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Diameter Maintenance and Extensions                       L. Morand, Ed.
(DIME)                                                       Orange Labs
Internet-Draft                                                V. Fajardo
Intended status: Informational
Expires: January 31, 2013                                  H. Tschofenig
                                                  Nokia Siemens Networks
                                                           July 30, 2012


                Diameter Applications Design Guidelines
                  draft-ietf-dime-app-design-guide-15

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.






























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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 January 31, 2013.

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.














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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Reusing existing Diameter applications . . . . . . . . . . . .  8
     4.1.  Adding a new command . . . . . . . . . . . . . . . . . . .  8
     4.2.  Deleting a command . . . . . . . . . . . . . . . . . . . .  9
     4.3.  Reusing existing commands  . . . . . . . . . . . . . . . .  9
       4.3.1.  Adding AVPs to a ommand  . . . . . . . . . . . . . . .  9
       4.3.2.  Deleting AVPs from a command . . . . . . . . . . . . . 11
     4.4.  Reusing existing AVPs  . . . . . . . . . . . . . . . . . . 12
       4.4.1.  Setting of the AVP flags . . . . . . . . . . . . . . . 12
       4.4.2.  Reuse of AVP of type Enumerated  . . . . . . . . . . . 12
   5.  Defining new Diameter applications . . . . . . . . . . . . . . 13
     5.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 13
     5.2.  Defining new commands  . . . . . . . . . . . . . . . . . . 13
     5.3.  Use of Application-Id in a message . . . . . . . . . . . . 14
     5.4.  Application specific Session State Machine . . . . . . . . 14
     5.5.  Session-Id AVP and session management  . . . . . . . . . . 15
     5.6.  AVPs defined as Boolean flag . . . . . . . . . . . . . . . 15
     5.7.  Application-specific message routing . . . . . . . . . . . 16
     5.8.  About Translation Agent  . . . . . . . . . . . . . . . . . 17
     5.9.  End-to-End applications capabilities exchange  . . . . . . 17
     5.10. Diameter accounting support  . . . . . . . . . . . . . . . 18
     5.11. Diameter security mechanisms . . . . . . . . . . . . . . . 20
   6.  Defining Generic Diameter Extensions . . . . . . . . . . . . . 22
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   9.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 26
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 27
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 28
     11.2. Informative References . . . . . . . . . . . . . . . . . . 28
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
















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1.  Introduction

   The Diameter Base protocol provides facilities to extend the Diameter
   Base protocol (see Section 1.3 of [I-D.ietf-dime-rfc3588bis]) 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
   [I-D.ietf-dime-rfc3588bis] 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 tradeoffs of design choices.











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2.  Terminology

   This document reuses the terminology used in
   [I-D.ietf-dime-rfc3588bis].















































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3.  Overview

   As designed, the Diameter Base protocol [I-D.ietf-dime-rfc3588bis]
   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.  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 [I-D.ietf-dime-rfc3588bis]).
   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
   [I-D.ietf-dime-rfc3588bis].  Extending Diameter can mean either the
   definition of a completly 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 [I-D.ietf-dime-rfc3588bis]
   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.


































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4.  Reusing existing Diameter applications

   When selecting the Diameter Base protocol to support new
   functionalities, protocol designers are advised to try to re-use as
   much as possible existing Diameter applications 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 an 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
   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.





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   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
      existing command's CCF syntax specification.  This would result in
      a monolithic and hard to manage applications supporting too many
      different functionalities and can cause interoperability issues
      between the different applications. .

4.2.  Deleting a command

   Although this process is not typical, removing a command to 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 which 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.

   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.
   [I-D.ietf-dime-rfc3588bis] 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.
   Therefore, if it is still recommended to re-use as much as possible
   existing commands, protocol designers can consider more easily the
   definition of a new command when it is a solution more suitable than
   twisting existing command use and applications.

4.3.1.  Adding AVPs to a ommand

   Based on the rules in [I-D.ietf-dime-rfc3588bis], AVPs that are added
   to an existing command can be categorized into:




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   o  Mandatory (to understand) AVPs.  As defined in
      [I-D.ietf-dime-rfc3588bis], these are AVPs with the M-bit flag
      set, which means that a Diameter node receiving are 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
      [I-D.ietf-dime-rfc3588bis], these are AVPs with the M-bit flag
      cleared, which mean that a Diameter node receiving these AVP 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 [I-D.ietf-dime-rfc3588bis].  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.  Here is a list of few
   common questions that application designers should wonder when trying
   to decide:


   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
      roundtrips, effectively changing the state machine of the
      application?

