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TCP, UDP and multicast

Dear Sirs,
we're testing TCP, UDP and multicast communication among some PC with the
following OS and ethernet cards:
1) a board similar to Axis Developer board, 100Mbit
        kernel 2.4.19
        Axis Developer Board LX release 2.1.0
2) Windows NT, 100Mbit
3) Linux, 10Mbit

TCP communication is ok, but we experienced some problems with UDP and

The Software code is very simple:
-- Sender code--
    send n bytes

-- Receiver code --
    receive n bytes
    printf # of bytes received

We used the same code on ETRAX and on Linux PC;  Windows' code is very

When we test TCP, printf continuously appear on the receiver side, no matter
which is the transmitter or the receiver.
When we use UDP or multicast communication, we notice different

a) If the receiver is Developer board and the sender is either NT PC or
Linux PC, the behaviour is the same observed during TCP test.

b) If the sender is Developer board and the receiver is NT PC, the frequency
of printf is lower and not constant: it seems that developer board sends
burst of data and then waits for a while.

We changed the amount of data sent on each "send()" from 4 bytes to 10Kbytes
(using multicast), but the behaviour is the same.

What could be the reason of these different performances? Any suggestion
would be appreciated.

Stefano Vicenzetto