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Diameter Applications Design Guidelines
draft-ietf-dime-app-design-guide-17

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7423.
Authors Lionel Morand , Victor Fajardo , Hannes Tschofenig
Last updated 2013-05-29
Replaces draft-fajardo-dime-app-design-guide
RFC stream Internet Engineering Task Force (IETF)
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Additional resources Mailing list discussion
Stream WG state WG Consensus: Waiting for Write-Up
Document shepherd Jouni Korhonen
IESG IESG state Became RFC 7423 (Best Current Practice)
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draft-ietf-dime-app-design-guide-17
Diameter Maintenance and Extensions (DIME)                L. Morand, Ed.
Internet-Draft                                               Orange Labs
Intended status: Informational                                V. Fajardo
Expires: November 30, 2013                                 H. Tschofenig
                                                  Nokia Siemens Networks
                                                            May 29, 2013

                Diameter Applications Design Guidelines
                  draft-ietf-dime-app-design-guide-17

Abstract

   The Diameter Base protocol provides facilities for protocol
   extensibility enabling to define new Diameter applications or modify
   existing applications.  This document is a companion document to the
   Diameter base protocol that further explains and clarifies the rules
   to extend the Diameter base protocol.  It is meant as a guidelines
   document and therefore it does not add, remove or change existing
   rules.

Requirements Language

   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].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 30, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Reusing Existing Diameter Applications  . . . . . . . . . . .   5
     4.1.  Adding a New Command  . . . . . . . . . . . . . . . . . .   5
     4.2.  Deleting an Existing Command  . . . . . . . . . . . . . .   6
     4.3.  Reusing Existing Commands . . . . . . . . . . . . . . . .   6
       4.3.1.  Adding AVPs to a Command  . . . . . . . . . . . . . .   7
       4.3.2.  Deleting AVPs from a Command  . . . . . . . . . . . .   8
     4.4.  Reusing Existing AVPs . . . . . . . . . . . . . . . . . .   9
       4.4.1.  Setting of the AVP Flags  . . . . . . . . . . . . . .   9
       4.4.2.  Reuse of AVP of Type Enumerated . . . . . . . . . . .   9
   5.  Defining New Diameter Applications  . . . . . . . . . . . . .   9
     5.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Defining New Commands . . . . . . . . . . . . . . . . . .  10
     5.3.  Use of Application-Id in a Message  . . . . . . . . . . .  11
     5.4.  Application-Specific Session State Machines . . . . . . .  11
     5.5.  Session-Id AVP and Session Management . . . . . . . . . .  11
     5.6.  AVPs Defined as Boolean Flag  . . . . . . . . . . . . . .  12
     5.7.  Application-Specific Message Routing  . . . . . . . . . .  13
     5.8.  About Translation Agent . . . . . . . . . . . . . . . . .  13
     5.9.  End-to-End Application Capabilities Exchange  . . . . . .  14
     5.10. Diameter Accounting Support . . . . . . . . . . . . . . .  15
     5.11. Diameter Security Mechanisms  . . . . . . . . . . . . . .  16
   6.  Defining Generic Diameter Extensions  . . . . . . . . . . . .  17
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  18
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     11.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

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   The Diameter base protocol provides facilities to extend the Diameter
   base protocol (see Section 1.3 of [RFC6733]) for supporting new
   functionalities.  In the context of this document, extending Diameter
   means one of the following:

   1.  Addition of a new functionality to an existing Diameter
       application without defining a new application.

   2.  Addition of a new functionality to an existing Diameter
       application that requires the definition of a new application.

   3.  The definition of a new Diameter application to provide a set of
       functionalities not supported by existing applications.

   4.  The definition of a new generic functionality that can be reused
       across different applications.

   All of these choices are design decisions that can be done by any
   combination of reusing existing or defining new commands, AVPs or AVP
   values.  However, application designers do not have total freedom
   when making their design.  A number of rules have been defined in
   [RFC6733] and place constraints on when an extension requires the
   allocation of a new Diameter application identifier or a new command
   code value.  The objective of this document is the following:

   o  Clarify updated Diameter extensibility rules in the Diameter base
      protocol.

   o  Clarify usage of certain Diameter functionalities that are not
      explicitly described in the Diameter Base specification.

   o  Discuss design choices and provide guidelines when defining new
      applications.

   o  Present trade-off of design choices.

2.  Terminology

   This document reuses the terminology used in [RFC6733].

3.  Overview

   As designed, the Diameter base protocol [RFC6733] can be seen as a
   two-layer protocol.  The lower layer is mainly responsible for
   managing connections between neighboring peers and for message
   routing.  The upper layer is where the Diameter applications reside.

