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

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
>

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