Diameter Maintanence and                                 V. Fajardo, Ed.
Extensions (DIME)                          Toshiba America Research Inc.
Internet-Draft                                                T. Asveren
Intended status: Informational                             Sonus Network
Expires: October 20, 2007                                  H. Tschofenig
                                           Siemens Networks GmbH & Co KG
                                                             G. McGregor
                                                             J. Loughney
                                                   Nokia Research Center
                                                          April 18, 2007

                Diameter Applications Design Guidelines

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   The Diameter Base protocol provides rules on how to extend Diameter

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   and to create new Diameter applications.  This is a companion
   document to clarify these rules.  This document does not intended to
   add, remove or change these rules, rather it helps protocol designers
   to extend Diameter.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Diameter Application Model . . . . . . . . . . . . . . . . . .  3
   4.  Rules on Diameter Extensibility  . . . . . . . . . . . . . . .  4
     4.1.  Rules on Extending Existing Applications . . . . . . . . .  5
   5.  Design Considerations  . . . . . . . . . . . . . . . . . . . .  7
     5.1.  Diameter Accounting Support  . . . . . . . . . . . . . . .  7
     5.2.  Generic Diameter Extensions  . . . . . . . . . . . . . . .  8
     5.3.  Updating an existing Application . . . . . . . . . . . . .  9
     5.4.  Use of optional AVPs . . . . . . . . . . . . . . . . . . . 10
     5.5.  Deleting AVPs from a Command ABNF  . . . . . . . . . . . . 10
     5.6.  Justifying the Allocation of Application-Id  . . . . . . . 11
     5.7.  Use of Application-Id in a Message . . . . . . . . . . . . 11
     5.8.  Support for Server Initiated Requests  . . . . . . . . . . 11
     5.9.  System Architecture and Deployment . . . . . . . . . . . . 12
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 15

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1.  Introduction

   The Diameter Base protocol document defines rules on how one would
   extend Diameter (see Section 1.2 of [1]).  In the context of this
   document, extending Diameter means that a new Diameter application is
   being defined which may or may not be based on an existing Diameter
   application.  A decision to define a new application would mean
   allocation of a new application ID.

   By themselves, the rules defined in the Diameter Base protocol are
   not necessarily comprehensive enough that one can easily derive good
   design decisions from them.  The effect of this can be seen in
   various attempts to extend Diameter where protocol designers have no
   clear answer on whether to even define a new application or not.  At
   worst, some existing Diameter applications that had purposely been
   derived from another existing application resulted in some in-
   appropriate design decision in which both applications are no longer
   interoperable in certain conditions.

   The intent of this document is to influence ongoing and future
   Diameter application design by providing the following content:

   o  Clarify existing Diameter extensibility rules present in the
      Diameter Base Protocol.

   o  Clarify usage of certain Diameter functionality which are not
      explicitly described in the Diameter Base specification.

   o  Discuss design choices when defining new applications.

   o  Present tradeoffs of design choices.

   Note that it is not always possible to offer a complete and concise
   answer to certain design choices.  There is, however, the belief that
   at a minimum, this document can be used as a guide to Diameter

2.  Terminology

   This document reuses the terminology used in [1].

3.  Diameter Application Model

   As it is currently interpreted and practiced, the Diameter Base
   protocol is a two-layer protocol.  The lower layer is mainly
   responsible for managing connections between neighboring peers and

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   for message routing.  The upper layer is where the Diameter
   applications reside.  This model is inline with a Diameter node
   having an application layer and a peer-to-peer delivery layer.  The
   Diameter Base protocol document completely 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 [1]).  The remainder
   of this document also treats a Diameter node as a single instance of
   a Diameter message delivery layer and one or more Diameter
   applications using it.

