Ethernet NAPs (was Re: Miami ...)

Thu Aug 23 14:20:04 UTC 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
>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 at 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.