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Re: [tcpm] Linux TCP Stack and Zero Window Probe

Neal Cardwell <ncardwell@google.com> Thu, 22 November 2018 18:22 UTC

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From: Neal Cardwell <ncardwell@google.com>
Date: Thu, 22 Nov 2018 13:21:52 -0500
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To: Kiruthika.Varadarajan@harman.com
Cc: "tcpm@ietf.org" <tcpm@ietf.org>, John Heffner <johnwheffner@gmail.com>
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Subject: Re: [tcpm] Linux TCP Stack and Zero Window Probe
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Hi,

This is expected. Linux TCP uses the same code to generate keepalive
probes and zero window probes. (Note that the comment for
tcp_write_wakeup() is "Initiate keepalive or window probe from
timer"). So, on the wire and divorced from context, the two kinds of
packets can look the same.

Given the context in your trace, with the receiver just having
advertised a receive window of 0 bytes, and with the probes starting
in the range of a plausible RTO, and exponentially backing off, those
look like zero window probes to me.

best,
neal

ps: this is a condensed version of the trace, with tcpdump -ttt
relative timestamps:

00:00:00.025099 cli > srv: S 3878781099:3878781099(0) win 65535
00:00:00.000506 srv > cli: S. 936571641:936571641(0) ack 3878781100
win 29200 <mss 1460>
00:00:00.012473 cli > srv: . 3878781100:3878781100(0) ack 936571642 win 65535
00:00:01.003173 srv > cli: P. 936571642:936571643(1) ack 3878781100 win 29200
00:00:00.013407 cli > srv: . 3878781100:3878781100(0) ack 936571643 win 0
00:00:01.208250 srv > cli: . 936571642:936571642(0) ack 3878781100 win 29200
00:00:00.431995 srv > cli: . 936571642:936571642(0) ack 3878781100 win 29200
00:00:00.887970 srv > cli: . 936571642:936571642(0) ack 3878781100 win 29200
00:00:01.728002 srv > cli: . 936571642:936571642(0) ack 3878781100 win 29200

On Wed, Nov 21, 2018 at 1:21 PM Varadarajan, Kiruthika
<Kiruthika.Varadarajan@harman.com> wrote:
>
> Hi Team,
>
> I have a question in Zero Window Probe in TCP.
>
> If the client sends Zero Window, according to RFC spec is it mandatory for the Linux TCP stack to send Zero Window Probe.
>
> In my stack I am not getting Zero Window Probe instead I am getting Keep Alive.
>
> I have attached my wireshark traces. Can you please confirm if this is the expected behavior from TCP stack.
>
>
>
> Thanks & Regards
>
> Kiruthika
>
>
>
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