Layer Four Traceroute (LFT) is a fast, multi-protocol traceroute engine, that also implements numerous other features including AS number lookups through regional Internet registries and other reliable sources, Loose Source Routing, firewall and load balancer detection, etc.
LFT sends various TCP SYN and FIN probes (differing from Van Jacobson's UDP-based method) or UDP probes utilizing the IP protocol time to live field and attempts to elicit an ICMP TIME_EXCEEDED response from each gateway along the path to some host.
LFT also listens for various TCP, UDP, and ICMP messages along the way to assist network managers in ascertaining per-protocol heuristic routing information, and can optionally retrieve various information about the networks it traverses.
The operation of layer four traceroute is described in detail in several prominent security books.
Renamed as a result of confusion with fast Fourier transforms, lft stands for layer four traceroute.