4.  Rules on Diameter Extensibility

   The general theme of Diameter extensibility is to reuse AVPs, AVP
   values, commands and applications as much as possible.  However,
   there are also rules for extending Diameter as specified in Section
   1.2 of [1].  As is, the rules apply to the scenario where one is
   trying to define a new Diameter application.  Defining a new Diameter
   application can be done by:

   Defining a completely new application

      This case applies to applications which have requirements that
      cannot be filled by existing applications and would require
      definition of new command(s), AVPs and 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 ([2] and [3]), Sh
      ([4] and [5]) etc .  Though some decisions may be clear, designers
      should also consider certain aspects of the application itself.
      Some of these are described in Section 5.  Applications design
      should also follow the theme of Diameter extensibility which
      advocates reuse of AVPs and AVP values as much as possible even in
      newly defined commands.  In certain cases where accounting will be
      used, the models described in Section 5.1 should be considered.

   Extending an existing application

      In this case, the requirements of the new applications are not
      completely unique and there are existing application's that can be
      reused to solve some or all of the application requirements.
      Thus, there is a greater likelihood of ambiguity on how much of
      the existing application can be reused, to what extent and what
      the implications for both the new and existing application.
      Section 4.1 discusses some of the issues in this case.

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4.1.  Rules on Extending Existing Applications

   The Diameter base protocol provides a clear set of rules on when one
   should define a new Diameter application.  In the context of this
   document, the rules are:

   Adding an AVP to a command ABNF of an existing application

      The rules are strict in the case where the AVP(s) to be added is
      mandatory to be understood and interpreted.  This means that the
      M-bit is set and the AVP(s) is required to exist in command ABNF.
      Note that this mandatory AVP rule applies to AVP(s) that either
      already exist in the same or in another application or the AVP(s)
      are yet to be defined.  In the latter case, the ambiguity arises
      when trying to decide whether the AVP(s) should be mandatory or
      not.  There are several questions that application designers
      should contemplate when trying to decide:

      *  Does the AVP(s) change the state machine of the application ?

      *  Would the presence of the AVP(s) cause additional message
         round-trips; effectively changing the state machine of the
         application ?

      *  Will the AVP be used to fulfill new required functionality ?

      *  Would the AVP be used to differentiate between old and new
         versions of the same application ?

      *  Will it 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 ?

      These questions are not comprehensive in any way but in all cases
      the semantics of the application must change to justify the use of
      mandatory AVPs.

      However, care should also be taken when opting for optional AVPs
      instead of mandatory AVPs simply to avoid allocating new
      applications.  Optional AVPs that fall into any of the categorical
      questions above would have consequences.  See Section 5.4 for

   Add a new AVP value to an to an existing AVP

      In this case, the rule applies to existing mandatory AVPs already
      present in a command ABNF where the semantics of the AVP changes.
      This means that the meaning or usage of the AVP has changed and

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      significantly affects the behavior of the application.  Although
      this case may be less common or seem more subtle, the exact same
      considerations given in the first scenario above apply here as

   Add a command to an existing application

      In this case, the rule applies to defining a new command for an
      existing application or importing an existing command from another
      application so as to inherit some or all of the functionality of
      that application.  In the first case, the decision is straight
      forward since this is typically a result of adding new
      functionality that does not yet exist.  The latter case would
      result in a new application but it has a more subtle issue such as
      deciding whether importing of commands and functionality is really
      better than simply using the existing application as it is in
      conjunction with any new application.

      A typical example would be the Diameter MIPv6 split scenario (see
      [6]) in which several application models would have been possible
      during the design phase; one model would reuse existing Diameter
      EAP application combined with a new Diameter MIPv6 application to
      form a complete authentication and authorization scheme and
      another would be to reuse Diameter EAP like commands within the
      new Diameter MIPv6 application to accomplish the same result.  In
      this case, the latter model was chosen which would permit the
      reuse of commands and/or AVPs from one application to another.
      Other applications such as Diameter QoS (see [7]) would likely
      face similar decisions.

      In general, it is difficult to come to a hard and fast guideline
      for this scenario so a case by case study of each application
      requirement should be applied.  Before importing a command,
      application designers should consider whether:

      *  The existing application can be reused as is without
         fundamental changes; i.e. an optional AVP is sufficient to
         indicate support for new optional functionality if any.  There
         are pitfalls to this as well.  See Section 5.4

      *  Reuse of existing applications would result in a distributed
         environment which may not be conducive to certain requirements
         of the applications; i.e. security and or deployment
         difficulties - because of Diameter routing, messages for
         different applications providing service to the same user may
         end up in different servers would then need to be co-related.
         This could mean extra signaling between application servers.  A
         typical example would be the initial proposal for Diameter

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         MIPv6 split scenario (see [6]) where authorization and
         authentication is separated.

