Network Stress Test Tool

I need a tool to stress test our network in between two machines.

The app should send out a packet to machine 2, machine 2 should send it back, machine 1 verifys the packet is correct, generates a new packet and the process starts all over.

Anyone know of such an app?

You want iperf. It's super tiny, cross platform, and dead simple to use.

If you are talking about 2 Windows boxes, I like NTTTCP:

http://www.microsoft.com/whdc/device/network/TCP_tool.mspx

NTttcp is a multithreaded, asynchronous application that sends and receives data between two or more endpoints and reports the network performance for the duration of the transfer. It is essentially a Winsock-based port of the ttcp tool that measures networking performance in terms of bytes transferred per second and CPU cycles per byte. Because it can be difficult to diagnose a system's overall performance without dividing the system into smaller subsystems, NTttcp allows users to narrow the focus of their testing and investigation to just the networking subsystem.