wdiff draft-ietf-dime-app-design-guide-17.txt draft-ietf-dime-app-design-guide-18.txt

Diameter Maintenance and Extensions (DIME) L. Morand, Ed.
Internet-Draft Orange Labs
Intended status: Informational V. Fajardo
Expires: November 30, December 08, 2013
H. Tschofenig
Nokia Siemens Networks
May 29,
June 06, 2013

Diameter Applications Design Guidelines
draft-ietf-dime-app-design-guide-17
draft-ietf-dime-app-design-guide-18

Abstract

The Diameter Base 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 Base protocol that further explains and clarifies the rules
to extend the Diameter base protocol. Diameter. It is meant as a guidelines document and
therefore it does not add, remove or change existing
rules.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described informative in [RFC2119]. 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 November 30, December 08, 2013.

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
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described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Reusing Existing Diameter Applications . . . . . . . . . . . 5
4.1. Adding a New Command . . . . . . . . . . . . . . . . . . 5
4.2. Deleting an Existing Command . . . . . . . . . . . . . . 6
4.3. Reusing Existing Commands . . . . . . . . . . . . . . . . 6
4.3.1. Adding AVPs to a Command . . . . . . . . . . . . . . 7 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 . . . . . . . . . . . 11 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 Defined as Boolean Flag . . . . . . . . . . . . . . . 12
5.7. Application-Specific Message Routing . . . . . . . . . . 13 12
5.8. About Translation Agent Agents . . . . . . . . . . . . . . . . . . . 13
5.9. End-to-End Application Capabilities Exchange . . . . . . 14
5.10. Diameter Accounting Support . . . . . . . . . . . . . . . 15 14
5.11. Diameter Security Mechanisms . . . . . . . . . . . . . . 16
6. Defining Generic Diameter Extensions . . . . . . . . . . . . 17 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18 17
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19 18
11. Informative References . . . . . . . . . . . . . . . . . . . . . . . . . 19
11.1. Normative References . . . . . . . . . . . . . . . . . . 19
11.2. Informative References . . . . . . . . . . . . . . . . . 19 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 20

1. Introduction

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

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

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

3. The definition of a an entirely new Diameter application to provide a set of
functionalities 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 total complete freedom
when making their design. A number of rules have been defined in
[RFC6733] and 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 updated the Diameter extensibility rules as defined in the
Diameter base protocol.

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

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

o Present trade-off of design choices.

2. Terminology

This document reuses the terminology used 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.

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

1. Defining new Diameter
application or the reuse of commands, AVPs and AVP values in any
combination for the purpose of inheriting

2. Creating new AVPs

3. Creating new commands

4. Creating new applications

As a main guiding principle, the features of an existing
Diameter application. The recommendation for re-using is: "try to re-use as
much as
possible existing implementations is meaningful possible!". It will reduce the time to finalize
specification writing, and it will lead to a smaller implementation
effort as most of well as reduce the
requirements defined need for a new application are likely already
fulfilled by existing applications.

However, testing. In general, it is
clever to avoid duplicate effort when reusing existing applications, there possible.

However, re-use is a greater
likelihood of ambiguity on how much of not appropriate when the existing application can
be enhanced without being distorted too much and therefore requiring functionality
does not fit the definition of a new application. requirement and/or the re-use leads to
ambiguity.

The impacts of impact on extending existing applications can be categorized as
follow: 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 to
for use in an existing command. Diameter implementations
supporting the existing application but not the new AVP will
simply ignore it, without major consequences on for the Diameter message
handling. In general, this includes everything
that is not covered by the next category. The standardization effort will be fairly small.

Major Extension: Enhancing the functional scope of an existing application in such a way that this implies backward compatible
change to the existing application and then requires the
definition of a new Diameter application.

Typical examples would be the creation of a new command for
providing functionality not supported by existing applications or
the definition of a new AVP with the M-bit set to carry be carried in an
existing command. For such extension, a significant specification
effort is required and a careful approach is recommended.

