The Cisco Stealthwatch system collects and analyzes network telemetry such as flow (NetFlow, sFlow, JFlow, IPFIX, etc.) from routers, switches, and firewalls to monitor network and user behavior. The system conducts sophisticated, proprietary analytics on network data to automatically detect abnormal behaviors that may signify an attack1. NetFlow is a protocol that provides information about network traffic flows, such as source and destination IP addresses, ports, protocols, bytes, packets, and timestamps. NetFlow data can be used to identify network anomalies, bandwidth usage, application performance, and security incidents2. References :=

Some possible references are:

1: Cisco Stealthwatch Improves Threat Defense with Network Visibility and Security Analytics, Zones, 3 2: What is NetFlow? Definition and Benefits, Cisco, 7