[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-fajardo-dime-app-design-guide) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 RFC 7423

Diameter Maintenance and Extensions (DIME)                L. Morand, Ed.
Internet-Draft                                               Orange Labs
Intended status: Informational                                V. Fajardo
Expires: December 08, 2013
                                                           H. Tschofenig
                                                  Nokia Siemens Networks
                                                           June 06, 2013


                Diameter Applications Design Guidelines
                  draft-ietf-dime-app-design-guide-18

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 Diameter.  It is meant as a guidelines document and
   therefore as informative in nature.

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 December 08, 2013.

Copyright Notice

   Copyright (c) 2013 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



Morand, et al.          Expires December 08, 2013               [Page 1]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


   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  . . . . . . . . . . . . . .   6
       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  . . . . . . . . . . .  10
     5.4.  Application-Specific Session State Machines . . . . . . .  11
     5.5.  Session-Id AVP and Session Management . . . . . . . . . .  11
     5.6.  Use of Enumerated Type AVPs . . . . . . . . . . . . . . .  12
     5.7.  Application-Specific Message Routing  . . . . . . . . . .  12
     5.8.  Translation Agents  . . . . . . . . . . . . . . . . . . .  13
     5.9.  End-to-End Application Capabilities Exchange  . . . . . .  14
     5.10. Diameter Accounting Support . . . . . . . . . . . . . . .  14
     5.11. Diameter Security Mechanisms  . . . . . . . . . . . . . .  16
   6.  Defining Generic Diameter Extensions  . . . . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  18
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  18
   11. Informative References  . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

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






Morand, et al.          Expires December 08, 2013               [Page 2]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


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

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

   3.  The definition of an entirely new Diameter application to offer
       functionality 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 complete freedom
   when making their design.  A number of rules have been defined in
   [RFC6733] that 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 the Diameter extensibility rules as defined in the
      Diameter base protocol.

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

   o  Present trade-off choices.

2.  Terminology

   This document reuses the terminology defined 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.
   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 Section 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.




Morand, et al.          Expires December 08, 2013               [Page 3]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


   The Diameter base protocol is designed to be extensible and the
   principles are described in the Section 1.3 of [RFC6733].  As a
   summary, Diameter can be extended by:

   1.  Defining new AVP values

   2.  Creating new AVPs

   3.  Creating new commands

   4.  Creating new applications

   As a main guiding principle, the recommendation is: "try to re-use as
   much as possible!".  It will reduce the time to finalize
   specification writing, and it will lead to a smaller implementation
   effort as well as reduce the need for testing.  In general, it is
   clever to avoid duplicate effort when possible.

   However, re-use is not appropriate when the existing functionality
   does not fit the new requirement and/or the re-use leads to
   ambiguity.

   The impact on extending existing applications can be categorized into
   two groups:

   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
      for use in an existing command.  Diameter implementations
      supporting the existing application but not the new AVP will
      simply ignore it, without consequences for the Diameter message
      handling.  The standardization effort will be fairly small.


   Major Extension:  Enhancing an application that 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 the M-bit set to be carried in an
      existing command.  For such extension, a significant specification
      effort is required and a careful approach is recommended.

   We would also like to remind that the definition of a new Diameter
   application and the definition of a new command should be something



Morand, et al.          Expires December 08, 2013               [Page 4]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


   to avoid as much as possible.  In the past, there has been some
   reluctance to define new commands and new applications.  With the
   modified extensibility rules provided by [RFC6733], registering new
   commands and new applications does not lead to additional overhead
   for the specification author in terms of standardization process.
   Registering new functionality (new commands, new AVPs, new
   applications, etc.) with IANA remains important to avoid namespace
   collisions, which will likely lead to deployment problems.

4.  Reusing Existing Diameter Applications

   An existing application may need to be enhanced to fulfill 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 impact.

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 Command Code Format (CCF) syntax 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 example considers the Diameter EAP application [RFC4072] and the
   Diameter NASREQ application [RFC4005].  When network access
   authentication using EAP is required, the Diameter EAP commands
   (Diameter-EAP-Request/Diameter-EAP-Answer) are used; otherwise the
   NASREQ application will be used.  When the Diameter EAP application
   is used, the accounting exchanges defined in Diameter NASREQ may be
   used.

   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:





Morand, et al.          Expires December 08, 2013               [Page 5]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


   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.

   Note: Importing commands too liberally could result in a monolithic
   and hard to manage application supporting too many different
   features.

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 (i.e.,
   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.

   From a historical point of view, it is worth to note that there was a
   strong recommendation to re-use existing commands in the [RFC3588] to
   prevent rapid depletion of code values available for vendor-specific
   commands.  However, [RFC6733] has relaxed the allocation policy and
   enlarged 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





Morand, et al.          Expires December 08, 2013               [Page 6]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


   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
      also 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 Command Code
      Format (CCF) syntax .

   o  Optional (to understand) AVPs.  As defined in [RFC6733], these are
      AVPs with the M-bit flag cleared.  A Diameter node receiving these
      AVPs can simply ignore them if it does not support them.

   The rules are strict in the case where the AVPs to be added are
   mandatory to understand, i.e., they have 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 about 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?

   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 to indicate that the
      message is for a new application?

   If the answer to at least one of the questions is "yes" 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.





Morand, et al.          Expires December 08, 2013               [Page 7]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


   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 depends 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 (regardless of the M-bit setting).

      In this case, a new command code and subsequently a new Diameter
      application have to be specified.

   o  Deleting an AVP, which has the M-bit set, and 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, which has the M-bit cleared, and 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.



Morand, et al.          Expires December 08, 2013               [Page 8]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


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, the characteristics of an AVP are tied to its
   role within an application and the commands.

