path: root/doc/rfc/rfc7423.txt

Internet Engineering Task Force (IETF)                    L. Morand, Ed.
Request for Comments: 7423                                   Orange Labs
BCP: 193                                                      V. Fajardo
Category: Best Current Practice                           Fluke Networks
ISSN: 2070-1721                                            H. Tschofenig
                                                           November 2014


                Diameter Applications Design Guidelines

Abstract

   The Diameter base protocol provides facilities for protocol
   extensibility enabling the definition of new Diameter applications or
   modification of existing applications.  This document is a companion
   document to the Diameter base protocol that further explains and
   clarifies the rules to extend Diameter.  Furthermore, this document
   provides guidelines to Diameter application designers reusing/
   defining Diameter applications or creating generic Diameter
   extensions.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7423.

















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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Reusing Existing Diameter Applications  . . . . . . . . . . .   6
     4.1.  Adding a New Command  . . . . . . . . . . . . . . . . . .   7
     4.2.  Deleting an Existing Command  . . . . . . . . . . . . . .   8
     4.3.  Reusing Existing Commands . . . . . . . . . . . . . . . .   8
       4.3.1.  Adding AVPs to a Command  . . . . . . . . . . . . . .   8
       4.3.2.  Deleting AVPs from a Command  . . . . . . . . . . . .  10
       4.3.3.  Changing the Flag Settings of AVP in Existing
               Commands  . . . . . . . . . . . . . . . . . . . . . .  11
     4.4.  Reusing Existing AVPs . . . . . . . . . . . . . . . . . .  11
       4.4.1.  Setting of the AVP Flags  . . . . . . . . . . . . . .  11
       4.4.2.  Reuse of AVP of Type Enumerated . . . . . . . . . . .  12
   5.  Defining New Diameter Applications  . . . . . . . . . . . . .  12
     5.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .  12
     5.2.  Defining New Commands . . . . . . . . . . . . . . . . . .  12
     5.3.  Use of Application Id in a Message  . . . . . . . . . . .  13
     5.4.  Application-Specific Session State Machines . . . . . . .  14
     5.5.  Session-Id AVP and Session Management . . . . . . . . . .  14
     5.6.  Use of Enumerated Type AVPs . . . . . . . . . . . . . . .  15
     5.7.  Application-Specific Message Routing  . . . . . . . . . .  17
     5.8.  Translation Agents  . . . . . . . . . . . . . . . . . . .  18
     5.9.  End-to-End Application Capabilities Exchange  . . . . . .  18
     5.10. Diameter Accounting Support . . . . . . . . . . . . . . .  19
     5.11. Diameter Security Mechanisms  . . . . . . . . . . . . . .  21
   6.  Defining Generic Diameter Extensions  . . . . . . . . . . . .  21
   7.  Guidelines for Registrations of Diameter Values . . . . . . .  23
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  25
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  25
   Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29














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1.  Introduction

   The Diameter base protocol [RFC6733] is intended to provide an
   Authentication, Authorization, and Accounting (AAA) framework for
   applications such as network access or IP mobility in both local and
   roaming situations.  This protocol provides the ability for Diameter
   peers to exchange messages carrying data in the form of Attribute-
   Value Pairs (AVPs).

   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:

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

   2.  The 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 extensions are design decisions that can be carried out
   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.









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2.  Terminology

   This document reuses the terminology defined in [RFC6733].
   Additionally, the following terms and acronyms are used in this
   application:

   Application:  Extension of the Diameter base protocol [RFC6733] via
      the addition of new commands or AVPs.  Each application is
      uniquely identified by an IANA-allocated application identifier
      value.

   Command:  Diameter request or answer carrying AVPs between Diameter
      endpoints.  Each command is uniquely identified by an IANA-
      allocated Command Code value and is described by a Command Code
      Format (CCF) for an application.

   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].

