Re: [RAM] Tunnelling & GigE jumbo frame size

RJ Atkinson <> Wed, 12 September 2007 17:43 UTC

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From: RJ Atkinson <>
Subject: Re: [RAM] Tunnelling & GigE jumbo frame size
Date: Wed, 12 Sep 2007 13:42:58 -0400
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On  12 Sep 2007, at 12:42, Iljitsch van Beijnum wrote:
> On 12-sep-2007, at 16:24, RJ Atkinson wrote:
>> Correction:
>> 	Just as "4K" MTU really means 4470 bytes, Jumbo Ethernet
>> 	sizes of "9K" really mean an IP MTU of 9180 bytes.  This
>> 	means the link MTU for Jumbo Ethernet is really:
>>            (9180 + sizeof(Ethernet header) + sizeof(VLAN tag))
> I think most people take it to mean 9000 bytes.

I strongly disagree -- speaking as someone who spends a lot of time
talking with A LOT of different Ethernet end users around the world,
including users with deployments on all continents.  The origin
for the requirement came from super-computing sites with ATM
deployments, and they drove all the vendors in the same direction
(and vendors that didn't comply with their definition lost
potential business as a result; economics tends to win in such cases).

> This is the only real jumboframe IP MTU size I've seen implemented
> in hosts.

You should look around more broadly then.

I've seen the jumbo MTU that I outlined in multiple versions of UNIX
and in other kinds of systems.  Moreover, the jumbo MTU is primarily
deployed by service providers and super-computer centers, and is
not often used by ordinary end systems (e.g. one's home computer).

I'm not current with MTU support in current Windows, but earlier
versions of Windows did support more than 9000 bytes on Gigabit

> However, routers and especially switches come with many different  
> maximum frame sizes. I've looked over some product literature and  
> compiled the following list: 1508, 1530, 1536, 1546, 1998, 2000,  
> 2018, 4464, 4470, 8092, 8192, 9000, 9176, 9180, 9216, 17976, 64000  
> and 65280.

As there is no standard, the maximum for a particular device will vary.
Most networking devices make this parameter *configurable*, below
some maximum value.  Varying maxima was not the question on the table,

The question on the table is *narrowly* "what is meant by 9K MTU ?".
I stand by the contents of my original note.


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