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Diameter Maintenance and Extensions L. Morand, Ed.
(DIME) Orange Labs
Internet-Draft V. Fajardo
Intended status: Informational
Expires: April 25, 2013 H. Tschofenig
Nokia Siemens Networks
October 22, 2012
Diameter Applications Design Guidelines
draft-ietf-dime-app-design-guide-16
Abstract
The Diameter Base protocol provides facilities for protocol
extensibility enabling to define new Diameter applications or modify
existing applications. This document is a companion document to the
Diameter base protocol that further explains and clarifies the rules
to extend the Diameter base protocol. It is meant as a guidelines
document and therefore it does not add, remove or change existing
rules.
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Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Reusing Existing Diameter Applications . . . . . . . . . . . . 8
4.1. Adding a New Command . . . . . . . . . . . . . . . . . . . 8
4.2. Deleting an Existing Command . . . . . . . . . . . . . . . 9
4.3. Reusing Existing Commands . . . . . . . . . . . . . . . . 9
4.3.1. Adding AVPs to a Command . . . . . . . . . . . . . . . 9
4.3.2. Deleting AVPs from a Command . . . . . . . . . . . . . 11
4.4. Reusing Existing AVPs . . . . . . . . . . . . . . . . . . 12
4.4.1. Setting of the AVP Flags . . . . . . . . . . . . . . . 12
4.4.2. Reuse of AVP of Type Enumerated . . . . . . . . . . . 12
5. Defining New Diameter Applications . . . . . . . . . . . . . . 13
5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 13
5.2. Defining New Commands . . . . . . . . . . . . . . . . . . 13
5.3. Use of Application-Id in a Message . . . . . . . . . . . . 14
5.4. Application-Specific Session State Machines . . . . . . . 14
5.5. Session-Id AVP and Session Management . . . . . . . . . . 15
5.6. AVPs Defined as Boolean Flag . . . . . . . . . . . . . . . 15
5.7. Application-Specific Message Routing . . . . . . . . . . . 16
5.8. About Translation Agent . . . . . . . . . . . . . . . . . 17
5.9. End-to-End Application Capabilities Exchange . . . . . . . 17
5.10. Diameter Accounting Support . . . . . . . . . . . . . . . 18
5.11. Diameter Security Mechanisms . . . . . . . . . . . . . . . 20
6. Defining Generic Diameter Extensions . . . . . . . . . . . . . 22
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
8. Security Considerations . . . . . . . . . . . . . . . . . . . 25
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 26
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
11.1. Normative References . . . . . . . . . . . . . . . . . . . 28
11.2. Informative References . . . . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
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1. Introduction
The Diameter base protocol provides facilities to extend the Diameter
base protocol (see Section 1.3 of [I-D.ietf-dime-rfc3588bis]) for
supporting new functionalities. In the context of this document,
extending Diameter means one of the following:
1. Addition of a new functionality to an existing Diameter
application without defining a new application.
2. Addition of a new functionality to an existing Diameter
application that requires the definition of a new application.
3. The definition of a new Diameter application to provide a set of
functionalities not supported by existing applications.
4. The definition of a new generic functionality that can be reused
across different applications.
All of these choices are design decisions that can be done by any
combination of reusing existing or defining new commands, AVPs or AVP
values. However, application designers do not have total freedom
when making their design. A number of rules have been defined in
[I-D.ietf-dime-rfc3588bis] and place constraints on when an extension
requires the allocation of a new Diameter application identifier or a
new command code value. The objective of this document is the
following:
o Clarify updated Diameter extensibility rules in the Diameter base
protocol.
o Clarify usage of certain Diameter functionalities that are not
explicitly described in the Diameter Base specification.
o Discuss design choices and provide guidelines when defining new
applications.
o Present trade-off of design choices.
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2. Terminology
This document reuses the terminology used in
[I-D.ietf-dime-rfc3588bis].
