title: MTU 1500: fragment packets after 1472 bytes
body: <p>I have couple of VMs running on Openstack. I have one data network gateway assigned into the switch with MTU=1500, where VMs has to running.   </p>
  
  <p>Also in Openstack, I have set the default MTU=1500 of the network. </p>
  
  <p>I am trying to ping from the VM to gateway (OR any ip on same network) of the switch with 1500 packetsize, it's de-fragmenting the packet. </p>
  
  <pre><code>localhost:~# ping -M do -s 1500 10.4.14.18 
  PING 10.4.14.18 (10.4.14.18) 1500(1528) bytes of data.
  From 10.4.14.17 icmp_seq=1 Frag needed and DF set (mtu = 1500)
  From 10.4.14.17 icmp_seq=1 Frag needed and DF set (mtu = 1500)
  From 10.4.14.17 icmp_seq=1 Frag needed and DF set (mtu = 1500)
  From 10.4.14.17 icmp_seq=1 Frag needed and DF set (mtu = 1500)
  From 10.4.14.17 icmp_seq=1 Frag needed and DF set (mtu = 1500)
  From 10.4.14.17 icmp_seq=1 Frag needed and DF set (mtu = 1500)
  From 10.4.14.17 icmp_seq=1 Frag needed and DF set (mtu = 1500)
  From 10.4.14.17 icmp_seq=1 Frag needed and DF set (mtu = 1500
  </code></pre>
  
  <p>Instead, If I am sending packets with smaller mtu (1472) it works,  </p>
  
  <pre><code>localhost:~# ping -M do -s 1472 10.4.14.18
  PING 10.4.14.18 (10.4.14.18) 1472(1500) bytes of data.
  1480 bytes from 10.4.14.18: icmp_seq=1 ttl=64 time=0.965 ms
  1480 bytes from 10.4.14.18: icmp_seq=2 ttl=64 time=0.515 ms
  ^C
  --- 10.4.14.18 ping statistics ---
  2 packets transmitted, 2 received, 0% packet loss, time 1875ms
  rtt min/avg/max/mdev = 0.515/0.740/0.965/0.225 ms
  </code></pre>
  
  <p>I can see 28 bytes of difference, i don't understand, where this 28 bytes are being utilised ?</p>