iperf UDP server fails with multiple clients

[Adhiraj <adhiraj --at-- linsyssoft.com>]

Hi, 

I am running iperf UDP test with 200 clients:

Server: iperf -s -u -w 256K -i 20 -D

Client: iperf -c <serv-ip> -u -w 256K -i 20 -P 200 -t 120

iperf version: 1.7.0
OS: Linux Fedora Core 2.6.11

Some time after the test starts, I get following messages:
[202] WARNING: did not receive ack of last datagram after 10 tries.
[202] Sent 10701 datagrams

for many threads on client machine and then the test halts with no
output on the console.

Why is this happening? Why it is waiting for acks in the first place if
its using UDP?

Thanks in advance,
Adhiraj.