tcp: RFC2988bis + taking RTT sample from 3WHS for the passive open side

This patch lowers the default initRTO from 3secs to 1sec per
RFC2988bis. It falls back to 3secs if the SYN or SYN-ACK packet
has been retransmitted, AND the TCP timestamp option is not on.

It also adds support to take RTT sample during 3WHS on the passive
open side, just like its active open counterpart, and uses it, if
valid, to seed the initRTO for the data transmission phase.

The patch also resets ssthresh to its initial default at the
beginning of the data transmission phase, and reduces cwnd to 1 if
there has been MORE THAN ONE retransmission during 3WHS per RFC5681.

Signed-off-by: H.K. Jerry Chu <hkchu@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 2 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index cda30ea354a2..149a415d1e0a 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -122,7 +122,13 @@ 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   */
+#define TCP_TIMEOUT_INIT ((unsigned)(1*HZ))     /* RFC2988bis initial RTO value */
+#define TCP_TIMEOUT_FALLBACK ((unsigned)(3*HZ)) /* RFC 1122 initial RTO value, now
+                                                 * used as a fallback RTO for the
+                                                 * initial data transmission if no
+                                                 * valid RTT sample has been acquired,
+                                                 * most likely due to retrans in 3WHS.
+                                                 */
 
 #define TCP_RESOURCE_PROBE_INTERVAL ((unsigned)(HZ/2U)) /* Maximal interval between probes
                                                          * for local resources.
@@ -295,7 +301,7 @@ static inline void tcp_synq_overflow(struct sock *sk)
 static inline int tcp_synq_no_recent_overflow(const struct sock *sk)
 {
         unsigned long last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
-        return time_after(jiffies, last_overflow + TCP_TIMEOUT_INIT);
+        return time_after(jiffies, last_overflow + TCP_TIMEOUT_FALLBACK);
 }
 
 extern struct proto tcp_prot;
@@ -508,6 +514,7 @@ extern void tcp_initialize_rcv_mss(struct sock *sk);
 extern int tcp_mtu_to_mss(struct sock *sk, int pmtu);
 extern int tcp_mss_to_mtu(struct sock *sk, int mss);
 extern void tcp_mtup_init(struct sock *sk);
+extern void tcp_valid_rtt_meas(struct sock *sk, u32 seq_rtt);
 
 static inline void tcp_bound_rto(const struct sock *sk)
 {