ECMP Packet Spraying

Overview

ECMP (Equal-Cost Multi-Path) Packet Spraying is the multipathing approach used by Ultra Ethernet: rather than making one routing decision per flow at network entry (which locks a flow to a single path for its duration), each individual packet is independently routed across all available equal-cost paths using a large number of per-packet entropy values. This fundamentally eliminates the two key failure modes of ordered flow-based load balancing: bandwidth taper (two flows randomly assigned to the same path and stuck there) and false contention (flows sharing a congested uplink with no mechanism to escape). With UEC's ECMP packet spraying combined with QuickAdapt congestion control, bandwidth-taper impact drops from 2x to approximately 25% and false contention is completely eliminated. "Bad" entropy values (paths experiencing congestion) are identified and excluded from future packets while "good" paths continue to be used automatically.