The rules outlined in

We would also like to remind that the section 1.3 definition of [RFC6733] indicate when an
extension requires a new command code to be registered Diameter
application and when the definition of a new
Diameter applications have to command should be defined. The subsequent sections
further explain and clarify something
to avoid as much as possible. In the rules past, there has been some
reluctance to extend define new commands and new applications. With the Diameter base
protocol. It is meant as a guidelines document
modified extensibility rules provided by [RFC6733], registering new
commands and therefore it new applications does not add, remove or change existing rules. 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

When selecting the Diameter base protocol to support new
functionalities, protocol designers are advised to reuse as much as
possible existing Diameter applications in order to simplify
standardization, implementation and avoid potential interoperability
issues. However,

An existing application needs may need to be adapted enhanced to support fulfill new
requirements and these modifications can be at the command level
and/or and/
or at the AVP level. The following sections describe the possible
modifications that can be performed on existing applications and
their related impacts. 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 CCF Command Code Format (CCF) syntax specification from another
application whereby the new application inherits some or all of the
functionality of the application where the command came from. In the
former case, the decision to create a new application is
straightforward since this is typically a result of adding a new
functionality that does not exist yet. For the latter, the decision
to create a new application will depend on whether importing the
command in a new application is more suitable than simply using the
existing application as it is in conjunction with any other
application. Therefore, a case by case study of each application
requirement should be applied.

An illustrative example is considers the command pair defined in Diameter EAP application [RFC4072] that can be re-used conjointly with any other
application (e.g. and the
Diameter NASREQ application [RFC4005]) as soon
as standard EAP-based [RFC4005]. When network access
authentication procedures need to be supported
by using EAP is required, the implementation. It may therefore not Diameter EAP commands
(Diameter-EAP-Request/Diameter-EAP-Answer) are used; otherwise the
NASREQ application will be required to import used. When the command pair in Diameter EAP application
is used, the new accounting exchanges defined application. 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:

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. e.g., new functionality introduced by the creation of
new optional AVPs.

o Care should be taken to avoid a liberal method of importing an
existing command's CCF syntax specification. This would

Note: Importing commands too liberally could result in a monolithic
and hard to manage application supporting too many different functionalities and can cause interoperability issues
between the different applications.
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
(DIAMETER_COMMAND_UNSUPPORTED). (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 the there was a
strong recommendation to re-use existing commands in the [RFC3588] was to
prevent rapid scarcity depletion of code values available for vendor-specific
commands. However, [RFC6733] relaxes the
policy with respect to has relaxed the allocation of command codes for vendor-
specific uses policy and enlarges
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 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 CCF Command Code
Format (CCF) syntax specification. .

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

The rules are strict in the case where the AVPs to be added are
mandatory to understand i.e. with 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 to set 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. meaning, i.e., be used to carry
application-related information as well as be used to indicate that the
message is for a new application?

When

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

If application designers are instead contemplating the use of
optional AVPs i.e. 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. i.e., to carry application
data as well as to indicate support for one or more features.
This has a tendency to introduce interpretation issues.

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

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

4.3.2. Deleting AVPs from a Command

The impacts of deleting an AVP from a command depend 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, whatever the setting specification (regardless of the M-bit set. This
means the definition of a new command. setting).

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

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

4.4. Reusing Existing AVPs

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

4.4.1. Setting of the AVP Flags

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

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

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

Application designers should also follow the theme of Diameter
extensibility, which in this case means 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. Though some decisions may be clear, designers
should also consider certain aspects of defining a new application.
Some of these aspects

Additional considerations are described in the following sections.

5.2. Defining New Commands

As a general recommendation, reusing as much as possible of existing
material is encouraged when defining new commands. Protocol
designers can thus usefully benefit from the experience gained with
the implementation of existing commands. This includes good
practices to reuse but also known mistakes commands should not to repeat. Therefore
it be defined from
scratch. It is advisable instead recommend to avoid the definition of a command from scratch and
rather take as an example re-use an existing command that would be
functionally close to command under definition.
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 if the context has slightly in a slighly changed
and some AVPs become obsolete.
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 in a way 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 that will allow 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
Application Id carried in all session-level messages must be the
application ID
Application Id of the application using those messages. This
includes the session-level messages defined in Diameter base
protocol, i.e., RAR/RAA, STR/STA, ASR/ASA and possibly ACR/ACA in the
coupled accounting model, see Section 5.10. Existing Some existing
specifications
may do not adhere to this rule for historical or other reasons.
However, this scheme guidance should be followed to avoid possible routing
problems for these messages. problems.

