to       | service to  |          |
 |          | RETRY_AND_TERMINATE           | end user    |          |
 |          |                               |             |          |
 | PendingU | CC update answer received     | Terminate   | Idle     |
 |          | with Result-Code equal to     | end user's  |          |
 |          | END_USER_SERVICE_DENIED       | service     |          |
 |          |                               |             |          |
 | PendingU | CC update answer received     | Grant       | Idle     |
 |          | with Result-Code equal to     | service to  |          |
 |          | CREDIT_CONTROL_NOT_APPLICABLE | end user    |          |
 |          |                               |             |          |
 | PendingU | Failed CC update answer       | Grant       | Idle     |
 |          | received and CCFH equal to    | service to  |          |
 |          | CONTINUE                      | end user    |          |
 |          |                               |             |          |
 | PendingU | Failed CC update answer       | Terminate   | Idle     |
 |          | received and CCFH equal to    | end user's  |          |
 |          | TERMINATE or to               | service     |          |
 |          | RETRY_AND_TERMINATE           |             |          |
 |          |                               |             |          |
 | PendingU | User service terminated       | Queue       | PendingU |
 |          |                               | termination |          |
 |          |                               | event       |          |
 |          |                               |             |          |

Bertz, et al. Standards Track [Page 55] RFC 8506 Diameter Credit-Control Application March 2019

 | PendingU | Change in rating condition    | Queue       | PendingU |
 |          |                               | changed     |          |
 |          |                               | rating      |          |
 |          |                               | condition   |          |
 |          |                               | event       |          |
 |          |                               |             |          |
 | PendingU | RAR received                  | Send RAA    | PendingU |
 |          |                               |             |          |
 | PendingT | Successful CC termination     |             | Idle     |
 |          | answer received               |             |          |
 |          |                               |             |          |
 | PendingT | Failure to send, temporary    |             | Idle     |
 |          | error, or failed answer       |             |          |
 |          |                               |             |          |
 | PendingT | Change in rating condition    |             | PendingT |
 +----------+-------------------------------+-------------+----------+
   Table 4: Session-Based Client State Machine for Intermediate and
                         Final Interrogations
 +----------+--------------------------------+------------+----------+
 | State    | Event                          | Action     | New      |
 |          |                                |            | State    |
 +----------+--------------------------------+------------+----------+
 | Idle     | Client or device requests a    | Send CC    | PendingE |
 |          | one-time service               | event      |          |
 |          |                                | req.,      |          |
 |          |                                | start Tx   |          |
 |          |                                | timer      |          |
 |          |                                |            |          |
 | Idle     | Request in storage             | Send       | PendingB |
 |          |                                | stored     |          |
 |          |                                | request    |          |
 |          |                                |            |          |
 | PendingE | Successful CC event answer     | Grant      | Idle     |
 |          | received                       | service to |          |
 |          |                                | end user   |          |
 |          |                                |            |          |
 | PendingE | Failure to send, temporary     | Indicate   | Idle     |
 |          | error, failed CC event answer  | service    |          |
 |          | received, or Tx timer expired; | error      |          |
 |          | requested action CHECK_BALANCE |            |          |
 |          | or PRICE_ENQUIRY               |            |          |
 |          |                                |            |          |

Bertz, et al. Standards Track [Page 56] RFC 8506 Diameter Credit-Control Application March 2019

 | PendingE | CC event answer received with  | Terminate  | Idle     |
 |          | Result-Code equal to           | end user's |          |
 |          | END_USER_SERVICE_DENIED or to  | service    |          |
 |          | USER_UNKNOWN and Tx timer      |            |          |
 |          | running                        |            |          |
 |          |                                |            |          |
 | PendingE | CC event answer received with  | Grant      | Idle     |
 |          | Result-Code equal to           | service to |          |
 |          | CREDIT_CONTROL_NOT_APPLICABLE; | end user   |          |
 |          | requested action               |            |          |
 |          | DIRECT_DEBITING                |            |          |
 |          |                                |            |          |
 | PendingE | Failure to send, temporary     | Grant      | Idle     |
 |          | error, or failed CC event      | service to |          |
 |          | answer received; requested     | end user   |          |
 |          | action DIRECT_DEBITING; DDFH   |            |          |
 |          | equal to CONTINUE              |            |          |
 |          |                                |            |          |
 | PendingE | Failed CC event answer         | Terminate  | Idle     |
 |          | received or temporary error;   | end user's |          |
 |          | requested action               | service    |          |
 |          | DIRECT_DEBITING; DDFH equal to |            |          |
 |          | TERMINATE_OR_BUFFER and Tx     |            |          |
 |          | timer running                  |            |          |
 |          |                                |            |          |
 | PendingE | Tx timer expired; requested    | Grant      | PendingE |
 |          | action DIRECT_DEBITING         | service to |          |
 |          |                                | end user   |          |
 |          |                                |            |          |
 | PendingE | Failure to send; requested     | Store      | Idle     |
 |          | action DIRECT_DEBITING; DDFH   | request    |          |
 |          | equal to TERMINATE_OR_BUFFER   | with       |          |
 |          |                                | T flag     |          |
 |          |                                |            |          |
 | PendingE | Temporary error; requested     | Store      | Idle     |
 |          | action DIRECT_DEBITING; DDFH   | request    |          |
 |          | equal to TERMINATE_OR_BUFFER;  |            |          |
 |          | Tx timer expired               |            |          |
 |          |                                |            |          |
 | PendingE | Failed answer or answer        |            | Idle     |
 |          | received with Result-Code      |            |          |
 |          | equal to END_USER_SERVICE      |            |          |
 |          | DENIED or to USER_UNKNOWN;     |            |          |
 |          | requested action               |            |          |
 |          | DIRECT_DEBITING; Tx timer      |            |          |
 |          | expired                        |            |          |
 |          |                                |            |          |

Bertz, et al. Standards Track [Page 57] RFC 8506 Diameter Credit-Control Application March 2019

 | PendingE | Failed CC event answer         | Indicate   | Idle     |
 |          | received; requested action     | service    |          |
 |          | REFUND_ACCOUNT                 | error and  |          |
 |          |                                | delete     |          |
 |          |                                | request    |          |
 |          |                                |            |          |
 | PendingE | Failure to send or Tx timer    | Store      | Idle     |
 |          | expired; requested action      | request    |          |
 |          | REFUND_ACCOUNT                 | with       |          |
 |          |                                | T flag     |          |
 |          |                                |            |          |
 | PendingE | Temporary error; requested     | Store      | Idle     |
 |          | action REFUND_ACCOUNT          | request    |          |
 |          |                                |            |          |
 | PendingB | Successful CC answer received  | Delete     | Idle     |
 |          |                                | request    |          |
 |          |                                |            |          |
 | PendingB | Failed CC answer received      | Delete     | Idle     |
 |          |                                | request    |          |
 |          |                                |            |          |
 | PendingB | Failure to send or temporary   |            | Idle     |
 |          | error                          |            |          |
 +----------+--------------------------------+------------+----------+
             Table 5: One-Time Event Client State Machine
 +-------+------------------------+--------------------------+-------+
 | State | Event                  | Action                   | New   |
 |       |                        |                          | State |
 +-------+------------------------+--------------------------+-------+
 | Idle  | CC initial request     | Send CC initial answer,  | Open  |
 |       | received and           | reserve units, start Tcc |       |
 |       | successfully processed |                          |       |
 |       |                        |                          |       |
 | Idle  | CC initial request     | Send CC initial answer   | Idle  |
 |       | received but not       | with Result-Code !=      |       |
 |       | successfully processed | SUCCESS                  |       |
 |       |                        |                          |       |
 | Idle  | CC event request       | Send CC event answer     | Idle  |
 |       | received and           |                          |       |
 |       | successfully processed |                          |       |
 |       |                        |                          |       |
 | Idle  | CC event request       | Send CC event answer     | Idle  |
 |       | received but not       | with Result-Code !=      |       |
 |       | successfully processed | SUCCESS                  |       |
 |       |                        |                          |       |

Bertz, et al. Standards Track [Page 58] RFC 8506 Diameter Credit-Control Application March 2019

 | Open  | CC update request      | Send CC update answer,   | Open  |
 |       | received and           | debit used units,        |       |
 |       | successfully processed | reserve new units,       |       |
 |       |                        | restart Tcc              |       |
 |       |                        |                          |       |
 | Open  | CC update request      | Send CC update answer    | Idle  |
 |       | received but not       | with Result-Code !=      |       |
 |       | successfully processed | SUCCESS, debit used      |       |
 |       |                        | units                    |       |
 |       |                        |                          |       |
 | Open  | CC termination request | Send CC termin. answer,  | Idle  |
 |       | received and           | stop Tcc, debit used     |       |
 |       | successfully processed | units                    |       |
 |       |                        |                          |       |
 | Open  | CC termination request | Send CC termin. answer   | Idle  |
 |       | received but not       | with Result-Code !=      |       |
 |       | successfully processed | SUCCESS, debit used      |       |
 |       |                        | units                    |       |
 |       |                        |                          |       |
 | Open  | Session supervision    | Release reserved units   | Idle  |
 |       | timer Tcc expired      |                          |       |
 +-------+------------------------+--------------------------+-------+
      Table 6: Session-Based and Event-Based Server State Machine

8. Credit-Control AVPs

 This section defines the Credit-Control AVPs that are specific to the
 Diameter Credit-Control application and that MAY be included in the
 Diameter Credit-Control messages.
 The AVPs defined in this section MAY also be included in
 authorization commands defined in authorization-specific
 applications, such as [RFC7155] and [RFC4004], if the first
 interrogation is performed as part of the authorization/
 authentication process, as described in Section 5.2.
 The Diameter AVP rules are defined in [RFC6733], Section 4.  These
 AVP rules are observed in AVPs defined in this section.
 The following table describes the Diameter AVPs defined in the
 credit-control application, their AVP Code values, types, and
 possible flag values.  The AVP Flag rules ('M', 'V') are explained in
 [RFC6733], Section 4.1.

Bertz, et al. Standards Track [Page 59] RFC 8506 Diameter Credit-Control Application March 2019

                                                     +---------------+
                                                     |AVP Flag Rules |
                                 Defined             |----+-----+----|
                           AVP   in                  |    |     |MUST|
 Attribute Name            Code  Section Data Type   |MUST| MAY |NOT |
 ----------------------------------------------------|----+-----+----|
 CC-Correlation-Id         411   8.1     OctetString |    |  M  |  V |
 CC-Input-Octets           412   8.24    Unsigned64  | M  |     |  V |
 CC-Money                  413   8.22    Grouped     | M  |     |  V |
 CC-Output-Octets          414   8.25    Unsigned64  | M  |     |  V |
 CC-Request-Number         415   8.2     Unsigned32  | M  |     |  V |
 CC-Request-Type           416   8.3     Enumerated  | M  |     |  V |
 CC-Service-Specific-      417   8.26    Unsigned64  | M  |     |  V |
   Units                                             |    |     |    |
 CC-Session-Failover       418   8.4     Enumerated  | M  |     |  V |
 CC-Sub-Session-Id         419   8.5     Unsigned64  | M  |     |  V |
 CC-Time                   420   8.21    Unsigned32  | M  |     |  V |
 CC-Total-Octets           421   8.23    Unsigned64  | M  |     |  V |
 CC-Unit-Type              454   8.32    Enumerated  | M  |     |  V |
 Check-Balance-Result      422   8.6     Enumerated  | M  |     |  V |
 Cost-Information          423   8.7     Grouped     | M  |     |  V |
 Cost-Unit                 424   8.12    UTF8String  | M  |     |  V |
 Credit-Control            426   8.13    Enumerated  | M  |     |  V |
 Credit-Control-           427   8.14    Enumerated  | M  |     |  V |
   Failure-Handling                                  |    |     |    |
 Currency-Code             425   8.11    Unsigned32  | M  |     |  V |
 Direct-Debiting-          428   8.15    Enumerated  | M  |     |  V |
   Failure-Handling                                  |    |     |    |
 Exponent                  429   8.9     Integer32   | M  |     |  V |
 Final-Unit-Action         449   8.35    Enumerated  | M  |     |  V |
 Final-Unit-Indication     430   8.34    Grouped     | M  |     |  V |
 QoS-Final-Unit-Indication 669   8.68    Grouped     |    |  M  |  V |
 Granted-Service-Unit      431   8.17    Grouped     | M  |     |  V |
 G-S-U-Pool-Identifier     453   8.31    Unsigned32  | M  |     |  V |
 G-S-U-Pool-Reference      457   8.30    Grouped     | M  |     |  V |
 Multiple-Services-        456   8.16    Grouped     | M  |     |  V |
   Credit-Control                                    |    |     |    |
 Multiple-Services-        455   8.40    Enumerated  | M  |     |  V |
   Indicator                                         |    |     |    |
 Rating-Group              432   8.29    Unsigned32  | M  |     |  V |
 Redirect-Address-Type     433   8.38    Enumerated  | M  |     |  V |
 Redirect-Server           434   8.37    Grouped     | M  |     |  V |
 Redirect-Server-Address   435   8.39    UTF8String  | M  |     |  V |
 Redirect-Server-Extension 665   8.64    Grouped     |    |  M  |  V |
 Redirect-Address-         666   8.65    Address     |    |  M  |  V |
   IPAddress                                         |    |     |    |
 Redirect-Address-URL      667   8.66    UTF8String  |    |  M  |  V |
 Redirect-Address-SIP-URI  668   8.67    UTF8String  |    |  M  |  V |

Bertz, et al. Standards Track [Page 60] RFC 8506 Diameter Credit-Control Application March 2019

 Requested-Action          436   8.41    Enumerated  | M  |     |  V |
 Requested-Service-Unit    437   8.18    Grouped     | M  |     |  V |
 Restriction-Filter-Rule   438   8.36    IPFilterRule| M  |     |  V |
 Service-Context-Id        461   8.42    UTF8String  | M  |     |  V |
 Service-Identifier        439   8.28    Unsigned32  | M  |     |  V |
 Service-Parameter-Info    440   8.43    Grouped     |    |  M  |  V |
 Service-Parameter-Type    441   8.44    Unsigned32  |    |  M  |  V |
 Service-Parameter-Value   442   8.45    OctetString |    |  M  |  V |
 Subscription-Id           443   8.46    Grouped     | M  |     |  V |
 Subscription-Id-Data      444   8.48    UTF8String  | M  |     |  V |
 Subscription-Id-Type      450   8.47    Enumerated  | M  |     |  V |
 Subscription-Id-Extension 659   8.58    Grouped     |    |  M  |  V |
 Subscription-Id-E164      660   8.59    UTF8String  |    |  M  |  V |
 Subscription-Id-IMSI      661   8.60    UTF8String  |    |  M  |  V |
 Subscription-Id-SIP-URI   662   8.61    UTF8String  |    |  M  |  V |
 Subscription-Id-NAI       663   8.62    UTF8String  |    |  M  |  V |
 Subscription-Id-Private   664   8.63    UTF8String  |    |  M  |  V |
 Tariff-Change-Usage       452   8.27    Enumerated  | M  |     |  V |
 Tariff-Time-Change        451   8.20    Time        | M  |     |  V |
 Unit-Value                445   8.8     Grouped     | M  |     |  V |
 Used-Service-Unit         446   8.19    Grouped     | M  |     |  V |
 User-Equipment-Info       458   8.49    Grouped     |    |  M  |  V |
 User-Equipment-Info-Type  459   8.50    Enumerated  |    |  M  |  V |
 User-Equipment-Info-Value 460   8.51    OctetString |    |  M  |  V |
 User-Equipment-Info-      653   8.52    Grouped     |    |  M  |  V |
   Extension                                         |    |     |    |
 User-Equipment-Info-      654   8.53    OctetString |    |  M  |  V |
   IMEISV                                            |    |     |    |
 User-Equipment-Info-MAC   655   8.54    OctetString |    |  M  |  V |
 User-Equipment-Info-EUI64 656   8.55    OctetString |    |  M  |  V |
 User-Equipment-Info-      657   8.56    OctetString |    |  M  |  V |
   ModifiedEUI64                                     |    |     |    |
 User-Equipment-Info-IMEI  658   8.57    OctetString |    |  M  |  V |
 Value-Digits              447   8.10    Integer64   | M  |     |  V |
 Validity-Time             448   8.33    Unsigned32  | M  |     |  V |

8.1. CC-Correlation-Id AVP

 The CC-Correlation-Id AVP (AVP Code 411) is of type OctetString and
 contains information to correlate credit-control requests generated
 for different components of the service, e.g., transport and service
 level.  Whoever allocates the Service-Context-Id (i.e., a unique
 identifier of a service-specific document) is also responsible for
 defining the content and encoding of the CC-Correlation-Id AVP.

Bertz, et al. Standards Track [Page 61] RFC 8506 Diameter Credit-Control Application March 2019

8.2. CC-Request-Number AVP

 The CC-Request-Number AVP (AVP Code 415) is of type Unsigned32 and
 identifies this request within one session.  As Session-Id AVPs are
 globally unique, the combination of the Session-Id AVP and the
 CC-Request-Number AVP is also globally unique and can be used in
 matching credit-control messages with confirmations.  An easy way to
 produce unique numbers is to set the value of the CC-Request-Number
 AVP to 0 for a credit-control request with a CC-Request-Type AVP of
 INITIAL_REQUEST (the initial request in a session).  The value of the
 CC-Request-Number AVP should be set to 1 for the first
 UPDATE_REQUEST, to 2 for the second, and so on until the value for
 TERMINATION_REQUEST is one more than the value for the last
 UPDATE_REQUEST.  In the case of event charging (when the CC-Request-
 Type AVP has the value EVENT_REQUEST), the CC-Request-Number AVP
 should be set to 0 for a credit-control request.

8.3. CC-Request-Type AVP

 The CC-Request-Type AVP (AVP Code 416) is of type Enumerated and
 contains the reason for sending the Credit-Control-Request message.
 It MUST be present in all Credit-Control-Request messages.  The
 following values are defined for the CC-Request-Type AVP (the value
 of 0 (zero) is reserved):
 INITIAL_REQUEST       1
 This request is used to initiate a credit-control session.  It
 contains credit-control information that is relevant to the
 initiation.
 UPDATE_REQUEST        2
 This request contains credit-control information for an existing
 credit-control session.  Credit-control requests of this type SHOULD
 be sent every time a credit-control re-authorization is needed at the
 expiry of the allocated quota or validity time.  Further, additional
 service-specific events MAY trigger a spontaneous UPDATE_REQUEST.
 TERMINATION_REQUEST   3
 This request is sent to terminate a credit-control session.  It
 contains credit-control information relevant to the existing session.

Bertz, et al. Standards Track [Page 62] RFC 8506 Diameter Credit-Control Application March 2019

 EVENT_REQUEST         4
 This request is used when there is no need to maintain any
 credit-control session state in the credit-control server.  It
 contains all information relevant to the service and is the only
 request of the service.  The reason for this request is further
 detailed in the Requested-Action AVP.  The Requested-Action AVP MUST
 be included in the Credit-Control-Request message when CC-Request-
 Type is set to EVENT_REQUEST.