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   This model is in line with a Diameter node having an application
   layer and a peer-to-peer delivery layer.  The Diameter base protocol
   document defines the architecture and behavior of the message
   delivery layer and then provides the framework for designing Diameter
   applications on the application layer.  This framework includes
   definitions of application sessions and accounting support (see
   Section 8 and 9 of [RFC6733]).  Accordingly, a Diameter node is seen
   in this document as a single instance of a Diameter message delivery
   layer and one or more Diameter applications using it.

   The Diameter base protocol is designed to be extensible and the
   principles are described in the section 1.3 of [RFC6733].  Extending
   Diameter can mean either the definition of a completely new Diameter
   application or the reuse of commands, AVPs and AVP values in any
   combination for the purpose of inheriting the features of an existing
   Diameter application.  The recommendation for re-using as much as
   possible existing implementations is meaningful as most of the
   requirements defined for a new application are likely already
   fulfilled by existing applications.

   However, when reusing existing applications, there is a greater
   likelihood of ambiguity on how much of the existing application can
   be enhanced without being distorted too much and therefore requiring
   the definition of a new application.

   The impacts of extending existing applications can be categorized as
   follow:

   Minor Extension:  Enhancing the functional scope of an existing
      application by the addition of optional features to support.  Such
      enhancement has no backward compatibility issue with the existing
      application.  A typical example would be the definition of a new
      optional AVP to use in an existing command.  Diameter
      implementations supporting the existing application but not the
      new AVP will simply ignore it, without major consequences on the
      Diameter message handling.  In general, this includes everything
      that is not covered by the next category.  The standardization
      effort will be fairly small.

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   Major Extension:  Enhancing the functional scope of an existing
      application in such a way that this implies backward compatible
      change to the existing application and then requires the
      definition of a new Diameter application.  Typical examples would
      be the creation of a new command for providing functionality not
      supported by existing applications or the definition of a new AVP
      with M-bit set to carry in an existing command.  For such
      extension, a significant specification effort is required and a
      careful approach is recommended.

   The rules outlined in the section 1.3 of [RFC6733] indicate when an
   extension requires a new command code to be registered and when new
   Diameter applications have to be defined.  The subsequent sections
   further explain and clarify the rules to extend the Diameter base
   protocol.  It is meant as a guidelines document and therefore it does
   not add, remove or change existing rules.

4.  Reusing Existing Diameter Applications

   When selecting the Diameter base protocol to support new
   functionalities, protocol designers are advised to reuse as much as
   possible existing Diameter applications in order to simplify
   standardization, implementation and avoid potential interoperability
   issues.  However, existing application needs to be adapted to support
   new requirements and these modifications can be at the command level
   and/or at the AVP level.  The following sections describe the
   possible modifications that can be performed on existing applications
   and their related impacts.

4.1.  Adding a New Command

   Adding a new command is considered as a major extension and requires
   a new Diameter application to be defined.  Adding a new command to an
   application means either defining a completely new command or
   importing the command's CCF syntax specification from another
   application whereby the new application inherits some or all of the
   functionality of the application where the command came from.  In the
   former case, the decision to create a new application is
   straightforward since this is typically a result of adding a new
   functionality that does not exist yet.  For the latter, the decision
   to create a new application will depend on whether importing the
   command in a new application is more suitable than simply using the
   existing application as it is in conjunction with any other
   application.  Therefore, a case by case study of each application
   requirement should be applied.

   An illustrative example is the command pair defined in Diameter EAP
   application [RFC4072] that can be re-used conjointly with any other

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   application (e.g.  the Diameter NASREQ application [RFC4005]) as soon
   as standard EAP-based authentication procedures need to be supported
   by the implementation.  It may therefore not be required to import
   the command pair in the new defined application.

   However, in general, it is difficult to come to a hard guideline, and
   so a case-by-case study of each application requirement should be
   applied.  Before adding or importing a command, application designers
   should consider the following:

   o  Can the new functionality be fulfilled by creating a new command
      independent from any existing command?  In this case, the
      resulting new application and the existing application can work
      independent of, but cooperating with each other.

   o  Can the existing command be reused without major extensions and
      therefore without the need for the definition of a new
      application, e.g.  new functionality introduced by the creation of
      new optional AVPs.

   o  Care should be taken to avoid a liberal method of importing an
      existing command's CCF syntax specification.  This would result in
      a monolithic and hard to manage application supporting too many
      different functionalities and can cause interoperability issues
      between the different applications.

