| Diameter Maintenance and Extensions (DIME) | L. Morand, Ed. |
| Internet-Draft | Orange Labs |
| Intended status: Informational | V. Fajardo |
| Expires: December 08, 2013 | H. Tschofenig |
| Nokia Siemens Networks | |
| June 06, 2013 |
Diameter Applications Design Guidelines
draft-ietf-dime-app-design-guide-18
The Diameter base protocol provides facilities for protocol extensibility enabling to define new Diameter applications or modify existing applications. This document is a companion document to the Diameter Base protocol that further explains and clarifies the rules to extend Diameter. It is meant as a guidelines document and therefore as informative in nature.
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The Diameter base protocol provides facilities to extend Diameter (see Section 1.3 of [RFC6733]) to support new functionality. In the context of this document, extending Diameter means one of the following:
All of these choices are design decisions that can be done by any combination of reusing existing or defining new commands, AVPs or AVP values. However, application designers do not have complete freedom when making their design. A number of rules have been defined in [RFC6733] that place constraints on when an extension requires the allocation of a new Diameter application identifier or a new command code value. The objective of this document is the following:
This document reuses the terminology defined in [RFC6733].
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 1.3 of [RFC6733]. As a summary, Diameter can be extended by:
As a main guiding principle, the recommendation is: "try to re-use as much as possible!". It will reduce the time to finalize specification writing, and it will lead to a smaller implementation effort as well as reduce the need for testing. In general, it is clever to avoid duplicate effort when possible.
However, re-use is not appropriate when the existing functionality does not fit the new requirement and/or the re-use leads to ambiguity.
The impact on extending existing applications can be categorized into two groups:
We would also like to remind that the definition of a new Diameter application and the definition of a new command should be something to avoid as much as possible. In the past, there has been some reluctance to define new commands and new applications. With the modified extensibility rules provided by [RFC6733], registering new commands and new applications does not lead to additional overhead for the specification author in terms of standardization process. Registering new functionality (new commands, new AVPs, new applications, etc.) with IANA remains important to avoid namespace collisions, which will likely lead to deployment problems.
An existing application may need to be enhanced to fulfill new requirements and these modifications can be at the command level and/or at the AVP level. The following sections describe the possible modifications that can be performed on existing applications and their related impact.
Adding a new command is considered as a major extension and requires a new Diameter application to be defined. Adding a new command to an application means either defining a completely new command or importing the command's Command Code Format (CCF) syntax from another application whereby the new application inherits some or all of the functionality of the application where the command came from. In the former case, the decision to create a new application is straightforward since this is typically a result of adding a new functionality that does not exist yet. For the latter, the decision to create a new application will depend on whether importing the command in a new application is more suitable than simply using the existing application as it is in conjunction with any other application. Therefore, a case by case study of each application requirement should be applied.
An example considers the Diameter EAP application [RFC4072] and the Diameter NASREQ application [RFC4005]. When network access authentication using EAP is required, the Diameter EAP commands (Diameter-EAP-Request/Diameter-EAP-Answer) are used; otherwise the NASREQ application will be used. When the Diameter EAP application is used, the accounting exchanges defined in Diameter NASREQ may be used.
However, in general, it is difficult to come to a hard guideline, and so a case-by-case study of each application requirement should be applied. Before adding or importing a command, application designers should consider the following:
Note: Importing commands too liberally could result in a monolithic and hard to manage application supporting too many different features.
Although this process is not typical, removing a command from an application requires a new Diameter application to be defined. This is due to the fact that the reception of the deleted command would systematically result in a protocol error (i.e., DIAMETER_COMMAND_UNSUPPORTED).
It is unusual to delete an existing command from an application for the sake of deleting it or the functionality it represents. This normally indicates of a flawed design. An exception might be if the intent of the deletion is to create a newer version of the same application that is somehow simpler than the previous version.
This section discusses rules in adding and/or deleting AVPs from an existing command of an existing application. The cases described in this section may not necessarily result in the creation of new applications.
From a historical point of view, it is worth to note that there was a strong recommendation to re-use existing commands in the [RFC3588] to prevent rapid depletion of code values available for vendor-specific commands. However, [RFC6733] has relaxed the allocation policy and enlarged the range of available code values for vendor-specific applications. Although reuse of existing commands is still recommended, protocol designers can consider defining a new command when it provides a solution more suitable than the twisting of an existing command's use and applications.