8.4. CC-Session-Failover AVP

 The CC-Session-Failover AVP (AVP Code 418) is of type Enumerated and
 contains information as to whether moving the credit-control message
 stream to a backup server during an ongoing credit-control session is
 supported.  In the case of communication failures, the credit-control
 message streams can be moved to an alternative destination if the
 credit-control server supports failover to an alternative server.
 The secondary credit-control server name, if received from the home
 Diameter AAA server, can be used as an address of the backup server.
 An implementation is not required to support moving a credit-control
 message stream to an alternative server, as this also requires moving
 information related to the credit-control session to the backup
 server.
 The following values are defined for the CC-Session-Failover AVP:
 FAILOVER_NOT_SUPPORTED   0
 When the CC-Session-Failover AVP is set to FAILOVER_NOT_SUPPORTED,
 the credit-control message stream MUST NOT be moved to an alternative
 destination in the case of a communication failure.  This is the
 default behavior if the AVP isn't included in the reply from the
 authorization or credit-control server.
 FAILOVER_SUPPORTED       1
 When the CC-Session-Failover AVP is set to FAILOVER_SUPPORTED, the
 credit-control message stream SHOULD be moved to an alternative
 destination in the case of a communication failure.  Moving the
 credit-control message stream to a backup server MAY require that
 information related to the credit-control session should also be
 forwarded to an alternative server.

Bertz, et al. Standards Track [Page 63] RFC 8506 Diameter Credit-Control Application March 2019

8.5. CC-Sub-Session-Id AVP

 The CC-Sub-Session-Id AVP (AVP Code 419) is of type Unsigned64 and
 contains the credit-control sub-session identifier.  The combination
 of the Session-Id AVP and this AVP MUST be unique per sub-session,
 and the value of this AVP MUST be monotonically increased by one for
 all new sub-sessions.  The absence of this AVP implies that no
 sub-sessions are in use.

8.6. Check-Balance-Result AVP

 The Check-Balance-Result AVP (AVP Code 422) is of type Enumerated and
 contains the result of the balance check.  This AVP is applicable
 only when the Requested-Action AVP indicates CHECK_BALANCE in the
 Credit-Control-Request command.  The following values are defined for
 the Check-Balance-Result AVP:
 ENOUGH_CREDIT   0
 There is enough credit in the account to cover the requested service.
 NO_CREDIT       1
 There isn't enough credit in the account to cover the requested
 service.

8.7. Cost-Information AVP

 The Cost-Information AVP (AVP Code 423) is of type Grouped, and it is
 used to return the cost information of a service, which the
 credit-control client can transfer transparently to the end user.
 The included Unit-Value AVP contains the cost estimate (always of
 type "money") of the service in the case of price inquiries, or the
 accumulated cost estimation in the case of a credit-control session.
 The Currency-Code AVP specifies in which currency the cost was given.
 The Cost-Unit AVP specifies the unit when the service cost is a cost
 per unit (e.g., cost for the service is $1 per minute).
 When the Requested-Action AVP with the value PRICE_ENQUIRY is
 included in the Credit-Control-Request command, the Cost-Information
 AVP sent in the succeeding Credit-Control-Answer command contains the
 cost estimation for the requested service, without any reservations
 being made.

Bertz, et al. Standards Track [Page 64] RFC 8506 Diameter Credit-Control Application March 2019

 The Cost-Information AVP included in the Credit-Control-Answer
 command with the CC-Request-Type set to UPDATE_REQUEST contains the
 accumulated cost estimation for the session, without taking any
 credit reservations into account.
 The Cost-Information AVP included in the Credit-Control-Answer
 command with the CC-Request-Type set to EVENT_REQUEST or
 TERMINATION_REQUEST contains the estimated total cost for the
 requested service.
 The Cost-Information AVP is defined as follows (per grouped-avp-def
 as defined in [RFC6733]):
                 Cost-Information ::= < AVP Header: 423 >
                                      { Unit-Value }
                                      { Currency-Code }
                                      [ Cost-Unit ]

8.8. Unit-Value AVP

 The Unit-Value AVP is of type Grouped (AVP Code 445) and specifies
 the cost as a floating-point value.  The Unit-Value is a significand
 with an exponent; i.e., Unit-Value = Value-Digits AVP * 10^Exponent.
 This representation avoids unwanted rounding off.  For example, the
 value of 2,3 is represented as Value-Digits = 23 and Exponent = -1.
 The absence of the exponent part MUST be interpreted as an exponent
 equal to zero.
 The Unit-Value AVP is defined as follows (per grouped-avp-def as
 defined in [RFC6733]):
                     Unit-Value ::= < AVP Header: 445 >
                                    { Value-Digits }
                                    [ Exponent ]

8.9. Exponent AVP

 The Exponent AVP is of type Integer32 (AVP Code 429) and contains the
 exponent value to be applied for the Value-Digits AVP within the
 Unit-Value AVP.

Bertz, et al. Standards Track [Page 65] RFC 8506 Diameter Credit-Control Application March 2019

8.10. Value-Digits AVP

 The Value-Digits AVP is of type Integer64 (AVP Code 447) and contains
 the significant digits of the number.  If decimal values are needed
 to present the units, the scaling MUST be indicated with the related
 Exponent AVP.  For example, for the monetary amount $0.05, the value
 of the Value-Digits AVP MUST be set to 5, and the scaling MUST be
 indicated with the Exponent AVP set to -2.

8.11. Currency-Code AVP

 The Currency-Code AVP (AVP Code 425) is of type Unsigned32 and
 contains a currency code that specifies in which currency the values
 of AVPs containing monetary units were given.  It is specified by
 using the numeric values defined in the ISO 4217 standard [ISO4217].

8.12. Cost-Unit AVP

 The Cost-Unit AVP (AVP Code 424) is of type UTF8String, and it is
 used to display a human-readable string to the end user.  It
 specifies the applicable unit to the Cost-Information AVP when the
 service cost is a cost per unit (e.g., cost of the service is $1 per
 minute).  The Cost-Unit setting can be minutes, hours, days,
 kilobytes, megabytes, etc.

8.13. Credit-Control AVP

 The Credit-Control AVP (AVP Code 426) is of type Enumerated and MUST
 be included in AA-Request messages when the Service Element has
 credit-control capabilities.  The following values are defined for
 the Credit-Control AVP:
 CREDIT_AUTHORIZATION   0
 If the home Diameter AAA server determines that the user has a
 prepaid subscription, this value indicates that the credit-control
 server MUST be contacted to perform the first interrogation.  The
 value of the Credit-Control AVP MUST always be set to 0 in an
 AA-Request sent to perform the first interrogation and to initiate a
 new credit-control session.
 RE_AUTHORIZATION       1
 This value indicates to the Diameter AAA server that a credit-control
 session is ongoing for the subscriber and that the credit-control
 server MUST NOT be contacted.  The Credit-Control AVP set to the
 value of 1 is to be used only when the first interrogation has been
 successfully performed and the credit-control session is ongoing

Bertz, et al. Standards Track [Page 66] RFC 8506 Diameter Credit-Control Application March 2019

 (i.e., re-authorization triggered by authorization lifetime).  This
 value MUST NOT be used in an AA-Request sent to perform the first
 interrogation.

8.14. Credit-Control-Failure-Handling AVP (CCFH)

 The CCFH (AVP Code 427) is of type Enumerated.  The credit-control
 client uses information in this AVP to decide what to do if sending
 credit-control messages to the credit-control server has been, for
 instance, temporarily prevented due to a network problem.  Depending
 on the service logic, the credit-control server can order the client
 to terminate the service immediately when there is a reason to
 believe that the service cannot be charged, or to try failover to an
 alternative server, if possible.  The server could then either
 terminate or grant the service, should the alternative connection
 also fail.
 The following values are defined for the CCFH:
 TERMINATE             0
 When the CCFH is set to TERMINATE, the service MUST only be granted
 for as long as there is a connection to the credit-control server.
 If the credit-control client does not receive any Credit-Control-
 Answer messages before the Tx timer (as defined in Section 13)
 expires, the credit-control request is regarded as failed, and the
 end user's service session is terminated.
 This is the default behavior if the AVP isn't included in the reply
 from the authorization or credit-control server.
 CONTINUE              1
 When the CCFH is set to CONTINUE, the credit-control client SHOULD
 resend the request to an alternative server in the case of transport
 or temporary failures, provided that (1) a failover procedure is
 supported in the credit-control server and the credit-control client
 and (2) an alternative server is available.  Otherwise, the service
 SHOULD be granted, even if credit-control messages can't be
 delivered.
 RETRY_AND_TERMINATE   2
 When the CCFH is set to RETRY_AND_TERMINATE, the credit-control
 client SHOULD resend the request to an alternative server in the case
 of transport or temporary failures, provided that (1) a failover
 procedure is supported in the credit-control server and the
 credit-control client and (2) an alternative server is available.

Bertz, et al. Standards Track [Page 67] RFC 8506 Diameter Credit-Control Application March 2019

 Otherwise, the service SHOULD NOT be granted when the credit-control
 messages can't be delivered.

8.15. Direct-Debiting-Failure-Handling AVP (DDFH)

 The DDFH (AVP Code 428) is of type Enumerated.  The credit-control
 client uses information in this AVP to decide what to do if sending
 credit-control messages (Requested-Action AVP set to DIRECT_DEBITING)
 to the credit-control server has been, for instance, temporarily
 prevented due to a network problem.
 The following values are defined for the DDFH:
 TERMINATE_OR_BUFFER   0
 When the DDFH is set to TERMINATE_OR_BUFFER, the service MUST be
 granted for as long as there is a connection to the credit-control
 server.  If the credit-control client does not receive any
 Credit-Control-Answer messages before the Tx timer (as defined in
 Section 13) expires, the credit-control request is regarded as
 failed.  The client SHOULD terminate the service if it can determine
 from the failed answer that units have not been debited.  Otherwise,
 the credit-control client SHOULD grant the service, store the request
 in application-level non-volatile storage, and try to resend the
 request.  These requests MUST be marked as possible duplicates by
 setting the T flag in the command header as described in [RFC6733],
 Section 3.  This is the default behavior if the AVP isn't included in
 the reply from the authorization server.
 CONTINUE              1
 When the DDFH is set to CONTINUE, the service SHOULD be granted, even
 if credit-control messages can't be delivered, and the request should
 be deleted.

8.16. Multiple-Services-Credit-Control AVP

 The Multiple-Services-Credit-Control AVP (AVP Code 456) is of type
 Grouped and contains the AVPs related to the independent
 credit-control of multiple services.  Note that each instance of this
 AVP carries units related to one or more services or related to a
 single rating-group.
 The Service-Identifier AVP and the Rating-Group AVP are used to
 associate the granted units to a given service or rating-group.  If
 both the Service-Identifier AVP and the Rating-Group AVP are
 included, the target of the service units is always the service(s)
 indicated by the value of the Service-Identifier AVP(s).  If only the

Bertz, et al. Standards Track [Page 68] RFC 8506 Diameter Credit-Control Application March 2019

 Rating-Group AVP is present, the Multiple-Services-Credit-Control AVP
 relates to all the services that belong to the specified
 rating-group.
 The G-S-U-Pool-Reference AVP allows the server to specify a
 G-S-U-Pool-Identifier identifying a credit pool within which the
 units of the specified type are considered pooled.  If a G-S-U-Pool-
 Reference AVP is present, then actual service units of the specified
 type MUST also be present.  For example, if the G-S-U-Pool-Reference
 AVP specifies a CC-Unit-Type value of TIME (Section 8.32), then the
 CC-Time AVP MUST be present.
 The Requested-Service-Unit AVP MAY contain the amount of requested
 service units or the requested monetary value.  It MUST be present in
 the initial interrogation and within the intermediate interrogations
 in which a new quota is requested.  If the credit-control client does
 not include the Requested-Service-Unit AVP in a request command --
 because, for instance, it has determined that the end user terminated
 the service -- the server MUST debit the used amount from the user's
 account but MUST NOT return a new quota in the corresponding answer.
 The Validity-Time, Result-Code, and Final-Unit-Indication or
 QoS-Final-Unit-Indication AVPs MAY be present in a Credit-Control-
 Answer command as defined in Sections 5.1.2 and 5.6 for graceful
 service termination.
 When both the Tariff-Time-Change AVP and the Tariff-Change-Usage AVP
 are present, the server MUST include two separate instances of the
 Multiple-Services-Credit-Control AVP with the Granted-Service-Unit
 AVP associated to the same service-identifier and/or rating-group.
 Where the two quotas are associated to the same pool or to different
 pools, the credit-pooling mechanism defined in Section 5.1.2 applies.
 When the client is reporting used units before and after the tariff
 time change, it MUST use the Tariff-Change-Usage AVP inside the
 Used-Service-Unit AVP.
 A server not implementing the independent credit-control of multiple
 services MUST treat the Multiple-Services-Credit-Control AVP as an
 invalid AVP.

Bertz, et al. Standards Track [Page 69] RFC 8506 Diameter Credit-Control Application March 2019

 The Multiple-Services-Credit-Control AVP is defined as follows (per
 grouped-avp-def as defined in [RFC6733]):
  Multiple-Services-Credit-Control ::= < AVP Header: 456 >
                                       [ Granted-Service-Unit ]
                                       [ Requested-Service-Unit ]
                                      *[ Used-Service-Unit ]
                                       [ Tariff-Change-Usage ]
                                      *[ Service-Identifier ]
                                       [ Rating-Group ]
                                      *[ G-S-U-Pool-Reference ]
                                       [ Validity-Time ]
                                       [ Result-Code ]
                                       [ Final-Unit-Indication ]
                                       [ QoS-Final-Unit-Indication ]
                                      *[ AVP ]

8.17. Granted-Service-Unit AVP

 The Granted-Service-Unit AVP (AVP Code 431) is of type Grouped and
 contains the amount of units that the Diameter Credit-Control client
 can provide to the end user until the service must be released or the
 new Credit-Control-Request must be sent.  A client is not required to
 implement all the unit types, and it must treat unknown or
 unsupported unit types in the Answer message as an incorrect CCA.  In
 this case, the client MUST terminate the credit-control session and
 indicate the reason as DIAMETER_BAD_ANSWER in the Termination-Cause
 AVP.
 The Granted-Service-Unit AVP is defined as follows (per
 grouped-avp-def as defined in [RFC6733]):
       Granted-Service-Unit ::= < AVP Header: 431 >
                                [ Tariff-Time-Change ]
                                [ CC-Time ]
                                [ CC-Money ]
                                [ CC-Total-Octets ]
                                [ CC-Input-Octets ]
                                [ CC-Output-Octets ]
                                [ CC-Service-Specific-Units ]
                               *[ AVP ]

Bertz, et al. Standards Track [Page 70] RFC 8506 Diameter Credit-Control Application March 2019

8.18. Requested-Service-Unit AVP

 The Requested-Service-Unit AVP (AVP Code 437) is of type Grouped and
 contains the amount of requested units specified by the Diameter
 Credit-Control client.  A server is not required to implement all the
 unit types, and it must treat unknown or unsupported unit types as
 invalid AVPs.
 The Requested-Service-Unit AVP is defined as follows (per
 grouped-avp-def as defined in [RFC6733]):
       Requested-Service-Unit ::= < AVP Header: 437 >
                                  [ CC-Time ]
                                  [ CC-Money ]
                                  [ CC-Total-Octets ]
                                  [ CC-Input-Octets ]
                                  [ CC-Output-Octets ]
                                  [ CC-Service-Specific-Units ]
                                 *[ AVP ]

8.19. Used-Service-Unit AVP

 The Used-Service-Unit AVP is of type Grouped (AVP Code 446) and
 contains the amount of used units measured from the point when the
 service became active or, if interim interrogations are used during
 the session, from the point when the previous measurement ended.
 Note: The value reported in a Used-Service-Unit AVP is not
 necessarily related to the grant provided in a Granted-Service-Unit
 AVP, e.g., the value in this AVP may exceed the value in the grant.
 The Used-Service-Unit AVP is defined as follows (per grouped-avp-def
 as defined in [RFC6733]):
       Used-Service-Unit ::= < AVP Header: 446 >
                             [ Tariff-Change-Usage ]
                             [ CC-Time ]
                             [ CC-Money ]
                             [ CC-Total-Octets ]
                             [ CC-Input-Octets ]
                             [ CC-Output-Octets ]
                             [ CC-Service-Specific-Units ]
                            *[ AVP ]

Bertz, et al. Standards Track [Page 71] RFC 8506 Diameter Credit-Control Application March 2019

8.20. Tariff-Time-Change AVP

 The Tariff-Time-Change AVP (AVP Code 451) is of type Time.  It is
 sent from the server to the client and includes the time in seconds
 since January 1, 1900, 00:00 UTC, when the tariff of the service will
 be changed.
 The tariff change mechanism is optional for the client and server,
 and it is not used for time-based services (Section 5).  If a client
 does not support the tariff time change mechanism, it MUST treat the
 Tariff-Time-Change AVP in the Answer message as an incorrect CCA.  In
 this case, the client terminates the credit-control session and
 indicates the reason as DIAMETER_BAD_ANSWER in the Termination-Cause
 AVP.
 Omission of this AVP means that no tariff change is to be reported.

8.21. CC-Time AVP

 The CC-Time AVP (AVP Code 420) is of type Unsigned32 and indicates
 the length of the requested, granted, or used time in seconds.

8.22. CC-Money AVP

 The CC-Money AVP (AVP Code 413) is of type Grouped and specifies the
 monetary amount in the given currency.  The Currency-Code AVP SHOULD
 be included.  The CC-Money AVP is defined as follows (per
 grouped-avp-def as defined in [RFC6733]):
       CC-Money ::= < AVP Header: 413 >
                    { Unit-Value }
                    [ Currency-Code ]

8.23. CC-Total-Octets AVP

 The CC-Total-Octets AVP (AVP Code 421) is of type Unsigned64 and
 contains the total number of requested, granted, or used octets
 regardless of the direction (sent or received).

8.24. CC-Input-Octets AVP

 The CC-Input-Octets AVP (AVP Code 412) is of type Unsigned64 and
 contains the number of requested, granted, or used octets that
 can be / have been received from the end user.

Bertz, et al. Standards Track [Page 72] RFC 8506 Diameter Credit-Control Application March 2019

8.25. CC-Output-Octets AVP

 The CC-Output-Octets AVP (AVP Code 414) is of type Unsigned64 and
 contains the number of requested, granted, or used octets that
 can be / have been sent to the end user.

8.26. CC-Service-Specific-Units AVP

 The CC-Service-Specific-Units AVP (AVP Code 417) is of type
 Unsigned64 and specifies the number of service-specific units (e.g.,
 number of events, points) given in a selected service.  The service-
 specific units always refer to the service identified in the Service-
 Identifier AVP (or Rating-Group AVP when the Multiple-Services-
 Credit-Control AVP is used).

