Outputs, Required

Description

Outputs, Required is a term used in 3GPP management specifications (23.700, 32.290) to define the mandatory output parameters that a network management operation must return upon execution. This concept is part of the service-based management architecture that 3GPP has developed for 5G and beyond, where management services expose operations with well-defined inputs and outputs. The term specifies which output parameters are required — as opposed to optional — ensuring that consumers of the management service can rely on receiving certain information.

In the context of 3GPP management, operations are defined as part of Management Services (MnS). Each management service, such as the Subscription Management Service or the Fault Management Service, exposes one or more operations. Each operation has an input signature (what the consumer provides) and an output signature (what the producer returns). The 'Outputs, Required' designation indicates which elements of the output signature are mandatory. If an operation completes successfully, the producer must include all required outputs in the response.

The specific example provided in the definition relates to subscription operations: when a subscription is accepted, the required outputs include the Subscription Correlation ID and the Expiry time (required if the subscription can be expired based on local policy). The Subscription Correlation ID is a unique identifier that the consumer can use to reference the subscription in subsequent operations, such as unsubscribing or modifying the subscription. The Expiry time indicates when the subscription will automatically terminate if not renewed, which is essential for managing resource consumption on both the consumer and producer sides.

This concept is part of the broader movement toward service-based management architectures in 3GPP. In earlier generations, management was primarily based on the ITU-T Telecommunications Management Network (TMN) model, with rigid interfaces and complex protocol stacks. The service-based approach, introduced in Rel-15 and refined in subsequent releases, simplifies management by exposing operations as RESTful services with JSON payloads. The 'Outputs, Required' concept ensures that these services have well-defined contracts, which is essential for multi-vendor interoperability.

The term appears in TS 23.700 (which covers study and normative work on management, orchestration, and charging) and TS 32.290 (which defines the 5G management services and their operations). These specifications are part of the 3GPP SA5 working group's portfolio, which is responsible for management, orchestration, and charging aspects of 3GPP systems. The required outputs concept is fundamental to the design of management service APIs, as it determines what information is guaranteed to be available to consumers.

From an implementation perspective, a management service producer must validate that it can provide all required outputs before accepting an operation request. If it cannot, it must reject the request with an appropriate error. This fail-fast behavior ensures that consumers do not proceed with assumptions about data availability that may prove false. The concept also has implications for testing and certification, as test suites must verify that all required outputs are present in successful responses.

Purpose & Motivation

The concept of Required Outputs exists to establish clear, enforceable contracts between management service producers and consumers in 3GPP networks. In a multi-vendor environment where management services from different suppliers must interoperate, ambiguity about what information will be returned is unacceptable. By designating certain outputs as required, the specifications ensure that consumers can write code that depends on those outputs being present, without defensive programming for every possible missing field.

The term addresses a fundamental challenge in management interface design: balancing flexibility with reliability. If all outputs were optional, consumers would need to handle every possible combination of present and absent fields, leading to complex and error-prone code. If all outputs were required, producers would be unable to optimize their implementations or handle edge cases gracefully. The Required Outputs concept provides a middle ground: a core set of outputs that must always be present, supplemented by optional outputs that may be included when applicable.

Historically, this concept emerged from the 3GPP's transition to service-based management architectures in Rel-15. Earlier management interfaces, such as those based on CORBA or SNMP, had similar concepts but were often less rigorously specified. The move to RESTful, JSON-based management services required a fresh look at interface contracts, and the Required Outputs concept was formalized to provide the necessary rigor. The evolution of the definition in Rel-19 to include specific required outputs for subscription operations reflects the maturation of these specifications as real-world deployment experience has been incorporated.