This paper presents an extensive statistical study and analysis of the effects of channel delays in the current (best-effort) Internet on underflow events in MPEG-4 video streaming. Two types of network delays are considered: end-to-end round-trip delays and delay jitter. Our data were collected in a seven-month real-time streaming experiment, which was conducted between a number of unicast dialup clients in more than 600 major U.S. cities and a backbone video server. Among other findings, our analysis shows that startup delays approximately 15-20 times the average roundtrip time (RTT) are required for the client to avoid 90% of late packets caused by delay jitter. Meanwhile, startup delays of only 3-4 times the average RTT are needed to achieve lost-packet recovery rates of 90% or more. Hence, a key finding of our study is that delay jitter represents a more challenging problem for video streaming applications than round-trip delays. We also show that the probability density function...