4.2.  Deleting an Existing Command

   Although this process is not typical, removing a command from an
   application requires a new Diameter application to be defined.  This
   is due to the fact that the reception of the deleted command would
   systematically result in a protocol error
   (DIAMETER_COMMAND_UNSUPPORTED).

   It is unusual to delete an existing command from an application for
   the sake of deleting it or the functionality it represents.  This
   normally indicates of a flawed design.  An exception might be if the
   intent of the deletion is to create a newer version of the same
   application that is somehow simpler than the previous version.

4.3.  Reusing Existing Commands

   This section discusses rules in adding and/or deleting AVPs from an
   existing command of an existing application.  The cases described in
   this section may not necessarily result in the creation of new
   applications.

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   It is worth to note that the strong recommendation to re-use existing
   commands in the [RFC3588] was to prevent rapid scarcity of code
   values available for vendor-specific commands.  [RFC6733] relaxes the
   policy with respect to the allocation of command codes for vendor-
   specific uses and enlarges the range of available code values for
   vendor-specific applications.  Although reuse of existing commands is
   still recommended, protocol designers can consider defining a new
   command when it provides a solution more suitable than the twisting
   of an existing command's use and applications.

4.3.1.  Adding AVPs to a Command

   Based on the rules in [RFC6733], AVPs that are added to an existing
   command can be categorized into:

   o  Mandatory (to understand) AVPs.  As defined in [RFC6733], these
      are AVPs with the M-bit flag set, which means that a Diameter node
      receiving them is required to understand not only their values but
      their semantics.  Failure to do so will cause an message handling
      error.  This is regardless of whether these AVPs are required or
      optional as specified by the command's CCF syntax specification.

   o  Optional (to understand) AVPs.  As defined in [RFC6733], these are
      AVPs with the M-bit flag cleared, which mean that a Diameter node
      receiving these AVPs can simply ignore them if not supported in
      the process of the received command.

   The rules are strict in the case where the AVPs to be added are
   mandatory to understand i.e.  with the M-bit set.  A mandatory AVP
   cannot be added to an existing command without defining a new
   Diameter application, as stated in [RFC6733].  This falls into the
   "Major Extensions" category.  Despite the clarity of the rule,
   ambiguity still arises when evaluating whether a new AVP being added
   should be mandatory to begin with.  Application designers should
   consider the following questions when deciding to set the M-bit for a
   new AVP:

   o  Would it be required for the receiving side to be able to process
      and understand the AVP and its content?

   o  Would the new AVPs change the state machine of the application?

   o  Would the presence of the new AVP lead to a different number of
      round-trips, effectively changing the state machine of the
      application?

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   o  Would the new AVP be used to differentiate between old and new
      versions of the same application whereby the two versions are not
      backward compatible?

   o  Would the new AVP have duality in meaning i.e.  be used to carry
      application-related information as well as be used to indicate
      that the message is for a new application?

   When one of the above questions can be answered in the affirmative
   then the M-bit has to be set for the new AVP.  This list of questions
   is non-exhaustive and other criteria can be taken into account in the
   decision process.

   If application designers are instead contemplating the use of
   optional AVPs i.e.  with the M-bit cleared, then the following are
   some of the pitfalls that should be avoided:

   o  Use of optional AVPs with intersecting meaning.  One AVP has
      partially the same usage and meaning as another AVP.  The presence
      of both can lead to confusion.

   o  An optional AVPs with dual purpose, i.e.  to carry application
      data as well as to indicate support for one or more features.
      This has a tendency to introduce interpretation issues.

   o  Adding one or more optional AVPs and indicating (usually within
      descriptive text for the command) that at least one of them has to
      be present in the command.  This essentially circumventing the
      ABNF and is equivalent to adding a mandatory AVP to the command.

   These practices generally result in interoperability issues and
   should be avoided as much as possible.

4.3.2.  Deleting AVPs from a Command

   The impacts of deleting an AVP from a command depend on its command
   code format specification and M-bit setting:

   o  Deleting an AVP that is indicated as { AVP } in the command's CCF
      syntax specification, whatever the setting of the M-bit set.  This
      means the definition of a new command.  In this case, a new
      command code and subsequently a new Diameter application have to
      be specified.

   o  Deleting an AVP with M-bit set that is indicated as [ AVP ] in the
      command's CCF syntax specification.  No new command code has to be
      specified but the definition of a new Diameter application is
      required.

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   o  Deleting an AVP with the M-bit cleared that is indicated as [ AVP
      ] in the command's CCF syntax specification.  In this case, the
      AVP can be deleted without consequences.

   If possible application designers should attempt the reuse the
   command's CCF syntax specification without modification and simply
   ignore (but not delete) any optional AVP that will not be used.  This
   is to maintain compatibility with existing applications that will not
   know about the new functionality as well as maintain the integrity of
   existing dictionaries.

