/usr/bin/nc

Ncat is a feature-packed networking utility which reads and writes data across networks from the command line

Useful to check network, by sending packets (e.g.  echo "test" | /usr/bin/nc -u 172.27.224.255 4000) on xterm,

using tcpdump (e.g.,  tcpdump -X port 4000) to verify.

iftop

($PACKAGE_TOP/iftop/iftop0.17/iftop)

Display the top list of remote hosts responsible for the most of network usage.

Using iftop you can identify which host is responsible for slowing down your network on a machine.

  /usr/sbin/nethogs

tool to monitor real-time network traffic bandwidth used by each process or application on a machine.

 /usr/sbin/lsof

e.g., /usr/sbin/lsof -i :port_number to find out what connects to the port_number

/bin/netstat

ttcp

Test network speed.   Built in /home/jingchen/bin/ttcp.