Date: Sat, 17 Nov 2007 00:42:54 -0500 (EST) From: Mike Andrews <mandrews@bit0.com> To: freebsd-current@freebsd.org Subject: bizarre em + TSO + MSS issue in RELENG_7 Message-ID: <20071117003504.R31357@mindcrime.int.bit0.com>
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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.
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