netdev - Re: [PATCH net-next 7/8] tcp: stronger sk

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Message-ID: <8f8836dd-c46f-403c-b478-a9e89dd62912@proxmox.com>
Date: Thu, 18 Dec 2025 10:31:15 +0100
From: Christian Ebner <c.ebner@...xmox.com>
To: Eric Dumazet <edumazet@...gle.com>, "David S . Miller"
 <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Neal Cardwell <ncardwell@...gle.com>
Cc: Simon Horman <horms@...nel.org>, Kuniyuki Iwashima <kuniyu@...gle.com>,
 Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org,
 eric.dumazet@...il.com, lkolbe@...iuswillert.com
Subject: Re: [PATCH net-next 7/8] tcp: stronger sk_rcvbuf checks

Hi,
to add some more information gained.

pcaps obtained via tcpdump of the traffic while in a stale state show 
the following recurring pattern:

41	0.705618	10.xx.xx.aa	10.xx.xx.bb	TCP	66	[TCP ZeroWindow] 8007 → 55554 
[ACK] Seq=1 Ack=28673 Win=0 Len=0 TSval=2656874280 TSecr=1348075902
42	0.705662	10.xx.xx.aa	10.xx.xx.bb	TCP	66	[TCP Window Update] 8007 → 
55554 [ACK] Seq=1 Ack=28673 Win=7 Len=0 TSval=2656874280 TSecr=1348075902
90	0.914606	10.xx.xx.bb	10.xx.xx.aa	TCP	7234	55554 → 8007 [PSH, ACK] 
Seq=28673 Ack=1 Win=139 Len=7168 TSval=1348076111 TSecr=2656874280

Output of `ss -tim` show the sockets being severely limited in buffer size:

ESTAB                          0                               0 

[::ffff:10.xx.xx.aa]:8007 
       [::ffff:10.xx.xx.bb]:55554
          skmem:(r0,rb7488,t0,tb332800,f0,w0,o0,bl0,d20) cubic 
wscale:10,10 rto:201 rtt:0.085/0.015 ato:40 mss:8948 pmtu:9000 
rcvmss:7168 advmss:8948 cwnd:10 bytes_sent:937478 bytes_acked:937478 
bytes_received:1295747055 segs_out:301010 segs_in:162410 
data_segs_out:1035 data_segs_in:161588 send 8.42Gbps lastsnd:3308 
lastrcv:191 lastack:191 pacing_rate 16.7Gbps delivery_rate 2.74Gbps 
delivered:1036 app_limited busy:437ms rcv_rtt:207.551 rcv_space:96242 
rcv_ssthresh:903417 minrtt:0.049 rcv_ooopack:23 snd_wnd:142336 rcv_wnd:7168

This would indicate that the buffer size not growing while in this 
state, therefore limiting the rcv_wnd?

Best regards,
Christian Ebner