How do I find out what hop is not forwarding traffic on a specific port?

[Phunkodelic <phunkodelic () gmail com>]

Looking for a way to find out who or what device is blocking traffic
on a specific port.  Sort of like a trace route tool where you could
designate the port to test with (UDP/TCP).  I realize that UDP may be
hard to do since it is a connectionsless protocol.

Anybody?  Thanks in advanced.

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