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3. Overview
As designed, the Diameter base protocol [I-D.ietf-dime-rfc3588bis]
can be seen as a two-layer protocol. The lower layer is mainly
responsible for managing connections between neighboring peers and
for message routing. The upper layer is where the Diameter
applications reside. This model is in line with a Diameter node
having an application layer and a peer-to-peer delivery layer. The
Diameter base protocol document defines the architecture and behavior
of the message delivery layer and then provides the framework for
designing Diameter applications on the application layer. This
framework includes definitions of application sessions and accounting
support (see Section 8 and 9 of [I-D.ietf-dime-rfc3588bis]).
Accordingly, a Diameter node is seen in this document as a single
instance of a Diameter message delivery layer and one or more
Diameter applications using it.
The Diameter base protocol is designed to be extensible and the
principles are described in the section 1.3 of
[I-D.ietf-dime-rfc3588bis]. Extending Diameter can mean either the
definition of a completely new Diameter application or the reuse of
commands, AVPs and AVP values in any combination for the purpose of
inheriting the features of an existing Diameter application. The
recommendation for re-using as much as possible existing
implementations is meaningful as most of the requirements defined for
a new application are likely already fulfilled by existing
applications.
However, when reusing existing applications, there is a greater
likelihood of ambiguity on how much of the existing application can
be enhanced without being distorted too much and therefore requiring
the definition of a new application.
The impacts of extending existing applications can be categorized as
follow:
Minor Extension: Enhancing the functional scope of an existing
application by the addition of optional features to support. Such
enhancement has no backward compatibility issue with the existing
application. A typical example would be the definition of a new
optional AVP to use in an existing command. Diameter
implementations supporting the existing application but not the
new AVP will simply ignore it, without major consequences on the
Diameter message handling. In general, this includes everything
that is not covered by the next category. The standardization
effort will be fairly small.
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Major Extension: Enhancing the functional scope of an existing
application in such a way that this implies backward compatible
change to the existing application and then requires the
definition of a new Diameter application. Typical examples would
be the creation of a new command for providing functionality not
supported by existing applications or the definition of a new AVP
with M-bit set to carry in an existing command. For such
extension, a significant specification effort is required and a
careful approach is recommended.
The rules outlined in the section 1.3 of [I-D.ietf-dime-rfc3588bis]
indicate when an extension requires a new command code to be
registered and when new Diameter applications have to be defined.
The subsequent sections further explain and clarify the rules to
extend the Diameter base protocol. It is meant as a guidelines
document and therefore it does not add, remove or change existing
rules.
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4. Reusing Existing Diameter Applications
When selecting the Diameter base protocol to support new
functionalities, protocol designers are advised to reuse as much as
possible existing Diameter applications in order to simplify
standardization, implementation and avoid potential interoperability
issues. However, existing application needs to be adapted to support
new requirements and these modifications can be at the command level
and/or at the AVP level. The following sections describe the
possible modifications that can be performed on existing applications
and their related impacts.
4.1. Adding a New Command
Adding a new command is considered as a major extension and requires
a new Diameter application to be defined. Adding a new command to an
application means either defining a completely new command or
importing the command's CCF syntax specification from another
application whereby the new application inherits some or all of the
functionality of the application where the command came from. In the
former case, the decision to create 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 the command pair defined in Diameter EAP
application [RFC4072] that can be re-used conjointly with any other
application (e.g. the Diameter NASREQ application [RFC4005]) as soon
as standard EAP-based authentication procedures need to be supported
by the implementation. It may therefore not be required to import
the command pair in the new defined application.
However, in general, it is difficult to come to a hard guideline, and
so a case-by-case study of each application requirement should be
applied. Before adding or importing a command, application designers
should consider the following:
o Can the new functionality be fulfilled by creating a new command
independent from any existing command? In this case, the
resulting new application and the existing application can work
independent of, but cooperating with each other.
