netdev - Re: A SACK block to the left of the ACK? (with ptr to raw trace)

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Message-ID: <CABrhC0kFEMOR7LTMFS4ktEgW0w+1v_vSP8xU_Vh8+iKmtEUb4A@mail.gmail.com>
Date:	Fri, 9 May 2014 14:21:12 -0400
From:	John Heffner <johnwheffner@...il.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	Netdev <netdev@...r.kernel.org>
Subject: Re: A SACK block to the left of the ACK? (with ptr to raw trace)

I didn't actually look at the trace, but offhand this sounds like
D-SACK. (http://www.ietf.org/rfc/rfc2883.txt)

  -John


On Fri, May 9, 2014 at 1:34 PM, Rick Jones <rick.jones2@...com> wrote:
> Hi -
>
> As part of looking at a customer issue, I've been running some netperf
> TCP_RR between a pair of instances running 3.2.0 with whatever stuff
> Canonical have backported into their -60 version.  While looking at packet
> traces I've seen the following odd SACK:
>
> 08:14:40.329583 IP 15.126.222.122.48130 > 10.0.0.3.12345: Flags [.], ack
> 63734, win 457, options [nop,nop,TS val 14282026 ecr 14255813,nop,nop,sack 1
> {63716:63717}], length 0
>
> I don't think that this "to the left of the ACK" SACK block actually caused
> anything heinous to happen but it does look odd and so I thought I might
> mention it to see if anyone else has seen it or if perhaps it is a known
> issue fixed in a later kernel.  The full tcpdump from one side is up at:
>
> ftp://ftp.netperf.org/rr_16.pcap.gz
>
> The netperf running was:
>
> ubuntu@...t-netperf-east-1-vm01:~$ netperf -l 60 -H zPet_NetPerf-East-2-vm01
> -t TCP_RR -- -b 16 -D -P ,12345
>
> So TCP_NODELAY was set (-D), and there were upwards of 17 segments in flight
> at any one time (-b 16 - 16 added to the default of one).
>
> The trace was taken at zPet_NetPerf-East-2-vm01.  As you might have guessed
> there is NAT involved - in both directions actually.  The node(s) on which
> this NAT is happening are running a 3.5.0-44 kernel.
>
> Here is one being sent from the side where the trace was being taken:
>
> 08:15:01.137718 IP 10.0.0.3.12345 > 15.126.222.122.48130: Flags [.], ack
> 218871, win 453, options [nop,nop,TS val 14261016 ecr 14287228,nop,nop,sack
> 1 {218854:218855}], length 0
>
> Which I suppose rules-out some odd NAT bug as the source of the "to the left
> of the ACK" SACKs since that was captured pre-NAT.
>
> happy benchmarking,
>
> rick jones
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