This International Standard specifies extensions to the API needed for support of the Command Link Transmission Unit (CLTU) service defined in Space Link Extension—Forward CLTU Service Specification, CCSDS 912.1-B-2. This International Standard defines extensions to the SLE API in terms of: a) the CLTU-specific functionality provided by API components; b) the CLTU-specific interfaces provided by API components; and c) the externally visible behavior associated with the CLTU 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 only interfaces and behavior that must be provided by implementations supporting the forward CLTU service in addition to the specification in Space Link Extension—Application Program Interface for Transfer Services—Core Specification, CCSDS 914.0-M-1. The scope and field of application are furthermore detailed in subclause 1.3 of the enclosed CCSDS publication.