This document is a specification of the Message Link Protocol (MLP). The MLP creates a transport service between session control entities via transport connections, and some additional session control service. It is based on virtual circuits a la X.25. It is concerned with transport connection establishment and termination, and handles resets and clears to provide a reliable transport service. Furthermore it provides synchronization support related to session establishment, termination and negotiation. The allocation of transmission resources to connections is also considered. It is assumed that the flow control capabilities of the X.25 virtual circuits can be used for transport connection flow control. If, for other transmission media, flow control may not be assumed, an additional function to provide flow control at the transport service level has to be introduced.