When does a UDP sendto() block?

This can happen if you filled up your socket buffer, but it is highly operating system dependent. Since UDP does not provide any guarantee your operating system can decide to do whatever it wants when your socket buffer is full: block or drop. You can try to increase SO_SNDBUF for temporary relief.

This can even depend on the fine tuning of your system, for instance it can also depend on the size of the TX ring in the driver of your network interface. There are a few discussions about this in the iperf mailing list, but you really want to discuss this with the developers of your operating system. Pay special attention to O_NONBLOCK and EAGAIN / EWOULDBLOCK.

This may be because your operating system is attempting to perform an ARP request in order to get the hardware address of the remote host.

Basically whenever a packet goes out, the header requires the IP address of the remote host and the MAC address of the remote host (or the first gateway to reach it). 192.168.1.34 and AB:32:24:64:F3:21.

Your "block" behavior could be that ARP is working.

I've heard in older versions of Windows (2k I think), that the 1st packet would sometimes get discarded if the request is taking too long and you're sending out too much data. A service pack probably fixed that since then.