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o Can the existing command be reused without major extensions and
therefore without the need for the definition of a new
application, e.g. new functionality introduced by the creation of
new optional AVPs.
o Care should be taken to avoid a liberal method of importing an
existing command's CCF syntax specification. This would result in
a monolithic and hard to manage application supporting too many
different functionalities and can cause interoperability issues
between the different applications.
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).
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.
It is worth to note that the strong recommendation to re-use existing
commands in the [RFC3588] was to prevent rapid scarcity of code
values available for vendor-specific commands.
[I-D.ietf-dime-rfc3588bis] relaxes the policy with respect to the
allocation of command codes for vendor-specific uses and enlarges the
range of available code values for vendor-specific applications.
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 [I-D.ietf-dime-rfc3588bis], AVPs that are added
to an existing command can be categorized into:
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o Mandatory (to understand) AVPs. As defined in
[I-D.ietf-dime-rfc3588bis], these are AVPs with the M-bit flag
set, which means that a Diameter node receiving them is required
to understand not only their values but their semantics. Failure
to do so will cause an message handling error. This is regardless
of whether these AVPs are required or optional as specified by the
command's CCF syntax specification.
o Optional (to understand) AVPs. As defined in
[I-D.ietf-dime-rfc3588bis], these are AVPs with the M-bit flag
cleared, which mean that a Diameter node receiving these AVPs can
simply ignore them if not supported in the process of the received
command.
The rules are strict in the case where the AVPs to be added are
mandatory to understand i.e. with the M-bit set. A mandatory AVP
cannot be added to an existing command without defining a new
Diameter application, as stated in [I-D.ietf-dime-rfc3588bis]. This
falls into the "Major Extensions" category. Despite the clarity of
the rule, ambiguity still arises when evaluating whether a new AVP
being added should be mandatory to begin with. Application designers
should consider the following questions when deciding to set the
M-bit for a new AVP:
o Would it be required for the receiving side to be able to process
and understand the AVP and its content?
o Would the new AVPs change the state machine of the application?
o Would the presence of the new AVP lead to a different number of
round-trips, effectively changing the state machine of the
application?
o Would the new AVP be used to differentiate between old and new
versions of the same application whereby the two versions are not
backward compatible?
o Would the new AVP have duality in meaning i.e. be used to carry
application-related information as well as be used to indicate
that the message is for a new application?
When one of the above questions can be answered in the affirmative
then the M-bit has to be set for the new AVP. This list of questions
is non-exhaustive and other criteria can 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, then the following are
some of the pitfalls that should be avoided:
o Use of optional AVPs with intersecting meaning. One AVP has
partially the same usage and meaning as another AVP. The presence
of both can lead to confusion.
o An optional AVPs with dual purpose, i.e. to carry application data
as well as to indicate support for one or more features. This has
a tendency to introduce interpretation issues.
o Adding one or more optional AVPs and indicating (usually within
descriptive text for the command) that at least one of them has to
be present in the command. This essentially circumventing the
ABNF and is equivalent to adding a mandatory AVP to the command.
These practices generally result in interoperability issues and
should be avoided as much as possible.
4.3.2. Deleting AVPs from a Command
The impacts of deleting an AVP from a command depend 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 of the M-bit set. This
means the definition of a new command. In this case, a new
command code and subsequently a new Diameter application have to
be specified.
o Deleting an AVP with M-bit set that is indicated as [ AVP ] in the
command's CCF syntax specification. No new command code has to be
specified but the definition of a new Diameter application is
required.
o Deleting an AVP with the M-bit cleared that is indicated as [ AVP
] in the command's CCF syntax specification. In this case, the
AVP can be deleted without consequences.
If possible application designers should attempt the reuse the
command's CCF syntax specification without modification and simply
ignore (but not delete) any optional AVP that will not be used. This
is to maintain compatibility with existing applications that will not
know about the new functionality as well as maintain the integrity of
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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 characteristics are tied to its
role within an application and command.
