FreeBSD discarding received packets > MTU

[Alfred Perlstein]

* David Christensen <davidch at broadcom.com> [070907 10:48] wrote:
> > > 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:

Internet ettiquette demands being gracious in what you accept.
The default policy of FreeBSD is to accept such packets.
This is a really weird bug to track down.
Other drivers support it.

This isn't worth making a stand over, unless you're trying
to hold users of YOUR driver hostage.

-- 
- Alfred Perlstein