The widespread use of firewalls on the modern Internet, many of the
packets that traceroute(8) sends out end up being filtered, making it
impossible to completely trace the path to the destination. However,
in many cases, these firewalls will permit inbound TCP packets to
specific ports that hosts sitting behind the firewall are listening for
connections on. By sending out TCP SYN packets instead of UDP or ICMP
ECHO packets, tcptraceroute is able to bypass the most common firewall
filters.