how to check network bandwidth usage of a port via tcpdump

Calculating network bandwidth usage

# documented in https://superuser.com/questions/356907/how-to-get-real-time-network-statistics-in-linux-with-kb-mb-bytes-format-and-for

Your application is probably sending packets to a specific UDP or TCP port number or to a specific IP-address.

You can therefore use something like TCPdump to capture that traffic.

TCPdump doesn't give you the real-time stats you desire but you can feed it's output to something that does (I'll try to update this answer with an answer later).

Update:

$ sudo tcpdump -i eth1 -l -e -n | ./netbps
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes
11:36:53    2143.33 Bps
11:37:03    1995.99 Bps
11:37:13    2008.35 Bps
11:37:23    1999.97 Bps
11:37:33    2083.32 Bps
131 packets captured
131 packets received by filter
0 packets dropped by kernel
I interrupted that after a minute by pressing Ctrl+C.

You'd need to add a suitable filter expression at the end of the tcpdump command to only include the traffic generated by your app (e.g. port 123)

The program netbps is this:

#!/usr/bin/perl
use strict;
use warnings;
use Time::HiRes;

my $reporting_interval = 10.0; # seconds
my $bytes_this_interval = 0;
my $start_time = [Time::HiRes::gettimeofday()];

STDOUT->autoflush(1);

while (<>) {
  if (/ length (\d+):/) {
    $bytes_this_interval += $1;
    my $elapsed_seconds = Time::HiRes::tv_interval($start_time);
    if ($elapsed_seconds > $reporting_interval) {
       my $bps = $bytes_this_interval / $elapsed_seconds;
       printf "%02d:%02d:%02d %10.2f Bps\n", (localtime())[2,1,0],$bps;
       $start_time = [Time::HiRes::gettimeofday()];
       $bytes_this_interval = 0;
    }
  }
}