DIAMETER
The RFC 3588 defines a core state machine for maintaining connections between peers and processing messages. This is part of the basic protocol functionality and all stacks should support it and as such abstract from the connectivity related operations.
Peer State Machine Part 1
Peer State Machine Part 2
Additionally, application specific state machines can be introduced either later or at a higher abstraction layer. The RFC 3588 defines an authorization and an accounting state machine.
Diameter Authorization State Machines (Client)
Diameter Authorization State Machines (Server)
Diameter Accounting State Machines (Client)
Diameter Accounting State Machines (Server)