The Space Link Extension (SLE) Reference Model identifies a set of SLE Transfer Services that enable missions to send forward space link data units to a spacecraft and to receive return space link data units from a spacecraft. A subset of these services is specified by the SLE Transfer Service Recommended Standards. The SLE Transfer Service Recommended Standards specify:

• the operations necessary to provide the transfer service;
• the parameter data associated with each operation;
• the behaviors that result from the invocation of each operation; and
• the relationship between, and the valid sequence of, the operations and resulting behaviors.

However, they deliberately do not specify the methods or technologies required for communications.

ISO 18440:2013 defines a protocol for transfer of SLE Protocol Data Units (PDUs) defined in the SLE Transfer Service Recommended Standards using the Internet protocols TCP (Transmission Control Protocol and IP (Internet Protocol) for data transfer and the Abstract Syntax Notation One (ASN.1) for data encoding. This protocol is referred to as the Internet SLE Protocol One (ISP1).

ISO 18440:2013 defines a protocol for transfer of SLE PDUs between an SLE user and an SLE provider system in terms of:

• the procedures used to establish and release associations;
• the messages exchanged on an established association;
• the procedures used to monitor the status of data communication connections; and
• the methods used to ensure that data are converted between different formats and representations on different platforms.

It does not specify:

• individual designs, implementations, or products;
• the configuration of the data communications infrastructure, including configuration of the TCP and IP protocols;
• the means by which addresses (IP addresses and TCP port numbers) are agreed, assigned, and communicated.

ISO 18440:2013 responds to the requirements imposed by the International Standards for SLE transfer services that were available when this International Standard was released. The protocol specified in ISO 18440:2013 conforms to the requirements on data communication services set forth in those International Standards.