4.4.  Reusing Existing AVPs

   This section discusses rules in reusing existing AVP when reusing an
   existing command or defining a new command in a new application.

4.4.1.  Setting of the AVP Flags

   When reusing AVPs in a new application, the AVP flag setting, such as
   the mandatory flag ('M'-bit), has to be re-evaluated for a new
   Diameter application and, if necessary, even for every command within
   the application.  In general, for AVPs defined outside of the
   Diameter base protocol, its mandatory characteristics are tied to its
   role within an application and command.

   All other AVP flags shall remain unchanged.

4.4.2.  Reuse of AVP of Type Enumerated

   When modifying the set of values supported by an AVP of type
   Enumerated, this means defining a new AVP.  Modifying the set of
   Enumerated values includes adding a value or deprecating the use of a
   value defined initially for the AVP.  Defining a new AVP will avoid
   interoperability issues.

5.  Defining New Diameter Applications

5.1.  Introduction

   The general recommendation for Diameter extensibility is to reuse
   commands, AVPs and AVP values as much as possible.  However, some of
   the extensibility rules described in the previous sections also apply
   to scenarios where a designer is trying to define a completely new
   Diameter application.

   This section discusses the case where new applications have
   requirements that cannot be filled by existing applications and would
   require definition of completely new commands, AVPs and/or AVP

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   values.  Typically, there is little ambiguity about the decision to
   create these types of applications.  Some examples are the interfaces
   defined for the IP Multimedia Subsystem of 3GPP, i.e.  Cx/Dx
   ([TS29.228] and [TS29.229]), Sh ([TS29.328] and [TS29.329]) etc.

   Application designers should also follow the theme of Diameter
   extensibility, which in this case means to import existing AVPs and
   AVP values for any newly defined commands.  In certain cases where
   accounting will be used, the models described in Section 5.10 should
   also be considered.  Though some decisions may be clear, designers
   should also consider certain aspects of defining a new application.
   Some of these aspects are described in following sections.

5.2.  Defining New Commands

   As a general recommendation, reusing as much as possible of existing
   material is encouraged when defining new commands.  Protocol
   designers can thus usefully benefit from the experience gained with
   the implementation of existing commands.  This includes good
   practices to reuse but also known mistakes not to repeat.  Therefore
   it is advisable to avoid the definition of a command from scratch and
   rather take as an example an existing command that would be
   functionally close to command under definition.

   Moreover, the new command's CCF should be carefully defined when
   considering applicability and extensibility of the application.  If
   most of the AVPs contained in the command are indicated as fixed or
   required, it might be difficult to reuse the same command and
   therefore the same application if the context has slightly changed
   and some AVPs become obsolete.  Defining a command with most of the
   AVPs indicated as optional must not be seen as a sub-optimal design
   introducing too much flexibility in the protocol.  The protocol
   designers are only advised to clearly state the condition of presence
   of these AVPs and properly define the corresponding behaviour of the
   Diameter nodes when these AVPs are absent from the command.

   In the same way, the CCF should be defined in a way that it will be
   possible to add any arbitrary optional AVPs with the M-bit cleared
   (including vendor-specific AVPs) without modifying the application.
   For this purpose, it is strongly recommended to add "* [AVP]" in the
   command's CCF that will allow the addition of any arbitrary AVP as
   described in [RFC6733].

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5.3.  Use of Application-Id in a Message

   When designing new applications, designers should specify that the
   application ID carried in all session-level messages must be the
   application ID of the application using those messages.  This
   includes the session-level messages defined in Diameter base
   protocol, i.e., RAR/RAA, STR/STA, ASR/ASA and possibly ACR/ACA in the
   coupled accounting model, see Section 5.10.  Existing specifications
   may not adhere to this rule for historical or other reasons.
   However, this scheme should be followed to avoid possible routing
   problems for these messages.

   In general, when a new application has been allocated with a new
   application id and it also reuses existing commands with or without
   modifications (Sec 4.1), it must use the newly allocated application
   id in the header and in all relevant application id AVPs (Auth-
   Application-Id or Acct-Application-Id) present in the commands
   message body.

   Additionally, application designs using Vendor-Specific-Application-
   Id AVP should not use the Vendor-Id AVP to further dissect or
   differentiate the vendor-specification application id.  Diameter
   routing is not based on the Vendor-Id.  As such, the Vendor-ID should
   not be used as an additional input for routing or delivery of
   messages.  In general, the Vendor-Id AVP is an informational AVP only
   and kept for backward compatibility reasons.

