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Date: Fri, 16 Dec 2011 15:01:28 +0700
From: Eugene Grosbein <egrosbein@rdtc.ru>
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To: Andrea Venturoli <ml@netfence.it>
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Cc: freebsd-net@freebsd.org
Subject: Re: Intel 82550 Pro/100 Ethernet and TSO troubles
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15.12.2011 02:33, Andrea Venturoli пишет:
> Hello.
> 
> I recently installed 8.2 with the following card:
> 
>  > dmesg
>  > ...
>> fxp0: <Intel 82550 Pro/100 Ethernet> port 0xdc00-0xdc3f mem 0xfebfb000-0xfebfbfff,0xfebc0000-0xfebdffff irq 20 at device 5.0 on pci4
>  > ...
>  > pciconv -lv
>  > ...
>> fxp0@pci0:4:5:0:        class=0x020000 card=0x00408086 chip=0x12298086 rev=0x0c hdr=0x00
>>     vendor     = 'Intel Corporation'
>>     device     = '82550/1/7/8/9 EtherExpress PRO/100(B) Ethernet Adapter'
>>     class      = network
>>     subclass   = ethernet
>  > ...
> 
> I had weird problems, especially due to ICMP/fragmentation/MTU/... issues.
> After hours spent in debugging, I noticed  tcpdump showing ICMP packets 
> with wrong checksums and that TSO4 was enabled.
> "ifconfig fxp0 -tso" seemed to solve everything.
> 
> 
> Don't know if it's a bug in the card, in the driver or what...
> Maybe it shouldn't be enabled by default?

Do you use NAT? man ipfw clearly states:

     ipfw nat is not compatible with
     the TCP segmentation offloading (TSO).  Thus, to reliably nat your net-
     work traffic, please disable TSO on your NICs using ifconfig(8)

Eugene Grosbein