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rfc:rfc6733

Internet Engineering Task Force (IETF) V. Fajardo, Ed. Request for Comments: 6733 Telcordia Technologies Obsoletes: 3588, 5719 J. Arkko Category: Standards Track Ericsson Research ISSN: 2070-1721 J. Loughney

                                                 Nokia Research Center
                                                          G. Zorn, Ed.
                                                           Network Zen
                                                          October 2012
                       Diameter Base Protocol

Abstract

 The Diameter base protocol is intended to provide an Authentication,
 Authorization, and Accounting (AAA) framework for applications such
 as network access or IP mobility in both local and roaming
 situations.  This document specifies the message format, transport,
 error reporting, accounting, and security services used by all
 Diameter applications.  The Diameter base protocol as defined in this
 document obsoletes RFC 3588 and RFC 5719, and it must be supported by
 all new Diameter 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 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6733.

Fajardo, et al. Standards Track [Page 1] RFC 6733 Diameter Base Protocol October 2012

Copyright Notice

 Copyright (c) 2012 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
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1. Introduction ....................................................7
    1.1. Diameter Protocol ..........................................9
         1.1.1. Description of the Document Set ....................10
         1.1.2. Conventions Used in This Document ..................11
         1.1.3. Changes from RFC 3588 ..............................11
    1.2. Terminology ...............................................12
    1.3. Approach to Extensibility .................................17
         1.3.1. Defining New AVP Values ............................18
         1.3.2. Creating New AVPs ..................................18
         1.3.3. Creating New Commands ..............................18
         1.3.4. Creating New Diameter Applications .................19
 2. Protocol Overview ..............................................20
    2.1. Transport .................................................22
         2.1.1. SCTP Guidelines ....................................23
    2.2. Securing Diameter Messages ................................24
    2.3. Diameter Application Compliance ...........................24
    2.4. Application Identifiers ...................................24
    2.5. Connections vs. Sessions ..................................25
    2.6. Peer Table ................................................26

Fajardo, et al. Standards Track [Page 2] RFC 6733 Diameter Base Protocol October 2012

    2.7. Routing Table .............................................27
    2.8. Role of Diameter Agents ...................................28
         2.8.1. Relay Agents .......................................30
         2.8.2. Proxy Agents .......................................31
         2.8.3. Redirect Agents ....................................31
         2.8.4. Translation Agents .................................32
    2.9. Diameter Path Authorization ...............................33
 3. Diameter Header ................................................34
    3.1. Command Codes .............................................37
    3.2. Command Code Format Specification .........................38
    3.3. Diameter Command Naming Conventions .......................40
 4. Diameter AVPs ..................................................40
    4.1. AVP Header ................................................41
         4.1.1. Optional Header Elements ...........................42
    4.2. Basic AVP Data Formats ....................................43
    4.3. Derived AVP Data Formats ..................................44
         4.3.1. Common Derived AVP Data Formats ....................44
    4.4. Grouped AVP Values ........................................51
         4.4.1. Example AVP with a Grouped Data Type ...............52
    4.5. Diameter Base Protocol AVPs ...............................55
 5. Diameter Peers .................................................58
    5.1. Peer Connections ..........................................58
    5.2. Diameter Peer Discovery ...................................59
    5.3. Capabilities Exchange .....................................60
         5.3.1. Capabilities-Exchange-Request ......................62
         5.3.2. Capabilities-Exchange-Answer .......................63
         5.3.3. Vendor-Id AVP ......................................63
         5.3.4. Firmware-Revision AVP ..............................64
         5.3.5. Host-IP-Address AVP ................................64
         5.3.6. Supported-Vendor-Id AVP ............................64
         5.3.7. Product-Name AVP ...................................64
    5.4. Disconnecting Peer Connections ............................64
         5.4.1. Disconnect-Peer-Request ............................65
         5.4.2. Disconnect-Peer-Answer .............................65
         5.4.3. Disconnect-Cause AVP ...............................66
    5.5. Transport Failure Detection ...............................66
         5.5.1. Device-Watchdog-Request ............................67
         5.5.2. Device-Watchdog-Answer .............................67
         5.5.3. Transport Failure Algorithm ........................67
         5.5.4. Failover and Failback Procedures ...................67
    5.6. Peer State Machine ........................................68
         5.6.1. Incoming Connections ...............................71
         5.6.2. Events .............................................71
         5.6.3. Actions ............................................72
         5.6.4. The Election Process ...............................74

Fajardo, et al. Standards Track [Page 3] RFC 6733 Diameter Base Protocol October 2012

 6. Diameter Message Processing ....................................74
    6.1. Diameter Request Routing Overview .........................74
         6.1.1. Originating a Request ..............................75
         6.1.2. Sending a Request ..................................76
         6.1.3. Receiving Requests .................................76
         6.1.4. Processing Local Requests ..........................76
         6.1.5. Request Forwarding .................................77
         6.1.6. Request Routing ....................................77
         6.1.7. Predictive Loop Avoidance ..........................77
         6.1.8. Redirecting Requests ...............................78
         6.1.9. Relaying and Proxying Requests .....................79
    6.2. Diameter Answer Processing ................................80
         6.2.1. Processing Received Answers ........................81
         6.2.2. Relaying and Proxying Answers ......................81
    6.3. Origin-Host AVP ...........................................81
    6.4. Origin-Realm AVP ..........................................82
    6.5. Destination-Host AVP ......................................82
    6.6. Destination-Realm AVP .....................................82
    6.7. Routing AVPs ..............................................83
         6.7.1. Route-Record AVP ...................................83
         6.7.2. Proxy-Info AVP .....................................83
         6.7.3. Proxy-Host AVP .....................................83
         6.7.4. Proxy-State AVP ....................................83
    6.8. Auth-Application-Id AVP ...................................83
    6.9. Acct-Application-Id AVP ...................................84
    6.10. Inband-Security-Id AVP ...................................84
    6.11. Vendor-Specific-Application-Id AVP .......................84
    6.12. Redirect-Host AVP ........................................85
    6.13. Redirect-Host-Usage AVP ..................................85
    6.14. Redirect-Max-Cache-Time AVP ..............................87
 7. Error Handling .................................................87
    7.1. Result-Code AVP ...........................................89
         7.1.1. Informational ......................................90
         7.1.2. Success ............................................90
         7.1.3. Protocol Errors ....................................90
         7.1.4. Transient Failures .................................92
         7.1.5. Permanent Failures .................................92
    7.2. Error Bit .................................................95
    7.3. Error-Message AVP .........................................96
    7.4. Error-Reporting-Host AVP ..................................96
    7.5. Failed-AVP AVP ............................................96
    7.6. Experimental-Result AVP ...................................97
    7.7. Experimental-Result-Code AVP ..............................97
 8. Diameter User Sessions .........................................98
    8.1. Authorization Session State Machine .......................99
    8.2. Accounting Session State Machine .........................104

Fajardo, et al. Standards Track [Page 4] RFC 6733 Diameter Base Protocol October 2012

    8.3. Server-Initiated Re-Auth .................................110
         8.3.1. Re-Auth-Request ...................................110
         8.3.2. Re-Auth-Answer ....................................110
    8.4. Session Termination ......................................111
         8.4.1. Session-Termination-Request .......................112
         8.4.2. Session-Termination-Answer ........................113
    8.5. Aborting a Session .......................................113
         8.5.1. Abort-Session-Request .............................114
         8.5.2. Abort-Session-Answer ..............................114
    8.6. Inferring Session Termination from Origin-State-Id .......115
    8.7. Auth-Request-Type AVP ....................................116
    8.8. Session-Id AVP ...........................................116
    8.9. Authorization-Lifetime AVP ...............................117
    8.10. Auth-Grace-Period AVP ...................................118
    8.11. Auth-Session-State AVP ..................................118
    8.12. Re-Auth-Request-Type AVP ................................118
    8.13. Session-Timeout AVP .....................................119
    8.14. User-Name AVP ...........................................119
    8.15. Termination-Cause AVP ...................................120
    8.16. Origin-State-Id AVP .....................................120
    8.17. Session-Binding AVP .....................................120
    8.18. Session-Server-Failover AVP .............................121
    8.19. Multi-Round-Time-Out AVP ................................122
    8.20. Class AVP ...............................................122
    8.21. Event-Timestamp AVP .....................................122
 9. Accounting ....................................................123
    9.1. Server Directed Model ....................................123
    9.2. Protocol Messages ........................................124
    9.3. Accounting Application Extension and Requirements ........124
    9.4. Fault Resilience .........................................125
    9.5. Accounting Records .......................................125
    9.6. Correlation of Accounting Records ........................126
    9.7. Accounting Command Codes .................................127
         9.7.1. Accounting-Request ................................127
         9.7.2. Accounting-Answer .................................128
    9.8. Accounting AVPs ..........................................129
         9.8.1. Accounting-Record-Type AVP ........................129
         9.8.2. Acct-Interim-Interval AVP .........................130
         9.8.3. Accounting-Record-Number AVP ......................131
         9.8.4. Acct-Session-Id AVP ...............................131
         9.8.5. Acct-Multi-Session-Id AVP .........................131
         9.8.6. Accounting-Sub-Session-Id AVP .....................131
         9.8.7. Accounting-Realtime-Required AVP ..................132
 10. AVP Occurrence Tables ........................................132
    10.1. Base Protocol Command AVP Table .........................133
    10.2. Accounting AVP Table ....................................134

Fajardo, et al. Standards Track [Page 5] RFC 6733 Diameter Base Protocol October 2012

 11. IANA Considerations ..........................................135
    11.1. AVP Header ..............................................135
         11.1.1. AVP Codes ........................................136
         11.1.2. AVP Flags ........................................136
    11.2. Diameter Header .........................................136
         11.2.1. Command Codes ....................................136
         11.2.2. Command Flags ....................................137
    11.3. AVP Values ..............................................137
         11.3.1. Experimental-Result-Code AVP .....................137
         11.3.2. Result-Code AVP Values ...........................137
         11.3.3. Accounting-Record-Type AVP Values ................137
         11.3.4. Termination-Cause AVP Values .....................137
         11.3.5. Redirect-Host-Usage AVP Values ...................137
         11.3.6. Session-Server-Failover AVP Values ...............137
         11.3.7. Session-Binding AVP Values .......................137
         11.3.8. Disconnect-Cause AVP Values ......................138
         11.3.9. Auth-Request-Type AVP Values .....................138
         11.3.10. Auth-Session-State AVP Values ...................138
         11.3.11. Re-Auth-Request-Type AVP Values .................138
         11.3.12. Accounting-Realtime-Required AVP Values .........138
         11.3.13. Inband-Security-Id AVP (code 299) ...............138
    11.4. _diameters Service Name and Port Number Registration ....138
    11.5. SCTP Payload Protocol Identifiers .......................139
    11.6. S-NAPTR Parameters ......................................139
 12. Diameter Protocol-Related Configurable Parameters ............139
 13. Security Considerations ......................................140
    13.1. TLS/TCP and DTLS/SCTP Usage .............................140
    13.2. Peer-to-Peer Considerations .............................141
    13.3. AVP Considerations ......................................141
 14. References ...................................................142
    14.1. Normative References ....................................142
    14.2. Informative References ..................................144
 Appendix A. Acknowledgements .....................................147
   A.1. This Document .............................................147
   A.2. RFC 3588 ..................................................148
 Appendix B. S-NAPTR Example ......................................148
 Appendix C. Duplicate Detection ..................................149
 Appendix D. Internationalized Domain Names .......................151

Fajardo, et al. Standards Track [Page 6] RFC 6733 Diameter Base Protocol October 2012

1. Introduction

 Authentication, Authorization, and Accounting (AAA) protocols such as
 TACACS [RFC1492] and RADIUS [RFC2865] were initially deployed to
 provide dial-up PPP [RFC1661] and terminal server access.  Over time,
 AAA support was needed on many new access technologies, the scale and
 complexity of AAA networks grew, and AAA was also used on new
 applications (such as voice over IP).  This led to new demands on AAA
 protocols.
 Network access requirements for AAA protocols are summarized in
 Aboba, et al. [RFC2989].  These include:
 Failover
    [RFC2865] does not define failover mechanisms and, as a result,
    failover behavior differs between implementations.  In order to
    provide well-defined failover behavior, Diameter supports
    application-layer acknowledgements and defines failover algorithms
    and the associated state machine.
 Transmission-level security
    RADIUS [RFC2865] defines an application-layer authentication and
    integrity scheme that is required only for use with response
    packets.  While [RFC2869] defines an additional authentication and
    integrity mechanism, use is only required during Extensible
    Authentication Protocol (EAP) [RFC3748] sessions.  While attribute
    hiding is supported, [RFC2865] does not provide support for per-
    packet confidentiality.  In accounting, [RFC2866] assumes that
    replay protection is provided by the backend billing server rather
    than within the protocol itself.
    While [RFC3162] defines the use of IPsec with RADIUS, support for
    IPsec is not required.  In order to provide universal support for
    transmission-level security, and enable both intra- and inter-
    domain AAA deployments, Diameter provides support for TLS/TCP and
    DTLS/SCTP.  Security is discussed in Section 13.
 Reliable transport
    RADIUS runs over UDP, and does not define retransmission behavior;
    as a result, reliability varies between implementations.  As
    described in [RFC2975], this is a major issue in accounting, where
    packet loss may translate directly into revenue loss.  In order to

Fajardo, et al. Standards Track [Page 7] RFC 6733 Diameter Base Protocol October 2012

    provide well-defined transport behavior, Diameter runs over
    reliable transport mechanisms (TCP, Stream Control Transmission
    Protocol (SCTP)) as defined in [RFC3539].
 Agent support
    RADIUS does not provide for explicit support for agents, including
    proxies, redirects, and relays.  Since the expected behavior is
    not defined, it varies between implementations.  Diameter defines
    agent behavior explicitly; this is described in Section 2.8.
 Server-initiated messages
    While server-initiated messages are defined in RADIUS [RFC5176],
    support is optional.  This makes it difficult to implement
    features such as unsolicited disconnect or re-authentication/
    re-authorization on demand across a heterogeneous deployment.  To
    address this issue, support for server-initiated messages is
    mandatory in Diameter.
 Transition support
    While Diameter does not share a common protocol data unit (PDU)
    with RADIUS, considerable effort has been expended in enabling
    backward compatibility with RADIUS so that the two protocols may
    be deployed in the same network.  Initially, it is expected that
    Diameter will be deployed within new network devices, as well as
    within gateways enabling communication between legacy RADIUS
    devices and Diameter agents.  This capability enables Diameter
    support to be added to legacy networks, by addition of a gateway
    or server speaking both RADIUS and Diameter.
 In addition to addressing the above requirements, Diameter also
 provides support for the following:
 Capability negotiation
    RADIUS does not support error messages, capability negotiation, or
    a mandatory/non-mandatory flag for attributes.  Since RADIUS
    clients and servers are not aware of each other's capabilities,
    they may not be able to successfully negotiate a mutually
    acceptable service or, in some cases, even be aware of what
    service has been implemented.  Diameter includes support for error
    handling (Section 7), capability negotiation (Section 5.3), and
    mandatory/non-mandatory Attribute-Value Pairs (AVPs)
    (Section 4.1).

Fajardo, et al. Standards Track [Page 8] RFC 6733 Diameter Base Protocol October 2012

 Peer discovery and configuration
    RADIUS implementations typically require that the name or address
    of servers or clients be manually configured, along with the
    corresponding shared secrets.  This results in a large
    administrative burden and creates the temptation to reuse the
    RADIUS shared secret, which can result in major security
    vulnerabilities if the Request Authenticator is not globally and
    temporally unique as required in [RFC2865].  Through DNS, Diameter
    enables dynamic discovery of peers (see Section 5.2).  Derivation
    of dynamic session keys is enabled via transmission-level
    security.
 Over time, the capabilities of Network Access Server (NAS) devices
 have increased substantially.  As a result, while Diameter is a
 considerably more sophisticated protocol than RADIUS, it remains
 feasible to implement it within embedded devices.

1.1. Diameter Protocol

 The Diameter base protocol provides the following facilities:
 o  Ability to exchange messages and deliver AVPs
 o  Capabilities negotiation
 o  Error notification
 o  Extensibility, required in [RFC2989], through addition of new
    applications, commands, and AVPs
 o  Basic services necessary for applications, such as the handling of
    user sessions or accounting
 All data delivered by the protocol is in the form of AVPs.  Some of
 these AVP values are used by the Diameter protocol itself, while
 others deliver data associated with particular applications that
 employ Diameter.  AVPs may be arbitrarily added to Diameter messages,
 the only restriction being that the Command Code Format (CCF)
 specification (Section 3.2) be satisfied.  AVPs are used by the base
 Diameter protocol to support the following required features:
 o  Transporting of user authentication information, for the purposes
    of enabling the Diameter server to authenticate the user
 o  Transporting of service-specific authorization information,
    between client and servers, allowing the peers to decide whether a
    user's access request should be granted

Fajardo, et al. Standards Track [Page 9] RFC 6733 Diameter Base Protocol October 2012

 o  Exchanging resource usage information, which may be used for
    accounting purposes, capacity planning, etc.
 o  Routing, relaying, proxying, and redirecting of Diameter messages
    through a server hierarchy
 The Diameter base protocol satisfies the minimum requirements for a
 AAA protocol, as specified by [RFC2989].  The base protocol may be
 used by itself for accounting purposes only, or it may be used with a
 Diameter application, such as Mobile IPv4 [RFC4004], or network
 access [RFC4005].  It is also possible for the base protocol to be
 extended for use in new applications, via the addition of new
 commands or AVPs.  The initial focus of Diameter was network access
 and accounting applications.  A truly generic AAA protocol used by
 many applications might provide functionality not provided by
 Diameter.  Therefore, it is imperative that the designers of new
 applications understand their requirements before using Diameter.
 See Section 1.3.4 for more information on Diameter applications.
 Any node can initiate a request.  In that sense, Diameter is a peer-
 to-peer protocol.  In this document, a Diameter client is a device at
 the edge of the network that performs access control, such as a
 Network Access Server (NAS) or a Foreign Agent (FA).  A Diameter
 client generates Diameter messages to request authentication,
 authorization, and accounting services for the user.  A Diameter
 agent is a node that does not provide local user authentication or
 authorization services; agents include proxies, redirects, and relay
 agents.  A Diameter server performs authentication and/or
 authorization of the user.  A Diameter node may act as an agent for
 certain requests while acting as a server for others.
 The Diameter protocol also supports server-initiated messages, such
 as a request to abort service to a particular user.

1.1.1. Description of the Document Set

 The Diameter specification consists of an updated version of the base
 protocol specification (this document) and the Transport Profile
 [RFC3539].  This document obsoletes both RFC 3588 and RFC 5719.  A
 summary of the base protocol updates included in this document can be
 found in Section 1.1.3.
 This document defines the base protocol specification for AAA, which
 includes support for accounting.  There are also a myriad of
 applications documents describing applications that use this base
 specification for Authentication, Authorization, and Accounting.
 These application documents specify how to use the Diameter protocol
 within the context of their application.

Fajardo, et al. Standards Track [Page 10] RFC 6733 Diameter Base Protocol October 2012

 The Transport Profile document [RFC3539] discusses transport layer
 issues that arise with AAA protocols and recommendations on how to
 overcome these issues.  This document also defines the Diameter
 failover algorithm and state machine.
 "Clarifications on the Routing of Diameter Request Based on the
 Username and the Realm" [RFC5729] defines specific behavior on how to
 route requests based on the content of the User-Name AVP (Attribute
 Value Pair).

1.1.2. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

1.1.3. Changes from RFC 3588

 This document obsoletes RFC 3588 but is fully backward compatible
 with that document.  The changes introduced in this document focus on
 fixing issues that have surfaced during the implementation of
 Diameter (RFC 3588).  An overview of some the major changes are given
 below.
 o  Deprecated the use of the Inband-Security AVP for negotiating
    Transport Layer Security (TLS) [RFC5246].  It has been generally
    considered that bootstrapping of TLS via Inband-Security AVP
    creates certain security risks because it does not completely
    protect the information carried in the CER/CEA (Capabilities-
    Exchange-Request/Capabilities-Exchange-Answer).  This version of
    Diameter adopts the common approach of defining a well-known
    secured port that peers should use when communicating via TLS/TCP
    and DTLS/SCTP.  This new approach augments the existing in-band
    security negotiation, but it does not completely replace it.  The
    old method is kept for backward compatibility reasons.
 o  Deprecated the exchange of CER/CEA messages in the open state.
    This feature was implied in the peer state machine table of RFC
    3588, but it was not clearly defined anywhere else in that
    document.  As work on this document progressed, it became clear
    that the multiplicity of meaning and use of Application-Id AVPs in
    the CER/CEA messages (and the messages themselves) is seen as an
    abuse of the Diameter extensibility rules and thus required
    simplification.  Capabilities exchange in the open state has been
    re-introduced in a separate specification [RFC6737], which clearly
    defines new commands for this feature.

Fajardo, et al. Standards Track [Page 11] RFC 6733 Diameter Base Protocol October 2012

 o  Simplified security requirements.  The use of a secured transport
    for exchanging Diameter messages remains mandatory.  However, TLS/
    TCP and DTLS/SCTP have become the primary methods of securing
    Diameter with IPsec as a secondary alternative.  See Section 13
    for details.  The support for the End-to-End security framework
    (E2E-Sequence AVP and 'P'-bit in the AVP header) has also been
    deprecated.
 o  Changed Diameter extensibility.  This includes fixes to the
    Diameter extensibility description (Section 1.3 and others) to
    better aid Diameter application designers; in addition, the new
    specification relaxes the policy with respect to the allocation of
    Command Codes for vendor-specific uses.
 o  Clarified Application Id usage.  Clarify the proper use of
    Application Id information, which can be found in multiple places
    within a Diameter message.  This includes correlating Application
    Ids found in the message headers and AVPs.  These changes also
    clearly specify the proper Application Id value to use for
    specific base protocol messages (ASR/ASA, STR/STA) as well as
    clarify the content and use of Vendor-Specific-Application-Id.
 o  Clarified routing fixes.  This document more clearly specifies
    what information (AVPs and Application Ids) can be used for making
    general routing decisions.  A rule for the prioritization of
    redirect routing criteria when multiple route entries are found
    via redirects has also been added (see Section 6.13).
 o  Simplified Diameter peer discovery.  The Diameter discovery
    process now supports only widely used discovery schemes; the rest
    have been deprecated (see Section 5.2 for details).
 There are many other miscellaneous fixes that have been introduced in
 this document that may not be considered significant, but they have
 value nonetheless.  Examples are removal of obsolete types, fixes to
 the state machine, clarification of the election process, message
 validation, fixes to Failed-AVP and Result-Code AVP values, etc.  All
 of the errata filed against RFC 3588 prior to the publication of this
 document have been addressed.  A comprehensive list of changes is not
 shown here for practical reasons.

1.2. Terminology

 AAA
    Authentication, Authorization, and Accounting.

Fajardo, et al. Standards Track [Page 12] RFC 6733 Diameter Base Protocol October 2012

 ABNF
    Augmented Backus-Naur Form [RFC5234].  A metalanguage with its own
    formal syntax and rules.  It is based on the Backus-Naur Form and
    is used to define message exchanges in a bi-directional
    communications protocol.
 Accounting
    The act of collecting information on resource usage for the
    purpose of capacity planning, auditing, billing, or cost
    allocation.
 Accounting Record
    An accounting record represents a summary of the resource
    consumption of a user over the entire session.  Accounting servers
    creating the accounting record may do so by processing interim
    accounting events or accounting events from several devices
    serving the same user.
 Authentication
    The act of verifying the identity of an entity (subject).
 Authorization
    The act of determining whether a requesting entity (subject) will
    be allowed access to a resource (object).
 Attribute-Value Pair (AVP)
    The Diameter protocol consists of a header followed by one or more
    Attribute-Value-Pairs (AVPs).  An AVP includes a header and is
    used to encapsulate protocol-specific data (e.g., routing
    information) as well as authentication, authorization, or
    accounting information.
 Command Code Format (CCF)
    A modified form of ABNF used to define Diameter commands (see
    Section 3.2).
 Diameter Agent
    A Diameter Agent is a Diameter node that provides relay, proxy,
    redirect, or translation services.

Fajardo, et al. Standards Track [Page 13] RFC 6733 Diameter Base Protocol October 2012

 Diameter Client
    A Diameter client is a Diameter node that supports Diameter client
    applications as well as the base protocol.  Diameter clients are
    often implemented in devices situated at the edge of a network and
    provide access control services for that network.  Typical
    examples of Diameter clients include the Network Access Server
    (NAS) and the Mobile IP Foreign Agent (FA).
 Diameter Node
    A Diameter node is a host process that implements the Diameter
    protocol and acts as either a client, an agent, or a server.
 Diameter Peer
    Two Diameter nodes sharing a direct TCP or SCTP transport
    connection are called Diameter peers.
 Diameter Server
    A Diameter server is a Diameter node that handles authentication,
    authorization, and accounting requests for a particular realm.  By
    its very nature, a Diameter server must support Diameter server
    applications in addition to the base protocol.
 Downstream
    Downstream is used to identify the direction of a particular
    Diameter message from the home server towards the Diameter client.
 Home Realm
    A Home Realm is the administrative domain with which the user
    maintains an account relationship.
 Home Server
    A Diameter server that serves the Home Realm.
 Interim Accounting
    An interim accounting message provides a snapshot of usage during
    a user's session.  Typically, it is implemented in order to
    provide for partial accounting of a user's session in case a
    device reboot or other network problem prevents the delivery of a
    session summary message or session record.

Fajardo, et al. Standards Track [Page 14] RFC 6733 Diameter Base Protocol October 2012

 Local Realm
    A local realm is the administrative domain providing services to a
    user.  An administrative domain may act as a local realm for
    certain users while being a home realm for others.
 Multi-session
    A multi-session represents a logical linking of several sessions.
    Multi-sessions are tracked by using the Acct-Multi-Session-Id.  An
    example of a multi-session would be a Multi-link PPP bundle.  Each
    leg of the bundle would be a session while the entire bundle would
    be a multi-session.
 Network Access Identifier
    The Network Access Identifier, or NAI [RFC4282], is used in the
    Diameter protocol to extract a user's identity and realm.  The
    identity is used to identify the user during authentication and/or
    authorization while the realm is used for message routing
    purposes.
 Proxy Agent or Proxy
    In addition to forwarding requests and responses, proxies make
    policy decisions relating to resource usage and provisioning.
    Typically, this is accomplished by tracking the state of NAS
    devices.  While proxies usually do not respond to client requests
    prior to receiving a response from the server, they may originate
    Reject messages in cases where policies are violated.  As a
    result, proxies need to understand the semantics of the messages
    passing through them, and they may not support all Diameter
    applications.
 Realm
    The string in the NAI that immediately follows the '@' character.
    NAI realm names are required to be unique and are piggybacked on
    the administration of the DNS namespace.  Diameter makes use of
    the realm, also loosely referred to as domain, to determine
    whether messages can be satisfied locally or whether they must be
    routed or redirected.  In RADIUS, realm names are not necessarily
    piggybacked on the DNS namespace but may be independent of it.

Fajardo, et al. Standards Track [Page 15] RFC 6733 Diameter Base Protocol October 2012

 Real-Time Accounting
    Real-time accounting involves the processing of information on
    resource usage within a defined time window.  Typically, time
    constraints are imposed in order to limit financial risk.  The
    Diameter Credit-Control Application [RFC4006] is an example of an
    application that defines real-time accounting functionality.
 Relay Agent or Relay
    Relays forward requests and responses based on routing-related
    AVPs and routing table entries.  Since relays do not make policy
    decisions, they do not examine or alter non-routing AVPs.  As a
    result, relays never originate messages, do not need to understand
    the semantics of messages or non-routing AVPs, and are capable of
    handling any Diameter application or message type.  Since relays
    make decisions based on information in routing AVPs and realm
    forwarding tables, they do not keep state on NAS resource usage or
    sessions in progress.
 Redirect Agent
    Rather than forwarding requests and responses between clients and
    servers, redirect agents refer clients to servers and allow them
    to communicate directly.  Since redirect agents do not sit in the
    forwarding path, they do not alter any AVPs transiting between
    client and server.  Redirect agents do not originate messages and
    are capable of handling any message type, although they may be
    configured only to redirect messages of certain types, while
    acting as relay or proxy agents for other types.  As with relay
    agents, redirect agents do not keep state with respect to sessions
    or NAS resources.
 Session
    A session is a related progression of events devoted to a
    particular activity.  Diameter application documents provide
    guidelines as to when a session begins and ends.  All Diameter
    packets with the same Session-Id are considered to be part of the
    same session.
 Stateful Agent
    A stateful agent is one that maintains session state information,
    by keeping track of all authorized active sessions.  Each
    authorized session is bound to a particular service, and its state
    is considered active either until it is notified otherwise or
    until expiration.

Fajardo, et al. Standards Track [Page 16] RFC 6733 Diameter Base Protocol October 2012

 Sub-session
    A sub-session represents a distinct service (e.g., QoS or data
    characteristics) provided to a given session.  These services may
    happen concurrently (e.g., simultaneous voice and data transfer
    during the same session) or serially.  These changes in sessions
    are tracked with the Accounting-Sub-Session-Id.
 Transaction State
    The Diameter protocol requires that agents maintain transaction
    state, which is used for failover purposes.  Transaction state
    implies that upon forwarding a request, the Hop-by-Hop Identifier
    is saved; the field is replaced with a locally unique identifier,
    which is restored to its original value when the corresponding
    answer is received.  The request's state is released upon receipt
    of the answer.  A stateless agent is one that only maintains
    transaction state.
 Translation Agent
    A translation agent (TLA in Figure 4) is a stateful Diameter node
    that performs protocol translation between Diameter and another
    AAA protocol, such as RADIUS.
 Upstream
    Upstream is used to identify the direction of a particular
    Diameter message from the Diameter client towards the home server.
 User
    The entity or device requesting or using some resource, in support
    of which a Diameter client has generated a request.

1.3. Approach to Extensibility

 The Diameter protocol is designed to be extensible, using several
 mechanisms, including:
 o  Defining new AVP values
 o  Creating new AVPs
 o  Creating new commands
 o  Creating new applications

Fajardo, et al. Standards Track [Page 17] RFC 6733 Diameter Base Protocol October 2012

 From the point of view of extensibility, Diameter authentication,
 authorization, and accounting applications are treated in the same
 way.
 Note: Protocol designers should try to reuse existing functionality,
 namely AVP values, AVPs, commands, and Diameter applications.  Reuse
 simplifies standardization and implementation.  To avoid potential
 interoperability issues, it is important to ensure that the semantics
 of the reused features are well understood.  Given that Diameter can
 also carry RADIUS attributes as Diameter AVPs, such reuse
 considerations also apply to existing RADIUS attributes that may be
 useful in a Diameter application.

1.3.1. Defining New AVP Values

 In order to allocate a new AVP value for AVPs defined in the Diameter
 base protocol, the IETF needs to approve a new RFC that describes the
 AVP value.  IANA considerations for these AVP values are discussed in
 Section 11.3.
 The allocation of AVP values for other AVPs is guided by the IANA
 considerations of the document that defines those AVPs.  Typically,
 allocation of new values for an AVP defined in an RFC would require
 IETF Review [RFC5226], whereas values for vendor-specific AVPs can be
 allocated by the vendor.

1.3.2. Creating New AVPs

 A new AVP being defined MUST use one of the data types listed in
 Sections 4.2 or 4.3.  If an appropriate derived data type is already
 defined, it SHOULD be used instead of a base data type to encourage
 reusability and good design practice.
 In the event that a logical grouping of AVPs is necessary, and
 multiple "groups" are possible in a given command, it is recommended
 that a Grouped AVP be used (see Section 4.4).
 The creation of new AVPs can happen in various ways.  The recommended
 approach is to define a new general-purpose AVP in a Standards Track
 RFC approved by the IETF.  However, as described in Section 11.1.1,
 there are other mechanisms.

1.3.3. Creating New Commands

 A new Command Code MUST be allocated when required AVPs (those
 indicated as {AVP} in the CCF definition) are added to, deleted from,
 or redefined in (for example, by changing a required AVP into an
 optional one) an existing command.

Fajardo, et al. Standards Track [Page 18] RFC 6733 Diameter Base Protocol October 2012

 Furthermore, if the transport characteristics of a command are
 changed (for example, with respect to the number of round trips
 required), a new Command Code MUST be registered.
 A change to the CCF of a command, such as described above, MUST
 result in the definition of a new Command Code.  This subsequently
 leads to the need to define a new Diameter application for any
 application that will use that new command.
 The IANA considerations for Command Codes are discussed in
 Section 3.1.

