netdev - [PATCH v2] tcp: make retransmitted SKB fit into the send window

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Date:   Fri,  8 Jul 2022 16:59:34 +0800
From:   Yonglong Li <liyonglong@...natelecom.cn>
To:     netdev@...r.kernel.org
Cc:     davem@...emloft.net, dsahern@...nel.org, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com, liyonglong@...natelecom.cn
Subject: [PATCH v2] tcp: make retransmitted SKB fit into the send window

From: lyl <liyonglong@...natelecom.cn>

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 not retransmit it.

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.

Signed-off-by: liyonglong <liyonglong@...natelecom.cn>
---
 net/ipv4/tcp_output.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 1c05443..7e1569e 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3176,7 +3176,12 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 	    TCP_SKB_CB(skb)->seq != tp->snd_una)
 		return -EAGAIN;
 
-	len = cur_mss * segs;
+	len = min_t(int, tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq, cur_mss * segs);
+
+	/* retransmit serves as a zero window probe. */
+	if (len == 0 && TCP_SKB_CB(skb)->seq == tp->snd_una)
+		len = cur_mss;
+
 	if (skb->len > len) {
 		if (tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb, len,
 				 cur_mss, GFP_ATOMIC))
-- 
1.8.3.1