tcp: early retransmit: delayed fast retransmit
Implementing the advanced early retransmit (sysctl_tcp_early_retrans==2).
Delays the fast retransmit by an interval of RTT/4. We borrow the
RTO timer to implement the delay. If we receive another ACK or send
a new packet, the timer is cancelled and restored to original RTO
value offset by time elapsed. When the delayed-ER timer fires,
we enter fast recovery and perform fast retransmit.
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 7859b41..d9b42c5be 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -372,7 +372,8 @@
repair : 1,
unused : 1;
u8 repair_queue;
- u8 do_early_retrans:1;/* Enable RFC5827 early-retransmit */
+ u8 do_early_retrans:1,/* Enable RFC5827 early-retransmit */
+ early_retrans_delayed:1; /* Delayed ER timer installed */
/* RTT measurement */
u32 srtt; /* smoothed round trip time << 3 */
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 685437a..5283aa4 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -500,6 +500,8 @@
/* tcp_input.c */
extern void tcp_cwnd_application_limited(struct sock *sk);
+extern void tcp_resume_early_retransmit(struct sock *sk);
+extern void tcp_rearm_rto(struct sock *sk);
/* tcp_timer.c */
extern void tcp_init_xmit_timers(struct sock *);
@@ -805,6 +807,7 @@
{
tp->do_early_retrans = sysctl_tcp_early_retrans &&
!sysctl_tcp_thin_dupack && sysctl_tcp_reordering == 3;
+ tp->early_retrans_delayed = 0;
}
static inline void tcp_disable_early_retrans(struct tcp_sock *tp)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index e042cab..7096790 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2344,6 +2344,27 @@
return tcp_is_fack(tp) ? tp->fackets_out : tp->sacked_out + 1;
}
+static bool tcp_pause_early_retransmit(struct sock *sk, int flag)
+{
+ struct tcp_sock *tp = tcp_sk(sk);
+ unsigned long delay;
+
+ /* Delay early retransmit and entering fast recovery for
+ * max(RTT/4, 2msec) unless ack has ECE mark, no RTT samples
+ * available, or RTO is scheduled to fire first.
+ */
+ if (sysctl_tcp_early_retrans < 2 || (flag & FLAG_ECE) || !tp->srtt)
+ return false;
+
+ delay = max_t(unsigned long, (tp->srtt >> 5), msecs_to_jiffies(2));
+ if (!time_after(inet_csk(sk)->icsk_timeout, (jiffies + delay)))
+ return false;
+
+ inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, delay, TCP_RTO_MAX);
+ tp->early_retrans_delayed = 1;
+ return true;
+}
+
static inline int tcp_skb_timedout(const struct sock *sk,
const struct sk_buff *skb)
{
@@ -2451,7 +2472,7 @@
* Main question: may we further continue forward transmission
* with the same cwnd?
*/
-static int tcp_time_to_recover(struct sock *sk)
+static int tcp_time_to_recover(struct sock *sk, int flag)
{
struct tcp_sock *tp = tcp_sk(sk);
__u32 packets_out;
@@ -2505,7 +2526,7 @@
if (tp->do_early_retrans && !tp->retrans_out && tp->sacked_out &&
(tp->packets_out == (tp->sacked_out + 1) && tp->packets_out < 4) &&
!tcp_may_send_now(sk))
- return 1;
+ return !tcp_pause_early_retransmit(sk, flag);
return 0;
}
@@ -3172,7 +3193,7 @@
if (icsk->icsk_ca_state <= TCP_CA_Disorder)
tcp_try_undo_dsack(sk);
- if (!tcp_time_to_recover(sk)) {
+ if (!tcp_time_to_recover(sk, flag)) {
tcp_try_to_open(sk, flag);
return;
}
@@ -3271,16 +3292,47 @@
/* Restart timer after forward progress on connection.
* RFC2988 recommends to restart timer to now+rto.
*/
-static void tcp_rearm_rto(struct sock *sk)
+void tcp_rearm_rto(struct sock *sk)
{
- const struct tcp_sock *tp = tcp_sk(sk);
+ struct tcp_sock *tp = tcp_sk(sk);
if (!tp->packets_out) {
inet_csk_clear_xmit_timer(sk, ICSK_TIME_RETRANS);
} else {
- inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
- inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
+ u32 rto = inet_csk(sk)->icsk_rto;
+ /* Offset the time elapsed after installing regular RTO */
+ if (tp->early_retrans_delayed) {
+ struct sk_buff *skb = tcp_write_queue_head(sk);
+ const u32 rto_time_stamp = TCP_SKB_CB(skb)->when + rto;
+ s32 delta = (s32)(rto_time_stamp - tcp_time_stamp);
+ /* delta may not be positive if the socket is locked
+ * when the delayed ER timer fires and is rescheduled.
+ */
+ if (delta > 0)
+ rto = delta;
+ }
+ inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, rto,
+ TCP_RTO_MAX);
}
+ tp->early_retrans_delayed = 0;
+}
+
+/* This function is called when the delayed ER timer fires. TCP enters
+ * fast recovery and performs fast-retransmit.
+ */
+void tcp_resume_early_retransmit(struct sock *sk)
+{
+ struct tcp_sock *tp = tcp_sk(sk);
+
+ tcp_rearm_rto(sk);
+
+ /* Stop if ER is disabled after the delayed ER timer is scheduled */
+ if (!tp->do_early_retrans)
+ return;
+
+ tcp_enter_recovery(sk, false);
+ tcp_update_scoreboard(sk, 1);
+ tcp_xmit_retransmit_queue(sk);
}
/* If we get here, the whole TSO packet has not been acked. */
@@ -3729,6 +3781,9 @@
if (after(ack, tp->snd_nxt))
goto invalid_ack;
+ if (tp->early_retrans_delayed)
+ tcp_rearm_rto(sk);
+
if (after(ack, prior_snd_una))
flag |= FLAG_SND_UNA_ADVANCED;
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 834e89f..d947330 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -78,9 +78,8 @@
tp->frto_counter = 3;
tp->packets_out += tcp_skb_pcount(skb);
- if (!prior_packets)
- inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
- inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
+ if (!prior_packets || tp->early_retrans_delayed)
+ tcp_rearm_rto(sk);
}
/* SND.NXT, if window was not shrunk.
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 34d4a02..e911e6c 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -319,6 +319,11 @@
struct tcp_sock *tp = tcp_sk(sk);
struct inet_connection_sock *icsk = inet_csk(sk);
+ if (tp->early_retrans_delayed) {
+ tcp_resume_early_retransmit(sk);
+ return;
+ }
+
if (!tp->packets_out)
goto out;