FreeBSD discarding received packets > MTU

[David Christensen]

> > It could certainly be argued by some that Cisco is not standards
> > compliant in this case for sending an oversized Ethernet frame
> > and expecting everyone to accept it.  Hardware has limitations
> > and assuming that all Ethernet controllers can support frames
> > greater than 1522 bytes is not reasonable.  Fortunately there is
> > a suitable workaround which is setting a larger MTU for the 
> > interface.  What size do you use?  How did you arrive at that
> > value?
> 
> I use 1550 to make it work in the test harness.
> 
> The trouble is that if I set the mtu to 1550, and the machine 
> talks to another
> such machine with it's mtu also set to 1550 then they 
> negotiate a maximum sized
> packet based on 1550, and the problem hits me again. This is 
> a web proxy 
> and that problem occurs when there are two layers of proxy 
> and one proxy talks to 
> another. I really just need it to to silently accept a packet some 
> 32 bytes or so larger than the stated MTU.
> 
> I see no reason for the driver to not do what the em driver 
> does and allow 
> itself to receive any packet up to the MCLBYTES size.
> 
> We only hit this problem recently because the data interfaces on our
> devices are usually em NICs and we only just recently started 
> allowing the 
> users to use the built in (on DELL 2950) bce interfaces for 
> this purpose.
> 

I'm not completely opposed to making such a change, but I don't want
to make a default change in the driver's behavior that other people 
may be depending upon (whether they are aware of it or not).  A
tunable driver value could be the answer but I'm not entirely sure
how it would fare in the hardware at the high end of MTU values such 
as 9000.

Dave