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
   Sections 8 and 9 of [RFC6733]).  Accordingly, a Diameter node is seen
   in this document as a single instance of a Diameter message delivery
   layer and one or more Diameter applications using it.

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

   1.  Defining new AVP values

   2.  Creating new AVPs

   3.  Creating new commands

   4.  Creating new applications




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   As a main guiding principle, application designers SHOULD comply with
   the following recommendation: "try to reuse 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, reuse is not appropriate when the existing functionality
   does not fit the new requirement and/or the reuse 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 it.
      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, as described in [RFC6733].  The standardization effort
      will be fairly small.

   Major Extension:  Enhancing an application that requires the
      definition of a new Diameter application.  Such enhancement causes
      a backward-compatibility issue with existing implementations
      supporting the 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 to be carried in an existing command
      with the M-bit set in the AVP flags (see Section 4.1 of [RFC6733]
      for definition of "M-bit").  For such an extension, a significant
      specification effort is required, and a careful approach is
      recommended.

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.






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4.1.  Adding a New Command

   Adding a new command to an existing application is considered to be a
   major extension and requires a new Diameter application to be
   defined, as stated in Section 1.3.4 of [RFC6733].  The need for a new
   application is because a Diameter node that is not upgraded to
   support the new command(s) within the (existing) application would
   reject any unknown command with the protocol error
   DIAMETER_COMMAND_UNSUPPORTED and cause the failure of the
   transaction.  The new application ensures that Diameter nodes only
   receive commands within the context of applications they support.

   Adding a new command 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 from which the command came.
   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.

   An example considers the Diameter Extensible Authentication Protocol
   (EAP) application [RFC4072] and the Diameter Network Access Server
   application [RFC7155].  When network access authentication using EAP
   is required, the Diameter EAP commands (Diameter-EAP-Request/
   Diameter-EAP-Answer) are used; otherwise, the Diameter Network Access
   Server application will be used.  When the Diameter EAP application
   is used, the accounting exchanges defined in the Diameter Network
   Access Server 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:

   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.




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   It is important to note that 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, and
   then it is considered as a major extension.  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.  An
   exception might be if the intent of the deletion is to create a newer
   variance of the same application that is somehow simpler than the
   application initially specified.

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 noting that there was a
   strong recommendation to reuse existing commands in [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

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

   o  Mandatory (to understand) AVPs.  As defined in [RFC6733], these
      are AVPs with the M-bit flag set in this command, which means that
      the Diameter node receiving them is required to understand not
      only their values but also their semantics.  Failure to do so will
      cause a message handling error: either an error message with the
      result-code set to DIAMETER_AVP_UNSUPPORTED if the AVP is not
      understood in a request or an application-specific error handling
      if the given AVP is in an answer.



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

   It is important to note that the definitions given above are
   independent of whether these AVPs are required or optional in the
   command as specified by the command's CCF syntax [RFC6733].

      NOTE: As stated in [RFC6733], the M-bit setting for a given AVP is
      relevant to an application and each command within that
      application that includes the AVP.

   The rules are strict in the case where the AVPs to be added in an
   exiting command are mandatory to understand, i.e., they have the
   M-bit set.  A mandatory AVP MUST NOT 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
      variances of the same application whereby the two variances 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 MUST be set for the new AVP, and a new Diameter application
   MUST be defined.  This list of questions is non-exhaustive, and other
   criteria MAY be taken into account in the decision process.







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   If application designers are instead contemplating the use of
   optional AVPs, i.e., with the M-bit cleared, there are still pitfalls
   that will cause interoperability problems; therefore, they must be
   avoided.  Some examples of these pitfalls are as follows:

   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  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 understood by the receiver of the command.  This would be
      equivalent to adding a mandatory AVP, i.e., an AVP with the M-bit
      set, to the command.

4.3.2.  Deleting AVPs from a Command

   Application designers may want to reuse an existing command, but some
   of the AVPs present in the command's CCF syntax specification may be
   irrelevant for the functionality foreseen to be supported by this
   command.  It may be then tempting to delete those AVPs from the
   command.