All other AVP flags shall remain unchanged.
4.4.2. Reuse of AVP of Type Enumerated
When modifying the set of values supported by an AVP of type
Enumerated, this means defining a new AVP. Modifying the set of
Enumerated values includes adding a value or deprecating the use of a
value defined initially for the AVP. Defining a new AVP will avoid
interoperability issues.
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5. Defining New Diameter Applications
5.1. Introduction
The general recommendation for Diameter extensibility is to reuse
commands, AVPs and AVP values as much as possible. However, some of
the extensibility rules described in the previous sections also apply
to scenarios where a designer is trying to define a completely new
Diameter application.
This section discusses the case where new applications have
requirements that cannot be filled by existing applications and would
require definition of completely new commands, AVPs and/or AVP
values. Typically, there is little ambiguity about the decision to
create these types of applications. Some examples are the interfaces
defined for the IP Multimedia Subsystem of 3GPP, i.e. Cx/Dx
([TS29.228] and [TS29.229]), Sh ([TS29.328] and [TS29.329]) etc.
Application designers should also follow the theme of Diameter
extensibility, which in this case means to import existing AVPs and
AVP values for any newly defined commands. In certain cases where
accounting will be used, the models described in Section 5.10 should
also be considered. Though some decisions may be clear, designers
should also consider certain aspects of defining a new application.
Some of these aspects are described in following sections.
5.2. Defining New Commands
As a general recommendation, reusing as much as possible of existing
material is encouraged when defining new commands. Protocol
designers can thus usefully benefit from the experience gained with
the implementation of existing commands. This includes good
practices to reuse but also known mistakes not to repeat. Therefore
it is advisable to avoid the definition of a command from scratch and
rather take as an example an existing command that would be
functionally close to command under definition.
Moreover, the new command's CCF should be carefully defined when
considering applicability and extensibility of the application. If
most of the AVPs contained in the command are indicated as fixed or
required, it might be difficult to reuse the same command and
therefore the same application if the context has slightly changed
and some AVPs become obsolete. Defining a command with most of the
AVPs indicated as optional must not be seen as a sub-optimal design
introducing too much flexibility in the protocol. The protocol
designers are only advised to clearly state the condition of presence
of these AVPs and properly define the corresponding behaviour of the
Diameter nodes when these AVPs are absent from the command.
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In the same way, the CCF should be defined in a way that it will be
possible to add any arbitrary optional AVPs with the M-bit cleared
(including vendor-specific AVPs) without modifying the application.
For this purpose, it is strongly recommended to add "* [AVP]" in the
command's CCF that will allow the addition of any arbitrary AVP as
described in [I-D.ietf-dime-rfc3588bis].
5.3. Use of Application-Id in a Message
When designing new applications, designers should specify that the
application ID carried in all session-level messages must be the
application ID of the application using those messages. This
includes the session-level messages defined in 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 specifications
may not adhere to this rule for historical or other reasons.
However, this scheme should be followed to avoid possible routing
problems for these messages.
In general, when a new application has been allocated with a new
application id and it also reuses existing commands with or without
modifications (Sec 4.1), it must use the newly allocated application
id in the header and in all relevant application id AVPs (Auth-
Application-Id or Acct-Application-Id) present in the commands
message body.
Additionally, application designs using
Vendor-Specific-Application-Id AVP should not use the Vendor-Id AVP
to further dissect or differentiate the vendor-specification
application id. Diameter routing is not based on the Vendor-Id. As
such, the Vendor-ID should not be used as an additional input for
routing or delivery of messages. In general, the Vendor-Id AVP is an
informational AVP only and kept for backward compatibility reasons.
5.4. Application-Specific Session State Machines
Section 8 of [I-D.ietf-dime-rfc3588bis] 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 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]).