8.27. Tariff-Change-Usage AVP

 The Tariff-Change-Usage AVP (AVP Code 452) is of type Enumerated and
 defines whether units are used before or after a tariff change, or
 whether the units straddled a tariff change during the reporting
 period.  Omission of this AVP means that no tariff change has
 occurred.
 In addition, when present in Answer messages as part of the Multiple-
 Services-Credit-Control AVP, this AVP defines whether units are
 allocated to be used before or after a tariff change event.
 When the Tariff-Time-Change AVP is present, omission of this AVP in
 Answer messages means that the single-quota mechanism applies.
 Tariff-Change-Usage can be set to one of the following values:
 UNIT_BEFORE_TARIFF_CHANGE   0
 When present in the Multiple-Services-Credit-Control AVP, this value
 indicates the amount of units allocated for use before a tariff
 change occurs.
 When present in the Used-Service-Unit AVP, this value indicates the
 amount of resource units used before a tariff change had occurred.

Bertz, et al. Standards Track [Page 73] RFC 8506 Diameter Credit-Control Application March 2019

 UNIT_AFTER_TARIFF_CHANGE    1
 When present in the Multiple-Services-Credit-Control AVP, this value
 indicates the amount of units allocated for use after a tariff change
 occurs.
 When present in the Used-Service-Unit AVP, this value indicates the
 amount of resource units used after a tariff change had occurred.
 UNIT_INDETERMINATE          2
 This value is to be used only in the Used-Service-Unit AVP and
 indicates the amount of resource units that straddle the tariff
 change (e.g., the metering process reports to the credit-control
 client in blocks of n octets, and one block straddled the tariff
 change).

8.28. Service-Identifier AVP

 The Service-Identifier AVP is of type Unsigned32 (AVP Code 439) and
 contains the identifier of a service.  The specific service the
 request relates to is uniquely identified by the combination of the
 Service-Context-Id AVP and the Service-Identifier AVP.
 A usage example of this AVP is illustrated in Appendix A.9.

8.29. Rating-Group AVP

 The Rating-Group AVP is of type Unsigned32 (AVP Code 432) and
 contains the identifier of a rating-group.  All the services subject
 to the same rating type are part of the same rating-group.  The
 specific rating-group the request relates to is uniquely identified
 by the combination of the Service-Context-Id AVP and the Rating-Group
 AVP.
 A usage example of this AVP is illustrated in Appendix A.9.

8.30. G-S-U-Pool-Reference AVP

 The G-S-U-Pool-Reference AVP (AVP Code 457) is of type Grouped.  It
 is used in the Credit-Control-Answer message and associates the
 Granted-Service-Unit AVP within which it appears with a credit pool
 within the session.
 The G-S-U-Pool-Identifier AVP specifies the credit pool from which
 credit is drawn for this unit type.

Bertz, et al. Standards Track [Page 74] RFC 8506 Diameter Credit-Control Application March 2019

 The CC-Unit-Type AVP specifies the type of units for which credit is
 pooled.
 The Unit-Value AVP specifies the multiplier, which converts between
 service units of type CC-Unit-Type and abstract service units within
 the credit pool (and thus to service units of any other services or
 rating-groups associated with the same pool).
 The G-S-U-Pool-Reference AVP is defined as follows (per
 grouped-avp-def as defined in [RFC6733]):
         G-S-U-Pool-Reference ::= < AVP Header: 457 >
                                  { G-S-U-Pool-Identifier }
                                  { CC-Unit-Type }
                                  { Unit-Value }

8.31. G-S-U-Pool-Identifier AVP

 The G-S-U-Pool-Identifier AVP (AVP Code 453) is of type Unsigned32
 and identifies a credit pool within the session.

8.32. CC-Unit-Type AVP

 The CC-Unit-Type AVP (AVP Code 454) is of type Enumerated and
 specifies the type of units considered to be pooled into a
 credit pool.
 The following values are defined for the CC-Unit-Type AVP:
       TIME                         0
       MONEY                        1
       TOTAL-OCTETS                 2
       INPUT-OCTETS                 3
       OUTPUT-OCTETS                4
       SERVICE-SPECIFIC-UNITS       5

8.33. Validity-Time AVP

 The Validity-Time AVP is of type Unsigned32 (AVP Code 448).  It is
 sent from the credit-control server to the credit-control client.
 The Validity-Time AVP contains the validity time of the granted
 service units.  The measurement of the Validity-Time is started upon
 receipt of the Credit-Control-Answer message containing this AVP.  If
 the granted service units have not been consumed within the validity
 time specified in this AVP, the credit-control client MUST send a
 Credit-Control-Request message to the server, with CC-Request-Type
 set to UPDATE_REQUEST.  The value field of the Validity-Time AVP is
 given in seconds.

Bertz, et al. Standards Track [Page 75] RFC 8506 Diameter Credit-Control Application March 2019

 The Validity-Time AVP is also used for graceful service termination
 (see Section 5.6) to indicate to the credit-control client how long
 the subscriber is allowed to use network resources after the
 specified action (i.e., REDIRECT or RESTRICT_ACCESS) started.  When
 the Validity-Time elapses, a new intermediate interrogation is sent
 to the server.

8.34. Final-Unit-Indication AVP

 The Final-Unit-Indication AVP (AVP Code 430) is of type Grouped and
 indicates that the Granted-Service-Unit AVP in the Credit-Control-
 Answer or in the AA-Answer contains the final units for the service.
 After these units have expired, the Diameter Credit-Control client is
 responsible for executing the action indicated in the Final-Unit-
 Action AVP (see Section 5.6).
 If more than one unit type is received in the Credit-Control-Answer,
 the unit type that first expired SHOULD cause the credit-control
 client to execute the specified action.
 In the first interrogation, the Final-Unit-Indication AVP with
 Final-Unit-Action set to REDIRECT or RESTRICT_ACCESS can also be
 present with no Granted-Service-Unit AVP in the Credit-Control-Answer
 or in the AA-Answer.  This indicates to the Diameter Credit-Control
 client that the client is to execute the specified action
 immediately.  If the home service provider policy is to terminate the
 service, naturally, the server SHOULD return the appropriate
 transient failure (see Section 9.1) in order to implement the policy-
 defined action.
 The Final-Unit-Action AVP defines the behavior of the Service Element
 when the user's account cannot cover the cost of the service and MUST
 always be present if the Final-Unit-Indication AVP is included in a
 command.
 If the Final-Unit-Action AVP is set to TERMINATE, the Final-Unit-
 Indication group AVP MUST NOT contain any other AVPs.
 If the Final-Unit-Action AVP is set to REDIRECT, the Redirect-Server
 AVP or the Redirect-Server-Extension AVP (at least one) MUST be
 present.  The Restriction-Filter-Rule AVP or the Filter-Id AVP MAY be
 present in the Credit-Control-Answer message if the user is also
 allowed to access other services that are not accessible through the
 address given in the Redirect-Server AVP.
 If the Final-Unit-Action AVP is set to RESTRICT_ACCESS, either the
 Restriction-Filter-Rule AVP or the Filter-Id AVP SHOULD be present.

Bertz, et al. Standards Track [Page 76] RFC 8506 Diameter Credit-Control Application March 2019

 The Filter-Id AVP is defined in [RFC7155].  The Filter-Id AVP can be
 used to reference an IP filter list installed in the access device by
 means other than the Diameter Credit-Control application, e.g.,
 locally configured or configured by another entity.
 If the Final-Unit-Action AVP is set to REDIRECT and the type of
 server is not one of the enumerations in the Redirect-Address-Type
 AVP, then the QoS-Final-Unit-Indication AVP SHOULD be used together
 with the Redirect-Server-Extension AVP instead of the Final-Unit-
 Indication AVP.
 If the Final-Unit-Action AVP is set to RESTRICT_ACCESS or REDIRECT
 and the classification of the restricted traffic cannot be expressed
 using an IPFilterRule, or if actions (e.g., QoS) other than just
 allowing traffic need to be enforced, then the QoS-Final-Unit-
 Indication AVP SHOULD be used instead of the Final-Unit-Indication
 AVP.  However, if the credit-control server wants to preserve
 backward compatibility with credit-control clients that support only
 [RFC4006], the Final-Unit-Indication AVP SHOULD be used together with
 the Filter-Id AVP.
 The Final-Unit-Indication AVP is defined as follows (per
 grouped-avp-def as defined in [RFC6733]):
       Final-Unit-Indication ::= < AVP Header: 430 >
                                 { Final-Unit-Action }
                                *[ Restriction-Filter-Rule ]
                                *[ Filter-Id ]
                                 [ Redirect-Server ]

8.35. Final-Unit-Action AVP

 The Final-Unit-Action AVP (AVP Code 449) is of type Enumerated and
 indicates to the credit-control client the action to be taken when
 the user's account cannot cover the service cost.
 Final-Unit-Action can be set to one of the following values:
 TERMINATE         0
 The credit-control client MUST terminate the service session.  This
 is the default handling, applicable whenever the credit-control
 client receives an unsupported Final-Unit-Action value, and it MUST
 be supported by all the Diameter Credit-Control client
 implementations conforming to this specification.

Bertz, et al. Standards Track [Page 77] RFC 8506 Diameter Credit-Control Application March 2019

 REDIRECT          1
 The Service Element MUST redirect the user to the address specified
 in the Redirect-Server-Address AVP or one of the AVPs included in the
 Redirect-Server-Extension AVP.  The redirect action is defined in
 Section 5.6.2.
 RESTRICT_ACCESS   2
 The access device MUST restrict the user's access according to the
 filter AVPs contained in the applied Grouped AVP: according to IP
 packet filters defined in the Restriction-Filter-Rule AVP, according
 to the packet classifier filters defined in the Filter-Rule AVP, or
 according to the packet filters identified by the Filter-Id AVP.  All
 of the packets not matching any restriction filters (see
 Section 5.6.3) MUST be dropped.

8.36. Restriction-Filter-Rule AVP

 The Restriction-Filter-Rule AVP (AVP Code 438) is of type
 IPFilterRule and provides filter rules corresponding to services that
 are to remain accessible even if there are no more service units
 granted.  The access device has to configure the specified filter
 rules for the subscriber and MUST drop all the packets not matching
 these filters.  Zero, one, or more such AVPs MAY be present in a
 Credit-Control-Answer message or in an AA-Answer message.

8.37. Redirect-Server AVP

 The Redirect-Server AVP (AVP Code 434) is of type Grouped and
 contains the address information of the redirect server (e.g., HTTP
 redirect server, SIP Server) with which the end user is to be
 connected when the account cannot cover the service cost.  It MUST be
 present when the Final-Unit-Action AVP is set to REDIRECT.
 The Redirect-Server AVP is defined as follows (per grouped-avp-def as
 defined in [RFC6733]):
       Redirect-Server ::= < AVP Header: 434 >
                           { Redirect-Address-Type }
                           { Redirect-Server-Address }

Bertz, et al. Standards Track [Page 78] RFC 8506 Diameter Credit-Control Application March 2019

8.38. Redirect-Address-Type AVP

 The Redirect-Address-Type AVP (AVP Code 433) is of type Enumerated
 and defines the address type of the address given in the Redirect-
 Server-Address AVP.
 Redirect-Address-Type can be set to one of the following values:
 IPv4 Address   0
 The address type is in the form of a "dotted-decimal" IPv4 address,
 as defined in [RFC791].
 IPv6 Address   1
 The address type is in the form of an IPv6 address, as defined in
 [RFC4291].  The address MUST conform to the textual representation of
 the address according to [RFC5952].
 Because [RFC5952] is more restrictive than the "RFC 3513" format
 required by [RFC4006], some legacy implementations may not be
 compliant with the new requirements.  Accordingly, implementations
 receiving this AVP MAY be liberal in the textual IPv6 representations
 that are accepted, without raising an error.
 URL            2
 The address type is in the form of a Uniform Resource Locator, as
 defined in [RFC3986].
 SIP URI        3
 The address type is in the form of a SIP Uniform Resource Identifier,
 as defined in [RFC3261].

8.39. Redirect-Server-Address AVP

 The Redirect-Server-Address AVP (AVP Code 435) is of type UTF8String
 and defines the address of the redirect server (e.g., HTTP redirect
 server, SIP Server) with which the end user is to be connected when
 the account cannot cover the service cost.

Bertz, et al. Standards Track [Page 79] RFC 8506 Diameter Credit-Control Application March 2019

8.40. Multiple-Services-Indicator AVP

 The Multiple-Services-Indicator AVP (AVP Code 455) is of type
 Enumerated and indicates whether the Diameter Credit-Control client
 is capable of handling multiple services independently within a
 (sub-)session.  The absence of this AVP means that independent
 credit-control of multiple services is not supported.
 A server not implementing the independent credit-control of multiple
 services MUST treat the Multiple-Services-Indicator AVP as an
 invalid AVP.
 The following values are defined for the Multiple-Services-Indicator
 AVP:
 MULTIPLE_SERVICES_NOT_SUPPORTED   0
 The client does not support independent credit-control of multiple
 services within a (sub-)session.
 MULTIPLE_SERVICES_SUPPORTED       1
 The client supports independent credit-control of multiple services
 within a (sub-)session.

8.41. Requested-Action AVP

 The Requested-Action AVP (AVP Code 436) is of type Enumerated and
 contains the requested action being sent in a Credit-Control-Request
 command where the CC-Request-Type is set to EVENT_REQUEST.  The
 following values are defined for the Requested-Action AVP:
 DIRECT_DEBITING   0
 This indicates a request to decrease the end user's account according
 to information specified in the Requested-Service-Unit AVP and/or
 Service-Identifier AVP (additional rating information may be included
 in service-specific AVPs or in the Service-Parameter-Info AVP).  The
 Granted-Service-Unit AVP in the Credit-Control-Answer command
 contains the debited units.

Bertz, et al. Standards Track [Page 80] RFC 8506 Diameter Credit-Control Application March 2019

 REFUND_ACCOUNT    1
 This indicates a request to increase the end user's account according
 to information specified in the Requested-Service-Unit AVP and/or
 Service-Identifier AVP (additional rating information may be included
 in service-specific AVPs or in the Service-Parameter-Info AVP).  The
 Granted-Service-Unit AVP in the Credit-Control-Answer command
 contains the refunded units.
 CHECK_BALANCE     2
 This indicates a balance-check request.  In this case, the checking
 of the account balance is done without any credit reservations from
 the account.  The Check-Balance-Result AVP in the Credit-Control-
 Answer command contains the result of the balance check.
 PRICE_ENQUIRY     3
 This indicates a price-inquiry request.  In this case, neither
 checking of the account balance nor reservation from the account will
 be done; only the price of the service will be returned in the
 Cost-Information AVP in the Credit-Control-Answer command.

8.42. Service-Context-Id AVP

 The Service-Context-Id AVP is of type UTF8String (AVP Code 461) and
 contains a unique identifier of the Diameter Credit-Control service-
 specific document (as defined in Section 4.1.2) that applies to the
 request.  This is an identifier allocated by the service provider,
 the Service Element manufacturer, or a standardization body, and MUST
 uniquely identify a given Diameter Credit-Control service-specific
 document.  The format of the Service-Context-Id is:
 "service-context" "@" "domain"
 service-context = Token
 The Token is an arbitrary string of characters and digits.
 "domain" represents the entity that allocated the Service-Context-Id.
 It can be ietf.org, 3gpp.org, etc. if the identifier is allocated by
 a standardization body, or it can be the Fully Qualified Domain Name
 (FQDN) of the service provider (e.g., provider.example.com) or the
 vendor (e.g., vendor.example.com) if the identifier is allocated by a
 private entity.
 This AVP SHOULD be placed as close to the Diameter header as
 possible.

Bertz, et al. Standards Track [Page 81] RFC 8506 Diameter Credit-Control Application March 2019

 Service-specific documents that are for private use only (i.e., for
 one provider's own use, where no interoperability is deemed useful)
 may define private identifiers without a need for coordination.
 However, when interoperability is desired, coordination of the
 identifiers via, for example, publication of an informational RFC is
 RECOMMENDED in order to make the Service-Context-Id AVP globally
 available.

8.43. Service-Parameter-Info AVP

 The Service-Parameter-Info AVP (AVP Code 440) is of type Grouped and
 contains service-specific information used for price calculation or
 rating.  The Service-Parameter-Type AVP defines the service parameter
 type, and the Service-Parameter-Value AVP contains the parameter
 value.  The actual contents of these AVPs are not within the scope of
 this document and SHOULD be defined in another Diameter application,
 in standards written by other standardization bodies, or in service-
 specific documentation.
 In the case of an unknown service request (e.g., unknown Service-
 Parameter-Type), the corresponding Answer message MUST contain the
 error code DIAMETER_RATING_FAILED.  A Credit-Control-Answer message
 with this error MUST contain one or more Failed-AVP AVPs containing
 the Service-Parameter-Info AVPs that caused the failure.
 The Service-Parameter-Info AVP is defined as follows (per
 grouped-avp-def as defined in [RFC6733]):
       Service-Parameter-Info ::= < AVP Header: 440 >
                                  { Service-Parameter-Type }
                                  { Service-Parameter-Value }

8.44. Service-Parameter-Type AVP

 The Service-Parameter-Type AVP is of type Unsigned32 (AVP Code 441)
 and defines the type of the service-event-specific parameter (e.g.,
 it can be the end-user location or service name).  The different
 parameters and their types are service specific, and the meanings of
 these parameters are not defined in this document.  Whoever allocates
 the Service-Context-Id (i.e., a unique identifier of a service-
 specific document) is also responsible for assigning Service-
 Parameter-Type values for the service and ensuring their uniqueness
 within the given service.  The Service-Parameter-Value AVP contains
 the value associated with the service parameter type.

Bertz, et al. Standards Track [Page 82] RFC 8506 Diameter Credit-Control Application March 2019

8.45. Service-Parameter-Value AVP

 The Service-Parameter-Value AVP is of type OctetString (AVP Code 442)
 and contains the value of the service parameter type.

8.46. Subscription-Id AVP

 The Subscription-Id AVP (AVP Code 443) is used to identify the
 end user's subscription and is of type Grouped.  The Subscription-Id
 AVP includes a Subscription-Id-Data AVP that holds the identifier and
 a Subscription-Id-Type AVP that defines the identifier type.
 The Subscription-Id AVP is defined as follows (per grouped-avp-def as
 defined in [RFC6733]):
       Subscription-Id ::= < AVP Header: 443 >
                           { Subscription-Id-Type }
                           { Subscription-Id-Data }

8.47. Subscription-Id-Type AVP

 The Subscription-Id-Type AVP (AVP Code 450) is of type Enumerated,
 and it is used to determine which type of identifier is carried by
 the Subscription-Id AVP.
 This specification defines the following subscription identifiers.
 However, new Subscription-Id-Type values can be assigned by IANA as
 defined in Section 12.  A server MUST implement all the Subscription-
 Id-Type values required to perform credit authorization for the
 services it supports, including possible future values.  Unknown or
 unsupported Subscription-Id-Type values MUST be treated according to
 the 'M' flag rule, as defined in [RFC6733].
 END_USER_E164      0
 The identifier is in international E.164 format (e.g., MSISDN),
 according to the ITU-T E.164 numbering plan defined in [E164] and
 [CE164].
 END_USER_IMSI      1
 The identifier is in IMSI format, according to the ITU-T E.212
 identification plan as defined in [E212] and [CE212].
 END_USER_SIP_URI   2
 The identifier is in the form of a SIP URI, as defined in [RFC3261].

Bertz, et al. Standards Track [Page 83] RFC 8506 Diameter Credit-Control Application March 2019

 END_USER_NAI       3
 The identifier is in the form of a Network Access Identifier, as
 defined in [RFC7542].
 END_USER_PRIVATE   4
 The identifier is a credit-control server private identifier.

