The OSI model defines 5 transport layer classes: TPO, TP1, TP2, TP3, and TP4.
• TPO and TP2 are used with perfect network layers.
• TP1 and TP3 are used with residual-error network layers.
• TP4 is used with unreliable network layers.
TPO: Simple class
TP 1: Basic error recovery class
TP2: Multiplexing class
TP3: Error recovery and Multiplexing class
TP4: Error detection and recovery class