Framework for Deriving Interface Data Schema from UML Information Models

Interface specifications are often generated as point solutions where the designer codes a particular interface from domain (problem space) concepts that may not be explicitly captured, may be defined using localized terminology that is subject to ambiguity in interpretation, and is highly focused on a particular use-case/application. The designer typically provides a representation of the interface schema in the form of a data schema (i.e., data structures conveyed over the interface), which only exposes the view of the domain relevant at that specific interface. As this data schema is a simple statement of the particular interface, it solely describes relationships relevant to the specific realization, having no inherent relationship to other interfaces in the system. Approaching the development of interface specifications on a per use-case/application basis tends to promote unnecessary variety through a proliferation of similar interfaces, resulting in unnecessary divergences that limit interoperability. It also risks confusion of representational artefacts with fundamental characteristics of the information to be conveyed across the interface. There is also a risk that conflicting representations of the same information may be generated. Finally, as each such interface appears to stand alone, it thereby fails to capture relationships with other aspects of the same (or different) domains that are not explicitly needed for the interface. This draft describes a framework for how a protocol specific data schema and the encoding used for the interface can be systematically derived from an underlying common information model, focusing upon the networking and forwarding domain. The benefit of using such an approach in the development of interface specifications is to promote convergence, interoperability, and efficiency.

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 RFC 2119.

An information model condenses domain knowledge and insights to provide a representation of its essential concepts, structures, and interrelationships. In capturing domain understanding, such a model offers a coherent and consistent terminology and structure, expresses the semantics of the domain, and interrelates all relevant aspects of the domain. It enables a consistent expression of information that improves interoperability between software components at interfaces derived from it. A "good" information model should capture domain best practices, and be designed to support domain variety as well as extensibility and evolution. Examples of domains include networking and forwarding, storage, etc. A single industry information model is the assembly of all domain information models, which inter-relate at "touch points". There may be several relevant views of any particular domain, depending upon the perspective of the viewer, all of which are interrelated and involve subsets of the information model, and none of which contradict each other. (It should be noted that one view provides the information model representation of the overall domain.) To form a particular (purpose-specific) view, some elements of the model may be pruned. Additionally, for efficiency, some systematic refactoring of the information model may also occur. In this draft, the term data schema is used in the context of either: (i) a specific protocol that is used to implement a purpose specific interface, or (ii) a programming language that is used to invoke a purpose specific API. Note that it is possible to map directly from the purpose specific information model to interface encoding. While a purpose specific interface/API is not a simple direct encoding of the information model of the overall domain, it is by its nature based on a relevant view of the information model of the domain (i.e., a purpose specific information model view). It must be completely and consistently traceable to this view and should use the associated domain terminology. Depending on its application, a particular view may lead to a number of encoded forms at various types of interfaces/APIs. The information model does not dictate the encoded form, which will depend upon such factors as necessary capability, interaction style, and programming language.

This section introduces the Unified Modeling Language (UML), which has been used to model application structure, behavior, and architecture (in addition to business process and data structure). It also provides references to existing and ongoing work on standard information models based on UML.

The information model is expressed in terms of the Unified Modeling Language (UML) , which was developed by the Object Management Group. It is a general-purpose modeling language in the field of software engineering. In 2000 the Unified Modeling Language was also accepted by the International Organization for Standardization (ISO) as an approved ISO standard . UML may be used in four ways: To define a set of objects (instantiated classes that, if organized, describe a data model) As an information model As a metamodel (used to create an information model) As a meta-metamodel UML defines a number of basic model elements (UML artefacts), such as object classes, attributes, associations, interfaces, operations, operation parameters, data types, etc. In order to assure a consistent and harmonized modelling approach, and to ensure uniformity in the application of UML to a problem domain, a subset of the basic model artefacts should be selected according to guidelines for creating an information model expressed in UML . The guidelines are generic; i.e., they are not specific to any particular domain that the information model is addressing, nor are they restricted to any particular protocol interface data schema.

Information models expressed in UML, primarily focused upon the networking and forwarding domain, have been, and are in the process of being, developed in ITU-T, TM Forum, NGMN, 3GPP, MEF, ONF, and others. ITU-T Recommendations are focused on understanding the telecommunications problem space and developing information models addressing network and network element considerations. Some examples of available standard ITU-T information models relevant to the networking and forwarding domain include: ITU-T G.874.1 (2012), Optical transport network: Protocol-neutral management information model for the network element view ITU-T G.8052/Y.1346 (2013), Protocol-neutral management information model for the Ethernet Transport capable network element ITU-T G.8152/Y.1375 (draft in progress), Protocol-neutral management information model for the MPLS-TP network element ITU-T G.gim (draft in progress), Generic protocol-neutral management Information Model for transport resources The above information models are developed from ITU-T Recommendations that define the respective transport technology functional models and management requirements. The TM Forum community has likewise developed extensive models of the same space from the network level management perspective . The basis for all functions made available to the network level management is defined in the protocol-neutral network element level management work done in ITU-T. Its models thus complement the ITU-T information models. In further collaboration with 3GPP, considerable joint effort has been devoted to develop a consistent and coherent approach to that space. The NGMN has published a document called Next Generation Converged Operations Requirements (NGCOR) , with the expressed purpose of taking these requirements into account when converged management interfaces for mobile and fixed networks are being standardized in the SDOs. An ongoing collaboration called the Multi-SDO Project on Converged Management is taking care that the requirements are considered during the specification of new interfaces. It includes participants from ETSI, NGMN, TMF, 3GPP, and other SDOs, equipment vendors, OS vendors and service providers.