5.  Design Considerations

   The following are some of the design considerations that apply to a
   Diameter application.

5.1.  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.  However, the lack of
   clarity in the base protocol document has prevented easy use the base
   accounting messages (ACR/ACA).  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 routing.  This means that
      accounting commands emanating from an application may be routed
      separately from the rest of the other application messages.  This
      also implies that the messages generally 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.

      From a Diameter architecture perspective, this model should be the
      typical design choice.  Note that when using this model, the
      accounting server must use the Acct-Application-Id AVP to
      determine which application is being accounted for.  Therefore,
      the application designer should specify the proper values used in
      Acct-Application-Id AVP when sending ACR messages.

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   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 is 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; i.e., via LDAP, XML
         etc.  This includes attempting to supporting 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 would be defining a new set of commands to carry application
   specific accounting records.

   Additionally, the application ID in the message header and
   Accounting-Application-Id AVP are populated depending on the
   accounting model used for a specific application, as described in
   [1].  Therefore, application designers have to specify the accounting
   model used to guarantee proper routing of accounting requests.

5.2.  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

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   examples include the extensions to support auditing and redundancy
   (see [8]), improvements in duplicate detection scheme (see [9]).

   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;
      i.e., use of proxy agents.  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.

5.3.  Updating an existing Application

   An application that is being upgraded must follow the same rules
   mentioned Section 4.  Even if the new version is fundamentally the
   same application, allocation of a new application ID is possible if
   it meets those criteria.

   Optional AVPs can also be used to indicate version differences.  If
   this approach is chosen, it is recommended that the optional AVP is
   used specifically to indicate version information only and nothing
   else.  Additionally, the use of too many optional AVPs to carry
   application enhancements should be avoided since such approach has a
   tendency to become unmanageable and introduce interoperability
   issues.  These pitfalls are discussed in Section 5.4

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   For the same reason, care should be taken in attempting to justify
   allocation of new application ID for every change.  The pitfalls of
   this approach is discussed in Section 5.6.

5.4.  Use of optional AVPs

   Problems arise when there is a tendency by applications designers to
   keep adding optional AVPs to an existing command so they can
   circumvent the extension rules in Section 4.  Some of the pitfalls
   that application designers should avoid are:

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

   o  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  Use of optional AVPs with a minimum occurrence of one(1) in the
      command ABNF.  This is generally contradictory.  Application
      designers should not use this scheme to circumvent definition of
      mandatory AVPs.

   All of these practices generally result in interoperability problems
   so they should be avoided as much as possible.

5.5.  Deleting AVPs from a Command ABNF

   Although this scenario is not as common, the deletion of AVPs from a
   command ABNF is significant when trying to extend an existing
   application.  Deletion can be categorized between deletion of
   mandatory and optional AVPs.

   In the unlikely event that an application designer would require that
   mandatory AVPs must be deleted then it constitutes a fundamental
   change to an existing application.  Though not specified in [1],
   deletion of mandatory would require the allocation of a new
   application since it dictates changes in the behavior and semantics
   of an application.

   The deletion of an optional AVP may not necessarily indicate
   allocation of a new application.  An optional AVP with a minimum
   occurrence of at least one(1) in the command ABNF would mean that the
   AVP is required and that if deleted, there would effectively be
   changes to the behavior of the application as well.  Such cases are
   highly dubious to begin with since those AVPs already exhibits
   properties of mandatory AVPs.  It should therefore fall into the

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   category of deleting mandatory AVPs.

   In other cases, it is recommended that application designers reuse
   the command ABNF as is and safely 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

5.6.  Justifying the Allocation of Application-Id

   Application designers should avoid justifying the allocation of
   application IDs for every change that is made to an existing
   application.  Proliferation of application ID can lead to confusion
   and an in-efficient use of the application ID namespaces.
   Application designers should always use Section 4 as a basis for
   justifying allocation of a new application ID.

