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Date: Fri, 8 Jul 2022 13:45:33 +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 v2] tcp: make retransmitted SKB fit into the send window
On Fri, Jul 8, 2022 at 10:59 AM Yonglong Li <liyonglong@...natelecom.cn> wrote:
>
> 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.
More exactly, 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.
>
> 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;
> +
I asked in my prior feeback to align len to a multiple of cur_mss.
Trying to send full MSS is likely helping receivers to not waste
memory and thus send us bigger RWIN.
Also I think the logic about zero window probe could be consolidated
in one place.
Also you forgot to change the part about tcp_retrans_try_collapse()
which I also mentioned in my feedback.
If we want to fix this problem for good, we need to take care of all cases...
Something like this:
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 18c913a2347a984ae8cf2793bb8991e59e5e94ab..2aa67cbfa1428a11b723263083ce48284762e306
100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3144,7 +3144,7 @@ int __tcp_retransmit_skb(struct sock *sk, struct
sk_buff *skb, int segs)
struct tcp_sock *tp = tcp_sk(sk);
unsigned int cur_mss;
int diff, len, err;
-
+ int avail_wnd;
/* Inconclusive MTU probe */
if (icsk->icsk_mtup.probe_size)
@@ -3167,16 +3167,24 @@ int __tcp_retransmit_skb(struct sock *sk,
struct sk_buff *skb, int segs)
cur_mss = tcp_current_mss(sk);
+ avail_wnd = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
/* If receiver has shrunk his window, and skb is out of
* new window, do not retransmit it. The exception is the
* case, when window is shrunk to zero. In this case
- * our retransmit serves as a zero window probe.
+ * our retransmit of one segment serves as a zero window probe.
*/
- if (!before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp)) &&
- TCP_SKB_CB(skb)->seq != tp->snd_una)
- return -EAGAIN;
+ if (avail_wnd <= 0) {
+ if (TCP_SKB_CB(skb)->seq != tp->snd_una)
+ return -EAGAIN;
+ avail_wnd = cur_mss;
+ }
len = cur_mss * segs;
+ if (len > avail_wnd) {
+ len = rounddown(avail_wnd, cur_mss);
+ if (!len)
+ len = avail_wnd;
+ }
if (skb->len > len) {
if (tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb, len,
cur_mss, GFP_ATOMIC))
@@ -3190,8 +3198,9 @@ int __tcp_retransmit_skb(struct sock *sk, struct
sk_buff *skb, int segs)
diff -= tcp_skb_pcount(skb);
if (diff)
tcp_adjust_pcount(sk, skb, diff);
- if (skb->len < cur_mss)
- tcp_retrans_try_collapse(sk, skb, cur_mss);
+ avail_wnd = min_t(int, avail_wnd, cur_mss);
+ if (skb->len < avail_wnd)
+ tcp_retrans_try_collapse(sk, skb, avail_wnd);
}
/* RFC3168, section 6.1.1.1. ECN fallback */
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