North American Network Operators Group

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Re: Ethernet NAPs (was Re: Miami ...)

  • From: RJ Atkinson
  • Date: Thu Aug 23 10:28:22 2001

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

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.

[email protected]

[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.