The National Bureau of Standards has developed service and design specifications for transport and session protocols for use in computer system and network procurements. These protocols reside in layers four and five of the International Organization for Standardization's (ISO) Reference Model for Open Systems Interconnection. This paper describes the services, interfaces, and internal behavior of the transport protocol. The transport (and session) protocol specifications were derived from the most recent developments within ISO on these protocols. Specific features were selected based on the needs of the agencies of the Federal Government within the United States, but they are consistent with the needs of any large organization engaged in the procurement or development of networks of heterogeneous computer systems.