diff options
| -rw-r--r-- | net/ipv4/tcp_input.c | 49 |
1 files changed, 32 insertions, 17 deletions
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index fe96e176d85a..561e5d404988 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -1236,22 +1236,22 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_ return flag; } +/* F-RTO can only be used if these conditions are satisfied: + * - there must be some unsent new data + * - the advertised window should allow sending it + */ int tcp_use_frto(const struct sock *sk) { const struct tcp_sock *tp = tcp_sk(sk); - /* F-RTO must be activated in sysctl and there must be some - * unsent new data, and the advertised window should allow - * sending it. - */ return (sysctl_tcp_frto && sk->sk_send_head && !after(TCP_SKB_CB(sk->sk_send_head)->end_seq, tp->snd_una + tp->snd_wnd)); } -/* RTO occurred, but do not yet enter loss state. Instead, transmit two new - * segments to see from the next ACKs whether any data was really missing. - * If the RTO was spurious, new ACKs should arrive. +/* RTO occurred, but do not yet enter Loss state. Instead, defer RTO + * recovery a bit and use heuristics in tcp_process_frto() to detect if + * the RTO was spurious. */ void tcp_enter_frto(struct sock *sk) { @@ -2489,6 +2489,30 @@ static void tcp_conservative_spur_to_response(struct tcp_sock *tp) tcp_moderate_cwnd(tp); } +/* F-RTO spurious RTO detection algorithm (RFC4138) + * + * F-RTO affects during two new ACKs following RTO. State (ACK number) is kept + * in frto_counter. When ACK advances window (but not to or beyond highest + * sequence sent before RTO): + * On First ACK, send two new segments out. + * On Second ACK, RTO was likely spurious. Do spurious response (response + * algorithm is not part of the F-RTO detection algorithm + * given in RFC4138 but can be selected separately). + * Otherwise (basically on duplicate ACK), RTO was (likely) caused by a loss + * and TCP falls back to conventional RTO recovery. + * + * Rationale: if the RTO was spurious, new ACKs should arrive from the + * original window even after we transmit two new data segments. + * + * F-RTO is implemented (mainly) in four functions: + * - tcp_use_frto() is used to determine if TCP is can use F-RTO + * - tcp_enter_frto() prepares TCP state on RTO if F-RTO is used, it is + * called when tcp_use_frto() showed green light + * - tcp_process_frto() handles incoming ACKs during F-RTO algorithm + * - tcp_enter_frto_loss() is called if there is not enough evidence + * to prove that the RTO is indeed spurious. It transfers the control + * from F-RTO to the conventional RTO recovery + */ static void tcp_process_frto(struct sock *sk, u32 prior_snd_una) { struct tcp_sock *tp = tcp_sk(sk); @@ -2497,25 +2521,16 @@ static void tcp_process_frto(struct sock *sk, u32 prior_snd_una) if (tp->snd_una == prior_snd_una || !before(tp->snd_una, tp->frto_highmark)) { - /* RTO was caused by loss, start retransmitting in - * go-back-N slow start - */ tcp_enter_frto_loss(sk); return; } if (tp->frto_counter == 1) { - /* First ACK after RTO advances the window: allow two new - * segments out. - */ tp->snd_cwnd = tcp_packets_in_flight(tp) + 2; - } else { + } else /* frto_counter == 2 */ { tcp_conservative_spur_to_response(tp); } - /* F-RTO affects on two new ACKs following RTO. - * At latest on third ACK the TCP behavior is back to normal. - */ tp->frto_counter = (tp->frto_counter + 1) % 3; } |