   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.

   If application designers are instead contemplating on the use of
   optional AVPs i.e. with the M-bit cleared, then the following are



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   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 applications
      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 AVPs 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

   When deleting an AVP from a command, the following cases need to be
   differentiated:


   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.

   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.




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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 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.




























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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
   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 pratices
   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.



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   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 [I-D.ietf-dime-rfc3588bis].

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 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 Machine

   Section 8 of [I-D.ietf-dime-rfc3588bis] provides session state
   machines for authentication, authorization and accounting (AAA)
   services.  When a new application is being defined that cannot
   clearly be categorized into any of these services it is recommended
   that the application itself define its own session state machine.
   The existing session state machines defined by
   [I-D.ietf-dime-rfc3588bis] is not intended for general use beyond AAA
   services, therefore any behavior not covered by that category would
   not fit well.  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]).



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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
   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 really 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 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 on the reception of the
   request.



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   However, AVPs of type Enumerated are too often used as 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 has to be
   considered as a sub-optimal design as this 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 that would cause in consequence backwards compatibility issues
   with existing implementations.

   Instead of defining Enumerated AVP when the AVP simply used as a
   Boolean flag, protocol designers are encouraged to rely on AVP
   defined in the form of a bit mask with the interpretation of the
   setting of each bit described in the relevant Diameter application
   specification.  Such AVPs can be reused and extended to multiplex
   several indications without major impact on the Diameter application.
   The bit-mask should be therefore long enough to leave room for future
   additions.  Examples of AVP defined as bit mask are the Session-
   Binding AVP defined in [I-D.ietf-dime-rfc3588bis] 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
   [I-D.ietf-dime-rfc3588bis] 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 specific routing function can be found the
   applications defined for the IP Multimedia Subsystem of 3GPP, i.e.
   Cx/Dx applications ([TS29.228] and [TS29.229]) in which the
   Subscriber Location Function (SLF) is defined a proxy agent (or
   enhanced Redirect agent) using 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



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   request, the routing of the answer should follow the reverse path of
   the request, as described in [I-D.ietf-dime-rfc3588bis], 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 Diameter agents in the request routing path (Relay or
   Proxy agents) will be able to correctly release the transaction state
   associated to the request upon receipt of the answer, avoiding thus
   unnecessary failover triggering due to non reception of the answer
   corresponding to the request.  Application designers are strongly
   recommended to not attempt to modify the answer routing principles
   described in [I-D.ietf-dime-rfc3588bis] when defining a new
   application.

5.8.  About Translation Agent

   As defined in [I-D.ietf-dime-rfc3588bis], a translation agent is a
   device that provides interworking between Diameter and another
   protocol (e.g.  RADIUS, TACACS+).

   In the specific case of RADIUS, it was initially foreseen that the
   translation function would have been straightforward to define and
   deploy by adopting few basic principles e.g. use of a shared range of
   code values for RADIUS attributes and Diameter AVPs, some guidelines
   on translation and management of key information (such as
   authentication parameter, routing/accounting or states), etc.  And
   all this material was put in the RFC 4005 ([RFC4005]) to be used as
   generic guideline for implementation of RADIUS-Diameter translation
   agent.

   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 will likely depend on the functionalities
   provided by the Diameter application under specification and a case-
   by-case analysis will be required.

   Therefore, when interoperability with RADIUS infrastructure is
   foreseen, protocol designers are advised that they cannot assume the
   availability of "standard" Diameter-to-RADIUS gateways agent and the
   required translation mechanism should be then specified along with
   the Diameter application.  And the recommendation in the case of
   RADIUS-Diameter interworking applies of course for any other kind of
   translation (e.g.  Diameter/MAP).

5.9.  End-to-End applications capabilities exchange

   New Diameter applications can rely on optional AVPs to exchange
   application specific capabilities and features.  These AVPs can be



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   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 can aid in the following cases:


   o  Formalizing the way new functionality is added to existing
      applications by announcing support for it.


   o  Applications that do not understand these AVP can discard it upon
      receipt.  In such case, senders of the AVP can also safely assume
      the receiving end-point does not support any functionality carried
      by the AVP if it is not present in subsequent responses.


   o  Useful in cases where deployment choices are offered and the
      generic design can be made available for a number of applications.


   Note that this list is not meant to be comprehensive.

   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.

