    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.

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    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
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       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.

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    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

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          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

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          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

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       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.

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       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.

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       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.

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    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.

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       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.

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    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 ]

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                                    [ 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 ]

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                                        [ 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.

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       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.

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       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.

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       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

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       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.

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       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.

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       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

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       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.

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       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).

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       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

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       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

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       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

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       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.

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       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

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    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

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       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]

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       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

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       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.

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       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.

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       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

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       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)

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       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.

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       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]

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                                                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

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       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.

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    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

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       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).

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    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

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       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.

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       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.

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       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

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       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.

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       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.

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       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.

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       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

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       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.

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       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.

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    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.

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       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.

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       +----------+-------------------------------+-------------+----------+
       | 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

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       +----------+-------------------------------+-------------+----------+
       | 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           |             |          |
       |          |                               |             |          |

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       | 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.        |          |

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       |          |                               |             |          |
       | 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       |          |
       |          |                               |             |          |

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    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               |            |          |
       |          |                                |            |          |

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       | 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                        |            |          |
       |          |                                |            |          |

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    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                  |       |
       |       |                        |                          |       |

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       | 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]

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                                                           +---------------+
                                                           |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 |

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       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.

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    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.

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       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.

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    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.

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       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.

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    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

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       (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.

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       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

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       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.

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       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 ]

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    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 ]

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    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.

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    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.

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       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.

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       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.

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       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.

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       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.

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       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 }

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    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.

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    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.

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       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.

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       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.

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    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].

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       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].

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       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.

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       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].

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    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].

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    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 ]

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    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

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       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].

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       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.

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    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.

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    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   |

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               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

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       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

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       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

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    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.

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            +-----------------------------------+------+--------------+
            | 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.

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    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.

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    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.

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    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].

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       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

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       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.

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    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.

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       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.

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    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.

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    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>.

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       [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.

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    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.

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    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  |                     |
         |                   |<--------------------|                     |
         :                   :                     :                     :
         :                   :                     :                     :

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         |(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

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       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.

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             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

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       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).

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    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).

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       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

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       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.

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    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

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       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

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       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

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       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).

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    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-

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       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.

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       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))                    |
         |                   |------------------------------------------->|

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         |                   |(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))                     |
         |                   |<-------------------------------------------|
         :                   :                                            :
         :                   :                                            :

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         |(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

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       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.

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       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).

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    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

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