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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>
This commit is contained in:
Yuchung Cheng 2013-09-03 14:14:35 -07:00 committed by David S. Miller
parent 430eda6d6d
commit 52f20e655d
1 changed files with 20 additions and 6 deletions
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26
net/ipv4/tcp_metrics.c
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@ -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