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I've been doing some reading about MTU and I just can't get my head around something. When we talk about MTU for ethernet, it's documentated that the MTU is 1500, however the frame size is larger after the Ethernet headers are added. So effectively, when people talk about the MTU for ethernet being 1500, they are referring to the payload.

Now then, I could accept the above statement if this was a standard case for every other protocol. But with TCP for example, the MTU is considered the payload + the IP header + the TCP header. If you compare this to Ethernet, the MTU was just the payload, but with TCP it's the payload + the headers.

Am I understanding this right, because this is really confusing to work out which protocols include the headers with the MTU, and which don't? I have a sneaking suspicion that it's just Ethernet that does not include the headers when talking about MTU, since it would be setting the standard for the maximum packet size for upper layer protocols (i.e. you could only ever have an IP packet with a total packet size that uses a 1500MTU including all the headers and everything). But I want to get some clarity on this topic.

I do realise that fragmentation can be used, but I want to know how to work this out without talking about fragmentation.

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You have hit what I have always considered a pain point in the naming here. Let me start with a small correction to what you said: technically speaking, the MTU is the maximum size of an L3PDU inside an Ethernet frame. The L3PDU is made up of IP headers and payload. So payload =/= MTU.

Now, when you talk about this informally, you often just say "payload", without clarifying whether you are talking about the payload of the Ethernet frame, or the effective payload of the L3PDU inside it. And this brings me to the issue: the confusion comes from the fact that some of the names actually overlap. When you talk about TCP MTU, you are effectively talking about the maximum packet size, which is a value that is independent from the physical Ethernet MTU, which remains 1500 bytes.

In summary, the real MTU is the physical one, i.e. the maximum size of the IP packet inside an Ethernet frame. When you talk about MTU with regards to other network layers, you are effectively talking about the maximum packet size, which doesn't necessarily have the same value of the physical MTU.

This article talks about this in a bit more detail, but here is an excerpt that is relevant to this question:

Higher-level network protocols like TCP/IP can be configured with a maximum packet size, a parameter independent of the physical layer MTU over which TCP/IP runs. Unfortunately, many network devices use the terms interchangeably. On both home broadband routers and Xbox Live enabled game consoles, for example, the parameter called MTU is in fact the maximum TCP packet size and not the physical MTU.

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  • Ok, you said that the MTU is the maximum L3PDU inside an Ethernet frame. What if there is another protocol like PPP between Ethernet and IP? So the encapsulation would be like this: TCP ---> IP ---> PPP ---> Ethernet. So the MTU is still going to be 1500, because Ethernet is the layer1 mode of transport? So from what you are saying, the entire L3PDU needs to not exceed 1500. However, since PPPoE adds 8 bytes of overhead, the actual L3PDU needs to not exceed 1492. When the PPP datagram is encapsulated as Ethernet payload, will the PPP datagram be small enough for the 1500 byte Ethernet MTU? – Steve Feb 13 '15 at 12:17
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- Hi Steve,

Telecom is a real difficult discipline because there are many wanted ambiguities.

MTU just means "maximum transmission unit", no more.

Now, consider that the payload of a MAC 802.3 frame is 1500 bytes in LAN.

So, the MTU of a IPv4 or IPv6 datagram encapsulated in a MAC 802.3 frame is 1500 bytes.

The MTU of a IPv4 or IPv6 datagram encapsulated in PPP, PPP in PPPoE, PPPoE in MAC 802.3, is 1492 bytes.

You can compose as you want. For example, the MTU of a PPP frame encapsulated in PPPoE, with PPPoE in a MAC 802.3 frame, is 1494 bytes.

I let you to define the MTU of a MAC 802.3 frame VLAN tagged.

Sometimes, people want to be more technical by forgetting important words.

Best regards, Michel

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