Java DatagramPacket (UDP) maximum send/recv buffer size

Question

In Java when using DatagramPacket suppose you have a byte[1024*1024] buffer. If you just pass that for the DatagramPacket when sending/receiving will a Java re

Answer 1

DatagramPacket is just a wrapper on a UDP based socket, so the usual UDP rules apply.

64 kilobytes is the theoretical maximum size of a complete IP datagram, but only 576 bytes are guaranteed to be routed. On any given network path, the link with the smallest Maximum Transmit Unit will determine the actual limit. (1500 bytes, less headers is the common maximum, but it is impossible to predict how many headers there will be so its safest to limit messages to around 1400 bytes.)

If you go over the MTU limit, IPv4 will automatically break the datagram up into fragments and reassemble them at the end, but only up to 64 kilobytes and only if all fragments make it through. If any fragment is lost, or if any device decides it doesn't like fragments, then the entire packet is lost.

As noted above, it is impossible to know in advance what the MTU of path will be. There are various algorithms for experimenting to find out, but many devices do not properly implement (or deliberately ignore) the necessary standards so it all comes down to trial and error. Or you can just guess 1400 bytes per message.

As for errors, if you try to send more bytes than the OS is configured to allow, you should get an EMSGSIZE error or its equivalent. If you send less than that but more than the network allows, the packet will just disappear.