The effect of layer-2 store-and-forward devices on per-hop capacity estimation

Tools such as pathchar, clink, and pchar attempt to measure the capacity of every layer-3 (L3) hop in a network path. These tools use the same underlying measurement methodology, which we refer to as Variable Packet Size (VPS) probing. The key assumption in VPS is that each L3 hop along a path increases the delay of a packet by a "serialization latency", which is the ratio of the packet size over that hop's capacity. Unfortunately, the capacity estimates of VPS tools are sometimes wrong. In this paper, we investigate the source of these errors, and show that the presence of layer-2 (L2) store-and-forward devices, such as Ethernet switches, have a detrimental effect on the accuracy of VPS tools. Specifically, each L2 store-and-forward device introduces an additional serialization latency in a packet's delay, which results in consistent underestimation of that L3 hop's capacity. We analyze this negative effect, deriving the measured capacity of an L3 hop as a function of the L2 link capacities at that hop. Experimental results in local, campus, and ISP networks verify the model, illustrating that L2 devices should be expected in networks of diverse type and size. Finally, we characterize some other sources of error in VPS tools, such as queueing delays, limited clock resolution, variation in ICMP generation delays, and error propagation along the measured path.