[Hampshire] network testing

Hi,

Does anyone know of any Linux tools to test TCP/IP networks.
If I have a redundant network, and I pull a cable, I wish to test the
time it takes to automatically fail over and network traffic resumes.
I wish to take these measurements on a per direction basis. I.e.
traffic might resume in one direction before it resumes in another.
I suppose I am looking for a tool that can send sequenced UDP packets
from a source to a destination, and the destination log which packets
were received.
I can then determine how long the network was down based on the amount
of packets lost.

Any ideas, or will I have to role my own tool?

James