5.4.  Application-Specific Session State Machines

   Section 8 of [RFC6733] provides session state machines for
   authentication, authorization and accounting (AAA) services and these
   session state machines are not intended to cover behavior outside of
   AAA.  If a new application cannot clearly be categorized into any of
   these AAA services, it is recommended that the application define its
   own session state machine.  Support for server-initiated request is a
   clear example where an application-specific session state machine
   would be needed, for example, the Rw interface for ITU-T push model
   (cf.[Q.3303.3]).

5.5.  Session-Id AVP and Session Management

   Diameter applications are usually designed with the aim of managing
   user sessions, e.g.  network access session (NASREQ application
   [RFC4005]) or specific service access session (Diameter SIP
   application [RFC4740]).  In the Diameter base protocol, the session
   management is based on the Session-Id AVP that it used to identify a
   given session and all the Diameter messages including the same
   Session-Id will be bound to the same session.  Diameter-based session

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   management also implies that both Diameter client and server (and
   potentially proxy agents in the diameter path) are maintaining
   session state information associated with the Session-Id contained in
   the Diameter messages.

   However, some applications may not need to rely on the Session-Id to
   identify and manage user sessions because other information can be
   used instead to correlate Diameter messages.  Indeed, the User-Name
   AVP or any other specific AVP can be present in every Diameter
   message and used therefore for message correlation.  There might even
   be applications for which the notion of Diameter session management
   would not be required at all.  For such applications, the Auth-
   Session-State AVP is usually set to NO_STATE_MAINTAINED in all the
   Diameter messages and these applications are therefore designed as a
   set of stand-alone transactions.  Even if an explicit access session
   termination is required, application-specific commands are defined
   and used instead of the Session-Termination-Request/Answer (STR/STA)
   or Abort-Session-Request/Answer (ASR/ASA) defined in the Diameter
   base protocol.  In such a case, the Session-Id is not significant.

   Based on these considerations, protocol designers should carefully
   appraise whether the application currently defined relies on the
   concept of session management and whether the Session-Id defined in
   the Diameter base protocol would be used for correlation of messages
   related to the same session.  If not, the protocol designers could
   decide to define application commands without the Session-Id AVP.  If
   any session management concept is supported by the application, the
   application documentation must clearly specify how the session is
   handled between client and server (as possibly Diameter agents in the
   path).

5.6.  AVPs Defined as Boolean Flag

   The type Enumerated was initially defined to provide a list of valid
   values for an AVP with their respective interpretation described in
   the specification.  For instance, AVPs of type Enumerated can be used
   to provide further information on the reason for the termination of a
   session or a specific action to perform upon the reception of the
   request.

   However, AVPs of type Enumerated are too often used as a simple
   Boolean flag, indicating for instance a specific permission or
   capability, and therefore only two values are defined e.g.  TRUE/
   FALSE, AUTORIZED/UNAUTHORIZED or SUPPORTED/UNSUPPORTED.  This is a
   sub-optimal design since it limits the extensibility of the
   application: any new capability/permission would have to be supported
   by a new AVP or new Enumerated value of the already defined AVP,
   causing backwards compatibility issues with existing implementations.

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   Instead of using an Enumerated AVP for a Boolean flag, protocol
   designers are encouraged to use Unsigned32 or Unsigned64 AVP type as
   bit mask whose bit settings are described in the relevant Diameter
   application specification.  Such AVPs can be reused and extended
   without major impact on the Diameter application.  The bit mask
   should leave room for future additions.  Examples of bit mask AVP are
   the Session-Binding AVP defined in [RFC6733] and the MIP6-Feature-
   Vector AVP defined in [RFC5447]

5.7.  Application-Specific Message Routing

   Diameter request message routing usually relies on the Destination-
   Realm AVP and the Application Id present in the request message
   header.  However, some applications may need to rely on the User-Name
   AVP or any other application-specific AVP present in the request to
   determine the final destination of a request e.g.  find the target
   AAA server hosting the authorization information for a given user
   when multiple AAA servers are addressable in the realm.

   In such a context, basic routing mechanisms described in [RFC6733]
   are not fully suitable, and additional application-level routing
   mechanisms have to be described in the application documentation to
   provide such specific AVP-based routing.  Such functionality will be
   basically hosted by an application-specific Proxy agent that will be
   responsible for routing decisions based on the received specific
   AVPs.

   Example of such application-specific routing functions can be found
   in the Cx/Dx applications ([TS29.228] and [TS29.229]) of the 3GPP IP
   Multimedia Subsystem, in which the proxy agent (Subscriber Location
   Function aka SLF) uses specific application-level identities found in
   the request to determine the final destination of the message.

