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

["Varadarajan, Kiruthika" <[e-mail]>]

Hi Neal,
 But RFC spec https://tools.ietf.org/html/rfc793#section-3.7  says after TCP-Zero Window from client, server must send 
 TCP Zero Window Probe, Hence in tcp_write_wakeup, tcp_xmit_probe_skb expect mib to be LINUX_MIB_TCPWINPROBE
  Are we not violating the spec?


Thanks & Regards
Kiruthika


-----Original Message-----
From: Neal Cardwell <[e-mail]> 
Sent: Thursday, November 22, 2018 11:52 PM
To: Varadarajan, Kiruthika <[e-mail]>
Cc: [e-mail]; John Heffner <[e-mail]>
Subject: [EXTERNAL] Re: [tcpm] Linux TCP Stack and Zero Window Probe

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 <[e-mail]> 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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