This section outlines the steps to be taken in deriving data schema from the information model specifications. Figure 1 below provides an overview of the structure of a common information model and how purpose specific IM views and data schema may be derived from it. The common information model is a library of model artefacts (objects, attributes, relationships, etc.) organized into a number of information model fragments, to facilitate the independent development of technology and application specific extensions. The core model fragment refers to information model artefacts that could be commonly used regardless of the specific technology or application domain. The forwarding technology specific model fragment refers to technology specific extensions; e.g., for OTN, Ethernet, SDH, etc. The application specific fragment refers to extensions for supporting particular applications. For purposes of navigability, the core information model fragment is itself structured as a series of modules. For example, a core foundation module on artefacts for identifiers and naming, a module on artefacts for forwarding aspects that are independent of the specific forwarding technologies, etc.

High-level methodology for deriving interface protocol specific data schema from UML information model The aforementioned guideline document also provides guidelines for creating a common information model view for a specific purpose. Guidelines are under development for conversion (mapping) from an UML information model into various protocol specific interface data schema/encodings such as REST/JSON, Netconf/YANG interface, etc. Applying such guidelines, protocol-specific interface data schema/encodings may be derived from existing, and emerging, standard UML information models addressing the forwarding and networking domains. Examples include MEF 38 and MEF 39, which provide YANG modules derived from UML information models G.8052 and MEF 7.1 for Service OAM Fault and Performance Monitoring, respectively. YANG models could be derived for OTN in a similar way using the G.874.1 UML information model as a base. The following subsections provide further elaboration of the high-level methodology described above.

A common information model includes the objects/packages, their properties (represented as attributes), and their relationships, etc. that are necessary to describe the domain for the applications being developed. It will be necessary to continually expand and refine the common model over time as new forwarding technologies, capabilities and applications are encompassed and new insights are gained. To allow these extensions to be made in a seamless manner, the common information model is structured into a number of model fragments. Core model fragment structured as a set of modules, each addressing a specific topic to allow for easier navigation. The artefacts in the core model fragment are technology and application neutral. Forwarding plane technology specific or application specific model fragments, which contain the artefacts (objects, attributes and associations) that relate solely the specific technology or application. This modelling approach enables application specific and forwarding plane technology specific extensions to be developed independently.

The next step is the development of a purpose specific information model view, which is a true subset of the common information model. To provide maximal reuse, the purpose specific view is developed in two steps: (1) pruning and refactoring to provide a purpose specific information model of the network to be managed, where only those artefacts that represent the capabilities that are both in scope and supported are included, and (2)defining the access rights for the various groups of users that will manage that network. Pruning to remove the objects/packages/attributes that are not required. Selecting the required object classes from the common IM (all mandatory attributes and packages must be included) Selecting the required conditional packages and optional attributes (note that, where appropriate, conditional packages and optional attributes may be declared mandatory) Removing any optional associations that are not required Refactoring to reduce association flexibility, such as: Reducing multiplicity (e.g., from [1..*] to [1]). When this results in a composition association of multiplicity [1] between a subordinate and superior object class, they can be combined into a single object class by moving the attributes of the superior class into the subordinate class. Where possible, reducing the depth of the inheritance (i.e., combining object classes by moving the attributes of the super class into the subclass). Adding reverse navigation, if useful for the client. The common IM only supports navigation from a subordinate object class to a superior object class. This allows new subordinate object classes to be added without any impact on the superior object class. In a network specific implementation it is frequently useful to be able to navigate the relationship between superior and subordinate object classes in both directions. Constraining attribute definitions. This can be done by reducing legal value ranges, defining which (if any) attributes should be read only (for all users), and/or defining constraints between attributes. Definition of access rights If only one group will use the network specific IM then this step is not required. If more than one group will use the network specific IM this optional step provides a profile for each user group to: Convert some attributes defined as read/write in the network specific IM to read only Remove the right to create/delete some or all object instances

A data schema (DS) is constructed by mapping of the purpose specific information model view into the DS together with the operations patterns from the common information model to provide the interface protocol specific operations and notifications. The operations should include data structures taken directly from the purpose specific information model view with no further adjustment. (Note that it is possible to map directly from the purpose specific information model to interface encoding). The development of the data schema should consider the following: The operations should act on the information in a way consistent with the modeled object lifecycle interdependency rules. Lifecycle dependencies to ensure sensible interface operation structuring and interface flow rules Usage of transaction approach style of interface to account for lifecycle dependencies of the model The operations should abide the attribute properties. Read only attributes (except those which are defined as setByCreate) should not be included in data related to creation of an object (e.g., not in createData) or in a specification of a desired object structure outcome. Usage of attribute value ranges, etc. to allow "effort" statement, optionality and negotiation to be supported by the interface.

This step encodes the purpose specific data schema or purpose specific information model into either a specific protocol that is used to implement a purpose specific interface or; a programming language that is used to invoke a purpose specific API. If the interface is encoded directly from the purpose specific information model then the interface operations must be added as described above.

This draft describes a modular and scalable approach for systematically deriving purpose and protocol specific interfaces from an underlying common information model, focusing upon the networking and forwarding domain. Building upon an underlying common information modeling description of network resources (functionality, capabilities, flexibility) is a key enabler to convergence and interoperability. It is also future proof in the sense that the emergence of new protocols becomes solely a non-disruptive mapping issue. It should be noted that not all domains require development of information model prior to solutions development; the domains where this is of greatest benefit involve networking domains requiring support for an enhanced level of control and network programmability.

This memo includes no request to IANA.

TBD