MTU Size?

I've read that the maximum size for the MTU is 1500 bytes.

Is this measured at the network Layer, and does it include the payload and headers?

Thanks

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Comments

chmorin

The MTU will vary depending on the media you are transporting in (IPv4 vs IPv6, 802.11, and others). For ethernet, it is generally 1500.

I am not 100% sure, but one would assume either network or data link layer layer.

My money is on Data-link MAC layer, as it provides error and flow control on more of a single packet basis, where network (ad LLC) would deal with frames and more complicated things.

Forsaken_GA

marioc wrote: »

I've read that the maximum size for the MTU is 1500 bytes.

Is this measured at the network Layer, and does it include the payload and headers?

Thanks

The maximum size of the layer 2 encapsulation determines the maximum possible size of the MTU. It can be adjusted at higher layers.

And yes, all payloads and headers are counted. This is why you occasionally need to manually lower the MTU when doing tunnelling, if you don't, when the tunnel goes to tack on it's headers, you might end up exceeding the maximum MTU

marioc

Many thanks for your posts.

Cyanic

To clarify this a bit as this can be confusing:

A typical Ethernet frame has a maximum size of 1518.

This is 14 bytes for the Ethernet header, 1500 bytes for IP and higher, and then another 4 at the end for the Ethernet frame CRC.

Just realize that when you say 1500 byte MTU you are talking IP/Network Layer and above.

astorrs

And just to screw you up, Jumbo Frames are anything with an MTU larger than 1500 (typically 9000) as you'd see in an iSCSI storage network for example.

But unless you're working on some of the large private research/higher-ed networks you won't have to worry about them traveling across your WAN.