1.3.4. Creating New Diameter Applications

 Every Diameter application specification MUST have an IANA-assigned
 Application Id (see Section 2.4).  The managed Application ID space
 is flat, and there is no relationship between different Diameter
 applications with respect to their Application Ids.  As such, there
 is no versioning support provided by these Application Ids
 themselves; every Diameter application is a standalone application.
 If the application has a relationship with other Diameter
 applications, such a relationship is not known to Diameter.
 Before describing the rules for creating new Diameter applications,
 it is important to discuss the semantics of the AVP occurrences as
 stated in the CCF and the M-bit flag (Section 4.1) for an AVP.  There
 is no relationship imposed between the two; they are set
 independently.
 o  The CCF indicates what AVPs are placed into a Diameter command by
    the sender of that command.  Often, since there are multiple modes
    of protocol interactions, many of the AVPs are indicated as
    optional.
 o  The M-bit allows the sender to indicate to the receiver whether or
    not understanding the semantics of an AVP and its content is
    mandatory.  If the M-bit is set by the sender and the receiver
    does not understand the AVP or the values carried within that AVP,
    then a failure is generated (see Section 7).
 It is the decision of the protocol designer when to develop a new
 Diameter application rather than extending Diameter in other ways.
 However, a new Diameter application MUST be created when one or more
 of the following criteria are met:

Fajardo, et al. Standards Track [Page 19] RFC 6733 Diameter Base Protocol October 2012

 M-bit Setting
    An AVP with the M-bit in the MUST column of the AVP flag table is
    added to an existing Command/Application.  An AVP with the M-bit
    in the MAY column of the AVP flag table is added to an existing
    Command/Application.
    Note: The M-bit setting for a given AVP is relevant to an
    Application and each command within that application that includes
    the AVP.  That is, if an AVP appears in two commands for
    application Foo and the M-bit settings are different in each
    command, then there should be two AVP flag tables describing when
    to set the M-bit.
 Commands
    A new command is used within the existing application because
    either an additional command is added, an existing command has
    been modified so that a new Command Code had to be registered, or
    a command has been deleted.
 AVP Flag bits
    If an existing application changes the meaning/semantics of its
    AVP Flags or adds new flag bits, then a new Diameter application
    MUST be created.
 If the CCF definition of a command allows it, an implementation may
 add arbitrary optional AVPs with the M-bit cleared (including vendor-
 specific AVPs) to that command without needing to define a new
 application.  Please refer to Section 11.1.1 for details.

2. Protocol Overview

 The base Diameter protocol concerns itself with establishing
 connections to peers, capabilities negotiation, how messages are sent
 and routed through peers, and how the connections are eventually torn
 down.  The base protocol also defines certain rules that apply to all
 message exchanges between Diameter nodes.
 Communication between Diameter peers begins with one peer sending a
 message to another Diameter peer.  The set of AVPs included in the
 message is determined by a particular Diameter application.  One AVP
 that is included to reference a user's session is the Session-Id.
 The initial request for authentication and/or authorization of a user
 would include the Session-Id AVP.  The Session-Id is then used in all
 subsequent messages to identify the user's session (see Section 8 for

Fajardo, et al. Standards Track [Page 20] RFC 6733 Diameter Base Protocol October 2012

 more information).  The communicating party may accept the request or
 reject it by returning an answer message with the Result-Code AVP set
 to indicate that an error occurred.  The specific behavior of the
 Diameter server or client receiving a request depends on the Diameter
 application employed.
 Session state (associated with a Session-Id) MUST be freed upon
 receipt of the Session-Termination-Request, Session-Termination-
 Answer, expiration of authorized service time in the Session-Timeout
 AVP, and according to rules established in a particular Diameter
 application.
 The base Diameter protocol may be used by itself for accounting
 applications.  For authentication and authorization, it is always
 extended for a particular application.
 Diameter clients MUST support the base protocol, which includes
 accounting.  In addition, they MUST fully support each Diameter
 application that is needed to implement the client's service, e.g.,
 Network Access Server Requirements (NASREQ) [RFC2881] and/or Mobile
 IPv4.  A Diameter client MUST be referred to as "Diameter X Client"
 where X is the application that it supports and not a "Diameter
 Client".
 Diameter servers MUST support the base protocol, which includes
 accounting.  In addition, they MUST fully support each Diameter
 application that is needed to implement the intended service, e.g.,
 NASREQ and/or Mobile IPv4.  A Diameter server MUST be referred to as
 "Diameter X Server" where X is the application that it supports, and
 not a "Diameter Server".
 Diameter relays and redirect agents are transparent to the Diameter
 applications, but they MUST support the Diameter base protocol, which
 includes accounting, and all Diameter applications.
 Diameter proxies MUST support the base protocol, which includes
 accounting.  In addition, they MUST fully support each Diameter
 application that is needed to implement proxied services, e.g.,
 NASREQ and/or Mobile IPv4.  A Diameter proxy MUST be referred to as
 "Diameter X Proxy" where X is the application which it supports, and
 not a "Diameter Proxy".

Fajardo, et al. Standards Track [Page 21] RFC 6733 Diameter Base Protocol October 2012

2.1. Transport

 The Diameter Transport profile is defined in [RFC3539].
 The base Diameter protocol is run on port 3868 for both TCP [RFC0793]
 and SCTP [RFC4960].  For TLS [RFC5246] and Datagram Transport Layer
 Security (DTLS) [RFC6347], a Diameter node that initiates a
 connection prior to any message exchanges MUST run on port 5658.  It
 is assumed that TLS is run on top of TCP when it is used, and DTLS is
 run on top of SCTP when it is used.
 If the Diameter peer does not support receiving TLS/TCP and DTLS/SCTP
 connections on port 5658 (i.e., the peer complies only with RFC
 3588), then the initiator MAY revert to using TCP or SCTP on port
 3868.  Note that this scheme is kept only for the purpose of backward
 compatibility and that there are inherent security vulnerabilities
 when the initial CER/CEA messages are sent unprotected (see
 Section 5.6).
 Diameter clients MUST support either TCP or SCTP; agents and servers
 SHOULD support both.
 A Diameter node MAY initiate connections from a source port other
 than the one that it declares it accepts incoming connections on, and
 it MUST always be prepared to receive connections on port 3868 for
 TCP or SCTP and port 5658 for TLS/TCP and DTLS/SCTP connections.
 When DNS-based peer discovery (Section 5.2) is used, the port numbers
 received from SRV records take precedence over the default ports
 (3868 and 5658).
 A given Diameter instance of the peer state machine MUST NOT use more
 than one transport connection to communicate with a given peer,
 unless multiple instances exist on the peer, in which, case a
 separate connection per process is allowed.
 When no transport connection exists with a peer, an attempt to
 connect SHOULD be made periodically.  This behavior is handled via
 the Tc timer (see Section 12 for details), whose recommended value is
 30 seconds.  There are certain exceptions to this rule, such as when
 a peer has terminated the transport connection stating that it does
 not wish to communicate.
 When connecting to a peer and either zero or more transports are
 specified, TLS SHOULD be tried first, followed by DTLS, then by TCP,
 and finally by SCTP.  See Section 5.2 for more information on peer
 discovery.

Fajardo, et al. Standards Track [Page 22] RFC 6733 Diameter Base Protocol October 2012

 Diameter implementations SHOULD be able to interpret ICMP protocol
 port unreachable messages as explicit indications that the server is
 not reachable, subject to security policy on trusting such messages.
 Further guidance regarding the treatment of ICMP errors can be found
 in [RFC5927] and [RFC5461].  Diameter implementations SHOULD also be
 able to interpret a reset from the transport and timed-out connection
 attempts.  If Diameter receives data from the lower layer that cannot
 be parsed or identified as a Diameter error made by the peer, the
 stream is compromised and cannot be recovered.  The transport
 connection MUST be closed using a RESET call (send a TCP RST bit) or
 an SCTP ABORT message (graceful closure is compromised).

2.1.1. SCTP Guidelines

 Diameter messages SHOULD be mapped into SCTP streams in a way that
 avoids head-of-the-line (HOL) blocking.  Among different ways of
 performing the mapping that fulfill this requirement it is
 RECOMMENDED that a Diameter node send every Diameter message (request
 or response) over stream zero with the unordered flag set.  However,
 Diameter nodes MAY select and implement other design alternatives for
 avoiding HOL blocking such as using multiple streams with the
 unordered flag cleared (as originally instructed in RFC 3588).  On
 the receiving side, a Diameter entity MUST be ready to receive
 Diameter messages over any stream, and it is free to return responses
 over a different stream.  This way, both sides manage the available
 streams in the sending direction, independently of the streams chosen
 by the other side to send a particular Diameter message.  These
 messages can be out-of-order and belong to different Diameter
 sessions.
 Out-of-order delivery has special concerns during a connection
 establishment and termination.  When a connection is established, the
 responder side sends a CEA message and moves to R-Open state as
 specified in Section 5.6.  If an application message is sent shortly
 after the CEA and delivered out-of-order, the initiator side, still
 in Wait-I-CEA state, will discard the application message and close
 the connection.  In order to avoid this race condition, the receiver
 side SHOULD NOT use out-of-order delivery methods until the first
 message has been received from the initiator, proving that it has
 moved to I-Open state.  To trigger such a message, the receiver side
 could send a DWR immediately after sending a CEA.  Upon reception of
 the corresponding DWA, the receiver side should start using out-of-
 order delivery methods to counter the HOL blocking.
 Another race condition may occur when DPR and DPA messages are used.
 Both DPR and DPA are small in size; thus, they may be delivered to
 the peer faster than application messages when an out-of-order
 delivery mechanism is used.  Therefore, it is possible that a DPR/DPA

Fajardo, et al. Standards Track [Page 23] RFC 6733 Diameter Base Protocol October 2012

 exchange completes while application messages are still in transit,
 resulting in a loss of these messages.  An implementation could
 mitigate this race condition, for example, using timers, and wait for
 a short period of time for pending application level messages to
 arrive before proceeding to disconnect the transport connection.
 Eventually, lost messages are handled by the retransmission mechanism
 described in Section 5.5.4.
 A Diameter agent SHOULD use dedicated payload protocol identifiers
 (PPIDs) for clear text and encrypted SCTP DATA chunks instead of only
 using the unspecified payload protocol identifier (value 0).  For
 this purpose, two PPID values are allocated: the PPID value 46 is for
 Diameter messages in clear text SCTP DATA chunks, and the PPID value
 47 is for Diameter messages in protected DTLS/SCTP DATA chunks.

2.2. Securing Diameter Messages

 Connections between Diameter peers SHOULD be protected by TLS/TCP and
 DTLS/SCTP.  All Diameter base protocol implementations MUST support
 the use of TLS/TCP and DTLS/SCTP.  If desired, alternative security
 mechanisms that are independent of Diameter, such as IPsec [RFC4301],
 can be deployed to secure connections between peers.  The Diameter
 protocol MUST NOT be used without one of TLS, DTLS, or IPsec.

2.3. Diameter Application Compliance

 Application Ids are advertised during the capabilities exchange phase
 (see Section 5.3).  Advertising support of an application implies
 that the sender supports the functionality specified in the
 respective Diameter application specification.
 Implementations MAY add arbitrary optional AVPs with the M-bit
 cleared (including vendor-specific AVPs) to a command defined in an
 application, but only if the command's CCF syntax specification
 allows for it.  Please refer to Section 11.1.1 for details.

2.4. Application Identifiers

 Each Diameter application MUST have an IANA-assigned Application ID.
 The base protocol does not require an Application Id since its
 support is mandatory.  During the capabilities exchange, Diameter
 nodes inform their peers of locally supported applications.
 Furthermore, all Diameter messages contain an Application Id, which
 is used in the message forwarding process.

Fajardo, et al. Standards Track [Page 24] RFC 6733 Diameter Base Protocol October 2012

 The following Application Id values are defined:
       Diameter common message       0
       Diameter base accounting      3
       Relay                         0xffffffff
 Relay and redirect agents MUST advertise the Relay Application ID,
 while all other Diameter nodes MUST advertise locally supported
 applications.  The receiver of a Capabilities Exchange message
 advertising relay service MUST assume that the sender supports all
 current and future applications.
 Diameter relay and proxy agents are responsible for finding an
 upstream server that supports the application of a particular
 message.  If none can be found, an error message is returned with the
 Result-Code AVP set to DIAMETER_UNABLE_TO_DELIVER.

2.5. Connections vs. Sessions

 This section attempts to provide the reader with an understanding of
 the difference between "connection" and "session", which are terms
 used extensively throughout this document.
 A connection refers to a transport-level connection between two peers
 that is used to send and receive Diameter messages.  A session is a
 logical concept at the application layer that exists between the
 Diameter client and the Diameter server; it is identified via the
 Session-Id AVP.
           +--------+          +-------+          +--------+
           | Client |          | Relay |          | Server |
           +--------+          +-------+          +--------+
                    <---------->       <---------->
                 peer connection A   peer connection B
                    <----------------------------->
                            User session x
              Figure 1: Diameter Connections and Sessions
 In the example provided in Figure 1, peer connection A is established
 between the client and the relay.  Peer connection B is established
 between the relay and the server.  User session X spans from the
 client via the relay to the server.  Each "user" of a service causes
 an auth request to be sent, with a unique session identifier.  Once
 accepted by the server, both the client and the server are aware of
 the session.

Fajardo, et al. Standards Track [Page 25] RFC 6733 Diameter Base Protocol October 2012

 It is important to note that there is no relationship between a
 connection and a session, and that Diameter messages for multiple
 sessions are all multiplexed through a single connection.  Also, note
 that Diameter messages pertaining to the session, both application-
 specific and those that are defined in this document such as ASR/ASA,
 RAR/RAA, and STR/STA, MUST carry the Application Id of the
 application.  Diameter messages pertaining to peer connection
 establishment and maintenance such as CER/CEA, DWR/DWA, and DPR/DPA
 MUST carry an Application Id of zero (0).

2.6. Peer Table

 The Diameter peer table is used in message forwarding and is
 referenced by the routing table.  A peer table entry contains the
 following fields:
 Host Identity
    Following the conventions described for the DiameterIdentity-
    derived AVP data format in Section 4.3.1, this field contains the
    contents of the Origin-Host (Section 6.3) AVP found in the CER or
    CEA message.
 StatusT
    This is the state of the peer entry, and it MUST match one of the
    values listed in Section 5.6.
 Static or Dynamic
    Specifies whether a peer entry was statically configured or
    dynamically discovered.
 Expiration Time
    Specifies the time at which dynamically discovered peer table
    entries are to be either refreshed or expired.  If public key
    certificates are used for Diameter security (e.g., with TLS), this
    value MUST NOT be greater than the expiry times in the relevant
    certificates.
 TLS/TCP and DTLS/SCTP Enabled
    Specifies whether TLS/TCP and DTLS/SCTP is to be used when
    communicating with the peer.
 Additional security information, when needed (e.g., keys,
 certificates).

Fajardo, et al. Standards Track [Page 26] RFC 6733 Diameter Base Protocol October 2012

2.7. Routing Table

 All Realm-Based routing lookups are performed against what is
 commonly known as the routing table (see Section 12).  Each routing
 table entry contains the following fields:
 Realm Name
    This is the field that MUST be used as a primary key in the
    routing table lookups.  Note that some implementations perform
    their lookups based on longest-match-from-the-right on the realm
    rather than requiring an exact match.
 Application Identifier
    An application is identified by an Application Id.  A route entry
    can have a different destination based on the Application Id in
    the message header.  This field MUST be used as a secondary key
    field in routing table lookups.
 Local Action
    The Local Action field is used to identify how a message should be
    treated.  The following actions are supported:
    1.  LOCAL - Diameter messages that can be satisfied locally and do
        not need to be routed to another Diameter entity.
    2.  RELAY - All Diameter messages that fall within this category
        MUST be routed to a next-hop Diameter entity that is indicated
        by the identifier described below.  Routing is done without
        modifying any non-routing AVPs.  See Section 6.1.9 for
        relaying guidelines.
    3.  PROXY - All Diameter messages that fall within this category
        MUST be routed to a next Diameter entity that is indicated by
        the identifier described below.  The local server MAY apply
        its local policies to the message by including new AVPs to the
        message prior to routing.  See Section 6.1.9 for proxying
        guidelines.
    4.  REDIRECT - Diameter messages that fall within this category
        MUST have the identity of the home Diameter server(s)
        appended, and returned to the sender of the message.  See
        Section 6.1.8 for redirection guidelines.

Fajardo, et al. Standards Track [Page 27] RFC 6733 Diameter Base Protocol October 2012

 Server Identifier
    The identity of one or more servers to which the message is to be
    routed.  This identity MUST also be present in the Host Identity
    field of the peer table (Section 2.6).  When the Local Action is
    set to RELAY or PROXY, this field contains the identity of the
    server(s) to which the message MUST be routed.  When the Local
    Action field is set to REDIRECT, this field contains the identity
    of one or more servers to which the message MUST be redirected.
 Static or Dynamic
    Specifies whether a route entry was statically configured or
    dynamically discovered.
 Expiration Time
    Specifies the time at which a dynamically discovered route table
    entry expires.  If public key certificates are used for Diameter
    security (e.g., with TLS), this value MUST NOT be greater than the
    expiry time in the relevant certificates.
 It is important to note that Diameter agents MUST support at least
 one of the LOCAL, RELAY, PROXY, or REDIRECT modes of operation.
 Agents do not need to support all modes of operation in order to
 conform with the protocol specification, but they MUST follow the
 protocol compliance guidelines in Section 2.  Relay agents and
 proxies MUST NOT reorder AVPs.
 The routing table MAY include a default entry that MUST be used for
 any requests not matching any of the other entries.  The routing
 table MAY consist of only such an entry.
 When a request is routed, the target server MUST have advertised the
 Application Id (see Section 2.4) for the given message or have
 advertised itself as a relay or proxy agent.  Otherwise, an error is
 returned with the Result-Code AVP set to DIAMETER_UNABLE_TO_DELIVER.

2.8. Role of Diameter Agents

 In addition to clients and servers, the Diameter protocol introduces
 relay, proxy, redirect, and translation agents, each of which is
 defined in Section 1.2.  Diameter agents are useful for several
 reasons:
 o  They can distribute administration of systems to a configurable
    grouping, including the maintenance of security associations.

Fajardo, et al. Standards Track [Page 28] RFC 6733 Diameter Base Protocol October 2012

 o  They can be used for concentration of requests from a number of
    co-located or distributed NAS equipment sets to a set of like user
    groups.
 o  They can do value-added processing to the requests or responses.
 o  They can be used for load balancing.
 o  A complex network will have multiple authentication sources, they
    can sort requests and forward towards the correct target.
 The Diameter protocol requires that agents maintain transaction
 state, which is used for failover purposes.  Transaction state
 implies that upon forwarding a request, its Hop-by-Hop Identifier is
 saved; the field is replaced with a locally unique identifier, which
 is restored to its original value when the corresponding answer is
 received.  The request's state is released upon receipt of the
 answer.  A stateless agent is one that only maintains transaction
 state.
 The Proxy-Info AVP allows stateless agents to add local state to a
 Diameter request, with the guarantee that the same state will be
 present in the answer.  However, the protocol's failover procedures
 require that agents maintain a copy of pending requests.
 A stateful agent is one that maintains session state information by
 keeping track of all authorized active sessions.  Each authorized
 session is bound to a particular service, and its state is considered
 active until either the agent is notified otherwise or the session
 expires.  Each authorized session has an expiration, which is
 communicated by Diameter servers via the Session-Timeout AVP.
 Maintaining session state may be useful in certain applications, such
 as:
 o  Protocol translation (e.g., RADIUS <-> Diameter)
 o  Limiting resources authorized to a particular user
 o  Per-user or per-transaction auditing
 A Diameter agent MAY act in a stateful manner for some requests and
 be stateless for others.  A Diameter implementation MAY act as one
 type of agent for some requests and as another type of agent for
 others.

Fajardo, et al. Standards Track [Page 29] RFC 6733 Diameter Base Protocol October 2012

2.8.1. Relay Agents

 Relay agents are Diameter agents that accept requests and route
 messages to other Diameter nodes based on information found in the
 messages (e.g., the value of the Destination-Realm AVP Section 6.6).
 This routing decision is performed using a list of supported realms
 and known peers.  This is known as the routing table, as is defined
 further in Section 2.7.
 Relays may, for example, be used to aggregate requests from multiple
 Network Access Servers (NASes) within a common geographical area
 (Point of Presence, POP).  The use of relays is advantageous since it
 eliminates the need for NASes to be configured with the necessary
 security information they would otherwise require to communicate with
 Diameter servers in other realms.  Likewise, this reduces the
 configuration load on Diameter servers that would otherwise be
 necessary when NASes are added, changed, or deleted.
 Relays modify Diameter messages by inserting and removing routing
 information, but they do not modify any other portion of a message.
 Relays SHOULD NOT maintain session state but MUST maintain
 transaction state.
     +------+    --------->     +------+     --------->    +------+
     |      |    1. Request     |      |     2. Request    |      |
     | NAS  |                   | DRL  |                   | HMS  |
     |      |    4. Answer      |      |     3. Answer     |      |
     +------+    <---------     +------+     <---------    +------+
  example.net                example.net                example.com
                Figure 2: Relaying of Diameter messages
 The example provided in Figure 2 depicts a request issued from a NAS,
 which is an access device, for the user bob@example.com.  Prior to
 issuing the request, the NAS performs a Diameter route lookup, using
 "example.com" as the key, and determines that the message is to be
 relayed to a DRL, which is a Diameter relay.  The DRL performs the
 same route lookup as the NAS, and relays the message to the HMS,
 which is example.com's home server.  The HMS identifies that the
 request can be locally supported (via the realm), processes the
 authentication and/or authorization request, and replies with an
 answer, which is routed back to the NAS using saved transaction
 state.
 Since relays do not perform any application-level processing, they
 provide relaying services for all Diameter applications; therefore,
 they MUST advertise the Relay Application Id.

Fajardo, et al. Standards Track [Page 30] RFC 6733 Diameter Base Protocol October 2012

2.8.2. Proxy Agents

 Similar to relays, proxy agents route Diameter messages using the
 Diameter routing table.  However, they differ since they modify
 messages to implement policy enforcement.  This requires that proxies
 maintain the state of their downstream peers (e.g., access devices)
 to enforce resource usage, provide admission control, and provide
 provisioning.
 Proxies may, for example, be used in call control centers or access
 ISPs that provide outsourced connections; they can monitor the number
 and type of ports in use and make allocation and admission decisions
 according to their configuration.
 Since enforcing policies requires an understanding of the service
 being provided, proxies MUST only advertise the Diameter applications
 they support.

2.8.3. Redirect Agents

 Redirect agents are useful in scenarios where the Diameter routing
 configuration needs to be centralized.  An example is a redirect
 agent that provides services to all members of a consortium, but does
 not wish to be burdened with relaying all messages between realms.
 This scenario is advantageous since it does not require that the
 consortium provide routing updates to its members when changes are
 made to a member's infrastructure.
 Since redirect agents do not relay messages, and only return an
 answer with the information necessary for Diameter agents to
 communicate directly, they do not modify messages.  Since redirect
 agents do not receive answer messages, they cannot maintain session
 state.
 The example provided in Figure 3 depicts a request issued from the
 access device, NAS, for the user bob@example.com.  The message is
 forwarded by the NAS to its relay, DRL, which does not have a routing
 entry in its Diameter routing table for example.com.  The DRL has a
 default route configured to DRD, which is a redirect agent that
 returns a redirect notification to DRL, as well as the HMS' contact
 information.  Upon receipt of the redirect notification, the DRL
 establishes a transport connection with the HMS, if one doesn't
 already exist, and forwards the request to it.

Fajardo, et al. Standards Track [Page 31] RFC 6733 Diameter Base Protocol October 2012

                                +------+
                                |      |
                                | DRD  |
                                |      |
                                +------+
                                 ^    |
                     2. Request  |    | 3. Redirection
                                 |    |    Notification
                                 |    v
     +------+    --------->     +------+     --------->    +------+
     |      |    1. Request     |      |     4. Request    |      |
     | NAS  |                   | DRL  |                   | HMS  |
     |      |    6. Answer      |      |     5. Answer     |      |
     +------+    <---------     +------+     <---------    +------+
    example.net                example.net               example.com
               Figure 3: Redirecting a Diameter Message
 Since redirect agents do not perform any application-level
 processing, they provide relaying services for all Diameter
 applications; therefore, they MUST advertise the Relay Application
 ID.

2.8.4. Translation Agents

 A translation agent is a device that provides translation between two
 protocols (e.g., RADIUS<->Diameter, TACACS+<->Diameter).  Translation
 agents are likely to be used as aggregation servers to communicate
 with a Diameter infrastructure, while allowing for the embedded
 systems to be migrated at a slower pace.
 Given that the Diameter protocol introduces the concept of long-lived
 authorized sessions, translation agents MUST be session stateful and
 MUST maintain transaction state.
 Translation of messages can only occur if the agent recognizes the
 application of a particular request; therefore, translation agents
 MUST only advertise their locally supported applications.
     +------+    --------->     +------+     --------->    +------+
     |      |  RADIUS Request   |      |  Diameter Request |      |
     | NAS  |                   | TLA  |                   | HMS  |
     |      |  RADIUS Answer    |      |  Diameter Answer  |      |
     +------+    <---------     +------+     <---------    +------+
    example.net                example.net               example.com
              Figure 4: Translation of RADIUS to Diameter

Fajardo, et al. Standards Track [Page 32] RFC 6733 Diameter Base Protocol October 2012

2.9. Diameter Path Authorization

 As noted in Section 2.2, Diameter provides transmission-level
 security for each connection using TLS/TCP and DTLS/SCTP.  Therefore,
 each connection can be authenticated and can be replay and integrity
 protected.
 In addition to authenticating each connection, the entire session
 MUST also be authorized.  Before initiating a connection, a Diameter
 peer MUST check that its peers are authorized to act in their roles.
 For example, a Diameter peer may be authentic, but that does not mean
 that it is authorized to act as a Diameter server advertising a set
 of Diameter applications.
 Prior to bringing up a connection, authorization checks are performed
 at each connection along the path.  Diameter capabilities negotiation
 (CER/CEA) also MUST be carried out, in order to determine what
 Diameter applications are supported by each peer.  Diameter sessions
 MUST be routed only through authorized nodes that have advertised
 support for the Diameter application required by the session.
 As noted in Section 6.1.9, a relay or proxy agent MUST append a
 Route-Record AVP to all requests forwarded.  The AVP contains the
 identity of the peer from which the request was received.
 The home Diameter server, prior to authorizing a session, MUST check
 the Route-Record AVPs to make sure that the route traversed by the
 request is acceptable.  For example, administrators within the home
 realm may not wish to honor requests that have been routed through an
 untrusted realm.  By authorizing a request, the home Diameter server
 is implicitly indicating its willingness to engage in the business
 transaction as specified by any contractual relationship between the
 server and the previous hop.  A DIAMETER_AUTHORIZATION_REJECTED error
 message (see Section 7.1.5) is sent if the route traversed by the
 request is unacceptable.
 A home realm may also wish to check that each accounting request
 message corresponds to a Diameter response authorizing the session.
 Accounting requests without corresponding authorization responses
 SHOULD be subjected to further scrutiny, as should accounting
 requests indicating a difference between the requested and provided
 service.
 Forwarding of an authorization response is considered evidence of a
 willingness to take on financial risk relative to the session.  A
 local realm may wish to limit this exposure, for example, by
 establishing credit limits for intermediate realms and refusing to
 accept responses that would violate those limits.  By issuing an

Fajardo, et al. Standards Track [Page 33] RFC 6733 Diameter Base Protocol October 2012

 accounting request corresponding to the authorization response, the
 local realm implicitly indicates its agreement to provide the service
 indicated in the authorization response.  If the service cannot be
 provided by the local realm, then a DIAMETER_UNABLE_TO_COMPLY error
 message MUST be sent within the accounting request; a Diameter client
 receiving an authorization response for a service that it cannot
 perform MUST NOT substitute an alternate service and then send
 accounting requests for the alternate service instead.

3. Diameter Header

 A summary of the Diameter header format is shown below.  The fields
 are transmitted in network byte order.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    Version    |                 Message Length                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Command Flags |                  Command Code                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Application-ID                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Hop-by-Hop Identifier                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      End-to-End Identifier                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  AVPs ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-
 Version
    This Version field MUST be set to 1 to indicate Diameter Version
    1.
  Message Length
    The Message Length field is three octets and indicates the length
    of the Diameter message including the header fields and the padded
    AVPs.  Thus, the Message Length field is always a multiple of 4.
 Command Flags
    The Command Flags field is eight bits.  The following bits are
    assigned:

Fajardo, et al. Standards Track [Page 34] RFC 6733 Diameter Base Protocol October 2012

        0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+
       |R P E T r r r r|
       +-+-+-+-+-+-+-+-+
    R(equest)
       If set, the message is a request.  If cleared, the message is
       an answer.
    P(roxiable)
       If set, the message MAY be proxied, relayed, or redirected.  If
       cleared, the message MUST be locally processed.
    E(rror)
       If set, the message contains a protocol error, and the message
       will not conform to the CCF described for this command.
       Messages with the 'E' bit set are commonly referred to as error
       messages.  This bit MUST NOT be set in request messages (see
       Section 7.2).
    T(Potentially retransmitted message)
       This flag is set after a link failover procedure, to aid the
       removal of duplicate requests.  It is set when resending
       requests not yet acknowledged, as an indication of a possible
       duplicate due to a link failure.  This bit MUST be cleared when
       sending a request for the first time; otherwise, the sender
       MUST set this flag.  Diameter agents only need to be concerned
       about the number of requests they send based on a single
       received request; retransmissions by other entities need not be
       tracked.  Diameter agents that receive a request with the T
       flag set, MUST keep the T flag set in the forwarded request.
       This flag MUST NOT be set if an error answer message (e.g., a
       protocol error) has been received for the earlier message.  It
       can be set only in cases where no answer has been received from
       the server for a request, and the request has been sent again.
       This flag MUST NOT be set in answer messages.
    r(eserved)
       These flag bits are reserved for future use; they MUST be set
       to zero and ignored by the receiver.

Fajardo, et al. Standards Track [Page 35] RFC 6733 Diameter Base Protocol October 2012

 Command Code
    The Command Code field is three octets and is used in order to
    communicate the command associated with the message.  The 24-bit
    address space is managed by IANA (see Section 3.1).  Command Code
    values 16,777,214 and 16,777,215 (hexadecimal values FFFFFE-
    FFFFFF) are reserved for experimental use (see Section 11.2).
 Application-ID
    Application-ID is four octets and is used to identify for which
    application the message is applicable.  The application can be an
    authentication application, an accounting application, or a
    vendor-specific application.
    The value of the Application-ID field in the header MUST be the
    same as any relevant Application-Id AVPs contained in the message.
 Hop-by-Hop Identifier
    The Hop-by-Hop Identifier is an unsigned 32-bit integer field (in
    network byte order) that aids in matching requests and replies.
    The sender MUST ensure that the Hop-by-Hop Identifier in a request
    is unique on a given connection at any given time, and it MAY
    attempt to ensure that the number is unique across reboots.  The
    sender of an answer message MUST ensure that the Hop-by-Hop
    Identifier field contains the same value that was found in the
    corresponding request.  The Hop-by-Hop Identifier is normally a
    monotonically increasing number, whose start value was randomly
    generated.  An answer message that is received with an unknown
    Hop-by-Hop Identifier MUST be discarded.
 End-to-End Identifier
    The End-to-End Identifier is an unsigned 32-bit integer field (in
    network byte order) that is used to detect duplicate messages.
    Upon reboot, implementations MAY set the high order 12 bits to
    contain the low order 12 bits of current time, and the low order
    20 bits to a random value.  Senders of request messages MUST
    insert a unique identifier on each message.  The identifier MUST
    remain locally unique for a period of at least 4 minutes, even
    across reboots.  The originator of an answer message MUST ensure
    that the End-to-End Identifier field contains the same value that
    was found in the corresponding request.  The End-to-End Identifier
    MUST NOT be modified by Diameter agents of any kind.  The
    combination of the Origin-Host AVP (Section 6.3) and this field is
    used to detect duplicates.  Duplicate requests SHOULD cause the
    same answer to be transmitted (modulo the Hop-by-Hop Identifier

Fajardo, et al. Standards Track [Page 36] RFC 6733 Diameter Base Protocol October 2012

    field and any routing AVPs that may be present), and they MUST NOT
    affect any state that was set when the original request was
    processed.  Duplicate answer messages that are to be locally
    consumed (see Section 6.2) SHOULD be silently discarded.
 AVPs
    AVPs are a method of encapsulating information relevant to the
    Diameter message.  See Section 4 for more information on AVPs.

3.1. Command Codes

 Each command Request/Answer pair is assigned a Command Code, and the
 sub-type (i.e., request or answer) is identified via the 'R' bit in
 the Command Flags field of the Diameter header.
 Every Diameter message MUST contain a Command Code in its header's
 Command Code field, which is used to determine the action that is to
 be taken for a particular message.  The following Command Codes are
 defined in the Diameter base protocol:
                                                 Section
  Command Name             Abbrev.    Code       Reference
    --------------------------------------------------------
    Abort-Session-Request     ASR       274           8.5.1
    Abort-Session-Answer      ASA       274           8.5.2
    Accounting-Request        ACR       271           9.7.1
    Accounting-Answer         ACA       271           9.7.2
    Capabilities-Exchange-    CER       257           5.3.1
       Request
    Capabilities-Exchange-    CEA       257           5.3.2
       Answer
    Device-Watchdog-Request   DWR       280           5.5.1
    Device-Watchdog-Answer    DWA       280           5.5.2
    Disconnect-Peer-Request   DPR       282           5.4.1
    Disconnect-Peer-Answer    DPA       282           5.4.2
    Re-Auth-Request           RAR       258           8.3.1
    Re-Auth-Answer            RAA       258           8.3.2
    Session-Termination-      STR       275           8.4.1
       Request
    Session-Termination-      STA       275           8.4.2
       Answer

Fajardo, et al. Standards Track [Page 37] RFC 6733 Diameter Base Protocol October 2012

3.2. Command Code Format Specification

 Every Command Code defined MUST include a corresponding Command Code
 Format (CCF) specification, which is used to define the AVPs that
 MUST or MAY be present when sending the message.  The following ABNF
 specifies the CCF used in the definition:
 command-def      = "<" command-name ">" "::=" diameter-message
 command-name     = diameter-name
 diameter-name    = ALPHA *(ALPHA / DIGIT / "-")
 diameter-message = header   *fixed  *required *optional
 header           = "<Diameter-Header:" command-id
                       [r-bit] [p-bit] [e-bit] [application-id]">"
 application-id   = 1*DIGIT
 command-id       = 1*DIGIT
                    ; The Command Code assigned to the command.
 r-bit            = ", REQ"
                    ; If present, the 'R' bit in the Command
                    ; Flags is set, indicating that the message
                    ; is a request as opposed to an answer.
 p-bit            = ", PXY"
                    ; If present, the 'P' bit in the Command
                    ; Flags is set, indicating that the message
                    ; is proxiable.
 e-bit            = ", ERR"
                    ; If present, the 'E' bit in the Command
                    ; Flags is set, indicating that the answer
                    ; message contains a Result-Code AVP in
                    ; the "protocol error" class.
 fixed            = [qual] "<" avp-spec ">"
                    ; Defines the fixed position of an AVP.
 required         = [qual] "{" avp-spec "}"
                    ; The AVP MUST be present and can appear
                    ; anywhere in the message.