Based on the rules in [RFC6733], AVPs that are added to an existing command can be categorized into:
The rules are strict in the case where the AVPs to be added are mandatory to understand, i.e., they have the M-bit set. A mandatory AVP cannot be added to an existing command without defining a new Diameter application, as stated in [RFC6733]. This falls into the "Major Extensions" category. Despite the clarity of the rule, ambiguity still arises when evaluating whether a new AVP being added should be mandatory to begin with. Application designers should consider the following questions when deciding about the M-bit for a new AVP:
If the answer to at least one of the questions is "yes" then the M-bit has to be set for the new AVP. This list of questions is non-exhaustive and other criteria can be taken into account in the decision process.
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:
These practices generally result in interoperability issues and should be avoided as much as possible.
The impacts of deleting an AVP from a command depends on its command code format specification and M-bit setting:
If possible, application designers should attempt the reuse the command's CCF syntax specification without modification and simply ignore (but not delete) any optional AVP that will not be used. This is to maintain compatibility with existing applications that will not know about the new functionality as well as maintain the integrity of existing dictionaries.
This section discusses rules in reusing existing AVP when reusing an existing command or defining a new command in a new application.
When reusing AVPs in a new application, the AVP flag setting, such as the mandatory flag ('M'-bit), has to be re-evaluated for a new Diameter application and, if necessary, even for every command within the application. In general, for AVPs defined outside of the Diameter base protocol, the characteristics of an AVP are tied to its role within an application and the commands.
All other AVP flags shall remain unchanged.
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.
This section discusses the case where new applications have requirements that cannot be fulfilled by existing applications and would require definition of completely new commands, AVPs and/or AVP values. Typically, there is little ambiguity about the decision to create these types of applications. Some examples are the interfaces defined for the IP Multimedia Subsystem of 3GPP, e.g., Cx/Dx ([TS29.228] and [TS29.229]), Sh ([TS29.328] and [TS29.329]) etc.
Application designers should try to import existing AVPs and AVP values for any newly defined commands. In certain cases where accounting will be used, the models described in Section 5.10 should also be considered.
Additional considerations are described in the following sections.
As a general recommendation, commands should not be defined from scratch. It is instead recommend to re-use an existing command offering similar functionality and use it as a starting point.
Moreover, the new command's CCF syntax specification should be carefully defined when considering applicability and extensibility of the application. If most of the AVPs contained in the command are indicated as fixed or required, it might be difficult to reuse the same command and therefore the same application in a slighly changed environment. Defining a command with most of the AVPs indicated as optional must not be seen as a sub-optimal design introducing too much flexibility in the protocol. The protocol designers are only advised to clearly state the condition of presence of these AVPs and properly define the corresponding behaviour of the Diameter nodes when these AVPs are absent from the command.
Note: As a hint for protocol designers, it is not sufficient to just look at the command's CCF syntax specification. It is also necessary to carefully read through the accompanying text in the specification.
In the same way, the CCF syntax specification should be defined such that it will be possible to add any arbitrary optional AVPs with the M-bit cleared (including vendor-specific AVPs) without modifying the application. For this purpose, it is strongly recommended to add "* [AVP]" in the command's CCF, which allows the addition of any arbitrary AVP as described in [RFC6733].
When designing new applications, designers should specify that the Application Id carried in all session-level messages must be the Application Id of the application using those messages. This includes the session-level messages defined in Diameter base protocol, i.e., RAR/RAA, STR/STA, ASR/ASA and possibly ACR/ACA in the coupled accounting model, see Section 5.10. Some existing specifications do not adhere to this rule for historical reasons. However, this guidance should be followed to avoid routing problems.
In general, when a new application has been allocated with a new Application Id and it also reuses existing commands with or without modifications, it must use the newly allocated Application Id in the header and in all relevant Application Id AVPs (Auth-Application-Id or Acct-Application-Id) present in the commands message body.
Additionally, application designs using Vendor-Specific-Application-Id AVP should not use the Vendor-Id AVP to further dissect or differentiate the vendor-specification Application Id. Diameter routing is not based on the Vendor-Id. As such, the Vendor-Id should not be used as an additional input for routing or delivery of messages. The Vendor-Id AVP is an informational AVP only and kept for backward compatibility reasons.