   The impacts of deleting an AVP from a command depends on its Command
   Code format specification and M-bit setting:

   o  Case 1: Deleting an AVP that is indicated as a required AVP (noted
      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, MUST be specified.

   o  Case 2: Deleting an AVP, which has the M-bit set, and is indicated
      as an optional AVP (noted as [AVP] in the command CCF) in the
      command's CCF syntax specification.

      In this case, no new Command Code has to be specified, but the
      definition of a new Diameter application is REQUIRED.

   o  Case 3: 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.



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

4.3.3.  Changing the Flag Settings of AVP in Existing Commands

   Although unusual, implementors may want to change the setting of the
   AVP flags a given AVP used in a command.

   Into an existing command, an AVP that was initially defined as a
   mandatory AVP to understand, i.e., an AVP with the M-bit flag set in
   the command MAY be safely turned to an optional AVP, i.e., with the
   M-bit cleared.  Any node supporting the existing application will
   still understand the AVP, whatever the setting of the M-bit.  On the
   contrary, an AVP initially defined as an optional AVP to understand,
   i.e., an AVP with the M-bit flag cleared in the command MUST NOT be
   changed into a mandatory AVP with the M-bit flag set without defining
   a new Diameter application.  Setting the M-bit for an AVP that was
   defined as an optional AVP is equivalent to adding a new mandatory
   AVP to an existing command, and the rules given in Section 4.3.1
   apply.

   All other AVP flags (V-bit, P-bit, reserved bits) MUST remain
   unchanged.

4.4.  Reusing Existing AVPs

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

4.4.1.  Setting of the AVP Flags

   When reusing existing AVPs in a new application, application
   designers MUST specify the setting of the M-bit flag for a new
   Diameter application and, if necessary, for every command of the
   application that can carry these AVPs.  In general, for AVPs defined
   outside of the Diameter base protocol, the characteristics of an AVP
   are tied to its role within a given application and the commands used
   in this application.

   All other AVP flags (V-bit, P-bit, reserved bits) MUST remain
   unchanged.






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4.4.2.  Reuse of AVP of Type Enumerated

   When reusing an AVP of type Enumerated in a command for a new
   application, it is RECOMMENDED to avoid modifying the set of valid
   values defined for this AVP.  Modifying the set of Enumerated values
   includes adding a value or deprecating the use of a value defined
   initially for the AVP.  Modifying the set of values will impact the
   application defining this AVP and all the applications using this
   AVP, causing potential interoperability issues: a value used by a
   peer that will not be recognized by all the nodes between the client
   and the server will cause an error response with the Result-Code AVP
   set to DIAMETER_INVALID_AVP_VALUE.  When the full range of values
   defined for this Enumerated AVP is not suitable for the new
   application, it is RECOMMENDED that a new AVP be defined to avoid
   backward-compatibility issues with existing implementations.

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.

5.2.  Defining New Commands

   As a general recommendation, commands SHOULD NOT be defined from
   scratch.  It is instead RECOMMENDED to reuse an existing command
   offering similar functionality and use it as a starting point.  Code
   reuse leads to a smaller implementation effort as well as reduces the
   need for testing.

   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 slightly



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   changed environment.  Defining a command with most of the AVPs
   indicated as optional is considered as a good design choice in many
   cases, despite the flexibility it introduces in the protocol.
   Protocol designers MUST clearly state the reasons why these optional
   AVPs might or might not be present and properly define the
   corresponding behavior 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, "* [AVP]" SHOULD be added in the
   command's CCF, which allows the addition of any arbitrary number of
   optional AVPs as described in [RFC6733].