Fajardo, et al. Standards Track [Page 38] RFC 6733 Diameter Base Protocol October 2012

 optional         = [qual] "[" avp-name "]"
                    ; The avp-name in the 'optional' rule cannot
                    ; evaluate to any AVP Name that is included
                    ; in a fixed or required rule.  The AVP can
                    ; appear anywhere in the message.
                    ;
                    ; NOTE:  "[" and "]" have a slightly different
                    ; meaning than in ABNF.  These braces
                    ; cannot be used to express optional fixed rules
                    ; (such as an optional ICV at the end).  To do
                    ; this, the convention is '0*1fixed'.
 qual             = [min] "*" [max]
                    ; See ABNF conventions, RFC 5234, Section 4.
                    ; The absence of any qualifier depends on
                    ; whether it precedes a fixed, required, or
                    ; optional rule.  If a fixed or required rule has
                    ; no qualifier, then exactly one such AVP MUST
                    ; be present.  If an optional rule has no
                    ; qualifier, then 0 or 1 such AVP may be
                    ; present.  If an optional rule has a qualifier,
                    ; then the value of min MUST be 0 if present.
 min              = 1*DIGIT
                    ; The minimum number of times the element may
                    ; be present.  If absent, the default value is 0
                    ; for fixed and optional rules and 1 for
                    ; required rules.  The value MUST be at least 1
                    ; for required rules.
 max              = 1*DIGIT
                    ; The maximum number of times the element may
                    ; be present.  If absent, the default value is
                    ; infinity.  A value of 0 implies the AVP MUST
                    ; NOT be present.
 avp-spec         = diameter-name
                    ; The avp-spec has to be an AVP Name, defined
                    ; in the base or extended Diameter
                    ; specifications.
 avp-name         = avp-spec / "AVP"
                    ; The string "AVP" stands for *any* arbitrary AVP
                    ; Name, not otherwise listed in that Command Code
                    ; definition.  The inclusion of this string
                    ; is recommended for all CCFs to allow for
                    ; extensibility.

Fajardo, et al. Standards Track [Page 39] RFC 6733 Diameter Base Protocol October 2012

 The following is a definition of a fictitious Command Code:
 Example-Request ::= < Diameter Header: 9999999, REQ, PXY >
                     { User-Name }
                  1* { Origin-Host }
                   * [ AVP ]

3.3. Diameter Command Naming Conventions

 Diameter command names typically includes one or more English words
 followed by the verb "Request" or "Answer".  Each English word is
 delimited by a hyphen.  A three-letter acronym for both the request
 and answer is also normally provided.
 An example is a message set used to terminate a session.  The command
 name is Session-Terminate-Request and Session-Terminate-Answer, while
 the acronyms are STR and STA, respectively.
 Both the request and the answer for a given command share the same
 Command Code.  The request is identified by the R(equest) bit in the
 Diameter header set to one (1), to ask that a particular action be
 performed, such as authorizing a user or terminating a session.  Once
 the receiver has completed the request, it issues the corresponding
 answer, which includes a result code that communicates one of the
 following:
 o  The request was successful
 o  The request failed
 o  An additional request has to be sent to provide information the
    peer requires prior to returning a successful or failed answer.
 o  The receiver could not process the request, but provides
    information about a Diameter peer that is able to satisfy the
    request, known as redirect.
 Additional information, encoded within AVPs, may also be included in
 answer messages.

4. Diameter AVPs

 Diameter AVPs carry specific authentication, accounting,
 authorization, and routing information as well as configuration
 details for the request and reply.

Fajardo, et al. Standards Track [Page 40] RFC 6733 Diameter Base Protocol October 2012

 Each AVP of type OctetString MUST be padded to align on a 32-bit
 boundary, while other AVP types align naturally.  A number of zero-
 valued bytes are added to the end of the AVP Data field until a word
 boundary is reached.  The length of the padding is not reflected in
 the AVP Length field.

4.1. AVP Header

 The fields in the AVP header MUST be sent in network byte order.  The
 format of the header is:
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           AVP Code                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |V M P r r r r r|                  AVP Length                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Vendor-ID (opt)                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    Data ...
    +-+-+-+-+-+-+-+-+
 AVP Code
    The AVP Code, combined with the Vendor-Id field, identifies the
    attribute uniquely.  AVP numbers 1 through 255 are reserved for
    reuse of RADIUS attributes, without setting the Vendor-Id field.
    AVP numbers 256 and above are used for Diameter, which are
    allocated by IANA (see Section 11.1.1).
 AVP Flags
    The AVP Flags field informs the receiver how each attribute must
    be handled.  New Diameter applications SHOULD NOT define
    additional AVP Flag bits.  However, note that new Diameter
    applications MAY define additional bits within the AVP header, and
    an unrecognized bit SHOULD be considered an error.  The sender of
    the AVP MUST set 'R' (reserved) bits to 0 and the receiver SHOULD
    ignore all 'R' (reserved) bits.  The 'P' bit has been reserved for
    future usage of end-to-end security.  At the time of writing,
    there are no end-to-end security mechanisms specified; therefore,
    the 'P' bit SHOULD be set to 0.
    The 'M' bit, known as the Mandatory bit, indicates whether the
    receiver of the AVP MUST parse and understand the semantics of the
    AVP including its content.  The receiving entity MUST return an
    appropriate error message if it receives an AVP that has the M-bit

Fajardo, et al. Standards Track [Page 41] RFC 6733 Diameter Base Protocol October 2012

    set but does not understand it.  An exception applies when the AVP
    is embedded within a Grouped AVP.  See Section 4.4 for details.
    Diameter relay and redirect agents MUST NOT reject messages with
    unrecognized AVPs.
    The 'M' bit MUST be set according to the rules defined in the
    application specification that introduces or reuses this AVP.
    Within a given application, the M-bit setting for an AVP is
    defined either for all command types or for each command type.
    AVPs with the 'M' bit cleared are informational only; a receiver
    that receives a message with such an AVP that is not supported, or
    whose value is not supported, MAY simply ignore the AVP.
    The 'V' bit, known as the Vendor-Specific bit, indicates whether
    the optional Vendor-ID field is present in the AVP header.  When
    set, the AVP Code belongs to the specific vendor code address
    space.
 AVP Length
    The AVP Length field is three octets, and indicates the number of
    octets in this AVP including the AVP Code field, AVP Length field,
    AVP Flags field, Vendor-ID field (if present), and the AVP Data
    field.  If a message is received with an invalid attribute length,
    the message MUST be rejected.

4.1.1. Optional Header Elements

 The AVP header contains one optional field.  This field is only
 present if the respective bit-flag is enabled.
 Vendor-ID
    The Vendor-ID field is present if the 'V' bit is set in the AVP
    Flags field.  The optional four-octet Vendor-ID field contains the
    IANA-assigned "SMI Network Management Private Enterprise Codes"
    [ENTERPRISE] value, encoded in network byte order.  Any vendors or
    standardization organizations that are also treated like vendors
    in the IANA-managed "SMI Network Management Private Enterprise
    Codes" space wishing to implement a vendor-specific Diameter AVP
    MUST use their own Vendor-ID along with their privately managed
    AVP address space, guaranteeing that they will not collide with
    any other vendor's vendor-specific AVP(s) or with future IETF
    AVPs.

Fajardo, et al. Standards Track [Page 42] RFC 6733 Diameter Base Protocol October 2012

    A Vendor-ID value of zero (0) corresponds to the IETF-adopted AVP
    values, as managed by IANA.  Since the absence of the Vendor-ID
    field implies that the AVP in question is not vendor specific,
    implementations MUST NOT use the value of zero (0) for the
    Vendor-ID field.

4.2. Basic AVP Data Formats

 The Data field is zero or more octets and contains information
 specific to the Attribute.  The format and length of the Data field
 is determined by the AVP Code and AVP Length fields.  The format of
 the Data field MUST be one of the following base data types or a data
 type derived from the base data types.  In the event that a new Basic
 AVP Data Format is needed, a new version of this RFC MUST be created.
 OctetString
    The data contains arbitrary data of variable length.  Unless
    otherwise noted, the AVP Length field MUST be set to at least 8
    (12 if the 'V' bit is enabled).  AVP values of this type that are
    not a multiple of 4 octets in length are followed by the necessary
    padding so that the next AVP (if any) will start on a 32-bit
    boundary.
 Integer32
    32-bit signed value, in network byte order.  The AVP Length field
    MUST be set to 12 (16 if the 'V' bit is enabled).
 Integer64
    64-bit signed value, in network byte order.  The AVP Length field
    MUST be set to 16 (20 if the 'V' bit is enabled).
 Unsigned32
    32-bit unsigned value, in network byte order.  The AVP Length
    field MUST be set to 12 (16 if the 'V' bit is enabled).
 Unsigned64
    64-bit unsigned value, in network byte order.  The AVP Length
    field MUST be set to 16 (20 if the 'V' bit is enabled).

Fajardo, et al. Standards Track [Page 43] RFC 6733 Diameter Base Protocol October 2012

 Float32
    This represents floating point values of single precision as
    described by [FLOATPOINT].  The 32-bit value is transmitted in
    network byte order.  The AVP Length field MUST be set to 12 (16 if
    the 'V' bit is enabled).
 Float64
    This represents floating point values of double precision as
    described by [FLOATPOINT].  The 64-bit value is transmitted in
    network byte order.  The AVP Length field MUST be set to 16 (20 if
    the 'V' bit is enabled).
 Grouped
    The Data field is specified as a sequence of AVPs.  These AVPs are
    concatenated -- including their headers and padding -- in the
    order in which they are specified and the result encapsulated in
    the Data field.  The AVP Length field is set to 8 (12 if the 'V'
    bit is enabled) plus the total length of all included AVPs,
    including their headers and padding.  Thus, the AVP Length field
    of an AVP of type Grouped is always a multiple of 4.

4.3. Derived AVP Data Formats

 In addition to using the Basic AVP Data Formats, applications may
 define data formats derived from the Basic AVP Data Formats.  An
 application that defines new Derived AVP Data Formats MUST include
 them in a section titled "Derived AVP Data Formats", using the same
 format as the definitions below.  Each new definition MUST be either
 defined or listed with a reference to the RFC that defines the
 format.

4.3.1. Common Derived AVP Data Formats

 The following are commonly used Derived AVP Data Formats.
 Address
    The Address format is derived from the OctetString Basic AVP
    Format.  It is a discriminated union representing, for example, a
    32-bit (IPv4) [RFC0791] or 128-bit (IPv6) [RFC4291] address, most
    significant octet first.  The first two octets of the Address AVP
    represent the AddressType, which contains an Address Family,
    defined in [IANAADFAM].  The AddressType is used to discriminate
    the content and format of the remaining octets.

Fajardo, et al. Standards Track [Page 44] RFC 6733 Diameter Base Protocol October 2012

 Time
    The Time format is derived from the OctetString Basic AVP Format.
    The string MUST contain four octets, in the same format as the
    first four bytes are in the NTP timestamp format.  The NTP
    timestamp format is defined in Section 3 of [RFC5905].
    This represents the number of seconds since 0h on 1 January 1900
    with respect to the Coordinated Universal Time (UTC).
    On 6h 28m 16s UTC, 7 February 2036, the time value will overflow.
    Simple Network Time Protocol (SNTP) [RFC5905] describes a
    procedure to extend the time to 2104.  This procedure MUST be
    supported by all Diameter nodes.
 UTF8String
    The UTF8String format is derived from the OctetString Basic AVP
    Format.  This is a human-readable string represented using the
    ISO/IEC IS 10646-1 character set, encoded as an OctetString using
    the UTF-8 transformation format [RFC3629].
    Since additional code points are added by amendments to the 10646
    standard from time to time, implementations MUST be prepared to
    encounter any code point from 0x00000001 to 0x7fffffff.  Byte
    sequences that do not correspond to the valid encoding of a code
    point into UTF-8 charset or are outside this range are prohibited.
    The use of control codes SHOULD be avoided.  When it is necessary
    to represent a new line, the control code sequence CR LF SHOULD be
    used.
    The use of leading or trailing white space SHOULD be avoided.
    For code points not directly supported by user interface hardware
    or software, an alternative means of entry and display, such as
    hexadecimal, MAY be provided.
    For information encoded in 7-bit US-ASCII, the UTF-8 charset is
    identical to the US-ASCII charset.
    UTF-8 may require multiple bytes to represent a single character /
    code point; thus, the length of a UTF8String in octets may be
    different from the number of characters encoded.
    Note that the AVP Length field of an UTF8String is measured in
    octets not characters.

Fajardo, et al. Standards Track [Page 45] RFC 6733 Diameter Base Protocol October 2012

 DiameterIdentity
    The DiameterIdentity format is derived from the OctetString Basic
    AVP Format.
                      DiameterIdentity  = FQDN/Realm
 The DiameterIdentity value is used to uniquely identify either:
  • A Diameter node for purposes of duplicate connection and

routing loop detection.

  • A Realm to determine whether messages can be satisfied locally

or whether they must be routed or redirected.

    When a DiameterIdentity value is used to identify a Diameter node,
    the contents of the string MUST be the Fully Qualified Domain Name
    (FQDN) of the Diameter node.  If multiple Diameter nodes run on
    the same host, each Diameter node MUST be assigned a unique
    DiameterIdentity.  If a Diameter node can be identified by several
    FQDNs, a single FQDN should be picked at startup and used as the
    only DiameterIdentity for that node, whatever the connection on
    which it is sent.  In this document, note that DiameterIdentity is
    in ASCII form in order to be compatible with existing DNS
    infrastructure.  See Appendix D for interactions between the
    Diameter protocol and Internationalized Domain Names (IDNs).
 DiameterURI
    The DiameterURI MUST follow the Uniform Resource Identifiers (RFC
    3986) syntax [RFC3986] rules specified below:
    "aaa://" FQDN [ port ] [ transport ] [ protocol ]
                    ; No transport security
    "aaas://" FQDN [ port ] [ transport ] [ protocol ]
                    ; Transport security used
    FQDN               = < Fully Qualified Domain Name >

Fajardo, et al. Standards Track [Page 46] RFC 6733 Diameter Base Protocol October 2012

    port               = ":" 1*DIGIT
                    ; One of the ports used to listen for
                    ; incoming connections.
                    ; If absent, the default Diameter port
                    ; (3868) is assumed if no transport
                    ; security is used and port 5658 when
                    ; transport security (TLS/TCP and DTLS/SCTP)
                    ; is used.
    transport          = ";transport=" transport-protocol
                    ; One of the transports used to listen
                    ; for incoming connections.  If absent,
                    ; the default protocol is assumed to be TCP.
                    ; UDP MUST NOT be used when the aaa-protocol
                    ; field is set to diameter.
    transport-protocol = ( "tcp" / "sctp" / "udp" )
    protocol           = ";protocol=" aaa-protocol
                    ; If absent, the default AAA protocol
                    ; is Diameter.
    aaa-protocol       = ( "diameter" / "radius" / "tacacs+" )
    The following are examples of valid Diameter host identities:
    aaa://host.example.com;transport=tcp
    aaa://host.example.com:6666;transport=tcp
    aaa://host.example.com;protocol=diameter
    aaa://host.example.com:6666;protocol=diameter
    aaa://host.example.com:6666;transport=tcp;protocol=diameter
    aaa://host.example.com:1813;transport=udp;protocol=radius
 Enumerated
    The Enumerated format is derived from the Integer32 Basic AVP
    Format.  The definition contains a list of valid values and their
    interpretation and is described in the Diameter application
    introducing the AVP.

Fajardo, et al. Standards Track [Page 47] RFC 6733 Diameter Base Protocol October 2012

 IPFilterRule
    The IPFilterRule format is derived from the OctetString Basic AVP
    Format and uses the ASCII charset.  The rule syntax is a modified
    subset of ipfw(8) from FreeBSD.  Packets may be filtered based on
    the following information that is associated with it:
          Direction                          (in or out)
          Source and destination IP address  (possibly masked)
          Protocol
          Source and destination port        (lists or ranges)
          TCP flags
          IP fragment flag
          IP options
          ICMP types
 Rules for the appropriate direction are evaluated in order, with the
 first matched rule terminating the evaluation.  Each packet is
 evaluated once.  If no rule matches, the packet is dropped if the
 last rule evaluated was a permit, and passed if the last rule was a
 deny.
 IPFilterRule filters MUST follow the format:
       action dir proto from src to dst [options]
       action       permit - Allow packets that match the rule.
                    deny   - Drop packets that match the rule.
       dir          "in" is from the terminal, "out" is to the
                    terminal.
       proto        An IP protocol specified by number.  The "ip"
                    keyword means any protocol will match.
       src and dst  <address/mask> [ports]
                    The <address/mask> may be specified as:
                    ipno       An IPv4 or IPv6 number in dotted-
                               quad or canonical IPv6 form.  Only
                               this exact IP number will match the
                               rule.

Fajardo, et al. Standards Track [Page 48] RFC 6733 Diameter Base Protocol October 2012

                    ipno/bits  An IP number as above with a mask
                               width of the form 192.0.2.10/24.  In
                               this case, all IP numbers from
                               192.0.2.0 to 192.0.2.255 will match.
                               The bit width MUST be valid for the
                               IP version, and the IP number MUST
                               NOT have bits set beyond the mask.
                               For a match to occur, the same IP
                               version must be present in the
                               packet that was used in describing
                               the IP address.  To test for a
                               particular IP version, the bits part
                               can be set to zero.  The keyword
                               "any" is 0.0.0.0/0 or the IPv6
                               equivalent.  The keyword "assigned"
                               is the address or set of addresses
                               assigned to the terminal.  For IPv4,
                               a typical first rule is often "deny
                               in ip! assigned".
                    The sense of the match can be inverted by
                    preceding an address with the not modifier (!),
                    causing all other addresses to be matched
                    instead.  This does not affect the selection of
                    port numbers.
                    With the TCP, UDP, and SCTP protocols, optional
                    ports may be specified as:
                       {port/port-port}[,ports[,...]]
                     The '-' notation specifies a range of ports
                    (including boundaries).
                    Fragmented packets that have a non-zero offset
                    (i.e., not the first fragment) will never match
                    a rule that has one or more port
                    specifications.  See the frag option for
                    details on matching fragmented packets.
       options:
          frag    Match if the packet is a fragment and this is not
                  the first fragment of the datagram.  frag may not
                  be used in conjunction with either tcpflags or
                  TCP/UDP port specifications.

Fajardo, et al. Standards Track [Page 49] RFC 6733 Diameter Base Protocol October 2012

          ipoptions spec
                  Match if the IP header contains the comma-separated
                  list of options specified in spec.  The
                  supported IP options are:
                  ssrr (strict source route), lsrr (loose source
                  route), rr (record packet route), and ts
                  (timestamp).  The absence of a particular option
                  may be denoted with a '!'.
          tcpoptions spec
                  Match if the TCP header contains the comma-separated
                  list of options specified in spec.  The
                  supported TCP options are:
                  mss (maximum segment size), window (tcp window
                  advertisement), sack (selective ack), ts (rfc1323
                  timestamp), and cc (rfc1644 t/tcp connection
                  count).  The absence of a particular option may
                  be denoted with a '!'.
          established
                  TCP packets only.  Match packets that have the RST
                  or ACK bits set.
          setup   TCP packets only.  Match packets that have the SYN
                  bit set but no ACK bit.
          tcpflags spec
                  TCP packets only.  Match if the TCP header
                  contains the comma-separated list of flags
                  specified in spec.  The supported TCP flags are:
                  fin, syn, rst, psh, ack, and urg.  The absence of a
                  particular flag may be denoted with a '!'.  A rule
                  that contains a tcpflags specification can never
                  match a fragmented packet that has a non-zero
                  offset.  See the frag option for details on
                  matching fragmented packets.
          icmptypes types
                  ICMP packets only.  Match if the ICMP type is in
                  the list types.  The list may be specified as any
                  combination of ranges or individual types
                  separated by commas.  Both the numeric values and
                  the symbolic values listed below can be used.  The
                  supported ICMP types are:

Fajardo, et al. Standards Track [Page 50] RFC 6733 Diameter Base Protocol October 2012

                  echo reply (0), destination unreachable (3),
                  source quench (4), redirect (5), echo request
                  (8), router advertisement (9), router
                  solicitation (10), time-to-live exceeded (11), IP
                  header bad (12), timestamp request (13),
                  timestamp reply (14), information request (15),
                  information reply (16), address mask request (17),
                  and address mask reply (18).
 There is one kind of packet that the access device MUST always
 discard, that is an IP fragment with a fragment offset of one.  This
 is a valid packet, but it only has one use, to try to circumvent
 firewalls.
 An access device that is unable to interpret or apply a deny rule
 MUST terminate the session.  An access device that is unable to
 interpret or apply a permit rule MAY apply a more restrictive rule.
 An access device MAY apply deny rules of its own before the supplied
 rules, for example to protect the access device owner's
 infrastructure.

4.4. Grouped AVP Values

 The Diameter protocol allows AVP values of type 'Grouped'.  This
 implies that the Data field is actually a sequence of AVPs.  It is
 possible to include an AVP with a Grouped type within a Grouped type,
 that is, to nest them.  AVPs within an AVP of type Grouped have the
 same padding requirements as non-Grouped AVPs, as defined in
 Section 4.4.
 The AVP Code numbering space of all AVPs included in a Grouped AVP is
 the same as for non-Grouped AVPs.  Receivers of a Grouped AVP that
 does not have the 'M' (mandatory) bit set and one or more of the
 encapsulated AVPs within the group has the 'M' (mandatory) bit set
 MAY simply be ignored if the Grouped AVP itself is unrecognized.  The
 rule applies even if the encapsulated AVP with its 'M' (mandatory)
 bit set is further encapsulated within other sub-groups, i.e., other
 Grouped AVPs embedded within the Grouped AVP.
 Every Grouped AVP definition MUST include a corresponding grammar,
 using ABNF [RFC5234] (with modifications), as defined below.
       grouped-avp-def  = "<" name ">" "::=" avp
       name-fmt         = ALPHA *(ALPHA / DIGIT / "-")

Fajardo, et al. Standards Track [Page 51] RFC 6733 Diameter Base Protocol October 2012

       name             = name-fmt
                          ; The name has to be the name of an AVP,
                          ; defined in the base or extended Diameter
                          ; specifications.
       avp              = header *fixed *required *optional
       header           = "<" "AVP-Header:" avpcode [vendor] ">"
       avpcode          = 1*DIGIT
                          ; The AVP Code assigned to the Grouped AVP.
       vendor           = 1*DIGIT
                          ; The Vendor-ID assigned to the Grouped AVP.
                          ; If absent, the default value of zero is
                          ; used.

4.4.1. Example AVP with a Grouped Data Type

 The Example-AVP (AVP Code 999999) is of type Grouped and is used to
 clarify how Grouped AVP values work.  The Grouped Data field has the
 following CCF grammar:
       Example-AVP  ::= < AVP Header: 999999 >
                        { Origin-Host }
                      1*{ Session-Id }
                       *[ AVP ]
    An Example-AVP with Grouped Data follows.
    The Origin-Host AVP (Section 6.3) is required.  In this case:
       Origin-Host = "example.com".
    One or more Session-Ids must follow.  Here there are two:
       Session-Id =
         "grump.example.com:33041;23432;893;0AF3B81"
       Session-Id =
         "grump.example.com:33054;23561;2358;0AF3B82"

Fajardo, et al. Standards Track [Page 52] RFC 6733 Diameter Base Protocol October 2012

    optional AVPs included are
       Recovery-Policy = <binary>
          2163bc1d0ad82371f6bc09484133c3f09ad74a0dd5346d54195a7cf0b35
          2cabc881839a4fdcfbc1769e2677a4c1fb499284c5f70b48f58503a45c5
          c2d6943f82d5930f2b7c1da640f476f0e9c9572a50db8ea6e51e1c2c7bd
          f8bb43dc995144b8dbe297ac739493946803e1cee3e15d9b765008a1b2a
          cf4ac777c80041d72c01e691cf751dbf86e85f509f3988e5875dc905119
          26841f00f0e29a6d1ddc1a842289d440268681e052b30fb638045f7779c
          1d873c784f054f688f5001559ecff64865ef975f3e60d2fd7966b8c7f92
       Futuristic-Acct-Record = <binary>
          fe19da5802acd98b07a5b86cb4d5d03f0314ab9ef1ad0b67111ff3b90a0
          57fe29620bf3585fd2dd9fcc38ce62f6cc208c6163c008f4258d1bc88b8
          17694a74ccad3ec69269461b14b2e7a4c111fb239e33714da207983f58c
          41d018d56fe938f3cbf089aac12a912a2f0d1923a9390e5f789cb2e5067
          d3427475e49968f841
 The data for the optional AVPs is represented in hexadecimal form
 since the format of these AVPs is not known at the time of definition
 of the Example-AVP group nor (likely) at the time when the example
 instance of this AVP is interpreted -- except by Diameter
 implementations that support the same set of AVPs.  The encoding
 example illustrates how padding is used and how length fields are
 calculated.  Also, note that AVPs may be present in the Grouped AVP
 value that the receiver cannot interpret (here, the Recover-Policy
 and Futuristic-Acct-Record AVPs).  The length of the Example-AVP is
 the sum of all the length of the member AVPs, including their
 padding, plus the Example-AVP header size.

Fajardo, et al. Standards Track [Page 53] RFC 6733 Diameter Base Protocol October 2012

 This AVP would be encoded as follows:
       0       1       2       3       4       5       6       7
    +-------+-------+-------+-------+-------+-------+-------+-------+
 0  |     Example AVP Header (AVP Code = 999999), Length = 496      |
    +-------+-------+-------+-------+-------+-------+-------+-------+
 8  |     Origin-Host AVP Header (AVP Code = 264), Length = 19      |
    +-------+-------+-------+-------+-------+-------+-------+-------+
 16 |  'e'  |  'x'  |  'a'  |  'm'  |  'p'  |  'l'  |  'e'  |  '.'  |
    +-------+-------+-------+-------+-------+-------+-------+-------+
 24 |  'c'  |  'o'  |  'm'  |Padding|     Session-Id AVP Header     |
    +-------+-------+-------+-------+-------+-------+-------+-------+
 32 | (AVP Code = 263), Length = 49 |  'g'  |  'r'  |  'u'  |  'm'  |
    +-------+-------+-------+-------+-------+-------+-------+-------+
                                  . . .
    +-------+-------+-------+-------+-------+-------+-------+-------+
 72 |  'F'  |  '3'  |  'B'  |  '8'  |  '1'  |Padding|Padding|Padding|
    +-------+-------+-------+-------+-------+-------+-------+-------+
 80 |     Session-Id AVP Header (AVP Code = 263), Length = 50       |
    +-------+-------+-------+-------+-------+-------+-------+-------+
 88 |  'g'  |  'r'  |  'u'  |  'm'  |  'p'  |  '.'  |  'e'  |  'x'  |
    +-------+-------+-------+-------+-------+-------+-------+-------+
                                 . . .
    +-------+-------+-------+-------+-------+-------+-------+-------+
 120|  '5'  |  '8'  |  ';'  |  '0'  |  'A'  |  'F'  |  '3'  |  'B'  |
    +-------+-------+-------+-------+-------+-------+-------+-------+
 128|  '8'  |  '2'  |Padding|Padding|  Recovery-Policy Header (AVP  |
    +-------+-------+-------+-------+-------+-------+-------+-------+
 136|  Code = 8341), Length = 223   | 0x21  | 0x63  | 0xbc  | 0x1d  |
    +-------+-------+-------+-------+-------+-------+-------+-------+
 144|  0x0a | 0xd8  | 0x23  | 0x71  | 0xf6  | 0xbc  | 0x09  | 0x48  |
    +-------+-------+-------+-------+-------+-------+-------+-------+
                                  . . .
    +-------+-------+-------+-------+-------+-------+-------+-------+
 352|  0x8c | 0x7f  | 0x92  |Padding| Futuristic-Acct-Record Header |
    +-------+-------+-------+-------+-------+-------+-------+-------+
 328|(AVP Code = 15930),Length = 137| 0xfe  | 0x19  | 0xda  | 0x58  |
    +-------+-------+-------+-------+-------+-------+-------+-------+
 336|  0x02 | 0xac  | 0xd9  | 0x8b  | 0x07  | 0xa5  | 0xb8  | 0xc6  |
    +-------+-------+-------+-------+-------+-------+-------+-------+
                                  . . .
    +-------+-------+-------+-------+-------+-------+-------+-------+
 488|  0xe4 | 0x99  | 0x68  | 0xf8  | 0x41  |Padding|Padding|Padding|
    +-------+-------+-------+-------+-------+-------+-------+-------+

Fajardo, et al. Standards Track [Page 54] RFC 6733 Diameter Base Protocol October 2012

4.5. Diameter Base Protocol AVPs

 The following table describes the Diameter AVPs defined in the base
 protocol, their AVP Code values, types, and possible flag values.
 Due to space constraints, the short form DiamIdent is used to
 represent DiameterIdentity.

Fajardo, et al. Standards Track [Page 55] RFC 6733 Diameter Base Protocol October 2012

                                          +----------+
                                          | AVP Flag |
                                          |  rules   |
                                          |----+-----|
                 AVP  Section             |    |MUST |
 Attribute Name  Code Defined  Data Type  |MUST| NOT |
 -----------------------------------------|----+-----|
 Acct-             85  9.8.2   Unsigned32 | M  |  V  |
   Interim-Interval                       |    |     |
 Accounting-      483  9.8.7   Enumerated | M  |  V  |
   Realtime-Required                      |    |     |
 Acct-            50   9.8.5   UTF8String | M  |  V  |
   Multi-Session-Id                       |    |     |
 Accounting-      485  9.8.3   Unsigned32 | M  |  V  |
   Record-Number                          |    |     |
 Accounting-      480  9.8.1   Enumerated | M  |  V  |
   Record-Type                            |    |     |
 Acct-             44  9.8.4   OctetString| M  |  V  |
  Session-Id                              |    |     |
 Accounting-      287  9.8.6   Unsigned64 | M  |  V  |
   Sub-Session-Id                         |    |     |
 Acct-            259  6.9     Unsigned32 | M  |  V  |
   Application-Id                         |    |     |
 Auth-            258  6.8     Unsigned32 | M  |  V  |
   Application-Id                         |    |     |
 Auth-Request-    274  8.7     Enumerated | M  |  V  |
    Type                                  |    |     |
 Authorization-   291  8.9     Unsigned32 | M  |  V  |
   Lifetime                               |    |     |
 Auth-Grace-      276  8.10    Unsigned32 | M  |  V  |
   Period                                 |    |     |
 Auth-Session-    277  8.11    Enumerated | M  |  V  |
   State                                  |    |     |
 Re-Auth-Request- 285  8.12    Enumerated | M  |  V  |
   Type                                   |    |     |
 Class             25  8.20    OctetString| M  |  V  |
 Destination-Host 293  6.5     DiamIdent  | M  |  V  |
 Destination-     283  6.6     DiamIdent  | M  |  V  |
   Realm                                  |    |     |
 Disconnect-Cause 273  5.4.3   Enumerated | M  |  V  |
 Error-Message    281  7.3     UTF8String |    | V,M |
 Error-Reporting- 294  7.4     DiamIdent  |    | V,M |
   Host                                   |    |     |
 Event-Timestamp   55  8.21    Time       | M  |  V  |
 Experimental-    297  7.6     Grouped    | M  |  V  |
    Result                                |    |     |
 -----------------------------------------|----+-----|

Fajardo, et al. Standards Track [Page 56] RFC 6733 Diameter Base Protocol October 2012

                                          +----------+
                                          | AVP Flag |
                                          |  rules   |
                                          |----+-----|
                 AVP  Section             |    |MUST |
 Attribute Name  Code Defined  Data Type  |MUST| NOT |
 -----------------------------------------|----+-----|
 Experimental-    298  7.7     Unsigned32 | M  |  V  |
    Result-Code                           |    |     |
 Failed-AVP       279  7.5     Grouped    | M  |  V  |
 Firmware-        267  5.3.4   Unsigned32 |    | V,M |
   Revision                               |    |     |
 Host-IP-Address  257  5.3.5   Address    | M  |  V  |
 Inband-Security                          | M  |  V  |
    -Id           299  6.10    Unsigned32 |    |     |
 Multi-Round-     272  8.19    Unsigned32 | M  |  V  |
   Time-Out                               |    |     |
 Origin-Host      264  6.3     DiamIdent  | M  |  V  |
 Origin-Realm     296  6.4     DiamIdent  | M  |  V  |
 Origin-State-Id  278  8.16    Unsigned32 | M  |  V  |
 Product-Name     269  5.3.7   UTF8String |    | V,M |
 Proxy-Host       280  6.7.3   DiamIdent  | M  |  V  |
 Proxy-Info       284  6.7.2   Grouped    | M  |  V  |
 Proxy-State       33  6.7.4   OctetString| M  |  V  |
 Redirect-Host    292  6.12    DiamURI    | M  |  V  |
 Redirect-Host-   261  6.13    Enumerated | M  |  V  |
    Usage                                 |    |     |
 Redirect-Max-    262  6.14    Unsigned32 | M  |  V  |
    Cache-Time                            |    |     |
 Result-Code      268  7.1     Unsigned32 | M  |  V  |
 Route-Record     282  6.7.1   DiamIdent  | M  |  V  |
 Session-Id       263  8.8     UTF8String | M  |  V  |
 Session-Timeout   27  8.13    Unsigned32 | M  |  V  |
 Session-Binding  270  8.17    Unsigned32 | M  |  V  |
 Session-Server-  271  8.18    Enumerated | M  |  V  |
   Failover                               |    |     |
 Supported-       265  5.3.6   Unsigned32 | M  |  V  |
   Vendor-Id                              |    |     |
 Termination-     295  8.15    Enumerated | M  |  V  |
    Cause                                 |    |     |
 User-Name          1  8.14    UTF8String | M  |  V  |
 Vendor-Id        266  5.3.3   Unsigned32 | M  |  V  |
 Vendor-Specific- 260  6.11    Grouped    | M  |  V  |
    Application-Id                        |    |     |
 -----------------------------------------|----+-----|

Fajardo, et al. Standards Track [Page 57] RFC 6733 Diameter Base Protocol October 2012

5. Diameter Peers

 This section describes how Diameter nodes establish connections and
 communicate with peers.

