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Commit 750ea2ba authored by Yuchung Cheng's avatar Yuchung Cheng Committed by David S. Miller
Browse files

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: default avatarYuchung Cheng <ycheng@google.com>
Acked-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent eed530b6

include/linux/tcp.h

+2 −1
Original line number Diff line number Diff line
@@ -372,7 +372,8 @@ struct tcp_sock { 
		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	*/

include/net/tcp.h

+3 −0
Original line number Diff line number Diff line
@@ -500,6 +500,8 @@ extern void tcp_send_delayed_ack(struct sock *sk); 


/* 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 @@ static inline void tcp_enable_early_retrans(struct tcp_sock *tp) 
{

	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)

net/ipv4/tcp_input.c

+62 −7
Original line number Diff line number Diff line
@@ -2344,6 +2344,27 @@ static inline int tcp_dupack_heuristics(const struct tcp_sock *tp) 
	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 @@ static inline int tcp_head_timedout(const struct sock *sk) 
 * 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 @@ static int tcp_time_to_recover(struct sock *sk) 
	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 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked, 
		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 @@ static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight) 
/* 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 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag) 
	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;

net/ipv4/tcp_output.c

+2 −3
Original line number Diff line number Diff line
@@ -78,9 +78,8 @@ static void tcp_event_new_data_sent(struct sock *sk, const struct sk_buff *skb) 
		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.

net/ipv4/tcp_timer.c

+5 −0
Original line number Diff line number Diff line
@@ -319,6 +319,11 @@ void tcp_retransmit_timer(struct sock *sk) 
	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;