Date: Fri, 16 Nov 2007 22:12:43 -0800 From: "Kip Macy" <kip.macy@gmail.com> To: "Mike Andrews" <mandrews@bit0.com> Cc: freebsd-current@freebsd.org Subject: Re: bizarre em + TSO + MSS issue in RELENG_7 Message-ID: <b1fa29170711162212ibefa571pa7ffca52ab39507a@mail.gmail.com> In-Reply-To: <20071117003504.R31357@mindcrime.int.bit0.com> References: <20071117003504.R31357@mindcrime.int.bit0.com>
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On Nov 16, 2007 9:42 PM, Mike Andrews <mandrews@bit0.com> wrote: > Has anyone run into problems with MSS not being respected when using TSO, > specifically on em cards? > > I've run into a weird 3-way interaction with TSO, users with small MTU's > (like 576), and a Cisco firewall that by default drops packets larger than > the MSS for that session. Most users can connect to our web servers > behind this firewall just fine, but a very few couldn't since we moved > from RELENG_6 to RELENG_7 about two weeks ago. One of them could connect > again after removing a Linksys box at his end that was clamping the MSS at > 536. Some of the others were on dialup which tends to use 576 MTU / 536 > MSS. The firewall was dropping a lot of *outbound* traffic to these users > because the MSS was exceeded -- ~1000 byte packets for a 536 MSS > connection. All of those users' problems disappeared after disabling TSO > on our end. Pretty weird. :) > > I'm quite happy running without TSO and I really don't expect this to get > looked at before 7.0-RELEASE -- I just thought the info that MSS appears > (to me anyway) to not always be respected might be useful for those > looking at TSO-related throughput issues. In our case throughput isn't > the issue, it's certain connections stalling out -- it actually behaves > sort of like (but isn't) a PMTUD issue. > > I've got a lot more detail for whoever wants it (Jack Vogel, probably) > including tcpdumps and firewall logs but I thought I'd put it out here > first in case it wasn't em-specific rather than clutter the list right > away, or if it's a known issue that's already fixed in HEAD or something. > This is with em driver version 6.5.3, 7.0-BETA2 from Nov 2. Hrrm. Either TCP isn't setting tso_segsz correctly, or em isn't honoring tso_segsz. Can you add some instrumentation to the TSO code in em to see if tso_segsz is being set to the MSS? -Kip
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