5.1. Peer Connections

 Connections between diameter peers are established using their valid
 DiameterIdentity.  A Diameter node initiating a connection to a peer
 MUST know the peer's DiameterIdentity.  Methods for discovering a
 Diameter peer can be found in Section 5.2.
 Although a Diameter node may have many possible peers with which it
 is able to communicate, it may not be economical to have an
 established connection to all of them.  At a minimum, a Diameter node
 SHOULD have an established connection with two peers per realm, known
 as the primary and secondary peers.  Of course, a node MAY have
 additional connections, if it is deemed necessary.  Typically, all
 messages for a realm are sent to the primary peer but, in the event
 that failover procedures are invoked, any pending requests are sent
 to the secondary peer.  However, implementations are free to load
 balance requests between a set of peers.
 Note that a given peer MAY act as a primary for a given realm while
 acting as a secondary for another realm.
 When a peer is deemed suspect, which could occur for various reasons,
 including not receiving a DWA within an allotted time frame, no new
 requests should be forwarded to the peer, but failover procedures are
 invoked.  When an active peer is moved to this mode, additional
 connections SHOULD be established to ensure that the necessary number
 of active connections exists.
 There are two ways that a peer is removed from the suspect peer list:
 1.  The peer is no longer reachable, causing the transport connection
     to be shut down.  The peer is moved to the closed state.
 2.  Three watchdog messages are exchanged with accepted round-trip
     times, and the connection to the peer is considered stabilized.
 In the event the peer being removed is either the primary or
 secondary, an alternate peer SHOULD replace the deleted peer and
 assume the role of either primary or secondary.

Fajardo, et al. Standards Track [Page 58] RFC 6733 Diameter Base Protocol October 2012

5.2. Diameter Peer Discovery

 Allowing for dynamic Diameter agent discovery makes possible simpler
 and more robust deployment of Diameter services.  In order to promote
 interoperable implementations of Diameter peer discovery, the
 following mechanisms (manual configuration and DNS) are described.
 These are based on existing IETF standards.  Both mechanisms MUST be
 supported by all Diameter implementations; either MAY be used.
 There are two cases where Diameter peer discovery may be performed.
 The first is when a Diameter client needs to discover a first-hop
 Diameter agent.  The second case is when a Diameter agent needs to
 discover another agent for further handling of a Diameter operation.
 In both cases, the following 'search order' is recommended:
 1.  The Diameter implementation consults its list of statically
     (manually) configured Diameter agent locations.  These will be
     used if they exist and respond.
 2.  The Diameter implementation performs a NAPTR query for a server
     in a particular realm.  The Diameter implementation has to know,
     in advance, in which realm to look for a Diameter agent.  This
     could be deduced, for example, from the 'realm' in an NAI on
     which a Diameter implementation needed to perform a Diameter
     operation.
     The NAPTR usage in Diameter follows the S-NAPTR DDDS application
     [RFC3958] in which the SERVICE field includes tags for the
     desired application and supported application protocol.  The
     application service tag for a Diameter application is 'aaa' and
     the supported application protocol tags are 'diameter.tcp',
     'diameter.sctp', 'diameter.dtls', or 'diameter.tls.tcp'
     [RFC6408].
     The client can follow the resolution process defined by the
     S-NAPTR DDDS [RFC3958] application to find a matching SRV, A, or
     AAAA record of a suitable peer.  The domain suffixes in the NAPTR
     replacement field SHOULD match the domain of the original query.
     An example can be found in Appendix B.
 3.  If no NAPTR records are found, the requester directly queries for
     one of the following SRV records: for Diameter over TCP, use
     "_diameter._tcp.realm"; for Diameter over TLS, use
     "_diameters._tcp.realm"; for Diameter over SCTP, use
     "_diameter._sctp.realm"; for Diameter over DTLS, use
     "_diameters._sctp.realm".  If SRV records are found, then the
     requester can perform address record query (A RR's and/or AAAA

Fajardo, et al. Standards Track [Page 59] RFC 6733 Diameter Base Protocol October 2012

     RR's) for the target hostname specified in the SRV records
     following the rules given in [RFC2782].  If no SRV records are
     found, the requester gives up.
 If the server is using a site certificate, the domain name in the
 NAPTR query and the domain name in the replacement field MUST both be
 valid based on the site certificate handed out by the server in the
 TLS/TCP and DTLS/SCTP or Internet Key Exchange Protocol (IKE)
 exchange.  Similarly, the domain name in the SRV query and the domain
 name in the target in the SRV record MUST both be valid based on the
 same site certificate.  Otherwise, an attacker could modify the DNS
 records to contain replacement values in a different domain, and the
 client could not validate whether this was the desired behavior or
 the result of an attack.
 Also, the Diameter peer MUST check to make sure that the discovered
 peers are authorized to act in its role.  Authentication via IKE or
 TLS/TCP and DTLS/SCTP, or validation of DNS RRs via DNSSEC is not
 sufficient to conclude this.  For example, a web server may have
 obtained a valid TLS/TCP and DTLS/SCTP certificate, and secured RRs
 may be included in the DNS, but this does not imply that it is
 authorized to act as a Diameter server.
 Authorization can be achieved, for example, by the configuration of a
 Diameter server Certification Authority (CA).  The server CA issues a
 certificate to the Diameter server, which includes an Object
 Identifier (OID) to indicate the subject is a Diameter server in the
 Extended Key Usage extension [RFC5280].  This certificate is then
 used during TLS/TCP, DTLS/SCTP, or IKE security negotiation.
 However, note that, at the time of writing, no Diameter server
 Certification Authorities exist.
 A dynamically discovered peer causes an entry in the peer table (see
 Section 2.6) to be created.  Note that entries created via DNS MUST
 expire (or be refreshed) within the DNS Time to Live (TTL).  If a
 peer is discovered outside of the local realm, a routing table entry
 (see Section 2.7) for the peer's realm is created.  The routing table
 entry's expiration MUST match the peer's expiration value.

5.3. Capabilities Exchange

 When two Diameter peers establish a transport connection, they MUST
 exchange the Capabilities Exchange messages, as specified in the peer
 state machine (see Section 5.6).  This message allows the discovery
 of a peer's identity and its capabilities (protocol version number,
 the identifiers of supported Diameter applications, security
 mechanisms, etc.).

Fajardo, et al. Standards Track [Page 60] RFC 6733 Diameter Base Protocol October 2012

 The receiver only issues commands to its peers that have advertised
 support for the Diameter application that defines the command.  A
 Diameter node MUST cache the supported Application Ids in order to
 ensure that unrecognized commands and/or AVPs are not unnecessarily
 sent to a peer.
 A receiver of a Capabilities-Exchange-Request (CER) message that does
 not have any applications in common with the sender MUST return a
 Capabilities-Exchange-Answer (CEA) with the Result-Code AVP set to
 DIAMETER_NO_COMMON_APPLICATION and SHOULD disconnect the transport
 layer connection.  Note that receiving a CER or CEA from a peer
 advertising itself as a relay (see Section 2.4) MUST be interpreted
 as having common applications with the peer.
 The receiver of the Capabilities-Exchange-Request (CER) MUST
 determine common applications by computing the intersection of its
 own set of supported Application Ids against all of the
 Application-Id AVPs (Auth-Application-Id, Acct-Application-Id, and
 Vendor-Specific-Application-Id) present in the CER.  The value of the
 Vendor-Id AVP in the Vendor-Specific-Application-Id MUST NOT be used
 during computation.  The sender of the Capabilities-Exchange-Answer
 (CEA) SHOULD include all of its supported applications as a hint to
 the receiver regarding all of its application capabilities.
 Diameter implementations SHOULD first attempt to establish a TLS/TCP
 and DTLS/SCTP connection prior to the CER/CEA exchange.  This
 protects the capabilities information of both peers.  To support
 older Diameter implementations that do not fully conform to this
 document, the transport security MAY still be negotiated via an
 Inband-Security AVP.  In this case, the receiver of a Capabilities-
 Exchange-Request (CER) message that does not have any security
 mechanisms in common with the sender MUST return a Capabilities-
 Exchange-Answer (CEA) with the Result-Code AVP set to
 DIAMETER_NO_COMMON_SECURITY and SHOULD disconnect the transport layer
 connection.
 CERs received from unknown peers MAY be silently discarded, or a CEA
 MAY be issued with the Result-Code AVP set to DIAMETER_UNKNOWN_PEER.
 In both cases, the transport connection is closed.  If the local
 policy permits receiving CERs from unknown hosts, a successful CEA
 MAY be returned.  If a CER from an unknown peer is answered with a
 successful CEA, the lifetime of the peer entry is equal to the
 lifetime of the transport connection.  In case of a transport
 failure, all the pending transactions destined to the unknown peer
 can be discarded.
 The CER and CEA messages MUST NOT be proxied, redirected, or relayed.

Fajardo, et al. Standards Track [Page 61] RFC 6733 Diameter Base Protocol October 2012

 Since the CER/CEA messages cannot be proxied, it is still possible
 that an upstream agent will receive a message for which it has no
 available peers to handle the application that corresponds to the
 Command Code.  In such instances, the 'E' bit is set in the answer
 message (Section 7) with the Result-Code AVP set to
 DIAMETER_UNABLE_TO_DELIVER to inform the downstream agent to take
 action (e.g., re-routing request to an alternate peer).
 With the exception of the Capabilities-Exchange-Request message, a
 message of type Request that includes the Auth-Application-Id or
 Acct-Application-Id AVPs, or a message with an application-specific
 Command Code MAY only be forwarded to a host that has explicitly
 advertised support for the application (or has advertised the Relay
 Application Id).

5.3.1. Capabilities-Exchange-Request

 The Capabilities-Exchange-Request (CER), indicated by the Command
 Code set to 257 and the Command Flags' 'R' bit set, is sent to
 exchange local capabilities.  Upon detection of a transport failure,
 this message MUST NOT be sent to an alternate peer.
 When Diameter is run over SCTP [RFC4960] or DTLS/SCTP [RFC6083],
 which allow for connections to span multiple interfaces and multiple
 IP addresses, the Capabilities-Exchange-Request message MUST contain
 one Host-IP-Address AVP for each potential IP address that MAY be
 locally used when transmitting Diameter messages.
    Message Format
       <CER> ::= < Diameter Header: 257, REQ >
                 { Origin-Host }
                 { Origin-Realm }
              1* { Host-IP-Address }
                 { Vendor-Id }
                 { Product-Name }
                 [ Origin-State-Id ]
               * [ Supported-Vendor-Id ]
               * [ Auth-Application-Id ]
               * [ Inband-Security-Id ]
               * [ Acct-Application-Id ]
               * [ Vendor-Specific-Application-Id ]
                 [ Firmware-Revision ]
               * [ AVP ]

Fajardo, et al. Standards Track [Page 62] RFC 6733 Diameter Base Protocol October 2012

5.3.2. Capabilities-Exchange-Answer

 The Capabilities-Exchange-Answer (CEA), indicated by the Command Code
 set to 257 and the Command Flags' 'R' bit cleared, is sent in
 response to a CER message.
 When Diameter is run over SCTP [RFC4960] or DTLS/SCTP [RFC6083],
 which allow connections to span multiple interfaces, hence, multiple
 IP addresses, the Capabilities-Exchange-Answer message MUST contain
 one Host-IP-Address AVP for each potential IP address that MAY be
 locally used when transmitting Diameter messages.
 Message Format
       <CEA> ::= < Diameter Header: 257 >
                 { Result-Code }
                 { Origin-Host }
                 { Origin-Realm }
              1* { Host-IP-Address }
                 { Vendor-Id }
                 { Product-Name }
                 [ Origin-State-Id ]
                 [ Error-Message ]
                 [ Failed-AVP ]
               * [ Supported-Vendor-Id ]
               * [ Auth-Application-Id ]
               * [ Inband-Security-Id ]
               * [ Acct-Application-Id ]
               * [ Vendor-Specific-Application-Id ]
                 [ Firmware-Revision ]
               * [ AVP ]

5.3.3. Vendor-Id AVP

 The Vendor-Id AVP (AVP Code 266) is of type Unsigned32 and contains
 the IANA "SMI Network Management Private Enterprise Codes"
 [ENTERPRISE] value assigned to the Diameter Software vendor.  It is
 envisioned that the combination of the Vendor-Id, Product-Name
 (Section 5.3.7), and Firmware-Revision (Section 5.3.4) AVPs may
 provide useful debugging information.
 A Vendor-Id value of zero in the CER or CEA message is reserved and
 indicates that this field is ignored.

Fajardo, et al. Standards Track [Page 63] RFC 6733 Diameter Base Protocol October 2012

5.3.4. Firmware-Revision AVP

 The Firmware-Revision AVP (AVP Code 267) is of type Unsigned32 and is
 used to inform a Diameter peer of the firmware revision of the
 issuing device.
 For devices that do not have a firmware revision (general-purpose
 computers running Diameter software modules, for instance), the
 revision of the Diameter software module may be reported instead.

5.3.5. Host-IP-Address AVP

 The Host-IP-Address AVP (AVP Code 257) is of type Address and is used
 to inform a Diameter peer of the sender's IP address.  All source
 addresses that a Diameter node expects to use with SCTP [RFC4960] or
 DTLS/SCTP [RFC6083] MUST be advertised in the CER and CEA messages by
 including a Host-IP-Address AVP for each address.

5.3.6. Supported-Vendor-Id AVP

 The Supported-Vendor-Id AVP (AVP Code 265) is of type Unsigned32 and
 contains the IANA "SMI Network Management Private Enterprise Codes"
 [ENTERPRISE] value assigned to a vendor other than the device vendor
 but including the application vendor.  This is used in the CER and
 CEA messages in order to inform the peer that the sender supports (a
 subset of) the Vendor-Specific AVPs defined by the vendor identified
 in this AVP.  The value of this AVP MUST NOT be set to zero.
 Multiple instances of this AVP containing the same value SHOULD NOT
 be sent.

5.3.7. Product-Name AVP

 The Product-Name AVP (AVP Code 269) is of type UTF8String and
 contains the vendor-assigned name for the product.  The Product-Name
 AVP SHOULD remain constant across firmware revisions for the same
 product.

5.4. Disconnecting Peer Connections

 When a Diameter node disconnects one of its transport connections,
 its peer cannot know the reason for the disconnect and will most
 likely assume that a connectivity problem occurred or that the peer
 has rebooted.  In these cases, the peer may periodically attempt to
 reconnect, as stated in Section 2.1.  In the event that the
 disconnect was a result of either a shortage of internal resources or
 simply that the node in question has no intentions of forwarding any
 Diameter messages to the peer in the foreseeable future, a periodic

Fajardo, et al. Standards Track [Page 64] RFC 6733 Diameter Base Protocol October 2012

 connection request would not be welcomed.  The Disconnection-Reason
 AVP contains the reason the Diameter node issued the Disconnect-Peer-
 Request message.
 The Disconnect-Peer-Request message is used by a Diameter node to
 inform its peer of its intent to disconnect the transport layer and
 that the peer shouldn't reconnect unless it has a valid reason to do
 so (e.g., message to be forwarded).  Upon receipt of the message, the
 Disconnect-Peer-Answer message is returned, which SHOULD contain an
 error if messages have recently been forwarded, and are likely in
 flight, which would otherwise cause a race condition.
 The receiver of the Disconnect-Peer-Answer message initiates the
 transport disconnect.  The sender of the Disconnect-Peer-Answer
 message should be able to detect the transport closure and clean up
 the connection.

5.4.1. Disconnect-Peer-Request

 The Disconnect-Peer-Request (DPR), indicated by the Command Code set
 to 282 and the Command Flags' 'R' bit set, is sent to a peer to
 inform it of its intentions to shut down the transport connection.
 Upon detection of a transport failure, this message MUST NOT be sent
 to an alternate peer.
    Message Format
       <DPR>  ::= < Diameter Header: 282, REQ >
                  { Origin-Host }
                  { Origin-Realm }
                  { Disconnect-Cause }
                * [ AVP ]

5.4.2. Disconnect-Peer-Answer

 The Disconnect-Peer-Answer (DPA), indicated by the Command Code set
 to 282 and the Command Flags' 'R' bit cleared, is sent as a response
 to the Disconnect-Peer-Request message.  Upon receipt of this
 message, the transport connection is shut down.

Fajardo, et al. Standards Track [Page 65] RFC 6733 Diameter Base Protocol October 2012

    Message Format
       <DPA>  ::= < Diameter Header: 282 >
                  { Result-Code }
                  { Origin-Host }
                  { Origin-Realm }
                  [ Error-Message ]
                  [ Failed-AVP ]
                * [ AVP ]

5.4.3. Disconnect-Cause AVP

 The Disconnect-Cause AVP (AVP Code 273) is of type Enumerated.  A
 Diameter node MUST include this AVP in the Disconnect-Peer-Request
 message to inform the peer of the reason for its intention to shut
 down the transport connection.  The following values are supported:
    REBOOTING                         0
       A scheduled reboot is imminent.  A receiver of a DPR with
       above result code MAY attempt reconnection.
    BUSY                              1
       The peer's internal resources are constrained, and it has
       determined that the transport connection needs to be closed.
       A receiver of a DPR with above result code SHOULD NOT attempt
       reconnection.
    DO_NOT_WANT_TO_TALK_TO_YOU        2
       The peer has determined that it does not see a need for the
       transport connection to exist, since it does not expect any
       messages to be exchanged in the near future.  A receiver of a
       DPR with above result code SHOULD NOT attempt reconnection.

5.5. Transport Failure Detection

 Given the nature of the Diameter protocol, it is recommended that
 transport failures be detected as soon as possible.  Detecting such
 failures will minimize the occurrence of messages sent to unavailable
 agents, resulting in unnecessary delays, and will provide better
 failover performance.  The Device-Watchdog-Request and Device-
 Watchdog-Answer messages, defined in this section, are used to pro-
 actively detect transport failures.

Fajardo, et al. Standards Track [Page 66] RFC 6733 Diameter Base Protocol October 2012

5.5.1. Device-Watchdog-Request

 The Device-Watchdog-Request (DWR), indicated by the Command Code set
 to 280 and the Command Flags' 'R' bit set, is sent to a peer when no
 traffic has been exchanged between two peers (see Section 5.5.3).
 Upon detection of a transport failure, this message MUST NOT be sent
 to an alternate peer.
    Message Format
       <DWR>  ::= < Diameter Header: 280, REQ >
                  { Origin-Host }
                  { Origin-Realm }
                  [ Origin-State-Id ]
                * [ AVP ]

5.5.2. Device-Watchdog-Answer

 The Device-Watchdog-Answer (DWA), indicated by the Command Code set
 to 280 and the Command Flags' 'R' bit cleared, is sent as a response
 to the Device-Watchdog-Request message.
    Message Format
       <DWA>  ::= < Diameter Header: 280 >
                  { Result-Code }
                  { Origin-Host }
                  { Origin-Realm }
                  [ Error-Message ]
                  [ Failed-AVP ]
                  [ Origin-State-Id ]
                * [ AVP ]

5.5.3. Transport Failure Algorithm

 The transport failure algorithm is defined in [RFC3539].  All
 Diameter implementations MUST support the algorithm defined in that
 specification in order to be compliant to the Diameter base protocol.

5.5.4. Failover and Failback Procedures

 In the event that a transport failure is detected with a peer, it is
 necessary for all pending request messages to be forwarded to an
 alternate agent, if possible.  This is commonly referred to as
 "failover".

Fajardo, et al. Standards Track [Page 67] RFC 6733 Diameter Base Protocol October 2012

 In order for a Diameter node to perform failover procedures, it is
 necessary for the node to maintain a pending message queue for a
 given peer.  When an answer message is received, the corresponding
 request is removed from the queue.  The Hop-by-Hop Identifier field
 is used to match the answer with the queued request.
 When a transport failure is detected, if possible, all messages in
 the queue are sent to an alternate agent with the T flag set.  On
 booting a Diameter client or agent, the T flag is also set on any
 remaining records in non-volatile storage that are still waiting to
 be transmitted.  An example of a case where it is not possible to
 forward the message to an alternate server is when the message has a
 fixed destination, and the unavailable peer is the message's final
 destination (see Destination-Host AVP).  Such an error requires that
 the agent return an answer message with the 'E' bit set and the
 Result-Code AVP set to DIAMETER_UNABLE_TO_DELIVER.
 It is important to note that multiple identical requests or answers
 MAY be received as a result of a failover.  The End-to-End Identifier
 field in the Diameter header along with the Origin-Host AVP MUST be
 used to identify duplicate messages.
 As described in Section 2.1, a connection request should be
 periodically attempted with the failed peer in order to re-establish
 the transport connection.  Once a connection has been successfully
 established, messages can once again be forwarded to the peer.  This
 is commonly referred to as "failback".

5.6. Peer State Machine

 This section contains a finite state machine that MUST be observed by
 all Diameter implementations.  Each Diameter node MUST follow the
 state machine described below when communicating with each peer.
 Multiple actions are separated by commas, and may continue on
 succeeding lines, as space requires.  Similarly, state and next state
 may also span multiple lines, as space requires.
 This state machine is closely coupled with the state machine
 described in [RFC3539], which is used to open, close, failover,
 probe, and reopen transport connections.  In particular, note that
 [RFC3539] requires the use of watchdog messages to probe connections.
 For Diameter, DWR and DWA messages are to be used.
 The I- prefix is used to represent the initiator (connecting)
 connection, while the R- prefix is used to represent the responder
 (listening) connection.  The lack of a prefix indicates that the
 event or action is the same regardless of the connection on which the
 event occurred.

Fajardo, et al. Standards Track [Page 68] RFC 6733 Diameter Base Protocol October 2012

 The stable states that a state machine may be in are Closed, I-Open,
 and R-Open; all other states are intermediate.  Note that I-Open and
 R-Open are equivalent except for whether the initiator or responder
 transport connection is used for communication.
 A CER message is always sent on the initiating connection immediately
 after the connection request is successfully completed.  In the case
 of an election, one of the two connections will shut down.  The
 responder connection will survive if the Origin-Host of the local
 Diameter entity is higher than that of the peer; the initiator
 connection will survive if the peer's Origin-Host is higher.  All
 subsequent messages are sent on the surviving connection.  Note that
 the results of an election on one peer are guaranteed to be the
 inverse of the results on the other.
 For TLS/TCP and DTLS/SCTP usage, a TLS/TCP and DTLS/SCTP handshake
 SHOULD begin when both ends are in the closed state prior to any
 Diameter message exchanges.  The TLS/TCP and DTLS/SCTP connection
 SHOULD be established before sending any CER or CEA message to secure
 and protect the capabilities information of both peers.  The TLS/TCP
 and DTLS/SCTP connection SHOULD be disconnected when the state
 machine moves to the closed state.  When connecting to responders
 that do not conform to this document (i.e., older Diameter
 implementations that are not prepared to received TLS/TCP and DTLS/
 SCTP connections in the closed state), the initial TLS/TCP and DTLS/
 SCTP connection attempt will fail.  The initiator MAY then attempt to
 connect via TCP or SCTP and initiate the TLS/TCP and DTLS/SCTP
 handshake when both ends are in the open state.  If the handshake is
 successful, all further messages will be sent via TLS/TCP and DTLS/
 SCTP.  If the handshake fails, both ends move to the closed state.
 The state machine constrains only the behavior of a Diameter
 implementation as seen by Diameter peers through events on the wire.
 Any implementation that produces equivalent results is considered
 compliant.

Fajardo, et al. Standards Track [Page 69] RFC 6733 Diameter Base Protocol October 2012

    state            event              action         next state
    -----------------------------------------------------------------
    Closed           Start            I-Snd-Conn-Req   Wait-Conn-Ack
                     R-Conn-CER       R-Accept,        R-Open
                                      Process-CER,
                                      R-Snd-CEA
    Wait-Conn-Ack    I-Rcv-Conn-Ack   I-Snd-CER        Wait-I-CEA
                     I-Rcv-Conn-Nack  Cleanup          Closed
                     R-Conn-CER       R-Accept,        Wait-Conn-Ack/
                                      Process-CER      Elect
                     Timeout          Error            Closed
    Wait-I-CEA       I-Rcv-CEA        Process-CEA      I-Open
                     R-Conn-CER       R-Accept,        Wait-Returns
                                      Process-CER,
                                      Elect
                     I-Peer-Disc      I-Disc           Closed
                     I-Rcv-Non-CEA    Error            Closed
                     Timeout          Error            Closed
    Wait-Conn-Ack/   I-Rcv-Conn-Ack   I-Snd-CER,Elect  Wait-Returns
    Elect            I-Rcv-Conn-Nack  R-Snd-CEA        R-Open
                     R-Peer-Disc      R-Disc           Wait-Conn-Ack
                     R-Conn-CER       R-Reject         Wait-Conn-Ack/
                                                       Elect
                     Timeout          Error            Closed
    Wait-Returns     Win-Election     I-Disc,R-Snd-CEA R-Open
                     I-Peer-Disc      I-Disc,          R-Open
                                      R-Snd-CEA
                     I-Rcv-CEA        R-Disc           I-Open
                     R-Peer-Disc      R-Disc           Wait-I-CEA
                     R-Conn-CER       R-Reject         Wait-Returns
                     Timeout          Error            Closed
    R-Open           Send-Message     R-Snd-Message    R-Open
                     R-Rcv-Message    Process          R-Open
                     R-Rcv-DWR        Process-DWR,     R-Open
                                      R-Snd-DWA
                     R-Rcv-DWA        Process-DWA      R-Open
                     R-Conn-CER       R-Reject         R-Open
                     Stop             R-Snd-DPR        Closing
                     R-Rcv-DPR        R-Snd-DPA        Closing
                     R-Peer-Disc      R-Disc           Closed

Fajardo, et al. Standards Track [Page 70] RFC 6733 Diameter Base Protocol October 2012

    I-Open           Send-Message     I-Snd-Message    I-Open
                     I-Rcv-Message    Process          I-Open
                     I-Rcv-DWR        Process-DWR,     I-Open
                                      I-Snd-DWA
                     I-Rcv-DWA        Process-DWA      I-Open
                     R-Conn-CER       R-Reject         I-Open
                     Stop             I-Snd-DPR        Closing
                     I-Rcv-DPR        I-Snd-DPA        Closing
                     I-Peer-Disc      I-Disc           Closed
    Closing          I-Rcv-DPA        I-Disc           Closed
                     R-Rcv-DPA        R-Disc           Closed
                     Timeout          Error            Closed
                     I-Peer-Disc      I-Disc           Closed
                     R-Peer-Disc      R-Disc           Closed

5.6.1. Incoming Connections

 When a connection request is received from a Diameter peer, it is
 not, in the general case, possible to know the identity of that peer
 until a CER is received from it.  This is because host and port
 determine the identity of a Diameter peer; the source port of an
 incoming connection is arbitrary.  Upon receipt of a CER, the
 identity of the connecting peer can be uniquely determined from the
 Origin-Host.
 For this reason, a Diameter peer must employ logic separate from the
 state machine to receive connection requests, accept them, and await
 the CER.  Once the CER arrives on a new connection, the Origin-Host
 that identifies the peer is used to locate the state machine
 associated with that peer, and the new connection and CER are passed
 to the state machine as an R-Conn-CER event.
 The logic that handles incoming connections SHOULD close and discard
 the connection if any message other than a CER arrives or if an
 implementation-defined timeout occurs prior to receipt of CER.
 Because handling of incoming connections up to and including receipt
 of a CER requires logic, separate from that of any individual state
 machine associated with a particular peer, it is described separately
 in this section rather than in the state machine above.

5.6.2. Events

 Transitions and actions in the automaton are caused by events.  In
 this section, we will ignore the I- and R- prefixes, since the actual
 event would be identical, but it would occur on one of two possible
 connections.

Fajardo, et al. Standards Track [Page 71] RFC 6733 Diameter Base Protocol October 2012

 Start          The Diameter application has signaled that a
                connection should be initiated with the peer.
 R-Conn-CER     An acknowledgement is received stating that the
                transport connection has been established, and the
                associated CER has arrived.
 Rcv-Conn-Ack   A positive acknowledgement is received confirming that
                the transport connection is established.
 Rcv-Conn-Nack  A negative acknowledgement was received stating that
                the transport connection was not established.
 Timeout        An application-defined timer has expired while waiting
                for some event.
 Rcv-CER        A CER message from the peer was received.
 Rcv-CEA        A CEA message from the peer was received.
 Rcv-Non-CEA    A message, other than a CEA, from the peer was
                received.
 Peer-Disc      A disconnection indication from the peer was received.
 Rcv-DPR        A DPR message from the peer was received.
 Rcv-DPA        A DPA message from the peer was received.
 Win-Election   An election was held, and the local node was the
                winner.
 Send-Message   A message is to be sent.
 Rcv-Message    A message other than CER, CEA, DPR, DPA, DWR, or DWA
                was received.
 Stop           The Diameter application has signaled that a
                connection should be terminated (e.g., on system
                shutdown).

5.6.3. Actions

 Actions in the automaton are caused by events and typically indicate
 the transmission of packets and/or an action to be taken on the
 connection.  In this section, we will ignore the I- and R- prefixes,
 since the actual action would be identical, but it would occur on one
 of two possible connections.

Fajardo, et al. Standards Track [Page 72] RFC 6733 Diameter Base Protocol October 2012

 Snd-Conn-Req   A transport connection is initiated with the peer.
 Accept         The incoming connection associated with the R-Conn-CER
                is accepted as the responder connection.
 Reject         The incoming connection associated with the R-Conn-CER
                is disconnected.
 Process-CER    The CER associated with the R-Conn-CER is processed.
 Snd-CER        A CER message is sent to the peer.
 Snd-CEA        A CEA message is sent to the peer.
 Cleanup        If necessary, the connection is shut down, and any
                local resources are freed.
 Error          The transport layer connection is disconnected,
                either politely or abortively, in response to
                an error condition.  Local resources are freed.
 Process-CEA    A received CEA is processed.
 Snd-DPR        A DPR message is sent to the peer.
 Snd-DPA        A DPA message is sent to the peer.
 Disc           The transport layer connection is disconnected,
                and local resources are freed.
 Elect          An election occurs (see Section 5.6.4 for more
                information).
 Snd-Message    A message is sent.
 Snd-DWR        A DWR message is sent.
 Snd-DWA        A DWA message is sent.
 Process-DWR    The DWR message is serviced.
 Process-DWA    The DWA message is serviced.
 Process        A message is serviced.

Fajardo, et al. Standards Track [Page 73] RFC 6733 Diameter Base Protocol October 2012

5.6.4. The Election Process

 The election is performed on the responder.  The responder compares
 the Origin-Host received in the CER with its own Origin-Host as two
 streams of octets.  If the local Origin-Host lexicographically
 succeeds the received Origin-Host, a Win-Election event is issued
 locally.  Diameter identities are in ASCII form; therefore, the
 lexical comparison is consistent with DNS case insensitivity, where
 octets that fall in the ASCII range 'a' through 'z' MUST compare
 equally to their uppercase counterparts between 'A' and 'Z'.  See
 Appendix D for interactions between the Diameter protocol and
 Internationalized Domain Name (IDNs).
 The winner of the election MUST close the connection it initiated.
 Historically, maintaining the responder side of a connection was more
 efficient than maintaining the initiator side.  However, current
 practices makes this distinction irrelevant.

6. Diameter Message Processing

 This section describes how Diameter requests and answers are created
 and processed.

6.1. Diameter Request Routing Overview

 A request is sent towards its final destination using one of the
 following three combinations of the Destination-Realm and
 Destination-Host AVPs:
 o  A request that is not able to be proxied (such as a CER) MUST NOT
    contain either Destination-Realm or Destination-Host AVPs.
 o  A request that needs to be sent to a home server serving a
    specific realm, but not to a specific server (such as the first
    request of a series of round trips), MUST contain a Destination-
    Realm AVP but MUST NOT contain a Destination-Host AVP.  For
    Diameter clients, the value of the Destination-Realm AVP MAY be
    extracted from the User-Name AVP, or other methods.
 o  Otherwise, a request that needs to be sent to a specific home
    server among those serving a given realm MUST contain both the
    Destination-Realm and Destination-Host AVPs.
 The Destination-Host AVP is used as described above when the
 destination of the request is fixed, which includes:
 o  Authentication requests that span multiple round trips.

