Thoughts on increasing MTUs on the internet

Thu Apr 12 11:55:01 UTC 2007

On Thu, Apr 12, 2007 at 01:03:45PM +0200, Iljitsch van Beijnum wrote:
> 
> On 12-apr-2007, at 12:02, Pierfrancesco Caci wrote:
> 
> >wouldn't that work only if the switch in the middle of your neat
> >office lan is a real switch (i.e. not flooding oversize packets to
> >hosts that can't handle them, possibly crashing their NIC drivers) and
> >it's itself capable of larger MTUs?
> 
> Well, yes, being compatible with stuff that doesn't support larger  
> packets pretty much goes without saying. I don't think there is any  
> need to worry about crashing drivers, packets that are longer than  
> they should are a common error condition that drivers are supposed to  
> handle without incident. (They often keep a "giant" count.)
> 
> A more common problem would be two hosts that support jumboframes  
> with a switch in the middle that doesn't. So it's necessary to test  
> for this and avoid excessive numbers or large packets when something  
> in the middle doesn't support them.

the internet is broken.. too many firewalls dropping icmp, too many hard coded systems that work for 'default' but dont actually allow for alternative parameters that should work according to the RFCs

if you can fix all that then it might work

alternatively if you can redesign path mtu discovery that might work too..

Martin Levy suggested this too me only two weeks ago, he had an idea of sending two packets initially - one 'default' and one at the higher mtu .. if the higher one gets dropped somewhere you can quickly spot it and revert to 'default' behaviour.

I think his explanation was more complicated but it was an interesting idea

Steve