netdev - Re: [PATCH v4] tcp: make retransmitted SKB fit into the send window

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Date:   Tue, 12 Jul 2022 13:53:39 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Yonglong Li <liyonglong@...natelecom.cn>
Cc:     netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH v4] tcp: make retransmitted SKB fit into the send window

On Mon, Jul 11, 2022 at 12:06 PM Yonglong Li <liyonglong@...natelecom.cn> wrote:
>
> current code of __tcp_retransmit_skb only check TCP_SKB_CB(skb)->seq
> in send window, and TCP_SKB_CB(skb)->seq_end maybe out of send window.
> If receiver has shrunk his window, and skb is out of new window,  it
> should retransmit a smaller portion of the payload.
>
> test packetdrill script:
>     0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
>    +0 fcntl(3, F_GETFL) = 0x2 (flags O_RDWR)
>    +0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
>
>    +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
>    +0 > S 0:0(0)  win 65535 <mss 1460,sackOK,TS val 100 ecr 0,nop,wscale 8>
>  +.05 < S. 0:0(0) ack 1 win 6000 <mss 1000,nop,nop,sackOK>
>    +0 > . 1:1(0) ack 1
>
>    +0 write(3, ..., 10000) = 10000
>
>    +0 > . 1:2001(2000) ack 1 win 65535
>    +0 > . 2001:4001(2000) ack 1 win 65535
>    +0 > . 4001:6001(2000) ack 1 win 65535
>
>  +.05 < . 1:1(0) ack 4001 win 1001
>
> and tcpdump show:
> 192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 1:2001, ack 1, win 65535, length 2000
> 192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 2001:4001, ack 1, win 65535, length 2000
> 192.168.226.67.55 > 192.0.2.1.8080: Flags [P.], seq 4001:5001, ack 1, win 65535, length 1000
> 192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 5001:6001, ack 1, win 65535, length 1000
> 192.0.2.1.8080 > 192.168.226.67.55: Flags [.], ack 4001, win 1001, length 0
> 192.168.226.67.55 > 192.0.2.1.8080: Flags [.], seq 5001:6001, ack 1, win 65535, length 1000
> 192.168.226.67.55 > 192.0.2.1.8080: Flags [P.], seq 4001:5001, ack 1, win 65535, length 1000
>
> when cient retract window to 1001, send window is [4001,5002],
> but TLP send 5001-6001 packet which is out of send window.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Yonglong Li <liyonglong@...natelecom.cn>

Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Thanks !