Fajardo, et al. Standards Track [Page 74] RFC 6733 Diameter Base Protocol October 2012

 o  A Diameter message that uses a security mechanism that makes use
    of a pre-established session key shared between the source and the
    final destination of the message.
 o  Server-initiated messages that MUST be received by a specific
    Diameter client (e.g., access device), such as the Abort-Session-
    Request message, which is used to request that a particular user's
    session be terminated.
 Note that an agent can only forward a request to a host described in
 the Destination-Host AVP if the host in question is included in its
 peer table (see Section 2.6).  Otherwise, the request is routed based
 on the Destination-Realm only (see Section 6.1.6).
 When a message is received, the message is processed in the following
 order:
 o  If the message is destined for the local host, the procedures
    listed in Section 6.1.4 are followed.
 o  If the message is intended for a Diameter peer with whom the local
    host is able to directly communicate, the procedures listed in
    Section 6.1.5 are followed.  This is known as "Request
    Forwarding".
 o  The procedure listed in Section 6.1.6 is followed, which is known
    as "Request Routing".
 o  If none of the above are successful, an answer is returned with
    the Result-Code set to DIAMETER_UNABLE_TO_DELIVER, with the 'E'
    bit set.
 For routing of Diameter messages to work within an administrative
 domain, all Diameter nodes within the realm MUST be peers.
 The overview contained in this section (6.1) is intended to provide
 general guidelines to Diameter developers.  Implementations are free
 to use different methods than the ones described here as long as they
 conform to the requirements specified in Sections 6.1.1 through
 6.1.9.  See Section 7 for more details on error handling.

6.1.1. Originating a Request

 When creating a request, in addition to any other procedures
 described in the application definition for that specific request,
 the following procedures MUST be followed:

Fajardo, et al. Standards Track [Page 75] RFC 6733 Diameter Base Protocol October 2012

 o  the Command Code is set to the appropriate value;
 o  the 'R' bit is set;
 o  the End-to-End Identifier is set to a locally unique value;
 o  the Origin-Host and Origin-Realm AVPs MUST be set to the
    appropriate values, used to identify the source of the message;
    and
 o  the Destination-Host and Destination-Realm AVPs MUST be set to the
    appropriate values, as described in Section 6.1.

6.1.2. Sending a Request

 When sending a request, originated either locally or as the result of
 a forwarding or routing operation, the following procedures SHOULD be
 followed:
 o  The Hop-by-Hop Identifier SHOULD be set to a locally unique value.
 o  The message SHOULD be saved in the list of pending requests.
 Other actions to perform on the message based on the particular role
 the agent is playing are described in the following sections.

6.1.3. Receiving Requests

 A relay or proxy agent MUST check for forwarding loops when receiving
 requests.  A loop is detected if the server finds its own identity in
 a Route-Record AVP.  When such an event occurs, the agent MUST answer
 with the Result-Code AVP set to DIAMETER_LOOP_DETECTED.

6.1.4. Processing Local Requests

 A request is known to be for local consumption when one of the
 following conditions occurs:
 o  The Destination-Host AVP contains the local host's identity;
 o  The Destination-Host AVP is not present, the Destination-Realm AVP
    contains a realm the server is configured to process locally, and
    the Diameter application is locally supported; or
 o  Both the Destination-Host and the Destination-Realm are not
    present.

Fajardo, et al. Standards Track [Page 76] RFC 6733 Diameter Base Protocol October 2012

 When a request is locally processed, the rules in Section 6.2 should
 be used to generate the corresponding answer.

6.1.5. Request Forwarding

 Request forwarding is done using the Diameter peer table.  The
 Diameter peer table contains all of the peers with which the local
 node is able to directly communicate.
 When a request is received, and the host encoded in the Destination-
 Host AVP is one that is present in the peer table, the message SHOULD
 be forwarded to the peer.

6.1.6. Request Routing

 Diameter request message routing is done via realms and Application
 Ids. A Diameter message that may be forwarded by Diameter agents
 (proxies, redirect agents, or relay agents) MUST include the target
 realm in the Destination-Realm AVP.  Request routing SHOULD rely on
 the Destination-Realm AVP and the Application Id present in the
 request message header to aid in the routing decision.  The realm MAY
 be retrieved from the User-Name AVP, which is in the form of a
 Network Access Identifier (NAI).  The realm portion of the NAI is
 inserted in the Destination-Realm AVP.
 Diameter agents MAY have a list of locally supported realms and
 applications, and they MAY have a list of externally supported realms
 and applications.  When a request is received that includes a realm
 and/or application that is not locally supported, the message is
 routed to the peer configured in the routing table (see Section 2.7).
 Realm names and Application Ids are the minimum supported routing
 criteria, additional information may be needed to support redirect
 semantics.

6.1.7. Predictive Loop Avoidance

 Before forwarding or routing a request, Diameter agents, in addition
 to performing the processing described in Section 6.1.3, SHOULD check
 for the presence of a candidate route's peer identity in any of the
 Route-Record AVPs.  In the event of the agent detecting the presence
 of a candidate route's peer identity in a Route-Record AVP, the agent
 MUST ignore such a route for the Diameter request message and attempt
 alternate routes if any exist.  In case all the candidate routes are
 eliminated by the above criteria, the agent SHOULD return a
 DIAMETER_UNABLE_TO_DELIVER message.

Fajardo, et al. Standards Track [Page 77] RFC 6733 Diameter Base Protocol October 2012

6.1.8. Redirecting Requests

 When a redirect agent receives a request whose routing entry is set
 to REDIRECT, it MUST reply with an answer message with the 'E' bit
 set, while maintaining the Hop-by-Hop Identifier in the header, and
 include the Result-Code AVP to DIAMETER_REDIRECT_INDICATION.  Each of
 the servers associated with the routing entry are added in a separate
 Redirect-Host AVP.
                   +------------------+
                   |     Diameter     |
                   |  Redirect Agent  |
                   +------------------+
                    ^    |    2. command + 'E' bit
     1. Request     |    |    Result-Code =
    joe@example.com |    |    DIAMETER_REDIRECT_INDICATION +
                    |    |    Redirect-Host AVP(s)
                    |    v
                +-------------+  3. Request  +-------------+
                | example.com |------------->| example.net |
                |    Relay    |              |   Diameter  |
                |    Agent    |<-------------|    Server   |
                +-------------+  4. Answer   +-------------+
                   Figure 5: Diameter Redirect Agent
 The receiver of an answer message with the 'E' bit set and the
 Result-Code AVP set to DIAMETER_REDIRECT_INDICATION uses the Hop-by-
 Hop Identifier in the Diameter header to identify the request in the
 pending message queue (see Section 5.5.4) that is to be redirected.
 If no transport connection exists with the new peer, one is created,
 and the request is sent directly to it.
 Multiple Redirect-Host AVPs are allowed.  The receiver of the answer
 message with the 'E' bit set selects exactly one of these hosts as
 the destination of the redirected message.
 When the Redirect-Host-Usage AVP included in the answer message has a
 non-zero value, a route entry for the redirect indications is created
 and cached by the receiver.  The redirect usage for such a route
 entry is set by the value of Redirect-Host-Usage AVP and the lifetime
 of the cached route entry is set by Redirect-Max-Cache-Time AVP
 value.
 It is possible that multiple redirect indications can create multiple
 cached route entries differing only in their redirect usage and the
 peer to forward messages to.  As an example, two(2) route entries
 that are created by two(2) redirect indications results in two(2)

Fajardo, et al. Standards Track [Page 78] RFC 6733 Diameter Base Protocol October 2012

 cached routes for the same realm and Application Id.  However, one
 has a redirect usage of ALL_SESSION, where matching requests will be
 forwarded to one peer; the other has a redirect usage of ALL_REALM,
 where request are forwarded to another peer.  Therefore, an incoming
 request that matches the realm and Application Id of both routes will
 need additional resolution.  In such a case, a routing precedence
 rule MUST be used against the redirect usage value to resolve the
 contention.  The precedence rule can be found in Section 6.13.

6.1.9. Relaying and Proxying Requests

 A relay or proxy agent MUST append a Route-Record AVP to all requests
 forwarded.  The AVP contains the identity of the peer from which the
 request was received.
 The Hop-by-Hop Identifier in the request is saved and replaced with a
 locally unique value.  The source of the request is also saved, which
 includes the IP address, port, and protocol.
 A relay or proxy agent MAY include the Proxy-Info AVP in requests if
 it requires access to any local state information when the
 corresponding response is received.  The Proxy-Info AVP has security
 implications as state information is distributed to other entities.
 As such, it is RECOMMENDED that the content of the Proxy-Info AVP be
 protected with cryptographic mechanisms, for example, by using a
 keyed message digest such as HMAC-SHA1 [RFC2104].  Such a mechanism,
 however, requires the management of keys, although only locally at
 the Diameter server.  Still, a full description of the management of
 the keys used to protect the Proxy-Info AVP is beyond the scope of
 this document.  Below is a list of common recommendations:
 o  The keys should be generated securely following the randomness
    recommendations in [RFC4086].
 o  The keys and cryptographic protection algorithms should be at
    least 128 bits in strength.
 o  The keys should not be used for any other purpose than generating
    and verifying instances of the Proxy-Info AVP.
 o  The keys should be changed regularly.
 o  The keys should be changed if the AVP format or cryptographic
    protection algorithms change.
 The message is then forwarded to the next hop, as identified in the
 routing table.

Fajardo, et al. Standards Track [Page 79] RFC 6733 Diameter Base Protocol October 2012

 Figure 6 provides an example of message routing using the procedures
 listed in these sections.
     (Origin-Host=nas.example.net)    (Origin-Host=nas.example.net)
     (Origin-Realm=example.net)       (Origin-Realm=example.net)
     (Destination-Realm=example.com)  (Destination-Realm=example.com)
                                      (Route-Record=nas.example.net)
    +------+      ------>      +------+      ------>      +------+
    |      |     (Request)     |      |      (Request)    |      |
    | NAS  +-------------------+ DRL  +-------------------+ HMS  |
    |      |                   |      |                   |      |
    +------+     <------       +------+     <------       +------+
   example.net    (Answer)   example.net     (Answer)   example.com
        (Origin-Host=hms.example.com)   (Origin-Host=hms.example.com)
        (Origin-Realm=example.com)      (Origin-Realm=example.com)
                Figure 6: Routing of Diameter messages
 Relay and proxy agents are not required to perform full inspection of
 incoming messages.  At a minimum, validation of the message header
 and relevant routing AVPs has to be done when relaying messages.
 Proxy agents may optionally perform more in-depth message validation
 for applications in which it is interested.

6.2. Diameter Answer Processing

 When a request is locally processed, the following procedures MUST be
 applied to create the associated answer, in addition to any
 additional procedures that MAY be discussed in the Diameter
 application defining the command:
 o  The same Hop-by-Hop Identifier in the request is used in the
    answer.
 o  The local host's identity is encoded in the Origin-Host AVP.
 o  The Destination-Host and Destination-Realm AVPs MUST NOT be
    present in the answer message.
 o  The Result-Code AVP is added with its value indicating success or
    failure.
 o  If the Session-Id is present in the request, it MUST be included
    in the answer.
 o  Any Proxy-Info AVPs in the request MUST be added to the answer
    message, in the same order they were present in the request.

Fajardo, et al. Standards Track [Page 80] RFC 6733 Diameter Base Protocol October 2012

 o  The 'P' bit is set to the same value as the one in the request.
 o  The same End-to-End identifier in the request is used in the
    answer.
 Note that the error messages (see Section 7) are also subjected to
 the above processing rules.

6.2.1. Processing Received Answers

 A Diameter client or proxy MUST match the Hop-by-Hop Identifier in an
 answer received against the list of pending requests.  The
 corresponding message should be removed from the list of pending
 requests.  It SHOULD ignore answers received that do not match a
 known Hop-by-Hop Identifier.

6.2.2. Relaying and Proxying Answers

 If the answer is for a request that was proxied or relayed, the agent
 MUST restore the original value of the Diameter header's Hop-by-Hop
 Identifier field.
 If the last Proxy-Info AVP in the message is targeted to the local
 Diameter server, the AVP MUST be removed before the answer is
 forwarded.
 If a relay or proxy agent receives an answer with a Result-Code AVP
 indicating a failure, it MUST NOT modify the contents of the AVP.
 Any additional local errors detected SHOULD be logged but not
 reflected in the Result-Code AVP.  If the agent receives an answer
 message with a Result-Code AVP indicating success, and it wishes to
 modify the AVP to indicate an error, it MUST modify the Result-Code
 AVP to contain the appropriate error in the message destined towards
 the access device as well as include the Error-Reporting-Host AVP; it
 MUST also issue an STR on behalf of the access device towards the
 Diameter server.
 The agent MUST then send the answer to the host that it received the
 original request from.

6.3. Origin-Host AVP

 The Origin-Host AVP (AVP Code 264) is of type DiameterIdentity, and
 it MUST be present in all Diameter messages.  This AVP identifies the
 endpoint that originated the Diameter message.  Relay agents MUST NOT
 modify this AVP.

Fajardo, et al. Standards Track [Page 81] RFC 6733 Diameter Base Protocol October 2012

 The value of the Origin-Host AVP is guaranteed to be unique within a
 single host.
 Note that the Origin-Host AVP may resolve to more than one address as
 the Diameter peer may support more than one address.
 This AVP SHOULD be placed as close to the Diameter header as
 possible.

6.4. Origin-Realm AVP

 The Origin-Realm AVP (AVP Code 296) is of type DiameterIdentity.
 This AVP contains the Realm of the originator of any Diameter message
 and MUST be present in all messages.
 This AVP SHOULD be placed as close to the Diameter header as
 possible.

6.5. Destination-Host AVP

 The Destination-Host AVP (AVP Code 293) is of type DiameterIdentity.
 This AVP MUST be present in all unsolicited agent initiated messages,
 MAY be present in request messages, and MUST NOT be present in answer
 messages.
 The absence of the Destination-Host AVP will cause a message to be
 sent to any Diameter server supporting the application within the
 realm specified in Destination-Realm AVP.
 This AVP SHOULD be placed as close to the Diameter header as
 possible.

6.6. Destination-Realm AVP

 The Destination-Realm AVP (AVP Code 283) is of type DiameterIdentity
 and contains the realm to which the message is to be routed.  The
 Destination-Realm AVP MUST NOT be present in answer messages.
 Diameter clients insert the realm portion of the User-Name AVP.
 Diameter servers initiating a request message use the value of the
 Origin-Realm AVP from a previous message received from the intended
 target host (unless it is known a priori).  When present, the
 Destination-Realm AVP is used to perform message routing decisions.
 The CCF for a request message that includes the Destination-Realm AVP
 SHOULD list the Destination-Realm AVP as a required AVP (an AVP
 indicated as {AVP}); otherwise, the message is inherently a non-
 routable message.

Fajardo, et al. Standards Track [Page 82] RFC 6733 Diameter Base Protocol October 2012

 This AVP SHOULD be placed as close to the Diameter header as
 possible.

6.7. Routing AVPs

 The AVPs defined in this section are Diameter AVPs used for routing
 purposes.  These AVPs change as Diameter messages are processed by
 agents.

6.7.1. Route-Record AVP

 The Route-Record AVP (AVP Code 282) is of type DiameterIdentity.  The
 identity added in this AVP MUST be the same as the one received in
 the Origin-Host of the Capabilities Exchange message.

6.7.2. Proxy-Info AVP

 The Proxy-Info AVP (AVP Code 284) is of type Grouped.  This AVP
 contains the identity and local state information of the Diameter
 node that creates and adds it to a message.  The Grouped Data field
 has the following CCF grammar:
       Proxy-Info ::= < AVP Header: 284 >
                      { Proxy-Host }
                      { Proxy-State }
                    * [ AVP ]

6.7.3. Proxy-Host AVP

 The Proxy-Host AVP (AVP Code 280) is of type DiameterIdentity.  This
 AVP contains the identity of the host that added the Proxy-Info AVP.

6.7.4. Proxy-State AVP

 The Proxy-State AVP (AVP Code 33) is of type OctetString.  It
 contains state information that would otherwise be stored at the
 Diameter entity that created it.  As such, this AVP MUST be treated
 as opaque data by other Diameter entities.

6.8. Auth-Application-Id AVP

 The Auth-Application-Id AVP (AVP Code 258) is of type Unsigned32 and
 is used in order to advertise support of the Authentication and
 Authorization portion of an application (see Section 2.4).  If
 present in a message other than CER and CEA, the value of the Auth-
 Application-Id AVP MUST match the Application Id present in the
 Diameter message header.

Fajardo, et al. Standards Track [Page 83] RFC 6733 Diameter Base Protocol October 2012

6.9. Acct-Application-Id AVP

 The Acct-Application-Id AVP (AVP Code 259) is of type Unsigned32 and
 is used in order to advertise support of the accounting portion of an
 application (see Section 2.4).  If present in a message other than
 CER and CEA, the value of the Acct-Application-Id AVP MUST match the
 Application Id present in the Diameter message header.

6.10. Inband-Security-Id AVP

 The Inband-Security-Id AVP (AVP Code 299) is of type Unsigned32 and
 is used in order to advertise support of the security portion of the
 application.  The use of this AVP in CER and CEA messages is NOT
 RECOMMENDED.  Instead, discovery of a Diameter entity's security
 capabilities can be done either through static configuration or via
 Diameter Peer Discovery as described in Section 5.2.
 The following values are supported:
 NO_INBAND_SECURITY 0
    This peer does not support TLS/TCP and DTLS/SCTP.  This is the
    default value, if the AVP is omitted.
 TLS 1
    This node supports TLS/TCP [RFC5246] and DTLS/SCTP [RFC6083]
    security.

6.11. Vendor-Specific-Application-Id AVP

 The Vendor-Specific-Application-Id AVP (AVP Code 260) is of type
 Grouped and is used to advertise support of a vendor-specific
 Diameter application.  Exactly one instance of either Auth-
 Application-Id or Acct-Application-Id AVP MUST be present.  The
 Application Id carried by either Auth-Application-Id or Acct-
 Application-Id AVP MUST comply with vendor-specific Application Id
 assignment described in Section 11.3.  It MUST also match the
 Application Id present in the Diameter header except when used in a
 CER or CEA message.
 The Vendor-Id AVP is an informational AVP pertaining to the vendor
 who may have authorship of the vendor-specific Diameter application.
 It MUST NOT be used as a means of defining a completely separate
 vendor-specific Application Id space.

Fajardo, et al. Standards Track [Page 84] RFC 6733 Diameter Base Protocol October 2012

 The Vendor-Specific-Application-Id AVP SHOULD be placed as close to
 the Diameter header as possible.
    AVP Format
    <Vendor-Specific-Application-Id> ::= < AVP Header: 260 >
                                         { Vendor-Id }
                                         [ Auth-Application-Id ]
                                         [ Acct-Application-Id ]
 A Vendor-Specific-Application-Id AVP MUST contain exactly one of
 either Auth-Application-Id or Acct-Application-Id.  If a Vendor-
 Specific-Application-Id is received without one of these two AVPs,
 then the recipient SHOULD issue an answer with a Result-Code set to
 DIAMETER_MISSING_AVP.  The answer SHOULD also include a Failed-AVP,
 which MUST contain an example of an Auth-Application-Id AVP and an
 Acct-Application-Id AVP.
 If a Vendor-Specific-Application-Id is received that contains both
 Auth-Application-Id and Acct-Application-Id, then the recipient MUST
 issue an answer with Result-Code set to
 DIAMETER_AVP_OCCURS_TOO_MANY_TIMES.  The answer MUST also include a
 Failed-AVP, which MUST contain the received Auth-Application-Id AVP
 and Acct-Application-Id AVP.

6.12. Redirect-Host AVP

 The Redirect-Host AVP (AVP Code 292) is of type DiameterURI.  One or
 more instances of this AVP MUST be present if the answer message's
 'E' bit is set and the Result-Code AVP is set to
 DIAMETER_REDIRECT_INDICATION.
 Upon receiving the above, the receiving Diameter node SHOULD forward
 the request directly to one of the hosts identified in these AVPs.
 The server contained in the selected Redirect-Host AVP SHOULD be used
 for all messages matching the criteria set by the Redirect-Host-Usage
 AVP.

6.13. Redirect-Host-Usage AVP

 The Redirect-Host-Usage AVP (AVP Code 261) is of type Enumerated.
 This AVP MAY be present in answer messages whose 'E' bit is set and
 the Result-Code AVP is set to DIAMETER_REDIRECT_INDICATION.
 When present, this AVP provides hints about how the routing entry
 resulting from the Redirect-Host is to be used.  The following values
 are supported:

Fajardo, et al. Standards Track [Page 85] RFC 6733 Diameter Base Protocol October 2012

 DONT_CACHE 0
    The host specified in the Redirect-Host AVP SHOULD NOT be cached.
    This is the default value.
 ALL_SESSION 1
    All messages within the same session, as defined by the same value
    of the Session-ID AVP SHOULD be sent to the host specified in the
    Redirect-Host AVP.
 ALL_REALM 2
    All messages destined for the realm requested SHOULD be sent to
    the host specified in the Redirect-Host AVP.
 REALM_AND_APPLICATION 3
    All messages for the application requested to the realm specified
    SHOULD be sent to the host specified in the Redirect-Host AVP.
 ALL_APPLICATION 4
    All messages for the application requested SHOULD be sent to the
    host specified in the Redirect-Host AVP.
 ALL_HOST 5
    All messages that would be sent to the host that generated the
    Redirect-Host SHOULD be sent to the host specified in the
    Redirect-Host AVP.
 ALL_USER 6
    All messages for the user requested SHOULD be sent to the host
    specified in the Redirect-Host AVP.
 When multiple cached routes are created by redirect indications and
 they differ only in redirect usage and peers to forward requests to
 (see Section 6.1.8), a precedence rule MUST be applied to the
 redirect usage values of the cached routes during normal routing to
 resolve contentions that may occur.  The precedence rule is the order
 that dictate which redirect usage should be considered before any
 other as they appear.  The order is as follows:

Fajardo, et al. Standards Track [Page 86] RFC 6733 Diameter Base Protocol October 2012

 1.  ALL_SESSION
 2.  ALL_USER
 3.  REALM_AND_APPLICATION
 4.  ALL_REALM
 5.  ALL_APPLICATION
 6.  ALL_HOST

6.14. Redirect-Max-Cache-Time AVP

 The Redirect-Max-Cache-Time AVP (AVP Code 262) is of type Unsigned32.
 This AVP MUST be present in answer messages whose 'E' bit is set,
 whose Result-Code AVP is set to DIAMETER_REDIRECT_INDICATION, and
 whose Redirect-Host-Usage AVP set to a non-zero value.
 This AVP contains the maximum number of seconds the peer and route
 table entries, created as a result of the Redirect-Host, SHOULD be
 cached.  Note that once a host is no longer reachable, any associated
 cache, peer, and routing table entries MUST be deleted.

7. Error Handling

 There are two different types of errors in Diameter; protocol errors
 and application errors.  A protocol error is one that occurs at the
 base protocol level and MAY require per-hop attention (e.g., a
 message routing error).  Application errors, on the other hand,
 generally occur due to a problem with a function specified in a
 Diameter application (e.g., user authentication, missing AVP).
 Result-Code AVP values that are used to report protocol errors MUST
 only be present in answer messages whose 'E' bit is set.  When a
 request message is received that causes a protocol error, an answer
 message is returned with the 'E' bit set, and the Result-Code AVP is
 set to the appropriate protocol error value.  As the answer is sent
 back towards the originator of the request, each proxy or relay agent
 MAY take action on the message.

Fajardo, et al. Standards Track [Page 87] RFC 6733 Diameter Base Protocol October 2012

                        1. Request        +---------+ Link Broken
              +-------------------------->|Diameter |----///----+
              |     +---------------------|         |           v
       +------+--+  | 2. answer + 'E' set | Relay 2 |     +--------+
       |Diameter |<-+ (Unable to Forward) +---------+     |Diameter|
       |         |                                        |  Home  |
       | Relay 1 |--+                     +---------+     | Server |
       +---------+  |   3. Request        |Diameter |     +--------+
                    +-------------------->|         |           ^
                                          | Relay 3 |-----------+
                                          +---------+
      Figure 7: Example of Protocol Error Causing Answer Message
 Figure 7 provides an example of a message forwarded upstream by a
 Diameter relay.  When the message is received by Relay 2, and it
 detects that it cannot forward the request to the home server, an
 answer message is returned with the 'E' bit set and the Result-Code
 AVP set to DIAMETER_UNABLE_TO_DELIVER.  Given that this error falls
 within the protocol error category, Relay 1 would take special
 action, and given the error, attempt to route the message through its
 alternate Relay 3.
          +---------+ 1. Request  +---------+ 2. Request  +---------+
          | Access  |------------>|Diameter |------------>|Diameter |
          |         |             |         |             |  Home   |
          | Device  |<------------|  Relay  |<------------| Server  |
          +---------+  4. Answer  +---------+  3. Answer  +---------+
                     (Missing AVP)           (Missing AVP)
         Figure 8: Example of Application Error Answer Message
 Figure 8 provides an example of a Diameter message that caused an
 application error.  When application errors occur, the Diameter
 entity reporting the error clears the 'R' bit in the Command Flags
 and adds the Result-Code AVP with the proper value.  Application
 errors do not require any proxy or relay agent involvement;
 therefore, the message would be forwarded back to the originator of
 the request.
 In the case where the answer message itself contains errors, any
 related session SHOULD be terminated by sending an STR or ASR
 message.  The Termination-Cause AVP in the STR MAY be filled with the
 appropriate value to indicate the cause of the error.  An application
 MAY also send an application-specific request instead of an STR or
 ASR message to signal the error in the case where no state is
 maintained or to allow for some form of error recovery with the
 corresponding Diameter entity.

Fajardo, et al. Standards Track [Page 88] RFC 6733 Diameter Base Protocol October 2012

 There are certain Result-Code AVP application errors that require
 additional AVPs to be present in the answer.  In these cases, the
 Diameter node that sets the Result-Code AVP to indicate the error
 MUST add the AVPs.  Examples are as follows:
 o  A request with an unrecognized AVP is received with the 'M' bit
    (Mandatory bit) set causes an answer to be sent with the Result-
    Code AVP set to DIAMETER_AVP_UNSUPPORTED and the Failed-AVP AVP
    containing the offending AVP.
 o  A request with an AVP that is received with an unrecognized value
    causes an answer to be returned with the Result-Code AVP set to
    DIAMETER_INVALID_AVP_VALUE, with the Failed-AVP AVP containing the
    AVP causing the error.
 o  A received command that is missing AVPs that are defined as
    required in the commands CCF; examples are AVPs indicated as
    {AVP}.  The receiver issues an answer with the Result-Code set to
    DIAMETER_MISSING_AVP and creates an AVP with the AVP Code and
    other fields set as expected in the missing AVP.  The created AVP
    is then added to the Failed-AVP AVP.
 The Result-Code AVP describes the error that the Diameter node
 encountered in its processing.  In case there are multiple errors,
 the Diameter node MUST report only the first error it encountered
 (detected possibly in some implementation-dependent order).  The
 specific errors that can be described by this AVP are described in
 the following section.

7.1. Result-Code AVP

 The Result-Code AVP (AVP Code 268) is of type Unsigned32 and
 indicates whether a particular request was completed successfully or
 an error occurred.  All Diameter answer messages in IETF-defined
 Diameter application specifications MUST include one Result-Code AVP.
 A non-successful Result-Code AVP (one containing a non-2xxx value
 other than DIAMETER_REDIRECT_INDICATION) MUST include the Error-
 Reporting-Host AVP if the host setting the Result-Code AVP is
 different from the identity encoded in the Origin-Host AVP.
 The Result-Code data field contains an IANA-managed 32-bit address
 space representing errors (see Section 11.3.2).  Diameter provides
 the following classes of errors, all identified by the thousands
 digit in the decimal notation:

Fajardo, et al. Standards Track [Page 89] RFC 6733 Diameter Base Protocol October 2012

 o  1xxx (Informational)
 o  2xxx (Success)
 o  3xxx (Protocol Errors)
 o  4xxx (Transient Failures)
 o  5xxx (Permanent Failure)
 An unrecognized class (one whose first digit is not defined in this
 section) MUST be handled as a permanent failure.

7.1.1. Informational

 Errors that fall within this category are used to inform the
 requester that a request could not be satisfied, and additional
 action is required on its part before access is granted.
 DIAMETER_MULTI_ROUND_AUTH 1001
    This informational error is returned by a Diameter server to
    inform the access device that the authentication mechanism being
    used requires multiple round trips, and a subsequent request needs
    to be issued in order for access to be granted.

7.1.2. Success

 Errors that fall within the Success category are used to inform a
 peer that a request has been successfully completed.
 DIAMETER_SUCCESS 2001
    The request was successfully completed.
 DIAMETER_LIMITED_SUCCESS 2002
    When returned, the request was successfully completed, but
    additional processing is required by the application in order to
    provide service to the user.

7.1.3. Protocol Errors

 Errors that fall within the Protocol Error category SHOULD be treated
 on a per-hop basis, and Diameter proxies MAY attempt to correct the
 error, if it is possible.  Note that these errors MUST only be used
 in answer messages whose 'E' bit is set.

Fajardo, et al. Standards Track [Page 90] RFC 6733 Diameter Base Protocol October 2012

 DIAMETER_COMMAND_UNSUPPORTED 3001
    This error code is used when a Diameter entity receives a message
    with a Command Code that it does not support.
 DIAMETER_UNABLE_TO_DELIVER 3002
    This error is given when Diameter cannot deliver the message to
    the destination, either because no host within the realm
    supporting the required application was available to process the
    request or because the Destination-Host AVP was given without the
    associated Destination-Realm AVP.
 DIAMETER_REALM_NOT_SERVED 3003
    The intended realm of the request is not recognized.
 DIAMETER_TOO_BUSY 3004
    When returned, a Diameter node SHOULD attempt to send the message
    to an alternate peer.  This error MUST only be used when a
    specific server is requested, and it cannot provide the requested
    service.
 DIAMETER_LOOP_DETECTED 3005
    An agent detected a loop while trying to get the message to the
    intended recipient.  The message MAY be sent to an alternate peer,
    if one is available, but the peer reporting the error has
    identified a configuration problem.
 DIAMETER_REDIRECT_INDICATION 3006
    A redirect agent has determined that the request could not be
    satisfied locally, and the initiator of the request SHOULD direct
    the request directly to the server, whose contact information has
    been added to the response.  When set, the Redirect-Host AVP MUST
    be present.
 DIAMETER_APPLICATION_UNSUPPORTED 3007
    A request was sent for an application that is not supported.
 DIAMETER_INVALID_HDR_BITS 3008
    A request was received whose bits in the Diameter header were set
    either to an invalid combination or to a value that is
    inconsistent with the Command Code's definition.

Fajardo, et al. Standards Track [Page 91] RFC 6733 Diameter Base Protocol October 2012

 DIAMETER_INVALID_AVP_BITS 3009
    A request was received that included an AVP whose flag bits are
    set to an unrecognized value or that is inconsistent with the
    AVP's definition.
 DIAMETER_UNKNOWN_PEER 3010
    A CER was received from an unknown peer.

7.1.4. Transient Failures

 Errors that fall within the transient failures category are used to
 inform a peer that the request could not be satisfied at the time it
 was received but MAY be able to satisfy the request in the future.
 Note that these errors MUST be used in answer messages whose 'E' bit
 is not set.
 DIAMETER_AUTHENTICATION_REJECTED 4001
    The authentication process for the user failed, most likely due to
    an invalid password used by the user.  Further attempts MUST only
    be tried after prompting the user for a new password.
 DIAMETER_OUT_OF_SPACE 4002
    A Diameter node received the accounting request but was unable to
    commit it to stable storage due to a temporary lack of space.
 ELECTION_LOST 4003
    The peer has determined that it has lost the election process and
    has therefore disconnected the transport connection.

7.1.5. Permanent Failures

 Errors that fall within the permanent failures category are used to
 inform the peer that the request failed and should not be attempted
 again.  Note that these errors SHOULD be used in answer messages
 whose 'E' bit is not set.  In error conditions where it is not
 possible or efficient to compose application-specific answer grammar,
 answer messages with the 'E' bit set and which comply to the grammar
 described in Section 7.2 MAY also be used for permanent errors.

Fajardo, et al. Standards Track [Page 92] RFC 6733 Diameter Base Protocol October 2012

 DIAMETER_AVP_UNSUPPORTED 5001
    The peer received a message that contained an AVP that is not
    recognized or supported and was marked with the 'M' (Mandatory)
    bit.  A Diameter message with this error MUST contain one or more
    Failed-AVP AVPs containing the AVPs that caused the failure.
 DIAMETER_UNKNOWN_SESSION_ID 5002
    The request contained an unknown Session-Id.
 DIAMETER_AUTHORIZATION_REJECTED 5003
    A request was received for which the user could not be authorized.
    This error could occur if the service requested is not permitted
    to the user.
 DIAMETER_INVALID_AVP_VALUE 5004
    The request contained an AVP with an invalid value in its data
    portion.  A Diameter message indicating this error MUST include
    the offending AVPs within a Failed-AVP AVP.
 DIAMETER_MISSING_AVP 5005
    The request did not contain an AVP that is required by the Command
    Code definition.  If this value is sent in the Result-Code AVP, a
    Failed-AVP AVP SHOULD be included in the message.  The Failed-AVP
    AVP MUST contain 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 zeroes.
 DIAMETER_RESOURCES_EXCEEDED 5006
    A request was received that cannot be authorized because the user
    has already expended allowed resources.  An example of this error
    condition is when a user that is restricted to one dial-up PPP
    port attempts to establish a second PPP connection.
 DIAMETER_CONTRADICTING_AVPS 5007
    The Home Diameter server has detected AVPs in the request that
    contradicted each other, and it is not willing to provide service
    to the user.  The Failed-AVP AVP MUST be present, which contain
    the AVPs that contradicted each other.

