From eed530b6c67624db3f2cf477bac7c4d005d8f7ba Mon Sep 17 00:00:00 2001 From: Yuchung Cheng Date: Wed, 2 May 2012 13:30:03 +0000 Subject: tcp: early retransmit This patch implements RFC 5827 early retransmit (ER) for TCP. It reduces DUPACK threshold (dupthresh) if outstanding packets are less than 4 to recover losses by fast recovery instead of timeout. While the algorithm is simple, small but frequent network reordering makes this feature dangerous: the connection repeatedly enter false recovery and degrade performance. Therefore we implement a mitigation suggested in the appendix of the RFC that delays entering fast recovery by a small interval, i.e., RTT/4. Currently ER is conservative and is disabled for the rest of the connection after the first reordering event. A large scale web server experiment on the performance impact of ER is summarized in section 6 of the paper "Proportional Rate Reduction for TCP”, IMC 2011. http://conferences.sigcomm.org/imc/2011/docs/p155.pdf Note that Linux has a similar feature called THIN_DUPACK. The differences are THIN_DUPACK do not mitigate reorderings and is only used after slow start. Currently ER is disabled if THIN_DUPACK is enabled. I would be happy to merge THIN_DUPACK feature with ER if people think it's a good idea. ER is enabled by sysctl_tcp_early_retrans: 0: Disables ER 1: Reduce dupthresh to packets_out - 1 when outstanding packets < 4. 2: (Default) reduce dupthresh like mode 1. In addition, delay entering fast recovery by RTT/4. Note: mode 2 is implemented in the third part of this patch series. Signed-off-by: Yuchung Cheng Acked-by: Neal Cardwell Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) (limited to 'net/ipv4/tcp_input.c') diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index be8e09d2c6b1..e042cabb695e 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -99,6 +99,7 @@ int sysctl_tcp_thin_dupack __read_mostly; int sysctl_tcp_moderate_rcvbuf __read_mostly = 1; int sysctl_tcp_abc __read_mostly; +int sysctl_tcp_early_retrans __read_mostly = 2; #define FLAG_DATA 0x01 /* Incoming frame contained data. */ #define FLAG_WIN_UPDATE 0x02 /* Incoming ACK was a window update. */ @@ -906,6 +907,7 @@ static void tcp_init_metrics(struct sock *sk) if (dst_metric(dst, RTAX_REORDERING) && tp->reordering != dst_metric(dst, RTAX_REORDERING)) { tcp_disable_fack(tp); + tcp_disable_early_retrans(tp); tp->reordering = dst_metric(dst, RTAX_REORDERING); } @@ -988,6 +990,9 @@ static void tcp_update_reordering(struct sock *sk, const int metric, #endif tcp_disable_fack(tp); } + + if (metric > 0) + tcp_disable_early_retrans(tp); } /* This must be called before lost_out is incremented */ @@ -2492,6 +2497,16 @@ static int tcp_time_to_recover(struct sock *sk) tcp_is_sack(tp) && !tcp_send_head(sk)) return 1; + /* Trick#6: TCP early retransmit, per RFC5827. To avoid spurious + * retransmissions due to small network reorderings, we implement + * Mitigation A.3 in the RFC and delay the retransmission for a short + * interval if appropriate. + */ + 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 0; } -- cgit v1.2.3-55-g7522