Capacity estimation effective in an IP network path is a key enabling technology for proactive and reactive resource management as well as intelligent planning in next generation IP networks.

In this paper we analyze the capacity measurements at different Internet destinations. Particularly, we use the variable packet size method (VPS) to measure the per-hop capacity in some network paths. This method is based on round trip time (RTT) measurements as well as on the Internet Control Message Protocol, ICMP. The RTT delay is approximated by gamma distribution. We observe how the measurement accuracy of this active probing is affected by factors such as layer 2 device latencies, limited clock resolution at the probing host or queuing delay. The before mentioned factors cause non linearity on the RTT curves obtained and consequently inaccuracies on estimation of path capacities. The use of robust regression to improve linear approximation in VPS method is proposed. As a result we obtain better qualitative and quantitative results.