Excessive Duplicate ACKs

[Tom Judge]

On 10/27/2010 01:27 PM, Sean Bruno wrote:
> We moved an application stack from BSD4(BOO!) to BSD7(YAY!) recently and
> got a great performance increase, so first:  GOOD JOB.
>
> Periodically, we are seeing strings of duplicate ACK being sent in
> <100uSec deltas.  I can't imagine that this should be happening, but
> there it is.  I've sanitized an example trace of a transaction,
> demonstrating an average case that had 8 dup ACKs, some less than 3
> microsends in delta!  sysctl -a also attached (and sanitized).
>

I saw something like this on FreeBSD 7.1 with an oldish slapd where the
daemon was sending very small frames very quickly and the receiver could
not keep up and was dropping the frames in the NIC firmware has the host
was not draining the receive buffer fast enough.

We used wireshark to analyze the flows and it was reporting lots of
duplicate ACKs in the capture.  Looking more closely at the ACK we saw
that all of the reported duplicates where actually SACK frames and
wireshark was not smart enough to look into the SACK option and was only
processing the ACK sequence number.  Later in the flow (microseconds
also) we where seeing a fast retransmission of the data between the
current ACK sequence number and the SACK start sequence.

Maybe you are seeing something similar?


Tom
 
> http://people.freebsd.org/~sbruno/dup_ack_collapsed.txt
>
> http://people.freebsd.org/~sbruno/sysctl_dup_acks.txt
>
>
> Sean
>
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-- 
TJU13-ARIN