5.3.  Use of Application Id in a Message

   When designing new applications, application designers SHOULD specify
   that the Application Id carried in all session-level messages is the
   Application Id of the application using those messages.  This
   includes the session-level messages defined in the Diameter base
   protocol, i.e., Re-Auth-Request (RAR) / Re-Auth-Answer (RAA),
   Session-Termination-Request (STR) / Session-Termination-Answer (STA),
   Abort-Session-Request (ASR) / Abort-Session-Answer (ASA), and
   possibly Accounting-Request (ACR) / Accounting Answer (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 by new applications to
   avoid routing problems.

   When a new application has been allocated with a new Application Id
   and it also reuses existing commands with or without modifications,
   the commands SHOULD 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 designers using a 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.  The Vendor-Id AVP is an informational AVP only and kept
   for backward compatibility reasons.



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5.4.  Application-Specific Session State Machines

   Section 8 of [RFC6733] provides session state machines for AAA
   services, and these session state machines are not intended to cover
   behavior outside of AAA.  If a new application cannot clearly be
   categorized into any of these AAA services, it is RECOMMENDED that
   the application define its own session state machine.  Support for a
   server-initiated request is a clear example where an application-
   specific session state machine would be needed, for example, the Rw
   interface for the 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 Server (NAS) application
   [RFC4005]) or a 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 the 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 the 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 STR/STA or ASR/
   ASA defined in the Diameter base protocol [RFC6733].  In such a case,
   the Session-Id is not significant.

   Based on these considerations, protocol designers should carefully
   appraise whether the Diameter application being defined relies on the
   session management specified in the Diameter base protocol:

   o  If it is, the Diameter command defined for the new application
      MUST include the Session-Id AVP defined in the Diameter base
      protocol [RFC6733], and the Session-Id AVP MUST be used for
      correlation of messages related to the same session.  Guidance on
      the use of the Auth-Session-State AVP is given in the Diameter
      base protocol [RFC6733].




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   o  Otherwise, because session management is not required or the
      application relies on its own session management mechanism,
      Diameter commands for the application need not include the
      Session-Id AVP.  If any specific session management concept is
      supported by the application, the application documentation MUST
      clearly specify how the session is handled between the client and
      server (and possibly Diameter agents in the path).  Moreover,
      because the application is not maintaining session state at the
      Diameter base protocol level, the Auth-Session-State AVP MUST be
      included in all Diameter commands for the application and MUST be
      set to NO_STATE_MAINTAINED.

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.

   As described in Section 4.4.2 above, defining an AVP of type
   Enumerated presents some limitations in terms of extensibility and
   reusability.  Indeed, the finite set of valid values defined in the
   definition of the AVP of type Enumerated cannot be modified in
   practice without causing backward-compatibility issues with existing
   implementations.  As a consequence, AVPs of type Enumerated MUST NOT
   be extended by adding new values to support new capabilities.
   Diameter protocol designers SHOULD carefully consider before defining
   an Enumerated AVP whether the set of values will remain unchanged or
   new values may be required in the near future.  If such an extension
   is foreseen or cannot be avoided, it is RECOMMENDED to define AVPs of
   type Unsigned32 or Unsigned64 in which the data field would contain
   an address space representing "values" that would have the same use
   of Enumerated values.  Whereas only the initial values defined at the
   definition of the AVP of type Enumerated are valid as described in
   Section 4.4.2, any value from the address space from 0 to 2^32 - 1
   for AVPs of type Unsigned32 or from 0 to 2^64 - 1 for AVPs of type
   Unsigned64 is valid at the Diameter base protocol level and will not
   cause interoperability issues for intermediary nodes between clients
   and servers.  Only clients and servers will be able to process the
   values at the application layer.









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   For illustration, an AVP describing possible access networks would be
   defined as follows:

    Access-Network-Type AVP (XXX) is of type Unsigned32 and
    contains a 32-bit address space representing types of access
    networks.  This application defines the following classes of access
    networks, all identified by the thousands digit in the decimal
    notation:

    o  1xxx (Mobile Access Networks)

    o  2xxx (Fixed Access Networks)

    o  3xxx (Wireless Access Networks)

    Values that fall within the Mobile Access Networks category are used
    to inform a peer that a request has been sent for a user attached to
    a mobile access network.  The following values are defined in this
    application:

    1001: 3GPP-GERAN

       The user is attached to a Global System for Mobile Communications
       (GSM) Enhanced Data rates for GSM Evolution (EDGE) Radio Access
       Network.

