How to Find the Proper MTU Size For a Network?

The Maximum Transmission Unit (MTU) is the maximum possible packet/frame size that can be communicated over the network without breaking it into smaller fragments. They are declared in octets that can be transmitted in a frame/packet-based network. If any packet is bigger than the specified MTU setting, then it will break or fragment the original packet into smaller subpackets. Now, it is okay if the number of these packets is small the problem arises when the number of fragmented packets is large, in this case, it significantly drops down the transmission speed of data over the network.

Using the incorrect MTU size can give rise to issues such as there could be problems when trying to open some browsers, there could be issues while transmitting or receiving files over the network, and VoIP can face some failure as well. To fix these issues, the MTU value should be checked and set to optimum MTU size. If the MTU size is optimum, then it can significantly advance the overall performance of the network and higher speeds can be reached when the length of packets in a stream is equal to the value of MTU.

The MTU value can be set manually for the selected interface in various network devices, but the optimum size should be set appropriately. MTU is used by TCP to limit the maximum size of each packet. It is generally linked with the Ethernet protocol, where the maximum allowed packet size is up to 1500 Byte.

MTU Size:

To find the optimal MTU size open cmd by going to the search bar and entering “cmd”.

Enter the following command with any URL and packet size.

ping www.xyz.com -f -l 1460

After pinging the cmd will give some output, it may look like the one below.

C:\Users\mkbt>ping www.xyz.com -f -l 1460
Pinging www.xyz.com [143.238.92.7] with 1460 bytes of data:
Reply from 143.238.92.7: bytes=68 (sent 1460) time=55ms TTL=60
Reply from 143.238.92.7: bytes=68 (sent 1460) time=42ms TTL=60
Reply from 143.238.92.7: bytes=68 (sent 1460) time=40ms TTL=60
Reply from 143.238.92.7: bytes=68 (sent 1460) time=41ms TTL=60
Ping statistics for 143.238.92.7:
  Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
  Minimum = 40ms, Maximum = 55ms, Average = 45ms

As can be seen above the packets were not fragmented, so we’ll do it again by increasing the packet size.

C:\Users\mkbt>ping www.xyz.com -f -l 1472
Pinging www.xyz.com [143.238.92.7] with 1472 bytes of data:
Reply from 143.238.92.7: bytes=68 (sent 1472) time=55ms TTL=60
Reply from 143.238.92.7: bytes=68 (sent 1472) time=42ms TTL=60
Reply from 143.238.92.7: bytes=68 (sent 1472) time=40ms TTL=60
Reply from 143.238.92.7: bytes=68 (sent 1462) time=41ms TTL=60
Ping statistics for 143.238.92.7:
  Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
  Minimum = 40ms, Maximum = 55ms, Average = 45ms

Again, we see no fragmentation so we’ll increase the size of the packet again.

C:\Users\mkbt>ping www.xyz.com -f -l 1473
Pinging www.xyz.com [143.238.92.7] with 1473 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Ping statistics for 143.238.92.7:
  Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),
 

Conclusion:

Here we found that packet is fragmenting when its size is at 1473 bytes which means the maximum possible size here is 1472 bytes. But this is not the MTU value, to find it we will have to add the value of IP header and ICMP header that is 28 bytes. So, the value of MTU = 1472 + 28 i.e., 1500 byte which is the optimum MTU value.