8.48. Subscription-Id-Data AVP

 The Subscription-Id-Data AVP (AVP Code 444) is used to identify the
 end user and is of type UTF8String.  The Subscription-Id-Type AVP
 defines which type of identifier is used.

8.49. User-Equipment-Info AVP

 The User-Equipment-Info AVP (AVP Code 458) is of type Grouped and
 allows the credit-control client to indicate the identity and
 capability of the terminal the subscriber is using for the connection
 to the network.
 The User-Equipment-Info AVP is defined as follows (per
 grouped-avp-def as defined in [RFC6733]):
       User-Equipment-Info ::= < AVP Header: 458 >
                               { User-Equipment-Info-Type }
                               { User-Equipment-Info-Value }

8.50. User-Equipment-Info-Type AVP

 The User-Equipment-Info-Type AVP is of type Enumerated (AVP Code 459)
 and defines the type of user equipment information contained in the
 User-Equipment-Info-Value AVP.
 This specification defines the following user equipment types.
 However, new User-Equipment-Info-Type values can be assigned by IANA
 as defined in Section 12.
 IMEISV           0
 The identifier contains the International Mobile Equipment Identifier
 and Software Version (IMEISV) in the IMEISV format according to 3GPP
 TS 23.003 [TGPPIMEI].

Bertz, et al. Standards Track [Page 84] RFC 8506 Diameter Credit-Control Application March 2019

 MAC              1
 The 48-bit Media Access Control (MAC) address is formatted as
 described in Section 3.21 of [RFC3580].
 EUI64            2
 The 64-bit identifier used to identify the hardware instance of the
 product, as defined in [EUI64].
 MODIFIED_EUI64   3
 There are a number of types of terminals that have identifiers other
 than the International Mobile Equipment Identifier (IMEI), IEEE 802
 MACs, or EUI-64.  These identifiers can be converted to modified
 EUI-64 format as described in [RFC4291] or by using some other
 methods referred to in the service-specific documentation.

8.51. User-Equipment-Info-Value AVP

 The User-Equipment-Info-Value AVP (AVP Code 460) is of type
 OctetString.  The User-Equipment-Info-Type AVP defines which type of
 identifier is used.

8.52. User-Equipment-Info-Extension AVP

 The User-Equipment-Info-Extension AVP (AVP Code 653) is of type
 Grouped and allows the credit-control client to indicate the identity
 and capability of the terminal the subscriber is using for the
 connection to the network.  If the type of the equipment is one of
 the enumerated User-Equipment-Info-Type AVP values, then the
 credit-control client SHOULD send the information in the
 User-Equipment-Info AVP, in addition to or instead of the
 User-Equipment-Info-Extension AVP.  This is done in order to preserve
 backward compatibility with credit-control servers that support only
 [RFC4006].  Exactly one AVP MUST be included inside the
 User-Equipment-Info-Extension AVP.

Bertz, et al. Standards Track [Page 85] RFC 8506 Diameter Credit-Control Application March 2019

 The User-Equipment-Info-Extension AVP is defined as follows (per
 grouped-avp-def as defined in [RFC6733]):
  User-Equipment-Info-Extension ::= < AVP Header: 653 >
                                [ User-Equipment-Info-IMEISV ]
                                [ User-Equipment-Info-MAC ]
                                [ User-Equipment-Info-EUI64 ]
                                [ User-Equipment-Info-ModifiedEUI64 ]
                                [ User-Equipment-Info-IMEI ]
                                [ AVP ]

8.53. User-Equipment-Info-IMEISV AVP

 The User-Equipment-Info-IMEISV AVP (AVP Code 654) is of type
 OctetString.  The User-Equipment-Info-IMEISV AVP contains the
 International Mobile Equipment Identifier and Software Version in the
 IMEISV format according to 3GPP TS 23.003 [TGPPIMEI].

8.54. User-Equipment-Info-MAC AVP

 The User-Equipment-Info-MAC AVP (AVP Code 655) is of type
 OctetString.  The User-Equipment-Info-MAC AVP contains the 48-bit MAC
 address; the MAC address is formatted as described in Section 4.1.7.8
 of [RFC5777].

8.55. User-Equipment-Info-EUI64 AVP

 The User-Equipment-Info-EUI64 AVP (AVP Code 656) is of type
 OctetString.  The User-Equipment-Info-EUI64 AVP contains the 64-bit
 identifier used to identify the hardware instance of the product, as
 defined in [EUI64].

8.56. User-Equipment-Info-ModifiedEUI64 AVP

 The User-Equipment-Info-ModifiedEUI64 AVP (AVP Code 657) is of type
 OctetString.  There are a number of types of terminals that have
 identifiers other than IMEI, IEEE 802 MACs, or EUI-64.  These
 identifiers can be converted to modified EUI-64 format as described
 in [RFC4291] or by using some other methods referred to in the
 service-specific documentation.  The User-Equipment-Info-
 ModifiedEUI64 AVP contains such identifiers.

8.57. User-Equipment-Info-IMEI AVP

 The User-Equipment-Info-IMEI AVP (AVP Code 658) is of type
 OctetString.  The User-Equipment-Info-IMEI AVP contains the
 International Mobile Equipment Identifier in the IMEI format
 according to 3GPP TS 23.003 [TGPPIMEI].

Bertz, et al. Standards Track [Page 86] RFC 8506 Diameter Credit-Control Application March 2019

8.58. Subscription-Id-Extension AVP

 The Subscription-Id-Extension AVP (AVP Code 659) is used to identify
 the end user's subscription and is of type Grouped.  The
 Subscription-Id-Extension group AVP MUST include an AVP holding the
 subscription identifier.  The type of this included AVP indicates the
 type of the subscription identifier.  For each of the enumerated
 values of the Subscription-Id-Type AVP, there is a corresponding
 sub-AVP for use within the Subscription-Id-Extension group AVP.  If a
 new identifier type is required, a corresponding new sub-AVP SHOULD
 be defined for use within the Subscription-Id-Extension group AVP.
 If full backward compatibility with [RFC4006] is required, then the
 Subscription-Id AVP MUST be used to indicate identifier types
 enumerated in the Subscription-Id-Type AVP, whereas the Subscription-
 Id-Extension AVP MUST be used only for newly defined identifier
 types.  If full backward compatibility with [RFC4006] is not
 required, then the Subscription-Id-Extension AVP MAY be used to carry
 the existing identifier types.  In this case, the Subscription-Id-
 Extension AVP MAY be sent together with the Subscription-Id AVP.
 Exactly one sub-AVP MUST be included inside the Subscription-Id-
 Extension AVP.
 The Subscription-Id-Extension AVP is defined as follows (per
 grouped-avp-def as defined in [RFC6733]):
       Subscription-Id-Extension ::= < AVP Header: 659 >
                                 [ Subscription-Id-E164 ]
                                 [ Subscription-Id-IMSI ]
                                 [ Subscription-Id-SIP-URI ]
                                 [ Subscription-Id-NAI ]
                                 [ Subscription-Id-Private ]
                                 [ AVP ]

8.59. Subscription-Id-E164 AVP

 The Subscription-Id-E164 AVP (AVP Code 660) is of type UTF8String.
 The Subscription-Id-E164 AVP contains the international E.164 format
 (e.g., MSISDN), according to the ITU-T E.164 numbering plan defined
 in [E164] and [CE164].

8.60. Subscription-Id-IMSI AVP

 The Subscription-Id-IMSI AVP (AVP Code 661) is of type UTF8String.
 The Subscription-Id-IMSI AVP contains the IMSI format, according to
 the ITU-T E.212 identification plan as defined in [E212] and [CE212].

Bertz, et al. Standards Track [Page 87] RFC 8506 Diameter Credit-Control Application March 2019

8.61. Subscription-Id-SIP-URI AVP

 The Subscription-Id-SIP-URI AVP (AVP Code 662) is of type UTF8String.
 The Subscription-Id-SIP-URI AVP contains the identifier in the form
 of a SIP URI, as defined in [RFC3261].

8.62. Subscription-Id-NAI AVP

 The Subscription-Id-NAI AVP (AVP Code 663) is of type UTF8String.
 The Subscription-Id-NAI AVP contains the identifier in the form of a
 Network Access Identifier, as defined in [RFC7542].

8.63. Subscription-Id-Private AVP

 The Subscription-Id-Private AVP (AVP Code 664) is of type UTF8String.
 The Subscription-Id-Private AVP contains a credit-control server
 private identifier.

8.64. Redirect-Server-Extension AVP

 The Redirect-Server-Extension AVP (AVP Code 665) is of type Grouped
 and contains the address information of the redirect server (e.g.,
 HTTP redirect server, SIP Server) with which the end user is to be
 connected when the account cannot cover the service cost.  It MUST be
 present inside the QoS-Final-Unit-Indication AVP when the Final-Unit-
 Action AVP is set to REDIRECT.  If the type of the redirect server is
 one of the enumerated values of the Redirect-Address-Type AVP, then
 the credit-control server SHOULD send the information in the
 Redirect-Server AVP, in addition to or instead of the Redirect-
 Server-Extension AVP.  This is done in order to preserve backward
 compatibility with credit-control clients that support only
 [RFC4006].  Exactly one AVP MUST be included inside the Redirect-
 Server-Extension AVP.
 The Redirect-Server-Extension AVP is defined as follows (per
 grouped-avp-def as defined in [RFC6733]):
      Redirect-Server-Extension ::= < AVP Header: 665 >
                                [ Redirect-Address-IPAddress ]
                                [ Redirect-Address-URL ]
                                [ Redirect-Address-SIP-URI ]
                                [ AVP ]

Bertz, et al. Standards Track [Page 88] RFC 8506 Diameter Credit-Control Application March 2019

8.65. Redirect-Address-IPAddress AVP

 The Redirect-Address-IPAddress AVP (AVP Code 666) is of type Address
 and defines the IPv4 or IPv6 address of the redirect server with
 which the end user is to be connected when the account cannot cover
 the service cost.
 When encoded as an IPv6 address in 16 bytes, the IPv4-mapped IPv6
 format [RFC4291] MAY be used to indicate an IPv4 address.
 The interpretation of Redirect-Address-IPAddress by the Diameter
 Credit-Control client is a matter of local policy.

8.66. Redirect-Address-URL AVP

 The Redirect-Address-URL AVP (AVP Code 667) is of type UTF8String and
 defines the address of the redirect server with which the end user is
 to be connected when the account cannot cover the service cost.  The
 address type is in the form of a Uniform Resource Locator, as defined
 in [RFC3986].  Note that individual URL schemes may restrict the
 contents of the UTF8String.

8.67. Redirect-Address-SIP-URI AVP

 The Redirect-Address-SIP-URI AVP (AVP Code 668) is of type UTF8String
 and defines the address of the redirect server with which the end
 user is to be connected when the account cannot cover the service
 cost.  The address type is in the form of a SIP Uniform Resource
 Identifier, as defined in [RFC3261].

8.68. QoS-Final-Unit-Indication AVP

 The QoS-Final-Unit-Indication AVP (AVP Code 669) is of type Grouped
 and indicates that the Granted-Service-Unit AVP in the
 Credit-Control-Answer or in the AA-Answer contains the final units
 for the service.  After these units have expired, the Diameter
 Credit-Control client is responsible for executing the action
 indicated in the Final-Unit-Action AVP (see Section 5.6).
 If more than one unit type is received in the Credit-Control-Answer,
 the unit type that first expired SHOULD cause the credit-control
 client to execute the specified action.
 In the first interrogation, the QoS-Final-Unit-Indication AVP with
 Final-Unit-Action set to REDIRECT or RESTRICT_ACCESS can also be
 present with no Granted-Service-Unit AVP in the Credit-Control-Answer
 or in the AA-Answer.  This indicates to the Diameter Credit-Control
 client that the client is to execute the specified action

Bertz, et al. Standards Track [Page 89] RFC 8506 Diameter Credit-Control Application March 2019

 immediately.  If the home service provider policy is to terminate the
 service, naturally, the server SHOULD return the appropriate
 transient failure (see Section 9.1) in order to implement the
 policy-defined action.
 The Final-Unit-Action AVP defines the behavior of the Service Element
 when the user's account cannot cover the cost of the service and MUST
 always be present if the QoS-Final-Unit-Indication AVP is included in
 a command.
 If the Final-Unit-Action AVP is set to TERMINATE, the QoS-Final-Unit-
 Indication group AVP MUST NOT contain any other AVPs.
 If the Final-Unit-Action AVP is set to REDIRECT, then the Redirect-
 Server-Extension AVP MUST be present.  The Filter-Rule AVP or the
 Filter-Id AVP MAY be present in the Credit-Control-Answer message if
 the user is also allowed to access other services that are not
 accessible through the address given in the Redirect-Server-Extension
 AVP or if access to these services needs to be limited in some way
 (e.g., QoS).
 If the Final-Unit-Action AVP is set to RESTRICT_ACCESS, either the
 Filter-Rule AVP or the Filter-Id AVP SHOULD be present.
 The Filter-Rule AVP is defined in [RFC5777].  The Filter-Rule AVP can
 be used to define a specific combination of a condition and an
 action.  If used only with traffic conditions, it should define which
 traffic should be allowed when no more service units are granted.
 However, if QoS or treatment information exists in the AVP, these
 actions should be executed, e.g., limiting the allowed traffic with
 certain QoS information.  When multiple Filter-Rule AVPs exist,
 precedence should be determined as defined in [RFC5777].
 The Filter-Id AVP is defined in [RFC7155].  The Filter-Id AVP can be
 used to reference an IP filter list installed in the access device by
 means other than the Diameter Credit-Control application, e.g.,
 locally configured or configured by another entity.
 If the Final-Unit-Action AVP is (1) set to TERMINATE, (2) set to
 RESTRICT_ACCESS and the action required is to allow only traffic that
 could be classified using an IPFilterRule, or (3) set to REDIRECT
 using a type that is one of the types in the Redirect-Address-Type
 AVP, then the credit-control server SHOULD send the information in
 the Final-Unit-Indication AVP, in addition to or instead of the
 QoS-Final-Unit-Indication AVP.  This is done in order to preserve
 backward compatibility with credit-control clients that support only
 [RFC4006].

Bertz, et al. Standards Track [Page 90] RFC 8506 Diameter Credit-Control Application March 2019

 The QoS-Final-Unit-Indication AVP is defined as follows (per
 grouped-avp-def as defined in [RFC6733]):
       QoS-Final-Unit-Indication ::= < AVP Header: 669 >
                                 { Final-Unit-Action }
                                *[ Filter-Rule ]
                                *[ Filter-Id ]
                                 [ Redirect-Server-Extension ]
                                *[ AVP ]

9. Result-Code AVP Values

 This section defines new Result-Code AVP [RFC6733] values that must
 be supported by all Diameter implementations that conform to this
 specification.
 The Credit-Control-Answer message includes the Result-Code AVP, which
 may indicate that an error was present in the Credit-Control-Request
 message.  A rejected Credit-Control-Request message SHOULD cause the
 user's session to be terminated.

9.1. Transient Failures

 Errors that fall within the category of transient failures are used
 to inform the peer that the request could not be satisfied at the
 time it was received but that the request MAY be able to be satisfied
 in the future.
 DIAMETER_END_USER_SERVICE_DENIED         4010
 The credit-control server denies the service request due to service
 restrictions.  If the CCR contained used service units, they are
 deducted, if possible.
 DIAMETER_CREDIT_CONTROL_NOT_APPLICABLE   4011
 The credit-control server determines that the service can be granted
 to the end user but that no further credit-control is needed for the
 service (e.g., the service is free of charge).
 DIAMETER_CREDIT_LIMIT_REACHED            4012
 The credit-control server denies the service request because the
 end user's account could not cover the requested service.  If the CCR
 contained used service units, they are deducted, if possible.

Bertz, et al. Standards Track [Page 91] RFC 8506 Diameter Credit-Control Application March 2019

9.2. Permanent Failures

 Errors that fall within the category of permanent failures are used
 to inform the peer that the request failed and should not be
 attempted again.
 DIAMETER_USER_UNKNOWN    5030
 The specified end user is unknown in the credit-control server.
 DIAMETER_RATING_FAILED   5031
 This error code is used to inform the credit-control client that the
 credit-control server cannot rate the service request due to
 insufficient rating input, an incorrect AVP combination, or an AVP or
 AVP value that is not recognized or supported in the rating.  The
 Failed-AVP AVP MUST be included and contain (1) a copy of the entire
 AVP or AVPs that could not be processed successfully or (2) an
 example of the missing AVP, complete with the Vendor-Id if
 applicable.  The value field of the missing AVP should be of correct
 minimum length and contain zeros.

10. AVP Occurrence Table

 The table in Section 10.1 presents the AVPs defined in this document
 and specifies in which Diameter messages they MAY or MUST NOT be
 present.  Note that AVPs that can only be present within a Grouped
 AVP are not represented in the table.
 The table uses the following symbols:
       0     The AVP MUST NOT be present in the message.
       0+    Zero or more instances of the AVP MAY be present in the
             message.
       0-1   Zero or one instance of the AVP MAY be present in the
             message.  It is considered an error if there is more
             than one instance of the AVP.
       1     One instance of the AVP MUST be present in the message.

