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What is MTU(Maximum Transmission Unit)?

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  • Last Updated : 31 Oct, 2021
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A maximum transmission unit also called as MTU, is a term used in networking and operating systems. It defines the largest size of the packet that can be transmitted as a single entity in a network connection. The size of the MTU dictates the amount of data that can be transmitted in bytes over a network. 



The larger MTU results in more data transmission during a single connection, therefore, reduces the overhead. On the other hand, the smaller MTU can be transferred faster, because of its size, thus reducing delay in the network. Therefore, the size of the MTU should be adjusted to optimize both the requirements. 
The default size of the maximum transmission unit is 1500 B, which is the Ethernet standard largest unit. 


  • Size of MTU is directly proportional to the amount of data transferred. Larger MTU size, larger chunk of data transmitted at once from the sender to the final recipient.
  • MTU size is based on the specifications of the network administration.
  • If size of MTU outweighs the capacity of router, it is re-transmitted again causing delay.
  • It is the optimal packet size of the network.

Working of MTU

Let us suppose the Internet’s Transmission Control Protocol(TCP) specified the size of MTU = 750 B, that is the maximum protocol data unit size that can be delivered from source to destination. In such a scenario, the following cases may arise: 

  • If the system sends packets larger than the size of MTU, that is packet size > 750 B in this case, then the system packet will be fragmented to smaller packets such that their size doesn’t exceed the largest packet size. The process of division of a large data packet into smaller chunks of data such that none of these chunks exceed the maximum frame size is called Fragmentation. These are later reassembled at the final client destination. 

  • If the system sends packets within MTU size, they are transmitted as a single frame in the network connection. However, packets much smaller than MTU may increase delay and cause network inefficiency. Reassembling packets in such a case is not required. 


The maximum transmission unit has the following applications: 

  • MTU is used over the internet, primarily by TCP to determine the optimal packet size.
  • It is associated with Ethernet protocol, and is referred as protocol data unit(PDU).
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