Re: [PATCH] tcp: fix marked lost packets not being retransmitted

[Yuchung Cheng <ycheng@google.com>]

On Tue, Jan 14, 2020 at 8:06 AM Eric Dumazet <edumazet@google.com> wrote:
>
> On Tue, Jan 14, 2020 at 1:24 AM Pengcheng Yang <yangpc@wangsu.com> wrote:
> >
> > When the packet pointed to by retransmit_skb_hint is unlinked by ACK,
> > retransmit_skb_hint will be set to NULL in tcp_clean_rtx_queue().
> > If packet loss is detected at this time, retransmit_skb_hint will be set
> > to point to the current packet loss in tcp_verify_retransmit_hint(),
> > then the packets that were previously marked lost but not retransmitted
> > due to the restriction of cwnd will be skipped and cannot be
> > retransmitted.
>
>
> "cannot be retransmittted"  sounds quite alarming.
>
> You meant they will eventually be retransmitted, or that the flow is
> completely frozen at this point ?
He probably means those lost packets will be skipped until a timeout
that reset hint pointer. nice fix this would save some RTOs.

>
> Thanks for the fix and test !
>
> (Not sure why you CC all these people having little TCP expertise btw)
>
> > To fix this, when retransmit_skb_hint is NULL, retransmit_skb_hint can
> > be reset only after all marked lost packets are retransmitted
> > (retrans_out >= lost_out), otherwise we need to traverse from
> > tcp_rtx_queue_head in tcp_xmit_retransmit_queue().
> >
> > Packetdrill to demonstrate:
> >
> > // Disable RACK and set max_reordering to keep things simple
> >     0 `sysctl -q net.ipv4.tcp_recovery=0`
> >    +0 `sysctl -q net.ipv4.tcp_max_reordering=3`
> >
> > // Establish a connection
> >    +0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
> >    +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
> >    +0 bind(3, ..., ...) = 0
> >    +0 listen(3, 1) = 0
> >
> >   +.1 < S 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
> >    +0 > S. 0:0(0) ack 1 <...>
> >  +.01 < . 1:1(0) ack 1 win 257
> >    +0 accept(3, ..., ...) = 4
> >
> > // Send 8 data segments
> >    +0 write(4, ..., 8000) = 8000
> >    +0 > P. 1:8001(8000) ack 1
> >
> > // Enter recovery and 1:3001 is marked lost
> >  +.01 < . 1:1(0) ack 1 win 257 <sack 3001:4001,nop,nop>
> >    +0 < . 1:1(0) ack 1 win 257 <sack 5001:6001 3001:4001,nop,nop>
> >    +0 < . 1:1(0) ack 1 win 257 <sack 5001:7001 3001:4001,nop,nop>
> >
> > // Retransmit 1:1001, now retransmit_skb_hint points to 1001:2001
> >    +0 > . 1:1001(1000) ack 1
> >
> > // 1001:2001 was ACKed causing retransmit_skb_hint to be set to NULL
> >  +.01 < . 1:1(0) ack 2001 win 257 <sack 5001:8001 3001:4001,nop,nop>
> > // Now retransmit_skb_hint points to 4001:5001 which is now marked lost
> >
> > // BUG: 2001:3001 was not retransmitted
> >    +0 > . 2001:3001(1000) ack 1
> >
> > Signed-off-by: Pengcheng Yang <yangpc@wangsu.com>
> > ---
> >  net/ipv4/tcp_input.c | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > index 0238b55..5347ab2 100644
> > --- a/net/ipv4/tcp_input.c
> > +++ b/net/ipv4/tcp_input.c
> > @@ -915,9 +915,10 @@ static void tcp_check_sack_reordering(struct sock *sk, const u32 low_seq,
> >  /* This must be called before lost_out is incremented */
> >  static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb)
> >  {
> > -       if (!tp->retransmit_skb_hint ||
> > -           before(TCP_SKB_CB(skb)->seq,
> > -                  TCP_SKB_CB(tp->retransmit_skb_hint)->seq))
> > +       if ((!tp->retransmit_skb_hint && tp->retrans_out >= tp->lost_out) ||
> > +           (tp->retransmit_skb_hint &&
> > +            before(TCP_SKB_CB(skb)->seq,
> > +                   TCP_SKB_CB(tp->retransmit_skb_hint)->seq)))
> >                 tp->retransmit_skb_hint = skb;
> >  }
> >
> > --
> > 1.8.3.1
> >