Path MTU Discovery using Socket option - IP_MTU & IP_MTU_DISCOVER

Question

I'm doing some experiments with path MTU discovery in Linux - TCP connection,Client and server are configured with different MTU.(MTU 1500 and 1000 respectively).

after TCP_CONNECT between server and client, i observe the MTU configured in client as follows using below command.

  getsockopt(iSocketId,IPPROTO_IP,IP_MTU,(char *)&socket_mtu, &size);
  printf("MTU --> %d\n",socket_mtu); 

I believe that getsocketopt return "Path MTU" and not the MTU of the client.

But i receive MTU - 1500 (Client MTU) instead of Path MTU : 1000.

Please give some idea to get PATH MTU using getsockopt.Examples are really appreciated.

Thanks in advance.

Answer 1

You may like to read how Path MTU Discovery works.

Path MTUs can be asymmetrical with different MTU value in different directions.

The fact that you set a low MTU on the client does not mean it cannot receive larger sized datagrams, it only means that it cannot send it.