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Date:	Wed, 14 Dec 2011 18:12:53 -0800
From:	Vijay Subramanian <>
To:	Rick Jones <>
Subject: Re: twice past the taps, thence out to net?

On 14 December 2011 11:27, Rick Jones <> wrote:
> While looking at "something else" with tcpdump/tcptrace, tcptrace emitted
> lots of notices about hardware duplicated packets being detected (same TCP
> sequence number and IP datagram ID).  Sure enough, if I go into the tcpdump
> trace (taken on the sender) I can find instances of what it was talking
> about, separated in time by rather less than I would expect to be the RTO,
> and often as not with few if any intervening arriving ACKs to trigger
> anything like fast retransmit.  And besides, those would have a different IP
> datagram ID no?
> I did manage to reproduce the issue with plain netperf tcp_stream tests. I
> had one sending system with 30 concurrent netperf tcp_stream tests to 30
> other receiving systems.  There are "hardware duplicates" in the sending
> trace, but no duplicate segments (that I can find thus far) in the two
> receiver side traces I took.  Of course that doesn't mean "conclusively"
> there were two actual sends but it suggests there werent.
> While I work through the "obtain permission" path to post the packet traces
> (don't ask...) I thought I would ask if anyone else has seen something
> similar.
> In this case, all the systems are running a 2.6.38-8 Ubuntu kernel (the same
> sorts of issues which delay my just putting the traces up on
> preclude a later kernel, and I've no other test systems :( ), with Intel
> 82576 interfaces being driven by:
> $ sudo ethtool -i eth0
> driver: igb
> version: 2.1.0-k2
> firmware-version: 1.8-2
> bus-info: 0000:05:00.0
> All the systems were connected to the same switch.

This may be of help.

Vijay Subramanian
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