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I was studying and came across the concept of MTU and I don't know how to differentiate it with the maximum length of a PDU.

I understood that the max length of a PDU is the maximum amount of information that a single PDU can carry measured in bytes and depends on the layer and the protocol used.

Per example, the max length of a ethernet frame is 1518 bytes and the max length of a IPV4 packet is 65535 bytes.

but when I read the MTU definition I could not see the difference.

I believe that the MTU differs from the maximum length in that the MTU only considers the maximum length of the payload.

For example, the MTU of the ethernet frame should be 1500 bytes and that of the IPV4 packet 65515 bytes.

Is this correct? or what is really the difference between these concepts?

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  • Education and homework are off limits here, however just from a quick Google search, there is this which explains the differences. learningnetwork.cisco.com/s/question/0D53i00000Kt7CX/mtu-vs-pdu
    – Jesse P.
    Jun 19, 2021 at 20:21
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    There are many questions here with answers relevant to your question, just search. For example, this, this, this, this, etc.
    – Ron Maupin ♦
    Jun 19, 2021 at 20:37
  • @Jesse P. Education are off limits here? So what is here?
    – ESCM
    Jun 19, 2021 at 21:26
  • @EduardoSebastian Education, as in things purely just for learning and not to solve an actual problem in a corporate or business environment, yes.
    – Jesse P.
    Jun 19, 2021 at 21:32

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Well, you are correct in the sense that the MTU represents the maximum payload of the frame. That is to say this is the maximum payload that an interface can transmit over the data link layer (Layer 2) at one time. In this regard the PDU is similar as this is the total size of the largest frame transmitted on the wire. Thus, the PDU is equal to the MTU plus the layer 2 frame header size.

Now if you are thinking of payload in terms of actual usable data this is even smaller still than the MTU as within the 1500-byte layer 2 payload of the frame the data will be further encapsulated by several higher-level protocols each of which taking up a portion of the payload available with their own headers containing essential metadata.

For instance with a 1518 byte ethernet frame on the wire carrying a TCP packet you will have the following:

  • Layer 2 Ethernet Frame: Header 18 bytes - Payload 1500 bytes
  • Layer 3 IP Datagram: Header 20-60 bytes - Payload 1440-1480 bytes
  • Layer 4 TCP Packet: Header 20-60 bytes - Payload 1380-1460 bytes

So, the actual payload could be at most 1460 bytes and as little as 1380 bytes of the full 1518 bytes on the wire. Also, this covers the usual straight forward case where the protocols are all nested neatly inside a lower-level protocol that supports it natively. Granted the maximum size for the TCP header assumes all options are present at their maximum size this could not actually happen in a genuine packet.

Other protocols do have different overheads for example UDP has a significantly smaller fixed header of 8 bytes. Of course, the reason why TCP requires more metadata is due to all the additional features it supports so there are trade-offs for reduced overheads.

Also note this all assumes a standard Ethernet frame as Ethernet too has extensions that will increase the PDU if used as the frame header will be larger, VLANs add 4 bytes for example.

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  • "Layer 2 Ethernet Frame: Header 18 bytes" It could actually be larger with things like VLANs (22 bytes). "Layer 3 IP Datagram: Header 20-24 bytes It is 20 to 60 bytes for IPv4, and a fixed 40 bytes for IPv6. Also, it may be worth mentioning that UDP has a fixed eight byte header.
    – Ron Maupin ♦
    Jun 19, 2021 at 21:19
  • Thanks yeah not sure where I got that number in my head for IP from now. I added a couple of notes about UDP and that extensions to the Ethernet standard like VLANs will increase the frame size if used. Don't want to overcomplicate matters bringing too many different scenarios and alternative protocols in though mostly picked one as an example to help clarify that PDU and MTU related to one layer only and thus MTU doesn't refer to the net payload as you cannot use it all. Well not practically unless you count pcaping the raw frames I guess.
    – MttJocy
    Jun 19, 2021 at 21:32

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