Protocol Data Units (PDU) At each layer,protocols are used to communicate ■( Control information is added to user data at each layer Transport layer may fragment user data Each fragment has a transport header added ■Destination SAP ■Sequence number Error detection code This gives a transport protocol data unitProtocol Data Units (PDU)  At each layer, protocols are used to communicate  Control information is added to user data at each layer  Transport layer may fragment user data  Each fragment has a transport header added  Destination SAP  Sequence number  Error detection code  This gives a transport protocol data unit