Section 8 of [RFC6733] provides session state machines for authentication, authorization and accounting (AAA) services and these session state machines are not intended to cover behavior outside of AAA. If a new application cannot clearly be categorized into any of these AAA services, it is recommended that the application defines its own session state machine. Support for server-initiated request is a clear example where an application-specific session state machine would be needed, for example, the Rw interface for ITU-T push model (cf.[Q.3303.3]).
Diameter applications are usually designed with the aim of managing user sessions (e.g., Diameter network access session (NASREQ) application [RFC4005]) or specific service access session (e.g., Diameter SIP application [RFC4740]). In the Diameter base protocol, session state is referenced using the Session-Id AVP. All Diameter messages that use the same Session-Id will be bound to the same session. Diameter-based session management also implies that both Diameter client and server (and potentially proxy agents along the path) maintain session state information.
However, some applications may not need to rely on the Session-Id to identify and manage sessions because other information can be used instead to correlate Diameter messages. Indeed, the User-Name AVP or any other specific AVP can be present in every Diameter message and used therefore for message correlation. Some applications might not require the notion of Diameter session concept at all. For such applications, the Auth-Session-State AVP is usually set to NO_STATE_MAINTAINED in all Diameter messages and these applications are therefore designed as a set of stand-alone transactions. Even if an explicit access session termination is required, application-specific commands are defined and used instead of the Session-Termination-Request/Answer (STR/STA) or Abort-Session-Request/Answer (ASR/ASA) defined in the Diameter base protocol. In such a case, the Session-Id is not significant.
Based on these considerations, protocol designers should carefully appraise whether the application currently defined relies on it's own session management concept or whether the Session-Id defined in the 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).
The type Enumerated was initially defined to provide a list of valid values for an AVP with their respective interpretation described in the specification. For instance, AVPs of type Enumerated can be used to provide further information on the reason for the termination of a session or a specific action to perform upon the reception of the request.
However, AVPs of type Enumerated are too often used as a simple Boolean flag, indicating for instance a specific permission or capability, and therefore only two values are defined, e.g., TRUE/FALSE, AUTORIZED/UNAUTHORIZED or SUPPORTED/UNSUPPORTED. This is a sub-optimal design since it limits the extensibility of the application: any new capability/permission would have to be supported by a new AVP or new Enumerated value of the already defined AVP, causing backwards compatibility issues with existing implementations.
Instead of using an Enumerated AVP for a Boolean flag, protocol designers are encouraged to use Unsigned32 or Unsigned64 AVP type as bit mask whose bit settings are described in the relevant Diameter application specification. Such AVPs can be reused and extended without major impact on the Diameter application. The bit mask should leave room for future additions. Examples of AVPs that use bit masks are the Session-Binding AVP defined in [RFC6733] and the MIP6-Feature-Vector AVP defined in [RFC5447].
Diameter request message routing usually relies on the Destination-Realm AVP and the Application Id present in the request message header. However, some applications may need to rely on the User-Name AVP or any other application-specific AVP present in the request to determine the final destination of a request, e.g., to find the target AAA server hosting the authorization information for a given user when multiple AAA servers are addressable in the realm.
In such a context, basic routing mechanisms described in [RFC6733] are not fully suitable, and additional application-level routing mechanisms have to be described in the application documentation to provide such specific AVP-based routing. Such functionality will be basically hosted by an application-specific proxy agent that will be responsible for routing decisions based on the received specific AVPs.
Examples of such application-specific routing functions can be found in the Cx/Dx applications ([TS29.228] and [TS29.229]) of the 3GPP IP Multimedia Subsystem, in which the proxy agent (Subscriber Location Function aka SLF) uses specific application-level identities found in the request to determine the final destination of the message.
Whatever the criteria used to establish the routing path of the request, the routing of the answer has to follow the reverse path of the request, as described in [RFC6733], with the answer being sent to the source of the received request, using transaction states and hop-by-hop identifier matching. In particular, this ensures that the Diameter Relay or Proxy agents in the request routing path will be able to release the transaction state upon receipt of the corresponding answer, avoiding unnecessary failover. Application designers are strongly dissuaded from modifying the answer-routing principles described in [RFC6733] when defining a new application.
As defined in [RFC6733], a translation agent is a device that provides interworking between Diameter and another protocol (e.g., RADIUS).
