• The communications between systems are split into seven different abstraction layers
• Data flow: transmit(high→low) → receive(low→high)

Layer 3: Network

• Protocol Data Unit(PDU): packet
• Protocol: IP
• Packetizing, Routing, Forwarding(Gateway → Subnet mask → Network)

Layer 4: Transport

• Protocol Data Unit(PDU): segment
• Protocol: TCP, UDP
• End to end communication

Layer 7: Application

• Protocol Data Unit(PDU): data
• Protocol: HTTP