tcp: better comments for RTO initiallization

Commit 1b7fdd2ab585("tcp: do not use cached RTT for RTT estimation")
removes important comments on how RTO is initialized and updated.
Hopefully this patch puts those information back.

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>

1 files changed, 20 insertions, 6 deletions

diff --git a/net/ipv4/tcp_metrics.c b/net/ipv4/tcp_metrics.c
index 273ed735cca2..4a22f3e715df 100644
--- a/net/ipv4/tcp_metrics.c
+++ b/net/ipv4/tcp_metrics.c
@@ -481,13 +481,27 @@ void tcp_init_metrics(struct sock *sk)
         crtt = tcp_metric_get_jiffies(tm, TCP_METRIC_RTT);
         rcu_read_unlock();
 reset:
+        /* The initial RTT measurement from the SYN/SYN-ACK is not ideal
+         * to seed the RTO for later data packets because SYN packets are
+         * small. Use the per-dst cached values to seed the RTO but keep
+         * the RTT estimator variables intact (e.g., srtt, mdev, rttvar).
+         * Later the RTO will be updated immediately upon obtaining the first
+         * data RTT sample (tcp_rtt_estimator()). Hence the cached RTT only
+         * influences the first RTO but not later RTT estimation.
+         *
+         * But if RTT is not available from the SYN (due to retransmits or
+         * syn cookies) or the cache, force a conservative 3secs timeout.
+         *
+         * A bit of theory. RTT is time passed after "normal" sized packet
+         * is sent until it is ACKed. In normal circumstances sending small
+         * packets force peer to delay ACKs and calculation is correct too.
+         * The algorithm is adaptive and, provided we follow specs, it
+         * NEVER underestimate RTT. BUT! If peer tries to make some clever
+         * tricks sort of "quick acks" for time long enough to decrease RTT
+         * to low value, and then abruptly stops to do it and starts to delay
+         * ACKs, wait for troubles.
+         */
         if (crtt > tp->srtt) {
-                /* Initial RTT (tp->srtt) from SYN usually don't measure
-                 * serialization delay on low BW links well so RTO may be
-                 * under-estimated. Stay conservative and seed RTO with
-                 * the RTTs from past data exchanges, using the same seeding
-                 * formula in tcp_rtt_estimator().
-                 */
                 inet_csk(sk)->icsk_rto = crtt + max(crtt >> 2, tcp_rto_min(sk));
         } else if (tp->srtt == 0) {
                 /* RFC6298: 5.7 We've failed to get a valid RTT sample from