Comment by withinboredom

Very rarely this is the case. From the application’s perspective yes. From a packet perspective… no. The interface is going to send packets and they’ll end up in a few buffers after going through some wires. If something goes wrong along the way, they’ll be retransmitted. But the packets don’t care about the response, except an acknowledgment the packets were received. If you send 4000 byte messages when the MTU is 9000, you’re wasting perfectly good capacity. If you had Nagle’s turned on, you’d send one 8040 byte packet. With Nagle’s you don’t have to worry about the MTU, you write your data to the kernel and the rest is magically handled for you.