Is your feature request related to a problem? Please describe.
I love nexttrace. but based on my (limited) experiences, there is no way (read 'an option') to make the source port number fixed/still during a path discoverty toward a target with TCP method. fixed/still source port is desired because almost every middle boxes (routers, switches, ...) today are doing per-flow load balancing where multiple equal-cost network paths are present. the source port is part of the flow identifier - so if it changes, the packet is regarded to belong to another flow. so the discovered path might be completely fake because the previous/next packet are in different flows. see https://paris-traceroute.net/about/ for an example.

Describe the solution you'd like
please add an option to make TCP source port fixed/still. this port should be either automatically picked or specified by the user.
many similar tools have such an option.

Describe alternatives you've considered
sorry, none on top of my mind.

Additional context
please at first make sure TCP method works in most cases (i have another request for this).