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Re: [RAM] Tunnelling & GigE jumbo frame size

On 12-sep-2007, at 19:42, RJ Atkinson wrote:

	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 [...] 

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

You should look around more broadly then.

(Oh wait: I once saw a 10 Gbit ethernet card in a couple of PowerMac G5s that did 16384 bytes.)

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).

Let me make several points at once. This is my computer at home, a Mac laptop:

$ sudo ifconfig en0 mtu 9180
Password:
$ ifconfig en0
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
supported media: autoselect 10baseT/UTP <half-duplex> 10baseT/UTP <full-duplex> 10baseT/UTP <full-duplex,hw-loopback> 10baseT/UTP <full-duplex,flow-control> 100baseTX <half-duplex> 100baseTX <full-duplex> 100baseTX <full-duplex,hw-loopback> 100baseTX <full-duplex,flow-control> 1000baseT <full-duplex> 1000baseT <full- duplex,hw-loopback> 1000baseT <full-duplex,flow-control> none
$ sudo ifconfig en0 mtu 9000
$ ifconfig en0
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 9000
$ ifconfig lo0
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384

In other words: it can do 9000 bytes but not 9180 and the default MTU on the loopback interface is 16834 so obviously 9000 is not a maximum imposed by the OS. And apparantly, it can't do half duplex for gigabit ethernet.

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,
however. 

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

Well obviously we shouldn't be saying "9k MTU" because it is too imprecise. Also, it's irrelevant unless we're trying to come up with a unified MTU size, which is something I am very much against, as it doesn't solve the problem but only moves the goalposts. We need to move to a situation where the capability to use larger packets is used where it exists and to the degree that it exists.

If we can increase the average packet size used on the internet by transmitting the same amount of data in fewer packets and routers/ switches are built such that they mostly use power when they are actually doing work, this will help with the power/heat issue.

But for the purposes of a jack up solution we just need a few dozen extra bytes to accommodate the new headers without triggering the breakage that ensues when 1500-byte packets can't be forwarded without fragmentation.

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