Networking: Still waiting for mbuf cluster

[Rolf.Schroedter at dlr.de (Rolf Schroedter)]

My RTEMS application communicates with a Linux-PC via connectionless UDP 
socket,
receiving and replying UDP packets every ~20 milliseconds.
After a few minutes the application stops communicating.
The application is still running, but no more UDP packets are received.

 From time to time a message is displayed at the console:
    "Still waiting for mbuf cluster."
With [ethereal] I see that correct UDP packets are coming from the PC.
Additionally the PC is sending ARP requests:
    "Who has 192.168.0.100 ? Tell 192.168.0.1"
which are also not answered by RTEMS.

What does this mean ?
How can the UDP communication be restarted ?

Any suggestions are highly appreciated.

Rolf.