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5.5. Session-Id AVP and Session Management
Diameter applications are usually designed with the aim of managing
user sessions, e.g. network access session (NASREQ application
[RFC4005]) or specific service access session (Diameter SIP
application [RFC4740]). In the Diameter base protocol, the session
management is based on the Session-Id AVP that it used to identify a
given session and all the Diameter messages including the same
Session-Id will be bound to the same session. Diameter-based session
management also implies that both Diameter client and server (and
potentially proxy agents in the diameter path) are maintaining
session state information associated with the Session-Id contained in
the Diameter messages.
However, some applications may not need to rely on the Session-Id to
identify and manage user sessions because other information can be
used instead to correlate Diameter messages. Indeed, the User-Name
AVP or any other specific AVP can be present in every Diameter
message and used therefore for message correlation. There might even
be applications for which the notion of Diameter session management
would not be required at all. For such applications, the Auth-
Session-State AVP is usually set to NO_STATE_MAINTAINED in all the
Diameter messages and these applications are therefore designed as a
set of stand-alone transactions. Even if an explicit access session
termination is required, application-specific commands are defined
and used instead of the Session-Termination-Request/Answer (STR/STA)
or Abort-Session-Request/Answer (ASR/ASA) defined in the Diameter
base protocol. In such a case, the Session-Id is not significant.
Based on these considerations, protocol designers should carefully
appraise whether the application currently defined relies on the
concept of session management and whether the Session-Id defined in
the Diameter base protocol would be used for correlation of messages
related to the same session. If not, the protocol designers could
decide to define application commands without the Session-Id AVP. If
any session management concept is supported by the application, the
application documentation must clearly specify how the session is
handled between client and server (as possibly Diameter agents in the
path).
5.6. AVPs Defined as Boolean Flag
The type Enumerated was initially defined to provide 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.
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However, AVPs of type Enumerated are too often used as a simple
Boolean flag, indicating for instance a specific permission or
capability, and therefore only two values are defined e.g. TRUE/
FALSE, AUTORIZED/UNAUTHORIZED or SUPPORTED/UNSUPPORTED. This is a
sub-optimal design since it limits the extensibility of the
application: any new capability/permission would have to be supported
by a new AVP or new Enumerated value of the already defined AVP,
causing backwards compatibility issues with existing implementations.
Instead of using an Enumerated AVP for a Boolean flag, protocol
designers are encouraged to use Unsigned32 or Unsigned64 AVP type as
bit mask whose bit settings are described in the relevant Diameter
application specification. Such AVPs can be reused and extended
without major impact on the Diameter application. The bit mask
should leave room for future additions. Examples of bit mask AVP are
the Session-Binding AVP defined in [I-D.ietf-dime-rfc3588bis] and the
MIP6-Feature-Vector AVP defined in [RFC5447]
5.7. Application-Specific Message Routing
Diameter request message routing usually relies on the Destination-
Realm AVP and the Application Id present in the request message
header. However, some applications may need to rely on the User-Name
AVP or any other application-specific AVP present in the request to
determine the final destination of a request e.g. find the target AAA
server hosting the authorization information for a given user when
multiple AAA servers are addressable in the realm.
In such a context, basic routing mechanisms described in
[I-D.ietf-dime-rfc3588bis] are not fully suitable, and additional
application-level routing mechanisms have to be described in the
application documentation to provide such specific AVP-based routing.
Such functionality will be basically hosted by an application-
specific Proxy agent that will be responsible for routing decisions
based on the received specific AVPs.
Example of such 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 follow the reverse path of
the request, as described in [I-D.ietf-dime-rfc3588bis], 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
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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
[I-D.ietf-dime-rfc3588bis] when defining a new application.
5.8. About Translation Agent
As defined in [I-D.ietf-dime-rfc3588bis], a translation agent is a
device that provides interworking between Diameter and another
protocol (e.g. RADIUS, TACACS+).