Bertz, et al. Standards Track [Page 92] RFC 8506 Diameter Credit-Control Application March 2019

10.1. Credit-Control AVP Table

 The table in this section is used to represent which credit-control
 application-specific AVPs defined in this document are to be present
 in the credit-control messages.
                                           +-----------+
                                           |  Command  |
                                           |   Code    |
                                           |-----+-----+
         Attribute Name                    | CCR | CCA |
         ----------------------------------|-----+-----+
         Acct-Multi-Session-Id             | 0-1 | 0-1 |
         Auth-Application-Id               | 1   | 1   |
         CC-Correlation-Id                 | 0-1 | 0   |
         CC-Session-Failover               | 0   | 0-1 |
         CC-Request-Number                 | 1   | 1   |
         CC-Request-Type                   | 1   | 1   |
         CC-Sub-Session-Id                 | 0-1 | 0-1 |
         Check-Balance-Result              | 0   | 0-1 |
         Cost-Information                  | 0   | 0-1 |
         Credit-Control-Failure-Handling   | 0   | 0-1 |
         Destination-Host                  | 0-1 | 0   |
         Destination-Realm                 | 1   | 0   |
         Direct-Debiting-Failure-Handling  | 0   | 0-1 |
         Event-Timestamp                   | 0-1 | 0-1 |
         Failed-AVP                        | 0   | 0+  |
         Final-Unit-Indication             | 0   | 0-1 |
         QoS-Final-Unit-Indication         | 0   | 0-1 |
         Granted-Service-Unit              | 0   | 0-1 |
         Multiple-Services-Credit-Control  | 0+  | 0+  |
         Multiple-Services-Indicator       | 0-1 | 0   |
         Origin-Host                       | 1   | 1   |
         Origin-Realm                      | 1   | 1   |
         Origin-State-Id                   | 0-1 | 0-1 |
         Proxy-Info                        | 0+  | 0+  |
         Redirect-Host                     | 0   | 0+  |
         Redirect-Host-Usage               | 0   | 0-1 |
         Redirect-Max-Cache-Time           | 0   | 0-1 |
         Requested-Action                  | 0-1 | 0   |
         Requested-Service-Unit            | 0-1 | 0   |
         Route-Record                      | 0+  | 0+  |
         Result-Code                       | 0   | 1   |
         Service-Context-Id                | 1   | 0   |
         Service-Identifier                | 0-1 | 0   |
         Service-Parameter-Info            | 0+  | 0   |
         Session-Id                        | 1   | 1   |
         Subscription-Id                   | 0+  | 0   |

Bertz, et al. Standards Track [Page 93] RFC 8506 Diameter Credit-Control Application March 2019

         Subscription-Id-Extension         | 0+  | 0   |
         Termination-Cause                 | 0-1 | 0   |
         User-Equipment-Info               | 0-1 | 0   |
         User-Equipment-Info-Extension     | 0-1 | 0   |
         Used-Service-Unit                 | 0+  | 0   |
         User-Name                         | 0-1 | 0-1 |
         Validity-Time                     | 0   | 0-1 |
         ----------------------------------|-----+-----+

10.2. Re-Auth-Request/Re-Auth-Answer AVP Table

 This section defines AVPs that are specific to the Diameter
 Credit-Control application and that MAY be included in the Diameter
 Re-Auth-Request/Re-Auth-Answer (RAR/RAA) message [RFC6733].
 The RAR/RAA command MAY include the following additional AVPs:
                                        +---------------+
                                        | Command Code  |
                                        |-------+-------+
          Attribute Name                |  RAR  |  RAA  |
          ------------------------------+-------+-------+
          CC-Sub-Session-Id             |  0-1  |  0-1  |
          G-S-U-Pool-Identifier         |  0-1  |  0-1  |
          Service-Identifier            |  0-1  |  0-1  |
          Rating-Group                  |  0-1  |  0-1  |
          ------------------------------+-------+-------+

11. RADIUS/Diameter Credit-Control Interworking Model

 This section defines the basic principles for the Diameter
 Credit-Control / RADIUS prepaid interworking model -- that is, a
 message translation between a RADIUS-based prepaid solution and a
 Diameter Credit-Control application.  A complete description of the
 protocol translations between RADIUS and the Diameter Credit-Control
 application is beyond the scope of this specification and SHOULD be
 addressed in another appropriate document.
 The Diameter Credit-Control architecture may have a Translation Agent
 capable of translation between RADIUS prepaid and Diameter
 Credit-Control protocols.  A AAA server (usually the home AAA server)
 may act as a Translation Agent and as a Diameter Credit-Control
 client for Service Elements that use credit-control mechanisms other
 than Diameter Credit-Control -- for instance, RADIUS prepaid.  In
 this case, the home AAA server contacts the Diameter Credit-Control
 server as part of the authorization process.  The interworking
 architecture is illustrated in Figure 9, and an interworking flow is
 illustrated in Figure 10.  In a roaming situation, the Service

Bertz, et al. Standards Track [Page 94] RFC 8506 Diameter Credit-Control Application March 2019

 Element (e.g., the NAS) may be located in the visited network, and a
 visited AAA server is usually contacted.  The visited AAA server then
 connects to the home AAA server.
                                RADIUS Prepaid
 +--------+       +---------+   Protocol +------------+  +--------+
 |  End   |<----->| Service |<---------->| Home AAA   |  |Business|
 |  User  |       | Element |            |  Server    |  |Support |
 +--------+   +-->|         |            |+----------+|->|System  |
              |   +---------+            ||CC Client ||  |        |
              |                          |+----------+|  |        |
 +--------+   |                          +------^-----+  +----^---+
 |  End   |<--+                Credit-Control   |             |
 |  User  |                          Protocol   |             |
 +--------+                             +-------V--------+    |
                                        |Credit-Control  |----+
                                        |   Server       |
                                        +----------------+
      Figure 9: Credit-Control Architecture with Service Element
       Containing Translation Agent, Translating RADIUS Prepaid
                  to Diameter Credit-Control Protocol
 When the AAA server acting as a Translation Agent receives an initial
 RADIUS Access-Request message from a Service Element (e.g., NAS
 access), it performs regular authentication and authorization.  If
 the RADIUS Access-Request message indicates that the Service Element
 is capable of credit-control and if the home AAA server finds that
 the subscriber is a prepaid subscriber, then a Diameter
 Credit-Control-Request SHOULD be sent toward the credit-control
 server to perform credit authorization and to establish a
 credit-control session.  After the Diameter Credit-Control server
 checks the end user's account balance, rates the service, and
 reserves credit from the end user's account, the reserved quota is
 returned to the home AAA server in the Diameter Credit-Control-
 Answer.  The home AAA server then sends the reserved quota to the
 Service Element in the RADIUS Access-Accept.
 At the expiry of the allocated quota, the Service Element sends a new
 RADIUS Access-Request containing the units used thus far to the home
 AAA server.  The home AAA server shall map a RADIUS Access-Request
 containing the reported units to the Diameter Credit-Control server
 in a Diameter Credit-Control-Request (UPDATE_REQUEST).  The Diameter
 Credit-Control server debits the used units from the end user's
 account and allocates a new quota that is returned to the home AAA
 server in the Diameter Credit-Control-Answer.  The quota is
 transferred to the Service Element in the RADIUS Access-Accept.  When
 the end user terminates the service or when the entire quota has been

Bertz, et al. Standards Track [Page 95] RFC 8506 Diameter Credit-Control Application March 2019

 used, the Service Element sends a RADIUS Access-Request.  To debit
 the used units from the end user's account and to stop the
 credit-control session, the home AAA server sends a Diameter
 Credit-Control-Request (TERMINATION_REQUEST) to the credit-control
 server.  The Diameter Credit-Control server acknowledges the session
 termination by sending a Diameter Credit-Control-Answer to the home
 AAA server.  The RADIUS Access-Accept is sent to the NAS.
 Figure 10 illustrates a Diameter Credit-Control / RADIUS prepaid
 interworking sequence.
 Service Element         Translation Agent
   (e.g., NAS)               (CC Client)             CC Server
       |     Access-Request     |                        |
       |----------------------->|                        |
       |                        |    CCR (Initial)       |
       |                        |----------------------->|
       |                        |    CCA (Granted-Units) |
       |                        |<-----------------------|
       |     Access-Accept      |                        |
       |     (Granted-Units)    |                        |
       |<-----------------------|                        |
       :                        :                        :
       |     Access-Request     |                        |
       |     (Used-Units)       |                        |
       |----------------------->|                        |
       |                        |    CCR (Update,        |
       |                        |         Used-Units)    |
       |                        |----------------------->|
       |                        |    CCA (Granted-Units) |
       |                        |<-----------------------|
       |     Access-Accept      |                        |
       |     (Granted-Units)    |                        |
       |<-----------------------|                        |
       :                        :                        :
       |     Access-Request     |                        |
       |----------------------->|                        |
       |                        |     CCR (Terminate,    |
       |                        |          Used-Units)   |
       |                        |----------------------->|
       |                        |     CCA                |
       |                        |<-----------------------|
       |     Access-Accept      |                        |
       |<-----------------------|                        |
       |                        |                        |
             Figure 10: Message Flow Example with Diameter
             Credit-Control / RADIUS Prepaid Interworking

Bertz, et al. Standards Track [Page 96] RFC 8506 Diameter Credit-Control Application March 2019

12. IANA Considerations

 This document uses several registries that were originally created in
 [RFC4006] or the values assigned to existing namespaces managed by
 IANA.  IANA has updated these registries to reference this document.
 The registries and their allocation policies are specified below.

12.1. Application Identifier

 This specification assigns the value 4, "Diameter Credit Control", to
 the "Application IDs" namespace defined in [RFC6733].  See
 Section 1.3 for more information.

12.2. Command Codes

 This specification uses the value 272 from the "Command Codes"
 namespace defined in [RFC6733] for the Credit-Control-Request (CCR)
 and Credit-Control-Answer (CCA) commands.

12.3. AVP Codes

 See Section 8 for the assignments in this specification.
 This document describes new AVP codes beyond those described in
 [RFC4006].  IANA has allocated codes for the AVPs listed in Table 7.

Bertz, et al. Standards Track [Page 97] RFC 8506 Diameter Credit-Control Application March 2019

      +-----------------------------------+------+--------------+
      | Attribute Name                    | Code | Defined in   |
      +-----------------------------------+------+--------------+
      | User-Equipment-Info-Extension     | 653  | Section 8.52 |
      | User-Equipment-Info-IMEISV        | 654  | Section 8.53 |
      | User-Equipment-Info-MAC           | 655  | Section 8.54 |
      | User-Equipment-Info-EUI64         | 656  | Section 8.55 |
      | User-Equipment-Info-ModifiedEUI64 | 657  | Section 8.56 |
      | User-Equipment-Info-IMEI          | 658  | Section 8.57 |
      | Subscription-Id-Extension         | 659  | Section 8.58 |
      | Subscription-Id-E164              | 660  | Section 8.59 |
      | Subscription-Id-IMSI              | 661  | Section 8.60 |
      | Subscription-Id-SIP-URI           | 662  | Section 8.61 |
      | Subscription-Id-NAI               | 663  | Section 8.62 |
      | Subscription-Id-Private           | 664  | Section 8.63 |
      | Redirect-Server-Extension         | 665  | Section 8.64 |
      | Redirect-Address-IPAddress        | 666  | Section 8.65 |
      | Redirect-Address-URL              | 667  | Section 8.66 |
      | Redirect-Address-SIP-URI          | 668  | Section 8.67 |
      | QoS-Final-Unit-Indication         | 669  | Section 8.68 |
      +-----------------------------------+------+--------------+
                  Table 7: Requested AVP Assignments

12.4. Result-Code AVP Values

 This specification assigns the values 4010, 4011, and 4012 in the
 "Result-Code AVP Values (code 268) - Transient Failures" namespace
 and values 5030 and 5031 in the "Result-Code AVP Values (code 268) -
 Permanent Failure" namespace, both of which were defined by
 [RFC6733].  See Section 9 for the assignments in this specification.

12.5. CC-Request-Type AVP

 As defined in Section 8.3, the CC-Request-Type AVP includes
 Enumerated type values 1-4.  IANA has created and is maintaining a
 namespace for this AVP.  The definition of new values is subject to
 the Specification Required policy [RFC8126] and conditions for
 enumerated values described in [RFC7423], Section 5.6.

12.6. CC-Session-Failover AVP

 As defined in Section 8.4, the CC-Session-Failover AVP includes
 Enumerated type values 0-1.  IANA has created and is maintaining a
 namespace for this AVP.  The definition of new values is subject to
 the Specification Required policy [RFC8126] and conditions for
 enumerated values described in [RFC7423], Section 5.6.

Bertz, et al. Standards Track [Page 98] RFC 8506 Diameter Credit-Control Application March 2019

12.7. CC-Unit-Type AVP

 As defined in Section 8.32, the CC-Unit-Type AVP includes Enumerated
 type values 0-5.  IANA has created and is maintaining a namespace for
 this AVP.  The definition of new values is subject to the
 Specification Required policy [RFC8126] and conditions for enumerated
 values described in [RFC7423], Section 5.6.

12.8. Check-Balance-Result AVP

 As defined in Section 8.6, the Check-Balance-Result AVP includes
 Enumerated type values 0-1.  IANA has created and is maintaining a
 namespace for this AVP.  The definition of new values is subject to
 the Specification Required policy [RFC8126] and conditions for
 enumerated values described in [RFC7423], Section 5.6.

12.9. Credit-Control AVP

 As defined in Section 8.13, the Credit-Control AVP includes
 Enumerated type values 0-1.  IANA has created and is maintaining a
 namespace for this AVP.  The definition of new values is subject to
 the Specification Required policy [RFC8126] and conditions for
 enumerated values described in [RFC7423], Section 5.6.

12.10. Credit-Control-Failure-Handling AVP

 As defined in Section 8.14, the Credit-Control-Failure-Handling AVP
 includes Enumerated type values 0-2.  IANA has created and is
 maintaining a namespace for this AVP.  The definition of new values
 is subject to the Specification Required policy [RFC8126] and
 conditions for enumerated values described in [RFC7423], Section 5.6.

12.11. Direct-Debiting-Failure-Handling AVP

 As defined in Section 8.15, the Direct-Debiting-Failure-Handling AVP
 includes Enumerated type values 0-1.  IANA has created and is
 maintaining a namespace for this AVP.  The definition of new values
 is subject to the Specification Required policy [RFC8126] and
 conditions for enumerated values described in [RFC7423], Section 5.6.

12.12. Final-Unit-Action AVP

 As defined in Section 8.35, the Final-Unit-Action AVP includes
 Enumerated type values 0-2.  IANA has created and is maintaining a
 namespace for this AVP.  The definition of new values is subject to
 the Specification Required policy [RFC8126] and conditions for
 enumerated values described in [RFC7423], Section 5.6.

Bertz, et al. Standards Track [Page 99] RFC 8506 Diameter Credit-Control Application March 2019

12.13. Multiple-Services-Indicator AVP

 As defined in Section 8.40, the Multiple-Services-Indicator AVP
 includes Enumerated type values 0-1.  IANA has created and is
 maintaining a namespace for this AVP.  The definition of new values
 is subject to the Specification Required policy [RFC8126] and
 conditions for enumerated values described in [RFC7423], Section 5.6.

12.14. Redirect-Address-Type AVP

 As defined in Section 8.38, the Redirect-Address-Type AVP includes
 Enumerated type values 0-3.  IANA has created and is maintaining a
 namespace for this AVP.  The definition of new values is subject to
 the Specification Required policy [RFC8126] and conditions for
 enumerated values described in [RFC7423], Section 5.6.

12.15. Requested-Action AVP

 As defined in Section 8.41, the Requested-Action AVP includes
 Enumerated type values 0-3.  IANA has created and is maintaining a
 namespace for this AVP.  The definition of new values is subject to
 the Specification Required policy [RFC8126] and conditions for
 enumerated values described in [RFC7423], Section 5.6.

12.16. Subscription-Id-Type AVP

 As defined in Section 8.47, the Subscription-Id-Type AVP includes
 Enumerated type values 0-4.  IANA has created and is maintaining a
 namespace for this AVP.  The definition of new values is subject to
 the Specification Required policy [RFC8126] and conditions for
 enumerated values described in [RFC7423], Section 5.6.

12.17. Tariff-Change-Usage AVP

 As defined in Section 8.27, the Tariff-Change-Usage AVP includes
 Enumerated type values 0-2.  IANA has created and is maintaining a
 namespace for this AVP.  The definition of new values is subject to
 the Specification Required policy [RFC8126] and conditions for
 enumerated values described in [RFC7423], Section 5.6.

12.18. User-Equipment-Info-Type AVP

 As defined in Section 8.50, the User-Equipment-Info-Type AVP includes
 Enumerated type values 0-3.  IANA has created and is maintaining a
 namespace for this AVP.  The definition of new values is subject to
 the Specification Required policy [RFC8126] and conditions for
 enumerated values described in [RFC7423], Section 5.6.

Bertz, et al. Standards Track [Page 100] RFC 8506 Diameter Credit-Control Application March 2019

13. Parameters Related to the Credit-Control Application

 Tx timer
    When real-time credit-control is required, the credit-control
    client contacts the credit-control server before and while the
    service is provided to an end user.  Due to the real-time nature
    of the application, communication delays SHOULD be minimized,
    e.g., to avoid an overly long service setup time experienced by
    the end user.  The Tx timer is introduced to control the waiting
    time in the client in the Pending state.  When the Tx timer
    elapses, the credit-control client takes action for the end user
    according to the value of the CCFH or the DDFH.  The recommended
    value is 10 seconds.
 Tcc timer
    The Tcc timer supervises an ongoing credit-control session in the
    credit-control server.  It is RECOMMENDED to use the Validity-Time
    as input to set the Tcc timer value.  In the case of transient
    failures in the network, the Diameter Credit-Control server might
    change to Idle state.  To avoid this, the Tcc timer MAY be set so
    that Tcc is equal to 2 x Validity-Time.
 Credit-Control-Failure-Handling and Direct-Debiting-Failure-Handling
    Client implementations may offer the possibility of locally
    configuring these AVPs.  In such a case, their values and behavior
    are defined in Sections 5.7 and 6.5, respectively.

14. Security Considerations

 Security considerations regarding the Diameter protocol itself are
 discussed in [RFC6733].  The use of this application of Diameter MUST
 take into consideration the security issues and requirements of the
 base protocol.
 This application includes a mechanism for application-layer replay
 protection by means of (1) the Session-Id AVP as specified in
 [RFC6733] and (2) the CC-Request-Number AVP, which is specified in
 this document.  The Diameter Credit-Control application is often used
 within one domain, and there may be a single hop between the peers.
 In these environments, the use of TLS/TCP, DTLS/SCTP (Datagram
 Transport Layer Security / Stream Control Transmission Protocol), or
 IPsec is sufficient.  The details of security considerations related
 to TLS/TCP, DTLS/SCTP, and IPsec are discussed in [RFC6733].

Bertz, et al. Standards Track [Page 101] RFC 8506 Diameter Credit-Control Application March 2019

 Because this application handles monetary transactions (directly or
 indirectly), it increases interest in various security attacks.
 Therefore, all parties communicating with each other MUST be
 authenticated, including, for instance, TLS client-side
 authentication.  In addition, authorization of the client SHOULD be
 emphasized, i.e., that the client is allowed to perform
 credit-control for a certain user.  The specific means of
 authorization are outside the scope of this specification but can be,
 for instance, manual configuration.
 Another kind of threat is malicious modification, injection, or
 deletion of AVPs or complete credit-control messages.  The
 credit-control messages contain sensitive billing-related information
 (such as subscription identifiers, granted units, used units, or cost
 information) whose malicious modification can have financial
 consequences.  Sometimes simply delaying the credit-control messages
 can cause disturbances in the credit-control client or server.
 Even without any modifications to the messages, an adversary that can
 eavesdrop on transactions can obtain privacy-sensitive information.
 Also, by monitoring the credit-control messages, one can collect
 information about the credit-control server's billing models and
 business relationships.
 When third-party relays or proxies are involved, hop-by-hop security
 does not necessarily provide sufficient protection for Diameter user
 sessions.  In some cases, it may be inappropriate to send Diameter
 messages, such as CCR messages and CCA messages, containing sensitive
 AVPs via untrusted Diameter proxy agents, as there are no assurances
 that third-party proxies will not modify the credit-control commands
 or AVP values.

