Diameter Maintenance and Extensions (DIME) L. Morand, Ed. Internet-Draft Orange Labs Intended status: Informational V. Fajardo Expires: November 30, 2013 H. Tschofenig Nokia Siemens Networks May 29, 2013 Diameter Applications Design Guidelines draft-ietf-dime-app-design-guide-17 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. 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 November 30, 2013. Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. Morand, et al. Expires November 30, 2013 [Page 1] Internet-Draft Diameter Applications Design Guidelines May 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 to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Reusing Existing Diameter Applications . . . . . . . . . . . 5 4.1. Adding a New Command . . . . . . . . . . . . . . . . . . 5 4.2. Deleting an Existing Command . . . . . . . . . . . . . . 6 4.3. Reusing Existing Commands . . . . . . . . . . . . . . . . 6 4.3.1. Adding AVPs to a Command . . . . . . . . . . . . . . 7 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 5.4. Application-Specific Session State Machines . . . . . . . 11 5.5. Session-Id AVP and Session Management . . . . . . . . . . 11 5.6. AVPs Defined as Boolean Flag . . . . . . . . . . . . . . 12 5.7. Application-Specific Message Routing . . . . . . . . . . 13 5.8. About Translation Agent . . . . . . . . . . . . . . . . . 13 5.9. End-to-End Application Capabilities Exchange . . . . . . 14 5.10. Diameter Accounting Support . . . . . . . . . . . . . . . 15 5.11. Diameter Security Mechanisms . . . . . . . . . . . . . . 16 6. Defining Generic Diameter Extensions . . . . . . . . . . . . 17 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 8. Security Considerations . . . . . . . . . . . . . . . . . . . 18 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 18 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 11.1. Normative References . . . . . . . . . . . . . . . . . . 19 11.2. Informative References . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 1. Introduction Morand, et al. Expires November 30, 2013 [Page 2] Internet-Draft Diameter Applications Design Guidelines May 2013 The Diameter base protocol provides facilities to extend the Diameter base protocol (see Section 1.3 of [RFC6733]) 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 [RFC6733] 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. 2. Terminology This document reuses the terminology used 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. Morand, et al. Expires November 30, 2013 [Page 3] Internet-Draft Diameter Applications Design Guidelines May 2013 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 [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]. 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. Morand, et al. Expires November 30, 2013 [Page 4] Internet-Draft Diameter Applications Design Guidelines May 2013 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 [RFC6733] 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. 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 Morand, et al. Expires November 30, 2013 [Page 5] Internet-Draft Diameter Applications Design Guidelines May 2013 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. 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. Morand, et al. Expires November 30, 2013 [Page 6] Internet-Draft Diameter Applications Design Guidelines May 2013 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. [RFC6733] 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 [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 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 [RFC6733], 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 [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 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? Morand, et al. Expires November 30, 2013 [Page 7] Internet-Draft Diameter Applications Design Guidelines May 2013 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. 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. Morand, et al. Expires November 30, 2013 [Page 8] Internet-Draft Diameter Applications Design Guidelines May 2013 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 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. 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 Morand, et al. Expires November 30, 2013 [Page 9] Internet-Draft Diameter Applications Design Guidelines May 2013 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. 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 [RFC6733]. Morand, et al. Expires November 30, 2013 [Page 10] Internet-Draft Diameter Applications Design Guidelines May 2013 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 [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 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. 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 Morand, et al. Expires November 30, 2013 [Page 11] Internet-Draft Diameter Applications Design Guidelines May 2013 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. 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. Morand, et al. Expires November 30, 2013 [Page 12] Internet-Draft Diameter Applications Design Guidelines May 2013 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 [RFC6733] and the MIP6-Feature- Vector AVP defined in [RFC5447] 5.7. Application-Specific Message Routing Diameter request message routing usually relies on the Destination- Realm AVP and the Application Id present in the request message header. However, some applications may need to rely on the User-Name AVP or any other application-specific AVP present in the request to determine the final destination of a request e.g. 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. 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 [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 Morand, et al. Expires November 30, 2013 [Page 13] Internet-Draft Diameter Applications Design Guidelines May 2013 As defined in [RFC6733], 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 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. Morand, et al. Expires November 30, 2013 [Page 14] Internet-Draft Diameter Applications Design Guidelines May 2013 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 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 Morand, et al. Expires November 30, 2013 [Page 15] Internet-Draft Diameter Applications Design Guidelines May 2013 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 methods might be defining a new set of commands to carry application- specific accounting records. 5.11. Diameter Security Mechanisms As specified in [RFC6733], the Diameter message exchange should be secured 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. The version 2 of IKE (IKEv2) [RFC5996] is recommended for performing mutual authentication and establishing and maintaining security associations (SAs). Morand, et al. Expires November 30, 2013 [Page 16] Internet-Draft Diameter Applications Design Guidelines May 2013 IKEv1 [RFC2409] was used in [RFC3588] and for easier migration from IKEv1 based implementations both RSA Digital Signatures and pre- shared keys should be used in IKEv2. However, if IKEv1 is used, implementers should follow the guidelines given in section 13.1 in RFC3588 [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 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 Morand, et al. Expires November 30, 2013 [Page 17] Internet-Draft Diameter Applications Design Guidelines May 2013 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 does not define nor address security-related protocols or schemes. 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 Morand, et al. Expires November 30, 2013 [Page 18] Internet-Draft Diameter Applications Design Guidelines May 2013 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 Morand, et al. Expires November 30, 2013 [Page 19] Internet-Draft Diameter Applications Design Guidelines May 2013 Physical Entity (PD-PE) and the Policy Enforcement Physical Entity (PE-PE): Diameter"", 2008. [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [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. [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", , . [TS29.229] Morand, et al. Expires November 30, 2013 [Page 20] Internet-Draft Diameter Applications Design Guidelines May 2013 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", , . [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", , . [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", , . 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 Morand, et al. 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