sting is a TCP-based network measurement tool that measures end-to-end
network path characteristics. sting is unique because it can estimate
one-way properties, such as loss rate, through careful manipulation and
observation of TCP behavior. In addition, using TCP allows sting to
leverage the existing Internet infrastructure -- any TCP server can be
used as a de facto measurement service -- and it avoids increasing
problems with ICMP-based network measurement (blocking, spoofing, rate
limiting, etc).

WWW: http://www.cs.washington.edu/homes/savage/sting/