Re: excessive TCP duplicate acks?

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

> On 2007-Jan-26 11:59:06 -0500, Andrew Gallatin  
> <gallatin_at_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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