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Commit 52f20e65 authored by Yuchung Cheng's avatar Yuchung Cheng Committed by David S. Miller
Browse files

tcp: better comments for RTO initiallization 



Commit 1b7fdd2a("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: default avatarYuchung Cheng <ycheng@google.com>
Acked-by: default avatarNeal Cardwell <ncardwell@google.com>
Acked-by: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 430eda6d

net/ipv4/tcp_metrics.c

+20 −6
Original line number Diff line number Diff line
@@ -481,13 +481,27 @@ void tcp_init_metrics(struct sock *sk) 
	crtt = tcp_metric_get_jiffies(tm, TCP_METRIC_RTT);

	rcu_read_unlock();

reset:

	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().

	/* 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) {

		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