An adaptive file delivery system and method transmits a data file, such as an audio-video file, over a network or collection of networks in segments, each segment transmitted during a different time period. Each time period has a transmission portion to transmit its associated file segment and a wait portion in which no further interaction with the network occurs regarding the transmitted segment. In some implementations, the duration of the transmission portion of each time period is sufficient to reach a steady-state throughput condition, which allows the traffic load status of the network or networks to be determined from rate measurements of file segment transmissions. The duration of the wait portion of each time period is at least long enough to limit the average rate of file segment transmission to adapt to network traffic load variations and avoid network congestion. Various techniques for measuring congestion are described.