In the case of RADIUS, it was initially thought that defining the
translation function would be straightforward by adopting few basic
principles e.g. use of a shared range of code values for RADIUS
attributes and Diameter AVPs. Guidelines for implementing a RADIUS-
Diameter translation agent were put into RFC 4005 ([RFC4005]).
However, it was acknowledged that such translation mechanism was not
so obvious and deeper protocol analysis was required to ensure
efficient interworking between RADIUS and Diameter. Moreover, the
interworking requirements depend on the functionalities provided by
the Diameter application under specification, and a case-by-case
analysis will be required.
Therefore, protocol designers cannot assume the availability of a
"standard" Diameter-to-RADIUS gateways agent when planning to
interoperate with the RADIUS infrastructure. They should specify the
required translation mechanism along with the Diameter application.
This recommendation applies for any kind of translation (e.g.
Diameter/MAP).
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 in [RFC5447] and [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 of these AVPs can discover the addional
functionalities supported the end-point orignating 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
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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 on the use of optional AVPs is not meant as a
generic mechanism to support extensibility of Diameter applications
with arbitrary functionalities. 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) possible models for using accounting:
Split Accounting Model
In this model, the accounting messages will use the Diameter base
accounting application ID (value of 3). The design implication
for this is that the accounting is treated as an independent
application, especially during Diameter routing. This means that
accounting commands emanating from an application may be routed
separately from the rest of the other application messages. This
may also imply that the messages end up in a central accounting
server. A split accounting model is a good design choice when:
* The application itself will not define its own unique
accounting commands.
* The overall system architecture permits the use of centralized
accounting for one or more Diameter applications.
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
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messages with applications and/or services being accounted for.
This may mean defining new AVPs, checking the presence, absence or
contents of existing AVPs, or checking the contents of the
accounting record itself. But in general, there is no clean and
generic scheme for sorting these messages. Therefore, the use of
this model is recommended only when all received accounting
messages can be clearly identified and sorted. For most cases,
the use of Coupled Accounting Model is recommended.
Coupled Accounting Model
In this model, the accounting messages will use the application ID
of the application using the accounting service. The design
implication for this is that the accounting messages are tightly
coupled with the application itself; meaning that accounting
messages will be routed like any other application messages. It
would then be the responsibility of the application server
(application entity receiving the ACR message) to send the
accounting records carried by the accounting messages to the
proper accounting server. The application server is also
responsible for formulating a proper response (ACA). A coupled
accounting model is a good design choice when:
* The system architecture or deployment will not provide an
accounting server that supports Diameter.
* The system architecture or deployment requires that the
accounting service for the specific application should be
handled by the application itself.
* The application server is provisioned to use a different
protocol to access the accounting server; e.g., via LDAP, SOAP
etc. This includes attempting to support older accounting
systems that are not Diameter aware.
In all cases above, there will generally be no direct Diameter
access to the accounting server.
These models provide a basis for using accounting messages.
Application designers may obviously deviate from these models
provided that the factors being addressed here have also been taken
into account. Though it is not recommended, examples of other
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methods might be defining a new set of commands to carry application-
specific accounting records.
5.11. Diameter Security Mechanisms
As specified in [I-D.ietf-dime-rfc3588bis], the Diameter message
exchange should be secured by using TLS/TCP or DTLS/SCTP. However,
IPsec can also be deployed to secure connections between Diameter
peers. When IPsec is used instead of TLS or DTLS, the following
recommendations apply.
IPsec ESP 5.3 [RFC4301] in transport mode with non-null encryption
and authentication algorithms is used to provide per-packet
authentication, integrity protection and confidentiality, and support
the replay protection mechanisms of IPsec. IKE is used for peer
authentication, negotiation of security associations, and key
management, using the IPsec DOI [RFC2407]. Peer authentication can
be achieved by using a pre-shared key, or certificate-based peer
authentication using digital signatures can be used as alternative.
Peer authentication using the public key encryption methods outlined
in IKE's Sections 5.2 and 5.3 [RFC2409] should not be used.
