UDP Response

Question

UDP doesnot sends any ack back, but will it send any response?

I have set up client server UDP program. If I give client to send data to non existent server then will client receive any response?

My assumption is as;

Client -->Broadcast server address (ARP) Server --> Reply to client with its mac address(ARP) Client sends data to server (UDP)

In any case Client will only receive ARP response. If server exists or not it will not get any UDP response?

Client is using sendto function to send data. We can get error information after sendto call.

So my question is how this info is available when client doesn't get any response. Error code can be get from WSAGetLastError.

I tried to send data to non existent host and sendto call succeeded . As per documentation it should fail with return value SOCKET_ERROR.

Any thoughts??

Answer 1

You can never receive an error, or notice for a UDP packet that did not reach destination.

Answer 2

The sendto call didn't fail. The datagram was sent to the destination.

The recipient of the datagram or some router on the way to it might return an error response (host unreachable, port unreachable, TTL exceeded). But the sendto call will be history by the time your system receives it. Some operating systems do provide a way to find out this occurred, often with a getsockopt call. But since you can't rely on getting an error reply anyway since it depends on network conditions you have no control over, it's generally best to ignore it.

Sensible protocols layered on top of UDP use replies. If you don't get a reply, then either the other end didn't get your datagram or the reply didn't make it back to you.