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 receive

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.