Confusion about UDP/IP and sendto/recvfrom return values

Question

I'm working with UDP sockets in C++ for the first time, and I'm not sure I understand how they work. I know that sendto/recvfrom and send/recv normally return the number of bytes actually sent or received. I've heard this value can be arbitrarily small (but at least 1), and depends on how much data is in the socket's buffer (when reading) or how much free space is left in the buffer (when writing).

If sendto and recvfrom only guarantee that 1 byte will be sent or received at a time, and datagrams can be received out of order, how can any UDP protocol remain coherent? Doesn't this imply that the bytes in a message can be arbitrarily shuffled when I receive them? Is there a way to guarantee that a message gets sent or received all at once?

Answer 1

It's a little stronger than that. UDP does deliver a full package; the buffer size can be arbitrarily small, but it has to include all the data sent in the packet. But there's also a size limit: if you want to send a lot of data, you have to break it into packets and be able to reassemble them yourself. It's also no guaranteed delivery, so you have to check to make sure everything comes through.

But since you can implement all of TCP with UDP, it has to be possible.

usually, what you do with UDP is you make small packets that are discrete.

Metaphorically, think of UDP like sending postcards and TCP like making a phone call. When you send a postcard, you have no guarantee of delivery, so you need to do something like have an acknowledgement come back. With a phone call, you know the connection exists, and you hear the answers right away.

Answer 2

No. With sendto you send out packets, which can contain down to a single byte. If you send 10 bytes as a single sendto call, these 10 bytes get sent into a single packet, which will be received coherent as you would expect.

Of course, if you decide to send those 10 bytes one by one, each of them with a sendto call, then indeed you send and receive 10 different packets (each one containing 1 byte), and they could be in arbitrary order.

It's similar to sending a book via postal service. You can package the book as a whole into a single box, or tear down every page and send each one as an individual letter. In the first case, the package is bulkier but you receive the book as a single, ordered entity. In the latter, each package is very light, but good luck reading that ;)

Answer 3

Actually you can send a UDP datagram of 0 bytes length. All that gets sent is the IP and UDP headers. The UDP recvfrom() on the other side will return with a length of 0. Unlike TCP this does not mean that the peer closed the connection because with UDP there is no "connection".