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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: PMTU-D: remember, your load balancer is broken
Sez <[email protected]> > The 576 value of the MS PPP MTU is merely a default - it can be > changed with a registry hack. Expecting the tens of millions of novice computer users to set their systems for a 1500 byte MTU is irrational. Those who are knowledgeable enough to do so are generally reducing it due to "speed up your modem" articles and programs which improve interactive performance at the expense of throughput. > Forcing excessive fragmentation/defragmentation is an effective DoS. Effective, but a fairly difficult problem to exploit. > I thought the frame-size limits for Gigabit Ethernet were 64-1518/1522 > bytes? And isn't that the limit on most host IP stacks for Ethernet media? > Or am I off in left field, here? IIRC, during development of the GigE spec, several vendors wished to increase the GigE MTU to ~9000 bytes. Due to the technical ramifications of bridging to low-MTU FastE segments, as well as inter-vendor politics, it didn't make it as part of the GigE spec but was later published as an optional extension. There are now several devices on the market that will do Jumbo frames on GigE. For instance, the Catalyst 6000 and GSR do: http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/sw_5_5/cmd_r efr/set_po_r.htm#xtocid661812 I know there are several other vendors who support Jumbo frames as well. > Finally, I would say that on any medium, <100% utilization in and of itself > isn't grounds for fiddling with the MTU. There are lots of other things to > look at, first. I hear consistent requests from server folks for higher MTUs; they claim the per-frame CPU hit is significant enough to warrant using Jumbo frames to increase throughput. The results clearly show that it helps. S | | Stephen Sprunk, K5SSS, CCIE #3723 :|: :|: Network Design Consultant, HCOE :|||: :|||: 14875 Landmark Blvd #400; Dallas, TX .:|||||||:..:|||||||:. Email: [email protected]
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