   Whatever the criteria used to establish the routing path of the
   request, the routing of the answer should follow the reverse path of
   the request, as described in [RFC6733], with the answer being sent to
   the source of the received request, using transaction states and hop-
   by-hop identifier matching.  In particular, this ensures that the
   Diameter Relay or Proxy agents in the request routing path will be
   able to release the transaction state upon receipt of the
   corresponding answer, avoiding unnecessary failover.  Application
   designers are strongly dissuaded from modifying the answer-routing
   principles described in [RFC6733] when defining a new application.

5.8.  About Translation Agent

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   As defined in [RFC6733], a translation agent is a device that
   provides interworking between Diameter and another protocol (e.g.
   RADIUS, TACACS+).

   In the case of RADIUS, it was initially thought that defining the
   translation function would be straightforward by adopting few basic
   principles e.g.  use of a shared range of code values for RADIUS
   attributes and Diameter AVPs.  Guidelines for implementing a RADIUS-
   Diameter translation agent were put into RFC 4005 ([RFC4005]).

   However, it was acknowledged that such translation mechanism was not
   so obvious and deeper protocol analysis was required to ensure
   efficient interworking between RADIUS and Diameter.  Moreover, the
   interworking requirements depend on the functionalities provided by
   the Diameter application under specification, and a case-by-case
   analysis will be required.

   Therefore, protocol designers cannot assume the availability of a
   "standard" Diameter-to-RADIUS gateways agent when planning to
   interoperate with the RADIUS infrastructure.  They should specify the
   required translation mechanism along with the Diameter application.
   This recommendation applies for any kind of translation (e.g.
   Diameter/MAP).

5.9.  End-to-End Application Capabilities Exchange

   New Diameter applications can rely on optional AVPs to exchange
   application-specific capabilities and features.  These AVPs can be
   exchanged on an end-to-end basis at the application layer.  Examples
   of this can be found in [RFC5447] and [RFC5777].

   The end-to-end capabilities AVPs formalize the addition of new
   optional functionality to existing applications by announcing support
   for it.  Applications that do not understand these AVPs can discard
   them upon receipt.  Recevers of these AVPs can discover the addional
   functionalities supported the end-point orignating the request and
   behave accordingly when processing the request.  Senders of these
   AVPs can safely assume the receiving end-point does not support any
   functionality carried by the AVP if it is not present in
   corresponding response.  This is useful in cases where deployment
   choices are offered, and the generic design can be made available for
   a number of applications.

   When used in a new application, protocol designers should clearly
   specify this end-to-end capabilities exchange and the corresponding
   behaviour of the Diameter nodes supporting the application.

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   It is also important to note that this end-to-end capabilities
   exchange relying on the use of optional AVPs is not meant as a
   generic mechanism to support extensibility of Diameter applications
   with arbitrary functionalities.  When the added features drastically
   change the Diameter application or when Diameter agents have to be
   upgraded to support the new features, a new application should be
   defined.

5.10.  Diameter Accounting Support

   Accounting can be treated as an auxiliary application that is used in
   support of other applications.  In most cases, accounting support is
   required when defining new applications.  This document provides
   two(2) possible models for using accounting:

   Split Accounting Model
      In this model, the accounting messages will use the Diameter base
      accounting application ID (value of 3).  The design implication
      for this is that the accounting is treated as an independent
      application, especially during Diameter routing.  This means that
      accounting commands emanating from an application may be routed
      separately from the rest of the other application messages.  This
      may also imply that the messages end up in a central accounting
      server.  A split accounting model is a good design choice when:

      *  The application itself will not define its own unique
         accounting commands.

      *  The overall system architecture permits the use of centralized
         accounting for one or more Diameter applications.
      Centralizing accounting may have advantages but there are also
      drawbacks.  The model assumes that the accounting server can
      differentiate received accounting messages.  Since the received
      accounting messages can be for any application and/or service, the
      accounting server has to have a method to match accounting
      messages with applications and/or services being accounted for.
      This may mean defining new AVPs, checking the presence, absence or
      contents of existing AVPs, or checking the contents of the
      accounting record itself.  But in general, there is no clean and
      generic scheme for sorting these messages.  Therefore, the use of
      this model is recommended only when all received accounting
      messages can be clearly identified and sorted.  For most cases,
      the use of Coupled Accounting Model is recommended.