Diameter implementations using IPsec as the security mechanism must
support both IKE Main Mode and Aggressive Mode. When pre-shared keys
are used for authentication, IKE Aggressive Mode should be used
instead of IKE Main Mode. When digital signatures are used for
authentication, either IKE Main Mode or IKE Aggressive Mode can be
used.
When digital signatures are used to achieve authentication, an IKE
negotiator should use IKE Certificate Request Payload(s) to specify
the certificate authority (or authorities) that are trusted in
accordance with its local policy. IKE negotiators should use
pertinent certificate revocation checks before accepting a PKI
certificate for use in IKE's authentication procedures.
The Phase 2 Quick Mode exchanges used to negotiate protection for
Diameter connections must explicitly carry the Identity Payload
fields (IDci and IDcr). The DOI provides for several types of
identification data. However, when used in conformant
implementations, each ID Payload must carry a single IP address and a
single non-zero port number, and must not use the IP Subnet or IP
Address Range formats. This allows the Phase 2 security association
to correspond to specific TCP and SCTP connections.
Since IPsec acceleration hardware may only be able to handle a
limited number of active IKE Phase 2 SAs, Phase 2 delete messages may
be sent for idle SAs, as a means of keeping the number of active
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Phase 2 SAs to a minimum. The receipt of an IKE Phase 2 delete
message should not be interpreted as a reason for tearing down a
Diameter connection. Rather, it is preferable to leave the
connection up, and if additional traffic is sent on it, to bring up
another IKE Phase 2 SA to protect it. This avoids the potential for
continually bringing connections up and down.
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6. Defining Generic Diameter Extensions
Generic Diameter extensions are AVPs, commands or applications that
are designed to support other Diameter applications. They are
auxiliary applications meant to improve or enhance the Diameter
protocol itself or Diameter applications/functionality. Some
examples include the extensions to support auditing and redundancy
(see [I-D.calhoun-diameter-res-mgmt]), improvements in duplicate
detection scheme (see [I-D.asveren-dime-dupcons]), and piggybacking
of QoS attributes (see [RFC5777]).
Since generic extensions can cover many aspects of Diameter and
Diameter applications, it is not possible to enumerate all the
probable scenarios in this document. However, some of the most
common considerations are as follows:
o Backward compatibility: Dealing with existing applications that do
not understand the new extension. Designers also have to make
sure that new extensions do not break expected message delivery
layer behavior.
o Forward compatibility: Making sure that the design will not
introduce undue restrictions for future applications. Future
applications attempting to support this feature should not have to
go through great lengths to implement any new extensions.
o 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
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specification must enumerate these applications and the scenarios
when the optional AVPs will be used. In the case where the optional
AVPs can be carried by any application, it is should be sufficient to
specify such a use case and perhaps provide specific examples of
applications using them.
In most cases, these optional AVPs piggybacked by applications would
be defined as a Grouped AVP and it would encapsulate all the
functionality of the generic extension. In practice, it is not
uncommon that the Grouped AVP will encapsulate an existing AVP that
has previously been defined as mandatory ('M'-bit set) e.g., 3GPP IMS
Cx/Dx interfaces ([TS29.228] and [TS29.229]).
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7. IANA Considerations
This document does not require actions by IANA.
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8. Security Considerations
This document provides guidelines and considerations for extending
Diameter and Diameter applications. It does not define nor address
security-related protocols or schemes.
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9. Contributors
The content of this document was influenced by a design team created
to revisit the Diameter extensibility rules. The team consisting of
the members listed below was formed in February 2008 and finished its
work in June 2008.
o Avi Lior
o Glen Zorn
o Jari Arkko
o Lionel Morand
o Mark Jones
o Victor Fajardo
o Tolga Asveren
o Jouni Korhonen
o Glenn McGregor
o Hannes Tschofenig
o Dave Frascone
We would like to thank Tolga Asveren, Glenn McGregor, and John
Loughney for their contributions as co-authors to earlier versions of
this document.
