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Re: [RAM] Tunnelling & GigE jumbo frame size

On 12-sep-2007, at 16:24, RJ Atkinson wrote:

Dino's notes all sound correct, with one exception. 

I'd like to support Pekka's point about internet exchanges, though.

The exception is a detail and does not alter his main point,
which is that by using Jumbo Ethernet, tunnelling
of 1518 byte Ethernet frames ought not create any
fragmentation issues.  However, just to be crystal clear
to anyone who isn't tracking Ethernet closely, here
is the summary analysis.

Please note that ethernet frame size / MTU may or may not include the Ethernet_II header (which doesn't conform to IEEE 802.3) and if it does, it may or may not include the frame check sequence. I'll use the IP MTU excluding ethernet headers unless otherwise specified.

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

Caveats:
	Jumbo Ethernet is explicitly not conformant to IEEE 802.3
	standards.  So all implementations that I'm aware of have
	a default MTU (i.e. out of the box) that is IEEE 802.3
	compliant.  Normally, users must explicitly configure the
	larger MTU size on each interface before it is enabled.


	1 GigE standards permit deployment with CSMA/CD, although
	I am not personally aware of any such deployments.  A GigE
	CSMA/CD deployment is likely not to work properly with
	an Ethernet frame size above that permitted by IEEE 802.3
	standards.  Switched Ethernet should be fine, however,
	and all actual 1 GigE deployments that I am aware of are
	switched and do not use CSMA/CD.

When gigabit ethernet was introduced the _minimum_ frame size (not IP MTU size) was (sort of) increased from 64 to 512 bytes when using CSMA/CD to make sure a frame would fill the entire collision domain even at the faster bit rate. However, the _maximum_ frame size was never increased because it would break old stuff. Lots of gigabit ethernet equipment supports larger frames, but few 10 or 100 Mbps implementations do. So having a larger frame size for GigE would make interoperation with older ethernet equipment a problem.

Although the issue comes up in the IEEE 802.3 working group every few years, no progress has been made here because the IEEE protocols simply don't have the hooks that make maximum frame size negotiation between ethernet systems possible. However, this is much easier at the IP level (especially IPv6), so I'm proposing that the IETF pick up the slack. See the discussion about my multi-mtu draft on the internet area list:

http://www1.ietf.org/mail-archive/web/int-area/current/index.html

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