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Date:      Sat, 3 Mar 2007 13:38:23 -0500
From:      Andre Guibert de Bruet <andy@siliconlandmark.com>
To:        Peter Jeremy <peterjeremy@optushome.com.au>
Cc:        freebsd-current@freebsd.org, Andrew Gallatin <gallatin@cs.duke.edu>
Subject:   Re: excessive TCP duplicate acks?
Message-ID:  <20DDE355-5F52-4E1F-B316-D933C816DE3B@siliconlandmark.com>
In-Reply-To: <20070303000125.GA9918@turion.vk2pj.dyndns.org>
References:  <17850.13146.266196.499166@grasshopper.cs.duke.edu> <20070303000125.GA9918@turion.vk2pj.dyndns.org>

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On Mar 2, 2007, at 7:01 PM, Peter Jeremy wrote:

> On 2007-Jan-26 11:59:06 -0500, Andrew Gallatin  
> <gallatin@cs.duke.edu> wrote:
>> When running some benchmarks, I noticed tons of duplicate acks  
>> showing
>> up in systat -tcp (thousands, or tens of thousands per second).
>
> Whilst investigating other problems, I've just seen the same on 6.2.
> The following trace was taken on 192.168.234.1, which is running
> 6.2-RELEASE/i386 (with ipfilter enabled) with fxp (Intel 82559) NICs.
> 192.168.234.64 is running 6.2-STABLE/amd64 from late January (no
> firewall active) with a bge (Broadcom BCM5705 A3, ASIC rev. 0x3003)
> NIC and checksum offloading enabled.
>
> The multiple SYN packets are due to a bug in the IPfilter state
> management, though it eventually allows a SYN through.  (And it is not
> totally unrealistic for multiple SYNs to be required before a SYN-ACK
> is received so this does not excuse the ACK flood).  Note that the
> duplicate ACKs are being sent from the host without a firewall so this
> does not appear to be related to ipfilter (or kern/102653).
>
> The common factor seems to be amd64.

I am seeing this problem on a machine with dual 64-bit Xeons in i386  
mode:

Vendor ID: "GenuineIntel"; CPUID level 5

Intel-specific functions:
Version 00000f34:
Type 0 - Original OEM
Family 15 - Pentium 4
Extended family 0
Model 3 - Intel Pentium 4 processor (generic) or newer
Stepping 4
Reserved 0

Extended brand string: "                  Intel(R) Xeon(TM) CPU 3.00GHz"
CLFLUSH instruction cache line size: 8
Hyper threading siblings: 1

Andy

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