[40825] in North American Network Operators' Group
Re: Ethernet NAPs (was Re: Miami ...)
daemon@ATHENA.MIT.EDU (RJ Atkinson)
Thu Aug 23 10:26:18 2001
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Date: Thu, 23 Aug 2001 10:20:04 -0400
To: jtk@aharp.is-net.depaul.edu
From: RJ Atkinson <rja@inet.org>
Cc: Nanog <nanog@merit.edu>
In-Reply-To: <3B850BAD.3F088A32@depaul.edu>
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At 09:57 23/08/01, John Kristoff wrote:
>Furthermore... Larger frames would be nice if all hosts supported them,
>but the problem is that the that most end hosts cannot and probably will
>not ever support so called jumbo frames.
WindowsNT/Windows2000 [1] and a lot of UNIX servers/hosts do support
9K frames today. Most GigE PCI NIC cards support them. Most of
the commodity GigE ASICs support the 9K MTU. You are correct that
not all hosts/servers support them today. In any event, any size
of jumbo Ethernet frame will only work over an all-switched layer-2
network. My guess is that the trend over time will be for more and
more hosts to support the ~9K MTU. YMMV.
>What does having 9K ethernet frame support at a NAP get us?
Some folks run their end-to-end network with a 9K MTU. So having
it at the NAP means they avoid potential fragmentation in their
network. Certainly my employer would prefer a WAN network provider
that supported the 9K MTU because it would improve NFS performance
among our several sites as compared with a smaller end-to-end MTU.
>Perhaps the one good approach to jumbo frames is to make use of the
>networking layer and ensure hosts are doing Path MTU discovery to avoid
>fragmentation.
Path MTU Discovery is curiously controversial in some circles.
My own experience is that PMTUD works well today (not necessarily
true 5 years ago). So I agree that ensuring Path MTU Discovery is
deployed is generally clever. Past experience is that many vocal
folks will disagree with this view.
Ran
rja@inet.org
[1] Someone at Microsoft has told me that use of ~9K frames is how
Microsoft got their high WinNT network throughput for the SuperComputing
conference demo a few years back. I'm also told the POS links used
in that demo had also been configured for a ~9K MTU and worked fine.
