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Subject[PATCH] tcp: Lower the initial RTO to 1s as per draft RFC 2988bis-02.
Date
Draft RFC 2988bis-02 recommends that the initial RTO be lowered
from 3 seconds down to 1 second, and that in case of a timeout
during the TCP 3WHS, the RTO should fallback to 3 seconds when
data transmission begins.

Signed-off-by: Benoit Sigoure <tsunanet@gmail.com>
---
Apologies for the spam, I sent this patch from the wrong address and without
sob'ing it. I build the Linux kernel in a 15G tmpfs (it's faster this way :D)
and I lost my .git/config after a reboot.

include/net/tcp.h | 5 ++++-
net/ipv4/tcp_input.c | 13 +++++++++----
2 files changed, 13 insertions(+), 5 deletions(-)
diff --git a/include/net/tcp.h b/include/net/tcp.h
index cda30ea..274d761 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -122,7 +122,10 @@ extern void tcp_time_wait(struct sock *sk, int state, int timeo);
#endif
#define TCP_RTO_MAX ((unsigned)(120*HZ))
#define TCP_RTO_MIN ((unsigned)(HZ/5))
-#define TCP_TIMEOUT_INIT ((unsigned)(3*HZ)) /* RFC 1122 initial RTO value */
+/* The next 2 values come from Draft RFC 2988bis-02. */
+#define TCP_TIMEOUT_INIT ((unsigned)(1*HZ)) /* initial RTO value */
+#define TCP_TIMEOUT_INIT_FALLBACK ((unsigned)(3*HZ)) /* initial RTO to fallback to when
+ * a timeout happens during the 3WHS. */

#define TCP_RESOURCE_PROBE_INTERVAL ((unsigned)(HZ/2U)) /* Maximal interval between probes
* for local resources.
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index bef9f04..a36bc35 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -868,6 +868,11 @@ static void tcp_init_metrics(struct sock *sk)
{
struct tcp_sock *tp = tcp_sk(sk);
struct dst_entry *dst = __sk_dst_get(sk);
+ /* If we had to retransmit anything during the 3WHS, use
+ * the initial fallback RTO as per draft RFC 2988bis-02.
+ */
+ int init_rto = inet_csk(sk)->icsk_retransmits ?
+ TCP_TIMEOUT_INIT_FALLBACK : TCP_TIMEOUT_INIT;

if (dst == NULL)
goto reset;
@@ -890,7 +895,7 @@ static void tcp_init_metrics(struct sock *sk)
if (dst_metric(dst, RTAX_RTT) == 0)
goto reset;

- if (!tp->srtt && dst_metric_rtt(dst, RTAX_RTT) < (TCP_TIMEOUT_INIT << 3))
+ if (!tp->srtt && dst_metric_rtt(dst, RTAX_RTT) < (init_rto << 3))
goto reset;

/* Initial rtt is determined from SYN,SYN-ACK.
@@ -916,7 +921,7 @@ static void tcp_init_metrics(struct sock *sk)
tp->mdev_max = tp->rttvar = max(tp->mdev, tcp_rto_min(sk));
}
tcp_set_rto(sk);
- if (inet_csk(sk)->icsk_rto < TCP_TIMEOUT_INIT && !tp->rx_opt.saw_tstamp) {
+ if (inet_csk(sk)->icsk_rto < init_rto && !tp->rx_opt.saw_tstamp) {
reset:
/* Play conservative. If timestamps are not
* supported, TCP will fail to recalculate correct
@@ -924,8 +929,8 @@ reset:
*/
if (!tp->rx_opt.saw_tstamp && tp->srtt) {
tp->srtt = 0;
- tp->mdev = tp->mdev_max = tp->rttvar = TCP_TIMEOUT_INIT;
- inet_csk(sk)->icsk_rto = TCP_TIMEOUT_INIT;
+ tp->mdev = tp->mdev_max = tp->rttvar = init_rto;
+ inet_csk(sk)->icsk_rto = init_rto;
}
}
tp->snd_cwnd = tcp_init_cwnd(tp, dst);
--
1.7.0.4


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