In the case of RADIUS, it was initially thought that defining the translation function would be straightforward by adopting few basic principles, e.g., by the use of a shared range of code values for RADIUS attributes and Diameter AVPs. Guidelines for implementing a RADIUS-Diameter translation agent were put into RFC 4005 ([RFC4005]).
However, it was acknowledged that such translation mechanism was not so obvious and deeper protocol analysis was required to ensure efficient interworking between RADIUS and Diameter. Moreover, the interworking requirements depend on the functionalities provided by the Diameter application under specification, and a case-by-case analysis will be required.
Therefore, protocol designers cannot assume the availability of a "standard" Diameter-to-RADIUS gateways agent when planning to interoperate with the RADIUS infrastructure. They should specify the required translation mechanism along with the Diameter application, if needed. This recommendation applies for any kind of translation.
New Diameter applications can rely on optional AVPs to exchange application-specific capabilities and features. These AVPs can be exchanged on an end-to-end basis at the application layer. Examples of this can be found with the MIP6-Feature-Vector AVP in [RFC5447] and the QoS-Capability AVP in [RFC5777].
The end-to-end capabilities AVPs formalize the addition of new optional functionality to existing applications by announcing support for it. Applications that do not understand these AVPs can discard them upon receipt. Receivers of these AVPs can discover the additional functionality supported by the end-point originating the request and behave accordingly when processing the request. Senders of these AVPs can safely assume the receiving end-point does not support any functionality carried by the AVP if it is not present in corresponding response. This is useful in cases where deployment choices are offered, and the generic design can be made available for a number of applications.
When used in a new application, protocol designers should clearly specify this end-to-end capabilities exchange and the corresponding behaviour of the Diameter nodes supporting the application.
It is also important to note that this end-to-end capabilities exchange relies on the use of optional AVPs is not meant as a generic mechanism to support extensibility of Diameter applications with arbitrary functionality. When the added features drastically change the Diameter application or when Diameter agents have to be upgraded to support the new features, a new application should be defined.
Accounting can be treated as an auxiliary application that is used in support of other applications. In most cases, accounting support is required when defining new applications. This document provides two possible models for using accounting:
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. Although it is not recommended, an application may define a new set of commands to carry application-specific accounting records.
As specified in [RFC6733], the Diameter message exchange should be secured between neighboring Diameter peers using TLS/TCP or DTLS/SCTP. However, IPsec can also be deployed to secure communication between Diameter peers. When IPsec is used instead of TLS or DTLS, the following recommendations apply.
IPsec ESP [RFC4301] in transport mode with non-null encryption and authentication algorithms is used to provide per-packet authentication, integrity protection and confidentiality, and support the replay protection mechanisms of IPsec. IKEv2 [RFC5996] is recommended for performing mutual authentication and for establishing and maintaining security associations (SAs).
IKEv1 [RFC2409] was used with RFC 3588 [RFC3588] and for easier migration from IKEv1 based implementations both RSA digital signatures and pre-shared keys should be supported in IKEv2. However, if IKEv1 is used, implementers should follow the guidelines given in Section 13.1 of RFC 3588 [RFC3588].
Generic Diameter extensions are AVPs, commands or applications that are designed to support other Diameter applications. They are auxiliary applications meant to improve or enhance the Diameter protocol itself or Diameter applications/functionality. Some examples include the extensions to support auditing and redundancy (see [I-D.calhoun-diameter-res-mgmt]), improvements in duplicate detection scheme (see [I-D.asveren-dime-dupcons]), and the support for QoS AVPs (see [RFC5777]).
Since generic extensions may cover many aspects of Diameter and Diameter applications, it is not possible to enumerate all scenarios. However, some of the most common considerations are as follows:
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]).
This document does not require actions by IANA.
This document provides guidelines and considerations for extending Diameter and Diameter applications. Although such an extension may related to a security functionality, the document does not explicitly give guidance on enhancing Diameter with respect to security.
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.
We would like to thank Tolga Asveren, Glenn McGregor, and John Loughney for their contributions as co-authors to earlier versions of this document.
We greatly appreciate the insight provided by Diameter implementers who have highlighted the issues and concerns being addressed by this document. The authors would also like to thank Jean Mahoney and Ben Campbell for their invaluable detailed review and comments on this document.