Receiving packets in UDP

Question

Let's say my program sends a 1000 bytes over the network (UDP). Does it guaranteed that the receiver will receive the 1000 bytes in one "batch"? Or perhaps he will need to perform sevral "reads" until he'll receive the entire message? if the later is true, how can i ensure that the order of the packets for the same message don't get "mixed up" (in order), or perhaps the protocol guarantees it?
Edit: that is, does it possible that my message will be split to sevral packets? (what if i try to send a 10000mb message, what happens then?)

Answer 1

Data sent using UDP is grouped in packets, so if you send x amount of bytes then IF the receiver receives the packet he'll receive x amount of bytes.

However, your packets might not even arrive, or they may arrive out of order.

Answer 2

UDP, unlike TCP, is not a reliable protocol. It provides no built in mechanism to ensure that the packets arrive in the proper order, or even arrive at all. That said, you can write your send/recv routines in a lock step fashion, where every time a packet is sent, the sender must wait to receive an ACK before sending again. If an ACK is not received after some specified timeout, the packet must be resent. This way you ensure that packets are received in the proper order. (For more information, check out the RFC for the TFTP protocol, which uses this strategy.)

Finally, if possible, you may want to consider using TCP instead.

Answer 3

The receiver will get the entire packet in one call. The packet length is limited, even in theory:

Length A 16-bit field that specifies the length in bytes of the entire datagram: header and data. The minimum length is 8 bytes since that's the length of the header. The field size sets a theoretical limit of 65,535 bytes (8 byte header + 65527 bytes of data) for a UDP datagram. The practical limit for the data length which is imposed by the underlying IPv4 protocol is 65,507 bytes.

However the real limit is much much lower, usually is safe to assume 512 bytes. See What is the largest Safe UDP Packet Size on the Internet.

Answer 4

You will get it all or nothing.

But there is no particular guarantee that you will receive packets exactly once in the order they were transmitted; packet loss, reordering and (less often) duplication are all possible.

There is a maximum frame size (of 65,507 bytes), send()ing packets of larger sizes will return an error.

You must provide enough buffer to receive the entire frame in one call.

UDP packets CAN be fragmented into multiple IP fragments, but the OS will drop an incomplete packet. This is therefore transparent to the application.

Answer 5

With UDP Lite you can request to receive partially corrupted packets. This can be useful for video and VoIP services.