    1002: 3GPP-UTRAN-FDD

       The user is attached to a Universal Mobile Telecommunications
       System (UMTS) access network that uses frequency-division
       duplexing for duplexing.

   Unlike Enumerated AVP, any new value can be added in the address
   space defined by this Unsigned32 AVP without modifying the definition
   of the AVP.  There is, therefore, no risk of backward-compatibility
   issues, especially when intermediate nodes may be present between
   Diameter endpoints.

   Along the same line, AVPs of type Enumerated are too often used as a
   simple Boolean flag, indicating, for instance, a specific permission
   or capability; therefore, only three values are defined, e.g., TRUE/
   FALSE, AUTHORIZED/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, with
   the backward-compatibility issues described above.  Instead of using
   an Enumerated AVP for a Boolean flag, protocol designers SHOULD use
   AVPs of type Unsigned32 or Unsigned64 in which the data field would



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   be defined as a 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

   As described in [RFC6733], a Diameter request that needs to be sent
   to a home server serving a specific realm, but not to a specific
   server (such as the first request of a series of round trips), will
   contain a Destination-Realm AVP and no Destination-Host AVP.

   For such a request, the message routing usually relies only 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 AVPs 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 MUST 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.

   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 MUST 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.  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.  Moreover, especially in roaming
   cases, proxy agents in the path must be able to apply local policies
   when receiving the answer from the server during authentication/
   authorization and/or accounting procedures and maintain up-to-date
   session state information by keeping track of all authorized active



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   sessions.  Therefore, application designers MUST NOT modify 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 AAA protocol, such
   as RADIUS.

   In the case of RADIUS, it was initially thought that defining the
   translation function would be straightforward by adopting a 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 the Diameter NAS
   Application [RFC4005].

   However, it was acknowledged that such a 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 is required.  As a consequence, all the material related to
   RADIUS-to-Diameter translation is removed from the new version of the
   Diameter NAS Application specification [RFC7155], which deprecates
   RFC 4005 [RFC4005].

   Therefore, protocol designers SHOULD NOT assume the availability of a
   "standard" Diameter-to-RADIUS gateway 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.

5.9.  End-to-End Application Capabilities Exchange

   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].

   End-to-end capabilities AVPs can be added as optional AVPs with the
   M-bit cleared to existing applications to announce support of new
   functionality.  Receivers that do not understand these AVPs or the
   AVP values can simply ignore them, as stated in [RFC6733].  When
   supported, receivers of these AVPs can discover the additional
   functionality supported by the Diameter endpoint originating the
   request and behave accordingly when processing the request.  Senders



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   of these AVPs can safely assume the receiving endpoint does not
   support any functionality carried by the AVP if it is not present in
   the 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, these end-to-end capabilities AVPs
   SHOULD be added as an optional AVP into the CCF of the commands used
   by the new application.  Protocol designers SHOULD clearly specify
   this end-to-end capabilities exchange and the corresponding behavior
   of the Diameter nodes supporting the application.

   It is also important to note that this end-to-end capabilities
   exchange relying on the use of optional AVPs is not meant as a
   generic mechanism to support extensibility of Diameter applications
   with arbitrary functionality.  When the added features drastically
   change the Diameter application or when Diameter agents must be
   upgraded to support the new features, a new application SHOULD be
   defined, as recommended in [RFC6733].

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:

      *  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



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      accounting server MUST 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.  One of these means could be to insert
      into the request sent to the accounting server an
      Auth-Application-Id AVP containing the identifier of the
      application for which the accounting request is sent.  But in
      general, there is no clean and generic scheme for sorting these
      messages.  Therefore, this model SHOULD NOT be used when all
      received accounting messages cannot be clearly identified and
      sorted.  For most cases, the use of the 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 MUST be provisioned to use a different
         protocol to access the accounting server, e.g., via the
         Lightweight Directory Access Protocol (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.

