Ethernet MTU

Michael Hamel mhamel at
Thu Mar 26 20:49:36 UTC 2009

> [ Please note replying to the last email on the list
>    and changing the subject stuffs up the threading.

Sorry, I forgot that the threading uses other headers..

> Michael Hamel wrote:
>> Has anyone used jumbo packets (MTU of ~9000) with the RTEMS  
>> networking  stack? I have a working gigabit network stack up on the  
>> PPC405EXr, but  I'm not seeing more than 17Mbytes a second  
>> throughput.
> How are you testing this ?

I have my own software running at both ends of the link; thats a one- 
way figure derived from blasting the host with data as fast as I can  
generate it. My reading indicates that thats about what I should  
expect without using jumbo packets as the turnaround overheads  
dominate gigabit transfers with the standard MTU.

I thought I might be able to use the SIOCSIFMTU ioctl() to change the  
MTU, but in if_ethersubr.c I find this:

		 * Set the interface MTU.
		if (ifr->ifr_mtu > ETHERMTU) {
			error = EINVAL;
		} else {
			ifp->if_mtu = ifr->ifr_mtu;

... which is kind of discouraging. Does anyone know why its clipping  
to ETHERMTU=1500 bytes?

"Not *another* shrubbery!"

Michael Hamel
ADInstruments Ltd, Dunedin, NZ

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