What syscalls does a UDP server need?

I previously described the syscalls needed for a multi-client TCP server. There are at least eight: socket, bind, listen, select, accept, recv, send, and close. This could be used to make an echo server: clients can open a TCP connection, and whatever bytes they send get echoed back.

Now let’s look at the simplest UDP server, and again make an echo server: clients can send a UDP datagram, and whatever they send gets echoed back.

1. udp_fd = socket(UDP): “OS, please create a UDP socket, and give me a file descriptor referencing it”.
2. bind(udp_fd, 9999): “OS, please point UDP port 9999 to the new UDP socket”.
3. msg, from = recvfrom(udp_fd): “OS, please put me to sleep until a UDP datagram is received, then give it to me”.
4. sendto(udp_fd, msg, from): “OS, please send this UDP datagram from my port to this remote address and port”.
5. close(udp_fd): “OS, I’m done listening for new datagrams. Stop listening on that port, and remove this file descriptor”.

Compared to the TCP server, we of course don’t get the features of TCP: messages are unreliable, unordered, and nothing monitors network congestion.

But a benefit is simplicity: we have no listen, select, or accept. We do not have to manage multiple connections, deal with splitting byte streams into individual messages, or work with select/kqueue/epoll.

The other benefit is flexibility: our communication may not fit into the TCP model very well.

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