From bc9f38c8328e10c22cb2016d6131ea36141c8d11 Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Mon, 29 Apr 2019 15:46:13 -0700 Subject: [PATCH] tcp: avoid unconditional congestion window undo on SYN retransmit Previously if an active TCP open has SYN timeout, it always undo the cwnd upon receiving the SYNACK. This is because tcp_clean_rtx_queue would reset tp->retrans_stamp when SYN is acked, which fools then tcp_try_undo_loss and tcp_packet_delayed. Addressing this issue is required to properly support undo for spurious SYN timeout. Fixing this is tricky -- for active TCP open tp->retrans_stamp records the time when the handshake starts, not the first retransmission time as the name may suggest. The simplest fix is for tcp_packet_delayed to ensure it is valid before comparing with other timestamp. One side effect of this change is active TCP Fast Open that incurred SYN timeout. Upon receiving a SYN-ACK that only acknowledged the SYN, it would immediately retransmit unacknowledged data in tcp_ack() because the data is marked lost after SYN timeout. But the retransmission would have an incorrect ack sequence number since rcv_nxt has not been updated yet tcp_rcv_synsent_state_process(), the retransmission needs to properly handed by tcp_rcv_fastopen_synack() like before. Signed-off-by: Yuchung Cheng Signed-off-by: Neal Cardwell Signed-off-by: Eric Dumazet Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 97671bff597a..e2cbfc3ffa3f 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2252,7 +2252,7 @@ static bool tcp_skb_spurious_retrans(const struct tcp_sock *tp, */ static inline bool tcp_packet_delayed(const struct tcp_sock *tp) { - return !tp->retrans_stamp || + return tp->retrans_stamp && tcp_tsopt_ecr_before(tp, tp->retrans_stamp); } @@ -3521,7 +3521,7 @@ static void tcp_xmit_recovery(struct sock *sk, int rexmit) { struct tcp_sock *tp = tcp_sk(sk); - if (rexmit == REXMIT_NONE) + if (rexmit == REXMIT_NONE || sk->sk_state == TCP_SYN_SENT) return; if (unlikely(rexmit == 2)) {