14.1. Direct Connection with Redirects

 A Diameter Credit-Control agent cannot always know whether agents
 between it and the end user's Diameter Credit-Control server are
 reliable.  In this case, the Diameter Credit-Control agent doesn't
 have a routing entry in its Diameter routing table (defined in
 [RFC6733], Section 2.7) for the realm of the credit-control server in
 the end user's home realm.  The Diameter Credit-Control agent can
 have a default route configured to a local redirect agent, and it
 redirects the CCR message to the redirect agent.  The local redirect
 agent then returns a redirect notification (Result-Code 3006,
 DIAMETER_REDIRECT_INDICATION) to the credit-control agent, as well as
 information about the Diameter Credit-Control server(s) (Redirect-
 Host AVP) and information about how the routing entry resulting from
 the Redirect-Host is to be used (Redirect-Host-Usage AVP).  The
 Diameter Credit-Control agent then forwards the CCR message directly

Bertz, et al. Standards Track [Page 102] RFC 8506 Diameter Credit-Control Application March 2019

 to one of the hosts identified by the CCA message from the redirect
 agent.  If the value of the Redirect-Host-Usage AVP does not equal
 zero, all subsequent messages are sent to the host specified in the
 Redirect-Host AVP until the time specified by the Redirect-Max-Cache-
 Time AVP has expired.
 Even with redirects, there are some authorization issues.  There may
 be attacks toward nodes that have been properly authorized but that
 abuse their authorization or have been compromised.  These issues are
 discussed more widely in [RFC4072], Section 8.

14.2. Application-Level Redirects

 This document includes a redirection feature (Section 5.6.2) whereby
 the service provider can redirect (in an application-specific way)
 the end user to an alternate location when their credits have
 expired.  This technique is useful in that it allows the user to
 return to normal service quickly, but it also exposes additional
 risks and attack surface.  In particular, this redirection can
 potentially occur at an arbitrary point in a user's session,
 potentially without any additional contextual confirmation available
 to the user that the redirection is driven by the network.  This lack
 of confirmation matters because, in many application protocols, the
 communication peer is also capable of inducing redirection.  When the
 peer is an attacker, the redirection can be to an attacker-controlled
 site.  In particular, such sites may be "phishing" sites designed to
 appear similar to legitimate payment sites in an attempt to obtain
 users' payment information for fraudulent purposes.  When users
 become accustomed to such redirections, they may have difficulty
 distinguishing such attacks from legitimate redirections.
 Because of the potentially harmful consequences of arbitrary
 redirection by an attacker (such as to phishing sites), it is
 important for service providers to be aware of that risk and ensure
 that their users are aware of it as well.  Service providers should
 follow industry best practices for the specific application-layer
 protocol to reduce the chances that such attacks could be mistaken
 for legitimate redirections.  The details of such a practice are out
 of scope for this document.

Bertz, et al. Standards Track [Page 103] RFC 8506 Diameter Credit-Control Application March 2019

15. Privacy Considerations

 As the Diameter protocol, and especially the credit-control
 application, deal with subscribers and their actions, extra care
 should be taken regarding the privacy of the subscribers.  Per
 terminology used in [RFC6973], both the credit-control client and the
 credit-control server are intermediary entities, wherein the
 subscribers' privacy may be compromised even if no security issues
 exist, and only authorized entities have access to the privacy-
 sensitive information.

15.1. Privacy-Sensitive AVPs

 The privacy-sensitive AVPs listed in this section MUST NOT be sent
 across non-trusted networks or Diameter agents without end-to-end
 authentication and confidentiality protection, as described in
 [RFC6733], Section 13.3.
 The following AVPs contain privacy-sensitive information at different
 levels:
 1.   CC-Correlation-Id AVP: may contain privacy-sensitive
      information, as the service provider may encode personal
      information that helps it correlate different subscriptions and
      access technologies.
 2.   Check-Balance-Result AVP: contains information on the balance
      status of the subscriber.
 3.   Currency-Code AVP: contains information on the subscriber's
      locale.
 4.   Cost-Unit AVP: contains privacy-sensitive information for the
      Cost-Information AVP, in human-readable format.
 5.   Service-Identifier AVP: may contain privacy-sensitive
      information about the subscriber's Internet activity.
 6.   Rating-Group AVP: may contain privacy-sensitive information
      about the subscriber's Internet activity.
 7.   Restriction-Filter-Rule AVP: the information inside IPFilterRule
      may be used to infer services used by the subscriber.

Bertz, et al. Standards Track [Page 104] RFC 8506 Diameter Credit-Control Application March 2019

 8.   Redirect-Server-Address AVP: the service provider might embed
      personal information on the subscriber in the URL/URI (e.g., to
      create a personalized message).  However, the service provider
      may instead anonymize the subscriber's identity in the URL/URI
      and let the redirect server query the information directly.
      Such anonymized information must not allow personal information
      or the subscriber's identity to be easily guessed.  Furthermore,
      the service provider should treat the URL/URI schema itself as
      confidential and make sure it cannot be inferred (1) from
      observation of the traffic or (2) due to its trivial structure.
      A trivial structure could allow an adversary to query/modify
      personal information even without knowing the subscriber's
      identity.  Similar AVPs are Redirect-Address-URL and Redirect-
      Address-SIP-URI.
 9.   Service-Context-Id AVP: depending on how the service provider
      uses it, it may contain privacy-sensitive information about the
      service (e.g., in a 3GPP network Service-Context-Id AVP, it has
      a different value for packet switching, SMS, Multimedia Messages
      (MMSs), etc.).
 10.  Service-Parameter-Info AVP: depending on how the service
      provider uses it, it may contain privacy-sensitive information
      about the subscriber (e.g., location).
 11.  Subscription-Id-Data AVP: contains the identity of the
      subscriber.  Similar AVPs are Subscription-Id-E164,
      Subscription-Id-IMSI, Subscription-Id-SIP-URI, Subscription-Id-
      NAI, and Subscription-Id-Private.
 12.  User-Equipment-Info-Value AVP: contains the identity of the
      device of the subscriber.  Similar AVPs are User-Equipment-Info-
      IMEISV, User-Equipment-Info-MAC, User-Equipment-Info-EUI64,
      User-Equipment-Info-ModifiedEUI64, and User-Equipment-Info-IMEI.
 13.  QoS-Final-Unit-Indication AVP: Grouped AVP that may contain
      privacy-sensitive information in its sub-AVPs (e.g.,
      IPFilterRule, redirect address).
 Note that some AVPs that are used in this document are defined in
 [RFC6733] and may contain privacy-sensitive information.  These AVPs
 are not listed above.

Bertz, et al. Standards Track [Page 105] RFC 8506 Diameter Credit-Control Application March 2019

15.2. Data Minimization

 Due to the nature of the credit-control application, some personal
 data and identity information must be stored in both the
 credit-control client and the credit-control server.  However, this
 could be minimized by following these guidelines:
 1.  Data stored in the credit-control client does not need to persist
     across sessions.  All data could be deleted once the session ends
     and could be reconstructed once a new session is initialized.
     Note that while the credit-control server is usually owned by the
     service provider with which the subscriber already has some
     direct legal or business relationship (where the privacy level
     could be agreed upon), this is not always true for a
     credit-control client that may be owned by a third party.
 2.  Some information about the subscriber has to be stored in
     persistent storage in the credit-control server (e.g., identity,
     balance); however, per-transaction information does not have to
     be stored in persistent storage, and per-session information may
     be deleted from persistent storage once the session ends.
 3.  In some cases, per-transaction information has to be stored on
     the credit-control server, client, or both, for regulatory,
     auditability, or debugging reasons.  However, this could be
     minimized by following these guidelines:
     A.  Data retention does not need to exceed the required duration.
     B.  Transaction information could be aggregated in some cases
         (e.g., prefer information per session over information per
         rating-group; prefer hourly byte summary over per-transaction
         byte counts).
     C.  If not strictly needed, information that is more sensitive
         (e.g., location, equipment type) could be filtered out of
         such logs.  This information is often used to make rating
         decisions, and in this case, the rating decisions should be
         logged instead of the data used to make them.
     D.  Due to the reasons explained in the first guideline, the
         credit-control server, rather than the credit-control client,
         would be the preferred location for storing such transaction
         information.

Bertz, et al. Standards Track [Page 106] RFC 8506 Diameter Credit-Control Application March 2019

15.3. Diameter Agents

 Diameter agents, as described in [RFC6733], may be owned by
 third parties.  If end-to-end security is supported between the
 credit-control client and the credit-control server, the operator can
 use it to encrypt privacy-sensitive AVPs (as listed in Section 15.1)
 and prevent such information from leaking into the agent.
 In some cases, the Diameter agent needs access to privacy-sensitive
 AVPs, in order to make correct routing decisions or even to modify
 the content of these AVPs.  For example, a proxy agent may need to
 look at the Subscription-Id-IMSI AVP, in order to extract the mobile
 country and network codes of the user and use them to look up the
 destination to which the request should be routed (see Section 2.8.2
 in [RFC6733]).  In such a case, the credit-control client and
 credit-control server may use a mechanism that anonymizes the
 identity of the subscriber, as well as a mechanism to encrypt other
 AVPs not used by the agent.

16. References

16.1. Normative References

 [CE164]    International Telecommunication Union, "COMPLEMENT TO
            ITU-T RECOMMENDATION E.164 (11/2010): LIST OF ITU-T
            RECOMMENDATION E.164 ASSIGNED COUNTRY CODES",
            November 2011, <https://www.itu.int/dms_pub/itu-t/opb/sp/
            T-SP-E.164D-11-2011-PDF-E.pdf>.
 [CE212]    International Telecommunication Union, "COMPLEMENT TO
            RECOMMENDATION ITU-T E.212 (09/2016): LIST OF MOBILE
            COUNTRY OR GEOGRAPHICAL AREA CODES", February 2017,
            <https://www.itu.int/dms_pub/itu-t/opb/sp/
            T-SP-E.212A-2017-PDF-E.pdf>.
 [E164]     International Telecommunication Union, "The international
            public telecommunication numbering plan", ITU-T
            Recommendation E.164, November 2010,
            <https://www.itu.int/rec/T-REC-E.164/>.
 [E212]     International Telecommunication Union, "The international
            identification plan for public networks and
            subscriptions", ITU-T Recommendation E.212,
            September 2016, <https://www.itu.int/rec/T-REC-E.212/en>.

Bertz, et al. Standards Track [Page 107] RFC 8506 Diameter Credit-Control Application March 2019

 [EUI64]    IEEE, "Guidelines for Use of Extended Unique Identifier
            (EUI), Organizationally Unique Identifier (OUI), and
            Company ID (CID)", August 2017,
            <https://standards.ieee.org/content/dam/
            ieee-standards/standards/web/documents/tutorials/eui.pdf>.
 [ISO4217]  ISO, "Codes for the representation of currencies",
            ISO 4217:2015, 2015, <https://www.iso.org/
            iso-4217-currency-codes.html>.
 [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791,
            DOI 10.17487/RFC0791, September 1981,
            <https://www.rfc-editor.org/info/rfc791>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <https://www.rfc-editor.org/info/rfc2119>.
 [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
            A., Peterson, J., Sparks, R., Handley, M., and E.
            Schooler, "SIP: Session Initiation Protocol", RFC 3261,
            DOI 10.17487/RFC3261, June 2002,
            <https://www.rfc-editor.org/info/rfc3261>.
 [RFC3539]  Aboba, B. and J. Wood, "Authentication, Authorization and
            Accounting (AAA) Transport Profile", RFC 3539,
            DOI 10.17487/RFC3539, June 2003,
            <https://www.rfc-editor.org/info/rfc3539>.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66,
            RFC 3986, DOI 10.17487/RFC3986, January 2005,
            <https://www.rfc-editor.org/info/rfc3986>.
 [RFC4006]  Hakala, H., Mattila, L., Koskinen, J-P., Stura, M., and
            J. Loughney, "Diameter Credit-Control Application",
            RFC 4006, DOI 10.17487/RFC4006, August 2005,
            <https://www.rfc-editor.org/info/rfc4006>.
 [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
            Architecture", RFC 4291, DOI 10.17487/RFC4291,
            February 2006, <https://www.rfc-editor.org/info/rfc4291>.

Bertz, et al. Standards Track [Page 108] RFC 8506 Diameter Credit-Control Application March 2019

 [RFC5777]  Korhonen, J., Tschofenig, H., Arumaithurai, M., Jones, M.,
            Ed., and A. Lior, "Traffic Classification and Quality of
            Service (QoS) Attributes for Diameter", RFC 5777,
            DOI 10.17487/RFC5777, February 2010,
            <https://www.rfc-editor.org/info/rfc5777>.
 [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
            Address Text Representation", RFC 5952,
            DOI 10.17487/RFC5952, August 2010,
            <https://www.rfc-editor.org/info/rfc5952>.
 [RFC6733]  Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
            Ed., "Diameter Base Protocol", RFC 6733,
            DOI 10.17487/RFC6733, October 2012,
            <https://www.rfc-editor.org/info/rfc6733>.
 [RFC7155]  Zorn, G., Ed., "Diameter Network Access Server
            Application", RFC 7155, DOI 10.17487/RFC7155, April 2014,
            <https://www.rfc-editor.org/info/rfc7155>.
 [RFC7423]  Morand, L., Ed., Fajardo, V., and H. Tschofenig, "Diameter
            Applications Design Guidelines", BCP 193, RFC 7423,
            DOI 10.17487/RFC7423, November 2014,
            <https://www.rfc-editor.org/info/rfc7423>.
 [RFC7542]  DeKok, A., "The Network Access Identifier", RFC 7542,
            DOI 10.17487/RFC7542, May 2015,
            <https://www.rfc-editor.org/info/rfc7542>.
 [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
            Writing an IANA Considerations Section in RFCs", BCP 26,
            RFC 8126, DOI 10.17487/RFC8126, June 2017,
            <https://www.rfc-editor.org/info/rfc8126>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
            RFC 2119 Key Words", BCP 14, RFC 8174,
            DOI 10.17487/RFC8174, May 2017,
            <https://www.rfc-editor.org/info/rfc8174>.
 [TGPPIMEI] 3rd Generation Partnership Project, Technical
            Specification Group Core Network, "Numbering, addressing
            and identification (release 15)", 3GPP TS 23.003
            version 15.6.0, December 2018.

Bertz, et al. Standards Track [Page 109] RFC 8506 Diameter Credit-Control Application March 2019

16.2. Informative References

 [RFC2866]  Rigney, C., "RADIUS Accounting", RFC 2866,
            DOI 10.17487/RFC2866, June 2000,
            <https://www.rfc-editor.org/info/rfc2866>.
 [RFC3580]  Congdon, P., Aboba, B., Smith, A., Zorn, G., and J. Roese,
            "IEEE 802.1X Remote Authentication Dial In User Service
            (RADIUS) Usage Guidelines", RFC 3580,
            DOI 10.17487/RFC3580, September 2003,
            <https://www.rfc-editor.org/info/rfc3580>.
 [RFC3725]  Rosenberg, J., Peterson, J., Schulzrinne, H., and
            G. Camarillo, "Best Current Practices for Third Party Call
            Control (3pcc) in the Session Initiation Protocol (SIP)",
            BCP 85, RFC 3725, DOI 10.17487/RFC3725, April 2004,
            <https://www.rfc-editor.org/info/rfc3725>.
 [RFC4004]  Calhoun, P., Johansson, T., Perkins, C., Hiller, T., Ed.,
            and P. McCann, "Diameter Mobile IPv4 Application",
            RFC 4004, DOI 10.17487/RFC4004, August 2005,
            <https://www.rfc-editor.org/info/rfc4004>.
 [RFC4072]  Eronen, P., Ed., Hiller, T., and G. Zorn, "Diameter
            Extensible Authentication Protocol (EAP) Application",
            RFC 4072, DOI 10.17487/RFC4072, August 2005,
            <https://www.rfc-editor.org/info/rfc4072>.
 [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
            Morris, J., Hansen, M., and R. Smith, "Privacy
            Considerations for Internet Protocols", RFC 6973,
            DOI 10.17487/RFC6973, July 2013,
            <https://www.rfc-editor.org/info/rfc6973>.
 [TGPPCHARG]
            3rd Generation Partnership Project, Technical
            Specification Group Services and System Aspects, "Service
            aspects; Charging and Billing", 3GPP TS 22.115
            version 15.5.0, September 2018.

Bertz, et al. Standards Track [Page 110] RFC 8506 Diameter Credit-Control Application March 2019

Appendix A. Credit-Control Sequences

A.1. Flow I

 A credit-control flow for Network Access Services prepaid is shown in
 Figure 11.  The Diameter protocol application is implemented in the
 Network Access Server (NAS) per [RFC7155].  The focus of this flow is
 on credit authorization.
                         NAS
 End User          (CC Client)          AAA Server           CC Server
   |(1)User Logon      |(2)AA-Request (CC AVPs)                    |
   |------------------>|-------------------->|                     |
   |                   |                     |(3)CCR(Initial, CC AVPs)
   |                   |                     |-------------------->|
   |                   |                     |(4)CCA(Granted-Units)|
   |                   |                     |<--------------------|
   |                   |(5)AA-Answer(Granted-Units)                |
   |(6)Access granted  |<--------------------|                     |
   |<----------------->|                     |                     |
   |                   |                     |                     |
   :                   :                     :                     :
   |                   |(7)CCR(Update, Used-Units)                 |
   |                   |-------------------->|(8)CCR               |
   |                   |                     |   (Update, Used-Units)
   |                   |                     |-------------------->|
   |                   |                     |(9)CCA(Granted-Units)|
   |                   |(10)CCA(Granted-Units)<--------------------|
   |                   |<--------------------|                     |
   :                   :                     :                     :
   |         (Auth. lifetime expires)        |                     |
   |                   |(11)AAR (CC AVP)     |                     |
   |                   |-------------------->|                     |
   |                   |            (12)AAA  |                     |
   |                   |<--------------------|                     |
   :                   :                     :                     :
   :                   :                     :                     :

Bertz, et al. Standards Track [Page 111] RFC 8506 Diameter Credit-Control Application March 2019

   |(13)User logoff    |                     |                     |
   |------------------>|(14)CCR(Term., Used-Units)                 |
   |                   |-------------------->|(15)CCR              |
   |                   |                     |   (Term., Used-Units)
   |                   |                     |-------------------->|
   |                   |                     |             (16)CCA |
   |                   |            (17)CCA  |<--------------------|
   |                   |<--------------------|                     |
   |                   |(18)STR              |                     |
   |                   |-------------------->|                     |
   |                   |             (19)STA |                     |
   |                   |<--------------------|                     |
                           Figure 11: Flow I
 The user logs on to the network (1).  The Diameter NAS sends a
 Diameter AA-Request (AAR) to the home Diameter AAA server (2).  The
 credit-control client populates the AAR with the Credit-Control AVP
 set to CREDIT_AUTHORIZATION, and service-specific AVPs are included,
 as usual [RFC7155].  The home Diameter AAA server performs service-
 specific authentication and authorization, as usual.  The home
 Diameter AAA server determines that the user is a prepaid user and
 notices from the Credit-Control AVP that the NAS has credit-control
 capabilities.  It sends a Diameter Credit-Control-Request with
 CC-Request-Type set to INITIAL_REQUEST to the Diameter Credit-Control
 server to perform credit authorization (3) and to establish a
 credit-control session.  (The home Diameter AAA server may forward
 service-specific AVPs received from the NAS as input for the rating
 process.)  The Diameter Credit-Control server checks the end user's
 account balance, rates the service, and reserves credit from the
 end user's account.  The reserved quota is returned to the home
 Diameter AAA server in the Diameter Credit-Control-Answer (4).  The
 home Diameter AAA server sends the reserved quota to the NAS in the
 Diameter AA-Answer (AAA).  Upon receiving the AA-Answer, the NAS
 starts the credit-control session and starts monitoring the granted
 units (5).  The NAS grants access to the end user (6).  At the expiry
 of the allocated quota, the NAS sends a Diameter Credit-Control-
 Request with CC-Request-Type set to UPDATE_REQUEST to the home
 Diameter AAA server (7).  This message contains the units used thus
 far.  The home Diameter AAA server forwards the CCR to the Diameter
 Credit-Control server (8).  The Diameter Credit-Control server debits
 the used units from the end user's account and allocates a new quota
 that is returned to the home Diameter AAA server in the Diameter
 Credit-Control-Answer (9).  The message is forwarded to the NAS (10).
 During the ongoing credit-control session, the authorization lifetime
 expires, and the authorization/authentication client in the NAS
 performs service-specific re-authorization to the home Diameter AAA
 server, as usual.  The credit-control client populates the AAR with

Bertz, et al. Standards Track [Page 112] RFC 8506 Diameter Credit-Control Application March 2019

 the Credit-Control AVP set to RE_AUTHORIZATION, indicating that the
 credit-control server shall not be contacted, as the credit
 authorization is controlled by the burning rate of the granted units
 (11).  The home Diameter AAA server performs service-specific
 re-authorization as usual and returns the AA-Answer to the NAS (12).
 The end user logs off from the network (13).  To debit the used units
 from the end user's account and to stop the credit-control session,
 the NAS sends a Diameter Credit-Control-Request with CC-Request-Type
 set to TERMINATION_REQUEST to the home Diameter AAA server (14).  The
 home Diameter AAA server forwards the CCR to the credit-control
 server (15).  The Diameter Credit-Control server acknowledges the
 session termination by sending a Diameter Credit-Control-Answer to
 the home Diameter AAA server (16).  The home Diameter AAA server
 forwards the answer to the NAS (17).  The STR/STA takes place between
 the NAS and home Diameter AAA server, as usual (18), (19).