5.7.  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 5.1.  Existing specifications may not
   adhere to this rule for historical or other reasons.  However, this
   scheme is followed to avoid possible routing problems for these

   Additionally, application designers using the Vendor-Specific-
   Application-Id AVP should note that the Vendor-Id AVP will not be
   used in any way by the Diameter message delivery layer.  Therefore
   its meaning and usage should be segregated only within the

5.8.  Support for Server Initiated Requests

   Section 8 of [1] implies that only client sessions can initiate a
   request messages.  Assuming that the Diameter client and server
   sessions can be de-coupled from application client and server
   entities, an application design requiring server initiated request
   can simply create a Diameter client session and the client entity can
   then initiate a corresponding Diameter server session.

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5.9.  System Architecture and Deployment

   The following are some of the architecture considerations that
   applications designers should contemplate when defining new

   o  For general AAA applications, Diameter requires more message
      exchanges for the same set of services compared to RADIUS.
      Therefore, application designers should consider scalability
      issues during the design process.

   o  Application design should be agnostic to any Diameter topology.
      Application designers should not always assume a particular
      Diameter topology; i.e., assume that there will always be
      application proxies in the path or assume that only intra-domain
      routing is applicable.

   o  Security Considerations.  Application designers should take into
      account that there is no end-to-end authentication built into

   o  Application design should consider some form of redundancy.
      Session state information is the primary data necessary for
      backup/recovering endpoints to continue processing for an
      previously existing session.  Carrying enough information in the
      messages to reconstruct session state facilitates redundant
      implementations and is highly recommended.

   o  Application design should segregate message delivery layer
      processing from application level processing.  An example is the
      use of timers to detect lack of a response for a previously sent
      requests.  Although the Diameter base protocol defines a watchdog
      timer Tw, its use on application level is discouraged since Tw is
      a hop-by-hop timer and it would not be relevant for end-to-end
      message delivery error detection.  In such a case, it is
      recommended that applications should define their own set of
      timers for such purpose.

6.  IANA Considerations

   This document does not require actions by IANA.

7.  Security Considerations

   This document does provides guidelines and considerations for
   extending Diameter and Diameter applications.  It does not define nor

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   address security related protocols or schemes.

8.  Acknowledgments

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

9.  References

9.1.  Normative References

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

   [2]  3GPP, "IMS Cx and Dx interfaces : signalling flows and message
        contents", 3GPP TS 29.228 Version 7.0.0 2006.

   [3]  3GPP, "IMS Cx and Dx interfaces based on the Diameter protocol;
        Protocol details", 3GPP TS 29.229 Version 7.0.0 2006.

   [4]  3GPP, "IMS Sh interface : signalling flows and message content",
        3GPP TS 29.328 Version 6.8.0 2005.

   [5]  3GPP, "IMS Sh interface based on the Diameter protocol; Protocol
        details", 3GPP TS 29.329 Version 6.6.0 2005.

   [6]  Bournelle, J., "Diameter Mobile IPv6: HA-to-AAAH support",
        draft-ietf-dime-mip6-split-01 (work in progress), October 2006.

   [7]  Zorn, G., "Diameter Quality of Service Application",
        draft-ietf-dime-diameter-qos-00 (work in progress),
        February 2007.

9.2.  Informative References

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

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

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Authors' Addresses

   Victor Fajardo (editor)
   Toshiba America Research Inc.
   One Telcordia Drive
   Piscataway, NJ 08854

   Email: vfajardo@tari.toshiba.com

   Tolga Asveren
   Sonus Network
   4400 Route 9 South
   Freehold, NJ, 07728

   Email: tasveren@sonusnet.com

   Hannes Tschofenig
   Siemens Networks GmbH & Co KG
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739

   Phone: +49 89 636 40390
   Email: Hannes.Tschofenig@siemens.com
   URI:   http://www.tschofenig.com

   Glenn McGregor

   Email: glenn@aaa.lucent.com

   John Loughney
   Nokia Research Center

   Email: john.loughney@nokia.com

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Full Copyright Statement

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