tcp: undo init congestion window on false SYNACK timeout

Linux implements RFC6298 and use an initial congestion window
of 1 upon establishing the connection if the SYNACK packet is
retransmitted 2 or more times. In cellular networks SYNACK timeouts
are often spurious if the wireless radio was dormant or idle. Also
some network path is longer than the default SYNACK timeout. In
both cases falsely starting with a minimal cwnd are detrimental
to performance.

This patch avoids doing so when the final ACK's TCP timestamp
indicates the original SYNACK was delivered. It remembers the
original SYNACK timestamp when SYNACK timeout has occurred and
re-uses the function to detect spurious SYN timeout conveniently.

Note that a server may receives multiple SYNs from and immediately
retransmits SYNACKs without any SYNACK timeout. This often happens
on when the client SYNs have timed out due to wireless delay
above. In this case since the server will still use the default
initial congestion (e.g. 10) because tp->undo_marker is reset in
tcp_init_metrics(). This is an intentional design because packets
are not lost but delayed.

This patch only covers regular TCP passive open. Fast Open is
supported in the next patch.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 30c6a42b1f5b49a5aacd5d32f2ee650c18d9f503..53b4c5a3113b8086eaa81cada85610b273cca40c 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6101,6 +6101,8 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
                         */
                        tcp_rearm_rto(sk);
                } else {
+                       tcp_try_undo_spurious_syn(sk);
+                       tp->retrans_stamp = 0;
                        tcp_init_transfer(sk, BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB);
                        tp->copied_seq = tp->rcv_nxt;
                }

diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index 79900f783e0d8a713712b0bf47700f16eebab4db..9c2a0d36fb200b04885cf90493274659640d561b 100644 (file)
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -522,6 +522,11 @@ struct sock *tcp_create_openreq_child(const struct sock *sk,
                newtp->rx_opt.ts_recent_stamp = 0;
                newtp->tcp_header_len = sizeof(struct tcphdr);
        }
+       if (req->num_timeout) {
+               newtp->undo_marker = treq->snt_isn;
+               newtp->retrans_stamp = div_u64(treq->snt_synack,
+                                              USEC_PER_SEC / TCP_TS_HZ);
+       }
        newtp->tsoffset = treq->ts_off;
 #ifdef CONFIG_TCP_MD5SIG
        newtp->md5sig_info = NULL;      /*XXX*/