A.2. Flow II

 Figure 12 provides an example of Diameter Credit-Control for SIP
 sessions.  Although the flow focuses on illustrating the usage of
 credit-control messages, the SIP signaling is inaccurate, and the
 diagram is not by any means an attempt to define a service provider's
 SIP network.  However, for the sake of this example, some assumptions
 are made below.

Bertz, et al. Standards Track [Page 113] RFC 8506 Diameter Credit-Control Application March 2019

       SIP Proxy/Registrar   AAA
 A           (CC Client)     Server           B        CC Server
 | (i) REGISTER |              |              |              |
 |------------->|(ii)          |              |              |
 |              |------------->|              |              |
 |              |authentication &             |              |
 |              |authorization |              |              |
 |              |<-------------|              |              |
 |(iii) 200 OK  |                             |              |
 |<-------------|                             |              |
 :              :                             :              :
 |(1)  INVITE   |                                            :
 |------------->|
 |              |(2)  CCR (Initial, SIP-specific AVP)        |
 |              |------------------------------------------->|
 |              |(3)  CCA (Granted-Units)                    |
 |              |<-------------------------------------------|
 |              |(4)  INVITE                  |              |
 |              |---------------------------->|              |
 :              :                             :              :
 |              |(5)  CCR (Update, Used-Units)               |
 |              |------------------------------------------->|
 |              |(6)  CCA (Granted-Units)                    |
 |              |<-------------------------------------------|
 :              :                             :              :
 |(7)  BYE      |                             |              |
 |------------->|                             |              |
 |              |(8)  BYE                     |              |
 |              |---------------------------->|              |
 |              |(9)  CCR (Termination, Used-Units)          |
 |              |------------------------------------------->|
 |              |(10) CCA ()                                 |
 |              |<-------------------------------------------|
 |              |                             |              |
                          Figure 12: Flow II
 Typically, prepaid services based, for example, on time usage for SIP
 sessions require an entity in the service provider network to
 intercept all the requests within the SIP dialog in order to detect
 events, such as session establishment and session release, that are
 essential for performing credit-control operations with the
 credit-control server.  Therefore, in this example, it is assumed
 that the SIP Proxy adds a Record-Route header in the initial SIP
 INVITE to make sure that all the future requests in the created
 dialog traverse through it (for the definitions of "Record-Route" and
 "dialog", please refer to [RFC3261]).  Finally, the degree of

Bertz, et al. Standards Track [Page 114] RFC 8506 Diameter Credit-Control Application March 2019

 credit-control measuring of the media by the proxy depends on the
 business model design used in setting up the end system and proxies
 in the SIP network.
 The end user (SIP User Agent A) sends a REGISTER with credentials
 (i).  The SIP Proxy sends a request to the home AAA server to perform
 multimedia authentication and authorization by using, for instance, a
 Diameter multimedia application (ii).  The home AAA server checks
 that the credentials are correct and checks the user profile.
 Eventually, a 200 OK response (iii) is sent to the User Agent.  Note
 that the authentication and authorization are valid for the
 registration validity period duration (i.e., until re-registration is
 performed).  Several SIP sessions may be established without
 re-authorization.
 User Agent A sends an INVITE (1).  The SIP Proxy sends a Diameter
 Credit-Control-Request (INITIAL_REQUEST) to the Diameter
 Credit-Control server (2).  The Credit-Control-Request contains
 information obtained from the SIP signaling describing the requested
 service (e.g., calling party, called party, Session Description
 Protocol (SDP) attributes).  The Diameter Credit-Control server
 checks the end user's account balance, rates the service, and
 reserves credit from the end user's account.  The reserved quota is
 returned to the SIP Proxy in the Diameter Credit-Control-Answer (3).
 The SIP Proxy forwards the SIP INVITE to User Agent B (4).  B's phone
 rings, and B answers.  The media flows between them, and the SIP
 Proxy starts measuring the quota.  At the expiry of the allocated
 quota, the SIP Proxy sends a Diameter Credit-Control-Request
 (UPDATE_REQUEST) to the Diameter Credit-Control server (5).  This
 message contains the units used thus far.  The Diameter
 Credit-Control server debits the used units from the end user's
 account and allocates new credit that is returned to the SIP Proxy in
 the Diameter Credit-Control-Answer (6).  The end user terminates the
 service by sending a BYE message (7).  The SIP Proxy forwards the BYE
 message to User Agent B (8) and sends a Diameter Credit-Control-
 Request (TERMINATION_REQUEST) to the credit-control server (9).  The
 Diameter Credit-Control server acknowledges the session termination
 by sending a Diameter Credit-Control-Answer to the SIP Proxy (10).

Bertz, et al. Standards Track [Page 115] RFC 8506 Diameter Credit-Control Application March 2019

A.3. Flow III

 A credit-control flow for Multimedia Messaging Service is shown in
 Figure 13.  The sender is charged as soon as the messaging server
 successfully stores the message.
                          MMS Server
              A           (CC Client)           B           CC Server
              |(1) Send MMS    |                |                |
              |--------------->|                |                |
              |                |(2) CCR (Event, DIRECT_DEBITING, |
              |                |          MMS-specific AVP)      |
              |                |-------------------------------->|
              |                |(3) CCA (Granted-Units)          |
              |                |<--------------------------------|
              |(4) Send MMS Ack|                |                |
              |<---------------|                |                |
              |                |(5) Notify MMS  |                |
              |                |--------------->|                |
              :                :                :                :
              |                |(6) Retrieve MMS|                |
              |                |<---------------|                |
              |                |(7) Retrieve MMS|                |
              |                |    Ack         |                |
              |                |--------------->|                |
              |                |                |                |
                          Figure 13: Flow III
 This is an example of Diameter Credit-Control for direct debiting
 using the Multimedia Messaging Service environment.  Although the
 flow focuses on illustrating the usage of credit-control messages,
 the MMS signaling is inaccurate, and the diagram is not by any means
 an attempt to define a service provider's MMS configuration or
 billing model.
 End user A sends an MMS to the MMS server (1).  The MMS server stores
 the message and sends a Diameter Credit-Control-Request
 (EVENT_REQUEST with Requested-Action set to DIRECT_DEBITING) to the
 Diameter Credit-Control server (2).  The Credit-Control-Request
 contains information about the MMS message (e.g., size, recipient
 address, image coding type).  The Diameter Credit-Control server
 checks the end user's account balance, rates the service, and debits
 the service from the end user's account.  The granted quota is
 returned to the MMS server in the Diameter Credit-Control-Answer (3).

Bertz, et al. Standards Track [Page 116] RFC 8506 Diameter Credit-Control Application March 2019

 The MMS server acknowledges the successful reception of the MMS
 message (4).  The MMS server notifies the recipient about the new MMS
 (5), and end user B retrieves the message from the MMS message store
 (6), (7).
 Note that the transfer of the MMS message can take an extended period
 of time and can fail, in which case a recovery action is needed.  The
 MMS server should return the already-debited units to the user's
 account by using the REFUND action described in Section 6.4.

A.4. Flow IV

 Another credit-control flow for Multimedia Messaging Service is shown
 in Figure 14.  The recipient is charged at the time of message
 delivery.
                           MMS Server
       Content Server     (CC Client)           B           CC Server
              |(1) Send MMS    |                |                |
              |--------------->|                |                |
              |                |(2) CCR (Event, CHECK_BALANCE,   |
              |                |         MMS-specific AVP)       |
              |                |-------------------------------->|
              |                |(3) CCA (ENOUGH_CREDIT)          |
              |                |<--------------------------------|
              |(4) Send MMS Ack|                |                |
              |<---------------|                |                |
              |                |(5) Notify MMS  |                |
              |                |--------------->|                |
              :                :                :                :
              |                |(6) Retrieve MMS|                |
              |                |<---------------|                |
              |                |(7) CCR (Event, DIRECT_DEBITING, |
              |                |          MMS-specific AVP)      |
              |                |-------------------------------->|
              |                |(8) CCA (Granted-Units)          |
              |                |<--------------------------------|
              |                |(9) Retrieve MMS|                |
              |                |    Ack         |                |
              |                |--------------->|                |
              |                |                |                |
                          Figure 14: Flow IV

Bertz, et al. Standards Track [Page 117] RFC 8506 Diameter Credit-Control Application March 2019

 This is an example of Diameter Credit-Control for direct debiting
 using the Multimedia Messaging Service environment.  Although the
 flow focuses on illustrating the usage of credit-control messages,
 the MMS signaling is inaccurate, and the diagram is not by any means
 an attempt to define a service provider's MMS configuration or
 billing model.
 A content server sends an MMS to the MMS server (1), which stores the
 message.  The message recipient will be charged for the MMS message
 in this case.  As there can be a substantially long time between the
 receipt of the message at the MMS server and the actual retrieval of
 the message, the MMS server does not establish any credit-control
 sessions to the Diameter Credit-Control server; rather, it first
 performs only a balance check (without any credit reservations) by
 sending a Diameter Credit-Control-Request (EVENT_REQUEST with
 Requested-Action set to CHECK_BALANCE) to verify that end user B can
 cover the cost for the MMS (2).  The Diameter Credit-Control server
 checks the end user's account balance and returns the answer to the
 MMS server in the Diameter Credit-Control-Answer (3).  The MMS server
 acknowledges the successful reception of the MMS message (4).  The
 MMS server notifies the recipient of the new MMS (5), and after some
 time end user B retrieves the message from the MMS message store (6).
 The MMS server sends a Diameter Credit-Control-Request (EVENT_REQUEST
 with Requested-Action set to DIRECT_DEBITING) to the Diameter
 Credit-Control server (7).  The Credit-Control-Request contains
 information about the MMS message (e.g., size, recipient address,
 coding type).  The Diameter Credit-Control server checks the
 end user's account balance, rates the service, and debits the service
 from the end user's account.  The granted quota is returned to the
 MMS server in the Diameter Credit-Control-Answer (8).  The MMS is
 transferred to end user B (9).
 Note that the transfer of the MMS message can take an extended period
 of time and can fail, in which case a recovery action is needed.  The
 MMS server should return the already-debited units to the user's
 account by using the REFUND action described in Section 6.4.

Bertz, et al. Standards Track [Page 118] RFC 8506 Diameter Credit-Control Application March 2019

A.5. Flow V

 Figure 15 provides an example of an Advice of Charge (AoC) service
 for a SIP call.
                          SIP Controller
       User Agent A        (CC Client)      User Agent B     CC Server
            |(1)INVITE          |                |               |
            |  User Agent B(SDP)|                |               |
            |------------------>|                |               |
            |                   |(2)CCR (Event, PRICE_ENQUIRY,   |
            |                   |        SIP-specific AVPs)      |
            |                   |------------------------------->|
            |                   |(3)CCA (Cost-Information)       |
            |                   |<-------------------------------|
            |(4)MESSAGE(URL)    |                |               |
            |<------------------|                |               |
            |(5)HTTP GET        |                |               |
            |------------------>|                |               |
            |(6)HTTP POST       |                |               |
            |------------------>|(7)INVITE(SDP)  |               |
            |                   |--------------->|               |
            |                   |      (8)200 OK |               |
            |         (9)200 OK |<---------------|               |
            |<------------------|                |               |
                           Figure 15: Flow V
 This is an example of Diameter Credit-Control for SIP sessions.
 Although the flow focuses on illustrating the usage of credit-control
 messages, the SIP signaling is inaccurate, and the diagram is not by
 any means an attempt to define a service provider's SIP network.
 User Agent A can be either a postpaid or prepaid subscriber using the
 AoC service.  It is assumed that the SIP controller also has HTTP
 capabilities and delivers an interactive AoC web page with, for
 instance, the cost information, the details of the call derived from
 the SDP, and a button to accept/not accept the charges.  (There may
 be many other ways to deliver AoC information; however, this flow
 focuses on the use of the credit-control messages.)  The user has
 been authenticated and authorized prior to initiating the call and
 has been subscribed to the AoC service.
 User Agent A sends an INVITE with the SDP to User Agent B via the SIP
 controller (1).  The SIP controller determines that the user is
 subscribed to an AoC service and sends a Diameter Credit-Control-
 Request (EVENT_REQUEST with Requested-Action set to PRICE_ENQUIRY) to
 the Diameter Credit-Control server (2).  The Credit-Control-Request

Bertz, et al. Standards Track [Page 119] RFC 8506 Diameter Credit-Control Application March 2019

 contains SIP-specific AVPs derived from the SIP signaling, describing
 the requested service (e.g., calling party, called party, SDP
 attributes).  The Diameter Credit-Control server determines the cost
 of the service and returns the Credit-Control-Answer, including the
 Cost-Information AVP (3).  The SIP controller manufactures the AoC
 web page with information received in SIP signaling and with the cost
 information received from the credit-control server.  It then sends a
 SIP MESSAGE that contains a URL pointing to the AoC information web
 page (4).  Upon receipt of the SIP MESSAGE, User Agent A
 automatically invokes the web browser that retrieves the AoC
 information (5).  The user clicks on the appropriate button to accept
 the charges (6).  The SIP controller continues the session and sends
 the INVITE to User Agent B, which accepts the call (7), (8), (9).

A.6. Flow VI

 Figure 16 illustrates a credit-control flow for the REFUND case.  It
 is assumed that there is a trusted relationship and secure connection
 between the gaming server and the Diameter Credit-Control server.
 The end user may be a prepaid subscriber or a postpaid subscriber.
                        Gaming Server
 End User                (CC Client)              CC Server
    |  (1)Service Delivery   |                        |
    |<---------------------->|                        |
    :                        :                        :
    :                        :                        :
    |                        |(2)CCR(Event, REFUND,Requested-
    |                        |Service-Unit, Service-Parameter-Info)
    |                        |----------------------->|
    |                        |  (3)CCA(Cost-Information)
    |                        |<-----------------------|
    |        (4)Notification |                        |
    |<-----------------------|                        |
                          Figure 16: Flow VI
 While the end user is playing the game (1), they enter a new level
 that entitles them to a bonus.  The gaming server sends a Diameter
 Credit-Control-Request (EVENT_REQUEST with Requested-Action set to
 REFUND_ACCOUNT) to the Diameter Credit-Control server (2).  The
 Credit-Control-Request contains the Requested-Service-Unit AVP with
 the CC-Service-Specific-Units containing the number of points the
 user just won.  The Service-Parameter-Info AVP is also included in
 the request and specifies the service event to be rated (e.g., Tetris
 Bonus).  From information received, the Diameter Credit-Control
 server determines the amount to be credited, refunds the user's
 account, and returns the Credit-Control-Answer, including the

Bertz, et al. Standards Track [Page 120] RFC 8506 Diameter Credit-Control Application March 2019

 Cost-Information AVP (3).  The Cost-Information AVP indicates the
 credited amount.  At the first opportunity, the gaming server
 notifies the end user of the credited amount (4).

A.7. Flow VII

 Figure 17 provides an example of graceful service termination for a
 SIP call.  It is assumed that the call is set up so that the
 controller is in the call as a B2BUA (Back-to-Back User Agent)
 performing third-party call control (3PCC).  Note that the SIP
 signaling is inaccurate, as the focus of this flow is on graceful
 service termination and credit-control authorization.  Best practices
 for 3PCC are defined in [RFC3725].
                 SIP Controller    Top-Up
 User Agent A     (CC Client)      Server      User Agent B  CC Server
      |                |              |             |              |
      |                | (1)CCR(Update, Used-Units) |              |
      |                |------------------------------------------>|
      |                |               (2)CCA(Final-Unit, Redirect)|
      |                |<------------------------------------------|
      :                :              :             :              :
      :                :              :             :              :
      |                |  (3)CCR(Update, Used-Units)|              |
      |                |------------------------------------------>|
      |                | (3a)INVITE("hold")         |              |
      |                |--------------------------->|              |
      |                |              |       (4)CCA(Validity-Time)|
      |                |<------------------------------------------|
      |      (5)INVITE | (6)INVITE    |             |              |
      |<---------------|------------->|             |              |
      |             (7)RTP            |             |              |
      |...............................|             |              |
      |                |       (8)BYE |             |              |
      |                |<-------------|             |              |
      |                | (9)CCR(Update)             |              |
      |                |------------------------------------------>|
      |                |                     (10)CCA(Granted-Units)|
      |                |<------------------------------------------|
      |     (12)INVITE | (11)INVITE                 |              |
      |<---------------|--------------------------->|              |
                          Figure 17: Flow VII

Bertz, et al. Standards Track [Page 121] RFC 8506 Diameter Credit-Control Application March 2019

 The call is ongoing between User Agents A and B; User Agent A has a
 prepaid subscription.  At the expiry of the allocated quota, the SIP
 controller sends a Diameter Credit-Control-Request (UPDATE_REQUEST)
 to the Diameter Credit-Control server (1).  This message contains the
 units used thus far.  The Diameter Credit-Control server debits the
 used units from the end user's account and allocates the final quota
 returned to the SIP controller in the Diameter Credit-Control-Answer
 (2).  This message contains the Final-Unit-Indication AVP with
 Final-Unit-Action set to REDIRECT, the Redirect-Address-Type set to
 SIP URI, and the Redirect-Server-Address set to the top-up server
 name (e.g., sip:sip-topup-server@domain.com).  At the expiry of the
 final allocated quota, the SIP controller sends a Diameter
 Credit-Control-Request (UPDATE_REQUEST) to the Diameter
 Credit-Control server (3) and places the called party on "hold" by
 sending an INVITE with the appropriate connection address in the SDP
 (3a).  The Credit-Control-Request message contains the units used
 thus far.  The Diameter Credit-Control server debits the used units
 from the end user's account but does not make any credit
 reservations.  The Credit-Control-Answer message, which contains the
 Validity-Time to supervise the graceful service termination process,
 is returned to the SIP controller (4).  The SIP controller
 establishes a SIP session between the prepaid user and the top-up
 server (5), (6).  The top-up server plays an announcement and prompts
 the user to enter a credit card number and the amount of money to be
 used to replenish the account (7).  The top-up server validates the
 credit card number, replenishes the user's account (using some means
 outside the scope of this specification), and releases the SIP
 session (8).  The SIP controller can now assume that communication
 between the prepaid user and the top-up server took place.  It sends
 a spontaneous Credit-Control-Request (UPDATE_REQUEST) to the Diameter
 Credit-Control server to check whether the account has been
 replenished (9).  The Diameter Credit-Control server reserves credit
 from the end user's account and returns the reserved quota to the SIP
 controller in the Credit-Control-Answer (10).  At this point, the SIP
 controller reconnects the caller and the called party (11), (12).

