METHOD TO FIND OPTIMUM MTU SIZE FOR WAN LINK

The Maximum Transmission Unit (MTU) is the largest size of a packet that can be sent over a network link without needing to be fragmented. This size includes the headers and the payload. MTU is typically measured in bytes and is an important parameter in network configuration because it affects the efficiency and performance of data transmission.

Ping Test with Don't Fragment Flag
Using a ping test with the Don't Fragment (DF) flag set is a common method to determine the largest packet size that can be sent over a network path without fragmentation. This test helps identify the MTU of the path. When the DF flag is set, if a packet is larger than the MTU of any link in the path, it will be discarded, and an ICMP "Fragmentation Needed" message will be sent back to the sender.

Why Add 28 Bytes?
When performing a ping test to determine the MTU, you need to account for the headers added by the network protocols:

IP Header: The IP header typically adds 20 bytes.
ICMP Header: The ICMP (Internet Control Message Protocol) header typically adds 8 bytes.
Therefore, the total overhead for the headers is 28 bytes (20 bytes for the IP header + 8 bytes for the ICMP header).

Determining the Maximum Data Size
When you perform the ping test with the DF flag:

Determine the maximum packet size that can be sent without fragmentation by incrementally increasing the size until you receive a "Fragmentation Needed" message.
Subtract the header overhead (28 bytes) from this maximum packet size to find the maximum payload size.
For example, if the largest packet size that does not get fragmented is 1500 bytes, the maximum data size you can send in a single packet is:

Maximum Data Size = MTU −Header Size
Maximum Data Size=MTU−Header Size
Maximum Data Size = 1500 bytes − 28 bytes = 1472 bytes
Maximum Data Size=1500bytes−28bytes=1472bytes

In our example Maximum Data Size Without Fragmentation is 1468
So the MTU is calculated as

MTU = Header Size + Maximum Data Size

MTU = 28 + 1468 = 1496

Summary
MTU is the maximum size of a packet that can be transmitted without fragmentation.
A ping test with the DF flag helps determine the path MTU.
28 bytes is added to account for the IP (20 bytes) and ICMP (8 bytes) headers.
Subtract 28 bytes from the maximum ping packet size to find the maximum data payload size.