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

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

5.4. Application-Specific Session State Machines

Section 8 of [RFC6733] provides session state machines for
authentication, authorization and accounting (AAA) services and these
session state machines are not intended to cover behavior outside of
AAA. If a new application cannot clearly be categorized into any of
these AAA services, it is recommended that the application define 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. sessions (e.g., Diameter network access session (NASREQ (NASREQ)
application [RFC4005]) or specific service access session (Diameter (e.g.,
Diameter SIP application [RFC4740]). In the Diameter base protocol, the
session
management state is based on referenced using the Session-Id AVP that it used to identify a
given session and all the AVP. All Diameter
messages including 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 in along the diameter
path) are maintaining maintain session state information associated with the Session-Id contained in
the Diameter messages. information.

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

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

5.6. Use of Enumerated Type AVPs Defined as Boolean Flag

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

However, AVPs of type Enumerated are too often used as a simple
Boolean flag, indicating for instance a specific permission or
capability, and therefore only two values are defined e.g. 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 mask AVP masks are the Session-Binding AVP defined in [RFC6733] and the MIP6-Feature-
Vector
MIP6-Feature-Vector AVP defined in [RFC5447] [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. 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
provide such specific AVP-based routing. Such functionality will be
basically hosted by an application-specific Proxy proxy agent that will be
responsible for routing decisions based on the received specific
AVPs.

Example

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 should 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. About Translation Agent Agents

As defined in [RFC6733], a translation agent is a device that
provides interworking between Diameter and another protocol (e.g.
RADIUS, TACACS+). (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.
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
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. application,
if needed. This recommendation applies for any kind of translation (e.g.
Diameter/MAP). translation.

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. Recevers Receivers of these AVPs can discover the addional
functionalities
additional functionality supported by the end-point orignating 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 relying relies on the use of optional AVPs is not meant as a generic
mechanism to support extensibility of Diameter applications with
arbitrary functionalities. 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(2) two
possible models for using accounting:

Split Accounting Model Model:

In this model, the accounting messages will use the Diameter base
accounting application ID Application Id (value of 3). The design implication
for this is that the accounting is treated as an independent
application, especially during 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 will does not define its own unique accounting
commands.

* The overall system architecture permits the use of centralized
accounting for one or more Diameter applications.

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

Coupled Accounting Model Model:

In this model, the accounting messages will use the application ID Application Id
of the application using the accounting service. The design
implication for this is that the accounting messages are tightly
coupled with the application itself; meaning that accounting
messages will be routed like any 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 will does not provide an
accounting server that supports Diameter.

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

* The application server is has to be provisioned to use a different
protocol to access the accounting server; server, e.g., via LDAP, SOAP
etc. This case includes attempting to 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
into account. Though Although it is not recommended, examples of other
methods might be defining an application may
define a new set of commands to carry application-
specific application-specific accounting
records.

5.11. Diameter Security Mechanisms

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

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

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

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

Backward compatibility: Dealing Compatibility:

With the design of generic extensions an protocol designer has to
consider with potential concerns about how existing applications that do
not understand
deal with the new extension. extension they do not understand. Designers
also have to make sure that new extensions do not break expected
message delivery layer behavior.

Forward compatibility: Making Compatibility:

Protocol designers need to make sure that the their design will not
introduce undue restrictions for future applications. Future
applications attempting to support this feature should not have to
go through great lengths to implement any new extensions.

Trade-off in signaling: 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
This document provides guidelines and considerations for extending
Diameter and Diameter applications. It Although such an extension may
related to a security functionality, the document does not define nor address
security-related protocols or schemes. 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 A. Jean Mahoney and Ben
Campbell for their invaluable detailed review and comments on this
document.

11. References

11.1. Normative References

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

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

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

11.2. Informative References

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

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

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