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-rw-r--r--net/ipv4/tcp_metrics.c26
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