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10. Acknowledgments
We greatly appreciate the insight provided by Diameter implementers
who have highlighted the issues and concerns being addressed by this
document. The authors would also like to thank A. Jean Mahoney and
Ben Campbell for their invaluable detailed review and comments on
this document.
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11. References
11.1. Normative References
[I-D.ietf-dime-rfc3588bis]
Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
"Diameter Base Protocol", draft-ietf-dime-rfc3588bis-34
(work in progress), June 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
11.2. Informative References
[I-D.asveren-dime-dupcons]
Asveren, T., "Diameter Duplicate Detection Cons.",
draft-asveren-dime-dupcons-00 (work in progress),
August 2006.
[I-D.calhoun-diameter-res-mgmt]
Calhoun, P., "Diameter Resource Management Extensions",
draft-calhoun-diameter-res-mgmt-08.txt (work in progress),
March 2001.
[Q.3303.3]
3rd Generation Partnership Project, "ITU-T Recommendation
Q.3303.3, "Resource control protocol no. 3 (rcp3):
Protocol at the Rw interface between the Policy Decision
Physical Entity (PD-PE) and the Policy Enforcement
Physical Entity (PE-PE): Diameter"", 2008.
[RFC2407] D. Piper, "The Internet IP Security Domain of
Interpretation for ISAKMP", 1998.
[RFC2409] D. Harkins and D. Carrel, "The Internet Key Exchange
(IKE)", 1998.
[RFC4005] P. Calhoun et al., "Diameter Network Access Server
Application", August 2005,
<http://www.rfc-editor.org/rfc/rfc4005.txt>.
[RFC4072] P. Eronen et al., "Diameter Extensible Authentication
Protocol (EAP) Application", August 2005,
<http://www.rfc-editor.org/rfc/rfc4072.txt>.
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[RFC4301] S. Kent and K. Seo, "Security Architecture for the
Internet Protocol", 2005.
[RFC4740] M. Garcia-Martin et al., "Diameter Session Initiation
Protocol (SIP) Application", November 2006,
<http://www.rfc-editor.org/rfc/rfc4740.txt>.
[RFC5447] J. Korhonen et al., "Diameter Mobile IPv6: Support for
Network Access Server to Diameter Server Interaction",
February 2009,
<http://www.rfc-editor.org/rfc/rfc5447.txt>.
[RFC5777] J. Korhonen et al., "Traffic Classification and Quality of
Service (QoS) Attributes for Diameter", 2010.
[TS29.228]
3rd Generation Partnership Project, "3GPP TS 29.228;
Technical Specification Group Core Network and Terminals;
IP Multimedia (IM) Subsystem Cx and Dx Interfaces;
Signalling flows and message contents",
<http://www.3gpp.org/ftp/Specs/html-info/29272.htm>.
[TS29.229]
3rd Generation Partnership Project, "3GPP TS 29.229;
Technical Specification Group Core Network and Terminals;
Cx and Dx interfaces based on the Diameter protocol;
Protocol details",
<http://www.3gpp.org/ftp/Specs/html-info/29229.htm>.
[TS29.328]
3rd Generation Partnership Project, "3GPP TS 29.328;
Technical Specification Group Core Network and Terminals;
IP Multimedia (IM) Subsystem Sh interface; signalling
flows and message content",
<http://www.3gpp.org/ftp/Specs/html-info/29328.htm>.
[TS29.329]
3rd Generation Partnership Project, "3GPP TS 29.329;
Technical Specification Group Core Network and Terminals;
Sh Interface based on the Diameter protocol; Protocol
details",
<http://www.3gpp.org/ftp/Specs/html-info/29329.htm>.
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Authors' Addresses
Lionel Morand (editor)
Orange Labs
Email: lionel.morand@orange.com
Victor Fajardo
Email: vf0213@gmail.com
Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
Email: Hannes.Tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
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