5.10.  Diameter accounting support

   Accounting can be treated as an auxiliary application which 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 generally end up in a central
      accounting server.  A split accounting model is a good design
      choice when:






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      *  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
      somehow differentiate received accounting messages.  Since the
      received accounting messages can be for any application and/or
      service, the accounting server has to be have a method to uniquely
      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 records 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

      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.





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      *  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 [I-D.ietf-dime-rfc3588bis], the Diameter message
   exchange should be secured by using TLS/TCP or DTLS/SCTP.  However,
   IPsec Additional security mechanisms such as 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.  IKE is used for peer
   authentication, negotiation of security associations, and key
   management, using the IPsec DOI [RFC2407].  Peer authentication can
   be achieved by using a pre-shared key or certificate-based peer
   authentication using digital signatures can be used as alternative.
   Peer authentication using the public key encryption methods outlined
   in IKE's Sections 5.2 and 5.3 [RFC2409] should not be used.

   Diameter implementations using IPsec as security mechanisms must
   support both IKE Main Mode and Aggressive Mode.  When pre-shared keys
   are used for authentication, IKE Aggressive Mode should be used
   instead of IKE Main Mode.  When digital signatures are used for
   authentication, either IKE Main Mode or IKE Aggressive Mode can be
   used.

   When digital signatures are used to achieve authentication, an IKE
   negotiator should use IKE Certificate Request Payload(s) to specify
   the certificate authority (or authorities) that are trusted in
   accordance with its local policy.  IKE negotiators should use
   pertinent certificate revocation checks before accepting a PKI



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   certificate for use in IKE's authentication procedures.

   The Phase 2 Quick Mode exchanges used to negotiate protection for
   Diameter connections must explicitly carry the Identity Payload
   fields (IDci and IDcr).  The DOI provides for several types of
   identification data.  However, when used in conformant
   implementations, each ID Payload must carry a single IP address and a
   single non-zero port number, and must not use the IP Subnet or IP
   Address Range formats.  This allows the Phase 2 security association
   to correspond to specific TCP and SCTP connections.

   Since IPsec acceleration hardware may only be able to handle a
   limited number of active IKE Phase 2 SAs, Phase 2 delete messages may
   be sent for idle SAs, as a means of keeping the number of active
   Phase 2 SAs to a minimum.  The receipt of an IKE Phase 2 delete
   message should not be interpreted as a reason for tearing down a
   Diameter connection.  Rather, it is preferable to leave the
   connection up, and if additional traffic is sent on it, to bring up
   another IKE Phase 2 SA to protect it.  This avoids the potential for
   continually bringing connections up and down.































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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  Tradeoffs 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, the drawback is that the timing of sending extension data
      will be tied to when the application would be sending 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 tradeoff 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
      tradeoffs based on the requirements of the extension.


   In practice, it is often the case that the generic extensions use
   optional AVPs because it's simple and not intrusive to the
   application that would carry it.  Peers that do not support the
   generic extensions need not 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



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   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]).







































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7.  IANA Considerations

   This document does not require actions by IANA.
















































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8.  Security Considerations

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














































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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

   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.


















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10.  Acknowledgments

   We greatly appreciate the insight provided by Diameter implementers
   who have highlighted the issues and concerns being addressed by this
   document.














































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11.  References

11.1.  Normative References

   [I-D.ietf-dime-rfc3588bis]
              Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
              "Diameter Base Protocol", draft-ietf-dime-rfc3588bis-34
              (work in progress), June 2012.

   [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.

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

   [RFC2407]  D. Piper, "The Internet IP Security Domain of
              Interpretation for ISAKMP", 1998.

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

   [RFC4005]  P. Calhoun et al., "Diameter Network Access Server
              Application", August 2005,
              <http://www.rfc-editor.org/rfc/rfc4005.txt>.

   [RFC4072]  P. Eronen et al., "Diameter Extensible Authentication
              Protocol (EAP) Application", August 2005,
              <http://www.rfc-editor.org/rfc/rfc4072.txt>.




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

   [RFC4740]  M. Garcia-Martin et al., "Diameter Session Initiation
              Protocol (SIP) Application", November 2006,
              <http://www.rfc-editor.org/rfc/rfc4740.txt>.

   [RFC5447]  J. Korhonen et al., "Diameter Mobile IPv6: Support for
              Network Access Server to Diameter Server Interaction",
              February 2009,
              <http://www.rfc-editor.org/rfc/rfc5447.txt>.

   [RFC5777]  J. Korhonen et al., "Traffic Classification and Quality of
              Service (QoS) Attributes for Diameter", 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]
              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>.









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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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