Jeff Buehler wrote:
> Higher MTu's run the risk of fragmenting packets, which slows down
> performance. So, you want the highest MTU possible for any given
> connection without fragmenting ("ping -f -l destination" will tell you
> if a packet has fragmented). Standard cable/dsl/t1 connections have
> MTU's of 1500. The ping to test this is actually "ping -f -l 1472"
> which equals an MTU of 1500 because of 28 extra bytes in the header (or
> something like that) so 1472 is actually an mtu of 1500.
The 1472 is MSS which is maximum segment size. I thought TCP/IP headers
were 40 bytes.
> Making things more complicated, OS's also set MTU for the adapter
> (ifconfig for *BSD, for Windows you will need to research registry
> changes or get a tool to modify the MTU).
Don't forget to reboot your 'doze box after changing MTU in registry ;)
> So, if you had a LAN composed of gigabit devices that support jumbo
> frames, you should get significantly improved performance by setting
> your MTU to 9000 for all of the devices in the OS. Then, for your WAN
I heard somewhere that 9000 was about the correct size to achieve 1Gbps
throughput on a 32 bit machine with a Gigabit adapter. My guess is that
most hardware won't support MTU much bigger than that for a while. I
believe another popular large MTU size is 4470, used by ATM and SDH.
> connection, you would almost certainly want and MTU of 1500 because that
> is likely the limit of the DSL/Cable/T1 line that you connect to.
Yep, I'm using iBurst wireless here (ZA) and they recommend 1352. Our
ADSL here typically uses 1492.
> I hope that helps.
Definitely! So it would seem logically that the less likely it is for
fragmentation to occur, the larger ones packets can be. I guess its just
a matter of finding an optimal value.