Date: Fri, 7 Sep 2007 12:47:49 -0700 From: Alfred Perlstein <alfred@freebsd.org> To: David Christensen <davidch@broadcom.com> Cc: FreeBSD Net <freebsd-net@freebsd.org>, Julian Elischer <julian@elischer.org> Subject: Re: FreeBSD discarding received packets > MTU Message-ID: <20070907194749.GD87451@elvis.mu.org> In-Reply-To: <09BFF2FA5EAB4A45B6655E151BBDD903051CBEB6@NT-IRVA-0750.brcm.ad.broadcom.com> References: <46E0632D.8070200@elischer.org> <46E07E74.5020204@elischer.org> <09BFF2FA5EAB4A45B6655E151BBDD9030501D5C0@NT-IRVA-0750.brcm.ad.broadcom.com> <46E0A5DB.3080404@elischer.org> <09BFF2FA5EAB4A45B6655E151BBDD903051CBEB6@NT-IRVA-0750.brcm.ad.broadcom.com>
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* David Christensen <davidch@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
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