   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

   This section discusses the case where new applications have
   requirements that cannot be fulfilled 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, e.g., Cx/Dx
   ([TS29.228] and [TS29.229]), Sh ([TS29.328] and [TS29.329]) etc.

   Application designers should try 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.

   Additional considerations are described in the following sections.









Morand, et al.          Expires December 08, 2013               [Page 9]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


5.2.  Defining New Commands

   As a general recommendation, commands should not be defined from
   scratch.  It is instead recommend to re-use an existing command
   offering similar functionality and use it as a starting point.

   Moreover, the new command's CCF syntax specification 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 in a slighly changed
   environment.  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.

   Note: As a hint for protocol designers, it is not sufficient to just
   look at the command's CCF syntax specification.  It is also necessary
   to carefully read through the accompanying text in the specification.

   In the same way, the CCF syntax specification should be defined such
   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, which allows the addition of any
   arbitrary AVP as described in [RFC6733].

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.  Some existing
   specifications do not adhere to this rule for historical reasons.
   However, this guidance should be followed to avoid routing problems.

   In general, when a new application has been allocated with a new
   Application Id and it also reuses existing commands with or without
   modifications, 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



Morand, et al.          Expires December 08, 2013              [Page 10]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


   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.  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 defines
   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., Diameter network access session (NASREQ)
   application [RFC4005]) or specific service access session (e.g.,
   Diameter SIP application [RFC4740]).  In the Diameter base protocol,
   session state is referenced using the Session-Id AVP.  All Diameter
   messages that use 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 along the
   path) maintain session state information.

   However, some applications may not need to rely on the Session-Id to
   identify and manage 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.  Some applications might not
   require the notion of Diameter session concept at all.  For such
   applications, the Auth-Session-State AVP is usually set to
   NO_STATE_MAINTAINED in all 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 it's own
   session management concept or whether the Session-Id defined in the



Morand, et al.          Expires December 08, 2013              [Page 11]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


   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.  Use of Enumerated Type AVPs

   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.

   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 AVPs that use
   bit masks 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., to 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



Morand, et al.          Expires December 08, 2013              [Page 12]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


   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.

   Examples 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 has to 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.  Translation Agents

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

   In the case of RADIUS, it was initially thought that defining the
   translation function would be straightforward by adopting few basic
   principles, e.g., by the 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,
   if needed.  This recommendation applies for any kind of translation.





Morand, et al.          Expires December 08, 2013              [Page 13]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


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 with the MIP6-Feature-Vector AVP in [RFC5447]
   and the QoS-Capability AVP in [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.  Receivers of these AVPs can discover the
   additional functionality supported by the end-point originating 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.

   It is also important to note that this end-to-end capabilities
   exchange relies on the use of optional AVPs is not meant as a generic
   mechanism to support extensibility of Diameter applications with
   arbitrary functionality.  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
   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 for 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:



Morand, et al.          Expires December 08, 2013              [Page 14]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


      *  The application itself does not define its own 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:

      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 the 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 does not provide an
         accounting server that supports Diameter.  Consequently, the
         application server has to be provisioned to use a different
         protocol to access the accounting server, e.g., via LDAP, SOAP
         etc.  This case includes the support of older accounting
         systems that are not Diameter aware.

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

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



Morand, et al.          Expires December 08, 2013              [Page 15]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


   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.  Although it is not recommended, an application may
   define 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 between neighboring Diameter peers using TLS/TCP or DTLS/
   SCTP.  However, IPsec can also be deployed to secure communication
   between Diameter peers.  When IPsec is used instead of TLS or DTLS,
   the following recommendations apply.

   IPsec ESP [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.  IKEv2 [RFC5996] is
   recommended for performing mutual authentication and for establishing
   and maintaining security associations (SAs).

   IKEv1 [RFC2409] was used with RFC 3588 [RFC3588] and for easier
   migration from IKEv1 based implementations both RSA digital
   signatures and pre-shared keys should be supported in IKEv2.
   However, if IKEv1 is used, implementers should follow the guidelines
   given in Section 13.1 of RFC 3588 [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 the support
   for QoS AVPs (see [RFC5777]).

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

   Backward Compatibility:

      With the design of generic extensions an protocol designer has to
      consider with potential concerns about how existing applications
      deal with the new extension they do not understand.  Designers



Morand, et al.          Expires December 08, 2013              [Page 16]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


      also have to make sure that new extensions do not break expected
      message delivery layer behavior.

   Forward Compatibility:

      Protocol designers need to make sure that their design will not
      introduce undue restrictions for future applications.

   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
   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




Morand, et al.          Expires December 08, 2013              [Page 17]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


   This document provides guidelines and considerations for extending
   Diameter and Diameter applications.  Although such an extension may
   related to a security functionality, the document does not explicitly
   give guidance on enhancing Diameter with respect to security.

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.

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 Jean Mahoney and Ben
   Campbell for their invaluable detailed review and comments on this
   document.

11.  Informative References




Morand, et al.          Expires December 08, 2013              [Page 18]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


   [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]  Piper, D., "The Internet IP Security Domain of
              Interpretation for ISAKMP", RFC 2407, November 1998.

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

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

   [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



Morand, et al.          Expires December 08, 2013              [Page 19]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


              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.

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

   [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>.

Authors' Addresses

   Lionel Morand (editor)
   Orange Labs
   38/40 rue du General Leclerc
   Issy-Les-Moulineaux Cedex 9  92794
   France

   Phone: +33145296257
   Email: lionel.morand@orange.com



Morand, et al.          Expires December 08, 2013              [Page 20]


Internet-Draft   Diameter Applications Design Guidelines       June 2013


   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





































Morand, et al.          Expires December 08, 2013              [Page 21]


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/