   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





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   into account.  As a general recommendation, application designers
   SHOULD NOT 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 Transport Layer
   Security (TLS) / TCP or Datagram Transport Layer Security (DTLS) /
   Stream Control Transmission Protocol (SCTP).  However, IPsec MAY also
   be deployed to secure communication between Diameter peers.  When
   IPsec is used instead of TLS or DTLS, the following recommendations
   apply.

   IPsec Encapsulating Security Payload (ESP) [RFC4301] in transport
   mode with non-null encryption and authentication algorithms MUST be
   used to provide per-packet authentication, integrity protection, and
   confidentiality and to support the replay protection mechanisms of
   IPsec.  Internet Key Exchange Protocol Version 2 (IKEv2) [RFC7296]
   SHOULD be used for performing mutual authentication and for
   establishing and maintaining security associations (SAs).

   Version 1 of IKE (IKEv1), defined in [RFC2409], was initially used
   for peer authentication, negotiation of security associations, and
   key management in RFC 3588 [RFC3588].  For easier migration from the
   obsoleted implementations based on IKEv1 to IKEv2, both RSA digital
   signatures and pre-shared keys SHOULD be supported in IKEv2.
   However, if IKEv1 is used, implementors 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 realm-based redirection of
   Diameter requests (see [RFC7075]), conveying a specific set of
   priority parameters influencing the distribution of resources (see
   [RFC6735]), and the support for QoS AVPs (see [RFC5777]).











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   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:

      When defining generic extensions designed to be supported by
      existing Diameter applications, protocol designers MUST consider
      the potential impacts of the introduction of the new extension on
      the behavior of the node that would not be yet upgraded to
      support/understand this new extension.  Designers MUST also ensure
      that new extensions do not break expected message delivery layer
      behavior.

   Forward Compatibility:

      Protocol designers MUST ensure 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 should be sufficient to
   specify such a use case and perhaps provide specific examples of
   applications using them.






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   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 IP
   Multimedia Subsystems (IMS) Cx/Dx interfaces ([TS29.228] and
   [TS29.229]).

7.  Guidelines for Registrations of Diameter Values

   As summarized in Section 3 of this document and further described in
   Section 1.3 of [RFC6733], there are four main ways to extend
   Diameter.  The process for defining new functionality slightly varies
   based on the different extensions.  This section provides protocol
   designers with some guidance regarding the definition of values for
   possible Diameter extensions and the necessary interaction with IANA
   to register the new functionality.

   a.  Defining New AVP Values

      The specifications defining AVPs and AVP values MUST provide
      guidance for defining new values and the corresponding policy for
      adding these values.  For example, RFC 5777 [RFC5777] defines the
      Treatment-Action AVP, which contains a list of valid values
      corresponding to predefined actions (drop, shape, mark, permit).
      This set of values can be extended following the Specification
      Required policy defined in [RFC5226].  As a second example, the
      Diameter base specification [RFC6733] defines the Result-Code AVP
      that contains a 32-bit address space used to identity possible
      errors.  According to Section 11.3.2 of [RFC6733], new values can
      be assigned by IANA via an IETF Review process [RFC5226].

   b.  Creating New AVPs

      Two different types of AVP Codes namespaces can be used to create
      a new AVP:

      *  IETF AVP Codes namespace.

      *  Vendor-specific AVP Codes namespace.

