This International Standard defines a C++ Application Program Interface (API) for CCSDS Space Link Extension (SLE) Transfer Services, which is independent of any specific technology used for communications between an SLE service user and an SLE service provider. This International Standard defines the Application Program Interface in terms of: a) the components that provide the services of the API; b) the functionality provided by each of the components; c) the interfaces provided by each of the components; and d) the externally visible behavior associated with the interfaces exported by the components. It does not specify: a) individual implementations or products; b) the internal design of the components; and c) the technology used for communications. This International Standard defines those aspects of the Application Program Interface which are common for all SLE service types or for a subset of the SLE service types, e.g. all return link services or all forward link services. It also defines a framework for specification of service type-specific elements of the API. Servicespecific aspects of the API are defined by supplemental Recommended Practice documents for SLE return link services and SLE forward link services. This International Standard for the Application Program Interface responds to the requirements imposed on such an API by the CCSDS SLE transfer service Recommended Standards that were available when this International Standard was released. The scope and field of application are furthermore detailed in subclause 1.3 of the enclosed CCSDS publication.