This part of ISO 10161 defines the protocol for an ILL application-service-element (ASE). It specifies the behaviour which must be exhibited by a system in order to take part in the provision of the ISO interlibrary loan service. It provides a formal statement of the rules of behaviour of each of the two or more entities participating in an ILL transaction. It specifies: a. the actions to be taken on receiving request service primitives issued by an ILL service-user; b. the actions to be taken on receiving application-protocol-data-units (APDUs); c. the actions to be taken as a result of events within the local system. It provides a specification (in clause 9) of the abstract syntax required to convey the ILL protocol APDUs. It states the conformance requirements to be met by implementors of this protocol (in clause 10). The scope of the ILL protocol is restricted to the interconnection of systems; it does not specify or restrict the possible implementation of interfaces within a computer system. Computer systems may range from stand-alone workstations to mainframes. This part of ISO 10161 is intended for use by libraries, information utilities such as union catalogue centres, and any other system which processes bibliographic information. These systems may participate in an interlibrary loan transaction in the role of requester (i.e. an initiator of ILL requests), responder (i.e. a provider of bibliographic material or information) and/or intermediary (i.e. an agent that acts on behalf of a requester to find suitable responders). Various interworking topologies are supported, ranging from simple two-party interactions, to multi-party interactions.