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.