Setting MTU

Steve Flynn anothermindbomb at
Mon Nov 6 13:22:47 UTC 2006

On 06/11/06, Dotan Cohen <dotancohen at> wrote:

>  I would ask your ISP why they have their kit running on 1400 MTU and thus
> > you having to mess around with your settings though. Seems a little odd
> in
> > this day and age.
> If you find this unusual, then I will ask them. Want to tell me what
> it should be set to, and the disadvantages and advantages of each
> setting? I want to be well armed when I ask them, not just some
> ignorant Parrot repeating what I've been told. Educate me a bit.

OK, the MTU is the Maximum Transmission Unit. It's a preset number of bytes
and effectively determine the largest packet which will be sent. Any packets
which are greater than 1500 bytes will be fragmented, or split into 2
smaller packets.

The standard MTU for Internet transmission (unless you're on specialised
hardware) is 1500 bytes. 8 bytes are normally reserved for packet overhead.

So, your router will pass through packets of up to 1500 bytes, and your NIC
will normally pass through packets of the same size. The only time you
should have to change this is when some other device down the line cannot
handle a 1500 byte packet which, if your ISP is requesting you send smaller
packets, it sounds like you have. From the numbers you've quoted they want
you to use an MTU of 1392, which implies something upstream from you can
only handle 1400 byte packets (1392 + 8bytes of packet red-tape). If you
start sending packets of this size, then that device will be ahppy but
obviously your throughput is hampered as you have to sent more packets to
transfer the same amount of data, thus "cramping" your throughput.

If you stick to sending 1500 byte packets, the rogue device will have to do
extra work to split the packets on your behalf.
If you drop down to 1392, as requested, your slightly impact your
throughput, but the rogue device has less work to do.

If they had requested a 1492 byte MTU it would be FAR more believable but
1392, to my limited knowledge, is an odd size. I would just double-check it
and confirm that someone hasn't made a typo!

Despair - It's always darkest just before it goes pitch black...
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