The concept of a flow is basic to NetFlow, which is defined as a unidirectional stream of packets from a source to destination. NetFlow considers a number of fields in packets being monitored including source IP address, destination IP address, source port number, destination port number, layer 3 protocol, and ToS (Type of Service). These fields are used by NetFlow to classify packets into separate flows. If a packet has a key field that’s different from another packet, the two packets are considered to belong to two different flows.