This Recommendation | International Standard defines in an abstract way the externally visible service provided by the Transport Layer in terms of: a) the primitive actions and events of the service; b) the parameter data associated with each primitive action and event; c) the relationship between, and the valid sequences of, these actions and events. The service defined in this Recommendation | International Standard is that which is provided by the Enhanced Communications Transport Protocol (in conjunction with the Network Service) and which may be used by any application protocol. The service can also be provided by other protocols possibly each supporting a subset of the services defined herein. The primitives specified in this Recommendation | International Standard support a connection-mode service and a connectionless service. In some cases of connectionless-mode service supporting enhanced communications, certain operations may also be necessary prior to the commencement of data transfer, e.g. agreement on quality of service. For the data transfer phase of either connection-mode or connectionless-mode services, there may be a range of dataordering characteristics. No implication is made in this Recommendation | International Standard regarding the inclusion or exclusion of any of the above characteristics given the service primitives specified herein.