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tcp: remove forward retransmit feature 

Forward retransmit is an esoteric feature in RFC3517 (condition(3)
in the NextSeg()). Basically if a packet is not considered lost by
the current criteria (# of dupacks etc), but the congestion window
has room for more packets, then retransmit this packet.

However it actually conflicts with the rest of recovery design. For
example, when reordering is detected we want to be conservative
in retransmitting packets but forward-retransmit feature would
break that to force more retransmission. Also the implementation is
fairly complicated inside the retransmission logic inducing extra
iterations in the write queue. With RACK losses are being detected
timely and this heuristic is no longer necessary. There this patch
removes the feature.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Yuchung Cheng 2017-01-12 22:11:38 -08:00 committed by David S. Miller
parent 89fe18e44f
commit 840a3cbe89
3 changed files with 3 additions and 64 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/tcp.h
View File

@ -307,7 +307,6 @@ struct tcp_sock {
*/
int lost_cnt_hint;
u32 retransmit_high; /* L-bits may be on up to this seqno */
u32 prior_ssthresh; /* ssthresh saved at recovery start */
u32 high_seq; /* snd_nxt at onset of congestion */

5
net/ipv4/tcp_input.c
View File

@ -916,10 +916,6 @@ static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb)
before(TCP_SKB_CB(skb)->seq,
TCP_SKB_CB(tp->retransmit_skb_hint)->seq))
tp->retransmit_skb_hint = skb;
if (!tp->lost_out ||
after(TCP_SKB_CB(skb)->end_seq, tp->retransmit_high))
tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
}
/* Sum the number of packets on the wire we have marked as lost.
@ -1983,7 +1979,6 @@ void tcp_enter_loss(struct sock *sk)
TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_ACKED;
TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
tp->lost_out += tcp_skb_pcount(skb);
tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
}
}
tcp_verify_left_out(tp);

61
net/ipv4/tcp_output.c
View File

@ -2831,36 +2831,6 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
return err;
}
/* Check if we forward retransmits are possible in the current
* window/congestion state.
*/
static bool tcp_can_forward_retransmit(struct sock *sk)
{
const struct inet_connection_sock *icsk = inet_csk(sk);
const struct tcp_sock *tp = tcp_sk(sk);
/* Forward retransmissions are possible only during Recovery. */
if (icsk->icsk_ca_state != TCP_CA_Recovery)
return false;
/* No forward retransmissions in Reno are possible. */
if (tcp_is_reno(tp))
return false;
/* Yeah, we have to make difficult choice between forward transmission
* and retransmission... Both ways have their merits...
*
* For now we do not retransmit anything, while we have some new
* segments to send. In the other cases, follow rule 3 for
* NextSeg() specified in RFC3517.
*/
if (tcp_may_send_now(sk))
return false;
return true;
}
/* This gets called after a retransmit timeout, and the initially
* retransmitted data is acknowledged. It tries to continue
* resending the rest of the retransmit queue, until either
@ -2875,24 +2845,16 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *skb;
struct sk_buff *hole = NULL;
u32 max_segs, last_lost;
u32 max_segs;
int mib_idx;
int fwd_rexmitting = 0;
if (!tp->packets_out)
return;
if (!tp->lost_out)
tp->retransmit_high = tp->snd_una;
if (tp->retransmit_skb_hint) {
skb = tp->retransmit_skb_hint;
last_lost = TCP_SKB_CB(skb)->end_seq;
if (after(last_lost, tp->retransmit_high))
last_lost = tp->retransmit_high;
} else {
skb = tcp_write_queue_head(sk);
last_lost = tp->snd_una;
}
max_segs = tcp_tso_segs(sk, tcp_current_mss(sk));
@ -2915,31 +2877,14 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
*/
segs = min_t(int, segs, max_segs);
if (fwd_rexmitting) {
begin_fwd:
if (!before(TCP_SKB_CB(skb)->seq, tcp_highest_sack_seq(tp)))
break;
mib_idx = LINUX_MIB_TCPFORWARDRETRANS;
} else if (!before(TCP_SKB_CB(skb)->seq, tp->retransmit_high)) {
tp->retransmit_high = last_lost;
if (!tcp_can_forward_retransmit(sk))
break;
/* Backtrack if necessary to non-L'ed skb */
if (hole) {
skb = hole;
hole = NULL;
}
fwd_rexmitting = 1;
goto begin_fwd;
if (tp->retrans_out >= tp->lost_out) {
break;
} else if (!(sacked & TCPCB_LOST)) {
if (!hole && !(sacked & (TCPCB_SACKED_RETRANS|TCPCB_SACKED_ACKED)))
hole = skb;
continue;
} else {
last_lost = TCP_SKB_CB(skb)->end_seq;
if (icsk->icsk_ca_state != TCP_CA_Loss)
mib_idx = LINUX_MIB_TCPFASTRETRANS;
else