      In the latter case, a vendor needs to be first assigned by IANA
      with a private enterprise number, which can be used within the
      Vendor-Id field of the vendor-specific AVP.  This enterprise
      number delimits a private namespace in which the vendor is
      responsible for vendor-specific AVP code value assignment.  The
      absence of a Vendor Id or a Vendor-Id value of zero (0) in the AVP
      header identifies standard AVPs from the IETF AVP Codes namespace



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      managed by IANA.  The allocation of code values from the IANA-
      managed namespace is conditioned by an Expert Review of the
      specification defining the AVPs or an IETF Review if a block of
      AVPs needs to be assigned.  Moreover, the remaining bits of the
      AVP Flags field of the AVP header are also assigned via Standards
      Action if the creation of new AVP flags is desired.

   c.  Creating New Commands

      Unlike the AVP Codes namespace, the Command Code namespace is
      flat, but the range of values is subdivided into three chunks with
      distinct IANA registration policies:

      *  A range of standard Command Code values that are allocated via
         IETF Review;

      *  A range of vendor-specific Command Code values that are
         allocated on a first-come, first-served basis; and

      *  A range of values reserved only for experimental and testing
         purposes.

      As for AVP flags, the remaining bits of the Command Flags field of
      the Diameter header are also assigned via a Standards Action to
      create new Command flags if required.

   d.  Creating New Applications

      Similarly, to the Command Code namespace, the Application-Id
      namespace is flat but divided into two distinct ranges:

      *  A range of values reserved for standard Application Ids,
         allocated after Expert Review of the specification defining the
         standard application.

      *  A range for values for vendor-specific applications, allocated
         by IANA on a first-come, first-served basis.

   The IANA AAA parameters page can be found at
   <http://www.iana.org/assignments/aaa-parameters>, and the enterprise
   number IANA page is available at <http://www.iana.org/assignments/
   enterprise-numbers>.  More details on the policies followed by IANA
   for namespace management (e.g., first-come, first-served; Expert
   Review; IETF Review; etc.) can be found in [RFC5226].







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      NOTE: When the same functionality/extension is used by more than
      one vendor, it is RECOMMENDED that a standard extension be
      defined.  Moreover, a vendor-specific extension SHOULD be
      registered to avoid interoperability issues in the same network.
      With this aim, the registration policy of a vendor-specific
      extension has been simplified with the publication of [RFC6733],
      and the namespace reserved for vendor-specific extensions is large
      enough to avoid exhaustion.

8.  Security Considerations

   This document provides guidelines and considerations for extending
   Diameter and Diameter applications.  Although such an extension may
   be related to a security functionality, the document does not
   explicitly give additional guidance on enhancing Diameter with
   respect to security.  However, as a general guideline, it is
   recommended that any Diameter extension SHOULD NOT break the security
   concept given in [RFC6733].  In particular, it is reiterated here
   that any command defined or reused in a new Diameter application
   SHOULD be secured by using TLS [RFC5246] or DTLS/SCTP [RFC6083] and
   MUST NOT be used without one of the following: TLS, DTLS, or IPsec
   [RFC4301].  When defining a new Diameter extension, any possible
   impact of the existing security principles described in [RFC6733]
   MUST be carefully appraised and documented in the Diameter
   application specification.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC6733]  Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
              "Diameter Base Protocol", RFC 6733, October 2012,
              <http://www.rfc-editor.org/info/rfc6733>.

9.2.  Informative References

   [Q.3303.3] International Telecommunications Union, "Resource control
              protocol No.  3: Protocols at the Rw interface between the
              policy decision physical entity (PD-PE) and a policy
              enforcement physical entity (PE-PE): Diameter profile
              version 3", ITU-T Recommendation Q.3303.3, August 2008.






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   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, November 1998,
              <http://xml.resource.org/public/rfc/info/rfc2409>.

   [RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
              Arkko, "Diameter Base Protocol", RFC 3588, September 2003,
              <http://www.rfc-editor.org/info/rfc3588>.

   [RFC4005]  Calhoun, P., Zorn, G., Spence, D., and D. Mitton,
              "Diameter Network Access Server Application", RFC 4005,
              August 2005, <http://www.rfc-editor.org/info/rfc4005>.

