[PATCH 3.12 08/19] tcp: fix SYNACK RTT estimation in Fast Open

Greg Kroah-Hartman
2013-11-18T18:41:32Z

From: Yuchung Cheng <ycheng@google.com>

[ Upstream commit bc15afa39ecc16f01c3389d15d8f6015a427fe85 ]

tp->lsndtime may not always be the SYNACK timestamp if a passive
Fast Open socket sends data before handshake completes. And if the
remote acknowledges both the data and the SYNACK, the RTT sample
is already taken in tcp_ack(), so no need to call
tcp_update_ack_rtt() in tcp_synack_rtt_meas() aagain.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/ipv4/tcp_input.c | 18 +++++++++++++-----
1 file changed, 13 insertions(+), 5 deletions(-)

--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2871,14 +2871,19 @@ static inline bool tcp_ack_update_rtt(st
}

/* Compute time elapsed between (last) SYNACK and the ACK completing 3WHS. */
-static void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req)
+static void tcp_synack_rtt_meas(struct sock *sk, const u32 synack_stamp)
{
struct tcp_sock *tp = tcp_sk(sk);
s32 seq_rtt = -1;

- if (tp->lsndtime && !tp->total_retrans)
- seq_rtt = tcp_time_stamp - tp->lsndtime;
- tcp_ack_update_rtt(sk, FLAG_SYN_ACKED, seq_rtt, -1);
+ if (synack_stamp && !tp->total_retrans)
+ seq_rtt = tcp_time_stamp - synack_stamp;
+
+ /* If the ACK acks both the SYNACK and the (Fast Open'd) data packets
+ * sent in SYN_RECV, SYNACK RTT is the smooth RTT computed in tcp_ack()
+ */
+ if (!tp->srtt)
+ tcp_ack_update_rtt(sk, FLAG_SYN_ACKED, seq_rtt, -1);
}

static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
@@ -5587,6 +5592,7 @@ int tcp_rcv_state_process(struct sock *s
struct request_sock *req;
int queued = 0;
bool acceptable;
+ u32 synack_stamp;

tp->rx_opt.saw_tstamp = 0;

@@ -5669,9 +5675,11 @@ int tcp_rcv_state_process(struct sock *s
* so release it.
*/
if (req) {
+ synack_stamp = tcp_rsk(req)->snt_synack;
tp->total_retrans = req->num_retrans;
reqsk_fastopen_remove(sk, req, false);
} else {
+ synack_stamp = tp->lsndtime;
/* Make sure socket is routed, for correct metrics. */
icsk->icsk_af_ops->rebuild_header(sk);
tcp_init_congestion_control(sk);
@@ -5694,7 +5702,7 @@ int tcp_rcv_state_process(struct sock *s
tp->snd_una = TCP_SKB_CB(skb)->ack_seq;
tp->snd_wnd = ntohs(th->window) << tp->rx_opt.snd_wscale;
tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
- tcp_synack_rtt_meas(sk, req);
+ tcp_synack_rtt_meas(sk, synack_stamp);

if (tp->rx_opt.tstamp_ok)
tp->advmss -= TCPOLEN_TSTAMP_ALIGNED;