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tcp: early retransmit: delayed fast retransmit

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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>
This commit is contained in:
Yuchung Cheng 2012-05-02 13:30:04 +00:00 committed by David S. Miller
parent eed530b6c6
commit 750ea2bafa
5 changed files with 74 additions and 11 deletions
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3
include/linux/tcp.h
View File

@ -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 */

3
include/net/tcp.h
View File

@ -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)

69
net/ipv4/tcp_input.c
View File

@ -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;

5
net/ipv4/tcp_output.c
View File

@ -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.

5
net/ipv4/tcp_timer.c
View File

@ -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;