This paper first presents an analysis model named flow-competing congestion model (FCCM) for this type of congestion. Based on FCCM, the paper derives the distribution of competing flows at the congested link, and analyzes the conditions under which the flow-competing congestion would happen. This paper also explores how much buffers a congested link requires to keep full link utilization when the flow-competing congestion occurs. This paper proves that when sizing buffers for a congested internet link with the aim of keeping full link utilization, the buffer requirement of the flow-competing congestion is not bigger than the minimum buffer requirement of the famous BSCL (buffer sizing for congested internet links) scheme.