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Date: Mon, 11 Jul 2022 08:39:49 +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 Mon, Jul 11, 2022 at 3:54 AM Yonglong Li <liyonglong@...natelecom.cn> wrote:
>
> Hi Eric,
> thank you for your feedback and suggestions. Sorry for miss part of your
> prior feedback. I will prepare a v3 patch as your suggestion.
>
> And anther your feedback about "need a Fixes: tag", I can not find any commit
> which is related to this patch. Do you have any suggestion about Fixes:tag?
This could be the following, because I do not think linux ever got this right.
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>
> On 7/8/2022 7:45 PM, Eric Dumazet wrote:
> > 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 */
> >
>
> --
> Li YongLong
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