Fajardo, et al. Standards Track [Page 93] RFC 6733 Diameter Base Protocol October 2012

 DIAMETER_AVP_NOT_ALLOWED 5008
    A message was received with an AVP that MUST NOT be present.  The
    Failed-AVP AVP MUST be included and contain a copy of the
    offending AVP.
 DIAMETER_AVP_OCCURS_TOO_MANY_TIMES 5009
    A message was received that included an AVP that appeared more
    often than permitted in the message definition.  The Failed-AVP
    AVP MUST be included and contain a copy of the first instance of
    the offending AVP that exceeded the maximum number of occurrences.
 DIAMETER_NO_COMMON_APPLICATION 5010
    This error is returned by a Diameter node that receives a CER
    whereby no applications are common between the CER sending peer
    and the CER receiving peer.
 DIAMETER_UNSUPPORTED_VERSION 5011
    This error is returned when a request was received, whose version
    number is unsupported.
 DIAMETER_UNABLE_TO_COMPLY 5012
    This error is returned when a request is rejected for unspecified
    reasons.
 DIAMETER_INVALID_BIT_IN_HEADER 5013
    This error is returned when a reserved bit in the Diameter header
    is set to one (1) or the bits in the Diameter header are set
    incorrectly.
 DIAMETER_INVALID_AVP_LENGTH 5014
    The request contained an AVP with an invalid length.  A Diameter
    message indicating this error MUST include the offending AVPs
    within a Failed-AVP AVP.  In cases where the erroneous AVP length
    value exceeds the message length or is less than the minimum AVP
    header length, it is sufficient to include the offending AVP
    header and a zero filled payload of the minimum required length
    for the payloads data type.  If the AVP is a Grouped AVP, the
    Grouped AVP header with an empty payload would be sufficient to
    indicate the offending AVP.  In the case where the offending AVP
    header cannot be fully decoded when the AVP length is less than

Fajardo, et al. Standards Track [Page 94] RFC 6733 Diameter Base Protocol October 2012

    the minimum AVP header length, it is sufficient to include an
    offending AVP header that is formulated by padding the incomplete
    AVP header with zero up to the minimum AVP header length.
 DIAMETER_INVALID_MESSAGE_LENGTH 5015
    This error is returned when a request is received with an invalid
    message length.
 DIAMETER_INVALID_AVP_BIT_COMBO 5016
    The request contained an AVP with which is not allowed to have the
    given value in the AVP Flags field.  A Diameter message indicating
    this error MUST include the offending AVPs within a Failed-AVP
    AVP.
 DIAMETER_NO_COMMON_SECURITY 5017
    This error is returned when a CER message is received, and there
    are no common security mechanisms supported between the peers.  A
    Capabilities-Exchange-Answer (CEA) message MUST be returned with
    the Result-Code AVP set to DIAMETER_NO_COMMON_SECURITY.

7.2. Error Bit

 The 'E' (Error Bit) in the Diameter header is set when the request
 caused a protocol-related error (see Section 7.1.3).  A message with
 the 'E' bit MUST NOT be sent as a response to an answer message.
 Note that a message with the 'E' bit set is still subjected to the
 processing rules defined in Section 6.2.  When set, the answer
 message will not conform to the CCF specification for the command;
 instead, it and will conform to the following CCF:
    Message Format
    <answer-message> ::= < Diameter Header: code, ERR [, PXY] >
                      0*1< Session-Id >
                         { Origin-Host }
                         { Origin-Realm }
                         { Result-Code }
                         [ Origin-State-Id ]
                         [ Error-Message ]
                         [ Error-Reporting-Host ]
                         [ Failed-AVP ]
                         [ Experimental-Result ]
                       * [ Proxy-Info ]
                       * [ AVP ]

Fajardo, et al. Standards Track [Page 95] RFC 6733 Diameter Base Protocol October 2012

 Note that the code used in the header is the same than the one found
 in the request message, but with the 'R' bit cleared and the 'E' bit
 set.  The 'P' bit in the header is set to the same value as the one
 found in the request message.

7.3. Error-Message AVP

 The Error-Message AVP (AVP Code 281) is of type UTF8String.  It MAY
 accompany a Result-Code AVP as a human-readable error message.  The
 Error-Message AVP is not intended to be useful in an environment
 where error messages are processed automatically.  It SHOULD NOT be
 expected that the content of this AVP be parsed by network entities.

7.4. Error-Reporting-Host AVP

 The Error-Reporting-Host AVP (AVP Code 294) is of type
 DiameterIdentity.  This AVP contains the identity of the Diameter
 host that sent the Result-Code AVP to a value other than 2001
 (Success), only if the host setting the Result-Code is different from
 the one encoded in the Origin-Host AVP.  This AVP is intended to be
 used for troubleshooting purposes, and it MUST be set when the
 Result-Code AVP indicates a failure.

7.5. Failed-AVP AVP

 The Failed-AVP AVP (AVP Code 279) is of type Grouped and provides
 debugging information in cases where a request is rejected or not
 fully processed due to erroneous information in a specific AVP.  The
 value of the Result-Code AVP will provide information on the reason
 for the Failed-AVP AVP.  A Diameter answer message SHOULD contain an
 instance of the Failed-AVP AVP that corresponds to the error
 indicated by the Result-Code AVP.  For practical purposes, this
 Failed-AVP would typically refer to the first AVP processing error
 that a Diameter node encounters.
 The possible reasons for this AVP are the presence of an improperly
 constructed AVP, an unsupported or unrecognized AVP, an invalid AVP
 value, the omission of a required AVP, the presence of an explicitly
 excluded AVP (see tables in Section 10) or the presence of two or
 more occurrences of an AVP that is restricted to 0, 1, or 0-1
 occurrences.
 A Diameter message SHOULD contain one Failed-AVP AVP, containing the
 entire AVP that could not be processed successfully.  If the failure
 reason is omission of a required AVP, an AVP with the missing AVP
 code, the missing Vendor-Id, and a zero-filled payload of the minimum
 required length for the omitted AVP will be added.  If the failure
 reason is an invalid AVP length where the reported length is less

Fajardo, et al. Standards Track [Page 96] RFC 6733 Diameter Base Protocol October 2012

 than the minimum AVP header length or greater than the reported
 message length, a copy of the offending AVP header and a zero-filled
 payload of the minimum required length SHOULD be added.
 In the case where the offending AVP is embedded within a Grouped AVP,
 the Failed-AVP MAY contain the grouped AVP, which in turn contains
 the single offending AVP.  The same method MAY be employed if the
 grouped AVP itself is embedded in yet another grouped AVP and so on.
 In this case, the Failed-AVP MAY contain the grouped AVP hierarchy up
 to the single offending AVP.  This enables the recipient to detect
 the location of the offending AVP when embedded in a group.
 AVP Format
       <Failed-AVP> ::= < AVP Header: 279 >
                     1* {AVP}

7.6. Experimental-Result AVP

 The Experimental-Result AVP (AVP Code 297) is of type Grouped, and
 indicates whether a particular vendor-specific request was completed
 successfully or whether an error occurred.  This AVP has the
 following structure:
 AVP Format
       Experimental-Result ::= < AVP Header: 297 >
                               { Vendor-Id }
                               { Experimental-Result-Code }
 The Vendor-Id AVP (see Section 5.3.3) in this grouped AVP identifies
 the vendor responsible for the assignment of the result code that
 follows.  All Diameter answer messages defined in vendor-specific
 applications MUST include either one Result-Code AVP or one
 Experimental-Result AVP.

7.7. Experimental-Result-Code AVP

 The Experimental-Result-Code AVP (AVP Code 298) is of type Unsigned32
 and contains a vendor-assigned value representing the result of
 processing the request.
 It is recommended that vendor-specific result codes follow the same
 conventions given for the Result-Code AVP regarding the different
 types of result codes and the handling of errors (for non-2xxx
 values).

Fajardo, et al. Standards Track [Page 97] RFC 6733 Diameter Base Protocol October 2012

8. Diameter User Sessions

 In general, Diameter can provide two different types of services to
 applications.  The first involves authentication and authorization,
 and it can optionally make use of accounting.  The second only makes
 use of accounting.
 When a service makes use of the authentication and/or authorization
 portion of an application, and a user requests access to the network,
 the Diameter client issues an auth request to its local server.  The
 auth request is defined in a service-specific Diameter application
 (e.g., NASREQ).  The request contains a Session-Id AVP, which is used
 in subsequent messages (e.g., subsequent authorization, accounting,
 etc.) relating to the user's session.  The Session-Id AVP is a means
 for the client and servers to correlate a Diameter message with a
 user session.
 When a Diameter server authorizes a user to implement network
 resources for a finite amount of time, and it is willing to extend
 the authorization via a future request, it MUST add the
 Authorization- Lifetime AVP to the answer message.  The
 Authorization-Lifetime AVP defines the maximum number of seconds a
 user MAY make use of the resources before another authorization
 request is expected by the server.  The Auth-Grace-Period AVP
 contains the number of seconds following the expiration of the
 Authorization-Lifetime, after which the server will release all state
 information related to the user's session.  Note that if payment for
 services is expected by the serving realm from the user's home realm,
 the Authorization-Lifetime AVP, combined with the Auth-Grace-Period
 AVP, implies the maximum length of the session for which the home
 realm is willing to be fiscally responsible.  Services provided past
 the expiration of the Authorization-Lifetime and Auth-Grace-Period
 AVPs are the responsibility of the access device.  Of course, the
 actual cost of services rendered is clearly outside the scope of the
 protocol.
 An access device that does not expect to send a re-authorization or a
 session termination request to the server MAY include the Auth-
 Session-State AVP with the value set to NO_STATE_MAINTAINED as a hint
 to the server.  If the server accepts the hint, it agrees that since
 no session termination message will be received once service to the
 user is terminated, it cannot maintain state for the session.  If the
 answer message from the server contains a different value in the
 Auth-Session-State AVP (or the default value if the AVP is absent),
 the access device MUST follow the server's directives.  Note that the
 value NO_STATE_MAINTAINED MUST NOT be set in subsequent re-
 authorization requests and answers.

Fajardo, et al. Standards Track [Page 98] RFC 6733 Diameter Base Protocol October 2012

 The base protocol does not include any authorization request
 messages, since these are largely application-specific and are
 defined in a Diameter application document.  However, the base
 protocol does define a set of messages that are used to terminate
 user sessions.  These are used to allow servers that maintain state
 information to free resources.
 When a service only makes use of the accounting portion of the
 Diameter protocol, even in combination with an application, the
 Session-Id is still used to identify user sessions.  However, the
 session termination messages are not used, since a session is
 signaled as being terminated by issuing an accounting stop message.
 Diameter may also be used for services that cannot be easily
 categorized as authentication, authorization, or accounting (e.g.,
 certain Third Generation Partnership Project Internet Multimedia
 System (3GPP IMS) interfaces).  In such cases, the finite state
 machine defined in subsequent sections may not be applicable.
 Therefore, the application itself MAY need to define its own finite
 state machine.  However, such application-specific state machines
 SHOULD follow the general state machine framework outlined in this
 document such as the use of Session-Id AVPs and the use of STR/STA,
 ASR/ASA messages for stateful sessions.

8.1. Authorization Session State Machine

 This section contains a set of finite state machines, which represent
 the life cycle of Diameter sessions and which MUST be observed by all
 Diameter implementations that make use of the authentication and/or
 authorization portion of a Diameter application.  The term "Service-
 Specific" below refers to a message defined in a Diameter application
 (e.g., Mobile IPv4, NASREQ).
 There are four different authorization session state machines
 supported in the Diameter base protocol.  The first two describe a
 session in which the server is maintaining session state, indicated
 by the value of the Auth-Session-State AVP (or its absence).  One
 describes the session from a client perspective, the other from a
 server perspective.  The second two state machines are used when the
 server does not maintain session state.  Here again, one describes
 the session from a client perspective, the other from a server
 perspective.
 When a session is moved to the Idle state, any resources that were
 allocated for the particular session must be released.  Any event not
 listed in the state machines MUST be considered an error condition,
 and an answer, if applicable, MUST be returned to the originator of
 the message.

Fajardo, et al. Standards Track [Page 99] RFC 6733 Diameter Base Protocol October 2012

 In the case that an application does not support re-auth, the state
 transitions related to server-initiated re-auth, when both client and
 server sessions maintain state (e.g., Send RAR, Pending, Receive
 RAA), MAY be ignored.
 In the state table, the event "Failure to send X" means that the
 Diameter agent is unable to send command X to the desired
 destination.  This could be due to the peer being down or due to the
 peer sending back a transient failure or temporary protocol error
 notification DIAMETER_TOO_BUSY or DIAMETER_LOOP_DETECTED in the
 Result-Code AVP of the corresponding Answer command.  The event 'X
 successfully sent' is the complement of 'Failure to send X'.
 The following state machine is observed by a client when state is
 maintained on the server:
                            CLIENT, STATEFUL
    State     Event                          Action       New State
    ---------------------------------------------------------------
    Idle      Client or device requests      Send         Pending
              access                         service-
                                             specific
                                             auth req
    Idle      ASR Received                   Send ASA     Idle
              for unknown session            with
                                             Result-Code =
                                             UNKNOWN_
                                             SESSION_ID
    Idle      RAR Received                   Send RAA     Idle
              for unknown session            with
                                             Result-Code =
                                             UNKNOWN_
                                             SESSION_ID
    Pending   Successful service-specific    Grant        Open
              authorization answer           Access
              received with default
              Auth-Session-State value
    Pending   Successful service-specific    Sent STR     Discon
              authorization answer received,
              but service not provided
    Pending   Error processing successful    Sent STR     Discon
              service-specific authorization
              answer

Fajardo, et al. Standards Track [Page 100] RFC 6733 Diameter Base Protocol October 2012

    Pending   Failed service-specific        Clean up     Idle
              authorization answer received
    Open      User or client device          Send         Open
              requests access to service     service-
                                             specific
                                             auth req
    Open      Successful service-specific    Provide      Open
              authorization answer received  service
    Open      Failed service-specific        Discon.      Idle
              authorization answer           user/device
              received.
    Open      RAR received and client will   Send RAA     Open
              perform subsequent re-auth     with
                                             Result-Code =
                                             SUCCESS
    Open      RAR received and client will   Send RAA     Idle
              not perform subsequent         with
              re-auth                        Result-Code !=
                                             SUCCESS,
                                             Discon.
                                             user/device
    Open      Session-Timeout expires on     Send STR     Discon
              access device
    Open      ASR received,                  Send ASA     Discon
              client will comply             with
              with request to end the        Result-Code =
              session                        = SUCCESS,
                                             Send STR.
    Open      ASR Received,                  Send ASA     Open
              client will not comply         with
              with request to end the        Result-Code !=
              session                        != SUCCESS
    Open      Authorization-Lifetime +       Send STR     Discon
              Auth-Grace-Period expires on
              access device
    Discon    ASR received                   Send ASA     Discon

Fajardo, et al. Standards Track [Page 101] RFC 6733 Diameter Base Protocol October 2012

    Discon    STA received                   Discon.      Idle
                                             user/device
 The following state machine is observed by a server when it is
 maintaining state for the session:
                           SERVER, STATEFUL
    State     Event                          Action       New State
    ---------------------------------------------------------------
    Idle      Service-specific authorization Send         Open
              request received, and          successful
              user is authorized             service-
                                             specific
                                             answer
    Idle      Service-specific authorization Send         Idle
              request received, and          failed
              user is not authorized         service-
                                             specific
                                             answer
    Open      Service-specific authorization Send         Open
              request received, and user     successful
              is authorized                  service-
                                             specific
                                             answer
    Open      Service-specific authorization Send         Idle
              request received, and user     failed
              is not authorized              service-
                                             specific
                                             answer,
                                             Clean up
    Open      Home server wants to confirm   Send RAR     Pending
              authentication and/or
              authorization of the user
    Pending   Received RAA with a failed     Clean up     Idle
              Result-Code
    Pending   Received RAA with Result-Code  Update       Open
              = SUCCESS                      session
    Open      Home server wants to           Send ASR     Discon
              terminate the service

Fajardo, et al. Standards Track [Page 102] RFC 6733 Diameter Base Protocol October 2012

    Open      Authorization-Lifetime (and    Clean up     Idle
              Auth-Grace-Period) expires
              on home server
    Open      Session-Timeout expires on     Clean up     Idle
              home server
    Discon    Failure to send ASR            Wait,        Discon
                                             resend ASR
    Discon    ASR successfully sent and      Clean up     Idle
              ASA Received with Result-Code
    Not       ASA Received                   None         No Change
    Discon
    Any       STR Received                   Send STA,    Idle
                                             Clean up
 The following state machine is observed by a client when state is not
 maintained on the server:
                            CLIENT, STATELESS
    State     Event                          Action       New State
    ---------------------------------------------------------------
    Idle      Client or device requests      Send         Pending
              access                         service-
                                             specific
                                             auth req
    Pending   Successful service-specific    Grant        Open
              authorization answer           access
              received with Auth-Session-
              State set to
              NO_STATE_MAINTAINED
    Pending   Failed service-specific        Clean up     Idle
              authorization answer
              received
    Open      Session-Timeout expires on     Discon.      Idle
              access device                  user/device
    Open      Service to user is terminated  Discon.      Idle
                                             user/device

Fajardo, et al. Standards Track [Page 103] RFC 6733 Diameter Base Protocol October 2012

 The following state machine is observed by a server when it is not
 maintaining state for the session:
                            SERVER, STATELESS
    State     Event                          Action       New State
    ---------------------------------------------------------------
    Idle      Service-specific authorization Send         Idle
              request received, and          service-
              successfully processed         specific
                                             answer

8.2. Accounting Session State Machine

 The following state machines MUST be supported for applications that
 have an accounting portion or that require only accounting services.
 The first state machine is to be observed by clients.
 See Section 9.7 for Accounting Command Codes and Section 9.8 for
 Accounting AVPs.
 The server side in the accounting state machine depends in some cases
 on the particular application.  The Diameter base protocol defines a
 default state machine that MUST be followed by all applications that
 have not specified other state machines.  This is the second state
 machine in this section described below.
 The default server side state machine requires the reception of
 accounting records in any order and at any time, and it does not
 place any standards requirement on the processing of these records.
 Implementations of Diameter may perform checking, ordering,
 correlation, fraud detection, and other tasks based on these records.
 AVPs may need to be inspected as a part of these tasks.  The tasks
 can happen either immediately after record reception or in a post-
 processing phase.  However, as these tasks are typically application
 or even policy dependent, they are not standardized by the Diameter
 specifications.  Applications MAY define requirements on when to
 accept accounting records based on the used value of Accounting-
 Realtime-Required AVP, credit-limit checks, and so on.
 However, the Diameter base protocol defines one optional server side
 state machine that MAY be followed by applications that require
 keeping track of the session state at the accounting server.  Note
 that such tracking is incompatible with the ability to sustain long
 duration connectivity problems.  Therefore, the use of this state
 machine is recommended only in applications where the value of the
 Accounting-Realtime-Required AVP is DELIVER_AND_GRANT; hence,
 accounting connectivity problems are required to cause the serviced
 user to be disconnected.  Otherwise, records produced by the client

Fajardo, et al. Standards Track [Page 104] RFC 6733 Diameter Base Protocol October 2012

 may be lost by the server, which no longer accepts them after the
 connectivity is re-established.  This state machine is the third
 state machine in this section.  The state machine is supervised by a
 supervision session timer Ts, whose value should be reasonably higher
 than the Acct_Interim_Interval value.  Ts MAY be set to two times the
 value of the Acct_Interim_Interval so as to avoid the accounting
 session in the Diameter server to change to Idle state in case of
 short transient network failure.
 Any event not listed in the state machines MUST be considered as an
 error condition, and a corresponding answer, if applicable, MUST be
 returned to the originator of the message.
 In the state table, the event "Failure to send" means that the
 Diameter client is unable to communicate with the desired
 destination.  This could be due to the peer being down, or due to the
 peer sending back a transient failure or temporary protocol error
 notification DIAMETER_OUT_OF_SPACE, DIAMETER_TOO_BUSY, or
 DIAMETER_LOOP_DETECTED in the Result-Code AVP of the Accounting
 Answer command.
 The event "Failed answer" means that the Diameter client received a
 non-transient failure notification in the Accounting Answer command.
 Note that the action "Disconnect user/dev" MUST also have an effect
 on the authorization session state table, e.g., cause the STR message
 to be sent, if the given application has both authentication/
 authorization and accounting portions.
 The states PendingS, PendingI, PendingL, PendingE, and PendingB stand
 for pending states to wait for an answer to an accounting request
 related to a Start, Interim, Stop, Event, or buffered record,
 respectively.
                          CLIENT, ACCOUNTING
    State     Event                          Action       New State
    ---------------------------------------------------------------
    Idle      Client or device requests      Send         PendingS
              access                         accounting
                                             start req.
    Idle      Client or device requests      Send         PendingE
              a one-time service             accounting
                                             event req
    Idle      Records in storage             Send         PendingB
                                             record

Fajardo, et al. Standards Track [Page 105] RFC 6733 Diameter Base Protocol October 2012

    PendingS  Successful accounting                       Open
              start answer received
    PendingS  Failure to send and buffer     Store        Open
              space available and real time  Start
              not equal to DELIVER_AND_GRANT Record
    PendingS  Failure to send and no buffer               Open
              space available and real time
              equal to GRANT_AND_LOSE
    PendingS  Failure to send and no         Disconnect   Idle
              buffer space available and     user/dev
              real time not equal to
              GRANT_AND_LOSE
    PendingS  Failed accounting start answer              Open
              received and real time equal
              to GRANT_AND_LOSE
    PendingS  Failed accounting start answer Disconnect   Idle
              received and real time not     user/dev
              equal to GRANT_AND_LOSE
    PendingS  User service terminated        Store        PendingS
                                             stop
                                             record
    Open      Interim interval elapses       Send         PendingI
                                             accounting
                                             interim
                                             record
    Open      User service terminated        Send         PendingL
                                             accounting
                                             stop req.
    PendingI  Successful accounting interim               Open
              answer received
    PendingI  Failure to send and (buffer    Store        Open
              space available or old         interim
              record can be overwritten)     record
              and real time not equal to
              DELIVER_AND_GRANT

Fajardo, et al. Standards Track [Page 106] RFC 6733 Diameter Base Protocol October 2012

    PendingI  Failure to send and no buffer               Open
              space available and real time
              equal to GRANT_AND_LOSE
    PendingI  Failure to send and no         Disconnect   Idle
              buffer space available and     user/dev
              real time not equal to
              GRANT_AND_LOSE
    PendingI  Failed accounting interim                   Open
              answer received and real time
              equal to GRANT_AND_LOSE
    PendingI  Failed accounting interim      Disconnect   Idle
              answer received and            user/dev
              real time not equal to
              GRANT_AND_LOSE
    PendingI  User service terminated        Store        PendingI
                                             stop
                                             record
    PendingE  Successful accounting                       Idle
              event answer received
    PendingE  Failure to send and buffer     Store        Idle
              space available                event
                                             record
    PendingE  Failure to send and no buffer               Idle
              space available
    PendingE  Failed accounting event answer              Idle
              received
    PendingB  Successful accounting answer   Delete       Idle
              received                       record
    PendingB  Failure to send                             Idle
    PendingB  Failed accounting answer       Delete       Idle
              received                       record
    PendingL  Successful accounting                       Idle
              stop answer received
    PendingL  Failure to send and buffer     Store        Idle
              space available                stop
                                             record

Fajardo, et al. Standards Track [Page 107] RFC 6733 Diameter Base Protocol October 2012

    PendingL  Failure to send and no buffer               Idle
              space available
    PendingL  Failed accounting stop answer               Idle
              received
                     SERVER, STATELESS ACCOUNTING
    State     Event                          Action       New State
    ---------------------------------------------------------------
    Idle      Accounting start request       Send         Idle
              received and successfully      accounting
              processed.                     start
                                             answer
    Idle      Accounting event request       Send         Idle
              received and successfully      accounting
              processed.                     event
                                             answer
    Idle      Interim record received        Send         Idle
              and successfully processed.    accounting
                                             interim
                                             answer
    Idle      Accounting stop request        Send         Idle
              received and successfully      accounting
              processed                      stop answer
    Idle      Accounting request received;   Send         Idle
              no space left to store         accounting
              records                        answer;
                                             Result-Code =
                                             OUT_OF_
                                             SPACE

Fajardo, et al. Standards Track [Page 108] RFC 6733 Diameter Base Protocol October 2012

                          SERVER, STATEFUL ACCOUNTING
    State     Event                          Action       New State
    ---------------------------------------------------------------
    Idle      Accounting start request       Send         Open
              received and successfully      accounting
              processed.                     start
                                             answer;
                                             Start Ts
    Idle      Accounting event request       Send         Idle
              received and successfully      accounting
              processed.                     event
                                             answer
    Idle      Accounting request received;   Send         Idle
              no space left to store         accounting
              records                        answer;
                                             Result-Code =
                                             OUT_OF_
                                             SPACE
    Open      Interim record received        Send         Open
              and successfully processed.    accounting
                                             interim
                                             answer;
                                             Restart Ts
    Open      Accounting stop request        Send         Idle
              received and successfully      accounting
              processed                      stop answer;
                                             Stop Ts
    Open      Accounting request received;   Send         Idle
              no space left to store         accounting
              records                        answer;
                                             Result-Code =
                                             OUT_OF_
                                             SPACE;
                                             Stop Ts
    Open      Session supervision timer Ts   Stop Ts      Idle
              expired

Fajardo, et al. Standards Track [Page 109] RFC 6733 Diameter Base Protocol October 2012

8.3. Server-Initiated Re-Auth

 A Diameter server may initiate a re-authentication and/or re-
 authorization service for a particular session by issuing a Re-Auth-
 Request (RAR).
 For example, for prepaid services, the Diameter server that
 originally authorized a session may need some confirmation that the
 user is still using the services.
 An access device that receives an RAR message with the Session-Id
 equal to a currently active session MUST initiate a re-auth towards
 the user, if the service supports this particular feature.  Each
 Diameter application MUST state whether server-initiated re-auth is
 supported, since some applications do not allow access devices to
 prompt the user for re-auth.

8.3.1. Re-Auth-Request

 The Re-Auth-Request (RAR), indicated by the Command Code set to 258
 and the message flags' 'R' bit set, may be sent by any server to the
 access device that is providing session service, to request that the
 user be re-authenticated and/or re-authorized.
  Message Format
       <RAR>  ::= < Diameter Header: 258, REQ, PXY >
                  < Session-Id >
                  { Origin-Host }
                  { Origin-Realm }
                  { Destination-Realm }
                  { Destination-Host }
                  { Auth-Application-Id }
                  { Re-Auth-Request-Type }
                  [ User-Name ]
                  [ Origin-State-Id ]
                * [ Proxy-Info ]
                * [ Route-Record ]
                * [ AVP ]

8.3.2. Re-Auth-Answer

 The Re-Auth-Answer (RAA), indicated by the Command Code set to 258
 and the message flags' 'R' bit clear, is sent in response to the RAR.
 The Result-Code AVP MUST be present, and it indicates the disposition
 of the request.

Fajardo, et al. Standards Track [Page 110] RFC 6733 Diameter Base Protocol October 2012

 A successful RAA message MUST be followed by an application-specific
 authentication and/or authorization message.
  Message Format
       <RAA>  ::= < Diameter Header: 258, PXY >
                  < Session-Id >
                  { Result-Code }
                  { Origin-Host }
                  { Origin-Realm }
                  [ User-Name ]
                  [ Origin-State-Id ]
                  [ Error-Message ]
                  [ Error-Reporting-Host ]
                  [ Failed-AVP ]
                * [ Redirect-Host ]
                  [ Redirect-Host-Usage ]
                  [ Redirect-Max-Cache-Time ]
                * [ Proxy-Info ]
                * [ AVP ]

8.4. Session Termination

 It is necessary for a Diameter server that authorized a session, for
 which it is maintaining state, to be notified when that session is no
 longer active, both for tracking purposes as well as to allow
 stateful agents to release any resources that they may have provided
 for the user's session.  For sessions whose state is not being
 maintained, this section is not used.
 When a user session that required Diameter authorization terminates,
 the access device that provided the service MUST issue a Session-
 Termination-Request (STR) message to the Diameter server that
 authorized the service, to notify it that the session is no longer
 active.  An STR MUST be issued when a user session terminates for any
 reason, including user logoff, expiration of Session-Timeout,
 administrative action, termination upon receipt of an Abort-Session-
 Request (see below), orderly shutdown of the access device, etc.
 The access device also MUST issue an STR for a session that was
 authorized but never actually started.  This could occur, for
 example, due to a sudden resource shortage in the access device, or
 because the access device is unwilling to provide the type of service
 requested in the authorization, or because the access device does not
 support a mandatory AVP returned in the authorization, etc.
 It is also possible that a session that was authorized is never
 actually started due to action of a proxy.  For example, a proxy may

Fajardo, et al. Standards Track [Page 111] RFC 6733 Diameter Base Protocol October 2012

 modify an authorization answer, converting the result from success to
 failure, prior to forwarding the message to the access device.  If
 the answer did not contain an Auth-Session-State AVP with the value
 NO_STATE_MAINTAINED, a proxy that causes an authorized session not to
 be started MUST issue an STR to the Diameter server that authorized
 the session, since the access device has no way of knowing that the
 session had been authorized.
 A Diameter server that receives an STR message MUST clean up
 resources (e.g., session state) associated with the Session-Id
 specified in the STR and return a Session-Termination-Answer.
 A Diameter server also MUST clean up resources when the Session-
 Timeout expires, or when the Authorization-Lifetime and the Auth-
 Grace-Period AVPs expire without receipt of a re-authorization
 request, regardless of whether an STR for that session is received.
 The access device is not expected to provide service beyond the
 expiration of these timers; thus, expiration of either of these
 timers implies that the access device may have unexpectedly shut
 down.

8.4.1. Session-Termination-Request

 The Session-Termination-Request (STR), indicated by the Command Code
 set to 275 and the Command Flags' 'R' bit set, is sent by a Diameter
 client or by a Diameter proxy to inform the Diameter server that an
 authenticated and/or authorized session is being terminated.
  Message Format
      <STR>  ::= < Diameter Header: 275, REQ, PXY >
                 < Session-Id >
                 { Origin-Host }
                 { Origin-Realm }
                 { Destination-Realm }
                 { Auth-Application-Id }
                 { Termination-Cause }
                 [ User-Name ]
                 [ Destination-Host ]
               * [ Class ]
                 [ Origin-State-Id ]
               * [ Proxy-Info ]
               * [ Route-Record ]
               * [ AVP ]

Fajardo, et al. Standards Track [Page 112] RFC 6733 Diameter Base Protocol October 2012

8.4.2. Session-Termination-Answer

 The Session-Termination-Answer (STA), indicated by the Command Code
 set to 275 and the message flags' 'R' bit clear, is sent by the
 Diameter server to acknowledge the notification that the session has
 been terminated.  The Result-Code AVP MUST be present, and it MAY
 contain an indication that an error occurred while servicing the STR.
 Upon sending or receipt of the STA, the Diameter server MUST release
 all resources for the session indicated by the Session-Id AVP.  Any
 intermediate server in the Proxy-Chain MAY also release any
 resources, if necessary.
  Message Format
       <STA> ::= < Diameter Header: 275, PXY >
                  < Session-Id >
                  { Result-Code }
                  { Origin-Host }
                  { Origin-Realm }
                  [ User-Name ]
                * [ Class ]
                  [ Error-Message ]
                  [ Error-Reporting-Host ]
                  [ Failed-AVP ]
                  [ Origin-State-Id ]
                * [ Redirect-Host ]
                  [ Redirect-Host-Usage ]
                  [ Redirect-Max-Cache-Time ]
                * [ Proxy-Info ]
                * [ AVP ]

8.5. Aborting a Session

 A Diameter server may request that the access device stop providing
 service for a particular session by issuing an Abort-Session-Request
 (ASR).
 For example, the Diameter server that originally authorized the
 session may be required to cause that session to be stopped for lack
 of credit or other reasons that were not anticipated when the session
 was first authorized.
 An access device that receives an ASR with Session-ID equal to a
 currently active session MAY stop the session.  Whether the access
 device stops the session or not is implementation and/or
 configuration dependent.  For example, an access device may honor
 ASRs from certain agents only.  In any case, the access device MUST

Fajardo, et al. Standards Track [Page 113] RFC 6733 Diameter Base Protocol October 2012

 respond with an Abort-Session-Answer, including a Result-Code AVP to
 indicate what action it took.

8.5.1. Abort-Session-Request

 The Abort-Session-Request (ASR), indicated by the Command Code set to
 274 and the message flags' 'R' bit set, may be sent by any Diameter
 server or any Diameter proxy to the access device that is providing
 session service, to request that the session identified by the
 Session-Id be stopped.
  Message Format
       <ASR>  ::= < Diameter Header: 274, REQ, PXY >
                  < Session-Id >
                  { Origin-Host }
                  { Origin-Realm }
                  { Destination-Realm }
                  { Destination-Host }
                  { Auth-Application-Id }
                  [ User-Name ]
                  [ Origin-State-Id ]
                * [ Proxy-Info ]
                * [ Route-Record ]
                * [ AVP ]

8.5.2. Abort-Session-Answer

 The Abort-Session-Answer (ASA), indicated by the Command Code set to
 274 and the message flags' 'R' bit clear, is sent in response to the
 ASR.  The Result-Code AVP MUST be present and indicates the
 disposition of the request.
 If the session identified by Session-Id in the ASR was successfully
 terminated, the Result-Code is set to DIAMETER_SUCCESS.  If the
 session is not currently active, the Result-Code is set to
 DIAMETER_UNKNOWN_SESSION_ID.  If the access device does not stop the
 session for any other reason, the Result-Code is set to
 DIAMETER_UNABLE_TO_COMPLY.

