Datagramsocket: how receive(...) handles fragmentation of a packet

Question

I came to know from my Professor that, a datagram packet sent using UDP socket gets fragmented in the lower layers and may arrive as multiple packets at the receiver end. For e.g, if I send a 1000 bytes data in a datagram packet, at the receiving end it might arrive as, say 2 bytes, 500 bytes, 12 bytes, and so on. Therefore, he suggested to do multiple receive(...) to receive the entire 1000 byte packet sent by the sender.

Later when I went through the Java documentation for datagram socket receive(...) and there is a line that reads as follows: "This method blocks until a datagram is received. ..." Does it mean that entire datagram packet is received and don't need to do multiple receive (even though it's the case in theory) when we use Java?

Pls. clarify. If multiple receive(...) for each packet is the only option to get around this problem, pls. give suggestions on how to do this.

Answer 1

Any call to receive() will give you an entire packet - the fragment handling happens in two layers below the socket. The fragmentation and defragmentation happens in the Network/Internet layer (IP), so the socket will never see the fragments but only receive entire and full UDP/TCP packets (only full packets gets sent to the listening port).

So, no, you do not need multiple receive() to get a single packet, but you should be aware that UDP is not reliable so if one fragment gets lost in the Network layer (and in some cases if it arrives out of order), you won't be able to get the packet.

You might also want to check the methods getReceiveBufferSize() and setReceiveBufferSize() if you're having trouble receiving packets - if the buffer size is smaller than the packet size, it's not guaranteed that you can receive the packet.