Elephant flow
A traffic flow of large volume and long duration — datacenter replication, video upload, backup. Opposite: mice flows (RPC calls, DNS, small HTTP responses). Elephants dominate bandwidth; mice dominate flow count.
Problem: one elephant fills the switch buffer → mice get latency spikes (buffer bloat). DCTCP (Data Center TCP) uses ECN markings to back off elephants early. Fair queueing per flow (fq_codel, CAKE in Linux) protects mice. Datacenter fabrics use ECMP per flow — if one elephant lands on a link while the rest balances for mice, that link becomes a hotspot. Spotify, Netflix scale requires elephant detection and separate scheduling.