   Coupled Accounting Model

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      In this model, the accounting messages will use the application ID
      of the application using the accounting service.  The design
      implication for this is that the accounting messages are tightly
      coupled with the application itself; meaning that accounting
      messages will be routed like any other application messages.  It
      would then be the responsibility of the application server
      (application entity receiving the ACR message) to send the
      accounting records carried by the accounting messages to the
      proper accounting server.  The application server is also
      responsible for formulating a proper response (ACA).  A coupled
      accounting model is a good design choice when:

      *  The system architecture or deployment will not provide an
         accounting server that supports Diameter.

      *  The system architecture or deployment requires that the
         accounting service for the specific application should be
         handled by the application itself.

      *  The application server is provisioned to use a different
         protocol to access the accounting server; e.g., via LDAP, SOAP
         etc.  This includes attempting to support older accounting
         systems that are not Diameter aware.

      In all cases above, there will generally be no direct Diameter
      access to the accounting server.

   These models provide a basis for using accounting messages.
   Application designers may obviously deviate from these models
   provided that the factors being addressed here have also been taken
   into account.  Though it is not recommended, examples of other
   methods might be defining a new set of commands to carry application-
   specific accounting records.

5.11.  Diameter Security Mechanisms

   As specified in [RFC6733], the Diameter message exchange should be
   secured by using TLS/TCP or DTLS/SCTP.  However, IPsec can also be
   deployed to secure connections between Diameter peers.  When IPsec is
   used instead of TLS or DTLS, the following recommendations apply.

   IPsec ESP 5.3 [RFC4301] in transport mode with non-null encryption
   and authentication algorithms is used to provide per-packet
   authentication, integrity protection and confidentiality, and support
   the replay protection mechanisms of IPsec.  The version 2 of IKE
   (IKEv2) [RFC5996] is recommended for performing mutual authentication
   and establishing and maintaining security associations (SAs).

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   IKEv1 [RFC2409] was used in [RFC3588] and for easier migration from
   IKEv1 based implementations both RSA Digital Signatures and pre-
   shared keys should be used in IKEv2.  However, if IKEv1 is used,
   implementers should follow the guidelines given in section 13.1 in
   RFC3588 [RFC3588].

6.  Defining Generic Diameter Extensions

   Generic Diameter extensions are AVPs, commands or applications that
   are designed to support other Diameter applications.  They are
   auxiliary applications meant to improve or enhance the Diameter
   protocol itself or Diameter applications/functionality.  Some
   examples include the extensions to support auditing and redundancy
   (see [I-D.calhoun-diameter-res-mgmt]), improvements in duplicate
   detection scheme (see [I-D.asveren-dime-dupcons]), and piggybacking
   of QoS attributes (see [RFC5777]).

   Since generic extensions can cover many aspects of Diameter and
   Diameter applications, it is not possible to enumerate all the
   probable scenarios in this document.  However, some of the most
   common considerations are as follows:

   o  Backward compatibility: Dealing with existing applications that do
      not understand the new extension.  Designers also have to make
      sure that new extensions do not break expected message delivery
      layer behavior.

   o  Forward compatibility: Making sure that the design will not
      introduce undue restrictions for future applications.  Future
      applications attempting to support this feature should not have to
      go through great lengths to implement any new extensions.

   o  Trade-off in signaling: Designers may have to choose between the
      use of optional AVPs piggybacked onto existing commands versus
      defining new commands and applications.  Optional AVPs are simpler
      to implement and may not need changes to existing applications.
      However, this ties the sending of extension data to the
      application's transmission of a message.  This has consequences if
      the application and the extensions have different timing
      requirements.  The use of commands and applications solves this
      issue, but the trade-off is the additional complexity of defining
      and deploying a new application.  It is left up to the designer to
      find a good balance among these trade-offs based on the
      requirements of the extension.

   In practice, generic extensions often use optional AVPs because they
   are simple and non-intrusive to the application that would carry
   them.  Peers that do not support the generic extensions need not

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   understand nor recognize these optional AVPs.  However, it is
   recommended that the authors of the extension specify the context or
   usage of the optional AVPs.  As an example, in the case that the AVP
   can be used only by a specific set of applications then the
   specification must enumerate these applications and the scenarios
   when the optional AVPs will be used.  In the case where the optional
   AVPs can be carried by any application, it is should be sufficient to
   specify such a use case and perhaps provide specific examples of
   applications using them.

   In most cases, these optional AVPs piggybacked by applications would
   be defined as a Grouped AVP and it would encapsulate all the
   functionality of the generic extension.  In practice, it is not
   uncommon that the Grouped AVP will encapsulate an existing AVP that
   has previously been defined as mandatory ('M'-bit set) e.g., 3GPP IMS
   Cx/Dx interfaces ([TS29.228] and [TS29.229]).

7.  IANA Considerations

   This document does not require actions by IANA.