Fajardo, et al. Standards Track [Page 114] RFC 6733 Diameter Base Protocol October 2012

  Message Format
       <ASA>  ::= < Diameter Header: 274, PXY >
                  < Session-Id >
                  { Result-Code }
                  { Origin-Host }
                  { Origin-Realm }
                  [ User-Name ]
                  [ Origin-State-Id ]
                  [ Error-Message ]
                  [ Error-Reporting-Host ]
                  [ Failed-AVP ]
                * [ Redirect-Host ]
                  [ Redirect-Host-Usage ]
                  [ Redirect-Max-Cache-Time ]
                * [ Proxy-Info ]
                * [ AVP ]

8.6. Inferring Session Termination from Origin-State-Id

 The Origin-State-Id is used to allow detection of terminated sessions
 for which no STR would have been issued, due to unanticipated
 shutdown of an access device.
 A Diameter client or access device increments the value of the
 Origin-State-Id every time it is started or powered up.  The new
 Origin-State-Id is then sent in the CER/CEA message immediately upon
 connection to the server.  The Diameter server receiving the new
 Origin-State-Id can determine whether the sending Diameter client had
 abruptly shut down by comparing the old value of the Origin-State-Id
 it has kept for that specific client is less than the new value and
 whether it has un-terminated sessions originating from that client.
 An access device can also include the Origin-State-Id in request
 messages other than the CER if there are relays or proxies in between
 the access device and the server.  In this case, however, the server
 cannot discover that the access device has been restarted unless and
 until it receives a new request from it.  Therefore, this mechanism
 is more opportunistic across proxies and relays.
 The Diameter server may assume that all sessions that were active
 prior to detection of a client restart have been terminated.  The
 Diameter server MAY clean up all session state associated with such
 lost sessions, and it MAY also issue STRs for all such lost sessions
 that were authorized on upstream servers, to allow session state to
 be cleaned up globally.

Fajardo, et al. Standards Track [Page 115] RFC 6733 Diameter Base Protocol October 2012

8.7. Auth-Request-Type AVP

 The Auth-Request-Type AVP (AVP Code 274) is of type Enumerated and is
 included in application-specific auth requests to inform the peers
 whether a user is to be authenticated only, authorized only, or both.
 Note any value other than both MAY cause RADIUS interoperability
 issues.  The following values are defined:
 AUTHENTICATE_ONLY 1
    The request being sent is for authentication only, and it MUST
    contain the relevant application-specific authentication AVPs that
    are needed by the Diameter server to authenticate the user.
 AUTHORIZE_ONLY 2
    The request being sent is for authorization only, and it MUST
    contain the application-specific authorization AVPs that are
    necessary to identify the service being requested/offered.
 AUTHORIZE_AUTHENTICATE 3
    The request contains a request for both authentication and
    authorization.  The request MUST include both the relevant
    application-specific authentication information and authorization
    information necessary to identify the service being requested/
    offered.

8.8. Session-Id AVP

 The Session-Id AVP (AVP Code 263) is of type UTF8String and is used
 to identify a specific session (see Section 8).  All messages
 pertaining to a specific session MUST include only one Session-Id
 AVP, and the same value MUST be used throughout the life of a
 session.  When present, the Session-Id SHOULD appear immediately
 following the Diameter header (see Section 3).
 The Session-Id MUST be globally and eternally unique, as it is meant
 to uniquely identify a user session without reference to any other
 information, and it may be needed to correlate historical
 authentication information with accounting information.  The
 Session-Id includes a mandatory portion and an implementation-defined
 portion; a recommended format for the implementation-defined portion
 is outlined below.
 The Session-Id MUST begin with the sender's identity encoded in the
 DiameterIdentity type (see Section 4.3.1).  The remainder of the
 Session-Id is delimited by a ";" character, and it MAY be any

Fajardo, et al. Standards Track [Page 116] RFC 6733 Diameter Base Protocol October 2012

 sequence that the client can guarantee to be eternally unique;
 however, the following format is recommended, (square brackets []
 indicate an optional element):
    <DiameterIdentity>;<high 32 bits>;<low 32 bits>[;<optional value>]
 <high 32 bits> and <low 32 bits> are decimal representations of the
 high and low 32 bits of a monotonically increasing 64-bit value.  The
 64-bit value is rendered in two part to simplify formatting by 32-bit
 processors.  At startup, the high 32 bits of the 64-bit value MAY be
 initialized to the time in NTP format [RFC5905], and the low 32 bits
 MAY be initialized to zero.  This will for practical purposes
 eliminate the possibility of overlapping Session-Ids after a reboot,
 assuming the reboot process takes longer than a second.
 Alternatively, an implementation MAY keep track of the increasing
 value in non-volatile memory.
 <optional value> is implementation specific, but it may include a
 modem's device Id, a Layer 2 address, timestamp, etc.
 Example, in which there is no optional value:
    accesspoint7.example.com;1876543210;523
 Example, in which there is an optional value:
   accesspoint7.example.com;1876543210;523;mobile@200.1.1.88
 The Session-Id is created by the Diameter application initiating the
 session, which, in most cases, is done by the client.  Note that a
 Session-Id MAY be used for both the authentication, authorization,
 and accounting commands of a given application.

8.9. Authorization-Lifetime AVP

 The Authorization-Lifetime AVP (AVP Code 291) is of type Unsigned32
 and contains the maximum number of seconds of service to be provided
 to the user before the user is to be re-authenticated and/or re-
 authorized.  Care should be taken when the Authorization-Lifetime
 value is determined, since a low, non-zero value could create
 significant Diameter traffic, which could congest both the network
 and the agents.
 A value of zero (0) means that immediate re-auth is necessary by the
 access device.  The absence of this AVP, or a value of all ones
 (meaning all bits in the 32-bit field are set to one) means no re-
 auth is expected.

Fajardo, et al. Standards Track [Page 117] RFC 6733 Diameter Base Protocol October 2012

 If both this AVP and the Session-Timeout AVP are present in a
 message, the value of the latter MUST NOT be smaller than the
 Authorization-Lifetime AVP.
 An Authorization-Lifetime AVP MAY be present in re-authorization
 messages, and it contains the number of seconds the user is
 authorized to receive service from the time the re-auth answer
 message is received by the access device.
 This AVP MAY be provided by the client as a hint of the maximum
 lifetime that it is willing to accept.  The server MUST return a
 value that is equal to, or smaller than, the one provided by the
 client.

8.10. Auth-Grace-Period AVP

 The Auth-Grace-Period AVP (AVP Code 276) is of type Unsigned32 and
 contains the number of seconds the Diameter server will wait
 following the expiration of the Authorization-Lifetime AVP before
 cleaning up resources for the session.

8.11. Auth-Session-State AVP

 The Auth-Session-State AVP (AVP Code 277) is of type Enumerated and
 specifies whether state is maintained for a particular session.  The
 client MAY include this AVP in requests as a hint to the server, but
 the value in the server's answer message is binding.  The following
 values are supported:
 STATE_MAINTAINED 0
    This value is used to specify that session state is being
    maintained, and the access device MUST issue a session termination
    message when service to the user is terminated.  This is the
    default value.
 NO_STATE_MAINTAINED 1
    This value is used to specify that no session termination messages
    will be sent by the access device upon expiration of the
    Authorization-Lifetime.

8.12. Re-Auth-Request-Type AVP

 The Re-Auth-Request-Type AVP (AVP Code 285) is of type Enumerated and
 is included in application-specific auth answers to inform the client
 of the action expected upon expiration of the Authorization-Lifetime.

Fajardo, et al. Standards Track [Page 118] RFC 6733 Diameter Base Protocol October 2012

 If the answer message contains an Authorization-Lifetime AVP with a
 positive value, the Re-Auth-Request-Type AVP MUST be present in an
 answer message.  The following values are defined:
 AUTHORIZE_ONLY 0
    An authorization only re-auth is expected upon expiration of the
    Authorization-Lifetime.  This is the default value if the AVP is
    not present in answer messages that include the Authorization-
    Lifetime.
 AUTHORIZE_AUTHENTICATE 1
    An authentication and authorization re-auth is expected upon
    expiration of the Authorization-Lifetime.

8.13. Session-Timeout AVP

 The Session-Timeout AVP (AVP Code 27) [RFC2865] is of type Unsigned32
 and contains the maximum number of seconds of service to be provided
 to the user before termination of the session.  When both the
 Session-Timeout and the Authorization-Lifetime AVPs are present in an
 answer message, the former MUST be equal to or greater than the value
 of the latter.
 A session that terminates on an access device due to the expiration
 of the Session-Timeout MUST cause an STR to be issued, unless both
 the access device and the home server had previously agreed that no
 session termination messages would be sent (see Section 8).
 A Session-Timeout AVP MAY be present in a re-authorization answer
 message, and it contains the remaining number of seconds from the
 beginning of the re-auth.
 A value of zero, or the absence of this AVP, means that this session
 has an unlimited number of seconds before termination.
 This AVP MAY be provided by the client as a hint of the maximum
 timeout that it is willing to accept.  However, the server MAY return
 a value that is equal to, or smaller than, the one provided by the
 client.

8.14. User-Name AVP

 The User-Name AVP (AVP Code 1) [RFC2865] is of type UTF8String, which
 contains the User-Name, in a format consistent with the NAI
 specification [RFC4282].

Fajardo, et al. Standards Track [Page 119] RFC 6733 Diameter Base Protocol October 2012

8.15. Termination-Cause AVP

 The Termination-Cause AVP (AVP Code 295) is of type Enumerated, and
 is used to indicate the reason why a session was terminated on the
 access device.  The currently assigned values for this AVP can be
 found in the IANA registry for Termination-Cause AVP Values
 [IANATCV].

8.16. Origin-State-Id AVP

 The Origin-State-Id AVP (AVP Code 278), of type Unsigned32, is a
 monotonically increasing value that is advanced whenever a Diameter
 entity restarts with loss of previous state, for example, upon
 reboot.  Origin-State-Id MAY be included in any Diameter message,
 including CER.
 A Diameter entity issuing this AVP MUST create a higher value for
 this AVP each time its state is reset.  A Diameter entity MAY set
 Origin-State-Id to the time of startup, or it MAY use an incrementing
 counter retained in non-volatile memory across restarts.
 The Origin-State-Id, if present, MUST reflect the state of the entity
 indicated by Origin-Host.  If a proxy modifies Origin-Host, it MUST
 either remove Origin-State-Id or modify it appropriately as well.
 Typically, Origin-State-Id is used by an access device that always
 starts up with no active sessions; that is, any session active prior
 to restart will have been lost.  By including Origin-State-Id in a
 message, it allows other Diameter entities to infer that sessions
 associated with a lower Origin-State-Id are no longer active.  If an
 access device does not intend for such inferences to be made, it MUST
 either not include Origin-State-Id in any message or set its value to
 0.

8.17. Session-Binding AVP

 The Session-Binding AVP (AVP Code 270) is of type Unsigned32, and it
 MAY be present in application-specific authorization answer messages.
 If present, this AVP MAY inform the Diameter client that all future
 application-specific re-auth and Session-Termination-Request messages
 for this session MUST be sent to the same authorization server.

Fajardo, et al. Standards Track [Page 120] RFC 6733 Diameter Base Protocol October 2012

 This field is a bit mask, and the following bits have been defined:
 RE_AUTH 1
    When set, future re-auth messages for this session MUST NOT
    include the Destination-Host AVP.  When cleared, the default
    value, the Destination-Host AVP MUST be present in all re-auth
    messages for this session.
 STR 2
    When set, the STR message for this session MUST NOT include the
    Destination-Host AVP.  When cleared, the default value, the
    Destination-Host AVP MUST be present in the STR message for this
    session.
 ACCOUNTING 4
    When set, all accounting messages for this session MUST NOT
    include the Destination-Host AVP.  When cleared, the default
    value, the Destination-Host AVP, if known, MUST be present in all
    accounting messages for this session.

8.18. Session-Server-Failover AVP

 The Session-Server-Failover AVP (AVP Code 271) is of type Enumerated
 and MAY be present in application-specific authorization answer
 messages that either do not include the Session-Binding AVP or
 include the Session-Binding AVP with any of the bits set to a zero
 value.  If present, this AVP MAY inform the Diameter client that if a
 re-auth or STR message fails due to a delivery problem, the Diameter
 client SHOULD issue a subsequent message without the Destination-Host
 AVP.  When absent, the default value is REFUSE_SERVICE.
 The following values are supported:
 REFUSE_SERVICE 0
    If either the re-auth or the STR message delivery fails, terminate
    service with the user and do not attempt any subsequent attempts.

Fajardo, et al. Standards Track [Page 121] RFC 6733 Diameter Base Protocol October 2012

 TRY_AGAIN 1
    If either the re-auth or the STR message delivery fails, resend
    the failed message without the Destination-Host AVP present.
 ALLOW_SERVICE 2
    If re-auth message delivery fails, assume that re-authorization
    succeeded.  If STR message delivery fails, terminate the session.
 TRY_AGAIN_ALLOW_SERVICE 3
    If either the re-auth or the STR message delivery fails, resend
    the failed message without the Destination-Host AVP present.  If
    the second delivery fails for re-auth, assume re-authorization
    succeeded.  If the second delivery fails for STR, terminate the
    session.

8.19. Multi-Round-Time-Out AVP

 The Multi-Round-Time-Out AVP (AVP Code 272) is of type Unsigned32 and
 SHOULD be present in application-specific authorization answer
 messages whose Result-Code AVP is set to DIAMETER_MULTI_ROUND_AUTH.
 This AVP contains the maximum number of seconds that the access
 device MUST provide the user in responding to an authentication
 request.

8.20. Class AVP

 The Class AVP (AVP Code 25) is of type OctetString and is used by
 Diameter servers to return state information to the access device.
 When one or more Class AVPs are present in application-specific
 authorization answer messages, they MUST be present in subsequent re-
 authorization, session termination and accounting messages.  Class
 AVPs found in a re-authorization answer message override the ones
 found in any previous authorization answer message.  Diameter server
 implementations SHOULD NOT return Class AVPs that require more than
 4096 bytes of storage on the Diameter client.  A Diameter client that
 receives Class AVPs whose size exceeds local available storage MUST
 terminate the session.

8.21. Event-Timestamp AVP

 The Event-Timestamp (AVP Code 55) is of type Time and MAY be included
 in an Accounting-Request and Accounting-Answer messages to record the
 time that the reported event occurred, in seconds since January 1,
 1900 00:00 UTC.

Fajardo, et al. Standards Track [Page 122] RFC 6733 Diameter Base Protocol October 2012

9. Accounting

 This accounting protocol is based on a server directed model with
 capabilities for real-time delivery of accounting information.
 Several fault resilience methods [RFC2975] have been built into the
 protocol in order minimize loss of accounting data in various fault
 situations and under different assumptions about the capabilities of
 the used devices.

9.1. Server Directed Model

 The server directed model means that the device generating the
 accounting data gets information from either the authorization server
 (if contacted) or the accounting server regarding the way accounting
 data shall be forwarded.  This information includes accounting record
 timeliness requirements.
 As discussed in [RFC2975], real-time transfer of accounting records
 is a requirement, such as the need to perform credit-limit checks and
 fraud detection.  Note that batch accounting is not a requirement,
 and is therefore not supported by Diameter.  Should batched
 accounting be required in the future, a new Diameter application will
 need to be created, or it could be handled using another protocol.
 Note, however, that even if at the Diameter layer, accounting
 requests are processed one by one; transport protocols used under
 Diameter typically batch several requests in the same packet under
 heavy traffic conditions.  This may be sufficient for many
 applications.
 The authorization server (chain) directs the selection of proper
 transfer strategy, based on its knowledge of the user and
 relationships of roaming partnerships.  The server (or agents) uses
 the Acct-Interim-Interval and Accounting-Realtime-Required AVPs to
 control the operation of the Diameter peer operating as a client.
 The Acct-Interim-Interval AVP, when present, instructs the Diameter
 node acting as a client to produce accounting records continuously
 even during a session.  Accounting-Realtime-Required AVP is used to
 control the behavior of the client when the transfer of accounting
 records from the Diameter client is delayed or unsuccessful.
 The Diameter accounting server MAY override the interim interval or
 the real-time requirements by including the Acct-Interim-Interval or
 Accounting-Realtime-Required AVP in the Accounting-Answer message.
 When one of these AVPs is present, the latest value received SHOULD
 be used in further accounting activities for the same session.

Fajardo, et al. Standards Track [Page 123] RFC 6733 Diameter Base Protocol October 2012

9.2. Protocol Messages

 A Diameter node that receives a successful authentication and/or
 authorization message from the Diameter server SHOULD collect
 accounting information for the session.  The Accounting-Request
 message is used to transmit the accounting information to the
 Diameter server, which MUST reply with the Accounting-Answer message
 to confirm reception.  The Accounting-Answer message includes the
 Result-Code AVP, which MAY indicate that an error was present in the
 accounting message.  The value of the Accounting-Realtime-Required
 AVP received earlier for the session in question may indicate that
 the user's session has to be terminated when a rejected Accounting-
 Request message was received.

9.3. Accounting Application Extension and Requirements

 Each Diameter application (e.g., NASREQ, Mobile IP) SHOULD define its
 service-specific AVPs that MUST be present in the Accounting-Request
 message in a section titled "Accounting AVPs".  The application MUST
 assume that the AVPs described in this document will be present in
 all Accounting messages, so only their respective service-specific
 AVPs need to be defined in that section.
 Applications have the option of using one or both of the following
 accounting application extension models:
 Split Accounting Service
    The accounting message will carry the Application Id of the
    Diameter base accounting application (see Section 2.4).
    Accounting messages may be routed to Diameter nodes other than the
    corresponding Diameter application.  These nodes might be
    centralized accounting servers that provide accounting service for
    multiple different Diameter applications.  These nodes MUST
    advertise the Diameter base accounting Application Id during
    capabilities exchange.
 Coupled Accounting Service
    The accounting message will carry the Application Id of the
    application that is using it.  The application itself will process
    the received accounting records or forward them to an accounting
    server.  There is no accounting application advertisement required
    during capabilities exchange, and the accounting messages will be
    routed the same way as any of the other application messages.
 In cases where an application does not define its own accounting
 service, it is preferred that the split accounting model be used.

Fajardo, et al. Standards Track [Page 124] RFC 6733 Diameter Base Protocol October 2012

9.4. Fault Resilience

 Diameter base protocol mechanisms are used to overcome small message
 loss and network faults of a temporary nature.
 Diameter peers acting as clients MUST implement the use of failover
 to guard against server failures and certain network failures.
 Diameter peers acting as agents or related off-line processing
 systems MUST detect duplicate accounting records caused by the
 sending of the same record to several servers and duplication of
 messages in transit.  This detection MUST be based on the inspection
 of the Session-Id and Accounting-Record-Number AVP pairs.  Appendix C
 discusses duplicate detection needs and implementation issues.
 Diameter clients MAY have non-volatile memory for the safe storage of
 accounting records over reboots or extended network failures, network
 partitions, and server failures.  If such memory is available, the
 client SHOULD store new accounting records there as soon as the
 records are created and until a positive acknowledgement of their
 reception from the Diameter server has been received.  Upon a reboot,
 the client MUST start sending the records in the non-volatile memory
 to the accounting server with the appropriate modifications in
 termination cause, session length, and other relevant information in
 the records.
 A further application of this protocol may include AVPs to control
 the maximum number of accounting records that may be stored in the
 Diameter client without committing them to the non-volatile memory or
 transferring them to the Diameter server.
 The client SHOULD NOT remove the accounting data from any of its
 memory areas before the correct Accounting-Answer has been received.
 The client MAY remove the oldest, undelivered, or as yet
 unacknowledged accounting data if it runs out of resources such as
 memory.  It is an implementation-dependent matter for the client to
 accept new sessions under this condition.

9.5. Accounting Records

 In all accounting records, the Session-Id AVP MUST be present; the
 User-Name AVP MUST be present if it is available to the Diameter
 client.
 Different types of accounting records are sent depending on the
 actual type of accounted service and the authorization server's
 directions for interim accounting.  If the accounted service is a

Fajardo, et al. Standards Track [Page 125] RFC 6733 Diameter Base Protocol October 2012

 one-time event, meaning that the start and stop of the event are
 simultaneous, then the Accounting-Record-Type AVP MUST be present and
 set to the value EVENT_RECORD.
 If the accounted service is of a measurable length, then the AVP MUST
 use the values START_RECORD, STOP_RECORD, and possibly,
 INTERIM_RECORD.  If the authorization server has not directed interim
 accounting to be enabled for the session, two accounting records MUST
 be generated for each service of type session.  When the initial
 Accounting-Request for a given session is sent, the Accounting-
 Record-Type AVP MUST be set to the value START_RECORD.  When the last
 Accounting-Request is sent, the value MUST be STOP_RECORD.
 If the authorization server has directed interim accounting to be
 enabled, the Diameter client MUST produce additional records between
 the START_RECORD and STOP_RECORD, marked INTERIM_RECORD.  The
 production of these records is directed by Acct-Interim-Interval as
 well as any re-authentication or re-authorization of the session.
 The Diameter client MUST overwrite any previous interim accounting
 records that are locally stored for delivery, if a new record is
 being generated for the same session.  This ensures that only one
 pending interim record can exist on an access device for any given
 session.
 A particular value of Accounting-Sub-Session-Id MUST appear only in
 one sequence of accounting records from a Diameter client, except for
 the purposes of retransmission.  The one sequence that is sent MUST
 be either one record with Accounting-Record-Type AVP set to the value
 EVENT_RECORD or several records starting with one having the value
 START_RECORD, followed by zero or more INTERIM_RECORDs and a single
 STOP_RECORD.  A particular Diameter application specification MUST
 define the type of sequences that MUST be used.

9.6. Correlation of Accounting Records

 If an application uses accounting messages, it can correlate
 accounting records with a specific application session by using the
 Session-Id of the particular application session in the accounting
 messages.  Accounting messages MAY also use a different Session-Id
 from that of the application sessions, in which case, other session-
 related information is needed to perform correlation.
 In cases where an application requires multiple accounting sub-
 sessions, an Accounting-Sub-Session-Id AVP is used to differentiate
 each sub-session.  The Session-Id would remain constant for all sub-
 sessions and is used to correlate all the sub-sessions to a
 particular application session.  Note that receiving a STOP_RECORD

Fajardo, et al. Standards Track [Page 126] RFC 6733 Diameter Base Protocol October 2012

 with no Accounting-Sub-Session-Id AVP when sub-sessions were
 originally used in the START_RECORD messages implies that all sub-
 sessions are terminated.
 There are also cases where an application needs to correlate multiple
 application sessions into a single accounting record; the accounting
 record may span multiple different Diameter applications and sessions
 used by the same user at a given time.  In such cases, the Acct-
 Multi-Session-Id AVP is used.  The Acct-Multi-Session-Id AVP SHOULD
 be signaled by the server to the access device (typically, during
 authorization) when it determines that a request belongs to an
 existing session.  The access device MUST then include the Acct-
 Multi-Session-Id AVP in all subsequent accounting messages.
 The Acct-Multi-Session-Id AVP MAY include the value of the original
 Session-Id.  Its contents are implementation specific, but the MUST
 be globally unique across other Acct-Multi-Session-Ids and MUST NOT
 change during the life of a session.
 A Diameter application document MUST define the exact concept of a
 session that is being accounted, and it MAY define the concept of a
 multi-session.  For instance, the NASREQ DIAMETER application treats
 a single PPP connection to a Network Access Server as one session and
 a set of Multilink PPP sessions as one multi-session.

9.7. Accounting Command Codes

 This section defines Command Code values that MUST be supported by
 all Diameter implementations that provide accounting services.

9.7.1. Accounting-Request

 The Accounting-Request (ACR) command, indicated by the Command Code
 field set to 271 and the Command Flags' 'R' bit set, is sent by a
 Diameter node, acting as a client, in order to exchange accounting
 information with a peer.
 In addition to the AVPs listed below, Accounting-Request messages
 SHOULD include service-specific accounting AVPs.

Fajardo, et al. Standards Track [Page 127] RFC 6733 Diameter Base Protocol October 2012

    Message Format
       <ACR> ::= < Diameter Header: 271, REQ, PXY >
                 < Session-Id >
                 { Origin-Host }
                 { Origin-Realm }
                 { Destination-Realm }
                 { Accounting-Record-Type }
                 { Accounting-Record-Number }
                 [ Acct-Application-Id ]
                 [ Vendor-Specific-Application-Id ]
                 [ User-Name ]
                 [ Destination-Host ]
                 [ Accounting-Sub-Session-Id ]
                 [ Acct-Session-Id ]
                 [ Acct-Multi-Session-Id ]
                 [ Acct-Interim-Interval ]
                 [ Accounting-Realtime-Required ]
                 [ Origin-State-Id ]
                 [ Event-Timestamp ]
               * [ Proxy-Info ]
               * [ Route-Record ]
               * [ AVP ]

9.7.2. Accounting-Answer

 The Accounting-Answer (ACA) command, indicated by the Command Code
 field set to 271 and the Command Flags' 'R' bit cleared, is used to
 acknowledge an Accounting-Request command.  The Accounting-Answer
 command contains the same Session-Id as the corresponding request.
 Only the target Diameter server, known as the home Diameter server,
 SHOULD respond with the Accounting-Answer command.
 In addition to the AVPs listed below, Accounting-Answer messages
 SHOULD include service-specific accounting AVPs.

Fajardo, et al. Standards Track [Page 128] RFC 6733 Diameter Base Protocol October 2012

    Message Format
       <ACA> ::= < Diameter Header: 271, PXY >
                 < Session-Id >
                 { Result-Code }
                 { Origin-Host }
                 { Origin-Realm }
                 { Accounting-Record-Type }
                 { Accounting-Record-Number }
                 [ Acct-Application-Id ]
                 [ Vendor-Specific-Application-Id ]
                 [ User-Name ]
                 [ Accounting-Sub-Session-Id ]
                 [ Acct-Session-Id ]
                 [ Acct-Multi-Session-Id ]
                 [ Error-Message ]
                 [ Error-Reporting-Host ]
                 [ Failed-AVP ]
                 [ Acct-Interim-Interval ]
                 [ Accounting-Realtime-Required ]
                 [ Origin-State-Id ]
                 [ Event-Timestamp ]
               * [ Proxy-Info ]
               * [ AVP ]

9.8. Accounting AVPs

 This section contains AVPs that describe accounting usage information
 related to a specific session.

9.8.1. Accounting-Record-Type AVP

 The Accounting-Record-Type AVP (AVP Code 480) is of type Enumerated
 and contains the type of accounting record being sent.  The following
 values are currently defined for the Accounting-Record-Type AVP:
 EVENT_RECORD 1
    An Accounting Event Record is used to indicate that a one-time
    event has occurred (meaning that the start and end of the event
    are simultaneous).  This record contains all information relevant
    to the service, and it is the only record of the service.

Fajardo, et al. Standards Track [Page 129] RFC 6733 Diameter Base Protocol October 2012

 START_RECORD 2
    Accounting Start, Interim, and Stop Records are used to indicate
    that a service of a measurable length has been given.  An
    Accounting Start Record is used to initiate an accounting session
    and contains accounting information that is relevant to the
    initiation of the session.
 INTERIM_RECORD 3
    An Interim Accounting Record contains cumulative accounting
    information for an existing accounting session.  Interim
    Accounting Records SHOULD be sent every time a re-authentication
    or re-authorization occurs.  Further, additional interim record
    triggers MAY be defined by application-specific Diameter
    applications.  The selection of whether to use INTERIM_RECORD
    records is done by the Acct-Interim-Interval AVP.
 STOP_RECORD 4
    An Accounting Stop Record is sent to terminate an accounting
    session and contains cumulative accounting information relevant to
    the existing session.

9.8.2. Acct-Interim-Interval AVP

 The Acct-Interim-Interval AVP (AVP Code 85) is of type Unsigned32 and
 is sent from the Diameter home authorization server to the Diameter
 client.  The client uses information in this AVP to decide how and
 when to produce accounting records.  With different values in this
 AVP, service sessions can result in one, two, or two+N accounting
 records, based on the needs of the home organization.  The following
 accounting record production behavior is directed by the inclusion of
 this AVP:
 1.  The omission of the Acct-Interim-Interval AVP or its inclusion
     with Value field set to 0 means that EVENT_RECORD, START_RECORD,
     and STOP_RECORD are produced, as appropriate for the service.
 2.  The inclusion of the AVP with Value field set to a non-zero value
     means that INTERIM_RECORD records MUST be produced between the
     START_RECORD and STOP_RECORD records.  The Value field of this
     AVP is the nominal interval between these records in seconds.
     The Diameter node that originates the accounting information,
     known as the client, MUST produce the first INTERIM_RECORD record
     roughly at the time when this nominal interval has elapsed from

Fajardo, et al. Standards Track [Page 130] RFC 6733 Diameter Base Protocol October 2012

     the START_RECORD, the next one again as the interval has elapsed
     once more, and so on until the session ends and a STOP_RECORD
     record is produced.
     The client MUST ensure that the interim record production times
     are randomized so that large accounting message storms are not
     created either among records or around a common service start
     time.

9.8.3. Accounting-Record-Number AVP

 The Accounting-Record-Number AVP (AVP Code 485) is of type Unsigned32
 and identifies this record within one session.  As Session-Id AVPs
 are globally unique, the combination of Session-Id and Accounting-
 Record-Number AVPs is also globally unique and can be used in
 matching accounting records with confirmations.  An easy way to
 produce unique numbers is to set the value to 0 for records of type
 EVENT_RECORD and START_RECORD and set the value to 1 for the first
 INTERIM_RECORD, 2 for the second, and so on until the value for
 STOP_RECORD is one more than for the last INTERIM_RECORD.

9.8.4. Acct-Session-Id AVP

 The Acct-Session-Id AVP (AVP Code 44) is of type OctetString is only
 used when RADIUS/Diameter translation occurs.  This AVP contains the
 contents of the RADIUS Acct-Session-Id attribute.

9.8.5. Acct-Multi-Session-Id AVP

 The Acct-Multi-Session-Id AVP (AVP Code 50) is of type UTF8String,
 following the format specified in Section 8.8.  The Acct-Multi-
 Session-Id AVP is used to link multiple related accounting sessions,
 where each session would have a unique Session-Id but the same Acct-
 Multi-Session-Id AVP.  This AVP MAY be returned by the Diameter
 server in an authorization answer, and it MUST be used in all
 accounting messages for the given session.

9.8.6. Accounting-Sub-Session-Id AVP

 The Accounting-Sub-Session-Id AVP (AVP Code 287) is of type
 Unsigned64 and contains the accounting sub-session identifier.  The
 combination of the Session-Id 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 no
 sub-sessions are in use, with the exception of an Accounting-Request
 whose Accounting-Record-Type is set to STOP_RECORD.  A STOP_RECORD
 message with no Accounting-Sub-Session-Id AVP present will signal the
 termination of all sub-sessions for a given Session-Id.

Fajardo, et al. Standards Track [Page 131] RFC 6733 Diameter Base Protocol October 2012

9.8.7. Accounting-Realtime-Required AVP

 The Accounting-Realtime-Required AVP (AVP Code 483) is of type
 Enumerated and is sent from the Diameter home authorization server to
 the Diameter client or in the Accounting-Answer from the accounting
 server.  The client uses information in this AVP to decide what to do
 if the sending of accounting records to the accounting server has
 been temporarily prevented due to, for instance, a network problem.
 DELIVER_AND_GRANT 1
    The AVP with Value field set to DELIVER_AND_GRANT means that the
    service MUST only be granted as long as there is a connection to
    an accounting server.  Note that the set of alternative accounting
    servers are treated as one server in this sense.  Having to move
    the accounting record stream to a backup server is not a reason to
    discontinue the service to the user.
 GRANT_AND_STORE 2
    The AVP with Value field set to GRANT_AND_STORE means that service
    SHOULD be granted if there is a connection, or as long as records
    can still be stored as described in Section 9.4.
    This is the default behavior if the AVP isn't included in the
    reply from the authorization server.
 GRANT_AND_LOSE 3
    The AVP with Value field set to GRANT_AND_LOSE means that service
    SHOULD be granted even if the records cannot be delivered or
    stored.

10. AVP Occurrence Tables

 The following tables present the AVPs defined in this document and
 specify in which Diameter messages they MAY or MAY NOT be present.
 AVPs that occur only inside a Grouped AVP are not shown in these
 tables.
 The tables use 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.

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 0-1   Zero or one instance of the AVP MAY be present in the message.
       It is considered an error if there are more than one instance
       of the AVP.
 1     One instance of the AVP MUST be present in the message.
 1+    At least one instance of the AVP MUST be present in the
       message.

