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I am currently studying networking at university and I have a question about MTUs and maximum frame sizes.

  1. What is the difference between the MTU and the maximum frame size? This Wikipedia article implies that although MTU is related to the maximum frame size, it is not quite the same. I am not entirely sure how it works in that case - does the linked layer set the maximum frame size and thereafter the network layer sets the MTU based on the max. frame size (which is smaller or equal to the max. frame size)?
  2. Can those two values even be different (i.e. MTU 1500 bytes and max. frame size 1600 bytes)?
  3. If they can be different, when are they different and why are they different?
  4. If they cannot be different, why use two different terms to describe the same thing? Why does the Wikipedia article imply that they are not quite the same?

2 Answers 2

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why use two different terms to describe the same thing?

They are not the same thing. The MTU is the size of the payload of the frame.

For example, ethernet has an MTU of 1500, which means that the packet that is the payload of the ethernet frame can be as large as 1500 octets. That does not count the ethernet header or FCS, which will add to the size of the frame (at least 14 octets for the header, and 4 octets for the FCS).

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They are not the same. MTU is the largest possible size of something that can be communicated in a single NETWORK LAYER transaction. For example, if you are using IP, then the IP header size plus the data field size should be ≤ MTU. MTU is also the maximum PAYLOAD of the frame.

The maximum frame size includes the ethernet header size and other things besides the payload, so it is larger than MTU.

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