8.  Security Considerations

   This document provides guidelines and considerations for extending
   Diameter and Diameter applications.  It does not define nor address
   security-related protocols or schemes.

9.  Contributors

   The content of this document was influenced by a design team created
   to revisit the Diameter extensibility rules.  The team consisting of
   the members listed below was formed in February 2008 and finished its
   work in June 2008.

   o  Avi Lior

   o  Glen Zorn

   o  Jari Arkko

   o  Lionel Morand

   o  Mark Jones

   o  Victor Fajardo

   o  Tolga Asveren

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   o  Jouni Korhonen

   o  Glenn McGregor

   o  Hannes Tschofenig

   o  Dave Frascone

   We would like to thank Tolga Asveren, Glenn McGregor, and John
   Loughney for their contributions as co-authors to earlier versions of
   this document.

10.  Acknowledgments

   We greatly appreciate the insight provided by Diameter implementers
   who have highlighted the issues and concerns being addressed by this
   document.  The authors would also like to thank A.  Jean Mahoney and
   Ben Campbell for their invaluable detailed review and comments on
   this document.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
              Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

   [RFC6733]  Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
              "Diameter Base Protocol", RFC 6733, October 2012.

11.2.  Informative References

   [I-D.asveren-dime-dupcons]
              Asveren, T., "Diameter Duplicate Detection Cons.", draft-
              asveren-dime-dupcons-00 (work in progress), August 2006.

   [I-D.calhoun-diameter-res-mgmt]
              Calhoun, P., "Diameter Resource Management Extensions",
              draft-calhoun-diameter-res-mgmt-08.txt (work in progress),
              March 2001.

   [Q.3303.3]
              3rd Generation Partnership Project, "ITU-T Recommendation
              Q.3303.3, "Resource control protocol no. 3 (rcp3):
              Protocol at the Rw interface between the Policy Decision

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              Physical Entity (PD-PE) and the Policy Enforcement
              Physical Entity (PE-PE): Diameter"", 2008.

   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, November 1998.

   [RFC4005]  Calhoun, P., Zorn, G., Spence, D., and D. Mitton,
              "Diameter Network Access Server Application", RFC 4005,
              August 2005.

   [RFC4072]  Eronen, P., Hiller, T., and G. Zorn, "Diameter Extensible
              Authentication Protocol (EAP) Application", RFC 4072,
              August 2005.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4740]  Garcia-Martin, M., Belinchon, M., Pallares-Lopez, M.,
              Canales-Valenzuela, C., and K. Tammi, "Diameter Session
              Initiation Protocol (SIP) Application", RFC 4740, November
              2006.

   [RFC5447]  Korhonen, J., Bournelle, J., Tschofenig, H., Perkins, C.,
              and K. Chowdhury, "Diameter Mobile IPv6: Support for
              Network Access Server to Diameter Server Interaction", RFC
              5447, February 2009.

   [RFC5777]  Korhonen, J., Tschofenig, H., Arumaithurai, M., Jones, M.,
              and A. Lior, "Traffic Classification and Quality of
              Service (QoS) Attributes for Diameter", RFC 5777, February
              2010.

   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
              5996, September 2010.

   [TS29.228]
              3rd Generation Partnership Project, "3GPP TS 29.228;
              Technical Specification Group Core Network and Terminals;
              IP Multimedia (IM) Subsystem Cx and Dx Interfaces;
              Signalling flows and message contents", ,
              <http://www.3gpp.org/ftp/Specs/html-info/29272.htm>.

   [TS29.229]

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              3rd Generation Partnership Project, "3GPP TS 29.229;
              Technical Specification Group Core Network and Terminals;
              Cx and Dx interfaces based on the Diameter protocol;
              Protocol details", ,
              <http://www.3gpp.org/ftp/Specs/html-info/29229.htm>.

   [TS29.328]
              3rd Generation Partnership Project, "3GPP TS 29.328;
              Technical Specification Group Core Network and Terminals;
              IP Multimedia (IM) Subsystem Sh interface; signalling
              flows and message content", ,
              <http://www.3gpp.org/ftp/Specs/html-info/29328.htm>.

   [TS29.329]
              3rd Generation Partnership Project, "3GPP TS 29.329;
              Technical Specification Group Core Network and Terminals;
              Sh Interface based on the Diameter protocol; Protocol
              details", ,
              <http://www.3gpp.org/ftp/Specs/html-info/29329.htm>.

Authors' Addresses

   Lionel Morand (editor)
   Orange Labs

   Email: lionel.morand@orange.com

   Victor Fajardo

   Email: vf0213@gmail.com

   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600
   Finland

   Phone: +358 (50) 4871445
   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at

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