[ale] PPP Perfromance

[Stephen Leonard]

On Fri, Oct 31, 2003 at 10:53:49AM -0500, Geoffrey wrote:
> Doug McNash wrote:
> >
> >
> >Some network traffic can't be fragmented but I am drawing a blank right 
> >now as to what besides ICMP that traffic is.
> >Packets get fragmented if they enter a network segment that has an MTU 
> >smaller than the packet size. There is a DO_NOT_FRAGMENT flag which if 
> >set causes the packet to be dropped and and ICMP UNREACHABLE error to be 
> >sent back if fragmentation is required somewhere along the line.  
> 
> Getting back to the mtu issue, I'm now curious as to whether that might 
> be part of the problem why I couldn't get to my mother-in-laws benefit 
> web site.  I've played with mtu a bit and found a couple things out. 
> Currently all my ethernet networks are set at mtu 1500.  I can easily 
> reduce this by simply:
> 
> ifconfig eth0 mtu 1000
> 
> But if I attempt to increase it, I get an error.  Me thinks this would 
> because there's a max set somewhere, although after running around in 
> /proc for a while, I couldn't find anything.  Anyone else have any ideas?

I think the max is dependent on the hardware, though I'm not sure
if there's a standard way to query the driver for this value.  I
remember some gigabit cards would allow you adjust the mtu up to
~9kB to create jumbo frames.  It definately increases throughput,
but requires specialized hardware (at least when I tried it).

stephen