10.1. Base Protocol Command AVP Table

 The table in this section is limited to the non-Accounting Command
 Codes defined in this specification.
                     +-----------------------------------------------+
                     |                  Command Code                 |
                     +---+---+---+---+---+---+---+---+---+---+---+---+
 Attribute Name      |CER|CEA|DPR|DPA|DWR|DWA|RAR|RAA|ASR|ASA|STR|STA|
 --------------------+---+---+---+---+---+---+---+---+---+---+---+---+
 Acct-Interim-       |0  |0  |0  |0  |0  |0  |0-1|0  |0  |0  |0  |0  |
   Interval          |   |   |   |   |   |   |   |   |   |   |   |   |
 Accounting-Realtime-|0  |0  |0  |0  |0  |0  |0-1|0  |0  |0  |0  |0  |
   Required          |   |   |   |   |   |   |   |   |   |   |   |   |
 Acct-Application-Id |0+ |0+ |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
 Auth-Application-Id |0+ |0+ |0  |0  |0  |0  |1  |0  |1  |0  |1  |0  |
 Auth-Grace-Period   |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
 Auth-Request-Type   |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
 Auth-Session-State  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
 Authorization-      |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
   Lifetime          |   |   |   |   |   |   |   |   |   |   |   |   |
 Class               |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0+ |0+ |
 Destination-Host    |0  |0  |0  |0  |0  |0  |1  |0  |1  |0  |0-1|0  |
 Destination-Realm   |0  |0  |0  |0  |0  |0  |1  |0  |1  |0  |1  |0  |
 Disconnect-Cause    |0  |0  |1  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
 Error-Message       |0  |0-1|0  |0-1|0  |0-1|0  |0-1|0  |0-1|0  |0-1|
 Error-Reporting-Host|0  |0  |0  |0  |0  |0  |0  |0-1|0  |0-1|0  |0-1|
 Failed-AVP          |0  |0-1|0  |0-1|0  |0-1|0  |0-1|0  |0-1|0  |0-1|
 Firmware-Revision   |0-1|0-1|0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
 Host-IP-Address     |1+ |1+ |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
 Inband-Security-Id  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
 Multi-Round-Time-Out|0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |

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 Origin-Host         |1  |1  |1  |1  |1  |1  |1  |1  |1  |1  |1  |1  |
 Origin-Realm        |1  |1  |1  |1  |1  |1  |1  |1  |1  |1  |1  |1  |
 Origin-State-Id     |0-1|0-1|0  |0  |0-1|0-1|0-1|0-1|0-1|0-1|0-1|0-1|
 Product-Name        |1  |1  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
 Proxy-Info          |0  |0  |0  |0  |0  |0  |0+ |0+ |0+ |0+ |0+ |0+ |
 Redirect-Host       |0  |0  |0  |0  |0  |0  |0  |0+ |0  |0+ |0  |0+ |
 Redirect-Host-Usage |0  |0  |0  |0  |0  |0  |0  |0-1|0  |0-1|0  |0-1|
 Redirect-Max-Cache- |0  |0  |0  |0  |0  |0  |0  |0-1|0  |0-1|0  |0-1|
   Time              |   |   |   |   |   |   |   |   |   |   |   |   |
 Result-Code         |0  |1  |0  |1  |0  |1  |0  |1  |0  |1  |0  |1  |
 Re-Auth-Request-Type|0  |0  |0  |0  |0  |0  |1  |0  |0  |0  |0  |0  |
 Route-Record        |0  |0  |0  |0  |0  |0  |0+ |0  |0+ |0  |0+ |0  |
 Session-Binding     |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
 Session-Id          |0  |0  |0  |0  |0  |0  |1  |1  |1  |1  |1  |1  |
 Session-Server-     |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
   Failover          |   |   |   |   |   |   |   |   |   |   |   |   |
 Session-Timeout     |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
 Supported-Vendor-Id |0+ |0+ |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
 Termination-Cause   |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |1  |0  |
 User-Name           |0  |0  |0  |0  |0  |0  |0-1|0-1|0-1|0-1|0-1|0-1|
 Vendor-Id           |1  |1  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
 Vendor-Specific-    |0+ |0+ |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
   Application-Id    |   |   |   |   |   |   |   |   |   |   |   |   |
 --------------------+---+---+---+---+---+---+---+---+---+---+---+---+

10.2. Accounting AVP Table

 The table in this section is used to represent which AVPs defined in
 this document are to be present in the Accounting messages.  These
 AVP occurrence requirements are guidelines, which may be expanded,
 and/or overridden by application-specific requirements in the
 Diameter applications documents.

Fajardo, et al. Standards Track [Page 134] RFC 6733 Diameter Base Protocol October 2012

                                  +-----------+
                                  |  Command  |
                                  |    Code   |
                                  +-----+-----+
    Attribute Name                | ACR | ACA |
    ------------------------------+-----+-----+
    Acct-Interim-Interval         | 0-1 | 0-1 |
    Acct-Multi-Session-Id         | 0-1 | 0-1 |
    Accounting-Record-Number      | 1   | 1   |
    Accounting-Record-Type        | 1   | 1   |
    Acct-Session-Id               | 0-1 | 0-1 |
    Accounting-Sub-Session-Id     | 0-1 | 0-1 |
    Accounting-Realtime-Required  | 0-1 | 0-1 |
    Acct-Application-Id           | 0-1 | 0-1 |
    Auth-Application-Id           | 0   | 0   |
    Class                         | 0+  | 0+  |
    Destination-Host              | 0-1 | 0   |
    Destination-Realm             | 1   | 0   |
    Error-Reporting-Host          | 0   | 0+  |
    Event-Timestamp               | 0-1 | 0-1 |
    Failed-AVP                    | 0   | 0-1 |
    Origin-Host                   | 1   | 1   |
    Origin-Realm                  | 1   | 1   |
    Proxy-Info                    | 0+  | 0+  |
    Route-Record                  | 0+  | 0   |
    Result-Code                   | 0   | 1   |
    Session-Id                    | 1   | 1   |
    Termination-Cause             | 0   | 0   |
    User-Name                     | 0-1 | 0-1 |
    Vendor-Specific-Application-Id| 0-1 | 0-1 |
    ------------------------------+-----+-----+

11. IANA Considerations

 This section provides guidance to the Internet Assigned Numbers
 Authority (IANA) regarding registration of values related to the
 Diameter protocol, in accordance with [RFC5226].  Existing IANA
 registries and assignments put in place by RFC 3588 remain the same
 unless explicitly updated or deprecated in this section.

11.1. AVP Header

 As defined in Section 4, the AVP header contains three fields that
 require IANA namespace management: the AVP Code, Vendor-ID, and Flags
 fields.

Fajardo, et al. Standards Track [Page 135] RFC 6733 Diameter Base Protocol October 2012

11.1.1. AVP Codes

 There are multiple namespaces.  Vendors can have their own AVP Codes
 namespace that will be identified by their Vendor-ID (also known as
 Enterprise-Number), and they control the assignments of their vendor-
 specific AVP Codes within their own namespace.  The absence of a
 Vendor-ID or a Vendor-ID value of zero (0) identifies the IETF AVP
 Codes namespace, which is under IANA control.  The AVP Codes and
 sometimes possible values in an AVP are controlled and maintained by
 IANA.  AVP Code 0 is not used.  AVP Codes 1-255 are managed
 separately as RADIUS Attribute Types.  Where a Vendor-Specific AVP is
 implemented by more than one vendor, allocation of global AVPs should
 be encouraged instead.
 AVPs may be allocated following Expert Review (by a Designated
 Expert) with Specification Required [RFC5226].  A block allocation
 (release of more than three AVPs at a time for a given purpose)
 requires IETF Review [RFC5226].

11.1.2. AVP Flags

 Section 4.1 describes the existing AVP Flags.  The remaining bits can
 only be assigned via a Standards Action [RFC5226].

11.2. Diameter Header

11.2.1. Command Codes

 For the Diameter header, the Command Code namespace allocation has
 changed.  The new allocation rules are as follows:
    The Command Code values 256 - 8,388,607 (0x100 to 0x7fffff) are
    for permanent, standard commands, allocated by IETF Review
    [RFC5226].
    The values 8,388,608 - 16,777,213 (0x800000 - 0xfffffd) are
    reserved for vendor-specific Command Codes, to be allocated on a
    First Come, First Served basis by IANA [RFC5226].  The request to
    IANA for a Vendor-Specific Command Code SHOULD include a reference
    to a publicly available specification that documents the command
    in sufficient detail to aid in interoperability between
    independent implementations.  If the specification cannot be made
    publicly available, the request for a vendor-specific Command Code
    MUST include the contact information of persons and/or entities
    responsible for authoring and maintaining the command.

Fajardo, et al. Standards Track [Page 136] RFC 6733 Diameter Base Protocol October 2012

    The values 16,777,214 and 16,777,215 (hexadecimal values 0xfffffe
    - 0xffffff) are reserved for experimental commands.  As these
    codes are only for experimental and testing purposes, no guarantee
    is made for interoperability between Diameter peers using
    experimental commands.

11.2.2. Command Flags

 Section 3 describes the existing Command Flags field.  The remaining
 bits can only be assigned via a Standards Action [RFC5226].

11.3. AVP Values

 For AVP values, the Experimental-Result-Code AVP value allocation has
 been added; see Section 11.3.1.  The old AVP value allocation rule,
 IETF Consensus, has been updated to IETF Review as per [RFC5226], and
 affected AVPs are listed as reminders.

11.3.1. Experimental-Result-Code AVP

 Values for this AVP are purely local to the indicated vendor, and no
 IANA registry is maintained for them.

11.3.2. Result-Code AVP Values

 New values are available for assignment via IETF Review [RFC5226].

11.3.3. Accounting-Record-Type AVP Values

 New values are available for assignment via IETF Review [RFC5226].

11.3.4. Termination-Cause AVP Values

 New values are available for assignment via IETF Review [RFC5226].

11.3.5. Redirect-Host-Usage AVP Values

 New values are available for assignment via IETF Review [RFC5226].

11.3.6. Session-Server-Failover AVP Values

 New values are available for assignment via IETF Review [RFC5226].

11.3.7. Session-Binding AVP Values

 New values are available for assignment via IETF Review [RFC5226].

Fajardo, et al. Standards Track [Page 137] RFC 6733 Diameter Base Protocol October 2012

11.3.8. Disconnect-Cause AVP Values

 New values are available for assignment via IETF Review [RFC5226].

11.3.9. Auth-Request-Type AVP Values

 New values are available for assignment via IETF Review [RFC5226].

11.3.10. Auth-Session-State AVP Values

 New values are available for assignment via IETF Review [RFC5226].

11.3.11. Re-Auth-Request-Type AVP Values

 New values are available for assignment via IETF Review [RFC5226].

11.3.12. Accounting-Realtime-Required AVP Values

 New values are available for assignment via IETF Review [RFC5226].

11.3.13. Inband-Security-Id AVP (code 299)

 The use of this AVP has been deprecated.

11.4. _diameters Service Name and Port Number Registration

 IANA has registered the "_diameters" service name and assigned port
 numbers for TLS/TCP and DTLS/SCTP according to the guidelines given
 in [RFC6335].
    Service Name:         _diameters
    Transport Protocols:  TCP, SCTP
    Assignee:             IESG <iesg@ietf.org>
    Contact:              IETF Chair <chair@ietf.org>
    Description:          Diameter over TLS/TCP and DTLS/SCTP
    Reference:            RFC 6733
    Port  Number:         5868, from the User Range

Fajardo, et al. Standards Track [Page 138] RFC 6733 Diameter Base Protocol October 2012

11.5. SCTP Payload Protocol Identifiers

 Two SCTP payload protocol identifiers have been registered in the
 SCTP Payload Protocol Identifiers registry:
  Value | SCTP Payload Protocol Identifier
 -------|-----------------------------------
   46   | Diameter in a SCTP DATA chunk
   47   | Diameter in a DTLS/SCTP DATA chunk

11.6. S-NAPTR Parameters

 The following tag has been registered in the S-NAPTR Application
 Protocol Tags registry:
 Tag                | Protocol
 -------------------|---------
 diameter.dtls.sctp | DTLS/SCTP

12. Diameter Protocol-Related Configurable Parameters

 This section contains the configurable parameters that are found
 throughout this document:
 Diameter Peer
    A Diameter entity MAY communicate with peers that are statically
    configured.  A statically configured Diameter peer would require
    that either the IP address or the fully qualified domain name
    (FQDN) be supplied, which would then be used to resolve through
    DNS.
 Routing Table
    A Diameter proxy server routes messages based on the realm portion
    of a Network Access Identifier (NAI).  The server MUST have a
    table of Realm Names, and the address of the peer to which the
    message must be forwarded.  The routing table MAY also include a
    "default route", which is typically used for all messages that
    cannot be locally processed.
 Tc timer
    The Tc timer controls the frequency that transport connection
    attempts are done to a peer with whom no active transport
    connection exists.  The recommended value is 30 seconds.

Fajardo, et al. Standards Track [Page 139] RFC 6733 Diameter Base Protocol October 2012

13. Security Considerations

 The Diameter base protocol messages SHOULD be secured by using TLS
 [RFC5246] or DTLS/SCTP [RFC6083].  Additional security mechanisms
 such as IPsec [RFC4301] MAY also be deployed to secure connections
 between peers.  However, all Diameter base protocol implementations
 MUST support the use of TLS/TCP and DTLS/SCTP, and the Diameter
 protocol MUST NOT be used without one of TLS, DTLS, or IPsec.
 If a Diameter connection is to be protected via TLS/TCP and DTLS/SCTP
 or IPsec, then TLS/TCP and DTLS/SCTP or IPsec/IKE SHOULD begin prior
 to any Diameter message exchange.  All security parameters for TLS/
 TCP and DTLS/SCTP or IPsec are configured independent of the Diameter
 protocol.  All Diameter messages will be sent through the TLS/TCP and
 DTLS/SCTP or IPsec connection after a successful setup.
 For TLS/TCP and DTLS/SCTP connections to be established in the open
 state, the CER/CEA exchange MUST include an Inband-Security-ID AVP
 with a value of TLS/TCP and DTLS/SCTP.  The TLS/TCP and DTLS/SCTP
 handshake will begin when both ends successfully reach the open
 state, after completion of the CER/CEA exchange.  If the TLS/TCP and
 DTLS/SCTP handshake is successful, all further messages will be sent
 via TLS/TCP and DTLS/SCTP.  If the handshake fails, both ends MUST
 move to the closed state.  See Section 13.1 for more details.

13.1. TLS/TCP and DTLS/SCTP Usage

 Diameter nodes using TLS/TCP and DTLS/SCTP for security MUST mutually
 authenticate as part of TLS/TCP and DTLS/SCTP session establishment.
 In order to ensure mutual authentication, the Diameter node acting as
 the TLS/TCP and DTLS/SCTP server MUST request a certificate from the
 Diameter node acting as TLS/TCP and DTLS/SCTP client, and the
 Diameter node acting as the TLS/TCP and DTLS/SCTP client MUST be
 prepared to supply a certificate on request.
 Diameter nodes MUST be able to negotiate the following TLS/TCP and
 DTLS/SCTP cipher suites:
       TLS_RSA_WITH_RC4_128_MD5
       TLS_RSA_WITH_RC4_128_SHA
       TLS_RSA_WITH_3DES_EDE_CBC_SHA
 Diameter nodes SHOULD be able to negotiate the following TLS/TCP and
 DTLS/SCTP cipher suite:
       TLS_RSA_WITH_AES_128_CBC_SHA

Fajardo, et al. Standards Track [Page 140] RFC 6733 Diameter Base Protocol October 2012

 Note that it is quite possible that support for the
 TLS_RSA_WITH_AES_128_CBC_SHA cipher suite will be REQUIRED at some
 future date.  Diameter nodes MAY negotiate other TLS/TCP and DTLS/
 SCTP cipher suites.
 If public key certificates are used for Diameter security (for
 example, with TLS), the value of the expiration times in the routing
 and peer tables MUST NOT be greater than the expiry time in the
 relevant certificates.

13.2. Peer-to-Peer Considerations

 As with any peer-to-peer protocol, proper configuration of the trust
 model within a Diameter peer is essential to security.  When
 certificates are used, it is necessary to configure the root
 certificate authorities trusted by the Diameter peer.  These root CAs
 are likely to be unique to Diameter usage and distinct from the root
 CAs that might be trusted for other purposes such as Web browsing.
 In general, it is expected that those root CAs will be configured so
 as to reflect the business relationships between the organization
 hosting the Diameter peer and other organizations.  As a result, a
 Diameter peer will typically not be configured to allow connectivity
 with any arbitrary peer.  With certificate authentication, Diameter
 peers may not be known beforehand and therefore peer discovery may be
 required.

13.3. AVP Considerations

 Diameter AVPs often contain security-sensitive data; for example,
 user passwords and location data, network addresses and cryptographic
 keys.  The following AVPs defined in this document are considered to
 be security-sensitive:
 o  Acct-Interim-Interval
 o  Accounting-Realtime-Required
 o  Acct-Multi-Session-Id
 o  Accounting-Record-Number
 o  Accounting-Record-Type
 o  Accounting-Session-Id
 o  Accounting-Sub-Session-Id
 o  Class

Fajardo, et al. Standards Track [Page 141] RFC 6733 Diameter Base Protocol October 2012

 o  Session-Id
 o  Session-Binding
 o  Session-Server-Failover
 o  User-Name
 Diameter messages containing these or any other AVPs considered to be
 security-sensitive MUST only be sent protected via mutually
 authenticated TLS or IPsec.  In addition, those messages MUST NOT be
 sent via intermediate nodes unless there is end-to-end security
 between the originator and recipient or the originator has locally
 trusted configuration that indicates that end-to-end security is not
 needed.  For example, end-to-end security may not be required in the
 case where an intermediary node is known to be operated as part of
 the same administrative domain as the endpoints so that an ability to
 successfully compromise the intermediary would imply a high
 probability of being able to compromise the endpoints as well.  Note
 that no end-to-end security mechanism is specified in this document.

14. References

14.1. Normative References

 [FLOATPOINT]
            Institute of Electrical and Electronics Engineers, "IEEE
            Standard for Binary Floating-Point Arithmetic, ANSI/IEEE
            Standard 754-1985", August 1985.
 [IANAADFAM]
            IANA, "Address Family Numbers",
            <http://www.iana.org/assignments/address-family-numbers>.
 [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
            September 1981.
 [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
            RFC 793, September 1981.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
            for Internationalized Domain Names in Applications
            (IDNA)", RFC 3492, March 2003.

Fajardo, et al. Standards Track [Page 142] RFC 6733 Diameter Base Protocol October 2012

 [RFC3539]  Aboba, B. and J. Wood, "Authentication, Authorization and
            Accounting (AAA) Transport Profile", RFC 3539, June 2003.
 [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
            10646", STD 63, RFC 3629, November 2003.
 [RFC3958]  Daigle, L. and A. Newton, "Domain-Based Application
            Service Location Using SRV RRs and the Dynamic Delegation
            Discovery Service (DDDS)", RFC 3958, January 2005.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66,
            RFC 3986, January 2005.
 [RFC4004]  Calhoun, P., Johansson, T., Perkins, C., Hiller, T., and
            P. McCann, "Diameter Mobile IPv4 Application", RFC 4004,
            August 2005.
 [RFC4005]  Calhoun, P., Zorn, G., Spence, D., and D. Mitton,
            "Diameter Network Access Server Application", RFC 4005,
            August 2005.
 [RFC4006]  Hakala, H., Mattila, L., Koskinen, J-P., Stura, M., and J.
            Loughney, "Diameter Credit-Control Application", RFC 4006,
            August 2005.
 [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
            Requirements for Security", BCP 106, RFC 4086, June 2005.
 [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
            Network Access Identifier", RFC 4282, December 2005.
 [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
            Architecture", RFC 4291, February 2006.
 [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
            RFC 4960, September 2007.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.
 [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246, August 2008.

Fajardo, et al. Standards Track [Page 143] RFC 6733 Diameter Base Protocol October 2012

 [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
            Housley, R., and W. Polk, "Internet X.509 Public Key
            Infrastructure Certificate and Certificate Revocation List
            (CRL) Profile", RFC 5280, May 2008.
 [RFC5729]  Korhonen, J., Jones, M., Morand, L., and T. Tsou,
            "Clarifications on the Routing of Diameter Requests Based
            on the Username and the Realm", RFC 5729, December 2009.
 [RFC5890]  Klensin, J., "Internationalized Domain Names for
            Applications (IDNA): Definitions and Document Framework",
            RFC 5890, August 2010.
 [RFC5891]  Klensin, J., "Internationalized Domain Names in
            Applications (IDNA): Protocol", RFC 5891, August 2010.
 [RFC6083]  Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
            Transport Layer Security (DTLS) for Stream Control
            Transmission Protocol (SCTP)", RFC 6083, January 2011.
 [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
            Security Version 1.2", RFC 6347, January 2012.
 [RFC6408]  Jones, M., Korhonen, J., and L. Morand, "Diameter
            Straightforward-Naming Authority Pointer (S-NAPTR) Usage",
            RFC 6408, November 2011.

14.2. Informative References

 [ENTERPRISE]  IANA, "SMI Network Management Private Enterprise
               Codes",
               <http://www.iana.org/assignments/enterprise-numbers>.
 [IANATCV]     IANA, "Termination-Cause AVP Values (code 295)",
               <http://www.iana.org/assignments/aaa-parameters/
               aaa-parameters.xml#aaa-parameters-16>.
 [RFC1492]     Finseth, C., "An Access Control Protocol, Sometimes
               Called TACACS", RFC 1492, July 1993.
 [RFC1661]     Simpson, W., "The Point-to-Point Protocol (PPP)",
               STD 51, RFC 1661, July 1994.
 [RFC2104]     Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
               Keyed-Hashing for Message Authentication", RFC 2104,
               February 1997.

Fajardo, et al. Standards Track [Page 144] RFC 6733 Diameter Base Protocol October 2012

 [RFC2782]     Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR
               for specifying the location of services (DNS SRV)",
               RFC 2782, February 2000.
 [RFC2865]     Rigney, C., Willens, S., Rubens, A., and W. Simpson,
               "Remote Authentication Dial In User Service (RADIUS)",
               RFC 2865, June 2000.
 [RFC2866]     Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
 [RFC2869]     Rigney, C., Willats, W., and P. Calhoun, "RADIUS
               Extensions", RFC 2869, June 2000.
 [RFC2881]     Mitton, D. and M. Beadles, "Network Access Server
               Requirements Next Generation (NASREQNG) NAS Model",
               RFC 2881, July 2000.
 [RFC2975]     Aboba, B., Arkko, J., and D. Harrington, "Introduction
               to Accounting Management", RFC 2975, October 2000.
 [RFC2989]     Aboba, B., Calhoun, P., Glass, S., Hiller, T., McCann,
               P., Shiino, H., Walsh, P., Zorn, G., Dommety, G.,
               Perkins, C., Patil, B., Mitton, D., Manning, S.,
               Beadles, M., Chen, X., Sivalingham, S., Hameed, A.,
               Munson, M., Jacobs, S., Lim, B., Hirschman, B., Hsu,
               R., Koo, H., Lipford, M., Campbell, E., Xu, Y., Baba,
               S., and E. Jaques, "Criteria for Evaluating AAA
               Protocols for Network Access", RFC 2989, November 2000.
 [RFC3162]     Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6",
               RFC 3162, August 2001.
 [RFC3748]     Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
               H. Levkowetz, "Extensible Authentication Protocol
               (EAP)", RFC 3748, June 2004.
 [RFC4301]     Kent, S. and K. Seo, "Security Architecture for the
               Internet Protocol", RFC 4301, December 2005.
 [RFC4690]     Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review
               and Recommendations for Internationalized Domain Names
               (IDNs)", RFC 4690, September 2006.
 [RFC5176]     Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
               Aboba, "Dynamic Authorization Extensions to Remote
               Authentication Dial In User Service (RADIUS)",
               RFC 5176, January 2008.

Fajardo, et al. Standards Track [Page 145] RFC 6733 Diameter Base Protocol October 2012

 [RFC5461]     Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
               February 2009.
 [RFC5905]     Mills, D., Martin, J., Burbank, J., and W. Kasch,
               "Network Time Protocol Version 4: Protocol and
               Algorithms Specification", RFC 5905, June 2010.
 [RFC5927]     Gont, F., "ICMP Attacks against TCP", RFC 5927,
               July 2010.
 [RFC6335]     Cotton, M., Eggert, L., Touch, J., Westerlund, M., and
               S. Cheshire, "Internet Assigned Numbers Authority
               (IANA) Procedures for the Management of the Service
               Name and Transport Protocol Port Number Registry",
               BCP 165, RFC 6335, August 2011.
 [RFC6737]     Kang, J. and G. Zorn, "The Diameter Capabilities Update
               Application", RFC 6737, October 2012.

Fajardo, et al. Standards Track [Page 146] RFC 6733 Diameter Base Protocol October 2012

Appendix A. Acknowledgements

A.1. This Document

 The authors would like to thank the following people that have
 provided proposals and contributions to this document:
 To Vishnu Ram and Satendra Gera for their contributions on
 capabilities updates, predictive loop avoidance, as well as many
 other technical proposals.  To Tolga Asveren for his insights and
 contributions on almost all of the proposed solutions incorporated
 into this document.  To Timothy Smith for helping on the capabilities
 Update and other topics.  To Tony Zhang for providing fixes to
 loopholes on composing Failed-AVPs as well as many other issues and
 topics.  To Jan Nordqvist for clearly stating the usage of
 Application Ids.  To Anders Kristensen for providing needed technical
 opinions.  To David Frascone for providing invaluable review of the
 document.  To Mark Jones for providing clarifying text on vendor
 command codes and other vendor-specific indicators.  To Victor
 Pascual and Sebastien Decugis for new text and recommendations on
 SCTP/DTLS.  To Jouni Korhonen for taking over the editing task and
 resolving last bits from versions 27 through 29.
 Special thanks to the Diameter extensibility design team, which
 helped resolve the tricky question of mandatory AVPs and ABNF
 semantics.  The members of this team are as follows:
 Avi Lior, Jari Arkko, Glen Zorn, Lionel Morand, Mark Jones, Tolga
 Asveren, Jouni Korhonen, and Glenn McGregor.
 Special thanks also to people who have provided invaluable comments
 and inputs especially in resolving controversial issues:
 Glen Zorn, Yoshihiro Ohba, Marco Stura, Stephen Farrel, Pete Resnick,
 Peter Saint-Andre, Robert Sparks, Krishna Prasad, Sean Turner, Barry
 Leiba, and Pasi Eronen.
 Finally, we would like to thank the original authors of this
 document:
 Pat Calhoun, John Loughney, Jari Arkko, Erik Guttman, and Glen Zorn.
 Their invaluable knowledge and experience has given us a robust and
 flexible AAA protocol that many people have seen great value in
 adopting.  We greatly appreciate their support and stewardship for
 the continued improvements of Diameter as a protocol.  We would also
 like to extend our gratitude to folks aside from the authors who have

Fajardo, et al. Standards Track [Page 147] RFC 6733 Diameter Base Protocol October 2012

 assisted and contributed to the original version of this document.
 Their efforts significantly contributed to the success of Diameter.

A.2. RFC 3588

 The authors would like to thank Nenad Trifunovic, Tony Johansson and
 Pankaj Patel for their participation in the pre-IETF Document Reading
 Party.  Allison Mankin, Jonathan Wood, and Bernard Aboba provided
 invaluable assistance in working out transport issues and this was
 also the case with Steven Bellovin in the security area.
 Paul Funk and David Mitton were instrumental in getting the Peer
 State Machine correct, and our deep thanks go to them for their time.
 Text in this document was also provided by Paul Funk, Mark Eklund,
 Mark Jones, and Dave Spence.  Jacques Caron provided many great
 comments as a result of a thorough review of the spec.
 The authors would also like to acknowledge the following people for
 their contribution in the development of the Diameter protocol:
 Allan C. Rubens, Haseeb Akhtar, William Bulley, Stephen Farrell,
 David Frascone, Daniel C. Fox, Lol Grant, Ignacio Goyret, Nancy
 Greene, Peter Heitman, Fredrik Johansson, Mark Jones, Martin Julien,
 Bob Kopacz, Paul Krumviede, Fergal Ladley, Ryan Moats, Victor Muslin,
 Kenneth Peirce, John Schnizlein, Sumit Vakil, John R. Vollbrecht, and
 Jeff Weisberg.
 Finally, Pat Calhoun would like to thank Sun Microsystems since most
 of the effort put into this document was done while he was in their
 employ.

Appendix B. S-NAPTR Example

 As an example, consider a client that wishes to resolve aaa:
 ex1.example.com.  The client performs a NAPTR query for that domain,
 and the following NAPTR records are returned:
  ;;        order pref flags service   regexp replacement
  IN NAPTR  50    50   "s"   "aaa:diameter.tls.tcp" ""
               _diameter._tls.ex1.example.com
  IN NAPTR  100   50   "s"   "aaa:diameter.tcp"     ""
               _aaa._tcp.ex1.example.com
  IN NAPTR  150   50   "s"   "aaa:diameter.sctp"    ""
               _diameter._sctp.ex1.example.com
 This indicates that the server supports TLS, TCP, and SCTP in that
 order.  If the client supports TLS, TLS will be used, targeted to a

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 host determined by an SRV lookup of _diameter._tls.ex1.example.com.
 That lookup would return:
  ;;       Priority  Weight  Port    Target
  IN SRV   0         1       5060    server1.ex1.example.com
  IN SRV   0         2       5060    server2.ex1.example.com
 As an alternative example, a client that wishes to resolve aaa:
 ex2.example.com.  The client performs a NAPTR query for that domain,
 and the following NAPTR records are returned:
  ;;        order pref flags service   regexp replacement
  IN NAPTR  150   50   "a"   "aaa:diameter.tls.tcp"  ""
               server1.ex2.example.com
  IN NAPTR  150   50   "a"   "aaa:diameter.tls.tcp"  ""
               server2.ex2.example.com
 This indicates that the server supports TCP available at the returned
 host names.

Appendix C. Duplicate Detection

 As described in Section 9.4, accounting record duplicate detection is
 based on session identifiers.  Duplicates can appear for various
 reasons:
 o  Failover to an alternate server.  Where close to real-time
    performance is required, failover thresholds need to be kept low.
    This may lead to an increased likelihood of duplicates.  Failover
    can occur at the client or within Diameter agents.
 o  Failure of a client or agent after sending a record from non-
    volatile memory, but prior to receipt of an application-layer ACK
    and deletion of the record to be sent.  This will result in
    retransmission of the record soon after the client or agent has
    rebooted.
 o  Duplicates received from RADIUS gateways.  Since the
    retransmission behavior of RADIUS is not defined within [RFC2865],
    the likelihood of duplication will vary according to the
    implementation.
 o  Implementation problems and misconfiguration.
 The T flag is used as an indication of an application-layer
 retransmission event, e.g., due to failover to an alternate server.
 It is defined only for request messages sent by Diameter clients or
 agents.  For instance, after a reboot, a client may not know whether

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 it has already tried to send the accounting records in its non-
 volatile memory before the reboot occurred.  Diameter servers MAY use
 the T flag as an aid when processing requests and detecting duplicate
 messages.  However, servers that do this MUST ensure that duplicates
 are found even when the first transmitted request arrives at the
 server after the retransmitted request.  It can be used only in cases
 where no answer has been received from the server for a request and
 the request is sent again, (e.g., due to a failover to an alternate
 peer, due to a recovered primary peer or due to a client re-sending a
 stored record from non-volatile memory such as after reboot of a
 client or agent).
 In some cases, the Diameter accounting server can delay the duplicate
 detection and accounting record processing until a post-processing
 phase takes place.  At that time records are likely to be sorted
 according to the included User-Name and duplicate elimination is easy
 in this case.  In other situations, it may be necessary to perform
 real-time duplicate detection, such as when credit limits are imposed
 or real-time fraud detection is desired.
 In general, only generation of duplicates due to failover or re-
 sending of records in non-volatile storage can be reliably detected
 by Diameter clients or agents.  In such cases, the Diameter client or
 agents can mark the message as a possible duplicate by setting the T
 flag.  Since the Diameter server is responsible for duplicate
 detection, it can choose whether or not to make use of the T flag, in
 order to optimize duplicate detection.  Since the T flag does not
 affect interoperability, and it may not be needed by some servers,
 generation of the T flag is REQUIRED for Diameter clients and agents,
 but it MAY be implemented by Diameter servers.
 As an example, it can be usually be assumed that duplicates appear
 within a time window of longest recorded network partition or device
 fault, perhaps a day.  So only records within this time window need
 to be looked at in the backward direction.  Secondly, hashing
 techniques or other schemes, such as the use of the T flag in the
 received messages, may be used to eliminate the need to do a full
 search even in this set except for rare cases.
 The following is an example of how the T flag may be used by the
 server to detect duplicate requests.
    A Diameter server MAY check the T flag of the received message to
    determine if the record is a possible duplicate.  If the T flag is
    set in the request message, the server searches for a duplicate
    within a configurable duplication time window backward and
    forward.  This limits database searching to those records where
    the T flag is set.  In a well-run network, network partitions and

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    device faults will presumably be rare events, so this approach
    represents a substantial optimization of the duplicate detection
    process.  During failover, it is possible for the original record
    to be received after the T-flag-marked record, due to differences
    in network delays experienced along the path by the original and
    duplicate transmissions.  The likelihood of this occurring
    increases as the failover interval is decreased.  In order to be
    able to detect duplicates that are out of order, the Diameter
    server should use backward and forward time windows when
    performing duplicate checking for the T-flag-marked request.  For
    example, in order to allow time for the original record to exit
    the network and be recorded by the accounting server, the Diameter
    server can delay processing records with the T flag set until a
    time period TIME_WAIT + RECORD_PROCESSING_TIME has elapsed after
    the closing of the original transport connection.  After this time
    period, it may check the T-flag-marked records against the
    database with relative assurance that the original records, if
    sent, have been received and recorded.

Appendix D. Internationalized Domain Names

 To be compatible with the existing DNS infrastructure and simplify
 host and domain name comparison, Diameter identities (FQDNs) are
 represented in ASCII form.  This allows the Diameter protocol to fall
 in-line with the DNS strategy of being transparent from the effects
 of Internationalized Domain Names (IDNs) by following the
 recommendations in [RFC4690] and [RFC5890].  Applications that
 provide support for IDNs outside of the Diameter protocol but
 interacting with it SHOULD use the representation and conversion
 framework described in [RFC5890], [RFC5891], and [RFC3492].

Fajardo, et al. Standards Track [Page 151] RFC 6733 Diameter Base Protocol October 2012

Authors' Addresses

 Victor Fajardo (editor)
 Telcordia Technologies
 One Telcordia Drive, 1S-222
 Piscataway, NJ  08854
 USA
 Phone: +1-908-421-1845
 EMail: vf0213@gmail.com
 Jari Arkko
 Ericsson Research
 02420 Jorvas
 Finland
 Phone: +358 40 5079256
 EMail: jari.arkko@ericsson.com
 John Loughney
 Nokia Research Center
 955 Page Mill Road
 Palo Alto, CA  94304
 US
 Phone: +1-650-283-8068
 EMail: john.loughney@nokia.com
 Glen Zorn (editor)
 Network Zen
 227/358 Thanon Sanphawut
 Bang Na, Bangkok  10260
 Thailand
 Phone: +66 (0) 87-0404617
 EMail: glenzorn@gmail.com

Fajardo, et al. Standards Track [Page 152]

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