tcp: prefer packet timing to TS-ECR for RTT

Prefer packet timings to TS-ecr for RTT measurements when both
sources are available. That's because broken middle-boxes and remote
peer can return packets with corrupted TS ECR fields. Similarly most
congestion controls that require RTT signals favor timing-based
sources as well. Also check for bad TS ECR values to avoid RTT
blow-ups. It has happened on production Web servers.

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

1 files changed, 18 insertions, 49 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index b531710596ec..c7398f05d12b 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2792,65 +2792,36 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
         tcp_xmit_retransmit_queue(sk);
 }
 
-void tcp_valid_rtt_meas(struct sock *sk, u32 seq_rtt)
+static inline void tcp_ack_update_rtt(struct sock *sk, const int flag,
+                                      s32 seq_rtt)
 {
-        tcp_rtt_estimator(sk, seq_rtt);
-        tcp_set_rto(sk);
-        inet_csk(sk)->icsk_backoff = 0;
-}
-EXPORT_SYMBOL(tcp_valid_rtt_meas);
+        const struct tcp_sock *tp = tcp_sk(sk);
+
+        /* Prefer RTT measured from ACK's timing to TS-ECR. This is because
+         * broken middle-boxes or peers may corrupt TS-ECR fields. But
+         * Karn's algorithm forbids taking RTT if some retransmitted data
+         * is acked (RFC6298).
+         */
+        if (flag & FLAG_RETRANS_DATA_ACKED)
+                seq_rtt = -1;
 
-/* Read draft-ietf-tcplw-high-performance before mucking
- * with this code. (Supersedes RFC1323)
- */
-static void tcp_ack_saw_tstamp(struct sock *sk, int flag)
-{
         /* RTTM Rule: A TSecr value received in a segment is used to
          * update the averaged RTT measurement only if the segment
          * acknowledges some new data, i.e., only if it advances the
          * left edge of the send window.
-         *
          * See draft-ietf-tcplw-high-performance-00, section 3.3.
-         * 1998/04/10 Andrey V. Savochkin <saw@msu.ru>
-         *
-         * Changed: reset backoff as soon as we see the first valid sample.
-         * If we do not, we get strongly overestimated rto. With timestamps
-         * samples are accepted even from very old segments: f.e., when rtt=1
-         * increases to 8, we retransmit 5 times and after 8 seconds delayed
-         * answer arrives rto becomes 120 seconds! If at least one of segments
-         * in window is lost... Voila.                          --ANK (010210)
          */
-        struct tcp_sock *tp = tcp_sk(sk);
+        if (seq_rtt < 0 && tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
+                seq_rtt = tcp_time_stamp - tp->rx_opt.rcv_tsecr;
 
-        tcp_valid_rtt_meas(sk, tcp_time_stamp - tp->rx_opt.rcv_tsecr);
-}
-
-static void tcp_ack_no_tstamp(struct sock *sk, u32 seq_rtt, int flag)
-{
-        /* We don't have a timestamp. Can only use
-         * packets that are not retransmitted to determine
-         * rtt estimates. Also, we must not reset the
-         * backoff for rto until we get a non-retransmitted
-         * packet. This allows us to deal with a situation
-         * where the network delay has increased suddenly.
-         * I.e. Karn's algorithm. (SIGCOMM '87, p5.)
-         */
-
-        if (flag & FLAG_RETRANS_DATA_ACKED)
+        if (seq_rtt < 0)
                 return;
 
-        tcp_valid_rtt_meas(sk, seq_rtt);
-}
+        tcp_rtt_estimator(sk, seq_rtt);
+        tcp_set_rto(sk);
 
-static inline void tcp_ack_update_rtt(struct sock *sk, const int flag,
-                                      const s32 seq_rtt)
-{
-        const struct tcp_sock *tp = tcp_sk(sk);
-        /* Note that peer MAY send zero echo. In this case it is ignored. (rfc1323) */
-        if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
-                tcp_ack_saw_tstamp(sk, flag);
-        else if (seq_rtt >= 0)
-                tcp_ack_no_tstamp(sk, seq_rtt, flag);
+        /* RFC6298: only reset backoff on valid RTT measurement. */
+        inet_csk(sk)->icsk_backoff = 0;
 }
 
 /* Compute time elapsed between (last) SYNACK and the ACK completing 3WHS. */
@@ -2989,8 +2960,6 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
                         if (sacked & TCPCB_SACKED_RETRANS)
                                 tp->retrans_out -= acked_pcount;
                         flag |= FLAG_RETRANS_DATA_ACKED;
-                        ca_seq_rtt = -1;
-                        seq_rtt = -1;
                 } else {
                         ca_seq_rtt = now - scb->when;
                         last_ackt = skb->tstamp;