Re: Re: [PATCH 2/3] tcp: Reduce SYN resend delay if a suspicous ACK is received

On Fri, 31 Jan 2020 07:01:21 -0800 Eric Dumazet [off-list ref] wrote:

On Fri, Jan 31, 2020 at 4:25 AM [off-list ref] wrote:

quoted

Signed-off-by: SeongJae Park <redacted>
---
 net/ipv4/tcp_input.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2a976f57f7e7..b168e29e1ad1 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c

@@ -5893,8 +5893,12 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
                 *        the segment and return)"
                 */
                if (!after(TCP_SKB_CB(skb)->ack_seq, tp->snd_una) ||
-                   after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt))
+                   after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
+                       /* Previous FIN/ACK or RST/ACK might be ignore. */
+                       inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
+                                                 TCP_ATO_MIN, TCP_RTO_MAX);

This is not what I suggested.

I suggested implementing a strategy where only the _first_ retransmit
would be done earlier.

So you need to look at the current counter of retransmit attempts,
then reset the timer if this SYN_SENT
socket never resent a SYN.

We do not want to trigger packet storms, if for some reason the remote
peer constantly sends
us the same packet.

You're right, I missed the important point, thank you for pointing it.  Among
retransmission related fields of 'tcp_sock', I think '->total_retrans' would
fit for this check.  How about below change?

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2a976f57f7e7..29fc0e4da931 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5893,8 +5893,14 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
                 *        the segment and return)"
                 */
                if (!after(TCP_SKB_CB(skb)->ack_seq, tp->snd_una) ||
-                   after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt))
+                   after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
+                       /* Previous FIN/ACK or RST/ACK might be ignored. */
+                       if (tp->total_retrans == 0)
+                               inet_csk_reset_xmit_timer(sk,
+                                               ICSK_TIME_RETRANS, TCP_ATO_MIN,
+                                               TCP_RTO_MAX);
                        goto reset_and_undo;
+               }

                if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
                    !between(tp->rx_opt.rcv_tsecr, tp->retrans_stamp,

Thanks,
SeongJae Park

Thanks.

quoted

                        goto reset_and_undo;
+               }

                if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
                    !between(tp->rx_opt.rcv_tsecr, tp->retrans_stamp,
--
2.17.1