perf.eval: bandwidth estimation

two main methodologies
Variable Packet Size (VPS) probing  
hop by hop
send many IP (UDP) pkts of varying size
use min RTT per size to filter out noise in b/w estimate 
implemented in pathchar, pchar, clink
not great for high b/w or busy links
need lot of probes
Packet Train Dispersion (PTD) probing 
end-to-end metrics
based on packet pair 
implemented in bprobe, cprobe, others
suggested change to TCP to base slow-start on disperson of first 3-4 ACKs
assumption: dispersion of long packet trains is inversely proportional to the available bandwidth
                 .......caida finds this assumption wrong...