Abstract: A communications infrastructure is provided with a service system that facilitates the setting up of data transfer operations between end systems connected to the infrastructure. When a first end system wishes to initiate a data transfer operation, it contacts the service system and establishes a transfer-operation characteristic for the proposed data transfer operation that is known both to itself and the service system. The first end system then contacts the target end system with which it wishes to effect a data transfer operation and passes it the aforesaid transfer-operation characteristic. The target end system uses the received transfer-operation characteristic to check with the service system that the proposed data transfer from the first end system is known and acceptable to the service system; only if this check is passed does the target end system proceed with the data transfer operation proposed by the first end system.