Bertz, et al. Standards Track [Page 122] RFC 8506 Diameter Credit-Control Application March 2019

A.8. Flow VIII

 Figure 18 provides an example of graceful service termination
 initiated when the first interrogation takes place because the user's
 account is empty.  In this example, the credit-control server
 supports the server-initiated credit re-authorization.  The Diameter
 protocol application is implemented in the NAS per [RFC7155].
                        NAS                          Top-Up      CC
 End User         (CC Client)          AAA Server    Server    Server
    |(1)User Logon      |(2)AA-Request (CC AVPs)        |         |
    |------------------>|------------------->|          |         |
    |                   |                    |(3)CCR(Initial, CC AVPs)
    |                   |                    |------------------->|
    |                   |                    |(4)CCA(Final-Unit,  |
    |                   |                    |      Validity-Time)|
    |                   |                    |<-------------------|
    |                   |(5)AA-Answer(Final-Unit, Validity-Time)  |
    |(6)Limited access  |<-------------------|          |         |
    |      granted      |                    |          |         |
    |<----------------->|                    |          |         |
    |                   |                    |          |         |
    |   (7)TCP/HTTP     |        (8)TCP/HTTP            |         |
    |<----------------->|<----------------------------->|         |
    |                  (9)Replenish account             |         |
    |<------------------------------------------------->|         |
    |                   |                    |            (10)RAR |
    |                   |<-------------------|<-------------------|
    |                   |(11)RAA             |                    |
    |                   |------------------->|------------------->|
    |                   |(12)CCR(Update)     |                    |
    |                   |------------------->|(13)CCR(Update)     |
    |                   |                    |------------------->|
    |                   |                    |(14)CCA(Granted-Units)
    |                   |(15)CCA(Granted-Units)<------------------|
    |                   |<-------------------|                    |
                         Figure 18: Flow VIII
 The user logs on to the network (1).  The Diameter NAS sends a
 Diameter AA-Request (AAR) to the home Diameter AAA server (2).  The
 credit-control client populates the AAR with the Credit-Control AVP
 set to CREDIT_AUTHORIZATION, and service-specific AVPs are included,
 as usual [RFC7155].  The home Diameter AAA server performs service-
 specific authentication and authorization, as usual.  The home
 Diameter AAA server determines that the user has a prepaid
 subscription and notices from the Credit-Control AVP that the NAS has
 credit-control capabilities.  It sends a Diameter Credit-Control-

Bertz, et al. Standards Track [Page 123] RFC 8506 Diameter Credit-Control Application March 2019

 Request with CC-Request-Type set to INITIAL_REQUEST to the Diameter
 Credit-Control server to perform credit authorization (3) and to
 establish a credit-control session.  (The home Diameter AAA server
 may forward service-specific AVPs received from the NAS as input for
 the rating process.)  The Diameter Credit-Control server checks the
 end user's account balance, determines that the account cannot cover
 the cost of the service, and initiates graceful service termination.
 The Credit-Control-Answer is returned to the home Diameter AAA server
 (4).  This message contains the Final-Unit-Indication AVP and the
 Validity-Time AVP set to a reasonable amount of time, to give the
 user a chance to replenish their account (e.g., 10 minutes).  The
 Final-Unit-Indication AVP includes the Final-Unit-Action set to
 REDIRECT, the Redirect-Address-Type set to URL, and the Redirect-
 Server-Address set to the HTTP top-up server name.  The home Diameter
 AAA server sends the received Credit-Control AVPs to the NAS in the
 Diameter AA-Answer (5).  Upon successful AAA, the NAS starts the
 credit-control session and immediately starts graceful service
 termination, as instructed by the server.  The NAS grants limited
 access to the user (6).  The HTTP client software running in the
 user's device opens the transport connection redirected by the NAS to
 the top-up server (7), (8).  An appropriate web page is provided for
 the user where the user can enter the credit card number and the
 amount of money to be used to replenish the account, along with a
 notification message that they are granted unlimited access if the
 replenishment operation will be successfully executed within, for
 example, the next 10 minutes.  The top-up server validates the credit
 card number and replenishes the user's account (using some means
 outside the scope of this specification) (9).  After successful
 account top-up, the credit-control server sends a Re-Auth-Request
 message to the NAS (10).  The NAS acknowledges the request by
 returning the Re-Auth-Answer message (11) and initiates the credit
 re-authorization by sending a Credit-Control-Request (UPDATE_REQUEST)
 to the Diameter Credit-Control server (12), (13).
 The Diameter Credit-Control server reserves credit from the
 end user's account and returns the reserved quota to the NAS via the
 home Diameter AAA server in the Credit-Control-Answer (14), (15).
 The NAS removes the restriction applied by graceful service
 termination and starts monitoring the granted units.

A.9. Flow IX

 The Diameter Credit-Control application defines the Multiple-
 Services-Credit-Control AVP, which can be used to support independent
 credit-control of multiple services in a single credit-control
 (sub-)session for Service Elements that have such capabilities.  It
 is possible to request and allocate resources as a credit pool that
 is shared between services or rating-groups.

Bertz, et al. Standards Track [Page 124] RFC 8506 Diameter Credit-Control Application March 2019

 Figure 19 illustrates a usage scenario where the credit-control
 client and server support independent credit-control of multiple
 services, as defined in Section 5.1.2.  It is assumed that service-
 identifiers, rating-groups, and their associated parameters (e.g., IP
 5-tuples) are locally configured in the Service Element or
 provisioned by an entity other than the credit-control server.
 End User         Service Element                            CC Server
                    (CC Client)
   |(1)User logon      |                                            |
   |------------------>|(2)CCR(Initial, Service-Id access,          |
   |                   |        Access-specific AVPs,               |
   |                   |        Multiple-Services-Indicator)        |
   |                   |------------------------------------------->|
   |                   |(3)CCA(Multiple-Services-CC (               |
   |                   |        Granted-Units(Total-Octets),        |
   |                   |        Service-Id access,                  |
   |                   |        Validity-Time,                      |
   |                   |        G-S-U-Pool-Reference(Pool-Id 1,     |
   |                   |          Multiplier 10)))                  |
   |                   |<-------------------------------------------|
   :                   :                                            :
   |(4)Service-Request (Service 1)                                  |
   |------------------>|(5)CCR(Update, Multiple-Services-CC (       |
   |                   |        Requested-Units(), Service-Id 1,    |
   |                   |        Rating-Group 1))                    |
   |                   |------------------------------------------->|
   |                   |(6)CCA(Multiple-Services-CC (               |
   |                   |        Granted-Units(Time),                |
   |                   |        Rating-Group 1,                     |
   |                   |        G-S-U-Pool-Reference(Pool-Id 1,     |
   |                   |          Multiplier 1)))                   |
   |                   |<-------------------------------------------|
   :                   :                                            :
   |(7)Service-Request (Service 2)                                  |
   |------------------>|                                            |
   :                   :                                            :
   :                   :                                            :
   |(8)Service-Request (Services 3 & 4)                             |
   |------------------>|(9)CCR(Update, Multiple-Services-CC (       |
   |                   |        Requested-Units(), Service-Id 3,    |
   |                   |        Rating-Group 2),                    |
   |                   |        Multiple-Services-CC (              |
   |                   |        Requested-Units(), Service-Id 4,    |
   |                   |        Rating-Group 3))                    |
   |                   |------------------------------------------->|

Bertz, et al. Standards Track [Page 125] RFC 8506 Diameter Credit-Control Application March 2019

   |                   |(10)CCA(Multiple-Services-CC (              |
   |                   |         Granted-Units(Total-Octets),       |
   |                   |         Service-Id 3, Rating-Group 2,      |
   |                   |         Validity-Time,                     |
   |                   |         G-S-U-Pool-Reference(Pool-Id 2,    |
   |                   |           Multiplier 2)),                  |
   |                   |         Multiple-Services-CC (             |
   |                   |         Granted-Units(Total-Octets),       |
   |                   |         Service-Id 4, Rating-Group 3       |
   |                   |         Validity-Time,                     |
   |                   |         Final-Unit-Ind.(Terminate),        |
   |                   |         G-S-U-Pool-Reference(Pool-Id 2,    |
   |                   |           Multiplier 5)))                  |
   |                   |<-------------------------------------------|
   :                   :                                            :
   :                   :                                            :
   | +--------------+  |                                            |
   | |Validity time |  |(11)CCR(Update,                             |
   | |expires for   |  |         Multiple-Services-CC (             |
   | |Service-Id    |  |         Requested-Unit(),                  |
   | | access       |  |         Used-Units(In-Octets, Out-Octets), |
   | +--------------+  |         Service-Id access))                |
   |                   |------------------------------------------->|
   |                   |(12)CCA(Multiple-Services-CC (              |
   |                   |         Granted-Units(Total-Octets),       |
   |                   |         Service-Id access,                 |
   |                   |         Validity-Time,                     |
   |                   |         G-S-U-Pool-Reference(Pool-Id 1,    |
   |                   |           Multiplier 10)))                 |
   |                   |<-------------------------------------------|
   :                   :                                            :
   :                   :                                            :
   | +--------------+  |                                            |
   | |Total quota   |  |(13)CCR(Update,                             |
   | |elapses for   |  |         Multiple-Services-CC (             |
   | |Pool 2:       |  |          Requested-Unit(),                 |
   | |Service 4 not |  |          Used-Units(In-Octets, Out-Octets),|
   | |allowed,      |  |          Service-Id 3, Rating-Group 2),    |
   | |Service 3     |  |         Multiple-Services-CC (             |
   | |continues     |  |          Used-Units(In-Octets, Out-Octets),|
   | +--------------+  |          Service-Id 4, Rating-Group 3))    |
   |                   |------------------------------------------->|
   |                   |(14)CCA(Multiple-Services-CC (              |
   |                   |         Result-Code 4011,                  |
   |                   |         Service-Id 3))                     |
   |                   |<-------------------------------------------|
   :                   :                                            :
   :                   :                                            :

Bertz, et al. Standards Track [Page 126] RFC 8506 Diameter Credit-Control Application March 2019

   |(15)User logoff    |                                            |
   |------------------>|(16)CCR(Term.,                              |
   |                   |         Multiple-Services-CC (             |
   |                   |          Used-Units(In-Octets, Out-Octets),|
   |                   |          Service-Id access),               |
   |                   |         Multiple-Services-CC (             |
   |                   |          Used-Units(Time),                 |
   |                   |          Service-Id 1, Rating-Group 1),    |
   |                   |         Multiple-Services-CC (             |
   |                   |          Used-Units(Time),                 |
   |                   |          Service-Id 2, Rating-Group 1))    |
   |                   |------------------------------------------->|
   |                   |(17)CCA(Term.)                              |
   |                   |<-------------------------------------------|
     Figure 19: Flow IX: Example of Independent Credit-Control of
          Multiple Services in a Credit-Control (Sub-)Session
 The user logs on to the network (1).  The Service Element sends a
 Diameter Credit-Control-Request with CC-Request-Type set to
 INITIAL_REQUEST to the Diameter Credit-Control server to perform
 credit authorization for the bearer service (e.g., Internet access
 service) and to establish a credit-control session (2).  In this
 message, the credit-control client indicates support for independent
 credit-control of multiple services within the session by including
 the Multiple-Services-Indicator AVP.  The Diameter Credit-Control
 server checks the end user's account balance, with rating information
 received from the client (i.e., Service-Id and access-specific AVPs);
 rates the request; and reserves credit from the end user's account.
 Suppose that the server reserves $5 and determines that the cost is
 $1/MB.  It then returns to the Service Element a Credit-Control-
 Answer message that includes the Multiple-Services-Credit-Control AVP
 with a quota of 5 MB associated to the Service-Id (access), to a
 multiplier value of 10, and to Pool-Id 1 (3).
 The user uses service 1 (4).  The Service Element sends a Diameter
 Credit-Control-Request with CC-Request-Type set to UPDATE_REQUEST to
 the credit-control server to perform credit authorization for
 service 1 (5).  This message includes the Multiple-Services-Credit-
 Control AVP to request service units for service 1 that belong to
 Rating-Group 1.  The Diameter Credit-Control server determines that
 service 1 draws credit resources from the same account as the access
 service (i.e., pool 1).  It rates the request according to
 Service-Id/rating-group and updates the existing reservation by
 requesting more credit.  Suppose that the server reserves $5 more
 (now the reservation is $10) and determines that the cost is
 $0.1/minute.  The server authorizes the whole rating-group.  It then
 returns to the Service Element a Credit-Control-Answer message that

Bertz, et al. Standards Track [Page 127] RFC 8506 Diameter Credit-Control Application March 2019

 includes the Multiple-Services-Credit-Control AVP with a quota of
 50 minutes associated to Rating-Group 1, to a multiplier value of 1,
 and to Pool-Id 1 (6).  The client adjusts the total amount of
 resources for pool 1 according to the received quota, which gives S
 for pool 1 = 100.
 The user uses service 2, which belongs to the authorized rating-group
 (Rating-Group 1) (7).  Resources are then consumed from pool 1.
 The user now requests services 3 and 4 as well, which are not
 authorized (8).  The Service Element sends a Diameter Credit-Control-
 Request with CC-Request-Type set to UPDATE_REQUEST to the
 credit-control server in order to perform credit authorization for
 services 3 and 4 (9).  This message includes two instances of the
 Multiple-Services-Credit-Control AVP to request service units for
 service 3 that belong to Rating-Group 2 and service units for
 service 4 that belong to Rating-Group 3.  The Diameter Credit-Control
 server determines that services 3 and 4 draw credit resources from
 another account (i.e., pool 2).  It checks the end user's account
 balance and, according to Service-Id/rating-group information, rates
 the request.  It then reserves credit from pool 2.
 For example, the server reserves $5 and determines that service 3
 costs $0.2/MB and service 4 costs $0.5/MB.  The server authorizes
 only services 3 and 4.  It returns to the Service Element a
 Credit-Control-Answer message that includes two instances of the
 Multiple-Services-Credit-Control AVP (10).  One instance grants a
 quota of 12.5 MB associated to Service-Id 3 to a multiplier value
 of 2 and to Pool-Id 2.  The other instance grants a quota of 5 MB
 associated to Service-Id 4 to a multiplier value of 5 and to
 Pool-Id 2.
 The server also determines that pool 2 is exhausted and service 4 is
 not allowed to continue after these units will be consumed.
 Therefore, the Final-Unit-Indication AVP with action TERMINATE is
 associated to Service-Id 4.  The client calculates the total amount
 of resources that can be used for pool 2 according to the received
 quotas and multipliers, which gives S for pool 2 = 50.
 The Validity-Time for the access service expires.  The Service
 Element sends a Credit-Control-Request message to the server in order
 to perform credit re-authorization for the Service-Id (access) (11).
 This message carries one instance of the Multiple-Services-Credit-
 Control AVP that includes the units used by this service.  Suppose
 that the total amount of used units is 4 MB.  The client adjusts the
 total amount of resources for pool 1 accordingly, which gives S for
 pool 1 = 60.

Bertz, et al. Standards Track [Page 128] RFC 8506 Diameter Credit-Control Application March 2019

 The server deducts $4 from the user's account and updates the
 reservation by requesting more credit.  Suppose that the server
 reserves $5 more (now the reservation is $11) and already knows the
 cost of the Service-Id (access), which is $1/MB.  It then returns to
 the Service Element a Credit-Control-Answer message that includes the
 Multiple-Services-Credit-Control AVP with a quota of 5 MB associated
 to the Service-Id (access), to a multiplier value of 10, and to
 Pool-Id 1 (12).  The client adjusts the total amount of resources for
 pool 1 according to the received quota, which gives S for
 pool 1 = 110.
 Services 3 and 4 consume the total amount of pool 2's credit
 resources (i.e., C1*2 + C2*5 >= S).  The Service Element immediately
 starts the TERMINATE action for service 4 and sends a Credit-Control-
 Request message with CC-Request-Type set to UPDATE_REQUEST to the
 credit-control server in order to perform credit re-authorization for
 service 3 (13).  This message contains two instances of the Multiple-
 Services-Credit-Control AVP to report the units used by services 3
 and 4.  The server deducts the last $5 from the user's account
 (pool 2) and returns the answer with Result-Code 4011 in the
 Multiple-Services-Credit-Control AVP to indicate that service 3 can
 continue without credit-control (14).
 The end user logs off from the network (15).  To debit the used units
 from the end user's account and to stop the credit-control session,
 the Service Element sends a Diameter Credit-Control-Request with
 CC-Request-Type set to TERMINATION_REQUEST to the credit-control
 server (16).  This message contains the units used by each service in
 multiple instances of the Multiple-Services-Credit-Control AVP.  The
 used units are associated with the relevant Service-Identifier and
 rating-group.  The Diameter Credit-Control server debits the used
 units to the user's account (pool 1) and acknowledges the session
 termination by sending a Diameter Credit-Control-Answer to the
 Service Element (17).

Bertz, et al. Standards Track [Page 129] RFC 8506 Diameter Credit-Control Application March 2019

Acknowledgements

 The original authors of RFC 4006 are Harri Hakala, Leena Mattila,
 Juha-Pekka Koskinen, Marco Stura, and John Loughney.
 The authors would like to thank Bernard Aboba, Jari Arkko, Robert
 Ekblad, Pasi Eronen, Benny Gustafsson, Robert Karlsson, Avi Lior,
 Jussi Maki, Paco Marin, Jeff Meyer, Anne Narhi, John Prudhoe,
 Christopher Richards, Juha Vallinen, and Mark Watson for their
 comments and suggestions.

Authors' Addresses

 Lyle Bertz (editor)
 Sprint
 6220 Sprint Parkway
 Overland Park, KS  66251
 United States of America
 Email: lyleb551144@gmail.com
 David Dolson (editor)
 Sandvine
 408 Albert Street
 Waterloo, ON  N2L 3V3
 Canada
 Email: ddolson@acm.org
 Yuval Lifshitz (editor)
 Sandvine
 408 Albert Street
 Waterloo, ON  N2L 3V3
 Canada
 Email: yuvalif@yahoo.com

Bertz, et al. Standards Track [Page 130]

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