   [RFC4072]  Eronen, P., Hiller, T., and G. Zorn, "Diameter Extensible
              Authentication Protocol (EAP) Application", RFC 4072,
              August 2005, <http://www.rfc-editor.org/info/rfc4072>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005,
              <http://www.rfc-editor.org/info/rfc4301>.

   [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, <http://www.rfc-editor.org/info/rfc4740>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008, <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [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,
              <http://www.rfc-editor.org/info/rfc5447>.

   [RFC5777]  Korhonen, J., Tschofenig, H., Arumaithurai, M., Jones, M.,
              and A. Lior, "Traffic Classification and Quality of
              Service (QoS) Attributes for Diameter", RFC 5777, February
              2010, <http://www.rfc-editor.org/info/rfc5777>.

   [RFC6083]  Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
              Transport Layer Security (DTLS) for Stream Control
              Transmission Protocol (SCTP)", RFC 6083, January 2011,
              <http://www.rfc-editor.org/info/rfc6083>.



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   [RFC6735]  Carlberg, K. and T. Taylor, "Diameter Priority Attribute-
              Value Pairs", RFC 6735, October 2012,
              <http://www.rfc-editor.org/info/rfc6735>.

   [RFC7075]  Tsou, T., Hao, R., and T. Taylor, "Realm-Based Redirection
              In Diameter", RFC 7075, November 2013,
              <http://www.rfc-editor.org/info/rfc7075>.

   [RFC7155]  Zorn, G., "Diameter Network Access Server Application",
              RFC 7155, April 2014,
              <http://www.rfc-editor.org/info/rfc7155>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, October 2014,
              <http://www.rfc-editor.org/info/rfc7296>.

   [TS29.228] 3rd Generation Partnership Project, "Technical
              Specification Group Core Network and Terminals; IP
              Multimedia (IM) Subsystem Cx and Dx Interfaces; Signalling
              flows and message contents", 3GPP TS 29.228, September
              2014, <http://www.3gpp.org/ftp/Specs/html-info/29228.htm>.

   [TS29.229] 3rd Generation Partnership Project, "Technical
              Specification Group Core Network and Terminals; Cx and Dx
              interfaces based on the Diameter protocol; Protocol
              details", 3GPP TS 29.229, September 2014,
              <http://www.3gpp.org/ftp/Specs/html-info/29229.htm>.

   [TS29.328] 3rd Generation Partnership Project, "Technical
              Specification Group Core Network and Terminals; IP
              Multimedia (IM) Subsystem Sh interface; Signalling flows
              and message contents", 3GPP TS 29.328, September 2014,
              <http://www.3gpp.org/ftp/Specs/html-info/29328.htm>.

   [TS29.329] 3rd Generation Partnership Project, "Technical
              Specification Group Core Network and Terminals; Sh
              Interface based on the Diameter protocol; Protocol
              details", 3GPP TS 29.329, September 2014,
              <http://www.3gpp.org/ftp/Specs/html-info/29329.htm>.











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Contributors

   The content of this document was influenced by a design team created
   to revisit the Diameter extensibility rules.  The team was formed in
   February 2008 and finished its work in June 2008.  In addition to
   those individuals listed in the Authors' Addresses section, the
   design team members were:

   o  Avi Lior

   o  Glen Zorn

   o  Jari Arkko

   o  Jouni Korhonen

   o  Mark Jones

   o  Tolga Asveren

   o  Glenn McGregor

   o  Dave Frascone

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

Acknowledgments

   We greatly appreciate the insight provided by Diameter implementors
   who have highlighted the issues and concerns being addressed by this
   document.  The authors would also like to thank Jean Mahoney, Ben
   Campbell, Sebastien Decugis, and Benoit Claise for their invaluable,
   detailed reviews and comments on this document.
















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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


   Victor Fajardo
   Fluke Networks

   EMail: vf0213@gmail.com


   Hannes Tschofenig
   Hall in Tirol  6060
   Austria

   EMail: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at



























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